Synthesis and catalytic properties of crystalline, microporous vanadium silicates with MEL structure

Abstract

A thermally stable, vanadium-containing silicate with the MEL structure has been prepared by hydrothermal crystallization. XRD and framework IR spectra of samples with three different Si/V ratios suggest the presence of V ions in framework positions wherein the concentration of SiOH groups is likely to be high. ESR spectra of the as-synthesized and reduced samples indicate that vanadium (in the tetravalent state) is in an atomically dispersed state, but not in Td symmetry positions. Only V 5÷ ions were present on calcination, and the V 4+ to V 5÷ transition was found to be reversible. The calcined samples contain exchangeable protons. These vanadium silicates are mildly acidic and are catalytically active in both acid-catalyzed (isomerisation of m-xylene) as well as oxidation (phenol to hydroquinone and catechol) reactions. A plausible environment of vanadium at defect sites in the MEL lattice is suggested.