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[Display omitted] • COF nanofilms are masked by loose polyamides to form heterostructured membranes. • The masked polyamides enhance membrane-forming ability of COF nanofilms. • The masking effect enables effective tune of the pore size. • The heterostructured membranes exhibit sharp selectivities to small molecules. Covalent organic frameworks (COFs) with uniform, tunable nanoporosities are promising alternatives to prepare nanofiltration membranes with homogeneous nanochannels for high-resolution molecular sieving. Due to the poor membrane-forming ability as well as the relatively inadequate strategies for pore engineering, thus far, the preparation of continuous and defect-free COF membranes with tunable pore sizes is still challenging. Herein, we demonstrate an ingenious strategy to prepare the heterostructured membranes featuring the loose polyamide networks masked COF nanofilms for precise sieving of small molecules. The loosely structured polyamides are created by the interfacial polymerization to mask the chemically stitched (chemically interlinked) ultrathin COF nanosheets. The masked polyamide with flexible, loose networks not only enhance the membrane-forming ability of the natively rigid ultrathin COF nanofilms, but also provide a masking effect that can effectively tune the pore size of the heterostructured COF membranes. Benefitting from the synergistic enhancement of the membrane-forming ability and the pore size tunability, thus-prepared membranes exhibit excellent molecular sieving performances toward various small molecules. This work is expected to provide a guidance for the rational design and controllable preparation of COF membranes for precise nanofiltration. [ABSTRACT FROM AUTHOR]

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]

Covalent organic frameworks (COFs), with their inherent merits of specific pore size and uniform channels, have been extensively employed to produce nanofiltration (NF) membranes. However, COF-based NF towards precise separations, especially for ion separations, are often unsatisfying due to the intercrystalline defects in selective layers. Herein, a pressure-modulated synthesis method has been explored to prepare crystalline and defect-free COF membranes on seeded substrates for fast NF. Amine and aldehyde solutions are separately placed with a level interval to create vertical hydraulic pressure, which can regulate the mobility of the amine monomers and thus results in the self-repairing of defects in the COF planes and remediation of intercrystalline gaps. The abundant nucleation sites on seeded substrates promote the confined growth of COF crystallites in top layers, leading to an ultrathin selective layer with improved permeance. The resultant COF membrane shows tight methyl orange (~90.4%) and Na 2 SO 4 (~63.6%) rejections with a pronounced water permeance of up to ~44.2 L m−2 h−1 bar−1, which is ~2–10 times higher than other NF membranes with similar rejections. This pressure-modulated synthetic strategy establishes not only the self-repairing synthesis of COFs but also the controllable preparation of defect-free COF-based NF membranes, thus enabling precise and fast separation of molecules and ions. Image 1 • A pressure-modulated synthesis method was built up to fabricate highly crystalline COF nanofiltration membranes. • The external pressure precisely regulates the mobility of precursors and eliminates the nonselective intercrystalline gaps. • Abundant nucleation sites promote the conformal growth of COF crystals. • The membranes exhibit excellent water permeance and rejections to objects with a size above 1.1 nm. [ABSTRACT FROM AUTHOR]

Microporous polyamide membranes that allow lithium ions to pass through easily but exclude magnesium ions hold vast potential in the lithium extraction from brine lakes. The efficacy, nevertheless, has long been limited by a trade-off that the magnesium/lithium selectivity is received at the expense of the permeability, arising from the highly cross-linked polyamide. This work describes that rationally lowering the internal density of polyamides by embedding functionalized covalent organic framework (COF) nanosheets can boost the selectivity and permeability simultaneously. A cationic COF, TpTG Cl , is used to build transport highways in membranes, which shows the double role that not only lowers the polyamide density in a controllable way, but also strengthens the positive charge property. The resulting membrane exhibits a large water permeance of 19.6 L m−2 h−1 bar−1, along with an excellent separation factor of up to 21.3 under a high magnesium/lithium ratio of 30. This engineering strategy offers a means by which charged and nanoporous two-dimensional frameworks can be leveraged as a booster for microporous membranes enabling fast and precise ion transports. [Display omitted] • COF nanosheets are embedded in polyamides to build transport highways. • Polyamide density is lowered and positive charge property is strengthened. • Membranes show large permeance and high Mg2+/Li+ separation factor. • Membranes have the ability to process highly concentrated Mg2+/Li+ brines. [ABSTRACT FROM AUTHOR]

Mixed matrix membranes (MMMs) have attracted everlasting attentions, as their large permeance and high selectivity can be accessible synchronously. Herein, we show that covalent organic framework (COF) nanofibers serving as the nanofillers are doing better than COF nanoparticles in boosting the separation performance of MMMs. The nanofibers and the nanoparticles are synthesized and doped into polysulfone matrices to prepare MMMs. The high porosity and relative hydrophilicity of TpPa nanofibers and nanoparticles enable rapid exchange of solvent and nonsolvent during the process of nonsolvent induced phase separation, thus leading to instantaneous demixing. Furthermore, compared with TpPa nanoparticles possessing small and irregular shapes, the one-dimensional structure of TpPa nanofibers weakens the entanglement of the surrounding polysulfone linear chains, resulting in much pronounced instantaneous demixing. In contrast with the nanoparticle-incorporated membranes, the porosity of nanofiber-incorporated membranes is significantly improved. The resultant nanofiber-incorporated membrane demonstrates a high water permeance of 424.4 L m−2 h−1 bar−1, along with a BSA rejection rate of 94.3%. This work clearly reveals that porous one-dimensional structures such as COF nanofibers are highly promising in the preparation of high-performance MMMs. [Display omitted] • TpPa nanofibers and nanoparticles serve as nanofillers for mixed matrix membranes. • Incorporation of nanofibers leads to much pronounced instantaneous demixing. • The porosity of nanofiber-incorporated membranes is greatly improved. • The resultant membrane exhibits large water permeance and high BSA rejection. [ABSTRACT FROM AUTHOR]