Plastic is an indispensable material with a global production reaching nearly 400 million tons in 20201. 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 waste2–4. Polystyrene is a kind of common synthetic plastic, which is often used to make various kinds of packaging, disposable tableware, and fillers, but its recycling rate is very low.
The traditional chemical recovery 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 reported5-8.
In our paper, we reported a heterogeneous g-C3N4 catalyzed oxidation of polystyrene to aromatic oxygenates. The catalytic transformation was performed at 150 ℃ in acetonitile solvent with visible light irradation. Highly efficient conversion of polystyrene to aromatic oxygenates such as benzoic acid was achieved (> 90% conversion, > 60% selectivity of aromatic oxygenates).
This study have shown that there is an induction period in the catalytic oxidation reaction. At this time, 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 further progress 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 Figure 1.
Figure 1. Proposed reaction mechanism of g-C3N4 catalyzed photooxidation of polystyrene.
On the basis of a preliminary understanding of the reaction mechanism, we designed a liquid-phase circulating reaction system to separate the reaction products at an appropriate stage of reaction to avoid excessive oxidation side reactions, and achieved the conversion of 0.5 g of commercial polystyrene foam pellets to 0.24 g of seperated pure benzoic acid, as shown in Figure 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).
Figure 2. Conversion of polystyrene foam pellets to pure benzoic acid.
For more information, please see our recent publication in Nature Communications: https://www.nature.com/articles/s41467-022-32510-x
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