Sequential imine exchange enables large-area, high-performance COF separation membranes

Membrane separation offers an energy-efficient route for resource recovery, purification, and pollution control. For fast and precise molecular separation, membrane materials require both highly ordered transport pathways and effective defect control. COFs are particularly attractive because their crystalline frameworks contain well-defined and periodically arranged pores. Yet this structural advantage also creates a fabrication challenge. Highly crystalline COFs are generally dominated by rigid frameworks, while weak interactions between neighboring crystalline domains can lead to intercrystalline defects and poor membrane continuity. In addition, imine bond formation occurs much faster than the subsequent structural rearrangement required for crystallization, creating a kinetic mismatch that can generate amorphous connections or structural discontinuities.

To address this problem, the researchers developed a sequential dynamic imine exchange strategy that decouples crystallization from membrane formation. In the first step, an aromatic amine serving as the COF framework building unit undergoes imine exchange with an energy gap of 1.77 kcal mol⁻¹. The relatively low energy gap enables highly reversible bond exchange and promotes rapid formation of an ordered crystalline framework. In the second step, hyperbranched polyethyleneimine (PEI), which contains abundant amino groups and has a higher exchange energy gap of 4.54 kcal mol⁻¹, participates in a slower exchange process. Its flexible branched structure connects neighboring crystallites and repairs intercrystalline defects, improving membrane continuity without sacrificing the established crystalline framework.

The imine exchange process also provides greater reversibility than conventional imine condensation. This enhanced reversibility helps the framework avoid kinetic trapping and facilitates rapid structural reorganization during crystallization. After only 3 h of imine exchange, the initially cast film was transformed into a highly crystalline COF membrane. With an appropriate PEI content, crystallinity was retained while membrane-forming ability was significantly improved. The simple casting and exchange process further enabled the preparation of membranes with areas exceeding 290 cm² under mild conditions.

Structural characterization revealed how PEI contributes to defect control. An appropriate amount of hyperbranched PEI promoted more uniform dispersion of crystalline seeds and strengthened the connections between adjacent COF crystallites. Its abundant amino groups and flexible chains helped reduce intercrystalline defects and stress concentration. As the PEI amount increased to 40 μL, the membrane showed improved mechanical performance, with a tensile strength of 18.9 MPa and an elongation at break of 56.2%. Excess PEI, however, disrupted ordered framework reconstruction and occupied accessible pores, demonstrating the importance of balancing crystalline order with flexible intercrystalline connections.

The optimized IELZU1-PEI membrane translated this structural control into strong nanofiltration performance. It achieved a water permeance of 344 ± 10 L m⁻² h⁻¹ bar⁻¹ while maintaining more than 99.9% rejection of Congo red. Methanol permeance reached 462 ± 22 L m⁻² h⁻¹ bar⁻¹. Separation tests with dye molecules of different molecular weights indicated that the dominant mechanism was size exclusion rather than simple adsorption. The membrane also maintained high dye rejection over 10 filtration cycles and showed no significant loss in rejection during 16 h of continuous operation.

The advantages became even more apparent in mixed-dye separation. Separation factors reached 714 for MO/CBB, 528 for MO/EBT, and 729 for AB25/CR mixtures. The strategy was also extended to other COF frameworks with theoretical pore sizes ranging from approximately 14 to 24 Å. These membranes maintained high crystallinity and showed pore-size-dependent transport behavior, demonstrating that sequential imine exchange is not limited to a single COF chemistry. Large membranes with areas above 290 cm² were also successfully prepared, supporting the scalability of the method.

By assigning crystallization and defect repair to two sequential imine-exchange processes, this work provides a practical route to reconcile high crystallinity with defect-free membrane formation. The strategy combines rapid fabrication, high permeance, strong molecular selectivity, and large-area processability, offering a versatile platform for the development of COF membranes for precise molecular separation and related membrane-based applications.

Title: Covalent Organic Framework Membranes through Sequential Imine Exchange for Precise Molecular Separation

Journal Name: Nano-Micro Letters

DOI: https://doi.org/10.1007/s40820-026-02287-5

URL: https://scholarcast.cast.org.cn/top-news/2104563854727942144