Znd. Eng. Chem. Res. 1991,30, 1139-1141
1139
Catalyst Evaluation for the Vapor-Phase Oxidation of p -Xylene Vinay Kumar* and P r e m
D.Grover
Department of Chemical Engineering, Indian Institute of Technology, Delhi 110016, India
Catalytic vapor-phase oxidation of p-xylene was studied in an isothermal plug flow reactor using mixed oxides as catalysts, viz., V-Ce-0, V-Fe-0, V-Mg-0, V-Co-0, and V-P-0. Among the catalysts investigated, only V-Fe-0 and V-Ce-0 exhibit sufficiently high activity with respect to products of oxidation. The activity of the various catalysts is in the following order: V-Fe-0 > V - C e O > Vz05> V-P-O > V-Mg-0 > V-Co-0. The catalyst V - C e O is found to be more selective for p-tolualdehyde than that of the most active catalyst V-Fe-0 in the temperature range 325-425 "C. Introduction Vanadium is a transition-metal oxide used for vaporphase oxidation of different compounds. Various oxides are also used with vanadium pentoxide for many reactions to get the optimum yield of the desired products. Vanadium pentoxide based catalysts have been employed extensively to study a number of reactions in the vapor phase, but only a few catalysts have been used to study the p-xylene reaction (Bhattacharya and Gulati, 1958, 1954; Mathur and Viswanath, 1974,1978; Parks and Allard, 1939; Trimm and Irshad, 1970). Since no systematic studies have been made to investigate the activity and the selectivity of vanadium pentoxides mixed with oxides of metals such as cobalt, iron, cerium, and magnesium and also oxide of phosphorous, we decided to undertake the study of activity and selectivity of these mixed oxide catalysts for p-xylene oxidation. Experimental Section The experimental setup consists of a S.S. tubular flow reactor (1 = 250 mm, d = 19 mm) and product collection assembly. The reactor is immersed in an electrically heated fluidized bed heater. A sheathed thermocouple is placed vertically in the center of the reactor to measure the temperature of the catalyst bed. Preheated vaporized xylene and air are passed through the catalyst bed of the reactor and reaction products are then collected in condensers and bubblers. The gas is collected in a gas sampler for analysis. The main reaction products of p-xylene oxidation are p-tolualdehyde, p-toluic acid, terephthalic acid, maleic anhydride, and carbon dioxide. The products, collected over water in bubblers, are cooled at 4 "C and filtered to remove p-toluic acid and terephthalic acid, as these become insoluble in water at such a low temperature. The filtrate containing maleic anhydride is estimated by titrating with potassium hydroxide. The filtration residue consisting of the remaining two acids, p-toluic acid and terephthalic acid, is treated with chloroform to separate terephthalic acid because it is completely insoluble in chloroform. Then the chloroform extract is titrated with alcoholic potassium hydroxide to determine the content of p-toluic acid. The residue from chloroform extraction is dried at 110 OC and weighed as terephthalic acid. The reported analytical procedure (Mathur and Viswanath, 1974; Bhattacharya and Gulati, 1958) for the analysis of p-tolualdehyde has been modified in the present study. This was necessitated because the acidic solution of dinitrophenylhydrazine (DNPH) gives precipitates in
* Present address: National Metallurgical Laboratory, NonFerrous Process Division, Jamshedpur 831007,India. To whom correspondence should be addressed.
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the aqueous solution that give erroneous results during quantitative analysis. In the modified procedure, propanol is used instead of water because DNPH does not precipitate in it. DNPH solution is added in the collected product to precipitate p-tolualdehyde hydrazone, which is dried to estimate the p-tolualdehyde. The gaseous product, C02,is estimated with barium hydroxide solution. The catalyst is prepared by impregnating purified pumice stones with the solution of ammonium metavanadate. The impregnated mass after 24 h of soaking is slowly heated till dryness. Finally, the catalyst is calcined in air in a muffle furnace for 6 h a t 550 O C . The method for preparing the mixed-oxide catalyst (80% Vz05 and 20% other oxide) is similar to that described for vanadium pentoxide. The vanadium pentoxide-iron oxide pellet was obtained by pelletizing the calcined V-Fe-0 catalyst in the form of a circular disk of 10-mm diameter and 2-mm thickness and then the required size is obtained after breaking the pellets. Results and Discussion Catalytic vapor-phase oxidation of p-xylene over mixed-oxides catalysts (V205:Me-0 = 4:l) has been studied under steady-state conditions at atmospheric pressure in a tubular fixed-bed reactor, and their catalytic activity and selectivity for p-tolualdehyde were studied to select the best catalyst by varying partial pressure of p-xylene and temperature. The various terms used in this study may be defined as follows: conversion (X)is expressed as the ratio of the moles of p-xylene reacted to the moles of p-xylene fed into the reactor, whereas yield (Y) is defined as the ratio of the moles of p-xylene consumed to form a particular product to the moles of p-xylene fed into the reactor at a given time. The ratio of moles of p-tolualdehyde produced per unit time to moles of p-xylene reacted per unit time is defined as selectivity (S), and reaction rate (r) is defined as the moles of p-xylene reacting per second per gram of the catalyst and eventually expressed as the activity of the catalyst. Activity of the Catalysts. The activity of the various catalysts for p-xylene oxidation was examined. In these experiments, the p-xylene rate was varied in the range 18.4 X 10-3-62.5 X g mol/h, while the temperature and the concentration of oxygen were kept constant at 400 "C and g mol/L, respectively. A comparison of the 8.59 X performance of these catalysts is shown in Figure 1. These results indicate that the activity of the Catalysts decreased markedly by the additions of 20% oxides of magnesium, cobalt, and phosphorous in the vanadium pentoxide. The remaining two catalysts, having additives as cerium oxide and iron oxide to vanadium pentoxide were found to be active over the range of the concentration of p-xylene 1991 American Chemical Society
1140 Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991
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Figure 2. Temperature effect on conversion, yields, and selectivity for oxidation of p-xylene over V-Fe-0 catalyst.
studied (6.13 X 10-6-20.83 X g mol/L) at a temperature of 400 "C. Figure 1indicates that the catalytic activity of V-Ce-0 (6.93 X 10-'-13.32 X g mol/s g) is higher to some extent compared to that of the pure vanadium pentoxide (6.84 X 10-7-11.84 X lo-' g mol/s g) in the concentration range of the investigation, whereas the activity of another catalyst (V-Fe-0) increased markedly with the incorporation of iron oxide in vanadium pentoxide. The activity of the catalysb studied is given in the decreasing order as V-Fe-0 > V-Ce-0 > VzO6 > V-P-0 > V-Mg-0 >
v-co-0
Effect of Temperature. The effects of temperature on the yield or the conversion of various products and selectivity of p-xylene to ptolualdehyde over V-Ce-0 and V - F A catalysts were examined in the temperature range 325-450 "C (Figures 2 and 3). As pumice-supported V-Fe-0 was found to be the most active catalyst, the pelletized form of the same was also used in order to see
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Figure 3. Temperature effect on conversion, yields, and selectivity for oxidation of p-xylene over V-Ce-0 catalyst.
its performance in the oxidation of p-xylene. The effect of temperature on the pelletized V-Fe-0 catalyst is shown in the Figure 2, which clearly indicates that the selectivity of the catalyst decreases sharply with a rise in temperature, which may be due to the high porosity of the pelletized form of V-Fe-0. With the V-Fe-0 having pumice as carrier, the selectivity of the supported catalyst decreases only from 71.37% to 65.03% by increasing the temperature from 325 to 450 "C, whereas selectivity falls to 41.57% from 68.20% for a pelletized catalyst with rise of temperature from 325-425 "C. The effect of reaction temperature (325-475 "C)on V-Ce-O catalyst shown in Figure 3 clearly indicates that the yield of each product increases with the rise of temperature. Although the conversion increases to 72.10% with rise of temperature to 475 O C , the selectivity decreases to 61.7% when the temperature is raised from 325 to 475 "C. By considering the yield of the desired product, viz., p-tolualdehyde for the p-xylene feed rate of 18.4 X lo9 to 36.5 X g mol/h at 400 "C, it is found that V-Ce-0 is more active as compared to V-Fe-0 catalyst. It can be observed that the selectivityof the V-Fe-0 decreases from 71.37% to 64.66% with the rise of temperature from 350 to 400 "C,whereas selectivity of V-Ce-0 decreases slightly from 72.99% to 71.16% in the temperature range 350-425 "C. Thus, in view of the better catalytic activity and selectivity for desired product during oxidation of p-xylene, the catalyst V-Ce-0 was considered for further studies to know the best composition of two metal oxides. Experimental results for the total conversion and selectivity for p-tolualdehyde over varying compositions of cerium oxide in vanadium pentoxide are shown in the Figure 4. It shows that the catalyst comprised of 20% cerium oxide has the maximum selectivity to p-tolualdehyde although the conversion to p-tolualdehyde is slightly less than that for the 40% cerium oxide. For above 40% cerium oxide in Vz06,both the yield and the selectivity to p-tolualdehyde decrease sharply. So the 20% cerium oxide and 80% vanadium pentoxide is found to be the most suitable composition. Conclusions From the analysis of the data, it is found that the in-
1141
Znd. Eng. Chem. Res. 1991, 30, 1141-1145
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However, V-Ce-0 is more selective for p-tolualdehyde than is V-Fe-0. Acknowledgment
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We express our sincere gratitude to the Director, National Metallurgical Laboratory, for providing necessary facilities in the preparation of the manuscript. Literature Cited Bhattacharya, S. K.; Gulati, I. B. Catalytic oxidation of ortho-xylene: activity of fused vanadium catalysts. Chem. Ind. (London) 1954,
- 20
0
20 Wt.%
LO of
60
80
100
c e r i u m Oxide
Figure 4. Effect of weight percent of cerium oxide on yield and selectivity for p-tolualdehyde over V-Ce-0 catalyst.
corporation of phosphorous, magnesium, and cobalt to vanadium pentoxide decreases the activity, whereas cerium and iron promote the activity of Vz06 markedly. The activity of the catalyst is found to be in the following order:
NO.13, 1425-1426. Bhattacharya, S. K.; Gulati, I. B. Catalytic vapour phase oxidation of xylenes. Znd. Eng. Chem. 1958,50, 1719-1726. Mathur, B. C.; Viswanath, D. S. Catalytic vapour-phase oxidation of p-xylene over tin vanadate. J . Catal. 1974, 32, 1-9. Mathur, B. C.; Viswanath, D. S. Vapour Phase oxidation of p-xylene. Can. J. Chem. Eng. 1978,56, 224-229. Parks, W. G.; Allard, C. E. Vapour phase catalytic oxidation of organic compounds. Znd. Eng. Chem. 1939,31,1162-1167. Trimm, D. L.; Irshad, M. The influence of electron directing effects on the catalytic oxidation of toluenes and xylenes. J. Catal. 1970, 18, 142-153.
Received for review February 19, 1990 Revised manuscript received July 23, 1990 Accepted November 27,1990
Effect of Solvents on Thermal Cracking of Model Compounds Typical of Coal Koji Chiba, Hideyuki Tagaya,* T a k u Yamauchi, and Shimio Sat0 Faculty of Engineering, Yamagata University, Yonezawa, Yamagata 992, Japan
Conversions of bibenzyl and dibenzyl ether as coal models depend on the nature of the solvent. When solvents that were poor hydrogen donors, but their dehydrogenated radicals were more stable than donors, were used, bibenzyl and dibenzyl ether conversion were markedly enhanced. Positive effects were observed by the mixing of tetralin with nondonor solvents. However, negative effects by such mixing were observed in the case of dibenzyl ether. Through this study, the importance of radical-induced decomposition on cracking of coal model compounds was suggested. Introduction Coal is regarded as a highly cross-linked macromolecular network consisting of a number of stable cluster units connected by cross links (Gray and Shah,1981; Lucht and Peppas, 1981). The structure is highly complex; therefore, a convenient way to study the reactions of various types of bonds is to simulate the structure by using a variety of model compounds (Benjamin et al., 1978; Allen and Gavalas, 1984). Bibenzyl is the simplest model for dimethylene connecting units in coal (Poutama, 1980). The cracking of bibenzyl has been described by a free-radical mechanism (Stein et al., 1982; Buchanan et al., 1986). It has been accepted that the hydrogen donating ability of the solvent plays the most important role in coal degradation (Chiba et al., 1987b);therefore, interests in bibenzyl as a coal model were mainly focused on thermal cracking of bibenzyl in good hydrogen donor solvents. Cronauer et al. (1978) reported the mechanism and kinetics of reactions between bibenzyl and a few non-hydrogen donor or hydrogen donor solvents. They and Panvelker et al.
(1982) concluded that the breakage of the carbon-carbon bond in bibenzyl occurs purely thermally and its rate is independent of the nature of the solvent present during the reaction. Their results imply that the bibenzyl free radical is very active and readily extracts hydrogen from any available solvents. However, a detailed discussion of the reactions taking place between bibenzyl and a variety of non-hydrogen donor solvents has not been published. In this paper we first briefly extend our previous communication (Chiba et al., 1985b) on the reactions between non-hydrogen donor solvents and bibenzyl. We also applied the results to the reactions between non-hydrogen donor solvents and dibenzyl ether. Experimental and Analytical Procedures The experiments were performed in a 100-mL magnetically stirred autoclave. The autoclave charged with bibenzyl(1.00 g) and solvent (or mixed solvents) (20.00 g) was filled with nitrogen at an initial pressure of 3 MPa. The autoclave was maintained at the reaction temperature
0888-5885/91/ 2630-1141$02.50/0 0 1991 American Chemical Society
