Oxidative coupling of methane: Influence of the phase composition of silica-based catalysts Original Research Article

A simple solid-phase synthesis technique was applied to prepare a series of silica-based composite materials. Each material contains tungsten, manganese and an alkali oxide. To determine the influence of the alkali nature on the catalytic properties of SiO2-based composites, a W–Mn/SiO2 material without any alkali was prepared. In addition, material containing tungsten, manganese and lithium without SiO2 was synthesized. The composites were studied by XRD, TPR, BET, XPS, SDB/SEM and EDS methods. A thorough analysis of the phase parameters, which were crystallized in alkali-doped W–Mn/SiO2 catalysts and determined by XRD, was performed for the first time. Most synthesized materials demonstrated higher catalytic activity in the oxidative coupling of methane (OCM) in comparison with the analogous materials prepared according the convenient wet impregnation method. The OCM reaction was carried out without any dilution of the methane–oxygen mixture. The results of catalytic tests were interpreted in relation to the catalyst silica-matrix phase composition. It was found that two polymorphic phases of SiO2 coexist in each composite material. These phases are cristobalite and quartz in Li- and Na-containing material or cristobalite and tridymite in K-, Rb- and Cs-containing materials. The hypothetical T–x phase diagrams of Me2O–SiO2 (where Me is an alkali metal) systems were constructed to explain the coexistence of two phases of SiO2 in the studied catalysts.