Thermo-regenerable azacrown ether MOF materials for enhanced selective removal of Na⁺ over K⁺ in water

The efficient separation of Na⁺ and K⁺ in water is essential for addressing challenges such as high-salinity water treatment and resource recovery. Biological ion channels play a crucial role in separating Na⁺ and K⁺. Current biomimetic channels successfully separate monovalent/divalent ions by mimicking gating mechanisms and ion site recognition, yet achieving high Na⁺/K⁺ selectivity remains a challenge. In this work, we report thermally regenerable metal-organic frameworks (MOFs) for the selective separation of Na⁺ from K⁺, with 18-crown-6 and its derivatives with one or two nitrogen heteroatoms as binding sites. Nitrogen heteroatom induced cavity distortion of crown ether, while aza-18-crown-6 proved the highest Na⁺ selectivity. The resultant pNICE1N-MIL-53 demonstrated a Na⁺ adsorption capacity of 2.55 mmol·g⁻¹, and a Na⁺/K⁺ selectivity ranged from 8.00 to 77.14 in synthetic brines with K/Na ratios of 1 to 50. It also showed high Na⁺ selectivity of 4.31-15.53 over other alkali and alkaline earth metal ions. Driven by the thermo-responsive segment of poly(N-isopropyl acrylamide) (pNIPAM), the adsorbent achieved a regeneration of >99% in 50℃ water and excellent cycling performance. This work provides a fine regulation strategy to design ion-selective materials with high selectivity and green regeneration.