Depolymerization Mechanism of Poly(ethylene terephthalate) in Supercritical Methanol

The kinetics of poly(ethylene terephthalate) (PET) depolymerization in supercritical methanol was investigated to develop a chemical recycling process for postconsumer PET bottles. PET with a high molecular weight (IV value (intrinsic viscosity) = 0.84) was depolymerized in a batch reactor at temperatures between 543 and 573 K under estimated pressures of 0.1−15 MPa. In addition to PET with high molecular weight, PET with low molecular weight, such as its oligomer (trimer), bis-hydroxyethyl terephthalate (BHET), and methyl-(2-hydroxyethyl) terephthalate (MHET), was used as a model reactant to clarify the depolymerization pathway of poly(ethylene terephthalate) in supercritical methanol. The reaction products were analyzed with size-exclusion chromatography, high-performance liquid chromatography, and high-performance liquid chromatography−mass spectrometry. The main products of each reaction were the monomers, dimethyl terephthalate (DMT) and ethylene glycol (EG). The depolymerization of high molecular weight PET to its oligomer was faster than that of the oligomer to its monomer. PET was depolymerized into DMT and EG through MHET. The kinetics study showed that the depolymerization of PET proceeds consecutively, where the step of the oligomer to its monomer would be a rate-determining step. MHET is a relatively stable intermediate in the depolymerization. The rate constants were estimated with simple reaction models. This kinetic model of PET depolymerization in supercritical methanol was proposed.