Molecular Recycling of Polyethylene Terephthalate (PET) Waste via Ytterbium Triflate-Catalyzed Hydrolysis

Abstract

Returning waste polyethylene terephthalate (PET) to its original monomers (e.g., terephthalic acid (TPA)) constitutes closed-loop molecular recycling. Hydrolysis can accomplish this conversion and ytterbium triflate (Yb(OTf)3) is an effective catalyst. PET hydrolysis with Yb(OTf)3 at 220 °C for 40 min resulted in 92% PET conversion and 66% TPA yield, whereas the uncatalyzed reaction produced just 0.1% TPA yield. We investigated the effects of temperature (160–250 °C), reaction time (6–960 min), catalyst loading (0.9–37 mol % relative to moles of PET repeat units), PET particle size, and PET:water w/w ratio (1:10–1:2) on Yb(OTf)3-catalyzed PET hydrolysis. The PET disappearance kinetics followed a rate law that was pseudo-first-order in PET with a temperature-dependent induction time. The Arrhenius plot showed two regimes. At 205 °C or greater, the activation energy was 54 ± 12 kJ/mol. This regime may reflect the catalytic kinetics for hydrolysis of PET in a softened, swollen, or molten state. The reaction order with respect to Yb(OTf)3 in this regime was 0.77 ± 0.02. For hydrolysis below 205 °C, the activation energy was 165 ± 13 kJ/mol. This regime may reflect the kinetics for PET in a more rigid solid-like state with diffusion of water and/or catalyst within PET being a rate-limiting process. Smaller PET particles reacted more quickly than larger ones in this regime, consistent with diffusion being rate limiting and with the rate being favored by increased PET surface area. Mono-2-hydroxyethyl terephthalate (MHET) was the key reaction intermediate. The PET:water w/w ratio played a crucial role in reaction outcomes, as the TPA yield decreased by about 40% when the ratio was reduced from 1:10 to 1:2.