(633d) Radical Initiated Thermal Depolymerization of Polystyrene

Chemical upcycling of waste plastics into fuels and chemicals has been performed via thermal processes such as hydrothermal liquefaction (HTL), which require severe operating conditions under aqueous conditions to depolymerize plastics (>400 °C and 25 MPa). Reducing thermal depolymerization severity would improve energy efficiency for converting waste plastics. Depolymerization has been shown to occur through radical initiated scission, where the radicals are formed from random scission or hydrogen abstraction¹. In this study, the impact of HTL with the coaddition of a radical initiator (termed HTL+) is examined to promote depolymerization while reducing severity.

Hydrogen peroxide was used as an external radical source to initiate β- scission of polystyrene. The effects of radical source loading and reaction temperature on oil yield and composition was examined. Addition of a radical initiator gave the most improvement in liquid yield at 350 °C, corresponding to an increase from 0% to ~90%. As the temperature was increased beyond 350 °C, non-initiated HTL yields increased monotonically while initiated HTL yields decreased. Peroxide loadings ranging from 3:1 to 10:1 on a carbon-oxidant basis were studied for a reaction temperature of 350 °C to analyze the effect of initiator on oil yields. Reducing initiator loading from 3:1 to 5:1 resulted in a decrease in oil yield and an increase in wax production indicative of partial depolymerization. Finally, the composition of the oils produced from HTL+ were analyzed using GC-MS, revealing that oils produced from initiated HTL include oxygenated compounds, such as benzaldehyde that are not present in samples obtained in the absence of initiator. These results confirm that radical initiated HTL is a new approach for plastic depolymerization that uses milder conditions than traditional HTL.

1. Guyot, A., Polymer Degradation and Stability 1986, 15 (3), 219-235