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 (Ds) 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 Ds among all the isomers.
This anomalous dependence of Ds as well
as the computed activation energies Ea are in excellent agreement with the predictions of the levitation
effect. These findings also explain the trends in Ea 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 Ds 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.