The acrolein and acrylonitrile synthesis over a bismuthmolybdate catalyst : kinetics and mechanismCitation for published version (APA):Lankhuijzen, S. P. (1979). The acrolein and acrylonitrile synthesis over a bismuth molybdate catalyst : kineticsand mechanism. Eindhoven: Technische Hogeschool Eindhoven. https://doi.org/10.6100/IR1200

DOI:10.6100/IR1200

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THE ACROLEIN AND ACRYLONITRILE SYNTHESIS OVER A BISMUTH MOLYBDATE CATALYST

' THE ACROLEIN AND ACRYLONITRILE SYNTHESIS

OVER A BISMUTH MOLYBDATE CATALYST

Kinetics and mechanism

PROEFSCHRIFT

TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAPPEN AAN DE TECHNISCHE HOGESCHOOL EINDHOVEN, OP GEZAG VAN DE RECTOR MAGNIFICUS, .PROF. DR. P. VAN DER LEEDEN, VOOR EEN COMMISSIE AANGEWEZEN DOOR HET COLLEGE VAN DEKANEN IN HET OPENBAAR TE· VERDEDIGEN OP

VRIJDAG 22 JUNI 1979 TE 16.00 UUR

DOOR

SIMON PIETER LANKHUIJZEN

GEBOREN TE BREDA

ORUK: WIBRO HELMOND

Dit proefschrift is goedgekeurd door de promotoren:

Prof. drs. H. s. van der Baan, le promotor

Prof. dr. G. C. A. Schuit, 2e promotor

Aan Anny

Aan Jannelies

Gerdiene

Joanne

Machteld

Han

CONTENTS

1. Introduction

1.1. General

1.2. Acrylonitrile manufacture

1

2

1.3. The mechanism of the oxidation and arnmoxidation of

propene 3

1.4. Aim and outline of the present investigation 4

2. Literature

2.1. Introduction

2.2. Kinetics

2.3. Adsorption of reactants and products

2.4. Hydrocarbon surface intermediates

2.5. Nitrogen containing surface intermediates origi-

nating from ammonia

2.6. ~ole of oxygen

2.7. Catalyst

2.8. Models

3. Apparatus and analysis

3.1. Introduction

3.2. The flow reactor system

3.2.1. Analysis

3.3. The thermobalance

3.4. The pulse reactor system

3.4.1. Analysis

3.5. Safety

3. 5 .1. Toxicity

3.5.2. Flammability and explosive ranges

4. The catalyst

4.1. Introduction

7

8

9

12

13

14

15

16

21

23

27

32

33

34

35

35

37

39

4.2. The structure of the bismuth molybdate catalyst 40

4.3. Catalyst preparation 43

5. Experimental methods

5.1. Introduction 47

5.2. The rate of reaction 48

5.3. Factors governing the reactor behavioUr 49

5.3.1. Plug flow 49

5.3.2. Temperature gradients 51

5.3.3. Catalyst dilution 53

5.3.4. Mass and heat transfer 54

5.3.5. Pressure drop 57

5.4. Data handling and analysis of errors 57

6. Results and discussion

6.1. Introduction 61

6.2. Flow experiments 62

6.2.1. Introduction 62 6.2.2. Preliminary experiments 62

6.2.3. The oxidation of propene to acrolein 64

6.2.3.1. Experiments at 673 K 64

6.2.3.2. Experiments at other temperatures 68

6.2.4. The oxidation of ammonia to nitrogen 69

6.2.5. The ammoxidation of acrolein to acrylonitrile 70

6.2.5.1. Preliminary experiments 71

6.2.5.2. Experiments at 673 K 72

6.2.6. The ammoxidation of propene to acrylonitrile 74

6.2.6.1. Introduction and preliminary ex-

periments 74

6.2.6.2. Experiments at 673 K 76

6.2.6.3. Experiments at other temperatures 79

6.2.6.4. Experiments at non-initial con-

ditions 80 6.3. Thermobalance experiments 84

6.3.1. Introduction 84

6.3.2. Preliminary experiments 85 6.3.3. Reduction of the catalyst with propene 85

6.3.4. Reduction of the catalyst with hydrogen 90

6.3.5. Reoxidation of a reduced catalyst 93

6.3.6. The reduction of the catalyst with mixtures

of propene, nitrogen and small quantities

of oxygen 94

6.4. Pulse experiments 6.4.1. Introduction 100

6.4.2. Preliminary experiments 101

6.4.3. Experiments with propene-helium mixtures 103

6.4.4. Experiments with mixtures of propene and

oxygen 105

6.4.5. Experiments with mixtures of ammonia and

helium 107

6.4.6. Experiments with mixtures of propene, ammo-

nia and helium 107

7. Final discussion

7.1. Introduction

7.2. The mechanism of the catalytic reaction 113

113

7.3. Adsorption and adsorption sites 115

7.3.1. The adsorption of ammonia 115

7.3.2. The adsorption of propene 117

7.3.3. The adsorption of acrolein 120

7.3.4. The role of the catalyst in the activation

of molecular oxygen

7.4. The formation of the main products

7.4.1. The formation of acrolein

7.4.2. The formation of nitrogen

7.4.3. The formation of acrylonitrile

7.4.4. The formation of water

7.5. The kinetic model

7.6. Selectivities in the acrylonitrile synthesis reac-

tion

List of symbols

summary

Samenvatting Dankwoord

121

125

125

128

129

130

130

134

139

143

147

150

CHAPTER 1

INTRODUCTION

1.1. General

Since acrolein was discovered in 1843 by Redtenbacher

(1) and acrylonitrile was synthesized fifty years later

by Moureu (2) these compounds remained laboratory chemi­

cals until the development of some large scale polymeri­

zation processes. Preparative methods were replaced by

commercial catalytic processes and expensive chemicals by

cheaper raw materials to obtain these important unsatura­

ted intermediates in chemical industry.

As a result of the development of certain heterogeneous

catalytic processes acrylonitrile is obtained nowadays

from propene, ammonia and air instead of acetylene and hydrogen cyanide. Contrary to the latter the former process

is based on catalytic oxidation, which is now a major tool

for the incorporation of carbonyl and nitrile groups in

hydrocarbons. The necessity of a catalytic oxidation re-'

action follows from thermodynamic calculations. The com-

plete oxidation of propene and ammonia to carbon dioxide

nitrogen and water at temperatures of interest is prefer­

red to the formation of acrylonitrile or acrolein.

An intensive research effort has been directed towards

the development of selective catalysts for the partial

oxidation of hydrocarbons to a number of products. This

has led to the development of a number of selective oxi­

dation catalysts made from transition metal oxides.

A major event in the history of oxidation catalysis was

the discovery of bismuth molybdate (3,4,5,6) as a selec­

tive catalyst for the partial oxidation of propene and

also in an one-step operation for the ammoxidation of pro­

pene. As other metal oxide combinations this catalyst is

1

very versatile, being effective in a number of processes,

e.g. the oxidative dehydrogenation of n•butenes and me­

thyl-butene to butadiene and isoprene and the formation of

aromatic carbonyls and nitriles.

Although in the 1960's the catalytic ammoxidation of propene over bismuth molybdate became a commercial process,

the basic understanding of the role of the catalyst lagged

far behind the technological knowledge of the process.

Many research studies have been carried out to characte­

rize the excellent catalytic properties of bismuth molyb­date based catalysts. These studies have increased the un­

derstanding of oxidation and.ammoxidation kinetics and

mechanisms, but the controlling parameters are not yet

completely understood.

Two basic principles however have become clear. Firstly

the oxygen atoms for the selective reaction come from the

catalyst. Secondly the metal oxide combinations must be

able to transfer oxygen by a redox reaction. The latter

requirement explains the suitability of transition metal

oxides in selective oxidation. The catalyst surface is

alternately in an oxidized and reduced state. Anion va­

cancies play an important role.

1.2. Acrylonitrile manufacture

The impressive growth of the acrylonitrile production

(average annual growth between 16 and 20% from 1960-1975)

has resulted from the manufacturing technology that could

use cheaper raw materials at high selectivity. Older com­

mercial processes were based on the reaction of acetylene with hydrogen cyanide or the reaction of ethylene oxide

with hydrogen cyanide followed by dehydration. Both pro­

cesses required more expensive starting materials and more * extensive safety measures than the modern (SOHIO) process

which is based on the direct ammoxidation of propene, according to

(r 1.1)

2

The other advantages of this process are the high selecti­

vity and the long lifetime and high activity of the cata­

lyst.

After the use of a bismuth molybdate catalyst promoted

by phosphorus (composition 50% Bi9 P Mo12 o52/50% Si02 )

and an uranium-antimony oxide catalyst (composition usb3 o10 > nowadays the socalled multicomponent molybdates {MCM)

catalyst (SOHIO 41) is the most important catalyst (7,8).

It is composed of a variety of elements like nickel, co­balt, iron, manganese, potassium, phosphorus but always

contains bismuth and molybdenum. Today SOHIO's process

accounts for over 95% of the world's installed capacity

of 2.4 106 metric tons/year.

Some of the older ammoxidation processes used multitu­

bular fixed bed reactors but all major modern processes

use fluidized bed reactors. The advantages of the fluid

bed are a better temperature control and a removal of the

limitations on propene and ammonia concentrations due to

the explosivity of the reactor feed (9), The process ope­

rates at 1-3 bar and 673-773 K. The main feature of the

process is the high conversion obtained on a once-through

basis in the fluid bed, thanks to the high selectivity of the catalyst.

Over the past two decades the rapidly expanding market

for acrylonitrile has shifted more and more from the acry­

lonitrile elastomers (NBR) to the acrylic fibers and re­

sins (ABS and SAN). High impact resistance, low porosity

acrylic copolymers with over 75% acrylonitrile are a re­cent development in the manufacture of bottles and con­

tainers. In the near future the demand for polyacrylamide

may capture a good deal of the acrylonitrile market (10).

1.3. The mechanism of the oxidation and ammoxidation of

propene

During the past ten years the commercial production of

acrylonitrile has been attended with an increasing number

of investigations dealing with the elucidation of the

3

mechanism of the ammoxidation reaction. Several reviews

have appeared about reaction mech~nisms of olefin oxida­

tion { 11-18) •

In literature relatively little attention has been paid

to the mechanism of the ammoxidation reaction itself. On

the other hand research has been focussed mainly on the

behaviour of the catalyst under various conditions that

differ considerably from commercial ones.

Generally a relatively simple test reaction is used for

that purpose. It has been stated that in commercial multi­

component catalysts bismuth molybdate performs the cataly­

tic role. We decided to investigate the ammoxidation reac­

tion itself in order to obtain further knowledge of the re­

action kinetics and insight in the mechanism of the acry­

lonitrile synthesis.

1.4. Aim and outline of the present investigation

It is the aim of this investigation to derive a reac­

tion model based on kinetic results for the catalytic

ammoxidation of propene. Unsupported y-bismuth molybdate

is used as a catalyst.

In chapter 2 a survey is given of the literature with

respect to the subject of this investigation.

The apparatus and the methods of analysis for studying

the kinetics are described in chapter 3.

Chapter 4 deals with the preparation and the properties

of the catalyst.

Chapter 5 is devoted to the experimental methods

applied in the kinetic studies of heterogeneous catalysis.

Chapter 6 deals with the kinetic experiments carried

out in the different reactors. The kinetics of the propene

oxidation and ammoxidation reactions are studied to de­

termine the conditions for the selective reaction proce­

dures. The significance of acrolein in the reaction model

is investigated by means of its ammoxidation to acryloni­

trile. Special attention is paid to the oxidation of ammo­

nia to nitrogen.

4

Thermobalance and pulse reactor experiments are per­

formed to investigate the reaction of propene and ammonia

with the catalyst in the absence of oxygen in the gas phase.

In this way the role of gasphase and catalyst oxygen is

studied during the catalytic reaction.

Finally in chapter 7 the reaction model for the diffe­

rent oxidation and ~oxidation reactions based on the

experimental results is given. In a final discussion a mechanistic model is proposed which may contribute to the

understanding of the catalytic activity of bismuth molyb­

date.

For the explanation of symbols, abbreviations and sub­

scripts see List of Symbols.

References

1. Redtenbacher, J., Anw. Chern. Liebigs 47, 114 (1843)

2. Moureu, c., Bull. Soc. Chim. Fr. ~ (3) 424 (1893)

3. Idol, J.D. (Standard Oil Co.), u.s. Pat. 2.904.580

(Sept. 15, 1959)

4. Callahan, J.L., Foreman, R.W., Veatch, F. (Standard

Oil Co.) u.s. Pat. 3.044.966 (July 17, 1962)

5. Veatch, F., Callahan, J.L., Idol, J.D., Milberger,

E.C., Chern. Engng. Progr. 56 (10) 65 (1960)

6. Callahan, J.L., Grasselli, R.K., Milberger, E.C., ' Strecker, H.A., Ind. Engng. Chern. Prod. Res. Dev. ~.

134 (1970)

7. Krabetz, R., Chern. Irig. Techn. 46, 1029 (1974)

8. Wolfs, M.W.J., Thesis Eindhoven (1974)

9. Anon, Hydroc. Proc. 56 (11) 124 (1977)

10. Pujado, P.R., Vora, B.V., Krueding, A.P., Hydroc.

Proc. 56 (5) 169 (1977)

11. Sachtler, W.M.H., Catal. Rev. !• 27 (1970)

12. Margolis, L.Ya., Catal. Rev. !• 241 (1973)

5

13. Hucknall, D.J., Selective oxidation of hydrocarbons. Acad. Press, London (1974)

14. Skarchenko, V.K., Russ. Chem. Rev.~~ 731 (1977)

15. Schuit, G.C.A., J. Less. Com. Met: 36, 329 · (1974)

16. Gates, B.C., Katzer, J.R., Schuit, G.C.A., Chemistry

of Catalytic Processes, Ch. IV, McGraw Hill N.Y. (1979)

17. van der Wiele, K., van den Berg, P.J., in Bamford

C.H. and Tipper C.H.F. (Eds.) Comprehensive Chemical

Kinetic~ Vol. 20 Complex Catalytic Processes, Chapter

2 1 12~ Elsevier Publ. Cy Amsterdam 1978

18. Keulks, G.W., Adv. Catal. 27, 183 (1978)

6

CHAPTER 2

LITERATURE

2.1. Introduction

During the past two decades more than 400 papers and

reviews have been published about the selective oxidation of olefins in general and the ammoxidation of propene

over bismuth molybdates or bismuth molybdate containing

catalysts in particular.

In this chapter we will give a brief literature survey

to situate the subject of our investigation. It is not our

aim however to add a new comprehensive review of the li­

terature to the excellent ones that have appeared already (1,2,3).

Catalytic oxidation reactions can be explained accor­

ding to two different mechanisms, viz.

a) the reduction-oxidation mechanism, proposed by Mars

and van Krevelen (4) operating in the higher tempe­

rature range;

b) the associative mechanism set up by Roiter (5) at

lower temperatures.

In the redox mechanism two separate steps are distinguish­

ed: in the first step the hydrocarbon is oxidized with

lattice oxygen whereas in the second step the reduced

oxide is reoxidized by oxygen of the gasphase. In the

associative mechanism a reaction between adsorbed oxygen

species and the hydrocarbon occurs. Evidence for the two

mechanisms is obtained from i~otopic exchange experiments,

as has been pointed out by Boreskov (6) and Winter (7,8)

and from catalyst reduction experiments carried out by

Batist et al (9) and Sachtler et al (10).

Bismuth molybdate catalysts show a high activity in com­

bination with a good selectivity both in the oxidation and

7

in the ammoxidation of propene. During the oxidation of

propene besides acrolein only small quantities of carbon dioxide, carbon monoxide, acetaldehyde and formaldehyde are

formed. Acrylonitrile is the main product of the propene

ammoxidation. Other products are acetonitrile, hydrogen cyanide, carbon dioxide and carbon monoxide whereas acrolein

is only a trace product. The stoichiometric equations are

(r 2.1)

(r 2.2)

2.2. Kinetics

Broadly there is a great similarity in the overall features of the oxidation and ammoxidation of propene over bismuth molybdate catalysts: the rates of oxidatiop and

ammoxidation are both first order with respect to propene

and zero order with respect to oxygen. The rate of ammoxi­

dation is zero order in ammonia (11,12).

Activation enthalpies for the formation of acrolein and

acrylonitrile show a considerable spread mainly caused by

the different catalysts and temperature ranges as can be seen in table 2.1.

Eact (kJ/mol) Bi/Mo T(K) Ref. C3H40 C3H3N mol mol-l

84 38 2/1 670 (13)

54 - 1/1 650-825 (14)

121 - 1/1 670-730 (15)

159 71 1/1 650-750 (16)

71 - .74/1 700-775. (17)

104 104 .74/1 670 (13)

84 75 BigPMo12o52/Si02 670-700 (12)

Table 2.1. Activation enthalpies (kJ mol-l) for the forma~ tion of acrolein and acrylonitrile over diffe­rent bismuth molybdate catalysts.

8

The rate of ammoxidation of acrolein, according to the

stoichiometric equation

(r 2.3)

is first order with respect to acrolein and zero order

both in ammonia and oxygen (12,13). The activation enthalpy is 29 kJ mol-l (12).

The rate of oxidation of ammonia over bismuth molybdate

according to the stoichiometric equation

+ (r 2 .4)

is first order with respect to ammonia and zero order with

respect to oxygen. The activation enthalpy is 155 kJ mol-l

( 18) •

If we compare the rate constant data presented by

Callahan et al (12) with those of Cathala et al (19)

carried out with slightly different catalysts it becomes

clear that the rate of propene ammoxidation at 700 K is

higher than the rate of propene oxidation. Callahan (12)

found the rate of acrolein ammoxidation at least twice as

high as the rate of propene ammoxidation. Contrary to

Shelstad et al (20), Callahan et al (12) conclude that

acrylonitrile is formed largely by a mechanism not in­

volving acrolein as a vapour phase intermediate.

2. 3. AdsorE,t'i,on of reactants an:d ·products

The adsorption of the reactants and products of the

ammoxidation of propene on the catalyst has been studied

by Matsuura et al (18,21,22), who investigated not only the adsorption behaviour of a fully oxidized but also that

of a partly reduced catalyst and of Bi2o 3 and Moo3 • Mat­

suura linked the adsorption data obtained at low pressures

and at temperatures between 325 and 475 K to the perform­ance of some oxidation catalysts at atmospheric pressure

and temperatures above 673 K in order to develop a

9

reaction mechanism. He distinguishes between two types of

adsorption viz. the socalled A-type and the B-type adsorp­tion.

The A-type is an activated, strong and slow adsorption, observed for butadiene, acrolein and ammonia on oxidized

Bi2Mo06 and for acrolein on Bi2o 3 • All adsorptions are of

the dual site type except the butadiene adsorption which

is a single site type. Enthalpies of adsorption are between 88 -1 .

and 100 kJ mol • Prereduction of the catalyst linearly de-creases the number of A-sites, so an A-site contains an

oxygen ion (OA). To allow for the two types of adsorption and for a similar adsorption of acrolein on Bi2o 3 it is

assumed that the A-site contains two anion vacancies (VBi)

located at two Bi-ions next to the oxygen ion OA. So the

A-site is VBiOAVBi' The B-type adsorption is a weak and fast adsorption

observed for butadiene, acrolein, olefins and ammonia.

This type of adsorption occurs on Bi 2Mo06 as well as on

Moo3 , but not on Bi2o 3 ~ On Bi2Moo6 all B-type adsorptions are of the dual site kind, except the ammonia adsorption. Enthalpies of adsorption are in the range of 25-50 kJ mol-l.

Previous reduction does not remove B .. sites, provided the

reduction temperature does not exceed 673 K. At tempera~

ture above 673 K Batist et al (23) found a rapid reduction

of the catalyst by butene-1 at degrees of reduction less than 8.3% and without loss of activity after reoxidation.

According to Matsuura (21) the reoxidation above 673 K is

first order in oxygen with an activation enthalpy of 72 kJ mol-1 •

The removal of B-sites, mentioned by Matsuura is pro­bably connected with some rearrangement in the solid viz.

the formation of metallic Bi in a separate phase. This

phenomenon has been mentioned by Batist et al '(24,25).

B-sites are claimed to be combinations of an anion vacancy

(VM0

) and two oxygen ions (OB). So the B-site is OBVMooB. The A-type adsorption of ammonia and acrolein is strong

and the enthalpies of adsorption are so high that desorption

can only occur at reaction tP;mperatures. Adsorption of.

10

oxygen on non reduced catalysts does not occur. However, the catalyst shows some reversible dissociation when . the gas phase oxygen partial pressure is lower than the

equilibrium oxygen pressure (i.e. p02

eq (673 K) = 1.3 10-8

bar). According to Matsuura (21) the adsorption of oxygen

on partially reduced catalysts at room temperature is

small, rapid and independent of the degree of reduction

and does not lead to complete reoxidation.

Between 373 and 673 K the rate of reoxidation is zero order in oxygen. The enthalpy of activation is 113 kJ mol-1 ,

a value found also by Batist et al (9) for the reoxidation

of reduced Bi 2Mo 2o9 • Above 673 K the rate of reoxidation

becomes first order with respect to oxygen and has an

enthalpy of activation of- about 72 kJ mol-l, depending on the degree of reduction. According to Matsuura the acro­

lein adsorption occurs on both A and B-sites. The strong and slow adsorption on site A, also observed on Bi2o3 is

a dual site adsorption. This acrolein adsorption fits the

adsorption model proposed by Sachtler et al (26}. This

dual site adsorption must influence the dual site adsorp­

tion of propene on site B, which needs also OB' This could

be verified experimentally as the weak propene adsorptiqn

decreased after a pretreatment of the catalyst with acro­

lein.

The adsorption behaviour of ammonia on an oxidized cata­lyst is very complicated. Matsuura (18) concludes that the

strong dual site adsorption is connected with the A-site,

with the donation of a proton to OB of the B-site. The ammonia adsorption on partially reduced catalysts is con­

nected with a reduced A-site.

The weak and dual site adsorption of propene on site B

decreases after a pretreatment of the catalyst with ammo­

nia. Experimental data of the strong adso~ption of ammonia

on Moo3 and Bi2o 3 are lacking because of the nitrogen for­matio.n already occuring at low temperatures. Kfivanek et

al (27) calculated the enthalpy of adsorption of propene under

reaction conditions at 440 K on bismuth molybdate to be 130 kJ mol-1 .

11

2.4. Hydrocarbon surface intermediates

By the use of isotopic labels it is established by

Sachtler et al (10), McCain et al (28) and Adams et al

(29,30) that the oxidation of propene over bismuth molyb­

date proceeds via the formation of the allylic interme­

diate which is negatively charged. According to Schuit (2)

the proton is donated to an 0 2- ion at the surface, expe­

rimentally confirmed by Beres et al (31), and the carbanion

is bonded to a metal ion at an anion vacancy. This mecha­

nism resembles that taking place during the chemisorption

of benzaldehyde on a Sno2-v2o 5 catalyst, studied by

Sachtler (32).

Recent molecular orbital calculations by Haber et al

(33) carried out for different transition-metal cations

support the postulate that the IT-bonding electrons are

transferred from the allylic intermediate to the Mo 6+ ion.

The Mo 6+ is reduced to Mo 5+ or Mo 4+ and the positive char­

ge on the c 3H5+-ion is concentrated on the terminal C­

atoms in a symmetrical distribution. After the transfer of 2-electrons the allylic intermediate is cr-bonded to an 0

as was confirmed by Kondo et al (34). Dozono et al (35)

studied the ammoxidation of 3-13c propene at 450°C in the

presence of bismuth molybdate. Half of 3-13c in the acry­

lonitrile was found to be in the CN-group. This points to

a symmetrical intermediate also in the acrylonitrile syn­

thesis. The appearance of 13c in both the methyl- and the

cyanogroup of acetonitrile, although not completely dis­

tributed (60/40 respectively) can only result from bond

rupture in the allylic intermediate rather than from the

breakage of a C = C bond in propene, acrylonitrile or

acrolein.

Further dehydrogenation must lead to a c 3H4-interme­

diate and proton donation to another o 2- ion. Adams et al

(29,30) suggested that the allylic intermediate undergoes

this hydrogen abstraction before the incorporation of

oxygen which has been experimentally confirmed by means of

kinetic isotope effect measurements. Cathala et al (19)

12

connected this step with a parallel bond rupture which

gives rise to degradation products. Daniel and Keulks (36)

reported at 725 K an enhanced conversion of propene in a

reactor having a large post-catalytic volume. It appeared

that a surface-initiated homogeneous gas phase reaction

caused the formation of side products. Without the post­

catalytic volume this formation disappeared, Recently

Kobayashi et al (37} have studied the mechanism of the

oxidation of propene by applying a transient response me­

thod. It was found that a stable surface intermediate exists which can be formed either from propene or from

acrolein. Further dehydrogenation of the c 3H4 intermediate is highly

unlikely. In the case of ammoxidation Cathala et al (19)

supposed that dehydrogenation occurs after the formation

of allylidene-imine (C 3H4NH). This was also suggested by

Grassel+i et al (38) for the ammoxidation catalysed by Usb3o 10 • 1

2.5. Nitrogen containin<J surface intermediates ori<Jinatin<J from ammonia

The NH2-intermediate follows from the adsorption expe­

riments of Matsuura {18). Ammonia is dissociatively ad­

sorbed, according to Matsuura donating a proton to an

oxygen ion of the B-site. Ammonia adsorption on a reduced

catalyst is supposed to occur preferentially on the anion

vacancy left after reduction. Matsuura (18) and Cathala

et al (19) drew for mechanistic reasons a parallel between the dehydrogenation of the allylic intermediate and the

amide group and supposed the formation of allylidene­

imine, synthesized by Bogdanovic et al (39), which proba­

bly has adsorption properties comparable with acrolein and

butadiene. Germain et al (40) classified the oxides that

catalyse the oxidation of ammonia and postulated that the

imine-intermediate is a substitute for the double bonded

oxygen ion. He classified Moo 3 and not Bi 2o 3 among the

oxides that show moderate oxidation activity for ammonia.

13

As mentioned already for the c3H4 intermediate

further dehydrogenation of the imine is supposed to be

very unlikely.

2.6. Role of o~gen

It is generally assumed that the o2- ion on the surface

of the oxide catalyst is responsible for the oxidation of

the hydrocarbon.

Reoxidation by gas phase oxygen leads to the formation of

o2- but needs four electrons for every oxygen molecule,

as follows from the equation

o2 + 4 e + 2 c {r 2.5)

Gates et al (2) suggest a more stepwise donation of

electrons, viz. the formation of some intermediate oxygen - 2- ~ species e.g. o2 1 o2 and o at lower temperatures.

In that region the Mars van Krevelen mechanism does not

apply as was indicated by Boreskov et al (41) and Sancier

et al (42). The evidence of these intermediates is esta­

blished by ESR spectroscopy (41). Van Hooff (44) suggested

that these intermediates lead to chain reactions. Haber

(45) assumed the oxygen intermediates to be electrophilic

reagents and the oxidizing species in the total oxidation

of hydrocarbons, whereas lattice oxygen ions are nucleo­

philic reagents with non oxidizing properties. Van Dillen

(46) investigated the existence of these species extensi­

vely. I 18 16 By means of 0 - 0 exchange, however, it is esta-

blished by Keulks (47) and Wragg et al (48) that bismuth

molybdate catalysts do not exchange with o2 at. temperatu­

.res below 773 K in the absence of an oxidation reaction. . 18

Keulks (47) suggested from experiments with o2 gas phase oxygen and Bi 2Mo16o6 that during the oxidation of propene

at 698 K the oxygen of about 500 layers participated in

the reaction and that these layers were oxidized by a

rapid diffusion of oxygen from the bulk of the catalyst

14

rather than by gas phase oxygen. However the gas phase oxy­

gen was gradually incorporated in the product. An imme­diate incorporation would be expected if the reaction with

catalyst oxygen was confined to the surface layer only on

which gas phase oxygen would be chemisorbed. Wragg et al

(48) with experiments at 748-773 K came to the same con­

clusion. As also 180 is gradually incorporated in the

carbon dioxide Keulks assumed that the selective and com­

plete oxidation of propene occurs at the same site.

Pendleton et al (49) studied the reaction between pro­

pene and 18o2 over bismuth molybdate between 623 and 673 K. They showed the incorporation of lattice oxygen into the

acrolein, whereas oxygen for the carbon dioxide formation

in that temperature region comes from both the gas phase

and the lattice. Keulks et al (50} however in a later in­

vestigation at 703 K concluded that there is no distinc­

tion be~ween the lattice oxygen incorporated into carbon

dioxide and into acrolein.

Sancier et al (42) determined the relative contribution

of sorbed and lattice oxygen during propene oxidation over

silica supported bismuth molybdate between 590 and 670 K in a pulse reactor and concluded that above 623 K lattice

oxygen becomes more important whereas below 623 K the mo­

bility of lattice oxygen is low and adsorbed oxygen takes

over the role. Recently van Oeffelen (51) found a rapid

increase of the electrical conductivity during the re­

duction of Bi2Mo1 • 02o6 • 06 with propene at 673 K. He

ascribed this phenomenom to the formation of bismuth metal

particles on the surface. Similar evidence was also ob­tained by Peacock et al (52). E.s.r.-signals due to Mo5+ were detected when the catalyst was exposed to propene

but these signals were absent when oxygen was added (53).

Sancier et al (54) and Burlamacchi et al (55) obtained

the same results.

2.7. Catalyst

Significant contributions to our understanding of the

15

excellent catalytic properties of bismuth molybdate and the

nature of the active phase have been made by Schuit, Ba­

tist and coworkers (2,9,56,58).

It would carry us too far to give a literature survey

about the structure of the active catalyst. We refer to

the recent review of Gates et al (2) and to chapter 4.

2.8. Models

Some authors have proposed models for the reaction me­

chanism of the oxidation or ammoxidation of propene. These

models are summarized in table 2.2 without detailed infor­

mation. In chapter 7 these models will be discussed.

16

REACTANTS/INTERMEDIATES

C3H6 C3H5 is is

C3H4 is

NH3 is

NH2 is

NH

is

c3a4o is

END PRODUCTS

C~H3NI H~O ~s ~s

REOXIDATION

site during to . reaction

re- 0-!absorbed formed formedladsorbedlformedlformedlformed formed! formed

with with

oxi- transfer dize from to

on I on on

VM.o

VMo4

VMo

VBi

VBi

2-Mo04

VMo..,.VBiiVBi

VMo~VBiiVBi

VMo

VBi

VBi

2-Mo04

VMo?

VMo

VBi

2-Mo04

on

VBi

VBi

on on

VBi VBi

VBi VBi

with

OBi

OBi

0Mo?

0Mo

OBi

0Mo

2-0Mo04

OBi

OBi

OBi

OBi

OBi

VBi

VM04

Mo4->-Bi

VBi Bi+Mo

VBi IBi+Mo

VMo IMo+Bi

0Moo4

2-IVBi Bi->-Mo0 3

REF.

* (18)

(2)

(53)

(59) (64)

(60) (61) ( 6 5)

( 62)

( 63)

Table 2.2. Different models for the reaction mechanisms of the oxidation and ammoxidation of

propene. * ~ See also 2.3.

~

1. Hucknall, D.J., Selective oxidation of hydrocarbons,

Road Press London (1974)

2. Gates, B.C., Katzer, ~.R., Schuit, G.C.A., Chemistry

of Catalytic Processes, McGraw Hill, Ch. 4 (1979)

3. Vander Wiele, K., van den Berg, P.J., in Bamford C.H.

and Tipper C.F.H. (Eds.), Comprehensive Chemical Ki­

netics, Vol. 20 Complex Catalytic Processes, Chapter

2, 123, Elsevier Publ. Cy. Amsterdam (1978)

4. Mars, J., van Krevelen, o.w., Chem. Eng. Sci. Suppl.

l, 41 (1954) 5. Roiter, V.A., Kin. i. Kat • .!_, 63 (1960)

6. Boreskov, G.K., Adv. Cat, 15, 285 (1964)

7. Winter, 8, Winter,

9. Batist,

G,C.A. I

E.R.S., Adv. Cat. 10, 196 (1958) E.R.S., J, Chem. Soc. A, 479 (1968)

Ph,A., Kapteijns, C.J., Lippens, B.C., Schuit,

J. Catal. z, 33 (1967) 10. Sachtler, W,M,H., Rec. Trav. Chim. 82, 243 (1963)

Sachtler, W.M,H., de Boer, N.H., Proc. 3rd Int. Congr.

Catal. Amsterdam 1964, Vol. I, 252, NH Publ. Co. Am­

sterdam ( 1965)

11. Adams, C.R., Voge, H,H., Morgan, C.Z., Armstrong, W.E.,

J. Catal. l• 379 (1964) 12. Callahan, J.L., Grasselli, R.K., Milberger, E.C.,

Strecker, H.A., Ind. Engng. Chem. Prod. Res. Dev. 1• 134 (1970)

13. Wragg, R.D., Ashmore, P.G., Hockey, J.A., J. Catal.

ll· 293 (1973) 14. Gorshkov, A.P., Kolchin, I.K., Gribov, J.M., Margolis,

L.Ya,, Kin. i. Kat. 2• 1086 (1968)

15. Keulks, G.W., Rosynek, M.P., Daniel, c., Ind. Engng.

Chem. Prod. Res. Dev. 1Q, 138 (1971)

16. Cathala, M., Germain, J.E., Bull, Soc. Chim. Fr. 2167,

2174 (1971)

17. Peacock, J.M., Parker, A,J,, Ashmore, P.G., Hockey,

J .A., J. Catal, g, 398 (1969)

18. Matsuura, I,, J. Catal. 1.11 420 (1974)

18

19. Cathala, M., Germain, J.E., Bull. Soc. Chim. Fr. 4114

(1970)

20. Shelstad, K.A., Chong, T.C., Can. J. Chern. Engng. 47,

597 (1969)

21. Matsuura, I., Schuit, G.C.A,, J. Catal. ~, 19 (1971)

22. Matsuura, I., Schuit, G.C,A., J. Catal. ~, 314 (1972)

23. Batist, Ph.A., Prette, H.J., Schuit, G.C.A,, J. Catal.

ll· 267 ( 1969) 24. Batist; Ph.A., Bouwens, J.F.H., Schuit, G.C.A., J.

Catal. 25, 1 ( 1972)

25. Batist, Ph.A., Lankhuijzen, S.P., J. Catal. 28, 496

(1973)

26. Sachtler, W.M.H., Dorgelo, G.J.H., Fahrenfort, J.,

Voorhoeve, R.J.H., Proc. 4th Int. Congr. Catal. (1968)

(1), 454 (1971)

27. Krivanek, M., Jiru, P., z. phys. Chemie, Leipzig 256,

(1) 153 (1975)

28. McCain, c.c., Gough, G, 1 Godin, G.W., Nature, Lond.

198, 989 (1963)

29. Adams, C.R., Jennings, T.J., J. Catal. ~, 63 (1963)

30. Adams, C.R., Jennings, T.J., J. Catal. l• 549 (1964)

31. Beres, J., Bruckman, K., Haber, J,, Janas, J., Bull.

Acad. Pol. Sci. Ser. Sc. Chim. 20, (8) 813 (1972)

32. Sachtler, W.M.H., Catal. Rev. ! (1) 27 (1970}

33. Haber, J., Sochacka, M., Grzybowska, B., Golzbiewski,

A., J. Mol. Catal. l• 35 (1975)

34. Kondo, T., Saito, s., Tamaru, K., J, Am. Chern. Soc.

2§_, 6857 (1974)

35. Dozono, T., Thomas, D.W., Wise, H., J. Chern. Soc. Far.

Transa~t. I 69, 620 (1973)

36. Daniel, C., Keulks, G.W., J. Catal. ~, 529 {1972)

37. Kobayashi, M. , Futaya, R. , J. Catal. 56, 73 ( 1979)

38. Grasselli, R.K., Suresh, D.D., J. Catal. 25, 273 (1972)

39. Bogdanovic, B., Velie, M., Angew. Chern. 79, 818 (1967)

40. Germain, J.E., Perez, R., Bull. Soc. Chim. Fr. 2042

(1972)

41. Boreskov, G.K., 2nd Jap. Sov. Catal. Sem. Tokyo (1973)

42. Sancier, K,M., Wentreck, P.R., Wise, H., J. Catal. 39,

141 (1975)

19

43. Lunsford, J.H., Catal. Rev. l• 135 (1973) 44. van Hooff, J.H.C., Thesis, Eindhoven (1968)

45. Haber, J., 4th Roermond Conf. on Catal. (1978)

46. van Dillen, A.J., Thesis, Utrecht (1977)

47. Keulks, G,W,, J. Catal. ~' 232 (1970) 48. Wragg, R.D., Ashmore, P.G., Hockey, J.A., J. Catal.

22, 19 (1971)

49. Pendleton, P., Taylor, D01 J. Chem. Soc. Far. Trans.--I

72, 1114 (1976)

50. Keulks, G.W., Krenzke, L.D., Proc. 6th Int. Congr.

Catal. ~' 806 (1977) 51. van Oeffelen, D.A.G., Thesis, Eindhoven (1978)

52. Peacock, J.M., Parker, A.J., Ashmore, P.G., Hockey,

J.A., J. Catal. ~' 387 (1969) 53. Peacock, J.M., Sharp, M.J., Parker, A.J., Ashmore,

P.G., Hockey, J.A., J. Catal. ~· 379 (1969)

54. Sancier, K.M., Dozono, T., Wise, H., J .• Catal. ll' 270 ( 1971)

55. Burlamacchi, L., Martini, G., Ferroni, E., J. Chem.

Soc. Far. Trans. I ~' 1586 (1972) 56. Bleijenberg, A.C.A.M., Lippens, B.C., Schuit, G.C.A.,

J. Catal. !1 481 (1965)

57. Batist, Ph.A., Lippens, B.C., Schuit; G.C.A., J. Catal.

1· 55 (1966) 58. Batist, Ph.A., der Kinderen, A., Leeuwenburgh, Y.,

Metz, F., J. Catal. 12, 45 (1968)

59. Haber, J., Grzybowska, B., J. Catal. 28, 489 (1973)

60. Otsubo, T., Miura, H,, Morikawa, Y., Shirasaki, T., J.

Catal. ~· 240 (1975) 61. Miura, H., Otsubo, T., Shirasaki, T., Morikawa, Y,,

J. Catal. 56, 84 (1979)

62. Trifiro, F., Kubelkova, L., Pasquon, I., J. Catal. ~'

121 (1970)

63. Sleight, A.W., Adv. Mat, Catal. (eds. J.J. Burton,

R.L. Garten) Acad. Press N.Y. (1976)

64. Grzybowska, B., Haber, J., Janas, J., J. Catal. ~'

150 (1977)

65. Dadyburjor, D.B., Ruckenstein, E., J. Phys. Chem. 82, 1563 ( 1978)

20

CHAPTER 3

APPARATUS AND ANALYSIS

3-. 1. Introduction

It is generally accepted that the catalytic activity

of bismuth molybdate is closely related to its oxidizing

properties. In the absence of molecular oxygen for short

periods the catalytic activity and selectivity in the

oxidation and ammoxidation of propene are not affected

i.e. a reduction-oxidation mechanism is operative.

To study the behaviour of bismuth molybdate under sta­tionary and non-stationary conditions three different

techniques have been used.

A. Reaction kinetics in a stationary state as carried

out in different plug flow fixed bed reactors,

operating under differential as well as under inte­

gral conditions~

B. The behaviour of bismuth molybdate as an oxidant and the reoxidation of partially reduced bismuth molyb­

date are studied in a thermobalance, acting as a

semi-batch reactor.

c. Additional information about the behaviour of the

catalyst under non-stationary conditions at a low

degree of reduction is gained with a pulse reactor

system.

In order to obtain reliable data the experiments have to meet certain requirements, such as:

- the experimental variables (temperature, flow and

reactant inlet concentrations) have to be measured

and controlled accurately~

- the concentration and temperature differences be­

tween the bulk gas phase and the catalyst surface

should be as small as possible1

21

- the chemical analysis has to provide for a mass ba­

lance over the whole range of experimental concentra­

tions;

- isothermicity has to be pursued as much as possible;

- as the residence time distribution of the reaction

mixture generally has an effect on the conversion

level and on the selectivity of the reaction and more­

over strongly depends upon the applied technique and

on the experimental variables this distribution

should be minimized and properly determined.

All reactors are connected to an on-line gaschromato­

graphic analysis system for the determination of the

reaction components. However, since. such a GLC-analysis

takes at least 15 minutes and has only a moderate sensiti­

vity it is less suitable for the examination of non-sta­

tionary processes in which rapid change of the reaction

rate occurs.

To gather information about the rate of oxygen deple­

tion of the oxidant, the thermobalance in combination with

a GLC apparatus with a flame ionization detector is suit­

able because it gives additional information about the

weight of the oxidant. Moreover this apparatus is useful

for the study of the catalyst reduction and for the oxi~

dation of previously reduced samples. However the thermo~

balance has the drawback that the flow around the catalyst

is poorly defined and one has to keep in mind that the

concentrations at the catalyst surface can differ consi­

derably from those in the bulk gasphase.

As our thermobalance is not resistant to ammonia vapour

the ammoxidation reactions could not be studied in this

apparatus. Additional information about these reactions

and about the behaviour of the catalyst has been obtained

with a pulse reactor. The pulse reactor is a good instru­

ment to detect small changes in the catalyst properties

but, unless the concentrations of the pulse in the reactor

and its residence time are carefully studied the kinetic

information leaves much to desire.

22

The conversion level at which one performs the kinetic

experiments with the various techniques is a compromise

between the low level necessary for the study of the ki­

netics at differential conditions and the higher level re­

quired for reliable analytical data.

As we deal with moderately or strongly exothermic re­

actions, the kinetic data can be affected by non-isother­

mic conditions in the fixed bed reactors. We have re­

pressed the axial and radial temperature gradients by

means of the dilution of the catalyst with silicon carbide

that has good heat conducting properties (A 673 K = 105 J s- 1 m- 1 K-1 ) (1). Although the commercial operation for

the acrylonitrile production takes place in a fluid bed

we have not used such reactors because of the unclear flow

pattern and the attrition of our unsupported catalyst.

3. 2. The flow reactor system

The flow reactor system used for the kinetic experi­

ments described in section 6.2.4 to 6.2.6 is shown in fi­

gure 3.1. It consists of

NH3

C3H6 M He

Figure 3.1 Flow reactor system.

r--Jf'---r;orm111'yncAL SYSTEM (SEE FIG. 3.3)

1. VAN OYCK MIXER 2. VAPORISER 3. CIRCULATION PUMP 4. REACTOR

(SEE ALSO FIG. 3.2) 5. OXYGEN ANALYSER SEl SELECTION VALVE

FEED/PRODUCT M ARTIFICIAL AIR

a) a gas mixing part in which carefu~~y controlled flows

of propene, ammonia, artificial air (20% vol o2

, 80%

vol He) and helium can be mixed in the desired compo-

23

sitions. For the experiments involving a liquid reac­

tant (acrolein) and for the determination of the sub­

stance specific correction factors of the liquids in

the analysis of the feed and the product composition

helium can be passed through a double-walled thermo­stated vaporizer filled with the pure component in

question. The desired partial pressure of the reactant

can be established by adjusting and controlling the

temperature of the vaporizer. It has been ascertained

that the rising heli~ bubbles were completely satura­

ted with vapour. We used the Fourier-number as a mea­

sure for the saturation of the dispersed phase

Dt Fo = r2 (3.J.)

with D is the molecular diffusion coef.ficient (m2 s-1),

tis the residence time of the bubble in the liquid (s),

r is the radius of the bubble (m). We found Fo > 4,

whereas already at Fo = .5 for Biot numbers >> 10 (no concentration gradient in the continuous phase), the

concentration distribution over the bubble is practi­

cally constant (2). Moreover we analysed the vapour

gaschromatographically at varying liquid levels in the vaporizer and we found a constant vapour concentration.

b) a tubular fixed bed reaator, which is made of AISI 321

stainless steel. Three reactors have been used for the

various reactions as can be seen in table 3.1.

24

Reactor B is shown in figure 3.2. An aluminium jacket

has been cast around the reactor tube to improve the

temperature profile in the reactor. This aluminium

jacket is divided in three sections that are indepen­

dently heated. The temperature is measured at eight places, three in the catalyst bed and five in the

siliconcarbide bed under and above the catalyst section.

The temperature is controlled at the three sections

within 1 K with Eurotherm thyristor controllers.

Reactor Catalyst SiC bed

Reaction type dia. length weight dia. weight height cat. lliiii lliiii g lliiii bed g lliiii

P-+ACO A 6 90 .6 • 5- • 85 1.7 0/ 53/ 0

a) jACO-+ACN B 20 340 1.5 1.0-1.2 25 50/ 50/120

INH3-+N2 B 20 340 7.5 1. 0-1.2 43 50/100/120 P-+ACN B 20 340 7.5 1. 0-1.2 43 50/100/120

IAco-+ACN c 11 110 .5 1.0-1.2 7 10/ 50/ 10

Table 3.1. Flow reactors: dimensions and fillings.

a) 50/50/120 means 50 mm SiC, 50 mm diluted catalyst,

120 mm SiC.

Under stationary reaction conditions the maximum

axial temperature differences over the whole reactor at comparable temperature and flow,were as shown in table 3.2.

Reactor Reaction t.Ta (K)

A P-+ACO 'V3

B ACO-+ACN 2 B P-+ACN 2

c ACO-+ACN 4

Table 3.2. Axial temperature differences in the reactor heart line.

These axial temperature differences are mainly due to

heat conduction to the colder inlet and outlet lines of the reactor,

25

II I ~ i

f;.

~ r

¥ ~

~ ~ ~

"' i~

fl-1-=~~~~~~:~~=--l ·~

H~.11

rg~~·i "'== • '

--'-.--J.I ~~"-'-'--· J

hfJ~(V/tf(l( .IVSI ZZt

{lllf¥,y·~s¥MMJrl«.

Figure 3.2 Flowreactor B.

26

Radial temperature profiles were measured in the cata­

lyst section of reactor B and C during the ammoxidation of acrolein when the greatest differences could occur

and a temperature difference of not more than 1 K was

found in the radial direction.

c) an analysis system.

The feed or the product stream is introduced by

means of sampling valves in the analysis system, which

will be dealt with in the next section. The feed and

product lines are heated electrically and the tempera­

ture of these lines is controlled at about 425 K to

prevent the condensation of water and hydrocarbons and

the polymerization of acrolein and acrylonitrile.

3.2.1. Anal:y:sis

All flow reactors are equipped with an on-line gas chroma­

tograph. With this apparatus we can determine quantitatively the components

oxygen

nitrogen

carbon monoxide

carbon dioxide

ammonia

water

formaldehyde

acetaldehyde

acetonitrile

acrolein

propene acrylonitrile

During the catalytic oxidation of propene we used at

fLrst only one GLC-apparatus with katharometer detection

(3). For the separation of the components the column tem­

perature had to be programmed in that case from 338 to

433 K with 12 K min-1 . With the introduction of ammonia

for the ammoxidation experiments however the reproducibi­lity of the temperature programmed analysis decreased.

Crozat and Germain (4) analysed ammonia and water on two

columns, i.e. on Porapak Q at 360 K one peak for NHj+H2o was obtained, whereas on a PEG column at the same tempera­

ture an inaccurate H2o determination was carried out.

With the introduction of two GLC's at constant tempera­

ture (5) i.e. one for the analysis of the low boiling com­

ponents and the other with a flame ionization detector for

the analysis of the combustible components we took advan­

tage of the better separation of the low boiling compo-

27

VENT

PRODUCT FEED

PRE SURE STABILIZER

VENT He

He

Figure 3.3 Scheme of the analytical system.

nents at a constant low column temperature, Moreover we could perform a greater number of analyses in a given

time. The system with two GLC's, schematically shown in

figure 3. 3, consists of a 4-way Whity-valve (S.E 1) for the

selection of the feed or product stream1 two 8-way Be.cker gas sampling valves S1 and S2 for the sampling of the gas stream and an 8-way Becker valve (SE 2) for the selection

of the columns during the analysis on GLC 1. The sampling

loops of S1 and S2 are .1 cm3 and 2 cro3 respectively.

28

Samples containing formaldehyde, propene, acetaldehyde,

acetonitrile, acrolein and acrylonitrile are analysed on the first gas chromatograph GLC-1, a Philips Pye series 104

gas chromatograph with flame ionization detector. The se­

cond gas chromatograph GLC-2, a Philips Pye series 44 with

katharometer detector is used for the analysis of oxygen, nitrogen and carbon monoxide by means of the separation on

a Molsieve l3X column and for the analysis of carbon di­

oxide, ammonia, water and propene on a Porapak Q4 column.

By means of a selection valve SE 2 the components separa­

ted on the Porapak Q4 column are detected in channel num­

ber 1, whereas then the carrier gas passes through channel

number 2. The components separated on the Molsieve 13X

Figure 3.4 Chroma­togram of an analysis on GLC-1.

a"'

Figure 3.5 Chro­matogram of an ana­lysis with Molsieve 13X on GLC-2,ch.2.

0

:rf''

Figure 3.6 Chroma­togram of an analysis with Porapak Q on GLC-2 ,ch .1.

29

column are detected in channel number 2, whereas the

carrier gas passes through channel number 1. As the pro­

pene peak is found in the chromatograms obtained with

GLC-1 as well as with GLC-2 a quantitative analysis of all

the components is feasible. For the prevention of a reac­tion between ammonia and acrolein in the Porapak Q4 column

of GLC-1 this column is preceded by a small column, filled

with docosanoic acid (c 21H43cooH, melting point 353 K)

which adsorbes ammonia completely. The only drawback is

the periodic regeneratio.n that is required for the Pora­

pak Q4 column of GLC-2.

The analysing conditions are summarized in table 3.3,

whereas the chromatograms are shown in figure 3.4, 3.5

and 3.6.

GLC-1: Philips Pye 104, temperature 523 K with flame

ionization detector. Hydrogen 30 cm3 min-1 •

Air 50 cm3 min-1 •

CoZumn: Porapak Q4, 50-80 mesh

length: 3,5 m, i.d. 2 mm

temperature: 423 K carrier gas flow: 25 cm3 min- 1 He

analysing time: 3 minutes

column material: glass

GLC-2: Philips Pye 44, temperatu~e 523 K with katharo­meter detector. Bridge current 150 m A. CoZumna: a) Molsieve 13X, particle size .5-.7 mm

length: 2 m, i.d. 4 mm

temperature: 298 K carrier gas flow: 25 cm3 min-1 He

b) Porapak Q4, 50-80 mesh

length: 2.75 m, i.d. 2 mm

temperature: 333 K carrier gas flow: 25 cm3 min-1 He/NH3 total analysing time: 15 minutes

Table 3.3. Analysing conditions for the flow reactor system.

30

At low mole fractions the peak area of a component in .

a chromatogram is proportional to its mole fraction and

the quantitative analysis of the diluted product mixture

can be carried out using the relation

(3.2)

with XA mole fraction of component A

XC H : mole fraction of propene A 3 6. peak area of component A A •

AC H : peak area of propene f 3 6. substance specific correction factor of A •

component A

This equation is based on the assumption that fc H = 1. In order to determine the f-values of the variou~ 6

components, propene-helium gasmixtures of different com­

positions are obtained with two plunger pumps (type Wosthoff) and analysed on the two gas chromatographs. The f­

values of the gaseous components are obtained in the same way by mixing with propene/helium gasmixtures in the range

of the experimental mole fractions. The f-values of the

liquid components can be determined with the thermostated

vaporizer already mentioned in section 3.2. The f-value

of ammonia is obtained by means of titration. Peak areas

are determined with Infotronics model CRS 208 electronic

integrators. The reliability of the f-values was checked

periodically, because of the continuous ageing of the

columns. Although temperature programming to 423 K had a

favourable effect on the lifetime of the Porapak Q4 co-

lumn the isothermal method is preferred, as was already

stated above.

The slight increase in the number of moles as a result

of the oxidation and ammoxidation of propene can be ne­

glected, especially because the reacting gas mixtures con­

tain at least 80% vol He.

31

For the stability and activity of the catalyst the

gas mixtures must contain oxygen and therefore we analysed the oxygen content of the product continuously by means

of a Servomex oxygen analyser.

3.3. The thermoba:la:nce

The experiments described in chapter 6, section'6.3 are

carried out in a Dupont series 900/950 thermobalance. This

apparatus is shown in f i,gure 3. 7. Mixtures of ni troqen and

~----------~--·vm

"z AA Cff; "z

1. Quartz glass furnace tube 8. BTS catalyst 2. Furnace 9. Mol sieve 3. Sample holder 10. van Oyck mixer 4. Thermocouple 11. Thermos tate 5. Balance housing s. Sampling valve 6. Photo voltaic cells AA. Artificial air 7. Counter weights Figure 3.7 Flow diagram thermobalance.

propene or nitrogen and hydrogen prepared as usual and care­

fully freed from oxygen over a bed of 120 gram of reduced * BTS catalyst and dried with 30 gram molsieve, are intra-

*

32

BTS stands for the reduced BASF R3-ll catalyst (30% wt Cu and promotor on carrier).

duced into the sample chamber of the thermobalance. This

sample chamber consists of a quartz tube with i.d. 2.1 em

heated by an electric furnace. The chamber is at atmos­

pheric pressure. The experiments are carried out under

isothermal conditions. The temperature is measured with a

chromel-alumel thermocouple placed just above the 12.5 x

8.4 x 1.20 mm quartz glass sample bucket usually con­

taining 75 mg of the oxidant sample. To avoid the presence

of reactants in the part of the balance where the weight

changes are recorded with a photoelectric cell, this side

of the system is continuously purged with nitrogen.

The accuracy of the temperature measurement is ± 1 K.

The sensitivity of the thermobalance is .01 mg, which

corresponds to an error in the degree of reduction of bis­

muth molybdate of .OS%. Before the reduction experiment is

carried out the thermobalance is carefully freed from

oxygen by means of flushing the system for 15 minutes at

room temperature with pure and dry nitrogen. Subsequently the balance is flushed with nitrogen at reaction tempera­

ture for one hour. We did not observe a weight

loss larger than .01 mg during this conditioning.period.

The effluent of the reduction experiment with propene

containing gas mixtures is analysed by means of gas chroma­tography as described in section 3.2.1 for the combustible

components.

3.4. The pulse reactor system

As shown in figure 3.8 a constant flow of helium passes

through the pulse reactor into an Hewlett Packard 5700 A

gas chromatograph with katharometer detector and further into a flame ionization detector. A pulse of a gasmixture

containing the reactants for the oxidation and the ammoxi­

dation reactions is injected closely before the pulse reactor inlet. After the reaction in the catalyst bed the pulse

is subsequently analysed. The carrier gas is carefully

freed from oxygen and dried as described in section 3.3.

33

r-----IVEMT

1. Pulse reactor 2. Furnace 3. BTS - catalyst 4. Molsieve 5. van Oyck mixer 6. Thermostate S.· Samling valve V. Switching valve

AA. Artificial air

Figure 3.8 Pulse reactor system.

Th~ pulse reactor D is a micro reactor, inner diameter

5 mm, length 14.6 mm made of AISI 316 stainless steel.

100 mg Bismuth molybdate, particle size .3-.5 mm is placed· between two plugs of quartz wool. The pulse reactor is

heated by means of an electric furnace. The temperature is

continuously recorded with a chromel-alumel thermocouple

in the midale of the fixed bed. The temperature of the

furnace is controlled with an Eurotherm thyristor con­

troller. The pulse volume is .155 cm3 NTP. The pressure

in the reactor is 2.5 bar and the carrier gas flow is

18 cm3 min-1 NTP. By means of an 8-way

Becker sampling valve s, which is switched pneumatically,

a pulse is introduced in the line to the reactor.

3.4.1. Analysis

The analysis of the pulse after reaction differs from that

of the flow reactor effluent because of the small sample

quantity, the maximum admissible pressure and the analysis

time. The separation of the components is obtained with a

temperature programmed Porapak Q4 column and the detection

occurs with a Hewlett Packard 5700 A katharometer. As the

quantities of the combustible p~oducts are very small the separated components subsequently pass through a flame

34

ionization detector of a Philips Pye 104 GLC for the determina­

tion of the combustible components. The analysing condi-

tions are summarized in table 3.4.

Column: Porapak Q4, 80-100 mesh

length: 2.8 m i.d. 2 mm

temperature programs and analysing times:

a) p~opene/helium

333-393 K with 2 K min-1

analysing time: 50 minutes

b) p~opene/oreygen/helium

16 minutes on 333 K, 333-393 K with 16 K min-1

8 minutes on 393 K

analysing time: 40 minutes

c) ammonia/helium

333 K constant temperature

analysing time: 10 minutes d) p~opene/ammonia/helium

16 minutes on 333 K, 333-423 K with 16 K min-1

16 minutes on 423 K

analysing time: 38 minutes

Katha~omete~-deteato~: HP 5700 A, temperature 523 K

bridge current 150 m A

Flame ionization-deteato~: Philips Pye 104, temperature

523 K Hydrogen: 30 cm3 min- 1

Air: 50 cm3 min-1

Table 3.4. Analysing conditions for the pulse reactor

system.

3.5. Safety

3. 5. 1. Toxicity

As acrolein and acrylonitrile are highly toxic sub­

stances (6), all experiments are carried out in a hood

35

with adequate exhaust ventilation. Due to its extreme

lachrymatory effect (the smelling limit is • 2 to • 4 ppm

(7)) acrolein serves as its own warning agent. It affects particularly the membranes of the eyes and respiratory

tract.

Acrylonitrile closely resembles hydrogen cyanide in its

toKic action. By inhibiting the respiratory enzymes of

tissue it renders the tissue cells incapable of oxygen

usage. In table 3.5 the Treshold Limit Values (time * ** weighted average) (TLC-TWA) and the LCLo values are

given.

TLV - TWA (8) LCLo (9)

ppm mg m- 3 ppm

c3H40 • 1 .25 150/10 min (inhalation human) 4. Qa) c3H3N 9.0 600/4 hrs (inhalation cat)

a) DuPont de Nemours has reduced the TLV-value to 2.0

ppm (I 0)

Table 3.5. Treshold Limit Values for acrolein and acrylo­

nitrile.

_We calculated the mean concentration of acrolein in the

hood when condensation and subsequent destruction would

have been omitted as .19 mg m- 3• For acrylonitrile a value

of .18 mg m- 3 would have applied. These values are ·smaller

than the adopted TWA-values.

*

**

36

TLV-TWA = "the time weighted average concentration for

a normal 8 hour workday to which all workers

may be repeatedly exposed, day after day,

without adverse effect" (8).

LCLo = "the lowest lethal concentration of a sub-

stance in air, which has been reported to

have caused death for a given period of ex­

posure" (11).

3.5.2. Flammability and explosive ranges

Most of the reaction components are flammable and have

explosive properties over wide ranges when mixed with air,

as can be seen in table 3.6. Propene is the main hazardous

substance, whereas acrolein and acrylonitrile are Class I

flammable liquids (6).

Flash point Ignition temp. Explosive range

K K vol %

C3H6 165 770 2 -11

NH3 924 15 - 28

c3H40 255 551 2.8 - 31

c3H4N 273 754 3.1 - 17

Table 3.6. Flammability and explosive ranges of the main

reaction components (6).

Therefore a flame extinguisher is included in the feed

line to the reactors. Strong ventilation is required.

References

1. Y.S. Touloukian ed., Thermophysical properties of

High Temperature Solid Materials Vol, 5, 125 (1967) The McMillan Cy., New York

2. H.A.C. Thijssen, Masstransfer Processes, Lecture-notes

6.605, 10,25 (1973) University of Technology, Eind­hoven

3. Verhaar L.A.Th., Lankhuijzen S.P., J, Chrom, Sci.~, 457 (1970)

37

4. Crozat, M., Germain, J.E., Bull. Soc. Chim. Fr. 3526 (1972)

5. A.P.B. Sommen, Int. Report TC (1975) University of Technology, Eindhoven

6. Sax, N.I., Dangerous Properties of Industrial Mate­

rials, 4th Ed., van Nostrand Reinhold Cy., N.Y. (1975)

7. Hommel, G., Handbuch der gefahrlichen Guter, 2 Aufl.,

Springer Verlag, Berlin (1973)

8. Association of American Governmental Industrial Hy­

gienists, Index TLV, Am. Ind. Hyg. Ass. Journ. 37, 721 (1976)

9. The International Technical Information Institute;

Toxic and Hazardous Industrial Safety Manual, Tokyo (1977)

10. Anon, Chern. Weekblad 1.1 (22) 1 (1977)

11. Registry of Toxic Effects of Chemical Substances, u.s.

38

Dept. of Health, Education and Welfare, NIOSH (1977) Cincinnatti, Ohio

CHAPTER 4

THE CATALYST

4.1. Introduction

It seems to be the fate of every catalyst to be repla­

ced by another more active and selective one. So the mul­

tiphase Cu-cu2o-cuo catalyst introduced in 1948 by Hearne and Adams (1), showing only a yield of about 50% in the

oxidation of propene to acrolein has.been superseded in the sixties by the superior bismuth molybdate catalyst.

Callahan et al (2) claimed this catalyst to be useful not

only for the oxidation of propene but also for the dehy­

drogenation of 1-butene to butadiene and even for the

ammoxidation of propene to acrylonitrile. Shortly after

the commercial realization of the acrylonitrile process

SOHIO developed its second process based on USb3o10 (31.

but nowadays these two component catalyst system have been

replaced by the so-called multi-component~molybdate (MCM)

catalyst which contains.besides bismuth and molybdenum a

variety of elements such as nickel, cobalt, iron, manga­nese, phosphorus and potassium.

Whichever oxide combination may be an active and selec~

tive catalyst for the incorporation of a hetero atom (0 or

N) or for the dehydrogenation of olefins, ~t became evi­

dent that the superior catalysts are all oxidic combina­

tions or compounds containing at least two different ele~

ments. One of these is always a transition metal and the

other belongs to the later row Sa elements. The group of

ammoxidation catalysts is characterized by a high selec­

tivity for partial oxidation and its ability to supply

oxygen as a reactant for a selective fissure of the C-H

and N-H bonds.

39

4.2. The structure of the bismuthmolybdate, catalyst

Of the two components of the catalyst of our investi­

gation, Bi2o 3 and Moo3 , the former shows at temperatures below 773 K a low ac~ivity, while Moo3 displays at these temperatures an even lower activity but a fair se­

lectivity at those temperatures. In combination by means

of a proper preparation method, however, a conspicuously

active and selective catalyst emerges.

Many attempts have b~en made to determine the structure

of the active phase. Series of catalysts have been prepa­red with varying Bi/Mo atomic ratios in the range of 2/3

to 2/1. All the catalyst samples were found to be selec­

tive but differently active (4). In this range three sta­

ble compounds were found, viz.

- the a.-phase (Bi2Mo3o12 ) with a monoclinic structure (a = 7.89, b = 11.70, c = 12.24 10""10 m, ~ =.116° l2'l .• This

structure is related to the structure of Scheelite, men­tioned by Mekhtiev (5). The x .. ray data of Aykan (6) were

in good agreement with those of E!leij~nberg et al {7)

although the pattern contained additional reflections,

which means that more than one phase was present. Single­

crystal studies by van den Elzen and Rieck (8) have con­firmed the monoclinic structure. The a.~phase is stable

and has a melting point of 949 K.

- the (3-phase (Bi2Mo2o 9) or the so .... called Erman phase (9)

has been studied in detail. However, there still remains a great deal of uncertainty with regard to its stability

in catalytic oxidation which depends on the applied pre~

parative technique. The solid state technique as used by

Erman (9) leads to different metastable phases such as

the high-temperature y'-bismuth molybdate in the tempera­ture region of catalytic activity in oxidation reactions.

40

The precipitation technique as used by Grzybowska et

al (10), Trifiro et al (ll) and Batist et al (4) (12) is

influenced by factors as pH, concentration of reagents and the treatment of the precipitate. Moreover the cal­cination temperature is an important factor, because

Batist (12) stated that already at temperatures up to

773 K the ~-phase slowly disproportionates into the a­

and the y-phase. It has been observed by Batist et al

(12)(13) that the pH during the precipitation has a

strong influence on the catalytic activity of the a­phase. This phenomenon is connected with the equilibrium

between the Mo-O-octahedra and - tetrahedra and it is

assumed that in these preparations the y-phase is always

present. The discussion about the structure of the ~­phase has not yet come to an end. As the Bi/Mo ratio 1/1

is frequently used in commercial catalysts the elucida­

tion of the nature of the active phase is an interesting

issue.

the y-phase (Bi2Mo06) has an x-ray pattern similar to

that of the rare mineral bismuthrnolybdate named Koechli~

nite, as reported by Zemann (14). It has a layer struc~

ture made up of (Bi 2o2 )~+ and (Moo2 )~+ sheets connected

by o 2- in the arrangement:

The structure of the mineral Koechlinite is orthorhombic . ~0 .

(a = 5.50, b = 16,24, c = 5.49 10 m) which was con-

firmed by van de Elzen and Rieck (15) who found the

following parameters (a= 5.487, b = 16.226, c = 5.506

10-10 m). The structure was described as alternating

layers of (Bio);n and (Moo 42->n perpendicular to the y

direction. In figure 4.1 and figure 4.2 the corner sha­

ring Me-octahedra and the Bi2o2-layer are shown respec­

tively. These figures are based on van den Elzen's data (15).

The (MoO~-)n layer consists of Mo6+ ions in octahe­

dral surrounding, the octahedra sharing corners in the

sheets and their apices point toward the (Bio);n layers.

The (Bio);n layers resemble the structure of BiOCl. In

Koechlinite the bond distances in the molybdenuro~xygen sheets are as follows: two at 1.76 10-10 m and two at

2.24 10-10 m. The molybdenum-oxygen bonds to the apex

41

Legend:

<D® apex oxygen ions

®® oxygen ions in Moo~+ layer

• Mo6+ ion

Figure 4.1 Cornersharing Mo-O-octahedra in koechlinite.

Figure 4,2 Bi 2o~+ layer in koechlinite.

42

Legend:

G) o2- ion

0 Bi 3+ ion

o 2- ions have intermediate lengths: 1.86 10""10 and 1.93

10""10 m. The bismuth ion is bonded to six oxygen ions:

the four Bi-0 distances in the BiO-layer range from 2.15

to 2.50 10-10 m. The distances to the apical oxygens of the Mo octahedra are 2.33 and 2.67 10-10 m. As pointed

out by Schuit (16) the structure of the y-bismuth molyb..,

date can therefore be regarded as an intermediate be­

tween one having a two dimensional Reo3 .... type of corner

sharing Moo6 octahedra and one having slightly distorted Moo 4 tetrahedra.

The y-phase is metastable and at temperature.s above

930 K it can be transformed to the y'~hase .with a te~ tragonal structure, reported by Blasse (17). The melting point is 1211 K (7), the density is 8.26 ~0 3 kg m~ 3 (6).

The catalytic activity of bismuth molybdate has been

connected by Schuit et al (4) with the presence of cornersharing oxo-molybdenum octahedra and it has been postulated that the active site for adsorption of pro..,.

pene is an oxygen anion vacancy on a mo~ybdenum ion still being present in a tetragonally pyramidal configu~

ration.

4.3. Catalyst preparation

y-Bismuth molybdate has been prepared according to the

method described by Batist et al (12), Batist (13) and

Konings et al (18) either starting with molybdic acid

(method A) or with ammonium heptamolybdate (method B). For

all preparations the basic chemicals were analysed care~ fully in order to be sure of the stoichiometry of the

catalyst. BismuthyJ nitrate (Merck, p.a.), containing 79.9

wt % Bi2o 3 , molybdic acid (BDH) with 87 .• 4% wt Moo3 and amrooniumheptaroolybdate (Merck, p.a.) containing 8~.8% wt

MoO 3 were used.

Method A

According to the method given by Konings et al (18) and based on the method described by Batist et al (12) 94.7 g

43

BiON03 are mixed with 27.3 g H2Moo4 and 2 liter distilled

water is added. Under vigorous stirring the slurry is

boiled continuously for 40 hours. The pH changes from 7 to 2 and within two hours the color of the suspension changes

from white into light yellow. After 10 hours the yellow

color becomes more intense. From time to time distilled

water is added to keep the volume of the slurry constant.

After filtration and drying at 393 K the material is cal­

cined for two hours at 773 K. Great care is taken to keep

the calcination tempera~ure constant. In this w~y an

highly active and selective catalyst is prepared with a

stoichiometric excess of 2 mole % Mo. The specific surface area, varying from 3.0 to 3.4 m2 g~~ was determined with

an areameter according to the BET-method, using nitro9en as the adsorbate. The x-ray pattern showed the d..,.yalues

and relative intensities (see table 4.1} which are charac..,_

teristic for y-bismuth molybdate as has been given by Ba­tist et al (12).

d rel. intensity d rel. intensity

8.20 10 2.48 ~0

4.55 < 5 2.27 < 5

3.78 5 1.94 ~5

3.16 100 1.93 20

2.76 25 1.65 20

2.70 15 1.64 ~5

2. 61. < 5 1.58 ~0

Table 4.1. x-ray pattern (d-values in 10-~0 m and relative

intensities) of the prepared bismuth molybdate.

The excess of 2 mole % Mo can be seen as a precaution, because from several investigations carried out by Schuit

and coworkers (12)(13)(18)(19) it became clear that the

activity of the catalyst decreases sharply when the over­

all Bi/Mo ratio > 2, whereas the activity is hardly in­

fluenced when Bi/Mo < 2.

44

Method B

According to Batist's method (13), 29.3 g (NH4

) Mo7e24 •

4 H2o are dissolved in 2 1 distilled water and the pH of

the solution is lowered to 2.5 by careful addition of

nitric acid, keeping the molybdenum in the octahedral co­

ordination. 94.7 g Bi0N03 are .added and under vigorous stirring the slurry is boiled continuously for 24 hours.

The same procedure is followed as at Method A. This cata­

lyst showed a similar activity and selectivity as the ca­talyst prepared according to method A and has the same

specific surface area. The main stoichiometric equation of

the catalyst preparation is

(r 4 ~.11

Batist (13) assumed that the mass swelling during the

slurry reaction (method B) is a result of the decomposi~

tion of the heptamolybdate ion into molybdic acid, accor~

ding to the equation

+

followed by a penetration of solved molybdenum oxide oc­

taeders into the layers of the solid BiON03 •

Both catalysts have the following stoichiometric bulk

composition: Bi2Mo1 • 02o6 • 06 as was checked by means of a.

weight analysis.

References

1. Hearne G.W., Adams M.L., u.s. Patent 2,451,485 (1948)

2. Callahan, J.L., Foreman, R.W., Veatch, F., u.s. Patent

3,044,966 (1962)

3. Callahan, J.L., Gertisser, B., u.s. Patent 3,198,750

(1965)

45

4. Batist, Ph.A'~ der Kinderen, A.H.W.M., Leeuwenburgh,

Y., Metz, F.A.M.G,, Schuit, G.C.A., J. Catal. g, 45

(1968)

5. Mekhtiev, K.M., Gamidov, R.S., Mamedov, Kh,S., Belov,

N.V., Dokl. Akad. Nauk. SSSR 162, 397 (1965)

6. Aykan, K., J. Catal. 12, 281 (1968)

7. Bleijenberg, A.C.A.M., Lippens, B.C., Schuit, G.C.A.,

J. Catal. i 1 581 (1965)

8. van den Elzen, A.F., Rieck, G.D., Acta Crystallogr.

Sect, B 29, 2433 (19.73)

9. Erman, L.Ya., Galgerin, E.L., Kolchin, I.K., Dobrzhan­

skii, G.F., Chermyshev, K.S., Russ. J. Inorg. Chem, ir 1174 (1964)

10. Grzybowska, B., Haber, J., Komorek, J., J. Catal. ~,

25 (1972)

11. Trifiro, F., Hoser, H., Searle, R.D., J. Catal. 25, ~2 (1972)

12. Batist, Ph.A., Bouwens, J.F.H., Schuit, G.C.A., J. Catal. 25, 1 (1972)

13. Batist, Pn.A., to be published

14. Zemann, J., Heidelberger Beitr. Mineral Petrogr. ~~

139 (1956)

15. van den Elzen, A.F., Rieck, G.D., Acta Crystallogr.

Sect. B 29, 2436 (1973)

16. Gates, B.C., Katzer, J.R., Schuit, G.C.A., Chemistry of Catalytic Processes,·. Chapter 4, McGraw Hill,

New York (1979)

17. Blasse, G., J. Inorg. Nucl. Chem. 28, 1124 (1966)

18. Konings, A.J.A., Creemers, H.J.M., Batist, Ph.A., J.

Catal. 41, 333 (1976)

19. van Oeffelen, D.A.G., Thesis, University of Technology,

Eindhoven (1978)

46

CHAPTER 5

EXPERIMENTAL METHODS

s·.l. Introduction

Because a catalyst increases the rate of a reaction ki­

netic experiments are among the key experimental methods

to investigate the behaviour of a catalyst. Kinetic mea­

surements however have their limitations as they do not

provide enough information for a complete description of

the catalytic reaction sequence, since the kinetics prima­

rily reflect the slowest step of the sequence of elemen­

tary reactions.

The requirements for our kinetic experiments are

- an accurate and fast analysis of all reactants and pro-

ducts;

- the absence of heat and mass transfer limitations;

- an isothermal operation of the catalyst bed;

- a well defined flow pattern in the reactor;

- a constant catalyst activity.

We have chosen the tubular fixed bed reactor because

the flow pattern is well defined, we have no catalyst

attrition and the isothermicity can be obtained by apply­

ing catalyst dilution. Reaction rate measurements in this

flow system were performed differentially and integrally.

A small fixed bed reactor was used ·for pulse experi­

ments. Valuable information can be obtained with this

reactor type, although reliable quantitative rate data can

only be extracted from these experiments for simple first

order kinetics.

A thermobalance is a powerful instrument in heteroge­

neous catalytic research. Because of the gasflow pattern

required the concentrations at the catalyst surface are

not well defined.

47

In this chapter the attention is directed to the main factors that determine the proper operation of the reactor

types that we have used during this investigation.

5.2. The rate of reaction

Before dealing with factors that determine the rate of formation of a product of a heterogeneous catalytic reac­

tion, this rate of formation will be defined in connection

with the definition of the rate of reaction.

When the specific mass of the reaction mixture does not change (p is constant) the specific rate of formation of

component Ai of a Chemical reaction

(5.1)

is defined as

(5.2)

in which W is the catalyst weight (kg) and F is the molar flow rate {mol s-1 ). This. definition is related to there­

commended definition for the rate of reaction by the IUPAC

(1) i.e. the rate of increase of the extent of the reac­tion~ of the reaction given by equation (5.1).

And

(5.3)

The specific rate of reaction of a heterogeneous catalytic reaction is defined as

(5.4)

with t the reaction time (s). However the stoichiometric

48

coefficients of an overall reaction are not defined unam­

biguously like the coefficients -of an .elementary reaction

and therefore ~ and the rate of a non-elementary reaction

are not exactly defined either. For that reason we prefer

the definition of the rate of formation as given in equa­tion (5.2) which can be calculated from measured data i.e.

the concentration, the amount of catalyst and the molar flow rate.

Note: The dimension of the rate of formation, [mol2 kg-1

m- 3 s-1], seems very complicated. If we would sub­

stitute for W (kg catalyst) the number of active

sites, expressed in moles Ncr, then our reaction time

would have the dimension (S) again and the specific

rate of formation would revert to its normal dimen­

sion [r] = [mol m- 3 s-1]. As the determination of Ncr

is not yet unambiguous we prefer the somewhat cum­

bersome but straightforward quantity as defined in

the text. We will often use the terms "rate of for­

mation" and "rate of consumption" instead of "spe­

cific rate of formation" and "specific rate of con­

sumption".

5. 3. Factors ·govern~n·g the re·a:ctor beha:Yiour

The interpretation of the kinetic results is very much

facilitated if the fluid flow in the reactor can be consi­

dered to be an ideal plug flow and if the kinetic data are

obtained in a chemically controlled regime under isother­

mal conditions •

.5. 3 .1. Plug flow

Danckwerts (2) pointed out that if the P~clet-number

for mass transport in axial direction based on the reactor

length

PemaL = iiL > 60 e:Da (5.5)

49

than the flow pattern will approach ideal plug flow. In

our fixed bed reactors the particle Reynolds number Redp is small and varies from .1 to 1.2. Edwards and Richardson

(3) have shown that at low Redp-values the axial disper­

sion is only caused by molecular diffusion, thus Da = Dmol" The Peclet-number based on the particle diameter is

(5.6)

From the correlation given by equation (5.6) we calculate

the value of Pemadp in the order of .5. This means that at low Reynolds-numbers where equation (5.6) holds, it can be

deduced from equation (5.5) that axial mixing can be ne­

glected if ~ > 120. This value is in line with the value p

of 100 calculated by Carberry and Wendel (4) for both an adiabatic and a fixed bed reactor based on an one dimen­

sional model for a first order reac'tion and with the value

of 150 suggested by Finlayson (5) which is a more conser­

vative value because the significance of axial heat trans­

fer is taken into account. Mears (6) derived for non-first

order kinetics a plug flow criterion

:k...> d p

20 n Co ln Pemadp

(5.7)

with n is the order of the reaction and C0

and Ce are the

inlet and outlet concentrations respectively. This crite­

rion shows that axial dispersion becomes more severe with increasing conversion and increasing order of the reaction.

Radial temperature gradients as a result of an exothermic

reaction are attended by radial dispersion, which further

improves the flow pattern in the direction of plug flow.

This improved flow pattern is known as the Taylor dispersion (7). During our experiments in the flow reac­tors the condition L/dp > 120 generally was satisfied. Measurements with respect to the residence time distribu­

tion in the pulse reactor showed that even in that small

reactor the axial dispersion was almost absent.

50

Cpanneling and maldistribution in small reactors be­

cause of wall effects can be dominant. Schwartz and Smith

(8) and Schertz and Bischoff (9) reported for reactor dia­

meter to particle diameter D/dp ratios < 10 that point

velocities one particle diameter from the wall were 2 to

10 times as high as at the center line. A radial tempera­

ture gradient as a result of an exothermic reaction gives

the highest figures bec~use of the corresponding radial

viscosity gradient. Carberry (10) and Bamford and Tipper

(11) recommend D/dp values > 10 for laboratory reactors.

5 •. 3. 2. Temperature 51radients

The reactions of our investigations are moderately or

strongly exothermic as can be seen in table 5.1.

stoichiometric equation (-l:IH~)6Zf K kJ mol

c3H6 + o2 + c3H4o + H2o 353 (r 5.1)

c3H6 + NH 3 + 1% 0 2 + c3H3N + 3 H2o 513 (r 5.2)

c3H4o + NH3 + ~ 02 + C3H3N + 2 H20 160 (r 5.3)

NH3 + ~ 0 2 + ~ N2 + 1~ H20 328 (r 5.4)

c3H6 + 4~ 0 2 + 3 C02 + 3 H20 1924 (r 5. 5)

C3H40 + 3% 0 2 + 3 C02 + 2 H20 1571 (r 5.6)

c3H3N + 3~ 0 2 + 3 C02 + % N2 + 1~ H20 1739 (r 5.7)

Table 5.1. Reaction enthalpies of the main reactions at

673 K in kJ mol-l of the first reactant in the

stoichiometric equation.

Therefore large reactor diameters must be avoided to pre­

vent large radial temperature differences in a cooled

reactor. Consequently D/d ratios > 30 as suggested by ' p

Bamford and Tipper (11) are detrimental for the isothermal

51

behaviour of the fixed bed reactor. Because the enthalpies

of the various reactions are different the heat produced

at comparable conditions depends strongly on the integral selectivity of the reaction concerned and on the overall

reaction rate. From the experimental data we calculated at

a reactant inlet concentration of 1 mol m- 3 for the ammoxi­

dation of propene at 673 K an initial heat production of

1.1 kJ kg- 1 s- 1 and for the amrnoxidation of acrolein 5,5 -1 -1 kJ kg s • This heat production at stationary conditions

causes the occurrence of temperature gradients, which can be divided in three types i.e.

- an intraparticle gradient within the catalyst particle;

- an interphase temperature difference between the exter-

nal surface of the particle and the adjacent gas;

- an interparticle temperature difference between the ca-

talyst particles.

According to Mears (12) the interparticle gradient has the

greatest heat transport resistance, followed by the inter­

phase and the intraparticle gradients. During our experi­

ments the intraparticle gradient is absent, as was calcu­

lated for the ammoxidation of propene by means of Ander~

son's criterion (13). This criterion requires

< 3 RT Ea

(5.8)

for a deviation from the isothermal rate less than 5%. (~Hr) is the absolute value of the reaction enthalpy, r is

the observed rate of reaction, AP is the thermal conducti­

vity of the particle and Ea is the activation enthalpy of the reaction. This criterion is amply satisfied, even for

the total oxidation reactions. The criterion for the onset

of an interphase heat transport limitation, given by Mears . ( 12) is

(5.9)

52

with h is the gas-solid heat transfer coefficient. During

our experiment this criterion is easily met and thus in­

terphase heat transfer is not influencing the experimen­

tal results.

The conditions in our fixed bed reactors make that in­

terparticle heat transport limitations can be present. As

the axial heat transport in laboratory reactors is small

compared with the radial transport, both the effective

radial thermal conductivity {Aer) and the heat transfer at the reactor wall (aw) are important parameters. According to de Wasch and Froment (14) who derived a model for ra•

dial heat traasfer with these two parameters, ~ and A w er depend on the Reynolds number and on the D/dp ratio. It will be clear that low Re-values and large bed diameters

cause radial temperature differences in integral fixed bed

reactors. Isothermicity is favoured by catalyst dilution

with inert solids that have a good thermal conductivity

and by small reactor diameters.

5.3.3. Catalyst dilution

By means of dilution of the catalyst with inert parti­

cles isothermal conditions can be obtained in integral

reactors. Caldwell and Calderbank (15) described strate­

gies for optimizing reactor performance by varying the

dilution ratio with axial distance. They used a dilution

ratio which decreased linearly with conversion. Mears (6)

argues that dilution is advantageous in minimizing radial

temperature gradients only if the reactor operates at

Reynolds numbers sufficiently low, for the effective ther­

mal conductivity to be relatively insensitive to the mass velocity.

The influence of catalyst dilution may be of a chemical

or a physical nature. The influence of a diluting material

on the kinetics of a catalytic reaction have been esta­

blished by Mosely and Good (16) and by Nix and Weisz (17).

Dilution can further have an effect on the residence

time distribution (with respect to the catalyst) and on

53

the temperature distribution. Bypassing at a high dilu­

tion ratio gives a lower conversion. If the dilution and

the catalyst particles have different particle size dis­

tributions the bed void fraction is influenced. Van den Bleek et al (18) developed a stochastic model that descri­

bes the influence of the bypassing effects on conversion.

Their criterion to determine the allowable degree of dilu­

tion

L b -d > 250 7 p v

(5.10)

where b is the volumetric dilution ratio and o is the re­lative experimental error in the conversion, ensures that

the bypassing effect will be an order-of-magnitude smaller

than o. This model has the drawback not to distinguish be­tween reactions that are influenced by diffusion limita­

tions and reactions that are not.

During our experiments with the ammoxidation of acro­

lein we found some dilution effect i.e. a decrease of the

conversion at higher dilution ratios, whereas with the

oxidation and ammoxidation of propene this phenomenom was not observed. This difference could be explained if we

considered the diffusion limitation of the reaction rate of the acrolein ammoxidation. In this case van den Bleek's

dilution criterion appeared to be insensitive.

5.3.4. Mass and heat transfer

An estimate of the mass and heat transfer coefficients

is required to decide whether external transport limita­tions influence the chemical reaction rate. The mass and

heat transfer coefficients kg and h are based on the film··

theory with the assumption that mass and heat transfer are caused by diffusion and conduction through a stagnant gas

film around the catalyst particle. As the film thickness depends on the Reynolds number, kg and h are similarly

54

related to Re. The Chilton and Colburn j 0 and jH relations

(19) offer the possibility to determine kg and h.

The following relations are often applied:

Yoshida et al (20) found for .01 < Re < 50

j 0 = .84 Re -.51 (5.11)

whereas Gamson et al (21) correlated jH and j0

(5.12)

For the mass transfer from the bulk to the catalyst sur­

face the overall rate of reactions rA is related to the

concentration difference ([A]b-[A]s> of the reactant be­tween the bulk gas phase and the catalyst surface,

according to

r = A (5.13)

in which kg is the mass transfer coefficient (m s-1), Sis the specific surface area of the catalyst (m2 kg-1), tfJ. is

the sphericity factor.

The Chilton and Colburn relation is

(5.14)

in which CfA is the concentration factor (mol m- 3), GM is the superficial molar flow rate (mol m- 2 s-1), pis the

-1 -1 viscosity of the fluid (kg m s ), Pf is the fluid den-sity (kg m~3) and Dmol is the molecular diffusivity of the reactant in the fluid (m2 s-1). As we know the j 0-value

from equation (5.11) kg can be calculated from equation

(5.14) and after substitution in equation (5.13)

C[A]b-[A]8

) is found. For the ammoxidation of propene we calculated at the most favourable conditions for mass

transfer limitation a concentration difference for propene between the bulk gas phase and the catalyst of 2.38 10-2

55

-3 mol m , which is 1.8% of the applied propene concentra-

tion. We conclude that the mass transfer limitation for

all propene ammoxidation experiments can be neglected. As the ammoxidation of acrolein is a very fast reaction in

comparison with the ammoxidation of propene, mass transfer

limitation of the overall rate can occur. We calculated

again at favourable conditions for mass transfer limita­tion an acrolein concentration difference of 1.16 10-1 mol

m- 3 which was 15% of the applied concentration of acrolein.

In a similar way the temperature differenc.e between the

catalyst surface and the bulk gas phase can be calculated,

using the following equations:

(5.15)

in which h is the heat transfer coefficient (J m- 2 s-1 K-1)

and (Ts-Tb) is the temperature difference (K) and .. the Chilton and ·Colburn relation for heat transfer

(5.16)

in which c is the heat capacity at constant pressure (J kg-1 K-~), Af is the thermal conductivity (J m- 1 s-1

K- 1 ) and us is the superficial velocity (m s-1) of the

fluid. jH is calculated from the equations (5.11) and

(5.12) and substituted in equation (5.16) whereas (Ts-Tb) is found from equation (5.15) after substitution of h.

For the flow reactor experiments at the most unfavourable conditions for heat transfer we calculated a temperature

difference between the catalyst surface and the gas.phase

of .15 K. Because the temperature measurements in a pulse reactor are less accurate than in a differential flow

reactor we have calcul~ted the temperature difference between the catalyst surface and the gas phase during the

pulse and found (Ts-Tb) to below .06 K. For the thermo­balance we calculated in a similar way a temperature dif­ference at the steady state below .04 K. We conclude that

56

the temperature measurements of the gas phase represent

very well the temperature at the catalyst surface,

A quantitative approach to the intraparticle heat and

mass transfer has been defined according to Thiele (22) in

terms of an effectiveness factor n, the ratio of the ob­served rate to the rate that would exist if all the cata­

lyst surface were equally accessible~ The calculation of n

according to Weisz and Hicks (23) has shown that in all cases this. parameter has a value very close to one. This

has been verified experimentally with catalyst particles

of different sizes.

5. 3. 5. Pr·essu:re drop

According to Ergun (24) the pressure drop over a fixed

bed depends on the particle diameter d , the void fraction . p e, the viscosity of the fluid p and the mass velocity G. As we carried out the experiment at atmospheric pressure

the measurements at differential conditions (high mass

velocities) could give appreciable pressure differences.

However at the maximum attainable mass velocity in flow reactor B a pressure drop over the reactor of 8.10- 3 bar

was found, whereas in the other flow reactors this diffe­

rence was smaller. We conclude that the influence of the

pressure variation on the rate of reaction can be neglec­ted.

5. 4. Data: handling' a:nd a:na'lysis of errors

The analytical data obtained by means of GLC can easily

be converted to concentrations. From these values quanti­

ties as conversion, space time and selectivities are cal­

culated. As a complete analysis was carried out we used the additional information of the atombalances to improve the experimental data, according to the method given by

van der Grinten (25) and swenker (26). The results of the

raw concentration calculations are used in the adjustment

computing program. If the experimental errors are normally

57

concentration rel. error standard

mol m- 3 % deviation flow -1 5 mol s ,10

uncorr. corr,

Feed

02 4.1640 4.1640 0 0

NH3 1.9213 1.9213 0 0

C3H6 1.4673 1.4673 0 0

Product

02 2.6156 2.6016 3 .0913

N2 .1295 .1330 10 .0151

co .0000 .0000 3 .0000

C02 .1492 .1487 3 .0052

NH 3 .8613 .9102 7 .0702

C3H6 .7301 • 7125 5 .0425

H2o 2.4170 2. 4796 10 .2813

·C3H4o .0000 .0000 10 .0000

C3H3N .6069 .6024 5 .0353

c2H3N ,0623 ,0625 19 .0138

X C3H6

.502 .514

XNH .552 .526 3

5 IPA .824 .798

5 INA .573 .595

W/F = 15.37 kg s mol-l

T = 673 K

Table 5.2. Uncorrected and corrected concentrations of an

ammoxidation experiment.

58

distributed the most probable value of the experimental

error can be calculated by means of a minimalisation of

the sum of squares of deviations. The problem of the non­

constant error vari.ance was solved by the use of appro­

priate weight factors, calculated from independent esti­

mates of the experimental error variances as a function of

experimental data, In this way the corrected concentrations

of all components could be calculated and plotted as a

function of space time. An example of raw and corrected

data is given in table 5.2. The constants of the kinetic equations were calculated by linearization of the equation

or by means of the Newton Raphson iteration method. The

difference between the experimental and model values were

minimalized with the procedure Minifun (27).

References

1. Manual of Symbols and Terminology for physico chemical

Quantities and Units, Appendix II, Part II, Hetero­

geneous Catalysis, Adv. Catal. 26, 351 (1977)

2. Danckwerts, P.V., Chern. Eng. Sci.~~ 1 (1953)

3. Edwards, M,F,, Richardson, J,F., Chern. Eng. Sci. 23,

U9 (1968)

4. Carberry, J.J., Wendel, M.M., A.I.Ch.E. Journ. 2• 129

(1963)

5. Finlayson, B.A., Catal. Rev. 10, 69 (1974)

6. Mears, D.E., Ind. Eng. Chern. Proc. Des. Dev. !Q, 541

(1971)

7. Taylor, G., Proc. Roy. Soc. (London) A219, 186 (1953)

8. Schwartz, C.E., Smith, J.M., Ind. Eng. Chern. 45, 1209

(1953)

9. Schertz, w.w., Bischoff, K.B., A.I.Ch.E. Journ. 15,

597 (1969) 10. Carberry, J.J., Ind. Eng. Chern. 56 (11), 39 (1964)

59

11. Bamford, C.H., Tipper, C.F.H., Comprehensive Chemical

Kinetics Vol. I. The practice of kinetics, Elsevier,

New York (1969)

12. Mears, D.E., J. Catal. £Q_, 127 (1971)

13, Anderson, J.B., Chern. Eng. Sci, 18, 147 (1963)

14. De Wasch, A.P., Froment, G.F., Chern. Eng. Sci. 27, 567

(1972)

15. Caldwell, A.D., Calderbank, P.H., Brit. Chern, Eng. 14,

1199 (1969)

16. Mosely, R.B., Good, G.M., J, Catal. !• 85 (1965)

17. Nix, P.S., Weisz, P.B., J, Catal. _!, 179 (1964}

18. Van den Bleek, C.M., van der Wiele, K., van den Berg,

P.J., Chern. Eng. Sci. 24, 681 (1969)

19. Chilton, T.H., Colburn, A.P., Ind. Eng. Chern. 1183

(1934)

20. Yoshida, F., Ramaswami, D., Hougen, O,A., A.I.Ch.E.

Journ. ~' 5 (1962}

21. Gamson, B.W., Thodos, G., Hougen, O.A., Trans. Am. Inst. Chern. Engrs. 39, 1 (1943)

22. Thiele, E.W., Ind. Eng, Chern. 21, 916 (1936)

23. Weisz, P.B., Hicks, J.S., Chern. Eng. Sci. 12, 265 (1962)

25. Vander Grinten, P.M.E.M., De Ingenieur ~ (21), 070

(1971)

26. Swenker, A.G., De Ingenieur ~ (21), 065 (1971)

27. Lootsma, F,A., Thesis, Eindhoven (1970)

60

CHAPTER 6

RESULTS AND DISCUSSION

6.1. Introduction

The aim of the experimental work presented in this

chapter is to develop a kinetic model for the catalytic

ammoxidation of propene, in order to contribute to the

elucidation of the reaction mechanism.

It is of technological significance to determine the

reaction kinetics of the acrylonitrile synthesis under con­

ditions that are relevant for industrial circumstances. Therefore continuously operated fixed bed reactors are

used under differential as well as under integral steady­

state conditions.

In the literature it is commonly accepted that the acry­

lonitrile formation.mainly occurs by a direct ammoxidation

of propene and that the oxidation of. propene, followed by

the ammoxidation of acrolein is of minor importance. Less

attention has been paid to the kinetics of the oxidation

of ammonia. Therefore the kinetics of this combination of

parallel and consecutive reactions were studied in detail.

The experimental programme carried out with the fixed

bed reactors involved the following catalytic reactions:

a) the oxidation of propene to acrolein;

b) the oxidation of ammonia to nitrogen;

c) the ammoxidation of acrolein to acrylonitrile;

d) the ammoxidation of propene to acrylonitrile.

The kinetics of many oxidation reactions are described

by models based on the Mars-van Krevelen mechanism. Cor­

respondingly the elucidation of the reaction mechanism of

the propene ammoxidation can be advanced by the study of

the role of bismuth molybdate as a reactant (i.e. an oxi­dant).

61

Two types of experiments at non-stationary conditions

have been applied to obtain information about the beha­

viour of bismuth molybdate, i.e.

1. thermogravimetric experiments in a thermobalance in

combination with GLC-analysis1

2. pulse reactor experiments.

In the thermobalance reduction kinetics of bismuth mo­

lybdate were studied using propene, propene-air mixtures

and hydrogen as reducing agents. Subsequently we investi­

gated the kinetics of reoxidation of reduced molybdate

with different oxygen containing gas mixtures. Special

attention has been given to the adsorption of acrolein on

the reduced catalyst.

The pulse reactor is used in order to obtain more in­

sight in the role of oxygen of the solid and the oxygen in

the gas phase in the different reactions and in the ini­

tial activity of the catalyst under specific conditions.

6.2. Flow experiments

6.2.1. Introduction

In chapter 5 we have shown that the chosen types of

fixed bed reactors can be used to investigate the reaction

kinetics of the acrylonitrile synthesis reactions, pro­vided one satisfies certain conditions. For a reliable

study of the reaction kinetics, one has to work under iso­

thermal conditions. With our strongly exothermic reactions

this condition can only be attained by sufficient dilution of the catalyst. We have always used silicon carbide as

the diluting agent. The influence of this dilution has

been discussed in chapter 5 and will be dealt with in this chapter.

6.2.2. Preliminary experiments

- Catalyst stability and aativity The activity of the catalyst could be maintained at a

62

constant level during the entire experimental period

provided that the gas phase. is not depleted in oxygen.

Loss of activity is connected with a chemical desinte­

gration of the bismuth molybdate due to reduction and formation of metallic bismuth droplets. Since a reoxida­

tion carried out after a period produces the two oxides

Bi2o3 and Mo03 , the activity and selectivity of the ca­talyst is impaired. Experiments carried out with a low

oxygen content in a quartz glass reactor showed that the

catalyst changed in color from light-yellow in the upper

part of the reactor where oxygen is present to greenish­

grey at the exit side of the reactor where all oxygen

had been consumed. A change in the oxygen content of the gas inlet stream was followed by a reversible change in

the color of the catalyst, provided the reduction time

was not too long. During the thermobalance experiments

we observed the same phenomenon as a .function of time. Reoxidation of a catalyst that has been reduced too far

results in.a loss of activity and selectivity. Especially

the integral selectivity for ammonia to acrylonitrile

remains low after reoxidation of a catalyst that has been

reduced too far. As the catalyst has a measurable oxy­

gen pressure at reaction temperatures a small stream of

air is carried over the catalyst bed at night between

the experiments, whereas the temperature was kept at the

experimental level.

- External and internal mass and heat transfer resistances

With respect to film diffusion limitation preliminary

experiments under steady state conditions have shown

that varying the gas flow at constant mass space velo­

city does not change the degree of conversion. Also a

calculation of the mass transfer coefficient for the

ammoxidation of propene at high temperatures showed only a very small difference between the surface and bulk gas phase concentration of propene.

From a calculation of the temperature difference be­

tween the catalyst particle and the gas phase we draw

63

the conclusion that this temperature difference is very

small and that the temperature measured in the gas phase

represents the temperature of the catalyst surface.

The absence of pore diffusion limitation was confir­

med by varying the average catalyst particle diameter,

i.e. no change of the degree of conversion was observed.

Also a calculation of the effectiveness factors n for

the different catalyst particles at isothermic condition,

according to Satterfield and Sherwood( 1) gives values

close to unity.

Axial and radial temperature profiles under steady

state conditions have been discussed in chapters 3 and 5.

- Catalyst dilution

As has been shown in chapters 3 and 5 great care has

been given to the selection of dilution materials for

controlling the temperature of the catalyst bed, thus

ascertaining a flat temperature profile over the reactor.

Application of silicon carbide was found to give a

strong decrease in carbon dioxide formation~ We ascribe

this to a suppression of the postcatalytic oxidation of

acrolein which can be noticed in empty reactors and to

a much smaller extent in a reactor filled with silicon

carbide.

6.2.3. The oxidation of propene to acrolein

6.2.3.1. Experiments at 673 K

The experimental data for the determination of the ki­

netic parameters were obtained with flow reactor A (see

chapter 3, Table 3.1). This small reactor was chosen in

order to reduce the postcatalytic volume, and consequently

the non catalytic after reactions. The reactor was filled

with a mixture of 600 mg Bi2Mo1 •02o6 •06 prepared according

to method A and 1.7 g silicon carbide. The particle dia­

meter range of the catalyst and of the silicon carbide was .50-.85 mm. The bed beight was 53 mm.

64

The rate equation based on the power rate law

(6 .1)

can at low conversions be simplified into

(6.2)

This simplification is allowed because these experiments

were carried out at contact times at which the concentra­

tions of propene and oxygen hardly change. The orders with

respect to propene and oxygen can thus be calculated from

plots of ln rc H versus ln (C3H6)0

and ln (02)0

respec-3 6

tively. [c3H6] and other concentrations refer here and in

the following to concentrations at reaction conditions.

When varying the inlet concentration of propene from .43-2.40 mol m- 3 at a constant inlet concentration of oxy­

gen of 1.97 mol m- 3 we observed a change in the reaction

order with respect to propene. In the range of .43-1.31

ln r

t

-4

-5

r = dP - awr

-1.0 :·5 +.5

- ln (P) 0 Figure 6.1 Rate of oxidation of propene as a function of the prope­ne inlet concentra3ion.

· (0)0

= 1.97 mol m- ·

ln {1-Xp)

t 0

-.05

-.10

T = 673K -.15

-.20

2 6

__,.,. W/F

Figure 6.2 ln(l-X0

) as a f~nction of space time W[~ (Kg s mol- ). _3 (P) 0=1.31 mol m {0)

0=1.97 mol m

65

.20

t .15

.10

.05

mol m-3 we found a constant

propene order of one as can

be concluded from the figures

6.1 and 6.2. Figure 6.3 con­

firms that under our experi­

mental conditions the reactor

behaves differentia:lly. At

propene concentrations higher

than 1.4 mol m- 3 the order

with respect to propene

for the initial rate of pro-

pene conversion decreases

significantly as is shown in Figure 6. 3 Conversion of propene figure 6 • 4 (curve c) • At 2. 4 as a func!fon of space time W/F 3

- W/F

(kg s mol ) • mol m- we calculated an or-(P)0=1.31 mol m-3

(0) 0 =1.97 mol m~3 der of • 6 •

r

t r=-~ .06

.05

.04

.03

.02

.01

.5 1.0 1.5 2.0

T (K) 0 723 ... 698 a 673 v 665 0 648

2.5

- (P)

3.0

0 Figure 6.4 Rate of oxidation of propene as a func!~on of the propene inlet concentration. (0)

0=1.97 mol m

Between oxygen concentrations of .58 and 4.39 mol m-3

at a constant propene concentration of 1.31 mol m- 3 no

change in reaction rate is observed and the order in oxy-

66

gen is therefore zero for that concentration range. The

initial rate of acrolein formation at low propene inlet

concentrations is also first order with respect to pro­

pene.'

As the decrease of the order with respect to propene

points either to a strong adsorption of propene or to an

adsorption of products (e.g. acrolein or water) we inves­

tigated the influence of acrolein and water on the rate of

reaction. The partial pressures of acrolein and water in

the inlet gas stream are of the same order as obtained in

situ from the oxidation of propene. We observed a decrease

of the conversion of propene when acrolein was introduced

but water had no effect on the conversion, as is shown in

table 6.1.

Temperature (K) 673 648

Feed (mol m- 3 )

C3H6 1.310 1.310 1.310 1.341 1.341

02 1.970 1.970 1.970 1.985 1,985

c3H40 - .091 - - .042

H20 - - .061 - -Product (mol m- 3 }

C3H6 1.235 1.259 1.234 1.289 1.316

c3H4o .073 .136 .073 .050 .065

co2 .007 .012 .007 .003 .004

co .002 .004 .002 ,001 .002

c 2H40 trace .002 trace - -H20 .078 .064 .142 .048 .029

Conversion

C3H6 (%) 5.7 3,9 5.8 3.9 1.9

Space time W/F

(kg s mol-l) 1.68 1.68 1.68 2.70 2.70

Integral select!-

vity of propene to

· acrolein SIPACO .97 .93 .96 ,96 .92

Table 6.1. Influence of acrolein and water on the conver­

sion of propene.

67

We conclude that acrolein hinders its own formation at

temperatures of 673 and below. As is illustrated by figure

6.5 the major part of the carbon dioxide and the carbon

monoxide formation occurs by means of a consecutive oxi­

dation of acrolein.

(C)

t 1.35 C H .

1.30 c3H60 3 4

0 C02 1.25 . co 1.20 0 C2H40

(c) 1.15

1.10 .10 t .20 .08

.15 .06

.10 .04

.05 .02

---+- W/F Figure 6.5 Product concentration as _1 a function of space time W/F (kg s mol ).

6.2.3.2. Experiments at other temperatures

The rate of oxidation of propene at 698 K and 723 K is

first order with respect to propene and zero order in oxy­

gen over the range of investigated concentrations and does

not show a substantial inhibition by acrolein as is shown

in figure 6.4 and 6.6. The rates of oxidation at 665 and

648 K are first order in propene only for low propene con­

centrations. We confirmed that the decrease of the order

at the higher concentrations is also caused by acrolein

inhibition in the same way as described in the previous

section. The overall activation enthalpy for the oxidation

of propene to acrolein increases from 62 kJ mol-l at tem­

peratures above 673 K to 102 kJ mol-l at temperatures be­

low 673 K as depicted in figure 6.7.

68

ln r

t r=-~

-3

-4

T (Kl • 723 A 698 • 673 • 665

62 kJ mol-l

-3

-4 -5 • 648

103 kJ mol-l

-.5 +,5 1.0

- ln (P)0

Figure 6.6 Rate of oxidation of propene as a function of the prope­ne inlet concen!3ation. (0)

0=1.97 mol m

1.4 1.45 1.5 1.55

-1000/T

Figure 6.7 Arrhenius plot for the oxidation of propene to acrolein.

6.2.4. The oxidation of ammonia to nitrogen

Under our experimental conditions the non catalytic ·

oxidation of ammonia in the flow reactor B1 either empty

or filled with the dilution material silicon carbide, can

be neglected. The products of the catalytic oxidation are

exclusively nitrogen and water.

In order to determine the kinetic parameters of the ca­

talytic oxidation of ammonia flow reactor B (see chapter

3, table 3.1) was filled with a mixture of 7.5 g of the

catalyst Bi2Mo1 •02o6 •06 , prepared according to method A and 43 g silipon carbide. This mixture was held by two

sections of SiC, 35 g under and 95 g above the diluted

catalyst bed.

The rate equation based on the power rate law

d [NH3] = kNH [NH3]a [o2]b

d (W/F) 3 (6.3)

can be simplified to

69

( 6. 4)

for differential conditions. As is shown in figure 6.8 and

6.9 the reaction rate is first order in ammonia and zero

order in oxygen.

1 n r

t -2 r=-Wf

T = 673 K

-3

- ln (N) 0 Figure 6.8 Rate of oxidation of ammonia as a function of the ammo­nia inlet conce~3ration. {0)

0=4.20 mol m

ln r

L2 r=-Wf

T = 673K

-3

4 4-----.----.-----r----~ 1.0 1.5 2.0 .5

-ln (0}0

Figur: 6.9 Rate of oxidation of ammon1a as a function of the oxy­gen inlet conce~3ration. (N)

0=1.55 mol m

Experiments carried out at 648, 698 and 723 K showed

the same kinetic relations. For the activation enthalpy a value of 159 kJ mol-l was found.

6,2,5. The ammoxidation of acrolein to acrylonitrile

6.2.5.1. Preliminary experiments

In literature the ideas of Shelstad et al (2), viz.

that acrolein is a necessary intermediate for the forma­

tion.of acrylonitrile, made way for the view of Callahan

et al (3) and Cathala et al (4) that mainly oxygen free

intermediates are precursors for acrylonitrile and that

the slow formation of acrolein by a parallel oxidation

reaction is followed by a fast ammoxidation reaction.

70

--main

slow~ / fast

Experimentally we found the ammoxidation of acrolein to be

very fast.at 673 K, as can be seen in table 6.2.

k (mol kg -1 -1 s )

C3H6 + c3H40 .025

NH3

+ N2 .018

C3H6 + c3H3N .035

C3H40 + c3H3N .42

Table 6.2. Overall initial rate constants with respect to

the product at 673 K for the oxidation and

ammoxidation of propene, the oxidation of

ammonia and the ammoxidation of acrolein.

The main products besides acrylonitrile are water, carbon

dioxide and nitrogen, with only traces of other products.

Experiments at temperatures well above 673 K showed an

almost total conversion of acrolein even at the lowest

contact times attainable in the flow reactors. Experiments

at temperatures below 673 K gave severe blockage of the

product lines and frequent upsets of the analytical system

because of the presence of acrolein. So the ammoxidation

of acrolein has been studied at 673 K only.

As the reaction rate at 673 K is high, special attention

was paid to the possibility of diffusion limitation. The

experimental data that are used in this section are not

influenced by diffusion nor by dilution of the catalyst

bed by inertmaterial.

71

6.2.5.2. Experiments at 673 K

The flow experiments were carried out at very low con­

tact times in the fixed bed reactors B and C (see chapter

3, table 3.1). This was done in order to investigate

whether the quantity W/F .(catalyst weight over flow rate)

was an unambiguous reaction parameter.

Reactor B was filled with a mixture of 1.5 g Bi2Mo1 •02o6 •06 prepared according to method A and 25 g silicon carbide.

This section was fixed by two sections of silicon carbide,

35 g under and 95 g above the catalyst section. Reactor C

was filled with a mixture of 500 mg Bi2Mo1 •02o6 •06 and 7 g

silicon carbide with 4 g SiC under and 12 g SiC above the

catalyst section. In both reactors the gas flow was in the

downward direction. The particle diameter range of the ca­

talyst and of the silicon carbide was 1.0-1.2 mm. The space time ranges were for reactor B: .6-3.6 kg s mol-l

-1 and for reactor C: .6-6.0 kg s mol •

From figure 6.10 and figure 6.11 we conclude that the

(0)

t 3.2

2.8

2.4

2.0

1.6

2

-3 (AC0)0

(N)0

mol m • .70 2.2~ • ,98 1.96 • 1.00 1.71

T = 673 K

4 5 -W/F

Figure 6.10 Oxygen concentration as a func!fon of space time W/F (kg s mol ). Ammoxidation of acrolein.

72

( N)

t 3.0

2.6

2.2

1.8

1.4

1.0

-3 (AC0)0

(0)0

mol m • ,,90' 2.80 • .78 2.80 • .70 3.10 ~ .98 2.67

T=673K

4 5 -W/F

Figure 6.11 Ammonia concentration as a funcHon of space time W/F (kg s mol ). Ammoxidation of acrolein.

rate of reaction is zero order with respect to oxygen as

well as to ammonia.

Figure 6.12 illustrates that the reaction order with

respect to acrolein is not constant but decreases from a

value close to unity at acrolein inlet concentrations up to 1.0 mol m- 3 to about .4 at an inlet concentration of

-3 1,45 mol m •

The mean integral selectivity of acrolein towards acry­

lontrile was .93.

In figure 6,13 we see that the formation of carbon

dioxide is not a simple function of W/F, but is inversely

proportional to the quantity of catalyst used. This leads

us to the assumption that this carbon dioxide formation

occurs mainly by a non catalytic combustion of acrolein

in the post catalytic part of the reactor, either in the

interstitial gas phase or at the silicon carbide surface.

This could be confirmed quantitatively by a shift of the

catalyst bed to the lower part of the reactor. We observed

a decrease of the formation of carbon dioxide.

xco 2

W . F ( N ~ (AC0)0 kg mol s-1 XCU'o mol m-3

t A .. 5 .52 .09 1.9 2.4 9 .5 .76 2.0 2.3

ln r • 1.0 .87 .29 2.2 2.3

- .6

• 1.5 1.25 .60 2.0 2.3 • 1.5 .85 .45 2.1 2.3

.02 • 1.5 1.25 - .. 45 2.6 1.8 t T = 673K

- .8 T 673K

-1.0

.01

-1.2

-1.4

-.4 -.2 +,2 +,4 2 3 4 6

___,..ln (AC0)0

- W/F Figure 6.12 Rate of ammoxidation Figure 6,13 Mole fraction of of acrolein as a function of the carbon dioxide as a function of acrolein inlet s~ncentration, _3 space time W/F (kg s mol- ). (N)

0=2.24 mol m (0)

0=3.10 mol m Ammoxidation of acrolein.

73

As compared to the ammoxidation of propene the formation

of nitrogen is now notably suppressed. The integral selec­

tivity of ammonia towards nitrogen is .09-.32 at inlet

molar ratios of ammonia to acrolein of 1.3-3.7 mol mol-1 •

For the propene ammoxidation the integral ammonia selecti­

vity towards nitrogen is at comparable conditions .60-.75

(see figure 6.24).

6.2.6. The ammoxidation of propene to acrylonitrile

6.2.6.1. Introduction and preliminary experiments

All reactions investigated in the preceding sections

appear to some extent during the ammoxidation of propene.

However for a selective catalyst it is expected that it

will not accelerate

- the parallel oxidation reactions of propene and ammonia

to acrolein, carbon dioxide, nitrogen and other oxida­

tion products7 - the consecutive oxidation of acrylonitrile and acrolein

to carbon dioxide and other oxidation products.

It will be clear from the foregoing that besides the

main products acrylonitrile and water we will have a num­

ber of byproducts. Of these nitrogen, carbon dioxide,

acetonitrile, carbon monoxide, acrolein and acetaldehyde

are of significance and generally found in modest quanti­

ties in the reaction mixtures. At high conversions also

small quantities of ethene and of hydrogen cyanide have

been detected.

During the oxidation of propene the catalyst remains

active and selective for long periods of time, provided

gas phase oxygen is present. We calculated that more than

10 4 mol propene per kg catalyst were converted without

loss of activity and selectivity.

Preliminary experiments with water vapour in the feed

did not show a significant improvement of the integral

selectivity of propene towards acrylonitrile. As regards ammonia we observed some improvement of its integral se-

74

lectivity towards acrylonitrile. This means that water has

some inhibiting influence on the oxidation of ammonia to

nitrogen. However as we already operate with a feed con­

sisting of four components i.e. the reactants and helium,

we decided not to add water vapour as a fifth one because

of the increasing intricacy of the flow control and hand­

ling of the analytical operations. Isothermal conditions are maintained when the catalyst

is diluted with silicon carbide. The influence of heat

transport on the axial temperature profile can be mini­

mized if we place the catalyst bed in the middle of the

reactor. However reactions in the post catalytic volume

filled with silicon carbide may not be excluded a priori.

As is shown in figure 6.14, experiments with and without

ln(l-Xp)

t 0

-.1 .,. at the bottom o in the middle

-.2

-.3

-.4

-.5 T = 693 K

-.6

.02 .OLJ .06 .08 .10 .12

-tres Figure 6.14 ln(l-X ) as a function of residence timet (s). Influence of the pos~tion of the catalyst bed in the reattBr on the conversion of propene. Ammoxidation of propene.

a post catalytic volume at 693 K gave the same conversion

figures, showing that no noticeable post catalytic reac­

tions of propene take place in this reactor.

75

6.2.6.2. Experiments at 673 K

The experimental data were obtained with flow reactor

B (see chapter 3, table 3.1). The reaction conditions are

summarized in table 6.3.

[c3H6] .43 - 2.47 mol m -3

0 -3 (NH31 .78- 2.70 mol m

0 -3 [02] 2.58 - 4.51 mol m

0 -1 [c3H6/NH3] .21 - 2.05 mol mol 0

Pressure 1.0 bar

W/F 2.4 - 32.4 kg s mol -1

Table 6.3. Reaction conditions for the ammoxidation of

propene in flow reactor B at 673 K.

The catalysts prepared according to method A and B (see

chapter 4, section 4.3) did not show a difference in ac­

tivity and selectivity. The product concentrations as a

function of the contact time W/F, which are shown in fi­

gure 6.15, are representative for the kinetic experiments

of the propene ammoxidation at 673 K.

The initial rate of reaction is zero order with respect to oxygen, provided the oxygen is present in stoichiometric

excess (see figure 6.16). This also holds for the oxida­

tion of propene and ammonia and for the ammoxidation of

acrolein, as is illustrated in section 6.2.3 to 6.2.5.

With respect to ammonia we observed a small decrease of

the rate of reaction at increasing ammonia concentrations.

For the rate of propene conversion we calculated an ammo­

nia order of -.08 to -.14 at inlet propene concentrations

of .78-2.33 mol m- 3 • This is shown in figure 6.17. The

same influence of the ammonia concentration is found for the rate of acrylonitrile formation.

76

(C)

p.o

10 20

• C3H6 a NH3

A C-jl~ v 0)2 o N2 o c2H3N

T = 673 K

30 40 -W/F

{C02) (C)

.·4ot .2ot

.36 .18

.32 .16

.28 .14

.24 .12

.20 .10

.16 .08

.12 .06

.08 .04

.04 .02

Figure 6.15 Concentration~ 1of reactants_and products as a function of space time WL~ (kg s mol ). Ammoxidat1on of propene. (0) =4.51 mol m -3 o ln r

0 (P}

0mol m

r

f t • .78 ( ) : i:~ r o =- %£f o

T • 673K -2.0

.08

-2.5 .06

-3.0

-3.5 .02 T • 673 K

-4.0 +---~--.----r---r--~

1 .2 .4 .6 .8 1.0

- (0) - ln (N) Figure 6.16 Initial rate 8f am- Figure 6.17 Initial rates of Rm­moxidation of propene as a function moxidation of propene as a function of the oxygen c22centration. _3 of the ammonia i~let concentration. (P)

0=1.49 mol m (N)

0=1.55 mol m {0}

0=4.26 mol m

77

The initial rates of acrylonitrile formation and of

propene conversion at low propene inlet concentrations are

first order with respect to propene and decrease only -3 slightly at propene concentrations above 1.50 mol m

See figure 6.18 and figure 6.22 for 673 K. We also found

this for the oxidation of propene to acrolein.

As is shown in figure 6.19 the rate ofnitrogen forma­

tion is again first order with respect to ammonia, as has

been found already for the oxidation of ammonia in section

ln r t ·2.0

-2.5

-3.0

-3.5

-4.0

-1

ln r

r=~ t d{N2) r= ifi17r

-3,0

T=673K

-3.5

-4.0

0 +1. 0 .5 1.0

-ln (P)0

-ln (N)0

Figure 6.18 Rate of formation of Figure 6,19 Rate of formation of acrylonitrile as a function of the nitrogen as a function of the am-propene inlet co~centration. -3 monia inlet concentration. {N)

0=2.06 mol m- (0)

0=4.18 mol m · Ammoxidation of propene.

6,2.4, The ammonia conversion as a function of contact

time W/F shows a high initial conversion of ammonia to ni­

trogen, especially at high molar ratios of ammonia/propene, as is shown in figure 6.20.

The rate of carbon dioxide formation at initial condi­

tions is a function of the concentration of propene as is

shown in figure 6.21. A decrease of the ammonia concentra­

tion at constant propene concentration increases the rate

of carbon dioxide formation somewhat. As acrylonitrile is

absent at initial conditions we conclude that some oxida­

tion of propene to carbon dioxide takes place.

78

(N2) ro t r -C(co2)) .30 (<Nl/(Plb <Nl 0 t . 4.70 2.02

o- dW/F o . 2.78 2.18 .25 a 1.05 1.56 .010

.90 2.14 !.91

.52 1.20 . T = 673K 1.12

.20 T = 673 K

.95 . .15 .''n (<Pl/(N~0

.005 .71

. 10 . .36

.~1

.05

10 20 30 40 .5 1.0 1.5 2.0 2.5

-- W/F -- (P) 0 Figure 6.20 Concentration of ni- Figure 6.21 Initial rate of car­trogen as a fu~ction of space time bon diGxide formation as a function W/F (kg s mol- ) at different molar of the propene inlet concentration ratios {N/P) at the reactor inlet. at different molar ratios. Ammoxidation°of propene. Ammoxidation of propene.

6.2.6.3. Experiments at other temperatures

Besides the experiments at 673 K, experiments were

carried out at 648, 698 and 723 K in flow reactor B with

the same variation in the feed composition as at 673 K (see table 6.3). As can be seen in figure 6.22 the initial

rate of reaction at 698 and 723 K is first order with res­

pect to propene for all investigated propene concentra­

tions. At 673 K and even more at 648 K the initial rate of

reaction is only first order in propene at low propene

concentrations. The decrease of the order at these tempe­

ratures is caused by product inhibition in the same way as

described for the oxidation of propene.

The initial rate of reaction is zero order with respect

to oxygen and shows a slightly negative order in ammonia

for all temperatures.

79

ln r0

t ·1.5

·2.0

·2.5

·3.0

-3.5

-1

T <K)

• 723 • 698 4 673 • 648

0 +1

- ln (P) 0

ln k.ACN t ·2

-3

-4

1.35

97 kJ mol-l

1.45 1.55

-1000/T Figure 6.22 moxidation at as a function concentration

Rate of propene am­initial conditions of the prop~~e inlet (P)

0 (mol m ) •

Figure 6.23 Arrhenius plot for the ammoxidation of propene to acrylonitrile.

The overall activation enthalpy for the arnmoxidation of -1 propene increases from 60 kJ mol at temperatures above

673 K to 97 kJ mol-l at temperatures below 673 K as is

shown in figure 6.23. These values are

those for the oxidation of propene (62 -1 . vely 103 kJ mol , see also 6.2.3.2).

comparable with -1 kJ mol respecti-

6.2.6.4. Experiments at non-initial conditions

- ConvePsion of ppopene

80

As we have seen in figure 6.18 and 6.22 the initial

rate of acrylonitrile formation at 673 K shows a de­

creasing order with respect to propene if propene con­

centrations are higher than 1.50 mol m- 3 • Because of the

high fractional selectivity of propene towards acryloni­

trile the same conclusion holds for the initial rate of

propene conversion. At increasing space times however

the rate of propene conversion decreases even when the

initial rate shows a zero order dependency in propene.

This means that the decrease of the reaction rate at

increasing space times is mainly caused by product inhi­

bition.

The integral selectivities of propene and ammonia to-.

wards acrylonitrile, SIPA and SINA' as a function of contact time are shown in figure 6.24. We mention the

marked different values, e.specially at initial conditions,

t 1.0

.8

.6

.4

.2

((N)/ (P)) 0

v 4.70 0 2.78 II 1.31 I> .90 0 .52

.,._~~~~ } C H T "' 673 K 3 6

10 20 30 40 - W/F

Figure 6.24 Integral selectivities of ammonia and propene to acrrlonitrile as a function of space time W/F (kg s mol ).

and the slight decrease of SIPA and the increase of SINA at increasing contact times. The integral selectivity

of.ammonia towards acrylonitrile is a function of the

molar ratio propene/ammonia at the reactor inlet, where­

as the integral selectivity of propene towards acryloni­

trile is almost independent of this ratio.

81

ConvePaion of ammonia and foPmation of nitPogen

As is shown in figure 6.20 the relatively high ini­

tial rate of nitrogen formation at 673 K is followed by

a sharp decrease at increasing space times, until an

almost constant conversion level is attained. This level

depends on the molar ratio ammonia/propene. The

strong decrease of the rate of nitrogen formation at

increasing space times points to the inhibition of this

reaction by a product. As can be seen from figure 6.24

the integral selectivity of ammonia towards acryloni­

trile increases markedly at increasing space times.

- ConvePaion of oxygen

82

The rate of oxygen conversion at 673 K is almost con­

stant at increasing space times and a 'function of the

molar ratio propene/ammonia at the inlet of the reactor

as can be seen in figure 6.25. According to the stoichio-

(0) (P)o ( N} o (mol m-3}

ho D 2,38 2.1q

~ "' 2.30 1.20 0 .78 2.19

18 ... .78 1.10 T = 673 K

16

14

12

10

10. 20 30 40 -w;F

Figure 6.25 Oxygen con~!ntration as a function of space time W/F (kg s mol ). Ammoxidation of propene.

metric equations for the acrylonitrile and the nitrogen

formation the oxygen consumption is the same for both

reactions. This means that the rate of acrylonitrile

formation at non initial conditions is higher than the

rate of nitrogen formation as was experimentally con­

firmed (see table 6.2). Most probably not the same sites are involved in the acrylonitrile and the nitrogen production.

- Formation of carbon dioreide As we have seen in figure 6.21 the initial rate of

carbon dioxide formation at 673 K depends on the molar

ratio propene/ammonia. This is in agreement with the fact

that at initial conditions the value of one for the

fractional selectivity of propene to acrylonitrile is

ln r

t -4 r _ dACON

- OA7t

T • 673K -5

-6

-7

-2 -1 +1 +2

- ln (P) 0 Figure 6.26 Rate of acetonitrile formation as a function of the pro" pene inlet concentration (P)

0mol m-3•

Ammoxidation of propene.

never attained. At in­

creasing space times at

low propene concentra­

tions the carbon dioxide

formation is almost com­

plete!~ caused by the

consecutive oxidation of

acrylonitrile, althou~h

some direct propene oxi­

dation remains.

- Formation of acryLonitrile Although initially

the rate of acetonitrile

formation at 673 K is

very small, because of

the high fractional se­

lectivity of propene to

acrylonitrile, we

found a first order dependency with respect to propene

as can be seen in figure 6.26. At increasing space times

acetonitrile formation increases and we found a first

order dependency of the rate with respect to acryloni­trile.

83

- Other products

Under the conditions of our investigation into the

kinetics of the propene ammoxidation at 673 K products like carbon monoxide, ethene, acetaldehyde and hydrogen

cyanide are produced in such low quantities that con­

clusions about their rate of formation would be specula­

tive.

6.3. Thermobalance experiments

Catalyst reduction and propene oxidation.

6.3.1. Introduction

In a thermobalance the weight changes of a given amount

of catalyst under various reaction conditions can be stu­

died as a function of reaction time. The weight changes

may be caused by reduction (oxygen depletion of the cata­

lyst) and by adsorption of reactants and products.

As discussed in section 6.1 and 6.2 the kinetics of the

different reactions present a rather complicated picture.

The results as regards the influence of the concentrations are summarized in table 6.4.

Order with respect to Reaction

C3H6 NH3 c3H40 02

* C3H6 -+- c3H40 1-0 - - 0 * c3H40 -+- c3H3N - 0 1-0 0

* C3H6 -+- c3H3N 1-0 0 - 0 NH3 -+- N2 - 1 - 0

* depends on temperature and concentration

of propene or acrolein respectively

Table 6,4. Orders of the initial rates of various reactions.

84

Under similar conditions the rates of the oxidation of

propene, of the oxidation of ammonia, of the ammoxidation

of propene and of the ammoxidation of acrolein increase in this order, as was shown in table 6.2.

Below 673 K acrolein and acrylonitrile hinder the oxi­

dation and ammoxidation of propene. Oxygen diffusion in the catalyst lattice to the reactive sites is also expec­ted to influence the overall rate of reaction.

6.3.2. Preliminary experiments

For all thermobalance experiments a catalyst with a specific surface of 7.3 m2 g-1 was used, which was obtained

according to method B, but with a calcination temperature

of 753 K. Neither changes in gas flow rate (from 1-70 to 250 cm3

min-1 NTP) nor changes in catalyst.quantity (from 45 to 125 mg) nor changes in particle size (from .s to 1.4 mm)

had any influence on the rate of reduction of the cata­

lyst. Calculations carried out according to the procedure of

Yoshida et al (5) showed that the tE;!mperature difference

across the boundary layer between the particle and the

gas phase can be neglected, so the temperature measured

above the sample is equal to the temperature of the parti­cle.

6.3.3. Reduction of the catalyst with propene

In table 6.5 the reaction conditions are given for the reduction of bismuth molybdate with mixtures of propene and nitrogen.

Figure 6.27 shows for a number of temperatures the weight of a sample as a function of time for the first 400

seconds of exposure to diluted propene (mole fraction xp = .085). More prolonged exposure causes the weight to

increase again, due to acrolein adsorption (see this chapter, section 6.3.6). In section 6.4.3 we will.offer

85

Gas flow rate

Amount of catalyst Particle size

Reaction temperature

Total pressure

210 cm3 min-1 NTP propene/nitrogen

75 mg

.50-.71 mm

648, 663, 673, 683, 693, 703 K

1 bar

Propene mole fractions: .059; .085; .145; .215

Table 6.5. Reaction conditions for the reduction of bis­

muth molybdate with propene.

w {mg) T (Kl .r

t • 643 t zoo HKl dW 6 703 r=- at • 693 • 683

75.0 v 673 150 • 648

100

50

74.0

1?1 300 400 .05 .10 .15 .zo .25

- t - {xp) 0 Figure 6.27 Weight of a catalyst Figure 6.28 Initial rate of weight sa~ple as a function of time(s). loss as a function of the mole frac-Thermobalance. (xp)

0=.085 tion of propene, Thermobalance.

arguments to show that on slightly reduced catalysts the

rate of acrolein adsorption is slow as compared to the

rate of catalyst reduction.

Figure 6.28 shows the initiat rate of weight loss as a

function of the propene mole fraction at different tempe-dW -1 -1 ratures. The rate of weight loss,- dt (mg kg s ), is

defined as the net change of weight (mg) of one kilogramme

of catalyst per second. The figure shows that up to mole

fractions of propene of about .10 the rate of weight loss

is first order in propene, but becomes zero order at pro­pene mole fractions above .20. It is reasonable to assume

86

that at propene mole fractions above .20 all surface sites

are engaged. The zero order cannot be ascribed to a limi­

tation due to oxygen transport as the energy of activation

of the catalyst reduction by propene is much lower than

that found for the oxygen diffusion, as we will show in

this section. If we use the surface density for the adsorp­

tion sites as given by Matsuura et al (6} the weight in­

crease for full propene coverage would be 1.6 ~g/75 mg

catalyst sample. The switching in of propene is accompa­

nied by some slight disturbancies in the weight indication,

that are of the same order of magnitude as this value.

We find, as shown in figure 6.29 that at initial con­

ditions and xp = .085 the energy of activation increases

ln r

t 5.0

3.

dW r = -at

1.50

• •=.00

1. 55

- 1000/T

3.0

2.0

Figure 6.29 Arrhenius plot for the rate of weight loss at·a·= 0 and a =.02. Reduction of bismuth molybdate with propene. Thermobalance. (xp)

0 =.085

from below 70 kJ mol-l at temperatures above 673 K to -1 104 kJ mol at temperatures below 673 K. This change is

also observed for the other propene mole fractions ~ .21.

At a degree of reduction ex = • 02 we find that the overall activation energy has increased to 158 kJ mol-l as is

87

shown by the curve for a = .02 in figure 6.29. The degree

of reduction a is defined as the fraction of the total

oxygen {six atoms per molecule bismuth molybdate) -that has

been removed i.e.

{6. 5)

This means that now another reaction step determines the

rate of reduction. We assume this to be the diffusion of

oxygen ions from the bulk to the surface of the catalyst

to reoxidize the reduced surface sites. If the diffusion

would be very fast the surface oxygen concentration would

be proportional to the bulk oxygen concentration and the

rate of reduction would be

dO - dt k 0 [Pj {6.6)

where 0 is the bulk concentration of oxygen. For constant

[P] we obtain from equation {6.6)

- ln

with k' = tion {6. 7)

0 k't 0 0

k [P] and t is

does not fit

the

the

{6.7)

reaction time. However equa­

observed rate of weight loss.

We conclude therefore that the oxygen diffusion influences the observed rate of reaction.

A simplified model for such a combined surface oxidation

reaction and oxygen diffusion has been given by Batist et

al {7). They arrive after certain simplifications to the relation

a = -A + Bt~

that is valid for intermediate values of a and t. In equation {6.8) A and B are

88

{ 6. 8)

B 2 S p D~ -~

'!1'; s

(6.9)

(6.10)

with S is the specific surface area, p the density of the

catalyst, D the diffusion coefficient of oxygen ions in

the lattice, k the rate constant of the chemical reaction, X a socalled jump distance of the order of the lattice

constant and t the reaction time. Although the more gene­

ral model of Steenhof de Jong (8) is based on the assump­

tion that the chemical reaction step and the diffusion

step are not necessarily equal in rate and that the solid

is not a semi-infinite flat plate, the resulting equation

is rather difficult to handle and we decided to apply

Batist's relation. As is shown in figure 6.30 relation

t a

.OS

.04

.03

.02

.01

.00

-.01

T • 673 K Xp

•. 146 •• 081 •• 059

-.02 +--~--....---,--..-----., 10 15 20 25

- vt Figure 6.30 Degree of reduction as a function of~ Thermobalance.

(6.8) holds at 673 K and at different mole fractions of

propene for .01 < a. < .045 and 6 <It < 15 s~. Nearly the

same ranges are valid at the other temperatures. The de-

89

viation at the upper limit is caused by the adsorption of

acrolein but the values of A and B are still clearly de­

fined. As we already know the value of the rate constant of

the chemical reaction on the catalyst surface and its

energy of activation we can use both the expression 6.9

and 6.10 to calculate the diffusion coefficient, its ener­

gy of activation and the jump distance. For the diffusion

coefficient at 673 K and xp = .085 we found 3.3 l0-17 m2

s-1 . This value is comparable with that given by Batist

et al (7) for a less active catalyst at 723 K viz. 2.4 10-17 m2 s-1.

We calculate an activation energy from B of 168 kJ mol-l

and from A of 164 kJ mol-1 , assuming a jump distance A that is not dependent on the temperature. For the mean jump distance A we found a value of 3.8 l0-10 m for the

temperatures between 673 and 703 K. From the crystal struc­

ture data determined by van den Elzen et al (9) (see fi­

gure 4.1) we calculated a distance between two apex oxygen

ions of 3.64 10-10 m, which is in good agreement with the

value of the jump distance calculated from the experiments.

The energy of activation for the diffusion found in this

way fits very well with the results from the rates of re­

duction at a= .02 viz. a value of 158 kJ mol-1 • We there­

fore conclude that even at a low degree of reduction the

diffusion of oxygen in the bulk has a major influence on

the overall rate of reduction of the catalyst by propene

only. The above confirms that here the model of Batist et

al describes the process accurately.

6.3.4. Reduction of the catalyst with hxdrogen

From experiments carried out with propene, artificial

air and water in the fixed bed reactor we found that water

does not inhibit the formation of acrolein (see table

6.1). This means that also during the reduction of the

catalyst in the thermobalance inhibition by water is pro­bably absent. The reduction of the catalyst with hydrogen

90

would therefore offer the possibility to investigate the

reactivity of the catalyst without the adsorption of acro­

lein.

In table 6.7 the reaction conditions are given.

Gas flow rate 210 cm3 min-1 NTP hydrogen/ni­

trogen

Amount of catalyst

Particle size

Reaction temperature

Total pressure

Hydrogen mole fraction

75 mg

.50-.71 mm

673, 703, 723 K

1 bar

.055~ .073~ .102

Table 6.7. Reaction conditions for the reduction of bis­

muth molybdate with hydrogen.

In figure 6.31 the influence of the hydrogen mole frac­

tion on the initial rate of reduction at 703 K is shown

and we conclude that the initial rate of reduction is

first order with respect to hydrogen. The reduction rate

ln r . t -0.5

-1.0

-1.5

_ dO r-- dt'

T = 703K

.OS

.03

.~II/ .Olr

T <Kl 0 723 6 703 • 673

(XHJ0=,073

q -2.5 -2.0 500 1000 1500 2000

-ln xH -t Figure 6.31 Initial rate of re- Figure 6.32 Degree of reduction duction (mmol kg-_s- } as a function as a function of time (s). Reduc;.. of the mole fraction of hydrogen. tion of bismuth molybdate with Thermobalance. · hydrogen. Thermobalance.

91

with hydrogen is much lower than with propene, as follows

from table 6.8.

dO -I -1 673 K 703 K -at {mmol kg s )

H2 .19 .37

C3H6 2.4 5.4

Table 6.8. Initial rates of reduction of bismuth molyb­

date with hydrogen and propene.

XH = Xp = .073.

At 703 and 723 K the degree of reduction is a linear

function of time for the reduction range studied. At 673 K

this linear relationship is somewhat affected as is illu­

strated in figure 6.32.

For the rate constant in the initial rate equation

(-~~) 0

(6.11)

we find an energy of activation of 85 kJ mol-l, as follows

from figure 6.33 (straight line). This value agrees with . -I

the value of 8I kJ mol given by Beres .et al {10). The

little curvature of the line in figure 6.32 for the re­

duction at lower temperatures is ascribed to influences of the oxygen diffusion on the rate of reduction at a > 0.

As the mean overall energy of activation for the diffu--1 sion (E0 = 163 kJ mol ) is very much larger than the

energy of activation for the reduction with hydrogen the

influence of the diffusion will be felt only at tempera­

tures below 673 K. This is illustrated in figure 6.33 by the dotted curve.

92

-1

-2

dO r=- df

1.35

(.l

• .00 • .02

(XHJ0=,073

'\ \ \

~

1.40 1.45 1.50

-1000/T

Figure 6.33 Arrhenius plot for the rate of reduction of bismuth molybdate with hydrogen. (xH)o = .073. Thermo­balance.

6.3.5. Reoxidation of a reduced catalyst

Reoxidation experiments were carried out under the

following conditions.

Ga_s flow rate

Amount of catalyst (before reduction)

210 cm3 min-I NTP oxygen/nitroger

75 mg

Reducing agent propene (8 mole % in helium)

Degree of reduction a max •• 08

Reoxidation temperature: 673, 693, 703 K

Mole fraction of oxygen: .07-.21

Total pressure 1 bar

Table 6.9. Reaction conditions for the reoxidation of re­

duced bismuth molybdate with oxygen.

93

The reoxidation of reduced bismuth molybdate is an ex­tremely fast reaction in comparison with the reduction

reaction. At 673 K and a = .073 the initial rate of reoxi­-1 -1 dation with air (x0 = .21) is 165 mmol 0 kg s , where-

2 as the maximum initial rate of reduction is only about 4 mmol 0 kg-1 s-1 •

The initial rate of reoxidation is first order with res­

pect to oxygen as can be seen in figure 6.34. The rate de-

_ dO ln r

T (K) r-- (ff

t -1 " 703 • 673

·2 01 .081

·3

.q

-5

·5 .q ·3 -2 -1

-ln x 0

Figure 6.34 Initial rate of reoxidation (mmol kg-1 s-1} of reduced bismuth molybdate (a. = .081) as a function of the mole fraction of gas phase oxygen. Thermobalance.

pends on the degree of reduction. We conclude that at

these temperatures oxygen diffusion limitation is absent.

The calculated energy of activation is 64 kJ mol-1 • This value agrees with the value of 70 kJ mol-l given by Mat­

suura and Schuit (1).

6.3.6. The reduction of the catalyst with mixtures of pro­

pene, nitrogen and small quantities of oxx~en

The experiments were carried out at 673 K and 703 K

with gas mixtures having a constant mole fraction of pro­

pene (xp = .08) and mole fractions of oxygen between .006

and .14.

94

r

t

6

4

2

(xo)o ~ ,140

dO •• 024 •• 012

r=Of v,006 T~673K

1200 2400 3600 lj8IJO 6000

-t Figure 6.35 Rate of oxygen consumption (mmol kg -ls -l) .as a function of time (s). (xp}

0=.08 Thermobalance.

In figure 6.35 the rate of oxygen consumption from the gas phase and the catalyst at 673 K is given as a function

of time for the different gas mixtures. These oxygen con­sumption data have been calculated from the chemical ana­

lysis of the feed and product stream. The initial rate of oxygen consumption is a function of the mole fraction of

oxygen in the gas phase, if x0 < .026.

We assume that under reducing conditions i.e. with a low oxygen partial pressure the number of active sites

decreases with the decrease in oxygen partial pressure (see also the pulse experiments, section 6.4}. With the

supply of gas phase oxygen the catalyst surface is acti­vated and consequently the initial rate of oxygen con­sumption increases. However with substoichiometric quan­

tities of oxygen a steady state is reached, whereby the

catalyst ends up in a somewhat reduced state. This was also concluded by van Oeffelen (12) from conductivity measurements.

From figure 6.35 we see that after about 30 minutes a

stationary condition is attained and that the rate of

oxygen consumption at the steady state depends on the mole fraction of oxygen in the gas phase.

95

After a reaction period of about 90 minutes the thermo­

balance was flushed with nitrogen for 18 hours at 673 K.

Mass spectrometric analysis showed that acrolein is the

only desorbing product. This means that during the flush­

ing period no further reduction of the catalyst takes

place. Subsequently the catalyst was reoxidized to the origi­

nal weight with artificial air, containing 5 mole % o2 and

95 mole % He. As we did not detect carbon dioxide during

the reoxidation we conclude that after the flushing ex­

periment no carbon deposit or carbon containing products

were present at the surface.

The experimental data are summarized in table 6.10.

weight increase weight decrease weight increase mole {mg/g} after 1~ (mg/g) after 18 {mg/g) after frac- hour reaction hour flushing reoxidation tion

02 673 K 703 K 673 K 703 K 673 K 703 K

* * * .006 2.47 2.93 3.24 5.50 .77 2.56 .012$ 1. 74 1. 31$ 2.29 3.27 • 55 1.96

.024 .77 .33 1.02 .80 .25 .47

.140 0 0 0 0 0 0

* A maximum weight decrease of .25 mg/g at 673 K and of 2.56 mg/g at 703 K was noticed after 5 minutes.

$ A maximum weight decrease of .28 mg/g at 703 K was noticed after 5 minutes.

Table 6.10. Changes of catalyst weight (mg/g) during pro­

pene oxidation, flushing and reoxidation at

673 K and 703 K and at xc H = .oa. 3 6

From the legend of table 6.10 it will be clear that after

an initial weight decrease during the first 5 minutes of

the experiment the weight increases and after a time a

96

net increase in weight results, caused by the adsorption

of the heavier acrolein molecules. The ultimate quantity

of acrolein adsorbed represents a steady state that is

i.a. determined by the degree of reduction of the catalyst

and by the rate of acrolein production. Without oxygen in

the gas phase the net weight decrease after 5 minutes was 6.5 mg kg-l at 673 K and 13.3 mg kg-1 at 703 K, as can be

calculated from figure 6.27.

As is shown in figure 6.36 the amount of acrolein ad­

sorbed under stationary conditions is proportional to the

degree of reduction a.

(ACO} .· a t 6

3

2

T (Kl . 673 . 703

.005 .01 .015 .• 02 .025 -a

Figure 6.36 Acrolein adsorbed (g kg-1) as a function of the degree of reduction a at stationary condi­tions. Thermobalance.

dO r=- at

ln r

t 2

T = 673K

0

-5 -4 -3 -2 -1

- ln x0 Figure 6.37 Rat!1of1oxygen con­sumption (mmol kg s ) as a func­tion of the mole fraction of oxygen in the gas phase. (xp)

0=.08

Thermobalance.

In figure 6.37 the rate of oxygen consumption under

stationary conditions is plotted as a function of the mole fraction of oxygen in the gas phase. The rate of oxygen consumption is first order with respect to oxygen provided

x0 < .024 and the order decreases to zero when x0 > .08.

From figure 6.27 it can be calculated that the initial

rate of weight loss at 673 K and xp = .085 without gas phase oxygen is 3.9 10-5 s-1 • From the chemical analysis

it follows that at initial conditions the selectivity of

97

propene towards acrolein is .96 and that the other product

is carbon dioxide (see also the flow experiments, section

6.2.3.1, table 6.1 and the pulse experiments, section

6,4.3, figure 6,40).

The initial rate of acrolein formation was calculated

from these data, according to the stoichiometric equations.

(r 6. 1)

(r 6.2)

and a value of 6.3 10-5 s-1 was obtained. From the flow

experiments with gas phase oxygen and the same propene

mole fraction the initial rates of acrolein formations

were calculated (see figux-e 6.35 and table 6.11).

xo (\~co) 0

s -1 10 5

.140 16.9

.024 15.2

• 012 10.7

.006 7.0

Table 6.11. Initial rates of acrolein formation as a

function of x0 at 673 K and xp = .08.

One can conclude from these figures that the initial rate

of acrolein formation in the absence of gas phase oxygen

is smaller than in its presence. From a calculation of the number of acrolein molecules

adsorbed per m2 after the experiment at 703 K with x0 =

,006, i.e. 8.5 1018 m-2 and the area of one acrolein mole­

cule (5 10-20 m2 ) it can be concluded that the reduced

catalyst surface is covered with 40 percent of a monolayer

of acrolein molecules.

98

From the data of table 6.10 and of figure 6.35 it can

be concluded that the amount of adsorbed acrolein is a

linear function of the rate of oxygen consumption under stationary conditions, as is shown in figure 6.38 for

673 K.

r

t 8

T '" 673 K

6

4

2

1 2 4

-(AGO) a

Figure 6.38 Rate of oxygen consumption (mmol kg-1 s-1) as a function of the amount of adsorbed ac~olein (g kg-1) at stationary conditions.. (xp}o = .oa, Thermobalance.

If only acrolein is desorbed by flushing the thermo­balance, as was experimentally confirmed by means of mass

spectroscopy and the weight increase by oxidation is only caused by oxygen addition, then at 673 K about .85 oxygen

atom replaces one acrolein molecule and at 703 K we have

about 1.9 oxygen atom per acrolein molecule. Both figures are independent of the mole fraction of oxygen during the

reaction with propene. As one oxygen atom has an area of

10-19 m2 the amount of oxygen atoms in a closed monolayer packing is 8.3 10-6 mol m- 2• From table 6.10, column 6,

the amount of oxygen used for the reoxidation of the cata­lyst after reduction and flushing at 673 K is 7 10-6 mol

m-2 • This means that at that temperature almost all the

surface oxygen atoms were removed. At 703 K also bulk oxygen must be removed.

99

From this experiment we conclude that the catalyst is

in a partly reduced state during the catalytic oxidation

of propene with substoichiometric quantities of oxygen,

even at a low propene conversion level.

6.4. Pulse experiments

6.4.1. Introduction

As we have seen the interpretation of the thermobalance

experiments was hampered by the fact that acrolein is ad­

sorbed on the reduced catalyst. However if we accounted

for this adsorption the experimental observations showed

themseives to be useful for the development of a reaction

model for the oxidation of propene. We decided to check

the conclusions by an investigation in a pulse reactor.

With this type of reactor we expected to obtain additional

information about the oxidation and ammoxidation reaction

under initial conditions.

However a pulse reactor is not a very appropriate de­

vice for a kinetic investigation for the following reasons:

1. For kinetic work the concentrations and the reaction

time must be known. But even with an ideal a-function

form of the input pulse the length of the pulse in the

reactor and the concentration distribution within the

pulse are rather difficult to assess;

2. Even when the information required sub 1 is available,

it is possible that the adsorption of the reactants is

not the rate determining step. If a fast adsorption

step is followed by a reaction step over a period which

can be longer than the residence time, the calculated

apparent reaction rate based on the contact time of the

pulse will be too high.

Therefore we checked the value of the residence time as

calculated from the inert gas flow by an experimental de­

termination of the pulse shape.

100

6.4.2. Preliminary experiments

The experimental data for the determination of the re­

sidence time distripution were obtained with reactor D (diameter 5 mm, length 14.6 mm) filled with 226 mg quartz

glass as inert material. The particle diameter amounted to

.3-.5 mm. The operating temperature was 673 K, the pres­

sure 2.5 bar. A sample of .155 cm3 NTP (6.45 ].!mol) of a gas mixture of propene and helium was pulsed in the helium

carrier gas flow (.3 cm3 s-1 NTP) by means of a sampling

valve S which is switched pneumatically. The response

signal was measured at three different places, i.e. at the sampling valve outlet (A), at the reactor inlet (B) and at

the reactor outlet {C) (see figure 6.39).

B

A

c PULSLOOP

ANALYSIS

Figure 6.39 Reactor system for the determination of the residence time of a pulse in the reactor. (A, B and C see text)

In order to minimize the contribution of the measuring in­strument to the overall residence time, we used a small

flame ionization detector and n:ot a more bulky thermal

conductivity cell. Consequently the residence time of an

ammonia pulse in the reactor could not be determined. The

flame ionization detector was directly connected to one of the places A, B or C.

101

The increase of the pulse width is mainly caused hy the

axial diffusion in the line between the valve and the reac­

tor and to a much smaller extent by the reactor itself. In particular valve V (see figure 6.39) and its connection to

the lines is responsible for a large increase of the re­

sidence time.

As can be seen in table 6.12 the mean residence time

calculated at half height is about 6.6 s and independent

of the mole fraction of propene at the inlet of the flow

system, Consequently the gas mixtures will be more diluted

with helium at the reactor inlet.

p C3H6

inlet system (bar) .105 .298 .502

p C3H6

inlet reactor (bar) .025 .059 .086

crB 2 (s2) 5.04 7~63 8.63

ac 2 (s2) 7.04 8.49 8.01

tres (s) 6.2 6.7 6.8

Table 6,12. Data from the determination of the residence

time of a pulse in the reactor at 673 K.

Total pressure 2.5 bar abs.

The adsorption of reactants and products during a pulse

could be determined by means of the separate analysis of

the pulse and of the adsorbed products originating from

that pulse. To do that the reactor was closed by valve V

(see figure 6.39) after passage of the first pulse into

the gas chromatograph. The adsorbed products were given

time to desorb into the reactor gas phase and thereafter

flushed into the gas chromatograph. In this way we obtai­

ned good carbon and nitrogen mass balances as can be seen from table 6.13.

From this table it follows that about 60% of the ammonia

remains adsorbed on the catalyst, As the product strip

102

c N

mmol kg -1 mmol kg -1

feed pulse 128.7 21.6

product pulse 15.6 5.4

unconverted pulse 110.0 3.5

pr<:>dU:ct strip 1.5 13.1 --- ---127.1 22.0

balance % 98.7 101.6

Table 6.13. Carbon and nitrogen mass balances for the re­

action of a pulse containing 67 mole % c3a6 and 33 mole % NH3•

contains mainly N2 as a nitrogen containing product we

conclude that ammonia is easily oxidized by oxygen from

the catalyst. The quantity of ammonia adsorbed is equiva­

lent to the number of surface Bi-sites if we use Matsu~ra's

data. This experiment further shows that of the first pul­

se very little carbon containing material is adsorbed. We

decided not to reoxidiz~ the catalyst sample after each

series of pulses but to take a new quantity of catalyst prepared according to method A, for every new series.

6.4.3. Experiments with propene-helium mixtures

The experiments were carried out at 673 K in reactor D,

with 100 mg catalyst. As is shown in figure 6.40 for pure

propene in the sampling valve, the propene conversion de­

creases from 8.3% at the first pulse to 5.0% at the tenth

puls~ with a sharp decrease from the first to the second

pulse. As from the second pulse onward the decay in con­version per pulse is small and constant, this decay is

ascribed to the oxygen loss of the catalyst.

103

0cat

t 12 ,.....,_ '" oC~qO ...... ..c

10 r\.__ oOCAT. .. •XC3"o

8 \ "'·._ T • 673 K

~.. ... " .....

6

4

2

.. .... _ ..............

.. '~ .... ~-.. ~ .... ....__ r-··1 / ~

2 4 6 8 10 12 14 16 18 -n p

Xp t

.10

.08

.06

.04

.02

Figure 6.40 Formation of acrolein· and_yarbon dioxide! consumption of catalyst oxygen (mmol kg } and convers1on of propene per pulse vs. pulse number np.

The rather great decrease in activity between the first

and the second pulse under almost all conditions makes it

reasonable to suppose that during the first pulse a number

of active sites are converted to less active and less se­

lective sites. The active sites are very selective in the

first pulse as can be seen in figure 6.40 from the low

carbon dioxide production in the first pulse.

From figure 6.40 we see that in the first pulse about

12 mmol oxygen/kg catalyst is removed. From the carbon

balance over that pulse we calculate that almost no acro­lein (less than 1 percent of the reduced sites) can be

adsorbed on these sites. This confirms again the slow ad­

sorption of carbon containing products on a catalyst that

is only reduced to a very small extent and that is only

for a short period exposed to acrolein. All the lines of

figure 6.40 bisect the abcissa at about 32 pulses, which

corresponds to a degree of reduction a = .02. This is in agreement with the thermobalance experiments. Due to the

increase in pulse width, partial pressures of propene

> .12 bar could not be attained with this pulse reactor

system. Consequently the zero-order dependency with res-

104

pect to propene of the rate of

in the thermobalance for Pc H

oxygen consumption as found

> .14 bar could only be 3 6

approached but not. obtained. The experimental data indi-

cate however that at a propene partial pressure of .115

almost full coverage of the active sites is attained, as

can be seen in table 6.14.

c3H6 partial pressures (bar) ocat-consumption 1st pulse

inlet system inlet reactor mmol 0 kg-1

.084 .021 4.0

.150 .035 6.9

.215 .047 7.3

.334 .066 9.5

.505 .087 11.1

1.000 .115 ll.5

Table 6.14 •. Catalyst reduction by propene/helium pulses as a function of the partial pressure of propene

at 673 K.

6.4.4. Experiments with mixtures of propene and. oxxgen

The experiments were carried out in reactor D at 673 K with a feed at the system inlet containing 50-99 mole %

propene and complementary percentages of oxygen. Due to

the increase of the pulse width the propene partial pres­sures at the reactor inlet were .086 to .114 bar, whereas

the oxygen partial pressures at the reactor inlet were .086 to .003 bar respectively. The total pressure was 2.5

bar. In figure 6.41 the amount of converted propene

(mmol kg-1 ) per pulse is shown as a function of the pulse

number. The lowest curve refers to the experiment without

gas phase oxygen. The conversion of propene per pulse in­

creases with the partial pressure of oxygen at the pulse

reactor inlet up to p0 = .075 bar for all pulses. The in-

105

p

t

18

14

12

T = 673 K

(p0 )0

bar •• 000 •• 003 •• 024 • ,045 •• 062 •• 075 •• 086

..,..---.:&:l..-#..---'t.-.... ~ ........................

' 10 ...... _•-.-- ____ ...._ 0 ·--...... - .....

2

2 4 6 8 10 12 14 16 18 20

- n p Figure 6.41 Amount of Pr~pene con­verted per pulse (mmol kg ) vs. pulse number n

0• Oxidation of prope­

ne with and wi~hout (6) gas phase oxygen.

ln ACO t3

-3

lstwthpulse

• • C#qO • • c~

T=673K

-2

- ln (p0)0 Figure 6.42 Rates of formation of acrolein and1carbon dioxide per pulse (mmol kg- ) as a function of the partial pressure of oxygen(bar) at the pulse reactor inlet.

crease of the conve~sion as explained for the thermoba­

lance experiments (see section 6.3.6) can be ascribed to

a decrease of the number of reduced sites.

The rate of formation of acrolein is a function of th~ partial pressureof oxygen as is illustrated in figure 6.42

for the first and the tenth pulse at low partial pressures of oxygen the oxygen is completely consumed. Consequently

at low partial pressures of oxygen the rate of acrolein

formation decreases in the last part of the pulse and the

catalyst becomes reduced to a certain extent. This causes

the rate of acrolein formation to decrease from the first

to the tenth pulse. The rate of formation of carbon dioxide is also a function of the partial pressure of oxygen for

all pulses. The increase of the rate from the first to the

tenth pulse at the same partial pressure of oxygen is pre­

sumably caused by an increase of the number of unselective

sites by reduction of the catalyst.

For all oxygen concentrations the selectivity of the first pulse was above .95.

106

6.4.5. Experiments with mixtures of ammonia and helium

Experiments were carried out in reactor D at 673 K

with mixtures containing 8-100 mole % ammonia at the sys-

tern inlet. The amount of catalyst is 100 mg. The conver­

sion of ammonia per pulse is constant for partial pressu­

res of ammonia at least until pNH < .086 bar, as can be 3 seen in table 6.15.

partial pressure conversion of NH3 ocatalyst-consumpti~n of NH3 (bar) XN (fraction) mmol 0 kg-1 pulse-

.020 .174 1.4

.037 .167 2.6

.086 .170 8.3

.11 .123 11.8

Table 6.15. Catalyst reduction by ammonia oxidation to N2 as a function of the partial pressure of NH 3 at 673 K.

This means that this reaction behaves in the pulse reactor

as a first order reaction (again at least until pNH = 3

.086 bar). This is in agreement with the flow reactor ex-periments (see section 6.2.4).

Contrary to the decrease in conversion for subsequent

pulses of propene here the first pulse fits completely in

the pattern of all the others.

6.4.6~ Experiments with mixtures of gropene, ammonia and

helium

The experiments were carried out with pulses of propene/

ammonia mixtures diluted with helium. Due to the increase

in pulse width the series of diluted pulses contained 94,

107

95 and 96.5 mole % helium. The ratio propene/ammonia of

the remaining 6, 5 and 3.5 mole % respectively of these

gas mixtures was varied from 1:9 to 9:1. The experiments can be divided into two groups. In the

first group (pulses containing 94 mole % helium, pHe = 2.35 bar) we investigated the conversion and product dis­

tribution as a·function of the pulse number for series of

16 pulses. In the second group (pulses containing 95 and

96.5 mole % helium, pHe = 2.385 and 2.413 bar respective­ly) we stopped each experiment after a few pulses because

our main interest was in the oxygen consumption by the

reactants and in the product distribution of the first

pulse.

Pulses of the first group of experiments showed quali­

tatively the same picture as pulses of diluted propene as

can be seen for figure 6.43. The decrease in conversion

108

p

t

1~

•• 000 •• 02~ • .!IllS •• 062 ... 075

.086

T = 673K

2 ~ 6 8 10 12 1~ 16 18 20

-n p

Figure 6.43 Amount of propene converted (mmol kg-1} per pulse vs. pulse number np. Ammoxidation of propene using bismuth molybdate as reactant.

between the first and second pulse is again ascribed to

the loss of special sites. But presumably due to the pre­

sence of ammonia we do not experience a loss in selecti­

vity. As compared to the reaction with propene only, the

maximum oxygen consumption rate per pulse is now about 5

times higher (see figure 6.43) and would continue (if a

linear extrapolation to a zero conversion rate would be

realistic) to a degree of reduction of almost 50%. We see from the sections 6 .4'. 3, 6. 4. 4 and 6. 4. 6 that

the rate of oxygen suppletion by the catalyst is greatly

enhanced in the presence of oxygen or ammonia. In this

connection we recall the observation of Sancier et al (13)

who mentioned that the production of c3H416o on a catalyst

containing 16o is enhanced by increasing the partial pres­

sure of 18o2 in pulse experiments. If all these phenomena

have the same cause it points to the assumption that the

adsorption of certain species enhance the supply of bulk 16 2- 18 2-oxygen to the reactive sites. In this respect 0 , 0

and NH2- could perform the same function.

ln ACN

t 3

2

15tlothpulse

C3H3N • a

c~ • • T = 673 K

-1

~ ~ ~

-ln (pN)o Figure·6.44 Rate of formation of acrylonitrile and·c,rbon dioxide per pulse (mmol kg- ) as a function of the partial pressure of ammonia (bar) at the pulse reactor inlet.

The rate of formation of

acrylonitrile is a function

of the partial pressure of

ammonia as is shown in fi­

gure 6.44 for the first and

the tenth pulse. The de­

crease and the rate of acry­

lonitrile and carbon dioxide

formation from the first to

the tenth pulse at the same

partial pressure of ammonia

is ascribed to the r·educ­

tion of the catalyst.

Compared with the oxida­

tion of propene the rate of

of carbon dioxide formation

is small as can be seen in

table 6.16.

109

co2 mmol kg -1 pulse -1

Pulse composition

c 3H6-oxidation c 3H6-ammoxidation

pulse number pulse number

X xo ~H3 1 10 1 10 C3H6 2

1.0 - - .2 1.2 - -.9 .1 1.1 2.4 - -.6 .4 6.7 7.6 - -.9 - • 1 - - .07 .07

.6 - .4 - - ,19 .09

Table 6.16. Rate of carbon dioxide formation at 673 K for

the oxidation and ammoxidation of propene.

The product distributions obtained from the first pulse of

the various propene/ammonia/helium gas mixtures fit in

0cat t 16

14

.02 .04 .06 .08 .10 .02 ,04 .06 .08 .

- Pp - Pp Figure 6.45 A~~unt of catalyst Figure 6.46 ~~unt of catalyst oxygen (mmol kg ) used during the oxygen (mmol kg ) used during the first pulse for the formation of first pulse for the formation of the main products as a function of the main products as a function of !he partial pressure of propene(bar)~the partial pressure of propene(bar}. PHe=2.385 bar Pp+PN=.l15 bar PHe=2.413 bar Pp+PN=.087 bar

110

with those obtained from the first propene/helium and

ammonia/helium pulses as is shown in figure 6.45 and fi­gure 6.46 where the rate of oxygen depletion of the cata­

lyst for the formation of the main products acrylonitrile,

acrolein, acetonitrile, carbon dioxide and nitrogen is

plotted as a function of the partial pressures of propene and ammonia for two values of the partial pressures of

helium.

References

1. Satterfield, C.N., Sherwood, T.K., "The role of diffu­

sion in catalysis", Addison-Wesley Publ. Cy. Inc., Reading (1963)

2. Shelstad, K.A., Chong, T.c., Can. J. Chem. Eng. !I• 598. (1969)

3. Callahan, J.L., Grasselli, R.K., Milberger, E.C.,

Strecker, H.A., Ind. Eng. Chem. Prod. Res. Dev. 2• 134

(1970)

4. Cathala, M., Germain, J.E., Bull. Chim. Soc. Fr. 2167

(1971)

5. Yoshida, F., Ramaswami, D., Hougen, O.A., A. I. Ch. E.

Journal ~. 5 (1962)

6. Matsuura, I., Schuit, G.C.A., J. Catal. 25, 314 (1972)

7. Batist, Ph.A., Kapteijns, c.J., Lippens, B.C., Schuit,

G.C.A., J. Catal. 2• 33 (1967)

8. Steenhof de Jong, J.G., Thesis, Eindhoven University of Technology (1972)

9. van den Elzen, A.F., Rieck, G.D., Acta Crystallogr. Sect. B. ~. 2433 (1973)

10. Beres, J., Bruckman, K., Haber, J., Janas, J., Bull. Acad. Pol. Sci. Ser. Sc. Chim. ~ (8) 813 (1972)

11. Matsuura, I., Schuit, G.C.A., J. Catal. 20, 19 (1971)

12. van Oeffelen, D.A.G., Thesis, Eindhoven University of Technology (1978)

13. Sancier, K.M., Wentreck, P.R., Wise, H., J. Catal. 39,

141 (1975)

111

112

CHAPTER 7

FINAL DISCUSSION

7.1. Introduction

It is the purpose of this chapter to put the experi­

mental results described in chapter 6 in a more general

context. We will do that with the aid of two models, a

mechanistic model and a kinetic model. The mechanistic model will be a sketch of the network

of elementary steps that we presume to take place on the

catalyst and that lead from the reactants to the main

products and byproducts. This model will be based on li­

terature data and arguments that can be developed from our

experimental work. It will remain sketchy and rather spe­

culative in parts.

The kinetic model will have to conform with the mecha­

nistic model but will contain the minimum number of the

reaction steps that are required to derive the kinetic

results described in chapter 6. The number of these re-.

action steps depends on the basic reaction concerned i.e.

- the oxidation of propene (P + ACO)

- the ammoxidation of pr.opene (P + ACN)

- the ammoxidation of acrolein (ACO + ACN)

- the oxidation of ammonia (N + N2}

Each sequence of reaction steps of a basic reaction in­

cludes a rate determining reaction step, which will be

connected with the experimental kinetics of the proper re­

action. The choice of the right rate determining steps

will be an important element in the derivation of the ki­

netic models for the four basic reactions.

7.2. The mechanism of the catalrtic reaction

The mechanistic model comprises i.a. the following concepts:

113

a) Adsorption equilibria between the reactants (products)

in the gas phase and at the catalyst surface follow the

Langmuir adsorption theory;

b) Secondly we include in this mechanistic model a Mars­van Krevelen redox mechanism i.e. the catalyst oxidizes

the adsorbed species and the gas phase oxygen subse­

quently oxidizes the reduced catalyst;

c) The active sites that are required for the various

elementary steps of the four basic reactions mentioned

in section 7.1 have to perform a number of functions

in order to catalyse the complete reaction sequence,

e.g. from propene, oxygen and ammonia to acrylonitrile

and water. In our mechanistic model we will endeavour to ascribe

elementary steps to the specific interaction of two sites.

To make this discussion more unambiguous we will use a

number of catalytic concepts that we have defined as

follows: - An aative site is a site, consisting of an coordinative

unsaturated atom, on which reactants or intermediates

can be adsorbed during the sequence of reaction steps;

- The expression active centre is used for a group of sur­

face atoms that together bind one adsorbant entity;

- A reactive ensemble is a group of active sites involved

in a particular reaction step; - A reactive agg!'egate is the system of reactive ensembles,

which is required for the formation of one product mole­

cule.

As the terms "active site" and "active centre" are often

used as synonyms, we prefer the term "reactive ensemble"

to "active centre", although according to the IUPAC ·(1)

both terms can be used to describe the place where a cata­lytic reaction step takes place.

For the Mars-van Krevelen mechanism the following ter­minology can be used:

- A redore component is a species or a vacancy required for

an elementary step in the redox mechanism;

- A redore ensemble is a set of redox components that can interact with each other;

114

- A ~edore system is the combination of redox ensembles re­quired for the restoration of the oxidation activity of

the catalyst after one product molecule has been formed. The main questions to be answered for the development

of the reaction mechanisms are:

a) How are propene, ammonia and acrolein activated, which are the reactive ensembles and how are the products

formed? b) What is. the role of the catalyst in the activation of

molecular oxygen and what is the nature of the selec­tive oxygen species?

Using the literature data presented in chapter 2 we focus our attention mainly on these questions.

From the extensive literature it became clear that on bismuth molybdate two different types of reactive ensem­

bles are present 1} a reactive ensemble connected with molybdenum;

2) a reactive ensemble connected with bismuth. In the following we will derive arguments from our experi­

ments to chose the active sites for propene and ammonia adsorption. The presence of ammonia as the third reactant

in the acrylonitrile synthesis is an important expedient

to achieve this object. In the following sections at first we will deal with the adsorption of ammonia and propene.

7.3. Adsorption and adsorEtion sites

7.3.1. Adsorption of ammonia

Although far more literature data are available for the adsorption of propene than for the adsorption of ammo­

nia, we begin our discussion with the latter compound as

the results will be useful for the discussion of the ad­

sorption of propene. Matsuura (2) assumed that NH2- species are formed on

an reactive ensemble A and adsorbed on Bi-ions. This .assumption resembles that of Grasselli et al (3) for the

activation of ammonia on the USb 3o10 catalyst during the

115

production of acrylonitrile. According to the latter 2-authors the proton is donated to the 0 ion of Matsuura's

a-ensemble, whereas Gates et al (4) postulated recently that the proton is ultimately donated to the o2- ion of

an A-ensemble. Weiss et al (5) suggested the formation of + an NH2 intermediate adsorbed on a molybdenum ion. In the

case of nitrogen production the formation of hydrazine as

an intermediate

step between an

Based on the

has been assumed according to a reaction + -NH2 and an NH2 intermediate.

kinetic .data of our investigation we

assume that the ammonia molecules are activated on t~o

different ensembtes: - first there is a nitrogen containing species (N

0) that

is required as a precursor for acrylonitrile. As the

rate of acrylonitrile formation is almost zero order in

ammonia we assume that the active centres that bind the

ammonia are fully occupied;

- secondly there is a nitrogen containing species (N1) that reacts with the species N

0 mentioned above to form

molecular nitrogen in a reaction that is first order in

ammonia. This can be understood if the degree of cove­

rage of N1 is proportional to the ammonia pressure.

As we further have shown that in fact there is a

slight inhibiting effect of ammonia on the rate of acry­

lonitrile formation we further assume that N1 and propene

compete for the same active centres. This then results in

the picture that ammonia is fully covering one type of

active centres, which we call "a" centres and is slightly

adsorbed on the other type of active centres, called the

"b" centres that also adsorb propene.

From the observation that reduction of the catalyst

leads to the formation of Bi0, a formation that can easily

be prevented by the presence of gas phase oxygen, we con­

clude that oxygen is adsorbed fast and completely at bis­

muth centres. In the pulse experiments we have seen that

the presence of ammonia acts in the same way as the pre­

sence of oxygen. From this we would conclude that also

ammonia gives a fast and complete adsorption at bismuth

116

centres. If this conclusion is right our "a" centre will

be connected with bismuth, like Matsuura's A centre, but

then the "b" centre at which propene adsorbs will be com­

parable to Matsuura's B centre.

7.3.2. Adsorption of propene

As_we have explained in chapter 2, it is now generally

assumed that the oxidation and the ammoxidation of pro­

pene proceed via the formation of a negatively charged allylic intermediate. The first abstracted proton is do­

nated to an oxygen ion at the surface, whereas the allylic

intermediate is bonded to a cation by means of a cr~-allyl

bond. The delocalized ~-bond system of an allylic inter­

mediate, according to the simple Ruckel molecular orbital

theory, has three orbitals

(non-bonding)

with +i p orbitals on carbon atom i. It is generally

assumed that the plane of the carbon atoms is parallel to

the surface of the catalyst. The bonding orbital is in

the centre of the chain, whereas only in non-bonding or­

bitals the electrons are localized at the terminal carbon atoms.

In the literature no agreement about· the adsorption

site for propene has yet been reached. Matsuura et al (6) (7) and Matsuura (2) postulated a model based on adsorp­

tion experiments that start with the dissociative adsorp-6+ 2-tion of propene on Mo and 0 ·• Haber et al (8) (9) use

two sets of experiments to support the opinion that the

bismuth ion activates propene. One set of experiments

carried out by Beres et al (10) consists of the reduction

of a, B and y bismuth molybdate with hydrogen, yielding

117

different energies of activation, which reflects the

difference in the enthalpy of chemisorption of hydrogen

on the oxygen ions because of the different coordination

of molybdenum in the three molybdates. With propene no

correlation of the energies of activation with the diffe­

rent coordination of molybdenum is obtained. In our

thermobalance experiments we also found a difference in

behaviour of hydrogen and propene in the reduction of our

catalyst. The rate of catalyst reduction with hydrogen

was low and represents a surface reaction,. whereas the

rate of reduction with propene was much higher and is

governed by the oxygen transport in the catalyst and does

not reflect the surface reaction.

Sakamoto et al (11), Swift et al {12), Haber et al (8)

and Boersma (13) have shown that propene can adsorb on

bismuth oxide and on bismuth containing oxides, yielding

i.a. 1,5-hexadiene. As however this reaction hardly takes

place below 773 K and during our experiments at tempera­

tures between 648 and 723 K 1.5-hexadiene was never found

we conclude that the adsorption discussed by these authors

does not occur in our experiments. Grzybowska et al (14) did pulse experiments with allyl

iodide, using the oxides of·bismuth and of molybdenum and

bismuth molybdate as oxidizing agents, whereas Gamid-Zade

et al (15) oxidized allyl bromide over these compounds

with gas phase oxygen. The first hydrogen abstraction,

generally supposed to be the rate determining step was

bypassed as allyl radicals were formed rapidly. Molybde­

num oxide, an inactive but selective catalyst for the

formation of acrolein from propene showed an increased

activity and especially at temperatures below 600 K a

high selectivity towards acrolein. On bismuth oxide non­

oxygenated products were found below 673 K and 1,5-hexa­

diene and propene were the main products. On bismuth

molybdate acrolein as well as non-oxygenated products

were obtained. The observation that only molybdenum oxide

produces acrolein shows that at least a part of the reac­tion takes place on reactive centres with molybdenum. The

118

fact that on bismuth molybdate acrolein and non-oxygenated

products are formed shows, when compared to the normal

propene oxidation that the bismuth related non-oxygenated

products are atypical for the reaction of propene on bis­

muth molybdate. It is also possible that gas phase allyl

radicals play a role in the 1,5-hexadiene formation. Ex­

perimental evidence for such a mechanism has been given

by Grasselli (16) who studied the reaction of azopropene

with bismuth molybdate at 600 K.

A model comparable with that of Matsuura has been pro-2-posed by Linn and Sleight (17). They used Moo4 groups

associated with a Bi-cation vacancy as active centres fo~

the propene adsorption. This leads to the formation of an

allylic intermediate on the Moo42- group and the donation

2-of a proton to the oxygen of a neighbouring Moo4 group.

Recently Matsuura (see (4) chapter 4) assumed that

Bi2Mo2o9 is the active phase and that the allyl formation

occurs on (Moo4 >4 clusters, according to the model pro­

posed by Sleight (18) which is based on a scheelite struc­

ture.

All models account for a dissociative adsorption, which

would lead to a half order dependency with respect to

propene if the two different active sites that form the

reactive ensemble are present in equal amounts and if the

adsorption is the rate determining step. However we have

found that the rates of acrolein and of acrylonitrile

formation are both first order with respect to propene for

the lower propene concentrations. This can only be achie­

ved if either a) the number of o 2- ions for the adsorption

of the proton is much larger than the number of cations

for the adsorption of the allylic intermediate, or b) the

number of o 2- ions is constant as a result of a rapid

regeneration at the temperatures of this investigation.

A high ratio of o 2- ions to cations would in Matsuura's

low temperature adsorption studies result in single site

adsorption kinetics. Actually he finds that both propene

and ammonia adsorb according to a dissociative kinetic

model.

119

According to Peacock et al {19) the allyl intermediate

is bonded to the Mo6+ ion. The bismuth ion is assumed to

keep the molybdenum ion in the highest oxidation state,

either by accepting an electron directly from the allyl

intermediate or via the Mo5+ ion.

Haber et al {20) showed by means of semi empirical

quantum-chemical calculations that the TI allyl interme­

diate is bonded to Mo6+ and that the TI allyl electrons

shift to the metal ion. These calculations are supported by the X-PS and U-PS measurements of Grzybowska et al {21)

that show the formation of Mo4+ and only in the later sta­

ges the reduction of Bi3+.

It will be clear that from the discussion of the ammo­

nia adsorption we also favour a propene adsorption con­

nected with molybdenum cations.

7.3.3. Adsorption of acrolein

The activation of acrolein has been studied by means

of adsorption experiments carried out by Matsuura {2) and

has been discussed by Batist et al (22). The experiments

showed that acrolein is adsorbed on bismuth molybdate via two

or three different processes, viz. a fast, weak adsorption

similar to the butadiene adsorption and connected with the

B-ensembles and a slow and strong adsorption comparable to

the strong adsorption of butadiene and connected with the A-ensembles. Moreover there is a marked but not well under­

stood adsorption on reduced bismuth molybdate. We assume

that the fast and weak adsorption of acrolein is the, for the propene conversion, slightly inhibiting adsorption

on molybdenum.

The slow and strong adsorption of acrolein, which is

non-inhibiting and reversible is assumed to occur on bis­muth.

The third type of adsorption on a reduced catalyst may

be connected with the coverage of a large part of the ca­talyst or with the formation of acrolein polymers.

These adsorptions have been noticed in our experiments

120

in the flow reactors (inhibition, section 6.2.3.1) and in

the thermobalance (experiments with substoichiometric

amounts of oxygen, section 6.3.6).

7.3.4. The role of the catalyst in the activities of

molecular oxygen

Much attention has been paid in literature to the under­

standing of the nature of the oxygen species that inter­

acts with the reactants. According to the redox mechanism

proposed by Mars and van Krevelen {23) lattice oxygen of

active catalysts can be involved in the oxidation of

hydrocarbons. Aykan (24) found for the ammoxidation of

propene in the absence of oxygen on bismuth molybdate in

a fluid bed reactor at 723 K initially the same activity

and selectivity as in the presence of molecular oxygen.

Due to the increasing reduction of the bismuth molybdate

catalyst the activity of the oxidant decreases.

In our pulse experiments we also found a decreased ac­

tivity with. increasing degree of reduction of the catalyst.

However we also found a higher initial activity in the

presence of gas phase oxygen than in the absence thereof.

The latter has been ascribed to the special conditions in

our pulse reactor.

As far as the structure of the catalyst surface is con­

cerned we assume, in accordance with the work of Grzybow­

ska et al (21) that during the reaction in the presence of

oxygen the surface Bi/0 and Mo/0 ratios are equal to

those of fresh samples and that only during reduction

changes in"the surface composition can take place.

From experiments carried out with labeled oxygen by

Keulks (25) and Wragg et al (26) in a recirculation reac­

tor1 by Sancier et al (27) in a pulse reactor; by Pendleton

et al (28) in a batch reactor and by Keulks et al (29) in

an integral flow reactor it can be concluded that at tem­

peratures above 648 K lattice oxygen is the only direct

source of oxygen for the formation of acrolein and acrylo­nitrile.

121

The hydrogenation steps and the ultimate selective oxi­

dation use oxygen ions to accomodate protons. After that

the products are desorbed and the active surface is main­

tained in a high degree of oxidation by diffusion of

oxygen ions through the lattice. This can only be realized

if the electrons produced can be transported simultaneous­

ly in the opposite direction via positive ions.

In order to explain how the gas phase oxygen reaches

the ensembles active in the propene oxidation we will

assume that a reaction sequence ends after the desorption 4+ of the product with the presence of Mo ions on the sur-

face. These can be reoxidized to Mo6+, according to the

overall redox reaction:

3 Mo4+ + 2 Bi3+ (r 7.1)

as was earlier proposed by van Oeffelen (30)". This redox

reaction is also in agreement with the work of Grzybowska

et al (21) who have shown that the reduction of Bi 3+ only 4+ takes place after the formation of Mo •

Although no data for the different bismuth molybdate

phases are available, some indications may be found from

the thermodynamic data for Moo2 , Bi2o 3 , Moo3 and Bi (31,

32, 33). The necessary cp-values were calculated as a

function of temperature. The results are summarized in table 7 .1.

r.b02 Bi2o3 r.b03 Bi

-587.9

-578.0

-745.2

-

50.0

151.5

77.8 56.7

-533.1 -584.2

-496.0 -533.1

-668.1 -739.8

- 16.9 21.8

103.2

247.9

147.0

100.0

-465.6

-393.4

-573.4

- 45.5

Table 7.1. Thermodynamic values at 298.15 and 673 K for

the redox components of the bismuth molybdate catalyst.

122

We calculated the standard Gibbs free energy 6G

of the redox reaction

+ + 3 Mo03 + 2 Bi

r,673,Bi2Q3

(r 7.2)

to be -21.0 kJ mol-1 • The melting point of bismuth is 544.5 -1

K with 6HM = 10.9 kJ mol • ~he th~ee other redox compo-

nents are solids in the temperature range of 298 to 673 K.

This means that at 673 K in the absence of molecular oxygen

bismuth atoms will form and can agglomerate to liquid

droplets and thus leave the solid lattice. Actually we

have observed under reducing conditions at 673 K a con­

densation of bismuth metal in those parts of the reactor

and outlet lines that were below 500 K (34). The equili­

brium reaction (r 7.1) is shifted to the right hand side

and the catalyst will loose its activity. This can be

counteracted by the presence of sufficient gas phase oxy­

gen, because then a second redox ensemble can become ac­

tive:

+ + (r 7.3)

For this second redox ensemble surface vacancies Dr at the

reoxidation site of the catalyst are required. These

vacancies and the Bi0 atoms result from the diffusion of

o2- and electrons respectively through the lattice, the

o 2- ions going from the reoxidation sites to the reaction

sites and the electrons in the reverse direction.Miura et

al (35) have found from temperature programmed reoxidation

experiments indications that the reoxidation at bismuth

sites is preferred, i.e. it takes place at a temperature

that is 150 K below the temperature at which Moo2 is re­

oxidized.

Much has been reported on the types of oxygen active in

propene oxidation and ammoxidation. Haber et al (9) rela­

ted the increase in the activation energy from 81 to 136

kJ mol-l for the reduction of the series ,_,f bismuth molyb­

dates with Bi/Mo ratios of 2, 1 and 2/3 respectively with

123

hydrogen, with the increase in tetrahedral surrounding of

molybdenum in these compounds. The fact that for propene

reduction the energies of activation were all the same

lead him to conclude that now oxygen from bismuth was ac­

tive. However, the rate of reduction with propene was

about 50 times higher than with hydrogen and consequently

a completely different mechanism may be rate determining.

Moreover it must be mentioned that the energies of acti­

vation for the reduction of Bi2o3 , Bi2Mo~~ and Mo03 were all found to be between 81 and 90 kJ mol · • Otsubo et al

(36) and Miura et al (37) prepared bismuth molybdates

(Bi/Mo = 2/1) by solid state reactions between bismuth

oxide and molybdenum oxide under vacuum at 823 K for 20

hour. The oxides were labeled by reduction and subsequent

reoxidation with 18o2 • The catalysts were reduced with

hydrogen and with propene. The 18o content of the water obtained from hydrogen reduction and of the· acrolein from

propene reduction was only initially somewhat higher for

the catalyst made with Bi218o3 • From these data the

authors conclude that the oxygen near bismuth is first

used in both reactions. This is not in agreement with the

work of Haber mentioned earlier, nor with the observation

of Grzybowska et al (21) who showed that the reduction of

bismuth is preceded by the reduction of molybdenum. In this connection it must be born in mind that the prolonged

vacuum treatment at high temperature may have lead to a

loss of slightly volatile molybdenum oxide and to a re­

duction of the catalyst. The work of Bleijenberg et al

(38) and of Batist et al (39) has shown that the solid state reaction of the oxides leads to rather inactive

catalysts and that a complete reaction between the oxides

is difficult to achieve.

The information discussed above leads us to believe

that there is a st.rong interaction between oxygen ions, which possibly makes the assignment of certain reactive

oxygen ions as belonging exclusively to either bismuth or to molybdenum problematic.

124

7.4. The formation of the main products

7.4.1. The formation of acrolein

In section 7.3.2 we concluded that the dissociative

adsorption of Mo6+o 2-. The

propene occurs on the reactive ensemble

Mo6+ ions has no d electrons and if the ne-s gative allylcarbanion is placed at the empty site elec-

trans can be shared by cation and ligand, involving a

charge displacement to the cation. Preferentially the two

electrons in the non-bonding allyl orbital, concentrated

on the terminal carbon atoms are transferred, which re­

sults in a partial positive charge on these atoms.

Additional information about the role of Mo6+ in the

oxidation of propene has been obtained by means of the

ESR technique applied by Peacock {19), Sancier et al {40)

and Burlamacchi et al {41). These experiments provide

evidence for the postulate that Mo6+ and o2- associated

with Mo6+ are essential for the propene oxidation and that

Mo5+ is an intermediate in the electron transfer process.

As the electron transfer process is not yet completely

understood we assume that two electrons are transferred to

one Mo6+ ion. This was experimentally confirmed by means

of X-PS measurements carried out by Grzybowska et al {21).

As has been suggested by Sachtler {42) the symmetric allyl intermediate can be of a transient nature only. By

means of the partial positive charges on the terminal car­

bon atoms a carbon-oxygen bond can be formed, resulting

in an asymmetric a-allyl compound. Since Sachtler et al

{43) found the terminal carbon atoms undistinguishable, a

socalled dynamic a-allyl intermediate has been postulated

and experimentally proven.by Kondo et al {44). This was

achieved by means of H-D exchange in the olefin when o2o was added to the reaction mixture. The allyl is then a

fluxional ligand i.e. it can move from one oxygen to an­

other presumably via a n-allyl intermediate on a cation.

According to Matsuura {7) the allyl intermediate moves

from this reactive ensemble B to the reactive ensemble A

125

("B-site" and "A-site" respectively in Matsuura's nomen­

clature) and this step is supposed to be the rate deter­

mining step. The abstraction of the second proton occurs

with the c3H4-intermediate staying on the reactive en-2-semble A whereas the proton is donated to an 0 of a

reactive ensemble B.

According to Haber et al {9) the second proton abstrac­tion takes place on Mo6+ and the proton is again donated

2-to an neighbouring 0 ion. Matsuura's model is composed

of more elementary steps than Haber's model and moreover

the intermediates shift in the opposite direction, where­

as the protons in Matsuura's case are ultimately donated

to his OA and in Haber's case to the oxygen near molyb­

denum (Matsuura's OB). In a third model put forward by Otsubo et al (36) and

Miura et al (37) the second hydrogen abstraction occurs

also near bismuth, whereas the oxygen near molybdenum is

supplied to the anion vacancy near bismuth after the for­

mation of the products. In this model the allyl interme­

diate is not shifted.

Although Peacock et al (19, 45) do.not give a detailed

model one can conclude from their discussion that they

adhere to a view opposite to that of Otsubo and Miura i.e.

all elementary steps of the acrolein formation take place

on the reactive ensemble near molybdenum and the oxygen

near bismuth is supplied to the anion vacancy near molyb­

denum. Linn and Sleight (17) donate also the second proton to

the oxygen of a neighbouring Moo42- group, whereas the

2- 2-C3H4 remains on the other Moo4 group. The formation of acrolein takes place by means of the donation of oxygen

2-from the Moo4 group. The electrons are donated to an

overlapping system of an empty Bi 6p conduction band and

to Mo 4d levels and are subsequently donated to an in­

coming oxygen molecule near bismuth.

As we show in table 7.2 in two of the five models the

allyl intermediate is shifted to the reactive ensemble

containing the other cation, whereas in the other three

126

models the allyl intermediate stays at the same reactive

ensemble.

liS 1-1 .II: +l ~ 0 tl ..c: ~ 1-1 .Q 0 tF> !I) <II ~ tl ..... +l .Q !I) liS <ll

~ Ill +l Q) .-1 ::= 0 llo til

1st proton abstraction + Bi Bi 2-(C3H5)-formation on Mo Mo Moo4

Shift of (C3HS) Bi Mo - - -2nd proton abstraction + (C3H4)-formation Bi Mo Bi Mo 2-on Moo4

Formation of c3H40 with 2-0 near resp. of (Sleight) Bi Mo Bi Mo Moo4

Formation of H2o with 0 2-near resp. of (Sleight) Bi Mo Bi Mo Moo4

a-diffusion from places

near Bi to - Mo - Mo Mo

a-diffusion from places - - Bi - -near Mo to

Reoxidation near Bi Bi Mo Bi Bi

Table 7.2. Scheme of the various reaction models as pre­sented in literature for the oxidation of pro­pene to acrolein over bismuth molybdate.

As we have shown that in the ammoxidation of propene one of.the types of reactive sites is fully covered by

ammonia or its derivatives we adhere to a non he.tero-shift model, such as that of Peacock et al. Thus for the aero-

127

lein formation we assume the following sequence of stoi-

chiometric equations, all connected with moJybdenum

C3H6 + Mo6+ + OS 2-

+ 6+ -Mo (C3H5) + (OSH) (r 7.4)

6+ Mo (C3H5 ) + Mo4+cc3

H5

)+ (r 7.5)

Mo4+(C3H

5)+ 2- + Mo4+ + (OsC3H5) (r 7.6) + OS +-

4+ + Mo {C3H5 ) + 2-OS +

4+ Mo (C3H4} + (OsH) (r 7. 7)

The equations (r 7.4 tor 7.7) have been discussed above. 4+ In order to proceed from the.Mo (c3H4) species a number

of sequences is possible: e.g. either the Mo4+ of the

surface complex is oxidized by Bi3+ or the c 3H4 species

transfers to a Mo6+ site. In both cases electron transfer

can follow according

+ (r 7. 8)

This cc3H4) 2+ species then reacts with anos2- ion to form

acrolein that subsequently desorbs. The adsorbed acrolein

on molybdenum accounts for the inhibiting effect of acro­

lein on the oxidation of propene.

There is also the possibility that we have a concerted

reaction,such as

+

(r 7. 9)

with subsequent desorption of acrolein and the same in­

hibiting effect.

In all sequences we will end with a Mo4+ species that

will be reoxidized by means of the redox reaction (r 7.1).

7.4.2. The formation of nitro9en

We have discussed in section 7.3.1 that the nitrogen

128

containing species N0

and N1 are formed on the active

centres connected with bismuth and molybdenum respectively.

We suppose these species to be formed from ~mmonia by means of two separate dehydrogenation sequences.

The sequence on the active centre containing molybdenum

is comparable with the dehydrogenation of propene as given by the equations (r 7.4) to (r 7.8) for propene and ends

4+ 2+ with the species Mo (NH) • The sequence on the active centre containing bismuth

ends with the formation of the species Bi3+(NH) 2-. This

assumption is in agreement with that of Matsuura (2). The

abstraction of two protons is also mentioned by Gates et ,

al (4). Matsuura (2) assumed that the nitrogen formation occurs on an A-ensemble from two similar species Bi3+(NH) 2-.

We however postulate a reaction between the two inter­mediates formed on the different reactive ensembles viz. Bi3+(NH) 2- and Mo4+(NH) 2+. The formation of two different

species on different active sites is supported by our kinetic experiments. After the formation of the interme­diate HN = NH proton abstraction and electron transfer leads to the formation of nitrogen. As was experimentally

found (see section 6.2.5.2) nitrogen formation is notably suppressed during the ammoxidation of acrolein. We a~cribe

4+ 2+ this phenomenon to the rapid formation of Mo (C3H4) and

Mo4+cc3H4NH) from acrolein, which results in a decrease

of the formation of the Mo4+(NH) 2+ intermediate.

7.4.3. The formation of acrylonitrile

Analogous to the.formation of acrolein Matsuura (2)

assumed that the greater part of the sequence that leads

to the formation of acrylonitrile from propene occurs o~ his A ensemble, whereas Gates et al (4) assume that the carbon-nitrogen bond is formed on this A ensemble before

the last two protons are abstracted. This also has been suggested by Cathala et al (46).

We also assume that acrylonitrile is formed via the formation of the intermediate c3H4NH, but from a

129

Mo4+cc3H4 ) 2+ and a Bi3+(NH) 2- species. This leads via a

rapid dehydrogenation to acrylonitrile, as was already

experimentally confirmed by Cathala et al (47), who found

the dehydrogenation of allyl amine over bismuth molybdate

to be a rapid and selective reaction.

The formation of acrylonitrile from acrolein at 673 K

is first order in acrolein for low acrolein concentrations

in the gas phase and zero order in ammonia. The rate of

acrolein formation is high in comparison with the rates of

oxidation and ammoxidation of propene, which points either

to a concerted reaction between N0

species and absorbed

acrolein or to a rapid formation of the intermediates 4+ 2+ 4+ .. Mo (C3H4) and Mo (C3H4NH). In th1.s respect we recall

the observation, mentioned in section 7.4.2 that the

nitrogen formation is notably suppressed, which means

that the intermediates are present on Mo4+. Further we mention the conclusion of Cathala et al (48) with respect

to the rapid formation of c3H4-species from acrolein. The

two possibilities are depicted in the scheme given in

section 7.5 by means of two dotted arrows.

7.4.4. The formation of water

The first order relation in propene for acrolein and

acrylonitrile formation requires that the proton abstrac­

tion step is irreversible, which can be explained if the

conversion of surface hydroxyl groups to water is very

fast. This is confirmed by the observation that water va­

pour does not inhibit the reaction. As discussed in sec­

tion 7.3.4 we have no further evidence to offer as to what cations the hydroxyl groups are related.

7.5. The kinetic model

The experimental data given in chapter 6 show that

under comparable stationary conditions in the flow reac­

tor the rate of propene oxidation is the smallest, direct­

ly followed by the rate of propene ammoxidation, whereas

130

the rate of acrolein ammoxidation is much greater as can be seen in table 6.2, which is copied here for easy refe-

renee:

k (mol kg -1 -1 s )

p + ACO 0.025 p + ACN 0.035

ACO + ACN 0.42

Table 6.2. Reaction rate constants at 673 K.

The ratio between the rate constant of the ammoxidation

of propene and that of the oxidation of propene (R1), gi­

ven in the literature and that calculated from this inves­

tigation are given in table 7.3. The same is done for the

ratio between the rate constant of the ammoxidation of

acrolein and that of the ammoxidation of propene (R2).

R1 R2 T (K)

Cathala et al (48) .8 5.4 733

Kolchin et al (49) 1.8 - 750

Callahan et al (50) 1.8 2 698

Wragg et al (51) 1.9 26 673

this investigation 1.4 12 673

Table 7.3. Ratios of rate constants of the various reac­

tions (for definition see text).

The bismuth molybdates used by these authors were dif­

ferent in composition or prepared in different ways. Only

Wragg et al used a catalyst which was prepared according

to the same method A as our catalyst. The ratios R1 and R2

131

are calculated for different temperatures and the spread

in the values is in the direction that can be derived from

the activation energies given in table 7.4. The ratio R2 decreases at increasing temperatures, whereas the ratio

R1 is less temperature dependent. The activation energies

show a considerable spread, which will be caused by the

use of different catalysts and by the different tempera­

ture ranges used for the calculation of Ea.

Ea (kJ mol-l) (T > 673 K)

p -~- ACO P -+ ACN ACO + ACN

Callahan et al (50) - 75 29

Wragg et al (51) 83a 38b -Cathala et al {52) 159c 71 -Monnier et al (53) 63 - -this investigation 62 60 -a temperature range unknown, value probably at 673 K b temperature range unknown c temperature range 657-754 K

Table 7.4. Activation energies of the three overall reac­

tions.

The activation energy for the propene oxidation given by

Cathala et al is very similar to the value of 163 kJ mol-l

that we calculated for the activation energy of oxygen

diffusion at temperatures below 673 K (see section 6.3.3).

although the experiments by Cathala were carried out above

as well as below 673 K.

If we restrict ourselves to the treatment of the kine­

tics of the three overall reactions given in table 7.4 we

conclude from the preceding sections that some of the

elementary steps occur in more than one of the three

132

overall reactions. This is shown in the following scheme,

which does not take account of all elementary steps nor

of the charges on the intermediates

1 2 3 4 5 C3H6 ~ (C3H6} a- (C3H5} a -_(C3H4} a:;:':: (C3H40} a~ C3H40

6 ~

7 ~

8 n

The first order in propene, both for oxidation and

ammoxidation requires that the surface coverage of the c3 species that enters the rate determining step is propor­

tional to the propene gas phase concentration. This means

that this surface species must be formed via a reversible

reaction from the gas phase propene and the degree of

coverage for this species is relatively low. This indi­

cates that reaction 2 of the scheme will be rate deter­

mining. The fact that the overall rate constant for the

ammoxidation reaction is 1.4 times the rate constant for

the oxidation reaction is ascribed to a higher coverage

of the surface with acrolein than with acrylonitrile. This

is supported by the fact that the rate of the ammoxidation

reaction at 673 K remains first order in propene up to a -3 propene gas phase concentration of about 3 mol m , where-

as for the oxidation reaction deviation from the first

order dependency already begins at a propene concentration

of about 1.4 mol m- 3 (see figure 6.6 and 6.18}. Moreover

the enthalpies of activation for oxidation and ammoxida­

tion have almost the same values i.e. 62-102 kJ mol-l and -1

60-97 kJ mol respectively for the same temperature

ranges (see figure 6.7 and 6.23}.

133

The observation that the rate of ammoxidation of acro­

lein is more than one order of magnitude higher than that

of propene leads us to the conclusions that the rate of

adsorption of acrolein must be much greater than the ad­

sorption of propene and further that the steps 7 and 8

also must be very fast and not rate determining in the

ammoxidation of propene. This is in agreement with the ob­

servation that the ammoxidation of acrolein is only first

order in acrolein for an acrolein gas phase concentration

below 1 mol m- 3 (see figure 6.12), and that the inhibition

by acrylonitrile is smaller than the inhibition by acro­

lein i.e. acrylonitrile desorbs much faster than acrolein.

As further little or no acrolein is found in the ammoxida­

tion of propene, provided ammonia is present in excess,

we also conclude that the steps 7 and 8 are fast as well

in comparison with the steps 4 and 5, see also section 7.4.3,

7.6. Selectivities in the acrylonitrile synthesis reaction

The almost constant and high value of the integral pro­

pene selectivity towards acrylonitrile (SIPA) and the

strong variation of the integral ammonia selectivity

(SINA)' as we have already mentioned in chapter 6, are

peculiar phenomena in this process. All other conditions

constant, the variation of SINA can be related to the molar ratio ammonia/propene Mat the reactor inlet (M ),

0 From experiments carried out at very short residence

times we calculated the initial rates of formation of the

nitrogen containing products at 673 K and from that the

fractional ammonia selectivity towards acrylonitrile

(SFNA) according to the definitions of the fractional

selectivity given by the IUPAC (1),

(7 .1)

could be calculated,

134

As shown in figure 7.1 between (SFNA) and (M/M+1) at

.6

(ACN} mol m-3

.4 A 0 0 .1 a ,2 9 .3

.2· T=673 K A

.2 .4 .6 .8 1.0 - M/M+l

Figure 7.1 Fractional selectivity of ammonia towards acrylonitrile (SFNA) as a function of M/M+l. Ammoxidation of propene.

the reactor inlet ~xists an almost linear relation. This can be interpreted as an illustration of the first order dependency in ammonia of the rate of nitrogen formation.

At increasing conversion i.e. increasing acrylonitrile

concentrations we calculated an increase of SFNA at the same (local) value of M in the flow reactor. The increase

of SFNA at decreasing M/(M+1) can be explained by the presented model. It is assumed that for the nitrogen for­

mation propene and ammonia compete for the same Mo-sites.

The increase of SFNA at increasing acrylonitrile concen­trations can be explained in the same way as set out in

135

section 7.4.2 for the influence of acrolein on the nitro­

gen formation.

As the rate of nitrogen formation is first order in

ammonia it will be clear that the greatest increase of

SFNA by acrylonitrile will be obtained at high M/(M+1)­values as was experimentally observed. However the frac­

tional selectivity of ammonia towards acrylonitrile never

exceeds the value of .8 under the experimental conditions.

References

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geneous Catalysis, Adv. Catai. ~, 351 (1977)

2. Matsuura, I., J. Catal. 11• 420 (1974)

3. Grasselli, R.K., Suresh, D.D., J. Catal. ~. 273 (1972)

4. Gates, B.C., Katzer, J,R. and Schuit, G.C.A., "Chemi­

stry of Catalytic Processes", chapter IV, McGraw Hill

N.Y. (1979)

5. Weiss, F., Marion, J.,. Metzger, J., Cognion, J .M.,

Kin. i. Kat. !!r 45 (1973)

6. Matsuura, I., Schuit, G.C.A., J. Catal. ~, 19 (1971)

7. Matsuura, I., Schuit, G.C.A., J. Catal. ~, 314 (1972)

8. Haber, J., z. Chern. ll• 241 (1973) 9. Haber, J., Grzybowska, B., J. Catal. .£!!, 489 (1973)

10. Beres, J., Bruckman, K., Haber, J., Janas, J., Bull.

Acad. Pol. Sci. Ser. Sci. Chim. ~, 813 (1972)

11. Sakamoto, T., Egashira, M., Seiyama, T., J. Catal. ~' 407 (1970)

12. Swift, H.E., Bozik, J.E., Ondrey, J.A., J. CataL £!_,

212 (1971)

13. Boersma, M.A.M., Thesis Eindhoven (1977)

14. Grzybowska, B., Haber, J., Janas, J., J. Catal. 49, 150 (1977)

15. Gamid-Zade, E.G., Kuliyev, A.R., Mamedov, E.A., Rizayev,

R.G., Solokovski, V.D., React. Kinet. Catal. Lett. l• i91 (1975)

136

16. Grasselli, R.K., 4th Roermond Conf. on Catal. (1978)

17. Linn, W.J., Sleight, A.W., Ann. N.Y. Acad. Sci.~' 22 (1976)

18. Sleight, A.w., in J.J. Burton and R.L. Garten (eds.)

"Advanced Materials in Catalysis" Acad. Press, N.Y •. in press

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Hockey, J.A., J. Catal. ~, 379 (1969) 20. Haber, J. 1 Sochacka, M., Grzybowska, B., Golzbiewski,

A., J. Molec. Catal. ! 1 35 (1975)

21. Grzybowska, B., Haber, J., Marczew-ski, w., Ungier, L.,

J. Catal. ~' 327 (1976) 22. Batist, Ph.A., Bouwens, J.F.H., Matsuura, I., J. Catal.

,g, 362 (1974)

23. Mars, P., van Krevelen, D.W., Chem. Eng. Sci. Suppl.

lt 41 (1954) 24. Aykan, K., J. Catal. 12, 281 (1968)

25. Keulks, G.W., J. Catal. ~' 232 (1970) 26. Wragg, R.D., Ashmore, P.G., Hockey, J.A., J. Catal.

g, 49 (.1971)

27. Sancier, K.M., Wentreck, P.R., Wise, H., J. Catal.

l2_, 141 (1975)

28. Pendleton, P., Taylor, D., J. Chem. Soc. Far. Trans.

I. E, 1114 (1976) 29. Keulks, G.W., Krenzke, L.D., Proc. 6th Int. Congr.

Catal. London ~. 806 (1977) 30. van Oeffelen, D.A.G., Thesis Eindhoven (1978)

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Kalorische zustandsgrossen, Springer Verlag Berlin 1961, p 226, 259

32. Gmelin's Handbuch der Anorganische Chemie 8. Aufl. 1964, Bismuth Syst. nr. 19, Erg. Band, 628-632, Verlag

Chemie GmbH Weinh,eim 33. JANAF Thermochemical Tables, 2nd ed., NSRDS-NBS 37,

US Government Printing Office, Washington DC 1971 34. Batist, Ph.A., Lankhuijzen, S.P., J. Catal. 28, 496

(.1973)

35. Miura, H., Morikawa,· Y. Shirasaki, T., J. Catal. 39, 22 (1975)

137

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J. Catal. 56, 84 (1979)

38. Bleijenberg, A.C.A.M., Lippens, B.C., Schuit, G.C.A.,

J. Catal. i• 581 (1965)

39. Batist, Ph.A., Lippens, B.C., Schuit, G.C.A., J. Catal. 2_, 55 (1966)

40. Sancier, K.M., Dozono, T., Wise, H., J. Catal. 23,

270 (1971)

41. Burlamacchi, L., Martini, G., Ferron!, E., J. Chern.

Soc. Far. Trans. I. ~, 1586 (1972)

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Catal. Amsterdam 1964, Vol. I, 252, NH Publ. Co.,

Amsterdam 1965

44. Kondo, T., Saito, s., Tamaru, K., J. Am. Chern. Soc.

96, 6857 (1974)

45. Peacock, J .M. I Parker, A.J •, Ashmore, p .G. I Hockey,

J .A., J. Catal. IS. 398 (1969)

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(1971)

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(1971)

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Neftekhimiya i• 301 (1964}

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Strecker, H.A., Ind. Engng. Chern. Prod. Res. Dev. !• 134 (1970)

51. Wragg, R.D. 1 Ashmore, P.G., Hockey, J .A. I J. Catal.

l!,, 293 (1973)

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138

Joint Meeting Am. Chern. Soc. and Chern. Soc. Japan

Honolulu, April 1-6 (1979)

List of symbols

Latin symbols

A

A

component of a chemical reaction

peak areas in chromatogram

A constant in a-It relation

[A],(A) concentration of A

a reaction order

Units

av B

surface area catalyst per bed volume m2 m-3

b

b

C, (C)

constant in a-It relation s-~ reaction order

volumetric dilution ratio

concentration

concentration factor

mol m-3

mol m-3 cf

cp D

heat capacity at constant pressure

diffusion coefficient

J kg-1 K-1

m2 s-1

Da

dp

Ea F

f

axial dispersion coefficient

particle diameter

activation enthalpy

molar gasflow rate

substance specific correction factor

2 -1 m s

m

kJ mol-l

mol s-1

G mass velocity kg m- 2 s-1

GM

t.Go

t.G o

superficial molar flow rate mol m-2 s-1

r

t.H o f

standard Gibbs free energy kJ mol-l

standard Gibbs free energy of reac-

tion kJ mol-l

standard enthalpy of formation kJ mol-l

standard enthalpy of reaction kJ mol-l t.H o r

h heat transfer coefficient J m- 2 s-1 K-l

j 0 Chilton and Colburn factor for

mass transfer

jH Chilton and Colburn factor for

heat transfer

K adsorption equilibrium constant

139

reaction rate constant

rate constant in eq. 6.9

mass transfer coefficient

length of catalyst bed

M molar ratio ammonia/propene

n number of moles

np pulse number

n reaction order

0 oxygen content of catalyst,

oxygen consumption per pulse

p pressure

R gas constant

r radius

r rate of reaction

rspec specific rate of reaction

S specific surface area

S selectivity

u

v w

standard entropy

temperature

residence time

reaction time

superficial gas velocity

anion vacancy

catalyst weight

X conversion

x mole fraction

Greek symbols

ct degree of reduction

ctw heat transfer coefficient

at the wall

t: porosity

n effectiveness factor

e fraction of sites

I. jump distance

140

2-n 3n-n -1 -1 mol m kg s -1 s

m s-1

m

mol

mmol kg-1

bar J mol-l K- 1

m

mol s-1

mol m- 3 s- 1

mol kg-l s-1

m2 kg-1

J mol-l K-1

K

s

s -1 m s

kg

m

A effective radial thermal conduct!-er -1 -1 K-1 vity J m s

-1 -1 K-1 Af thermal conductivity of fluid J m s

-1 s-1 K-1 thermal conductivity of particle Jm '-p -1 -1 ll dynamic viscosity kg m s

\) coefficient in stoichiometric

equation

~ extent of reaction

p density cr2 variance

41 sphericity factor

Abbpeviations

A acrylonitrile

ABS acrylonitrile-butadiene-styrene

ACO ACON

ACN BET

copolymer

acrolein acetonitrile

acrylonitrile

Brunauer-Emmet-Teller (determina-

tion method of catalyst surface area)

Fo Fourier number

H hydrogen IUPAC International Union of Pure and

Applied Chemistry LCLo lowest lethal concentration

N ammonia

NBR nitrile-butyl-rubber NTP normal temperature and pressure

o oxygen P propene

P~ P~clet number ppm parts per million

mol kg m-3

s2

ppm

Re = ~. Reynolds number (equation 5.11) avq>u

141

pud Redp = ~ Reynolds number with ref. to particle

SAN

Sc SOHIO

TLV

TWA

Subsar>ipts

a

b

cat e

eq

f

F

I

L

0

p

r

s

t

Symbol

c

142

diameter

styrene-acrylonitrile copolymer

Schmidt number Standard Oil Company of Ohio

Treshold Limit Value

time weighted average

activation, adsorbed, axial

bulk

catalyst

effective, reactor outlet equilibrium

fluid

fractional

integral

liquid state, bed length

initial value, reactor inlet

particle, pressure, pulse

reaction, reoxidation, radial

surface, superficial at time t

2-0 vacancy

ppm

Summary

This thesis deals with the synthesis of acrolein and

acrylonitrile over a bismuth molybdate catalyst. During

the last twenty years catalysts based on this compound

played an important role both in the commercial production

of acrylonitrile and in catalytic research with respect to

the elucidation of the mechanism of the selective oxida­

tion of olefins over oxide catalysts.

Bismuth molybdate is not only a catalyst for the oxi­

dation and ammoxidation of propene but catalyzes at the

same time also the oxidation of ammonia to nitrogen and

the ammoxidation of acrolein to acrylonitrile.

In order to contribute to the elucidation of the reac­

tion mechanism we have investigated the kinetics of these

four reactions. The experimental work can be divided into

two parts:

1. The determination of the kinetics of these four reac­

tions in continuously operating fixed bed flow reactors

under differential and integral conditlons at tempera­

tures between 648 and 723 K and at atmospheric pressure;

2. An investigation into the behaviour.of bismuth molyb­

date acting as oxidant in the absence of oxygen and in

the presence of substoichiometric amounts of oxygen ' and ammonia. These reactions have been studied in a

thermobalance and in a socalled pulse reactor.

At temperatures above 673 K and in the range of the

concentrations of this investigation the oxidation and

ammoxidation reactions are all first order in the organic

reactant and zero order in oxygen. At temperatures below

673 K the order in the organic reactant decreases with

increasing concentrations. At these temperatures the rate

of propene conversion is also inhibited by acrolein. The

ammoxidation of propene has a slightly negative order in

ammonia. The catalyst shows initially a high activity for

the oxidation of ammonia to nitrogen, which reaction is

first order in ammonia. The ammoxidation of acrolein is

zero order in ammonia. The activation enthalpies for the

143

oxidation and the ammoxidation of propene are very similar -1 . .

and increase from about 60 kJ mol at temperatures above 673 K to about 100 kJ mol-l at temperatures below 673 K.

Experiments in a thermobalance in the same temperature

range have shown that the initial rate of catalyst weight

loss is a function of the propene concentration. At tem­

peratures below 673 K even at a low degree of reduction

the oxygen diffusion in the catalyst has a major influence

on the overall rate of reduction of the catalyst. The ad­

sorption of acrolein on a partially reduced catalyst

appears to be a relatively slow process. The reoxidation

of a reduced catalyst with gas phase oxygen is an extreme­

ly fast reaction and is first order in oxygen at tempera­

tures above 673 K. The oxidation of propene with substoi­

chiometric amounts of oxygen and at low propene conversion

can be carried out under stationary conditions with the catalyst operating in a partially reduced state, Under

these conditions the rate of reaction is first order in

oxygen for low oxygen concentrations and the amount of

acrolein adsorbed is a function of the degree of reduction and of the temperature.

Pulse experiments with propene-helium gas mixtures con­

firmed the adsorption behaviour of partially reduced bis­

muth molybdate for acrolein. When small quantities of

oxygen are added the rate of acrolein formation per pulse is again a function of the partial pressure of oxygen.

From experiments with ammonia-propene-helium gas mixtures

without oxygen it became clear that the rate of oxygen

suppletion by the catalyst is enhanced in the presence of

ammonia in the same way as in the presence of gas phase . 2- 2-

oxygen and it has been assumed that 0 and NH species perform the same function.

The presence of ammonia as the third reactant in the acrylonitrile synthesis is an important expedient to de­

rive a mechanistic model from the experiments and from

literature data. We have concluded that ammonia molecules

are activated on two different active centres in two different ways. The activation for the ammoxidation of

144

propene and acrolein occurs on the active centre connected with bismuth, whereas that for the oxidation of ammonia

occurs on two different active centres connected with

molybdenum and with bismuth. From this we have concluded that propene is activated on molybdenum. The activation of oxygen takes place after the reduction of the catalyst

to some extent, according to the reduction-oxidation me­

chanism proposed by Mars and van Krevelen and is related

to the diffusion of oxygen in the catalyst. From the kinetic data we have concluded that the rate

determining step of the oxida~ion and ammoxidation of

propene is the abstraction of the first proton of propene.

The acrolein ammoxidation may either proceed via a con­

certed mechanism or via an adsorbed c3H4 intermediate.

145

146

Samenvatting

In dit proefschrift wordt de synthase van acroleine en

acrylonitril behandeld. Bismuth molybdaat wordt gebruikt

als katalysator. Gedurende de laatste twintig jaar hebben

katalysatoren op basis van deze verbinding niet alleen

een belangrijke rol gespeeld in deproduktie van acrylo­

nitril maar ook heeft bismuth molybdaat in het fundamen­

teel katalytisch onderzoek naar het mechanisme van de

selectieve oxidatie van olefinen met behulp van oxidische

katalysatoren een centraleplaats ingenomen.

Bismuth molybdaat is niet alleen een katalysator voor

de oxidatie en ammoxidatie van propeen, maar versnelt

tegelijkertijd ook de oxidatie van ammoniak tot stikstof

en de ammoxidatie van acroleine tot acrylonitril.

Om een bijdrage te leveren aan de opheldering van het

reactiemechanisme hebben we deze vier reacties aan een

kinetisch onderzoek onderworpen. Het experimentele werk

kan in twee stukken worden onderverdeeld:

1. De bepaling van de kinetiek van deze vier reacties in

continu werkende reactoren met een vast katalysatorbed

zowel onder differentiele als onder integrale kondities

bij temperaturen tussen 648 en 723 K en onder atmos­

ferische druk;

2. Een onderzoek naar het gedrag van bismuth molybdaat

als oxidatiemiddel in afwezigheid van zuurstof en bij

gebruik van ondermaat zuurstof of ammoniak. Deze reac­

ties zijn bestudeerd met gebruikmaking van een thermo­

balans en een zogenaamde pulsreaktor.

Bij temperaturen boven 673 K zijn in het onderzochte

concentratie gebied de oxidatie en ammoxidatie reacties

eerste orde in de organische reaktant en nulde orde in

zuurstof. Beneden 673 K daalt de orde in de organische

reaktant bij toenemende concentraties. Bij deze tempera­

turen wordt de omzettingvan propeen door acroleine geremd.

De ammoxidatie van propeen vertoont een kleine negatieve

orde in ammoniak. De katalysator is initieel zeer actief

in het oxideren van ammoniak tot stikstof, een reactie die

117

eerste orde in anunoniak is. De anunoxidatie van acroleine

is nulde orde in ammoniak. De activeringsenthalpieen voor

de oxidatie en de ammoxidatie van propeen zijn vrijwel

gelijk en nemen toe van ongeveer 60 kJ mol-l bij tempera•

turen hoven 673 K tot ongeveer 100 kJ mol-l bij tempera­

turen beneden 673 K.

Experimenten met propeen-helium gasmengsels in het­

zelfde temperatuurgebied in een thermobalans hebben aan­

getoond dat de initiele snelheid waarmee het katalysator

gewicht afneemt een functie is van de propeen concentra­

tie. Bij temperaturen beneden 673 K en zelfs bij lage

reduktiegraden heeft de zuurstofdiffusie in de katalysator

een grote invloed op de reduktiesnelheid van de katalysa­

tor. De adsorptie van acroleine op een gedeeltelijk gere­

duceerde katalysator blijkt vrij langzaam te verlopen. De

reoxidatie van een gereduceerde katalysator met zuurstof

is een zeer snelle reactie die bij temperaturen hoven 673

K een eerste orde gedrag in zuurstof vertoont. De oxida­

tie van propeen met ondermaat zuurstof kan bij een lage

propeen conversie stationnair verlopen, waarbij de kata­

lysator zich in een gedeeltelijk gereduceerde toestand

bevindt. Onder deze omstandigheden is de reactie bij lage.

zuurstofconcentraties eerste orde in zuurstof. De gead­

sorbeerde hoeveelheid acroleine onder stationnaire om­

standigheden is een funktie van de reduktiegraad en van

de temperatuur.

Puls experimenten met propeen-helium gasmengsels hebben

bevestigd dat gedeeltelijk gereduceerd bismuth molybdaat

acroleine adsorbeert. Als kleine hoeveelheden zuurstof

worden meegepulst is de vormingssnelheid van acroleine een

funktie van de partiele druk van zuurstof. Experimenten

met ammoniak-propeen-helium gasmengsels zonder zuurstof

hebben aangetoond dat de zuurstof afgiftesnelheid van de

katalysator in aanwezigheid van ammoniak op dezelfde wijze

wordt verhoogd als die in aanwezigheid van zuurstof zonder

ammoniak tijdens de oxidatie van propeen. Er wordt verondersteld dat o2- en NH2- species dezelfde funktie

vervullen bij de synthese van acroleine respectievelijk acrylonitril.

148

De aanwezigheid van ammoriiak als de derde reaktant in

de acrylonitril synthase is een belangrijk middel om een

mechanistisch model af te leiden uit de experimenten en de literatuurgegevens. We hebben geconcludeerd dat ammo­

niak op twee verschillende manieren wordt geactiveerd. De

activering van ammoniak in de ammoxidatie van propeen en acroleine gebeurt op een "active centre" samenhangend met

bismuth, terwijl die voor de oxidatie van ammoniak tot

stikstof op twee verschillende "active centres" plaats­

vindt, nl. op een bismuth- en op een molybdeen-centre.

Propeen wordt op het laatste "active centre" geactiveerd. De activering van zuurstof vindt plaats nadat de kataly­

sator enigszins gereduceerd is volgens het Mars-van Kre­

velen redox mechanisme en de inbouw van zuurstof in de

produkten geschiedt via een diffusie door het katalysator rooster.

Uit de kinetische gegevens hebben we eveneens gecon­

cludeerd dat de snelheidsbepalende stap van de oxidatie

en ammoxidatie van propeen de eerste proton afsplitsing

van propeen is. De ammoxidatie van acroleine verloopt via een concerted mechanisme of via een geadsorbeerd c3H4 intermediair.

149

Dankwoord

Het onderzoek weergegeven in dit proefschrift is tot

stand gekomen da~ij de medewerking van velen. In het

bijzonder geldt dit voor de leden van de vakgroep Chemi­

sche Technologie van de Afdeling der Scheikundige Techno­

logie. Ik dank speciaal de heren D. Francois, J. van

Hettema, w. van Lith, G. van de Put, A. Sommen, L. Verhaar

en R. van der Wey, die steeds bereid waren de experimenten

te laten slagen.

Veel dank ben ik verschuldigd aan de afstudeerders, de

heren E. van Poelvoorde, P. Derks, J. van Laarhoven, P.

Linders, J. Voet, A. Tjang, R. van Collenburg, N. Willem­

sen, P. Evertse, P. Florack, F. Hautus, A. de Laet, F.

Kroes, H. Houben, F. Douven, J. Spork, P. Oostveen en H.

van Liempt, die enthousiast aan het onderzoek hebben mee­

gewerkt. Anderen die hebben bijgedragen dank ik in de

persoon van de heer R. Poulina, die tijdens twee stages

metingen heeft verricht.

Mijn dank gaat uit naar mijn oud-collega's van de werk­

groep Gasfase reacties dr. J. Steenhof de Jong, dr. H.

Heynen en dr. M. Boersma met wie ik over dit onderzoek

interessante discussies h.Elb gevoerd. De heer Ph. Batist, die op kenmerkende wijze dit werk gestimuleerd heeft dank

ik zeer. Voorts dank ik dr. G. Visser voor de wijze waarop hij heeft bijgedragen tot de verdieping van het inzicht in

de katalysator structuur en mijn collega's van andere vak­

groepen voor de nuttige wetenschappelijke informatie die

zij mij hebben verschaft.

Bij de vormgeving van het proefschrift heb ik de onmis­

bare steun gekregen van de heren W. van Lith en R. van der

Wey. Mevr. E. Eichhorn-Meijers heeft op concientieuze

wijze het typewerk verricht. Hen allen dank ik zeer.

Gaarne maak ik van deze gelegenheid gebruik om ook

anderen, buiten mijn gezin en mijn werkverband, te danken

voor hun stimulerende bijdragen.

150

STELLINGEN

1 Otsubo et al en Miura et al houden - gelet op de kondi­

ties waaronder zij de katalysatoren bereiden - bij de

verklaring van het mechanisme van de acroleine synthese

over bismuth molybdaat onvoldoende rekening met de mo­

gelijke aanwezigheid van vrije oxiden en met de beweeg­

lijkheid van de zuurstof in de katalysator.

T. Otsubo, H. Miura, Y. Morikawa, T. Shirasaki,

J. Catal. 240 {1975)

H. Miura, T. Otsubo, T. Shirasaki, Y. Morikawa,

J. Catal. 56, 84 (1979)

Dit proefschrift, hoofdstuk 7

2 Door voorbij te gaan aan o.a. het sterisch effekt van

de substituenten komen van Krevelen en Chermin met hun

berekeningsmethode voor de plafond temperatuur van een

polymerisatie reactie voor verschillende op dezelfde

plaats gesubstitueerde monomeren tot dezelfde uitkomst,

hetgeen in strijd is met de experimenteel gevonden

waarden.

D.W. van Krevelen, Properties of Polymers, 2nd ed.

Elsevier, Amsterdam 1976

R.W. Lenz, Organic Chemistry of Synthetic High

Polymers, Interscience, New York 1967

J. :&J:"andru:;>, E.H. Immergut, Polymer Handbook,

John Wiley, New York 1965

3 De onduidelijkheid met betrekking tot het begrip selec­

tiviteit in de chemie is door de IUPAC slechts gedeel­

telijk weggenomen. Het onderdeel integrale selectivi­

teit voor een reactie uitgevoerd in een continu bedreven

reactor moet nog worden gedefinieerd.

Manual of Symbols and Terminology for physico che­

mical Quantities and Units, Appendix II, Part II,

Heterogeneous Catalysis, Pure & Appl. Chern~ 46, 71 (1976)

4 Voor een juistE;! toepassing van primaire, droge batte­

rijen is het noodzakelijk, zowel ten aanzien van energie

gebruik als ten aanzien van milieu aspekten dat de eti­

kettering in plaats van de weinig zeggende informatie

"medium duty", "transistor batterij" of "long life" dui­

delijke gegevens verstrekt over de scheikundige samen­

stelling, kapaciteit, belastbaarheid en houdbaarheid van

de batterij.

Consumentengids 24, 259 (1976)

Consumentengids 25, 14 (1977)

5 De interpretatie van de veranderingen in het 13c NMR

spectrum van 1-buteen door chernisorptie op zeolieten en

de vertaling daarvan naar elektronen dichtheden c.q. verschuivingen leidt tot verkeerde conclusies.

D. Denney, W.M. Mastikhin, s. Narnba, J. Turkevich,

J. Phys, Chern. 82, 1752 (1978)

6 De bewering van Wauters et al dat gesilyleerde polyolen

in het algerneen geen (geprotoneerde) molekulaire ionen

geven in de chernische ionisatie rnassaspektrornetrie met isobutaan als reaktiegas, is onjuist.

E. Wauters, F. Vangaever, P. Sandra, M. Verzele,

J. Chrornatogr. 170, 133 {1979)

A.C. Schoots, F.E. Mikkers, C.A.M.G. Cramers,

s. Ringoir, J. Chromatogr. Biomedical Appl. (ver­schijnt binnenkort)

7 Minstens vier van de diffusiepaden in het systeem

Cu-Ni-Zn die Wirtz en Dayananda presenteren als resul­

taat van hun onderzoek zijn op grond van de wet van be­

houd van rnassa onmogelijk. Hun theoretische beschouwingen omtrent het verloop van de diffusie in dit systeern komen

hierdoor op losse schroeven te staan.

L.E. Wirtz, M.A. Dayananda, Metal. Trans. A 8A, 567 {1977)

8 Aangezien vele chemische en fysische wegen voor milieu­

belastende stoffen naar de oceaan leiden, verdient het

aanbeveling de bestudering van bodem-, lucht- en water­

verontreiniging uit te voeren in samenhang met de

oceanografie.

9 Hoewel het een goed gebruik is experimenten reeksen

zorgvuldig te plannen, moet de betekenis van het gelso­

leerde, ad hoc experiment niet worden onderschat.

P.C. Sander, Intreerede, T.H. Eindhoven (1978)

10 Door de toepassing van zig-zag kanalen in een centri­

fugaal-zifter wordt het principe van het zuivere meer­

trapsproces, zoals dit geldt in een zig-zag zifter in

het zwaartekrachtsveld, nadelig beinvloed.

F. Kaiser, Chern. Ing. Techn. ~~ 273 (1963)

M.M.G. Senden, Proefschrift, T.H. Eindhoven (1979)

11 De bewering dat de kostprijs voor gas verkregen door

ondergrondse vergassing van steenkoollagen met 80 % kan

worden gereduceerd door verdubbeling van de afstand der

boorgaten,is bij gebrek aan uitkomsten van een onder­

zoek naar de invloed van deze afstand op het rendement,

ongegrond.

P.N. Thompson, Endeavour~ (2), 93 (1978)

12 Kinderen met een lichte achterstand in de ontwikkeling

van de fijne motoriek kunnen baat vinden bij het be­

spelen van een muziekinstrument, waarbij de gewenste

toonhoogte wordt verkregen door een juiste plaatsing

van de vingers. Vooral een zelfgemaakte bamboefluit

is hiervoor geschikt, omdat bij het intoneren de

vingers in volgorde bij het spel betrokken raken.

13 De periodieke betrouwbaarheidstest van de sirenes in

de gebouwen van de afdeling der Scheikundige Technolo­

gie onder werktijd is zonder een evacuatie van het

personeel ongewenst, aangezien deze test nadelige bij­

verschijnselen zoals gehoor irritatie, gehoor bescha­diging of verlies van arbeidstijd veroorzaakt.

Eindhoven, 22 juni 1979 S.P. Lankhuijzen