Advancements and applications of three-dimensional covalent organic frameworks
Abstract
Covalent organic frameworks (COFs) represent an emerging class of crystalline porous polymers characterized by their pre-designed interconnected structures formed via dynamic covalent bonds. These materials have garnered widespread attention in recent years. While applications of two-dimensional (2D) COFs have been extensively investigated since 2005, their practicality has been impeded by their limited specific surface area and the robust
Keywords
INTRODUCTION
Covalent organic frameworks (COFs) represent an emerging class of constant and porous crystalline polymers intricately woven together by multifunctional organic building units[1]. In 2007, El-Kaderi et al. pioneered the design of a groundbreaking three-dimensional (3D) porous COF by interconnecting pre-designed organic building blocks[2]. Subsequently, researchers have embarked on developing new 3D COFs using various organic building units and conducting comprehensive investigations into their applications[3]. What is particularly intriguing is that when compared to two-dimensional (2D) counterparts, 3D COFs boast higher surface areas, uniform interpenetrating channels, and a greater number of exposed active sites, rendering them more competitive across various application fields[4]. Furthermore, 3D COFs with precisely defined crystal structures can be functionalized through a multitude of methods, making them an ideal platform for research focused on structural regulation and the elucidation of structure-function relationships[5].
However, despite the great potential of 3D COFs in various application fields, the literature on their applications remains limited (less than 200 reported studies)[6]. This limitation stems from several factors. Firstly, the absence of π-π stacking interactions and the presence of large internal void spaces pose significant challenges in crystallizing 3D COF structures[7]. Additionally, the topologies and conformations of 3D COFs can vary considerably, making their structures challenging to elucidate. The limited availability of diverse 3D organic building blocks, coupled with their inherent instability, further restricts the range of synthesizable 3D COFs[8]. Furthermore, these frameworks typically exhibit small pore sizes and even incorporate interpenetrating channels. This complexity makes their functionalization through post-modification methods more challenging[9]. Consequently, they currently find application in only a handful of specific areas, such as gas storage, separation, chemical sensing, catalysis, batteries, and so on[10]. For example, El-Kaderi et al. first used 3D COFs for gas adsorption in 2007[2]. Fang et al. employed them for catalysis for the first time in 2014[11].
In addition, the thermal stability of 3D COFs exceeds that of 2D COFs characterized solely by π-π stacking, thanks to their intrinsic interpenetrating structure. Recent studies have emphasized the influence of linker types on the thermal stability of 2D COFs[12]. Specifically, borate-linked COFs exhibit superior thermal stability compared to their imine-linked counterparts. Simultaneously, networks with smaller pores demonstrate enhanced thermal stability than those with larger pores. Additionally, functionalized COFs generally exhibit lower thermal stability than non-functionalized ones. While these findings have not been explicitly validated in the context of 3D COFs, they provide valuable insights for reference.
In spite of these challenges, 3D COFs continue to stand as an appealing platform for further applications owing to their distinctive properties and ability to provide intricately designed pore structures[13]. Furthermore, they feature an abundance of void spaces, resulting in a larger surface area and readily accessible active sites, greatly benefiting practical applications[14]. Surprisingly, over the past decade, while numerous reviews have discussed the applications of 2D COFs, there has been limited exploration of the potential applications of 3D COFs[15]. This review seeks to comprehensively introduce all reported applications of 3D COFs across various fields and delineate the current challenges and prospects for expanding the utilization of 3D COFs.
DESIGN AND SYNTHESIS OF 3D COFS
Features of 3D COFs
In comparison to 2D COFs, 3D COFs exhibit unique characteristics that amplify their versatility across diverse applications. A prominent attribute is the presence of interpenetrating channels in 3D COFs, a feature absent in their 2D counterparts[5]. These channels not only fortify the stability of 3D COFs but also further diminish pore size[16]. Recent reports suggest that the degree of interpenetration in these channels can be regulated to some extent[17]. Additionally, the specific surface area of 3D COFs significantly surpasses that of 2D COFs, with numerous 3D COF materials reporting specific surface areas exceeding 2,000 m2·g-2 - a rarity in 2D COFs[18]. Furthermore, 3D COFs exhibit lower densities, exemplified by JUC-640-H, boasting a density of only 0.106 cm3·g-1, the highest among crystalline materials[19]. This heightened specific surface area facilitates the exposure of more functional groups within the pores, promoting not only the post-functionalization of COFs but also expanding their applications in catalytic energy storage and other fields.
Topology
Topology plays a pivotal role in the design of 3D COFs as it determines the size and distribution of pores within the 3D network[20]. Since the pioneering report by El-Kaderi et al. in 2007, showcasing the first instance of a 3D COF, more than 20 new 3D topologies have been unveiled as shown in Scheme 1[2]. Among them, excluding eight specific ones - namely srs[21], fjh[22], she[23], ffc[24], tbo[25], pto[26], mhq-z[26], and nbo[27] - all others incorporate spatial building blocks. Tetrahedral building blocks are the most prevalent spatial construction units, encompassing examples such as dia[28], pts[29], ctn[2], bor[2], qtz[30], lon-b[31] and ljh[32]. Alongside tetrahedral blocks, triptycene building blocks are also commonly employed in the creation of 3D COFs, resulting in successful topologies such as stp[33], acs[34], ceq[34,35] and hea[36]. Recently, eight-node building blocks have also been harnessed to craft new topologies, such as bcu[37], pcb[38], flu[39], scu[40] and
Scheme 1. Topologies in 3D COFs. 3D: Three-dimensional; COFs: covalent organic frameworks.
Linkage
Linkage is a fundamental requirement for the construction of COFs. Traditionally, COF linkers have been known for their rigidity and dynamism. However, as research has advanced, some less reversible linkages have also been applied in COF synthesis[3]. It is important to note that while new linkages have been reported in the realm of 2D COFs, not all of them seamlessly translate to the fabrication of 3D COFs[45]. This limitation often arises from the need to distort certain linkages to conform to the spatial configuration of 3D COFs. Scheme 2 illustrates the successful incorporation of linkages in 3D COFs, including boryl-based linkages such as boroxine[2], boronate ester[2], spiroborate[35] and borosilicate[46], along with nitrile-based linkages such as imine[28], amide[47], imide[48], trazine[49] and azodioxy[50]. Additionally, there have been instances where 3D COFs were ingeniously constructed using two distinct linkages. In addition to the previously mentioned bond types, other robust covalent bonds, such as olefins[51] and β-ketoenamines[52], have been developed to enhance the stability of 3D COFs.
Scheme 2. Linkages in 3D COFs. 3D: Three-dimensional; COFs: covalent organic frameworks.
Synthesis
While numerous methods are available for synthesizing 2D COFs, only a select few, such as solvothermal heating, ionic heating, microwave synthesis, ultrasonic-assisted techniques, and plasma-induced methods, have successfully been adapted for the production of 3D COFs[53]. To date, the vast majority of 3D COFs have been synthesized using solvothermal methods[3]. Given the diverse building blocks chosen for COFs, various parameters of solvothermal synthesis, including solvent choice, temperature, catalyst, reaction time, and more, must be meticulously explored. In addition to solvothermal techniques, ionic thermography has garnered attention from researchers due to its gentle synthesis conditions and rapid reaction times[54]. In this approach, ionic liquids serve as both solvents and catalysts for COF synthesis, enabling the swift production of COFs at ambient temperatures.
Microwave synthesis and ultrasonic-assisted synthesis have recently garnered significant attention as alternative methods for synthesizing COFs compared to traditional solvothermal heat. Among these, the microwave method typically employs microwave radiation in the range of 1 mm to 1 m[55], generated by a microwave reactor, and finds widespread application in organic synthesis. Despite the relatively low energy of microwaves, insufficient for breaking most chemical bonds, the high-frequency electric field accompanying microwaves compels molecules or ions in the reaction mixture to lose energy through collisions, leading to efficient internal heating[56]. Consequently, the efficiency of heat energy absorption by molecules exceeds that of traditional ovens and oil baths, markedly reducing reaction time and potentially enhancing COF yield and crystallinity. Leveraging the unique properties of microwave synthesis, it may hold substantial application value in COF synthesis, akin to organic synthesis. Microwave synthesis, first reported by Campbell et al. in 2009, offers a significantly shorter reaction time compared to traditional solvothermal methods while preserving the crystallinity and specific surface area of COFs[57].
The ultrasonic synthesis method utilizes ultrasonic waves in the frequency range of 20 kHz to 1 MHz to generate bubbles and collapses in a liquid medium[58]. Although the energy of ultrasound is weak and insufficient for altering the vibration or spin state of electrons in atoms, the moment of bubble formation or collapse may be accompanied by instantaneous heat or pressure, facilitating chemical reactions under these extreme conditions[59]. Zhao et al. recently first introduced an ultrasound-assisted synthesis method for 3D COFs in 2022[60].
In 2021, He et al. developed a plasma-induced synthesis technique for 3D COFs, which boasts advantages such as low energy consumption, rapid synthesis, and no heating requirements, all while ensuring the crystallinity and high specific surface area of COFs[61]. In addition to polycrystalline synthesis methods, techniques for synthesizing 3D COF single crystals and films have also been developed. The first 3D single-crystal COF was reported by Beaudoin et al. in 2013,[50] while Zhang et al. reported a smaller single crystal linked by imine bonds around the same time[62]. In 2018, the same research group proposed a strategy for growing large-size (10-60 μm) 3D COF single crystals using aniline as a growth regulator[31]. In 2019, Evans prepared a series of 2D and 3D COF monocrystals via boroxine links, and colloidal suspension of COFs was stabilized with the addition of a co-solvent containing nitrile[63]. In 2022, Peng et al. demonstrated the rapid growth of large-size (110 μm) 3D COF single crystals at the millimeter level using supercritical carbon dioxide (CO2) as a solvent[64]. Apart from single crystal synthesis, achieving 3D COF films poses equally significant challenges. The first 3D COF membrane was grown in situ by Lu et al. on an α-Al2O3 substrate[65], and a similar approach was adopted by Fu et al. on a polyaniline-modified silicon substrate[66]. More recently, Yang et al. successfully synthesized an ultra-thin (13 nm) 3D COF crystal film using a two-phase interface method, enabling production at a centimeter scale[67].
Functionalization of 3D COFs
Generally, three strategies are employed for COF functionalization. The first entails pre-modifying functional groups on the monomers before using these functionalized monomers to synthesize COFs. This method ensures enough COF functional groups with predetermined positions and distributions, thereby enhancing material properties. However, challenges in crystallization and a reduction in specific surface area may arise due to spatial hindrance during COF synthesis on the monomer side chain[68]. The second method involves pre-synthesizing COFs while retaining some functional groups on the side chain for further reactions. COFs with these functional groups can react with monomeric small molecules and undergo modifications on the side chains. This approach preserves high COF crystallinity but may lead to lower functionalization degrees and diminished material performance. Ensuring chemical stability during further reactions is crucial for the COF framework in this method. It is important to note that active sites may not be uniformly distributed, and the modification amount may be uncontrollable, hindering the construction of structure-activity relationships. Several mild covalent transformations for 3D COF modifications have been explored, including thiol-alkene coupling reactions, copper-mediated azide-alkyne cycloaddition, and ring-opening reactions. For instance, Bunck and Dichtel reported the first example of 3D COF modification based on a 3D boroxane-linked COF-102-allyl group[69]. The last method of functionalizing COFs involves intrinsically introducing functionalized substances into COFs, such as in-situ growth of metal nanoparticles or composite materials achieving COF growth on other materials (e.g., graphene or MXene)[70]. Ex-situ methods can also be employed to introduce functionalized metal ions or non-metallic particles into COF pores. This approach uses COF as a carrier, ensuring uniform dispersion of functionalized substances, preventing agglomeration, and preserving performance[71]. However, this method is closely tied to COF porosity and its ability to interact with functionalized substances. Additionally, due to the uncertainty of modification, understanding the structure-activity relationship becomes challenging.
APPLICATIONS OF 3D COFS
Uptake, storage and separation
Capture and adsorption of carbon dioxide
CO2 is a major greenhouse gas and a potential carbon source, making its capture a key environmental concern[72]. Additionally, 3D COFs have emerged as ideal platforms for capturing CO2, offering potential solutions to mitigate the issues associated with the greenhouse effect[73]. In 2009, Furukawa et al. designed two novel 3D COFs, COF-102 and COF-103, and for the first time, measured their CO2 capture capacities[74]. COF-102 exhibited a high CO2 capacity of 1,200 mg·g-1, while COF-103 reached 1,190 mg·g-1 at 298 K and 55 bar, surpassing the performance of 2D COFs, which typically do not exceed 1,010 mg·g-1. Furthermore, research by Li et al. introduced 3D COFs with two types of covalent linkages as shown in Figure 1A and B. DL-COF-1 and DL-COF-2 demonstrated CO2 uptake capacities of 136 and 111 cm3·g-1, respectively, at 273 K and 1 bar[75]. This group also developed metal-free 3D ionic COFs, 3D-ionic-COF-1 and 3D-ionic-COF-2, with CO2 capacities of 161 mg·g-1 at 273 K and 93 mg·g-1 at 298 K for 3D-ionic-COF-1, and 133 mg·g-1 at 273 K and 76 mg·g-1 at 298 K for 3D-ionic-COF-2[76]. Additionally, Guan et al. identified a novel 3D azine-linked COF, 3D-HNU5, with excellent CO2 uptake performance, achieving a capacity of 123.1 mg·g-1 at 273 K[77]. In 2020, Zhu et al. synthesized the first 3D organic cage COF, named
Figure 1. (A and B) Schematic diagram of Preparing DL-COFs structures with dual for gas adsorption; (C) Adsorption isotherms for H2, CO2, and CH4 on DL-COF-1; (D) DL-COF-2. Reproduced from ref[75], with permission from the American Chemical Society, Copyright 2016. COFs: Covalent organic frameworks.
Adsorption and storage of hydrogen
Air pollution and global warming have become significant challenges confronting humanity, necessitating the proactive pursuit of clean energy and storage solutions[83]. Hydrogen (H2) energy has garnered considerable attention as a promising green energy source[84]. However, it presents substantial challenges in terms of transportation and storage[85]. Characterized by their exceptionally high surface area, well-defined structures, and abundant open 3D channels, 3D COFs emerge as ideal candidates for H2 storage[86]. In 2009, Furukawa and Yaghi conducted measurements of H2 storage capacity for both 2D and 3D COFs[74]. Intriguingly, 3D COFs, specifically COF-102 and COF-103, exhibited remarkable saturation H2 uptake values of 72.4 and 70.5 mg·g-1 at 77 K and 35 bar, respectively. This underscores the distinct advantages of 3D COFs over their 2D counterparts. In 2012, Kalidindi et al. successfully integrated palladium (Pd) nanoparticles into COF-102, resulting in a more than two-fold enhancement in H2 storage capacity at room temperature and 20 bar[87]. However, Campbell et al. synthesized COF-102 using a mild microwave thermal method, yielding a disappointingly low hydrogen uptake of less than 1.7 wt% at 1 bar and 77.3 K[57]. Meanwhile, the DL-COF, prepared by Li et al., exhibited H2 storage capacities of 234 cm3·g-1 for DL-COF-1 and 193 cm3·g-1 for DL-COF-2 at 77 K and 1 bar [Figure 1C][75]. Similar excellent performances were shown by JUC-568 and JUC-569, in which JUC-568, in particular, demonstrated an impressive hydrogen uptake of 274 cm3·g-1 at 77 K and 1 bar[34]. Furthermore, Yu et al. designed JUC-596 with a hea topology, showcasing exceptional performance in H2 adsorption, reaching an impressive 2.72 wt% (305 cm3·g-1) at 77 K and 1 bar[36]. This accomplishment positions JUC-596 among the best porous organic materials for H2 storage reported to date. In the same year, Li et al. reported similar findings[88]. Additionally, Liao et al. designed two 3D imine-based COFs featuring rare bcu nets, named JUC-588 and JUC-589, which exhibited H2 uptake capacities of 245 and 211 cm3·g-1, respectively, at 77 K and 1 bar[89].
Capture and adsorption of methane
Methane (CH4) is considered a relatively clean energy carrier and is being explored as a potential alternative to oil due to increasing demand and its potential to reduce carbon emissions[90]. However, CH4 also has a significant drawback - its low energy density and its classification as a greenhouse gas[91]. As a result, there has been a surge in research focused on developing effective materials for CH4 adsorption[92]. Additionally, 3D COFs have garnered significant attention as promising candidates for CH4 storage[1]. This is primarily due to their low density, large surface area, and substantial void spaces. However, research in this field remains relatively limited[2-5]. A groundbreaking study for the CH4 adsorption by 3D COFs was demonstrated, measured under various conditions. Furukawa et al. investigated two 3D COFs, namely COF-102 and COF-103[74]. Notably, at 298 K and 35 bar, COF-102 exhibited exceptional CH4 adsorption capacity, reaching 187 mg·g-1, while COF-103 achieved 175 mg·g-1. Even under increased pressure at 85 bar, COF-102 maintained robust performance with an adsorption capacity of 243 mg·g-1, while COF-103 reached 229 mg·g-1. Meanwhile, Ma et al. achieved a notable breakthrough by successfully synthesizing a novel 3D Metal-Covalent Organic Frameworks (MCOF) named MCOF-1[93]. Under standard conditions of 298 K and 1 bar, MCOF-1 displayed impressive adsorption capacities for various gases, including CH4 (9 mL·g-1), C2H4 (36 mL·g-1), C2H6 (44 mL·g-1), and C3H8 (55 mL·g-1). Both DL-COF-1 and DL-COF-2 exhibited substantial CH4 adsorption capacities at 273 K and 1 bar, specifically 36 and 30 mL·g-1 [Figure 1C][75]. These materials exhibited substantial CH4 adsorption capacities at 273 K and 1 bar, specifically 36 and 30 mL·g-1 [Figure 1D]. Similarly, both JUC-568 and JUC-569 have also been explored for CH4 adsorption[34]. JUC-568 demonstrated higher CH4 adsorption capacity at 273 and 298 K, reaching 48 and 32 mL·g-1, respectively, in comparison to JUC-569, which achieved 19 mL·g-1 at 273 K and 11 mL·g-1 at 298 K. Furthermore, the same research group introduced a novel triptycene-based hea topology in 2022, and they investigated the CH4 adsorption performance of both 3D COFs. Likewise, the triptycene-based 3D COFs (JUC-596 and JUC-597) are also promising for CH4 adsorber exhibiting a maximum adsorption capacity of 31 mL·g-1 at 273 K and
Capture of iodine
As global oil resources continue to deplete, nuclear energy has emerged as a green and environmentally friendly alternative that has garnered increasing attention over the years[94]. However, the presence of radioactive iodine isotopes (129I and 131I) in nuclear waste poses a major challenge[95]. These isotopes are known to sublimate easily into the gas phase, making them a significant source of radioactive vapor waste. Furthermore, they tend to dissolve readily in water, leading to adverse ecological consequences. Consequently, there has been a growing focus on finding efficient methods to capture and manage iodine in recent years[96]. In 2018, Wang et al. introduced a groundbreaking 3D COF (COF-DL229) characterized by a unique eight-fold interwoven diamond net structure, specifically designed for the purpose of removing iodine vapor. Impressively, this innovative material achieved an exceptional I2 uptake capacity of
Figure 2. (A) Schematic diagram of the building unit structure of JUC-560 and JUC-561; (B) Schematic diagram of the structure of JUC-560 and JUC-561. Extended structures of mesoporous (C) 2D JUC-560 and (D) 3D JUC-561. 2D sql and 3D ffc net for (E) JUC-560 and (F) JUC-561 respectively; (G) Iodine uptake of JUC-560 as a function of exposure time at 75 °C and ambient pressure; (H) Iodine uptake of JUC-561 as a function of exposure time at 75 °C and ambient pressure; (I) Iodine retention of JUC-560 after capturing iodine upon exposure to air at 25 °C and ambient pressure; (J) Iodine retention of JUC-561 after capturing iodine upon exposure to air at 25 °C and ambient pressure; (K) Recyclability of JUC-560 in iodine adsorption; (L) Recyclability of JUC-561 in iodine adsorption. Reproduced with permission from ref[99], Copyright 2021, Royal Society of Chemistry. 2D: Two-dimensional; 3D: three-dimensional.
Extraction of ionic pollutants
As industrial processes develop, ion-containing wastewater with harmful environmental effects is inevitably generated during production[102]. How to effectively treat harmful ions in industrial wastewater and recover valuable metal ions from wastewater before discharge has become a recent research hotspot[103]. Furthermore, 3D COFs have the potential to adsorb ions due to their high specific surface area and the absence of any metallic elements[104]. Uranium is a crucial fuel for nuclear reactions, and when leaked into the environment, it poses significant health risks to humans[105]. Therefore, 3D COFs have also garnered widespread attention as adsorbents for uranium ions[106]. Cui et al. ingeniously designed a salen-functionalized 3D COF called tetrakis(4-aminophenyl)methane (TAPM)-3,3’-dihydroxy-[1,1’-biphenyl]-4,4’-dicarbaldehyde (DHBD) for concurrent UO22+ extraction, achieving a uranium extraction capacity of 955.3 mg·g-1[107]. In 2023, the same group designed a novel sp2 carbon-linked 3D COF, named tetrakis(4-formylphenyl)methane (TFPM)-2,2’-(1,4-phenylene)diacetonitrile (PDAN)-amidoxime (AO), which serves as a platform for uranium extraction from seawater with an adsorption capacity of 4,685 mg·g-1[108]. Chen
Capture of other organic carbon
Organic carbon capture, including the adsorption of organic vapors, dyes, pesticides, antibiotics, and other organic compounds, has only seen sporadic exploration regarding the application of 3D COFs[119]. Moroni
Sustained release of drug
Sustained drug release is a pivotal strategy aimed at reducing dosing frequency and the overall drug dosage which helps maintain a stable drug concentration in the bloodstream, ultimately mitigating the toxic side effects of medications[128]. Fang et al. made a groundbreaking effort by designing and synthesizing the initial set of 3D polyimide COFs. They also embarked on using 3D COFs for controlled drug delivery for the first time [Figure 3][48]. During drug delivery, the 3D COFs exhibited a remarkable drug loading capacity and precise control over release rates. In 2022, the same research group achieved another milestone, utilizing sp2 carbon-linked 3D COFs (JUC-580 and JUC-581) for sustained drug release and fluorescence imaging. The results demonstrated that JUC-581 not only serves as a promising material for drug delivery and sustained release but also excels as an excellent fluorescent dye for cellular imaging[129]. During the same year, the research team introduced a novel 3D COF named TUS-84. This structure featured a two-fold interpenetrating scu-c net, which was employed for the delivery and sustained release of ibuprofen. Impressively, about 12% of the ibuprofen was delivered within a 24-hour period. Extended and controlled drug release holds the potential to reduce dosing frequency and effectively manage long-term pain[130]. In the subsequent year, another research group presented TUS-64, a 3D COF based on a 4-connected porphyrin topology[131]. TUS-64 boasts an exceptionally low density and an extraordinarily large pore size within its 3D COF structure. These features make it a promising candidate for drug delivery and controlled release applications. Meanwhile, Wan et al. engineered a 3D COF with tetrathiafulvalene ligation designed for horseradish peroxidase. This sustained enzyme release process significantly prolongs the enzyme activity, particularly during enzyme prodrug therapy[132].
Figure 3. (A) Schematic diagram of the structures of PI-COFs for drug adsorption; (B) UV-vis spectra of ibuprofen in simulated body fluid at different concentrations; (C) Release profiles of ibuprofen -loaded 3D PI-COFs. Reproduced with permission from ref[48], Copyright 2015 from American Chemical Society. COFs: Covalent organic frameworks; 3D: three-dimensional.
Separation of gas
Efficiently recovering H2 from refinery waste gases containing H2, CO2, and CH4 is a significant industrial goal in the pursuit of clean energy generation and the reduction of CO2 emissions[104]. This endeavor aims to continually decrease CO2 emissions, contributing to a cleaner environment. Additionally, the separation and capture of CO2 from gas mixtures, such as flue gas, hold paramount importance in reducing carbon emissions[105]. In 2015, Lu et al. achieved a groundbreaking milestone by successfully cultivating 3D COF membranes on the surface of porous α-Al2O3, marking a breakthrough in using 3D COF for hydrogen and nitrogen methane separation. The resulting 3D COF-320 membrane exhibited an impressive hydrogen permeance of 5.67 × 10-7 mol·m-2·s·Pa-1[65]. Fu et al. made notable contributions by fabricating COF-MOF composite membranes, enhancing the selectivity and separation of H2/CO2 gases. Notably, the separation factors for H2/CO2 mixtures were 12.6 and 13.5 for [COF-300]-[Zn2(bdc)2(dabco)] and [COF-300]-[ZIF-8] composite membranes, respectively[66]. Another pioneering achievement was the work of Guan et al., who utilized ionic liquids at room temperature and pressure to synthesize a series of 3D-IL-COFs for CO2/N2 and CO2/CH4 separation[54]. These 3D COFs exhibited exceptionally high carbon dioxide capture rates at 298 K and 1 bar (5.34%, 7.61%, and 4.93%) due to strong ionic dipole interactions. They also observed high CO2/N2 separation (24.6, 24.0, and 24.4 for 3D-IL-COF-1, 3D-IL-COF-2, and 3D-IL-COF-3, respectively,) and CO2/CH4 adsorption selectivity (23.1 in 3D-IL-COF-1, 22.3 in 3D-IL-COF-2, and 21.5 in 3D-IL-COF-3). Gao et al. reported three 3D-TPB-COFs with five-fold interpenetrated pts topology, which exhibited significantly higher CO2 adsorption capacity than N2 (approximately 90 cm3·g-1) at 273 K and 1 bar[80]. Cheng et al. ingeniously incorporated COF-300 into 6FDA-DAM and Pebax to create mixed matrix membranes for gas separation[133]. This innovation resulted in a 52% increase in CO2 permeability for 6FDA-DAM and a 57% increase for Pebax, thereby enhancing gas separation efficiency. As demonstrated in Figure 4, Li et al. introduced a novel synthetic strategy for fabricating COF membranes, including 2D N-COF and 3D COF-300 membranes. Both membrane types exhibited outstanding selectivity for the mixture of H2/CO2 mixed gas, with a selectivity of 13.8 for the 2D COF membrane and 11 for the 3D COF membrane[134]. Ji et al. investigated three isostructural 3D-OC-COFs by introducing various substituted ligands through a novel in situ acid-base neutralization strategy for the first time[135]. Among the three COFs, 3D-OC-COF-OH demonstrated outstanding CO2 capacity (89.2 cm3·g-1 at 273 K and 51.5 cm3·g-1 at 298 K) and relatively superior separation performance in CO2/CH4 or CO2/N2 mixed gas. Furthermore, Yang et al. introduced a novel 3D COF via imine condensation reaction using a symmetric corrole linker for the first time[136]. Notably, this COF exhibited higher selectivity for CO compared to N2, O2, and CO2 compared to other porous materials. Fu et al. developed a dense and continuous 2,5-dimethoxyterephthalaldehyde (DMTA)-COF membrane containing ether oxygen functional groups. This membrane was covalently attached to a polyaniline-modified porous alumina substrate through in situ synthesis at room temperature and pressure. The H2/CO2 two-component separation factor was 8.3, corresponding to an H2 permeation flux of
Figure 4. (A) Equimolar binary gas mixture separation performance of the N-COF Membrane; (B) Equimolar binary gas mixture separation performance of the COF-300 membrane; (C) Long-term test Results for the N-COF membrane using an equimolar H2/CO2 mixture at room temperature; (D) Long-term test results for the COF-300 membrane using an equimolar H2/CO2 mixture at room temperature; (E) Selectivity and permeance of the N-COF membrane as functions of temperature; (F) Selectivity and permeance of the COF-300 membrane as functions of temperature; (G) Schematic diagram of COF membrane structure for gas separation. Reproduced with Permission from ref[134], Copyright 2023, Wiley-VCH. COF: Covalent organic framework.
Oil-water and dye separations
Water pollution, closely intertwined with human survival, has garnered increasing attention as a critical environmental issue[145]. The efficient removal of contaminants, such as oils and dyes, from water has become a focal point of research efforts[146]. Shi et al. have developed a 3D COF membrane with anti-swelling properties, utilizing interpenetrating imine bonds within its channels[147].
This membrane has exhibited remarkable stability, maintaining its performance for approximately 1,000 h when subjected to high-concentration feed. In another approach, Mohammed et al. utilized blocks containing halogen side chains to construct 3D-COF membranes with inherent hydrophobic characteristics. These membranes proved to be highly effective in demulsifying oil-water mixtures, achieving an impressive gravitational flux rate of 1,536 L·m-2·h-1[148]. Ma et al. engineered a hydrophobic 3D COF by introducing isopropyl groups into its structure. The 3D COF displayed exceptional stability, achieving separation efficiencies exceeding 99% and maintaining separation performance over at least 30 cycles when dealing with oil/water mixtures across a broad pH range[149].
Desalination and different ion separation
Desalination is of great significance for regions suffering from severe water scarcity[150]. Additionally, there are situations where it is essential to efficiently and selectively separate different ions from solutions[151]. Sun et al. constructed a 3D COF TAPM-Tp/nylon 6 membrane using a layer-by-layer approach, primarily for preliminary exploration of dye wastewater purification[152]. The TAPM-Tp/nylon 6 membrane exhibited negligible rejection rates for NaCl, Na2SO4, MgCl2, and MgSO4 (< 3.4%), while maintaining a high rejection rate for positively charged dyes. Shi et al. developed a 3D-COOH-COF Membrane designed for selective ion capture [Figure 5][153]. This membrane demonstrated exceptional selectivities, reaching up to 766 for K+/
Figure 5. (A and B) Rates of ion permeation and the corresponding selectivity of the membranes; (C) Schematic diagram illustrating the selective ion capture by the 3D-COOH-COF; (D) Schematic diagram of 3D-OH-COF. Reproduced with permission from ref[153], Copyright 2021, American Chemical Society. 3D: Three-dimensional; COF: covalent organic framework.
Chromatographic separation
Considering the high stability, insolubility in common solvents, and excellent adsorption selectivity of 3D COFs, these materials have the potential to become stationary phases in HPLC[155]. Han et al. synthesized the first 3D chiral COFs with a 4-fold interpenetrated structure through imine linkages[156]. Both chiral COFs (CCOF 5 and CCOF 6) can serve as stationary phases for HPLC, enabling efficient separation of racemic alcohols [Figure 6]. Porous materials containing metals have been shown to have some ability to separate aromatic hydrocarbon mixtures[157]. In 2019, Huang et al. developed a zinc (Zn)-coordinated 3D Salen COF as a stationary phase in an HPLC column for separating ethylbenzene and xylene isomers[68]. Qian et al. used a layer-by-layer assembly method to create COF- 300@SiO2 composite as a stationary phase for HPLC. This phase efficiently and precisely separated isomers such as nitrophenol, nitroaniline, and aminophenol[158]. In 2022, the same group reported JNU-5, a 3D COF containing amino functional groups with a 9-fold-interpenetrated dia net, as a stationary phase for separating xylene isomers[159]. In the same year, they introduced hydroxyl-functionalized 3D COF JNU-6 with a 5-fold interpenetrating diamond net for fast-selective solid-phase extraction of organophosphorus pesticides. This combination, along with gas chromatography-flame thermion detection, was used to detect four selected organophosphorus pesticides with detection limits as low as 3.0-15.0 ng·mL-1[160]. Subsequently, the group reported an imine-based 3D COF with a four-fold interpenetrating dia net structure, prepared by a solvothermal method, as a stationary phase for HPLC. This COF efficiently separates acidic phenol mixtures and basic aniline mixtures[161]. In the following year, the same group reported single-crystal COF (SCOF-303) with different particle sizes, where a particle size of about 0.4 μm provided excellent separation of xylene isomers (with a resolution of 2.26-3.52 and efficiency of 7,879 plates·m-1)[162]. Zong et al. synthesized 3D TpTAM by ultrasound to evaluate its potential as a stationary phase in capillary columns. They tested its performance in separating four fluoroquinolones[163]. In 2023, the same research group successfully synthesized JUC-515, introduced it into a capillary column, and thoroughly studied its effects on the isolation of fluoroquinolones, achieving excellent resolution (> 1.5)[164]. Niu et al. pioneered the in-situ growth of COF-300 on open-tubular capillary electrochromatography as a separation medium[165]. This medium efficiently separates organic compounds such as styrene, benzene, ethylbenzene, propylbenzene, naphthalene, methylbenzene, and 1-methylnaphthalene. Moroni et al. selected imine-linked 3D COFs, including COF-300 (COF-300-rt and COF-300-st), for the adsorption and separation of benzene and cyclohexane[120]. These COFs adsorb more benzene (251, 221, and 214 cm3·g-1 STP) than cyclohexane (175, 133, and 164 cm3·g-1 STP) at 298 K and
Figure 6. HPLC separation of (A) racemic 1-phenyl-2-propanol; (B) 1-phenyl-1-pentanol; (C) 1-phenyl-1-propanol; and (D) 1-(4-bromophenyl) ethanol on CCOF 5 (blue line) and CCOF 6 (red line) packed columns, respectively, using a hexane/isopropyl alcohol (v/v = 99:1) mobile phase at a flow rate of 0.2 mL/min; (E) Schematic diagram of preparing CCOF 5 and CCOF 6. Reproduced with permission from ref[156], Copyright 2019, American Chemical Society. HPLC: High-performance liquid chromatography.
Heterogeneous catalysis
Thermal catalysis
Characterized by their abundant, continuous, and uniformly open channels, 3D COFs play a crucial role in facilitating mass transfer during catalysis. This feature positions them as a highly promising platform for catalytic applications[166]. Additionally, these 3D networks are insoluble and exceptionally stable, enabling easy separation from products or reactants and allowing for multiple recycling cycles[167]. Moreover, their continuous and uniform pores exhibit a certain degree of size selectivity. Through pre-modification or post-modification methods, active sites can be precisely anchored to the specific locations within these channels. This capability provides an ideal platform for conducting in-depth studies of catalytic mechanisms[168].
Thermal catalysis serves as a fundamental method for various organic reactions[169]. Fang et al. synthesized two innovative 3D imine-connected COFs, BF-COF-1 and BF-COF-2, and explored their application in the Knoevenagel condensation reaction[11]. BF-COF-1 exhibited excellent catalytic activity, achieving a high conversion rate of approximately 96%, while BF-COF-2 showed an even higher conversion rate of around 98%. Both BF-COFs displayed highly efficient size selectivity and recyclability, marking a pioneering use of 3D COFs as a catalytic platform. Meanwhile, 3D DL-COFs prepared by the same group underwent hydrolysis through acetal catalysis, facilitated by the acidic site of the boron oxide group, followed by Knoevenagel condensation catalyzed by the imine-bonded basic site[76]. Subsequently, the group developed two 3D radical-based COFs via a bottom-up approach (JUC-565 and JUC-566) containing radical moieties
Figure 7. (A) Schematic diagram of preparing JUC-565 and JUC-566; (B) Yield vs. time curve for aerobic oxidation controlled by the addition or removal of COF crystals; (C) Yield vs. time curves for aerobic oxidation with substrates of different sizes; (D) Recyclability study of aerobic oxidation with 4-nitrobenzyl alcohol as the substrate. Reproduced with permission from ref[170], Copyright 2021 Wiley-VCH. COF: Covalent organic framework.
Photocatalysis
In comparison to traditional thermal catalysis, photocatalytic technology requires only low-density solar energy and does not necessitate additional energy input during the process, rendering it an environmentally friendly green technology[181]. Meng et al. introduced the synthesis of two porphyrin-based materials, a 2D COF (2D-PdPor-COF) and a 3D COF (3D-PdPor-COF), and explored their applications as photocatalysts[182]. In contrast to 2D-PdPor-COF, 3D-PdPor-COF exhibited higher catalytic selectivity and efficiency in the catalytic reaction of sulfide oxidation to sulfoxide. Kumar et al. designed a 3D COF with a nine-fold interpenetrating dia net containing bipyridine functional groups[79]. This 3D COF can be employed as a photocatalyst for the oxidation of cyanide in an aqueous solution after chelation with ruthenium ions. Chao et al. developed a 3D non-interpenetrating fjh COF named 1,3,6,8-tetrakis-(aminophenyl)pyrene (TAPPy)- tris-formylphenylamine (TFPA) containing pyrene with mesoporous channels, displaying excellent catalytic activity for photo redox thiol-ene cross-coupling reactions and photoredox thiol radical-mediated acylation of amines into amides[183]. Jin et al. designed and synthesized two highly crystalline 3D COFs (NKCOF-25-H and NKCOF-25-Ni) through an [8 + 4] approach with new scu topologies as photocatalysts for the cycloaddition of tert-aniline and maleimide under visible light[40]. Wang et al. prepared Fe3O4/COF-300 composites for effective photocatalysis in dye degradation in wastewater under 365 nm violet light irradiation[184]. Xu et al. constructed a new 3D COF named 5,10,15,20-tetra(4-aminophenyl)porphyrin (TAPP)-hexa(4-formylphenyl)benzene (HFPB)-COF) containing porphyrin units with a non-interpenetrated she net, providing active sites for the photocatalytic α-functionalization of aldehydes and CO2 reduction reactions[23]. Yu et al. reported a 3D COF named N,N,N,N-tetrakis(4-aminophenyl)-1,4-benzenediamine and 5,5’-(2,1,3-benzothiadiazole-4,7-diyl) bis[2-thiophenecarboxaldehyde] (TPDT)-COF with a seven-fold interpenetrating structure[185]. Notably, the type of product obtained by photocatalysis (cis-stilbene or phenanthrene) can be altered by regulating the reaction gas environment, with a selectivity exceeding 99%. The same group also reported a series of 3D MCOFs based on covalent bonding between Ru(bpy)32+/Fe(bpy)32+ for efficient photocatalytic hydrogen production. Among them, Rubpy-ZnP achieved the highest H2 generation rate (30,338 μmol·g-1·h-1), making it one of the most effective COF-based catalysts[186]. As shown in Figure 8, Ding et al. designed and synthesized a series of 3D COFs based on triptycene and porphyrin with an stp net, referred to as JUC-640-M, for photocatalysis[19]. Among them, JUC-640-Co was employed for photocatalytic CO2 reduction and achieved an extremely high carbon monoxide yield (15.1 mmol·g-1·h-1) with ultra-high selectivity (94.4%). Dong et al. conducted similar research and developed π-conjugated 3D COFs (pNJU-318 and pNJU-319Fe) containing a hexabenzo-trinaphthalene structure for photocatalytic carbon dioxide reduction. Among them, pNJU-319Fe efficiently reduced carbon dioxide to carbon monoxide under visible light, with a yield of up to 68.8 μmol·g-1·h-1[187]. Wu et al. reported a 3D COF with a tty net, which displayed a high production rate (1,081 μmol·g-1·h-1) for the photocatalytic production of hydrogen peroxide[41]. Lin et al. reported two Cu-porphyrin-based 3D COFs as heterogeneous photocatalysts to produce singlet oxygen under illumination[188]. Another porphyrin-linked 3D COF (COF-366) was designed and synthesized by Hynek et al.[189]. COF-366 can continuously produce singlet oxygen under visible light, inactivate bacteria, and achieve antibacterial effects. Zhang et al. constructed a porphyrin-based 3D COF with copper coordination named Cu@COF-1,3,5,7-Tetrakis(4-aminophenyl)adamantane and 5,10,15,20-tetrakis(4-benzaldehyde)porphyrin (TATB) in 2022 for use as a photocatalyst[190]. Cu@COF-TATB could produce singlet oxygen to induce immunogenic cell death (ICD) in cancer cells at 635 nm. Yan et al. reported a 3D fluorescent COF photocatalyst for the generation of singlet oxygen, which can control the capture and desorption of singlet oxygen to regulate the fluorescence intensity of the COF[191]. In 2021, Zhang et al. reported a series of ICD-active 3D COFs (COF-607, COF-608, and COF-609) to achieve the production of reactive oxygen species, resulting in excellent ICD induction efficacy[192]. Among them, COF-609 was tested in mice with breast cancer, and after 110 days of treatment, the mice had a survival rate of over 95%.
Figure 8. (A) Schematic diagram of preparing JUC-640-M; (B) Comparison of the photocatalytic activity of JUC-640-M under the following conditions: 3 mg of COF, 5 mg of Ru(bpy)3Cl2·6H2O, 50 mg of BIH, 8 mL of MeCN, 12 mL of water, 300 W Xe lamp (≥
Electrocatalysis
In response to the rapid societal development, the human demand for energy is surging[193], but this heightened need, coupled with the excessive utilization of fossil fuels, has led to severe environmental pollution and an impending energy crisis[194]. Addressing these issues necessitates an improvement in energy efficiency and the exploration of cleaner energy sources[195]. In this context, electrochemical conversion processes have garnered significant attention due to their inherent advantages of high efficiency and minimal environmental impact[196]. These electrochemical processes encompass a range of reactions, including the oxygen reduction (ORR), oxygen evolution (OER), hydrogen evolution (HER), carbon dioxide reduction (CO2RR), and nitrogen reduction reactions (NRR)[197], all of which rely on efficient catalysts to enable their efficient execution[198]. Liu et al. have undertaken the design and synthesis of two 3D COFs based on porphyrin, namely PCOF-1 and PCOF-2[199]. Notably, PCOF-1-Co exhibited an overpotential of 473 mV at a current density of 10 mA·cm-2. Similarly, Gong et al. introduced a porphyrin-based 3D COF featuring a stp topology for electrocatalytic OER. Subsequent metallization with cobalt ions resulted in JUT-1@Co, which showcased a low overpotential of 295 mV at 10 mA·cm-2[200]. Furthermore, Meng et al. developed a 3D porphyrin-based COF, which, after pyrolysis and coordination with iron and nickel ions, achieved a low overpotential of 328 mV at 10 mA·cm-2 in 0.1 M KOH for OER[201]. Tavakoli et al. designed a 3D COF incorporating porphyrin groups through an electropolymerization process, demonstrating an impressive electron transfer number of approximately 3.97 for ORR[202]. Additionally, Zhou et al. devised a catalyst composed of pyrolyzed COF-300-derived carbons loaded with platinum for the ORR, revealing a half-wave potential and onset potential of 0.85 and 0.98 V vs. reversible hydrogen electrode (RHE), respectively[203]. Li et al. achieved a significant milestone by introducing the first woven 3D COF, COF-112, as a catalyst[204]. Upon cobalt ion anchoring (E1/2 = 0.76 V vs. RHE and n = 3.86), COF-112Co demonstrated exceptional ORR catalytic properties. Bao et al. successfully designed a fully conjugated 3D COF, Beijing University of Chemical Technology (BUCT)-COF-11, which, when combined with carbon nanotubes (CNTs), served as a high-efficiency, metal-free ORR electrocatalyst. The resulting BUCT-COF-11/CNT exhibited outstanding ORR activity, featuring a low half-wave potential of 0.72 V (vs. RHE)[205]. Chi et al. synthesized a 3D Co-porphyrin-based COF, for CO2RR, achieving a remarkable CO faradaic efficiency of 92.4% at 0.6 V vs. RHE[206]. Lastly, Han et al. constructed 3D imide-bonded COFs with a pts network for CO2RR, with CoPc-PI-COF-3 exhibiting a faradaic efficiency exceeding 88% for the conversion of CO2 to CO, [Figure 9][207].
Figure 9. (A) LSV curves of CoPc-PI-COF-3 in CO2 and Ar saturated electrolyte; (B) Faradaic efficiency diagrams of different COFs electrodes; (C) Scan rate dependence of cyclic voltammetry response of CoPc-PI-COF-3; (D) Partial CO current density of different COFs electrodes; (E) jCO of 3D COF electrocatalyst and of Por/Pc-containing reticular materials performed in an H-type electrochemical cell; (F) Durability performance of 3D COF electrode at -0.80 V; (G) Schematic diagram of preparing MPc-PI-COF-3 with pts net. Reproduced with permission from ref[207], Copyright 2022, Wiley-VCH. COFs: Covalent organic frameworks; 3D: three-dimensional.
Carrier conduction and energy storage devices
Conductive
Due to its high degree of conjugation, 3D COF is regarded as a promising semiconductor material and exhibits excellent carrier mobility, making it a potential candidate for conducting materials[208]. Li et al. reported the first instance of a 3D conductive COF[209]. They doped I2 into a TTF-based 3D COF to achieve a high conductivity of 1.4 × 10-2 S·cm-1 at 120 °C. The strategy of enhancing the conductivity of 3D COF through I2 doping has also been successfully employed by Li et al.[210]. They used two different monomers with the same core to construct a 3D imine COF with lvt net, and the electrical conductivity reached as high as 1.5 × 10-4 S/cm. Wang et al. designed and synthesized a fully conjugated 3D COF termed BUCT-COF-1, which achieved an electron mobility of 2.75 cm2·V-1·s-1[211]. The same research group also reported a sp2 carbon-conjugated 3D COF (BUCT- COF-4) for the first time in the same year. It exhibited an electron mobility of 1.97 cm2·V-1·s-1 for the bare-COF and 2.62 cm2·V-1·s-1 after iodine doping[51]. Additionally, Yang
Ion transport
Ion conductors play a crucial role in the electrolyte separators of energy storage devices, with lithium (Li)-ion and hydrogen-ion conduction being particularly significant in Li-ion batteries and fuel cells, attracting widespread attention[213]. Zhang et al. synthesized a 3D COF (CD-COF-Li) containing γ-cyclodextrin (γ-CD) for Li+ transport. Impressively, it exhibited a high conductivity of up to 2.7 mS·cm-1 after doping with Li ions, making it one of the highest conductivities among crystalline organic porous materials at that time[42]. Furthermore, Wang et al. designed and synthesized a 3D COF with hydroxyl groups as a solid-state electrolyte. When combined with plastic crystals, these materials exhibited outstanding Li-ion conductivity (0.64 mS·cm-1) and Li-ion transference numbers (0.7) at room temperature[214]. In the same year, Wu et al. reported three polyethylene glycol (PEG)-functionalized 3D COFs with different lengths of PEG chains. When combined with lithium bis((trifluoromethyl)sulfonyl)azanide (Li TFSI), these 3D COFs achieved an impressive ionic conductivity of 0.36 mS·cm-1 at 260 °C[215]. Moreover, Fan et al. fabricated a free-standing 3D COF membrane for H+ ion transport, which achieved a remarkable conductivity of 0.650 S·cm-1 at
Figure 10. (A) The I-V curves of JUC-590-MMM before and after activation with different concentrations of Li+; (B) The concentration-dependent changes in gating ratio (ILi+/I) of JUC-590-MMM at -1 and +1 V; (C) I-V curves of JUC-590-MMM after activation with different metal ions at -1 and +1 V; (D) The gating ratios of JUC-590-MMM after activation with different metal ions at -1 and +1 V; (E) Schematic diagram of preparing JUC-590 and JUC-591. Reproduced with permission from ref[217], Copyright 2022, Wiley-VCH.
Energy storage devices
To date, 3D COFs have found successful applications in various energy storage devices, including Li batteries, Zn batteries, supercapacitors, and solar cells[218]. Yu et al. reported the use of 3D polyoxometalate COFs with a 3-fold interpenetrated dia net structure as anode materials for Li-ion batteries. These COFs exhibited a high reversible capacity exceeding 550 mA·h·g-1 and demonstrated impressive cycling stability, even after undergoing more than 500 cycles[219]. Schon et al. engineered a 3D framework based on perylene imide (PDI) for constructing a lithium battery cathode. This cathode achieved a high capacity of
Figure 11. (A) Galvanostatic charge/discharge profiles of the S@3D-flu-COF electrode at various rates; (B) Rate performance of the S@3D-flu-COF and S@C electrodes at various rates; (C) Cycling stability of the S@3D-flu-COF and S@C electrodes at 0.5 C; (D) Cycling stability of the S@3D-flu-COF electrode at 5.0 C; (E) Comparison of the rate performances of COF-based sulfur host materials; (F) CVs of the S@3D-flu-COF electrode at scan rates ranging from 0.1 to 0.5 mV·s-1; (G) Li+ diffusion coefficient of S@3D-flu-COF and S@C electrodes; (H) Nyquist plots of the S@3D-flu-COF and S@C electrodes; (I) Schematic diagram of preparing 3D-flu-COF with flu net. Reproduced with permission from ref[39], Copyright 2022, American Chemical Society. 3D: Three-dimensional; COF: covalent organic framework.
Fluorescence and sensors
Fluorescence sensing
Fluorescent COFs enhance the fluorescence properties of COF materials by incorporating chromophores such as triazine, pyrene, s-tetrazine, and triphenylene[229]. These fluorescent COFs can be utilized as sensors to detect persistent organic pollutants and analytes, including metal ions, organic explosives, fullerene derivatives, or biomolecules that significantly affect the fluorescence intensity of substances[230]. Lin et al. pioneered the design and synthesis of the first fluorescent pyrene-based 3D COF (3D-Py-COF) for explosive detection[29]. This fluorescent COF enabled sensitive detection of picric acids, exhibiting a quenching degree of 20% at a picric acid concentration of 75 ppm. The same research group also introduced a fluorescent 3D COF [3D-tetraphenylethylene (TPE)-COF] with a cluster-induced luminescent agent building block in 2018[231]. Notably, 3D-TPE-COF emits yellow fluorescence upon excitation, with a quantum yield of approximately 20%. Coating it on a commercial blue light-emitting diode can convert it into a white light-emitting diode with no significant intensity attenuation for 1,200 h. In 2023, the same group reported another 3D fluorescent COF designed for the detection of trace antibiotics in water. This COF gradually quenches as the amount of nitrofurazone (a typical antibiotic) increases, with a maximum quenching efficiency of up to 93%[232]. Yuan et al. developed a 3D porous aromatic frameworks (PAF) named PAF-15 with an absolute quantum yield of fluorescence of up to 14% in methylene chloride, applied for the molecular detection of explosives[233]. Li et al. created a COOH-functionalized 3D-COF used for adsorption and fluorescence detection of 9,10-phenanthraquinone after europium ions coordinated with it[234]. In 2021, Wu et al. prepared two fully conjugated 3D COFs from a saddle-like molecule containing thiophene as a tetrahedral node[235]. This research revealed that the prepared fluorescent COFs exhibit selective sensitivity to picric acids. Wang et al. reported a bipyridine-based 3D COF for fluorescence sensing of Co2+. This COF serves as a fluorescent probe, displaying low detection limits and good recovery in cobalt ion detection[236]. Wei et al. constructed a 3D fluorescent COF (dynaCOF) with an acceptor-donor-acceptor (A-D-A) π coupling system for the detection of various organic vapors. This fluorescence sensor offers high sensitivity and low detection limits and maintains satisfactory stability and moisture resistance[237]. Chi et al. prepared COF-300 loaded with gold nanoparticles, followed by CsPbBr3 quantum dots, as a fluorescence sensor for the determination of benzo(a)pyrene in cooking oil[238].
Other sensors and optics
Zhao et al. pioneered the design and synthesis of two 3D COFs completed with hydrazone motifs as pH-triggered molecular switches. They explored applications of these COFs in stimulus-responsive drug delivery systems[239]. At a pH of 4.8, the release of Ara-C increases nearly three-fold compared to a pH of 7.4. This enhancement holds significant promise for improving drug targeting and minimizing side effects. The same research group also created two dynamic 3D COFs, named JUC-635 and JUC-636, based on aggregation-induced emission or aggregation-induced quenching chromophores. As depicted in Figure 12, these COFs were employed for the first time to investigate pressure-induced color changes using diamond anvil pool technology[240]. Notably, JUC-635 maintains its fluorescence even under high pressure up to
Figure 12. (A-C) constitute the monomers of JUC-635 and JUC-636; (D and E) Schematic diagram of JUC-635 and JUC-636. and JUC-636; (F) The two COFs have 10-fold interpenetrated dia net; (G) These materials were further used to study the pressure change behavior, demonstrating structural transformations in response to mechanical stimulation by DAC techniques. Reproduced with permission from ref[240], Copyright 2023, Wiley-VCH. COFs: Covalent organic frameworks.
CONCLUSION AND OUTLOOK
Since El-Kaderi et al. first reported 3D COFs in 2007, these materials have received considerable attention from researchers due to their high specific surface area, adjustable pore size, and interpenetrating pore structures[2]. Currently, 3D COFs have found successful applications in various fields, including for adsorption, separation, heterogeneous catalysis, electrochemistry, energy storages, and sensors. The structure-activity relationship is paramount for the practical application of COFs. Researchers consistently strive to finely control their performance through optimal structural design. COF materials exhibit excellent crystallinity and stability, allowing the construction of an ideal periodic structure using different building block elements. This characteristic positions COFs as an ideal carrier material for establishing a structure-activity relationship. Consequently, crystallinity becomes a prerequisite for COFs to transition into industrial application production.
The synthesis of highly crystalline COF frameworks typically involves reversible covalent bonds, which, while offering some advantages, also comes with limitations. These bonds are prone to decomposition under extreme conditions, significantly restricting their application scenarios. Conversely, the use of irreversible covalent bonds can yield highly stable COF materials, but often at the expense of poor crystallinity, hindering their ability to achieve high performance. Additionally, the strict deoxidation operation required for COF material synthesis imposes constraints on macro-preparation and large-scale applications. As a result, the synthesis strategy and efficient preparation of highly stable and crystalline COF materials pose a major challenge in the practical application of this field. Addressing this issue will become another significant obstacle in the development of COFs, especially for 3D COFs.
When compared to 2D COFs, 3D COFs still exhibit certain limitations. Firstly, due to the limited diversity of building blocks available for constructing 3D COFs, the number of reported 3D COFs remains relatively low, at less than 200, which hampers the exploration of their full potential in various applications. Secondly, the synthesis methods for 3D COFs are comparatively restricted, and they lack the strong π-π stacking interactions that are prevalent in 2D COFs, making it more challenging to achieve highly crystalline materials. Stability is also a significant challenge for 3D COFs. Hence, designing highly stable 3D COFs, by means with stable linkage, shall explore more. On the other hand, while the interpenetrating pore structure can enhance stability to some extent, it may lead to reduced exposure of functional groups within the pores and low specific surface area. Finally, the current applications of 3D COFs primarily revolve around adsorption, separation, sensing, catalysis, and energy storage, but many of these studies are relatively superficial. Further research into machine and system regulation is needed, requiring more attention from researchers. Moreover, expanding the application of 3D COFs into broader fields is essential. In summary, we have outlined the research progress of 3D COFs in their existing applications. This article emphasizes the potential applications for functional 3D COFs, encompassing separation, adsorption, heterogeneous catalysis, energy storage, and sensing. We believe that this review can offer valuable guidance for the synthesis and functionalization of functional 3D COFs in the future.
DECLARATIONS
Authors’ contributions
Responsible for the overall design, direction and supervision of the project: Fang Q
Performed the writing of the manuscript: Wang R
Discussed the results and contributed to the writing of the manuscript: Zhao J, Qiu S
Availability of data and materials
Not applicable.
Financial support and sponsorship
This work was supported by National Key R&D Program of China (2021YFF0500504 and 2022YFB3704900), National Natural Science Foundation of China (22025504, 21621001, and 22105082), the SINOPEC Research Institute of Petroleum Processing, “111” project (BP0719036 and B17020), and the program for JLU Science and Technology Innovative Research Team.
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2024.
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Wang, R.; Zhao, J.; Fang, Q.; Qiu, S. Advancements and applications of three-dimensional covalent organic frameworks. Chem. Synth. 2024, 4, 29. http://dx.doi.org/10.20517/cs.2023.61
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