1490
Energy & Fuels 2002, 16, 1490-1498
Elucidation of Hydrogen Transfer between Coal and Tritiated Organic Solvent Atsushi Ishihara,* I. Putu Sutrisna, Toru Miura, Masaru Saito, Eika Weihua Qian, and Toshiaki Kabe Department of Chemical Engineering, Tokyo University of Agriculture and Technology, 2-24-16, Nakacho, Koganei, Tokyo 184-8588, Japan Received March 28, 2002
The hydrogen transfer behavior of five different ranks of coals on their reaction with the tritiumlabeled toluene or tetralin in the temperature range from 200 to 400 °C for 5-360 min has been investigated. The behavior of hydrogen in functional groups was also traced. The extensive hydrogen transfer to coal occurred with tetralin even at 300 °C, while only a small amount of hydrogen was transferred from toluene. The hydrogen exchange of functional groups occurred with tetralin, although it was difficult for hydrogen exchange to take place with toluene. The transfer of hydrogen/tritium from the tritiated tetralin to the coals occurred not only by the conversion of tetralin into naphthalene but also by hydrogen exchange. This showed that although tetralin can readily be dehydrogenated to the aromatic, the tetralyl radicals formed by the reaction with coal radicals are also reactive as hydrogen acceptors. On the other hand, some structural elements existing in the coals should be good hydrogen donors, which transfer hydrogen to the tetralyl radicals. The enhancement of hydrogen transfer reactions occurred with the lower rank Illinois No. 6, Wandoan, and Beulah-Zap coals, compared to the higher rank Upper Freeport and Pocahontas No. 3 coals because of the existence of a relatively large amount of thermally labile or sensitive oxygen functional groups in these coals.
Introduction It is well accepted that hydrogen transfer reactions are a necessary step for the conversion of intractable coal molecules into useful liquid and soluble products. The hydrogen transfer reaction can occur between coal molecules by redistribution of the hydrogen atoms available in the coal molecules, as can be found in coal pyrolysis. On the other hand, addition of hydrogen atoms from the hydrogen donor solvents or/and gasphase H2 are required in most processes for direct coal liquefaction processes. In general, hydrogen donor solvents, such as tetralin and 9,10-dihydrophenanthrene, can readily be dehydrogenated to the aromatic compounds and hence give hydrogen atoms to the reaction system. This behavior is used as a measurement of the degree of hydrogen atoms transferred from the solvents in many works of the coal liquefaction reaction. However, it also can be recognized that radicals derived from the solvents by reaction with coal radicals may be reactive as hydrogen abstractors. Thus, the hydrogen exchange reactions between solvent and transferable hydrogen atoms available in coal would proceed. This feature has been revealed by hydrogen-deuterium exchange, as reported by King and Stock.1 They have shown in the study of hydrogen exchange reaction between IL coal and tetralin-d12, that the exchange reaction with the coal occurs rapidly in the 1-position * Corresponding author. Tel. +81-42-388-7228. Fax: +81-42-3887228. E-mail:
[email protected]. (1) King, H.-H.; Stock, L. M. Fuel 1982, 61, 257.
and at a slower rate in the 2-position and aromatic of tetralin-d12. The deuterium-tracer techniques have been exploited to provide a detailed understanding of mechanistic pathways of hydrogen transfer reactions operating at the molecular level during coal liquefaction in donor media.1-7 On the other hand, we have reported that tritium tracer techniques were effective in tracing quantitatively the hydrogen in coal liquefaction.8-10 In these works, it was shown that the quantitative analysis of hydrogen transfer in coal could be achieved through the hydrogen exchange reactions among coal, gas phase, and solvent as well as hydrogen addition. Recently, we have also reported that the tritium tracer methods are effective in determining the amount of hydrogen in the functional groups of coal, and we examine the reactivity of hydrogen in coal through the reaction of coal with (2) Stock, L. M. In Chemistry of Coal Conversion; Schlorsberg, R. H., Ed.; Plenum Press: New York, 1985; Chapter 6. (3) Franz, J. A.; Camaioni, D. M. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1981, 26, 106. (4) Cronauer, D. C.; Mcneil, R. I.; Young, D. C.; Ruberto, R. G. Fuel 1982, 61, 610. (5) Wilson, M. A.; Vassalo, A. M.; Collin, P. J. Fuel Process. Technol. 1984, 8, 213. (6) Collin, P. J.; Wilson, M. A. Fuel 1983, 62, 1243. (7) Skowronski, R. P.; Ratto, J. J.; Goldberg, I. B.; Heredy, L. A. Fuel 1984, 63, 440. (8) Kabe, T.; Kimura, K.; Kameyama, H.; Ishihara, A.; Yamamoto, K. Energy Fuels 1990, 4, 201. (9) Kabe, T.; Horimatsu, T.; Ishihara, A.; Kameyama, H.; Yamamoto, K. Energy Fuels 1991, 5, 459. (10) Kabe, T.; Ishihara, A.; Daita, Y. Ind. Eng. Chem. Res. 1991, 30, 1755.
10.1021/ef020074r CCC: $22.00 © 2002 American Chemical Society Published on Web 10/10/2002
H Transfer between Coal and Tritiated Organic Solvent Table 1. Analyses of Coals Useda ultimate (%, daf) ND WN IL UF POC
% dry
C
H
N
S
O
total mineral
pyrite
72.9 76.9 77.7 85.5 91.1
4.8 6.7 5.0 4.7 4.4
1.2 1.1 1.4 1.6 1.3
0.7 0.3 2.4 0.7 0.5
20.4 15.0 13.6 7.5 2.7
8.7
0.3
18.1 15.3 5.5
5.5 3.4 0.1
a Abbreviations: ND: North Dakota Beulah-Zap, WN: Wandoan, IL: Illinois No. 6, UF: Upper Freeport, POC: Pocahontas No. 3.
tritiated water and gaseous hydrogen in a pulse flow reactor or in a batch reactor.11-15 Many interesting works have been reported in the field of hydrogen transfer reactions that occur in coal liquefaction,2 but very little attention has been given to the contribution of hydrogen transfer from the solvent to coal as a result of the hydrogen exchange reaction, since it should be difficult to estimate this contribution when the reaction is carried out with 1H-donor solvent. In the present study, we investigate the hydrogen transfer behavior of five different ranks of coals on their reactions with tritium-labeled toluene or tetralin. The hydrogen transfer of coals with tritiated tetralin under undestructive conditions at lower temperature was briefly reported.16 In this work, the extents of hydrogen transfer from tritiated tetralin to the coals under the wider range of conditions including destructive conditions at higher temperature are presented. The degree of hydrogen transfer consisting of both hydrogen addition by the conversion of tetralin into naphthalene and the exchange reaction was estimated by tritium transfer. The behavior of functional groups in coals was traced in detail. The reaction of coal with tritiated toluene was also discussed. Experimental Section Materials. Four kinds of Argonne Premium Coal Samples (Beulah-Zap (ND), Illinois No. 6 (IL), Upper Freeport (UF), and Pocahontas No. 3 (POC)) were obtained in 5 g ampules (
