High Catalytic Performance of MoO3-Bi2SiO5/SiO2 for the Gas-Phase Epoxidation of Propylene by Molecular Oxygen

MoO3-Bi2SiO5/SiO2 catalysts with a Mo/Bi molar ratio of 5, prepared by a two-step hydrothermal and simple impregnation method, were investigated for the epoxidation of propylene by O-2 and characterized by XRD, N-2 absorption-desorption isotherms, thermogravimetric analysis (TGA), temperature-programmed reduction, NH3 temperature-programmed desorption (TPD), and IR, Raman, and X-ray photoelectron spectroscopy (XPS). On MoO3-Bi2SiO5/SiO2 with Mo/Bi=5 calcined at 723K, a propylene conversion of 21.99% and a propylene oxide selectivity of 55.14% were obtained at 0.15MPa, 673K, and flow rates of C3H6/O-2/N-2=1/4/20cm(3)min(-1). XRD, IR spectroscopy, and XPS results show that Bi2SiO5 and MoO3 are crystalline nanoparticles. NH3-TPD results indicate that the surface acid sites are necessary for the high catalytic activity. The results of TGA and N-2 absorption-desorption isotherms reveal that a reasonable calcination temperature is 723K. The reaction mechanism of propylene epoxidation on MoO3-Bi2SiO5/SiO2 catalysis is hypothesized to involve an allylic radical generated at the molybdenum oxide species and the activation of O-2 at the bismuth oxide cations.