Separation of Xylene Isomers through Multiple Metal Site Interactions in Metal–Organic Frameworks

Purification of the
C8 alkylaromatics o-xylene, m-xylene, p-xylene, and
ethylbenzene remains among the most challenging industrial separations,
due to the similar shapes, boiling points, and polarities of these
molecules. Herein, we report the evaluation of the metal–organic
frameworks Co2(dobdc) (dobdc4– = 2,5-dioxido-1,4-benzenedicarboxylate)
and Co2(m-dobdc) (m-dobdc4– = 4,6-dioxido-1,3-benzenedicarboxylate) for the separation
of xylene isomers using single-component adsorption isotherms and
multicomponent breakthrough measurements. Remarkably, Co2(dobdc) distinguishes among all four molecules, with binding affinities
that follow the trend o-xylene > ethylbenzene
> m-xylene > p-xylene. Multicomponent
liquid-phase
adsorption measurements further demonstrate that Co2(dobdc)
maintains this selectivity over a wide range of concentrations. Structural
characterization by single-crystal X-ray diffraction reveals that
both frameworks facilitate the separation through the extent of interaction
between each C8 guest molecule with two adjacent cobalt­(II)
centers, as well as the ability of each isomer to pack within the
framework pores. Moreover, counter to the presumed rigidity of the
M2(dobdc) structure, Co2(dobdc) exhibits an
unexpected structural distortion in the presence of either o-xylene or ethylbenzene that enables the accommodation
of additional guest molecules.

C8烷基芳烃（C8 alkylaromatics）涵盖邻二甲苯(o-xylene)、间二甲苯(m-xylene)、对二甲苯(p-xylene)与乙苯(ethylbenzene)，其纯化至今仍是工业分离领域极具挑战性的任务之一，根源在于这类分子的形状、沸点与极性均高度相似。本研究采用单组分吸附等温线与多组分穿透曲线测试方法，评估了金属有机框架（metal–organic frameworks, MOFs）Co₂(dobdc)（dobdc⁴⁻ = 2,5-二氧代-1,4-苯二甲酸根）与Co₂(m-dobdc)（m-dobdc⁴⁻ = 4,6-二氧代-1,3-苯二甲酸根）用于二甲苯异构体分离的性能。值得注意的是，Co₂(dobdc)可有效区分这四种组分，其结合亲和力遵循邻二甲苯(o-xylene) > 乙苯(ethylbenzene) > 间二甲苯(m-xylene) > 对二甲苯(p-xylene)的顺序。多组分液相吸附实验进一步证实，Co₂(dobdc)在宽浓度范围内均可维持该分离选择性。通过单晶X射线衍射（single-crystal X-ray diffraction）开展的结构表征显示，两种框架材料均通过双重机制实现分离：一是各C8客体分子与相邻两个钴(II)中心的相互作用强度，二是各异构体在框架孔道内的装填适配能力。此外，与此前学界预设的M₂(dobdc)结构刚性相悖，Co₂(dobdc)在邻二甲苯或乙苯存在时会发生意想不到的结构畸变，从而能够容纳额外的客体分子。