Spectroscopic studies of vanadium-substituted zeolitic silicates of MFI topology

Abstract

Samples of silicalite-1 with vanadium ions incorporated in stable framework positions (KVS-5) have been synthesized using several different compounds of V. The materials were investigated by XRD, chemical analysis and SEM–EPM techniques and UV–VIS, EPR, NMR, IR and Raman spectroscopy. One group of the samples contained up to 0.1 wt. % of V only, predominantly in a tetrahedral coordination and in two oxidation states V⁴⁺(violet) and V⁵⁺(colourless). The other group consisted of materials with about 1 wt. % of V, coordinated mostly in square pyramids with vanadyl groups (dirty green to blue–grey). The pyramids occur in two frequent 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 and the pyramids transform to strongly distorted octahedra complemented by water molecules (yellow). All these transformations, as well as the redox reaction V⁴⁺↔ V⁵⁺, are reversible. In all coordinations, the 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 groups with non-exchangeable Na ions. A model is proposed for transformations of V complexes.