First author by Junjie Du, Lin Zeng
Corresponding author: Zeng Jie
Communication unit: University of Science and Technology of China
Paper DOI: https://doi.org/10.1038/s41565-023-01429-9
Full text at a glance
Plastic pollution has become a global threat due to the proliferation of medical waste, personal protective equipment and takeaway packaging. To address this, sustainable and economically viable plastic recycling methods should avoid the use of consumable materials such as co-reactive agents or solvents. In this paper, the authors report that Ru nanoparticles loaded on HZSM-5 zeolite can be used to catalyze solvent-free and hydrogen-free upcycling recovery of high-density polyethylene to form separable straight-chain (C1 to C6) and cyclic (C7 to C15) hydrocarbons. Among them, high-value monocyclic hydrocarbons accounted for 60.3 mol% of the total yield. Through mechanistic studies, it is found that the formation of C=C bonds by dehydrogenation of polymer chains occurs at the Ru site and acid site of HZSM-5, while at the acid site, carbocation ions are produced by protonation of C=C bonds. Therefore, cyclization processes can be facilitated by optimizing Ru and acid sites, which requires the simultaneous presence of C=C bonds and carbocation ions at appropriate distances on the molecular chain, providing high activity and cyclic hydrocarbon selectivity.
Background
The problem of plastic pollution has become a serious challenge on a global scale. At present, most plastic waste is not recycled, but is incinerated and disposed of in landfills. According to preliminary estimates, 12,000 metric tons of plastic waste will end up in landfills or the natural environment by 2050. Therefore, urgent action is urgently needed to recycle plastic waste. Currently, most recycling processes face certain challenges, including high costs for recycling, sorting and processing. Plastics obtained by traditional mechanical recycling methods perform less well and cost more than virgin plastics that are mass-produced in centralized plants. Alternatives to mechanical methods are chemical recycling, which recovers the original monomer subunits for subsequent repolymerization, or selectively breaks down plastic waste into high value-added chemicals. Traditional energy-intensive pyrolysis methods typically produce large amounts of unwanted light hydrocarbons (C1 to C4), tar, and coke at 400–600°C. Catalytic hydrogenolysis can not only reduce the reaction temperature, but also improve the selectivity of target products such as hydrocarbons and low molecular weight waxes. However, these methods often require the consumption of large amounts of coreactants, such as hydrogen and olefins. A socially sustainable and economically viable approach is to use recyclable materials such as catalysts rather than consumables such as coreactants and solvents, and upgrade plastic waste to high-value products such as benzene, toluene and xylene.
Recent studies have found a solvent-free and hydrogen-free method for selectively breaking down polyethylene (PE) into high-value long-chain alkyl aromatics and alkyl naphthenes, known as tandem hydrodecomposition-aromatization. In this process, the hydrogenation, hydrogenation, and ring-opening steps on Pt/γ-Al2O3 consume the H2 molecule. These H2 molecules are generated in situ by closed-loop and dehydroaromatization, so no external input of H2 is required. However, the catalytic activity of Pt/γ-Al2O3 is not satisfactory, especially for the cleavage of HDPE. In addition, many linear and cyclic products have the same number of carbon atoms, which makes subsequent separation difficult. Therefore, there is still a lot of room for improvement of this catalyst to increase activity and obtain more separable products.
Graphic analysis
Figure 1. Catalytic performance of Ru/HZSM-5(300) in HDPE upcycling. a, photographs of liquid phase products (i), solid residues (ii), powder HDPE (iii) and Ru/HZSM-5 (300) (iv). b, Hydrocarbon selectivity of volatiles/gases and liquid phase products formed by Ru/HZSM-5(300) at 280°C, 24 h in HDPE upcycling. c, Time-dependent GPC analysis of solids residues generated by Ru/HZSM-5(300) at 280°C in HDPE upcycling. d, time course of HDPE conversion and product selectivity on Ru/HZSM-5(300) at 280°C in HDPE upcycling.
Figure 2. On Ru/HZSM-5 (300), the reaction route of HDPE upcycling.
Figure 3. The role of Ru/HZSM-5(300) in HDPE upcycling. a-c, HZSM-5 (300) and Ru/HZSM-5 (300) in HDPE upcycling, at 280°C, yield of volatiles/gases, liquid phase products and insoluble hydrocarbons (a), selectivity of volatiles/gas and liquid phase products (b), and GPC analysis of solid residues (c) after 24 h. Yield (d) and selectivity (e) of monocyclic aromatic hydrocarbons (d) and selectivity (e) of d,e, HZSM-5(300) and Ru/HZSM-5(300) at 280°C, 24 h cyclohexane conversion. f, in HDPE upcycling, reference catalyst Ru/SiO2 for 24 h at 280°C, selectivity of volatiles/gases and liquid phase products.
Figure 4. The role of zeolite in HDPE upcycling. a-f, yields of volatiles/gases, liquid phase products and insoluble hydrocarbons (a, d), selectivity of volatiles/gases and liquid phase products (b, e), and GPC analysis of solid residues (c, f); For HDPE upcyclings at Ru/HZSM-5 (25), Ru/HZSM-5 (80), Ru/HZSM-5 (200), and Ru/HZSM-5 (300), for 24 h at 280°C (a- c); and Ru/SAPO-34, Ru/USY, and Ru/HZSM-5(300) for 24 h at 280°C (d–f).
Figure 5. Stability evaluation of Ru/HZSM-5(300) in PE upcycling. a–d, volatiles/gases, liquid phase products and insoluble hydrocarbons yields (a, c), and volatiles/gas and liquid phase product selectivity (b, d); On Ru/HZSM-5 (300), three consecutive HDPE upcycle tests were performed for 24 h at 280°C (a,b); and upcycling tests of HDPE and LDPE for 24 h at 280°C on Ru/HZSM-5 (300) (c,d).
Summary and outlook
Overall, this article reports that Ru/HZSM-5 (300) can selectively upgrade HDPE. In the absence of solvents or external hydrogen, it is capable of generating separable linear and cyclic hydrocarbon distributions. The recovery of valuable monocyclic hydrocarbons from gaseous and liquid fractions accounted for 60.3 mol% of the total yield. In addition, the catalyst shows high stability and robustness in the upcycling of different commercial grades of polyethylene. This work offers a potential avenue for plastic waste recycling without any consumable materials. These value-added products may provide more economic benefits for plastic recycling. In addition, the results of this study have contributed to the understanding of the catalytic mechanism of plastic upcycling.