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Energy & Fuels 1997, 11, 998-1002
Structural Rearrangement of Strained Coals John W. Larsen,*,† Robert A. Flowers, II,‡ and Peter J. Hall§ Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015, and Exxon Research and Engineering Company, Rt. 22 East, Clinton Township, Annandale, New Jersey 08801
Gary Carlson Sandia National Laboratories, Fuel Science Department, Albuquerque, New Mexico 87185-0710 Received January 21, 1997X
Native coals are strained and glassy. When coals are swollen, this strain is relieved as the coal structure rearranges to a lower free energy and more highly noncovalently associated state. Four coals ranging in carbon content from 77% C to 84% C were warmed in the weak swelling solvent chlorobenzene at 132 °C for 2 weeks and samples were withdrawn at intervals. After evaporation of the chlorobenzene, the pyridine extractability of the treated coals had decreased, sometimes by a factor of 2. The pyridine swelling of Pittsburgh No. 8 coal was sharply reduced. The extractability and swelling decreases with time demonstrate that changes in coal structure occurred with the rearranged coal being more associated. This increased association is not due to hydrogen bond formation because pyridine is known to break most if not all of the hydrogen bonds which occur in coals. The rearranged Pittsburgh No. 8 coal was studied by differential scanning calorimetry. Over the 2 week chlorobenzene reflux period, the heat capacity decreased by a factor of 2, demonstrating that the coal rearranged to a more highly associated, more rigid structure. X-ray diffraction studies show enhanced intensity for a regular structural feature occurring at about 20 Å with no other alterations, including the aromatic face to face stacking. The observation that the rearrangement occurs in a day or two in pyridine at room temperature and the absence of a decrease in the radical population argue against increases in covalent bonding as the source of the observed changes. We believe the driving force for the rearrangement is the release of stored elastic strain. Coal swelling provides the macromolecule with the opportunity to undergo conformational rearrangements and to adopt a lower free energy more highly associated structure. The behavior of the high-rank Upper Freeport coal is opposite in direction to the lower rank coals. It apparently rearranges to a less associated structure.
Introduction Data have been published recently demonstrating that native coals are anisotropically strained.1 In three solvents, chlorobenzene, tetrahydrofuran (THF), and pyridine, a set of six coals and lignites swelled approximately twice as much perpendicular to the bedding plane as they did parallel to the bedding plane on their first exposure to these solvents.1 When the solvents were removed, the shape of the coal particles had changed. They were expanded perpendicular to the bedding plane and contracted parallel to the bedding plane. This new shape is their lowest free energy structure. This is shown clearly by the observation that subsequent swellings are isotropic. When fresh solvent is added, the coal returns to its original swollen volume. When the solvent is removed, it returns to the rearranged shape, demonstrating that this is the thermodynamically most * Corresponding author. † Lehigh University and Exxon Research and Engineering Co. ‡ Lehigh University. Current address: Department of Chemistry, The University of Toledo, Toledo, OH 43606. § Exxon Research and Engineering Co. Current address: Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow, UK. X Abstract published in Advance ACS Abstracts, August 1, 1997. (1) Cody, G. D., Jr.; Larsen, J. W.; Siskin, M. Energy Fuels 1988, 2, 340-344.
S0887-0624(97)00014-5 CCC: $14.00
Figure 1. Diagram of coal one-time irreversible swelling.
stable state. This behavior is shown diagrammatically in Figure 1 and the relevant data are contained in Table 1. The data in Table 1 demonstrate that coals as mined are strained. They are compressed perpendicular to the bedding plane and expanded in the bedding plane. There are two possible sources for the strain. The most reasonable is the occurrence of very slow creep over © 1997 American Chemical Society
Structural Rearrangement of Strained Coals
Energy & Fuels, Vol. 11, No. 5, 1997 999
Table 1. Ratio of the Linear Expansion of Coal Thin Sections Perpendicular and Parallel to the Bedding Plane When Swollen by Pyridine at Room Temperature (Data from Ref 1)
coal
first swelling ratio perpendicular/ parallel
second swelling ratio perpendicular/ parallel
Pittsburgh No. 8 Illinois No. 6 Cerrjon Wandoan
1.27 1.21 1.18 1.13
0.95 1.00 0.98 1.03
millions of years due to the immense pressure of the seam overburden. The coal is compressed perpendicular to the bedding plane and forced to flow in a direction parallel to the bedding plane. When the coal is mined, this strain cannot spontaneously be relieved because the coal is a glassy solid and the macromolecular chain segments have very limited freedom of motion.2,3 We prefer this explanation for the origin of the strain in coals. The other possible origin of the strain is that the coal remains glassy and locked in position while it undergoes significant chemical alteration during the coalification process. The most stable configuration for the young lignite is preserved in the old bituminous coal despite the occurrence of many chemical changes. This original conformation is no longer the most stable. The first explanation is the more reasonable because it is difficult to see how this second mechanism would generate an anistropically strained coal. In fact, the extent of anisotropic strain may provide a valuable indicator of the Paleostress to which the seam has been subjected. This topic is currently under investigation. We wished to learn more about the nature of the transformation from strained to unstrained state. If a coal is fully swollen in a solvent, it becomes rubbery.4 The rubbery coal is free to adopt its most favorable conformation in the swollen state. As the solvent is slowly removed, the coal will collapse upon itself and in so doing, if solvent removal is sufficiently slow, adopt its lowest free energy conformation. If a coal is only partly swollen, this relaxation to the lowest free energy state should proceed more slowly because of the smaller free volume available to the coal macromolecular segments and greater coal-coal interactions. In other words, the activation energy for the rearrangement process should decrease with increased swelling and the rearrangement will proceed more rapidly or at a lower temperature. Results and Discussion We chose to monitor the coal rearrangement by heating the coal in excess chlorobenzene. In this work, it will be necessary to compare the starting coal and the rearranged coal. It was necessary to choose a solvent which would not extract much material from the coal and whose complete removal from the coal could be verified. Chlorobenzene is an excellent choice because (1) it is a poor extracting solvent, but an efficient modestly polar, non-hydrogen-bonding swelling solvent, (2) its presence in the coal can be detected by an increase (2) Brenner, D. Proc. Int. Cont. Coal Sci. (Glockauf, Essen) 1981, 163. (3) Lucht, L. M.; Larson, J. M.; Peppas, N. A. Energy Fuels 1987, 1, 56-58. (4) Brenner, D. Fuel 1984, 63, 1324-1328; 1985, 64, 167-173.
Table 2. Pyridine Soxhlet Extraction Yields (wt %) of Various Coals, Pittsburgh No. 8 Coal-Pyridine Solvent Swelling after Heating at 115 °C in Chlorobenzene, and Coal-Pyridine Solvent Swelling after 1 day in Pyridine at Room Temperature
Illinois Pittsburgh PSOCWandoan No. 6 No. 8 1336 % C (daf) starting coal 1 day 2 days 4 days 7 days 8 days 14 days 1 day in pyridine (room temp)
77.3 18 10 10 11 11
79.9 29 19 19 20 21
83.8 44 41 38 32 32
84.1 31 28 23 24 28
12 10
15 23
28 32
22 21
pyridine swelling ratio, Pittsburgh No. 8 2.4 2.0 1.9 1.5
Table 3. Elemental Analyses of Pittsburgh No. 8 Coal Refluxed in Chlorobenzene reflux time (days)
%C
%H
%N
%S
% Cl
% H2O
% ash
0 2 4 8 12
74.92 75.45 74.47 74.71 75.57
4.73 4.83 4.34 4.52 4.43
1.49 1.45 1.45 1.66 1.49
2.57 2.42 2.99 2.64 2.82
0.13 0.31 0.17 0.15 0.29
1.27 1.38 0.59 0.85 0.96
8.49 8.00 8.68 8.55 8.15
in its chlorine content, and (3) it provides a convenient rate of coal rearrangement at a reasonable temperature. Four different coals were placed in chlorobenzene under dry nitrogen and the chlorobenzene was brought to reflux. After times ranging from 1 to 14 days (see Table 2), chlorobenzene was removed from the coal by rotary evaporation followed by room temperature evacuation until the samples reached constant weight. Note that this procedure will not remove anything from the coal that is less volatile than chlorobenzene because the coal and solvent are not separated before the solvent is evaporated. Pyridine extraction yields were then determined by Soxhlet extraction and solvent swellings were measured by the accepted literature procedure.5 Elemental analysis demonstrated that no chlorobenzene remained behind in the coal since the chlorine contents were the same before and after this treatment. The data are contained in Table 3. The scatter in the measured % Cl is similar to the scatter in the data for other elements and the variation in % Cl may be experimental error. Assuming that it is not, the maximum chlorobenzene uptake (for the 2 day reflux samples) is 10 mg of chlorobenzene in 100 g of coal (0.01 wt %). The variations in coal properties do not track the changes in % Cl. Retained chlorobenzene is not responsible for the observed changes in coal properties. Only the data for Pittsburgh No. 8 coal will be discussed in detail. The other coals behave in a generally similar way though the trends are not as regular as they are with Pittsburgh No. 8 coal. As the reflux time in chlorobenzene increases, the swelling and extractability of the coal in pyridine decrease. Decreased swelling indicates an increase in the cross-link density.5 The cross-links could be either covalent or noncovalent associations unbroken by pyridine. Decreased extractability could be due to increased binding of the extractables to the insoluble network or greatly reduced diffusion rates through the network. Both of these physical changes indicate increased association. This increase in association (effective cross-link density) cannot be due to the formation of new hydrogen bonds
1000 Energy & Fuels, Vol. 11, No. 5, 1997
since pyridine is thought to break essentially all of the coal-coal hydrogen bonds.5,6 We do not believe it is reasonable to assign this decreased swelling to an increase in the covalent crosslink density. That requires formation of new covalent bonds. The radicals in coal are known to be quite unreactive7 and ESR spectroscopy does not reveal any significant loss in radical concentration. Specifically, the spin density of Pittsburgh No. 8 coal increased from 4.5 × 1017 spins/g after 2 days refluxing to 9.3 × 1017 spins/g after 12 days refluxing.8 The functional groups contained in structures such as those proposed by Shinn and Nomura are not expected to undergo any chemical reaction in refluxing chlorobenzene.9,10 For the lower rank coals, it is conceivable that carboxylic acid groups could react with phenolic hydroxyls to form esters, but this process cannot explain the trends observed with Pittsburgh No. 8 and PSOC-1336 coals which contain no carboxylic acid groups. The best explanation for the swelling decrease is a rearrangement of the coal to a more highly noncovalently associated structure. These interactions must be sufficiently strong that they are not broken by pyridine. Nevertheless, convalent bond formation has not been disproven; it is the least reasonable of the two alternatives. The pyridine extraction data shown in Table 2 parallels the solvent swelling data. The longer the coals are treated in chlorobenzene, the less material is extracted from them by pyridine. The explanation offered for this is the same as that for the swelling reduction. The coals are undergoing rearrangement to a more highly noncovalently associated state. This leads to a diminution in the extract yield by two different mechanisms. It reduces the amount of extract because many of the formerly extracted molecules are now interacting more strongly with the macromolecular network, too strongly to be removed by pyridine. In addition, as the network becomes more associated and swells less, mass transport of large molecules through the network will be increasingly hindered and sometimes made impossible. This also reduces extractability. There is abundant evidence that there can exist in coals noncovalent interactions that are not broken by pyridine. In our laboratories, we have observed precipitation of solid material from and increases in the molecular weight of coal extracts that were very carefully stored in pyridine in the dark under inert gas. Clearly, some association occurs. Nishioka has published several lines of evidence demonstrating that highrank coals contain noncovalent interactions which are not disrupted evey by refluxing pyridine.11 All of the coals in Table 2 behave similarly to Pittsburgh No. 8 and we propose that the same phenomenon is occurring in each case. The magnitude of (5) Larsen, J. W.; Green, T. K.; Kovac, J. J. Org. Chem. 1985, 50, 4729-4735. (6) (a) Larsen, J. W.; Baskar, A. J. Energy Fuels 1987, 2, 232-233. (b) Larsen, J. W.; Mohammadi, M. Energy Fuels 1990, 4, 107-110. (7) Flowers, R. A., II; Gebhard, L. A.; Larsen, J. W.; Silbernagel, B. G. Energy Fuels 1989, 3, 762-764. (8) We are grateful to Dr. Bernie Silbernagel and Layce Gebhard of Exxon Research and Engineering Co. for these data. (9) Shinn, J. H. Fuel 1984, 63, 1187-1196. (10) Nomura, M.; Matsubayashi, K.; Ida, T.; Murata, S. Fuel Proc. Technol. 1992, 31, 169. (11) Nishioka, M.; Larsen, J. W. Energy Fuels 1990, 4, 100-106. Bendale, P. G.; Zeli, R. A.; Nishioka, M. Fuel 1994, 73, 251-255 and references therein. (12) Sakurovs, R.; Lynch, L. J.; Maher, T. P.; Banerjee, R. N. Energy Fuels 1987, 1, 167-172.
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Figure 2. Effect of refluxing in chlorobenzene on the heat capacity of Pittsburgh No. 8 coal.
the change in extractability differs from coal to coal. The amount of deformation is expected to be a function of the pressure exerted by the overburden, the temperature of the coal, and the length of time under pressure. The extractability decrease is expected to be least in those coals which have been subject to the lowest amount of deformation, that is, those which had the lowest overburden pressure and/or lowest temperatures for the shortest time. We have already noted that this structural relaxation rearrangement alters the shape of coal particles.1 It is expected to alter other physical properties as well and we elected to probe the heat capacity of the coal in a set of differential scanning calorimetric (DSC) measurements. These measurements on Pittsburgh No. 8 coal are shown in Figure 2. The outstanding feature of these data is that the heat capacity of the coal is reduced by about a factor of 2 by refluxing in chlorobenzene. A lower heat capacity is diagnostic of a less flexible, more rigid structure which does not have as much freedom to “wiggle” as it is warmed. These data show that the rearranged state of the coal is more rigid and more highly associated though they reveal nothing about the nature of that association. The phase change occurring at about 360 K is interesting. Changes in the degree of association of coals, whether covalent or noncovalent, will cause changes in the temperature of a phase transition. If this is a glass to rubber transition and we have decreased the free energy of the glassy state, then the glass to rubber transition temperature should shift. Because the temperature does not shift, the observed heat capacity change is not a glass to rubber transition. We do not have structure data that allow us to assign the origin of this transition. Because we are interested in the changes in aromaticaromatic interactions as this rearrangement proceeds, we chose to look at some of these coal samples using X-ray diffraction. We were particularly interested in seeing whether the X-ray diffraction provided evidence for increased face-to-face arrangements of aromatic rings. The relevant spectra are shown in Figure 3. There is no shift nor is there any change in intensity of the 002 peak assigned to face-to-face aromatic-aromatic interactions. The rearrangement does not affect it. The principal change in the X-ray diffraction is the increased intensity of a peak which corresponds to a regular structure of about 20 Å in size. This 20 Å feature has been noted before and has not been assigned.13 We cannot assign it. It may play a significant role in (13) Van Krevelen, D. W. E. Coal, 3rd ed.; Elsevier: New York, 1993, pp 233-235.
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Energy & Fuels, Vol. 11, No. 5, 1997 1001
Figure 3. Effect of refluxing in chlorobenzene on X-ray scattering from Pittsburgh No. 8 coal.
associative interactions in coals. It is not far from the distance across a substituted phenanthrene system. So far, we have discussed only the behavior of coals swollen and warmed in chlorobenzene. Table 2 also contains data gathered by swelling the coal in pyridine at room temperature and then removing the pyridine by evaporation in a manner identical to that used with chlorobenzene. That is, the extractable material was left behind in the coal and not removed. It is stunning that relatively brief room temperature treatment with pyridine brings the coals to a state very similar to that resulting from a 2 week treatment in warm chlorobenzene. Swelling in pyridine is much greater than in chlorobenzene and in pyridine the coals become rubbery at room temperature. When fully swollen in pyridine, even at room temperature the coal rearrangement proceeds relatively quickly. When exposed to any swelling solvent, coals immediately begin to rearrange. The rate of that rearrangement will depend on the extent to which the coals are swollen and the temperature of the system. Greater swelling and higher temperature lead to greater rearrangement rates. It is obvious from the data shown that these rearrangements have very significant effects on the coal structure: reducing swelling, reducing extractability, reducing heat capacity, and altering the structure as revealed by X-ray diffraction. We are not the only workers who have observed coal rearrangements induced by solvent penetration of coals. The first paper proposing the existence of this rearrangement was published by Hsieh and Duda in 1987.14 They studied the absorption of toluene vapors by a coal and showed that the absorption process irreversibly altered the coal. Recently, Suuberg has studied the effects of heating and solvent (pyridine/CS2) treatment on the room temperature solvent swelling and elastic properties of Upper Freeport and Pocahontas coals.15,16 Both undergo significant changes to a more relaxed less associated structure. We will present our own results on Upper Freeport coal shortly. It is clear from these data that these high-rank coals rearrange whether swollen or heated, but that the results of the rearrangement is a less associated structure, opposite from that which we observe with lower rank coals. Nishioka has published a long series of papers in which he has demonstrated that there exist in high-rank coals disruptable noncovalent interactions, although his extension of this observation to the assertion that noncovalent interac(14) Hsieh, S. T.; Duda, J. L. Fuel 1987, 66, 170-178. (15) Yun, Y.; Suuberg, E. M. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1992, 37(2), 856-865. (16) Yun, Y.; Suuberg, E. M. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38(2), 489-494.
Figure 4. Effect of refluxing in chlorobenzene on the heat capacity of Upper Freeport coal.
tions are the principal associative interaction in coals is fatally flawed because there have not yet been any measurements which differentiate covalent from noncovalent interactions in these coals.11 The behavior of Upper Freeport coal is so different from the others that it deserves separate discussion. Perhaps the best view of the differences comes from a comparison of the effect of refluxing in chlorobenzene on the heat capacities of Pittsburgh No. 8 coal (Figure 2) and Upper Freeport coal (Figure 4). Where the heat capacity of Pittsburgh coal decreases continuously, Upper Freeport at first (8 days) decreases and then increases significantly. Refluxing in chlorobenzene appears to induce a structural relaxation, a rearrangement to a less rigid less associated structure. Pyridine extraction and drying also changes the heat capacity of coals as shown by the three sets of data in Figure 5. The heat capacities of Illinois No. 6 coal and Pittsburgh No. 8 coal are decreased while that of Upper Freeport increases. These independent experiments confirm the observation reported in Figure 4. If our interpretation of these data is correct, then refluxing Upper Freeport coal in chlorobenzene should increase pyridine extractability and swelling. The data are contained in Table 4. There is a clear increase in extractability over the 12 day reflux time. It is tempting to associate the extractability decrease occurring after 2 days with the heat capacity decreases observed after 4 days. Both indicate a “tighter” structure is a transient intermediate which passes into a “looser” less highly associated structure. The swelling data (Table 4) are scattered and the changes are small. We are not willing to draw any conclusions from them. Finally, the X-ray scattering changes (Figure 6) are opposite to those observed with Pittsburgh No. 8 coal. The interaction distances increase and the structure becomes looser after refluxing in chlorobenzene. Refluxing has no effect on the unpaired electron density. It is 6.1 × 1018 spins/g after 2 days and 6.4 × 1018 spins/g after 12 days.8 Yun and Suuberg15 reported that heating Upper Freeport coal to 350 °C under N2 increased the pyridine swelling ratio from 1.14 to 2.17. The swelling ratio in THF increased from 1.08 to 1.82. They also report solvent swelling relaxes the structure of this coal. Mechanical property measurements have revealed an irreversible transition above 200 °C.16 Solvent swelling of coals induces structural rearrangements. Surprisingly, coals of different rank respond in different directions. Upper Freeport coal
1002 Energy & Fuels, Vol. 11, No. 5, 1997
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Figure 6. Effect of refluxing in chlorobenzene on the X-ray scattering for Upper Freeport coal.
terization of these rearrangements and its effects on coal properties, behaviors, and reactivities. Experimental Section
Figure 5. Effect of pyridine extraction and drying on the heat capacities of (a, top) Illinois No. 6 coal, (b, middle) Pittsburgh No. 8 coal, and (c, bottom) Upper Freeport coal. Table 4. Pyridine Soxhlet Extraction Yields (wt %) and Swelling Ratio for Upper Freeport Coal Refluxed in Chlorobenzene at 115 °C time (days)
extraction yield
swelling ratio
0 2 4 8 12
32 25 36 38 39
1.2 1.1 1.3 1.5 1.2
(88.1% C dmmf) rearranges to a looser, less highly associated structure. The lower rank coals studied all rearrange in the opposite sense, to a more highly associated “tighter” structure. For the lower rank coals, the driving force is the release of strain and greater associative interactions. With Upper Freeport, we can only speculate that strain release and entropy changes are the driving forces. The structure changes are large enough to promise significant effects on coal chemistry. We hope to continue our investigation into the charac-
All coal samples were dried at room temperature under vacuum until they reached constant weight. Chlorobenzene was obtained from Aldrich and distilled over MgSO4. All solvents were purged with dry nitrogen via a gas dispersion tube and sealed with a septum until use. A typical reflux experiment involved weighing 10 g of coal into a tared 250 mL round bottom in a vacuum atmosphere drybox. The round bottom was sealed with a septum and removed from the drybox, and 150 mL of chlorobenzene was transferred to the flask via a canula. A reflux condenser fitted with a nitrogen line (vented to a bubbler) was attached to the round bottom containing the coal benzene mixture. The mixture was refluxed for the appropriate time (1-14 days) and cooled, and the chlorobenzene was removed via rotary evaporation. After chlorobenzene evaporation, the round bottom was back-filled with dry nitrogen, sealed, and placed in a vacuum oven at room temperature. The sample was weighed periodically until constant weight was reached. Constant weight was always obtained within 1 week. The samples were then analyzed for chlorine content by elemental analysis (Galbraith Laboratories) to assure that no chlorobenzene was contained in the treated coal sample. Differential scanning calorimetry was performed on Mettler DSC 30 equipment. Temperature calibration was by the melting points of indium, lead, and zinc standards. Temperatures are accurate to (0.5 K. Enthalpy calibration was by integrating the melting endotherm of an indium standard supplied by Mettler. It was estimated that enthalpies were accurate to (0.05 J/g. Standard aluminum pans were used with two pin holes to allow evaporation of water. The sample size was 10 mg. The coal was spread in a monolayer over the base of the aluminum pans to maximize heat transfer to the coal. DSC was performed at 10 K/min with a nitrogen carrier. In DSC investigations of coal, the sensor design is important and a Mettler glass sensor was used. The wide-angle X-ray scattering was performed on a Rigaku diffractometer using a copper target. The sample thickness was 2 mm. The powdered coal samples were placed between two sheets of Kapton film.
Acknowledgment. We are grateful to the U.S. Department of Energy for partial support of this work and to Exxon Research and Engineering Co. for partial support and for permission to publish this work. EF970014Z
