Recent advances in pyrolysis upcycling of waste plastics into hydrocarbon fuels on biochar-based catalysts

Abstract

Waste plastics, positioned between biomass and fossil fuels, hold great potential for valorization through upcycling technologies. However, it is still challenging to produce high-quality energy and chemicals from real-word commingling waste plastics. Catalytic pyrolysis demonstrates strong adaptability to mixed feedstocks, and the development of specialized catalysts is key to enhancing product quality. Among various catalysts, biochar-based catalysts stand out for their simple preparation and remarkable resistance to coke deposition. In this context, the catalytic pyrolysis of waste plastics based on biochar-based catalysts was comprehensively and statistically analyzed by considering plastics feedstocks, pyrolysis reactors, reaction conditions, and catalyst development. This paper critically examines the mechanisms governing catalytic reaction during pyrolysis and analyzes the current challenges aimed at improving the yield and selectivity in the production of target products. In comparison to in-situ, the oil yield from ex-situ and microwave catalytic pyrolysis is notably higher. The coconut shell-based biochar with polymetallic modification under higher catalyst-to-feedstock ratios and lower reaction temperatures are more conducive to producing liquid oils. The free radical generation, carbocation reactions, and hydrogen transfer are the main mechanisms of waste plastics pyrolysis catalyzed by biochar-based catalysts. The Bronsted and Lewis acid centers on biochar-based catalysts, enhanced by biochar design, metal modification, and microwave heating, are key to determining the above reactions. Nonetheless, more efforts should be taken to design outstanding biochar-based catalysts and conversion systems by considering the environmental impacts and economic feasibility from intelligent and life-cycle aspects for scaling up to promote the sustainability and socio-economic benefits.