Controlling Product Selectivity in Oxidative Coupling of Methane by Identifying and Regulating Oxygen Species

ConspectusThe direct oxidation of methane, which is the main component of natural gas, shale gas, methane clathrates, and biogas, to value-added products is an economically attractive and environmentally friendly alternative to strongly endothermic methane steam reforming to synthesis gas (CO/H₂). Among the different routes, the oxidative coupling of methane (OCM) to ethylene/ethane (C₂-hydrocarbons) is the most promising one. A key limiting factor is insufficiently high selectivity to C₂-hydrocarbons due to their overoxidation to carbon oxides (CO_x) at industrially relevant degrees of methane conversion. Although it is generally agreed that both selective and unselective reactions are initiated by oxygen species on the surface of catalysts, the kind, role, and origin of these species remain elusive, which hampers the tailored design of catalysts.In this Account, we summarize our recent progress in understanding how product selectivity in the OCM reaction can be tuned by controlling the type of oxygen species through catalyst composition or reaction conditions. The combination of in situ time- and temperature-resolved catalyst characterization with transient kinetic methods, i.e., temporal analysis of products (TAP) and steady-state isotopic transient kinetic analysis (SSITKA), has been proven to be effective for understanding the origin and role of oxygen species involved in selective and unselective pathways. We also present strategies for regulating the concentrations of selective and unselective oxygen species. For the Mn-M(M = Na, K, Rb, or Cs)₂WO₄ system, the electronegativity of the alkali metal was found to influence the ability of the catalysts to form selective oxygen species from gas-phase oxygen. The binding strength of atomic oxygen species is a key parameter for hindering the oxidation of methane to CO_x over Gd₂O₃-based catalysts. This property can be adjusted by using a metal oxide promoter. The nature and concentration of different oxygen species can also be controlled through the use of steam or an alternative oxidizing agent, N₂O, and by performing the OCM reaction in a chemical looping mode, i.e., by alternating between CH₄- and air-containing feeds. Using steam in the latter option enabled us to largely enhance the productivity of C₂-hydrocarbons, thus making this technology more attractive for large-scale applications. The knowledge summarized in this Account is expected to present insights for further studies in the development of selective catalysts for various alkane oxidation reactions and in the optimization of reactor operation.