Ligand-Aided Glycolysis of PET Using Functionalized Silica-Supported Fe₂O₃ Nanoparticles

The development of efficient catalysts for the chemical recycling of poly(ethylene terephthalate) (PET) is essential to tackling the global issue of plastic waste. There has been intense interest in heterogeneous catalysts as a sustainable catalyst system for PET depolymerization, having the advantage of easy separation and reuse after the reaction. In this work, we explore heterogeneous catalyst design by comparing metal-ion (Fe³⁺) and metal-oxide nanoparticle (Fe₂O₃ NP) catalysts immobilized on mesoporous silica (SiO₂) functionalized with different N-containing amine ligands. Quantitative solid-state nuclear magnetic resonance (NMR) spectroscopy confirms successful grafting and elucidates the bonding mode of the organic ligands on the SiO₂ surface. The surface amine ligands act as organocatalysts, enhancing the catalytic activity of the active metal species. The Fe₂O₃ NP catalysts in the presence of organic ligands outperform bare Fe₂O₃ NPs, Fe³⁺-ion-immobilized catalysts and homogeneous FeCl₃ salts, with equivalent Fe loading. X-ray photoelectron spectroscopy analysis indicates charge transfer between the amine ligands and Fe₂O₃ NPs and the electron-donating ability of the N groups and hydrogen bonding may also play a role in the higher performance of the amine-ligand-assisted Fe₂O₃ NP catalysts. Density functional theory (DFT) calculations also reveal that the reactivity of the ion-immobilized catalysts is strongly correlated to the ligand-metal binding energy and that the products in the glycolysis reaction catalyzed by the NP catalysts are stabilized, showing a significant exergonic character compared to single ion-immobilized Fe³⁺ ions.