Hydrogen peroxide (H2O2) is an important green oxidizing agent, but the main anthraquinone process for production thereof suffers high energy consumption and large safety risks. Artificial photosynthesis H2O2 from water and oxygen features safe, environmentally friendly and energy-saving characteristics and has gradually become a research focus. Covalent organic frameworks (COFs) have been widely used in the photocatalytic production of H2O2 for their high specific surface area, good photocatalytic performance and structural tunability. This review summarizes the recent research progress in the field of COFs photocatalytic production of H2O2, discussing the reaction mechanisms for the production of H2O2 through oxygen reduction, water oxidation, and dual-channel processes. It introduces methods to improve the photocatalytic production of H2O2 by regulating the optical bandgap, enhancing charge separation capability, and improving carrier mobility of COFs through structural design and functional group modification. These methods contribute to the design of efficient, stable, and sustainable COFs for photocatalytic production of H2O2.
Contents
1 Introduction
2 Hydrogen peroxide production by ORR pathway
2.1 Direct one-step two-electron oxygen reduction mechanism
As an important oxidant, hydrogen peroxide （H2O2） is an environmentally friendly and widely used chemical raw material and fine chemicals[1][2]. It can be used in pulp bleaching, sewage treatment, food and medicine, metallurgy and other fields[3][4][5]. According to statistics, China's H2O2 production capacity is increasing year by year, and the global production capacity is expected to reach 5.7 million tons in 2027[6].
The production processes of H2O2 mainly include sulfate electrolysis, isopropanol oxidation, etc. Electrolysis method has the advantages of simple process, high product purity and low impurities, but it is not suitable for large-scale production because of its high consumption of precious metals[7]. Isopropanol oxidation does not require other catalysts, but it consumes a large amount of raw materials and is not competitive[8]. The oxygen cathode reduction method has the advantages of low production cost, simple equipment and no pollution, but the concentration of the prepared H2O2 product is low and the method is not practical[9]. The direct hydrogen-oxygen synthesis method has high atom utilization rate and low production cost, but the potential safety hazard of mixing hydrogen and oxygen has not been solved yet[10]. At present, the mature production method in industry is the anthraquinone method, but there are still the following problems in the large-scale production of high-concentration products, such as the production process is cumbersome, easy to consume a lot of energy, more by-products and difficult to deal with[11].
In contrast, photocatalytic technology has the advantages of environmental friendliness, sustainable energy saving and high catalytic performance, which is conducive to the green production of H2O2. However, most of the reported photocatalysts still have some problems, such as narrow light absorption range, low absorption utilization, and easy and rapid recombination of photogenerated electrons and holes. Therefore, the development of new photocatalysts has always been the focus and focus of scientific research.
Covalent Organic Frameworks (Covalent Organic Frameworks, COFs), as a new type of photocatalyst, have the characteristics of regular structure, narrow pore size distribution, high surface area and π-conjugated structure[12]. The structure-tunable performance of COFs enables the targeted design of photocatalysis, and the high specific surface area and porosity enable the introduction and exposure of more catalytic sites. Therefore, more and more COFs have been successfully used in photocatalytic CO2 reduction, water splitting and other fields. However, the study of COFs in photocatalytic production of H2O2 is still relatively small.
In this paper, based on the mechanism of H2O2 synthesis, the research progress in the field of photocatalytic production of H2O2 by COFs in recent years is summarized. Photocatalytic production of H2O2 usually occurs through the following three Reaction pathways: Oxidation-Reduction Reaction (ORR), Water-Oxidation reaction (WOR), and a dual-channel pathway coupling ORR and WOR. We hope that this paper can provide some reference for researchers in related fields, and promote the development of COFs materials in the field of photocatalytic production of H2O2.
2 Oxygen reduction (ORR) to produce hydrogen peroxide
The production of H2O2 via the photocatalytic oxygen reduction pathway has gradually become a research hotspot[13⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~30]. Compared with traditional photocatalysts, COFs materials have tunable composition and band structure at the molecular and nanoscale levels (Fig. 1), and the photocatalytic performance of COFs materials can be significantly improved by strategies such as element doping, metal loading, and the introduction of active sites.
Based on the mechanism of photocatalytic oxygen reduction to produce H2O2 and starting from the structural design of COFs, this section will illustrate the research progress of COFs materials in the field of photocatalytic oxygen reduction to produce H2O2 through a series of typical examples.
Two-electron ORR is the most widely studied H2O2 generation pathway in photocatalysis (e.g., Fig. 2). According to the electron transfer process, it can be generally divided into a one-step two-electron path (formula 1) and a two-step one-electron path (formulas 2 to 4).
O2＋2e-＋2H＋→H2O2 E0=+0.68 VNHE
O2＋e-→·O2- E0=-0.33 VNHE
·O2- ＋H＋→*OOH
*OOH ＋H＋＋e-→H2O2 E0 = +1.44 VNHE
e- represents photogenerated electrons; The H+ represents a proton; ·O2- represents superoxide radical; * OOH represents adsorbed OOH radical;
2.1 Direct one-step two-electron oxygen reduction mechanism
Photocatalyzed ORR is a promising strategy for H2O2 synthesis, and the one-step two-electron pathway in particular has great potential in achieving high efficiency and selectivity.
In November 2020, Thomas and Van Der Voort et al. Synthesized two kinds of COFs linked by imine bonds (Figure 3A)[31]. Visible light irradiation forms electron hole carriers on COFs, ethanol uses holes to produce acetaldehyde and protons, and molecular oxygen uses electrons from COF and protons from alcohol conversion to produce H2O2（ Fig. 3 B) with a yield of 91.00~234.52μmol·h-1·g-1. This report is the first time to use COFs materials for oxygen reduction to produce H2O2, which provides a new catalyst option for photocatalytic production of H2O2.
Han et al. Synthesized S-COFs and FS-COFs by introducing sulfone units (Fig. 3C, d)[32]. The simulation results show that FS-COFs form a yeager-type conformation with strong oxygen adsorption ability, which accelerates the separation of photogenerated electron-hole pairs, enhances the protonation of COFs, and promotes the adsorption of O2. Compared with the unmodified C-COFs, the conformational transition changes the path of H2O2 production from two-step one-electron oxygen reduction to one-step two-electron oxygen reduction, and the H2O2 yield reaches 3904.2μmol·h-1·g-1.
At present, there are few reports on the generation of H2O2 through the one-step two-electron ORR pathway, and the mechanism of photocatalytic generation of H2O2 is still focused on the two-step one-electron ORR pathway.
The introduction of metal active sites is an important strategy to regulate the photocatalytic performance of COFs. Jiang et al. Obtained CoPc-BTM-COF and CoPc-DAB-COF by introducing a metal Co atom as the active center of ORR (Fig. 4A)[33]. The formation of the *OOH intermediate species was confirmed and the two-step one-electron oxygen reduction pathway was revealed by electron paramagnetic resonance and in situ infrared spectroscopy (Fig. 4 B – e). The introduction of metal atom active sites greatly promoted the adsorption of oxygen, and the H2O2 yield of CoPc-BTM-COF reached 2096μmol·h-1·g-1. Gu et al. Reported the first 3D-Ti-based COF with spn topology (TiCOF-spn), which has strong visible light absorption ability and exhibits certain H2O2 photocatalytic activity （489.94μmol·g-1·h-1） This study expands the topology of 3D COFs and broadens the application of metal-containing COFs in the field of photocatalytic oxygen reduction to produce H2O2[17]. Although the introduction of metal active sites can improve the photocatalytic performance of COFs to a certain extent, its application is limited due to the high cost of raw materials and the environmental pollution.
Comparatively speaking, non-metallic active sites have been widely studied because of their controllable catalytic ability and low cost. Gu et al. Rationally designed a covalent organic framework Bpy-TAPT retaining the polar aldehyde group and dual active sites (bipyridine and triazine) (Figure 5A) with a H2O2 yield of 4038μmol·h-1·g-1（ Figure 5B)[34]. Its two-step one-electron oxygen reduction mechanism is shown in Fig. 5 C, d ：h+ oxidizes H2O to O2 at the bipyridine site,e- is reduced at the triazine ring to O2 to produce ·O2-, which is then converted to H2O2 by e- reduction. The introduction of double active sites enhances the charge generation, and the electron-rich aldehyde group effectively promotes the separation of charge carriers and the adsorption of O2/H+.
Alkynyl-modified COFs have attracted much attention in the field of photocatalysis. The COFs constructed by Kong et al. Through strong π − π conjugated polycyclic aromatic benzene and acetylene units have excellent photoresponse performance, and the H2O2 yield is 1240μmol·h-1·g-1[35]. In addition, the research on new active centers of COFs has never stopped. Van Der Voort et al. Prepared COFs containing pyrene structure and explored their photocatalytic performance, and the yield of Van Der Voort was 1242μmol·h-1·g-1[24]. Wang et al. Successfully synthesized three kinds of azole COFs materials, including thiazole, oxazole and imidazole, and constructed a more accessible charge transfer channel through the donor-π-acceptor structure, thus effectively inhibiting the recombination of photoexcited charges[36].
The structural tunability of COFs enables multiple choices of building units. Cooper et al. Have done a lot of work in the structural design and synthesis of COFs, revealing the influence of building units of COFs materials on photocatalytic H2O2[37]. In addition, ligand functionalization is also one of the effective strategies to regulate the properties of COFs. Han et al. Developed a partially fluorinated, metal-free, imine-linked two-dimensional triazine-based covalent organic framework （TF50-COF） photocatalyst[38]. Abundant Lewis acid sites were generated by fluorine substitution to adjust the electron distribution of adjacent carbon atoms, which provided highly active sites for O2 adsorption and broadened the visible light response range of the catalyst, and the H2O2 yield was 1739μmol·h-1·g-1. In addition, they modified COFs materials by cyano groups to form a D-π-A structure, which promoted the adsorption of oxygen and the response to visible light, and significantly improved the charge separation efficiency[39]. The synergistic effect caused by this strategy promoted the activation of O2 from ·O2- to 1O2 under visible light irradiation, and the H2O2 yield reached 2623μmol·h-1·g-1.
Wang et al. Synthesized functionalized TAPB-PDA-X via 1,3,5-tris (4-aminophenyl) benzene and terephthalaldehyde with different functional groups （X=H2,OH,OCH3 and CH3）[40]. The COF modified by hydroxyl functional groups can effectively capture holes and reduce the recombination of photogenerated carriers, and the H2O2 yield reaches 2117.6μmol·g-1·h-1. The ligand functionalization strategy is simple and feasible, which promotes the application of COFs materials in the field of photocatalytic production of H2O2.
Overall, the introduction of metal or non-metal active sites can improve the photocatalytic performance of COFs materials, further affect the process of oxygen adsorption and electron transfer, and ultimately enhance the yield of H2O2. In addition, from the point of view of the material itself, the photocatalytic performance can be greatly improved by the selection of building units and ligand functionalization strategies, which also makes the research of COFs materials in the field of photocatalytic production of H2O2 possible.
3 Water oxidation (WOR) to produce hydrogen peroxide
Compared with ORR, WOR is an alternative pathway for the conversion of H2O to H2O2 using light-induced production of h+. WOR contains the following paths:
2 H2O + 2 h+→ H2O2 + 2 H+, E0= + 1.78 VNHE
H2O + h+→ H+ + ·OH, E0= + 2.732 VNHE
OH- + h+→ ·OH, E0= + 1.9 VNHE
·OH + ·OH→H2O2
h+ represents photogenerated holes; e- represents photogenerated electrons; ·O2- represents superoxide radical; *OOH represents the adsorbed OOH radical; ; OH represents hydroxyl radical; The OH- comes from the ionization of water;
In WOR, the h+ generated by the photoinduced catalyst realizes the direct conversion of H2O to H2O2 through one-step two-electron oxidation (as shown in Equation 5). In addition, it can also indirectly produce H2O2 by two-step one-electron oxidation (such as formula 6~8）：h+ first forms hydroxyl radical with H2O, and then through the coupling of hydroxyl radical. Theoretically, the valence band energy of the catalyst is required to reach 1.78 eV for one-step two-electron oxidation and 2.732 eV for two-step one-electron oxidation.
Yu and Li et al. Designed four isostructural hydrazone-linked COFs using the hydrophilic hydrazone structure as the active site for water oxidation (Figure 6A)[41]. The possible mechanism of WOR process is as follows: the electrons generated by photoexcitation can increase the polarity of the hydrazone bond and promote the hydration of water and hydrazone bond. Upon photoinduced deprotonation, the hydroxyl radical formed directly couples to produce H2O2（ Fig. 6B, C), reaching a yield of 1665μmol·h-1·g-1 in pure water.
4 Double-channel hydrogen peroxide production
In most catalytic systems for ORR to produce H2O2, the oxidation potential is too low to realize the WOR process, which leads to the failure of photogenerated holes to be consumed in time, resulting in photogenerated charge recombination, low charge utilization efficiency, and weak H2O2 production performance.
Different from other reaction pathways of photosynthetic H2O2, the two-pass pathway of direct two-electron water oxidation coupled with direct two-electron oxygen reduction can achieve 100% atom utilization and high energy conversion efficiency, so it is urgent to develop polymeric catalysts capable of synthesizing H2O2 through this pathway.
In COFs materials, in order to realize the WOR path, it is an ideal process to control the band structure by designing the structural unit and bonding mode, and then realize the dual-channel path photocatalytic production of H2O2 (Fig. 7).
Xu et al. First reported the two-channel synthesis of alkynyl-functionalized covalent triazine framework materials H2O2（ as shown in Fig. 8 a, B)[42]. The theoretical calculation results show that the alkynyl structural unit as the active site of WOR can significantly reduce the Gibbs free energy of the hydroxyl intermediate state (Fig. 8C), and the ORR on the coupled triazine unit realizes the two-channel path synthesis H2O2. On this basis, they designed the heptazine ring unit (Figure 8 d) to deepen the oxidation potential of the material[43]. The ORR occurs at the heptazine ring unit, while the WOR occurs at the alkynyl unit, achieving 100% atom utilization efficiency as well as 0.78% solar-to-chemical energy conversion efficiency during the photochemical synthesis of H2O2. Studies have shown that the benzene ring is the active center of WOR[44]. The triazine ring and benzene ring were further integrated onto heptazinyl COF to give HEP-TAPT-COF and HEP-TABB-COF (Fig. 8E)[45]. Comparatively speaking, HEP-TAPT-COF exhibits higher photocatalytic efficiency for H2O2 production due to its dual ORR active sites.
Ma et al. Prepared bipyridyl COF (COF-TfpBpy) from 1,3,5-triformylphloroglucinol (Tfp) and 2,2 ′ -bipyridine-5,5 ′ -diamine (Bpy) (Fig. 9 a ~ e)[46]. In situ infrared measurements, as well as DFT calculations, revealed the mechanism of its two-channel pathway for H2O2 production: under illumination, water molecules undergo one-step two-electron oxidation, leaving two protonated pyridine substituents （PyH+）, and oxygen molecules adsorb on the PyH+ substituent to form an internal peroxide intermediate (N − H − O − O − H − N), which is converted to H2O2 by two-electron reduction, realizing a H2O2 production process with 100% atom utilization efficiency.
These methods have shown excellent performance in photocatalytic production of H2O2, but there is still a long way to go for industrial application. In the actual industrial production, it is often necessary to provide high concentration of H2O2, which requires that the designed COFs materials can maintain high performance to produce H2O2 for a long time. Lan and others have made breakthroughs in this field[47]. TTF-BT-COF (Figure 9 f), with the oxidation group tetrathiafulvalene (TTF) and the reduction group benzothiazole (BT) as the reaction center, has a H2O2 photosynthesis yield of ~276 000μmol·h-1·g-1 without sacrificial agents, and its H2O2 concentration reaches about 18.7 wt% during the long catalytic reaction, which has great industrial application prospects.
Only about 2.5% of global water resources are freshwater resources. Therefore, how to use seawater resources is a research hotspot. Tang et al. Constructed two thiophene-containing covalent organic frameworks (TD-COF and TT-COF) (Figure 9 G), using only seawater and air as reactants, and achieved H2O2 photosynthetic yields of 3364μmol·h-1·g-1 and 2890μmol·h-1·g-1[48]. This provides a new idea for the design of new catalysts for photocatalytic production of H2O2.
The catalytic performance of COFs is not only determined by the chemical structure of their building units, but also can be optimized by constructing highly connected 3D COFs with topological structure. Zhang et al. First designed and synthesized an octaaldehyde difluorophenyl monomer (FBTA-8CHO) as an 8-connected cubic node, which provided more options for the design and construction of various unique 3D COFs and laid the foundation for the efficient preparation of various 3D COFs for photocatalytic production of H2O2[49].
Modification with different functional groups can change the light absorption range of COFs materials, affect the charge distribution of structural units and provide more reactive sites to improve the performance of COFs in photocatalytic H2O2 production. Shen et al. Prepared thioether-modified triazine-based COF (TDB-COF) via Schiff base reaction (Figure 10A)[50]. The experimental results show that compared with TFPT-COF without thioether group modification, the modification of thioether group broadens the visible light absorption range, regulates the band structure which is more conducive to H2O2 production, and reduces the Gibbs free energy of intermediates in WOR process and ORR process (as shown in Fig. 10 B, C), so that TDB-COF shows 723.5μmol·h-1·g-1 H2O2 production without any sacrificial agent or cocatalyst.
To sum up, the WOR process can be realized by (1) designing structural units to increase the oxidation potential of COF; (2) designing the "D-A" structure by using the affinity of different units to electrons to improve the separation ability of photogenerated charges; (3) changing the bonding mode to enhance the activity of adjacent units; (4) Introducing functional group modification, changing charge distribution and other strategies to enhance the ability of COF photocatalytic production of H2O2.
Table 1 is a summary of the application of COFs materials to photocatalytic hydrogen peroxide production through the ORR pathway.
Table 2 is a summary of the application of COFs materials to photocatalytic hydrogen peroxide production through WOR and dual-channel pathways.
5 Conclusion and outlook
Covalent organic frameworks (COFs), as a new type of functional polymer porous materials, have been widely studied in photocatalysis, membrane separation, and other fields because of their advantages such as high specific surface area, regular and diverse structures, controllable structural design, appropriate optical band gap, and high physical and chemical stability. In the field of photocatalysis, it provides a green and environmentally friendly route for the production of H2O2, which meets the needs of the current dual-carbon development strategy.
(1) At present, there are many studies on ORR pathway, but there is a lack of more detailed studies on WOR pathway. Photocatalytic production of H2O2 via the WOR pathway remains difficult. To solve this problem, it is necessary to find new building blocks and linkers, introduce specific active groups, and design new COF materials with deeper oxidation potential to realize the WOR process and improve the photocatalytic performance and atom utilization efficiency of COF.
(2) Photocatalytic production of H2O2 is still a long way from industrial application. The photostability and catalytic stability of the material itself are still the key research contents.
(3) In addition to exploring direct photocatalytic H2O2 production methods, we can also consider the direct use of covalent organic frameworks for photocatalytic H2O2 production and its tandem reactions, such as photocatalytic in situ H2O2 production and its use in organic synthesis, photodynamic therapy, photoenzyme tandem reactions, etc., so that they can efficiently produce and use H2O2 in situ.
SunJ M, SekharJena H, KrishnarajC, SinghRawat K, AbednatanziS, ChakrabortyJ, LaemontA, LiuW L, ChenH, LiuY Y, LeusK, VrielinckH, Van Speybroeck V, Van Der Voort P. Angew. Chem. Int. Ed., 2023, 62(19): 2216719.
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