1H NMR studies on the acidity of zeolites

Abstract

1H NMR, performed on sealed samples, provides dynamic methods to study the acidity of zeolites. Among high resolution NMR techniques, the magic angle spinning (MAS) is the best developed. It inquires into the nature and concentration of OH groups of different types in fully desorbed samples, identifying the bridging acidic sites and framework defects, as well as extraframework species. The chemical shift of a fully desorbed zeolite alone, however, is not enough to measure its acid strength, interaction with a base is necessary. In such conditions, chemical exchange between H atoms, from the acidic site and from the base, causes coalescence of these H atom resonance positions and no useful measurement can be made. Broad-line NMR in "rigid lattice" conditions (4K) can be used to identify and quantify the quenched species formed when a base (containing a small number of H atoms) and a zeolite Brφnsted acid site interact. A scale of Brφnsted acid strength has been proposed, which is discussed in terms of the nature, the composition and defects of the zeolitic framework.