Structural uniformity of toluene-insolubles from heat-treated coal tar pitch as determined by solid state ¹³C NMR spectroscopy

A suite of coal tar pitches (CTPs) thermally treated to varying degrees and their toluene-insoluble (TI) fractions have been characterized by solid state C-13 nuclear magnetic resonance (NMR). The nonprotonated aromatic carbon concentrations estimated from dipolar dephasing experiments using cross polarization with a relatively long contact time (5 ms) and the quantitatively reliable single-pulse excitation (SPE) technique were in good agreement and these concentrations were used to deduce the average ring sizes. Complementary information has been obtained from X-ray diffraction and thermogravimetry analysis. An increase in the degree of condensation for the aromatic structure was evident for the whole pitches upon heat treatment, consistent with the increasing concentrations of TIs and published liquid chromatographic results on the toluene-soluble fractions. In contrast, the proportions of nonprotonated and bridgehead/internal aromatic carbons remained constant over the heat treatment range for the TIs, despite the increase in TI concentration of the CTPs with temperature. Thus, the growth in aromatic ring size appears to be limited by the heat treatment temperature of 380 degrees C used here. Interestingly, the TIs obtained from the parent tar are more condensed than those from the pitches mainly due to the higher concentration of quinoline-insolubles present.