Development of metal-free photocatalysts for converting a diverse range of plastics and mixed plastics into high-value compounds at ambient conditions

Abstract

The current invention relates to compounds comprising carbon nitride with tri-coordinated nitrogen vacancies and oxygen-linked heptazine units. The current invention further relates to methods of synthesizing the compounds, methods of upcycling plastics, and use of the compound in upcycling plastic.

Description

[0001] DEVELOPMENT OF METAL-FREE PHOTOCATALYSTS FOR CONVERTING A DIVERSE RANGE OF PLASTICS AND MIXED PLASTICS INTO HIGH-VALUE COMPOUNDS AT

[0002] AMBIENT CONDITIONS

[0003] Field of Invention

[0004] The current invention relates to compounds comprising carbon nitride with tri-coordinated nitrogen vacancies and oxygen-linked heptazine units, methods of synthesizing such compounds, methods of upcycling plastics, and use of the compound in upcycling plastic. The present invention has particular, but not exclusive, application to upcycling plastic into value- added chemicals.

[0005] Background

[0006] The listing or discussion of a prior-published document in this specification shouid not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

[0007] The annual production of plastics has grown exponentially from less than 2 million tons in 1950s to 368 million tons in 2019 (Sc / . Adv. 2017, 3, e1700782, Science 2021 , 373, 43-47), and the production is estimated to double within the next two decades {Palgrave Common. 2019, 5, 6). Despite the massive production of plastics, only 9% of global plastic waste had been recycled in 2019, 19% had been incinerated, while the remaining 72% had been accumulated in the environment, ending up in landfills, uncontrolled dumpsites, combusted in open areas, or leaked into the natural surroundings including waterways and oceans (Organization for Economic Co-operation and Development (OECD) (2022). Plastic Waste Management Challenges). The accumulation of plastic in the environment not only contributes significantly to pollution but also represents untapped carbon resources. Proper exploitation and utilization of plastic waste as resources for chemicals synthesis and energy sources can simultaneously address several issues including: (i) reducing environmental pollution, (ii) conserving the finite fossil fuels resources, (iii) mitigating climate change by offsetting carbon emissions, and (iv) promoting circular economy. Through transformation of plastic waste into fuels, valuable chemicals, and monomers of plastics, we can minimize environmental waste, recover material value, and reduce reliance on virgin resources, supporting sustainable development by closing the loop on plastic usage. Typically, recycling a tonne of plastic conserves approximately 130 million kilojoules (kJ), equivalent to the energy produced by burning 22 barrels of oil. Developing advanced and efficient recycling approach for global plastic waste management could save around 3.5 billion barrels oil, leading to an annual financial saving of about US$ 176 billion {Nat. Rev. Chem. 2017, 1, 46).

[0008] Plastic recycling involves four key approaches: primary (closed-loop), secondary (mechanical), tertiary (chemical), and energy recovery (incineration) {Compos. Part B Eng. 2017, 115, 409- 422). Most recycling depends on primary recycling, needing near-pristine, clean, and singleuse plastics. Reprocessed plastics often have lower quality than virgin materials, with decreased molecular weights and inferior thermal and mechanical properties. As a result, global plastic solid waste has surged to 150 million tonnes annually. According to the US Environmental Protection Agency (EPA), 87% of global plastic waste belongs to the six most common categories labelled with resin codes 1-6, which are code 1 : polyethylene terephthalate (PET), code 2: high-density polyethylene (HDPE), code 3: polyvinyl chloride (PVC), code 4: low-density polyethylene (LDPE), code 5: polypropylene (PP), code 6: polystyrene (PS) and code 7 belongs to other plastics not included above. Non-biodegradable plastics can persist for 100 to 1000 years in natural environments, contributing to severe environmental pollution and ecological issues. Given the escalating environmental concerns and the need to repurpose the carbon content in plastic waste, the development of efficient technologies for upcycling plastic waste - transformation of plastic waste into valuable compounds, is a pressing necessity. However, this is of a great challenge, because plastics are typically connected by highly stable and kinetically inert C-C bonds which are difficult to break, and it requires selective cleavage of C-C bonds at opportune sites to chemically transform plastic waste into desired small molecules.

[0009] Currently, pyrolysis stands as the most used method for chemically recycling plastic waste, though it demands significant energy to heat materials to 700-1000 °C and generates considerable greenhouse gas emissions {Prog. Energy Combust. Set. 2022, 93, 101021 ). Due to the random cleavage of C-C bonds, pyrolysis process produces a mix of hydrocarbons with low values and selectivity, also potentially including toxic and pernicious gases due to incomplete combustion {Adv. Mater. 2021 , 33, 2005192). Mechanical recycling, another common method, requires clean and sorted plastics, with mixed or contaminated plastics posing significant challenges and increasing operational costs due to the need for sorting and cleaning. Mechanical recycling can only handle certain types of plastics, whereas most conventional plastics like LDPE, PS, and PVC, are unsuitable for mechanical recycling because of their soft properties that clog the machineries or the highly corrosive gases they release will damage the equipment. Mechanical recycling also results in lower quality outputs as compared to virgin plastics. Chemical recycling, on the other hand, shows promise for dealing with complex plastic wastes like co-polymers by transforming them through chemical reactions. Yet, this field remains under explored, particularly for mixed plastics, as the inclusion of materials like PVC complicates the process by potentially damaging the reaction system or poisoning of catalysts through chlorine or hydrochloric acid (HCI) emissions. Besides, most research works still center on processing single-component plastics, as the co-upcycling of various plastics poses a tremendous challenge. In addition, the majority of existing studies necessitate harsh reaction conditions, typically involving temperatures ranging from 150 to 300°C. This results in considerable energy consumption and emissions of greenhouse gases. Furthermore, chemical recycling processes conducted at room temperature often require prolonged reaction times, sometimes up to five days. Additionally, most developed catalysts are metal-based, incorporating expensive noble metals such as platinum (Pt) and ruthenium (Ru), toxic metals like cadmium (Cd) and nickel (Ni), or base metals such as vanadium (V) (Science 2020, 370, 437-441 ; J. Am. Chem. Soc. 2023, 145. 22836-22844; Energy Environ. Sci. 2018, 11, 2853- 2857; J. Am. Chem. Soc. 2019, 141, 15201-15210; Chem 2023, 9, 2683-2700). Apart from their high costs, these metal-based photocatalysts can have significant environmental impacts throughout their lifecycle, including land degradation, water pollution, and habitat destruction resulting from mining activities for metal extraction. Moreover, improper handling and disposal of metal compounds can pose toxicity risks to aquatic and terrestrial ecosystems. In contrast, metal free catalysts offer numerous advantages, including cost-effectiveness, environmental friendliness, and sustainability. Due to the absence of metal compounds, non-metal catalysts are generally non-toxic or less toxic and are more feasible for large-scale production for practical applications. Despite the versatility and eco-friendliness of non-metal catalysts, research on metal-free catalysts for plastic upcycling remains limited, and this area of research remains largely unexplored to date.

[0010] Generally, carbon dioxide (CO2) and water (H2O) are the primary products from the photocatalytic mineralization of plastics (Adv. Energy Mater. 2022, 12, 2200435). Since CO2 is a greenhouse gas and the H2O generated from plastics is typically contaminated and of low value, the formation of these products is undesirable. In this context, plastic upcycling, which involves converting plastics into more valuable products, emerges as a more sustainable and promising solution.

[0011] As traditional pyrolysis requires heating to 700-1000 °C, this prompted extensive research efforts in developing low-temperature methods for plastics transformation. Scott’s group reported the use of Pt / y-AkOa, an expensive noble-metal-loaded catalyst, to convert polyethylene (PE) into liquid alkylaromatics at 280°C (Science, 2020, 370, 437-441 ). Following that, the Chen group reported on solar thermal catalysis, combining solar energy and heat to drive plastic transformation reactions. With the assistance of solar energy, they lowered the depolymerization temperature of polyesters to 150 °C (Matter, 2022, 5, 1305— 1317). While these methods significantly reduce the required temperature for plastics upcycling, these processes still involve elevated temperatures that require substantial energy input and produce greenhouse gases.

[0012] Photocatalysis, with its ability to drive reactions at ambient temperature, appears to be a promising approach. In a pioneering work by Reisner’s group, they employed cadmium-based CdS / CdOx quantum dots to upcycle plastics including polyurethane (PUR), polylactic acid (PLA), and PET into formate, acetate, lactate, and pyruvate at room temperature and atmospheric pressure (Energy Environ. Sci. 2018, 1 1 , 2853-2857). However, pre-treatment of plastics was required, involving dispersion in a strong alkaline aqueous solution (10 M NaOH) to partially hydrolyze the plastics into monomers before photocatalytic reactions. Additionally, the use of Cd-based photocatalysts impedes commercialization as Cd is a highly toxic metal with severe environmental impacts. In response, the use of less-toxic photocatalysts for plastic conversion is desirable. Remarkably, Soo’s group demonstrated the use of commercially sourced vanadium (III) acetylacetonate, V(O)(acac)2 as an efficient photocatalyst to upcycle a wide range of plastics, covering resin codes 2-7 into platform chemicals (Chem, 2023, 9, 2683-2700). Nonetheless, the reactions were relatively slow, requiring 4-7 days, and the homogenous photocatalytic system impedes the separation and recovery of the catalyst for repeated usage. In addition, the use of metal-based photocatalysts still poses the risk of metal ions leaching into the environment. In this regard, metal-free photocatalysts have emerged as better candidates. Mclnnes’s group successfully upcycled PS into benzoic acid, formic acid, and acetophenone using p-toluenesulfonic acid (pTsOH H2O) as the catalyst (J. Am. Chem. Soc. 2022, 144, 6532). However, benzene, a highly toxic and carcinogenic chemical, was used as the solvent for the reaction. In a separate work, Ma’s group focused on PS upcycling using graphitic carbon nitride as the photocatalyst (Nat. Common. 2022, 13, 4809). Other than producing large amount of unwanted CO2, the catalyst-to-plastic ratio is impractical for real-world applications, in which 50 mg of catalyst was required to process just 20 mg of PS. Moreover, all the mentioned works can only process one type of plastic in each reaction, highlighting the challenges of chemically upcycling mixed plastics, although it is highly desirable.

[0013] Li’s group reported photocatalytic upcycling of mixed plastics with three components (polypropylene (PP), PET, and polyethylene (PE)) into methane (CH4) (J. Am. Chem. Soc. 2023, 145, 22836-22844). However, this process requires high-temperature pre-treatment of plastic waste by H2at 520 °C and 1 .5 bar. Additionally, the catalyst used is a single-atom Ru catalyst, an expensive noble-metal-based catalyst, and large-scale synthesis of single-atom catalysts is generally challenging due to the requirement of strict control over reaction conditions for catalyst synthesis and the low catalyst formation yield. In another study, Ma’s group developed a method to process PVC and PET simultaneously ( / Vat. Sustain. 2023, 6, 1685-1692). This achievement, however, involved the use of metal-based ZnCI2as a Lewis acid catalyst and a toxic and environmentally detrimental chlorine-containing ionic liquid as the catalyst / solvent.

[0014] According to the 2018 report by Mckinsey, reusing the carbon sources in plastic waste could lead to a profit-pool growth of up to $60 billion by the year 2030 (McKinsey (2018). How plastics waste could transform the chemical industry. Available at: https: / / www.mckinsey.com / industries / chemicals / our-insights / how-plastics-wasterecycling- could-transform-the-chemical-industry [Accessed on: 16 February 2024]), demonstrating the enormous economic potential of plastic upcycling technology. Moreover, repurposing plastic waste could also save costs for landfills, considering that Semakau landfill, the only landfill site in Singapore, is projected to reach its capacity limit by 2035 (The National Environment Agency. The Zero Waste Masterplan. Available at: https: / / www.mse.gov.sg / resources / zero- waste-masterplan.pdf [Accessed on: 16 February 2024]). The economic feasibility of plastic upcycling is further supported by increasing market demand for the chemicals that are produced from plastics. The market for the main product, formic acid, is valued at US$ 1.01 billion in 2022 and anticipated to grow at a compound annual growth rate (CAGR) of over 5.1 % from 2023 to 2032 (Global Market Insights (2023). Formic Acid Market Size, Share & Trend Analysis Report - 2032. Available at: https: / / www.gminsights.com / industry- analysis / formicacid-market [Accessed on: 18 February 2024]). The applications of formic acid span across leather production, textile, agriculture, and chemical manufacturing, with a notable use as an antibacterial agent and preservative in animal feed. Besides, formic acid is also a liquid organic hydrogen carrier (LOHC) which serves as a hydrogen storage medium that aligns with the increasing interest in clean energy solutions Adv. Sustain. Syst. 2018, 2, 1700161 ). The market for benzoic acid is valued at US$ 1.01 billion and expected to reach US$ 1 .69 billion in 2032, with a CAGR of 5.3% (Future Market Insights (2024). Benzoic acid market by production method, application, end-use & region. Available at: https: / / www.futuremarketinsights.com / reports / benzoicacid-market [Accessed on: 18 February 2024). This is fuelled by its widespread use in the pharmaceutical and food additives industries. It is often employed as a preservative in processed foods and beverages. In addition, the demand for benzoic acid is also boosted by its application in non-phthalate plasticizers, replacing phthalate plasticizers in Europe and North America due to stringent government regulations. The global acetic acid market size is forecasted to reach US$ 23.02 billion by 2030, growing at a CAGR of 7.5% from 2023 to 2030. Its major applications include the production of vinyl acetate monomer, acetate esters, and acetic anhydride, which are pivotal in manufacturing photographic films, paints and coatings, and various pharmaceuticals (Grand View Research (2023). Acetic acid market size, share & trends analysis report by application. Available at: https: / / www.grandviewresearch.com / industryanalysis / acetic-acid-market [Accessed on: 18 February 2024]).

[0015] For acetophenone, the global market is forecasted to reach US$ 285.1 million by 2027, growing at a CAGR of 6.5% from 2022-2027, driven by rising demands in fragrances, cosmetics, resins, and food and beverages sectors (Research and Markets (2021 ). Acetophenone market. Available at: https: / / www.researchandmarkets.com / reports / 3820912 / acetophenone-market [Accessed on: 18 February 2024]). Owing to its sweet-smelling properties, it is notably used in perfumes, lotions, and soaps. Furthermore, its application in the pharmaceutical sector as an ingredient for cosmetic formulations is set to contribute to market growth. The global ethylene glycol market size reached US$ 44.4 billion in 2022, and it is expected to reach US$ 57.9 billion by 2028, with a CAGR of 4.52% from 2022 to 2028 (Research and Markets (2024). Ethylene glycol market: Global industry trends, share, size, growth, opportunity and forecast 2023-2028. Available at: https: / / www.researchandmarkets.eom / report / ethylene-glycol#tag-pos-1 [Accessed on: 19 February 2024]). Ethylene glycol is often employed as an industrial coolant and antifreeze. Besides, it also serves as a key ingredient for the synthesis of PET resin, polyester fibers, and fiberglass. It is widely utilized in various consumer products including hydraulic brake fluids, automative antifreeze, plastics, solvents, films, paints, and cosmetics. The global market size for terephthalic acid was valued at US$ 87.1 billion in 2022 and is projected to rise to US$ 119.2 billion by 2028, with a CAGR of 5.19% from 2023 to 2028 (Research and Markets (2023). Purified terephthalic acid market: Global industry trends, share, size, growth, opportunity and forecast 2023-2028. Available at: https: / / www.researchandmarkets.com / reports / 5642313 / purified-terephthalic-acidmarket- global#tag-pos-4 [Accessed on: 19 February 2024]). Other than ethylene glycol, terephthalic acid is another key material for the production of PET resin that can be utilized in the packaging industry for the production of beverage bottles, containers, and packaging materials. In addition, the polyester fiber segment accounts for a significant portion of the market demand for terephthalic acid. Polyester fibers are widely used in the textile industry for the production of clothing, carpets, upholstery, and industrial fabrics. The expansive applications and increasing market sizes of these chemicals generated from plastic upcycling underscore their vast potential in commercial industries. While significant improvements have been observed in this rigorous research area in recent years, these methods required metal-based catalysts, high temperatures, long reaction times, yet they were only able to deliver low product yields while having limitations such as restricted substrate scopes and incapability of processing mixed plastics. It is apparent that a major breakthrough is still needed to overcome the current challenges and move forward to potential commercialization. As such, there is a need to address the current limitations of conventional plastic upcycling processes and to explore metal-free catalysts for plastic upcycling. The present invention aims to address at least some of the limitations of existing methods and processes.

[0016] Summary of Invention

[0017] In this invention, a metal-free photocatalyst capable of upcycling a diverse range of plastics, covering the entire range of resin codes 1 -7, into value-added chemicals was developed. It has been surprisingly found that the developed metal-free photocatalyst, a surface defect- engineered graphitic carbon nitride with tri-coordinated nitrogen vacancies and oxygen-linked heptazine units incredibly facilitated photocatalytic plastic upcycling. In addition to conventional plastics, the current invention can also upcycle co-polymers, mixed plastics, post-consumer plastic wastes, and contaminated real-life plastic waste at ambient conditions - room temperature and atmospheric pressure.

[0018] To the best of our knowledge, this is the first work that successfully realized photocatalytic upcycling of the entire spectrum of resin codes 1 -7 (PET, HDPE, PVC, LDPE, PP, PS, polyvinyl acetate (PVAc) and ethylene vinyl acetate (EVA)) into value-added chemicals. More importantly, the current invention can also process mixed plastics and post-consumer contaminated plastic waste without deterioration in the photoactivity. This achievement was realized through the carefully designed photocatalyst, a surface defect-engineered graphitic carbon nitride with tri-coordinated nitrogen vacancies and oxygen-linked heptazine units. In contrast, pristine carbon nitride without structural modification demonstrated minimal photoactivity for plastic conversion under our reaction conditions (room temperature and atmospheric pressure), highlighting the significance of the nanoscale engineering of the materials and the robustness of the developed photocatalyst.

[0019] Aspects and embodiments of the invention will now be described by reference to the following numbered clauses. 1. A compound comprising carbon nitride with tri-coordinated nitrogen vacancies and oxygen-linked heptazine units.

[0020] 2. The compound according to Clause 1 , wherein the compound has a specific surface area of from about 90 to about 180 m2 / g, such as from about 100 to about 130 m2 / g, such as about 120.35 m2 / g.

[0021] 3. The compound according to Clause 1 or Clause 2, wherein the compound has a plurality of pores having a diameter of about 1 nm to about 120 nm, such as from about 5 nm to about 100 nm.

[0022] 4. The compound according to any one of Clauses 1 to 3, wherein the compound has an optical band gap of from 2.0 eV to 3.5 eV, such as from 2.0 eV to 2.80 eV, such as 2.40 eV.

[0023] 5. The compound according to any one of Clauses 1 to 4, wherein the compound has a conduction band minimum (CBM) of from about -1 .0 V to about -1 .25 V versus NHE, such as from about -1.15 V to about -1.2 V versus NHE, such as about -1.18 V versus NHE.

[0024] 6. The compound according to any one of Clauses 1 to 5, wherein the compound has a valence band maximum (VBM) of from about 1 .0 V to about 1 .7 V versus NHE, such as from about 1 .1 V to about 1 .3 V versus NHE, such as about 1 .22 V versus NHE.

[0025] 7. The compound according to any one of Clauses 1 to 6, wherein the compound has an Urbach energy of from about 0.1 eV to about 0.25 eV, such as from about 0.15 to about 0.20 eV, such as about 0.19 eV.

[0026] 8. The compound according to any one of Clauses 1 to 7, wherein the compound is capable of at least partially converting a plastic into smaller molecules.

[0027] 9. The compound according to any one of Clauses 8, wherein the compound is capable of at least partially converting the plastic into smaller molecules upon irradiation with light.

[0028] 10. The compound according to Clause 8 or Clause 9, wherein the plastic comprises plastic materials selected from one or more types of plastics classified under resin codes 1 - 11. The compound according to any one of Clauses 1 to 10, wherein the compound is metal-free.

[0029] 12. A method of synthesizing the compound according to any one of Clauses 1 to 1 1 , comprising the following steps:

[0030] (a) mixing a nitrogen-containing precursor and an ammonium salt to form a first mixture;

[0031] (b) calcinating the first mixture for a first period of time at a first temperature;

[0032] (c) cooling the mixture from step (b) to room temperature and grinding the mixture to form a second mixture; and

[0033] (d) calcinating the second mixture for a second period of time at a second temperature to obtain the compound.

[0034] 13. The method according to Clause 12, wherein the nitrogen-containing precursor is a nitrogen-rich precursor, optionally wherein the nitrogen-containing precursor is selected from one or more of the group consisting of urea, thiourea, melamine, cyanamide, dicyandiamide, and guanidine hydrochloride.

[0035] 14. The method according to Clause 12, wherein the nitrogen-containing precursor is urea.

[0036] 15. The method according to any one of Clauses 12 to 14, wherein the ammonium salt is selected from one or more of the group consisting of ammonium acetate, ammonium bicarbonate, ammonium formate, ammonium carbonate, and ammonium oxalate.

[0037] 16. The method according to any one of Clauses 12 to 14, wherein the ammonium salt is ammonium acetate.

[0038] 17. The method according to any one of Clauses 12 to 16, wherein the first period of time is from about 2 hours to about 5 hours, such as from about 3.5 hours to about 4.5 hours, such as about 4 hours, and / or the first temperature is from about 500 °C to about 600 °C, such as from about 545 °C to about 560 °C, such as about 550 °C.

[0039] 18. The method according to any one of Clauses 12 to 17, wherein the second period of time is from about 1 hour to about 3 hours, such as from about 1 .5 hours to about 2.5 hours, such as about 2 hours, and / or the second temperature is from about 500 °C to about 600 °C, such as from about 545 °C to about 560 °C, such as about 550 °C. 19. The method according to any one of Clauses 12 to 18, wherein step (b) has a first ramping rate of from about 2 °C / min to about 10 °C / min, such as about 5 °C / min.

[0040] 20. The method according to any one of Clauses 12 to 19, wherein step (d) has a second ramping rate of from about 2 °C / min to about 10 °C / min, such as about 5 °C / min.

[0041] 21 . The method according to any one of Clauses 12 to 20, wherein step (d) is under air or an oxygen-enriched atmosphere.

[0042] 22. A method of upcycling plastics, comprising the following steps:

[0043] (i) providing a plastic, a solvent system, and the compound according to any one of Clauses 1 to 1 1 to form a reaction mixture; and

[0044] (ii) irradiating the reaction mixture with a light source for a third period of time, such that the plastic is at least partially converted into smaller molecules.

[0045] 23. The method according to Clause 22, wherein the plastic comprises plastic materials selected from one or more types of plastics classified under resin codes 1 - 7.

[0046] 24. The method according to Clause 22 or Clause 23, wherein the plastic comprises plastics classified under resin code 1 , i.e. polyethylene terephthalate (PET), and the solvent system is an aqueous solution comprising an alkali metal hydroxide.

[0047] 25. The method according to Clause 22 or Clause 23, wherein the plastic comprises one or more plastics classified under resin codes 2 - 7, and the solvent system comprises one or more organic solvents or an aqueous solution that comprises an alkali metal hydroxide.

[0048] 26. The method according to Clause 22 or Clause 23, wherein the plastic comprises plastics classified under resin code 1 , i.e. polyethylene terephthalate (PET), and one or more plastics classified under resin codes 2 - 7, and the solvent system comprises one or more organic solvents and / or an aqueous solution that comprises an alkali metal hydroxide.

[0049] 27. The method according to Clauses 24 to 26, wherein the alkali metal hydroxide is selected from one or more of the group consisting of sodium hydroxide (NaOH), potassium hydroxide (KOH), lithium hydroxide (LiOH), preferably wherein the alkali metal hydroxide is NaOH. 28. The method according to Clause 27, wherein the aqueous solution has a concentration of from about 0.5M to about 12M, such as from about 1 M to about 10M, such as about 1 M, about 2M, about 5M, or about 10M, preferably about 1 M.

[0050] 29. The method according to Clause 25 or Clause 26, wherein the one or more organic solvents is selected from one or more of the group consisting of 1 ,2-dichloroethane (DCE), acetonitrile, dichloromethane (DCM), ethyl acetate, acetone, dimethyl sulfoxide (DMSO), dimethylformamide (DMF), 1 ,2-dichlorobenzene, tetrahydrofuran (THF), and 2- methyltetrahydrofuran (2-MeTHF).

[0051] 30. The method according to any one of Clauses 22 to 23 and 25 to 29, comprising an additional step (ia) prior to step (I):

[0052] (la) melting a plastic; wherein the plastic comprises plastics classified under one or more of the following resin codes:

[0053] (I) resin code 2, i.e. high-density polyethylene (HDPE),

[0054] (II) resin code 4, i.e. low-density polyethylene (LDPE), and / or

[0055] (III) resin code 7, i.e. other plastics not classified under resin codes 1-6; or (ia) converting a plastic into a powder.

[0056] 31 . The method according to any one of Clauses 22 to 30, wherein the light source is one or more of the group consisting of sunlight, a Xenon lamp, a fluorescent lamp, a LED light, a visible light source, a white light source, and a blue light source, preferably a blue light source with a wavelength of 456 nm or 448 nm.

[0057] 32. The method according to any one of Clauses 22 to 31 , wherein the smaller molecules formed is one or more of the group consisting of formic acid, benzoic acid, acetic acid, acetophenone, ethylene glycol, and terephthalic acid.

[0058] 33. A use of the compound according to any one of Clauses 1 to 11 in upcycling plastic.

[0059] Drawings

[0060] FIG. 1 depicts a simple illustration on the setup for photocatalytic upcycling of plastics disclosed herein.

[0061] FIG. 2 depicts (A) the production of small-molecules from PS over CN (pristine graphitic carbon nitride), VN-CN (carbon nitride with artificially induced tri-coordinated nitrogen vacancies), and O-VN-CN (carbon nitride with the co-existence of tri-coordinated nitrogen vacancies and oxygen-linked heptazine units) after 24 h of reaction - control experiments were performed in the case of (i) without light, (ii) without O2, (iii) without catalyst; (B) product yields attained from the entire range of plastics, covering resin codes 1-7 and Styrofoam™ takeaway food container (both clean and oil-contaminated for comparison) over O-VN-CN, the product yields were quantified after a reaction duration of 24 h for all substrates, except for EVA, where the readings were taken after 48 h of reaction; (C) products generated from single-element (a) PVC, (b) PP, and (c) PS after 24 h reaction over O-VN-CN, panel (d) shows the total yield obtained by summing the yields from panels (a) to (c), panel (e) depicts the yield amount generated from a mixture of PVC, PP, and PS after 24 h reaction. (CN: pristine graphitic carbon nitride, VN-CN: carbon nitride with artificially induced tri-coordinated nitrogen vacancies, O-V -CN: carbon nitride with the co-existence of tri-coordinated nitrogen vacancies and oxygen-linked heptazine units).

[0062] FIG. 3 depicts the comparison of the representative works in the literature with the current invention on plastic upcycling conversion. (A) Conversion of PS into cumene, xylene, benzene, and indane; reaction condition: H2(0.3 MPa), Ru / Nb2O5, H2O. 200 °C); the method required high pressure, high temperatures, and rare expensive metal catalyst; reference: Angew, Chem. Int. Ed. 2021 , 60, 5527). (B) Conversion of PS into benzoic acid, benzoyl chloride, benzaldehyde, and acetophenone; reaction condition: O2(1 atm), FeC , acetone, room temperature, white light, 20 h; the method resulted in low product yields (total of 23%); reference: J. Am. Chem. Soc. 2022, 144, 5745. (C) Conversion of PS into benzoic acid, formic acid, and acetophenone; reaction condition: pTsOH-H2O, benzene / MeCN, O2(1 atm), room temperature, purple light, 15 h; the method involved highly toxic and carcinogenic solvent; reference: J. Am. Chem. Soc. 2022, 144, 6532. (D) Conversion of PE into liquid alkylaromatics; reaction condition: Pt / y-AhOa, 280 °C, 24 h; the method required high temperatures and expensive noble metal catalyst; reference: Science, 2020, 370, 437-441 . (E) Conversion of polymers of resin codes 2-7 into benzoic acid, formic acid, and acetophenone; reaction condition: O2(1 atm), V(O)(acac)2, DCM, room temperature, white light, 5 days; the method involved is a slow reaction (requiring 4-7 days), required toxic metalbased catalyst, and the catalyst was difficult to separate from reaction mixture and recovered as the method was a homogeneous system; reference: Chem. 2023, 9, 2683. (F) Current invention: an example of conversion of PS into benzoic acid (41 .6% yield), formic acid (49.3% yield), and acetophenone (8.5% yield); reaction condition: O2(1 atm), O-VN-CN, DCE, room temperature, blue light, 24 h; the method works well at ambient temperature and pressure, the reaction involved was driven by light, involving metal-free catalyst made from cheap and abundant precursors. FIG. 4 depicts the transmission electron microscopy (TEM) images of (A) CN and (B-D) O- VN-CN. (E) STEM (Scanning Transmission Electron Microscopy) image of O-VN-CN and the corresponding energy-dispersive X-ray spectroscopy (EDX) mapping showing (F) C, (G) N, and (H) O elements.

[0063] FIG. 5 depicts (A) nitrogen adsorption-desorption isotherms of CN (circle), VN-CN (diamond), and O-VN-CN (triangle) (inset shows the respective pore size distribution); (B) X-ray diffraction (XRD) spectra; (C) survey X-ray photoelectron spectroscopy (XPS) spectra; (D) high- resolution N 1 s spectra of CN, VN-CN, and O-VN-CN; and schematic atomic model of (E) CN and (F) O-VN-CN.

[0064] FIG. 6 depicts (A) ultraviolet-visible (UV-vis) absorption spectra; (B) Tauc plots; (C) Mott- Schottky plots of CN, VN-CN, and O-VN-CN; (D) Urbach plot of VN-CN and O-VN-CN; and combination of Urbach and Tauc plots for (E) O-VN-CN and (F) VN-CN.

[0065] FIG. 7 depicts the electronic band structures of CN, VN-CN, and O-VN-CN.

[0066] FIG. 8 depicts the plausible reaction mechanism occurred during plastic upcycling disclosed herein.

[0067] FIG. 9 depicts (A) transient photocurrent responses; (B) Nyquist plots; and (C) room temperature steady-state photoluminescence (PL) spectra of CN, VN-CN, and O-VN-CN.

[0068] FIG. 10 depicts a schematic illustration for the conversion of plastic waste into fuel and useful chemicals over catalyst.

[0069] Description

[0070] In a first aspect of the invention, there is provided a compound comprising carbon nitride with tri-coordinated nitrogen vacancies and oxygen-linked heptazine units.

[0071] In embodiments herein, the word “comprising” may be interpreted as requiring the features mentioned, but not limiting the presence of other features. Alternatively, the word “comprising” may also relate to the situation where only the components / features listed are intended to be present (e.g. the word “comprising” may be replaced by the phrases “consists of” or “consists essentially of”). It is explicitly contemplated that both the broader and narrower interpretations can be applied to all aspects and embodiments of the present invention. In other words, the word “comprising” and synonyms thereof may be replaced by the phrase “consisting of” or the phrase “consists essentially of’ or synonyms thereof and vice versa.

[0072] The phrase, “consists essentially of’ and its pseudonyms may be interpreted herein to refer to a material where minor impurities may be present. For example, the material may be greater than or equal to 90% pure, such as greater than 95% pure, such as greater than 97% pure, such as greater than 99% pure, such as greaterthan 99.9% pure, such as greater than 99.99% pure, such as greater than 99.999% pure, such as 100% pure.

[0073] As used herein, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a composition” includes mixtures of two or more such compositions, reference to “a first repeating unit” includes a plurality of said repeating units and does not exclude the possibility of further (different) repeating units also being present, and the like.

[0074] When used herein, the term “carbon nitride” is intended to refer to a material comprising primarily carbon and nitrogen atoms. Pure carbon nitride consists of only carbon and nitrogen atoms. It will be appreciated that carbon nitride may be modified to include other atoms, such as oxygen or hydrogen, whilst remaining primarily formed of carbon and nitrogen atoms, and therefore still referred to as carbon nitride.

[0075] When used herein, the phrase “tri-coordinated nitrogen vacancies” is intended to refer to defect structures in the compound as described herein where a nitrogen atom may be missing and the neighbouring three atoms which would normally bond with the nitrogen atom may remain under-coordinated. As will be appreciated, the phrase may be used interchangeably with other phrases including “3-coordinated nitrogen vacancies”, “C3-coordinated nitrogen vacancies”, “N3c”, and “N-C3”.

[0076] When used herein, the phrase “oxygen-linked heptazine units” is intended to refer to units of heptazine rings being connected to each other via one or more oxygen atoms.

[0077] The compound may be a catalyst. The compound may be a photocatalyst. The compound has particular application to the catalytic breakdown of plastics into smaller molecules. The catalytic breakdown is able to occur at ambient temperatures and pressures. Ambient temperatures may also be referred to as room temperature. Ambient temperatures may be from about 15 °C to about 30 °C, such as from about 20 °C to about 25 °C, such as about 23 °C.

[0078] In embodiments of the invention that may be mentioned herein, the compound may have a specific surface area of from about 90 to about 180 m2 / g, such as from about 100 to about 130 m2 / g, such as about 120.35 m2 / g. The specific surface area may be determined via measurement of the adsorption of a gas, such as nitrogen, at liquid nitrogen temperatures, such as around 77 K. In the present invention, the N2 adsorption-desorption isotherm was performed at 77 K using a Micrometrics ASAP 2020 ™.

[0079] For the avoidance of doubt, it is explicitly contemplated that where a number of numerical ranges related to the same feature are cited herein, that the end points for each range are intended to be combined in any order to provide further contemplated (and implicitly disclosed) ranges. Thus, in relation to the above related numerical ranges, there is disclosed specific surface area of: from about 90 to about 120.35 m2 / g, from about 90 to about 130 m2 / g, from about 90 to about 180 m2 / g; from about 100 to about 120.35 m2 / g, from about 100 to about 130 m2 / g; from about 100 to about 180 m2 / g; and from about 120.35 to about 130 m2 / g, from about 120.35 to about 180 m2 / g.

[0080] In embodiments of the invention that may be mentioned herein, the compound may have a plurality of pores having a diameter of about 1 nm to about 120 nm, such as 1 nm to about 100 nm, such as from about 5 nm to about 120 nm, such as from about 5 nm to about 100 nm.

[0081] In embodiments of the invention that may be mentioned herein, the compound may have an optical band gap of from 2.0 eV to 3.5 eV, such as from 2.0 eV to 2.80 eV, such as from 2.0 eV to 2.40 eV, such as from 2.40 eV to 3.5 eV, such as from 2.40 eV to 2.80 eV, such as 2.40 eV.

[0082] In embodiments of the invention that may be mentioned herein, the compound may have a conduction band minimum (CBM) of from about -1 .0 V to about -1 .25 V versus NHE (Normal Hydrogen Electrode), -1 .0 V to about -1 .2 V versus NHE, such as from about -1 .15 V to about -1 .25 V versus NHE, -1.18 V to about -1.2 V versus NHE, such as from about -1.15 V to about -1.18 V versus NHE, such as from about -1.15 V to about -1.2 V versus NHE, such as about -1.18 V versus NHE. In embodiments of the invention that may be mentioned herein, the compound may have a valence band maximum (VBM) of from about 1 .0 V to about 1 .7 V versus NHE, such as from about 1 .1 V to about 1 .7 V versus NHE, such as from about 1 .0 V to 1 .3 V versus NHE, such as from about 1 .1 V to about 1 .3 V versus NHE, such as about 1 .22 V versus NHE.

[0083] In embodiments of the invention that may be mentioned herein, the compound may have an Urbach energy of from about 0.1 eV to about 0.25 eV, such as from about 0.1 eV to about 0.20 eV, such as from 0.15 eV to about 0.25 eV, such as from about 0.1 eV to about 0.19 eV, such as from about 0.15 to about 0.20 eV, such as from about 0.15 eV to about 0.19 eV, such as about 0.19 eV to about 0.20 eV, such as about 0.19 eV.

[0084] In particular embodiments of the invention that may be mentioned herein, the compound may be capable of at least partially converting a plastic into smaller molecules.

[0085] In particular embodiments of the invention that may be mentioned herein, the compound may be capable of at least partially converting a plastic into smaller molecules upon irradiation by light.

[0086] When used herein, the term “smaller molecules” is intended to refer to molecules which have a lower molecular weight than the polymers from which they are obtained. For example, the smaller molecules may include any carbon-containing compound with a molecular weight of less than about 200 g / mol. For example, the smaller molecules may be one or more of the group consisting of formic acid, benzoic acid, acetic acid, acetophenone, ethylene glycol, and terephthalic acid. Such smaller molecules may have increased value as compared to the plastic waste, such that the catalytic breakdown of the plastics upcycles the plastics into more valuable compounds.

[0087] When used herein, the term “irradiation by light” is intended to refer to irradiating, illuminating, or exposing the compound as described herein with light energy or photons from a light source. Without wishing to be bound by scientific theory, it is believed that the energy from the light provides energy to allow the reaction to proceed. The light source described herein may be selected from one or more of the group consisting of sunlight, a Xenon lamp, a fluorescent lamp, a LED light, a visible light source, a white light source, and a blue light source. Preferably, the light source may be a blue light source with a wavelength of 456 nm or 448 nm.

[0088] When used herein, the term “plastic” is intended to refer to any plastic material containing at least one polymeric material. In certain embodiments that may be mentioned herein, the plastic comprises plastic materials selected from one or more types of plastics classified under resin codes 1 - 7. As will be appreciated, resin codes 1 - 7 represents the categories of plastics - resin code 1 belongs to polyethylene terephthalate (PET), resin code 2 belongs to high-density polyethylene (HDPE), resin code 3 belongs to polyvinyl chloride (PVC), resin code 4 belongs to low-density polyethylene (LDPE), resin code 5 belongs to polypropylene (PP), resin code 6 belongs to polystyrene (PS), and resin code 7 belongs to other plastics not classified under resin code 1 - 6.

[0089] In embodiments of the invention that may be mentioned herein, the compound may be metal- free.

[0090] When used herein, the term “metal-free” is intended to refer to a material which do not consist of any metal. As will be appreciated, the term “metal-free” may also be interpreted herein to refer to a compound where minor impurities of metal may be present. For example, the compound may be greater than or equal to 90% free of metal, such as greater than 95% free of metal, such as greater than 97% free of metal, such as greater than 99% free of metal, such as greater than 99.9% free of metal, such as greater than 99.99% free of metal, such as greater than 99.999% free of metal, such as 100% free of metal. Metals used in catalysts can be expensive, such as platinum or ruthenium, or can be toxic, such as nickel or cadmium. The present invention provides for a metal-free compound which therefore does not include the limitations of metal-based catalysts, whilst also allowing for the upcycling of plastics under mild conditions, such as atmospheric pressure and ambient temperatures.

[0091] In a second aspect of the invention, there is provided a method of synthesizing a compound as described herein, the method comprising the steps of:

[0092] (a) mixing a nitrogen-containing precursor and an ammonium salt to form a first mixture;

[0093] (b) calcinating the first mixture for a first period of time at a first temperature;

[0094] (c) cooling the mixture from step (b) to room temperature and grinding the mixture to form a second mixture; and

[0095] (d) calcinating the second mixture for a second period of time at a second temperature to obtain the compound.

[0096] In embodiments of said method of synthesizing a compound as described herein, the nitrogenprecursor may be any suitable nitrogen-containing precursor, for example nitrogen-rich precursor. For example, the nitrogen-precursor may be selected from one or more of the group consisting of urea, thiourea, melamine, cyanamide, dicyandiamide, and guanidine hydrochloride. More particularly, the nitrogen-containing precursor may be urea. In embodiments of said method of synthesizing a compound as described herein, the ammonium salt may be any suitable salt containing an ammonium cation. A suitable ammonium salt is one which provides a compound according to the first aspect of the present disclosure when processed in accordance with the second aspect of the present disclosure. For example, the ammonium salt may be selected from one or more of the group consisting of ammonium acetate, ammonium bicarbonate, ammonium formate, ammonium carbonate, and ammonium oxalate. More particularly, the ammonium salt may be ammonium acetate.

[0097] In embodiments of said method of synthesizing a compound as described herein, any suitable first period of time and / or first temperature may be used. For example, the first period of time may be a period of from about 2 hours to about 5 hours, such as from about 2 hours to about 4.5 hours, such as from about 3.5 hours to about 5 hours, such as from about 2 hours to about 4 hours, such as about 3.5 hours to about 4.5 hours, such as from about 3.5 hours to about 4 hours, such as about 4 hours, and / or the first temperature may be from about 500 °C to about 600 °C, such as from about 500 °C to about 560 °C, such as from 545 °C to about 600 °C, such as from about 500 °C to about 550 °C, such as from about 550 °C to 600 °C, such as about 545 °C to about 560 °C, such as about 550 °C to about 560 °C, such as about 550 °C.

[0098] In embodiments of said method of synthesizing a compound as described herein, any suitable second period of time and / or second temperature may be used. For example, the second period of time is from about 1 hour to about 3 hours, such as from about 1 hour to about 2.5 hours, such as from about 1 .5 hours to about 3 hours, such as from about 1 hour to 2 hours, such as from about 1 .5 hours to about 2.5 hours, such as from about 2 hours to about 3 hours, such as from about 1 .5 hours to about 2 hours, such as from about 2 hours to about 2.5 hours, such as about 2 hours, and / or the second temperature may be from about 500 °C to about 600 °C, such as from about 500 °C to about 560 °C, such as from about 545°C to about 600°C, such as from about 500°C to about 550°C, such as from about 550 °C to about 600°C, such as from about 545 °C to about 560 °C, such as from about 545 °C to about 550°C, such as from about 550 °C to about 560°C, such as about 550 °C.

[0099] In embodiments of said method of synthesizing a compound as described herein, any suitable first ramping rate may be used. For example, the first ramping rate may involve an increase in temperature of from about 2 °C / min to about 10 °C / min, such as from about 2 °C / min to about 5°C / min, such as from about 5 °C / min to about 10 °C / min, such as about 5 °C / min, until the first temperature mentioned above is reached. In embodiments of said method of synthesizing a compound as described herein, any suitable second ramping rate may be used. For example, the second ramping rate may involve an increase in temperature of from about 2 °C / min to about 10 °C / min, such as from about 2 °C / min to about 5°C / min, such as from about 5 °C / min to about 10 °C / min, such as about 5 °C / min, until the second temperature mentioned in the above is reached.

[0100] In embodiments of said method of synthesizing a compound as described herein, step (d) of said method may be under air or an oxygen-enriched atmosphere. When used herein, the term “oxygen-enriched atmosphere” is intended to refer to a gaseous atmosphere comprising oxygen at a higher concentration than found in air.

[0101] In a third aspect of the invention, there is provided a method of upcycling plastics, the method comprises the steps of:

[0102] (i) providing a plastic, a solvent system, and a compound as described herein to form a reaction mixture; and

[0103] (ii) irradiating the reaction mixture with a light source for a third period of time, such that the plastic is at least partially converted into smaller molecules.

[0104] In embodiments of said method of upcycling plastics, the plastic may be plastic materials selected from one or more types of plastics classified under resin codes 1 - 7.

[0105] In embodiments of said method of upcycling plastics, the solvent system may comprise one or more suitable solvents, for example one or more organic solvents and / or an aqueous solvent. An aqueous solvent may be preferred when condensation polymers, such as those classified in resin code 1 , are being processed. An organic solvent may be preferred when addition polymers, such as those classified in resin codes 2 to 6, are being processed.

[0106] When used herein, the term “organic solvents” is intended to refer to carbon-based liquids capable of dissolving or dispersing other substances. The organic solvents described herein may be selected from one or more of the group consisting of 1 ,2-dichloroethane (DCE), acetonitrile, dichloromethane (DCM), ethyl acetate, acetone, dimethyl sulfoxide (DMSO), dimethylformamide (DMF), 1 ,2-dichlorobenzene, tetrahydrofuran (THF), and 2- methyltetrahydrofuran (2-MeTHF). More particularly, the organic solvent may be DCE.

[0107] When used herein, the term “aqueous solution” is intended to refer to a homogeneous mixture with one or more solutes being dissolved in water. The aqueous solution described herein may comprise an alkali metal hydroxide. For example, the alkali metal hydroxide may be any alkali metal hydroxide with a suitable pKa value. More particularly, the alkali metal hydroxide may be selected from one or more of the group consisting of sodium hydroxide (NaOH), potassium hydroxide (KOH), lithium hydroxide (LiOH). Preferably, the alkali metal hydroxide may be NaOH.

[0108] The aqueous solution described herein may have a concentration of from about 0.5M to about 12M, such as from about 1 M to about 10M, such as about 1 M, about 2M, about 5M, or about 10M, preferably about 1 M.

[0109] For the avoidance of doubt, it is explicitly contemplated that where a number of numerical ranges related to the same feature are cited herein, that the end points for each range are intended to be combined in any order to provide further contemplated (and implicitly disclosed) ranges. Thus, in relation to the above related numerical ranges, there is disclosed concentration of aqueous solution of: from about 0.5M to about 12M, from about 0.5M to about 10M, from about 0.5M to about 5M, from about 0.5M to about 2M; from about 1 M to about 12M, from about 1 M to about 10M, from about 1 M to about 5M, from about 1 M to about 2M; from about 2M to about 12M, from about 2M to about 10M, from about 2M to about 5M; from about 5M to about 12M, from about 5M to about 10M; from about 10M to about 12M.

[0110] In embodiments of said method of upcycling plastics, any suitable third period of time may be used. For example, the third period of time may be a period of from about 1 hour to about 120 hours, from about 1 hour to about 72 hours, such as from about 6 hours to about 60 hours, such as from about 12 hours to about 48 hours, such as from about 18 hours to about 30 hours, such as about 24 hours, such as about 48 hours.

[0111] For the avoidance of doubt, it is explicitly contemplated that where a number of numerical ranges related to the same feature are cited herein, that the end points for each range are intended to be combined in any order to provide further contemplated (and implicitly disclosed) ranges. Thus, in relation to the above related numerical ranges, there is disclosed third period of time of: from about 1 hour to about 120 hours, from about 1 hour to about 72 hours, from about 1 hour to about 60 hours, from about 1 hour to about to 48 hours, from about 1 hour to about 30 hours, from about 1 hour to about 24 hours; such as from about 6 hours to about 60 hours, from about 6 hours to about to 48 hours, from about 6 hours to about 30 hours, from about 6 hours to about 24 hours; such as from about 12 hours to about 48 hours, from about 12 hours to about 30 hours, from about 12 hours to about 24 hours; such as from about 18 hours to about 30 hours, from about 18 hours to about 24 hours; such as from about 24 hours.

[0112] In particular embodiments of said method of upcycling plastics, the plastic may be plastics classified under resin code 1 , i.e. polyethylene terephthalate (PET), and the solvent system may be an aqueous solution comprising an alkali metal hydroxide.

[0113] In particular embodiments of said method of upcycling plastics, the plastic may comprise one or more plastics classified under resin codes 2 - 7, and the solvent system may comprise one or more organic solvents or an aqueous solution that comprises an alkali metal hydroxide.

[0114] In particular embodiments of said method of upcycling plastics, the plastic may comprise plastics classified under resin code 1 , i.e., polyethylene terephthalate (PET) and one or more plastics classified under resin codes 2 - 7, and the solvent system may comprise one or more organic solvents and / or an aqueous solution that comprises an alkali metal hydroxide. Condensation polymers (resin code 1 ) may be processed in an aqueous solution. Addition polymers (resin codes 2 to 6) may be processed in an organic solvent. Resin code 7 includes condensation polymers and addition polymers, so a suitable solvent system may be selected depending on the nature of such polymers.

[0115] In yet further embodiments of said method of upcycling plastics, said method may comprise an additional step (ia) prior to step (i):

[0116] (ia) melting a plastic; wherein the plastic comprises plastics classified under one or more of the following resin codes:

[0117] (I) resin code 2, i.e. high-density polyethylene (HDPE),

[0118] (II) resin code 4, i.e. low-density polyethylene (LDPE), and / or

[0119] (III) resin code 7, i.e. other plastics not classified under resin codes 1-6; or (ia) converting a plastic into a powder.

[0120] The plastic may be converted into a powder by subjecting the plastic to one or more of the processes consisting of mechanical grinding, ball milling, hammer milling, jet milling, crushing, pulverization, shredding, and cryogenic grinding. In embodiments of said method of upcycling plastics, the light source may be one or more of the group consisting of sunlight, a Xenon lamp, a fluorescent lamp, a LED light, a visible light source, a white light source, and a blue light source, preferably a blue light source with a wavelength of 456 nm or 448 nm.

[0121] In embodiments of said method of upcycling plastics, the smaller molecules formed may be one or more of the group consisting of formic acid, benzoic acid, acetic acid, acetophenone, ethylene glycol, and terephthalic acid.

[0122] The method of upcycling plastics may include providing an oxygen source to the reaction mixture. The oxygen source may be a gaseous oxygen source, such as air.

[0123] In a fourth aspect of the invention, there is provided a use of the compound as described herein in upcycling plastic.

[0124] Further aspects and embodiments of the invention are described in the following numbered statements.

[0125] 1 . A compound comprising defect-engineered carbon nitride with tri-coordinated nitrogen vacancies and oxygen-linked heptazine units.

[0126] 2. A method of synthesizing the compound according to Statement 1 , comprising:

[0127] (a) mixing urea and an ammonium salt (e.g., ammonium acetate) and grinding the mixture to form a first ground mixture;

[0128] (b) calcinating the first ground mixture for a first period of time (3.5 hours to 4.5 hours, in particular 4 hours) at a first temperature (545 °C to 560 °C, in particular 550 °C);

[0129] (c) cooling the mixture from step (b) to room temperature and further grinding the mixture to form a second ground mixture;

[0130] (d) calcinating the second ground mixture for a second period of time (1 .5 hours to 2.5 hours, in particular 2 hours) at a second temperature (545 °C to 560 °C, in particular 550 °C) to obtain the compound.

[0131] 3. A method to upcycle plastics comprising:

[0132] (i) providing a plastic, a solvent and the compound according to Statement 1 to form a reaction mixture;

[0133] (ii) irradiating the reaction mixture for a period of time to form small molecules. 4. The method according to Statement 3, wherein the plastic comprises plastics of one or more of resin codes 1 - 7.

[0134] 5. The method according to Statement 4, wherein when the plastic comprises plastics of resin code 1 (polyethylene terephthalate (PET)), the solvent is aqueous NaOH (e.g., 1 M NaOH).

[0135] 6. The method according to Statement 4, wherein when the plastic comprises plastics of one or more of resin codes 2 -7, the solvent is one or more of 1 ,2-dischloroethane, acetonitrile and dichloromethane.

[0136] 7. The method according to any one of Statements 3 - 6, wherein the reaction mixture is irradiated with white light or more particularly, blue light.

[0137] 8. The method according to any one of Statements 3 - 7, wherein the small molecules formed is one or more of formic acid, benzoic acid, acetic acid, acetophenone, ethylene glycol and terephthalic acid.

[0138] The process of natural decomposing non-biodegradable plastics is generally estimated to require 100 to 1000 years. The current invention provides great improvement with this process being shortened to require only a day. The current invention allows plastic upcycling into useful products, such as the following, which can be repurposed for other industrial uses:

[0139] • formic acid: commonly used as liquid organic hydrogen carrier (LOHC), which serves as a type of clean fuel;

[0140] • benzoic acid: commonly used as food preservatives;

[0141] • acetic acid: commonly used as food additive, solvents, and antiseptics;

[0142] • acetophenone: commonly used as fragrance;

[0143] • terephthalic acid and ethylene glycol: commonly used as monomer of PET synthesis.

[0144] Some advantages of the current invention over existing catalysts, materials, or methods may include one or more of the following.

[0145] (1 ) Owing to the metal-free nature, the current invention does not lead to any secondary pollution which originates from the metal. (2) The entire conversion process may be carried out at ambient conditions - room temperature and atmospheric pressure, with visible light as the only source of energy input to drive the conversion, in contrast to the literature work that require elevated temperatures for reactions to proceed.

[0146] (3) Despite the elegant design of the photocatalysts claimed herein, the raw materials, particularly urea and ammonium acetate, needed for the synthesis of the photocatalyst are cheap, abundant, and readily available.

[0147] (4) The synthesis of the photocatalysts involves simple calcination and the production of the photocatalyst can be scaled up easily.

[0148] The catalysts developed herein required for driving photocatalytic upcycling of plastics are made up of cheap and abundant raw materials - such as urea and ammonium acetate. The price for urea was US$ 700 and US$ 358 per metric ton in 2022 and 2023, respectively (The World Bank. \Norld Bank Commodity Price Data (The Pink Sheet), Annual Prices. Available at: https: / / www.worldbank.Org / en / research / commodity-markets#1 [Accessed on: 22 February 2024]), and it is predicted by The World Bank that the price will further decrease to US$ 315 per metric ton in 2024 and US$ 300 per metric ton in 2025 (The World Bank (2024). World Bank commodity price forecasts. Available at: https: / / www.worldbank.Org / en / research / commodity-markets#3 [Accessed on: 22 February 2024]). Other than urea, ammonium acetate serves as another raw material necessary for synthesizing our catalyst. Despite its slightly higher price compared to urea, ranging from US$ 820 to 900 per metric ton, the amount of ammonium acetate required is relatively low, i.e. , 5 wt.% with respect to urea, making the raw material cost for the synthesis of our catalyst very low (Made-in-China (2024). Food additives 99% ammonium acetate wholesale price. Available at: https: / / pulisichem1 .en.made-inchina.com / product / LmzUoeblYWVp / China-Food- Additives-99-Ammonium-Acetate-Wholesale-Price-CAS-No-631 -61 -8.html [Accessed on: 22 February 2024]), especially compared to those metal-based photocatalysts reported in the literature. The readily availability and cost-effectiveness of the raw materials for the production of the disclosed catalysts further bolster the economic feasibility of the plastic upcycling projects.

[0149] Typically, photocatalytic transformation of plastics in heterogenous system involves three primary processes, including (i) photoabsorption, (ii) separation and transportation of photogenerated charge carriers, and (iii) reactants activation and conversion. Herein, the designed catalysts allow this photocatalytic system to fully optimize the aforementioned key processes. The catalysts claimed herein are particularly advantageous over the pristine carbon nitride without surface modification. Owing to the highly stable C-C bonds present in plastics, the pristine graphitic carbon nitride demonstrated minimal photocatalytic activity for transforming plastics under our experimental conditions, manifesting the robustness of the surface defect-engineered photocatalysts that facilitate the generation, separation, and transportation of photogenerated charge carriers to participate in reactions.

[0150] The current invention holds potential to serve as a green and sustainable solution to one of the world's most pressing environmental challenges. Furthermore, the facilely synthesis and cost-effectiveness of the claimed photocatalysts make them feasible options for large-scale application, potentially revolutionizing waste management practices and contributing to the circular economy. The current invention also opens new avenues for the development of photocatalytic materials and processes that can address the growing global plastic pollution crisis while also harnessing the untapped potential of plastics as resources for chemical production.

[0151] Further aspects and embodiments of the invention will now be described by reference to the following non-limiting examples.

[0152] Examples

[0153] Materials

[0154] For the synthesis of photocatalysts, urea (> 99.0%) and ammonium acetate (> 97.0%) of analytical grade were purchased from Sigma Aldrich. Plastic resins (HDPE, PVC, LDPE, PP, PS, PVAc, and EVA), 1 ,2 dichloroethane (DCE), and sodium hydroxide (NaOH) were also supplied by Sigma Aldrich and used as received without any purification. Oxygen (99.8% purity) was procured from Leeden National Oxygen Ltd. Deionized water (>18 MQ cm resistivity) was used in the experiments wherever mentioned. Real-life plastic wastes (PET water bottle and Styrofoam™ food container) were recovered from the dustbins in the campus of Nanyang Technological University, Singapore.

[0155] Example 1. Synthesis of Photocatalysts

[0156] In this Example, we prepared three types of metal-free photocatalysts, which are pristine graphitic carbon nitride (CN), carbon nitride with artificially induced tri-coordinated nitrogen vacancies (VN-CN), and carbon nitride with the co-existence of tri-coordinated nitrogen vacancies and oxygen-linked heptazine units (O-VN-CN). Synthesis of O-VN-CN

[0157] The surface-engineered carbon nitride with tri-coordinated nitrogen vacancies and O-linked heptazine units (denoted as O-VN-CN) was facilely prepared as follows: In a typical synthesis, 40 g of urea was mixed with 2 g of ammonium acetate by grinding until the solid mixture was homogeneous. The resulting solid was placed into porcelain crucibles, covered, and transferred to a muffle furnace before being subjected to calcination at 550 °C for 4 h in air with a ramping rate of 5 °C / min. After the furnace cooled down naturally to room temperature, the solids were ground and underwent a second-step heat treatment in air at the same ramping rate and temperature for 2 h. Finally, the sample was collected and ground.

[0158] Synthesis of VN-CN

[0159] For comparison, the control sample with nitrogen vacancies but without O-doping (denoted as N-CN) was synthesized using the procedure above, except that it was only subjected to one- step calcination.

[0160] Synthesis of Pristine Graphitic Carbon Nitride (CN)

[0161] Another control sample, pristine carbon nitride without surface modifications (denoted as CN) was synthesized following the same procedures as VN-CN, except that ammonium acetate was not added.

[0162] Example 2. Photocatalytic Transformation of Plastics

[0163] The photocatalytic plastic conversion experiments were performed using CN, VN-CN, and O- VN-CN using PS as the substrate for initial screening. The photoreactions were conducted at ambient condition - room temperature and atmospheric pressure in an 02-rich atmosphere, with blue light as the only source of input energy to drive the reaction.

[0164] Photocatalytic Plastic Conversion Experiments

[0165] In a typical experiment, 300 pmol of the single repeat unit of plastic resin was added to a 15 mL Schlenk tube containing 5 mL of DCE solvent and 5 mg of our developed photocatalyst. The Schlenk tube was then connected to a balloon filled with O2, and the reaction mixture was subjected to blue LEDs irradiation (two units, 50 W, Amax = 448 nm) with continuous stirring at room temperature (FIG. 1 ). The product yields were quantified after 24 h of reaction for all plastics, except that 48 h for EVA. A series of plastic resins, including HDPE, PVC, LDPE, PS, PP, EVA, and PVAc, were tested under identical experimental conditions. Typically, PS, PVC, PP, EVA, and PVAc are readily soluble in DCE, whereas HDPE and LDPE were melted prior to the photocatalytic experiments. Different from the aforementioned plastic resins, which are addition polymers, PET is a condensation polymer. To upcycle PET, PET drinking water bottle was first cut into small pieces, then cooled using liquid nitrogen for 30 min to make them brittle before transferring them to a 15 mL ball milling jar charged with one 5 mm stainless steel ball. After ball milling at 30 Hz for 90 min, PET in powder form was obtained. Unlike plastic resin codes 2-7, the photocatalytic transformation process of PET was performed in an aqueous solution. In a typical PET upcycling experiment, 300 pmol of the single repeat unit (68.4 mg) of PET powder was added to a 15 mL Schlenk tube containing 5 mg of photocatalyst and 5 mL of water with 1 M NaOH. Apart from the different solvent, the experimental setup for PET upcycling was similar to that of addition polymers, and the product yields were quantified after 24 h of reaction.

[0166] In addition to processing single-element plastics, this example also explored mixed plastics to attest to the general applicability and versatility of our technology. For mixed plastics upcycling, PS, PP, and PVC (0.3 mmol each) were transferred to the same Schlenk tube containing 5 mL of DCE and 5 mg of photocatalyst. While there were more plastics loaded into the Schenk tube, the catalyst loading and the volume of solvent remained unchanged. Similarly, photoreactions were performed under identical conditions, and the product yields were quantified after 24 h. Post-consumer plastics such as contaminated Styrofoam™ food containers and drinking water bottles were recovered from the dustbins in the campus of Nanyang Technological University, Singapore.

[0167] Results and Discussions

[0168] After the photocatalytic reaction, we observed the formation of small molecules including formic acid, benzoic acid, and acetophenone, indicating selective cleavage of the C-C bonds in the PS. Among these three photocatalysts, O-VN-CN exhibited the highest product yields, in which 148 pmol (49.3%) of formic acid, 125 pmol of benzoic acid (41.6%), and 25.6 pmol (8.5%) of acetophenone were produced from PS after 24 h of visible light illumination (FIG. 2A). This demonstrates a significant 2.3-fold and 4.7-fold enhancement in benzoic acid yield compared to VN-CN and GN, respectively. More importantly, the formation of acetophenone was detected only in the reaction using the O-VN-CN photocatalyst.

[0169] To confirm the products were generated from the photocatalytic reactions, a series of control experiments were carried out under three different conditions: (i) without light irradiation, (ii) without oxygen, and (iii) without photocatalyst. In all cases, negligible amount of products was formed (FIG. 2A), attesting that the photocatalyst, oxygen, and light are prerequisites for the formation of formic acid, benzoic acid, and acetophenone. To expand the substrate scope of our study, we systematically tested a diverse range of plastics, including condensation polymer (PET) and addition polymers (HDPE, PVC, LDPE, PP, PS, PVAc, EVA). Remarkably, our carefully engineered O-VN-CN photocatalyst demonstrated good performance in transforming all the above-mentioned polymers into value- added small molecules, including versatile platform chemicals, liquid organic hydrogen carrier (LOHC), and monomers for plastic synthesis. Particularly, formic acid, a LOHC that could be fed directly into fuel cell for energy production, is the main product generated from most plastics (FIG. 2B). To the best of our knowledge, this is the first research work capable of photocatalytically converting the entire spectrum of plastics, covering resin codes 1 -7 into value-added compounds. This was incredibly achieved through a facilely synthesized, inexpensive, and metal-free photocatalyst at mild reaction conditions, demonstrating a green and sustainable approach to plastic waste management. Remarkably, our photocatalytic system could also tolerate real-life plastic waste contaminated with oil, showing no noticeable deterioration in the product yields generated from the contaminated Styrofoam™ takeaway box compared to the clean one. In addition, our photocatalytic system also exhibited excellent photoactivities towards PET drinking bottle upcycling, as real plastics often contain fillers, antioxidants, or cross-linkers that could complicate or impede the photoconversion process. Contrary to the literature that necessitates pre-treatment of PET in highly-alkaline condition (10 M NaOH) at 40 °C for 24 h prior to the photocatalytic reaction (Energy Environ. Sci. 2018, 11, 2853-2857), our photocatalytic reaction can transform PET in a single step, requiring a significantly lower concentration of NaOH (1 M).

[0170] As widely accepted, upcycling mixed plastics presents a significant challenge, with most existing research focusing solely on the photocatalytic conversion of single-component plastics. For instance, the presence of PVC in mixed plastic waste complicates the transformation process because the released chlorine or HCI can damage the photocatalytic system by poisoning the catalysts. This necessitates the separation and sorting of plastic waste, greatly increasing processing costs. On the contrary, we demonstrated that our developed O-VN-CN photocatalyst can upcycle mixed plastics consisting of PS, PP, and PVC simultaneously, without any deterioration in the product yields compared to the aggregate yield obtained by summing the yields from the individual plastics (FIG. 2C).

[0171] Since PS is the most researched plastic in photocatalytic conversion, we have compared our work with some representative works in the literature. Using O-VN-CN as the photocatalyst, the product yields from this work surpass those reported in the literature (FIG. 3). Example 3. Characterization of Photocatalysts

[0172] To elucidate the superior performance of the photocatalysts, we systematically conducted a series of materials characterization to divulge the chemical, physical, structural, electronic, and optical properties of our developed photocatalysts, aiming to uncover the reasons behind the photocatalytic enhancement.

[0173] Transmission Electron Microscopy (TEM) and Energy Dispersive X-Ray Spectroscopy (EDX) The surface morphology and elemental composition of our developed samples were revealed through transmission electron microscopy (TEM) images and energy-dispersive X-ray spectroscopy (EDX) mapping using a FEI Tecnai G2 20 S-Twin operating at an accelerating voltage of 200 kV. For the preparation of TEM specimens, the solid samples were dispersed in ethanol, and a drop of the sample suspension was placed onto the copper grid.

[0174] Brunauer-Emmett-Teller (BET) Measurements

[0175] To determine the specific surface area of the samples, multipoint Brunauer-Emmett-Teller (BET) N2 adsorption-desorption isotherm was performed at 77 K using a Micrometrics ASAP 2020. Prior to the analysis, the samples were degassed at 150 °C for 8 h to remove adsorbed species.

[0176] X-Ray Diffraction (XRD) Measurements

[0177] The powder X-ray diffraction (XRD) patterns of the samples were recorded on an X-ray diffractometer (Broker D8 Discover) with Ni-filtered Cu Ka radiation (A = 0.15406 nm).

[0178] X-Ray Photoelectron Spectroscopy (XPS)

[0179] The chemical environment of the samples was analyzed using X-ray photoelectron spectroscopy (XPS) with a Phoibos 100 spectrometer (SPECS, Germany) with a monochromatic Al-Ka X-ray source. Prior to deconvolution, the binding energies were calibrated by setting the C 1 s signal from adventitious carbon to 284.6 eV.

[0180] Ultraviolet-Visible (UV-vis) Spectroscopy

[0181] The optical properties of the photocatalysts were elucidated from an Agilent Cary 100 Ultraviolet-Visible (UV-vis) spectrophotometer equipped with an integrated sphere, with a scanning range of 200-800 nm.

[0182] Steady-State Photoluminescence (PL) Spectroscopy The recombination of photogenerated electron-hole pairs were investigated using a fluorescent spectrometer (Perkin Elmer LS55) to record the steady-state photoluminescence (PL) spectra of the samples.

[0183] Example 4. Photoelectrochemical Properties of Photocatalysts

[0184] The photoelectrochemical properties of the samples were assessed by electrical impedance spectroscopy (EIS) Nyquist plots, photocurrent responses and Mott-Schottky analysis of the samples using a Metrohm Autolab electrochemical workstation. The setup involved a standard three-electrode quartz cell, comprising a Pt rod and Ag / AgCI saturated with KCI as counter and reference electrodes, respectively. The working electrode was prepared by coating a fluorine-doped tin oxide (FTO) coated glass slide with the sample suspension in ethanol, covering an electroactive area of 1 cm x 1 cm. Throughout the experiments, these three electrodes were immersed in an electrolyte comprised of 0.5 M Na2SO at pH 7. The visible light source was provided by a 500 W Xenon arc lamp (CHF-XM-500 W) equipped with a UV cut-off filter to eliminate wavelengths below 400 nm.

[0185] Results and Discussions

[0186] The morphology of our developed photocatalysts was disclosed by TEM. As shown in FIG. 4A, the pristine CN formed by calcination of urea exhibited the characteristic stacked layered structure of graphitic carbon nitride. With the addition of ammonium acetate precursor, followed by two-step heating, the O-VN-CN appeared as exfoliated nanosheets with rough and curled edges (FIG. 4B and 4G). In addition, the nearly transparent appearance of O-VN-CN suggests its ultrathin thickness, which facilitates the transportation of photogenerated charge carriers from the interior to the surface for redox reactions to take place. Besides, the rough and curved edges, formed as a result of minimizing surface energy and van der Waals forces to maintain structural stability, are beneficial for photoreactions as these rough edges can serve as surface active sites. Furthermore, numerous pores were observed on the exfoliated nanosheets (FIG. 4D), indicating the pivotal role of ammonium acetate as a gas bubble template, in which the pores were generated by the gases released during the acylation reaction between urea and ammonium acetate, along with the delamination of stacked sheets to enlarge the surface exposure for photoreactions.

[0187] The EDX mapping of O-VN-CN reveals the co-existence of C, N, and O elements, which are distributed homogeneously throughout the sample (FIG. 4E-4H). Since graphitic carbon nitride should contain only C and N elements, the presence of additional O elements suggests that the introduction of ammonium acetate in the thermal polymerization process of urea leads to the introduction of oxygen atom to the structure. The porous, ultrathin structure of O-VN-CN endows them good textual properties, as evidenced by the BET analysis. The BET specific surface areas and pore size distributions of our developed photocatalysts were determined by the nitrogen adsorption-desorption isotherms. Based on FIG. 5A, all the samples exhibited Type III isotherm with an H3 hysteresis loop, demonstrating the mesoporous characteristics of our samples with larger abundance of small pores. The specific surface areas of GN, VN-CN, and O-VN-CN were estimated as 33.35, 70.12, and 120.35 m2 / g, respectively. This indicates the specific surface area of engineered O-VN-CN is 3.6-fold larger than the pristine CN, thereby offering more active sites for surface reactions to take place. As shown in the Barrett-Joyner-Halenda (BJH) pore size distribution in FIG. 5A inset, there are larger abundance of small pores present in the O-VN-CN compared to CN and VN-CN, which are beneficial for the photocatalytic reactions.

[0188] The crystallographic structures of our developed samples were disclosed by X-ray diffraction (XRD). As shown in FIG. 5B, two distinct XRD peaks at 12.9° and 27.3° were observed, corresponding to the in-plane structure of repeated heptazine units (100) and the interlayer arrangement of zr-conjugated C-N heterocycles (002) in g-CsN4, respectively {Adv. Fund Mater. 2021 , 31, 2010763). Compared to bulk CN, both VN-CN and O-VN-CN exhibited weaker XRD peaks, manifesting the formation of exfoliated nanosheets during the second-step heat treatment in the catalysts synthesis. It is worth noting that O-VN-CN exhibited the lowest peak intensity among the samples, along with a slight shift of the peak at 27.3° towards the lower angle region. This suggests the disruption of the in-plane structure due to the insertion of foreign oxygen atoms into O-VN-CN, and the replacement of nitrogen atoms with larger-sized oxygen atoms lead to the increased interlayer distance {ACS Appt. Mater. Interfaces 2023, 15, 53371-53381 ).

[0189] To elucidate the atomic chemical environment of the photocatalysts, XPS analysis was performed. The XPS survey spectrum (FIG. 5C) revealed the co-existence of C, N, and O elements in the O-VN-CN sample, well agreeing with the chemical composition of the sample attained from EDX mapping. FIG. 5D shows the high-resolution N 1 s spectra of the samples. Three characteristic peaks centered at 401.30, 400.10, and 398.70 eV can be assigned to amino functional groups (C-NFF, x= 1 , 2), tertiary N (N-Cs, N3c), and sp2hybridized N bonded to C (C=N-C, N2c), respectively. It is worth noting that the normalized area of N-C3 peak reduced from 0.32 for the pristine CN to 0.26 and 0.23 for O-CN and O-CN-UT, respectively. This suggests N3c is the most probable nitrogen-deficient site (FIG. 5E), allowing O atoms to be doped into the lattice by substituting the N atoms. Since tri-coordinated N site was replaced with foreign O atom, the O atom serves as a bridge to connect the heptazine units (FIG. 5F). The aforementioned XPS analyses provide indisputable evidence for the successful formation of tri-coordinated N vacancies along with O-linked heptazine structures in O-VN-CN, providing prerequisites for investigating the roles of these artificially induced defects played during photocatalytic transformation of plastics.

[0190] The effects of these opportune defects on the optical properties of the photocatalysts were first investigated. Based on the UV-Vis absorption spectra in FIG. 6A, the optimal O-VN-CN photocatalyst displays not only the inherent optical absorption between 200 and 450 nm that arises from the characteristic TT-TT* electron transitions in the heterocyclic aromatics, but also displays a slight uplift of the absorption tail between 450 and 600 nm that is ascribable to the n-TT* electron transitions triggered by the lone pairs of electrons within the graphitic carbon nitride framework {Adv. Funct. Mater. 2021 , 31, 2010763). In comparison to pristine ON, both VN-CN and O-VN-CN displayed significant red-shifting of the absorption edge, where the absorption edge extended from 451 nm for CN to 510 and 538 nm for VN-CN and O-VN-CN, respectively. The enhanced photoabsorption along with the stark alteration of sample colour from cream to dark yellow is a classical phenomenon attributed to the sub-band excitation of electrons from the midgap states to the conduction band (CB), which manifests the alteration of electronic band structures in VN-CN and O-VN-CN {Chem. Commun. 2016, 52, 14242- 14245). From the Tauc plots in FIG. 6B, the bandgaps of CN, VN-CN, and O-VN-CN were estimated as 2.80, 2.66, and 2.40 eV, respectively. This suggests the presence of nitrogen vacancies led to the bandgap narrowing in VN-CN, and the doping of O atoms to bridge the triazine frameworks further reduced the bandgap of O-VN-CN, allowing a broader range of solar spectrum to be harnessed for the generation of more electron-hole pairs to participate in photocatalytic applications (FIG. 7).

[0191] Since the band edge positions greatly affect the separation and transportation of photogenerated electron-hole pairs, the electronic band structures of our developed photocatalysts are systematically investigated to elucidate the flowchart of charge carriers. Based on the Mott-Schottky analysis in FIG. 60, positive slopes were observed for all samples, indicating that the intrinsic n-type semiconductor characteristic of graphitic carbon nitride remained unchanged after the introduction of nitrogen defects and oxygen doping. For n-type semiconductors, the Mott-Schottky equation is given by Eq. (1 ): where C is the capacitance of the space charge region, e is the electron charge, E is the dielectric constant of the photocatalyst, E» is the vacuum permittivity, ND is the number of donors, is the applied voltage, Ea> is the flat band potential, is Boltzmann’s constant, and 7" is the absolute temperature. From Eq. (1 ), plotting 1 / CZversus E should yield a straight line, from which the Efbof the photocatalyst can be determined by the intercept of the extrapolated straight line with X-axis. From FIG. 6C, the Efbof ON, VN-CN, and O-VN-CN were estimated as -0.93, -0.93, and -1 .08 V vs. Ag / AgCI, respectively, corresponding to -0.73, -0.73, and - 0.88 V vs. NHE, using the conversion formula (ENHE = EAg / Agci + 0.197 V). According to the rule of thumb, the CB minimum (CBM) of n-type semiconductor is typically -0.3 V from Efb. Thus, the CBM for CN, VN-CN, and O-VN-CN were inferred to be -1 .03, -1.03, and -1.18 V vs. NHE, respectively. Combining the bandgap energies determined from Tauc plots, the valence band maximum (VBM) of ca. 1.77, 1.63, and 1.22 V vs. NHE for CN, VN-CN, and O-VN-CN, respectively. Based on the UV-Vis absorption spectra in FIG. 6A, both VN-CN and O-VN-CN exhibited absorption tails, indicating the presence of vacancy-induced midgap states that position judiciously below the CB of the photocatalysts. The energy levels of the band tails, often referred to as Urbach tails, can be ascertained using the Urbach plot, described by the following expressions in Eq. (2) and Eq. (3):

[0192] Eq. (2) Eq. (3) where a is an absorption coefficient, a0is a constant, hv is the incident photon energy, and Eu denotes the Urbach energy, which can be approximated from the inverse of slope for the In a vs. hv. From the Urbach plots in FIG. 6D, the Eu values were determined as 0.17 and 0.19 eV for VN-CN and O-VN-CN, respectively, corresponding to the gap from CBM to the midgap states. The relatively small Eu values indicates the close distance of the midgap states to the CBM, signifying the midgap states serve as shallow trap states which not only facilitate the separation of photogenerated charge carriers, but also allow a lower energy excitation pathway for secondary electron excitation by harnessing the long-wavelength photons. The presence of shallow trap states in VN-CN and O-VN-CN were further confirmed by accessing the transition energy (E() by extrapolating the Tauc plot to the X-axis (FIG. 6E). In good agreement to Eu evaluation, the exact levels for the defect states were attained from Etanalysis, further affirming the extrinsic sub-bands induced by tri-coordinated N3c defects located well below the CB of VN-CN and O-VN-CN. Since the vacancies-induced defect states are located not far below the CB, this suggests that these shallow trap states can serve as excellent sites for trapping the photogenerated electrons, allowing the O2 molecules to be spontaneously activated to superoxide radicals (*O2_) via one-electron oxygen reduction. In addition, the upshifting of the CB level of VN-CN and O-VN-CN is highly desirable, rendering stronger reduction power for ’02“ formation ( o2 / -o2~ = -0.33 vs. NHE), where these •O2_ radicals play pivotal roles in cleaving the bonds in plastics (FIG. 8). Reactions driven by photogenerated charge carriers and radicals are summarised as follow (Eq. (4) to Eq. (6)):

[0193] The exceptional photocatalytic activity of our developed O-VN-CN catalyst has sparked our interest in exploring how the O-linked heptazine units and tri-coordinated nitrogen vacancies assisted in photocatalytic reactions. To delve into this, we systematically carried out a series of characterizations to disclose the kinetics of photogenerated electron hole-pairs. Firstly, transient photocurrent responses of our developed samples were investigated under periodic visible light exposure with alternating on-off cycles. This method involves measuring the intensity of photocurrent responses arising from the movement of photogenerated electrons towards the back contact, whereas the photogenerated holes transfer to the photocatalystelectrolyte interface which are then captured by the electrolyte’s reduced species (Chem. Eng. J. 2019, 372, 1183-1 193). Given the electrodes and electrolyte were prepared identically for each measurement, the photocurrent intensity serves as a direct indicator of the charge carriers generation, transportation, and separation efficiency. From FIG. 9A, GN demonstrated poor photocurrent responses, suggesting the intrinsic rapid recombination of photogenerated charge carriers in pristine g-CsIS . With the presence of tri-coordinated nitrogen vacancies that serve as the shallow trapping sites, VN-CN displayed higher photocurrent intensity, while the co-existence of O-linked triazine units and nitrogen defects boosted the photocurrent responses of O-VN-CN.

[0194] Aside from the improved photocurrent responses that suggest more photogenerated charge carriers to take part in the photocatalytic reactions, the synergistic roles of internal electric field originated from the O-linked triazine units and nitrogen defects were divulged by the electrical impedance spectroscopy (EIS) Nyquist plots. Based on the EIS Nyquist plots in FIG. 9B, a similar trend to photocurrent responses was observed, in which O-VN-CN displayed smallest arc radius, followed by VN-CN and lastly CN. Since a smaller arc radius of the Nyquist plots indicates a lower resistance for electron transfer and an improved separation of photogenerated electron-hole pairs that facilitates the interfacial charge transfer, the presence of the opportune defects and foreign atoms in O-VN-CN greatly boost the migration and separation of charge carriers. The much depressed Nyquist arc radius of O-VN-CN suggests its superior interfacial charge transfer efficiency. The separation efficiency of the photogenerated charge carriers was further elucidated by steady-state photoluminescence (PL) spectroscopy. Under an excitation wavelength of 350 nm, CN exhibited a strong emission peak centered at 451 nm, as shown in FIG. 90. This emission wavelength coincides with the absorption edge of CN as determined from the UV- Vis spectra in FIG. 6A, demonstrating intrinsic band-to-band recombination of GN. The observed fluorescence emission arises from energy dissipation corresponding to the bandgap between the CB and VB of CN. For VN-CN, a significant suppression of the emission peak was observed. This suggests that the presence of nitrogen vacancies that act as trapping sites for charge carriers. Consequently, the intrinsic radiative recombination of electron-hole pairs was greatly inhibited. Furthermore, with the co-existence of nitrogen vacancies and foreign oxygen atoms doping, strong PL quenching was observed for O-VN-CN, demonstrating their synergistic role in retarding the recombination of photogenerated charge carriers. In comparison to CN, significant red-shifting of emission peaks was observed for VN-CN and O- VN-CN, offering further evidence for the enhanced light harvesting ability of the defect- engineered samples.

Claims

Claims1. A compound comprising carbon nitride with tri-coordinated nitrogen vacancies and oxygen-linked heptazine units.

2. The compound according to Claim 1 , wherein the compound has a specific surface area of from about 90 to about 180 m2 / g, such as from about 100 to about 130 m2 / g, such as about 120.35 m2 / g.

3. The compound according to Claim 1 or Claim 2, wherein the compound has a plurality of pores having a diameter of about 1 nm to about 120 nm, such as from about 5 nm to about 100 nm.

4. The compound according to any one of Claims 1 to 3, wherein the compound has an optical band gap of from 2.0 eV to 3.5 eV, such as from 2.0 eV to 2.80 eV, such as 2.40 eV.

5. The compound according to any one of Claims 1 to 4, wherein the compound has a conduction band minimum (CBM) of from about -1 .0 V to about -1 .25 V versus NHE, such as from about -1.15 V to about -1.2 V versus NHE, such as about -1.18 V versus NHE.

6. The compound according to any one of Claims 1 to 5, wherein the compound has a valence band maximum (VBM) of from about 1 .0 V to about 1 .7 V versus NHE, such as from about 1 .1 V to about 1 .3 V versus NHE, such as about 1 .22 V versus NHE.

7. The compound according to any one of Claims 1 to 6, wherein the compound has an Urbach energy of from about 0.1 eV to about 0.25 eV, such as from about 0.15 to about 0.20 eV, such as about 0.19 eV.

8. The compound according to any one of Claims 1 to 7, wherein the compound is capable of at least partially converting a plastic into smaller molecules.

9. The compound according to any one of Claims 8, wherein the compound is capable of at least partially converting the plastic into smaller molecules upon irradiation with light.

10. The compound according to Claim 8 or Claim 9, wherein the plastic comprises plastic materials selected from one or more types of plastics classified under resin codes 1 - 7.11 . The compound according to any one of Claims 1 to 10, wherein the compound is metal- free.

12. A method of synthesizing the compound according to any one of Claims 1 to 1 1 , comprising the following steps:(a) mixing a nitrogen-containing precursor and an ammonium salt to form a first mixture;(b) calcinating the first mixture for a first period of time at a first temperature;(c) cooling the mixture from step (b) to room temperature and grinding the mixture to form a second mixture; and(d) calcinating the second mixture for a second period of time at a second temperature to obtain the compound.

13. The method according to Claim 12, wherein the nitrogen-containing precursor is a nitrogen-rich precursor, optionally wherein the nitrogen-containing precursor is selected from one or more of the group consisting of urea, thiourea, melamine, cyanamide, dicyandiamide, and guanidine hydrochloride.

14. The method according to Claim 12, wherein the nitrogen-containing precursor is urea.

15. The method according to any one of Claims 12 to 14, wherein the ammonium salt is selected from one or more of the group consisting of ammonium acetate, ammonium bicarbonate, ammonium formate, ammonium carbonate, and ammonium oxalate.

16. The method according to any one of Claims 12 to 14, wherein the ammonium salt is ammonium acetate.

17. The method according to any one of Claims 12 to 16, wherein the first period of time is from about 2 hours to about 5 hours, such as from about 3.5 hours to about 4.5 hours, such as about 4 hours, and / or the first temperature is from about 500 °C to about 600 °C, such as from about 545 °C to about 560 °C, such as about 550 °C.

18. The method according to any one of Claims 12 to 17, wherein the second period of time is from about 1 hour to about 3 hours, such as from about 1 .5 hours to about 2.5 hours, such as about 2 hours, and / or the second temperature is from about 500 °C to about 600 °C, such as from about 545 °C to about 560 °C, such as about 550 °C.

19. The method according to any one of Claims 12 to 18, wherein step (b) has a first ramping rate of from about 2 °C / min to about 10 °C / min, such as about 5 °C / min.

20. The method according to any one of Claims 1 to 19, wherein step (d) has a second ramping rate of from about 2 °C / min to about 10 °C / min, such as about 5 °C / min.21 . The method according to any one of Claims 12 to 20, wherein step (d) is under air or an oxygen-enriched atmosphere.

22. A method of upcycling plastics, comprising the following steps:(i) providing a plastic, a solvent system, and the compound according to any one of Claims 1 to 1 1 to form a reaction mixture; and(ii) irradiating the reaction mixture with a light source for a third period of time, such that the plastic is at least partially converted into smaller molecules.

23. The method according to Claim 22, wherein the plastic comprises plastic materials selected from one or more types of plastics classified under resin codes 1 - 7.

24. The method according to Claim 22 or Claim 23, wherein the plastic comprises plastics classified under resin code 1 , i.e. polyethylene terephthalate (PET), and the solvent system is an aqueous solution comprising an alkali metal hydroxide.

25. The method according to Claim 22 or Claim 23, wherein the plastic comprises one or more plastics classified under resin codes 2 - 7, and the solvent system comprises one or more organic solvents and / or an aqueous solution that comprises an alkali metal hydroxide.

26. The method according to Claim 22 or Claim 23, wherein the plastic comprises plastics classified under resin code 1 , i.e. polyethylene terephthalate (PET) and one or more plastics classified under resin codes 2 - 7, and the solvent system comprises one or more organic solvents and / or an aqueous solution that comprises an alkali metal hydroxide.

27. The method according to Claims 24 to 26, wherein the alkali metal hydroxide is selected from one or more of the group consisting of sodium hydroxide (NaOH), potassium hydroxide (KOH), lithium hydroxide (LiOH), preferably wherein the alkali metal hydroxide is NaOH.

28. The method according to Claim 27, wherein the aqueous solution has a concentration of from about 0.5M to about 12M, such as from about 1 M to about 10M, such as about 1 M, about 2M, about 5M, or about 10M, preferably about 1 M.

29. The method according to Claim 25 or Claim 26, wherein the one or more organic solvents is selected from one or more of the group consisting of 1 ,2-dichloroethane (DCE), acetonitrile, dichloromethane (DCM), ethyl acetate, acetone, dimethyl sulfoxide (DMSO), dimethylformamide (DMF), 1 ,2-dichlorobenzene, tetrahydrofuran (THF), and 2- methyltetrahydrofuran (2-MeTHF).

30. The method according to any one of Claims 22 to 23 and 25 to 29, comprising an additional step (ia) prior to step (i):(ia) melting a plastic; wherein the plastic comprises plastics classified under one or more of the following resin codes:(I) resin code 2, i.e. high density polyethylene (HDPE),(II) resin code 4, i.e. low density polyethylene (LDPE), and / or(III) resin code 7, i.e. other plastics not classified under resin codes 1-6; or (ia) converting a plastic into a powder.31 . The method according to any one of Claims 22 to 30, wherein the light source is one or more of the group consisting of sunlight, a Xenon lamp, a fluorescent lamp, an LED light, a visible light source, a white light source, and a blue light source, preferably a blue light source with a wavelength of 456 nm or 448 nm.

32. The method according to any one of Claims 22 to 31 , wherein the smaller molecules formed is one or more of the group consisting of formic acid, benzoic acid, acetic acid, acetophenone, ethylene glycol, and terephthalic acid.

33. A use of the compound according to any one of Claims 1 to 1 1 in upcycling plastic.

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