Structural Uniformity of Toluene-Insolubles from Heat-Treated Coal Tar Pitch As Determined by Solid State 13 C NMR Spectroscopy John M. Andre´sen, Carlos A. Luengo, † Sabino R. Moinelo, ‡ Roberto Garcia, ‡ and Colin E. Snape* Department of Pure & Applied Chemistry, University of Strathclyde, 295, Cathedral Street, Glasgow G1 1XL, Scotland, U.K. Received August 14, 1997. Revised Manuscript Received February 25, 1998 A suite of coal tar pitches (CTPs) thermally treated to varying degrees and their toluene- insoluble (TI) fractions have been characterized by solid state 13 C nuclear magnetic resonance (NMR). The nonprotonated aromatic carbon concentrations estimated from dipolar dephasing experimentsusing 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 °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. Introduction Thermal and other pretreatments ofcoal tar pitch (CTP) are being widely investigated as a means to (i) better meet the binder specifications for the production of carbon electrodes from petroleum coke in the alumi- num industry where the current trend is to use pitches with high softening points (ca. 130 °C) 1 and (ii) increase the extent of mesophaseformation on subsequent carbonization in relation to the production of high- performance carbons. 2-4 Raising the treatment tem- perature gives rise to increasing softening points with a concomitant reduction in solubility. 5,6 The extensive characterization studies on coal tar and pitch using a range of chromatographic and spectro- scopic techniques have been summarized in a number of review articles. 7-10 Although the increase in the degree of condensation of the structure for the toluene- soluble fractions of pitch has been well documented using high-pressure liquid chromatography (HPLC), 11 little work has been carried out on the toluene-in- solubles (TIs), 12 particularly in view of these being the major precursors of mesophase. 13,14 The bulk compositions of CTPs and their insoluble fractions can be assessed using carbon skeletal param- eters but, although solid state 13 C NMR has been used in a number of studies, 15-18 the quantitative reliability of the technique for pitches has not been demonstrated convincingly. As for coals 19 where the inherent prob- lems with the cross-polarization (CP) technique con- † Depto. Fı ´sica Aplicada, Universidade Estadual de Campinas, C.P. 6165, Campinas, SP, Brazil. ‡ Instituto Nacional del Carbo´n, CSIC, Apartado 73, 33080 Oviedo, Spain. (1) McCormack, R. L.; Jha, M. C. Energy Fuels 1994, 8, 388-394. (2) Marsh, H.; Menendez, R. Fuel Process. Technol. 1988, 20, 269- 296. (3) Weishauptova, Z.; Medek J.; Rada, M. Fuel 1994, 73, 177-182. (4) Matsumoto, M.; Higuchi, M.; Tomioka, T.; Sunago, H. Fuel 1994, 73, 237-242. (5) Ferna´ ndez, J. J.; Figueiras, A.; Granada, M.; Bermejo, J.; Mene´ndez, R. Carbon 1995, 33, 295-307. (6) Alvarez, R.; Dı ´ez, M. A.; Garcı ´a, R.; Gonza´ lez de Andre´s, A. I.; Snape, C. E.; Moinelo, S. R. Energy Fuels 1993, 7, 953-959. (7) Bartle, K. D. Rev. Pure Appl. Chem. 1972, 22, 79-113. (8) Bartle, K. D.; Collin, G.; Stadelhofer, J. W.; Zander, M. J. Chem. Technol. Biotechnol. 1979, 29, 531-551. (9) Zander, M. 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Energy Fuels 1993, 7, 639-644. 524 Energy & Fuels 1998, 12, 524-530 S0887-0624(97)00145-X CCC: $15.00© 1998 American Chemical Society Published on Web 04/11/1998