Characterization of PP and PE Waste Pyrolysis Oils by Ultrahigh-Resolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Increasing recycling rates of plastic waste is necessary to achieve a sustainable and climate-neutral chemical industry. For polyolefin waste, corresponding to 60% of plastic waste, chemical recycling via thermal pyrolysis is the most promising process. However, the hydrocarbon composition of these pyrolysis oils differs from conventional fossil-based feedstocks as they are heavier and more unsaturated. GC × GC-FID is the most prevalent characterization method for the analysis of these complex hydrocarbon mixtures but fails to discern heavy unsaturated, aromatic compounds. An up-and-coming technique to fully characterize those analytically challenging heavy fractions is ultrahigh-resolution Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) coupled with soft ionization techniques, such as atmospheric pressure photoionization and atmospheric pressure chemical ionization. In this work, FT-ICR MS has been employed to analyze both real PE and PP postconsumer waste pyrolysis oils, which allowed to provide additional insights into the pyrolysis reaction pathways of both polyolefin types. FT-ICR MS identifies heavy hydrocarbons, up to C85, and discerns a wide range of complex polycyclic aromatic hydrocarbons with up to seven aromatic rings. These hepta-aromatics were not found in PP, which only revealed penta-aromatics; this complies with the reaction mechanism proposed in the literature. Moreover, the polypropylene (PP) pyrolysis oil displayed clear signs of depolymerization reactions occurring during pyrolysis, both for the formation of olefins and diolefins. Here, FT-ICR MS identified heavier, unsaturated, and highly aromatic hydrocarbons, whereas GC × GC-FID quantified saturated and less complex unsaturated components. These observations highlight the added benefit of combining GC × GC-FID and FT-ICR MS data to completely characterize plastic pyrolysis oils and understand pyrolysis reaction pathways.