Elucidating the dealumination mechanism of zeolite H-Y by solid-state NMR spectroscopy

Abstract

Abstract Hydrothermally treated zeolites H-Y were investigated by XRD, 29Si MAS NMR, 27A1 MQMAS NMR, high-field 27A1 MAS NMR, and 1H MAS NMR spectroscopy. Upon adsorption of ammonia on the hydrothermally treated materials, a reversible change of octahedrally coordinated to tetrahedrally coordinated aluminum atoms was found. Quantitative 29Si, 27A1, and 1H MAS NMR measurements indicated that this coordination change is accompanied by the formation of bridging OH groups (SiOHAl) in the dehydrated materials, while only a weak decrease in the amount of silanol (SiOH) groups and no systematic change of A10H groups occurred. Based on these results, a model for the reversible coordination change of aluminum atoms in the framework of hydrothermally treated zeolites H-Y is proposed assuming local structures consisting of threefold-coordinated framework aluminum atoms with SiO− defect sites in their vicinity, which are coordinated to extra-framework aluminum species. After adsorption of ammonia at the threefold-coordinated framework aluminum atoms, the SiO− defect sites are healed to Si-O-Al− bridges leading to a transformation of the threefold-coordinated aluminum atoms to tetrahedrally coordinated atoms. Upon thermal decomposition of the ammonium ions formed at these Si-O-Al− bridges, SiOHAl groups occur.