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 exi stence 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 sp ecies in the Co/ZSM-5 sample, which was prepared by precipitating impr egnation of cobalt oxide on ZSM-5, were therefore established. TPR stu dies indicated that both framework cobalt and cobalt silicate-like spe cies required temperatures higher than 973 K to be reduced. The struct ure of Co-ZSM-5 was partially destroyed after reduction at temperature s 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-l ike species in Co/ZSM-5 at 1023 K did not affect the zeolitic structur e. 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 latte r was obtained from the reduction of cobalt oxide under pretreatment c onditions. The acidity and shape selectivity of ZSM-5 accounted for th e product selectivity.