Sequential Solvent Extraction of Argonne Premium Coal Samples

Five coals from the Argonne Premium Coal Sample program, ranging in rank from sub-bituminous to low volatile bituminous, before and following artificial weathering, were exhaustively extracted with solvents in the sequence: toluene, tetrahydrofuran, N,N-dimethyl-formamide, and pyridine. Sequential solvent extraction reveals more insight into the coal structure than single solvent extraction because fractions are isolated on the basis of both molecular size and polarity. The recovered extracts and residue from the extraction are monitored by FT-IR spectroscopy, elemental analysis, phenol content, solvent swelling, and Gel Permeation Chromatography analysis (GPC). The extraction results show that for fresh coals, the total extract yields are coal rank dependent, but trends are less clear for oxidized coals. A correlation between the total extract yields and the molecular weights of the extracts are found by GPC analysis. FT-IR spectra of extracts reveal changes with coal rank and upon oxidation in more detail than spectra of the whole coals. Fresh and oxidized coals of middle rank are less rich in carbonyl groups. The oxygen content in fresh subbituminous coal is dominated by acid groups. Higher rank coals (up to 89% C daf) exhibit more diverse oxygen-containing function groups than lower rank coals. Air oxidation of higher rank coals brings complicated changes in FT-IR spectra of extracts. For high volatile bituminous coals, it is concluded that the loss of extractability upon air oxidation is due to the formation of ether linkages