(513f) Structure and Catalysis of V-Zsm5 Prepared by Vapor Exchange of Vanadium (V) Trichloride Oxide

The structure of VO_x/H-ZSM5 catalysts prepared using VOCl₃ precursors was determined using complementary spectroscopic and site titration methods. VOCl₃ exposure at 373 K and subsequent hydrolysis led to the stoichiometric removal of protons in H-ZSM5 up to V/Al_f ratios of unity. Isolated monomers were detected at V/Al_f less than 0.4 by a Raman band at 1065 cm^-1 assigned to VO₂⁺ by molecular simulations, while a band 1044 cm^-1, assigned to [O=V-O₂-V=O]²⁺ dimers, became apparent at higher V/Al_f. Neither V₂O₅ nor aqueous V⁵⁺ precursors exchanged with protons in H-ZSM5, except at silanols prevalent at external surfaces. VOCl₃ forms volatile monomers, which unlike V₂O₅ or aqueous V-oxo ions, diffuse within 10-ring channels and react with acidic protons. V-ZSM5 prepared via sublimation of VOCl₃ were active in oxidative dehydrogenation of propane at 673 K. Dehydrogenation rates did not depend on V/Al_f ratios (<0.4) and increased with VO_x loading at higher V/Al_f ratios, which led to the formation of V-O-V bonds in exchanged [O=V-O₂-V=O]²⁺ dimers. VO_x/ZSM-5 samples were less reducible in H₂ than V₂O₅, but treatment in C₂H₄/CH₄ mixtures at 973 K led to the formation of VC_x species active in the non-oxidative conversion of CH₄ to benzene with high selectivity.