Characterization of Bronsted and Lewis acidity in zeolites by solid-state NMR and the recent progress in the REDOR technique

Because of their use as catalysts, the volume of papers published on the na ture of the acid sites in zeolites is impressive. The number of acid sites is correctly estimated and their chemical nature is known. There are howeve r numerous domains still largely unexplored, such as the structural descrip tion (topology) of the disordered surface of a real catalyst. Si-29 and Al- 27 REDOR NMR from chemisorbed ammonia provide the spatial arrangement of th e base with respect to either the Si/Al distribution in the lattice or the coordination in the nonframework Al nano-particles, shedding light on the n ature of the Bronsted and Lewis sites. In particular they explain the main features ruling the strength of the Bronsted sites and the 'dispersion' of Lewis sites, that is the ratio of the number of sites to the number of nonf ramework aluminum. Copyright (C) 1999 John Wiley & Sons, Ltd.