STRUCTURAL UNIFORMITY OF TOLUENE-INSOLUBLES FROM HEAT-TREATED COAL-TAR PITCH AS DETERMINED BY SOLID-STATE C-13 NMR-SPECTROSCOPY

A suite of coal tar pitches (CTPs) thermally treated to varying degree s and their toluene-insoluble (TI) fractions have been characterized b y solid state C-13 nuclear magnetic resonance (NMR). The nonprotonated aromatic carbon concentrations estimated from dipolar dephasing exper iments using cross polarization with a relatively long contact time (5 ms) and the quantitatively reliable single-pulse excitation (SPE) tec hnique were in good agreement and these concentrations were used to de duce the average ring sizes. Complementary information has been obtain ed from X-ray diffraction and thermogravimetry analysis. An increase i n the degree of condensation for the aromatic structure was evident fo r the whole pitches upon heat treatment, consistent with the increasin g concentrations of TIs and published liquid chromatographic results o n the toluene-soluble fractions. In contrast, the proportions of nonpr otonated and bridgehead/internal aromatic carbons remained constant ov er the heat treatment range for the TIs, despite the increase in TI co ncentration 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 pare nt tar are more condensed than those from the pitches mainly due to th e higher concentration of quinoline-insolubles present.