Hydrogen bonding of methanol with bridged OH groups of zeolites:: Ab initio calculation, 1H NMR and FTIR studies

FTIR spectroscopy at ambient temperature, broad-line (1)H NMR spectroscopy at 4 K, and magic angle spinning (MAS) spectroscopy at ambient temperature, together with ab initio calculations at the Hartree-Fock and the second-order perturbation theory levels on the skeleton fragment HO-SiH(2)-O-SiH(2)-OH-AlH(2)-O-SiH(2)-OH are used to study interaction complexes of methanol-d(3) with bridging hydroxyls of zeolites (represented by H-mordenite and HZSM-5). The two-site neutral hydrogen-bonded methanal complex with bridging hydroxyl Si-OH-Al and the zeolite skeletal oxygen, which is predicted by the theory, is confirmed by the experimental observations provided that the number of adsorbed molecules is less than the number of bridging OH groups (Theta(OH) < 100%). Simulation of (1)H broad-line NMR indicates an average distance 193-200 pm between the zeolite and methanol hydrogens in magnetic interaction, which is in a reasonable agreement with ab initio calculations value 198.5-195 pm. The MAS NMR signal assigned to the exchange between methanol and bridging hydroxyls at very low OH coverage is found at 8.6 ppm, which is below the value reported for methoxonium ion. Using the Fermi resonance theory, the seven bands of OH groups observed in the infrared spectra allows us to determine the fundamental stretching and bending vibrations of bridging hydroxyls (1810-1790, 1375-1370, 880-866 cm(-1)) and OH groups of the methanal (3560-3555, 1375 cm(-1)) in the surface complex. The ab initio calculations on a large zeolite fragment show new possibilities of the localization of methanol on the zeolite surface. Nevertheless, compared to the theoretical calculations, the experiment still proves that the zeolitic hydrogen is much closer to the center between methanol and zeolite oxygens, and the methanal OH group is only little affected by the skeletal oxygen. The clusters of methanol adsorbed on bridging hydroxyls and affected by skeletal oxygens appear in methanol excess ((Theta(OH) 100-280%). The zeolite increases the charge of the cluster and the hydrogen bonding, which is higher than that for the liquid methanol.