Kinetics of Polyester Methanolysis by a Consecutive Reaction Model

Methanolysis of polyethylene terephthalate (PET) particles was carried out to obtain dimethyl terephthalate (DMT) and ethylene glycol (EG) under subcritical condition, with zinc acetate as a catalyst. The effects of key operational variables, such as reaction temperature and the methanol (MeOH)/PET weight ratio, were investigated to optimize the DMT yield. The product yield increased with higher amounts of methanol, reaching a plateau at a MeOH/PET weight ratio of 5. A kinetic model was developed to describe methanolysis, which involves a series of reactions: random scission of PET into dissolvable solid oligomers, dissolution of the solid oligomers into liquid oligomers, and degradation of these oligomers into monomers. Continuous distribution kinetics and a shrinking core model were applied to develop the multiple-step kinetics for depolymerization of PET particles in methanol. The model’s predicted values showed good agreement with the experimental results. Kinetic analysis revealed that the heterogeneous reactions, including random scission and oligomer dissolution, are rate-limiting and highly temperature-sensitive. In contrast, the homogeneous reaction, which degrades the liquid oligomers into monomers, is faster but limited by equilibrium, which is relatively insensitive to temperature. These results indicate that increasing the reaction temperature is essential for enhancing the rate of the heterogeneous reaction. Meanwhile, for improving the yield of the homogeneous reaction, increasing the MeOH/PET ratio is much more effective than raising the temperature. Therefore, simplifying the heterogeneous reaction steps─by reducing PET particle size or incorporating cosolvents─enables subcritical methanolysis to proceed effectively at relatively lower temperatures