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Energy & Fuels 1996, 10, 726-732
Carbonization Behavior of Hydrotreated Coal Tar Pitch Containing Fine Molybdenum and Ruthenium Particles Atsushi Ishihara,* Kunio Kawashima, Xiangsheng Wang, Hiroaki Shono,† and Toshiaki Kabe Department of Applied Chemistry, Faculty of Technology, Tokyo University of Agriculture & Technology, Nakacho, Koganei, Tokyo 184, Japan Received October 2, 1995X
In the carbonization of hydrotreated coal tar pitch containing fine molybdenum and ruthenium particles, a tritium tracer method revealed that prior addition of fine molybdenum and ruthenium particles into the pitch can selectively catalyze the condensation of low molecular weight hydrocarbons at temperatures below 600 °C. This is because they prevent the release of those molecules by vaporization and result in an increase in the carbonization yield of coal tar pitch and the hydrogen content of resultant cokes. The carbonization yield with ruthenium particles was higher than that with molybdenum particles. Observations by a transmission electron micrograph showed that the ruthenium particles (10 nm even at 1000 °C) were smaller than molybdenum particles and were dispersed uniformly throughout the hydrogenated pitch.
Introduction The carbonization process is one of the most important methods for producing high-performance carbon fibers (HPCF) from coal tar pitch because the mesophase transformation occurring in the early stage of the pitch carbonization is a key step in determining the physical and chemical properties of the resulting coke and carbon products.1 Hydrotreatment is a particularly effective technique for preparing a mesophase pitch from a coal tar pitch,2 since this process can modify the fluidity and melting point of the pitch in such a manner to facilitate the manufacture of the mesophase in the subsequent carbonization process and to improve the optical texture of the resulting carbon products. The role played by the hydrogen contained in the pitch in these contrasting processes of hydrotreatment and carbonization (a dehydrogenation process) has not yet been clarified. On the other hand, it has been reported that the carbonization behavior of various types of pitch is affected by the presence of particulate matters in the pitch, such as carbon blacks, natural graphite and mica, carbon felt, and silica.3 For instance, Obara et al. investigated the carbonization of silica-containing pitch by means of in situ electron spin resonance spectroscopy and found that the spin concentration of pitch increases with an increase in the silica content.4 Kuo et al. reported that particulate matter (diameter < 1 µm) in pitch significantly affected the carbonization behavior † Mineral Fiber Research Laboratory, Nitto Boseki Co. Ltd, Higashi Gonome, Fukushima, Fukushima 960, Japan. X Abstract published in Advance ACS Abstracts, April 1, 1996. (1) Yamada, Y.; Shiraishi, M.; Furuta, T.; Yamakawa, T.; Sanada, Y. Bull. Chem. Soc. Jpn. 1984, 57, 3027. (2) Mochida, I.; Korai, Y.; Fujitsu, H.; Takeshita, K.; Komatsubara, Y.; Koba, K. Fuel 1981, 60, 1083. (3) (a) Bradford, D.; Greenhaugh, E.; Kingshott, R.; Senior, A.; Bailey, P. A. Proceedings of the 3rd International Conference on Carbon and Graphite; SCI: London, 1970; p 520. (b) Forrest, M.; Marsh, H. Fuel 1983, 62, 612. (c) Tanaka, H.; Maruyama, K.; Yasuda, E.; Kimura, S. Tanso 1986, 86, 107. (4) Obara, T.; Yokono, T.; Sanada, Y.; Marsh, H. Fuel 1985, 64, 995.
0887-0624/96/2510-0726$12.00/0
and decreased the optical texture in the resultant coke.5 It has been reported that most elements act as catalysts in the graphitization of carbon.6,7 However, there have been few studies on the effect of such metallic elements or compounds on dehydrogenation and polycondensation during the low-temperature carbonization of heavy hydrocarbons. Recently, the authors have reported the hydrotreatment8 and the carbonization9 of coal tar pitches for highperformance carbon fibers in which tritium tracer methods were used in order to elucidate the behavior of hydrogen in coal tar pitch. In these studies, it was possible to quantitatively estimate the mobility of hydrogen in pitch during hydrotreatment and carbonization. The authors also investigated the effect of fine transition metallic particles on the hydrotreatment10 of pitches using tritium tracer methods. It was found that fine metal particles derived from metal carbonyls are more active than conventional supported catalysts in promoting the transfer of hydrogen from the gas phase to pitch molecules and, in addition, can inhibit the formation of light fractions by hydrocracking. Furthermore, the carbonization11 of pitches containing fine molybdenum particles was also performed. Although the addition of molybdenum selectively catalyzed the dehydrogenation process and led to an increase in the (5) Kuo, K.; Marsh, H.; Broughton, D., Fuel, 1987, 66, 1544. (6) Oya, A.,Tanso, 1980, 102, 118. (7) Ishikawa, T.; Magari, S.; Mizutani, S. Tanso 1965, 42, 7. (8) (a) Okuyama, S.; Shono, H.; Ishihara, A.; Kabe T. J. Jpn. Pet. Inst. 1990, 33 (3), 181. (b) Shono, H.; Marumoto, M.; Ishihara, A.; Kabe T. J. Jpn. Pet. Inst. 1990, 33 (5), 299. (c) Wang, X.; Ishihara, A.; Shono, H.; Matsumoto, M.; Kabe, T. Fuel Process. Technol. 1994, 38, 69. (d) Takeuchi, M.; Shono, H.; Wang, X.; Ishihara, A.; Kabe T. J. Jpn. Pet. Inst. 1994, 37 (2), 136. (9) (a) Kabe, T.; Wang, X.; Ishihara, A.; Shono, H. Chem. Lett. 1990, 1235. (b) Wang, X.; Matsumoto, M.; Shono, H.; Ishihara, A.; Kabe, T. J. Jpn. Pet. Inst. 1991, 34, 314. (c) Wang, X.; Ishihara, A.; Shono, H.; Kabe, T. Fuel Process. Technol. 1994, 38, 45. (10) (a) Ishihara, A.; Wang, X.; Shono, H.; Kabe T. Energy Fuels 1993, 7, 334. (b) Wang, X.; Ishihara, A.; Satou, T.; Shono, H.; Kabe, T. J. Jpn. Pet. Inst. 1992, 35, 451. (11) Ishihara, A.; Wang, X.; Shono, H.; Kabe T. Ind. Eng. Chem. Res. 1993, 32, 1723.
© 1996 American Chemical Society
Hydrotreated Coal Tar Pitch
Energy & Fuels, Vol. 10, No. 3, 1996 727
Table 1. Composition and Properties of Coal Tar Pitcha elemental analysis
properties
C H N O (diff) FC SP QI TI ash (wt %) (wt %) (wt %) (wt %) C/H (wt %) (°C) (wt %) (wt %) (wt %) 93.6
4.5
0.9
1.0
1.73 56.4 88.8
9.2
30.7
0.07
a
FC, fixed carbon; SP, softening point; QI, quinoline-insoluble fraction; TI, toluene-insoluble fraction.
carbonization yield, the effect of fine metal particles on the carbonization of a hydrotreated pitch was not investigated. Hydrotreatment has a considerable influence on the carbonization of pitches. To determine the behavior of hydrogen incorporated into pitch, the hydrogen needs to be labeled by tritium. In the present study, the hydrotreated coal tar pitch was prepared by reacting the coal tar pitch with tritiated tetralin, which led to tritium being incorporated into the pitch by hydrogen addition. The carbonization of the hydrotreated pitches containing fine molybdenum and ruthenium particles was investigated in order to estimate the effect of fine metal particles on carbonization and to clarify both the mechanism underlying the process and the role of hydrotreatment. Figure 1. Flow diagram of fractionation.
Experimental Section Materials. Table 1 lists the characteristics of the coal tar pitch employed in this study (Nitto Boseki Co. Ltd.) (this is the starting material in the production of HPCF). Commercial Mo(CO)6 and Ru3(CO)12 were used as precursors for the preparation of pitches containing fine molybdenum and ruthenium particles. Nitrogen (99.99%) was supplied by Tohei Chemicals Co. Ltd. Tritiated tetralin was prepared by treating tetralin with tritiated water in the presence of Pt/Al2O3. The initial radioactivity of tritiated tetralin was adjusted to 2.2 × 105 dpm/g. Hydrotreatment of Coal Tar Pitch by Tritiated Tetralin. Hydrotreatment of coal tar pitch with tritiated tetralin was performed in a 350 mL autoclave with an inner glass tube equipped with a mechanical stirrer assembly. Pitch (30 g) and tritiated tetralin (60 g) were charged into the autoclave, and this was then heated to 400 °C under a nitrogen atmosphere at a rate of 10 °C/min and maintained at this temperature for 2 h. After the reaction was complete, the mixture was separated in the manner indicated in Figure 1. Gas was collected in a vessel and was analyzed by gas chromatography with a thermal conductivity detector. The reaction mixture was distilled initially under atmospheric pressure into naphtha (
