Metal Octoates as Lewis Acid Catalysts for the Hydrolytic Depolymerization of PET in Tetrahydrofuran

Abstract

Chemical recycling of polyethylene terephthalate (PET) with decomposition into monomers suitable for resynthesizing primary polymer is a critical task for achieving carbon neutrality in the polyester industry. This paper presents a study comparing PET depolymerization solvolysis in tetrahydrofuran medium using a series of metal octoates (Co, Mn, Zn, Ca, Ba, Li, Zr, Cu, Pb). The resulting terephthalic acid (TPA) was characterized using a combination of methods (1H NMR, thermogravimetric analysis, differential scanning calorimetry, and Raman spectroscopy). In an aprotic tetrahydrofuran (THF) medium, a comparative performance of commercial metal octoates under fixed metal-mass loading was conducted for the first time: the leadership of Co (83.7%) and Pb (78.4%) is explained by the lability of their coordination sphere, while the anomalously low activity of Zr (17.5%) is associated with steric blocking of the metal active center. In the present work, the influence of technological parameters on the depolymerization process using the most active catalyst, cobalt octoate, was studied. It was found that increasing the reaction time from 1 to 6 h leads to an increase in PET conversion from 27% to ~85%, and in TPA yield from 14% to ~84%. A study of the influence of water content in the reaction mixture showed that increasing the water concentration to 20 wt.% intensifies hydrolysis, promoting an increase in the yield of TPA to ~80%; however, a further increase in the water content (up to 30 wt.%) does not lead to a significant change in the yield, which indicates the achievement of conditions providing the highest observed isolated TPA yield within the tested range for the reaction in this system. The obtained results create a scientific basis for the rational design of catalytic systems for the depolymerization of PET, adapted to the different qualities of secondary raw materials and the technological requirements of industrial chemical recycling.