PET Depolymerization via Glycolysis

A high-pressure hydrothermal autoclave reactor (100 mL) equipped with a Teflon vessel, a pressure and temperature monitor, and a magnetic stirrer was used to conduct glycolysis reactions. In a typical glycolysis reaction, the required amounts of PET, EG, and Cat-600, Cat-800, or Cat-1000 were weighed and added to the Teflon vessel, which was then inserted into the reactor. The reactor was closed and heated from room temperature to 200 °C, taking about 2 h, and was kept at 200 °C for the required amount of time (Scheme 1). The heating was then turned off, and the reaction was cooled to around 75 °C, at which point the reactor was opened and the reaction mixture was visually inspected. In the cases with high BHET yields, the reaction mixture was a one-phase liquid solution at 70–75 °C with the white catalyst dispersed in it (Figure ​Figure22a). The dispersed catalyst was removed from the hot solution by vacuum filtration. However, preheating the funnel is necessary to prevent BHET and oligomers from precipitating onto the inner walls of the filtration apparatus because their solubility in EG is temperature-dependent. Upon cooling the filtered reaction mixture to room temperature, the BHET and the other possible products, i.e., PET oligomers, were precipitated as a white solid (Figure ​Figure22b). A small portion of this two-phase mixture was vacuum-filtered to remove the excess EG and vacuum-dried at 65 °C overnight to be used as a crude reaction mixture for reaction characterization and BHET yield measurements via HPLC. The reaction mixture was poured into cold water and kept at 4 °C overnight for the BHET to be crystallized, which then was filtered and vacuum-dried at 65 °C (Figure ​Figure22c). When the PET conversion is incomplete, in which there are remaining deformed flakes of PET, the hot reaction mixture is filtered through a hot funnel to separate and dry the unreacted PET. The weight of this unreacted PET was then measured, and the PET conversion was calculated using the following equation (eq 1)

Reaction mixture of PET glycolysis at 70 °C containing the dispersed catalyst Cat-800 (a), the reaction mixture after cooling to room temperature (b), and BHET crystals from recrystallization in cold water (c).