Toward better understanding of the catalytic action of acidic zeolites: investigation in methane and ethane activation and transformation

Studies of cracking reactions of alkanes with three or more carbon atoms have been central to the development of our understanding of the catalytic action of acidic zeolites, which are important catalysts in petrochemical and chemical industries. However, the mechanisms of the simplest cracking reactions, that is, the cracking of the C–H and C–C bonds in methane and ethane, have only been studied theoretically, not experimentally. Here we show that ethane is converted over a H-MFI zeolite at 510–550 °C with formation of such primary products as ethene, hydrogen, methane, and propane. To explain these results, we suggest and consider two catalytic cycles of the reaction. The first cycle involves protolytic cracking of the C–H bond with formation of hydrogen and ethoxide group, the latter decomposing into ethene and the zeolite acid site. We propose that the second cycle is initiated by the protolytic cracking of the C–C bond that results in formation of methane and a methoxide group as an intermediate. We theorize that this reacts with ethane molecules regenerating the zeolite acid site and producing (i) methane and ethene via hydrogen transfer and (ii) propane via a C–C bond formation reaction. Both suggested catalytic cycles are fully supported by the kinetic results of this study and are in a good agreement with recent theoretical work. We also demonstrate that the cracking of the C–H bond in methane (that could proceed via methoxide intermediate) does not occur over H-MFI zeolites up to 700 °C most likely because of a high activation energy. The proposed involvement of methoxide groups, as active intermediates, in ethane transformation provides a basis and excellent opportunity for theoretical studies of such interesting reactions as hydrogen transfer and C–C bond formation with participation of these surface species.