The nature of vanadium in vanado-silicate (MFI) molecular sieves: influence of synthesis methods

Abstract

The influence of synthesis in acidic or alkaline media on the incorporation of vanadium in the MFI (ZSM-5) lattice has been studied. An alkaline gel favors the incorporation of vanadium in the MFI framework, while an acidic gel does not. EPR and analytical data indicate that mostly V⁴⁺species are present in an octahedral (Oh) environment in the as-synthesized V-MFI prepared in acidic media, which latter transform into polymeric V⁵⁺species on calcination. In the alkaline media, vanadium is incorporated mostly as V⁵⁺in a distorted Td environment. Thermal analysis and XRD data indicate that phase transformation from orthorhombic to monoclinic takes place during the calcination process only when the sample is prepared from acidic medium. It is found that the sample prepared from alkaline medium contains Si-OH groups from defect centers while such groups are absent in the sample prepared in acidic medium. It appears that V-ions are present in the defect sites in the MFI framework in the sample prepared from alkaline medium.