1H/27Al TRAPDOR NMR studies on aluminum species in dealuminated zeolites.

Aluminum species in several dealuminated zeolites (ultrastable HY, HZSM-5 and mordenite) were investigated in detail by means of the newly introduced 1H/27Al TRAPDOR method in combination with 27Al MAS NMR, and the quadrupole coupling constants (Q[CC]s) for aluminum atoms associated with these species were obtained. A signal at ca. 6.8 ppm, due to water molecules adsorbed on Lewis acid sites, was observed in the 1H MAS spectra for all the three zeolites. The TRAPDOR NMR provides direct evidence that there is a strong interaction between the adsorbed water molecules and the aluminum atoms of the Lewis-acid sites. The Q(CC) values for this aluminum species of 8.3, 6.7 and 11.3 MHz were determined from the TRAPDOR profiles for the ultrastable HY, HZSM-5 and mordenite zeolites, respectively. The Q(CC)s calculated from the TRAPDOR curves are usually larger than 10 MHz for both Bronsted-acid sites (SiOHAI) and non-framework aluminum species in the three zeolites. Three narrow peaks at 54, 30 and 0 ppm are separately superimposed on a broad hump in the 27Al MAS spectra of the three dehydrated zeolites, while the latter is associated with the 'NMR invisible' Al. The NMR experimental results suggest that the three kinds of aluminum species (non-framework aluminum species, Bronsted- and Lewis-acid sites) are all responsible for the resonance of the broad hump in dehydrated zeolites, which makes it difficult to explain the 27Al MAS spectra. Fortunately, the TRAPDOR NMR provides a direct method for individually studying different aluminum species with large Q(CC)s via their dipolar coupling to nearby proton nuclei.