Catalytic recycling of PET waste bottles into a value-added amide monomer using a heterogeneous niobium pentoxide nanocatalyst

Abstract

Using a diverse heterogeneous nanocatalyst, aminolysis represents a promising approach for the chemical recycling of discarded PET waste bottles into a valuable monomer bis(2-hydroxyethyl) terephthalamide (BHETA). This study reports the solution combustion synthesis of a nanostructured Nb₂O₅ material for the catalytic aminolysis of PET waste bottles using ethanolamine. The Nb₂O₅ nanocatalyst calcined at 450 °C (Nb₂O₅-450) exhibited robust catalytic performance with a 92% isolated yield of the BHETA monomer and complete PET conversion under mild conditions compared with several homogeneous and heterogeneous catalysts. The Nb₂O₅-450 nanocatalyst has a unique morphology with both nanosheet and nanorod particles. The Nb₂O₅-450 nanocatalyst, possessing strong acid sites and more oxygen vacancies as estimated by NH₃-TPD and O 1s XPS analyses, respectively, induced electron deficiency in the carbonyl carbon of PET. This electron-deficient characteristic facilitated the aminolysis reaction, wherein ethanolamine attacked the carbonyl carbon, initiating the reaction toward the formation of BHETA. The purity and structure of BHETA were confirmed through NMR, FT-IR, TGA/DSC, and powder XRD techniques. The 1 wt% Nb₂O₅ catalyst exhibited reasonably good catalytic reusability for up to five cycles. The characterization of the Nb₂O₅-450 nanocatalyst before and after the reaction highlighted its structural stability, affirming the sustainable nature of the catalyst for valorizing PET waste into value-added monomers.