Enabling resource circularity through thermo-catalytic and solvent-based conversion of waste plastics

Abstract

The recent rise in plastic pollution has led to a growing environmental burden, motivating new and effective methods for circular repurposing of “end-of-use” plastics. In this review, we highlight recent advances in thermochemical and catalytic pathways toward circularity of plastics utilization; specifically, hydroconversion, solvent conversion, and catalytic conversion without solvent or gaseous reagent. We present advances in the design of supported metal catalysts (Pt, Ru, Zr) for the hydroconversion of plastics, especially polyolefins (POs) and polyesters. We deduce mechanistic insights from hydroconversion reactions toward realizing economic circularity. We also review two solvent treatments: solvolysis of condensation polymers and solvent extraction for composite polymers. Last, we discuss advances in hydrocarbon conversion, without solvent or gaseous reagent, to catalytically depolymerize POs. We highlight the challenges and envision the path forward in optimal catalyst and process design that will enable the development of chemical upcycling technologies for building a circular plastic economy. Potentially irreversible environmental damage is a deleterious consequence of our industrial development, with plastic pollution being a key contributor. To avert this crisis, building pathways towards a sustainable circular plastic economy is critical. To this end, chemical upcycling and recycling strategies have shown promise for mitigating plastics pollution and easing our dependence on fossil sources for their production. This review presents recent advances in catalytic methods for converting waste plastics, develops mechanistic insights, and highlights challenges and opportunities associated with these approaches. Three specific pathways are emphasized, namely, hydroconversion, solvent conversion, and catalytic conversion without solvent or gaseous reagent. The realization of chemical upcycling will facilitate the United Nations’ 2030 Sustainable Development Goals, including battling climate change, preserving life on land and under water, and generating affordable and clean energy. On the path to address plastic pollution, thermochemical and catalytic processes have shown promising results for polymer recycling and upcycling. The classes of depolymerization reactions presented here have been reviewed in detail and tabulated for comparison, conveying the potential of hydroconversion and solvolysis in the transition to a circular plastics economy. The challenges deduced here portray the gaps this community needs to address to meet the broader environmental challenge.