HYDROGEN-BONDING OF METHANOL WITH BRIDGED OH GROUPS OF ZEOLITES - AB-INITIO CALCULATION, H-1-NMR AND FTIR STUDIES

FTIR spectroscopy at ambient temperature, broad-line H-1 NMR spectrosc opy at 4 K, and magic angle spinning (MAS) spectroscopy at ambient tem perature, together with ab initio calculations at the Hartree-Fock and the second-order perturbation theory levels on the skeleton fragment HO-SiH2-O-SiH2-OH-AlH2-O-SiH2-OH are used to study interaction complex es of methanol-d(3) with bridging hydroxyls of zeolites (represented b y H-mordenite and HZSM-5). The two-site neutral hydrogen-bonded methan al complex with bridging hydroxyl Si-OH-Al and the zeolite skeletal ox ygen, which is predicted by the theory, is confirmed by the experiment al observations provided that the number of adsorbed molecules is less than the number of bridging OH groups (Theta(OH) < 100%). Simulation of H-1 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 b ridging hydroxyls at very low OH coverage is found at 8.6 ppm, which i s below the value reported for methoxonium ion. Using the Fermi resona nce theory, the seven bands of OH groups observed in the infrared spec tra allows us to determine the fundamental stretching and bending vibr ations of bridging hydroxyls (1810-1790, 1375-1370, 880-866 cm(-1)) an d OH groups of the methanal (3560-3555, 1375 cm(-1)) in the surface co mplex. 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 experimen t still proves that the zeolitic hydrogen is much closer to the center between methanol and zeolite oxygens, and the methanal OH group is on ly little affected by the skeletal oxygen. The clusters of methanol ad sorbed on bridging hydroxyls and affected by skeletal oxygens appear i n methanol excess ((Theta(OH) 100-280%). The zeolite increases the cha rge of the cluster and the hydrogen bonding, which is higher than that for the liquid methanol.