OXIDATION OF TOLUENE AND XYLENE
OVER BISMUTH MOLYBDATE CATALYSTS
K. VAN DER WIELE
Z ; ^ ^ ^ y c 3 /
P1797
5401
I IC10053
78607
BIBLIOTHEEK TU Delff P 1797 5401 537860OXIDATION OF TOLUENE AND XYLENE
OVER BISMUTH MOLYBDATE CATALYSTS
P R O E F S C H R I F T
TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN
DE TECHNISCHE WETENSCHAPPEN AAN DE
TECH-NISCHE HOGESCHOOL DELFT, OP GEZAG VAN DE
RECTOR MAGNIFICUS PROF.DR.IR. H. VAN BEKKUM,
VOOR KEN COMMISSIE AANGEWEZEN DOOR HET
COLLEGE VAN DEKANEN TE VERDEDIGEN OP
WOENSDAG 7 JULI 1976 TE 14.00 UUR
DOOR
KEES VAN DER WIELE
f-jC^-J
6^Vo /
SCHEIKUNDIG INGENIEUR
GEBOREN TE ZWOLLE
DIT PROEFSCHRIFT IS GOEDGEKEURD DOOR DE PROMOTOR
P R O F . D R S . P.J. VAN DEN BERG
Laboratorium voor Chemische Technologie
Technische Hogeschool Delft
Aan alien die ertoe hebben bijgedragen dat de "Delftse tijd" voor mij een bijzonder prettige en leerzame tijd is geweest, waaraan ik nog lang goede herinneringen zal behouden.
D A N K B E T U I G I N G
De totstandkoming van dit proefschrift is mogelijk gemaakt door de inzet van vele personeelsleden van de Technische Hogeschool. In het bijzonder geldt het de servicegroepen en -diensten van het Labo-ratorium voor Chemische Technologie, en de Instrumentmakerij van de Afdeling der Scheikundige Technologie. Met name wil ik danken de Keren Piet Verbooijen en Loek Peffer voor hun voortdurende assisten-tie ten aanzien van analyse-apparatuur en de Heer M.J. Huisman voor zijn hulpvaardigheid in en ora de laboratoriumzaal.
Voorts ben ik dankbaar voor de vruchtbare discussies en de ple-e
zierige samenwerkmg met collega Albert Gerritsen en de 5 jaars-studenten Jan Grootendorst, Wim Both, Fred Platzek, Johan Kooper, Arie van Dongen, Karel Warmenhoven, Jan Verra, Fred Koekoek, Henk Hogervorst en Ab Rijkeboer.
Voor het gereedmaken van het manuscript wil ik mijn bijzondere erkentelijkheid betuigen aan Mevrouw Flory Molenaar-Mathijsen voor het typewerk, en aan de Keren Wim Jongeleen en Koos Kamps voor het tekenwerk en het fotografisch verkleinen van figuren en teksten.
Tenslotte wil ik graag mijn waardering kenbaar maken voor de onmisbare bijdragen van Akzo Chemie Nederland B.V. (Amsterdam) en de Heer Ph.A. Batist (Technische Hogeschool Eindhoven), die de kataly-sat,oren voor dit onderzoek ter beschikking stelden.
SUMMARY
The selective catalytic oxidation of hydrocarbons with air or oxygen is one of the most important methods to introduce reactive groups into hydrocar-bon molecules and thus to convert hydrocarhydrocar-bons into basic materials for numer-ous chemical syntheses. In particular the gas-phase oxidation and ammoxidation of olefins have great industrial importance. The development of similar pro-cesses to convert relatively cheap methyl benzenes into valuable aromatic aldehydes or acids may be very interesting in view of the increasing demand for aromatic compounds, particularly for fibres and plastics.
The possibility of selective side chain oxidation of aromatics by means of a mild oxidation catalyst is investigated in this thesis. Bismuth molybdate is selected as the catalyst for this study because of its outstanding quali-ties for the oxidation of propylene and isobutene to unsatured aldehydes, and in view of the analogy between vinylic and aromatic methyl groups.
The experimental work consists of two parts:
1. A study of the catalyst reactivity with respect to the reactants toluene and air, either mixed or separate, particularly after partial reduction of the catalyst. The experiments are carried out in a pulse reactor at 500 C and a pressure of 1.5 atm gauge.
2. A kinetic study of the oxidation of toluene and the xylenes with air. The experiments are carried out in a continuously operated fixed-bed reactor at 400 - 500 C, atmospheric pressure and space times below 4 s.
In both parts of the experimental work two catalysts are used: a pure bismuth molybdate catalyst and a commercial ammoxidation catalyst; the latter contains silica and some phosphate in addition to bismuth molybdate.
The main reactions occurring are aldehyde formation, complete combustion and dealkylation. Dealkylation is relatively unimportant for the oxidation of toluene, in which case the reaction scheme is given by:
BENZALDEHYDE
TOLUENE ^ CO, CO 2
(toluene, benzene) are formed besides the main product, tolualdehyde. Moreover the dialdehyde (terephthalaldehyde) is formed in the case of p-xylene.
The kinetics of the reactions in the above scheme can be adequately des-cribed by first order equations for the reacting components, based on a redox mechanism (Mars-Van Krevelen). The ratio of the rate constants of the reac-tions 1, 2 and 3 is approximately 1 : 1 : 6 , which implies that even at low conversion about 50% of the toluene is combusted. Some improvement is achieved by addition of water to the feed. The xylene oxidation kinetics are essen-tially similar to those of the toluene oxidation. The oxidation rate and acti-vation energy, however, are considerably larger for o- and p-xylene than for m-xylene and toluene.
Pulse experiments have demonstrated that the toluene oxidation also pro-ceeds in the absence of gas-phase oxygen. The selectivity is approximately the same as in the presence of gas-phase oxygen. Remarkably, catalyst reduc-tion and the presence or absence of gas-phase oxygen have a different effect on the oxidation. The latter parameter primarily influences the rate of oxida-tion, while the degree of reduction influences the selectivity: above a cer-tain degree of reduction, which depends on the catalyst used, dealkylation is the main reaction.
A hypothesis concerning the mechanism has been derived from the combined results of the pulse experiments and the kinetic study. This hypothesis explains the high percentage of combustion in spite of the mildness of the catalyst. With respect to the oxygen transfer, the presence of two types of lattice oxygen is suggested to explain the behaviour of reduced catalysts.
C O N T E N T S P8
S U M M A R Y 5 C H A P T E R 1 I N T R O D U C T I O N 9
1.1 Catalytic oxidation in general 9 1.2 Side chain oxidation of aromatic hydrocarbons 10
1.3 Types of bismuth molybdate catalysts 10
1.4 Aim and scope of the thesis 11 CHAPTER 2 T O L U E N E O X I D A T I O N PART I - PULSE E X P E R I M E N T S 12
2.1 Introduction 12 2.2 Materials and catalyst 12
2.3 Apparatus 13 2.4 Pulse experiments with an oxygen-free carrier gas 15
2.4.1 Toluene oxidation by a completely oxidised catalyst 15 2.4.2 Catalyst reduction by hydrogen and carbon monoxide 17
2.4.3 Reoxidation of the catalyst with air 18 2.4.4 Oxidation of toluene by a partially reduced catalyst 19
2.4.4.1 Experiments after catalyst reduction with hydrogen 19 2.4.4.2 Experiments after catalyst reduction with carbon monoxide 23 2.4.4.3 Experiments after catalyst reduction with
hydrogen and partial reoxidation by air 24 2.4.5 Oxidation of toluene with air over a partially reduced 25
catalyst
2.4.6 Experiments with the Ketjen A catalyst 27 2.4.7 Summary of results and discussion 29 2.5 Pulse experiments with air as the carrier gas 33
2.5.1 Determination of activation energies 34 2.5.2 Influence of the toluene concentration on conversion 37
and selectivities
CHAPTER 3 T O L U E N E O X I D A T I O N PART II - C O N T I N U O U S F L O W 38 E X P E R I M E N T S
3.1 Introduction 38 3.2 Materials and catalysts 39
3.3 Apparatus 39 3.4 Qualitative experiments 41
3.5 Catalyst activation and stability 42 3.6 Introductory kinetic experiments 44
3.7 Design of a kinetic model 47 3.8 Experimental evaluation of the model 52
3.8.1 Experimental programme 52 3.8.2 Method of estimation of rate constants 52
3.8.3 Results of regression calculations 55 3.8.4 Considerations on model adequacy 58 3.9 Toluene oxidation in the presence of water vapour 63
3.10 Discussion 64 3.10.1 Optimum conditions for benzaldehyde production 64
3.10.2 Reaction scheme and product spectrum 64
3.10.3 Oxidation kinetics 65 CHAPTER 4 O X I D A T I O N OF X Y L E N E S 67
4.1 Introduction 67 4.2 Apparatus, materials and catalyst 67
4.3 Introductory experiments 68 4.3.1 Determination of activation energies 70
4.4 Experiments with varying space time and oxygen partial pressure 73
4.5 Kinetic modeling 76 4.5.1 Regression calculations 77
4.5.2 Results of regression calculations 79
4.6 Discussion 83 CHAPTER 5 FINAL D I S C U S S I O N 85
5.1 Catalyst reduction related to the structure of the catalyst 85
5.1.1 Structure of bismuth molybdate 85
5.1.2 Catalyst reduction 86 5.2 Mechanism of the oxidation 90
5.2.1 Reaction between oxygen and the catalyst 90 5.2.2 Reaction between toluene (xylene) and the catalyst 93
5.3 Conclusion 97 R E F E R E N C E S 98 S A M E N V A T T I N G 100 T O E L I C H T I N G OP HET O N D E R W E R P VAN DIT P R O E F S C H R I F T VOOR 102
B E L A N G S T E L L E N D E N I E T - V A K G E N O T E N
CHAPTER 1
I N T R O D U C T I O N
1.1 C a t a l y t i c o x i d a t i o n in general
Oxidation of hydrocarbons is one of the oldest large scale processes in the history of industrial organic chemistry and one of the first areas where catalysts were applied in gas-phase reactions (43). The necessity for the application of catalysts to achieve selectivity is clear from the thermodynam-ics of oxidation: in general combustion to carbon oxides and water is the preferable reaction, even at low oxygen/hydrocarbon ratio.
Selective oxidation of hydrocarbons, and in particular the heterogeneous-ly cataheterogeneous-lysed gas-phase oxidation is a very attractive method to convert rela-tively inert hydrocarbons into basic products for chemical synthesis, because the process is straightforward and the second reactant (air) is free avail-able.
The use of metal oxide catalysts has already been proposed at the end of the last century (43^53). One of the early technical processes in this field is the oxidation of naphtalene to phthalic acid anhydride over V.O , realized before World War II and still of commercial importance today. The superior quality of modified V„0_ catalyst or aompound oxides (e.g. tin vanadate (62)) has been known since the beginning of this century. A major breakthrough, how-ever, occurred in the 1950's when it was discovered in the Sohio laboratories that certain molybdates, and in particular bismuth molybdate were active and highly selective catalysts for the oxidation of propylene to acroleine (70, 84). This discovery was followed by the finding that propylene can be oxidized in one step to acrylonitrile by a mixture of oxygen and ammonia over the same catalyst.
These discoveries have initiated research programmes all over the world in industry as well as at universities to uncover similar valuable processes, and to investigate the fundamental aspects of catalysis by metal oxides (4,36, 42,52,60,79). Numerous studies have been devoted to correlations between
selectivity and activity of oxidation catalysts and fundamental properties like reducibility (47), heat of formation of the metal oxides (6-8,80), semi-conductor properties (26,87), exchange with gas-phase oxygen (12,83,86) , etc.
Although these studies have enormously increased the insight in oxidation mechanisms, selection of a suitable catalyst on a theoretical basis is still
problematic, especially for the compound oxide catalysts. One basic principle of selective oxidation, however, has become clear: the oxygen that reacts with the hydrocarbon molecule must be chemically bonded to the catalyst, because loosely bonded oxygen (sorbed oxygen) in general causes non-selective oxida-tion. As a consequence the metal oxide must be able to transfer oxygen by a redox process, which explains the suitability of transition metal oxides in selective oxidation processes.
1.2 Side chain oxidation of aromatic hydrocarbons
The selective side chain oxidation of aromatic hydrocarbons to aldehydes or acids by a gas-phase process has not received much attention in the last decades*. Such processes may be very attractive, however, provided that a good selectivity is obtained: the difference in price between feed (e.g. toluene, p-xylene) and product (e.g. benzaldehyde, terephthalic acid) is large and an increasing demand for aromatic products in general can be expected in the field of high quality fibres and plastics.
In this thesis the oxidation of toluene and xylene are studied. The major part of the study concerns toluene, which represents the simplest model compo-nent for aromatic side chain oxidation.
Bismuth molybdate catalysts are selected for this study for three rea-sons. Firstly the analogy between the methyl group in toluene and propylene** suggest that bismuth molybdate might be an effective catalyst for both. Sec-ondly molybdenum based catalysts are known to be practically inactive for the oxidation of benzene, in contrast with vanadium based catalysts. This inacti-vity indicates that molybdates are too mild to enable a successful attack on the aromatic nucleus. In the third place the use of a catalyst of which much is known from other studies, is advantageous for the understanding of its behaviour in a new process.
1.3 Types of bismuth molybdate catalysts
Three main types of bismuth molybdate catalysts can be discerned: a. Pure bismuth molybdate catalysts, only containing Bi, Mo and 0.
* Except for the o-xylene oxidation to phthalic acid anhydride (36,42). ** The dissociation energy for H-atom abstraction from the methyl group, for
example, is 85 kcal/mole for both, compared with about 100 kcal/mole for other hydrogen atoms in these molecules.
b. Supported bismuth molybdate catalysts, containing also silica and some P. c. Multicomponent catalysts, which contain a variety of metal oxides besides
those of Mo and Bi.
The first group of catalysts is only used in laboratory studies, due to its poor mechanical strength. The second group of catalysts is the one that appeared in the first patents (81) and has been widely used in industry to manifacture acrylonitrile and acroleine. These catalysts are now replaced by the multicomponent catalysts. This group will not receive attention in this thesis: their nature is rather complicated, while the catalytic properties are essentially the same. (Small improvements, however, are of great commercial importance, witness the fact that a continuous stream of at least two patent applications per week appears today in the "Central Patent Index" concerning multicomponent catalysts for olefin oxidation or ammoxidation.) Specimen of both the groups a. and b. were used in this study.
1.4 Aim and scope of the thesis
The aim of this thesis is to investigate the feasibility of selective aromatic side chain oxidation over bismuth molybdate catalysts, and to get a better insight in complex oxidation kinetics and mechanisms.
The kinetics of the oxidation of the model component toluene are pro-foundly studied to determine the optimum conditions for selective oxidation (Chapter 3 ) .
Pulse experiments are performed to study the reaction of toluene with the catalyst in absence of air, and to determine whether a regenerative process might have interesting aspects. Moreover, pulse experiments are carried out with catalysts reduced to a well defined degree, in order to investigate the possible significance of catalyst reduction for the steady state toluene/air oxidation process. The experiments may also provide information about the oxi-dation mechanism (Chapter 2 ) .
The kinetics of the oxidation of xylenes is studied to investigate the applicability of the toluene oxidation kinetics for aromatic side chain oxida-tion in general, and to determine whether unexpected products might be formed in the xylene oxidation (Chapter 4 ) .
In the final Chapter (Chapter 5) the correlation between results pre-sented in previous chapters is re-examined, and an attempt is made to indicate the essential pathways of the methylbenzene oxidation over bismuth molybdate from a mechanistic point of view.
CHAPTER 2
T O L U E N E O X I D A T I O N PART I - PULSE E X P E R I M E N T S
2. 1 I n t r o d u c t i o n
The pulse experiments described in this chapter were carried out in part preceding the experiments with continuous flow (Chapter 3) and in part paral-lel to them. The aim of the pulse experiments is mainly the qualitative inves-tigation of reactions between the catalyst and the reactants (mixed or sepa-rately) under various conditions. The conditions that were varied are the oxi-dation state of the catalyst, the composition of the pulse and the reactor temperature.
The advantage of pulse experiments relative to continuous flow experi-ments is that the reaction between the catalyst (oxidized or partially reduced) and individual components involved in the oxidation reaction can be studied. Information is thus obtained that can provide more insight in the mechanism of the catalytic oxidation process.
The experimental work consists of two parts. In the first part an oxygen-free carrier gas is used, and the experiments, for the greater part concern partially reduced catalysts. In the second part air is used as the carrier gas. Thus the catalyst remains in the oxidized state, and because of the pres-ence of gas-phase oxygen the results are closely related to those of experi-ments with continuous flow of reactants.
2.2 M a t e r i a l s and c a t a l y s t s
All of the gases (helium, nitrogen, hydrogen, air, carbon monoxide, car-bon dioxide) were obtained from cylinders (research grade). Helium and nitro-gen, used as the carrier gases, were purified with a copper catalyst (BASF R3-11) at 120 C to reduce the oxygen concentration from the 3 ppm level to below 0.03 ppm. For the gases used in the pulses no purification was found necessary. Analytical grade toluene and benzaldehyde were used.
Two catalysts were studied:
a. Pure bismuth molybdate
This catalyst was prepared in the Laboratory of Prof.dr. G.C.A. Schuit
(Eindhoven University of Technology). The method of preparation is reported by Batist et al (19). The atomic ratio Bi/Mo was 2/1.3, which implies some excess of MoO with respect to koechlinite (Bi„0 -MoO.). The mechanical strength of the particles was poor. A sieve fraction of 0.35 - 0.65 mm, with a specific
2
surface area of about 3 m /g, was used.
b. Commercial bismuth molybdate
This catalyst, the "KETJEN A" ammoxidation catalyst used in the SOHIO acrylonitrile process, was supplied by AKZO CHEMIE NEDERLAND BV. It consists of a mixture of oxides of bismuth, molybdenum and phosphorus, and silica (atomic ratio Bi/Mo/P/Si approximately 2/2.5/0.2/14, according to information of the manufacturer). Irregular shaped particles were crushed and sieved. A sieve fraction of 0.35 - 0.65 mm was used. The specific surface area was
2
approximately 12 ra /g (BET method, nitrogen adsorption). This value is not very significant however, because it includes an unknown amount of inactive silica surface.
Most of the experiments were performed on catalyst (a); some experiments however, were carried out on both catalysts.
2.3 Apparatus
A diagrammatic flowsheet of the apparatus is shown in Figure 2-1. Except for the control unit for gas flows the entire apparatus is made of stainless steel 315.
1. gas flow control 2. saturator 3. pulse valve (2.3 ml) 4. pulse valve (0.46 ml) 5. injection assembly 6. switch valve 7. reactor 8. analysis system
Figure 2-1 Diagrammatic flowsheet of the apparatus
13 1
1
2 L^*
H
Z'
_
. ^ r ^
4 , 1 • l 3 ^ 1p
D
^ 6-1 — • — o • '
The reactor consists of a tube (volume 1 ml, inner diameter 6 mm) which is externally heated by a fluidized bed of sand particles. The catalyst (500 -700 mg) is placed between two plugs of quartz wool. A constant flow of carrier gas passes through the reactor into the analysis system.
The pulse injection system consists of two injection valves, a syringe injection assembly and a switch valve, placed together in an air thermostat (130 C ) . The three valves are of the same type: pneumatically operated 6-way valves with strengthened Teflon sliders. One injection valve (sample volume 0.46 ml) serves for the toluene containing pulse gas, while the other (sample volume 2.3 ml) serves for injection of oxidizing and reducing gases (air, hydrogen, carbon monoxide). Syringes are used for special purposes. Toluene is introduced into the pulse gas stream by passing this stream through a satura-tor contained in a thermostat. The switch valve controls the injection of the pulse before or after the reactor, which enables analysis with or without reaction.
-^
1. silicon gum column 2. Porapak Q column 3. mol. sieves column
4. split control valve 5. flame ionisation detector 6,7. thermal conduct, detectors
Figure 2-2 Analysis system
The analysis system allows quantitative determination of benzene, tolu-ene, benzaldehyde, nitrogen, carbon monoxide, carbon dioxide and water. The system consists of three chromatographic columns with inner diameters of 4 mm (Figure 2-2). One half of the analysis stream is passed through column 1 (silicone gum, 1 m, 150 C) and a flame ionisation detector to determine the aromatic products. The other half is passed through the columns 2 and 3, connected in series, and followed each by a thermal conductivity detector. Column 2 (Porapak Q, 2 m, 70 C) absorbs aromatic compounds and separates carbon dioxide, water and a gas mixture composed of oxygen,nitrogen and carbon monoxide. This mixture is further separated into its three components by col-umn 3 (molecular sieves, Linde 5A, 4 m, 70 C ) . The electrical signals produc-ed by the three detection instruments were fproduc-ed to digital integrators (Info-tronics model 203) for accurate peak surface determination.
2.4 Pulse e x p e r i m e n t s with an o x y g e n - f r e e carrier gas
This paragraph describes experiments in which helium was used as the carrier gas. The experimental work and the detailed results concerning reac-tions with the completely oxidized catalyst, catalyst reduction and reoxida-tion, and various reactions with the reduced catalyst are reported separately, and followed by a summary of the main results together with a general d i s c u s -sion. The pure bismuth molybdate catalyst is used in all experiments reported, unless otherwise stated.
Partially reduced catalysts are characterized by a degree of reduction, which is defined as the amount of oxygen removed from the catalyst divided by the total amount of oxygen associated with molybdenum or bismuth in the fully oxidized catalyst. The latter is assumed to contain bismuth and molybdenum in the form of Mo0„ and Bi^O only.
2.4.1 Toluene o x i d a t i o n by a completely oxidized catalyst
To bring the catalyst into the oxidized state air pulses (2.3 m l ) were passed through the reactor. This procedure is always necessary, because regardless of the preceeding experiments, the catalyst may slowly lose some oxygen in the oxygen-free helium atmosphere.
Three types of experiments were carried o u t , in which the following pulses were injected:
a. pulses containing a toluene/air mixture
b. pulses containing a toluene/nitrogen mixture, injected alternately with air pulses (to reoxidize the catalyst)
0. pulses containing a toluene/nitrogen m i x t u r e , without intermediate oxygen supply CATALYST: CARRIER GAS: TEMPERATURE: PRESSURE: PULSE VOLUME TOLUENE IN PULSE: TIME BETWEEN PULSES:
pure bismuth molybdate, 500 mg helium, 70 ral/min (STP) 506 °C 2.5 ata 0.46 ml 4 mol% (0.7 u mole) 1 5 min
Table 2-1 Reaction of toluene with the oxidized catalyst: Experimental conditions
PULSE TYPE a. b. c . toluene/air toluene/nitrogen (alternating with air) toluene/nitrogen (constant level) CONVERSION (%) OXYGEN 29 TOLUENE 27 14 7.5 SELECTIVITY BENZALD. 32 33 31 BENZENE 3 6 7
(%)
CO^ 49 32 28 CO 16 30 34Table 2-2 Reaction of toluene with the oxidized catalyst: Results
The experimental conditions are given in Table 2-1. The results of these experiments are summarized in Table 2-2. The differences shown in this Table are remarkable. The conversion (activity) strongly decreases in the direction a-b-o and the product composition also changes somewhat. In the presence of gas-phase oxygen (a) relatively more C0„ and less CO and benzene are produced than in its absence {b and a). The selectivity with respect to benzaldehyde however, is about the same.
The a- and i-experiments give constant results from the beginning (no differences between the first and following pulses), the c-experiments how-ever, give constant results only after some pulses have passed over the cata-lyst: After the first pulse (identical to a fc-experiment) the conversion rapidly decreases and then becomes almost constant. A typical example is given
40 ;: 30 u 0 2 20 10
-^
^ x 1 L 1 ^ A a Benzaldeh X Benzene 0 Toluene 1 0 02 0 04 >— Degree of catalyst reduction (%)Figure 2-3 Reaction of toluene/N2 p u l s e s with the oxidized c a t a l y s t 16
in Figure 2-3. The conversion and selectivities are plotted as a function of the average degree of reduction instead of the pulse number, for comparison with later experiments, and to show that catalyst reduction takes place, but only to a very small degree. (The degree of reduction is averaged between the situations before and after passage of the pulse.)
The decreasing conversion is not merely caused by dynamic conditions on or in the catalyst. This fact was checked by carrying out the same experiments
(c) with intervals of 60 minutes instead of 15. The final conversion level was a bit higher in that case (+ 2 % ) , but still much lower than the initial con-version.
2.4.2 Catalyst reduction by hydrogen and carbon m o n o x i d e
In order to study reactions of toluene and oxygen with reduced catalysts, catalyst reduction has to be performed beforehand and with a suitable reduc-ing gas. With hydrogen as a first choice reductions were carried out by pass-ing pulses (2.3 ml) over the catalyst. Conditions were the same as for the toluene oxidation experiments (see Table 2-1). The conversion of hydrogen (and the consumption of oxygen) were calculated from the water production. The con-version level was low, so that an almost homogeneous reduction of the whole catalyst bed can be assumed. The hydrogen conversion was found to depend on the degree of reduction as shown in Figure 2-4. Remarkably the conversion ini-tially increases, reaches a maximum at about 2% catalyst reduction and then slowly decreases. These observations have not been caused by initial absorb-tion of water by the catalyst. The same results were obtained with nitrogen
c o 0 v L > T3 C 1 U 5 o o 1 2 Degree of catalyst reduction (%)
Figure 2-4 C a t a l y s t reduction by hydrogen
carrier gas, in which case the conversion was calculated from the hydrogen consumption (and not from the water production).
Carbon monoxide was the second gas that was tested for suitability for catalyst reduction. The reduction was performed in a similar way to that of the reduction with hydrogen. The results, however, are completely different. The conversion as a function of the degree of reduction is depicted in Figure 2-5. The conversion curve resembles the catalyst reduction with toluene (Fig-ure 2-3), sharply declining in the beginning, and ending at a low level (here about 1.5%). This low conversion makes carbon monoxide less suitable for cata-lyst reduction then hydrogen: large numbers of pulses are necessary to achieve a certain degree of reduction, while the increase in degree of reduction per pulse cannot be determined with sufficient accuracy.
4 3 3 c o in I-o u O u 1 0 0 0 2 0 4 0 6 • Degree of catalyst reduction (%)
Figure 2-5 Catalyst reduction by carbon monoxide
2.4.3 R e o x i d a t i o n of the c a t a l y s t with air
Reoxidation of partly reduced catalysts was achieved by passing air pulses (2.3 ml) over the catalyst. The conditions were the same as described above. A typical result for a series of pulses is given in Figure 2-6, which must be read from right to left, because each oxygen pulse decreases the degree of reduction. The reaction of oxygen with the catalyst is very fast,
o o 100 • 75- 50-X O 25 1 2 3 Degree of catalyst reduction (°/.)
Figure 2-6 Catalyst reoxidation by air pulses
and initially 100% of the oxygen in the pulse is consumed. Then the catalyst seems almost saturated, and the oxygen conversion rapidly decreases to values below 10%. The last amount of oxygen necessary to oxidize the catalyst com-pletely is absorbed very slowly, indicating diffusional resistance and rear-rangement reactions in the solid. The exact degree of reduction at which the conversion starts to decrease and the steepness of the curve depend on the initial degree of reduction. The lower this initial value, the steeper is the conversion decrease, and the lower the value at which it occurs.
It is clear from the course of the reoxidation that this process does not uniformly decrease the degree of reduction throughout the entire catalyst bed. Therefore a partially reoxidized catalyst and a partially reduced catalyst may differ considerably at the same (average) degree of reduction. In a partially reoxidized catalyst bed three regions can be expected: a first part which is almost completely oxidized, a small transition area, and finally a region which still has the initial degree of reduction.
The total amount of oxygen that was consumed by the reduced catalyst was in good agreement with the amount that was removed by reduction with hydrogen. Furthermore the oxidation properties of the catalyst were not changed by reduction and subsequent reoxidation. Therefore this reduction and reoxidation can be considered to be perfectly reversible processes. When carbon monoxide was used for catalyst reduction, however, the catalyst properties seemed per-manently changed in some cases.
2.4.4 Oxidation of toluene by a partially reduced c a t a l y s t
The experiments were carried out by passing pulses of a toluene/nitrogen mixture (0.46 ml) over the catalyst, which was previously reduced with hydro-gen or carbon monoxide pulses (2.3 m l ) , and in some cases partially reoxidized with air pulses (2.3 ml). Conditions are the same as previously mentioned (see Table 2-1); in some cases a different temperature was used, which is then indicated in the Figure concerned.
The results of these toluene oxidations appeared not only to depend on the degree of catalyst reduction, but in many cases also on the catalyst his-tory, that is, what happened to the catalyst before the particular degree of reduction was achieved. Therefore three types of experiments will be sepa-rately reported.
2.4.4.1 Experiments after catalyst reduction with hydrogen
within this group of experiments the catalyst history is of minor impor-tance. Experiments have been performed with alternation of toluene/nitrogen pulses and hydrogen pulses, with alternation of series of these pulses, and with an initial large number of hydrogen pulses to directly achieve a high degree of reduction.
The results of all these experiments quantitatively fit one picture. Two typical examples are presented in the Figures 2-7 and 2-8. Figure 2-7 shows the result of continually alternating three toluene/nitrogen pulses and one hydrogen pulse. The reduction caused by the toluene pulse was accounted for, as can be seen in this Figure. Figure 2-8 shows the result of an experiment which started with four hydrogen pulse and continued with alternating pulses of toluene/nitrogen and hydrogen, until a high degree of reduction had been reached. Both Figures show the marked influence of catalyst reduction on the selectivities. Below 0.4% reduction conversion and selectivities are almost constant, and the results coincide with those obtained with toluene/nitrogen pulses alone (Figure 2-3). Then, with increasing degree of reduction important changes are observed: the toluene conversion very slowly decreases; the benz-aldehyde selectivity increases a little, reaches a maximum (at about 0.5% reduction), then rapidly falls and finally becomes zero; the benzene selecti-vity rises continuously. At high degrees of reduction (3% and higher) the benzene production is almost equal to the toluene conversion. Furthermore, only carbon monoxide and water are formed, in approximately the same
quan-6 0 A Benzaldehyde X Benzene o Toluene 4 0 o u Si' i ^ 2 0 0 3 0 6 — Degree of catalyst reduction (°/o)
Figure 2-7 Oxidation of toluene/N2 pulses as a function of the degree of reduction
(Reduction by hydrogen pulses)
100 •
X. Benzene
A Benzaldehyde o Toluene
1 2 Degree of catalyst reduction ('W
Figure 2-8 Oxidation of toluene/N2 pulses as a function of the degree of reduction
(Reduction by hydrogen pulses)
titles as benzene, according to the reaction:
< ^ CH3 . 2 0^^^ ^ < g ) . CO . H^O
Figure 2-7 clearly shows that for the production of benzaldehyde from toluene a catalyst reduction greater than 0.6% must be avoided.
In order to study the dynamic situation that occurs after catalyst reduc-tion some series of experiments were carried out in which a very short time interval occurred between the hydrogen pulse and the toluene pulse. Figure 2-9 gives an example of such an experiment. Hydrogen pulses alternating with two three or more toluene pulses were led over the catalyst. The time between pulses was 15 minutes, except for the toluene pulse subsequent to a hydrogen pulse, where the interval was only 10 seconds. The broken curves in Figure 2-9 connect the data points for these toluene pulses, while the solid curves con-nect the remainder of the data points. The latter are exactly the same as the curves in Figure 2-7. The broken curves show that in the dynamic situation the conversion is considerably lower while the selectivities with respect to benz-aldehyde and benzene are only slightly altered: the change of the selectivi-ties at increasing degree of reduction appears to be more gradual now; the production of C0„ (not shown here) is somewhat smaller in the dynamic situa-tion.
A,» Benzaldehyde •,• Benzene
O 0 3 0 6 0 9 ^ Degree of catalyst reduction C%)
Figure 2-9 Oxidation of toluene/N? pulses as a function
of the degree of reduction (Reduction by hydrogen pulses) toluene pulse immediately after H„ pulse (10 sec) measurements with usual interval time (15 min)
These results cannot be explained by simply assuming that immediately after the reaction with hydrogen the degree of reduction at the catalyst sur-face IS high. That would explain the low conversion, but strongly altered selectivities would then also be expected. It seems that the removal of some kind of surface oxygen influences the conversion and the CO production, while the selectivities with respect to benzaldehyde and benzene depend mainly on the degree of lattice (bulk) reduction.
2.4.4.2 E x p e r i m e n t s after c a t a l y s t r e d u c t i o n with carbon m o n o x i d e
Because carbon monoxide was considered less suitable than hydrogen as reducing agent (see section 2.4.2) only a limited number of experiments was carried out with catalysts reduced by this gas. These experiments are never-theless interesting because they differ considerably from the H experiments. This IS shown in the Figures 2-10 and 2-11, representing experiments which started with a catalyst reduced by a large number of CO pulses and which con-tinued with injection of toluene/nitrogen pulses. While after reduction with hydrogen subsequent toluene pulses all give the same conversion and selecti-vities, significant changes are observed in this case with respect to the selectivities. Initially the treatment with CO pulses seems to have caused
£ 1/ a 3 0 •t ij 1 20 1/) Conversio n o 0
—
A Benzaldehyde X Benzene 0 Toluene~
_ X ^ ^ / > < " ~ - ~ - - x . /\J n o ^ ^' * ' ' = " 1 1 0 20 O 23 O 26Degree of catalyst reduction (°/o)
3 0 2 0 10 -A Benzaldehyde X Benzene o Toluene 0 15 O 18
— Degree of catalyst reduction
Iigure 2-10 Oxidation of toluene/N2 pulses after reduction with carbon monoxide Example 1
Figure 2-11 Oxidation of toluene/N2 pulses after reduction with carbon monoxide Example 2
considerable reduction of the catalyst because hardly any benzaldehyde and relatively much benzene are formed. The picture rapidly changes, however, when more toluene/nitrogen pulses are supplied, and the selectivities then tend to reach values that are observed after reduction with hydrogen. The catalyst seems to recover from the treatment with CO; reaction of the catalyst with toluene seems essential for the "recovery process", since it was found that waiting a time of two hours after the reduction with CO had a much smaller effect than the injection of two toluene pulses separated by an interval of only 15 minutes.
2.4.4.3 E x p e r i m e n t s after catalyst r e d u c t i o n with hydrogen
and partial r e o x i d a t i o n by air
Different results are again found at the same (average) degrees of reduc-tion. A typical example is presented in Figure 2-12. In this case the catalyst was first reduced with hydrogen pulses to a degree of about 5%. Then pulses of toluene/nitrogen (0.46 ml) and air (2.3 ml) were alternately injected. The toluene conversion is considerably higher than after reduction with hydrogen to the same degree (Figure 2-8). The selectivity picture, however, is very much the same, although some more C0„ is formed. Therefore the conclusion here is the same as for the situation occurring immediately after reduction with hydrogen: It seems that some kind of surface oxygen (apparently present here in a large amount) determines the conversion, while the selectivity pattern
regarding benzaldehyde and benzene depends on the degree of reduction only. Remarkably however, the situation here, with a relative excess of oxygen at the surface, is not very dynamic. The interval time between pulses was 15 minutes, and increasing this time does not significantly influence the results.
2.4.5 O x i d a t i o n of toluene with air over a p a r t i a l l y reduced
c a t a l y s t
The experiments were performed in the same way as described in paragraph 2.4.4, except that now toluene/air pulses are used instead of toluene/nitrogen pulses. Only hydrogen pulses (2.3 ml) were used to reduce the catalyst. The nature of these experiments is rather complicated, because two reactions take place at the same time: oxidation of the catalyst with air and reaction between toluene and oxygen of the catalyst surface. The net result for the catalyst is a decrease in the degree of reduction.
A typical example of the results of a series of experiments in which tol-uene/air pulses were injected during reduction with hydrogen as well as during oxidation is presented in the Figures 2-13 en 2-13 . Figure 2-13 shows the conversions of oxygen and toluene, Figure 2-13 the selectivities. The data points for the toluene/air pulses that were injected during the course of the reduction with hydrogen pulses are connected with broken lines. They should be read from left to right. The solid lines connect data points for oxidation and should be read from right to left. The oxidation is caused merely by the oxy-gen in the toluene/air pulses. No other pulses were injected after the maximum degree of reduction was reached. The differences between the broken curves and the solid curves can be ascribed to a different distribution of the degree of reduction in the catalyst bed, which was explained in section 2.4.3. Apart from relatively small differences the selectivity pattern is the same as before, and it is apparently not influenced by the presence of gas-phase oxy-gen. The conversion of toluene, however, reaches values corresponding to a completely oxidized catalyst, immediately when the reoxidation starts. More-over, a remarkable maximum was observed at 0.5% catalyst reduction, at which the conversion is as much as 4% higher than the final value at complete reoxi-dation. The oxygen conversion pattern corresponds to previous reoxidation experiments (section 2.4.3). However, the conversion decreases more gradually here because of the lower pulse volume (0.46 instead of 2.3 ml air), and because in this case the whole reoxidation takes much more time, a factor
o~ C c 1" OJ 0} X o o u
^
1 0 0 7 5 h O 2 5-—
/
- "^Z^ o - ^ ^ ' ^ y ^^^
^^
1 1*
1 C J) « > 3 C O 6 0 9 2 5 2 0 1 5r-^
^ -—J^~" ~o^^ 1 1 1 0 3 0 6 0 9 D e g r e e of c a t a l y s t r e d u c t i o n (°/o)a. Toluene and oxygen c o n v e r s i o n
4 0 := 3 0 Z, 2 0 -10
-•^^
~
J * * * * * * * " * - % < t ^ ~ * - ^ - o 0 - - ^ ^ * ~ ~ ~ ~ - » ^ • , o B e n z a l d e h y d e '^., ^ \ A,A B e n z e n e V , V ^ > A ^ ^ , ^ ^ --'^''^ c^^^^"^ A - , - - ^ z ^ ^ ^ ^ - " ' * ' ^ ' 1 1 1 0 0 3 0 6 0 9 • D e g r e e of c a t a l y s t r e d u c t i o n (%) b. SelectivitiesFigure 2-13 Oxidation of toluene/air pulses during reduction with hydrogen pulses ( ) and during reoxidation ( )
which favours the contribution of slow diffusion processes to the oxygen con-sumption.
2.4.6 Experiments with the Ketjen A catalyst
The results of various experiments are qualitatively similar to the results with the pure bismuth molybdate catalyst. Quantitatively there are several differences. One of the reasons not to report all these differences m detail here is the fact that many difficulties were encountered concerning the reproducibility of the experiments. The catalyst properties often appeared to have changed after a series of experiments, not only m the case of a fresh catalyst, but also with a catalyst that had been pre-treated and used in the continuous flow reactor.
The main differences between the Ketjen A catalyst and the pure bismuth molybdate concern the activity and the dependency of the catalyst properties on the degree of reduction. The Ketjen A catalyst is more active, and the amount of catalyst in the reactor had to be halved m order to achieve a com-parable conversion level. This means that the amount of MoO. + Bi„0 is con-siderably smaller, taking also into account that 40% (wt) of the Ketjen A cat-alyst consists of silica. Two typical examples of results obtained with this catalyst are worthy of being reported:
Reaat^on of toluene yyith the oxydvzed Ketjen A catalyst
Toluene/air and toluene/nitrogen pulses were led over a completely oxi-dized catalyst, analogous to the a- and fc-experiments reported m section 2.4.1. The experimental conditions are given m Table 2-3, the results in
CATALYST: CARRIER GAS: TEMPERATURE: PRESSURE: PULSE VOLUME TOLUENE IN PULSE: TIME BETWEEN PULSES:
250 mg, diluted with quartz (1 : 1), aged m the continuous flow reactor helium, 70 ml/min (STP) 517 °C 2.5 ata 0.46 ml 8 mol% (1.5 u mole) 12 min
Table 2-3 Reaction of toluene pulses with the oxidized Ketjen A catalyst:
Experimental conditions
PULSE TYPE a. toluene/air b. toluene/N2 (first pulse*) c. toluene/N2 (second pulse) CONVERSION (%) OXYGEN 43 (S9) TOLUENE 20 (27) 1 (14) 6 (7.5) SELECTIVITY (%) BENZALD 18 (22) 23 (S3) 17 (21) BENZENE 4.5 (3) 7 (6) 6 (7)
co^
40 (49) 25 (32) 25 (28) CO 37 (16) 45 (30) 52 (34)* after air pulse
Table 2-4 Reactions of toluene pulses with the oxidized cata-lyst. Results for the Ketjen A catalyst and for pure bismuth molybdate (in italics)
Table 2-4; the numbers in italics are the corresponding values for the pure bismuth molybdate, taken from Table 2-2. The table of results shows that the Ketjen A catalyst is less selective with respect to benzaldehyde formation, while more CO is produced. The reason for these differences, which are much smaller in the case of the continuous oxidation experiments, is not clear. The difference in conversion between a and b in Table 2-4 is comparatively large, while the difference between b and a is small. This result can be ascribed to the relatively strong reduction that already occurs during the passage of the first toluene pulse. (The increase in degree of reduction per pulse in this experiment was about 5 times larger than in the pure bismuth molybdate exper-iments reported here for comparison.) Apart from these differences both cata-lysts apparently display the same trends in results for these three pulse types.
Reaction of toluene with the reduced Ketjen A catalyst
Pulses of hydrogen and toluene were fed alternately to the catalyst. Sev-eral experiments of the same kind were performed, and the resulting average conversion and selectivity pattern is depicted in Figure 2-14. Considerable variation was observed in the levels of ttie benzaldehyde and benzene selecti-vities; the rapid decrease in the benzaldehyde selectivity, however, occurred between 35% and 40% catalyst reduction in all experiments. This high value is remarkably different from results with the pure bismuth molybdate catalyst, in which case this decrease takes place below 1% degree of reduction. Although a certain difference could be expected because the Ketjen A catalyst contains a larger excess of MoO , the magnitude of the difference is still surprising. A discussion about possible active structures in the bismuth molybdate catalyst will follow in Chapter 5.
O 4) U I/)
20 40 Degree of catalyst reduction (7.)
Figure 2-14 Oxidation of toluene/N„ pulses (alternated with hydrogen pulses) by the Ketjen A catalyst
20 40 Degree of catalyst reduction (%)
Figure 2-15 Reduction of the Ketjen A catalyst by hydrogen; conversion versus degree of catalyst reduction
2.4.7 Summary of results and discussion
T h e influence of v a r i o u s conditions on the activity and the selectivity
of the toluene oxidation has clearly demonstrated that the catalyst exhibits
at least two distinct functions. One function is related to the rate of the
oxidation and probably also to the formation of products of complete o x i d a
tion, while the other function determines whether side chain oxidation results in benzaldehyde or benzene formation.
This concept will be further developed now, assuming two types of oxygen: 0 responsible for the reaction rate and 0 responsible for the selectivity in
R o side chain oxidation. The following qualities will be attached to these types of oxygen as a hypothesis:
is a surface oxygen anion that is involved in the rate determining step of the oxidation, which is presumably the dissociative adsorption of toluene on two 0-atoms.
is consumed in the non-selective oxidation and/or the production of water. can be formed in two ways:
1) from gas-phase oxygen; this reaction is fast and will always give approximately the same, maximum amount of 0 .
2) from 0 ; this reaction is much slower, moreover it does not proceed further than an "equilibrium level", proportional to the 0 concentra-tion, and far below the maximum amount of 0
R"
0„ is a lattice oxygen anion at the surface, identical to the mobile oxygen ions in the catalyst bulk that can be removed by catalyst reduction; its
GAS - PHASE OXYGEN
v e r y fast • t slow
O p ~
1
0.
m o d e r a t e l y fast, only proceeding t o equilibr level t t OXIDATION PRODUCTS TOLUENE adsorption side chain oxidation complete o x i d a t i o n O j - vacancy B E N Z - BENZENE ALDEHYDE R e l a t i o n between d i f f e r e n t kinds of oxygenAction of the two t y p e s of c a t a l y s t oxygen
Figure 2-16 Schematic representation of the hypothesis concerning two types of catalytic functions
concentration at the surface is proportionally decreasing with increasing degree of reduction.
0„ favours the formation of benzaldehyde, while 0 vacancies effect the
ben-S o zene production.
0 can "be formed in two ways:
1) from gas-phase oxygen; this reaction is fast. 2) from 0 ; this reaction is very slow.
K
The relations between the different kinds of oxygen and their action in the toluene oxidation as described above, is illustrated in the Figures 2-16 and 2-16 . To check whether this hypothesis can explain all the previously report-ed results, these will now be passreport-ed in review.
Toluene oxidation over a completely oxidized catalyst
Three levels of conversion were reported (Table 2-2 and Figure 2-3). In the case of toluene/air pulses (a) the highest conversion is obtained. Because of the presence of gas-phase oxygen, the concentration of 0 is high here and
R
remains high during the passage of the pulse, as the consumption of 0 is R immediately compensated for by replenishment from the gas-phase. In the case of injection of toluene/nitrogen pulses and oxygen pulses alternately (b), the 0 concentration is high when the toluene/nitrogen pulse enters the catalyst bed, but may be considerably lower during and after passage of the pulse. (The
calculated degree of reduction after passage of one pulse is 0.02%, which means an average reduction during passage of 0.01%.) Continuous feeding of
toluene/nitrogen pulses (c) further decreases the 0 concentration and results R
in a decreasing conversion until a constant level is reached, which is the equilibrium level established by conversion of 0 in 0 . The production of benzene is very low compared with the benzaldehyde production in all these cases (a - c ) ; this phenomenon is explained by the low degree of reduction and hence the high ratio of 0 to 0 -vacancies.
Toluene oxidation over a catalyst reduced with hydrogen
The conversion slightly decreases with increasing degree of reduction, because the equilibrium level of 0 decreases (Figures 2-7 and 2-8). A considerably lower conversion is found immediately after reduction with hydrogen (Figure 2-9). Apparently 0 was removed by the reduction, and the time was too short
for complete regeneration of 0 from 0 . The ratio in which benzaldehyde and R b
benzene are produced depends only on the degree of reduction, which determines the 0 concentration.
Toluene oxidation over a catalyst reduced with hydrogen and partially
reoxi-dized by air
Reoxidation with air pulses regenerates 0 and therefore conversions are found R
at the level of a oxidized catalyst (Figure 2-12). Because the reoxidation starts at a high degree of reduction and because the rate of reoxidation is very fast, the oxygen pulses will initially oxidize only a part of the cata-lyst bed and therefore 0 can be regenerated only as far as the oxygen pene-trates in the catalyst bed before being entirely consumed. This explains why the conversion level does not immediately attain the final high level, but gradually increases with decreasing degree of reduction.
Toluene oxidation over a reduced catalyst in presence of gas-phase oxygen
The observed conversions (Figures 2-13 and 2-14) have a level similar to that for a oxidized catalyst in the presence of air (Table 2-2). 0 is regenerated immediately after consumption, and therefore the concentration is constantly high and effects a high conversion. However, at high degrees of reduction the gas-phase oxygen is already consumed in the first part of the catalyst bed, just as in the previous case. For this reason there is no gas-phase oxygen in
the rest of the catalyst bed, in which case 0 is present only at the (low) equilibrium concentration. Starting at a high degree of reduction, the conver-sion therefore is relatively low and increases until oxygen reaches the end of the catalyst bed (observation of an oxygen conversion less than 100%). The reason that, at further decreasing degree of reduction, the conversion slight-ly decreases instead of remaining constant is not very clear. It may be that the energy produced by the catalyst reoxidation, which is proportional to the oxygen conversion, somehow stimulates the toluene oxidation. The ratio in which benzaldehyde and benzene are produced is again predominantly a function of the degree of reduction and hence explained by the ratio of 0 ions and 0 vacancies.
This review has so far demonstrated that regarding the (total) toluene conversion and the selectivity of the side chain oxidation the results can be satisfactory explained in a qualitative way by the hypothesis presented.
NR^ 1 1 2 12 9 1 7{ 4 8 5 3 6 PULSE GAS COMPONENTS air, toluene air, toluene air, toluene air, toluene air, toluene air, toluene nitrogen, toluene air, toluene air, toluene air, toluene (+ benzaldehyde) air, benzaldehyde air, benzaldehyde MOLE FRACTION HYDROCARBON 0.004 0.014 0.016 0.017 0.030 0.032 0.032 0.034 0.056 0.030 (0.003) 0.003 0.006 a series nr, which indicates sequence of b toluene (or benzaldehyde) conversion a c selectivity with respect to benzaldehy
CONVERSION'' 38 27.5 29 29 24.5 24.5 23 23.5 22 24 65 52 SELECTIVITY'^ 28 38 30 39 37 40 38 36 43
—
—
experimentation t 500 °C ie at 500 °C ^A 18.8 20.2 18.2 19.6 16.4 18.5 18.5 17.5 17.0 18.1 14.9 13,6Table 2-5 Summary of experiments and results
in which c is a proportionality constant without further significance. The coefficient E /R in this lineair relationship between In [- In (1 - C)] and 1/T was calculated for each series by ordinary lineair regression.
Inspection of the results reveals a number of interesting points, which will be each discussed in some detail:
a. The same activation energies were found for all toluene oxidation experi-ments. The average value is 18 kcal/mole with a standard deviation of 1 kcal/mole. Deviations can be ascribed to experimental error.
b. The conversion of toluene is not entirely independent of the toluene con-centration in the pulse. In particular at very low concon-centrations an increased conversion is observed. This dependency was further investigated and is reported in the next section.
c. The absence of oxygen in the toluene pulse causes some decrease in conver-sion (series No. 7 ) . Compared with earlier results (section 2.4.1, Table 2-2) the decrease is very small. This may be caused by diffusion of oxygen from surrounding air into the pulse.
d. The presence of benzaldehyde in the toluene/air pulse does not inhibit the toluene oxidation, as the same reaction rates were found in series No. 5
• 1 0 UJ) J. -1 C IZJ -2 1-2 1 3 1-4 ^ ^ (K-1)
Figure 2-17 Arrhenius plot
and No. 1.
e. The benzaldehyde oxidation is much faster than the toluene oxidation. There is also some influence of the concentration on the conversion here. The activation energy is lower than for the toluene oxidation, and more-over, it is not constant. This fact can be seen in Figure 2-17, which is an Arrhenius plot according to Equation (2.2), in which toluene oxidation
(series No. 1 and 4) and benzaldehyde oxidation (series No. 3 and 6) are compared. The data points for the toluene oxidation are adequately fitted by straight lines (except for the data points below 430 C ) . However, the benzaldehyde results clearly deviate from a lineair relation and are bet-ter represented by the broken curves. One reason for deviation may be the very large temperature range that is concerned here, while the Arrhenius relation has only a limited applicability and other parameters than the activation energy may attribute to the temperature dependence of the reac-tion rate. It is clear from Figure 2-17 that the apparent activareac-tion energy for the benzaldehyde oxidation increases with increasing tempera-tures.
The activation energies for the toluene and benzaldehyde reactions can be con-sidered equal in the region 480 - 550 C.
o, a Toluene X , A Benzaldehyde
2 . 5 . 2 I n f l u e n c e o f t o l u e n e c o n c e n t r a t i o n o n c o n v e r s i o n and s e l e c t i v i t i e s
One series of experiments was carried out in which the mole fraction toluene in the pulse was varied between 0.002 and 0.14, at a constant reaction temperature of 507 C. The results are presented in Figure 2-18. In the region of high concentrations (mole fraction > 0.03) conversion (and selectivities) are almost constant, in agreement with the assumed first order dependence. However, at small concentrations a strong increase in conversion is observed. It seems as if separate from the first order reaction, a zero order reaction also takes place (zero order with respect to toluene). The difference between the observed conversions and the final conversion level at high concentration (19%) is a variable percentage, but turns out to be an almost constant abso-lute amount of toluene (0.028 p mol toluene, corresponding to a mole fraction of 0.0014). This second reaction may be an unselective oxidation, because the selectivity to benzaldehyde tends to decrease at very low toluene concentra-tions. However, the data do not provide decisive evidence regarding this point because of relative large experimental errors in this region.
I 60
u a> o I/) ° 40 tn L. > C o u 20 0 0 0 05 0 10 0-15 • Toluene mole fraction m pulseFigure 2-18 Influence of toluene concentration in the pulse on conversion and selectivity
,
(
_4>^ A AV
- x x - x " — 4 ^#~-.-^
— 9 1 "^ 0 Toluene A Benzaldehyde X Benzene — i' A 5^^-^^-X 1 A —0 1CHAPTER 3
T O L U E N E O X I D A T I O N PART II - C O N T I N U O U S FLOW E X P E R I M E N T S
3 . 1 Introducti on
The aim of the experimental work presented in this chapter is twofold. Firstly, the reaction kinetics of the oxidation of toluene with dry air over bismuth molybdate are considered under circumstances which are relevant for industrial application. Therefore the experiments were carried out in a con-tinuously operated fixed-bed micro-reactor, and reaction rates were measured in the form of integral rates. One of the catalysts used in these investiga-tions is a commercial ammoxidation catalyst. A relatively high temperature level (450 - 550 C) was chosen to include high conversion levels (60 - 100%) of the reactants.
In the second place attention is given to general aspects of the kinet-ics in relation to properties of bismuth molybdates, and in relation to other oxidation reactions of aromatic hydrocarbons.
The experimental programme consists of four parts:
Qualitative experiments. These experiments concern the determination of the main reactions and reaction products involved. The importance of mass trans-fer limitations is investigated and conditions for isothermal operation are determined. Furthermore catalyst activity and stability are studied. Introductory kinetic experiments. To select an appropriate type of kinetic model, experiments are carried out at various conditions which are critical
for model design and discrimination.
Kinetic experiments. Several series of experiments are carried out to estimate the parameters in the selected model and to evaluate the model by comparison of measured data and values calculated by the model. Statistical methods are used for a quantitative approach. The number of experiments needed and the character of the experimental programme depend on the type of model, the number of parameters and the required precision of the parameter estimates in relation to experimental error. Moreover they particularly depend on the main goal of the evaluation, which may be the maximum accuracy of estimated parameters, to use the model for prediction purposes or examination of the influence of some important process variables in detail, and comparison of measured and predicted dependencies. Here the emphasis has been on the
the model.
Experiments with water in the feed. From patent literature (40) it is well
known that large amounts of water are often added in hydrocarbon oxidation and ammoxidation, mainly to improve the catalyst stability. Some experiments are carried out here to investigate the influence of water on conversion and se-lectivity.
3.2 M a t e r i a l s and c a t a l y s t s
All gases (helium, nitrogen, air) were obtained from cylinders (high pu-rity) and no further purification was found necessary. Analytical grade tolu-ene, benzene and benzaldehyde were used.
The same two catalysts were studied that are reported in the previous chapter. A somewhat larger sieve fraction was used in most of the experiments
(0.65 - 0.85 m m ) . Crushed quartz of the same sieve fraction served as diluting agent.
3.3 A p p a r a t u s
A diagrammatic flowsheet of the apparatus is shown in Fig. 3.1. Except for the control unit for gas flows the entire apparatus is made of stainless steel 316.
-®--*— 3
•6 — *
-TIT
1. gas flow control 2. saturator 3. switch valve 4. reactor 5. condenser 6. sampling valve 7. analysis system
8. thermo couple connection
1. isolating material 2. electrical heating 3. fluid bed
4. air inlet chamber 5. reactor tube
(inner diam. 6 mm) 6. catalyst
7. quartz wool 8. distributor plate 9. reactor outlet tube
(inner diam. 3 mm)
Figure 3-2 Detail of reactor construction
The reactor consists of a tube (volume 3 ml, inner diameter 6 m m ) , which is externally heated by a fluidized bed of silicon particles to assure good heat exchange and isothermal reaction conditions. A thin thermocouple (0.5 mm) is mounted in the axis of the catalyst bed to register possible temperature differences between reactor wall and axis. To avoid non-catalytic reactions, initiated by the catalytic reaction, as were reported by Daniel and Keulks (38), special attention has been given to minimize the "post-catalytic volume" (see Figure 3-2). The diameter of the reactor tube decreases to 3 mm, immedi-ately under the catalyst bed, and a very quick cooling of the reactor product stream to about 300 C is achieved because this tube passes through the dis-tributer plate and through the air inlet chamber of the fluid bed.
Toluene, benzene, benzaldehyde and water are all introduced to the feed gas stream in a similar way, i.e. by passing a part of the gas stream through, a saturator contained in a thermostat. Feed and product streams can be sampled during continuous operation by a combination of two valves (both pneumatically operated 6-way valves with strengthened Teflon sliders) and one sample loop.
The valves are places in an air thermostat at 200 OC to avoid condensation of reaction products. In a condensor (- 77 °C) hydrocarbon vapours from the reac-tor product stream can be trapped for sensitive qualitative analysis of oxida-tion products.
The analysis system is identical to the system used for the pulse experi-ments and described in the previous chapter (Figure 2-2). Helium is used as
3
carrier gas, at a total flow rate of 40 cm /min (STP), which demands a pressure of about 1 atm gauge. The nitrogen peak is used as an internal standard, since nitrogen is not affected m the oxidation reactions.
3.4 Q u a l i t a t i v e e x p e r i m e n t s
The products of catalytic and thermal oxidation have been studied under various conditions. Catalytic oxidation of toluene led to benzaldehyde, ben-zene and carbon oxides as the main products, besides traces of anthraqumone and benzoic acid. An example of the product distribution is given m Table 3-1. TEMP PRESSURE SPACE TIME 500 °C 1 ATM 1.5 s FEED toluene oxygen nitrogen molZ 7.6 19 5 72.9 PRODUCT toluene benzald. benzene mol% 5 0 0.76 0.08 C02 CO H2O 02 N2 8 4 2 2 7 2 5.6 70.7
Table 3-1 Example of the product distribution
Thermal oxidation of toluene, measured in the empty reactor under condi-tions comparable to the catalytic experiments, was negligible. The catalytic oxidation of benzaldehyde was faster than the toluene oxidation. The same products were formed; anthraqumone however was found m much larger quanti-ties The thermal oxidation of benzaldehyde was slow compared with the cata-lytic oxidation. Remarkably, only carbon oxides were found. Both catacata-lytic and thermal oxidation of benzene were negligible under toluene oxidation condi-tions .
Qualitatively there was not much difference in catalytic behaviour be-tween the supported and the pure bismuth molybdate catalyst. About the same selectivities and activities were measured.
As to external and internal mass transfer limitations, criteria are given in the literature to determine on the basis of global reaction rates whether or not physical processes can be neglected in the kinetics (,70), For the
toluene oxidation experiments the calculations have shown that mass transfer limitations are unimportant. Moreover experiments were carried out with dif-ferent particle sizes. No influence of the particle size on conversion was found.
Because of the large heats of reaction (80 kcal/mol for the formation of benzaldehyde from toluene and 900 kcal/mole for the total combustion of tolu-ene) attention was given to possible, undesirable temperature gradients in the catalyst bed. Some experiments were carried out under severe conditions (rela-tively high feed rate of toluene and high conversion). Axial temperature pro-files were measured, of which an example is given in Figure 3-3, showing a maximum temperature difference of 16 C between reactor axis and wall. Under normal conditions, and with a diluted catalyst (50% quartz or more) tempera-ture differences are insignificant and therefore the reactor can be assumed isothermal.
3.5 C a t a l y s t a c t i v i t y and s t a b i l i t y
The catalysts were active without any pre-treatment. There was no signif-icant difference in activity when the catalyst was pre-treated by an air stream at various temperatures up to 570 C and during 24 hours. However, a significant decrease in activity occurred during the first period of experi-mentation with a fresh catalyst. A typical example is shown in Figure 3-4. After a period of 30 hours the activity was practically constant. The length of this period and the final activity level were dependent on the temperature and the ratio W/F (catalyst weight/mass flow toluene fed to the reactor). Therefore a 50 hours run at standard conditions (T = 500 C, W/F = 24 sec) was used as the catalyst pre-treatment before kinetic investigations were started.
As Figure 3-4 shows, the benzene formation is only important on a fresh catalyst. The benzene formation steadily decreases and becomes less then 1% after 50 hours. Therefore benzene has been ignored in the kinetic modelling of the toluene oxidation.
Decreasing conversion and increasing selectivity towards benzaldehyde may both be effects of decreasing activity and can be explained without assuming other changes in the catalyst properties, according to the kinetic model that is described later in this chapter.
0 5 1 0 Relative length in catalyst bed Experimental conditions: catalyst: No. 1, Ig diluent: quartz, Ig bed length: 8 cm toluene: 0.11 atm conversion: 25Z space time: 1 sec
Figure 3-3 Example of a temperature profile in the catalyst bed
7 Toluene X Benzaldehyde o Benzene
Figure 3-4 Deactivation of a fresh catalyst
3.6 Introductory kinetic e x p e r i m e n t s
To determine a proper reaction kinetic scheme for the toluene oxidation, investigations were made concerning the importance of the benzaldehyde oxida-tion and concerning the possible direct oxidaoxida-tion of toluene to carbon oxides.
The importance of the benzaldehyde oxidation is shown in Figure 3-5, re-presenting two experiments in which the feed consisted of air and relatively small amounts of toluene or benzaldehyde. Space time was varied, and because of the excess of oxygen (air) the reaction rates can be assumed not to depend on the oxygen partial pressure. The straight lines in Figure 3-5 therefore demonstrate that the reaction rates are first order with respect to the aro-matic reactant. The ratio of the slopes of these lines indicates that benzal-dehyde is oxidized approximately three times faster than toluene.
The existence of the direct oxidation of toluene to carbon oxides has become clear from a series of experiments represented in Figures 3-6 and 3-7. Toluene was oxidized with air at various space times and three temperature
levels. Figure 3-6 shows the dependence of selectivity on conversion. Remark-ably all measurements are fitted by one curve. The form of the curve leads to a first conclusion: 40 - 50% of the toluene is directly combusted, since the initial selectivity is 50 - 60%. Therefore the simplest kinetic scheme for the toluene oxidation is given by:
BENZALDEHYDE
2 ^
TOLUENE ^ • CARBON OXIDES
The curve in Figure 3-6 is a calculated curve: the ratio of the first order rate constants k : k„ : k was set to the value 0.55 : 0.45 : 3, to produce the best fit. The fact that all measurements fit the same curve leads to a second conclusion: the activation energies of the three reactions have approx-imately the same value, since the influence of temperature on the selectivity-conversion pattern is small. This conclusion is in agreement with the results of the pulse experiments reported in Chapter 2.
The reaction kinetics cannot be described by first order dependency in aromatic reactants only. Figure 3-8 shows the results of the oxidation of tol-uene at a relative low air/toltol-uene ratio. Now no linear plot is found as in Figure 3-5. Hence oxygen must play a role in the kinetics. The same can be concluded from the oxidation of toluene at varying oxygen partial pressures
