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 neighbor Al sites. The location and structure of exchanged Co cations were probed using X-ray absorption spectroscopy (XAS) and temperature-programmed reduction (TPR). No H2 consumption was detected up to 1273 K during TPR in any of the Co/H-ZSM5 samples, consistent with the absence of CoOx crystallites, which reduce at approx. 800 K. In situ near-edge X-ray absorption studies confirmed that Co species remain as divalent cations during exposure to H2 or C3H8 at 773 K. Near-edge and fine structure analysis detected Co2+ cations with similar structure in all Co/H-ZSM5 samples (Co/Al < 0.22), and Co coordination changes from octahedral to tetrahedral upon sample dehydration at 773 K in He. Radial structure functions showed weak contributions from the first and second shells around Co. This reflects the nonuniform nature of the distance and orientation in Al-Al next-nearest neighbor sites in ZSM5.