Synthesis, Structure, and Catalytic Reactivity of Isolated V5+-Oxo Species Prepared by Sublimation of VOCl3 onto H-ZSM5

Abstract

Isolated and uniform V(5+)-oxo species were grafted onto H-ZSM5 at V/Al(f) ratios of 0.2-1 via sublimation of VOCl(3) precursors. These methods avoid the restricted diffusion of solvated oligomers in aqueous exchange, which leads to poorly dispersed V(2)O(5) at external zeolite surfaces. Sublimation methods led to stable and active V-ZSM5 catalysts for oxidative dehydrogenation (ODH) reactions; they led to an order of magnitude increase in primary C(2)H(6) ODH rates compared with impregnated ZSM5 catalysts at similar V/Al(f) ratios and showed similar activity to impregnated VO(x)/Al(2)O(3). The structure of grafted V(5+)-oxo species was probed using spectroscopic and titration methods. Infrared spectra in the OH region and isotopic exchange of D(2) with residual OH groups showed that exposure to VOCl(3(g)) at 473 K led to stoichiometric replacement of H(+) by each (VOCl(2))(+) species. Raman spectra supported by Density Functional Theory electronic structure and frequency calculations showed that, at V/Al(f) < 0.5, hydrolysis and subsequent dehydration led to the predominant formation of (VO(2))(+) species coordinated to one Al site with single-site catalytic behavior (0.7-0.9 x 10(-3) mol C(2)H(4) V(-1) s(-1), 673 K). At higher V/Al(f) ratios, simulation of extended X-ray absorption fine structure spectra indicated that V(2)O(4)(2+) dimers coexisted with VO(2)(+) monomers and led to an enhancement in ODH rates as a result of bridging V-O-V (1.3 x 10(-3) mol C(2)H(4) V(-1) s(-1)). These V(5+)-oxo species form via initial reactions between VOCl(3(g)) and OH groups to form HCl((g)), hydrolysis of grafted (VOCl(2))(+) to form HCl((g)) and (VO(OH)(2))(+), and intramolecular and intermolecular condensation to form monomers and dimers, respective with the concurrent evolution of H(2)O. Raman and X-ray spectroscopies did not detect crystalline V(2)O(5) at V/Al(f) ratios of 0.2-1, but V(2)O(5) crystals were apparent in samples prepared by impregnation or physical mixtures of V(2)O(5)/H-ZSM5. Framework Al atoms and zeolite crystal structures are maintained during VOCl(3) treatment and subsequent hydrolysis; (27)Al and (29)Si MAS NMR showed that these synthetic protocols removes <10% of the framework Al atoms (Al(f)).