Compositional Analysis of Oxygenates and Hydrocarbons in Waste and Virgin Polyolefin Pyrolysis Oils by Ultrahigh-Resolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

The linear plastic lifecycle is unsustainable. Mechanical recycling of mixed plastic waste remains challenging, making chemical recycling necessary. Polyolefins, the largest share of plastic waste, can be chemically recycled through thermal pyrolysis. However, the impact of the feedstock type on pyrolysis oil composition remains unclear. Only very advanced analytical techniques allow to assess the detailed composition of these oils, which is crucial to evaluate their economic potential. Therefore, in this work, the hydrocarbon and oxygenate contents of three pyrolysis oils derived from postconsumer waste polyethylene, polypropylene, mixed polyolefins, and virgin polyethylene are characterized by ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry. Both positive-ion atmospheric-pressure photoionization and negative electrospray ionization were employed to identify oxygenates and hydrocarbons. It was found that the presence of trace polymers, metals, and polymer defects in postconsumer waste oils resulted in a higher fraction of complex polycyclic aromatic hydrocarbons compared to virgin pyrolysis oils. Furthermore, clear sources of oxygenates (polymer additives, trace polymers, or organic residues) could be identified in all four samples. Suspected thermal dissociation products of polymer additives such as Irganox 1010 and diethylhexyl phthalate (DEHP) were observed, indicating the various reactions occurring upon pyrolysis of these complex blends. The fact that a variety of additive-derived contaminants were found in pyrolysis oil from virgin polyolefins further indicates that pyrolysis oil contaminants do not exclusively accumulate during the plastics' lifetime but could potentially be limited on the manufacturing side. This unprecedented level of molecular detail in the compositional analysis of plastic pyrolysis oils will aid in the development of improved recycling strategies, which can help close the loop to a circularized lifecycle for polyolefin waste.