Photothermal recycling of waste polyolefin plastics into liquid fuels with high selectivity under solvent-free conditions

The excessive use of polyolefin plastics, encompassing low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP), has resulted in an environmental crisis. Over 8 billion tons of plastics have been produced, with approximately 80% accumulating in landfills or polluting natural environments, posing a significant ecological threat, particularly due to the accumulation of microplastics. Polyolefins constitute a substantial portion (~57%) of global plastic production, with annual production reaching ~220 million tons and projected to quadruple by 2050. Current global recycling rates remain disappointingly low (5% for LDPE, 10% for HDPE, and <1% for PP), representing a massive untapped resource with an estimated annual value exceeding US$100 billion if efficiently recycled. Traditional recycling methods are energy-intensive due to the inert nature of C–C and C–H bonds in polyolefins. Pyrolysis requires harsh conditions (>500 °C), while advanced hydrogenolysis, although operating at lower temperatures (~300 °C), still demands external heat. This research addresses the need for sustainable and low-cost polyolefin recycling by exploring the potential of solar energy. While photocatalytic degradation using TiO₂, ZnO, and other catalysts shows promise, the rates are currently too low for practical application. This study investigates photothermal catalysis, combining the advantages of thermal and photocatalysis, to efficiently recycle polyolefin plastics using concentrated sunlight or a xenon lamp as the energy source. The hypothesis is that photothermal recycling, leveraging a catalyst's ability to absorb light and induce local heating above the polymers' melting points, will enhance catalyst-plastic contact and facilitate C–C and C–H bond scission, leading to the production of valuable liquid fuels.

Existing literature highlights the challenges associated with polyolefin recycling. Traditional methods such as pyrolysis are energy-intensive, requiring high temperatures to break down the robust C-C and C-H bonds. Advanced hydrogenolysis offers a lower-temperature alternative but still necessitates external heat input. Photocatalytic approaches utilizing materials like TiO2, ZnO, and NiAl2O4 spinels have shown potential for plastic mineralization under UV or visible light, yet their reaction rates are insufficient for large-scale application. Recent research has demonstrated the potential of alkaline hydrolysis and photocatalytic oxidation of certain plastics to yield monomers, which can be further converted into organic chemicals. However, these methods primarily focus on specific polymer types and may not be broadly applicable to polyolefins. The study builds upon the growing interest in photothermal catalysis, where catalysts absorb light across the UV-Vis-NIR spectrum, leading to rapid local heating and initiating thermal catalytic reactions. This approach combines the benefits of thermal and photocatalysis, potentially enhancing efficiency. The authors posit that photothermal catalysis, by achieving temperatures above the melting points of polyolefins, will improve catalyst-polymer contact and promote C-C and C-H bond scission, leading to efficient conversion into valuable products.

This research developed a solvent-free photothermal method for recycling polyolefin plastics using a Ru/TiO₂ catalyst. The catalyst, synthesized by impregnating TiO₂ (Degussa P25) with RuCl₃ and subsequent reduction, exhibited strong light absorption across the UV-Vis-NIR range, enabling efficient photothermal heating. Characterizations using techniques like XRD, TEM, and EDX confirmed the uniform dispersion of ~2.5 nm Ru nanoparticles on the TiO₂ support. The crucial aspect of intimate catalyst-plastic contact was addressed by demonstrating that photothermal heating melts the plastics, leading to better wetting of the catalyst compared to traditional photocatalytic approaches. Contact angle measurements verified this enhanced contact at higher temperatures. A custom-built photothermal reactor was used, incorporating a Xe lamp (or concentrated sunlight) as the light source, a thermocouple for temperature monitoring, and ports for gas sampling and product collection. Experiments involved mixing the catalyst with various polyolefin plastics (LDPE, HDPE, UHMWPE, PP, and commercial LDPE bags) and subjecting them to photothermal treatment under controlled temperature (200–350 °C) and pressure (1–40 bar) conditions in a H₂/Ar or H₂/N₂ atmosphere. Product analysis included gas chromatography (GC) for gaseous products (C₁-C₄), and high-temperature gas chromatography (HTGC) and ¹H NMR for liquid/waxy products (C₅+). Gel permeation chromatography (GPC) was employed to analyze the molecular weight changes of the polymers during degradation. Mechanistic studies involved varying light irradiation regimes (UV-Vis, Vis, NIR) to determine the contribution of different wavelengths to the degradation process. Control experiments were conducted under thermal conditions (without light irradiation) and using individual catalyst components (Ru and TiO₂) to identify the active sites. High-pressure experiments were performed using a modified reactor to optimize the product distribution by varying pressure. Finally, the scalability of the method was tested using a larger-scale (5 g) reaction, and the efficiency was demonstrated using concentrated sunlight.

The photothermal recycling system demonstrated remarkable efficiency in converting various polyolefin plastics into valuable products. Under optimized conditions (300 °C, 1 bar H₂/Ar, 20 h), nearly complete degradation of LDPE (95%) was achieved, significantly surpassing thermal degradation (7.8%) and photolysis (negligible). The molecular weight of LDPE decreased drastically during photothermal treatment, indicating efficient C-C bond scission. The main products at these conditions were methane (selectivity >90% in gaseous products) and liquid/waxy fuels with a C₂₇-centered distribution. The photothermal approach also efficiently recycled HDPE, UHMWPE, and PP, with degradation percentages exceeding 87%. Commercial LDPE bags were also successfully recycled with a 97.3% degradation rate. Mechanistic studies revealed that UV light (λ < 365 nm) plays a crucial role in activating the LDPE chains, creating reaction sites for the Ru nanoparticles to catalyze C-C bond scission. Vis and NIR light mainly contributed to local heating, melting the plastics and enhancing catalyst-polymer contact. Experiments using individual components confirmed Ru nanoparticles as the primary active sites for C-C bond scission. By increasing the pressure to 30 bar at 220 °C, the selectivity to valuable liquid fuels (86% gasoline- and diesel-range hydrocarbons, C₅–C₂₁) increased significantly, reaching almost 100% liquid products within 3 h. The system also demonstrated scalability, achieving 87% selectivity to gasoline and diesel range hydrocarbons in a 5 g scale reaction. Finally, successful recycling was demonstrated using concentrated sunlight, showcasing the potential for solar-driven plastic waste valorization.

The findings address the research question by demonstrating the feasibility and efficiency of photothermal catalysis for polyolefin recycling. The synergistic effect of UV-activated polymer chains and the Ru-catalyzed C-C bond scission under photothermal conditions leads to a significantly improved recycling rate compared to conventional methods. The high selectivity to liquid fuels, especially under elevated pressures, showcases the potential for converting waste plastics into valuable energy sources. The use of abundant solar energy further enhances the sustainability of the process. The successful application of this technology across various polyolefin types and even commercial LDPE bags demonstrates its potential for broad applicability in addressing the global plastic waste problem. The results significantly advance the field of plastic recycling by offering a promising alternative to energy-intensive conventional methods, promoting a circular economy approach and contributing to environmental sustainability.

This study presents a highly efficient and sustainable photothermal catalytic system for recycling polyolefin plastics into valuable liquid fuels. The method successfully leverages the synergistic effects of UV light activation, Ru-catalyzed hydrogenolysis, and photothermal heating to achieve near-complete conversion of various polyolefin waste streams into useful products. The system's adaptability to different polyolefin types and its demonstrated scalability and potential for solar energy utilization open exciting avenues for large-scale plastic waste management. Future research directions include exploring alternative catalysts, optimizing reaction parameters further, and conducting comprehensive life cycle assessments to fully evaluate the environmental and economic impacts of this technology.

While the photothermal recycling system demonstrated high efficiency, some limitations exist. The current study mainly focused on LDPE and other common polyolefins; further research is needed to explore its effectiveness on other plastic types, including complex copolymers and plastics containing additives or fillers that might interfere with the process. The influence of various additives commonly found in commercial plastics on the efficiency and selectivity of the process should be studied more extensively. The long-term stability and durability of the Ru/TiO₂ catalyst under continuous operation need further investigation. More comprehensive techno-economic analysis at industrial scale is needed to assess the full economic viability of this technology.