Enabling Solution Processable COFs through Suppression of Precipitation during Solvothermal Synthesis

Abstract

Covalent organic frameworks (COFs) are crystalline, nanoporous materials of interest for various applications, but current COF synthetic routes lead to insoluble aggregates which precludes processing for practical implementation. Here, we report a COF synthesis method that produces a stable, homogeneous suspension of crystalline COF nanoparticles that enables the preparation of COF monoliths, membranes, and films using conventional solution-processing techniques. Our approach involves the use of a polar solvent, diacid catalyst, and slow reagent mixing procedure at elevated temperatures which altogether enable access to crystalline COF nanoparticle suspension that does not aggregate or precipitate when kept at elevated temperatures. On cooling, the suspension undergoes a thermoreversible gelation transition to produce crystalline and highly porous COF materials. We further show that the modified synthesis approach is compatible with various COF chemistries, including both large-and small-pore imine COFs, hydrazone-linked COFs, and COFs with rhombic and hexagonal topologies, and in each case, we demonstrate that the final product has excellent crystallinity and porosity. The final materials contain both micro-and macropores, and the total porosity can be tuned through variation of sample annealing. Dynamic light scattering measurements reveal the presence of COF nanoparticles that grow with time at room temperature, transitioning from a homogeneous suspension to a gel. Finally, we prepare imine COF membranes and measure their rejection of polyethylene glycol (PEG) polymers and oligomers, and these measurements exhibit size-dependent rejection and adsorption of PEG solutes. This work demonstrates a versatile processing strategy to create crystalline and porous COF materials using solution-processing techniques and will greatly advance the development of COFs for various applications.

Squaraine-linked zwitterionic COF modified LLZTO nanoparticles for high performance polymer composite electrolytes in Li–S batteries

“…33 FTIR spectroscopy exhibited the characteristic stretching vibration of the triazine ring (1501 and 1358 cm −1 ), 34 and CvO (1799 cm −1 ), 35 CvC (1646 cm −1 ) 36 and CvN groups (1667 cm −1 ). 37 The peak at 3415 cm −1 represents the stretching vibration mode of N-H, which confirmed the formation of imine linkages (Fig. S2b †).…”

Aqueous Sol–Gel Synthesis and Shaping of Covalent Organic Frameworks

“…Since the first report of covalent organic frameworks (COFs) by Yaghi et al in 2005, the unique structures and attractive properties of the COFs have received increasing attention . Due to their robust nature and abundant porosities, COFs have broad application prospects in the fields of separation, − catalysis, − and optoelectronic devices. − At present, the majority of the COFs were prepared by the solvothermal method, which requires the use of toxic organic solvents, pressurized tubes, and tedious and complicated procedures. − The ionothermal method is an alternative route to prepare COFs with high crystallinity; however, elevated reaction temperatures and harsh conditions were needed. − Alternatively, the solvent-free mechanochemical method has been investigated, where lower specific surface areas and moderate crystallinity of COFs were observed compared to the solvothermal analogues. − Although significant advances have been achieved in recent years, the synthetic methodology of COFs has always been a stumbling block that restricts further development of this material. Therefore, the exploration and development of facile, efficient, and universal preparation methods are imminent and highly beneficial.…”

Synthesis and Additive Manufacturing of Hydrazone‐Linked Covalent Organic Framework Aerogels

“…A number of recent studies have reported the production of COF gels, foams, and aerogels as part of novel processing strategies [26–31] . For example, thermoreversible COF sol‐gel formulations enable casting from solution to produce free‐standing membranes or thin films [28] .…”

“…In particular, hydrazone COFs are more stable in acidic and basic environments and may ultimately provide a more robust material for applications in these conditions [45] . However, work focused on novel processing methods of hydrazones is limited and no prior study has reported additive manufacturing of hydrazone COFs [28,46] . Developing strategies for hydrazone COF aerogels and gels could provide access to unique applications which require greater chemical stability.…”

“…A number of recent studies have reported the production of COF gels, foams, and aerogels as part of novel processing strategies. [26][27][28][29][30][31] For example, thermoreversible COF sol-gel formulations enable casting from solution to produce freestanding membranes or thin films. [28] Furthermore, these gelation strategies can be used for the fabrication of microstructured COFs through printing and additive manufacturing.…”