Theoretical study on adsorption and proton exchange reaction of H 2O on H-form zeolite

Abstract

The mechanism of water adsorptions onto the H-form zeolite, as well as the following proton exchange reaction, has been studied by ab initio molecular orbital calculations at the Hartree-Fock and the 2nd-order Møller Plesset levels with basis sets of double-zeta plus polarization quality. The H-form zeolite is represented by clusters consisting of 3 ∼ 8 SiO 4(AlO 4) units. Some adsorption structures are located, and the normal mode analyses are performed for the respective structures to verify their geometric character and to determine the vibrational frequencies. The adsorption energies indicate that two types of complexes, i.e., a protonated form and a H-bonded form, are evenly probable as surface species, whereas the comparison of the calculated harmonic frequencies with the experimentally observed ones suggests that the adsorbed water forms the H-bonded complex. The potential energy surface related to the water adsorption and proton exchange processes is extensively investigated in terms of energy profiles along some energetically optimum paths.