890 I n d . Eng. Chem. Res. 1989, 28, 890-894

Esters. J . Chem. SOC., Perkin Trans. 1, 1982, 993-997. Tundo, P.; Angeletti, E.; Rubbo, M.; Venturello, P. Crystal Growth

of Alkali Metal Halides During the Gas-Liquid Phase-Transfer Catalysis. J. Chem. SOC., Perkin Trans. 2 1983a, 493-495.

Tundo, P.; Angeletti, E.; Venturello, P. Gas-Liquid Phase Transfer Catalysis of Phenyl Ethers and Sulphides with Carbonate as a Base and Carbowax as a Catalyst. J . Chem. SOC., Perkin Trans.

Tundo, P.; Angeletti, E.; Venturello, P. Catalytic Interconversion of Alkyl Halides by Gas-Liquid Phase Transfer Catalysis. J. Chem. SOC., Perkin Trans. 2 1983c, 485-491.

Tundo, P.; Venturello, P.; Angeletti, E. Gas-Liquid Phase Transfer Catalysis: Catalytic and Continuous Transesterification Reaction. J . Org. Chem. 1983d, 48, 4106-4109.

Tundo, P.; Trotta, F.; Canavesi, R. Procedimento per 1’Eterificazione dei Fenoli e Tiofenoli con Carbonati Alchilici in Condizioni di Catalisi di Trasferimento di Fase Gas-Liqudo. Ital. Pat. Appl. 19971 A/86, 1986.

Tundo, P.; Venturello, P.; Angeletti, E. Malonic, Acetoacetic and Acetylacetonic Alkylation Reactions by Gas-Liquid Phase Transfer Catalysis. J . Chem. SOC., Perkin Trans. I 1987a,

Tundo, P.; Venturello, P.; Angeletti, E. Continuous Conversion of Alcohols into Alkyl Halides by Gas-Liquid Phase Transfer Cata- lysis (GL-PTC). J . Chem. Soc., Perkin Trans. 1 1987b,

1 1983b, 1137-1141.

2159-2162.

2157-2158.

Tundo, P.; Trotta, F.; Angeletti, E.; Venturello, P. Eur. Patent Appl. 1 045 308, 1987c; U.S. Patent Appl. 31,598 March 30, 1987c; Canada Patent Appl. 533,654 April 2 ,1987~; Japan Patent Appl. 62 080 875 April 1, 1987c (Consiglio Nazionale delle Ricerche, Rome).

Tundo, P.; Trotta, F.; Moraglio, G. Kinetic Study on Exchange Re- action of Alkyl Halides under Gas-Liquid Phase Transfer Cata- lysis. J . Chem. SOC., Perkin Trans. 2 1988a, 1709-1712.

Tundo, P.; Trotta, F.; Moraglio, G.; Ligorati, F. Continuous Flow Processes under Gas-Liquid Phase Transfer Catalysis (GL-PTC) Conditions: The Reaction of Dialkyl Carbonates with Phenols, Alcohols, and Mercaptans. Ind. Eng. Chem. Res. 1988b, 27,

Villadsen, J.; Lievbjerg, H. Supported Liquid-Phase Catalysis. Ca- tal. Rev.-Sci. Eng. 1978, 17, 203-272.

Villadsen, J.; Lievbjerg, H. Supported Liquid-Phase Catalysis. Nato Adu. Study Ser. 1981,51, 541-576.

Weber, W. P.; Gokel, G. W. Phase Transfer Catalysis in Organic Synthesis; Springer Verlag: West Berlin, 1977.

Yanagida, S.; Takahashi, K.; Okahara, M. Metal-ion Complexation of Noncyclic Polyoxy(ethy1ene) Derivatives I. Solvent Extracion and Alkaline Earth Metal Thiocianates and Alkaline Iodides. Bull. Chem. SOC. Jpn. 1977,50, 1386-1390.

1565-1571.

Received for reuiew August 12, 1988 Accepted January 24, 1989

KINETICS AND CATALYSIS

Kinetics of Toluene Alkylation with Methanol on HZSM-8 Zeolite Catalyst

Y. S. Bhat, A. B. Halgeri, and T. S. R. Prasada Rao* Research Centre, Indian Petrochemicals Corporation Limited, Baroda 391 346, India

A detailed kinetic study of the alkylation of toluene with methanol on HZSM-8 zeolite was carried out in a fixed bed reactor. The catalyst showed high activity, selectivity, and stability. An attempt has been made to relate the overall changes taking place during the reaction. The experimental data were analyzed, and a reaction mechanism was proposed based on the product pattern. The Langmuir-Hinshelwood-Hougen-Watson model with dual-site mechanism was used to derive the rate equation. The unknown parameters in the rate equations were estimated by a nonlinear regression method. The kinetic model based on the proposed reaction mechanism predicted the conversion values, which are in close agreement with the experimentally observed values.

The selective preparation of p-xylene by alkylation of toluene with methanol over modified zeolite catalyst has been the subject of many investigations in view of its commercial importance as a raw material for dimethyl terephthalate and purified terephthalic acid (Chen and Garwood, 1978). Much attention has been focused on ZSM-&type zeolites since these materials were effective for alkylation due to its high activity and predominant shape selectivity (Yashima et al., 1980). By adjustment of the acid activity, diffusion parameters, and reaction conditions, high p-xylene selectivity has been achieved in toluene alkylation (Young et al., 1982). Borade et al. (1986) have demonstrated the selective formation of p-xylene over isomorphous substituted metallosilicate of ZSM-5 struc- ture.

* To whom correspondence should be addressed.

Enormous work has been reported on ZSM-5-type zeolites, and as compared to it, very little attention has been given to ZSM-&type zeolite. Only a few patents (Mobil Oil Corp., 1971; Vybihal, 1983; Chen et al., 1972) have been reported on the synthesis and application of ZSM-8 as a catalyst. The adsorption capacity of ZSM-8 for water, ammonia, and aliphatic hydrocarbons has been studied (Levinbuk et al., 1979). Park and Hakze (1985) have investigated selective formation of p-ethyltoluene on ZSM-8. It is not yet clear whether the two zeolites ZSM-8 and ZSM-5 have similar structures (Lechert, 1984; Jacobs and Martens, 1987).

Akolekar and Choudhary (1987) have reported that ZSM-8 is somewhat similar to ZSM-5 in its physical properties like sorption capacity, crystal density, and also catalytic properties. However, they observed higher rates of deactivation in the channels of ZSM-8 and concluded

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Ind. Eng. Chem. Res., Vol. 28, No. 7, 1989 891

0 - Toluene I 0.06

o -Toluene into p - x 0 02

Process t i m e , hr

Figure 1 . Activity and selectivity of HZSM-8 zeolite.

that there is a small but significant difference in the channel structure of the two zeolites. Halgeri and Prasada Rao (1987) observed a higher selectivity for p-xylene on ZSM-8 as compared to ZSM-5 for toluene alkylation with methanol. They also interpreted the difference in selec- tivity on the basis of a slight difference in the channel structure of the two zeolites. It was considered of great interest to study this reaction in detail with the following specific objectives: (1) to study the kinetics of toluene alkylation with methanol on ZSM-8 catalyst, (ii) to propose a reaction mechanism for toluene alkylation, (iii) to derive a kinetic model based on the reaction mechanism, and (iv) to estimate the unknown parameters in the kinetic model.

Experimental Section The ZSM-8 zeolite was synthesized by the hydrothermal

crystallization technique as described in Chu's (1973) patent. One-hundred percent crystalline product was obtained from the batch composition 4.8(TEA)20-18.2- (Na20)-117Si02-A1203-3800H20. The H form of the zeolite was obtained by ion exchange with ammonium ions followed by deammination at 450 "C in air.

The catalytic experiments were carried out in a fixed bed, continuous downflow, cylindrical glass reactor. Two grams of the catalyst was charged into the reactor, and the runs were taken at atmospheric pressure. Before the ex- perimental runs were started, the catalyst was activated for 3 h at 400 "C in air and 1 h in hydrogen. The mixture of reactants was introduced by a Sage syringe pump and evaporated in a vaporizer before contacting the catalyst. The reaction products were analyzed by a Varian Vista 6000 gas chromatograph, and the details of analytical procedure were reported previously (Halgeri and Prasada Rao, 1985).

Results and Discussion In order to test that reliable and reproducible kinetic

data would be obtained, the activity and stability of HZSM-8 zeolite were tested for the toluene alkylation reaction, and the results are presented in Figure 1. It is obvious from this figure that the total conversion of toluene and the conversion of toluene into p-xylene remained the same even after 30 h time on stream. The catalyst did not show any sign of deactivation. I t was also observed that the catalyst was restored to its original activity even after successive regeneration.

Experiments were carried out in the region free of inter- and intraparticle diffusion effects. In order to estimate the external diffusional effects, runs were carried out with constant WIF but with varying feed rates in each run. From the results, the liquid feed rate range, with which there is negligible mass transfer, was found. In all our

Table I. Conversion of Toluene and Conversion of Toluene into Different Products at 400 "C

conversions of

W I h T into T into T into T into ( a h)/mol T PX MX OX TMB

4.28 0.0844 0.0597 0.0080 0.0065 0.0100 8.56 0.1240 0.0850 0.0128 0.0082 0.0179

16.69 0.1660 0.0914 0.0203 0.0107 0.0434 33.76 0.2179 0.1153 0.0379 0.0159 0.0485 69.72 0.2319 0.1195 0.0537 0.0240 0.0353

kinetic runs, feed rates falling in this range were used. To test the intraparticle diffusion limitation, experiments were conducted by varying the particle size and keeping the same WIF. The results were used to find the particle size range with which there is no diffusion limitation. The particle size employed in this study falls within the in- traparticle diffusion free range.

In case of zeolite-catalyzed reactions, two types of dif- fusion process are to be considered: (i) micropore inside the zeolite crystal, and (ii) macropore between catalyst pellet particles. The above experiments for intraparticle diffusion limitations show only the absence of diffusion in macropores. To evaluate the micropore diffusional resistance, the crystal size of the zeolite is to be changed by modifying the synthesis conditions. As the channel dimensions of ZSM-8 are comparable with the kinetic diameters of hydrocarbon molecules like toluene and xylenes, micropore diffusion cannot be ruled out. Hence, the kinetic parameters presented here include the diffu- sional effects, if any. Similar kinetic studies have been reported for toluene disproportionation on ZSM-5 catalyst (Chang et al., 1987; Nayak and Riekert, 1986) for ethyla- tion of toluene (Lee and Wang, 19851, isomerization of ethylbenzene and m-xylene (Hsu et al., 1988), and nick- el-aluminum-deficient mordenite (Bhavikatti and Pat- wardhan, 1981).

Kinetic runs were carried out a t three temperatures, viz., 400, 425, and 450 "C. At each temperature, WIFT was varied by changing the liquid feed rate. Hydrogen gas was used as a carrier gas in all the experiments. In all these runs, the molar ratio of toluene to methanol was kept a t 2, whereas the molar ratio of liquid hydrocarbon feed to hydrogen was maintained at 0.5. These ratios were chosen on the basis of our preliminary work. Data collected during the kinetic runs at 400 "C are presented in Table I. A similar product pattern was also observed at elevated temperatures of 425 and 450 "C but are not presented here. In Table I, the conversion of toluene and the conversion of toluene into different products are defined as conversion of toluene =

moles of toluene converted/moles of toluene fed conversion of toluene into product =

moles of product formed/moles of toluene fed

The effect of space-time WIFT on toluene conversion at three different temperatures is represented in Figure 2. I t is observed that toluene conversion increases with an increase in space-time at all temperatures. The selectivity for p-xylene at various toluene conversions is shown in Figure 3. It is evident from this figure that the selectivity for p-xylene is as high as 78% at lower toluene conversion and at higher temperature. The selectivity decreases with an increase in conversion at the same temperature. However, a t the same toluene conversion, the selectivity also increases with an increase in temperature. These observations, along with earlier work (Yashima et al., 1980; Young et al., 1982) on a similar type of zeolite catalyst, have led to the conclusion that p-xylene is a primary

892 Ind. Eng. Chem. Res., Vol. 28, No. 7 , 1989

->I- d

0 10 20 30 LO 50 60 70

' w / F ~ , 9 h r / m O l

Figure 2. Toluene conversion a t different W / F p

% Toluene conversion

Figure 3. Selectivity for p-xylene a t different toluene conversions.

product of toluene alkylation. Theoretically the alkylation of toluene is expected to give p-xylene and o-xylene as primary products, but because of stereospecificity of HZSM-8, p-xylene is formed more selectively. The se- lectivity for p-xylene decreases due to its isomerization, leading to a mixture of three xylenes. The secondary alkylation reactions involving xylenes and methanol which form trimethylbenzene occur on the external surface of the zeolite. Hence, toluene alkylation with methanol can be represented by the following reaction mechanism:

C H 3 I

I

k2 \";..

(gH3

It is experimentally observed that methanol is com- pletely converted. The side reactions of methanol forming products other than xylenes and trimethylbenzenes are ignored in the above reaction mechanism.

Since the active sites are protonic type in nature (in the HZM-8 framework), they are more energetically uniform than the protonic amorphous material such as Si02-A1203. Langmuir adsorption isotherms were assumed to hold good. The Langmuir-Hinshelwood-Hougen-Watson model with dual-site mechanism was used to derive the rate equations for various steps in the above proposed reaction mechanism; while deriving the rate equations, it was assumed that the surface reaction is rate controlling. The rate equations are given below: rate of disappearance of toluene = rT = dxT/dT =

klPTPM /z rate of formation of p-xylene = rpx = dxpx/dT =

[k iPTPM - k z ( P p x - PMX/K~) - ~ Q P X P M I /z2 rate of formation of n-xylene = r M X = dxM/dT =

[k2(PPX - PMX/K2) - k3(PMX - -

k$MXPMI / z z rate of formation of o-xylene = rox = dxox/dT =

rate of formation of trimethylbenzenes = rTMB =

[ ~ ~ ( P M x - Pox/&) - k6PoxP~1 /z

dXTMB/dT = [ k 4 P P X P M + I Z 8 M X p M + k6POXPMI/Z2 (5)

KoxPox + KTMB~TMB where the equilibrium constants are given by the equations

K, = k2/k-2 (6)

K3 = k3/k.., ( 7 )

Z = 1 + KTPT + KMPM + KpXPpX + K M X P M X +

These equilibrium constants used in the parameter esti- mation are calculated from the thermodynamic data. The partial pressures in eq 1-5 are related to the conversions by the relationships P T = (1 - xT)P/(1 + X T + 0.5 + 4.5) =

(1 - X T ) / ( ~ + XT)

Ppx = xpxP/(6 + XT)

Pox = xoxP/(6 + XT)

PMX = X ~ x p / ( 6 +- XT)

PTMB = X T M B P / ( ~ + XT)

P M = [0.5 - ( X T + ZTMB)]P / (6 -k XT)

where 0.5 is the molar ratio of methanol to toluene and 4.5 is the molar ratio of hydrogen to toluene.

The rate eq 1-5 contain 12 unknown parameters, viz., kl-k6, KT, KM, Kpx, K M X , KOX, and KTm. These param- eters were estimated by treating each temperature data separately. As the rate equations are nonlinear with re- spect to unknown parameters, a nonlinear regression program based on Marquard's algorithm was used to es- timate the unknown kinetic and adsorption constants. Equations 1-5 were integrated by using a fourth-order Runge-Kutta program to the space-time limits a t which experimental xT, xpx, x m , xox, and x T ~ are known. The parameters were estimated by minimizing the objective function:

n

dJ = XCYL - 9J2 1=l

Ind. Eng. Chem. Res., Vol. 28, No. 7 , 1989 893

020 - A

? n

Table 11. Kinetic Constants Estimated by Nonlinear Regression Method

- O b s e r v e d o - P r e d i c t e d

temp const, mol/(g atm h) 400 "C 425 "C 450 "C

kl 1.862 2.762 3.91 k2 0.578 0.672 0.898 k3 1.321 1.710 2.332 k4 0.80 0.983 1.246 k5 9.813 11.765 14.51 k6 1.018 1.118 1.345

Table 111. Adsorption Constants Estimated by Nonlinear Regression Method

temp const, atm-' 400 "C 425 "C 450 "C

KT 0.2409 0.1829 0.1346 KM 0.6260 0.5154 0.4285 KPX 4.5301 3.8974 3.28 KMX 18.174 15.324 13.56 Kox 40.311 35.066 33.20 KTMB 314.10 265.90 215.0

Table IV. Activation Energy and Frequency Factor for the Kinetic Constants const, mol/(g atm h) E, kJ/mol k,, mol/(g atm h)

k , 60.52 f 5.35 9.0196 X lo4 k2 35.37 i 4.10 3.1538 X lo2 k3 45.92 f 4.28 4.7807 X lo3 k4 35.83 * 2.17 4.8015 X k5 32.86 f 2.76 2.7688 X lo3 k6 29.80 f 1.63 1.9045 X lo2

where n is the number of responses, yi is the experimental conversion, and f i is the predicted conversion. Tables I1 and I11 respectively give the kinetic and adsorption con- stants estimated by the nonlinear regression method.

It is obvious from the above Tables I1 and I11 that with an increase in temperature the kinetic constants increase, which is the right trend for these parameters. Similarly as expected, the adsorption constants show a decreasing trend with an increase in temperature.

The kinetic constants evaluated at different tempera- tures were used to determine the activation energy and frequency factor using the Arrhenius relationship k = k , exp(-E/RT). A least-squares fitting program was used to evaluate the slope and intercept from In k versus 1 /T values. From the slope and intercept, the activation energy and frequency factor are calculated and presented in Table IV. From this table, it can be seen that the alkylation of toluene is slower than the isomerization of xylenes since the activation energy for alkylation is higher (60.52 kJ/ mol) than for isomerization (35.37 kJ/mol). In the case of alkylation, both toluene and methanol have to move to the adjacent sites before reaction takes place, whereas isomerization can take place on the same site where xylenes are formed. This observation supports the earlier findings of Young et al. (1982) on ZSM-5-type catalyst. Also, the activation energy for the xylene isomerization steps in the present study agrees very well with the value reported by Hsu et al. (1988) for the isomerization of xylenes on ZSM-5. Judging by the magnitude of activation energies, it appears that kl and k , are in the transition regime of diffusion and reaction, and all other rate constants are controlled by intrinsic diffusion.

The predicted conversion values are obtained by nu- merically integrating eq 1-5 using a fourth-order Runge- Kutta program and the parameters determined previously. Figure 4 compares the experimental and the predicted toluene conversions. A good correlation is observed be- tween the experimentally observed and predicted con-

a s 010 01s 020 02s o Toluene conversion

30

Figure 5. Comparison of observed and predicted conversion of toluene into p-xylene a t different toluene conversions.

version values. This demonstrates that the proposed ki- netic model fits well into our experimental observations. It is further supported by the evidence in Figure 5 , which shows a close agreement between the experimental and predicted conversions of toluene into p-xylene at 400 and 425 "C. These results substantiate that alkylation of toluene with methanol follows the proposed reaction mechanism and can be represented by the rate eq 1-5.

Conclusions A systematic kinetic study on alkylation of toluene on

HZSM-8 catalyst has been made for the first time. An attempt was made to relate the changes occurring during the reaction. Kinetic data were obtained by employing a fixed bed integral reactor a t temperatures in the range 400-450 "C at atmospheric pressure. The catalyst showed good activity and stability for alkylation of toluene with methanol. The selectivity for p-xylene was as high as 78% a t lower toluene conversion and higher temperature.

From the product distribution pattern, a reaction mechanism was proposed for the alkylation. The rate equations for this mechanism were derived on the basis of Langmuir-Hinshelwood-Hougen-Watson model with dual-site mechanism. A nonlinear regression computer program was used to estimate the unknown parameters in the rate equations. The activation energy and frequency factor were evaluated by using an Arrhenius relationship.

The proposed reaction mechanism was confirmed by a trend shown by the kinetic and adsorption constants with temperatures. Activation energy values suggest that al-

894 I n d . Eng. Chem. Res . 1989, 28, 894-899

kylation of toluene with methanol to p-xylene is slower than the isomerization of xylenes. The kinetic model based on the proposed reaction mechanism predicts the con- version values comparable with the experimental conver- sions.

Acknowledgment

The authors are grateful to the Management of Indian Petrochemicals Corporation Limited for permission to publish this paper and also are grateful to S. J. Thanki for her skilled assistance.

Nomenclature

T = toluene PX = p-xylene M X = m-xylene OX = o-xylene TMB = trimethylbenzene rT, r M X , rex, r T M B = rates P T , P M X , Ppx, POX, PTMB, P M = partial pressures, atm P = total pressure, atm K,, K, = equilibrium constants KT, K M x , KM, KpX, Kox, KTm = adsorption constants, atm-' XT, xpx, XMX, XOX, x T M B = conversions T = W/F,. space-time, (g h)/mol k , , k,, k , , k , , k,, k , = kinetic constants, mol/(h atm g) E = activation energy, kcal/mol k , = frequency factor, mol/(g atm h) R = gas constant, 1.987 cal/(mol K ) T = reaction temperature, K W = catalyst weight, g

Registry No. Toluene, 108-88-3; methanol, 67-56-1.

Literature Cited Akolekar, D. B.; Choudhary, V. R. J . Catal. 1987, 105,416. Bhavikatti, S. S.; Patwardhan, S. R. Ind. Eng. Chem. Prod. Res. Deu.

1981, 20, 106. Borade, R. B.; Halgeri, A. B.; Prasada Rao, T. S. R. Proceedings of

Seventh International Conference, Kodensha, Tokyo; Elsevier: Amsterdam, 1986; p 851.

Chang, Jen-Ray; Sheu, Fong-Chang; Cheng, Ying-Ming; Wu, Jung- Chung Appl. Catal. 1987, 33, 39.

Chen, N. Y.; Garwood, W. E. J . Catal. 1978, 52, 453. Chen, N. Y.; Lucki, S. J.; Garwood, W. E. U S . Patent 3700585, 1972. Chu, P. U.S. Patent 3709979, 1973. Gabelica, Z.; Deroune, E. G.; Blom, N. Appl. Catal. 1983, 5 , 109. Hakze, C.; Ahn, B. J . ; Park, S. E. Proceedings of Eighth Interna-

tional Congress on Catalysis; Verlag Chemie: Weinheim, 1984;

Halgeri, A. B.; Prasada Rao, T. S. R. In Studies in Surface Science and Catalysis; Drzaj, B., Hocevar, S., Pejovnik, S., Eds.; Elsevier: Amsterdam, 1985; p 667.

Halgeri, A. B.; Prasada Rao, T. S. R. Proceedings of Eighth National Symposium on Catalysis; Allied Traders: New Delhi, 1987; p 703.

Hsu, Y. S., Lee, T. Y.; Hy, H. C. Ind. Eng. Chem. Res. 1988,27, 942. Jacobs, P. A.; Martens, J . A. In Synthesis of High Silica Alumino-

Lechert, H. NATO ASI Ser., Ser. E 1984, 80, 151. Lee, Biing-Jye; Wang, Ikai Ind. Eng. Prod. Res. Deu. 1985,24, 201. Levinbuk, M. I.; Khadzhiev, S. N.; Limova, T. V.; Meged, N. F.;

Topchieva, K. V.; Mironova, L. P. Vestn. Mosk. Univ Ser. 2: Khim. 1979, 20(2), 177.

p IV-555.

silicate Zeolites; Elsevier: Amsterdam, 1987; p 191.

Mobil Oil Corp. The Netherlands Patent 7014807, 1971. Nayak, V. S.; Riekert, L. Appl. Catal. 1986, 23, 403. Park, S . E.; Hakze, C. Chem. Eng. Commun. 1985, 34(1-6), 137. Vybihal, J. Czechoslovakian CS 211981, 1983. Yashima, T.; Sakaguchi, Y.; Namba, S. Proceedings of Seventh In-

ternational congress on Catalysis, Kodensha, Tokyo; Elsevier: Amsterdam, 1980; p A52-1.

Young, L. B.; Butter, S. A,; Keading, W. W. J . Catal. 1982, 765, 418.

Received f o r review October 27, 1988 Accepted February 19, 1989

Wet Oxidation of Oxygen- and Nitrogen-Containing Organic Compounds Catalyzed by Cobalt(II1) Oxide

Masato M. Ito, Kazuyuki Akita, and Hakuai Inoue* Department of Chemical Engineering, Faculty of Engineering, T h e University of Tokyo, Hongo, Bunkyo-ku , Tokyo 113, Japan

Co203-catalyzed oxidation of a variety of organic compounds containing oxygen and nitrogen atoms was carried out in water under oxygen atmosphere a t 200 "C in an autoclave, and the time course of C 0 2 formation was measured. The carbon number-initial rate profiles for alcohols, amines, and carboxylic acids are similar to one another except for the case of C1 compounds. Bifunctional compounds are much more readily oxidized t o give C 0 2 than monofunctional ones. Analysis of the products dissolved in water shows that nitrogen-containing molecules readily decomposed to give ammonia. It also suggests that acetic acid is an important intermediate in the oxidation of C2 and larger monofunctional compounds. Based on a simplified reaction scheme, which involves the fast @-cleavage of intermediate radicals, a set of kinetic parameters are determined. Simulation of the kinetics using these parameters afforded a fairly good agreement with the observed time course of CO:, formation.

Introduction

Wet oxidation, proposed and developed by Zimmermann (1958), has been successfully applied to the oxidative de- composition of organic and/or inorganic materials in wastewater to reduce its COD. However, it has the defect of (1) requiring high oxygen pressure (50-100 atm) as well

0~88-5885/89/2628-0894$01.50/0

as high temperature (ca. 250 "C) and (2) being difficult in decomposing ammonia and lower carboxylic acids formed by partial decomposition of organic compounds (Shogenji. 1968, and the references cited therein).

Catalytic wet oxidation recently attracted attention because of its potential utility as an alternate method for wastewater treatment at lower temperatures and pressures.

0 1989 American Chemical Society