Crystalline nanosheets of three-dimensional supramolecular frameworks with uniform thickness and high stability

Fabricating three dimensional (3D) supramolecular frameworks (SMFs) into stable crystalline nanosheets remains a great challenge due to the homogeneous and weak inter-building block interactions along 3D directions. Herein, crystalline nanosheets of a 3D SMF with a uniform thickness of 4.8 ± 0.1 nm immobilized with Pt nanocrystals on the surface (Q[8]/Pt NSs) were fabricated via the solid-liquid reaction between cucurbit[8]uril/H2PtCl6 single crystals and hydrazine hydrate with the help of gas and heat yielded during the reaction process. A series of experiments and theoretical calculations reveal the ultrahigh stability of Q[8]/Pt NSs due to the high density hydrogen bonding interaction among neighboring Q[8] molecules. This in turn endows Q[8]/Pt NSs with excellent photocatalytic and continuous thermocatalytic CO oxidation performance, representing the thus-far reported best Pt nano-material-based catalysts.

Aminal-Linked Covalent Organic Framework Membranes Achieve Superior Ion Selectivity

High-salinity wastewater treatment is perceived as a global water resource recycling challenge that must be addressed to achieve zero discharge. Monovalent/divalent salt separation using membrane technology provides a promising strategy for sulfate removal from chlor-alkali brine. However, existing desalination membranes often show low water permeance and insufficient ion selectivity. Herein, an aminal-linked covalent organic framework (COF) membrane featuring a regular long-range pore size of 7 Å and achieving superior ion selectivity is reported, in which a uniform COF layer with subnanosized channels is assembled by the chemical splicing of 1,4-phthalaldehyde (TPA)-piperazine (PZ) COF through an amidation reaction with trimesoyl chloride (TMC). The chemically spliced TPA-PZ (sTPA-PZ) membrane maintains an inherent pore structure and exhibits a water permeance of 13.1 L m-2 h-1 bar-1, a Na2SO4 rejection of 99.1%, and a Cl-/SO4 2- separation factor of 66 for mixed-salt separation, which outperforms all state-of-the-art COF-based membranes reported. Furthermore, the single-stage treatment of NaCl/Na2SO4 mixed-salt separation achieves a high NaCl purity of above 95% and a recovery rate of ≈60%, offering great potential for industrial application in monovalent/divalent salt separation and wastewater resource utilization. Therefore, the aminal-linked COF membrane developed in this work provides a new research avenue for designing smart/advanced membrane materials for angstrom-scale separations.

3D Covalent Organic Frameworks from Design, Synthesis to Applications in Optoelectronics

Covalent organic frameworks (COFs), a new class of crystalline materials connected by covalent bonds, have been developed rapidly in the past decades. However, the research on COFs is mainly focused on two-dimensional (2D) COFs, and the research on three-dimensional (3D) COFs is still in the initial stage. In 2D COFs, the covalent bonds exist only in the 2D flakes and can form 1D channels, which hinder the charge transport to some extent. In contrast, 3D COFs have a more complex pore structure and thus exhibit higher specific surface area and richer active sites, which greatly enhance the 3D charge carrier transport. Therefore, compared to 2D COFs, 3D COFs have stronger applicability in energy storage and conversion, sensing, and optoelectronics. In this review, it is first introduced the design principles for 3D COFs, and in particular summarize the development of conjugated building blocks in 3D COFs, with a special focus on their application in optoelectronics. Subsequently, the preparation of 3D COF powders and thin films and methods to improve the stability and functionalization of 3D COFs are summarized. Moreover, the applications of 3D COFs in electronics are outlined. Finally, conclusions and future research directions for 3D COFs are presented.

Organic Solvent Nanofiltration in Pharmaceutical Applications

Separation and purification in organic solvents are indispensable procedures in pharmaceutical manufacturing. However, they still heavily rely on the conventional separation technologies of distillation and chromatography, resulting in high energy and massive solvent consumption. As an alternative, organic solvent nanofiltration (OSN) offers the benefits of low energy consumption, low solid waste generation, and easy scale-up and incorporation into continuous processes. Thus, there is a growing interest in employing membrane technology in the pharmaceutical area to improve process sustainability and energy efficiency. This Review comprehensively summarizes the recent progress (especially the last 10 years) of organic solvent nanofiltration and its applications in the pharmaceutical industry, including the concentration and purification of active pharmaceutical ingredients, homogeneous catalyst recovery, solvent exchange and recovery, and OSN-assisted peptide/oligonucleotide synthesis. Furthermore, the challenges and future perspectives of membrane technology in pharmaceutical applications are discussed in detail.

Fine-tuning the pore environment of ultramicroporous three-dimensional covalent organic frameworks for efficient one-step ethylene purification

The construction of functional three-dimensional covalent organic frameworks (3D COFs) for gas separation, specifically for the efficient removal of ethane (C2H6) from ethylene (C2H4), is significant but challenging due to their similar physicochemical properties. In this study, we demonstrate fine-tuning the pore environment of ultramicroporous 3D COFs to achieve efficient one-step C2H4 purification. By choosing our previously reported 3D-TPB-COF-H as a reference material, we rationally design and synthesize an isostructural 3D COF (3D-TPP-COF) containing pyridine units. Impressively, compared with 3D-TPB-COF-H, 3D-TPP-COF exhibits both high C2H6 adsorption capacity (110.4 cm3 g-1 at 293 K and 1 bar) and good C2H6/C2H4 selectivity (1.8), due to the formation of additional C-H···N interactions between pyridine groups and C2H6. To our knowledge, this performance surpasses all other reported COFs and is even comparable to some benchmark porous materials. In addition, dynamic breakthrough experiments reveal that 3D-TPP-COF can be used as a robust absorbent to produce high-purity C2H4 directly from a C2H6/C2H4 mixture. This study provides important guidance for the rational design of 3D COFs for efficient gas separation.

Recent advances in the fabrication of organic solvent nanofiltration membranes using covalent/metal organic frameworks

Organic solvent nanofiltration (OSN) has evolved as a vital technological frontier with paramount significance in the separation and purification of organic solvents. Its implication is particularly prominent in industries such as pharmaceuticals, petrochemicals, and environmental remediation. This comprehensive review, meticulously navigates through the current state of research in OSN membranes, unveiling both the critical challenges and promising opportunities that beckon further exploration. The central focus of this review is on the unique utilization of covalent organic frameworks (COFs) and metal-organic frameworks (MOFs) in OSN membrane design, leveraging their distinctive structural attributes-tunable porosity, robust chemical stability, and molecular sieving capabilities. These qualities position them as exceptional candidates for crafting membranes tailored to the intricacies of organic solvent environments. Our investigation extends into the fundamental principles that render COFs and MOFs adept in OSN applications, dissecting their varied fabrication methods while offering insights into the advantages and limitations of each. Moreover, we address environmental and sustainability considerations in the use of COF and MOF-based OSN membranes. Furthermore, we meticulously present the latest advancements and innovations in this burgeoning field, charting a course toward potential future directions and emerging research areas. By underscoring the challenges awaiting exploration, this review not only provides a panoramic view of the current OSN landscape but also lays the groundwork for the evolution of efficient and sustainable OSN technologies, specifically harnessing the unique attributes of COFs and MOFs.

Polymer carbon nitride nanosheet-based lamellar membranes inspired by "couple hardness with softness" for ultrafast molecular separation in organic solvents

Membranes with ultrafast molecular separation ability in organic solvents can offer unprecedented opportunities for efficient and low-cost solvent recovery in industry. Herein, a graphene-like polymer carbon nitride nanosheet (PCNN) with a low-friction surface was applied as the main membrane building block to boost the ultrafast transport of the solvent. Meanwhile, inspired by the concept of "couple hardness with softness", soft and flexible graphene oxide (GO) was chosen to fix the random stack of the rigid PCNN and tailor the lamellar structure of the PCNN membrane. The optimal PCNN/GO lamellar membrane shows a remarkable methanol permeance of 435.5 L m-2 h-1 bar-1 (four times higher than that of the GO membrane) while maintaining a high rejection for reactive black (RB, 98.9% in ethanol). Molecular dynamics simulations were conducted to elucidate the ultrafast transport mechanism of the PCNN/GO membrane. This study reveals that PCNN is a promising building block for lamellar membranes and may open up new avenues for high-performance molecular separation membranes.

Ultra-fast supercritically solvothermal polymerization for large single-crystalline covalent organic frameworks

Crystalline polymer materials, e.g., hyper-crosslinked polystyrene, conjugate microporous polymers and covalent organic frameworks, are used as catalyst carriers, organic electronic devices and molecular sieves. Their properties and applications are highly dependent on their crystallinity. An efficient polymerization strategy for the rapid preparation of highly or single-crystalline materials is beneficial not only to structure-property studies but also to practical applications. However, polymerization usually leads to the formation of amorphous or poorly crystalline products with small grain sizes. It has been a challenging task to efficiently and precisely assemble organic molecules into a single crystal through polymerization. To address this issue, we developed a supercritically solvothermal method that uses supercritical carbon dioxide (sc-CO2) as the reaction medium for polymerization. Sc-CO2 accelerates crystal growth due to its high diffusivity and low viscosity compared with traditional organic solvents. Six covalent organic frameworks with different topologies, linkages and crystal structures are synthesized by this method. The as-synthesized products feature polarized photoluminescence and second-harmonic generation, indicating their high-quality single-crystal nature. This method holds advantages such as rapid growth rate, high productivity, easy accessibility, industrial compatibility and environmental friendliness. In this protocol, we provide a step-by-step procedure including preparation of monomer dispersion, polymerization in sc-CO2, purification and characterization of the single crystals. By following this protocol, it takes 1-5 min to grow sub-mm-sized single crystals by polymerization. The procedure takes ~4 h from preparation of monomer dispersion and polymerization in sc-CO2 to purification and drying of the product.

Recent advances of pure/independent covalent organic framework membrane materials: preparation, properties and separation applications

Covalent organic frameworks (COF) are porous crystalline polymers connected by covalent bonds. Due to their inherent high specific surface area, tunable pore size, and good stability, they have attracted extensive attention from researchers. In recent years, COF membrane materials developed rapidly, and a large amount of research work has been presented on the preparation methods, properties, and applications of COF membranes. This review focuses on the research on independent/pure continuous COF membranes. First, based on the membrane formation mechanism, COF membrane preparation methods are categorized into two main groups: bottom-up and top-down. Four methods are presented, namely, solvothermal, interfacial polymerization, steam-assisted conversion, and layer by layer. Then, the aperture, hydrophilicity/hydrophobicity and surface charge properties of COF membranes are summarized and outlined. According to the application directions of gas separation, water treatment, organic solvent nanofiltration, pervaporation and energy, the latest research results of COF membranes are presented. Finally, the challenges and future directions of COF membranes are summarized and an outlook provided. It is hoped that this work will inspire and motivate researchers in related fields.

Large-Area Soluble Covalent Organic Framework Oligomer Coating for Organic Solution Nanofiltration Membranes

Covalent organic frameworks (COFs) are a family of engaging membrane materials for molecular separation, which remain challenging to fabricate in the form of thin-film composite membranes due to slow crystal growth and insoluble powder. Here, an additive approach is presented to construct COF-based thin-film composite membranes in 10 min via COF oligomer coating onto poly(ether ether ketone) (PEEK）ultrafiltration membranes. By the virtue of ultra-thin liquid phase and liquid-solid interface-confined assembly, the COF oligomers are fast stacked up and grow along the interface with the solvent evaporation. Benefiting from the low out-plane resistance of COFs, COF@PEEK composite membranes exhibit high solvent permeances in a negative correlation with solvent viscosity. The well-defined pore structures enable high molecular sieving ability (Mw = 300 g mol-1 ). Besides, the COF@PEEK composite membranes possess excellent mechanical integrities and steadily operate for over 150 h in the condition of high-pressure cross flow. This work not only exemplifies the high-efficiency and scale-up preparation of COF-based thin-film composite membranes but also provides a new strategy for COF membrane processing.

2D Covalent Organic Framework Membranes for Liquid-Phase Molecular Separations: State of the Field, Common Pitfalls, and Future Opportunities

2D covalent organic frameworks (2D COFs) are attractive candidates for next-generation membranes due to their robust linkages and uniform, tunable pores. Many publications have claimed to achieve selective molecular transport through COF pores, but reported performance metrics for similar networks vary dramatically, and in several cases the reported experiments are inadequate to support such conclusions. These issues require a reevaluation of the literature. Published examples of 2D COF membranes for liquid-phase separations can be broadly divided into two categories, each with common performance characteristics: polycrystalline COF films (most >1 µm thick) and weakly crystalline or amorphous films (most <500 nm thick). Neither category has demonstrated consistent relationships between the designed COF pore structure and separation performance, suggesting that these imperfect materials do not sieve molecules through uniform pores. In this perspective, rigorous practices for evaluating COF membrane structures and separation performance are described, which will facilitate their development toward molecularly precise membranes capable of performing previously unrealized chemical separations. In the absence of this more rigorous standard of proof, reports of COF-based membranes should be treated with skepticism. As methods to control 2D polymerization improve, precise 2D polymer membranes may exhibit exquisite and energy efficient performance relevant for contemporary separation challenges.

Flexible Covalent Organic Network with Ordered Honeycomb Nanoarchitecture for Molecular Separations

Membranes with precisely defined nanostructure are desirable for energy-efficient molecular separations. The emergence of membranes with honeycomb lattice or topological nanopores is of fundamental importance. The tailor-made nanostructure and morphology may have huge potential to resolve the longstanding bottlenecks in membrane science and technology. Herein, inspired by honeycomb architecture, we demonstrate an effective and scalable route based on interfacial polymerization (IP) to generate flexible and ordered covalent organic network (CON) membranes for liquid-phase molecular separations. The aperture size of a CON membrane can be reasonably designed through the strong covalent bond between molecular building blocks. The fabricated CON membrane formed by IP showed an obviously size-dependent sieving of molecules, yielding a stepwise conversion from low rejection to the expected high rejection. Moreover, the CON membrane was also found to have the sieving capability for tetracycline and ciprofloxacin, ascribed to the effect of size exclusion by an ordered single-nanoscale channel (<1 nm). This approach provides a viable strategy for creating target-sized channels from molecular-level design and demonstrates their potential for accurate molecular separations.

From 3D to 2D: Directional Morphological Evolution of a Three-Dimensional Covalent Organic Framework

The morphology of materials has a huge impact on their properties and functions; however, the precise control and direct evolution toward specific morphologies remains challenging. Herein, we outline a novel strategy for the morphology modulation of covalent organic frameworks based on COF-300 with the diamond structure, which usually exhibits a three-dimensional shuttle morphology. A monofunctional structural regulator has been designed to break the continuity of the three-dimensional structure. As the proportion of the monofunctional structural regulator increases, the morphology of COF-300 shows a directional evolution from a shuttle morphology to a two-dimensional nanosheet, while still retaining the consistency of the crystal structure. Our study reports the first two-dimensional nanosheet based on a three-dimensional structured COF to date and will inspire future research into the traced morphological evolution in materials by predesign.

Chemically Asymmetric Polymers Manipulate the Crystallization of Two-Dimensional Covalent Organic Frameworks to Synthesize Processable Nanosheets

Nanosheets derived from two-dimensional covalent organic frameworks (2D COFs) are increasingly desirable in various fields. While breakthroughs in the chemical and physical delamination of 2D COFs are rising, precisely regulating the growth of the COF nanosheets has not been realized yet. Herein, we report an effective strategy of polymer-manipulated crystallization to accurately control the growth of COF nanosheets. Chemically asymmetric polyvinylpyrrolidone (PVP) is developed as the manipulator that selectively interacts with the aldehydes and (100) facet to induce anisotropic growth of COFs. The number of PVP constitutional units determines this specific interaction, leading to molecularly thin but thickness-controllable nanosheets with excellent dispersity. We process these nanosheets into robust A4-sized membranes for ultraselective molecular separation. The membrane intercalated with long-chain PVP demonstrates largely improved performance, surpassing the reported COF membranes. This work reports a strategy for anisotropically crystallizing 2D COFs to yield processable nanosheets toward practical applications.

Continuous Covalent Organic Frameworks Membranes: From Preparation Strategies to Applications

Covalent organic frameworks (COFs) are porous crystalline polymeric materials formed by the covalent bonding of organic units. The abundant organic units library gives the COFs species diversity, easily tuned pore channels, and pore sizes. In addition, the periodic arrangement of organic units endows COFs regular and highly connected pore channels, which has led to the rapid development of COFs in membrane separations. Continuous defect-free and high crystallinity of COF membranes is the key to their application in separations, which is the most important issue to be addressed in the research. This review article describes the linkage types of covalent bonds, synthesis methods, and pore size regulation strategies of COFs materials. Further, the preparation strategies of continuous COFs membranes are highlighted, including layer-by-layer (LBL) stacking, in situ growth, interfacial polymerization (IP), and solvent casting. The applications in separation fields of continuous COFs membranes are also discussed, including gas separation, water treatment, organic solvent nanofiltration, ion conduction, and energy battery membranes. Finally, the research results are summarized and the future prospect for the development of COFs membranes are outlined. More attention may be paid to the large-scale preparation of COFs membranes and the development of conductive COFs membranes in future research.

Carbon-doped metal oxide interfacial nanofilms for ultrafast and precise separation of molecules

Membranes with molecular-sized, high-density nanopores, which are stable under industrially relevant conditions, are needed to decrease energy consumption for separations. Interfacial polymerization has demonstrated its potential for large-scale production of organic membranes, such as polyamide desalination membranes. We report an analogous ultrafast interfacial process to generate inorganic, nanoporous carbon-doped metal oxide (CDTO) nanofilms for precise molecular separation. For a given pore size, these nanofilms have 2 to 10 times higher pore density (assuming the same tortuosity) than reported and commercial organic solvent nanofiltration membranes, yielding ultra-high solvent permeance, even if they are thicker. Owing to exceptional mechanical, chemical, and thermal stabilities, CDTO nanofilms with designable, rigid nanopores exhibited long-term stable and efficient organic separation under harsh conditions.

Recent progress of covalent organic frameworks membranes: Design, synthesis, and application in water treatment

To date, significant efforts have been devoted to eliminating hazardous components to purify wastewater through the development of various nanomaterials. Covalent organic frameworks (COFs), an important branch of the porous crystalline family, possess the peculiarity of ultrahigh surface area, adjustable pore size, and facile functionality. Exciting studies from design fabrication to potential applications in water treatment by COF-based membranes (COMs) have emerged. This review summarizes various preparation strategies and synthesis mechanisms for COMs, including layer-by-layer stacking, in situ growth, interfacial polymerization, and electrochemical synthesis, and briefly describes the advanced characterization techniques for COMs. Moreover, the application of COMs in heavy metal removal, dye separation, purification of radionuclides, pollutant detection, sea water desalination, and so on, is described and discussed. Finally, the perspectives on future opportunities for designing COMs in water purification have been proposed.