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Sherwood, T. K., Pigford, R. L., Wilke, C. R., "Mass Transfer", pp 554-561, McGraw-Hill, New York, N.Y., 1975. Shunk, F. A., "Constltution of Binary Alloys", 2nd Supplement, pp 515-517, McGraw-Hill, New York, N.Y., 1969. Thomas, J. M., Thomas, W. J., "Introduction to the Principles of Heterogeneous Catalysts", pp 29-32, Academic Press, New York, N.Y., 1967. Uken, A. H., Bartholomew, C. H., manuscript in preparation, 1979. Vannice, M. A,, J. Catal., 37, 449 (1975). Vannice, M. A,, J . Catal., 50, 228 (1977). Weatherbee, G. D., Ami, G. A., Barthdomew, C. H., "Nickel BimetaHic Catalysts for Methanatbn of CO: Effects of Presswe, H,O, H$ and Carbon Deposition on Activity", paper presented at the 85th National AIChE Meeting, Philadelphia, Pa., June 4-8, 1978. Wise, H., Gikis, B. J., Isakson, W. E., McCarty, J. G., Sancier, K. M., Schechter, S., Wentrcek, P. R., Wood, B. J., "Sulfur Poisonlng of Catalysts: A Study of Activity Decay in Methanol Synthesis and Fischer-Tropsch Catalysis", Final Report PERC-0060-8 (ERDA), Sept 30, 1977. Zutshi, P. K., Mahadevan, T. N., Tabnta, 17, 1014 (1970)
Bartholomew, C. H., Pannell, R. B., "Kinetics of CO Hydrogenation over Nickel and Nickel Alloy Catalysts", presented at the Centennial Meeting of the American Chemical Society, San Francisco, Calif., Aug 29-Sept 2, 1976. Bartholomew, C. H., Weatherbee, G. D., Jarvi, G. A., J . Catal., 1979, in press. Benard, J., Catal. Rev., 3 , 93 (1969). Dalla Betta, R. A., Piken, A. G., Shelef, M., J . Catal., 40, 173 (1975). Gustafsson, L., Talanta, 4, 227 (1960a). Gustafsson, L., Talanta, 4, 236 (1960b). "CRC Handbook of Chemistry & Physics", CRC Press, Inc., West Palm Beach, Fla.. 1978. Herinion, E. F. G., Woodward. L. A., Trans. Faraday SOC..35, 958 (1939). Kabe, T., Yamadaya, S., Oba, M., Miki, Y., Int. Chem. Eng., 12, 366 (1972). Massoth, F. E . , J . Catal., 30, 204 (1973). Massoth, F. E., J. Catal., 36, 164 (1975). Oliphant, J. L., Fowler, R. W., Pannell, R. B., Bartholomew, C. H., J. Catal., 51, 229 (1978). Oudar, J., "Sulfur Adsorption and Poisoning of Metallic Catalysts", paper presented at the Conference on Catalyst Deactivation and Poisoning, Berkeley, Calif., May 24-26, 1978. Pew, J. H., "Chemical Engineer's Handbook", 4th ed, McGraw-Hill, New York, N.Y., 1963, pp 3-138 and 3-139. Schult, G. C. A., Gates, B. C., AIChEJ., 10, 417 (1973).
Received for review March 19, 1979 Accepted September 4, 1979
Reaction Mechanism of Ethylbenzene Isomerization Karl-H. Robschlager and Erhard G. Chrlstoffel' Department of Chemistry, Ruhr-Univers#at Bochum, 4630 Bochum 7, West Germany
Isomerization of ethylbenzene in the gaseous phase over bifunctional R/AI.$& and R-zedie catalysts was investigated in a microcatalyticfixed bed reactor using the pulse method. Over the R/AI,03 catalyst, where the carrier displays only weak acidity, xylenes are formed from ethylbenzene mainly via the route ethylbenzene-ethylcyclohexene1,2-methylethylcycbpntene- 1,2dimethyIcyck1hexene-o-xylene, which corresponds to predictions from a bifunctional mechanism in which skeletal rearrangements of tertiary carbocations only are considered to be the ratedetermining steps. Over Pt-zeolite catalysts with higher acidity, skeletal isomerizationsin which secondary carbocations are involved contribute-in addition to the route mentioned above-to the measured product distributions.
Introduction The main source of industrial production of xylenes is the C8-aromatic cut of the product stream of catalytic reforming plants with a typical distribution of about 17 wt % ethylbenzene (EBz), 18 wt % p-xylene, 43 wt % m-xylene, and 22 wt % o-xylene. From this feed the particularly important p- and o-xylene are obtained by a process consisting of an isomerization and a thermic separation plant. Isomerization of xylenes takes place either in the gaseous phase over acidic catalysts such as, for instance, amorphous and crystalline aluminosilicates, or in the liquid phase over HF/BF3 (Allen and Yats, 1959; Brown and Jungk, 1955) or H-mordenite (Norman et al., 1976). For this acid catalyzed reaction three different mechanisms are discussed (Poutsma, 1976): an intramolecular 1,2 methyl shift in benzenium ions (Olah et al., 1964; Cortes and Corma, 1978), an intermolecular transalkylation (Allen and Yats, 1960; Lanewala and Bolton, 1969), and a dealkylation/ alkylation mechanism. The contribution of each mechanism depends on the reaction conditions. Ethylbenzene does not isomerize over acidic catalysts (Lien, 1954). As was shown by Pitts et al. (1955), isomerization of ethylbenzene proceeds via hydrogenated intermediates over bifunctional catalysts. Recently, Gnep and Guisnet (1977) reported that the main product of ethylbenzene isomerization over weakly acidic Pt/Alz03-F catalysts is o-xylene, while m- and p-xylene are formed in smaller amounts. Based on this product distribution and on the additional experimental result that over the acidic catalyst carrier 0019-7890/79/1218-0347$01.00/0
ethylcyclohexane isomerization yielding dimethylcyclohexanes does not proceed, they postulate a reaction mechanism involving nonclassical carbonium ions via ethylcyclohexene-1-ethyl-2-methylcyclopentene-1,Zdimethylcyclohexene to o-xylene, which further isomerizes to mand p-xylene. The present investigation was carried out to check the mechanism of ethylbenzene isomerization more thoroughly by examining the product distributions of the conversions of the intermediates postulated by Gnep and Guisnet (1977) and of other C8 naphthenes, which may also be intermediates. The reactions were carried out over several Pt/carrier catalysts and acidic carriers with differing acidity. Experimental Methods (a) Materials. The aromatics and cycloalkanes were purchased from Fluka AG and Merck; cycloalkenes were produced by Grignard reaction followed by dehydration. The purity of all feed products was >99 %. The following catalysts were used: a commercial 0.5 wt ?& Pt-zeolite isomerization catalyst (A), a 0.5 wt % Pt-zeolite, a 0.5 wt % Pt/Al2O3, and the acidic carriers of these catalysts (provided by Kali-Chemie AG). (b) Apparatus and Operating Conditions. The experiments were carried out in a pulse microreactor described earlier (Christoffel and Robschliiger, 1978). Products were analyzed by gas chromatography with a 100-m squalane capillary column. In order to reduce the hydrogenolysis activity the Pt/A1203catalyst was pretreated a t 530 " C under H2flow and is in the following termed 0 1979 Amerlcan Chemical Society
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Table I. Catalytic Activity and Selectivity of t h e Catalysts Used for Conversions of Methylcyclopentane, Methylcyclopentene, Cyclohexane, and Cyclohexenea catalyst : feed:
MCP
Pt-zeolite MCP' CH
A MCP
CH
MCP
product
3.44 10.91 0.18 G-C, DMB 0.40 0.58 2MP 1.67 2.96 3MP 1.13 2.16 1.23 2.20 0.01 nc, MCP 44.78 34.00 8.11 BZ 45.98 46.45 89.41 CH + CH= 0.90 0.76 2.29 MCP' 0.38 0.30
Pt/Al,O, MCP'
CH
zeolite MCP'
MCP
A1203 MCP MCP'
CHS
mol %
65.46 0.47 0.03 0.08 1.10 13.05 0.02 0.01 8.19 0.01 7.30 0.12 0.08 0.99 98.40 97.30 3.89 99.42 0.30 1.30 99.72 0.11 0.28 0.02 1.03 0.80
32.60 23.18 0.02 0.82 0.06 0.11 0.09 0.07 0.09 0.38 0.08 0.02 99.80 55.14 11.76 99.64 6.50 1.06 39.72 0.08 0.40 1.41 17.19 3.21 0.02 9.70 7.51 0.18 89.10 0.11
a Reaction conditions: temp, 400 "C; pressure, 1.8 bar; catalyst, 120 mg; carrier gas (H,) velocity, 150 mL/min; pulse width, 0.1 pL. Abbreviations: M = methyl; E = ethyl; P = propane; B = butane; P = pentane; P' = pentene; H = hexane; H= = hexene; Bz = benzene; D = d i ; T = tri; C = cyc1o;I = iso; C l . . . 8 = hydrocarbons with 1. . .8carbons.
"partially aged catalyst". Zeolite catalysts were heated no further than to the reaction temperature chosen because they lost their isomerization activity at 530 "C as well. The reproducibility of product concentrations obtained over various charges of the individual catalysts was below 5% of the actual conversion. Results In the following the individual catalysts are characterized first on the basis of the product distributions of the reactions of methylcyclopentane (MCP). Then the results of measurements of the temperature dependence of the isomerization of EBz and product distributions of the reactions of EBz, ethylcyclohexene (ECH=), ethylcyclohexane (ECH), 1,2-methylethylcyclopentene(1,2MECP=), 1,3-methylethylcyclopentene(1,3MECP'), 1,l-dimethylcyclohexane (l,lDMCH), propylcyclopentane (PCP), 1,2dimethylcyclohexane (1,2DMCH), and o-xylene are presented. A. Activity and Selectivity of the Catalysts Used. Over nonacidic Pt/carrier catalysts ( p-xylene > o-xylene. As in the xylene distribution from 1.2MECP= conversion o-xylene is the main dehydroisomerization product of ECH' conversion. The experimental results discussed so far do not agree with predictions from a classical carbenium ion mechanism. From 1.3MECP= two tertiary carbenium ions are formed by proton addition. These tertiary carbenium ions undergo skeletal rearrangements yielding m- and p-xylene via secondary carbenium ions and EBz via a primary carbenium ion (Scheme 1). In the case of 1.3MECP' dehydroisomerization the classic carbenium ion mechanism predicts m- and p-xylene as main products and small amounts of EBz, which is in agreement with our experimental results. I t does not account, however, for the measured xylene concentrations from 1.2MECP' dehydroisomerization where m- and o-xylene are predicted as principal products and an EBz concentration which is in the same order of magnitude as from conversion of 1.
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Scheme 111
%-&d
6 0
+
Scheme IV
Scheme V
Scheme VI
-
/
I
U 3MECP= (Scheme 11). The results presented in Table IX reveal that o-xylene is the main product of 1.2MECP' dehydroisomerization and EBz is formed in much higher concentrations than from 1.3MECP= conversion. These experimental results can be explained, however, by assuming protonated cyclopropanes as intermediates of skeletal isomerization (Olah, 1975). Different amounts of EBz are predicted for dehydroisomerization of the methylethylcyclopentenes, because in the case of 1.3MECP' a secondary carbenium ion is transposed into a secondary one (Scheme 111),whereas in the case of 1.2MECP= a tertiary carbenium ion is transposed into a tertiary one (Scheme IV). Because of this the isomerization of 1.2MECP' yielding EBz is a faster reaction than the isomerization of 1.3MECP' into EBz, since more stable intermediates are involved. In all cases isomerization of cycloalkanes proceeds without formation of energetically unfavored primary carbenium ions.
Similar arguments apply to the formation of the xylenes from the MECP=. In this case the conversion of 1. 2MECP= yielding o-xylene is the only one where only tertiary carbenium ions are transposed into tertiary ones (Scheme V), while all other skeletal rearrangements of methylethylcyclopentenyl structures into dimethylcyclohexenyl structures and vice versa involve at least one secondary carbenium ion. The formation of o-xylene as the principal product of EBz isomerization is due to the fact that the reaction sequence EBz-ECH=-1.2MECP'-o-xylene is the only one in the total C8 five-ring and six-ring system which only involves skeletal rearrangements of tertiary carbenium ions (Scheme VI). m- and p-xylene are either formed by isomerization of o-xylene involving benzenium ions or by isomerization of naphthenes involving secondary carbenium ions. In the case of ethylbenzene isomerization over Pt/ A1,0,, these consecutive and parallel reactions are slow in comparison to the skeletal rearrangements via tertiary carbenium ions only. In the case of ethylbenzene isomerization over Pt-zeolite catalysts with a high isomerization activity of the acidic centers consecutive and parallel isomerizations contribute to a larger extent to the obtained product distributions. Acknowledgment The authors would like to thank the Deutsche Forschungsgemeinschaft for a grant t~ K.-H.R. and for financial support. Literature Cited Allen, R.; Yats, L. D. J. Am. Chem. SOC. 1959, 81, 5209. Allen, R. H.; Yats, L. D. J. Am. Chem. SOC. IWO, 8 2 , 4853. Brandenberaer. - S. G.: Callender. W. C.: Meerbott, W. K. J. Catal. 1978. 42. 282. Brown, H.; Jungk, H. J. Am. Chem. SOC.1955, 77, 5579. Christoffel, E. Chem. Ztg. 1978, 702, 391. Christoffel, E.; Robschliiger, K. H. Ind. Eng. Chem. Prod. Res. Dev. 1978, 17. 331. Cortes, A.;Corma, A. J. J. Catal. 1978, 57, 338. Gnep, N. S.;Guisnet, M. Bull. SOC. Chim. Fr. 1977, 5 - 6 , 429. Lanewala, M. A.; Bolton, A. P. J. Org. Chem. 1969, 3 4 , 3107. Lien, A. P. 125th National Meting of the American Chemical Soclety, Kansas City, Mo., 1954. Maire, G.; Plouidy, G.; Prudhomme. J. C.; Gault, F. G. J. Catal. 1965, 4 , 556. Norman, G. H.; Shigemura, D. S.;Hopper, J. R. Ind. Eng. Cbem. Prod. Res. D e v . 1976, 15, 41. Olah, G. A.; Meyer, M. W.; Overshiek, N. A. J. Org. Chem. 1964, 29, 2313. Olah, G. A. "Carbokationen und elektrophile Reaktionen", Veriag Chemie, Weinheim, 1975, p 128. Pines, H.; Shaw, A. W. J. Am. Chem. SOC. 1957, 79, 1474. Pitts, P. M.; Connor, J. E.; Leun, L. N. Ind. Eng. Chem. 1955, 4 7 , 770. Poutsma, M. L. ACSMoncgr. 1976, No. 171, 496. Smith, R. L.; Naro, P. A.; Slkestri, J. A. J . Catal. 1971, 20,359.
Received for review March 1, 1979 Accepted August 10, 1979
