Towards solvothermal upcycling of mixed plastic wastes: Depolymerization pathways of waste plastics in sub- and supercritical toluene

Abstract

Solvothermal liquefaction (STL) is a thermochemical conversion process, where a waste feedstock is treated with sub-and supercritical solvents. The key objective of the study was to investigate how a one-step STL using toluene as a solvent can degrade hard-to-recycle waste plastics (#5–#7). Three different plastic wastes namely, polypropylene (#5), polystyrene (#6), and polyurethane (#7), and their equal mixture (by weight, also referred here as mixed plastic wastes) were solvothermally liquefied by toluene in a 7 mL pressure bomb at 300, 350, and 400 °C for 3, 6, and 9 h in order to determine the effect of temperature and time, respectively. The liquid products were separated from the solid residue and further analyzed in terms of the STL conversion, change in elemental compositions via ultimate analysis, boiling point distribution via thermogravimetric analyzer (TGA), alteration of chemical bonds via proton nuclear magnetic resonance spectroscopy (1H NMR), and degradation products via gas chromatography mass spectroscopy (GCMS). The results showed that the STL conversion increase with the increase of reaction temperature and time. From the elemental analysis, it can be predicted that the higher heating values of the crude products are between 30 and 45 MJ/kg. Boiling point distribution showed that the production of lower hydrocarbon (C8–C20) increased significantly from about 10% at 300 °C to 80% at 400 °C. Additionally, product distribution showed that the aromaticity increased with the increase of residence time where the main products are benzene and styrene like products. Overall, it was observed that mixed plastic wastes have a synergistic effect on the degradation products.