NSF Award
2309899
With the accumulation of plastics in the environment being one of the most pressing societal challenges today, the development of methods to improve plastics recycling is crucial. A major challenge to plastics recycling is developing processes that are cost effective and yield polymers with properties that are identical to or better than those of the original plastic. While the use of biological catalysts for polymer recycling and upcycling has generated significant interest, research has been restricted to reactions that take place in water. Due to the limitations of water as a medium for such reactions, new approaches that improve the efficiency and conversion of plastic waste using biologically based methods are needed. The overall aim of this work is to explore the use of natural enzymes to deconstruct common single-use plastics such as polyethylene terephthalate (PET – commonly used in clothing fibers and liquid/food containers) in the absence of bulk water. This work will specifically leverage the dramatically improved properties of enzymes in “dry” environments, including the ability to catalyze a range of reactions that are not possible in aqueous media. This research will investigate enzymes that potentially can degrade PET in new ways that facilitate the repolymerization of the reaction products, which is critical for recycling. This effort will lead to a detailed understanding of the reaction parameters that impact enzyme efficiency in converting PET waste into useful, recycled plastics building blocks. This research will broaden participation in STEM research by engaging a diverse group of undergraduate and high school students in this research program. Participants will be recruited through a variety of programs, including the Summer Multicultural Access to Research Training (SMART) and STEM Routes programs at CU Boulder and Miami University. Moreover, meaningful teaching modules will be developed that can be incorporated into K-12 curricula on the biodegradation of materials and biocatalysis.<br/><br/>The objective of this proposal is to develop a novel biocatalytic process for the deconstruction of polyethylene terephthalate (PET) plastics based on the alcoholysis of ester bonds in the polymer backbone by lipase. Of specific interest is correlating the kinetics of PET alcoholysis with the thermodynamics of the depolymerization reaction as well as the solvent properties, which enable the rate of alcoholysis to be fine-tuned. A major advantage of this approach over the hydrolysis of PET is that the use of nonaqueous media may promote swelling of the polymer, thereby increasing plasticity of the polymer chains. This in turn will increase the accessibility of the ester linkages in the polymer backbone and thus may significantly enhance the rate of biocatalytic conversion. Additionally, the use of nonaqueous solvents over water increases the ease of separation of the reaction products and can enhance the thermostability of the enzymes. These advantages potentially will eliminate the need for the high reaction temperatures as well as energy intensive pretreatment steps, such as melt extrusion and microgrinding, used in current PET recycling approaches. This work will specifically test the hypothesis that esterases can catalyze the alcoholysis of PET in nonaqueous media, and that the rate of alcoholysis can be controlled by varying the thermodynamic equilibrium water activity of the reaction as well as the solvent properties and choice of alcohol as the nucleophile. These studies will use lipase, including cutinase, as a model esterase since it is already well established that lipases can catalyze alcoholysis reactions in anhydrous media and bind to PET. Additionally, to correlate the kinetics of PET alcoholysis with the thermodynamics of the reaction, water activity will be controlled using salt hydrates. Finally, as part of this effort, rational strategies will be developed to reduce diffusional limitations of lipase in nonaqueous solvents, including ion pairing with surfactants. While this effort will focus on PET recycling, the proposed approach may be applicable in recycling of other synthetic polyesters. Moreover, the fundamental understanding of depolymerization reaction mechanisms developed by this this research program also may lead to new ways to chemically modify polyesters to improve their utility and recyclability.<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.
