Effect of the local structures of V-oxides in MCM-41 on the photocatalytic properties for the partial oxidation of methane to methanol

Vanadium-containing MCM-41 mesoporous molecular sieves with tetrahedrally coordinated VO4 species have been prepared by two direct synthesis methods under acidic and basic conditions as well as impregnation method. V-MCM-41 prepared in acidic solution and imp-V/MCM-41 exhibit high activity and methanol selectivity for the photocatalytic partial oxidation of methane with NO under UV irradiation at 295 K. However, V-MCM-41 prepared in basic solution shows only a low activity for the coupling reaction of methane. Photoluminescence, UV–vis, photoadsorption and ESR measurements reveal that V-MCM-41 prepared in acidic solution contains VO4 species exposed on a silica surface, while the catalyst prepared in basic solution contains VO4 species confined within a silica layer. The exposed tetrahedral VO4 species act as active sites for the formation of methanol. Furthermore, it is found that the V4+ species and methyl radicals formed by UV irradiation of the exposed VO4 species with methane interact easily with NO, resulting in the selective formation of methanol.