TRANSITION-STATE STUDIES OF XENON AND SF6 DIFFUSION IN SILICALITE

Self-diffusion coefficients for infinitely dilute spherical sorbate molecules in the zeolite silicalite are computed by transition-state theory. The diffusion process is modeled as a series of uncorrelated jumps between potential minima (sites) determined from a Lennard-Jones representation of the silicalite lattice. Rate constants for jumping between different sites within the lattice are computed by transition-state theory and by the dynamically corrected transition-state theory of Voter and Doll for both a flexible and a rigid zeolite lattice model. Diffusivities are then determined from the rate constants by generating continuous-time/discrete-space Monte Carlo random walks. The computed diffusivities are shown to be in good agreement with molecular dynamics calculations performed on an identical model in the infinite dilution limit at low temperatures and with available experimental results. The transition-state theory and dynamically corrected transition-state theory methods afford computational savings of up to 2 orders of magnitude relative to full molecular dynamics simulations. Shortcomings in the various algorithms and zeolite models are discussed.