Morphology-tuned MnOx/TiO2 nanocatalysts for recycling PET plastic waste using biomass-derived ethylene glycol

Abstract

This study presents a decisive role of TiO₂ morphology on the catalytic activity of MnO_x/TiO₂ nanomaterials for the chemical recycling of PET waste bottles using biomass-derived ethylene glycol to produce a valuable monomer, bis(2-hydroxyethyl) terephthalate (BHET). Three types of MnO_x/TiO₂ nanocatalysts were prepared by varying the TiO₂ morphology (nanosheets: NS, nanotubes: NT, and nanorods: NR). The combination of MnO_x nanoparticles and TiO₂ nanorods (MnO_x/TiO₂-NR) showed significantly enhanced catalytic activity in PET glycolysis, with a 91% isolated yield of BHET at 190 °C in 3 h, whereas 74% and 82% yields of BHET were attained with MnO_x/TiO₂-NS and MnO_x/TiO₂-NT nanocatalysts, respectively. The morphology of TiO₂ and the uniform dispersion of MnO_x on TiO₂-NR were confirmed by electron microscopic analysis. The MnO_x/TiO₂-NR catalyst contains optimum basic sites, which play a key role, along with surface hydroxyl species and Mn³⁺/Mn²⁺ species, in activating ethylene glycol and PET/its oligomers towards BHET formation. The excellent stability of the MnO_x/TiO₂-NR nanocatalyst, as confirmed by the hot-filtration test, good catalytic reusability up to four cycles, non-toxic nature, and the low cost of the MnO_x/TiO₂ materials indicate the practical feasibility of the developed catalytic protocol for the plastic recycling industry.