Continuous Covalent Organic Frameworks Membranes: From Preparation Strategies to Applications

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.

Advanced Microporous Framework Membranes for Sustainable Separation

Advancements in membrane-based separation hinge on the design of materials that transcend conventional limitations. Microporous materials, including metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), macrocycles, and porous organic cages (POCs) offer unprecedented control over pore architecture, chemical functionality, and transport properties, making them promising candidates for next-generation membrane technologies. The well-defined and tunable micropores provide a pathway to directly address the permeability-selectivity trade-off inherent in conventional polymer membranes. Here, this review explores the latest advancements in these four representative microporous membranes, emphasizing their breakthroughs in hydrocarbon separation, liquid-phase molecular sieving, and ion-selective transport, particularly focusing on their structure-performance relationships. While their tailored structures enable exceptional performance, practical adoption requires overcoming hurdles in scalability, durability, and compatibility with industrial processes. By offering insights into membrane structure optimization and innovative design strategies, this review provides a roadmap for advancing microporous membranes from laboratory innovation to real-world implementation, ultimately supporting global sustainability goals through energy-efficient separation processes.

Covalent Organic Framework Nanofilm-Assisted Laser Desorption Ionization Mass Spectrometry for Rapid Screening of Parabens in Personal Care Products

RATIONAL

People are widely exposed to parabens in their daily life, but parabens are endocrine disrupting chemicals that pose a threat to human health. Therefore, establishing a rapid screening method to enhance monitoring of parabens is necessary. Herein, a covalent organic framework (COF) nanofilm-assisted laser desorption ionization mass spectrometry (LDI-MS) method was established to screen parabens in personal care products (PCPs).

METHODS

TAPB-TFPB-COF nanofilm was synthesized on indium tin oxide (ITO) glass and used as LDI-MS substrates. To observe the practicability of TAPB-TFPB-COF nanofilm-assisted LDI-MS, the results of this method for analyzing small molecules such as parabens, estrogens, and bisphenols were compared with those of the conventional organic matrix 9-aminoacridine (9-AA), and the reproducibility and detection limit were further verified. Finally, the method was applied to screen parabens in PCPs.

RESULTS

TAPB-TFPB-COF nanofilm-assisted LDI-MS analyzed small molecules such as parabens, estrogens, and bisphenols with higher mass spectral signals and cleaner mass spectral backgrounds compared with 9-AA. Meanwhile, the method analyzed methylparaben (MeP) with high reproducibility (RSD = 6.96%) and low detection limit (1.64 μM) and performed well for rapid screening of parabens in PCPs.

CONCLUSION

TAPB-TFPB-COF nanofilm-assisted LDI-MS for analyzing small molecules such as parabens, estrogens, and amino acids offered the advantages of rapid analysis, a clean background, and good reproducibility. The method was successfully applied to detecting parabens in PCPs, demonstrating the practical utility of LDI-MS based on TAPB-TFPB-COF nanofilm for analyzing parabens in complex samples.

Pioneering the Future: Principles, Advances, and Challenges in Organic Electrodes for Aqueous Ammonium-Ion Batteries

Aqueous ammonium-ion (NH4 +) batteries (AAIBs) have recently been considered as attractive alternatives for next-generation large-scale energy storage systems, on account of their cost-effectiveness, nonflammability, less corrosive, small hydrated ionic radius, and rapid NH4 + diffusion kinetics. In addition, the tetrahedral structure of NH4 + exhibits preferential orientation characteristics, resulting in a different electrochemical storage mechanism from spherical charge carriers such as Li+, Na+, and K+. Therefore, unlocking the NH4 +-ion storage mechanisms in host electrode materials is pivotal to advancing the design of high-performance AAIBs. Organic materials, with their customizable, flexible, and stable molecular structures, along with their ease of recycling and disposal, offer tremendous potential. However, the development of cutting-edge organic electrode materials specifically for ammonium-ion storage in AAIBs remains an exciting, yet largely untapped, frontier. This review systematically explores the interaction mechanisms between NH4 + ions and organic electrode materials, such as electrostatic interactions including hydrogen bonding. It also highlights the application of diverse organic electrode materials, such as small molecules, conducting polymers, covalent organic frameworks (COFs), and organic-inorganic hybrids in AAIBs. Lastly, the review addresses the key challenges and future perspectives of organic-material-based AAIBs, aiming to push the boundaries of cutting-edge aqueous energy storage systems.

Design of Self-Standing Chiral Covalent-Organic Framework Nanochannel Membrane for Enantioselective Sensing

Nanochannel membranes are promising materials for enantioselective sensing. However, it is difficult to make a compromise between the selectivity and permeability in traditional nanochannel membranes. Therefore, new types of nanochannel membranes with high enantioselectivity and excellent permeability should be explored for chiral analysis. Here, asymmetric catalysis strategy is reported for interfacial polymerization synthesis of chiral covalent-organic framework (cCOF) nanochannel membrane for enantioselective sensing. Chiral phenylethylamine (S/R-PEA) and 2,4,6-triformylphloroglucinol (TP) are used to prepare chiral TP monomer. 4,4',4″-triaminotriphenylamine (TAPA) is then condensed with chiral TP to obtain cCOF nanochannel membrane via a C═N Schiff-base reaction. The molar ratio of TP to S/R-PEA is adjusted so that S/R-PEA is bound to the aldehyde only or both the aldehyde and hydroxyl groups on TP to obtain chiral-induced COF (cCOF-1) or both chiral-induced and modified COF (cCOF-2) nanochannel membrane, respectively. The prepared cCOF-2 nanochannel membrane showed two times more selectivity for limonene enantiomers than cCOF-1 nanochannel membrane. Furthermore, cCOF-2 nanochannel platform exhibited excellent sensing performance for other chiral molecules such as limonene, propanediol, methylbutyric acid, ibuprofen, and naproxen (limits of detection of 19-42 ng L-1, enantiomer excess of 63.6-86.3%). This work provides a promising way to develop cCOF-based nanochannel enantioselective sensor.

Photoenzyme Coupling System: Covalent Organic Frameworks In Situ Production of Hydrogen Peroxide Cascaded with Unspecific Peroxygenase to Achieve C-H Bonds Selective Activation

As an efficient, sustainable, and environmentally friendly semiconductor material, covalent organic frameworks (COFs) can generate hydrogen peroxide (H2O2) by photocatalysis, attracting wide attention in recent years. Herein, the effects of hydroxyl, methoxyl, and vinyl groups of imide-linked two-dimensional (2D) COFs on the photocatalytic production of H2O2 were studied theoretically and experimentally. The introduction of vinyl groups greatly promotes the photogenerated charge separation and migration of COFs, providing more oxygen adsorption sites, stronger proton affinity, and lower intermediate binding energy, which effectively facilitates the rapid conversion of oxygen to H2O2. Further, we have integrated the properties of the photocatalytic in situ generation of H2O2 by COFs and the continuous consumption of H2O2 by unspecific peroxygenases (UPOs) to construct a mild and simple photoenzyme coupling system that can achieve selective activation of C-H bonds without the need of any external oxidants or sacrificial agents. This simple, stable, and compatible photoenzyme system avoids irreversible enzyme damage caused by excessive exogenous H2O2 and the utilization of sacrificial agents, thus providing an efficient and green pathway for fine chemical synthesis. This system not only breaks the restriction of continuous exogenous H2O2 supplementation on the UPO catalytic system but also provides a new practical application direction for semiconductor photocatalytic H2O2 production.

De Novo Design of Aminopropyl Quaternary Ammonium-Functionalized Covalent Organic Frameworks for Enhanced Polybenzimidazole Anion Exchange Membranes

Quaternary ammonium functionalized covalent organic frameworks (COFs) have great potential to enhance hydroxide transport owing to crystalline ordered 1D nanochannels, however, suffer from limited quaternary ammonium functional monomers and poor membrane-forming ability. In this work, a novel aminopropyl quaternary ammonium-functionalized COF (DCOF) is designed and synthesized via a bottom-up strategy. The self-supporting DCOF membrane exhibits high crystallinity with a dense and orderly arrangement of quaternary ammonium groups (IEC, 2.07 mmol g-1), achieving a high hydroxide conductivity of 172.5 mS cm-1 and an extremely low water swelling of 5.3% at 80 °C. The exfoliated DCOF colloidal suspension is further incorporated into quaternary ammonium di-cation grafted polybenzimidazoles (DPBI) matrix. Molecular simulations reveal strong electrostatic and van der Waals interfacial interactions between DCOF and DPBI, which enable a high doping content of 20 wt.% and interconnected ionic channels through the surface and nanochannels of the DCOF. The DCOF/DPBI-20% membrane exhibits a tensile strength of 29.7 MPa, a hydroxide conductivity of 135.3 mS cm-1, and a low swelling ratio of 37.2% at 80 °C. A H2/O2 single cell assembled with the membrane reaches a peak power density of 323 mW cm- 2, surpassing most recently reported COF-based membranes.

Assembly-Dissociation-Reconstruction Synthesis of Covalent Organic Framework Membranes with High Continuity for Efficient CO2 Separation

Covalent organic frameworks (COFs), at the forefront of porous materials, hold tremendous potential in membrane separation; however, achieving high continuity in COF membranes remains crucial for efficient gas separation. Here, we present a unique approach termed assembly-dissociation-reconstruction for fabricating COF membranes tailored for CO2/N2 separation. A parent COF is designed from two-node aldehyde and three-node amine monomers and dissociated to high-aspect-ratio nanosheets. Subsequently, COF nanosheets are orderly reconstructed into a crack-free membrane by surface reaction under water evaporation. The membrane exhibits high crystallinity, open pores and a strong affinity for CO2 adsorption over N2, resulting in CO2 permeance exceeding 1060 GPU and CO2/N2 selectivity surpassing 30.6. The efficacy of this strategy offers valuable guidance for the precise fabrication of gas-separation membranes.

Recent Trends in Polymer Membranes: Fabrication Technique, Characterization, Functionalization, and Applications in Environmental Science (Part I)

Polymer membranes have gained significant attention in recent years due to their pivotal role in addressing various environmental challenges such as water purification, gas separation, and pollutant removal [...].

Covalent Organic Framework-Based Materials for Advanced Lithium Metal Batteries

Lithium metal batteries (LMBs), with high energy densities, are strong contenders for the next generation of energy storage systems. Nevertheless, the unregulated growth of lithium dendrites and the unstable solid electrolyte interphase (SEI) significantly hamper their cycling efficiency and raise serious safety concerns, rendering LMBs unfeasible for real-world implementation. Covalent organic frameworks (COFs) and their derivatives have emerged as multifunctional materials with significant potential for addressing the inherent problems of the anode electrode of the lithium metal. This potential stems from their abundant metal-affine functional groups, internal channels, and widely tunable architecture. The original COFs, their derivatives, and COF-based composites can effectively guide the uniform deposition of lithium ions by enhancing conductivity, transport efficiency, and mechanical strength, thereby mitigating the issue of lithium dendrite growth. This review provides a comprehensive analysis of COF-based and derived materials employed for mitigating the challenges posed by lithium dendrites in LMB. Additionally, we present prospects and recommendations for the design and engineering of materials and architectures that can render LMBs feasible for practical applications.

Covalent organic frameworks and their composites for rechargeable batteries

Covalent organic frameworks (COFs), characterized by well-ordered pores, large specific surface area, good stability, high precision, and flexible design, are a promising material for batteries and have received extensive attention from researchers in recent years. Compared with inorganic materials, COFs can construct elastic frameworks with better structural stability, and their chemical compositions and structures can be precisely adjusted and functionalized at the molecular level, providing an open pathway for the convenient transfer of ions. In this review, the energy storage mechanism and unique superiority of COFs and COF composites as electrodes, separators and electrolytes for rechargeable batteries are discussed in detail. Special emphasis is placed on the relationship between the establishment of COF structures and their electrochemical performance in different batteries. Finally, this review summarizes the challenges and prospects of COFs and COF composites in battery equipment.

Self-standing perylene diimide covalent organic framework membranes for trace TMA sensing at room temperature

The unprecedented demand for highly selective, real-time monitoring and low-power gas sensors used in food quality control has been driven by the increasing popularity of the Internet of Things (IoT). Herein, the self-standing perylene diimide based covalent organic framework membranes (COFMPDI-THSTZ) were prepared via liquid-liquid interfacial synthesis method. By incorporating the perylene diimide monomer into the COFM through molecular engineering, COFMPDI-THSTZ based sensor demonstrated an outstanding trimethylamine (TMA)-sensing performance at room temperature. Benefited from the TMA-accessible self-standing membrane morphology, π-electron delocalization effect, and extensive surface area with continuous nanochannels, the specific and highly sensitive TMA measurement has been achieved within the range of 0.03-400 ppm, with an exceptional theoretical detection limit as low as 10 ppb. Moreover, the primary internal mechanism of COFMPDI-THSTZ for this efficient TMA detection was investigated through in-situ FT-IR spectra, thereby directly elucidating that the chemisorption interaction of oxygen modulated the depletion layers on sensing material surface, resulting in alterations in sensor resistance upon exposure to the target gas. For practical usage, COFMPDI-THSTZ based sensor exhibited exceptional real-time in-situ sensing capabilities, further confirmed their potential for application in dynamic prediction evaluation of marine fish products and quality monitoring in IoT.

Thiol-Ene Click Reaction in Constructing Liquid Separation Membranes for Water Treatment

In the evolving landscape of water treatment, membrane technology has ascended to an instrumental role, underscored by its unmatched efficacy and ubiquity. Diverse synthesis and modification techniques are employed to fabricate state-of-the-art liquid separation membranes. Click reactions, distinguished by their rapid kinetics, minimal byproduct generation, and simple reaction condition, emerge as a potent paradigm for devising eco-functional materials. While the metal-free thiol-ene click reaction is acknowledged as a viable approach for membrane material innovation, a systematic elucidation of its applicability in liquid separation membrane development remains conspicuously absent. This review elucidates the pre-functionalization strategies of substrate materials tailored for thiol-ene reactions, notably highlighting thiolation and introducing unsaturated moieties. The consequential implications of thiol-ene reactions on membrane properties-including trade-off effect, surface wettability, and antifouling property-are discussed. The application of thiol-ene reaction in fabricating various liquid separation membranes for different water treatment processes, including wastewater treatment, oil/water separation, and ion separation, are reviewed. Finally, the prospects of thiol-ene reaction in designing novel liquid separation membrane, including pre-functionalization, products prediction, and solute-solute separation membrane, are proposed. This review endeavors to furnish invaluable insights, paving the way for expanding the horizons of thiol-ene reaction application in liquid separation membrane fabrication.

Ultrathin Free-Standing Porous Aromatic Framework Membranes for Efficient Anion Transport

Porous aromatic frameworks (PAFs) show promising potential in anionic conduction due to their high stability and customizable functionality. However, the insolubility of most PAFs presents a significant challenge in their processing into membranes and subsequent applications. In this study, continuous PAF membranes with adjustable thickness were successfully created using liquid-solid interfacial polymerization. The rigid backbone and the stable C-C coupling endow PAF membrane with superior chemical and dimensional stabilities over most conventional polymer membranes. Different quaternary ammonium functionalities were anchored to the backbone through flexible alkyl chains with tunable length. The optimal PAF membrane exhibited an OH- conductivity of 356.6 mS ⋅ cm-1 at 80 °C and 98 % relative humidity. Additionally, the PAF membrane exhibited outstanding alkaline stability, retaining 95 % of its OH- conductivity after 1000 hours in 1 M NaOH. To the best of our knowledge, this is the first application of PAF materials in anion exchange membranes, achieving the highest OH- conductivity and exceptional chemical/dimensional stability. This work provides the possibility for the potential of PAF materials in anionic conductive membranes.

Ion-Concentration-Hopping Heterolayer Gel for Ultrahigh Gradient Energy Conversion

Conventional solid ion channel systems relying on single one- or two-dimensional confined nanochannels enabled selective and ultrafast convective ion transport. However, due to intrinsic solid channel stacking, these systems often face pore-pore polarization and ion concentration blockage, thereby restricting their efficiency in macroscale ion transport. Here, we constructed a soft heterolayer-gel system that integrated an ion-selective hydrogel layer with a water-barrier organogel layer, achieving ultrahigh cation selectivity and flux and effectively providing high-efficiency gradient energy conversion on a macroscale order of magnitude. Specifically, the hydrogel layer featured an unconfined 3D network, where the fluctuations of highly hydrated polyelectrolyte chains driven by thermal dynamics enhanced cation selectivity and mitigated transfer energy barriers. Such chain fluctuation mechanisms facilitated ion-cluster internal transmission, thereby enhancing ion concentration hopping for more efficient ion-selective transport. Compared to the existing rigid nanochannel-based gradient energy conversion systems, such a heterogel-based power generator exhibited a record power density of 192.90 and 1.07 W/m2 at the square micrometer scale and square centimeter scale, respectively (under a 500-fold artificial solution). We anticipate that such heterolayer gels would be a promising candidate for energy separation and storage applications.

From Sheep Track to Motorway: Supramolecular-Mediated 2D Nanofluidic Channels for Ultrafast Water Transport

Atomic thick 2D materials hold great potential as building blocks to construct highly permeable membranes, yet the permeability of laminar 2D material membranes is still limited by their irregularity sheep track-like interlayer channels. Herein, a supramolecular-mediated strategy to induce the regular assembly of high-throughput 2D nanofluidic channels based on host-guest interactions is proposed. Inspired by the characteristics of motorways, supramolecular-mediated ultrathin 2D membranes with broad and continuous regular water transport channels are successfully constructed using graphene oxide (GO) as an example. The prepared membrane achieves an ultrahigh water permeability (369.94 LMH bar-1) more than six times higher than that of the original membranes while maintaining dye rejection above 98.5%, which outperforms the reported 2D membranes. Characterization and simulation results show that the introduction of hyaluronate-grafted β-cyclodextrin not only expands the interlayer channels of GO membranes but also enables the membranes to operate stably under harsh conditions with the help of host-guest interactions. This universal supramolecular assembly strategy provides new opportunities for the preparation of 2D membranes with high separation performance and reliable and stable nanofluidic channels.

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.

Methanol recovery: potential of nanolaminate organic solvent nanofiltration (OSN) membranes

Researchers have made a significant breakthrough by merging the energy-saving attribute of organic solvent nanofiltration (OSN) with the remarkable solvent permeance and solute rejection of two-dimensional (2D) laminated membranes. This innovative approach brings forth a new era of sustainable and cost-effective separation techniques, presenting a promising solution to the issue of industrial solvents contaminating the environment. This development paves the way for new opportunities in building a sustainable future. Specifically, our mini-review has cast a spotlight on the separation and recovery of methanol-a solvent abundantly used in industrial processes. We systematically evaluated a diverse array of free-standing 2D nanolaminate OSN membranes. The analysis encompasses the assessment of pure methanol permeance, solute rejection capabilities, and the simultaneous evaluation of methanol permeance and solute rejection performance. Notably, this study sheds light on the considerable potential of 2D laminated OSN membranes in revolutionizing separation processes for the industrial use of methanol.

Covalent Organic Framework-derived Composite Membranes for Water Treatment

Water treatment has experienced a surge in the adoption of membrane separation technology. Covalent organic frameworks (COFs), a class of metal-free and open-framework materials, have emerged as potential membrane materials owing to their interconnected periodic porosity, tunability, and chemical stability. However, the challenges associated with processing COF powders into self-standing membranes have spurred the emergence of COF composite membranes. This review article highlights the rationale behind developing COF composite membranes and their categories, including mixed matrix membranes (MMMs) and thin film composite (TFC) membranes. The common fabrication techniques of each category are presented. In addition, the influence of COF additives on the performance of the resultant composite membranes is systematically discussed, with a focus on the recent progress in applying COF composite membranes in the separation of different categories of water pollutants, including organic ions/molecules, toxic solvents, proteins, toxic heavy metals, and radionuclides.

Explorative Image Analysis of Methylene Blue Interactions with Gelatin in Polypropylene Nonwoven Fabric Membranes: A Potential Future Tool for the Characterization of the Diffusion Process

This manuscript explores the interaction between methylene blue dye and gelatin within a membrane using spectroscopy and image analysis. Emphasis is placed on methylene blue's unique properties, specifically its ability to oscillate between two distinct resonance states, each with unique light absorption characteristics. Image analysis serves as a tool for examining dye diffusion and absorption. The results indicate a correlation between dye concentrations and membrane thickness. Thinner layers exhibit a consistent dye concentration, implying an even distribution of the dye during the diffusion process. However, thicker layers display varying concentrations at different edges, suggesting the establishment of a diffusion gradient. Moreover, the authors observe an increased concentration of gelatin at the peripheries rather than at the center, possibly due to the swelling of the dried sample and a potential water concentration gradient. The manuscript concludes by suggesting image analysis as a practical alternative to spectral analysis, particularly for detecting whether methylene blue has been adsorbed onto the macromolecular network. These findings significantly enhance the understanding of the complex interactions between methylene blue and gelatin in a membrane and lay a solid foundation for future research in this field.