Interplay of entropic and memory effects in diffusion of methane in silicalite zeolites

The role of entropic effects in methane distribution and transport in silicalite zeolites is studied using molecular dynamics in the limit of infinite dilution or small loading. Diffusive behavior and its anisotropy is assessed as a function of temperature where we find both an Arrhenius regime above 250 K and deviations thereof below such temperature. Using a previous probabilistic model, geometrical correlations or memory effects are evidenced and are shown to be enhanced as temperature is reduced. Deviations from Arrhenius behavior are concomitant with entropic effects. We find that, the preference of methane towards presence at intersections or channel centers changes at a threshold temperature. A discrete transition is found from a channel-center preferred phase, at low temperatures, versus an intersection preferred phase at high temperatures with evidence of hysteresis effects. Such entropic effects are also reflected, in diffusive transport, as non-Arrhenius-type behavior. A model based on accessible volume as a function of energy agrees with the simulated transition lending new insight into zeolite cavity design.