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Energy & Fuels 1997, 11, 1288-1292
Elucidation of Hydrogen Mobility in Coal Using a Tritium Tracer Method. 1. Hydrogen Exchange Reaction of Coal with Tritiated Water Weihua Qian, Atsushi Ishihara, Hiroaki Fujimura, Masaru Saito, Masazumi Godo, and Toshiaki Kabe* Department of Applied Chemistry, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei, Tokyo 184, Japan Received May 28, 1997. Revised Manuscript Received August 19, 1997X
The hydrogen exchange of coals with water was investigated using a tritium tracer method to estimate the mobility of hydrogen in coals. The reactions at several temperatures between 50 and 300 °C were carried out using a glass batch reactor and a pulse flow reactor, respectively. At lower temperatures, the ratio of hydrogen exchange of coal with water increased with a decrease in rank of coals and tended to change with respect to the content of functional groups such as hydroxyl group, thiol, amino group, and carboxylic acid in coal. From the results obtained from the hydrogen exchange of model compounds of the functional groups present in coal, it is proposed that the hydrogen only in the functional group was exchangeable at lower temperature while the hydrogen in aromatic ring substituted by functional groups also became exchangeable at 300 °C. It was found that the use of the pulse flow reactor as well as the glass batch reactor was very useful facile and convenient methods to trace the hydrogen exchange between coal and water.
Introduction The heteroatom functionality in coal such as hydroxyl group, thiol, and amino group, etc. plays a critical role in the processing of coal because they constitute the more polar fraction of the coal and stabilize free radicals.1,2 Consequently, it is very important to know the forms in which they appear in coal and their accurate content present in coal to construct the very complex structure model of the coal and to develop coal conversion techniques. There are in principle two kinds of methods to investigate the chemical structure of coal. One is to attempt breaking down the coal macromolecules into representative fragments and then to deduce the initial structure of the coal from the structure identified from such fragment. The other is the direct nondestructive characterization of coal in its original form in the solid state spectroscopic methods.3 Generally, the oxygen group is primarily present in the form of hydroxyl group and ether group and a little in carbonyl group and carboxylic acid. Fourier transform infrared was available for the measurement of oxygen functional groups.4,5 The heteroatom nitrogen is mainly present in the form of pyrrolic and pyridinic nitrogen, and X-ray photoelectron spectroscopy (XPS) has been recently used to quantify their content.6-9 The X Abstract published in Advance ACS Abstracts, October 1, 1997. (1) Attar, A.; Hendrickson, G. G. In Coal Structure; Meyers, R. A., Ed.; Academic Press: New York, 1982; p 131. (2) Shinn, J. H. Fuel 1984, 63, 1176. (3) Haenel, M. W. Fuel 1992, 71, 1211. (4) Solomon, P. R.; Serio, M. A.; Despande, G. V.; Kroo, E. Energy Fuels 1990, 4, 42. (5) Martin, K. A.; Chao, S. S. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1988, 33 (3), 17. (6) Berkowits, N. The chemistry of Coal; Coal Science and Technology, 7; Elsevier: Amsterdam, 1985. (7) Wallace, S.; Bartle, K. D.; Perry, D. L. Fuel 1989, 68, 1450. (8) Stock, L. M.; Wolny, R.; Bal, B. Energy Fuels 1989, 3, 651. (9) Derbyshire, F. Fuel 1991, 70, 276.
S0887-0624(97)00076-5 CCC: $14.00
organic sulfur in coal appears mainly in thiophenic heterocycles and aliphatic sulfides and it appears in smaller quantities in thiol, thiophenols, and diaryl sulfides. XPS and X-ray absorption near-edge structure (XANES) spectroscopy were appropriate methods for the determination of the proportions of thiophenic and aliphatic-sulfidic sulfur in coal.10-12 However, discrepancies between two sets obtained from different researchers need to be resolved before the data can be used with great confidence. Also, the available results are somewhat limited in scope, not being an exhaustive analyses of functional groups found in coal. Hence, many studies are only qualitative or semiquantitative. Recently, we reported that tritium tracer methods are effective to quantify the mobility of hydrogen in coal under coal liquefaction conditions.13,14 In these works, we have performed the reactions of coals with tritiated molecular hydrogen where the hydrogen exchange as well as the hydrogen addition is estimated quantitatively. Especially, we found that in the reaction of Wandoan coal (subbituminous coal) with tritiated water, water can be regarded as a proton donor rather than a hydrogen atom donor.15 Werstiuk and Ju reported that in the protium-deuterium exchange of heteroaromatics with deuterium oxide in the neutral condition and at low temperature, the hydrogen exchange between the (10) Gorgaty, M. L.; George, G. N.; Kelemen, S. R. Fuel 1990, 69, 1065. (11) Gorgaty, M. L.; George, G. N.; Kelemen, S. R.; Sansone, M. Energy Fuels 1991, 5, 93. (12) Kelemen, S. R.; Gorgaty, M. L.; George, G. N.; Kwiatek, P. J.; Sansone, M. Fuel 1991, 70, 396. (13) Kabe, T.; Ishihara, A.; Daita, Y. Ind. Eng. Chem. Res. 1991, 30, 1755. (14) Ishihara, A.; Morita, S.; Kabe, T. Fuel 1995, 74, 63. (15) Ishihara, A.; Takaoka, H.; Nakajima, E.; Imai, Y.; Kabe, T. Energy Fuels 1993, 7, 362.
© 1997 American Chemical Society
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Energy & Fuels, Vol. 11, No. 6, 1997 1289
Table 1. Ultimate Analysis of Coals Used (% daf) coala
C
H
N
S
O
NDb WA ILb UFb POCb
72.94 76.9 77.67 85.50 91.05
4.83 6.7 5.00 4.70 4.44
1.15 1.1 1.37 1.55 1.33
0.70 0.3 2.38 0.74 0.50
20.38 15.0 13.58 7.51 2.68
(L) (SB) (HVB) (MVB) (LVB)
a Abbreviations: ND ) Beulah-Zap, WA ) Wandoan, IL ) Illinois No. 6, UF ) Upper Freeport, POC ) Pocahontas No. 3, L ) lignite, SB ) subbituminous coal, HVB ) high-volatile bituminous coal, MVB ) medium-volatile bituminous coal, LVB ) lowvolatile bituminous coal. b Coals of the Argonne Premium Coal Sample Program.
aromatic hydrogen and water scarcely occurred.16 Therefore, it can be considered that the relatively acidic hydrogen such as the hydrogen in the hydroxyl group is preferably exchanged through protium-tritium with tritiated water at lower temperature. This means that the behavior of hydrogen in the functional groups of coal can be determined by use of the isotope tracer method. Thus, the correlation of behavior of hydrogen at lower temperature and the content of functional group in coal may be determined by using the isotope tracer method. In the present study, we investigated the hydrogen exchange reaction of various Argonne coals with tritiated water. The hydrogen in functional group of the coals was determined and a comparison with Wandoan coal was also carried out by using a glass batch reactor. Further, a method of pulse tritium tracer was also developed to determine the amount of tritium more easily than the conventional method using a batch reactor. Experimental Section Materials. Four kinds of Argonne Premium Coal Samples were obtained in 5 g ampules ( 99.99%). All scintillator solvents for the measurement of radioactivity were purchased from Packard Japan Co. Ltd. Reaction Procedure 1. One gram of coal and 1 g tritiated water (initial radioactivity 106 dpm) were added into a 25 mL Pyrex glass reactor. After the mixture was degassed in vacuum via three freeze-pump-thaw cycles, the reactor was immersed into an oil bath and the reaction mixture was stirred with a magnetic stirrer. The reaction temperature were 50 and 100 °C, and the reaction times were 1-24 h. After the reaction, the reaction mixture was separated to tritiated water and coal with a vacuum line (6 h). The effect of exchange temperature on HER of coal is shown in Figure 3. Although much time was taken to reach the equilibrium of hydrogen exchange at 50 °C than at 100 °C, HER even at 50 °C after reaction for 12 h also reached 7.8%, which is as well representative of the HER as the 100 °C experiment. Thus, it is likely that for the reaction, the rate-limiting step is not the speed of the exchange reaction between water and coal but the diffusion of water into the coal. That is, temperature in fact affects the diffusion rate of the water into the coal. When the hydrogen exchange with tritiated water were carried out with other coals, similar results were obtained. It was observed that HERs for all coals in reactions for over 6 h at 100 °C trended toward a constant value independent of drying of the coal before reaction. The constant value of HER for each coal was obtained for more than 6 h of the reaction time and are presented in Table 3. In order to investigate the isotope effect in the hydrogen exchange reaction, the exchange reaction of coal with deuterium oxide was also carried out at 100 °C. The concentration of proton in recovered water after reaction was determined by means of 1H NMR. HER was calculated from the increase in the amount of proton in recovered water (deuterium oxide) before and after reaction. The results are also presented in Table 3. On comparing with the results obtained with tritiated water, no significant difference between two sets of HERs was observed. This indicates that the isotope effect can be neglected in these hydrogen exchange reactions. Hydrogen Exchange of Model Compounds with Tritiated Water. Further, a series of heteroatom compounds such as phenol, naphthol, toluidine, and indole etc. are regarded as model compounds of the
Figure 3. Effect of temperature on hydrogen exchange (Illinois No. 6 coal, as received). Table 3. Hydrogen Exchange Ratio of Coal with Water (%, 100 °C, Batch Reactor) coal
ND
WA
IL
UF
POC
HER with tritiated water HER with deuterium oxide
19.2 21.5
7.87 x
7.77 8.90
2.84 2.90
1.60 2.94
Table 4. Hydrogen Exchange Ratio of Model Compounds with Tritiated Water (%, 100 °C, 6 h, Batch Reactor) compounds
ratio Aa
ratio Bb
phenol naphthol toluidine indole phenanthrene
17.0 13.1 21.8 15.0 0.13
16.7 12.5 22.2 14.3 0.00
a Hydrogen exchange ratio obtained from exchange reaction with tritiated water. b Ratio of hydrogen in functional group to total hydrogen in each compound.
functional groups present in coal and the hydrogen exchange of these compounds with tritiated water was conducted at 100 °C to identify the position of exchanged hydrogen. In addition, phenanthrene was also used as a model compound of nonsubstituted aromatic ring. The results are presented in Table 4. Since hydrogen in phenanthrene was hardly exchanged with water, the aromatic hydrogen in coal was considered not to be exchanged with the hydrogen in water. In contrast to this, all heteroatom compounds could readily exchange hydrogen with water and HERs of these compounds were approximately the same as ratios of hydrogen in functional groups to total hydrogen derived from stoichiometry of the model compounds. Acid- and basecatalyzed deuterium incorporation into aromatic nucleus
Hydrogen Mobility in Coal
Energy & Fuels, Vol. 11, No. 6, 1997 1291
Figure 4. Change in radioactivity of introduced pulse of tritiated water (Illinois No. 6 coal, 200 °C).
has been reported in the literature.17-19 Ingold et al. showed that the reaction of phenol with deuterium oxide in the presence of NaOH exchanged three nuclear hydrogen atoms in an aromatic ring after 30-40 days at 100 °C. It seems not to agree with our results. This is not a surprise because in the present study, neutral water was used and the reaction time was not so long. In a recent study on the protium-deuterium exchange of heteroaromatics with D2O in the neutral condition, Werstiuk and Ju reported that the hydrogen in aromatic ring of phenol was scarcely exchanged by deuterium at 165 °C for 24 h.16 This is in good agreement with our results. Therefore, it is considered that hydrogen in the functional groups of the coal such as hydroxyl, carboxylic acid, etc. are rapidly exchanged through the proton exchange between water and coal and that the effect of temperature would be less for this type of ion-exchange reaction because the rate of ion-exchange reaction is generally very rapid. Hence, it is suggested that HER of coal at lower temperature represents the amount of hydrogen in the functional groups of coal. Hydrogen Exchange Reaction in a Pulse Flow Reactor. The pulse flow reactor was used to investigate the hydrogen exchange reaction of coal with water at higher temperature. Figure 4 shows the change in radioactivity of tritiated water in a recovered pulse with the number of introduced pulse when a pulse of tritiated water (8 µL) with a constant radioactivity (9100 dpm/ pulse) was introduced into Illinois No. 6 coal at 200 °C every 30 min. After the first pulse was introduced, the radioactivity of the recovered pulse was only 580 dpm. This indicates that some tritium in tritiated water was incorporated into coal. Further, the radioactivity in the recovered pulse increased with the number of introduced pulses and approached a constant value (9100 dpm) at the seventh pulse. In contrast to this, the amount of recovered water, which was monitored by the TCD equipped at the outlet of coal packed column, approximately remained constant for every introduced pulse. This indicates that the decrease in the radioactivity of the introduced pulse cannot be attributed to the adsorption/desorption of water in the coal but to the hydrogen exchange between the tritiated water and the coal. Meantime, it can be considered that hydrogen exchange between the tritiated water and the coal reached equilibrium after introduction of the seventh (17) Thomas, A. F. Deuterium Labeling in Organic Chemistry; Appleton-Century-Crofts: New York, 1971; p 204. (18) Ingold, C. K.; Raisin, C. G.; Wilson, C. L. J. Chem. Soc. 1936, 1637. (19) Calf, G. E.; Garnett, J. L. Adv. Heterocycl. Chem. 1973, 15, 137.
Figure 5. Hydrogen exchange ratio in exchange reaction of coal with tritiated water in the batch or the pulse reactor: (O) batch reactor, (b) pulse flow reactor. Table 5. Hydrogen Exchange Ratio of Coal with Tritiated Water in the Pulse Flow Reactor (%) coal
100 °C
200 °C
300 °C
ND WA IL UF POC
16.9 6.4 7.0 2.0 0.8
17.7 7.6 7.9 2.6 1.50
18.7 12.0 11.9 4.9 2.6
pulse. According to eqs 1 and 2, the amount of exchanged hydrogen and HER was determined from the difference in the radioactivity between introduced and recovered pulse or from the radioactivity of tritium incorporated into coal obtained by combustion of coal. The results are presented in Table 5. When other coals were used, similar results were obtained and are also presented in Table 5. HERs of Beulah Zap, Wandoan, Upper Freeport, and Pocahontas coals at 100 °C were 16.9, 6.4, 2.0, and 0.8%, respectively. The results using the pulse reactor at 100 °C were compared with those using the batch reactor in Figure 5. There is no significant difference in HERs between two types of reactors, although the results obtained in the pulse reactor are slightly less than that obtained in the batch reactor. It is well-known that the diffusion rate of material in a flow system is much faster than that in the batch system. Thus, the exchange reaction in the pulse reactor could approach an equilibrium state although the reaction time was much shorter than that in the batch reactor. This further indicates that the hydrogen exchange rate with water is very rapid and that the diffusion rate is the limiting step of the hydrogen exchange. In addition, the consistency of data between the two reactors also shows that the extent of hydrogen exchange into the wall of the glass reactor in the batch method is negligible in the present study. Effect of Rank of Coal and Temperature on HER. As mentioned above, the hydrogen exchange reaction of coal with water at low temperature primarily proceeds through the proton exchange between coal and water. Through the review of a lot of literature on distribution of oxygen functional groups in coals, Attar and Hendrickson developed an empirical correlation between the distribution of oxygen functional groups and the ultimate analysis of coals.1 According to this correlation, the contents of the hydroxyl group in each coal were estimated. Further, Solomon et al. determined the contents of hydroxyl groups in Argonne Premium Coals using FT-IR.3,20 The maximum content of hy-
1292 Energy & Fuels, Vol. 11, No. 6, 1997
Figure 6. Comparison of hydrogen exchange ratio with content of hydroxyl group.
droxyl group can also be calculated from the analytical data listed in Table 1 assuming that all oxygen in coals is present in the form of hydroxyl groups. In Figure 6 HERs of coals with tritiated water at 100 °C were compared with the ratios of hydrogen in the hydroxyl group to total hydrogen in coal calculated by the several methods mentioned above. It is observed that HER decreases with an increase in rank of coals. HER for the high-rank coal (Pocahontas No. 3) is very close to HER calculated from ratio of hydrogen in hydroxyl group whereas HER for the low-rank coal (Beulah-Zap) is more than the calculated one. The results shows that, in the high-rank coal, the hydrogen exchanged with water may be hydrogen of the hydroxyl group because the content of other functional group is very low. On the other hand, there are nitrogen and sulfur functional groups in coals such as amino and thiophenyl group and hydrogen in these group as well as hydrogen in hydroxyl group could readily be exchanged through ion exchange. This can be verified by the results shown in Table 4. It indicates the presence of COOH, SH, and NH, etc. groups in the low-rank coals. In addition, dihydric phenols, aminophenols, and hydroxylthiophenols are more abundant in the low-rank coal, especially in lignite, because of its poor coalification. The hydrogen in the aromatic ring of these functional groups may be more mobile and may be exchanged with water in relatively mild condition. This may also cause difference in HER and the hydroxyl content for Beulah-Zap coal. All HERs obtained in the batch reactor or the pulse flow reactor are summarized in Figure 7. Figure 7 shows the effect of temperature on the ratio of hydrogen exchange of coal. HERs for all coals hardly changed up to 200 °C; however, they slightly increased at 300 °C. This indicates that other hydrogen in coal rather than hydrogen in functional groups were exchanged with (20) Solomon, P. R.; Hamblen, D. G.; Yu, Z.; Serio, M. A. Fuel 199l, 69, 754.
Qian et al.
Figure 7. Effect of temperature on hydrogen exchange ratio. Batch reactor: (0) ND, (4) WA, (O) IL, (]) UF, (+) POC. Pulse reactor: (9) ND, (2) WA, (b) IL, ([) UF, (×) POC.
hydrogen in water. In our previous paper, it was proposed that, in the hydrogen exchange reaction of coal and coal related compounds with tritiated water, aromatic hydrogen in an aromatic ring substituted by such a functional group as hydroxyl group in coal would be exchangeable. It was shown that only hydrogen in hydroxyl group was exchangeable at 100 °C while hydrogen at para or ortho position of 1-naphthol became exchangeable at 300 °C.15 Therefore, in the present paper using the pulse flow reactor, it also appears that a part of hydrogen in aromatic rings substituted by functional groups as well as hydrogen in the functional groups was exchanged at 300 °C. Conclusions The radioisotope tracer method using the pulse flow reactor as well as the batch reactor provided a more accurate and efficient approach elucidating the behavior of hydrogen and determining the content of hydrogen related to functional groups in coals. The hydrogen exchange ratios of coal with water in the reactions lasting more than 6 h approached a constant value independent of the drying of coal before reactions. At lower temperatures, HER of coal with water increased with a decrease in rank of coals and tended to change with respect to the content of functional groups such as hydroxyl group, thiol, and amino group, etc. in coals. From the results obtained in the hydrogen exchange of model compounds with water, it was proposed that only hydrogen in functional groups was exchangeable at lower temperature while hydrogen in aromatic rings substituted by functional groups also became exchangeable at 300 °C. Acknowledgment. This study was supported by the Research for the Future Project of the Japan Society for the Promotion of Science (the 148 Committee on Coal Utilization Technology) under Contract JSPSRFTF96R14801. EF970076P
