Detailed Group-Type Characterization of Plastic-Waste Pyrolysis Oils: By Comprehensive Two-Dimensional Gas Chromatography Including Linear, Branched, and Di-Olefins

Abstract

Plastic-waste pyrolysis oils contain large amounts of linear, branched, and di-olefinic compounds. This makes it not obvious to determine the detailed group-type composition in particular to the presence of substantial amounts of N-, S-, and O-containing heteroatomic compounds. The thorough evaluation of different column combinations for two-dimensional gas chromatography (GC × GC), i.e., non-polar × polar and polar × non-polar, revealed that the second combination had the best performance, as indicated by the bi-dimensional resolution of the selected key compounds. By coupling the GC × GC to multiple detectors, such as the flame ionization detector (FID), a sulfur chemiluminescence detector (SCD), a nitrogen chemiluminescence detector (NCD), and a mass spectrometer (MS), the identification and quantification were possible of hydrocarbon, oxygen-, sulfur-, and nitrogen-containing compounds in both naphtha (C5–C11) and diesel fractions (C7–C23) originating from plastic-waste pyrolysis oils. Group-type quantification showed that large amounts of α-olefins (36.39 wt%, 35.08 wt%), iso-olefins (8.77 wt%, 9.06 wt%), and diolefins (4.21 wt%, 4.20 wt%) were present. Furthermore, oxygen-containing compounds (alcohols, ketones, and ethers) could be distinguished from abundant hydrocarbon matrix, by employing Stabilwax as the first column and Rxi-5ms as the second column. Ppm levels of sulfides, thiophenes, and pyridines could also be quantified by the use of selective SCD and NCD detectors.

Opportunities and challenges for the application of post-consumer plastic waste pyrolysis oils as steam cracker feedstocks: To decontaminate or not to decontaminate?

“…Non-sulfur/nitrogen compounds are converted into non-chemiluminescent CO 2 and H 2 O in the oxidative combustion step ( Yan, 2002 ). GC × GC coupled to SCD and NCD have been applied for complex hydrocarbon samples in various publications and have proven to be valuable techniques for the analysis of trace amounts of sulfur ( Dijkmans et al, 2015 , Djokic et al, 2017 , Ruiz-Guerrero et al, 2006 , Hua et al, 2003 , Blomberg et al, 2004 ) and nitrogen ( Adam et al, 2009 , Adam et al, 2007 , Lissitsyna et al, 2013 , Ristic et al, 2016 , Dijkmans et al, 2015 , Wang et al, 2004 , Toraman et al, 2016 , Dao Thi et al, 2021 ), respectively (see Fig. 7 ).…”

“…In order to improve the separation of heteroatomic compounds such as sulfur and nitrogen containing compounds as well as oxygenates from the dominant complex hydrocarbon matrix it is possible to use reversed-phase GC × GC using the polar column first, followed by the non-polar column. This specific mechanism was used by ( Dao Thi et al, 2021 ) and ( Toraman et al, 2014 ) who reported improved accuracy for the characterization of plastic waste pyrolysis oils.

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“…Furthermore, the choice of columns for the first and second dimension separation in GC × GC has a substantial impact on the analytical accuracy. Different column combinations including reverse-phase (polar/non-polar instead of non-polar/polar) have been tested, making the identification of highly complex hydrocarbon matrices possible ( Dao Thi et al, 2021 , Toraman et al, 2014 ). This way it is also possible to distinguish between α-olefins, branched olefins and diolefins present in the pyrolysis oils ( Dao Thi et al, 2021 ).…”

“…Different column combinations including reverse-phase (polar/non-polar instead of non-polar/polar) have been tested, making the identification of highly complex hydrocarbon matrices possible ( Dao Thi et al, 2021 , Toraman et al, 2014 ). This way it is also possible to distinguish between α-olefins, branched olefins and diolefins present in the pyrolysis oils ( Dao Thi et al, 2021 ). Furthermore, in order to assess plastic waste pyrolysis oils as a steam cracking feedstock from an industry point-of-view, standardized methods are of highest importance to increase and guarantee comparability of compositional data.…”

“…This can be explained by the presence of PS in the waste since its pyrolysis yields mostly (C 8 ) mono-aromatics as confirmed elsewhere ( Toraman et al, 2014 ). In the recently published study by ( Dao Thi et al, 2021 ), two different fractions of plastic waste pyrolysis oils were thoroughly analyzed using GC × GC coupled to various detectors including the assessment of different column combinations. The authors reported around 10 wt% of aromatics in the light fraction (C 5 – C 11 ) while the heavy fraction (C 7 – C 23 ) only contained very small amounts (<1 wt%) of aromatics.…”