Concentration and chainlength dependence of the diffusivity of alkanes in zeolites studied with MD simulations

Molecular dynamics simulations are used to study the self-diffusion of linear and branched alkanes in the zeolites Mordenite, ZSM-5, Ferrierite, and ZSM-22. A rigid zeolite model and a united atom model for the alkanes was used in these simulations. Even at maximum loading, no single-file diffusion behavior of the time dependency of the mean-square displacement was observed in the one-dimensional zeolites Mordenite and ZSM-22. This might be the result of the low activation barriers for the molecular motion in the channels, causing the molecules not to move in a "hopping-like" motion. Both the diffusion constant and activation energy strongly depend on zeolite loading. At high loadings, the motion is predominantly determined by the alkane-alkane collisions. The chainlength dependency of the diffusion constant and activation energy seems to indicate the occurrence of a resonant diffusion mechanism. These effects are stronger in the medium-pore zeolites, as the molecules are more constrained in these systems. Diffusion of i-butane and 2-methyl-pentane in Mordenite was found to be less than 1 order of magnitude slower than their linear counterparts, and diffusion barriers are only 4.7 and 6.3 kJ/mol, respectively. In ZSM-5 and ZSM-22, where the bulky iso-alkanes fit tightly into the pores, differences an as large as 3 orders of magnitude, and the activation energies are in the order of 30 kJ/mol.