Polystyrene (PS) is widely used in daily life due to its advantages such as good thermal stability, light weight and low price. For example, as food packaging boxes, foam incubators, and building materials, etc. According to statistics, PS plastic produced annually accounts for 6% of the global market share of plastic products, but its recycling rate after waste is less than 1%. At present, waste PS accounts for about one-third of the total amount of waste plastics in landfills, which brings serious pollution to the earth's ecological environment and poses a potential threat to the health of animals and humans. Converting waste PS plastics into high-value aromatic hydrocarbons is a very promising means of recycling waste plastics. At present, there are still many challenges in the chemical recovery of PS, such as the catalyst is easy to deposition and deactivate at high temperatures, the need to use organic solvents or homogeneous catalysts, and the low conversion rate. Therefore, it is of great significance to design the reaction process to optimize the chemical recovery of PS to prepare high-value alkylbenzene.
Recently, Professor Zeng Jie and Associate Professor Li Hongliang of the University of Science and Technology of China reported a new PS degradation pathway, the construction of ruthenium-supported silica catalyst (Ru/SiO2) for the efficient degradation of methanol in situ hydrogen production to assist the efficient degradation of PS to prepare alkylbenzene depolymerization" in Angew. Chem. Int. Ed.》（DOI：/10.1002/anie.202404952）。 After a 6 h reaction at 280°C, the yield of the liquid phase product reached 93.2 wt%. Among them, the main products are high-value alkylbenzene (monocyclic aromatic hydrocarbons and diphenylalkanes) with a selectivity of 84.3 wt%.
Comparison of the performance of Ru/SiO2-catalyzed PS hydrogenolysis, PS pyrolysis and methanol-assisted PS degradation reaction after 6 h at 280°C (Image source: Angew. Chem. Int. Ed.）
The degradation of PS by pyrolysis faces the problems of slow reaction rate and catalyst carbon deposition. At the same time, the recovery of benzene rings was limited by the fused rings and biphenyls generated by the dehydroaromagenation of the PS backbone. However, PS hydrogenolysis faces the problem of over-hydrogenation of benzene rings. Through theoretical analysis, the authors found that methanol as a hydrogen carrier assisted PS degradation was better than pyrolysis. At the same time, the hydrogen species produced by the in-situ decomposition of methanol can inhibit the aromatization of the PS backbone, thereby avoiding the formation of carbon deposition. Compared with hydrogenation, the low hydrogen partial pressure generated by the in-situ decomposition of methanol inhibits the overhydrogenation of the benzene ring, thereby improving the selectivity of alkylbenzene products. The experimental results showed that the yield of liquid phase products (49.6 wt% for PS pyrolysis, 41.6 wt% for PS hydrogenation, 93.2 wt% for methanol-assisted PS degradation), and the selectivity of monocyclic aromatic hydrocarbons and diphenylalkanes (74.7 wt% for PS pyrolysis, 51.4 wt% for PS hydrogenation, and 84.3 wt% for methanol-assisted PS degradation) were significantly improved after the introduction of methanol.
Ru/SiO2 catalyzes the reaction route of methanol-assisted PS degradation (Image source: Angew. Chem. Int. Ed.）
This work evaluated the catalytic performance of Ru/SiO2 for the degradation of commercially discarded PS, including test tubes, measuring cups, bottles, and styrofoam. The above samples were cut into centimeter-sized fragments and catalyzed at 280°C for 6 hours, and all kinds of commercially discarded PS were almost completely degraded. Among them, the yield of liquid phase products after degrading test tubes, measuring cups, bottles and foam plastics reached 93.5 wt%, 95.6% wt%, 97.6% wt% and 97.9% wt, respectively, proving the universality of Ru/SiO2-catalyzed methanol-assisted PS degradation strategy for commercial waste plastic recycling.
Effects of reaction time, reaction temperature and different commercial waste PS on the degradation performance of Ru/SiO2 catalyzed methanol-assisted PS (Image source: Angew. Chem. Int. Ed.）
brief summary
Through the construction of a new reaction path and the analysis of the reaction mechanism, the researchers proposed a strategy for the preparation of alkylbenzene by in-situ hydrogen production of methanol to degrade PS, which realized the efficient degradation of waste PS and helped to explore a lower energy consumption and more efficient PS recovery process in the future.
Source: Frontiers of Polymer Science