Efficient Methanolysis of PET Catalyzed by Nonmetallic Deep Eutectic Solvents

The methanolysis of polyethylene terephthalate (PET) can address the environmental pollution and resource waste caused by waste PET, and the obtained monomer dimethyl terephthalate (DMT) is easy to separate and purify. However, conventional methanolysis of PET usually uses metal-containing catalysts, resulting in metal residues in the degradation products, which affects subsequent high-value utilization. Meantime, the reaction conditions of high temperature and high pressure promotes the occurrence of side reactions and reduces the yield of the main products. Herein, the nonmetallic deep eutectic solvent (DES) 1,5-diazabicyclo [4.3.0]-5-nonene (DBN)/Phenol was employed as a catalyst for PET methanolysis. 100% PET conversion and 95.3% DMT yield were obtained at 130 °C for 1 h. The possible reaction mechanism was investigated by combining NMR, FT-IR, GPC analysis, and DFT calculations. DBN and phenol were connected through N–H–O hydrogen bonding, where the hydrogen atom interacted with the carbonyl oxygen of PET and the oxygen atom activated the hydroxyl group of methanol. Additionally, GPC analysis showed that the dissolution and degradation of PET were simultaneous, which enhanced mass transfer and facilitated the reaction. The kinetic studies showed that PET methanolysis followed the first-order kinetic model with an activation energy of 103.3 kJ/mol, which was lower than the literature-reported value. This research provides an efficient nonmetallic deep eutectic solvent catalyst for PET methanolysis, which solves the problem of metal residues in the degradation products and reduces the reaction temperature. It is of great significance to simplify the purification steps and improve the economy of regeneration.