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.
Valorizing lignin into valuable chemicals requires the selective breaking of a large number of C–O bonds found in the β–O–4 structure. In this study, we present a one-pot strategy that utilizes two NiO/TiO2 catalysts for the photocatalytic depolymerization of β–O–4 alcohols. The S-1.6%NiO/TiO2 catalyst is used for the dehydrogenation of β–O–4 alcohols at 450 nm. Then, the L-0.8%NiO/TiO2 catalyst is employed for the hydrogenolysis of β–O–4 ketones at 365 nm. Remarkably, the dehydrogenation–hydrogenolysis tandem reaction can be conducted in one pot, which affords high yields of ketones and phenols (up to 89%) from β–O–4 alcohols. Additionally, the L-0.8%NiO/TiO2 catalyst prepared in this article is the smallest NiO/TiO2 photocatalyst reported to date, and the size of NiO in the S-1.6%NiO/TiO2 catalyst is even small enough to enter the lattice of TiO2. We successfully regulated the reaction selectivity by controlling the loading method and the amount of NiO. Our work offers a promising solution for efficiently cleaving target bonds in lignin and provides insights into the design of efficient photocatalysts.
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