Wet oxidation of polyethylene under elevated oxygen pressure: revisiting product selectivity

Abstract

Wet oxidative upcycling of polyethylene is commonly discussed in terms of dicarboxylic-acid formation, yet the parameters of catalyst-free aqueous oxidation that ensure product selectivity remain poorly defined. Here we show for the first time that wet oxidation of two commercial polyethylene grades, low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE), in pressurized H₂O/O₂ mixture at 150 °C proceeds with acetic acid as the dominant liquid-phase product throughout the reaction, while dicarboxylic acids remain secondary products. This observation challenges the widespread diacid-centered view of polyethylene wet oxidation and indicates that acetic acid is not merely a late overoxidation product formed from initially generated diacids. We have conducted a detailed study of the products in all phases, including the gas phase, and compiled a carbon balance for the process. We also show that polymer microstructure primarily influences the early-stage oxidation kinetics, whereas both feedstocks converge to similar product distributions at later stages. Iodometric titration reveals hydroperoxide accumulation at early times confirming the radical pathway of the process. FTIR analysis reveals a rapid loss of polyethylene crystallinity and the formation of highly oxidized oligomeric intermediates, considered to be the link between the solid polymer and soluble acids. Using obtained data, the temporal conditions for converting polyethylene into recoverable carboxylic acids with optimal yields using this type of wet oxidation under mild conditions were determined. The results provide insight into the process kinetics, which is essential to develop scalable strategies for the upcycling of polyolefin waste.