Effect of alkali-metal promoter on silica-supported copper catalysts in benzyl alcohol oxidation

Abstract

The effect of alkali metals, such as Na, added to silica-supported copper (Cu/SiO₂) catalysts has been studied in the gas-phase catalytic oxidation of benzyl alcohol. The main products were benzaldehyde as a partial oxidation product and carbon dioxide as a complete oxidation product. The silica-supported copper catalysts were prepared using both ion-exchange and impregnation methods. The catalyst prepared by the impregnation method (Cu_imp/SiO₂) had a higher oxidation activity than (Cu_ex/SiO₂) prepared by the ion-exchange method. The effects of reaction temperature and the amount of supported Cu on the oxidation activity were investigated using the catalysts both with and without added Na. The addition of alkali metal to the silica-supported copper catalysts prepared by both methods was found to promote the oxidation activity, particularly the partial oxidation activity. The coordination and redox properties of the Cu/SiO₂ catalysts, with and without alkali metal, were investigated using diffuse reflectance (DR) spectroscopy and temperature-programmed reduction (TPR) measurements. It is suggested, based on the DR spectra, that the alkali-metal added to the Cu/SiO₂ catalysts decreases the electronegativity of the supported Cu species; this decrease makes dissociative oxygen adsorption easier. The other role of the added alkali metal was thought to be to remove the oxidic Cu species which do not have catalytic activity for the partial oxidation.