Efficient glycolysis of used PET bottles into a high-quality valuable monomer using a shape-engineered MnOx nanocatalyst†

Abstract

The chemical recycling of used polyethylene terephthalate (PET) bottles, a widely used plastic in the modern world, to obtain valuable monomers offers a promising solution to address post-consumer plastic-related environmental concerns. In this study, we have developed an efficient heterogeneous catalytic approach using a shape-engineered manganese oxide (MnO_x) nanocatalyst with a well-defined rod morphology to facilitate the glycolysis of PET with biomass-derived ethylene glycol to produce a high-quality bis(2-hydroxyethyl) terephthalate (BHET) valuable monomer under mild conditions. The nanorod morphology of the MnO_x material, specifically the MnO_x calcined at 500 °C (MnO_x-500), exhibited remarkable catalytic efficiency in converting used PET bottles into BHET. At a temperature of 180 °C for 3 h, the MnO_x-500 nanocatalyst achieved a complete conversion of PET with a 86% isolated yield of BHET, surpassing the performance of various metal oxides, such as CeO₂, TiO₂, and Nb₂O₅. Qualitative analysis of the isolated BHET monomer crystals was conducted using NMR, FT-IR, HR-MS, and powder XRD, along with assessments of thermal stability through TGA and DSC studies. Furthermore, the study demonstrated the catalyst's stability and reusability, suggesting the practical application potential of this methodology. The structure–activity correlation, revealed through comprehensive characterization of the nanostructured MnO_x materials, highlighted the crucial role of the oxygen vacancy defects and the acidic properties in the MnO_x-500 nanocatalyst for efficient PET glycolysis to obtain the desired BHET monomer.