Hydrothermal Conversion of Poly(ethylene naphthalate) to Naphthalene Dicarboxylic Acid and Ethylene Glycol in the Presence of Trimethylamine

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Poly(ethylene naphthalate) (PEN) pellets were converted to monomers (naphthalene dicarboxylic acid (NDC) and ethylene glycol (EG)) in an aqueous amine solution under hydrothermal conditions using a small semi-batch reactor. Three amines (methylamine, dimethylamine, and trimethylamine) were evaluated as reactants for depolymerization of PEN and compared with ammonia. The total yield of both monomers (NDC+EG) on the basis of the carbon mass of the initial PEN pellets, at 493 K and 5.0 MPa, for a reaction time of 50 min, using an amine concentration of 0.6 mol/kg was highest with trimethylamine (97.2%) followed by methylamine (86.6%) and dimethylamine (89.8%) in succession. The differences in the total yields resulted primarily from the differences in the NDC yields, and were affected by the amine species, although the EG yield was similar with all amines. The yields were higher than 95% of the theoretical value with all evaluated amines. The difference in the yields may be affected by the stabilities of the intermediate products with the primary and secondary amines in the aqueous solutions. The reaction rates with respect to the unreacted mass of PEN were represented by the surface reaction model, i.e., 2/3rd order reaction kinetics, as previously observed for poly(ethylene terephthalate) in aqueous solutions of ammonia and amine at low concentrations. The apparent rate constant from the surface reaction model was proportional to the amine concentration.

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