H-1 AL-27 TRAPDOR NMR-STUDIES ON ALUMINUM SPECIES IN DEALUMINATED ZEOLITES/

Aluminum species in several dealuminated zeolites (ultrastable HY, HZS M-5 and mordenite) were investigated in detail by means of the newly i ntroduced H-1/Al-27 TRAPDOR method in combination with Al-27 MAS NMR, and the quadrupole coupling constants (Q(CC)s) for aluminum atoms asso ciated with these species were obtained. A signal at ca. 6.8 ppm, due to water molecules adsorbed on Lewis acid sites, was observed in tile H-1 MAS spectra for all the three zeolites. The TRAPDOR NMR provides d irect 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 de termined from the TRAPDOR profiles for the ultrastable HY, HZSM-5 and mordenite zeolites, respectively. The Q(CC)s calculated from the TRAPD OR curves are usually larger than 10 MHz for both Bronsted-acid sites (SiOHAI) and non-framework aluminum species in the three zeolites. Thr ee narrow peaks at 54, 30 and 0 ppm are separately superimposed on a b road hump in the Al-27 MAS spectra of the three dehydrated zeolites, w hile the latter is associated with the 'NMR invisible' Al. The NMR exp erimental results suggest that the three kinds of aluminum species (no n-framework aluminum species, Bronsted-and Lewis-acid sites) are all r esponsible for the resonance of the broad hump in dehydrated zeolites, which makes it difficult to explain the Al-27 MAS spectra. Fortunatel y, the TRAPDOR NMR provides a direct method for individually studying different aluminum species with large Q(CC)s via their dipolar couplin g to nearby proton nuclei. Published by Elsevier Science B.V.