VANADIUM-MODIFIED ZEOLITE WITH THE STRUCTURE OF ZSM-12 - EPR AND NMR-STUDIES

A new crystalline vanadium-containing silicate with the zeolite ZSM-12 structure has been prepared by hydrothermal synthesis and characterized by EPR and NMR methods. EPR studies show that vanadium is present in the as-synthesized zeolite in two different states. EPR as well as H-1 and Si-29 NMR data suggest that the incorporation of the vanadium into the lattice occurs through interactions with hydroxyl groups at defect sites. According to V-51 NMR the vanadium after calcination is present in tetrahedral oxygen coordination and atomically dispersed throughout the lattice. Vanadium in the lattice of the zeolite can be easily converted from V(5+) to V(4+) and vice versa. EPR of the zeolites after calcination and subsequent reduction showed that vanadium was present in the lattice mainly in distorted tetrahedral coordination, and only a small part was present in the axially symmetric octahedral coordination state.