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Embedding nanofillers into polyamide (PA) matrices is considered as a simple yet effective strategy for boosting the performances of thin-film nanocomposite (TFN) nanofiltration (NF) membranes. However, the NF performances are usually compromised by the inadequate interface compatibility between nanofillers and PA matrices as well as the anisotropic orientation of nanofillers. Herein, we propose a distinctive fabrication strategy for TFN membranes featuring excellent compatibility and highly-aligned microstructures. The reactive covalent organic framework (COF) nanosheets are chemically stitched at oil-water interfaces to form continuous nanofilms, and the PA matrices are then in-situ formed to encapsulate them, giving the TFN membranes. The stitched nanofilms contain two types of perpendicularly oriented pores: intrinsic apertures in COF nanosheets, and gaps among adjacent nanosheets. Together with the abundant reactive sites, the stitched nanofilms not only mediate the controllable synthesis of PA matrices, but also are covalently fused into the matrices. The aligned pores of the stitched nanosheets are capable of sieving salt ions, endowing TFN membranes with high separation precision and fast water permeation. This work demonstrates a strategy to control the orientation of COF nanosheets, which is highly desired in diverse fields not limited to membrane separation. Image 1 • Highly-aligned COF nanosheets are embedded into the PA matrices. • COF nanosheets are chemically stitched to form ultrathin nanofilms. • The stitched nanosheets function as a reaction mediator and a performance enabler. • The composite membranes exhibit a boosted water permeance and enhanced selectivity. [ABSTRACT FROM AUTHOR]

Tight ultrafiltration (TUF) membranes featuring specific pore sizes are increasingly developed to bridge the gap between nanofiltration and ultrafiltration. So far, a wealth of efforts has been devoted to tackle the limitation found in TUF, but there still lacks a facile accessibility to upgrade TUF membranes with simultaneously improved permeance and selectivity. Herein, we report a large-pore covalent organic framework (LP-COF) as the building material to prepare ultra-permeable TUF membranes for fast separations. The LP-COF layers with an exceptionally large aperture size of up to ~3.6 nm are synthesized on macroporous substrates through a unidirectional diffusion method. The resultant LP-COF layers show a moderate crystallinity and low thickness down to ~60 nm, and allow ultrafast water permeation. Surprisingly, the optimal LP-COF membrane exhibits an unprecedented water permeance of ~3147 L m−2 h−1 MPa−1 with a high Congo red rejection (~92.6%), which is basically unchanged after several cycles of filtration. Moreover, the large-pore channels enable an unimpeded pass of ions, thus affording the membrane an excellent dye/salt separation competence, and largely exceeding state-of-the-art membranes. Therefore, this work opens up a new opportunity in producing high-performance TUF membranes by large-pore COFs for rapid and precise separations. [Display omitted] • A large-pore covalent organic framework (LP-COF) is designed to prepare TUF membranes. • Local nucleation and interfacial polymerization collectively promote the growth of ultrathin LP-COF layers. • Ordered channels of LP-COF membranes promise fast separations with high selectivities. • The optimized LP-COF membrane shows a preeminent dye/salt separation performance. • The extraction of nanoparticles from aggressive solvents is realized by the LP-COF membrane. [ABSTRACT FROM AUTHOR]