The self-diffusion of ethane in cation-free zeolites was investigated at various temperatures. The diffusivity decreased with increasing temperature between 150 and 300K for the low loading of one molecule per cage. The temperature dependence was found to be same for the rate of cage-to-cage crossing. The activation entropy of the rate was analyzed in order to explain the observations. The entropic barrier for cage-to-cage motion increased because heating the system moved ethane away from eight-ring windows.

On Entropic Barriers for Diffusion in Zeolites: a Molecular Dynamics Study. Schüring, A., Auerbach, S.M., Fritzsche, S., Haberlandt, R.: Journal of Chemical Physics, 2002, 116[24], 10890-4