Colorless BHET obtained from PET by modified mesoporous catalyst ZnO/SBA-15

Abstract

Bis(2-hydroxyethyl terephthalate) (BHET) obtained by poly(ethylene terephthalate) (PET) catalytic glycolysis often has a slight color, which results in the subsequent preparation of repolymerized PET (rPET) with a darker color. The darker rPET is unable to be utilized in a high value. In order to obtain BHET with good chroma, a series of mesoporous catalysts were proposed and synthesized. Compared with traditional catalysts, the developed catalysts have obvious advantages to obtain BHET with a good chroma. The chromaticity values of BHET obtained through ZnO/SBA-15 catalyst are 96.64, 0.16 and 1.39 (L, a and b), and the chromaticity values of rPET after polymerization are 93.69, 0.86 and 1.31. The 5% ZnO/SBA-15 catalyst displayed prominent capabilities to degrade PET into BHET. The key factors influencing the reaction were further studied. Under the conditions of reaction temperature 197 °C, reaction time 1 h, mass ratio of Cat./PET 5% and mass ratio of ethylene glycol (EG)/PET 4:1, the catalytic efficiency reached the optimal one where conversion of PET is 100% and yield of BHET is 91.0%. The PET degradation kinetic was studied and conforms to the first order reaction kinetics. The reaction mechanism was deduced by in situ infrared reaction and theoretical calculation. The mechanism of ester bond breaking catalyzed by ZnO/SBA-15 was proposed. Mesoporous catalyst still has excellent catalytic activity through multiple recycling.