Catalytic hydrothermal conversion of end-of-life plastic waste in near supercritical water

Abstract

Upcycling of plastic waste to low carbon fuels remains technologically challenging. This study presents near-supercritical catalytic hydrothermal conversion of automotive, marine, agricultural and model plastic wastes to hydrogen and value-added products. At 350 °C, 3000–3300 psi and 1 h of reaction time using a sodium hydroxide catalyst, the hydrothermal conversion of plastic wastes produced between 23 and 76 g of hydrogen per kg of feedstock with few other gases present. The activation energy of the plastic gasification reaction was found to be 6.38–6.83 kJ/mol through the 1 h unsteady state hydrothermal ramping at average of 5.5 °C/min. The highly contaminated, heterogeneous automotive fluff (AF) containing polymeric and natural fiber waste, had the highest hydrogen production of 76 g(H₂)/kg(feed) and was further evaluated with additional alkali and non-precious metal catalysts. Alkali catalysts increased hydrogen production in the following order: NaOH > KOH > K₂CO₃ and were more effective than non-precious metal catalysts whose order was Fe/MCM41 > Ni/SiAl > Fe/ZSM-5. The use of non-precious metal and no catalyst led to mainly CO and CO₂ production, reaching 1251 g(CO₂)/kg(feed) with no hydrogen produced using the Fe/ZSM-5 catalyst. Valuable chemical products including phenol, cresols, guaiacol, and straight chain alkanes dominated the oil phase. The solid analysis revealed new and enhanced peak intensities in the hydrochar functional groups that were absent in the feedstocks. Regular spherical, hexagonal, and cylindrical structural surface morphologies showed varying amounts of carbon and inorganic elements. This work highlights a viable technology for safe upcycling of challenging waste.