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Energy & Fuels 1997, 11, 724-729
Elucidation of Mechanism of Coal Liquefaction Using Tritium and 35S Tracer Methods Masazumi Godo, Atsushi Ishihara, and Toshiaki Kabe* Department of Applied Chemistry, Faculty of Technology, Tokyo University of Agriculture and Technology, Nakacho, Koganei, Tokyo 184, Japan Received September 10, 1996. Revised Manuscript Received January 23, 1997X
The reactivity of hydrogen in tetralin and in Taiheiyo coal in the presence of iron catalysts (pyrrhotite and iron pentacarbonyl (Fe(CO)5)) and sulfur were investigated using tritium and 35S tracer methods under liquefaction conditions. Coal liquefaction was performed for 30 min at an initial pressure of 5.9 MPa and a temperature of 400 °C in the presence of tetralin solvent and tritiated hydrogen, with and without a catalyst and 35S-labeled sulfur. The rate of coal conversion and the tritium distribution in coal were higher when Fe(CO)5 and sulfur were used than were obtained with conventional pyrrhotite. Further, the coal products obtained with the use of Fe(CO)5 and sulfur were lighter than those produced in the presence of added pyrrhotite. The results suggest that the catalyst derived from Fe(CO)5 and sulfur was successfully dispersed in the coal and directly acted on the coal to increase both the rate of coal conversion and the tritium transfer from the gas phase to the coal. The iron catalysts greatly promoted the hydrogen exchange between the gas phase and the tetralin solvent in the absence of coal. Part of the added sulfur participated in the sulfur exchange reaction with the pyrrhotite catalyst. The addition of sulfur increased the amount of hydrogen incorporated into the coal. This suggests that sulfur promoted the formation of tetralyl radicals during the hydrogen transfer from the solvent to the coal.
Introduction Iron-sulfur catalyst systems have been successfully employed for direct hydrogenation during coal liquefaction on a commercial scale.1 Even today, they are preferred because they are simple to use and because of economic reasons. Iron-sulfur catalyst systems have been widely investigated by several authors.2-11 Montano et al. suggested that the highest conversion of coal to liquid products was associated with a pyrrhotite which had the largest number of vacancies. Moreover, both H2S and pyrrhotite appeared to play a significant role in the coal liquefaction process.2,4 Recently, much attention has been focused on the advantages of dispersed catalysts in direct coal liquefaction.12-19 Unsupported dispersed catalysts can offer good contact beAbstract published in Advance ACS Abstracts, April 1, 1997. (1) Wu, W. R. K.; Storch, H. H. Hydrogenation of Coal and Tar. Bull.sU. S., Bureau Mines 1968, 633. (2) Montano, P. A.; Granoff, B. Fuel 1980, 59, 214. (3) Cypres, R.; Ghodsi, M.; Stocq, R. Fuel 1981, 60, 247. (4) Montano, P. A.; Bommannavar, A. S.; Shah, V. Fuel 1981, 60, 703. (5) Baldwin, R. M.; Vinciguerra, S. Fuel 1983, 62, 498. (6) Lambert, J. M., Jr. Fuel 1982, 61, 777. (7) Stenberg, V. I.; Ogawa, T.; Willson, W. G.; Miller, D. Fuel 1983, 62, 1487. (8) Mukherjee, D. K.; Mitra, J. R. Fuel 1984, 63, 722. (9) Trewhella, M. J. Fuel 1987, 66, 1315. (10) Kamiya, Y.; Nobukatsu, T.; Futamura, S. Fuel Process. Technol. 1988, 18, 1. (11) Yokoyama, S.; Yamamoto, M.; Yoshida, R.; Maekawa, Y.; Kotanigawa, T. Fuel 1991, 70, 163. (12) Weller, S. Energy Fuels 1994, 8, 415. (13) Derbyshire, F. CHEMTECH 1990, 20, 439. (14) Suzuki, T.; Yamada, O.; Fujita. K.; Takegami, Y.; Watanabe, Y. Chem. Lett. 1982, 1467. (15) Suzuki, T.; Yamada, O.; Takehashi, Y.; Watanabe, Y. Fuel Process. Technol. 1985, 10, 33. (16) Watanabe, Y.; Yamada, O.; Fujita, K.; Takegami, Y.; Suzuki, T. Fuel 1984, 63, 752. X
S0887-0624(96)00149-1 CCC: $14.00
tween the coal and the catalyst. There have been reports that the use of Fe(CO)5 in the presence of added sulfur increased the rate of coal conversion in solvents such as tetralin and 1-methylnaphthalene.14-17 Suzuki et al. suggested that the higher catalytic activity was due to the finely dispersed state of the pyrrhotite which was formed when Fe(CO)5 and molecular sulfur were added during the reaction.20 In most studies, it was recognized that the active form of iron-sulfur catalysts was pyrrhotite. An alternative interpretation was that the catalyst was actually H2S produced from the reduction of pyrite.6,8 It is important to elucidate the catalytic roles played by highly dispersed catalysts and sulfur which are added during coal liquefaction, although these are not well defined. On the other hand, since the reactions involved in coal liquefaction include hydrocracking and hydrogenation by molecular hydrogen and donor solvents, a number of attempts have been made to elucidate the mechanism of hydrogen transfer occurring during coal liquefaction in the presence of solvents.21-27 A useful method for clarifying the mechanism of hydrogen transfer in coal (17) Montano, P. A.; Stenberg, V. I.; Sweeny, P. J. Phys. Chem. 1986, 90, 156. (18) Hirschon, A. S.; Wilson, R, R. B. Coal Science II; American Chemical Society: Washington, DC, 1991; p 273. (19) Song, C.; Saini, A. K. Energy Fuels 1995, 9, 188. (20) Suzuki, T.; Yamada, H.; Sears, P. L.; Watanabe, Y. Energy Fuels 1989, 3, 707. (21) Billmers, R.; Griffith, L. L.; Stein, S. E. J. Phys. Chem. 1986, 90, 517. (22) Camaioni, D. M.; Autrey, S. T.; Franz, J. A. J. Phys. Chem. 1993, 97, 5791. (23) Autrey, S. T.; Alborn, E. A.; Franz, J. A.; Camaioni, D. M. Energy Fuels 1995, 9, 420. (24) McMillen, D. F.; Malhotra, R.; Chang, S. J.; Nigenda, S. E. In International Coference on Coal Science, Proceedings; Pergamon Press: Sydney, 1985; p 91.
© 1997 American Chemical Society
Mechanism of Coal Liquefaction
liquefaction is to utilize isotopes such as deuterium and tritium tracers.28-35 Recently, the authors have reported that tritium and carbon-14 tracer techniques were effective in quantitatively monitoring the hydrogen during coal liquefaction.36-43 These reports showed that the hydrogen mobility of coal and coal-related compounds could be quantitatively analyzed using the hydrogen exchange reactions occurring between coal, the gas phase, and the solvent, as well as by considering the hydrogen addition reactions. The current study of coal liquefaction using tritium attempted to clarify the reactivities of hydrogen in tetralin and in Taiheiyo coal in the presence of conventional and dispersed iron catalysts (pyrrhotite and Fe(CO)5) and in the presence of sulfur. Further, 35S was also used to determine the behavior of the added sulfur. Experimental Section Materials. Taiheiyo coal (C, 77.2%; H, 6.7%; N, 1.1%; S, 0.2%; O, 14.9daf%; ash, 16.1daf%) was used as a raw coal. Pyrrhotite was synthesized from Fe2O3 and sulfur under a hydrogen atmosphere. Reagents of commercially available materials were used without further purification. Tritiated molecular hydrogen was obtained by the electrolysis of tritiated water using an HG-225 hydrogen generator (Gaskuro Kogyo Inc.). Gaseous hydrogen was supplied by Tohei Chemical. Liquefaction Procedure. Coal liquefaction was performed in a 350 mL autoclave with a glass liner using coal (25 g), tetralin solvent (75 g), catalyst (0 or 5 g), and 35S-labeled sulfur (0, 1, or 2 g, about 1 000 000 dpm), which was pressurized with tritiated hydrogen (about 1 000 000 dpm) at an initial pressure of 5.9 MPa. The autoclave was heated at a rate of 10 C/min and maintained at 400 °C for 30 min. The products were separated as follows: naphtha (distillate under 200 °C), light oil (distillate between 204 and 350 °C), solvent refined coal (SRC ) tetrahydrofuran (THF) extract from the distillation residue over 350 °C), oil (hexane-soluble of SRC), asphaltenes (hexane-insoluble and benzene-soluble of SRC), preasphaltenes (benzene-insoluble and THF-soluble of SRC), and a residue (THF extraction residue). Gaseous products, naphtha, the solvent, and the light oil were analyzed by gas chromatography to obtain more precise separation data. H2S in the gas was absorbed by bubbling through a lead acetate solution; (25) McMillen, D. F.; Malhotra, R.; Tse, D. S. Energy Fuels 1991, 5, 179. (26) Malhotra, R.; McMillen, D. F. Energy Fuels 1993, 7, 227. (27) Murakata, T. M.; Saito, Y.; Yoshikawa, T.; Suzuki, T.; Sato, S. Fuel 1993, 34, 1436. (28) Fu, Y. C.; Blaustein, B. D. Chem. Ind. 1967, 1257. (29) Franz, J. A.; Camaioni, D. M. Fuel 1980, 59, 803. (30) Brower, K. P. J. Org. Chem. 1982, 47, 1889. (31) Schweighardt, F. K.; Bockrath, B. C.; Friedel, R. A.; Retkofsky, H. L. Anal. Chem. 1976, 48, 1254. (32) Cronauer, D. C.; Mcneil, R. I.; Young, D. C.; Ruberto, R. G. Fuel 1982, 61, 610. (33) Skowronski, R. P.; Ratto, J. J.; Goldberg, I. B.; Heredy, L. A. Fuel 1984, 63, 440. (34) King, H. H.; Stock, L. M. Fuel 1982, 61, 257. (35) Collin, P. J.; Wilson M. A. Fuel 1983, 62, 1243. (36) Kabe, T.; Nitoh, O.; Funatsu, E.; Yamamoto, K. Fuel Process. Technol. 1986, 14, 91. (37) Kabe, T.; Nitoh, O.; Marumoto, M.; Kawakami, A.; Yamamoto, Y. Fuel 1987, 66, 1321. (38) Kabe, T.; Nitoh, O.; Funatsu, E.; Yamamoto, K. Fuel 1987, 66, 1326. (39) Kabe, T.; Kimura, K.; Kameyama, H.; Ishihara, A.; Yamamoto, K. Energy Fuels 1990, 4, 201. (40) Kabe, T.; Horimatsu, T.; Ishihara, A.; Kameyama, H.; Yamamoto, K. Energy Fuels 1991, 5, 459. (41) Kabe, T.; Ishihara, A.; Daita, Y. Ind. Eng. Chem. Res. 1991, 30, 1755. (42) Ishihara, A.; Takaoka, H.; Nakajima, E.; Imai, Y.; Kabe, T. Energy Fuels 1993, 7, 362. (43) Ishihara, A.; Morita, S.; Kabe, T. Fuel 1995, 74, 63.
Energy & Fuels, Vol. 11, No. 3, 1997 725 the PbS produced was filtered, and the amount of H2S recovered was calculated from the recovered PbS. Measurement of Radioactivity and Calculation of the Hydrogen Exchange Ratio. The radioactivities of the products were determined by liquid scintillation counting.44-46 Colorless or lightly colored products were directly dissolved into a scintillator, while the colored liquid, solid, and gaseous products were oxidized to H2O and SOX in order to avoid color quenching. The radioactivity of tritiated hydrogen sulfide ([3H]H2S) was determined by measuring the radioactivity of the acetic acid solution which was formed by the previously mentioned reaction of H2S with lead acetate solution. Each sample was dissolved into a scintillator reagent (14 mL) (Monophase S for water and acetic acid solution, Instaflour for an organic sample; Packard Japan), and the radioactivity of the resulting solution was measured with a liquid scintillation counter (Aloka LSC-1050). The amount of hydrogen exchanged and the amount of hydrogen transferred between the gas phase and the solvent/coal were calculated from the distribution of tritium and changes in the composition of the products. The amount of hydrogen exchanged between the gas phase and the solvent in the absence of coal was calculated on the basis of eq 1. Hex(GTS) is the amount of hydrogen exchanged between the gas phase and the solvent. In these calculations, it was assumed that this hydrogen exchange reached equilibrium when the reaction reached completion. When hydrogen in the gas phase was added to the solvent, the amount involved was subtracted from Hex(GTS). In the presence of coal, the amounts of hydrogen transferred from the gas phase to coal and the solvent were defined on the basis of eqs 2 and 3 and estimated from the amounts of tritium transferred from the gas phase to the coal and the solvent. Since the tritium transfer proceeds by both hydrogen addition and exchange, the amounts of hydrogen transferred are given by the sum of the amounts of hydrogen added and exchanged. The isotope effect was regarded as small and was thus ignored in these calculations because most of the reactions were performed at a comparatively high temperature. Recently, it was reported elsewhere that the isotope effect was negligible under the conditions of coal liquefaction.47
Hex(GTS) ) Hgas(Rsolvent/Rgas) - Hadt
(1)
Htrf(GfC) ) Hint(Rcoal/Rint)
(2)
Htrf(GfS) ) Hint(Rsolvent/Rint)
(3)
where Hex(GTS) is the amount of hydrogen exchanged between the gas phase and the solvent, Htrf(GfC) is the amount of hydrogen transferred from the gas phase to the coal, Htrf(GfS) is the amount of hydrogen transferred from the gas phase to the solvent, Hadt is the amount of hydrogen added from the gas phase to the solvent after the reaction, Hgas is the amount of hydrogen in the gas phase after the reaction, Hint is the initial amount of hydrogen in the gas phase, Rsolvent is the radioactivity in the solvent after the reaction, Rgas is the radioactivity in the gas phase after the reaction, Rcoal is the radioactivity in the coal after the reaction, and Rint is the initial radioactivity in the gas phase (total radioactivity).
Results and Discussion Reaction of Tetralin with Tritiated Hydrogen in the Absence of Coal. Tetralin is one of the most (44) Kobayashi, Y.; Maudsley, D. V. Biological Applications of Liquid Scintillation Counting; Academic Press: New York, 1974. (45) Horocks, D. L. Application of Liquid Scintillation Counting; Academic Press: New York, 1974. (46) Crook, M., Johnson P., Eds. Liquid Scintillation Counting; Heyden and Son: London, 1977; Vol. 4. (47) Kamo, T.; Steer, J. G.; Muehlenbachs, K. Int. Conf. Coal Sci., Proc. 1993, 415.
726 Energy & Fuels, Vol. 11, No. 3, 1997
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Figure 1. Products in the exchange reaction between tetralin and tritiated hydrogen.
Figure 2. Tritium distributions in the exchange reaction between tetralin and tritiated hydrogen. Table 1. Hydrogen Transfer between Gas Phase and Solvent run 1 2 3 4 5 6
catalyst
amt of sulfur (g) 1
pyrrhotite pyrrhotite Fe(CO)5 Fe(CO)5
1 1 2
amt of H exchanged (g)
amt of H2S generated (g)
0.013 0.156 2.032 2.428 0.275 0.410
0.000 0.800 0.121 1.011 0.012 1.041
simple and convenient model compounds because it contains both an aromatic ring and a naphthene ring and because it can serve as a hydrogen donor solvent. Before investigating the complex reaction with coal, the reaction of tetralin with tritiated hydrogen was performed in the absence of coal. Figures 1 and 2, respectively, show the products and tritium distributions in the exchange reaction between tetralin and tritiated hydrogen. Table 1 shows the amount of hydrogen exchanged between the gas phase and the solvent. Although naphthalene (NP), 1-methylindan (MI), and n-butylbenzene (BB) were produced in the absence of a catalyst and sulfur (run 1), the yield of each product was very low, with the virtual absence of hydrogen exchange. When sulfur was added (run 2), the yield of NP increased significantly (from 0.7 to 3.0%), with most of the added sulfur converted into hydrogen sulfide (Table 1). If all the added sulfur reacted with tetralin to produce hydrogen sulfide, it would correspond to the production of 2.7% of the NP. These results indicate that sulfur promotes the dehydrogenation of tetralin to produce naphthalene. Further, the yield of MI also increased from 0.3 to 0.6% and the yield of BB
from 0.1 to 0.3%; the tritium distribution in the solvent increased from 0.6 to 7.7%. When pyrrhotite was added (run 3), the yields of NP, MI, and BB were respectively 2.0, 0.7, and 0.4%, and the tritium distribution in the solvent was 59%. These values therefore increased significantly, but the amount of H2S generated was very small. When pyrrhotite and sulfur were added simultaneously (run 4), the tritium distribution in the solvent amounted to 64%. This was very close to the equilibrium value of 82% which was calculated on the assumption that the hydrogen atoms were completely and randomly dispersed between the gas phase and the solvent. It was considered that the independent effects of the sulfur and pyrrhotite would considerably increase the amount of hydrogen exchange. When Fe(CO)5 and sulfur were used (runs 5 and 6), the yields of MI and BB were not as high as occurred with pyrrhotite and/or sulfur. The tritium distributions in the solvent in runs 5 and 6 increased to 16% and 27%, respectively, significantly greater than in the absence of catalysts (runs 1 and 2). However, these values were smaller than those obtained with pyrrhotite. In the case of Fe(CO)5, part of the added sulfur produced iron sulfide, with the remainder being converted into H2S (Table 1). After the reaction in the presence of pyrrhotite and Fe(CO)5, iron sulfide was recovered from the inside of the autoclave. Figure 3a,b shows scanning electron micrographs (SEMs) of the iron sulfide which was recovered after the reaction in the presence of pyrrhotite (run 4) and Fe(CO)5 (run 6). In comparison with the pyrrhotite reaction, the particle size of the iron sulfide recovered from the reaction with Fe(CO)5 and
Mechanism of Coal Liquefaction
Energy & Fuels, Vol. 11, No. 3, 1997 727
gaseous hydrogen, tetralin may collide with not only itself but also tritiated hydrogen to give a tetralyl radical (eq 4). The hydrogen exchange between tetralyl radicals and tritiated hydrogen or tritiated hydrogen sulfide can be assumed to proceed via eq 5 depending on the concentration of tetralyl radicals, which also controls the formation of MI in eq 6. The products were MI by isomerization, BB by hydrocracking, and the main product NP by dehydrogenation. Decalin by disproportionation was not formed. This is consistent with a previously reported result.48,49 Hooper et al. reported that BB was formed by the thermal dissociation of tetralin as shown in eq 7. Concerning the hydrogen
Figure 3. Scanning electron micrographs of recovered iron sulfide: (a) run 4 (in the presence of pyrrhotite and sulfur); (b) run 6 (in the presence of Fe(CO)5 and sulfur).
sulfur was larger and more uniform. Further, many large fragments had plain surfaces. It was assumed that these fragments had been deposited on the inside wall of the autoclave. Although Fe(CO)5 is one of the most highly dispersed iron catalysts used in direct coal liquefaction, the particle size of the iron sulfide recovered in the absence of coal was about 1.2 µm, which was larger than the particles of pyrrhotite (about 0.6 µm). It seems that in comparison with pyrrhotite an increase in particle size results in a decrease in the amount of hydrogen exchanged. Reaction Mechanism of Tetralin with Tritiated Hydrogen. In previous reports, the authors assumed that the formation of products and the hydrogen transfer from the tetralin, proceeded via a tetralyl radical, which acted as an intermediate in the conversion and the hydrogen exchange of tetralin.39,41,43 When radicals generated in coal react with tetralin in the system, a tetralyl radical may be formed easily. However, if coal is not included in the system, a tetralyl radical is difficult to generate. In the system of tetralin and
exchange reaction of tetralin, studies using deuterium have been reported extensively. Skowronski et al. reported that, in hydrogen exchange between tetralind12 and hydrogen in coal at 400 °C, 1 h in a shaken a autoclave system, protium was incorporated into HR (66%), Hβ (23%), and Haromatic (11%) positions in tetralin and that HR absorption of the recovered naphthalene in 1H nuclear magnetic resonance (NMR) was approximately seven times as intense as Hβ absorption.33 Collin and Wilson showed from their intensive nucleus enhancement by polarization transfer (INEPT) and gated spin echo (GASPE) NMR study that, in the reaction of tetralin, consists of molecules that were nondeuterated at HR and/or Hβ positions. In their NMR measurement, the intensity of the signal at the HR position was larger than that at the Hβ position.35 These reports represent that the 1-tetralyl radical appears to be a more important intermediate than the 2-tetralyl radical in the conversion and the hydrogen exchange of tetralin, although it has been proposed that 2-tetralyl radical may be an intermediate in the formation of MI.50 Therefore, the formation of the 1-tetralyl radical in this system may be the rate-determining step for both the conversion of tetralin and the hydrogen exchange. The authors previously reported that the rate of conversion of tetralin in the presence of H2S was nearly equal to that in the absence of H2S.51 In contrast to the result with H2S, it was considered that sulfur promoted the formation of tetralyl radicals. The process of radical formation may be represented by eqs 8-11, which is consistent with the fact that sulfur promotes naphthalene formation. Because sulfur promotes the formation of tetralyl radicals, the concentration of tetralyl radicals would increase. As a result, the formation of products (48) Hooper, R. J.; Battaerd, H. A.; Evans, D. G. Fuel 1979, 58, 132. (49) Jan J. V.; Antonius P. G. K.; Herman, B. Fuel 1984, 63, 334. (50) Franz, J. A.; Camaioni, D. M. J. Org. Chem. 1980, 45, 5247. (51) Godo, M.; Saito, M.; Sashara, J.; Ishihara, A.; Kabe, T. Energy Fuels, submitted for publication.
728 Energy & Fuels, Vol. 11, No. 3, 1997
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Figure 4. Products from coal in Taiheiyo coal liquefaction using gaseous tritium.
Figure 5. Tritium distributions in Taiheiyo coal liquefaction using gaseous tritium.
was promoted by sulfur. When pyrrhotite was added, the yields of products increased and the tritium distribution in the solvent increased significantly. Autrey et al. have suggested that FeS catalysts are reduced by donor solvent.52 It was thought that pyrrhotite promoted not only the formation of the tetralyl radicals in eq 12 but also the dissociation of the tritiated hydrogen molecules in eq 13.
Effects of Sulfur and Catalysts on the Liquefaction of Taiheiyo Coal. The liquefaction of Taiheiyo coal with tritiated gaseous hydrogen was performed in the presence of catalysts and sulfur. Figure 4 shows the distribution of the products from coal. Without the catalyst and sulfur (run 7), the product yield () 100 residue) was 74% and the SRC was 71%. In the (52) Autrey, T.; Linehan, J. C.; Camaioni, D. M.; Kaune, L. E.; Watrb, H. M.; Franz, J. A. Catal. Today 1996, 31, 105.
presence of sulfur (run 8), although there was an increase in the residue and light fractions (such as light oil), the product yields decreased. This suggests that sulfur simultaneously promoted both the thermal decomposition and polycondensation of coal. In the presence of pyrrhotite or Fe(CO)5, the product yields in liquefaction increased to more than 80%; the use of sulfur also increased the product yields. When Fe(CO)5 and 2 g of sulfur were added (run 12), the product yields reached 89%, which was higher than those obtained with pyrrhotite. In the presence of Fe(CO)5 and sulfur, the yield of preasphaltene decreased and the yields of oil and asphaltene increased in comparison with the use of pyrrhotite. This observed that Fe(CO)5 and sulfur could be used to obtain products with a lighter composition. The result indicated that the liquefaction activity of Fe(CO)5 was higher than that of pyrrhotite. Figure 5 shows the tritium distributions in Taiheiyo coal liquefaction. Compared with the absence of sulfur (run 7), the addition of sulfur (run 8) increased the tritium distribution in the solvent increased and decreased it in coal. The use of pyrrhotite only (run 9) increased the distributions in both coal and the solvent. When pyrrhotite and sulfur were both added (run 10), the tritium distribution in the solvent increased and that in coal decreased. It was believed that the sulfur promoted the hydrogen transfer from the gas phase to the solvent and that the reaction mechanism was the same as that occurring in the absence of coal (as described above). In contrast, when Fe(CO)5 and sulfur were used, the tritium distribution in the coal increased in spite of the presence of sulfur, which suggested that the catalyst derived from Fe(CO)5 and sulfur acted directly on the coal and increased both the rate of coal conversion and the tritium transfer to coal. The amounts of hydrogen transferred from the gas
Mechanism of Coal Liquefaction
Figure 6.
35
Energy & Fuels, Vol. 11, No. 3, 1997 729
S distribution in Taiheiyo coal liquefaction.
Table 2. Hydrogen Transfer between Gas Phase, Solvent, and Coal amt of H transferred (g) run 7 8 9 10 11 12
catalyst
amt of sulfur (g) 1
pyrrhotite pyrrhotite Fe(CO)5 Fe(CO)5
1 1 2
from gas to solvent
from gas to coal
amt of H added to coal (g)
0.229 0.301 0.320 0.510 0.187 0.213
0.129 0.116 0.328 0.207 0.335 0.397
0.264 0.335 0.330 0.348 0.354 0.415
to the solvent and from the gas to the coal, and the amount of hydrogen incorporated into the coal, are listed in Table 2. The amount of hydrogen incorporated into coal increased with the addition of catalysts and sulfur. In the absence of a catalyst (runs 7 and 8) and in the presence of pyrrhotite (runs 9 and 10), the amounts of hydrogen transferred from the gas to the coal in the presence of sulfur were less than those in the absence of sulfur. However, the amount of hydrogen incorporated into the coal in the presence of sulfur was greater than when sulfur was absent. It is therefore likely that sulfur promotes the hydrogen addition from the solvent to the coal. On the other hand, in the case of Fe(CO)5, the amounts of hydrogen transferred from the gas to the solvent were less than those when pyrrhotite was used, whereas the amounts of transferred from the gas to the coal were greater; in addition, the amount of hydrogen incorporated into the coal increased significantly. This showed that Fe(CO)5 was effective in transferring the hydrogen from the gas to the coal and suggests that the iron sulfide generated from Fe(CO)5 was dispersed successfully on the coal particles and was more effective in transferring the hydrogen from the gas to the coal than in transferring it from the gas to the solvent. Behavior of Added Sulfur. To trace the behavior of added sulfur, the reactions were conducted using 35Slabeled sulfur. Figure 6 shows the 35S distributions in the Taiheiyo coal liquefaction. In run 4, using pyrrhotite and sulfur but no coal, 9% of the added sulfur was transferred into the catalyst after the reaction, corresponding to 5% of the sulfur atoms in the pyrrhotite. This shows that a sulfur exchange reaction occurred between the added sulfur and pyrrhotite. In the presence of coal (run 10), it is likely that a similar sulfur exchange would occur. In the presence of Fe(CO)5 and 1 g of sulfur (runs 5 and 11), most of the 35S was distributed into the THFI fraction. The amount of sulfur used (1 g) was not enough to produce H2S. In the presence of Fe(CO)5 and 2 g of sulfur (runs 6 and 12), one-half of the 35S was distributed into the THFI
Figure 7. Possible mechanism of sulfur exchange.
fraction and almost all the remaining 35S was distributed into H2S; the distribution into the solvent and the coal was negligible. The pyrrhotite catalyst is a nonstoichiometric iron sulfide which has a number of vacancies or defects on the catalyst surface. A possible mechanism for the sulfur exchange reaction is shown in Figure 7. The added sulfur produced [35S]H2S (H2S labeled by 35S), which was then assumed to dissociate into H and [35S]SH groups (SH group labeled by 35S) on the surface of the pyrrhotite catalyst. [32S]Sulfur in pyrrhotite would generate [32S]SH, which would be in a state of equilibrium with [32S]H2S. The H and SH groups are intermediates in the sulfur exchange reaction; this hydrogen atom may contribute to the promotion of the hydrogen transfer into tetralyl radicals and coal radicals. This would increase the amount of hydrogen exchange and addition via the radical reaction mechanism. Conclusions The investigation of the effects of sulfur and catalysts (pyrrhotite and Fe(CO)5) on Taiheiyo coal liquefaction using tritium and 35S tracer methods led to the following conclusions: (1) The hydrogen exchange reaction between gaseous hydrogen and tetralin was promoted by the addition of sulfur. (2) In the absence of coal, pyrrhotite had a greater catalytic effect on hydrogen exchange between the gas phase and tetralin than did Fe(CO)5. This was believed to be due to the small particle size of the recovered iron sulfide. (3) In the presence of coal, the catalyst derived from Fe(CO)5 and sulfur appeared to be highly dispersed in coal and to act directly on the coal. The highly dispersed catalyst therefore greatly promoted both the coal conversion and the tritium transfer to coal, and the resulting product were lighter. (4) Part of the added sulfur participated in a sulfur exchange reaction with the pyrrhotite catalyst. EF960149P
