A pair of strongly reductive and oxidative photocatalysts for the general upcycling of biomass derivatives and plastic wastes

Abstract

Tailoring photocatalysts to achieve both strongly reductive and oxidative properties from a common scaffold remains challenging. Herein, we report the development of a pair of photocatalysts, isothiatruxene (ITS) and isosulfonyltruxene (ITSO₂), by modulating the valence states of heteroatoms. ITS exhibits highly reducing power, facilitating selective cleavage of C–O bonds in biomass derivatives despite their negative redox potentials. Upon oxidation of ITS, the resultant ITSO₂ demonstrates strongly oxidizing capacity, enabling metal-free and acid-free upcycling of plastic wastes even with their high redox potentials. By immobilizing ITS and ITSO₂ on polystyrene and oxidized lignin supports, respectively, we have developed recyclable photocatalysts that drive multiple catalytic cycles with high efficiency. Gram-scale upcycling of plastic is achieved by integrating photoredox catalysis with flow chemistry. Mechanistic studies reveal that the excited states of ITS and ITSO₂ can directly activate inert substrates, correlating with their strong redox properties. The newly introduced pair of photocatalysts, characterized by their metal-free nature, concise synthesis via trimerization, and dual photocatalytic capabilities encompassing both strongly reducing and oxidizing properties, show great potential for a wide range of applications. Furthermore, this study presents a sustainable catalytic strategy for synthesizing high-value aromatic compounds directly from biomass derivatives and plastic wastes.

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Introduction

Photoredox catalysis has emerged as a powerful tool for enabling previously challenging transformations through the selective activation of compounds via radical processes.^1–11 Central to this approach are photocatalysts (PCs), which, upon irradiation, attain short-lived yet highly reactive excited states that mediate subsequent single-electron transfer (SET) events. The development of PCs with highly reducing^12–34 or oxidizing^35–41 capacities is of particular significance due to their ability to activate inert substrates. However, modulating PCs to simultaneously achieve both high reduction and oxidation potentials from a common scaffold is challenging. In this regard, altering the ligands around the metal center represents a flexible strategy to tune the redox properties.⁴² For instance, modifying electron-donating ligands [e.g., fac-Ir(ppy)₃: E_1/2(PC˙⁺/PC*) = −1.73 V vs. SCE] or electron-withdrawing ligands {e.g., Ir[dF(CF₃)ppy]₂(dtbbpy)PF₆: E_1/2(PC*/PC˙⁻) = +1.21 V vs. SCE} around iridium complexes can access ambivalent reducing and oxidizing capacities (Fig. 1A). Alternatively, the photophysical properties of organic PCs can be readily tuned by altering the donor or acceptor substituents around their structural cores.⁴³ For example, systematic modifications of 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) have identified that the excited-state redox potentials for the strongly reducing 3DPA2FBN and the potent oxidizing 3CzClIPN are −1.60 V and + 1.56 V (vs. SCE), respectively (Fig. 1B). Despite these advances, the development of PCs that can simultaneously achieve even stronger reducing and oxidizing capacities for activating more inert substrates remains underexplored.

Fig. 1 Strategies for tailoring photocatalysts to achieve both strongly reductive and oxidative properties (potentials are given versus the saturated calomel electrode (SCE)).

The over-reliance on non-renewable fossil fuels has led to rapid resource depletion and significant environmental issues. Meanwhile, the large-scale accumulation of plastic wastes in landfills and ecosystems has caused severe environmental, safety, and health problems, as well as substantial resource waste. Therefore, the development of approaches for the sustainable utilization of renewable biomass^44–49 and plastic wastes^50–67 has become a critical priority to address these alarming dilemmas. However, the inert chemical bonds within biomass and plastics pose significant challenges for their upcycling into value-added chemicals. For instance, lignin, a key biomass component, contains multiple interunit linkages, with the β-O-4 bond (BDE = 274 kJ mol⁻¹) being predominant in both softwood and hardwood varieties.⁶⁸ Traditional methods for lignin depolymerization suffer from stringent conditions and intensive energy input. Similarly, polystyrene (PS), a major plastic, is resistant to degradation due to its strong C–C bonds (BDE = 356 kJ mol⁻¹).⁶⁹ Conventional thermochemical treatment processes often require precious metal catalysts and harsh reaction conditions, such as high pressure or elevated temperature. Recently, photocatalysis has emerged as an attractive and effective tool to convert PS into small valuable molecules. However, most photocatalytic upcycling methods employ metal-based catalysts or acid additives to facilitate the degradation process. Therefore, the development of metal-free and environmentally friendly strategies for the upcycling of biomass derivatives^70,71 and plastic wastes^72–88 is highly desirable.

Herein, we introduce a pair of highly reductive and oxidative photocatalysts, isothiatruxene (ITS) and isosulfonyltruxene (ITSO₂), which are derived from the same starting materials through modulation of heteroatom valence states (Fig. 1C). Isothiatruxene (ITS) is readily synthesized via direct trimerization of benzothiophene, and a single-step oxidation with m-CPBA converts it to isosulfonyltruxene (ITSO₂).^89–91 Photophysical characterization reveals that ITS possesses strong reducing power , whereas ITSO₂ exhibits significant oxidizing ability . Notably, the combined redox potential window of ITS and ITSO₂ exceeds that of conventional photocatalysts. To address the challenges in upcycling biomass derivatives and plastic wastes, we explored whether the combination of ITS and ITSO₂ could provide a general strategy for cleaving inert bonds in these materials under mild visible-light conditions. This approach would enable the direct synthesis of valuable aromatic building blocks, such as acetophenone, phenol, and benzoic acid, from biomass derivatives and plastic wastes. It was found that ITS photocatalysis can efficiently cleave C–O bonds in oxidized lignin models, vanillin-derived crosslinked polymer, and native oxidized lignin at room temperature under reductive conditions, despite the highly negative redox potentials of ketone moieties [e.g., E_1/2 = −2.11 V vs. SCE for acetophenone].⁹² Similarly, ITSO₂ photocatalysis converts polystyrene and real-world plastics into benzoic acid under oxidative conditions, even with the high redox potentials of arene groups [e.g., E_1/2 = +2.36 V vs. SCE for toluene].⁹² The recyclability of catalysts is achieved by introducing polystyrene-supported ITS and oxidized lignin-supported ITSO₂, thereby enhancing the practicality. Importantly, this protocol is amenable to continuous flow processing, facilitating gram-scale upcycling under ambient conditions. Mechanistic studies correlate with the strongly reducing and oxidizing properties of this pair of PCs, in which inert substrates can be directly activated by their excited states.

Results and discussion

Both isothiatruxene (ITS) and isosulfonyltruxene (ITSO₂) were rapidly synthesized from the same starting material. While ITS was prepared through 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)-enabled trimerization of benzothiophene, subsequent oxidation of ITS with meta-chloroperoxybenzoic acid (m-CPBA) afforded ITSO₂ in a single step (Fig. 2A).⁹³ X-ray crystallographic analysis further confirmed the heteroatom valence states in ITSO₂.⁹³ The UV/vis absorption spectrum of ITS exhibits a maximum at 389 nm, with a tailed absorption edge extending into the visible region. Comparatively, ITSO₂ shows enhanced visible-light absorption with a notable bathochromic shift (Fig. 2B). The emission maximum of ITSO₂ was consistently more red-shifted than that of ITS (Fig. 2C). The intersection points of the normalized absorbance and emission spectra were determined to be 394 nm for ITS and 405 nm for ITSO₂, corresponding to zero–zero vibrational state excitation energies (E_0,0) of 3.15 eV and 3.06 eV, respectively (Fig. S4 and S12 in the ESI†). Cyclic voltammetry measurements provided the ground-state potentials (E_1/2) of ITS and ITSO₂ (Fig. S5–S8 and S13–S15 in the ESI†). Based on these potentials and the E_0,0 values, the excited-state oxidation potential of ITS and the excited-state reduction potential of ITSO₂ were calculated to be −2.17 V and +2.39 V, respectively.⁹³ The fluorescence lifetime decay curves revealed excited-state lifetimes of 1.23 ns for ITS and 1.70 ns for ITSO₂ (Fig. 2D). Collectively, these results demonstrate that this pair of photocatalysts exhibits both strongly reducing and oxidizing capacities through the simple tuning of heteroatom valence states.

Fig. 2 Development of a pair of strongly reductive and oxidative photocatalysts: ITS and ITSO₂.

Owing to the strongly reducing ability of ITS, its capacity to cleave oxidized β-O-4 lignin models was investigated (left column, Fig. 3A). With DIPEA and HCOOH as additives, the reductive C–O bond cleavage of model substrate (1a) proceeded smoothly in toluene under blue LED irradiation, affording the expected products p-methoxyacetophenone (2a) and 4-biphenylol (3a) in 89% yield and 90% yield, respectively (entry 1). Notably, the choice of solvent significantly influenced the outcome. While dichloromethane provided comparable results, inferior yields of 2a and 3a were achieved in the presence of methanol (entries 2 and 3). When the solvent was changed to dimethyl sulfoxide, acetonitrile, acetone, or ethyl acetate, the p-methoxyacetophenone (2a) product underwent a secondary pinacol-type coupling, resulting in the formation of 1,2-diol 4a instead (entries 4–7). The absence of either HCOOH or DIPEA led to slightly decreased yields of 2a and 3a (entries 8 and 9). Control experiments further demonstrated that both ITS and visible light are essential for this transformation (entries 10 and 11).

Fig. 3 Photocatalytic upcycling of biomass derivatives enabled by ITS.

Under the optimized reaction conditions, the scope of β-O-4 ketones was evaluated (right column, Fig. 3A). When oxidized β-O-4 lignin models bearing diverse substituents (–Ph, –Cl, –^tBu, –OMe) at the para-position of the phenol moieties were tested under the photocatalysis of ITS, selective C–O bond cleavage was uniformly achieved, delivering p-methoxyacetophenone (2a) and functionalized phenols (3a–d) in good yields (entries 1–4). Similarly, a naphthalene-derived substrate 1e underwent efficient C–O bond cleavage, affording p-methoxyacetophenone (2a) and naphthalen-2-ol (3e) in 90% and 88% yields, respectively (entry 5). This method is also applicable to substrates (1f–h) bearing di-electron-donating MeO– groups on the aromatic ketones, despite their more negative redox potentials (entries 6–8). Substrates 1i–l were tolerated, leading to the corresponding acetophenones and phenols with high efficiency (entries 9–12).

The catalytic utility of ITS was subsequently probed in the degradation of biomass-derived crosslinked polymer (Fig. 3B). As an elegant precedence, biomass-derived crosslinked polymer 5, which was designed and synthesized from vanillin, can be programmed at ∼300 nm to degrade with ∼60% recovery of the monomer 4.⁹⁴ This method mainly relied on the excited state reactivity of ketone moieties under ultraviolet light. In a complementary fashion, we envisioned that ITS catalysis may provide a catalytic and mechanistically distinct single-electron-transfer (SET) approach by directly harnessing abundant visible light. To our delight, by utilizing ITS as the photocatalyst, DIPEA as the stoichiometric reductant in concert with formic acid, and dichloromethane as the solvent, the desired C–O cleavage product 4 was obtained in 75% yield under irradiation with blue LEDs (Fig. 3B). As such, this method offers a complementary sustainable strategy to address the issue of degradability.

Encouraged by these results, we sought to explore the potential of ITS to catalyze the depolymerization of native oxidized lignin (6), which is particularly challenging due to its highly robust and complex structure (Fig. 3C). Irradiating a reaction mixture containing 20.0 mg of oxidized lignin, 2.0 mg of ITS photocatalyst, and DIPEA/HCOOH additives in dichloromethane with blue LEDs for 48 hours resulted in an organic-soluble fraction, along with an insoluble solid fraction. Gratifyingly, gas chromatography-mass spectrometry (GC-MS) analysis of the soluble fraction identified at least three monomeric products: 3-(4-hydroxy-3,5-dimethoxyphenyl)acrylaldehyde (M1, 0.87 wt%), 4-ethyl-2,6-dimethoxy-3-methylphenol (M2, 0.2 wt%), and 4-ethyl-2,6-dimethoxyphenol (M3, 0.1 wt%).⁹³ The formation of these products can be attributed to the cleavage of β-O-4 linkages within the oxidized lignin structure. Consistently, heteronuclear single quantum coherence (HSQC) spectroscopy of the insoluble solid fraction showed the disappearance of characteristic signals corresponding to β-O-4 linkages, further confirming their cleavage during the depolymerization process.⁹³

On the other hand, ITSO₂ was attempted for the upcycling of polystyrene (PS) due to its strongly oxidizing capacity (Fig. 4A). Unlike previous studies using HAT photocatalysts to abstract hydrogen atoms from the benzylic position for the subsequent cleavage of PS,⁶³ photocatalysis with ITSO₂ may offer a complementary upcycling strategy by directly oxidizing non-activated arene groups. Initially, commercial PS, with a mass average molecular weight (M_w) of 560 966 g mol⁻¹, a number average molecular weight (M_n) of 198 240 g mol⁻¹, and a polydispersity index (PDI) of 2.83, was tested (left column, Fig. 4A). Irradiating PS in the presence of ITSO₂ in 1,2-dichloroethane under an oxygen atmosphere using blue LEDs for 24 hours resulted in a transparent, clear solution, from which benzoic acid was isolated in 57% yield (entry 1). The gel permeation chromatography (GPC) analysis of the reaction mixture after photocatalytic upcycling confirmed the complete degradation of PS (Fig. S19 in the ESI†). Optimization of the reaction conditions revealed that solvents play a crucial role in this upcycling process. While dichloromethane, ethyl acetate, and acetone were effective solvents, no reaction occurred when acetonitrile, dimethyl sulfoxide, methanol, tetrahydrofuran, or toluene were used (entries 2–5). Control experiments confirmed that both the photocatalyst and visible light are essential (entries 6 and 7). This upcycling protocol is appealing due to its extremely mild reaction conditions, high selectivity for converting PS into benzoic acid, and the absence of any metals or acids.

Fig. 4 Photocatalytic upcycling of plastic wastes enabled by ITSO₂.

To demonstrate the generality of the oxidative upcycling strategy, several plastics were investigated under the optimized reaction conditions (right column, Fig. 4A). General-purpose polystyrene (GPPS), a rigid and transparent polymer, was efficiently converted into benzoic acid with 56% yield under the photocatalysis of ITSO₂. Expandable polystyrene (EPS), an elastic polymer containing azodicarbonamide as a foaming agent for enhanced impact resistance, underwent degradation to afford benzoic acid in 56% yield. High-impact polystyrene (HIPS), a graft copolymer composed of styrene and butadiene with notable impact strength, was successfully transformed into benzoic acid with 56% yield. Styrene–butadiene–acrylonitrile copolymers, including styrene–acrylonitrile (SAN) and acrylonitrile–butadiene–styrene (ABS), are widely utilized for their superior heat resistance and stability. Due to their chemical recalcitrance, the degradation of SAN and ABS did not occur under standard reaction conditions. However, when 4,5,6,7-tetrachloro-2-hydroxy-isoindole-1,3-dione (TCNHPI) was employed as an additive, SAN and ABS were effectively upcycled into benzoic acid in 54% yield and 48% yield, respectively. It was proposed that TCNHPI could serve as a hydrogen atom transfer agent to facilitate the degradation process (Tables S4, S5 and Fig. S33 in the ESI†).⁹³ Intriguingly, this protocol is applicable to the upcycling of mixed plastics. A mixture of PS, GPPS, EPS, HIPS, SAN, and ABS can be uniformly converted into benzoic acid in 50% yield under the photocatalysis of ITSO₂ (Fig. 4A).

Next, PS waste from our daily life was investigated (Fig. 4B). Despite the presence of various additives in real-life plastics, this upcycling method demonstrated excellent compatibility with a wide range of plastic waste materials. As illustrated in Fig. 4B, all selected real-life plastics underwent smooth degradation into benzoic acid with high selectivity. Specifically, PS waste, including laboratory weighing boats, yogurt boxes, rulers, and plastic forks, was successfully converted into the desired benzoic acid under standard conditions (50–55% yields). Additionally, EPS waste, such as foam boxes and shockproof foam, was effectively processed, producing benzoic acid in 57% yield and 56% yield, respectively. ABS waste from items like magic cubes, table tennis balls, plastic fans, and toy vehicles was also compatible with this protocol, delivering benzoic acid in 45–49% yields. Similarly, SAN waste from plastic cup was upcycled into benzoic acid in 46% yield.

The feasibility of ITSO₂ photocatalysis for aerobic degradation of commercial PS, mixed PS, and PS waste from daily life has been demonstrated. To explore the scalability of this upcycling protocol, a continuous flow photoreactor was implemented (Fig. 4C). Using this flow technology, 1.04 g of commercial polystyrene was successfully converted into benzoic acid in 50% yield. More intriguingly, a gram-scale mixture of real-life plastics, including yogurt boxes, rulers, plastic forks, foam boxes, and shockproof foam, underwent upcycling to benzoic acid in 41% yield. These results clearly demonstrate the practicality and scalability of the ITSO₂ photocatalysis protocol for the upcycling of plastic wastes.

To achieve the recyclability, the immobilization of ITS and ITSO₂ on polymer supports has been explored. With the growing emphasis on the valorization of biomass derivatives and plastic waste, aforementioned studies have focused on converting oxidized lignin and polystyrene into value-added small molecules. As an alternative approach for repurposing these materials, we hypothesize that polystyrene and oxidized lignin can be employed as supports for the development of recyclable photocatalysts. To this end, ITS and ITSO₂ were immobilized on polystyrene and oxidized lignin supports, respectively.⁹³ As depicted in Fig. 5A, ITS@PS was synthesized through the substitution reaction of ITS-derived carboxylic acid (14) with polymer (chloromethyl)polystyrene (15). Pleasingly, sequential photocatalytic cleavage of five varieties of oxidized β-O-4 lignin models was achieved using a single batch of ITS@PS added at the beginning of the reaction sequence, with comparable efficiency to that of homogeneous ITS. Notably, the heterogeneous photocatalyst can be easily separated from the reaction mixture via filtration and washing, then dried and reused directly without further purification.⁹³ Similarly, ITSO₂@oxidized lignin was prepared via the condensation reaction of ITSO₂-derived carboxylic acid (16) with primary alcohols present in oxidized lignin (6). The resultant ITSO₂@oxidized lignin was found to catalyze the aerobic degradation of commercial PS for at least five cycles without an apparent decrease in the yields (Fig. 5B). Thus, ITS@PS and ITSO₂@oxidized lignin represent a new class of recoverable and reusable photocatalysts that enable the upcycling of biomass derivatives and plastic wastes in a sustainable fashion.

Fig. 5 Development of recyclable polymer-supported ITS and ITSO₂.

To elucidate the mechanism, a series of Stern–Volmer quenching experiments were conducted based on each component involved in the cleavage of the oxidized β-O-4 lignin model 1a (Fig. 6A). The results demonstrated that the excited state of ITS was significantly quenched by the ketone-containing substrate 1a, whereas no significant quenching was observed with DIPEA or HCOOH. These findings support the hypothesis that the excited state of ITS (ITS*) can directly facilitate SET reduction of ketone compounds, consistent with its favorable redox potential . Regarding the photocatalytic upcycling of polystyrene, isopropylbenzene (17) was used as a model substrate to probe the mechanism. Stern–Volmer quenching experiments involving 17 confirmed that the excited state of ITSO₂ can undergo SET oxidation of unactivated arenes. This is attributed to the strongly oxidizing capacity of , which contrasts with the mechanism of commonly used HAT photocatalysts.⁶³ Thus, ITS and ITSO₂ are highly reducing and oxidizing photocatalysts, respectively, capable of activating inert substrates directly. Besides, electron paramagnetic resonance (EPR) experiments were conducted using dimethylpyridine N-oxide (DMPO) or 2,2,6,6-tetramethylpiperidine (TEMP) as the free radical spin-trapping reagent (Fig. S30 in the ESI†). Upon irradiation, characteristic signals with six lines were observed, which match well with the DMPO–carbon-centered radical adduct under ITS photocatalysis. Meanwhile, the typical signals of DMPO–O₂˙⁻ and TEMP–¹O₂ were both detected, indicating the generation of superoxide radical and singlet oxygen species under ITSO₂ photocatalysis.⁹³

Fig. 6 Mechanistic studies.

As illustrated in Fig. 6B, plausible mechanism is proposed using the cleavage of oxidized β-O-4 lignin model 1a and the upcycling of polystyrene as the illustrative examples.⁹³ Upon irradiation, ITS reaches its excited state (ITS*). Oxidative quenching of ITS* by 1a generates ITS˙⁺ and the corresponding ketyl radical (I), which fragments to yield the α-carbonyl radical II and 4-biphenylol (3a) after protonation. Subsequently, DIPEA donates an electron to convert ITS˙⁺ back to ITS, thereby completing the photocatalytic cycle of ITS. Meanwhile, p-methoxyacetophenone (2a) is obtained by accepting both electron and proton from DIPEA˙⁺. On the other hand, undergoes SET oxidation of PS, affording the radical cation intermediate III and ITSO₂˙⁻. ITSO₂˙⁻ is regenerated to ITSO₂ through oxidation by O₂, thus closing the photocatalytic cycle of ITSO₂. The resultant superoxide radical anion O₂˙⁻ abstracts a proton from intermediate III to form a benzylic radical intermediate IV, which then reacts with oxygen to induce the upcycling process, ultimately yielding benzoic acid (8). Alternatively, the degradation can proceed via an energy transfer mechanism involving singlet oxygen as the key intermediate.⁷⁴

Conclusions

In summary, we have developed a pair of photocatalysts, isothiatruxene (ITS) and isosulfonyltruxene (ITSO₂), through precise modulation of the valence states of heteroatoms. These photocatalysts are distinguished by their metal-free nature, cost-effectiveness, and facile synthesis via trimerization. More importantly, ITS and ITSO₂ exhibit both strongly reducing and oxidizing capacities, with a combined redox potential window that exceeds those of conventional photocatalysts. Under ITS catalysis, photocatalytic reductive cleavage of C–O bonds in oxidized lignin models, vanillin-derived crosslinked polymer, and native oxidized lignin is achieved at room temperature. Conversely, ITSO₂ catalysis enables photocatalytic oxidative upcycling of polystyrene and real-life plastics to benzoic acid in a metal-free and acid-free fashion. The implementation of flow technology allows for gram-scale upcycling of polystyrene. In addition, the recyclability of these photocatalysts is accomplished by immobilizing ITS and ITSO₂ on polystyrene and oxidized lignin supports, respectively. Consistent with their strong redox capacities, mechanistic investigations confirm that the excited states of ITS and ITSO₂ can directly activate biomass derivatives and plastic wastes, despite the demanding redox potentials. This study not only presents a novel strategy to access two significant opposing redox properties by tuning the valence states of heteroatoms, but also demonstrates a sustainable catalytic approach for the synthesis of value-added aromatic compounds from biomass derivatives and real-life plastic wastes. It is anticipated that this pair of photocatalysts may lead to the discovery of new challenging transformations.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

X. Z. conceived the project. H. C., X. C., X. S., W.-X. S., and S.-C. C. performed the experiments. H. C., X. C., and X. Z. cowrote the paper.

Conflicts of interest

A patent has been filed on this work.

Acknowledgements

We thank the National Natural Science Foundation of China (grant no. 22071024 and 22271047), and the Natural Science Foundation of Fujian Province (grant no. 2024J01290) for generous financial support. We also thank Prof. Chenguang Liu (Fuzhou University) for the assistance with flow photoreactor. The image of a tree in the graphical abstract and Figure 1 is reproduced from H. Jiang, M. Liu, X. Lian, M. Zhu and F. Zhang, Angew. Chem. Int. Ed., 2024, 63, e202318850, https://doi.org/10.1002/anie.202318850, with permission

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Footnotes

† Electronic supplementary information (ESI) available: Experimental and copies of NMR spectra. CCDC 2412529 and 2412532. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5sc03457d

‡ These authors contributed equally to this work.

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