Upcycling of polyethylene and alkanes in a CO2 DBD plasma

Low-temperature plasmas provide a pathway for upcycling waste polyethylene and captured CO₂ into more valuable chemical feedstocks such as organic acids. However, the chemical mechanism of plasma-assisted polymer processing is yet to be elucidated, including the detailed pathways and the coupling of bulk, surface, and gas-phase reactions. We use a 20-kHz dielectric barrier discharge (DBD) plasma sampled downstream by molecular beam electron-ionization mass spectrometry to compare a polyethylene/CO₂ plasma to several prototypical gaseous alkane/CO₂ and alcohol/CO₂ plasmas. Pentane/CO₂ plasma provides a qualitatively similar mass spectrum to that of polyethylene, with similar levels of oxygen-containing organics and a comparable distribution of carbon chain lengths extending to at least C₁₁. The presence of long chains (when starting with pentane) as well as acetone and other few-carbon molecules indicates competition between bond-breaking and bond-forming pathways. During the model alkane and alcohol experiments, we adjusted flow parameters and plasma conditions to elucidate the effects of residence time and discover the elementary chemical initiation steps of the gas-phase reaction network, which is part of the multi-phase plasma-assisted polyethylene upcycling.