Configurational entropy effects during sorption of hexane isomers in silicalite

Santilli (J. Catal. 99, 335-341, 1986) has presented experimental data for sorption of hexane isomers, n-hexane (n-C6), 3-methylpentane (3MP), and 2,2-dimethylbutane (22DMB), in various zeolites. The experimental data on the loadings of the isomers at various temperatures using silicalite are particularly intriguing; 3MP and 22DMB are both seen to exhibit maxima in the sorption loadings with increasing temperature. We rationalise the experimental results by performing configurational-bias Monte Carlo simulations of sorption isotherms at various temperatures and pressures. While the linear n-C₆ can locate at either the straight or zig-zag channels, configurational effects make the branched isomers locate only at the intersections. At a total mixture loading of four molecules per unit cell, all the intersections are fully occupied. Both branched isomers 3MP and 22DMB demand an extra "push" to locate within the channel interiors. Configurational entropy effects cause these branched isomers to be progressively "replaced" by n-C₆ in order to achieve higher mixture loadings. The net result is that the branched isomers exhibit maxima at a total mixture loading of four molecules per unit cell. We discuss how configurational entropy effects can be exploited to separate hexane isomers with high selectivities.