(A) study on the hydrogenation of carbon monoxide with Cobalt/Y-Zeolite and Nickel/Y-Zeolite catalysts

Abstract

Co/Y-zeolite catalysts were prepared by three different preparation methods, i.e., ion-exchange(IE), carbonyl complex-impregnation(CI) and excess-water(EW), and their catalytic behaviors for CO hydrogenation were investigated. The distribution of cobalt metal and the catalytic properties of the catalysts were greatly influenced by the preparation methods. In the EW catalysts large cobalt metals were mainly located at the exterior surface of the zeolite crystals, while in the IE and CI catalysts small cobalt metals were restricted into the pores of the zeolite. Owing to the structure-sensitive nature of CO hydrogenation over cobalt catalysts, the activity of the EW catalysts was significantly higher than that of the IE and CI catalysts. In the IE catalysts the activity increased and the olefin production decreased with increasing cobalt loading and reduction temperature, which was ascribed to an electronic modification of cobalt metals through the interaction with Bronsted acid sites. That is, partial migration of free electrons from metal atoms to the proton of strongly acidic OH groups was believed to result in electron-deficiency in metal. In addition branched hydrocarbons were produced in significant amounts on the Bronsted acid sites which were formed through the reduction of $CO^{2+} + H_2 -->Co^0 + 2H^+$. In the CI catalysts, where cobalt metal was introduced through the decomposition of cobalt carbonyl, the electronic modification with cobalt loading and reduction temperature was hardly expected. Accordingly the activity and olefin production varied only slightly with cobalt loading and reduction temperature, and branched hydrocarbons were produced in negligible amounts. A further study showed that carbon-containing reaction intermediates propagate easily on the cobalt metal surface in the CI catalysts, and the CI catalysts were most favorable for the production of higher hydrocarbons, mainly $C_3$ and $C_4$. Catalyst deactivation by carbon deposits was ...