Molecular indicators of the Oligocene Bobov Dol coal organic matter composition from bitumen analysis and preparative off-line thermochemolysis

• Extraction and thermochemolysis reveal different lipid pools in coal organic matter.
• Linear esters are potential structural bonds at coalification level Ro = 0.42–0.43%.
• Lignin building blocks are still preserved for identification in Late Oligocene coal.
• Vanillic structures denote conifers as the coal-forming vegetation in the peat bog.

Bobov Dol subbituminous coal from Bulgaria was studied by chloroform extraction and subsequent TMAH thermally assisted hydrolysis and methylation. The molecular composition of the “free” bitumen was compared with the products of residue pyrolysis. GC/MS separation registered lipids, i.e. n-alkanes and terpenoids, mainly sesqui- and diterpenoids, predominantly aromatized. Pyrolysis released additional portions of n-alkanes, regular isoprenoids, n-fatty acids, hopanes, etc. Direct extraction of “free” bitumen with chloroform and TMAH thermally assisted hydrolysis and methylation of the residue attained somewhat different lipid pools of coal organic matter.The results of TMAH thermally assisted hydrolysis and methylation demonstrated that, despite the relatively advanced geological age of the sample (Late Oligocene) some lignin remnants were still preserved. The following species were registered: 3-methoxy-4-hydroxybenzaldehyde, 3-methoxy-4-hydroxyacetophenon and 3-methoxy-4-hydroxy benzoic acid. By analogy with the lignin structure of modern trees, the abundance of vanillic structures indicated conifers as the predominant coal-forming vegetation in the Bobov Dol peat bog.Fatty acids in “free” bitumen and in the products of thermochemolysis were compared and certain differences were depicted. The presence in the pyrolysate of linear mono- and dicarboxylic fatty acids, mainly of higher plant origin, revealed the role of ester linkages in coal organic matter structuring at an Ro = 0.42–0.43% maturity.The experimental data denoted that lipid components and conifer lignin had considerable contribution to Bobov Dol coal organic matter.