31st Annual SSRL Users' Meeting

The formation and reactivity of ZSM-5 supported Re catalysts were studied using Re L_I and L_III edge X-ray absorption spectroscopy. These catalysts demonstrated unprecedented methane and propane pyrolysis rates and high methanol oxidation activity and selectivity to dimethoxymethane and methyl formate at low temperature (373°K). Materials were prepared by thermal treatment of rhenium heptoxide and H-ZSM-5 physical mixtures with Re/Alf molar ratios less than unity at 723°K in air. The LI near edge spectra evolved during thermal treatment to resemble tetrahedral ReO₄- domains. Temperature programmed reduction in H₂ suggested the ReO₄- species were extraframework ReO₃⁺ bonded to a framework (Si-O-Al)- that consume 3.5 H₂/Re during reduction to Re0 at 723°K. The stoichiometric reduction of extraframework oxygen generated 3.0 H₂O/Re, and the balance 0.5 H₂/Re replaced ReO₃+ at exchange sites to form Si-OH-Al. These materials resemble the organometallic complex methyltrioxorhenium that is used in olefin epoxidation reactions. ZSM-5 provides shape selectivity and an anchor for ReO₃⁺ to prevent volatilization at elevated temperatures. Re L_III near edge spectra confirm the complete reduction to Re0 in H₂ at 723°K. Simulations of the k3-weighted extended X-ray absorption fine structure of Re0-ZSM-5 in H₂ at 723°K or CH₄ at 950°K indicated the presence of 0.6 nm Re metal clusters, similar to the diameter of ZSM-5 channels (0.5-0.55 nm) suggesting that clusters lie within zeolitic pores.