ADSORPTION OF METHANOL ON BRONSTED ACID SITES IN ZEOLITE H-ZSM-5 INVESTIGATED BY MULTINUCLEAR SOLID-STATE NMR-SPECTROSCOPY

Adsorbate complexes formed by adsorption of methanol on zeolite H-ZSM- 5 have been investigated by H-1, H-2, C-13, and Al-27 NMR spectroscopy at temperatures between T = 425 K and T = 85 K. The data derived were explained by hydrogen-bonded neutral methanol molecules and partially protonated methanol clusters consisting of one and three methanol mol ecules per SiOHAl group, respectively. At T greater than or equal to 2 95 K, the hydroxyl protons of adsorbate complexes formed by one methan ol molecule per SiOHAl group exhibit a high mobility and undergo a rap id chemical exchange. The H-1 MAS NMR shifts (delta(1H) = 4.1 and 14.2 ppm) of the OH groups after adsorption of CD3OH and the H-2 quadrupol e coupling constants (QCC = 230 and 140 kHz) of the OD groups after ad sorption of CH3OD, obtained at T less than or equal to 153 K, correspo nd to those of rigid and hydrogen-bended molecules. The Al-27 quadrupo le coupling constants of AlO4 tetrahedra involved in the local structu re of bridging OH groups interacting with one and three methanol molec ules (QCC = 8.0 and 2.5 MHz, respectively) depend on the number of ads orbate molecules and indicate a partial deprotonation of SiOHAl groups in the complexes formed by three methanol molecules per SiOHAl group.