A hierarchical approach to the molecular modeling of diffusion and adsorption at nonzero loading in microporous materials

A new hierarchical approach is presented for the modeling of small molecules at nonzero concentrations in microporous materials. This approach is complementary to other methods recently appearing in the literature; it is targeted for systems with pores that are well defined, large enough to host multiple molecules, and energetically uncorrugated in the interior. Statistical mechanical partition functions are calculated on molecular-level models and used as input to coarse-grained models, to predict both adsorption isotherms and self-diffusion coefficients. Certain physically reasonable simplifying approximations are employed to make the partition functions tractable. The approach is demonstrated on the model system of methane in siliceous zeolite ZK4 at 300 K, and the results are judged in comparison to those from traditional grand canonical Monte Carlo and molecular dynamics simulations. The adsorption isotherm is predicted to a high degree of accuracy across a large pressure range. The predicted trends in the self-diffusion coefficient are in qualitative agreement with the molecular dynamics results, but there is some quantitative disagreement at the lowest and highest adsorbate loadings.