Disintegrative supercritical fluid extraction (SFE) of brown coal using supercritical dichloromethane

摘要

A Rhenish brown coal was extracted with dichloromethane under supercritical conditions. Extraction pressure and temperature increased rapidly after passing critical point of dichloromethane indicating the rapid disintegration of both solvent and coal. After fractionating the extract in hydrocarbon classes analyses using gas chromatography and gas chromatography-mass spectrometry were performed. Comparing these fractions with those from the original coal significant changes in hydrocarbon assemblages were obvious. The n-alkane distribution shifted from the higher homologues to those with lower carbon numbers indicating pyrolysis of paraffins. Pyrolysis was also indicated by the appearance of bibenzyl, naphthaline and phenanthrene and their alkylated homologues not existing in the original sample. Further some interesting reactions of the solvent were observed. First, the insertion of methylene groups (-CH_2-) into oxygen-hydrogen bonds of carboxyl groups or oxygen-carbon bonds of esters took place indicated by the appearance of methyl, ethyl, propyl, butyl, pentyl and hexyl esters of fatty acids not existing in the original brown coal. Second, methylated sulfur species like alkylated tri- and tetrasulfides were identified. Both classes of substances are supposed to be the reaction products of dichloromethane, which decomposes rapidly under supercritical conditions, and fatty acids or their methyl esters as well as cyclo-octasulfur (S_8), which are present in the brown coal.