THE NATURE OF VANADIUM IN VANADO-SILICATE (MFI) MOLECULAR-SIEVES - INFLUENCE OF SYNTHESIS METHODS

The influence of synthesis in acidic or alkaline media on the incorpor ation of vanadium in the MFI (ZSM-5) lattice has been studied. An alka line gel favors the incorporation of vanadium in the MFI framework, wh ile an acidic gel does not. EPR and analytical data indicate that most ly V4+ species are present in an octahedral (Oh) environment in the as -synthesized V-MFI prepared in acidic media, which latter transform in to polymeric V5+ species on calcination. In the alkaline media, vanadi um is incorporated mostly as V5+ in a distorted Td environment. Therma l analysis and XRD data indicate that phase transformation from orthor hombic to monoclinic takes place during the calcination process only w hen the sample is prepared from acidic medium. It is found that the sa mple prepared from alkaline medium contains Si-OH groups from defect c enters while such groups are absent in the sample prepared in acidic m edium. It appears that V-ions are present in the defect sites in the M FI framework in the sample prepared from alkaline medium. (C) 1996 Aca demic Press, Inc.