Propylene oxide synthesis and selective oxidation over supported metal oxide photocatalysts with molecular oxygen

Recent progress of photocatalytic epoxidation of propylene with molecular oxygen is reviewed. Highly dispersed metal oxide photocatalysts, such as silica-supported V2O5, Nb2O5, and TiO2 with low loading, promote steady-state propylene oxide formation with high selectivity (> 40%). The isolated tetrahedrally-coordinated species in the ligand-to-metal charge-transferred state is the photocatalytic active site. The photo-induced hole center localized on the lattice oxygen plays a significant role for the photocatalytic epoxidation. In the case of isolated TiO4 tetrahedra on silica, an electrophilic oxygen species is generated by the reaction between the hole center and molecular oxygen. The addition of alkali ions to surface VO4 tetrahedra improves the photocatalytic epoxidation ability of V2O5/SiO2 with low loading. The key step in the selective photocatalytic oxidation over isolated VO4 tetrahedra is the electron transfer from a hydrocarbon to the photo-induced hole center on the lattice oxygen, which is inserted in the photogenerated products.