Multinuclear solid-state NMR studies of Brmsted sites in zeolites

Two NMR techniques, zH MAS NMR and echo Fourier 27AI NMR, have been applied for the first time to the investigation of Bronsted sites in dehydrated hydrogen forms ofzeolites. The “AI signal of SiOHAl sites, which was “NMR-invisible” until now, yields a quadrupole coupling constant C,,= 16 MHz for the dehydrated zeolite H-ZSM-5. The values of C,, determined from the ZH MAS NMR spectra of SiODAl sites increase with the framework aluminium content of the zeolite from 208 kHz (H-ZSM5) to 236 kHz (H-X and H-Y) due to the decreased acid strength of the bridging OH groups. The state of the art of the two techniques, zH MAS NMR and ‘H MAS NMR, applied to the study ofbare Brensted sites, is compared.