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Article

Isomeric Anthraquinone-Based Covalent Organic Frameworks for Boosting Photocatalytic Hydrogen Peroxide Generation

by
Shengrong Yan
1,2,3,
Songhu Shi
1,
Wenhao Liu
1,
Fang Duan
1,*,
Shuanglong Lu
1 and
Mingqing Chen
1,*
1
Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, China
2
School of Environmental and Biological Engineering, Nantong College of Science and Technology, Nantong 226007, China
3
Jiangsu Engineering Research Center of Environmental Functional Materials and Pollution Control, Nantong 226007, China
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(6), 556; https://doi.org/10.3390/catal15060556
Submission received: 30 March 2025 / Revised: 3 May 2025 / Accepted: 12 May 2025 / Published: 3 June 2025
(This article belongs to the Special Issue Nanostructured Photocatalysts for Hydrogen Production)
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Abstract

:
Utilizing isomeric monomers to construct covalent organic frameworks (COFs) could easily and precisely regulate their structure in order to raise the photocatalytic performance towards two-step single-electron oxygen reduction reaction (ORR) to hydrogen peroxide (H2O2). Herein, isomeric anthraquinone (AQ)-based COFs (designated as 1,4-DQTP and 2,6-DQTP) were successfully fabricated through a simple yet effective one-step solvothermal synthesis approach, only utilizing isomeric monomers with alterations in the catalysts. Specifically, the black 1,4-DQTP displayed a high photocatalytic H2O2 production rate of 865.4 µmol g−1 h−1, with 2.44-fold enhancement compared to 2,6-DQTP (354.7 µmol g−1 h−1). Through a series of experiments such as electron paramagnetic resonance (EPR) spectroscopy and the free radical quenching experiments, as well as density functional theory (DFT) calculations, the photocatalytic mechanism revealed that compared with 2,6-DQTP, 1,4-DQTP possessed a stronger and broader visible light absorption capacity, and thus generated more photogenerated e−-h+ pairs. Ultimately, more photogenerated electrons were enriched on the AQ motif via a more apparent electron push–pull effect, which provided a stable transfer channel for e− and thus facilitated the generation of superoxide anion radical intermediates (•O2−). On the other hand, the negative charge region of AQ’s carbonyl group evidently overlapped with that of TP, indicating that 1,4-DQTP had a higher chemical affinity for the uptake of protons, and thus afforded a more favorable hydrogen donation for H+. As a consequence, the rational design of COFs utilizing isomeric monomers could synergistically raise the proton-coupled electron transfer (PCET) kinetics for two-step single-electron ORR to H2O2 under visible light illumination. This work provides some insights for the design and fabrication of COFs through rational isomer engineering to modulate their photocatalytic activities.

1. Introduction

Since hydrogen peroxide (H2O2) was first recognized by Thenard in 1818, H2O2, serving as a versatile green oxidant and a potential energy carrier of industrial significance, has been one of the most important chemicals in the world, and it has diverse applications spanning chemical synthesis, pulp and paper bleaching, medical sterilization, textile industry, metallurgy, military, electronics industry, wastewater treatment, and so on [1]. In the past, the electrolytic method was widely utilized for the continuous production of H2O2. However, due to its high energy demands and expensive cost, it has been substituted by the anthraquinone oxidation process invented in the 1940s [2]. The industrial synthesis of H2O2 is as follows: firstly, 2-alkylanthraquinone is dissolved by heavy aromatics, 2-alkylanthraquinone is subsequently reduced to 2-alkylanthrahydroquinone under H2 catalyzed by palladium catalyst, and lastly, 2-alkylanthrahydroquinone is oxidized to 2-alkylanthraquinone while O2 is reduced to H2O2 [3]. Therefore, anthraquinone (AQ) plays an important role in the industrial production of H2O2.
Due to -O-O- bond of H2O2 with a low bond energy, metal ions can accelerate the decomposition of photocatalyst-generated H2O2 [4]. Therefore, metal-free photocatalysts such as g-C3N4 and supramolecules are encouraged to generate H2O2 from a sacrificial-agent-free system [5,6]. Inspired by the industrial anthraquinone oxidation process to fabricate H2O2, researchers have introduced the AQ structure into metal-free polymeric photocatalysts to investigate their photosynthesis of H2O2 [7]. Serving as a co-catalyst for improving reduction selectivity [8], the electron acceptors with the strong electron withdrawal capacity [9,10], or O2 reduction centers [11,12], AQ motifs in the polymeric photocatalysts play a pivotal role in non-sacrificial H2O2 production. At the same time, integrating the AQ-like structure into the polymeric photocatalysts to generate H2O2 has also been reported. For instance, a covalently crosslinked polyimide aerogel photocatalyst (PI-BD-TPB) featuring a reductive C=O group [13] or the conjugated organic polymers (PQTEE-COP) containing phenanthrenequinone redox centers [14] could conduct visible-light-driven H2O2 production from H2O and O2 without any additives. Notably, phenanthrenequinone-modified conjugated polymer (PQ-AB) could improve the conversion efficiency of •O2− to H2O2 through the directional enrichment of photogenerated charges at sites of •O2− reduction and the stabilization of •O2− [15].
Covalent organic frameworks (COFs) are crystalline organic materials connected by covalent bonds. Due to their pre-designed structures, adjustable optical and electronic properties, high chemical and thermal stability, and so on, COFs have been proven to be promising photocatalysts for the production of H2O2 [16]. Therefore, researchers have also inserted AQ or AQ-like structures into COFs’ backbone for the photosynthesis of H2O2. For instance, AQ centers’ modified DQTb-COFs could inhibit the in situ decomposition of H2O2 and thus boost photocatalytic O2-to-H2O2 synthesis [17]. Accumulating photo-electrons on the C=O active site of polyimide COFs (PB-COF) with an AQ-like structure could boost H2O2 photosynthesis [18]. More significantly, researchers have further adopted the corresponding strategies to improve the photocatalytic performance of AQ-based COFs to produce H2O2. Constructing the multicomponent COF-Tfp-BpyDaaq [19], possessing the highest content of AQ motifs in TpAQ-COF by controlling the aldehyde-amine condensation time [20], integrating β-ketoenamine linkages into keto-form anthraquinone covalent organic frameworks (Kf-AQ) for substituting the enol-to-keto tautomerism [21], or fabricating donor–acceptor COFs or donor–π–acceptor COFs [22,23] could be capable of efficient photocatalytic H2O2 production in the absence of organic sacrificial agents. However, to the best of our knowledge, there have been few studies reported to date on regulating the electron push–pull effect and the proton trapping capability by constructing isomeric AQ-based COFs with identical composition but divergent linkage sites to raise the proton-coupled electron transfer (PCET) kinetics of the two-step single-electron ORR to H2O2 under visible light illumination.
Constructing COFs through rational isomer engineering can easily regulate their structures [24]. Furthermore, the well-defined structure of isomeric AQ-based COFs provides molecular-level insights into the mechanistic understanding of the photocatalytic H2O2 generation from earth-abundant H2O and O2 in air. Herein, through a simple one-step solvothermal synthesis process, a pair of AQ-based COFs (denoted as 1,4-DQTP and 2,6-DQTP) were fabricated via the Schiff base condensation reaction of 2,4,6-triformylphloroglucinol (TP) with 1,4-diaminoanthraquinone (1,4-DQ) or 2,6-diaminoanthraquinone (2,6-DQ), respectively (Figure 1). Through a series of experiments including electron paramagnetic resonance (EPR) spectroscopy and free radical quenching experiments, as well as density functional theory (DFT) calculations, the photocatalytic mechanism revealed that compared with the purple 2,6-DQTP, on the one hand, owing to a more apparent electron push–pull effect, ultimately, more photogenerated electrons were enriched on the AQ motif of the black 1,4-DQTP, promoting the formation of superoxide anion radical intermediates (•O2−); on the other hand, the negative charge region of AQ’s carbonyl group obviously overlapped with that of TP; 1,4-DQTP had a higher chemical affinity for the uptake of protons, which significantly improved the photocatalytic performance by jointly raising the PCET kinetics for the two-step, single-electron ORR to H2O2. This work provides new insights to construct isomeric AQ-based COFs for promoting photocatalytic H2O2 production.

2. Results and Discussion

2.1. Structural and Morphological Characterization

The crystal phases of the as-obtained COFs were first resolved utilizing powder X-ray diffraction (PXRD) measurements. According to Figure 2a and Table S1, the simulated PXRD pattern of 1,4-DQTP with an eclipsed AA stacking mode aligns with the experimental data to a large extent. Particularly, the broad peak at 26.5° is caused by π-π stacking in certain regions of the 1,4-DQTP molecular structure [25]. In Figure 2d and Table S2, the as-synthesized 2,6-DQTP presents a series of characteristic signals at 3.4°, 6.0°, and 7.0°, assigned to (100), (110), and (200), respectively, which is well matched with the existing literature [26]. In order to further gain the chemical structures of the as-prepared COFs, a Fourier transform infrared (FT-IR) spectrometer was employed. The new characteristic peaks (around 1200–1250 cm−1) are both attributed to the C-N bonds in the β-ketoenamine form, together with the significantly attenuated absorption peaks of -NH2 at ~3380 cm−1 and -CHO at ~1640 cm−1 (Figure 2b,e), implying the successful preparation of both 1,4-DQTP and 2,6-DQTP via the Schiff base condensation reaction [27]. This was further demonstrated by the 13C ssNMR spectrum (Figure 2c,f). As for both 1,4-DQTP and 2,6-DQTP, distinctive resonance peaks designated as a, b, and c with black color belong to the carbons in the C=O, C=C, and C-N bonds in the β-ketoenamine form, respectively; resonance peaks marked as j are assigned to the carbons of the carbonyl (C=O) originated from AQ; and weak resonance peaks denoted as a, b, and c with red color are ascribed to the carbons in the C-O, C-C, and imine bonds (C=N) in the enol–imine form, respectively [28]. As shown in Figure S1a, the chemical shift at 3.95 ppm in the solid-state 1H NMR spectra is indexed to the enol-form hydrogen (C-OH), while the chemical shift at 14.40 ppm is assigned to the keto-form hydrogen (N-H). Because the keto-form hydrogen of 1,4-DQTP forms an intramolecular hydrogen bond with the carbonyl oxygen in its AQ structure, its chemical shift moves towards the low field. The characteristic chemical shift at 4.01 ppm for the enol-form hydrogen is also observed for 2,6-DQTP in Figure S1b, which is closely in accordance with the reported literature [21]. In summary, the coexistence of the β-ketoenamine and enol–imine tautomerism in both 1,4-DQTP and 2,6-DQTP is confirmed via the characterizations of FT-IR, 13C ssNMR spectra, and solid-state 1H NMR spectra.
X-ray photoelectron spectroscopy (XPS) was employed to analyze the electronic state and elemental composition on the surface of 1,4-DQTP and 2,6-DQTP. As shown in Figure S2, C, O, and N elements are all present in the XPS survey spectrum of both 1,4-DQTP and 2,6-DQTP. Notably, the C 1s XPS spectrum of 1,4-DQTP in Figure 3a is deconvoluted into three peaks at binding energies of 288.1 eV (C=O bond), 286.2 eV (C-O bond), and 284.8 eV (C-C bond), respectively [21]. The C 1s XPS of 2,6-DQTP in Figure 3d is similar to that of 1,4-DQTP. The N 1s peak is also divided into three peaks for 1,4-DQTP at 399.3, 399.8, and 404.4 eV in Figure 3b, which are attributed to the C=N (enol–imine form), C-N-H (β-ketoenamine form), and π-π*, respectively [20]. As displayed in Figure 3e, the N 1s XPS of 2,6-DQTP is nearly the same as that of 1,4-DQTP. As for the O 1s spectrum in Figure 3c, three sets of peaks (C=O (β-ketoenamine form), C-OH (enol-imine form), and absorbed O2/H2O) are observed at the binding energies of 531.0, 532.8, and 536.0 eV for 1,4-DQTP [28]. According to Figure 3f, the O 1s XPS spectrum of 2,6-DQTP is almost consistent with that of 1,4-DQTP, except the peak of O2/H2O. Briefly, the coexistence of the β-ketoenamine and enol–imine tautomeric forms in both 1,4-DQTP and 2,6-DQTP is further verified by XPS.
The morphologies and structures of 1,4-DQTP and 2,6-DQTP were analyzed via a scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that 1,4-DQTP exhibits a flower-like lamellar structure, while 2,6-DQTP presents a rod-like structure (Figure S3a,b). The as-synthesized two COF materials both possess irregular two-dimensional sheet-like stacking structures (Figure S3c,d). Additionally, clear lattice fringes can be found in 2,6-DQTP, indicating that it has good crystallinity and porosity. The elemental composition and distribution were examined through the analysis of the energy dispersive X-ray (EDX) spectrometer. As can be seen from Figure S4, C, N, and O are presented and evenly distributed in both 1,4-DQTP and 2,6-DQTP, which is consistent with the results of XPS. The specific surface area and pore size distribution of 1,4-DQTP and 2,6-DQTP were meticulously determined by the N2 adsorption–desorption isotherms. As shown in Figure S5, although 1,4-DQTP and 2,6-DQTP both possess a mesoporous structure calculated via the nonlocal density function theory (NLDFT) method, the BET surface area of 2,6-DQTP (392 m2/g) representing a typical type I adsorption isotherm profile is larger than that of 1,4-DQTP (237 m2/g) exhibiting a classic type II pattern. In addition, the thermal stability of the obtained 1,4-DQTP and 2,6-DQTP was assessed using thermogravimetric analysis (TGA) within an inert N2 atmosphere. From Figure S6 it can be observed that the as-synthesized 1,4-DQTP and 2,6-DQTP both have good thermal stability.

2.2. Optical and Photoelectrochemical Properties

In order to determine the band alignment of the as-prepared samples, UV-Vis absorption spectroscopy and Mott–Schottky (M-S) tests were carried out. As depicted in Figure 4a, 1,4-DQTP displays a black color, corresponding to its strong and wide visible light absorption, and thus its absorption band edge exhibits a 115 nm red shift compared with the purple 2,6-DQTP [29]. As illustrated in Figure 4b, the optical band gap (Eg) was calculated using the Kubelka–Munk function and the Tauc plots, and for 1,4-DQTP (Eg = 1.58 eV), it is obviously narrower than that of 2,6-DQTP (Eg = 1.83 eV) [30]. Subsequently, density functional theory (DFT) calculations were performed to investigate the localizations of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of 1,4-DQTP and 2,6-DQTP. As presented in Figure 4c, the HOMO orbit electron distribution of 2,6-DQTP is almost delocalized across the entire molecular framework, while its LUMO orbit is mainly localized on the AQ motif. As for 1,4-DQTP in Figure 4d, its HOMO and LUMO orbits mainly consist of the TP motif and another AQ motif, respectively. Therefore, more electrons of 1,4-DQTP could jump from TP to AQ during the photo-activated process, indicating that a more apparent electron push–pull effect triggers stronger and broader visible light absorption and thus creates a narrower band gap semiconductor compared to 2,6-DQTP [31]. Additionally, from the electrochemical M-S plots in Figure S7, the semiconductor types and the flat band potentials can be obtained. The M-S plots for 1,4-DQTP and 2,6-DQTP both exhibit positive slopes, which are typical for n-type semiconductors [32]. The flat band potentials (Ef) of 1,4-DQTP and 2,6-DQTP are −0.90 and −1.15 V vs. Ag/AgCl electrode, and thus their conduction band minimums (CBMs) are calculated to be −0.70 and −0.95 V (vs. NHE). According to Eg = EVB − ECB, the valence band maximums (VBMs) of 1,4-DQTP and 2,6-DQTP are calculated to be 0.88 and 0.88 V (vs. NHE), respectively [33]. The energetic band configurations of 1,4-DQTP and 2,6-DQTP are schematically shown in Figure S8. From the thermodynamical viewpoint, the CBMs of 1,4-DQTP and 2,6-DQTP both lie above the potential for the two-electron reduction of O2, demonstrating that their reduction potential is feasible for the photocatalytic reduction of O2 to generate •O2− intermediates, which can further transform into H2O2 [34].
To gain insights into the photogenerated electron transfer rate, the responses of transient photocurrents for the as-prepared two COFs were analyzed using the CHI 660E electrochemical workstation. As illustrated in Figure 5a, more photogenerated electrons divert from 1,4-DQTP to the counter electrode via the peripheral circuit, as evidenced by its higher transient photocurrent density [35]. The electrochemical impedance spectra (EIS) of 1,4-DQTP and 2,6-DQTP are shown in Figure 5b, from which 1,4-DQTP exhibits the lower charge transfer resistance (Rct) at the photocatalyst/electrolyte interface, suggesting the faster diffusion mobility of photogenerated electrons on 1,4-DQTP than those on 2,6-DQTP [36]. In addition, as shown in Figure 5c, the photoluminescence (PL) spectrum intensity of 1,4-DQTP is significantly weakened compared with that of 2,6-DQTP, indicating that 1,4-DQTP efficiently inhibits the recombination of photogenerated electron–hole pairs and facilitates the photoexcited electrons’ transfer/migration to catalytic active sites for the succeeding H2O2 production [37]. According to Figure 5d, the average fluorescent lifetimes of 1,4-DQTP and 2,6-DQTP are calculated to be 1.51 and 1.37 ns, respectively. The longer fluorescence life expectancy of 1,4-DQTP implies that 1,4-DQTP has a longer photogenerated carrier diffusion time, and that the prolonged charge lifetime is significantly beneficial for participation in photocatalytic H2O2 generation [38]. It is also worth noting that 1,4-DQTP (4.23 Debye) has a larger dipole moment than 2,6-DQTP (4.04 Debye), due to a more apparent push–pull electronic effect (Figure S9), resulting in a stronger internal electric field for 1,4-DQTP, which can further facilitate charge separation and transfer. Based on the above analysis, compared with 2,6-DQTP, 1,4-DQTP could accelerate the photoinduced electron–hole pairs’ separation and transfer, and thus avoid their recombination.

2.3. Photocatalytic H2O2 Production and Its Mechanism

The photocatalytic activities of 1,4-DQTP and 2,6-DQTP in the generation of H2O2 were evaluated in O2-saturated aqueous solution without adding any sacrificial agents under visible light irradiation. The quantitative determination of H2O2 was obtained by using the potassium titanium (IV) oxalate method (Figure S10). As shown in Figure 6a, the kinetic profiles display that H2O2 accumulation increases gradually by prolonging the visible light irradiation time. Specifically, photocatalytic H2O2 production over 1,4-DQTP maintains a better linear growth within 3 h compared to 2,6-DQTP, implying that its photocatalytic generation of H2O2 is a stable and continuous process. As illustrated in Figure S11, 1,4-DQTP displays a high photocatalytic H2O2 production rate of 865.4 µmol g−1 h−1, with 2.44-fold enhancement compared to 2,6-DQTP (354.7 µmol g−1 h−1). The apparent quantum yield (AQY) was measured to appraise the conversion efficiency of photons during H2O2 production on 1,4-DQTP under monochromatic light illumination (Figure S12). The AQY values are closely in accordance with the light absorption of 1,4-DQTP. The highest AQY value appears at 600 nm and reaches 3.29%. According to Figure 6b, after five consecutive photocatalytic cyclic experiments, no apparent deactivation was observed in 1,4-DQTP. Moreover, no significant changes could be found in the PXRD pattern and FT-IR spectrum of 1,4-DQTP after successive photocatalytic H2O2 production (Figure S13). Therefore, the as-prepared 1,4-DQTP shows good durability during photocatalytic H2O2 generation.
In order to investigate the mechanism of 1,4-DQTP in the H2O2 photosynthesis reaction, the control experiments under diverse conditions, as well as the free radical quenching experiments, were reasonably conducted (Figure 6c,d). In dark conditions or without adding any photocatalyst, no H2O2 can be produced, suggesting that visible light or the photocatalyst is essential for photocatalytic H2O2 production. When the photocatalytic reaction is carried out under the air conditions, the photocatalytic performance of 1,4-DQTP in producing H2O2 significantly decreases, and when O2 is replaced by argon (Ar), almost no H2O2 can be detected, indicating that the oxygen-rich atmosphere is critical for photocatalytic H2O2 production, and that 1,4-DQTP under visible light irradiation mainly generates H2O2 by reducing O2 [39]. Furthermore, tert-butanol (t-BA), ethylene diamine tetraacetic acid (EDTA), AgNO3, and p-benzoquinone (p-BQ) are used as the trapping agents of •OH, h+, e− and •O2−, respectively. The introduction of t-BA shows a slight influence on photocatalytic H2O2 production, indicating that •OH is not the intermediate for the photocatalytic ORR. However, the photocatalytic H2O2 production rate increases by adding EDTA to trap holes, thus generating more electrons for the photocatalytic ORR. Particularly, photocatalytic H2O2 efficiency is significantly affected in the presence of AgNO3 or p-BQ, which demonstrates that the vital active species responsible for photocatalytic H2O2 production are e− and •O2−. In brief, as for 1,4-DQTP, the above free radical quenching experiments verify the sequential two-step single-electron route of the ORR [40].
To further confirm the photocatalytic H2O2 generation pathway, electron paramagnetic resonance (EPR) spectroscopy using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the probe and the electrochemical rotating disk electrode (RDE) over 1,4-DQTP were carried out. In the EPR spectra of Figure 6e, the •O2− signal of 1,4-DQTP cannot be detected in dark conditions. Upon exposure to visible light, 1,4-DQTP reveals detectable DMPO-•O2− signals under the experimental condition. Moreover, the longer the extension of the light time is, the stronger the DMPO-•O2− signal, which implies that more •O2− radicals are produced during the process of photocatalytic H2O2 production; thus, H2O2 production is dominated by the single-electron transfer process, with •O2− as the intermediate [41]. As shown in Figure 6f and Figure S14, the average number of transferred electrons (n) of 1,4-DQTP is calculated to be 2.37 by plotting the curves with the Koutecky–Levich method, further demonstrating that the H2O2 production of 1,4-DQTP under visible light irradiation is dominated by a two-electron ORR pathway [42]. In conclusion, combined with the energy band alignment, the control experiments under diverse conditions, the free radical quenching experiments, the EPR spectra, and the determination of the n value for ORR, it can be confirmed that 1,4-DQTP produces H2O2 from pure water and oxygen under visible light irradiation via two-step, single-electron ORR to H2O2 [43].
Based on the aforementioned experimental results and DFT simulations, a possible mechanism for photocatalytic H2O2 production catalyzed by 1,4-DQTP has been proposed in Figure 7. As can be seen, the electrostatic potential (Figure 7a,b) and Mulliken charge distribution (Figures S15 and S16) analyses show that the molecular electrostatic potentials of 1,4-DQTP and 2,6-DQTP are both negative in the region near the carbonyl group (C=O). More significantly, the negative charge region of AQ’s carbonyl group obviously overlaps with that of TP in 1,4-DQTP, demonstrating that 1,4-DQTP has a higher chemical affinity for the uptake of protons [26]. Meanwhile, as for the AQ motif in 1,4-DQTP, the observed increase in the positive Mulliken charge on the AQ carbonyl carbon is indicative of a greater accumulation of electrons on these carbon atoms, indicating that the AQ carbonyl carbons are active sites for O2 adsorption and activation, promoting •O2− formation [15]. When the black 1,4-DQTP is irradiated under visible light, more photoexcited electrons and holes can be generated via the dissociation of excitons (electron–hole pairs). Due to a more apparent electron push–pull effect, on the one hand, photogenerated holes almost accumulate in the TP motif, and then could be consumed by a 4h+ water oxidation reaction. On the other hand, photoexcited electrons almost accumulate in another AQ motif. Subsequently, the AQ carbonyl group capturing H+ could be converted into the AQ C-OH, while the AQ carbonyl carbon in 1,4-DQTP could adsorb and activate O2 to generate •O2−. Subsequently, •O2− with H+ forms *OOH through the first PCET step, and then generates *HOOH via the second PCET step. Finally, photocatalytic H2O2 production is accomplished via the desorption of *HOOH, while AQ C-OH is changed back into the AQ carbonyl group (Figure 7c) [36].

3. Materials and Methods

The experimental details of the chemicals and reagents, characterization methods, thermogravimetric analysis (TGA), apparent quantum yield (AQY) measurements, photoelectrochemical measurements, electron paramagnetic resonance (EPR) spectroscopy measurements, and rotating disk electrode (RDE) measurements can be found in the Supplementary Materials.

3.1. Synthesis of COFs

Firstly, 2,6-DQTP was prepared according to a previous protocol [28]. TP (0.3 mmol, 63 mg) and 2,6-DQ (0.45 mmol, 107.1 mg) were dissolved into 3 mL mixed solvent of trimethylbenzene and 1,4-dioxane (v/v trimethylbenzene:1,4-dioxane = 1:1) and sonicated for 30 min, respectively. Then, the TP solution and 2,6-DQ solution were mixed into a 10 mL Pyrex tube and sonicated for another 10 min. Next, 6 M CH3COOH aqueous solution (0.5 mL) as a catalyst was added into the Pyrex tube (Chongqing Xinweier Glass Co., Ltd. Chongqing, China). Subsequently, the Pyrex tube was degassed via the freeze–pump–thaw technique three times, vacuum-sealed, and then heated at 120 °C for 3 days. After being cooled to room temperature, the precipitate was isolated via filtration and washed with methyl alcohol (3 × 50 mL), dichloromethane (3 × 50 mL), and acetone (3 × 50 mL) in turn. The purple solid was further purified using a Soxhlet extractor (Shanghai Joyn Electronic Co., Ltd. Shanghai, China) and then obtained via freeze drying for 24 h. Similarly, keeping the other conditions unchanged, the black 1,4-DQTP was successfully prepared by only replacing 2,6-DQ and 6 M CH3COOH aqueous solution with 1,4-DQ and 6 M CF3COOH aqueous solution, respectively.

3.2. Photocatalytic H2O2 Production

Under visible light irradiation, the photocatalytic performance of H2O2 production was tested. Typically, the as-obtained photocatalyst (5 mg) and 30 mL of deionized water were added into the reactor, respectively. Then, the above-mentioned solution was sonicated for 10 min. After that, oxygen was continuously bubbled to maintain the saturated state in the system, and then the reactor was illuminated by turning on the light source (300 W Xenon lamp with 420 nm cut-off filter, Beijing PerfectLight Technology Co., Ltd. Beijing, China). In order to control the reaction temperature, a circulating water system was used, and the temperature was controlled at 25 °C. The potassium titanium (IV) oxalate method was used to quantify the concentration of H2O2 [44], in which 2 mL of the suspension was withdrawn and passed through 0.45 µm micro-porous filter. Then, 1.5 mL of the above filtrate was injected into a test tube, and 1 mL of 0.02 M potassium titanium (IV) oxalate solution was added and shaken to mix the solution for 6 min. The change in color of the mixed solution from clear to yellow is indicative of the formation of titanium peroxides. The absorbance at 400 nm was quantified using a UV-Visible spectrophotometer, enabling the calculation of the H2O2 concentration according to the Lambert–Beer law.

3.3. Computational Details

All density functional theory (DFT) calculations were carried out on the basis of B3LYP/6-31G (D) with the Gaussian 09 D.0l software package. All structures were optimized, vibration analysis was conducted, and there was no virtual frequency in the steady state. Multiwfn (version 3.8) and VMD (version 1.9.3) were used to conduct the wave function analysis and visualization [45].

4. Conclusions

In summary, through a one-step solvothermal method, a pair of isomeric AQ-based COFs were constructed successfully for photocatalytic H2O2 production in a sacrificial-agent-free system. The black 1,4-DQTP displayed better photocatalytic performance in H2O2 production, with a production rate of 865.4 µmol g−1 h−1. Through the corresponding experiments and DFT calculations, a possible mechanism for photocatalytic H2O2 production over 1,4-DQTP was proposed. Compared to 2,6-DQTP, 1,4-DQTP possessed a stronger and broader visible light absorption capacity and thereby generated more photogenerated e−-h+ pairs. Ultimately, more photogenerated electrons were enriched on the AQ motif via a more apparent electron push–pull effect, which provided a stable transfer channel for e− in the PCET process of indirect 2e− ORR. On the other hand, the negative charge region of AQ’s carbonyl group obviously overlapped with that of TP, indicating that 1,4-DQTP had a higher chemical affinity for the uptake of protons and could afford more H+ in the PCET process of indirect 2e− ORR. As a result, the rational design of COFs based on isomer engineering could synergistically raise the PCET kinetics for two-step, single-electron ORR to H2O2 under visible light illumination. This work provides a good reference for designing efficient COFs through rational isomer engineering in the photocatalytic synthesis of H2O2 only from solar energy, water, and O2.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15060556/s1. Figure S1: Solid-state 1H NMR spectra of 1,4-DQTP (a) and 2,6-DQTP (b); Figure S2: The survey spectra of XPS spectra over 1,4-DQTP and 2,6-DQTP; Figure S3: (a) SEM image and (c) TEM image of 1,4-DQTP; (b) SEM image and (d) TEM image of 2,6-DQTP; Figure S4: The SEM elemental mappings of 1,4-DQTP (a) and 2,6-DQTP (b); Figure S5: (a) N2 adsorption–desorption isotherms and (b) the pore size distributions of 1,4-DQTP and 2,6-DQTP; Figure S6: TGA curves of 1,4-DQTP and 2,6-DQTP; Figure S7: Mott–Schottky plots of 1,4-DQTP (a) and 2,6-DQTP (b); Figure S8: Schematic diagram of the band alignment of 1,4-DQTP and 2,6-DQTP; Figure S9: The dipole moments of 1,4-DQTP (a) and 2,6-DQTP (b); Figure S10: Standard curve of H2O2 obtained via the potassium titanium (IV) oxalate method; Figure S11: Photocatalytic production rate of H2O2 over 1,4-DQTP and 2,6-DQTP; Figure S12: Wavelength-dependent AQY of photocatalytic H2O2 production on 1,4-DQTP; Figure S13: (a) PXRD patterns and (b) FT-IR spectra before and after five cycles over 1,4-DQTP; Figure S14: Linear sweep RDE voltammograms of 1,4-DQTP measured at different rotating speeds; Figure S15: Mulliken charge distribution of 1,4-DQTP; Figure S16: Mulliken charge distribution of 2,6-DQTP. Table S1: Fractional atomic coordinates for the unit cell of 1,4-DQTP; Table S2: Fractional atomic coordinates for the unit cell of 2,6-DQTP.

Author Contributions

Conceptualization, S.Y. and F.D.; methodology, S.Y.; software, S.S.; validation, S.Y., S.S. and W.L.; formal analysis, S.Y.; investigation, S.Y.; resources, F.D.; data curation, S.S.; writing—original draft preparation, S.Y.; writing—review and editing, F.D. and S.L.; visualization, W.L.; supervision, M.C.; project administration, F.D.; funding acquisition, F.D. and S.L. All authors have read and agreed to the published version of the manuscript.

Funding

The research was funded by the National Key Research and Development Program of China (No. 2023YFF1105200) and the National Natural Science Foundation of China (No. 52173201).

Data Availability Statement

The data will be made available upon request.

Acknowledgments

This work was financially supported by the Postgraduate Research & Practice Innovation Program of Jiangsu Province (No. KYCX18_1808), and the High-end Training Program for Teachers’ Professional Leaders in Higher Vocational Colleges of Jiangsu Province in 2023 (No. 2023GRFX045).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic illustration of the synthesis of 2,6-DQTP and 1,4-DQTP through rational isomer engineering.
Figure 1. Schematic illustration of the synthesis of 2,6-DQTP and 1,4-DQTP through rational isomer engineering.
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Figure 2. (a) PXRD pattern, (b) FT-IR spectra, and (c) 13C ssNMR spectra of 1,4-DQTP; (d) PXRD pattern, (e) FT-IR spectra, and (f) 13C ssNMR spectra of 2,6-DQTP.
Figure 2. (a) PXRD pattern, (b) FT-IR spectra, and (c) 13C ssNMR spectra of 1,4-DQTP; (d) PXRD pattern, (e) FT-IR spectra, and (f) 13C ssNMR spectra of 2,6-DQTP.
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Figure 3. (a) C 1s, (b) N 1s, and (c) O 1s XPS spectra of 1,4-DQTP; (d) C 1s, (e) N 1s, and (f) O 1s XPS spectra of 2,6-DQTP.
Figure 3. (a) C 1s, (b) N 1s, and (c) O 1s XPS spectra of 1,4-DQTP; (d) C 1s, (e) N 1s, and (f) O 1s XPS spectra of 2,6-DQTP.
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Figure 4. (a) UV-Vis absorption spectra and (b) Tauc plots of 1,4-DQTP and 2,6-DQTP; distribution of HOMO and LUMO orbits over (c) 2,6-DQTP and (d) 1,4-DQTP.
Figure 4. (a) UV-Vis absorption spectra and (b) Tauc plots of 1,4-DQTP and 2,6-DQTP; distribution of HOMO and LUMO orbits over (c) 2,6-DQTP and (d) 1,4-DQTP.
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Figure 5. (a) Transient photocurrent responses, (b) EIS Nyquist plots, (c) PL spectra, and (d) the fluorescence decay curves of 1,4-DQTP and 2,6-DQTP.
Figure 5. (a) Transient photocurrent responses, (b) EIS Nyquist plots, (c) PL spectra, and (d) the fluorescence decay curves of 1,4-DQTP and 2,6-DQTP.
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Figure 6. (a) Kinetic profiles of photocatalytic H2O2 production over 1,4-DQTP and 2,6-DQTP. (b) Cyclic experiments of photocatalytic H2O2 production for 1,4-DQTP. (c) Photocatalytic H2O2 production over 1,4-DQTP under various conditions. (d) The free radical quenching experiments of 1,4-DQTP (the pH values of standard solution, e− trapping solution, •O2− trapping solution, •OH trapping solution, and h+ trapping solution are 5.69, 4.05, 5.58, 5.61, and 3.87, respectively). (e) EPR spectra of DMPO-•O2− by 1,4-DQTP. (f) Koutecky–Levich plots obtained from RDE measurement of 1,4-DQTP.
Figure 6. (a) Kinetic profiles of photocatalytic H2O2 production over 1,4-DQTP and 2,6-DQTP. (b) Cyclic experiments of photocatalytic H2O2 production for 1,4-DQTP. (c) Photocatalytic H2O2 production over 1,4-DQTP under various conditions. (d) The free radical quenching experiments of 1,4-DQTP (the pH values of standard solution, e− trapping solution, •O2− trapping solution, •OH trapping solution, and h+ trapping solution are 5.69, 4.05, 5.58, 5.61, and 3.87, respectively). (e) EPR spectra of DMPO-•O2− by 1,4-DQTP. (f) Koutecky–Levich plots obtained from RDE measurement of 1,4-DQTP.
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Figure 7. (a) The electrostatic surface potential of 2,6-DQTP. (b) The electrostatic surface potential of 1,4-DQTP. (c) The proposed photocatalytic mechanism of H2O2 production over 1,4-DQTP.
Figure 7. (a) The electrostatic surface potential of 2,6-DQTP. (b) The electrostatic surface potential of 1,4-DQTP. (c) The proposed photocatalytic mechanism of H2O2 production over 1,4-DQTP.
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Yan, S.; Shi, S.; Liu, W.; Duan, F.; Lu, S.; Chen, M. Isomeric Anthraquinone-Based Covalent Organic Frameworks for Boosting Photocatalytic Hydrogen Peroxide Generation. Catalysts 2025, 15, 556. https://doi.org/10.3390/catal15060556

AMA Style

Yan S, Shi S, Liu W, Duan F, Lu S, Chen M. Isomeric Anthraquinone-Based Covalent Organic Frameworks for Boosting Photocatalytic Hydrogen Peroxide Generation. Catalysts. 2025; 15(6):556. https://doi.org/10.3390/catal15060556

Chicago/Turabian Style

Yan, Shengrong, Songhu Shi, Wenhao Liu, Fang Duan, Shuanglong Lu, and Mingqing Chen. 2025. "Isomeric Anthraquinone-Based Covalent Organic Frameworks for Boosting Photocatalytic Hydrogen Peroxide Generation" Catalysts 15, no. 6: 556. https://doi.org/10.3390/catal15060556

APA Style

Yan, S., Shi, S., Liu, W., Duan, F., Lu, S., & Chen, M. (2025). Isomeric Anthraquinone-Based Covalent Organic Frameworks for Boosting Photocatalytic Hydrogen Peroxide Generation. Catalysts, 15(6), 556. https://doi.org/10.3390/catal15060556

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