Catalysis Letters Vol. 72, No. 3-4, 2001 221 The methylation of benzene with methane over zeolite catalysts: effect of hydrocarbon impurities Moses O. Adebajo ∗ , Russell F. Howe and Mervyn A. Long School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia E-mail: funmi_adebajo@hotmail.com Received 24 August 2000; accepted 2 January 2001 Trace amounts of ethylene impurities in the methane used for benzene methylation over ZSM-5 and H-beta catalysts are found to enhance benzene conversions and selectivity to ethylbenzene. However, methane containing ethane impurities yielded negligible benzene conversion in the absence of oxygen over CoZSM-5 while a much higher conversion is obtained over H-beta due to cracking of benzene as previously reported. KEY WORDS: methane activation; benzene methylation; ZSM-5; H-beta; hydrocarbon impurities 1. Introduction Considerable interest is presently shown in the conver- sion of methane, the major component of natural gas, to transportable liquid fuels and feedstocks in addition to its previous main use for heating purposes by combustion. The methylation of aromatic compounds with methane is one possible new route for methane conversion that has been de- scribed recently in the literature. For example, the methy- lation of naphthalene, toluene and phenol with methane over various metal substituted aluminophosphate molecu- lar sieves in a high pressure batch reactor has been reported [1,2]. The conversion of benzene in the presence of methane over zeolite catalysts in both high pressure flow reactor [3] and batch reactor [4] has also been described. However, the high pressure flow reactor study [3] showed, through the use of 13 C labelled methane, that the methyl groups in the methylated aromatic products did not originate from the methane reactant. In our more recent investigation of the methylation of benzene with methane at 400 ◦ C over ZSM-5 catalysts in a high pressure batch reactor [5], we demonstrated clearly that the presence of oxygen was required for the reaction to proceed to yield the methylated products. Thus, a two- step mechanism involving the intermediate formation of methanol by partial oxidation of methane followed by the methylation of benzene with methanol, was postulated. Fur- ther support to this mechanism has also been reported in our later investigation [6] which showed excellent correla- tion between the activity of the catalysts used for both the benzene methylation with methanol and benzene methyla- tion with methane in the presence of oxygen, in a high pres- sure batch reactor. In this paper, we report the results of our preliminary in- vestigation of the effect of hydrocarbon impurities (namely, ∗ To whom correspondence should be addressed. ethane and ethylene) on the methylation of benzene with methane over ZSM-5 catalysts in a batch reactor. 2. Experimental Commercial samples of HZSM-5 (PQ Corporation, SiO 2 / Al 2 O 3 = 35) and H-beta (PQ Corporation, SiO 2 /Al 2 O 3 = 35) were used in this study. A Co-exchanged ZSM-5 cata- lyst which was prepared by conventional ion exchange with aqueous cobalt nitrate was also used. All the reactions were carried out at 400 ◦ C in a 71 ml Parr reactor fitted with a glass liner. In each experiment, 55 mg of catalyst and 465 μl of benzene were placed in the reactor which was then charged with 6.9 MPa chemically pure (>99.0% pure) or ul- trahigh purity (>99.97% pure) methane. The details of the experimental procedures are similar to those published re- cently [5]. In order to study the effect of ethane and ethylene impu- rities on the benzene methylation reaction, the reactor (al- ready loaded with the catalyst and benzene) was chilled in a chloroform slush bath and flushed several times, first with nitrogen gas to remove residual air and then with the ethane (or ethylene), prior to charging with high pressure ultrahigh purity methane. The amount of the ethane (or ethylene) in- troduced into the reactor was about 2.1% of the composition of the gases in the reactor excluding benzene. The details of the GC and GC/MS analysis of reaction products and measurement of zeolite catalyst acidities us- ing FTIR of adsorbed pyridine are similar to those described elsewhere [6]. 3. Results and discussion Table 1 summarises the results of the benzene methyla- tion experiments over both HZSM-5 and CoZSM-5 in the 1011-372X/01/0400-0221$19.50/0  2001 Plenum Publishing Corporation