Thermally regenerable metal-organic framework with high monovalent metal ion selectivity

Separation of monovalent metal ions is essential for metal extraction and mining such as lithium salt production for battery manufacturing and other industrial applications, but it is challenging. In this study, we develop a cation selective and thermally regenerable metal-organic framework (MOF) for efficient separation of monovalent metal ions. Specifically, a crosslinked poly(N-isopropylacrylamide-co-acryloylamidobenzo-18-crown-6) functionalized MOF-808 (pNCE/MOF-808) preferentially adsorb Na⁺ and K⁺, leaving Li⁺ behind at ambient temperature, due to the size-match effect, ion dehydration effect, and stability constant of cation-benzo-18-crown-6 complex. It shows high adsorption capacity of 1.04 mmol g⁻¹ and excellent selectivity up to 29.4 for Na⁺/Li⁺ and 34.4 for K⁺/Li⁺ in the mixed solution contained 0.5 M LiCl, 0.5 M NaCl, and 0.5 M KCl. Importantly, pNCE/MOF-808 with adsorbed Na⁺ and K⁺ ions could be efficiently regenerated at mild temperature (e.g., 45 °C) in water. This work firstly provides an effective way to functionalize MOFs with thermoresponsive property for high ion selectivity and chemical-free regeneration. pNCE/MOF-808 holds promise for further development for environmentally friendly monovalent ion separations such as lithium ion purification.