REDUCTION BEHAVIOR AND CATALYTIC PROPERTIES OF COBALT CONTAINING ZSM-5 ZEOLITES

The reduction behaviors and catalytic properties of cobalt containing ZSM-5 zeolites prepared by various methods were compared. Temperature-programmed reduction (TPR), X-ray diffraction and IR spectroscopy were the main techniques used for the study of reduction behavior. The existence of framework cobalt and small amounts of cobalt oxide in Co-ZSM-5 samples, which were prepared by incorporation of different amounts of cobalt into the zeolite synthesis gels, and cobalt silicate-like species in the Co/ZSM-5 sample, which was prepared by precipitating impregnation of cobalt oxide on ZSM-5, were therefore established. TPR studies indicated that both framework cobalt and cobalt silicate-like species required temperatures higher than 973 K to be reduced. The structure of Co-ZSM-5 was partially destroyed after reduction at temperatures higher than 1023 K, due to the removal of framework cobalt from the ZSM-5 structure. On the other hand, the reduction of cobalt silicate-like species in Co/ZSM-5 at 1023 K did not affect the zeolitic structure. As the cobalt loading in CoZSM-5 was increased, increasing amounts of extra-framework cobalt oxide were formed and well dispersed in the porous structure of zeolite. The catalytic activities of these cobalt-containing zeolites in carbon monoxide hydrogenation were found to be directly proportional to the amount of cobalt metal present. The latter was obtained from the reduction of cobalt oxide under pretreatment conditions. The acidity and shape selectivity of ZSM-5 accounted for the product selectivity.