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

Abstract The Maxwell–Stefan (M–S) diffusivities Đ i of a variety of gases (He, Ne, Ar, Kr, H 2 , N 2 , CO 2 , CH 4 ) 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 Đ i are strongly dependent on q i . For a given molecule the Đ 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 Đ 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 Đ i for strongly confined molecules, such as Kr and CH 4 , 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 H 2 , correlation effects are significant even for LTA, CHA and DDR.

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