Development of Eco-Friendly and Sustainable PET Glycolysis Using Sodium Alkoxides as Catalysts

Abstract

The massive accumulation of postconsumer polyethylene terephthalate (PET) waste in the environment demands enhancement in the recycling rate to tackle global pollution. At the same time, efficient recycling will contribute to a future circular economy. PET glycolysis has emerged as a promising chemical recycling method that turns PET into a widely used monomer, bis(2-hydroxyethyl)terephthalate (BHET), and adds value to waste. For reasons of comparability, all reported studies on PET glycolysis so far used the same work-up procedure to separate residual PET from the BHET product: Addition of hot water to the reaction mixture, filtration to remove residual PET, and crystallization of BHET from the filtrate by cooling. Water not only destroys the catalyst but also has to be removed to reuse ethylene glycol. We herein report a new "green" glycolysis approach without the need for an anti-solvent using simple sodium alkoxides (MeONa and EtONa) as catalysts. A response surface methodology (RSM) based on the Box−Behnken design was applied to optimize the reaction parameters. Under optimum conditions, the results demonstrated the validity of the optimization. EG was successfully recycled and PET conversion was found close to the initial recycling run. The optimum recipe was applied to analyze the tolerance of the catalysts, MeONa and EtONa, in depolymerizing colored PET waste and mixed PET waste. The findings indicate that both catalysts can effectively break down both types of waste under eco-friendly glycolysis conditions, with MeONa resulting in higher PET conversion. This demonstrates that BHET precipitation is achievable without the use of water, reusing the EG and maintaining catalyst activity for mixed PET waste. These outcomes offer a promising basis for further developing a new and more efficient PET recycling technology.

Polyethylene Terephthalate (PET) Recycled by Catalytic Glycolysis: A Bridge toward Circular Economy Principles

“…Among the previously discussed tertiary recycling methods (also depicted in Figure 8 and Figure 9 ), glycolysis emerges as one of the most promising techniques for depolymerizing PET on an industrial scale [ 45 ]. Glycolysis offers a significant enhancement in closed-loop and weighted circularity, allowing for the production of virgin PET to be replaced directly with pristine recycled PET [ 46 , 47 , 125 ]. Hence, an examination of PET bottles’ recycling, spanning from 2020 to 2049, demonstrates that the implementation of chemical recycling via glycolysis, coupled with enhanced collection systems facilitated by recycling centers, will markedly enhance the circularity of PET bottles [ 126 ].…”

“…Most of the homogenous catalysts (encompassing metal acetates [ 146 , 147 , 148 , 149 , 150 ], chlorides [ 151 , 152 ], alkoxides [ 46 ], hydroxides [ 153 ], carbonates [ 154 , 155 ], sulfates [ 156 ], and phosphates [ 157 ]) necessitate an additional unit operation in the chemical process, such as distillation. Zinc acetate (Zn(OAc) 2 ) has been widely employed as a catalyst for the glycolysis of PET, demonstrating its effectiveness for polyesters degradation (82 documents found on Scopus when we searched for “PET AND glycolysis AND zinc AND acetate”, March 2024).…”

“…Based on recent studies (Scopus database), glycolysis is seen as an effective method for PET chemical recycling because it operates under milder conditions, uses less-volatile solvents, and yields a relatively pure monomer [ 45 , 46 , 47 , 48 ]. Consequently, by employing ethylene glycol (EG) as a solvent, one can add value to waste by transforming PET into the commonly utilized monomer, bis(2-hydroxyethyl) terephthalate (BHET) [ 44 , 47 ].…”