Heterogeneous oxidative upcycling of polystyrene plastics to benzoic acid under air conditions

Abstract

Plastics with inter-monomer C–C bond linkages comprise more than 70% of all plastics production, but their chemical recycling and upcycling usually require harsh conditions due to the inertness of C–C bonds, hindering their utilization efficiency. Here, we develop an oxidative upcycling strategy to convert polystyrene (PS) waste into benzoic acid over a NiO/TiO₂ catalyst. This system is carried out in an environmentally friendly manner in an aqueous phase by using air as the oxidant, and up to 51.1% carbon yield of benzoic acid is obtained at 200 °C, 1 MPa air and 18 h. The conversion of real-life PS plastics is also successfully demonstrated. The reaction mechanism is further investigated by capturing radicals and intermediates during the oxidative reaction, confirming ·O₂⁻ radicals as the reactive oxygen species. A possible oxidative mechanism was proposed: the ·O₂⁻ radicals first activated the C–H bonds in the aliphatic portion of PS to generate carbonyl groups or CC bonds; then, through attack of the weak CC bonds, the polymer was constantly depolymerized to smaller oxygenated oligomers, dimers and finally the target product, benzoic acid. This work has provided a promising and green polystyrene upcycling strategy.