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

Purification of the C 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 Co(dobdc) (dobdc = 2,5-dioxido-1,4-benzenedicarboxylate) and Co( m-dobdc) ( m-dobdc = 4,6-dioxido-1,3-benzenedicarboxylate) for the separation of xylene isomers using single-component adsorption isotherms and multicomponent breakthrough measurements. Remarkably, Co(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 Co(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 C 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 M(dobdc) structure, Co(dobdc) exhibits an unexpected structural distortion in the presence of either o-xylene or ethylbenzene that enables the accommodation of additional guest molecules.