AL-27-H-1 REDOR NMR AND AL-27 SPIN-ECHO EDITING - A NEW WAY TO CHARACTERIZE BRONSTED AND LEWIS ACIDITY IN ZEOLITES

A new approach to the NMR characterization of acid zeolites is describ ed. The approach is based on (i) the quantitative comparison of the ro tational echo double-resonance (REDOR) evolution curves obtained by on e-pulse excitation and by cross-polarization excitation and (ii) the s pin-echo editing technique. The procedure that is described allows a ' 'reconstruction'' of Al-27 NMR spectra corresponding to either Lewis s ites or nonframework (NF) Al. The approach is applied to the character ization of the dealuminated mordenite DHM600 with chemisorbed ammonia as a proton reservoir for polarization transfer, The second moment ana lysis suggests that the NF Lewis sites are constituted by a pair of Al atoms having chemisorbed NH3. The procedure provides a means for obta ining framework Si/Al ratios. The values obtained are more reliable th an ones determined by the standard Si-29 MAS NMR method, since a possi ble coupling between NF species and the framework is taken into accoun t.