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

The Maxwell-Stefan (M-S) diffusivities D-i of a variety of gases (He, Ne, Ar, Kr, H-2, N-2, 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 H-2, correlation effects are significant even for LTA, CHA and DDR. (C) 2007 Elsevier Inc. All rights reserved.