Qualitative and quantitative analysis of acid properties for solid acids by solid-state nuclear magnetic resonance spectroscopy

Silica-alumina-based solid acid is one of the most popular heterogeneous catalysts utilized in the petrochemical process and biorefining. The acid density, strength, and accessibility of catalysts have dominant effects on their catalytic performance and process efficiency. Unlike liquid acids, the acidity formed on most solid acids is caused by the electronegativity of the coordinated atoms. Herein the local structure and coordination of the acid sites are directly related to their acidity. Solid-state nuclear magnetic resonance (SSNMR) spectroscopy has been approved as a powerful technique to characterize both the acidity and the local structure of solid acids in both qualitative and quantitative manner. This review article will focus on the recent progress in experimental approaches and applications using SSNMR spectroscopy to characterize silica-alumina catalysts: (1) experimental approaches for characterizing the properties of acid sites, (2) characterization of the local structure and the acidic properties of Brønsted and Lewis acid sites, and (3) the reactivity of acid sites in acid-catalyzed reactions via hydrogen-deuterium exchange experiments using in situ SSNMR. This review article provides a routine approach for the study of solid acid catalysts and improves the understanding of the structure-acidity relationship for the design of new catalysts.

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