Natural-sunlight-harvesting benzothiadiazole-based molecular photocatalyst for H₂O₂ production: a recyclable homogeneous biphasic system

Abstract

Photocatalytic hydrogen peroxide (H₂O₂) synthesis offers a sustainable route for solar-to-chemical energy conversion to address the increasing global energy demand. However, developing an efficient and sustainable photocatalytic system for H₂O₂ synthesis remains a critical bottleneck. Herein, we report a unique toluene–water two-phase system for highly efficient H₂O₂ synthesis, employing a benzothiadiazole-based small organic photocatalyst, Py–BT–Th. Py–BT–Th shows a high H₂O₂ production rate of 107.18 mmol g⁻¹ h⁻¹, with a turnover frequency of ∼32 h⁻¹, under natural sunlight irradiation in the toluene–water biphasic system, which uses triethylamine as a sacrificial agent. The biphasic system promotes H₂O₂ mass transport, leading to rapid kinetics across the toluene–water interface. Experimental studies reveal that the superior photocatalytic performance of the catalyst Py–BT–Th is attributed to its donor–acceptor-type molecular architecture, which promotes efficient exciton separation, high molar absorptivity (ε = 12.27 × 10³ M⁻¹ cm⁻¹), long exciton lifetime (12.9 ns) and broad light-harvesting capacity. Scavenger and in situ EPR spectroscopic studies confirm that Py–BT–Th produces H₂O₂ by following a dual mechanistic route: (1) indirect two-step one-electron ORR via an electron transfer pathway and (2) singlet-oxygen generation via an energy transfer pathway. In both the routes, ˙O₂⁻ is the key intermediate that ultimately gets converted into hydrogen peroxide, in association with the oxidation of triethylamine by photogenerated holes. Overall, this work disentangles the dilemma associated with small organic photocatalysts for H₂O₂ synthesis through a recyclable toluene–water biphasic system.

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Broader context

Hydrogen peroxide (H₂O₂) is a well-known green oxidant exploited in various practical applications; and its global demand is increasing and expected to reach 5.7 million tons by 2028. Currently, more than 95% of H₂O₂ is produced through the traditional anthraquinone (AQ) process; however, the AQ process generates harmful organic waste and poses a substantial threat to environmental sustainability. This traditional process is also highly energy-expensive and tedious, involving multiple steps of purification. Therefore, developing a green and sustainable route for H₂O₂ synthesis is extremely desirable. With abundant resources, such as water, sunlight, and air, and an appropriate photocatalyst, H₂O₂ synthesis can be a promising solution to the energy catastrophe. However, developing a highly efficient, robust, and sustainable photocatalytic system for H₂O₂ synthesis remains a critical bottleneck. In this context, we develop a unique toluene–water biphasic system for the efficient photocatalytic synthesis of H₂O₂, employing a benzothiadiazole-based small organic molecular photocatalyst, Py–BT–Th. Py–BT–Th shows a high H₂O₂ production rate of 107.18 mmol g⁻¹ h⁻¹, with a turnover frequency of ∼32 h⁻¹, under natural sunlight irradiation in the toluene–water biphasic system, using triethylamine as a sacrificial agent. The findings of this work disentangle the dilemma associated with small organic photocatalysts for H₂O₂ production through reaction-media engineering. Overall, this work presents a promising strategy for the green and efficient synthesis of H₂O₂ using a recyclable two-phase system under natural sunlight.

Introduction

Hydrogen peroxide (H₂O₂) is a well-known green oxidant used in various applications spanning organic synthesis, wastewater treatment, disinfection, healthcare and chemical engineering.^1–3 Hence, the global demand for H₂O₂ is increasing rapidly, and it is estimated to reach 5.7 million tons by 2028.⁴ Compared with H₂, H₂O₂ is a more convenient form for energy transmission due to its high solubility in water.⁵ The H₂O₂ one-compartment fuel cell exhibits an open circuit voltage of +1.09 V vs. NHE, comparable with that of the H₂ fuel cell (+1.23 V vs. NHE), releasing only green byproducts, such as oxygen and water.^6,7 The primary industrial process of hydrogen peroxide production is the anthraquinone process, which is a highly energy-expensive and environmentally adverse process, generating an ample amount of hazardous organic waste.^8,9 Therefore, developing a green and sustainable route for H₂O₂ synthesis is extremely desirable.

Photocatalytic H₂O₂ synthesis from pure water (H₂O) and O₂ occurs via solar energy. Although it follows a green route for solar-to-chemical energy conversion, it is a thermodynamic uphill process requiring an input energy of 117 kJ mol⁻¹.¹⁰ H₂O₂ production from water and oxygen under photocatalytic conditions is usually achieved using two pathways: through a two-step one-electron oxygen reduction reaction (2-Step ORR: O₂ + e⁻ → ˙O₂⁻ and ˙O₂⁻ + e⁻ + 2H⁺ → H₂O₂) and through a two-electron water oxidation reaction (WOR: 2H₂O – 2e⁻ → H₂O₂ + 2H⁺) pathway.¹¹ The accumulation of superoxide radical anions (˙O₂⁻) and photogenerated holes (h⁺) in the reaction medium actually promotes the corrosion of the photocatalyst and decomposition of H₂O₂, ultimately resulting in a decline in catalytic efficiency.^12,13 Therefore, designing and developing a unique reaction system to resolve these challenges is of utmost importance. A suitable water-organic two-phase system can be a promising solution to safeguard the catalyst. It simultaneously helps to minimize H₂O₂ decomposition by effectively suppressing the accumulation of reactive oxygen species.^14,15

To enhance the photocatalytic efficiency and durability of the catalyst, employing the oxidation of organic substances, rather than the water oxidation reaction (WOR), is a promising strategy. However, the inherently sluggish kinetics of the water oxidation reaction (WOR) limit both the catalytic efficiency and apparent quantum yield (AQY).^16,17 The two-phase water-benzyl alcohol system has been widely studied, as it remarkably enhances the photocatalytic efficiency of the formation of H₂O₂. However, H₂O₂ formation is facilitated by the oxidation of benzyl alcohol to benzaldehyde, instead of through the WOR.^18,19 The major limitation of the water-benzyl alcohol two-phase system is associated with the relatively high solubility of benzyl alcohol in the aqueous phase, which consequently leads to contamination of the H₂O₂ solution.²⁰ Additionally, autocatalytic peroxide formation is quite an undemanding process that occurs with benzyl alcohol under irradiated conditions without a photocatalyst, which ultimately leads to an overestimation of the photocatalytic efficiency of the catalyst.^21,22 Actual intrinsic photocatalytic efficacy can be more accurately assessed using a sacrificial donor that does not follow an autocatalysis pathway under irradiated conditions. Such a sacrificial donor can be an amine, which can serve as both electron and proton donor simultaneously.

Over the years, several photocatalytic materials have been explored for H₂O₂ production, such as inorganic semiconductors (ISs),^23–25 conjugated microporous polymers (CMPs),^26–29 metal organic frameworks (MOFs),^30,31 covalent organic frameworks (COFs),^32–34 polymeric carbon nitrides (PCNs),^35–37 metal organic cages (MOCs),³⁸ and resins³⁹ (as represented in Fig. 1a). These heterogeneous materials have been extensively explored for photocatalytic hydrogen peroxide (H₂O₂) production due to their leads in the areas of separation, surface area, redox active sites, charge separation, and photostability.⁴⁰ Some of the recent reports utilise small organic photocatalysts for H₂O₂ synthesis via different strategies, such as the supramolecular self-assembly^41–44 of small organic molecules and the incorporation of organic dye molecules as ligands coordinated to metal ions.⁴⁵ In 2022, Zhenhui Kang and co-workers reported the synthesis of H₂O₂ using 9,10-bis(ethynyl)anthracene (DAn) as an organophotocatlyst.⁴⁶ More recently, Fan Zhang and co-workers reported polycyclic aromatic hydrocarbons (PAHs) as a small organic photocatalyst for H₂O₂ synthesis.⁴⁷ In another very recent report, the Motonori Watanabe group explored a melamine-phthalimide donor–acceptor-type molecule for H₂O₂ production under visible light using ethanol (EtOH) as a sacrificial donor.⁴⁸ However, it is still noteworthy that small organic photocatalysts (SOPs) remain in their early stages of development for photocatalytic H₂O₂ production. The application of SOPs in photocatalytic hydrogen peroxide synthesis is limited due to their inherent hydrophobic character. The hydrophobicity of such molecules makes them underexplored in aqueous solutions during H₂O₂ synthesis from pure water and O₂. These organic materials have considerable potential to enhance efficiency and can therefore be a fascinating choice for photocatalytic H₂O₂ production owing to several advantages, such as broad light-absorption range, tunable electronic structure, adaptable redox sites, and easy synthesis and functionalization.⁴⁹

Fig. 1 (a) Schematic of the reported photocatalytic materials for H₂O₂ synthesis. (b) Photocatalytic H₂O₂ synthesis using the Py–BT–Th catalyst in a recyclable homogeneous toluene–water biphasic system.

Benzothiadiazole-based donor–acceptor (D–A)-type skeletons are preferred over other materials due to the strong electron-withdrawing character of the BT core, facilitating efficient charge separation and migration, broad light absorption range, and tunable excited state dynamics.⁵⁰ These characteristics render D–A-type benzothiadiazole-based architectures a pertinent material for photocatalytic applications. BT-based materials have been utilized in various applications, such as hydrogen production,⁵¹ organic transformation,⁵² biomedical imaging,⁵³ environmental remediation,⁵⁴ optoelectronic devices,⁵⁵ photovoltaic cells,⁵⁶ and dye-sensitized solar cells.⁵⁷ Most of the benzothiadiazole-containing materials are characterized by an excited state with a long lifetime and reduction potential of more than −0.6 eV. These properties facilitate the generation of reactive oxygen species (ROS) by the oxygen reduction reaction (ORR) pathway, and hence, these materials can be surfaced as promising photocatalysts for ORR.^58,59

Here, we develop a unique toluene–water biphasic system for efficient photocatalytic H₂O₂ synthesis, employing a benzothiadiazole-based small organic photocatalyst, Py–BT–Th, (structure shown in Fig. 1b) and trimethylamine (TEA) as a sacrificial donor. In this toluene–water two-phase system, the toluene phase contains both the sacrificial donor and the catalyst, while the H₂O₂ formed accumulates in the aqueous phase. The catalyst and trimethylamine are completely soluble in the toluene phase, and therefore, the efficient separation of the catalyst from the aqueous solution of hydrogen peroxide can be facilitated (see Fig. 1b). The easy separation of the H₂O₂ layer would foster the reusability and recyclability of the homogeneous catalyst. The donor–acceptor-type (D–A-type) architecture of Py–BT–Th promotes the efficient exciton separation and light absorption capacity of the catalyst with a high excited-state reduction potential and lifetime. Py–BT–Th demonstrates a high photocatalytic H₂O₂ production rate of 107.18 mmol g⁻¹ h⁻¹ in a toluene–water biphasic system, with a turnover frequency of ∼32 h⁻¹, under natural sunlight irradiation in an oxygen atmosphere. The sacrificial agent hinders the electron–hole pair recombination by providing electrons to the holes and enables the efficient H₂O₂ synthesis. In situ EPR spectroscopic study and scavenger study altogether corroborate that the Py–BT–Th photocatalyst follows a dual mechanistic route for H₂O₂ synthesis, with superoxide radical anion (˙O₂⁻) as the key intermediate. The current work, therefore, would provide a promising strategy for green and efficient photocatalytic H₂O₂ synthesis with a recyclable though homogeneous two-phase system under natural sunlight.

Experimental

Materials

4,7-Dibromo-2,1,3-Benzothiadiazole (98%), 1,4-diazabicyclo[2.2.2]octane (DABCO, 98%) and Pd(PPh₃)₄ were purchased from TCI Chemicals. Sodium sulfate (Na₂SO₄, 99%), tert-butyl alcohol, isopropanol, 30% H₂O₂ (w/v), and deuterated chloroform (CDCl₃) were obtained from CDH Fine Chemicals. Potassium iodide (KI, 99%), TEMP (2,2,6,6-tetramethyl-4-piperidone), p-benzoquinone (98%), 2-(tributylstannyl)thiophene, and DMPO (5,5-dimethyl-1-pyrroline N-oxide) were acquired from Sigma-Aldrich. K₂CO₃ was sourced from the Merck suppliers. Hexane and ethyl acetate were purchased from Qualigens Chemicals for synthesis and purification. Spectroscopy-grade solvents, including acetonitrile, hexane, toluene, dioxane, THF, triethylamine, and DMSO, were provided by Spectrochem. H₂O₂ detection test strips were purchased from Bartovation (0–100 ppm). All the chemicals and solvents were used as received without any further purification.

Methods

Synthetic procedure of the Py–BT–Th photocatalyst. The first step of the photocatalyst synthesis involved the Suzuki–Miyaura coupling reaction protocol (Scheme S1, SI), in which a 25-mL Schlenk tube was charged with 4,7-dibromobenzo[c]-1,2,5-thiadiazole (588 mg, 2 mmol), pyridin-2-ylboronic acid (246 mg, 2 mmol), and 5 mol% of Pd(PPh₃)₄ catalyst (115 mg, 0.1 mmol) in dioxane solvent. Then, the reaction mixture was heated at 90 °C for 1 hour under a nitrogen atmosphere, and subsequently, 2 M K₂CO₃ was added to the reaction solution. The reaction was then carried out for 18 hours at the same temperature. The workup was accomplished after the completion of the reaction, and the crude reaction mixture was purified through column chromatography using an ethyl acetate/hexane (6 : 4) solution. Then, the compound was dried over Na₂SO₄, resulting in a light-yellow compound (321 mg, 55%).

¹H NMR (400 MHz, chloroform-d) δ 8.82–8.77 (m, 2H), 8.00 (d, J = 7.6 Hz, 1H), 7.90–7.87 (m, 2H), 7.71 (d, J = 7.6 Hz, 1H) (Fig. S7, SI).

¹³C NMR (101 MHz, Chloroform-d) δ 153.96, 152.47, 150.18, 143.98, 132.13, 130.86, 128.94, 123.50, 115.44, 77.37 (Fig. S8, SI).

ESI-HRMS: m/z = 293.9521 [M+H]⁺ and calculated [M+H]⁺ = 293.9518 (Fig. S11, SI).

The second step involves the Stille coupling reaction (Scheme S2, SI). Py–BT–Br (300 mg, ∼1 mmol), tributyl(2-thienyl) stannane (381 µL, 1.2 mmol), and 5 mol% of Pd(PPh₃)₄ in THF solvent were added to the Schlenk tube and heated at 90 °C for 12 hours under magnetic stirring in N₂ atmosphere. After the completion of the reaction, the reaction mixture was subjected to workup, and the resulting reaction mixture was purified using column chromatography in ethyl acetate/hexane (4 : 6) solution. Then, the obtained compound was dried over Na₂SO₄, resulting in an isolated orangish-yellow compound (266 mg, 90%).

¹H NMR (400 MHz, chloroform-d) δ 8.85–8.76 (m, 2H), 8.21 (dd, J = 3.7, 1.1 Hz, 1H), 8.04–7.97 (m, 3H), 7.88 (d, J = 7.5 Hz, 1H), 7.54 (dd, J = 5.1, 1.1 Hz, 1H), 7.26 (dd, J = 5.1, 3.7 Hz, 1H) (Fig. S9, SI).

¹³C NMR (101 MHz, chloroform-d) δ 153.36, 152.74, 149.42, 145.18, 138.85, 129.09, 128.99, 128.50, 128.43, 128.21, 127.74, 125.37, 123.62, 77.36 (Fig. S10, SI).

ESI-HRMS: m/z = 298.0286 [M+H]⁺ and calculated [M+H]⁺ = 298.0284 (Fig. S12, SI).

General procedure for the photocatalytic H₂O₂ synthesis. In a 15-mL Schlenk tube, 8 mL of deionised water was added, followed by 2 mL of toluene containing 100 µg of photocatalyst Py–BT–Th and 10 µL of triethylamine. Toluene and water formed a two-phase system, with the organic phase comprising both the sacrificial donor and the catalyst. After that, the reaction solution was irradiated for 2 hours using a 20 W white LED bulb or Natural sunlight, keeping the reaction at room temperature. Upon the completion of the reaction, 2 mL of the aqueous phase was extracted for H₂O₂ quantification using the iodometry method (detailed procedure for quantification is discussed in the SI).

Results and discussion

A benzothiadiazole-based photocatalyst, Py–BT–Th, was synthesized in two steps via Suzuki–Miyaura and Stille coupling reactions to form carbon–carbon bonds between the acceptor (A) and donor (D) unit. Being a strong acceptor, benzothiadiazole (BT), when combined with donors like thiophene (Th) and pyridine (Py), affords an unsymmetrical donor–acceptor type of architecture of Py–BT–Th. The molecule with a high dipole moment is perceived to exhibit high molar absorptivity, broad visible-light absorption, and efficient exciton separation, which are crucial for photocatalytic H₂O₂ production.

The basic structure of the photocatalyst was characterized using various analytical techniques, including ¹H and ¹³C NMR, Fourier transform infrared (FTIR), and XPS spectroscopy. In the ¹H and ¹³C NMR spectra (shown in Fig. S9 and S10), the catalyst displayed all peaks in good agreement with the structure. The FTIR spectrum of Py–BT–Th is shown in Fig. S13. The peak at 3055–2854 cm⁻¹ corresponded to C–H stretching vibrations, and the peaks at 1720, 1265 cm⁻¹ corresponded to the pyridine (Py) unit, whereas the peaks at 1589 and 1180 cm⁻¹ were due to the benzothiadiazole (BT) moiety. Using XPS, the structure of the Py–BT–Th catalyst was further elucidated. XPS survey and high-resolution XPS analyses results are shown in Fig. 2, which confirm the presence of C, N, and S atoms and affirm the structure of catalyst Py–BT–Th.^20,60,61

Fig. 2 (a) XPS survey spectra of the Py–BT–Th catalyst. High-resolution XPS signals of (b) C 1s (c) N 1s and (d) S 2p.

Photophysical and electrochemical characterization

The light absorption properties of the photocatalyst, Py–BT–Th, were first analysed by UV-visible spectroscopy, as shown in Fig. 3a. UV-vis analysis showed that the catalyst absorbed in the range of 200–470 nm, with the absorption maximum appearing at 406 nm in acetonitrile. This peak could be assigned to the intramolecular charge transfer transition (ICT) from the donor to the acceptor unit, based on the reported literature for a similar kind of benzothiadiazole-based compounds.^62,63 High molar extinction coefficient (ε = 12.27 × 10³ M⁻¹ cm⁻¹) showcases the excellent light-harvesting ability of the catalyst in the visible region. To further confirm the ICT characteristics of the catalyst, we studied the effect of solvents with a range of polarity on the absorption and photoluminescence properties of the catalyst (see Fig. S5a and S5b). The absorption maxima at 406 nm exhibited a slight change when the solvent polarity varied. In contrast, the fluorescence maxima appearing at 530 nm in acetonitrile solvent manifested a red shift when the solvent polarity varied from low to high. The Lippert–Mataga plot demonstrated a relatively high Stokes shift with increasing solvent polarity, indicating the ICT character of that peak, as shown in Fig. S5c. The optical bandgap of Py–BT–Th was estimated to be 2.77 eV from the Tauc plot shown in Fig. 3a inset, as transformed from the UV-visible spectrum.

Fig. 3 (a) UV-visible spectrum of the Py–BT–Th catalyst in acetonitrile solvent. Inset: Tauc plot for the HOMO–LUMO energy gap calculation. (b) Experimentally derived HOMO–LUMO band position alignment of the catalyst and the oxidation potential of TEA, with partially reduced oxygen species potentials from literature. (c) Photoluminescence spectra of the Py–BT–Th catalyst at different TEA concentrations. (d) Photoluminescence spectra of the Py–BT–Th catalyst in acetonitrile solvent under different atmospheres at λ_ex = 406 nm. (e) PL lifetime spectra of the Py–BT–Th catalyst at λ_em = 530 nm. (f) Photocurrent response data of the acceptor (BT) and D–A-type photocatalyst (Py–BT–Th).

The redox potential of the catalyst from the HOMO–LUMO band position was determined by cyclic voltammetry. Catalyst Py–BT–Th exhibited oxidation and reduction potentials at +1.92 V and −0.88 V vs. NHE, respectively (see Fig. S6c). The appropriate alignment of the redox potential made the catalyst suitable for the photocatalytic H₂O₂ production through ORR and the simultaneous oxidation of the sacrificial agent. The oxidation potential of the catalyst was adequate for the oxidation of the chosen sacrificial agent, TEA, to TEA⁺ (+0.85 V vs. NHE) by the photogenerated holes (h⁺), and the reduction potential was also enough for ORR O₂/˙O₂⁻ (−0.35 V vs. NHE), as shown in Fig. 3b.⁶⁴

The exciton separation efficiency of the catalyst was analyzed through photoluminescence and time-resolved fluorescence spectroscopy; the higher the photoluminescence intensity, the greater the rate of recombination of the electron–hole pair. The Py–BT–Ph catalyst in acetonitrile showed high fluorescence intensity at 530 nm in a nitrogen atmosphere, establishing the strong propensity of the recombination of the electron–hole pair. However, in the presence of air and oxygen atmosphere, there was a decrease in the intensity, as shown in Fig. 3d. These observations indicated that there may be some extent of separation of exciton in the presence of oxygen and air atmosphere through either the oxygen reduction reaction (ORR) or triplet–triplet energy transfer to oxygen molecules in the excited state or both for the catalyst. The time-resolved fluorescence measurement further supported these results, as the exciton lifetime was shorter in the oxygen and air atmosphere, showing the facilitated exciton separation, as depicted in Fig. 3e. Photocurrent measurements were performed for the acceptor, benzothiadiazole (BT), unit and the D–A-type photocatalyst, Py–BT–Th (shown in Fig. 3f). Py–BT–Th exhibited a markedly enhanced photocurrent density relative to the only acceptor (BT) unit. The pronounced increase in photocurrent density confirmed that the combination of the donor and acceptor moieties significantly facilitated charge-carrier generation. To further improve exciton dissociation in the Py–BT–Th organic photocatalyst, we have utilized triethylamine (TEA) as a sacrificial agent, which demonstrated a substantial and steady decrease in the luminescence intensity with increasing concentration (shown in Fig. 3c). TEA consumed the holes and reduced the electron–hole pair recombination significantly, which was consequently manifested by the high Stern–Volmer quenching constant, K_sv = 41 M⁻¹ (shown in Fig. S5d).

Photocatalytic H₂O₂ synthesis by Py–BT–Th

Photocatalytic activity of the as-synthesized catalyst, Py–BT–Th, was estimated on the H₂O₂ production in an air atmosphere in a toluene–water two-phase system under visible light and natural sunlight irradiation, using triethylamine (TEA) as a sacrificial donor (digital photograph of the reaction setup is shown in Fig. S2, SI). Then, we systematically investigated the photocatalytic efficiency by varying various parameters, such as the toluene-to-water ratio, the amount of sacrificial agent used, and catalyst concentration. The amount of H₂O₂ produced was determined using iodometry by observing the change in absorbance at 352 nm; the detailed procedure is provided in the SI.^60,65 Fig. 4a demonstrates the effect of the toluene-to-water ratio on the photocatalytic activity. We found that 10 µL of sacrificial agent was sufficient to generate H₂O₂ efficiently under 2 hours of visible light irradiation (Fig. 4b), and subsequently, the catalyst dosage for the H₂O₂ production was also optimized (shown in Fig. 4c). Under the optimized condition of a toluene : water (2 mL : 8 mL) solvent system with 10 µL of TEA and 100 µg of the Py–BT–Th catalyst under mechanical stirring and 2 min of oxygen bubbling under a closed condition, the H₂O₂ production rates as high as 107.18 mmol g⁻¹ h⁻¹ and 61.51 mmol g⁻¹ h⁻¹ were observed with natural sunlight (average intensity: 66.5 mW cm⁻²) and a 20 W white LED bulb (intensity: 5.32 mW cm⁻²) as light sources for 2 hours, respectively. The observed H₂O₂ production rate was high, and it surpassed those of several reported photocatalysts based on benzothiadiazole and other materials (refer to Tables S1 and S2 and Fig. 4e). Afterwards, the time-dependent H₂O₂ production was examined under optimized photocatalytic conditions, resulting in the accumulation of H₂O₂ in the aqueous phase, with the irradiation time prolonged, as shown in Fig. 4d.

Fig. 4 (a) Effect of the toluene-to-water ratio on the photocatalytic activity. Reaction conditions: 100 µg of catalyst, 10 µL of TEA, and 2 h LED bulb irradiation time. (b) Effect of amount of triethylamine on the photocatalytic activity. Reaction conditions: 100 µg of catalyst, 2 h irradiation time, toluene : water (2 mL : 8 mL). (c) Effect of the catalyst dosage on the H₂O₂ production rate. Reaction conditions: 10 µL of TEA, toluene : water (2 mL : 8 mL), 2 h irradiation time. (d) Time-dependent H₂O₂ production. Reaction conditions: toluene : water (2 mL : 8 mL), 10 µL of TEA, 100 µg of catalyst. (e) Comparison of the photocatalytic H₂O₂ production rate with those of reported benzothiadiazole-based photocatalysts and other photocatalytic materials. Light source: 20 W white LED bulb (intensity: 5.32 mW cm⁻²) and natural sunlight (average intensity: 65.5 mW cm⁻²). All optimization reactions were performed under an air atmosphere and without any mechanical stirring. The error bars indicate the standard deviation of the three independent replicate experiments.

Control experiment, scavenger and EPR studies

We conducted several control experiments to elucidate the photocatalytic mechanistic pathway (Fig. 5a). First, we examined the H₂O₂ production under a nitrogen atmosphere, where the catalyst exhibited a very low H₂O₂ production rate. In contrast, under an oxygen atmosphere and without mechanical stirring, there was a stark rise in the H₂O₂ production rate. This indicated that oxygen was predominantly involved in the H₂O₂ production pathway. Subsequently, H₂O₂ synthesis was performed under an oxygen atmosphere with continuous stirring, as shown in Fig. 5a. A further increase in photocatalytic efficiency was observed, indicating that agitation facilitated the dispersion of the toluene phase into the aqueous phase. Subsequently, the efficient dispersion enhanced the interfacial contact between droplets of water and toluene, containing the photocatalyst and sacrificial donor. This improved mass transfer efficiency across interfaces and promoted more efficient H₂O₂ formation. The advantage of the biphasic system over the monophasic system was demonstrated by the control experiments performed in both the monophasic (toluene only) and the biphasic (toluene/water) systems. The results showed that a significantly high amount of H₂O₂ was generated in the biphasic system, and the amount was nearly double that produced in the monophasic system, as illustrated in Fig. S3. The importance of the photocatalyst was also proven, as without the catalyst, absolutely no production of H₂O₂ was observed. Moreover, no photocatalytic activity was observed without a sacrificial agent, indicating that the catalyst was inactive for the water oxidation reaction (WOR); H₂O₂ production solely occurred through the oxidation of triethylamine (TEA) by photogenerated holes. The lack of activity of the catalyst in the dark indicated the necessity of light to drive the photocatalysis process. After the set of control experiments, we performed the scavenger study to understand the photocatalytic H₂O₂ generation mechanism (shown in Fig. 5b). In the presence of an electron scavenger, AgNO₃, and a hole scavenger, KI, a decrease in the hydrogen peroxide production rate for both cases suggested that photocatalytic H₂O₂ production occurred due to the photogenerated electron–hole pair. Then, we added the p-benzoquinone (PBQ) as a trapping agent for superoxide radical anion (˙O₂⁻); there was a 65% decrease in the catalytic activity compared with that of the control reaction. The result indicated that ˙O₂⁻ was the key intermediate for the photocatalytic H₂O₂ generation via an indirect two-step single-electron ORR pathway. In the presence of a singlet oxygen (¹O₂) scavenger, 1,4-diazabicyclo[2.2.2]octane (DABCO), though to a low extent, 31% decrease in the activity was noted, demonstrating that H₂O₂ production also occurred via a ¹O₂ intermediate. However, the singlet oxygen was not the dominant intermediate in comparison with the superoxide radical anion (˙O₂⁻), as the relative contribution indicated that the electron transfer pathway made a major contribution over the energy transfer pathway. The generation of the superoxide radical anion and singlet oxygen as intermediates during the H₂O₂ synthesis was also confirmed via in situ EPR spectroscopy experiment, using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and 2,2,6,6-tetramethyl-4-piperidone (TEMP) as trapping agents of ˙O₂⁻ and ¹O₂, respectively (see Fig. 5c and d).

Fig. 5 (a) H₂O₂ production rate under different control conditions. Control 1 : 100 µg of catalyst, 10 µL of TEA, 2 hours, toluene : water (2 mL : 8 mL), 20-W white LED bulb, under air atmosphere, without stirring. Control 2 : 100 µg of catalyst, 10 µL of TEA, 2 hours, toluene : water (2 mL : 8 mL), natural sunlight, under air atmosphere, without stirring. (b) H₂O₂ production under various scavengers. Control reaction: 100 µg of catalyst, 10 µL of TEA, 2 hours, toluene : water (2 mL : 8 mL), 20-W white LED bulb, under atmospheric air, without stirring. (c and d) In situ EPR spectra of superoxide radical anion (˙O₂⁻) and singlet oxygen (¹O₂). [No reaction (N.R.)]. The error bars indicate the standard deviation of the three independent replicate experiments.

Reusability, recycling and scale-up study

The reusability of the Py–BT–Th catalyst was tested for five consecutive cycles, though the catalytic process was purely homogeneous (as shown in Fig. 6c, and the complete procedure of reusability is explained in the SI). The catalyst exhibited moderately good reusability with an 18% reduction in activity compared with the fresh catalytic cycle. In addition, the long-term photocatalytic activity of the catalyst was assessed over 12 hours (shown in Fig. S4a), and the catalyst exhibited H₂O₂ production of ∼204 mmol g⁻¹ after 12 hours. We recovered the catalyst (procedure provided in the SI) and performed UV-visible and photoluminescence spectroscopic analyses. The analyses confirmed the stability of the catalyst after 12 hours of long-term photocatalytic activity, as no change was observed in the UV-visible and fluorescence spectra (shown in Fig. S4b–e) before and after 12 hours of photocatalysis. The high photocatalytic efficiency and stability of Py–BT–Th provided a promising strategy for H₂O₂ synthesis and its collection via a two-phase system. Then, we performed the onsite scale-up reaction, and the catalyst produced (2270 µM) 0.32 L of aqueous H₂O₂ solution within 2 hours under natural sunlight irradiation. This was conducted to probe the real-world application of the toluene–water biphasic system. After the scale-up reaction, we recovered the catalyst and performed ¹H-NMR analysis, which confirmed that the catalyst retained its structural integrity after the scale-up reaction (as shown in Fig. 6a and b). This work may present a potentially transformative approach for industrial-level photocatalytic H₂O₂ production using a small organic photocatalyst in a reusable homogeneous two-phase system (schematic for onsite scale-up H₂O₂ production is shown in Fig. 7a).

Fig. 6 (a and b) ¹H-NMR spectra of the recycled Py–BT–Th catalyst before and after scale-up photocatalytic H₂O₂ synthesis. (c) Catalyst reusability tests up to five cycles. (d) Left side of the photograph showing the H₂O₂ detection using a hydrogen peroxide test strip and the right side of the photograph showing the H₂O₂ production in toluene–water two-phase reaction under natural sunlight.

Fig. 7 (a) Schematic of the on-site scale-up H₂O₂ production (Inset: Photograph of the scale-up reaction). Reaction conditions: 4 mg of catalyst, 400 µL of TEA, 2 hours of natural sunlight, Toluene : water (80 mL : 320 mL), 5 minutes of oxygen bubbling with mechanical stirring. (b) Schematic of the photocatalytic H₂O₂ synthesis mechanism using the Py–BT–Th catalyst.

Photocatalytic H₂O₂ synthesis mechanistic insight

With these insights gathered by the control and scavenger studies, based on the reported literature,^66–70 the plausible photocatalytic H₂O₂ generation mechanism using the Py–BT–Th catalyst, following both electron and energy-transfer processes, is summarized in Fig. 7b. Py–BT–Th, under photoexcitation conditions, generated the electron–hole pair, which was efficiently separated and transferred over the donor–acceptor part of the catalyst due to the D–A-type structure. The spatially separated electron–hole pairs act as an independent redox centre to drive oxidation and reduction reactions concurrently. The photogenerated excited-state (S₁) electron followed two pathways: (1) the excited electron was transferred to the molecular oxygen (³O₂) and generated the superoxide radical anion through the oxygen reduction reaction (ORR, O₂ + e⁻ → ˙O₂⁻) process, while the holes oxidized the triethylamine (TEA) to triethylamine radical cation (TEA˙⁺). (2) The excited electron was transferred to the triplet state (T₁) via an intersystem crossing. Following that, the triplet molecular oxygen (³O₂) interacted with the excited triplet state (T₁) of photocatalyst Py–BT–Th and generated the singlet oxygen (¹O₂) via an energy-transfer pathway. Then, the intermediate singlet oxygen (¹O₂) may also accept an electron and get converted into the superoxide radical anion (˙O₂⁻). Subsequently, the superoxide radical anion undergoes stepwise proton and electron abstraction and finally yields the hydrogen peroxide (H₂O₂) molecule in both pathways, as illustrated in Fig. 7b.

Conclusion

In this work, we developed a new toluene–water biphasic system to synthesize H₂O₂ photocatalytically, employing the benzothiadiazole-based small molecule, Py–BT–Th, as a catalyst. Photocatalyst Py–BT–Th, in the presence of triethylamine as a sacrificial donor, exhibited a highly efficient H₂O₂ production rate of 107.18 mmol g⁻¹ h⁻¹, with a turnover frequency of ∼32 h⁻¹, which was achieved under 2 hours of natural-sunlight irradiation in an oxygen atmosphere. The high efficacy of the photocatalyst was attributed to its donor–acceptor-type (D–A-type) push–pull structure, promoting exciton separation and migration, as well as excellent light harvesting, and was characterised by its high molar extinction coefficient. Control experiments, in situ EPR, and scavenger studies revealed that photocatalytic H₂O₂ production occurred via two mechanistic pathways: a two-step one-electron oxygen reduction reaction (ORR) and singlet oxygen formation; however, both involved the superoxide radical anion (˙O₂⁻) as the key intermediate. The toluene–water two-phase system efficiently suppressed the accumulation of holes and superoxide radical anion in the reaction medium and reduced the decay of the photocatalyst and H₂O₂ significantly, ultimately leading to a high H₂O₂ production rate. The photocatalyst, Py–BT–Th, exhibited good reusability and stability over a 12-hour reaction time, and the easy separation of the catalyst from the H₂O₂ aqueous solution made this methodology for the toluene–water biphasic system essentially viable. The easy separation of the catalyst even shaped the homogeneous catalyst into a recyclable form. Altogether, this work provides a successful example of a benzothiadiazole-based small organic photocatalyst for H₂O₂ synthesis in a biphasic manner through reaction media engineering, which, in the future, can lead to a new perspective towards sustainable and green H₂O₂ synthesis.

Author contributions

A. S. conceived the idea. A. S. and S. R. conceptualized and designed the overall research work. A. S. and A. M. S. performed the experiments, formal analysis, and data acquisition. A. S. did the writing and editing of the manuscript. S. R. did the supervision, funding acquisition, formal analysis, data acquisition, and writing and editing of the manuscript. [Ajeet Singh (A. S.), Aditya Mohan Shukla (A. M. S.), Saumi Ray (S. R.)].

Conflicts of interest

The authors declare no conflict of interest.

Data availability

The data supporting the findings of this article have been included as part of the supplementary information (SI). Supplementary information: instrumentation, scheme for Py-BT-Th synthesis, H₂O₂ detection and calibration plot & TOF calculations, digital photograph of the reaction setup, biphasic vs monophasic H₂O₂ synthesis, reusability, scale-up, long-term photocatalytic and recyclability procedure, PL, UV-Visible spectra, Lippert Matage & Stern-Volmer plot of Py-BT-Th, cyclic voltammogram, ¹H and ¹³C NMR spectra of the Py-BT-Br and Py-BT-Th, HRMS data of Py-BT-Br and Py-BT-Th, FTIR, SEM images of Py-BT-Th,¹H NMR of the aqueous layer and water contact angle measurement, comparison of photocatalytic H₂O₂ production rate with literature. See DOI: https://doi.org/10.1039/d6ey00008h.

Acknowledgements

A. S. and A. M. S. acknowledge the institute research fellowship of BITS Pilani, Pilani campus. S. R. is thankful to the BITS-Pilani CDRF-Saumi Ray-2023 (C1/23/165) and DST-SERB (CRG/2021/004126) projects for financial assistance. The authors acknowledge the DST-FIST for the HRMS and other instrumentation facilities at BITS-Pilani, Pilani campus. They would like to thank the CRF-IIT Delhi for the EPR facility. The authors gratefully acknowledge Dr Mrinmoyee Basu for assisting with the light intensity and photocurrent response measurements. The authors would like to thank Dr Avik Kumar Pati for the helpful discussion on the mechanistic part. They would like to thank the Department of Chemistry, BITS Pilani, Pilani Campus, for the infrastructure and instrumentation facilities.

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