A method was developed to distinguish between hydrolysis and pyrolysis pathways in the steam degradation of various polyesters. Selectivity was shown to be strongly influenced by the polyester structure.

The proportions of pyrolysis and hydrolysis in poly(ethylene terephthalate) (PET) degradation were determined from the decomposition of PET in 18 O-labeled steam, producing labeled and unlabeled terephthalic acid. At a 100 vol % steam concentration at 400°C, 74% of the ester bonds were pyrolyzed and 26% were hydrolyzed, as determined by gas chromatographymass spectrometry. In addition, thermogravimetric analysis revealed the activation energy to be 199 kJ mol ¹1 under pyrolytic conditions, decreasing with increasing steam concentration, and reaching a minimum of 132 kJ mol ¹1 at 75 vol %.Poly(ethylene terephthalate) (PET) is the most commonly used polyester resin for the production of various items such as bottles, sheets, films, and fibers. Solvolysis, a frequently used chemical recycling method, enables monomer recovery from PET but suffers the drawback of requiring fairly neat and additive-free spent PET.1 Hydrolysis in the gas phase can overcome this obstacle, but the competitive process of pyrolysis reduces the terephthalic acid (TPA) yield.2 The recovered TPA can be used as a feedstock for PET synthesis or converted to benzene in the presence of CaO for use as a petrochemical feedstock.3 Kinetic information regarding both pyrolysis and hydrolysis might, therefore, shed light on the conditions necessary to achieve optimal TPA yields. Martín-Gullón et al. 4 and Saha et al.5 investigated the kinetics of the pyrolysis of PET, resulting in a good correlation between experiments and calculations. The kinetics of the hydrolysis was mainly investigated using subcritical conditions. 69 Very little is known about the behavior of PET in a steam atmosphere. 10 Degradation kinetics has been studied in different steam atmospheres, but the reaction mechanism was not considered. Thus, it is still not clear how pyrolysis influences hydrolysis, and vice versa. The development of a new approach for determining the proportions of pyrolysis and hydrolysis is, therefore, needed. Such an approach will be useful in kinetics studies and will enable assessment of the maximal TPA yields.In this study, PET was decomposed in an 18 O-labeled steam atmosphere (H 2 18 O). Scheme 1 shows the pyrolysis and hydrolysis reactions of PET in the presence of H 2 18 O. It can be seen that the hydrolysis-derived carboxy groups contain 18 O isotopes. In contrast, carboxy groups derived from the initial pyrolysis step are obtained by intramolecular elimination, which leaves the isotope ratio intact.Crushed PET bottles (>250¯m) were used for all experiments. H 2 18 O was mixed with water purified by ion exchange in a ratio of 1:9 to prepare a 10% H 2 18 O solution. Steam containing H 2 18O was generated in a quartz tube heated by an electric furnace. A 200-mg PET sample was placed in a perforated quartz-glass sample holder and suspended outside the heating zone in the reactor. After reaching the experimental conditions, the sample holder was pushed into the hot reactor zone for 30 min at 400°C and a gas flow of 300 mL min ¹1 at various steam concentr...