VERIFICATION AND QUANTITATIVE-DETERMINATION OF A NEW-TYPE OF BRONSTED ACID SITES IN H-ZSM-5 BY H-1 MAGIC-ANGLE-SPINNING NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY

The existence of a new type of bridging hydroxyl group characterized by an additional interaction with the zeolitic framework has been postulated for the first time by Zholobenko et al. [Zeolites, 8 (1988) 175] on the basis of diffuse reflectance infrared (IR) spectroscopic measurements on H-ZSM-5 and mordenites. The aim of the present paper is to give evidence for a corresponding signal in the H-1 magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra of H-ZSM-5 and to determine the concentration of these hydroxyl groups. Bridging hydroxyl groups influenced by an additional interaction with the zeolitic framework exhibit the same strength of acidity as ''free'' bridging hydroxyl groups as could be shown by the study of their interaction with adsorbed CO molecules.