MULTINUCLEAR NMR-STUDIES OF ACID SITES IN ZEOLITES

H-2 MAS NMR, echo Al-27 NMR and in addition H-1 and Si-29 MAS NMR have been applied to study acid sites of dehydrated zeolites. The Al-27 Signal of Si-OH-Al sites, which has been considered ''NMR-invisible'' in the past, yields a quadrupole coupling constant C-QCC = 16 MHz for zeolite H-ZSM-5 [1-3]. Now, non-framework aluminum species could also be observed in dealuminated and dehydrated zeolites H-ZSM-5 giving C-QCC approximate to 9 MHz. The two-dimensional H-2 MAS NMR spectra of the dehydrated and deuterated zeolites give information about both the chemical shift and the first-order quadrupole shift of the acid sites.