Phenol hydroxyl-modified imine-based covalent organic frameworks for enhanced solar-driven generation of H₂O₂via hydrogen bonds

Abstract

Photosynthesis of H₂O₂ has been considered an eco-friendly strategy. However, the concentration of H₂O₂ reported in earlier studies is far from the industrial requirement. Herein, we present a strategy of employing phenolic hydroxyl-modified imine-based covalent organic frameworks (COFs) as catalysts for enhancing the photosynthesis of H₂O₂ in a benzyl alcohol (BA)/water system. H₂O₂ production rate was 19 times that by unmodified imine-based COFs, and the H₂O₂ concentration reached 380 mM with a record rate of 61.3 M h⁻¹ under simulated solar irradiation. In addition, the selective oxidation of BA into benzaldehyde was achieved, indicating the potential for industrial applications. The phenolic hydroxylic group played an important role, as indicated by the result of experiments and DFT calculations. First, intermolecular hydrogen bonding between the phenolic hydroxyl group and BA facilitated electron transfer, thereby lowering the energy barrier for H₂O₂ generation. Second, intramolecular hydrogen bonding between the phenolic hydroxyl group and imine increased the energy barriers of H₂O₂ decomposition and ensured catalyst stability. Overall, our research highlights the critical role of hydrogen bonding in the H atom of C=NH in imine-based COFs in augmenting the photocatalytic activity.

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Introduction

Hydrogen peroxide (H₂O₂) is placed among the top 100 chemicals worldwide since it is extensively used as a green oxidant in industrial processes and environmental treatment.^1,2 Although the anthraquinone method is widely used for commercial H₂O₂ production,³ the method is complex, energy-intensive, and generates toxic byproducts. Thus, it is necessary and imperative to develop eco-friendly strategies for H₂O₂ production that consumes less energy and generates fewer toxic by-products. In addition, the photosynthesis of H₂O₂ is a potential candidate to meet the above-mentioned requirement.

Recent studies have shown that TiO₂ and its composites,^4–7 g-C₃N₄,^8–13 and metal organic frameworks (MOFs)^14–16 facilitate eco-friendly photosynthesis of H₂O₂ generally via a 2 e⁻ oxygen reduction reaction (ORR).¹⁷ In addition, the mechanism can be illustrated as follows: both electron and positive hole pairs are generated on the semiconductor surface, and then, oxygen is reduced by the photogenerated electron to form a superoxide radical (O₂˙⁻). Afterwards, an ·OOH intermediate is formed. Finally, the intermediate (·OOH) gains another electron and a proton to form H₂O₂,¹⁸ while the sacrificial agent is oxidized by holes. Despite significant advancements in this field, the reported strategies still have several drawbacks. For instance, some catalysts exhibit poor stability,^19,20 while others produce insufficient H₂O₂ concentration (μM to mM).¹⁸ Additionally, photosynthesized H₂O₂ is decomposed into hydroxyl radicals (·OH) during the photocatalytic process.^21,22 Several studies have demonstrated that the decomposition rate of the generated H₂O₂ increases with the increase in the produced H₂O₂ concentration, which makes the concentration of H₂O₂ to not be further improved. Therefore, it is necessary to develop new strategies to enhance the H₂O₂ production rate by suppressing H₂O₂ decomposition and maintain the stability of photocatalysts. Besides the design of photocatalysts, the sacrificial agent is important for the enhancement of H₂O₂ generation because the sacrificial agent can improve the separation of electron–hole pairs. Benzyl alcohol (BA) displays a lower solubility in water than water-soluble sacrificial agents such as i-propanol (IPA) and methanol. As a result, it is easier to separate BA from H₂O₂.^7,15,19,23 However, when H₂O₂ was decomposed into ·OH on the catalyst surface, BA could act as a radical scavenger for ·OH, leading to a lower H₂O₂ concentration. Moreover, the oxidative product of BA could be benzaldehyde or benzoic acid. Therefore, there is a need to design new catalysts to suppress the effect of BA on the decomposition of H₂O₂ and achieve the selective oxidation of BA.

Covalent organic frameworks (COFs), which are characterized by their highly crystalline, customizable, and stable structures, have been used as catalysts. There have been some studies on their application in H₂O₂ photosynthesis.^19,24–27 Among this study, we focus on the potential of utilizing phenolic hydroxyl-modified imine-based COFs as catalysts for the photosynthesis of H₂O₂. Accordingly, a photocatalyst has been developed by combining 1,1,2,2-tetrakis(4-aminophenyl)ethene (TAE) and 2,5-dihydroxyterephthalaldehyde (DaOH),²⁸ which is named TAE-DaOH. As an aggregation-induced emission (AIE) molecule with a large conjugated structure, TAE enhances the light conversion efficiency of H₂O₂ photosynthesis.²⁹ When incorporated into the COF structure, TAE is immobilized in COFs, thereby extending the conjugated structure of COFs and improving the separation of photogenerated electron–hole pairs (Fig. 1). It has been demonstrated that DaOH is a promising candidate to improve the stability of imine bonds in COFs.²⁸ More importantly, the phenolic hydroxyl groups present in TAE-DaOH can play a key role in the enhancement of catalytic activity via hydrogen bonding. The intermolecular hydrogen bonds between BA and the phenolic hydroxyl groups improve the conversion between iminol and ketoenimine,³⁰ which increases H₂O₂ photocatalytic production. Meanwhile, the intramolecular hydrogen bonds between the phenol and the imine enhance the stability of the imine bond^31–33 and inhibit the H₂O₂ decomposition. In addition, the autocatalysis of benzaldehyde results in selective production of benzaldehyde as well as extremely high concentration of H₂O₂.

Fig. 1 Schematic of the structure of TAE-COFs; red, blue, grey, and white spheres represent oxygen, nitrogen, carbon, and hydrogen, respectively.

Experimental section

Synthesis of TAE-ben²⁸

A Pyrex tube was charged with 1,1,2,2-tetrakis(4-aminophenyl)ethene (30 mg, 0.0764 mmol) and terephthalaldehyde (20.5 mg, 0.153 mmol), followed by the addition of 1 mL 1,4-dioxane. Then, the mixture was subjected to sonic treatment for 5 min to make sure the solid fully dispersed. After that, 0.1 mL 6 M acetic aqueous solution was added to the suspension, followed by sonication for 5 min. Afterwards, the reactor was frozen by liquid N₂, and degasified by bubbling N₂. Subsequently, the tube was placed in an oven (DHG-960NHPG) to be heated at 120 °C for 3 d. A dark yellow precipitate was obtained and washed with EtOH, EA, acetone, water, and EtOH. Then, the solid was further purified by Soxhlet extraction (solution: THF and DCM, respectively). Finally, the product was dried in vacuum at 80 °C for 1 night (yield: 85%).

Synthesis of TAE-DaOH

1,1,2,2-Tetrakis(4-aminophenyl)ethene (30 mg, 0.0764 mmol), 2,5-dihydroxyterephthalaldehyde (25.4 mg, 0.153 mmol), and 1 mL 1,4-dioxane were added into a Pyrex tube, followed by sonic treatment for 5 min. Afterwards, 0.1 mL 6 M acetic aqueous solution was added and the mixture was sonicated for 5 min. Subsequently, the tube was frozen by liquid N₂, and degasified by N₂. Afterwards, the mixture was heated at 120 °C for 3 d. A red-orange solid was obtained, followed by washing with EtOH, EA, acetone, water, and EtOH. Then, the solid was under Soxhlet extraction by THF and DCM, respectively. Finally, the product was obtained after evaporation under vacuum at 80 °C for 1 night (yield: 90%).

Synthesis of TAE-DaOH-am

The amorphous TAE-DaOH was synthesized similarly to TAE-DaOH but without 0.1 mL 6 M acetic aqueous solution (yield: 96%) (Scheme 1).

Scheme 1 Synthesis of TAE COFs.

Photocatalytic experiment in a diphase system

First, 10 mg catalyst was dispersed in different ratio mixtures of water and benzyl alcohol (v : v = 9 : 1 or 5 : 5, 10 mL). After the ultrasonication process, the suspension was placed under a Xe lamp upon illumination in air. Finally, H₂O₂ was detected by iodometry. A circulating water system was used to control the temperature.

Scavenger trapping experiment

First, 10 mg catalyst was added into 10 mL aqueous solution with different radical scavengers (25% IPA, 10 mg benzoquinone, or 5 mM AgNO₃) in a 60 mL quartz vessel, followed by ultrasonication for 10 min for full dispersion of catalysts in solutions. Afterwards, the mixture was under illumination of a Xe lamp for 1 h. and the yield of H₂O₂ was detected by iodometry.

Electron paramagnetic resonance (EPR) detection for TAE-DaOH

The radical generated by TAE-DaOH was trapped by DMPO via EPR. Then, 30 μL DMPO in 1 mL DMSO was mixed with 0.5 mL suspension of TAE-DaOH/MeOH (v : v = 1 : 1). The mixture (300 μL) was added into the tube. EPR was conducted upon the light irradiation with a 300 W Xe lamp under the air condition.

EPR detection for BA

First, 20 μL DMPO was dispersed in 300 μL BA. The mixture was added to the EPR tube for detection.

In situ infra-red (IR)

TAE-DaOH was placed in the in situ cell, followed by degasification by N₂. Subsequently, O₂ and water vapor were bubbled to the cell, recording the IR absorption as baseline. Then, the IR spectra were recorded after 30 min under dark conditions, followed by the measurement upon illumination for 10 min, 20 min, and 60 min, respectively.

Photocurrent measurement under different conditions

First, 4 mg catalyst was dispersed into 1 mL EtOH, followed by the addition of 10 μL Nafion. Subsequently, the mixture was sonicated for 20 min to make sure the solid is fully dispersed. Afterwards, 300 μL suspension was dropped onto the conductive glass (ITO) and the dropping area was 1 × 1 cm². The electrolyte was 0.1 M Na₂SO₄ aqueous solution. As for the photocurrent of TAE-DaOH + BA and TAE-DaOH + benzaldehyde, BA or benzaldehyde was added into the electrolyte solution, followed by sonic treatment. Afterwards, the solution was used as an electrolyte.

Measurement of surface potential

First, 4 mg catalyst was dispersed in a HCl aqueous solution with different pH values ranging from 3 to 7. After sonication treatment, the surface potential was measured using a Malven ZS 90.

Experiment to evaluate the effect of benzaldehyde

A little benzaldehyde (20 μL, 200 mM in BA) was added into the initial mixture of 9 mL water and 1 mL benzyl alcohol and 10 mg TAE-DaOH.

Solar experiment

A 25 mL flask charged with 5 mL water, 5 mL BA, and 10 mg catalyst was placed under solar light from 9 am to 5 am (totally 8 h, place: radiochemical lab, Chengdu China, weather: sunny, 23–32 °C). The produced H₂O₂ was detected every 1 h.

Recycling experiment

After each run, the used catalyst was washed with EtOH, water, and EtOH, followed by drying under vacuum at 80 °C.

Results and discussion

Photochemical property

Both optical and photochemical properties are extremely important for photocatalysts. TAE-DaOH displays a wide light absorption range from 200 to 610 nm with a bandgap value of 2.20 eV (Fig. S7†).³⁴ The conduction band (CB) and valent band (VB) values determined by Mott–Schottky measurements are −0.516 V and 1.684 V, respectively.³⁵ Fig. S8 and S9† reveal that the band structures of TAE-ben and amorphous TAE-DaOH (TAE-DaOH-am) are sufficient for H₂O₂ photosynthesis (E_O2/H2O = 0.82 V vs. NHE, E_O2/·–O2 = −0.33 Vvs. NHE).

First, fluorescence spectroscopy (Fig. S10c†) shows that amorphous TAE-DaOH produces intense emissions ranging between 430 and 600 nm, while crystalline TAE-DaOH does not exhibit fluorescence, indicating that charge transfer is enhanced by the higher crystallinity of COFs (TAE-DaOH).^36,37 Furthermore, the H₂O₂ production rate using amorphous TAE-DaOH is much poorer (0.24 mM h⁻¹) than the highly crystalline COFs (crystal TAE-DaOH), thus confirming that crystalline TAE-DaOH is much more effective for H₂O₂ production (Fig. S10d†). The result further demonstrates that the crystallinity of AIE-based COFs plays a very important role in the suppression of fluorescence and improvement of charge separation.

Second, phenolic hydroxyl causes the conversion reactions between iminol and ketoenimine in the COFs (TAE-DaOH), which enhances electron transfer, as confirmed by the photocurrent experiment and electrochemical impedance spectroscopy (EIS). The photocurrent of TAE-DaOH is much higher than that of TAE-ben after illumination (Fig. S10a†), indicating that the former has a better electron transfer ability. A similar result is obtained via EIS, and TAE-DaOH shows a lower charge-transfer resistance than that of TAE-ben (Fig. S10b†).

Performance of hydrogen peroxide photosynthesis in a diphasic system

First, the contact angle of TAE-DaOH was measured (TAE-DaOH: 129° vs. TAE-ben: 128°), as shown in Fig. S11.† Then, we use the COFs (TAE-DaOH) as photocatalysts in a diphasic system, Fig. 2a shows that 380 mM of H₂O₂ is produced after six hours of illumination under air, with the maximum production rate of 61.3 mM h⁻¹ in a 5/5 water/BA system. Fig. 2b shows that the production rate of benzaldehyde is consistent with that of H₂O₂. The H₂O₂ production rate increases with the increase in the concentration of benzaldehyde, which indicates the occurrence of an autocatalytic reaction of benzaldehyde (the selective generation of benzaldehyde was demonstrated by 1HNMR and HPLC shown in Fig. S25†). The catalytic activity of TAE-DaOH in the diphasic system (7.2 mM h⁻¹) is much higher than that observed in pure water (1.1 mM h⁻¹) (Fig. 2c). In order to investigate the effect of the ratio water/BA, we tested two different ratios, 9/1 and 5/5. The results show that the yield of H₂O₂ is similar (61.3 mM h⁻¹ in 5/5 water/BA vs. 7.2 mM h⁻¹ in 9/1 water/BA). We also carried out the pH experiment to investigate the effect of pH (Fig. S19c†), and the yield of H₂O₂ from different pH values is very close (4.1 mM h⁻¹ at pH = 7, 4.2 mM h⁻¹ at pH = 5, 4.4 mM h⁻¹ at pH = 4, and 4.8 mM h⁻¹ at pH = 3), which means that pH or protons have little influence on the TAE-DaOH system. After five cycles, TAE-DaOH still has a good performance (372 mM), while the reactivity of TAE-ben decreases dramatically after 3 runs (Fig. S19d†). Furthermore, Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and powder X-ray diffraction (PXRD) are used to evaluate the stability of TAE-DaOH. Fig. S13–S15† indicate that the valence states of C, O, and N remain unchanged. Similar results have been obtained using scanning electron microscopy (Fig. S16†), PXRD (Fig. S17†) and FT-IR spectroscopy (Fig. S18†), indicating that TAE-DaOH is stable and not oxidized during the photocatalytic process. The solar experiment has been performed, as shown in Fig. 2d and S19b,† and the maximum H₂O₂ production reaches 203 mM after 8 h of solar irradiation (higher than almost reported photocatalysts), which suggests that TAE-DaOH has potential for large-scale applications.

Fig. 2 Photocatalytic performance when BA is used as a sacrificial agent (m(cat)/V(solvent) = 1 g L⁻¹, pH = 7, V = 10 mL, atmosphere: air, light condition: air mass (AM) 1.5 G). (a) Kinetic curve associated with H₂O₂ production. (b) Benzaldehyde and H₂O₂ concentrations (V(BA)/V(water) = 5/5 mL). (c) H₂O₂ concentrations produced under different conditions (V(BA)/V(water) = 1/9 mL). (d) Results of the solar experiment (V(BA)/V(water) = 5/5 mL).

Photocatalytic mechanism

Oxygen reduction. The photocatalytic performance for H₂O₂ production has been evaluated in pure water under different atmospheres. No H₂O₂ is detected under N₂ (Fig. S20†), whereas the H₂O₂ production under O₂ (1.81 mM h⁻¹) is 1.98 times that (0.91 mM h⁻¹) observed under air, indicating that O₂ is extremely important for H₂O₂ production.

Radical trapping experiments were conducted by using IPA and benzoquinone (BQ) as scavengers of hydroxyl radical (·OH) and superoxide radical (O₂˙⁻), respectively (Fig. S20d†). The H₂O₂ production (almost 0 mM h⁻¹) is suppressed after adding BQ, indicating the involvement of O₂˙⁻ in the ORR. In contrast, the H₂O₂ produced in the aqueous IPA solution (2.01 mM) is much higher than that generated in pure water, thus indicating the absence of ·OH during the process. After adding an aqueous solution of 5.0 mM AgNO₃ (Ag ion is an electron acceptor), H₂O₂ production (0.7 mM h⁻¹) is suppressed, suggesting that photogenerated electrons play an essential role in the ORR. The generated radical is identified using the EPR spectrum (Fig. S20e†). Only the characteristic peaks of O₂˙⁻ are observed after illumination.

In situ FT-IR spectroscopy (Fig. S21†) reveals that upon irradiation, new characteristic peaks are found at 1580 cm⁻¹ and 1720 cm⁻¹, which correspond to C=C_BQ and C=O, respectively, and reveals the formation of ketoenimine. The peaks at 900 cm⁻¹, 951 cm⁻¹, and 1110 cm⁻¹ correspond to the O–O, O₂˙⁻, and ·O–OH intermediates, respectively.^24,25 The above-mentioned results indicate that O₂ is adsorbed onto the catalyst surface to form O₂˙⁻, which is followed by the formation of the ·O–OH intermediate and finally H₂O₂ (Fig. 3a).

Fig. 3 (a) Conversion reactions that occur during the photocatalytic process. (b) Conversion reactions between iminol and ketoenimine occurring under different conditions. (c) Photocurrent of TAE-DaOH in a 0.1 M Na₂SO₄ solution and in a mixture of BA and 0.1 M Na₂SO₄. (d) H₂O₂ production achieved with or without adding benzaldehyde; inset: illustration of hydrogen transfer between benzaldehyde and BA. (e) Comparison of the catalytic performances of TAE-DaOH, TAE-ben at pH = 3, and TAE-ben at pH = 7; inset: structure of catalysts. (f) Oxidation of COFs during the photocatalytic process.

Intermolecular hydrogen bond between phenolic hydroxyl group and BA. According to the reported studies, hydrogen bonding with the phenolic hydroxyl group accelerates the reversible conversion reactions between iminol and ketoenimine,^30,38 resulting in a stronger electron transfer (Fig. 3b). Accordingly, FT-IR spectroscopy was performed to demonstrate the interaction (hydrogen bonding) between BA and TAE-DaOH, during the photocatalytic reaction shown in Fig. S23c.† Two new peaks at 1108 cm⁻¹ and 1177 cm⁻¹ are observed in the mixture containing TAE-DaOH and BA, which was caused by the intermolecular hydrogen bonding (the red shift of v(C–O) from 1050 cm⁻¹ to1108 cm⁻¹). This result is also confirmed by using photocurrent measurements (Fig. 3c). The photocurrent is much stronger in the BA/water system than in pure water, indicating that faster conversion reactions occur between iminol and ketoenimine due to the intermolecular hydrogen bonding between TAE-DaOH and BA. Consequently, the electron transfer is enhanced and ·OOH is formed faster, which greatly enhances H₂O₂ production.

The density functional theory (DFT) calculations (Fig. 4) indicate the key steps involved in the TAE-DaOH or TAE-ben system, which include the formation of ·OOH species via TS1 and the extraction of a hydrogen atom from BA via TS2. The energy curve of the TAE-ben system is higher than that of the TAE-DaOH system. Notably, TAE-ben has a high energy barrier (>189 kJ mol⁻¹) at TS1, which makes it difficult for the reaction to occur under mild conditions. Conversely, the TAE-DaOH system is predicted to have a lower energy barrier of approximately 126 kJ mol⁻¹ at TS1, thereby allowing the reaction to occur under present conditions. This may be because the higher acidity of phenol in the TAE-DaOH system facilitates both electron and proton transfer to form ·OOH. A natural bond orbital (NBO) analysis indicates that a charge of approximately 0.5 e⁻ is accumulated on the O₂ molecule at TS1 for both the TAE-DaOH and TAE-ben systems, suggesting that O₂ exhibits some negative ionic properties (O₂˙⁻) during the formation of ·OOH species.

Fig. 4 Reaction mechanism and energy (in kJ mol⁻¹) diagram for both systems. Bond lengths are in angstroms; white, red, blue, and grey spheres represent hydrogen, oxygen, nitrogen, and carbon, respectively.

Although the reactivity of the produced benzaldehyde is significantly higher than that of BA, it does not undergo further oxidation to form benzoic acid, which was proved by the following experiment. When benzaldehyde is added in a diphasic system (BA/water), the H₂O₂ production tends to increase after illumination (Fig. 3d), indicating that benzaldehyde can accelerate the photocatalytic process. Furthermore, the kinetic curve changed from an exponential curve (blue) to a zero-order curve (red) after the addition of benzaldehyde, suggesting that benzaldehyde acts as an auto-catalyst. Previous studies have suggested that benzaldehyde reacts with the produced ·OOH to form a benzoyl radical, which could extract hydrogen from BA³⁹ and then formed both benzaldehyde and α-hydroxybenzyl radicals (Fig. 3d). As a result, the selective oxidation of BA into benzaldehyde is achieved. The above-mentioned result has been corroborated by EPR (Fig. S23d†) and the photocurrent analysis (Fig. S24b†). Photocurrent increases significantly after the addition of benzaldehyde when compared to that case after the addition of BA. Furthermore, the characteristic peaks of α-hydroxybenzyl radicals were observed through EPR.

Intramolecular hydrogen bonding between the phenol and imine suppresses H₂O₂ decomposition. As shown in Fig. 3e, the H₂O₂ content produced using TAE-DaOH is 19 times that generated using TAE-ben. However, the H₂O₂ production rate decreases with the increase in H₂O₂ concentration when TAE-ben is used as the photocatalyst (Fig. S26b†), suggesting that H₂O₂ decomposes rapidly on the TAE-ben surface.⁴⁰ The intramolecular hydrogen bonds between the hydrogen and the imine bond in TAE-DaOH protect the imine bond from oxidizing. The imine bonds are also weakened and stabilized after they undergo protonation.⁴¹ According to zeta potential measurements (Fig. S26e†), imines in TAE-ben are protonated at pH = 3. In addition, the H₂O₂ production at pH = 3 is much higher than that at other higher pH values; meanwhile, the trend in the generation of H₂O₂ changed (Fig. S26b and c†). Despite protonation, the amount of H₂O₂ produced using TAE-DaOH is higher than that produced using the protonated TAE-ben (Fig. 3e). The yield of H₂O₂ using different catalysts (TAE-DaOH > protonated TAE-ben > TAE-ben) is consistent with the turn of imine bond of different catalysts (TAE-DaOH < protonated TAE-ben < TAE-ben), which indicates that more weakened imine bonds due to hydrogen bonding suppress the decomposition of H₂O₂.⁴² The result is further confirmed by using a scavenger trapping experiment (Fig. S27†). The H₂O₂ production decreases from 0.28 mM h⁻¹ to 0.10 mM h⁻¹ after adding IPA to the TAE-ben system, indicating that H₂O₂ is decomposed into ·OH in the TAE-ben system.⁴³ However, no decrease in the H₂O₂ production is observed after adding IPA to the TAE-DaOH system (Fig. S20d†). Based on these results, intermolecular hydrogen bonding can effectively suppress the decomposition of H₂O₂.

As mentioned above, TAE-DaOH does not undergo oxidation. However, the imine bonds of TAE-ben are oxidized due to the strong oxidizing ability of ·OH.⁴¹ The result is confirmed using XPS and FT-IR spectroscopy (Fig. S28–S32†). The characteristic peak of C=O is significantly enhanced after illumination (Fig. S29†), indicating the formation of an amide bond. Based on O 1s XPS, a new amide peak is observed at 538.9 eV, which suggests that the imine bonds undergo oxidation. Fig. S33† shows the peak shift from 2.75° to 2.60° in the PXRD pattern, while means imine COFs are oxidized to amide COFs. Nevertheless, these changes are not observed in TAE-DaOH. Above all, it can be concluded that intramolecular hydrogen bonding stabilizes the imine bond, thereby inhibiting catalyst oxidation.

As shown in Fig. S36,† DFT calculations also indicate that the TAE-ben catalyst can be oxidized by the resulting H₂O₂, owing to its relatively small energy barrier (68.7 kJ mol⁻¹). Meanwhile, the oxidation of TAE-DaOH is hindered due to its larger energy barrier (168.2 kJ mol⁻¹). This implies that TAE-ben is unstable during H₂O₂ production and cannot steadily convert O₂ into H₂O₂; these findings are consistent with the observations made in previous sections. In addition, a spin contamination 〈s²〉 of 0.983 and a longer O–O bond (2.272 Å) of the key transition state in the TAE-ben system impart the H₂O₂ moiety with typical diradical properties (·OH), thereby indicating that H₂O₂ is easily decomposed into ·OH on the TAE-ben surface. In the TAE-DaOH system, the H₂O₂ moiety maintained its close-shell electron configuration, which included a shorter O–O bond of 1.908 Å in the transition state. The calculations reveal that the hydrogen bond in TAE-DaOH suppresses the H₂O₂ decomposition during the photocatalytic process, thus ensuring a high H₂O₂ yield (details are given in ESI†).

Reaction mechanism. Based on the above-mentioned analyses, the entire reaction mechanism can be illustrated as follows: upon light irradiation, the catalyst donates an electron to oxygen to form O₂˙⁻ (1). The resulting radical then extracts a proton from phenol to form the ·OOH intermediate, while TAE-DaOH is transformed from an iminol to a ketoenimine (2). The intermediate subsequently extracts a hydrogen atom from BA to generate H₂O₂ and an α-hydroxybenzyl radical (3). The intermolecular hydrogen bonding between TAE-DaOH and BA then accelerates the conversion of ketoenimine to iminol (4), while the α-hydroxybenzyl radical is oxidized to benzaldehyde. Finally, the produced benzaldehyde acts as an auto-catalyst that exerts a stronger effect (path II in Fig. 5) on the overall recycling process, resulting in a higher H₂O₂ production. In contrast, when TAE-ben is used as the photocatalyst, H₂O₂ is decomposed into ·OH, which then combines with α-hydroxybenzyl radicals to form benzaldehyde and H₂O rather than H₂O₂. Meanwhile, TAE-ben is oxidized to amide-COF. Thus, despite benzaldehyde causing autocatalysis, the H₂O₂ production by using TAE-ben is significantly lower than that realized with TAE-DaOH.

Fig. 5 Mechanism governing the photocatalytic generation of H₂O₂via TAE-DaOH or TAE-ben. White, red, blue, and grey spheres represent hydrogen, oxygen, nitrogen, and carbon, respectively.

Potential of industrial application. During the process of traditional anthraquinone method, the generated H₂O₂ would need to be enriched to meet the industrial requirement. Before enrichment, the concentration of H₂O₂ is roughly 5% wt. In our catalytic system, the generated H₂O₂ can reach up to 2.2% (w.t.) for 10 h illumination. Therefore, the photocatalytic system can combine with the traditional enrichment system to produce industrial H₂O₂. Furthermore, the 2.2% (w.t.) H₂O₂ meets the requirement for the disinfection or other usage.

Conclusion

In summary, we demonstrated that TAE-DaOH, a type of crystalline COF photocatalyst, is highly efficient in the photocatalysis of H₂O₂ in a two-phase system. The key feature of TAE-DaOH is that it comprises phenolic hydroxyl groups. The hydroxyl groups form intermolecular hydrogen bonds with BA, which facilitates the electron transfer between them. Another important aspect of the phenolic hydroxylic groups is the intramolecular hydrogen bond between the imine bond and the phenolic hydroxyl group, which stabilizes the imine bond, suppresses the H₂O₂ decomposition, and thus produces a higher H₂O₂ concentration. Based on DFT calculations, the phenolic hydroxyl group not only lowers the energy barrier associated with H₂O₂ production, but also suppresses the H₂O₂ decomposition. In addition, the produced benzaldehyde is highly effective in forming radicals, which accelerate the H₂O₂ production rate. These properties synergistically ensure that TAE-DaOH is a highly efficient photocatalyst for the photosynthesis of H₂O₂. Accordingly, in the TAE-DaOH system, the H₂O₂ concentration can go up to 380 mM, while the maximum H₂O₂ production rate equals 61.3 mM h⁻¹ (higher than previous reported catalysts) and is 19 times as that in the TAE-ben system. Overall, the results of this study have important implications for the development of novel photocatalysts that can be used for sustainable energy production in industrial applications, for example, production of industrial H₂O₂, disinfection, and other applications.

Data availability

The data supporting this article are available upon request.

Author contributions

Lang Chen: data curation and writing; Song Qin: DFT calculation; Jiahui Hang: methodology; Bo Chen: methodology; Jinyang Kang: editing; Yang Zhao: data curation; Shanyong Chen: editing; Yongdong Jin: supervision; Hongjian Yan: editing; Yuanhua Wang: supervision; Chuanqin Xia: supervision, funding acquisition, writing – review and editing.

Conflicts of interest

We have no known competing financial interests with others.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 22176136, U21A20296, 22125605, U1732107) and the International Collaboration Project of Science and Technology Program of Sichuan Province, China. No. 2023YFH0098 and 2021YFH0170). We thank the Comprehensive training platform of specialized laboratory (College of Chemistry, Sichuan University) and Analytical & Testing Center of Sichuan University for technical support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cy01096e

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