Diffusion of binary mixtures across zeolite membranes: entropy effects on permeation selectivity

Diffusion of binary mixtures across zeolite membranes is strongly influenced by the sorption behaviour of the individual components. When the components in the mixture have significantly different molecular sizes, size entropy effects tend to favour the sorption of the smaller species at high molecular loadings. This is the case, for example, for adsorption of methane and n-butane in silicalite. For mixtures of linear and branched alkanes having the same number of carbon atoms, configurational entropy effects tend to favour the sorption of the linear alkane becuase these molecules “pack” more efficiently inside the ordered zeolite structure. In estimating the permeation of mixtures across zeolite membranes, both size and configurational entropy effects need to be properly accounted for. A model to estimate the permeation fluxes across zeolite membranes is developed by combining the Real Adsorbed Solution Theory to describe the mixture sorption, and the Maxwell-Stefan diffusion equations. The utility of the model is demonstrated by means of two case studies: (1) permeation of methane and n-butane, and (2) permeation of n-hexane and 2,2 dimethylbutane across a silicalite membrane.