SOLID-STATE NUCLEAR-MAGNETIC-RESONANCE STUDIES OF ACID SITES IN ZEOLITES

H-2 Magic-angle spinning nuclear magnetic resonance (MAS NMR), echo Al-27 NMR, two-dimensional (2D) echo H-1 MAS NMR and H-1, Al-27 and Si-29 MAS NMR have been applied to study the acid sites of dehydrated zeolites. The quadrupole coupling constants C-QCC determined from H-2 MAS NMR spectra of Si-OH-Al sites increase with the framework aluminium content of the zeolites 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 Al-27 signal of the Si-OH-Al sites, which was considered ''NMR-invisible'' in the past, yields C-QCC=16 MHz for zeolite H-ZSM-5. The majority of non-framework aluminium species could also be observed in dealuminated and dehydrated zeolites H-ZSM-5 giving C(QCC)approximate to 9 MHz. 2D echo H-1 MAS NMR spectra yield values of 16-40 ms for the lower limits of the lifetime of hydroxyl species at room temperature. A lifetime of more than 40 ms was obtained from echo H-2 MAS NMR spectra for protonated sites giving a signal at ca. 6.5 ppm in partially dealuminated and weakly rehydrated samples.