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.

研究人员通过分子动力学模拟（Molecular Dynamic Simulations），获取了Y型分子筛（Zeolite Y）内五种己烷异构体的吸附与扩散性能。
该类异构体的自扩散系数（self-diffusivities，简称Ds）随分子直径呈现反常依赖关系：尺寸更大的双支链己烷异构体2,2-二甲基丁烷，在所有异构体中拥有最高的自扩散系数。
该自扩散系数的反常依赖特性，以及计算得到的活化能（activation energies，简称Ea），与悬浮效应（levitation effect）的预测结果高度吻合。
上述研究发现同时解释了巴西亚等人（Bárcia et al.）在实验研究中观测到的BEA型分子筛（Zeolite BEA）内己烷异构体活化能的变化趋势。
不同己烷异构体从Y型分子筛色谱柱的流出顺序，取决于其在不同温度区间内的自扩散系数排序。
正己烷会通过提升邻位交叉构象（gauche conformer）的占比，降低其在Y型分子筛笼（Zeolite Y cage）的12元环窗口（12-ring window）处遭遇的能垒，进而发生构象弯曲。