Solubility limitations in the determination of molecular mass distributions of coal liquefaction and hydrocracking products: 1-methyl-2-pyrrolidinone as mobile phase in size exclusion chromatography

The use of tetrehydrofuran (THF) as mobile phase in size exclusion chromatography (SEC) has been found to lead to partial loss of sample and to give anomalous results in the characterization of a liquefaction extract and its hydrocracking products. The problem has been resolved by using NMP (1-methyl-2-pyrrolidinone) as mobile phase in SEC, showing significant fractions of sample eluting at the exclusion limit of an identical SEC column. This fraction has not previously been observed in SEC chromatograms obtained in THF. Comparison of SEC chromatograms obtained by UV-absorption and UV-fluorescence detection (in NMP) suggests that the material observed at the exclusion limit of the column corresponds to larger, more complex polynuclear aromatic ring systems than those present in material separated by the column. In NMP, samples produced during progressively higher temperature hydrocracking experiments eluted, as expected, at longer times, indicating progressive molecular size reduction with increasing intensity of the reaction. These data are consistent with the W-fluorescence spectra and TGA-derived boiling point distributions of the set of samples. A twofold mechanism for loss of material in THE-based SEC may be proposed: (i) not all the sample dissolves in THE and (ii) some of the larger/more polar molecules apparently soluble in THF tend to deposit on column packings and do not elute through the column. Considerable caution therefore appears necessary in using THF as mobile phase in SEC work for the characterization of complex coal-derived liquids.