Insight into molecular structural features of Naomaohu lignite via sequential density gradient separation with multiple technical strategies

A comprehensive understanding of the organic structural characteristics of lignite is essential for its clean conversion and efficient utilization. However, such understanding remains ambiguous at the molecular level, primarily due to the intrinsic heterogeneity of lignite and the lack of systematic characterization using complementary analytical techniques. In this study, we address this challenge by employing a sequential density gradient separation (SDGS) method that preserves the major detectable organic structural features of Naomaohu lignite (NL), eliminates the intermingling effects of organic species with varying degrees of condensation, and removes the interference of inorganics in the analysis of its organic characteristics. We demonstrate that the density of organic species in NL predominantly ranges from 1.3 to 1.4 g cm −1 , whereas its organic structure is characterized by aliphatic linkages connecting 2–3 aromatic rings, and O-, N-, and S-containing species are mainly present as > C-O- bridging bonds, pyrrolic, and sulfones, respectively. Notably, the low-density SDGS samples are enriched in aliphatic carbons with weak- and medium-strength covalent bonds (C al -X-, X denotes H, N, S, or > C al ), while the high-density SDGS samples, which are abundant in well-ordered aromatic rings, are more susceptible to thermal condensation and semi-coke formation. Our research underscores that for abundant and representative NL resources, elucidating the structural and compositional characteristics of NL organic species through non-destructive strategies could facilitate value-added and advanced utilization of lignite resources.