Aqueous-Phase Photothermal Co-Catalyzed Oxidation of Polystyrene to Monoaromatic Products: Mn3O4 Phase Regulation Promotes Oxygen Activation

Abstract The widespread use of polystyrene (PS) has resulted in severe environmental concerns, highlighting the urgency of developing efficient upcycling technologies. In this study, the crystalline structure of MnOx catalysts was optimized by modulating the synthesis method and calcination temperature. Specifically, the 350MnOx-SG nanosheets, prepared via a sol–gel method followed by secondary calcination at 350 °C, exhibited a pure Mn3O4 crystalline phase and demonstrated exceptional O2 activation capability. Based on this, an efficient aqueous-phase photothermal co-catalytic system was successfully established to upgrade PS into high-value benzoic acid (BA) and other monoaromatic products. Mechanistic investigations revealed that the 350MnOx-SG nanosheets are rich in oxygen vacancies (OV), which significantly promote the activation of O2 to generate reactive oxygen species (ROS), primarily superoxide radical (·O2–) and singlet oxygen (1O2). Furthermore, the mesoporous architecture of the catalyst effectively confined the reaction intermediates, while the photothermal synergy significantly accelerated the reaction kinetics. Under optimized conditions, this system achieved a remarkable conversion of PS to monoaromatic products with a yield of 66% and a BA yield of 53%. A simple hydrothermal pretreatment to weaken the C–C bonds in PS further enhanced the efficiency and afforded monoaromatic products with a yield of up to 76%, and a scaled-up reaction demonstrated that this reaction system held promise for upgrading hundred-milligram-scale PS to BA (55%). This work provides a promising strategy for the resource utilization of PS.