Harnessing crown ethers in separation membranes toward energy and environmental sustainability.

Membrane separation technology, as a green and efficient separation method, has been widely applied in chemical industry, energy conversion, water purification, environmental and other fields. The core challenge of precise membrane separation lies in developing membrane materials with high selectivity, high permeability and long-term stability. Crown ethers, as typical macrocyclic compounds with unique cyclic cavity structures, exhibit remarkable potential in precise separation owing to their molecular recognition capability, structural tunability, and chemical stability. Nevertheless, crown ethers suffer from inherent structural limitations that hinder their standalone membrane-forming capability, thus restricting their practical applications in membrane processes. In recent years, researchers have made breakthroughs by incorporating crown ethers into polymers, metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), and two-dimensional (2D) materials to construct hybrid membranes. This strategy not only compensates for the deficiencies of pure crown ether membranes, but also couples selective molecular recognition with efficient transport pathways, offering a general supramolecular approach to alleviate the permeability-selectivity trade-off in membrane technology. This review summarizes recent advances in crown ether-based membranes for precise membrane separation, focusing on four aspects: the intrinsic advantages of crown ethers for separation, the membrane fabrication strategies, underlying separation mechanisms, and their typical application scenarios. Finally, the challenges and future development trends in this field are prospected, aiming to provide insights for the further advancement and implementation of crown ethers in precise membrane separation.