Preparation of two series of VxSiBeta zeolite catalysts with V centres in framework and extra-framework positions and their application in selective oxidation of methanol

Abstract

Two series of V-containing Beta zeolites have been prepared by contacting of the aqueous NH4VO3 solution with two SiBeta zeolites at pH = 2.5 and 6, respectively. Because of the presence in NH4VO3 solution at pH = 2.5 mainly monomeric VO2+ ions, vanadium have been easily incorporated into framework of SiBeta zeolite as pseudo-tetrahedral non hydroxylated (SiO)3V = O and hydroxylated (SiO)2(OH)V = O species. The moderate nucleophilicity of the basic vanadyl oxygen of the latter species play important role in methanol oxidation toward formaldehyde. The selectivity toward formaldehyde on this series of VxSiBeta(I) increases with the amount of pseudo-tetrahedral hydroxylated (SiO)2(HO)V = O species, which act as either redox or acidic and basic centres. In contrast at pH = 6, the aqueous NH4VO3 solution is expected to contain both mononuclear and polynuclear V ions, thus it is more difficult to incorporate V species in SiBeta at this condition as shown by FT-IR and NMR data. The absence of pseudo-tetrahedral V species in framework position of V0.6SiBeta(II) is probably responsible for lack of activity of this catalyst in methanol oxidation. The appearance of this species in the series of VxSiBeta(II) zeolites for high V content leads to their activity in methanol oxidation toward formaldehyde.