Influence of isotherm inflection on diffusion in silicalite

Adsorption isotherms of benzene, p-xylene, n-hexane, II-heptane and branche d alkanes in silicalite show inflection behaviour; this behaviour is adequa tely modelled using a dual-site Langmuir model. In this model we make a dis tinction between two sites with different sorption characteristics: (I)site A which refer to the intersections between the straight channels and the z ig-zag channels, and site B which refers to the channel interiors (straight or zig-zag channels). Using the Maxwell-Stefan theory of diffusion in zeol ites, the influence of the isotherm inflection on the Fick diffusivity of p ure components in silicalite is shown to be characterized by two extrema. F or 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 C arlo simulations. The Maxwell-Stefan theory is used to demonstrate the poss ibility of separating the hydrocarbon isomers by permeation across a silica lite membrane. (C) 1999 Elsevier Science Ltd. All rights reserved.