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

摘要

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℃, in air, followed by evacuation at 20℃, 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 VO~(2+) complexes formed during solid phase reaction. At least two types VO~(2+) centres were identified in H-ZSM-5 zeolite. One of them being ascribed to dehydrated VO~(2+) complexes in cationic sites, with C_(4v) 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 σ-bond and increasing ionic character for the out-of-plane π bond, have been found on rehydrated samples. For a low content of oxide (≤5% wt V_2O_5) in a mixture the thermal stability, T = 1070℃, of the zeolite phase is not affected by the presence of the reaction products.