Incorporating the loading dependence of the Maxwell-Stefan diffusivity in the modeling of CH4 and CO2 permeation across zeolite membranes

Molecular dynamics (MD) simulations were carried out in order to gain some generic insights into the loading dependence of the Maxwell-Stefan (MS) diffusivity D-i of CH4 and CO2 in different zeolite topologies that can be divided into three classes consisting of (a) intersecting channels, (b) one-dimensional channels, and (c) cages separated by windows. The MD simulations demonstrate that D-i is generally loading dependent and reduces to zero at saturation loading. Furthermore, the loading dependence of D-i shows a qualitatively different trend for CO2 than for CH4. The loading dependence of the MS diffusivity is described using the model of Reed and Ehrlich (Surf. Sci. 1981, 102, 588-609), which accounts for the reduction in the energy barrier for hopping of molecules between adsorption sites due to intermolecular interactions. A unary permeation model accounting for the loading dependence is developed, and published data on permeation of CH4 and CO2 across MFI, CHA, and DDR membranes are used for quantitative validation. Implications for mixture permeation are also discussed.