Introduction of vanadium species in β zeolite by solid-state reaction: spectroscopic study of V speciation and molecular mechanism

V-containing β zeolites were prepared by solid-state reaction between V2O5 and β zeolite. The zeolite structure was analysed by XRD and N2 physisorption. The V speciation was studied by chemical analysis and different spectroscopies (FT-IR, 27Al-NMR, UV-Vis, EPR, photoluminescence). After calcination of V2O5-β zeolite mechanical mixtures at 500°C, three kinds of V species were identified: (i) oligomeric vanadates with octahedral V5+ easily removed by treatment with NH4OAc, (ii) isolated vanadyl (V=O)2+ ions in axially distorted octahedral or square pyramidal environment, interacting with framework and/or extraframework Al nuclei and (iii) isolated V5+ in tetrahedral and octahedral environments, localized in framework defect sites. The amount of the latter species is higher when water vapor is present during calcination and when parent β zeolite contains a high concentration of defect sites generated by a strong acid pretreatment. Isolated V5+ are easily reduced to tetrahedral V4+ or to square pyramidal (V=O)2+. Possible models of the mechanism of formation of V species by solid-state reaction and further reduction are proposed.