Ind. Eng. Chem. Res. 1993,32, 2930-2933
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RESEARCH NOTES Deep Oxidation of Toluene on Perovskite Catalyst Chen-Chang Chang and Hung-Shan Weng' Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan 70101, R.O.C.
The deep oxidation of toluene over Lao.&0.2CoOa catalyst in the range of 239-255 "C was investigated by employing a fixed-bed flow reactor. Several kinetic models for the reaction mechanism and the rate expression have been tested. It is found that a Langmuir-Hinshelwood model incorporating a dual-site mechanism with competitive adsorption of atomic oxygen and toluene is suitable for the cases of higher partial pressure of oxygen, lower partial pressure of toluene, and low conversion, but not suitable for the case of high conversion obtained at high temperature. A semiempirical equation is proposed t o correlate the data of conversion and reaction temperature.
Introduction Toluene is used as a raw material in manufacturing benzoic acid, benzaldehyde, explosives, dyes, and many other organiccompounds and, as a solvent, in the extraction of various plant compounds. Unfortunately, toluene may induce mild macrocytic anemia in those working with it. It is narcotic in high concentrations. Several methods (carbon adsorption, catalytic incineration, thermal incineration, etc.) have been developed to avoid emission of this solvent (Jennings et al., 1985). The oxidation of toluene on various catalysts, including simple and complex oxides, has been introduced by Golodets (1983) and Spivey (1987). The reaction mechanisms of the initial steps in the oxidation of toluene have been discussed by Germain and Laugier (1972) and Andersson (1986). Charlot (1934) found that metal catalysts such as Ni, Pt, and Pd can actively accelerate the deep oxidation of this hydrocarbon. Although the simple oxides of Co, Cu, Mn, and Ni demonstrate high activity, their kinetics have not been adequately studied. The oxidation of toluene over complex oxide catalysts was also investigated by several researchers (Parks and Yula, 1941; Downie et al., 1961; Volynkin, 1966; Van der Wiele and Van den Berg, 1975). Their results show that the mechanism of toluene oxidation is the same over both simple and complex oxide catalysts. Due to their relatively inexpensive cost, good thermal stability, and high activity, perovskites (one type of mixed oxides) have recently been employed in the catalytic incineration of volatile organic compounds and treatment of automotive exhaust. Madhok (1986) has investigated the partial oxidation of toluene over lanthanum cobaltite (LaCoOs). He has found that the oxidation rate is first order with respect to toluene. Although the oxidation of toluene has been studied over various catalysts, most studies concerned the selective or partial oxidation. In addition, the kinetics of the deep oxidation of toluene on perovskites have not been studied. The present study was undertaken to investigate the nature of the adsorbed species formed on the catalyst surface through an interpretation of their kinetic data and to evaluate various kinetic models in order to derive the rate
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expression. A semiempirical equation was also used to correlate the experimental data of conversion and temperature.
Experimental Section Catalyst Preparation. It has been reported that La0.8Sr0.2CoO3,among the cobaltite perovskites, was found to be the most active and its activity was comparative to or higher than those of the Pt, Pd, and Ni Catalysts in oxidation of propane, methane, and carbon monoxide (Nakamura et al., 1980). We chose this type of perovskite as catalyst for the oxidation of toluene in the present study. The preparation procedures and characterizations were described in the precedingpaper (Chang and Weng, 1992). Activity Test. The measurement of catalytic activity was carried out in a differential fixed-bed flow reactor, consisting of a piece of 318-in. stainless steel tubing packed with 0 . 1 0 . 5 0 g of perovskite-type oxides (40-48 mesh) and diluted by a 4-fold excess of mullite. The limitations of external mass transfer and intraparticle diffusion are negligible. While a flow of air was passed through the reactor at a rate of 100 mL/min, the oxides were activated at 400 OC for 40 min, after which the temperature of the bed was decreased to 200 "C. Toluene, along with a mixture of oxygen and nitrogen at a rate of 132 mL/min as the carrier gas, was fed into the reactor system via a saturator at l-atm pressure. The reactant stream bypassed the reactor during the coolingperiod (about 1.5 h). When the input concentration of toluene rose to a steady value, the reactant stream was switched to the reactor. The activity of perovskite-type oxides was measured by the conversion of toluene oxidation. To investigate the effects of reaction temperature, toluene concentration, and oxygen partial pressure, the fixed-bed reactor was operated at low conversions over the following range of conditions: 239-255 "C, partial pressure of oxygen 0.015-0.197 atm, and partial pressure of toluene 0.000 67-0.0052 atm. The deactivation of oxides is not obvious when conversion is low (Figure 1). The kinetic data were taken when the conversion decreased to a steady value (about 4 h). Water vapor was fed via an another saturator when we investigated the effect of water vapor on the activity of this catalyst. The reaction mixture was analyzed by an on-line gas chromatograph (GC) (Shimadzu-8APF) with a flame ionization detector and a 2-m column packed with Porapak Q (80/100 mesh). 0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 2931
' o c o - - 0 4 - 0 - 0 ~ n -
12 16 20 Time on stream ( h r l Figure 1. Effect of time on stream on conversion in oxidation of toluene (0.0017 atm) on 0.50 g of L~o.sS~O.ZCOOS catalyst (Po, 0.197 atm). Reaction temperature: (0) 230, (0) 266 OC. 0
4
8
Reaction temperature ( oc I Figure 2. Dependence of reaction temperature on conversion in oxidation of toluene on 0.10 g of Lao.sSro.zCoOa catalyst. (a) (Po, 0.197 atm) h (0) 0.0052, (0) 0.0029, (A)0.0017, ( 0 )O.Oo0 67 atm. (b)(PrO.002 atm) Po,: ( 0 )0.197, ( 0 )0.098, (A)0.038, ( 0 )0.015 atm.
The conversion of toluene oxidation X was defined as follows:
and the reaction rate was calculated from the following equation:
r = - -m -T dW
dX - ciu dW
Since we used a differential reactor, the reaction rate can be evaluated from the following approximate equation:
r = CivX/W
(3) -0
Results and Discussion Effect of Water Vapor. The effect of water vapor on the catalytic oxidation of hydrocarbons has been investigated by several researchers (Germain and Peuch, 1969; Arnold and Sundaresan, 1988;Zhu and Andersson, 1989; Huang, 1992). The addition of water vapor to the feed in the oxidation of butane over a vanadium phosphate catalyst led to an enhancement in the selectivity and a decline in the activity (Arnold and Sundaresan, 1988). The addition of water vapor improved both activity and selectivity for benzoic acid in the catalytic oxidation of toluene over bulk v205and monolayer-type 2w t % ' V/Ti02 catalysts (Zhu and Andersson, 1989). Note however that neither benzene oxidation kinetics over a V - M A catalyst a t 370-440 "C (Germain and Peuch, 1969)nor nitric oxide reduction kinetics on perovskite catalysts (Huang, 1992) are influenced by water addition. In the present study, we also found that the rate of toluene oxidation is independent of water vapor concentration. This might be due to high catalytic activity (deep oxidation) and weak adsorption of the water molecules. Effects of Operating Conditions. The temperature dependence of toluene oxidation over Lao.aSr0.2CoOa catalyst at different concentrations of toluene and oxygen is shown in Figure 2. Note that toluene concentration is much more influential on toluene oxidation than is oxygen concentration. The relationships between reaction rate and partial pressures of toluene and oxygen are plotted in Figure 3. Fittings of Various Kinetic Models. Kinetic models, including the power law model (Trimm and Irshad, 1970), Mars and van Krevelen model (1954),and the Langmuir-
0
20
4.0 6.0
I
PT x 1000 (atml
Figure 3. Dependence of partial pressures of toluene and oxygen on toluene oxidation rate on 0.10 g of Lao.eSro.&oOa catalyst. (a) (PO, 0.197 atm) ( 0 )225, (0) 249, (A)244, ( 0 )239 OC. (b) (I+ 0.002 atm) (13 256, ( 0 )249, (A)244, ( 0 )239 OC.
Hinshelwood model with several reaction mechanisms, were examined for a best fit to the data obtained in the above experiments based on the values of correlation coefficient. The results show that the Langmuir-Hinshelwood model with the following rate expression gives the best fit:
This model corresponds to a reaction mechanism in which the molecular oxygen is dissociated into atomic oxygen when it is adsorbed on an active site. The adsorbed oxygen atom then attracts the nearby toluene that has been adsorbed on the same type of active site and undergoes surface reaction. The rate expression we obtained is consistent with those obtained for complete degradation (deep oxidation) of some substituted toluenes (xylene and chlorotoluene isomers) but is different from that obtained for partial oxidation of toluene over Moos (Trimm and Irshad, 1970). After the values of KO2,KT, and k were obtained by the linear regression method, eq 4 was substituted into eq 2 and a fourth-order Runge-Kutta method was used in solving eq 2 in order to calculate the theoretical values of
2932 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 IO0
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Reaction temperature ( " C )
Figure 4. Comparisons of predicted and experimental conversions in oxidation of toluene on 0.50 g of Lao.~Sro.zCoOscatalyst: (a) at various partial pressures of oxygen (Pr0.002 atm) ( 0 ) 0.197, (0) 0.098, (A)0.038 atm; (b) at various partial pressures of toluene (Po, 0.197atm) (0) 0.002 93, (0) 0.002 23atm. (-) Langmuir-Hinshelwood model predictions.
conversion a t various reaction temperatures. Comparisons of predicted and experimental conversions obtained by an isothermal integral fixed-bed reactor at various partial pressures of oxygen and toluene are shown in Figure 4. In Figure 4a the deviation is enlarged when oxygen is deficient. When the partial pressure of toluene is varied, the deviation is nearly unchanged (Figure 4b). Thus we can conclude that the model is suitable only for higher partial pressure of oxygen, lower partial pressure of toluene, and low conversion. In all cases, the model is not suitable for high conversions obtained at high operating temperatures. Note that external mass-transfer resistance becomes significant a t high temperatures. SemiempiricalFitting. Seeing the shape of the curve is similar to that of the plot of conversion versus reciprocal of absolute temperature for a CSTR (Hill, 19771, we tried to use the following equation to fit the data of conversion and temperature.
Note that, a t larger values of T, the value of a equals c because X should equal unity. As shown in Figure 5, the data of conversion and temperature can be well fitted by the above equation. This simple technique provides an easy and useful approach to describe the relationship between conversion and reaction temperature and thus can be used in the future for designing industrial catalytic incinerators provided that the dependences of a and b on the operating conditions, including the space velocity and reactant concentration, are evaluated in advance.
Conclusions A Langmuir-Hinshelwood model incorporating dualsite mechanism with competitive adsorption of atomic oxygen and toluene was found to be suitable for higher partial pressure of oxygen, lower partial pressure of toluene, and low conversion, but not suitable for high conversion obtained at high temperature. For reasons of simplicity, we suggest an easy semiempirical equation that can well delineate the relationship between conversion and reaction temperature.
Reaction temperature ( O C ) Figure 5. Comparisons of predicted and experimental conversions in oxidation of toluene on 0.50 g of Leo.aSro.zCoO~catalyst (a) at various partial pressures of oxygen (Pr 0.002 a h ) (0) 0.197, ( 0 ) 0.098, (A)0.038 atm; (b) at various partial pressures of toluene (Po, 0.197 atm) ( 0 ) 0.002 93, (0)0.002 23 atm. (-) semiempirical correlations.
Acknowledgment This work was supported by a grant from the Union Chemical Laboratories, Industrial Technology Research Institute, Republic of China.
Nomenclature a, b,
c = constants
Ci = inlet or bypass toluene concentration, mol (cm9-l C, = outlet toluene concentrationfrom the catalyst bed, mol (cm3)-' F = molar flow rate, mol s-1 k = rate constant for toluene oxidation, mol (g-s.atm)-l Kj = apparent adsorption equilibrium constant for species j Pj = partial pressure of species j , atm r = rate of toluene oxidation, mol (g.s)-' T = absolute temperature, K u = gas feed rate, cm3 s-l W = catalyst weight, g X = conversion of toluene oxidation, % Subscripts 02 = oxygen T = toluene
Literature Cited Andersson, S. L. T. Reaction Networks in the Catalytic Vapor-Phase Oxidation of Toluene and Xylene. J. Catal. 1986, 98, 138-149. Amold,E. W.III;Sundaresan,S.EffectofWaterVaporontheActivity and Selectivity Characteristics of a Vanadium Phosphate Catalyst towards Butane Oxidation. Appl. Catal. 1988,41, 225-239. Chang, C. C.; Weng, H. S. An Improvement on the Preparation of La-Sr-Co Perowkite by the Oxalate Method. Znd. Eng. Chem. Res. 1992,31, 1615-1621. Charlot, G. The Catalytic Oxidation of Vaporized Organic Compounds. Ann. Chim. 1934,11,415-490. Downie, J.; Shelstad, K. A.; Graydon, W. F. Kinetics of the Vaporphase Oxidation of Toluene over a Vanadium Catalyst. Can. J. Chem. Eng. 1961,39, 201-204. Germain, J. E.; Peuch, J. C. Kinetics of Catalytic Benzene Oxidation on Vanadium-molybdenum Mixed Oxides. 111. Selectivity, Activitv. and ComDoeition ofthe Catalvst. Bull. SOC. Chim.Fr. 1969. 1844-1851.
Germain. J. E.: Laueier. R. Kinetics of the Catalvtic Oxidation of Aromatic HydrocLbons. Bull. SOC.Chim. Fr."1972, 541-648. Golodets, G. I. In Heterogeneous Catalytic Reactions Involving Molecular Oxygen; Studies in Surface Science and Catalysis 15; Elsevier: New York, 1983; pp 668-680.
Ind. Eng. Chem. Res., Vol. 32, No. 11,1993 2933
Hill,C. G., Jr. An Introduction to Chemical Engineering Kinetics and Reuctor Design.; Wiley: New York, 1977; pp 37b371.
Hwaug, C. F. Kinetic Studies of the Catalytic Reduction of Nitric Oxide on Peromkite Catalpta. Master’s Thesis, Department of Chemical Eneineering, National Chena - Kuna - University, Tainan,
Taiwan,ROC,1992. Jenaings, M. S.; Krohn, N.E.; Berry,R. S.; Palazzolo, M. A,;Parks, R. M.: Fidler, K. K. In Catalytic Incineration for Control of Volatile Orgu& CompoundEmissions.; Noyea Publications: Park Ridge, NJ, 1985; pp 61-90. Madhok, K. L. Oxidation of Toluene on Lanthanum Cobaltite Peromkite (LaCoOa)Catalyst. React. Kinet. Catal. Lett. 1986,30, 186192. Mars, P.; van Krevelen, D. W. Oxidations Carried Out by Means of Vanadium OxideCatalysts. Chem.Eng. Sci. 1954,3, specialsuppl., 41-59. Nakamura, T.; Misono, M.;Uchijima, T.; Yoneda, Y. Catalytic Activitiesof Peromkite-Type Compoundsfor Oxidation Reactions. Nippon Kugaku Kuishi 1980,167+1684. Parks, W. G.; Yula, R. W. Vapor-Phaee CatalyticOxidationof Organic Compounds. Znd. Eng. Chem. 1941,33,891-897.
Spivey, J. J. Complete Catalytic Oxidation of Volatile Organice.Id. Eng. Chem. Res. 1987,26,2166-2180. Trimm, D. L.; Irshad, M. T h e Influence of Electron Directing Effects on the Catalytic Oxidation of Toluene6 and Xylenee. J. Catal. 1970,18,142-153.
Van der Wiele, K.;Van den Berg, P. J. Oxidation of Toluene over Bismuth Molybdate Catalysta. J. Catal. 1976,39 (3), 437-448. Volynkin, N. 1. Catalytic Vapor-Gaa Oxidation of Toluene to Benzaldehyde by Atmospheric Oxygen in Presence of Solid Catalyata. Zh. Prikl. Khim. 1966,39 (12), 2783-2787. Zhu, J.;Andersson, S. L. T. Effect of Water ontheCatalyticOxidation of Toluene over Vanadium Oxide Catalysts. Appl. Cutul. 1989, 63.251-262.
Received for review February 1, 1993 Revised manuscript received June 29, 1993 Accepted July 27, 1993. e
Abstract published in Advance ACS Abstracts, September
15, 1993.
