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Energy & Fuels 2002, 16, 6-11
The Nature of the Aggregated Structure of Upper Freeport Coal Toshimasa Takanohashi,* Hiroyuki Kawashima, and Takahiro Yoshida Institute for Energy Utilization, National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8569, Japan
Masashi Iino Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan Received June 14, 2001
Data for the sorption and interaction of alcohols with the coal suggested Upper Freeport coal has a large number of micropores into which relatively bulky reagents could diffuse only marginally; micropores may be molecular voids in the aggregated. The irreversible structural relaxation observed on differential scanning calorimeter (DSC) thermograms at 350 °C is probably very similar in nature to that caused by extraction with highly efficient mixed solvents such as carbon disulfide/N-methyl pyrrolidinone (CS2/NMP). Upper Freeport coal must be greatly relaxed (swollen) before solvents have access. A molecular dynamics simulation of relaxation at several temperatures was carried out on a model structure for the aggregated coal. A large change in the volume of the model structure was found between 350 °C and 400 °C, the region in which the irreversible peak was observed in the DSC thermograms. Moreover, in agreement with the experimental results, the structural changes caused by “heating” in the simulation were not reversible. These results suggested that irreversible structural relaxation caused by heating is the result of thermal stabilization of the strained structure of the raw coal.
Introduction At room temperature, Upper Freeport coal, an Argonne Premium sample (APCS-1), gives a very high extraction yield (60 wt %, daf) with a carbon disulfide/ N-methyl-2-pyrrolidinone (CS2/NMP, 1:1 v:v) mixed solvent.1-3 The extraction yield is increased by addition of small amounts of compounds such as tetracyanoethylene (TCNE) and p-phenylenediamine.4-7 On the basis of these results, we have suggested that Upper Freeport coal does not have an extensive covalently cross-linked network but consists instead of associations of coal molecules that are solublized without breaking covalent bonds.5,6,8 The precise nature of the aggregated structure and aggregation/disaggregation dynamics, which affect process operations such as reaction and transportation, are not well understood. In differential scanning calorimetry (DSC) measurements on Upper Freeport coal, Yun and Suuberg found * Author to whom correspondence should be addressed. (1) Iino, M.; Takanohashi, T.; Ohsuga, H.; Toda, K. Fuel 1988, 67, 1639. (2) Iino, M.; Takanohashi, T.; Obara, S.; Tsueta, H.; Sanokawa, Y. Fuel 1989, 68, 1588. (3) Takanohashi, T.; Iino, M. Energy Fuels 1990, 4, 452. (4) Sanokawa, Y.; Takanohashi, T.; Iino, M. Fuel 1990, 69, 1577. (5) Ishizuka, T.; Takanohashi, T.; Ito, O.; Iino, M. Fuel 1993, 72, 579. (6) Liu, H.-T.; Ishizuka, T.; Takanohashi, T.; Iino, M. Energy Fuels 1993, 7, 1108. (7) Dyrkacz, G. R.; Bloomquist, C. A. A. Energy Fuels 2000, 14, 513. (8) Takanohashi, T.; Iino, M.; Nishioka, M. Energy Fuels 1995, 9, 788.
an endothermic peak around 350 °C.9 Otake and Suuberg also reported10 that preheating coals to 350 °C before solvent treatment increased the rate of swelling. They suggested that the DSC peak is caused by structural relaxation of coal and that thermally dissociable coal-coal interactions may be responsible in part for the rigidity of the original sample. These results indicate that the aggregated structure of Upper Freeport coal can be relaxed by mild heat treatment; at such temperature that decomposition reactions little take place. In a previous part of the present paper, the aggregated-structure of Upper Freeport is reviewed on the basis of recent published results, and in the latter the results of molecular dynamics (MD) simulation are described; we carried out MD simulations to investigate whether relaxation of the aggregated structure occurs, and what structural changes take place during relaxation. The results of simulation are compared to those of DSC experiments. Aggregated Structure of Upper Freeport Coal Diffusion of Solvents into Aggregated Structure. We have probed the structure of Upper Freeport raw coal using three experimental techniques: sorption of alcohol vapors, interaction between the coal and solvents using inverse liquid chromatography (ILC), and solvent swelling. (9) Yun, Y.; Suuberg, E. M. Fuel 1993, 72, 1245. (10) Otake, Y.; Suuberg, E. M. Energy Fuels 1997, 11, 1155.
10.1021/ef0101255 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/03/2001
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Figure 1. Sorption isotherm of various alcohols at 30 °C.13
Alcohol Sorption. Isotherms for absorption of methanol, ethanol, 1-propanol, and 1-butanol by Upper Freeport coal are shown in Figure 1.13 While the sorptions for methanol and ethanol varied as expected based on the bulk of the alkyl group, the sorption behavior for 1-propanol and 1-butanol was small and practically identical over the entire range of relative pressures. The difference in adsorption for the alcohols suggests that the size of the pores in Upper Freeport coal is relatively small, and that even modestly large groups such as 1-propyl and 1-butyl are sterically hindered from entering the pores. Interactions between the Coal and Various Organic Solvents. These studies were conducted using an ILC technique in which the coal was used as the stationary phase. All solvents, including the good solvents pyridine, NMP, and tri-aromatics, gave relatively low capacity factors (the increment of the elution volume of the probe relative to the elution volume of the carrier solvent) and some gave negative values, indicating that the interaction between the solvents and Upper Freeport coal is small compared to that of lower-rank coals such as Illinois No. 6 and Beulah-Zap coals. Capacity factors for straight-chain alcohols with increasing carbon number are shown in Figure 2.14 For high-rank Upper Freeport and Pocahontas No. 3 coals, the capacity factors greatly decreased for one-three carbons, after which there was essentially no change. The alcohols with more than three-carbon alkyl groups hardly penetrate coal bulk. These results suggest that the diameters of the micropores of Upper Freeport and Pocahontas No. 3 coals are in the range of 4-7 Å. Such fine micropores maybe are voids between the molecules of the aggregated structure. Solvent Swelling. The swelling ratios of Upper Freeport raw coal are relatively low in ordinary solvents such as pyridine, THF, methanol, and benzene. Larsen et al.15 proposed that pores in Argonne premium coals are (11) Mayo, S. L.; Olafson, B. D.; Goddard, W. A., III. J. Phys. Chem. 1990, 94, 8897. (12) Takanohashi, T.; Kawashima, H. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 2000, 45, 238. (13) Takanohashi, T.; Terao, Y.; Yoshida, T.; Iino, M. Energy Fuels 2000, 14, 915. (14) Takanohashi, T.; Nakano, K.; Yamada, O.; Kaiho, M.; Ishizuka, A.; Mashimo, K. Energy Fuels 2000, 14, 720. (15) Larsen, J. W.; Hall, P.; Wernett, P. C. Energy Fuels 1995, 9, 324.
Figure 2. Capacity factors of straight-chain alcohols against the number of alkyl groups on the alcohols.14 Toluene was used as carrier solvent. Table 1. Ultimate and Proximate Analyses of Coals ultimate analysis, wt % (daf) coal
C
H
N
Pocahontas No.3 Upper Freeport Piitsburgh No.8 Illinois No.6 Beulah-Zap
89.7 86.2 82.6 76.9 71.6
4.5 5.1 5.5 5.5 4.8
1.1 1.9 2.1 1.9 1.0
a
S
Oa
0.7 4.0 2.2 4.6 2.4 7.4 5.6 10.1 0.9 21.7
proximate analysis, wt % (db) VM
ash
FC
17.6 28.2 38.3 38.6 42.2
4.8 13.1 8.7 15.0 9.6
77.6 58.7 53.0 46.4 48.2
By difference.
Figure 3. Plots of the extraction yield and swelling ratio with the CS2/NMP mixed solvent against NMP vol % in the mixed solvent.16
isolated and can be reached only by diffusion through the solid. Our data for high-rank coals are in agreement with this model. Table 1 presents ultimate and proximate analyses of the coals mentioned above. We reported that its extract fractions and residue obtained from the CS2/NMP extraction showed higher sorption of alcohols,13 stronger interaction between the fractions and solvents, and higher swelling ratios8 than those for the raw coal. These results suggest that significant structural changes such as cross-linking and porosity changes occurred by the CS2/NMP extraction,
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Figure 4. DSC thermograms of the extraction residues with different extraction yields: (s) first scan, (- - -) second scan, (- - -) third scan. Extraction yield: (a) 18%, (b) 43%, (c) 62%, (d) 66%, (e) 50%, (f) 35%, (g) 20%, and (h) 3%.16
resulting in high diffusibility into the extract fractions and residue. The details of structural changes are described below. Relaxation of the Aggregated Structure. Data for the effect of solvent or heat on extraction yield, solvent swelling, and DSC thermograms have been reported. Extraction Yield. The extraction yields of Upper Freeport coal are plotted against CS2 vol % in the mixed CS2/NMP solvent in Figure 3.16 Yields with NMP or CS2 alone were 18% and 3%, respectively, while the mixed solvent gave higher extraction yields and showed a significant synergistic effect, as shown in Figure 3. The effect of solvent composition on the swelling ratio is also shown in Figure 3. The same trends were found for both procedures. We performed DSC on residues obtained from CS2/ NMP mixed-solvent extractions of Upper Freeport coal; thermograms of the residues are shown in Figure 4. Residues from extractions that gave relatively low yields of 18 (a), 20 (g), and 3 (h) wt % (daf), respectively, had a peak at 350 °C similar to that found in the raw coal.16 In contrast, the peak at 350 °C is not present in residues
from samples with high extraction yields of 44 (b), 63 (c), 67 (d), 51 (e), and 35 (f) wt % (daf), respectively. These results suggest that solvent extraction relaxes the aggregated structure of Upper Freeport coal at room temperature in a manner analogous to relaxation caused by heat treatment at 350 °C. Therefore, in the case of low extraction yield residues (a, g, h), there is limited relaxation of the aggregated structure, which produces low extraction yields. This is consistent with the observation that pyridine-Soxhlet-extracted coal also showed the endothermic peak.9 We also found17 that coals preheated in the range 375-400 °C before extraction, where softening starts for caking coals, gave higher extraction yields with the CS2/NMP (1:1) mixed solvent than obtained at room temperature; for Upper Freeport coal the extraction yield with CS2/NMP mixed solvent at room temperature after heat treatment at 380 °C was 78% compared to 60% without the heat treatment. The softening temperature for Upper Freeport coal is 373 °C, which shows that the coal begins to soften after the appearance of the exothermic peak in the DSC thermogram.
(16) Takanohashi, T.; Terao, Y.; Iino, M.; Yun, Y.; Suuberg, E. M. Energy Fuels 1999, 13, 506.
(17) Takanohashi, T.; Yoshida, T.; Iino, M.; Katoh, K.; Fukada, K. Energy Fuels 1998, 12, 913.
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Figure 5. Model structure consisting of seven different molecules.
Characterization of extracts and residues suggests that no significant reactions occur between the components of the mixed solvents and the coal.1 Therefore, as seen in Figures 3 and 4, solvent compositions that gave high swelling ratios also gave high extraction yields and caused the endothermic peak to disappear. This means that the relaxation of the aggregated structure caused by the dissolution of noncovalent bonds is most probably responsible for the peak. We have suggested5,6,8 that the high extraction yields of Upper Freeport coal are attributed to dissociation of π-π interactions and chargetransfer interactions and not to the breaking of covalent bonds. A better solvent such as CS2/NMP mixed solvent may more efficiently relax the aggregated structure of coal18 and dissolve extractable substances. Therefore, diffusion of solvent into Upper Freeport coal requires that the coal is swollen significantly, which in all likelihood also relaxes the aggregated structure. Model for an Aggregated Coal Structure. A model structure for aggregated Upper Freeport coal, shown in Figure 5, is composed of seven molecules with a continuous molecular weight distribution from light to heavy fractions.12 It is noted that this structure is significantly different from the widely accepted “twophase model” structure19,20 that consists of a small amount of low-molecular-weight components trapped in a covalently bound cross-linked network. The aggregated model structure supports the “monophase concept” proposed by Nishioka and Gorbaty.21 Figure 6a 12 shows the minimum energy conformation for model molecules in the 53.6 Å × 55.8 Å × 4.2 Å unit cell. An anisotropic associated structure was obtained. Figure 6b12 shows two cells enclosing the model structure at minimum energy. Aromatic rings seem to interact with one another perpendicular to the bedding plane. Cody et al.22 have reported anisotropic swelling behavior for bituminous coals: the swelling ratio was (18) Suuberg, E. M.; Otake, Y.; Langner, M. J.; Leung, K. T.; Milosavljevic, I. Energy Fuels 1994, 8, 1247. (19) Given, P. H.; Marzec, A.; Barton, W. A.; Lynch, L. J.; Gerstein, B. C. Fuel 1986, 65, 155. (20) Derbyshire, F.; Marzec, A.; Schulten, H.-R.; Wilson, M. A.; Davis, A.; Tekely, P.; Delpuech, J.-J.; Jurkiewicz, A.; Bronnimann, C. E.; Wind, R. A.; Maciel, G. E.; Narayan, R.; Bartle, K.; Snape, C. Fuel 1989, 68, 1091. (21) Nishioka, M.; Gorbaty, M. L. Energy Fuels 1990, 4, 70.
Figure 6. Molecular model for Upper Freeport coal in a basic cell (a) and two cells enclosing the model in the energyminimum state (b).
greater perpendicular to the bedding plane than parallel to it. When a powerful solvent such as the CS2/NMP mixed solvent is used for extraction of Upper Freeport coal,2,3 it can relax the strained structure by breaking aromaticaromatic interactions and solvating the molecules released from the gross structure, resulting in the higher extraction yields. While the extraction yield with pyridine at room temperature is only 2.8 wt % (daf), the amount of pyridine-soluble material obtained from fractionation of a whole CS2/NMP extract is 29 wt % (daf),1 which indicates that a considerable amount of solvent-soluble component remains in the raw coal even when pyridine is used for extraction, consistent with an unrelaxed aggregated structure among coal components. Molecular Dynamics Simulation MD simulation techniques were used to investigate structural changes during relaxation by heat and aggregation/disaggregation dynamics. Cerius2 software package (version 4.0, Molecular Simulation Inc.) was run on an OCTANE graphic work station (Silicon Graphics Inc.). The DREIDING 2.02 method was used for the force field calculations.11 The model of aggregated structure shown in Figure 6 was used. The MD calculation was conducted under periodic boundary condition at 100-450 °C. The type of MD was “Constant NPT”, i.e., under constantpressure and -temperature condition. The charges were also updated every 0.01 ps during MD. With the MD calculation, the size of cell and the potential energy are changeable. The changes in the average volume of cell (22) Cody, J. G. D.; Larsen, J. W.; Siskin, M. Energy Fuels 1988, 2, 340.
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Figure 7. Simulation procedure to investigate the reversibility of structural relaxation.
Figure 9. The conformations before (a) and after (b) the structural relaxation at 400 °C.
Figure 8. Change in the average volume cell at 100-450 °C by MD calculation.
including the structure and the total energy during the MD calculation were studied at each temperature. Reversibility of the structural change by heat was investigated by the simulation procedure as shown in Figure 7. After the MD calculation at 400 °C, subsequent MD calculation at 100 °C was again carried out for the structure relaxed at 400 °C. Total energies in the system before and after the MD at 400 °C were compared. Structural Relaxation Figure 8 shows changes in the average volume of the unit cell during MD calculation at each temperature. A significant change in structure occurred in the range of 350-400 °C, in agreement with the DSC peak at 350 °C found by Yun and Suuberg.9 The peak was also reported to disappear on the second and third scans; structural relaxation was irreversible. To investigate the reversibility of structural changes, the structure obtained at 400 °C was recalculated at 100 °C. The structures before (a) and after (b) relaxation at 400 °C are shown in Figure 9. The significant structural change (relaxation) was found at 400 °C and not restored at 100 °C. In addition, the potential energies of the aggregated structures at 100 °C with and without structural relaxation at 400 °C were quite different: 1229 and 1255 kcal/mol, respectively. These results show that structural relaxation of the raw coal is
irreversible, and strongly suggests that the irreversible structural change found in the MD procedure corresponds to the irreversible DSC peak observed at 350 °C. While, for the extract fractions and residue obtained from the CS2/NMP extraction such irreversible changes were not observed by either the MD procedure or the DSC measurement, indicating that such irreversible behavior is a characteristic of the raw coal. The extract fractions and residue have the most stable conformation, since the relaxation has already occurred during the extraction. As described above, the raw coal has an anisotropic structure and higher strain energy. Larsen reported23 that the driving force for the rearrangement (relaxation) is the release of stored elastic strain. Thus, release of strain in Upper Freeport coal at 400 °C decreases the potential energy to a structure that is more stable. Conclusion Upper Freeport raw coal has a large number of micropores (
