HIGH-TEMPERATURE INTERACTION OF VANADIUM PENTOXIDE WITH H-ZSM-5 ZEOLITE - ESR AND IR STUDY

ESR, IR, UV-vis spectroscopies and temperature-programmed desorption of ammonia (TPDA) have been used to study the high-temperature interaction of vanadium pentoxide with the H-ZSM-5 zeolite at 720 K in a nitrogen stream. For comparison, the ESR parameters of the VO2+ ion exchanged ZSM-5 zeolite were investigated. It appears that part of the vanadyl ions formed as a result of V2O5 reduction moves predominantly into the zeolite cationic sites, replacing strong acid OH groups, the other part is supported on a zeolite via terminal Si-OH groups. Both types of vanadyl complexes exhibit a square pyramid (C4v) coordination with \B2> as the ground state. Adsorption of water or ammonia causes changes of the ligand field symmetry of VO2+ ions. The resulting square bipyramid complexes produce elongation and thus weakening of the V = O bond and, on the other hand, shortening and strengthening of the V-O bonds in the basal plane. The bonding coefficients beta(1)* and epsilon(pi)* obtained from the ESR spectra revealed that the average ionic character of the vanadium bonding in VO2+ complexes increases with the content of vanadium in the mixtures, approaching that of ion-exchanged VO2+ complexes.