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

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/TiO 2 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 ·O 2 − radicals as the reactive oxygen species. A possible oxidative mechanism was proposed: the ·O 2 − radicals first activated the C-H bonds in the aliphatic portion of PS to generate carbonyl groups or C&z.dbd;C bonds; then, through attack of the weak C&z.dbd;C 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. A heterogeneous oxidative upcycling strategy converting polystyrene (PS) waste into benzoic acid.