Study of diffusion of guest molecules through zeolite pores using methods of quantum chemistry, computer simulations, and pulse field gradient-nuclear magnetic resonance experiments

Abstract

Quantum chemical calculations at Hartree Fock and the second order Moller-Plesset perturbation levels with a basis set of 6-31G* have been performed to investigate water/silicalite-1 and methane/silicalite-1 interactions. The silicalite crystal structure has been represented by three fragments, in which the chemical compositions are O[30]Si[22]H[44] and O[35]Si[29]H[58]. The results indicate how the water molecule moves and turns, "rolling movement", in order to search for the optimal route via diffusion through the center of the silicalite pore. Preferential binding sites with the corresponding binding energies have been intensively evaluated for water molecules but not for methane. Subsequently, water/silicalite-1 and methane/silicalite-1 potential functions have been developed from 1,032 fragment-water and 150 fragment-methane interactions, respectively. The newly developed pair potentials have been applied for series of molecular dynamics simulations in order to evaluate structural and dynamics properties of water and methane molecules in silicalite-1. The results show that structures and diffusion coefficients of guest molecules in silicalite-1 channels change dramatically as a function of loading and temperature. In addition, a formation of 'low density water cluster' has been detected. The PFG NMR diffusion measurements of water in silicalite-1 samples have been also performed and the results are in satisfactory agreement with those obtained from the simulations.