CO2-Enhanced methanolysis: A sustainable route for the depolymerization of PLA and PET-rich mixed plastic waste

Methanolysis of waste poly(lactic acid) (PLA) to produce the non-toxic, biodegradable solvent methyl lactate (MLA) is a promising strategy for waste management. We introduce an efficient, catalyst-free PLA methanolysis approach enhanced by a CO₂ atmosphere, achieving nearly 100 % PLA conversion and a 95.5 % MLA yield. These results represent a significant increase in both conversion and yield compared to reactions conducted without CO₂. Through analytical methods and density functional theory (DFT) calculations, we elucidate the reaction pathways, showing that CO₂ critically enhances the ester exchange between methanol and PLA chains. This enhancement is attributed to CO₂'s role in stabilizing transition states and promoting ester bond scission by forming additional hydrogen bonds, which facilitates efficient MLA formation. Furthermore, this approach's viability for real-world recycling is demonstrated with commercial PLA products, achieving 53.9 % PLA conversion and 17.0 % MLA yield. When applied to mixed plastic waste containing PLA and PET, the process achieves nearly complete PLA conversion with a 79.4 % MLA yield, while also producing the PET-derived product DMT, which can be isolated along with MLA via simple precipitation and distillation. This research presents a sustainable and highly tolerant solution for the chemical recycling of both pure and mixed PLA waste streams.