Selective liquid-phase oxidation of alcohols catalyzed by a silver-based catalyst promoted by the presence of ceria

摘要

A number of silver catalysts supported on SiO_2, Al_2O_3, Celite, CeO_2, kaolin, MgO, and activated carbon were screened for their catalytic activity in the selective liquid-phase oxidation of benzyl alcohol using a special screening approach. For this purpose 5-6 catalyst samples were mixed and tested simultaneously. When a high catalytic conversion (>30% over 2 h) was found the number of catalyst components was reduced in the following tests. Thereby, a collaborative effect between a physical mixture of ceria nanoparticles and silver-impregnated silica (10 wt.% Ag-SiO_2) was found. The catalytic activity was highly dependent on the silver loading, the amount of ceria, and especially the calcination procedure. Transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray absorption spectroscopy (XANES, EXAFS) demonstrate that silver is mainly present as metallic particles. This is also supported by in situ XAS experiments. Oxygen species incorporated in the silver lattice appear to be important for the catalytic oxidation of the alcohol for which a preliminary mechanism is presented. The application of the catalyst was extended to the oxidation of a wide range of primary and secondary alcohols. Compared to palladium and gold catalysts, the new silver catalyst performed similarly or even superior in the presence of CeO_2. In addition, the presence of ceria increased the catalytic activity of all investigated catalysts.