H-1 BROAD-LINE AND MAS NMR - APPLICATION TO THE STUDY OF ACID SITES OF DESILICATED ZEOLITE ZSM-5

Broad-line H-1 NMR study of desilicated zeolite ZSM-5 was carried out as a function of the number of adsorbed water molecules in amounts low er than or equal to that of the Bronsted acid sites. The dissociation coefficient of the acid OH groups, currently associated with the acid strength, was shown not to be affected by the selective removal of Si from the zeolite ZSM-5 framework which resulted in more Bronsted acid sites per unit surface area. On the other hand, by using MAS NMR, brid ging Bronsted acid sites hydrogen-bonded to the zeolite framework were identified on the ''anhydrous'' surface of the desilicated ZSM-5. Mor eover, MAS NMR spectra of the desilicated zeolite partially rehydrated showed the presence of some Lewis acid sites.