1H broad-line and MAS NMR: application to the study of acid sites of desilicated zeolite ZSM-5

Broad-line¹H NMR study of desilicated zeolite ZSM-5 was carried out as a function of the number of adsorbed water molecules in amounts lower than or equal to that of the Brønsted 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 Brønsted acid sites per unit surface area. On the other hand, by using MAS NMR, bridging Brønsted acid sites hydrogen-bonded to the zeolite framework were identified on the anhydrous surface of the desilicated ZSM-5. Moreover, MAS NMR spectra of the desilicated zeolite partially rehydrated showed the presence of some Lewis acid sites.