Efficient Calculation of Gas Diffusivity in Single-Component and Binary Mixtures of Spherical Adsorbates in Flexible 8MR Zeolites

We use two methods, the changing snapshot method and transition state theory (TST)/snapshot method, to characterize the effects of zeolite framework flexibility on diffusion of spherical molecules in 8MR zeolites. These methods are applied to noble gases (Ne, Ar, Kr, Xe, and Rn) and CF4. We demonstrate the effect of the zeolite framework flexibility on diffusion by considering the size and loading of adsorbates and temperature. In both the methods, we approximate flexible structures as a set of discrete rigid snapshots obtained from molecular dynamics simulations of an empty framework. We show that the diffusivities predicted with these two efficient methods agree with direct MD simulations in the fully flexible structures. We studied in detail how the framework flexibility affects the loading dependence of diffusion. By looking at the computational costs, we demonstrated that both the methods are orders of magnitude more efficient than the fully flexible simulations. We then apply the changing snapshot method to binary mixtures of adsorbates to obtain accurate binary diffusivities and binary selectivities.