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

Light alkane conversion to aromatics requires sequential dehydrogenation steps and acid-catalyzed reactions of propene intermediates. Dehydrogenation reactions proceed via elementary steps that involve C-H bond activation and disposal of hydrogen atoms formed in these reactions. Hydrogen removal can occur via bimolecular or surface-mediated transfer to adsorbed hydrocarbons or by recombinative desorption to form H{dollar}sb2{dollar}. C-H bond activation steps are quasi-equilibrated and hydrogen removal limits aromatization rates and selectivity on H-ZSM5. The presence of exchanged Zn cations increases the rate of hydrogen removal steps by introducing "portholes" for recombinative desorption of H-atoms as H{dollar}sb2{dollar}.; The rates of C-H activation, recombinative hydrogen desorption, and propane chemical conversion were concurrently measured from isotopic redistribution and chemical conversion during reactions of {dollar}rm Csb3Hsb8/Csb3Dsb8{dollar} and {dollar}rm Dsb2/Csb3Hsb8{dollar} mixtures. C-H bond activation rates are fast during steady-state propane conversion and occur at similar rates on H-ZSM5 and Zn/H-ZSM5 and negligible on Na-ZSM5, consistent with the predominant role of Zn in recombinative desorption of H-atoms. Exchanged cations increase the rates of propane conversion to aromatics, of propene conversion to aromatics, of recombinative hydrogen desorption, and of incorporation of D-atoms from D{dollar}sb2{dollar} into reaction products. These data are consistent with quasi-equilibrated acid-catalyzed C-H activation and rate-determining hydrogen removal steps, as also confirmed by the kinetic hydrogen dependence and by {dollar}rm Hsb2/Dsb2{dollar} isotopic equilibration measurements.; In-situ Zn K-edge X-ray absorption studies show that aqueous ion exchange with H-ZMS5 (Si/Al = 14.5) leads to isolated Zn{dollar}sp{lcub}+2{rcub}{dollar} cations at exchange sites. Temperature programmed reduction studies show that Zn{dollar}sp{lcub}+2{rcub}{dollar} cations in Zn/H-ZSM5 do not reduce to zero-valent species during propane reactions at 773 K. High temperature condensation reactions of initial (ZnOH){dollar}sp+{dollar} species with acidic OH groups appear to lead to the formation of the active Zn{dollar}sp{lcub}+2{rcub}{dollar} cations interacting with two Al sites (O{dollar}sp-{dollar}-Zn{dollar}sp{lcub}+2{rcub}{dollar}-O{dollar}sp-{dollar}). Titration of Bronsted acid sites with Na{dollar}sp+{dollar} cations, however, prevent condensation and Zn sites remain as (ZnOH){dollar}sp+{dollar}.