Large-Scale Synthesis of Covalent Organic Frameworks: Challenges and Opportunities

Covalent Organic Framework Membranes: Synthesis Strategies and Separation Applications

Covalent organic frameworks (COFs) have emerged as highly promising materials for membrane separations due to their high porosity, tunable pore sizes, ordered crystalline structures, and exceptional chemical stability. With these features, COF membranes possess greater selectivity and permeability than conventional materials, making them the preferred choice in various fields, including membrane separations. Fascinating research endeavors have emerged encompassing fabrication strategies for COF-based membranes and their diverse separation applications. Hence, this review summarizes the latest advancements in COF synthesis, including COF powders and continuous COF-based membranes and their applications in separation membranes. Special consideration was given to regulation strategies for the performance optimization of COF membranes in separation applications, such as pore size, hydrophilicity/hydrophobicity, surface charge, crystallinity, and stability. Furthermore, applications of COF membranes in water treatment, metal ion separation, organic solvent nanofiltration, and gas separation are comprehensively reviewed. Finally, the research results and future prospects for the development of COF membranes are discussed. Future research may be focused on the following key directions: (1) single-crystal COF fabrication, (2) cost-effective membrane preparation, (3) subnanometer pore engineering, (4) advanced characterization techniques, and (5) AI-assisted development.

Synergistic Computational and Experimental Investigation of Covalent Organic Frameworks for Efficient Alcohol Dehydration

Covalent organic frameworks (COFs), a promising class of nanoporous materials, have received significant attention for membrane separation. Currently, several COFs are reported for alcohol dehydration, but they are not efficient owing to the pervasive challenge to separate small-sized molecular mixture. Herein, first we have computationally explored a series of COFs with different functionality and aperture size as pervaporation (PV) membrane and identified a novel COF for efficient dehydration of water/alcohol mixtures (90 wt % IPA, 90 wt % n-butanol and 90 wt % t-butanol). Subsequently, the best-performing COF was experimentally synthesized and characterized, and its sorption properties were correlated with computational results. Molecular dynamics (MD) simulations revealed that solvent permeation fluxes are predominantly influenced by the pore aperture of COFs, and larger pore aperture exhibits higher flux. Conversely, the separation factor is primarily determined by the polarity of the pore functional groups. Among the tested COF membranes, TpPa-1-OC3H6OCH3 demonstrated superior performance, surpassing the current state-of-the-art membranes. The activation energy (Ea) for water permeation in alcohol mixtures through TpPa-1-OC3H6OCH3 is mostly governed by water-alcohol interactions. Furthermore, experimental evaluation of the COFs indicated a plate-like morphology for TpPa-1-OC3H6OCH3 which ascertained a 2D-sheet-like structure. TpPa-1 showed greater sorption than TpPa-1-OC3H6OCH3 with all of the solvents tested owing to the inability of the solvent molecules to enter the relatively small pores in the later COF. This is in accordance with the MD simulation predictions, which indicated that the solvent molecules cannot penetrate the small pores of TpPa-1-OC3H6OCH3. This work synergistically integrates computational and experimental approaches to develop novel COFs with superior performance compared to previously reported PV membranes, paving the way for advanced membranes for sustainable solvent recovery.

Industrialization of Covalent Organic Frameworks

Covalent organic frameworks (COFs) have attracted broad interest because of their well-defined, customizable, highly stable, and porous structures. COFs have shown significant potential for various practical applications, such as gas storage/purification, drug purification, water treatment, catalysis, and battery applications. Scaling up COFs is highly desirable to meet industrial application demands but is hindered by the limitations of synthesis methods and the high cost of reactants. Recently, emerging green synthesis methods, such as mechanochemical synthesis and flux synthesis, have offered promising solutions to these challenges (e.g., ton-scale production of COFs has been achieved by companies recently). This Perspective provides an overview of the state of the art with respect to the industrial production of COFs and discusses factors influencing the large-scale production of COFs. Directions and opportunities for improving the performance and sustainability of COFs toward industrial applications are also emphasized.

Covalent organic frameworks and related innovative materials in chiral separation and recognition

Chiral recognition and enantioseparation are of paramount importance in various fields, including pharmaceuticals, agrochemicals, and material science. Covalent organic frameworks (COFs) have emerged as promising materials for chiral separation due to their unique structural features and tunable properties. This review provided a comprehensive overview of recent progress in the application of COFs and related innovative materials for chiral separation and recognition. Various strategies were analyzed for the design and synthesis of chiral COFs, including the incorporation of chiral building blocks, post-synthetic modification, and the integration of chiral selectors. The applications of chiral COFs in chromatographic techniques, membrane separations, and other emerging methods were critically evaluated with the emphasis on their advantages and limitations. Additionally, the review summarized the potential of combining COFs with other nanomaterials, such as metal-organic frameworks (MOFs) and nanoparticles, to enhance chiral recognition and separation performance. The fundamental principles and mechanisms of chiral recognition were discussed, highlighting the role of chiral selectors and their interactions with enantiomers. Finally, current challenges and future perspectives in this field were discussed, providing insights into the development of more efficient and versatile chiral separation systems based on COFs and related materials.

Continuous flow synthesis and post-synthetic conversion of single-crystalline covalent organic frameworks

The synthesis and scale-up of high quality covalent organic frameworks (COFs) remains a challenge due to slow kinetics of the reversible bond formation and the need for precise control of reaction conditions. Here we report the rapid synthesis of faceted single crystals of two-dimensional (2D) COFs using a continuous flow reaction process. Two imine linked materials were polymerized to the hexagonal CF-TAPB-DMPDA and the rhombic CF-TAPPy-PDA COF, respectively. The reaction conditions were optimized to produce single crystals of micrometer size, which notably formed when the reaction was cooling to room temperature. This indicated a growth mechanism consistent with the fusion of smaller COF particles. The optimized conditions were used to demonstrate the scalability of the continuous approach by synthesizing high quality, faceted COFs at a rate of more than 1 g h-1. The materials showed high crystallinity and porosity with surface areas exceeding 2000 m2 g-1. Additionally, the versatility of the continuous flow reaction approach was demonstrated on a post-synthetic single crystal to single crystal demethylation of CF-TAPB-DMPDA to afford a hydroxyl functionalized COF CF-TAPB-DHPDA. Throughout the modification process, the material maintained its hexagonal morphology, crystallinity, and porosity. This work reports the first example of synthesizing and post-synthetically modifying imine linked COF single crystals in continuous flow and will prove a first step towards scaling high quality COFs to industrial levels.

Breaking Dynamic Behavior in 3D Covalent Organic Framework with Pre-Locked Linker Strategy

Due to their large surface area and pore volume, three-dimensional covalent organic frameworks (3D COFs) have emerged as competitive porous materials. However, structural dynamic behavior, often observed in imine-linked 3D COFs, could potentially unlock their potential application in gas storage. Herein, we showed how a pre-locked linker strategy introduces breaking dynamic behavior in 3D COFs. A predesigned planar linker-based 3,8-diamino-6-phenylphenanthridine (DPP) was prepared to produce non-dynamic 3D JUC-595, as the benzylideneamine moiety in DPP locked the linker flexibility and restricted the molecular bond rotation of the imine linkages. Upon solvent inclusion and release, the PXRD profile of JUC-595 remained intake, while JUC-594 with a flexible benzidine linker experienced crystal transformation due to framework contraction-expansion. As a result, the activated JUC-595 achieved higher surface areas (754 m2 g-1) than that of JUC-594 (548 m2 g-1). Furthermore, improved CO2 and CH4 storages were also seen in JUC-595 compared with JUC-594. Impressively, JUC-595 recorded a high normalized H2 storage capacity that surpassed other reported high-surface area 3D COFs. This works shows important insights on manipulating the structural properties of 3D COF to tune gas storage performance.