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.