Nitrogen-Rich Angstrom Channels within Covalent Triazine Framework Membrane Enable Efficient Acid Recovery

Membranes tailored for selective H⁺ transfer are highly demanded in various fields such as acid recovery and proton exchange membranes. Emerging framework materials featuring permanent micropores present more competitive selectivity than traditional polymeric membranes. However, it remains challenging to construct angstrom channels for more precise ion separations. Herein, we demonstrate the modulation of the nitrogen-rich angstrom channels within a covalent triazine framework (CTF) membrane by a mix-monomer copolymerization strategy, in which one monomer provides defect-free angstrom channels and another offers plentiful nitrogen sites. The abundant nitrogen sites with strong affinity for H⁺ facilitate fast H⁺ diffusion, and their high protonation level in acid solution imparts positive charge, enabling efficient Fe²⁺ retention via Donnan exclusion. The optimized CTF membrane achieves a H⁺ dialysis coefficient of 1.5 × 10^-3 m/h and a separation factor of 11,242 for H⁺/Fe²⁺ mixtures. The ion selectivity outperforms most reported membranes benefiting from its highly confined channels. Additionally, the robust stability of the triazine groups guarantees consecutive operation in aggressive acidic solutions. This work presents an effective approach for modulating proton transport efficiency through membranes and its potential applications in acid recovery.