Multifactor Optimization of Solvolytic Recycling of Polyethylene Terephthalate in Designer Deep Eutectic Solvents

A circular economy for polyethylene terephthalate (PET) aims to achieve complete material recirculation and reduce dependence on fossil-based feedstocks. Among recycling strategies, depolymerization offers the greatest potential for true circularity. This work investigates deep eutectic solvent (DES)-assisted PET depolymerization using a tetrabutylammonium bromide/sulfolane (TBABr/Sulf) system. A four-factor, three-level Box–Behnken design was employed, where reaction time, NaOH concentration, PET/solvent ratio, and water content were selected as independent variables, with PET weight loss as the response. The optimized conditions of 35 min reaction time, 5.9 wt% NaOH, a PET/solvent ratio (g/g) of 0.061, and a 1:1 water-to-DES ratio achieved 97% PET depolymerization and 93.8% TPA recovery. The process achieved a relatively low PET/DES (g/g) utilization ratio of 0.12 and required at least 40% less NaOH than other DES-based alkaline hydrolysis reported in the literature. The identity and quality of the recovered TPA were confirmed by FTIR, NMR, DSC, and elemental analysis. Under microwave-assisted conditions, TBABr/Sulf was successfully reused over three consecutive cycles while maintaining 100% depolymerization efficiency with consistent TPA quality. Collectively, these results demonstrate the technical potential of the proposed DES-assisted PET depolymerization process, particularly in terms of reduced solvent and alkaline requirements, shortened reaction time, and demonstrated solvent recyclability.