Adsorption of Methanol on Brønsted 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 1 H, 2 H, 13 C, and 27 Al 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 molecules per SiOHAl group, respectively. At T ≥ 295 K, the hydroxyl protons of adsorbate complexes formed by one methanol molecule per SiOHAl group exhibit a high mobility and undergo a rapid chemical exchange. The 1 H MAS NMR shifts (δ 1H = 4.1 and 14.2 ppm) of the OH groups after adsorption of CD 3 OH and the 2 H quadrupole coupling constants (QCC = 230 and 140 kHz) of the OD groups after adsorption of CH 3 OD, obtained at T ≤ 153 K, correspond to those of rigid and hydrogen-bonded molecules. The 27 Al quadrupole coupling constants of AlO 4 tetrahedra involved in the local structure of bridging OH groups interacting with one and three methanol molecules (QCC = 8.0 and 2.5 MHz, respectively) depend on the number of adsorbate molecules and indicate a partial deprotonation of SiOHAl groups in the complexes formed by three methanol molecules per SiOHAl group.