Heteroatom-Mediated Covalent Organic Framework Membranes With Nitrogen-Enriched Nanochannels for Efficient Desalination.

Covalent organic framework (COF) membranes synthesized via scalable interfacial polymerization (IP) show significant promise for water purification and desalination. However, random monomer diffusion and rapid polymerization kinetics often result in limited crystallinity and disordered pore architectures within COF membranes, compromising their separation efficacy. Here we address this challenge by introducing a heteroatom precursor engineering strategy to develop ultra-microporous, highly crystalline COF membranes tailored for monovalent ion rejection. The strategic nitrogen substitution within the aromatic ring of hydrazide monomers enables gradient reduction of their oil-water partition coefficients and reactivity, thereby finely reducing the interfacial reaction kinetics. This inhibited diffusion-reaction behavior facilitates the self-healing of nascent polymeric chains into ordered networked structures, yielding crystalline nanofilms with controlled ultrafine pore width (0.40-0.68 nm). Augmented by hetero-nitrogen-induced electrostatic interaction and hydrogen-bond recognition, the resulting ultra-microporous COF (Tp-PDHz) membranes exhibited exceptional monovalent-ion removal capability (e.g., RNaCl = 97.5%, RLiCl = 98.0%), and a high water permeance of 1.2 L m-2 h-1 bar-1. Molecular dynamics simulations reveal that nitrogen-enriched nanochannels accelerate water transport via reinforced hydrogen-bond networks while simultaneously increasing the energy barrier for ion permeation. This work provides a molecular-level roadmap for the rational design of high-performance COF membranes in next-generation water desalination and purification.