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 1H and 27Al double-quantum magic-angle spinning (DQ-MAS) NMR spectroscopy. 1H DQ-MAS NMR revealed the detailed spatial proximities among Brønsted and Lewis acid sites, while 27Al 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 (Brønsted acid sites) in both H–MOR and H–ZSM-5 zeolites. The result from 13C CP/MAS NMR of adsorbed acetone demonstrated that the spatial proximities between Brønsted and Lewis acid sites lead to a synergy effect that can remarkably enhance the Brønsted acid strength of the dealuminated zeolites. On the basis of these experimental results, we proposed a model to describe the Brønsted/Lewis acid synergy in the dealuminated zeolites.