Ordering Covalent-Organic Frameworks toward Next-Generation Nanofiltration

ConspectusNanofiltration (NF), regulating the pass and rejection of molecules and ions through membranes carrying sub-2 nm pores, has been used in numerous applications from water softening and Li⁺ extraction to pharmaceutical purification and petroleum fractionation. The presently used NF membranes are mostly made of cross-linked polyamides developed over 60 years ago, inherently suffering from tortuous transport paths and scattering pore sizes. Hopefully, covalent-organic frameworks (COFs) are opening the door to the next generation of NF and the corresponding membranes. Their inherent monodispersive sub-2 nm pore channels, customizable pore chemistry, and good chemical and thermal stability promise unprecedented selective and fast transport of molecules and ions, mitigating the frustrated "trade-off" effect between permeability and selectivity in traditional polyamide NF membranes.We first performed molecular dynamic simulations on COFs to decode how the pore structure and chemistry govern molecular and ionic transport behaviors. By analyzing the synergistic effect of pore-entrance sieving and pore-interior diffusion, we identified the critical factors affecting mass transfer such as pore sizes and geometry and charge density and surprisingly discovered that ideally structured COFs exhibit remarkable water permeance 1-2 orders of magnitude higher than traditional polyamide NF membranes. However, experimentally prepared thin films of aggregated COF crystallites usually contain defects, amorphous regions, and poorly defined grain boundaries, significantly weakening the selectivity of nanofiltration. Considering that COFs are practically condensation polymers, we envision that their structural imperfection can be fixed or minimized by regulating the condensation reactions or by changing the conformation of the formed polymer chains/networks. We manipulated the nucleation and crystallization kinetics via the controlled release of activators and temperature-swing synthesis. Monomers and crystallites can fully preorganize before epitaxial growth, ultimately forming highly crystallized, continuous films. Furthermore, we reorganized COF skeletons into the favored face-on orientation by vapor annealing to increase the mobility of the COF segments. The long-desired perpendicular alignment of COF films was thus realized, enabling sharpened rejection and simultaneously enhanced permeance compared with those of randomly oriented counterparts.To realize adaptive control over mass transport in specific NF applications, we incorporated dangling azobenzene units into COF skeletons as intrapore groups. Light activation induces precise regulation of pore sizes at the angstrom magnitude in a continuous manner. Furthermore, azobenzene isomerization allows drastic changes in intrapore polarity, which exerts a profound effect on the intermolecular affinity through dipole interactions. These distinctive features enable COF membranes to smartly sieve molecules or ions with close kinetic diameters.We conclude this Account by discussing the remaining challenges and future directions to be explored to push COFs from potential game changers to real leading players in the field of nanofiltration. This Account aims to provide insights into advancing next-generation NF membranes and may inspire growing interest in multidisciplines crossing reticular chemistry, porous materials, polymer science, and separation technologies.