Thermal oxo-degradation of plastic wastes to valuable compounds

As plastic production and environmental accumulation continues to grow, developing new technologies for upcycling plastic wastes is critical. A chemical recycling technology in commercial development is pyrolysis, which involves rapidly heating plastics without oxygen present to convert plastics into condensable products such as wax and oil. These products contain a wide variety of chemicals, some of which can be used as building blocks for new plastic products or chemicals. Pyrolysis has shown potential for developing a circular economy for plastics, but scaling up pyrolysis technology can be challenging due to heat transfer limitations of typical pyrolysis reactors. This work advances thermal oxo-degradation (TOD) as an alternative to conventional pyrolysis. TOD differs from the typical pyrolysis process by introducing air into the thermal reaction. The main benefits of oxygen are to increase the rate of thermal depolymerization of waste plastics and to provide some of the enthalpy for the reaction without dramatically impacting desired product yields. This research defines TOD as a separate regime from gasification, which has the goal of producing syngas. Alternatively, the herein reported studies use TOD to create energy-dense condensable products. Both virgin and post-consumer waste high-density polyethylene and polypropylene show enthalpies for pyrolysis greater than 2100 J g-1. Comparison of TOD to pyrolysis showed that partial oxidation reactions in TOD release heat in the reactor, helping to manage the endothermic enthalpy load for vapor-phase cracking reactions. By manipulating reaction variables such as temperature, equivalence ratio, and vapor residence time, the products of TOD can either be majority oxygenated molecules (e.g. alcohols, aldehydes, carboxylic acids) or majority hydrocarbon molecules (paraffins, olefins, and diolefins). These products are condensed as waxes and oils. Oxygenated waxes from TOD can be used as the sole carbon source for bioconversion by yeasts. Because hydrocarbon waxes and oils from TOD resemble those produced from pyrolysis, hydrocarbon-rich TOD waxes and oils can be catalytically upgraded to light olefins for reintegration into the hydrocarbon economy. This research shows that TOD can be applied as a chemical upcycling technique that overcomes heat transfer limitations of pyrolysis, enabling easier scaling. Applying TOD to the plastics upcycling can help mitigate plastic waste accumulation in our landfills and achieve a plastics circular economy.