Ind. Eng. Chem. Res. 2007, 46, 4421-4425
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Production of para-Cymene from Alkylation of Toluene with Propylene in Supercritical CO2 over Shape-Selective HZSM-5 Pellets Chih-Chieh Yu and Chung-Sung Tan* Department of Chemical Engineering, National Tsing Hua UniVersity, Hsichu, Taiwan 30013, Republic of China
Pellets consisting of highly shape-selective HZSM-5 crystals prepared by chemical liquid deposition using tetraethyl orthosilicate (TEOS) as the silicon precursor were observed to be an effective catalyst for the alkylation of toluene with propylene in supercritical carbon dioxide. The p-cymene selectivity at a temperature of 523 K, a pressure of 11.72 MPa, a weight hourly space velocity of 4.56 (g of toluene)/(g of catalyst)/h, and a molar ratio of toluene to propylene of 7.7 was as high as 98%, indicating a pronounced shape-screening effect. The experimental results also indicated that propylene cracking could be significantly reduced compared to that under atmospheric pressure operation, indicating that supercritical CO2 was an effective carrier for alkylation. When binder alumina was used in the preparation of pellets, chemical liquid deposition with a proper amount of TEOS could be applied right after the compression and calcination of the prepared shapeselective HZSM-5 crystals. With a proper ratio of TEOS to pellet, p-cymene yield and selectivity could be sufficiently close to those using the compressed pellets not containing alumina. Introduction para-Cymene is a valuable aromatic compound that can be produced from alkylation of toluene with propylene
over solid acid catalysts, such as Y- and β-zeolite as well as silica-alumina crystals.1,2 Unfortunately, because of the cogeneration of other cymene isomers in the alkylation, a p-cymene selectivity >60% is hard to obtain as a result of the larger pores of zeolite crystals and the occurrence of the isomerization of p-cymene to m- and o-cymene at acid sites near the pore mouth for the case that more p-cymene can diffuse out of shape-selective pores. One of the means to enhance the selectivity of p-isomer is to use chemical liquid deposition to narrow the pore opening and to alter acidic sites near the pore mouth.3-5 The pore opening has been proven to be gradually narrowed as the deposition agent is increased.3 A p-cymene selectivity >90% could be achieved for the alkylation of toluene with propylene at atmospheric pressure when HZSM-5 pellets treated by chemical liquid deposition using SiCl4 as silicon precursor were employed as the catalyst.5 However, severe propylene cracking was observed. Because supercritical carbon dioxide possesses some unique characteristics and physicochemical properties, it has been * To whom correspondence should be addressed. Tel.: 886-3-5721189. Fax: 886-3-5721684. E-mail:
[email protected].
regarded as a green solvent and extensively used as a carrier in alkylation.6-9 The use of supercritical carbon dioxide may offer the following advantages for alkylation: enhancement of reaction rate and selectivity, enhancement of mass and heat transfer rates, and an increase in catalyst life due to extraction of coke precursor.10,11 Using Si-deposited HZEM-5 pellets as the catalyst and supercritical CO2 as the solvent for the alkylation of toluene with propylene, Kuo and Tan12 observed that the p-cymene yield could be doubled over that under atmospheric-pressure operation. A higher yield was attributed to a longer retention time and a reduction of cracking of propylene in supercritical carbon dioxide. The p-cymene selectivity at the most appropriate operation conditions was observed to be 90%, the same as that under atmospheric-pressure operation. The mass transfer in the Si-deposited HZEM-5 pellets involved the transport of reactants and products in both intercrystalline mesopores and crystalline micropores. The alkylation was mainly carried out in the crystalline micropores because more active sites were present in them. Because of the presence of acidic binder that causes the isomerization of p-cymene to m- or o-cymene and possible nonuniform Si-deposition onto crystals in pellets, a higher p-cymene selectivity is, therefore, expected if selective deposition of silicon on HZSM-5 crystals rather than pellets is carried out. In this study, HZSM-5 crystals modified by the chemical liquid deposition using tetraethyl orthosilicate (TEOS) as the silicon precursor were prepared first. They were then used as the catalyst to perform the alkylation of toluene with propylene in supercritical carbon dioxide. Because pellets are commonly used in a commercial process, HZSM-5 pellets consisting of the selectively Si-deposited crystals were prepared as well by compressing crystals with and without addition of the binder alumina. To eliminate the acidity effect of the binder alumina, the pellets were treated with chemical liquid deposition with TEOS right after the compression. From the measured p-cymene yield and selectivity at different operation variables including temperature, pressure, weight hourly space velocity (WHSV) of toluene, and molar ratio of toluene to propylene, the validity of the prepared HZSM-5 pellets for para-selectivity could be examined and the most appropriate operation conditions for
10.1021/ie060639x CCC: $37.00 © 2007 American Chemical Society Published on Web 05/12/2007
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generating high p-cymene yield and selectivity could also be determined. The results obtained at the most appropriate operation conditions were then compared to those reported in our previous work.5,12 Experimental Section Toluene of a minimum purity of 99% and propylene of a minimum purity of 99.5% were purchased from Mallinckrodt and Air Products and Chemicals, respectively. The standards used in gas chromatography (GC) analysis including cymene isomers (Aldrich Chemical) and p-n-propyltoluene (p-NPT, Tokyo Chemical) were research-grade reagents. All chemicals were used without further purification. ZSM-5 crystals with an average particle size of 3.5 nm, a pore size of 0.55 nm, a Si/Al of 25, and a surface area of 376 m2/g were purchased from Zeolyst International. The HZSM-5 crystals were generated by deamination in air for 3 h. The selectively Si-deposited HZSM-5 crystals, i.e., silica deposited near the pore mouth of the crystals, were prepared by chemical liquid deposition with TEOS (Acros, purity of 98%). This was done by adding 4 g of the HZSM-5 crystals containing 7% of water into a solution consisting of 6.4 mL of TEOS and 32 mL of hexane. The mixture was stirred at 303 K for 5 h, after which the crystals were filtered out and 20 mL of hexane was used to flush the crystals to remove TEOS retained. After drying at 333 K and calcination at 773 K for 6 h, these HZSM-5 crystals were ready for alkylation. To form 0.15 cm × 0.15 cm cylindricalshape pellets, the modified crystals were compressed at 5000 kg with and without addition of the binder alumina. The experimental apparatus used in this study was the same as that used by Kuo and Tan.12 A stainless steel 316 tube with an inside diameter of 1.58 cm and a length of 73 cm served as the reactor. In each experiment, about 4 g of the resultant HZSM-5 pellets mixed with 34 g of 12-20 mesh inert ceramic was loaded into the reactor. Around 70 g of the 12-20 mesh ceramic was packed above the catalyst packing for premixing and preheating of the inlet streams, and 90 g of the same size ceramic was 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 the catalyst packing and three thermocouples located on the wall of the reactor were employed for recording temperatures during the operation. 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 air. Carbon dioxide (San Fu Chemical, purity of 99.95%) was compressed and sent to a surge tank by a minipump (Milton Roy). At the beginning, only supercritical carbon dioxide passed through the reactor. When its temperature, pressure, and flow rate became steady, it was allowed to mix with toluene. The liquid toluene fed to the reactor was delivered and controlled by a syringe pump (Isco, 260D). Before mixing, toluene was heated. A preheating coil with a length of 2 m was employed to ensure a complete mixing of toluene with supercritical carbon dioxide and an achievement of the desired temperature of the mixture. When temperature, pressure, and flow rate of the supercritical carbon dioxide and toluene mixture became steady, propylene was allowed to flow into the reactor. The pressure and flow rate of propylene were controlled by a syringe pump (Isco, 100DX). 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 light hydrocarbons and carbon dioxide were separated from the high boiling point compounds
Figure 1. p-Cymene yield and selectivity for the HZSM-5 crystals modified by adding different amounts of TEOS in chemical liquid deposition for a temperature of 523 K, a pressure of 11.72 MPa, a WHSV of 4.56 (g of toluene)/(g of catalyst)/h, and a molar ratio of toluene to propylene of 7.7.
such as toluene, cymene isomers, and other aromatics. The phase separator was place in a cold trap in which the temperature was controlled at about 268 K. The compositions of the gas and liquid streams leaving the separator were analyzed by two flame ionization detector (FID) gas chromatographers (Varian, 3400CX, and China Chromatography, 8900). The total volume of the gas stream was determined via a wet gas meter (Shinagawa, WNK1B). From the measured compositions and volumes of the gas and liquid streams, the total mass in the reactor effluent stream could be determined. Results and Discussion The p-cymene yield and selectivity were defined as the molar ratio of p-cymene to the propylene fed and the molar ratio of p-cymene to cymene isomers, respectively. Figure 1 shows the p-cymene yield and selectivity for the modified HZSM-5 crystals prepared by adding different amounts of TEOS in chemical liquid deposition for the alkylation operated at 523 K, 11.72 MPa, a WHSV of 4.56 (g of toluene)/(g of catalyst)/s, and a molar ratio of toluene to propylene of 7.7. It is noted that p-cymene yield and selectivity at the above-mentioned operating conditions and the other operating conditions were observed to be steady in a 10 h operation with a deviation of
