Characterization of Biomarkers and Structural Features of Condensed Aromatics in Xianfeng Lignite

Xianfeng lignite (XL) was sequentially extracted under ultrasonication at room temperature with petroleum ether, carbon disulfide (CDS), methanol, acetone, and isometric CDS/acetone mixed solvent to afford extracts 1–5, respectively. The mixed solvent-inextractable portion was sequentially extracted with cyclohexane, benzene, 1-methylnaphthalene, methanol, and ethanol at 320 °C to afford extracts 6–10, respectively. The extracts were analyzed with a gas chromatography/mass spectrometer (GC/MS) to characterize biomarkers in XL. The biomarkers were significantly enriched in extracts 1 and 6. They can be classified into a series of n-alkanes, isoprenoid alkanes, terpenoids, n-alkenes, methyl alkanones, n-alkylbenzenes, n-alkyltoluenes, and n-alkyl-p-xylenes. The biomarker distributions provided important information on the main origin of organic matter in XL. Related mechanisms for the formation of biomarkers during coalification were discussed. The residue from sequential thermal extraction was subjected to ruthenium-ion-catalyzed oxidation along with subsequent product analyses with GC/MS and direct analysis in a real-time ionization source coupled to a time-of-flight mass spectrometer (DARTIS/TOFMS) to understand its structural features. The results show that the residue is rich in condensed aromatics (CAs) and methyl is the dominant alkyl side chain on aromatic rings. The aromatic rings in the residue are mainly connected by −(CH2)3– and −CHCH3(CH2)2–. DARTIS/TOFMS analysis suggests that CAs with alkyl-substituted biphenyl and alkyl-substituted phenylbiphenyl skeletons also exist in the residue. This investigation provides an effective approach for understanding biomarkers and the structural features of the macromolecular network in XL.