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

Authors: 
Lacheen, H. S., University of California, Berkeley
Iglesia, E., University of California at Berkeley


The structure of VOx/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/Alf ratios of unity. Isolated monomers were detected at V/Alf 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/Alf. 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/Alf ratios (<0.4) and increased with VOx loading at higher V/Alf ratios, which led to the formation of V-O-V bonds in exchanged [O=V-O2-V=O]2+ dimers. VOx/ZSM-5 samples were less reducible in H2 than V2O5, but treatment in C2H4/CH4 mixtures at 973 K led to the formation of VCx species active in the non-oxidative conversion of CH4 to benzene with high selectivity.