Energy & Fuels 2001, 15, 801-806
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Coal Dehydrogenation Using Quinones or Sulfur John W. Larsen,* Murat Azik, Andrzej Lapucha, and Shang Li Department of Chemistry, 6 E. Packer Avenue, Lehigh University, Bethlehem, Pennsylvania 18015
Koh Kidena and Masakatsu Nomura Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565, Japan Received August 7, 2000. Revised Manuscript Received March 8, 2001
Seven coals have been dehydrogenated with sulfur in refluxing o-dichlorobenzene with the rate of reaction being followed by analysis of the H2S swept from the reaction by an N2 stream. Dehydrogenation using benzoquinone was not successful due to benzoquinone incorporation into the coal. The chief structure change on dehydrogenation with sulfur is loss of aliphatic hydrogen with a concomitant increase in coal aromaticity. Methyl hydrogen is lost. There is spectroscopic evidence for the formation of carboxyl which is contradicted by the fact that the equilibrium thermodynamics of their formation by the oxidizing agent sulfur is unfavorable. The rate of H2S formation drops sharply early in the reaction demonstrating that reactive hydrogen is selectively removed. Unfortunately, we could not determine whether the selectivity is due to high hydrogen chemical reactivity or easy hydrogen accessibility. Sulfur incorporation is low initially and increases as the reaction progresses. Dehydrogenation reduces the pyridine swelling of the coals.
Introduction Dehydrogenation is an important reaction during coal coking, during the later stages of coalification, and may play a role in coal direct liquefaction.1-3 Despite its importance, there are few studies of chemical coal dehydrogenation and fewer investigations of the behavior of deliberately dehydrogenated coals. Driven by our curiosity about the effect of removing the most readily donated (the most easily oxidized) hydrogen from coals on the coking and direct liquefaction behavior of coals, we sought a method for chemically dehydrogenating coals. The method must be as mild as possible so as to minimize changes in coal structure. It is also necessary to be able to control dehydrogenation reactivity so that a selected amount of hydrogen can be removed. It must therefore be possible to measure the amount of hydrogen being removed. Finally, the reagent being used must not be incorporated into the coal. All of these criteria are not satisfied by the method we have developed, but the method is good enough to be useful. Refluxing coals with sulfur in o-dichlorobenzene and measuring the H2S produced is a satisfactory, but not perfect, dehydrogenation method. The use of quinones as dehydrogenation reagents is not satisfactory. In a classic study, Wender and co-workers used 1% Pd on calcium carbonate catalyst in refluxing phenanthridine to dehydrogenate 36 coals.4 The overwhelming * Author to whom correspondence should be addressed. (1) Howard, J. B. Fundamentals of Coal Pyrolysis and Hydropyrolysis in Chemistry of Coal Utilization, 2nd Suppl. Vol.; Elliot, M. A., Ed.; Wiley-Interscience: New York, 1981. (2) Van Krevelen, D. W. Coal, 3rd Ed.; Elsevier: New York 1993. (3) Neavel, R. C. Fuel 1976, 55, 237-242.
gaseous product was hydrogen measured volumetrically. In more recent work, CoMo, SnCl2, Fe2O3, and ammonium heptamolybdate have been used.5,6 These methods will not suit our purpose because the hydrogen evolution may be controlled by contact between the solid catalyst and the solid coal. Such contact may remove all of the reactive hydrogen in one place leaving more reactive hydrogen in another site untouched because it does not contact the solid catalyst. In so far as possible, the dehydrogenating reagent must have access to all of the coal so that hydrogen evolution is controlled by reactivity, not accessibility. Polynuclear aromatic hydrocarbons have been used to dehydrogenate coals.7 The reaction is hydrogen transfer to form a partially hydrogenated aromatic system, for example anthracene going to first 9,10dihydroanthracene and then subsequently to 1,2,3,4tetrahydroanthracene. These compounds have several advantages. One is a high affinity for coals that will minimize accessibility problems. Another is that the reactivity range can be controlled by the choice of the aromatic system used. For our purposes, the use of aromatic hydrogen acceptors has one severe problem. Both the starting aromatic compounds and their hydrogenated products may be covalently bonded to the coal. (4) Reggel, L.; Wender, I.; Raymond, R. Science 1962, 137, 681682. Fuel 1964, 43, 229-233; 1968, 47, 373-389; 1970, 49, 281-286; 1970, 49, 287-288; 1971, 50, 152-156; 1973, 52, 162-163. (5) Kim, J.-J.; Weller, S. W. Fuel Proc. Technol. 1985, 11, 205-209. (6) Chowdhury, P. B.; Rudra, S. R.; Samuel, P.; Mukherjee, D. K. Fuel Proc. Technol. 1989, 21, 201-208. (7) Kidena, K.; Murata, S.; Nomura, M. Energy Fuels 1996, 10, 672678. Yokono, T.; Takahashi, N.; Sanada, Y. Energy Fuels 1987, 1, 360362.
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If this occurs, the added hydrocarbon cannot be distinguished from the coal. The most that can be done is to use isotopically labeled compounds so that the extent of incorporation can be measured. Because of problems with incorporation, we sought other methods. A variety of additional techniques have been used to dehydrogenate coals. The reagents used include Nbromosuccinimide/base, halogens, air, and triphenylmethyl perchlorate.8-11 Mild air oxidation (weathering) is especially important and well studied. The initial site of oxygen attack is benzylic positions and the most easily donotable hydrogen reacts first.12 Loss of this hydrogen is associated with the familiar decreases in fluidity caused by exposure to air.13 Accurately measuring the water produced in order to know how much hydrogen is removed from the coal by O2 and problems due to overoxidation (CO2 formation) will be so difficult that we chose not to use mild air oxidation. As expected, the reaction mechanism is that of autoxidation.14 There is a good review of the dehydrogenation of polycyclic hydroaromatic compounds and the methods used there will also be effective for many aliphatic structures, especially those involving benzylic carbons.15 The structures in which we are most interested are the most reactive hydrogen donors and in coal these are probably hydroaromatic systems. Two of the standard methods for dehydrogenation are attractive for use with coals, have been used, and are the subjects of this paper: dehydrogenation with quinones and dehydrogenation with sulfur. Quinones are believed to dehydrogenate using an ionic mechanism while sulfur dehydrogenation probably involves a radical mechanism.15 Quinones have been used to dehydrogenate coals, the product being hydroquinones.8 The reaction was carried out in refluxing N,N-dimethylformamide (DMF, bp 153 °C) and the amount of products, both quinone and hydroquinone, were measured using polarography. The reaction proceeded rapidly taking only about 8 h. This system has two serious drawbacks. One is that DMF and benzoquinone react slowly at reflux and the other is that the mass balance never closed. With one coal, when 80% of the benzoquinone had reacted, only 40% hydroquinone had formed.8 At its boiling point, DMF decomposes slightly into carbon monoxide and diethylamine and this process is catalyzed by both acids and bases.16 Amines react readily with benzoquinone. This provides an explanation for the reaction of benzoquinone with refluxing DMF and, if DMF decomposition is catalyzed by coals, for the large loss of benzoquinone. It seemed worthwhile to try this mild oxidant in another solvent, one that will not react with benzoquinone. In related chemistry, benzophenone has been used to dehydrogenate coals and this reaction is the basis for (8) Peover, M. E. J. Chem. Soc. 1960, 5020-5026. (9) Dicker, P. H.; Flagg, M. K.; Gaines, A. F.; Martin, T. G. J. Appl. Chem. 1963, 13, 444-454. (10) Pinchin, F. J. Fuel 1958, 37, 293-298. (11) Mazumdar, B. K.; Chakrabartty, S. K.; Saha, M.; Aront, K. S.; Lahiri, A. Fuel 1959, 38, 469-482. (12) Lopez, D.; Sanada, Y.; Mondragon, F. Fuel 1998, 77, 16231628. (13) Clemens, A. H.; Matheson, T. W. Fuel 1992, 71, 193-197, and references therin. (14) Clemens, A. H.; Matheson, T. W.; Rogers, D. E. Fuel 1991, 70, 215-221. (15) Fu, P. P.; Harvey, R. G. Chem. Rev. 1978, 78, 317-361. (16) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of Laboratory Chemicals; Pergamon: New York, 1966; p 143.
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the claim that coals contain some hydrogen more readily donatable than that of tetralin.17,18 The product of the dehydrogenation is diphenylmethane. The reaction was studied at 400 °C in benzene and product analysis was by gas chromatography. Mass balances were not given but presumably were good. Hydrogen transfer from hydroaromatics to benzophenone has also been studied as part of an investigation of “hydrogen shuttlers”.19 We have not used benzophenone because the high temperatures required may cause side reactions. Sulfur is the oxidant most frequently used to dehydrogenate coals. Mazumdar used the amount of hydrogen removed by sulfur as a measure of the amount of alicyclic hydrogen in coals.20 But it has been shown that heating aromatic hydrocarbons with sulfur generates H2S thus vitiating his conclusions.21 A subsequent careful study of coal and model compound dehydrogenation by sulfur using IR to quantify hydrogen removal confirmed Mazumdar’s results.9 These conflicting results have not been reconciled. It has been shown that the amount of H2S formed on reacting hydrogen donor coal liquefaction solvents with sulfur correlates with the amount of donor hydrogen measured by NMR.22,23 The method has been used for both coal liquefaction solvents and for anthracene oil.24-26 Experimental Section Dehydrogenation with Benzoquinone. All compounds were obtained from Fisher Scientific and used as received. 0.1 g of Illinois No. 6 coal (100 mesh, Argonne Premium Coal) was stirred with 0.3 g of benzoquinone in 5 mL of chlorobenzene or 1,2-dichlorobenzene for between 0.3 and 24 h at temperatures between 100 °C and 180 °C. Benzoquinone is stable at 130 °C in chlorobenzene and at 180 °C in 1,2-dichlorobenzene for 24 h. Hydroquinone is not soluble in these solvents at room temperature. Both of these solvents extracted no more than 0.2% of the coal. After heating the reaction mixture for the desired time, it was cooled, about 0.1 g of naphthalene internal standard was added together with 10 mL of ethanol. This mixture was shaken, centrifuged, and a sample was carefully withdrawn from above the coal for analysis by gas chromatography. An HP 5880A gas chromatograph with a FID detector was used with a Supelco 15m SPB-20 column. The carrier gas was He at 1.0 mL/min at a split ratio of 100:1 and the temperature was programmed from 100 to 140 °C. Dehydrogenation with Sulfur. The weighed sample of dried coal (about 5 g) was refluxed in a solution of an equal (17) Collins, C. J.; Raaen, V. F.; Benjamin, B. M.; Kabalka, G. W. Fuel 1977, 56, 107-108. (18) Raaen, V. F.; Roark, W. H. Fuel 1978, 57, 650-651. (19) Chiba, K.; Tagaya, H.; Suzuki, Tohru; Suzuki, Takashi Bull. Chem. Soc. Jpn. 1987, 60, 2669-2670. Chiba, K.; Tagaya, H.; Suzuki, T.; Sato, S. Bull. Chem. Soc. Jpn. 1991, 64, 1034-1036. (20) Mazumdar, B. K.; Chakrabartty, S. K.; Lahiri, A. Fuel 1962, 41, 129-139. Mazumdar, B. K.; Chakrabartty, S. K.; De, N. G.; Ganguly, S.; Lahiri, A. Fuel 1962, 41, 121-128. Mazumdar, B. K.; Chakrabartty, S. K.; Lahiri, A. Fuel 1959, 38, 112-114. Mazumdar, B. K.; Choudhury, S. S.; Chakrabartty, S. K.; Lahiri, A. J. Sci. Ind. Res. 1958, 17B, 509-511. (21) Van Krevelen, D. W.; Goedkoop, M. L.; Palmen, P. H. G. Fuel 1959, 38, 256. (22) Aiura, M.; Masunaga, T.; Moriya, K.; Kageyama, Y. Fuel 1984, 63, 1138-1142.Prepr. Pap.s (23) Bate, K.; Harrison, G. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1989, 34 (3), 839-845. (24) Harrison, G.; Ross, A. B. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1997, 42 (1), 142-145. (25) Fernandez, L.; Granda, M.; Bermejo, J.; Menendez, R.; Bernad, P. Energy Fuels 1998, 12, 949-957. (26) Rahimi, P. M.; Dawson, W. H.; Kelly, J. F. Fuel 1991, 70, 9599.
Coal Dehydrogenation Using Quinones or Sulfur
Figure 1. Percent of Illinois No. 6 coal hydrogen removed by heating with benzoquinone (hydroquinone formation) at 100 °C (4), 115 °C (O), and 130 °C (0) in chlorobenzene and at 180 °C (b) in 1,2-dichlorobenzene. weight of sulfur in 150 mL of 1,2-dichlorobenzene through which a slow stream of dry N2 gas was passed. The H2S formed was removed in the gas stream and trapped by bubbling the gas through a 2 M aqueous solution of NaOH from which aliquots were withdrawn for analysis. The concentration of Na2S was measured by UV at 229 nm.. NMR Spectra. Solid sate 13C NMR spectra of coals or reacted coal samples were recorded on a Chemagnetics CMX300 using a 13C frequency of 75.55 MHz. An 80-120 mg sample was placed in a 5 mm diameter rotor. Single pulse excitation was used. Magic angle rotation at 10.5 kHz was used to remove the effects of anisotropy. The pulse width was 1.5 µs (45° pulse) and the delay time was 100 s. More than 2000 accumulations were used for each sample. The resulting spectra were treated with Spinsight (3.5.2) and Mac FID 5.4. To estimate the carbon distribution of the samples, the spectra were deconvoluted into 14 Gaussian curves by using the chemical shift positions listed in Table 4. Tetrakistrimethylsilylsilane (TKS) was added as an internal standard for both the chemical shift and peak areas.27 IR Spectra. Spectra were recorded using the Diffuse Reflectance (DRIFT) technique and a Mattson Serius FTIR spectrometer. To prepare the samples, 300 mg each of coal and dry KBr were separately and individually ground for 3 min in a Wig-L-Bug. Then about 1% (wt) of the ground coal was added to the KBr and the mixture was ground for 3 min in a WigL-Bug. All operations were carried out in a dry N2 filled Vacuum Atmospheres Glovebox. Spectra were recorded using Harrick’s praying mantis diffuse reflectance optics and KubelkaMunk data processing.
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Figure 2. Benzoquinone-hydroquinone mass balance during the dehydrogenation of Illinois No. 6 coal at 100 °C (4), 115 °C (O), and 130 °C (0) in chlorobenzene and at 180 °C (b) in 1,2-dichlorobenzene.
Figure 3. Benzoquinone conversion to hydroquinone (O) and incorporation into Illinois No. 6 coal (b) during dehydrogenation.
Argonne Premium Illinois No. 6 coal was dehydrogenated by heating with benzoquinone in chlorobenzene at 100 °C, 115 °C, and 130 °C and with 1,2-dichlorobenzene solvent at 180 °C. Both the amounts of remaining benzoquinone and the product hydroquinone were analyzed by gas chromatography using added naphthalene as standard so that the absolute amounts could be determined. Missing material was assumed to have been incorporated into the coal. Figure 1 shows the amount of hydrogen (as a % of the hydrogen in the coal) removed as a function of time under these different conditions. Rapid removal is observed only at 180 °C. It seems to level out at 34% hydrogen removal. As
observed by Peover, the benzoquinone-hydroquinone mass balance does not close demonstrating that one of these compounds or a reaction intermediate in the dehydrogenation process reacts with the coal.8 Figure 2 shows the material loss. It seems to be associated with dehydrogenation, but as Figure 3 shows, incorporation increases faster than dehydrogenation. Our first attempts to dehydrogenate coals with sulfur used refluxing chlorobenzene as the solvent. At this temperature (132 °C), H2S formation is too slow to be useful. Replacing chlorobenzene with o-dichlorobenzene (bp 180 °C) led to satisfactory reaction rates as shown in Figures 4 and 5. Similar plots for 5 other coals can be found in ref 28. Note that there was an error in ref 28 and the legends for Figures 1 and 2 should be interchanged. Elemental analyses (Table 1 and ref 28) revealed small amounts of sulfur incorporation early in the reaction and undesirably large amounts late in the dehydrogenation. The effects of dehydrogenation on the macromolecular structure were investigated by following the changes in coal swelling in pyridine (Table 2), a measurement that should reveal changes in crosslinking. The IR spectra of Pittsburgh No. 8 coal and that coal at 5 different dehydrogenation levels are shown in the
(27) Franz, J. A.; Garcia, R.; Linehan, J. C.; Love, G. D.; Snape, C. E. Energy Fuels 1992, 6, 598-602.
(28) Larsen, J. W.; Li, S.; Nomura, M. Energy Fuels 1998, 12, 830831.
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Figure 4. Amount and rate of hydrogen lost from Pittstone M (a) coal by oxidation with sulfur in refluxing o-dichlorobenzene.
Figure 6. 8 coal.
13
C NMR spectra of dehydrogenated Pittsburgh No.
Table 3. Relative Areas of Aliphatic (Al) and Aromatic (Ar) C-H Stretching Vibrations
Figure 5. Amount and rate (%H2/h) of hydrogen lost from Zao Zhuang coal by oxidation with sulfur in refluxing odichlorobenzene. Table 1. Elemental Analyses of Coals and Dehydrogenated Coals and the Mole Ratio of Sulfur Incorporated to Hydrogen Removed sample
%C
%H
%S
H/C
Pittstone-M
77.9 73.4 71.6 60.8 79.5 69.6 59.2 73.0 63.2 54.3 54.3
4.9 4.3 3.7 2.5 4.6 3.8 3.2 5.1 3.7 2.7 2.5
0.87 4.25 8.43 23.32 0.53 10.53 14.63 0.52 8.3 18.58 21.89
0.75 0.70 0.62 0.49 0.69 0.66 0.65 0.84 0.70 0.60 0.55
Goonyella Workworth
-2.3%H -9.0%H -37%H -11.3%H -20.5%H -9.0%H -22.5%H -26.9%H
S/H 0.18 0.20 0.29 0.39 0.31 0.17 0.24 0.26
Table 2. Equilibrium Volumetric Swelling Ratios (Q) in Pyridine at Room Temperature as a Function of the % of Coal Hydrogen Losta Pittsburgh %H lost No. 8 Q Pittstone-M %H lost Q Workworth %H lost Q Goonyella %H lost Q Wyodak %H lost Q Zao Zhuang %H lost Q
0 2.53 6.80 14.1 19.7 22.7 38.8 47.8 2.2 2.0 1.9 1.6 1.7 1.6 1.3 1.3 0 36.5 1.9 1.3 0 9.0 22.5 26.9 2.0 1.5 1.3 1.2 0 4.17 11.3 20.5 1.6 1.3 1.2 1.0 0 25.1 2.3 1.6 0 40.9 1.5 1.3
Supporting Information. There are two clear changes. As expected, the amount of aliphatic hydrogen compared to the amount of aromatic hydrogen decreases. The same behavior is seen with Zao Zhuang coal. Changes in the relative areas of the two peaks were measured
Pittsburgh No. 8
Zao Zhuang
%H lost
Al/Ar
0 2.53 6.80 19.7 22.7 0 40.9
17.4 11.4 12.1 7.99 7.84 12.2 1.83
by integrating the peaks taking 3000 cm-1 as the dividing line between them. These changes in relative area are presented in Table 3. The other change is the growth of a shoulder at 1696 cm-1. Assigning this to an aryl ketone would be reasonable except that the accompanying strong band between 1075 and 1225 cm-1 is not present.29 Its frequency is a bit high for a benzoquinone whose normal range is 1660-1690 cm-1.29 It is typical of an aromatic carboxylic acid.29 The coal sample that has lost 19.7% of its hydrogen shows a strong peak at 959 cm-1 that is not present in any of the other spectra. Dehydrogenated Zao Zhuang coal also shows a peak in the carbonyl region, this one at 1702 cm-1, presumably due to the same functional group. The 13C NMR spectra of Pittsburgh No. 8 coal and several dehydrogenated samples of this coal were recorded (Figure 6). Each sample was run three times and the spectra were deconvoluted to show the changes in chemical types present. Deconvolution is not a precise process, especially because of the range of chemical shifts possible for the various structures. Accordingly only the larger changes in composition are significant. For the more intense peaks, the scatter in the data was usually within (0.5% C. The data are summarized in Table 4. The data in the table make it clear that the expected aromatization is occurring as aliphatic struc(29) Bellamy, L. J. The Infrared Spectra of Complex Molecules, 2nd ed.; John Wiley & Sons: New York, 1964.
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Table 4. Carbon Functional Group Analysis of Pittsburgh No. 8 Coal and Its Dehydrogenated Products Expressed as %C functional group COO Ar-O Ar-C Ar-C,H Ar-H Al-O CH2 CH3 fa
chem. shift (ppm) 186, 176 167, 153 140 127 114 54, 75, 93 31, 41 13, 20
coal -2.53%H -6.8%H -19.7%H -22.7%H 0.7 8.0 15.9 36 14.5 2.6 13.9 7.5 0.75
2.0 10.0 16.2 40.8 14.7 2.2 10.3 5.7 0.80
1.5 9.7 17.3 38.2 14.5 2.9 10.1 5.7 0.80
1.9 10.5 17.2 38 14.9 2.4 9.4 5.5 0.81
1.3 9.0 18.3 41.9 15.7 1.7 7.4 4.8 0.85
tures are lost while aromaticity increases. Methyl groups are lost. There is no carbonyl except for a trace in the starting coal. There appears to be an increase in carboxyl most of which occurs in the early stages of reaction. The IR and NMR spectra are consistent and show what seems to be the formation of carboxyl, but the peaks are small. Discussion Selective dehydrogenation of coals using benzoquinone is not a satisfactory method. The reason is quinone incorporation. Peover speculated that this was due to Diels-Alder reactions with coals.9 In his system, some of the losses were undoubtedly due to benzoquinone reactions with solvent decomposition products. This is most unlikely in our work, yet benzoquinone or hydroquinone is still lost. While Diels-Alder reactions of coals seem to be possible,30 we are not willing to invoke them here without positive evidence. It is clear that dehydrogenation enables the coal-benzoquinone (or hydroquinone, it could be either) reaction, whatever it may be. This point may be important. If dehydrogenation increases some coal reactions, perhaps its occurrence can be controlled to maximize (or minimize) the desirable (or undesirable) reactions. For our purposes, the selective removal of the most reactive hydrogen, this reaction is unsuitable and we moved on to another candidate. Sulfur is a mild oxidant that has been used neat to dehydrogenate coals producing H2S that can be measured. In a weakly interacting coal-swelling solvent, it might be capable of selective dehydrogenation. We first tried chlorobenzene as solvent, but at its reflux temperature (132 °C), H2S production is too slow to be useful. Using o-dichlorobenzene as solvent at reflux (180 °C) led to H2S production at useful rates (Figures 4 and 5 and ref 28). Sulfur incorporation occurs. As the last column in Table 1 shows, early in the reaction 1 sulfur is incorporated for every 5.5 hydrogens removed from Pittstone and Workworth coals and at twice that rate for Goonyella coal. Sulfur incorporation increases relative to dehydrogenation as the reaction proceeds. The removal of the most reactive hydrogen can be achieved with the introduction of only modest amounts of sulfur. As more hydrogen is removed, the coals swell less in pyridine. The two possible explanations for this are an (30) Larsen, J. W.; Quay, D. M.; Roberts, J. E. Energy Fuels 1998, 12, 856-863.
increase in the covalent cross-link density or an increase in noncovalent coal-coal associative interactions. Both are reasonable explanations and we cannot choose between them. Radical substitution reactions by radical intermediates formed during the dehydrogenation would increase the coal’s cross-link density leading to swelling decreases. The formation of disulfide linkages is also possible. As the aromatic character of the coal increases with increasing dehydrogentation, coal-coal noncovalent interactions will increase leading to decreased swelling. The coal is behaving as expected, but the cause of this cannot uniquely be assigned. As expected, the rates of H2S formation decrease with time (Figures 4 and 5 and ref 28). The rates were calculated by fitting a polynomial to plots of %H removed vs time and then taking the first derivative of that curve. Because coals contain many chemically distinct hydrogens and the most reactive should be removed first, a steady decrease in reactivity is expected. The ratio of rates at 1 h to rates at 200 h ranges from 3.2 to 20. For Pittsburgh No. 8 coal, after 200 h the reactivity has decreased by 20-fold and after 300 h by 34-fold. This is a remarkably small reduction. It may be due to the relative reaction rates of the chemically different hydrogens in the coal or the rate may be mass transport controlled or it may be some unfortunate combination of the two. It is a measure of our poor understanding of this material that we cannot distinguish between these. The observed drop in rate corresponds to a change in activation energy of only about 3 kcal/mol. If hydrogen reactivity is controlling the kinetics, all of the hydrogens lost must have similar bond energies. The other alternative is the operation in coals of an unexpected reaction mechanism that is insensitive to C-H bond strengths. Neither alternative is attractive. The small changes in the rates of H2S formation are strong evidence against a radical bond cleavage reaction being the rate determining step. But, the initial sharp increase in fa from 0.75 to 0.80 with the removal of 2.53% of the hydrogen suggests a chemical-based selectivity. It is possible that the rate-determining step is not chemical, but slow mass transport of sulfur into the coal. In this case, the fast initial rate of H2S formation comes from the immediately accessible near-surface hydrogens and the reaction then slows to the diffusion rate into the coals. We did not expect these coals to be glassy when swollen with dichlorobenzene at 180 °C, but have not done the experiments necessary to check this and have not had the opportunity to test this idea by going to a better swelling solvent at the same temperature. If diffusion is the rate-controlling step, H2S formation should follow approximately (time)1/2.31 It does not, providing evidence against a diffusion-controlled process. Two different kinetic behaviors are observed. In one, there is an initial rapid drop in the reaction rate with half of the total hydrogen loss occurring in 50 h or less. The rate then remains about constant or decreases slowly over several hundred hours. This could be due to a pool of hydrogens with similar reactivity or two coal regions one easily accessible to sulfur and the other less accessible. It is known that 1/4 to 1/3 of some coals is (31) Ritger, P. L.; Peppas, N. A. Fuel 1987 66, 815-826.
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not accessible to and is not swollen by pyridine and a similar two-phase situation may exist in dichlorobenzene-swollen coals.32,33 Candidates for the most reactive hydrogen begin with hydroaromatics and proceed to other oxidizable benzylic structures. The least reactive hydrogens are those on aromatic structures. The condensation of aromatics to form polynuclear aromatics caused by sulfur oxidation is known and provides a candidate large pool of hydrogens of similar reactivity. As already mentioned, it is difficult to explain the observed 20- to 30-fold reactivity variation using hydrogens as different in oxidative reactivity as benzylic and aromatic. There may be changes in reaction mechanism or the rate-determining step may be physical, not chemical. There is another kinetic pattern. The hydrogen loss rate decreases in 60 h or so to a broad plateau which eventually drops off rapidly to 0. The rates on all of the plateaus are similar, around 6 × 10-4% H lost per hour. This suggests that hydrogens in similar structures are reacting. There is one coal that exhibits a rate increase at long time. Some structure change must be involved. We cannot even say whether this is physical or chemical, though an explanation based on physical structure changes is more believable. Information about the reaction chemistry can be obtained from the IR and NMR spectra of the starting and dehydrogenated coals. The most obvious feature of both the IR and NMR results is the decrease in aliphatic relative to aromatic hydrogen. This confirms the obvious supposition that a principal reaction is the aromatization of hydroaroaromatic structures. Sulfur is incorporated. Its effect on 13C chemical shifts is sufficiently small to make NMR determination of the product structures impossible. The IR spectra provide no information except for the absence of an S-H stretching band at 2550-2560 cm-1. The IR spectra show the development of a peak at 1695 cm-1. The NMR spectra show the development of the carboxyl group. Both spectra can best be explained by the formation of ArCOOH. This presents great (32) Norinaga, K.; Iino, M.; Cody, G. D. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem 1999, 45 (2), 352-356. (33) Yang, X.; Larsen, J. W.; Silbernagel, B. G. Energy Fuels 1993, 7, 439-445.
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difficulty. There is no source in the reaction of the oxygen necessary to form the carboxylic acid and the oxidation of a benzylic methyl group to a carboxylic acid by sulfur and water as the oxygen source is thermodynamically impossible being endothermic by nearly 20 kcal/mol.34 Another possibility, the conversion of hydroquinones to quinones does not require any additional oxygen, but is more than 30 kcal/mol endothermic.34 Overcoming the thermodynamic barrier by removing the H2S product as it is formed seems improbable. It also requires rather a large population of hydroquinone structures in the coal. It is most unlikely that there is enough adventitious water in the system at a temperature of 180 °C to provide the oxygen necessary for the reaction. The spectroscopic evidence for formation of carboxyl groups depends on the presence of two peaks. The assignments are reasonable, but given the thermodynamic data, must be questioned. No certain conclusion can be reached. This study of a relatively simple coal reaction has led to a pair of conundrums: complex reaction kinetics and production of a functional group whose formation is thermodynamically difficult. Sulfur is useful for the controlled dehydrogenation of coals, but we do not know whether the control is by reactivity or accessibility though the later seems more likely. It is perhaps not surprising that we have uncovered puzzles. The organic reactivity of coals with mild reagents under mild conditions has not been much studied and the material itself remains a poorly understood reaction medium. Acknowledgment. Grateful acknowledgment is made to the donors of the Petroleum Research Foundation administered by the American Chemical Society for partial support of this research. Partial support by the Exxon Education Foundation is also gratefully acknowledged. Supporting Information Available: Supporting Information Available: IR spectra of Pittsburgh No. 8 coal and that coal at 5 different dehydrogenation levels. This material is available free of charge via the Internet at http://pubs.acs.org. EF000175U (34) Stull, D. R.; Westrum, E. F., Jr.; Sinke, G. C. The Chemical Thermodynamics of Organic Compounds; John Wiley and Sons: New York, 1969.
