Qualitative and quantitative analysis of Brønsted and Lewis acid sites in zeolites: A combined probe-assisted 1H MAS NMR and NH3-TPD investigation

Abstract

Zeolite is one of the most important heterogeneous catalysts in acid catalytic reactions. Considering that the catalytic behaviors of zeolites are mostly related to their acidic characteristics, extensive attention has been attracted to the measurements of acid type, strength and concentration in zeolites. Numerous techniques including Fourier-transform infrared spectroscopy (FTIR), probe-assisted 1H, 13C and 31P magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) as well as temperature programmed desorption of ammonia (NH3-TPD) have been developed for determining the acid sites. Nevertheless, a single approach is defective to characterize the acid sites comprehensively. Herein, combining the probe-assisted (e.g., NH3 and CD3CN) 1H MAS NMR and NH3-TPD, the acid sites in different zeolites including the acid type, density and strength were determined. The commonly utilized NH3-TPD to determine the acid strength of zeolite samples with different topologies should be rigorously considered owing to zeolite confinement effect. Controlling the desorption temperature of NH3 probe molecules, the acid type (i.e., Brønsted acid sites (BAS) and Lewis acid sites (LAS)) and the corresponding density could be determined by NH3 probe-assisted 1H MAS NMR spectroscopy, while the acid strength could be investigated via CD3CN probe-assisted 1H MAS NMR spectroscopy.