Adsorptive separation of xenon/krypton mixtures using ligand controls in a zirconium-based metal-organic framework

A Zr-based metal-organic framework (MOF), UiO-66, is considered to be one of the most stable MOFs and has recently been shown to be a good candidate for adsorptive separation of Xe/Kr mixtures. Since the ability to impart chemical functionalities is an attractive characteristic of MOFs, herein, we prepared a series of functionalized UiO-66 materials (UiO-66-NH2, UiO-66-(OMe) 2, UiO-66-NH2(OMe)(2), and UiO-66-F-4) by incorporating several polar functionalities into UiO-66. We found that series of electron-rich and electron-deficient UiO-66 materials have stronger interactions with Xe and Kr atoms than pristine UiO-66. We also found that Xe/Kr selectivity increases with the electron density of the ligand. Although there was a reduction in surface area, UiO66- NH2(OMe) 2 exhibited the highest Xe/Kr Henry's constant ratio (14.4), possibly due to having the highest electron density as well as an unsymmetrical arrangement of functional groups of the ligands. Furthermore, the breakthrough experiments show that UiO-66-NH2(OMe)(2) can effectively separate Xe from Kr under dynamic mixture conditions and can be easily regenerated under a mild regeneration condition.