ESR STUDY OF VANADIUM-DOPED ALUMINA-PILLARED AND TITANIA-PILLARED MONTMORILLONITES

Vanadium-doped alumina- and/or titania-pillared montmorillonites obtai ned by various preparative procedures have been investigated with the aid of ESR. The ESR signals of air-dried samples are characteristic of immobilized VO2+ ions, even after storage in a wet atmosphere. Compar ison with model spectra shows that these species are bound to the pill ars. Estimation of the V=O bond strength from the experimentally deter mined ESR parameters shows that in the titania-pillared montmorillonit e this bond is stronger than in alumina-pillared samples. In all cases the unpaired electron is strongly localized at the vanadium centre. A certain degree of delocalization into ligand orbitals is observed for the vanadyl ions present in the (V-Ti)-PILC (pillared clay) sample ob tained by calcination of the co-pillared precursor. A hypothesis is ad vanced that these species are responsible for the unique catalytic pro perties of this sample.