NSF Award
2419181
Environmental accumulation of postconsumer plastics, including poly (ethylene terephthalate) (PET), is a significant ecological concern. Various chemical recycling technologies are being actively developed to address plastic waste management issues and their threat to the environment. Methods that use chemical solutions can facilitate the recovery of building blocks called monomers. These building blocks can be used to synthesize new-like PET or generate other materials and chemicals. However, moving the target products through in the solutions can be challenging, especially at temperatures below the melting point of PET. At these temperatures the solutions can become thick and or the feedstock is less soluble in most conventional solvents. Therefore, this research program will investigate how CO2-H2O mixtures can be used as a tunable media to reduce these challenges and better break-down PET into its monomeric compounds for recycling/ upcycling. The research will be conducted by a team of researchers located at the University of Kansas and Northwestern University in collaboration with researchers at Washington University in St. Louis and SLAC National Accelerator Laboratory. The technical and environmental impacts of this project are significant, given the millions of tons of waste plastics that must be recycled or chemically transformed into valuable products. The researchers will receive cross-disciplinary training in science and engineering, attend communication workshops and conduct outreach activities, and work collaboratively in partnering universities and National laboratories.<br/><br/>This project is built around the high-media tunability offered by near-supercritical and supercritical CO2 (scCO2) to minimize the limitations of chemical recycling processes. The limitations encompass poor accessibility of catalysts and reagents in a viscous semi-crystalline polymer matrix and thermodynamic barriers pertaining to polymer morphology and structure. These impact the reactivity and selectivity during solvent-based deconstruction of waste plastics. The two primary aims of this project are (1) to elucidate the impact of CO2 on PET polymer morphology, phase behavior, and reagent transport during acid-catalyzed hydrolysis, and (2) to understand the deconstruction of semi-crystalline PET in CO2-tunable media mechanistically. Advanced ex situ and in situ/ operando analytical techniques, such as small- and wide-angle X-ray scattering (SAXS/WAXS) and Magic Angle Spinning-Nuclear Magnetic Resonance (MAS-NMR) Spectroscopy will be leveraged to generate fundamental knowledge on how CO2 impacts polymer phase behavior, morphological and thermal properties as well as solvent transport at varying temperatures and pressures. A detailed kinetic Monte Carlo model will be developed to provide insights into the reaction and transport mechanisms of hydrolytic deconstruction of PET into monomers in the presence of CO2. This work will also put forward a state-of-the-art methodology blueprint, which can be adapted to expand multi-scale mechanistic studies to other chemical recycling strategies for post-consumer plastics.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
