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. (c) 2013 Elsevier B.V. All rights reserved.