Energy & Fuels 1994,8,846-850
846
Conversion of Benzene to Substituted Aromatic Products over Zeolite Catalysts at Elevated Pressures Eric M. Kennedy,' Ferenc Lonyi,*Todd H. Ballinger, Michael P. Rosynek, and Jack H. Lunsford' Department of Chemistry, Texas A & M University, College Station, Texas 77843 Received February 1, 1994. Revised Manuscript Received April 29, 1994"
The conversion of benzene to toluene and other substituted aromatic products has been investigated at elevated pressure and temperature. Over sodium-free H-Y and H-beta zeolites at 400 "C, 700 psi, and a methane/benzene feed ratio of 811, a wide range of methylated aromatic hydrocarbons was produced, with toluene (ca.50%) being the principal component. Over H-beta zeolite, C)4 and COz, when co-fed with the benzene reactant, enhanced the yield of aromatic products in comparison to co-fed N2 or H2. Isotopic tracer experiments, utilizing WH4 co-feed, demonstrated that methane reactant does not become incorporated into the "methylated" aromatic products over zeolite beta and that all of the observed hydrocarbon products were derived solely from the benzene reactant. Alkenes that are subsequently reacted with this surface species produce methylated homologues. The nature of The conversion of methane into more valuable hydrothe surface CH, species produced over supported Ni carbon products has been the subject of intensive research catalysts has been investigated by XPS and was found to during the past decade. In particular, the oxidative be carbonaceous, but not necessarily graphiticS8 coupling of methane (OCM) has been explored as a Both indirect and direct alkylation of olefins and commercially viable route to convert natural gas into aromatics with methane have also been reported over ethylene.' The OCM reaction is believed to occur via the superacid and modified molecular sieve catalyst^.^^^ In gas-phase coupling of methyl radicals, which are formed order to facilitate methane adsorption, elevated pressures when a methaneloxygen mixture reacts at high temperahave often been employed, although some investigators tures (>600 "C) over metal oxide catalysts.2 Alkylation have reported high yields of methylated products at only reactions, in which methane reacts with a co-fed hydro1atm and at temperatures as low as 100"C using superacid carbon moiety, have been less extensively studied as a ~atalysts.~ Long and co-workers'3 demonstrated that means of methane utilization. substituted aluminophosphate molecular sieves catalyzed One approach to methane alkylation reactions has been the methylation of naphthalene at 400 "C under elevated to adopt reaction conditions similar to those that are pressures of methane. This group has subsequently effective in OCM reactions. It has been proposed that confirmed, using 13CH4, that the methyl groups in the under these conditions methyl radicals may couple with substituted naphthalene products were derived from an activated co-fed hydrocarbon to produce methylated methane.14 products. Methylations of acetylene,3 ethane: and toluAlthough it is often assumed that the methane reactant ene5 have been reported using this approach. A surfacebecomes directly incorporated into the products during based alkylation process has also been investigated, in such "methylation" reactions, it is important to verify which methane reacts with a group VI11 transition metal, conclusively that co-fed methane actually participates in such as nickel or ruthenium, to form surface CH,specie~.~?~ the reaction by adding 13CH4to the reaction mixture, for example, and observing the presence of 13Cin the products. t Present address: Department of Chemical Engineering; Yale UniRecent applications of this technique have confirmed the versity; New Haven, CT 06520. methylation of naphthalene14 and revealed the nonPreaentaddrees: CentralReaearchInatituteforChemiatry,Hungarian Academy of Sciences P.O.Box 17, H-1525, Budapest, Hungary. methylation of propane.15 In the present study, we have Abstract published in Advance ACS Abstracts, June 1, 1994. used '3CH4 to investigate the apparent alkylation of (1) Sokolovskii, V. D.; Mamedov, E. A. Catal. Today 1992, 14, 415. benzene with methane to form toluene at 400 "C and 700 (2) Lunsford, J. H. In New Frontiers in Catalysis. Proceedings of the 10thInternational Congresson Catalysis;Guai, L.,Solymosi,F., Tetenyi, psi over a series of microporous aluminophosphates and P., E&.; Elsevier: Budapest, 1992; Part A, p 103.
Introduction
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(3) (a) Trukhacheva, N. P.; Noekova, N. F. React. Kinet. Catal. Lett. 1988,37,151. (b) Belavin,B. V.; Krasova,E. T.;Kushch,S. D.; Morvaekii, A. P.; Ratnikov, V. I.; SavchenkoV. I.; Tairova, G. G. Neftekhimiya 1989, 29, 746. (4) Wada, K.; Watanabe, Y.; Saitoh, F.; Suzuki, T. Appl. Catal. A 1992,88,23. (5) (a) Kim, H.; Suh, H. M.; Paik, H. Appl. Catal. A 1992,87,115. (b) Suzuki, T.;Wada, K.; Watanabe, Y. Ind. Eng. Chem.Res. 1991,30,1719. (c) Osada, Y.; Ogasawara, 5.;Fukushima, T.; Shikada, T.; Ikariya, T. J . Chem. SOC.,Chem. Commun. 1990, 1434. (d) Osada, Y.; Okino, N.; Ogasawara, S.;Fukushima, T.; Shikada, T.; Ikariya, T. Chem. Lett. 1990, 281. (e) Osada, Y.; Enomoto,K.; Fukushima,T.; Ogamwara,S.;Shikada, T.; Ikariya, T. J. Chem. Soc., Chem. Commun. 1989,1156. (6) Lijffler, I. D.; Maier, W. F.; Andrade, J. G.; Thies, I.; Scheyer, P. R. J . Chem. SOC.,Chem. Commun.1984, 1177.
(7) Koerta, T.;van Santen,R. A. J. Chem. SOC.,Chem. Commun.1992, 345. (8) Ovallee, C.; Leon,V.; Reyee, S.; Rosa,F. J. Catal. 1991,129,368. (9) Olah, G. A. European Patent Application 73673,1983. (10) S m e l l , M. S.; Cooks, M. Stud. Surf. Sci. Catal. 1988,36,433. (11) (a) Wahh, D. E.; Hans, 5.;Palenno, R. E. J. Chem. SOC.,Chem. Commun. 1991, 1259. (b) Han, S.; Martenek, D. J.; Palermo, R. E.; Pearson, J. A.; Walah, D. E. J. Catal. 1992,136,578. (12) Wang, D.; Huang, M.; Jiang, Y. Fenzi Cuihw 1991,5,164. (13) He, S. J. X.; Long, M. A.; Attalla, M. I.; Wileon, M. A. Energy Fuek 1992,6,498. (14) He, S. J. X.; Long, M. A.; Attalla, M. I.; Wilson, M. A. Energy Fuek 1994,8, 286. (15) Iglesia, E.; Baumgartner, J. E. Catal. Lett. 1993,21, 55.
0 1994 American Chemical Society 000~-0624/94/2500-0~6~~4.50l0
Conversion of C& to Substituted Products
Energy & Fuels, Vol. 8, No. 4, 1994 047
pressure
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Figure 1. Schematic diagram of high-pressure reaction system. Thenntmuple
zeolitecatalysts. Although toluene was formed over certain of the zeolites under these reaction conditions, isotopic labeling experimentsusing WH4 have demonstrated that the toluene product in these cases was not formed by the reaction of benzene with methane. Experimental Section All catalysts were obtained from commercial sources, except the SAPO-37, which was synthesized using a method described previously.16 Ion exchanges were performed by mixing 80 mL of a 0.1 M solution of the desired cation with 20 g of catalyst at 80 "C for 16 h. The ion-exchanged catalyst was then washed in deionized distilled water and dried for 2 h at 140 "C. Catalysts were pressed, crushed, and sieved to 20-40-mesh granules, and then activated in situ under flowing N2 at 1 atm. The 1.0-g catalyst samples used for all experiments were activated by heating from ambient temperatureto 400 "C in 100"C increments over at least 4 h, with a final heating at 400 "C for 110 h. A schematic diagram of the high-pressure stainless steel reaction systemis presented in Figure 1. All gaseswere introduced into the manifold through particulate filters using Brooks Model 5850C mass flow controllers. All reactant/diluent gases, uiz. CHI, C02, C2&, H2, and N2 (>99.9%),as well asthe isotopically labeled 13CH4(9 L of 99% isotopic purity at 1800 psig, obtained from Cambridge Isotopes)were used without further purification. The benzene (99.9 % , Aldrich spectroscopic grade) flow rate was controlled by an ISCO Model LC-2600 HPLC pump. All lines downstream from the point of benzene introduction into the gas stream were heated at 1170 "C. A pressure gauge mounted upstream from the reactor was used to monitor the reactor pressure. A schematic diagram of the catalytic reactor is shown in Figure 2. The reactor consisted of a stainless steel tube with a fusedquartz liner. The methane/benzene gas mixture entered the vertically orientedreactor from the top and flowed down through the catalyst bed, which was positioned in the middle of the reactor (16)Maistriau, L.; Dumont, N.; Nagy, J. B.; Gabelica, Z.; Derouane, E. G. Stud. Surf. Sei. Catal. 1989,52 209. The catalyst was crystallized
without stirring at 200 "C for 16 h.
Figure 2. Schematic diagram of catalytic reactor and furnace. to minimize heat gradients and held in place by quartz chips and quartz wool. The temperature was monitored by a chromel/ alumel thermocouple sheathed in a quartz thermowell. Standard reaction conditionsfor all experiments were 400 "C and 700 psi, with a co-feed gas consisting of CHI and CeHe in an 8 1 molar ratio at a total flow rate of 32 mL/min (NTP). Upon exiting the reactor, the product gases passed through a back pressure valve and into an on-line gas chromatograph. A heated gas sampling valve, mounted in a Carle Model 111gas chromatograph,was used to sample the reaction products. Gas analysis was achieved using a Porapak Q column at 100"C, which separated CHI, C2H6, C2H4, and C3+ gases, and a 5A molecular sieve column, which separated H2, N2, and CHI. No C2+ nonaromatic products were detected over any of the catalysts tested. Liquid effluents, uiz., benzene, toluene, ethylbenzene, and xylenes, were analyzed on-line using a 5% Bentone 34 on Chromosorb W-AW column at 100 "C. An in-line cold trap at 0 "C was also used to condense the liquid products (benzene, toluene, etc.) for subsequent off-line analyses. A bubble flow meter mounted after the ice trap was used to monitor total gas flow rates. GC/MS analyses of the collected liquid effluents were performed using a Hewlett-Packard Model 5890 Series I1 gas chromatograph equipped with a Model 5971 mass-sensitive detector. Products were separated on a 100% dimethylpolysiloxane capillary column by maintainingthe column temperature at 60 "C for 5 min and then ramping the temperature to 200 "C at 20 OC/min. Mass spectral intensities were checked daily using standards and were found to deviate by less than 2 % .
Results and Discussion For the reaction involving direct methylation of benzene to toluene
AG at the standard reaction conditions employed in this investigation, uiz., 400 "C and 700 psi, is +10.2 kcal/mol.
Kennedy et al.
848 Energy & Fuels, Vol. 8, No. 4,1994 Table 1. Activities of Catalysts for Conversion of Methane/Benzene to Toluene. max rate hydrogen/toluene of toluene product production ratio catalystb (pmol/min)c (mol/mol) quartz chips 0 Si02 gel 0 AlPO-5 0 SAPO-34 0 SAPO-37 0 cu/sAPo-34 0 Cu/AlPO-5 0 H-mordenite 0 H-ZSM-5' 1.3 3 H-ZSM-5' 1.1 3 Mg-ZSM-5' 1.0 3 CU-ZSM-5' 0 H-Y 2.7 3.1 Pt-Y 0 H-beta 7.0 3.4 Na-betaf 0 -
Reaction conditions: 400 OC, 700 psi, C H J C A feed ratio = 8/1. 0 Rate at ca. 90 min after beginningbenzene feed. Si/Al= 150/1. e Si/Al = 15/1. f Exchanged three times with 1.0 M NaNOs. a
* 1.0 g of 20-42-mesh granules (air dried).
This value corresponds to a maximum toluene formation rate of 14 pmollmin for a methane/benzene feed ratio of 8/1 flowing at 32 mL/min.17 Thus, if reaction 1 occurs and achieves equilibrium, the maximum single-pass conversion of benzene that could be obtained under the reaction conditions employed is ca. 7.5%. Catalytic Behaviors. Table 1summarizesthe results obtained with all of the Catalysts examined for the methane/benzenereaction. The empty reactor, containing only fused quartz chips, produced no observable products under the reaction conditions employed. Similarly, nonacidic materials, such as Si02 gel and AlPO-5, as well as the mildly acidic SAPO-34, SAPO-37, Cu/SAPO-34, and Cu/AlPO-5 and the large pore zeolite H-mordenite also exhibited no measurable activity for toluene production. Certain zeolites, however, produced detectable amounts of toluene and hydrogen products. T w o H-ZSMd zeolites, of differing Si/A1ratios, produced relatively low yields of toluene, uiz., 1.1-1.3 pmol/min, correspondingto
