Molecular and geochemical characteristics of lignite in different plant sources evaluated using unsupervised analyses

Three lignites were subjected to sequential thermal dissolution using cyclohexane, methanol, and ethanol as the solvent to acquire soluble fractions (SFs). Comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry (GC x GC/TOF MS) was used to separate and reveal complex molecular composition of SFs. Low polar compounds such as chain alkanes (CAs) and arenes were enriched by cyclohexane. Strong polar compounds like phenols were concentrated by methanol, and partial phenols and CAs were also detected in the ethanol extracts. Meanwhile, organic nitrogen compounds (ONCs) and organic sulfur compounds (OSCs) such as pyridines, quinolines and thiophenes were identified in SFs. In all SFs, pyridines have the highest relative abundance, followed by quinolines compared with other ONCs, and thiophenes were only detected in cyclohexane and methanol SFs. In addition, halogen-containing compounds reflecting the geochemical characteristics of lignite were also detected, and their total relative abundances were 0.544%, 1.591% and 5.101% in the SFs of Shengli (SL), Xiheishan (XHS), and Xiaolongtan (XLT), respectively, which provided evidence for the evolution of coal. Two unsupervised analysis methods, hierarchical cluster analysis and principal components analysis, were efficient in clustering components according to molecular characteristics, and visualized detailed similarities and differences among the compounds in SFs.