English

• 0.   Introduction

  In the past century, the extensive extraction and use of fossil fuels such as oil, coal, and natural gas have led to a significant amount of CO₂ being emitted into the atmosphere, disrupting the original "carbon cycle" primarily driven by biological metabolism, and causing a surge in CO₂ concentration in the atmosphere^[1-2]. This has resulted in irreversible impacts on the Earth′s environment. Against the backdrop of global energy shortages and severe environmental degradation, directly converting CO₂ into various carbon-based hydrocarbons can achieve "negative emissions" of carbon^[3], and is also a strategy that serves two purposes: it not only alleviates excessive CO₂ emissions but also provides energy and industrial raw materials for human development.

  Therefore, the strategy of reducing CO₂ to valuable chemicals is most beneficial for achieving lower carbon emissions and mitigating the greenhouse effect^[4-9]. Due to the high thermodynamic stability of CO₂ molecules, a significant amount of energy is required for CO₂ conversion. This can typically be achieved through techniques such as photocatalytic reduction^[10-23] and electrochemical reduction^[24-28]. Among these, photocatalytic CO₂ reduction utilizes solar energy as the sole energy source to convert CO₂ and water into hydrocarbons directly. Electrocatalytic CO₂ reduction utilizes external voltage as the primary energy source to achieve the catalytic conversion of CO₂ at an appropriate negative potential.

  Covalent organic framework (COF) is a two-dimensional (2D) or three-dimensional (3D) crystalline porous organic polymer composed of organic building blocks connected by covalent bonds^[29-37]. Currently, chemists have synthesized numerous COF materials with diverse topological structures and functions using various synthesis strategies^[38-44]. Their unique structure and chemical stability make them have broad application prospects in separation^[45-53], catalysis^[54-59], energy and environmental science^[60-69], biomedical^[70-79], solar cell light energy conversion^[80-84], proton conduction^[85-88], semiconductors^[89-90], photoluminescence^[91-93], enantiomer separation^[94-97], adsorption of radioactive elements^[98], detection and removal of pollutants^[99-100], and other fields. On the other hand, as a class of organic porous materials, COFs do not contain metals and therefore cannot meet the requirements of catalytic applications involving metal catalysts. Therefore, the design and synthesis of metalized COFs can not only make up for the shortcomings of COFs in this regard but also greatly broaden the application range of COFs in the field of catalysis. The so-called metallization process, which involves combining metal ions with COFs, includes direct synthesis and post-synthetic modification approaches. In the direct synthesis of metallized COFs, monomers containing metal complexes can be constructed to precisely control the position and quantity of metals within the COFs. However, due to the slight differences in the physical and chemical properties of metallized and non-metallized monomers, it is necessary to optimize the synthesis conditions multiple times during the construction of COFs. In addition, metal complexes may not be stable under certain harsh reaction conditions. Currently, the post-synthetic modification method provides new ideas for the synthesis of metallized COFs. In this approach, metals are introduced into the COFs through direct coordination or monomer exchange, and their content is controlled by adjusting the concentration and the time of the reaction system, while the COFs retain their original framework structure. According to the location of the metal, the introduction of metal active centers can be divided into three categories: (1) the metal coordinates with the COF building blocks in the form of ions; (2) the metal coordinates with the COF linking units in the form of ions; (3) the metal is embedded in the COFs in the form of nanoparticles.

  Our research group has been engaged in the synthesis, design, and performance research of COFs for a long time. We have also achieved certain research results in metal catalysis^[101-102]. A novel NHC-AuCl-COF was prepared under solvothermal conditions^[101]. Subsequently, NHC-AuSbF₆-COF was prepared by ion exchange from NHC-AuCl-COF (Fig.1). As a heterogeneous catalyst, NHC-AuCl-COF can efficiently catalyze the carboxylation reaction of terminal alkynes and CO₂ under mild conditions. Compared with NHC-AuCl-COF, NHC-AuSbF₆-COF exhibited higher catalytic activity in the hydration reaction of terminal alkynes. A chiral COF (CCOF) with an imine linkage was constructed by direct polymerization^[102]. The obtained (S)-NHC-Au-SA-COF had dual catalytic active sites, which can efficiently catalyze the asymmetric series reaction of aryl methanol oxidation, aldol condensation with heterogeneous catalysis (Fig.2).

  Figure 1

  

  Figure 1.  Synthesis process and catalytic application of NHC-AuCl-COF and NHC-AuSbF₆-COF^[101]

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  Figure 2

  

  Figure 2.  Synthesis process and asymmetric catalytic application of (S)-NHC-Au-SA-COF^[102]

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  Therefore, the research and design of metallized COFs is gradually becoming one of the research hotspots in recent years. It is imperative to design novel, efficient, and highly selective metallized COFs. Currently, copper (Cu), as one of the most common catalysts capable of converting CO₂, has long attracted widespread attention from researchers^[103-104]. Cu has a moderate adsorption energy for *CO intermediates, which is beneficial for maintaining the balance between adsorption and desorption on the catalyst surface. Therefore, Cu sites embedded in the COF can promote the adsorption and activation of CO₂, thereby lowering the energy barriers for the formation of different intermediates and improving the CO₂ reduction performance of the catalyst. In this review, we present the research progress on Cu-based COF^[105-107], including the synthesis methods of Cu-based COFs and applications in the fields of electrocatalysis and photocatalysis.

  1.   Electrocatalytic CO₂ reduction

  Researchers generally agree that the electrocatalytic CO₂ reduction reaction (CO₂RR) consists of four main steps: (1) CO₂ molecules are chemisorbed on the cathode catalyst surface; (2) CO₂ molecules are activated into intermediate free radicals *CO₂^-; (3) multiple electron/proton transfers occur to form hydrocarbons; (4) products desorb from the electrocatalyst surface. Mechanistic studies of electrocatalytic CO₂RR have found that the rate and product distribution of the catalytic reaction are mainly affected by the active sites, surface properties, and electron transfer effects of the catalyst. Among them, the characteristics and number of active sites, as well as their adsorption capacity for reaction substrates, directly determine the activity of the catalyst; while the physical and chemical properties of the catalyst surface affect the adsorption and desorption behavior of reactants and products on the catalyst surface, thereby affecting the rate and direction of the catalytic reaction; the electron transfer effect is related to the migration and distribution of charges during the catalytic process and is also a key factor determining catalytic efficiency. Therefore, the design of novel, efficient, and highly selective electrocatalysts is particularly important. Cu-based COFs with regular structures can be used for electrocatalytic CO₂RR due to the following characteristics: (1) the porous structure increases the contact speed between the substrate and the active sites; (2) abundant active sites promote the adsorption and conversion of CO₂; (3) the rational arrangement between building units and active sites makes the electron transfer and mass transfer properties adjustable. In this section, we will elucidate the importance of Cu-based COFs in electrocatalytic CO₂RR.

  1.1   C₂H₄ as the main reduction product

  Currently, Cu is considered an effective electrocatalyst for producing C₂₊ products via CO₂RR. Its moderate adsorption energy for intermediates allows these intermediates to remain on the catalyst surface with sufficient opportunities for C—C coupling with neighboring intermediates, which is the rate-determining step for generating C₂H₄. To achieve efficient conversion of CO₂ to multi-carbon products, researchers have made significant efforts to enhance both the activity and selectivity of CO₂ conversion to C₂H₄.

  Huang et al. stated that the coordination structure of materials can affect the selectivity of reactions^[108]. They prepared Cu-PyCA-MCOF from copper-pyrazolate complex and 2,4,6-tris(4-aminophenyl)-1,3,5-triazine under solvothermal conditions. Afterwards, nanosheet Cu-PyCAOH-MCOF was obtained by treating Cu-PyCA-MCOF with H₂O₂ at room temperature (Fig.3a). Although Cu-PyCA-MCOF and Cu-PyCAOH-MCOF had similar topological structures, they exhibited significantly different selectivity in CO₂RR (Fig.3b). When Cu-PyCA-MCOF was used as a catalyst, the main product of electrocatalytic CO₂RR was CH₄, while Cu-PyCAOH-MCOF promoted the formation of more valuable chemical C₂H₄. On the one hand, this is attributed to the advantage of the nanosheet structure in exposing the Cu sites of Cu-PyCAOH-MCOF, which effectively contact the electrolyte and CO₂ from a direction perpendicular to the plane of the trinuclear Cu cluster. On the other hand, the coordination of oxygen species (μ₃-O, OH^-/H₂O) not only changes the charge density distribution of Cu centers of Cu-PyCAOH-MCOF, reducing the formation energy of *COOH, but also affects the attack direction of CO₂. Thus, it effectively promotes the C—C coupling reaction and efficiently converts CO₂ into C₂H₄. This work opens a new avenue for improving the conversion performance of C₂₊ products in CO₂RR by adjusting the coordination structure of Cu-based electrocatalysts.

  Figure 3

  

  Figure 3.  (a) Schematic representation of the synthesis of Cu-PyCA-MCOF and Cu-PyCAOH-MACOF; Views of (b) Cu-PyCA-MCOF and (c) Cu-PyCAOH-MCOF structural models (C: gray, N: blue, O: red, Cu: pink, and H: white)^[108]

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  1.2   Acetate as the main reduction product

  In electrocatalytic CO₂RR, the selectivity for acetate is typically lower than that for ethylene. This is attributed to the different pathways for CO₂ conversion to ethylene and acetate, respectively. CO₂ conversion to ethylene involves C—C coupling of *CO with *CO or *CHO, a process that requires the synergistic effect of adjacent catalytic sites. However, CO₂ conversion to acetate requires C—C coupling of *CH₃ and CO₂, for which single or isolated active sites are beneficial. Benefiting from uniform and isolated active sites, Cu-based COFs exhibit great potential in efficient electrocatalytic CO₂ reduction for acetate production.

  Liao et al. prepared a stable and conductive 2D COF, named PcCu-TFPN, with a singular copper phthalocyanine center (Fig.4)^[109]. PcCu-TFPN was synthesized by reacting Cu(Ⅱ) 2,3,9,10,16,17,23,24-octahydroxyphthalocyanine (PcCu-(OH)₈) with 2,3,5,6-tetrafluorobenzonitrile (TFPN) in a vacuum at 150 ℃ for 3 d. PcCu-TFPN exhibited an excellent selectivity in electrocatalytic CO₂RR for acetate [Faraday efficiency (FE): 90.3%], with a current density of 12.5 mA·cm^-2 at -0.8 V (vs RHE). Due to PcCu-TFPN being a single active site catalyst, *CO species can be converted into *CH₃ through hydrogenation reaction, which then couples with CO₂ to form acetate. To certify this hypothesis, the authors synthesized a metal organic framework, PcCu-CuO, which consists of PcCu-(OH)₈ ligands and square planar CuO₄ nodes, where CuO₄ is the second active site. The results showed that the main product of PcCu-CuO was C₂H₄, while PcCu-TFPN promoted the reaction to mainly produce C₂H₄ and C₂H₅OH. These results indicate that with a second active site that generates CO or in the presence of CO gas molecules, the isolated copper-phthalocyanine active site will catalyze the reduction of CO₂ to produce C₂H₄ and C₂H₅OH in the CO₂ atmosphere. To further investigate the catalytic mechanism of acetate production of PcCu-TFPN, the authors analyzed and compared two recently reported catalysts with similar isolated Cu-N₄ catalytic sites, namely the single-atom Cu catalyst (CuSAC) and Cu-porphyrin-based COF (Cu-porphyrin). Among them, the CO₂RR product of CuSAC was CO, and the product of Cu-porphyrin was CH₄. This may be due to the unstable adsorption of *CO intermediates in CuSAC, leading to desorption of *CO instead of further hydrogenation. The energy barrier in Cu-porphyrin (0.29 eV) for the formation of *OOCCH₃ by the combination of *CH₃ and CO₂ was much higher than that in PcCu-TFPN (0.55 eV), indicating that Cu-porphyrin is more conducive to the production of CH₄.

  Figure 4

  

  Figure 4.  Schematic representation of the synthesis of PcCu-TFPN^[109]

  Reaction conditions: dimethylacetamide, 1,3,5-trimethylbenzene, 150 ℃, 3 d, 85% yield.

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  1.3   CH₄ as the main reduction product

  CH₄ is a crucial strategic energy source. It is cleaner than coal and oil and plays an indispensable role in the current energy system. However, the electroreduction of CO₂ to methane is a complex process. This process faces two major challenges: the need to activate water molecules to provide protons and competition with the strong hydrogen evolution side reaction. Researchers have successfully created a unique microenvironment by integrating Cu into a COF. This environment efficiently activates both CO₂ and water, and stabilizes reaction intermediates, thereby effectively guiding the reaction towards methane production while suppressing hydrogen generation.

  Shao et al. developed a tandem electrocatalyst composed of Cu single atoms (SAs) and nanoclusters (NCs) to achieve the conversion of CO₂ to CH₄ (Fig. 5a)^[110]. Firstly, COF grew on a glassy carbon electrode by in-situ polymerization using 2,6-diaminoanthraquinone and 2,4,6-triformylphloroglucinol. Afterwards, different types of Cu aggregates were deposited onto the electrode by an electrodeposition method. When an electrochemical reduction potential of 0.12 V (vs Ag/AgCl) was applied to the electrode for 100 s, Cu SA was formed in the COF network (referred to as SA Cu/COF). As the electrodeposition time increased, many Cu atoms aggregated to form Cu NCs (referred to as NC-SA Cu/COF). The FEs of products at different potentials indicate that the catalytically active sites in the CO₂RR are Cu atoms, rather than the COF. After 1 h of catalysis, the morphology of COF retained its original appearance, and no other products were detected in the electrolyte after reaction. Interestingly, although NC-SA Cu/COF and SA Cu/COF had similar CO₂ reduction current densities (Fig.5b), there were significant differences in catalytic products. This indicated that Cu NCs of NC-SA Cu/COF play a crucial role in the CO₂RR, altering the pathway of CO₂ conversion to CH₄. This may be related to the synergistic effect between SA Cu/COF and Cu NCs, where CO generated on SA Cu/COF is rapidly transferred to the nearby Cu NCs and is subsequently reduced to CH₄. In addition, coordinated K⁺ ions can promote CO₂ adsorption activation and H₂O dissociation by stabilizing the more electronegative O atoms in the adsorbed material.

  Figure 5

  

  Figure 5.  (a) A Structural model of NC‐SA Cu/COF (Grey, blue, red, and brown spheres represent C, N, O, and Cu atoms, respectively, and H atoms are omitted for clarity); (b) CO₂ reduction current densities on NC-SA Cu/COF, SA Cu/COF, and pure COF^[110]

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  Lan et al. designed and synthesized cyanide-containing COFs to uniformly support and encapsulate metal NCs^[111]. Firstly, 2,3,9,10,16,17,23,24-octahydroxyphthalocyaninato copper (CuPc) underwent a condensation reaction with tetrafluorophthalonitrile (TFPN) to obtain a Cu-phthalocyanine COF (CuPc-COF). Afterwards, CuPc-COF and hexahydrate copper perchlorate were mixed by the hydrothermal method. Then Cu(Ⅱ) ions were reduced using hydrazine hydrate to obtain the final product Cu-NC@CuPc-COF (Fig.6a). During this process, the coordination interaction between —CN and Cu in the COF pores guided the formation of Cu-NCs, and the confinement effect resulted in their uniform dispersion. The diffraction peaks in the powder X-ray diffraction (PXRD) pattern indicate that COFs had high crystallinity, but no obvious diffraction peaks of Cu-NCs were observed in the Cu-NC@CuPc-COF PXRD pattern. The porosity and surface areas of COFs were studied through nitrogen adsorption-desorption experiments at 77 K. Compared with CuPc-COF, the surface area and pore size of Cu-NC@CuPc-COF had significantly decreased. These results indicated that Cu-NCs of approximately the same size occupied the pores of COF, and these Cu-NCs were extremely small and uniformly dispersed. Cu-NC@CuPc-COF as a gas diffusion electrode, achieved the FE (CH₄) of ca. 74.3% at a potential of 1.0 V (vs RHE), along with a current density of 538.31 mA·cm^-2 at a potential of 1.2 V (vs RHE), which is significantly higher than the corresponding values of most existing advanced electrocatalysts and exceeds the relevant industrial current density (200 mA·cm^-2) (Fig.6b). The in-situ Fourier transform infrared (FTIR) spectroscopy results showed that the Cu atoms and Cu-NCs in the composite material have a synergistic effect on the catalytic active sites during CO₂ reduction. This work provides a new paradigm for designing highly active CO₂ electrocatalytic reduction catalysts using pore confinement effects and synergistic strategies.

  Figure 6

  

  Figure 6.  (a) Synthesis and structure illustrations of Cu-NC@CuPc-COF; (b) Catalytic application in CO₂ reduction of Cu-NC@CuPc-COF

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  Wang′s group modulated the microenvironment of Cu nanoparticles (Cu NPs) using an ionomer of COF nanosheets containing heteroatoms and sulfonic acid groups, meeting the requirements for CO₂ reduction at industrial-grade current densities^[112]. First, NUS9 nanosheets were prepared using 2,5-diaminobenzenesulfonic acid and 1, 3, 5-triformylphloroglucinol (Fig.7a). PXRD patterns of NUS9 showed two significant peaks at 4.9° and 27° (Fig.7b). Subsequently, a mixture obtained by mixing Cu NPs with the NUS9 ionomer (named Cu-NUS9) was coated on a gas diffusion layer to construct a catalyst layer. The catalyst layer was tested in a three-electrode flow cell using 0.5 mol·L^-1 potassium sulfate as an acidic electrolyte. Experiments showed that in the CO₂ reduction to CH₄ process catalyzed by Cu-NUS9, FE was 66%, and the current density was 296 mA·cm^-2. In addition, Cu-NUS9 could operate continuously for 9 h at a current density of 200 mA·cm^-2 and maintain good stability. The authors also elucidated the mechanism of the main product transition in the CO₂RR through experiments and theoretical calculations (Fig.8). They stated that the porous structure of the Cu-NUS9 nanosheets improved the adsorption and conversion rate of CO₂, while the high charge structure and carbonyl groups in the framework were beneficial for cation enrichment. These synergistic effects enabled the CO₂RR to proceed over a wide pH range.

  Figure 7

  

  Figure 7.  (a) Chemical structure of NUS9; (b) Typical PXRD patterns of NUS9 nanosheets with an eclipsed stacking model^[112]

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  Figure 8

  

  Figure 8.  Schematic representation of Cu-NUS9 in electrocatalytic CO₂RR^[112]

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  Lan et al. synthesized anthraquinone-based COFs with a one-dimensional superstructure via a condensation reaction, investigated the formation mechanism of the anthraquinone-based COF morphology, and explored their CO₂RR performance^[113]. First, AAn-COF nanofibers with a one-dimensional superstructure were prepared using a Schiff-base condensation reaction between 1,3,5-triformylphloroglucinol and 1,5-diaminoanthraquinone (Fig.9). To investigate the formation mechanism of the nanofibers, the authors recorded the morphological changes of AAn-COF at different reaction times (Fig.10). Small nanoparticles appeared after 1 h of reaction, and then the small nanoparticles gradually grew larger and stacked to produce nanosheets. Then, the size of the nanosheets increased, and nanofibers began to grow on their surface as the reaction time was extended. When the reaction proceeded for 24 h, all the nanosheets were transformed into uniform nanofibers. As the reaction time continued to increase, the surface of the nanofibers began to become rough, gradually forming a mace-like nanofiber. This process adopted a bottom-up self-templating mechanism. For comparison, the authors synthesized OH-AAn-COF hollow tubes, performed similar structural characterization, and studied the formation mechanism. Unlike the formation mechanism of AAn-COF, the synthesis process of OH-AAn-COF involved a nanosheet curling mechanism. In addition, AAn-COF and OH-AAn-COF were separately added to an aqueous solution of Cu(OAc)₂ and refluxed for 72 h, yielding AAn-COF-Cu and OH-AAn-COF-Cu, respectively. AAn-COF-Cu and OH-AAn-COF-Cu exhibited excellent catalytic activity in CO₂RR. Specifically, FE_CH4 of OH-AAn-COF-Cu was 61% (99.5 mA·cm^-2, 1.0 V). FE_CH4 of AAn-COF-Cu was 77% (128.1 mA·cm^-2, 0.9 V). This FE_CH4 was relatively high among reported crystalline COFs. This work provides a general method for designing COF electrocatalysts with different morphologies in the CO₂RR process.

  Figure 9

  

  Figure 9.  Schematic of the synthesis and structure of AAn-COF-Cu^[113]

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  Figure 10

  

  Figure 10.  Schematic representation of AAn-COF obtained from different reaction times of (a) 6 h, (b) 12 h, (c) 18 h, and (d) 72 h, respectively^[113]

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  Lan′s group proposed a strategy based on functionalized exfoliants that can simultaneously modify bulk COFs and exfoliate them into functionalized nanosheets, while also exploring their electrocatalytic performance for CO₂RR^[114]. This strategy exposes more active sites in the exfoliated COFs, resulting in a larger surface area, which significantly enhances the contact area between reactants and intermediates, as well as the selectivity of the reaction towards CH₄. First, Cu-Tph-COF-OH was prepared under solvothermal conditions, and then Cu-Tph-COF-Dct was synthesized in the presence of the functionalized exfoliant (2,4-diamino-6-chloro-1,3,5-triazine, Dct) (Fig.11). It was shown that the PXRD patterns of Cu-Tph-COF-Dct and Cu-Tph-COF-OH were essentially consistent. In CO₂ adsorption experiments at 273 and 298 K, Cu-Tph-COF-Dct exhibited a higher CO₂ absorption capacity compared to Cu-Tph-COF-OH. TEM tests revealed that the morphology of Cu-Tph-COF-Dct was nanosheets with a thickness of approximately 3.8 nm. The authors conducted electrochemical tests in a flow cell, where at -0.90 V, FE_CH4 was 80%, with a current density of -220.0 mA·cm^-2. Notably, the FE_CH4 of Cu-Tph-COF-Dct remained above 68% over a wide potential range from -0.8 to -1.0 V. Additionally, the catalytic mechanism of CO₂RR and the influence of Dct on the reaction were systematically studied through theoretical calculations. The amino and triazine structures in the Dct group enhanced the availability of CO₂ and regulated the coverage of *CO on the catalyst surface, thereby effectively promoting the conversion of CO₂ to CH₄. This work provides a very important method for generating valuable chemicals through CO₂RR.

  Figure 11

  

  Figure 11.  Schematic preparation of Cu-Tph-COF-Dct^[114]

  Reaction conditions: 1,4-dioxane, triethylamine, 90 ℃, 6 h.

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  2.   Photocatalytic CO₂ reduction

  Generally, the photocatalytic CO₂RR process is similar to the electrocatalytic CO₂RR, including four aspects: (1) the photocatalyst is photoexcited to generate carrier electron-hole pairs; (2) efficient separation and transfer of electron-hole pairs; (3) adsorption and activation of CO₂; (4) separated electrons and holes are transferred to the catalyst surface and participate in redox reactions. According to the above basic principles, an ideal photocatalyst for CO₂ reduction needs to meet the following conditions: (1) strong light absorption capacity; (2) large specific surface area; (3) suitable band structure; (4) abundant active sites; (5) high electron-hole pair separation efficiency and charge transfer efficiency; (6) strong CO₂ adsorption capacity. Fortunately, Cu-based COFs have effective photocatalytic active sites. When they act as photocatalysts for CO₂RR, they can meet the above requirements because they have the following advantages: (1) the ordered arrangement and combination of various building units and active sites make the band structure and active sites adjustable; (2) high surface area and porous structure increase the possibility of contact between the substrate and active sites; (3) the conjugated and heterojunction structures existing in the framework are conducive to the separation and migration of electron/hole pairs; (4) multiple active sites can improve the absorption and activation of CO₂. This section will discuss the photocatalytic structure-activity relationship of Cu-based COFs with photocatalytically active sites.

  2.1   CH₄ as the main reduction product

  Cu-based COF photocatalytic reduction of CO₂ to CH₄ is a cutting-edge research direction in the field of artificial photosynthesis. This technology cleverly combines the Cu active center with the designable structure of COFs, achieving highly selective conversion of CO₂. Although challenges remain in catalytic efficiency, material stability, and preparation costs, this innovative strategy provides an important theoretical basis and technical pathway for developing efficient solar fuel conversion systems.

  Lan et al. synthesized Bpy-COF using the typical solvothermal method through the imine condensation reaction of 1,3,5-tris(4-aminophenyl) triazine and 2,2″-bipyridine-5,5″-dicarboxaldehyde^[115]. Then, they used the bipyridine unit in Bpy-COF to connect a single non-precious metal Cu site, obtaining Cu-Bpy-COF (Fig.12a). The prepared Cu-Bpy-COF exhibited high selectivity and efficiency in aqueous photocatalytic CO₂ reduction without photosensitizers. Experiments and DFT calculations have shown that single Cu sites can act as active centers, promoting the activation and adsorption of CO₂ molecules, thereby achieving almost 100% selective directional conversion of CO₂ to CO or CH₄ (Fig.12b). It is also found that the triethylamine has a dual function, serving as both a sacrificial agent and a proton transfer catalyst for CO₂ reduction. It can capture protons from H₂O to further reduce *CO to CH₄ due to its lower dissociation energy.

  Figure 12

  

  Figure 12.  (a) Schematic representation of Bpy-COF and M-Bpy-COF; (b) Proposed reaction mechanism for photocatalytic CO₂ conversion over Cu-Bpy-COF^[115]

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  2.2   CO as the main reduction product

  Photocatalytic conversion of CO₂ into high-value carbon monoxide was one of the key technologies for achieving the "dual carbon" goals. Cu-based COFs offer an innovative solution for this: their ordered channels can efficiently concentrate CO₂, while precisely integrated Cu sites not only promote photogenerated charge separation but also significantly reduce the CO₂ activation energy barrier, preferentially generating CO rather than deeply reduced CH₄. Through molecular-level structural design, the adsorption energy of intermediates can be optimized, thereby achieving efficient and highly selective production of CO, providing a very promising research path for solar-driven carbon resource conversion.

  Jing et al. successfully synthesized a mesoporous donor-acceptor pyridine-based COF (pCOF) with broad-spectrum light absorption properties using the solvothermal method, and subsequently prepared ultrathin pCOF nanosheets in the solvent of 1-methyl-2-pyrrolidinone using the ultrasonic exfoliation method^[116]. Then, ionic liquid portions were covalently attached to the pyridine N of pCOF through an aromatic N-quaternization reaction to obtain im-pCOF. Subsequently, using CuCl₂ as the metal source and EmimBr-derived NHC ligand as the anchor site, a single Cu(Ⅱ) atom was successfully fixed in the pores of pCOF (denoted as Cu(Ⅱ)-im-pCOF). Further reduction treatment of Cu(Ⅱ)-im-pCOF with ascorbic acid resulted in the formation of a single Cu(Ⅰ) atom (Fig.13). The catalytic effect of the obtained Cu(Ⅰ)-im-pCOF is superior to that of a single Cu(Ⅱ) atomic photocatalyst. The CO₂ conversion rate of Cu(Ⅰ)-im-pCOF in pure water was 184.8 μmol·g^-1·h^-1, and the selectivity toward CO₂ reduction to produce CO was approximately 100%. It is worth noting that during the reduction process of low-concentration CO₂ (CO₂/N₂ with a volume ratio of 10%), the photocatalyst also maintained 87.5% of its inherent photoactivity. Compared with single Cu(Ⅱ) atoms, single Cu(Ⅰ) atoms had higher electron capture and CO₂ adsorption/activation capabilities. Further study the overall electron dynamics by combining fs- and μs-transient absorption spectroscopy (TAS). It has been confirmed that a single Cu(Ⅰ) atom can rapidly extract electrons from the electron-rich region of pCOF along the NHC ligand at an electron transfer rate of 3×10⁹ s^-1 and promote the adsorption and activation of CO₂ (Fig.14a, 14b). Through in-situ isotope μs-TAS measurement, the electron transfer efficiency could reach 60.4% and 31.1% in the reduction process of pure CO₂ and diluted CO₂, respectively (Fig.14c-14e). This work provides a research solution for the application of photocatalysts based on COF in energy conversion.

  Figure 13

  

  Figure 13.  Schematic illustration for the preparation process of Cu(Ⅱ)-im-pCOF and Cu(Ⅰ)-im-pCOF^[116]

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  Figure 14

  

  Figure 14.  (a) fs-TAS spectra and (b) fs-TAS kinetic plots at 900 nm of pCOF, im-pCOF, and Cu(Ⅰ)-im-pCOF; μs-TAS spectra of pCOF, im-pCOF, and Cu(Ⅰ)-im-pCOF under different CO₂ environments of (c) H₂O bubbled with ¹²CO₂, (d) H₂O bubbled with ¹³CO₂, and (e) H₂O bubbled with 10%CO₂/90%N₂^[116]

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  Yang′s group synthesized DhaTph using 5,10,15,20-tetra(4-aminophenyl)-porphyrin (Tph) and 2,5-dihydroxyterephthal-aldehyde (Dha) as monomers^[117]. When synthesizing DhaTph-Cu, Tph was first complexed with Cu(CH₃COO)₂ to form a complex and then subjected to a condensation reaction under similar conditions. The hollow DhaTph Tubes and DhaTph-Cu Tubes were prepared under solvothermal conditions by a bottom-up template-free method using 2,4,6-trimethylbenzaldehyde (TBA) as a competitive reagent (Fig.15). To investigate the formation mechanism of COF tubular structures, the authors recorded the changes in morphology of DhaTph Tubes over time. Fig.16 shows SEM images at different time intervals during the formation process of DhaTph Tubes. When the system reacted for 0.5 h, a solid rectangular prism with a smooth surface was displayed. As time went on, the surface of the rectangular prism became rough, and some microcrystals appeared on the surface. The overflowing microcrystals gradually form large aggregates, forming hollow tubular structures within the rectangular prism. As the reaction time increased further, the outer tube wall became thinner and thinner. The formation mechanism of hollow DhaTph Tubes was speculated based on Ostwald maturation. In addition, under full light irradiation (200≤λ≤1 000 nm), without any cocatalysts or hole scavengers, the yield of the main product CO in the photocatalytic CO₂RR over DhaTph-Cu Tubes was 15.9 μmol·g^-1·h^-1, which was about two and three times higher than that of DhaTph Tubes and DhaTph COF under the same experimental conditions. DhaTph-Cu Tubes exhibited excellent photostability, and even after three consecutive uses, the CO precipitation rate remained high at approximately 13.1 μmol·g^-1·h^-1. The CH₄ production of DhaTph-Cu Tubes was 13.7 μmol·g^-1·h^-1, which was superior to the other two materials. They stated that due to the tubular structure and Cu(Ⅱ) anchoring on the porphyrin ring, DhaTph-Cu Tubes had rapid separation and transfer ability, thus exhibiting efficient photocatalytic activity.

  Figure 15

  

  Figure 15.  Synthetic procedures of DhaTph Tubes and DhaTph-Cu Tubes^[117]

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  Figure 16

  

  Figure 16.  SEM images of the intimal Tph+TBA (a), DhaTph Tubes prepared with different reaction times of 0 h (b), 0.5 h (c), 2 h (d), 5 h (e), 15 h (f), 35 h (g), and 72 h (h)^[117]

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  Xu et al. obtained DHTA-TTA 2D COF by the solvothermal method using 2,5-dimethoxybenzene-1,4-dicarboxaldehyde (DHTA) and 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)trianiline (TTA)^[118]. Then, Cu-COF was prepared by mixing DHTA-TTA 2D COF and CuCl₂ in acetonitrile (Fig.17a, 17b). The mass fraction of Cu in Cu-COF was 0.2%. The PXRD patterns of Cu-COF were similar to that of DHTA-TTA 2D COF, although the peak intensity became weaker (Fig.17c). The CO₂ adsorption experiment at an ambient pressure of 25 ℃ showed that the Brunauer-Emmett-Teller (BET) specific surface area of Cu-COF was slightly lower than that of COF, but its CO₂ adsorption capacity was higher, which was related to the Cu—O/N bonds in Cu-COF. In addition, Cu—O/N as the active center can selectively reduce CO₂ to CO under visible light (> 420 nm) irradiation and in the absence of any photosensitizer. The activity and selectivity of Cu-COF did not show significant deactivation during the fifth use, and no Cu-derived nanoclusters or nanoparticles were observed on Cu-COF. To further investigate the process of photocatalytic reduction of CO₂, the author studied the reaction mechanism through UV-visible absorption spectroscopy, the photoluminescence transient time-resolved decay measurement, and in situ electron paramagnetic resonance experiments. The above experimental results indicated that Cu-COF had good photothermal stability.

  Figure 17

  

  Figure 17.  (a) Synthetic scheme of DHTA-TTA 2D COF and Cu-COF; (b) Structural models of DHTA-TTA 2D COF and Cu-COF (Grey, blue, red, and yellow spheres represent C, N, O, and Cu atoms, respectively; H atoms are omitted for clarity); (c) PXRD patterns of DHTA-TTA 2D COF^[118]

  Reaction conditions: 1,2-dichlorobenzene, 1-butanol, 120 ℃, 36 h, CuCl₂; In c: comparison between the experimental (black) and Pawley refined (red) profiles, the simulated pattern for AA stacking mode (purple), the refinement differences (blue), and experimental PXRD patterns for Cu-COF (dark cyan).

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  2.3   HCOOH as the main reduction product

  Photocatalytic CO₂ reduction to produce easily storable liquid HCOOH is one of the ideal routes to achieve carbon resource recycling. Cu-based COFs have demonstrated unique potential in this field: their highly designable pore environment can precisely anchor Cu active sites and effectively stabilize the key *OCHO intermediate in the HCOOH production pathway through specific interactions between the skeleton functional groups and reaction intermediates, thereby significantly improving reaction activity and HCOOH selectivity. This strategy provides a blueprint for developing efficient and specific photocatalysts for CO₂-to-HCOOH conversion.

  Kim′s group prepared ethynyl-linked photogate catalyst ZnCu-COF through polymerization reaction between Zn(Ⅱ) 5,10,15,20-tetrakis (4-bromophenyl)porphyrin (ZnBPP) and Cu(Ⅱ) 5,15-bis(pentafluorophenyl)-10,20-bis(4-ethynylphenyl)porphyrin (CuFPP), and characterized the properties of the catalyst using commonly used characterization techniques^[119]. The mass fractions of Zn (3.06%) and Cu (5.93%) in the material were analyzed using inductively coupled plasma-optical emission spectrometry (ICP-OES). For comparison, the author also synthesized single-metal COFs (Zn-COF and Cu-COF) to investigate the role of Zn and Cu sites in electronics during the photocatalytic process. Photoelectrochemical characterizations indicate that the excellent photocatalytic performance is attributed to the synergistic effect between Zn/Cu structures, which facilitates efficient charge transfer within the framework. The AB-stacked structure results in a periodic arrangement of multiple Zn and Cu active centers in the skeleton. Each Zn-porphyrin photoresponsive center is connected to four Cu-porphyrin catalytic units to form a square structure, which promotes energy and electron transfer (Fig.18). The energy transfer and electron diffusion within ZnCu-COF occur from the ZnBPP compartment to the CuFPP compartment. Cu atom centers enrich the electrons of zinc photocenters through electron diffusion, which affects the energy of Cu3d orbitals and determines the electronic state dispersion of Cu3d orbitals under the excited states. When ZnCu-COF was used as a catalyst, the yields of HCOOH, CO, and H₂ were 138.6, 12.7, and 4.3 μmol·g^-1·h^-1, respectively, with HCOOH selectivity of 89.1%, which was superior to that of the Zn-COF and Cu-COF. By manipulating the orbital energy/interacting orientation between Cu3d and C2p_z orbital of HCOOH, Cu sites promote CO₂ absorption and nucleophilic reaction of *COOH intermediates, resulting in higher HCOOH selectivity of ZnCu-COF than that of Zn-COF and Cu-COF.

  Figure 18

  

  Figure 18.  Synthetic scheme of ZnCu-COF^[119]

  Reaction conditions: triethylamine, tetrahydrofuran, triphenylphosphine, Pd(Ⅱ) acetate, cuprous iodide, 130 ℃, 120 h.

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  3.   Conclusions and outlook

  Developing photochemical and electrochemical reduction conversion technologies for CO₂ is not only an effective way to reduce carbon emissions and realize the resource utilization of pollutants, but also of great significance in promoting green economic growth and sustainable development. As an emerging organic crystalline functional material, Cu-based COFs have been widely applied in the field of CO₂ reduction, exhibiting many excellent properties in photocatalytic and electrocatalytic applications. Here, we summarize the research progress of Cu-based COFs in this area, covering Cu-based COFs synthesis strategies and examples of electrocatalytic and photocatalytic CO₂ reduction conversion.

  Cu-based COFs possess large specific surface areas, providing abundant active sites for photocatalytic and electrocatalytic reactions, effectively enhancing the apparent activity of the catalytic process. Furthermore, their unique electronic structure offers special electron transport and distribution capabilities during CO₂ reduction, thereby modulating the energy barriers and kinetics of the reaction process and enabling precise control over the catalytic process. More importantly, because the framework structure of Cu-based COFs is composed of strong covalent bonds, they can usually maintain their structural stability under water, organic solvents, bases, acids, reducing, and oxidizing conditions, thus extending the lifespan of the catalyst. Therefore, Cu-based COF catalysts, with their excellent performance, demonstrate great application potential and have become an important frontier area in current catalytic science.

  With the advancement of research, chemists have constructed numerous COF materials with diverse topologies and functionalities through various synthetic strategies. Continuous progress in characterization techniques and theoretical calculations has opened innovative pathways for elucidating the structure-property relationship, significantly deepening researchers′ understanding and design capabilities for high-performance materials. The breakthrough advancements in artificial intelligence technology are reshaping the paradigm of new material development, and its powerful data analysis and model prediction capabilities are opening unprecedented innovative space for designing high-performance, multifunctional materials. Considering the enormous development potential of COFs, we sincerely hope that this review can inspire future researchers and motivate them to accelerate the leapfrog development of COFs from laboratory-based fundamental research to industrial-scale CO₂ utilization applications.

  
  
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  Figure 1  Synthesis process and catalytic application of NHC-AuCl-COF and NHC-AuSbF₆-COF^[101]

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  Figure 2  Synthesis process and asymmetric catalytic application of (S)-NHC-Au-SA-COF^[102]

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  Figure 3  (a) Schematic representation of the synthesis of Cu-PyCA-MCOF and Cu-PyCAOH-MACOF; Views of (b) Cu-PyCA-MCOF and (c) Cu-PyCAOH-MCOF structural models (C: gray, N: blue, O: red, Cu: pink, and H: white)^[108]

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  Figure 4  Schematic representation of the synthesis of PcCu-TFPN^[109]

  Reaction conditions: dimethylacetamide, 1,3,5-trimethylbenzene, 150 ℃, 3 d, 85% yield.

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  Figure 5  (a) A Structural model of NC‐SA Cu/COF (Grey, blue, red, and brown spheres represent C, N, O, and Cu atoms, respectively, and H atoms are omitted for clarity); (b) CO₂ reduction current densities on NC-SA Cu/COF, SA Cu/COF, and pure COF^[110]

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  Figure 6  (a) Synthesis and structure illustrations of Cu-NC@CuPc-COF; (b) Catalytic application in CO₂ reduction of Cu-NC@CuPc-COF

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  Figure 7  (a) Chemical structure of NUS9; (b) Typical PXRD patterns of NUS9 nanosheets with an eclipsed stacking model^[112]

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  Figure 8  Schematic representation of Cu-NUS9 in electrocatalytic CO₂RR^[112]

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  Figure 9  Schematic of the synthesis and structure of AAn-COF-Cu^[113]

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  Figure 10  Schematic representation of AAn-COF obtained from different reaction times of (a) 6 h, (b) 12 h, (c) 18 h, and (d) 72 h, respectively^[113]

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  Figure 11  Schematic preparation of Cu-Tph-COF-Dct^[114]

  Reaction conditions: 1,4-dioxane, triethylamine, 90 ℃, 6 h.

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  Figure 12  (a) Schematic representation of Bpy-COF and M-Bpy-COF; (b) Proposed reaction mechanism for photocatalytic CO₂ conversion over Cu-Bpy-COF^[115]

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  Figure 13  Schematic illustration for the preparation process of Cu(Ⅱ)-im-pCOF and Cu(Ⅰ)-im-pCOF^[116]

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  Figure 14  (a) fs-TAS spectra and (b) fs-TAS kinetic plots at 900 nm of pCOF, im-pCOF, and Cu(Ⅰ)-im-pCOF; μs-TAS spectra of pCOF, im-pCOF, and Cu(Ⅰ)-im-pCOF under different CO₂ environments of (c) H₂O bubbled with ¹²CO₂, (d) H₂O bubbled with ¹³CO₂, and (e) H₂O bubbled with 10%CO₂/90%N₂^[116]

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  Figure 15  Synthetic procedures of DhaTph Tubes and DhaTph-Cu Tubes^[117]

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  Figure 16  SEM images of the intimal Tph+TBA (a), DhaTph Tubes prepared with different reaction times of 0 h (b), 0.5 h (c), 2 h (d), 5 h (e), 15 h (f), 35 h (g), and 72 h (h)^[117]

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  Figure 17  (a) Synthetic scheme of DHTA-TTA 2D COF and Cu-COF; (b) Structural models of DHTA-TTA 2D COF and Cu-COF (Grey, blue, red, and yellow spheres represent C, N, O, and Cu atoms, respectively; H atoms are omitted for clarity); (c) PXRD patterns of DHTA-TTA 2D COF^[118]

  Reaction conditions: 1,2-dichlorobenzene, 1-butanol, 120 ℃, 36 h, CuCl₂; In c: comparison between the experimental (black) and Pawley refined (red) profiles, the simulated pattern for AA stacking mode (purple), the refinement differences (blue), and experimental PXRD patterns for Cu-COF (dark cyan).

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  Figure 18  Synthetic scheme of ZnCu-COF^[119]

  Reaction conditions: triethylamine, tetrahydrofuran, triphenylphosphine, Pd(Ⅱ) acetate, cuprous iodide, 130 ℃, 120 h.

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