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 described. The approach is based on (i) the quantitative comparison of the rotational echo double-resonance (REDOR) evolution curves obtained by one-pulse excitation and by cross-polarization excitation and (ii) the spin-echo editing technique. The procedure that is described allows a ''reconstruction'' of Al-27 NMR spectra corresponding to either Lewis sites or nonframework (NF) Al. The approach is applied to the characterization of the dealuminated mordenite DHM600 with chemisorbed ammonia as a proton reservoir for polarization transfer, The second moment analysis suggests that the NF Lewis sites are constituted by a pair of Al atoms having chemisorbed NH3. The procedure provides a means for obtaining framework Si/Al ratios. The values obtained are more reliable than ones determined by the standard Si-29 MAS NMR method, since a possible coupling between NF species and the framework is taken into account.