Switching Adsorbent Layered Material that Enables Stepwise Capture of C₈ Aromatics via Single-Crystal-to-Single-Crystal Transformations

Separation of the C₈ aromatic isomers, xylenes (PX, MX, and OX) and ethylbenzene (EB), is important to the petrochemical industry. Whereas physisorptive separation is an energy-efficient alternative to current processes, such as distillation, physisorbents do not generally exhibit strong C₈ selectivity. Herein, we report the mixed-linker square lattice (sql) coordination network [Zn₂(sba)₂(bis)]_n·mDMF (sql-4,5-Zn, H₂sba or 4 = 4,4′-sulfonyldibenzoic acid, bis or 5 = trans-4,4′-bis(1-imidazolyl)stilbene) and its C₈ sorption properties. sql-4,5-Zn was found to exhibit high uptake capacity for liquid C₈ aromatics (∼20.2 wt %), and to the best of our knowledge, it is the first sorbent to exhibit selectivity for PX, EB, and MX over OX for binary, ternary, and quaternary mixtures from gas chromatography. Single-crystal structures of narrow-pore, intermediate-pore, and large-pore phases provided insight into the phase transformations, which were enabled by flexibility of the linker ligands and changes in the square grid geometry and interlayer distances. This work adds to the library of two-dimensional coordination networks that exhibit high uptake, thanks to clay-like expansion, and strong selectivity, thanks to shape-selective binding sites, for C₈ isomers.