Energy & Fuels 1993, 7, 469-472
469
CAMD Study of Coal Model Molecules. 2. Density Simulation for Four Japanese Coals Satoru Murata and Masakatsu Nomura* Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565, Japan
Kazuo Nakamura* Fundamental Research Laboratory, Osaka Gas Co., Ltd., 19-9,6-Chome, Torishima, Konohana-ku, Osaka 554, Japan
Haruo Kumagai and Yuzo Sanada Metals Research Institute, Faculty of Engineering, Hokkaido University, Sapporo 060, Japan Received December 14, 1992. Revised Manuscript Received March 18, 1993
The physical densities of previously proposed model structures of four Japanese coals [Tempoku, Taiheiyo, Akabira, and Yubaricoals; carbon content (wt % ,daf) 71.5,77.9,81.2,and 86.7 and physical density (g.cm-9 1.37, 1.27, 1.28, and 1.24, respectively] were calculated using CAMD (computeraided molecular design) software. The calculated densities (g.cm-9 followed the sequence Akabira coal 1.03 < Yubari coal 1.11 < Taiheiyo coal 1.22 < Tempoku coal 1.29, increasing as the carbon content of the coal structures decreased, except for the Akabira coal structure. A modification of the bridge structure in the Akabira coal model afforded a calculated density of 1.15. Thus, the correlation between the carbon contents and the physical densities for the original coals could be reasonably reproduced.
Introduction Computer-aided molecular design (CAMD) has been employed mainly in biological chemistry including drug design and protein modeling. Recently, this methodology has begun to be applied to the area of fuel science by constructing coal model macromolecules.lJ In a previous paper? we proposed an improved method to determine the physical density of coal model molecules using CAMD software in which intermolecular interactions and void volume are taken into consideration. The method was applied to simplified coal models which were composed of polyaromatic hydrocarbons and polymethylene chains. It was found that the calculated density depended markedly on the length of the methylene chain and the size of the aromatic moiety of the models. The purpose of the present work is to investigate the applicability of the above method to models more closely related to existing coals. A series of models for Japanese coals [Tempoku, Taiheiyo, Akabira, and Yubari coals; carbon content (wt %, daf) 71.5, 77.9,81.2, and 86.7 and physical density (g.cm-9 1.37, 1.27, 1.28, and 1.24, respectively] which were proposed previously by Iwata et al.4t5 was chosen (Figure l),and simulation of their physical densities was attempted by our CAMD method. The results are described herein. (l)Carlson, G.A. Prepr. Pap.-Am. Chem. SOC.,Fuel Chem. Diu. 1991,36,398. Carbon, G.A.; Granoff, B. In Coal Science II; Schobert, H. H. Bartle, K. D., Lynch,L. J., E&.; ACS Symp. Ser. 461; American Chemical Society: Washington,DC, 1991; 159. Carbon,G. A. (1991Znt. Coni. C o d Sci. hoc. 1991, No. 24. (2) Faulon, J. L.; Vandenbroucke, M.; Drappier, J. M.; Behar, F.; Romero, M. Adu. Org. Geochem. 1989,16,981. (3) Nakamura, K.; Murata, S.; Nomura, M. Energy Fuels, in press. (4) Iwata, K.; Itoh, H.; Ouchi, K. Fuel Process. TechnoZ. 1980,3,221. (5) Iwata, K. Doctoral Thesis. Studieson average chemical structure of coal, Hokkaido University, 1983.
a) 4 H P To&00"2HO OH
OH
bH
?H
CH2
I
I
4& o
CH2
I
Figure
1. Structures of the coal models proposed by
Iwata et
al.' for (a) Tempoku, (b) Taiheiyo, (c) Akabira, and (d) Yubari
coals.
Met hod The CAMD study was carried out by using a TITAN 760V graphic workstation (Kubota Computer Inc.) with PolyGraf software (version 3.0, Molecular Simulations Inc.) as described
0887-0624/93/2507-0469$04.00/00 1993 American Chemical Society
470 Energy & Fuels, Vol. 7, No.4,1993
Murata et al.
Table I. Degree of Oligomerization, Molecular Formula, Molecular Weight, Carbon Content, and Number of Hydroxyl Groups of the Coal Model Molecules TemDoku Taiheivo Akabira Yubari deg of 7 7 6 4 oligomerizn mol formula C147H142036 c 1 4 7 ~ 1 4 2 ~ 2C1 I ~ ~ O Cldl42012 I B mol w t 2469 2245 2381 2497 carbon 71.5 78.7 81.7 86.6 content (% ) no.ofOH 21 14 12 8
in a preceding paper.3 The software allows treatment of relatively large molecules containing up to 20 O00 atoms and is capable of calculating the most stable structures with the minimum conformational energies using AMBER, MM2, and DREIDINGgforce fields. In this study, the DREIDING force field was used. PolyGraf also allowsthe use of periodic boundary conditions (PBC). Under periodicboundary conditions,a specific molecule is assumed to be surrounded in three dimensions by identical molecules. Thus, a molecular group system including intermolecular interactions and void volume can be treated. The procedure for the calculation of physical density of the model molecules is described briefly. At first, the model molecule was input, and ita potential energy was then optimized. The conformation having the lowest potential energy was extracted as a best conformer. This model molecule was enclosed in cell (periodic boundary conditions). For this PBC calculation, hydrogen atoms attached to carbon or oxygen atoms were treated as included in each carbon or oxygen group such as methine, methylene, methyl or hydroxyl group according to DREIDING's method. Molecular mechanics calculation was then carried out in order to reduce the potential energy of the system up to ita minimum value, by using a cell volume of which the true density can be calculated.
Results and Discussion Optimizationof the Structure of the Coal Models. The model molecules for the four Japanese coals proposed by Iwata et al.4 are oligomers composed of the unit structures shown in Figure 1. They were constructed on the basis of lH NMR, elemental analysis, and hydroxyl group analysis of their liquefied products. In this study, each model molecule is assumed to have a molecular weight of approximately 2500 (Table I). Thus, the models for Tempoku, Taiheiyo, Akabira, and Yubari coals used for the calculation are heptamer, heptamer, hexamer, and tetramer of the given unit structures, respectively. The carbon contents of the models are 71.5,77.9,81.2, and 86.7 wt % ,respectively, and are consistent with those observed for the four ~0als.4 Table I1 summarizes the results for the energy calculations without periodic boundary conditions. Figure 2 shows the most stable conformational structure for the Tempoku coal model as a representative one. The calculation for each model molecule was carried out by considering that it is isolated in a vacuum as is usual in CAMD. The DREIDING force field method employed calculates the energy, E, as a linear combination of covalently-bondedinteractions (E,, stretch; Eb, bend; Et, torsion; and Ei, inversion energies) and noncovalently bonded interactions (Ev,van der Waals; E,, electrostatic; and Eh, hydrogen bond energies), Le., E = E, + E b + Et + Ei + E, + Ee + Each energy value obtained is also given in Table 11. In order to obtain the normalized potential energy for each model molecule, the total energy (6) Mayo, S. L.; Olafmn, B. D.; Goddard In, W.A. J. Phys. Chem. 1990,94,8897.
Figure 2. The most stable conformational structure of the Tempoku coal model.
was divided by the total number of atoms. The total potential energy per atom (kcal-mol-') was found to increase as the rank of the coals becomes higher. The major factors affecting the total potential energy appear to be the electrostatic and hydrogen bonding energies, Ee and Eh. As the rank of the coals (Le., the carbon contents) decreases, these energies tend to large negative values. This may be due to the fact that the lower rank coals have relatively larger numbers of hydroxyl groups and ether oxygen atoms per molecule. According to the suggestion of the one reviewer, we examined the relationship between total potential energy and the heat of combustion of these coals. Sugimura et aL7 and Krevelen et aL8 had reported that, as coal rank increases, the heat of combustion at first increases, reaches a maximum value at 90% of carbon, and then decreases. Direct comparison of the value of the heat of combustion with the potential energy calculated in this paper could not be undertaken; however, it is interesting to note that the tendencies of these two values are quite similar. Estimation of Physical Density of the Coal Models. Figure 3 shows the relationships between the total potential energy and the physical density of the coal models calculated under the periodic boundary conditions by inputting the structural and energy data obtained for each single oligomeric model molecule. As is seen, there is a minimum potential energy for each model which corresponds to the optimized density. The physical densities for the models thus obtained are recorded in Table I11 along with the corresponding values previously measured experimentally for the original coals using sink-float method. The observed densities of the coal specimens (7) Sugimura,H.; Osawa, Y.;Hatami, M.;Sato, S.;Honda, H.Nenryo Kyokaishi 1966,45,199. (8) Bangham, D. H.;van Krevelen, D. W.Fuel 1954,33,348.
CAMD Study of Coal Model Molecules
Energy I% Fuels, Vol. 7, No. 4, 1993 471
Table 11. Potential Energy of the Coal Model Molecules Tempoku 206.4 164.0 53.8 64.4 44.8 1.0 42.4 311.3 -160.0 -108.9 0.64
total potential energy (kcal/mol) covalently bonded energy stretch bend torsion inversion noncovalently bonded energy van der Waals electrostatic hydrogen bond total potential energy per atom (kcal-mol-l) a Modified
Taiheiyo 204.7 131.8 49.7 42.7 37.9 1.5 72.9 260.6 -138.3 -49.4 0.66
Akabira 371.1 169.8 67.3 52.5 49.0 1.0 201.3 337.5 -103.3 -32.9 1.14
Akabiraa 384.7 179.0 56.7 80.0 41.6 0.7 205.7 282.3 -60.1 -16.5 1.18
Yubari 583.5 242.0 101.4 73.2 65.1 2.3 341.5 407.4 -49.2 -16.7 1.75 ~~
~
structure of Akabira coal.
1.0 I 70
I 75
80
85
90
Carbon content of coal (%)
Figure 4. Relationship between the carbon content and the physical density; observed values (a),calculated values of the models (H), and calculated value of the modified Akabira coal model (A). 0.2
0.6
1 .o
-
1.4
Physical density (g ~ r n . ~ ) Figure 3. Relationships between the total potential energy and thephysicaldensityofthecoalmodelsfor Tempoku (O),Taiheiyo (a),Akabira (O), and Yubari (H) coals. Table 111. Physical Density of the Coal Models physical density coal calculated measured Tempoku 1.29 1.37 Taiheiyo 1.22 1.27 Akabira 1.03 1.28 Akabiraa 1.15 Yubari 1.11 1.24 a Modified
CHZ
I
structure of Akabira coal.
follows the sequence Tempoku > Taiheiyo N, Akabira > Yubari, increasing as the carbon content decreases. This is consistent with the general observation that the density of coals decreases as the coal rank increases up to carbon contents of 85-87%.9 The results for the calculations appear to be well in harmony with the trend except the Akabira coal model (Figure 4). In the previous paper, we demonstrated that the simulated physical density of polymeric hydrocarbons composed of polyaromatics and polymethylene chains markedly depends on the length of the chains and the number of the aromatic rings: these functions may greatly affect the flexibility of the polymeric molecules and thus affect the density by influencingthe ability of the molecules to pack efficiently to minimize the density. Each of the coal models used has two aromatic moieties whose ring numbers depend on the rank of the original coals and which are connected by a -CHzOCHz- chain, (9) Meyers, R. A. Cool Structure; Academic Press: New York, 1982: pp 54-80.
OH
CH2
I Figure 5. Modification of the Akabira coal model. except for the Akabiracoal model (Figure 1). The Akabira coal model has a shorter chain [-CH20-]. We considered that the shortness of the chain could be a major reason for the disagreement of the calculated model density with carbon content-density correlation. Consequmtly, a modification of the Akabira coal model was made as illustrated in Figure 5 (refer to Table I1 for energy terms of the revised Akabira coal). The modified model has a -CHZOCHrchains,as have the other models, while carbon content, molecular weight, and aromaticity are unchanged. Using the revised structure, a calculated density of 1.15 was obtained. The value is between those for Taiheiyo and Yubari coal models, which suggests that the change of the bridge structure may be appropriate. The change
472 Energy & Fuels, Vol. 7, No. 4, 1993
in density appears to be attributable to the increased flexibility of the modified model molecule. In connection with the above argument, it should be pointed out that the Tempoku model is very similar to the Taiheiyo model, while there is a considerable difference in the two calculated densities. It may be reasonable to consider that the origin derives mainly from the difference in the linkages connecting the unit structures. The former model has a -CH2CH20CH2- group and the latter has a shorter linkage of -(CH2)3-. Therefore, the Tempoku coal oligomer model is expected to be more flexible than the Taiheiyo coal model, giving the higher calculated density. In summary, the density-carbon content correlation for the four Japanese coals could be successfully simulated by using the C A M D method we previously proposed. While the coal models employed in this study, which were suggested by Iwata et al., are rather simple, they appear to represent the characteristics of the original coals reasonably, at least for the simulation of the coal density, although the Akabira coal model had to be somewhat modified. This also suggesb that, in constructing coal
Murata et al.
models, it is important to build up the linkage connecting aromatic moieties carefully. The calculated densities were low by0.05-0.13 compared with the corresponding original coals. The measured density of coals is known to vary in a range of about 0.1 depending on the method of the measurement and because of their inhomogeneous natures.lOJ1 One of reviewers pointed out the fact that these calculations systematically underestimate the coal density deserves comment. The reason for this is that it may be considered that native coals are more strained and more compacted due to their coalificationprocess. Nevertheless, this procedure is considered to be very useful as an evaluation method for the adequateness of coal models constructed. Simulation of other physical properties of coals including solvent swelling of coal models using the CAMD technique is also in progress in our laboratory. Acknowledgment. We thank Dr. K. Iwata and his colleagues for the coal model molecules. (10)Franklin, R. E. Tram.Faraday SOC.1949,45,274. (11) Toda, Y. h e 2 1972,52, 199.
