The team led by Ding Ma and Meng Wang at the College of Chemistry and Molecular Engineering, Peking University have developed a heterogeneous photocatalytic method to oxidize polystyrene to aromatic oxygenates in liquid phase in the presence of a graphitic carbon nitride (g-C3N4) catalyst under visible light irradiation. The reaction mechanism is also investigated with various characterization methods and proceeds via polystyrene activation to form hydroxyl and carbonyl groups over its backbone via C―H bond oxidation which is followed by oxidative bond breakage via C―C activation and further oxidation processes to aromatic oxygenates. The research article was published online by the journal Nature Communications on August 16, 2022, with the title "Catalytic Oxidation of Polystyrene to Aromatic Oxygenates over a Graphitic Carbon Nitride Catalyst".
Plastic is an indispensable material with a global production reaching nearly 400 million tons in 2020. In the past, the post-consumer plastics were considered as garbage. It is estimated that more than 80% of waste plastics are directly discarded, landfilled or incinerated after use, without effective recycling, resulting in serious environmental pollution and resource waste. Polystyrene is a common synthetic plastic, which is often used to produce various kinds of packaging, disposable tableware, and fillers, but its recycling rate is very low. The traditional chemical recycling method of polystyrene is to obtain mixed aromatics by catalytic pyrolysis at a temperature above 300 °C. Recently, new processes of converting polystyrene into benzoic acid using a homogeneous catalyst were reported.
In this work, a heterogeneous graphitic carbon nitride (g-C3N4) catalyzed photo-oxidation of polystyrene to aromatic oxygenates was reported. The catalytic transformation was performed at 150 °C in acetonitrile solvent with visible light irradiation. Highly efficient conversion of polystyrene to aromatic oxygenates such as benzoic acid was achieved (> 90% conversion, > 60% selectivity of aromatic oxygenates).
An induction period was observed in the catalytic oxidation reaction. In induction period, random partial oxidation first occurs on polystyrene, and active groups such as hydroxyl and carbonyl are introduced into its backbone, while the formation rate of small molecular products is slow. In the subsequent stage of the reaction, the activated C―C bond adjacent to the oxygen-containing group is cleaved, and finally small molecular products such as benzaldehyde, acetophenone, and benzoic acid are obtained, as shown in Fig. 1.
Fig. 1 Proposed reaction mechanism of g-C3N4 catalyzed photo-oxidation of polystyrene.
On the basis of a preliminary understanding of the reaction mechanism, a liquid-phase circulatory reaction system was designed to remove the reaction products at an appropriate stage of reaction to prevent the over-oxidation. The conversion of 0.5 g of commercial polystyrene foam pellets to 0.24 g of separated pure benzoic acid was achieved by the liquid-phase circulatory reaction system, as shown in Fig. 2. In addition, the solubility and catalytic performance of polystyrene plastics can be further improved by certain oxidative pretreatment. By changing the weight hourly space velocity (WHSV) of the polystyrene solution in the liquid-phase circulating reaction system, the liquid-phase product selection can also be altered (50% benzaldehyde or 74% benzoic acid).
Fig. 2 Conversion of polystyrene foam pellets to pure benzoic acid.
Ruochen Cao, a Ph.D. student from the College of Chemistry and Molecular Engineering, Peking University, is the first author of this paper. Prof. Ding Ma and Assoc. Prof. Meng Wang, from the College of Chemistry and Molecular Engineering, Peking University, are the co-corresponding authors. This work received financial support from the National Natural Science Foundation of China, the Key Research Program of Nanjing IPE Institute of Green Manufacturing Industry, the Beijing National Laboratory for Molecular Sciences in China.
Original link for the paper: https://www.nature.com/articles/s41467-022-32510-x