Influence of isotherm inflection on diffusion in silicalite

Abstract

Asorption isotherms of benzene, p-xylene, n-hexane, n-heptane and branched alkanes in silicalite show inflection behaviour; this behaviour is adequately modelled using a dual-site Langmuir model. In this model we make a distinction between two sites with different sorption characteristics: (1) site A which refer to the intersections between the straight channels and the zig-zag channels, and site B which refers to the channel interiors (straight or zig-zag channels). Using the Maxwell-Stefan theory of diffusion in zeolites, the influence of the isotherm inflection on the Fick diffusivity of pure components in silicalite is shown to be characterized by two extrema. For a mixture of n-hexane and 3-methyl pentane, the dual-site Langmuir model for the mixture predicts a curious maximum in the loading of the branched alkane; this mixture behaviour is confirmed by Configurational-Bias Monte Carlo simulations. The Maxwell-Stefan theory is used to demonstrate the possibility of separating the hydrocarbon isomers by permeation across a silicalite membrane.