Urea-Based Eutectic Catalysts for PET Recycling in the Presence of Residual PEF: Experimental, Spectroscopic, and DFT Insights

The widespread use of plastics has played a major role in driving the economic progress worldwide, with an annual production exceeding 430 million tons in 2024. Nevertheless, only a small portion (c.a. 15%) undergoes recycling. In that regard, poly(ethylene terephthalate) (PET) have been used extensively and it is the most chemically recycled polyester worldwide. Given the imminent market introduction of poly(ethylene 2,5-furandicarboxylate) (PEF) it is important to evaluate the impact on the recycling stream of PET due to their potential mixture. Recently, some approaches using eutectic solvents (ES) have been advantageously explored for poly(ethylene terephthalate) (PET) chemical recycling.

Despite the enormous importance and impact on PET recycling and the eminent PEF introduction in the markets, this is the first study reporting the structural, thermal and thermo-mechanical properties of the recycled copolymers rPET-co-PEF. In this work, the ability of a greener chemical recycling system using ES (Urea:Zinc acetate) as catalyst was tested in the chemical recycling of PET with PEF residual molar percentages, considering 2%, 5% and 10%. The thermal and thermo-mechanical properties of the rPET-co-PEFs revealed very similar thermal behaviour compared to virgin PET.

Moreover, the depolymerization mechanism was investigated through density functional theory (DFT) calculations, revealing that hydrogen bonding between the urea amine groups and the ester moieties plays a major role in reducing the reaction activation energy, thereby confirming the catalytic effect of the urea-based eutectic solvent . Inelastic neutron scattering (INS) spectroscopy was employed to support the DFT findings, providing clear evidence of a two-step depolymerization process. The combined DFT and INS results demonstrated the crucial role of urea in lowering the depolymerization activation energy.