STRUCTURAL CHARACTERIZATION OF COAL-TAR AND PETROLEUM PITCHES

''Average structure'' data for coal-tar and petroleum (Ashland A-240) pitches are reported, based on NMR spectroscopy combined with elemental and molecular weight analyses. The average structure of both coal-tar and A-240 pitches indicates that they have similar sized aromatic units. The major difference is the much higher degree of aromatic substitution in petroleum pitch. The ''average'' petroleum pitch molecule contains two methyl and one ethyl group, with one in two molecules containing a ring-joining methylene group. However, the average coal-tar pitch molecule contains only one methyl and one ring-joining methylene group for every two molecules. The nitrogen content is higher and the sulfur lower in coal-tar pitch compared to petroleum pitch. Sequential solvent fractionation of a coal-tar pitch into six fractions was carried out. The THF soluble/toluene insoluble fraction (average MW = 835) contains more highly condensed aromatic ring systems and significantly different alkyl substituents than the lower molecular weight fractions. About 60% of the aliphatic carbon in this higher molecular weight fraction is in -CH2CH3 or >CHCH3 groups, while in the lower molecular weight fractions -CH2- and -CH3 groups predominate. Molecular weight and NMR data are also given on high-softening point, commercially available petroleum pitches which are produced from A-240 using a wiped film evaporator. The molecular weight distribution of the nitrogen-containing constituents of coal-tar pitch is discussed.