Detailed analysis of olefins and diolefins in hydrotreated plastic waste pyrolysis oils by GC-VUV

Recently, the hydrotreatment of plastic waste pyrolysis oils has been considered a promising route to remove heteroatoms and unsaturates from plastic pyrolysis oils to be used as steam cracking feedstock. As plastic waste pyrolysis oils differ vastly in composition from conventional crude oils, accurately identifying the different molecular species present in the mixture is essential to acquire further insights into their hydrogenation pathways. This work used gas chromatography coupled to a vacuum ultraviolet detector (GC-VUV) to obtain accurate quantification of (di)olefins compared to more conventional methods. It was found that there is no difference in intrinsic hydrogenation rates between diolefins and olefins, at least for chain lengths above C₁₀. Moreover, an explicit dependency of the carbon chain length is identified on the hydrogenation rate for both olefins and diolefins with optimal hydrogenation rates up to C₁₅. This is a consequence of the competition between hydrogenation and cracking reactions. Shorter unsaturated chains are thus formed because intrapore diffusional limitations prevent the hydrogenation of larger species. In addition, the degree of substitution (non-branched, methyl, dimethyl, trimethyl, etc.) of the unsaturated molecules mattered concerning the hydrogenation rate. Linear olefins were more rapidly hydrogenated than branched components. Apart from hydrogenation, skeletal isomerization reactions are also important, especially at high olefin conversions. These fundamental insights demonstrate the capability of GC-VUV to analyze hydrotreated plastic pyrolysis oils and provide essential knowledge for designing and optimizing the hydrotreatment processes of plastic waste pyrolysis oils.