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

Abstract

This study presents the decisive role of TiO2 morphology on the catalytic activity of MnOx/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 MnOx/TiO2 nanocatalysts were prepared by varying the TiO2 morphology (nanosheets: NS, nanotubes: NT, and nanorods: NR). The combination of MnOx nanoparticles and TiO2 nanorods (MnOx/TiO2-NR) showed significantly enhanced catalytic activity in PET glycolysis, with a 91% isolated yield to BHET at 190 oC for 3 h, whereas 74% and 82% yields of BHET were attained with MnOx/TiO2-NS and MnOx/TiO2-NT nanocatalysts, respectively. The morphology of TiO2 and the uniform dispersion of MnOx on TiO2-NR were confirmed by electron microscopic analysis. The MnOx/TiO2-NR catalyst contains the optimum basic sites, which played 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 MnOx/TiO2-NR nanocatalyst, as confirmed by the hot-filtration test, good catalytic reusability up to four cycles, and non-toxic nature as well as low cost of the MnOx/TiO2 materials indicate the practical feasibility of the developed catalytic protocol for the plastic recycling industry.