Effects of Water and Air on the Reaction Pathways in the Hydrothermal Liquefaction of Plastics

This study establishes the role of water in the hydrothermal liquefaction (HTL) of plastics. 1-dodecene is employed as a model compound for polyolefin fragments resulting from chain cracking. The presence of a terminal double bond enables potential hydration and oxidation reactions to be assessed. Comparative experiments are conducted under identical conditions of temperature using pyrolysis and HTL in the presence and absence of air. The resulting product distributions provide critical insight into the operative reaction pathways, specifically highlighting the function of water as a reaction medium. Importantly, although hydrogen exchange and hydrogen abstraction are well-established, water does not appear to react oxidatively with the hydrocarbon substrate, as no incorporation of oxygen is observed when operating under an argon atmosphere. When operating under air, oxygenated compounds are detected in the products, particularly as light alcohols, aldehydes, and acids, consistent with the established supercritical water oxidation reaction pathways.