Breathing-Assisted Selective Adsorption of C₈ Alkyl Aromatics in Zn-Based Metal-Organic Frameworks

The breathing phenomenon in metal-organic frameworks (MOFs) has revealed supramolecular host-guest interactions that could be beneficial for chemical separation in numerous industrial applications. The cost-effective purification of C8 alkyl aromatics such as o-xylene, m-xylene, p-xylene, and ethylbenzene remains challenging owing to their similar molecular structures, boiling points, kinetic diameters, polarities, etc. Herein, we report two Zn-based pillar-bilayered MOFs, denoted [Zn2 (aip)2 (pillar)] (aip=5-aminoisophthalic acid; pillar: bpy=4,4'-bipyridine or bpe=1,2-bis(4-pyridyl)ethane) that exhibit a breathing effect depending on the adsorbed guest molecules. Guest-dependent sorption studies in organic solvents such as N,N-dimethylformamide, methanol, benzene, and water vapor display reversible structural flexibility through the breathing effect in both framework compounds. The experiments conducted on C8 -alkyl aromatics resulting in both MOF compounds can access these isomers in the shrunken pores, and thereby expand the pore size by framework breathing. In C8 binary mixtures, these Zn-MOFs exhibit selective sorption properties based on the different interactions between guest C8 aromatics and the framework structure.