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Templates for the synthesis of zeolites
Huimin Luan, Qinming Wu, Jianping Wu, Xiangju Meng*, Feng-Shou Xiao*
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Qualitative and quantitative analysis of Brønsted and Lewis acid sites in zeolites: A combined probe-assisted 1H MAS NMR and NH3-TPD investigation

Runze Liu, Yankai Bian, Weili Dai*

Chin. J. Struct. Chem., 2024, 43: 100250. DOI: 10.1016/j.cjsc.2024.100250

April 15, 2024

1H MAS NMR; NH3-TPD; Brønsted acid sites; Lewis acid sites; Zeolites

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 attentions have 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 MAS (nuclear magnetic resonance spectroscopy) 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. According to controlling the desorption temperature of NH3 probe molecules, the acid type (i.e., Brønsted and Lewis acid sites) 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.
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