Impact of Acid Sites in ZSM-5 Zeolites on the Selective Catalytic Oxi-Upcycling of High-Density Polyethylene to Dicarboxylic Acid under Mild Conditions

The catalytic oxidation strategy has shown great potential in the upcycling of polyethylene (PE) into valuable chemicals that contain oxygenated groups. The development of inexpensive and efficient catalysts is highly required but remains challenging. Herein, we report that ZSM-5 zeolites could be used as an effective catalyst for the selective catalytic oxi-upcycling of high-density polyethylene (HDPE) into dicarboxylic acids within the C 4 –C 17 range at 160 °C and 1.5 MPa air pressure with a reaction duration of 16 h. Among the three tested ZSM-5 zeolites (Si/Al = 38, 70, and 170), the sample with a Si/Al ratio of 38 exhibited the highest acid site density and, consequently, the best catalytic activity. Based on the model molecular reactions, detected intermediate species, and theoretical calculation, we propose that Bro̷nsted acid sites synergize with oxygen to drive the oxidation of PE via a carbenium ion (C + ) mediated mechanism, which is beyond the conventional pathway of direct oxidation of PE molecular chains by a free radical mechanism. Besides, the ZSM-5 zeolite has the potential to be applied broadly in the oxi-upcycling of different PE feedstocks and PE plastic wastes.