Methanol oxidation on VSiBEA zeolites: Influence of V content on the catalytic properties

This study deals with the influence of V content on the catalytic properties of VxSiBEA zeolites in the oxidation of methanol. The samples are prepared following a postsynthesis method reported earlier (S. Dzwigaj, M.J. Peltre, P. Massiani, A. Davidson, M. Che, T. Sen, S. Sivasanker, Chem. Commun. (1998) 87). The incorporation of isolated mononuclear V(V) into the framework of SiBEA is evidenced by XRD, FTIR, diffuse reflectance UV–vis and NMR. It is found that, for low V content, V(V) ions are in pseudo-tetrahedral coordination only, either in nonhydroxylated (SiO)3Vdouble bond; length as m-dashO or hydroxylated (SiO)2(OH)Vdouble bond; length as m-dashO species in framework position. For higher V content, additional species appear in extraframework position with vanadium in octahedral coordination. FTIR investigations of pyridine adsorption show that strong Brønsted and Lewis acidic centres are present in SiBEA leading to dimethyl ether only, in methanol oxidation. Upon incorporation of vanadium into the BEA framework, Lewis acidic (V5+) and basic (O2−) centres are generated with simultaneous appearance of partial oxidation products mainly, whose total concentration increases with vanadium content. These results suggest that those centres are responsible for the oxidation activity of VxSiBEA zeolites. The selectivity toward formaldehyde increases with the amount of vanadium present as pseudo-tetrahedral hydroxylated (SiO)2(HO)Vdouble bond; length as m-dashO species. This selectivity is suggested to be related to the moderate nucleophilicity of the basic vanadyl oxygen (Vdouble bond; length as m-dashO) of (SiO)2(HO)Vdouble bond; length as m-dashO species.