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 H-1/Al-27 TRAPDOR method in combination with Al-27 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 H-1 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 Al-27 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 Al-27 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. Published by Elsevier Science B.V.