Studies on activation factors for oxidative coupling of methane over lithium-based silicate/germanate catalysts

Abstract

Several oxides containing group 14 elements were characterized to investigate catalytic activity factors and the reaction mechanism for oxidative coupling of methane (OCM). Lithium-based silicate Li₂SiO₃ and germanate Li₂GeO₃ showed much higher selectivities for C₂ hydrocarbons than their Sn-based counterpart Li₂SnO₃. Our XPS study suggested that strong metal–oxygen bonds in silicates and germanates could be of importance to effectively suppress the deep oxidation of methane at the catalyst surface, resulting in enhanced C₂ selectivities. In contrast, weakly bound surface oxygen of Sn-based oxides is subject to direct oxygenation of methane to form CO_x as by-products, emphasizing the importance of metal–oxygen bonding strength. To achieve high OCM catalytic activity, effective methane activation by strong basicity is also required, and the local environment around oxygen sites could be a key factor to enhance CH₄ conversion. Based on the chemical/structural viewpoints, we found that silicate and germanate with higher Li contents, Li₄SiO₄ and Li₄GeO₄, are significantly OCM active, comparable to the previously reported catalyst Li₂CaSiO₄.