Acid sites topology in aluminas and zeolites from high-resolution solid-state NMR

Abstract

High-resolution solid-state nuclear magnetic resonance (NMR) spectroscopy is recognized as one of the most powerful tools in characterization of acid catalysts. Hundreds of research papers and tens of reviews and monographs appear each year. It is evidently an impossible task not only to discuss all recent contributions but even to mention each of them. Therefore, we restricted ourselves to a rather narrow (but fast developing) field, namely, the characterization of surface topology of real catalysts by heteronuclear dipolar interaction between magnetic nuclei. The common desire of all spectroscopists to obtain spectra with the best possible resolution competes with the general trend to obtain the greatest amount of information on systems. In this review we outline experimental approaches to this problem and describe several editing techniques which introduce new dimension into conventional NMR. The discussion is furnished with examples from current literature, however the emphasis is given to our recent publications in which the introduction of new experimental ideas resulted in deeper insight into the nature of surface acidity. In particular, two problems are discussed in detail, namely, how to observe selectively surface species (cross-polarization experiments) and how to retrieve dipolar contribution from the overall lineshape (double-resonance spin-echo experiments). The results obtained from these experiments allowed us to reach important conclusions concerning the nature of Lewis acidity in aluminas and the role of isolated Al atoms in formation of Bronsted sites in zeolites.