Vertical covalent organic framework interlayer channels enable ultraselective molecular separations

Mechanistic understanding into the formation and growth of imine-linked two-dimensional (2D) covalent organic frameworks (COFs) is needed to improve their materials quality and access larger crystallite sizes, both of which limit the promise of 2D COFs and 2D polymerization techniques. Here we report a previously unknown temperature-dependent depolymerization of colloidal 2D imine-linked COFs, which offers a new means to improve their crystallinity. 2D COF colloids form at room temperature but then depolymerize when their reaction mixtures are heated to 90 °C. As the solutions are cooled back to room temperature, the 2D COFs repolymerize and crystallize with improved crystallinity and porosity, as characterized by X-ray diffraction, infrared spectroscopy and N2 porosimetry. The evolution of COF crystallinity during the solvothermal depolymerization and repolymerization processes was characterized by in situ wide angle X-ray scattering, and the concentrations of free COF monomers as a function of temperature were quantified by variable temperature 1H NMR spectroscopy. The ability of a 2D COF to depolymerize under these conditions depends on both the identity of the COF and its initial materials quality. For one network formed at room temperature (TAPB-PDA COF), a first depolymerization process is nearly complete, and the repolymerization yields materials with dramatically enhanced crystallinity and surface area. Already recrystallized materials partially depolymerize upon heating their reaction mixtures a second time. A related 2D COF (TAPB-DMTA COF) forms initially with improved crystallinity compared to TAPB-PDA COF and then partially depolymerizes upon heating. These results suggest that both high materials quality and network-dependent properties, such as interlayer interaction strength, influence the extent to which 2D COFs resist depolymerization. These findings offer a new means to recrystallize or solvent anneal 2D COFs and may ultimately inform crystallization conditions for obtaining large-area imine-linked two-dimensional polymers from solution.

Two-dimensional (2D) covalent organic frameworks (COFs) represent a unique structural paradigm owing to their intriguing properties. The diverse chemical tunability and good charge carrier mobility make these materials suitable for electronics and optoelectronic applications. However, the poor conductivity and processability restrict their full potential in device fabrication. Several strategies were followed to improve the conductivity in 2D COFs, including redox-active linkers and doping. Herein, we report four 2D COFs, by modulating the substituents on one of the monomeric units. Interestingly, less-bulky groups (H and OH) yielded a Kagome lattice in ETTA-TA- and ETTA-DHTA-COFs, whereas square-grid networks were obtained for ETTA-An-ph- and ETTA-BPTA-COFs with bulky substituents (anthracene and propargylic moieties). Further, we investigated the effect of the topology and the functionalities of these COFs on electrical conductivity. The current–voltage (I–V) measurements reveal that ETTA-DHTA-COF with a Kagome lattice exhibits higher electrical conductivity of 1.3 × 10–4 S cm–1 compared to other congeners (square-grid and Kagome-type COFs) and many reported undoped COFs. The trade-off between the frameworks’ topology (Kagome vs square-grid) and the functionalities (H, anthracene vs OH, oxy-propargyl) exhibits the synergetic role in stimulating the conductivities in the resulting COFs. This strategy of switching framework topology and the effect of functional groups on electrical conductivity unveil a unique approach to fine-tuning the electronic properties in the 2D COFs.

Sol-gel processing of a covalent organic framework for the generation of hierarchically porous monolithic adsorbents

Covalent organic framework (COF) membrane holds great promise in water treat-ment. Improving the antifouling property of COF membrane is critical for practical application while rare investigations have been reported. Grafting fluorinated chains on the COF membrane surface is expected an effective strategy but quite challenging due to the lack of grafting sites. In this work, the defect engineering strategy is adopted to generate free amino groups as grafting sites through the Schiff-base reaction between amine monomer and mixed aldehyde monomers, then perfluoroalkyl chains are grafted on the COF membrane surface through the reaction between the free amino groups and the perfluorooctanoyl chloride. The content of perfluoroalkyl chains can be regulated and optimized by controlling the amount of free amino groups. The fluorinated COF membrane shows superior antifouling performance with a significantly increased flux recovery ratio and reduced flux decline ratio against oil/water emulsions and humic acid (FRR ≈ 98%, DRt = 10%). Furthermore, the fluorinated COF membrane exhibits high water permeance up to ≈115 L m-2 h-1 bar-1 while acquiring a high salt/dye selective factor. This work affords an effective approach to the development of antifouling, high-separation-performance COF membranes, and other kinds of organic molecular sieve membranes.

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

Two-dimensional (2D) covalent organic frameworks (COFs) are composed of structurally precise, permanently porous, layered macromolecular sheets, which are traditionally synthesized as polycrystalline solids with crystalline domain lengths smaller than 100 nm. Here, we polymerize imine-linked 2D COFs as suspensions of faceted single crystals in as little as 5 min at moderate temperature and ambient pressure. Single crystals of two imine-linked 2D COFs were prepared, consisting of a rhombic 2D COF (TAPPy-PDA) and a hexagonal 2D COF (TAPB-DMPDA). The sizes of TAPPy-PDA and TAPB-DMPDA crystals were tuned from 720 nm to 4 μm and 450 nm to 20 μm in width, respectively. High-resolution transmission electron microscopy revealed that the COF crystals consist of layered, 2D polymers comprising single-crystalline domains. Continuous rotation electron diffraction resolved the unit cell and crystal structure of both COFs, which are single-crystalline in the a-b plane but disordered in the stacking c dimension. Single crystals of both COFs were incorporated into gas chromatography separation columns and exhibited unusual selective retention of cyclohexane over benzene, with single-crystalline TAPPy-PDA significantly outperforming single-crystalline TAPB-DMPDA. Polycrystalline TAPPy-PDA exhibited no separation, while polycrystalline TAPB-DMPDA exhibited poor separation and the opposite order of elution, retaining benzene more than cyclohexane, indicating the importance of improved material quality for COFs to exhibit properties that derive from their precise, crystalline structures. This work represents the first example of synthesizing imine-linked 2D COF single crystals at ambient pressure and short reaction times and demonstrates the promise of high-quality COFs for molecular separations.

ConspectusAs one of the most attractive members in the porous materials family, covalent organic frameworks (COFs) have been reported thousands of times since their first discovery in 2005, covering their design, synthesis, and applications. However, an overwhelming majority of these COFs are based on two-dimensional (2D) topologies while three-dimensional (3D) COFs are numbered fewer than 100 up to date. In fact, baring enhanced specific surface area, interconnected channels, well-exposed functional moieties, and highly adjustable structures, 3D COFs are often more competitive in various application fields like adsorption, separation, chemical sensing, and heterogeneous catalysis compared with their 2D counterparts. However, significant crystallization problems and poor chemical stabilities, which might be attributed to the highly void frameworks and the absence of π-π stacking, have raised severe limitations over the research and application of 3D COFs. To solve these problems, more elaborate synthesis regulations or more moderate functionalization conditions are required. More importantly, the strategies for enhancing chemical stabilities of 3D COFs are of vital importance for their further development and practical applications.In this Account, we review the design principles, functional approaches, and stability regulation methods toward functional 3D COFs. We begin the discussion with some essential elements in the construction of 3D COF structures, including topologies, interpenetrations, linkages, and synthetic methods. After that, we focus on several strategies for the functionalization of 3D COFs, including in situ approaches (utilizing in situ generated COF linkages as the active sites), bottom-up synthesis (embedding functional moieties from predesigned building blocks), and postsynthesis modification (covalent modification or metalation of pristine frameworks). At last, we highlight some approaches toward the durable amplification of 3D COFs, which is highly important for framework functionalization and practical application. This target could be achieved through not only the introduction of some extra strengthening force, such as hydrophobic effects, coulomb repulsion, and steric hindrance effects, but also the utilization of robust linkages, which could enhance the stability from material nature.Due to their high surface area, various interpenetrated channels, multifarious functionalities, and promising stabilities, 3D COFs demonstrated excellent performance and have great potential in a wide range of application fields including adsorption and separation, heterogeneous catalysis, energy storage, and so on. Although the development of these materials has been limited by serious crystallization problems and stability restriction, great efforts have been devoted by researchers in the past decade, and a mass of strategies have been developed in synthesis control, functionalization regulation, and stability enhancement for 3D COFs. We expect 3D COFs to be practically utilized in the future with further advances in the design, preparation, and functionalization of these materials.

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.

Polyamide⊥TpPa-PVDF membranes for aliphatic-aromatic solvent separations

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.

Three-dimensional (3D) covalent organic frameworks (COFs) possess higher surface areas, more abundant pore channels, and lower density compared to their two-dimensional counterparts which makes the development of 3D COFs interesting from a fundamental and practical point of view. However, the construction of highly crystalline 3D COF remains challenging. At the same time, the choice of topologies in 3D COFs is limited by the crystallization problem, the lack of availability of suitable building blocks with appropriate reactivity and symmetries, and the difficulties in crystalline structure determination. Herein, we report two highly crystalline 3D COFs with pto and mhq-z topologies designed by rationally selecting rectangular-planar and trigonal-planar building blocks with appropriate conformational strains. The pto 3D COFs show a large pore size of 46 Å with an extremely low calculated density. The mhq-z net topology is solely constructed from totally face-enclosed organic polyhedra displaying a precise uniform micropore size of 1.0 nm. The 3D COFs show a high CO2 adsorption capacity at room temperature and can potentially serve as promising carbon capture adsorbents. This work expands the choice of accessible 3D COF topologies, enriching the structural versatility of COFs.

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 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.