Probing the Spatial Proximities among Acid Sites in Dealuminated H-Y Zeolite by Solid-State NMR Spectroscopy

Abstract

A comprehensive study has been made to probe the spatial proximities among different acid sites in dealuminated H-Y zeolites modified with various degrees of calcination, steam, and acid treatments by using a variety of different solid-state NMR techniques, including multinuclear MAS NMR and two-dimensional 1H double-quantum (DQ) MAS NMR spectroscopy. The effects of dealumination treatments on the nature, concentration, and location of extraframework Al species in H-Y zeolites were followed by 1H DQ MAS NMR of hydroxyl protons in conjunction with 1H, 27Al, and 29Si MAS NMR results. It was found that the extraframework AlOH species (Lewis acid sites) are always in close proximity to the bridging AlOHSi hydroxyls (Brønsted acid sites) on the framework of dealuminated H-Y zeolites prepared by thermal and hydrothermal treatments, indicating the presence of a Brønsted/Lewis acid synergy effect. However, such an effect is absent in acid-treated H-Y zeolites, as also confirmed by 13C CP/MAS NMR of adsorbed 2-13C-acetone.