MULTINUCLEAR SOLID-STATE NMR-STUDIES OF BRONSTED SITES IN ZEOLITES

Two NMR techniques, H-2 MAS NMR and echo Fourier Al-27 NMR, have been applied for the first time to the investigation of Bronsted sites in dehydrated hydrogen forms of zeolites. The Al-27 signal of SiOHAl sites, which was ''NMR-invisible'' until now, yields a quadrupole coupling constant C(qcc) = 16 MHz for the dehydrated zeolite H-ZSM-5. The values of C(qcc) determined from the H-2 MAS NMR spectra of SiODAl sites increase with the framework aluminium content of the zeolite from 208 kHz (H-ZSM-5) 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, H-2 MAS NMR and H-1 MAS NMR, applied to the study of bare Bronsted sites, is compared.