Dynamically corrected transition state theory calculations of self-diffusion in anisotropic nanoporous materials.
We apply the dynamically corrected transition state theory to confinements with complex structures. This method is able to compute self-diffusion coefficients for adsorbate-adsorbent systems far beyond the time scales accessible to molecular dynamics. Two example cage/window-type confinements are examined: ethane in ERI- and CHA-type zeolites. In ERI-type zeolites, each hop in the z direction is preceded by a hop in xy direction and diffusion is anisotropic. The lattice for CHA-type zeolite is a rhombohedral Bravais lattice, and diffusion can be considered isotropic in practice. The anisotropic behavior of ERI-type cages reverses with loading, i.e., at low loading the diffusion in the z direction is two times faster than in the xy direction, while for higher loadings this changes to a z diffusivity that is more than two times slower. At low loading the diffusion is impeded by the eight-ring windows, i.e., the exits out of the cage to the next, but at higher loadings the barrier is formed by the center of the cages.
Dubbeldam
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Van't Hoff Institute for Molecular Sciences, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands. dubbelda@science.uva.nl
Beerdsen
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Calero
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Smit
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