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

Abstract

Titrimetric analysis, ⁵¹V 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. ²⁹Si 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 (T_d) 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 H₂ 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 H₂. 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.