On the Inflection in the Concentration Dependence of the Maxwell−Stefan Diffusivity of CF 4 in MFI Zeolite

The recently published data on the Maxwell−Stefan diffusivity Đ of CF 4 in MFI zeolite ( J. Phys. Chem. B 2004, 108, 10613) shows an inflection at a loading ϑ = 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 ϑ = 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 Đ 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.