Conversion of Methane over Ag+-exchanged Zeolite in the Presence of Ethene

Abstract

Methane is shown to react with ethene over silver-exchanged zeolites at around 673 K to form higher hydrocarbons. Methane conversion of 13.2% is achieved at 673 K over Ag–ZSM−5 catalyst. Under these conditions, H–ZSM−5 does not catalyze the methane conversion, only ethene being converted into higher hydrocarbons. Zeolites with extra-framework metal cations such as In and Ga also activate methane in the presence of ethene. Using 13C-labeled methane as a reactant, propene is shown to be a primary product from methane and ethane. 13C atoms were not found in benzene molecules produced, indicating that benzene is entirely originated from ethane. On the other hand, in toluene, 13C atoms are found in both the methyl group and the aromatic ring. This implies that toluene is formed by the reaction of propene with butenes formed by the dimerization of ethene, and also by the reaction of benzene with methane. The latter path was confirmed by direct reaction of 13CH4 with benzene. In this case, 13C atoms are found only in methyl groups of toluene produced. The heterolytic dissociation of methane over Ag+-exchanged zeolites is proposed as a reaction mechanism for the catalytic conversion of methane, leading to the formation of silver hydride and CH3δ+ species, which reacts with ethene and benzene to form propene and toluene, respectively. The conversion of methane over zeolites loaded with metal cations other than Ag+ is also described. The reaction of methane with benzene over indium-loaded ZSM−5 afforded toluene and xylenes in yields of up to 7.6% and 0.9% at 623 K when the reaction was carried out in a flow reactor.