Crown Ether-Functionalized Covalent Organic Framework Membranes for Precise Monovalent Cation Separation

Membrane-based separation offers a highly promising paradigm for the precise extraction of monovalent cations, which is critical for bioinspired engineering and advanced ion-separation applications. However, efficient discrimination among these cations remains profoundly challenging owing to their nearly identical valence states and comparable subnanometer hydrated diameters. Herein, we report the rational design and fabrication of crown ether (CE)-functionalized covalent organic framework (COF) membranes that achieve efficient mono/monovalent cation separation while maintaining rapid ion transport. The integration of CE moieties into the framework architectures plays a dual role: narrowing the channels to the angstrom scale while simultaneously serving as specific recognition sites for target cations through host–guest interactions. Consequently, the optimal CE-functionalized TFP-TDA-2 COF membrane exhibits an exceptional K+/Na+ selectivity of 64 with a K+ permeation rate of 41.2 mmol m–2 h–1, surpassing previously reported COF membranes. Theoretical simulations reveal that, while the COF channels provide rapid transport pathway, synergistic CE recognition substantially lowers the thermodynamic dehydration and translocation barrier for K+ relative to Na+. Furthermore, the modular nature of this strategy enables the specific recognition of other targeted cations through precise side-chain engineering by covalently anchoring different crown ether derivatives. This study demonstrates an effective structural engineering strategy for efficient monovalent cation separation, offering an approach for the development of next-generation, single-ion-selective membranes for broad real-world applications.