On the nature of gallium species in gallium-modified mordenite and MFI zeolites. A comparative DRIFT study of carbon monoxide adsorption and hydrogen dissociation

Abstract

The results of a DRIFT study of carbon monoxide molecular adsorption and hydrogen dissociative adsorption on gallium-modified mordenite and MFI (ZSM-5) zeolites are presented. It was found that in the reduced gallium-modified mordenite (Ga-MOR) both Ga³⁺ and Ga⁺ exchanged cations are present and can be detected by CO adsorption. Ga³⁺ cations in Ga-MOR dissociatively adsorb molecular hydrogen at elevated temperatures, resulting in the formation of gallium hydride species and acidic hydroxyl groups. In the reduced Ga-MFI evacuated at 823 K under medium vacuum conditions only Ga⁺ exchanged intrazeolite cations were detected. It was found, however, that Ga³⁺ intrazeolite exchanged cations which form upon high-temperature disproportionation of Ga⁺ cations in the reduced Ga-MFI and Ga-MOR can be stabilized by high-temperature oxidation of these zeolites.