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Computer Simulation of Solvent Swelling of Coal Molecules: Effect of Different Solvents Toshimasa Takanohashi* National Institute for Resources and Environment, Tsukuba 305-8569, Japan
Kazuo Nakamura Fundamental Research Laboratories, Osaka Gas Co., Ltd., Osaka 554-0051, Japan
Yuki Terao and Masashi Iino Institute for Chemical Reaction Science, Tohoku University, Sendai 980-8577, Japan Received July 7, 1999. Revised Manuscript Received November 2, 1999
We used a molecular mechanics/molecular dynamics computational method to simulate solvent swelling of the pyridine-insoluble (PI) fraction obtained from extraction of Upper Freeport bituminous coal, an Argonne premium coal sample. The effects of benzene and cyclohexane on swelling were examined and compared with previous results for methanol. A model structure for the PI fraction was placed in a periodic boundary cell. As solvent molecules were introduced into the cell, the potential energy of the PI-solvent system decreased and the volume of the cell increased up to the limiting number of solvent moleculess9 for benzene and 2 for cyclohexanes that could be added to produce stable structures. The contribution of the electrostatic interaction to the decrease in the total energy of the PI-solvent system was larger than those of the hydrogen bond and van der Waals interactions, regardless of the solvent used. Swelling ratios estimated from the ratio of the weight increase with the number of the solvent molecules introduced were in good agreement with the ratio determined experimentally from sorption data, and were much lower than ratios obtained from volumetric swelling measurements.
Introduction Computer-aided molecular design has been widely used to construct model structures for complex fossil fuels.1-8 Recently, simulation techniques were used in attempts to elucidate coal structure-property relations.7,9-14 Nakamura and co-workers have estimated * Corresponding author. Telephone: +81 (0)298 58 8441. Fax: +81 (0)298 58 8408. E-mail:
[email protected] (T. Takanohashi). (1) Carlson, G. A. Energy Fuels 1992, 6, 771. (2) Nomura, M.; Matsubayashi, K.; Ida, T.; Murata, S. Fuel Proc. Technol. 1992, 31, 169. (3) Faulon, J. L.; Vandenbroucke, M.; Drappier, J. M.; Behar, F.; Romero, M. Adv. Org. Geochem. 1990, 16, 981. (4) Faulon, J. L.; Hatcher, P. G.; Wenzel, K. A. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1992, 37, 900. (5) Hatcher, P. G.; Faulon, J.-L.; Wenzel, K. A.; Cody, G. D. Energy Fuels 1992, 6, 813. (6) Nakamura, K.; Takanohashi, T.; Iino, M.; Kumagai, H.; Satou, M.; Yokoyama, S.; Sanada, Y. Energy Fuels 1995, 9, 1003. (7) Takanohashi, T.; Iino, M.; Nakamura, K. Energy Fuels 1994, 8, 395. (8) Murgich, J.; M., J. R.; Aray, Y. Energy Fuels 1996, 10, 68. (9) Nakamura, K.; Murata, S.; Nomura, M. Energy Fuels 1993, 7, 347. (10) Murata, S.; Nomura, M.; Nakamura, K. Energy Fuels 1993, 7, 469. (11) Dong, T.-L.; Murata, S.; Miura, M.; Nomura, M.; Nakamura, K. Energy Fuels 1993, 7, 1123. (12) Takanohashi, T.; Iino, M.; Nakamura, K. Energy Fuels 1998, 12, 1168. (13) Takanohashi, T.; Nakamura, K.; Iino, M. Energy Fuels 1999, 13, 922. (14) Kumagai, H.; Norinaga, K.; Hayashi, J.-i.; Chiba, T. Inter. Symp. Adv. Energy Technol. 1998, 61.
the physical density of coal model structures.9-11 We reported that the most stable energy-minimized conformations of model coal extract constituents associated through noncovalent interactions.7 The interaction of coal associates with several solvents was simulated; pyridine disrupted the interaction in coal associates, but benzene or methanol had no effect even though both interacted with the associates.12 These results suggest that hydrogen bonds and aromatic-aromatic interactions form strongly associated structures, and that if one interaction is disrupted, the remaining interactions can hold the associated structure together.12 Strong interactions such as hydrogen bonds, π-π and electrostatic interactions contribute to the network structure of coals.15-19 Solvents cause coal networks to swell. The swelling behavior of coals is not fully understood, especially the effects of solvent on the cross-linked structure of coals. Recently we proposed a new simulation method to evaluate solvent swelling of the solvent-soluble constitu(15) Larsen, J. W.; Green, T. K.; Kovac, J. J. Org. Chem. 1985, 50, 4729. (16) Nishioka, M.; Larsen, J. W. Energy Fuels 1990, 4, 100. (17) Cody, G. D.; Davis, A.; Hatcher, P. G. Energy Fuels 1993, 7, 463. (18) Iino, M.; Takanohashi, T.; Ohkawa, T.; Yanagida, T. Fuel 1991, 70, 1236. (19) Suuberg, E. M.; Otake, Y.; Langner, M. J.; Leung, K. T.; Milosavljevic, I. Energy Fuels 1994, 8, 1247.
10.1021/ef990147f CCC: $19.00 © 2000 American Chemical Society Published on Web 01/21/2000
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Figure 1. A model structure of the PI fraction constructed on the basis of its ultimate analysis and structural parameters. Table 1. Ultimate Analysis and Structural Parameters for Upper Freeport Raw Coal and Its PI Fraction raw coal PI, observed PI, model
C%
H%
N%
S%
O%
OH%
Mna
fab
Har/Carc
σd
H/C
86.2 85.8 85.9
5.1 5.0 5.0
1.9 2.1 1.9
2.2 1.1 1.4
4.6 6.0 5.8
2.0 3.6
2210 2208
0.79 0.80
0.71 0.70
0.48 0.41
0.71 0.70 0.70
a M , average molecular weight. b f , aromaticity. c H /C , degree of condensation of aromatic rings. n a ar ar rings.
ent from Upper Freeport bituminous coal, and reported13 that the swelling ratio in methanol estimated by the method was consistent with the value observed from a methanol vapor sorption measurement. The simulation showed that electrostatic interactions between methanol and coal were much more important than hydrogen bond formation and van der Waals interactions. It has been reported that nonpolar solvents such as benzene and cyclohexane cause the solvent-soluble constituent to swell.20,21 In the present study, the effects of benzene and cyclohexane on the swelling of the pyridine-insoluble fraction of Upper Freeport coal were simulated. Swelling ratios were estimated from the data generated by the simulation and compared with experimentally determined values. Experimental Section Sorption Isotherm. The procedure for Upper Freeport bituminous coal22 has been reported.23 The coal sample was (20) Fujiwara, M.; Ohsuga, H.; Takanohashi, T.; Iino, M. Energy Fuels 1992, 6, 859. (21) Takanohashi, T.; Iino, M.; Nishioka, M. Energy Fuels 1995, 9, 788.
d
σ, degree of substitution of aromatic
extracted at room temperature with a carbon disulfide/Nmethyl-2-pyrrolidinone (1:1 v/v) mixed solvent system. The extraction yield was 59.4 wt % (daf). The initial extract was further fractionated with acetone and pyridine into acetonesoluble, acetone-insoluble-pyridine-soluble, and pyridineinsoluble (PI) fractions. The PI fraction, which is 30.0 wt % (daf) on the basis of raw coal, was used to determine the sorption isotherm as described.24 Methanol, benzene, and cyclohexane were used as the sorption solvents. Volumetric Swelling. The procedure for Upper Freeport bituminous coal has been reported.20 The PI fraction was used as the sample, and the equilibrium swelling value was measured at 25 °C. An equilibrium swelling value was attained within 30 min. Simulation Methods. A model structure (Figure 1) based on the structural analysis (Table 1) of the PI fraction was used. The model structure is one of several average structures that match the analytical data. The density (1.20 g/cm3) of the model structure estimated from the simulation was in rather good agreement with the measured value (1.25 g/cm3).13 The simulation procedure has been reported.13 The Cerius 2 software package (version 3.5, Molecular Simulation Inc.) (22) Vorres, K. S. Energy Fuels 1990, 4, 420. (23) Takanohashi, T.; Iino, M. Energy Fuels 1990, 4, 452. (24) Takanohashi, T.; Terao, Y.; Iino, M. Fuel 2000, 79, 349.
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Figure 3. Changes in total energy and volume (a), and interaction energy (b) of the PI-methanol system.13
Figure 2. Flowchart of simulation procedure. was run on an Indigo 2 graphic workstation (Silicon Graphics Inc.). The DREIDING 2.02 method25 was used for force-field calculations. The charge distribution of the model structure was determined by using the method of charge equilibration proposed by Rappe´ and Goddard.26 The potential energy for an arbitrary geometry of a molecule is expressed as a combination of bonded torsions, which depend on the covalent bonds of the structure, and nonbonded interactions, which depend only on the distance between atoms. The bonded terms consist of bond length torsion (Eb), bond angle torsion (Eθ), dihedral angle torsion (Eφ), and inversion (Ei), while the nonbonded terms consist of van der Waals (Evdw), electrostatic (Eel), and hydrogen bond (Ehb) energies. The total energy E is the simple sum of these energies,
E ) Eb + Eθ + Eφ + Ei + Evdw + Eel + Ehb
(1)
The flowchart for simulation of solvent swelling is shown in Figure 2. The details of the procedure have been reported.13 In brief, the model structure was placed in a periodic boundary cell and the energy-minimized conformation in the cell was calculated with molecular mechanics/molecular dynamics (MM/MD) methods. One solvent molecule was placed into the space of the PI model structure; charge modification was carried out, and the energy-minimized conformation of the PI model-solvent structure was found using MM/MD. Successive (25) Mayo, S. L.; Olafson, B. D.; Goddard, W. A., III. J. Phys. Chem. 1990, 94, 8897. (26) Rappe´, A. K.; Goddard, W. A., III. J. Phys. Chem. 1991, 95, 3358.
solvent molecules were added stepwise to the most stable preceding model-solvent structure and the computational cycle repeated. Changes in volume and total energy of the system were determined with addition of each solvent molecule.
Results and Discussion When a solvent molecule is introduced into the stable PI model unit cell, the interaction energy (negative) between the solvent and model molecule depends on the affinity between the solvent and model structure and corresponds to the mixing energy between them. The unit cell expands and the total energy of the PI model structure increases. The structure holds the original stable conformation, which corresponds to the elastic energy of the original PI structure. When the difference in energy between structures with the successive addition of solvent molecules is negative, the additional solvent molecule is interacting favorably with the preceding structure. The result for the PI model-methanol system has been reported.13 Changes in the total energy of the system and the volume of the cell as a function of the number of methanols added are shown in Figure 3a.13 When more than 14 molecules of methanol were added, the total energy changed little, which indicates that swelling attains an equilibrium.13 Figure 3b shows changes in the interaction energy of PI-methanol as a function of the number of methanols added, i.e., (total
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Figure 4. Changes in total energy and volume (a), and interaction energy (b) of the PI-benzene system.
Figure 5. Changes in total energy and volume (a), and interaction energy (b) of the PI-cyclohexane system.
energy) - (energy of PI) - (energy of methanol). The change in interaction energy showed a tendency similar to that in the total energy as shown in Figure 3a, indicating that swelling is caused by the PI-methanol interaction itself. The results for benzene and cyclohexane are shown in Figures 4 and 5. The total energy, volume, and mass of the PI model-solvent system changed as successive solvent molecules were added. In the case of benzene, the total energy of the system decreased monotonically until 5 molecules of benzene had been added, increased slightly until 9 molecules had been added, and then showed a tendency to increase (Figure 4a). The volume of the cell increased monotonically until 9 molecules were added, after which the rate of increase was greater, as observed in the case of methanol. These results suggest that a further increase in the volume with addition of benzene from 9 to 10 would be not be favorable because there is a difference in the total energy of 10-25 kcal/mol between 9 and 11 benzene molecules, and further increases in volume will not occur in the actual swelling system. In addition, the interaction energy of PI-benzene itself decreased greatly until 5 molecules added, after which the rate of decrease was smaller (Figure 4b). The interaction of PI-benzene can be small compared to that of PI-methanol, although the rate of volume increase by benzene (Figure 4a) was greater than that by methanol (Figure 3a). The stable
structure of the model including 9 molecules of benzene (represented as spheres) is shown in Figure 6. Benzene interacts with several sites in the PI model, especially with the aromatic rings. In contrast, the total energy decreased slightly with addition of 2 molecules of cyclohexane, and then increased greatly as more cyclohexanes were added. The volume of the cell increased monotonically (Figure 5a). These results indicate that only 2 molecules of cyclohexane can be added. The interaction energy of PIcyclohexane was the smallest of all solvents used here, as shown in Figure 5b. Changes in each nonbonded energy term and the total energy as a function of the number of solvent molecules introduced are shown in Figures 7-9, for methanol,13 benzene, and cyclohexane, respectively. For methanol the electrostatic term decreased dramatically compared with the other terms, which suggests that the electrostatic interaction between methanol and the coal model structure is much more important than hydrogen bond formation.13 The electrostatic term decreased slightly with the number of benzene molecules, although benzene does not seem to interact with polar sites in the PI model structure (Figure 8). The van der Waals term varied slightly and the hydrogen bond term was essentially unchanged. These results indicate that the van der Waals interaction between benzene molecules and the aromatic rings in the PI model and the electrostatic
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Figure 6. The equilibrium state of swelling of the PI-benzene system. The PI model incorporates 9 benzene molecules.
Figure 7. Changes in nonbonded interaction energy of the PI-methanol system; the total energy is included for comparison.13
Figure 8. Changes in nonbonded interaction energy of the PI-benzene system; the total energy is included for comparison.
interaction among the polar sites in the PI model, which occur because of a change in conformation, are important in this system. Figure 9 shows that the decrease in the total energy found for the introduction of 2
molecules of cyclohexane is also caused by electrostatic effects, which can be explained by a decrease in the electrostatic energy among the polar sites in the PI model, similar to the case of benzene.
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Figure 9. Changes in nonbonded interaction energy of the PI-cyclohexane system; the total energy is included for comparison.
Figure 10. Methanol sorption isotherm of the PI fraction at 30 °C,24 and the lines curve-fitted with the Langmuir-Henry dual mode equation.
These results suggest that the volume increase in the unit cell caused by the introduction of up to 9 benzene or 2 cyclohexane molecules represents the swelling behavior of coal in each solvent, because the total energy decreases as a result of the interaction among the PI model and solvent. Swelling may attain equilibrium because additional solvent molecules do not affect the energy. The sorption isotherm at 30 °C of the PI from Upper Freeport coal is shown in Figures 10-12 for methanol,24 benzene, and cyclohexane, respectively. For methanol and benzene, the lines through the data points were fit with the Langmuir-Henry dual-mode eq 124,27,28
C ) CH + CD ) C′H bp/(1 + bp) + kDp
(1)
where C is the total sorption (mmol/g-sample), CH is adsorption on the surface, CD is dissolution into bulk, C′H is the pore saturation constant, b is the pore affinity constant, p is the relative vapor pressure (P/P0), and kD is the Henry’s dissolution constant. For methanol and benzene, data were fit by the Langmuir-Henry equation, as shown in Figures 10 and 11. Therefore, the (27) Green, T. K.; Selby, T. D. Energy Fuels 1994, 8, 213. (28) Shimizu, K.; Takanohashi, T.; Iino, M. Energy Fuels 1998, 12, 891.
Figure 11. Benzene sorption isotherm of the PI fraction at 30 °C, and the lines curve-fitted with the Langmuir-Henry dual mode equation.
Figure 12. Cyclohexane sorption isotherm of the PI fraction at 30 °C. Table 2. The Swelling Values Estimated from the Simulation and the Observed Ones from Vapor Sorption and Volumetric Swelling Measurements swelling ratio (-)
methanol
benzene
cyclohexane
from volumetric method estimated, Vn/V0 from sorption data estimated, Mn/M0
1.54 1.09 1.15 1.20
1.84 1.30 1.32 1.32
1.20 1.09 1.11a 1.08
a
Calculated from the total amount of sorption.
amount absorbed, which would contribute to swelling, was estimated from the second term in eq 1; the values are 3.6 and 4.1 (mmol/g-coal) for methanol and benzene, respectively. For cyclohexane the data were not fit by the equation, and the total amount sorbed (1.3 (mmol/ g-coal)) was used. As a result, the ratio of the weight increase (the amount of absorption per weight of the PI feed) was calculated (Table 2). The volumetric swelling ratios of the PI fraction in the solvents have been reported29 and are listed in Table 2. The weight or volume swelling ratios estimated from the simulation are listed in Table 2. For all solvents, the weight swelling ratios estimated from the simulation are in good agreement with the experimental values, while for all cases the estimated ratio of volume increase was (29) Takanohashi, T.; Iino, M. Energy Fuels 1991, 5, 708.
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much lower than the value obtained with the volumetric method. The swelling of coal in the liquid phase is affected by extraction of solvent-soluble components and relaxation of the macromolecular structure, which would influence the swelling ratio. Conclusions The effect of benzene and cyclohexane on the swelling of the solvent-soluble constituent of Upper Freeport coal was simulated using MM/MD methods; results were compared with a previous study using methanol as solvent. As solvent molecules are introduced into the boundary cell, the potential energy in the PI-solvent system decreased and the volume of the cell increased up to a limiting number of solvent molecules that were
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different for each solvent. The contribution of the electrostatic interaction to the decrease in the total energy of the PI-solvent system was larger than those of the hydrogen bond and van der Waals interaction, regardless of the solvent used. The weight swelling ratios estimated from the simulation were in good agreement with the value obtained from sorption data. The simulation method appears to be a valid representation for solvent swelling of coal in the vapor phase. Acknowledgment. This work has been carried out as one of “Research for the Future” project of the Japan Society for the Promotion of Science (JSPS) through the 148 committee on coal utilization technology of JSPS. EF990147F
