Zeolites H-[Ga]ZSM-5 and H-ZSM-5. A comparative study of the introduction of transition-metal cations by a solid-state reaction

Abstract

A comparative electron spin resonance study of the solid-state reaction of Cu, Fe, Cr or V compounds with H-ZSM-5 and H-[Ga]ZSM-5 has been carried out. The protonic (acid) sites in H-[Ga]ZSM-5 are less powerful and less thermostable ‘traps’ for migrating cationic species than those in H-ZSM-5. In contrast to the case of H-ZSM-5, the sites in H-[Ga]ZSM-5 are not strong enough to stabilize the isolated vanadyl species. However Cu^II, Fe^III and Cr^v ions which migrate from the outer surface of the zeolite crystals are coordinated in the cationic positions of H-[Ga]ZSM-5. The majority of the ions in H-[Ga]ZSM-5, like those in H-ZSM-5, are located as isolated coordinatively unsaturated cationic species accessible to gas-phase molecules. The presence of superhyperfine splitting in the e.s.r. spectra of Cr^v provides evidence of an electronic interaction between the unpaired electron of Cr^v and the nuclear spin of lattice Al³⁺ or Ga³⁺ ions.