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Evidence for the associated structure of bituminous coal
Evidence for the associated bituminous coal Masaharu
structure
of
Nishioka”
BCR National Laboratory, 500 William Pitt Way, Pittsburgh, (Received 26 March 7992; revised 2 November 1992)
PA 15238,
USA
Previous studies have shown that coal swelling is not reversible and highly dependent on coal concentration in a solvent. This rules out the conventional coal structural model of a covalently cross-linked three-dimensional network. Many coal molecules may be physically associated by relatively strong intra- and intermolecular interactions. Coal swelling in selected solvents was examined for fractionated samples of two high volatile C bituminous coals to investigate the role of physical associations in coal structure. The
dependence of coal concentration on swelling in tetrahydrofuran and toluene was similar for pyridine soluble and insoluble fractions. Solvent swelling of pyridine solubles was smaller than that of pyridine insolubles. Pyridine extract was mixed with iodine and further fractionated into pyridine soluble and insoluble fractions. The swelling ratio of the pyridine solubles was much smaller than that of the pyridine insolubles. The difference between swelling with toluene and with tetrahydrofuran was larger for pyridine insolubles than for pyridine solubles. All these results indicate that significant portions of coal molecules are not a three-dimensional network, but are physically associated. (Keywords: solvent swelling; associated structure; coal)
A series of reviews concerning the molecular nature of coal is availablelp3. Among them, a cross-linked threedimensional macromolecular model occluding some solvent-extractable components has been widely accepted for the last thirty years since van Krevelen4 reviewed the ‘polymeric’ character of coal in 1961. Wiser’s’ and Shinn’@ models would be representative examples of the model. As their models show, it is thought that aromatic ring systems are linked by carbon or heteroatom bridges. Relatively strong intra- and intermolecular (secondary) interactions play an important role in the structure of all ranks of coal, and their abundances are far more significant than generally believed7-9. Pyridine is one of the best solvents for coal, but can disrupt only a limited fraction of these relatively strong interactions. The importance of physical associations due to these relatively strong interactions in coal should not be underestimated”. The extent to which coal molecules may be covalently cross-linked (A) and/or physically associated (B) is illustrated in Figure I. It is the author’s opinion that direct evidence to elucidate the real state of the physical structure has not been obtained. Model A should exhibit rubber-like elasticity and be reversible in solvent swelling as do cross-linked polymeric materials. Whereas, Model B may not be reversible in solvent swelling, because interacted sites are not permanently fixed during swelling and deswelling. Irreversibility of coal swelling has been observed for high and medium volatile bituminous coals”. These coals showed distinctively different irreversibility, which is consistent with different behaviour on solvent extractability of these coals.
* Present address: Viking Systems International, 2070 William Pitt Way, Pittsburgh,
PA 15238, USA
0016-2361/93/12/171946 0 1993 Butterworth-Heinemann
Ltd.
Coal based on Model A should not change its swelling ratio upon changing coal concentration, while coal based on Model B may change its swelling ratio because complexes with relatively strong secondary interactions dissociate to some extent at low coal concentration. The degree of change is due to the magnitude of change in dissociative equilibria. It has been shown previouslyi that volumetric solvent swelling is highly dependent on coal concentration, and is significantly enhanced at low concentration. Caution is advised on ‘coal concentration’. Most of the coal samples do not dissolve in solvents. The system studied may be regarded as a slurry. However, concentration in a coal-solvent system cannot be clearly defined, as discussed in Reference 12. In the previous and present papers, ‘coal concentration’, coal-solvent mass ratio (C/S), is used as an index of concentration. For these reasons Model A, for which all cross-linkings are covalently bonded, can be ruled out. Relatively strong interactions must play an important role as ‘cross-links’. As disruption energies of physical associates and covalent bonds generally differ by one to two orders of magnitude, a clear distinction between the covalently bonded (chemical) cross-links and relatively strong secondary interactions (physical cross-links) is necessary for coal technology. If physical associations predominate, then all coal properties are due to the strong function of secondary interactions. If covalent cross-links predominate, then selective bond cleavages must be the essential process for coal upgrading. The first step to characterize polymeric material is to study interactions between the material and various solvents by using extraction, fractionation and swelling etc. As many portions of coal are undissolved in any solvent, solvent swelling would be a very useful measure for coal characterization. In this paper, solvent swelling is utilized to investigate the physical structure of coal.
Fuel 1993
Volume
72 Number
12
1719
Associated
structure
of bituminous
coal:
M. Nishioka
RESULTS AND DISCUSSION Comparison
of solvent swelling between
and associated
A Figure 1 Illustration (B) models
B of the conventional
network (A) and the associated
The dependence of coal concentration on swelling in selected solvents is studied for fractionated coal samples. EXPERIMENTAL American Chemical Society reagents and h.p.1.c. grade solvents were used. Tetrahydrofuran (THF) was distilled before use, and the other solvents were used without purification. Coal samples were obtained from the Premium Sample Program at Argonne National Laboratory’ 3 and the Pennsylvania State University Coal Bank. Coal particles of - 100 mesh size were used and their analyses are shown in Table I. The symbols given in Tuble I will be used for the Argonne Premium coal samples. Samples (2-5 g) were extracted under a nitrogen atmosphere with 200 ml pyridine for 3 days in Soxhlet apparatus, and then residues were Soxhlet rinsed with methanol for 24 h. Extracts and residues were dried in a vacuum oven at 95°C overnight. For the iodine treatmenta, about 50wt% of iodine versus the extract was added in pyridine solution after Soxhlet extraction. The mixture was magnetically stirred for 0.5 h. Pyridine was evaporated with a rotary evaporator, methanol was added, and the mixture was scratched out and filtered. The sample was further Soxhlet rinsed with methanol for 24 h and dried under the above conditions. The experimental procedure of coal swelling is based on an earlier reported method”,12. The measurements were performed in disposable Wintrobe tubes (Fisher Scientific, Pittsburgh, USA) of 3 mm inner diameter and 115 mm in length, with graduations in 1 mm divisions. After placing three weighted coal samples into the tubes, these tubes were centrifuged for 5 min (at 3600 rev min- ’ in a 30 cm diameter horizontal rotor). Bulk density (BD) of each coal under these conditions was determined by the average height centrifuged. Each initial height before solvent swelling was calculated from the mass of the samples using BD. Solvent volume was measured and added to each weighed sample, and the content was vigorously stirred with a thin rod. The tubes were centrifuged for 5 min at 3600revmin-‘. Generally, the heights of swollen samples were promptly read. The volumetric swelling ratio (Q) was calculated by the difference between the heights of samples. The specific swelling ratio (Q’) defined by the following equation12 is used in this paper. Q’ = Q/BD
Fuel 1993
The importance of each contributions from physical and chemical cross-links may be estimated by the dependence of coal concentration on swelling of fractionated samples. Although pyridine extract (PS) is partially dissolved, it is swollen in poorer solvent than pyridine. Associated sites in PS behave as physical cross-links. Physical cross-links should demonstrate dissociative equilibria under a certain condition, as represented in Figure 2. As it is thought that associated sites partially dissociates at low coal concentration; enormous numbers of equilibrium states are possible for PS, because the extract is a mixture of high molecular mass compounds with polyfunctionality, and several physical cross-links in a molecule are possible. For pyridine residue (PI), the possibility of all the covalent bonds forming cross-links can be ruled out, as discussed in the introduction (residue 1 in Figure 2). If PI is cross-linked both physically and chemically (residue 2 in Fiyure 2), PI should show dissociative equilibria to some Table 1
Elemental
analyses
of coals used (w&X) Element
(daf)
Coal symbol
Coal
H,O
Ash
C
H
N
S
ND _ IL UF
North Dakota PSOC-1491 Illinois No. 6 Upper Freeport
32.2 11.5 8.0 1.1
6.6 15.2 14.3 13.0
72.9 82.4 77.7 85.5
4.8 5.1 5.0 4.7
1.2 1.7 1.4 1.6
0.9 3.4 5.7 2.7
Extract
1-i 11 H H 14 2
2
z:z
Z
z:z
1 -
1 -
z:i!
Z
Z
Z
z:z
Z
Z
Residue X
1
X
(1)
The value Q’ corresponds to the swollen volume (ml) of 1 g sample. Alternatively, Q’ can be directly calculated in terms of the height of a swollen sample and mass of a sample. BD of each sample is shown in this paper to be compared to the conventional value Q.
1720
the network
models
Volume
72 Number
12
Figure 2 Dissociative equilibria for extract and residue (Z:Z and -X- represent physical and chemical cross-links, respectively)
Associated
extent. However, possible numbers of equilibrium states should be compared much less to those of the extract. This means that swelling of PI is enhanced at low sample concentration, but the enhancement in swelling of PI is not so great as that of PS. The difference between the change in equilibria of PS and PI, by changing their concentration, should be very significant. Therefore, Model A should show a significantly different dependence of coal concentration on equilibria for these samples. Whereas, Model B should show no such significant difference. Physical cross-links are solvated to some extent under certain conditions. However, as chemical cross-links are permanently bonded in a non-reactive solvent, Model A predicts that the swelling value of PI will be less than that of PS at nearly the same sample concentration. On the basis of Model B, molecular size would be an important factor for solubilization, as seen for many polymeric materials. The average molecular size of PI should be larger than that of the extract as shown in Figure 3. Therefore, the apparent average molecular mass between physical cross-links in PI would be larger than that in PS. Accordingly, Model B predicts that the swelling value of PI is larger than that of PS. A better solvent can disrupt more physical cross-links than a poorer solvent. Therefore, on the basis of Model A, the swelling ratio of PS is more enhanced than that of PI when a better solvent is used. The apparent molecular mass between physical cross-links in PI is larger than that in PS for Model B. If these physical cross-links are disrupted with a better solvent, the increment in apparent molecular mass between physical
Coal
Increase
in Mw
-
1
Pyridine extraction
Y
L:
Extract
A
Residue
A
Increase -
in Mw
Increase -
in Mw
‘2
N
1 Pyridine extraction I
._
LLLh_._ Extract
B
Residue
B
Figure 3 Schematic diagram of molecular mass distributions of each fraction obtained from pyridine Soxhlet extraction (I,, iodine; M,, molecular weight)
structure
of bituminous
coal: M. Nishioka
Table 2 Comparison of predicted solvent swelling for the network (Model A) and the associated (Model B) models of coal Solvent
swelling
Reswelling Dependence of coal concentration Dependence of coal concentration for fractionated samples Dependence of fractionation on swelling ratio (Q) Dependendence of solvent for fractionated samples
Model A
Model B
Reversible None or small
Irreversible Significant
Significantly different
Similar
Qsow~er ’ Qinso,uwerQro~es< Qinso,ub,cs Larger for solubles than insolubles
Larger for insolubles solubles
than
cross-links is expected to be larger for PI than for PS. Therefore, on the basis of Model B, the swelling ratio of PI is generally more enhanced than that of PS when a better solvent is used. The comparisons discussed above are summarized in Table 2. The first and second comparisons have already been reported llsl* . It is thought that the importance of physical and chemical cross-links is estimated by studying solvent swelling of fractionated samples. The last three comparisons will be examined here. Efects
of solvents on swelling
The effect of solvents on swelling was examined before studying solvent swelling of the fractionated samples. Swelling ratios of various samples from three different ranks of coals [lignite (ND), high volatile bituminous coal (IL) and medium volatile bituminous coal (UF)] were measured in various solvents. Table 3 shows the specific swelling ratios (Q’) at initial time and after 24 h. As solvent swelling is highly concentration-dependent, as previously reported’*, the swelling rate is likely to be dependent on coal concentration. Also, solvent swelling needs to be measured at very low concentration, with continuous mixing for a given period of time. A specially designed apparatus will be necessary to measure accurate solvent swelling in the future. However, in this study, swelling was measured at a given coal-solvent ratio (C/S = 4.4 + 0.2 wt%) with mixing only at initial time. Swellings in pyridine of ND and IL coals both at room temperature and 70°C are the same, but swellings of raw and dried UF coals at room temperature are smaller than those at 70°C. Two mixed solvents, 1 M of tetrabutylammonium hydroxide (TBAH) in methanol/pyridine (25/75 ~01%) and N-methyl-2-pyrrolidinone (NMP)/ carbon disulphide (50/50v01%), swelled these three coals more than pyridine did in most cases. These results mean that coal in pyridine at room temperature associates to some degree. The swelling rate of lower rank coal becomes slow by dryingi4. The dried ND coal, and its PI, swell slowly in all solvents used except in toluene as shown in Table 3. However, the effect of moisture on coal swelling for the IL coal is not so great as that for the ND coal. Better solvents than pyridine (TBAH/pyridine and NMP/CS,) promptly swell even the dried IL coal and its PI; but THF, which is a poorer solvent than pyridine, slowly swells even the raw IL coal and its PS. Swelling in THF of PI and PS from two high volatile C bituminous coals will be studied in the following sections. The swelling
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1721
Associated Table 3
structure
Specific swelling
of bituminous ratios
in various
coal: M. Nishioka
solvents
of three different
ranks
of coals at the initial time and after 24 h (C/S = 4.4kO.2
wt%)
Specific swelling ratio, Q’ ND Coal solvent
IL
Initial time
After 24 h
Initial time
Toluene
1.3
1.3
Tetrahydrofuran
1.6
1.7
Pyridine
(25°C)
1.8
Pyridine
(70°C)
UF After 24 h
Initial time
After 24 h
1.2
1.2
1.2
1.2
1.9
2.3
1.2
1.2
1.9
2.7
2.7
1.2
1.2
Raw coal
_
1.9
_
2.8
_
1.6
TBAH/pyridine”
3.3
3.9
3.5
3.7
1.3
2.0
NMP/CS,*
2.2
3.0
3.1
3.3
1.4
2.0
1.1
Dried coal Toluene
1.3
1.3
1.2
1.3
1.1
Tetrahydrofuran
1.4
2.0
2.1
2.4
1.1
1.1
Pyridine
(25°C)
1.5
2.4
2.4
2.8
1.3
1.3
Pyridine
(70°C)
_
2.2
_
2.9
_
1.6
TBAH/pyridine”
2.0
4.5
3.8
3.8
1.3
1.8
NMP/CSzb
1.4
2.0
3.3
3.3
1.3
1.3
Residue
(PI)
Toluene
1.3
1.3
1.6
1.8
2.1
2.1
Tetrahydrofuran
1.4
1.7
2.6
2.1
2.5
2.4
Pyridine
(25°C)
1.6
2.1
2.9
3.0
2.5
2.3
Pyridine
(70°C)
_
2.1
_
3.0
_
2.2
TBAH/pyridine”
2.3
4.0
3.5
4.1
3.7
4.5
NMP/CS,*
1.4
1.8
2.8
3.1
3.1
2.8
Extract
(PS)
Toluene
1.4
1.3
1.4
1.4
1.3
1.3
Tetrahydrofuran
c
c
1.6
2.1
1.3
2.1
“TBAH/pyridine: 1 M of tetraammonium hydroxide solution in methanol b NMP/CS,: N-methyl-2-pyrrolidinone (50 vol)/CS, (50 vol) ‘Swelling was not measured because of large solubility of the sample
values are measured promptly after swelling, because of experimental difficulty as discussed above. Although ultimate solvent swelling under an ideal condition has not been obtained, THF swelling is significantly different for the samples of PI and PS from the IL coal both at the initial time and after 24 h under the conditions studied (Table 3). The difference in THF swelling between PI and PS at the initial time will be considered when investigating the coal structural model. Dependence of coal concentration on swelling for fractionated samples
Illinois No. 6 coal was fractionated into Extraxt A and Residue A by Soxhlet extraction with pyridine (Figure 3). Extract A was further fractionated into Extract B and Residue B by iodine treatment, followed by Soxhlet extraction with pyridine. Residue B (yield: 60 wt% of Extract A) is thought to be composed of relatively high molecular mass associated with the charge-transfer interaction in Extract A. When a solvent contains solute to some extent, solvent activity is not 1.0. Unfortunately, the coal has not been characterized yet, so as to estimate solvent activity under the conditions usually used for coal swelling. Reference 12 considers how significantly solubles in a solvent affected coal swelling no matter how great the solvent activity was. Four lines of evidence” showed that
1722
Fuel 1993
Volume
72 Number
12
(25 vol)/pyridine
(75 vol)
solubles (at least up to 30 wt%) did not significantly affect solvent swelling of a high volatile bituminous coal. Even if a sample contained 10 wt% pyridine solubles, swelling ratios in pyridine changed only to -0.1. THF dissolves less than 5 wt% of Extracts A and B under these conditions. Therefore, the effect of a small extent of solubles in THF can be ignored for this study. Swelling ratios of these samples were measured with THF. These results are shown in Figure 4. The swelling ratio of Extract B was very small and could not be measured at its low concentration, and the dependence of coal concentration was not determined. The dependence of coal concentration on swelling is nearly equal for Extract A, Residues A and B. This is against the prediction derived for Model A as discussed above. Dependence of fractionation of swelling ratio
The specific swelling ratios, Q’, in THF of Residue A are significantly larger than those of Extract A as shown in Figure 4. The difference between Extract B and Residue B is more remarkable. The swelling values in THF of PS and PI from another high volatile C bituminous coal, PSOC-1491, are shown in Figure 5. The specific swelling ratios, Q’, of the PI are also larger than those of the PS. These results are again consistent with Model B as discussed above. The specific swelling ratios, Q’, in THF of these native coals are also shown in Figures 4 and 5.
Associated structure of bituminous coal: M. Nishioka
3.0 -
3.0
I
1
I
I
AA A A
2.5 -
0
2.5
A
l
b b
2.0
00 -a
A
A
0
A
0
2.0 -
A 0
0
O
0 0
1.5 -
1.5 n
0
n n
1.0
A
0
l
5
10
15
Coal/Solvent
0
0
0
n
.._
0
0
20
ht/wt
25
0
5
IO Coal/Solvent
%)
Figure 4 Specific swelling ratios (Q’) of: 0, Extract A (BD=O.Sl); n , Extract B (BD=0.96); 0, Residue A (BD=O.Sl); 0, Residue B (BD = 0.94); and, A, the native sample (BD =0.90) of Illinois No. 6 coal with tetrahydrofuran (see Figure jl for the sample name)
15 (wt/wt
20
Figure 6 The specific swelling ratios (Q’) oE 0, Extract A; 0, Residue A with toluene; and, A, Residue A with pyridine for Illinois No. 6 coal (see Figure 3 for the sample name)
Dependence of solvent on swellingforfractionated
2.5
O” 0
ti
0
“0A
2.0
0
0
0
DA
A Cl
samples
The effect of other solvents on swelling were examined for the fractionated samples. Figure 6 shows the specific swelling ratios, Q’, in toluene and pyridine of Extract A and Residue A from the IL coal. The specific swelling ratios, Q’, in toluene of Residue A are larger than those of Extract A. The dependence of concentration on the Q’ values of Residue A in pyridine are approximately equal to those in THF. These results are consistent with Model B, even if different solvents are used for swelling. The specific swelling ratios at high coal concentration of Residue A are 2.2 in THF and 1.4 in toluene. The values of Extract A are 1.7 and 1.3, respectively. Therefore, the difference of swelling with poor and better solvents is larger for PI than PS. This result again supports Model B as discussed above.
3.0
AA
25
%)
qA
1.5
CONCLUSION
1.0 0
5
10 Coal/Solvent
15 hvt/wt
Figure 5 Specific swelling ratios (Q’) oE 0, pyridine 0, pyridine residue (BD = 1.0); and, A, the native of PSOC-1491 coal with tetrahydrofuran
20
25
%1 extract (BD= 1.0); sample (BD = 1.0)
The swelling values of the two native coals are close to those of their extracts. This may be rationalized as follows on the basis of Model B. The number of physical cross-links between surrounding molecules would be larger when a sample contains molecules with small molecular mass, because the coordination numbers with smaller coal molecules are expected to be larger. If good solvents are used for coal swelling, molecules with small molecular mass are readily dissolved in the solvent. In this case, coal swelling may be different from PS swelling. In fact, swelling in pyridine of the PSOC-1491 coal was nearly equal to that of PI I2 . Consequently, all the results obtained from fractionated samples are reasonable for Model B.
The following effects on solvent swelling of high volatile C bituminous coals were investigated using their fractionated samples: concentration, swelling ratios and solvents. Pyridine-Soxhlet extraction was simply chosen to fractionate these coals. As Table 2 shows, Models A and B in Figure I should result in distinctively different swelling. It is obvious that all results obtained in this work coincide with the predictions for Model B shown in Table 2. Therefore, it can be concluded that significant portions of coal molecules in these two coals are not three-dimensional networks, but are associated.
REFERENCES 1
2
3
Green, T., Kovac, J., Brenner, D. and Larsen, J. W. in ‘Coal Structure’ (Ed. R. A. Meyers), Academic Press, New York, 1982, Chapter 6, pp. 199-282 Given, P. H. in ‘Coal Science’ (Eds M. L. Gorbaty, J. W. Larsen and I. Wender), Academic Press, New York, 1984, Vol. 3, pp. 63-252 Berkowitz, N. in ‘Polynuclear Aromatic Compounds’ (Ed. L. B. Ebert), Advances in Chemistry 217, American Chemical Society, Washington, DC, 1987, Chapter 13, pp. 217-233
Fuel 1993
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1723
Associated structure of bituminous coal: /VI. Nishioka 4 5
6 7 8
1724
van Krevelen. D. W. in ‘Coal’, Elsevier, New York. 1961. o. 440 Wiser, W. H: in ‘Proceedings of the’ Electric Power Research Institute Conference on Coal Catalvsis’. Palo Alto. California. _ 1973, p. 3 Shinn, J. H. Fuel 1984, 63, 1187 Nishioka, M. and Larsen, J. W. Energy & Fuels 1990, 4, 100 Nishioka, M., Gebhard, L. A. and Silbernagel, B. G. Fuel 1991, 70, 341
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9 10 11 12 13 14
Nishioka, M. Energy & Fuels 1991, 5, 487 Nishioka, M. Fue1-i992, 71, 941 Nishioka. M. Fuel 1993, 72. 997 Nishioka, M. Fuel 1993,72; 1001 Vorres, K. S. Energy & Fuels 1990, 4, 420 Otake, Y. and Suuberg, E. M. Am. Chem. Sot. Div. Fuel Gem. Prep. 1988, 33(4), 898
structure
of
Nishioka”
BCR National Laboratory, 500 William Pitt Way, Pittsburgh, (Received 26 March 7992; revised 2 November 1992)
PA 15238,
USA
Previous studies have shown that coal swelling is not reversible and highly dependent on coal concentration in a solvent. This rules out the conventional coal structural model of a covalently cross-linked three-dimensional network. Many coal molecules may be physically associated by relatively strong intra- and intermolecular interactions. Coal swelling in selected solvents was examined for fractionated samples of two high volatile C bituminous coals to investigate the role of physical associations in coal structure. The
dependence of coal concentration on swelling in tetrahydrofuran and toluene was similar for pyridine soluble and insoluble fractions. Solvent swelling of pyridine solubles was smaller than that of pyridine insolubles. Pyridine extract was mixed with iodine and further fractionated into pyridine soluble and insoluble fractions. The swelling ratio of the pyridine solubles was much smaller than that of the pyridine insolubles. The difference between swelling with toluene and with tetrahydrofuran was larger for pyridine insolubles than for pyridine solubles. All these results indicate that significant portions of coal molecules are not a three-dimensional network, but are physically associated. (Keywords: solvent swelling; associated structure; coal)
A series of reviews concerning the molecular nature of coal is availablelp3. Among them, a cross-linked threedimensional macromolecular model occluding some solvent-extractable components has been widely accepted for the last thirty years since van Krevelen4 reviewed the ‘polymeric’ character of coal in 1961. Wiser’s’ and Shinn’@ models would be representative examples of the model. As their models show, it is thought that aromatic ring systems are linked by carbon or heteroatom bridges. Relatively strong intra- and intermolecular (secondary) interactions play an important role in the structure of all ranks of coal, and their abundances are far more significant than generally believed7-9. Pyridine is one of the best solvents for coal, but can disrupt only a limited fraction of these relatively strong interactions. The importance of physical associations due to these relatively strong interactions in coal should not be underestimated”. The extent to which coal molecules may be covalently cross-linked (A) and/or physically associated (B) is illustrated in Figure I. It is the author’s opinion that direct evidence to elucidate the real state of the physical structure has not been obtained. Model A should exhibit rubber-like elasticity and be reversible in solvent swelling as do cross-linked polymeric materials. Whereas, Model B may not be reversible in solvent swelling, because interacted sites are not permanently fixed during swelling and deswelling. Irreversibility of coal swelling has been observed for high and medium volatile bituminous coals”. These coals showed distinctively different irreversibility, which is consistent with different behaviour on solvent extractability of these coals.
* Present address: Viking Systems International, 2070 William Pitt Way, Pittsburgh,
PA 15238, USA
0016-2361/93/12/171946 0 1993 Butterworth-Heinemann
Ltd.
Coal based on Model A should not change its swelling ratio upon changing coal concentration, while coal based on Model B may change its swelling ratio because complexes with relatively strong secondary interactions dissociate to some extent at low coal concentration. The degree of change is due to the magnitude of change in dissociative equilibria. It has been shown previouslyi that volumetric solvent swelling is highly dependent on coal concentration, and is significantly enhanced at low concentration. Caution is advised on ‘coal concentration’. Most of the coal samples do not dissolve in solvents. The system studied may be regarded as a slurry. However, concentration in a coal-solvent system cannot be clearly defined, as discussed in Reference 12. In the previous and present papers, ‘coal concentration’, coal-solvent mass ratio (C/S), is used as an index of concentration. For these reasons Model A, for which all cross-linkings are covalently bonded, can be ruled out. Relatively strong interactions must play an important role as ‘cross-links’. As disruption energies of physical associates and covalent bonds generally differ by one to two orders of magnitude, a clear distinction between the covalently bonded (chemical) cross-links and relatively strong secondary interactions (physical cross-links) is necessary for coal technology. If physical associations predominate, then all coal properties are due to the strong function of secondary interactions. If covalent cross-links predominate, then selective bond cleavages must be the essential process for coal upgrading. The first step to characterize polymeric material is to study interactions between the material and various solvents by using extraction, fractionation and swelling etc. As many portions of coal are undissolved in any solvent, solvent swelling would be a very useful measure for coal characterization. In this paper, solvent swelling is utilized to investigate the physical structure of coal.
Fuel 1993
Volume
72 Number
12
1719
Associated
structure
of bituminous
coal:
M. Nishioka
RESULTS AND DISCUSSION Comparison
of solvent swelling between
and associated
A Figure 1 Illustration (B) models
B of the conventional
network (A) and the associated
The dependence of coal concentration on swelling in selected solvents is studied for fractionated coal samples. EXPERIMENTAL American Chemical Society reagents and h.p.1.c. grade solvents were used. Tetrahydrofuran (THF) was distilled before use, and the other solvents were used without purification. Coal samples were obtained from the Premium Sample Program at Argonne National Laboratory’ 3 and the Pennsylvania State University Coal Bank. Coal particles of - 100 mesh size were used and their analyses are shown in Table I. The symbols given in Tuble I will be used for the Argonne Premium coal samples. Samples (2-5 g) were extracted under a nitrogen atmosphere with 200 ml pyridine for 3 days in Soxhlet apparatus, and then residues were Soxhlet rinsed with methanol for 24 h. Extracts and residues were dried in a vacuum oven at 95°C overnight. For the iodine treatmenta, about 50wt% of iodine versus the extract was added in pyridine solution after Soxhlet extraction. The mixture was magnetically stirred for 0.5 h. Pyridine was evaporated with a rotary evaporator, methanol was added, and the mixture was scratched out and filtered. The sample was further Soxhlet rinsed with methanol for 24 h and dried under the above conditions. The experimental procedure of coal swelling is based on an earlier reported method”,12. The measurements were performed in disposable Wintrobe tubes (Fisher Scientific, Pittsburgh, USA) of 3 mm inner diameter and 115 mm in length, with graduations in 1 mm divisions. After placing three weighted coal samples into the tubes, these tubes were centrifuged for 5 min (at 3600 rev min- ’ in a 30 cm diameter horizontal rotor). Bulk density (BD) of each coal under these conditions was determined by the average height centrifuged. Each initial height before solvent swelling was calculated from the mass of the samples using BD. Solvent volume was measured and added to each weighed sample, and the content was vigorously stirred with a thin rod. The tubes were centrifuged for 5 min at 3600revmin-‘. Generally, the heights of swollen samples were promptly read. The volumetric swelling ratio (Q) was calculated by the difference between the heights of samples. The specific swelling ratio (Q’) defined by the following equation12 is used in this paper. Q’ = Q/BD
Fuel 1993
The importance of each contributions from physical and chemical cross-links may be estimated by the dependence of coal concentration on swelling of fractionated samples. Although pyridine extract (PS) is partially dissolved, it is swollen in poorer solvent than pyridine. Associated sites in PS behave as physical cross-links. Physical cross-links should demonstrate dissociative equilibria under a certain condition, as represented in Figure 2. As it is thought that associated sites partially dissociates at low coal concentration; enormous numbers of equilibrium states are possible for PS, because the extract is a mixture of high molecular mass compounds with polyfunctionality, and several physical cross-links in a molecule are possible. For pyridine residue (PI), the possibility of all the covalent bonds forming cross-links can be ruled out, as discussed in the introduction (residue 1 in Figure 2). If PI is cross-linked both physically and chemically (residue 2 in Fiyure 2), PI should show dissociative equilibria to some Table 1
Elemental
analyses
of coals used (w&X) Element
(daf)
Coal symbol
Coal
H,O
Ash
C
H
N
S
ND _ IL UF
North Dakota PSOC-1491 Illinois No. 6 Upper Freeport
32.2 11.5 8.0 1.1
6.6 15.2 14.3 13.0
72.9 82.4 77.7 85.5
4.8 5.1 5.0 4.7
1.2 1.7 1.4 1.6
0.9 3.4 5.7 2.7
Extract
1-i 11 H H 14 2
2
z:z
Z
z:z
1 -
1 -
z:i!
Z
Z
Z
z:z
Z
Z
Residue X
1
X
(1)
The value Q’ corresponds to the swollen volume (ml) of 1 g sample. Alternatively, Q’ can be directly calculated in terms of the height of a swollen sample and mass of a sample. BD of each sample is shown in this paper to be compared to the conventional value Q.
1720
the network
models
Volume
72 Number
12
Figure 2 Dissociative equilibria for extract and residue (Z:Z and -X- represent physical and chemical cross-links, respectively)
Associated
extent. However, possible numbers of equilibrium states should be compared much less to those of the extract. This means that swelling of PI is enhanced at low sample concentration, but the enhancement in swelling of PI is not so great as that of PS. The difference between the change in equilibria of PS and PI, by changing their concentration, should be very significant. Therefore, Model A should show a significantly different dependence of coal concentration on equilibria for these samples. Whereas, Model B should show no such significant difference. Physical cross-links are solvated to some extent under certain conditions. However, as chemical cross-links are permanently bonded in a non-reactive solvent, Model A predicts that the swelling value of PI will be less than that of PS at nearly the same sample concentration. On the basis of Model B, molecular size would be an important factor for solubilization, as seen for many polymeric materials. The average molecular size of PI should be larger than that of the extract as shown in Figure 3. Therefore, the apparent average molecular mass between physical cross-links in PI would be larger than that in PS. Accordingly, Model B predicts that the swelling value of PI is larger than that of PS. A better solvent can disrupt more physical cross-links than a poorer solvent. Therefore, on the basis of Model A, the swelling ratio of PS is more enhanced than that of PI when a better solvent is used. The apparent molecular mass between physical cross-links in PI is larger than that in PS for Model B. If these physical cross-links are disrupted with a better solvent, the increment in apparent molecular mass between physical
Coal
Increase
in Mw
-
1
Pyridine extraction
Y
L:
Extract
A
Residue
A
Increase -
in Mw
Increase -
in Mw
‘2
N
1 Pyridine extraction I
._
LLLh_._ Extract
B
Residue
B
Figure 3 Schematic diagram of molecular mass distributions of each fraction obtained from pyridine Soxhlet extraction (I,, iodine; M,, molecular weight)
structure
of bituminous
coal: M. Nishioka
Table 2 Comparison of predicted solvent swelling for the network (Model A) and the associated (Model B) models of coal Solvent
swelling
Reswelling Dependence of coal concentration Dependence of coal concentration for fractionated samples Dependence of fractionation on swelling ratio (Q) Dependendence of solvent for fractionated samples
Model A
Model B
Reversible None or small
Irreversible Significant
Significantly different
Similar
Qsow~er ’ Qinso,uwerQro~es< Qinso,ub,cs Larger for solubles than insolubles
Larger for insolubles solubles
than
cross-links is expected to be larger for PI than for PS. Therefore, on the basis of Model B, the swelling ratio of PI is generally more enhanced than that of PS when a better solvent is used. The comparisons discussed above are summarized in Table 2. The first and second comparisons have already been reported llsl* . It is thought that the importance of physical and chemical cross-links is estimated by studying solvent swelling of fractionated samples. The last three comparisons will be examined here. Efects
of solvents on swelling
The effect of solvents on swelling was examined before studying solvent swelling of the fractionated samples. Swelling ratios of various samples from three different ranks of coals [lignite (ND), high volatile bituminous coal (IL) and medium volatile bituminous coal (UF)] were measured in various solvents. Table 3 shows the specific swelling ratios (Q’) at initial time and after 24 h. As solvent swelling is highly concentration-dependent, as previously reported’*, the swelling rate is likely to be dependent on coal concentration. Also, solvent swelling needs to be measured at very low concentration, with continuous mixing for a given period of time. A specially designed apparatus will be necessary to measure accurate solvent swelling in the future. However, in this study, swelling was measured at a given coal-solvent ratio (C/S = 4.4 + 0.2 wt%) with mixing only at initial time. Swellings in pyridine of ND and IL coals both at room temperature and 70°C are the same, but swellings of raw and dried UF coals at room temperature are smaller than those at 70°C. Two mixed solvents, 1 M of tetrabutylammonium hydroxide (TBAH) in methanol/pyridine (25/75 ~01%) and N-methyl-2-pyrrolidinone (NMP)/ carbon disulphide (50/50v01%), swelled these three coals more than pyridine did in most cases. These results mean that coal in pyridine at room temperature associates to some degree. The swelling rate of lower rank coal becomes slow by dryingi4. The dried ND coal, and its PI, swell slowly in all solvents used except in toluene as shown in Table 3. However, the effect of moisture on coal swelling for the IL coal is not so great as that for the ND coal. Better solvents than pyridine (TBAH/pyridine and NMP/CS,) promptly swell even the dried IL coal and its PI; but THF, which is a poorer solvent than pyridine, slowly swells even the raw IL coal and its PS. Swelling in THF of PI and PS from two high volatile C bituminous coals will be studied in the following sections. The swelling
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12
1721
Associated Table 3
structure
Specific swelling
of bituminous ratios
in various
coal: M. Nishioka
solvents
of three different
ranks
of coals at the initial time and after 24 h (C/S = 4.4kO.2
wt%)
Specific swelling ratio, Q’ ND Coal solvent
IL
Initial time
After 24 h
Initial time
Toluene
1.3
1.3
Tetrahydrofuran
1.6
1.7
Pyridine
(25°C)
1.8
Pyridine
(70°C)
UF After 24 h
Initial time
After 24 h
1.2
1.2
1.2
1.2
1.9
2.3
1.2
1.2
1.9
2.7
2.7
1.2
1.2
Raw coal
_
1.9
_
2.8
_
1.6
TBAH/pyridine”
3.3
3.9
3.5
3.7
1.3
2.0
NMP/CS,*
2.2
3.0
3.1
3.3
1.4
2.0
1.1
Dried coal Toluene
1.3
1.3
1.2
1.3
1.1
Tetrahydrofuran
1.4
2.0
2.1
2.4
1.1
1.1
Pyridine
(25°C)
1.5
2.4
2.4
2.8
1.3
1.3
Pyridine
(70°C)
_
2.2
_
2.9
_
1.6
TBAH/pyridine”
2.0
4.5
3.8
3.8
1.3
1.8
NMP/CSzb
1.4
2.0
3.3
3.3
1.3
1.3
Residue
(PI)
Toluene
1.3
1.3
1.6
1.8
2.1
2.1
Tetrahydrofuran
1.4
1.7
2.6
2.1
2.5
2.4
Pyridine
(25°C)
1.6
2.1
2.9
3.0
2.5
2.3
Pyridine
(70°C)
_
2.1
_
3.0
_
2.2
TBAH/pyridine”
2.3
4.0
3.5
4.1
3.7
4.5
NMP/CS,*
1.4
1.8
2.8
3.1
3.1
2.8
Extract
(PS)
Toluene
1.4
1.3
1.4
1.4
1.3
1.3
Tetrahydrofuran
c
c
1.6
2.1
1.3
2.1
“TBAH/pyridine: 1 M of tetraammonium hydroxide solution in methanol b NMP/CS,: N-methyl-2-pyrrolidinone (50 vol)/CS, (50 vol) ‘Swelling was not measured because of large solubility of the sample
values are measured promptly after swelling, because of experimental difficulty as discussed above. Although ultimate solvent swelling under an ideal condition has not been obtained, THF swelling is significantly different for the samples of PI and PS from the IL coal both at the initial time and after 24 h under the conditions studied (Table 3). The difference in THF swelling between PI and PS at the initial time will be considered when investigating the coal structural model. Dependence of coal concentration on swelling for fractionated samples
Illinois No. 6 coal was fractionated into Extraxt A and Residue A by Soxhlet extraction with pyridine (Figure 3). Extract A was further fractionated into Extract B and Residue B by iodine treatment, followed by Soxhlet extraction with pyridine. Residue B (yield: 60 wt% of Extract A) is thought to be composed of relatively high molecular mass associated with the charge-transfer interaction in Extract A. When a solvent contains solute to some extent, solvent activity is not 1.0. Unfortunately, the coal has not been characterized yet, so as to estimate solvent activity under the conditions usually used for coal swelling. Reference 12 considers how significantly solubles in a solvent affected coal swelling no matter how great the solvent activity was. Four lines of evidence” showed that
1722
Fuel 1993
Volume
72 Number
12
(25 vol)/pyridine
(75 vol)
solubles (at least up to 30 wt%) did not significantly affect solvent swelling of a high volatile bituminous coal. Even if a sample contained 10 wt% pyridine solubles, swelling ratios in pyridine changed only to -0.1. THF dissolves less than 5 wt% of Extracts A and B under these conditions. Therefore, the effect of a small extent of solubles in THF can be ignored for this study. Swelling ratios of these samples were measured with THF. These results are shown in Figure 4. The swelling ratio of Extract B was very small and could not be measured at its low concentration, and the dependence of coal concentration was not determined. The dependence of coal concentration on swelling is nearly equal for Extract A, Residues A and B. This is against the prediction derived for Model A as discussed above. Dependence of fractionation of swelling ratio
The specific swelling ratios, Q’, in THF of Residue A are significantly larger than those of Extract A as shown in Figure 4. The difference between Extract B and Residue B is more remarkable. The swelling values in THF of PS and PI from another high volatile C bituminous coal, PSOC-1491, are shown in Figure 5. The specific swelling ratios, Q’, of the PI are also larger than those of the PS. These results are again consistent with Model B as discussed above. The specific swelling ratios, Q’, in THF of these native coals are also shown in Figures 4 and 5.
Associated structure of bituminous coal: M. Nishioka
3.0 -
3.0
I
1
I
I
AA A A
2.5 -
0
2.5
A
l
b b
2.0
00 -a
A
A
0
A
0
2.0 -
A 0
0
O
0 0
1.5 -
1.5 n
0
n n
1.0
A
0
l
5
10
15
Coal/Solvent
0
0
0
n
.._
0
0
20
ht/wt
25
0
5
IO Coal/Solvent
%)
Figure 4 Specific swelling ratios (Q’) of: 0, Extract A (BD=O.Sl); n , Extract B (BD=0.96); 0, Residue A (BD=O.Sl); 0, Residue B (BD = 0.94); and, A, the native sample (BD =0.90) of Illinois No. 6 coal with tetrahydrofuran (see Figure jl for the sample name)
15 (wt/wt
20
Figure 6 The specific swelling ratios (Q’) oE 0, Extract A; 0, Residue A with toluene; and, A, Residue A with pyridine for Illinois No. 6 coal (see Figure 3 for the sample name)
Dependence of solvent on swellingforfractionated
2.5
O” 0
ti
0
“0A
2.0
0
0
0
DA
A Cl
samples
The effect of other solvents on swelling were examined for the fractionated samples. Figure 6 shows the specific swelling ratios, Q’, in toluene and pyridine of Extract A and Residue A from the IL coal. The specific swelling ratios, Q’, in toluene of Residue A are larger than those of Extract A. The dependence of concentration on the Q’ values of Residue A in pyridine are approximately equal to those in THF. These results are consistent with Model B, even if different solvents are used for swelling. The specific swelling ratios at high coal concentration of Residue A are 2.2 in THF and 1.4 in toluene. The values of Extract A are 1.7 and 1.3, respectively. Therefore, the difference of swelling with poor and better solvents is larger for PI than PS. This result again supports Model B as discussed above.
3.0
AA
25
%)
qA
1.5
CONCLUSION
1.0 0
5
10 Coal/Solvent
15 hvt/wt
Figure 5 Specific swelling ratios (Q’) oE 0, pyridine 0, pyridine residue (BD = 1.0); and, A, the native of PSOC-1491 coal with tetrahydrofuran
20
25
%1 extract (BD= 1.0); sample (BD = 1.0)
The swelling values of the two native coals are close to those of their extracts. This may be rationalized as follows on the basis of Model B. The number of physical cross-links between surrounding molecules would be larger when a sample contains molecules with small molecular mass, because the coordination numbers with smaller coal molecules are expected to be larger. If good solvents are used for coal swelling, molecules with small molecular mass are readily dissolved in the solvent. In this case, coal swelling may be different from PS swelling. In fact, swelling in pyridine of the PSOC-1491 coal was nearly equal to that of PI I2 . Consequently, all the results obtained from fractionated samples are reasonable for Model B.
The following effects on solvent swelling of high volatile C bituminous coals were investigated using their fractionated samples: concentration, swelling ratios and solvents. Pyridine-Soxhlet extraction was simply chosen to fractionate these coals. As Table 2 shows, Models A and B in Figure I should result in distinctively different swelling. It is obvious that all results obtained in this work coincide with the predictions for Model B shown in Table 2. Therefore, it can be concluded that significant portions of coal molecules in these two coals are not three-dimensional networks, but are associated.
REFERENCES 1
2
3
Green, T., Kovac, J., Brenner, D. and Larsen, J. W. in ‘Coal Structure’ (Ed. R. A. Meyers), Academic Press, New York, 1982, Chapter 6, pp. 199-282 Given, P. H. in ‘Coal Science’ (Eds M. L. Gorbaty, J. W. Larsen and I. Wender), Academic Press, New York, 1984, Vol. 3, pp. 63-252 Berkowitz, N. in ‘Polynuclear Aromatic Compounds’ (Ed. L. B. Ebert), Advances in Chemistry 217, American Chemical Society, Washington, DC, 1987, Chapter 13, pp. 217-233
Fuel 1993
Volume
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12
1723
Associated structure of bituminous coal: /VI. Nishioka 4 5
6 7 8
1724
van Krevelen. D. W. in ‘Coal’, Elsevier, New York. 1961. o. 440 Wiser, W. H: in ‘Proceedings of the’ Electric Power Research Institute Conference on Coal Catalvsis’. Palo Alto. California. _ 1973, p. 3 Shinn, J. H. Fuel 1984, 63, 1187 Nishioka, M. and Larsen, J. W. Energy & Fuels 1990, 4, 100 Nishioka, M., Gebhard, L. A. and Silbernagel, B. G. Fuel 1991, 70, 341
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9 10 11 12 13 14
Nishioka, M. Energy & Fuels 1991, 5, 487 Nishioka, M. Fue1-i992, 71, 941 Nishioka. M. Fuel 1993, 72. 997 Nishioka, M. Fuel 1993,72; 1001 Vorres, K. S. Energy & Fuels 1990, 4, 420 Otake, Y. and Suuberg, E. M. Am. Chem. Sot. Div. Fuel Gem. Prep. 1988, 33(4), 898