A General Group-Protection Synthesis Strategy to Fabricate Covalent Organic Framework Gels.

Fabricating insoluble and infusible porous materials into gels for advanced applications is of great importance but has formidable challenges. Here, we present a general, facile, and scalable protocol to fabricate covalent organic framework (COF) gels using a group-protection synthesis strategy. To prove the generality of this strategy, we successfully prepared 10 types of COF organohydrogels with high crystallinity, porosity, good mechanical properties, and excellent solvent and freezing resistance. Notably, these COF organohydrogels can easily transform into hydrogels, organogels, and aerogels, breaking the gaps between different types of COF gels. An in-depth mechanism investigation unveils that the group-protection strategy effectively slows down the formation rate and regulates the morphology of COFs, benefiting the formation of cross-linked nanofibers/nanosheets to produce COF gels. We also find that the hydrogen bond network formed by the organic/water binary solvent and functional groups in the COF skeletons plays a vital role in creating organohydrogels and maintaining frost resistance and solvent resistance. As an application demonstration, COF gels installed with photoresponsive azobenzene groups show excellent solar energy absorption, photothermal conversion, and water transmission performances, demonstrating great potential in solar desalination. This work enriches the synthesis toolboxes for COF gels and expands the application scope of COFs.

Bottom-Up Synthesis of Covalent Organic Frameworks with Quasi-Three-Dimensional Integrated Architecture via Interlayer Cross-Linking.

Developing strategies to enhance the structural robustness of covalent organic frameworks (COFs) is of great importance. Here, we rationally design and synthesize a class of cross-linked COFs (CCOFs), in which the two-dimensional (2D) COF layers are anchored and connected by polyethylene glycol (PEG) or alkyl chains through covalent bonds. The bottom-up fabrication of these CCOFs is achieved by the condensation of cross-linked aldehyde monomers and tritopic amino monomers. All the synthesized CCOFs possess high crystallinity and porosity, and enhanced structural robustness surpassing the typical 2D COFs, which means that they cannot be exfoliated under ultrasonication and grinding due to the cross-linking effect. Furthermore, the cross-linked patterns of PEG units are uncovered by experimental results and Monte Carlo molecular dynamics simulations. It is found that all CCOFs are dominated by vertical cross-layer (interlayer) connections (clearly observed in high-resolution transmission electron microscopy images), allowing them to form quasi-three-dimensional (quasi-3D) structures. This work bridges the gap between 2D COFs and 3D COFs and provides an efficient way to improve the interlayered stability of COFs.

Solvent-Free Synthesis of C=N Linked Two-Dimensional Covalent Organic Frameworks.

Covalent organic frameworks (COFs) are an emerging class of porous crystalline polymers with well-defined structures and tunable functionalities, which have fascinating applications in a wide range of fields. However, the synthetic procedures of COFs are mainly confined to the solvothermal synthesis method which usually requires harsh experimental conditions. In this work, an effective solvent-free synthesis method to construct a series of two-dimensional (2D) COFs including imine-linked, azine-linked, ß-ketoenamine-linked, which avoids the complicated solvent screening process and most of the disadvantages of solvothermal methods is developed. The crystallinity and porosity of these COFs are comparable to those prepared by traditional solvothermal routes. What's more, the advantages of the solvent-free method enable the production of gram-scale of those 2D COFs through a one-pot reaction, demonstrating high industrial application potentials.

Pore Geometry and Surface Engineering of Covalent Organic Frameworks for Anhydrous Proton Conduction.

Developing new materials for anhydrous proton conduction under high-temperature conditions is significant and challenging. Herein, we create a series of highly crystalline covalent organic frameworks (COFs) via a pore engineering approach. We simultaneously engineer the pore geometry (generating concave dodecagonal nanopores) and pore surface (installing multiple functional groups such as -C=N-, -OH, -N=N- and -CF3 ) to improve the utilization efficiency and host-guest interaction of proton carriers, hence benefiting the enhancement of anhydrous proton conduction. Upon loading with H3 PO4 , COFs can realize a proton conductivity of 2.33×10-2  S cm-1 under anhydrous conditions, among the highest values of all COF materials. These materials demonstrate good stability and maintain high proton conductivity over a wide temperature range (80-160 °C). This work paves a new way for designing COFs for anhydrous proton conduction applications, which shows great potential as high-temperature proton exchange membranes.

Rationally Fabricating Three-Dimensional Covalent Organic Frameworks for Propyne/Propylene Separation.

Efficient propyne/propylene separation to obtain polymer-grade propylene is a crucial and challenging process in industrial production, but it has not yet been realized in the covalent organic framework (COF) field. Addressing this challenge, we synthesize two three-dimensional COF adsorbents via a [8 + 4] construction approach based on an octatopic aldehyde monomer. Upon using the continuous rotation electron diffraction technique and structural simulation, both COFs are successfully determined as rare flu topology. Various characterization techniques prove that both COFs exhibit high crystallinity, high porosity, and good stability. Attributed to their interconnected micropores and nonpolar pore environment, these COFs can efficiently remove trace amounts of propyne from the propyne/propylene (1/99, and 0.1/99.9, v/v) mixture to obtain high-purity propylene (>99.99%), validated by dynamic breakthrough experiments. This work paves a new avenue for propyne/propylene separation using COFs as highly efficient adsorbents.

Grotthuss Proton-Conductive Covalent Organic Frameworks for Efficient Proton Pseudocapacitors.

Herein, we describe the synthesis of two highly crystalline, robust, hydrophilic covalent organic frameworks (COFs) that display intrinsic proton conduction by the Grotthuss mechanism. The enriched redox-active azo groups in the COFs can undergo a proton-coupled electron transfer reaction for energy storage, making the COFs ideal candidates for pseudocapacitance electrode materials. After in situ hybridization with carbon nanotubes, the composite exhibited a high three-electrode specific capacitance of 440 F g-1 at the current density of 0.5 A g-1 , among the highest for COF-based supercapacitors, and can retain 90 % capacitance even after 10 000 charge-discharge cycles. This is the first example using Grotthuss proton-conductive organic materials to create pseudocapacitors that exhibited both high power density and energy density. The assembled asymmetric two-electrode supercapacitor showed a maximum energy density of 71 Wh kg-1 with a maximum power density of 42 kW kg-1 , surpassing that of all reported COF-based systems.