Chemical elucidation of recycled plastic pyrolysis oils by means of GC×GC-PI-TOF-MS and GC-VUV

Abstract

Detailed chemical characterization of the composition of pyrolysis oils produced from plastics is a mandatory step in understanding the recycling process. Even though the chemical composition of pyrolysis plastic waste oils shows similarities with fossil-based oils, e.g. PIONA related compounds, the relative proportions of the chemical classes are significantly different, resulting in important changes in the global properties of these fluids. In this context, an analytical method based on comprehensive two-dimensional gas chromatography (GC×GC), coupled with photoionization (PI) – time-of-flight mass spectrometry (TOF MS) was developed to obtain a detailed chemical characterization of plastic pyrolysis-oil. A group-type analysis was achieved thanks to the detection of molecular ions by soft ionization PI-TOF MS. Moreover, olefins were successfully distinguished from naphthenes by in-depth consideration of MS spectra, even though such compounds elute very close in the 2D chromatographic separation space. A part of the olefins identified by PI was further confirmed by GC coupled to vacuum ultraviolet detection (VUV) since olefins VUV spectra differ from other compounds. The overall combination of the entire set of data of this multi-modal approach was valuable in studying the pyrolyzed plastic oil composition. It was furthermore applied to monitor the efficiency of upgrading processes that were applied to the oils to remove specific contaminants such as the diolefins. This approach has the potential to support the production of plastic pyrolysis oils that could efficiently be reintroduced in the market to sustain economic circularity needs.