(516b) Improved Hydrothermal Depolymerization of Polystyrene with Substoichiometric Hydrogen Peroxide Addition

Plastics production has more than doubled over the last two decades, largely due to their desirable properties that include low cost, versatility, and durability, which also lead to poor biodegradability. When combined with the low recyclability of plastics, in particular polystyrene that has a global recycled rate of less than 1%, results in widespread environmental pollution. Hydrothermal liquefaction has been recently shown as effective in chemically depolymerizing waste plastics into smaller fragments at elevated temperatures and pressures, resulting in an oil phase containing a wide array of smaller organic precursors often used as fuels and/or commodity chemicals. HTL requires harsh conditions to generate the radicals thought necessary to propagate chain reaction. We hypothesized that the addition of a labile radical initiator such as hydrogen peroxide would allow HTL to be performed at milder conditions than is typical. In this work, hydrothermal liquefaction (HTL) experiments were conducted on polystyrene (PS) in a high-pressure vessel at operating temperatures of 325 – 400 ˚C, autogenic pressures and 0 – 12 wt% aqueous solutions of a hydrogen peroxide (H₂O₂) radical initiator. The resulting products formed were either a solid plastic phase, a waxy phase from incomplete depolymerization, a carbon rich oil phase and a gas product. When operated at 350 ˚C without a radical source, the product from HTL was nearly all solid PS residue but shifts towards 90% oil yield with the addition of 12 wt% H₂O₂ solution. Control tests with molecular oxygen confirmed the role of hydrogen peroxide as not only a source of oxidant during the reaction but also a radical source carrier. Single dimension GC analysis revealed styrene as the dominant product in the absence of initiator and a complex mixture of styrene and additional oxygenated and non-oxygenated one-ring compounds in the product obtained with initiator.

Additional control tests and product characterization provide further mechanistic insight. Specifically, comprehensive two-dimensional gas chromatography with a high-resolution mass spectrometer (GC×GC-HRT) provides improved product separation and detection obtained with radical initiator, which confirmed the production of benzaldehyde and acetophenone as major oxygenated products. Other nonoxygenated products are also observed with a-methylstyrene being a prominent oil product under HTL conditions without H₂O₂. Secondary single ring oxygenated products included benzeneacetaldehyde, a-methyl-benzeneacetaldehyde, dimethyl-acetal-benzaldehyde. Two-ring and three-ring compounds were formed as secondary products both in the presence and absence of radical initiator. Notably, unlike the one-ring compounds, the two-ring compounds were non-oxygenated both in the presence and absence of the radical initiator. The lack of oxygenated dimers and trimers in all liquefaction treatments suggests that typical condensation routes of oxygenated product monomers are unlikely and that these products result from mid-chain beta scission of PS. The relative increased abundance of dimers and trimers with the addition of H₂O₂ is consistent with an increase in radical promoted scission of the PS polymer chain. The combination of solid and oil characterization is informative on the role of both water and radicals during PS hydrothermal processing. The comparative experimental results with hydrogen peroxide addition reveal a reduced process severity necessary to generate fuel precursors from polystyrene and these analyses shed light on the role of the radicals on the depolymerization mechanism of polystyrene depolymerization.