Magnetically recoverable cobalt oxide nanoparticle catalyst for PET glycolysis

Abstract

In spite of its many applications and desirable properties, the non-biodegradable nature of polyethylene terephthalate (PET) raises concerns about the accumulation of post-consumer products. Recycling these end-of-life polymers is the most effective solution to address this issue. Mechanical recycling, the primary commercial recycling method for PET, often produces materials with inferior properties, and hence chemical recycling, or tertiary recycling, is considered a viable alternative. Glycolysis, which is a transesterification reaction in which the PET molecule is depolymerized to its monomer, bis(hydroxyethyl) terephthalate (BHET), has gained significant commercial interest due to its mild reaction conditions. In the present work, magnetic cobalt oxide nanoparticles (CONP) were synthesized using different methods: NaBH₄ reduction route, hydrothermal route, and combustion route, and experimented as catalysts for the glycolysis of PET. The catalysts produced were characterized using FTIR, SEM-EDS, surface area and XRD techniques. Glycolysis of PET was performed with the CONP catalysts obtained from each synthesis method and the resulting BHET was analysed using DSC, HPLC, and NMR techniques. Among the catalysts, CONP prepared via the NaBH₄ route showed the best performance, achieving a BHET yield of 97% with just 1% catalyst at 180 °C within a reaction time of 2 hours. The effects of various reaction conditions, including temperature, reaction time, and the PET/EG ratio, were also investigated. Notably, the CONP–NaBH₄ catalyst is magnetically separable from the reaction mixture after the process and can be reused multiple times without considerable loss in efficiency. CONP–NaBH₄ is structurally distinct from other cobalt oxides, featuring a core–shell structure with a nanocrystalline cobalt boride core and an amorphous cobalt oxide, oxyhydroxide, hydroxide shell. This was established by STEM, HRTEM, XPS, Raman, and FTIR analyses. The amorphous cobalt oxide shell drives catalytic activity, while the cobalt boride core imparts ferromagnetism. Superior catalytic performance is attributed to its high surface area, porosity, and presence of surface hydroxyl groups.