Energy & Fuels 1994,8, 286-287
286
Methylation of Naphthalene by Methane-13Cover Copper-Exchanged Silicoaluminophosphate Simon J. X. He and Mervyn A. Long* School of Chemistry, University of New South Wales, Kensington, N S W 2033, Australia
Motaez I. Attalla and Michael A. Wilson* CSIRO Division of Coal and Energy Technology, P.O. Box 136, North Ryde, N S W 2113, Australia Received June 14, 1993. Revised Manuscript Received October 15, 1993 We report the formation of methylnaphthalene-13Cfrom naphthalene and methaneJ3C at 6.8 MPa and 400 OC catalyzed by Cu-exchanged SAPO-5. The result demonstrates unequivocally that methane provides the C atom for the methylated products. The ability of microporous metal-SAPO catalysts to bring about direct methylation of naphthalene by methane gas was recently reported.' Blank experiments with nitrogen replacing methane demonstrated that methane was necessary to produce substantial yields of the methylated products and hence it was concluded that the methane was the source of the added methyl substituent. However, earlier 13C studies2s3of the methylation of benzene and some other hydrocarbons with methane over nickel on silica and alumina catalysts showed that the methyl was largely derived from sources other than methane gas with those catalysts, and carbonaceousspecies on the surface were thought to supply part of the carbon for methylation. Thus, with the present metal-SAPO catalysts, it was necessary to investigate whether the methyl group incorporated during methylation of naphthalenel might have originated from a series of side reactions of naphthalene itself, involving cracking, hydrogenolysis, etc. The present result dispels these doubts and shows that methylation with methane as the methyl source is a reality over microporous Cu-exchanged SAPO5. The details of the experimental methods of the present work are similar to those previously published,' the main variation being that methane-'3C (98% isotopic abundance) was substituted for natural abundance methane and a smaller reactor (to conserve isotopic reagent) was used. In brief, catalyst (26 mg) and naphthalene (198mg) were combined inside a glass liner (58 mL) in a stainless steel autoclave and methane (98% 13C) was added to a pressure of 6.8 MPa at room temperature. The temperature of the reactor was raised to 400 "C and held for 4 h with rocking. In each case, the catalyst was Cuexchanged SAPO-5 whose preparation, properties, and structure have been described elsewhere.lq4 Analysis by GC-mass spectrometry of the products from the reaction of methane-'3C with naphthalene gave mle intensities (shown in Table 1)in the mass spectra of the (1)He, S. J. X.; Long, M. A.; Attalla, M. I.; Wilson, M. A. Energy Fuels, 1992, 6, 498.
(2) Loffler, I . D.; Maier, W. F.; Andrade, J. G.; Thies, I.; Schleyer, P. v. R. J. Chem. Soc., Chem. Commun. 1984, 1177. (3) Tanaka, K; Yaegashi, I.; Aomura,K. J . Chem. Soc., Chem. Commun.
1982,938. (4) Lok, B. M.; Messina, C. A.; Patton, R. L.; Gajek, T. R.; Cannan, T. R.; Flanigen, E. M. US.Patent 4,440,871 (1984).
0887-0624/94~2508-0286$04.5010
Table 1 relative mass sDectrd Deak intensities ~~
product 1-methylnaphthalene mJe = 141 m l e = 142 mJe = 143 m/e = 144 2-methylnaphthalene m i e = 141 mJe = 142 mJe = 143 m i e = 144 dimethylnaphthalene mJe = 155 mJe = 156 mJe = 157 mJe = 158 m l e = 159
methane-% reaction 96 100 14
~~~
methane-1V reaction
-
-
141 100 12
91 100 12 -
29 124 100 11
32 100 12
25 40
-
86 100 24
GC peaks corresponding to the two monomethylnaphthalenes and the principal dimethylnaphthalene (identification of the particular isomer unknown). The results for a parallel reaction with natural abundance methane are included. Intensities of the expected parent ions are normalized to 100 in each case. It is clear from these results that the methylnaphthalenes from reaction with the l3C reagent are substantially one mass unit heavier and the dimethylnaphthalene substantially two mass units heavier. The most reliable estimate is that the ratio of the yield of 13C to 12C methylnaphthalenes from the reaction with the methane-'3C is 73 f 10 % . I t is concluded that the origin of the additional carbon atoms in methylated products is dominantly the methane gas. 'H NMR spectra of the reaction products were also obtained. Since 13C-labeled molecules show lH-l3C J coupling, an estimate of the 13C/12Cratio in the methyl groups in the sample can be obtained from comparison of the J-coupled methyl resonance and the unsplit methyl resonance. This yields an estimate of 73% 13C in the methyl groups of the products of the 13Cexperiment. The majority of l3C atoms, other than natural abundance, thus reside in the methyl substituents of products frorn that experiment. It has been pointed out that the thermodynamics of the reaction of methane with naphthalene to yield methylnaphthalene and hydrogen is such that the equilibrium lies heavily in favor of the reactants at methane pressures around 1 atm. However, under the reaction conditions selected in this work where the methane is in large excess, 1994 American Chemical Society
Communications calculations based on standard free enthalpies and entropies estimate the formation of methylnaphthalene to be about 18%at equi1ibrium.l In the current experiments, attempts were not made to maximize naphthalene conversions. Typical conversions of naphthalene were 1-2 %, with total selectivities for the methylnaphthalenes of 8595 % , but combined yields of monomethylnaphthalenes of about 18%have been reported for previous experiments at 400 OC under different conditions.' The mechanism of formation of the methylated product may involve catalytic activation of either the methane or the aromatic substrate, or both molecules. Some brief (5) Iton, L.E.; Desjardins,C. J. A.; Maroni, V. A. Zeolites 1989,9,535. (6) Iton, L.E.; Maroni, V. A. US. Patent 5,068,485 (1991). (7) Han, S.;Palermo, R.; Pearson, J.; Walsh, D. E. US. Patent 5,023,392 (1991). (8) Ernst, S.; Weitkamp, J. In Natural Gas Conversion. In Studies in Surface Science and Catalysis; Holemen, A,, Ed.;Elsevier: Amsterdam, 1991; p 25.
Energy & Fuels, Vol. 8, No. 1, 1994 287 reports have indicated that oxidative coupling of methane may occur at temperatures as low as 400 "C over Co-substituted AlP0,536or metal' or metal oxide8loaded zeolites. While coupling of methane to C-2 or higher hydrocarbons was not observed in the current reaction system, the results reported above, together with these other studies, support the contention that methane itself is activated by catalyst systems of this type. Methylation of an aromatic substrate is then one possible reaction outcome of that activation. The conclusion that appropriate microporous catalysts activate methane raises the possibility of finding new uses of methane derived from natural gas or other sources.
Acknowledgment. We thank Dr. J. Brophy and Lisa Roach for GUMS analyses. The Australian Research Council and NERDDC are acknowledged for financial assistance.
