Maxwell-Stefan diffusivities, Di, of gases in all-silica zeolites, MFI, AFI, FAU, CHA, DDR and LTA, were determined using molecular dynamics simulations for a range of molar loadings, qi. In all cases the Di were strongly dependent upon qi. For a given molecule, the Di versus qi behavior depended upon the zeolite structure and could exhibit either a decreasing or increasing trend; dictated by the molecular dimensions. For diffusion in AFI, FAU and MFI, the Di commonly decreased with qi for all molecules. For zeolites such as CHA, DDR and LTA, which consisted of cages separated by narrow windows, the Di for strongly confined molecules such as Kr and CH4 commonly showed an increase with qi; reaching a maximum before decreasing by a few orders of magnitude as the saturation loading was approached. For binary mixtures, correlation effects caused the more mobile species to be slowed down, and the slower species to be speeded-up. The Maxwell-Stefan equations provided a convenient framework for quantifying these effects. For a given molecule, the correlation effects were dependent upon the zeolite structure, pore size and connectivity. Correlation effects were particularly strong in AFI, FAU and MFI, but were relatively weak in LTA, CHA and DDR because the narrow windows permitted the passage of only one molecule at a time. The correlation effects also depended upon the degree of confinement within a given zeolite. For weak confinement, as in the case of small molecules such as He, Ne and H2, correlation effects were significant even for LTA, CHA and DDR.

Insights into Diffusion of Gases in Zeolites Gained from Molecular Dynamics Simulations. Krishna, R., van Baten, J.M.: Microporous and Mesoporous Materials, 2008, 109[1-3], 91-108