Application of Free Energy Calculations at an Ultrahigh Temperature for Estimation of Molecular Diffusivities and Permeabilities in Zeolite Nanopores at an Ambient Temperature

Abstract

Molecular diffusivities and gas permeabilities through zeolite nanopores, which have been difficult to simulate directly from conventional molecular dynamics (MD), were estimated at an ambient temperature by performing the free energy calculation at an ultrahigh temperature. In this method, the hopping rate of a guest molecule is calculated based on transition state theory. Using these hopping rates, molecular self-diffusivities for a CH(4)/CF(4) binary mixture through an LTA-type zeolite, as well as those for each single component, are calculated at 300 K. The diffusivities of CF(4) are in the order of ca. 10(-14) m(2)/s at 300 K and thus are within an extremely slow molecular diffusion regime. Gas permeabilities of each single component at 300 K are also estimated by combining these calculated diffusivities with Fick's first law. For predicting CH(4) permeabilities, nonequilibrium MD is also applied for comparison, giving results within the same order, ca. 10(-12) molm/m(2)sPa. This methodology dramatically reduces computational time when predicting molecular diffusivity and gas permeability.