Vapor-phase epoxidation of propylene using H_2/O_2 mixture over gold catalysts supported on non-porous and mesoporous titania-silica:effect of preparation conditions and pretreatments prior to reaction

摘要

Ti-doped nonporous silica was used as a support to prepare nanometer gold catalysts by the deposition-precipitation method for the direct vapor-phase epoxidation of propylene with O_2 and H_2.Both the catalysts dried under vacuum at room temperature and the catalysts calcined at 573 K in air exhibited catalytic activity to form PO even at a temperature as low as 323 K.The calcined catalysts were more stable and more active at higher temperatures than the dried catalysts.Simple filtration after aging the suspension resulted in the catalysts more active and selective but less stable than the complete washing of the solid precursors.Pretreatment in argon appreciably increased the catalytic activity and H_2 efficiency over the Au catalysts deposited on Ti-doped nonporous silica,which was opposite to the case of Au/Ti-MCM catalysts.Similar deactivation rate of Au catalysts supported on a variety of titania-silica supports with different porosities suggests that quick deactivation of Au catalysts within hours is mainly affected by the surface properties rather than the pore structure and diffusion limitation of the supports.