STUDY OF HIGH-SILICA H-ZSM-5 ACIDITY BY H-1-NMR TECHNIQUES USING WATER AS BASE

The Bronsted acidity of high-silica H-ZSM-5 zeolites (Si/Al=74 and 180 ) has been characterized by H-1 NMR, 4 K broad-line and 300 K high-res olution magic angle spinning. It is compared to that of other zeolites already ranked by their ''acidity coefficient'' X-a, which is the con centration of hydroxonium ions per Bronsted site formed when a water m olecule interacts with each Bronsted site. The general behaviour of H- ZSM-5 acidic sites, in the presence of increasing concentrations of wa ter, differs from that of H-faujasites or H-mordenites. Moreover, the chi(a) values of the two samples described in this work are much highe r than for H-ZSM-5 with an Si/Al ratio of 39, being 0.77 and 0.95 (+/- 15%) for Si/Al values of 74 and 180, respectively. (C) 1998 Elsevier S cience B.V. All rights reserved.