Comparison of FAM mixing to single-pulse mixing in 17O 3Q- and 5Q-MAS NMR of oxygen sites in zeolites

Fast amplitude modulation (FAM) shifted-echo pulse sequences for 3Q- and 5Q-MAS NMR experiments were compared for resolution of 17O on chemically and/or crystallographically distinct framework oxygen sites (i.e., Si–O–Si, Si–O–Al) in zeolites 13X, 4A, and a natural analcime (Na1.01Al1.01Si1.99O6·H2O). Significantly better sensitivity is afforded by fast amplitude modulation sequences. Resolution among different sites is also better in 5Q-MAS than in 3Q-MAS spectra, although sites with large quadrupolar coupling constants are more efficiently excited and reconverted in the latter experiments. No Al–O–Al linkages were detected in these zeolites.