The molybdenum species of MoO3/SiO2 and their catalytic activities for the epoxidation of propylene with cumene hydroperoxide

The MoO3/SiO2 catalysts containing different surface molybdenum species were prepared by a sol–gel method, and the effects of the preparation condition and MoO3 loading on the surface molybdenum species and property of MoO3/SiO2 were studied. The XRD, FT-IR, UV–vis and Raman spectroscopies were used to characterize the surface molybdenum species, and temperature-programmed desorption of NH3 adsorbed on a catalyst was used to detect the surface acidic properties. The results show that, there were the dispersed polymolybdate, α-MoO3, β-MoO3, monomeric molybdenum species and silicomolybdic acid on the MoO3/SiO2 catalyst, and their distributions and subsistence states were affected by the preparation condition and MoO3 loading. Different molybdenum species exhibit different catalytic activities for the epoxidation of propylene with cumene hydroperoxide. In the 15 wt% MoO3/SiO2 catalyst synthesized at pH 9.1 and dried appropriately, there are the small size β-MoO3 and monomeric molybdenum species that they are mainly effective catalyst components for the epoxidation of propylene. Using this catalyst, the ∼100% conversion of cumene hydroperoxide and ∼100% selectivity to propylene oxide can be obtained in the tert-butyl alcohol solvent at 2.6 MPa and 80 °C for 4 h.