Structure and function of metal cations in light alkane reactions catalyzed by modified H-ZSM5

The rate of propane dehydrocyclodimerization to form C-6+ aromatics is limited by a sequence of irreversible dehydrogenation reactions leading to propene, higher alkenes, dienes, trienes, and aromatics. Quasi-equilibrated acid-catalyzed cracking, oligomerization, and cyclization reactions of alkene intermediates occur in sequence with these dehydrogenation reactions. Each dehydrogenation reaction is in turn limited by the rate of elementary steps that dispose of H-atoms formed in C-H bond activation steps. The rate of C-H bond activation, recombinative hydrogen desorption, and propane chemical conversion have been measured from the rates of isotopic redistribution and chemical conversion during reactions of C3H8/C3D8 and D-2/C3H8 mixtures on H-ZSM5, Ga/H-ZSM5, and Zn/H-ZSM5. Isotopic studies show that C-H activation steps are fast during steady-state propane dehydrocyclodimerization on H-ZSM5, Ga/H-ZSM5, and Zn/H-ZSM5. Ga and Zn species increase the rates of propane chemical conversion, recombinative hydrogen desorption, and deuterium incorporation from D-2 into reaction products. Disposal of hydrogen formed in C-H bond activation steps occurs by transfer of H-atoms to unsaturated species to form alkanes or to Ga and Zn species, which catalyze the recombinative desorption of H-atoms to form dihydrogen (H-2). The sequential release of several H-atoms during a propane dehydrocyclodimerization turnover limits the rate and selectivity of this reaction on H-ZSM5. In-situ X-ray absorption studies suggest that Ga and Zn species reside at cation exchange sites as monomeric cations and that recombinative desorption involve reduction-oxidation cycles of such cations during each dehydrocyclodimerization turnover. These monomeric species form directly during exchange of Zn ions from solution onto H-ZSM5. Ga3+ species, however, do not exchange directly from solution onto H-ZSM5, but instead form extrazeolitic Ga2O3 crystals. Ion exchange occurs during subsequent contact with propane or hydrogen at 700-800 K via vapor phase exchange of volatile Ga1+ species.