Fluorinated Covalent Organic Framework Antifouling Nanofiltration Membranes Through Defect Engineering.

This work addresses the long-standing challenge of insufficient permselectivity in two-dimensional (2D) covalent organic framework (COF) membranes by developing a generalizable and potentially scalable self-templating interfacial assembly method to synthesize vertically aligned 2D COF polycrystalline nanofilms. Experiments and molecular simulations reveal that molecular amphiphilicity-driven spontaneous confinement and directional alignment of framework-building units at the organic/aqueous interface instigate the vertical growth of COF monolayers. The superior sieving ability of sub–4 angstrom interlayer nanochannels enables vertically aligned COF membranes to efficiently intercept salt ions, electrically neutral small molecules, and hazardous chemicals, outperforming conventional COF membranes. The implemented strategy can be extended to other building blocks and framework topologies, opening up avenues for fine-tuning the pore structure of 2D COF membranes.

PTSA-mediated interfacial catalytic polymerization of crystalline dense covalent organic framework membranes for enhanced desalination

Ultrathin covalent organic framework (COF) membranes are urgently demanded in molecular/ionic separations. Herein, we reported an electrochemical interfacial polymerization strategy to fabricate ultrathin COF membranes with thickness of 85 nm, by actively manipulate self‐healing effect and self‐inhibiting effect. The resulting COF membrane exhibited superior performance in brine desalination with the permeation flux of 92 kg m−2 h−1 and the rejection of 99.96 %. Our electrochemical interfacial polymerization strategy enriches the fabrication approach of COF membranes and facilitates the rational design of ultrathin membranes.

Covalent organic frameworks (COFs) are highly porous crystalline polymers with uniform pores and large surface areas. Combined with their modular design principle and excellent properties, COFs are an ideal candidate for separation membranes. Liquid-liquid interfacial polymerization is a well-known approach to synthesize membranes by reacting two monomers at the interface. However, volatile organic solvents are usually used, which may disturb the liquid-liquid interface and affect the COF membrane crystallinity due to solvent evaporation. Simultaneously, the domain size of the organic solvent-water interface, named the reaction zone, can hardly be regulated, and the diffusion control of monomers for favorable crystallinity is only achieved in the water phase. These drawbacks may limit the widespread applications of liquid-liquid interfacial polymerization to synthesize diverse COF membranes with different functionalities. Here, we report a facile strategy to synthesize a series of imine-linked freestanding COF membranes with different thicknesses and morphologies at tunable ionic liquid (IL)-H2O interfaces. Due to the H-bonding of the catalysts with amine monomers and the high viscosity of the ILs, the diffusion of the monomers was simultaneously controlled in water and in ILs. This resulted in the exceptionally high crystallinity of freestanding COF membranes with a Brunauer-Emmett-Teller (BET) surface area up to 4.3 times of that synthesized at a dichloromethane-H2O interface. By varying the alkyl chain length of cations in the ILs, the interfacial region size and interfacial tension could be regulated to further improve the crystallinity of the COF membranes. As a result, the as-fabricated COF membranes exhibited ultrahigh permeance toward water and organic solvents and excellent selective rejection of dyes.

Hydrated-ion transport and desolvation dominate energy transfer and ionic selectivity in membrane separation, electrochemical energy storage, and catalytic systems, whereas achieving fast ion conduction with low hydration remains highly challenging. In this work, we discover that pyridinyl-based covalent organic framework (COF) membranes enable the fast transport of hydrated ions with efficient desolvation. This originates from a soft Lewis acid-mediated competitive coordination mechanism, where pyridinyl groups partially displace hydration shells. The ordered channels with optimized coordination not only stabilize desolvated ions but also provide continuous hopping pathways for ion migration, resulting in rapid ion transport with reduced desolvation barriers. As a proof-of-concept application, the pyridinyl COF membranes were studied in aqueous zinc batteries. Electrochemical tests reveal that partial zinc ion desolvation lowers the thermodynamic barrier for zinc nucleation, while rapid ion transport balances interfacial reaction kinetics, effectively suppressing dendrite growth and parasitic reactions. Consequently, a zinc anode paired with pyridyl COF membranes exhibits reversible stripping/plating over 2000 cycles in a conventional ZnSO4 electrolyte without additives, outperforming most of the current aqueous battery separators. This work demonstrates the unique desolvation transport behavior of hydrated ions in pyridinyl COF membranes and provides new insights for the rational design of COFs for electrochemical energy storage.

Nanofluidic ion transport holds high promise in bio‐sensing and energy conversion applications. However, smart nanofluidic devices with high ion flux and modulable ion transport capabilities remain to be realised. Herein, we demonstrate smart nanofluidic devices based on oriented two‐dimensional covalent organic framework (2D COF) membranes with vertically aligned nanochannel arrays that achieved a 2–3 orders of magnitude higher ion flux compared with that of conventional single‐channel nanofluidic devices. The surface‐charge‐governed ion conductance is dominant for electrolyte concentration up to 0.01 M. Moreover, owing to the customisable pH‐responsivity of imine and phenol hydroxyl groups, the COF‐DT membranes attained an actively modulable ion transport with a high pH‐gating on/off ratio of ≈100. The customisable structure and rich chemistry of COF materials will offer a promising platform for manufacturing nanofluidic devices with modifiable ion/molecular transport features.

Membrane-based nanofiltration has the potential to revolutionize the large-scale treatment of organic solvents in various applications. However, the widely used commercial membranes suffer from low permeability, narrow structural tunability, and limited chemical resistance. Here, we report a strategy for fabricating covalent organic framework (COF) membranes with solvent-responsive structural flexibility. The interlayer shifting of these COF membranes in polar organic solvents results in sub-nanopores with high selectivity. High rejection rates (>99%), high permeance (>15 kilogram meter−2 hour−1 bar−1), and excellent organic solvent resistance of these smart COF membranes are achieved for a diverse array of nanofiltration applications.

Membrane nanofiltration provides a sustainable and energy-efficient platform for precise molecular separation. However, highly permselective and chemically stable membrane materials capable of operating under harsh conditions are currently lacking. Here we report a generalizable monomer-solvent dual engineering strategy that enables the one-step synthesis of chemically robust thiazole-linked polycrystalline covalent organic framework (COF) membranes via scalable interfacial polymerization under ambient conditions for ultraselective molecular separation. The fully π-conjugated aromatic skeleton and the spatially exposed heteroatoms on the irreversible thiazole linkages establish a lone-pair electron network, which not only forms an atomic hydration layer to protect the framework but also confers long-range regulation of electrostatic interactions. The thiazole-linked COF membranes exhibit remarkable structural stability in strong acids (e.g., 12 M HCl), good resistance to organic solvents and chlorine, and high pharmaceutical desalination permselectivity, achieving ion/pharmaceutical separation factors up to 690. This versatile thiazole-linked framework structure offers potential for the development of chemically stable aromatic conjugated COF membranes for diverse vital applications.

Covalent-organic framework (COF) membranes are increasingly used for many potential applications including ion separation, fuel cells, and ion batteries. It is of central importance to fundamentally and quantitatively understand ion transport in COF membranes. In this study, a series of COF membranes is designed with different densities and arrangements of functional groups and subsequently utilize molecular simulation to provide microscopic insights into ion transport in these membranes. The membrane with a single-sided layer exhibits the highest chloride ion (Cl-) conductivity of 77.2 mScm-1 at 30°C. Replacing the single-sided layer with a double-sided layer or changing layer arrangement leads to a decrease in Cl- conductivity up to 33% or 53%, respectively. It is revealed that the electrostatic repulsion between ions serves as a driving force to facilitate ion transport and the positions of functional groups determine the direction of electrostatic repulsion. Furthermore, the ordered pores generate concentrated ions and allow rapid ion transport. This study offers bottom-up inspiration on the design of new COF membranes with moderate density and proper arrangement of functional groups to achieve high ion conductivity.

Conferring efficient alcohol dehydration to covalent organic framework membranes via post-synthetic linker exchange

Covalent organic frameworks (COFs) build a versatile platform for constructing separation membranes, affording them disruptive candidates for nanoscale molecule/ion sieving. However, the reported aperture of COFs mainly concentrates on 0.8–10.0 nm, leaving a daunting challenge in fabrication of ultramicroporous (<0.7 nm) COF membranes. Herein, the channel partition of COF membranes is elaborately implemented by taking advantage of the copper‐catalyzed azide–alkyne cycloaddition (CuAAC) based on click chemistry. Two kinds of novel alkynyl COF membranes bearing symmetric alkynyl monomers are predesigned and synthesized. The alkynyl groups can react with dual‐azide molecules to form triazole ring bridge, which allows the partition of hexagonal or tetragonal channels. Moreover, the π‐electron‐rich structures of triazole ring offer cation‐philic or anion‐repulsive region, showing great potentials in precise ion separations. Consequently, the superior performance is obtained in separating ternary/binary ion mixtures: Li + /Mg 2+ /(K + ) and F − /SO 4 2− /(Cl − ). Example of dealing with a K + /Li + /Mg 2+ ternary mixture, the optimum p ‐TpPa‐C≡CH membrane exhibits an actual selectivity of 101.2 for K + /Mg 2+ and 30.9 for Li + /Mg 2+ . The strong adaptability and wide range of application scenarios of our COF membranes exceed the ever‐reported membranes. This work provides a fundamental to comprehending the channel manipulation mechanisms and advances the development of high‐performance ion separation membranes.

High-throughput computational screening of Covalent−Organic framework membranes for helium purification

Dual-activation interfacial polymerization based anionic covalent organic framework nanofiltration membrane for high-flux dye separation

Covalent organic frameworks (COFs) are a disruptive material platform for various novel applications including nanofiltration for water purification due to their excellent physicochemical features. Nevertheless, the currently available approaches for preparing COF membranes need stringent synthesis conditions, prolonged fabrication time, and tedious post-processing, leading to poor productivity. Herein, a simple and efficient layer-by-layer stacking assembly strategy is developed based on electrophoretic deposition (EPD) to rapidly generate ionic COF membranes due to the uniform driving force for nanosheet assembly. A new two-cell EPD design avoids the usual EPD problems such as bubbles and acidic/alkaline microenvironments in the near-electrode region in aqueous EPD processes. Ultrathin COF membranes with homogenous structures can be produced within several minutes. Consequently, the prepared COF membranes exhibit outstanding permselectivity and possess good stability and anti-pressure ability due to their uniform architecture and unique chemical composition.

Covalent organic frameworks (COFs) have been widely used in gas separation due to their permanent open porosity, highly ordered crystalline structures and excellent processability. The pore structure of a membrane determines its performance in gas separation. This review highlights the importance of pore chemistry in COF membranes with gas-separation function. The latest advancements of various strategies are presented to regulate the pore structures of COF membranes, which are clearly illustrated by the representative examples. The application of gas separation by COF membranes is reviewed. The separation mechanisms are systematically summarized, with a focus on correlating the structure (pore size, wall, and dimensionality) and the performance (gas permeability and selectivity). The future challenges and prospective of COF membranes in the field of gas separation are briefly described, aiming to provide guidance for the potential development of next-generation membrane in the gas separation field.