Synthesis and Investigation of Phase-pure M1 MoVTeNbOx Catalysts for Selective Oxidation of Propane to Acrylic Acid [original]

„Synthesis and Investigation of Phase-pure M1

MoVTeNbOx Catalysts for Selective Oxidation of

Propane to Acrylic Acid”

vorgelegt von

Dipl.- Ing. Chemikerin

Almudena Celaya Sanfiz

aus Lugo (Spanien)

Von der Fakultät II – Mathematik und Naturwissenschaften

der Technischen Universität Berlin

zur Erlangung des akademischen Grades

Doktorin der Ingenierwissenschaften

- Dr. Dipl.-Ing. -

genehmigte Dissertation

Promotionsausschuss:

Vorsitzender: Prof. Dr. R. Schomäcker

Berichter/Gutachter: Prof. Dr. R. Schlögl

Berichter/Gutachter: Prof. Dr. M. Lerch

Tag der wissenschaftliche Aussprache: 30. Mai 2008

Berlin 2008

D 83

Danksagung

Die vorliegende Arbeit wurde in der Zeit von Januar 2004 bis Dezember 2007 in der

Abteilung Anorganische Chemie am Fritz-Haber-Institut der Max-Planck-Gesellschaft in

Berlin und während zweier Aufenthalte am Combinatorial Technology and Catalysis

Research Centre (COMBICAT), University Malaya in Kuala Lumpur/Malaysia

angefertigt. Bei allen, die mich in dieser Zeit unterstützt und so zum Entstehen dieser

Arbeit beigetragen haben, möchte ich mich herzlich bedanken.

Zunächst möchte ich den beiden Gutachtern dieser Arbeit danken, Prof. Dr. Robert

Schlögl für die interessante Themenstellung und seine Unterstützung durch hilfreiche

Anmerkungen und Diskussionen und Prof. Dr. Martin Lerch für seine Bereitschaft, diese

Arbeit zu begutachten.

Prof. Dr. Reinhard Schomäcker danke ich, dass er sich so schnell bereit erklärt hat, den

Vorsitz des Promotionsausschusses zu übernehmen.

Mein besonderer Dank gilt auch Dr. Annette Trunschke für die gute Zusammenarbeit,

Motivation und Förderung und für ihre stete Diskussionsbereitschaft und konstruktive

Kritik.

Weiterhin möchte ich all denen danken, die durch ihre Zusammenarbeit und ihre

Unterstützung diese Arbeit ermöglicht haben: Dr. Andreas Furche für die

thermogravimetrischen Analysen; Frau Dodo Lee für die Hilfe bei der Präparation am

COMBICAT; Dr. Eva Rödel für die EXAFS-Untersuchung; Frau Edith Kitzelmann für

die XRD Messungen; Dr. Frank Girgsdies für die Phasenzusammensetzungsanalysen

(XRD&TOPAS); Frau Gisela Lorenz für die zahlreichen BET Messungen; Dr. Ming-

Hong Looi und Dr. Sharifah B. A. Hamid für die katalytische Untersuchung am

COMBICAT; Herr Peter Schnörch und Dr. Detre Teschner für die in-situ XPS

Experimente; Dr. Olaf Timpe für die außergewöhnlichen chemischen Ideen und die

vielen anregenden Diskussionen und Dr. Thomas W. Hansen für die Analysen der

Mikrostruktur (TEM), der elementaren Zusammensetzung (EDX) und der Morphologie

(SEM) der Katalysatoren.

Den jetzigen und ehemaligen Mitgliedern der Arbeitsgruppe „Präparation“ sowie allen

Mitarbeitern der Abteilung Anorganische Chemie danke ich für die angenehme

Arbeitsatmosphäre und die kontinuierliche Unterstützung.

Desde luego, llego al final de este doctorado gracias al gran apoyo y ayuda recibida de

Philipp, y como no al cariño y al soporte moral que me otorgan mi madre y mis

hermanos. Igualmente, quiero agradecer a mis dos amigas incondicionales de Berlín

Maria y Georgia por sus ánimos constantes y su alegría contagiosa.

Synthesis and Investigation of phase-pure M1 MoVTeNbOx Catalysts for Selective

Oxidation of Propane to Acrylic Acid

by Almudena Celaya Sanfiz

Abstract

Hydrothermal synthesis of MoVTeNbOx catalysts has been investigated. It has been

shown that the phase composition of the crystalline catalyst strongly depends on the

conditions of the hydrothermal process. The X-ray amorphous product of the

hydrothermal synthesis shows a long-range order of structural motifs in [001] direction.

Subsequent heat treatment of the precursor in inert atmosphere leads to crystallization.

The target structure is an orthorhombic bronze-like structure, denominated as M1. M1

phase has been associated with high activity and selectivity in the direct oxidation of

propane to acrylic acid. Other phases formed are, e.g. the orthorhombic M2 phase

(isostructural to KW3O9 phase), Mo5O14, V0.95Mo0.97O5 and TeMo5O16. Independent of

the technical parameters of the autoclave used, phase-pure M1 has been successfully

synthesized by optimization of the hydrothermal conditions (e.g. temperature and

reaction time). As evidenced by SEM/EDX, precursor materials of M1 catalysts are

characterized by a fairly homogeneous distribution of the elements and a significant

higher Nb-content compared to that reported in the literature. According to EXAFS

analysis, Nb is located into pentagonal bi-pyramidal units but also into octahedral

coordinated positions, which is in contradiction to the established structural model.

Furthermore, it was shown by XRD and TG/MS that the presence of ammonium

containing phases in the precursor leads to the formation of phase mixture after the heat

treatment.

Additionally, the impact of phase composition on the catalytic behavior of

hydrothermally prepared MoVTeNbOx catalysts in the selective oxidation of propane to

acrylic acid has been investigated. Phase cooperation of M1 and M2 phase was not found

in this contribution. For phase-pure M1 catalysts, the propane conversion increases with

increasing the specific surface area. Other properties, such as chemical composition and

aspect ratio of the needle-like M1 crystallites may influence the catalytic activity as well.

In the present study the role of the (001) plane of the M1 phase in the propane oxidation

is addressed by providing a M1 model catalyst that exposes preferentially this plane.

The molecular structure and dynamic nature of the active moiety on the M1 surface under

conditions of propane oxidation, is the key for understanding the catalytic behaviour of

these catalysts. X-ray photoelectron spectroscopic experiments in presence of the

reactants propane, oxygen and water indicated re-distribution of the elements at the

catalyst surface, in response to changes in the gas mixture. Furthermore, phase-pure M1

showed a reasonable stability under reaction conditions in laboratory-scale experiments,

which is of a great importance for their potential industrial application.

The composition of multi-metal oxide catalysts has been varied. Metal substitution and/or

addition of suitable diluents to the Mo-V-X-Nb (X=Te, P, Bi) oxides has been performed.

The catalysts have been investigated in the selective oxidation of propane to acrylic acid.

The results underlined the exceptional role of M1 phase. On the other hand, it has been

proven that M1 is not necessarily required for acrylic acid formation.

MoVTeNbOx Katalysatoren für die selektive Oxidation von Propan zu Acrylsäure

Synthese und Untersuchung von phasenreinem M1

vorgelegt von Almudena Celaya Sanfiz

Zusammenfassung

Die Hydrothermalsynthese von MoVTeNbOx Katalysatoren ist untersucht worden. Es hat

sich gezeigt, dass die Phasenzusammensetzung des kristallinen Katalysators stark von

den Bedingungen des Hydrothermalprozesses (Temperature, Reaktionszeit) abhängt. Das

Diffraktogramm des amorphen Syntheseprodukts (Precursor) zeigt, dass bereits hier eine

langreichweitige Ordnung von strukturellen Elementen in [001] Richtung vorliegt. Die

anschließende Temperung des Precursors in inerter Atmosphäre führt zur Kristallisation.

Die Zielstruktur ist eine orthorhombische, bronzeartige Phase, die als M1 bezeichnet wird

und die mit der hohen Aktivität und Selektivität von MoVTeNb-oxid Katalysatoren in der

direkten Oxidation von Propan zu Acrylsäure in Verbindung gebracht wird. Andere

Phasen, die gebildet werden, sind z. B. die orthorombische M2 Phase (isostrukturell mit

KW3O9), Mo5O14, V0.95Mo0.97O5 und TeMo5O16. Wie mittels SEM/EDX gezeigt,

zeichnen sich Precursor Materialien von M1 Katalysatoren durch eine homogene

Verteilung der Elemente und einen, im Vergleich zu Literaturangaben, erhöhten Nb-

Gehalt aus. Basierend auf der EXAFS Analyse von phasenreinem M1 liegt Nb sowohl in

pentagonal bipyramidaler Koordination, als auch oktaedrisch koordiniert vor, was im

Widerspruch zu publizierten Strukturmodellen steht. Außerdem wurde durch XRD und

TG/MS gezeigt, dass die Anwesenheit von ammoniumhaltigen Phasen im Precursor zur

Bildung von Phasengemischen während der thermischen Behandlung führt.

Desweiteren ist der Einfluss der Phasenzusammensetzung auf die katalytischen

Eigenschaften von hydrothermal hergestellten MoVTeNbOx Katalysatoren in der

Oxidation von Propan zu Acrylsäure untersucht worden. Eine Kooperation zwischen M1

und M2 Phase wurde in diesem Beitrag nicht gefunden. Für phasenreine M1

Katalysatoren, nimmt der Propanumsatz mit der Erhöhung der spezifischen Oberfläche

zu. In der vorliegenden Untersuchung wird die Rolle der (001) Netzebene der M1-Phase

in der Propanoxidation untersucht. Dazu wurde ein Modellkatalysator hergestellt, bei

dem vorzugsweise (001) Ebenen exponiert sind, während die Seitenflächen der Nadeln

mit SiO2 passiviert wurden. Die Kenntnis der molekularen Struktur und der dynamischen

Eigenschaften der M1 Oberfläche unter den Bedingungen der Propanoxidation ist der

Schlüssel, um das katalytische Verhalten dieser Katalysatoren zu verstehen. Photo-

elektronenspektroskopische Experimente in Anwesenheit von Propan, Sauerstoff und

Wasser zeigten, dass sich die chemische Zusammensetzung der Oberfläche stark von der

des Volumens unterscheidet und dass diese darüber hinaus von der Zusammensetzung

der Gasphase abhängt. Außerdem ist phasenreines M1 in der Propanoxidation über 50

Stunden stabil, was von Bedeutung für eine potentielle industrielle Anwendung des

Katalysators ist.

Desweiteren wurde die Zusammensetzung der Multimetalloxidkatalysatoren variiert und

SiO2, ZrO2, und Cr2O3 als Verdünnungsmittel zu MoVXNbOx (X=Te, P, Bi) zugegeben.

Die Katalysatoren wurden in der Oxidation von Propan zu Acrylsäure getestet. Die

Ergebnisse unterstreichen die außergewöhnliche Bedeutung der M1 Phase für die

selektive Aktivierung von Propan. Andererseits konnte gezeigt werden, dass die

kristalline M1-Phase für die Acrylsäurebildung nicht notwendigerweise erforderlich ist.

III

List of Abbreviations

ab plane (001) plane

at.- % atome percentage

BESSY Berliner Elektronenspeicherring – Gesellschaft für

Synchrotronstrahlung

BET Brunauer, Emmett and Teller method

c direction [001] direction

∆fH° standard enthalpy of formation

Ea activation energy

EDX Energy Dispersive X-ray Analysis

EXAFS Extended X-ray Absorption Fine Structure

FT Fourier Transform

GHSV Gas Hour Space Velocity

HRTEM High Resolution Transmission Electron Microscopy

HPCs Heteropoly Compounds

i. d. internal diameter

i. e. that is

ICP Inductively Coupled Plasma

ICSD Inorganic Crystal Structure Database

IR Infrared spectroscopy

JEMS Jet Engine Maintenance Simulator

LEIS Low Energy Ion Scattering

MS Mass Spectrometry

m/e mass/charge ratio

MMOs multi-metal oxides

p pressure

rpm revolutions per minute

SEM Scanning Electron Microscopy

Sl specific surface area of the lateral of the needles

Ss specific surface area of the cross-section of the needles

STP Standard Temperature and Pressure

T temperature

TCD Temperature Control Device

TEM Transmission Electron Microscopy

TG Thermogravimetry

tsynthesis time of synthesis

UV/VIS Ultraviolet/Visible Spectroscopy

vol.-% volume percentage

vs very strong

vs. versus

w weak

XAFS X-ray Absorption Fine Structure

XAS X-ray Absorption Spectroscopy

XPS X-ray Photoelectron Spectroscopy

XRD X-ray Diffraction

Table of content

INTRODUCTION .................................................................................................. 1

1 Catalysis ................................................................................................... 1

1.1 Basic concepts ............................................................................................ 1

1.2 Catalytic oxidation of hydrocarbons ......................................................... 4

2 Acrylic acid ............................................................................................... 5

2.1 Applications and demand ........................................................................... 6

2.2 Industrial manufacture ................................................................................ 7

2.3 Future Technologies ................................................................................... 8

3 Mo-V-Te-Nb mixed oxide catalyst ........................................................... 9

3.1 Historical development ............................................................................. 10

3.2 Bulk structure of Mo-V-Te-Nb mixed oxides ........................................... 10

3.3 Synthesis methods .................................................................................... 12

3.3.1 Precipitation-evaporation method (dry-up method) ....................... 13

3.3.2 Hydrothermal synthesis .................................................................... 13

4 Motivation and Objectives ..................................................................... 14

5 References .............................................................................................. 17

CHAPTER 1- Preparation of phase-pure M1 Mo-V-Te-Nb oxide catalysts by

hydrothermal synthesis – Influence of reaction parameters on the structure

and morphology. ............................................................................................... 20

Abstract .......................................................................................................... 20

1 Introduction ............................................................................................ 21

2 Experimental ........................................................................................... 24

2.1 Preparation of Mo-V-Te-Nb mixed oxides ............................................... 24

2.2 Characterization ........................................................................................ 26

3 Results and discussion ......................................................................... 28

3.1 Molecular species in the initial suspension ............................................ 28

3.2 Hydrothermal synthesis ............................................................................ 34

3.3 Homogeneity and microstructure of the precursor materials ............... 41

3.4 Development of short-range order during hydrothermal synthesis and

activation ............................................................................................................... 42

Table of content

3.5 Post-treatment of multi-phase precursors .............................................. 45

3.6 Activation ................................................................................................... 46

4 Summary and conclusions .................................................................... 48

5 Acknowledgments ................................................................................. 50

6 References .............................................................................................. 50

CHAPTER 2- Investigation of catalytic behavior of M1-phase catalyst in the

selective oxidation of propane to acrylic acid. Correlation approach

between catalytic performance and surface/bulk properties. ....................... 54

Abstract .......................................................................................................... 54

1 Introduction ............................................................................................ 55

2 Experimental ........................................................................................... 58

2.1 Catalysts preparation ................................................................................ 58

2.2 Activity measurements ............................................................................. 59

2.3 Catalysts characterization ........................................................................ 59

3 Results .................................................................................................... 61

3.1 General properties of the Mo-V-Te-Nb mixed oxides ............................. 61

3. 2 Selective oxidation of propane ................................................................ 62

3.3 Characterization of phase-pure M1 catalysts ......................................... 65

3.3.1 Morphology and shape analysis .......................................................... 65

3.3.2 Microstructure ....................................................................................... 68

3.3.3 Elemental dynamics and composition at the catalytic surface ........ 70

3.3.4 Properties of the phase-pure catalysts after propane oxidation ...... 73

4 Discussion and conclusions ................................................................. 75

5 Acknowledgments ................................................................................. 81

6 References .............................................................................................. 81

CHAPTER 3- How important is the (001) plane of M1 for selective oxidation

of propane to acrylic acid ................................................................................ 84

Abstract .......................................................................................................... 84

1 Introduction ............................................................................................ 85

2 Experimental ........................................................................................... 87

2.1 Preparation of M1 ...................................................................................... 87

2.2 Sylilation of M1 .......................................................................................... 88

Table of content

VII

2.3 Activity measurements ............................................................................. 89

2.4 Catalysts characterization ........................................................................ 89

3 Results .................................................................................................... 92

3.1 Silylation of M1 .......................................................................................... 93

3.2 Phase composition of the Mo-V-Te-Nb oxides ....................................... 95

3.3 Microstructure of the model catalysts ..................................................... 96

3.4 Catalytic properties of the model catalysts in propane oxidation ...... 101

3.5 LEIS after catalysis.................................................................................. 102

4 Discussion ............................................................................................ 104

5 Conclusions .......................................................................................... 110

6 Acknowledgments ............................................................................... 111

7 References ............................................................................................ 111

CHAPTER 4- New synthesis routes of MMO catalysts by dilution of Mo-V-X-

Nb (X=Te, Bi, and P) mixed oxides with SiO2, Cr2O3 or ZrO2 for the oxidation

of propane to acrylic acid. ............................................................................. 114

Abstract ........................................................................................................ 114

1 Introduction .......................................................................................... 115

2 Experimental ......................................................................................... 118

2.1 Catalysis preparation .............................................................................. 118

2.2 Activity measurements ........................................................................... 121

2.3 Catalyst characterization ........................................................................ 121

3 Results .................................................................................................. 122

3.1 Bulk structure of precursors and catalysts .......................................... 122

3.2 Catalytic performance ............................................................................. 133

4 Discussion ............................................................................................ 138

5 Conclusion ............................................................................................ 147

6 Acknowledgments ............................................................................... 150

7 References ............................................................................................ 150

CONCLUSION AND OUTLOOK ...................................................................... 153

APPENDIX ....................................................................................................... 157

Curriculum Vitae .......................................................................................... 157

Publication Index ......................................................................................... 159

Introduction

INTRODUCTION

Steady increase of demand for manufactured products implicates a rise of world

production of chemical intermediates. High amount of these valuable intermediates are

industrially manufactured by means of chemical processes involving olefins. Since the

raw materials constitute a large portion of the cost of production of the most commodity

chemicals, low cost and abundant starting materials are of high importance. Therefore,

the potential utilization of alkanes, which are more abundant and cheaper than the

corresponding olefins, as feedstock in the industrial production of the organic chemicals,

has recently generated a high interest. However, it is well-known that saturated

hydrocarbons have a lower reactivity than unsaturated hydrocarbons, because of the little

polarity of their C-H bonds and the absence of high polarizability associated with the π-

bonding of the olefins. Design and development of new types of solid catalyst capable to

activate and convert alkanes into desirable oxygen containing compounds in a selective

manner at reasonable temperatures are therefore, challenging tasks.

1 Catalysis

Catalysis plays a remarkable role in industrial and environmental chemistry. More

than 60% of all chemical products (including fuels, commodity and fine chemicals) and

90% of chemical processes are based on catalysis.

1.1 Basic concepts

In a reacting chemical system, two considerations are of importance: (i) the maximum

attainable yield of products under specified conditions (chemical equilibrium), which is

Introduction

matter of chemical thermodynamics and (ii) the rate of the reaction, i.e. how fast the

equilibrium situation is attained. The answer of the last consideration is the concern of

chemical kinetics. A chemical process is economically feasible if both, yields and rates

are favorable. It is well-known that the rates of many chemical reactions can be affected

by the presence of “alien” material – not included in the stoichiometric equation of the

reaction. This material is denominated as catalyst and it is defined as a substance which

increases the rate (i.e. the rate coefficient) at which a chemical reaction approaches

equilibrium, without being consumed in the process (excluding physical deterioration

and/or deactivation during use). As described either by the collision theory or by the

transition-state theory [1], this phenomenon occurs by lowering the activation energy of

the catalyzed reaction due to interaction with the catalyst (Figure 1). Lower activation

energies and/or higher selectivities are achieved when the reactants find different

pathways across the potential barrier separating them from the product state.

Accordingly, a catalyst also has the effect of lowering the temperature at which a given

rate is achieved, which is of a great advantage for many practical applications.

In heterogeneous catalysis the catalyst is in a different phase than the reactants, being the

material state of the catalyst usually solid. The interaction between the reactants and the

solid occurs on the surface of the catalyst. Therefore, heterogeneous catalysis is related to

the specific chemical properties of the surface of the catalyst. However, the surface

properties are determined by the chemistry of the bulk solid, and some useful insight into

the catalytic activities of surfaces is also gained from knowledge of the bulk properties of

the solid.

Introduction

catalyzed

reaction

Reaction coordinate

Potential energy

Reactants

Products

non-catalyzed

reaction

-∆H

catalyzed

reaction

Reaction coordinate

Potential energy

Reactants

Products

non-catalyzed

reaction

-∆H

Figure 1. Potential-energy profile for an exothermic reaction, showing the lower

activation energy of the catalyzed reaction.

The chemical interaction during chemisorption of the reacting molecule on the surface of

the catalyst lowers the activation energy that is needed in the non-catalyzed reactions.

Two basic possible mechanisms are known in heterogeneous catalysts, depending on the

mode of adsorption of the reactants: (i) Langmuir-Hinshelwood mechanism, which

proposes that both reactants adsorb and the adsorbed reactants react on the surface of the

catalyst, and (ii) Eley-Rideal mechanism, which proposes that only one reactant adsorbs

and the other reacts with it directly, without adsorbing. One or more of the reactants

diffuse to the catalyst surface and chemisorb on the active centers of the surface.

Balanced bond strength in the adsorbate complex leads to activation and product

formation by rearrangement of chemical bonds without poisoning the catalyst surface.

Finally, products desorb and diffuse through the pore system away from the solid surface.

The described mass transport of reactants and products from one phase to another can

sometimes be the limiting step of the reaction.

Introduction

1.2 Catalytic oxidation of hydrocarbons

Selective catalytic oxidation is a common method of functionalizing hydrocarbon

molecules. In the chemical industry more than 60 % of the products obtained by catalytic

routes are products of oxidation. Basically all monomers used in manufacturing artificial

fibers and plastics are obtained by selective catalytic oxidation processes. Oxidation is

not only essential in the production of a wide range of materials needed in the modern

society but also in the removal of undesired by-products. Environmental pollution control

may be achieved, e. g., by total catalytic oxidation.

In the particular case of selective oxidation of hydrocarbons involving metal oxide based

catalyst, one possible reaction mechanism is the Mars and van Krevelen mechanism [2].

According to this mechanism, the oxidation of adsorbed hydrocarbons proceeds through

lattice oxygen leaving reduced metal sites. The regeneration of the lattice oxygen

vacancies occurs via reoxidation with molecular oxygen of the gas feed. Consequently,

selective oxidation reaction is a dynamic process with respect to the catalyst. As far as the

organic molecule is concerned, usually one or several H atom abstractions, O atom

insertions and electron transfers are involved. Generally, the catalysts appropriate for

selective oxidation reactions have the following common properties, summarized as the

so-called “7 pillars” of the selective oxidation catalysis [3]:

(i) lattice oxygen,

(ii) metal-oxygen bonds,

(iii) host structure,

(iv) redox properties,

(v) multifunctional active sites,

Introduction

(vi) site isolation and

(vii) cooperation of phases.

Lattice oxygen, [O2-] (i), is more nucleophilic (less oxidizing power) than molecular

oxygen delivered from the gas phase, reacting more selectively. The strength of the

metal-oxygen bond (ii) of active oxygen determines the oxidizing power of lattice

oxygen, being the intermediate strength the appropriate for selective oxidation. The host

structure (iii) accommodates the appropriate metal-bond strength and the lattice oxygen,

permitting the transfer of the electrons and vacancies as well as lattice oxygen diffusion.

Once the lattice oxygen oxidizes the adsorbed hydrocarbon, the remaining reduced

catalyst has to be re-oxidized (iv). Generally, the resulting vacancy is replenished via

dioxygen of the gas phase, completing thus the redox cycle. The reactive surface lattice

oxygen has to be isolated in defined groupings (vi) to achieve selectivity, and to avoid

total combustion. As above-mentioned several functions are involved in the catalytic

cycle: chemisorption of the reactant, abstraction of hydrogen, insertion of oxygen and

desorption of the product. Therefore, multifunctionality of the active sites (v) of these

catalysts is important. In case that the host structure of a single phase catalyst cannot

incorporate such functions, intimate phase cooperation (vii) is required.

2 Acrylic acid

Acrylic acid is a widely used monomer for polymerization and its demand increases

every year. Currently, acrylic acid is industrially produced by a two-stage oxidation

process starting from propylene. Replacement of propylene by propane may provide an

economic alternative for this commercial process. However, catalysts that are active and

Introduction

selective enough to make this challenging process, reasonable from an economic point of

view, have not yet been found.

2.1 Applications and demand

Acrylic acid, CH2=CHCOOH, is the simplest unsaturated carboxylic acid possessing both

a double bond (-C=C-) and a carboxyl group (-COOH).

Crude acrylic acid (99.7-99.8 % purity) is manufactured in a two stage oxidation process

starting from propylene. By-products formed are maleic anhydride, propionic acid, acetic

acid, furfural and sometimes traces of allyl acrylate and benzaldehyde. Crude acrylic acid

is used in the production of commodity acrylates including methyl, ethyl, n-butyl, and 2-

ethylhexyl acrylates. These acrylates are used to produce, e.g., coatings, adhesives,

sealants, textiles, fibres, varnishes and polishes. Both crystallization and distillation are

applied to purify crude acrylic acid. In its pure form, acrylic acid is a clear, colorless

liquid with a characteristic acrid odor, presenting the following chemical properties:

melting point at 285 K, boiling point at 412 K and ΔfH°gas = −330.7 ± 4.2 kJ·mol−1 [4].

Glacial acrylic acid normally contains about 200 ppm of methyl ethyl hydrochinone to

prevent polymerization during storage and transportation. Acrylic acid in its glacial form

is used to produce homo- and copolymers of acrylic and methacrylic acid (“polyacrylic

acid”), superabsorbent polymers (based on sodium polyacrylate) and detergent co-

builders (chelating agents for the removal of alkaline earth ions).

The global crude acrylic acid market reached around 3.2 million tons by the end of 2005.

Since 1999, the market has grown at an average annual rate of around 4.6 percent. The

forecast global demand growth lies in the range of 3.4 percent.

Introduction

2.2 Industrial manufacture

Acrylic acid synthesis dates back to the mid nineteenth century. However, the large scale

production of acrylic acid did not begin until the 1930s. Acrylic acid has been produced

in commercial processes starting from acetylene, ethylene or propylene, respectively. In

the last century, acrylic acid was mainly manufactured by means of an acetylene-based

process (1) developed by Reppe in the 1930s in Germany and commercialized by the

BASF in Ludwigshafen, Germany, in the 1950s.

The BASF plant at Ludwigshafen, Germany, was the last acetylene-based acrylic acid

plant to close in the 1990s, with the start-up of a new propylene-based plant in Antwerp,

Belgium.

Today, acrylic acid is manufactured worldwide from propylene in two steps via acrolein

in a gas phase oxidation process. The first stage is the oxidation of propylene to acrolein

using promoted molybdenum-bismuth catalyst (promoters, e.g., Fe, Co, W, and Pt) at a

reaction temperature of about 623 K ± 50 K. In the second stage, acrolein is passed over

a promoted molybdenum-vanadium oxide catalyst (promoters, e.g., W, Cu, Sn, and Sb) at

lower temperatures of about 543 K. The two oxidation steps are strongly exothermic.

33-473 K, 4.5-8 MPa

catalyst: Ni(CO)4-CuBr-THF

C2H2 + H2O + CO CH2CHCOOH

catalyst: Ni(CO)4-CuBr-THF

433-473 K, 4.5-8 MPa

Step 1 catalyst:

promoted molybdenum-bismuth system

T = 623 K ± 50 K

ΔH = -340.8 KJ/Mol

H2C=CHCH3 + O2 H2C=CHCHO + H2O

(1)

Introduction

There is considerable variation in plant design. Reactors can be of tandem (two reactors

in series) or combination design. In the latter case, the reactor consists of one tube sheet

divided into two reaction zones operating under different conditions. The overall acrylic

acid yield reaches 87 % [5]. Producers of acrylic acid are e. g., BASF, Rohm & Haas,

Dow and Nippon Shokubai.

2.3 Future Technologies

Current research activities are directed to substitute propylene as feedstock in the

production of acrylic acid, focusing on the following processes:

• biotransformation processes (e.g., using lactic acid as an intermediate)

• oxidation of propane via propylene (oxidative dehydrogenation) and acrolein

(three stage process), or

• direct oxidation of propane to acrylic acid in one step.

Due to the evident attractiveness of using propane as raw material, particularly, the latter

pathway appears as potential route to acrylic acid in the future.

Step 2

H2C=CHCHO + 1/2O2

catalyst:

promoted molybdenum-vanadium system

T = 543 K

ΔH = -254.1 KJ/Mol

H2C=CHCOOH

CH3CH2CH3 + 2 O2 T = 673

catalyst:

promoted molybdenum-vanadium system

H2C=CHCOOH + 2 H2O

ΔH° ~ -709.64 KJ/Mol

Introduction

However, the direct catalytic oxidation of propane provides numerous challenges that

face the further technical and commercial process development. Development of an

efficient catalyst for this reaction is one of the major problems. The main difficulty

regarding the selective conversion of propane is related to its low reactivity. Without

proper catalysts, activation of propane needs significantly high temperatures (about 773

K). However, at such high temperatures, acrylic acid is not stable and totally oxidized

products (COx) are obtained. The catalyst should activate stable C-H bonds of the

saturated hydrocarbon at relatively low temperatures and selectively transform it into the

desired oxygenated product avoiding its further oxidation. Furthermore, this reaction

requires the transfer of more electrons (8 vs. 6) or oxygen atoms (4 vs. 3), as compared to

the oxidative reaction starting form propylene.

Several types of catalysts, such as vanadium phosphorous oxides (VPO), heteropoly

compounds (HPCs), and multi-component mixed oxides (MMOs) [6-8] have been

investigated for this reaction.

3 Mo-V-Te-Nb mixed oxide catalyst

Reducible mixed metal oxides catalysts are currently the most promising catalytic

materials for the selective oxidation of propane to acrylic acid. Among them, Mo-V-Te-

CH3CH2CH3 + 4 O2- H2C=CHCOOH + 2 H2O + 8e-

CH3CH2CH3 + 2 O2 H2C=CHCOOH + 2 H2O

2 O2 + 8e- 4 O2-

Introduction

Nb mixed metal oxides, originally patented by Mitsubishi, achieve the maximum yields

of acrylic acid of about 50 % [9].

3.1 Historical development

The application of mixed metal oxides as catalysts for propane oxidation to acrylic acid

began in the late 1970s with Mo-V-Nb mixed oxides, previously reported as a catalyst for

ethane oxidation [10]. Propane could be activated at 573 K producing only acetic acid,

acetaldehyde, and carbon oxides. In 1993, Te was incorporated into the system and the

patent concerning use of this system for ammoxidation of propane was published by

Mitsubishi. In Table 1, several MMO catalysts applied in the oxidation of propane are

shown. The highest yields have been obtained with Mo-V-Te-Nb mixed oxides.

Table 1

Propane oxidation over MMO catalysts

Catalyst TReaction [K] XC3H8 [%] SAA [%] YAA [%] Reference

BiMo12V5Nb0.5SbKOx 673 18 28 5 11

Mo1V0.3Te0.23Nb0.12On 653 84 63 53 12

Mo1V0.3Te0.23Nb0.12On 653 80 60 48 9, 13

Mo1V0.3Te0.23Nb0.12On 663 71 59 42 14

Mo1V0.44Te0.10On 653 36.2 46.6 17 15

Mo1V0.25Te0.11Nb0.12On 653 33.4 62.4 21 15

Mo1V0.3Sb0.16Nb0.05On 653 50 32 16 16

Mo1V0.3Sb0-0.1Nb0.1On 673 39-40 43-42 17 17

Mo1V0.3Sb0.25Nb0.12

0.013On 693 39 64 25 18

Mo1V0.18Sb0.15

0.005On 653 27 31 8.4 19

Ni1Mo1.51Te0.01P0.02Ox 733 12 23 3 20, 21

Te-Ni-Mo-O 693 40 41 16 22

3.2 Bulk structure of Mo-V-Te-Nb mixed oxides

Mo-V-Te-Nb oxide catalysts consist of mainly two orthorhombic phases, denoted as M1

(ICSD [55097], [23]) and M2 (ICSD [55098], [23]) [24], which are isostructural with

Introduction

Cs0.7(Mo2.3Nb2.7)O14 and KW3O9, respectively. Both structures consist of basically the

same layered structure [23], in which networks of corner-shared MO6 (M=Mo, V, (Nb))

octahedra form slabs in the ab plane. The slabs are also connected by corner oxygen

atoms forming linear infinite chains of octahedra along the c direction (Figure 2a).

However, the arrangement of octahedra in the ab plane differs between both structures. In

the M1 structure (Figure 2b), the network of MO6 (M=Mo5+, Mo6+, V4+, V5+) octahedra

generates pentagonal, hexagonal and heptagonal rings. The hexagonal and the heptagonal

rings are reported to be partially occupied (80 % and 20 %, respectively) by TeO (Te4+

with a lone pair of electrons) units. Niobium (Nb5+) has been reported to be exclusively

located in the center of the pentagonal bipyramids. In the M2 structure (Figure 2c) the

arrangement of octahedra in the ab plane is simpler. All MO6 (M=Mo6+, V4+, Nb5+)

octahedra are corner sharing forming hexagonal rings, which are fully occupied by TeO

units. Lattice parameters and formula of the unit cell of both structures are shown in the

Figure 2. The M1 structure is richer in Te than M2, while M1 is especially poor in Nb.

Introduction

Figure 2. Crystal structures of Mo-V-Te-Nb oxides [23]. Schematic view of the M1

phase perpendicular to c direction (a), arrangement of octahedra in the ab plane of the M1

structure (b) and arrangement of octahedra in the ab plane of the M2 structure (c).

3.3 Synthesis methods

The most common preparation method of Mo-V-Te-Nb mixed oxide catalysts is the so-

called “dry-method” [9, 25-28], which combines a precipitation and drying procedure

Mo1V0.32Te0.42Nb0.08O4.60

a=12.6 Å, b=7.3 Å, c=4.0 Å

Space group: Pmm2

b) c)

Mo1V0.15Te0.12Nb0.13O3.71

a=21.1 Å, b=26.6 Å, c=4.0 Å

Space group: Pba2

Introduction

(freeze-dry, rotavap or spray-dry). Hydrothermal synthesis has also been applied [15, 29-

32] in the preparation of these catalysts.

3.3.1 Precipitation-evaporation method (dry-up method)

The starting point of this method is the preparation of an aqueous suspension containing

the metals in the required proportions to some extent in precipitated form. Generally,

ammonium heptamolybdate, ammonium metavanadate, telluric acid, and ammonium

niobium oxalate, have been used as starting salts. The slurry obtained is then spray dried.

The resulting material is calcined first in air at 548 K in order to eliminate the counter

ions of the starting salts. Finally, crystallization of the X-ray amorphous calcined material

is carried out in inert gas at 873 K. The disadvantage of this method is that crystallization

usually leads to phase mixtures.

3.3.2 Hydrothermal synthesis

The hydrothermal synthesis method starts also from an aqueous mixture of the

corresponding salts. As in the dry-up method, ammonium heptamolybdate, telluric acid,

and ammonium niobium oxalate are used. However, instead of ammonium metavanadate,

vanadyl sulfate (VOSO4) has been used in the hydrothermal synthesis. The initial

mixture, which is partially precipitated, is introduced into the autoclave. The standard

hydrothermal conditions generally applied are: T = 448 K (autogeneous pressure ~ 9 bar)

and tsynthesis = 48 hours. The precursor obtained after the hydrothermal process is then

activated in inert gas at 873 K, resulting in a crystalline material.

Introduction

4 Motivation and Objectives

Direct synthesis of acrylic acid from propane is an alternative, challenging

approach in view of displacing the current industrial two-stage oxidation process starting

from propylene, reducing thus the price of the feedstock and the complexity of the

process. Among the catalysts proposed to date for the selective oxidation of propane,

mixed metal oxides of Mo, V, Te, and Nb, containing mainly two phases, designated as

M1 and M2 phase [24], have shown the most promising catalytic properties [9].

However, this process has not yet reached a stage of commercialization. Deeper

understanding of the characteristics of these materials with regard to catalytic

applications is required to enable the design of efficient industrial catalysts.

The M1 phase has been suggested to be responsible for activation of propane and its

selective conversion to acrylic acid. However, the nature of the active sites in selective

oxidation of propane to acrylic acid and the potential cooperation of the M2 phase are

still actively debated in the literature [24, 27, 28, 34-44]. Mixtures of M1 and M2 have

been suggested to improve the yield of acrylic acid for operation at high propane

conversions [24, 27, 34-42]. It is assumed that the M2 phase contributes to a slightly

enhanced performance by converting the unreacted propylene generated on the M1 phase,

to acrylic acid. In contrast to that, the M1 phase has been reported to achieve better

catalytic performance than mixture of phases [43].

The unique catalytic properties of M1 have been attributed by Ueda et al. [45] and further

by Grasselli et al. [38] to the specific arrangement of the elements in the ab plane (cross-

section of the needle-like M1 crystals). This conclusion was drawn from experiments

based on comparative catalytic tests of M1 materials before and after grinding. An

Introduction

enhanced catalytic performance was exclusively attributed to the increased surface area

of the basal plane presumably obtained after grinding [35, 45, 46] without providing any

evidence for that and disregarding further impact of mechanical treatment, e.g., on nature

and concentration of defects [47]. Based on the crystal structure of the M1 phase [23],

Grasselli et al. has proposed a reaction mechanism of propane oxidation to acrylic acid

[36], considering the atomic arrangement in the basal plane of the M1 structure as a rigid

ensemble of active sites [36, 38]. Thereby, Grasselli et al. have been implicitly assumed

that the chemical and structural assembly of the proposed active sites comprises an

integral part of the catalyst surface. Contrary, it has been shown by Wagner et al. [44]

that the surface of M1 is significantly different from the well-defined bulk structure. The

M1 crystals are completely covered by a structurally disordered layer exhibiting no long-

range order. Moreover, chemical and structural rearrangements of the catalyst surface as

an effect of the chemical potential of the reaction gas mixture have not been considered

in the hypotheses reported by Grasseli et al.. Generally, little is known regarding the

surface properties of the MoVTeNbOx catalyst, particularly under operation conditions.

Consequently, the mode of action of this promising catalyst as well as the reaction

mechanism in the selective oxidation of propane to acrylic acid is still far from being

understood.

López-Nieto et al. [48] have also proposed a reaction mechanism based on catalytic

studies performed at different reaction temperatures and contact times on MoVTeNbOx

catalysts. However, all the studied catalysts were composed of phase mixtures; thereby it

is difficult to delineate the role played by the respective phases. Therefore, the reported

correlations between structure and catalytic performance and consequently the suggested

Introduction

reaction mechanism may be misleading. Access to single phases followed by a systematic

investigation of each particular phase, is evidently required in order to shed some light to

the catalytic behavior of such complex catalysts.

Therefore, the focus of the present contribution is a detailed investigation of the phase-

pure M1 MoVTeNbOx catalyst. Links between the M1 properties and its catalytic

behavior will be discussed. However, as it has been previously shown [29, 31-32, 49-50],

accessibility to phase-pure M1 materials is not a trivial task. Therefore, in the first step,

the hydrothermal synthesis of the M1 phase has been investigated, achieving thus an

improved control over the phase composition of the Mo-V-Te-Nb oxides finally obtained.

Bulk and surface properties of the resulting phase-pure M1 catalysts have been examined.

Specifically, a M1 model catalyst that preferentially exposes the ab plane to the reactants

has been prepared, enabling thus the investigation of the catalytic relevance of this certain

crystallographic plane. Moreover, in order to examine the dynamic nature of the M1

catalysts, in-situ XPS analysis has been carried out under different gas mixtures at

reaction temperature.

Additionally, different mixed metal oxide catalysts (diluted and undiluted Mo-V-X-Nb

(X=Te, Bi, P) oxides), exhibiting various final phase composition and different degrees

of crystallinity, were synthesized and tested in propane oxidation. Comparing the

catalytic properties of such catalysts with those of the phase-pure M1 catalysts, the

significance of crystallinity in general, and the specific function of the M1 phase in

particular will be discussed.

Introduction

5 References

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Introduction

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Toby, T. Vogt, Zeitschrift für Kristallographie 219 (2004), 3, 152.

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4, 47.

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Chapter 1

CHAPTER 1- Preparation of phase-pure M1 Mo-V-Te-Nb oxide catalysts by

hydrothermal synthesis – Influence of reaction parameters on the structure

and morphology.

Abstract

This work presents a detailed investigation of the preparation of MoVTeNbOx catalysts

by hydrothermal synthesis. Phase-pure synthesis of M1 has been achieved applying the

metals in a molar ratio Mo/V/Te/Nb = 1/0.25/0.23/0.12. Raman, UV/Vis spectroscopy,

and SEM/EDX analysis show that the elements are inhomogeneously distributed in the

initial suspension that is formed after mixing the metal salts in an aqueous medium. Iso-

and heteropoly anions of molybdenum, free telluric acid as well as supra-molecular

polyoxometalate clusters are observed in the solution, whereas all metals have been

found in the precipitate. Complete rearrangement of molecular building blocks under

hydrothermal conditions is essential for formation of phase-pure materials. Optimized

synthesis conditions with respect to temperature and time result in the formation of a

precursor consisting of nano-structured M1 characterized by an extended periodic

organization in the [001] direction and a fairly homogeneous distribution of the elements.

Residual ammonium containing supra-molecular species in the precursor result in the

formation of phase mixtures during the subsequent crystallization by heat treatment in

inert gas. Phase-pure M1 exhibits a distinct degree of flexibility with respect to the

chemical composition that becomes obvious by incorporating Nb not exclusively into

pentagonal bi-pyramidal units, but also into octahedral coordinated positions as shown by

EXAFS. Anisotropic growth of the needle-like M1 crystals has been observed during the

final heat treatment performed at 873-923 K in inert atmosphere disclosing a potential

method to control the catalytic properties of MoVTeNbOx catalysts.

Keywords: Hydrothermal synthesis, MoVTeNbOx catalyst, M1 phase, propane oxidation

Chapter 1

1 Introduction

Multi-metal oxides based on molybdenum, vanadium, tellurium, and niobium have been

reported to achieve outstanding performance in the direct oxidation of propane to acrylic

acid [1]. Despite their considerable chemical complexity, such materials basically consist

of two orthorhombic phases known as M1 and M2 [2]. Minority phases like

(Mo0.93V0.07)5O14, MoO3, and TeMo5O16 [3, 4] are also formed depending on the

preparation method and the activation procedure applied. In the M1 phase (ICSD 55097

[5]), corner-linked MO6 (M=Mo, V) octahedrons are assembled forming 6- and, 7-

membered rings, partially accommodating TeO units [6-9]. Niobium has been postulated

to be exclusively located in the center of a MO7 pentagonal bipyramidal unit sharing

edges with the surrounding octahedrons [6, 9]. The (001) planes are congruently stacked

along the [001] direction, forming a bronze-like structure similar to the structure of

Cs0.7(Nb2.7W2.3)O14 (ICSD 67974 [5]) [10]. The M2 phase differs from M1 by the

absence of the pentagonal bi-pyramidal unit, and the 7-membered ring [6]. The formulae

of the refined unit cells have been determined to be Mo7.8V1.2Te0.937NbO28.9 for M1 and

Mo4.31V1.36Te1.81Nb0.33O19.81 for M2, respectively [6].

Symbiosis between the two phases has been suggested to be responsible for maximum

yield of acrylic acid or acrylonitrile in the oxidation or ammoxidation of propane,

respectively [11, 12]. Selective oxidation of propane has been assigned to M1. M2 on its

own does not activate propane, but selectively oxidizes the propylene intermediate.

Correlations between phase composition and catalytic properties are still actively debated

[13-16]. However, it has been generally accepted that the high selectivity towards acrylic

acid is associated with the M1 phase [3, 17]. Based on chemical experience, the unique

Chapter 1

catalytic properties of M1 have been attributed to the specific geometric arrangement of

the metal centers in the (001) plane [18, 19]. However, the molecular structure and

dynamic nature of the active moiety on the M1 surface under conditions of propane

oxidation, (673 K and up to 50 vol.- % steam in the feed) still remain to be elucidated

experimentally. For that purpose, synthesis of phase-pure M1 in larger batches is a basic

prerequisite.

Generally, two different preparation methods have been used to synthesize Mo-V-Te-Nb

mixed oxides. By precipitation and fast evaporation of water from the precipitate

suspended in the mother liquor, orange colored precursors are obtained. Subsequent

calcination in air followed by annealing in inert atmosphere at high temperatures usually

results in catalysts composed of several phases [2, 14, 20]. Purification by post-treatment

with nitric acid [16] or H2O2 [21], gives access to phase-pure M1. However, the chemical

composition of M1 might be affected by leaching [15].

Hydrothermal synthesis represents an alternative synthesis route to Mo-V-Te-Nb oxide

catalysts [22]. Usually the synthesis is performed at T = 448 K in nitrogen atmosphere

applying a synthesis time of 48 h [23-29].

López-Nieto and co-workers conducted the synthesis under these conditions in a Teflon-

lined stainless-steel autoclave using different metal-containing compounds and various

nominal metal stoichiometries [23, 24, 27-29]. Generally, phase mixtures including

Mo5TeO16, MoO3, M5O14 (M=Mo, V, Nb), Te2M20O57 (M=Mo, V, Nb), and Te0.33MO3.33

(M=Mo, V, Nb) were obtained after heat treatment for 2 h at 873 K in nitrogen of the

hydrothermally prepared precursor. The latter two phases correspond to M1 and M2,

respectively, as originally denominated by Ushikubu et al. [2]. M1 has been achieved as

Chapter 1

the predominant phase using a nominal metal stoichiometry of Mo/V/Te/Nb =

1/0.2/0.17/0.17 and oxalate/molybdenum molar ratios in the range between 0.2 and 0.6

[29]. The as-synthesized metal stoichiometry of the M1-rich materials correspond to

Mo/V/Te/Nb = 1/0.20-0.27/0.19-0.22/0.19-0.21.

Applying a synthesis temperature of 448 K and a synthesis time of 48 h, Ueda and co-

workers prepared orthorhombic Mo-V-Te-Nb mixed oxides in a stainless steel autoclave

equipped with a 70 ml Teflon inner beaker without stirring using a nominal stoichiometry

of the metals of Mo/V/Te/Nb = 1/0.3/0.16/0.12 and a molybdenum concentration of

0.76 M [25]. The XRD patterns of the resulting dark blue powder indicate the formation

of a basically amorphous material. However, one sharp peak close to 30° 2θ probably due

to elemental tellurium has also been observed. Heat treatment of the precursor in nitrogen

at 873 K for 2 h resulted in the formation of a crystalline solid. The XRD patterns

displayed all reflections of the orthorhombic M1 phase. Polycrystalline orthorhombic

Mo-V-Te [30] and Mo-V [31] oxides have also been prepared operating at the same

synthesis temperature, but applying different metal ratios, molybdenum concentrations,

and synthesis times, as well as separating undesired phases by manual sorting [30].

Selective synthesis of phase-pure M1 requires precise control of preparation parameters,

such as temperature, metal salt concentration, metal stoichiometry, batch size, pressure,

and redox potential of the synthesis mixture. The latter is determined by pH, solvent,

organic additives, and the nature of metal salt precursors. Moreover, different

characteristics of a specific autoclave, e.g. wall material, volume, and heating/cooling

regime have substantial influence on the crystallization behavior. These factors are well

known in the synthesis of crystalline alumosilicates but have not yet been considered in

Chapter 1

Table 1

Specifications of the autoclaves used

Parameter Autoclave 1 (A1) Autoclave 2 (A2)

Volume 200 ml 300 ml

Wall material Hastelloy C276 Teflon

Stirring 250 rpm no control

Cooling water cooling (1.6 K/min) manually (6.0 K/min)

the preparation of Mo-V-Te-Nb oxides. In the present work, a systematic investigation of

the hydrothermal preparation procedure of Mo-V-Te-Nb mixed oxides has been

undertaken. The arrangement of molecular building blocks in aqueous solution has been

studied by Raman and UV/Vis spectroscopy. The development of short-range order

during hydrothermal synthesis has been investigated by X-ray absorption spectroscopy,

while the bulk and microstructure of precursors and heat-treated crystalline synthesis

products have been analyzed by X-ray diffraction and electron microscopy, respectively.

The hydrothermal synthesis was revealed to be the crucial synthesis step in the formation

of nano-structured M1. The morphology of the final catalyst is governed by the

conditions of the thermal treatment.

2 Experimental

2.1 Preparation of Mo-V-Te-Nb mixed oxides

Mo-V-Te-Nb mixed metal oxide catalysts have been prepared by hydrothermal synthesis

using two different autoclaves. The technical parameters of these autoclaves are

summarized in Table 1. The general preparation procedure is illustrated in Scheme 1.

Ammonium heptamolybdate (NH4)6Mo7O24·4H2O (Merck), vanadyl sulfate

VOSO4·5H2O (Riedel-deHäen), telluric acid Te(OH)6 (Aldrich), and ammonium niobium

Chapter 1

Initial Reaction Mixture

Molar ratio Mo/V/Te/Nb = 1/0.25/0.23/0.12

cool to 313 K

Hydrothermal Synthesis

(A1 or A2)

383-448K

24-144h

Washing, Filtration, Drying (air, 353 K, 16 h)

Hydrothermal

post-treatment

T = 773 K, p = 20 MPa

atmosphere: Ar

Precursor

(NH4)6Mo7O24 (52 mmol Mo / 100 ml H2O)

T = 353 K

VOSO4(s)

(

13 mmol V

)

C4H14NO14Nb (6.5 mmol Nb / 50 ml H2O)

T = 313 K

Activation

(calcination in air, 498-548 K, 1h)

heat treatment in Ar, 873-923K, 2h

Te(OH)6(s)

(

12 mmol Te

)

MoVTeNb mixed oxide catalyst

Scheme 1

Hydrothermal synthesis of MoVTeNbOx

oxalate (NH4)[NbO(C2O4)2(H2O)2]·3H2O (Aldrich) were used as starting materials to

prepare the initial suspension that contains the metals in a molar ratio of Mo/V/Te/Nb =

1/0.25/0.23/0.12. After replacing residual air by nitrogen, hydrothermal synthesis was

carried out at temperatures between 373 and 448 K for 24 to 144 h, producing

autogeneous pressures between 4

and 9 bars. The obtained

suspensions were centrifuged for

20 minutes; the precipitate was

washed with 100 ml of bidistilled

water for 10 minutes and filtered

under vacuum. Finally, the solid

material was dried in a muffle

furnace at 353 K for 16 hours in

air, resulting in the precursor

material, referred to as “P”

followed by a consecutive

number.

Starting from the precursors,

crystalline products have been

obtained by heat treatment of 3 g

of the precursor in inert gas with

a flow of 100 ml/min for 2 hours

at 873 or 923 K (heating rate 15

Chapter 1

K/min), either with or without previous calcination in synthetic air (100 ml/min) for 1

hour at 548 or 598 K (heating rate 10 K/min). The heat treatments were carried out in a

rotating oven. The crystalline reaction products are denominated as “C” followed by the

consecutive number of the corresponding precursor.

2.2 Characterization

Molecular species in the initial solutions and precipitates were studied by Raman

spectroscopy performed on a Labram I (Dilor) instrument equipped with a confocal

microscope (Olympus). A notch filter (Kaiser Optical) was applied to cut off the laser-

line and the Rayleigh scattering up to 150 cm–1. The spectrometer is equipped with a

CCD camera (1024*298 diodes) that is Peltier cooled to 243 K to reduce the thermal

noise. A He/Ne laser (Melles Griot) was used to excite the Raman scattering at 632 nm.

Using a slit width of 200 μm and a 1800 grating gives a spectral resolution of 2.5 cm-1.

For the solution experiments the laser beam was directed through the glass reaction

vessel into the solution. UV/Vis spectra have been recorded on a PerkinElmer Lambda

950 UV/Vis-NIR spectrometer.

Phase composition of the catalysts was determined by X-ray diffraction performed on a

STOE STADI-P transmission diffractometer equipped with a focussing primary Ge (111)

monochromator and a position sensitive detector, using Cu-Kα1 radiation. The diffraction

patterns of the activated materials were analyzed with the “TOPAS” software (v.2.1,

Bruker AXS).

The short-range order in the M1 precursor and in crystalline M1 has been investigated by

X-ray absorption spectroscopy. The XAS measurements were performed in transmission

mode at the Mo K edge (19.999 keV), Nb K edge (18.986 keV), V K edge (5.465 keV),

Chapter 1

and Te LIII edge (4.314 keV) at beamlines X1 and E4 at the Hamburger

Synchrotronstrahlungslabor, HASYLAB. For investigation of Mo and Nb, 30 mg of

boron nitride was mixed with ca. 8 mg sample, ground, and pressed into a pellet of 5 mm

in diameter under one ton of pressure. For investigation of V and Te, 100 mg of

polyethylene was mixed with 5 mg sample, ground and pressed at a force of 2 tons into a

pellet 13 mm in diameter. The resulting edge jump amounted to Δμ ~ 0.5 at the Mo K

edge and Nb K edge, Δμ~0.02 at the V K edge, and Δμ ~ 0.1 at the Te LIII edge. Data

processing and analysis were performed with the software package WinXAS 3.1 [32].

Morphology studies and shape analysis were performed using scanning electron

microscopy. A Hitachi S-5200 with a PGT Spirit EDX system and a Hitachi S-4800 with

an EDAX Genesis EDX detector were used. EDX studies in the SEMs were carried out

with an accelerating voltage of 10 kV while images were acquired at 2 kV to optimize

surface resolution. For SEM investigations, the samples were deposited on carbon tape

without any pretreatment. From the SEM images, size distributions of the M1 needles

have been obtained by measuring the lengths and diameters of more than 300 M1

needles.

Nitrogen physisorption at 77 K was measured using an AUTOSORB-1-C

physisorption/chemisorption analyzer (Quantachrome). Specific surface areas have been

calculated from the adsorption isotherms using the BET method.

Thermal analysis was performed using a STA 449C Jupiter apparatus (Netzsch).

Precursors have been heated in He atmosphere (flow rate 100 ml/min) applying a heating

rate of 10 K/min. The produced gases were analyzed using an OmniStar quadrupole mass

spectrometer (Pfeiffer Vacuum).

Chapter 1

3 Results and discussion

3.1 Molecular species in the initial suspension

Metal salt concentration and metal stoichiometry strongly influence the phase

composition of molybdenum based mixed oxides prepared by hydrothermal synthesis

[22, 24, 25, 28, 29, 31, 33]. These parameters have been kept constant, because the

present study mainly addresses the thermodynamic and kinetic parameters of M1

synthesis. The preparation procedure (Scheme 1) differs slightly from the routines

described in the literature [22]. Ammonium heptamolybdate was dissolved in bidistilled

water at 353 K, resulting in a colorless solution with a pH of 5.3. Subsequently, vanadyl

sulfate as a powder was added at 353 K. The color of the mixture (pH = 3.05) changed

into dark violet. Finally, solid telluric acid was added at the same temperature, forming a

light brown slurry (pH = 2.18). Afterwards, the MoVTe slurry was cooled to 313 K and a

solution of ammonium niobium oxalate in bidistilled water (313 K, pH = 0.8) was added,

resulting in further precipitation. Before introduction into the autoclave, the dispersion

(313 K, pH = 1.69) was stirred for 10 minutes. The observation of color changes and

precipitation processes during the preparation procedure described above reflects the

formation and rearrangement reactions of molecular building blocks happening in this

early stage of the synthesis. Raman spectra of reference solutions and the binary MoV

solution are shown in Figure 1. The spectrum of the colorless telluric acid solution (296

K, pH = 3.7, [Te] = 0.138 M) exhibits a single peak at 644 cm-1 due to ν(Te-O) vibrations

(Figure 1a) [34]. The aqueous solution of vanadyl sulfate (298 K, pH = 1.86, [V] = 0.198

M) (Figure 1b) displays a strong band at 980 and a shoulder at 996 cm-1 associated to

superimposed V=O and S=O stretching vibrations of solvated vanadyl ions [35]. The

Raman spectrum of ammonium heptamolybdate in aqueous solution (333 K, pH = 5.2,

Chapter 1

600 700 800 900 1000

(d)

980

937

874

824

570

919

942

937

893

996

980

644

Intensity [a.u.]

Raman shift [cm-1]

(a)

(b)

(c)

(e)

Figure 1. Raman spectra of aqueous solutions of (a) Te(OH)6 (296 K, pH = 3.7, [Te] =

0.138 M), (b) VOSO4·5H2O (298 K, pH = 1.86, [V] = 0.198 M), (c) (NH4)6Mo7O24·4H2O

(333 K, pH = 5.2, [Mo] = 0.6 M), (d) NH4)[NbO(C2O4)2(H2O)2]·3H2O (296 K, pH = 0.8,

[Nb] = 0.5 M), and (e) (NH4)6Mo7O24·4H2O + VOSO4·5H2O ( T = 298 K, [Mo] = 0.33

M, molar ratio Mo/V = 1/0.03).

[Mo] = 0.6 M) is characterized by bands at 937 and 893 cm-1 due to the presence of

[Mo7O24]6- species (Figure 1c) [36]. Bands at 942, 919, and 570 cm-1 assigned to Nb=O

and Nb-O stretching vibrations, respectively, were observed with the ammonium niobium

oxalate solution (296 K, pH = 0.8, [Nb] = 0.5 M) (Figure 1d) [37].

When vanadium and tellurium are added successively to the solution of ammonium

heptamolybdate, significant changes in the Raman spectra are observed. Figure 1e shows

the Raman spectrum taken after addition of 0.11 mmol vanadium as vanadyl sulfate in 10

ml bidistilled water (Mo/V = 1/0.03). The bands due to ammonium heptamolybdate are

Chapter 1

400 500 600 700 800

0.0

0.5

1.0

1.5

2.0

2.5

3.0 375

Absorbance [%]

λ [nm]

(a)

(b)

(c)

(d)

530 688

460

Figure 2. UV/Vis spectra of aqueous solutions of (a)

(NH4)6Mo7O24·4H2O (298 K, pH=5.2, [Mo]=0.3 M),

(b) (NH4)6Mo7O24·4H2O + VOSO4·5H2O (T=298 K,

pH=3.05, [Mo]=0.3 M, molar ratio Mo/V=1/0.25),

reduced in intensity and two new bands at 824 and 874 cm-1 appear. The band at 874 cm-1

is reminiscent of the peaks observed in Raman spectra of supra-molecular

polyoxomolybdates or mixed molybdenum-vanadium clusters [38, 39]. Such clusters are

composed of {(Mo)Mo5} structural units, which can be regarded as structural building

block of the M1 structure. These units consist of a central pentagonal bipyramidally

coordinated Mo atom surrounded by five edge-sharing MoO6 octahedrons. The UV/Vis

spectrum of the supra-molecular polyoxometalate cluster MoVI72VIV30 in aqueous solution

is characterized by two bands at 689 (w) and 510 (vs) nm [39]. A related spectrum has

also been observed during preparation of a mixed Mo-V oxide [40]. The binary MoV

solution shows similar

bands at about 690 and

530 nm (Figure 2b)

providing further

evidence for the

formation of mixed MoV

polyoxometalates.

The Raman spectrum of

the final binary solution

that contains molybdenum

and vanadium in a molar

ratio of Mo/V = 1/0.25

Chapter 1

200 400 600 800 1000

(d)

225

310

360

927

812

790

780

965

Intensity [a.u.]

Raman shift [cm-1]

1010 996

980

946

874

644

(a)

(b)

(c)

Figure 3. Raman spectra of the (a) MoVTe filtrate, (b) MoVTeNb filtrate,

(not shown) is of very low quality due to the dark color of the solution. However, the

general spectroscopic patterns and, therefore, the nature of the molecular species in

solution are not changed with increasing vanadium concentration.

After the addition of tellurium, precipitation occurs, making interpretation of the Raman

spectrum difficult due to superposition of bands originating from both solution and

precipitate. Therefore, the solid was removed by filtration.

The Raman spectrum of the MoVTe filtrate is shown in Figure 3a. In addition to the

bands at 874 and 980 cm-1 due to the presence of polyoxometalate clusters and vanadyl

sulfate, respectively, bands at 644, 946 and 1010 cm-1 appear. The band at 946 cm-1 could

Chapter 1

be attributed to the formation of [TeMo6O24]6- Anderson-type heteropolyanions [14, 41,

42]. The band at 1010 cm-1 may indicate partial substitution of Mo by V in the Anderson

anion [42], which is also supported by a shift of the absorption maximum in the UV/Vis

spectrum to lower energies (Figure 2c). Molybdo-tellurates containing vanadium have

been reported to show absorption below 400 nm in the UV/Vis spectrum [42]. However,

due to similar band positions and intensity ratios in the Raman spectra of [Mo7O24]6- and

[TeMo6O24]6-, the coexistence of heptamolybdate anions in the solution cannot be

excluded. On the other hand, mixed MoV/MoVI polyoxomolybdate clusters, such as

(NH4)42[MoVI72MoV60O372(CH3COO)30 (H2O)72] ca. 300 H2O ca. 10 CH3COONH4 exhibit

bands at 215, 260, and 450 nm in the UV/Vis spectrum [38]. The spectrum of the MoVTe

filtrate (Figure 2c) showing a band at 450 nm and absorption below 300 nm, could

therefore also be interpreted in terms of the formation of molybdenum-blue-type clusters.

The latter interpretation would be in agreement with the existence of the band at 874 cm-1

in the Raman spectrum, which is also characteristic for such clusters [38]. Finally, the

peak at 644 cm-1 indicates the presence of free telluric acid in the MoVTe filtrate.

The dried white precipitate that has been separated from the MoVTe filtrate was also

analyzed by Raman spectroscopy (Figure 3c) and SEM-EDX (Figure 4a, Table 2). A

Mo/Te molar ratio of ~ 6 was measured at different spots of the white solid (Figure 4a),

which is consistent with the metal ratio in an Anderson-type heteropolyanion

[TeMo6O24]6-. Particles containing substantial amounts of vanadium have also been

detected. The Raman spectrum of the solid shows bands at 225, 310, 360, 780, 790, 812,

927, and 965 cm-1.

Chapter 1

After addition of the ammonium niobium oxalate solution to the ternary MoVTe mixture,

further precipitation occurs. Raman spectra of the filtrated solution and the precipitate

have been recorded separately (Figure 3b and 3d). The Raman spectrum of the

MoVTeNb solution shows similarities to that of the MoVTe solution (Figure 3a). Bands

due to Nb-O stretching vibrations are absent in the spectrum of the filtrate, indicating that

most of the added niobium was precipitated. Figure 3d shows the Raman spectrum of the

MoVTeNb precipitate that is similar to the spectrum of the MoVTe precipitate.

In summary, Raman and UV/Vis spectroscopy, together with SEM-EDX analysis

indicate that the individual elements are distributed rather inhomogeneously in the initial

suspension. Although the interpretation of Raman and UV/Vis spectra of the complex

mixture is not unambiguous, giant polyoxometalate clusters containing the pentagonal

bipyramidal structure, iso- and heteropolyanions of molybdenum and free telluric acid are

1 2

2 3

(a)

(b) (c)

Figure 4. SEM images of (a) the MoVTe precipitate obtained after filtration of the

ternary slurry (synthesis with molar ratio Mo/V/Te = 1/0.25/0.23), and (b and c) of the

MoVTeNb precipitate obtained after filtration of the final mixture (synthesis with

molar ratio Mo/V/Te/Nb = 1/0.25/0.23/0.12). Elemental composition at the spots

marked is shown in Table 2.

Chapter 1

supposed to be present in the solution. The pre-formed precipitate contains all the

elements.

3.2 Hydrothermal synthesis

Catalyst synthesis aimed at the formation of a desired crystal structure is preferably

performed under hydrothermal conditions. Although the structural rearrangements

occurring during hydrothermal synthesis are concealed by the “black box” autoclave, the

synthesis can be optimized by applying different reaction conditions and appropriate

adjustment of the chemical properties of the reaction medium. Moreover, it is well known

that technical parameters of a specific autoclave, such as wall material, vessel size,

stirring system, cooling and heating facilities, exert significant influence by affecting heat

transfer, wall effects, homogeneity, and crystallization behavior. Until now, less attention

has been paid to the hydrothermal conditions in preparation of Mo-V-Te-Nb mixed

oxides. However, these parameters might have a crucial influence on phase and chemical

composition of the resulting catalysts and finally on their catalytic properties, particularly

due to the considerable chemical flexibility of the orthorhombic M1 structure [22].

Table 2

Elemental analysis (at.-%) of the MoVTe and MoVTeNb precipitate by EDX at

different spots. SEM images of both materials are shown in Figure 4

Spot Mo V Te Nb Mo/Te

ratio

MoVTe

precipitate

1 67 26 7 0 9.0

2 84 2 14 0 5.6

3 85 2 13 0 6.5

MoVTeNb

precipitate

1 80 1 15 14 5.3

2 66 8 13 13 5.1

3 54 7 13 26 4.2

Chapter 1

In the following, the impact of temperature, reaction time, and technical parameters of the

autoclave on M1 synthesis has been studied. For this purpose, the initial reaction mixture,

Table 3

Hydrothermal conditions and phase composition of the activated catalysts; the nominal

stoichiometry applied in hydrothermal synthesis corresponds to Mo/V/Te/Nb =

1/0.25/0.23/0.124, with the exception of P7 (Mo/V/Te/Nb = 1/0.25/0.15/0.124)

Precursor/

Catalyst Code Autoclave

Hydrothermal

conditions

Phase composition

[%]

P1/C1 1760/1761

A1 448 K, 48 h M1 100

P2/C2 929/939

P3/C3 1885/1886

P4/C4 1422/1434

P5/C5 1422/1650

P6/C6 2431/2501

448 K, 48 h

M1 58

M2 37

Mo5O14 5

P7/C7 2445/2488

M1 47

M2 11

Mo5O14 25

V0.95Mo0.97O5 17

P8/C8 3961/3984 403 K, 24 h

M1 53

M2 14

Mo5O14 23

TeMo5O16 10

P9/C9 3791/3792 403 K, 48 h

M1 51

M2 23

Mo5O14 26

P10/C10 3241/3673 403 K, 96 h M1 100

P11/C11 3648/3779 403 K, 144 h M1 90

M2 10

P12/C12 3665/3695 383 K, 96 h

M2 29

Mo5O14 60

TeMo5O16 11

P13/C13 3777/3778 423 K, 96 h M1 86

M2 14

P14/C14 3298/3303 448 K, 96 h

M1 38

M2 41

Mo5O14 8

V0.95Mo0.97O5 13

Chapter 1

Figure 5. Phase composition of catalysts prepared in

A2 at different temperatures; synthesis time = 96 h.

100

383 403 423 448

Phase composition [%]

Tsynthesis [K]

Mo5O14

V0.95Mo0.97O5

TeMo5O16

prepared as described above, was transferred into two different autoclaves, referred to as

A1 and A2, respectively. The technical parameters of the two autoclaves differing in wall

material, batch size and cooling system are summarized in Table 1. An overview of the

experimental conditions applied and the final phase composition of the activated mixed

oxides obtained is given in Table 3. Precursor materials are denominated as “P” and the

corresponding activated catalysts are denominated as “C”.

The phase composition of the final crystalline material differs depending on autoclave,

synthesis temperature and reaction time used (Table 3). Applying standard hydrothermal

conditions (T = 448 K, t = 48

h) in autoclave A1, precursor

materials P1–P5 have been

obtained, which could repro-

ducibly be crystallized into

phase-pure M1 (C1-C5). If the

same reaction conditions were

applied in autoclave A2, the

synthesis results in a precursor

of a phase mixture (P6/C6 in

Table 3). However, synthesis of phase-pure M1 succeeded in A2 after optimization of

the reaction temperature. The latter was varied between 383 and 448 K (autogeneous

pressures between 4 and 9 bar) (Figure 5), keeping the synthesis time constant at 96 h.

The optimum synthesis temperature in A2 was found to be 403 K (P10). At this

temperature, phase-pure M1 has been obtained after activation (C10). Below this

Chapter 1

100

tsynthesis [h]

14496

Phase composition [%]

Mo5O14

TeMo5O16

24 48

Figure 6. Phase composition of catalysts prepared

in A2 at 403 K and different synthesis times.

temperature, M1 is not formed, but M2, Mo5O14, and TeMo5O16. Increasing the

temperature further to 423 and 448 K, results in the formation of M1/M2 (C13), and

M1/M2/Mo5O14/V0.95Mo0.97O5 phase mixtures (C14), respectively.

Furthermore, the synthesis time of the hydrothermal treatment has been varied between

24 and 144 h, keeping the optimized synthesis temperature constant at 403 K (Figure 6).

The complexity of the crystalline phase mixtures decreases with increasing reaction time

resulting in phase-pure M1 after 96 h (C10). However, an extended reaction time of 144

h, again results in a M1/M2 phase mixture (C11).

The diffraction patterns of the various precursor materials obtained under the different

hydrothermal reaction conditions show different characteristics. Generally, XRD patterns

of the precursor materials

synthesized in A1 exhibit a broad

peak at 22º 2θ, a peak of very

low intensity at 45º 2θ, and two

ill-defined features around 8º and

27º 2θ (see for example P2 in

Figure 7a). The reflection at 22°

2θ indicates the presence of long-

range ordering of either, M1, M2,

or a Mo5O14-type phase in the

[001] direction. Ordering along other crystallographic directions is hardly developed as

indicated by the width of the other features, which are observed at diffraction angles

where the reflections of the M1 structure are expected (Figure 8) [5, 8]. Activation of the

Chapter 1

precursor prepared in A1 leads to highly crystalline phase-pure M1 (see for example C2

in Figure 8a). The XRD patterns of crystalline M1 materials C1-C5 fit satisfactorily with

the structural model of M1 refined by DeSanto et al. (ICSD 55097) [5], showing

differences only with respect to the peak intensities, which indicates deviant metal site

occupancy (Figure 8c).

In addition to the peaks described above for the M1 precursor materials, sharp peaks at

8.3°, 9.7º, 10.0°, 12.4º, 18.6º, 24.9º, 27.1º, 27.6º, 30.4º, and 38.3º 2θ appear in the XRD

patterns of precursor materials leading to phase mixtures of M1, M2, M5O14-type

(M=Mo, V and/or Nb) structures, and/or V0.95Mo0.97O5 (Table 3, Figure 7b-h). The peak

10 20 30 40 50 60

M1: 47

M2: 11

M5O14: 25

V0.95Mo0.97O5:17

M1: 58

M2: 37

M5O14: 5

M1: 38

M2: 41

M5O14: 8

V0.95Mo0.97O5:13

M1: 53

M2: 14

M5O14: 23

TeMo5O16: 10

♦

∗

M1: 100

M1: 51

M2: 23

M5O14: 26

M1: 86

M2: 14

M1: 90

M2: 10

♦

⎨

Intensity [a. u.]

⎨

∗∗

∗

⎨

∗

•

2θ

•

∗

(a)

⎨

•

∗

∗⎨

(h)

(g)

•(c)

♦

∗∗

⎨

(f)

(e)

(d)

∗

(b)

Figure 7. XRD patterns of precursors, (a) P2, (b) P11, (c) P13, (d) P6, (e) P9,

(f) P8, (g) P14, and (h) P7.

● (NH4)8(V19O41(OH)9)(H2O)11, ♦ (NH4)6Mo8O27·4H2O, * M1 precursor.

The resulting phase-composition of each precursor after activation is indicated

right in the figure in [%].

Chapter 1

Figure 8. XRD patterns of the phase-pure M1

catalyst C2: (a) measured, (b) calculated based on

[5], and (c) difference a-b; the lattice parameters of

C2 are a = 21.2044(23), b = 26.6785(30) and c =

4.00466(33).

10 20 30 40 50 60

2θ

Intensity [a. u.]

(c)

(b)

(a)

020 120

210

001

180

181

002

at 8.3º 2θ may be assigned to a supra-molecular vanadium compound

(NH4)8(V19O41(OH)9)(H2O)11 (ICSD 063213) [43], and the peak at 10.0° 2θ may indicate

the formation of ammonium octamolybdate (NH4)6Mo8O27·4H2O (ICSD 2017) [44]. The

peaks at 9.7º, 12.4º, 18.6º, and 30.4º 2θ can be assigned to (NH4)2(Mo4O13) (ICSD

068562), whereas the peaks at 27.6º and 38.3º 2θ are due to the presence of elemental

tellurium. Furthermore, the presence of Nb2Te4O13 (ICSD 90371) cannot be excluded due

to the presence of additional peaks at 24.9º and 27.1º 2θ.

Precursors P8, P9, P11, and P13 (Figure 7b, c, e, and f) have identical XRD fingerprints,

differing only in the intensity of the peak at 8.3º 2θ. As mentioned above, this peak may

originate from a residual supra-molecular vanadium compound with NH4+ as the counter

ion. The variety of phases in the activated catalysts increases with increasing intensity of

this peak in the patterns of the

precursor.

Figure 7g and h show a slightly

different XRD fingerprint with a

peak at 10.0° 2θ assigned to

crystalline ammonium octamo-

lybdate. The presence of this

phase in the precursor may

correlate with the formation of

V0.95Mo0.97O5, which is obtained

after the activation process in

both cases (C14, C7).

Chapter 1

400 500 600 700 800

100

0.0

1.0x10-9

2.0x10-9

3.0x10-9

4.0x10-9

5.0x10-9

6.0x10-9

Weight loss [%]

Temperature [K]

P-1 ≈ 15%

Ion current

(a)

400 500 600 700 800

100

0.0 3.0x10-9 6.0x10-9

≈ 7%

Ion current

Weight loss [%]

Temperature [K]

m/e

(b)

400 500 600 700 800

0.0 5.0x10-11 1.0x10-10 1.5x10-10

Ion current

Temperature [K]

m/e

Figure 9. TG-MS of precursor materials; (a) M1 precursor P4 and (b) precursor of

a phase mixture P8.

The presence of ammonium containing phases in the precursor materials leading to phase

mixtures after activation is supported by thermoanalysis of the M1 precursor P4 and the

multi-phase precursor P8, respectively. Two steps of mass loss appear in the TG curves

of both precursors (Figure 9). The mass loss of 15 % of P4 is exclusively due to release

of water. Traces of nitrogen containing compounds were not detected. In contrast, the

mass loss of 7 % observed in case of P8 is partially due to desorption of ammonia. The

two losses observed at 388 K and 578 K are mainly associated with the release of water

Chapter 1

and oxygen. However, the second mass loss is also due to the decomposition of residual

nitrogen containing compounds (N2 (m/e = 28), NO (m/e = 30)). Traces of CO2 (m/e =

44) are released at 490 K and 690 K.

Obviously, nanostructure and phase composition of the precursor material predetermine

the final phase composition of the catalyst obtained after activation in inert gas at high

temperatures. The entire disruption of the metal-ligand coordination in the initial metal

salts and complete rearrangement of the coordination geometry around the central metal

atoms are presumably necessary requirements for crystallization of phase-pure M1 during

the subsequent activation step. As long as, e.g., ammonium containing phases are

observed in the precursor, the crystallization into a phase-pure M1 catalyst fails and

additional phases are formed.

3.3 Homogeneity and microstructure of the precursor materials

Phase-pure crystalline mixed oxides characteristically exhibit a high spatial homogeneity

in their chemical composition. The homogeneous distribution of elements in phase-pure

M1 is already reflected in the precursor as evidenced by EDX in the scanning electron

microscopy. The standard deviation of the elemental analysis by EDX including

numerous spots is lowest for the precursor of single-phase M1 (Figure 10a). As

expected, the standard deviation increases with increasing phase variety. Therefore, in

addition to the XRD patterns, the analysis of the local bulk elemental distribution can

also be used in evaluating the potential of as-synthesized precursor materials in view of

the expected phase purity after activation. Compared to precursors leading to phase

mixtures, the M1 precursor shows a strikingly increased niobium content. This increased

Nb content is also evident from SEM/EDX analysis of the activated catalyst (Figure

Chapter 1

100

Elemental composition [at.- %]

Mo VTe Nb

(a)

100

Elemental composition [at.- %]

NbMo VTe

(b)

Figure 10. Elemental composition of (a) single-, bi- and multi-phase precursors

and (b) single, bi, multi-phase activated catalysts as determined by EDX.

● phase-pure M1; ▲ bi-phase materials; ■ multi-phase materials

The bars represent the standard deviation.

10b). Since the activated material is highly crystalline, the presence of Nb in amorphous

fractions of the material can be excluded. The metal stoichiometry in the activated M1

corresponds to Mo6.24V1.41Te1.76Nb2.35Ox for the catalyst C1. Compared to the unit cell

formula normalized to niobium Mo7.8V1.2Te0.94Nb1O28.9 given in the literature [5], the Nb

content of M1 prepared in this work is more than two times higher than in the M1 phase

analyzed by DeSanto et al.. This increased Nb content is exclusively observed for the

phase-pure M1 catalysts, but not for the phase mixtures and their precursors.

3.4 Development of short-range order during hydrothermal synthesis and

activation

The M1 structure contains 13 crystallographic metal sites (Figure 11). According to

combined neutron and synchrotron powder diffraction data, the four metals occupy

certain sites either preferred or exclusively [5]. EXAFS analysis has been performed on

Chapter 1

M 13

M 3

M 1

M 7

M 9

M 4

M 12

M 6

M 2

M 11

M 5

M 10

Figure 11. Classification of average metal-metal

distances in the M1 structure [6] (oxygen positions

not shown): red: 2.9-3.5 Å, blue: 3.5-4.0 Å, green:

4.0-4.5, black: 4.5-5.0 Å.

phase-pure M1 C1. The data quality obtained at the V K edge and Te LIII edge was not

sufficient for reliable analysis, but local coordination of Mo and Nb in both precursor and

activated materials have been

investigated. Of particular

interest is the metal-metal

shell in the EXAFS spectra,

because this is directly

correlated to the distance

between neighboring metals

and characteristic for the

coordination geometry. An

approximate classification of

metal-metal distances in the

M1 structure is given in

Figure 11. As observed in

the EXAFS spectra at the Mo

and Nb K edges, the

characteristic bond length distribution of the metal-oxygen and the metal-metal shell of

the M1 phase is already established in the precursor (Figure 12). The pseudo-radial

distribution functions shown in Figure 12 are not phase-shift corrected, and, therefore,

the peaks are shifted by approximately -0.4 Å with respect to the crystallographic

distances. The Fourier transform of the EXAFS signal at the Nb K edge of the precursor

material has a large peak at about 3 Å that corresponds to the short metal-metal distance

Chapter 1

0246

0.00

0.05

FT(χ(k)*k3)

R [Å]

(a)

0246

0.00

0.05

FT(χ(k)*k3)

R [Å]

(b)

Figure 12. Fourier transformed (not phase-shift corrected) of (a) the Nb K edge χ(k)*k3

and (b) the Mo K edge χ(k)*k3 of the precursor (gray) and activated phase-pure M1

(black). The peaks are shifted by approx. -0.4 Å with respect to crystallographic

distances.

between the pentagonal bipyramidal coordination and the center of the surrounding

octahedrons (Figure 12a). In agreement with XRD, the higher amplitude in the metal-

metal shells of the activated material compared to the precursor can be attributed to

ongoing crystallization. At the Mo K edge (Figure 12b), a second peak is observed at

about 3.8 Å, which corresponds to longer metal-metal distances, typical for either

distances between central atoms in octahedrons, or the interlayer Mo-M distance in [001]

direction. In EXAFS spectra measured at the Nb K edge, the strong amplitude in the

metal-metal shell around the Nb centers at about 3 Å indicates the presence of many short

metal-metal distances implying preferential pentagonal bipyramidal coordination of

niobium. Additionally, a broad shoulder appears at about 3.8 Å, which is present in the

precursor and more pronounced in the activated material (Figure 12a). This shoulder is

much stronger than in simulations assuming exclusive pentagonal bi-pyramidal

coordination of Nb. Taking the high niobium content of C1 into account, which is

inconsistent with the unit cell formula of a M1 containing Nb only in pentagonal

Chapter 1

Figure 13. XRD patterns of (a) a multi-phase precursor (catalyst code 2392) that

crystallizes during heat treatment in Ar at 873 K into 55 % M1, 31 % M2, and

14% Mo5O14, (b) the precursor after hydrothermal post-treatment at 773 K in

presence of steam (catalyst code 2902) and (c) the activated steam-treated material

(catalyst code 3057).

10 20 30 40 50 60

(c)

2θ

(a)

002

181

180

002

181

180

001 001 001

210

020 120

Intensity [a. u.]

(b)

020120

210

bipyramidal positions [5, 8], EXAFS supplementary indicates that Nb is additionally

located in octahedral positions.

3.5 Post-treatment of multi-phase precursors

Decomposition of the metal salts and complete rearrangement of molecular building

blocks has been shown to be essential in hydrothermal synthesis of phase-pure M1.

Control of process parameters is required to achieve this. Under standard conditions (T =

448 K, t = 48 h), the reconstruction succeeds more or less efficiently depending on the

technical parameters of the autoclave. In Figure 13a, diffraction patterns of a multi-phase

precursor with a content of residual ammonium octamolybdate is shown. In the absence

of oxygen, this precursor has been subjected to steam at 773 K and a pressure of 20 MPa

for 2 hours. The resulting material shows the typical X-ray diffraction patterns of nano-

structured M1 (Figure 13b). Performing the usual thermal activation in argon at 673 K,

Chapter 1

Table 4

Influence of the heat treatment on BET surface area of crystalline

phase-pure M1 catalysts synthesized in A1

Catalyst Calcination in

synthetic air Activation in Ar BET

[m2/g]

C1 548K, 1h 873K, 2h 1

C2 - 873K, 2h 2

C4 598K, 1h 923K, 2h 4

the steam-treated precursor crystallizes into phase-pure M1 (Figure 13c) indicating a

comparatively high thermodynamic stability of M1 under the high temperature and high

pressure applied, compared to other phases, such as M5O14-type (M=Mo, V and/or Nb)

structures, or M2.

3.6 Activation

Activation in inert atmosphere at high temperatures is necessary, on the one hand, to

obtain long-range order of the material and, on the other hand, to create catalytic activity.

In the present work, different activation conditions have been applied to precursors

prepared in A1 in order to study the influence of the activation conditions on the

microstructure of the final catalyst. Irrespective of the temperature applied in the thermal

treatment in an inert atmosphere (873 or 923 K), and whether preceding calcination in air

(548 or 498 K) was performed, the bulk of the final catalyst is exclusively composed of

M1, confirming the observation described above that the M1 phase is established during

hydrothermal synthesis. To a certain extent, the specific surface area may be adjusted by

the thermal activation parameters (Table 4). Different from the general expectation, the

BET surface area increases with increasing activation temperature. In Figure 14, SEM

images of two phase-pure M1 catalysts activated at 873 or 923 K, respectively, are

Chapter 1

(b) (a)

Figure 14. SEM images of phase-pure catalysts treated at different temperatures;

(a) calcination in air at 598 K followed by activation in argon at 923 K (C5) and

(b) calcination in air at 548 K followed by activation in argon at 873 K (C1).

0 500 1000 1500 2000 2500

0.00

0.05

0.10

0.15

0.20

0.25

C-5

C-1

Fraction

Length [nm]

(a)

0 100 200 300 400

0.0

0.1

0.2

0.3

0.4

0.5

C-5

C-1

Fraction

Diameter [nm]

(b)

Figure 15. Shape analysis of two single-phase catalysts heat-treated at different

temperatures: distribution of (a) length and (b) the diameter of the M1 needles.

● single-phase activated at lower temperatures (C1), ■ single-phase activated at

higher temperatures (C5).

shown. From these images, the typical needle-shape morphology of the M1 crystals is

evident. The results of a shape-analysis based on measuring approximately 400 needles

are presented in Figure 15. The distribution of the length of the needles in the two

catalysts (Figure 15a) is very similar. Evidently, in the present experiments the length

was predetermined by the hydrothermal synthesis. This observation is consistent with the

Chapter 1

XRD results indicating that long-range ordering in the [001] direction is already

established in the precursor material. Further growth of the needles in the [001] direction

during the activation step cannot be excluded. However, in the temperature range applied

(873-923 K), no influence of the temperature on the length of the needles was observed.

In contrast, an anisotropic growth of the basal (001) plane has been observed. This is

reflected in an increased mean diameter of the needles in phase-pure M1 activated at

higher temperature (Figure 15b). Apparently, the parameters of the heat treatment could

be used to control the aspect ratio in the final phase-pure catalyst, which may have

implications on the catalytic activity in propane oxidation. However, the effect observed

for the two catalysts needs further systematic elucidation.

4 Summary and conclusions

In the last decade, MoVTeNbOx catalysts, and specifically the M1 phase have generated

great academic interest due to the exceptional catalytic performance achieved in the

selective oxidation of propane to acrylic acid. The accessibility of phase-pure M1 on a

large scale is a key issue that needs to be solved in order to systematically investigate the

catalytic properties of M1 and to elucidate the function of its structure in propane

activation and oxygenate formation [3, 12, 15, 17, 24]. Hydrothermal synthesis of the

chemically and structurally complex M1 phase requires precise control of the preparation

parameters. In this study, the influence of reaction temperature, reaction time, and the

technical parameters of the autoclave on morphology, and bulk and local structure of the

resulting MoVTeNbOx catalysts has been investigated, achieving a better understanding

of the formation mechanism of the M1 phase, and enabling an improved control over the

final catalyst structure.

Chapter 1

Hydrothermal reaction is the crucial step in the synthesis of M1. The individual elements

are inhomogeneously distributed in the reaction mixture initially introduced into the

autoclave as shown by Raman and UV/Vis spectroscopy together with SEM-EDX

analysis. In solution, iso- and heteropolyanions of molybdenum and giant

polyoxometalate clusters have been identified. Pre-precipitation is not necessarily

detrimental for formation of phase-pure M1. Investigation of the resulting precursor

materials proves that the structural elements required for crystallization of M1 are already

established in the course of the hydrothermal process. The reaction could take place by a

dissolution-precipitation mechanism involving dissolution or rearrangement of polyoxo

metalate building blocks followed by precipitation of an ill-crystallized nano-structured

product from solution. A complete reorganization of counter-ion-containing phases is

essential, which has been shown to be accelerated in presence of steam at very high

temperatures and pressures. Residual ammonium containing supra-molecular species in

the precursor result in formation of phase mixtures during the subsequent heat treatment.

Precursor materials of phase-pure M1 catalysts show long-range order in the [001]

direction as confirmed by XRD and EXAFS analysis. Ordering along the other

crystallographic directions is less developed. Crystallization occurs during the subsequent

heat treatment of the precursor in an inert gas in the temperature range between 823 and

923 K. The spatial homogeneity of elements in crystalline M1 is already reflected in the

corresponding precursor. Remarkably, phase-pure M1 prepared in the present work

shows a niobium content twice as high as the Nb content in the M1 phase analyzed by

DeSanto et al. [6], which confirms observations that the M1 structure has a considerable

chemical flexibility. There is some indication that the specifics of the needle-like

Chapter 1

morphology of the final catalyst can be controlled by the conditions of the activation

process. An anisotropic growth of the basal (001) plane of the M1-phase has been

observed, which is reflected in an increased mean diameter of the needles in materials

activated at higher temperature.

In summary, precise control of the hydrothermal reaction conditions is required to obtain

the desired crystal structure. The proper conditions can be achieved by optimizing

particularly reaction temperature and reaction time. Technical parameters of the

autoclave influencing the hydrothermal synthesis have to be taken into account. The

present study shows that application of identical reaction parameters does not necessarily

mean that identical materials with comparable catalytic properties will be produced when

the reaction is performed in different autoclaves. This has to be taken into account when

the catalytic behavior of MoVTeNbOx catalysts is compared. The availability of in-situ

techniques is required and currently under development to monitor the hydrothermal

reaction inside the autoclave and to generate quantitative kinetic information.

5 Acknowledgments

The authors thank Mrs. Gisela Lorenz, Mrs. Edith Kitzelmann, and Mrs Gisela Weinberg

for technical assistance. Dr. Andreas Furche is acknowledged for performing the thermal

analysis. We are grateful to HASYLAB/Hamburg for providing beam time for this work.

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Chapter 2

CHAPTER 2- Investigation of catalytic behavior of M1-phase catalyst in the

selective oxidation of propane to acrylic acid. Correlation approach

between catalytic performance and surface/bulk properties.

Abstract

Hydrothermally synthesized single-, bi-, and multi-phase MoVTeNbOx catalysts were

studied in the selective oxidation of propane to acrylic acid. Phase-pure M1 catalysts

showed high formation rates of acrylic acid. Phase cooperation between the M1 and M2

phases has not been observed. Single-phase M1 catalysts show different catalytic

properties, which are mainly related to differences in the BET surface areas. Shape

analysis of the needle-like M1 crystals has been presented as a suitable method to

elucidate the correlation between the surface area of a certain crystallographic plane, such

as the ab plane, and the formation rate of acrylic acid. In-situ XPS experiments under

different reaction media over time confirmed the elemental re-distribution at the catalytic

surface. Tellurium enrichment at the expense of molybdenum is observed at the surface

of phase-pure M1 depending on the reactant mixture. Depth profile analysis of the

catalytic surface reveals tellurium migration to the surface from subsurface layers in

response to changes in the reactive gas mixture. Furthermore, phase-pure M1 exhibited

reasonable stability under reaction conditions, which is of a great importance for any

potential industrial application.

Keywords: MoVTeNbOx catalyst, M1-phase, hydrothermal synthesis, in-situ XPS, SEM-

EDX, TEM, propane oxidation

Chapter 2

Scheme 1. M1

hase

[

]

1 Introduction

Direct selective oxidation of propane into acrylic acid is an alternative and

challenging approach [1-3] with the view of displacing the current industrial two-stage

oxidation process, in which propylene is used as feedstock [4, 5]. Efficient catalysts for

such a reaction should be able simultaneously to activate the stable C-H bonds of the

saturated hydrocarbon, provide lattice oxygen atoms and accommodate the suitable acid-

base properties, which may control the desorption of the desired reaction product

avoiding its further oxidation. Among all catalysts proposed to date [1-3], Mo-V-Te-Nb

mixed metal oxides have been found to be the most promising, achieving yields of acrylic

acid of about 50 % [6]. These multi-component metal oxide catalysts consist of mainly

two orthorhombic phases called “M1” and “M2” [7]. In the M1 phase (Scheme 1),

corner-linked MO6 (M=Mo, V) octahedrons

are two-dimensionally assembled in the ab

plane forming 6- and, 7-membered rings

which are partially occupied by TeO units

[8, 9]. Niobium has been reported to be

exclusively located in the centre of a MO7

pentagonal bipyramidal motif sharing edges

with the surrounding octahedrons [8]. The

ab planes are congruently stacked along the c direction by sharing corner oxygen atoms,

resulting in a needle-like crystal morphology in which the ab planes are perpendicular to

the length axis. The M2 phase is less complex [8]. In this structure, only 6-membered

rings are formed by arrangement of corner-linked octahedrons in the ab plane. The

Chapter 2

formula of the refined unit cell has been determined to be Mo7.8V1.2NbTe0.937O28.9 for M1

and Mo4.31V1.36Te1.81Nb0.33O19.81 for M2, respectively [8].

It is generally accepted that propane activation and the high selectivity to acrylic acid are

due to the M1 phase [10-12]. It has been suggested that the ab plane in particular is

responsible for this high activity. This conclusion was drawn from experiments based on

mechanical grinding of M1 phase catalysts [13-15]. It was postulated that this mechanical

treatment resulted in exclusively the creation of new surface belonging to the ab plane.

Grasseli et al. have proposed a propane oxidation mechanism based on structural

considerations (Scheme 2), supposing that the ab plane of the M1 phase is the carrier of

the active centers, and general concepts of organic chemistry [16]. According to this

mechanism, the first step is the activation and abstraction of a methylene hydrogen atom

from propane at a V5+ site. Subsequently, α-H abstraction and oxygen insertion occur on

Te4+ and Mo6+ sites, respectively. The resulting σ-O-allylic species is attacked by water,

leading to a surface bound acrolein hemiacetal intermediate which is then readily further

oxidized and desorbed as acrylic acid. The function of Nb is attributed to spatial

separation of the active centers - according to the concept of site isolation. Increased

selectivity to acrylic acid is thus obtained by minimizing unwanted total oxidation. On

the other hand, the specific arrangement of transition metal atoms in the model suggested

by Grasselli et al. is, however, not essential since catalysts of completely different

chemical composition, e.g. without Te, show catalytic activity in selective oxidation of

propane to acrylic acid [17, Chapter 4]. Moreover, the mechanism reported by Grasselli

et al. [16] is based on the addition of half an atom of oxygen (1/4 O2), which is physically

meaningless.

Chapter 2

1/2 O2

Te3+

O O

1/2 H2O

Mo6+

Te3+

O O

Mo6+

■

1/4 O2

3/4 O2

3/2 HO2

1/4 O2

1/2 H2O

Mo6+

Te3+

O O

Mo6+

■

V5+

Mo6+

Te4+

O O

Mo6+

+ H2O

1/4 O2

1/2 H2O

Mo6+

O O

Mo5+

O O

O H

Scheme 2. Mechanism of the selective oxidation of propane to acrylic acid reported by Grasselli et al. [16].

Chapter 2

On the other hand, the M2 phase is regarded as being unable to activate propane but as

being efficient in the oxidation of propylene to acrylic acid [10]. Correlations between

phase composition and catalytic performance are still actively debated [7, 10, 11, 15, 16,

18-26]. It has been suggested that a mixture of M1 and M2 is required for operation at

high propane conversions. Phase cooperation was reported in case of bi-phase catalysts

synthesized in one pot [7, 10, 11, 15, 18, 19], as well as in case of intimate physical

mixtures of both orthorhombic single-phases [16, 20-23]. On the contrary,

hydrothermally synthesized phase-pure M1 was found to produce higher acrylic acid

yields compared to physical mixture of M1 with M2 prepared by hydrothermal synthesis

and solid-state reaction, respectively [24].

The present contribution is an attempt to shed some light on the debate regarding the

correlation between phase composition and catalytic properties. For this propose, single-

phase M1 and multi-phase MoVTeNbOx catalysts have been prepared by hydrothermal

synthesis and studied in the selective oxidation of propane to acrylic acid. Phase-pure M1

proved to be highly active and selective to acrylic acid. Therefore, in this study a series of

differently prepared phase-pure M1 catalysts was investigated. The bulk and surface

properties were examined with emphasis on surface composition under different reaction

conditions and morphological analysis was carried out in order to elucidate whether the

exposure of a certain crystal plane correlates with catalytic performance.

2 Experimental

2.1 Catalysts preparation

The catalysts have been prepared by hydrothermal synthesis as described previously [27].

Ammonium heptamolybdate (NH4)6Mo7O24·4H2O (Merck), vanadyl sulfate

Chapter 2

VOSO4·5H2O (Riedel-deHäen), telluric acid Te(OH)6 (Aldrich) and ammonium niobium

oxalate C10H8N2O33Nb2 (Aldrich) were used as starting materials. A slurry was prepared

by mixing the aqueous solutions of the metal salts in a Mo/V/Te/Nb molar ratio of

1/0.25/0.23/0.124. After the hydrothermal synthesis at 403-448 K during 24-48 h, the

dark blue slurry obtained was filtered, washed with bidistillated water and dried for 16

hours at 353 K. Starting from the dried precursors, 3 g of the crystalline products were

obtained by heat-treatment in inert gas with a flow of 100 ml/min for 2 h at 873 K or at

923 K (ramp 15°/min), either with or without previous calcination in air for 1 h at 548 K

or at 598 K (ramp 10°/min). The activation was carried out in a rotating furnace.

2.2 Activity measurements

For testing the catalysts in selective oxidation of propane to acrylic acid, 0.5 g of the

material was loaded into a quartz fixed bed reactor with an inner diameter of 4 mm. The

feed was composed of propane/oxygen/nitrogen/steam in a molar ratio of 2.8/6.4/50.8/40.

A GHSV of 1200 h-1 and a reaction temperature of 673 K have been applied. The

products were analyzed by gas chromatography. Inorganic gases and C1-C3

hydrocarbons have been analyzed with a TCD detector using a molecular sieve column

and a Porapak Q column, respectively, for separation. Oxygenated products have been

detected applying a HP-FFAP column and a flame ionization detector.

2.3 Catalysts characterization

Bulk and microstructure were studied by X-ray Diffraction (XRD) and Transmission

Electron Microscopy (TEM). XRD measurements were performed with a STOE STADI-

P transmission diffractometer equipped with a focusing primary Ge (111)

Chapter 2

monochromator and a position sensitive detector, using Cu-Kα1 radiation. For data

analysis, the program TOPAS (v.2.1, Bruker AXS) was used to fit the diffraction patterns

of the activated materials. Transmission electron microscopy was carried out on a Philips

CM 200 FEG TEM operated at 200 kV. The microscope is equipped with a Gatan CCD

camera for image acquisition and an EDAX Genesis EDX system. For TEM, the

specimens were prepared by dry dispersing the catalyst powder on a standard copper grid

coated with holey carbon film. HRTEM image simulations were calculated using JEMS.

Morphology studies and shape analysis were performed applying Scanning Electron

Microscopy (SEM). A Hitachi S-5200 with a PGT Spirit EDX system and a Hitachi S-

4800 with an EDAX Genesis EDX detector were used. EDX (Energy Dispersive X-ray)

studies in the SEMs were carried out with an accelerating voltage of 10 kV while images

were acquired at 2 kV to optimize surface resolution. For SEM investigations the samples

were deposited on carbon tape. From the SEM images, the length and diameter of more

than 300 M1 needles was measured for each sample. Based on these measurements, the

surface area was separately calculated for the lateral surface area, and the surface area of

the ab facets assuming that the needles can be approximated as cylinders and that all

material is in the form of needles.

A depth profile of the elemental composition was studied by X-ray Photoelectron

Spectroscopy (XPS). By varying the kinetic energy between 170 eV and 470 eV,

information depths of ca. 0.5 and ca. 1.5 nm were examined. The experiments have been

performed in the presence of 0.3-0.5 mbar of the reaction mixtures, at the beamline U49-

/2-PGM1, at the synchrotron source BESSY II (Berlin, Germany). Details of the setup

have been published earlier [29].

Chapter 2

Specific surface areas of the catalysts were measured by nitrogen physisorption using an

AUTOSORB-1-C physisorption/chemisorption analyzer. All the samples were degassed

at 353 K for 2 hours prior to nitrogen adsorption. Generally, 11 points have been

considered in the linear range of the adsorption isotherm to calculate the surface area

according to the BET method.

3 Results

3.1 General properties of the Mo-V-Te-Nb mixed oxides

Single-, bi-, and multi-phase Mo-V-Te-Nb mixed oxide catalysts have been prepared by

hydrothermal synthesis at

different temperatures and

reaction times [27]. Phase

composition as determined by

XRD and the specific surface

area of the prepared catalysts

are shown in Table 1. Single-

phase M1 materials, denomi-

nated as “SP”, have a specific

surface area between 1 and 4

m2/g. The specific surface area

decreases in the sequence, SP-1

< SP-2 < SP-3 < SP-4 < SP-5.

Bi- and multi-phase materials,

denominated as “MP”, contain

Table 1

Notation, phase composition and specific

surface areas of the prepared catalysts

ID Notation

Phase composition

[%]

BET

[m2/g]

1761 SP-1 M1 100 1.0

1886 SP-2 M1 100 1.4

939 SP-3 M1 100 2.0

1650AR SP-4 M1 100 2.5

1434 SP-5 M1 100 4.0

3906 MP-1

M1 85

M2 15 1.0

3982 MP-2

M1 59

M2 33

V0.95Mo0.97O5 8

3.2

1464 MP-3

M1 58

M2 42 5.1

3984 MP-4

M1 53

M2 14

Mo5O14 23

TeMo5O16 10

0.8

3303 MP-5

M1 38

M2 41

Mo5O14 8

V0.95Mo0.97O5 13

6.3

3695 MP-6 M2 32

Mo5O14 68 1.0

Chapter 2

different amounts of M1. In addition to the M1 phase, other phases, such as M2,

V0.95Mo0.97O5, M5O14 (M=Mo, V) or TeMo5O16 are present. The M1 content decreases

with increasing number of the samples notation. The bulk composition of the phase-pure

catalysts is summarized in Table 2.

3. 2 Selective oxidation of propane

The hydrothermally synthesized single-, bi-, and multi-phase MoVTeNbOx catalysts have

been studied in the selective oxidation of propane to acrylic acid. Propane conversion,

selectivity to acrylic acid and acrylic acid yield of all tested catalysts are shown in the

Table 3. The phase-pure M1 catalysts (SP-1-5) convert propane with ca. 70-80 %

selectivity into acrylic acid. Such value is significantly higher as compared with those

reported in the literature (73 % vs. 62 % at comparable propane conversion of ~ 35 %)

[28]. The activity and the formation rate of acrylic acid per unit of catalyst mass, as well

Table 2

Bulk composition of phase-pure M1 catalysts as determined by EDX before and after

propane oxidation. The numbers in parenthesis represent molar ratios of the metals

normalized to molybdenum

Notation Before/After

reaction Mo [%] V [%] Te [%] Nb [%]

SP-1 Before reaction 53

1.9 12

0.9 15

2.6 20 ± 1.7

After reaction 52

2.7 14

2.0 14

3.2 20 ± 4.1

SP-2 Before reaction 58

3.0 13

1.1 12

2.6 17 ± 1.6

After reaction 55

1.0 14

0.8 13

1.3 18 ± 0.8

SP-3 Before reaction

4.3 15

1.3 11

2.8 23 ± 5.1

SP-4 After reaction

0.9 15

0.5 10

1.6 17 ± 1.4

SP-5

Before reaction 54 16 10 20

Average composition

before reaction

(1)

(0.24)

(0.37)

Average composition

After reaction

(1)

(0.25)

(0.22)

(0.35)

* ICP

Chapter 2

as, per unit surface area are also included in Table 3. M1 catalysts show similar but not

identical values of their activity calculated per unit surface area, indicating that the

activity of these catalysts is mainly but not exclusively dependent on their specific

surface area. As it is also shown in Table 3, by normalizing the activity and the formation

rate of acrylic acid to the specific surface area of the catalysts it is revealed that phase-

pure M1 catalysts show higher activity and enhanced formation rates as compared with

the mixed-phase materials. In Figure 1, the corresponding catalytic data are plotted

against the M1 content of the catalysts. Both the trend of the normalized propane

consumption as well as the trend of the normalized formation rate of acrylic acid increase

with increasing M1 content.

In Figure 2, the catalytic behavior of single-phase catalyst (SP-1) and multi-phase

catalysts with different M1 content (MP-1, MP-2 and MP-5) are plotted against the time

on stream. The single-phase catalysts have been tested for 9 hours, whereas the other

Table 3

Catalytic properties of Mo-V-Te-Nb mixed oxides in the selective oxidation of propane to

acrylic acid

Notation XC3H8

[%]

SAA

[%]

YAA

[%]

Rate [mmol/gca

·h] Rate [mmol/m2·h]

C3H8

consumption

Formation

of AA

C3H8

consumption

Formation

of AA

SP-1 18 70 13.0 0.27 0.19 0.27 0.19

SP-3 38 73 28.0 0.57 0.42 0.29 0.21

SP-4 56 79 44.0 0.84 0.66 0.34 0.27

SP-5 52 79 41.0 0.78 0.62 0.20 0.15

MP-1 9 62 5.3 0.14 0.08 0.14 0.09

MP-2 33 58 18.9 0.50 0.29 0.15 0.09

MP-3 61 47 29.0 0.92 0.43 0.18 0.08

MP-4 6 65 3.6 0.09 0.06 0.11 0.07

MP-5 19 49 9.3 0.29 0.14 0.05 0.02

MP-6 2 44 0.6 0.03 0.01 0.03 0.01

Chapter 2

Figure 1. Rates of propane consumption and acrylic acid formation per unit of surface area

of single-, bi-, and multi-phase MoVTeNbOx catalysts containing different amounts of M1.

0 20 40 60 80 100

0.00

0.05

0.10

0.15

0.20

0.25

0.00

0.05

0.10

0.15

0.20

0.25

0.30

propane consumption rate

Formation rate of acrylic acid

[mmolAA/h⋅m2]

Propane consumption rate

[mmolC3H8/h⋅m2]

M1-content [%]

formation rate of acrylic acid

catalysts were subjected to the feed for 50 hours. All the catalysts are stable under

reaction conditions. No deactivation of the catalysts is observed during the reaction times

applied. It should be noted at this point that propylene has never been observed as a

reaction product on the phase-pure M1 catalysts under the conditions applied in this

study. The bulk and surface characterization of the post-reaction samples will be

presented in section 3.3.4.

As was mentioned above, the discrepancies observed between the catalytic activities of

the phase-pure M1 catalysts are not exclusively due to the differences in their specific

surface areas. Therefore, the bulk and surface properties of these materials were

investigated in order to evaluate the effects of bulk and surface composition as well as

morphological factors on the catalytic properties.

Chapter 2

200 300 400 500

100

XC3H8

SAA

SAceA

SCO2

SCO

time [min]

(a) SP-1

M1 (100 %)

0 500 1000 1500 2000 2500 3000

100

XC3H8

SAA

SC3H6

SAceA

SCO2

SCO

time [min]

(b) MP-1

M1(85%), M2 (15%)

0 500 1000 1500 2000 2500 3000

100

time [min]

XC3H8

SAA

SC3H6

SAceA

SCO2

SCO

MP-2

M1(59%), M2 (33%), V0.95Mo0.97O5 (8%)

(c)

0 500 1000 1500 2000 2500 3000

100 XC3H8

SAA

SC3H6

SAceA

SCO2

SCO

time [min]

MP-5

M1 (38%), M2 (41%),

Mo5O14 (8%), V0.95Mo0.97O5 (13%)

(d)

Figure 2. Catalytic behavior of (a) SP-1 (b) MP-1 (c) MP-2 and (d) MP-5 against

time on stream.

3.3 Characterization of phase-pure M1 catalysts

3.3.1 Morphology and shape analysis

SEM images of the phase-pure MoVTeNbOx catalyst (Figure 3) revealed that the

material mainly crystallized in a high aspect ratio needle-like morphology. The needle-

shaped crystals are not isolated entities. They form agglomerates and the crystals are in

Chapter 2

(

)

(a)

Figure 3. SEM images of phase-pure M1 materials, (a) needle shape and

(b) steps on the sides of the needles parallel to c direction.

Figure 4. HRTEM

(view of the steps).

some cases fused together (Figure 3a). This high aspect ratio structure possesses a

limited amount of exposed ab plane (section surface, Ss) whereas the surface area is

mainly accounted for by the sides of the needles (lateral surface, Sl). Along the sides,

steps of different dimensions parallel to the length axis (c direction) are observed. The

largest steps amount tens of nanometers comprising several unit cells of M1 (arrows in

Figure 3b). On the terraces, contrast variations are

observed which are assumed to be steps of sub-unit cell

dimensions. In the HRTEM (Figure 4), these surface

steps are indeed revealed as crystallographic steps

propagating along the length axis of the needle structures.

Such steps provide sites of high coordination traditionally

assumed to be centers of catalytic activity.

The diameter and length of the needles of different M1 materials were determined from

SEM images. From this data, the aspect ratio was calculated as the ratio of diameter to

length. Table 4 summarizes the measurements from the samples investigated in this

Chapter 2

study. The materials were analyzed after propane oxidation in the fixed bed reactor over

9 hours time on stream (SP-1AR and SP-4AR). Length distributions of both materials

overlap, whereas the diameter distributions differ from each other considerably (Figure15

of chapter 1). SP-4R shows a bigger mean diameter of the needles as compared with SP-

1R (Table 4).

Based on the shape analysis of the materials, two different surface areas were calculated:

the surface areas of the ab plane (Ss) and the surface areas of the sides of the needles (Sl)

(Table 4). These calculations were done under the assumption that (i) the material is 100

% crystalline, (ii) each crystal is cylindrically needle shaped, and (iii) the density of the

material is the density of the unit cell reported in the literature (4.4 g/cm3) [8]. XRD

patterns (not shown) and SEM images (Figure 3) confirm the high crystallinity of the

materials, justifying the first postulate. SEM images of the needles (Figure 3) show that a

cylindrical shape is just a rough estimation that leads especially to an underestimation of

the surface area of the sides of the needles. In case of the ab planes, the calculation error

is significantly smaller, justifying the second postulate. As observed from Table 2, the

elemental composition of the synthesized materials differs from that reported in the

Table 4

Shape analyses of three phase-pure catalysts and their corresponding propane

consumption rate and formation rate of acrylic acid

Notation NP Length

[nm]

Diameter

[nm]

[m2/g]

Rate [mmol/gca

·h]

C3H8

consumption

Formation

of AA

SP-1AR 407 437 ± 308 75 ± 46 0.6 6.0 0.27 0.19

SP-4AR 389 411 ± 276 152 ± 71 0.9 4.4 0.84 0.66

NP: number of particles analysed

Chapter 2

literature [8]. However, these discrepancies mainly involve Mo and Nb. As the atomic

mass of these elements is close, these discrepancies are expected to have only a minor

influence on the real density justifying the third postulate. In Table 4, the catalytic

properties of the two samples are compiled together with the calculated surface areas of

the ab planes and the sides of the needles. The catalytic activity of the M1-phase catalysts

decreases in the following sequence: SP-1 < SP-4. The corresponding calculated S

s of

both catalysts follow the same trend (Table 4). However, correlation between catalytic

properties and the calculated Ss can not be confirmed, since only two catalysts were

investigated. Morphological analyses of a series of phase-pure M1 catalysts are required.

Furthermore, functionality of the ab plane of the M1 phase in the selective oxidation of

propane is thoroughly investigated in our next work [30]. Such investigation is based on

the preparation of a model M1 catalyst, which exposes preferentially MoVTeNbOx

surface area, belonging to the ab plane of this structure.

3.3.2 Microstructure

The microstructure of the phase-pure M1 was studied by HRTEM. Figure 5a shows the

M1 phase of MoVTeNbOx viewed in the direction of 〈001〉 zone axis. The observed

structure matches that reported in the literature [8], with arrangements of octahedra

forming pentagonal (Figure 5b), heptagonal (Figure 5c) and hexagonal rings (Figure

5d). The contrast variation between the heptagonal and hexagonal channels indicates the

difference in tellurium occupancy. Lattice spacings observed in the images agree with

those of the M1 phase reported by DeSanto et al [8]. Figure 6 shows the HRTEM image

of a M1 needle viewed along the 〈127〉 direction. No contrast variations are observed

along the needle indicating that it is of uniform thickness. Structures of different

Chapter 2

(b)

(c)

(a)

(d)

Figure 5. Microstructure examined by TEM, (a) ab plane of M1-phase; Structure

features – overlapping simulation with experimental- of (b) pentagonal bipyramides,

Figure 6. HRTEM image of M1

overlapped with that of M1 simulation.

thicknesses were simulated by using the JEMS program. In the inset of the HRTEM

image, a simulation of the thickness of 1.6 nm is shown in the same orientation as in the

experimental image. The contrast variations on the simulated image closely match that of

the experimental image, confirming the simulated thickness.

As previously reported [25], Mo-V-Te-Nb

mixed oxides show a characteristic complex

termination clearly observed in the HRTEM

images. To verify that the formation of this

terminal layer is not an artifact of the

electron beam, a region of the sample was

irradiated with electrons over an extended

period of time (Figure 7). The thickness of

the surface layer was measured at different times and plotted as a function of irradiation

time (Figure 8). A constant growth rate of the surface layer (~ 0.06 nm·min-1) was

Chapter 2

Figure 8. Thickness of the complex

termination of the M1 phase against

the exposure time of the beam.

(

)

(

)

0 5 10 15 20 25 30 35

0.5

1.0

1.5

2.0

2.5

3.0

Thickness [nm]

Time [min]

Figure 7. TEM images after (a) 1 minute (b) 31

minutes exposure time.

observed, indicating that the layer is in part an effect of the electron beam. However,

extrapolating to t = 0, a positive thickness value is obtained, indicating that there is a

terminating structure different than that of the bulk M1 structure prior to electron beam

exposure. The thickness of this surface layer is roughly 0.7 nm, which is within the range

of surface sensitive X-ray photoelectron spectroscopy.

3.3.3 Elemental dynamics and composition at the catalytic surface

The terminating layer that covers the surface of M1 crystallites has been investigated by

in-situ X-ray photoelectron spectroscopy in the presence of a reactant gas mixture. A

depth profile of single-phase M1 catalyst was analyzed by applying different excitation

energies, corresponding to a depth of approximately 0.5 nm (denominated as surface) or

1.5 nm (denominated as subsurface), respectively. In-situ experiments were performed on

SP-1 in presence of 0.25-0.3 mbar of the reactants (Figure 9) in order to elucidate the

influence of the reaction medium on the surface composition. Additionally, SP-1 has

been also investigated after the catalytic test in a fixed bed reactor at 1 bar (SP-1AR), in

the presence of 0.3 mbar of oxygen at 298 K (Figure 9).

Chapter 2

298K623K

623K

298K

Elemental composition

of surface (0.5 nm) [at.- %]

C3H6, O2

C3H8, O2, H2OC3H8, O2

(a) SP-1

- used catalyst*-

Elemental composition

of subsurface (1.5 nm) [at.- %]

298K

623K

C3H6, O2

623K

C3H8, O2, H2O

623K

C3H8, O2

293K

(b) SP-1

- used catalyst*-

* Reaction conditions: C3H8/O2/N2/H2O(v) = 0.85/1.9/15.2/12 (molar ratio %)

GHSV = 1200 h-1, T = 673 K and P = 1 bar

Figure 9. In-situ XPS results at different layer depth, (a) surface (information

depth of ca. 0.5 nm) and (b) subsurface (information depth of ca. 1.5 nm).

The surface of SP-1 in 0.3 mbar of O2 before admission of propane is mainly composed

of molybdenum and tellurium. Furthermore, a considerable enrichment of tellurium at the

expense of niobium is observed at the surface in comparison to the bulk (Table 2). After

admission of propane, the surface concentration of tellurium is enhanced at the expense

of molybdenum, which is even more noticeable in the presence of steam. As the steam is

removed from the feed and propane is replaced by propylene keeping the temperature

constant at 623 K, the tellurium fraction decreases in favor of molybdenum, close to the

initial composition measured in O2. The concentration of vanadium and niobium remain

relatively constant under the different reaction media. Consequently, a reversible

enrichment of the M1 surface with tellurium is observed in presence of the reactants

(C3H8, O2 and H2O at 623 K). Such tellurium enrichment at the surface is also observed

in the used SP-1 (SP-1AR).

Chapter 2

surface surface subsurface subsurface

100

Elemental composition

[at.- %]

SP-3

SP-1

SP-3

SP-1

Figure 10. Depth profile composition of two single-phase catalysts (SP-1 and SP-3).

The subsurface composition of the M1 catalyst is similar to the surface composition. In

contrast to what occurs on the surface, the tellurium content decreases after the addition

of propane to the oxidative atmosphere in favor of molybdenum. Apparently, the

subsurface supplies tellurium to the surface and is, therefore, depleted in tellurium.

However, following the addition of steam the tellurium content increases again at the

expense of molybdenum, as is the case at the surface. The observations reflect the

mobility of tellurium that migrates through the hexagonal and heptagonal channels

towards the surface, particularly in wet reaction atmosphere conditions.

In Figure 10, the depth profiles of two single-phase catalysts (SP-1 and SP-3) that show

different catalytic properties are compared in presence of 0.3 mbar of O2. SP-3 is more

active and shows a higher formation rate of acrylic acid than SP-1 (Table 3). The

enhanced tellurium content on the surface as well as on the subsurface, observed in the

Chapter 2

20 40

2 θ

(b)

Intensity [a. u.]

(a)

20 40 60

2 θ

(b)

Intensity [a. u.]

(a)

Figure 11. XRD patterns of (a) SP-1 and (b) SP-2; before (black line) and after

reaction (grey line).

more active catalyst (SP-3) might suggest that tellurium is essential for an improved

catalytic efficiency.

3.3.4 Properties of the phase-pure catalysts after propane oxidation

Bulk structure, elemental composition and morphology of used SP-1 and SP-2 have been

investigated after 9 hours time on stream. In both cases the reaction was performed at 673

K and the feed was composed of propane/oxygen/nitrogen/steam in a molar ratio of

2.8/6.4/50.8/40. However, different GHSVs, 1200 h-1 and 4800 h-1, were applied.

XRD patterns of both catalysts before and after reaction are shown in Figure 11. No

evident changes in the bulk structure are observed after the catalytic reaction even in case

of the more severe conditions (SP-2). However, SP-2 shows sharper reflections after

reaction, indicative of an increase on crystallinity and/or growth of the crystals during

reaction. Last affirmation is in agreement with the crystallite size (Crystallite Size

Chapter 2

SP-2 SP-2AR

Crystallite Size Lorentz [nm] 70(4) 72(2)

Crystallite Size Gauss [nm] 42(2) 93(5)

Table 5

Crystallite size values of SP-2 before and after reaction calculated from XRD data.

The standard deviations are indicated in brackets

Lattice

parameters SP-1 SP-1AR SP-2 SP-2AR

a [Å] 21.234(6) 21.208(5) 21.199(3) 21.1957(16)

b [Å] 26.749(7) 26.740(6) 26.710(4) 26.700(2)

c [Å] 4.0030(7) 4.0016(6) 4.0131(4) 4.0108(3)

Table 6

Lattice parameters of the samples SP-1 and SP-2 before and after reaction. The standard

deviations are indicated in brackets.

Lorentz/Gauss) calculated from the XRD data (Table 5). The unit cell sizes (lattice

parameters) of both M1 materials have not suffered significant modifications after

reaction as it is observed in Table 6.

Bulk elemental composition also remains unchanged (Figure 12). The shape analyses of

the catalyst tested at 1200 h-1 (SP-1) before and after reaction (Figure 13), confirmed that

the dimensions of the needles, the diameter (Figure 13a) as well as the length (Figure

13b), are not modified during catalytic reaction. In case of the catalyst exposed to high

space velocities (SP-2), the inspection of the SEM images indicates that the morphology

also remains unchanged after the catalytic reaction (Figure 14).

The absence of bulk structural and morphological changes indicates a structural stability

of the M1 phase under the reaction conditions applied.

Chapter 2

4 Discussion and conclusions

Catalysts composed exclusively of the M1 phase show a higher catalytic activity

per unit surface area (Figure 1, Table 3) and a higher selectivity to acrylic acid (Table 3)

compared to bi-, and multi-phase catalysts. However, normalization of the surface area

shown in Table 3 and Figure 1 is problematic, since different phases present in the

multi-phase catalysts could contribute differently to the total specific surface area of the

catalyst. Thus, correlating the catalytic data per unit surface area with the M1 content of

the catalysts may be misleading. This potential inconsistency could be the reason for the

lack of a linear trend in the catalytic properties versus M1 content. In any case, no

evidence of M1/M2 phase cooperation, as it is reported in the literature [7, 10, 11, 15, 16,

18-23], was observed in the present contribution.

100

SP-2

Elemental composition

[at.-%]

Mo VTe Nb

SP-1

Figure 12. Elemental composition of SP-1 (■) and SP-2 (▼), before reaction (filled

symbol) and after reaction (empty symbol).Reaction conditions: C3H8: O2: N2: steam

(% molar) = 0.85: 1.9: 15.2: 12; T = 673 K and a GHSV of 1200 h-1 and 4800 h-1 for

SP-1 and SP-2, respectively.

Chapter 2

0 100 200 300 400

0.0

0.1

0.2

0.3

0.4

0.5

0.6 SP-1

SP-1AR

Fraction

Diameter [nm]

(a)

0 250 500 750 1000 1250 1500

0.00

0.05

0.10

0.15

0.20

0.25

SP-1

SP-1AR

Fraction

Length [nm]

(b)

Figure 13. Shape analysis distribution of SP-1 before and after reaction: (a) diameter

distribution and (b) length distribution.

The SP-1 catalyst, composed exclusively of M1 phase, and the MP-5 catalyst, composed

of a mixture of phases including M1 and M2 phases, showed a propane conversion of ca.

20 % (Figure 2a and 2d). The selectivity to total oxidation products was rather similar

for both catalysts, and comprised ca. 16 %. As expected, SP-1 showed higher selectivity

to acrylic acid as compared with MP-5. Remarkably, the spectrum of the other partial

oxidation products was different in both cases. Whereas no propylene was formed over

SP-1, substantial amounts of propylene were detected in the final products obtained over

Figure 14. SEM images of SP-2 (a) before and (b) after reaction.

Chapter 2

CH3-CH2-CH3

CH2=CH-CH3

[CH2=CH-CH2-OH]

CH2=CH-CHO

CH2=CH-COOH

[CH3-CH2-CH2OH]

CH3-CH2-CHO

CH3-CH2-COOH

CH3-CH(OH)-CH3

CH3-CO-CH3

CH3-COOH

+ 1/2 O2

- H2O

+ 1/2 O2

- H2O

+ 1/2 O2

- H2O

+ 1/2 O2

- H2O

+ 1/2 O2 + 1/2 O2

+ 1/2 O2

+ O2

- H2O

+H2O

+ 1/2 O2

Scheme 3. Pathways of the selective oxidation of propane.

MP-5. These observations are inconsistent with the reported phase cooperation theory [7,

10, 11, 15, 16, 18-23], in which it is assumed that the M2 phase cooperates with the M1

phase by converting the propylene intermediate into acrylic acid. Moreover, acetic acid is

one of the main products on MP-5, indicating that propylene formation may proceed via

different reaction pathways (Scheme 3) on the multi-phase catalysts. In case of SP-1,

only traces of acetic acid are detected.

The highest catalytic activities and formation rates of acrylic acid per unit surface area as

well as per unit of catalytic mass have been achieved by materials composed exclusively

of the M1 phase. On the other hand, the phase-pure materials differ slightly with respect

to their intrinsic catalytic properties. Consequently, the differences in specific surface

Chapter 2

areas of the M1 catalysts are not exclusively responsible for the different formation rates

of acrylic acid.

The chemical composition of the bulk (Table 2) varies slightly and shows significant

discrepancies with the composition of the M1 structural model reported in the literature

[8] as well as the hydrothermally synthesized phase-pure catalysts reported by Ueda et

al. [9]. Obviously, the M1 structure copes with a certain chemical flexibility. Changes in

the chemical composition of the phase affect the nature of the active sites at the surface

and, therefore, the catalytic properties. Consequently, catalysts with identical phase

composition, such as phase-pure M1 materials, could exhibit different catalytic

properties. This fact has to be taken into account when carrying out comparative studies

in order to elucidate the role of the different structures. On the other hand, there is no

correlation between the chemical composition of the bulk of the phase-pure M1 catalysts

studied in the present work and the formation rate of acrylic acid.

The phase-pure materials show the typical needle-like morphology of this crystal

structure. However, shape analysis based on the SEM projections shows that the

dimensions of the needles, in particular the cross-sectional diameters, vary widely (Figure

15 of chapter 1). It has been suggested in the literature that the active centers of the

catalysts are exclusively located on the surface of the basal ab plane (cross-section of the

needles) [13-15], and that the sides of the needles are inactive in the selective oxidation

of propane. However, such investigations were based on the assumption that mechanical

treatment of the M1 materials exclusively increased the surface area of the exposed ab

plane by means of breaking the needles perpendicularly to the needle length axis.

Experimental evidence for corresponding changes in the morphology of the milled

Chapter 2

catalysts has not been provided in the literature. In the present contribution,

morphological analysis of the M1 phase catalysts has been presented as a suitable method

to investigate the impact of the exposed ab plane on the catalytic performance. M1

materials that show identical bulk properties (structure and elemental composition) and

are identically mechanically treated for the preparation of the sieve fraction, are

appropriated candidates for those studies. It has to be mentioned that while the surface of

the cross-section of the needles is flat, the sides of the needles are characterized by micro-

and nano-step, which may increase the surface area of the needle, substantially.

Differences in the morphology of the sides of the needles have not been considered in our

simplified approach, assuming an ideal cylindrical shape of the crystals. Therefore, the

contribution of the sides of the needles may have been underestimated. Investigation of a

series of phase-pure M1 materials by applying the above-mentioned method is required in

order to elucidate if a certain plane of this structure correlates with the catalytic properties

of the material.

As revealed by HRTEM, the surface of the M1 crystals is covered by a structurally

disordered layer, exhibiting no long-range order (Figure 6 and 7). The thickness of the

terminating layer has been estimated to approximately 0.7 nm (Figure 7 and 8). Since

the catalytic reaction takes place on the surface of M1, investigation of this surface layer

is essential for understanding the functionality of these catalysts. In-situ XPS experiments

show that the surface composition (information depth of ca. 0.5 nm) of the catalysts

exhibit an increased content of tellurium at the expense of molybdenum as the catalyst is

exposed to a gas mixture of propane and oxygen (C3H8/O2 = 0.5 molar ratio) at a

temperature of 623 K. The tellurium content at the surface increases even more as steam

Chapter 2

is added to the feed. The latter reaction atmosphere is closer to the reaction conditions

applied in the fixed bed reactor. On the other hand, the concentrations of vanadium and

niobium remain rather constant under the different reaction conditions. An elemental

composition on the surface that deviates from the bulk composition is confirmed by the

XPS analyses of the quenched catalyst collected after the catalytic test in a fixed bed

reactor at 1 bar. The surface dynamics of M1 are clearly reflected in these results.

Tellurium may play an important role in the formation of acrylic acid. The higher

tellurium content detected in the more active catalyst (SP-3) might explain the slight

discrepancies in the catalytic properties of these phase-pure materials. However, the high

Te content observed on the catalytic surface under the reaction conditions may also be

explained by the formation of isolated tellurium particles, taking no part in the catalytic

active centers. Further investigations to confirm the active involvement of the surface

tellurium in the catalytic reaction are currently being carried out.

In conclusion, hydrothermally synthesized phase-pure M1 MoVTeNbOx catalysts show

the highest activity and the highest formation rate of acrylic acid in the selective

oxidation of propane to acrylic acid as compared with MoVTeNbOx catalysts composed

by a mixture of phases. Therefore, no evidence of phase-cooperation is found in this

contribution. Catalytic activity of phase-pure M1 materials depends mainly on the

specific surface area, but also on the surface properties of the catalysts in question. In the

presence of the reactant gas mixture, the surface of M1 is mainly composed of tellurium

and molybdenum in a stoichiometric ratio that is not compatible with the M1

stoichiometry. Surface dynamics of these catalysts are confirmed by in-situ XPS

experiments, resulting in tellurium enrichment at the surface as the catalyst is exposed to

Chapter 2

reaction conditions (propane/oxygen) and even more distinctly as steam is presented to

the atmosphere. Such results suggest a remarkable role of tellurium in the catalytic

reaction. Tellurium is assumed [16] to be involved in hydrogen abstraction reactions from

propane and from propylene.

The high stability detected for these single-phase catalysts under the real reaction

conditions is of a great importance for their potential industrial application in the future.

In this contribution, it has also been described, a suitable method based on shape analysis

of the M1 catalysts in order to elucidate the catalytic function of the ab plane of this

structure.

5 Acknowledgments

The authors thank Dr. Olaf Timpe for helpful discussions, Edith Kitzelmann for

conducting the XRD measurements, and Gisela Lorenz for carrying out the nitrogen

adsorption measurements.

6 References

[1] E. K. Novakova, V. C. Védrine, Propane Selective Oxidation to Propene and

Oxygenates on Metal Oxides in J. L. G. Fierro, Metal Oxides, Chemistry and

Applications, CRC Press (2006), 414.

[2] G. Centi, F. Cavani, F. Cavani, F. Trifiro, Selective Oxidation by Heterogeneous

Catalysis, Kluwer Academic/Plenum Publishers (2001), 363.

[3] M. M. Lin, Applied Catalysis A: General 207 (2001), 1.

[4] M. Tanimoto, H. Himeji-shi, I. Mihara, H. Aboshi-ku, T. Kawajiri, H. Himeji-shi,

EP 0711745 B1 (1996); Nippon Shokubai.

[5] A. Tenten, H. Hibst, F.-G. Martin, L. Marosi, V. Kohl, DE 4405514 A1 (1995);

BASF.

Chapter 2

[6] T. Ushikubo, H. Nakamura, Y. Koyasu, S. Wajiki, US Patent 5 380 933 (1995);

Mitsubishi Kasei Corporation.

[7] T. Ushikubo, K. Oshima, A. Kayou, M. Hatano, Studies in Surface Science and

Catalysis 112 (1997), 473.

[8] P. DeSanto, D. J. Buttrey, R. K. Grasselli, C. G. Lugmair, A. F. Volpe, B. H.

Toby, T. Vogt, Zeitschrift für Kristallographie 219 (2004), 152.

[9] H. Murayama, D. Vitry, W. Ueda, G. Fuchs, M. Anne, J. L. Dubois, Applied

Catalysis A: General 318 (2007), 137.

[10] M. Baca, A. Pigamo, J. L. Dubois, J. M. M. Millet, Topics in Catalysis 23 (2003),

1-4, 39.

[11] J. M. Oliver, J. M. López-Nieto, P. Botella, Catalysis Today 96 (2004), 241.

[12] W. Ueda, D. Vitry, T. Katou, Catalysis Today 96 (2004), 235.

[13] K. Oshihara, T. Hisano, W. Ueda, Topics in Catalysis 15 (2001), 153.

[14] V. V. Guliants, R. Bhandari, R. S. Soman, O. Guerrero-Pérez, M. A. Bañares,

Applied Catalysis A: General 274 (2004), 213.

[15] R. K. Grasselli, D. J. Buttrey, P. DeSanto, Jr., J. D. Burrington, C. G. Lugmair, A.

F. Volpe, Jr., T. Weingand, Catalysis Today 91–92 (2004), 251.

[16] R. K. Grasselli, Catalysis Today 99 (2005), 23.

[17] C. Hess, M. H. Looi, S. B. A. Hamid, R. Schlögl, Chemical Communications

(2006), 451.

[18] T. Ushikubo,

Catalysis Today 57 (2000), 331.

[19] R. K. Grasselli, J. D. Burrington, D. J. Buttrey, P. DeSanto Jr., C. G. Lugmair, A.

F. Volpe Jr., T. Weingand, Topics in Catalysis 23 (2003), 1–4, 5.

[20] J. Holmberg, R. K. Grasselli, A. Andersson, Topics in Catalysis 23 (2003), 1-4,

55.

[21] J. Holmberg, R. K. Grasselli, A. Andersson, Applied Catalysis A: General 270

(2004), 121.

[22] R. K. Grasselli, D. J. Buttrey, J. D. Burrington, A. Andersson, J. Holmberg, W.

Ueda, J. Kubo, C. G. Lugmair, A. F. Volpe Jr., Topics in Catalysis 38 (2006), 1-3,

Chapter 2

[23] E. García-González, J. M. López-Nieto, P. Botella, M. M. Gónzalez-Calbet,

Chemistry of Materials 14 (2002), 4416.

[24] D. Vitry, Y. Morikawa, J. L. Dubois, W. Ueda, Topics in Catalysis 23 (2003), 1-

4, 47.

[25] J. B. Wagner, O. Timpe, F. A. Hamid, A. Trunschke, U. Wild, D. S. Su, R. K.

Widi, S. B. A. Hamid, R. Schlögl, Topics in Catalysis 38 (2006), 51.

[26] P. Beato, A. Blume, F. Girgsdies, R. E. Jentoft, R. Schlögl, O. Timpe, A.

Trunschke, G. Weinberg, Q. Basher, F. A. Hamid, S. B. A. Hamid, E. Omar, L.

Mohd Salim, Applied Catalysis A: General 307 (2006), 137.

[27] Preparation of phase-pure M1 MoVTeNb oxide catalysts by hydrothermal

synthesis – Influence of reaction parameters on structure and morphology.

A. Celaya Sanfiz, T. W. Hansen, E. Rödel, O. Timpe, A. Trunschke, R. Schlögl,

(accepted for publication in Topics in Catalysis).

[28] W. Ueda, D. Vitry, T. Kato, N. Watanabe, Y. Endo, Research on Chemical

Intermediates 32 (2006), 3–4, 217.

[29] H. Bluhm, M. Hävecker, A. Knop-Gericke, E. Kleimenov, R. Schlögl, D.

Teschner, V. I. Bukhtiyarov, D. F. Ogletree, M. Salmeron, Journal of Physical

Chemistry B 108 (2004), 14340.

[30] How important is the (001) plane of M1 for selective oxidation of propane to

acrylic acid.

A. Celaya Sanfiz, F. Girgsdies, T. W. Hansen, A. Trunschke, R. Schlögl, A.

Knoester, H. H. Brongersma, M. H. Looi, S. B. A. Hamid, (submitted in Journal

of Catalysis).

Chapter 3

CHAPTER 3- How important is the (001) plane of M1 for selective oxidation

of propane to acrylic acid

Abstract

The role of the (001) crystallographic plane of the M1 phase of Mo-V-Te-Nb mixed

oxide catalysts in selective oxidation of propane to acrylic acid has been addressed by

investigating a phase-pure M1 material preferentially exposing this surface. A model

catalyst has been prepared by complete silylation of M1 followed by breakage of the SiO2

covered needles. Using this approach, the reactivity of the M1 (001) surface has been

investigated by combining a micro-reactor study of propane oxidation with High-

Sensitivity Low Energy Ion Scattering (HS-LEIS). Scanning electron microscopy (SEM)

and transmission electron microscopy (TEM) have been used to study shape and

microstructure of the model system and to verify the surface exposure of the model

catalyst. The specific formation rate of acrylic acid on the model catalyst is similar to that

on the phase-pure M1 reference material indicating that the (001) plane of the M1 crystal

structure does not possess enhanced catalytic properties compared to the lateral surface of

M1 needles in propane oxidation.

Keywords

Mo-V-Te-Nb oxide catalyst, M1, propane oxidation, propane ammoxidation, low-energy

ion scattering, LEIS

Chapter 3

1 Introduction

Selective oxidation of light alkanes requires multi-functional catalysts capable of

simultaneous activation of C-H bonds, insertion of oxygen atoms, and prevention of COx

formation by further oxidation of intermediates or desired reaction products, which are

usually much more reactive than the inactive reactant. The necessary functional diversity

is implemented in chemically and structurally complex MoVTeNbOx catalysts, which

show high activities and selectivities in partial (amm)oxidation of propane to acrylic acid

and acrylonitrile, respectively [1, 2]. Usually, these catalysts consist of phase mixtures

composed of the so-called “M1” and “M2” phases [3], as well as minor amounts of

phases, like Mo5O14-type structures or binary Mo-V and Mo-Te oxides. Propane

activation and high selectivities towards acrylic acid and acrylonitrile are generally

attributed to the presence of the orthorhombic M1 phase [4-6]. The M1 structure (ICSD

55097) consists of corner-sharing MO6 octahedrons, (M=Mo,V), which are assembled in

the (001) plane forming characteristic hexagonal and heptagonal rings hosting Te-O

units. Niobium is preferentially located in pentagonal bipyramidal environment [7-9].

However, due to a certain chemical flexibility of the phase, octahedral positions may also

be occupied by Nb in M1 with higher Nb content [10]. A bronze like channel structure is

established by stacking layers of the polyhedrons in the [001] direction resulting in a

needle-like crystal morphology in which the (001) planes are arranged perpendicular to

the length axis of the needle. It has been suggested that the (001) planes of the M1 phase

contain the active and selective surface sites for selective oxidation reactions [11-14].

This claim has stimulated research on the structure and properties of the M1 surface by

Low Energy Ion Scattering (LEIS) [15, 16]. One of the unique features of LEIS analysis

Chapter 3

is that it gives the atomic composition of the outermost atoms of a surface. These outer

atoms are precisely the atoms that largely control the catalytic properties of the solid. It

has been shown before [17] that in cases where conventional surface analytic techniques,

such as XPS, do not show correlation with the catalytic activity, the extreme surface

sensitivity of LEIS gives a direct relationship between composition and catalysis. In a

recent review [18], the underlying principles and quantification of LEIS are described.

The general absence of matrix effects enables the use of simple reference samples for the

quantification of the surface composition.

The present study addresses the origin of the positive effect of grinding on the catalytic

activity of MoVTeNbOx in the selective oxidation of propane to acrylic acid. Due to the

needle-shape of the particles, the surface of (001) planes comprises only a minor fraction

of the total surface area. Thus, increased activity after grinding of M1 needles has been

attributed to the generation of additional (001) surface area [11, 12]. Even though an

increased specific surface area is observed [12], the downside of the grinding process is

the well-known effect of mechanical treatment on the nature and concentration of defects

on the overall surface of transition metal oxides [19].

Therefore, a different strategy illustrated in Scheme 1 has been pursued. A batch of

crystalline, phase-pure M1 prepared by hydrothermal synthesis was divided into two

parts. One part was fully coated with silica. The complete coverage of the mixed metal

oxide surface by SiO2 was verified by HS-LEIS. Both, the coated material and the non-

coated reference M1, were pressed into pellets, crushed and sieved in order to prepare

sieve fractions, which were then loaded into a micro-reactor and studied in the partial

oxidation of propane to acrylic acid. The preparation of the sieve fraction represents a

Chapter 3

silylation

M1 after reaction

silylated M1

after reaction

pressing

sieving

propane

oxidation

Scheme 1

Silylation and mechanical treatment of phase-pure M1

gentle mechanical treatment that generates new M1 surface, which has been quantified by

LEIS for the silica-coated catalyst [18] and by nitrogen adsorption for the reference

material. Special attention has been paid to a comprehensive microstructural

characterization of the model catalysts before and after the propane oxidation. Scanning

electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that

complete coverage of M1 followed by breakage of the SiO2-covered needles generates a

model catalyst that predominantly exposes (001) planes (Scheme 1). In addition, LEIS

was applied to compare the chemical composition of the newly formed M1 surface of the

coated catalyst with that of the total surface of the M1 reference. Based on the results of

the micro-reactor and LEIS study, the relevance of particular surface terminations will be

discussed in view of the catalytic properties of phase-pure M1 catalysts in partial

oxidation of propane to acrylic acid.

2 Experimental

2.1 Preparation of M1

A phase-pure M1 catalyst with nominal atomic ratio Mo/V/Te/Nb = 1/0.3/0.23/0.13 has

been prepared starting from 18.71 g MoO3 (Fluka) suspended in 300 ml bidistilled water.

Chapter 3

After adding 19.90 g oxalic acid at 363 K a light yellowish solution has been obtained.

The mixture was stirred for 25 minutes and cooled down to 313 K. Afterwards, 6.86 g of

telluric acid (Sigma-Aldrich) dissolved in 30 ml bidistilled water at 313 K was added to

the molybdenum oxalate solution. A vanadium solution was synthesized by carefully

mixing 7.05 g oxalic acid and 3.55 g V2O5 (Riedel-de-Haën) in 30 ml bidistilled water at

338 K. The resulting blue solution was cooled down to 313 K and added to the binary

MoTe solution. Finally, a solution of 7.35 g ammonium niobium oxalate (Sigma-Aldrich)

in 30 ml bidistilled water was added at 313 K to the previous mixture. The clear

quaternary solution was stirred for 10 min and spray-dried in a Büchi B-191 spray-dryer

at an inlet temperature of 423 K. The delivery rate of the pump and the aspirator were

tuned to an outlet temperature of 383 K. The spray-dried material was calcined in flowing

air at 548 K (heating rate 5 K/min) for one hour. The calcined mixed oxide was then

heated for two hours at 773 K (heating rate 5 K/min) and 20 MPa in the presence of

steam. The resulting solid was finally crystallized to M1 in flowing argon at 873 K

(heating rate 15 K/min) for another two hours (catalyst ID 3030). The atomic ratio of the

metals in the final catalyst, Mo/V/Te/Nb = 1/0.34/0.08/0.14, as determined by EDX, was

close to the nominal ratio, with the exception of the reduced tellurium content due to

evaporation of elemental Te during thermal activation of the catalyst

2.2 Sylilation of M1

Silylation of M1 has been performed by addition of 5.8 g HSi(OEt)3 (Sigma-Aldrich) to a

suspension of 0.5 g phase-pure M1 (catalyst 3030) in 100 ml toluol. The mixture was

kept for 16 hours at 384 K under reflux. Cross-linking of anchored silanol groups has

been achieved by treating the resulting solid in a mixture of 90 ml ethanol, 10 ml of

Chapter 3

bidistilled water and 1 ml concentrated sulphuric acid for four hours under reflux. The

two-step procedure has been repeated three times in order to assure complete silylation of

the entire M1 surface (catalyst ID 3159).

2.3 Activity measurements

For testing of the catalysts in selective oxidation of propane to acrylic acid, 0.5 g of each

material was pelletized (8 ton on a round, 3-cm-diameter surface) and sieved to 200–400

μm. This shape-forming procedure, referred to as mechanical treatment, causes partial

disruption of the needles, changing the size distribution of the primary particles in the

original M1 and making catalytic testing of completely silylated M1 impossible. But pure

silica (Aerosil 300) was shown to be inactive in propane oxidation under the reaction

conditions applied. The sieve fractions of the mechanically treated M1 and silylated M1

were loaded into quartz reactors with a diameter of 4 mm. The feed was composed of

propane/oxygen/nitrogen/steam in a molar ratio of 0.85/1.9/15.2/12. The reaction was

performed at atmospheric pressure, a GHSV of 4800 h−1, and a reaction temperature of

673 K. The products were analyzed by gas chromatography. A molecular sieve column

and a Porapak column coupled with a thermal conductivity detector were used to analyze

O2, N2, CO, CO2, and hydrocarbons (C1–C3). The oxygenated products were analyzed

with a HP-FFAP column coupled with a flame ionization detector. The detection limit of

the individual products depended on the detector used but was generally >0.2 vol.-%. A

mass balance of 100 ±10% was calculated.

2.4 Catalysts characterization

X-ray diffraction

Chapter 3

The phase-purity of the materials was verified by X-ray diffraction (XRD). The

measurements were performed with a STOE STADI-P transmission diffractometer

equipped with a focusing primary Ge (111) monochromator and a position sensitive

detector, using Cu-Kα1 radiation. For data analysis, the program Topas (v.2.1, Bruker

AXS) was used to fit the diffraction pattern of the activated materials.

Electron microscopy

The microstructure of the catalysts was investigated by TEM using a Philips CM 200

FEG transmission electron microscope operated at 200 kV, equipped with a Gatan CCD

camera for image acquisition and an EDAX Genesis EDX system. Morphology studies

and shape analysis were performed by SEM, using a Hitachi S-5200 with a PGT Spirit

EDX system and a Hitachi S-4800 with an EDAX Genesis EDX detector. EDX studies in

the SEMs were carried out with an accelerating voltage of 10 kV, with images acquired at

2 kV to optimize surface resolution. For TEM, the specimens were prepared by dry

dispersing the catalyst powder on a standard copper grid coated with holey carbon film.

For SEM investigations, the samples were deposited on carbon tape. The shape of the M1

needles was analyzed by measuring the length and diameter of more than 200 needles on

SEM images. Based on the average length and diameter of the needles, the total surface

area and the surface area of the (001) plane were calculated for an average needle for

each catalyst (Table 1). These calculations were done under the assumptions that the

material was 100% crystalline and each crystal was cylindrically shaped. Furthermore,

the cylinder was assumed to have a circular basal plane. A SEM-derived specific surface

area was calculated using the surface area of the average needle and the crystallographic

density of M1 (4.4 g/cm3 [8]).

Chapter 3

Nitrogen adsorption

The specific surface areas of the catalysts were measured using an AUTOSORB-1-C

physisorption/chemisorption analyzer (Quantachrome). Eleven points in the linear range

of the nitrogen adsorption isotherm measured at 77 K have been used to calculate the

BET surface area. Prior to adsorption, the samples have been degassed at 353 K for 2

hours.

High-Sensitivity LEIS

The Calipso LEIS uses a double toroidal analyzer, which combines a large acceptance

angle with parallel energy analysis of the backscattered ions. This gives orders of

magnitude higher sensitivity (HS-LEIS) than that of conventional LEIS equipment [20].

The required ion dose for analysis is so low that HS-LEIS analysis is essentially non-

destructive ("static").

The extreme surface sensitivity of LEIS requires that organic contaminations due to

transport and storage as well as carbonaceous deposits from the catalytic reactions are

removed from the catalysts before analysis. This was done with an oxygen atom source

(Oxford Applied Research, type MPD 21) which produces O-atoms of thermal energy. In

contrast to molecular oxygen and ozone, the chemical energy of these O-atoms enables

the complete removal of organic contaminations at room temperature without sputtering

the surface. All samples were cleaned in this way (200 W, 5 min) before analysis.

Prolonged treatment had no influence on the LEIS spectra, thus confirming the

cleanliness of the samples.

Calibration of the LEIS signals

Chapter 3

500 1000 1500 2000 2500 3000

100

150

200

Yield [Cts/nC]

Energy [eV]

V2O5

Nb2O5

MoO3

TeO2

500 1000 1500 2000 2500 3000

SiO2

V2O5

Yield [Cts/nC]

Energy [eV]

Figure 1. 3 keV He+ scattering.

References SiO2 and V2O5. Figure 2. 5 keV Ne+ scattering.

References V2O5 , Nb2O5 , MoO3 and TeO2.

It has been shown previously that roughness has a very small effect on the LEIS signals

[21]. Thus, the precise dispersion is not of great importance for a good reference for

calibrating the LEIS signals of the catalysts. Thus, high-purity powder samples of SiO2,

V2O5, Nb2O5, MoO3, and TeO2 were used as references to quantify the surface

concentrations of Si, V, Nb, Mo, and Te. Because the surfaces were cleaned with O

atoms, the cations in the outer surface were in their highest oxidation state (e.g., TeO3 for

Te). The LEIS spectra are given in Figures 1 and 2. The lighter elements (Si, V) were

quantified with 3 keV 4He+ ions; the heavier elements (V, Nb, Mo and Te), with 5 keV

20Ne+ ions. As can be seen in Figures 1 and 2, the SiO2 surface was contaminated with a

trace of iron oxide, whereas the TeO2 contained some cobalt or nickel oxide (mass 59).

Even with the heavier Ne ions, there was an overlap of the Nb and Mo peaks. The

elements are neighbors in the periodic table, and their isotopes (Nb: 93; Mo: 92–100)

overlap. For the catalysts, the relative concentrations of Nb and Mo were determined by

Chapter 3

curve fitting. The large width of the Te peak originates from isotopic broadening (atomic

mass 122–130). Because these masses are very different from those of the other elements,

Te can be readily quantified. V can be analyzed using both He and Ne ions, and the two

analyses give very similar results. The values given for V are the average of the two

measurements. LEIS calibration gives the fractions of the surface that are composed of

the various oxides. To obtain atomic concentrations, these fractions must be corrected for

the surface densities of the cations in the references. These surface densities have been

estimated as (ρ·NAv/M)2/3, where ρ is the bulk density, NAv is Avogadro’s number, and

M is the molar mass. It has been shown previously [22] that this estimate agrees closely

with the estimate from the surface unit-cell taking the close-packed surface plane, which

is dominant on powders. For V2O5, Nb2O5, MoO3, and TeO3 these densities are 9.96,

9.36, 7.28, and 7.68 (×1014) metal atoms/cm2, respectively.

3 Results

3.1 Silylation of M1

The surface of a MoVTeNbOx mixed oxide composed mainly of the M1 phase has been

covered with a layer of silica by silylation using HSi(OEt)3. The silylation was optimized

using LEIS. Figure 3 and Figure 4 show the LEIS spectra of the catalyst before and

after silylation according to the process described in Section 2.2. The 3 keV He+

spectrum before silylation shows the peaks for O, V and a combined peak for Nb, Mo and

Te. In addition, some Na contamination is observed. The 5 keV Ne+ spectrum gives a

better mass resolution for the heavier elements. It shows the peaks for V, Te and a broad

peak due to Nb and Mo. After silylation, only the peaks for O and Si (He spectrum) are

Chapter 3

500 1000 1500 2000 2500 3000

Nb/Mo/Te

Yield [Cts/nC]

Energy [eV]

silylated

silylated x 10

nonsilylated

silylated x 10

500 1000 1500 2000 2500 3000

100

150

Yield [Cts/nC]

Energy [eV]

nonsilylated

silylated

Nb/Mo

Figure 3. 3 keV He+ scattering.

Non-silylated, silylated and silylated x 10.

Figure 4. 5 keV Ne+ scattering.

Non-silylated and silylated.

present. No peaks are observed in the Ne spectrum. This confirms the successful

complete silylation of the catalyst.

In addition to the peaks in the He spectrum there is a background extending to higher

energies. This background is due to ions that were backscattered by Nb, Mo/Te in deeper

layers. The most likely process being that the ions were neutralized at the first interaction

with the surface, lost some energy along the ingoing (straggling) trajectory, backscattered

in a hard binary collision with a Nb, Mo, or Te atom, and lost again some energy on the

straggling way back to the surface. Since in LEIS only backscattered ions are detected, it

is necessary that the backscattered He atoms are reionized in an interaction with a surface

atom, just before leaving the surface. For 3 keV He+ ions scattered by this type of oxides

the straggling energy loss contributes about 160 eV for backscattering from a depth of 1

nm. The shape of the 10x magnified background thus gives information on the thickness

distribution of the silica coating. It shows that a small fraction of the coating is only just

Chapter 3

10 20 30 40 50 60

(c)

2 θ

(a)

Intensity [a. u.]

(b)

(d)

(e)

Figure 5. XRD patterns of (a) as synthesized M1, and (b) calculated patterns

assuming phase-pure M1 [8]; XRD patterns of (c) M1 after propane oxidation,

and of silylated M1 (d) before, and (e) after propane oxidation.

covered (1-2 atoms thick), while the maximum thickness is about 2 nm. Consistently,

TEM (see below) shows that the silica layer is not homogeneous and its thickness varies.

3.2 Phase composition of the Mo-V-Te-Nb oxides

In regard to the bulk structure, a material of high phase purity has been exposed to the

silylating agent (Figure 5a), as it is evident from comparing the measured XRD patterns

with the patterns calculated under the assumption that the solid is composed exclusively

of M1 [8] (dotted line, Figure 5b). Within the method´s range of accuracy, the material is

phase-pure. The phase composition of the mixed oxide remained unchanged after

catalytic reaction (Figure 5c) as well as after the silylation procedure (Figure 5d, and e).

The materials are highly crystalline, which is also confirmed by TEM.

Chapter 3

(

)

(

)

(

)

(

)

Figure 6. SEM images of phase-pure M1 (a) before and (b) after propane

oxidation; SEM images of silylated M1 (c) before and (d) after propane oxidation.

3.3 Microstructure of the model catalysts

The typical needle-shape morphology of the original phase-pure M1 material is observed

in the SEM image shown in Figure 6a. Only very few particles are found that clearly

differ in morphology indicating the presence of traces of other phases not detectable by

XRD, or minor amounts of amorphous fractions. The needle-shape morphology is

maintained after catalytic test (Figure 6b) and after silylation (Figure 6c). In the latter

material, some spherical particles are observed that have been assigned to pure SiO2 by

EDX analysis. The needle-like shape of the silylated M1 particles (Figure 6c) is also not

substantially affected by propane oxidation (Figure 6d).

Shape analysis of the original M1 catalyst revealed a broad distribution of the needle

length with a maximum between 100 and 400 nm (Figure 7a). The abundance of shorter

needles, especially those with length shorter than 400 nm, clearly increases after propane

oxidation (Figure 7b), which is attributed to breakage of the needles due to the

Chapter 3

0 200 400 600 800 1000 1200

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

a) as synthesized M1

b) M1 after propane oxidation

c) Silylated M1 after propane oxidation

Frequency

Length [nm]

Figure 7. Distribution of the needle length (a) in as synthesized phase-pure M1,

(b) in M1 after propane oxidation, and (c) in silylated M1 after propane oxidation.

mechanical manipulation during preparation of the sieve fraction before the catalytic test.

A similar length distribution has been observed with the silylated catalyst after the

catalytic reaction (Figure 7c), indicating a comparable impact of mechanical treatment

for silylated and non-silylated M1.

Table 1 summarizes mean needle length, mean needle diameter and mean

diameter/length ratio of the needles obtained by shape analysis of about 300 needles for

the original M1 material, the M1 catalyst after propane oxidation and the silylated M1

after the catalytic testing. As the M1 needles are naturally crystals with a preferred

growth direction perpendicular to the {001} planes, a preferential breaking perpendicular

to the length axis would be expected during mechanical treatment. This preferential

breaking is reflected in a slight increase of the mean diameter/length ratio from 0.56 for

the original M1 catalyst before propane oxidation to 0.59 and 0.61 for the nonsilylated

Chapter 3

and the silylated M1, respectively, after propane oxidation. Assuming that all needles

have cylindrical geometry with circular basal planes and mean geometric parameters

(diameter, length) as given in Table 1, the total surface area and the surface area of the

(001) planes of an average needle have been calculated for the three model catalysts

(Table 1). Taking into account a unit cell density of M1 of 4.4 g/cm3 [8], specific surface

areas can be calculated that agree well with the BET surface areas measured (Table 1),

confirming high crystallinity of the materials.

If each needle was disrupted once, an increase in surface area of about 20% is expected

from the cylindrical geometry. The actual increase in BET surface area of the

nonsilylated M1 before and after propane oxidation of 13% agrees well with the increase

in specific surface area of 14% calculated based on the shape analysis, confirming the

reliability of the method. The BET surface area of the silylated M1 was not measured

since falsification due to the presence of silica particles was expected. However,

considering the similar size distribution of nonsilylated and silylated needles after the

catalytic testing (Figure 7), an increase in surface area of 13% could be expected for the

silylated catalyst as well.

In summary, combined shape and BET surface area analysis clearly indicate a

preferential disruption of the M1 needles perpendicular to the [001] direction by

mechanical treatment before catalytic testing. Comparing nonsilylated and silylated M1

needles, the similar impact of the mechanical treatment on particle morphology provides

further evidence for the applicability of the present approach.

Chapter 3

Table 1

Results of particle shape analysis based on SEM images and BET surface areas measured by nitrogen adsorption

Catalyst

Number

particles

Mean

length

(nm)

Mean

diameter

(nm)

Mean

aspect

ratio3

BET

surface

area

(m2/g)

Calculated3

total surface

area of one

needle (nm2)

Calculated3

surface area of

the (001) plane of

one needle (nm2)

Calculated3

specific surface

area (m2/g)

M11 379 336±189 184

99 0.56 6.7 2.47 105 0.53 105 6.3

M12 241 277±119 164

85 0.59 7.6 1.85 105 0.42 105 7.2

silyl. M12 298 275±114 167

71 0.61 n. d. 1.88 105 0.44 105 7.1

1 M1 as synthesized

2 catalyst after propane oxidation

3 calculation under consideration of mean diameter, mean length and under assumption of cylindrical geometry with circular basal plane

Table 2

Catalytic properties of M1 and silylated M1 in propane oxidation to acrylic acid (AA) after 32 h time on stream at 673 K

Catalyst XC3H8

[%]

SAA

[%]

YAA

[%]

Activity

[mmolC3H8/h·gcat] Activity

[mmol C3H8/h·m2exp.surf.] Formation rate AA

[mmolAA /h·gcat] Formation rate AA

[mmolAA /h·m2exp.surf.]

M1 49 73 36 2.94 0.39 2.15 0.28

Silylated

M1 9 100 9 0.54 0.24 0.54 0.24

Chapter 3

100

(b)

(c)

Figure 8. HRTEM images of (a) a silylated M1 needle disrupted along the (001)

plane, of (b) a completely silylated M1 needle and of (c) a silylated M1 needle with

silica scratched from the sides.

Provided that no silica is removed from the sides of the needles, the new surface area

formed by mechanical treatment of completely silylated M1 is mainly composed of (001)

planes (scheme 1). Needles that exclusively expose the (001) plane, have been detected

by high-resolution transmission electron microscopy in the silylated M1 catalyst after the

catalytic test reaction (Figure 8a). The image in Figure 8a shows one needle with the

sides covered by silica and with the cross-section plane free of SiO2 confirming that the

needles can break perpendicular to the length axis. Furthermore, needles completely

covered by a layer of silica are also observed (Figure 8b), indicating that not every

Chapter 3

101

needle is broken. This is in agreement with the results of shape analysis and nitrogen

adsorption. The possibility of partial removal of silica from the sides of the needles

cannot be excluded, as evidenced by the TEM image shown in Figure 8c.

3.4 Catalytic properties of the model catalysts in propane oxidation

Pure silica shows no catalytic activity in propane oxidation under the reaction conditions

applied. Thus, completely silylated M1 is expected to be inactive. But, experimental

verification of this statement for a completely silylated model catalyst is impossible.

Mechanical manipulation of the catalyst by pressing and sieving is required to prepare a

sieve fraction before catalytic testing. This causes partial damage of aggregates and even

of the primary particles, resulting in exposure of a new MoVTeNbOx surface in both the

silylated as well as in case of the nonsilylated reference catalysts (Scheme 1).

Table 2 compares the catalytic performance of the two mechanically treated catalysts.

Propane oxidation was performed in parallel reactors applying the same temperature,

space velocity, and feed composition. Under steady-state conditions, the conversion of

propane is fivefold greater over the reference M1 than over the silylated model catalyst.

Whereas only acrylic acid is formed over silylated M1, the selectivity is lower for the

reference material producing 12% CO2 and 15% CO as byproducts. These differences in

selectivity most likely are related to the different propane conversion levels. The carbon

oxides may be formed in consecutive reactions, thus lowering the selectivity to acrylic

acid over the reference M1. Table 2 gives average rates. Due to the high conversion of

the nonsilylated reference catalyst, the average consumption rate of propane is

underestimated for this catalyst. Nevertheless, it is fivefold higher than that of the

silylated catalyst. The greater MoVTeNbOx surface area that is exposed to the reactants

Chapter 3

102

500 1000 1500 2000 2500 3000

Co/Ni

Nb/Mo

Yield [Cts/nC]

Energy [eV]

nonsilylated

silylated

500 1000 1500 2000 2500 3000

Nb/Mo/Te

Yield [Cts/nC]

Energy [eV]

nonsilylated

silylated

Figure 10. Background-subtracted 5 keV 20Ne+

spectra from catalytically tested samples

Figure 9. Background-subtracted 3 keV 4He+

spectra from catalytically tested samples.

in case of the nonsilylated reference catalyst is a possible explanation for this finding.

The surface area of the reference M1 was measured by nitrogen adsorption (Table 1).

LEIS was used to determine the accessible MoVTeNbOx surface area of the silylated

catalyst, as described in the following section.

3.5 LEIS after catalysis

To properly compare between the catalysts, the silylated and nonsilylated samples were

both investigated with LEIS after the same mechanical treatment (pelletizing, crushing,

sieving) and catalytic testing (see Scheme 1). Figures 9 and 10 show the He and Ne

spectra after background subtraction. Due to partial disruption of the needles, significant

signals can be seen for V, Nb, Mo, and Te in the silylated sample. Again, some Co/Ni

contamination is present. The surface fractions of the elements in Figures 9 and 10 can

be quantified using the reference spectra in Figures 1 and 2. It is found that the sum of

the surface fractions of V2O5, Nb2O5, MoO3 and TeO3 in the treated silylated sample

amounts to 30% of that in treated nonsilylated sample. The other part of M1 (70%)

Chapter 3

103

remains covered by SiO2. The 30% thus represents the new surface generated by the

mechanical treatment.

To obtain the atomic surface concentrations, the surface fractions of the oxides must be

corrected for the differing metal atoms/cm2 in these oxides. Table 3 gives the atomic

compositions (in parentheses normalized to Mo) for the treated nonsilylated sample and

the new surface of the silylated sample, along with the results of the bulk analyses by

EDX for comparison. In agreement with earlier studies [15], the chemical composition of

the outermost surface layer differs significantly from that of the bulk. The LEIS

measurement of the M1 reference provides important information, including the basal

plane and the lateral surface, showing an enrichment of tellurium at the expense of

vanadium. The situation on the freshly formed surface of the silylated model catalyst,

which is dominated by the (001) plane, is different, with an increase of tellurium at the

expense of molybdenum. No depth profile and no differences between the catalysts with

respect to the Nb concentration are noted. Figure 11 also illustrates the clear differences

Table 3

Metal content of the bulk (EDX) and topmost layer (LEIS) in at.-%. The numbers in

parenthesis represent molar ratios of the metals normalized to molybdenum

Catalyst Method Mo V Te Nb

M11 EDX 64 (1) 22 (0.34) 5 (0.08) 9 (0.14)

LEIS

M12 EDX 64 (1) 20 (0.31) 6 (0.09) 10 (0.16)

LEIS 65 (1) 12 (0.18) 12 (0.18) 11 (0.17)

silylated M11 EDX 63 (1) 22 (0.35) 6 (0.09) 9 (0.14)

LEIS 0 0 0 0

silylated M12 EDX 62 (1) 21 (0.34) 6 (0.09) 11 (0.18)

LEIS 55 (1) 20 (0.37) 15 (0.27) 10 (0.17)

1 as synthesized

2 catalyst after propane oxidation

Chapter 3

104

500 1000 1500 2000 2500 3000

0.0

0.2

0.4

0.6

0.8

1.0

Co/Ni

Nb/Mo

Yield [Cts/nC]

Energy [eV]

nonsilylated

silylated

Figure 11. Normalised 5 keV 20Ne+ spectra (Mo=1.00) from catalytically tested

sample (after subtraction of background).

in the surface compositions, with the Ne spectra normalized on the maximum of the

Nb/Mo peak.

4 Discussion

It has been well documented that the M1 phase occurring in MoVTeNb mixed oxide

catalysts possesses a key function in selective oxidation and ammoxidation of propane [6,

14, 23]. The crystal structure hosts four different metals, three of them with variable

oxidation state, meeting the requirements for catalyzing redox reactions by exposing or

bearing structural arrangements at the surface that easily meet the demands of propane

activation and the transfer of 8 electrons in its selective oxidation to acrylic acid or in its

ammoxidation to acrylonitrile, respectively. Surface and sub-surface reduction and

reoxidation of M1 crystals without structural collapse seem to be possible [24]. The

Chapter 3

105

occupancy of the 13 metal sites in the unit cell of the crystal structure is variable [9, 10,

14]. Accordingly, the catalytic properties may be controlled by synthesis or post-

treatment procedures. Crystallinity seems to be required to obtain catalytic activity for

selective (amm)oxidation of propane [6, 25, 26]. The possible role of defects remains an

open question [24, 27]. Typically, nanocrystalline M1, composed of fine needles of a few

hundred nanometers in length, exhibits excellent catalytic behavior.

The phase-pure M1 synthesized in the present study exhibits the corresponding

morphology and has very small amounts of impurity phases or amorphous fractions, as

evidenced by XRD (Figure 5) and electron microscopy (Figure 6a). Compared with

published M1 stoichiometries, (e.g., Mo1V0.15Te0.12Nb0.13Ox [8] or Mo1V0.23Te0.11Nb0.14Ox

[9]), the present material is characterized by increased V content and a reduced Te

content (Table 3), which might be the reason for its exceptional catalytic behavior (Table

2). The average formation rate of acrylic acid measured in the stationary state after 32

hours on stream is 2.15 mmol/h·gcat, which exceeds results reported for M1 in the

literature [28]. Due to its phase purity and catalytic activity the present material was

chosen to prepare a model catalyst preferentially exposing the (001) plane. Based on

grinding experiments, and under the assumption that the structural characteristics and the

chemical composition of the surface do not differ from the bulk even under reaction

conditions, it was previously concluded that the catalytically active sites in propane

oxidation [11, 12] or ammoxidation [14, 26] are located on terminating (001) planes. This

assumption is not consistent with room temperature LEIS measurements that give an

indication of gradients in elemental composition between surface and bulk of M1 [29].

Surface texturing of catalyst particles also has been revealed by high-resolution TEM on

Chapter 3

106

the surface of high-performing MoVTeNbOx catalysts that have been leached with

solvents [27]. In any case, a well-defined and rigid arrangement of structural units may be

rather unlikely at reaction temperature and in the presence of steam in the feed.

Exploring this question by in-situ spectroscopic methods is quite challenging, however.

Thus, in the present study we used propane oxidation itself as a probe reaction to

ascertain the specific catalytic activity of the (001) plane compared with the integral

activity of the entire M1 surface. For this purpose, the M1 crystals were covered

completely by a thin layer of silica. HS-LEIS confirms coverage of the whole mixed

oxide surface (Figure 4). The thickness of the layer ranges from a few monolayers to

maximum 20 nm, as verified by LEIS and TEM. Neither the particle size distribution of

the original M1 (Table 1) nor its chemical composition (Table 3) is affected by the

silylation procedure. But the powders were pelletized, crushed, and sieved before being

introduced into the catalytic test reactor. Such a gentle mechanical treatment leads to

partial disruption of needles and formation of fresh MoVTeNbOx surface. For the

silylated M1, the newly emerging MoVTeNb oxide surface was veryfied by HS-LEIS

(Figures 9 and 10). For the pure, nonsilylated M1, these morphological changes should

be reflected in an increased specific surface area as measured by nitrogen adsorption.

Under the assumption that the mechanical treatment leads exclusively to a breakage of

needles perpendicular to the [001] direction, the difference corresponds to the surface of

newly formed (001) planes. In the present experiment, the difference in the specific

surface area is 13.4% (Table 1). A quite similar result (14.3%) is obtained if the mean

particle size determined based on the shape analysis by SEM and the bulk density of M1

(4.4 g/cm3 [8]) are used to calculate the increase in surface area of pure, nonsilylated M1

Chapter 3

107

(Table 1), satisfactorily confirming the reliability of the shape analysis and

demonstrating that the mechanical treatment applied is associated mainly with breakage

of needles and not with other damage. The relevance of shape analysis is important,

because in the silylated catalyst, BET surface area measurements cannot be relied on for

calculation due to an expected increase in surface area caused by the possibly not smooth

SiO2 layer and the occasional formation of pure silica particles.

Both the original M1 needles and the silylated needles have a low probability of breaking

(Table 1, Figure 7), and long needles break more frequently than short needles (Figure

7). Figure 7 confirms that nonsilylated and silylated needles break in a similar way,

because the two materials have essentially the same particle size distribution after the

catalytic test. For the silylated catalyst, the increase in surface area (i.e., the yield in

newly formed basal plane surface area) corresponds to 12.7% when the mean particle size

determined by shape analysis is used for the calculation (Table 1).

In summary, based on shape analysis and/or nitrogen adsorption before and after catalytic

test, the breakage of the M1 needles benefits in newly formed (001) surface area of

approximately 15% both for original and silylated M1.

As an independent method, LEIS analysis was applied to determine the freshly formed

MoVTeNbOx surface of the silylated catalyst. This fraction corresponds to 30% of the

surface of the original M1. This value is twice as high as the increase calculated from

shape analysis, implying that more than silica-free (001) planes were generated. HRTEM

(Figure 8c) shows that some silica was scratched from the sides of the needles as well.

Assuming that the approximate 15% increase in surface area calculated from shape

analysis (Table 1) corresponds exclusively to the newly formed silica-free (001) planes,

Chapter 3

108

the ratio of the basal plane area versus other MoVTeNbOx surface area (i.e., lateral

surface area) is about 1 in the silylated M1. In the original M1, the entire surface of the

needles is exposed to the gas phase during propane oxidation and, thus the ratio of (001)

surface area to lateral surface area corresponds to 0.25 (Table 1). Consequently, even

though other surface planes also are to some extent exposed to the reactants in the

silylated M1 during propane oxidation, we can conclude that the silylated, mechanically

treated M1 catalyst represents a suitable model system for studying the catalytic

properties of the (001) plane, since half of the exposed MoVTeNbOx surface is composed

of (001) planes.

Table 2 compares the catalytic performance of M1 and silylated M1. As expected, the

conversion of silylated M1 is much lower than that of M1, because less MoVTeNbOx

surface is accessible in the former catalyst. Based on the information obtained from the

HS-LEIS study indicating that actually 30% more MoVTeNbOx surface is available in

the silylated catalyst, consumption rates of propane normalized to the area of exposed

MoVTeNbOx surface were calculated considering 7.6 m2/g MoVTeNbOx surface for M1

(BET surface area given in Table 1) and a 2.3 m2/g MoVTeNbOx surface for silylated

M1 (0.30 x 7.6 m2/g). The rates given are quite similar for both catalysts, meaning that

the intrinsic catalytic properties of the entire M1 surface are similar to the catalytic

behavior of a surface composed of 50% (001) planes. This finding cates some doubt on

the uniqueness of the basal plane of M1.

The result is especially surprising since the chemical composition of the newly formed

M1 surface of the silylated catalyst (which can be regarded at least partially, as the inner

surface of M1) has a different chemical composition than the average surface

Chapter 3

109

composition of M1, as shown by the LEIS experiment. Table 3 compares the bulk

composition measured by EDX and the surface composition measured by HS-LEIS.

Whereas the EDX bulk compositions of the samples are almost the same, confirming that

the silylation procedure does not affect the bulk composition, the HS-LEIS surface

composition of the newly-generated surface shows relatively large differences,

specifically an increased concentration of tellurium at the expense of molybdenum. But,

prior to the present LEIS measurements, a mild pre-treatment with oxygen atoms at room

temperature was necessary to eliminate environmental contamination from the reaction,

transport, and storage. This procedure, which converts all the surface metal ions into their

highest oxidation states, may be connected with structural changes. Although the local

structure is affected, it is unclear whether the elemental composition changes. However,

the surface reconstruction holds for the entire surface of M1, thus justifying a comparison

of the basal plane with the total surface in terms of elemental composition.

In any case, these uncertainties do not affect the findings that the intrinsic catalytic

properties of the basal plane of M1 in the selective oxidation of propane to acrylic acid do

not differ much from those of the lateral surface of the M1 needles. If we accept this

result, then the question arises as to why increased catalytic activity and improved

selectivity to acrylic acid were observed in previous studies after grinding of M1 [11, 12].

The details of the grinding procedure were not given in those reports; however, most

likely, the mechanical treatment was less gentle than that in the present study and

probably generated a more defect-rich material along with extra basal planes. Another

possible explanation could be that contaminations, such as residues from the preparation

procedure, which are common on catalyst surfaces, were removed by extended grinding.

Chapter 3

110

Breaking the needles and scratching deposited impurities away from the sides of the

needles would expose fresh, uncontaminated, thus active MoVTeNb oxide surface.

5 Conclusions

The model studies presented here suggest that a distinguished lattice plane of the M1

crystal structure, the (001) plane, is not solely responsible for its outstanding catalytic

activity and selectivity in the partial oxidation of propane to acrylic acid. The chemical

and structural natures of the active ensembles on the catalyst surface remain unknown.

The unique crystal structure of the M1 phase certainly plays an essential role in the

selective (amm)oxidation of propane, because amorphous material or MoVTeNb oxide

comprising different phases cannot compare with phase-pure crystalline M1. All

experimental experience to date suggests that this may differ from the situation for

related mixed oxides, such as nanostructured MoVW oxides, which have been reported to

be active in propylene oxidation in a semicrystalline state [30]. Similar intrinsic reactivity

irrespective of the terminating lattice plane implies that related active ensembles of

metal-oxo clusters are exposed to the reactants on the entire surface of the M1 needles.

The lateral surface of the needles accounts for the main part of the surface area of M1

(80%). The stepped morphology of the latter surface may generate similar metal-oxo

arrangements as on the surface of the basal plane of the needles. HR-TEM studies

together with targeted synthesis of M1 showing different microstructure are currently in

progress to verify this assumption.

Chapter 3

111

6 Acknowledgments

The authors thank Dr. Olaf Timpe for helpful discussions, Dr. Frank Girgsdies for

performing phase analysis of the catalysts, Edith Kitzelmann for conducting the XRD

measurements, and Kilian Klaeden, and Gisela Lorenz for carrying out the nitrogen

adsorption measurements.

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Chapter 4

114

CHAPTER 4- New synthesis routes of MMO catalysts by dilution of Mo-V-X-

Nb (X=Te, Bi, and P) mixed oxides with SiO2, Cr2O3 or ZrO2 for the oxidation

of propane to acrylic acid.

Abstract

Metal substitution and/or dilution of the up-to-date most active catalyst MoVTeNbOx in

the selective oxidation of propane to acrylic acid was investigated in an exploratory

study. The structural consequences of (i) substituting the “in-channel” element (Te vs. P

or Bi), and (ii) diluting the multimetal oxide (MMO) by using SiO2 (Aerosil 300), ZrO2,

and Cr2O3 as diluents, were studied applying a parallel screening approach. Hydrothermal

synthesis was used for preparation of the MMO catalysts.

The hydrothermal synthesis of the undiluted Mo-V-X-Nb (X=Te, P, Bi) materials

resulted in the formation of crystalline catalysts composed of various phases. The

catalytically relevant M1 phase was only obtained in case of the Mo-V-Te-Nb oxide.

Cr2O3 as diluent did not interfere in phase formation, while SiO2 prevented the formation

of the M1 phase in the diluted Mo-V-Te-Nb oxide. ZrO2 reacted with molybdenum oxide

under formation of a more stable zirconium molybdate.

The undiluted Mo-V-Te-Nb oxide catalyst, mainly composed of the M1 phase, showed

the highest activity and the highest formation rate of acrylic acid normalized to the

specific surface area of the catalyst. Unexpectedly, all materials showed substantial

catalytic performance, irrespective of their chemical and phase composition. These

results underline on one side the exceptional role of the M1 phase but also prove that this

phase is not essential for the selective oxidation of propane to acrylic acid. Moreover, the

observations presented in this work support the hypothesis that the active sites on the

surface of crystalline M1 can be associated with high-energy sites created from the bulk

under reaction conditions.

Keywords: multi-metal oxide catalysts, Mo-V-Te-Nb oxide, tellurium, phosphor,

bismuth, SiO2, ZrO2, Cr2O3, propane oxidation, M1 phase, BiVO4

Chapter 4

115

1 Introduction

Catalysts for alkane oxidation are usually complex multimetal oxides (MMO)

containing molybdenum and vanadium oxide as essential constituents [1-3]. Particularly

for selective oxidation of propane to acrylic acid, mixed oxides of molybdenum,

vanadium, niobium, and tellurium have been identified in extensive compositional

searches as the most promising catalysts [3], reaching yields of acrylic acid of about 50

%. Currently, acrylic acid is industrially produced in a two-step oxidation process starting

from propene [4-5] over promoted bismuth molybdate and Mo-V mixed oxide catalysts

achieving yields up to 86 %. Despite the lower price of propane compared to propene, the

yield of acrylic acid in the direct oxidation of propane is still below the profitability

threshold at actual commercial conditions. Thus, further improvement of catalysts for

selective activation of alkanes is desired.

MoVTeNbOx catalysts mainly consist of two orthorhombic phases called “M1” and

“M2” [6]. The structural and chemical complexity of M1 seems to cope with the

challenging task of propane activation [7, 8], while M2 is regarded to be active and

selective in oxidation of propene to acrylic acid [7, 9]. The M2 phase is richer in

tellurium than M1. It was postulated that Te is an active element in propene conversion

[8], but its beneficial effect found empirically may also be related to the generation of Te-

free active sites. Specifically, tellurium seems to play a critical role with respect to phase

formation. It was assumed that Te acts structure-directing and structure-stabilizing

regarding the M1 phase. However, Ueda and co-workers showed that a Mo-V oxide with

a related orthorhombic structure may also be obtained by crystallization in inert

atmosphere at 773 K in absence of tellurium [10], proposing that NH4+ serves as

Chapter 4

116

structure-directing component [11]. A structure stabilizing role has been attributed to Te

based on the observation that the precursor material of the ternary Mo-V-Te oxide treated

at 873 K crystallizes in the desired orthorhombic structure, whereas the binary Mo-V

system decomposes into other phases at such high temperatures [10-12]. Orthorhombic

MoVOx and MoVTeOx systems show similar activity in the partial oxidation of propane

to acrylic acid. However, the tellurium containing catalyst is significantly more selective

to acrylic acid [11-12]. Therefore, tellurium seems to play an essential role in view of the

specific catalytic properties of these catalysts. However, the concentration of tellurium on

the surface under operating conditions is poorly defined. Another disadvantage consists

in the volatility of elemental tellurium at temperatures higher than 723 K that causes

substantial Te loss during thermal activation of the catalyst and contamination of reactors

and exhaust systems. The substitution of this element is, therefore, not only of interest for

a deeper understanding of the role of tellurium in catalysis, but also for the prevention of

tellurium contamination during catalyst preparation and operation.

Recently, tellurium has been substituted by a number of metals in hydrothermally

synthesized Mo-V-X ternary oxides with either X=Al, Fe, Cr, Ti [13], or X=Al, Ga, Bi,

Sb, Te [14] and Mo-V-X-Nb quaternary oxides with (X=Te, Sb) [15]. The orthorhombic

M1 structure was achieved only through substitution of Te by Sb in the ternary as well as

in the quaternary system [14, 15]. However, none of the substituted catalysts reached the

efficiency of tellurium-containing materials. Tellurium was also partly replaced by

cesium in the M2 structure. The maximum tellurium substitution was 30 % [9]. With

such a catalyst, improved activity for the ammoxidation of propene to acrylonitrile was

observed.

Chapter 4

117

Dilution of the active metal oxides could also be an option for improving the productivity

of multi-metal oxide catalysts. Thus, dispersion of M1 crystallites on a support resulting

in an increased number of active sites exposed to the gas phase might be achieved.

However, enhanced dispersion of the active phase does not always compensate the

dilution in terms of activity per unit surface area [16]. Furthermore, dilution of the

catalyst is also highly interesting for preventing the occurrence of “hot spots” under

reaction conditions resulting in sintering and phase transformations, such as the formation

of the detrimental but stable α-MoO3. Dispersion of the active MMO crystals by dilution

impedes the growth of crystallites increasing in this way the resistance of the catalyst

under reaction conditions. Silica, alumina, titania, zirconia, and niobium oxide have been

studied as diluents or supports for multimetal oxides in selective oxidation reactions [16-

18]. MCM41 was used to prepare highly dispersed Mo-V-Te oxide catalysts for direct

oxidation of propane to acrolein [19]. In all these cases, the MMO were prepared by

precipitation and evaporation.

Hydrothermal synthesis is an approved preparation method for crystalline Mo-V-Te-Nb

mixed oxides. In the present exploratory study, hydrothermal synthesis was applied to

substitute tellurium with phosphor or bismuth both in presence and in absence of silica,

zirconia, or chromia as diluents. The consequences on phase formation and catalytic

properties are studied. Based on the extended parameter field, the function of crystallinity

in selective oxidation of propane to acrylic acid is discussed. Although Mo-V-Te-Nb

oxide containing the M1 phase was still proven to be excellent, alternative compositions

with great potential for optimization are disclosed.

Chapter 4

118

2 Experimental

2.1 Catalysis preparation

Diluted and undiluted Mo-V-X-Nb (X=Te, Bi, P) mixed metal oxides were prepared by

hydrothermal synthesis in a high-throughput (HT) autoclave system. The HT autoclave

system is composed of 12 parallel reactors with volume of 140 ml each. The

hydrothermal synthesis was carried out at 448 K applying reaction times between 10

minutes and 24 hours. SiO2 (Aerosil 300, Degussa), Cr2O3 (Merck) and ZrO2 (MEL

Chemicals) were used as diluents. MoO3 was purchased from Merck and all other

chemicals were purchased from Aldrich and used without further purification.

a) Mo-V-X-Nb-O (X=Te and Bi) mixed oxides

Initially, 2.59 g of molybdenum trioxide (MoO3) were dispersed in 45 ml bidistilled

water at 353 K. Afterwards, 0.95 g of telluric acid (Te(OH)6 ) or 1.65 g of bismuth nitrate

(Bi(NO3)3), respectively, were added to this slurry. Separately, 0.5 g of vanadium

pentoxide (V2O5) was dispersed in 22.5 ml bidistilled water at 353 K. The latter slurry

was added to the first mixture and stirred for 5 minutes. Finally, 0.94 g of ammonium

niobium oxalate ((NH4)[NbO(C2O4)2(H2O)2]·3H2O) was dissolved in 22.5 ml bidistilled

water at 353 K and added to the previous mixture. The resulting molar ratio of the active

metal oxides corresponds to the stoichiometry Mo1V0.3X0.23Nb0.125, X=Te, Bi. The slurry

was then stirred for 10 minutes at 353 K and the diluent was added. The amount of

diluent comprised 50 wt.-%, based on mass of the stoichiometric oxides.

b) Mo-V-P-Nb-O mixed oxide

12-Molybdophosphoric acid, H3(PMo12O40)·nH2O, was synthesized by dissolving 103.64

g MoO3 using 4 ml phosphoric acid (85 %, ρ = 1.7 kg/l) in bidistilled H2O and filling up

Chapter 4

119

with bidistilled H2O to a total volume of 1 l. Thereafter, 38.8 ml of the synthesized

molybdophosphoric acid solution and another 0.35 ml H3PO4 were mixed and further

diluted with bidistilled water to a total volume of approximately 45 ml. Then, the solution

was heated to 353 K. Separately, 0.5 g vanadium oxide (V2O5) was dispersed in 22.5 ml

bidistilled water at 353 K. The latter mixture was added to the first solution and stirred

for 5 minutes. Finally, 0.94 g of ammonium niobium oxalate ((NH4)[NbO(C2O4)2(H2O)2]

·3H2O was dissolved in 22.5 ml bidistilled water at 353 K and added to the previous

mixture. The resulting molar ratio of the active metal oxides corresponds to the

stoichiometry of Mo1V0.3P0.27Nb0.125. The slurry was stirred for 10 minutes at 353 K and

then the diluent was added in a mass ratio of 1:1 with respect to the weight of the mixture

of the stoichiometric oxides.

Subsequently, the slurries were introduced into the autoclaves. Residual air was replaced

by bubbling nitrogen for 5 minutes through the suspension. After hydrothermal synthesis,

the resulting gel was filtered, washed and dried at 353 K for 16 hours, resulting in the

precursor materials. An identical synthesis procedure was followed to prepare reference

Mo-V-X-Nb (X=Te, P, Bi) mixed oxide catalysts without diluent. Starting from the

precursors, the final catalysts were obtained by heat treatment in inert gas with a flow of

50 ml/min for 2 h at 873 K (undiluted precursors) or 923 K (diluted precursors), heating

rate 15 K/min, with preceding calcination in air for 1 h at 548 K (undiluted precursors) or

598 K (diluted precursors), heating rate 10 K/min. Sample denomination and preparation

parameters are summarized in Table 1.

Chapter 4

120

Table 1

Characteristics of MoVXNbO – Y mixed oxides (X=Te, Bi, P; Y=Aerosil 300 (A), ZrO2 (Z),

Cr2O3 (Cr))

Notation

(precursor

/catalyst)

X Y

Hydrothermal

synthesis time

[h]

Phases composition (XRD)

Precursor Catalyst

TeA-1/6 992/1151 Te

Aerosil

300 1/6

α-MoO3

V2O5

SiO2

TeA-1/2 988/1143 Te

Aerosil

300 1/2 α-MoO3

SiO2

TeA-2 986/1139 Te

Aerosil

300 2 α-MoO3

SiO2

TeA-24 1034/1183 Te

Aerosil

300 24

peak at 22º

traces of α-MoO3

SiO2

Mo5-x(V/Nb)xO14

PA-1/6 954/1128 P

Aerosil

300 1/6 unknown phase

SiO2

PA-1/2 988/1143 P

Aerosil

300 1/2 unknown phase

SiO2

PA-2 987/1141 P

Aerosil

300 2 unknown phase

SiO2

PA-24 1040/1135 P

Aerosil

300 24 unknown phase

SiO2

MoOPO4

(weak peaks)

amorphous

TeZ-1/6 1013/1167 Te ZrO2 1/6 α-MoO3

ZrO2

TeZ-1/2 1011/1163 Te ZrO2 1/2 α-MoO3

ZrO2

TeZ-2 1007/1155 Te ZrO2 2

peak at 22º

α-MoO3 (traces)

ZrO2

TeZ-24 990/1147 Te ZrO2 24 peak at 22º

ZrO2

m-ZrO2

α-Zr(MoO4)2,

peak at 22º

BiCr-24 1049/1201 Bi Cr2O3 24

Bi0.82(V0.45Mo0.55)

Cr2O3

Mo5-x(V/Nb)xO14

Bi0.82(V0.45Mo0.55)

extra peaks: 26-28°

Te-24 1048/1112 Te - 24

α-MoO3

peak at 22º

small extra peaks

α-MoO3

Mo5-x(V/Nb)xO14

P-24 1052/1118 P - 24 unknown phase MoOPO4

~ VOMoO4 (traces)

Bi-24 1050/1114 Bi - 24 peak at 22º

Bi0.82(V0.45Mo0.55)

Mo5-x(V/Nb)xO14

extra peaks: 26-28°

Chapter 4

121

2.2 Activity measurements

Selective oxidation of propane to acrylic acid was carried out in a setup with twelve

parallel fixed bed quartz reactors (i.d., 4 mm; length, 225 mm), working at atmospheric

pressure. Catalyst samples (sieve fraction: 0.24 to 0.45 mm particle size) were introduced

into each reactor tube. The feed flow rate was fixed at a gas hourly space velocity

(GHSV) of 1200 h-1 (at STP) with standard catalytic bed volume of 0.5 ml. In the feed, a

propane/oxygen/nitrogen/steam ratio of 0.85-1/1.9-6.7/15.2-18/9-12 vol.-% was applied.

The reaction was carried out at 673 K. The products were analyzed by gas

chromatography. Inorganic gases and C1-C3 hydrocarbons were analyzed with a TCD

detector using a molecular sieve column and a Porapak Q column, respectively, for

separation. Oxygenated products were detected applying a HP-FFAP column and a flame

ionization detector.

2.3 Catalyst characterization

XRD measurements were performed with a STOE STADI-P transmission diffractometer

equipped with a focusing primary Ge (111) monochromator and a position sensitive

detector, using Cu-Kα1 radiation (λ = 1.54 Å). For data analysis, the program TOPAS

(v.2.1, Bruker AXS) was used to fit the diffraction patterns of the activated materials.

Specific surface areas of the catalysts were measured with an AUTOSORB-1-C

physisorption/chemisorption analyzer applying the BET method. All the samples were

degassed in vacuum at 353 K for 2 hours prior to analysis.

Chapter 4

122

100

150

10 20 30 40 50 60

500

1000

22-1 12-2

12-1

116

024

202

113

110

104

012

212

221

202

220

022

112 20-2

102

21-1

021

200 020

002

111

11-1

110

011

(c)

100

2 θ

(a)

Intensity

(b)

Figure 1. XRD patterns of the diluents: (a) SiO2 (Aerosil 300), (b) ZrO2 and (c) Cr2O3.

3 Results

3.1 Bulk structure of precursors and catalysts

The phase composition of the as-synthesized materials (precursors) and activated

materials (catalysts) was analyzed by X-ray diffraction. For reference, the XRD patterns

of the oxides used as diluents are shown in Figure 1. Chromia with a corundum structure

(ICSD 250078) and monoclinic zirconia (ICSD 89426) are more or less crystalline,

whereas the silica is a X-ray amorphous powder.

a) Mo-V- Te- Nb oxides diluted with SiO2

The X-ray diffractograms of Mo-V-Te-Nb oxide precursors diluted with silica,

hydrothermally synthesized for 1/6, 1/2, 2 and 24 hours, are shown in Figure 2. As

expected, after a short synthesis time of 1/6 h, no complete reaction of the educts was

Chapter 4

123

10 20 30 40 50 60

1000

2000

3000

4000

200

400

600

500

1000

1500

100

200

300

400

500

+⏐

(d)

(c)

(b)

2θ

Intensity

(a)

⏐⏐⏐+

+++

⏐

⏐⏐

+++

Figure 2. XRD patterns of MoVTeNb/SiO2 precursor materials prepared with a

synthesis time of (a) 1/6 h, (b) 1/2 h, (c) 2 h and (d) 24 h. Diffraction peaks are marked

as follows: α-MoO3 (+), V2O5 (׀), peak 22º 2θ→d = 4 Å (*).

achieved. The corresponding XRD patterns show sharp peaks due to α-MoO3 (ICSD

35076) and to V2O5 (ICSD 60767) (Figure 2a). The broad halo between 15 and 30° 2θ is

characteristic for SiO2 (Figure 1a). The reflections of the α-MoO3 structure are still

present in the diffractograms of the precursors obtained after synthesis times of 1/2 and 2

h (Figure 2b and 2c). However, the latter diffractrogram shows a preferred orientation of

Chapter 4

124

the α-MoO3 crystals in [010] direction. The XRD fingerprint of the as-synthesized oxide

changes after a hydrothermal synthesis time of 24 hours. The sharp peaks of the α-MoO3

structure disappear. However, residual α-MoO3 cannot be ruled out considering the

presence of weak peaks observable between 25° and 30° 2θ. Additionally, a sharp peak

appears at 22° 2θ (Figure 2d). Basically, the diffraction patterns shown in Figure 2d

resemble the patterns of crystalline M1. In fact, simulated broadening of all peaks in the

calculated diffractogram of M1 (ICSD 55097) [20] except from the sharp peak at 2θ =

22.2° results in a patterns that is in good agreement with Figure 2d. Shear structures,

which are characterized by a high order of corner-sharing MO6 octahedra in the

crystallographic c direction, which is the growth direction of the needle-like M1 crystals,

and without long-range order in the ab plane were also reported in the literature for

mixed metal oxides that are able to form oxidic bronze-like structures [21]. An example

is semi-crystalline (Mo,V,W)5O14 that crystallizes during heat treatment at 693 K in He

into a Mo5O14-type structure (ICSD 27202) [22-24] For crystalline orthorhombic M2

(ICSD 55098) [20], the most intense reflection is also the peak at 2θ = 22.2° that

represents again long-range order of corner-sharing MO6 (M=Mo, V, Nb) units in the

crystallographic c direction corresponding to a distance between the {001} lattice planes

of d = 4 Å.

Thermal treatment of the precursor TeA-24 at 873 K in Ar results in crystallization of the

Mo5-x(V,Nb)xO14 structure [24] (Figure 3a) as evident by the presence of the

characteristic peaks of this phase (Table 2). Crystalline α-MoO3 is identified as the

second phase in this catalyst (Figure 3a). The M1 phase was identified as the main

constituent of the reference, Te-24, prepared in absence of the diluent (Figure 3b). The

Chapter 4

125

10 20 30 40 50 60

1000

2000

3000

4000

1000

2000

3000

4000

2 θ

(a)

♦

♦**

(b)

Intensity

♦

Figure 3. XRD patterns of activated MoVTeNb oxide catalysts; (a) diluted with

SiO2 and (b) in absence of diluent. Diffraction peaks are marked as follows:

MoO3 (+), M1 (*), M5O14 (M=Mo, V or/and Nb) (♦).

XRD patterns of the orthorhombic M1 phase show characteristic peaks at 6.6°, 7.9°, 9.0°,

22.2°, 27.3° and 45.3° 2θ, with the peak located at 22.2° 2θ being the most intense line.

In addition to M1, α-MoO3 and Mo5-x(V,Nb)xO14-type phases are also present. It should

be mentioned that phase-pure M1 has been prepared successfully by hydrothermal

synthesis over 48 h starting from ammonium heptamolybdate and vanadyl sulfate [25].

Therefore, it may be assumed that the short hydrothermal synthesis time (24 h) applied in

the present work may be responsible for the incomplete formation of M1.

Chapter 4

126

Table 2

XRD peaks due to the Mo5-x(V,Nb)xO14 phase

present in the activated TeA-24 catalyst

XRD peaks [2θ] Indexing

7.7º 200

8.6º 210

22.5º 530

23.2º 600

24.8º 540

27.3º 710, 550

31.3º 810, 740

33.8º 541

39.0º 811

45.0º 970, 1130

In summary, the presence of SiO2 in the hydrothermal preparation of Mo-V-Te-Nb mixed

oxide catalysts suppresses the formation of the M1-phase. By applying other preparation

methods such as evaporation [16] or precipitation [17], the M1-phase was reported to be

successfully obtained despite dilution with SiO2. It is thus rather the choice of synthesis

parameters and not a property of the diluent that prevented successful preparation of

diluted M1 in phase-pure quality.

b) Mo-V- Te- Nb oxide diluted with ZrO2

By applying zirconia instead of Aerosil 300 as diluent, the XRD patterns of the Te-

system precursors (Figure 4) show the characteristic reflection patterns of monoclinic

zirconia (ICSD 89426), Figure 1b. In the XRD patterns of the precursors prepared using

short hydrothermal synthesis times of 1/6 and 1/2 hours (Figure 4a, 4b), peaks belonging

to non-converted MoO3 and a few peaks belonging to V2O5 are observed. Moreover, both

diffractrograms show a preferred orientation of the α-MoO3 crystals in [010] direction.

Traces MoO3 are also detected in the precursor synthesized for 2 hours (Figure 4c). In

this diffractogram, an additional weak peak emerges at 22º 2θ. As mentioned above, this

Chapter 4

127

10 20 30 40 50 60

500

1000

1500

500

1000

1500

2000

500

1000

1500

2000

500

⎥

2 θ

(a)

⎥

(b)

o++

⎥

(c)

o+o

+oo

Intensity

(d)

Figure 4. XRD patterns of MoVTeNb/ZrO2 precursor materials prepared with a

synthesis time of (a) 1/6 h, (b) 1/2 h, (c) 2h and (d) 24h. Diffraction peaks are marked

as follows: α-MoO3 (+), peak 22º 2θ→d = 4 Å (*), m-ZrO2 (o).

peak could be assigned to the (001) reflection of M1, M2 or Mo5O14-type phases or to a

semi-crystalline oxidic bronze structure (structural analogue in the family of the shear

structures). For the material, hydrothermally synthesized for 24 hours, similar XRD

patterns were obtained. However, this pattern did no longer show any evidence of the α-

MoO3 phase (Figure 4d). Additionally, it is observed that the color of the as-synthesized

Chapter 4

128

10 20 30 40 50 60

1000

2000

3000

4000

5000

∇∇∇

∇

∇∇∇

∇

2 θ

Intensity

∇

Figure 5. XRD patterns of activated MoVTeNb oxide catalyst diluted with

ZrO2. Diffraction peaks are marked as follows: m-ZrO2 (o), α-Zr(MoO4)2 (∆),

peak 22º 2θ→d = 4 Å (*).

materials gets darker by increasing the synthesis time in the autoclave, which could be

due to a decrease in either particle size or the band gap.

The TeZ-24 precursor was activated in Ar at 873 K generating the phase α-Zr(MoO4)2

(ICSD 59144) (Figure 5). Evidently, zirconia does not represent an inert diluent under

the present preparation conditions. It reacts with molybdenum at high temperature, in

accordance with previously published results [26]. For zirconia supported MoO3, it has

been reported that at a high surface density of molybdenum (> 5 Mo/nm2), α-Zr(MoO4)2

is formed above 773 K in air [26]. The reflections of monoclinic zirconia and the

additional peak at 22º 2θ remain in the diffractogram after the heat treatment step.

Chapter 4

129

10 20 30 40 50

200

(a)

2 θ

Intensity

∨

(b)

∨

∨(c)

∨

(d)

Figure 6. XRD patterns of MoVPNb/SiO2 precursor materials prepared with a

synthesis time of (a) 1/6 h, (b) 1/2 h, (c) 2 h, and (d) 24 h. Diffraction peaks are

marked as follows: unknown phase (V).

However, lower intensity of the peak at 22º 2θ is observed as compared with that of the

corresponding precursor material (TeZ-24) (Figure 4d).

c) Mo-V- P- Nb oxide diluted with SiO2

The precursors diluted with SiO2 and synthesized for 1/6 and 1/2 hours (Figure 6a and

6b) are XRD amorphous displaying a broad halo between 15º and 30º 2θ and a weak

Chapter 4

130

peak at about 13º 2θ. After hydrothermal synthesis for 2 and 24 hours, the XRD patterns

(Figure 6c and 6d) of the corresponding precursor show the peak at about 13º, whose

intensity increases with increasing synthesis time, and two additional reflections around

28º and 45º 2θ. None of these reflections could be assigned to a specific phase. After

thermal treatment of the precursor PA-24 in Ar, no phase crystallization is achieved

(Figure 7a). Only a few weak peaks at about 25.5º, 28.8º and 29.4º 2θ, possibly

belonging to the MoOPO4 (ICSD 24894) structure, are observed. The bulk structure of

the reference, P-24, prepared in absence of the diluent contains the crystalline MoOPO4

10 20 30 40 50 60

100



2 θ

(a)



(b)

Intensity

Figure 7. XRD patterns of activated MoVPNb oxide catalysts; (a) diluted with SiO2,

and (b) in absence of diluent. Diffraction peaks are marked as follows: MoOPO4 (⁪)

and VOMoO4 (^).

Chapter 4

131

phase. Additional peaks at about 19º, 20.9°, 26.8°, 28.5º 2θ could be assigned to a phase

related to VOMoO4 (ICSD 27315), presenting a value of the lattice parameter a slightly

different from that usually observed for this phase (Figure 7b). No M1-phase formation

was obtained by replacement of Te by P.

Again, SiO2 prevents crystallization of the phases present in the corresponding undiluted

system (Figure 7a).

c) Mo-V-Bi-Nb oxides diluted with Cr2O3

For the Mo-V-Bi-Nb mixed oxide, the only synthesis time applied was 24 h. The XRD

10 20 30 40 50 60

500

1000

1500

100

200

(a)

2 θ

Intensity

•

••

•

°°

***

(b)

°•°

•

♦

♦•

♦

♦••

•

♦

♦♦

(c)

•

♦

♦•

•

••

•

♦

♦•

♦

♦•

♦

♦♦

♦

♦♦

♦

♦•

•

Figure 8. XRD patterns of MoVBiNb oxide catalysts synthesized for 24 h;

(a) precursor BiCr-24, (b) activated BiCr-24 catalyst and (c) Bi-24 catalyst in

absence of diluent. Diffraction peaks are marked as follows: M1 (*), Cr2O3 (°),

Bi0.82(V0.45Mo0.55)O4 (●), Mo5-x(V,Nb)xO14 (♦).

Chapter 4

132

patterns of the BiCr-24 precursor show the typical peaks of M1 in the lower 2θ interval

(6.6°, 7.9° and 9.0° 2θ) and the peak at 22º and 45º 2θ, which are also typical for this

phase (Figure 8a). Moreover, the crystalline phase, Bi0.82(V0.45Mo0.55)O4 (ICSD 24939),

is also present before the heat treatment. This phase is also observed in the bulk structure

of the activated BiCr-24 material together with the Mo5-x(V,Nb)xO14 structure (ICSD

27202). However, the M1 phase is no longer observed after activation. It seems that the

M1 phase decomposes during the thermal treatment. Formation of unknown phases

cannot be excluded since additional XRD reflections between 26º and 28° 2θ are

observed (Figure 8b). As expected, the characteristic peaks belonging to the diluent,

Cr2O3 (ICSD 250078), Figure 1c, are observed in the precursor as well as in the activated

BiCr-24 catalyst (Figure 8a and 8b).

The reference Bi-24 catalyst shows a phase composition identical to that of the diluted

activated catalyst BiCr-24, with peaks due to Bi0.82(V0.45Mo0.55)O4 and Mo5-x(V,Nb)xO14,

the only difference being the peaks belonging to the diluent Cr2O3 (Figure 8c).

Table 3

Lattice parameters, cell volume, and crystallite size of the phases present in the undiluted Bi-

24 and diluted BiCr-24 catalysts and of pure Cr2O3. The standard deviations of the values are

indicated in brackets.

Bi-24 BiCr-24

Lattice

Parameters

[nm]

Cell

Volume

[nm3]

Crystallite

size

LVol-IB*

[nm]

Lattice

Parameters

[nm]

Cell

Volume

[nm3]

Crystallite

size

[nm]

Bi0.82(V0.45Mo0.55)O4

a=5.19666(32)

316.24(5) 46.43(88)

a=5.22234(43)

320.03(7) 49.2(21)

b=5.19666(32) b=5.22234(43)

c=11.7104(10) c=11.7344(18)

Mo5-x(V,Nb)xO14

a=22.8131(22)

2085.6(5) 45.1(31)

a=22.8226(45)

2088.4(9) 44.2(97) b=22.8131(22) b=22.8226(45)

c=4.00742(48) c=4.00946(92)

Cr2O3

Cr2O3-diluent BiCr-24

a=4.95651(17)

289.31(2) -

a=4.95878(14)

289.485(19) -

b=4.95651(17) b=4.95878(14)

c=13.59805(52) c=13.59396(42)

Chapter 4

133

Additional peaks between 26º and 28° 2θ are also observed in the undiluted catalyst. Also

in this case, no M1-phase was obtained by replacement of Te by Bi. Dilution with

chromia results in a significant increase of the cell volume of the Bi0.82(V0.45Mo0.55)O4

structure. The changes in the lattice parameters of the Mo5-x(V,Nb)xO14 structure is within

the error of the measurement. The crystallites size of the Bi0.82(V0.45Mo0.55)O4 and Mo5-

x(V,Nb)xO14 is comparable before and after dilution (Table 3).

3.2 Catalytic performance

Table 4 and 5 show the catalytic behavior of selected catalysts: TeA-series, PA-series,

TeZ-series, and BiCr-24 in selective oxidation of propane to acrylic acid at 673 K and a

contact time (W/F) between 560 and 704 gcat·h·molC3H8-1. The undiluted reference

materials oxides, Te-24, P-24 and Bi-24 have also been tested under comparable

conditions (same catalyst mass).

The consumption rate of propane and the formation rate of acrylic acid were normalized

either to the BET surface area, or to the mass of active metal oxide. These values

represent a “formal” approach, since no dispersion measurements are available. The

catalytic performance of the catalysts prepared by hydrothermal synthesis for 24 hours

are compared in Figure 9 (normalized to the BET surface area) and Figure 10

(normalized to the mass of active metal oxide). Despite differences with respect to

catalyst composition (active metal oxides and diluents), phase composition, and surface

area, all tested catalysts showed catalytic activity for propane oxidation and formation of

acrylic acid (Table 4 and 5). As it is shown in Figure 9, the activity normalized to the

surface area of the catalysts decreases as follows: Te-24 > P-24 > Bi-24 > TeZ-24 >

BiCr-24 > TeA-24 > PA-24. The undiluted Mo-V-Te-Nb oxide catalyst is the most active

Chapter 4

134

material. Diluted catalysts are less active than their undiluted counterparts. The higher

specific surface area of the diluted catalysts involves not only the specific surface area of

the corresponding active mixed metal oxide but also that of the diluent, which might be

not completely covered by the active phase. Therefore, and irrespective of the variations

with respect to elemental and phase compositions between diluted and undiluted

catalysts, a lower activity per unit of surface area is calculated for the diluted systems.

Especially in the case of PA-24, showing the lowest activity per unit of surface area, the

Table 4

Conversion of propane and selectivity to acrylic acid in the oxidation of propane over diluted

and non-diluted catalysts. The rates of propane consumption and acrylic acid formation are

normalized to the mass of the active Mo-V-X-Nb (X=Te, Bi, P) oxide

Notation W/F*

[gcat ·h·molC3H8-1] XC3H8

[%] SAA

[%]

Rate [10-3 mmol·gAM-1·h-1]

C3H8

consumption Formation

of AA

TeA-1/6 704 5 94 142 134

TeA-1/2 704 4 99 114 113

TeA-2 560 8 83 286 237

TeA-24 704 7 83 199 165

PA-1/6 560 5 85 179 152

PA-1/2 704 4 99 114 113

PA-2 704 9 57 256 146

PA-24 560 5 82 179 146

TeZ-1/6 666 12 22 360 79

TeZ-1/2 666 16 29 480 139

TeZ-2 666 21 33 631 208

TeZ-24 560 24 32 857 274

BiCr-24 666 5 86 150 129

Te-24 666 19 38 285 108

P-24 666 17 26 255 66

Bi-24 704 9 75 128 96

* contact time (gcat·h·molC3H8-1)

T = 673 K; GHSV = 1200 h-1 (STP: T = 273 K, p = 1 atm)

AM: active metal oxide

Chapter 4

135

Table 5

Specific surface area, propane consumption rate, and rate of acrylic acid formation norma-

lized to the specific surface area of catalysts prepared at 24 h hydrothermal synthesis time

Notation

SBET

[m2·g-1]

W/F*

[gcat ·h·molC3H8-1]

Rate [10-3 mmol·m-2·h-1]

C3H8

consumption Formation

of AA

TeA-24 27 704 3.7 3.1

PA-24 185 560 0.5 0.4

TeZ-24 11 560 39.0 12.5

BiCr-24 2 666 37.6 32.3

Te-24 1.5 666 190.2 72.3

P-24 2.6 666 98.2 25.6

Bi-24 2.1 704 60.9 45.7

* contact time (gcat·h·molC3H8-1)

T = 673 K; GHSV = 1200 h-1 (STP: T = 273 K, p = 1 atm)

increase in BET surface area is about seventy times compared to that of the undiluted P-

24.

This shows that the extent of “dispersion” of the active phase is quite different for the

catalysts studied and no conclusion about a specific effect of dilution can be drawn from

these data. The morphology seems to be not that of a homogeneously dispersed shell of

active phase on a diluent particle.

In Figure 10, the rate of propane consumption and the formation rate of acrylic acid

calculated per mass of active mixed metal oxide are compared. TeZ-24 is the most active

catalyst. Specifically, TeZ-24 is more active than Te-24 per mass of active mixed metal

oxide. A large fraction of the enhanced activity is due to combustion which may occur on

the ZrO2 support or which may be related to the presence of the zirconium molybdate

phase. TeZ-24 operates at a similar selectivity to acrylic acid compared to that of Te-24

(32 % vs. 38 %) at comparable propane conversion of ~ 20 % (Table 4). Consequently,

Chapter 4

136

100

120

140

160

180

200

100

150

200

250

Bi-24P-24Te-24BiCr-24TeZ-24PA-24

TeA-24

Formation rate of acrylic acid

[10-3 mmol m-2 h-1]

Consumption rate of C3H8

Formation rate of C3H4O2

Consumption rate of propane

[10-3 mmol m-2 h-1]

Catalyst

Figure 9. Rate of propane consumption, and rate of acrylic acid formation

normalized to the specific surface area.

the formation rate of acrylic acid normalized to the mass of active mixed metal oxide is

significantly higher for TeZ-24 than for Te-24 (Table 4 and Figure 10). Moreover, TeZ-

24 also shows an enhanced formation rate normalized to the mass of the catalyst (Table

6) as compared with the corresponding undiluted catalyst. With all other examples a

similar, albeit less drastic beneficial effect of dilution is demonstrated from the data.

The influence of the hydrothermal synthesis time in preparation of the TeZ-catalysts on

the catalytic performance is shown in Figure 11. Longer synthesis time in the autoclave

leads to higher rates of propane consumption and acrylic acid formation calculated per

mass of active metal oxides. However, the fraction of by-products is also much higher for

the TeZ-24 catalyst than for the undiluted Te-system, which is not desirable. On the other

hand, it should also be noted that the ratio rAA/rC3H8 increases with increasing

hydrothermal synthesis time. Therefore, the application of longer synthesis times could

lead to the formation of catalysts showing higher selectivities to acrylic acid.

Chapter 4

137

In summary, the dilution of Mo-V-Bi-Nb mixed oxide with Cr2O3 slightly improves the

activity and the acrylic acid formation rate per of active mixed metal oxide (Figure 10).

Cr2O3 does not act as an oxidation catalyst by itself but, in fact, it seems to dilute the

active component without much functional modification.

The redox-inactive diluent SiO2 is also beneficial for the acrylic acid productivity per

mass of active mixed metal oxide. Figure 10 reveals that in particular the selectivity to

acrylic acid is improved for both, the Te and the P catalyst. Besides the dilution function,

it is speculated that silica neutralizes some phase impurities such as molybdenum oxide

that otherwise would catalyze total oxidation. From comparison of Figure 9 and Figure

10 it becomes apparent that the dispersion effect is small as compared to the

“neutralization” effect, for which the high surface area may be instrumental.

100

200

300

400

500

600

700

800

100

200

300

400

500

600

700

800

900

Consumption rate of C3H8

Formation rate of C3H4O2

Consumption rate of propane

[10-3 mmol gAM

-1 h-1]

Formation rate of acrylic acid

[10-3 mmol gAM

-1 h-1]

Bi-24

P-24

Te-24BiCr-24TeZ-24PA-24

TeA-24

Catalyst

Figure 10. Rate of propane consumption, and rate of acrylic acid formation

normalized to the mass of active mixed metal oxide.

Chapter 4

138

4 Discussion

In the present study, the effect of dilution of multi-metal oxides Mo-V-X-Nb (X=Te, P,

Bi) with SiO2 (Aerosil 300), ZrO2 and Cr2O3 was studied with regards to phase

compositions and the properties in selective oxidation of propane to acrylic acid.

The undiluted multi-metal oxide catalysts, Mo-V-X-Nb (X=Te, P, Bi), are composed of

different crystalline phases. The classical Mo-V-Te-Nb oxide catalyst mainly consists of

the M1 phase. Furthermore, α-MoO3, and Mo5-x(V,Nb)xO14 were identified as minority

phases. On the other hand, no M1 was obtained in the activated catalyst, when Te was

substituted by either P or Bi. A crystalline MoOPO4 phase is identified as main

component in the Mo-V-P-Nb oxide, whereas two crystalline phases,

Bi0.82(V0.45Mo0.55)O4 and Mo5-x(V,Nb)xO14, constitute the bulk of the Mo-V-Bi-Nb oxide

(Scheme 1). Despite of their different elemental and phase composition, comparable

100

200

300

400

500

600

700

800

900

100

200

300

400

500

600

700

800

900

Consumption rate of C3H8

Formation rate of C3H4O2

Formation rate of acrylic acid

[10-3 mmol gAM

-1 h-1]

Consumption rate of propane

[10-3 mmol gAM

-1 h-1]

TeZ-24

TeZ-2

TeZ-1/2

TeZ-1/6

Catalyst

Figure 11. Rates of propane consumption, and rate of acrylic acid

formation normalized to the mass of active metal oxide in the TeZ-series.

Chapter 4

139

formation rates of acrylic acid per mass of catalyst were found for these three structurally

different undiluted catalysts in the screening experiments. This observation clearly

indicates that the highly crystalline M1 phase is not necessarily required to convert

propane into acrylic acid. Moreover, the Bi containing catalyst produces acrylic acid with

similar rate compared to the Te containing catalyst (0.096 vs. 0.108 mmol·g-1·h-1)

indicating that Te cannot be an essential ingredient for acrylic acid formation. These

observations are not in contradiction to the extensive mechanistic considerations by

Grasselli et al. [27], since all catalysts contain vanadium, which has been considered to

be responsible for alkane activation. Furthermore, this is in line with the finding that also

VPO catalysts [28, 29], comprising structural building blocks different from that of M1

or Mo5O14, effectively catalyze propane conversion. This implicates that the frequently

used argument about the superior performance of M1 (see also below) is not a good

support for the claimed chemical complexity of the active site as described on the (001)

plane of M1, as no attempts were made to optimize the chemical potential of the reactants

to the different material classes studied.

The Te containing catalyst, consisting mainly of the M1 phase, was diluted either with

SiO2 or with ZrO2. Both additives cannot be considered as “inert” diluents, since different

phase compositions of the diluted catalysts compared to the undiluted reference materials

were obtained and the formation of the M1 phase was suppressed in both cases. Zirconia

reacts with molybdenum by forming a Mo-Zr mixed oxide, which is more stable than

M1. The phase α-Zr(MoO4)2 (ICSD 59144) was generated. The reaction between zirconia

and molybdenum oxide may have occurred at the high activation temperature of 873 K

that is necessary to crystallize the M1 phase from its nanostructured precursor. Increasing

Chapter 4

140

fractions of such a nano-crystalline M1 precursor are probably formed with increasing

hydrothermal synthesis time, as it becomes obvious from Figure 4. However, the data

quality of the diffractograms that have been measured for the purpose of screening the

phase composition does not allow to draw any conclusions on the impact of the

hydrothermal synthesis on the microstructure of the diluent ZrO2.

c b

Scheme 1. Schematic representation of the unit cells of (a) BiVO4

(isostructural with Bi0.82(V0.45Mo0.55)O4), (b) M1, and (c) Mo5O14.

b) c)

Chapter 4

141

Among all diluted catalysts, the dilution of the Te-system with ZrO2 yielded a strong

improvement in the propane consumption rate calculated per mass of active component

as well as calculated per mass of the catalyst (Table 4). A discussion about dispersion of

the active material is not appropriate, since the phases of the diluted and undiluted Mo-V-

Te-Nb oxide catalysts differ and no reliable dispersion measurements were available. The

enhanced propane consumption rate per mass catalyst of TeZ-24 might be due to the

presence of the α-Zr(MoO4)2 phase. This phase has been reported to be active in propane

oxidative dehydrogenation (ODH) at 703 K [26]. Although the reaction conditions of the

present study (T = 673 K and presence of 40 % steam in the feed) differ drastically from

the ODH conditions, Zr-O-Mo sites may contribute to propane consumption. It is noted

that the diluent m-ZrO2 is inert under the reaction conditions applied. The formation rate

of acrylic acid normalized to the mass of active metal oxide as well as normalized to the

mass of the catalyst is higher for TeZ-24 compared to that of Te-24. This points to an

additional function of the diluent that may be related to the weak peak at 22º 2θ observed

in the XRD patterns of this catalyst. In the orthorhombic M1 structure, this peak indicates

the presence of regular stacking of metal-oxygen polyhedra in [001] direction. The

formation of M1 crystallites with a flat, disk-like morphology instead of the usual needle-

like morphology may explain the very weak XRD signal of the M1 phase.

Zirconia was the most reactive diluent used here in the sense that a competitive formation

of a stable compound (zirconium molybdate) with the essential constituent Mo prevented

to a large extent the formation of the precursor to the desired M1 phase that may be

present as minority phase in a distorted form in the active catalyst.

Chapter 4

142

Silica as diluent exerts a different influence on the phase formation. With its large surface

area and the supposed defect structure of the Aerosil material this diluent seems to bind

Te such that it is not available for M1 formation leading to the “parent” structure Mo5O14

and some MoO3 (Figure 3). On the P-system that did not form the M1 phase in the

undiluted state, an unknown effect on the phase formation was found for the silica diluent

that did work as an agent preventing or retarding crystallization.

Chromia seems to be the most inert diluent in the series used here, as it did not notably

interfere with the phase inventory. The comparison of the lattice parameters of the

crystalline phases in the pure and in the diluted form gives, however, clear evidence for a

modification of the defect state of one of these phases, namely Bi0.82(V0.45Mo0.55)O4

(Table 3). It is assumed that chromium oxide did not interfere with the precursor

formation in the autoclave but modified the defect state of the oxygen sub-lattice in the

activated Bi0.82(V0.45Mo0.55)O4 phase.

In summary, it became clear that the concept of diluting crystalline, phase-pure M1 by an

“inert” oxide is not realizable. However, in all cases, some indication of dispersion was

found. This effect is strongly superimposed by structural modifications of the active

phase due to the presence of the diluent. Either capturing of a structural ingredient, or

spriting of the intended guest species for the M1 channel structure, or modification of the

defect disposition of a non-M1 active phase were effective means of catalyst

modification.

The diversity of modifications hampers a meaningful comparative discussion of the

results. It is obvious, however, that the M1 phase is not the only carrier of the active sites.

Chapter 4

143

The results of this study are considered as path-finding experiments and suggest

directions of further improvement of the systems.

As it is facile to improve a poor performance and very difficult to further optimize a

highly advanced system, it is useful to validate the state of the art in propane oxidation to

acrylic acid before investing into the necessary substantial optimisation of the systems

presented here. To this end a survey of literature data presented in Table 6 was collected

from those sources allowing a comparison of rates. Regarding the yield of acrylic acid, it

appears that the classical Mo-V-Te system is by far the best choice reaching up to 50 %

yield. This compares reasonably to the patent literature where maximum yields slightly

above 50 % yield [44] were claimed. It further occurs that no other catalyst system comes

close to that performance. The comparison is only semi-quantitative as the experimental

conditions and hence the reactant chemical potentials were significantly different in the

studies and no attempts are documented that for any of the systems an optimization of the

process conditions was carried out.

In this situation it seems appropriate to relate the discussion to the rates of acrylic acid

formation spreading for the catalysts described in Table 6 over about two orders of

magnitude with an average of about 0.5 mmolAA/gcat·h. This value indicates a still low

productivity of all the complex MMO systems. The large spread in rates for nominally

iso-structural M1 systems claimed as active materials in the studies indicates that either

activity is not an intrinsic property of M1 but of its defect disposition. Alternatively, the

presence of known or undetected additional phases affects the performance, a conjecture

being likely in view of the many reaction steps and consecutive processes possibly

involved in the target transformation of propane to acrylic acid.

Chapter 4

144

0 102030405060708090100

100 M1 (chapter 2)

VPO

HPA

MMO

50% yield

XC3H8[%]

SAA[%]

STYmax (chapter 2) = 0.15 gAAgcat

-1h-1

STYmax (ref.) = 0.12 gAAgcat

-1h-1

Figure 12. Conversion of propane versus selectivity of acrylic acid of the

catalysts prepared in this work and the catalysts reported in the literature.

It is informative to compare the rates found in Table 6 to values quoted for another

complex alkane transformation, namely the formation of maleic anhydride from butane

over VPP systems also active in propane oxidation. Under comparable process conditions

(GHSV = 1500 h-1, feed: n-C4H10/O2/He = 1.6/18/80.4, T = 673 K) a performance of 65

% conversion at a selectivity of 69 % maleic anhydride was reported [45] relating to a

rate of maleic anhydride formation of 0.97 mmolMA/gcat·h. This comparison reveals that

purely from the rates achieved, the very best MMO systems for propane oxidation are

just as good as the highly optimized VPP system.

In Figure 12 the reference data and the present results are compared in a conversion-

selectivity diagram showing for reference the 50 % yield trajectory. The situation for

propane activation looks less promising from this diagram. It occurs that almost all

Chapter 4

145

systems reported have a serious selectivity problem if we consider the selectivity for

maleic anhydride of about 70 % as reference. It becomes clear that the high-rate systems

from Table 6 are all unacceptable from their selectivity. VPP systems and the catalysts

from the present study form a family of highly selective albeit poorly active catalysts. It

is remarkable that they all are not of the M1 structure. Few data points near the 50 %

yield trajectory relate all to phase-pure M1 catalysts. It may be concluded that phase-pure

M1 is indeed the best existing catalyst system. Its performance is, however, very severely

affected by phase impurities being they X-ray crystalline or not. Also the two nominal

Mo-V-Te systems of the present study follow this observation. The argument that the

highly selective systems are just good as their conversion is so low is not the only

answer, as there are many Mo-V-Te systems in Figure 12 with comparable low

conversion but much worse selectivity. We conclude that it may be worthwhile to start an

optimisation of non-M1 systems as they offer the prospect to highly selective propane

oxidation systems.

The rate data for acrylic acid formation collected in this work and from reference data of

the literature are graphically displayed in Figure 13. The lines labelled a-d are guides to

the eye and may not be misunderstood as fits. Here a correlation of the rate per unit mass

versus the rate per unit surface area is chosen. Would the catalysts all contain the same

active sites in differing dispersions then we would expect a linear correlation. The spread

of data as well as the poor statistics do not allow a conclusive interpretation of the data in

Figure 13 but some trends may be deduced. First we state that Te-free systems are not

very productive. Second it becomes clear that the systems from the present study are

reasonably active per unit surface area but inefficient per unit mass as expected from the

Chapter 4

146

0 50 100 150 200 250 300

400

800

1200

1600

2000

2400

2800 diluted (this work)

undiluted (this work)

Te-free system (reference)

Te-systems (reference)

Formation rate of acrylic acid

[10-3 mmol g-1 h-1]

Formation rate of acrylic acid

[10-3 mmol m-2 h-1]

Figure 13. Formation rate of acrylic acid normalized to the catalyst mass versus

formation rate of acrylic acid normalized to the surface area of the catalysts prepared

in this work, and of catalysts reported in the literature.

principle of dilution of active mass by inert materials. Line (c) may serve as extrapolation

of the potential of this development. The reference systems with a comparable rate per

unit surface area but with substantial better productivity per mass fall into an opening

data field limited by the lines (a) and (b). The data statistics is not yet good enough to

distinguish between families of correlations and an area of correlation maybe broadened

by the spread in reaction conditions.

Very markedly different is the family of data for highly pure M1 systems grouping

around line (d). The fact that this line shows no variation of the area-specific activity

suggests that this rate is an intrinsic property of the pure M1 system. The active sites

Chapter 4

147

should thus be an integral component of the M1 phase. Their high-performance operation

can only be realized if no interference is allowed of the initial synthesis of acrylic acid

with consecutive oxidation processes. The formation rate for acrylic acid is the highest

for all systems studied. The only variable in this family is the surface area of the bulk

catalysts that exhibit no dispersion effect. It is conceivable that the intersection between

lines (a) and (d) marks the maximum evolution of activity of the M1 system that can be

obtained within the crystal chemistry of the Mo-V-Te system.

From this plot is seems less desirable to optimize the systems described in the results

section of this paper but rather to concentrate on the optimisation of the clean and phase-

pure M1 system. It is reassuring that the data points along line (d) designate systems from

two different groups namely that of Ueda and of the authors of this study giving

confidence that the performance level is an intrinsic property of the M1 phase without

strong interferences of details of the preparation scheme of an individual research team.

5 Conclusion

Such a consequence of the analysis of the data from Figure 13 needs to be contrasted

with the shortcoming of the M1 system concerning its stability and lifetime as catalyst.

This critical parameter for application is not reflected in Figure 12 and Figure 13. It is

known, however, that Te tends to volatilize setting free Mo-species and so degrading the

catalyst under operation conditions. If this detrimental property of the M1 system cannot

be removed then it may be a promising fallback strategy to develop the systems of the

present study. Both the synthesis parameters (hydrothermal synthesis and activation) and

the process conditions need to be optimized before a discussion on the potential of the

highly selective non-M1 systems can be started.

Chapter 4

148

The present work has shown again that the M1 phase in pure form is an excellent catalyst

for the target reaction that cannot be rivalled by other systems. The work further showed

the critical role of crystalline phases in the hydrothermal precursor that tend to strongly

interfere with the formation of the target phase. This observations may form the basis of

another explanation for the “synergy effect” [46] claimed for M1/M2 phase mixtures

obtained from high throughput synthesis. Here, M2 may acts as “weak” diluent, such as

Cr2O3 in the present study, dispersing the active M1 without interfering with the

precursor chemistry of the M1 contribution due to its compositional and structural

similarity with the M1 phase. The present study has opened up some alternative synthesis

pathways to possibly more stable albeit still worse performing catalyst systems. A critical

analysis of the publicly reported performance data of the target reaction reveals that it

may be useful both from selectivity arguments and from challenge of a stable catalyst

system to invest in optimization of the present alternative catalyst formulations rather

than to only further improve the performance of phase-pure M1. M1, however, seems to

be intrinsically active for the reaction rendering it the preferred target for fundamental

studies on the mode of operation of propane oxidation over MMO catalysts.

Chapter 4

149

Table 6 Catalytic performance of catalysts for the selective oxidation of propane to acrylic acid

Catalyst Feed Flow rate

Cat[g]/

Vcat[ml]

[°C]

XC3

[%]

SAA

[%]

YAA

[%]

Formation rate

[mmolAA

/gca

Reference

V1P1.15Te0.1–0.15O C3/O2/H2O=1.86/76/22.15 C3+O2=274ml/min

H2O=0.195 mol/h 40/- 390 30 30 10.5 0.0418 28

V1P1.1O C3/O2/H2O=1.08/35.18/63.73 6.9 s 10/10 420 46.8 32 14.54 0.036 29

(PyH)3PMo12O40 C3/O2/H2O/N2=20/10/20/50 50ml/min 3/- 340 7.5 29 2 0.178 30, 31

H1.26Cs2.5Fe0.08PVMo11O40 C3/O2/N2=30/40/30 (vol%) 15cm3/min 1/- 380 47 28 13 1.5 32

MoV0.3Te0.23Nb0.12O

C3/O2/inert/H2O(ml/min)=0.6/2/7.4/8.8 1.9s 0.61/- 380 80 61 49 1.08 3

MoV0.44Te0.10On C3/O2/H2O/N2 = 8/10/45/37 20 ml/min 0.5/- 380 36.2 46.6 17 1.4 33

MoV0.25Te0.11Nb0.12O

C3/O2/H2O/N2 =8/10/45/37 20 ml/min 0.5/- 380 33.4 62.4 21 1.7 33

Te-Ni-Mo-O C3/O2/H2O/N2 =12/8/40/40 1500 ml (gcat h)-1 1/- 420 30 55 17 1.4 34

MoV0.45Te0.17Nb0.12O

C3/O2/H2O/He=4/8/30/58 205 gca

h(molC3)-1 0.3-3/- 380 35.2 54.3 19.1 0.93 35

MoV0.26Te0.11Nb0.12O

C3/O2/H2O/N2/Ne=5/10/45/35/5 30ml/min 0.5/- 380 33 55 18.15 0.43 36

MoV0.28Sb0.13Nb0.15O

C3/O2/H2O/N2/Ne=5/10/45/35/5 30ml/min 0.5/- 380 35 49 17.15 0.41 36

MoV0.30Te0.23Nb0.11O

C3/O2/H2O/N2/Ne=5/10/45/35/5 30ml/min 0.5/- 380 30 58 17.4 0.42 36

MoV0.30Sb0.15Nb0.10O

C3/O2/H2O/N2/Ne=5/10/45/35/5 30ml/min 0.5/- 380 34 41 14 0.33 36

MoV0.36Te0.17Nb0.12O

C3/O2/H2O/He=4/8/30/58 100gca

h(molC3)-1 -/- 400 21.5 58.5 12.6 1.2 37

MoV0.3Te0.23Nb0.12O

C3/O2/H2O/N2=2/6/50/42 20-80 Ncm3/s 215-340/- 360 38 65 24.7 0.908 38

MoV0.17Te0.22Nb0.15O

C3/O2/H2O/He=4/8/30/58 50cm3min-1 0.5-2.5/- 380 21.1 33.2 7 0.14 39

MoV0.25Te0.23Nb0.10O

C3/O2/H2O/N2/He=6/10/43/36/5 50cm3min-1 0.5-2.5/- 380 58 52 30 0.27 39

Mo12W2Bi1Co5.5Fe3Si1.6K0.08On C3/O2=80/20 56 l/h 200/- 430 10 14 1.4 0.035 40

MoV0.3Sb0.25Nb1.12O

C3/air/H2O=3.3/50/46.7 2.4 s 35/30 390 36 29 10.44 0.20 41

MoV0.25Te0.23Nb0.124O

C3/O2/N2/H2O=2.8/6.4/50.8/40 1200 h-1 0.5/0.5 400 56 79 44.0 0.66 42

MoV0.25Te0.23Nb0.124On (P-123) C3/O2/N2/H2O=2.8/6.4/50.8/40 1200 h-1 0.5/0.5 400 24 24 5.6 0.0864 43

MoV0.25Te0.23Nb0.124O

(P-123) C3/O2/N2/H2O=2.8/6.4/50.8/40 4800h-1 0.5/0.5 400 4 100 4 0.24 43

MoV0.25Te0.23Nb0.124O

(S0) C3/O2/N2/H2O=2.8/6.4/50.8/40 4800h-1 0.5/0.5 400 18 27 4.8 0.288 43

Mo1V0.3Te0.23Nb0.125/SiO2 C3/O2/N2/H2O=2.65/19.76/47.78/29.81 1200 h-1 0.5/0.5 400 7 83 5.81 0.083 this work

Mo1V0.3Te0.23Nb0.125/ZrO2 C3/O2/N2/H2O= 3.33/6.66/60/30 1200 h-1 0.5/0.5 400 24 32 7.68 0.137 this work

Mo1V0.3Bi0.23Nb0.125/Cr2O3 C3/O2/N2/H2O= 2.8/6.4/50.8/40 1200 h-1 0.5/0.5 400 5 86 4.3 0.065 this work

Mo1V0.3P0.27Nb0.125/SiO2 C3/O2/N2/H2O= 3.33/6.66/60/30 1200 h-1 0.5/0.5 400 5 82 4.1 0.073 this work

Mo1V0.3Te0.23Nb0.125 C3/O2/N2/H2O= 2.8/6.4/50.8/40 1200 h-1 0.5/0.5 400 19 38 7.22 0.108 this work

Mo1V0.3Bi0.23Nb0.125 C3/O2/N2/H2O= 2.65/19.76/47.78/29.81 1200 h-1 0.5/0.5 400 9 75 6.75 0.096 this work

Mo1V0.3P0.27Nb0.125 C3/O2/N2/H2O= 2.8/6.4/50.8/40 1200 h-1 0.5/0.5 400 17 26 4.42 0.066 this work

Chapter 4

150

6 Acknowledgments

The authors thank Mrs. Gisela Lorenz, and Mrs. Edith Kitzelmann for technical

assistance and Dr. Olaf Timpe for helpful discussions.

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Conclusions and Outlook

153

CONCLUSION AND OUTLOOK

Hydrothermal synthesis of phase-pure M1 MoVTeNbOx catalysts was

systematically investigated in the present work achieving a better understanding of the

formation mechanism of the M1 phase, and an improved control over the final catalyst

structure. Moreover, links between bulk and surface properties of the M1 phase and its

catalytic behavior in the selective oxidation of propane to acrylic acid are discussed.

Preparation of phase-pure M1 was shown to be not trivial. Hydrothermal synthesis

enables access to the chemical and structural complex M1 phase. However, precise

control of the preparation parameters is required. Phase-pure M1 synthesis was achieved

in this work by applying a preparative stoichiometry of the metals of Mo1V0.25Te0.23Nb0.12

and a molybdenum concentration of 0.25 M. Depending on the technical parameters of

the autoclave used, optimization of hydrothermal conditions, particularly, temperature

and synthesis time is necessary to obtain M1 precursors. SEM-EDX analysis, Raman and

UV/VIS spectroscopy show that the metals (Mo, V, Te and Nb) are inhomogeneously

distributed in the initial aqueous suspension of the metal salt solution and initially formed

precipitates, which is introduced into the autoclave. However, the precursor materials

obtained after the hydrothermal reaction already exhibit the chemical composition of

phase-pure M1. M1 precursors show a fairly homogeneous spatial distribution of the

elements with a remarkably high Nb content and a long-range order of metal-oxygen

polyhedra in the crystallographic c direction (XRD, EXAFS). Obviously, the phase-

purity of the Mo-V-Te-Nb oxide is determined by prearrangement of structural elements

established in course of the hydrothermal process, being the essential step in the

Conclusions and Outlook

154

preparation of phase-pure M1. Further crystallization of the precursor occurs during the

subsequent heat treatment in an inert gas at 823-923 K.

It was shown that the presence of ammonium containing phases in the precursor leads to

the formation of phase mixtures after activation (XRD, TG/MS).

No phase cooperation between M1 and M2 phase was found in this work. Phase-pure M1

catalysts showed a better catalytic performance in the selective oxidation of propane to

acrylic acid than mixtures of phases. The catalytic performance of phase-pure M1

synthesized in this work is comparable to that reported in the literature and in patents. M1

converts propane with about 75 % selectivity into acrylic acid whereas the propane

conversion is mainly dependent on the specific surface area. The M1 phase is

characterized by a certain flexibility with respect to the chemical composition. However,

any systematic correlation between the bulk concentration of a specific element and the

catalytic properties was not observed within the variations studied in the present work.

Moreover, it could be shown that M1 can be prepared with different morphology. The

aspect ratio of the needle-like crystals has been changed by applying different

pretreatment temperatures. However, it seems that the unique catalytic properties of the

M1 phase are not related to the surface area of the crystallographic ab plane. Moreover, it

was shown that M1 catalysts with different composition of the topmost surface layer

(LEIS) show similar catalytic behavior. These observations are in agreement with a

dynamic nature of the active moiety on the surface of M1 under reaction conditions. The

surface of M1, which was confirmed to be covered by an approximately 0.7 nm thick,

structurally disordered layer (TEM), was investigated by in-situ XPS in presence of

different gas mixtures at pressures below 1 mbar, and at high temperatures (623 K).

Conclusions and Outlook

155

These experiments indicated the re-distribution of the elements at the catalyst surface in

response to changes in the composition of the gas phase. The tellurium content at the

surface increases in presence of steam in the feed. The ex-situ comparison of spent

catalysts also revealed an increased Te concentration at the surface, the higher the activity

the higher the Te concentration. Furthermore, phase-pure M1 showed a reasonable

stability under reaction conditions, which is of a great importance for its potential

industrial application.

Additionally, new synthesis routes to multi-metal oxide catalysts were also proposed.

Metal substitution and/or addition of suitable diluents to the Mo-V-X-Nb (X=Te, P, Bi)

oxides were prepared and investigated in the selective oxidation of propane to acrylic

acid. The results underlined the exceptional role that is attributed to the M1 phase but

also proved that M1 is not necessarily required for acrylic acid formation. This supports

the hypothesis presented in the present work that the active ensembles are generated on

the surface of the catalyst under reaction conditions. Contrary to findings previously

reported, the arrangement of atoms like in the crystalline structure of the M1 phase

should not be considered as structural holder of the active centers, but more likely as their

precursor.

The lack of a comprehensive understanding of the catalytic behaviour of these catalysts

will render impossible a further improvement of the Mo-V-Te-Nb mixed oxide catalysts

and/or the development of new solid catalysts suitable for acrylic acid production starting

from propane. Hence, in-situ methods, as for example in-situ Raman, in-situ IR and in-

situ XRD, should be applied under conditions of propane oxidation in presence of C3H8,

O2, N2 and H2O at T = 673 K and p = 1 bar with the purpose of getting a realistic view of

Conclusions and Outlook

156

the catalyst bulk and surface in contact with the gas phase. Stability of phase-pure M1

should also be confirmed in a long-term catalytic test experiment. It is important to

monitor possible sintering of the M1 crystallites or phase transformation during time on

stream by in-situ XRD. Additionally, further development of the new potential synthesis

routes of the multi-metal oxides presented in this work (diluted and undiluted Mo-V-X-

Nb (X=Te, P, Bi) oxides), together with the optimization of the reaction conditions for

these alternative catalysts, are of high interest.

Appendix

157

Dissertation: „Synthesis and Investigation of phase-pure M1

MoVTeNbOx Catalysts for Selective Oxidation of Propane to Acrylic

Acid”, Fritz-Haber-Institut der Max-Planck Gesellschaft, Berlin,

unter der Leitung von Prof. Dr. R. Schlögl und Dr. A. Trunschke.

Studienaustausch (Erasmus-Programm) mit der Universität

Kaiserslautern. Fachrichtung Chemieingenieurwesen: neuntes und

zehntes Semester.

Diplomarbeit: „Simulation von Sorptionszyklen in halbtechnischen

Aktivkohleschüttungen“; Note: Sehr Gut mit Auszeichnung; Lehrstuhl

für Thermische Verfahrenstechnik von Prof. Dipl.-Ing. Dr. techn. Hans-

Jörg Bart.

Studium an der Universität Oviedo (Spanien). Fachrichtung

Chemieingenieurwesen: Erstes bis achtes Semester.

APPENDIX

Curriculum Vitae

Almudena Celaya Sanfiz

Persönliche Angaben:

Geburtsdatum: 16-März-1978

Geburtsort: Incio (Lugo)

Staatsangehörigkeit: spanisch

Akademische Ausbildung:

seit 01/2004

10/2000-10/2001

10/1996-10/2000

Schulausbildung:

1991 – 1996

1981 – 1991

Gymnasium “Instituto Marqués de Casariego”, Tapia de Casariego

(Spanien).

Abitur mit der Gesamtnote Sehr Gut mit Auszeichnung.

Grundschule „Colegio Principe de Asturias“, Tapia de Casariego

(Spanien).

Grundschulabschluss mit der Gesamtnote Sehr Gut

Appendix

158

Sprachkenntnisse:

Spanisch

Deutsch

Englisch

Griechisch

Berufserfahrung:

09/2002 – 12/2003

01/2002 – 07/2002

01/2001 – 07/2001

Berlin 2008

Muttersprache

Fließend in Wort und Schrift. DSH (Deutsche Sprachprüfung für den

Hochschulzugang).

Fließend in Wort und Schrift. Zertifikat der offiziellen Sprachschule

in Spanien (10 Semester). Irland: zwei 4-wöchige Sommerkurse.

Grundkenntnisse

Wissenschaftliche Mitarbeiterin am Forschungszentrum Karlsruhe,

Institut für Technische Chemie, Abteilung Thermische

Abfallbehandlung; im Rahmen eines EU-Projektes mit dem Thema:

„Konzepte modularer Kleinkraftwerke zur dezentralen Kraft-Wärme-

Nutzung mit biogenen Festbrenn-stoffen“.

Wissenschaftliche Hilfskraft am Lehrstuhl für Thermische

Verfahrenstechnik von Prof. Dipl.-Ing. Dr. techn. Hans-Jörg Bart.

Wissenschaftliche Hilfskraft am Lehrstuhl für Thermische

Verfahrenstechnik von Prof. Dipl.-Ing. Dr. techn. Hans-Jörg Bart.

Appendix

159

Publication Index

Articles

A novel synthetic route to mesostructured MoVTeNb mixed oxide

A. Sakthivel1, A. Trunschke1, T. W. Hansen1, A. Celaya Sanfiz1, O. Timpe1, F.

Girgsdies1, R. E. Jentoft1, F. Mothar1, R. Schlögl1; in preparation.

Preparation of phase-pure M1 MoVTeNb oxide catalysts by hydrothermal synthesis

– Influence of reaction parameters on structure and morphology

A. Celaya Sanfiz1, T. W. Hansen1, F. Girgsdies1, O. Timpe1, E. Rödel1, R. Ressler2, A.

Trunschke1, R. Schlögl1; accepted for publication in Topics in Catalysis.

Investigation of catalytic behavior of M1-phase catalyst in the selective oxidation of

propane to acrylic acid. Correlation approach between catalytic performance and

surface/bulk properties.

A. Celaya Sanfiz1, T. W. Hansen1, F. Girgsdies1, P. Schnörch1, D. Teschnner1, A.

Trunschke1, R. Schlögl1, M. H. Looi3, S. B. A. Hamid3; in preparation.

How important is the (001) plane of M1 for selective oxidation of propane to acrylic

acid.

A. Celaya Sanfiz1, T. W. Hansen1, A. Sakthivel1, A. Trunschke1, R. Schlögl1, A.

Knoester4, H. H. Brongersma4, M. H. Looi3, S. B. A. Hamid3, Journal of Catalysis 258

(2008), 35.

New synthesis routes of MMO catalysts by dilution of Mo-V-X-Nb (X=Te, Bi, and P)

mixed oxides with SiO2, Cr2O3 or ZrO2 for the oxidation of propane to acrylic acid.

A. Celaya Sanfiz1, F. Girgsdies1, A. Trunschke1, R. Schlögl1, M. H. Looi3, S. B. A.

Hamid3; in preparation.

1 Fritz Haber Institute of the Max Planck Society, Department of Inorganic Chemistry,

14195 Berlin, Germany.

2 Technische Universität Berlin, Institute of Chemistry, 10623 Berlin, Germany.

3 COMBICAT, University Malaya, 50603 Kuala Lumpur, Malaysia.

4 Calipso B.V., 5600 MB Eindhoven, The Netherlands.

Appendix

160

Posters

Preparation and Characterization of Mo-V-M-Nb-O (M=Te, P, Bi) Catalysts

prepared by Hydrothermal Synthesis

A. Celaya Sanfiz, J.B. Wagner, A. Trunschke, R. Schlögl, S.T. Lee, M.H. Looi, S.B.A.

Hamid, XXXVIII. Jahrestreffen Deutscher Katalytiker, March 16-18, 2005,

Weimar/Germany.

Oxidación Selectiva de Propano a Ácido Acrílico sobre Catalizadores de Óxidos de

Mo-V-Te-Nb preparados por el Método Hidrotermal

A. Celaya Sanfiz, J.B. Wagner, A. Trunschke, R. Schlögl, S.T. Lee, M.H. Looi, S.B.A.

Hamid, SECAT´05, Reunión de la Sociedad Española de Catálisis, June 27-29, 2005,

Madrid/Spain.

Mo-V-Te-Nb-O Mixed Metal Oxides –“Diluted” and “Undiluted”- Prepared by

Hydrothermal Synthesis for Selective Oxidation of Propane to Acrylic Acid

A. Celaya Sanfiz, J.B. Wagner, A. Trunschke, R. Schlögl, S.T. Lee, M. H. Looi, S.B.A.

Hamid, EuropaCat-VII, August 28-September 01, 2005, Sofia/Bulgaria.

Selective Oxidation of Propane to Acrylic Acid on Mo-V-Te-Nb-O Mixed Oxides

Catalysts Prepared by Hydrothermal Synthesis

A. Celaya Sanfiz, J.B. Wagner, A. Trunschke, R. Schlögl, S.T. Lee, M. H. Looi, S.B.A.

Hamid, 5WCOC, The World Congress of Oxidation of Catalysis, September 25-30, 2005,

Sapporo/Japan.

The Effect of Thermal Pre-treatment Parameters on Structural and Catalytic

Properties of MoVTeNbOx Catalysts

A. Celaya-Sanfiz, A. Blume, O. Timpe, R. Jentoft, F. Girgsdies, A. Trunschke, R.

Schlögl, XXXIX. Jahrestreffen Deutscher Katalytiker, March 15-17, 2006,

Weimar/Germany.

Microstructure, Bulk and Surface Properties of Single-phase MoVTeNb Oxide

Catalysts

A. Celaya Sanfiz, F. Girgsdies, T. W. Hansen, P. Schnörch, E. Rödel, A. Trunschke, R.

Schlögl, M. H. Looi, S. B. A. Hamid, 40 Jahrestreffen Deutscher Katalytiker, March 14-

16, 2007, Weimar/Germany.

Synthesis of Single-phase M1 MoVTeNbOx Catalysts – Optimization of

Hydrothermal Reaction Conditions

A. Celaya Sanfiz, F. Girgsdies, T. W. Hansen, E. Rödel, P. Schnörch, A. Trunschke, R.

Schlögl, EuropaCat-VIII, August 26-31, 2007, Turku/Finland.

Appendix

161

Selective Oxidation of Propane over MoVTeNb Oxide Catalysts Prepared by

Hydrothermal Synthesis

A. Celaya Sanfiz, F. Girgsdies, T. W. Hansen, P. Schnörch, D. Teschner, A. Trunschke,

R. Schlögl, M. H. Looi, S. B. A. Hamid, 41 Jahrestreffen Deutscher Katalytiker, February

27-29, 2008, Weimar/Germany.