Hexane Isomers in Faujasite: Anomalous Diffusion and Kinetic Separation

Adsorption and diffusion properties of the five hexane isomers in zeolite Y have been obtained from molecular dynamic simulations. The self-diffusivities (D_s) of the isomers exhibit an anomalous dependence on their molecular diameter, where the larger-sized doubly branched hexane isomer 2,2- dimethylbutane shows the maximum D_s among all the isomers. This anomalous dependence of D_s as well as the computed activation energies E_a are in excellent agreement with the predictions of the levitation effect. These findings also explain the trends in E_a of hexane isomers in zeolite BEA, which was observed by Bárcia et al. in their experimental study. The order of exit of different isomers from a zeolite Y column depends on the order of their D_s at different temperature ranges. n-Hexane shows a tendency to bend by increasing its gauche conformer population in order to reduce the energetic barrier it experiences at the 12-ring window of the zeolite Y cage.