(154ak) Upcycling Plastics to Value-Added Chemicals Via Electrified Spatiotemporal Heating

Nearly 9 billion metric tons of plastic have been produced in the human history, among which about 2/3 have become waste that can result in long-term negative environmental and ecological impact. For example, over 14 million tons of plastic waste end up in the ocean every year, threatening biodiversity and wildlife. Moreover, microplastics and its microfibers have been often detected in air and municipal drinking water, causing long-term health concerns. With the continuing heavy demand for plastic products as well as the aforementioned issues, it is urgent to develop efficient plastic recycling strategies to ensure a sustainable future. Among all thermochemical plastic recycling routes, depolymerization is a promising one as it can turn waste plastic into constituent monomers for further polymerization or as chemical feedstocks for manufacturing. However, many commodity plastics cannot be depolymerized with high selectivity and yield, as it is challenging to control the reaction progress and pathway using conventional thermochemical treatment. Although catalysts can be applied to improve selectivity, they are prone to performance degradation, which limits the system durability.

In this talk, I will demonstrate a catalyst-free depolymerization method via the pyrolysis reaction that can convert model polyolefin and polyester plastics to their monomers with high yield (Nature 2023, accepted). This pyrolysis process is realized by two critical features: (1) a spatial temperature gradient and (2) a temporal heating profile. The spatial temperature gradient is enabled by a bilayer structure made of porous carbon felts, in which the top layer is electrified to conduct heat down to the underlying reactor layer and plastic feed. The resulted temperature gradient promotes continuous melting, wicking, vaporization and reaction of the plastic as it experiences increasing temperatures when traveling upward, enabling a high depolymerization degree. In the meantime, we apply pulsed electrical heating through the top heater layer, creating a temporal heating profile that features periodic high temperatures (e.g., ~600 °C) to enable high reaction rate and conversion, yet the transient heating duration (e.g., 0.11 s) can reduce unwanted side products and keep the reaction under far-from-equilibrium conditions. With this process and setup, we converted polypropylene and polyethylene terephthalate to their monomers via the pyrolysis reaction. We achieved monomer yields of ~40%, which are among the highest compared to conventional thermochemical methods even with optimized catalysts. Owing to the high tunability of the electrified heating (temperature, timescale), this approach potentially offers a unique platform to convert plastic waste to a range of value-added chemicals.