Silica-Supported Silver Nanoparticles with Surface Oxygen Species as a Reusable Catalyst for Alkylation of Arenes

摘要

A series of silica-supported silver catalysts with various Ag loading (3-30 wt%) and different treatment (oxidation and subsequent reduction by H2) were prepared, and their local structures were characterized by extended X-ray absorption fine structure (EXAFS) analysis. In oxidized catalysts, silver were mainly present as small nanoparticles with surface oxygen atoms, and the number of surface oxygen atoms decreased with increased silver loading. The H2 reduction of these samples resulted in a removal of the surface oxygen atoms. The structure-activity relationship was studied for the alkyla- tion of anisole with benzyl alcohol and styrene. Ag~I oxide, Ag~I ion, bulk silver metal, and silica-supported silver nanoparticles were non-active species. In contrast, silica-supported silver nanoparticles with the surface oxygen atoms acted as an effective heterogeneous catalyst for the alkylation of arenes with alcohols and styrenes. Kinetic studies in alkylation of anisole with alcohols showed that the cleavage of the α-C-H bond of alcohol was the rate-limiting step. It is proposed that the surface oxygen adjacent to the silver surface atoms plays an important role in the dissociation of the α-C-H bond.