Nonthermal transport of small sorbates in zeolites: Chaotic dynamics and long jumps

Abstract

In some molecular systems, the dominant driving force for transport is not thermal noise from lattice vibration or other sources, but low-dimensional deterministic chaos. We consider this deterministic transport for an example of diffusion of methane sorbate in zeolite AlPO4-5. In this system, the chaotic motion of the sorbate is due to nonlinear coupling between its longitudinal and azimuthal degrees of freedom. Assuming ergodicity of the sorbate motion, we develop a quantitative RRKM-type theory for the sorbate transport. The theoretical predictions for the escape rate of the sorbate from the zeolite cage are in good agreement with molecular dynamics simulations. We observe that, in addition to ergodic mixing of the sorbate degrees of freedom when the sorbate is trapped inside a zeolite cage, long ballistic flights are an important aspect of the sorbate dynamics. We investigate the complicated interplay between the ergodic trapping and the multisite flights, where the ergodicity assumption breaks down, with a kinetic Monte Carlo scheme which offers a diffusivity estimate that includes contribution from ballistic flights.