Oxidative coupling of methane: Mechanism and kinetics

详细

The mechanism of the oxidative coupling of methane (OCM), a reaction discovered in the early 1980s, and the development of approaches to kinetic description of this reaction are analyzed. In addition to being of potential practical significance as a single-step synthetic route from methane to ethylene, a valuable chemical and petrochemical semiproduct, OCM was the first heterogeneous–homogeneous process of this hitherto unknown type. Its distinctive feature is that it is impossible to separately determine the methane conversion via the main pathway (into C₂ hydrocarbons) in the homogeneous phase and on the catalyst surface, because reactive intermediates (methyl radicals) result from the interaction of methane molecules with active sites of the surface, while the primary OCM product (ethane) results from the recombination of these radicals in the gas phase. The correlation between the phenomenological regularities of the OCM process and its mechanism, the correlation between the nature of the active sites and the mechanism of activation of methane and oxygen, and the possibility of constructing a self-consistent kinetic model taking into account the specific features of the process—redox nature of the active sites and the free-radical character of the major intermediate compounds—are analyzed in detail.