Trace-Element Partitioning between Fractions of Coal Liquids during Column Chromatography and Solvent Separation

Abstract

A coal tar pitch, a coal liquefaction extract, and a low-temperature coal tar have been fractionated by molecular mass, using column chromatography, and the fractions have been analyzed for trace-element content. The solvents used for sequential extraction were acetonitrile, pyridine, and 1-methyl-2-pyrrolidinone (NMP). Trace elements were determined by inductively coupled plasma−mass spectroscopy (ICP−MS) after digestion of the liquids and fractions in a microwave bomb, using nitric acid and hydrogen peroxide. The mercury content was determined using a Leco model AMA254 analyzer. The larger portion of the trace elements analyzed have been found to associate preferentially with fractions that have been shown by size exclusion chromatography to contain the largest molecules. Some of the largest-molecular-mass material adhered to the silica that was used for fractionations. Trace-element mass balances for fractions separated by column chromatography were very poor, because of higher concentrations of trace elements in the largest organic molecules that were held onto the silica. One of the samples, the coal tar pitch, was fractionated by solvent solubility, without contact with filtration media. The method led to somewhat less-sharp molecular-mass separations; however, trace-element analyses of these fractions gave much-improved mass balances. Structural data from this work and previous characterizations suggest that, within larger molecules, increasingly large polycyclic aromatic (PCA) ring systems are being held together by a variety of aliphatic and alicyclic bridging structures. In the absence of mineral matter or other solids, it is thought that the high trace-element concentrations represented organic associations with these complex molecules.