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.