Incorporation of vanadium in zeolite lattices: Studies of the MEL (ZSM-11) system

Titrimetric analysis, V-51 liquid state NMR, and ESR data indicate that both V5+ (mostly HVO42-) and V4+ species (VO(OH)(3)(-)) are present in precursor gels used in the synthesis of V-MEL molecular sieves. Si-29 NMR studies of the gels indicate that the V5+ species interact with the silicate species. Two different V species have been identified in as-synthesized V-MEL samples. The concentration of the two species depends on the vanadium input and the synthesis atmosphere (air or He). The first one is incorporated in the lattice and is not extracted by a solution of 1 N NH4OAc after calcination. The second one is a loosely bound V species. The former species is V5+ in a tetrahedral (T-d) environment (lattice), and the latter species is V4+ in an octahedral (O-h) environment. The V4+ species transforms into V5+ species (two types) with a symmetrical tetrahedral (Td) environment of the orthovanadate type during calcination in the presence of air. The first species (V5+ incorporated in the lattice) is insensitive to hydration/dehydration and is easily reduced by H-2 at 573 K with a change in coordination from tetrahedral (T-d) to Square pyramidal, while the V5+ (formed from oxidation of V4+ in the as-synthesized samples) is sensitive to hydration/dehydration and is difficult to reduce by Hz. Only the reducible V species are active in the oxidation of toluene and phenol. Unit cell expansion due to vanadium incorporation is related to the nonextractable vanadium present in the calcined samples. Models for V species present in the gel and the different V-MEL samples are proposed.