Potassium Carbonate as a Low-Cost and Highly Active Solid Base Catalyst for Low-Temperature Methanolysis of Polycarbonate

Abstract

As the demand for polycarbonate (PC) plastic increases over the years, the development of a chemical recycling system to produce virgin-like-quality monomers is indispensable not only to attain completely sustainable cycles but also to contribute to the decrease in global plastic pollution. Herein, potassium carbonate (K₂CO₃) was used as a low-cost, readily available, and highly active solid base catalyst for low-temperature PC methanolysis in the presence of THF as a solvent, producing highly pure and crystalline bisphenol A (BPA) and with a catalytic performance higher than group IIA metal oxides (MgO, CaO, and SrO) and some group IA metal carbonates (NaHCO₃, KHCO₃, and Na₂CO₃). THF was the best solvent in aiding the reaction owing to it having a similar polar parameter (δ_p) to that of PC according to Hansen solubility parameters. By the reaction over the catalyst, 100% PC conversion, 97% BPA yield, and 86% dimethyl carbonate yield were achieved within just 20 min at 60 °C. The catalyst possessed an apparent activation energy (E_a) of 52.3 kJ mol^-1, which is the lowest value so far for heterogeneous catalysts, while the mechanistic study revealed that the reaction proceeded via the methoxide pathway. The reusability test demonstrated that the catalyst was reusable at least four times. Furthermore, this catalytic system was successfully applied to actual post-consumer PC wastes and polyesters, including polyethylene terephthalate (PET) and polylactic acid (PLA).

Keywords: Bisphenol A; Methanolysis; Polycarbonate; Potassium carbonate; Tetrahydrofuran.