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Mar 11, 2003 - The alkylation of toluene with isopropyl alcohol under the operations in gaseous nitrogen, supercritical CO2, and liquid and supercriti...
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Ind. Eng. Chem. Res. 2003, 42, 1334-1340

Alkylation of Toluene with Isopropyl Alcohol over Chemical Liquid Deposition Modified HZSM-5 under Atmospheric and Supercritical Operations Tai-Chen Chiang, Jui-Chi Chan, and Chung-Sung Tan* Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan 300, ROC

The alkylation of toluene with isopropyl alcohol under the operations in gaseous nitrogen, supercritical CO2, and liquid and supercritical toluene over the parent and modified HZSM-5 pellets was carried out in a packed bed. The chemical liquid deposition technique with the deposition agent SiCl4 was used to modify the HZSM-5 pellets. The experimental results at all of the operations indicated that the modification was essential to enhance the p-cymene selectivity and yield. Because of the limitation of the solubility of water in supercritical CO2, the operating pressure far away from the CO2 critical pressure was required to effectively remove water from the catalyst pellets. The p-cymene selectivity and yield were found to be of 94% and 84%, respectively, at a temperature of 543 K, a pressure of 17.23 MPa, a weight hourly space velocity of 6.5 (g of toluene)/(g of catalyst)/h, and a molar ratio of toluene to isopropyl alcohol of 11 using the modified HZSM-5 pellets. The p-cymene selectivity at this operation condition was slightly higher than that under atmospheric pressure operation (92%), but the p-cymene yield was much enhanced compared to that under atmospheric pressure operation (51%). For the operations in liquid and supercritical toluene, a steady-state operation was not achieved, resulting from an inefficient removal of water from the catalyst pellets. Besides, more side products were generated when the operating temperatures exceeded the toluene critical temperature. Introduction Because of the abundance and low price of toluene, the attempt to convert toluene to more valuable aromatic compounds such as benzene and xylene has received widespread attention.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. Besides the homogeneous Friedel-Crafts catalysts, the solid acid catalysts can also be used to produce p-cymene via alkylation of toluene with isopropyl alcohol.4-8 When MFI zeolites were used as the catalysts, though high p-cymene selectivity could be obtained, a significant amount of the side product n-propyltoluene was also generated.6 Wichterlova and Cejka7 used HZSM-5, H-mordenite, and H-Y zeolites to reduce the side products. Their results showed that HZSM-5 could result in a higher p-cymene selectivity compared to H-mordenite and H-Y zeolites. This was attributed to the fact that the channel dimensions of HZSM-5 were approximately the same as the molecular dimensions of many aromatic molecules.9 According to these studies, it seems that ZSM-5 is a proper catalyst regarding paraselectivity in alkylation, despite the fact that n-propyltoluene may also be formed.8 In general, the dimensions of isomers are sufficiently close, the pore opening of HZEM-5 may not be small enough to make a desired separation. Selective poisoning of the ZSM-5 crystals therefore was proposed to improve the selectivity of the para isomer.10-12 One of the poisoning means was to use chemical liquid deposi* To whom correspondence should be addressed. Tel: 8863-572-1189.Fax: 886-3-572-1684.E-mail: [email protected].

tion (CLD) in which pore-opening sizes of ZSM-5 zeolite crystal were controlled using metal halides as the deposition agents.12 This kind of poisoning has shown a great improvement for the production of p-cymene in the alkylation of toluene with propylene under atmospheric and supercritical operations.13,14 Because supercritical CO2 (Tc ) 304.2 K and Pc ) 7.38 MPa) possesses many unique characteristics and physicochemical properties, such as nonflammable, nontoxic, tunable extraction power with 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 CO2 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 catalyst life due to the extraction of the coke precursors, and easier separation of products from solution after reaction.15-19 Besides carbon dioxide, supercritical propane, isobutene, and ethane exhibit similar features as well when they are used as solvents. For the alkylation of toluene with propylene, Kuo and Tan14 observed that the p-cymene yield using supercritical CO2 as the carrier could be doubled over that under atmospheric operation. A higher yield was attributed to a longer retention time under supercritical operations. Besides, the cracking of propylene in supercritical CO2 was found to reduce significantly. Though p-cymene could be produced effectively in supercritical CO2 using propylene as the reactant, cracking and safety are the issues that need more attention. The objective of this study is to demonstrate the feasibility of the alkylation of toluene with isopropyl alcohol in supercritical CO2 over the HZSM-5 pellets modified by the CLD of SiCl4. The operation took

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advantage of the special properties possessed by supercritical CO2 and the modification of the HZSM-5 pellets. To make a comparison, the best p-cymene selectivity and yield were also determined under the atmospheric operation using nitrogen as the carrier. Because of the limitation of the solubility of toluene and isopropyl alcohol in supercritical CO2, the alkylation in liquid and supercritical toluene in which toluene acted not only as a reactant but also as a carrier was also performed in this study. The operation variables including temperature, pressure, weight hourly space velocity of toluene (WHSV), and molar ratio of toluene to isopropyl alcohol were varied to see their effects on p-cymene yield and selectivity. From the comparison in p-cymene selectivity and yield among the operations in gaseous nitrogen, supercritical CO2, and liquid and supercritical toluene, the most appropriate operation could then be suggested. Experimental Section Toluene and isopropyl alcohol of a minimum purity of 99.7% were purchased from Mallinckrodt Inc. The standards used in the gas chromatographic (GC) analysis including 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 supplied by the Taiwan Styrene Monomer Co. They were in cylindrical shape possessing a diameter of 1.5 mm, a length of 1.5 mm, a Si/Al of 22.3, a Brunauer-Emmett-Teller (BET) surface area of 335 m2/g, and a binder alumina of 20%. The modified catalyst, silica deposited on HZSM-5 pellets, was prepared by the CLD of SiCl4.14 It was done by adding 4.0 g of the parent HZSM-5 pellets containing 14% of water into a solution containing 4 mL of SiCl4 and 16 mL of hexane. The mixture was stirred at 303 K for 5 h. Then the catalyst pellets were filtered out, and 20 mL of hexane was used to wash the pellets 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 BET surface area of the modified HZSM-5 pellets was found to reduce to 280 m2/g. 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 was served as the reactor. About 4.0 g of the modified HZSM-5 pellets mixed with 34 g of 12-20 mesh inert ceramic was loaded in the reactor. About 86 and 102 g of 12-20 mesh ceramic were packed above and below the catalyst packing, respectively. The reactor was placed in an electric furnace that was equipped with three temperature controllers. An inserted thermocouple located at the middle of the catalyst packing and three thermocouples located on the wall of the reactor 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 catalyst pellets in the reactor were preheated with air at 673 K for 4 h and then were purged with nitrogen for at least 6 h to remove any residual air. For the operations at supercritical conditions, CO2 with a purity of 99.7% (San Fu Chemical Co.) was compressed and sent to a surge tank by a metering pump (Milton LDC). At the beginning, only supercritical CO2 passed through the reactor. When its temperature, pressure, and flow rate reached steady state, it was allowed to mix with the liquid mixture of toluene and

Figure 1. Experimental apparatus for the alkylation of toluene with isopropyl alcohol: (1) CO2 cylinder; (2) heat exchanger; (3) filter; (4) metering pump; (5) surge tank; (6) syringe pump; (7) air cylinder; (8) N2 cylinder; (9) toluene + isopropyl alcohol; (10) flowmeter; (11) heating tape; (12) reactor; (13) electric furnace; (14) phase separator; (15) cold trap; (16) gas sampling valve; (17) wet test meter. T ) thermocouple. P ) pressure indicator.

isopropyl alcohol. A syringe pump (ISCO 260D) was used to pressurize and deliver the liquid mixture and to control the flow rate of that mixture. Before mixing, the CO2 and liquid streams were heated. To ensure that the liquid mixture could be completely dissolved in CO2 and the operation temperature could reach the desired one, the mixture of supercritical CO2, toluene, and isopropyl alcohol flowed through a preheating coil with a length of 2 m before it entered the reactor. The mole fraction of toluene in supercritical CO2 was about 20% below the equilibrium solubility at each operating temperature and pressure. The Peng-Robinson equation of state with the interaction parameter proposed by Ng and Robinson20 was used to evaluate the equilibrium solubility of toluene in CO2. The effluent stream of the reactor was expanded with a metering valve. It then passed through a cooling coil and flowed into a phase separator where C1 to C5 light hydrocarbons and CO2 were separated from the high boiling point compounds such as toluene, isopropyl alcohol, cymene isomers, and other aromatics. The phase separator was placed in a cold trap whose temperature was at about 273 K. The compositions in the gas and liquid streams leaving the separator were analyzed by two flame ionization detector gas chromatographs (Varian 3400CX and China Chromatography 8900). The GC column for the gas stream was a 240 cm length and 3.1 mm outside diameter stainless steel tube packed with 23% SP-1700 on 80/100 Chromosorb, and the GC column for the liquid stream was a 60 m length and 0.25 mm inside diameter capillary tube coated with a 0.25 µm of Supelcowax 10. The total volume of the gas stream leaving the separator was determined via a wet test meter. 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. For the operations at atmospheric pressure, nitrogen with a purity of 99.7% was used as the carrier. For the

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Figure 2. p-Cymene selectivity and yield at various temperatures and WHSV for a feed rate of isopropyl alcohol of 1.54 g/h under atmospheric pressure operations.

operations in liquid and supercritical toluene, only the mixture of toluene and isopropyl alcohol was allowed to enter the reactor. The operation procedures and sample collection for these two operations were similar to those using supercritical CO2 as the carrier. Results and Discussion Cymene isomers were observed to be the major products for the alkylation of toluene with isopropyl alcohol over the original and modified HZSM-5 pellets. In addition to cymene isomers, p-NPT was also formed. The reactions are expressed by

For the alkylation over the modified HZSM-5 pellets under atmospheric pressure operations, the experiments were carried out over a temperature range from 498 to 533 K, a WHSV range from 4.6 to 8.0 (g of toluene)/(g of catalyst)/h, and a molar ratio of toluene to isopropyl alcohol range from 7.7 to 13.8. Some experimental results obtained at a fixed feed rate of isopropyl alcohol are shown in Figure 2. It is seen from Figure 2 that the most appropriate operating conditions regarding pcymene selectivity and yield were at a temperature of 523 K, a WHSV of 6.5 (g of toluene)/(g of catalyst)/h, and a molar ratio of toluene to isopropyl alcohol of 11.

The p-cymene yield was defined as the molar ratio of p-cymene to the fed isopropyl alcohol, and the p-cymene selectivity was defined as the molar ratio of p-cymene to cymene isomers. The p-cymene selectivity of 92% and the p-cymene yield of 51% at these conditions were then chosen as the comparison basis. It is noted here that the p-cymene selectivity and yield using the parent HZSM-5 pellets were at least 20% lower than those using the modified HZSM-5 pellets. The reproducibility tests under the operations in gaseous nitrogen and supercritical CO2 were all performed. The data on the p-cymene selectivity and yield could be reproduced with a deviation of less than 2.0%. The total mass not including nitrogen and CO2 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 5%, indicating a satisfactory mass balance in each run. From the GC analyses, only a trace amount of propylene was detected in the effluent gas stream from the phase separator for the operations in gaseous nitrogen, supercritical CO2, and liquid and supercritical toluene. Though a very small amount of propylene was detected, this verified the mechanism that the reaction was proceeded through the activation of propylene generated from the dehydration of isopropyl alcohol over the catalyst active sites.5 The absence of the other C1 to C3 light hydrocarbons also indicated the advantage of using isopropyl alcohol as the reactant compared to propylene as concerned with cracking. To ensure that no cracking occurred, only toluene or isopropyl alcohol in nitrogen and supercritical CO2 was allowed to pass through the catalyst bed at the temperatures equal to and less than 543 K. No light hydrocarbons except propylene were detected as well. Cymene isomers, p-NPT, and some other aromatics were the major components detected in the liquid phase for all of the operations. The product distributions in liquid for some operations in gaseous nitrogen, supercritical CO2, and supercritical toluene are exhibited in Table 1. It is noted here that benzene and xylene were not detected when only toluene passed through the reactor, indicating that no disproportionation occurred over the present catalyst pellets. Because the p-cymene yield was calculated based on the amount of isopropyl alcohol fed to the reactor, the feed rate of isopropyl alcohol in supercritical CO2 was therefore maintained as the same as that under the atmospheric pressure operations to make a fair comparison. Under this circumstance, a change in WHSV resulted in a change of the molar ratio of toluene to isopropyl alcohol under supercritical CO2 operations. Figures 3 and 4 illustrate the p-cymene selectivity and yield, respectively, for various WHSV under the operating conditions at 523 K and 11.72 MPa as well as at 543 K and 17.23 MPa. It can be seen that both the p-cymene selectivity and yield were decreased with time and were lower than those at atmospheric operations for the operating conditions at 523 K and 11.72 MPa. The decrease in selectivity and yield was attributed to the fact that the other product, water, could not be removed rapidly from the catalyst pellets right after it was generated because of the limitation of the equilibrium solubility of water in supercritical CO2. The effect of the resided water on selectivity and yield was also verified by carrying out the experiments as follows. After a certain period of alkylation time, only super-

Ind. Eng. Chem. Res., Vol. 42, No. 7, 2003 1337 Table 1. Product Distributions at the Fifth Hour for Some Operations in Gaseous Nitrogen, Supercritical CO2, and Supercritical Toluene T (K)

P (MPa)

carrier

WHSV (g/g/h)

ratioa

p-cymene (wt %)

m-cymene (wt %)

o-cymene (wt %)

p-NPT (wt %)

others (wt %)

523 523 523 543 543 543 523 593 593 603

0.101 0.101 0.101 17.23 15.85 17.23 17.23 7.58 6.20 4.83

N2 N2 N2 CO2 CO2 CO2 CO2 toluene toluene toluene

4.6 6.5 8 6.5 6.5 5.5 6.5 22.6 22.6 22.6

7.7 11.0 13.6 11.0 11.0 9.3 11.0 7.0 7.0 7.0

70.5 67.1 67.9 88.9 86.1 84.4 72.9 62.9 64.1 52.2

4.2 3.5 3.7 4.7 4.9 5.6 14.0 9.3 8.3 12.1

1.9 2.4 3.2 1.4 1.7 2.3 3.2 6.6 6.8 11.4

3.4 6.4 8.1 3.1 3.4 4.4 1.5 5.7 7.1 10.9

20.0 20.6 17.1 1.9 3.9 3.3 8.4 15.5 13.7 13.4

a

Ratio represents the molar ratio of toluene to isopropyl alcohol.

Figure 3. p-Cymene selectivity at various WHSV for a feed rate of isopropyl alcohol of 1.54 g/h under the operations in supercritical CO2.

critical CO2 was allowed to enter the reactor. Within a 4 h flow of supercritical CO2, an observable amount of water was collected from the effluent stream. After that, the alkylation was allowed to occur again by feeding the mixture of toluene and isopropyl alcohol under supercritical CO2 operations. The p-cymene selectivity and yield were found to restore as before. It is also seen from Figures 3 and 4 that both p-cymene selectivity and yield were low at low WHSV. If the mechanism for this alkylation at high pressures was similar to that at atmospheric pressure, a carbonium ion of propylene generated from the dehydration of isopropyl alcohol should be formed prior to proceed the reaction.5 Because the feed rate of isopropyl alcohol was fixed under supercritical CO2 operations, a lower WHSV represented a lower molar ratio of toluene to isopropyl alcohol. Under this situation, more isopropyl alcohol could access the active sites and consequently more water was formed. If the water could not be removed from the pellets once it was generated, both the pcymene selectivity and yield were reduced as a result. For the operating conditions at 543 K and 17.23 MPa, the resultant water could be completely dissolved into supercritical CO2 according to the equilibrium solubility

Figure 4. p-Cymene yield at various WHSV for a feed rate of isopropyl alcohol of 1.54 g/h under the operations in supercritical CO2.

data.21 This was also verified by the fact that no water was collected in the supercritical CO2 stream in the runs in which only supercritical CO2 was allowed to enter the reactor after a certain period of the alkylation time. Under this circumstance, the p-cymene selectivity was comparable to and even higher than that under atmospheric operation, as shown in Figure 3. Similar to the alkylation of toluene with propylene under atmospheric and supercritical conditions,13,14 there existed an optimal WHSV as 6.5 (g of toluene)/(g of catalyst)/h for the present alkylation. The presence of an optimal WHSV resulted from the competition of toluene and isopropyl alcohol accessing the catalyst active sites. However, the effect of WHSV was not as pronounced for this alkylation as compared to the alkylation of toluene with propylene14 because no cracking of isopropyl alcohol occurred. It can be seen from Figure 4 that the p-cymene yield was 84%, about 70% higher than that under atmospheric pressure operation. This was attributed to a longer retention time under supercritical CO2 operations due to a higher density possessed by supercritical CO2 compared to the atmospheric nitrogen. The ratio of the density of supercritical CO2 at 543 K and 17.23

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Figure 5. p-Cymene selectivity in supercritical CO2 at different temperatures and pressures for a WHSV of 6.5 (g of toluene)/(g of catalyst)/h and a molar ratio of toluene to isopropyl alcohol of 11.

Figure 6. p-Cymene yield in supercritical CO2 at different temperatures and pressures for a WHSV of 6.5 (g of toluene)/(g of catalyst)/h and a molar ratio of toluene to isopropyl alcohol of 11.

MPa to that of nitrogen at atmospheric pressure was about 190, indicating a large difference in retention time between these two operations. In Figures 5 and 6 show that a higher pressure resulted in higher p-cymene selectivity and yield under isothermal operation. This was due to an increase in the retention time and an increase in the dissolution rate of water in CO2. When the pressure was maintained as a constant, a higher temperature favored both p-cymene selectivity and yield. This was probably because more of the generated water could be removed at higher temperatures. Besides, more kinetic energy possessed by p-cymene molecules at higher temperatures allowed them to more easily diffuse out of the catalyst pellets compared to m- and o-cymene molecules because of the existence of shape selectivity of the pore openings. From Figures 5 and 6, it is seen that the operating conditions at 543 K and 17.23 MPa resulted in a p-cymene selectivity of 94% and a p-cymene yield of 84%, which were the highest compared to the other

Figure 7. p-Cymene selectivity at a WHSV of 22.6 (g of toluene)/ (g of catalyst)/h and a molar ratio of toluene to isopropyl alcohol of 7 under the operations in liquid and supercritical toluene.

operating conditions. While a further increase in pressure and/or temperature might generate more p-cymene because of the ease of removal of water, the operation cost would also increase. The operating conditions at 543 K and 17.23 MPa were therefore suggested. It is also noted here that the p-cymene selectivity and yield over the parent HZSM-5 pellets at 543 K and 17.23 MPa were 79% and 77%, respectively, indicating that the modification of the catalyst pellets was essential to enhance the production rate of p-cymene in the alkylation of toluene with isopropyl alcohol. Because of the limitation of the equilibrium solubility of toluene in supercritical CO2, the alkylation was also proceeded in liquid and supercritical toluene (Tc ) 592 K and Pc ) 4.11 MPa) to increase the feed of the reactants. In the preliminary runs, it was found that the most appropriate molar ratio of toluene to isopropyl alcohol at 603 K, 4.83 MPa, and a WHSV of 22.6 (g of toluene)/(g of catalyst)/h was 7. This ratio was therefore chosen to make a comparison for the operations in supercritical CO2 and toluene. Figure 7 illustrates that the p-cymene selectivities under supercritical toluene operations with a WHSV of 22.6 (g of toluene)/(g of catalyst)/h were all less than 90% and decreased with time, mainly resulting from the immiscibility of toluene with water and more kinetic energy possessed by each component. The residing water in the catalyst pellets definitely reduced the p-cymene selectivity just as the operation in supercritical CO2 at relatively lower pressures did. Because less water could be removed by liquid toluene, the water effect on selectivity was therefore more pronounced for the operation in liquid toluene, as shown in Figure 7. Because the temperatures under operations in supercritical toluene were higher than those under supercritical CO2 operations, an increase in kinetic energy of m- and o-cymene molecules at higher temperatures would allow those molecules to overcome the shape hindrance of the pore openings and to more easily diffuse out of the pellets. As a result, p-cymene selectivity was not comparable to that for the operations in supercritical CO2. In addition to the lower p-cymene selectivity, the weight percent of cymene isomers in the liquid products was also observed to decrease under supercritical toluene operations, as

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Figure 8. p-Cymene formation rates at a WHSV of 22.6 (g of toluene)/(g of catalyst)/h under liquid and supercritical toluene operations and a WHSV of 6.5 (g of toluene)/(g of catalyst)/h under supercritical CO2 operations for a molar ratio of toluene to isopropyl alcohol of 7.

shown in Table 1. This indicates that more side reactions occurred under supercritical toluene operations. Because WHSV under liquid and supercritical toluene operations was higher than that under supercritical CO2 operations, more isopropyl alcohol should be fed to the reactor for the former operations when the molar ratio of toluene to isopropyl alcohol was the same for these operations. Under this situation, the p-cymene formation rate, (mol of p-cymene)/(g of catalyst)/h, instead of the p-cymene yield was used for the comparison. It is seen from Figure 8 that there are very low p-cymene formation rates under liquid toluene operations. This was because the generated water was hard to remove from the catalyst pellets. For the operation in supercritical toluene, a steady p-cymene formation rate was not achieved, probably because an uncertain amount of water remained in the catalyst pellets. While the p-cymene formation rates were higher than that under supercritical CO2 operation, the increased ratio in p-cymene formation rate was found to be close to those of the fed isopropyl alcohol. On the basis of the obtained data on p-cymene selectivity and yield, the operations in liquid and supercritical toluene were not recommended. Conclusion The formation of p-cymene from the alkylation of toluene with isopropyl alcohol over the parent and modified HZSM-5 pellets under the operations in gaseous nitrogen, supercritical CO2, and liquid and supercritical toluene was investigated in this study. The experimental results indicated that the reduction of the pore opening of HZSM-5 pellets via the CLD modification resulted in a significant enhancement in both p-cymene selectivity and yield. For the operations in liquid and supercritical toluene, the p-cymene selectivity and yield were observed to vary with time because the generated water could not be effectively removed from the catalyst pellets. Besides, more side products were formed under supercritical toluene operations compared to those under supercritical CO2 operations because of

the higher operating temperatures. On the basis of these observations, the operations in liquid and supercritical toluene were not suggested. There existed a proper combination of temperature, pressure, toluene WHSV, and molar ratio of toluene to isopropyl alcohol regarding p-cymene selectivity and yield for the operations in gaseous nitrogen and supercritical CO2. The p-cymene yield under supercritical CO2 operations was found to be 70% higher than that under atmospheric pressure operation, while the p-cymene selectivity could be maintained at 94%. The increase in yield was attributed to a longer retention time under supercritical CO2 operations. The obtained results indicated that the operation in supercritical CO2 was superior to that under atmospheric pressure operation. The pressure required for a steady operation at 543 K for the alkylation was 17.28 MPa, about 5.6 MPa higher than that for the alkylation of toluene with propylene in supercritical CO2. This was due to the need to remove the generated water from the catalyst pellets. No isopropyl alcohol was found to crack, indicating a more effective use of isopropyl alcohol over propylene that was cracked in a significant extent even under supercritical CO2 operations. Acknowledgment Financial support from the National Science Council of ROC (Grant 90-2623-7-007-028) is gratefully acknowledged. Literature Cited (1) Hatch, L. F.; Mater, S. From Hydrocarbons to Petrochemicals. Hydrocarbon Process. 1979, 58, 189. (2) Kaeding, W. W.; Chu, C.; Young, L. B.; Weinstein, B.; Butter, S. A. Selective Alkylation of Toluene with Methanol to Produce Para-Xylene. J. Catal. 1981, 67, 159. (3) Kim, J. O.; Kunieda, T.; Niwa, M. Generation of ShapeSelectivity of P-Xylene Formation in the Synthesized ZSM-5 Zeolites. J. Catal. 1998, 173, 433. (4) Cejka, J.; Kapustin, G. A.; Wichterlova, B. Factor Controlling Iso/N- and Para- Selectivity in the Alkylation of Toluene with Isopropyl alcohol on Molecular Sieves. Appl. Catal. A 1994, 108, 187. (5) Reddy, K. S. N.; Rao, B. S.; Shiralkar, V. P. Selective Formation of Cymenes over Large Pore Zeolites. Appl. Catal. A 1995, 121, 191. (6) Parikh, P. A.; Subrahmanyam, N.; Bhat, Y. S.; Halgeri, A. B. Toluene Isopropylation over Zeolite b and Metallosilicates of MFI Structure. Appl. Catal. A 1992, 90, 1. (7) Wichterlova, B.; Cejka, J. Mechanism of N-Propyltoluene Formation in C3 Alkylation of Toluene: The Effect of Zeolite Structural Type. J. Catal. 1994, 146, 523. (8) C ˇ ejka, J.; Wichterlova´, B. Acid-Catalyzed Synthesis of Monoand Dialkyl Benzenes over Zeolites: Activities, Zeolite Topology, and Reaction Mechanisms. Catal. Rev. 2002, 44, 375. (9) Young, L. B.; Butter, S. A.; Kaeding, W. W. Shape Selective Reactions with Zeolite Catalysts, III. Selectivity in Xylene Isomerization, Toluene-Methanol Alkylation, and Toluene Disproportionation over ZSM-5 Zeolite Catalysts. J. Catal. 1982, 76, 418. (10) Niwa, M.; Itoh, H.; Kato, S.; Hattori, T.; Murakami, Y. Modification of H-Mordenite by a Vapor-phase Deposition Method. J. Chem. Soc., Chem. Commun. 1982, 15, 819. (11) Wang, I.; Ay, C. L. Para-Selectivity of Dialkylbenzenes over Modified HZSM-5 by Vapor Phase Deposition of Silica. Appl. Catal. 1989, 54, 257. (12) Yue, Y. H.; Tang, Y.; Liu, Y.; Gao, Z. Chemical Liquid Deposition Zeolites with Controlled Pore-Opening Size and ShapeSelective Separation of Isomers. Ind. Eng. Chem. Res. 1996, 35, 430.

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(13) Wang, S. Y.; Tan, C. S. Alkylation of Toluene with Propylene to Produce Para-Cymene. J. Chin. Inst. Chem. Eng. 2001, 32, 319. (14) Kuo, T. W.; Tan, C. S. Alkylation of Toluene with Propylene in Supercritical Carbon Dioxide over Chemical Liquid Deposition HZSM-5 Pellets. J. Chin. Inst. Chem. Eng. 2001, 40, 4724. (15) Subramanian, B.; McHugh, M. A. Reactions in Supercritical FluidssA Review. Ind. Eng. Chem. Process Des. Dev. 1985, 25, 1. (16) Savage, P. E.; Gopalan, S.; Mizan, T. I.; Martino, C. J.; Brock, E. E. Reactions at Supercritical Conditions: Applications and Fundamentals. AIChE J. 1995, 41, 1723. (17) Gao, Y.; Shi, Y. F.; Zhu, Z. N.; Yuan, W. K. Coking Mechanism of Zeolite for Supercritical Fluid Alkylation of Benzene. The 3rd International Symposium on High-Pressure Chemical Engineering; Elsevier Science: Zurich, Switzerland, 1996; p 151.

(18) Hitzler, M. G.; Smail, F. R.; Ross, S. K.; Poliakoff, M. Friedel-Crafts Alkylation in Supercritical Fluids: Continuous, Selective and Clean. Chem. Commun. 1998, 359. (19) Baiker, A. Supercritical Fluids in Heterogeneous Catalysis. Chem. Rev. 1999, 99, 453. (20) Ng, H. J.; Robinson, D. B. Equilibrium Phase Properties of the Toluene-Carbon Dioxide System. J. Chem. Eng. Data 1978, 23, 325. (21) Fenghour, A.; Wakeham, W. A. Densities of (Water + Carbon Dioxide) in the Temperature Range 415 K to 700 K and Pressures up to 35 MPa. J. Chem. Thermodyn. 1996, 28, 433.

Received for review August 15, 2002 Revised manuscript received January 22, 2003 Accepted January 30, 2003 IE0206305