Selective oxidative upgrade of waste polystyrene plastics by nitric acid to produce benzoic acid†

Polystyrene plastic is a widely used artificial material, but there is still no efficient, economic or environmentally friendly recycling method for waste polystyrene plastic, which has caused serious environmental pollution and a waste of resources. Herein, the method of nitric acid oxidative upgrade is used to convert polystyrene plastic into a high-value chemical raw material, benzoic acid. The yield can reach nearly 90% at 180 °C within 3 h, and the purity of the product is more than 95%. In this process, nitric acid is decomposed by heat to generate nitrogen dioxide and oxygen, which react with the carbon-centered radicals formed at weak sites on the long chain of polystyrene and further generate peroxy radicals and hydroxyl radicals. The formation of oxygen-containing functional groups promoted the fracture of the C–C bond and eventually formed benzoic acid. In addition, this method also has good treatment effects on real-life polystyrene plastic products. This research provides a new method for the recycling and high-value utilization of waste polystyrene plastics. The intrinsic material value of polymers can be maintained by recycling functional chemicals through oxidative upgrade.