470
Energy & Fuels 1997, 11, 470-476
Elucidation of Coal Liquefaction Mechanism Using a Tritium Tracer Method. Effect of H2S and H2O on Hydrogen Exchange Reaction of Tetralin with Tritiated Molecular Hydrogen Masazumi Godo, Masaru Saito, Jyunichi Sasahara, Atsushi Ishihara, and Toshiaki Kabe* Department of Applied Chemistry, Faculty of Technology, Tokyo University of Agriculture and Technology, Nakacho, Koganei, Tokyo 184, Japan Received July 29, 1996. Revised Manuscript Received December 20, 1996X
To elucidate the effect of H2S and H2O on hydrogen exchange reactions between tetralin and gaseous hydrogen, the reactions of tetralin with tritiated hydrogen in the presence of H2S or H2O were performed under practical coal liquefaction conditions. The amounts of hydrogen exchange between tetralin and tritiated hydrogen were estimated from hydrogen and tritium balance. The conversions of tetralin in the presence and absence of H2S at 400 °C for 300 min were 4.1 and 3.9%, respectively, while the hydrogen exchange ratio (HER) of tetralin remarkably increased from 4.6% to 40.4% in the presence of H2S and reached 71% at 400 °C for 600 min. It was suggested that the hydrogen exchange mechanism of tetralin with hydrogen in the presence of H2S would include a different mechanism from that in the absence of H2S. It was considered that the H2S molecule acted as both an initiating species and an HS• radical and rapidly abstracted and added hydrogen atom reversibly, promoting the hydrogen exchange in both an aromatic ring and a naphthene ring. In addition to the radical mechanism, the electrophilic exchange with a proton formed from hydrogen sulfide was proposed for the hydrogen exchange mechanism. On the other hand, the conversion and HER of tetralin in the presence of H2O at 400 °C for 300 min were 1.7 and 1.3%, respectively. These values were smaller than those in the absence of H2O. It was considered that H2O inhibited the formation of tetralyl radicals and the hydrogen transfer from tritiated hydrogen to tetralyl radicals. The tritium distribution in H2S reached equilibrium at the initial stage of the reaction, and the hydrogen exchange reaction between gaseous hydrogen and H2S rapidly occurred. However, it was estimated that the hydrogen exchange reaction between gaseous hydrogen and H2O was slower than that with H2S.
Introduction The total sulfur or pyrite in coal correlates with an increase in coal conversion.1,2 The pyrite is rapidly transformed into pyrrhotite, and H2S is produced from the reduction of pyrite under coal liquefaction conditions.3-5 It has been suggested that H2S generated from pyrite is the catalyst for coal liquefaction.6,7 On the other hand, H2O is a major product in coal thermolysis and a large amount of H2O generates under rather mild coal liquefaction conditions. Since the reactions in coal liquefaction proceed with hydrocracking and hydrogenation by molecular hydrogen and donor solvents, the hydrogen transfer mechanism may be extremely influenced by the presence of H2S and H2O. At the present time, however, the role of H2S and H2O in the hydrogen transfer in coal liquefaction is not wellX Abstract published in Advance ACS Abstracts, February 15, 1997. (1) Abdel-Baset, M. B.; Yazab, R. F.; Given, P. H. Fuel 1978, 57, 89. (2) Montano, P. A.; Granoff, B. Fuel 1980, 59, 214. (3) Montano, P. A.; Bommannavar, A. S.; Shah, V. Fuel 1981, 60, 703. (4) Keisch, B.; Gibbon, G. A.; Akhtar, S. Fuel Process. Technol. 1978, 1, 269. (5) Harris, L. A.; Kennedy, C. R.; Yust, C. S. Fuel 1979, 58, 59. (6) Lambert, J. M., Jr. Fuel 1982, 61, 777. (7) Mukherjee, D. K.; Mitra, J. R. Fuel 1984, 63, 722.
S0887-0624(96)00119-3 CCC: $14.00
defined because the complex nature of coal and its derived products prevents a thorough understanding of the reaction mechanism. A useful method to elucidate the hydrogen transfer mechanism in coal liquefaction is to utilize isotopes, such as deuterium and tritium.8-23 Although a deute(8) Fu, Y. C.; Blaustein, B. D. Chem. Ind. 1967, 1257. (9) Franz, J. A.; Camaioni, D. M. Fuel 1980, 59, 803. (10) Brower, K. P. J. Org. Chem. 1982, 47, 1889. (11) Schweighardt, F. K.; Bockrath, B. C.; Friedel, R. A.; Retkofsky, H. L. Anal. Chem. 1976, 48, 1254. (12) Cronauer, D. C.; Mcneil, R. I.; Young, D. C.; Ruberto, R. G. Fuel 1982, 61, 610. (13) Skowronski, R. P.; Ratto, J. J.; Goldberg, I. B.; Heredy, L. A. Fuel 1984, 63, 440. (14) King, H. H.; Stock, L. M. Fuel 1982, 61, 257. (15) Collin, P. J.; Wilson, M. A. Fuel 1983, 62, 1243. (16) Kabe, T.; Nitoh, O.; Funatsu, E.; Yamamoto, K. Fuel Process. Technol. 1986, 14, 91. (17) Kabe, T.; Nitoh, O.; Marumoto, M.; Kawakami, A.; Yamamoto, Y. Fuel 1987, 66, 1321. (18) Kabe, T.; Nitoh, O.; Funatsu, E.; Yamamoto, K. Fuel 1987, 66, 1326. (19) Kabe, T.; Kimura, K.; Kameyama, H.; Ishihara, A.; Yamamoto, K. Energy Fuels 1990, 4, 201. (20) Kabe, T.; Horimatsu, T.; Ishihara, A.; Kameyama, H.; Yamamoto, K. Energy Fuels 1991, 5, 459. (21) Kabe, T.; Ishihara, A.; Daita, Y. Ind. Eng. Chem. Res. 1991, 30, 1755. (22) Ishihara, A.; Takaoka, H.; Nakajima, E.; Imai, Y.; Kabe, T. Energy Fuels 1993, 7, 362. (23) Ishihara, A.; Morita, S.; Kabe, T. Fuel 1995, 74, 63.
© 1997 American Chemical Society
Coal Liquefaction Mechanism
rium tracer was effective in tracing reactive sites in coal and coal model compounds, there are few samples that enable quantitative analysis of hydrogen mobility in coal, because of poor solubility of coal products and the difficulty of quantification of the deuterium tracer.8-15 We have already reported that tritium and carbon-14 tracer techniques were effective to trace quantitatively the behavior of hydrogen in coal liquefaction.16-23 In these works, it was shown that the quantitative analysis of hydrogen mobility of coal and coal-related compounds could be given through the hydrogen exchange reactions among coal, gas phase, and solvent as well as the hydrogen addition. In the course of our study, we were interested in determining the role of H2S and H2O in the liquefaction using a model compound. Tetralin is one of the most interesting and convenient model compounds, because it has an aromatic ring and a naphthene ring in its structure and it can serve as an effective hydrogen donor solvent. The main purpose of this work is to examine the effect of H2S and H2O on the hydrogen exchange reactions of tetralin with tritiated molecular hydrogen. Experimental Section
Energy & Fuels, Vol. 11, No. 2, 1997 471 Table 1. Tritium Distribution and Hydrogen Exchange after Reaction of Solvent with Tritiated Gaseous Hydrogen in the Presence of H2Sa reaction conditions temp (°C) time (min)
(24) Kobayashi, Y.; Maudsley, D. V. Biological Applications of Liquid Scintillation Counting; Academic Press: New York, 1974. (25) Horocks, D. L. Application of Liquid Scintillation Counting; Academic Press: New York, 1974. (26) Crook, M., Johnson, P., Eds. Liquid Scintillation Counting; Heyden and Son: London, 1977; Vol. 4.
Rgas
Rsolvent
RH2S
amt of H exchange (g)
350 350 350 350
0 120 300 600
970399 698145 657364 557070
5351 278668 321407 423907
24250 23187 21228 19023
0.007 0.510 0.733 0.965
375 375 375 375
0 120 300 600
958337 609885 511158 320718
16879 371042 472690 668533
24784 19073 16152 10749
0.023 0.785 1.300 2.814
400 400 400 400
0 120 300 600
936601 436463 320130 216579
32246 547088 666864 777250
31153 16449 13007 7681
0.045 1.629 2.756 4.892
a Total readioactivities were normalized to 106 dpm. Tritium recovery was 100 ( 5%. Initial amount of hydrogen in gas phase was 1.3 g. Initial amount of hydrogen in solvent was 6.8 g.
Table 2. Tritium Distribution and Hydrogen Exchange after Reaction of Solvent with Tritiated Gaseous Hydrogen in the Presence of H2Oa reactions conditions temp (°C) time (min)
Materials. A commercial guaranteed reagent tetralin was used without further purification. Gaseous hydrogen and H2S were supplied from Tohei Chemical. Tritiated molecular hydrogen was obtained by electrolysis of tritiated water using an HG-225 hydrogen generator supplied from Gaskuro Kogyo Inc. Reaction Procedure. A 350 mL autoclave with a glass liner was charged with 75 g of tetralin. It was then charged with tritiated hydrogen (initial total radioactivity, ca. 1 000 000dpm) in addition of 5.0 mol % H2S or 20 mol % H2O, at an initial pressure of 5.9 MPa. The autoclave was stirred with a mechanical stirrer (500 rpm), heated at a heating rate of 10 °C/min, and maintained at the chosen reaction temperature (350-425 °C) for 0-10 h. After the reaction, the reaction mixture separated into gas and liquid products. H2S in the gas was absorbed by bubbling through a lead acetate solution. PbS was filtrated, and the amount of H2S was calculated from the recovered PbS. The gas and liquid products were analyzed by a gas chromatography (GC) with FID to get more precise compositions. For the measurement of radioactivities of these products, liquid scintillation counting was applied.24-26 The liquid products were directly dissolved into a scintillator. The gas products were oxidized into tritiated water ([3H]H2O) with copper oxide at 800 °C to measure the radioactivity of the solution. The radioactivity of tritiated hydrogen sulfide ([3H]H2S) was determined by measuring the radioactivity of the acetic acid solution which was formed by the reaction of H2S with a lead acetate solution as mentioned above. Each sample was dissolved into 14 mL of a scintillator reagent (Monophase S for water and acetic acid solution, Instafluor for an organic sample; Packard Japan), and the radioactivity of the obtained solution was measured with a liquid scintillation counter (Aloka LSC-1050). Calculation of Hydrogen Exchange Ratio (HER). All tritium data appear in Tables 1 and 2. All of the experimental points related to hydrogen exchange were obtained and calculated on the basis of such data. The HER described in this paper is the ratio of hydrogen exchanged in a tetralin solvent to total hydrogen in the solvent. HER between the
tritium distribution (dpm)
tritium distribution (dpm) Rgas
Rsolvent
R H 2O
amt of H exchange (g)
375 375 375
0 120 300
986734 897816 803031
2649 23128 32740
10618 79056 164229
0.0036 0.0346 0.0549
400 400 400
0 120 300
983478 854083 790114
7304 23866 48142
9219 122051 161744
0.0102 0.0392 0.0837
425 425 425
0 120 300
978577 816579 728661
8284 46653 111849
13138 136768 159490
0.0113 0.0780 0.2063
a Total radioactivities were normalized to 106 dpm. Tritium recovery was 100 + 5%. Initial amount of hydrogen in gas phase was 1.3 g. Initial amount of hydrogen in solvent was 6.8 g.
solvent and gaseous hydrogen was calculated on the basis of eq 1. In eq 1, Hex(GTS) is the amount of hydrogen exchanged in solvent in the reaction of solvent with gaseous hydrogen. Hex(GTS) was calculated on the basis of eq 2. In these calculations, it was assumed that the hydrogen exchange between solvent and gaseous hydrogen reached equilibrium at the end of reaction. The tritium concentration in exchanged hydrogen of solvent was equal to that in gaseous hydrogen. When hydrogen addition from the gas phase to the coal or solvent occurred, the amount of added hydrogen from the gas phase to solvent (Hadt), which was equal to the decrease in molecular hydrogen, was subtracted from Hex(GTS). The decrease in molecular hydrogen was determined by change in the partial pressure. The partial pressure of gaseous products determined by GC analysis was subtracted from the final pressure in the gas phase. The isotope effect was regarded as small and was ignored in these calculations because most of the reactions were performed at comparatively high temperature. Recently, it was reported that the isotope effect was negligible under coal liquefaction conditions.27
HER ) Hex(GTS)/Hsolvent
(1)
Hex(GTS) ) HgasRsolvent/Rgas - Hadt
(2)
HER is the hydrogen exchange ratio; Hex(GTS) is the amount of exchanged hydrogen in solvent with gaseous hydrogen; Hsolvent is the amount of hydrogen in solvent; Hgas is the amount of hydrogen in gas phase; Rsolvent is the radioactivity in solvent (27) Kamo, T.; Steer, J. G.; Muehlenbachs, K. Proceedings, International Conference on Coal Science; Elsevier: Banff, 1993; p 415.
472 Energy & Fuels, Vol. 11, No. 2, 1997
Figure 1. Effect of reaction time on the product yield, reaction temperature 400 °C. Reaction in the presence of H2S: b, naphthalene; 2, n-butylbenzene; 9, 1-methylindan. Reaction in the presence of H2O: y, naphthalene; 5, n-butylbenzene; *, 1-methylindan. Reaction in the absence of H2S and H2O: O, naphthalene; 4, n-butylbenzene; 0, 1-methylindan.
Godo et al.
Figure 2. Effect of reaction time on the conversion of tetralin. Reaction in the presence of H2S: +, 350 °C; 9, 375 °C; 2, 400 °C. Reaction in the presence of H2O: *, 375 °C; 5, 400 °C; y, 425 °C. Reaction in the absence of H2S and H2O: 0, 375 °C; 4, 400 °C; O, 425 °C.
after reaction; Rgas is the radioactivity in gas phase after reaction; Hadt is the amount of hydrogen added from gas phase to solvent after reaction.
Results and Discussion Reaction of Tetralin with Tritiated Hydrogen in the Presence of H2S. The reactions of tetralin with tritiated hydrogen were performed under the conditions of 350-400 °C in the presence of H2S. The yields of products are plotted against the reaction time in Figure 1. The reaction products from tetralin were 1-methylindan (MI) by isomerization, naphthalene (NP) by dehydrogenation, and n-butylbenzene (BB) by hydrocracking. They increased monotonously with an elapse of time, and the yields of MI, NP, and BB in the presence of H2S at 400 °C for 300 min were 2.3, 0.7, and 0.9%, respectively. Decalin by disproportionation was not formed under these conditions. The results in the absence of H2S are also plotted in Figure 1, and the yields of MI, NP, and BB at 400 °C for 300 min were 2.1, 0.8, and 1.1%, respectively. The amounts of each product in the presence of H2S were close to those in the absence of H2S. In the reaction of tetralin with tritiated hydrogen in the presence of H2S, the conversion and HER of tetralin were plotted against reaction time in Figures 2 and 3 and compared with those in the reaction in the absence of H2S. As shown in Figure 2, the conversions of tetralin in the presence and absence of H2S at 400 °C for 300 min were 4.1 and 3.9%, respectively. These values are very close to each other. As shown in Figure 3, the HERs of tetralin in the presence and absence of H2S increased gradually with an elapse of time and reached 40.4 and 4.6%, respectively, at 400 °C for 300 min. The HER in the presence of H2S was about 10 times higher than that in the absence of H2S at 375 and 400 °C and reached 71% at 400 °C for 600 min. First-order plots of these data for the conversion and hydrogen exchange are shown in Figure 4. All plots were approximated by straight lines. The result indicated that these reactions could be treated as first-order reactions. The rate constants of conversion and hydrogen exchange could be determined from the slopes of
Figure 3. Effect of reaction time on the HER. Reaction in the presence of H2S: +, 350 °C; 9, 375 °C; 2, 400 °C. Reaction in the presence of H2O: *, 375 °C; 5, 400 °C; y, 425 °C. Reaction in the absence of H2S and H2O: 0, 375 °C; 4, 400 °C; O, 425 °C.
first-order plots. As shown in Table 3, the rate constants of tetralin conversion in the presence and absence of H2S at 375 and 400 °C were not different significantly. On the other hand, as shown in Table 4, the rate constants of hydrogen exchange reactions in the presence of H2S were about 12-15 times higher than those in the absence of H2S. Figure 5 shows Arrhenius plots of the rate constants of tetralin conversion and hydrogen exchange between tetralin and tritiated hydrogen in the absence and presence of H2S. Activation energies of the tetralin conversion in the absence and presence of H2S in Table 5 were 33 ( 1 and 35 ( 1 kcal/mol, respectively. These values are very close to each other. This suggested that the conversion of tetralin in the absence and presence of H2S depended on the same reaction mechanism and that the presence of H2S did not affect the activation energies. Activation energies of the hydrogen exchange in the absence and presence of H2S were 30 ( 1 and 34 ( 1 kcal/mol, respectively. These values are a little different from each other. This suggested that the hydrogen exchange of tetralin in the absence and presence of H2S would proceed with different mechanisms.
Coal Liquefaction Mechanism
Energy & Fuels, Vol. 11, No. 2, 1997 473 Table 4. Rate Constants (min-1) of Hydrogen Exchange Reaction temp reaction
350 °C
375 °C
400 °C
425 °C
tetralin tritiated H2 2.70 × 10-4 9.00 × 10-4 1.97 × 10-3 H2S tetralin tritiated H2 H2S tetralin tritiated H2
Figure 4. First-order plots of conversion of tetralin and HER. (a) Conversion of tetralin. Reaction in the presence of H2S: +, 350 °C; 9, 375 °C; 2, 400 °C. Reaction in the presence of H2O: *, 375 °C; 5, 400 °C; y, 425 °C. Reaction in the absence of H2S and H2O: 0, 375 °C; 4, 400 °C; O, 425 °C. (b) HER. Reaction in the presence of H2S: +, 350 °C; 9, 375 °C; 2, 400 °C. Reaction in the presence of H2O: *, 375 °C; 5, 400 °C; y, 425 °C. Reaction in the absence of H2S and H2O: 0, 375 °C; 4, 400 °C; O, 425 °C. Table 3. Rate Constants (min-1) of Tetralin Conversion temp reaction
350 °C
375 °C
400 °C
425 °C
tetralin tritiated H2 1.45 × 10-5 3.37 × 10-5 1.19 × 10-4 H2S tetralin tritiated H2 H2S tetralin tritiated H2
2.32 × 10-5 5.48 × 10-5 1.69 × 10-4 3.85 × 10-5 1.15 × 10-4 2.40 × 10-4
Reaction of Tetralin with Tritiated Hydrogen in the Presence of H2O. The reactions of tetralin with tritiated hydrogen were performed under the conditions of 375-425 °C in the presence of H2O. The yields of products are also plotted against the reaction time in Figure 1. The yields of MI, NP, and BB at 400 °C for 300 min in the presence of H2O were 0.9, 0.2, and 0.4%, respectively. These values were less than half of those in the absence of H2O. Under every condition in the absence and presence of H2O, the ratios among MI, NP, and BB were not different significantly. In the reaction of tetralin with tritiated hydrogen in the presence of H2O, the conversion and HER of tetralin were also plotted against reaction time in Figures 2 and 3. The conversions of tetralin and HER in the presence
2.62 × 10-5 4.85 × 10-5 1.02 × 10-4 6.10 × 10-5 1.65 × 10-4 3.33 × 10-4
Figure 5. Arrhenius plots of rate constants of tetralin conversion and hydrogen exchange. Reaction in the presence of H2S: b, tetralin conversion; 2, hydrogen exchange. Reaction in the presence of H2O: y, tetralin conversion; 5, hydrogen exchange. Reaction in the absence of H2S and H2O: O, tetralin conversion; 4, hydrogen exchange. Table 5. Activation Energies in the Reaction of Tetralin with Tritiated Hydrogen and H2S/H2O conversion (kcal/mol)
H exchange (kcal/mol)
tetralin tritiated H2 H2S
35 ( 1
34 ( 1
tetralin tritiated H2 H2O
35 ( 1
24 ( 1
33 ( 1
30 ( 1
reaction
tetralin tritiated H2
of H2O at 400 °C for 300 min were 1.7 and 1.3%, respectively. The HER at 375-425 °C in the presence of H2O was about one-third of that in the absence of H2O. First-order plots of these data for the conversion and hydrogen exchange are also shown in Figure 4. As shown in Tables 3 and 4, both of the rate constants of the tetralin conversion and hydrogen exchange in the presence of H2O were smaller than those in the absence of H2O at 375-425 °C. Figure 5 also shows Arrhenius plots of the rate constants of tetralin conversion and hydrogen exchange between tetralin and tritiated hydrogen in the presence of H2O. The activation energy of tetralin conversion in the presence of H2O in Table 5 was 35 ( 1 kcal/mol. This value was very close to that in the absence of H2O. This suggested that the conversion of tetralin in the presence of H2O proceeded with the reaction mechanism which the conversion of tetralin in the absence of H2O depended on. The activation energy of the hydrogen exchange in the presence of H2O
474 Energy & Fuels, Vol. 11, No. 2, 1997
Figure 6. Effect of reaction time on the tritium concentration, reaction in the presence of H2S: O, tritiated hydrogen; 4, H2S; 0, tetralin. Reaction temperature: (a) 375 °C; (b) 400 °C; (c) 425 °C.
was 24 ( 1 kcal/mol. In contrast to the results from the conversion of tetralin, this value was different from that in the absence of H2O. This result shows that the hydrogen exchange of tetralin in the presence and absence of H2O would proceed with different mechanisms. Tritium Distribution in the Presence of H2S and H2O. Changes in the tritium distribution in the pres-
Godo et al.
ence of H2S among gaseous hydrogen, tetralin, and H2S at 350, 375, and 400 °C with reaction time are shown in Figure 6. In this figure, dotted lines show the equilibrium value, which was calculated on the assumption that hydrogen atoms were completely scrambled between gaseous hydrogen, tetralin, and H2S. The tritium initially included in gaseous hydrogen decreased monotonously with the reaction of time and slopes became steep with a rise in temperature. On the other hand, the tritium concentration in tetralin solvent increased monotonously with an elapse of time and slopes also became steep with a rise in reaction temperature. The tritium concentration in H2S increased rapidly, showed the maximum value beyond the calculated equilibrium value at 0 min, and decreased with the same tendency as the gaseous hydrogen molecule. In the reaction at 400 °C, the tritium concentration in H2S was very close to that in gaseous hydrogen at 0 min. This result shows that the hydrogen exchange reaction between gaseous hydrogen and H2S proceeded rapidly even during the heating period. At 400 °C for 600 min, the tritium distribution among gaseous hydrogen, tetralin solvent, and H2S approached the calculated equilibrium value. Change in the tritium distribution in the presence of H2O among gaseous hydrogen, tetralin, and H2O at 375, 400, and 425 °C with reaction time is shown in Figure 7. Since the rate of hydrogen exchange between gaseous hydrogen and tetralin in the presence of H2O was extremely smaller than that in the presence of H2S, the decreases of tritium in gaseous hydrogen and the increases of tritium in tetralin solvent were slower than those in the presence of H2S. The tritium concentration in H2O at every temperature increased gradually and was close to that in gaseous hydrogen at 300 min. However, the increasing rate of tritium concentration in H2O was slower than that in H2S obviously. Reaction Mechanism in the Presence of H2S and H2O. The tritium distributions of H2S in Figure 6 showed that the hydrogen exchange reaction between gaseous hydrogen and H2S could proceed rapidly under coal liquefaction conditions. A possible mechanism of the hydrogen exchange reaction between gaseous hydrogen and H2S is shown in eqs 3-6. In eq 3, H2S produces H• and SH• radicals by the thermal dissociation under these reaction conditions. The formed H• and SH• radicals will react with a tritiated hydrogen molecule and tritium atom to form tritiated HTS and HT in eqs 4-6. It is well-known that H2S played an important role in producing H• from molecular hydrogen through eq 3.28 In previous works, we assumed that the formations of naphthalene and 1-methylindan and the hydrogen transfer from tetralin proceeded with a radical reaction mechanism.9,13,21,23 The reaction mechanism was described by assuming a tetralyl radical as an intermediate in the conversion and hydrogen exchange of tetralin. 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 gaseous hydrogen, tetralin may collide with not only itself but also tritiated hydrogen to give a tetralyl radical in eq 7. The hydrogen exchange between tetralyl (28) Thomas, M. G.; Padrick, T. D.; Stohl, F. V. Fuel 1982, 61, 761.
Coal Liquefaction Mechanism
radicals and tritiated hydrogen can be assumed to proceed through eq 8 depending on the concentration of tetralyl radicals, which also controls the formation of 1-methylindan and naphthalene in eq 9. Therefore, the formation of the tetralyl radical in this system may be the rate-determining step for both the conversion of tetralin and the hydrogen exchange. The HER between tetralin and tritiated hydrogen in the presence of H2S was about 10 times higher than that in the absence of H2S. This result of increase of hydrogen exchange cannot be explained only by this radical reaction mechanism. Skowronski et al. reported that, in the hydrogen exchange between tetralin-d12 and hydrogen in coal at 400 °C for 60 min in a shaken autoclave system, protium was incorporated into Halpha, Hbeta, and Haromatic positions in tetralin at the ratio 66: 23:11.13 In our results, at 400 °C for 600 min, the HER of tetralin in the presence of H2S reached 71%. This value shows that not only R-hydrogen in a naphthene ring but also hydrogen in an aromatic ring and β-hydrogen in a naphthene ring become more exchangeable in the presence of H2S. Stenberg et al. reported that hydrogen donor ability of H2S was indicated by the stoichiometry of the bibenzyl conversion and the favorably low bond dissociation energies of H2S compared with H2.29 The bond energy of H2 is greater than that of most C-H bonds, whereas that for H2S is not. The most likely and energetically feasible early initiation (29) Sondreal, E. A.; Wilson, W. G.; Stenberg, V. I. Fuel 1982, 61, 925.
Energy & Fuels, Vol. 11, No. 2, 1997 475
Figure 7. Effect of reaction time on the tritium concentration, reaction in the presence of H2O: O, tritiated hydrogen; 4, H2O; 0, tetralin. Reaction temperature: (a) 375 °C; (b) 400 °C; (c) 425 °C.
step between tetralin and H2S is the hydrogen addition from H2S to tetralin, promoting the hydrogen exchange in an aromatic ring, as shown in eq 10. In the reaction of formation of NP, 1,2-dihydronaphthalene (DHN) as an intermediate is produced by the abstraction reaction of tetralin. It is a very facile reaction with the tritiated hydrogen sulfide (HTS) to give 2-tritio-1-tetralyl radical, which is an intermediate leading to tritium incorporation in the β-position of tetralin, as shown in eq 11.
476 Energy & Fuels, Vol. 11, No. 2, 1997
Concerning the hydrogen exchange in an aromatic ring, there may be another hydrogen exchange mechanism through an electrophilic substitution, which does not proceed through a tetralyl radical. We reported that hydrogen at ortho and para positions in phenol exchanged with water via an electrophilic substitution.22 The HER of tetralin in the reaction of tetralin with water is much smaller than that with gaseous hydrogen.30 In contrast, hydrogen in an aromatic ring of tetralin may be exchanged via an electrophilic substitution with tritiated hydrogen sulfide as shown in eqs 12 and 13 because H2S is more acidic than water or phenol (H2S + H2O ) H3O+ + HS-: K ) 1 × 10-7; phenol, Ka ) 1.1 × 10-10). The tritium distributions of H2O in Figure 7 showed that the hydrogen exchange reaction between gaseous hydrogen and H2O was much slower than that with H2S. The difference of bond dissociation energy between H2S and H2O would result in the difference of the rate of hydrogen exchange with gaseous hydrogen (H2S, 90 kcal/mol; H2O, 119 kcal/mol). The conversion of tetralin in the presence of H2O decreased in comparison with those in the absence of H2O. It was suggested that H2O inhibited the formation of tetralyl radical. The rate (30) Kabe, T.; Takaoka, H.; Yamamoto, K.; Ishihara, A.; Horimatsu, T. Proceedings, International Conference on Coal Science; Elsevier: Tokyo, 1989; p 157.
Godo et al.
constant of hydrogen exchange between tetralin and tritiated hydrogen in the presence of H2O was about one-third of that in the absence of H2O. However, that of tetralin conversion in the presence of H2O was about half of that in the absence of H2O. This indicated that H2O inhibited more strongly the hydrogen exchange than the conversion of tetralin. It was suggested that H2O inhibited not only the formation of tetralyl radicals but also the hydrogen transfer from tritiated hydrogen to tetralyl radicals. Conclusion The hydrogen exchange mechanism between tetralin and gaseous hydrogen was investigated in the presence of H2S and H2O. It was clarified that H2S enhanced the hydrogen exchange reaction while H2O inhibited it. The thermal reaction of tetralin was not affected significantly by H2S. It was considered that the H2S molecule acted as both an initiating species and an HS• radical, promoting the hydrogen exchange in both an aromatic ring and a naphthene ring. It was suggested that the hydrogen exchange reaction of tetralin with tritiated hydrogen in the presence of H2S would proceed not only through a tetralyl radical but also through an electrophilic substitution. EF960119M
