Characterization and Source Tracing of Nitrogenates and Halogens in Polyolefin Waste Pyrolysis Oils by Ultrahigh-Resolution FT-ICR MS

As the global tide of plastic waste surges, the need for scalable, effective recycling technologies has never been more urgent. Among emerging strategies, chemical recycling, particularly thermal pyrolysis, stands out as a promising approach for converting complex plastic waste streams, such as polyolefins, into valuable petrochemical feedstocks. However, the use of these pyrolysis oils is currently hampered by the presence of heteroatomic and halogen-containing species, which promote corrosion, fouling, coking and catalyst deactivation in downstream equipment. Detailed knowledge of those contaminants is required to design efficient purification techniques. Therefore, in this study, we employed ultrahigh-resolution 21 T Fourier transform ion cyclotron resonance mass spectrometry to determine the molecular composition of nitrogenates, nitrogen oxides, and halogens across a diverse set of pyrolysis oils. These pyrolysis oils originate from virgin and waste polyethylene, waste polypropylene, and waste mixed polyolefins. Combining positive and negative electrospray ionization facilitated the selective ionization of basic and acidic species, respectively. The remarkable similarities between virgin and waste pyrolysis oils revealed that a substantial fraction of the heteroatomic compounds originates from additives and/or processing agents. In addition to specific contaminants, pyrolysis oils also contain an abundance of aromatic compounds, which act as important “scavengers” of nitrogen and oxygen atoms. Among the halogens, chlorine dominated, with only traces of fluorine and no bromine. Chlorine was primarily present in specific additives and their corresponding breakdown products. Secondary reactions between chlorinated compounds and the hydrocarbon matrix appeared to be less important during thermal pyrolysis. Moreover, no dioxins were detected in any of the pyrolysis oils. The extraordinary level of detail in the results provides new fundamental insights into the formation, persistence, and possible origin of heteroatomic species in plastic waste pyrolysis oils, offering a molecular foundation for additive redesign, improved decontamination strategies and a cleaner path forward for chemical recycling.