Kinetic investigation oft he heterogeneously catalysed selective

Fakultät für Chemie und Biochemie

Ruhr-Universität Bochum

Kinetic Investigation of the Heterogeneously

Catalysed Selective Oxidation of Propene to Acrolein

on Molybdenum Based Mixed Oxides

vorgelegt von

Thomas Franzke

aus Bochum

zur Erlangung des Grades

eines Doktors der Naturwissenschaften

Bochum, Dezember 2010

Diese Arbeit wurde im Zeitraum von November 2006 bis Oktober 2010 am Lehrstuhl für

Technische Chemie der Ruhr-Universität Bochum angefertigt.

Betreuender Hochschullehrer: Prof. Dr. Martin Muhler

Erster Gutachter: Prof. Dr. Martin Muhler

Zweiter Gutachter: Prof. Dr. Wolfgang Grünert

Danksagung

Herrn Prof. Dr. Martin Muhler danke ich für die vertrauensvolle Bereitstellung des The-

mas, sowie für anregende Diskussionen in zahlreichen Besprechungen. Weiterhin bin ich

ihm für die weitgehenden Freiheiten im Zusammenhang mit der Entstehung dieser Arbeit

zu Dank verpichtet.

Herrn Prof. Dr. Wolfgang Grünert danke ich für die Übernahme des Korreferats, sowie

für die Möglichkeit XPS- und ISS-Messungen in seiner Arbeitsgruppe durchführen zu

können.

Bei Frau Dr. Wilma Busser, Frau Dipl.-Chem. Dagmar Scholz und Herrn Dr. Volker

Hagen bedanke ich mich herzlichst für die freundliche Zusammenarbeit auf den Gebieten

der angewandten Massenspektrometrie und Gaschromatographie.

Herrn Dipl.-Ing. Bruno Otto, Herrn Dipl.-Ing. Horst Otto und Herrn Heinrich-Josef Pfeif-

fer möchte ich für die Unterstützung in technischen Fragen, insbesondere der Umsetzung

des Anlagenneubaus und der Instandhaltung danken.

Bei Herrn Dr. Thomas Reinecke bedanke ich mich für die Durchführung der XRD Mes-

sungen und die hervorragende Einarbeitung in die dazugehörige Theorie und Praxis.

Frau Susanne Buse, Frau Lina Freitag, Frau Kirsten Keppler und Frau Sigrid Plischke gilt

mein Dank für die Durchführung zahlreicher charakterisierender Messungen, die Eingang

in diese Arbeit gefunden haben.

Herrn Dr. Uwe Cremer, Frau Dr. Katharina Horstmann, Herrn Dr. Josef Macht, Herrn

Dr. Andreas Raichle und Herrn Dr. Frank Rosowski danke ich für die vielen interessanten

Diskussionen und Anregungen, die diese Arbeit befördert haben.

Bei den Mitarbeitern des Lehrstuhls für Technische Chemie bedanke ich mich für fachliche

Diskussionen und das stimmige Arbeitsklima in der Zeit, die wir zusammen hier verbracht

haben.

I

Contents

List of symbols and abbreviations VII

1. Introduction 1

2. Literature Review 5

2.1. Acrolein . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1.1. Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1.2. Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1.3. Applications and reactions . . . . . . . . . . . . . . . . . . . . . . 7

2.2. Mechanism of the heterogeneously catalysed selective oxidation of propene

to acrolein . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.2.1. Formation of acrolein . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.2.2. Formation of byproducts . . . . . . . . . . . . . . . . . . . . . . . 13

2.3. Multicomponent catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.4. Kinetics of the heterogeneously catalysed selective oxidation of propene to

acrolein . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3. Experimental 23

3.1. Transient kinetic experiments . . . . . . . . . . . . . . . . . . . . . . . . 23

3.1.1. Experimental background . . . . . . . . . . . . . . . . . . . . . . 23

3.1.2. Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.1.3. Experimentation . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.2. Steady-state kinetic experiments . . . . . . . . . . . . . . . . . . . . . . . 28

3.2.1. Experimental background . . . . . . . . . . . . . . . . . . . . . . 29

3.2.2. Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.2.3. Experimentation . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

4. Model Catalysts for Selective Propene Oxidation Probed by Redox Cy-

cling 37

4.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

III

Contents

4.2. Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

4.2.1. Catalyst preparation . . . . . . . . . . . . . . . . . . . . . . . . . 38

4.2.2. Characterisation . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

4.2.3. Temperature-programmed experiments . . . . . . . . . . . . . . . 41

4.3. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42



4.4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

4.5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

5. Optimisation of mixed oxide catalysts 63

5.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

5.2. Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65




5.2.4. Catalytic activity measurements . . . . . . . . . . . . . . . . . . . 67

5.3. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67



5.3.3. Catalytic activity measurements . . . . . . . . . . . . . . . . . . . 74

5.4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

5.5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

6. Kinetics of propene oxidation over multicomponent mixed oxide cata-

lysts 87

6.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

6.2. Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88



6.2.3. Kinetic experiments . . . . . . . . . . . . . . . . . . . . . . . . . . 90

6.3. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

6.3.1. Preliminary considerations . . . . . . . . . . . . . . . . . . . . . . 90

6.3.2. Kinetics of the selective oxidation of propene . . . . . . . . . . . . 92

6.3.3. Evaluation of kinetic data . . . . . . . . . . . . . . . . . . . . . . 105

6.4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

6.5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

IV

Contents

7. Concluding remarks 113

List of Tables 127

List of Figures 129

A. Figures 133

B. Determination of mass and heat transfer coecients 149

B.1. Diusion coecient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

B.2. Mass transfer coecient . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

B.3. Heat transfer coecient . . . . . . . . . . . . . . . . . . . . . . . . . . . 150

V

List of Symbols and abbreviations

Latin symbols

Symbol Denition Unit

A frequency factor s−1

Ai peak area of GC signal of component i −bp boiling point C

ci,g gas phase concentration of substance i molm−3

di inner diameter m

Di,eff eective diusivity of substance i m2 s−1

dp particle diameter m

dp mean (geometric) particle diameter m

EA activation energy kJmol−1

h heat transfer coecient Wm−2 K−1

∆HR reaction enthalpy kJmol−1

K adsorption constant −K characteristic number −k rate constant varying

kg mass transfer coecient ms−1

m mass ow kg s−1

mcat catalyst mass kg

N number of species −ni amount of substance i mol

ni molar ow rate of substance i mol s−1

Nu Nusselt number -

p pressure Pa

pi partial pressure Pa

Pr Prandtl number -

continued on next page

VII

List of symbols and abbreviations


R gas constant Jmol−1 K−1

Re Reynolds number -

reff eective reaction rate molm−3 s−1

S selectivity −Sc Schmidt number -

Sh Sherwood number -

SBET BET specic surface area m2 g−1

t time s

TR reaction temperature C

Tcalc calcination temperature C

THuttig Hüttig temperature C

Tmelt melting point C

TTammann Tammann temperature C

X conversion −Y yield −yi mole fraction of substance i −

Greek symbols


α degree of reduction -

β heating rate Kmin−1

β stoichiometric coecient -

ϵ bed porosity -

ϵ catalyst porosity -

η eectiveness factor -

λ stoichiometric coecient -

ν stoichiometric factor -

Θ coverage -

τ modied residence time kg smol−1

VIII

List of symbols and abbreviations

Abbreviations

Abbreviation Denition

ACA acrylic acid

ACR acrolein

BET Brunauer-Emmett-Teller

BJH Barret-Joyner-Halenda

EDX energy-dispersive X-ray analysis

GC gas chromatograph

GHSV gas-hourly-space-velocity

ICP-OES inductively coupled plasma optical emission spectrometry

IR infrared

ISS ion scattering spectroscopy

MMO multicomponent (multielement) mixed oxide

ODH oxidative dehydrogenation

PFR plug-ow reactor

RRF relative response factor

SEM scanning electron microscopy

SSITKA steady-state isotopic transient kinetic analysis

STP standard temperature and pressure

TAP temporal analysis of products

TCD thermal conductivity detector

TLV threshold limit value

TPD temperature-programmed desorption

TPO temperature-programmed oxidation

TPR temperature-programmed reduction

TEM transmission electron microscopy

WHSV weight-hourly-space-velocity

XPS X-ray photoelectron spectroscopy

XRD X-ray diraction

IX

1. Introduction

It is remarkable that in chemical industry a great variety of materials with tailored pro-

perties is produced from a manageable number of feedstocks and intermediates. Acrolein,

the simplest unsaturated aldehyde, is such a versatile organic intermediate. By a multiple-

stage reaction acrolein is transformed into the amino acid methionine, which is used as

an animal feed supplement. An alternative route is its oxidation to acrylic acid, which is

the basis for acrylates and superabsorbent polymers.

In general the incentive for research in chemical industry is cost reduction, which is

achieved by an increased eciency of the production processes. Acrolein itself is nowadays

exclusively produced by the selective oxidation of propene, and a multitude of industrial

processes is competing for this valuable feedstock. In fact, the increasing demand for

propene is believed to result in a shortage of propene, which will generally lead to higher

prices. Besides material also energy eciency and emission reduction are factors of gro-

wing importance. This is mainly due to increasing prices for primary energy and high

environmental specications in most industrialised countries, which require cost-intensive

end-of-pipe technology. However, facing the growing awareness of the population for en-

vironmental issues, the promotion of sustainable or environmentally acceptable processes

will also add to the corporate image.

Since the scale of production of these chemicals is of the order of some million tons

per year, the possible savings justify the investment in research, and catalysis is a key

technology in the production of bulk chemicals. Catalysts in general accelerate chemical

transformations by providing an alternative reaction mechanism, which lowers the overall

activation energy of the process. In the oxidation of hydrocarbons this is the key to

selectivity, since thermodynamically the total oxidation of hydrocarbons to CO2 is always

most favourable.

The direct synthesis of acrolein by the selective and heterogeneously catalysed oxida-

tion of propene was rst reported by Shell in 1948 using cuprous oxide (Cu2O) as the

active catalyst phase. However, it was not until 1959 when the discovery of the outstan-

ding activity and selectivity of bismuth molybdate-based catalysts triggered a massive

increase in research activities in industry and academia. In the following years this was

mainly directed at elucidating key features of the reaction mechanism and developing

1

1. Introduction

new catalytic materials. These eorts yielded a great diversity of active and selective

catalysts comprising mainly binary and ternary oxides containing molybdenum and anti-

mony. Simultaneously, the mechanistic studies enabled the establishment of a conclusive

reaction mechanism for acrolein production and also for some of the byproducts of pro-

pene oxidation. Several studies aimed at the identication of active ensembles in the

catalyst structure.

Another milestone in catalyst development was the recognition of the importance of a

multiphasic nature of catalysts for selective allylic oxidation, which is due to a pronounced

synergetic eect between the constituent phases. Today's commercial acrolein catalysts

are multiphase systems, at the least containing 4-5 main and transition group metals

as major elements forming complex oxides and solid solutions. Additionally, numerous

main group elements are usually included as structural and electronic promoters. New

mechanistic investigations were hence directed at identifying the nature of the observed

synergy. The synergy eect was found to be linked to the interphase mobility of lattice

oxygen. Numerous models of the active structure were proposed relating these ndings

to the results of improved physico-chemical characterisation of the catalyst material.

Although signicant progress has been made in the last decades the working mechanism

of multicomponent catalysts is still not completely understood and there is some debate

on the nature of the active site. Furthermore, the eorts of research can hardly keep

pace with the rapid development in industry, introducing new structures and materials.

Consequently, the role of promoters and additives has attracted little attention.

Recently, a catalyst system representing a modication of the traditional multiphase

systems was reported to be highly active and selective in the oxidation of propene to

acrolein [1, 2]. The modication consists in the replacement of bismuth molybdate by

bismuth tungstate. This nding prompts a number of new, interesting questions and

oers an alternative approach to some of those addressed by the previous research. A key

feature of this new type of oxidation catalyst is the absence of a crystalline phase that

has formerly been reported as being active and selective on its own. Bismuth tungstates

have been tested as catalysts for the selective oxidation of propene before, but were found

to exhibit poor activity and selectivity to acrolein [3]. It is thus inferred that either the

synergy observed is not simply due to an interphase oxygen transport or that a massive

structural rearrangement takes place, which has not yet been reported by the authors.

This calls for an investigation of the reaction mechanism, since the classic mechanisms for

multicomponent systems assume that the active site is present on the bismuth molybdate

phase.

The main objective of the present work was a comparative study of multicomponent

mixed oxide (MMO) catalysts being molybdate- or tungstate-based. Due to the mar-

2

1. Introduction

ked structural complexity of these systems the approach followed here is primarily based

on transient and steady-state kinetic investigations. In the rst stage, cyclic reduction

and reoxidation experiments were developed as a tool to characterise single phase model

catalysts and MMO systems with respect to oxygen mobility and stability in the redox

cycle. In a second step, mixed oxide catalysts based on either bismuth molybdate or

tungstate were prepared and optimised. The kinetics of propene oxidation were investi-

gated under varying reaction conditions over selected catalysts. Since the microkinetics

reect the reaction mechanism, the comparison of the kinetic ngerprint of molybdate-

and tungstate-based catalysts was expected to yield information on the nature of the

active site and the working mechanism of multicomponent mixed oxide catalysts.

3

4

2. Literature Review

2.1. Acrolein

2.1.1. Properties

At ambient conditions acrolein is a colourless liquid with low viscosity [4, 5]. As a low-

boiling aldehyde it is lacramatory and possesses a penetrative odour. Due to the conjuga-

tion of the vinyl and aldehyde group it is extremely reactive, which in turn is the reason

for its toxicity. Some important properties of acrolein are summarised in Table 2.1. The

data show that at ambient conditions the vapour pressure of acrolein exceeds both the

lower ammability limit and the threshold limit value. This demonstrates the need for

comprehensive safety precautions in the handling of acrolein.

2.1.2. Production

First plans to commercialise acrolein production via dehydration of glycerol were abando-

ned due to the low eciency of the process and high glycerol costs [7]. Recently, the topic

has attracted new attention due to the increase in glycerol availability as a consequence

of growing biodiesel production [8].

In 1942 Degussa introduced the rst industrial process for acrolein production by

condensation of formaldehyde and acetaldehyde. The reaction was carried out in tubu-

lar reactors at temperatures of 300− 320 C in the presence of silica-supported sodium

silicate catalysts. By recycling of unreacted aldehydes yields up to 80 % were obtained.

In the mid-forties Shell developed a process for the direct oxidation of propene

(Eq. 2.1) based on supported copper catalysts. This process and similar ones opera-

ted at temperatures between 370− 400 C and low conversion requiring high recycling

streams. The redox potential of the gas phase was crucial in this process in order to keep

the catalyst in its active state. Yields reported for this process range from 68− 81 %.

CH2−−CHCH3 +O2 −→ CH2−−CHCHO+ H2O (2.1)

5


Table 2.1.: Physical properties and toxicology of acrolein (from [5]).

M , [g/mol ] 56.06bp (101.3 kPa), [C ] 52.69Vapour pressure (20 C), [kPa ] 29.3Viscosity (20 C), [mPa s ] 0.35Solubility in water (20 C), [g/kg ] 260

Flammability limits in air, [vol%]lower 2.8upper 31

LC50 (inhalation, 4 h), [mg/L ]* 0.018LD50 (skin), [mg/kg ]* 200TLV, [mg/m3 ]* 0.2

* from [6]

The major breakthrough was the discovery of the outstanding activity and selectivity

of complex oxides and in particular of bismuth molybdates in selective oxidation reactions

of hydrocarbons by SOHIO in 1959. This type of catalyst is characterised by a higher

tolerance towards varying reaction conditions as compared to the copper-based systems.

Accordingly, high yields can be achieved in single pass operation. Further development

resulted in increasingly active and selective catalysts by incorporation of additional ele-

ments. The basis of modern catalyst systems is composed of a minimum of four transition

metals present in at least two complex oxide phases. Virtually all commercial processes

are nowadays based on this catalyst concept and yields up to 91 % for acrolein and 97 %

for acrolein and acrylic acid have been reported.

Typically, processes operate at 300− 400 C and pressures of 1.5− 2.5 kPa. The feed

gas contains 5− 10 % propene, which is mixed with air and a diluent, which is steam or

o-gas from the process. High inert concentrations and steam are necessary in order to

avoid the ammability limits (FL) of propene (LFL298 = 2.4 % and UFL298 = 10.3 % in

air). The oxygen/propene molar ratio is set to a value around 1.6, because lower values

result in reduction of the catalyst limiting eectiveness and catalyst lifetime.

Fig. 2.1 shows a simplied ow sheet of the production process for isolated acrolein. The

reactants are fed to a multitubular xed bed reactor (a). The reaction heat is dissipated

by a molten salt bath circulating around the individual tubes. After a contact time of

1.5− 3.5 s the euent stream is quenched in order to prevent homogeneous reactions in

the postcatalytic zone. In a rst purication step the product stream is scrubbed with

water to remove acrylic and acetic acid and polymers (b). In the subsequent absorber

(c) the condensable products, mainly acrolein and acetaldehyde, are absorbed in cold

6


Steam

Air

Propene

Quencher

Steam

Inert gas

Purge

Acids andpolymers

lightfraction

Acrolein

d ecba

e

Figure 2.1.: Flow scheme for acrolein production by propene oxidation. a) Reactor;b) Scrubber; c) Absorber; d) Desorber; e) Fractionators.

water. The o-gas at the column head contains nitrogen, oxygen, carbon oxides, propene

and propane. Due to the high inert content it can be partly refed to the reactor. To

avoid accumulation the rest is purged and combusted. Crude acrolein is then recovered

from the aqueous solution in a desorption column (d). The bottom stream is refed to

the previous column as the absorbent. In two following distillations acrolein is further

puried by removing low and high-boiling by-products.

2.1.3. Applications and reactions

Because of its high reactivity acrolein exhibits a rich organic chemistry, which is described

in detail in [4] and [5]. Furthermore, the synthesis of a number of speciality chemicals

from acrolein and their application is reported. In low concentrations acrolein is used as

a broad-spectrum biocide to control growth of aquatic organisms in recirculating water

systems. However, the most important applications of acrolein are the oxidation to acrylic

acid and the multiple-stage synthesis of methionine.

The polymers of acrylic acid and acrylates (esters of acrylic acid) have a wide range

of applications in the paint, paper and textile industry. In the presence of molecular

oxygen and suitable catalysts acrolein is oxidised to acrylic acid as the second step of the

two-stage process for acrylic acid production (Eq. 2.2).

7


C3H4O+ 12O2

Mo-V-W-O−−−−−−→ C3H4O2 (2.2)

The process is carried out in an integrated plant. The euent of the rst reactor

producing acrolein from propene is mixed with air and fed to the second stage without

further purication. The second reactor operates at lower temperatures of 250− 300 C.

Similar to the selective oxidation of propene, mixed metal oxides and solid solutions based

on molybdenum are used as catalysts for this reaction, the key component in this system

being vanadium [9]. The two-step operation has enabled the individual optimisation of

both steps with respect to catalyst systems and reaction conditions. Recently, the single

phase Mo-V-Te-Nb catalyst systems, which are able to directly transform propane into

acrylic acid, have attracted a lot of attention [10]. The approach looks promising, because

propane is a relatively cheap and abundant feedstock.

Methionine is an essential proteinogenic amino acid. Since it is not produced in mam-

mals, including humans, it has to be ingested. Therefore, synthetic methionine is a

common feed supplement in animal breeding. It is produced in a three-stage synthesis

from acrolein, methyl mercaptan, hydrogen cyanide and ammonia:

C3H4O+ CH3SH + HCN+ NH3 −→ D,L−CH3SCH2CH2CH(NH2)COOH (2.3)

By addition of methyl mercaptan to the C-C double bond of acrolein methyl-mercapto-

propionaldehyde is obtained. Subsequent to chain elongation by hydrogen cyanide at the

aldehyde function, the reaction with ammonium carbonate yields a hydantoin, which is

then hydrolised yielding racemic methionine.

Among the potential reactions of acrolein, those presented in the following are conside-

red to be relevant to the present study. The uncatalysed thermal dimerisation reaction ac-

cording to the Diels-Alder mechanism yields 3,4-dihydro-2H-pyran-2-carboxaldehyde.

This and higher oligomers have high boiling points giving rise to condensation and pos-

sibly polymerisation in downstream piping. Furthermore, they usually escape analysis

by gas chromatographic (GC) applications designed for separation and identication of

lower-boiling compounds. The polymerisation of acrolein is triggered by heating, expo-

sure to light and radical or ionic initiators. This is important in view of pipe blocking and

controlled polymerisation in order to limit the acrolein concentration in the exhaust of

laboratory-scale plants. In acidic environments the aldehyde group of acrolein undergoes

acetalisation. In a heterogeneously catalysed reaction the catalyst surface may provide

8


OH groups and an acidic environment. Thus, acetals and hemiacetals were proposed as

adsorbate structures for acrolein on oxidation catalysts [1113].

2.2. Mechanism of the heterogeneously catalysed

selective oxidation of propene to acrolein

The mechanism of acrolein formation from propene has been extensively studied, and the

ndings are summarised in several reviews [1419]. It is observed that the focus of research

has shifted over the three decades, in which the topic has been exploited intensively. The

rst mechanistic studies concentrated on the elementary steps of the transformation of

propene into acrolein disregarding the role of the catalyst. These investigations typically

involved the use of isotopically labelled reactants. The need for bifunctionality of the

catalyst, that is, hydrogen abstraction and oxygen insertion, together with the discovery

of binary oxides then raised the question for the structure of the active site and the

nature of the active oxygen species. Last but not least, the attention was drawn to

multicomponent oxide catalysts in an attempt to identify the role of the dierent phases.

Among others, isotopic transients proved a suitable tool in these investigations.

2.2.1. Formation of acrolein

On copper-based catalysts the formation of acrolein was found to depend critically on the

redox potential of the gas phase and thus the extent of reduction of the catalyst [4,15,16].

Deviations from the optimum resulted in either a lack or a surplus of active oxygen, both

eecting non-selective oxidation. The nding that the limitation of oxygen activity was

a prerequisite for selectivity has been conceptualised in the term site isolation [15, 20].

In contrast, bismuth molybdates and other complex oxides exhibited relatively little

dependence of the selectivity on the gas-phase redox potential. Furthermore, the reaction

was found to proceed selectively even in the absence of molecular oxygen in the gas phase,

resulting in a substantial reduction of the catalyst material [21]. It thus appeared that

the source of oxygen for the selective transformation of propene to acrolein was indeed

lattice oxygen of the catalyst. Molecular oxygen was accordingly necessary to replenish

the lattice vacancies. This reduction-reoxidation cycle has become well-known as the

Mars-van Krevelen mechanism.

Evidence for the participation of lattice oxygen was provided by the Keulks group

[22,23],Wragg et al. [24], and later byMoro-Oka et al. [17] using isotopically labelled

oxygen. Oxidising propene in the presence of 18O2 over a non-labelled catalyst, they

found up to 100 % of the catalyst lattice oxygen in the reaction products. However, the

9


extent of lattice oxygen participation was found to be temperature-dependent with only

part of the lattice oxygen being available at low temperatures (350 C) [22,23,25].

By using 13C and 14C labelled and deuterated propene, researchers of the Shell group

showed that the formation of acrolein proceeds initially via the abstraction of a hydrogen

atom from the methyl group and the formation of a symmetric allyl species [2628]. The

existence of an allyl adsorbed as π- or σ-species was veried by Davydov et al. [29] on

a gallium molybdate catalyst by IR spectroscopy.

Most likely, the allyl then reacts with lattice oxygen to form a σ-oxo-allyl species

(O−CH2−CH−−CH2) prior to undergoing a second hydrogen abstraction in the allyl posi-

tion. Earlier studies though indicated that the second hydrogen abstraction should occur

before the incorporation of oxygen [16,22,23]. It was shown that deuterium-labelled allyl

species (e.g. CD2−CD−CH2) did not react with equal probability on either the deutera-

ted or non-deuterated side. However, Burrington et al. [30] used isotopically labelled

allyl alcohol as a molecular precursor for the σ-oxo-allyl species and showed that a rapid

equilibration of this species occurred, thereby accounting for the observed deuterium dis-

tribution in the previous studies. It has further been argued that the second abstraction

of hydrogen from the resonance stabilised allyl should be more dicult than the rst,

which would thus not be rate-determining [19].

A secondary reaction pathway leading to the formation of acrolein was proposed by

Daniel and Keulks [31]. Using a reactor with a variable post-catalytic reaction vo-

lume, they were able to detect the occurrence of surface-initiated homogeneous reactions.

Analogous to the mechanism of propene oxidation on supported noble metals [32], they

suggested a hydroperoxide as the reaction intermediate. This was either to react ho-

mogeneously to give propene oxide or to decompose to acrolein at the catalyst surface.

However, later investigations showed that this mechanism was negligible under the usual

reaction conditions [22].

Despite the applicability of comprehensive concepts in oxidation catalysis, it should

be noted that the mechanism of well-understood reactions may depend on the catalyst

employed. In a recent series of contributions Zhao and Wachs studied the oxidation of

propene on a monolayer of vanadium oxide supported on Nb2O5 [33, 34]. Their ndings

indicate that on the active and selective single site catalyst it is the second hydrogen

abstraction, which is rate-determining and which precedes the incorporation of oxygen.

Obviously, the limitation of active oxygen species at monolayer or sub-monolayer coverage

requires an alternative reaction mechanism.

In order to elucidate the nature of the active site on bismuth molybdate catalysts se-

veral researchers studied the oxidation of propene on the constituent oxides Bi2O3 and

MoO3. They observed that in the absence of oxygen bismuth oxide converts propene

10


to 1,5-hexadiene. On the other hand, molybdenum oxide exhibits some selectivity to

acrolein, but is poorly active. These results indicate that the rate-determining hydro-

gen abstraction is associated with a bismuth-containing site and the insertion of oxygen

with a molybdenum-containing site. Thus, in the absence of molybdenum the allyl spe-

cies formed on bismuth oxide combine to give 1,5-hexadiene. In order to conrm the

oxygen-inserting function of molybdenum it was tempting to study the interaction bet-

ween molybdenum oxide and some reactants that readily form allyl species. It was found

that allyl species formed from decomposition of azopropene and allyl halides were selecti-

vely transformed into acrolein on pure molybdenum oxide [15,35]. Further corroboration

results from an investigation by Miura et al. [36]. These authors studied the oxdidative

dehydrogenation of 1-butene to butadiene using model catalyst exhibiting single structu-

ral features of γ-Bi2MoO6. They found out that bismuth oxyhalides possessing Bi2O2+

2

layers selectively oxidised butene, while La2MoO6 having MoO 2

4 layers was only active

in isomerisation and deep oxidation. This also indicates that hydrogen abstraction from

alkenes is facilitated by bismuth-containing sites.

The role of dierent oxygen species in the structure of complex oxides has also been

studied by Raman spectroscopy using isotopic tracers [3739]. The common approach is

the removal of lattice oxygen by suitable reductants and its subsequent replacement by18O, which results in a shift of the respective vibrational modes. Glaeser et al. [38]

showed by this method that prereduction by 1-butene of a γ-Bi2MoO6 catalyst produced

no band shifts in the frequency region associated with Mo-O stretching modes. With

catalysts prereduced by propene, however, immediate shifts of the respective Raman

signals were observed. Similarly, Ono and Ogata [39] detected that oxygen exchange

occurred preferentially at those Mo polyhedra in α-Bi2Mo3O12 having adjacent bismuth

ions. This supports the perception that oxygen anions bridging bismuth and molybdenum

are the active species for hydrogen abstraction.

In a similar approach Ueda et al. [40] reduced γ-Bi2MoO6 catalysts by either propene

or 1-butene. The samples were then reoxidised by 18O2 and the oxidation of propene was

carried out in the absence of gas-phase oxygen. Substantial amounts of 18O were found

in the reaction products, when the prereduction had been carried out using propene,

but only traces were found when the reductant had been 1-butene. In another set of

experiments catalysts were labelled with 18O by oxidising either propene or 1-butene in

the presence of 18O2 under steady-state conditions. Oxidation of propene on the former

catalyst immediately yielded labelled products, while on the latter a delayed incorporation

of labelled oxygen into the products was observed. These results are consistent with

the concept that two dierent oxygen species are involved in hydrogen abstraction and

11


oxygen insertion. However, they indicate that an interlayer oxygen transport by diusion

is possible.

Recently, the reduction-reoxidation behaviour of bismuth molybdates was studied by

Ayame et al. [41, 42] using in-situ X-ray photoelectron spectroscopy (XPS). The results

showed that bismuth was not reduced in the process, but time-dependent evolution of

Mo 5+ and Mo 4+ in equal amounts was observed. Reoxidation of the former was found

to proceed faster at low temperatures, while higher temperatures favoured the stepwise

reoxidation of the latter. The ndings support the perception that the rst hydrogen

abstraction from propene is brought about by a lattice oxygen bridging bismuth and

molybdenum (Mo 6+−O−Bi 3+), while the oxygen incorporated into acrolein is doubly

bound to molybdenum (molybdenyl species). Furthermore, the authors conclude that

the reoxidation sites are vacancies bridging bismuth and molybdenum. In an earlier

contribution Okamoto et al. studied the reduction of γ-Bi2MoO6 by hydrogen, 1-butene

and allyl alcohol [43]. Reduction by hydrogen and allyl alcohol yielded only reduced

molybdenum and bismuth species, respectively. When 1-butene was used as reductant

both bismuth and molybdenum were reduced, whereas oxygen was preferentially removed

from the bismuth layers.

Based on ab initio calculations Jang and Goddard [44] report that the most favou-

rable mechanism over small Bi/Mo model clusters requires the formation of a BiV species

for the rate-determining hydrogen abstraction. The allyl species generated is then ad-

sorbed on a site consisting of two adjacent MoVI centres, where it is transformed into

acrolein leaving the Mo sites reduced to Mo IV and MoV.

Grasselli et al. [45] proposed a reaction mechanism for acrolein formation on bismuth

molybdates, which is consistent with most of the aforementioned observations. Recently,

Grasselli [46] suggested a revised mechanism on the basis of the theory study by Jang

and Goddard [44], which is schematically shown in Fig. 2.2. The results can thus be

summarised as follows:

• An α-hydrogen is abstracted from propene by an oxygen associated with bismuth.

• The allyl is adsorbed as a π-species on a site consisting of two adjacent di-oxo MoVI

species.

• The π-species is reversibly transformed into a σ-oxo allyl species by reaction with

a molybdenyl species.

• A second hydrogen abstraction occurring by action of the adjacent MoVI species

leads to the formation of acrolein.

12


H

H

O

[O]H O2

Bi Mo

O

OO

O

OO

3+ 6+Mo

OO

6+

OO O

Bi Mo

O

OO

O

OO

3+ 6+Mo

OO

6+

OO O

Bi Mo

O

OO

O

OOH

2+ 6+Mo

OO

6+

OO O

O

Bi Mo

OO

O

OOH

2+ 5+Mo

OO

6+

OO O

Bi Mo

OO

O

OOH

2+ 4+Mo

OHO

5+

OO O

Figure 2.2.: Mechanism of acrolein formation [46].

• Two surface hydroxyls formed by the reaction are eliminated as water leaving an

oxygen vacancy. The oxygen vacancies are then relled by active oxygen species

from the lattice or the gas phase depending on the reaction conditions.

It is noted that no nal agreement regarding the electronic state of the species involved

has been reached. However, Bettahar et al. [18] argued that the discussion is in parts

unnecessary, if the redox and acid-base steps at the surface are in equilibrium. This also

reects the perception of a dynamic and adaptive catalyst system.

2.2.2. Formation of byproducts

Like any selective oxidation process the oxidation of propene to acrolein gives rise to a

number of byproducts. The mechanisms leading to these products are naturally less well-

understood than that yielding acrolein. This is primarily due to the focus on the main

reaction and experimental limitations like dierential conditions or analysis methods. The

most important by-products regarding yield and their recognition in the open literature

are acrylic acid, CO, CO2, acetaldehyde, formaldehyde and acetic acid [5, 47]. However,

also the formation of methanol, formic acid, ethene, allyl alcohol, acetone, propene oxide,

13


propionaldehyde, propionic acid and oligomers of the aforementioned have been reported

in the literature.

Among the by-products of acrolein formation, acrylic acid is sticking out as the target

product in the two-step-process for acrylic acid production. Detailed kinetic data ob-

tained at integral conversion levels revealed that it is formed in a consecutive reaction

from acrolein [1,47]. Using a multicomponent mixed oxide catalyst designed for acrolein

production from propene, Boreskov et al. [48] and Gorshkov et al. [49] were able to

show that the main product of acrolein oxidation on this type of catalyst was acrylic acid

along with COx and acetaldehyde.

In most studies carbon oxides or at least CO2 have been reported to be the major

by-products of propene oxidation. However, dierent conclusions regarding their origin

have been drawn. Both parallel and consecutive reaction pathways have been proposed,

which could be attributed to greatly varying reaction conditions. Furthermore, several

mechanisms involving dierent active sites, oxygen species and adsorption geometries of

reaction intermediates have been put forward.

On the basis of kinetic investigations it has been established that carbon oxides are

indeed formed via parallel and consecutive reactions depending on the reaction conditions

[1, 47]. At low temperatures the parallel reaction mechanism dominates, while at high

temperatures and conversion levels consecutive pathways become increasingly important

under steady-state conditions.

Haber et al. [50] studied the oxidation of propylene over various molybdenum oxide-

based catalysts and attributed the occurrence of non-selective deep oxidation to the pre-

sence of electrophilic oxygen species on the surface, which originate from the reduction

of molecular oxygen. However, Krenzke and Keulks [22,51] demonstrated that lattice

oxygen is equally incorporated into acrolein and CO2 on bismuth molybdate catalysts.

Since oxygen exchange has been shown to be negligible on these catalysts [16], this proves

that COx is also produced involving nucleophilic oxygen species.

By means of transient kinetic investigations partly involving isotopically labelled oxy-

gen, several researchers found hydrocarbon species to be accumulated on the catalyst

surface under steady-state conditions [23, 5254]. Thereafter, an alternative mechanism

for the formation of carbon oxides is the continuous oxidation of this carbonaceous spe-

cies by either lattice oxygen [23] or oxygen from the gas phase [53, 54]. At the same

time, this mechanism explains the absence of parallel pathways of COx formation in

non-steady-state kinetic investigations.

Davydov et al. [29] studied the oxidation of propene over copper- and molybdenum-

based catalysts by infrared (IR) spectroscopy and found propene to be either reversibly

adsorbed as π- or σ-allyl species or irreversibly adsorbed as π- or carboxylate complex.

14


The latter are believed to lead to the products of total oxidation, while the former yield

acrolein.

By oxidising the dierent oxygen-containing products of propene oxidation over a bis-

muth molybdate catalyst, Gorshkov et al. [49] concluded that carbon monoxide is pri-

marily formed from consecutive reactions of aldehydes, while the oxidation of acids yields

mainly CO2. Furthermore, their ndings indicate that all aldehydes and acids formed

in the oxidation of propene may be degraded to CO and CO2. On the other hand, the

oxidation of CO to CO2 was shown to be negligible on bismuth- and molybdenum-based

catalysts [49,55,56].

Relatively little information is available regarding the formation of acetaldehyde, for-

maldehyde and acetic acid. The results of Redlingshöfer et al. [47] indicate that

acetaldehyde is formed in a parallel reaction pathway directly from propene. Weiss et

al. [13] and Bettahar et al. [18] suggest its direct formation by rupture of the C-C

double bond of propene as the result of the attack of an electrophilic oxygen species. On

the other hand, the results of Gorshkov et al. [57] indicate that acetaldehyde may also

be formed in a consecutive reaction from acrolein. McCain and Godin [58] studied the

oxidation of propene in a reactor with variable post-catalytic volume and detected the

formation of acetaldehyde from propene and acrolein in a surface-initiated homogeneous

reaction. The formation of formaldehyde is ascribed to the oxidation of acrolein in a

consecutive reaction pathway. Acetic acid is also formed in a consecutive reaction, most

likely by the oxidation of acetaldehyde [47,49].

2.3. Multicomponent catalysts

Virtually all commercial processes in operation for the selective oxidation of propene to

acrolein utilise multicomponent metal oxide catalysts. In addition to bismuth and mo-

lybdenum, these catalysts contain substantial amounts of di- and trivalent transition and

main group metals (see table 2.2). Furthermore, dierent main group elements usually

constitute a minor part of the catalyst mass. A compilation of catalyst formulations from

the patent literature is provided in [5]. Due to its multicomponent nature this highly ac-

tive and selective catalyst system is always composed of several phases. The resulting

structural complexity is a major drawback for investigations, because it is beyond the

performance of most of today's analytical methods. Nevertheless, some light has been

shed on the role of the main components and their interaction in the working catalyst.

Still, alkali and other main group promotors have attracted little attention in the open

literature. In analogy to other catalyst systems, it is inferred that these species are

15


Table 2.2.: Composition of multicomponent catalysts (from [17]).

component mol %

Mo6+ 50-55Bi3+ 3-7M2+ 30-35 Co, Ni, Fe, Mg, Mn, ...M3+ 8-15 Fe, Cr, AlM+ K, Na, Cs, Tl, ...X Sb, Nb, V, W, Te, ...Y P, B

present to modify sorption properties and as structural promotors increasing mechanical

and chemical stability.

Structural characterisation of multicomponent catalysts by X-ray diraction (XRD)

reveals that these are composed of bismuth molybdates and the molybdates of the di-

and trivalent cations, such as CoMoO4 and Fe2(MoO4)3. As compared to simple bis-

muth molybdates, MMO catalysts exhibit signicantly higher specic surface areas [17].

Catalytic tests considering the reaction rates per unit surface area indicate that a speci-

c interaction between the bismuth molybdate phase and the supporting phase occurs,

when the latter contains di- and trivalent cations, especially iron. Thus, in the absence

of trivalent components the activity of multicomponent catalysts per unit surface area

is always lower than that of bismuth molybdates. However, when iron is present, the

surface-related activity surpasses that of pure bismuth molybdates considerably [59]. It

is therefore obvious that the improved performance of MMO catalysts is not solely due to

dispersion of the active phase on a high surface area support, but originates from a strong

interaction between the two phases. The promoting eect of mixed iron/cobalt molyb-

dates on the activity of bismuth molybdates is often referred to as phase cooperation or

synergy eect.

The interaction of bismuth molybdates and mixed iron and cobalt molybdates has

thoroughly been studied by Japanese researchers [17, 59] by means of an 18O2 tracer

technique similar to that reported by Keulks et al. [22, 56]. Using isotopically non-

labelled catalysts they found that the extent of lattice oxygen incorporation into the

selective and non-selective oxidation products was limited to the amount associated with

the bismuth molybdate phase in case the support phase contained no trivalent metal

oxides. In the catalyst systems with incorporated iron, also the lattice oxygen of the

support phase was found to participate in the reaction. Thus, the authors were able

to establish that the extent of lattice oxygen participation is correlated to the catalytic

activity.

16


In a detailed investigation of mechanical mixtures of bismuth molybdates and iron

cobalt molybdates Millet et al. [60] studied the inuence of phase composition on

catalytic activity. They found that incorporation of a critical amount of iron into CoMoO4

eected a dramatic increase in electronic conductivity of the support phase, which in turn

is correlated to the activity of physically mixed catalysts. Furthermore, characterisation

by XPS and electron microscopy indicated that in the course of reaction spreading of the

bismuth molybdate phase over the support phase occurs. The authors conclude that the

morphological changes providing intimate contact between the phases are a prerequisite

for the synergy eect, which is based on the interphase exchange of electrons and anions.

Several authors have asserted that the dierent elements in multicomponent catalysts

are not homogeneously distributed. By XPS analysis of a series of model catalysts of

the general formula Mo12Bi0-1Co8Fe3Ox, Moro-Oka and Ueda [17] showed that bis-

muth and molybdenum are enriched at the surface of the catalyst, while iron and cobalt

are mainly situated in the catalyst bulk. The same was observed by Prasada Rao and

Krishnamurthy [61] for catalysts of the composition Mg11-xFexBiMoOn. These ndings

are further corroborated by the results of He et al. [62], who detected a surface enrich-

ment of bismuth and molybdenum on a similar catalyst by TEM and EDX (transmission

electron microscopy; energy-dispersive X-ray analysis).

Based on the aforementioned several active structure models and working mechanisms

have been proposed (quod vide Ref. [17]). Most of these models have in common that

the activation of molecular oxygen is brought about by the iron- or cobalt-containing

phase. The reduced oxygen species then diuses to the bismuth molybdate phase, where

it restores active sites reduced by the reaction with propene. A schematic representation

of this mechanism is shown in Fig. 2.3. According to Moro-Oka et al. and Matsuura

et al. [17] oxygen is dissociatively adsorbed on the iron-doped cobalt molybdate phase

and incorporated into the lattice. Oxygen transport through the bulk then occurs, which

is facilitated by vacancies induced by the replacement of divalent ions by Fe 3+. Delmon

and coworkers [63] in their remote control mechanism suggest the spillover of oxygen ad-

sorbed on a phase capable of activating molecular oxygen onto a second phase selectively

oxidising hydrocarbons.

2.4. Kinetics of the heterogeneously catalysed selective

oxidation of propene to acrolein

Two principal motivations for the investigation of the kinetics of a heterogeneously cata-

lysed reaction can be dierentiated. In reaction engineering a profound knowledge of the

17


Fe Co MoOx 1-x 4O2

OL

OL

CO, CO2

C H3 6 C H O3 4

oxygen vacancy

O2Oads

Bi M O2 x 3x+3

(a)(b)

Figure 2.3.: Working mechanism of multicomponent mixed oxide catalysts for acroleinformation. a) Bulk oxygen diusion [17]; b) spillover oxygen [63].

dynamics of the chemical transformations taking place in the catalyst bed is the funda-

mental basis in view of an ecient and safe operation of the reactor. On the other hand,

the microkinetics reect the elementary mechanistic steps taking place at the catalyst

surface, which is valuable information for the optimisation or development of catalytic

materials. Generally, the two objectives require a dierent approach.

For the dimensioning of an industrial process quantitative information on the reaction

kinetics are indispensable. The modelling and the prediction of the reactor dynamics re-

quire the solution of the mass and heat balances of the system. The input of the reactor

model therefore includes the parameters of the contributing transport processes and the

microkinetics coupling mass and heat balance. Ideally, a microkinetic model comprises all

elementary steps of a reaction including sorption processes and surface intermediates (in-

trinsic kinetics). The determination of those and the related kinetic parameters requires

extensive and time-consuming investigations. The approach is therefore usually limited

to relatively simple reactive systems. However, the suciently accurate description of an

industrial reactor does not necessitate a precise representation of the elementary steps

taking place at the surface of a catalyst. In certain cases simplications may be justi-

ed limiting the number of species to be considered without aecting the validity of the

model assumptions. In favourable cases the system may even be simulated by a pseudo-

homogeneous model. It is obvious that no mechanistic information is implemented in

such a conception.

On the other hand, the optimisation of heterogeneous catalysts requires information on

the nature of the active ensemble and the reaction intermediates involved in the rate- or

product-determining steps in order to modify the related catalyst properties. In general,

such information cannot be derived directly from kinetic investigations alone due to the

complex interaction of dierent elementary steps. A mechanistic model is acceptable in

case the kinetic equations derived thereof accurately reproduce the data obtained. The

achievable accuracy is also a function of the number of model parameters. Therefore,

18


additional evidence for the occurrence of a mechanism from complementary experimental

techniques (e.g. spectroscopy) is needed.

Power-law or hyperbolic rate equations are commonly used for the description of reac-

tion kinetics. These models are convenient for the interpolation of kinetic data, but

predictions extending the examined range of conditions can be problematic. Therefore,

this approach is limited to cases, where the parameter space is suciently covered by

experimental data.

In addition, a variety of rate equations based on mechanistic models has been sug-

gested. The most prominent example is probably the Mars-van Krevelen rate equation,

which is based on the famous redox mechanism established by these authors. It implies

that the rate of formation of the selective oxidation product is dened by the rate of

either catalyst reduction or reoxidation. However, in a recent analysis of the derived rate

equation Vannice [64] noted that it does not precisely reect the mechanism proposed

by Mars and van Krevelen. Nevertheless, he acknowledged that the rate equation

may suciently simulate reaction data, in case certain presumptions are met. Similarly,

Monnier and Keulks [23] described the reaction rate of propene on a β-Bi2Mo2O9

catalyst by the coupled kinetics of catalyst reduction and reoxidation.

Several authors have reported an optimum in acrolein selectivity at intermediate tem-

peratures over dierent molybdate-based catalysts [47,54,65]. Similar to previous sugges-

tions catalyst reduction and reoxidaton were described as elementary steps in the catalytic

cycle by these authors. However, Drochner et al. [54] suggested the non-selective total

oxidation of propene to occur by irreversible adsorption of propene on reduced sites re-

sulting in coke formation and its continuous burn-o by gas-phase oxygen. Consequently,

the optimum in acrolein selectivity may be explained in terms of a slow reoxidation of

active sites at low temperatures favouring coke formation.

So far, numerous studies have been published describing the kinetics of propene oxida-

tion to acrolein on dierent catalytic systems and experimental designs and under varying

reaction conditions. The catalyst systems examined comprise copper-based catalysts [66],

bismuth molybdates [22,23,57] and other complex oxides [51,67,68], and multicomponent

systems [1, 2, 47, 48, 52]. Conventional xed bed reactors are commonly used [57, 67, 68],

but also dierential [22,23,51] and multi-tap reactors [1,2], recirculating systems [48,66],

wall reactors [47] and TAP (temporal analysis of products) studies [52, 69] are on the

record. In view of the variety in this (incomplete) summary, it is not surprising that

quite a number of mechanisms and reaction networks have been proposed. The ndings

presented in the following attempt to express the common denominator of these reports.

Several authors have reported a rst-order dependence of the rate of acrolein formation

on the partial pressure of propene and zero-order dependence for oxygen at temperatures

19


of 400 C and higher. In a series of contributions comparing the reaction rates of deu-

terated and non-labelled propenes kinetic isotope eects were observed, when the allyl

position was partly or fully deuterated. These ndings are consistent with the abstraction

of an allylic hydrogen being the rate-determining step in the oxidation of propene on bis-

muth molybdate-based catalysts under these conditions. However, at lower temperatures

deviations from the aforementioned characteristics are observed.

The Keulks group studied the oxidation of propene on dierent bismuth molybdate

catalysts by classical kinetic measurements and using isotopic tracers [22, 23, 51]. They

found the reaction orders w.r.t. propene and oxygen to be temperature-dependent in the

range of 350 C to 450 C. Accordingly, at high temperatures rst-order dependence on

the propene partial pressure and zero-order dependence on the oxygen partial pressure

were observed in accordance with previous ndings. However, at lower temperatures the

propene dependence changed to zero, and a partial oxygen dependence was found. The

evaluation of the corresponding activation energies revealed that a break in the Arrhe-

nius plots occurred. The transition temperature matched with the temperature range, in

which the reaction orders changed. Analogous characteristics have also been reported by

other authors [17, 47]. Interestingly, the apparent activation energy at higher tempera-

tures amounted to less than half of that at lower temperatures. Thus, the results cannot

be rationalised in terms of an additional reaction pathway prevailing at higher tempera-

tures. This behaviour must be due to drastic alterations in the reaction mechanism or

(reversible) structural changes in the catalytic material. Brazdil et al. [70] observed

a similar temperature-dependence of activation energies for the reoxidation of bismuth

molybdates and a multicomponent system reduced by propene/ammonia mixtures. Low

activation barriers were found for the reoxidation of surface vacancies or at high tempera-

tures. At low temperatures bulk reoxidation was strongly activated. In accordance with

the ndings of the Keulks group this indicates that the reoxidation is rate-limiting at

low temperatures.

In another series of experiments propene was oxidised, in which either the methyl or

the methylene group were fully deuterated [23]. While the latter exhibited no eect at all,

a kinetic isotope eect was observed in case of the former. Nevertheless, the full kinetic

isotope eect was only found at 450 C, and partial kinetic isotope eects were detected

at 400 C and 350 C. When instead the reaction was carried out using 18O2 as oxidant

at 350 C, again a weak isotope eect was observed. The results indicate that at high

temperatures the abstraction of an allylic hydrogen is indeed rate-determining, while at

low temperatures oxygen participates in the slow step. Based on the observation that

the reaction initially proceeds at similar rates in the absence and presence of oxygen, the

authors concluded that the reaction is controlled by the redox kinetics of the catalyst

20


material. Correspondingly, they assume that catalyst reduction and reoxidation are the

rate-determining steps in the high- and low-temperature regime, respectively.

Although steam is in many cases added to the reaction gas mixture as a diluent, the

role of water has seldom been examined systematically. Saleh-Alhamad et al. [7173]

and Redlingshöfer et al. [47] both reported that water enhances the rate of acrolein

formation and suppresses the formation of carbon oxides. Furthermore, the authors claim

that water promotes the formation of acrylic acid from acrolein. The active role of water

was suggested to originate from a blocking of hyperactive sites, which cause non-selective

oxidation. Additionally, it was supposed that water acts as a co-oxidant keeping the

surface in highly oxidised state thus facilitating the desorption of oxygenates.

With lots of studies performed at low conversion levels or low partial pressures, very

few reports mention the inhibition of acrolein formation by reaction (by-)products. In

the absence of any systematic investigation of the topic, however, conclusive results can

not be presented.

21

22

3. Experimental

3.1. Transient kinetic experiments

Transient kinetic studies are a preferred tool in heterogeneous catalysis research, since

they usually provide fast and easy access to valuable mechanistic information such as

coverage and nature of reactive species. Temperature-programmed reduction (TPR) and

oxidation (TPO) are techniques widely employed in heterogeneous catalysis research.

These are generally utilised to probe the redox properties of solids, e.g. for nding the

optimum reduction or calcination temperatures for the activation of catalysts. This is

usually done by subjecting the samples to a controlled temperature programme in owing

hydrogen or oxygen.

When studying selective oxidation catalysts it can be advantageous to use the reactant

as the reducing agent. In the Mars-van Krevelen mechanism the catalyst undergoes

reduction by the hydrocarbon and is subsequently reoxidized by gas-phase oxygen. It

is therefore tempting to decouple the individual steps of this redox cycle by running

successive TPR and TPO experiments.

3.1.1. Experimental background

In standard temperature-programmed reduction and oxidation experiments the samples

react either with hydrogen or oxygen, while the temperature increases linearly in time.

The techniques thus allow for the determination of rates of reduction and oxidation,

if certain experimental conditions are fullled. The apparent rates may be aected by

several transport mechanisms, determining the nal shape of hydrogen or oxygen uptake

traces and the temperature of the maximum rate of reduction or oxidation.

It is recommended to conduct the experiments under dierential conditions, i.e. high

linear gas velocity and low reductant or oxidant conversion. This prevents concentration

gradients across the catalyst bed and the obscuring of the obtained data by outer mass

and heat transfer limitations. On the other hand, the uptake must be detectable with

sucient accuracy. Therefore, the determination of appropriate experimental parameters

is a challenging task. The parametric sensitivity of the method has been studied in detail

23

3. Experimental

by Monti and Baiker [74]. They provide a criterion ensuring the generation of usable

kinetic data.

K =ncat

V · c0(3.1)

Here, ncat is the amount of sample, V is the volumetric ow rate and c0 is the concen-

tration of the reductant in the gas phase. Malet and Caballero also include the

heating rate β in their characteristic number [75]. Both groups suggest a certain range

of their factors aording evaluable signals.

The quantitative treatment of the reduction process provided by Wimmers et al. [76]

formalises the rate expression as follows:

r = dα/dt = kred (T ) f(α)f′(pH2 , pH2O) (3.2)

with,

α degree of conversion of the metal oxide,

pH2 gas phase concentration of hydrogen,

pH2O gas phase concentration of water,

kred (T ) reduction rate constant,

f(α) function describing the dependence of the reduction rate on the

concentration of metal oxide and the reduction mechanism.

f ′(pH2 , pH2O) function describing the dependence of the reduction rate on the

concentration of hydrogen and water in the gas phase.

The reduction rate is thus described by one temperature-dependent term (kred) and

three concentration-dependent terms (f(α), f ′(pH2 , pH2O)). At dierential conditions,

which are usually attempted for temperature-programmed experiments, the gas phase-

dependent term f ′(pH2 , pH2O) can be regarded as constant. This simplies Equation 3.2

to:

dα/dt = k′red (T ) f(α) (3.3)

Introducing a constant heating rate β = dT/dt and describing the temperature depen-

dence of k′red by the Arrhenius ansatz yields:

24

3. Experimental

dα

dT=

A

βexp (−EA/RT ) f(α) (3.4)

It is obvious that with increasing temperature the temperature-dependent rate constant

increases, while the concentration of the reducible solid decreases in the course of the

reaction. Generally, this results in a maximum of the rate of reduction [76]. A common

application is the determination of the temperature of the maximum reduction rate at

dierent heating rates, which allows for the calculation of the activation energy of reduc-

tion [77]. However, the reduction of a solid is usually not a single-step reaction as might

be assumed based on the simplicity of the above rate equations, but comprises the ad-

sorption and activation of the reductant, diusion of either the reducing agent or lattice

oxygen (vacancies), the actual redox reaction and the nucleation of the reduced phase.

Several models representing dierent slow steps are established [78]. The mechanistic

information is implemented in the term f(α), so that the comparison of experimental

signals with simulated curves gives an indication of the reduction mechanism.

It should be noted that it is tacitly assumed that the reduction occurs instantaneously

upon adsorption of the reductant. Mathematically, this means that:

r =dα

dt= − c · ν dpH2

dt(3.5)

where c is a factor of proportionality and ν is the stoichiometric factor of the reduction

reaction.

It is evident that the situation is more complicated when a hydrocarbon is used as the

reductant, which possibly gives rise to the formation of coke or strong adsorption of car-

bonaceous species. Hence, the rate of reduction cannot be equated with the consumption

of the reductant. The information on the degree of reduction can therefore rather be

deduced from the evolution of oxygen-containing products. Thus, in analogy to Eq. 3.5

it assumed that:

dα

dt∝ d (

∑λipi)

dt(3.6)

where λi is the stoichiometric factor of oxygen in the component i, and pi is the partial

pressure of this component. This in turn requires that product desorption and diusion

are not the limiting steps.

25

3. Experimental

PI

FIC FIC FIC FIC FIC FIC

FIC

PI

PI

PI

PI

PI

PI

PI

PI

PI

PI

PI

PI

PI

PI

PI

TIC2

TI2

V1

Reaktor 2

TIC1

TI1

V2

Reaktor 1Schutzreaktor

PI

QMS

Hochdruck-einheit

FIC

PI

V3 V4

V5

V8

V6 PS

E1

E2

E3

E4

E5

E7

E6

TG

Gasmischeinheit

1 2 3 4 5EE3

1 2 3 4

5

8

7

6

He C H /He63 O2/He O2 KWS NO/He N2

Dosierventil

Figure 3.1.: Flow diagram of the laboratory setup for temperature-programmed ex-periments.

3.1.2. Experimental setup

The temperature-programmed experiments were conducted in a ow setup shown sche-

matically in Fig.3.1. The setup consists of a gas supply section with several gas lines,

two in-line catalytic microreactors, which can be individually operated, and a quadrupole

mass spectrometer for online gas analysis.

The whole setup is constructed of stainless steel tubing. The lines downstream of

the reactors consist of Silcosteel R⃝ tubing (Restek). The reactant gases are stored in

gas cylinders. Helium (99.9999 %), 1 % propene (99.98 %) in helium (99.9999 %), and

1 % oxygen (99.995 %) in helium (99.9999 %) were obtained from Air Liquide. Thermal

mass ow controllers (MFCs) Bronkhorst EL-Flow F are used for precise gas dosing.

The temperature-programmed experiments are carried out in a U-shaped tubular reactor

(Reactor 1) constructed of glass-lined tubing (SGE; inner diameter di = 4 mm). The

temperature of the catalyst sample is determined by a thermocouple, which can be pla-

ced inside the catalytic bed. The reactor is heated by a tubular furnace (Carbolite

MTF 12/38/250). The tubing downstream of the reactor is heated to 120 C by heating

tapes in order to prevent adsorption of products. Fast online gas analysis is accomplished

by a quadrupole mass spectrometer GAM 445 (Balzers).

26

3. Experimental

3.1.3. Experimentation

Experimental procedure

Prior to the catalytic testing the gas supply and analysis section were calibrated. The

mass ow controllers were calibrated using a Drycal Dener 220 (Bios) with integrated

temperature and pressure compensation. High purity gas mixtures were used for the

calibration of the mass spectrometer. For calibration of liquids, a helium gas ow was

saturated using a thermostated two-chamber saturator.

For the temperature-programmed experiments 150 mg of sample present as sieve frac-

tion 250− 355 µm were placed in the catalytic microreactor between two quartz wool

plugs. The position was adjusted so that the thermocouple was in the middle of the

catalyst bed. Prior to reduction/reoxidation cycles a TPD (temperature-programmed

desorption) experiment was carried out in order to remove physisorbed species and to

establish a dened initial state of all tested materials. The samples were heated in owing

helium (50 mlmin−1) up to 250 C at a heating rate of 10 Kmin−1. The nal temperature

was kept for 1 h before cooling to room temperature. For TPR and TPO experiments

the reactor was ushed with either propene or oxygen for 15 min at room temperature.

Then the temperature was raised to 420 C applying a heating rate of 5 Kmin−1. The

temperature was then maintained constant for either 5 h (TPR) or 3 h (TPO).

Data evaluation

The mass spectrometer records the mole fractions of the calibrated components and the

reaction temperature with a temporal resolution of 4.3 min−1. Since the temperature

is recorded simultaneously, the respective values have to be corrected for the time the

euents need to travel from the reactor to the mass spectrometer. In order to determine

the degree of reduction of the sample the mole fractions have to be converted into an

extensive variable. Provided that the reactor euents are treated as an ideal gas, the

integral amount ∆n of any product i detected by the mass spectrometer is given by

Eq. 3.7.

∆ni =pV

RT

tend∫t0

yi dt (3.7)

where p is the pressure, V is the volumetric ow rate, R is the gas constant, T is the

temperature, yi is the mole fraction of component i, and t0 and tend are the start and the

end of the experiment, respectively.

27

3. Experimental

The mass balance of an element h in any system is given by Eq. 3.8.

N∑i=1

∆niβh,i = 0 (3.8)

Here, N denotes the total number of species i, ∆ni the change of the amount of

substance and βh,i is the coecient of element h in the species i.

The explicit oxygen mass balance for the considered system can thus be written as:

∆nO,L +∆nO,C3H4O +∆nO,CO + 2 ·∆nO,CO2 +∆nO,H2O + 2 ·∆nO,O2 = 0 (3.9)

The index L introduced for the accumulation term signies that this can be equated

by the lattice oxygen of the catalyst sample. It is implied that the contribution of other

species is negligible, which has been conrmed by GC analysis. On the basis of this the

degree of reduction α of the sample is dened as:

α =∆nO,L

nO,tot

(3.10)

where nO,tot is the total amount of oxygen in the sample in its initial (oxidised) state.

Furthermore, the integral selectivity S is redened according to Eq. 3.11.

S =∆nC3H4O

∆nC3H4O + 1/3 ·∆nCO + 1/3 ·∆nCO2

(3.11)

The element mass balances for hydrogen and carbon could not be determined readily,

since the propene consumption could not always be measured with sucient accuracy.

3.2. Steady-state kinetic experiments

Steady-state kinetic studies in laboratory-scale microreactors are a suitable means of

measuring the kinetics of a heterogeneously-catalysed reaction. Since the overall reac-

tion rates are determined by the dynamics of elementary reaction steps, they represent

a macroscopic, reduced image of the reaction mechanism. This may be resolved by ad-

ditional information from spectrosopic characterisation methods. A major issue in the

heterogeneously catalysed oxidation of hydrocarbons is the selectivity to the target pro-

28

3. Experimental

duct, which is lowered due to the formation of undesired organic byproducts and CO and

CO2. In order to limit the inuence of side reactions it is indispensable to understand

the mechanism and the conditions favouring their progression.

3.2.1. Experimental background

Heterogeneously catalysed reactions generally comprise a sequence of diusion, sorption

and reaction steps. The ad- and desorption of reactants and products and the reaction

of adsorbed (and possibly gas phase) species is dened as the microkinetics. Apart from

the chemical reaction transport phenomena can have a decisive impact on the reaction

kinetics. The interaction of chemical processes with mass transport and heat transport

are hence understood as the macrokinetics of the reaction.

The reason for the inuence of mass and heat transport can be illustrated by looking at

the temperature- and a concentration-dependence of a simple chemical reaction. The rate

of any reactive elementary step can be factorised in a temperature- and a concentration-

dependent term according to collision theory (Eq. 3.12).

r = k(T )f(pi,Θi) = A exp(−EA/RT )f(pi,Θi) (3.12)

Here, k is the rate constant which is ideally described by the frequency factor A and

the exponential term, with activation barrier EA, gas constant R and temperature T . piand Θi are reactant partial pressure and adsorbate coverage, respectively.

In order to detect the microkinetics of a catalytic reactions it is therefore necessary

to consider temperature and concentration gradients within the reactor. Since the as-

sessment of these inuences is dicult or even impossible, it is advisable for laboratory

purposes to seek a (radial) gradient-free operation of the reactor, which may then be

treated as an ideal PFR (plug-ow reactor).

3.2.2. Experimental setup

The laboratory-scale setup constructed to meet the requirements for kinetic investiga-

tions and testing of heterogeneous selective oxidation catalysts is shown schematically in

Fig. 3.2. The main features of the apparatus are:

• gas supply

• catalytic microreactor

• GC analytics

29

3. Experimental

FIC

FIC

FIC

FIC

FIC

TI2

TIC1

GC

Reactor 1

Absorber

PI PI

PI PI

PI PI

PI PI

PI PI

Ne/He

C H /He63

O2/He

He

H2

PI

T = 100 - 200 °C

V1

V2

V3 V4

V5

FIC

PI PI

PI PI

Figure 3.2.: Flow diagram of the laboratory setup for steady-state kinetic experi-ments.

The gas supply section provides the gaseous components propene, oxygen, helium as

inert gas and balance and neon as internal standard for the GC-analysis. Optional co-

feeds of water or other liquid components are achieved by saturating the gas ow with the

desired component at an appropriate temperature. Additionally, helium and hydrogen are

provided for purging and leak testing. The reactant stream is passed through the reactor,

which is heated by a stainless steel jacket oven. The euent stream passes through

the sample loop of the GC analytics. The automatic actuator of the sampling valve is

controlled by the GC. Before going to the vent, the product stream is passed through an

absorber containing an aqueous solution of sodium nitrite (0.2 mol L−1), which promotes

the polymerisation of the acrolein.

The whole setup is constructed of stainless steel tubing (1/8 ). Downstream of the

reactor Silcosteel tubing (Restek) was used for construction. In order to prevent the

condensation of the co-feeds the section between the saturator and the reactor is heated

to 100 C. Due to the high tendency of acrolein and acrylic acid to spontaneous poli-

merisation and due to the high boiling point of acrylic acid, the tubing and armatures

downstream of the reactor and the sample loop are heated to 200 C.

The control of the gas supply, the reactor heating and the GC analysis is computer-

controlled, thus enabling a largely automatic experimentation.

Gas supply

The reactants propene and oxygen and the internal standard neon are provided as gas

mixtures with helium (purity 99.9999 %) by Air Liquide. Concentrations and puritites

are 20 % propene (99.95 %), 40 % oxygen (99.995 %) and 5 % neon (99.99 %). The gases

are dosed by thermal massow controllers (Bronckhorst El-Flow F200) with a range of

30

3. Experimental

Thermocouple

Sample

Quartz wool

a b

Reactor

Oven half shells

Figure 3.3.: a) Catalytic microreactor and oven. b) Cross sectional view of reactorwith catalyst bed.

20− 50 mlmin−1. The reactants are mixed in countercurrent (turbulent) ow in modied

three-way valves (Nupro DL). Helium and hydrogen (purities 99.999 %, Air Liquide) are

also supplied by thermal mass ow controllers (Brooks 5850 TR). A series of 4-port, 2-

position Valco valves (Vici) enables the rapid switching between the dierent streams.

The ow controllers are protected by particle lters (7 µm) upstream and check valves

(1 psi) downstream of the controller. In the same way two diaphragm valves are connected

to each gas line.

The Valco valve V4 connects two ports for the installation of a saturator or additional

gas lines. A two-stage saturator, which is contained in a bath thermostat, is used for co-

feeds that are liquid at ambient conditions. External gas supplies are joint via a three-way

armature to the connecting ports.

Catalytic microreactor

The microreactor is a U-shaped tubular reactor constructed of glass-lined tubing (SGE, di= 4 mm). It is enclosed by two stainless steel half shells, each heated by a heating rod. A

schematic representation of reactor and oven is given in Fig. 3.3. The sample is mounted

as a packed bed in between two quartz wool plugs to keep it in place. A thermocouple

is placed in the middle of the catalyst bed. The output of the heating rods is regulated

by a PID controller (Eurotherm 2416). The control variable is the temperature of the

catalyst bed. A second thermocouple is installed in the stainless steel block in order to

detect possible temperature gradients between reactor and oven.

31

3. Experimental

Porapak QS

Porapak QS

Molsieve 13X

TCD 1

Restrictioncarrier ref.

carrier sample

sample in

sample out

DELSI GC11SHIMADZU GC14A

TCD 2

vent sample

vent ref.

V1 V2

Figure 3.4.: Schematic drawing of the GC application.

GC analytics

The detection of the components of the product stream of the selective oxidation of

propene to acrolein, including the most important byproducts, is accomplished by online

gas chromatography. The application is a tandem setup consisting of a Shimadzu GC-

14B and a Delsi GC11, each equipped with thermal conductivity detectors (TCD). It is

operated as a twin-column system. The carrier gas (He) for both lines is supplied by the

mechanical ow controller CFC-14PM (Shimadzu). The carrier gas purity is 99.999 %,

which is is further improved by passing the gas through an OxisorbR⃝ cartridge (Linde).

The column setup is depicted in Fig. 3.4. The sample loop (V1) is continuously purged

by the product stream. By switching the 6-port Valco valve, its content is injected onto

the sample line of the GC application.

The rst column is a packed column with stationary phase Porapak QS (length l =

2.5 m; di = 2 mm). Separation of CO2, H2O, ethylene, propene and C1-, C2- and C3

hydrocarbons and oxygenates is accomplished by temperature-programmed operation

(see Fig. A.1). The permanent gases (Ne, O2, CO) are separated on the downstream

column with Molsieve 13X packing (l = 2 m; di = 2 mm) operated at 30 C. Due to

the strong adsorption of the other components in the product stream, these do not elute

from the molsieve column, which therefore requires regeneration at regular intervals. The

column is regenerated by switching the 4-port Valco valve (V2), thus reversing the carrier

gas ow, and heating the molsieve column to 100 C.

The components are identied by their characteristic retention times, which are listed

in Table 3.1 together with the response factors. Quantitative analysis is carried out using

the method of the internal standard (IS) [79]. This guarantees precise analysis in case of

pressure or temperature variations in the sample loop, or increasing or decreasing reaction

volume. The method requires the determination of relative response factors (RRF ) of

the components i, which are dened by Eq. 3.13:

32

3. Experimental

Table 3.1.: Retention times and response factors

component retention time RRF[min ] [-]

neon 1.6 1.000oxygen 1.9 1.670carbon monoxide 2.3 1.790

carbon dioxide 1.9 2.801ethylene 2.6 2.889water 4.6 1.419formaldehyde 5.9 3.010propene 8.0 3.621acetaldehyde 16.6 2.143acrolein 23.3 3.649acetic acid 27.8 2.284acrylic acid 37.9 2.208

RRFi =ni ·AIS

nIS ·Ai

(3.13)

Here, ni and Ai denote the molar ow of the analyte and the peak area of the component

in the chromatogram, respectively. The calibration is thus performed by analysis of a

stream containing the internal standard and the component i in well-known amounts and

determination of the detector response in terms of peak areas. For product analysis a

dened amount of the internal standard is added to the make-up gas. The partial streams

of the reaction products are then obtained by rearranging Eq. 3.13.

3.2.3. Experimentation

The mass ow controllers and the GC analytics were carefully calibrated before carrying

out any experiments. A Drycal Dener 220 (Bios) with automatic temperature and

pressure compensation was used for the calibration of the ow controllers. All ow data

refer to standard temperature and pressure (STP (IUPAC); T = 0 C, p = 100 kPa).

The gas chromatograph was calibrated according to the method presented in section 3.2.2

utilising high-purity gas mixtures.

For catalyst testing or kinetic experiments 250− 500 mg of catalyst were loaded into

the reactor. The particle size fraction of 250− 355 µm was chosen in order to reduce the

pressure drop and at the same time to avoid wall eects [80]. The resulting bed height

of 20− 40 mm furthermore satises the established criteria for plug-ow.

33

3. Experimental

Table 3.2.: Parameter space covered by kinetic experiments. Standard parameters areunderlined.

parameter value

TR [C ] 300, 320, 340, 360, 380τmod [kg smol−1 ] 5− 45 (incr. 5)yC3H6 [ - ] 1, 2.5, 4, 5.5, 7yO2 [ - ] 6.25, 9.5, 12.75, 16yH2O/yC3H6 [ - ] 0, 0.5, 1, 1.5

The reactor was tested for leakages by purging with hydrogen at room temperature and

checking the anges with a hydrogen-sensitive detector. After switching back to helium

(20 mlmin−1) the temperature was increased up to 300 C at a heating rate of 5 Kmin−1.

After this the reactant gas mixture was dosed and the temperature was increased at a

rate of 1 Kmin−1 up to the desired value.

Catalytic testing

For the screening of mixed oxide catalysts a fast testing procedure was applied. The

catalyst mass used was 500 mg. The reactant concentrations were 5 % propene and 9.5 %

oxygen. In order to comply with the conditions applied in industry a weight-hourly-space-

velocity (WHSV; ratio of propene mass ow mC3H6 and catalyst mass mcat) of 0.1 h−1

was applied corresponding to a total ow rate of 9.1 mlmin−1. The reaction temperature

was set to 320 C, and the catalytic activity was determined after 96 h time-on-stream.

Kinetic experiments

In the kinetic experiments it was attempted to cover a wide range of experimental condi-

tions. Table 3.2 summarises the inuencing factors examined and the ranges for these

parameters. The amount of catalyst used in these experiments was always 250 mg. The

standard concentrations of propene and oxygen, expressed as mole fraction, were 5.5 %

and 9.5 %, respectively. In order to study the inuence of the partial pressure of propene

and oxygen on the reaction rate, these were individually varied while keeping the other

concentration constant. The concentration range covered was 1− 7 % for propene and

6.25− 16 % for oxygen. The water co-feed was adjusted in dened relation to the pro-

pene partial pressure and the reaction temperature (TR) was varied between 300 C and

380 C. The residence time was expressed as:

34

3. Experimental

τmod =mcat

ntot

(3.14)

where ntot is the total molar ow rate. The minimum and maximum value of the

modied residence time τmod were 5 kg smol−1 and 45 kg smol−1, respectively. This cor-

responds to a gas-hourly-space-velocity (GHSV) of ca. 1800− 16000 h−1.

Data evaluation

The euent partial streams were determined from the gas chromatographic analysis and

the feed stream of the internal standard neon according to Eq. 3.13. The euent streams

are the basis for the calculation of conversion X (Eq. 3.15), selectivity S (Eq. 3.16) and

product yield Y (Eq. 3.17).

Xi =ni,0 − ni

ni,0

(3.15)

Sk =nk − nk,0

ni,0 − ni

·∣∣∣∣ νiνk∣∣∣∣ (3.16)

Yk =nk − nk,0

ni,0

·∣∣∣∣ νiνk∣∣∣∣ (3.17)

The values n and n0 are the euent and initial molar low rates and ν is the stoichiome-

tric coecient. The indices i and k denote reactants and reaction products, respectively.

Product selectivities and yields always refer to propene.

35

36

4. Model Catalysts for Selective

Propene Oxidation Probed by

Redox Cycling

4.1. Introduction

Although the concept has been known for a long time, catalysts based on bismuth and

molybdenum are still the state-of-the-art technology for the production of acrolein from

propene [5]. Beside bismuth and molybdenum, cobalt and even more iron have been

found to be indispensable key components of the active catalyst state. It is well-known

that three crystalline phases in the Bi-Mo-O ternary system, namely α-Bi2Mo3O12, β-

Bi2Mo2O9, and γ-Bi2MoO6, are ecient catalysts for the oxidation of propene to acro-

lein [14, 16, 19]. Conversely, iron/cobalt molybdates, which are the major component

of multicomponent catalysts, have been reported to be inactive and non-selective [60].

The superior activity of the compound system has been attributed to a synergy eect.

Although there is some indication of an enrichment of bismuth and molybdenum at the

surface [60, 61] or even a core-shell structure [62] the problem eludes a resolution due to

the high degree of structural complexity.

It is established for acrolein catalysts that the working mechanism is of the Mars-

van Krevelen type in a certain range of conditions. The concept of utilising lattice

oxygen for the reaction and splitting the catalytic cycle into separate reduction and reoxi-

dation steps provides potential for reaction engineering and catalyst research. Reactor

conceptions providing separate vessels for the two steps and circulating uid bed reactors

(riser-regenerator) have been proposed [81].

Brazdil et al. [70] studied the redox kinetics of bismuth- and molybdenum-based cata-

lysts and found dierent rankings for reduction and reoxidation, which they attributed to

structural peculiarities of the oxide catalysts. Furthermore, fundamental dierences were

observed for the reoxidation of surface and bulk vacancies. Similarly, Hutchings and

coworkers [82] applied cycles of TPR and TPO experiments to characterise the structural

stability of bismuth molybdate catalysts. Miura et al. [83] reoxidised model catalysts

37

4. Model Catalysts for Selective Propene Oxidation Probed by Redox Cycling

based on bismuth and molybdenum prereduced by hydrogen and were thus able to assign

the signals obtained to dierent reduced species. Besselmann et al. [84] studied the

reoxidation of titania-supported vanadium oxide catalysts, which had been reduced by

reaction with toluene and p-xylene. By applying fast on-line mass spectrometry they

were able to detect the products of coke burn-o. By solving the oxygen mass balance

this enabled the distinction between catalyst reoxidation and coke oxidation for a catalyst

prereduced under conditions relevant to the catalytic cycle.

It is deduced that a good catalyst should possess intermediate redox properties. Facile

reduction is required to enable low-temperature operation and high selectivity, but also

the ongoing of the catalytic cycle by reasonable reoxidation rates has to be ensured.

Therefore, cyclic temperature-programmed reduction and reoxidation experiments are a

suitable technique for studying the activity of these catalysts.

Recently, the substitution of the bismuth molybdate moiety of MMO catalysts by bis-

muth tungstates was implemented [1, 85]. Previously, tungsten had been successfully

applied as a structural promoter in molybdenum-based oxidation catalysts [86, 87]. Ho-

wever, bismuth tungstates were reported to be poorly active and selective for propene

oxidation [3].

In order to elucidate the role of the dierent constituents of multicomponent mixed

oxide catalysts, several bismuth molybdates and tungstates and iron molybdate-based

model catalysts have been extensively studied by cyclic temperature-programmed re-

duction and reoxidation measurements. The results are complemented by a thorough

physicochemical characterisation allowing to correlate activity and structural properties.

4.2. Experimental

4.2.1. Catalyst preparation

Two types of precursors for the catalytically active state have been prepared. In the

following the bismuth-based precursors will be denoted as active components and the

iron molybdate-based precursors as promoting components. Table 4.1 lists the model

catalysts prepared in this study and summarises important properties.

The bismuth-based model catalysts used in this study were prepared by a spray-drying

routine described in detail in Ref. [85]. Precursor solutions were obtained by dissolving

the desired amounts of suitable bismuth and molybdenum or tungsten salts in water.

Insoluble precursors were introduced by dispersing a ne powder (dp ≤ 1 µm) of the

respective compound in the solution. The aqueous solution was spray-dried and the dry

mass was subjected to a thermal treatment at a temperature specied. The resultant

38


Table

4.1.:Com

positionandsurfaceproperties

ofmodel

catalysts.

Label

Adenotes

active

constituents

andlabel

Ppromoter

constituents

ofthecatalyticallyactive

state.

Elem.comp.[M

1/M

2(/M

3)]

phases

detected

label

sample

Tca

lc1

SBET

2

theor.

ICP-O

ES

XPS

byXRD

A1

Bi 2Mo 3O12

740

0.4

0.67

0.65

0.71

Bi 2Mo 3O12,MoO

3,Bi 2O3

A2

Bi 2Mo 4O15

550

1.8

0.5

0.57

0.42

Bi 2Mo 3O12,MoO

3

A3

Bi 2W

2O9

800

1.1

1.0

1.09

1.04

Bi 2W

2O9,Bi 2WO6

A4

Bi 2WO6

620

3.0

2.0

2.13

-Bi 2WO6,Bi 14W

2O27,Bi 2W

2O9,WO3

A5

Bi 2W

4O15

830

1.0

0.5

0.49

0.51

Bi 2W

2O9,WO3

P1

Fe 3Co 4Mo 1

2O43

450

9.1

1/2.3/4

1/2.3/3.7

1/3.6/12

Co xFe 1-xMoO

4,Fe 2Mo 3O12,MoO

3

P2

Fe 3Co 7Mo 1

2O46

450

14.0

--

-1[C]

2[m

2g−1]

39


samples were ground to give a particle size 2 µm ≤ d50 ≤ 3 µm. The specication d50

denotes that 50 % of the total particle volume consists of particles with a diameter smaller

than indicated.

The precursors for the mixed iron and cobalt molybdate catalysts were synthesised by

a spray-drying routine described in Ref. [85]. Aqueous precursor solutions were obtained

by mixing suitable iron, cobalt and molybdenum precursors in water, so that each of the

starting materials passed through a degree of dispersion characterised by d90 ≤ 5 µm,

indicating that 90 % of the total particle volume of the respective precursor consisted of

particles with a diameter smaller than 5 µm. The precursor solutions/suspensions were

dried using a spray-dryer. The powder samples obtained from the spray-drying process

were calcined in a rotary furnace in a owing air-like (20 % oxygen, 80 % nitrogen)

gas mixture. A multistep temperature programme was applied in order to carefully

remove or decompose ammonium nitrate contained in the precursor (Fig. A.2). The nal

temperature of 450 C was kept for 10 h.

For catalytic tests the samples were pressed to tablets and sieved to obtain the size

fraction of 250− 355 µm.

4.2.2. Characterisation

XRD

X-ray diraction patterns were recorded using a Panalytical MPD diractometer equip-

ped with a Cu X-ray source, 0.5 divergent and antiscatter slits, a 0.2 mm high receiving

slit, incident and diracted beam 0.04 rad soller slits, and a secondary graphite monochro-

mator. The 2Θ range covered was from 5 to 60 with a step width of 0.03. Qualitative

phase analysis was carried out using the X'Pert Line software (Panalytical) together with

powder diraction les (PDFs) from the International Centre of Diraction Data (ICDD).

SEM and EDX

Scanning electron micrographs were obtained using a LEO 1530 Gemini scanning electron

microscope equipped with an INCA X-ray microanalysis system for energy-dispersive X-

ray analysis analysis. The standard secondary electron detector was used for imaging.

N2physisorption

Adsorption data were obtained with volumetric Quantachrome Autosorb-1-MP and

Autosorb-1-C setups. Specic surface areas of the samples were determined by static

40


nitrogen or krypton adsorption at −196 C and data evaluation according to the multi-

point BET method. Pore size distributions were determined by evaluating the desorption

branch of the nitrogen isotherm according to the BJH method. Standard pretreatment

procedure was heating in vacuo at 300 C for 2 h.

XPS and ISS

X-ray photoelectron spectroscopy and ion scattering spectroscopy experiments were per-

formed with a Leybold surface analysis system equipped with a XR 50 X-ray source

(Specs) with an Al/Mg twin anode, an IQE 12/38 ion source (Specs), a ood gun FG

15/40 (Specs), and an EA 10/100 electron (ion) analyser with multichannel detection.

XP spectra were recorded using Al Kα radiation (1486.6 eV, 12.25 kV × 20 mA), with the

analyser in the xed pass energy mode (pass energy = 82 eV). CasaXPS (Casa Software

Ltd.) was used for spectra processing. Binding energies were calibrated with reference

to the C 1s binding energy of 284.5 eV. Surface atomic ratios were calculated from in-

tegral signal intensities applying a Shirley-type background, X-ray satellite subtraction

and Scoeld interaction cross sections. ISS sputter series were performed with 2000 eV

He+ ions with the analyser in the xed pass energy mode (pass energy = 195 eV). The

ood gun was used to avoid sample charging. Recording of the spectra was started upon

moving the fresh sample into the beam. Qualitative analysis of surface atomic ratios was

accomplished comparing integral intensities using a linear background.

4.2.3. Temperature-programmed experiments

The temperature-programmed experiments were performed in a conventional ow setup

equipped with thermal mass ow controllers (Bronkhorst) and a GAM 445 quadrupole

mass spectrometer. For standard experiments 150 mg of the sample were placed bet-

ween two quartz wool plugs in a U-shaped catalytic microreactor constructed of glass-

lined tubing (SGE). The gases applied were He (99.9999 %), 1 % C3H6 (99.98 %) in He

(99.9999 %) and 1 % O2 (99.995 %) in He (99.9999 %).

Prior to the reduction and reoxidation experiments the samples were pretreated in a

stream of 50 mlmin−1 helium at a temperature of 250 C for 1 h in order to remove any

physisorbed species. For a typical TPR (TPO) experiment the reactor was purged with

50 mlmin−1 propene (oxygen) at room temperature for 15 min. The sample temperature

was then increased to 420 C at a heating rate of 5 Kmin−1. The nal temperature was

maintained for 5 h (TPR) and 3 h (TPO), respectively.

41


4.3. Results


The X-ray diraction patterns of the fresh samples and after reduction by propene are

shown in Figure 4.1. The A1 sample is composed of α-Bi2Mo3O12 (JCPDS 78-2420)

along with traces of γ-Bi2MoO6 (JCPDS 84-0787), as indicated by the characteristic

line at 28.24, MoO3 (JCPDS 89-5108) and potentially Bi2O3 (JCPDS 77-2008). Upon

reduction new reexes appear at 26.08 and 37.06, which are attributed to the formation

of MoO2 (JCPDS 65-1273) and the intensity of those associated with the γ-Bi2MoO6

phase increases signicantly.

The XRD pattern of the fresh A2 sample exhibits reexes of α-bismuth molybdate, α-

MoO3 and γ-bismuth molybdate. After reduction the reexes of MoO3 have disappeared

almost completely, and the characteristic lines of MoO2 are observed. At the same time,

the ratio of the principal lines of the α- and γ-bismuth molybdate at 28.05 and 28.24

decreases drastically indicating the formation of the latter at the expense of the former.

The A3 sample shows mainly the reexes of Bi2W2O9 (JCPDS 33-0221) and a minor

contamination by russelite (Bi2WO6, JCPDS 73-2020). The reduction of the sample does

neither eect the disappearance of reexes nor the occurrence of new lines. However,

changes in the intensity of the signals are observed, which are due to the measurement

procedure. In the Bragg-Brentano geometry the beam footprint, i.e. the area that is

irradiated by the incident beam, is angle-dependent. Since the amount of sample available

for XRD measurements after reduction is limited, the relative irradiated volume increases

with increasing diraction angle.

The major crystalline phase in the A4 sample is Bi2WO6 as indicated by the reexes

at 28.31 and 32.68, 32.80 and 32.93. Furthermore, traces of Bi2W2O9, Bi14W2O27

(JCPDS 39-0061) and WO3 (JCPDS 89-4476) are contained. New reexes at 22.47,

27.17, 37.95 and 39.62 arise upon reduction by propene. These can unequivocally be

assigned to metallic bismuth (JCPDS 85-1329). At the same time, the intensity of the

reexes at 27.55 and 31.95 decreases signicantly.

The fresh A5 sample is composed of Bi2W2O9 and WO3. Apparently, the composition

and crystalline structure are not changed due to the temperature-programmed reduction

of the sample. The systematic changes in the relative intensities are again caused by the

measurement setup.

In all cases the composition of the fresh sample reects the thermodynamic equilibrium

[8890] except for minor impurities. Upon loss of oxygen, the α-bismuth molybdate phase

in the molybdenum-based catalysts decomposes to the γ-phase and MoO2. The β-phase

42


10 20 30 40 10 20 30 40

10 20 30 40 10 20 30 40

10 20 30 40

Bi2MoO

6 MoO

2

pristine

pristinepristine

reduced

reducedreduced

reduced

pristinepristine

reduced

A5 - Bi2W

4O

15

A4 - Bi2WO

6A3 - Bi

2W

2O

9

A2 - Bi2Mo

4O

15A1 - Bi

2Mo

3O

12

inte

nsity

/ a.

u.

2

Bi2MoO

6 MoO

2

inte

nsity

/ a.

u.

2

inte

nsity

/ a.

u.

2

Bi (cub.)

inte

nsity

/ a.

u.

2

inte

nsity

/ a.

u.

2

Figure 4.1.: X-ray diractograms of active components before and after reduction bypropene.

43


10 20 30 40 10 20 30 40

MoO3

MoO2

pristinepristine

reduced

P2 - Fe3Co

7Mo

12O

46P1 - Fe

3Co

4Mo

12O

43in

tens

ity /

a.u.

2

inte

nsity

/ a.

u.

2

Figure 4.2.: X-ray diractograms of the support components before and after reduc-tion by propene.

of the Bi-Mo-O system is not observed, because it is unstable at the temperatures of the

reduction experiments. MoO3 is also reduced to MoO2 under the experimental conditions

as has been previously shown in several studies [91,92].

Relatively little bulk structural transformations are observed with the bismuth tungs-

tates after propene TPR. This is especially the case for the Bi2W2O9 phase and tungsten

oxide. For the high surface area tungstate A4 the amount of reduced bismuth species is

sucient for the formation of small bismuth metal crystallites.

The reduction by 1-butene causes considerable structural changes in both bismuth

molybdates and tungstates (Figures A.3 and A.4). The α-phase of the A2 sample is com-

pletely decomposed to γ-Bi2MoO6 and MoO2 and further to metallic bismuth. Although

the A3 sample largely retains its Bi2W2O9 structure, signicant amounts of metallic

bismuth are obtained under these conditions.

The diractograms of the samples P1 and P2 are displayed in Figure 4.2. Both samples

are mixtures of iron and cobalt molybdates or solid solutions of the type FexCo1-xMoO4

(cp. JCPDS 89-6590), ferric molybdate (Fe2Mo3O12) (JCPDS 83-1701) and α-MoO3. The

patterns exhibit a slightly increasing background in the 2Θ range of 20− 35 indicating

some amorphous constituents. Due to the relatively high molybdenum content the P1

sample contains considerably more free molybdenum oxide than the P2 sample.

Figure 4.3 shows the scanning electron micrographs of the samples A1 to A5. It can

be stated that in all cases the morphology of the particles and the size distribution are

indicative of the production process. The overview micrograph of sample A1 (Fig. 4.3b)

reveals that the particles are non-uniform in size and shape. Large irregularly shaped

particles of up to 100 µm diameter are present along with smaller nanoscaled particles.

The smaller particles partly decorate the larger particles. The particle shape is caused by

cleavage and abrasion in the grinding process. The large crystallites were formed due to

44


the high calcination temperature of this sample. The EDX analysis yielded a Bi/Mo ratio

of 0.70, which is in accordance with the intended stoichiometry. The overview micrographs

of the other samples (Fig. 4.3d, e, g, j) show that these exhibit more uniform particle

size distributions. The A2 sample is composed of rather spherical particles, which are

approximately 1 µm in diameter (Fig. 4.3c). The composition as determined by EDX

(0.49) matches perfectly with the sample stoichiometry. The Bi2W2O9 phase of the

A3 sample forms small cuboidal crystallites of micron to submicron size partly present in

larger agglomerates. The Bi/W molar ratio of 0.92 is close to the nominal ratio (1 : 1.05).

Figure 4.3i reveals that the A5 sample is composed of two type of particles. By spatially

resolved EDX analysis it was shown that the rod-shaped particles in the upper part of the

image are Bi2W2O9 crystallites (Bi/W molar ratio of 0.8), while the spherical particles

highlighted in the lower part consist of WO3 (Bi/W molar ratio of 0.1).

The micrographs of the samples P1 and P2 are displayed in Figure 4.4. It is noticed that

the samples exhibit a similar microstructure, while the morphology of the agglomerates

is largely dierent. Both are composed of primary crystallites of submicron size forming

larger, macroporous agglomerates. While the P1 sample exhibits large, irregularly shaped

agglomerates, the P2 sample consists of hollow spheres, which are typical for the spray-

drying process. The size of the latter is in the range of 10− 50 µm.


TPR experiments

The model catalysts can be grouped according to their elemental composition. The

molybdenum-based active components are both capable of selectively oxidising propene

to acrolein in the absence of gas-phase oxygen, but largely dier in the degree of reduction

obtained in the reaction time. In contrast, the tungsten-based model catalysts produced

almost no acrolein, and at least the A3 and A5 samples exhibit hardly any reducibility.

The designated support components are very active, i.e. they are severely reduced, but

propene is mostly oxidised to non-selective products. Consequently, a total of three

reduction/reoxidation cycles was performed with the reducible and/or selective catalysts

in order to observe changes induced by the redox cycling. Accordingly, two cycles were

sucient for characterising the tungstate samples.

The information obtained from the quantication of the TPR experiments is summa-

rised in Table 4.2. The results of the TPR experiments with the bismuth molybdate

samples applying subsequent reduction/reoxidation cycles are displayed in Figure 4.5.

For the A1 sample a marked activation is observed from TPR 1 to the subsequent TPR

experiments. In the rst run the selectivity to acrolein is relatively low and its forma-

45


20 µm

20 µm 200 µm

4 µm

2 µm

4 µm

20 µm

2 µm

20 µm

a b

c d

e f

g h

i j

10 µm

Figure 4.3.: SEM images of active components (left column: detail; right column:overview). a,b) A1: Bi2Mo3O12; c,d) A2: Bi2Mo4O15; e,f) A3: Bi2W2O9;g,h) A4: Bi2WO6; i,j) A5: Bi2W4O15.

46


a b

c d

1 µm 200 µm

100 µm2 µm

Figure 4.4.: SEM images of promoting components (left column: detail; right column:overview). a,b) P1: Fe3Co4Mo12O43; c,d) P2: Fe3Co7Mo12O46.

tion only starts at higher temperatures as compared to that of COx. In the following

reduction cycles the formation of selective and non-selective oxidation products starts si-

multaneously. At the same time the selectivity and the reducibility increase. Only subtle

changes are observed from TPR 2 to TPR 3. Considering the rst TPR experiment

after an oxidising pretreatment (i.e. TPO; Fig. A.5) it is reasonable to assume that the

deviations in the rst cycles are not caused by some surface contamination removed by

the intermediate TPO, but rather by some structural rearrangement due to the reduction

of the sample in the rst TPR. Based on the catalyst mass the degree of reduction is

comparatively low. However, the rate of reduction depends to some extent on the specic

surface area. Furthermore, the reduction process was found to continue at nearly constant

rate after several hours. Accordingly, the amount of lattice oxygen released by the sample

is equal to several theoretical monolayers (Table 4.2). The α-bismuth molybdate can thus

be classied as reducible under the chosen conditions.

The A2 sample exhibits a behaviour similar to that of the A1 sample concerning the

onset of acrolein formation and the reducibility. Again, the retardation of acrolein for-

mation is found irrespective of the pretreatment, i.e. the rst TPR experiment resembles

that of the TPO/TPR cycle (cp. Fig. A.6). The amount of lattice oxygen released in-

creases from cycle to cycle, and so does the initial rate of reduction as indicated by the

growing intensity and the sharpening of the signals. Additionally, a minute but discer-

nible signal for acrolein is observed between 200 and 350 C, which is not accompanied

by the concomitant formation of COx (inset in Fig. A.6b). The oxygen incorporated into

47


Table 4.2.: Quantication of TPR experiments of model catalysts.

∆nO,L Selectivity % CO/CO2

sample[µmol

g ]monolayers

Acrolein CO2 CO ratio

Bi2Mo3O12 148 / 177* 22 / 27 56 / 72 44 / 23 0 / 4 0 / 0.18Bi2Mo4O15 1423 / 1572 48 / 53 66 / 69 26 / 22 9 / 9 0.34 / 0.40

Bi2W2O9 68 4 29 50 21 0.43Bi2WO6 304 6 10 81 9 0.11Bi2W4O15 68 4 20 46 33 0.69

Fe3Co4Mo12O43 2892 / 2667 19 / 18 37 / 35 36 / 36 27 / 29 0.76 / 0.80Fe3Co7Mo12O46 1565 / 1227 7 / 5 34 / 46 37 / 34 29 / 20 0.79 / 0.59

* rst cycle/average of last two cycles

0 1 2 3 4 5

0,000

0,001

0,002

0,003

0,004

Acrolein CO2 CO H2O OL

-300

-200

-100

0

100

0 1 2 3 4 5

0,00

0,01

0,02

0,03

0,04


-2000

-1500

-1000

-500

0

500

0

100

200

300

400

0

100

200

300

400b) A2 - Bi2Mo

4O

15

a) A1 - Bi2Mo

3O

12

C3H

4O TPR 1 CO

2 TPR 1

C3H

4O TPR 2 CO

2 TPR 2

C3H

4O TPR 3 CO

2 TPR 3

efflu

ent m

ole

fract

ion

/ %

time / h

TPR 1 TPR 2 TPR 3

n i mca

t-1 /

µmol

g-1

0,5 1,0 1,5 2,0

0,000

0,001

0,002

time / h

efflu

ent m

ole

fract

ion

/ %

time / h

n i mca

t-1 /

µmol

g-1

tem

pera

ture

/ °C

tem

pera

ture

/ °C

Figure 4.5.: TPR experiments of bismuth molybdate-based model catalysts. a) A1:

Bi2Mo3O12, b) A2: Bi2Mo4O15. Symbols: rst cycle: () acrolein, (H)CO2; second cycle: () acrolein, () CO2; third cycle: () acrolein,(H) CO2.

48


the products at this stage of the experiment clearly amounts to less than one monolayer.

It is assumed that this signal corresponds to the reduction of active and selective surface

sites.

Additional reduction experiments performed with the bismuth molybdate samples

using 1-butene as the reductant exhibit that the relative reduction rates are approxi-

mately proportional to the surface area of the samples (Figs. A.12 and A.13). This

corroborates the assumption that the surface microstructure and the particular condi-

tions of the reduction process by propene are responsible for the lower activity of the A1

sample in anaerobic propene oxidation rather than the mere surface area. The results

from the ISS sputter series indicate an enrichment of bismuth in the near-surface volume

of the A1 sample (Fig. A.15) as do the XPS results (Table 4.1). This is not observed for

the A2 sample (Fig. A.17).

The bismuth tungstates exhibit very similar characteristics in the propene TPR expe-

riments (Fig. 4.6). In all cases the main carbon-containing product is CO2. In addition,

signicant amounts of water are formed. Except for the A4 sample, for which a prolonged

formation of total oxidation products is observed under isothermal reduction conditions,

the reduction proceeds in a relatively narrow time frame. As a consequence, the degree

of reduction attained is very low. For the A4 sample this applies, too, when the amount

of lattice oxygen released is related to the unit surface area of the sample (see Table 4.2).

The A3 sample was also subjected to reduction by 1-butene (Fig. A.14). It is found that

butene is selectively transformed into butadiene with only minor amounts of CO2 being

produced. The higher degree of reduction obtained by the reductant 1-butene has already

been indicated by the respective XRD results (cp. sec. 4.3.1). The results indicate that

bismuth tungstates are active and selective catalysts for the oxidative dehydrogenation

(ODH) of olens. However, it is noticed that the degree of reduction does not reach

the level attained by the bismuth molybdates, which demonstrates the higher structural

stability of the tungstates.

The key reactions of propene oxidation yielding acrolein, CO and CO2 involve the

formation of one equivalent of water per turnover. It is obvious that this is not balanced

in the reduction of the bismuth tungstates, and that carbon must therefore be retained

by the catalyst. It is therefore concluded that the deposition of carbonaceous species

is the major process taking place during propene TPR experiments with the tungstate

phases.

The mixed iron/cobalt molybdates exhibit a fair activity in the anaerobic oxidation

of propene (Fig.4.7). The main products are carbon oxides and the CO/CO2 ratio is

remarkably high. The reducibility of the P1 sample exceeds that of P2 by a factor of

2, which is due to the broad isothermal signal observed for this sample and which is

49


0 1 2 3 4 5

0,000

0,002

0,004

0,006

0,008

0,010


-300

-200

-100

0

100

0 1 2 3 4 5

0,000

0,002

0,004

0,006

0,008

0,010


-300

-200

-100

0

100

0 1 2 3 4 5

0,000

0,002

0,004

0,006

0,008

0,010


-300

-200

-100

0

100

0

100

200

300

400

0

100

200

300

400

0

100

200

300

400c) A5 - Bi2W

4O

15

b) A4 - Bi2WO

6

a) A3 - Bi2W

2O

9

C3H

4O TPR 1 CO

2 TPR 1

C3H

4O TPR 2 CO

2 TPR 2

efflu

ent m

ole

fract

ion

/ %

time / h

TPR 1 TPR 2

n i mca

t-1 /

µmol

g-1

efflu

ent m

ole

fract

ion

/ %

time / h

n i mca

t-1 /

µmol

g-1

efflu

ent m

ole

fract

ion

/ %

time / h

n i mca

t-1 /

µmol

g-1

tem

pera

ture

/ °C

tem

pera

ture

/ °C

tem

pera

ture

/ °C

Figure 4.6.: TPR experiments of bismuth tungstate based model catalysts. a) A3:

Bi2W2O9, b) A4: Bi2WO6, c) A5: Bi2W4O15. Symbols: rst cycle: ()acrolein, (H) CO2; second cycle: () acrolein, () CO2.

50


absent in the TPR prole of sample P2. Although based on catalyst mass the support

components show maximum activity, the oxygen mobility is still lower than that of the

bismuth molybdates, as indicated by the number of oxygen monolayers participating in

the reaction (Table 4.2).

The onset of reduction is approximately at 200 C for both samples. The reduction

rate steadily increases, until the maximum temperature is reached and then decreases

sharply. For the P1 sample a broad signal is observed during the isothermal dwell, which

indicates an autocatalytic reduction process. Signicant alterations of the TPR traces in

the second cycle are observed for both samples, while, at the same time, they exhibit a

high degree of congruence in the rst signal. The total amount of oxygen incorporated

into the gaseous reaction products slightly decreases from the rst to the second cycle.

Conversely, only minute dierences are observed between the second and third cycle. It

is thus believed that the initial state of the catalyst is not stable under the conditions of

redox cycling, but the stable structure evolves due to the reduction and reoxidation of

the samples.

The rst signal can be ascribed to the reduction of the mixed iron/cobalt molybdate

(FexCo1-xMoO4), being the main constituent of both samples. The broad signal observed

for the P1 sample is assigned to the reduction of bulk MoO3 by comparison with the

TPR spectra of pure α-MoO3 (Fig. A.19). This is supported by the XRD pattern of the

reduced sample, which shows complete transformation of MoO3 to MoO2.

TPO experiments

Figure 4.8 displays the temperature-programmed reoxidation experiments with the bis-

muth molybdate model catalysts (second TPO of OR cycles). In both cases the formation

of carbon dioxide is observed starting at a temperature of ca. 250 C. Simultaneously,

small amounts of CO and water are formed. The signals culminate at 410 C and then

rapidly decline. The amount of carbon oxides evolving from the A2 sample exceeds that

of the A1 sample by almost one order of magnitude. The oxygen uptake of the A1 sample

is characterised by a single signal at 420 C. The deconvolution of the signal by subtrac-

tion of the contribution from coke burn-o reveals a shoulder at ca. 320 C preceding

the major uptake signal. The reoxidation outlasts the formation of carbon oxides and

also its maximum rate is reached later in the experiment. For the A2 sample a small

but relevant oxygen uptake peak is observed at 175 C. Since this is not accompanied

by the formation of oxygen containing products, it is likely due to catalyst reoxidation.

The main signal again culminates at 420 C and shows the same characteristics as the

A1 sample except for the absence of the shoulder at 320 C.

51


0 1 2 3 4 5

0,00

0,01

0,02

0,03

0,04

0,05

0,06


-3000

-2000

-1000

0

1000

0 1 2 3 4 5

0,00

0,01

0,02

0,03

0,04

0,05

0,06


-3000

-2000

-1000

0

1000

0

100

200

300

400

0

100

200

300

400

C3H

4O TPR 1 CO

2 TPR 1

C3H

4O TPR 2 CO

2 TPR 2

C3H

4O TPR 3 CO

2 TPR 3

b) P2 - Fe3Co

7Mo

12O

46

a) P1 - Fe3Co

4Mo

12O

43

efflu

ent m

ole

fract

ion

/ %

time / h

TPR 1 TPR 2 TPR 3n i mca

t-1 /

µmol

g-1

efflu

ent m

ole

fract

ion

/ %

time / h

n i mca

t-1 /

µmol

g-1

tem

pera

ture

/ °C

tem

pera

ture

/ °C

Figure 4.7.: TPR experiments of iron molybdate based model catalysts. a) P1:

Fe3Co4Mo12O43, b) P2: Fe3Co7Mo12O46. Symbols: rst cycle: () acro-lein, (H) CO2; second cycle: () acrolein, () CO2; third cycle: ()acrolein, (H) CO2.

Table 4.3.: Quantication of TPO experiments of model catalysts.

∆nO,L ∆nC,D C/Hsample

[µmolg ] [µmol

g ] [µmolm2 ] ratio

Bi2Mo3O12 251 51 122 5.3Bi2Mo4O15 1173 191 105 4.2

Bi2W2O9 52 55 50 5.9Bi2WO6 401 172 57 6.0Bi2W4O15 66 86 85 10.1

Fe3Co4Mo12O43 2141 744 82 3.9Fe3Co7Mo12O46 1569 759 54 3.3

52


0 1 2 3

0,00

0,01

0,02

0,03

0,04

0,05

0,06

CO2 CO H2O O2 CD OL

-200

-100

0

100

200

300

0 1 2 3

0,0

0,1

0,2

0,3

CO2 CO H2O O2 CD OL

-1000

-500

0

500

1000

1500

0

100

200

300

400

0

100

200

300

400b) A2 - Bi2Mo

4O

15

a) A1 - Bi2Mo

3O

12

CO CO

2 H

2O

O2,total

O2,reox

efflu

ent m

ole

fract

ion

/ %

time / h

CDO2

CDO2

TPO 2

n i mca

t-1 /

µmol

g-1

efflu

ent m

ole

fract

ion

/ %

time / h

n i mca

t-1 /

µmol

g-1

tem

pera

ture

/ °C

tem

pera

ture

/ °C

Figure 4.8.: TPO experiments of bismuth molybdate based model catalysts. a) A1:

Bi2Mo3O12, b) A2: Bi2Mo4O15. Symbols: () CO, () CO2, (N)water, ( ) total oxygen uptake, ( ) catalyst reoxidation.

The TPO of bismuth molybdates conrms the assumption that carbonaceous spe-

cies are deposited on the catalyst surface during the reduction experiments. The low

hydrogen content of the deposits and the absence of gaseous hydrogen in the TPR expe-

riments indicate that these are formed by oxidative dehydrogenation (ODH) of propene

and condensation of the resulting species. Thus, the coking of the catalyst during re-

duction requires lattice oxygen. This is also reected in the amount of carbon deposited

per unit area of catalyst surface (see Table 4.3). Thereafter, coking is strongest on the

reducible bismuth molybdates, which supply the lattice oxygen necessary for the ODH

reaction.

The oxygen uptake during reoxidation is believed to be the result of fast oxygen re-

duction and incorporation into the lattice. Consequently, the signals are assigned to

the oxidation of reduced species present on the catalyst after a TPR experiment. The

main oxygen uptake occurs at 420 C for both samples. By XRD analysis of the reduced

A1 and A2 samples, MoO2 was identied as the only crystalline phase, in which either

bismuth or molybdenum is not in its highest oxidation state. Further, the reoxidation

of MoO2 obtained from reduction of MoO3 by propene yielded a similar signal in that

temperature range (Fig. A.20). Thus, it is reasonable to assign this signal to the reoxi-

dation of Mo 4+ to Mo 6+. Miura et al. [83] studied the reoxidation of hydrogen-reduced

53


bismuth and molybdenum oxides. For a 3.8 % reduced MoO3 sample, they found a single

peak at 410 C, which is in accordance with the results of the present study. For more

severely reduced samples or MoO2 they report a second peak at 320 C. This was also

observed in Fig. A.20 and is probably the reason for the shoulder in Fig. 4.8a. However,

it is absent in the TPO of the A2 sample, although this is more severely reduced and also

the presence of MoO2 has been evidenced by XRD.

The reoxidation of a 3.9 % reduced Bi2O3 sample was reported to give a peak at

180 C [83]. Similarly, slightly reduced (2.1 −3.5 %) γ-Bi2MoO6 gave peaks at 158 C

and 170 C [83,93]. It is therefore highly likely that the signal observed at 175 C for the

A2 sample is due to the oxidation of some Bi 0 species. These species must be present

in a highly dispersed state, since no reexes of metallic bismuth were found in the XRD

pattern. The low-temperature signal is in a temperature range, where the oxygen mobility

was found to be limited for the fully oxidised samples.

The tungstate-based catalysts exhibit a slightly diering reoxidation behaviour

(Fig. 4.9). The burn-o of the carbonaceous deposits formed during reduction results

in a tapered signal, the maximum coinciding with the end of the heating ramp. Accor-

dingly, the culmination is caused by the constant temperature-dependent rate constant

and the decrease in the amount of carbon available for oxidation. It is implied that the

coke formed on the tungstates is not as readily oxidised as that formed on the bismuth

molybdates. This is probably due to the fact that hard coke is found on the tungstates,

while the deposits on the molybdate-based catalysts exhibit a lower carbon-to-hydrogen

ratio (Table 4.3). The amount of carbon-containing products formed is highest for the

A4 sample, which also exhibited the highest reducibility in the reduction cycle. The

situation is dierent, when the amount per unit surface area is considered. In this case

coking is strongest on the A5 sample. Assuming a graphite-like structure of the deposits

the coverage amounts to roughly one monolayer for the tungstate samples.

The oxygen uptake signal is very similar for the A3 and A5 sample. Both show a

minor peak at ca. 170 C and another stronger signal at 420 C. The comparison with

the amount of oxygen consumed for the removal of the carbonaceous deposits reveals that

the second signal is solely due to coke burn-o, while the former can be attributed to

catalyst reoxidation. In accordance with the considerations in the previous paragraphs

the peak at 170 C is assigned to the reoxidation of dispersed or isolated Bi 0 species.

A more complex signal is observed for the A4 sample. Deconvolution of the signal shows

that the reoxidation gives two peaks at 230 C and 290 C, respectively. It has been shown

that the TPO proles of Bi2O3-x samples depend on the degree of reduction [83]. The

emergence of peaks in the temperature range of 230 C to 300 C has been assigned to

54


0 1 2 3

0,00

0,01

0,02

0,03

0,04

CO2 CO H2O O2 CD OL

-100

-50

0

50

100

0 1 2 3

0,00

0,02

0,04

0,06

0,08

0,10

CO2 CO H2O O2 CD OL

-400

-200

0

200

400

0 1 2 3

0,00

0,01

0,02

0,03

0,04

CO2 CO H2O O2 CD OL

-150

-100

-50

0

50

100

0

100

200

300

400

0

100

200

300

400

0

100

200

300

400

a) A3 - Bi2W

2O

9

c) A5 - Bi2W

4O

15

b) A4 - Bi2WO

6

CO CO

2 H

2O

O2,total

O2,reox

efflu

ent m

ole

fract

ion

/ %

time / h

CD

CDO2

TPO 1

n i mca

t-1 /

µmol

g-1

efflu

ent m

ole

fract

ion

/ %

time / h

O2

n i mca

t-1 /

µmol

g-1

efflu

ent m

ole

fract

ion

/ %

time / h

CDO2

n i mca

t-1 /

µmol

g-1

tem

pera

ture

/ °C

tem

pera

ture

/ °C

tem

pera

ture

/ °C

Figure 4.9.: TPO experiments of bismuth tungstate based model catalysts. a) A3:

Bi2W2O9, b) A4: Bi2WO6, b) A5: Bi2W4O15. Symbols: () CO, ()CO2, (N) water, ( ) total oxygen uptake, ( ) catalyst reoxidation.

the reoxidation of agglomerated particles of metallic bismuth. In fact, these have been

shown to be present in the reduced A4 sample by XRD.

The TPO proles obtained with the propene-reduced P1 and P2 samples are displayed

in Figure 4.10. The carbonaceous deposits detected on the support components by reoxi-

dation exhibit a relatively low C/H ratio. Like the bismuth molybdates and unlike the

tungstate samples, the maximum rate of carbon oxide formation is clearly attained during

the temperature ramping at ca. 400 C.

A single asymmetric peak at 410 C and 420 C, respectively, is produced for the oxygen

uptake on both samples. The asymmetry is caused by a pronounced fronting and the

onset can be estimated to be at ca. 160 C. Subtraction of the oxygen consumed for

coke burn-o does not signicantly change the signal shape for the P1 sample, but the

55


0 1 2 3

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

CO2 CO H2O O2 CD OL

-2000

-1000

0

1000

2000

0 1 2 3

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

CO2 CO H2O O2 CD OL

-2000

-1000

0

1000

2000

0

100

200

300

400

0

100

200

300

400b) P2 - Fe3Co

7Mo

12O

46

a) P1 - Fe3Co

4Mo

12O

43

CO CO

2 H

2O

O2,total

O2,reox

efflu

ent m

ole

fract

ion

/ %

time / h

CDO2

TPO 1

n i mca

t-1 /

µmol

g-1

efflu

ent m

ole

fract

ion

/ %

time / h

CDOL

n i mca

t-1 /

µmol

g-1

tem

pera

ture

/ °C

tem

pera

ture

/ °C

Figure 4.10.: TPO experiments of iron molybdate based model catalysts. a) P1:

Fe3Co4Mo12O43, b) P2: Fe3Co7Mo12O46. Symbols: () CO, () CO2,(N) water, ( ) total oxygen uptake, ( ) catalyst reoxidation.

shoulder/fronting develops clearly. In analogy to the bismuth molybdate samples the

high temperature signal can be assigned to the Mo 4+→Mo 6+ reoxidation. For the P2

sample this peak is signicantly less pronounced indicating only a minor reduction of the

Mo moiety of this sample. This is also expected from the TPR results, which showed

quantitative reduction of the MoO3 contained in the P1 sample to MoO2. The broad low-

temperature signal is observed for both catalysts. A similar signal has been observed in

the reoxidation of heavily reduced molybdenum oxides (Fig. A.20, [83]). However, since

the signal has the same magnitude for both samples, whereas the P2 sample contains only

minor amounts of free MoO3, it is more likely that the signal is caused by the reoxidation

of reduced species in the mixed β-FexCo1-xMoO4 phase.

4.4. Discussion

The results from the cyclic reduction and reoxidation experiments allow for the classica-

tion of the model catalysts according to their reducibility and selectivity in the anaerobic

oxidation of propene. Thereafter, bismuth molybdates are active and selective catalysts

for this reaction. Conversely, both bismuth tungstates and mixed iron/cobalt molyb-

dates are incapable of selective oxidation and bulk reduction, but provide only surface

56


and some subsurface oxygen. Nevertheless, due to the high specic surface area of the

latter this amounts to a substantial lattice oxygen reservoir. Quantitative reoxidation of

bulk and surface oxygen vacancies was shown to be possible at temperatures of 420 C.

This indicates that a Mars-van Krevelen-like redox mechanism is feasible only with

bismuth molybdate catalysts, since numerous sublayers of lattice oxygen can be supplied

for the selective formation of acrolein and the vacancies thus formed can be reoxidised at

reasonable conditions.

The results are in full agreement with literature data on the steady-state activity of

bismuth molybdates, and iron/cobalt molybdates. Krenzke and Keulks [22] reported

that at temperatures of 400− 450 C up to 50 monolayers of oxygen of a Bi2Mo3O12

catalyst participated in the formation of acrolein and COx under SSITKA (steady-state

isotopic transient kinetic analysis) conditions. This agrees very well with the results ob-

tained for the A1 and A2 sample. Millet et al. [60] studied the activity of iron/cobalt

molybdates of dierent composition and structure and found them to be poorly active

and non-selective for acrolein formation. There is relatively little information available

regarding the activity of bismuth tungstates in propene oxidation. Trifiro and cowor-

kers [3] report that Bi2W2O9 is inactive as an oxidation catalyst under the conditions

of propene (amm)oxidation, which is in agreement with the present study. The authors

highlight the isomerising activity of WO3, but they suggest an acidic functionality of

WO3, which is hence unsuitable for the selective propene activation. The high selectivity

to acrolein and acrylonitrile, which the authors found for a russelite (Bi2WO6) catalyst, is

somehow in contrast with the results presented here. This must therefore be attributed to

the dierent experimental conditions, which indicates that selective oxidation may occur

by action of a Langmuir-Hinshelwood-type mechanism.

The approach of combining cyclic reduction and reoxidation experiments with suitable

ex-situ characterisation allows to retrace basic aspects of the redox process. For a che-

mist the reduction of the oxide and the concomitant formation of acrolein and COx is

a sequence of acid-base and redox reactions. From an engineering point of view the re-

duction of a catalyst by a hydrocarbon is generally a gas-solid reaction. The elementary

steps of the reaction correspond to those of a heterogeneously catalysed reaction. As

hydrocarbons are usually too large to penetrate into interstitials and the bulk of the

catalyst, the reduction process is sustained by diusion of oxygen from the bulk to the

surface. Additionally, the reaction (by-)products may aect the eective reduction rate in

dierent ways. First, the formation of reduced phases alters the diusion and adsorption

properties of the sample as a whole by providing new pathways and sites. Second, the

formation of coke or irreversibly adsorbed species can directly limit the accessibility of

the surface.

57


The reduction is initiated by the activation of the substrate. In the case of olen

oxidation, this is brought about by abstraction of a hydrogen to form an allylic interme-

diate. The results indicate that mixed bismuth oxides can eciently play this role. It

is well-known that bismuth molybdates are capable of the oxidative dehydrogenation of

butene [94, 95], but here bismuth tungstate Bi2W2O9 was also shown to be a selective

catalyst for this reaction. Miura et al. [36] suggested that the Bi2O2+

2 layers are res-

ponsible for the hydrogen abstraction in the oxidative dehydrogenation of 1-butene over

bismuth oxohalides and Bi2MoO6. This agrees with the results of the present study, since

Bi2WO6 and Bi2W2O9 exhibit the same structural motif. However, it is absent from the

crystal structures of α-Bi2Mo3O12 and β-Bi2Mo2O9, which are also excellent catalysts for

olen activation. Therefore, it can be doubted that the precise structural arrangement

is required for hydrogen abstraction, but rather a similar local environment. Also iron

oxides and iron molybdates have been reported to be active and selective catalysts for

ODH reactions [96,97].

The formation of acrolein from the allyl intermediate requires an electron transfer

from the allyl to the metal cation and the insertion of an oxygen from the lattice into

the allylic intermediate or the formation of a σ-oxo-allyl species. Subsequently, a second

hydrogen is abstracted from the allyl intermediate and acrolein desorbs leaving an oxygen

vacancy. Two hydroxyl groups will eliminate water and form another oxygen vacancy.

For the continuation of the reduction process, the two vacancies thus formed have to be

regenerated by oxygen diusion from the bulk.

It is observed that molybdenum in bismuth molybdate Bi2Mo3O12 readily transfers

its oxygen to form acrolein and possesses a suciently high oxygen mobility to restore

active surface sites. This can be deduced from the high acrolein selectivity in the TPR

experiments and from the signal in the TPO, which is assigned to the reoxidation of

Mo 4+. It was found that the reduction of bismuth molybdates yields only Mo 4+, but

no Bi 0 except for a small fraction observed for the severely reduced A2 sample. This

is in accordance with Ayame et al. [42], who exclusively found Mo 4+ and Mo 5+ on α-

Bi2Mo3O12 reduced by propene jets in an in-situ XPS setup. This somehow contradicts

the Grasselli mechanism suggesting a reduced bismuth species as an intermediate of the

redox cycle and the observation that bismuth is crucial for the initial hydrogen abstraction

[20,46]. However, the TPO experiments with the bismuth-based model catalysts indicate

that reduced bismuth is more readily reoxidised than molybdenum, especially when it is

nely dispersed. Since the temperature at which reoxidation of bismuth occurs (170 C)

is far lower than the reduction temperature, it is likely that oxygen vacancies formed

in the vicinity of bismuth are replenished by transfer of bulk oxygen associated with

molybdenum. This is easily accepted, since sucient oxygen mobility at the temperature

58


required for bismuth reoxidation has been proven by the reduction experiments. Critical

vacancy accumulation then results in structural disintegration and nucleation of Bi2MoO6

and MoO2.

For the bismuth tungstates no signicant formation of acrolein is observed, which in-

dicates that oxygen bound to either bismuth or tungsten is unsuitable for insertion into

the allyl species. The formation of an allyl species is plausible due to the results obtained

with butene as the reductant. The participation of bismuth, which is believed to result

in the generation of an allyl species, is also evidenced by the reoxidation experiments

and the XRD results of the A4 sample. Traces of acrolein found in TPR experiments

could stem from surface-initiated homogeneous reactions as proposed by Daniel and

Keulks [31], but this would require loosely bonded oxygen species to form a hydrope-

roxide intermediate, which is unlikely under the conditions of the TPR experiments. It

can be assumed that allyl species that do not undergo oxygen insertion oligomerise and

are further dehydrogenated to form coke. It was reported that the oxidation of propene

over bismuth oxide yielded only 1,5-hexadiene [15], which is the product of the dimerisa-

tion of allyl species. The diene may easily be dehydrogenated to form polyaromatics as

precursors of coke formation. However, no fragments of mass 81 amu were detected by the

mass spectrometer, which demonstrates that the coke precursors are not released from

the surface. The dehydrogenation (and coking) reaction can be traced back by comparing

water and acrolein and COx formation. According to the reaction stoichiometry coking

occurs when the amount of water exceeds the summed-up carbon content of acrolein and

carbon oxides. This is denitely and moreover immediately the case for all tungstates

(cp. Figs. A.5-A.11), which means that coke forms already in the initial phase of the

reaction. For the molybdate samples the amount of water only surpasses the threshold

signicantly after severe reduction, which signies that reduced metal sites are probably

active centres for coke formation on bismuth molybdates. This is in full agreement with

the observations of Fehlings et al. [53, 54].

For the tungstate samples the reduction process soon stops after the consumption of

some monolayers of oxygen. In the TPO experiments carbon residues amounting to a

theoretical monolayer were found on the surface of the reduced tungstates, which could

restrict its accessibility. Yet, the formation of coke per unit surface area was found to

be most severe on the bismuth molybdates, which proved to be reducible. It is therefore

unlikely that the formation of coke decisively hinders the ad- and desorption of reactants

and products. Consequently, the dierent reactivity must be caused by the intrinsic

oxygen mobility under the reaction conditions.

A simple method to assess the mobility of ions in the crystal lattice is the evaluation of

the Tammann and Hüttig temperatures [98]. The Tammann temperature indicates a

59


Table 4.4.: Melting point, Tammann and Hüttig temperatures (in C ) of modelcatalysts.

sample Tmelt1 TTammann THuttig

Bi2Mo3O12

Bi2Mo4O15

6602 192 6

Bi2WO6 10803 403 133

Bi2W2O9

Bi2W4O15

9253 326 86

Fe3Co4Mo12O43

Fe3Co7Mo12O46

11704 448 160

1 major phase2 from [89]3 from [90]4 from [99]

level of mobility that is sucient for bulk to surface mass transport, whereas the Hüttig

temperature characterises conditions at which the mobility of surface species suces

to cause sintering or agglomeration. Common estimates are TTammann ≈ 0.5 ·T bulkmelt[K]

and THuttig ≈ 0.3 ·T bulkmelt[K], respectively. Table 4.4 summarises the values of the model

catalysts studied.

Although these values do not match perfectly the results obtained in the reduction

experiments, they indicate that mobilising the bulk oxygen anions of the tungstate and

iron/cobalt molybdate samples is signicantly more dicult than activating those of the

bismuth molybdates. However, the conductivity of oxides and the oxygen mobility are

decisively aected by structural factors, such as ionicity, symmetry and defects [100].

Further evidence for the role of bismuth and molybdenum in the selective oxidation

of propene using complex oxide catalysts is derived from the surface characterisation

by means of XPS and ISS. The striking dierence in the reducibility of the A1 and A2

sample being exposed to either propene or 1-butene can be rationalised in terms of the

surface enrichment with bismuth detected in case of the A1 sample. In a recent study,

van Well et al. [101] showed that the surface concentration of Bi has a marked eect

on the activity of Bi2MoO6 catalysts for the selective oxidation of propene. Thereafter,

surface enrichment of bismuth, which may be caused by prolonged calcination or high

calcination temperatures, results in poor activity and selectivity. As discussed before, the

formation of acrolein requires an environment providing bismuth and molybdenum sites

to bring about the abstraction of hydrogen and the oxygen insertion, respectively. The

dominance of bismuth in the surface layer as determined for the A1 sample suciently

generates allyl species, but due to the low molybdenum site density these are only slowly

60


converted into acrolein or can act as precursors of coke formation (vide supra). For 1-

butene oxidative dehydrogenation the molybdenum site concentration is less decisive, and

in fact, the surface-related reduction rate is higher for the A1 sample in this case. The

present results are therefore fully consistent with those of van Well et al. [101].

The TPR traces of both bismuth molybdates exhibit maxima at isothermal conditions.

Especially in the case of the A2 sample a signicant acceleration of the reduction rate is

observed in consecutive cycles. The maximum at isothermal conditions is indicative of

a nucleation-controlled reduction process [77,78]. The reduction products of both model

catalysts were found to be γ-Bi2MoO6 and MoO2. It is probable that a certain fraction

of the Bi2MoO6 formed will not react with MoO3 in the reoxidation process. These seed

crystallites may then accelerate the nucleation-controlled reduction in the subsequent

TPR experiment.

4.5. Conclusion

Cyclic reduction and reoxidation was successfully applied to study the activity of dierent

precursor samples in the oxidation of propene to acrolein. Quantitative evaluation of the

experiments revealed that cyclic operation restored the initial degree of oxidation of the

samples, but deviations in reducibility and selectivity indicated slight structural changes.

Bismuth molybdates were found to be active and selective catalysts in accordance with

literature data. The rate of reduction on these catalysts is strongly dependent on the

surface structure. The enrichment of the surface with bismuth eects a reduced acti-

vity for anaerobic propene oxidation. Both bulk reduction and reoxidation proceed at

temperatures of 300− 400 C. Bismuth tungstates are incapable of anaerobic propene

oxidation. This is due to the lattice oxygen associated with tungsten being too strongly

bound, which makes it unsuitable for insertion into the allylic intermediate. Iron co-

balt molybdates were found to be reduced by propene but exhibited little selectivity to

acrolein.

It was shown that the formation of acrolein requires bismuth and molybdenum in their

highest oxidation state. Molybdenum was shown to be reduced to Mo IV and reoxidation

to MoVI restores the active state. Bismuth participates in the hydrogen abstraction.

Reduction occurring in the course of this is compensated by a fast redox reaction with

surrounding MoVI species.

61

62

5. Optimisation of mixed oxide

catalysts

5.1. Introduction

Complex and mixed oxides hold an outstanding position in the selective heterogeneous

oxidation of hydrocarbons [102]. Usually, it is observed that the more demanding chemi-

cal transformations require a higher degree of complexity of composition and structure of

the catalyst (catalyst multifunctionality) [103]. Single oxides and supported metal oxide

species can be ecient catalysts for simple dehydrogenation reactions [96,104]. The selec-

tive transformation of propane to acrylic acid in turn requires not less than 4 transition

and main group metals in a precise structural arrangement [46, 105]. The demand for

multiple functions in a catalyst can be satised by combining dierent oxides, and the

resulting formulations range from mixed to complex oxides and solid solutions.

Multiple synergistic eects arise from the combination of dierent oxides and these

have attracted considerable attention in the selective oxidation of C3- and C4-olens.

Complex bismuth and molybdenum oxides were the breakthrough technology for the

(amm)oxidation of propene to acrolein or acrylonitrile [19]. Going one step back, it was

shown that physical mixtures of Bi2O3 and MoO3 produced an improved performance

as compared to the action of the single oxides [15]. In fact, the combination of the two

oxides was the prerequisite for selectivity to acrolein. This was attributed to the fact

that bismuth oxide is capable of dehydrogenation, while molybdenum oxide facilitates

oxygen insertion. Complex bismuth molybdates perfected the principle by providing the

dierent functionalities on a molecular scale.

In the Bi-Mo-O ternary system three crystalline phases were found to be active and

selective for the (amm)oxidation of propene. However, no nal agreement on the relative

activity of these phases was reached [19]. Later on several authors reported a new type

of synergy eect that evolved when dierent bismuth molybdate phases were brought

into contact [106, 107]. The catalysts in which either the α- or β-phase were present

in a mixed phase together with γ-Bi2MoO6 exhibited signicantly higher activity and

selectivity, which was attributed to an oxygen spill-over from the latter onto the surface

63

5. Optimisation of mixed oxide catalysts

of the α- or β-bismuth molybdate according to the remote-control mechanism. This

mechanistic concept was developed by Delmon and co-workers [108110] in order to

rationalise similar observations on bismuth phospho-molybdates and mixed tin antimony

oxides.

An even more eective way to facilitate the synergy between the bismuth molybdate

and the alleged oxygen donor phase was the incorporation of iron into the catalyst system.

Although quaternary Bi-Fe-Mo-O phases are known, multiphase systems in which iron

was incorporated into a solid solution of a metal molybdate (MIIMoO4, M = Co, Ni)

proved to be most ecient. It is reckoned that all commercial catalysts for acrolein

production are based on this building principle [5].

The origin of the synergy in multiphase systems has been investigated intensively and

interphase oxygen transport was shown to occur under certain conditions [17]. Several

studies mention that cooperative eects are observed for physically or mechanically mixed

samples already [60,109,110]. However, the investigation of the kind of contact facilitated

by unit operations such as mixing, grinding and calcination could easily be beyond the

possibilities of standard characterisation techniques.

In a recent contribution Breiter and Lintz [2] reported on a strong synergy eect

operating with bismuth molybdate-free catalysts for the selective oxidation of isobutene

to methacrolein. Their catalyst system was a mixture of bismuth tungstate, tungsten

oxide, iron cobalt molybdate and molybdenum oxide. It was shown that the presence

of a (XRD-) crystalline selective phase (e.g. bismuth molybdate) was not necessary to

achieve activity and selectivity in this modied multiphase system.

The vast majority of catalysts described in the open literature is prepared by co-

precipitation and in none of these studies the role of tungsten is investigated, which is

included in catalyst formulations described in German and Japanese patent literature [5].

In order to shed some light on the nature of the synergy and the active ensemble in this

new type of catalyst, a comparison with the bismuth molybdate system seems promising.

A facile way to access mechanistic information are steady-state and transient kinetic

investigations.

The objective of this study was the optimisation of multicomponent mixed oxide ca-

talysts obtained from physical mixtures of either bismuth molybdate or tungstate and

iron/cobalt molybdate. This was to ensure comparable performance for the steady-state

kinetic experiments. Additionally, a deeper understanding of the formation of the active

catalyst and the underlying processes was to be obtained. From previous reports [60] and

the results of transient experiments (see Chapter 4) it is evident that the performance of

these catalysts depends on a variety of factors such as stoichiometry of the precursors,

composition and pretreatment temperature.

64


The catalysts examined in this study were optimised with respect to their active com-

ponent/support component ratio, pretreatment temperature and preparation procedure.

In consideration of previous ndings the compositional range studied was 15 to 35 wt. %

of the respective active component material and the pretreatment temperature was varied

between 450 C and 480 C. The increments were 10 wt. % and 10 K, respectively. The

thoroughly mixed precursors were either calcined in a rotating tube furnace, in a xed

bed tube furnace or pelletised prior to calcination in a tube furnace.

5.2. Experimental


The mixed oxide catalyst were prepared from physical mixtures of suitable precursor

oxides. The precursors were obtained by spray drying and dierent post-drying treatment.

The mixed oxides comprise a bismuth-based complex oxide, in the following denoted as

active component, and an iron-, cobalt- and molybdenum-based oxide, in the following

denoted as promoter component. The active component precursors used in this study

have been prepared by a spray-drying routine described in more detail in Ref. [85]. Both

the tungsten-based mixed oxide (label A5) and the molybdenum-based (label A2) had a

Bi/M (M = W,Mo) ratio of 0.5 The spray-dried powders were calcined at 830 C (W)

and 550 C (Mo), respectively, and ground to particle size 2 µm ≤ dp50 ≤ 3 µm. The

promoter component (label P2) has been prepared by a similar spray-drying routine. The

composition was set to yield the formal stoichiometry Fe3Co7Mo12Ox. The dried powder

was used for the preparation of mixed oxide catalysts without further treatment.

The physical mixtures of the precursor oxides were obtained by thoroughly mixing

the desired amounts of powder in an analysis mill IKA A10 in an argon atmosphere,

excluding any humidity in order to avoid agglomeration of the particles. The sequence

of the subsequent preparation steps was varied and is part of the optimisation procedure

including calcination, pelletising and grinding. The calcination was carried out in a rotary

or tubular furnace in a owing synthetic air (20 % oxygen, 80 % nitrogen) gas mixture.

A multistep temperature programme (Fig. A.2) was applied in order to carefully remove

or decompose ammonia nitrate contained in the precursor. The nal temperature of the

calcination process was also part of the optimisation stragtegy. The powder samples were

pelletised using a standard pellet die (msscientic, di = 13 mm) and press. For catalytic

tests the catalyst tablets obtained from the pelletising routine were ground to obtain the

size fraction of 250− 355 µm.

65



X-ray diraction

XRD patterns were recorded using a Panalytical MPD diractometer equipped with a Cu

X-ray source, 0.5 divergent and antiscatter slits, a 0.2 mm high receiving slit, incident

and diracted beam 0.04 rad soller slits, and a secondary graphite monochromator. The

2Θ range covered was from 5 to 60 with a step width of 0.03. Qualitative phase ana-

lysis was carried out using the X'Pert Line software (Panalytical) together with powder

diraction les (PDFs) from the International Centre of Diraction Data (ICDD).

Scanning electron microscopy and energy-dispersive X-ray analysis

Electron micrographs were obtained on a LEO 1530 Gemini scanning electron microscope

equipped with an INCA X-ray microanalysis system for EDX analysis. The standard

secondary electron detector was used for imaging.

N2physisorption

N2 physisorption data were obtained on a Quantachrome Autosorb-1-MP and a Autosorb-

1-C apparatus. Specic surface areas of the samples were determined by static nitrogen

or krypton adsorption at −196 C and data evaluation according to the multipoint BET

method. Pore size distributions were determined by evaluating the desorption branch

of the nitrogen isotherm according to the BJH method. The standard pretreatment

procedure was heating in vacuo at 300 C for 2 h.


The temperature-programmed experiments were performed in a conventional ow setup

equipped with thermal mass ow controllers (Bronkhorst) and a GAM 445 quadrupole

mass spectrometer (Balzers). For standard experiments 150 mg of the sample were placed

between two quartz wool plugs in a U-shaped catalytic microreactor constructed of glass-

lined tubing (SGE). Gases supplied were He (99.9999 %), 1 % C3H6 (99.98 %) in He

(99.9999 %) and 1 % O2 (99.995 %) in He (99.9999 %).

Prior to the reduction and reoxidation experiments the samples were pretreated in a

stream of 50 mlmin−1 helium at a temperature of 250 C for 1 h in order to remove any

physisorbed species. For a typical TPR (TPO) experiment the reactor was purged with

propene (oxygen) at room temperature for 15 min. The sample temperature was then

increased to 420 C at a heating rate of 5 Kmin−1. The nal temperature was maintained

for 5 h (TPR) or 3 h (TPO).

66


5.2.4. Catalytic activity measurements

The steady-state activity measurements were performed in an all stainless steel ow setup

equipped with thermal mass ow controllers. The catalytic microreactor was constructed

of glass-lined tubing (di = 4 mm). Product analysis was realised using a Shimadzu

GC-14B and Delsi GC11 gas chromatograph equipped with two TCD and Porapak QS

(8′ × 1/8′′) and Molsieve 13X (6′ × 1/8′′) columns, respectively.

For the experiments 500 mg of catalyst were used at a WHSV (weight-hourly-space

velocity; ratio of propene mass ow catalyst massmC3H4

mcat) of 0.1 h−1. The feed composition

was 5.0 % propene and 9.5 % oxygen. The balance was helium and neon as internal

standard for the GC application. Activities were measured at 320 C after a period of

time on stream of 96 h.

5.3. Results


A total of 14 catalysts was prepared and catalytically tested. Table 5.1 summarises the

specic surface areas and the phase composition of the catalyst samples. The samples are

characterised by type and ratio of the active component, the calcination temperature and

the preparation procedure. The prexes Mo and W denote whether the active component

is molybdate- or tungstate-based, respectively. The rst pair of digits describes the

active component/promoter component ratio of the calcined sample (A/(A+P) in wt-%).

TG-MS analysis (thermogravimetry-mass spectrometry) revealed a mass loss of 40 % of

the promoter component precursor during calcination, which was used as the basis for

calculating the precursor mixtures. The next digits give the calcination temperature in

degrees Celsius. The sux P signies that the sample was pelletised prior to calcination

using a press load of 4 t (1 t indicated), while RT and FB denote calcination in a rotary

kiln and xed bed, respectively.

Due to the low amount of sample used for the catalytic testing, the amounts recovered

were mostly too small for post-catalytic characterisation. Especially the determination

of the surface area by nitrogen physisorption was not possible due to instrumental limi-

tations. X-ray diraction data were obtained using specimen dispersed on a silicon wafer

to eliminate reexes of the sample holder. This procedure induces angle-dependent shifts

in the intensity ratio, which may therefore not be interpreted.

The phase composition of the samples was examined by X-ray diraction. The catalysts

are composed of several phases exhibiting structures of relatively low symmetry giving

rise to a high number of reexes. This makes the unambiguous identication especially

67


Table

5.1.:Com

position

andspeci

csurface

areasof

catalyst

samples.

sample

SBETa

phasecom

position

(XRD)

prec

urso

rcomponents

A2

1.8Bi2 M

o3 O

12 ,MoO

3

A5

1.0Bi2 W

2 O9 ,WO3

P2(ca

lcined

450

C)

14.0Cox Fe1-x M

oO4 ,Fe2 M

o3 O

12 ,MoO

3

tungsta

te-ba

sedsamples

W25-460-R

T9.9

Bi2 W

2 O9 ,WO3 ,β-Cox Fe1-x M

oO4 ,Fe2 M

o3 O

12 ,MoO

3

W25-460-F

B12.5

Bi2 W


oO4 ,Fe2 M

o3 O

12 ,MoO

3

W25-460-P

8.9Bi2 W


oO4 ,α-CoM

oO4Fe2 M

o3 O

12 ,MoO

3

W15-460-P

Bi2 W


oO4 ,α-CoM

oO4Fe2 M

o3 O

12 ,MoO

3

W35-460-P

7.1Bi2 W


oO4 ,α-CoM

oO4Fe2 M

o3 O

12 ,MoO

3

W35-470-P

6.0Bi2 W


oO4 ,α-CoM

oO4Fe2 M

o3 O

12 ,MoO

3

W35-480-P

5.5Bi2 W


oO4 ,α-CoM

oO4Fe2 M

o3 O

12 ,MoO

3

W35-470-P

1t6.5

Bi2 W


oO4 ,α-CoM

oO4Fe2 M

o3 O

12 ,MoO

3

molybdate-ba

sedsamples

Mo15-460-P

7.2Bi2 M

o3 O

12 ,MoO

3 ,β-Cox Fe1-x M

oO4 ,Fe2 M

o3 O

12 ,α-CoM

oO4

Mo25-460-P

7.3Bi2 M

o3 O

12 ,MoO

3 ,β-Cox Fe1-x M

oO4 ,Fe2 M

o3 O

12 ,α-CoM

oO4

Mo35-460-P

5.8Bi2 M

o3 O

12 ,MoO

3 ,β-Cox Fe1-x M

oO4 ,Fe2 M

o3 O

12 ,α-CoM

oO4

Mo25-450-P

Bi2 M

o3 O

12 ,MoO

3 ,β-Cox Fe1-x M

oO4 ,Fe2 M

o3 O

12 ,α-CoM

oO4

Mo25-470-P

5.4Bi2 M

o3 O

12 ,MoO

3 ,β-Cox Fe1-x M

oO4 ,Fe2 M

o3 O

12 ,α-CoM

oO4

Mo25-460-P

1t8.2

Bi2 M

o3 O

12 ,MoO

3 ,β-Cox Fe1-x M

oO4 ,Fe2 M

o3 O

12 ,α-CoM

oO4

am

2g−1

68


10 20 30 40

Fe0.3

Co0.7

MoO4

Bi2W

2O

9

-Fe(Co)MoO4

W25-460P

W25-460

inte

nsity

/ a.

u.

2

Figure 5.1.: X-ray diractograms of catalysts W25-460 and W25-460-P before reac-tion.

of minor phases dicult. Fig. 5.1 compares the diractograms of the samples W25-

460 and W25-460-P. Both show the characteristic pattern of Bi2W2O9 (JCPDS 33-0221)

and WO3 (JCPDS 83-0950) the constituents of the active component bismuth tungs-

ten oxide. Furthermore, an iron cobalt molybdate (Fe0.3Co0.7MoO4; JCPDS 89-6590) is

readily identied. In minor amounts molybdenum oxide MoO3 (JCPDS 75-0912) and

Fe(III) molybdate Fe2(MoO4)3 (JCPDS 83-1701) are present in both samples. The reex

at 18.35 in the X-ray diractogram of the W25-460 sample indicates traces of magnetite

Fe3O4 (JCPDS 74-1910). A distinction of the sample W25-460-P and all samples prepa-

red via pelletisation prior to calcination is the occurrence of the reex at 14.14, which

is characteristic of the α-modication of iron and cobalt molybdate (FeMoO4: JCPDS

22-1115; CoMoO4: JCPDS 73-1331).

Increasing the amount of the active compound results in the increase of the respective

reexes, but does not alter the general phase composition (see Figs. A.22 and A.23).

Also does the variation of the calcination temperature in the range from 450− 480 C

not eect changes in the phase composition (see Fig. A.24). XRD patterns of selected

samples were recorded after the catalytic tests (not shown). In all cases only the phases

present prior to catalysis and no new phases were detected. The results indicate that the

catalysts are essentially stable under reaction conditions.

In order to study the formation of the catalyst and the interaction of the phases, SEM

micrographs documenting the dierent preparation steps were obtained (Fig. 5.2). Ad-

69


ditionally, by using the elemental mapping function of the microscope, it was conrmed

that within the accuracy of the method (resolution ≈ 3 µm) the components are homo-

geneously distributed in the sampled volume. The precursors can easily be distinguished

by their dierent morphologies. The A5 sample (Fig. 5.2a) is composed of small (sub-

micron) crystallites or aggregates of these. The particle size distribution appears to be

rather narrow. The uncalcined promoter component P2 (Fig. 5.2b) consists of regular

spheres of 10− 60 µm exhibiting a rough surface. The micrograph of the physical mix-

ture of the two phases indicates that the morphology of the P2 sample is not severely

altered by the milling process (Fig. 5.2c). The spherical particles of the P7 sample are

clearly recognised, which are partly decorated by smaller particles consisting of the A5

particles and abrasives of the P2 sample. The surface of the larger particles of the pro-

moter compound appears smoothed after calcination in the rotary furnace (Fig. 5.2d).

However, individual particles are still identied, indicating that composite formation is

not complete.

The BET surface areas of the catalyst series are summarised in Tables 5.3 and 5.4. It

was observed that pelletising the sample prior to calcination reduces the surface area,

which is due to the increase of contact area. Increasing the content of the active com-

ponent tends to result in decreasing surface areas because of the signicantly lower specic

surface area of the bismuth metallates. Higher calcination temperatures lead to sintering,

presumably of the support compound, which has been demonstrated to proceed drasti-

cally in this temperature range (cp. Fig. A.25). The BJH analysis revealed that the

catalysts have no mesopores.


The characteristics of the reduction of the A2, A5 and P2 component by propene have

been previously described (Chapter 4). It was shown that only the A2 component is

active and selective for the anaerobic oxidation of propene to acrolein. However, based

on the catalyst mass the P2 component is able to supply substantial amounts of oxygen

to the non-selective oxidation of propene, whereas the A5 component is neither active

nor selective.

In Figure 5.3 the TPR proles of the W25-460-P sample is shown exemplarily for

the mixed oxide catalysts. The onset-temperature for the formation of CO, CO2 and

acrolein is ca. 175 C. On the pure P2 sample COx formation starts at roughly the same

temperature, while acrolein formation is only observed at temperatures as high as 250 C

(Fig. 4.7a). The acrolein signal of sample W25-460-P is composed of two peaks, where

70


20 µm

a

20 µm

20 µm 20 µm

c

b

d

Figure 5.2.: SEM micrographs of a) active component A5 (Bi2W4O15), b) promo-ter component P2 (Fe3Co7Mo12O46), c) physical mixture of 25 wt% A5and 75 wt% P2 (based on oxide compounds) and d) same sample aftercalcination at 460 C.

71


0 1 2 3 4 5

0,000

0,005

0,010

0,015

0,020

0,025

0,030


-1000

-500

0

500

0

100

200

300

400

C3H

4O

CO CO

2 H

2O

W25-460-P

efflu

ent m

ole

fract

ion

/ %

time / h

OL

TPR 1 (RO cycle)n i mca

t-1 /

µmol

g-1

tem

pera

ture

/ °C

Figure 5.3.: TPR of catalyst W25-460-P. Symbols: () acrolein, () CO, (H) CO2,() water.

Table 5.2.: Quantication of TPR experiments of mixed oxide catalysts.

∆nO,L Selectivity % CO/CO2

sample[µmol

g ] Acrolein CO2 CO ratio

P2 - Fe3Co7Mo12O46 1565 34 37 29 0.79

W25-460 1147 37 40 23 0.58W25-460P 1065 54 30 16 0.54W35-470P 879 64 27 10 0.37Mo25-460P 1457 58 29 13 0.45

the smaller, low-temperature peaks appears as a shoulder at ca. 280 C. In contrast,

only a single peak is observed for the P2 component.

The TPR proles of selected samples are displayed in Figs. A.26, A.27 and A.28. The

derived quantitative results are presented in Table 5.2. Some general trends can be iden-

tied: the amount of lattice oxygen incorporated into the reaction products is the same

for the P2 component and the physical mixture W25-460 (based on its P2 moeity), but

is signicantly less for those samples that have been prepared via pelletisation prior to

calcination. The selectivity parameters of the W25-460 sample are very close to those of

the support compound indicating that there is little interaction between the constituent

phases. Conversely, the selectivity is increased due to the alternative preparation method.

Also, the higher content of active component and/or the increased calcination tempera-

ture add to this eect as demonstrated by the comparison of W25-460P and W35-470P.

The same tendency is observed for the CO/CO2 ratio, which decreases considerably going

from the pure promoter compound to the most selective system.

In Fig. 5.4 the acrolein signals of the TPR experiments of the tested catalysts are

displayed. The graph of the promoter compound P2 has been scaled by factor 0.75 for

better comparison. It is seen that both the low-temperature signal and the main signal

72


0 1 2 3 4 5

0,000

0,005

0,010

0,015

0,020

0,025

0

100

200

300

400

100 200 300 400

0,000

0,001

0,002

0,003

0,004

0,005

efflu

ent m

ole

fract

ion

/ %

temperature / °C

P2 W25-460 W25-460P W35-470P Mo25-460P

efflu

ent a

crol

ein

mol

e fra

ctio

n / %

time / h

Figure 5.4.: Acrolein formation during TPR experiments. Graph of promoter com-

pound P2 has been scaled by factor 0.75 for comparison. Symbols: ( )P2 - Fe3Co7Mo12O46, () W25-460, () W25-460P, (H) W35-470P,() Mo25-460P.

are aected by the preparation method, the catalyst composition and the calcination

temperature. The inset shows that the low-temperature signal is weakly developed in the

physical mixture. With those catalysts prepared by pelletisation prior to calcination the

signal is shifted to lower temperatures and visible as a dened shoulder. For the bismuth

molybdate-based sample Mo25-460P an individual peak at 240 C is observed.

The promoter compound and the physical mixture Mo25-460 exhibit a main signal of

comparable intensity. The peak intensity of the other catalysts is considerably higher

and increases in the order: W25-460P < W35-470P < Mo25-460P. For these samples the

decrease in acrolein formation after reaching its maximum is also less steep. The broad

signal arising in the isothermal part of the spectrum of the Mo25-460P sample indicates

the presence of MoO3, which is contained in the active component A2. The respective

CO2 signals show a slight high-temperature shift, following the general tendency described

above, which is primarily due to the decreased CO2 selectivity (Fig. A.29).

The amount of oxygen released in the TPR peaks is depicted in Fig. 5.5. For this pur-

pose, the contributions from the pure active and support components were subtracted.

The results indicate that the TPR prole of the physical mixture is merely described by

superposition of the TPR proles of its constituents. However, the intensity of the low-

temperature peak is comparable to that of the other systems. For the high-temperature

73


W25-460 W25-460-P W35-470-P Mo25-460-P0

20

40

60

80

0

2

4

6

8

10

1st peak 2nd peak

acro

lein

yie

ld /

µmol

g-1

acro

lein

yie

ld /

µmol

m-2

Figure 5.5.: Acrolein formation due to phase cooperation eects during TPR expe-

riments. Symbols: () low-temperature peak, () high-temperaturepeak.

signal it is worth noting that the maximum net yield is obtained with the W35-470P cata-

lyst. This result implies that the high activity of the Mo25-460P sample is rather caused

by the high reducibility of the bismuth molybdate phase than by the phase interaction.

Nevertheless, this sample is by far the most active in the low-temperature regime.

5.3.3. Catalytic activity measurements

The catalytic activity data resulting from the optimisation are summarised in Tables 5.3

and 5.4. The variable catalyst preparation was examined for the tungstate-based system

only, and the results were assumed to be applicable for the molybdate-based catalysts.

The objective was the maximisation of the combined yields of acrolein (ACR) and acrylic

acid (ACA) as a function of the variables composition and calcination temperature. It

was presumed that the inuencing factors are decoupled within the range of parameters

studied, i.e. the result is not aected by the order of the optimisation steps.

The inuence of catalyst preparation

Fig. 5.6 shows the results of the activity measurements of catalyst samples obtained via

dierent preparation techniques. These involved calcination of the physical mixture in a

74


Table 5.3.: Catalytic results of tungstate-based catalysts under standard reactionconditions.

SBET Conversion, selectivity % Yield % CO/CO2

sample[m2 g−1 ] Propene Acrolein COx ACR + ACA ratio

W25-460-RT 9.9 38.2 29.4 62.3 12.2 0.77W25-460-FB 12.5 38.1 34.6 58.1 14.1 0.76W25-460-P 8.9 34.6 56.8 39.0 20.1 0.59W15-460-P 33.9 44.3 49.5 15.7 0.68W35-460-P 7.1 36.8 68.5 28.0 25.7 0.53W35-470-P 6.0 53.3 77.2 18.4 42.5 0.48W35-480-P 5.5 48.9 79.2 18.8 39.0 0.48W35-470-P1t 6.5 66.7 75.8 15.9 54.8 0.49

Table 5.4.: Catalytic results of molybdate-based catalysts under standard reactionconditions.

SBET Conversion, selectivity % Yield % CO/CO2

sample[m2 g−1 ] Propene Acrolein COx ACR + ACA ratio

Mo15-460-P 7.2 80.8 78.9 15.8 66.7 0.45Mo25-460-P 7.3 79.2 82.9 12.2 68.8 0.42Mo35-460-P 5.8 70.8 84.1 11.7 61.7 0.41Mo25-450-P 80.1 80.5 12.4 68.8 0.44Mo25-470-P 5.4 74.1 83.9 11.6 64.8 0.40Mo25-460-P1t 8.2 88.9 79.6 11.8 77.2 0.44

75


rotating tube fixed bed pelletised

10

20

30

40

50

60

70

rotating tube fixed bed pelletised0,0

0,5

1,0

1,5

2,0

2,5

3,0

X(C3H

6) X(O

2) S(C

3H

4O)

S(COx) Y(C

3H

4O + C

3H

4O

2)

conv

ersi

on, s

elec

tivity

, yie

ld /

%

S(C3H

4O

2)

S(C2H

4O)

S(C2H

4O

2)

sele

ctiv

ity /

%

Figure 5.6.: The inuence of the preparation on the catalytic activity of tungstate-based mixed oxide catalysts.

rotary furnace, in xed-bed operation and pelletisation of the physical mixtures prior to

calcination.

It is seen that increasing cohesion of the particles during the calcination process re-

duces the activity of the catalyst in terms of propene and especially oxygen conversion.

Simultaneously, a marked increase in selectivity to acrolein is observed. However, the

dierence between the samples calcined in the rotary furnace and in xed bed congu-

ration are rather small. Accordingly, the pelletised sample exhibits the highest yield for

acrolein and acrylic acid.

The inuence of pelletisation also becomes evident when regarding the selectivities to

acrylic and acetic acid. These products are obtained in signicantly lower amounts, while

the selectivity to acetaldehyde remains almost constant. This is remarkable, since the

acids are presumably the products of the consecutive oxidation reactions of the corres-

ponding aldehydes. Therefore, the selectivity to acrylic acid is expected to increase at

higher acrolein yields. The converse eect demonstrates that these reactions are eecti-

vely suppressed on selective catalysts.

The inuence of catalyst composition

The inuence of the ratio of the constituent phases on the catalytic activity is shown in

Figs. 5.7 and 5.8. All catalysts were prepared by pelletisation prior to calcination. The

results indicate that the catalyst composition is a decisive parameter for mixed metal

oxide catalysts. For the tungstate-based catalysts, a steady increase in performance is

observed upon increasing the content of active component up to 35 wt%. Interestingly,

the activity increases slightly despite the decrease in specic surface area, which is caused

76


15 20 25 30 3510

20

30

40

50

60

70

80

15 20 25 30 350,0

0,5

1,0

1,5

2,0

2,5 X(C

3H

6) X(O

2) S(C

3H

4O)

S(COx) Y(C

3H

4O + C

3H

4O

2)

wt% Bi2W4O15

conv

ersi

on, s

elec

tivity

, yie

ld /

% S(C

3H

4O

2)

S(C2H

4O)

S(C2H

4O

2)

wt% Bi2W4O15

sele

ctiv

ity /

%Figure 5.7.: The inuence of catalyst composition on catalytic activity of tungstate-

based mixed oxide catalysts.

by the higher tungstate content. In general, the characteristics of varying composition are

similar to those of the previous optimisation step. The enhancement of acrolein selectivity

is correlated with a decrease in the CO/CO2 ratio and the reduction of acid formation.

The impact of varying the content of the molybdate based active component in mixed

oxide catalysts is less pronounced. A shallow optimum at 25 wt% bismuth molybdate

is though found. The propene conversion decreases at higher active component content,

while at the same time the selectivity increases, which results in the maximum of acrolein

and acrylic acid yield.

The inuence of the pretreatment temperature

The variation of the pretreatment temperature produces yield maxima of the selective

oxidation products (Figs. 5.9 and 5.10). For the tungstate-based catalysts the propene

conversion runs through a maximum in the temperature range 460− 480 C. This is

attributed to the opposing contributions of phase cooperation on the one hand and the

decrease in surface area on the other hand. The selectivity to acrolein is increasing throu-

ghout, but seems to level o at higher temperatures, so that it cannot compensate the

loss in activity. The optimum temperature determined for the tungstate based catalyst

is 470 C. The increase in selectivity is accompanied by a reduction of the CO/CO2

ratio and a decrease in acid formation. The maximum in acrylic acid selectivity can be

attributed to the massive increase in acrolein yield at 470 C.

The optimal calcination temperature for the mixed oxide catalysts with molybdate-

based active component is 460 C, which is somewhat lower than that for the tungstate-

77


15 20 25 30 350

20

40

60

80

100

15 20 25 30 350

1

2

3

4

5

X(C3H

6) X(O

2) S(C

3H

4O)

S(COx) Y(C

3H

4O + C

3H

4O

2)

wt% Bi2Mo4O15

conv

ersi

on, s

elec

tivity

, yie

ld /

%

S(C3H

4O

2)

S(C2H

4O)

S(C2H

4O

2)

wt% Bi2Mo4O15

sele

ctiv

ity /

%

Figure 5.8.: The inuence of catalyst composition on catalytic activity of molybdate-based mixed oxide catalysts.

460 470 4800

20

40

60

80

100

460 470 4800,0

0,5

1,0

1,5

2,0

2,5

3,0 X(C

3H

6) X(O

2) S(C

3H

4O)

S(COx) Y(C

3H

4O + C

3H

4O

2)

calcination temperature / °C

conv

ersi

on, s

elec

tivity

, yie

ld /

%

S(C3H

4O

2)

S(C2H

4O)

S(C2H

4O

2)


sele

ctiv

ity /

%

Figure 5.9.: The inuence of calcination temperature on catalytic activity oftungstate-based mixed oxide catalysts.

78


450 460 4700

20

40

60

80

100

450 460 4700

1

2

3

4

5

6

X(C3H

6) X(O

2) S(C

3H

4O)

S(COx) Y(C

3H

4O + C

3H

4O

2)


conv

ersi

on, s

elec

tivity

, yie

ld /

%

S(C3H

4O

2)

S(C2H

4O)

S(C2H

4O

2)


sele

ctiv

ity /

%

Figure 5.10.: The inuence of calcination temperature on catalytic activity ofmolybdate-based mixed oxide catalysts.

based catalyst system. The inuence of increasing the pretreatment temperature is simi-

lar, but less well pronounced. The selectivity can be improved by raising the calcination

temperature, aecting mainly acrolein, COx and acrylic acid selectivity. The conversion

of propene, however, decreases and seems to drop away at higher temperatures.

5.4. Discussion

The results clearly emphasise the importance of an intimate contact of the support and the

active component. Furthermore, it was found that the composition and the calcination

temperature aect the activity of multicomponent oxide catalysts prepared via physical

mixtures of precursor oxides. The magnitude of inuence of these parameters was dierent

depending on the active component material, which is discussed later.

The microscopic characterisation of the samples revealed that the calcination of the

physical mixture did not facilitate the contact of the promoter and the active component.

This is certainly an explanation for the poor performance in selective propene oxida-

tion, since the promoter compound and the tungstate-based active component are both

non-selective. The physical mixtures calcined in the rotary furnace and in the xed bed

were both pelletised and ground prior to the activity test, which should also ensure suf-

cient interaction. Nevertheless, only the pelletisation of the sample prior to calcination

signicantly improves the catalyst performance. This implies that the calcination, which

is carried out at temperatures higher than the reaction (460 C vs. 320 C), decisively

aects the catalyst structure and is a strong indication that the active structure is not

simply a mechanical mixture.

79


The results of X-ray diraction indicate that the major dierence between pelleti-

sed samples and those calcined with only loose interaction is the occurrence of the

α-modication of iron molybdate or isotypic cobalt molybdate. However, Millet et

al. [60] studied mechanical mixtures of iron/cobalt molybdates and dierent bismuth

molybdates and found that the mixed oxide catalysts exhibited much higher activity

when the iron/cobalt molybdate was β-type. The transition between the two phases oc-

curs at a temperature of roughly 400 C, but the β-phase was reported to be metastable

at lower temperatures, especially when quenched [111,112]. Due to the low cooling rates

applied at the end of the calcination process a mixture of high- and low-temperature mo-

dication could be expected. Nevertheless, the samples prepared without pelletisation

of the precursor oxides contain only the β-phase. The enthalpy change of the transition

between the two polymorphs is very low and the α → β phase change can be induced by

grinding or by applying pressure at room temperature [111]. It is therefore likely that

the grinding of the samples of the P-series causes the transition of the structure of the

Fe-Co-Mo-O solid solution from the β-phase to the α-phase.

The XRD patterns recorded show exclusively the bismuth metallate phases and phases

obtained from the decomposition/calcination of the promoter compound precursor. The

formation of complex oxides or solid solutions involving ion exchange between active and

promoter compound was not observed except for a sample subjected to severe reaction

conditions and extensive time-on-stream. It is thus concluded that the formation of a bulk

bismuth molybdate is expendable for activity and selectivity suggesting that the active

structure is X-ray-amorphous. Alternatively, it could be constituted by the crystalline

phases present, which would imply a complicated mechanism requiring interphase spill-

over of reactive species (vide infra).

Valuable information on the nature of the interaction of the promoter and active com-

ponent is deduced from the temperature-programmed reduction experiments. The low-

temperature acrolein signal proves the formation of active sites that oxidise propene to

acrolein with high selectivity. The temperature range, in which these sites are reduced

(150− 350 C), is well below the reduction temperature for the bulk oxides, while the

upper limit compares well with the operating temperatures of the industrial processes.

In turn, the increase of the high-temperature acrolein signal, which is associated with the

reduction of the support component, indicates that these sites are in the vicinity of the

promoter component.

The light-o temperature of reduction (high-temperature signal) is ca. 350 C. The

quantication of concomitantly formed acrolein, COx and water proves that the oxygen

responsible for the main reduction peak originates at least to some extent from subsurface

layers. The parallelism of the dierent product signals further indicates that the source of

80


oxygen is the same for all components. Based on the comparison with the TPR of the P2

sample (Fig. 4.7) it is concluded that the oxygen is supplied by the promoter component.

The increased selective utilisation of lattice oxygen for the samples W25-460P, W35-

470P and Mo25-460P can therefore be ascribed to the regeneration of the previously

reduced active sites by diusion of oxygen of the promoter component. The amount

of oxygen participating in this process depends on the catalyst topology, that is, the

spatial arrangement of active sites and (sacricial) reoxidation sites. Ueda et al. [59,113]

showed that at slightly higher temperatures than those applied here (450 C) considerable

amounts of lattice oxygen originating from the complex iron cobalt molybdenum oxides

are selectively incorporated into acrolein under steady-state conditions.

It is obvious that the number of active sites formed depends on the extent of interac-

tion between the catalyst components, especially for the tungstate-based system, which

possesses no immanent activity. A marked increase of the (low-T) signal is therefore

observed for those catalysts, in which intimate contact was ensured by pelletising the

physical mixtures before calcination. Additionally, the formation of this contact seems to

be a function of the active component material, i.e. the bismuth source, and potentially

the calcination temperature.

A considerable dierence is observed when comparing the performance of mixed oxide

catalysts with tungstate- and molybdate-based active component and their response to

changes in the preparation process. All catalysts with molybdenum-based active com-

ponent exhibit a high performance level relative to the tungstate-based samples. Both

propene conversion and acrolein selectivity are appreciably high at the lowest pretreat-

ment temperature tested. This reveals that the performance-enhancing interaction of pro-

moter and active component is easily achieved in the temperature range studied. On the

other hand, a signicant boost in activity and selectivity requires elevated temperatures

for the tungstate-based system. Unfortunately, the deactivation at high temperatures,

which is probably due to sintering and/or formation of inactive phases, counteracts this

improvement. The eect of calcination temperature on catalyst structure and activity is

shown schematically in Fig. 5.11.

For the investigated system mixed oxide phases of all components are known. There-

fore, it is accepted that increasing the calcination temperature intensies the interaction

of the phases. The extent of this interaction may range from surface wetting to mixed

oxide formation corresponding to the thermodynamic equilibrium. According to Knö-

zinger [98] the activation temperature of the related processes can be estimated from

the melting points of the respective phases. The melting point of the bismuth tungs-

tate Bi2W2O9 (Tmelt = 925 C) is signicantly higher than that of α-bismuth molybdate

(Tmelt = 660 C). The higher calcination temperature required for the activation of the

81


BET surface area

calcination temperature

è

è

formaton of undesired

phases (e.g.

Bi FeMo O , Fe Mo O )

sintering

3 2 12 2 3 12

èno interaction

between phases

Figure 5.11.: Model of the inuence of calcination temperature on catalytic activityof mixed oxide catalysts.

tungstate-based catalyst can thus be rationalised. The much lower melting point of the

bismuth molybdate explains the relatively low impact of calcination temperature in the

molybdate-based system.

Regarding the inuence of catalyst composition a simple correlation is oered for the

mixed oxide catalysts with bismuth tungstate as active component, which is displayed in

Fig. 5.12. At low loading of the active component the predominant part of the promoter

compound particles is not involved in agglomerates suitable for phase cooperation eects.

Therefore, the performance exhibits the characteristics of the pure promoter compound,

i.e. high CO/CO2 ratio and poor selectivity. Increasing the active component content

results in the formation of sucient contacts. This leads to a signicantly improved

selectivity and even improved activity, which is supposedly due to the newly created

active sites being reduced at lower temperatures. At very high loadings a saturation of

the promoter compound is achieved and the surplus active component particles are present

as inactive spectator species. The resulting catalysts maintain a high selectivity, but lose

activity based on the catalyst mass. This is consistent with the results of Breiter and

Lintz [2] for a series of similar catalysts prepared via a slurry method.

The implications are generally the same for mixed oxide catalysts with bismuth

molybdate-based active component, but the results are less clear, because bismuth mo-

lybdate itself is an active and selective oxidation catalyst. Millet et al. [60] studied

the whole compositional range for a very similar catalyst and reported a maximum of

activity in the range examined here (15− 35 wt% Bi2Mo3O12). Their pure promoter and

active component both exhibited very low activity.

Three possible scenarios accounting for the formation of active sites are discussed in

the following:

82


pure active phasepure support phase

è

è

part of inactive tungstate

is not interacting with

molybdate phase

low active catalyst

è

è

influence of promoting

phase dominant

non-selective catalyst

precu

rso

r

mix

ture

calc

ined

cata

lyst

Figure 5.12.: Model of the inuence of catalyst composition on catalytic activity ofmixed oxide catalysts.

• phase boundaries as active sites (mechanical mixture),

• formation of a (new) selective phase,

• modication of the surface of the existing oxides.

It is recalled that the characterisation by XRD excluded crystalline phases from scena-

rio No. 2. The Grasselli mechanism assigns hydrogen abstraction to an oxygen species

associated with bismuth and oxygen insertion to a molybdenyl species. At the phase

boundary of the bismuth tungstate and the iron cobalt molybdate or molybdenum oxide

both species are present. The reaction might therefore proceed via activation of the pro-

pene on the tungstate and subsequent surface diusion and spill-over onto the molybdate

phase.

Table 5.5 recalls the amount of acrolein selectively produced in the rst TPR signal.

The active site concentration determined as the amount of oxygen selectively incorpora-

ted into acrolein and concomitantly formed water in the low-temperature reduction peak

totals approximately 0.1 monolayers or rather 1018 atomsm−2 based on the total catalyst

surface. If the assumption holds that bismuth and molybdenum are required in close

proximity in order to facilitate the selective transformation of propene into acrolein, the

reaction would be expected to occur at the interface of the active and promoter compound

of the physical mixtures of bismuth tungstate and iron cobalt molybdate. A rough esti-

mate of the number of interface sites is provided by assuming the reaction to occur on the

perimeter of spherical bismuth metallate particles. The number of particles is calculated

from the active compound content, the crystallographic density of the metallate and the

83


Table 5.5.: Acrolein formation during rst TPR signal and BET surface areas of se-lected catalysts.

SBET acrolein formationsample

[m2 g−1 ] [µmol g−1 ] [molecules m−2]

W25-460-P 9.9 5.3 3.2 · 1017W25-460-P 8.9 7.8 5.3 · 1017W25-460-P 6.0 8.3 8.3 · 1017Mo25-460-P 7.3 16.3 1.3 · 1018

mean particle diameter dp ≈ 0.8 µm determined from electron micrographs (sec. 4). The

value thus obtained for the number of interface sites is 1015 atomsm−2. This value is too

small by three orders of magnitude. Even when assuming a certain roughness of the grain

boundaries this can not account for the activity observed.

Millet et al. [60], using SEM and X-ray photoelectron spectroscopy, observed a wet-

ting interaction between mixed iron/cobalt molybdates and dierent bismuth molybdates.

The spreading of the bismuth tungstate phase over the promoter compound would cer-

tainly lead to an increase in interphase boundary. Yet, a degree of dispersion sucient to

account for the observed active site density would imply complete wetting of the promoter

compound by the bismuth metallate, since approximately 10 % of the total oxygen sur-

face atoms are active. This can be ruled out on the basis of the XRD patterns of catalysts

before and after reaction, which clearly demonstrate that the crystallinity of the active

component precursors remains unaltered. Consequently, the possibility of the physical

mixture being the active structure for selective acrolein formation can be excluded.

In case that the total surface area of the bismuth metallate phases amounting to

0.25− 0.35 m2 per gram of mixed oxide catalyst (cp. Tables 5.1 and 5.5) is taken as

the basis, the amount of surface oxygen atoms (4 5 · 1017 atoms m−2) compares quite

well with the estimated number of active sites. Thus, it appears likely that the sur-

face of the bismuth metallate phase is modied by the molybdate promoter (iron cobalt

molybdate). A feasible mechanism for this modication might be the reverse wetting

interaction, i.e. the spreading of the support over the active component. However, the

complete wetting would reduce the accessible surface area of the active component, which

in turn would diminish the active site density. Therefore, it appears more likely that single

molybdate species migrate onto the surface of the active component, which could then

generate active ensembles as those proposed by the Grasselli mechanism.

The spreading or wetting of oxides is a well-known phenomenon in oxidation catalysis

in general and in molybdenum-based systems in particular [114]. These wetting interac-

tions are driven by surface free energy minimisation. Knözinger and co-workers ( [115]

84


and ref. cited therein) did extensive research on the wetting of dierent surfaces by mo-

lybdenum oxide. They found out that molybdenum is transported over several hundred

micrometers by thermal treatment at temperatures of 800 K, but also temperatures of

670− 770 K have been reported to be sucient for spreading to occur.

Gas phase transport is another mechanism that is eective for molybdenum oxides. It

is well-known that in the presence of water MoO3 forms volatile MoO2(OH)2 species [116].

Actually, the loss of molybdenum from the matrix of oxidation catalysts has frequently

been discussed as a cause of catalyst deactivation [19, 117]. Markgraf et al. [118]

detected the formation of polymolybdate species as a consequence of MoO3 spreading over

alumina in the presence of water. Recently, Schlögl [102] suggested similar molecular

assemblies as the active site on selective oxidation catalysts. For bulk oxide systems

these are supposed to segregate from the surrounding matrix to account for site isolation.

Routray et al. [117] suggested a monolayer of MoO3 supported on bulk Fe2Mo3O12 to

be the active surface structure on methanol oxidation catalysts.

The bismuth molybdate active component fulls the demand for proximity of bismuth

and molybdenum, which, however, does not exhibit a pronounced low-temperature peak.

This could possibly indicate an additional electronic eect of supporting the bismuth

molybdate on the iron cobalt molybdate as suggested by Moro-Oka and Ueda [17].

Based on the above considerations it is proposed that the active state consists of a

matrix constituted by the phases of the pure precursor oxides. Furthermore, junctions

between the oxides provide for oxygen exchange as at medium to high temperatures. The

active site itself is an X-ray amorphous surface structure being either an extensive top

layer of a complex oxide or consisting of isolated molecular ensembles.

5.5. Conclusion

It was shown that physical mixtures of bismuth tungstate and iron cobalt molybdates are

essentially inactive and non-selective in the oxidation of propene to acrolein. A strong

synergistic eect is observed when the intimate contact of the constituent phases is assu-

red by pelletisation prior to thermal treatment indicating that the active site formation

depends on interphase junctions or mass transport. Pelletisation prior to catalytic tes-

ting alone did not eect high activity because the reaction temperature was too low to

result in the formation of the active state. The formation of the active state was found

to be a strongly activated process, especially for tungstate-based catalysts, as indicated

by the high temperature dependence of the catalyst performance. Beside the formation

of the active state, the phase interaction involves interphase oxygen transport at elevated

temperatures and potentially electronic eects at lower temperatures.

85


It could thus be demonstrated that the active state of the multicomponent mixed

oxide catalyst system is neither a crystalline phase nor a purely mechanical mixture.

Furthermore, it is concluded that it is an X-ray amorphous structure, which strongly

interacts with the promoting oxide FexCo1-xMoO4.

86

6. Kinetics of propene oxidation over

multicomponent mixed oxide

catalysts

6.1. Introduction

The heterogeneously catalysed gas phase oxidation of propene is the only large-scale pro-

cess for the production of acrolein and acrylic acid [119]. Modern catalysts and processes

provide yields up to 96 %, apparently leaving little space for quantum leaps in catalyst

development. Additionally, the high eciency is a hurdle for the implementation of alter-

native and sustainable processes [5, 119]. However, in the production of bulk chemicals

still minute optimisations of the production process can generate massive gain. Poten-

tial for improvement of complex industrial processes can base upon dierent approaches.

Catalyst development can eect higher space-time-yields increasing the eciency of the

plant. In selective oxidation reactions minor improvements of the selectivity to the tar-

get product increase the yield of value-added products while at the same time reducing

expenses for waste removal and product purication. But also increased catalyst stabi-

lity is a factor since it reduces both, costs for catalyst material and shutdown times for

changing catalyst charges. Besides, the exploitation of the intrinsic activity, i.e. the full

potential, of the catalytic material requires optimised operating conditions. In this case,

the intrinsic activity may be equated with the microkinetics which therefore has central

signicance for the objective discussed.

Steady-state kinetic measurements are an important tool in elucidating the microkine-

tics of a reaction. The method permits virtually direct access to mechanistic information,

provided that care is taken that the data collected are not obscured by uncontrollable

gradients and transport phenomena. Nevertheless, the detailed microkinetic analysis of

a complex reaction network is an extensive matter requiring a multi-method approach.

The catalysed oxidation of propene to acrolein has been the subject of numerous kinetic

studies so far [1, 2, 22, 23, 48, 52, 69, 120]. Detailed information on the mechanism of

acrolein formation has thus been obtained. Naturally, the formation of byproducts has

87

6. Kinetics of propene oxidation over multicomponent mixed oxide catalysts

attracted less attention and the perception of reaction mechanisms varies to some extent.

However,Monnier andKeulks [23] and more recently Fehlings et al. [53] and Le et al.

[121] oered explanations how some of the inconsistencies could be removed, considering

dierent catalyst systems and operating conditions.

Still, there are open questions remaining and continuing development raises new ques-

tions. Looking at catalyst formulations from the patent literature it is noticed that these

contain numerous main group and rare earth elements as dopants and promoters [5].

These are introduced to increase the selectivity or the stability of the catalyst, but their

functionality remains widely unclear. Progress has been made in the understanding of

the promoting action of iron cobalt molybdate solid solutions, but especially in the low-

temperature range the validity of resultant models has scarcely been tested [17]. Another

promoter that is incorporated into mixed oxide catalysts in stoichiometric amounts giving

rise to oxide and complex oxide formation is tungsten. The role of tungsten in Mo-V-

W mixed oxides for acrolein oxidation to acrylic acid has been studied in some detail.

Thereafter, tungsten may act as a structural promoter stabilising active suboxides of mo-

lybdenum and vanadium [86]. Endres et al. [87] report that small amounts of tungsten

increase stability and the availability of lattice oxygen in this type of catalyst.

In the present study the kinetics of propene oxidation were investigated on a tungsten-

and a molybdenum-based multicomponent mixed oxide catalyst. Information on the role

of tungsten was to be deduced from the comparison of the microkinetics reecting the

mechanism.

6.2. Experimental


Multicomponent mixed oxide catalysts were prepared by a solid-state synthesis from

suitable complex oxide precursors. The precursors were obtained by spray drying and

dierent post-drying treatment. One of the precursors is a complex oxide or mixed

oxide denoted as active component, being either a bismuth molybdate (Bi2Mo4O15) or a

bismuth tungstate (Bi2W2O9 or Bi2W4O15). The other precursor is a mixed iron-, cobalt-

and molybdenum-based oxide with the composition Fe3Co7Mo12O46, in the following

denoted as promoter component. The precursor oxides used in this study were prepared

by a spray-drying routine described in more detail in Ref. [85].

The physical mixtures of the precursor oxides were obtained by thoroughly mixing

the desired amounts of powder in an analysis mill IKA A10 in an argon atmosphere,

excluding any humidity in order to avoid agglomeration of the particles. The mixtures

88


Table 6.1.: Composition and surface properties of model catalysts. A denotes the ac-tive constituent and P the promoter constituent of the catalytically activestate.

sample active comp. A/(A+P) 1 Tcalc2 SBET

3 phases detected by XRD

Mo25-460-P Bi2Mo4O15 25 460 8.2 Bi2Mo3O12, MoO3,CoxFe1-xMoO4, Fe2Mo3O12

W35-470-P Bi2W4O15 35 470 6.5 Bi2W2O9, WO3,CoxFe1-xMoO4, Fe2Mo3O12

1 [wt%], 2 [C ], 3 [m2 g−1 ]

were pressed to tablets in a pellet die (di = 13 mm) applying a load of 1 t. The pellets

were calcined in a tubular furnace in owing synthetic air (20 % oxygen, 80 % nitrogen).

The temperature programme used is shown in Fig. A.2. The nal temperature was 460 C

for the catalyst with molybdate-based active component and 470 C for the catalyst with

tungstate-based active component. The pellets were ground to obtain the size fraction

250− 355 µm, which was used for the kinetic investigations.

Table 6.1 summarises the information on the composition and important properties of

the catalysts studied in the kinetic investigations. The optimisation of the composition

and the temperature of thermal treatment resulted in catalysts with comparable activity

and selectivity.


X-ray diraction

XRD patterns were recorded using a Panalytical MPD diractometer equipped with a Cu

X-ray source, 0.5 divergent and antiscatter slits, a 0.2 mm high receiving slit, incident

and diracted beam 0.04 rad soller slits, and a secondary graphite monochromator. The

2Θ range covered was from 5 to 60 with a step width of 0.03. Qualitative phase ana-

lysis was carried out using the X'Pert Line software (Panalytical) together with powder

diraction les (PDFs) from the International Centre of Diraction Data (ICDD).

N2physisorption

Adsorption data were obtained on Quantachrome Autosorp-1-MP and Autosorp-1-C ap-

paratus. Specic surface areas of the samples were determined by static nitrogen or

krypton adsorption at −196 C and data evaluation according to the multipoint BET

method. Pore size distributions were determined by evaluating the desorption branch of

89


the nitrogen isotherm according to the BJH method. For pretreatment the samples were

heated in vacuo at 300 C for 2 h.

6.2.3. Kinetic experiments

Experimental setup

The kinetic experiments were performed in a stainless steel ow setup. Propene, oxy-

gen, helium and neon as internal standard were dosed by thermal mass ow controllers

(Bronkhorst). Liquid co-feeds were dosed by saturating the gas stream by passing it

through a thermostatic two-stage saturator. A U-shaped microreactor constructed of

glass-lined tubing (SGE; di = 4 mm) was heated by a stainless steel jacket oven. The

product gas mixture was analysed by online gas chromatography (Shimadzu GC-14B and

Delsi GC11). A Molsieve 13X column was used for separating the permanent gases and a

Porapak QS column for the remaining reactants, respectively. Product peaks were detec-

ted by two thermal conductivity detectors. Carbon, hydrogen and oxygen were balanced

within ±5 %, and usually much better.

For the experiments the reactor was loaded with 250 mg of catalyst, which were placed

between two quartz wool plugs resulting in a total bed height of 30 mm.

The kinetic measurements covered a broad range of experimental conditions. All ex-

periments were performed at atmospheric pressure. The standard feed composition was

5.5 % propene and 9.5 % oxygen. The modied residence time (τ = mcat/ntot) was va-

ried between 5 and 45 kg smol−1, which resulted in a reasonable conversion range. The

reaction temperature was varied from 300− 380 C. Higher temperatures were omitted

in order to prevent catalyst deactivation. Reactant concentrations were varied between

1− 7 % for propene and 6.25− 16 % for oxygen, respectively, while keeping the other

reactant at standard concentration. In order to test the inuence of the water co-feed,

the saturator temperature was adjusted so that the water/propene molar ratio was bet-

ween 0.0 and 1.5. Depending on the reaction conditions and the time required to reach

steady state, the displayed values represent the average of 46 consecutive analyses.

6.3. Results

6.3.1. Preliminary considerations

The obtained data were checked for mass and heat transport limitations by applying

suitable criteria assuming worst-case conditions. Diusivity and mass and heat transfer

coecients were determined as specied in Chapter B.

90


Regarding interphase mass transport an eectiveness factor of η ≥ 0.95 is guaranteed

for a rst order reaction according to Mears [122], if the inequation Eq. 6.1 is satised.

reffdp2 (1− ϵ) kgci,g

< 0.15 (6.1)

The maximum eective reaction rate reff estimated for 380 C and ca. 13 % conversion

is 7.5 mol s−1m−3. With particle diameter dp = 355 µm, bed porosity ϵ = 0.5 , a propene

concentration of 2.4 molm−3 and a gas to particle mass transfer coecient of 0.03 m s−1

the resultant value is 0.04 Thus, interphase mass transport limitations can be excluded.

The Weisz-Prater criterion (Eq. 6.2) was calculated to assess the inuence of intrapar-

ticle mass transport.

reffd2p

4 (1− ϵ)Di,effci,g< 0.6 (6.2)

The eective diusivity Di,eff was determined from the binary diusivity of propene

in helium calculated according to Hirschfelder et al. [123, 124] and adverse particle

porosity and tortuosity factors. The relation takes a value of 0.08 , which rules out the

possibility of pore diusion limitation being in accordance with previous experimental

evidence under comparable conditions [120].

Heat transfer limitations bear a great potential to obscure kinetic data in oxidation

reactions due to the high reaction enthalpies. As intraparticle heat transport is usually

much faster than solid to gas heat transfer in laboratory applications, catalyst eective-

ness of 0.95 ≤ η ≤ 1.05 is guaranteed if:

(−∆HR) reffdp2 (1− ϵ)hTg

· EA

RTg

< 0.15 (6.3)

The reaction enthalpy ∆HR is a function of selectivity. For simplicity and in order not

to underestimate the eect of heat transfer a 2/1 ratio of acrolein and CO2 was chosen

to maximise ∆HR. An activation energy EA of 40 kJmol−1 was adopted from literature

data [120] for the maximum gas phase temperature Tg of 380 C. The heat transfer

coecient h was calculated on the basis of the lowest ow rates applied.

Using the respective parameters the left side of Eq. 6.3 yields a value of 0.22 , indicating

that data obtained at high temperature and high residence time are potentially inuenced

by heat transfer limitation. However, using less rigorous parameters for selectivity, heat

91


transfer and reaction rate, which describe most of the data obtained, the inequation is

easily fullled.

6.3.2. Kinetics of the selective oxidation of propene

In the kinetic investigations the multicomponent catalyst with tungsten-based active

component exhibited a higher long-term stability and therefore enabled the determination

of comprehensive reaction kinetics. Therefore, the results of this catalyst system are

displayed in the following unless otherwise noted.

Main reaction

Inuence of residence time Fig. 6.1 shows the dependence of propene conversion

on the modied residence time. As expected, the conversion increases with increasing

residence time and reaction temperature. At temperatures up to 340 C the increase is

linear. Conversely, at temperatures of 360 C and higher a levelling-o of the degree

of conversion curves is observed, which clearly indicates a non-zero reaction order. The

dashed lines in Fig. 6.1 represent ts assuming net zero order kinetics for T ≤ 340 C

and rst order kinetics for T ≥ 360 C. According to the congruence of experimental

and simulated conversion, this represents a good description. It should be noted that

due to the low degree of conversion at 300 C and 320 C these data can be equally well

described by rst order kinetics. The behaviour hints at the complexity of the underlying

microkinetics.

The data obtained at low residence times indicate that the acceleration of the reac-

tion rate with increasing temperature levels o at approximately 360 C. Below this

temperature an increase by 20 K eects a triplication of the conversion level. At higher

temperature the increase is signicantly lower, even when considering the higher conver-

sion at 360 C and 380 C.

The selectivity to acrolein as a function of propene conversion is displayed in Fig. 6.2.

The comparison of the tungstate-based and the molybdate-based catalyst shows that

very similar selectivities are obtained at identical temperature and comparable degrees

of conversion. At low temperatures and thus at low conversion the data scatter some-

what, but a general trend of decreasing selectivity at increasing conversion is observed.

This is strongest at high temperatures and indicative of consecutive reactions of the pro-

duct acrolein. It is furthermore remarkable that the initial selectivity obtained from the

extrapolation of the curves to zero conversion has its maximum at T ≥ 360 C.

92


0 10 20 30 40 500

20

40

60

80

100 300 °C 320 °C 340 °C 360 °C 380 °C

prop

ene

conv

ersi

on /

%

mod / kg s mol-1

Figure 6.1.: The dependence of propene conversion on residence time for catalystBAW35-70.

0 20 40 60 80 10060

70

80

90

100

BAW / BAM 300 °C 320 °C 340 °C 360 °C 380 °C

acro

lein

sel

ectiv

ity /

%

propene conversion / %

Figure 6.2.: The dependence of acrolein selectivity on propene conversion. Filledsymbols: BAW35-70; open symbols: Mo25-460-P.

93


6 8 10 12 14 160

20

40

60

80

100

300 °C 320 °C 340 °C 360 °C 380 °C

prop

ene

conv

ersi

on /

%

oxygen mole fraction / %

Figure 6.3.: The dependence of propene conversion on oxygen partial pressure forcatalyst BAW35-70.

Inuence of oxygen partial pressure When increasing the oxygen concentration

in the feed while keeping that of propene constant, a nearly linear increase of propene

conversion is observed for the whole temperature range (Fig. 6.3). Considering the ap-

parent acceleration of the reaction rate, a marked increase is brought about in the low-

temperature range. Below 340 C the conversion more than doubles in the concentration

interval examined. At higher temperatures the eect is less pronounced.

Increasing the oxygen partial pressure is also benecial for the acrolein selectivity

(Fig. 6.4). Despite the substantial increase in propene conversion, which would be expec-

ted to promote consecutive reactions of the intermediate acrolein, a gain in selectivity is

observed up to temperatures of 340 C. The minor decrease in selectivity at 360 C and

380 C can be attributed to consecutive pathways in accordance with the results obtained

at varying residence time.

Inuence of water co-feed The response of the catalyst system to the addition of

steam to the feed exhibits a similar temperature dependence as the variation of the oxygen

concentration (Fig. 6.5). A strong eect of water addition is observed at temperatures up

to 340− 360 C. At 380 C only a minor increase in propene conversion is detected. It

should be noted though that due to the high conversion level under these conditions the

94


6 8 10 12 14 1675

80

85

90

95

100

300 °C; 30 kg s mol-1 320 °C; 30 kg s mol-1

340 °C; 20 kg s mol-1 360 °C; 10 kg s mol-1

380 °C; 10 kg s mol-1

acro

lein

sel

ectiv

ity /

%


Figure 6.4.: The dependence of acrolein selectivity on oxygen partial pressure for ca-talyst W35-470-P.

amount of water produced from selective and non-selective oxidation of propene exceeds

the amount added at the rst stage. It is also seen that the lowest water-to-propene ratio

tested already generates the positive eect, but higher amounts of water do not induce

further improvement.

The dependence of the selectivity to acrolein at varying water co-feeds ts the scheme

described for propene conversion (Fig. 6.6). At H2O/propene ratios exceeding 0.5 no

signicant changes are observed. Up to 340 C reaction temperature a substantial increase

in acrolein selectivity is eected by the water co-feed. In contrast, the selectivity decreases

when water is added at higher temperatures. This could, however, be due to consecutive

reactions, which are enhanced because of the higher propene conversion.

Acrylic acid formation

Fig. 6.7 compares the formation of acrylic acid on the tungstate- and the molybdate-based

catalyst systems. Acrylic acid is only detected at degrees of conversion higher than 30 %.

The selectivity is found to increase with increasing conversion and also with increasing

temperature at constant conversion. For the temperature range covered the estimated

initial selectivity is always zero, which is a strong indication for acrylic acid being formed

95


0,0 0,5 1,0 1,50

20

40

60

80

100

300 °C; 30 kg s mol-1 320 °C; 30 kg s mol-1

340 °C; 20 kg s mol-1 360 °C; 10 kg s mol-1

380 °C; 10 kg s mol-1

prop

ene

conv

ersi

on /

%

H2O/propene molar ratio

Figure 6.5.: The inuence of the water co-feed on propene conversion for catalystW35-470-P.

0,0 0,5 1,0 1,580

85

90

95

100

300 °C; 30 kg s mol-1 320 °C; 30 kg s mol-1

340 °C; 20 kg s mol-1 360 °C; 10 kg s mol-1

380 °C; 10 kg s mol-1

acro

lein

sel

ectiv

ity /

%


Figure 6.6.: The inuence of the water co-feed on acrolein selectivity for catalystW35-470-P.

96


0 20 40 60 80 100

0

4

8

12

16

20 BAW / BAM 300 °C 320 °C 340 °C 360 °C 380 °C

acry

lic a

cid

sele

ctiv

ity /

%


Figure 6.7.: The dependence of acrylic acid formation on propene conversion. Filledsymbols: W35-470-P; open symbols: Mo25-460-P.

in a consecutive reaction from acrolein. This is corroborated by the sum of selectivities

to acrolein and acrylic acid (Fig. A.31).

The inuence of varying oxygen and water partial pressures on acrylic acid selectivity

for the tungstate-based catalyst is depicted in Fig. 6.8. A slight increase in selectivity

is observed upon raising the oxygen concentration. However, when values are compared

to those obtained at comparable conversion (cp. Fig. 6.7), the selectivity is always lower

or the same in the presence of excess oxygen. The same holds true for the acrylic acid

selectivity when water is present in the feed.

Carbon oxide formation

The formation of CO and CO2 exhibited analogous kinetics under the various reaction

conditions studied. Consequently, a joint presentation of the results for these components

is chosen.

Figures 6.9 and 6.10 display how the carbon oxide selectivity depends on propene

conversion in the examined temperature range. The selectivity decreases with increa-

sing reaction temperature when regarding similar conversion levels. The magnitude of

this eect is slightly dierent for carbon monoxide and carbon dioxide, resulting in a

97


6 8 10 12 14 16

0

2

4

6

8

0,0 0,5 1,0 1,5

0

2

4

6

8

300 °C; 30 kg s mol-1

320 °C; 30 kg s mol-1

340 °C; 20 kg s mol-1

360 °C; 10 kg s mol-1

380 °C; 10 kg s mol-1

ac

rylic

aci

d se

lect

ivity

/ %



acry

lic a

cid

sele

ctiv

ity /

%Figure 6.8.: Left: The dependence of acrylic acid formation on oxygen partial pres-

sure. Right: The dependence of acrylic acid formation on water partialpressure.

temperature-dependent CO/CO2 ratio. At 300C this amounts to 0.4 , while at 380 C

it is already 0.7

Up to 340 C the selectivity is found to be independent of conversion at a given tempe-

rature. At higher temperatures an increase in selectivity is observed, when the conversion

surpasses approximately 50 %. In this case also the CO/CO2 ratio increases. The results

are interpreted in terms of dierent reaction pathways leading to CO and CO2. At low

temperatures and conversion the constant selectivities indicate that the carbon oxides

are part of a reaction network consisting of parallel reactions. The increase in selectivity

at high temperatures and conversion levels is due to additional consecutive reactions of

oxygenated primary products. These secondary reactions have a higher tendency to form

CO.

Increasing the oxygen partial pressure results in decreasing COx selectivities for tem-

peratures up to 340 C (Figs. 6.11 and 6.12). This complies with the nding that acrolein

formation is enhanced under these conditions. At higher temperatures no distinct eect

of increasing the oxygen concentration is observed.

Water aects the formation of the carbon oxides in a similar way as oxygen (Figs. 6.11

and 6.12). Stoichiometric co-feeds with respect to propene eect a signicant decrease

in COx selectivity at temperatures up to 340 C, whereas higher H2O/C3H6 ratios have

relatively little impact on carbon oxide selectivities. At temperatures above 360 C no

eect is observed at all.

98


0 20 40 60 80 1001

2

3

4

5

6

7

BAW / BAM 300 °C 320 °C 340 °C 360 °C 380 °C

CO

sel

ectiv

ity /

%


Figure 6.9.: The dependence of CO selectivity on propene conversion. Filled symbols:W35-470-P; open symbols: Mo25-460-P.

0 20 40 60 80 1002

4

6

8

10

12

14

16

BAW / BAM 300 °C 320 °C 340 °C 360 °C 380 °C

CO

2 sel

ectiv

ity /

%


Figure 6.10.: The dependence of CO2 on propene conversion. Filled symbols: W35-470-P; open symbols: Mo25-460-P.

99


6 8 10 12 14 160

1

2

3

4

5

6

0,0 0,5 1,0 1,50

1

2

3

4

5

6

300 °C; 30 kg s mol-1

320 °C; 30 kg s mol-1

340 °C; 20 kg s mol-1

360 °C; 10 kg s mol-1

380 °C; 10 kg s mol-1

CO

sel

ectiv

ity /

%



CO

sel

ectiv

ity /

%

Figure 6.11.: Left: The dependence of CO formation on oxygen partial pressure.Right: The dependence of CO formation on water partial pressure.

6 8 10 12 14 160

4

8

12

16

0,0 0,5 1,0 1,50

4

8

12

16

300 °C; 30 kg s mol-1

320 °C; 30 kg s mol-1

340 °C; 20 kg s mol-1

360 °C; 10 kg s mol-1

380 °C; 10 kg s mol-1

CO

2 sel

ectiv

ity /

%



CO

2 sel

ectiv

ity /

%

Figure 6.12.: Left: The dependence of CO2 formation on oxygen partial pressure.Right: The dependence of CO2 formation on water partial pressure.

100


0 20 40 60 80 100

0,0

0,5

1,0

1,5

2,0 BAW / BAM 300 °C 320 °C 340 °C 360 °C 380 °C

acet

alde

hyde

sel

ectiv

ity /

%


Figure 6.13.: The dependence of acetaldehyde formation on propene conversion.Filled symbols: W35-470-P; open symbols: Mo25-460-P.

Acetaldehyde and acetic acid formation

Acetaldehyde and acetic acid are both obtained in very small yields (Figs. 6.13 and 6.14),

which makes an unambiguous assignment of possible reaction pathways dicult. From

the extrapolation of data obtained at reasonable signal intensity an initial selectivity

of 0.5− 1.0 % is estimated for acetaldehyde indicating that it is directly formed from

propene in a parallel reaction. However, the steady increase in acetaldehyde selectivity

that is at least seen at T ≥ 340 C suggests an additional secondary pathway possibly

from acrolein or acrylic acid. It is dicult to identify temperature eects, but it seems

as if acetaldehyde selectivity decreases at increasing temperature. This parallels the

temperature eect observed for carbon oxide formation.

Acetic acid formation exhibits a similar temperature and conversion dependence as

acetaldehyde except for the grain selectivity being zero. This signies that this product

is exclusively formed in a secondary reaction. It is reasonable to assume that acetaldehyde

is the primary source of acetic acid, since it simply involves the insertion of one oxygen

atom.

Figures 6.15 and 6.16 illustrate the results of varying inuent concentrations of oxygen

and water. In both cases a general trend of increasing selectivity can be identied. This

should again be evaluated as a function of the degree of conversion, since the selectivity

101


0 20 40 60 80 100

0,0

0,2

0,4

0,6

0,8

1,0BAW / BAM

300 °C 320 °C 340 °C 360 °C 380 °C

acet

ic a

cid

sele

ctiv

ity /

%


Figure 6.14.: The dependence of acetic acid formation on propene conversion. Filledsymbols: W35-470-P; open symbols: Mo25-460-P.

of both products was shown to be conversion-dependent. The deviations between data

obtained under identical conditions indicate that the absolute error is too high to allow

for quantitative discrimination, especially at low temperatures. However, the selectivity

range is not decisively expanded, which would justify further investigations.

Formaldehyde formation

The formation of formaldehyde is only detected at degrees of conversion higher than

40 % (Fig. 6.17). The selectivity increases markedly upon increasing conversion and

temperature. It is therefore concluded that formaldehyde is a secondary product. The

relatively low selectivity and the concomitant strong increase in acrylic acid selectivity

do not allow a straightforward identication of the reaction pathway.

Figure 6.18 shows the results of the variation of the oxygen and water partial pressure

in the feed. It appears that increasing the concentration of either component eecti-

vely suppresses the formation of formaldehyde. Regarding the inuence of oxygen this

becomes apparent when considering the selectivity obtained at comparable conversion

during residence time variation. It was hardly possible to determine the inuence of the

water co-feed, because formaldehyde concentrations were close to the detection limit.

102


6 8 10 12 14 160,0

0,5

1,0

1,5

0,0 0,5 1,0 1,50,0

0,5

1,0

1,5

300 °C; 30 kg s mol-1

320 °C; 30 kg s mol-1

340 °C; 20 kg s mol-1

360 °C; 10 kg s mol-1

380 °C; 10 kg s mol-1

acet

alde

hyde

sel

ectiv

ity /

%



acet

alde

hyde

sel

ectiv

ity /

%

Figure 6.15.: Left: The dependence of acetaldehyde formation on oxygen partial pres-sure. Right: The dependence of acetaldehyde formation on water partialpressure.

6 8 10 12 14 16

0,00

0,05

0,10

0,15

0,20

0,25

0,0 0,5 1,0 1,5

0,0

0,1

0,2

0,3

0,4

0,5 300 °C; 30 kg s mol-1

320 °C; 30 kg s mol-1

340 °C; 20 kg s mol-1

360 °C; 10 kg s mol-1

380 °C; 10 kg s mol-1

acet

ic a

cid

sele

ctiv

ity /

%



acet

ic a

cid

sele

ctiv

ity /

%

Figure 6.16.: Left: The dependence of acetic acid formation on oxygen partial pres-sure. Right: The dependence of acetic acid formation on water partialpressure.

103


0 20 40 60 80 100

0,0

0,2

0,4

0,6

0,8

1,0 BAW / BAM 300 °C 320 °C 340 °C 360 °C 380 °C

form

alde

hyde

sel

ectiv

ity /

%


Figure 6.17.: The dependence of formaldehyde formation on propene conversion.Filled symbols: W35-470-P; open symbols: Mo25-460-P.

6 8 10 12 14 16

0,0

0,1

0,2

0,3

0,4

0,0 0,5 1,0 1,5

0,0

0,1

0,2

0,3

0,4

form

alde

hyde

sel

ectiv

ity /

%


300 °C; 30 kg s mol-1

320 °C; 30 kg s mol-1

340 °C; 20 kg s mol-1

360 °C; 10 kg s mol-1

380 °C; 10 kg s mol-1


form

alde

hyde

sel

ectiv

ity /

%

Figure 6.18.: Left: The dependence of formaldehyde formation on oxygen partial pres-sure. Right: The dependence of formaldehyde formation on water par-tial pressure.

104


1,6 1,8 2,0 2,2 2,4 2,6 2,8 3,0

-18

-17

-16

-15

-14

1,6 1,8 2,0 2,2 2,4 2,6 2,8 3,0

-20

-19

-18

-17

-16

-15

1,6 1,8 2,0 2,2 2,4 2,6 2,8 3,0

-19

-18

-17

-16

c)b)a)

m = 0,50

m = 0,30

m = 0,95

m = 0,88

m = 0,87

ln

(A

cr) /

-

ln y(O2) / -

m = 0,66

m = 0,64

m = 0,77

m = 0,80

m = 0,70

ln

(CO

) / -

ln y(O2) / -

300 °C 320 °C 340 °C 360 °C 380 °C

m = 0,53

m = 0,46

m = 0,64

m = 0,66

m = 0,64

ln

(CO

2) / -

ln y(O2) / -

Figure 6.19.: Determination of formal reaction orders with respect to oxygen for a)acrolein formation, b) CO formation and c) CO2 formation for catalystW35-470-P.

6.3.3. Evaluation of kinetic data

The experiments were designed in order to obtain kinetic data in the temperature range

relevant to conditions in polytropic industrial reactors. This included the attempt to

study the formation of byproducts which are usually only formed in detectable amounts at

integral conversion levels. Due to the seemingly high activation energy and instrumental

limitations it was not possible to produce purely dierential conditions in the whole

temperature range, which is the most convenient way to assess kinetic parameters like

activation energies and reaction orders. Thus, the reaction order with respect to oxygen

and propene determined should give a rough estimate of the extent to which the reaction

rate is aected by either reactant.

Figures 6.19 and 6.20 display the reaction orders for acrolein and COx formation with

respect to oxygen and propene tted on the basis of a power-law rate expression. The

results disclose a positive order for oxygen in the temperature range studied. Up to

340 C it is close to unity for acrolein and a bit lower for CO and CO2 as deducible

from the increase in acrolein selectivity at increasing oxygen partial pressure. At higher

temperatures the values decrease. At higher temperatures the slope decreases signicantly

for acrolein formation and is more or less constant for COx. This is not a kinetic eect,

but rather due the high conversion level and the contribution of consecutive reactions of

acrolein reducing the apparent rate of acrolein formation and increasing that for CO and

CO2.

The values obtained from the variation of the inuent concentration of propene in-

dicate a strong inhibition of acrolein formation by propene at temperatures ≤ 340 C.

105


1,2 1,4 1,6 1,8 2,0

-18

-17

-16

-15

-14

1,2 1,4 1,6 1,8 2,0

-19,5

-19,0

-18,5

-18,0

-17,5

-17,0

-16,5

1,2 1,4 1,6 1,8 2,0

-19,0

-18,5

-18,0

-17,5

-17,0

-16,5

-16,0

-15,5

-15,0

m = -0,06

m = 0,53

m = -0,78

m = -0,70

m = -0,74

ln

(A

cr) /

-

ln y(C3H6) / -

m = 0,04

m = 0,12

m = -0,11

m = -0,16

m = -0,26

ln

(CO

) / -

ln y(C3H6) / -

c)b)a) 300 °C 320 °C 340 °C 360 °C 380 °C

m = 0,44

m = 0,48

m = 0,13

m = -0,07

m = -0,12

ln

(CO

2) / -

ln y(C3H6) / -

Figure 6.20.: Determination of formal reaction orders with respect to propene for a)acrolein formation, b) CO formation and c) CO2 formation for catalystW35-470-P.

Conversely, the eect is limited for COx formation, indicating that propene blocks sites

for acrolein formation at the respective conditions but not those for deep oxidation. Like

for oxygen an alteration of the inuence of the reactant is observed at higher tempera-

tures, which has to be analysed critically due to the high conversion level. For T = 360 C

and 380 C zero and half order are found, respectively. It should be noted that the net

reaction orders are thus very close to those assumed in the tting of propene conversion

(cp. Fig. 6.1). This relation is feasible due to the high selectivity and the stoichiometry

of the main reaction.

Similar characteristics have been interpreted by dierent authors in terms of a change

in the rate-determining step of acrolein formation [23, 47, 51]. Presuming that the dis-

tinctively dierent behaviour of the catalyst in the temperature range below and above

approximately 350 C is due to this eect, the activation energy of these steps is estimated

from the rate constants determined from the tting of the overall propene conversion (see

Fig. 6.1). In the absence of extensive data obtained at dierent temperatures the values

only represent a rough estimate. The tting of the data to a linearised Arrhenius ansatz

yields an extremely high activation energy of 173 kJmol−1 in the low temperature range

and 60 kJmol−1 at high temperatures. The estimated kinetic values are summarised in

Table 6.2.

106


Table 6.2.: Kinetic constants for catalyst W35-470-P.

temperature reaction order activation energycatalyst

[C ] C3H6 O2 [kJmol−1 ]

W35-470-P 300 -0.7 0.9320 -0.7 0.9 173340 -0.8 1.0

360 -0.1 0.5380 0.5 0.3

60

6.4. Discussion

Reaction network

It has been briey discussed that acrolein, CO, CO2 and acetaldehyde are primary pro-

ducts being formed directly from acrolein. This is readily inferred from the nite initial

selectivities observed for these components. The fact that their ratio is constant in a

broad conversion range and especially at low temperatures further corroborates a parallel

reaction scheme and indicates that the net reaction orders of the dierent pathways do

not vary greatly.

Accordingly, the products acrylic acid, acetic acid and formaldehyde are formed in

consecutive reactions as deduced from their initial selectivities and conversion dependence

of selectivity. It is reasonable to assume that acrylic acid is derived from acrolein and

acetic acid from acetaldehyde. The assignment is so far fully in line with literature

reports of a reaction network established under similar experimental conditions [47]. The

concurrent increase in COx selectivity and decrease in acrolein selectivity indicates that

the larger fraction of consecutive total oxidation happens in the acrolein/acrylic acid

branch of the reaction network.

It is not possible to unambiguously determine the intermediate oxygenate of formalde-

hyde formation. Weiss et al. [13] suggested its formation from propene by rupture of the

C−−C double bond yielding equivalent amounts of acetaldehyde and formaldehyde. This

can be excluded, since formaldehyde denitely behaves like a secondary product. Gor-

shkov et al. [49] reported formaldehyde being a major product of acetaldehyde oxidation

and a byproduct of acrolein oxidation under severe reaction conditions (T = 460 C) on

a bismuth molybdate catalyst. Regarding possible secondary pathways, the amount of

acetaldehyde formed is too low to account for the formaldehyde yield observed at high

conversion. It is therefore concluded that it must form from either acrolein or acrylic acid,

either by scission of the double bond involving electrophilic oxygen or by splitting-o of

107


CH =CH-CH2 3

CH =CH-CHO2 CH =CH-COOH2

CH -CHO3 CH -COOH3

CO, CO2

HCHO

Figure 6.21.: Reaction network of the oxidation of propene on multicomponent mixedoxide catalysts.

the carbonyl group of acrolein. Reduction of the carboxyl function of acrylic acid can be

regarded as highly unlikely.

A schematic representation of the reaction network based on the above considerations

is depicted in Fig. 6.21. The prevalent reaction pathways have been highlighted. It

is assumed that all reaction products may eventually be degraded to COx. Additional

experiments (Figs. A.32 and A.33) showed that CO oxidation to CO2 under various

reaction conditions is extremely slow at temperatures below 400 C and therefore does

not contribute to the observed product distribution.

Reaction kinetics

Several interesting mechanistic features have been observed in the kinetic investigations.

In particular, a superordinate principle of changing kinetic characteristics at a transition

temperature of about 350 C was found. Similar phenomena have been observed by

dierent researchers for bismuth molybdates [22, 23, 51, 70] and multicomponent mixed

oxides [17,47,65,70].

The overall conversion of propene was shown to be suciently described by zero and

rst order kinetics under and beyond the transition temperature, respectively. It should

be remembered though that the overall conversion is dominated by the selective oxidation

to acrolein. A great variety of scenarios can possibly account for zero order kinetics in

heterogeneously catalysed reactions, but generally it requires the consideration of adsorp-

tion kinetics. The survey of the inuence of the reactants on the reaction rates suggests

108


that the apparent zero order is caused by an inhibiting action of propene counterbalancing

the reaction rate enhancing eect of oxygen.

For a simple bimolecular reaction of two adsorbed species (propene, oxygen) the reac-

tion rate assuming Langmuir adsorption is given by:

r =kp1p2

(1 +K1p1 +K2p2)2 (6.4)

where p1, p2 are the partial pressures of propene and oxygen and K1 and K2 the

respective adsorption constants. If one of the reactants is strongly adsorbed, i.e. K1p1 >>

1 +K2p2, the expression reduces to:

r =k′p2p1

(6.5)

If the ratio p2/p1 is (nearly) constant over the whole conversion range, that means,

the feed is stoichiometric with respect to the net reaction, this eects overall zero order

kinetics.

Several authors suggest a two-site mechanism for multicomponent acrolein catalysts,

with oxygen activation being facilitated by the support compound and alkene oxidation

happening on a suitable supported phase. If the reaction sequence involves the reaction

of adsorbate species on distinct sites, zero order kinetics are expected when both sites are

saturated. However, the scenario presented above, accounting for the observed specic

reaction orders is also valid for a two-site mechanism if propene and spill-over oxygen are

competing for the same site.

In a series of contributionsKeulks et al. [22,23,51] observed a temperature dependence

of the reaction order with respect to propene and oxygen. While the dependence of the

reaction rate on propene increased with increasing temperature, the opposite was the case

for oxygen. At the lowest and highest temperatures investigated the orders of propene and

oxygen were zero, respectively. In the same temperature range a break in the Arrhenius

plot of the reaction rate constants was observed. The authors came to the conclusion

that the kinetics of acrolein formation can be described by the coupled kinetics of catalyst

reduction and reoxidation. Accordingly, both processes exhibit a distinctively dierent

temperature dependence having the eect that either the one or the other is rate-limiting

in a certain temperature range. Redlingshöfer et al. [47] studied the propene oxidation

on a multicomponent system in an integral reactor. The satisfactory modelling of their

109


data was only achieved by assuming temperature-dependent reaction orders as suggested

by Keulks and coworkers.

Contrary to the above, only Salah-Alhamad et al. [73] detected the inhibition of

acrolein formation on a Sb-Sn-V-O catalyst at high propene-to-oxygen ratios. Neverthe-

less, alsoMonnier and Keulks [23] found the propene dependence of acrolein formation

to be a function of the C3H6/O2 ratio, tending to zero at low temperatures and high pro-

pene partial pressures.

The results presented here also indicate that the positive inuence of oxygen levels

o at high reaction temperatures. The temperature range covered and the limited high-

temperature stability of the catalysts did not allow to perform a complete analysis on

the basis of dierential kinetic data. However, the variation of propene partial pressure

at 380 C indicates a positive reaction order. It is therefore expected that the reason for

the observed kinetics is the same as in previous studies.

Another astonishing feature of the mixed oxide catalysts is the optimum of acrolein

selectivity at high temperatures. Since this is mainly balanced by reduced total oxidation

it generally indicates a lower activation energy for the latter. It is useful, though, to

consider the propene and oxygen dependence of COx formation as well in order to get an

impression of the process. The apparent reaction order with respect to oxygen is indeed

lower for CO and CO2 as compared to acrolein. This is surprising, because the formation

of carbon oxides requires more oxygen than the selective oxidation to acrolein. Still, the

apparent reaction orders, which are in fact the essence of multiple elementary steps, do

not vary greatly. It is therefore likely that the rate-determining step is not dissimilar

for the dierent products, but the distribution of active sites changes as a function of

temperature. This is reasonable since surface heterogeneity due to e.g. defect formation

is certainly a function of temperature.

Fehlings et al. [53, 54] suggested a model, which is chemically and kinetically

consistent with the results obtained. They assume that selective oxidation of propene to

acrolein generally occurs on completely oxidised sites, i.e. with the participation of lat-

tice oxygen. Alternatively, propene may also adsorb irreversibly on reduced sites, which

would be the reaction pathway to carbon oxides. This is reasonable since literature data

clearly indicates that selective oxidation requires MoVI and Bi III or even BiV species [46].

On the other hand, interaction of propene with reduced metal sites gives rise to strong

adsorption and π-backbonding which will lead to breaking of C−C bonds and deep oxi-

dation. The observed temperature dependence is then in accordance with the results of

Keulks and coworkers [22,23,51] and Brazdil et al. [70] since reoxidation of the cata-

lyst proceeds much faster at higher temperatures eecting a higher degree of oxidation

in the steady-state.

110


Since the rate of reoxidation is a function of oxygen partial pressure the increase results

in reduced carbon oxide formation at constant temperature. Also does the temperature

where the eect of surplus oxygen diminishes and the optimum selectivity is observed

coincide with the temperature range where bulk oxygen or spill-over oxygen from the

support compound become available (cp. Chapter 4). However, in this study it was

shown by transient experiments (TPO) that coke burn-o yielded almost exclusively

CO2, which suggests that carbon oxides formed in the steady state do not originate from

the same hard coke precursor.

Only few accounts of the inuence of water on propene oxidation on mixed oxide

catalysts exist, although it is added to the feed in several selective oxidation reactions

[18, 47, 72, 73, 125]. It is accepted that high water partial pressures will eect a higher

degree of surface hydroxylation. In several studies oxygen exchange between gas phase

water molecules and catalyst lattice oxygen was proven by isotopic labelling. It has

been suggested that surface hydroxyls could play the role of basic oxygen species in

hydrocarbon activation [125]. Several authors observed increased acrylic acid formation

in the presence of water [47, 72, 73], which was not found to take place in the present

study. It has been stated that water may act as co-oxidant keeping the surface in a

high oxidation state [47, 77]. Although this could not be proved by detecting hydrogen

evolution, it is possible that water binds to reduced sites preventing strong adsorption

of propene and resultant total oxidation. This would also eect a higher mean degree of

oxidation due to the high oxygen consumption of deep oxidation reactions.

Nature of the active site

The most striking feature of the comparison of the kinetics of the tungstate- and the

molybdate-based catalyst is the virtually perfect congruence of the dependence of the

selectivities on conversion. In a complex reaction network consisting of parallel and

consecutive pathways as identied in the present case this signies that the relative rates

of these reactions are the same. This can only be interpreted in terms of identical micro-

kinetics for the oxidation of propene on these catalysts.

Due to the compositional dierence and the decisively diering behaviour of bismuth

molybdate and bismuth tungstate, a mere additive mode of operation can be excluded

in concordance with previous considerations (cp. Chapter 5). It is moreover likely that

the same type of active site forms in the mixed oxides under calcination and/or reaction

conditions. The variable activity can be rationalised by a dierent active site density of

the catalyst material, which is readily accepted in view of the varying ratio of active and

promoter component.

111


6.5. Conclusion

Two factors have been identied to limit catalyst activity in the investigated system.

First, a strong inhibition of the reaction by propene was observed, which may be cau-

sed by propene and oxygen competing for the same adsorption site. Second, oxygen

activation or reoxidation of the catalyst was found to be rate-limiting in the chosen para-

meter space. Since the desorption of propene and reaction products is facilitated by high

oxidation states of surface cations, the optimisation of properties related to the latter

should result in an improved overall performance. The main focus of catalyst develop-

ment should therefore aim at improving the oxygen activation and interphase transport.

Since it has been demonstrated that at intermediate temperature oxygen supply by the

support compound is signicant, the limiting factors are phase interaction and oxide

anion conductivity.

On the other hand, implications for the operation of commercial reactors arise from

the complex reaction kinetics. Accordingly, it is desirable to operate at a relatively

high temperature. However, due to the growing importance of non-selective oxidation at

the high degrees of conversion that are attempted, the control of the axial temperature

prole should be useful. Industrial xed bed reactors used for highly exothermic reactions

usually exhibit hotspots due to the insucient heat transfer. In this special case it might

accidentally provide optimal reaction conditions, i.e. high temperatures at low conversion

followed by lower temperatures preventing consecutive oxidation at higher degrees of

conversion.

There is also a strong indication that high oxygen partial pressures are benecial both

for the acrolein selectivity and the space-time-yield. Increasing the oxygen concentra-

tion is, however, problematic because this produces ammable mixtures of propene and

oxygen. A solution to this might be the distributed feed of oxygen along the reactor

guaranteeing high reaction rates and selectivity while at the same time avoiding the

ammability limits.

The two dierent catalyst systems exhibit the same kinetic ngerprint and further-

more similar catalytic activity. This indicates very strongly that the active sites and the

reaction mechanism over the two catalysts are the same. This in turn rules out the neces-

sity of bulk bismuth molybdate as active material for the selective oxidation of propene.

These results provide the basis for a deeper understanding of the structure of active sites

on this type of catalyst.

112

7. Concluding remarks

The main objective of this work was the determination of the microkinetics and mechanis-

tic aspects of the oxidation of propene to acrolein on multicomponent mixed oxide cata-

lysts. By comparison of systems based on dierent oxides information on the interaction

of the constituent phases was to be obtained. Reduction and reoxidation experiments

were performed with those phases constituting the integral part of industrial catalyst

systems, namely bismuth molybdates, bismuth tungstates and iron cobalt molybdates.

From these precursors multicomponent mixed oxide catalysts were obtained by a solid

state synthesis route, and the dierent preparation steps were optimised. A molybdate-

and a tungstate-based catalyst system were characterised by detailed steady-state kinetic

investigations.

The characteristic TPR proles of the precursor oxides showed that bismuth

molybdate-based model catalysts selectively produce acrolein upon reduction in propene.

TPO experiments in cyclic operation revealed that molybdenum is reduced to Mo 4+ in

the course of the TPR. Furthermore, reduction experiments using 1-butene strongly sug-

gested that bismuth participates in activating hydrogen abstraction from propene, but is

rapidly reoxidised by Mo 6+-species. Bismuth tungstates showed no activity at all, but

tend to coking by oxidative dehydrogenation of the propene reductant. The mixed iron

cobalt molybdenum oxides exhibited an appreciable potential for surface reduction, but

little selectivity to acrolein and a remarkably high CO/CO2 ratio. Bismuth and molyb-

denum are thus identied as being the key components in binary oxide catalysts for the

selective oxidation of propene to acrolein accounting for activity and selectivity.

A strong synergy was observed when combining inactive bismuth tungstate and iron

cobalt molybdate resulting in highly ecient and selective catalysts. The synergy is ac-

companied by a masking of the support phase suppressing undesirable COx formation

and decreasing the characteristic CO/CO2 ratio. The catalytic activity of the samples

was found to depend critically on the contact between the precursor phases. Further

important parameters are the elemental composition and the activation temperature of

the catalysts. The dependence of the optimum calcination temperature on the bismuth

source material is ascribed to the diering thermal stability of the oxide structures. Cha-

racterisation by TPR revealed that oxygen is supplied to the active site by the iron cobalt

113

7. Concluding remarks

molybdate at temperatures exceeding 350 C and that this eect, too, is a function of

the pretreatment temperature. Due to this and by considering the active site density de-

termined from transient kinetic experiments and the textural properties of the precursor

oxides, the mechanical mixture can be excluded as the active structure. Since no "new"

phases were detected by XRD, it was concluded that the active structure is amorphous

and consists of molybdenum oxide-based sites. The weak temperature dependence of the

activation of the bismuth molybdate-based catalyst suggests a similar local order as in

the active α-Bi2Mo3O12 phase.

This conclusion is further corroborated by the results of the comparative kinetic study

of the molybdate- and the tungstate-based catalyst systems. The extraordinary agree-

ment of the selectivity-conversion plots indicates that the dynamics of the complex pro-

pene oxidation network are the same, which is interpreted in terms of identical active

sites and mechanisms prevailing on both catalysts.

The reaction network of parallel and consecutive reactions was elucidated by compre-

hensive steady-state kinetic investigations. It was shown that propene is directly conver-

ted into acrolein, acetaldehyde and carbon oxides. Additionally, acrylic acid, acetic acid,

formaldehyde and carbon oxides are derived from the former in secondary reactions. The

general dependence of selectivity on temperature and gas phase redox potential exhibits

interesting features in view of the fact that intermediate temperatures (ca. 360 C) and

high oxygen partial pressures favour selective acrolein formation. It is therefore concluded

that the selective utilisation of propene and the prevention of non-selective readsorption

of acrolein requires the active sites to be in their highest oxidation state. This is provided

for by high reoxidation rates, which are a function of the oxygen partial pressure. In

addition, also the transient kinetic experiments of the mixed oxide catalysts indicated

that severe coke formation proceeds rather on the reduced catalyst surfaces, which is

the supposed source of steady-state carbon oxide formation. The optimum in selectivity

observed at intermediate temperatures must therefore be assessed in view of the transient

kinetic experiments demonstrating that this temperature coincides with the availability

of spill-over oxygen from the supporting iron cobalt molybdate phase.

114

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H. Knözinger. Transport mechanisms during spreading of MoO3 on Al2O3 supports

investigated by photoelectron spectromicroscopy. J. Chem. Phys., 112:54405446,

2000.

[116] P. Serp, P. Kalck, and R. Feurer. Chemical vapor deposition methods for the

controlled preparation of supported catalytic materials. Chem. Rev., 102:3085

3128, 2002.

[117] K. Routray, W. Zhou, C.J. Kiely, W. Grünert, and I.E. Wachs. Origin of the

synergistic interaction between MoO3 and iron molybdate for the selective oxidation

of methanol to formaldehyde. J. Catal., 275:8498, 2010.

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persion on supported catalysts using ion scattering and Raman spectroscopy. Surf.

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[121] M.T. Le, W.J.M. van Well, P. Stoltze, I. van Driessche, and S. Hoste. Synergy

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selective oxidation of propylene to arcrolein. Appl. Catal. A, 282:189194, 2005.

124

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[122] D.E. Mears. On the relative importance of intraparticle and interphase transport

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vapor in the oxidation of propene to acrylic acid on a mixed oxide catalyst. React.

Kinet. Catal. Lett., 4:389395, 1976.

125

126

List of Tables

2.1. Physical properties and toxicology of acrolein. . . . . . . . . . . . . . . . 6

2.2. Composition of multicomponent catalysts. . . . . . . . . . . . . . . . . . 16

3.1. Retention times and response factors . . . . . . . . . . . . . . . . . . . . 33

3.2. Parameter space covered by kinetic experiments. Standard parameters are

underlined. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

4.1. Composition and surface properties of model catalysts. . . . . . . . . . . 39

4.2. Quantication of TPR experiments of model catalysts. . . . . . . . . . . 48

4.3. Quantication of TPO experiments of model catalysts. . . . . . . . . . . 52

4.4. Melting point, Tammann and Hüttig temperatures (in C ) of model

catalysts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

5.1. Composition and specic surface areas of catalyst samples. . . . . . . . . 68

5.2. Quantication of TPR experiments of mixed oxide catalysts. . . . . . . . 72

5.3. Catalytic results of tungstate-based catalysts under standard reaction

conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

5.4. Catalytic results of molybdate-based catalysts under standard reaction

conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

5.5. Acrolein formation during rst TPR signal and BET surface areas of se-

lected catalysts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

6.1. Composition and properties of multicomponent mixed oxide catalysts. . . 89

6.2. Kinetic constants for catalyst W35-470-P. . . . . . . . . . . . . . . . . . 107

127

128

List of Figures

2.1. Flow scheme for acrolein production by propene oxidation. . . . . . . . . 7

2.2. Mechanism of acrolein formation. . . . . . . . . . . . . . . . . . . . . . . 13

2.3. Working mechanism of multicomponent mixed oxide catalysts for acrolein

formation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.1. Flow diagram of the laboratory setup for temperature-programmed expe-

riments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.2. Flow diagram of the laboratory setup for steady-state kinetic experiments. 30

3.3. Catalytic microreactor and oven. . . . . . . . . . . . . . . . . . . . . . . 31

3.4. Schematic drawing of the GC application. . . . . . . . . . . . . . . . . . 32

4.1. X-ray diractograms of active components before and after reduction by

propene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

4.2. X-ray diractograms of the support components before and after reduction

by propene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

4.3. SEM images of active components. . . . . . . . . . . . . . . . . . . . . . 46

4.4. SEM images of promoting components. . . . . . . . . . . . . . . . . . . . 47

4.5. TPR experiments with bismuth molybdate-based model catalysts. . . . . 48

4.6. TPR experiments with bismuth tungstate-based model catalysts. . . . . . 50

4.7. TPR experiments with iron molybdate-based model catalysts. . . . . . . 52

4.8. TPO experiments with bismuth molybdate-based model catalysts. . . . . 53

4.9. TPO experiments with bismuth tungstate-based model catalysts. . . . . 55

4.10. TPO experiments with iron molybdate-based model catalysts. . . . . . . 56

5.1. X-ray diractograms of catalysts W25-460 and W25-460-P before reaction. 69

5.2. SEM micrographs of mixed oxide catalysts. . . . . . . . . . . . . . . . . . 71

5.3. TPR of catalyst W25-460-P. . . . . . . . . . . . . . . . . . . . . . . . . . 72

5.4. Acrolein formation during TPR experiments. . . . . . . . . . . . . . . . . 73

5.5. Net acrolein formation due to phase cooperation eects during TPR expe-

riments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

129

List of Figures

5.6. The inuence of the preparation on the catalytic activity of tungstate-

based mixed oxide catalysts. . . . . . . . . . . . . . . . . . . . . . . . . . 76

5.7. The inuence of catalyst composition on catalytic activity of tungstate-


5.8. The inuence of catalyst composition on catalytic activity of molybdate-


5.9. The inuence of calcination temperature on catalytic activity of tungstate-


5.10. The inuence of calcination temperature on catalytic activity of

molybdate-based mixed oxide catalysts. . . . . . . . . . . . . . . . . . . . 79

5.11. Model of the inuence of calcination temperature on catalytic activity of

mixed oxide catalysts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

5.12. Model of the inuence of catalyst composition on catalytic activity of


6.1. The dependence of propene conversion on residence time for catalyst

BAW35-70. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

6.2. The dependence of acrolein selectivity on propene conversion. Filled sym-

bols: BAW35-70; open symbols: Mo25-460-P. . . . . . . . . . . . . . . . 93

6.3. The dependence of propene conversion on oxygen partial pressure for ca-

talyst BAW35-70. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

6.4. The dependence of acrolein selectivity on oxygen partial pressure for cata-

lyst W35-470-P. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

6.5. The inuence of the water co-feed on propene conversion for catalyst W35-

470-P. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

6.6. The inuence of the water co-feed on acrolein selectivity for catalyst W35-

470-P. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

6.7. The dependence of acrylic acid formation on propene conversion. Filled

symbols: W35-470-P; open symbols: Mo25-460-P. . . . . . . . . . . . . . 97

6.8. Left: The dependence of acrylic acid formation on oxygen partial pressure.

Right: The dependence of acrylic acid formation on water partial pressure. 98

6.9. The dependence of CO selectivity on propene conversion. Filled symbols:

W35-470-P; open symbols: Mo25-460-P. . . . . . . . . . . . . . . . . . . 99

6.10. The dependence of CO2 on propene conversion. Filled symbols: W35-470-

P; open symbols: Mo25-460-P. . . . . . . . . . . . . . . . . . . . . . . . . 99

6.11. Left: The dependence of CO formation on oxygen partial pressure. Right:

The dependence of CO formation on water partial pressure. . . . . . . . 100

130

List of Figures

6.12. Left: The dependence of CO2 formation on oxygen partial pressure. Right:

The dependence of CO2 formation on water partial pressure. . . . . . . . 100

6.13. The dependence of acetaldehyde formation on propene conversion. Filled


6.14. The dependence of acetic acid formation on propene conversion. Filled


6.15. Left: The dependence of acetaldehyde formation on oxygen partial pres-

sure. Right: The dependence of acetaldehyde formation on water partial

pressure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

6.16. Left: The dependence of acetic acid formation on oxygen partial pressure.

Right: The dependence of acetic acid formation on water partial pressure. 103

6.17. The dependence of formaldehyde formation on propene conversion. Filled


6.18. Left: The dependence of formaldehyde formation on oxygen partial pres-

sure. Right: The dependence of formaldehyde formation on water partial

pressure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

6.19. Determination of formal reaction orders with respect to oxygen for a) acro-

lein formation, b) CO formation and c) CO2 formation for catalyst W35-

470-P. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

6.20. Determination of formal reaction orders with respect to propene for a)

acrolein formation, b) CO formation and c) CO2 formation for catalyst

W35-470-P. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

6.21. Reaction network of the oxidation of propene on multicomponent mixed

oxide catalysts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

A.1. Temperature programme of the Shimadzu GC14-B column oven. . . . . . 133

A.2. Temperature programme for the calcination of iron/cobalt molybdate-

based catalysts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

A.3. X-ray diractogram of A2 (Bi2Mo4O15) phase after reduction by 1-butene. 134

A.4. X-ray diractogram of A3 (Bi2W2O9) phase after reduction by 1-butene. 134

A.5. TPR traces of sample A1 after oxidising pretreatment (TPO) . . . . . . . 134





A.10.TPR traces of sample P1 after oxidising pretreatment (TPO) . . . . . . . 136

A.11.TPR traces of sample P2 after oxidising pretreatment (TPO) . . . . . . . 136

131

List of Figures

A.12.(1-butene) TPR traces of sample A1. . . . . . . . . . . . . . . . . . . . . 137



A.15.Depth prole of Bi/Mo surface ratio of sample A1 as determined from ISS

sputter series (cp. Fig. A.16). . . . . . . . . . . . . . . . . . . . . . . . . 138

A.16.Ion scattering spectra of sample A1 (rst 29 of 100 scans). . . . . . . . . 138

A.17.Depth prole of Bi/Mo surface ratio of sample A2 as determined from ISS

sputter series (cp. Fig. A.18). . . . . . . . . . . . . . . . . . . . . . . . . 139

A.18.Ion scattering spectra of sample A2 (rst 29 of 100 scans). . . . . . . . . 139

A.19.TPR traces of α-MoO3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140

A.20.TPO traces of α-MoO3. . . . . . . . . . . . . . . . . . . . . . . . . . . . 140

A.21.X-ray diractograms of P2 sample (Fe3Co7Mo12O46) calcined at dierent

temperatures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141

A.22.X-ray diractograms of bismuth tungstate-based catalysts with dierent

active component/promoter component weight ratio. . . . . . . . . . . . 142

A.23.X-ray diractograms of bismuth molybdate-based catalysts with dierent

active component/promoter component weight ratio. . . . . . . . . . . . 142

A.24.X-ray diractograms of bismuth molybdate-based catalysts calcined at dif-

ferent temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

A.25.BET surface area of the P2 sample as a function of calcination temperature.143

A.26.TPR of catalyst W25-460. . . . . . . . . . . . . . . . . . . . . . . . . . . 144

A.27.TPR of catalyst W35-470-P. . . . . . . . . . . . . . . . . . . . . . . . . . 144

A.28.TPR of catalyst Mo25-460-P. . . . . . . . . . . . . . . . . . . . . . . . . 144

A.29.CO2 formation during TPR experiments. . . . . . . . . . . . . . . . . . . 145

A.30.The inuence of pelletising pressure on catalytic activity of tungstate based


A.31.The summation of acrolein and acrylic acid selectivity as a function of

propene conversion. Filled symbols: BAW35-70; open symbols: BAM25-60. 146

A.32.Temperature-programmed CO-oxidation reaction with catalyst Mo25-460-

P. Reaction conditions as indicated. . . . . . . . . . . . . . . . . . . . . . 146

A.33.CO-TPR experiment with catalyst Mo25-460-P. Reaction conditions as

indicated. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

132

A. Figures

0 10 20 30 40 50 60

100

120

140

160

180

GC-14B

tem

pera

ture

/ °C

time / min

Figure A.1.: Temperature programme of the Shimadzu GC14-B column oven.

0 200 400 600 800 1000 12000

100

200

300

400

500

T = 450 °C = 2 K/min

t = 600 min

T = 250 °C = 1.5 K/min

t = 60 minT = 205 °C = 0.4 K/min

t = 120 minT = 185 °C = 1.5 K/min

t = 60 min

tem

pera

ture

/ °C

time / min

Figure A.2.: Temperature programme for the calcination of iron/cobalt molybdate-based catalysts.

133

A. Figures

10 20 30 40

A2 - Bi2Mo4O15 Bi MoO

2 Bi

2MoO

6

inte

nsity

/ a.

u.

2

Figure A.3.: X-ray diractogram of A2 (Bi2Mo4O15) phase after reduction by 1-butene.

10 20 30 40

A3 - Bi2W2O9 Bi2W

2O

9 Bi Bi

2WO

6

inte

nsity

/ a.

u.

2

Figure A.4.: X-ray diractogram of A3 (Bi2W2O9) phase after reduction by 1-butene.

0 1 2 3 4 5

0,000

0,001

0,002

0,003

0,004


-300

-200

-100

0

100

0

100

200

300

400A1 - Bi

2Mo

3O

12

C3H

4O

CO CO

2 H

2O

efflu

ent m

ole

fract

ion

/ %

time / h

OL

TPR 1 (ORO cycle)

n i mca

t-1 /

µmol

g-1

tem

pera

ture

/ °C

Figure A.5.: TPR traces of sample A1 after oxidising pretreatment (TPO). Symbols:

() acrolein, () CO, () CO2, (N) water.

134

A. Figures

0 1 2 3 4 5

0,000

0,005

0,010

0,015

0,020

0,025


-1500

-1000

-500

0

500

0

100

200

300

400A2 - Bi

2Mo

4O

15 C

3H

4O

CO CO

2 H

2O

efflu

ent m

ole

fract

ion

/ %

time / h

OL

TPR 1 (ORO cycle)n i mca

t-1 /

µmol

g-1

tem

pera

ture

/ °C



0 1 2 3 4 5

0,00

0,01

0,02

0,03

0,04


-100

-50

0

50

0

100

200

300

400

C3H

4O

CO CO

2 H

2O

A3 - Bi2W

2O

9

efflu

ent m

ole

fract

ion

/ %

time / h

OL

TPR 1 (ORO cycle)

n i mca

t-1 /

µmol

g-1

tem

pera

ture

/ °C



0 1 2 3 4 5

0,00

0,01

0,02

0,03

0,04


-300

-200

-100

0

100

200

0

100

200

300

400

C3H

4O

CO CO

2 H

2O

A4 - Bi2WO

6

efflu

ent m

ole

fract

ion

/ %

time / h

OL

TPR 1 (ORO cycle)

n i mca

t-1 /

µmol

g-1

tem

pera

ture

/ °C



135

A. Figures

0 1 2 3 4 5

0,00

0,01

0,02

0,03

0,04


-100

-50

0

50

0

100

200

300

400

C3H

4O

CO CO

2 H

2O

A5 - Bi2W

4O

15

efflu

ent m

ole

fract

ion

/ %

time / h

OL

TPR 1 (ORO cycle)

n i mca

t-1 /

µmol

g-1

tem

pera

ture

/ °C



0 1 2 3 4 5

0,00

0,01

0,02

0,03

0,04

0,05

0,06


-3000

-2000

-1000

0

1000

0

100

200

300

400 C3H

4O

CO CO

2 H

2O

P1 - Fe3Co

4Mo

12O

43

efflu

ent m

ole

fract

ion

/ %

time / h

OL


t-1 /

µmol

g-1

tem

pera

ture

/ °C

Figure A.10.: TPR traces of sample P1 after oxidising pretreatment (TPO). Symbols:


0 1 2 3 4 5

0,00

0,01

0,02

0,03

0,04

0,05

0,06


-1500

-1000

-500

0

500

0

100

200

300

400

C3H

4O

CO CO

2 H

2O

P2 - Fe3Co

7Mo

12O

46

efflu

ent m

ole

fract

ion

/ %

time / h

OL


t-1 /

µmol

g-1

tem

pera

ture

/ °C

Figure A.11.: TPR traces of sample P2 after oxidising pretreatment (TPO). Symbols:


136

A. Figures

0 1 2 3 4 5

0,00

0,05

0,10

0,15

0,20

0,25

0,30

Butadien CO2 H2O OL

-3000

-2000

-1000

0

1000

2000

0

100

200

300

400A1 - Bi

2Mo

3O

12

C4H

6 CO

2 H

2O

efflu

ent m

ole

fract

ion

/ %

time / h

OL

TPR 1

n i mca

t-1 /

µmol

g-1

tem

pera

ture

/ °C

Figure A.12.: (1-butene) TPR traces of sample A1.

0 1 2 3 4 5

0,00

0,05

0,10

0,15

0,20

0,25

0,30

Butadien CO2 H2O OL

-6000

-4000

-2000

0

2000

4000

0

100

200

300

400 C

4H

6 CO

2 H

2O

A2 - Bi2Mo

4O

15

efflu

ent m

ole

fract

ion

/ %

time / h

OL

TPR 1

n i mca

t-1 /

µmol

g-1

tem

pera

ture

/ °C


0 1 2 3 4

0,000

0,005

0,010

0,015

0,020

Butadien CO2 H2O OL

-200

-100

0

100

200

0

100

200

300

400 C

4H

6 CO

2 H

2O

A3 - Bi2W

2O

9

efflu

ent m

ole

fract

ion

/ %

time / h

OL

TPR 1

n i mca

t-1 /

µmol

g-1

tem

pera

ture

/ °C


137

A. Figures

0 10 20 30 40 50 600,2

0,4

0,6

0,8

1,0

A1: Bi2Mo

3O

12

He+ 2 keV

F Bi/F

Mo /

-

scan number

Figure A.15.: Depth prole of Bi/Mo surface ratio of sample A1 as determined fromISS sputter series (cp. Fig. A.16).

1600 1700 1800 1900

0

200

400

600

800

1000

1200

scan25scan20

scan15scan10

scan5

A1: Bi2Mo3O12

He+ 2 keV

kinetic energy / eV

coun

ts /

s-1

Figure A.16.: Ion scattering spectra of sample A1 (rst 29 of 100 scans).

138

A. Figures

0 10 20 30 40 50 600,0

0,2

0,4

0,6

0,8

1,0

A2: Bi2Mo

4O

15

He+ 2 keV

F Bi/F

Mo /

-

scan number

Figure A.17.: Depth prole of Bi/Mo surface ratio of sample A2 as determined fromISS sputter series (cp. Fig. A.18).

1600 1700 1800 1900

0

200

400

600

800

1000

scan25scan20

scan15scan10

scan5

A2: Bi2Mo4O15

He+ 2 keV

kinetic energy / eV

coun

ts /

s-1

Figure A.18.: Ion scattering spectra of sample A2 (rst 29 of 100 scans).

139

A. Figures

0 1 2 3 4 5 6 7

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0

100

200

300

400MoO

3; C

3H

6-TPR cycle 1

C3H

4O

CO CO

2 H

2O

H2

degree of reduction

ef

fluen

t mol

e fra

ctio

n / %

/ n O

/nO

,0

time / h

tem

pera

ture

/ °C

Figure A.19.: TPR traces of α-MoO3.

0 1 2 3 4

0,0

0,2

0,4

0,6

0,8

CO CO

2 H

2O

O2,total

O2,reox

efflu

ent m

ole

fract

ion

/ %

time / h

0

100

200

300

400MoO3

O2 TPO cycle 1

tem

pera

ture

/ °C

Figure A.20.: TPO traces of α-MoO3.

140

A. Figures

10 20 30 40

Fe2Mo

3O

12

T = 250 °C

T = 350 °C

T = 450 °C

T = 550 °C

P2 - Fe3Co7Mo12O46

inte

nsity

/ a.

u.

2Figure A.21.: X-ray diractograms of P2 sample (Fe3Co7Mo12O46) calcined at dif-

ferent temperatures.

141

A. Figures

10 20 30 40

W15-460P

W25-460P

W35-460P

Bi2W

2O

9in

tens

ity /

a.u.

2

Figure A.22.: X-ray diractograms of bismuth tungstate-based catalysts with dif-ferent active component/promoter component weight ratio.

10 20 30 40

Mo15-460P

Mo25-460P

Mo35-460P

Bi2Mo

3O

12

inte

nsity

/ a.

u.

2

Figure A.23.: X-ray diractograms of bismuth molybdate-based catalysts with dif-ferent active component/promoter component weight ratio.

142

A. Figures

10 20 30 40

Mo25-450P

Mo25-460P

Mo25-470P

inte

nsity

/ a.

u.

2

Figure A.24.: X-ray diractograms of bismuth molybdate-based catalysts calcined atdierent temperature.

200 300 400 500 60010

15

20

25

30

P2 - Fe3Co

7Mo

12O

46sintering seriesT = 250 - 550 °C

BE

T su

rface

are

a / m

2 g-1


Figure A.25.: BET surface area of the P2 sample as a function of calcination tempe-rature.

143

A. Figures

0 1 2 3 4 5

0,00

0,01

0,02

0,03

0,04


-1000

-500

0

500

0

100

200

300

400

C3H

4O

CO CO

2 H

2O

W25-460

efflu

ent m

ole

fract

ion

/ %

time / h

OL

TPR 1 (RO cycle)

n i mca

t-1 /

µmol

g-1

Figure A.26.: TPR of catalyst W25-460. Symbols: () acrolein, () CO, (H) CO2,() water.

0 1 2 3 4 5

0,000

0,005

0,010

0,015

0,020

0,025


-1000

-500

0

500

0

100

200

300

400

C3H

4O

CO CO

2 H

2O

W35-470-P

efflu

ent m

ole

fract

ion

/ %

time / h

OL


t-1 /

µmol

g-1

Figure A.27.: TPR of catalyst W35-470-P. Symbols: () acrolein, () CO, (H)CO2, () water.

0 1 2 3 4 5

0,00

0,01

0,02

0,03

0,04


-1500

-1000

-500

0

500

0

100

200

300

400Mo25-460-P

C3H

4O

CO CO

2 H

2O

efflu

ent m

ole

fract

ion

/ %

time / h

OL


t-1 /

µmol

g-1

tem

pera

ture

/ °C

Figure A.28.: TPR of catalyst Mo25-460-P. Symbols: () acrolein, () CO, (H)CO2, () water.

144

A. Figures

0 1 2 3

0,00

0,01

0,02

0,03

0,04

0

100

200

300

400 P2 BAW25-60 BAW25-60P BAW35-70P BAM25-60P

efflu

ent C

O2 m

ole

fract

ion

/ %

time / h

Figure A.29.: CO2 formation during TPR experiments. Graph of promoter com-

pound P2 has been scaled by factor 0.75 for comparison. Symbols:

( ) P2 - Fe3Co7Mo12O46, () W25-460, () W25-460P, (H) W35-470P, () Mo25-460P.

1 2 3 410

20

30

40

50

60

70

80

90

1 2 3 40

2

4

6

8

pelletising pressure / t

X(C3H

6) X(O

2) S(C

3H

4O)

S(COx) Y(C

3H

4O + C

3H

4O

2)

conv

ersi

on, s

elec

tivity

, yie

ld /

%

S(C3H

4O

2)

S(C2H

4O)

S(C2H

4O

2)

pelletising pressure / t

sele

ctiv

ity /

%

Figure A.30.: The inuence of pelletising pressure on catalytic activity of tungstatebased mixed oxide catalysts.

145

A. Figures

0 20 40 60 80 10070

80

90

100

BAW / BAM 300 °C 320 °C 340 °C 360 °C 380 °C

acro

lein

sel

ectiv

ity /

%


Figure A.31.: The summation of acrolein and acrylic acid selectivity as a functionof propene conversion. Filled symbols: BAW35-70; open symbols:BAM25-60.

50 100 150 200 250 300 350

0

2

4

6

8

10 Mo25-460-P

heating cooling

CO-oxidation5 % CO, 5% O

2, bal. Ar

50 ml/min (STP)RT --> 350 °C, 2 K/min

conv

ersi

on /

%

temperature / °C

Figure A.32.: Temperature-programmed CO-oxidation reaction with catalyst Mo25-460-P. Reaction conditions as indicated.

146

A. Figures

0 2000 4000 6000 8000 100000

20

40

60

80

100

0

100

200

300

400Mo25-460-P

CO-TPR5 % CO/Ar50 ml/min (STP)RT --> 400 °C; 3 K/min

conv

ersi

on /

%

time / s

T /

°C

Figure A.33.: CO-TPR experiment with catalyst Mo25-460-P. Reaction conditions asindicated.

147

148

B. Determination of mass and heat

transfer coecients

B.1. Diusion coecient

The diusivity of a component k in a mixture is approximated on the basis of binary

diusion coecients Dki [123]:

Dk = (1− xk) ·

(N∑i=k

xi

Dki

)−1

(B.1)

where xi is the mole fraction of component i.

According to Hirschfelder the binary diusion coecients can be calculated using

tabulated viscosity data [123].

Dki =0.0018583 T 3/2 [(M1 +M2) /M1M2]

0.5

p ·σ212 ·Ω

(cm2 · s−1

)(B.2)

T temperature [K ],

Mi molar mass [kgmol−1 ],

p pressure [105 Pa],

Ω collision integral,

σ12 Lennard-Jones force constant

B.2. Mass transfer coecient

The characteristic numbers for the estimation of the mass transfer occurring at the phase

boundary are the Reynolds number, Schmidt number and Sherwood number:

Re =u0 · dpνf

(B.3)

149

B. Determination of mass and heat transfer coecients

Sc =νfDk

(B.4)

Sh =ki · dpDk

(B.5)

where u0 is the (relative) linear velocity of the uid, dp the particle diameter, νf the

kinematic viscosity, and ki the mass transfer coecient.

Equations B.6 and B.7 are empirical relations of these numbers accounting for the ow

conditions in the laboratory reactor.

jm =Sh

Re ·Sc1/3(B.6)

jm =1.15√

ϵRe−0.5 (B.7)

B.3. Heat transfer coecient

Analogous to Schmidt number and Sherwood number are Prandtl number (Eq. B.8) and

Nusselt number (Eq. B.9) for heat transfer.

Pr =ηf · cpλ

(B.8)

Nu =hi · dpλ

(B.9)

where νf is the dynamic viscosity of the uid, cp the heat capacity, λ the thermal

conductivity, and hi the interphase heat transfer coecient. The respective values for

gas mixtures can be calculated from the pure substance properties using appropriate

correlations [124].

Due to the similarity of mass and heat transfer analogous relations for the characteristic

numbers are provided:

jh =Nu

Re ·Pr1/3(B.10)

jh =1.15√

ϵRe−0.5 (B.11)

150

Documents

Kinetic investigation oft he heterogeneously catalysed selective