Insights into diffusion of gases in zeolites gained from molecular dynamics simulations

Abstract

The Maxwell-Stefan (M-S) diffusivities D_i of a variety of gases (He, Ne, Ar, Kr, H₂, N₂, CO₂, CH₄) in six different all-silica zeolite structures (MFI, AFI, FAU, CHA, DDR and LTA) have been determined using Molecular Dynamics (MD) simulations for a range of molar loadings, q_i. In all cases the D_i are strongly dependent on q_i. For a given molecule the D_i vs. q_i behavior depends on the zeolite structure and can exhibit either a decreasing or increasing trend, dictated by molecular dimensions. For diffusion within the AFI, FAU and MFI the D_i commonly decreases with q_i for all molecules. For zeolites such as CHA, DDR and LTA that consist of cages separated by narrow windows the D_i for strongly confined molecules, such as Kr and CH₄, commonly shows an increase with q_i, reaching a maximum before decreasing by a few orders of magnitude as saturation loading is approached. For binary mixtures, correlation effects cause the more mobile species to be slowed down, and the tardier species to be speeded-up; the Maxwell–Stefan equations provide a convenient framework for quantifying these effects. For a given molecule, correlation effects are dependent on the zeolite structure, pore size and connectivity. Correlation effects are particularly strong in AFI, FAU and MFI; they are relatively weak in LTA, CHA and DDR because the narrow windows allow the passage of only one molecule at a time. Correlation effects also depend on the degree of confinement within a given zeolite. For weak confinement, as is the case for small molecules such as He, Ne and H₂, correlation effects are significant even for LTA, CHA and DDR.