Structural characterization of the thermal extracts of lignite

• The thermal extract mainly consists of normal alkane and normal fatty acid ester.
• Higher normal alkanes are intrinsic small molecular compounds in XL.
• Most of normal fatty acid esters are intrinsic components of XL.

Thermal extraction (TE) with nonspecific solvent at high temperature is a potential technology to separate organic materials from coal, especially low-rank coal such as lignite. In this paper, thermal extract (TES) of Xianfeng lignite (XL) in toluene/methanol (3:1, volume) mixed solvent at 300 °C was separated into different sub-fractions by the method of column chromatography combined with Soxhlet extraction in tetrahydrofuran (THF). These sub-fractions were characterized by element analysis, FTIR, 1H NMR and GC/MS. 78 compounds including C12–30 higher aliphatic hydrocarbons (HAHs), aromatic hydrocarbons (AHs), C17–27 fatty acid methyl esters (FAMEs) and other heteroatomic compounds were identified from the TES. As two groups of predominant components, HAHs and FAMEs are the intrinsic components in XL except for small amount of FAMEs produced by esterification and transesterification reactions in the TE process. Further, the mechanism of TE was also speculated by the characterization results of TES. As a result, the TE with nonspecific solvent at high temperature can not only improve the extract yield of organic materials, but also obtain chemicals such as HAHs and FAMEs from lignite.