EPR STUDY OF INTERACTION OF VANADIUM PENTOXIDE WITH H-ZSM-5 ZEOLITE

The EPR spectroscopy has been used to study the interaction of vanadium pentoxide with H-ZSM-5 zeolite, supplied by ICITPR-Ploiesti, Romania, occurring at the calcination of the mixture at temperatures from 100 to 1000 degrees C, in air, followed by evacuation at 20 degrees C, as well as for those rehydrated at room temperature. The EPR method is also used to follow the thermal stability of the investigated zeolite, the framework alteration and the nature of VO2+ complexes formed during solid phase reaction. At least two types VO2+ centres were identified in H-ZSM-5 zeolite. One of them being ascribed to dehydrated VO2+ complexes in cationic sites, with C-4r ligand field symmetry and the second one corresponding to the hydrated complex with a square bipyramidal symmetry, D-4h. Increasing covalent character, for the in-plane sigma-bond and increasing ionic character for the out-of-plane pi bond, have been found on rehydrated samples. For a low content of oxide (less than or equal to 5% wt V2O5) in a mixture the thermal stability, T = 1070 degrees C, of the zeolite phase is not affected by the presence of the reaction products.