Precision-Engineered Crystalline Covalent Organic Framework Membranes with Staggered ABC Stacking for High-Performance Desalination

Covalent organic framework (COF) membranes hold immense potential for aqueous separations, yet their inherently large pore apertures and insufficient film crystallinity often limit their performance, particularly in challenging applications like water desalination. Here, we address these limitations by introducing an acid-modulated interfacial synthesis (AMIS) strategy to precisely engineer an ultramicroporous, highly crystalline Turing COF membrane. A detailed mechanistic investigation reveals that acetic acid forms hydrogen-bonded adducts with the hydrophilic aliphatic linker, oxalyl dihydrazide (ODH), finely tuning both its reactivity and diffusivity during Schiff base condensation with the linker, 1,3,5-triformylphloroglucinol (Tp). This modulated reaction-diffusion behavior not only facilitates the formation of a unique stripe-patterned Turing architecture but also enables sufficient defect self-correction via reaction retardation, yielding a COF film with high crystallinity. The resultant aliphatic ODH-COF membranes exhibit a unique ABC stacking mode and a sub-6-Å pore aperture, validated by experimental data and simulations. These characteristics, working in concert, enable the ODH-COF membranes to achieve record-high NaCl rejection of 99.7% with a water permeance of 0.82 L m^-2 h^-1 bar^-1, surpassing previously reported state-of-the-art COF membranes in pressure-driven separation processes. Coupled with robust fouling resistance and long-term stability, this work substantially advances COF membrane technology for sustainable and efficient water management.