Metal-Free Photocatalysts for the Conversion of Plastic Wastes into Valuable Chemicals and Fuels at Ambient Conditions

Plastic has significantly enhanced the convenience of modern life. As one of the most robust synthetic materials, plastics offer a range of advantageous properties, including lightness, durability, affordability, and wide applicability, leading to their ever-increasing demand and production over the past decades. However, only 9% of global plastic waste had been recycled in 2019, 19% had been incinerated, while the remaining 72% of plastics remained in the environment. This results in severe environmental pollution, and the current unsustainable trajectory signifies the urgent need for innovation plastic recycling technologies that not only mitigate environmental issues but also valorize plastic waste as a resource.

To date, most plastic products are made of non-biodegradable polymers, including polystyrene (PS), polypropylene (PP), polyvinyl chloride (PVC), polyethylene (PE), and polyethylene terephthalate (PET). These materials can persist for 100 to 1000 years in natural environments, contributing to severe environmental pollution and ecological issues. Although some plastics, such as polycarbonate (PC), polyurethane (PU), and polylactic acid (PLA) are labelled as biodegradable, the decomposition of such plastics require specific conditions like microorganisms, enzymes, or light exposure for their decomposition into water (H₂O) and carbon dioxide (CO₂), a greenhouse gas. 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 a great challenge, as plastics are typically composed of long-chain organic molecules with highly stable and kinetically inert C–C bonds, requiring selective cleavage to chemically transform plastic waste into its monomer or value-added small molecules.

In this study, we developed a green approach to convert the unwanted plastic wastes into energy-rich fuels and high-value chemicals through the development of metal-free organo-photocatalysts with the co-existence of tri-coordinatied nitrogen vacancies and oxygen-linked heptazine units. This photocatalyst was designed to act as chemical scissors, selectively cleaving the highly stable and kinetically inert C–C bonds in biomass and plastics, thereby transforming them into commercially valuable small molecules. Based on our preliminary studies, we have successfully transformed a wide range of plastics, from resin codes 1–7, including conventional plastics, co-polymers, real-life plastic wastes, mixed plastics, and contaminated waste plastics into value-added platform chemicals and liquid organic hydrogen carrier, serving as a form of fuel. This was incredibly achieved without requiring any pre-treatment of the plastics, unlike other methods documented in the literature that typically require sorting, cleaning, or pre-treatment of plastic wastes in strong alkaline or acidic condition. Remarkably, our innovative photocatalytic chemical reactions can take place at room temperature and atmospheric pressure, contrasting with the most established plastic conversion method of pyrolysis, which requires elevated reaction temperatures (160–1000^oC). To the best of our knowledge, no existing research has yet demonstrated the capability to photocatalytically upcycle such a diverse array of plastics under ambient conditions. Our project aligns with global objectives for sustainable waste management, aiming to mitigate environmental pollution and promote a circular economy by harnessing the latent value in plastic wastes for waste-to-treasure conversion.