NMR Spectroscopic Techniques for Determining Acidity and Basicity

Abstract

Solid acids and bases have found numerous applications in heterogeneous catalysis. However, the characterization of the acidic and basic properties of solids is relatively complicated compared to that of liquids. This contribution summarizes the state of the art in the spectroscopic determination of acidity and basicity by NMR methods, especially with respect to zeolites. Zeolites are porous inorganic crystals built from TO4 tetrahedra. In original zeolites, T represents Si or Al. The latter atoms may be replaced to some extent by other elements like B, Ga, Be, Ti etc. Other classes of microporous materials are based, e.g. on AlPO4 compounds. The ability of zeolites to act as acid catalysts had already been described in the early 1960s and has remained an interesting subject of research. In recent years, zeolites have also found increasing interest as basic catalysts. The presence of both acidic and basic sites is responsible for the broad applicability of zeolites as catalysts.