Supercritical fluid extracts from brown coal lithotypes and their group components—molecular composition of non-polar compounds

Abstract

The two Tertiary brown coals, i.e. xylitic brown coal (XBC) and humodetritous brown coal (HBC), from the ‘Turów’ and ‘Konin’ deposits in Poland and their group components were investigated on their behaviour to Soxhlet and supercritical fluid (SFE process) extractions. The raw coals were Soxhlet pre-extracted to remove unbound lipids (bitumens). Humic acids, lignin and cellulose were obtained from the pre-extracted XBC, while humic acids and residual coal were separated from the HBC. Raw coals and their group components were extracted under supercritical conditions with toluene (653 K, 8 MPa, 1 h). The yield of non-polar fractions from the Soxhlet and supercritical extracts were determined and analysed by gas chromatography–mass spectrometry (GC–MS). The diversity in biomarker compositions ( n-alkanes, n-alkylbenzenes, iso-alkanes, fatty acids, diterpanes and pentacyclic triterpanes and their aromatised derivatives, methylphenanthrenes) of the supercritical fluid extracts (SFEs) were compared and contrasted with the Soxhlet extracts. The biomarker geochemical indices in Soxhlet extracts are typical of low maturity, while these released by SFE process have characteristic of a higher maturation level. The Soxhlet extracts of both coals show a dominance of 17α(H),21β(H)-homohopane with the precursor configuration of 22 R. The pentacyclic triterpanes found in the SFEs are diverse structures, i.e. 17α(H),21β(H)-hopanes, moretanes and small amounts of benzohopanes, as well as geochemically unaltered homohopane with the biological configuration of 17β(H),21β(H)-(22 R). Isopimarane is the characteristic of SFEs from humic acids of the coals. Partially aromatised pentacyclic triterpanes related to α- and β-amyrin are characteristic of free lipids and SFEs of humic acid fractions from both coals. The diversity in the phenanthro[4,5- bcd]furan, methylphenanthrenes and 2-methylanthracene compositions is dependent on chemical nature of the brown coal group component.