Inflection in the loading dependence of the Maxwell-Stefan diffusivity of iso-butane in MFI zeolite

Configurational-bias Monte Carlo simulations of the adsorption isotherm for iso-butane (iC4) in MFI zeolite shows a strong inflection at a loading Θ = 4 molecules per unit cell. The consequence of the isotherm inflection on the loading dependence of the Maxwell-Stefan (M-S) diffusivity, Ð, was investigated using infra-red microscopy (IRM). The experimental data show that Ð decreases nearly linearly as Θ is increased to 4; further increase in Θ results in a sharp increase in Ð, with a cusp at Θ = 4. Kinetic Monte Carlo (KMC) simulations are used to rationalize the experimental observations. The simulations indicate that significant inter-molecular repulsions are present for Θ > 4.