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

International audienceTwo series of V-containing Beta zeolites have been prepared by contacting of the aqueous NH 4 VO 3 solution with two SiBeta zeolites at pH = 2.5 and 6, respectively. Because of the presence in NH 4 VO 3 solution at pH = 2.5 mainly monomeric VO 2 + ions, vanadium have been easily incorporated into framework of SiBeta zeolite as pseudo-tetrahedral non hydroxylated (SiO) 3 V = 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 V x SiBeta(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 NH 4 VO 3 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 V 0.6 SiBeta(II) is probably responsible for lack of activity of this catalyst in methanol oxidation. The appearance of this species in the series of V x SiBeta(II) zeolites for high V content leads to their activity in methanol oxidation toward formaldehyde