Epoxidation of olefins on M-SiO_2 (M=Ti, Fe, V) catalysts with highly isolated transition metal ions preparaed by ion beam implantation

摘要

M-SiO_2 (M=Ti, V, Fe) catlaysts with highly isolated transition metal ions were prepared by ion beam imoplantation and the catalysts were tested for the epoxidation of styrene. 1-octene and cyclohexene using dilute H_2O_2 (30 percent) as oxidant. Ti-SiO_2 shows the highest epoxide selectivity, while Fe-SiO_2 exhibits the highest ativity among the three catalysts. Epoxide selectivity can approach 100 percent for the expoxidation of styrene on Ti-SiO_2 catalyst at 298 K. The nature of active sites in the catalysts was significantly changed after the calcination. The activities and selectivities of the catalysts are mainly due to the isolated surface transition metal ions derived from the ion beam implantation. It is assumed that there are at least two types of active sites, M (alpha) (isolated M~(n+) and M(beta) (aggregated metal oxide nanoparticles) on the catalyst surface, and the good activity and selectivity attributed mainly to the M (alpha) sites.