SPECTROSCOPIC STUDIES OF VANADIUM-SUBSTITUTED ZEOLITIC SILICATES OF MFI TOPOLOGY

Samples of silicalite-1 with vanadium ions incorporated in stable fram ework positions (KVS-5) have been synthesized using several different compounds of V. The materials were investigated by XRD, chemical analy sis and SEM-EPM techniques and UV-VIS, EPR, NMR, IR and Raman spectros copy. One group of the samples contained up to 0.1 wt.% of V only, pre dominantly in a tetrahedral coordination and in two oxidation states V 4+ (violet) and V5+ (colourless). The other group consisted of materia ls with about 1 wt.% of V, coordinated mostly in square pyramids with vanadyl groups (dirty green to blue-grey). The pyramids occur in two f requent configurations and one that is rare. The different shapes are probably due to various distributions of stronger and weaker V-O bonds . After calcination, vanadium is fully oxidized to V5+ without leaving its framework position. The tetrahedral coordination remains stable a nd the pyramids transform to strongly distorted octahedra complemented by water molecules (yellow). All these transformations, as well as th e redox reaction V4+ <-> V5+, are reversible. In all coordinations, th e V centres remain neutral and contain, in addition to the framework V -O bonds, either a type of non-acidic OH group or, more likely, ONa gr oups with non-exchangeable Na ions. A model is proposed for transforma tions of V complexes.