Green and large-scale production of covalent organic framework nanofiltration membranes | Communications Materials

Communications Materials volume 6, Article number: 61 (2025) Cite this article

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Covalent organic frameworks (COFs) are porous, crystalline materials formed through the covalent bonding of organic units. They are characterized by high surface areas and tunable pore sizes, making them promising candidates for membrane separation. Typically, COFs have pore sizes ranging from 1 to 3 nm, which is ideal for nanofiltration applications. Over the last decade, COF membranes (COFMs) have rapidly advanced and demonstrate significant potential in nanofiltration. However, challenges remain in achieving environmentally friendly and scalable fabrication. This Review addresses the key challenges and opportunities for green and large-scale production of COFMs. We critically evaluate the advantages and disadvantages of various laboratory fabrication methods and propose strategies for eco-friendly and scalable preparation, focusing on green solvents, large-area production, and production efficiency. Finally, we offer our perspectives on synthesizing COFMs sustainably at large-scale.

Covalent organic frameworks (COFs) represent a class of crystalline, porous polymers constructed through the covalent linkage of organic building blocks1. The synthesis of COFs typically involves the condensation of monomers, such as amines and aldehydes or boronic acids, through imine or boronate ester linkages, respectively. These reactions are carefully controlled to form extended two/three-dimensional (2D/3D) framework structures with specific topology. COF materials have attracted considerable interest due to their high surface area, tunable pore size, excellent stability, and the ability to be synthesized with a variety of functional groups, making them a promising candidate for next-generation separation membranes. In general, a membrane-based separation process heavily depends on the pore size of membrane materials2. According to the pore limited diameter and the largest cavity diameter in the COF crystallographic database3, most COF materials have a pore size of 1 ~ 3 nm (Fig. 1a), which means that the most suitable application for COF materials in membrane separation is nanofiltration (NF)2 (Fig. 1b). Analyzing the publications, NF dominates the applications of COF membranes (COFMs) with 67.5% share (Fig. 1c). Hence, this review focuses on the COF-based NF membranes.

a Pore size distribution of 1242 COFs from the crystallographic database3. b Membrane processes for water purification and desalination. c Research activities relating to COFMs. d Timeline of the milestones for COF nanofiltration membranes4,5,6,7,8,9,10,11,12,13,14,15,16,18,19,20,21,22,24,25,26,27,28,29,31,32,33,34,35,36,37,38,39,40,41,96,97,98,99,100,101. e Increasing scientific interest in COF materials and COFMs.

Figure 1d presents a historical timeline showcasing key breakthroughs of COFMs innovations in the NF field. Following the invention of 2D COFs in 20054, and the development of 3D COFs in 20075, the development of membranes using this novel porous COF material is inevitable. Inspired by metal–organic framework membranes, the first supported COFM was produced by the microwave heating method in 2014, but its application in membrane separation was not presented6. In 2015, the first computational prediction of COFs in membrane separation demonstrated the promising future of COFMs in desalination7. In subsequent years, pure COFMs were rapidly developed for NF, and COFs were also blended with polymers to create mixed matrix membranes (MMMs) which offer superior properties compared to pure polymers. In 2017, pure COFMs synthesized via an eco-friendly solid-state mixing method8 and a scalable interfacial crystallization approach9,10 were utilized for NF to separate dye molecules from water. In view of the good compatibility of pure organic COF materials and polyamide (PA) materials, the enhanced desalination performance of PA membranes was obtained via incorporating COF in the interfacial polymerization (IP) process11. In 2018, we synthesized a 400 nm thick imine-linked COFM on alumina tubes via in-situ solvothermal methods for fast water purification12. Thin COFMs ( ~ 60 nm) for organic solvent nanofiltration (OSN) were formed using Langmuir-Blodgett deposition of COF nanofilms synthesized at the water-air interface by Lai and colleagues in the same year13. COFMs synthesized at liquid-liquid or liquid-air interfaces should be transferred to porous substrates to enhance their mechanical properties for separation. To eliminate the transfer process, IP directly on polymeric substrates has been developed by Wang and colleagues, using traditional IP methods for scale-up preparation of PA membranes, to synthesize COF-based membranes14. In addition, the initial COFMs assembled from COF nanosheets were prepared in that year for selective molecular and ionic sieving15.

In the following years, the development of COFMs focuses on the following two aspects. On the one hand, to achieve the fabrication of high-quality COFMs, many different synthetic approaches have been developed, for example, diffusion controlled synthesis16,17, solid-vapor18 and ionic liquids (ILs)-water19 interface synthesis, electrophoretic deposition (EPD)20, phase switching approach21, photo-tailored method22, electrochemical polymerization (ECP)23,24, and interface self-assembly method25. On the other hand, to expand the application area of COFMs through pore engineering26,27,28,29,30,31,32,33,34,35,36,37 and stacking model adjustment19,25,38. As mentioned above, most COF materials have a pore size of 1 ~ 3 nm, limiting their effectiveness in separating small-sized substances, particularly inorganic ions. Pore engineering, involving both pre-synthesis monomer design28,29,30,31,32,33 and post-synthesis pore modification34,35,36, imparts unique properties to COF pore walls28,29,31,32,33,34,35,36,37 and more narrowed nanochannel26,27. Functional groups integrated into the pore walls can selectively interact with specific species for ion separation28,29,30,31,32,33,35,36,37 or introduce charged groups to enhance electrostatic repulsion for desalination34. Adjusting the stacking model of adjacent COF layers is an alternative strategy to narrow the pore size of COFMs, improving their NF performances. For example, Ma and colleagues developed COFMs with a pore size of ~0.6 nm by altering the stacking mode from AA to AB. The AB-stacked COFMs, made of highly ordered nanoflakes, exhibit exceptional performance in OSN, water treatment, and gas separation38.

In addition to the two primary aspects mentioned, some branching but significant advancements have been made in recent years to enhance the development of COF NF membranes. For instance, Chung and colleagues adjusted pore polarity to optimize the OSN performance of COFMs39. Wang and colleagues optimized the orientation of 2D COFMs through solvent vapor annealing for ion separation40, and designed 3D COFMs with narrowed pore sizes for effective separation of small drug molecules and ions36,41,42. We developed a rapid EPD method to assemble ionic COF nanosheets, facilitating the construction of ultrathin COFMs for NF20. This approach demonstrates the potential of COFMs for large-scale production, utilizing a scalable bottom-up synthesis of COF nanosheets at room temperature and an efficient EPD assembly process that takes just a few minutes. In contrast, the Wang23 and Jiang24 groups employed an ECP method to synthesize COFMs for NF, but their fabrication process takes several hours. Very recently, Meng and colleagues reported a new green and scalable scraping-assisted IP (SAIP) technique for fabricating COFMs19. This method uses ILs instead of traditional toxic organic solvents, improving the greenness of the membrane formation process thus facilitating large-scale production.

The development of COFMs demonstrates growing research interest in these membrane materials (Fig. 1e), which are expected to become a next generation of practical membranes. However, the evolution from laboratory research to industrial production is bound to face many thorny problems such as solvent, membrane area, production efficiency, and so on. In this review, we focus on seeking a way for green and scaled-up preparation of COFMs based on existing preparation technologies. In the following sections, we will briefly introduce the basic features of COFMs and then discuss in detail the green and scale-up preparation, application of large-area COFMs for nanofiltration, as well as challenges and prospects.

As discussed above, pore size is usually critical in membrane-based separation. For COFMs, it can be adjusted through monomer and topology design, stacking mode regulation, and pore wall modification. Monomers, as fundamental building units, vary in molecular size and topology, directly influencing the apertures and geometry of the synthesized COF materials (Fig. 2a). Moreover, the choice of monomer can also dictate the chemical functionality and stability of the resulting COF materials, which are essential for specific separation applications43. For instance, monomers with rigid, planar structures tend to form more robust and ordered frameworks with well-defined pore sizes, while flexible monomers may lead to more dynamic or amorphous structures44,45. The topology design, which refers to the spatial arrangement of monomers within the framework, further refines the pore architecture46. Common topologies such as 2D hexagonal or 3D cubic frameworks offer distinct advantages in terms of pore accessibility and mechanical strength. Stacking mode regulation, particularly in 2D COFs, allows for the tuning of interlayer distances via spatial resistances47 or restacking technology48,49,50, which can significantly impact the overall porosity and pore size (Fig. 2b). This approach is commonly used in COFMs to create angstrom-scale pores for ion sieving and gas separation, overcoming pore size limitations of COFs and expanding their application in membrane separation. However, ensuring uniform pore size remains challenging through this strategy, particularly with restacking technology. Finally, pore wall modification through post-synthetic functionalization or pre-modified monomer introduces additional chemical moieties that not only can reduce pore size of COFMs but also can enhance selectivity towards specific molecules through improve the host-guest interactions with the target species51 (Fig. 2c). This strategic incorporation of functional groups can also influence the overall stability and hydrophilic/hydrophobicity of the COFMs, making them more adaptable to various applications. For instance, hydrophilic moieties can facilitate the entry of water molecules into the COFMs pores. Furthermore, the introduction of charged or polar functionalities can facilitate specific interactions, such as hydrogen bonding or electrostatic attraction, which are crucial for the selective separation of target molecules or ions. By tailoring the chemical environment within the pores, the properties of COFMs can be fine-tuned to achieve optimal separation performances. This approach underscores the versatility of COFMs and their potential to address complex challenges in material science and engineering. Together, these strategies provide a comprehensive toolkit for tailoring COFMs to meet the precise requirements of various membrane-based separation processes.

Main pore engineering strategies of a Monomer and topology design, b Stacking mode regulation, and c Pore wall modification. Main separation mechanism of COF nanofiltration membranes including d Size exclusion, e Electrostatic interaction, f Bonding interaction and g Solvation as well as h–k corresponding case studies. Fig. h reproduced with permission from ref. 27 (copyright Royal Society of Chemistry, 2020), Fig. i reproduced with permission from ref. 20 (copyright Wiley-VCH, 2022), Fig. j reproduced with permission from ref. 37 (copyright Wiley-VCH, 2021), Fig. k reproduced with permission from ref. 33 (copyright Proc. Natl. Acad. Sci. USA, 2024).

COFMs, as a class of porous separation membranes, utilize mechanisms such as physical size exclusion (Fig. 2d) and chemical interactions—including electrostatic interaction (Fig. 2e), bonding interactions (Fig. 2f), solvation (Fig. 2g), dielectric effects, and chemical adsorption—to achieve nanofiltration. Size exclusion, driven by the precise designability of COFMs’ pore structures, plays a dominant role, blocking molecules larger than the pore size while allowing smaller ones to pass (Fig. 2d). For instance, Lai and colleagues demonstrated that structural modifications can control the pore size of COFMs, directly impacting molecular separation performance13,27. Using the Langmuir-Blodgett method, they synthesized three ultrathin COFMs with varying carbon side chain lengths to adjust pore size. These membranes exhibited systematic changes in flux and molecular weight cut-off during OSN (Fig. 2h).

Chemical interactions offer greater diversity than physical size screening. Electrostatic interaction becomes particularly significant in the scenarios of separating charged species, which significantly enhance selectivity towards target species due to the electrostatic repulsion (Fig. 2e). For example, negatively charged COFMs prepared by EPD effectively reject negatively charged dye molecules despite their slightly smaller size than the COFMs’ pores, while neutral molecules of the same molecular weight pass through20 (Fig. 2i). In addition, COFMs decorated with charged moieties in the pore wall also successfully achieve the ion sieving29,30,31,32,35,52,53,54,55 and desalination56,57,58,59 via the electrostatic interaction mechanism. Bonding interactions, such as hydrogen bonding, dipole interactions, π-π stacking, van der Waals forces, and stereoselective interactions, enhance selective recognition of specific species like metal ions or chiral molecules through tailored functional group modifications (Fig. 2f). For instance, COFMs with sub-2-nanometer channels and abundant hydrogen bonding sites exhibit high permeability for monovalent cations (K+, Na+, and Li+) but low permeability for multivalent cations (Mg2+) due to stronger hydrogen bonding interactions between the hydrated multivalent cations and the COF channel walls37, resulting in high selectivity (Fig. 2j). Solvation effect modulates transport kinetics by influencing the hydration states of both membrane channels and permeating species (Fig. 2g). For instance, Sun and colleagues demonstrated the enhance ion selectivity by tuning the pore solvation abilities of COFMs, which were manipulated by adjusting the lengths of oligoether segments attached to the pore channels33. This study displayed that increasing the length of the oligoether chain facilitated ion transport, but the COFM with oligoether chains containing two ethylene oxide units exhibited the highest separation factor for Li+/Mg2+, achieving an exceptional selectivity of up to 1352. This was attributed to the most pronounced discrepancy in transmembrane energy barrier between Li+ and Mg2+ in this specific COFMs (Fig. 2k). This study provides insights into selective ion transport within confined nano-spaces and valuable design principles for developing highly selective COFMs. Notably, these mechanisms often operate synergistically-the ordered nanochannels may amplify chemical interactions through confined mass transfer process.

To objectively evaluate covalent organic framework membranes (COFMs), a comprehensive comparison was conducted between COFs and other common membrane materials - metal-organic frameworks (MOFs), hydrogen-bonded organic frameworks (HOFs), polymers of intrinsic microporosity (PIMs), zeolites, traditional polymers, and layered 2D materials - focusing on pore structure, crystallinity, stability, diversity, designability, processing, key advantages, and limitations (Table 1). COFs feature highly ordered and tunable pores (typically 1 ~ 3 nm), enabling precise molecular sieving for selective separation of small molecules or ions. Their chemical versatility allows functional group modifications to enhance properties like hydrophilicity, antifouling, or target-specific interactions. COFMs also demonstrate superior stability in harsh environments (e.g., organic solvents, acids/bases) compared to conventional polymers, making them suitable for diverse separation applications. However, challenges remain in green and scale-up preparation of defect-free COFMs. Additionally, pore alignment and intergrowth issues in thin-film configurations can compromise permeability and selectivity. While COFMs theoretically allow pore-size engineering, achieving sub-nanometer precision ( < 1 nm) for gas separation and ion sieving is difficult, limiting their competitiveness with other advanced membranes. Overall, COFMs show promise for high-precision separations but require breakthroughs in green and scalable fabrication, as well as sub-nanopore control, to bridge the gap between lab research and industrial adoption.

Over the past decade, various methods for fabricating COFMs have been developed43,60,61, mainly categorized into in situ growth (Fig. 3a)12, IP at free interface (Fig. 3b)9, IP at porous substrate (Fig. 3c)14, layer-by-layer (LBL) stacking of COF nanosheets (Fig. 3d)62, and solution casting of COF-based MMMs (Fig. 3e)63. As comprehensive reviews on these methods are available, we will not reiterate their details here43,60,61. An analysis based on six criteria - ease of operation, preparation efficiency, membrane quality, energy-saving, scalability, and environmental friendliness - highlights the respective advantages and disadvantages of these methods (Table 2, Fig. 3f~j). On this basis, we believe that such factors like solvents, membrane area, production efficiency, energy consumption, mainly limit the green and scale-up preparation of COFMs. This part will discuss, therefore, the impact of these factors on the green and scale-up preparation of COFMs.

a In situ growth. b IP at free interface. c IP at porous substrate. d LBL stacking. e Solution casting. f–j Evaluation of corresponding methods from six criteria of ease of operation, preparation efficiency, membrane quality, energy-saving, scalability, and environmental friendliness.

Increasing awareness of the harmful effects of toxic solvents has led to a growing interest in non-toxic, eco-friendly alternatives for membrane preparation64. The advantages of using green solvents are illustrated in Fig. 4a. Based on the analysis of COFM preparation methods, most modern COFM fabrication techniques depend on conventional solvents like mesitylene, 1,4-dioxane, 1,2-dichlorobenzene, dichloromethane, and acetonitrile. However, these solvents pose serious risks to human health and the environment. To mitigate the environmental and health risks linked to conventional solvents, it is essential to explore eco-friendly alternatives. The ideal green solvent should effectively dissolve the selected COF monomers, matching the performance of traditional solvents while reducing harmful impacts on the environment and human health. In general, green solvents used for membrane fabrication can be divided into water, bio-sourced solvents, green synthetic organic solvents, ILs, deep eutectic solvents (DESs), and supercritical fluids (Fig. 4b)64,65. Due to the increasing interest in green chemistry, there are also some initial attempt to synthesize COF materials or COFMs using green solvents like water8,59,66, bio-sourced solvents67, green synthetic organic solvents68, ILs19,69,70 and DESs71,72, supercritical fluids73. Specially, the synthesis time of COF materials using DESs71 and ILs69 can be reduced to several hours, even to several minutes, thus greatly improving the fabrication efficiency. Additionally, solvent-free synthesis of COFMs is possible since COF materials have been produced using this method74. However, the process requires prolonged high-temperature heating, which restricts scalability. The essential reason may be that the COF synthesis process requires solvents or elevated temperatures to facilitate the diffusion of monomers for their organized assembly into a crystalline framework. Consequently, suitable solvents or prolonged high-temperature heating are generally complementary.

a Advantages of green solvents. b Commonly used green solvents for synthesizing COFMs or COF materials. c Scalable method to prepare self-standing porous COFMs utilizing small amounts of water as solvent. Reproduced with permission from ref. 8 (copyright Wiley-VCH, 2017). d Fabrication of COFMs at an aqueous two-phase interface. Reproduced with permission from ref. 59 (copyright Springer Nature, 2022). e LBL synthesis of COFMs using ethanol as solvent. Reproduced with permission from ref. 75 (copyright American Chemical Society, 2018). f Fabrication of COFMs using SAIP with ILs. Reproduced with permission from ref. 19 (copyright Wiley-VCH, 2023).

Figure 4c presents a simple and scalable method for the preparation of self-standing porous COFMs using a small amount of water as solvent, demonstrating their potential for selective molecular sieving in nanofiltration applications8. However, the long-term programmed temperature control seems not to be the solution for green fabrication of COFMs. Wang and colleagues presented a novel method for the preparation of COFMs using water as a green solvent in an aqueous two-phase interfacial assembly process (Fig. 4d), which offers the advantage of being environmentally friendly; however, it requires a relatively long synthesis time to achieve optimal membrane formation59. Shi and colleagues introduced a LBL synthesis method for COFMs using ethanol as a green solvent, which offers advantages such as enhanced selectivity and high water permeance (Fig. 4e)75. However, the effectiveness of ethanol in dissolving different COF monomers warrants further investigation. Gao and colleagues presented a green method to synthesize highly crystalline COFMs using ILs−water interfaces70. Traditional liquid−liquid IP often utilizes toxic and volatile organic solvents, which can negatively impact crystallinity. In this research, the authors replace these solvents with various ILs, achieving better control over the polymerization process due to their low volatility and tunable properties. By controlling the diffusion rates of monomers through the dual-diffusion mechanism, they successfully fabricated COFMs with crystallinities surpassing those produced from dichloromethane−water interfaces, significantly enhancing their potential for applications in molecular separation. On this basis, Pan et al.19 presented a novel green synthesis method for scalable production of COFMs using SAIP with ILs (Fig. 4f). Traditional methods face challenges due to non-uniformity and reliance on toxic solvents. The SAIP technique allows for fabricating a TpPa COFMs with an area of 19×25 cm2 and a thickness of 78 nm in just 2 minutes with a permeance of 48.1 L m−2 h−1 bar−1 and superior separation efficiency for antibiotic desalination. This environmentally friendly approach minimizes solvent use and opens new avenues for industrial applications in water purification and pharmaceutical separations.

These studies show that using green solvents to prepare COFMs can reduce the use of toxic chemicals, minimize waste, and lower energy consumption, while also enhancing their fabrication efficiency through shorter synthesis times, thus contributing to a safer and more sustainable membrane production process. However, the types of green solvents for fabricating COFMs and their applicability to synthesize various COFMs require further exploration.

COFMs can be prepared in the lab using various methods. However, producing large-area COFMs remains challenging due to device size limitations. Some efforts have achieved membrane areas ranging from several to thousands square centimeters (Fig. 5a) using methods like in situ growth, IP, and LBL stacking of nanosheets.

a Comparison of the COFMs areas reported in literatures8,9,14,19,22,41,76,77,78,79,80,81,95,102,103,104,105,106,107,108. b Fabrication of large-area COFMs through an IP approach at liquid-solid interface as well as the corresponding macroscopic and microscopic morphology of COFMs. Reproduced with permission from ref. 77 (copyright Wiley-VCH, 2024). c Fabrication of large-area COFMs through a photo-tailored IP approach at liquid-liquid interface and the corresponding digital photo of COFMs. Reproduced with permission from ref. 22 (copyright Springer Nature, 2022). d Fabrication of large-area COF films through an IP approach at air-liquid interface as well as the corresponding digital photo and mechanical performance testing of COFMs. Reproduced with permission from ref. 79 (copyright Wiley-VCH, 2023). e Fabrication of large-area COFMs through a spray-coating approach as well as the corresponding digital photo and separation performance of COFMs. Reproduced with permission from ref. 81 (copyright American Chemical Society, 2023).

In 2017, Kandambeth et al.8 developed a straightforward and scalable method to create large-area ( ~ 25 cm2), self-standing COFMs by baking a mixture of organic linkers and co-reagents with a programmed temperature control (Fig. 4c). The resultant membranes demonstrated impressive solvent permeance, significantly outperforming traditional polymeric membranes, making them suitable for wastewater treatment and recovery of valuable organic compounds. However, the heating process may limit the fabrication of COFMs with greater membrane area in the lab owing to the heating device size limitations.

In contrary, there is no heating process for preparing COFMs using the IP method, which is per se beneficial to fabricate large-area membranes. For instance, we introduced a novel method for the synthesis of COF-based membranes with areas up to 44 cm2 by IP directly on polymeric substrates in 201814. Through a moderate reaction rate between monomer pairs in corresponding aqueous and organic solutions, a conformal growth of COF crystallites directly composited with polysulfone (PSF) ultrafiltration substrates can be achieved within 1 minute. The synthesis parameters are optimized, and the synthesized COF/PSF membrane shows a good NF performance. This convenient IP process is expected to facilitate the scale up and real-world application of COFMs. Thereafter, Wang’s groups76 and Sun’s groups77 employed IP at liquid-solid interfaces to synthesize COFMs on the inner walls of glass tubes (Fig. 5b), producing membrane areas of 201 cm2 and 33 cm2, respectively. The COFMs composed of vertically aligned channels with a hydrophilicity gradient by engineering defects in COF films through the removal of imine bonds exhibit superior performance in membrane distillation76. While the COFMs that can dynamically adjust surface charge, perform well in osmotic energy harvesting77. Shevate et al.78 synthesized large-area (64 cm2), ultrathin (24 nm) COFMs using a modified Langmuir-Blodgett method at liquid-liquid interface. This supported COFM shows a correlation between pore tortuosity and membrane thickness, allowing accurate predictions of solvent fluxes based on the COFMs structural properties, which highlights the potential of engineered COFMs for OSN. Further, Yuan et al.22 employed a two-step procedure based on the IP to prepare hetero-crystalline COFMs with membrane areas up to 116 cm2 (Fig. 5c). The membranes feature high-crystalline and low-crystalline. Using a two-step procedure, the first step involves a dark reaction to form high-crystalline regions, followed by a photo reaction to create low-crystalline regions that seal defects. The resulting COFMs demonstrate remarkable organic solvent permeance up to 44 times higher than conventional membranes, thus showcasing potential for advanced applications in precise separations. Additionally, amorphous and free-standing COF films with a record area of 3000 cm2 were synthesized at a liquid-air interface by polymerizing meso-benzohydrazide-substituted metal porphyrins with tris-aldehyde linkers79 (Fig. 5d). These films demonstrate remarkable mechanical stability and structural integrity. However, these COFMs prepared at solid-liquid and liquid-liquid interface require cumbersome transfer processes to substrates for practical separation applications. Ensuring a strong bond between the COF layer and substrate can be challenging. In addition, the membrane area is heavily limited by the container size used for IP. The newly developed technology of SAIP with ILs should be able to solve these problems because the large-area (475 cm2) COFMs can be produced directly on substrates within 2 minutes, indicating the future direction for COFMs research.

COF materials are typically insoluble or non-molten solid powders, making them difficult to reprocess. Batch preparation of COF nanosheets can partially mitigate this issue and facilitate the rapid production of COFMs. However, thick membranes made solely from COF nanosheets may develop large cracks upon drying80. Introducing small amounts of polymer can significantly enhance the processability and mechanical properties of COF nanosheets, as long-chain polymers act like glue, binding adjacent nanosheets effectively together. For example, Ju et al.81 presented a novel approach to synthesize large-scale, processable COF nanosheets using the polymer polyvinylpyrrolidone (PVP)-manipulated crystallization (Fig. 5e). This method induces anisotropic growth and gives nanosheets in a remarkable 72% yield with controllable thickness between 2 to 8 nm. Then, a scalable spray-coating technique was employed to fabricate A4-sized ( ~ 624 cm2) COFMs from these nanosheets. The prepared COFMs display high mechanical stability and impressive molecular separation performance. The separation performance of small randomly sampled membrane pieces taken from a large-area membrane showed good consistency, demonstrating the homogeneity of the membrane material. Similarly, Tian et al.80 fabricated flexible COFMs using a molecular weaving strategy that combines hydroxyl-functionalized polymers with 2D TpPa-SO3H COF nanosheets. This approach enhances the membranes’ flexibility and mechanical strength while it effectively reduces pore size, thus facilitating large-scale production ( ~ 70 cm2) and addressing previous limitations in size-sieving capabilities and processability. The resulting membranes show promise for industrial gas separations. In summary, the methods of IP and nanosheet assemble have the potential of achieving the large-area preparation of COFMs (Fig. 5a).

The production efficiency (η) is typically defined as the effective membrane area produced per unit time:

where \(A\) is membrane area produced, \({T}_{{total}}\) is total production time (including fixed and variable times).

where \({T}_{0}\) is the fixed time including equipment preheating, cleaning, initialization, which is time independent of membrane area. \({T}_{v}\) is the variable time including coating, drying, which is time dependent of membrane area. \(v\) is the production rate (m2 h−1).

Substituting total time into the efficiency formula gives:

From Eq. (3), it can be found that for small \(A\), the fixed time \({T}_{0}\) dominates the process, resulting in an efficiency of \(\eta \approx \frac{A}{{T}_{0}}\), which indicates low efficiency. In contrast, when \(A\) is large, the variable time \({T}_{v}\) becomes dominating, leading to an efficiency of \(\eta \approx v\), which approaches the production rate \(v\). This shift highlights the different factors influencing efficiency depending on the scale of \(A\). Overall, membrane production efficiency \(\eta\) can be significantly improved by optimizing the production rate \(v\) and minimizing fixed time \({T}_{0}\), especially for large-scale production. Hence, the roll-to-roll continuous production is critical to improve the production efficiency of preparing COFMs. However, the reports of continuous production of COFMs are rare.

Fabrication methods with high production efficiency have the potential to push scale-up preparation of COFMs. In this review, we simplify Eq. (3) to \(\eta =\frac{A}{{T}_{v}}\) due to the lack of information of fixed time \({T}_{0}\) in the lab research to evaluate the common fabrication methods of COFMs. As shown in Fig. 6a, most reports have a preparation efficiency less than 3 cm2 h−1 due to the long duration of fabricating COFMs. In this review, we will focus on reports related to preparation efficiencies higher than 3 cm2 h−1. For example, Jiang’s groups82 and Wang’s groups83 utilized a two-step microwave-assisted method and a solvothermal synthesis method, thus achieving process efficiencies of 3.1 cm2 h−1 and 7.3 cm2 h−1, respectively, for fabricating COFMs. Compared to traditional methods that require 2-3 days for growing COFMs, these approaches significantly reduce preparation duration and improve process efficiencies. However, the synthesis process conducts at high temperature and requires a confined space, limiting the potential of large-area fabrication. Similarly, the chemical vapor deposition method84 also faces this challenge despite achieving a high efficiency of 9.5 cm2 h−1. Comparatively, IP is notably more efficient for producing COFMs. For instance, Su et al.85 used this method and achieved a production efficiency of 19.1 cm2 h−1 to form a defect-free COF layer on microfiltration substrates. Further, using ultrafiltration membranes as substrates can significantly enhance the production efficiency up to 2200 cm2 h−1 (Fig. 6b), as the defect-free COF layers form more easily on small-pore substrates14. In addition, the newly developed SAIP technology also enables the rapid preparation of COFMs with a production efficiency of 15833 cm2 h−1 (Fig. 4f), showcasing its green credentials, high efficiency, and capability for large-area production19.

a Comparison of high-efficiency preparation of COFMs using different methods described in literature8,9,12,13,14,18,19,20,23,24,39,82,83,84,85,109,110,111,112,113. b Rapid fabrication of COFMs through an IP approach direct on the porous substrate and the corresponding digital photos of the COFM. Reproduced with permission from ref. 14 (copyright Elsevier, 2018). c Rapid fabrication of COFMs through an EPD approach. Reproduced with permission from ref. 20 (copyright Wiley-VCH, 2022). d Continuous fabrication of COF fiber through a wet spinning approach using the ionic COF nanosheets dispersion as starting materials and the corresponding application in osmotic energy harvesting. Reproduced with permission from ref. 89 (copyright Wiley-VCH, 2024).

In the context of large-scale preparation of COF nanosheets81,86,87,88, the rapid assemble technology supports the fabrication efficiency of COFMs. For example, we utilized EPD to assemble ionic COF nanosheets (Fig. 6c) and increased the preparation efficiencies of up to 49 cm2 h−1 in COFMs fabrication20. However, this method also has hurdles to achieve large-area preparation of COFMs due to equipment size limitations. Thrillingly, the recently developed wet spinning method can continuously produce COF fibers using ionic COF nanosheet dispersions as starting materials (Fig. 6d), showing noteworthy preparation efficiency89. The prepared meter-long COF fiber displayed impressive power densities 70.2 W m−2 in the application of osmotic energy harvesting. More importantly, this work inspired us that COF flat and hollow fiber membranes can be continuously prepared using COF nanosheet dispersions with a suitable technology.

According to the above analysis, we believe that COFMs have the potential to achieve green and scale-up production although there are still many issues that need to be addressed. For example, the bottom-up IP method showcases greatly high preparation efficiency14, which means that by combining this technology with a roll-to-roll production process, it is possible to achieve continuous production of COFMs (Fig. 7a). Further issues like suitable green solvents, surface properties of substrate, and process parameters require additional studies to obtain high-quality membranes. Besides, batch preparation of COF nanosheets can improve the reproducibility of COFs, accelerating the scale-up preparation of COFMs from a top-down aspect. Integrating coating technologies, such as slot-die coating (Fig. 7b) and spray-coating (Fig. 7c), the roll-to-roll production process can also achieve a continuous preparation of COFMs. In the nanosheet assembly process, special attention must be given to the parameters, such as viscosity of the COF nanosheet dispersion, volumetric flow rate, substrate velocity, coating gap, and slot gap, as these factors influence membrane thickness, orderliness, and uniformity. Moreover, another significant aspect to consider is the universality of this technology for fabricating COFMs with different pore size and defined separation characteristics, to meet the demand of different application scenarios for COFMs.

The integration of a IP, b slot-die coating, and c spray-coating within a roll-to-roll production process.

Additionally, integrating advanced characterization techniques will be essential to optimize the structure and properties of COFMs. Understanding the relationship between the structure and the functionality of these materials will allow for improved design and validation of their performance in various applications. Advanced spectroscopy and microscopy can provide insights into the molecular arrangement and mechanical integrity, which are critical for utilizing the advantage of regular pores of COFMs in separation. Furthermore, environmental assessment, energy-consume assessment and economic accounting are necessary for the scale-up preparation of COFMs, which requires the collaborative efforts of people from different professional backgrounds to accomplish. Collaboration between academia and industry can expedite the transition from laboratory-scale experiments to commercial production. Establishing partnerships can facilitate knowledge transfer, resource sharing, and the development of pilot projects that bridge the gap between research and real-world application, which will greatly accelerate the practical application of COFMs in various scenarios.

In summary, although achieving green and scale-up preparation of COFMs poses significant challenges, the potential benefits are appealing. Ongoing research and development are essential to address these obstacles and fully realize the intrinsic advantages of COFMs for highly efficient NF separation.

As the pore size of non-modified COFs is between 1 ~ 3 nm, COFMs are widely used in NF processes. The state-of-the-art COFMs with membrane area large than 40 cm2 are usually used for the removal of organic micropollutant from water or organic solvents. To evaluate the performance of COFMs, two types of organic micropollutants - dyes and antibiotics - are assessed, as summarized in Table 3. Since separation of dye molecules can be easier detected e.g. by ultraviolet-visible spectroscope, their separation is widely studied as a substitute for pharmaceuticals, hormones, antibiotics, pesticides. However, their molecular weight (Dalton) is often slightly lower than that of dyes. Large-area COFMs can effectively separate various dye molecules larger than 1.5 nm, such as chrome black T (461 Da), indigo carmine (466 Da), Congo red (696 Da), reactive red 24 (788 Da), methyl blue (800 Da), brilliant blue (828 Da), Evans blue (961 Da), rose Bengal (1018 Da), alcian blue 8GX (1299 Da), and direct red 80 (1373 Da), achieving a rejection rate above 92%. In contrast, smaller molecules like acid orange 7 (350 Da), nitroaniline (138 Da), saffranin O (351 Da), and methyl orange (327 Da) have a rejection rate of usually less than 30%. The varying rejection rates for different molecules can be explained by classical size screening, and potential electrostatic repulsion for COFMs that possess a strong charge property. Different to the rejection rate, the water permeance of different COFMs is mainly attributed to the pore size and geometry. For example, the TpPa-Bz/AAO membrane shows much higher water permeance than a membrane made from TpPa-Py/AAO because the pore size of TpPa-Bz (2.05 nm) is larger than that of TpPa-Py (1.4 nm)78. Size-dependent separation enables the design and preparation of COFMs with specific pore sizes tailored to the dimensions of the components to be separated, thus facilitating high-flux separations. Similarly, large-area COFMs prepared by SAIP exhibit excellent separation performance in antibiotic desalination through size screening. These membranes reject various antibiotics, including norfloxacin, tetracycline, and adriamycin, with rejection rates exceeding 95%, while permitting smaller salt ions to pass, characterized by rejection rates below 18.3%. This results in a NaCl/ antibiotic separation factor of 41.8. This study demonstrates the considerable potential of COFMs for drug separation, purification, and the concentration of valuable agricultural products such as fruit and vegetable nutrients. Additionally, by modulating the pore size, structure, and property of COFMs, they are expected to be suitable for small molecule and ion separations.

COFMs can contribute or even solve an acute problem of mankind: The separation of drugs in the effluent of wastewater purification plants. As an example, while the anti-pain drugs aspirin and ibuprofene become perfectly removed, diclofenac passes the sewage plant almost unchanged. Ozonization, active coal adsorption – or membrane separation are discussed as the 4th clarification stage. Using the vacuum-assisted self-assembly method, Li’s group prepared a thin film composite membrane by stacking COF nanosheets on a predesigned ceramic hollow fiber90. The resultant COF composite membrane showed high rejection for the five environmentally persistent pharmaceuticals diclofenac, sulfamethoxazole, ketoprofen, naproxen, and ibuprofen governed by electrostatic repulsion and steric exclusion. In an interesting approach a 3D COF was grown on a 2D COF forming a bi-layered COFM membranes for efficient OSN. Ethanol was separated from various bioactive pharmaceuticals and molecules like rifampicin, vitamin B12 or bacitracin91. Intrinsically charged covalent organic frameworks (COFs) afford specific ionic nanochannels for mass transport, and thus become promising platforms to design membranes with unique selectivity92. However, these ion-paired COFMs realize the controlled release of bioactive pharmaceuticals too. Recent studies have shown that COFMs exhibit solvent-responsive behaviors, enabling them to create narrower pore sizes in certain solvents and effectively separate pharmaceutical ingredients93,94. Additionally, 3D COFMs, with their narrower pore sizes resulting from complex topology and structural interpenetration, exhibit robust capabilities in separating active drugs41,57. Hence, the most promising application of COFMs could be drug separation and purification.

In summary, we present a comprehensive overview of green and scale-up preparation of COFMs for NF. Recent advances in fabrication methods and production efficiency have led to many breakthroughs in highly effective and large-area preparation of COFMs, demonstrating their great potential for industrial NF. Various green solvents and preparation methods open new opportunities for green and scale-up fabrication of COFMs. The prepared large-area COFMs with regular pore channels and robust membrane structure displayed excellent NF performances for the removal of organic micropollutants. However, the gap between research and real-world application of COFMs is still great. In order to accelerate the process from laboratory-scale experiments to commercial production, the following issues are worth considering:

The high cost of monomers for COFMs will limit their large-scale production. To overcome this problem, researchers should seek for cheaper alternatives or more efficient synthesis methods to lower the preparation costs. There is currently no research on a continuous process for scaling up COFM fabrication. Combining high-efficiency traditional methods including IP and coating technology with roll-to-roll techniques is essential for continuous production, necessitating collaboration between academia and industry to optimize membrane structure and processing parameters. Additionally, more research is needed on membrane geometries for practical applications, such as spiral wound or hollow fibers. A membrane module for COFMs should be designed to maximize packing density fore high fluxes. Integrating membrane fabrication with separation measurement in the module design could help to address issues related to packing and sealing.

Enhance the separation performance when scaling-up COFMs. The high production efficiency shortens the time for monomer polymerization and crystallization during COFMs synthesis. While some reports indicate that highly crystalline COF materials can be achieved quickly using high-efficiency catalysis, it is generally understood that COF materials typically require longer crystallization periods. To facilitate the scale-up production of COFMs, the development of low-cost, efficient catalysts is essential to improve crystallinity. Furthermore, when preparing COFMs through nanosheet re-stacking, careful control of the assembly process is necessary to ensure uniformity and regularity, which improve the effectiveness of the separation process. Last but not the least, it is vital to enhance the anti-pollution capabilities of COFMs and explore low-energy regeneration methods, as membrane materials are susceptible to contamination during the separation process. These properties including regularity, stability, and extendibility of COFs can only be achieved under appropriate production conditions, which are subject to strict requirements regarding environmental compatibility.

To mitigate the environmental impact and equipment corrosion through hazardous solvents, the adoption of green solvents is essential for the large-scale production of COFMs. Currently, research on fabricating COFMs using green solvents is limited. Additionally, some green solvents have low volatility, high viscosity, and high boiling points, which can lead to significant solvent residues in COF pores, adversely affecting membrane permeability. Conventional membrane activation methods may not be effective, making the development of new activation techniques crucial for the green fabrication of COFMs. Future studies should prioritize the comprehensive screening of suitable green solvents for COFMs synthesis, utilizing machine learning technology for high-throughput evaluations based on synthesis efficiency, membrane structure, and process simplification.

Broaden the application of large-area COFMs through evaluation of their performance under industry-relevant conditions, which can guide the design of membrane structures and modules. Aside from NF, COFMs have demonstrated potential in reverse osmosis and gas separation under lab conditions. To adapt COFMs for smaller-sized object separations like gases, it is essential to precisely adjust pore sizes through monomer pre-design, post-synthetic modification, or stacking pattern regulation. Additionally, lab separation tests often simulate operational conditions, neglecting the impact of complex components present in real industrial environments on membrane structure and performance. Future research should focus on the separation capability of COFMs under these relevant conditions.

Environmental assessment, energy consumption analysis, and economic accounting are crucial for scaling up COFMs. A comprehensive life cycle impact evaluation is necessary to ensure COFMs align with sustainable development goals, demanding in-depth analysis of resource extraction, manufacturing, and end-of-life scenarios. Embracing a circular economy will reduce environmental impacts, improve resource efficiency, and minimize waste. Successfully transitioning from laboratory experiments to commercial production requires the collaborative effort of researchers, engineers, economists, entrepreneurs, and others from diverse backgrounds.

In conclusion, challenges and opportunities often coexist, and addressing these challenges can bring new opportunities for the green and scalable preparation of COFMs.

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We acknowledge the National Key R&D Program of China (grant No. 2023YFB3810700), Natural Science Foundation of China (grant No. 22422809, 22408200, 22138005, 22141001, and 22378226), Postdoctoral Fellowship Program of CPSF (GZC20231260), the China Postdoctoral Science Foundation (2024T170473) and the Shuimu Tsinghua Scholar Program of Tsinghua University (2023SM089).

Beijing Key Laboratory for Membrane Materials and Engineering, Department of Chemical Engineering, Tsinghua University, Beijing, 100084, China

Rui Wang, Li Ding & Haihui Wang

Guangdong Provincial Key Lab of Green Chemical Product Technology, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, China

Jian Xue

College of Chemical Engineering, Beijing University of Chemical Technology, Beijing, 100029, China

Hongwei Fan

Institute of Physical Chemistry and Electrochemistry, Leibniz University of Hannover, Callinstrasse 3A, 30167, Hannover, Germany

Jürgen Caro

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All authors, including R. Wang, L. Ding, J. Xue, H. Fan, J. Caro, and H. Wang, discussed the outline of the review. R. Wang wrote the draft of the manuscript, all authors revised the manuscript in several iteration loops. All authors have read and approved the final version of the manuscript for publication.

Correspondence to Li Ding, Jürgen Caro or Haihui Wang.

The authors declare no competing interests.

Communications Materials thanks Kuang-Jung Hsu and Qi Liu for their contribution to the peer review of this work. Primary Handling Editors: Natalia Shustova and Jet-Sing Lee. A peer review file is available.

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Wang, R., Ding, L., Xue, J. et al. Green and large-scale production of covalent organic framework nanofiltration membranes. Commun Mater 6, 61 (2025). https://doi.org/10.1038/s43246-025-00780-9

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Received: 26 December 2024

Accepted: 14 March 2025

Published: 01 April 2025

DOI: https://doi.org/10.1038/s43246-025-00780-9

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