The advantages of plastics, such as low cost, long durability, and chemical versatility, have led to their use in various sectors of society. Yet, the omnipresence of plastics has been recognized to be a double-edged sword, as most of them ended up in landfill or leaked into the natural environment as waste. The plastic waste can persist in the environment for many decades, and some of them are leaked into the water system as microplastics, leading to serious environmental and health risks. It is therefore imperative to design socially- and industrially-viable approaches for plastic waste management.
Among the emerging techniques, catalytic upcycling of plastics stands out that can transform waste into value-added products, allowing synchronous environmental remediation and chemical production. However, its industry-grade practice is challenging since the economic value of the corresponding products cannot recover the costs of the upcycling process. Polyolefins constitute most of the global plastics produced and discarded currently, and their monomers are linked by inert C−C bonds. Upcycling of polyolefin waste has been hampered by the need for relatively high temperatures to crack the inert bonds at high rates and conversion yields. In our recent work, we designed an ambient, sustainable, and coreactant-free solar thermal route to upcycle the polyolefin waste into high-value hydrocarbons, operating under the notably mild temperature of 55 °C that generated photothermally at just 4 suns (400 mW/cm2). The soft solar reaction conditions and simplicity of the chemistry collectively bode well for developing an energy and environmentally sustainable polymer upcycling technology.
Our strategy utilized Cu/two-dimensional (2D) Si as the catalyst, combining the outstanding dehydrogenation activity of metallic Cu nanoparticles and excellent solar light absorption properties of silicon nanosheets. The Cu/2D Si heterostructure catalysts were prepared by encapsulating copper nanoparticles in stacked 2D Si. The chloroaluminateionic ionic liquid, [C4Py]Cl-AlCl3, was diluted in chloroform as the solvent to establish a highly polar reaction environment, stabilizing the ionic intermediates of plastic cracking and thereby lowering the reaction barriers. This strategy enables rapid and total conversion of polyethylene to C3 to C26 hydrocarbons under a low photothermally raised temperature of 55 °C within merely 6 hours (Figure 1). Our mechanistic studies proposed a selective polyolefin cracking pathway involving two β-scissions of C–C bonds, a rapid intramolecular cyclization, without terminal C–C cleavage. The distinctive feature generated products with a distinct distribution consists of valuable alkanes (C3-C7) and cyclic hydrocarbons (C8-C26). In the process, the Cu nanoparticles serve as the active sites for the dehydrogenation process, and 2D Si promotes light absorption, which concertedly enhances the conversion efficiency.
The surprising catalytic performance motivated us to further explore the economic feasibility of our study. We were lucky to get supervision from Professor Ozin, one of the most world-renowned and experienced scientists in Nanochemistry, who raised a critical question about the sustainability feature of this work: instead of using sunlight to drive the photothermal catalytic process, why not use thermal catalysis powered by renewable electricity to drive the reaction? This question is very thought-provoking since, from a larger perspective of solar energy utilization, the conversion efficiency of photocatalysis has always been much lower than that of solar cells. For example, for one of the most established fields, solar-driven water splitting, the photovoltaic-assisted electrolysis (PV-E) route can achieve solar-to-hydrogen (STH) conversion efficiencies of more than 30% (Bai, X., et al., iScience 24, 102056 (2021)). As far as we know, the highest STH conversion efficiency of particulate photocatalytic systems is 9% achieved under about 70 °C (Zhou, P., et al. Nature 613, 66–70 (2023)), while other systems operating at room temperature are typically around 1%. Given the consideration, we compared the energy consumption of our solar thermal upcycling system and a thermal upcycling system powered by a solar cell with similar catalytic performance. Notably, estimations show that our solar thermal system only requires about a quarter of the solar power (23.76 kJ) of the thermal system (91.61 kJ), signifying its superiority. We think this is because of the unique localized heat effect of the photothermal process, which generates intense local heating at the nanostructured catalyst surface, enabling fast reaction rates and minimizing energy consumption.
Another surprising fact is that the versatility of our solar thermal system in the upcycling of various real-world polyolefin waste, and the related explorations are inspired by Nature Editor, Dr. Claire Hansell and reviewers in the peer review process (Figure 2). The real-world plastics investigated in our work included the six most common plastics (PET, HDPE, PVC, LDPE, PP, and PS), and combinations thereof. The system demonstrated effective conversions for various post-consumer LDPE, HDPE, and PP, whether individually or in mixed form. The effects of PVC, PS, and PET as impurities on upcycling the mixed polyolefin plastics were also revealed. And we propose a facile and scalable slicing-separation-conversion protocol to separate polyolefins from other heavier-than-water polymers, such as PET, PVC, and PS, via a simple sink-float method in water, achieving a total solution for upcycling of polyolefins from real-world mixed wastes. Besides, based on techno-economic analysis and life cycle assessment of a conceptual upcycling facility, our technology yields economic feasibility in certain regions and 30% reduced greenhouse gas emissions compared with non-renewable fossil-derived processes.
For more information, please refer to our recent publication in Nature Catalysis, “Ambient Solar Thermal Catalysis for Polyolefin Upcycling Using Copper Encapsulated in Silicon Nanosheets and Chloroaluminate Ionic Liquid”. The blog was drafted by Chuanwang Xing and edited by Geoffrey Ozin.