Efficient Ti-MOF catalyst for sustainable synthesis and closed-loop recycling of bio-based PEF with low metal residues

The closed-loop recycling of plastics is crucial for polymer sustainability. Bio-based PEF can be depolymerized via glycolysis, but catalysts often exhibit low efficiency. In this work, a titanium-based metal-organic framework (MOF), MIL-125, was synthesized with optimized metal-to-ligand ratios (MIL-125-5 with a Ti/PTA ratio of 1:5) to achieve a high specific surface area of 1719.6 m 2 /g and abundant Lewis acid sites. This catalyst demonstrated exceptional performance in PEF polymerization, yielding a polyester with a M n of 3.7 ×10 4 g/mol, a PDI of 1.9, and low metal residue (176.6 ppm Ti content), under mild conditions (210 o C for esterification and 230 o C for polycondensation). Comprehensive structural and catalytic characterization of MIL-125 was conducted through a combination of advanced techniques. XPS confirmed the existence of crystallographically distinct Ti 3+ /Ti 4+ active sites. The N 2 adsorption-desorption isotherms indicate the presence of microporous and mesoporous structures, with a pore volume of 0.65 cm 3 /g. while TGA revealed exceptional thermal stability exceeding 300 o C. Furthermore, NH 3 -TPD and EPR spectroscopy revealed abundant Lewis acid sites and oxygen vacancies, respectively. These structural features demonstrated a strong correlation with catalytic activity, as further validated through DFT calculations. Remarkably, the same MIL-125 catalyst facilitated closed-loop recycling by enabling depolymerization of PEF via glycolysis at 180 o C and atmospheric pressure, followed by repolymerization without catalyst separation or purification. This process produced recycled PEF (rPEF) with an Mn of 2.7×10 4 g/mol and properties comparable to virgin PEF, underscoring the catalyst's reusability and efficiency. This study establishes MIL-125 as a green, high-performance catalyst for sustainable polyester production and recycling.