Structure and functional properties of ... · Structure and functional properties of...

Structure and functional properties of

heteropolyoxomolybdates supported on silica

SBA-15

vorgelegt von

Dipl.-Chem.

Rafael Zubrzycki

geb. in Berent

von der Fakultät II - Mathematik und Naturwissenschaften

der Technischen Universität Berlin

zur Erlangung des akademischem Grades

Doktor der Naturwissenschaften

-Dr. rer. nat.-

genehmigte Dissertation

Promotionsausschuss

Vorsitzender: Prof. Dr. rer. nat Thomas Friedrich

Berichter/Gutachter: Prof. Dr. rer. nat. Thorsten Ressler

Berichter Gutachter: Prof. Dr. rer. nat. Malte Behrens

Tag der wissenschaftlichen Aussprache: 20. März 2015

Berlin 2015

Abstract

Heteropolyoxomolybdates with Keggin structure (HPOM) were supported on SBA-15 and

introduced as model catalysts for investigating structure-property correlations during

selective propene oxidation. The chemical composition of the HPOM was varied by

substituting molybdenum with vanadium or tungsten. Subsequently, the various

heteropolyoxomolybdates were supported on nanostructured silica SBA-15. Additionally,

unsubstituted HPOM were deposited on SBA-15 with different pore radii. Unsupported

and supported heteropolyoxomolybdates were characterized by ex situ techniques yielding

a detailed knowledge about structure and chemical composition of the model catalysts.

Afterwards, the unsupported and supported heteropolyoxomolybdates were characterized

by in situ techniques and tested for their catalytic properties in the partial oxidation of

propene. HPOM supported on SBA-15 were investigated to elucidate the influence of

addenda atoms, the silanol groups of SBA-15, the pore radii of SBA-15, and the HPOM

loading on the resulting structures forming during propene oxidation conditions.

The initial Keggin structure was retained after supporting HPOM on SBA-15. The removal

of adsorbed water and a following dehydroxylation of silanol groups of SBA-15 lead to a

destabilizing effect on the Keggin ion during propene oxidation conditions. Subsequently,

the HPOM supported on SBA-15 formed a mixture of [MoOx] and [(V,W)Ox] species on

the support material under catalytic conditions. The [MoO6] units were influenced by the

structural evolution of neighboring [VO6] and [WO6] units of the initial Keggin ion

structure. The structural evolution of the [MoOx] and [(V,W)Ox] species lead to

predominantly tetrahedral [MoO4] and [VO4] units in vanadium substituted HPOM and to

predominantly octahedral [MoO6] and [WO6] units in tungsten substituted HPOM. The

formation of [MoO4] units or [MoO6] depended on the degree of vanadium or tungsten

substitution. The resulted [MOx] (M = V, W) units were in close vicinity to the [MoOx]

species. The various structures resulting for supported HPOM exhibited an influence on

the catalytic activity. The reaction rates at similar propene conversions for supported

HPOM decreased with higher [MoO4]/[MoO6] ratio. The higher reaction rate resulted in an

increased formation of total oxidation products. Hence, samples with an increased

[MoO4]/[MoO6] ratio exhibited an increased selectivity towards C3 oxidation products.

Zusammenfassung

Heteropolyoxomolybdate mit Keggin Struktur (HPOM) geträgert auf SBA-15 wurden als

Modellkatalysatoren für die selektive Propenoxidation verwendet und hinsichtlich ihrer

Struktur-Eigenschafts-Beziehungen untersucht. Die chemische Zusammensetzung der

HPOM wurde durch Substitution von Molybdän mit den sog. Addenda-Atomen Vanadium

oder Wolfram variiert. Anschließend wurden die verschiedenen Heteropolyoxomolybdate

auf SBA-15 geträgert. Zusätzlich wurden unsubstituierte HPOM auf SBA-15 mit

unterschiedlichen Porenradien geträgert. Die ungeträgerten und geträgerten HPOM wurden

charakterisiert, um detaillierte Informationen über die Struktur und die chemische

Zusammensetzung der Modellkatalysatoren zu erhalten. Danach wurden die ungeträgerten

und geträgerten HPOM unter Reaktionsbedingungen charakterisiert und auf ihre

katalytischen Eigenschaften bei der partiellen Oxidation von Propen getestet. Die auf SBA-

15 geträgerten HPOM wurden untersucht, um den Einfluss der Addenda-Atome, der

Silanolgruppen des SBA-15, der unterschiedlichen Porenradien des SBA- 15 und der

HPOM-Beladung auf die sich unter Propenoxidationsbedingungen bildenden Strukturen

aufzuklären.

Die Kegginstruktur blieb nach der Trägerung der HPOM auf SBA-15 erhalten. Die

Entfernung von adsorbiertem Wasser und eine folgende Dehydroxylierung der

Silanolgruppen des SBA-15 führten zu einer Destabilisierung der Keggin-Ionen unter

Propenoxidationsbedingungen. Anschließend bildeten die geträgerten HPOM unter

katalytischen Bedingungen eine Mischung aus [MoOx]- und [(V,W)Ox]-Spezies auf dem

Trägermaterial. Die [MoO6]-Einheiten wurden durch die strukturelle Entwicklung der

benachbarten [VO6]- und [WO6]-Einheiten aus der ursprünglichen Kegginstruktur

beeinflusst. Die Strukturentwicklung der [MoOx]- und [(V,W)Ox]-Spezies führte zu

überwiegend tetraedrischen [MoO4]- und [VO4]-Einheiten in den vanadiumsubstituierten

HPOM und zu überwiegend oktaedrischen [MoO6]- und [WO6]-Einheiten in den

wolframsubstitutierten HPOM. Die Bildung der [MoO4]- oder [MoO6]-Einheiten waren

von der Anzahl der Addenda-Atome pro Keggin-Ion abhängig. Die [MOx]-Einheiten

(M = V, W) befanden sich in unmittelbarer Nähe zu den [MoOx]-Einheiten. Die

verschiedenen Strukturen, die sich aus den geträgerten HPOM bildeten, zeigten einen

Einfluss auf die katalytische Aktivität. Die Reaktionsrate bei ähnlichen Propenumsätzen

nahm für die geträgerte HPOM mit höherem [MoO4]/[MoO6] Verhältnis zu. Die höhere

Reaktionsgrate führten zu einer erhöhten Bildung von Totaloxidationsprodukten. Die

Proben mit einem erhöhten [MoO4]/[MoO6] Verhältnis zeigte eine erhöhte Selektivität

gegen C3 Oxidationsprodukten.

VII

Contents

Abstract ................................................................................................................................ III

Zusammenfassung ................................................................................................................ V

Contents .............................................................................................................................. VII

Abbreviations ....................................................................................................................... X

1 Introduction ................................................................................................................ 1

1.1 Motivation .................................................................................................................. 1

1.2 Heteropolyoxomolybdates in partial oxidation reactions .......................................... 3

1.3 Supported heteropolyoxomolybdates partial oxidation reactions .............................. 5

1.4 Outline of the work .................................................................................................... 7

2 Characterization Methods .......................................................................................... 8

2.1 Structural Characterization ........................................................................................ 8

2.1.1 Powder X-ray diffraction ........................................................................................... 8

2.1.2 Vibrational spectroscopy ........................................................................................... 9

2.1.3 Physisorption ........................................................................................................... 10

2.1.4 X-ray absorption spectroscopy ................................................................................ 11

2.1.5 Nuclear magnetic resonance spectroscopy .............................................................. 13

2.2 Element Analysis ..................................................................................................... 14

2.2.1 X-ray fluorescence (XRF) spectroscopy.................................................................. 14

2.2.2 Atomic absorption spectroscopy (AAS) .................................................................. 15

2.3 Thermal analysis ...................................................................................................... 15

2.4 Catalytic Characterization ....................................................................................... 15

3 Charaterization of bulk P(V,W)xMo12-x (x = 0, 1 ,2) ............................................... 17

3.1 Sample Preparation .................................................................................................. 17

3.2 Sample characterization ........................................................................................... 19

3.3 Ex situ characterization of P(V,W)xMo12-x (x = 0, 1, 2) .......................................... 24

3.3.1 Quantification of metal loading by XRF ................................................................. 24

3.3.2 Long-range structure of as-prepared P(V,W)xMo12-x (x = 0, 1, 2) .......................... 24

3.4 Short-range order structural characterization of P(V,W)xMo12-x (x = 0, 1, 2) ......... 26

3.5 In situ Characterization of bulk heteropolyacids ..................................................... 32

3.5.1 In situ XRD of PMo12-x(V,W)x x = 0, 1, 2 during oxidation conditions .................. 32

3.5.2 Functional characterization of bulk HPOM ............................................................. 36

VIII

3.6 Summary.................................................................................................................. 41

4 Characterization of P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) (10 wt.% Mo) ............... 42

4.1 Sample Preparation .................................................................................................. 42

4.2 Sample characterization ........................................................................................... 43

4.3 Results of the Characterization ................................................................................ 45

4.3.1 Long-range structure of as-prepared P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) ............ 45

4.3.2 Short-range order structural characterization of as-prepared P(V,W)xMo12-x- ...........

SBA-15 (x = 0, 1, 2) ................................................................................................ 48

4.4 Conclusion ............................................................................................................... 55

5 Characterization of PVxMo12-x-SBA-15 (x = 1, 2) under catalytic conditions ........ 56

5.1 Experimental............................................................................................................ 56

5.1.1 Sample Characterization .......................................................................................... 56

5.1.2 Sample preparation .................................................................................................. 59

5.2 Structural characterization of PVxMo12-x-SBA-15 (x = 1, 2) under ...........................

catalytic conditions .................................................................................................. 59

5.2.1 Local structure in activated PVxMo12-x-SBA-15 (x = 0, 1, 2) and a ...........................

reference V2Mo10Ox-SBA-15 under catalytic conditions ........................................ 62

5.2.2 Local structure of P in activated PV2Mo10SBA-15 under catalytic conditions ....... 68

5.2.3 Structure directing effects of vanadium and the support material on the structure ....

of activated PV2Mo10-SBA-15 under catalytic conditions ..................................... 70

5.3 Functional characterization of PVxMo12-x-SBA-15 (x = 0, 1, 2) ............................. 71

5.3.1 Reducibility ............................................................................................................. 71

5.3.2 Catalytic performance ............................................................................................. 72

5.3.3 Influence of phosphorus species on catalytic activity ............................................. 74

5.4 Summary.................................................................................................................. 76

6 Characterization of PWxMo12-x-SBA-15 (x = 1, 2) under catalytic conditions ....... 77

6.1 Experimental............................................................................................................ 77

6.1.1 Sample Characterization .......................................................................................... 77

6.1.2 Sample preparation .................................................................................................. 80

6.2 Structural evolution of PWxMo12-x-SBA-15 (x = 1, 2) under catalytic conditions . 80

6.2.1 Local structure in activated PWxMo12-x-SBA-15 (x = 0, 1, 2) and a ..........................

reference W2Mo10Ox-SBA-15 under catalytic conditions ....................................... 86

6.2.2 Comparison of the local structure around Mo centers in act. PW2Mo10-SBA-15 ......

and a reference act. W2Mo10Ox-SBA-15 under catalytic conditions ....................... 90

IX

6.3 Functional characterization of PWxMo12-x-SBA-15 (x= 1, 2) ................................. 94

6.3.1 Reducibility .............................................................................................................. 94

6.3.2 Catalytic performance .............................................................................................. 95

6.4 Summary .................................................................................................................. 98

7 Characterization of PMo12 supported on SBA-15 with tailored pore radii ............. 99

7.1 Experimental .......................................................................................................... 100

7.2 Structure of the support materials .......................................................................... 104

7.3 Characterization of PMo12-SBA-15 (10, 14, 19 nm) ............................................. 106

7.4 Structural evolution of PMo12- SBA-15 (10, 14, 19 nm) under catalytic ...................

conditions ............................................................................................................... 108

7.5 Functional characterization of PVxMo12-x-SBA-15 (x= 1, 2) ................................ 113

7.5.1 Influence of the resulting structures to catalytic activity ....................................... 113

7.6 Summary ................................................................................................................ 115

8 Characterization of PVMo11 supported on SBA-15 with different metal loading . 116

8.1 Experimental .......................................................................................................... 117

8.2 Characterization of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, and .......................

1 wt.% Mo) ............................................................................................................ 120

8.3 Structural evolution of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, and .................

1 wt.% Mo) under catalytic conditions .................................................................. 123

8.4 Functional characterization of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, ............

and 1 wt.% Mo) ..................................................................................................... 128

8.4.1 Reducibility ............................................................................................................ 128

8.4.2 Influence of the resulting structure on catalytic activity........................................ 129

8.5 Summary ................................................................................................................ 132

9 General discussion and Summary .......................................................................... 133

9.1 Structure directing effect of the support material .................................................. 133

9.2 Structure directing effects of the addenda atoms ................................................... 135

9.3 Structure activity relationships .............................................................................. 137

10 Conclusions ............................................................................................................ 142

11 References .............................................................................................................. 148

12 Appendix ................................................................................................................ 164

Danksagung ........................................................................................................................ XII

X

Abbreviations

act. activated

AHM ammonium heptamolybdate

BET Brunauer-Emmet-Teller

BJH Barrett-Joyner-Halenda

cf. compare (Latin "confer")

DTG derivative thermogravimetric

e.g. or example (Latin "exempli gratia")

eq. equation

et al. and others (Latin "et alii")

EXAFS extended X-ray absorption fine structure

exp. experimental

FID flame ionization detector

FT Fourier transformed

HASYLAB Hamburg Synchrotron Radiation Laboratory

HPOM heteropolyoxomolybdate with Keggin structure

i.e. that is (Latin "id est")

IR infrared

IUPAC International Union of Pure and Applied Chemistry

m/e mass-charge ratio

MoO3-SBA-15 molybdenum oxides supported on SBA-15

nom. nominal

Norm. normalized

PMo12 H3[PMo12O40

PVMo11 H4[PVMo11O40]

PV2Mo10 H5[PV2Mo10O40]

PWMo11 H3[PWMo11O40]

PW2Mo10 H3[PW2Mo10O40]

PZC point of zero charge

RT room temperature

SBA-15 mesoporous silica (Santa Barbara amorphous type material No. 15)

SDA structure-directing agent

TA thermal analysis

XI

TCD thermal conductivity detector

TG thermogravimetry

V2Mo10Ox-SBA-15 vanadium and molybdenum oxides supported on SBA-15

W2Mo10Ox-SBA-15 tungsten and molybdenum oxides supported on SBA-15

wt.% weight percent

XAFS X-ray absorption fine structure

XANES X-ray absorption near edge structure

XAS X-ray absorption spectroscopy

XRD X-ray diffraction

XRF X-ray fluorescence

1

1 Introduction

1.1 Motivation

Heteregenous catalyzed reactions play a fundamental role in the production of industrial

organic chemicals and intermediates. Selective catalytic oxidation processes generate

approximately one quarter of the value produced world wide by catalytic processes.[1] The

products include such intermediates as acrolein, acrylic acid, acrylonitrile, methacrylic

acid, MTBE, maleic anhydride, phthalic anhydride, ethylene and propylene oxide.[2] One

important industrial process is the selective oxidation of propene towards acrolein and

acrylic acid.[3,4] Acrylic acid is an important raw material in the fine chemicals industry.

The acid and its esters are important monomers for the preparation of polymers and are

used in the manufacture of paints and adhesives, in the treatment of paper and textile, as

well as superabsorbent.[5] The industrial production of acrylic acid is a two step process.

In the first reaction, propene is oxidized using a bismuth molybdate based catalyst resulting

in the production of acrolein. In the second reaction, acrolein is oxidized using a bismuth

molybdate based catalyst mixed with additional metal oxides with transitions metals such

as vanadium or tungsten[3]:

propene

acrolein

acrylic acid

The product yield of acrylic acid in this process is about 90%.[6] Increasing the product

yield using new or improved catalyst is of particular interest, because the industrial

processes can be made more economical and sustainable. Molybdenum based catalyst are

of particular interest as they are often used in partial oxidation catalysts for industrial

application.[7] The catalyst may be improved by varying the chemical complexity.

Additional metals such as W, Nb, or V stabilize characteristic crystallographic structures

which lead to oxidation catalysts with improved activity and selectivity.[8,9] However, the

influence of structural variety and chemical complexity in the mixed oxide systems on

catalytic performance is difficult to distinguish. Moreover, the functionality of individual

metal centers or particular structural motifs of these highly active mixed oxide catalysts

2

can hardly be determined. Hence, model systems are required which combine structural

invariance with compositional variety or vice versa.[10–12] Heteropolyoxomolybdates

(HPOM) with Keggin structure exhibit a broad compositional range while maintaining

their characteristic structural motif.[13–16] Therefore, substituting Mo atoms with addenda

atoms (i.e. V, W, Nb) make Keggin type HPOM suitable model system to study structure

activity relationships. A challenge within the study of structure-activity relationships of

those catalysts is to distinguish between the bulk and surface structures of this materials.

Catalytic reaction occurs on the crystalline surface of the bulk compounds. Therefore, the

majority of the bulk compound is not involved in the catalytic reaction. This leads to an

analytical problem, because the average of the bulk compound is measured with most

analytical methods. Therefore, a study of structure-activity relationships of surface

structures corresponding to the "real" catalyst is not feasible. An approach to solve this

analytic problem is the use of supported metal oxide catalysts.[17,18] Supported catalytic

species possesses high dispersions and an improved surface to bulk ratio. Hence,

differentiating between bulk and surface structures is no longer necessary. Therefore,

structure activity relationships can be readily deduced from the characteristic oxide species

observed on the support material under catalytic reaction conditions. Suitable support

materials for catalysts posseses a large surface area and a homogenous internal pore

structure with sufficiently large pores. The precursors are ideally highly dispersed on the

support material. Hence, all the centers are accessible on the surface and are involved in

the catalytic reaction. Furthermore, the support may interact with the precursor to stabilize

particular structural motifs without affecting the catalytic reaction.[19–21] Nanostructured

SiO2 materials such as SBA-15 represent suitable support systems for metal oxide

catalysts.[22–25] Additionally, the mechanical, thermal, and hydrothermal stability

improves SBA-15 as support material during catalytic conditions like high temperatures or

in the presence of steam.[26] The deposition of vanadium or tungsten substituted HPOM

on SBA-15 lead to well dispersed HPOM Keggin ions. HPOM supported on SBA-15 can

be used to vary the chemical composition while maintaining good accessibility of the

supported molybdenum based catalyst.[27] Therefore, a study of structure-activity

relationships of surface structures corresponding to the "real" catalyst was possible.

3

1.2 Heteropolyoxomolybdates in partial oxidation reactions

The first characterization of the Keggin structure was performed by J.F- Keggin at 1934

using XRD.[28] Fig. 1-1 depicts the Keggin ion structure typically represented by the

formula [XM12O40]x-8

where X is the central atom (Si4+

, P5+

, etc.), x is the oxidation state

and M is the metal ion (Mo6+

or W6+

). The smallest structural units of the Keggin structure

are metal-oxygen octahedra (MO6) and are called primary structure. The octahedra are

arrranged in four M3O13 (triad) groups surrounding the central tetrahedron XO4. The

Keggin ion structure is called secondary structure and the arrangement of the Keggin ion in

a crystal structure represents the tertiary structure.[29]

Heteropolyoxomolybdates with Keggin structure are able to catalyze a variety of reactions

as homogeneous or heterogeneous catalyst (Table 1-1). Thus, HPOM are part of current

investigations in catalysis research.[30–33] Substituted HPOM were intensely investigated

in the past with regard to partial oxidation reactions.[13–16,34,35] Li et al. showed, that

introducing vanadium in the Keggin ion structure of HPOM lead to an enhanced

P

Mo

O

Keggin structure triad

Fig. 1-1: (left) Triad of the Keggin structure (Mo3O13); (right) Keggin structure (secondary

structure).

4

Table 1-1: Summary of reactions, which are catalyzed by Heteropolyoxomolybdates with Keggin

structure.

reactions catalysing by HPOM[34–38]

isomerisation of alkanes polymerisation of THF

MeOH to olefins Diels-Alder Reaction

alkylation of paraffins oxidation of alkanes

oligomerisation of alkenes oxidation of alkenes

Friedel-Crafts Acylation hydrogenation of alkenes

Beckmann rearrangement methacrolein to methacrylic acid

catalytic activity in partial oxidation of propane.[36] Bondavera et al. investigated the

influence of vanadium on the catalytic activity in ammoxidation of methylpyrazine.[35]

The vanadium substituted HPOM showed an enhanced catalytic activity depending on the

degree of substitution. Comparable correlations between catalytic activity and the degree

of vanadium substitution were found for the oxidation of acrolein, isobutylene, and

isobutane.[37–39] Ressler et al. investigated in various studies the structural evolution of

vanadium substituted HPOM during propene oxidation.[13–16] H4[PVMo11O40] (PVMo11)

loses crystal water during treatment under propene oxidation conditions in the temperature

range from 373 to 573 K.[15] The release of crystal water is followed by partial

decomposition, reduction of the average Mo valence, and formation of cubic HPOM

(Mox[PVMo11-xO40]) at 573 K. The formation of cubic Mox[PVMo11-xO40] with Mo centers

outside the Keggin ion structure and V centers remaining in a lacunary Keggin ion

coincides with the onset of catalytic activity.[15] Niobium substituted HPOM

(H4[PNbMo11O40]) formed the characteristic cubic HPOM structure, similar to the

structural evolution of H3[PMo12O40], H4[PVMo11O40], and H5[PV2Mo10O40].[14] In

contrast to H3+x[PVxMo12-xO40] (x = 0, 1, 2) the lacunary Keggin ion decompose rapidly

towards MoO3 at about 673 K. The decomposition process correlated with a decrease in

catalytic activity.[14] Mestl et al. investigated the thermal induced decomposition of

PVMo11.[40] Upon loss of crystal water vanadyl and molybdenyl species are expelled from

the Keggin ion structure forming the lacunary Keggin ion. This defective structure further

disintegrated to triads and finally condensed to the thermodinamically stable MoO3.[40]

5

Therefore, it may be assumed, that the vanadium and niobium substitution in HPOM have

a structure directing effect, stabilizing a structure active in oxidation of propene. However,

the role of addenda atoms is still under discussion, because addenda atoms may have both

a structure directing effect and/ or a functional effect during propene oxidation conditions.

1.3 Supported heteropolyoxomolybdates in partial oxidation reactions

Support material

Mesoporous material SBA-15 was fist synthesized in 1998.[22,23] The SBA-15 structure

composed of hexagonal channels has a high surface area and a narrow pore size

distribution. Supramolecular aggregates are used for the synthesis of mesoporous systems

as structure-directing agents (SDAs).[42,43] In the synthesis of SBA-15 a block copolymer

is used as SDA.[22,23] The self-organization of the block copolymer promotes the

formation of a silica based inorganic network around the organic aggregates. Afterwards,

the organic template is removed through a calcination process. Fig. 1-2 illustrates a

schematic representation of the preparation of SBA-15.

The surface area and pore width are tailored by the preparation procedure. The typical

synthesis of SBA-15 leads to surface areas between 600 and 1000 m2/g and pore diameters

between 5 and 10 nm.[22] The pore radius is tunable with swelling agents resulting in pore

radii up to 50 nm.[46] The swelling agents are for example benzene, 1,3,5,-

Fig. 1-2: Schematic representation of the preparation of SBA-15 (adapted from [45]).

Globe

Micelle

Rod-Shaped

Micelle

Liquid-Crystalline

Phase

Organic-Inorganic

Composite

Mesoporous

Material

Calcination

6

trimethybenzene, decane, and gelatin.[47–50] The swelling agents enrich in the

hydrophobic chains of the surfactants in the micelles and expand the micelles resulting in a

larger diameter. The important conditions during the synthesis of SBA-15 with larger pores

are the initial synthesis temperature, the amount of swelling agent, and the hydrothermal

treatment time and temperature.[51] Generally, the pore diameter increases with lower

initial synthesis temperature influencing the formation of the micelles. The expansion of

the micelles is limited, because mesocellular foams with spherical mesopores are formed,

when higher relative amounts of swelling agents are used.[49] Further parameters for

adjusting the pore radius of SBA-15 are the hydrothermal treatment time and temperature.

The increase in the hydrothermal treatment temperature allows to achieve larger pore sizes

in a shorter period of time.[22,46] Disadvantages in changing the hydrothermal conditions

are long hydrothermal treatment times and high temperatures (e.g. 2 days at 130 C) that

lead to merging of adjacent cylindrical mesopores.[51]

Supported heteropolyoxomolybdates in partial oxidation reactions

Industrial applications and investigations of supported tungstate or molybdate heteropoly

acids with Keggin ion structure have been recently reviewed.[52,53] Various authors

reported, that the Keggin ion structure of supported tungstate or molybdate heteropoly

acids retained intact after deposition on silica, titania or zirconia based support

materials.[54–58,27] For H3[PMo12O40] supported on ZrO2 (PMo12-ZrO2) Devassy et al.

investigated the nature of the phosphorous species depending on Keggin loading and

calcination temperature.[39] They, reported a decomposition of the HPOM to oxide

species at temperatures above 723 K. The thermal stability of H3[PW12O40] supported on

ZrO2 (PW12-ZrO2) was investigated by López-Salinas et al.. The structural behaviour of

PW12-ZrO2 during calcination was comparable to that of PMo12-ZrO2. PW12-ZrO2

decomposed at temperatures above 773 K to form the corresponding supported oxides.[46]

Ressler et al. reported for H4[PVMo11O40] supported on SBA-15 (PVMo11-SBA-15) a

decomposition under propene oxidation conditions above 573 K resulting in Mo oxide

species.[27] The resulting molybdenum oxide species are comparable to that of

molybdenum oxide species synthesized from an ammonium hepta molybdate (AHM)

precursor. Both molybdenum oxide species on SBA-15 revealed comparable structural

motifs during treatment in propene oxidation conditions.[59] Results of the Mo K edge

7

XANES analysis of activated PVMo11-SBA-15 indicated tetrahedrally coordinated MoOx

species. A Comparison with references afforded about 50% of tetrahedrally coordinated

MoOx species on SBA-15.[27] Ressler et al. assumed that the [MoO6] units exhibited a

connectivity similar to that of the building blocks of MoO3. The MoO4 units may be

isolated or connected to other MoOx species on the surface of SBA-15.[27] Therefore, no

stable HPOM supported on SBA-15 could be obtained on SBA-15 under reactions

conditions. The role and structural evolution of V and P in PVMo11-SBA-15 under

catalytic conditions remained largely unknown.

1.4 Outline of the work

The objective of this work is to elucidate the role and structural evolution of Mo, V, W and

P in HPOM supported on SBA-15 under catalytic conditions. Additionally, the influence of

the support material SBA-15 on the stability of the supported HPOM is investigated with

respect to the role of dehyrdation processes of the SBA-15, the pore radii of SBA-15 and

HPOM loading. Therefore, the chemical composition of HPOM is varied by substituting

molybdenum with vanadium or tungsten. Subsequently, the various

heteropolyoxomolybdates are supported with different loading on nanostructured silica

SBA-15. Additionally, unsubstituted HPOM are deposited on SBA-15 with different pore

radii. The unsupported and supported heteropolyoxomolybdates are characterized by ex

situ techniques ensuring a detailed knowledge about structure and chemical composition of

the model catalysts. Afterwards, the unsupported and supported heteropolyoxomolybdates

are characterized by in situ techniques and tested for their catalytic properties in the partial

oxidation of propene. HPOM supported on SBA-15 are intensively investigated to clarify

the influence of the silanol groups of SBA-15, the pore radii of SBA-15, and the HPOM

loading on SBA-15 on the structures forming during propene oxidation conditions.

Additionally, the influence of the addenda atoms of the substituted HPOM supported on

SBA-15 on the resulting structures is investigated during propene oxidation conditions.

The characterization of the structural evolution especially of the addenda atoms (V, W) and

the heteroatom (P) focus on identifying the metal oxide structure and correlating the

structure with catalytic activity.

8

2 Characterization Methods

2.1 Structural Characterization

2.1.1 Powder X-ray diffraction

X-ray diffraction (XRD) is used for determining the long-range order structures of the

synthesized bulk samples in this work. For that, powder samples are irradiated with

monochromatic X-ray photons. The X-ray photons are inelastically and elastically

scattered by the electrons of atoms arranged in a periodic structure.[60] The X-ray photons

that are elastically scattered by the electrons of atoms are used for determining the crystal

structure. The scattered X-ray wave interfere constructively or destructively depending on

the distance between the lattice planes and the angle (θ), between the incident X-rays and

the lattice planes. The Bragg equation described the detectable X-ray photons resulting

from constructive interference as a relationship between the lattice spacing (d), and the

angle (θ), between incident X-rays and lattice plane.

nλ = 2d sin θ n = 1, 2, ... (2.1)

with:

n diffraction order

λ wavelength of the X-ray photons

d lattice spacing

θ angle between incident X-rays and lattice planes

Fig. 2-1 depicts a schematic representation of the scatted X-ray waves and interfere

constructively. Constructive interference takes place only when sum of the path lengths of

+ is an integer factor of the wavelength, λ. Diffraction peaks can be characterized by

the Miller indices, which describe the corresponding lattice planes. Detailed information

about data analyzing and structure refinement can be found elsewhere.[61,62]

9

2.1.2 Vibrational spectroscopy

The advantage of Infrared (IR) spectroscopy is that the method can be used to analyze

amorphous compounds. Amorphous compounds could not be analyzed by XRD.

Therefore, IR spectroscopy is chosen for excluding additional amorphous compounds

besides the crystalline compounds in the synthesized bulk samples. In solid states the

atoms in a crystal structure vibrate internally by changing the interatomic distances. This

vibrations lead to transitions that are within the range of the infrared radiation (0.7μm-

1000μm). The wavelength of the IR radiation is varied and the decrease of intensity is

measured.[63,64] A prerequisite for the absorption of IR radiation is a change in the dipole

moment in molecules or structural motifs in solid state compounds. Thus, the electric field

of the electromagnetic radiation couples to the dipole moment in the structure, resulting in

an absorption of the electromagnetic radiation. The observed vibration are called modes.

Detailed information can be found elsewhere.[68,69]

Similar to IR spectroscopy different vibrational modes are also observed in Raman

spectroscopy. In contrast to IR spectroscopy that uses the absorption resulting by various

vibrational modes at different irradiation energies, another method of excitation is selected

in Raman spectroscopy. In Raman spectroscopy the sample is irradiated with

monochromatic radiation, usually with a laser.[64,65] The wavelength of the radiation

should be chosen outside the absorption range of electronic transitions of the analyte

lattice planes

d A B

C

X-rays

Fig. 2-1: X-ray photons strikes the ordered lattice at an angle . X-ray photons are scattered and

interfere constructively in direction given by the Bragg equation (equation 2.6).

http://de.pons.com/%C3%BCbersetzung/englisch-deutsch/prerequisite

10

excluding electronic absorption effects at the excitation wavelength. In Raman

spectroscopy, the electromagnetic radiation induces a dipole moment in the molecule

achieving higher vibration levels. Hence, linear and homoatomic structural motifs can be

studied. Thus, Raman spectroscopy is complementary to IR spectroscopy. There are three

possible types of interactions subdivided into two categories, the Raman and Rayleigh

scattering. In the Rayleigh scattering, a photon with the energy causes a transition to a

higher virtual energy state of the structural motif and relaxes to a vibrational level of the

ground electronic state. The absorbed and emitted energy is equal in this process resulting

in an elastic scattering interaction. The excitation occur from any vibrational states

depending on the Boltzmann distribution. At room temperature, a excitation from the

vibrational ground state and the first excited vibrational state of the electronic ground state

is usual. In Raman scattering, the incident energy is different from the given energy

. Thus, Raman scattering is an inelastic scattering interaction. The energy

difference is located above (anti-Stokes radiation) and below (Stokes radiation) of the

incident monochromatic electromagnetic radiation.[63,69]

2.1.3 Physisorption

Physisorption measurements are used for determining the surface area and pore structure of

the used support material SBA-15. Physisorption (physical adsorption) denoted an

attaching gas to a surface, which was bound by van der Waals interactions at this. The gas

molecules are denoted as adsorbate and the surface as an adsorbent.[66,67] A dynamic

equilibrium is established between the adsorbent and adsorbate from the gas phase. In this

case, under isothermal conditions, the number of adsorbed molecules n depends on the

equilibrium pressure p of the gas. This relationship can be expressed by the following

equation.The physisorption isotherm can be obtained by plotting the relationship shown in

equation 2.2. The isotherms are divided into six classes by IUPAC.[68] The relevant type

of isotherms for this work is the type IV isotherm. The type IV isotherm has a specific

hysteresis curve, which is characteristic for mesoporous materials. The adsorption of the

monolayer and multi-layer on the walls of the mesopores is following by capillary

condensation in the mesopores of the adsorbent.

11

with:

number of adsorbed molecules

equilibrium pressure

saturation pressure of the adsorptive

The specific surface area of the investigated materials can be determined by the

Brunauer-Emmer-Teller (BET) method from the measurements of the isotherms.[69] In

addition to the specific surface area, the pore size distribution can be determined in the

mesoporous material from the capillary condensation. This method is the Barret-Joyner-

Halenda (BJH) method and is derived from the Kelvin equation.[70] In this case, the

emptying of pores is considered by stepwise lowering of the relative pressure , in

which the thinning of the multilayer film is taken into account. Detailed informations can

be found elsewhere.[66–70]

2.1.4 X-ray absorption spectroscopy

The X-ray absorption fine structure was first discovered in the 1920s. X-ray absorption

fine structure spectrocopy was interesting for the analysis of atoms in biological molecules,

catalysts, and amorphous materials since the presence of particle accelerators (synchrotron)

in the 1970`s.[71,72] X-Ray Absorption Spectroscopy (XAS) is an analytical method for

elucidating electronic properties and the local structure of the absorber atoms.[73,74] The

particular advantage of XAS is the applicability of the method even in the absence of long-

range order. These property is especially relevant for supported catalysts. XAS is used in

this work to determine the structural evolution of the Mo, V, and W centers in supported

HPOM during propene oxidation conditions.

Absorption of X-rays at the core-level binding energy leads to excitation of an electron

from a core level. Excitation of the electron at the core-level binding energy lead to a sharp

rise in absorption, which is denoted as absorption edge. The energy for the excitation is

specific for the absorber atom. XAS can be subdivided in two regions, X-ray near edge

structure (XANES) and the extended X-ray absorption fine structure (EXAFS).

12

The XANES region is typically located in a 50 eV range around the absorption edge. This

region contains information about the element specific oxidation states and coordination

geometries around the absorber atom.[75,76] The XANES pre-edge region resulted

through the excitation of an electron from a core level (K, L, M, depending on the

observed absorption edge) to an unoccupied state. The resulting dipole transitions obey the

following selection rules: the spin quantum number cannot be changed during the

transition Δs = 0, the orbital quantum number l and the total spin j have to change (Δl,

Δj) = ± 1. Therefore, electronic transitions from the s orbital of the K shell to a higher p

orbital and from the p orbital of the L shell to higher s or d orbitals are typical. The final

state of the transition can be also a hybridized orbital. Therfore, transitions from the s

orbital of the K shell to pd hybridized orbitals are possible due to the partially different

19.9 20.0 20.1 20.2 20.3 20.4 20.5 20.6

0

1

E [keV]

Ab

sorp

tion

[μd

]

EXAFS XANES

absorber atom

scattering atom

dipol transitions

Fig. 2-2: X-ray absorption spectrum with a schematic representation of the processes at the

absorption edge,. XANES region: Absorption of an X-ray photon and excitation to a higher

unoccupied level (dipole transition). EXAFS region: Representation of the fine structure of the

absorption edge resulting from the interference of the outgoing photoelectron wave from the

absorber atom with the incoming photoelectron wave resulting by backscaterring of neighbored

atoms.

13

orbital character. Transitions, that are forbidden in principle in quantum mechanics, occur

also in a non-centrosymmetric structure geometry, as in the tetrahedral geometry.

Additionally, transitions take place in non centrosymmetric structures such as a distorted

octahedral structure. Thus, the shape of the XANES region is caused by the local electronic

and geometric structure of the absorbing atom. All quantum mechanically forbidden but

occurring transitions are characterized by a pre-edge peak in the K edge/LI edge for the

investigated elements in this work (Mo, V, W). In octahedrons no pre-edge peak is

expected. The highest intensity of the pre-edge peak are observed for a tetrahedral

geometry. Therefore, a mixed coordination geometry (tetrahedral and octahedral) can be

quantified by a linear combination of reference spectra of tetrahedral and octahedral

coordinated references.

In the EXAFS region the generated photoelectron interact with the electron density of

adjacent atoms. The outgoing electron wave from the absorber atom reaches the

neighboring atoms and will be scattered back.[76] The incoming spherical electron waves

interferes with the outgoing photoelectron wave resulting in an oscillation of the absorption

coefficient and in the fine structure in the absorption spectrum (Fig. 2-2). The absorption

coefficient can be extracted from the EXAFS region of the absorption spectrum. The

oscillatory part can be separated from the atomic absorption of a free atom and is denoted

as EXAFS function . The EXAFS function contains information about the

coordination number, element specific backscattering amplitude, and the mean-squared

displacement of the neighboring atoms around the absorber atom. The EXAFS function

χ(k)can be transferred to a pseudo radial distribution function FT (χ(k)) through a Fourier

transformation. Detailed explanations can be found in Ref. [76].

2.1.5 Nuclear magnetic resonance spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy is an analytical method to elucidate

structures in solid states and molecules. NMR spectroscopy is used in this work for

determining the local structure of phosphorus in the activated supported HPOM after

treatment under propene oxidation conditions. NMR spectroscopy could be used for atoms

with a nuclear spin I ≠ 0. If such a nucleus is in an external magnetic field, the nuclear

level split as function of the nuclear spin. The lower nuclear level can be excited with

14

electromagnetic radiation resulting in transition between the energy levels. The transition

corresponds to the Larmor frequency and the chemical and electronic environment of the

atom, resulting in the resonant frequency . Thereby, the resonant frequencies change

depending on the local structure of the atom. The resulting NMR spectrum is normalized in

the abscissa to a standard substance. The normalization results in a device-nonspecific

scale, the chemical shift . NMR spectra of liquid or gas phase exhibit narrow signals with

characteristic splitting signals caused by coupling to other nuclei in the environment. This

results from the rapid rotation of the molecules in all directions and leads to averaging of

all orientation-dependent spin interactions. In the solid-state NMR spectroscopy rather

broad signals are obtained that show a characteristic shape. This broad signals result due to

the different orientation of the structural motifs in space, caused by an orientation-

dependent spin interactions. The MAS technology (Magic Angle Spinning) averages these

anisotropic spin interactions. Therefore, the sample is placed at an angle of = 54.7° to the

magnetic field and rotates around its own axis. With this construction, anisotropic

interactions are averaged and discrete signals obtained. These signals are comparable to

signals in liquid or gaseous phase.

2.2 Element Analysis

2.2.1 X-ray fluorescence (XRF) spectroscopy

XRF spectroscopy is particularly suitable for the quantitative analysis of metals in metal

oxides such as used in this work. Irradiation of the sample with X-ray photons results in

removal of an electron from a core shell. The resulted core hole is filled by an electron

from a higher level, which results in the emission of an X-ray photon. The energy of the

resulting fluorescence photons is described by Mosley`s law. The energy of the emitted

fluorescence photons is characteristic for each element and can be used to identify the

elemental composition of the sample. Detailed explanation can be found in Ref. [65].

15

2.2.2 Atomic absorption spectroscopy (AAS)

AAS is used in this work for the quantitative analysis of phosphorus. XRF spectroscopy is

due to overlapping of the main P peak and a Mo peak of limited use. For the

measurements, the samples are dissolved in a suitable solvent. Afterwards, the solution is

transferred in the atomizer.[77] Flames or electrothermal (graphite tube) atomizer are

typical atomizer using in AAS. In this work a pyrocoated graphite tube was used as

atomizer. A matrix modifier and the sample were injected into the graphite tube. The

graphite tube was heated up to 2873 K resulting in atomization of the phosphorus species.

Phosphorus atoms are irradadiated with a Hollow Cathode Lamp (HCL) containg small

amounts of phosphorus The HCL emmited the line spectrum of the element of interest

(here phosphorus). This lead to an excitation of the valence electrons in the atoms of the

element of interest resulting in an absorption of the line spectrum of the HCL. The

absorption is proportional to the amount of the element of interest and could be used for

quantitative analysis. Detailed information can be found elsewhere.[64]

2.3 Thermal analysis

Thermal analysis consist of analytical methods, which detect physical or chemical

properties of a substance or substance mixture as a function of temperature or time, while

the sample undergoes a controlled temperature program.[78] Thermogravimetry (TG) is

used mainly for the investigation of decomposition processes, thermal stability, or

dehydration processes.[79] The sample weight is monitored using a sensitive

thermobalance as a function of time or temperature. The first derivative of the TG signal

corresponds to the differentiated thermogravimetric curve, the DTG signal. The DTG

signal facilitates the identification of mass decreases due to the resulting maxima.

2.4 Catalytic Characterization

Quantitative catalysis measurements were performed using a fixed bed laboratory reactor

connected to an online gas chromatography system. Gas chromatography is a suitable

16

chromatographic method for the separation of analyte mixtures in gas phase.[80] A

detailed description of various reactor types and the individual assets of each setup can be

found elsewhere.[81,82] The used fixed-bed reactor consisted of a SiO2 tube. Reactants are

passed through the catalyst bed. At the outlet of the reactor, reaction products are analyzed

by gas chromatography. Therefore, the sample is injected in the inert gaseous mobile phase

(carrier gas). The sample in the carrier gas stream are passed through the stationary phase.

The interaction between the sample and the mobile or stationary phase strongly depends on

their physical and chemical properties. The larger the interaction with the stationary phase,

the slower the migration velocity of the sample components. A low interaction with the

stationary phase shorts retention time of the sample components. Therefore, the adsorption

on the stationary phase or dissolving in the mobile phase depends on whether and how a

sample component interacts with the stationary phase. The different migration velocities of

the current sample components resulted in various times at which the individual

compounds passed through the stationary phase. The various components can finally be

analyzed as discrete bands with suitable detectors. The retention time is the time each

component requires to move from the point of injection to the detector. The resulting bands

are detected as a function of retention time. Thus, individual sample components can be

qualitatively and quantitatively analyzed in this way.[83,84]

17

3 Charaterization of bulk P(V,W)xMo12-x (x = 0, 1 ,2)

Heteropolyoxomolybdates (HPOM) of the Keggin type (e.g., H3[PMo12O40]) are active

catalysts for the partial oxidation of alkanes and alkenes.[7,85–88] HPOM with Keggin

structure exhibit a broad compositional range while maintaining their characteristic

structural motifs.[14–16] Substituting Mo atoms with addenda atoms (i.e. V, W, Nb) make

Keggin type HPOM suitable model systems to study structure-activity relationships. Thus,

HPOM have been frequently studied as active catalysts for selective oxidation

reactions.[89] Therefore, H3[PMo12O40] (PMo12), H4[PVMo11O40] (PVMo11),

H5[PV2Mo10O40] (PV2Mo10), H3[PWMo11O40] (PWMo11), and H3[PW2Mo10O40]

(PW2Mo10) were synthesized as model catalysts in selective propene oxidation. For

elucidating structure activity correlations of model systems for catalytic investigations, a

detailed knowledge about structure and chemical composition is indispensable. Therefore,

various characterization methods, such as XRF, AAS, XRD, IR, Raman and XAS are

necessary for a sufficient characterization of the used catalyst systems. In this chapter, in

addition to ex situ characterization, in situ XRD at oxidizing conditions P(V,W)xMo12-x

(x = 0, 1 ,2) was performed. Catalytic testing revealed the functional properties of

P(V,W)xMo12-x (x = 0, 1 ,2) under propene oxidation conditions.

3.1 Sample Preparation

Preparation of H3[PMo12O40]

19.72 g MoO3 (Sigma Aldrich) was dissolved in 650 ml water and heated under reflux. 95

ml of 0.12 M phosphoric acid were added dropwise to the reaction mixture. The resulting

suspension was heated for 3 h and kept at 298 K for 24 h until a clear yellow solution was

obtained. The remainder was filtered of and the volume of the resulting yellow solution

was reduced to ~30 ml using an evaporator. H3[PMo12O40] crystallized during storage at

277 K for several days.

18

Preparation of H3+x[PVxMo12-x] (x=1,2)

H4[PVMo11O40] was prepared as follows. 17.85 g MoO3 (Sigma Aldrich) and 1.02 g V2O5

(Sigma Aldrich) were dissolved in 650 ml water and heated under reflux. 95 ml of 0.12 M

phosphoric acid were added dropwise to the reaction mixture. The resulting suspension

was heated for 3 h and kept at 298 K for 24 h until a clear orange solution was obtained.

The remainder was filtered of and the volume of the resulting orange solution was reduced

to ~30 ml using an evaporator. H4[PVMo11O40] crystallized during storage at 277 K for

several days.

H5[PV2Mo12O40] was prepared as follows. 16.89 g MoO3 (Sigma Aldrich) and 2.13 g V2O5



was heated for 3 h and kept at 298 K for 24 h until a red solution was obtained. The

remainder was filtered of and the volume of the resulting red solution was reduced to ~30

ml using an evaporator. H5[PV5Mo12O40] crystallized during storage at 277 K for several

days.

Preparation of H3[PWxMo12-x] (x=1,2)

H3[PWMo11O40] was prepared as follows. 3.12 g Na2WO4 · H2O (Merck) were dissolved

in 50 ml water. The aqueous solution of Na2WO4 was passed through an ion-exchange

resin (Ion exchanger I (Merck)).[90] The resulted sol of tungsten acid and 15 g MoO3



was heated for 3 h and kept at 298 K for 24 h until a clear yellow solution was obtained.

The remainder was filtered of and the volume of the resulting yellow solution was reduced

to ~30 ml using an evaporator. H3[PWMo11O40] crystallized during storage at 277 K for

several days.

H3[PW2Mo10O40] was prepared as follows. 3.97 g Na2WO4 · H2O (Merck) were dissolved

in 50 ml water. The aqueous solution of Na2WO4 was passed through an ion-exchange

resin (Ion exchanger I (Merck)).[90] The resulted sol of tungsten acid and to 13 g MoO3



19

was heated for 3 h and kept at 298 K for 24 h until a clear yellow solution was obtained.

The remainder was filtered of and the volume of the resulting yellow solution was reduced

to ~30 ml using an evaporator. H3[PW2Mo11O40] crystallized during storage at 277 K for

several days.

3.2 Sample characterization

X-Ray Fluorescence Analysis

Elemental analysis by X-ray fluorescence spectroscopy was performed on an X-ray

spectrometer (AXIOS, 2.4 kW model, PANalytical) equipped with a Rh K alpha source, a

gas flow detector and a scintillation detector. 60-80 mg of the samples were diluted with

wax (Hoechst wax C micropowder, Merck) at a ratio of 1:1 and pressed into 13 mm

pellets. Quantification was performed by standardless analysis with the SuperQ 5 software

package (PANalytical).

Atom Absorption Spectroscopy (AAS)

45 mg of P(V,W)xMo12-x (x = 0, 2) obtained before and after treatment under propene

oxidation condition (5% propene + 5% O2 in He; 723 K; 0 h and 12 h time on stream) were

diluted with aqueous ammonia to 10 ml. Measurements were performed with the Perkin

Elmer 1100B AAS-spectrometer equipped with pyrocoated graphite tubes, including a

platform, and an AS700 autosampler. Extinction was measured for 4 s (Table 3-1) at

213.6 nm using a super hollow cathode lamp (HCL) of Photron at 35 mA. Background

correction was achieved by alteration of D2 lamp radiation and HCL radiation,

respectively. Background was subtracted from peak area of extinction. Each sample was

measured 3 times. Therefore, 10 µl of LaCl3 solution as matrix modifier (Roth ≥ 99.9 %,

100 mg/l) were injected, and subsequently 90 µl of sample solution were added.[77] The

applied temperature program is presented in Table 3-1. The, syringe of autosampler was

purged before and after each step with diluted nitric acid.

20

Table 3-1: AAS measurement program with applied temperatures [K], heating durations [s],

dwelling times [s], and internal gas purges [ml min-1

(Ar)].

step temperature

setpoint [K]

time to reach

setpoint [s]

dwelling

time [s]

internal gas purge

[ml min-1

(Ar])

drying 363 7 5 300

drying 393 3 30 300

pyrolysis 573 3 20 300

pyrolysis 1673 5 40 300

pyrolysis 1673 1 5 0

atomization/measurement 2873 0 4 0

cleaning 2873 1 8 200

cooling RT fast - 300

X-ray absorption spectroscopy (XAS)

Transmission XAS experiments were performed at the Mo K edge (19.999 keV) at

beamline X, V K edge (5.465 keV) and W LIII edge (10.204 keV) at beamline C at the

Hamburg Synchrotron Radiation Laboratory, HASYLAB, using a Si(311) double crystal

monochromator at beamline X and a Si(111) double crystal monochromator at beamline C.

X-ray absorption fine structure (XAFS) analysis was performed using the software

package WinXAS v3.2..[91] Background subtraction and normalization were carried out

by fitting linear polynomials and 3rd degree polynomials to the pre-edge and to the post-

edge region of an absorption spectrum, respectively. The extended X-ray absorption fine

structure (EXAFS) χ(k) was extracted by using cubic splines to obtain a smooth atomic

background μ0(k). The FT(χ(k)·k3), often referred to as pseudo radial distribution function,

was calculated by Fourier transforming the k3-weighted experimental χ(k) function,

multiplied by a Bessel window, into the R space. EXAFS data analysis was performed

using theoretical backscattering phases and amplitudes calculated with the ab-initio

multiple-scattering code FEFF7.[92] Structural data employed in the analyses were taken

from the Inorganic Crystal Structure Database (ICSD).

The modified H3[PMo12O40] (ICSD 209 [14,93]) model structure was modified by

substituting a Mo for either V or W (V for PVxMo12-x; W for PWxMo12 (x = 1, 2)). Single

scattering and multiple scattering paths of the model structure were calculated up to 6.0 Å

21

with a lower limit of 4.0% in amplitude with respect to the strongest backscattering path.

EXAFS refinements were performed in R space simultaneously to magnitude and

imaginary part of a Fourier transformed k3-weighted and k

1-weighted experimental χ(k)

using the standard EXAFS formula.[94] This procedure reduces the correlation between

the various XAFS fitting parameters. Structural parameters allowed to vary in the

refinement were (i) disorder parameter σ2

of selected single-scattering paths assuming a

symmetrical pair-distribution function and (ii) distances of selected single-scattering paths.

The statistical significance of the fitting procedure employed was carefully evaluated in

three steps as outlined in.[95] The procedures accounts for recommendations of the

International X-ray Absorption Society on criteria and error reports.[96] First, the number

of independent parameters (Nind) was calculated according to the Nyquist theorem Nind =

2/Π · ΔR· Δk + 2. In all cases, the number of free running parameters in the refinements

was well below Nind. Second, confidence limits were calculated for each individual

parameter. In the corresponding procedure, one parameter was successively varied by a

certain percentage (i.e. 0.05% for R and 5% for σ2) and the refinement was restarted with

this parameter kept invariant. The parameter was repeatedly increased or decreased until

the fit residual exceed the original fit residual by more than 5%. Eventually, the confidence

limit of the parameter was obtained from linear interpolation between the last and second

last increment for an increase in fit residual of 5%. This procedure was consecutively

performed for each fitting parameter. Third, a F test was performed to assess the

significance of the effect of additional parameters on the fit residual.[97]

Powder X-ray diffraction (XRD)

Ex situ XRD measurements were conducted on an X’Pert PRO MPD diffractometer

(Panalytical, θ-θ geometry), using Cu K alpha radiation and a solid-state multi-channel

PIXcel detector. Wide-angle scans (5-90° 2θ, variable slits) were measured in reflection

mode using a silicon sample holder.

In situ XRD measurements were conducted on a STOE diffractometer (θ-θ Mode) using an

Anton Paar in situ cell. Thermal stability tests were conducted in 20 % O2 in He (total flow

100 ml/min) in a temperature range from 323 K to 723 K. The gas phase composition at

the cell outlet was continuously monitored using a non-calibrated mass spectrometer in a

22

multiple ion detection mode (Pfeiffer Omnistar). Phase analysis was performed using the

X´Pert Highscore Plus software package (Panalytical).

IR Spectroscopy

IR spectra were recorded on a Magna System 750 of Nicolet in a wavenumber range of

400 – 4000 cm-1

. Samples were pressed into pellets of 13 mm in diameter after diluting

with KBr.

Raman Spectroscopy

Raman spectra were recorded on a FT-RAMAN spectrometer (RFS 100, Bruker). A

Nd:YAG laser with a wavelength of 1064 nm was used for excitation. Samples were

measured in a glass sample holder with a resolution of 1 cm-1

. The laser power at the

sample position was adjusted to 150 mW. All measurement consisted of 200 scans for each

sample. Scans were averaged for improvement of the signal-to-noise ratio.

Catalytic testing - selective propene oxidation


connected to an online gas chromatography system (Varian CP-3800) and a non-calibrated

mass spectrometer (Pfeiffer Omnistar). The fixed-bed reactor consisted of a SiO2 tube (30

cm length, 9 mm inner diameter) placed vertically in a tube furnace. In order to achieve a

constant volume and to exclude thermal effects, catalysts samples (~ 38 mg) were diluted

with boron nitride (Alfa Aesar, 99.5%) to result in an overall sample mass of 375 mg.

Additionally, samples were prepared for XRD annd AAS measurements. Therefore, pure

PMo12 (98 mg), PV2Mo10 (55 mg), and PW2Mo10 (55 mg) was placed in a SiO2 tube (30

cm length, 3 mm inner diameter) and fixed between two layers of quartz wool and treated

under catalytic conditions

For catalytic testing in selective propene oxidation a mixture of 5% propene (Linde Gas,

10% propene (3.5) in He (5.0)) and 5% oxygen (Linde Gas, 20% O2 (5.0) in He (5.0)) in

helium (Air Liquide, 6.0) was used in a temperature range of 293-723 K Reactant gas flow

rates of oxygen, propene, and helium were adjusted with separate mass flow controllers

23

(Bronhorst) to a total flow of 40 ml/min. All gas lines and valves were preheated to 473 K.

Hydrocarbons and oxygenated reaction products were analyzed using a Carbowax capillary

column connected to an AL2O3/MAPD column or a fused silica restriction (25 m x

0.32 mm) each connected to a flame ionization detector. O2, CO, and CO2 were separated

using a Hayesep Q (2 m x 1/8``) and a Hayesep T packed column (0.5 m x 1/8``) as

precolumns combined with a back flush. For separation, a Hayesep Q packed column (0.5

m x 1/8``) was connected via a molsieve (1.5 m x 1/8``) to a thermal conductivity detector

(TCD).

Conversion, product selectivity, and reaction rate were calculated by the following

equations:

Conversion:

(3.1)

Selectivity:

(3.2)

reaction rate:

(3.3)

with:

volume fraction

stoichiometric factor

desired product

volume flow

mass of molybdenum

24

3.3 Ex situ characterization of P(V,W)xMo12-x (x = 0, 1, 2)

3.3.1 Quantification of metal loading by XRF

Quantitative analysis of P(V,W)xMo12-x (x = 0, 1, 2) was performed to verify the synthesis

process. Results of quantitative XRF measurements and nominal composition of

P(V,W)xMo12-x (x = 0, 1, 2) are summarized in Table 3-2. Experimental composition

corresponded very well to the nominal composition and confirmed a successful synthesis.

Table 3-2: Results of quantitative XRF measurements and nominal composition of P(V,W)xMo12-x

(x = 0, 1, 2).

elements

H P Mo V W O

PVMo11 nom.. wt.% 0.23 1.75 59.25 2.86 - 35.93

PVMo11 exp. wt.% - 0.75 62.07 2.24 - 34.48

PV2Mo10 nom. wt.% 0.29 1.78 55.22 5.86 - 36.84

PV2Mo10 exp.. wt.% - 0.89 58.62 4.91 - 35.14

PWMo11 nom. wt.% 0.16 1.62 55.16 - 9.61 33.45

PWMo11 exp.wt.% - 0.81 56.60 - 9.98 32.60

PW2Mo10 nom. wt.% 0.15 1.55 47.94 - 18.38 31.98

PW2Mo10 exp.wt.% - 0.66 50.73 - 17.35 31.27

3.3.2 Long-range structure of as-prepared P(V,W)xMo12-x (x = 0, 1, 2)

Long-range order structural analysis of P(V,W)xMo12-x (x = 0, 1, 2) was performed using

X-ray powder diffraction (XRD). Fig. 3-1 shows the XRD powder pattern together with

the theoretical pattern from structure refinement. H3[PMo12O40]·13H2O (ICSD 31128) was

used as model for the refinement.[98] The samples P(V,W)xMo12-x (x = 0, 1, 2) were

synthesized as 13 hydrate. Vanadyl or molybdenyl salts of the corresponding

heteropolyacids typically crystallize in the cubic crystal systems.[99,100] Other Hydrates,

for example 6 H2O, 30 H2O of the heteropolyacids, lead also to a cubic crystal

system.[101,102]

25

Therefore, the 13 hydrate, crystallizing in a triclinic crystal system, was chosen as desired

compound to ensure the incorporation of the addenda atoms (V, W) in the Keggin ion and

excluding the formation of the corresponding salt compounds. For the refinement, the

molybdenum atoms were statistically replaced by the addenda atoms (V, W) depending on

the degree of substitution in the model structure. Atomic coordinates were kept invariant.

All samples showed the typical pattern of the 13 hydrate of HPOM.[98] The observed

deviations are explained by different crystal water content due to grinding of the sample.

Table 3-3 summarized the lattice parameters of the refinements. The volumes of the unit

cells of the corresponding PVxMo12-x (x = 1, 2) samples indicated slightly decreased

Inte

nsity

Inte

nsity

Inte

nsity

Inte

nsity

Inte

nsity

10 20 30 40 50 60 70 80

Diffraction angle 2θ [°]

PV2Mo10

PMo12

PVMo11

PWMo11

PW2Mo10

experiment refinement difference

Fig. 3-1: XRD pattern of PMo12, PVMo11, PV2Mo10, PWMo11, and PW2Mo10 together with the

XRD structure refinement of the 13 hydrate phase.

26

volumes with higher degrees of vanadium substitution. The volumes of the unit cells of the

corresponding PWxMo12-x (x = 1, 2) samples were nearly independent of the tungsten

degree of substitution. A possible simplified explanation for the different behaviour of V

and W centers substituting for Mo in Keggin type HPOM may be the considerably

different ion radii of V (68 pm) and W (74 pm) in a six-fold coordination compared to Mo

(74 pm).[103] The smaller ionic radius of V resulted in a decreased unit cell in contrast to

W with identical ionic radius compared to Mo.

Table 3-3: Lattice parameters resulting from a refinement for PMo12, PVMo11, PV2Mo10, PWMo11,

and PW2Mo10.

Sample PMo12 PVMo11 PV2Mo10 PWMo11 PW2Mo10

Space group P P P P P

Lattice parameters

a (Å) 13.546 13.551 13.566 13.531 13.577

b (Å) 14.088 14.058 14.022 14.047 14.051

c (Å) 14.134 14.145 14.126 14.109 14.135

alpha (°) 60.616 60.542 60.584 60.784 60.656

beta (°) 67.871 67.619 67.524 67.919 67.752

gamma (°) 70.213 70.143 70.189 70.346 70.210

V (Å3) 2137.213 2130.313 2124.298 2129.766 2136.110

GOF 2.52 2.98 2.79 2.97 3.38

3.4 Short-range order structural characterization of P(V,W)xMo12-x (x = 0, 1,

2)

IR-/RAMAN-Spectroscopy

IR and Raman spectra of P(V,W)xMo12-x (x = 0, 1, 2) are shown in Fig. 3-2. All samples

exhibited identical IR and Raman bands and were comparable to bands known from the

literature for H3[PMo12O40].[104–106] Therefore, it may be assumed that the Keggin ion

structure existed for all samples without significant influence of the addenda atoms (V, W).

27

The IR and Raman signals are summarized in the Appendix (Table A 1; Table A 2).

H3+x[PVxMo12-xO40] (x = 1, 2) showed two shoulders in the IR spectra at ~1080 cm-1

(νas (P-Oi)) and ~980 cm-1

(νas (V-Ot)) which could be assigned to V incorporated in the

Keggin ion.[107,108] Additionally, the IR signals for νas (P-Oi) (~1064 cm

-1) and νas (M-

Ot) (~977 cm-1

) shifted to lower wavenumbers for vanadium substituted and to higher

wavenumbers for tungsten substituted HPOM. This confirmed the assumption of

incorporated addenda atoms (V, W) in the HPOM.[108–110] The absence of a band in the

Raman spectra at 1034 cm-1

related to a vanadyl cation suggested the V in the secondary

structure.[111–113]. Therefore, it may be assumed that no further structural motifs except

of the Keggin ion was formed independent of the degree of substitution.

X-ray Absorption Spectroscopy

Mo K edge analysis of P(V,W)xMo12-x (x = 0, 1, 2)

Fig. 3-3 depicts the theoretical and experimental Mo K edge FT(χ(k)·k3) of P(V,W)xMo12-x

(x = 0, 1, 2). The very similar shape of Mo K edge FT(χ(k)·k3) of P(V,W)xMo12-x

1400 1200 1000 800 600 400

Tra

nsm

issio

n

Wavenumber [cm-1]

1000 800 600 400 200

Inte

nsity

Wavenumber [cm-1]

Fig. 3-2: IR (left) and Raman spectra (right) of PMo12, PVMo11, PV2Mo10, PWMo11, and PW2Mo10.

PV2Mo10

PMo12

PVMo11

PWMo11

PW2Mo10

PV2Mo10

PMo12

PVMo11

PWMo11

PW2Mo10

28

(x = 0, 1, 2) indicated a similar local structure of the Mo centers in P(V,W)xMo12-x

(x = 0, 1, 2). For a more detailed analysis, theoretical phase and amplitudes were calculated

for Mo-O and Mo-Mo distances and used for EXAFS refinement. The results of the

refinements of the Mo K edge FT(χ(k)·k3) of P(V,W)xMo12-x (x = 0, 1, 2) are summarized

in Table 3-4. The shapes of Mo K edge FT(χ(k)·k3) of P(V,W)xMo12-x (x = 0, 1, 2) were

comparable to that of PVxMo12-x (x = 0, 1, 2) investigated by Ressler et al..[13,14,16]

Substituting PMo12 lead to decreasing amplitudes in the FT(χ(k)·k3) with higher amount of

addenda atoms in the Keggin structures. The diminished amplitudes were accounted for by

an increased disorder parameters σ2 depending on the degree of substitution. Probably the

decreased amplitudes resulted from a distortion of the [(V,W)O]6 units in substituted

Keggin ions based on PMo12. The M-O and Mo-Mo distances were comparable for all

P(V,W)xMo12-x (x = 0, 1, 2) samples. This indicated an incorporation of addenda V and W

atoms into the Mo based Keggin ion without influencing the structure. Hence, V and W

were suitable elements for substituting Mo atoms in HPOM.

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0 1 2 3 4 5 6 R [Ǻ]

FT

(χ(k

)·k

3)

Fig. 3-3: Theoretical (dotted) and experimental (solid) Mo K edge FT(χ(k)·k3) of as prepared

PMo12, PVMo11, PV2Mo10, PWMo11, and PW2Mo10.

PV2Mo10

PMo12

PVMo11

PWMo11

PW2Mo10

29

Table 3-4: Type and number (N), and XAFS disorder parameters (σ2) of atoms at distance R from

the Mo atoms in as prepared P(V,W)xMo12-x (x = 0, 1, 2). Experimental parameters were obtained

from a refinement of H3[PMo12O40] model structure (ICSD 209 [14,93]) to the experimental Mo K

edge XAFS χ(k) of P(V,W)xMo12-x (x = 0, 1, 2) (k range from 3.0-13.7.0 Å-1

, R range from 0.9 to

4.0 Å, E0 = ~1.7, residuals ~11.3-20.0 Nind = 22, Nfree = 9). Subscript c indicates parameters that

were correlated in the refinement.

Keggin model PMo12 PVMo11 PV2Mo10

N R(Å) R(Å) σ2(Å

2) R(Å) σ

2(Å

2) R(Å) σ

2(Å

2)

Mo-O 1 1.68 1.64 0.0024 1.65 0.0039 1.66 0.0052

Mo-O 2 1.91 1.78 0.0030c 1.78 0.0043c 1.77 0.0059c

Mo-O 2 1.92 1.95 0.0030c 1.95 0.0043c 1.94 0.0059c

Mo-O 1 2.43 2.40 0.0006 2.40 0.0011 2.40 0.0051

Mo-Mo 2 3.42 3.42 0.0054c 3.43 0.0068c 3.43 0.0076c

Mo-Mo 2 3.71 3.73 0.0054c 3.73 0.0068c 3.72 0.0076c

Keggin model PMo12 PWMo11 PW2Mo10


2) R(Å) σ

2(Å

2) R(Å) σ

2(Å

2)

Mo-O 1 1.68 1.64 0.0024 1.63 0.0036 1.62 0.0026

Mo-O 2 1.91 1.78 0.0030c 1.77 0.0039c 1.77 0.0033c

Mo-O 2 1.92 1.95 0.0030c 1.94 0.0039c 1.94 0.0033c

Mo-O 1 2.43 2.40 0.0006 2.40 0.0020 2.41 0.0019

Mo-Mo 2 3.42 3.42 0.0054c 3.42 0.0082c 3.43 0.0094c

Mo-Mo 2 3.71 3.73 0.0054c 3.73 0.0082c 3.74 0.0094c

V K edge analysis of PVxMo12-x (x = 1, 2)

Fig. 3-4 depicts the theoretical and experimental V K edge FT(χ(k)·k3) of PVMo11 and

PV2Mo10. The very similar shape of the FT(χ(k)·k3) indicated similar local structures

around the V centers in the unsupported HPOM Keggin structure. A detailed structure

analysis was performed by EXAFS. The shapes of the V K edge FT(χ(k)·k3) of PVMo11

and PV2Mo10 were comparable to the V K edge FT(χ(k)·k3) of PVMo11 and PV2Mo10

30

investigated by Ressler et al..[16,14] The results of the refinement of the V K edge

FT(χ(k)·k3) of PVMo11 and PV2Mo10 are shown in Table 3-5.. All V-O and V-Mo distances

were comparable for PVxMo12-x (x = 1, 2). The disorder parameters (σ2) for all V-O and


the V atoms in as prepared PVMo11 and PV2Mo10. Experimental parameters were obtained from a

refinement of H3[PMo12O40] model structure (ICSD 209 [14,93]) to the experimental V K edge

XAFS χ(k) of PV2Mo12-SBA-15 (k range from 3.0-11.0 Å-1

, R range from 0.9 to 4.0 Å, E0 = 0.6,

residuals 9.1-11.7; Nind = 16, Nfree = 9). Subscript c indicates parameters that were correlated in the

refinement.

Keggin model PVMo11 PV2Mo10


2) R(Å) σ

2(Å

2)

V-O 1 1.68 1.63 0.0078 1.62 0.0041

V-O 2 1.91 1.94 0.0154c 1.95 0.0120c

V-O 2 1.92 1.95 0.0154c 1.95 0.0120c

V-O 1 2.43 2.38 0.0178 2.38 0.0093

V-Mo 2 3.42 3.36 0.0173c 3.34 0.0151c

V-Mo 2 3.71 3.78 0.0173c 3.74 0.0151c

-0.05

0.00

0.05

0.10

0 1 2 3 4 5 6

FT

(χ(k

)·k

3)

R [Ǻ]

PV2Mo10

PVMo11

Fig. 3-4: Theoretical (dotted) and experimental (solid) V K edge FT(χ(k)·k3) of as prepared

PVMo11 and PV2Mo10.x.

31

V-Mo distances were decreased for PV2Mo10 compared PVMo11. This decreased disorder

parameters (σ2) indicated a lower degree of distortion of the [VO6] units incorporated in the

Keggin ion for PVMo11.

W K edge analysis of PWxMo12-x (x = 1, 2)

EXAFS analysis of PWxMo12-x (x = 1, 2) rarely have been reported in the literature.

Therefore, W LIII edge FT(χ(k)·k3) of PWxMo12-x (x = 1, 2) were compared to H3[PW12O40]

(PW12) as reference to exclude the formation of PW12. The results confirmed the

assumption, that tungsten was incorporated in the HPOM. Fig. 3-5 (left) depicts the

theoretical and experimental W LIII edge FT(χ(k)·k3) of PWxMo12-x (x = 1, 2) and the

experimental W LIII edge FT(χ(k)·k3) of PW12. All EXAFS spectra showed comparable

shapes, indicating a comparable structure around the W centers The W LIII edge χ(k)·k3 of

PW12 exhibited distinct differences compared to the W LIII edge χ(k)·k

3 of PWxMo12-x

(x = 1, 2) in the range above 9 Å-1

. This indicated a slightly different structure or other

backscattering atoms around the W centers. W LIII edge FT(χ(k)·k3) of PWxMo12-x

R [Ǻ]

0 1 2 3 4 5 6

Fig. 3-5: (left) Theoretical (dotted) and experimental (solid) W K edge FT(χ(k)·k3) and (right)

χ(k)·k3 of as prepared PW12, PWMo11, and PW2Mo10.

FT

(χ(k

)·k

3)

PW2Mo10

PWMo11

0.00

0.05

0.10

0.15

0.20

0.25 PW12

4 6 8 10

χ(k

)·k

3

-4

-2

0

2

4

6

8

10

12

14

k [Ǻ-1]

32

(x = 1, 2) could be sufficiently simulated using only W-Mo distances (Table 3-6). Hence,

the formation of predominantly PW12 was excluded. All disorder parameters (σ2) in the

EXAFS refinement for PWxMo12-x (x = 1, 2) were nearly identical indicating a comparable

degree of distortion.

Table 3-6: Type and number (N), and XAFS disorder paramters (σ2) of atoms at distance R from

the Mo atoms in as prepared PWMo11 and PW2Mo10. Experimental parameters were obtained from

a refinement of H3[PMo12O40] model structure (ICSD 209 [14,93]) to the experimental W LIII edge

XAFS χ(k) of PWxMo12-x (x = 1, 2) (k range from 3.4-11.5 Å-1

, R range from 0.9 Å to 3.8 Å, E0 =

3.2, residuals 9.1-13.2 Nind = 8, Nfree = 16)

Keggin model PWMo11-SBA-15 PW2Mo10-SBA-15


2) R(Å) σ

2(Å

2)

W-O 1 1.68 1.69 0.0024 1.72 0.0025

W-O 2 1.91 1.82 0.0025 1.83 0.0026

W-O 2 1.92 1.94 0.0025 1.95 0.0026

W-O 1 2.43 2.32 0.0028 2.29 0.0028

W-Mo 2 3.42 3.46 0.0047 3.46 0.0045

W-Mo 2 3.71 3.69 0.0047 3.69 0.0045

3.5 In situ Characterization of bulk heteropolyacids

3.5.1 In situ XRD of PMo12-x(V,W)x x = 0, 1, 2 during oxidation conditions

For elucidating structure-activity relationships, an analysis of the structural evolution

during oxidizing conditions was useful. Fig. 3-6 depicts selected in situ powder pattern of

PMo12 during treatment in 20% oxygen He (323 K to 723 K). At 323 K the typical pattern

peaks for H3[PMo12O40]·3-8 H2O were identified.[114,115] Between 373 K and 473 K a

dehydration of the HPOM lead to H3[PMo12O40] without constitutional water.[116,117]

The dehydration process continued until 673 K, where the anhydrous [PMo12O38.5] and β-

MoO3 were found.[117,34,118] Appendix Fig. A 1depicts the ion current m/e = 18 during

temperature programmed oxidation representing the amount of water. The ion current

increased between 498 K and 648 K confirming the loss of constitutional water. At

33

723 K a mixture of α-MoO3 + β-MoO3 with a higher concentration of the metastabile β-

MoO3 was identified.[118] Fig. 3-7 shows the in situ powder pattern of PVMo11 and

PV2Mo10 during treatment in 20% oxygen in He (323 K to 723 K). The powder pattern at

323 K shows for both PVMo11 and PV2Mo11 a hydrated structure (3-8 hydrate).[18,19]

Subsequently a dehydration of the water of crystallization resulted in PVxMo12-x (x = 1, 2)

without water of crystallization. Between 473 K and 573 K for PVMo11 and 523 K for

PV2Mo10 a mixture of H3-x[PVxMo12-xO40] (x = 1, 2) and the anhydrous [PVxMo12-xO38.5-

0.5x] (x = 1, 2) was found. Above 623 K (for PVMo11) and 573 K (for PV2Mo10) only the

[PVxMo12-xO38.5-0.5x] (x = 1, 2) phase was identified.[117] At 673 K a mixture of

[PV2Mo10O37.5], α-MoO3, and β-MoO3 could be detected for PV2Mo10. At 723 K only

MoO3 was found for both PVMo11 and PV2Mo10. The phase composition of the two MoO3

phase was different to that of PMo12 at 723 K. The amount of β-MoO3 decreased with

higher degree of vanadium substitution resulting in mainly α-MoO3 for PV2Mo10.

Fig. 3-8 shows the in situ powder pattern of PWMo11 and PW2Mo10 during treatment in

20% oxygen in He (323 K to 723 K). The powder pattern at 323 K shows for both PVMo11

and PV2Mo11 a hydrated structure (3-8 hydrate) comparable to H3-x[PVxMo12-xO40]

(x = 0, 1, 2).[18,19] Subsequently dehydration of water of crystallization resulted in

10 15 20 25 30 35 40 45

723 K

673 K

623 K

573 K

523 K

473 K

423 K

373 K

323 K

α-MoO3 + β-MoO3

[PMo12O38.5] + β-MoO3

H3[PMo12O40]+ [PMo12O38.5]

H3[PMo12O40]+ [PMo12O38.5]

H3[PMo12O40]+ [PMo12O38.5]

H3[PMo12O40]

H3[PMo12O40]

H3[PMo12O40]

H3[PMo12O40]*3-8 H2O


Inte

nsitiy

Fig. 3-6: Selected in situ powder pattern during treatment in 20% oxygen in He (temperature

range 323 K to 723 K) of H3[PMo12O40] · n H2O.

34

H3[PWxMo12-xO40] (x = 1, 2) without water of crystallization. Between 473 K and 623 K

for PWMo11 and PW2Mo10 a mixture of H3[PWxMo12-xO40] (x = 1, 2) and the anhydrous

[PWxMo12-xO38.5] (x = 1, 2) was detectable. In contrast to H3-x[PVxMo12-xO40] (x = 0, 1, 2)

no separated anhydrous [PWxMo12-xO38.5] (x = 1, 2) phase was found. At 673 K a mixture

of [PWxMo12-xO38.5] (x = 1, 2) and at 623 K (for PVMo11) and β-MoO3 was found. At

723 K only MoO3 was found for both PWMo11 and PW2Mo10. This phase composition of

the MoO3 phase were different to that of PMo12, PVMo11, and PV2Mo10. The amount of β-

MoO3 increased with higher degree of tungsten substitution resulting in mainly β-MoO3

for PWMo11 and PW2Mo10.

Both MoO3 modifications (α-MoO3, and β-MoO3) possess slight different structures. The

smallest structural motif in α-MoO3, and β-MoO3 is a distorted MoO6 octahedron.[119]

α-MoO3 possesses a characteristic layer structure, with edges MoO6 octahedra sharing

10 15 20 25 30 35 40 45


Inte

nsitiy

Inte

nsitiy

10 15 20 25 30 35 40 45


H3+x[PVxMo12-xO40] · 3-8 H2O (x = 1, 2)

H3+x[PVxMo12-xO40] (x = 1, 2)

H3+x[PVxMo12-xO40]+ [PVxMo12-xO40-1.5·x]

(x = 1, 2)

[PVxMo12-xO40-1.5·x] (x = 1, 2)

[PV2Mo10O37.5] + MoO3

MoO3

723 K

673 K

623 K

573 K

523 K

473 K

423 K

373 K

323 K

723 K

673 K

623 K

573 K

523 K

473 K

423 K

373 K

323 K

Fig. 3-7: Selected in situ powder pattern during treatment in 20% oxygen in (temperature range

323 K to 723 K) of H4[PVMo11O40] · n H2O (left) and H5[PV2Mo10O40] · n H2O (right).

PVMo11 PV2Mo10

35

within the layer. The layers in α-MoO3 are connected via van der Waals interaction of

corner-sharing MoO6 octahedra.[120] Similar to ReO3 the MoO6 octahedra in β-MoO3 are

exclusively corner-shared.[121] An explanation for the favored formation of α-MoO3

depending on the degree of vanadium substitution in PVxMo12-x (x = 1, 2) could be

Pauling`s 3rd rule.[122] This rule states, that the stability of an ionic structure decreases

with higher amount of corner-sharing polyhedra. The distance between the neighboring

MoO6 octahedra in α-MoO3 is smaller than that of edge-sharing MoO6 octahedra in

β-MoO3.[123] Therefore, the energy loss due to the incorporation of V5+

compared to Mo6+

was compensated by the formation of predominantly edge-sharing α-MoO3. W6+

and Mo6+

have an identical charge and the consideration of a lower charge was irrelevant.

That is the reason why tungsten substituted HPOM lead to the formation of predominantly

β-MoO3. Additionally, many tertiary Mo/V or rather Mo/W as well as V/W/Mo mixed

oxides are known in the literature.[124–126]The high miscibility of the mixed metal oxides

Inte

nsitiy

Inte

nsitiy

10 15 20 25 30 35 40 45


10 15 20 25 30 35 40 45


H3[PWxMo12-xO40] · 3-8 H2O (x = 1, 2)

H3 [PWxMo12-xO40] (x = 1, 2)

H3[PWxMo12-xO40]+ [PWxMo12-xO38.5] (x = 1, 2)

[PWxMo10O38.5] (x = 1, 2)+ MoO3

MoO3

723 K

673 K

623 K

573 K

523 K

473 K

423 K

373 K

323 K

723 K

673 K

623 K

573 K

523 K

473 K

423 K

373 K

323 K

Fig. 3-8: Selected in situ powder pattern during treatment in 20% oxygen in (temperature range

323 K to 723 K) of H3[PWMo11O40] · n H2O (left) and H3[PW2Mo10O40] · n H2O (right).

PWMo11 PW2Mo10

36

may be explained by the comparable ion radii in a six fold coordination and the preferred

formation of MO6 (x = V, W, Mo) octahedra.[125,126] A further structure stabilizing

effect of addenda atoms (V, W, Nb) in the synthesis of mixed molybdenum oxides with

Mo5O14-type structure has been shown.[127] Therefore, it may be assumed that the

addenda atoms (V, W) possessed a structure directing effect during decomposition of

HPOM under oxidizing conditions. The resulting structures depended on the addenda

atom. Hence, vanadium substituted PVxMo12-x (x = 1, 2) favored the formation of α-MoO3

with edge-shared MO6 octahedra (Mo, V). Tungsten substituted PWxMo12-x (x = 1, 2) lead

to the formation of β -MoO3 with corner-shared MO6 octahedra (Mo, W).

3.5.2 Functional characterization of bulk HPOM

Catalytic testing of P(V, W)xMo12-x (x = 0, 1, 2)

Reaction rates and selectivities of P(V,W)xMo12-x (x = 0, 1, 2) in propene oxidation at

723 K are shown in Fig. 3-9. Reaction rates for of P(V,W)xMo12-x (x = 0, 1, 2) were

calculated for similar propene oxidation conditions (4-5 % propene conversion). Reaction

rates for PVxMo12-x (x = 0, 1, 2) were different depending of the degree of vanadium and

tungsten substitution. The reaction rates for PVxMo12-x (x = 0, 1, 2) increased with higher

0

2

4

6

8

10

12

14

0

20

40

60

80

100

a b c e d

acrylic acid

acetic acid acrolein

acetone

acetaldehyde

CO

CO2 propionaldehyde

Se

lectivity [%

]

Re

actio

n r

ate

[µ

mo

l(p

roen

e)g

-1(M

o)s

-1]

Fig. 3-9: Reaction rate (µmol(propene)·g-1

(Mo)·s-1

) and selectivity of (a) PV2Mo10, (b) PVMo11,

(c) PMo12, (d) PWMo11, and (e) PW2Mo10 in 5% propene and 5% oxygen in He at 723 K.

37

degree of vanadium substitution from 5.8 µmol(propene)g-1

(Mo)s-1

(PMo12) to 12.3

µmol(propene)g-1

(Mo)s-1

(PV2Mo10). The tungsten substituted samples showed an

increased reaction rate to 7.4 µmol(propene)g-1

(Mo)s-1

too compared to PMo12. The

influence on the catalytic activity was higher for the vanadium substituted samples

PVxMo12-x (x = 1, 2) in contrast to the tungsten substituted samples PWxMo12-x (x = 1, 2).

Selectivities towards acrolein decreased with higher degree of vanadium substitution from

29% (PMo12) to 22% (PV2Mo10). In contrast to the selectivities of PVxMo12-x (x =' 1, 2),

the selectivities towards acrolein for both tungsten substituted samples (27%) were

comparable to that of unsubstituted PMo12 (29%). Selectivities towards propionaldehyde

and acetaldehyde decreased with higher degree of substitution independent of the addenda

atoms (V or W). Additionally, the selectivities towards acetic acid increased with higher

degree of vanadium substitution. In contrast to that PWxMo12-x (x = 1, 2) showed not

significant selectivities towards acetic acid (below 1%). The amount of total oxidation

products (CO; CO2) increased with higher degree of substitution. The distribution of the

total oxidation products was different depending to the nature and degree of substitution.

Compared to the product distribution of unsubstituted PMo12, PVxMo12-x (x = 1, 2) and

PWxMo12-x (x = 1, 2) showed an increased production of CO and CO2, respectively.

Influence of phosphorus and structure to the catalytic activity

In situ XRD characterization (chapter 3.5.1) showed an influence of the addenda atoms in

HPOM on the resulting structures during thermal treatment under oxidizing conditions. A

structural characterization during propene oxidation at temperatures above 698 K was not

feasible due to coke formation in the in situ cell of the XRD. Therefore, the reactor for

catalytic testing is used for catalytic treatment (5% propene and 5% O2 at 723 K; 0 h and

12h time on stream) of pure PMo12, PV2Mo12 and PW2Mo10. Subsequently, the samples

were investigated with XRD after treatment (0 h and 12 h time on stream) to obtain

structural information. Additional, the samples were investigated with AAS to quantify the

content of phosphorus. Fig. 3-10 depicts the evolution of the propene conversion for

PMo12, PV2Mo10, and PW2Mo10 up to 12 h time on stream at 723 K. The sample weights of

PMo12, PV2Mo10, and PW2Mo10 were chosen to reach a comparable propene conversion

(11.8-13.5) at 723 K with time on stream 0 h (c.f. chapter 3.2) All three samples exhibited

a decreased activity with longer time on stream. The degree of deactivation was higher for

38

PMo12 than for PV2Mo10 which again was higher than that of PW2Mo10. One explanation

for the deactivation process is an enrichment of phosphorous on the surface of bulk

HPOM. Phosphorus containing catalysts (i.e. VPO, FePO, MoPO) play a crucial role as

oxidation catalysts.[128] Millet et al. showed that adding small amounts of phosphoric acid

to the feed showed positive effects on long-term stability and catalytic performance of

FePO catalysts during ODH of isobutyric acid into methacrylic acid.[129] The phosphorus

source was needed to maintain a constant P/Fe ratio at the surface of the catalyst.[129]

Another example for the relevance of P in oxidation reactions are VPO catalysts.[128,130]

VPO catalysts showed migration of phosphorus species to the surface and a decreasing

amount of phosphorus in the catalyst during water vapor treatment. The excess of

phosphorus on the surface may suppress oxidation of VPO catalysts hindering formation of

active sites for oxidation reactions. Subsequently, hydrolysis of P-O-P or P-O-V groups

resulted in a removal of phosphate groups on the surface and an increasing

activity.[128,130] The quantification of phosphorus in all samples (P(V,W)xMo12-x

x = 0, 2) showed that the content of phosphorus was similar before and during treatment

under catalytic conditions (0 h and 12 h time on stream) (Appendix Fig. A 4). Therefore,

removal of phosphate groups from the HPOM was excluded. Hence, XRD structural

analysis of the treated (time on stream 0 h and 12 h) samples was used to elucidate

Fig. 3-10: propene conversion for PMo12 ( ), PV2Mo10 ( ), and PW2Mo10 ( ) during catalytic testing

(5% propene and 5% O2 at 723 K; 12h time on stream).

0 10 20 30 40

323

423

523

623

Cycles

T [

K]

0

2

4

6

8

10

12

14

pro

pe

ne

co

nvers

ion [%

]

723

0 h 12 h Time on stream

39

structural differences (Fig. 3-11). Structural differences may be also responsible for the

different deactivation processes and/or different reaction rates and selectivities (Fig. 3-9).

XRD powder pattern for each sample measured at 0 h and at 12 h were nearly identical.

XRD pattern peaks of PMo12 (0 h and 12 h time on stream) corresponded to that of α-

MoO3 (c.f. Appendix Fig. A 2).[41] α-MoO3 is the thermodynamically stable modification

of molybdenum oxides in their highest oxidation state (+6) and indicated an oxidizing

character of the catalytic gas composition.[131,132] Kühn et al. showed that the treatment

of α-MoO3 under propene oxidation conditions had no influence on the structure.[133]

Therefore, it may be assumed that PMo12 decomposed during propene oxidation conditions

at 723 K to the thermodynamically stable α-MoO3 which lead in a poor catalytic activity

(c.f. Fig. 3-9).[41]

The resulting structure for PV2Mo10 (0 h and 12 h time on stream) was probable a mixture

of lacunary Keggin ions and Keggin ions. Ressler et al. described for PV2Mo10 a dynamic

behaviour by isothermally switching from propene (reducing) to an oxidizing (propene and

oxgen) and back to propene (reducing) conditions at 723 K. This in situ XAS experiments

showed the formation of a short vanadium-molybdenum distance of about 2.8 Å for the

0h 12h

Fig. 3-11: XRD powder pattern of PMo12, PV2Mo10, and PW2Mo10 after treatment in 5% propene

and 5% oxygen in He at 723 K with time on stream 0 h (left) and 12 h (right).

Inte

nsitiy

10 20 30 40 50 60 70 80

Diffraction angle 2Ɵ [°]

PMo12

PV2Mo10

PW2Mo10

Inte

nsitiy

10 20 30 40 50 60 70 80


PMo12

PV2Mo10

PW2Mo10

40

reduced state. The oxidized state exhibits a longer distance of the vanadium center to an

extra-Keggin molybdenum center at 3.2 Å.[16] The results suggested a mixture of at least

two sites around two different V centers.[16] Therefore, it may be assumed, that the

resulted structure for PV2Mo10 treated during catalytic conditions (5% propene + 5%

oxygen in He at 723 K) corresponded to a mixture of lacunary Keggin ions and Keggin

ions. Hence, a final structural solution was not feasible. Nevertheless, the mixture of the

various structures may be responsible for the enhanced catalytic activity (c.f. Fig. 3-9).

XRD pattern of PW2Mo10 after catalytic treatment (0 h and 12 h time on stream) showed

broad peaks indicating an amorphous or less crystalline compound. A mixture of α-MoO3

and Mo17O47 could be indentified from the XRD pattern (Appendix Fig. A 3).

Molybdenum centers in Mo17O47 have an average valence of ~ +5.5. This indicated, that

the degree of reduction was higher for PW2Mo10 than for PMo12. Hence, tungsten lead to

an increased reducibility of PW2Mo10 during propene oxidation conditions.

The different structures formed during catalytic conditions (5% propene + 5% oxygen in

He at 723 K) depended on the substituted element (V, Mo). Therefore a structure directing

effect of the addenda atoms to structures formed during thermal treatment under catalytic

may be assumed. The different structures resulting during catalytic conditions (5% propene

+ 5% oxygen in He at 723 K) were an explanation for the different catalytic behaviours of

P(V,W)xMo12-x ( x = 0, 1, 2). α-MoO3 resulting from PMo12 showed the lowest catalytic

activity in propene oxidation. The mixture of lacunary Keggin ions and Keggin ions

resulting for PV2Mo10 and the mixture of α-MoO3 and Mo17O47 resulting for PW2Mo10

showed an increased catalytic activity in propene oxidation.

41

3.6 Summary

P(V,W)xMo12-x (x = 0, 1, 2) were examined by a combination of various characterization

techniques. The synthesis of P(V,W)xMo12-x (x = 0, 1, 2) lead to HPOM with the desired

Keggin type structure and chemical composition. P(V,W)xMo12-x (x = 0, 1, 2) crystallized

as 13 hydrate in a triclinic crystal system. The 13 hydrate structure of the HPOM indicated

the incorporation of the addenda atoms (V, W) in the Keggin ion. Hence, the formation of

the corresponding salt compounds was excluded. The volume of the unit cell decreased

with higher vanadium substitution because of the smaller ionic radius of V in a six-fold

coordination in contrast to W with an identical ionic radius compared to Mo. The results of

the IR and Raman measurements confirmed, that the addenda atoms (V, W) were

incorporated in the Keggin ion. Peaks in the IR and Raman spectra indicating additional

structure motifs except for the Keggin ion structure were not found. Results of the EXAFS

refinements indicated, that the addenda atoms (V, W) were incorporated in the Keggin ion

independent of the degree of substitution.

In situ XRD pattern of P(V,W)xMo12-x (x = 0, 1, 2) showed that the addenda atoms have an

structure directing effect during decomposition under oxidizing conditions. Vanadium

substitution in HPOM (PVxMo12-x (x = 1, 2)) lead to an increased formation of

predominantly α-MoO3 depending on the degree of vanadium substitution. Conversely,

tungsten substituted PWxMo12-x (x = 1, 2) exhibited an increased formation of β-MoO3

depending on the degree of tungsten substitution.

Catalytic tests revealed an increased activity for the substituted HPOM (P(V,W)xMo12-x

(x = 1, 2)) depending on the degree of substitution. The addenda atoms (V, W) had a

structure directing effect to the structures forming during propene oxidation conditions.

Unsubstituted PMo12 formed α-MoO3 with poor catalytic activity in propene oxidation.

Vanadium substitution probably lead to a mixture of lacunary Keggin ions and Keggin

ions, resulting in an enhanced catalytic activity. Tungsten substituted HPOM (P(W)xMo12-x

(x = 1, 2)) formed a mixture of α-MoO3 and Mo17O47 with an enhanced catalytic activity.

Hence, the different structures resulting during catalytic condition and the different

chemical compositions (P(V,W)xMo12-x (x = 0, 1, 2)) seemed to be responsible for the

different catalytic activities and selectivities during propene oxidation.

42

4 Characterization of P(V,W)xMo12-x-SBA-15 (x = 0, 1,

2) (10 wt.% Mo)

For elucidating structure activity correlations of model systems for catalytic investigations,

a detailed knowledge about structure and chemical composition is indispensable.

Therefore, various characterization methods, such as XRF, XRD, Nitrogen Physisorption,

and XAS are necessary for a sufficient characterization of the used catalyst systems. In this

chapter it will be shown that the initial Keggin structure was retained after supporting of

P(V,W)xMo12-x (x = 0, 1, 2) onto SBA-15. Furthermore it should be ensured, that the

supporting process lead to the desired metal loadings of 10 wt.% Mo for the current model

catalyst. Additionally, an analysis of the structure of the supported model catalyst was

carry out to confirm well dispersed Keggin ion structure motifs on the support material.

4.1 Sample Preparation

Preparation of the support material silica SBA-15

Silica SBA-15 was prepared according to Ref. [23]. 16.2 g of triblock copolymer (Aldrich,

P123) were dissolved in 294 g water and 8.8 g hydrochloric acid at 308 K and stirred for

24 h. After addition of 32 g tetraethyl orthosilicate for 24 h, the reaction mixture was

stirred for 24 h at 373 K. The resulting gel was transferred to a glass bottle and the closed

bottle was heated to 388 K for 24 h. Subsequently, the suspension was filtered and washed

with a mixture of H2O/EtOH (100:5). The resulting white powder was dried at 378 K for

3 h and calcined at 453 K for 3 h and at 823 K for 5 h. Three batches of silica SBA-15

were used for P(V, W)xMo12-x-SBA-15 (10 wt. % Mo).

Preparation of HPOM supported on silica SBA-15

HPOM (Chapter 3.1) were supported on SBA-15 via incipient wetness. The amount of

molybdenum was adjusted to 10 wt.%. Therefore an aqueous solution of HPOM was used.

43

4.2 Sample characterization








Physisorption measurements

Nitrogen physisorption isotherms were measured at 77 K on a BEL Mini II volumetric

sorption analyzer (BEL Japan, Inc.). The silica SBA-15 sample was treated under vacuum

at 368 K for about 20 min and at 448 K for about 17 h before starting the measurement.

Data processing was performed using the BELMaster V.5.2.3.0 software package. The

specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method in

the relative pressure range of 0.03-0.24 assuming an area of 0.162 nm2

per N2

molecule.[69] The adsorption branch of the isotherm was used to calculate pore size

distribution and cumulative pore area according to the method of Barrett, Joyner, and

Halenda (BJH).[70]


XRD measurements were conducted on an X’Pert PRO MPD diffractometer (Panalytical,

θ-θ geometry), using Cu K alpha radiation and a solid-state multi-channel PIXcel detector.

Wide-angle scans (5-90° 2θ, variable slits) were measured in reflection mode using a

silicon sample holder. Small-angle scans (0.4-6.0° 2θ, fixed slits) were collected in

transmission mode with the sample spread between two layers of Kapton foil.

44



beamline X, V K edge (5.465 keV) and W LIII edge (10.204 keV) at beamline C at the

Hamburg Synchrotron Radiation Laboratory, HASYLAB, using a Si(311) double crystal

monochromator at beamline X and a Si(111) double crystal monochromator at beamline C.












The modified H3[PMo12O40] (ICSD 209 [14,93]) model structure was modified by

substituting a Mo for either V or W (V for PVxMo12-x; W for PWxMo12 (x = 1, 2)) .Single

scattering and multiple scattering paths of the model structure were calculated up to 6.0 Å










Detailed information about the fitting procedure are described in chapter 3.2.

45

4.3 Results of the Characterization

Quantification of metal loading XRF

Quantitative analysis of P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) was performed to verify the

supporting process. Results of quantitative XRF measurements and nominal composition

of P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) was summarized in Table 4-1. Experimental

composition corresponded very well to the nominal composition and confirmed a

successful supporting process. For simplification of the nomenclature the samples were

still denoted as P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) (10 wt.% Mo).

Table 4-1: Results of quantitative XRF measurements and nominal composition of P(V,W)xMo12-x-

SBA-15 (x = 0, 1, 2).

Elements

H P Mo V W O Si

PMo12-SBA-15 nom. wt.% 0.03 0.27 10.01 - - 50.37 39.32

PMo12-SBA-15 exp. wt.% - 0.33 9.89 - - 50.30 39.41

PVMo11-SBA-15 nom. wt.% 0.04 0.29 10.00 0.48 - 50.33 38.85

PVMo11-SBA-15 exp. wt.% - 0.42 9.15 0.42 - 50.44 39.46

PV2Mo10-SBA-15 nom. wt.% 0.05 0.32 9.99 1.06 - 48.99 37.15

PV2Mo10-SBA-15 exp. wt.% - 0.50 9.86 1.1 - 50.13 38.13

PWMo11-SBA-15 nom. wt.% 0.03 0.29 9.99 - 1.74 49.67 38.28

PWMo11-SBA-15 exp. wt.% - 0.43 9.92 - 1.83 49.57 38.24

PW2Mo10-SBA-15 nom. wt.% 0.03 0.32 10.02 - 3.84 48.81 36.97

PW2Mo10-SBA-15 exp. wt.% - 0.43 9.80 - 3.94 48.74 37.09

4.3.1 Long-range structure of as-prepared P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2)

The long-range structure of as-prepared SBA-15 and P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2)

were investigated by low-angle and wide-angle X-ray diffraction (Fig. 4-1). At small

angles SBA-15 and P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) exhibited the characteristic

patterns ((10l, (11l) and (20l)) representing the hexagonal pore structure of nanostructured

46

SBA-15. The lattice constant of the hexagonal unit cell of a = 12.1 nm of SBA-15 was

determined from the (10l) peak (Fig. 4-1; left). Lattice constants of the hexagonal unit cell

resulting for P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) were nearly identical to unsupported

SBA-15. Supporting P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) on SBA-15 showed no influence

to the pore structure of the support material. Wide-angle X-ray diffraction patterns of a

SBA-15 and P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) with a loading of 10 wt.% Mo, showed

no long-ranged ordered molybdenum oxide species (Fig. 4-1; right). This indicated

sufficiently dispersed Keggin ions without formation of extended crystalline HPOM

structures.

1 2 3

10 20 30 40 50 60 70 80

Diffraction angle 2Ɵ [°] Diffraction angle 2Ɵ [°]

Inte

nsity

Inte

nsity

SBA-15

PMo12-SBA-15

PVMo11-SBA-15

PV2Mo10-SBA-15

(10l)

(11l)

(20l)

PWMo11-SBA-15

PW2Mo10-SBA-15

SBA-15

PMo12-SBA-15

PVMo11-SBA-15

PV2Mo10-SBA-15

PWMo11-SBA-15

PW2Mo10-SBA-15

Fig. 4-1: Low-angle (left) and wide-angle (right) X-ray diffraction patterns of SBA-15, PMo12-

SBA-15, PVMo11-SBA-15, PV2Mo10-SBA-15, PWMo11-SBA-15, PW2Mo10-SBA-15. (10l), (11l),

and (20l) reflections are indicated.

47

Nitrogen physisorption

Nitrogen adsorption/desorption isotherms and pore distributions (BJH) of SBA-15 and

PMo12-SBA-15 are shown in Fig. 4-2. The isotherms of SBA-15 and PMo12-SBA-15 were

of type IV indicative of mesoporous materials. Adsorption and desorption branches were

nearly parallel in SBA-15 and PMo12-SBA-15 as expected for regularly shaped pores. N2

physisorption isotherms of PMo12-SBA-15 resembled that of the original SBA-15.

Constrictions of the pores due to deposition of PMo12 on SBA-15 could be excluded. Pore

radius of the supported material decreased slightly. Therefore, the results of nitrogen

physisorption measurements indicated, that PMo12 was sufficient dispersed on the support

material without influence on the pore structure.

SBA-15 possessed a BET surface area of 843 m2/g. BJH calculations of pore size

distributions resulted in a pore diameter of dBJH = 10.6 nm. Given the pore diameter and

the lattice constant (a = 12.1 nm), a wall thickness of the SBA-15 material used amounted

to ~2 nm. PMo12-SBA-15 was chosen exemplary to elucidated an influence on pore

diameter and surface area of the support material after deposition. PVMo11-SBA-15,

PV2Mo10-SBA-15, PWMo11-SBA-15 and, PW2Mo10-SBA-15 were not analyzed by

0.0 0.2 0.4 0.6 0.8 1.0

0

200

400

600

800

Vo

lum

e [m

l g

-1]

Relative Pressure p/p0

5 7 9 11 13 15

0

0.1

0.2

0.3

0.4

Pore Diameter [nm]

dV

/dp [m

l nm

-1g

-1]

Fig. 4-2: Nitrogen physisorption isotherms of silica SBA-15 (square) and PMo12-SBA-15 (circle),

and pore distributions of silica SBA-15 (square) and PMo12-SBA-15 (circle), (inset).

48

nitrogen physisorption because of the required pretreatment (0.5 h at 450°C in He) which

leads to a dehydration and dehydroxylation process concomitant with structural changes of

the orginate Keggin structure. Therefore the result was used as indicator to exclude a

significant influence on the pore structure.

Table 4-2 summarizes specific surface area aBET, external surface area aEXT, area

corresponding to the mesopores aMeso, pore diameter dpore, and mesopore volume VMeso of

SBA-15 and PMo12-SBA-15

Table 4-2: Specific surface area aBET (calculated by BET method), external surface area aEXT

(calculated as the difference between aBET and aMeso), area corresponding to the mesopores aMeso,

pore diameter dBJH (calculated by BJH method), mesopore volume VMeso, of SBA-15 and PMo12-

SBA-15.

aBET (m2/g) aExt (m

2/g) aMeso (m

2/g) dBJH (nm) VMeso (cm

3/g)

SBA-15 843 145 698 10.6 1.233

PMo12-SBA-15 603 67 536 9.2 0.940

4.3.2 Short-range order structural characterization of as-prepared P(V,W)xMo12-x-SBA-

15 (x = 0, 1, 2)

X-Ray Absorption Spectroscopy

XAS was chosen as suitable spectroscopic method to analyze the structure of the Keggin

ion motif on the support material. XRD was unsuitable for a structural analysis, because no

long-ranged ordered molybdenum oxide species (Fig. 4-1; right) were expected. IR- and

RAMAN-spectroscopy were not chosen as analyzing method as well. The IR- and

RAMAN spectra of P(V,W)xMo12-x (x = 0, 1, 2) were superimposed with the excess of

SBA-15. Additionally, XAS is an element specific method and suitable for analyzing

substituted compounds.

49

Mo K edge analysis of P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2)

Fig. 4-3 (left) depicts the Mo K edge XANES spectra of P(V,W)xMo12-x-SBA-15

(x = 0, 1, 2) and (right) the theoretical and experimental Mo K edge FT(χ(k)·k3) of

P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2). A comparison of the Mo K edge XANES spectra

confirmed identical structural motifs in P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2). Analysis of

the Mo K edge position (Fig. 4-3, left; broken line) yielded an average valence of ~6 (cf.

Appendix Fig. A 6) of the substituted supported HPOM.[134] Therefore, substitution and

the supporting process did not have an influence on the average valence of Mo in

P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2). The very similar shape of Mo K edge FT(χ(k)·k3) of

P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) indicated a similar local structure around the Mo

centers in the unsupported and supported HPOM Keggin structure. P(V,W)xMo12-x-SBA-

15 (x = 0, 1, 2). The results of the refinements of the Mo K edge FT(χ(k)·k3) of

0 1 2 3 4 5 6 -0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

PMo12-SBA-15

PVMo11-SBA-15

PV2Mo10-SBA-15

PWMo11-SBA-15

PW2Mo10-SBA-15

R [Ǻ]

20.0 20.1 20.2

PMo12-SBA-15

PVMo11-SBA-15

PV2Mo10-SBA-15

PWMo11-SBA-15

PW2Mo10-SBA-15

No

rma

lize

d a

bsorp

tion

Photon energy [keV]

FT

(χ(k)·k

3)

Fig. 4-3: (left) Mo K edge XANES with indicated Mo K position (broken line) and (right)

theoretical (dotted) and experimental (solid) Mo K edge FT(χ(k)·k3) of as prepared PMo12-SBA-15,

PVMo11-SBA-15, PV2Mo10-SBA-15, PWMo11-SBA-15, PW2Mo10-SBA-15.

50

P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) are shown in Table 4-3. Substituting PMo12 lead to

decreasing amplitudes in the FT(χ(k)·k3) with higher amount of addenda atoms in the

Keggin structures. The diminished amplitudes were discernible in all increased disorder

parameters σ2 with substitution degree. Probably the decreased amplitudes resulted from a

distortion of the [(V,W)O]6 units in substituted Keggin ions based on PMo12. Comparable

results were found in the unsupported P(V,W)xMo12-x (x = 0, 1, 2) (cf. chapter 3)


the Mo atoms in as prepared P(V, W)xMo12-x-SBA-15 (x= 0, 1, 2). Experimental parameters were

obtained from a refinement of H3[PMo12O40] model structure (ICSD 209 [14,93]) to the

experimental Mo K edge XAFS χ(k) of P(W,V)xMo12-x-SBA-15 (x = 0, 1, 2) (k range from 3.0-13.7

Å-1

, R range from 0.9 to 4.0 Å, E0 = ~1.7, residuals ~11.3-20.0 Nind = 22, Nfree = 9). Subscript c

indicates parameters that were correlated in the refinement.

Keggin model PMo12-SBA-15 PVMo11-SBA-15 PV2Mo10-SBA-15


2) R(Å) σ

2(Å

2) R(Å) σ

2(Å

2)

Mo-O 1 1.68 1.64 0.0025 1.64 0.0035 1.66 0.0053

Mo-O 2 1.91 1.78 0.0033c 1.78 0.0035c 1.77 0.0067c

Mo-O 2 1.92 1.95 0.0033c 1.95 0.0035c 1.94 0.0067c

Mo-O 1 2.43 2.40 0.0008 2.40 0.0006 2.39 0.0008

Mo-Mo 2 3.42 3.42 0.0052c 3.43 0.0057c 3.43 0.0076c

Mo-Mo 2 3.71 3.74 0.0052c 3.73 0.0057c 3.72 0.0076c

Keggin model PMo12-SBA-15 PWMo11-SBA-15 PW2Mo10-SBA-15


2) R(Å) σ

2(Å

2) R(Å) σ

2(Å

2)

Mo-O 1 1.68 1.64 0.0025 1.64 0.0028 1.62 0.0026

Mo-O 2 1.91 1.78 0.0033c 1.77 0.0038c 1.77 0.0033c

Mo-O 2 1.92 1.95 0.0033c 1.94 0.0038c 1.94 0.0033c

Mo-O 1 2.43 2.40 0.0008 2.40 0.0013 2.41 0.0019

Mo-Mo 2 3.42 3.42 0.0052c 3.42 0.0073c 3.43 0.0094c

Mo-Mo 2 3.71 3.74 0.0052c 3.74 0.0073c 3.73 0.0094c

51

V K edge analysis of PV2Mo10-SBA-15 (x = 1, 2)

Fig. 4-4 (left) depicts the V K edge XANES spectra of PV2Mo10 and PV2Mo10-SBA-15

and (right) the theoretical and experimental V K edge FT(χ(k)·k3) of PV2Mo10 and

PV2Mo10-SBA-15. XAS analysis at the V K edge for PVMo11-SBA-15 was hardly feasible

due to the low content of V (~0.5 wt.%) beside Mo (~10 wt.%). Hence only an XAS

analysis at V K edge for PV2Mo10-SBA-15 was performed and compared to unsupported

PV2Mo10. V K edge XANES spectra of PV2Mo10 -SBA-15 were identical with V K edge

XANES spectra of PV2Mo10 (Fig. 4-4, left). Comparing the pre-edge peak at the V K edge

of PV2Mo10SBA-15 and vanadium oxide as reference compound indicated an average V

valence between 4 and 5 (Appendix Fig. A 5). Results of the Mo and V K edge XANES

spectra indicate a successful deposition of heteropolyoxomolybdates on the support

material. The very similar shape of the FT(χ(k)·k3) indicated similar local structure around

the V and Mo centers in the unsupported and supported HPOM Keggin structure. A

detailed structure analysis was performed by EXAFS. The results of the refinement of the

V K edge FT(χ(k)·k3) of PV2Mo10-SBA-15 are shown in Table 4-4. Supporting PV2Mo10

No

rma

lize

d a

bsorp

tion

0.00

0.05

0.10

FT

(χ(k)·k

3)

R [Ǻ]

0 1 2 3 4 5 6 5.60

0.0

0.5

1.0

1.5

5.45 5.50 5.55

Photon energy [keV]

PV2Mo10

PV2Mo10-SBA-15

PV2Mo10

PV2Mo10-SBA-15

Fig. 4-4: (left) V K edge XANES of PV2Mo10-SBA-15 and PV2Mo10; (right) Theoretical (dotted)

and experimental (solid) V K edge FT(χ(k)·k3) of as prepared PV2Mo10-SBA-15 and PV2Mo10.

52

on SBA-15 resulted in a decreased amplitude of the corresponding V K edge FT(χ(k)·k3).

Conversely, the Mo K edge FT(χ(k)·k3) of PVxMo12-x-SBA-15 (x = 0, 1, 2) showed a minor

increasing amplitude in the range between 2.5-3.8 Å. This increase in amplitude of the

FT(χ(k)·k3) was previously reported for PVMo11 and PVMo11-SBA-15.[27] Eventually, the

Mo and V K edge FT(χ(k)·k3) confirmed that the Keggin ion structure motifs were

maintained upon supporting PVxMo12-x on SBA-15.


the V atoms in as prepared PV2Mo12-SBA-15. Experimental parameters were obtained from a

refinement of H3[PMo12O40] model structure (ICSD 209 [14,93]) to the experimental V K edge

XAFS χ(k) of PV2Mo12-SBA-15 (k range from 3.0-11.0 Å-1

, R range from 0.9 to 4.0 Å, E0 = 0.6,

residuals 13.8; Nind = 16, Nfree = 9). Subscript c indicates parameters that were correlated in the

refinement.

Keggin model PV2Mo10-SBA-15


2)

V-O 1 1.68 1.60 0.0053

V-O 2 1.91 1.94 0.0017c

V-O 2 1.92 1.95 0.0017c

V-O 1 2.43 2.43 0.0156

V-Mo 2 3.42 3.32 0.0178c

V-Mo 2 3.71 3.71 0.0178c

53

W LIII edge analysis of PWxMo12-x-SBA-15 (x = 1, 2)

Fig. 4-5 (left) depicts the W LIII edge XANES spectra of PWMo11-SBA-15 and PW2Mo10-

SBA-15 and (right) the theoretical and experimental W LIII K edge FT(χ(k)·k3) of

PWMo11-SBA-15 and PW2Mo10-SBA-15. XANES spectra of PWxMo12-x-SBA-15 (x= 1,

2) exhibited increased white lines comparing to bulk PWxMo12-x (x= 1, 2) (cf. chapter 0)

The change of white line intensities indicated a change of the absorption properties.

Therefore, it could be assumed, that the supported Keggin ions were well dispersed on the

SBA-15 causing an increased whiteline. The very similar shape of W LIII edge EXAFS

spectra of PWxMo12-x-SBA-15 (x = 1, 2) FT(χ(k)·k3) indicated a similar local structure of

the W centers in the unsupported and supported HPOM Keggin structure. Comparable to

the white line height, the amplitudes representing the W-Mo distances were increased.

Thus, a dispersion effect could be determined from the amplitudes or rather from decreased

disorder parameters σ2 representing the heights of the amplitudes compared to bulk

PWxMo12-x (x = 1, 2). The results confirmed the results of the vanadium substituted

R [Ǻ] Photon energy [keV]

No

rma

lize

d a

bsorp

tion

FT

(χ(k

)·k

3)

10.2 10.3 10.4

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 1 2 3 4 5 6

0.00

0.05

0.10

0.15

PW2Mo10-SBA-15

PWMo11-SBA-15 PW2Mo10-SBA-15

PWMo11-SBA-15

Fig. 4-5: (left) W LIII edge XANES of PW2Mo10-SBA-15 and PWMo11-SBA-15; (right)

Theoretical (dotted) and experimental (solid) W K edge FT(χ(k)·k3) of as prepared PW2Mo10-SBA-

15 and PWMo11-SBA-15.

54

PVxMo12-x (x = 1, 2). The supporting process did not have an influence on the Keggin ion

structure on the support material.


the Mo atoms in as prepared PWxMo11-x-SBA-15 (x=1, 2). Experimental parameters were obtained

from a refinement of H3[PMo12O40] model structure (ICSD 209 [14,93]) to the experimental W LIII

edge XAFS χ(k) of PWxMo12-x-SBA-15 (x = 1, 2) (k range from 3.4-11.5 Å-1

, R range from 0.9 Å to

3.8 Å, E0= ~ 3.2, residuals ~9.1-13.5 Nind = 8, Nfree = 16). Subscript c indicates parameters that


Keggin model PWMo11-SBA-15 PW2Mo10-SBA-15


2) R(Å) σ

2(Å

2)

W-O 1 1.68 1.71 0.0018 1.72 0.0017

W-O 2 1.91 1.82 0.0019 1.83 0.0018

W-O 2 1.92 1.95 0.0019 1.95 0.0018

W-O 1 2.43 2.31 0.0020 2.29 0.0020

W-Mo 2 3.42 3.46 0.0049 3.47 0.0035

W-Mo 2 3.71 3.70 0.0049 3.69 0.0036

55

4.4 Conclusion

P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) were examined by a combination of various

characterization techniques. The supporting process of P(V,W)xMo12-x (x = 0, 1, 2) on

SBA-15 via incipient wetness lead to the desired metal loadings of 10 wt.% Mo for the

current model catalyst. P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) were sufficiently dispersed on

the support material without any influence on the pore structure of the support material.

Supporting P(V,W)xMo12-x on SBA-15 resulted in regular and well dispersed Keggin ions

on the support material. The formation of extended crystalline HPOM structures could be

excluded. Therefore, substituting Mo atoms with addenda atoms (i.e. V, W) make Keggin

type heteropolyoxomolybdates suitable model systems to study structure activity

relationships. Supported catalytic species posses high dispersions and an improved surface

to bulk ratio. Therefore, structure activity relationships can be readily deduced from the

characteristic oxide species observed on the support material under catalytic conditions.

Hence, in the following chapters (5-6) P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) with a loading

of 10 wt.% Mo were investigated under catalytic conditions and structure activity

relationships presented.

56

5 Characterization of PVxMo12-x-SBA-15 (x = 1, 2)


Keggin type H4[PVMo11O40] has been reported to exhibit a pronounced interaction effect

with SBA-15 as support material.[27] This effect resulted in a further decreased thermal

stability of the supported Keggin ions compared to the bulk materials. Under catalytic

conditions PVMo11-SBA-15 formed a mixture of tetrahedrally and octahedrally

coordinated [MoO4] and [MoO6] units.[27] However, the role and structural evolution of V

and P in PVxMo12-x-SBA-15 (x = 0, 1, 2) under catalytic conditions remained largely

unknown. In this chapter in situ X-ray absorption spectroscopy investigations at the Mo K

edge of PVxMo12-x-SBA-15 (x = 0, 1, 2) and at the V K edge of PV2Mo10-SBA-15 during

propene oxidation conditions were performed. Moreover, 31

P MAS NMR measurement of

PV2Mo10-SBA-15 and the reference H3PO4 supported on SBA-15 (denoted as H3PO4-

SBA-15) after catalytic oxidation with propene are described. Correlations between

structural evolution of [MoOx] and [VOx] units and performance under catalytic conditions

will be described. Additionally, the obtained structures and catalytic performances were

compared to a suitable supported reference material.

5.1 Experimental

5.1.1 Sample Characterization

31

P-NMR measurement

31P MAS NMR spectra were recorded on a Bruker Avance 400 spectrometer (

31P: 161.92

MHz) using a 4 mm double resonance HX MAS probe. Data collection used a 90° pulse

with a relaxation delay of 60 s under a MAS rotation of 12 kHz. Spectra were referenced to

85% H3PO4 in aqueous solution using solid NH4H2PO4 (δ = 0.81 ppm) as a secondary

reference.

57



beamline X at the Hamburg Synchrotron Radiation Laboratory, HASYLAB, using a

Si(311) double crystal monochromator. Transmission XAS experiments at the V K edge

(5.465 keV) were also performed at HASYLAB, using a Si(111) double crystal

monochromator at beamline C. In situ experiments were conducted in a flow reactor at

atmospheric pressure (5 vol% oxygen in He, total flow ~30 ml/min, temperature range

from 303 to 723 K, heating rate 4 K/min). The gas phase composition at the cell outlet was

continuously monitored using a non-calibrated mass spectrometer in a multiple ion

detection mode (Omnistar from Pfeiffer).












Single scattering paths in the hexagonal MoO3 model structure (ICSD 75417 [135]) and a

modified Na2MoO4 structure (ICSD 24312 [136]) were calculated up to 6.0 Å with a lower

limit of 4.0% in amplitude with respect to the strongest backscattering path. EXAFS

refinements were performed in R space simultaneously to magnitude and imaginary part of

a Fourier transformed k3-weighted and k

1-weighted experimental χ(k) using the standard

EXAFS formula.[94] This procedure reduces the correlation between the various XAFS

fitting parameters. Structural parameters allowed to vary in the refinement were (i)

disorder parameter σ2

of selected single-scattering paths assuming a symmetrical pair-

distribution function and (ii) distances of selected single-scattering paths.


58




Wide-angle scans (5-90° 2θ, variable slits) were measured in reflection mode using a

silicon sample holder.

Temperature programmed reduction

Temperature programmed reduction (TPR) was performed with a catalysts analyzer from

BEL Japan Inc. equipped with a silica glass tube reactor. Samples were placed on silica

wool inside the reactor next to a thermocouple. A gas flow (5 % H2 in Ar) of 60 ml/min

was adjusted during reaction. A heating rate of 8 K / min to 973 K was used while H2

consumption was measured with a TCD unit. All samples were pretreated with a gas flow

of 60 ml/min Ar at 393 K for about 45 min before starting the measurement. For

measurements 37.2 mg PMo12-SBA-15, 40.4 mg PVMo11-SBA-15, and 39.8 PV2Mo10-

SBA-15 were used.







with boron nitride (Alfa Aesar, 99.5%) to result in an overall sample mass of 375 mg. For

catalytic testing in selective propene oxidation a mixture of 5% propene (Linde Gas, 10%

propene (3.5) in He (5.0)) and 5% oxygen (Linde Gas, 20% O2 (5.0) in He (5.0)) in helium

(Air Liquide, 6.0) was used in a temperature range of 293-723 K Reactant gas flow rates of

oxygen, propene, and helium were adjusted with separate mass flow controllers



column connected to an AL2O3/MAPD column or a fused silica restriction (25 m·0.32 mm

59

each) connected to a flame ionization detector. O2, CO, and CO2 were separated using a

Hayesep Q (2 m x 1/8``) and a Hayesep T packed column (0.5 m x 1/8``) as precolumns

combined with a back flush. For separation, a Hayesep Q packed column (0.5 m x 1/8``)

was connected via a molsieve (1.5 m x 1/8``) to a thermal conductivity detector (TCD).

Details about the calculation of conversion, selectivity, and reaction rate are described in

chapter 3.2.

5.1.2 Sample preparation

PVxMo12-x (x = 0, 1, 2) supported SBA-15 were prepared as described in chapter 4.1.

A reference material (denoted as V2Mo10Ox-SBA-15) was prepared as follows.

232.1 mg (NH4)6Mo7O24·4H2O and 29.3 mg (NH4)6V10O28·6H2O were dissolved in water

and were deposited via incipient wetness on 1 g SBA-15 to obtain metal loading of 10

wt.% Mo and 1 wt.% V. The sample was dried for 18 h at room temperature and calcined

for 3 h at 773 K. H3PO4-SBA-15 was prepared by depositing 1.1 ml of 0.12 M phosphoric

acid on 1 g of silica SBA-15.

5.2 Structural characterization of PVxMo12-x-SBA-15 (x = 1, 2) under

catalytic conditions

In situ XANES analysis

PVxMo12-x-SBA-15 (x = 1, 2) was investigated by in situ XAS in propene oxidation

conditions. Fig. 5-1 depicts the evolution of molybdenum XANES spectra of PVMo11-

SBA-15 during temperature-programmed treatment in 5% propene and 5% oxygen. XAS

analysis at V K edge for PVMo11-SBA-15 was hardly feasible due to the low content of V

(~0.5 wt.%) beside Mo (~10 wt.%).

Fig. 5-2 shows the evolution of vanadium (a) and molybdenum (b) XANES spectra of

PV2Mo10-SBA-15 during temperature-programmed treatment in 5% propene and 5%

oxygen. The pre-edge peak features can be employed to elucidate the local structure

around the metal centers. Using the pre-edge peak height sufficed to quantify the

contribution of tetrahedral [MO4] and distorted [MO6] (M = V, Mo) units present during

60

thermal treatment of the catalysts. The pre-edge peak heights of in situ V and Mo K edge

XANES spectra at 298 K (Fig. 5-2) were attributed to the distorted [MO6] (M = V, Mo)

building units of the Keggin ion with the metal centers in their highest oxidation

states.[14,133] Mo K edge XANES spectra of PVxMo12-x-SBA-15 (x = 1, 2) and V K edge

XANES spectra of PV2Mo10-SBA-15 remained unchanged within the temperature range

from 298 K through 473 K. Hence, the Keggin structure appeared to be stable up to 473 K.

Between 473-648 K the pre-edge peak height increased with temperature. This indicated

5.46 5.48

5.5 5.52

5.54

373

473

673

T [K]

573

Photon energy

[keV]

V K edge

No

rma

lize

d

abso

rptio

n

No

rma

lize

d

abso

rptio

n

20.20

20.00

00

20.05 20.10

20.15

373

473

673

573

Photon energy

[keV]

T [K]

Mo K edge

Fig. 5-2: (left) in situ V K edge XANES spectra and (right) in situ Mo K edge XANES spectra of

PV2Mo10-SBA-15 during temperature-programmed treatment in 5% propene and 5% oxygen in

helium in a temperature range between 300 K and 723 K.

20.00 20.05

20.10 20.15

20.20

373 473

573 673

Photon energy

[keV]

No

rma

lize

d

abso

rptio

n

T [K]

Fig. 5-1: in situ Mo K edge XANES spectra of PVMo11-SBA-15 during temperature-programmed

treatment in 5% propene and 5% oxygen in helium in a temperature range between 300 K and

723 K.

61

structural changes from octahedral to tetrahedral [MOx] (M = V, Mo) units during thermal

treatment under catalytic conditions comparable to unsubstituted PMo12-SBA-15 (chapter

7.4).[95,59] Evolution of the normalized pre-edge peak height together with the ion

currents of H2O, CO, CO2, acrolein, and acetone during oxidation of propene are shown in

Fig. 5-3, (right). In situ Mo K edge FT(χ(k)·k3) (Fig. 5-3, left) indicated that the Keggin

structure was intact up to 473 K. Structural changes between 473 K and 648 K as observed

in the Mo K edge FT(χ(k)·k3) coincided with the evolution of pre-edge height of V and Mo

K edge of PV2Mo10-SBA-15. No structural changes in the FT(χ(k)·k3) of Mo could be

determined above 648 K. Moreover, changes in pre-edge heights of V and Mo occurred on

the same time scale thereby confirming the incorporation of V centers in the Keggin

structure. The onset of catalytic activity and the formation of various selective oxidation

products coincided with the detected structural changes. Apparently, the catalytically

active species formed during thermal activation from the original Keggin structure under

reaction conditions. These mainly consisted of V and Mo species centers in a particular

tetrahedral coordination. The evolution of the [MoO4]/[MoO6] ratio of PVxMo12-x-SBA-15

(x = 0, 1, 2) based on a linear combination of bulk MoO3 and bulk Na2MoO4 (cf. chapter

No

rm. p

re e

dg

e p

ea

k

heig

ht

heig

ht

373 473 573 673

No

rm. io

n c

urr

ent

m/e=18 (H2O) m/e=28 (CO) m/e=44 (CO2) m/e=56 (acroleine) m/e=58 (acetone)

pre edge peak height V

pre edge peak height Mo

T [K]

T [K]

0.04

0.06

6 0 2 4 373 473

573 673

R [Ǻ]

FT

(χ(k

)·k

3)

0.02

Fig. 5-3: (left) Evolution of Mo K FT(χ(k)·k3) of PV2Mo10-SBA-15 during thermal treatment in 5%

propene and 5% oxygen in helium in the temperature range from 303 to 723 K (4 K/min); (right)

evolution of normalized ion current of H2O (m/e 18), CO (m/e 28), CO2 (m/e 44), acroleine (m/e

56), and acetone (m/e 58), and normalized pre-edge height of V and Mo K edge of PV2Mo10-SBA-

15 during thermal treatment in 5% propene and 5% oxygen in helium in the temperature range from

303 to 723 K (4 K/min).

62

7.4) during propene oxidation conditions was shown in Fig. 5-4. A comparison of the

structural changes of PVxMo12-x-SBA-15 (x = 0, 1, 2) indicated identical onset

temperatures. An increase of tetrahedral [MoO4] units with V substitution was determined.

5.2.1 Local structure in activated PVxMo12-x-SBA-15 (x = 0, 1, 2) and a reference

V2Mo10Ox-SBA-15 under catalytic conditions

Local structure of Mo centers in act. PVxMo12-x-SBA-15 (x = 0, 1, 2)

Fig. 5-5 (left) shows the Mo K edge FT(χ(k)·k3) of act. PMo12-SBA-15, act. PVMo11-SBA-

15, and act. PV2Mo10-SBA-15 after thermal treatment under catalytic conditions at 723 K.

The Mo K edge FT(χ(k)·k3) were nearly similar for all three PVxMo12-x-SBA-15

(x = 0, 1, 2) and exhibited features similar to that of previously reported dehydrated

molybdenum oxides and HPOM supported on SBA-15.[27,59] Minor differences between

the three PVxMo12-x-SBA-15 (x = 0, 1, 2) are marked in the Mo K edge FT(χ(k)·k3) and the

Mo K edge χ(k)·k3. For a more detailed analysis hexagonal MoO3 was used as structural

model. Theoretical XAFS phases and amplitudes were calculated for Mo-O and Mo-Mo

distances and used for EXAFS refinement. The results of the refinement are shown in

Table 5-1. The first coordination sphere of the Mo K edge FT(χ(k)·k3) of as prepared

300 400 500 600 700

0

10

20

30

40

50

60

[MoO

4]/[M

oO

6] ra

tio

[%

]

Fig. 5-4: Quantification of the [MoO4]/[MoO6] ratio of PMo12-SBA-15, PVMo11-SBA-15, and

PV2Mo10-SBA-15 during thermal treatment under propene oxidation conditions.

T [K]

PMo12-SBA-15 PVMo11-SBA-15 PV2Mo10-SBA-15

63

PVxMo12-x-SBA-15 (x = 1, 2) (cf. chapter 4.3) exhibited differences compared to act.

PVxMo12-x-SBA-15 (x = 1, 2). The first peak in the FT(χ(k)·k3) originated mainly from the

tetrahedral species on the SBA-15 support and could be sufficiently simulated using four

Mo-O distances. These four distances sufficiently accounted for the minor amount

octahedral [MoO6] species. Confirming the results of the XANES analysis 1st and 2nd

disorder parameters (1st-σ2, 2nd-σ

2) were higher for act. PMo12-SBA-15 and indicated a

decreasing amount of tetrahedral structural motifs compared to act. PVxMo12-x-SBA-15 (x

= 1, 2). In addition, the 4th disorder parameter (4th-σ2) was smaller than the disorder

parameters for act. PMo12-SBA-15. This disorder parameter mainly represented the

fraction of octahedral [MoO6] species. Hence, the reduced disorder parameter indicated an

increasing amount of octahedral structural motifs in act. PMo12-SBA-15 compared to act.

PVxMo12-x-SBA-15 (x = 1, 2). Therefore a structure directing effect of addenda vanadium

resulting in an increased concentration of tetrahedral [MoO4] units under propene

oxidation conditions was determined. A distinct peak at ~3 Å in the FT(χ(k)·k3) indicated

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0 1 2 3 4 5 6

PVMo10-SBA-15

PV2Mo10-SBA-15

PMo12-SBA-15

R [Ǻ]

FT

(χ(k

)·k

3)

4 6 8 10 12 14 16

-4

0

4

8

12

k [Ǻ]-1

χ(k)·

k3

Fig. 5-5: (left) Mo K edge FT(χ(k)·k3) and (right) Mo K edge χ(k)·k

3of act. PMo12-SBA-15, act.

PVMo11-SBA-15, and act. PV2Mo10-SBA-15 after thermal treatment under propene oxidation

conditions at 723 K.

64

the formation of dimeric or oligomeric [MoxOy] units on SBA-15. Hence, only isolated

tetrahedral [MoO4] units can be excluded as major molybdenum oxide species.[137] The

disorder parameters σ2

of the Mo-Mo distances for act. PVxMo12-x-SBA-15 (x = 0, 1, 2)

were nearly identical indicating a comparable degree of oligomerization of Mo species on

silica SBA-15 independent of the V substitution in contrast to PMo12 supported on SBA-15

with larger pore radii (chapter 7.4).


the Mo atoms in act. PVxMo12-x -SBA-15 (x = 0, 1, 2). Experimental parameters were obtained

from a refinement of a hexagonal MoO3 model structure (ICSD 75417 [135]) to the experimental

Mo K edge XAFS χ(k) of act PVxMo12-x -SBA-15 (x = 0, 1, 2) (k range from 3.4-16.0 Å-1

, R range

from 0.9 to 4.0 Å, E0 = -5.2, residuals ~12.3-12.8 Nind = 26, Nfree = 12). Subscript c indicates

parameters that were correlated in the refinement.

hex-MoO3

model

act. PMo12-

SBA-15

act. PVMo11-

SBA-15

act. PV2Mo11-

SBA-15


2) R(Å) σ

2(Å

2) R(Å) σ

2(Å

2)

Mo-O 2 1.67 1.67 0.0015 1.67 0.0012 1.67 0.0013

Mo-O 2 1.96 1.89 0.0038c 1.89 0.0034c 1.88 0.0034c

Mo-O 1 2.20 2.19 0.0038c 2.18 0.0034c 2.18 0.0034c

Mo-O 1 2.38 2.35 0.0011 2.36 0.0014 2.34 0.0017

Mo-Mo 2 3.31 3.49 0.0068c 3.50 0.0066c 3.49 0.0061c

Mo-Mo 2 3.73 3.63 0.0068c 3.63 0.0066c 3.63 0.0061c

Mo-Mo 2 4.03 3.73 0.0100 3.75 0.0100 3.75 0.0100

Comparison of the local structure around the Mo centers in act. PV2Mo10-SBA-15

and a reference act. V2Mo10Ox-SBA-15 under catalytic conditions

Fig. 5-6 shows the Mo K edge FT(χ(k)·k3) of act. PV2Mo10-SBA-15 and activated

V2Mo10Ox-SBA-15 after thermal treatment under catalytic conditions. Linear combinations

of the XANES spectra of Na2MoO4 and MoO3 references were used to determine the

amount of tetrahedral [MoO4] and octahedral [MoO6] units in act. PV2Mo10-SBA-15 and

act. V2Mo10Ox-SBA-15. Apparently, act. PV2Mo10-SBA-15 consisted of a mixture of

tetrahedral [MoO4] and octahedral [MoO6] units in a ratio of 1:1. For act. V2Mo10Ox-SBA-

65

15 a ratio of 1:3 was found. A comparison of the pseudo radial distribution function of act.

V2Mo10Ox-SBA-15 and act. PV2Mo10-SBA-15 confirmed the results of the XANES

analysis. The first peak of the Mo K edge FT(χ(k)·k3) of act. V2Mo10Ox-SBA-15 exhibited

differences compared to that of act. PV2Mo10-SBA-15. The first peak in both FT(χ(k)·k3)

originated from the tetrahedral and octahedral species on the SBA-15 support and could be

sufficiently simulated using four Mo-O distances accounting for the amount of octahedral

[MoO6] species (Table 5-2). The 1st and 2nd disorder parameters (1st-σ2, 2nd-σ

2) were

higher for act. V2Mo10Ox-SBA-15 and indicated a decreasing concentration of tetrahedral

[MoO4] units. The third Mo-O distance is considerably shorter than the distance in act.

PV2Mo10-SBA-15. In addition, the 4th disorder parameter (4th-σ2) is smaller than the

disorder parameter for act. PV2Mo10-SBA-15. This disorder parameter mainly represented

the fraction of octahedral MoO6 units. Hence, the reduced disorder parameter indicated an

increasing amount of octahedral structural motifs in act. V2Mo10Ox-SBA-15 compared to

act. PV2Mo10-SBA-15. Furthermore, the amplitude in the Mo K edge FT(χ(k)·k3) of act.

V2Mo10Ox-SBA-15 at higher Mo-Mo shells resembled the shape of α-MoO3. The Mo-Mo

distance at ~3.3 Ǻ is characteristic for α-MoO3. These results confirmed the existence of

crystalline α-MoO3 which was also identified by XRD before thermal treatment under

catalytic conditions (Fig. 5-6, right). Estimating the amount of α-MoO3 from the amplitude

Fig. 5-6: (left) Mo K edge FT(χ(k)·k3) of activated PV2Mo10-SBA-15 and activated V2Mo10Ox-

SBA-15 after thermal treatment in 5% propene and 5% oxygen in helium at 723 K; (right) XRD

of as prepared PV2Mo10-SBA-15, as prepared V2Mo10Ox-SBA-15, and simulated MoO3.

-0.04

0.00

0.04

0.08

0 1 2 3 4 5 6

R [Ǻ]

act. V2Mo10Ox -SBA-15 act. PV2Mo12-SBA-15

10 20 30 40 50 60 70

Inte

nsity

FT

(χ(k

)·k

3)

MoO3

PV2Mo10-SBA-15

V2Mo10Ox-SBA-15


66

at ~3.3 Ǻ (not phase corrected) in the pseudo radial distribution function yielded an

amount of about ~20%.


the Mo atoms in act. PV2Mo10-SBA-15 and act. V2Mo10Ox-SBA-15. Experimental parameters were

obtained from a refinement of a hexagonal MoO3 model structure (ICSD 75417 [135]) to the

experimental Mo K edge XAFS χ(k) of act. PV2Mo10-SBA-15 (k range from 3.6-16.0 Å-1

, R range

from 0.9 to 4.0 Å, E0v= ~ -5.2, residual ~12.8 Nind = 27, Nfree =12) and act. V2Mo10Ox-SBA-15 (k

range from 3.6-16.0 Å-1

, R range from 0.9 to 4.0 Å, E0 = ~ -0.4, residual ~ 10.9, Nind = 26, Nfree =14).

Subscript c indicates parameters that were correlated in the refinement.

Type act. PV2Mo11-SBA-15 Type act. V2Mo10Ox-SBA-15

N R(Ǻ) σ2(Ǻ

2) N R(Ǻ) σ

2(Ǻ

2)

Mo-O 2 1.67 0.0013 Mo-O 2 1.68 0.0023

Mo-O 2 1.88 0.0034c Mo-O 2 1.90c 0.0042c

Mo-O 1 2.18 0.0034c Mo-O 1 2.11c 0.0042c

Mo-O 1 2.34 0.0017 Mo-O 1 2.34 0.0001

Mo-Mo - - - Mo-Mo 1 3.14 0.0061c

Mo-Mo 2 3.49 0.0061c Mo-Mo 1 3.28 0.0061c

Mo-Mo 2 3.63 0.0061c Mo-Mo 2 3.71 0.0057

Mo-Mo 2 3.75 0.0100 Mo-Mo 2 3.92 0.0113

Comparison of the local structure around V centers in act. PVxMo12-x-SBA-15

(x = 0, 1, 2) and a reference act. V2Mo10Ox-SBA-15 under catalytic conditions

The evolution of the local structure around V centers in the supported catalysts and

reference materials differed from that of the Mo centers. Fig. 5-7 shows the V K edge

FT(χ(k)·k3) of act. PV2Mo10-SBA-15 (left) and act. V2Mo10Ox-SBA-15 (right). The

amplitudes at distances between 3-4 Ǻ indicated different scattering atoms. Single

scattering paths of a Na2MoO4 structure (ICSD 24312 [136]) were used for the EXAFS

refinement of act. V2Mo10Ox-SBA-15 and act. PV2Mo10-SBA-15.The theoretical model for

act. PV2Mo10-SBA-15 based on Na2Mo2O7 with replaced Mo atoms with V atoms per

formula unit. The theoretical model structure for act. PV2Mo10-SBA-15 based on the

67

Na2Mo2O7 structure with two replaced Mo atoms by V atoms per formula unit. Results of

the refinements for the V K edge FT(χ(k)·k3) are given in Table 5-3. The distances between

1-2 Ǻ corresponded to a tetrahedral [VO4] unit. Distances R and disorder parameters σ2

were nearly identical for act. PV2Mo10-SBA-15 and act. V2Mo10Ox-SBA-15 (Fig. 5-7). The

first Mo coordination sphere corresponded to a mixture of octahedral and

tetrahedral [MoOx] species. In contrast to the first Mo-O peak with six individual Mo-O

distances, the first V-O peak could be sufficiently simulated using two V-O distances. The

two V-O distances sufficiently accounted for the tetrahedral [VO4] species. Silica atoms

from the support were found at a distance of ~2.55 Ǻ. Additionally, a V-Mo distance was

identified in the V K edge FT(χ(k)·k3) of act. PV2Mo10-SBA-15. V-O and V-V distances in

act. V2Mo10Ox-SBA-15 were very similar to those in dehydrated VxOy-SBA-15

synthesized with a butylammonium decavanadate precursor.[95]

Assuming only V-V distances resulted in a sufficient agreement between experimental and

theoretical spectra in contrast to act. PV2Mo10-SBA-15. This indicated that [VOx] and

[MoOx] species were not present in close vicinity to each other. The results of Mo K and V

K edge analysis of act. PV2Mo10-SBA-15 and act. V2Mo10Ox-SBA-15 confirmed that only

act. PV2Mo10-SBA-15 contained supported V-O-Mo mixed oxides structural motifs

forming under catalytic conditions.

Active sites of selective oxidation catalysts often consist of multiple metal atoms.[137]

Synthesis routes of supported ternary oxides with different metal oxide precursors rarely

have been reported. Vanadium substituted Keggin ions enabled the synthesis of connected

-0.02

0.00

0.02

FT

(χ(k

)·k

3)

0 1 2 3 4 5 6

Experiment Theory

R [Ǻ] 0 1 2 3 4 5 6

-0.02

0.00

0.02

0.04

FT

(χ(k

)·k

3)

R [Ǻ]

Experiment Theory

Fig. 5-7: V K edge FT(χ(k)·k3) of (left) act. PV2Mo10-SBA-15 and (right) act. V2Mo10Ox-SBA-15

after thermal treatment in 5% propene and 5% oxygen in helium at 723 K.

act. PV2Mo10-SBA-15 act. V2Mo10Ox-SBA-15

68

[VOx] and [MoOx] species not readily available from physically mixed precursors.

Apparently, the proximity of vanadium and molybdenum in the Keggin precursors is a

prerequisite for obtaining connected metal oxide species on a support material.


the V atoms in act. PV2Mo10-SBA-15 and act. V2Mo10Ox-SBA-15. Experimental paramters were

obtained from a refinement of a modified [Mo2O7]2-

model system (ICSD 24312 [136]) were Mo is

replaced by V and Si additional added compared to the experimental V K edge XAFS χ(k) of act.

PV2Mo10-SBA-15 (k range from 3.0-10.0 Å-1

, R range from 0.9 to 3.8 Å, E0 = ~8.8, residual ~ 6.0

Nind = 13, Nfree =8) and act. V2Mo10Ox -SBA-15 (k range from 3.0-10.0 Å-1

, R range from 0.9 to 3.8

Å, E0 = ~8.8, residual ~ 11.3, Nind = 13, Nfree =8). Subscript c indicates parameters that were

correlated and f that were fixed in the refinement.

Type act. PV2Mo11-SBA-15 Type act. V2Mo10Ox-SBA-15

N R(Ǻ) σ2(Ǻ

2) N R(Ǻ) σ

2(Ǻ

2)

V-O 2 1.83 0.0183 V-O 2 1.82 0.0160

V-O 2 1.83c 0.0183c V-O 2 1.82c 0.0160c

V-Si 1 2.54 0.0172 V-Si 1 2.55 0.0094

V-O 1 2.91 0.005f V-O 1 2.95 0.0046f

V-O 1 3.10 0.0246 V-V 1 3.30 0.0089

V-Mo 1 3.60 0.0221 V-V 1 3.58 0.0089c

5.2.2 Local structure of P in activated PV2Mo10SBA-15 under catalytic conditions

Fig. 5-8 shows 31

P-MAS-NMR measurements of as-prepared and act. PV2Mo10-SBA-15 in

comparison to as-prepared and activated reference H3PO4-SBA-15. The 31

P MAS NMR

spectrum of PV2Mo10-SBA-15 resembled that of bulk PVMo11.[138] This confirmed that

the majority of P centers was located in Keggin ions supported on SBA-15.[139,140] The

peak in the spectrum of H3PO4-SBA-15 at 0.8 ppm could be assigned to molecular H3PO4.

In the spectrum of act. H3PO4-SBA-15 four pronounced peaks can be seen at chemical

shifts of 0.8, -10.8, 22.8, and -35.9 ppm. Zhi-qiang Zhang et al. reported similar results for

SiO2 impregnated with H3PO4.[140,141] Accordingly, the peak at 0.1 ppm is characteristic

for H3PO4, while the peaks at -10.8 and -22.8 ppm were attributed to terminal and internal

69

phosphate groups of condensed phosphates, respectively.[141] Krawietz et al. assigned the

peak at -35.9 ppm to silicon hydrogen tripolyposphate (SiHP3O10, -35 ppm).[142] For

H3[PMo12O40] supported on ZrO2 (PMo12-ZrO2) Devassy et al. investigated the nature of

phosphorous species depending on Keggin loading and calcination temperature.[54]

PMo12-ZrO2 exhibited a comparable broadening of the peaks in the 31

P MAS NMR

spectrum with increasing calcination temperature. Decomposition of the HPOM to the

oxide species was observed at temperatures above 723 K. Thermal stability of

H3[PW12O40] supported on ZrO2 (PW12-ZrO2) was investigated by López-Salinas et al..

The structural behaviour of PW12-ZrO2 during calcination was comparable to that of

PMo12-ZrO2. PW12-ZrO2 decomposed at temperatures above 773 K to form the

corresponding supported oxides.[56] The authors assigned an additional peak at -30 ppm to

phosphorous oxides exhibiting P-O-P motifs. In the 31

P MAS NMR spectra of act.

PV2Mo10-SBA-15 studied here, a broad resonance indicated structural rearrangement and a

40

(ppm)

100 80 60 20 0 -20 -40 -60 -80 -100

act. H3PO4-SBA-15

H3PO4-SBA-15 n

orm

. in

ten

sity

100 80 60 40 20 0 -20 -40 -60 -80 -100

(ppm)

act. PV2Mo10-SBA-15

PV2Mo10-SBA-15

no

rm.

inte

nsity

Fig. 5-8: 31

P MAS NMR spectra of asprepared H3PO4-SBA-15, PV2Mo10-SBA-15, and thermal

treated under catalytic conditions (5% propene and 5% oxygen in He) at 723 K act. H3PO4-SBA-

15 and act. PV2Mo10-SBA-15.

70

partial decomposition of the Keggin ions during thermal treatment under catalytic

conditions. Moreover, the 31

P MAS NMR spectra of act. PV2Mo10-SBA-15 resembled that

of a VPO-SBA-15 sample treated under oxidative and reductive conditions.[143] A broad

resonance observed for the VPO sample between -12 and -38 ppm was attributed to

various vanadyl orthophosphates phases (-8.4 to 21.2 ppm) and phosphorus bound to the

SBA-15 support (ca. -38 ppm). Comparable structural motifs can be assumed for act.

PV2Mo10-SBA-15. However, formation of phosphorous oxide or SiHP3O10 exhibiting

linked P-O-P structures could be excluded. In total, the 31

P MAS NMR results indicated a

variety of structural motifs in the activated samples studied here. Apparently, phosphorus

remained connected to the molybdate- and/or vanadate-species of the [(Mo,V)Ox] units

during propene oxidation conditions.

5.2.3 Structure directing effects of vanadium and the support material on the structure of

activated PV2Mo10-SBA-15 under catalytic conditions

Characteristic differences were revealed by comparing the structural evolution of bulk

HPOM during thermal treatment to that of supported HPOM. In bulk HPOM the Keggin

ion partially decomposes under catalytic conditions to form a lacunary Keggin anion.[13]

In this process Mo cations migrate on extra Keggin sites while remaining coordinated to

the resulting lacunary Keggin anion.[13] Driving force for the formation of lacunary

Keggin anions may be the relaxation of the Keggin structure at elevated temperature upon

removal of structural water. Eventually, this leads to the formation of more extended oxide

structures. These structural changes at temperatures above 573 K are accompanied by

reduction of the metal centers.[16,13] Vanadium incorprated in bulk HPOM acts as a

structural promoter facilitating the formation of the active (Mo, V) oxide phase under

catalytic conditions. The incorporated V centers result in a pronounced destabilization and

accelerated decomposition of the Keggin ion at elevated temperatures.[14,117] The

structural characteristics of model systems like MoOx-SBA-15 and VOx-SBA-15 depend

on their hydration states.[144,95] A comparable effect could be responsible for the

structural evolution of HPOM supported on SBA-15. Adsorbed water and silanol groups

from the support may possess a structure stabilizing effect on the Keggin ion. This effect

would be comparable to that of water of crystallization and constitutional water in bulk

71

HPOM under ambient conditions.[117] Vansant et al. reported that amorphous silica

showed dehydroxylation of silanol groups between 473-673 K resulting in a decrease of a

silanol density from 4.6 OH/nm2 (473 K) to 2.3 OH/nm

2 (673 K).[145] TG measurement

of PVMo11-SBA-15 showed a mass loss of about 2 wt.% between 473 and 673 K which

correlated with the temperature range of structural rearrangement of PVMo11-SBA-15

under catalytic conditions (c.f. Chapter 8.2; Fig. 8-1).

Thus, desorption of water and dehydroxylation of silanol groups may be responsible for the

formation of act. PVxMo12-x-SBA-15 (x = 0, 1, 2). SBA-15 seemed to possess a directing

effect on the formation of activated (Mo, V, P)Ox structures depending on the treatment

conditions (i.e. temperature, gas composition). Apparently, the thermal stability of Keggin

ions supported on SBA-15 was significantly decreased. While vanadium had a minor

influence on the thermal stability, the interaction with the support material appeared to be

more important. Nevertheless, vanadium still had a distinct structure directing effect to

form V-O-Mo mixed structures under catalytic conditions. The presence of tetrahedral

[VO4] species lead to an increasing ratio of tetrahedral [MoO4] to octahedral [MoO6]

species. Compared to SBA-15 other support materials exhibit different structure directing

effects depending on the acidity of the surface.[25,146,147] For instance, mainly isolated

[MoO4] units existed on an alkaline MgO support in agreement with the behaviour of Mo

oxides in alkaline solution.[137] Here, the acidic surface of silica SBA-15 resulted in

mainly linked M-O-M (M = Mo, V) species again corresponding to the behaviour of

vanadates and molybdates in acidic solutions.[37,148]

5.3 Functional characterization of PVxMo12-x-SBA-15 (x = 0, 1, 2)

5.3.1 Reducibility

Fig. 5-9 shows the H2 TPR profiles of PVxMo12-x-SBA-15 (x = 0, 1, 2). The resulted

H2 TPR profiles revealed one sharp (~800 K) and a very broad (873- 973 K) reduction

peak. The shapes of the H2 TPR profiles were nearly identical, just the reduction

temperatures slightly increased with the vanadium content from 790 K for PMo12-SBA-15

to 808 K for PV2Mo10-SBA-15. The H2 TPR profiles at ~800 K were comparable to

molybdenum oxide supported on SBA-15 with Mo loadings between 9.5 wt.% and 13.3

72

wt.%.[149,150] Lou et al. assigned the sharp reduction peak from oligomeric MoOx

species or small MoOx clusters and the broaded signal above ~800 K to the reduction of

monomeric MoOx species.[150] Vanadium oxide supported on SBA-15 with a loading

between 1.0 wt.% V and 4.5 wt.% V reduced between 769 K and 799 K depended on the

dispersion degree.[151] Hence, the reducibility were comparable to molybdenum oxides

supported on SBA-15. A comparison of vanadium and molybdenum oxides supported on

Al2O3 with similar metal surface density showed that the temperature of the maximum of

H2-consumption were slightly higher (~10 K) for the supported vanadium oxides.[152] A

comparable intrinsic effect of the metals could be responsible for the shift to higher

reduction temperatures with higher vanadium loading. However, no significant changes in

the reducibility depending on the vanadium substitution degree were detected.

5.3.2 Catalytic performance

Reaction rates and selectivities of PMo12-SBA-15, PVMo11-SBA-15, PV2Mo10-SBA-15,

V2Mo10Ox-SBA-15, and bulk PV2Mo10 in propene oxidation at 723 K are shown in Fig.

5-10. Reaction rates for PVxMo12-x-SBA-15 (x = 0, 1, 2) were calculated for similar

propene oxidation conditions (~ 14-16% propene conversion). The propene conversion for

bulk PV2Mo10 (~ 3%) was lower due to the strongly decreased catalytic activity. Adjusting

H2 c

onsu

mp

tio

n

T [K]

373 473 573 673 773 873 973

790 K

800 K

808 K

PMo12-SBA-15

PVMo11-SBA-15

PV2Mo10-SBA-15

Fig. 5-9: Temperature.programmed reduction (H2-TPR) of PMo12-SBA-15, PVMo11-SBA-15, and

PV2Mo10-SBA-15 measured at a heating rate of 8 Kmin-1

5% H2 in Ar.

73

to similar propene oxidation conditions for the low active sample would lead to an large

volume and thermal effects. Hence, comparing of the catalytic performance of PVxMo12-x-

SBA-15 (x = 0, 1, 2) to that of the low active sample needs to be done carefully.

Reaction rates for PVxMo12-x-SBA-15 (x = 0, 1, 2) were similar and independent of the

degree of vanadium substitution. Selectivities for CO increased at the expense of

acetaldehyd with higher degree of vanadium substitution. The results of the catalytic

performance of bulk PVxMo12-x (x = 0, 1, 2) showed strong increased reaction

rates for vanadium substituted bulk HPOM. Selectivities towards oxygenates, especially

acetaldehyd decreased at the expense of CO with increased vanadium substitution degree.

PV2Mo10 was chosen as bulk HPOM for comparing the product distribution (i.e. acrylic

acid, acetic acid, acrolein, acetone, propionaldehyde, acetaldehyde, CO, and CO2) to

PV2Mo10-SBA-15. While, PV2Mo10 showed a slightly increased selectivity towards

acrolein, PV2Mo10-SBA-15 exhibited an increased selectivity towards acetic acid. Total

oxidation products CO and CO2 amounted to about ~55% in the resulting oxidation

products. Conversely, the reaction rates of PV2Mo10 and PV2Mo10-SBA-15 exhibited

considerable differences. The catalytic activity of PV2Mo10-SBA-15 was four times higher

than that of PV2Mo10. Apparently, higher dispersion and an improved surface to bulk ratio

of Keggin ions resulted in a much increased activity at comparable selectivity. Structural

0

20

40

60

80

100

0

10

20

30

40

50

60

70

a b c e d

acrylic acid


acetone

acetaldehyde

CO

CO2 propionaldehyde

Se

lectivity [%

]

Re

actio

n r

ate

[µ

mo

l(p

rope

ne

)g-1(M

o)s

-1]

Fig. 5-10: Reaction rate (µmol(propene)g-1

(Mo)s-1

) and selectivity of (a) PMo12-SBA-15, (b)

PVMo11-SBA-15, (c) PV2Mo10-SBA-15, (d) V2Mo10Ox-SBA-15, and (e) bulk PV2Mo10 in 5%

propene and 5% oxygen in He at 723 K.

74

analysis of act. PVxMo12-x-SBA-15 (x = 0, 1, 2) revealed an increased concentration of

tetrahedral [MoO4] units at comparable degree of oligomerization. Apparently, the

additional [VOx] species in act. PVxMo12-x-SBA-15 (x = 0, 1, 2) lead to new

multifunctional active sites, resulting in a different product distribution without influence

to the reaction rates.

In contrast to the HPOM samples, V2Mo10Ox-SBA-15 showed a decreasing activity and a

different product distribution compared to PV2Mo10-SBA-15. While, the amount of total

oxidation products in the gas phase was considerably lower, an increasing selectivity to

acetaldehyde was determined. The structural analysis indicated that act. V2Mo10Ox-SBA-

15 possessed an increased amount of higher oligomerized Mo species and a decreased

content of tetrahedral [MoO4] units. It was shown earlier, that higher oligomerized V and

Mo species showed an increased selectivity towards oxidations products.[137,153]

Additionally, the majority of [VOx] and [MoOx] species in act. V2Mo10Ox-SBA-15 did not

seem to be directly connected to each other. Local separation of the [VOx] and [MoOx]

species may be responsible for the increased concentration of acetaldehyde, which is

mainly formed by vanadium based catalysts in contrast to molybdenum based

catalysts.[1,31,50] Apparently, the new multifunctional active site consisting of connected

[VO4] and [MoOx] units lead to an increased amount of total oxidation products for act.

PV2Mo10-SBA-15 in contrast to not connected [VO4] and [MoOx] units in act. V2Mo10Ox-

SBA-15. Furthermore, availability of dimeric or oligomeric [(V,Mo)Ox] units increased the

selectivity towards oxygenates in contrast to isolated [MoO4] units [47,50]. Hence,

connected [VO4] and [MoOx] units and the general degree of oligomerization of

[(V,Mo)Ox] units influenced the catalytic activity and selectivity towards propen oxidation.

5.3.3 Influence of phosphorus species on catalytic activity

Phosphorus containing catalysts (i.e. VPO, FePO, MoPO) play a crucial role as oxidation

catalysts.[128] Adding small amounts of phosphoric acid to the feed showed positive

effects on long-term stability and catalytic performance of FePO catalysts during ODH of

isobutyric acid into methacrylic acid. The phosphorus source was needed to maintain a

constant P/Fe ratio at the surface of the catalyst.[129] VPO catalysts showed migration of

phosphorus species to the surface and a decreasing amount of phosphorus in the catalyst

75

during water vapor treatment. The excess of phosphorus on the surface suppressed

oxidation of VPO catalysts and hindered formation of active sites for oxidation reactions.

Subsequently, hydrolysis of P-O-P or P-O-V groups resulted in a removal of phosphate

groups on the surface and an increasing activity.[128,130] Moreover, adding V and P to

MoOx based catalysts for ODH of ethane afforded an increasing selectivity and conversion

towards ethane. Haddad et al. suggested synergistic effects between structurally related

oxides like (V,Mo)5O14 and (V, Mo)PO phases to be responsible for the enhancened

catalytic performance.[154] Here, the formation of water as byproduct during oxidation of

propene may have favored the migration of phosphorus species under catalytic conditions.

The different surface to bulk ratios of PV2Mo10 and PV2Mo10-SBA-15 could lead to a

different migration and hydrolysis of phosphate groups in the materials. The available

Keggins in PV2Mo10-SBA-15 were located on the surface of the support material.

Therefore, an enrichment of phosphate groups was not possible for PV2Mo10-SBA-15. An

enrichment of phosphate groups on the surface of bulk PV2Mo10 would results in a higher

P/M (M = Mo, V) with a possible influence on catalytic activity and selectivity. However,

the comparable selectivity of bulk PV2Mo10 and PV2Mo10-SBA-15 (Fig. 5-10) was

indicative of similar active centers despite different P/M (M = Mo, V) ratios. Therefore,

the increased catalytic activity of PV2Mo10-SBA-15 was attributed to a higher dispersion

and an improved surface to bulk ratio of supported Keggin ions.

76

5.4 Summary

The structural evolution of PVxMo12-x-SBA-15 (x = 0, 1, 2) and a mixture of V and Mo

oxides supported on SBA-15 during propene oxidation conditions was examined by in situ

X-ray absorption spectroscopy at the Mo K and V K edge. Additionally, 31

P MAS NMR

measurements of supported PV2Mo10-SBA-15 and H3PO4-SBA-15 after catalytic reaction

were performed. During thermal treatment under propene oxidation conditions PVxMo12-x-

SBA-15 (x = 0, 1, 2) formed a mixture of mainly tetrahedral [MoOx] and [VOx] units.

Changes in the local structure around the V centers coincided with structural changes of

the Mo centers and the onset of catalytic activity. The concentration of tetrahedral [MoO4]

units correlated with the degree of vanadium substitution without affecting to the degree of

oligomerization of the [MoxOy] and [VxOy] species. Apparently, the mainly tetrahedral

[MoOx] and [VOx] units were in close vicinity and able to interacted under catalytic

conditions. The new multifunctional active site consisting of connected [VO4] and [MoOx]

units lead to an increased amount of total oxidation products without influencing the

reaction rate. Conversely, structural analysis of activated reference V2Mo10Ox-SBA-15

synthesized with individual V and Mo precursors indicated that [VOx] and [MoOx] species

were mostly separated from each other on the surface of SBA-15. Moreover, activated

V2Mo10Ox-SBA-15 possessed an increased amount of higher oligomerized [MoxOy]

species and a decreased content of tetrahedral [MoO4] units. This may explain the observed

increased selectivity towards partial oxidations products. The structural environment of

phosphorus in PV2Mo10-SBA-15 under catalytic conditions corresponded to a mixture of

various species. Phosphorus was linked to both the SBA-15 support via P-O-Si bonds and

to the Mo or V centers of the [MoOx] or [VOx] units. In total, supported vanadium

substituted Keggin ions are suitable precursors to synthesize connected [VOx] and [MoOx]

species on SBA-15. Apparently, the proximity of vanadium and molybdenum in the

Keggin precursors a prerequisite for obtaining connected metal oxide species.

77

6 Characterization of PWxMo12-x-SBA-15 (x = 1, 2)


Keggin type H4[PVMo11O40] has been reported to exhibit a pronounced interaction effect

with SBA-15 as support material.[14] This effect resulted in a further decreased thermal

stability of the supported Keggin ions compared to the bulk materials. PVMo11-SBA-15

formed a mixture of tetrahedrally and octahedrally coordinated [MoO4] and [MoO6] units

during propene oxidation.[14] The structural evolution and role of tungsten in PWxMo12-x-

SBA-15 (x = 1, 2) during propene oxidation were not part of previous investigations.

Therefore, a first structural and functional characterization were necessary to elucidated

structure activity of supported molybdenum oxide based model catalysts. In this chapter in

situ X-ray absorption spectroscopy investigations at the LIII-LI edges of PWxMo12-x-SBA-

15 (x = 1, 2) during propene oxidation conditions were performed. Correlations between

structural evolution of [MoOx] and [WOx] units and performance under catalytic conditions

will be described. Additionally, the obtained structures and catalytic performances were

compared to a suitable supported reference material.

6.1 Experimental

6.1.1 Sample Characterization



beamline at X and W LIII-LI edges (10.204-12.098 keV) at beamline C at the Hamburg

Synchrotron Radiation Laboratory, HASYLAB. Using a Si(311) double crystal

monochromator at Beamline X for the Mo K edge and a Si(111) double crystal

monochromator at Beamline C for the W LIII-LI edges. In situ experiments were conducted

in a flow reactor at atmospheric pressure (5 vol% oxygen in He, total flow ~30 ml/min,

temperature range from 303 to 723 K, heating rate 4 K/min). The gas phase composition at

the cell outlet was continuously monitored using a non-calibrated mass spectrometer in a

multiple ion detection mode (Omnistar from Pfeiffer).

78












Single scattering paths in the hexagonal MoO3 model structure (ICSD 75417 [135]) and a

modified H3[PMo12O40] structure (ICSD 209 [14,93]) were calculated up to 6.0 Å with a

lower limit of 4.0% in amplitude with respect to the strongest backscattering path. EXAFS

refinements were performed in R space simultaneously to magnitude and imaginary part of

a Fourier transformed k3-weighted and k

1-weighted experimental χ(k) using the standard

EXAFS formula.[94] This procedure reduces the correlation between the various XAFS

fitting parameters. Structural parameters allowed to vary in the refinement were (i)

disorder parameter σ2

of selected single-scattering paths assuming a symmetrical pair-

distribution function and (ii) distances of selected single-scattering paths.





Wide-angle scans (5-90° 2θ, variable slits) were collected in reflection mode using a

silicon sample holder. Small-angle scans (0.4-6.0° 2θ, fixed slits) were measured in


79






consumption was measured with a TCD unit. All samples were pretreated with a gas flow

of 60 ml/min Ar at 393 K for about 45 min before starting the measurement. For

measurements 37.2 mg PMo12-SBA-15, 33.4 mg PWMo11-SBA-15, and 34.3 mg

PW2Mo10-SBA-15 were used.







with boron nitride (Alfa Aesar, 99.5%) to result in an overall sample mass of 375 mg. For

catalytic testing in selective propene oxidation a mixture of 5% propene (Linde Gas, 10%

propene (3.5) in He (5.0)) and 5% oxygen (Linde Gas, 20% O2 (5.0) in He (5.0)) in helium

(Air Liquide, 6.0) was used in a temperature range of 293-723 K Reactant gas flow rates of

oxygen, propene, and helium were adjusted with separate mass flow controllers



column connected to an AL2O3/MAPD column or a fused silica restriction (25 m·0.32 mm

each) connected to a flame ionization detector. O2, CO, and CO2 were separated using a

Hayesep Q (2 m x 1/8``) and a Hayesep T packed column (0.5 m x 1/8``) as precolumns

combined with a back flush. For separation, a Hayesep Q packed column (0.5 m x 1/8``)

was connected via a molsieve (1.5 m x 1/8``) to a thermal conductivity detector (TCD).

Details about the calculation of conversion, selectivity, and reaction rate are described in

chapter 3.2.

80

6.1.2 Sample preparation

PWxMo12-x (x=1,2) supported SBA-15 were prepared as described in chapter 4.1.

A reference material (denoted as W2Mo10Ox-SBA-15) was prepared as follows. 241.0 mg

(NH4)6Mo7O24·4H2O and 29.3 mg (NH4)6W12O39·xH2O were dissolved in water and were

deposited via incipient wetness on 1 g SBA-15 to obtain metal loading of 10 wt.% Mo and

3.8 wt.% W. The sample was dried for 18 h at room temperature and calcined for 3 h at

773 K.

6.2 Structural evolution of PWxMo12-x-SBA-15 (x = 1, 2) under catalytic

conditions

In situ XANES analysis

PWxMo12-x-SBA-15 (x = 1, 2) was investigated by in situ XAS in propene oxidation

conditions. Fig. 6-1 shows the evolution of molybdenum Mo K edge XANES spectra of

PW2Mo10-SBA-15 during temperature-programmed treatment in 5% propene and 5%

oxygen. Mo K edge XANES spectra of PWxMo12-x-SBA-15 (x = 1, 2) remained unchanged

within the temperature range from 298 K through 473 K comparable to PVxMo12-x-SBA-15

20.00

20.05

20.10

20.15

20.20

373

473

573 673

No

rma

lize

d

abso

rptio

n

T [K]

Photon energy [keV]

Fig. 6-1: in situ Mo K edge XANES spectra of PW2Mo10-SBA-15 during temperature-programmed


723 K.

81

(x = 0, 1, 2) (cf. chapter 5.2). Hence, the Keggin ion appeared to be stable up to 473 K

independent of the degree of substitution. Between 473-648 K the pre-edge peak height

increased during thermal treatment under catalytic conditions.[15,23] This increasing pre-

edge peak corresponded to a structural rearrangement to tetrahedral [MoO4] species. Linear

combinations of the XANES spectra of MoO3 and bulk Na2MoO4 references were used to

determine the amount of tetrahedral [MoO4] and octahedral [MoO6] units of PWxMo12-x-

SBA-15 (x = 0, 1, 2) during propene oxidation conditions (cf. chapter 7.4). The evolution

of the tetrahedral [MoO4] to octahedral [MoO6] ratio of PWxMo12-x-SBA-15 (x = 0, 1, 2)

during propene oxidation conditions was shown in Fig. 6-2. In contrast to PVxMo12-x-

SBA-15 (x = 1, 2) the W substitution lead to decreased concentration of tetrahedral

[MoO4]

species. Hence, tungsten substitution lead to mostly octahedral [MoO6] species resulting

during propene oxidation conditions. Fig. 6-3 depicts the W LIII and LI edge XANES

spectra of PWxMo12-x-SBA-15 (x = 1, 2) during temperature-programmed treatment in 5%

propene and 5% oxygen. Compared to the onset temperature of 473 K of the structural

rearrangement of the [VOx] and [MoOx] units for PVxMo12-x-SBA-15 (x = 1, 2) (c.f. 5.2), a

delayed structural change of the initial octahedral [WO6] units was observed. The onset of

structural changes increased to 550 K for PW2Mo12-SBA-15 and to 623 K for PWMo11-

SBA-15. The X-ray absorption W LIII edge corresponds to electron transitions from 2p3/2

Fig. 6-2: Quantification of the [MoO4]/[MoO6] ratio of PMo12-SBA-15, PWMo11-SBA-15, and

PW2Mo10-SBA-15 during thermal treatment under propene oxidation conditions.

0

10

20

30

40

50

60

300 400 500 600 700

T [K]

[MoO

4]/[M

oO

6] ra

tio

[%

]

PMo12-SBA-15 PWMo11-SBA-15 PW2Mo10-SBA-15

82

orbital to vacant 5d orbitals and to the vacuum level.[155] The contribution of the possible

p-s transitions is ca. 50 times weaker.[156] Hence, especially the white line W LIII edge

reflects the electronic state of the vacant states of the absorbing atoms. The white line in

the W LIII edge is assigned to the 5d orbital split by the ligand field. The orbital split by an

Fig. 6-4: (a) W LIII edge XANES spectra of PW2Mo10-SBA-15 and (b) 2nd derivates of W LIII

edge XANES spectra of PW2Mo10-SBA-15; ( ) experiment, ( ) fitting function, and ( )fitting

peaks.

5d

10.18 10.20 10.22 10.24

No

rma

lize

d

abso

rptio

n

2nd

de

riva

tive

s

Photon energy [keV]

(a)

(b)

2p3/2

eg

t2g

d-orbital

splitting

Fig. 6-3: in situ W LIII edge (left) and W LI edge (right) XANES spectra of PW2Mo10-SBA-15

during temperature-programmed treatment in 5% propene and 5% oxygen in helium in a

temperature range between 300 K and 723 K.

T [K]

No

rma

lize

d

abso

rptio

n

12.05 12.15

12.25 12.35

373 473

573 673 10.35

10.15 10.20

10.25 10.30

373

473 573

673

T [K]

No

rma

lize

d

abso

rptio

n

Photon energy [keV]

Photon energy [keV]

83

octahedral ligand field is stronger than in a tetrahedral ligand field.[157,158] Therefore, an

analysis of the white line was suitable to elucidate the electronic structure of the possible

structural motifs. Fig. 6-4 shows an example for a white line analysis. The 2nd derivates of

the W LIII edge spectra resulted in a splitted peak representing the electron transitions from

2p3/2 to split 5d states (t2g and eg orbitals). The difference in energy position of the splitted

peaks corresponds to the energy difference between eg and t2g. For elucidating the energy

difference, the 2nd derivates of two Lorentz functions were used to simulated the 2nd

derivate of the W LIII edge spectra. The resulted positions of the two minima were used to

calculate the energy difference.

W LI edge XANES spectra represent the transition from the 2s orbital and have "K edge

character". Hence, the W LI edge XANES spectra may be interpreted comparable to a K

edge spectrum. A change in the pre edge peak height may correspond to a structural

rearrangement. Fig. 6-5 shows the results of the analysis of W LIII and LI edge XANES

spectra meausred during propene oxidation conditions. XAS analysis at W LI edge for

PWMo11-SBA-15 was hardly feasible due to the low content of W (~1.8 wt.%) beside Mo

(~10 wt.%). The structural changes of the [WO6] in PVxMo12-x-SBA-15 (x = 1, 2) were

delayed compared to [VOx] units in PVxMo12-x-SBA-15 (x = 1, 2) during propene

oxidation conditions (cf. chapter 5.2). [WO6] species in PWMo11-SBA-15 and in

PW2Mo10-SBA-15 changed their local structure above 620 K and 560 K, respectively. This

( )

W L

I pre

ed

ge p

ea

k h

eig

ht

T [K]

300 400 500 600 700

( )

En

erg

y G

ap [e

V]

2.5

2.6

2.7

2.8

2.9

3.0

3.1

3.2

0.360

0.365

0.370

0.375

0.380

0.385

0.390

0.395

0.400 PWMo11-SBA-15 PW2Mo10-SBA-15

Fig. 6-5: (square) Evolution of the energy gap of PWMo11-SBA-15 and PW2Mo10-SBA-15 during

thermal treatment under propene oxidation conditions; (cycle) evolution of the pre edge height of W

LI edge XANES spectra of PW2Mo10-SBA-15 during thermal treatment under propene oxidation

conditions.

84

temperatures corresponded to the temperatures where the major structural rearrangement of

the [MoOx] species were finished. For elucidating, the type of structural rearrangement, a

detailed analysis of the 2nd derivates of the W LIII edge XANES spectra was necessary.

Fig. 6-6 shows the 2nd derivates of the W LIII edge XANES spectra of PW2Mo10-SBA-15

before and after propene oxidation conditions and during propene oxidation at 723 K.

Additionally, the 2nd derivates of the W LIII edge XANES spectra of WO3 and Na2WO4

were used as references. Unexpectedly the 2nd derivates of the W LIII edge XANES

spectra of PW2Mo10-SBA-15 after propene oxidation conditions exhibited an increased

energy gap in contrast to the derivates of the W LIII edge XANES spectra of PW2Mo10-

SBA-15 during propene oxidation conditions at 723 K. An analysis of the ratio of the areas

of the Lorentz functions used for the refinement revealed that the ligand field under

propene oxidation conditions was octahedral. Comparable result was obtained for

PWMo11-SBA-15. An identification of the type of ligand field of the unknown structure

motif was possible, because the X-ray absorption intensity is t2g:eg = 3:2 for an octahedral

[WO6] and eg:t2g = 2:3 for a tetrahedral [WO4] unit.[157] Table 6-1 summarizes the results

of the detailed W LIII edge analysis. Hence, the initial octahedral [WO6] units persisted

Fig. 6-6: Second derivates of W LIII edge XANES spectra of (left) WO3 and Na2WO4 and (right)

PW2Mo10-SBA-15 before, at 723 K, and after propene oxidation; experiment ( ), fitting function

( ), and fitting peaks ( ).

10.19 10.20 10.21 10.22 10.23 10.24

2nd D

erivative

s

10.19 10.20 10.21 10.22 10.23 10.24

2nd D

erivative

s

WO3

Na2WO4

PW2Mo10SBA-15

before

at 723 K

after

Photon energy [keV] Photon energy [keV]

85

during propene oxidation conditions in contrast to the [MoO4] and [MoO6] units in

PWxMo12-x-SBA-15 (x = 0, 1, 2) and vanadium substituted PVxMo12-x-SBA-15 (x = 1, 2)

(cf. chapter 5.2). Nevertheless structural rearrangements for PWMo11-SBA-15 (620-723 K)

and PW2Mo10-SBA-15 (560-723K) were identified and could be assigned to a reversible

distortion of the octahedral [WO6] units under propene oxidation conditions. The reversible

distortion of the [WO6] units explained the delayed onset temperature of the structural

rearrangement as well. The octahedral [WO6] seemed to influenced the structural

rearrangements of the octahedral [MoO6] to tetrahedral [MoO4] units under catalytic

conditions. Therefore, a shift to increased onset temperatures of the distortion process

depending on the decreased degree of W substitution was detectable.

Table 6-1: Peak positions of the fitting Lorentz peaks, splitted peak energy (difference of the peak

positions of the fitting Lorentz peaks), quotient of the fitting peak areas (peak 1area/ peak 2area), and

the resulting ligand field of PW2Mo10-SBA-15 before and after propene oxidation conditions and at

723 K at propene oxidation conditions and the references WO3 and Na2WO4..

Peak 1 [keV] Peak 2 [keV] splitted peak

energy [eV]

peak 1area /

peak 2area

Resulted

ligand field

PW2Mo10-SBA-15

(before) 10.2151 10.2183 3.2 1.38 Oh

PW2Mo10-SBA-15

(723 K) 10.2153 10.2179 2.6 1.52 Oh

PW2Mo10-SBA-15

(after) 10.2152 10.2183 3.1 1.37 Oh

WO3 10.2147 10.2187 4.0 1.29 Oh

Na2WO4 10.2143 10.2167 2.4 0.68 Td

The interaction between the [MoOx] and [WO6] units resulted in a structure directing effect

to mostly octahedral [MoO6] units under propene oxidation conditions depending on the

degree of W substitution. The structure directing effect of the addenda tungsten atoms

differed from the structure directing of addenda vanadium in supported HPOM. In

PVxMo12-x-SBA-15 (x = 1, 2), the [MoO6] units were influenced by the neighboring [VO6]

86

units of the initial Keggin ion structure resulting in mostly tetrahedral [MoO4] and [VO4]

units depending on the degree of V substitution during thermal treatment under propene

oxidation conditions (cf. chapter 5.2).

6.2.1 Local structure in activated PWxMo12-x-SBA-15 (x = 0, 1, 2) and a reference

W2Mo10Ox-SBA-15 under catalytic conditions

Local structure around the Mo centers in act. PWxMo12-x-SBA-15 (x = 0, 1, 2)

Fig. 5-5 (left) shows the Mo K edge FT(χ(k)·k3) of act. PMo12-SBA-15, act. PWMo11-

SBA-15, and act. PW2Mo10-SBA-15 after thermal treatment under propene oxidation

conditions at 723 K. The resulted Mo K edge FT(χ(k)·k3) of PWxMo12-x-SBA-15 ( x = 0, 1,

2) were similar. Minor differences are marked in the Mo K edge FT(χ(k)·k3) and the Mo K

edge χ(k)·k3. For a more detailed analysis hexagonal MoO3 was used as structural model.

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0 1 2 3 4 5 6

R [Ǻ]

FT

(χ(k

)·k

3)

4 6 8 10 12 14 16

-4

0

4

8

12

k [Ǻ]-1

χ(k)·

k3

PWMo10-SBA-15

PW2Mo10-SBA-15

PMo12-SBA-15

Fig. 6-7: (left) Mo K edge FT(χ(k)·k3) and (right) Mo K edge χ(k)·k

3 of act. PMo12-SBA-15, act.

PWMo11-SBA-15, and act. PW2Mo10-SBA-15 after thermal treatment under propene oxidation

conditions at 723 K.

87

XAFS phases and amplitudes were calculated for Mo-O and Mo-Mo distances and used for

EXAFS refinement. The results of the refinement are shown in Table 6-2. The first peak of

Mo K edge FT(χ(k)·k3) of as prepared PWxMo12-x-SBA-15 (x = 1, 2) (cf. chapter 4.3)

exhibited differences compared to act. PWxMo12-x-SBA-15 (x = 1, 2). The first peak in the

FT(χ(k)·k3) originated from tetrahedral and octahedral [MoOx] species on the SBA-15

support and could be sufficiently simulated using four Mo-O distances. These four

distances sufficiently accounted for the minor amount of octahedral [MoO6] species.

Confirming the results of the XANES analysis 1st and 2nd disorder parameters (1st-σ2,

2nd-σ2) were higher for act. PWMo11-SBA-15 and PW2Mo10-SBA-15 indicating a

decreasing amount of tetrahedral structural motifs compared to act. PVxMo12-x-SBA-15

(x = 0, 1, 2). In addition, the 4th disorder parameters were smaller than the disorder

parameter for act. PMo12-SBA-15. This disorder parameter represented mainly the fraction

of octahedral [MoO6] species. Hence, the reduced disorder parameter indicated an

increasing amount of octahedral structural motifs in act. PWxMo12-x-SBA-15 (x = 1, 2)

compared to act. PMo12 -SBA-15. Therefore a structure directing effect of addenda

tungsten resulting in a decreased ratio of tetrahedral [MoO4] to octahedral [MoO6] under


the Mo atoms in act. PWxMo12-x -SBA-15 (x = 0, 1, 2). Experimental parameters were obtained


Mo K edge XAFS χ(k) of act. PWxMo12-x -SBA-15 (x = 0, 1, 2) (k range from 3.4-16.0 Å-1

, R range

from 0.9 to 4.0 Å, E0= ~ -5.2, residuals ~12.1-14.7 Nind = 26, Nfree =12). Subscript c indicates

parameters that were correlated and f fixed in the refinement.

hex-MoO3

model

act. PMo12-

SBA-15

act. PWMo11-

SBA-15

act. PW2Mo10-

SBA-15


2) R(Å) σ

2(Å

2) R(Å) σ

2(Å

2)

Mo-O 2 1.67 1.67 0.0015 1.67 0.0019 1.68 0.0024

Mo-O 2 1.96 1.89 0.0038c 1.90 0.0042c 1.90 0.0045c

Mo-O 1 2.20 2.19 0.0038c 2.18 0.0042c 2.16 0.0045c

Mo-O 1 2.38 2.35 0.0011 2.35 0.0009 2.35 0.0008

Mo-Mo 2 3.31 3.49 0.0068c 3.49 0.0077c 3.49 0.0072c

Mo-Mo 2 3.73 3.63 0.0068c 3.63 0.0077c 3.63f 0.0072c

Mo-Mo 2 4.03 3.73 0.0100 3.72 0.0103 3.74 0.0095

88

propene oxidation conditions was determined. The distinct peak at ~3 Å (not phase

corrected) in the FT(χ(k)·k3) indicated the formation of dimeric or oligomeric [MoxOy]

units on SBA-15. Therefore, isolated octahedral [MoO6] and tetrahedral [MoO4] units can

be excluded as major molybdenum oxide species.

Local structure around the W centers in act. PWxMo12-x-SBA-15 (x = 1, 2)

Fig. 6-8 (left) shows the W LIII edge FT(χ(k)·k3) of act. PW2Mo10-SBA-15 at 723 K during

propene oxidation conditions. Fig. 6-8 (right) depicts the W LIII edge FT(χ(k)·k3) of act.

PWMo11-SBA-15, and act. PW2Mo10-SBA-15 after thermal treatment under propene

oxidation conditions at 723 K. Comparing the W LIII edge FT(χ(k)·k3) of act. PW2Mo10-

SBA-15 at 723 K and after propene oxidation conditions resulted in significant differences.

For a detailed structure analysis theoretical XAFS phases and amplitudes were calculated

for W-O, W-Si and W-Mo distances and used for EXAFS refinement. The used theoretical

Fig. 6-8: Experimental (solid) and theoretical (dashed) W LIII edge FT(χ(k)·k3) of act. (left)

PW2Mo10-SBA-15 during thermal treatment in 5% propene and 5% oxygen in helium at 723 K;

(right) act. PWMo11-SBA-15 and act. PW2Mo10-SBA-15 after thermal treatment in 5% propene and

5% oxygen in helium at 723 K.

-0.02

0.00

0.02

0.04

0 1 2 3 4 5 6

0.00

0.05

0.10

FT

(χ(k

)·k

3)

R [Ǻ]

PW2Mo10-SBA-15

PWMo11-SBA-15

0 1 2 3 4 5 6

FT

(χ(k

)·k

3)

R [Ǻ]

89

model system based on modified H3[PMo12O40] (ICSD 209 [14,93]) where P was replaced

by Si. Thus, the model structure corresponded to a former triad of the Keggin ion with a Si

bond. The result of the refinements are summarized in Table 6-3. The shapes of W LIII

edge FT(χ(k)·k3) of as prepared PWxMo12-x-SBA-15 (x = 1, 2) (cf. chapter 4.3) were

different to that of act. PWxMo12-x-SBA-15 (x = 1, 2). This results confirmed the

assumption of structural changes of [WOx] units in PWxMo12-x-SBA-15 (x = 1, 2) during

propene oxidation conditions. The first peak of W LIII edge FT(χ(k)·k3) of act. PWxMo12-x-

SBA-15 (x = 1, 2) could be sufficiently simulated using three W-O distances. These

distances sufficiently accounted for the amount of octahedral [WO6] species. The

refinement of the W LIII edge FT(χ(k)·k3) of PW2Mo10-SBA-15 during propene oxidation

at 723 K indicated two decreased disorder parameters (1st-σ2, 2nd-σ

2) and one increased

disorder parameter (3rd-σ2) compared to PW2Mo10-SBA-15 after catalytic conditions. The

resulting distances of the first shell of both W LIII edge FT(χ(k)·k3) were nearly identical.

Hence, the different disorder parameters corroborated a distorted arrangement of the

octahedral [WO6] units. Generally, disorder parameters will increase linear with

temperature, if a structural rearrangement can be excluded.[159,13] Hence the identified


the Mo atoms in act. PWMo11-SBA-15, act. PW2Mo10-SBA-15 after and act. PW2Mo10-SBA-15

during thermal treatment in 5% propene and 5% oxygen in helium at 723 K. Experimental

paramters were obtained from a refinement of modified H3[PMo12O40] (ICSD 209 [14,93]) model

structure to the experimental W LIII edge XAFS χ(k) of act. PWxMo12-x-SBA-15 (x = 1, 2) (k range

from 3.0-13.6 Å-1

, R range from 1.0 to 3.8 Å, E0 = ~ -4.5 residual ~12.9 Nind = 10, Nfree = 20).


act. PWMo11-

SBA-15

act. PW2Mo11-

SBA-15 (723 K)

act. PW2Mo11-

SBA-15


2) R(Å) σ

2(Å

2) R(Å) σ

2(Å

2)

W-O 2 1.68 1.79 0.0048c 1.76 0.0055c 1.77 0.0065c

W-O 2 1.91 1.69 0.0048c 1.69 0.0055c 1.71 0.0065c

W-O 2 1.92 1.89 0.0021 1.89 0.0032 1.89 0.0026

W-Mo 2 3.42 3.56 0.0117c 3.53 0.0145c 3.53 0.0087c

W-Si 1 3.10 3.08 0.0057 3.11 0.0063 3.06 0.0034

W-Mo 2 3.71 3.78 0.0117c 3.80 0.0145c 3.74 0.0087c

90

changes in the first disorder parameters (1st σ2, 2nd σ

2 and 3rd σ

2) were due to various

distorted arrangement of the octahedral [WO6] units. The 2nd W-Mo distance was

increased for act. PW2Mo10-SBA-15 at 723 K compared to act. PW2Mo10-SBA-15 after

thermal treatment. Thus, it may be assumed that a different binding state of the W-Mo

bonds existed at 723 K and after thermal treatment. A similar feature could be indentified

in the W-Si bond. The resulted structural motifs especially the shorter 3rd W-O distance,

indicated that the connection to phosphorus was broken as well. Nevertheless, the triad of

the original Keggin ion may persisted resulting in connected edge- and corner-sharing

octahedral [(W, Mo)O6] units. The [(W, Mo)O6] units were additionally connected to the

support material. Typical distances for edge-sharing tungsten oxide compounds and

corner-sharing H3[PW12O40], (NH4)10H2W12O42·4H2O, and WO3 were 3.4-3.6 Å and 3.7-

3.9 Å, respectively.[28,160,161] Therefore, the resulting W-Mo distances between 3.53-

3.80 Å in act. PW2Mo10-SBA-15 corresponded to both edge- and corner-sharing units.

Ross-Medgarden et al. found for WO3 supported on SiO2 a comparable structure motif.

The resulted structure after dehydration conditions corresponded to a Si containing Keggin

type cluster with corner- and edge-shared [WO6] units on the support material with an

interacting bond to Si.[162] Therefore, a comparable structural motif of W substituted

heteropolyoxomolybdates on SBA-15 resulting under propene oxidation conditions was

expected.

6.2.2 Comparison of the local structure around Mo centers in act. PW2Mo10-SBA-15 and

a reference act. W2Mo10Ox-SBA-15 under catalytic conditions

Fig. 6-9 shows the Mo K edge FT(χ(k)·k3) of act. PW2Mo10-SBA-15 and act. W2Mo10Ox-

SBA-15 after thermal treatment under catalytic conditions. The first peak of the Mo K

edge FT(χ(k)·k3) of act. W2Mo10Ox-SBA-15 resembled that of act. PW2Mo10-SBA-15. In

contrast to the reference V2Mo10Ox-SBA-15 no Mo-Mo distance at ~3.3 Ǻ (not phase

corrected) indicating crystalline α-MoO3 could be detected. Hence the resulted [MoxOy]

species seemed to be very well dispersed on the support material SBA-15. The first peak in

both FT(χ(k)·k3) originated from tetrahedral [MoO4] and octahedral [MoO6] species on the

SBA-15 support and could be sufficiently simulated using four Mo-O distances. These four

distances sufficiently accounted for the amount of octahedral [MoO6] species. Linear

91

combinations of the XANES spectra of Na2MoO4 and MoO3 references were used to

determine the amount of tetrahedral [MoO4] and octahedral [MoO6] units in act.

PW2Mo10-SBA-15 and act. W2Mo10Ox-SBA-15. Apparently, act. W2Mo10Ox-SBA-15

consisted of a mixture of increased tetrahedral [MoO4] and decreased octahedral [MoO6]

units. For act. W2Mo10Ox-SBA-15 and for act. PW2Mo10-SBA-15 a [MoO4]:[MoO6] ratio

of 3:2 and 1:4. were found, respectively. A comparison of the pseudo radial distribution

function of act. W2Mo10Ox-SBA-15 and act. PW2Mo10-SBA-15 confirmed the results of

the XANES analysis (Table 6-4). The 1st and 2nd disorder parameters (1st-σ2, 2nd-σ

2)

were higher for act. PW2Mo10-SBA-15 and indicated a decreasing amount of tetrahedral

MoO4 units. In addition, the 4th disorder parameter (4th-σ2) of is smaller than the disorder

parameter for act. W2Mo10Ox-SBA-15. This disorder parameter mainly represented the

fraction of octahedral MoO6 units. Therefore, the reduced disorder parameter indicated an

increasing amount of octahedral structural motifs in act. PW2Mo10-SBA-15 compared to

act. W2Mo10Ox-SBA-15. Furthermore, the Mo-Mo distances and disorder parameters were

comparable for act. PW2Mo10-SBA-15 and act. W2Mo10Ox-SBA-15 indicating well

dispersed [MoOx] units. A significant amount of crystalline MoO3 compared to the

reference act. V2Mo10Ox-SBA-15 (cf. chapter 5.2.1) could be excluded. The ratio of

0 1 2 3 4 5 6

-0.05

0.00

0.05

0.10

0.15

0.20

FT

(χ(k

)·k

3)

R [Ǻ]

Fig. 6-9: Mo K edge FT(χ(k)·k3) of act. PWMo11-SBA-15 and act. W2Mo10Ox-SBA-15 after

thermal treatment in 5% propene and 5% oxygen in helium at 723 K.

act. PW2Mo10-SBA-15

act. W2Mo10Ox -SBA-15

92

[MoO4]/[MoO4] units was increased indicating a favored interaction of the [MoxOy]

species with the support material SBA-15 in contrast to act. PW2Mo10-SBA-15.

Table 6-4: Type and number (N), and XAFS disorder paramters (σ2) of atoms at distance R from

the Mo atoms in act. PWxMo12-x -SBA-15 (x = 0, 1, 2). Experimental parameters were obtained


Mo K edge XAFS χ(k) of act. PWxMo12-x -SBA-15 (x = 0, 1, 2) (k range from 3.4-16.0 Å-1

, R range

from 0.9 to 4.0 Å, E0= ~ -5.2, residuals ~12.1-14.7 Nind = 26, Nfree = 12). Subscript c indicates

parameters that were correlated and f fixed in the refinement.

hex-MoO3

model

act. PW2Mo12-

SBA-15

act. W2Mo10Ox-

SBA-15


2) R(Å) σ

2(Å

2)

Mo-O 2 1.67 1.68 0.0024 1.67 0.0017

Mo-O 2 1.96 1.90 0.0045c 1.88 0.0041c

Mo-O 1 2.20 2.16 0.0045c 2.18 0.0041c

Mo-O 1 2.38 2.35 0.0008 2.35 0.0013

Mo-Mo 2 3.31 3.49 0.0072c 3.47 0.0072c

Mo-Mo 2 3.73 3.63f 0.0072c 3.61 0.0072c

Mo-Mo 2 4.03 3.74 0.0095 3.70 0.0099

Comparison of the local structure around the W centers in act. PW2Mo10-SBA-15 and a

reference act. W2Mo10Ox-SBA-15 under catalytic conditions

Fig. 6-10 (left) depicts the W LIII edge FT(χ(k)·k3) of act. PW2Mo10-SBA-15, act.

W2Mo10Ox -SBA-15 after thermal treatment under propene oxidation conditions at 723 K,

and monoclinic WO3. Different structural motifs could be assumed comparing the shapes

of the W LIII edge FT(χ(k)·k3) of act. PW2Mo10-SBA-15 and act. W2Mo10Ox -SBA-15. W

LIII edge FT(χ(k)·k3) and χ(k)·k

3 of act.W2Mo10Ox -SBA-15 were nearly identical to

monoclinic WO3. Thus, the predominant [WOx] species seemed to be crystalline

monoclinic WO3. XRD (Fig. 6-11) of the reference W2Mo10Ox-SBA-15 before thermal

treatment confirmed this assumption. Comparable to the reference V2Mo10Ox-SBA-15 (cf.

chapter 5.2.1) and in contrast to act. PW2Mo10-SBA-15 the [WOx] and [MoOx] units were

not in a close vicinity. Active sites of selective oxidation catalysts are often multifunctional

93

and consist of multiple metal centers.[28] Synthesis routes of supported ternary oxides

with different metal oxide precursors rarely have been reported. Hence, the synthesis of

supported tungsten substituted Keggin ions enabled the synthesis of connected [WOx] and

[MoOx] species on SBA-15 comparable to the vanadium substituted PVxMo12-x-SBA-15

10 20 30 40 50 60 70 80

Diffraction angle 2Ɵ[°]

Inte

nsity

WO3

W2Mo10Ox -SBA-15

PW2Mo10-SBA-15

Fig. 6-11: XRD of as prepared PW2Mo10-SBA-15, as prepared W2Mo10Ox-SBA-15, and

monoclinic WO3.

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0 1 2 3 4 5 6

R [Ǻ]

FT

(χ(k

)·k

3)

-5

0

5

10

15

20

25

30

χ(k)·

k3

k [Ǻ]-1

4 6 8 10 12

Fig. 6-10: (left) W LIII edge FT(χ(k)·k3) and (right) W LIII edge χ(k)·k

3 of act. PW2Mo10-SBA-15

(green), act. W2Mo10Ox-SBA-15 (red) after thermal treatment under propene oxidation conditions

at 723 K and monoclinic WO3 (blue) as reference.

WO3

act. W2Mo10Ox -SBA-15

act. PW2Mo10-SBA-15

94

(x = 1, 2). Therefore the results of the structural characterization showed that the proximity

of tungsten and molybdenum in the Keggin precursors was necessary to obtain connected

metal oxide species on a support material.

6.3 Functional characterization of PWxMo12-x-SBA-15 (x= 1, 2)

6.3.1 Reducibility

Fig. 6-12 shows the H2TPR profiles of PWxMo12-x-SBA-15 (x = 0, 1, 2). The resulted H2

TPR profiles revealed one sharp (~800 K) and a very broad (873- 973 K) reduction peak

comparable to PVxMo12-x-SBA-15 (x = 0, 1, 2) (cf. chapter 5.3.1). The shapes of the H2

TPR profiles were nearly identical. The reduction temperatures increased slightly

with the degree of tungsten substitution from 790 K for PMo12-SBA-15 to 815 K for

PW2Mo10-SBA-15. The H2 TPR profiles at ~800 K were comparable to molybdenum

oxides supported on SBA-15 with Mo loadings between 9.5 wt.% and 13.3 wt.%.[149,150]

Lou et al. assigned the sharp reduction peak to oligomeric [MoxOy] species or small MoOx

clusters and the broaded signal above ~800 K to the reduction of monomeric [MoOx]

species (cf. chapter 5.3.1).[150] Tungsten oxide reduced at temperatures between 1063-

1273 K. Tungsten oxide supported on SiO2 with a loading of 8.0 wt.% W had a reduction

H2 c

onsu

mp

tio

n

T [K]

373 473 573 673 773 873 973

790 K

808 K

815 K

PMo12-SBA-15

PWMo11-SBA-15

PW2Mo10-SBA-15

Fig. 6-12: Temperature programmed reduction (H2 TPR) of PMo12-SBA-15, PWMo11-SBA-15,

and PW2Mo10-SBA-15 measured at a heating rate of 8 Kmin-1

5% H2 in Ar.

95

temperature of 804 K and 1073 K .[163,164] The reduction peak at the higher temperature

(1073 K) was ascribed to the reduction of well dispersed tungsten species.[165] Comparing

the typical reduction temperatures of supported tungsten oxides to molybdenum oxides,

slightly higher reduction temperatures for supported tungsten oxides were determined.

Therefore, the slight increase of reduction temperature with the degree of tungsten

substitution degree may be interpreted as intrinsic effect of the addenda tungsten,

comparable to vanadium substituted PVxMo12-x-SBA-15 (x = 0, 1, 2) (cf. chapter 5.3.1).

However, slight changes in the reducibility depending on the degree of tungsten

substitution were detected.

6.3.2 Catalytic performance

Reaction rate and selectivity of PMo12-SBA-15, PWMo11-SBA-15, PW2Mo10-SBA-15,

W2Mo10Ox-SBA-15, and bulk PW2Mo10 in propene oxidation at 723 K are shown in Fig.

6-13. Reaction rates for PWxMo12-x-SBA-15 (x = 0, 1, 2) were calculated for similar

propene oxidation conditions (~ 14-17% propene conversion). The propene conversion for

bulk PW2Mo10 (~ 3%) and W2Mo10Ox-SBA-15 (~ 2%) were lower due to the strong

decreased catalytic activity. Adjusting to similar propene oxidation conditions for the low

active samples would lead to a large volume of the samples and thermal effects. Hence, the

comparison of the catalytic performance between PWxMo12-x-SBA-15 (x = 0, 1, 2) and the

low active samples has to be done carefully.

Reaction rates for PWxMo12-x-SBA-15 (x = 0, 1, 2) slightly increased with the degree of

tungsten substitution in contrast to constant reaction rates for vanadium substituted

PVxMo12-x-SBA-15 (x = 1, 2) (cf. chapter 5.3.2). Selectivities for CO increased at the

expense of those of acetaldehyd with higher degree of tungsten substitution. Structural

analysis of act. PWxMo12-x-SBA-15 (x = 0, 1, 2) revealed a decreased concentration of

tetrahedral [MoO4] units as a function of tungsten substitution. The degree of

oligomerization of [MoxOy] seemed to be comparable in all act. PWxMo12-x-SBA-15 (x =

0, 1, 2) samples. Therefore, the additional [WO6] species in act. PWxMo12-x-SBA-15 (x =

0, 1, 2) may lead to new multifunctional active sites, resulting in a slightly different

product distribution and increased reaction rates. This [WO6] species in act. PWxMo12-x-

SBA-15 (x = 0, 1, 2) showed a structure directing effect towards formation of octahedral

96

[MoO6] units. Hence, due to the compositional and structural variety structure-activity

correlation remained vague.

The reaction rate of PW2Mo10-SBA-15 strongly increased due to the improved surface to

bulk ratio compared to bulk PW2Mo10. While, PW2Mo10 showed a slightly increased

selectivity towards acrolein, PW2Mo10-SBA-15 exhibited an increased selectivity towards

acetic acid. Total oxidation products CO and CO2 amounted to about ~55% in the resulting

oxidation products. Apparently, higher dispersion and an improved surface to bulk ratio of

Keggin ions resulted in a much increased activity at comparable selectivity similarly to

PVxMo12-x-SBA-15 (x = 1, 2) (cf. chapter 5.3.2).

In contrast to the supported HPOM samples, W2Mo10Ox-SBA-15 showed a strongly

decreased activity. The product distribution of W2Mo10Ox-SBA-15 was comparable to

unsubstituted act. PMo12-SBA-15. The amount of total oxidation products in the gas phase

was slightly lower and an increasing selectivity to acetaldehyde and acrolein was

determined compared to act. PWxMo12-x-SBA-15 (x = 1, 2). The structural analysis

indicated that act. W2Mo10Ox-SBA-15 possessed an increased amount of oligomerized

[MoxOy] species and an increased content of tetrahedral [MoO4] units. Additionally, the

predominant tungsten oxide species seemed to be crystalline monoclinic WO3 which may

not significantly participate in the catalytic reaction. Apparently, the new multifunctional

0

20

40

60

80

100

0

10

20

30

40

50

60

70

a b c e d

acrylic acid acetic acid acrolein

acetone

acetaldehyd

e

CO CO2 propionaldehyd

e

Se

lectivity [

%]

rea

ctio

n r

ate

[µ

mo

l(pro

pe

ne

)g-1

(Mo

)s-1

]

Fig. 6-13: Reaction rate (µmol(propene)·g-1

(Mo)·s-1

) and selectivity of (a) PMo12-SBA-15, (b)

PWMo11-SBA-15, (c) PW2Mo10-SBA-15, (d) W2Mo10Ox-SBA-15, and (e) bulk PW2Mo10 in 5%


97

active site resulting from connected [WO6] and [MoxOy] units lead to an increasing amount

of total oxidation products for act. PW2Mo10-SBA-15 in contrast to not connected [WO6]

and [MoxOy] units in act. W2Mo10Ox-SBA-15.

Combining the results of the structural and functional characterization, the increase of

octahedral [MoO6] units resulting from tungsten substitution lead to an enhanced catalytic

activity and slightly different selectivity. The reference act. W2Mo10Ox-SBA-15 with

decreased octahedral [MoO6] units and crystalline WO3 exhibited a decreased catalytic

activity at comparable selectivity compared to unsubstituted act. PMo12-SBA-15.

Apparently, oligomerized octahedral [MoO6] species were the catalytically active sites in

propene oxidation. The various selectivities as a function of tungsten substitution in act.

PWxMo12-x-SBA-15 (x = 0, 1, 2) may be caused by new multifunctional active sites

consisting of connected [WO6] and [MoxOy] species. Apparently, tetrahedral [MoO4]

species were not involved in selective propene oxidation which confirmed the results of

previous studies.[137]

98

6.4 Summary

Structural evolution of PWxMo12-x-SBA-15 (x = 0, 1, 2) and a reference W2Mo10Ox-SBA-

15 during propene oxidation conditions were examined by in situ X-ray absorption

spectroscopy at the Mo K and W K edges. During thermal treatment under propene

oxidation conditions PWxMo12-x-SBA-15 (x = 1, 2) formed a mixture of mainly octahedral

[MoOx] and [WO6] units. Changes in the local structure around the W centers were

delayed compared to the structural changes of the Mo centers depending on the degree of

tungsten substitution. The delayed structural rearrangement corresponded to the

temperatures where the major structural rearrangement of the [MoxOy] species was

finished. The octahedral [WO6] units distorted during propene oxidation conditions.This

influenced the structural changes of the [MoxOy] species resulting in mainly octahedral

[MoO6] units depending on the degree of tungsten substitution. The degree of

oligomerization of the [MoxOy] species for all act. PWxMo12-x-SBA-15 (x = 0, 1, 2) was

comparable and independent of the degree of tungsten substitution. Apparently, the mainly

octahedral [MoOx] and [WO6] units were in close vicinity and able to interacted under

catalytic conditions. The new multifunctional active site resulting due to connected [WO6]

and [MoOx] units lead to an increased reaction rate and increased amount of total oxidation

products. Conversely, structural analysis of activated reference W2Mo10Ox-SBA-15

synthesized with individual W and Mo precursors indicated that [WO6] and [MoOx]

species were mostly separated from each other on the surface of SBA-15. Moreover,

activated W2Mo10Ox-SBA-15 possessed a decreased amount of oligomerized [MoxOy]

species and an increased content of tetrahedral [MoO4] units. Additionally, the

predominantly tungsten oxide species seemed to be crystalline monoclinic WO3 and may

not be involved in the catalytic reaction. This may explain the strongly decreased reaction

rates and similar selectivities towards partial oxidations products compared to

unsubstituted PMo12-SBA-15. In total, supported tungsten substituted Keggin ions are

suitable precursors to synthesize connected [WO6] and [MoOx] species on SBA-15.

Apparently, the proximity of tungsten and molybdenum in the Keggin precursors is a

prerequisite for obtaining connected metal oxide species.

99

7 Characterization of PMo12 supported on SBA-15 with

tailored pore radii

Nanostructured SiO2 materials such as SBA-15 represent suitable support systems for

oxide catalysts.[22-25] Studies on H4[PVMo11O40] supported on SBA-15 revealed a

structure directing effect of the silica support on the stability of the resulting Mo oxide

species.[27] H4[PVMo11O40] supported on SBA-15 formed a mixture of tetrahedrally and

octahedrally coordinated and linked [MoO4] and [MoO6] units under catalytic

conditions.[27] Previous studies have shown that catalytic activity and selectivity scales

with both the concentration and the degree of oligomerization of tetrahedral [MoO4] and

octahedral [MoO6] units at the surface.[137] Isolated [MoO4] units supported on MgO

were nearly inactive for propene oxidation. The catalytic activity and selectivity towards

oxygenates increased with increasing amount of [MoxOy] species.[137] Previously, the

degree of oligomerization was adjusted by either varying the metal loading or altering the

surface acidity of the support material.[25,153,166,167] In addition, only few other

characteristics of supported model systems are conceivable to alter the connectivity of

supported MoOx species. A complimentary approach may be varying the pore radii of the

support material. This could lead to modified structure directing effects on supported

HPOM at constant metal oxide loading and identical surface acidity. Subsequently, the

resulting [MoxOy] structures on tailored SBA-15 may be used to further elucidated

structure-activity relationships.

A study with tailored SBA-15 as support material was necessary elucidating the

catalytically active structural motifs. Therefore, H3[PMo12O40] was supported on SBA-15

with modified pore radii (10, 14, 19 nm). The Samples were prepared with a surface

coverage of 1 Keggin ion per 13 nm2 independent of the pore radii. PMo12-SBA-15 (10, 14,

19 nm) were treated under propene oxidation conditions. In situ X-ray absorption

spectroscopy investigations at the Mo K edge of PMo12 supported on SBA-15 with

different pore radii (10, 14, 19 nm) during catalytic conditions are presented. A detailed

analysis of the structures resulting under catalytic conditions is performed and correlated

with the catalytic activity and product distribution towards propene oxidation.

Additionall,y a comparison of the thermal stability of the supported samples was

established.

100

7.1 Experimental

Sample Characterization








Physisorption measurements

Nitrogen physisorption isotherms were measured at 77 K on a BEL Mini II volumetric

sorption analyzer (BEL Japan, Inc.). Silica SBA-15 samples were treated under vacuum at

368 K for about 20 min and at 448 K for about 17 h before starting the measurement. Data

processing was performed using the BELMaster V.5.2.3.0 software package. The specific

surface area was calculated using the Brunauer–Emmett–Teller (BET) method in the

relative pressure range of 0.03-0.24 assuming an area of 0.162 nm2

per N2 molecule.[69]

The adsorption branch of the isotherm was used to calculate pore size distribution and

cumulative pore area according to the method of Barrett, Joyner, and Halenda (BJH).[70]




Wide-angle scans (5-90° 2θ, variable slits) were collected in reflection mode using a

silicon sample holder. Small-angle scans (0.4-6.0° 2θ, fixed slits) were collected in


101

Thermal analysis

Thermogravimetric (TG) measurements were conducted using a SSC 5200 from Seiko

Instruments. The gas flow through the sample compartment was adjusted to 100 ml/min

(20% O2 and 80% He). Samples were measured with a rate of 2 K/min in the range from

298 K to 823 K.




Si(311) double crystal monochromator. In situ experiments were conducted in a flow

reactor at atmospheric pressure (5 vol% oxygen in He, total flow ~30 ml/min, temperature

range from 303 to 723 K, heating rate 4 K/min). The gas phase composition at the cell

outlet was continuously monitored using a non-calibrated mass spectrometer in a multiple

ion detection mode (Omnistar from Pfeiffer).



by fitting linear polynomials and 3rd

degree polynomials to the pre-edge and post-edge

region of an absorption spectrum, respectively. The extended X-ray absorption fine


background μ0(k) The FT(χ(k)·k3), often referred to as pseudo radial distribution function,






Single scattering and multiple scattering paths in the H3[PMo12O40] (ICSD 209 [14,93])

and hexagonal MoO3 (ICSD 75417 [135]) model structure was calculated up to 6.0 Å with

a lower limit of 4.0% in amplitude with respect to the strongest backscattering path.





102










cm length, 9 mm inner diameter) placed vertically in a tube furnance. In order to achieve a

constant volume and to exclude thermal effects, catalysts samples (~ 38-76 mg) were

diluted with boron nitride (Alfa Aesar, 99.5%) to result in an overall sample mass of 375

mg. For catalytic testing in selective propene oxidation a mixture of 5% propene (Linde

Gas, 10% propene (3.5) in He (5.0)) and 5% oxygen (Linde Gas, 20% O2 (5.0) in He (5.0))

in helium (Air Liquide, 6.0) was used in a temperature range of 293-723 K Reactant gas

flow rates of oxygen, propene, and helium were adjusted with separate mass flow

controllers (Bronhorst) to a total flow of 40 ml/min. All gas lines and valves were

preheated to 473 K. Hydrocarbons and oxygenated reaction products were analyzed using

a Carbowax capillary column connected to an AL2O3/MAPD column or a fused silica

restriction (25 m·0.32 mm each) connected to a flame ionization detector. O2, CO, and CO2

were separated using a Hayesep Q (2 m x 1/8``) and a Hayesep T packed column (0.5 m x

1/8``) as precolumns combined with a back flush. For separation, a Hayesep Q packed

column (0.5 m x 1/8``) was connected via a molsieve (1.5 m x 1/8``) to a thermal

conductivity detector (TCD). Details about the calculation of conversion, selectivity, and

reaction rate are described in chapter 3.2.

Sample preparation

Silica SBA-15 samples with a pore diameter of ~10 nm was prepared according to Ref.

[22]. 16.2 g of triblock copolymer (Aldrich, P123) were dissolved in 294 g water and 8.8 g

hydrochloric acid at 308 K and stirred for 24 h. After addition of 32 g tetraethyl

orthosilicate, the reaction mixture was stirred for 24 h at 373 K. The resulting gel was

103

transferred to a glass bottle and the closed bottle was heated to 388 K for 24 h.

Subsequently, the suspension was filtered by vacuum filtration and washed with a mixture

of H2O/EtOH (100:5).

Silica SBA-15 with large pores were prepared according to Refs. [46],[51]. Silica SBA-15

with a pore diameter ~14 nm (or ~19 nm) was prepared as follows. 9.6 g of triblock

copolymer (Aldrich, P123) were dissolved in 336 ml hydrochloric acid (1.3 M) at 288 K

(or 290 K). After addition of 0.108 g ammonium fluoride the solution was stirred for 16 h

(or 24 h). 20.7 g tetraethyl orthosilicate and 8.08 g 1,3,5-triisopropylbenzene were added to

the solution. The resulting gel was transferred to a glass bottle and the closed bottle was

heated to 393 K (or 373 K) for 29 h (or 48 h). Subsequently, the suspension was filtered by

vacuum filtration and washed with a mixture of EtOH/HCl/H2O (100:10:100). The

resulting white powders were dried at 378 K for 3 h and calcined at 453 K for 3 h and at

823 K for 5 h.

H3[PMo12O40] was prepared as described in chapter 3.1. H3[PMo12O40] was supported on

SBA-15 via incipient wetness. The amount of molybdenum was adjusted to 10 wt.%, 6.7

wt.%, and 5.2 wt.% on SBA-15 (10, 14, 19 nm).

104

7.2 Structure of the support materials

Pore size distributions and specific surface areas of the synthesized support materials were

calculated from N2 adsorption/desorption isotherms. SBA-15 (10 nm), SBA-15 (14 nm),

and SBA-15 (19 nm) showed typical type IV isotherms indicative of mesoporous

materials. Adsorption and desorption branches in hysteresis range were nearly parallel for

SBA-15 (10 nm) and SBA-15 (14 nm) indicating regular shaped pores Fig. 7-1. N2

isotherm for SBA-15 (19 nm) showed a slight broadening of the hysteresis loop, indicating

the development of minor constrictions. BET surface areas were calculated from

physisorption data. The tailored SBA-15 samples exhibited areas between 400 and

850 m2/g. Specific surface area aBET (calculated by BET method), external surface area aEXT

(calculated as the difference between aBET and aMeso), and area corresponding to the

mesopores aMeso are summarized in Table 7-1. Fig. 7-1 (inset) shows the pore size

distribution derived from BJH analysis resulting in three different pore diameters of ~10,

~14, and ~19 nm. Small-angle X-ray diffraction patterns of the tailored SBA-15 are

presented in Fig. 7-2 (left). The second derivates of small-angle X-ray diffraction patterns

are shown for clarity in Fig. 7-2 (right). SBA-15 (10 nm) and SBA-15 (14 nm) exhibited

Vo

lum

e [m

l g

-1]

0

200

400

600

800

5 10 15 20 25

dV

/dp [m

l nm

-1g

-1]

rrrd

p

dp [nm]

Relative Pressure p/p0

0.2 0.4 0.6 0.8 1.0 0.0

Fig. 7-1: Nitrogen physisorption isotherms of silica SBA-15 (10 nm) (square), SBA-15 (14 nm)

(circle), and SBA-15 (19 nm) (triangle) and pore distributions of of silica SBA-15 (10 nm)

(square), SBA-15 (14 nm) (circle), and SBA-15 (19 nm) (triangle)(inset).

105

the typical patterns with low-angle 10l, 11l, and 20l peaks corresponding to the two-

dimensional hexagonal symmetry.

Small-angle X-ray diffraction pattern of SBA-15 (19 nm) showed one peak at low values

of 2Ɵ. The lattice spacings d10l, derived from the Bragg equation (eq. 2.1.1), and unit cell

constants a0, corresponding to the hexagonal pore arrangement, are given in Table 7-1. The

received structural parameter of the tailored SBA-15 confirmed a successful synthesis of

mesoporous SiO2 materials with different pore size distributions, and high surface areas.

Table 7-1: Specific surface area aBET (calculated by BET method), external surface area aEXT

(calculated as the difference between aBET and aMeso), area corresponding to the mesopores aMeso,

pore diameter dpore (calculated by BJH method), mesopore volume VMeso, d10l-values (derived from

low-angle XRD), unit cell constants a0 (corresponding to the hexagonal pore arrangement) of SBA-

15 (10 nm), SBA-15 (14 nm), and SBA-15 (19 nm).

aBET

(m2/g)

aExt

m2/g)

aMeso

(m2/g)

dBJH

(nm) VMeso (cm

3/g)

d10l

( nm)

a0

(nm)

SBA-15 (10 nm) 843 145 698 10.3 1.233 10.52 12.14

SBA-15 (14 nm) 525 83 442 13.8 1.344 12.52 14.46

SBA-15 (19 nm) 395 50 345 18.5 0.957 14.77 17.05

-2.0 -1.5 -1.0 -0.5 0.5 1.0 1.5 2.0

SBA-15 (10nm) SBA-15 (14nm) SBA-15 (19nm)

No

rm. in

ten

sity

-1.0 -0.5 0.5 1.0

2nd

de

riva

te

norm

. in

tensity

Diffraction angle 2Ɵ [°] Diffraction angle 2Ɵ [°]

Fig. 7-2: (left) Low-angle X-ray diffraction patterns and (right) 2nd derivates of the low-angle X-

ray diffraction patterns SBA-15 (10 nm), SBA-15 (14 nm), and SBA-15 (19 nm).

106

7.3 Characterization of PMo12-SBA-15 (10, 14, 19 nm)

Local structure around theMo centers in PMo12-SBA-15 (10, 14, 19 nm)

Fig. 7-3 shows the theoretical and experimental Mo K edge FT(χ(k)·k3) of PMo12-SBA-15

(10, 14, 19 nm). The shapes of the FT(χ(k)·k3) resembled that of bulk PMo12 indicating a

similar local structure around the Mo centers in supported and unsupported HPOM Keggin

ions. For a more detailed structural analysis H3[PMo12O40] Keggin ions (ICSD 209

[14,93]) was chosen as model structure. Comparing the distances R and disorder

parameters σ2 of PMo12-SBA-15 (10, 14, 19 nm) supported on SBA-15 with different pore

radii exhibited no significant differences between the initial Keggin ion structure and

Keggin ions supported on SBA-15 (chapter 3) structure. The good agreement between

theory and experiment for PMo12-SBA-15 (10, 14, 19 nm) confirmed the Keggin ion

structure upon supporting PMo12 on SBA-15.[27]

R [Å]

FT

(χ(k)·k

3)

0 1 2 3 4 5 6 -0.05

0.00

0.05

0.10

0.15

0.20

0.25

PMo12-SBA-15 (19 nm)



Fig. 7-3: Theoretical (dotted) and experimental (solid) Mo K edge FT(χ(k)·k3) of PMo12 supported

on SBA-15 (10 nm), SBA-15 (14 nm), and SBA-15 (19 nm).

107


the Mo atoms in as prepared PMo12-SBA-15 (10, 14, 19 nm). Experimental parameters were

obtained from a refinement of H3[PMo12O40] model structure (ICSD 209 [14,93]) to the

experimental Mo K edge XAFS χ(k) of PMo12-SBA-15 (10, 14, 19 nm) (k range from 3.0-13.7 Å-1

,

R range from 0.9 to 4.0 Å, E0= ~ 2.3, residuals ~11.3-12.5 Nind = 22, Nfree = 9). Subscript c indicates

parameters that were correlated in the refinement.

Keggin model

PMo12-SBA-15

(10nm)

PMo12-SBA-15

(14nm)

PMo12-SBA-15

(20nm)


2) R(Å) σ

2(Å

2) R(Å) σ

2(Å

2)

Mo-O 1 1.68 1.65 0.0025 1.65 0.0029 1.65

0.0021

Mo-O 2 1.91 1.78c 0.0033c 1.78c 0.0040c 1.79c 0.0032c

Mo-O 2 1.92 1.95c 0.0033c 1.95c 0.0040c 1.94c 0.0032c

Mo-O 1 2.43 2.40 0.0008 2.39 0.0010 2.40 0.0007

Mo-Mo 2 3.42 3.42 0.0052c 3.41 0.0056c 3.42 0.0054c

Mo-Mo 2 3.71 3.74 0.0052c 3.74 0.0056c 3.74 0.0054c

Thermal stability of PMo12 supported on SBA-15 with different pore radii

Fig. 7-4 depicts the measured thermogravimetric data of PMo12-SBA-15 (10, 14, 19 nm) in

20% O2 in He. The mass loss between 303 K and 373 K was ascribed to desorption of

physically adsorbed water on the surface of the materials. Relative mass loss decreased for

samples with large pore diameter. Afterwards a nearly constant mass in between 373 K and

448 K could be detected. The temperature range 448-523 K showed a mass loss of ~1%

(PMo12-SBA-15 (11 nm)), ~0.7% (PMo12-SBA-15 (14 nm)), and ~0.5% (PMo12-SBA-15

(19 nm)). Sample mass between 523 K and 823 K did not change for all samples.

Comparable behaviour was shown for pure silica samples in vacuum.[168] Silica

dehydrated between room temperature and 453 K followed by the dehydroxylation process

of silanol groups between 453 K and 673 K. This resulted in the formation of siloxane

groups and a decrease of silanol density from 4.6 OH/nm2 (473 K) to 2.3 OH/nm

2

(673 K).[145,168]

Comparing dehydration and dehydroxylation processes of supported HPOM and bulk

HPOM revealed a correlation between dehydration and thermal stability of the Keggin ion.

Bulk PMo12 loses 1.5 molecules of constitutional water between 673-713 K

108

under dry air conditions. This decomposition is accompanied by the formation of

MoO3.[117] Structures resulting for supported HPOM after thermal treatment under

oxidizing conditions were comparable to stabilized two dimensional hexagonal MoO3 on

SBA-15.[27,59] It has been shown that structural characteristics of supported model

systems like MoOx-SBA-15 and VOx-SBA-15 depended mainly on their hydration states

and previous calcination processes.[95,144] A comparable effect may be responsible for

the structural evolution of HPOM supported on SBA-15. Adsorbed water and silanol

groups from the support material may possess a structure stabilizing effect on the Keggin

ion. This effect would be comparable to that of water of crystallization and constitutional

water in bulk HPOM under ambient conditions.[117] Therefore, dehydroxylation of SBA-

15 may be the driving force for the structural decomposition of the Keggin ion resulting in

the formation of Mo oxide species on SBA-15.

7.4 Structural evolution of PMo12- SBA-15 (10, 14, 19 nm) under catalytic

conditions

PMo12-SBA-15 (10, 14, 19 nm) samples were investigated by in situ XAS under catalytic

conditions. Fig. 7-5 shows the evolution of molybdenum XANES spectra of PMo12-SBA-

15 during temperature-programmed treatment in 5% propene and 5% oxygen. The

T [K]

No

rma

lize

d M

ass [%

] N

orm

aliz

ed M

ass [%

]

373 473 573 673 773

90

92

94

96

98

100 PMo12-SBA-15 (10 nm) PMo12-SBA-15 (14 nm) PMo12-SBA-15 (19 nm)

Fig. 7-4: Thermograms of PMo12-SBA-15 (10 nm), PMo12-SBA-15 (14 nm), and PMo12-SBA-15

(19 nm) at 20% O2 in He.

109

resulting structures exhibited an increased concentration of tetrahedral [MoO4] units. The

pre-edge peak features in the Mo K edge XANES spectra can be employed to elucidate the

local structure around the Mo center. Using the pre-edge peak height sufficed to quantify

the contribution of tetrahedral [MoO4] and distorted [MoO6] units present under catalytic

conditions. Fig. 7-6 showed the Mo K edge XANES spectra of PMo12-SBA-15 (14 nm)

and a spectrum calculated from a linear combination of bulk MoO3 and bulk

20.0 20.1 20.2 0.00

0.25

0.50

0.75

1.00

Photon energy [keV]

Na2MoO4

MoO3

No

rma

lize

d a

bsorp

tion

Fig. 7-6: Refinement of the sum (dotted) of XANES spectra of references MoO3 and Na2MoO4

(dashed) to Mo K edge XANES spectrum of activated PMo12-SBA-15 (14 nm) after thermal

treatment under propene oxidation conditions at 723 K.

20.00

20.05

20.10 20.15

20.2

373

473

573

673

Photon energy

T [K]

No

rma

lize

d

abso

rptio

n

Fig. 7-5: in situ Mo K edge XANES spectra of PMo12-SBA-15 during temperature-programmed


723 K.

110

Na2MoO4. The linear combination represented the amount of distorted [MoO6] and

tetrahedral [MoO4] units. Quantitative evolution of the structural units was used to

visualize changes in the structure of PMo12-SBA-15 (10, 14, 19 nm) during temperature

programmed treatment in 5% propene and 5% oxygen. Fig. 7-7 depicts the calculated

concentration of tetrahedral [MoO4] units during thermal treatment under catalytic

conditions. No significant structural changes of PMo12-SBA-15 (10, 14, 19 nm) could be

detected in the temperature range between 303 K and 448 K. Apparently, the Keggin

structure was stable on silica SBA-15 in the temperature range 303-448 K. Subsequently,

concentration of tetrahedral [MoO4] units considerably increased in the temperature range

between 448 K and 598 K for PMo12-SBA-15 (10, 14, 19 nm). The onset of structural

rearrangement was identical for all PMo12-SBA-15 (10, 14, 19 nm) samples and

independent of the pore radii of the support materials. The stability of the Keggin ion

seemed to depend only on the nature of the support material. The structural evolution in the

temperature range (448-598 K) correlated to the dehydration and dehydroxylation process

of SiO2 under oxidizing conditions (20% O2 in He). Apparently, dehydroxylation process

was also the driving force for the structural rearrangement of the PMo12-SBA-15 (10, 14,

19 nm) samples under catalytic conditions. At a temperature of about 598 K a higher

amount of the tetrahedral [MoO4] units for PMo12-SBA-15 (14, 19 nm) could be detected

compared to conventional PMo12-SBA-15 (10 nm) (Fig. 7-7). The concentration of

Fig. 7-7: Evolution of MoO4/MoO6 ratio of PMo12-SBA-15 (10 nm), PMo12-SBA-15 (14 nm), and

PMo12-SBA-15 (19 nm) during thermal treatment under propene oxidation conditions.

300 400 500 600 700

0

20

40

60

80

PMo12-SBA-15 (10nm) PMo12-SBA-15 (14nm) PMo12-SBA-15 (19nm)

[MoO

4]/[M

oO

6] ra

tio

[%

]

T [K]

111

tetrahedral [MoO4] units for PMo12-SBA-15 (14, 19 nm) amounted to ~65% compared to

~45% for PMo12.SBA-15 (10 nm). Hence, the concentration of tetrahedral [MoO4] units

increased with the larger pore radii of the support material. Quantification of tetrahedral

[MoO4] and octahedral [MoO6] units in the temperature range between 598 K and 723 K

confirmed this assumption. The [MoO4] concentration of PMo12-SBA-15 (14 nm) and

PMo12-SBA-15 (19 nm) were comparable and reached the highest concentration with 75%

[MoO4] units at 723 K. PMo12-SBA-15 (10 nm) reach a maximum of tetrahedral [MoO4]

units (~50%) at 657 K. Subsequently, a decreasing concentration of tetrahedral [MoO4]

units to 40% at 723 K was determined.

Influence of the pore radii to the resulting structure of [MoxOy] species

Fig. 7-8 shows the Mo K edge FT(χ(k)·k3) of activated PMo12-SBA-15 (10, 19 nm) after

thermal treatment under propene oxidation conditions. The FT(χ(k)·k3) of act. PMo12-SBA-

15 exhibited features similar to that of previously reported dehydrated molybdenum oxides

and HPOM supported on SBA-15.[27,59] For a more detailed structural analysis

hexagonal MoO3 was chosen as model structure. Theoretical XAFS phases and amplitudes

were calculated for Mo-O and Mo-Mo distances and used for EXAFS refinement. The

results of the refinement are given in Table 7-3. The first peak of Mo K edge FT(χ(k)·k3) of

act. PMo12-SBA-15 (10 nm) exhibited differences compared to act. PMo12-SBA-15 (14,

19 nm). The first peak in the FT(χ(k)·k3) originated mainly from the tetrahedral species on

-0.08

-0.04

0.00

0.04

0.08

FT

(χ(k

)·k

3

0 1 2 3 4 5 6


R [Å]


Fig. 7-8: Mo K edge FT(χ(k)·k3) of activated PMo12-SBA-15 (10 nm) and activated PMo12-SBA-

15 (19 nm) after thermal treatment under propene oxidation conditions at 723 K.

112

the SBA-15 support and could be sufficiently simulated using four Mo-O distances. These

four distances sufficiently accounted for the minor amount of octahedral [MoO6] species.

The 1st and 2nd disorder parameters (1st-σ2, 2nd-σ

2) were higher for act. PMo12-SBA-15

(10 nm) and indicated a lower amount of tetrahedral structural units. Additionally, the 4th

disorder parameter (4th-σ2) was smaller than that of act. PMo12-SBA-15 (14, 19 nm) with

larger pores. This disorder parameter mainly represented the fraction of octahedral [MoO6]

species, and corresponded to increasing amount of octahedral structural motifs in act.

PMo12-SBA-15 (10 nm) compared to act. PMo12-SBA-15 (14, 19 nm).

A distinct peak at ~3 Å in the FT(χ(k)·k3) indicated a significant amount of dimeric or

oligomeric [MoxOy] units on SBA-15 (10, 14, 19 nm) independent of the pore radii. Hence,

isolated tetrahedral [MoO4] units can be excluded as major molybdenum oxide

species.[137] The obtained Mo-Mo distances were identical for act. PMo12-SBA-15 (10,

14, 19 nm) samples and, thus, were independent of the pore radii. The disorder parameters

σ2

of the Mo-Mo distances for act. PMo12-SBA-15 (10 nm) were slightly increased

compared to act. PMo12-SBA-15 (14, 19 nm). This indicated a decreased oligomerization

degree of Mo silica SBA-15 with larger pore diameters. Apparently, the


the Mo atoms in act. PMo12-SBA-15 (10, 14, 19 nm). Experimental parameters were obtained from

a refinement of a hexagonal MoO3 model structure (ICSD 75417 [135]) to the experimental Mo K

edge XAFS χ(k) of act. PMo12-SBA-15 (10, 14, 19 nm) (k range from 3.4-16.0 Å-1

, R range from

0.9 to 4.0 Å, E0= ~ -5.2, residuals ~12.5 Nind = 26, Nfree =12). Subscript c indicates parameters that


hex-MoO3

model

act. PMo12-

SBA-15 (10nm)

act. PMo12-SBA-

15 (14nm)

act. PMo12-SBA-

15 (20nm)


2) R(Å) σ

2(Å

2) R(Å) σ

2(Å

2)

Mo-O 2 1.67 1.67 0.0015 1.67 0.0009 1.67 0.0009

Mo-O 2 1.96 1.89 0.0038c 1.88 0.0024c 1.88 0.0024c

Mo-O 1 2.20 2.19 0.0038c 2.17 0.0024c 2.17 0.0024c

Mo-O 1 2.38 2.35 0.0011 2.34 0.0030 2.34 0.0029

Mo-Mo 2 3.31 3.49 0.0068c 3.49 0.0054c 3.49 0.0058c

Mo-Mo 2 3.73 3.63 0.0068c 3.62 0.0054c 3.62 0.0058c

Mo-Mo 2 4.03 3.73 0.0100 3.73 0.0089 3.72 0.0096

113

concentration of the [MoxOy] species reached a minimum on the large pore samples

PMo12-SBA-15 (14, 19 nm). These large pore SBA-15 (dMeso = 14-19 nm) materials

possessed a larger angle of inclination because of the decreased curvature of the pores in

these samples. This may reduce the contact area between the decomposing Keggin ions

eventually resulting in a higher concentration of dispersed and isolated oxide species.

Conversely, thermal decomposition of Keggin ions on small pore support materials was

prone to result in connected [MoxOy] species. The Mo-O, Mo-Mo distances and disorder

parameters for act. PMo12-SBA-15 (14 nm) and act. PMo12-SBA-15 (19 nm) were nearly

identical and different from those of PMo12-SBA-15 (10 nm). This confirmed the results of

the quantification of tetrahedral [MoO4] (~75%) and distorted [MoO6] (~25%) units,

present on large pore materials under catalytic conditions. Apparently, the formation of

oligomeric [MoxOy] units mostly consisting of tetrahedral [MoO4] units depended on the

pore radius of the silica SBA-15. Hence, act. PMo12-SBA-15 (10 nm) with smaller pores

favored the formation of more extended structures on the support material.

7.5 Functional characterization of PVxMo12-x-SBA-15 (x= 1, 2)

7.5.1 Influence of the resulting structures to catalytic activity

PMo12-SBA-15 (10, 14, 19 nm) samples were tested under catalytic conditions for

selective propene oxidation. Fig. 7-9 shows a comparison of the selectivities towards the

oxidation products and reaction rates. The oxidation product distributions were comparable

for all three PMo12-SBA-15 (10, 14, 19 nm) samples. The oxidation product distributions

were comparable for all three PMo12-SBA-15 (10, 14, 19 nm) samples. The reaction rates

for PMo12-SBA-15 (14 nm) and PMo12-SBA-15 (19 nm) were also nearly identical. In

contrast to the samples with larger pores, PMo12-SBA-15 (10 nm) showed a ~14% higher

reaction rate during propene oxidation. The theoretical Mo coverage of 0.9 Mo/nm2 was

similar for all PMo12-SBA-15 (10, 14, 19 nm) samples. However, a significant difference

between all SBA-15 (10, 14, 19 nm) materials was the curvature of the surface in the

pores. The pore structure of mesoporous SBA-15 corresponds to that of hollow cylinders.

Thus, the curvature of the walls of these cylinders decreases with higher pore radius.

Therefore, arrangement of spherical Keggin ions on an area along the inner surface of

pores with different pore radii leads to a decreasing distance between the spheres at smaller

114

pore radius. Hence, assuming a volume of 1 nm3 per Keggin ion and considerating the

various curvatures for SBA-15 (10, 14, 19 nm) lead to an increased effective coverage at

smaller pore radius. Therefore, the increased effective distance of the Keggin ion on

PMo12-SBA-15 (14 nm) and PMo12-SBA-15 (19 nm) resulted in a lower concentration of

[MoxOy] units compared to PMo12-SBA-15 (10 nm). Hence, the catalytic activity in

propene oxidation increased with higher concentration of [MoxOy] units under catalytic

conditions. [MoxOy] units orginating from PMo12-SBA-15 (10 nm) resulted in an enhanced

catalytic activity without significant influence on the product distribution. Therefore, a

higher concentration of [MoxOy] units at similar loadings improved the catalytic activity

towards propene oxidation. Comparable results have been shown for PVMo11 supported on

SiO2 (Aerosil 300: 295 m2g

-1, Nippon Aerosil Co., Ltd.) with different loadings during

oxidation of methacrolein.[167] Selective propene oxidation requires the transfer of more

than two electrons. Therefore, [MoxOy] sites are necessary to selectively oxidize the

propene molecule.[37,169,170] Catalytic activity of supported vanadium oxide based

catalysts depended also on the concentration of [VxOy] units comparable to

[MoxOy].[153,171] Therefore, [MoxOy] units seemed to be necessary for catalytic activity

while their concentration increased with the effective coverage.

0

20

40

60

80

100

30

40

50

60

70

Se

lectivity [%

]

reactio

n r

ate

µm

ol(

pro

pen

e)g

-1(M

o)s

-1

acrylic acid


acetone

acetaldehyde

CO

CO2 propionaldehyde

10 nm 14 nm 20 nm


(Mo)s-1

) and selectivity of PMo12-SBA-15 (10, 14,

20 nm) in 5% propene and 5% oxygen in He at 723 K.

115

7.6 Summary

Structural evolution of H3[PMo12O40] supported on SBA-15 (PMo12-SBA-15) with

different pore radii (10, 14, 19 nm) was examined by in situ X-ray absorption spectroscopy

investigations at the Mo K edge during propene oxidation conditions. Large pore SBA-15

was successfully used as support material for molybdenum based oxidation catalysts.

Supporting heteropolyoxo molybdates on large pore SBA-15 resulted in regular Keggin

ions on the support material. During thermal treatment in propene oxidation conditions

PMo12-SBA-15 (10, 14, 19 nm) formed a mixture of mostly tetrahedral [MoO4] and

octahedral [MoO6] units. The onset temperature of structural changes of PMo12-SBA-15

(10, 14, 19 nm) during thermal treatment in propene oxidation conditions was largely

independent of the pore size of SBA-15. The stability of the Keggin ions depended mostly

on the nature of the support. Apparently, the dehydroxylation of silanol groups of the

support material was the driving force for the structural instability of the Keggin ion. The

resulting [MoxOy] structures present under catalysis conditions depended on the pore size

of the support material. A higher concentration of octahedral [MoO6] units and higher

oligomerized [MoxOy] units was detected for act. PMo12-SBA-15 (10 nm) compared to act.

PMo12-SBA-15 (14, 19 nm). The higher concentration of [MoxOy] units present in act.

PMo12-SBA-15 (10 nm) resulted in an increased catalytic activity compared to to activated

PMo12-SBA-15 (14, 19 nm) with a lower concetration of [MoxOy] units. Selectivities

towards oxidation products during propene oxidation were comparable and largely

independent of the pore radii of act. PMo12-SBA-15 (10, 14, 19 nm). Apparently, tailoring

the pore radius of silica SBA-15 permitted to prepare Mo oxide model systems to

investigate correlations between activity and structure of characteristic oxide species at

similar loadings.

116

8 Characterization of PVMo11 supported on SBA-15

with different metal loading

H4[PVMo11O40] supported on SBA-15 forms a mixture of tetrahedrally and octahedrally

coordinated and linked [MoO4] and [MoO6] units under catalytic conditions.[27]

Supposingly, the catalytic activity and selectivity towards oxygenates increases with

increasing amount of linked [MoxOy] species.[27] Accordingly, isolated [MoO4] units

supported on MgO were nearly inactive for propene oxidation.[137] The degree of

oligomerization may be varied by increasing or decreasing the metal loading or by altering

the surface acidity of the support material.[25,148,153] Additionally, the variation of pore

radii of the support material showed various structure directing effects of supported HPOM

at constant metal oxide loading and identical surface acidity (cf. chapter 7). A higher

concentration of octahedral [MoO6] and [MoxOy] units for PMo12-SBA-15 with smaller

pore radius could be detected compared to PMo12-SBA-15 with larger pores. While, the

catalytic activity increased with the amount of [MoxOy], the selectivities towards oxidation

products during propene oxidation conditions were comparable and independent of the

pore radius. This indicated similar active sites in act. PMo12-SBA-15 with various pore

radii. For elucidating structure activity correlations a study with varied metal loading was

necessary to established the catalytic active structure motifs. Variation of metal loading of

PVMo11-SBA-15 could lead to different structure directing effects during propene

oxidation conditions. Therefore, H4[PVMo11O40] was supported on SBA-15 with different

Mo loading (1 wt.% Mo, 5 wt.% Mo, and 10 wt.% Mo) to elucidate the resulting structure

under propene oxidation conditions and the influence on the catalytic activity. Hence,

PVMo11-SBA-15 was treated under propene oxidation conditions. In situ X-ray absorption

spectroscopy investigations at the Mo K edge of PVMo11 supported on SBA-15 with

different Mo loading) under catalytic conditions were conducted. A detailed analysis of the

structures present under catalytic conditions was performed and correlated with the

catalytic activity and product distribution towards propene oxidation.

117

8.1 Experimental

Sample Characterization











Si(311) double crystal monochromator. In situ experiments were conducted in a flow

reactor at atmospheric pressure (5 vol% oxygen in He, total flow ~30 ml/min, temperature

range from 303 to 723 K, heating rate 4 K/min). The gas phase composition at the cell

outlet was continuously monitored using a non-calibrated mass spectrometer in a multiple

ion detection mode (Omnistar from Pfeiffer).



by fitting linear polynomials and 3rd degree polynomials to the pre-edge and post-edge

region of an absorption spectrum, respectively. The extended X-ray absorption fine


background μ0(k) The FT(χ(k)·k3), often referred to as pseudo radial distribution function,






118

Single scattering and multiple scattering paths in the H3[PMo12O40] (ICSD 209 [14,93])

and hexagonal MoO3 (ICSD 75417 [135]) model structures were calculated up to 6.0 Å
















consumption was measured with a TCD unit. All samples were treated with a gas flow of

60 ml/min Ar at 393 K for about 45 min before starting the measurement. For

measurements 37.2 mg PVMo11-SBA-15 (10 wt. % Mo), 35.0 mg PVMo11-SBA-15 (5 wt.

% Mo), and 34.5 mg PVMo11-SBA-15 (1 wt. % Mo), were used.






constant volume and to exclude thermal effects, catalysts samples were diluted with SBA-

15 and boron nitride (Alfa Aesar, 99.5%). For catalytic testing in selective propene

oxidation a mixture of 5% propene (Linde Gas, 10% propene (3.5) in He (5.0)) and 5%

oxygen (Linde Gas, 20% O2 (5.0) in He (5.0)) in helium (Air Liquide, 6.0) was used in a

temperature range of 293-723 K Reactant gas flow rates of oxygen, propene, and helium

119

were adjusted with separate mass flow controllers (Bronhorst) to a total flow of 40 ml/min.

All gas lines and valves were preheated to 473 K. Hydrocarbons and oxygenated reaction

products were analyzed using a Carbowax capillary column connected to an AL2O3/MAPD

column or a fused silica restriction (25 m·0.32 mm each) connected to a flame ionization

detector. O2, CO, and CO2 were separated using a Hayesep Q (2 m x 1/8``) and a Hayesep

T packed column (0.5 m x 1/8``) as precolumns combined with a back flush. For

separation, a Hayesep Q packed column (0.5 m x 1/8``) was connected via a molsieve (1.5

m x 1/8``) to a thermal conductivity detector (TCD).

Sample preparation

Silica SBA-15 was prepared according to [22,23] as described in chapter 4.1. PVMo11-

SBA-15 (10 wt.% Mo, 5 wt.% Mo, and 1 wt.% Mo) was prepared via incipient wetness.

The amount of molybdenum was adjusted to 10 wt.%, 5 wt.% Mo, and 1 wt.% Mo.

Therefore an aqueous solution of HPOM was used.

120

8.2 Characterization of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, and 1

wt.% Mo)

Quantification of metal loading XRF

Quantitative analysis of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, and 1 wt.% Mo) were

performed to verify the supporting process. Results of quantitative XRF measurements and

nominal compositions of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, and 1 wt.% Mo)

were summarized in Table 8-1.

Table 8-1: Results of quantitative XRF measurements and nominal composition of PVMo11-SBA-

15 (10 wt.% Mo, 5 wt.% Mo, and 1 wt.% Mo).

elements

H P Mo V O Si

PVMo11-SBA-15 nom. wt.% 0.04 0.29 10.00 0.48 50.33 38.85

PVMo11-SBA-15 exp. wt.% - 0.42 9.15 0.42 50.44 39.46

PVMo11-SBA-15 nom. wt.% 0.02 0.15 4.99 0.24 51.79 42.80

PVMo11-SBA-15 exp. wt.% - 0.19 4.37 0.24 51.93 43.28

PVMo11-SBA-15 nom. wt.% 0.00 0.03 1.00 0.05 52.96 45.96

PVMo11-SBA-15 exp.. wt.% - 0.03 0.83 0.04 53.00 46.10

Experimental composition corresponded well to the nominal composition and confirmed a

successful supporting process. For simplification of the nomenclature the samples were

still denoted as PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, and 1 wt.% Mo).

Thermal stability of PVMo11 supported on SBA-15 with various metal loading (10

wt.% Mo, 5 wt.% Mo, and 1 wt.% Mo)

Fig. 8-1 depicts the measured thermogravimetric data of PVMo11-SBA-15 (10 wt.% Mo, 5

wt.% Mo, and 1 wt.% Mo) in 20% O2 in He. The mass loss between 303 K and 373 K was

ascribed to desorption of physically adsorbed water on the surface of the supported

materials. The loss of adsorb water delayed with higher Mo loading. The absolute mass

121

loss in this temperature range was between 8-12 wt.% for all PVMo11-SBA-15 (10 wt.%

Mo, 5 wt.% Mo, and 1 wt.% Mo) samples. The samples were not pretreated. Thus, the

amount of physically adsorbed water on the surface of the samples may depend on the

storage conditions. Afterwards a nearly constant mass between 373 K and 448 K could be

detected. The mass showed a loss for PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo) in

temperature range 448-523 K. The mass of PVMo11-SBA-15 (1 wt.% Mo) was nearly

constant in this temperature range. Subsequently, between 523 K and 823 K the mass

slightly decreased for all samples. Pure silica showed a comparable behaviour in vacuum.

Silica dehydrated between room temperature and 453 K followed by the dehydroxylation

process of silanol groups. This resulted in the formation of siloxane groups.[145,168]

Adsorbed water and silanol groups from the support material may possess a structure

stabilizing effect on the Keggion ion. This effect would be comparable to that of water of

crystallization and constitutional water in bulk HPOM.[117] It has been shown that

structural characteristics of supported model systems like MoOx-SBA-15 and VOx-SBA-15

depended mainly on their hydration states and previous calcination processes.[59,95]

Therefore, the delayed desorption of physically adsorbed water at higher Mo loadings, may

indicative a variable stability of the Keggin ion.

T [K]

Norm

aliz

ed

Mass [

%]

Norm

aliz

ed

Mass [

%]

373 473 573 673 773

Fig. 8-1: Thermograms of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, and 1 wt.% Mo) at 20% O2

in He.

10 wt.% Mo

5 wt.% Mo

1 wt.% Mo

86

88

90

92

94

96

98

100

122

Structure of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, and 1 wt.% Mo)

Fig. 8-2 shows the theoretical and experimental Mo K edge FT(χ(k)·k3) of PVMo11-SBA-

15 (10 wt.% Mo, 5 wt.% Mo, and 1 wt.% Mo). The shapes of the FT(χ(k)·k3) resembled

that of bulk PVMo11 indicating similar local structure around the Mo centers in supported

and unsupported HPOM Keggin structure. For a more detailed structural analysis, the

H3[PMo12O40] Keggin structure was chosen as model structure. A comparison of the

distances R and disorder parameters σ2 of PVMo11 supported on SBA-15 with different Mo

loadings (10 wt.% Mo, 5 wt.% Mo) exhibited no significant differences between the initial

Keggin ion structure and Keggin ions supported on SBA-15. PVMo11-SBA-15 (1 wt.%

Mo) exhibited increased 2nd and 3rd Mo-O distances compared to PVMo11-SBA-15 (10

wt.% Mo, 5 wt.% Mo).This distances corresponded to the edge- and corner-sharing

octahedral [MoO6] units. Additionally, the 2nd Mo-Mo distance was slightly increased

confirming the increased Mo-O distances. The various Mo-O and Mo-Mo distances may

indicate a slightly different binding state compared to PVMo11-SBA-15 (10 wt.% Mo, 5

wt.% Mo) possibly orginated from a stronger interaction of the Keggin ions with the

PVMo11-SBA-15 (1wt.% Mo)

R [Å]

FT

(χ(k)·k

3)

0 1 2 3 4 5 6 -0.05

0.00

0.05

0.10

0.15

0.20

0.25

Fig. 8-2: Theoretical (dotted) and experimental (solid) Mo K edge FT(χ(k)·k3) of PVMo11

supported on SBA-15 (10wt.% Mo, 5wt.% Mo, 1wt.% Mo).



123

support material SBA-15. However, the good agreement between theory and experiment

for PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo) confirmed the maintained Keggin ion

structure upon supporting PVMo11 on SBA-15.[27]


the Mo atoms in as prepared PVMo11-SBA-15 (10wt.% Mo, 5wt.% Mo, 1wt.% Mo). Experimental

parameters were obtained from a refinement of H3[PMo12O40] model structure (ICSD 209 [14,93])

to the experimental Mo K edge XAFS χ(k) of PVMo11-SBA-15 (10wt.% Mo, 5wt.% Mo, 1wt.%

Mo) (k range from 3.0-13.7 Å-1

, R range from 0.9 to 4.0 Å, E0 = ~ 1.7, residuals ~13.3-18.0 Nind =

22, Nfree = 9). Subscript c indicates parameters that were correlated in the refinement.

Keggin model

PVMo11-SBA-

15 (10wt.%

Mo) Mo)

PVMo11-SBA-15

(5wt.% Mo)

PVMo11-SBA-15

(1wt.%Mo)


2) R(Å) σ

2(Å

2) R(Å) σ

2(Å

2)

Mo-O 1 1.68 1.64 0.0035 1.65 0.0036 1.66

0.0004

Mo-O 2 1.91 1.78 0.0035c 1.78 0.0051c 1.82c 0.0030c

Mo-O 2 1.92 1.95 0.0035c 1.96 0.0051c 1.99c 0.0030c

Mo-O 1 2.43 2.40 0.0006 2.39 0.0010 2.38 0.0014

Mo-Mo 2 3.42 3.43 0.0057c 3.43 0.0063c 3.42 0.0066c

Mo-Mo 2 3.71 3.73 0.0057c 3.72 0.0063c 3.74 0.0066c

8.3 Structural evolution of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, and

1 wt.% Mo) under catalytic conditions

PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, and 1 wt.% Mo) samples were investigated by

in situ XAS under catalytic conditions. Fig. 8-3 shows the evolution of molybdenum

XANES spectra of PVMo11-SBA-15 (5 wt.% Mo, 1 wt.% Mo) samples during

temperature-programmed treatment in 5% propene and 5% oxygen. The resulting spectra

exhibited an increasing pre-edge peak with higher temperatures. The pre-edge peak

features in the Mo K edge XANES spectra can be employed to elucidate the local structure

around the Mo center. This increasing pre-edge peak correspond to structural changes from

octahedral [MoO6] species to partial tetrahedral [MoO4] species. The evolution of the ratio

of tetrahedral [MoO4] to octahedral [MoO6] units of PVMo11-SBA-15 (10 wt.% Mo, 5

124

wt.% Mo, and 1 wt.% Mo) based on a linear combination of bulk MoO3 and bulk

Na2MoO4 (cf. chapter 7.4) during propene oxidation conditions was shown in Fig. 8-4. In

contrast to P(V,W)xMo12-x-SBA-15 (x = 1, 2) (cf. chapter 5.2, 6.2) with identical Mo

loading and to PMo12-SBA-15 (10, 14, 19 nm) (cf. chapter 7.4) with tailored pore radii, the

onset temperatures the structural changes decreased with lower Mo loading. Structural

changes of PVMo11-SBA-15 (1 wt.% Mo) could be detected above 400 K and of PVMo11-

SBA-15 (5 wt.% Mo) above 425 K. Apparently, the Keggin ions in PVMo11-SBA-15 (10

Fig. 8-4: Quantification of the tetrahedral MoO4 ratio of ( )PVMo11-SBA-15 (10 wt.% Mo),

( )PVMo11-SBA-15 (5 wt.% Mo), and ( ) PMo11-SBA-15 (1 wt.% Mo) during thermal

treatment under propene oxidation conditions.

300 400 500 600 700

0

20

40

60

80

[MoO

4]/[M

oO

6] ra

tio

[%

]

T [K]

20 20.05

20.10 20.15

Photon energy [keV]

No

rma

lize

d

abso

rptio

n

T [K] 373 473

573 673

20 20.05

20.10 20.15

Photon energy [keV]

No

rma

lize

d

abso

rptio

n

T [K] 373 473

573 673

Fig. 8-3: in situ Mo K edge XANES spectra of (left) PVMo11-SBA-15 (5 wt.% Mo) and (right)

PVMo11-SBA-15 (1 wt.% Mo) during temperature-programmed treatment in 5% propene and 5%

oxygen in helium in a temperature range between 300 K and 723 K.

125

wt.% Mo) were stable on silica SBA-15 in the temperature range 303-448 K. Hence, the

stability of the Keggin ion increased with loadings from 1 wt. Mo % to 10 wt. Mo %.

Subsequently, concentration of tetrahedral [MoO4] units for all PVMo11-SBA-15 (10 wt.%

Mo, 5 wt.% Mo, and 1 wt.% Mo) considerably increased with higher temperature.

However, the temperatures of the structural evolution to mainly tetrahedral [MoO4] units

correlated to the dehydration and dehydroxylation process of SiO2 under oxidizing

conditions (20% O2 in He). Therefore, dehydroxylation was the driving force for the

structural rearrangement of the PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, and 1 wt.%

Mo) samples under catalytic conditions. This stability seemed to depend only on the nature

of the support material comparable to PMo12-SBA-15 (10, 14, 19 nm) (cf. chapter 7.4).

The structural rearrangement finished at ~ 550 K (PVMo11-SBA-15 (1 wt.% Mo)) and 598

K (PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo). Hence, higher metal loadings resulted in

increased temperatures, where the structural change were finished. Subsequently, the

concentration of tetrahedral [MoO4] units for PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo,

1 wt.% Mo) was constant and reached the highest concentration with ~75% [MoO4] units

at 723 K. PVMo11-SBA-15 (10 wt.% Mo) reach the highest concentration with ~50%

[MoO4] units at 723 K. Therefore, an increasing metal loading may lead to a decreased

amount of tetrahedral [MoO4] units.

Influence of metal loading to the resulting structure of [MoxOy] species

Fig. 8-5 shows the Mo K edge FT(χ(k)·k3) of activated PVMo11-SBA-15 (10 wt.% Mo, 5

wt.% Mo, 1 wt.% Mo) after thermal treatment under propene oxidation conditions. The

FT(χ(k)·k3) of act. PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, 1 wt.% Mo) exhibited

features similar to that of dehydrated molybdenum oxides and HPOM supported on SBA-

15.[27,59] For a more detailed structural analysis hexagonal MoO3 was chosen as model

structure. Theoretical XAFS phases and amplitudes were calculated for Mo-O and Mo-Mo

distances and used for EXAFS refinement. The results of the refinement are shown in Fig.

8-5. The first peak in the Mo K edge FT(χ(k)·k3) of act. activated PVMo11-SBA-15 (10

wt.% Mo) exhibited differences compared to that of act. activated PVMo11-SBA-15 (5

wt.% Mo, 1 wt.% Mo). The first peak in the FT(χ(k)·k3) originated mainly from the

tetrahedral [MoO4] species on the SBA-15 support and could be sufficiently simulated

126

using four Mo-O distances. These four distances sufficiently accounted for the minor

amount of octahedral [MoO6] species. The 1st and 2nd disorder parameters (1st-σ2, 2nd-σ

2)

were higher for act. PVMo11-SBA-15 (10 wt.% Mo) and indicated a decreasing amount of

tetrahedral [MoO4] units. Additionally, the 4th disorder parameter (4th-σ2) was smaller

than the disorder parameters for act. activated PVMo11-SBA-15 (5 wt.% Mo, 1 wt.% Mo)

with lower metal loading. This disorder parameter mainly represented the fraction of

octahedral [MoO6] species. Hence, the reduced disorder parameter indicated an increasing

amount of octahedral structural motifs in act. activated PVMo11-SBA-15 (10 wt.% Mo)

compared to act. PVMo11-SBA-15 (5 wt.% Mo, 1 wt.% Mo) and comparable to act.

PMo12-SBA-15 (14, 19 nm) (cf. chapter 7.3). The distinct peak at ~3 Å in the FT(χ(k)·k3)

indicated the formation of dimeric or oligomeric [MoOx] units on SBA-15 independent of

metal loading. The obtained Mo-Mo distances were comparable for act. PVMo11-SBA-15

(10 wt.% Mo, 5 wt.% Mo) samples and largely independent of the metal loading.

Additionaly, the disorder parameters of the Mo-Mo distances were lower for act. PVMo11-

SBA-15 (5 wt.% Mo) compared to act. PVMo11-SBA-15 (10 wt.% Mo) indicating a lower

oligomerization degree for the [MoxOy] species in act. PVMo11-SBA-15 (5 wt.% Mo). The

third Mo-Mo distance for act. PVMo11-SBA-15 (1 wt.% Mo) was increased compared to

act. PVMo12-SBA-15 (1wt.% Mo)



-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

FT

(χ(k)·k

3)

R [Å] 0 1 2 3 4 5 6

Fig. 8-5: Theoretical (dotted) and experimental (solid) Mo K edge FT(χ(k)·k3) of activated PVMo11

supported on SBA-15 (10wt.% Mo, 5wt.% Mo, 1wt.% Mo).

127


the Mo atoms in act. PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, 1 wt.% Mo). Experimental

parameters were obtained from a refinement of a hexagonal MoO3 model structure (ICSD 75417

[135]) to the experimental Mo K edge XAFS χ(k) of act. PMo12-SBA-15 (10, 14, 19 nm) (k range

from 3.6-16.0 Å-1

, R range from 0.9 to 4.0 Å, E0 = ~ -5.2, residuals ~12 Nind = 26, Nfree = 12).


hex-MoO3

model

act. PVMo11-SBA-

15 (10wt.% Mo)

act. PVMo11-SBA-

15 (5wt.% Mo)

act. PVMo11-SBA-

15 (1wt.%Mo)


2) R(Å) σ

2(Å

2) R(Å) σ

2(Å

2)

Mo-O 2 1.67 1.67 0.0012 1.67 0.0007 1.66 0.0007

Mo-O 2 1.96 1.89 0.0034c 1.87 0.0024c 1.86 0.0029c

Mo-O 1 2.20 2.18 0.0034c 2.18 0.0024c 2.17 0.0029c

Mo-O 1 2.38 2.36 0.0014 2.34 0.0024 2.35 0.0026

Mo-Mo 2 3.31 3.50 0.0066c 3.49 0.0049c 3.49 0.0051c

Mo-Mo 2 3.73 3.63 0.0066c 3.62 0.0049c 3.64 0.0051c

Mo-Mo 2 4.03 3.75 0.0100 3.75 0.0087 3.79 0.0118

act. PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo). This indicated a slightly different binding

state of Mo species on silica SBA-15 with lower metal loading. Apparently, the degree of

oligomerization of Mo of the [MoxOy] species reached a minimum on the samples with

lower metal loadings. This was comparable to PMo12-SBA-15 (14, 19 nm) samples with

large pores (cf. chapter 7.3). Additionally, the act. PVMo11-SBA-15 (1 wt.% Mo) showed a

slight different binding state compared to samples with higher metal loading. Therefore,

the Mo-O distances and disorder parameters for act. PVMo11-SBA-15 (5 wt.% Mo) and

act. PVMo11-SBA-15 (1 wt.% Mo) were nearly identical and different from those of act.

PVMo11-SBA-15 (10 wt.% Mo). The comparable Mo-O distances and disorder parameters

for act. PVMo11-SBA-15 (5 wt.% Mo) and act. PVMo11-SBA-15 (1 wt.% Mo) confirmed

the results of the quantification of tetrahedral [MoO4] and distorted [MoO6] units. The

quantification resulted in ~70% tetrahedral [MoO4] for both act. PVMo11-SBA-15 (5 wt.%

Mo) and act. PVMo11-SBA-15 (1 wt.% Mo) present under catalytic conditions. A

comparable quantification was found for molybdenum oxide supported on SBA-15 with a

Mo loading of 5.5wt.%.[59] The slightly increased Mo-Mo distances in act. PVMo11-SBA-

15 (1 wt.% Mo) may indicate a further decreased degree of oligomerization. Therefore, the

128

formation of dimeric or oligomeric [MoxOy] units mostly consisting of tetrahedral [MoO4]

units depended on metal loading on silica SBA-15. Hence, act. PVMo11-SBA-15 (10 wt.%

Mo) favored the formation of more extended structures on the support material due to

higher metal loading.

8.4 Functional characterization of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.%

Mo, and 1 wt.% Mo)

8.4.1 Reducibility

Fig. 8-6 shows the H2 TPR profiles of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, 1 wt.%

Mo). The resulted H2 TPR profiles of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo)

revealed one sharp reduction peak (~800 K) and a very broad signal after the sharp peak

comparable to PVxMo12-x-SBA-15 (x = 0, 1, 2) (cf. chapter 5.3.1). The H2 consumption for

PVMo11-SBA-15 (5 wt.% Mo) was due to lower metal loading compared to PVMo11-SBA-

15 (10 wt.% Mo). Additionally, the peak height of the sharp peak decreased for PVMo11-

SBA-15 (5 wt.% Mo) compared to the broaded signal above ~800 K. The H2 TPR profile

of PVMo11-SBA-15 (1 wt.% Mo) exhibited only a broad signal above 748 K. The H2 TPR

profiles of PVMo11-SBA-15 (10 wt.% Mo) were comparable to molybdenum oxides

supported on SBA-15 with Mo loadings between 9.5 wt.% and 13.3 wt.%.[149,150] Lou et

al. assigned the sharp reduction peak to oligomeric MoOx species or small MoOx clusters.

H2 TPR of PVMo11-SBA-15 (10 wt.% Mo) with the reduced sharp peak compared to the

broaded signal above ~800 K indicated a decreased degree of oligomerization comparable

to molybdenum oxide supported on SBA-15 with a Mo loading of 6.6 wt.%.[149] H2 TPR

profile of PVMo11-SBA-15 (1 wt.% Mo) was different from H2 TPR profiles of supported

molybdenum oxides with low Mo loadings (~2 wt.%). Typically, molybdenum oxides with

Mo loadings below 5 wt.% exhibited a shift of the reduction temperature to higher

temperatures (> 1000 K) indicating the reduction of predominantly monomeric MoOx

species.[149,150] Therefore, the broad signal above 748 K may corresponded dimeric or

oligomeric [(V,Mo)xOy] species. This species were lower oligomerized than the PVMo11-

SBA-15 (10 wt.% Mo, 5 wt.% Mo) samples. Apparently, mostly of the triads ([(V,Mo)xOy]

species) of the initial Keggin ion persisted during temperature programmed reduction.

129

Comparing the typical reduction temperatures of supported molybdenum oxides and

PVMo11-SBA-15 with different metal loading, a slightly higher degree of oligomerization

for PVMo11-SBA-15 with higher metal loading was determined.[150] Therefore, the

decreased height of the sharp peak compared to the broad signal above ~800 K PVMo11-

SBA-15 (5 wt.% Mo) indicated a decreased oligomeric [(V,Mo)xOy] species. Apparently,

the degree of oligomerization reached a minimum for PVMo11-SBA-15 (1 wt.% Mo),

indicated oligomerized [(V,Mo)xOy] species. In contrast to that, supported molybdenum

oxide synthesized from a AHM precursor lead to predominantly monomeric MoOx species

with Mo loadings below 2wt.%.[149,150]

8.4.2 Influence of the resulting structure on catalytic activity

Reaction rates and selectivities of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, 1 wt.% Mo)

in propene oxidation at 723 K are shown in Fig. 8-7. The reaction rates of PVMo11-SBA-

15 (5 wt.% Mo, 1 wt.%) were measured under various propene conversion conditions. The

propene conversion was varied to achieve a constant sample volume and to exclude

thermal effects. Therefore, PVMo11-SBA-15 (10 wt.% Mo) was mechanically diluted with

H2 c

on

su

mptio

n

T [K]

373 473 573 673 773 873 973

10 wt.% Mo

5 wt.% Mo 1 wt.% Mo

Fig. 8-6: Temperature programmed reduction (H2 TPR) of PVMo11-SBA-15 (10 wt.% Mo),

PVMo11-SBA-15 (5 wt.% Mo), and PVMo11-SBA-15 (1 wt.% Mo) measured at a heating rate of 8

Kmin-1

5% H2 in Ar.

130

SBA-15 to concentrations of 5 wt.% Mo and 1 wt.% Mo to achieve sample volumes

comparable to PVMo11-SBA-15 (5 wt.% Mo, 1 wt.% Mo).

The reaction rate of PVMo11-SBA-15 (10 wt.% Mo) (35.6 µmol/g(Mo)s) at

isoconversional conditions was slightly increased compared to that of PVMo11-SBA-15 (5

wt.% Mo) (34.1 µmol/g(Mo)s). The oxidation product distributions were comparable for

PVMo11-SBA-15 (5 wt.% Mo, 1 wt.% Mo) samples. The reaction rate for PVMo11-SBA-

15 (10 wt.% Mo) (29.9 µmol/g(Mo)s) at isoconversional conditions was also slightly

increased compared to that of PVMo11-SBA-15 (1 wt.% Mo) (28.8 µmol/g(Mo)s). The

product distribution of PVMo11-SBA-15 (10 wt.% Mo) was different from that of PVMo11-

SBA-15 (1 wt.% Mo). Selectivities towards acrolein and propionaldehyd increased and the

amount of total oxidation products decreased for PVMo11-SBA-15 (1 wt.% Mo) compared

to PVMo11-SBA-15 (10 wt.% Mo). Hence, an influence of the Mo loading on the catalytic

properties may be assumed. Thus, samples with lower Mo loading exhibited a decreased

catalytic activity comparable to PMo12-SBA-15 (10 nm, 14 nm, 20 nm) (cf. chapter 7.5.1).

This indicated a lower degree of oligomerization for samples with lower Mo loadings

0

20

40

60

80

100

a b c e d

acrylic acid acetic acid acrolein

acetone

acetaldehyd

e

CO CO2 propionaldehyd

e

Se

lectivity [

%]

rea

ctio

n r

ate

[µ

mo

l(pro

pe

ne

)g-1

(Mo

)s-1

]


(Mo)s-1

) and selectivity at different propene conversions

of (a) PVMo11-SBA-15 (10 wt.% Mo), (b) PVMo11-SBA-15 (10 wt.% Mo), (c) PVMo11-SBA-15

(5 wt.% Mo), (d) PVMo11-SBA-15 (10 wt.% Mo), and (e) PVMo11-SBA-15 (10 wt.% Mo) in 5%


0 5 10 15 20 25 30 35 40 45 50 55

14.1% 7.7% 8.3% 5.2% 3.7% propene conversion

131

confirming the results of the XAS analysis and TPR of act. PVMo11-SBA-15 (10 wt.% Mo,

5 wt.% Mo, 1 wt.% Mo). Nevertheless the catalytic activity in propene oxidation increased

with higher degree of oligomerization of the [(V,Mo)xOy] units under catalytic conditions.

Compared to PVMo11-SBA-15 (5 wt.% Mo) [(V,Mo)xOy] units orginating from PVMo11-

SBA-15 (10 wt.% Mo) resulted in an enhanced catalytic activity without significant

influence on the product distribution. The different product distribution of PVMo11-SBA-

15 (1 wt.% Mo) may result from slightly different binding states of the [(V,Mo)xOy] units.

The [(V,Mo)xOy] species resulting from PVMo11-SBA-15 (1 wt.% Mo) lead to an

increased selectivity towards acrolein and propionaldehyd. Grasseli et al. discussed the

role of site isolation i.e. the spatial separation of active sites on the surface of a

heterogenous catalyst.[170,172] A comparable effect could be responsible for the enhanced

selectivity towards partial oxidation products in act. PVMo11-SBA-15 (1 wt.% Mo).

Hence, may be assumed, that the [(V,Mo)xOy] species resulting in act. PVMo11-SBA-15 (1

wt.% Mo) improved the favorable number of active oxygens. Apparently, this active

oxygen species seemed to be bridged M-O-M (M = V, Mo) oxygen. Isolated [VOx] and

[MoOx] species would result mostly in total combustion (~ 90%). [137,153]

132

8.5 Summary

Structural evolution of H4[PVMo11O40] supported on SBA-15 (PVMo11-SBA-15)

with various Mo loadings (10 wt.% Mo, 5 wt.% Mo, 1 wt.% Mo) was examined by in situ

X-ray absorption spectroscopy investigations at the Mo K edge during propene oxidation

conditions. Supporting heteropolyoxo molybdates on SBA-15 with different Mo loadings

resulted in regular Keggin ions on the support material. During thermal treatment in

propene oxidation conditions the molybdenum oxide species on SBA-15 formed a mixture

of octahedral [MoO6] and mostly tetrahedral [MoO4] units. The ratio of the tetrahedral

[MoO4] to octahedral [MoO6] units increased with lower Mo loading. The onset

temperature of structural changes of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, 1 wt.%

Mo) during thermal treatment in propene oxidation conditions increased with higher Mo

loading. A delayed desorption of physically adsorbed water in PVMo11-SBA-15 with

higher Mo loading was indicative of a varied stability of the Keggin ion. Hence, desorption

of physically adsorbed water and dehydroxylation of silanol groups of the support material

were the driving forces for the structural instability of the Keggin ion. The instability lead

to the formation of act.PVMo11-SBA-15. The resulting [(V,Mo)xOy] structures present

under catalysis conditions depended on Mo loading. A higher concentration of octahedral

[MoO6] units together with higher oligomerized [(V,Mo)xOy] species could be detected for

higher Mo loading. The higher oligomerized [(V,Mo)xOy] species present in act. PVMo11-

SBA-15 (10 wt.% Mo) showed a slight increased catalytic activity compared to lower

oligomerized [(V,Mo)xOy] species in act. PVMo11-SBA-15 (5 wt.% Mo, 1 wt.% Mo). The

selectivities towards oxidation products during propene oxidation conditions were

comparable and independent of the Mo loading (10 wt.% Mo, 5 wt.% Mo) indicating the

same active sites in act. PVMo11-SBA-15. The product distribution for PVMo11-SBA-15

(1 wt.% Mo) was different from those of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo).

Selectivities towards acrolein and propionaldehyd increased and total oxidation products

decreased for PVMo11-SBA-15 (1 wt.% Mo) compared to PVMo11-SBA-15 (10 wt.% Mo,

5 wt.% Mo). The different product distribution of PVMo11-SBA-15 (1 wt.% Mo) may

result from a slightly different binding state of the [(V,Mo)xOy] species. This different

binding state of the [(V,Mo)xOy] species may lead to an increased selectivity towards

partial oxidation products.

133

9 General discussion and Summary

9.1 Structure directing effect of the support material

SBA-15 as support material had a significant influence on the structures of supported

species that formed during thermal treatment. Generally, the structures of model systems

such as MoOx-SBA-15, VOx-SBA-15, and WOx depend on their hydration

states.[17,59,95,144,149,173,174] SBA-15 or rather SiO2 adsorbed water at ambient

conditions. This water is removed at temperatures above 423 K.[168,175] In this work

HPOM were supported via incipient wetness with an aqueous solution on the support

material (cf. Chapter 4.1). Therefore, it may be assumed that the HPOM were incorporated

in a matrix of adsorbed water comparable to higher hydrates of bulk HPOM or to an

aqueous solution. The removal of adsorbed water and the following dehydroxylation of

silanol groups during thermal treatment lead to a destabilizing effect on the Keggin

ion.[145] This effect would be comparable to that of the removal of water of crystallization

and constitutional water in bulk HPOM. The removal of constitutional water in bulk

HPOM leads to a decomposition resulting in MoO3 during thermal treatment.[116] TG

measurement of PVMo11-SBA-15 showed a mass loss of about 2 wt.% between 473 and

673 K during oxidizing conditions indicating the dehydroxylation of silanol groups (c.f.

Chapter 8.2; Fig. 8-1). The destabilizing effect on the Keggin ion was independent of the

addenda atoms (V,W) in P(V,W)x-SBA-15 (x = 0, 1, 2), the pore radii of the SBA-15, and

HPOM loading.

The structures forming during thermal treatment in propene oxidation conditions depended

on the nature of SiO2, pore radii of the SBA-15, and HPOM loading. According to Wachs

et al. the structure of supported metal oxides is correlated to the net pH at the point of zero

charge (pzc) of the oxide support.[166] SiO2 has a pH of 3.9 at pzc. Therefore, the low pH

at pzc of SiO2 leads to mainly linked M-O-M (M = Mo, V, W) species corresponding to

the behaviour of molybdates, vanadates, and wolframates in acidic solutions.[37,148,176]

Compared to SBA-15, other support materials exhibit different structure directing effects

depending on the acidity of the surface.[69–71] For instance, mainly isolated [MoO4] and

[VO4] units existed on an alkaline MgO support in agreement with the behaviour of

molybdates and vanadates in alkaline solution.[137,153]

134

The pore radius also had a significant influence on the structures formed during thermal

treatment under propene oxidation conditions. Fig. 9-1 depicts a two dimensional

schematic representation of pores with different radii. Squares in the pores represent the

Keggin ions. The radius (r1) of the smaller pore is half of the pore radius of the second

pore (r2 = 2r1). Doubling of the pore radius lead to a doubling of the circumference of the

pore and the number of squares representing the Keggin ions. Nevertheless, the effective

distance D between the squares is clearly smaller in the smaller pore (D1) than in the larger

pore (D2). Therefore, thermal treatment under propene oxidation conditions lead to lower

oligomerized [MoOx] species and an increased [MoO4]/[MoO6] ratio at smaller pore radii

as shown in Chapter 7.

The HPOM loading has a further influence on the structures formed during thermal

treatment under propene oxidation conditions (Chapter 8). This effect was comparable to

the effect of SBA-15 with different pore radii. The surface coverage for samples with

lower HPOM loading was decreased resulting in less extended species on the support

material. Hence, the degree of oligomerization of the [MoOx] species was decreased and

the [MoO4]/[MoO6] ratio was increased at lower HPOM loading. This decreasing degree of

oligomerization at lower metal loading has been shown for various supported metal

oxides.[59,95,137,149,153,177,178]

r1 2r1 = r2

D1

D2

D1 < D2

Fig. 9-1: Two dimensional schematic representation of pores with squares representing the Keggin

ions.

135

9.2 Structure directing effects of the addenda atoms

Both bulk HPOM and supported HPOM exhibited a structure directing effect of addenda

atoms. The structures forming during thermal treatment under oxidizing and propene

oxidation conditions for bulk HPOM depended on the degree of substitution and type of

addenda atoms (V, W). Bulk HPOM decompose during thermal treatment under oxidizing

conditions resulting in MoO3. The loss of constitutional water depended also on the degree

of substitution and type of addenda atoms (V, W). Vanadium substitution lead to decreased

decomposition temperatures of 573 K for PV2Mo10, 623 K for PVMo11, and 673 K for

unsubstituted PMo12 K according to the literature.[35,179] Tungsten substitution had no

influence on the decomposition temperatures resulting in the loss of constitutional water.

Subsequently, the HPOM without constitutional water decomposed to MoO3. The resulting

modifications were α-MoO3 and β-MoO3. Vanadium substituted HPOM (PVxMo12-x x = 1,

2) favored the formation of α-MoO3 and tungsten substituted HPOM (PWxMo12-x (x = 1,

2)) favored the formation of β-MoO3 (c.f. Chapter 3.5). An explanation of the structure

directing effect are the different charges and ion radii of V5+

, W6+

, and Mo6+

resulting in

the edge-shared structure of α-MoO3 for (PVxMo12-x (x = 1, 2)) and corner-shared structure

of β-MoO3 for PWxMo12-x x = 1, 2 according to Pauling`s rules.[122]

During thermal treatment under propene oxidation conditions, bulk HPOM (PVxMo12-x x =

0, 1, 2) decomposed to various structures depending on the type of addenda atoms (V, W).

PMo12 decomposed to α-MoO3 as the thermodynamically stable modification of

molybdenum oxides in their highest oxidation state (+ 6). PV2Mo10 decomposed during

thermal treatment in propene oxidation conditions at 723 K into a mixture of various

structures. Ressler et al. performed in situ XAS measurements of PVMo11 during propene

oxidation conditions.[14] In this process Mo cations migrate on extra Keggin sites while

remaining coordinated to the resulting lacunary Keggin anion.[13] Driving force for the

formation of lacunary Keggin anions may be the relaxation of the Keggin structure at

elevated temperature upon removal of structural water. These structural changes at

temperatures above 573 K are accompanied by reduction of the molybdenum

centers.[13,14] Subsequently, the reduced Mo centers reoxidized upon 723 K.[13,14] In

another study on PV2Mo10 Ressler et al. described a dynamic behaviour by isothermally

switching from propene (reducing) to oxidizing (propene and oxygen) and back to propene

(reducing) conditions at 723 K. The results of the in situ XAS experiment revealed for the

136

reduced state the formation of a short vanadium-molybdenum distance of about 2.8 Å. The

oxidized state exhibited a longer distance of the vanadium center to an extra-Keggin

molybdenum center at 3.2 Å.[16] The results suggested a mixture of at least two sites

around two different V centers.[16] Therefore, it may be assumed, that the structures

formed during treatment of PV2Mo10 under propene oxidation conditions corresponded to a

mixture of various structures comparable to lacunary Keggin ions or Keggin ions.

PW2Mo10 decomposed during thermal treatment in propene oxidation conditions at 723 K

to a mixture of α-MoO3 and Mo17O47.[41,44] Molybdenum in Mo17O47 has an average

valence of ~ +5.5 which indicated, that the degree of reduction was higher for PW2Mo10

than for PMo12. Hence, tungsten lead to an increased reducibility of PW2Mo10 during

propene oxidation conditions.

The addenda atoms (V, W) in substituted HPOM supported on SBA-15 (P(V,W)xMo12-x-

SBA-15 (x = 0, 1, 2)) exhibited also a structure directing effect on the structures forming

during thermal treatment in propene oxidation conditions at 723 K. Vanadium lead to an

increasing [MoO4]/[MoO6] ratio in contrast to tungsten substituted HPOM supported on

SBA-15 (Fig. 9-2). The typical structure resulting for dehydrated molybdenum oxides

supported on SiO2 was a mixture of tetrahedral [MoO4] and octahedral [MoO6]

units.[27,59,149] Dehydrated vanadium oxides supported on SiO2 lead to the formation of

predominantly [VO4] units.[17,25,95] Therefore, it may be assumed, that during the

decomposition process, neighboring [MoO6] units were influenced by [VO6] units of the

20

40

60

80 PV2Mo10-SBA-15 PVMo11-SBA-15 PMo12-SBA-15 PWMo11-SBA-15 PW2Mo10-SBA-15

[MoO

4]/[M

oO

6] ra

tio

[%

]

Fig. 9-2: [MoO4]/[MoO6] ratio of P(V,W)xMo12-SBA-15 (x = 0, 1, 2) during thermal treatment

under propene oxidation condition (5% propene + 5% O2 in He) at 723 K.

137

initial Keggin ion structure. This influence lead to tetrahedral [MoO4] and [VO4] units

during thermal treatment under propene oxidation conditions (c.f. Chapter 5.2). Hence, the

formation of [MoO4] units depended on the degree of vanadium substitution.

Dehydrated tungsten oxides or H3[PW12O40] supported on SiO2 corresponded to a Si

containing Keggin type cluster with corner- and edge-shared [WO6] units on the support

material.[58,162,180] Therefore, additional [WO6] species in PWxMo12-x-SBA-15 (x = 1,

2) influenced the Mo species of the initial Keggin ion structure resulting in predominantly

[MoO6] and [WO6] units during thermal treatment under propene oxidation conditions (c.f.

Chapter 6.2). Both in vanadium and in tungsten substituted supported HPOM

(P(V,W)xMo12-x-SBA-15 (x = 1, 2)) the resulting [MOx] (M = V, W) units were in close

vicinity to the [MoOx] species.

9.3 Structure activity relationships

The different structures forming under catalytic conditions (5% propene + 5% oxygen in

He at 723 K) for bulk P(V,W)xMo12-x (x = 0, 1, 2) depended on the substituted element

(V, W). The different structures forming during catalytic conditions correlated with the

different catalytic behaviours of P(V,W)xMo12-x ( x = 0, 1, 2). Fig. 9-3 depicts the resulting

reaction rates and selectivities towards C3 oxidation products and CO, CO2. Both

vanadium substituted PVxMo12-x (x = 1, 2) and tungsten substituted PWMo12-x (x = 1, 2)

showed increased reaction rates compared to unsubstituted PMo12. The significant

differences between the samples were the resulting structures. α-MoO3 resulting from

PMo12 showed the lowest catalytic activity in propene oxidation. The lacunary Keggin and

Keggin ion resulting from PV2Mo10 and the mixture of α-MoO3 and Mo17O47 resulting

from PW2Mo10 showed an increased catalytic activity during propene oxidation. Therefore,

it may be assumed that the different catalytic activities of the various structures depended

on the type of addenda atom (V, W) and the degree of substitution. The resulting structures

for the substituted HPOM indicated also a lower average valence of molybdenum for the

lacunary Keggin and Keggin ion resulting from PV2Mo10 and the mixture of α-MoO3 and

Mo17O47 resulting from PW2Mo10 (c.f. Chapter 3.5.2).

138

Vanadium centers in substituted HPOM (PVxMo12-x (x = 1, 2)) can reversibly change their

oxidation state from V5+

to V4+

without significant destabilization of the lacunary Keggin

or intact Keggin ion.[15,16] Additionally, reduced V4+

has an ion radius of 72 pm which is

comparable to that of Mo6+

(74 pm). The similar ion radius of V4+

may stabilize partially

reduced intermediates.[103,181] Ressler et al. described in various XANES studies, that

the Mo centers in bulk HPOM reduced between 573 K and 723 K to an average valence of

~ 5.85 during propene oxidation conditions. Subsequently, the Mo centers reoxidized

above 723 K.[13–16] The ion radii of reduced Mo5+

centers (75 pm) and W5+

centers

(76 pm) were larger than the ion radius of V4+

(72 pm) [103,181]. In particular at elevated

temperatures the larger ion radii of reduced Mo5+

and W5+

centers destabilized the Keggin

ion structure resulting in α-MoO3 or the corresponding partially reduced Mo17O47.

Therefore, the reduction process during propene oxidation conditions above 573 K may

lead to a destabilization of the lacunary Keggin ions or Keggin ions. Hence, the increased

catalytic activity may result from the significantly different structures of PVxMo12-x

(x = 1, 2) in contrast to PMo12 (Fig. 9-3). The partial reduced Mo17O47 phase resulting from

PWxMo12-x (x = 1, 2) was stabilized by tungsten. Tungsten centers are able to occupy up to

30% of the sites of molybdenum atoms on Mo17O47.[182] Therefore, the formation of a

mixture of α-MoO3 and Mo17O47 exhibited more reduced Mo centers than

4

5

6

7

8

9

10

11

12

13

PV2Mo10

PVMo11

PMo12

PWMo11

PW2Mo10

reactio

n r

ate

µm

ol(p

rop

ene)g

-1(M

o)s

-1

20 25 30 35 40 45 50 55 60 65 70 75 80

Se

lectivity [%

]

CO+CO2

C3 oxidation products

Fig. 9-3: (left) Reaction rates (µmol(propene)/g(Mo)) and (right) selectivities towards C3 oxidation

products and CO+CO2 of bulk P(V,W)xMo12-x (x = 0, 1, 2) in 5% propene and 5% oxygen in He at

723 K.

PV2Mo10

PVMo11

PMo12

PWMo11

PW2Mo10

139

α-MoO3 resulting from unsubstituted PMo12. This partially reduced mixture of α-MoO3

and Mo17O47 may lead to the enhanced catalytic activity.

The different structures and average valences of Mo resulting from P(V,W)xMo12-x

(x = 0, 1, 2) may also lead to different selectivities (Fig. 9-3). Both structure and average

valence of the resulting compounds have an influence on the product distribution.[2,4,183]

Therefore, explaining the various selectivities was hindered by simultaneously varying

composition and structure. Reduced metal centers may be particularly effective in

activating gas-phase oxygen, resulting in an oversupply of surface-bond electrophilic

oxygen. Generally, oxidation reactions proceed in two steps. The first step is the reduction

of the catalyst with propene at the expense of lattice oxygen. The second step is the

reoxidation of the catalyst with gas phase oxygen.[4] The reoxidation process is described

by a series of reactions starting with adsorption of gas phase oxygen and eventually leading

to the formation of lattice oxygen. If the reoxidation process is slow, propene may be

attacked by nonselective oxygen species.[4] The reduced metal centers indicated, that the

reaction rate of the reduction process was higher compared to that of the oxidation process.

Hence, surface-bond electrophilic oxygen is prone to further oxidize propene or acrolein to

CO2.[4,16,184] Therefore, it may be assumed, that reduced metal centers were responsible

for the decreased selectivity towards C3 oxidation products and increased formation of

total oxidation products.

Fig. 9-4 (left) depicts reaction rates as a function of the [MoO4]/[MoO6] ratio resulting for

act. P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) and PMo12-SBA-15 (14 nm, 19 nm) (coverage of

1 Keggin ion per 13 nm2) during propene oxidation (5% propene + 5% O2 in He; 723 K).

The reaction rates were determined for similar propene conversions. The reaction rate

decreased with higher [MoO4]/[MoO6] ratio independent of the addenda atoms (V,W) or

pore radii of SBA-15. Additionally, the [MoO4]/[MoO6] ratio was an indicator for the

degree of oligomerization. In all samples with higher [MoO4]/[MoO6] ratio, a decreased

degree of oligomerization was assumed. This result confirmed the assumption, that

selective oxidation takes place only in the presence of bridging M-O-M bonds.[4] The

increased degree of oligomerization leads to a higher concentration of Mo-O-Mo bonds

resulting in an increased reaction rate. The increased reaction rate resulted in increased

total combustion (Fig. 9-4). The samples with a higher [MoO4]/[MoO6] ratio and a

decreased degree of oligomerization exhibited an increased selectivity towards C3

140

oxidation products. Hence, reaction rates and selectivity towards the desired C3 oxidation

products competed with each other. Therefore, it may be assumed that the excess of

bridging M-O-M (M = Mo, V, W) lead to overoxidation according to the

literature.[2,4,170]

Addenda atoms had an additional influence on the reaction rates and selectivities for act.

P(V,W)xMo12-x-SBA-15 (x = 1, 2). It was shown for both act PVxMo12-x-SBA-15 (x = 1, 2)

and act. PWxMo12-x-SBA-15 (x = 1, 2), that the reaction rates and selectivities differed

from that of references synthesized with individual metal (Mo, V, W) precursors (c.f.

Chapter 5.3.2, 6.3.2). Structural analysis revealed that the resulting [MOx] (M = V, W)

species in act. (P(V,W)xMo12-x-SBA-15 (x = 1, 2)) were in close vicinity to the [MoOx]

species. Conversely, the [MOx] (M = V, W) species in act. V2Mo10Ox-SBA-15 and act.

W2Mo12Ox-SBA-15 were mostly separated from the [MoOx] species Apparently, the

neighboring [MOx] (M = V, W) units and [MoOx] units in act. (P(V,W)xMo12-x-SBA-15 (x

= 1, 2)) resulted in an increased formation of CO and CO2. Various mixed metal oxides

exhibited an increased formation of CO and CO2 with higher chemical complexity [185–

187]. Probably, the neighboring [MOx] (M = V, W) units and [MoOx] units were able to

activate gas-phase oxygen, resulting in an oversupply of surface-bond electrophilic

oxygen. This electrophilic oxygen is prone to further oxidize propene or acrolein to CO2

20

25

30

35

40

45

50

55

60

10 20 30 40 50 60 70 80 90

reactio

n r

ate

µm

ol(p

rop

ene)g

-1(M

o)s

-1

Se

lectivity [

%]

40

45

50

55

60

65

70

20 30 40 50 60 70 80

[MoO4]/[MoO6] ratio [%]

[MoO4]/[MoO6] ratio [%]

CO+CO2

C3 oxidation products

2V V

2W

W

2W

2W

W

W

2V

2V V

V

Fig. 9-4: (left) Reaction rates and (right) selectivities towards C3 oxidation products and CO+CO2

as a function of the [MoO4]/[MoO6] ratio of act. P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) in 5%

propene and 5% oxygen in He at 723 K. The type of addenda atoms (V,W) and degree of

substitution (x = 1, 2) are marked.

141

comparable to the effect of particularly reduced metal centers in bulk oxides.[8,118,121]

The references act. V2Mo10Ox-SBA-15 and act. W2Mo12Ox-SBA-15 with mostly separated

[MOx] (M = V, W) and [MoOx] species exhibited an increased selectivity towards C3

oxidation products and a decreased formation of CO and CO2. The neighboring [MOx]

(M = V, W) units and [MoOx] units in act. P(V,W)xMo12-x-SBA-15 (x = 1, 2) with higher

chemical complexity exhibit an increased formation of CO2 and CO according to the

literature.[185–187]

142

10 Conclusions

Introduction

Understanding structure-activity correlations of functional materials is an important issue

in catalysis and materials science. Often model systems are investigated. For elucidating

structure activity correlations of model systems for catalytic investigations, a detailed

knowledge about structure and chemical composition is indispensable. Thus, various

characterization methods are necessary for a sufficient characterization of the catalyst

systems. Heteropolyoxomolybdates (HPOM) of the Keggin type exhibit a broad

compositional range while maintaining their characteristic structural motifs. Therefore,

H3[PMo12O40] (PMo12), H4[PVMo11O40] (PVMo11), H5[PV2Mo10O40] (PV2Mo10),

H3[PWMo11O40] (PWMo11), and H3[PW2Mo10O40] (PW2Mo10) were synthesized as model

catalysts in selective propene oxidation. P(V,W)xMo12-x (x = 0, 1 ,2) were supported on

SBA-15. The initial Keggin structure of P(V,W)xMo12-x (x = 0, 1 ,2) was retained after the

supporting process. In situ XAS investigations under propene oxidation conditions of

P(V,W)xMo12-x-SBA-15 (x = 0, 1 ,2) were conducted. Catalytic testing elucidated the

functional properties of P(V,W)xMo12-x-SBA-15 (x = 0, 1 ,2) during propene oxidation

conditions. A detailed analysis of the structures formed under catalytic conditions was

conducted and correlated with the catalytic activity and product distribution towards

propene oxidation.

Synthesis of bulk HPOM and supported HPOM

The synthesis of P(V,W)xMo12-x (x = 0, 1, 2) lead to HPOM with the desired Keggin type

structure and chemical composition. P(V,W)xMo12-x (x = 0, 1, 2) crystallized as 13 hydrate

in a triclinic crystal system. The 13 hydrate structure of the HPOM ensured the

incorporation of the addenda atoms (V, W) in the Keggin ion. The volume of the unit cell

decreased with higher vanadium substitution because of the smaller ionic radius of V in a

six-fold coordination. W with an identical ionic radius compared to Mo had no influence

on the volume of the unit cell in contrast to the vanadium substitution. The results of IR

and Raman measurements confirmed, that the addenda atoms (V, W) were incorporated in

143

the Keggin ion. The IR and Raman spectra exhibited the typical peaks of the Keggin ion

structure. The EXAFS refinements indicated, that the addenda atoms (V, W) were

incorporated in the Keggin ion independent of the degree of substitution.

XRD and physisorption measurements of the tailored SBA-15 (10 to 19 nm) confirmed a

successful synthesis of mesoporous SiO2 materials with different pore size distributions,

high specific areas, and the typical pore structure of SBA-15. Supporting P(V,W)xMo12-x

(x = 0, 1, 2) on SBA-15 (10 to 20 nm pore radius) via incipient wetness lead to the desired

metal loadings (1 to 10 wt. Mo or rather 1 Keggin ion per 130 to 13 nm2). P(V,W)xMo12-x-

SBA-15 (x = 0, 1, 2) were sufficiently dispersed on the support material without affecting

the pore structure of the support material. The formation of extended crystalline HPOM

structures could be excluded.

Structure directing effect of SBA-15

SBA-15 as support material had a significant influence on the structures formed during

thermal treatment. SBA-15 or rather SiO2 adsorbed water at ambient conditions. This water

is removed at temperatures above 423 K. The removal of adsorbed water and the following

dehydroxylation of silanol group lead to a destabilizing effect on the Keggin ion.

Therefore, it may be assumed that the HPOM were incorporated in a matrix of physisorbed

water comparable to higher hydrates of bulk HPOM or to an aqueous solution. This effect

would be comparable to that of the removal of water of crystallization and constitutional

water in bulk HPOM. The removal of constitutional water in bulk HPOM results in

decomposition and formation of MoO3 during thermal treatment. The destabilizing effect

on the Keggin ion was independent of the addenda atoms (V,W) in P(V,W)x-SBA-15

(x = 0, 1, 2), the pore radii of the SBA-15, and HPOM loading. Hence, the stability of

Keggin ions supported on SBA-15 was significantly decreased compared to bulk HPOM,

with decomposition temperatures between 623 K and 713 K.

The resulting structures forming during thermal treatment in propene oxidation conditions

depended on the nature of SiO2, pore radii of the SBA-15, and HPOM loading. The

structure of supported metal oxides is correlated to the net pH at the point of zero charge

(pzc) of the oxide support. SiO2 have a pH of 3.9 at pzc. Therefore, the low pH at pzc of

SiO2 lead to mainly linked M-O-M (M = Mo, V, W) species corresponding to the

behaviour of molybdates, vanadates and wolframates in acidic solutions.

144

The pore radius had also a significant effect on the structures formed during thermal

treatment under propene oxidation conditions. A higher concentration of octahedral

[MoO6] units and higher oligomerized [MoxOy] units resulted for act. PMo12-SBA-15

(10 nm) compared to act. PMo12-SBA-15 (14, 19 nm). Enlarging the pore radii lead to an

increased effective distances between the Keggin ions. The effective distances were clearly

smaller in the smaller pores than in the larger pores. Therefore, the structures forming

during thermal treatment under propene oxidation conditions consisted of lower

oligomerized [MoOx] species and an increased [MoO4]/[MoO6] ratio with smaller pore

radius. Apparently, tailoring the pore radius of silica SBA-15 permitted to prepare Mo

oxide model systems to investigate correlations between activity and structure of

characteristic oxide species at similar loadings.

The HPOM loading had a further influence on the structures forming during thermal

treatment under propene oxidation conditions. The surface coverage for samples with

lower HPOM loading was decreased resulting in less extended species on the support

material. Hence, the degree of oligomerization of the [MoOx] species was decreased and

the [MoO4]/[MoO6] ratio was increased with lower HPOM loading. This effect was

comparable to the effect resulting for SBA-15 with different pore radii.

Structure directing effect of the addenda atoms

The addenda atoms exhibited structure directing effect in both bulk HPOM and supported

HPOM. The structures forming during thermal treatment under oxidizing and propene

oxidation conditions for bulk HPOM depended on the degree of substitution and type of

addenda atoms (V, W). Bulk HPOM decomposed during thermal treatment under

oxidizing conditions resulting in MoO3. The release of constitutional water of the HPOM

depended also on the degree of substitution and type of addenda atoms (V, W). Vanadium

substitution lead to decreased decomposition temperatures of 573 K for PV2Mo10, of 623 K

for PVMo11, and of 673 K for unsubstituted PMo12. Tungsten substitution had no influence

on the release of constitutional water of the HPOM. Subsequently, the HPOM without

constitutional water decomposed to MoO3. Vanadium substitution in HPOM (PVxMo12-x (x

= 1, 2)) lead to an increased formation of predominantly α-MoO3 depending on the degree

of vanadium substitution. Conversely, tungsten substituted PWxMo12-x (x = 1, 2) resulted in

an increased formation of β-MoO3 depending on the degree of tungsten substitution. The

145

different charges and ion radii of V5+

, W6+

, and Mo6+

were responsible for the structure

directing effect resulting in the rather edge-shared structure α-MoO3 for (PVxMo12-x (x = 1,

2)) and corner-shared structure β-MoO3 for PWxMo12-x x = 1, 2 according to Pauling`s

rules.

Bulk HPOM (PVxMo12-x x = 0, 1, 2) decomposed during thermal treatment in propene

oxidation conditions to various structures depending on the type of addenda atoms (V, W).

The Mo centers in bulk HPOM partially reduced between 573 K and 723 K to an average

valence of ~ 5.85 and reoxidized above 723 K during propene oxidation conditions. The

ion radii of reduced Mo5+

centers (75 pm) and W5+

centers (76 pm) were larger than the ion

radius of V4+

(72 pm). In particular at elevated temperatures the larger ion radii of reduced

Mo5+

centers and W5+

centers destabilized the Keggin ion structure. Therefore, PMo12

decomposed to the thermodynamically stable modification of molybdenum oxides, α-

MoO3. PW2Mo10 decomposed during thermal treatment in propene oxidation conditions to

a mixture of α-MoO3 and Mo17O47. In contrast to PWxMo12-x (x = 0, 1, 2), PV2Mo10

decomposed during thermal treatment in propene oxidation conditions at 723 K to a

mixture of various structures. The ion radius of reduced V4+

(72 pm) was comparable to

the ion radius of Mo6+

(74 pm) stabilizing partially reduced intermediates of the initial

Keggin ion structure. Vanadium substitution lead probably to a mixture of lacunary Keggin

ion and Keggin ions.

The addenda atoms (V, W) in substituted HPOM supported on SBA-15 (P(V,W)xMo12-x-

SBA-15 (x = 0, 1, 2)) exhibited also a structure directing effect on the structures forming

during thermal treatment in propene oxidation conditions at 723 K. Vanadium substituted

HPOM lead to an increasing [MoO4]/[MoO6] ratio whereas tungsten substituted HPOM to

a decreasing [MoO4]/[MoO6] ratio of the resulting [MOx] (M = Mo, V, W) species

supported on SBA-15. The [MoO6] units were influenced by the neighboring [VO6] units

and [WO6] units of the initial Keggin ion structure resulting in tetrahedral [MoO4] and

[VO4] units and octahedral [MoO6] and [WO6] units during thermal treatment under

propene oxidation conditions. Hence, the formation of [MoO4] or [MoO6] units depended

on the degree of vanadium or tungsten substitution. Both in vanadium and in tungsten

substituted supported HPOM (P(V,W)xMo12-x-SBA-15 (x = 1, 2)) the resulting [MOx] (M

= V, W) units were in close vicinity to the [MoOx] species.

146

Structure-activity correlations

The different structures forming under catalytic conditions (5% propene + 5% oxygen in

He at 723 K) for bulk P(V,W)xMo12-x (x = 0, 1, 2) depended on the addenda atoms (V, W).

Both vanadium substituted PVxMo12-x (x = 1, 2) and tungsten substituted PWMo12-x

(x = 1, 2) showed increased reaction rates compared to unsubstituted PMo12. α-MoO3

resulting from PMo12 showed the lowest catalytic activity in propene oxidation. The

mixture of lacunary Keggin ions and Keggin ions resulting from PV2Mo10 and the mixture

of α-MoO3 and Mo17O47 resulting from PW2Mo10 showed an increased catalytic activity

during propene oxidation. The resulting structures for the substituted HPOM indicated also

a lower average valence of molybdenum compared to unsubstituted HPOM. Hence, both

the structure and the average valence of the metal centers were responsible for the different

catalytic behaviour. The various structures and average valences of Mo resulting from

P(V,W)xMo12-x (x = 0, 1, 2) may also lead to various selectivities. Both PVxMo12-x (x = 1,

2) and PWxMo12-x (x = 1, 2) lead to an increased formation of total oxidation products and

a decreased selectivity towards C3 oxidation products depending on the degree of

substitution. The reduced metal centers in the mixture of lacunary Keggin ions and Keggin

ions resulting from PV2Mo10 and the mixture of α-MoO3 and Mo17O47 resulting from

PW2Mo10 may be particularly effective in activating gas-phase oxygen, resulting in an

oversupply of surface-bond electrophilic oxygen. This electrophilic oxygen is prone to

further oxidize propene or acrolein to CO2. Therefore, it may be assumed, that the reduced

average valence in the structures resulting from P(V,W)xMo12-x (x = 1, 2) was responsible

for the decreased selectivity towards C3 oxidation products and higher formation of total

oxidation products.

The various structures resulting for supported HPOM exhibited also an influence on the

catalytic activity. The reaction rates at similar propene conversion for supported HPOM

decreased with higher [MoO4]/[MoO6] ratio for act. P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2).

Therefore, the concentration of [MoO4] and [MoO6] correlated with the catalytic activity.

Additionally, the [MoO4]/[MoO6] ratio was an indicator for the degree of oligomerization.

In samples with higher [MoO4]/[MoO6] ratios a decreased degree of oligomerization is

assumed. These results confirmed the assumption, that selective oxidation takes place only

in the presence of bridging M-O-M bonds. The increased degree of oligomerization lead to

147

a higher concentration of Mo-O-Mo bonds resulting in an increased catalytic activity.

However, the higher reaction rate resulted in an increased formation of total oxidation

products. Samples with an increased [MoO4]/[MoO6] ratio exhibited in an increased

selectivity towards C3 oxidation products. Hence, reaction rate and selectivity towards the

desired C3 oxidation products competed with each other.

The addenda atoms had an additional influence on the reaction rates and selectivities for

act. P(V,W)xMo12-x-SBA-15 (x = 1, 2). The reaction rates and selectivities of both act

PVxMo12-x-SBA-15 (x = 1, 2) and act. PWxMo12-x-SBA-15 (x = 1, 2) were different from

the reaction rates and selectivities of the references synthesized with individual metal (Mo,

V, W) precursors. Structural analysis revealed that the resulting [MOx] (M = V, W) species

in act. (P(V,W)xMo12-x-SBA-15 (x = 1, 2)) were in close vicinity to the [MoOx] species.

Conversely, the [MOx] (M = V, W) species in the references act. V2Mo10Ox-SBA-15 and

act. W2Mo12Ox-SBA-15 were mostly separated from the [MoOx] species Apparently, the

neighboring [MOx] (M = V, W) and [MoOx] species resulted in an increased formation of

CO and CO2. Various mixed metal oxides exhibited an increased formation of CO and CO2

with higher chemical complexity. Probably, the new multifunctional active site was able to

activate gas-phase oxygen, resulting in an oversupply of surface-bond electrophilic

oxygen. This electrophilic oxygen is prone to further oxidize propene or acrolein to CO2 .

The references act. V2Mo10Ox-SBA-15 and act. W2Mo12Ox-SBA-15 with mostly separated

[MOx] (M = V, W) and [MoOx] species exhibited an increased selectivity towards C3

oxidation products and a decreased formation of CO and CO2.

148

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12 Appendix

Table A 1: Wave numbers [cm-1

] of the meausured IR vibration bands for P(V,W)xMo12-x

(x = 0, 1, 2)(Chapter 3.4).

PMo12 PVMo11 PV2Mo10 PWMo11 PW2Mo10 Assignment

1064 (s) 1062 (s) 1060 (s) 1066 (s) 1068 (s) νas (P-O), νas (Mo-Ot)

(asymmetric coupling)

961 (vs) 960 (vs) 959 (vs) 964 (vs) 964 (vs) νas (P-O), νas (Mo-Ot)

(symmetric coupling)

869 (m) 865 (m) 862 (m) 870 (m) 874 (m) νas (Mo-Oe-Mo)

781 (vs) 779 (vs) 779 (vs) 784 (vs) 783 (vs) νas (Mo-Oe-Mo)

Table A 2: Wave numbers [cm-1

] of the meausured Raman vibration bands for P(V,W)xMo12-x

(x = 0, 1, 2) )(Chapter 3.4).

PMo12 PVMo11 PV2Mo10 PWMo11 PW2Mo10

999 (vs) 997 (vs) 1001 (vs) 996 (vs) 1001 (vs) νas (Mo-Ot)

972 (sh) 973 (sh) 974 (sh) 982 (sh) 982 (sh) νas (Mo-Ot)

909 (vw) 904 (vw) 904 (vw) 888 (vw) 888 (vw) νas (Mo-Oe-Mo)

611 (w) 609 (w) 617 (w) 608 (w) 608 (w) νas (Mo-Oc-Mo),

δ (Mo-Oc-Mo)

- 497 (vw) 507 (vw) 497 (vw) 498 (vw) δ (Mo-Oc-Mo)

- 460 (vw) 454 (vw) 460 (vw) 463 (vw) δ (Mo-Oc-Mo)

368 (vw) 370 (vw) 372 (vw) 370 (vw) 366 (vw) δ (Mo-Oe-Mo)

250 (m) 252 (m) 256 (m) 252 (m) 248 (m) δ (Oc-Mo-Oc),

δ (Oe-Mo-Oe')

227 (m) 224 (m) 224 (m) 224 (m) 231 (m) δ (Mo-Oe-Mo)

156 (m) 160 (m) 161 (m) 160 (m) 159 (m) δ (Mo-O-Mo),

δ (O-Mo-O)

111 (m) 112 (m) 113 (m) 112 (m) 108 (m) δ (Oc-Mo-Ot)

84 (w) 85 (w) 85 (w) 85 (w) 83 (w) δ (Mo-O-Mo),

δ (O-Mo-O)

165

Fig. A 2: XRD powder pattern of PMo12 after thermal treatment during catalytic conditions

:conditions (5% propene + 5% oxygen in He at 723 K) and diffraction peaks of bulk reference

material α-MoO3 (ICSD 76365 [129]).

10 20 30 40 50 60 70 80

Inte

nsity

Diffraction angle [2Ɵ]

α-MoO3

No

rma

lize

d io

n c

urr

ent

m/e = 18 (H2O)

temperature

0 1000 2000 3000 4000 5000 6000 0

100

200

300

400

tem

pera

ture

[K

]

Cycle

Fig. A 1: Ion current m/e = 18 corresponding to water and temperature steps of in situ XRD

meausurements of PMo12 during oxidation conditions (20% O2 in He; RT-723 K).

166

0.15

0.20

0.25

0.30

0.35

0.40

0h 12h

0h 12h

PW2Mo10 PV2Mo10

Ab

sorp

tion

PMo12

0h 12h

Fig. A 4: AAS absorption of phosphorus in PMo12, PVMo11, and PV2Mo10 before and after

treatment during propene oxidation conditions (5% propene + 5% O2 in He; 723 K; 0h and 12h

time on stream).

10 20 30 40 50 60 70 80

α-MoO3

Mo17O47

Fig. A 3: XRD powder pattern of PW2Mo10 after thermal treatment during catalytic conditions

:conditions (5% propene + 5% oxygen in He at 723 K) and diffraction peaks for bulk reference

materials α-MoO3 (ICSD 76365 [41]) and Mo17O47 (ICSD 36098 [44]).

Inte

nsity

Diffraction angle [2Ɵ]

167

5.45 5.50 5.55 5.60

0.0

0.5

1.0

1.5

2.0

2.5

No

rma

lize

d a

bsorp

tion

Photon energy [ke'V]

PV2Mo10-SBA-15

PV2Mo10

V2O5

VO2

Fig. A 5: V K edge XANES of PV2Mo10-SBA-15, PV2Mo10 and the references V2O5, VO2.

0 5 10 15 20 25 30 35 40

0.0

1.0

2.0

3.0

4.0

5.0

6.0

Mo

avera

ge

va

lence

Mo K edge, (eV)

Mo oxide references

PMo12-SBA-15

PVMo11-SBA-15

PV2Mo10-SBA-15

PWMo11-SBA-15

PW2Mo10-SBA-15

Fit Curve

Mo

MoO2

Mo4O11

MoO3

Fig. A 6: Mo average valence of the Mo oxide references and P(V,W)xMo12-x-SBA-15 ( x = 0, 1, 2)

as a function of the Mo K edge position.

XII

Danksagung

Ich bedanke mich bei Herrn Prof. Dr. Thorsten Ressler für die interessante

wissenschaftliche Fragestellung. Insbesondere danke ich ihm auch für die exzellente

fachliche Betreuung während der gesamten Zeit meiner Forschungstätigkeit in seinem

Arbeitskreis. Ich danke außerdem Herrn Prof. Dr. Malte Behrens für die Anfertigung des

Zweitgutachtens.

Mein besonderer Dank gilt der gesamten Arbeitsgruppe Ressler für die angenehme und

freundschaftliche Arbeitsatmosphäre. Ich danke vor allem Gregor Koch, Alexander Müller,

Sven Kühn und Dr. Juliane Scholz für ihre stete Diskussionsbereitschaft. Bei Alexander

Hahn und Dr. Thomas Christoph Rödel bedanke ich mich für die wissenschaftliche und

technische Unterstützung bei der Durchführung zahlreicher Experimente. Ich danke auch

Semiha Schwarz für die technische Hilfestellung bei der Durchführung von TG-

Messungen und die zahlreichen Ratschläge innerhalb- und außerhalb des

Forschungsthemas. Besonders will ich mich an dieser Stelle bei Dr. Anke Walter

bedanken, die mich als Forschungspraktikant und Diplomand in zahlreiche Methoden der

analytischen Chemie eingeführt und mich in dieser Zeit für die Arbeit in der

instrumentellen Analytik und Katalyseforschung exzellent vorbereitet hat. Darüber hinaus

bedanke ich mich bei Lars Eggers, Mario Willoweit, Tina Somnitz und Larissa Braun, die

mich im Rahmen ihrer Bachelorarbeiten unterstützt haben.

Dem gesamten Arbeitskreis Lerch danke ich für die Aufnahmen der

Weitwinkelbeugungsdaten. Ich bedanke mich bei den Arbeitskreisen Grohmann und Lerch

für die freundliche Atmosphäre und tatkräftige Unterstützung im Syntheselabor.

Ich bedanke mich bei allen Mitgliedern des Instituts für Chemie der TU Berlin, die mich

bei meiner Arbeit unterstützt haben.

Dem DESY und dem HASYLAB in Hamburg sei für die Bereitstellung zahlreicher

Messzeiten gedankt. Bei der Deutschen Forschungsgemeinschaft (DFG) bedanke ich mich

für die finanzielle Unterstützung.

Ich danke meiner Frau Hanna und meiner kleinen Tochter Nell für die familäre Ablenkung

nebem dem Forschungsalltag und die uneingeschränkte Unterstützung und

Rücksichtnahme während der Anfertigung dieser Arbeit.

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