Covalent organic frameworks (COFs) are an emerging class of crystalline porous polymers with highly tuneable structures and functionalities. COFs have been proposed as ideal materials for applications in the energy-intensive field of molecular separation due to their notable intrinsic features such as low density, exceptional stability, high surface area, and readily adjustable pore size and chemical environment. This review attempts to highlight the key advancements made in the synthesis of COFs for diverse separation applications such as water treatment or the separation of gas mixtures and organic molecules, including chiral and isomeric compounds. Methods proposed for the fabrication of COF-based columns and continuous membranes for practical applications are also discussed in detail. Finally, a perspective regarding the remaining challenges and future directions for COF research in the field of separation has also been presented.

Covalent organic frameworks for separation applications.

Summary Covalent organic frameworks (COFs) are a new type of crystalline porous polymers known for chemical stability, excellent structural regularity, robust framework, and inherent porosity, making them promising materials for capturing various types of pollutants from aqueous solutions. This review thoroughly presents the recent progress and advances of COFs and COF-based materials as superior adsorbents for the efficient removal of toxic heavy metal ions, radionuclides, and organic pollutants. Information about the interaction mechanisms between various pollutants and COF-based materials are summarized from the macroscopic and microscopic standpoints, including batch experiments, theoretical calculations, and advanced spectroscopy analysis. The adsorption properties of various COF-based materials are assessed and compared with other widely used adsorbents. Several commonly used strategies to enhance COF-based materials’ adsorption performance and the relationship between structural property and sorption ability are also discussed. Finally, a summary and perspective on the opportunities and challenges of COFs and COF-based materials are proposed to provide some inspiring information on designing and fabricating COFs and COF-based materials for environmental pollution management.

Orderly Porous Covalent Organic Frameworks-based Materials: Superior Adsorbents for Pollutants Removal from Aqueous Solutions

Covalent organic frameworks (COFs) for environmental applications

Thinking the future of membranes: Perspectives for advanced and new membrane materials and manufacturing processes

Covalent organic frameworks (COFs) are a promising category of porous materials possessing extensive chemical tunability, high porosity, ordered arrangements at a molecular level, and considerable chemical stability. Despite these advantages, the application of COFs as membrane materials for gas separation is limited by their relatively large pore apertures (typically >0.5 nm), which exceed the sieving requirements for most gases whose kinetic diameters are less than 0.4 nm. Herein, we report the fabrication of ultrathin two-dimensional (2D) membranes through layer-by-layer (LbL) assembly of two kinds of ionic covalent organic nanosheets (iCONs) with different pore sizes and opposite charges. Because of the staggered packing of iCONs with strong electrostatic interactions, the resultant membranes exhibit features of reduced aperture size, optimized stacking pattern, and compact dense structure without sacrificing thickness control, which are suitable for molecular sieving gas separation. One of the hybrid membranes, TpEBr@TpPa-SO3Na with a thickness of 41 nm, shows a H2 permeance of 2566 gas permeation units (GPUs) and a H2/CO2 separation factor of 22.6 at 423 K, surpassing the recent Robeson upper bound along with long-term hydrothermal stability. This strategy provides not only a high-performance H2 separation membrane candidate but also an inspiration for pore engineering of COF or 2D porous polymer membranes.

Ultrathin Two-Dimensional Membranes Assembled by Ionic Covalent Organic Nanosheets with Reduced Apertures for Gas Separation

Covalent organic frameworks (COFs) hold great promise in molecular separations owing to their robust, ordered and tunable porous network structures. Currently, the pore size of COFs is usually much larger than most small molecules. Meanwhile, the weak interlamellar interaction between COF nanosheets impedes the preparation of defect-free membranes. Herein, we report a series of COF membranes through a mixed-dimensional assembly of 2D COF nanosheets and 1D cellulose nanofibers (CNFs). The pore size of 0.45–1.0 nm is acquired from the sheltering effect of CNFs, rendering membranes precise molecular sieving ability, besides the multiple interactions between COFs and CNFs elevate membrane stability. Accordingly, the membranes exhibit a flux of 8.53 kg m−2 h−1 with a separation factor of 3876 for n-butanol dehydration, and high permeance of 42.8 L m−2 h−1 bar−1 with a rejection of 96.8% for Na2SO4 removal. Our mixed-dimensional design may inspire the fabrication and application of COF membranes. The fabrication of defect-free covalent organic framework (COF) membranes for the separation of small molecules is challenging. Here, the authors report robust COF membranes with precise molecular sieving through a mixed-dimensional assembly, exhibiting high performance for alcohol dehydration and salt rejection.

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Covalent organic framework membranes through a mixed-dimensional assembly for molecular separations.

Covalent organic frameworks (COFs) with designable architectures and superior stability have afforded intriguing opportunities to develop advanced separation membranes. Currently, COF membranes are rarely used in molecular separations due to their relatively large pore sizes. Herein, we report a vapor–liquid interfacial synthesis approach to in situ generate an azine-linked COF layer onto a pre-assembled carboxyl-functionalized COF layer at room temperature, acquiring an ultrathin heterostructured COF membrane with narrowly distributed interfacial sub-nanopores. Attributed to the covalent linkages between the two COF layers, the narrowly distributed interfacial sub-nanopores of ∼0.39 nm within the heterostructured COF membranes were formed, which endow the membranes with distinct interfacial molecular sieving capacity. The resulting COF membrane with a thickness of 28 nm exhibits a superior separation factor of 4464 and a high permeation flux of 14.35 kg m−2 h−1 for water/n-butanol separations. The design of ultrathin heterostructured COF membranes with interfacial molecular sieving capacity may facilitate the development of framework nanomaterial based membranes.

Yumeng Zhao

Papers

Covalent organic framework membranes through a mixed-dimensional assembly for molecular separations.

Ultrathin heterostructured covalent organic framework membranes with interfacial molecular sieving capacity for fast water-selective permeation