Identification of Organic Sulfur Compounds in Supercritical Extracts from Polish Lignite

Abstract

Supercritical fluid extraction (SFE) of a Bełchatów lignite (Ro = 0.26%) was carried out with four different solvents, i.e., a toluene, 2-propanol, toluene/2-propanol, and toluene/THF mixtures, at 360 °C and 10 MPa in an apparatus with continuous flow of solvent. The type of solvent has an influence not only on the extract yield but also on the sulfur extractability. For the 2-propanol/toluene mixture the highest process yield was obtained, 43.1%. The toluene/THF mixture was shown to be the most effective solvent in terms of the extraction of organosulfur compounds (OSC). Gas chromatography−mass spectrometry (GC-MS) was used to study the distribution of OSC in the extract obtained on the SFE process. The results demonstrate that the preparation of a sample prior to GC-MS analysis has a great influence on the OSC distribution of the extract. For the raw extract with solvent, aliphatic and aromatic thiols, aliphatic sulfides, polysulfides, and the C1−C4 alkylthiophenes were detected, whereas only diphenyl sulfide, benzothiophene, and dibenzothiophene were identified after solvent removal by evaporating and final drying of the sample in a vacuum oven. Methyl thiol and methyl mono-, di-, and trisulfides seem to be originally present in the lignite. The presence of thiol and mono-, di-, and trisulfides substituted with an isopropyl group in the extract can be explained as due to the reaction of elemental sulfur with the 2-propanol used for the extraction