²⁷Al−H REDOR NMR and ²⁷Al Spin-Echo Editing:  A New Way To Characterize Brönsted 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 27 Al 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 NH 3 . The procedure provides a means for obtaining framework Si/Al ratios. The values obtained are more reliable than ones determined by the standard 29 Si MAS NMR method, since a possible coupling between NF species and the framework is taken into account.