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.
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