Alkylation of Toluene with Propylene in Supercritical Carbon Dioxide

Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan 300, ROC. Ind. Eng. Chem. Res. , 2001, 40 (22), pp 4724–4730...
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Ind. Eng. Chem. Res. 2001, 40, 4724-4730

KINETICS, CATALYSIS, AND REACTION ENGINEERING Alkylation of Toluene with Propylene in Supercritical Carbon Dioxide over Chemical Liquid Deposition HZSM-5 Pellets Te-Wen Kuo and Chung-Sung Tan* Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan 300, ROC

Alkylation of toluene with propylene in supercritical carbon dioxide over the parent and modified HZSM-5 pellets was carried out in a packed bed in this study. The modification was done by adding a mixture of SiCl4 and hexane to the parent HZSM-5 pellets containing 14% water. The experimental data showed that much more p-cymene could be yielded using the modified HZSM-5 pellets, exhibiting shape-screening character. When the toluene feed and the molar ratio of toluene to propylene in supercritical carbon dioxide were kept the same as that in the gas phase, it was found that 50% more of p-cymene could be generated and the cracking of propylene was significantly reduced under supercritical operations, indicating more efficient operation and more effective utilization of propylene. From a systematic study of the effects of operating variables on p-cymene yield and selectivity for the use of 4 g of the modified HZSM-5 pellets, the most appropriate operating conditions were found to be a temperature of 523 K, a pressure of 11.72 MPa, a weight hourly space velocity of 4.6 (g of toluene)/(g of catalyst)/h, and a molar ratio of toluene to propylene of 7.7. These appropriate conditions were influenced by the reaction mechanism and solubilizing capability of supercritical carbon dioxide toward reactants and products. Introduction Toluene is a major product in the catalytic reforming process. Because of its abundance and low price, many attempts have been made to convert it to more valuable aromatic compounds such as benzene and xylene.1-3 Because p-cymene has been extensively used for the production of pesticides, fungicides, flavors, and heating media, it seems that an alternative utilization of toluene is to convert it into p-cymene. p-Cymene can be produced by alkylation of toluene with propylene or 2-propanol over the Friedel-Crafts catalysts, such as AlCl3, BF3, or H2SO4, but the needs of high catalyst concentration and treatment of the disposed catalyst as well as the corrosion of the solution always cause some problems. Because solid acid catalysts, such as Y and β zeolites, MCM-41, and ZSM-5, exhibit high activity and versatility in alkylation and may avoid the above-mentioned problems, they are considered to be candidates for this alkylation reaction.4-9 Flockhart et al.4 employed Y zeolite to carry out alkylation of toluene with propylene at atmospheric pressure. They observed that the degree of ion exchange and the calcination temperature strongly affected the distribution of cymene isomers. Under any combination of catalyst and the operating conditions, they found that a p-cymene selectivity of greater than 60% was hard to achieve. When mesoporous silica-aluminas were used as catalysts, though cymene was observed to be the major product, a sufficiently high p-cymene selectivity * To whom correspondence should be addressed. Tel: 8863-572-1189. Fax: 886-3-5721684. E-mail: [email protected].

could not be achieved because of the larger pores of the zeolites.9 When 2-propanol was used in the alkylation, though a high p-cymene selectivity could be obtained using MFI zeolites, a significant amount of the side product n-propyltoluene was also formed.10 Wichterlova and Cejka11 used HZSM-5, H-mordenite, and H-Y zeolites to reduce the production of side products. Their results showed that HZSM-5 could result in a higher p-cymene selectivity compared to those of H-mordenite and H-Y zeolites. This was because the channel dimensions of HZSM-5 were approximately the same as the molecular dimensions of many aromatic molecules.12 According to these studies, it seems that ZSM-5 is a proper catalyst regarding paraselectivity in the alkylation. In general, the dimensions of isomers are sufficiently close, and the pore opening of zeolite may not be small enough to make a desired separation. Selective poisoning of the surface of ZSM-5 crystals was thus used to improve the selectivity of the para isomer.13-15 One of the poisoning means is to use chemical vapor deposition (CVD) of silicon alkoxide to alter the pore opening and acidic sites of the zeolite crystals.13 For alkylation of toluene with ethylene, the selectivity of p-ethyltoluene was significantly enhanced using this kind of modified zeolite crystal.14 However, scaling up in a CVD process may cause a problem. To overcome this drawback, the chemical liquid deposition (CLD) was proposed in which the pore-opening sizes of the ZSM-5 zeolite crystal were controlled by using metal halides as a depositing agent.15 Recently, Wang and Tan16 used the HZSM-5 pellets modified by CLD of SiCl4 to carry out alkylation of toluene with propylene. Their results showed that a

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p-cymene selectivity exceeding 90% and a high pcymene yield could be achieved. However, severe propylene cracking occurred in this operation, leading to low p-cymene yield. Because supercritical carbon dioxide possesses many unique characteristics and physicochemical properties, such as nonflammability, nontoxicity, extraction power tunability by temperature and pressure, and a higher mass-transfer rate compared to organic solvents, it has been regarded as a green solvent to substitute conventional organic solvents. The use of supercritical carbon dioxide as a solvent may offer the following advantages for chemical reactions: enhancement of the reaction rate and selectivity, enhancement of mass- and heat-transfer rates, an increase in the catalyst life due to the extraction of the coke precursors, and easier separation of the products from the solution after reaction.17-19 Besides carbon dioxide, supercritical propane, isobutene, and ethane also exhibit similar features when they are used as solvents. Fan et al.20 studied alkylation of supercritical isopentane and isobutane with isobutene over Y zeolite. The paraffin acted not only as a reactant but also as a carrier. The higher catalytic activity and the longer catalyst lifetime compared to the operations in liquid and gas phases were observed. These authors attributed the improvement to the removal of the deactivated oligomers from active sites of the catalyst by a supercritical solvent. Hitzler et al.21 found a 100% selectivity toward monoalkylated products in the alkylation of mesitylene and anisole with propylene and 2-propanol in supercritical propylene and carbon dioxide over a polysiloxane-supported solid acid catalyst. Clark and Subramaniam22 compared the results for the alkylation of isobutene with 1-butene over solid acid catalysts, such as microporous zeolites and mesoporous sulfated zirconia, in supercritical, gas, and liquid phases. Though the conversion of 1-butene in supercritical carbon dioxide was lower than that in the gas-phase operation, a steady production of trimethylpentanes and dimethylhexanes in supercritical carbon dioxide in a 2-day operation was observed that was not possible in gas- and liquid-phase operations because of a significant decay of catalyst. For the alkylation of benzene with ethylene over Y zeolite, Gao et al.23 found that both the catalyst deactivation rate and the formation rate of byproducts were reduced in the presence of supercritical carbon dioxide compared to the operations in gas and liquid phases. They attributed these results to a faster removal of the product ethylbenzene from the catalyst surface due to a high diffusion rate of ethylbenzene in supercritical carbon dioxide. This desorption hindered the subsequent isomerization of ethylbenzene to xylene at the surface of the catalyst. In this study, the alkylation of toluene with propylene in supercritical carbon dioxide over the HZSM-5 pellets modified by the CLD of SiCl4 was investigated. This operation takes advantage of the special properties of supercritical CO2 that may enhance the yield and catalyst lifetime via a high mass-transfer rate and its extraction power toward coke precursor. In the study the effects of the operation variables including temperature, pressure, weight hourly space velocity (WHSV) of toluene, and molar ratio of toluene to propylene on p-cymene selectivity and yield were examined and discussed. A comparison between the presently obtained results and those obtained at atmospheric pressrue16

Figure 1. Experimental apparatus for alkylation at supercritical conditions.

was made to see the feasibility for the use of supercritical carbon dioxide as a solvent in the alkylation. Experimental Section Toluene of a minimum purity of 99.7% was purchased from Mallinckrodt Inc. The standards used in gas chromatographic (GC) analysis, such as cymene isomers (Aldrich Chemical Co.) and p-n-propyltoluene (p-NPT, Tokyo Chemical Co.), were research-grade reagents. All chemicals were used without further purification. The parent HZSM-5 pellets were in cylindrical form having a diameter of 1.5 mm, a length of 1.5 mm, and a Si/Al ratio of 22.3. The modified catalyst, silica deposited on HZSM-5, was prepared by the CLD of SiCl4.16 It was done by adding 4.0 g of the parent HZSM-5 pellets containing 14% water into a solution containing 4 mL of SiCl4 and 16 mL of hexane. Wang and Tan16 observed that 14% water present in the parent HZSM-5 pellets could provide the highest yield and p-cymene selectivity for this alkylation. The mixture was stirred at 303 K for 5 h. Then the solid catalyst was filtered out, and 20 mL of hexane was used to wash the solid catalyst to remove SiCl4 retained. After that, the modified HZSM-5 pellets were dried at 333 K for 5 h and calcined at 773 K for 6 h. The experimental apparatus used in this study is illustrated in Figure 1. A stainless steel 316 tube with an inside diameter of 1.77 cm and a length of 70 cm served as the reactor. In each experiment, about 4.0 g of the newly modified HZSM-5 pellets mixed with 34 g of a 12-20 mesh inert ceramic was loaded into the reactor. Around 86 g of the 12-20 mesh ceramic was packed above the catalyst packing for preheating of the inlet stream, and 102 g of the same-size ceramic was also packed below the catalyst packing. The reactor was placed in an electric furnace that was equipped with three temperature controllers. An inserted thermocouple located at the middle of catalyst packing and three thermocouples located on the wall of the reactor

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were employed for recording temperatures during the operation. The variation in temperature at each location was observed to be less than 2.0 K. Prior to each run, the reactor was preheated with air at 673 K for 4 h and then was purged with nitrogen for at least 6 h to remove any residual air. Carbon dioxide with a purity of 99.7% (San Fu Chemical Co.) and propylene with a purity of 99.5% (Air Products and Chemicals, Inc.) were compressed and sent to two surge tanks by two metering pumps (Milton Roy and LDC). At the beginning, only supercritical carbon dioxide passed through the reactor. When its temperature, pressure, and flow rate reached steady state, it was allowed to mix with toluene. A syringe pump (Isco 260D) was used to pressurize and deliver toluene and to control the flow rate of toluene. Before mixing, toluene was heated. To ensure that toluene could be completely dissolved in carbon dioxide and the temperature of the mixture could reach the desired one, the supercritical carbon dioxide and toluene mixture flowed through a preheating coil with a length of 2 m before they entered the reactor. The mole fraction of toluene in supercritical carbon dioxide was at least 20% below the equilibrium solubility at each operating temperature and pressure. The Peng-Robinson equation of state with an interaction parameter provided by Ng and Robinson24 was used to evaluate the equilibrium solubility of toluene in supercritical carbon dioxide. When the temperature, pressure, and flow rate of the supercritical carbon dioxide and toluene mixture became steady, propylene was allowed to enter the supercritical carbon dioxide stream prior to mixing with toluene. The pressurized propylene leaving the surge tank possessed the same pressure as carbon dioxide. The pressure and flow rate of propylene were controlled with a syringe pump (Isco 100DX). The effluent gas stream of the reactor was first expanded with a metering valve. It then passed through a cooling coil and flowed into a phase separator where C1-C5 light hydrocarbons, unreacted propylene, and carbon dioxide were separated from the high boiling point compounds such as toluene, cymene isomers, and other aromatics. The phase separator was placed in a cold trap whose temperature was at about 268 K. The compositions in the gas and liquid streams leaving the separator were analyzed by two flame ionization detection GCs (Varian 3400CX and China Chromatography 8900). The total volume of the gas stream leaving the separator was determined via a wet testmeter. From the measured compositions and volumes of the gas and liquid streams leaving the separator, the total mass in the reactor effluent stream could then be determined. In each run, it was found that steady operation could be reached within 2.5 h, so that the results after 3 h were reported in the remaining of text. Results and Discussion For the operations at atmospheric pressure, Wang and Tan16 found that the most appropriate operating conditions for the p-cymene selectivity and yield over the modified HZSM-5 pellets were at a temperature of 523 K, a WHSV of 4.6 (g of toluene)/(g of catalyst)/h, and a molar ratio of toluene to propylene of 7.7. These conditions were thus chosen as the comparison basis at the beginning. The p-cymene and cymene isomers yields were defined as the molar ratio of p-cymene and cymene isomers to the fed propylene, and the p-cymene selectiv-

Figure 2. Contents of cymene isomers in liquid products at various pressures for a temperature of 523 K, a WHSV of 4.6 (g of toluene)/(g of catalyst)/h, and a molar ratio of toluene to propylene of 7.7.

ity was defined as the molar ratio of p-cymene to cymene isomers. The reproducibility tests at these and almost all of the other operating conditions under gas-phase and supercritical operations were performed. The data on the p-cymene yield and selectivity could be reproduced with a deviation of less than 8%. The total mass in the reactor effluent stream was found to agree well with that in the inlet streams. The difference between these two masses was always less than 7%, indicating a satisfactory mass balance in each run. From the GC analyses, it was seen that the alkylation at supercritical conditions gave a product distribution of C1-C5 light hydrocarbons, cymene isomers, p-NPT, and some other aromatics. Light hydrocarbons mainly resulted from the cracking of propylene. It was verified from the runs when only toluene or propylene in supercritical carbon dioxide was allowed to pass through the catalyst bed at temperatures equal to and less than 573 K. No C1-C5 light hydrocarbons were detected when only toluene flowed through the reactor. However, this was not the case for propylene; about 73% of propylene was found to crack at 523 K and 11.72 MPa, and the extent of cracking decreased with increasing pressure. It is also noted that benzene and xylene were not detected when only toluene passed through the reactor, indicating that no disproportionation occurred. Figure 2 shows that cymene isomers were the major liquid products at supercritical conditions and more cymene isomers could be yielded compared to the operation at atmospheric pressure. The experimental data also showed that the cymene isomers and p-cymene yields at 523 K and 11.72 MPa were quite steady in a 15-h operation, with a deviation of less than 2%. However, this was not the case for the atmospheric pressure operation in which the difference between the yields at the first hour and the fifteenth hour was observed to be 25%. For pressures equal to or less than 11.72 MPa, cymene isomers, p-NPT, and heavy aromatics were detected and the contents of cymene isomers in liquid products varied from 85 to 95%. For pressures higher than 11.72 MPa, methanol and formaldehyde were also detected and the contents of cymene isomers dropped to about 70%.

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Figure 3. Cymene isomers yields at various pressures for a temperature of 523 K, a WHSV of 4.6 (g of toluene)/(g of catalyst)/ h, and a molar ratio of toluene to propylene of 7.7.

Figure 3 shows the measured cymene isomers yields at various pressures for a temperature of 523 K, a WHSV of 4.6 (g of toluene)/(g of catalyst)/h, and a molar ratio of toluene to propylene of 7.7 in the feed. It can be seen from Figure 3 that the operations at supercritical conditions could result in more cymene isomers compared to the atmospheric pressure operation and the highest cymene isomers yield occurred at 11.72 MPa. One of the advantages of using supercritical fluid as the solvent is its extraction capability for coke precursors. If this were the case, more active sites would be available for the alkylation. The GC analysis showed that propylene was almost consumed at 523 K and 11.72 MPa; the data in Figures 2 and 3 therefore indicated that ca. 20% propylene was cracked at this condition. Compared to ca. 40% cracking of propylene under atmospheric pressure operation and 73% cracking for the flow containing only propylene at 523 K and 11.72 MPa, a pronounced improvement in the utilization of propylene and a significant prevention of formation of coke on the catalyst under supercritical operations were exhibited. According to the proposed reaction mechanism for the alkylation of toluene with propylene under gas-phase operation,4 a carbonium ion of propylene should be formed first to proceed the reaction. If this were also the case at supercritical conditions, the above observation indicated that more active sites would be available for the alkylation resulting from the dissolution of coke precursors in supercritical carbon dioxide and the reaction rate of the adsorbed propylene with toluene was faster than the cracking rate of the adsorbed propylene under supercritical operations. Figures 4 and 5 show the p-cymene yield and the p-cymene selectivity, respectively, for a temperature of 523 K, a WHSV of 4.6 (g of toluene)/(g of catalyst)/h, and a molar ratio of toluene to propylene of 7.7. Compared to the p-cymene yield using the parent HZSM-5 pellets, it is obvious from Figure 4 that a modification of HZSM-5 pellets with CLD was beneficial to yield more p-cymene resulting from a change in the pore opening. It can also be seen from Figure 4 that the operations at supercritical conditions could result in higher p-cymene yields over the atmospheric pressure operation. At the same WHSV of toluene, the retention

Figure 4. p-Cymene yields at various pressures for a temperature of 523 K, a WHSV of 4.6 (g of toluene)/(g of catalyst)/h, and a molar ratio of toluene to propylene of 7.7.

Figure 5. p-Cymene selectivity at various pressures for a temperature of 523 K, a WHSV of 4.6 (g of toluene)/(g of catalyst)/ h, and a molar ratio of toluene to propylene of 7.7.

time under supercritical operations was longer than that under the gas-phase operation because of the higher densities possessed by compressed carbon dioxide. Under these circumstances, the p-cymene yield was enhanced, resulting from a longer time to carry out the alkylation. While high p-cymene yields were exhibited under supercritical operations, an optimal pressure yielding the highest p-cymene yield was observed to exist at 11.72 MPa. From the phase diagram of the carbon dioxide-toluene system,24 it is interesting to note that the optimal pressure is located near the critical locus. A possible reason to enhance the p-cymene yield with pressure under supercritical operations was an increase in the retention time due to an increase in the pressure. However, this effect was not so pronounced over 11.72 MPa because there was no big difference in the densities at these pressures. On the other hand, supercritical carbon dioxide was an effective desorbent when pressure increased;25,26 adsorption isotherm thus decreased with increasing pressure. Under this situation, the adsorption of propylene and toluene from supercritical carbon dioxide became less at higher

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pressures and the subsequent surface reaction on catalyst active sites was therefore reduced. The cracking data might be evidence to support this point. The extent of propylene cracking was found to decrease as the pressure increased. These opposite effects therefore resulted in the existence of an optimal pressure regarding p-cymene yield. From Figure 5 it can be seen that p-cymene selectivities at supercritical and atmospheric conditions were all over 75% using the modified HZSM-5 pellets. When the parent HZSM-5 pellets were used, the p-cymene selectivity was observed to be lower than those using the modified HZSM-5 pellets, indicating that the modification could indeed improve p-cymene selectivity. The reason for yielding higher p-cymene selectivity using the modified HZSM-5 pellets was mainly attributed to a decrease in the pore opening and a reduction of the isomerization of p-cymene to m- and/or o-cymene near the pore mouth because of the deposition of Si on the catalyst surface. Though all p-cymene yields at supercritical conditions were higher than that at atmospheric pressure as shown in Figure 4, only the p-cymene selectivity at 11.72 MPa was comparable to that at atmospheric pressure. While the pore opening was still favorable to the passage of p-cymene compared to o- and m-cymene under supercritical operations, an increase in the diffusion resistance at high pressures exhibited the opposite effect. An increased diffusion resistance not only enhanced the possibility for isomerization due to a longer retention time of p-cymene in the pores but also reduced the possibility of the diffusion of p-cymene out of the pores because of the enhanced collision with other species. Under these circumstances, the p-cymene selectivity decreased at very high pressures. When the pressure was close to the critical pressure of carbon dioxide, some carbon dioxide molecules would access solute to form a cluster. The number of carbon dioxide molecules in the cluster depended on the temperature and pressure. Though the operating temperature was largely apart from the critical temperature of carbon dioxide, small clusters might still exist at pressures sufficiently close to the critical pressure of carbon dioxide. Under these circumstances, the diffusion of cymene in the pores was hindered by the existence of clusters. Because the size of the cluster decreased with increasing pressure, less p-cymene selectivity was thus exhibited at 6.89 and 8.96 MPa compared to that at 11.72 MPa. When the modified HZSM-5 pellets were used and the WHSV and molar ratio of toluene to propylene were maintained at 4.6 (g of toluene)/(g of catalyst)/h and 7.7, respectively, the temperature resulting in the highest p-cymene yield and selectivity existed at about 523 K in each isobaric operation, as shown in Figures 6 and 7. These results accompanied with those in Figures 4 and 5 indicated that the most appropriate pressure for this alkylation was at 11.72 MPa. When the temperature exceeded 523 K, both the p-cymene yield and selectivity dropped compared to that at 523 K. The lower selectivity might be due to a thermal expansion of the pores and an increase in the kinetic energy of the molecules. Under these circumstances, more m- and o-cymene molecules could diffuse out from the pores. The reason for the lower yield might be explained as follows. The activation energy of a secondary carbonium ion was generally lower than that of a primary carbonium ion; therefore, the formation of cymene was

Figure 6. Temperature dependence of p-cymene yield for the modified HZSM-5 pellets, a WHSV of 4.6 (g of toluene)/(g of catalyst)/h, and a molar ratio of toluene to propylene of 7.7 (the data were taken at the 5th h).

Figure 7. Temperature dependence of p-cymene selectivity for the modified HZSM-5 pellets, a WHSV of 4.6 (g of toluene)/(g of catalyst)/h, and a molar ratio of toluene to propylene of 7.7 (the data were taken at the 5th h).

predominant over that of p-NPT at lower temperatures. When the temperature was raised, sufficient energy could overcome the activation energy barrier of a primary carbonium ion, and as a result more p-NPT could be produced. The GC analyses showed that the contents of p-NPT in liquid products were 15.2, 13.8, 6.2, 6.2, and 1.3% for the temperatures of 548, 533, 523, 513, and 498 K, respectively. While the p-cymene selectivity at 498 and 513 K were comparable to that at 523 K, the yields were much lower, as shown in Figures 6 and 7. This was because heavy aromatics produced at 498 and 513 K did not possess sufficient energy to diffuse out from the pores; as a result they occupied active sites and decreased the p-cymene yield. Regarding the p-cymene yield and selectivity, the tem-

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the p-cymene yield was observed to decrease. From the comparison of p-cymene yields at different WHSV, it might be concluded that a WHSV of 4.6 (g of toluene)/ (g of catalyst)/h was the best choice for this alkylation reaction. An optimal molar ratio of toluene to propylene for the p-cymene yield was also observed to exist when the operation was at a temperature of 523 K, a pressure of 11.72 MPa, and a WHSV of 4.6 (g of toluene)/(g of catalyst)/h, as shown in Figure 9. Because more propylene was fed to the reactor when the ratio was low, more cracking of propylene therefore reduced the catalyst active sites, resulting in a decrease in the p-cymene yield. When the molar ratio increased to a certain level, less propylene was adsorbed to form carbonium ions that were necessary to make the alkylation continue. Under this condition, less p-cymene was generated. From Figure 9, it can be seen that a molar ratio of 7.7 was the most appropriate choice. Conclusion Figure 8. WHSV dependence of p-cymene yield and selectivity for the modified HZSM-5 pellets, a temperature of 523 K, a pressure of 11.72 MPa, and a molar ratio of toluene to propylene of 7.7 (the data were taken at the 5th h).

Figure 9. Molar ratio of toluene to propylene dependence of p-cymene yields and selectivity for the modified HZSM-5 pellets, a temperature of 523 K, a pressure of 11.72 MPa, and a WHSV of 4.6 (g of toluene)/(g of catalyst)/h (the data were taken at the 5th h).

perature at 523 K was concluded to be the most appropriate one to carry out the present alkylation reaction. For a temperature of 523 K, a pressure of 11.72 MPa, and a molar ratio of toluene to propylene of 7.7, Figure 8 shows that the most appropriate WHSV for the p-cymene yield and selectivity was about 4.6 (g of toluene)/(g of catalyst)/h, the same as that under the atmospheric pressure operation. At lower WHSV, less toluene could participate in alkylation. As a consequence, less p-cymene yield was exhibited. When the WHSV was raised from 4.6 to 6.0 or 7.0 (g of toluene)/ (g of catalyst)/h, though more toluene could participate in alkylation, active sites available to adsorb propylene and to generate carbonium ion were reduced because of the occupation of toluene. Under these circumstances,

To produce the desired product p-cymene, alkylation of toluene with propylene in supercritical carbon dioxide over HZSM-5 pellets was performed in a packed-bed reactor. Both the parent HZSM-5 pellets and the HZSM-5 pellets modified by CLD of SiCl4 were used in the study. The experimental results indicated that the reduction of the pore opening of HZSM-5 pellets via CLD modification was beneficial to the p-cymene yield and selectivity. When the modified HZSM-5 was employed and the operation was at a temperature of 523 K, a pressure of 11.72 MPa, a WHSV of 4.6 (g of toluene)/(g of catalyst)/h, and a molar ratio of toluene to propylene of 7.7, the p-cymene yield was found to be much higher than that under atmospheric pressure operation and the p-cymene selectivity could be as high as 90%. A higher yield was attributed to the longer retention time under supercritical operations and the solubilizing capability of supercritical carbon dioxide toward coke precursors. In addition to a high yield, the extent of propylene cracking was also found to decrease significantly under supercritical operations. This finding exhibited more effective utilization of propylene at the operation in the presence of supercritical carbon dioxide. Over the presently studied ranges of temperature, pressure, WHSV of toluene, and molar ratio of toluene to propylene, the experimental results showed the existence of an optimal combination of those variables. It was believed that both the dissolution power of supercritical carbon dioxide toward reactants and products and the reaction mechanism affected the location of the optimal operation variables. Finally, it is interesting to note that the optimal operating condition, i.e., at 523 K and 11.72 MPa, is located near the critical locus of the CO2-toluene mixture. Acknowledgment Financial support from the National Science Council of ROC (Grant NSC89-2214-E-007-023) is gratefully acknowledged. Literature Cited (1) Hatch, L. F.; Mater, S. From Hydrocarbons to Petrochemicals. Hydrocarbon Process. 1979, 58, 189.

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Received for review June 1, 2001 Revised manuscript received August 22, 2001 Accepted August 27, 2001 IE0104868