Incorporation of Vanadium in Zeolite Lattices:  Studies of the MEL (ZSM-11) System

Titrimetric analysis, 51 V liquid state NMR, and ESR data indicate that both V 5+ (mostly HVO 4 2- ) and V 4+ species (VO(OH) 3 - ) are present in precursor gels used in the synthesis of V-MEL molecular sieves. 29 Si NMR studies of the gels indicate that the V 5+ 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 NH 4 OAc after calcination. The second one is a loosely bound V species. The former species is V 5+ in a tetrahedral ( T d ) environment (lattice), and the latter species is V 4+ in an octahedral ( O h ) environment. The V 4+ species transforms into V 5+ species (two types) with a symmetrical tetrahedral ( T d ) environment of the orthovanadate type during calcination in the presence of air. The first species (V 5+ 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 V 5+ (formed from oxidation of V 4+ in the as-synthesized samples) is sensitive to hydration/dehydration and is difficult to reduce by H 2 . 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.