Transport Diffusivities of CH4, CF4, He, Ne, Ar, Xe, and SF6 in Silicalite from Atomistic Simulations

We have used atomistic simulations to examine the adsorption isotherms, self diffusivity, and transport diffusivity of seven light gases, CH 4, CF 4, He, Ne, Ar, Xe, and SF 6, adsorbed as single-components in silicalite at room temperature. By using equilibrium molecular dynamics, the self and transport diffusivities are computed simultaneously. For each species the self diffusivity decreases as pore loading is increased due to steric hindrance from other adsorbed molecules. In contrast, the transport diffusivity is an increasing function of pore loading for each species. Our results are the most extensive collection of transport diffusivities determined from atomistic modeling of adsorption in a zeolite to date, and they allow us to examine the accuracy of several common approximations to the loading-dependent diffusivities. Carefully converged results for the anisotropy of diffusion of CH 4, CF 4, He, Ne, Ar, Xe, and SF 6 in silicalite are presented. We discuss the implications of our results for understanding self and transport diffusivities in mesoporous materials and for multi-component mixtures in microporous materials.