Hydrogen Bonding of Methanol with Bridged OH Groups of Zeolites: Ab Initio Calculation, 1H NMR and FTIR Studies

Abstract

FTIR spectroscopy at ambient temperature, broad-line 1H 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−SiH2−O−SiH2−OH−AlH2−O−SiH2−OH are used to study interaction complexes of methanol-d3 with bridging hydroxyls of zeolites (represented by H-mordenite and HZSM-5). The two-site neutral hydrogen-bonded methanol 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 (ϑ(OH) < 100%). Simulation of 1H 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 ...