BRONSTED ACID SITES IN ZEOLITES CHARACTERIZED BY MULTINUCLEAR SOLID-STATE NMR-SPECTROSCOPY

Nearly all atoms contributing to the local structure of Bronsted acid sites in zeolites exhibit isotopes accessible for multinuclear solid-s tate nuclear magnetic resonance (NMR) investigations. Therefore, in th e last 15 years, NMR spectroscopy has found numerous applications for the determination of the types of hydroxyl proton in zeolites, of thei r concentration, accessibility, and mobility, and for the characteriza tion of their acid strength and local structure. It allows the study o f the role of hydroxyl groups in the formation of adsorbate complexes and in heterogeneously catalyzed reactions. Meanwhile, NMR spectroscop y belongs to the most powerful techniques for the characterization of Bronsted acid sites in zeolites and related materials. The basis of th is success is the invention of new sample preparation techniques, exte rnal magnetic fields with high-flux densities, effective line-narrowin g methods, andnew two-dimensional experiments, making the detection of highly resolved solid-state NMR spectra and the separation of spectra l parameters possible. This article gives a review of these techniques and a summary of the most important applications of multinuclear soli d-state NMR spectroscopy for the characterization of Bronsted acid sit es in dehydrated zeolites.