Construction of a Model Structure for Upper Freeport Coal Using 13C

Energy & Fuels 2002, 16, 379-387

379

Construction of a Model Structure for Upper Freeport Coal Using 13C NMR Chemical Shift Calculations Toshimasa Takanohashi* and Hiroyuki Kawashima Institute for Energy Utilization, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8569, Japan Received May 30, 2001. Revised Manuscript Received December 4, 2001

Upper Freeport coal was extracted in the high yield at room temperature with a carbon disulfide/ N-methyl-2-pyrrolidinone (CS2/NMP) mixed solvent. A solid-state 13C NMR spectrum was measured for the mixed-solvent insoluble residue. On the basis of the spectrum, a model structure for the residue was estimated using chemical shift calculations and structural data, including the ultimate analysis, and was then iterated until spectra calculated for the model matched the experimentally obtained spectrum in the excellent way. In earlier work, the original extract had been further fractionated with acetone and pyridine, and 1H NMR spectra and the elemental composition had been obtained on the resulting subfractions. These data were used to construct model structures for each fraction. With these structures from the earlier work and the residue’s structure based on the 13C NMR spectrum, a seven-molecule associated model structure for Upper Freeport raw coal was constructed; the structure had a continuous molecular weight distribution from the lightest to the heaviest extract fraction, allowing us to explain the high extraction yields (60-85 wt % (daf)) for the coal even at room temperature. The lowest energy conformation of the model structure was determined using a molecular mechanics-molecular dynamics method under periodic boundary condition. A gross anisotropic structure was produced. The physical density estimated from the energy-minimized conformation of this structure was 1.28 g/cm3, in good agreement with the experimentally determined value of 1.30 g/cm3. The average distance between the aromatic planes was calculated 4.1 Å, a little higher than the reported value of 3.6 Å.

Introduction Upper Freeport coal, an Argonne premium standard, gives a very high extraction yield (60 wt % (daf)) with a carbon disulfide/N-methyl-2-pyrrolidinone (CS2/NMP) mixed solvent at room temperature.1-3 The extraction yield was increased (78-85 wt % (daf)) by addition of small amounts of compounds such as tetracyanoethylene (TCNE) and p-phenylenediamine.4-7 On the basis of these results, we suggested that Upper Freeport coal consists of associations of coal molecules that are solublized without breaking covalent bonds, and that the coal does not have an extensive covalently crosslinked network. The molecular structures of the components that form the three-dimensional conformation of coal are not clear, however. As found for extraction residues, coal extracts swell in organic solvents such as methanol and benzene.8-10 Figure 1. Extraction and fractional procedures. * Author to whom correspondence should be addressed. E-mail: [email protected]. jp. (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) Green, T. K.; Chamberlin, J. M.; Lopez-Froedge, L. Prepr. Paps Am. Chem. Soc., Div. Fuel Chem. 1989, 34, 759.

Takanohashi et al. suggested9-11 that cooperative noncovalent interactions may form physical cross-links in extracts that are soluble in strong solvents such as CS2/ NMP and insoluble in weak solvents such as methanol and benzene, which is the source of swelling in the (9) Fujiwara, M.; Ohsuga, H.; Takanohashi, T.; Iino, M. Energy Fuels 1992, 6, 859. (10) Takanohashi, T.; Iino, M.; Nishioka, M. Energy Fuels 1995, 9, 788. (11) Takanohashi, T.; Iino, M.; Nakamura, K. Energy Fuels 1998, 12, 1168.

10.1021/ef0101154 CCC: $22.00 © 2002 American Chemical Society Published on Web 02/09/2002

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Energy & Fuels, Vol. 16, No. 2, 2002

Takanohashi and Kawashima

Table 1. Analyses of Coal, Extraction with CS2/NMP Mixed Solvent, and Fractionation of the Extracta ultimate analysis, wt % (daf)

proximate analysis, wt % (db)

C

H

N

S

Ob

86.2

5.1

1.9

2.2

4.6

extraction yield, wt %, daf 59.4 a

VM

ash

FC

28.2

13.1

58.7

fraction yield, wt %, daf AS

PS

PI

7.3

22.1

30.0

Ref 2. b By difference.

Figure 2. Flow chart of calculation procedure.

poorer solvent. The swelling ratio of coals also correlated with the heat of immersion of coal in various solvents,12 indicating that the enthalpy of interaction of specifically interacting solvents with surface functionalities primarily determines the swelling behavior. Computer simulation of the interactions of coal extract associates with solvents showed that cooperative interactions through hydrogen bonds and aromatic-aromatic interactions at several sites form a strongly associated structure, and that dissociation occurs in stronger solvents.11 Because no bond breaking seemed to occur during the room-temperature extraction of Upper Freeport coal,1,3 model structures that reflect the original chemical structures were constructed on the basis of information obtained from 1H NMR spectra and the ultimate analysis of the fractions obtained by acetone and pyridine fractionation of the original mixed-solvent extract.11 The residue from the initial extraction is insoluble in ordinary solvents, 1H NMR spectra could not be obtained, and therefore a model structure for the residue could not be constructed. Solid-state 13C NMR is an attractive tool for the characterization of coal because carbon atom hybridiza(12) Suuberg, E. M.; Otake, Y.; Langner, M. J.; Leung, K. T.; Milosavljevic, I. Energy Fuels 1994, 8, 1247.

Figure 3. Model structures for extract fractions: (a) AS, (b) PS, and (c) PI.14

Figure 4. Aromatic structural elements for MI.

tion can be elucidated directly. Recently, a method has been used to calculate 13C NMR chemical shifts for materials such as polymers. Thomas et al. showed13 that 13C chemical shifts calculated for some substituted pyridines were in good agreement with the experimental

Model Structure for Upper Freeport Coal

Energy & Fuels, Vol. 16, No. 2, 2002 381

Table 2. Structural Parameters and Molecular Formula of the Extraction Residue C% observed model a

81.7 81.2

H% 4.7 4.8

N% 1.8 2.0

S% 5.5 5.7

O% 6.3 6.3

H/C 0.69 0.71

O/C 0.058 0.058

Mna

fab

molecular formula

0.81c 2796d

Average molecular weight. b Ratio of aromatic carbons to total carbons. c From 13C NMR.

d

0.85

C189H133N4O11S5

Total molecular weight of two molecules.

values. We used this method to construct the average model structure of extract fractions obtained from the room-temperature extraction of Upper Freeport coal, and suggested model structures that fit the actual NMR spectra.14 These preliminary results are expanded and refined in the present study. A structural model for the extraction residue was devised using 13C NMR chemical shift calculations and analytical data.2,5 With this structure and the model structures of the extract fractions reported previously,14 a total model structure for the raw coal was constructed. A three-dimensional model structure of Upper Freeport coal was determined using molecular dynamics-molecular mechanics (MM-MD) methods. Experimental Methods Sample Preparation. Upper Freeport coal was obtained in ampules (5 g of -150 µm).15 The sample was extracted exhaustively with a carbon disulfide/N-methyl-2-pyrrolidinone (CS2/NMP) mixed solvent (1:1 v/v) at room temperature.2 The extract obtained was further fractionated with acetone and pyridine into acetone-soluble (AS), acetone-insoluble and pyridine-soluble (PS), and pyridine-insoluble (PI) fractions (Figure 1). The PS, PI, and the original mixed solvent-insoluble residue (MI) were washed with acetone, and the AS was washed with acetone-water solutions (1:4 v/v). All fractions were dried in a vacuum oven at 80 °C for 12 h. The extraction yields and fraction distributions are listed in Table 1. NMR Measurements. Solid-state 13C NMR spectra were measured by the Single Pulse Excitation/Magic Angle Spinning (SPE/MAS) method on a Chemmagnetics CMX-300 NMR spectrometer operating at a 13C resonance frequency of 75.46 MHz. Approximately 100 mg of the MI sample were packed in the sample rotor. All spectra were acquired employing a 1H 90° pulse length of 4 µs. This was combined with a magic angle-spinning rate of 10 kHz. Repetition rates of 60 s were used for all samples. For each spectrum, 2000 scans were accumulated. The chemical shifts were calibrated with respect to tetramethylsilane using the peak of the methyl group on hexamethylbenzene at 17.4 ppm as the external standard. NMR Chemical Shift Calculation. Chemical shift calculations were carried out using the Advanced Chemistry Development Laboratory’s ACD/CNMR software. Up to 256 carbon atoms may be used. The chemical shift of model structures is calculated by searching for similar sub-structural fragments with the corresponding experimental shift value in the database (600 000 chemical shifts of 50 000 compounds) and evaluating the chemical shift value, taking into account intramolecular interactions. The calculated 13C NMR spectra of the model structures were obtained by considering an adequate line width for each peak and summing, which confirms that all peaks are Gaussian-type.14

Figure 5. Aromatic structures for MI. Table 3. The Amount of Each Atom Type for the Extraction Residue on the Basis of Ultimate Analysis and Structural Data atom type

amount

bond type

carbon carboxyl C aromatic C aliphatic C

2 159 28

oxygen OH, COOH -OCdO

6 4 1

phenol + carboxyl ether carbonyl

nitrogen pyridinic N pyrrole N

3 1

pyridinic pyrrole

sulfur aromatic S aliphatic S

4 1

thiophenic sulfide

as shown in Figure 4 side-chain + naphthenic + methylene

total: C189N4 O11 S5

The Cerius2 software package (version 4.2, Molecular Simulation Inc.) was run on an OCTANE graphic

work station (Silicon Graphics Inc). The DREIDING 2.02 method16 was used for the force field calculations. The periodic boundary condition (PBC) was used to elucidate the physical properties of the model structure.17 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

(13) Thomas, S.; Bruhl, I.; Heilman, D.; Kleinpeter, E. J. Chem. Int. Comput. Sci. 1997, 37, 726. (14) Kawashima, H.; Takanohashi, T. Energy Fuels 2001, 15, 591. (15) Vorres, K. S. Energy Fuels 1990, 4, 420.

(16) Mayo, S. L.; Olafson, B. D.; Goddard, W. A., III. J. Phys. Chem. 1990, 94, 8897. (17) Takanohashi, T.; Nakamura, K.; Iino, M. Energy Fuels 1999, 13, 922.

Simulation Methods

382



Figure 6. A model structure (model A) for MI.

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)

Figure 2 shows a flowchart of the calculation procedure. The size of the cell containing initial structures was 76.8 Å × 58.3 Å × 12.9 Å. The charge distribution was determined using the method of charge equilibra-

tion proposed by Rappe´ and Goddard,18 and the energyminimized conformation was calculated with MM-MD methods.11 MD was performed under constant-pressure and -temperature condition. Charges were updated every 0.01 ps during MD. With the MD calculation, the size of the cell and the total energy were decreased mainly because of interactions of molecules between cells. The MM-MD calculation was conducted repeatedly. The physical density of model molecules was estimated using periodic boundary conditions.19-21 Densi(18) Rappe, A. K.; Goddard, W. A., III. J. Phys. Chem. 1991, 95, 3358.



Table 4. Chemical Shifts Calculated for the Carbons on the Model Structure in Figure 6 carbon no. 107 106 102 26 101 84 125 196 88 105 83 150 195 146 197 85 151 80 79 100 104 145 174 184 87 175 126 86 203 194 208 190 43 17 94 90 48 58 95 114 149 31 19 62 64 60 21 4 42 51

CHn

chem. shifts

conf. limits

carbon no.

CHn

chem. shifts

conf. limits

carbon no.

CHn

chem.. shifts

conf. limits

carbon no.

CHn

chem. shifts

conf. limits

CH3 CH3 CH3 CH2 CH2 CH2 CH3 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH CH2 CH CH3 CH CH CH CH C CH CH C C C C CH C CH C CH C CH CH

13.93 13.97 14 20.92 22.3 22.36 22.88 23.43 23.44 24.84 25.42 26.54 30.03 30.23 30.3 31.54 34.52 34.61 36.68 36.95 37.08 37.95 38.62 39.62 40.38 41.85 43.37 43.77 44.33 45.97 55.8 94.3 94.53 102.6 104.13 104.16 108.85 109.42 109.99 110.61 111.96 112.16 113.55 113.56 114.15 114.92 116.24 117.92 118.84 118.86

1 0.4 0.5 3.4 1.4 3.5 3 3.1 0.9 0.6 2.8 2.3 3.5 2.9 0.3 3.4 4.8 0.9 2.6 0.8 2.6 1 2.5 2.8 4.7 2.6 0.2 3.8 3.3 3.6 0.2 3.2 4.1 6.2 2 2.7 5.1 5.3 4.8 5.3 2.9 3.2 3.6 2.4 6.1 4.4 2.1 3.7 9.4 0.5

180 160 65 186 34 29 129 161 10 67 38 49 200 71 120 117 69 204 202 75 181 22 110 13 52 73 171 76 179 11 156 15 159 12 158 89 33 99 96 23 164 116 141 8 133 177 188 93 166 9

CH CH CH C CH CH C CH C CH C CH CH C CH CH C C C CH CH CH C C CH CH CH CH CH CH CH C C CH C C C CH C C C C CH C C CH C CH C C

118.97 119.7 120.02 120.19 121.2 121.23 121.33 121.64 121.77 121.9 122.4 122.51 122.59 122.63 122.74 122.82 123.04 123.25 123.43 123.5 123.86 123.94 124.01 124.09 124.16 124.2 124.68 124.72 124.86 125.05 125.16 125.26 125.67 125.97 126.12 126.18 126.33 126.33 126.46 126.89 126.93 127.02 127.06 127.09 127.25 127.31 127.63 127.68 127.87 128.16

1.1 1.1 6.4 4.9 4.9 5.6 4.3 3.7 3.4 4.9 3.9 5 2.1 3.7 1.5 6.3 2.6 4.5 10.4 2.5 1.5 2.9 3 3.4 0.7 6.6 5.3 2.9 0.7 3.7 2.6 8.4 2 3.3 3.5 2.9 1.4 5 2.3 2.6 3.1 2.5 1 1.7 3.1 5 4.1 2.2 1.8 2.8

191 97 74 152 46 54 25 124 59 53 135 172 199 136 173 154 123 162 176 153 119 39 40 66 167 32 14 192 72 30 138 91 201 68 82 147 122 198 121 169 178 142 137 2 187 37 165 81 193 163

C C CH C C CH C CH C CH C C CH CH C C C C C C C C C C C C C C C C C C CH C C C CH C CH C CH CH CH C CH C C C C CH

128.31 128.35 128.48 128.59 128.65 128.93 128.98 129.11 129.47 129.5 129.58 129.77 129.86 129.91 130.63 130.94 131.05 131.2 131.5 131.5 131.58 131.67 131.68 131.73 131.84 131.85 132.19 132.33 132.42 132.49 132.85 132.89 133.25 133.49 133.57 133.7 134.31 134.34 134.4 134.63 134.65 135.56 135.6 135.66 136.05 136.25 136.31 136.52 136.61 137.1

3.2 4.5 3.8 13.2 5.5 0.7 4.1 5.5 3.2 0.8 0.6 0.2 1.9 5.1 4 2.4 0.9 3.8 10.3 7.1 14.8 1.4 1.8 3.1 1.2 1.3 7.5 1.2 0.9 4.7 5.1 3.4 1.5 2.5 5.7 4.9 4.3 1.8 1 2.5 8.6 2.7 5.2 7.2 7.5 6.7 1.8 2.7 1.9 9.6

168 128 61 5 139 98 18 134 70 50 118 170 57 41 115 111 24 157 130 109 27 45 28 63 7 189 132 16 140 44 113 127 3 20 148 143 36 205 77

C C C C CH C CH C CH C C C C C C CH C CH C C C C C C CH C CH C C C C C C C C C C C CH

138.02 138.08 138.16 138.29 138.29 138.34 138.62 139.16 139.28 139.83 140.4 141.55 142.92 143.17 143.9 145.08 145.19 145.73 146.7 147.73 147.95 148 148.36 149.16 149.31 150.92 151.1 153.2 153.93 154.25 154.35 154.54 157.07 158.54 160.48 167.9 167.99 169.31 193.21

2.3 2.7 3.2 0.2 1.8 2.5 4.9 3 8.7 0.8 0.6 6.9 3.3 3.2 4.1 3.6 5.2 5.4 2.3 7.2 19.8 7.1 3.5 1.9 3.1 2 2.9 2.5 7.7 12.1 1.2 4 14.3 2.3 6.3 1 2.7 2 3.8

ties estimated for styrene and coal models using this method are in good agreement with experimentally determined values. Construction of a Model Structure for the Extraction Residue Aromatic Ring (Unit) Structure. 1H NMR spectra and the ultimate analyses have been reported for the AS, PS, and PI extract fractions.2,3 For each fraction, structural parameters (fa, aromaticity; Har/Car, degree of aromatic ring condensation; σ, degree of aromatic ring substitution; and n, average length of aliphatic side chain) were determined using the modified Brown(19) Nakamura, K.; Murata, S.; Nomura, M. Energy Fuels 1993, 7, 347. (20) Murata, S.; Nomura, M.; Nakamura, K. Energy Fuels 1993, 7, 469. (21) Dong, T.-L.; Murata, S.; Miura, M.; Nomura, M.; Nakamura, K. Energy Fuels 1993, 7, 1123.

Ladner equation22 based on ultimate analysis and 1H NMR. The values of Har/Car for all fractions were 0.680.69, corresponding to 3-4 aromatic ring size.2 For the construction of model structures for each fraction, chemical shift calculations were carried out using the CNMR predictor software. First, the chemical shifts of all the carbons in the model structure constructed on the basis of structural parameters were calculated. Next, the 13C NMR spectra were calculated by considering an adequate line width for each peak and summing, supporting that all were Gaussian peaks. The model structures were modified after comparison with actual spectra, and the chemical shifts were calculated again for the revised structures. By repeating this process several times, we determined the best fitting structures. Finally, the model structures for the three fractions were successfully constructed as shown in Figure 3.14 (22) Kanda, N.; Itoh, H.; Yokoyama, S.; Ouchi, K. Fuel 1978, 57, 676.

384



Figure 7. Another model structure (model B) for MI.

We have reported5,6 that addition of 1% of tetracyanoethylene (TCNE) to the CS2/ NMP mixed solvent enhanced greatly the extraction yield of Upper Freeport coal, and that the original residue became part of the PI fraction; in fact, the amount of increase in the PI fraction corresponded precisely to the decrease in the amount of the MI.5 TCNE is rapidly converted to the pentacyanopropenide anion in NMP, the species that causes the increase in the extraction yield.7,23,24 We assumed that the PI and MI have similar chemical structures. Thus, we considered that ring structures of the MI are similar to those of PI, and consist primarily of structures with 3-5 rings. Models of unit structures are shown in Figure 4. Cross-Linked Structure. It has been widely accepted that coal has a covalently bound cross-linked network.25,26 It is unlikely, however, that Upper Freeport coal has such a network structure because of the high extraction yields that are obtained with the CS2/ NMP1-3 and the CS2/NMP/TCNE5-7 mixed solvent at room temperature. Therefore, the authors assumed on the analogy to the above-mentioned observation, that the MI fraction has not necessarily a three-dimensional covalent network. Model Structures for the MI. The ultimate analysis and molecular formulas for MI are listed in Table 2. It was assumed that the model structure for MI is composed of two different molecules similar to those assumed for the extract fractions (Figure 3). The unit structures in Figure 4 were randomly connected to form two molecules that are shown in Figure 5. On the basis of ultimate analysis and structural data, the amount of each atom type is determined and summarized in Table (23) Chen, C.; Kurose, H.; Iino, M. Energy Fuels 1999, 13, 1180. (24) Takahashi, K.; Norinaga, K.; Masui, Y.; Iino, M. Energy Fuels 2001, 15, 141. (25) Given, P. H.; Marzec, A.; Barton, W. A.; Lynch, L. J.; Gerstein, B. C. Fuel 1986, 65, 155. (26) 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.

Figure 8. The calculated and observed 13C NMR spectra: (a) model A and (b) model B.

3. Thus, aliphatic carbon, oxygen, nitrogen, and sulfur atoms were attached to the aromatic clusters. The 13C NMR spectrum calculated for the model structures was compared with the observed one. The model structure was changed several times until the calculated spectrum almost fit the observed one. One representative model structure (model A) is shown in Figure 6, and the chemical shifts calculated for all carbons in the model structure are shown in Table 4. As shown in Table 2, the value of aromaticity fa



Figure 9. Seven coal molecules put randomly in a rectangular cell.

calculated for the model A was 0.85, slightly higher than that (0.81) determined from 13C NMR. The difference may be attributed to the inner carbons that could not be detected in the solid-state NMR. Another model structure (model B, same fa as model A) was constructed as shown in Figure 7. The model consists of one big molecule, in which every aromatic unit connected with one another through covalent bonds; the structure was obtained by replacing three naphthenic aliphatic carbons (C-195, C-196, and C-197) in Figure 6 with three ring-joining methylene carbons in Figure 7. For two model structures in Figures 6 and 7, the calculated 13C NMR spectra are shown in Figure 8, together with the observed one. For the model B (Figure 8 b), a significant difference at 35-50 ppm between the calculated and observed spectra was seen, while, for the model A (Figure 8 a) the calculated spectrum was in relatively good agreement with the observed one. Therefore, the model A was selected as the final model structure of the extraction residue (MI). Calculation of a Molecular Model for Upper Freeport Raw Coal Associated Structure. We constructed27 threedimensionally a model structure of Zao Zhuang bituminous coal using computer-aided molecular design by (27) Nakamura, K.; Takanohashi, T.; Iino, M.; Kumagai, H.; Satou, M.; Yokoyama, S.; Sanada, Y. Energy Fuels 1995, 9, 1003.

assuming an anisotropic model structure formed from a periodic boundary cell. The same method was used to construct a model structure with a molecular weight of about 6700 daltons for Upper Freeport coal.2 Model structures for all fractions including MI of Upper Freeport coal were randomly placed in a rectangular cell (Figure 9) to give an associated structure of seven molecules (AS 1; PS and PI, 2 each; and the residue (MI), 2 molecules) with a continuous molecular weight distribution from AS to MI. This structure is significantly different from the widely accepted25,26 “two-phase model” structure that consists of a covalently bound cross-linked network and a small amount of lowmolecular-weight component trapped in the network. Recently, Mathews, Hatcher, and Scaroni constructed28 a model structure for Upper Freeport vitrinite by using a computational program “SIGNATURE”.29-31 They are trying to construct a representative model to explain several reactions such as carbonization and liquefaction, and the values of He density and CO2 surface estimated for the model structure were in agreement with experimentally obtained ones.28 The structure consisted of a covalently cross-linked macromolecule, in disagreement (28) Mathews, J. P.; Hatcher, P. G.; Scaroni, A. W. Energy Fuels 2001, 15, 863. (29) Faulon, J.-L.; Carlson, G. A.; Hatcher, P. G. Energy Fuels 1993, 7, 1062. (30) Faulon, J.-L.; Hatcher, P. G.; Carlson, G. A.; Wenzel, K. A. Fuel Process. Technol. 1993, 34, 277. (31) Faulon, J.-L.; Mathews, J. P.; Carlson, G. A.; Hatcher, P. G. Energy Fuels 1994, 8, 408.

386


Figure 10. Plot of calculated density versus total energy for the molecular model of Upper Freeport coal.


with the model proposed in the present study. The nature of cross-linking structure of Upper Freeport coal should be further clarified in details. For the molecular assemblage, energy-minimum conformation was evaluated using the MM-MD procedure. In the procedure, the physical density of the model structure can be calculated from the volume and total weight of model molecules. The volume was calculated from the difference between the whole volume of the cell and the void volumesthe accessible volume of water to the model structure in the cell based on the molecular volume of water. The change in total energy with the physical density is shown in Figure 10. The total energy plateaued at around 1.27 g/cm3. The size of the cell was reduced manually, and MM-MD calculation was carried out; this procedure was repeated several times. Finally, the energy-minimum point was observed at a density

Figure 11. A molecular model for Upper Freeport coal in a basic cell; (a) shadow area shows accessible area of water molecule, and (b) two cells enclosing the model in the energy-minimum state.


Figure 12. Distribution of micropores for the model structure.

of 1.28 g/cm3 (Figure 10), in good agreement with the experimentally obtained value of 1.30 g/cm3. Figure 11 a shows the conformation at minimum energy for the model molecules in the cell. The size of cell was 53.6 Å × 55.8 Å × 4.2 Å. An anisotropic associated structure was obtained. Cody et al. have reported32 anisotropic swelling behavior for bituminous coals: the swelling ratio was greater perpendicular to the bedding plane than parallel to it. Shadow area in Figure 11 (a) shows accessible area of water molecule. By changing the diameter of adsorbed probe in this simulation, distribution of micropores for the model structure was estimated, and the result is shown in Figure 12. In this simulation, only microporosity can be evaluated because the bulk structure is simulated by using a periodic boundary condition. Figure 12 shows that there seem to be many micropores smaller than 6 Å, and micropores larger than 7 Å are only 12.4% of the total ones. On the basis of the results of inverse liquid chromatography33 and vapor sorption34 using alcohols with different alkyl group bulk, we reported that Upper Freeport coal has a large number of micropores into which relatively bulky reagents could diffuse only marginally. The developed microporosity for the model structure of Upper Freeport coal can explain those experimental results. Figure 11b shows two cells enclosing the model structure at minimum energy. Aromatic rings seem to (32) Cody, J. G. D.; Larsen, J. W.; Siskin, M. Energy Fuels 1988, 2, 340. (33) Takanohashi, T.; Nakano, K.; Yamada, O.; Kaiho, M.; Ishizuka, A.; Mashimo, K. Energy Fuels 2000, 14, 720. (34) Takanohashi, T.; Terao, Y.; Yoshida, T.; Iino, M. Energy Fuels 2000, 14, 915.


interact with one another perpendicular to the bedding plane. The distribution of the distances between aromatic clusters in the model structure was 3.5-5.5 Å, and the average distance was 4.1 Å. X-ray diffraction data of Wertz and Bissell showed35 that the average distance between the polycyclic aromatic planes of Upper Freeport coal is 3.6 Å. Thus the value obtained here is slightly higher than that measured by X-ray. Large distances (>4.5 Å) observed at sites of more strained structures in the model seem to increase the average value. Recently, Li et al. reported36 that heat treatment of Pocahontas No. 3 and Pittsburgh No. 8 coals in NMP/ hexahydroanthracene mixed solvents at 300 °C gave high dissolution yields of 90 wt % and 84 wt % (daf), respectively. The authors suggest two possible explanations for the result. One is that relatively weak covalent bonds including ether linkage might be cleaved at 300 °C, although the amount of such weak bonds may be small. Another explanation is that Pocahontas No. 3 and Pittsburgh No. 8 coals may also have an associated structure similar to that described here for Upper Freeport coal. Structural analyses and molecular modeling for other coals will be continued. Conclusions An associated model structure of Upper Freeport raw coal with a continuous molecular-weight distribution from the lightest extract fraction to the extraction residue (MI) was constructed, and the conformation of a seven-molecule raw coal model was determined using MM-MD methods under periodic boundary condition. The resulting associated structure for Upper Freeport, which is not covalently cross-linked, allowed us to explain the high extraction yields even at room temperature. The physical density estimated from the energy-minimized conformation of this structure was 1.28 g/cm3, in good agreement with the experimentally determined value of 1.30 g/cm3. The average distance between the aromatic planes was calculated 4.1 Å, slightly higher than the reported value of 3.6 Å. Acknowledgment. This work has been carried out as one of “Research for the Future” projects of the Japan Society for the Promotion of Science (JSPS) through the 148th committee on coal utilization technology of JSPS. EF0101154 (35) Wertz, D. L.; Bissell, M. Energy Fuels 1994, 8, 613. (36) Li, C.; Ashida, S.; Iino, M.; Takanohashi, T. Energy Fuels 2000, 14, 190.

Construction of a Model Structure for Upper Freeport Coal Using 13C

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