(337r) Microwave-Assisted Depolymerization and Upcycling of Plastic Waste to High-Value Products

Plastic waste accumulation, particularly polyolefins and PET from single-use plastics is a pressing environmental issue. Mechanical recycling falls short in managing these materials effectively, leading to increased interest in chemical depolymerization and upcycling methods like pyrolysis, solvolysis, hydrogenolysis, and hydrocracking. However, the high energy demands and subsequent carbon dioxide emissions associated with these processes is a significant challenge. Furthermore, many of these processes currently employ catalysts that are either expensive or not selective to high-value products, and there is a need for better alternatives to make these processes more sustainable.

In this work, we demonstrate that coupling microwave (MW) heating with suitable heterogeneous catalysts in appropriate reactor configurations can result in rapid depolymerization of polymers such as PET and polyethylene, allowing to overcome the energy-related challenge of conventional heating (CH), at the same time achieving high selectivities to valuable products. First, we demonstrate that tuning the morphology of ZnO materials can result in high activity for MW-assisted glycolysis of PET, achieving complete conversion and >95% yields of bis(2- hydroxyethyl) terephthalate (BHET), its monomer, in minutes. We then illustrate how reactor design and tuning acidity/porosity of solid acids can selectivity produce olefins (>88%) via MW-slurry pyrolysis of polyethylene in seconds. We also demonstrate the influence of different types of acid sites on the catalyst's activity for LDPE pyrolysis. Lastly, we compare the performances of our MW-slurry pyrolysis with CH-pyrolysis and demonstrate how the volumetric heating of MWs can improve performance significantly by reducing coke formation.