Generation of paramagnetic vanadium species in ZSM-5 and investigation of their interaction with framework aluminium nuclei by electron spin resonance and electron spin echo modulation

Abstract

Paramagnetic vanadium species generated by solid-state reactions of V₂O₅ in H- and Ca-ZSM-5 zeolites were investigated by ESR and ESEM spectroscopies. Heating conditions, in air, in vacuum and in H₂ were shown to be controlling factors for the generation of paramagnetic vanadium species. ESR results showed that (1) the paramagnetic vanadium species were more accessible to H₂O than O₂ and (2) the yield depended upon the cationic form of ZSM-5, heating conditions, and water adsorption. The yields were greater in H-ZSM-5 than in Ca-ZSM-5. The yields were largest in H₂, less in air and least in vacuum. Hydrated samples gave greater yields than dehydrated samples. ESEM results indicated that framework Al was located within 0.5 nm of the paramagnetic vanadium species.