Response Surface Optimization of Acidification Parameters and Solvent Recyclability for Deep Eutectic Solvent (DES)-Based Depolymerization of Polyethylene Terephthalate

In the context of polyethylene terephthalate (PET) depolymerization, growing attention has been directed toward the use of alternative capable of functioning as both solvent and catalyst. These dual-function systems offer notable advantages, including reduced energy requirements, simplified processing, and enhanced sustainability. From a process optimization perspective, three key stages govern overall energy and material efficiency: (i) PET depolymerization, (ii) the subsequent acidification step for the recovery of terephthalic acid (TPA), and (iii) solvent recovery for reuse in subsequent depolymerization cycles. The depolymerization step has been widely studied, however, the latter two stages remain underexplored. To enhance process sustainability, the acidification stage was optimized by minimizing acid and water consumption while maintaining high product quality. A Box– Behnken Design (BBD) within the framework of Response Surface Methodology (RSM) was employed to optimize key parameters, including acid volume, acid concentration, and water content for dilution. Model predictions using tetrabutylammonium bromide and sulfolane-based DES suggested that 13 mL of water, 2.5 mL of HCl, and 36 wt% acid concentration would yield 79.7% TPA, while laboratory validation achieved 83.0%, corresponding to a mean error of 4.1%. In terms of solvent recovery, the DES was successfully reused for up to three consecutive cycles without significant structural degradation or loss of solvolytic performance. The recovered TPA is a high-value monomer with broad market potential, and Fourier Transform Infrared (FT-IR) spectroscopy analysis confirms its quality as comparable to that of virgin material. This enhances its appeal for supporting a circular and sustainable plastics economy.