On the inflection in the concentration dependence of the Maxwell-Stefan diffusivity of CF4 in MFI zeolite

The recently published data on the Maxwell-Stefan diffusivity D of CF4 in MFI zeolite (J. Phys. Chem. B 2004, 108, 10613) shows an inflection at a loading Theta = 12 molecules per unit cell, corresponding to the inflection in the adsorption isotherm. We investigate the underlying reasons by performing kinetic Monte Carlo simulations and show that at Theta = 12, the molecular traffic along the zigzag channels comes to a virtual stand-still and further transport occurs only along the straight channels of MFI The inflection behavior of D is likely to be a generic characteristic of transport in nanoporous materials in which the adsorption of the diffusant occurs on multiple sites with widely different strengths.