Structural characteristics of coal tar-derived isotropic pitches and their soluble fractions

Three coal tar-derived isotropic pitches obtained by air blowing reaction and two parent pitches obtained by vacuum distillation were extracted with benzene and carbon disulfide, respectively. The hydrogen distribution of the soluble fractions was assessed by IR spectroscopy and hydrogen-1 NMR. A method developed by the authors to determine the aromaticity index by IR spectroscopy was more reliable than carbon-13 NMR method to differentiate the aromatic portion from the aliphatic one in the pitches. Average structural parameters of CS₂ solubles of the three air blown pitches indicated the substantial differences in molecular size and shape. The 002-band of XRD clearly showed the existence of stacking of aromatic planes in the three air blown pitches at room temperature. Analyses of IR and XRD profiles on the whole pitches suggested that three air blown pitches are different in aromaticity and crystallinity, although their softening points, H/C ratios, and solubilities are almost the same.