Structure and Properties of Cobalt-Exchanged H-ZSM5 Catalysts for Dehydrogenation and Dehydrocyclization of Alkanes

Abstract

Co/H-ZSM5 catalysts with Co/Al ratios of 0.09−0.22 were prepared by aqueous exchange. Turnover rates for propane conversion to propene and to C6−C8 aromatics on these catalysts are about 10-fold higher than on H-ZSM5. The selectivities to propene, aromatics, and H2 are also higher on Co/H-ZSM5 than on H-ZSM5. The rate of D2 exchange with OH groups increases with increasing Co/Al ratio, suggesting that Co cations catalyze D2 dissociative chemisorption steps that limit the rate of isotopic exchange. Co cations also catalyze hydrogen recombinative desorption steps, which limit the rate of propane dehydrogenation and aromatization reactions. The density of residual zeolitic hydroxyls was measured by D2−OH isotopic exchange and by changes in the intensity of OH infrared bands as a function of Co content. D2−OH and infrared measurements showed that Co2+ cations replace 1.1−1.3 zeolitic protons, suggesting the predominant presence of Co2+−O−Co2+ dimers, with some Co2+ monomers, each bridging two next-nearest nei...