Depolymerization behavior polyesters (poly(ethylene terephthalate) (PET) and poly(ethylene 2,6-naphthalene dicarboxylate) (PEN)) in sub- and supercritical water was investigated for the purpose of monomer recycling. In case of the depolymerization of PET, the yield of terephthalic acid (TPA) increased to 90% with increasing reaction temperature up to 693 K while the maximum yield of ethylene glycol (EG) achieved was only 60% at 573 K. For PEN depolymerization, the yield of 2,6-naphthalene dicarboxylic acid (2,6-NPA) also increased to 97% with increasing reaction temperature up to 693 K and the maximum yield of EG obtained was also 60% at 573 K. The higher yield of 2,6-NPA than that of TPA was ascribed to the higher stability of 2,6-NPA than that of TPA. The lower yields of EG than the corresponding monomers (TPA and 2,6,-NPA) in the decomposition of the two kinds of polyesters were caused by the catalytic dehydroxylation of EG by protons derived from dicarboxylic acids.
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