The research group of Xuefeng Jiang at East China Normal University has long focused on the scientific research of upgrading and degrading waste plastics. Relying on a self-built photocatalytic reaction instrumentation platform, they have established a room-temperature and ambient-pressure catalytic upgrading and recycling system covering various strongly inert polymers such as polyester, polyphenylene sulfide, polysulfone, and gem-dimethyl plastics. Recently, in collaboration with Yujie Song of Hainan University, they achieved the efficient conversion of polystyrene to high-purity benzoic acid under room-temperature, ambient-pressure, and visible light conditions using a homogeneous/multiphase synergistic dynamic enhanced photocatalysis strategy. The article was published as an open access Research Article in CCS Chemistry, the flagship journal of the Chinese Chemical Society.
Background information:
Polystyrene, with a global annual production exceeding 30 million tons, has a recycling rate of less than 1%, and 30% is landfilled, posing a risk of soil and water pollution. While its natural environmental stability has lasted for centuries, the waste generated after its use poses a serious ecological safety hazard. In its chemical molecular structure, the dense steric hindrance caused by the massive C-C and C-H superposition bonds and the numerous π-π interactions of the benzene rings means that traditional degradation methods such as pyrolysis or hydrogenolysis require extremely high energy and produce products lacking selectivity. How to achieve selective breaking of C–C/C–H bonds in polystyrene under mild conditions? How to further directionally functionalize it? How to obtain high-value-added chemicals with high selectivity? These are the bottleneck problems facing the circular economy and circular science.
Highlights of this article:
Through precise coupling and regulation of photochemistry and photophysics, the authors utilized a self-designed parallel photoreactor (PhotoSyn-YL) to systematically optimize key parameters such as catalyst, light wavelength, atmosphere, and solvent. They discovered that the activity of g-C₃N₄ doped with various metals (Ce, Au, Pd, Cu, Fe, Co, Ni, and K) was lower than that of pristine g-C₃N₄. Under optimal conditions (room temperature, ambient pressure, O₂, visible light 430-435 nm), using non-metallic g-C₃N₄ as the sole photocatalyst, the yield of polystyrene degraded to benzoic acid could reach 70%.
Under these optimal conditions, this method is applicable to various common polystyrene products in daily life: takeout containers, yogurt cups, medical devices, and laboratory consumables. The catalytic system effectively degrades these products and demonstrates good applicability to a variety of styrene-based copolymers. Notably, the copolymer degradation efficiency is highly sensitive to adjacent comonomer units, further confirming the unique advantage of this system in selectively activating the C–H bonds of polystyrene.
In terms of practical application potential, this system demonstrated excellent catalyst stability and reproducibility. In six consecutive cycles, the catalyst maintained extremely high reactivity with almost no yield decline. X- ray diffraction and Fourier transform infrared spectroscopy results before and after the reaction further confirmed that the catalyst's lattice structure and surface functional groups remained intact, without structural collapse or disintegration.
Subsequent gram-scale scale-up studies involved three-phase reactions involving solid-liquid-gas, where common photofluidic equipment could not meet experimental requirements. They utilized a self-developed cascaded micro-reactor (Photo-CSTR) (Org. Process Res. Dev. 2024, 28, 1683-1689) to precisely control the solid/liquid mixing rate and the oxygen gaseous flow rate, achieving precise synergistic matching of mass transfer, heat transfer, and light transfer. Operating under continuous flow conditions, they successfully converted 2.06 grams of real waste polystyrene into 1.15 grams of high-purity benzoic acid. Furthermore, using a large-scale automated photofluidic reactor (PhotoSyn-YX), they achieved kilogram-scale continuous degradation, validating the industrial scale-up potential of this strategy.
Typically, photocatalysts face a persistent "activity-stability paradox" after prolonged operation, but this method exhibits a "the longer it's used, the stronger it becomes" catalytic characteristic. In evaluating catalyst stability, the authors found that as the reaction proceeded (12 and 36 hours), the reaction rate gradually increased from an initial 11.60 μmol/h to 16.02 μmol/h, with the catalytic efficiency increasing by a remarkable 38%. Combined with XPS, EPR, and DFT theoretical calculations, this indicates that thermodynamically stable two-coordinate nitrogen vacancies were continuously generated in situ on the catalyst surface during the reaction.
Combining UV-Vis diffuse reflectance spectroscopy and a series of photoelectrochemical characterizations, the team comprehensively revealed the chemical mechanism of this photocatalyst's "self-evolution". These in-situ generated nitrogen defects play the role of shallow electron traps in the band structure: 1) They effectively broaden the visible light absorption range of the material, significantly narrowing the optical band gap from 2.81 eV to 2.57 eV. Band structure measurements show a significant negative shift in the conduction band bottom potential (from -0.34 V to -0.42 V), providing a more abundant thermodynamic driving force for the single-electron reduction of oxygen; 2) In terms of carrier dynamics, steady-state and transient photoluminescence spectroscopy further confirms that the introduction of defects effectively suppresses ineffective electron-hole radiative recombination, significantly shortening the average carrier lifetime from 4.19 ns to 3.52 ns, indicating that photogenerated electrons are rapidly and efficiently trapped by defect sites; and 3) The combination of significantly reduced interfacial charge transfer impedance and significantly enhanced photocurrent response, along with this series of photoelectric property optimizations, greatly promotes the rapid transfer of interfacial electrons to oxygen and accelerates the efficient generation of superoxide radicals.
Through a series of free radical capture experiments, gas chromatography-mass spectrometry (successfully capturing key chlorinated adducts), cyclic voltammetry potential analysis, and condition control experiments, the authors clearly elucidated the reaction mechanism of the system: dichloromethane (DCM) not only plays the role of a traditional solvent but also generates highly active chlorine radicals (•Cl) under the oxidation of photogenerated holes. The system thus exhibits a "spatiotemporally decoupled" catalytic operation mode: at the homogeneous level, •Cl, as a hydrogen atom transfer reagent, rapidly diffuses, successfully overcoming the mass transfer resistance at the solid-liquid interface, "penetrating" into the polystyrene polymer chain for continuous HAT, oxidation, and cleavage, thereby depolymerizing; at the heterogeneous level, the pyrolysis products of DCM simultaneously chemically etch the photocatalyst, dynamically inducing highly active two-coordinate nitrogen defects on the g-C₃N₄ solid surface. This surface reconstruction drives the "self-evolution" of the catalyst; the N defects, acting as shallow electron traps, significantly enhance the efficiency of interfacial electron transfer to O2 and the generation rate of superoxide radicals, thus comprehensively accelerating the efficient degradation of polystyrene. Homogeneous radical initiation and multiphase defect engineering work together to form this closed-loop, self-reinforcing catalytic degradation mode.
Summary and Outlook:
This research provides a feasible technical path for the low-energy-consumption and high-efficiency chemical degradation and recycling of polystyrene. The established homogeneous/multiphase synergistic dynamic interface engineering strategy also provides new ideas for catalyst design and reaction system construction in the field of photocatalytic plastics upgrading and recycling. From the development of an independently developed photoreaction instrument coupling platform to the systematic elucidation of the catalytic mechanism, a new research paradigm is gradually being promoted that integrates basic research with practical applications.
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About the journal: CCS Chemistry is the Chinese Chemical Society's flagship publication, established to serve as the preeminent international chemistry journal published in China. It is an English language journal that covers all areas of chemistry and the chemical sciences, including groundbreaking concepts, mechanisms, methods, materials, reactions, and applications. All articles are diamond open access, with no fees for authors or readers. More information can be found at https://www.chinesechemsoc.org/journal/ccschem .
About the Chinese Chemical Society: The Chinese Chemical Society (CCS) is an academic organization formed by Chinese chemists of their own accord with the purpose of uniting Chinese chemists at home and abroad to promote the development of chemistry in China. The CCS was founded during a meeting of preeminent chemists in Nanjing on August 4, 1932. It currently has more than 120,000 individual members and 184 organizational members. There are 7 Divisions covering the major areas of chemistry: physical, inorganic, organic, polymer, analytical, applied and chemical education, as well as 31 Commissions, including catalysis, computational chemistry, photochemistry, electrochemistry, organic solid chemistry, environmental chemistry, and many other sub-fields of the chemical sciences. The CCS also has 10 committees, including the Woman's Chemists Committee and Young Chemists Committee. More information can be found at https://www.chinesechemsoc.org/ .
