Partial oxidation of methane to formaldehyde over W/SiO2 catalysts

Two samples of silica with different surface area have been used in the preparation of silica-supported tungsten oxide catalysts with W loading ranging from 0.8 to 15.8 wt%. Both catalyst series have been tested in the partial oxidation of methane to formaldehyde with molecular oxygen at atmospheric pressure. It has been shown that both surface area and tungsten loading affect the nature and dispersion of the tungsten oxide species and, therefore, the catalytic behavior of the samples. Poor dispersion and high reducibility of the tungsten oxide phase is observed for the low surface area catalysts, thus leading to a high activity per unit area in methane conversion. High surface catalysts show a well-dispersed and hardly reducible tungsten oxide overlayer that covers the active sites of the silica surface. Consequently, a negative effect on the catalytic activity is observed. Formaldehyde production has been related to the density and activity of three types of active sites present on the W/SiO2 catalysts: active sites of silica surface, terminal W=O sites and W-O-W functionalities. These results agree with those previously reported by the authors for W/HZSM-5 catalysts. (C) 1999 Elsevier Science B.V. All rights reserved.