Brønsted/Lewis Acid Synergy in H–ZSM-5 and H–MOR Zeolites Studied by 1H and 27Al DQ-MAS Solid-State NMR Spectroscopy

Abstract

The spatial proximities of acid sites in dealuminated H-MOR and H-ZSM-5 zeolites have been comprehensively studied by two-dimensional H-1 and Al-27 double-quantum magic-angle spinning (DQ-MAS) NMR spectroscopy. H-1 DQ-MAS NMR revealed the detailed spatial proxirnities among Bronsted and Lewis acid sites, while Al-27 DQ-MAS NMR probed the detailed spatial proximities among various aluminum species in the dealuminated zeolites. It was found that all of the extra-framework aluminum species (EFAL, Lewis acid sites) are always in close proximity to the framework aluminum (Bronsted acid sites) in both H-MOR and H-ZSM-5 zeolites. The result from C-13 CP/MAS NMR of adsorbed acetone demonstrated that the spatial proximities between Bronsted and Lewis acid sites lead to a synergy effect that can remarkably enhance the Bronsted acid strength of the dealuminated zeolites. On the basis of these experimental results, we proposed a model to describe the Bronsted/Lewis acid synergy in the dealuminated zeolites.