340
Energy & Fuels 1988,2,340-344
Anisotropic Solvent Swelling of Coals George D. Cody, Jr., John W. Larsen,* and Michael Siskin* Corporate Research Science Laboratory, Exxon Research & Engineering Company, Route 22E, Clinton Township, Annandale, New Jersey 08801 Received August 24,1987. Revised Manuscript Received November 28, 1987
The swelling of six coals has been measured parallel to and perpendicular to the bedding plane in chlorobenzene, THF, and pyridine. The swelling of all coals in all solvents is anisotropic, being greater perpendicular to the bedding plane than parallel to it. Coals possess anisotropic structures and appear to be more highly cross-linked in the bedding plane than perpendicular to it. The solvent swellings have been measured as a function of time and pass through a maximum due to the formation of a metastable intermediate state. The time required to reach this maximum is different for swelling parallel to and perpendicular to the bedding plane and is a function of coal rank. Once swollen, coals from which most of the solvent (pyridine) has been removed do not return to their original shape but are smaller when measured in the bedding plane and are larger normal to it. Further swellings and solvent removals cause no further shape changes. Mined coals are therefore strained and will relax to their equilibrium state when transformed to a rubbery state by solvent swelling.
Introduction Solvent swelling has been used extensively to investigate the macromolecular network properties of coals.'-* Significant structural insight has been derived from these measurements. In all but two studies, the techniques used have been such that the swelling values were averaged over all directions. Brenner has reported the qualitative observation that solvent swelling is greater perpendicular to the bedding plane than parallel to it.7 The linear expansion of six coals when swollen with methanol has been reported.* In all cases, expansion perpendicular to the bedding plane exceeded that in the bedding plane, usually by a large amount. During their formation, coals experience enormous directional pressure due to gravity and overburden. This may have structural consequences, and the existence of molecular orientation due to directional pressures has been proposed? The bedding plane is a convenient reference since the pressure was exerted normal to it. Anisotropic pressures may induce anisotropic macromolecular structures or packing, which in turn may lead to anisotropic solvent swelling. The solvent swelling may be different parallel to and perpendicular to the bedding plane. We report here the first detailed study of anisotropic solvent swelling. Our observation of anisotropic swelling of coals by solvents was anticipated. It has been noted that coal swelling is anisotropic without providing any quantitative data7 It is well established that coal mechanical properties are different perpendicular to and parallel to the bedding
plane.lOJ' There exists a direct relationship between mechanical properties and solvent swelling since both are functions of the network elasticity.13J4 If the anisotropy in mechanical properties is due to molecular structure, then swelling should be anisotropic. Bulk mechanical property measurements have suffered from the presence of cracks and cleats present in all coals, and it has not been determined whether the anisotropy of the mechanical properties was due to the presence of these macroscopic flaws or the fundamental properties of the material."J2 Two principal forces control the geometry of aromatic stacks in coals. One is pressure, which will favor minimum volume arrangements. The other is the potential energy of the aromatic-aromatic interactions, which leads to orientations that do not minimize volume. The actual (most stable) orientation will be the best compromise between the two and will thus depend on the pressure. The effects of pressure and the resulting minimum volume conformations are easily visualized and will not be discussed. Aromatics stacked face-to-face minimize volume. Our knowledge of the geometry required for the most favorable aromatiearomatic interactions will be summarized very briefly because it is important and because it has been largely ignored within the coal community. Gas phase and calculated geometries must be considered because only these are independent of the packing forces present in crystals and amorphous solids. Benzene is the most thoroughly studied system and clearly shows that the face-to-faceparallel stack is an unfavorable geometry and is probably repulsive at all distance~.'~Favored are "T"
(1)Green, T.;Kovac, J.; Brenner, D.; h e n , J. W. In Coal Structure; Meyers, R. A., Ed., Academic: New York, 1982. (2)Larsen, J. W.; Green, T. K.; Kovac, J. J. Org. Chem. 1986,50, 4729-4735. (3)Peppas, N. A.; Lucht, L. M. Chem. Eng. Commun. 1984, 30, 291-310 and references therein. (4)Suuberg, E.M.;Lee, D.; Larsen, J. W. Fuel 1986,64,1668-1671. ( 5 ) Green, T. K.; West, T. A. Fuel 1986,65,298-299. (6) Bockrath, B. C.; Illig, E. G.; Wassell-Bridger,W. D. Energy Fuels 1987,1,226-227. (7)Brenner, D.Fuel, 1984,63,1324-1328. (8)Bangham, D.H.;Maggs, F. A. P. Proceedings of the Conference on Ultra-fine Structure of Coals and Cokes; British Coal Utilization Research Association: London, 1943. (9)Skripchenko, G.B. Solid Fuel Chen. (E&. T r a w l . ) 1984,18, 15-23.
(10)van Krevelen, D.W. Coal; Elsevier Scientific: New York, 1981. (11)Szwilski, A. B. Znt. J. Rock Mech. Min. Sci. Geomech. Abstr. 1984,21,3-12. (12)Klepaczko, J. R.;Hsu, T. R.; and Baasim, M. N. Can. Geotech. J. 1984,21,203-212. (13)Szwilski,A. B. Min.Sci. Technol. 1986,2,181-189. (14)Treloar, L. R. G. The Physics of Rubber Elasticity, 3rd ed.; Oxford University Press: New York, 1975. (15) Cox, E. G.; Cruickshank, D. W. J.; Smith, J. A. h o c . R. Soc. London, A 1968,247,1-21. Hopkins, J. B.; Powers, D. E.; Smalley, R. E. J. Phys. Chem. 1981,85, 3739-3742. Langridge-Smith, P. R. R.; Brumbaugh, C. A.; Levy, D. H.J. Phys. Chem. 1981,85, 3742-3746. Singh, J.; Thornton, J. M. FEES Lett. 1986,191,1-6. Pawliszyn, J.; Szczesniak, M. M.; Scheiner, S. J. Chem. Phys. 1984,88, 1726-1730. Karlstrom, G.; Linse, P.; Wallqvist, A.; Jonsson, B. J. Am. Chem. SOC. 1983,105,3777-3782.
0887-0624/88/2502-0340$01.50/00 1988 American Chemical Society
Solvent Swelling of Coals
Energy & Fuels, Vol. 2, No. 3, 1988 341
interactions (structure 1). Calculations on larger aromatic
. 2
1
"T"
interact ion
"herringbone" stack
systems and the geometry of aromatics in biological systems show a preference for similar interactions and avoidance of face-to-face s t a ~ k s . ' ~ JCalculations ~ have not been carried out on aggregates containing more than two molecules, so we must consider crystals to learn the effecta of cooperative interactions. However, crystal geometries are strongly affected by lattice forces that may cover up the smaller dipolar interactions. For aromatics larger than benzene, a herringbone stacking pattern (structure 2) is favored and is obviously a compromise between favorable "T" interactions and an efficient space-fing structure.l' The effects of high pressure on a reacting macromolecular network can be qualitatively visualized. Aromatic systems interact most favorably when their planes are oriented perpendicularly or nearly so.1617Parallel stacking is not energetically preferred at atmospheric pressure. The effect of great pressure will be to impose strong steric factors on the equilibrium packing of the molecules. Interactions that are favorable but do not effectively fill space will be overwhelmed at high pressures by the driving force to minimize volume. For planar aromatic systems, the atmospheric tendency toward perpendicular alignment will be replaced by a tendency toward the space-filling parallel stacks at high pressures. The alignment of these stacks with the bedding plane seems reasonable. If such interactions are locked into the glassy coal during its formation, mined coals newly at atmospheric pressure may be in a strained state. The motion of the cod macromolecules is too slow to allow the most favorable configurations to be adopted. Alternatively, the coals could be random at the molecular scale yet contain larger anisotropically oriented structures.
Experimental Section The following coals were used in this study: Big Brown Texas lignite, Smith seam subbituminous c, Wandoan subbituminous a or b, Cerrejon high-volatile bituminous c, Illinois No. 6 highvolatile bituminous c, and Pittsburgh No. 8 high-volatile bituminous a. Analyses of these coals can be found in Table I. All of these coals were stored under dry nitrogen and were not dried before use. Preparation of Uncontaminated Thin-Section Samples. Uncontaminated thin section samples were prepared by using a soluble adhesive to hold the sample to a petrographic slide during the grinding process following the procedure of Brenner.' The only variation to his procedure was the use of a nitrogen atmosphere in sample preparation. The samples were removed from the slide by dissolving the adhesive in excess n-hexane and then were transfered to another bath of n-hexane to ensure complete dissolution of the adhesive. High-purity chlorobenzene, tetrahydrofuran, and pyridine were the solvents used. The T H F was distilled to remove the inhibitor. Each thin section sample was on the order of 25-50 mmz in area and was approximately 12 pm thick. Several approximately 1mm* pieces were cut from each present sample for swelling studies. (16)Miller, J. H.;Mallard,W. G.; Smyth, K.C. J. Phys. Chem. 1984, 88,4963-4970. (17)Cruickshank, D. W. J. Acta Crystallogr. 1957, 10, 504-508. Cmickshank, D.W. J. Acta Crystallogr. 1956,9,915-923.Burns, D.M.; Iball, J. Proc. R. SOC.London, A 1955,227,2W214.Burns, D.M.; ball, J. h o c . R. SOC. London, A 1960,257,491-514.Iball, J.; Scrimgeour, S. N. J. Chem. Soc., Perkin Trans. 2 1974, 1445-1448. Wieckowski, T.; Krygowski, T. M.Can. J. Chem. 1981,59,1622-1629.
Table I. coal Big Brown Wyodakb Wandoan Cerrejon Ill. No. 6 Pitt. No.8
Elemental Analyses and Rank rank %C %H %N 1.2 lignite 63.3 4.9 subbit c 67.0 4.7 0.8 subbit b 0.9 69.5 5.6 HvB c (1) 81.4 5.4 1.4 HvB c (2) 68.4 5.2 1.3 HvB a 1.5 83.9 5.7
Dry Basis.
* Wyodak = Smith seam.
e
of Coals Studied' % S %Oc %ash 1.2 16.5 12.9 0.5 21.6 5.4 0.6 13.4 10.1 0.6 10.6 0.6 3.1 10.8 11.2 1.5 2.0 5.4
By difference.
Table 11. Typical Solvent-Swelling Values (cm) for Wandoan Coal Measured on Photomicrographs chlorobenzene THF pyridine dry wet ratio dry wet ratio dry wet ratio 5.13 4.30 4.25 5.30 5.10
5.22 4.47 4.42 5.49 5.32
1.02 1.04 1.04 1.04 1.04
Parallel (811) 5.06 5.96 1.18 3.41 3.99 1.17 5.59 6.61 1.18 3.22 3.79 1.18 4.05 4.75 1.17
4.85 4.25 3.45 3.05 4.41
5.92 5.11 4.15 3.62 5.41
1.22 1.20 1.20 1.19 1.23
3.11 4.15 4.88 5.18 4.69
3.33 4.45 5.22 5.59 5.08
1.07 1.07 1.07 1.08 1.08
Perpendicular (QL) 4.34 5.57 1.28 5.45 5.51 7.11 1.29 4.12 5.47 7.01 1.28 4.47 6.25 8.05 1.29 4.64 4.89 6.29 1.29 4.80
7.50 5.73 6.15 6.33 6.45
1.38 1.39 1.38 1.36 1.34
i
n
!
140
3
-e
3
B
I3O
IZ0 110
ABC
ABC
ABC
Lig.
Sub-c
Sub-b
ABC
ABC
HVc (1) HVC (2)
ABC Hva
Figure 1. Swelling of coals in three solvents parallel to and perpendicular to the bedding plan: (A) PhC1; (B)THF; (C) CSHSN. Swelling Procedure. Reference photomicrographs were obtained prior to immersing each sample of the respective solvents. Each sample was placed in a 1-02 ointment jar, which was filled to approximately three-fourths volume with solvent. The jars were sealed under a nitrogen atmosphere and left untouched until swelling equilibrium was reached. The samples were then placed on a 2 X 3 in. glass slide and covered with a coverslip. To ensure that the sample was not being squeezed by the coverslip, a frame of coverslips was used for support. Photomicrographs were then obtained of the solvent-swollen coal thin-section samples. Singular pairs of points that define lines parallel to and perpendicular to the bedding plane were identified in the photographs of the unswollen coal samples. The same pairs of points were then located on the photomicrographs of the swollen samples, measured, and ratioed to the unswollen distance values to yield percent linear expansion parallel to and perpendicular to the bedding plane. The average values of the linear expansion perpendicular to and parallel to the bedding plane in the three solvents used are reported in Figure 1. Typical reproducibility is shown by the data in Table 11. Swelling is independent of grinding direction. Swelling Kinetics. The measurement of linear expansion as a function of time was identical with the equilibrated swelling anisotropy measurements except that sequential photomicrographs were used to continuously document physical changes after exposure of the coal to the solvent (pyridine in all cases). After equilibration, the sample was deswelled by evaporating the pyridine under flowing nitrogen. The sample was then stored
Cody et al.
342 Energy & Fuels, Vol. 2, No. 3, 1988 Table 111. Comparison of Solvent-Swelling Ratios ( Q JQ in Coal Thin Sections QJQii PhCl THF CsHsN Big Brown lignite 1.08 1.07 1.14 Smith seam 1.04 1.07 1.12 Wandoan 1.04 1.09 1.13 Cerrejon 1.03 1.11 1.18 Ill. No. 6 1.05 1.19 1.20 Pitt. No. 8 1.02 1.22 1.27
Swollen In Pyrldlne
Swollen In Pyridine
140
Perpendicular lo Bedding
Second Cycle
1 iT
Parallel to Beddins
I Parallel to Bedding
Y
e
$
100 160 Flmt Cycle
-/
Illlnole NO.. 6 Swollen In Pyridine
C
Plnrburg NO. 8
D
Swollen In Pyridine
Second Cycle
Table IV. Comparison of Solvent-Swelling Ratio Measurements on Bulk Samples (Dried Coals) and on Thin Sections' Ill. No. 6 Big Brown lig Smith seam solvent thin sect bulk thin sect bulk thin sect bulk pyridine 2.35 2Ab 1.79 2.0 2.04 2.4 THF 1.98 2.1 1.30 1.7 1.59 1.7 CBH6Cl 1.08 1.17 1.2 1.12 1.3 Except aa indicated, all data on bulk swelling are from: Shawver, S. Ph.D. Thesis, University of Tennessee, 1986. bData from ref 2. in dry nitrogen for at least 24 h, allowing most of the pyridine to evaporate (it is not possible to remove all the pyridine, even under relatively high vacuum).12 Photomicrographs were again obtained to record changes occurring due to solvent removal. A second swelling cycle was run in a manner identical with the first. Sequential photomicrographs were similarly used to document the physical changes as a function of time.
Results Figure 1shows the swelling of six coals in chlorobenzene, tetrahydrofuran (THF), and pyridine measured both parallel to and perpendicular to the bedding plane. The swellings are anisotropic for all coals in all solventa with greater swelling occurring perpendicular to the bedding plane than parallel to it. Within this behavior, several general trends are recognized. Swelling is always greatest in pyridine, next in THF, and least in chlorobenzene. This behavior has been observed before and was rationalized by invoking the strength of the solvents as hydrogen bond acceptors.2 The degree of swelling anisotropy increases with solvent hydrogen bond acceptor strength (pyridine > THF > chlorobenzene) and also with coal rank for THF and pyridine (see Table 111). The structural anisotropy of coals is clearly manifest in these measurements. As a check on the validity of these measurements, the total volumetric expansion of the coal can be calculated from the linear expansions in three directions and compared with solvent-swelling measurements made by others on the same coals (see Table IV) using the volumetric swelling technique. Only a limited data set is available, and the data were not obtained on identical samples, since those used for the bulk swelling were dried. Nevertheless, the results are quite similar and give confidence in the correctness of the measurements. In addition to the measurement of the equilibrium solvent swelling, the rate at which four of the coals reached that equilibrium has been measured. The data are shown in Figure 2. Two kinetic runs on the same coal thin section are shown for each of the coals. After the first run, which was the sample's first exposure to the pyridine swelling liquid, the pyridine was removed by drying in a stream of dry nitrogen. The same sample was again exposed to pyridine and another swelling rate measured. Except for the lowest rank coal studied, all show the formation of a metastable excess swelling on the first exposure to pyridine. Such behavior was reported earlier by Peppasl8 and observed by Brenner.lg The rate of formation
110 100
10
30
50
VTiE
70
90
VTiG
Figure 2. Kinetics of thin-section coal swelling in pyridine. Table V. Percent Size Changes in Coals after Pyridine Swelling coal parallel perpendicular vol change py extr 82 109 -27 -13 Pitt. No. 8 Ill. No. 6 90 109 -12 -16, -19 96 115 +6 -8 Cerrejon 97 106 0 -6 Wandoan
and disappearance of the metastable swollen state is different parallel to and perpendicular to the bedding plane. This metastable structure is not formed on any swelling except the first, and the same equilibrium is reached on repetitive swellings. The time required to reach the maximum metastable swelling (perpendicular) increases with coal rank; Wandoan, none; Cerrejon, 0.34 h; Illinois No. 6, 0.55 h; Pittsburgh No. 8, 0.75 h. This is the same order of increase as the swelling asymmetry. After swelling and removal of the pyridine, the shape of the coal thin section has changed. It has expanded perpendicular to the bedding plane and shrunk parallel to it. Since some pyridine remains in the coal and an unknown amount of material (bitumen) has been extracted, an overall size change is expected. This is calculated and compared with the room-temperature pyridine extractibility of the coal in Table V. Repeated pyridine swellings return the coal to this new size.
Discussion The data presented here reveal clearly the asymmetric character of a variety of coals, and this property is probably present to varying degrees in all coals. It is particularly impressive in this series because several of the coals are of such low rank. Even lignites are strongly anisotropic. This raises forcibly the issue of the origin of the anisotropy. One candidate is the pressures to which coals are subjected during their formation and maturation. However, organic sediments that will eventually lead to coals will also be anisotropic under the influence of gravity. Disk-shaped things will tend to lie flat, not on edge. A forest floor is not isotropically packed. This is a familiar example; we do not mean to suggest that coals are formed from modern forest floors. The observation that the asymmetry increases with rank shows that the coalification process is at least enhancing anisotropy. Whether it is present initially is not revealed by these data. The anisotropy could be due to the stacking of aromatics parallel to the bedding plane. In another paper we present evidence against this occurrence.20 Any structure capable (18) Peppas, N. A.; Larsen, J. M.; Lucht, L. M.; Sinclair, G. W. Proc.-Znt. Conf. C o d S C ~1983, ., 1983, 280. (19) Personal communication.
Solvent Swelling of Coals
of greater bonding density in the bedding plane rather than perpendicular to it will fit the solvent-swelling data since the anisotropy observed by solvent swelling is a bulk effect. While we do not support the classical “micelle model” for coals, disklike micelles more strongly linked at their edges than a t their surfaces would fit these data. Residual cellular structure is a possibility though it is hard to see how this would be enhanced by coalification. The data presented here unequivocally demonstrate the existence of an anisotropic structure but do not reveal its origin. We do not wish to speculate on its origin at this time and are engaged in several different probes of coals’ structural asymmetry. Two factors control the swelling of a macromolecular network by a solvent. One is the interaction between the macromolecule and the solvent, usually expressed as the Flory x parameter. The other factor is the cross-link density of the network, which also controls its mechanical properties. The solvent molecule is small compared to the macromolecular segments, and ita interaction with the those segments will be isotropic. Even chlorobenzene, which does not undergo specific interactions with coals,2l results in anisotropic swellings. The anisotropic swelling must be due to an anisotropic distribution of cross-links. Since they swell less in the bedding plane than perpendicular to it, these coals are more bonded in the bedding plane than they are perpendicular to it. Since the cross-link densities of coals are anisotropic, their mechanical properties will also be anisotropic. The anisotropy of coal mechanical properties is well established and is an important factor in mine design and operation. Mechanical property measurements made on bulk samples are always complicated by the presence of cleats and cracks, and until now, it has proven impossible to determine unequivocally whether the observed anisotropic mechanical properties were due to the flaws in the sample or were an intrinsic property of the material. The swelling measurements make it clear that coals are inherently mechanically anisotropic and would be so in crack- and cleat-free specimens. The swelling anisotropy indicates anisotropy in cross-link density. This will translate directly into mechanical property anisotropy since mechanical properties are governed by the cross-link density. Homogeneous, fault-free samples of coals should be stronger in the bedding plane than perpendicular to it. The three solvents probe different aspects of coal structure. Chlorobenzene breaks no coal-coal hydrogen bonds and probes the network with hydrogen bonds intact.21 Pyridine breaks most, if not all, coal-coal hydrogen bonds, leaving a network held together primarily by covalent bonds. There is evidence that, in coals of this low rank, London interactions (stacking interactions) are not important structural forces in the presence of pyridine.22 The perpendicular/parallel swelling ratios are highest in pyridine and lowest in chlorobenzene, indicating a highly anisotropic arrangement of covalent bonds. With pyridine, the effects of hydrogen bonds are smallest and the anisotropy is greatest. The highly anisotropic arrangement of covalent bonds apparently (but not actually) is reduced by the hydrogen bonds. The conclusion that the density of hydrogen bonds is greater perpendicular to the bedding plane than in it seems to follows directly. The intermediate swelling anisotropy observed with THF is consistent with this since THF is known to break only a portion of coal-coal hydrogen bonds.21 Unfortunately for this simple (20) Cody, G.; Lareen, J. W.; Siskin, M., manuscript in preparation. (21) Laraen, J. W.; Baskar, A. J. Energy Fuels 1987, I, 230-232. (22) Quinga, E. M. Y.; Larsen, J. W. Energy Fuels 1987,1,300-304.
Energy & Fuels, Vol. 2, No. 3, 1988 343
picture, evidence from other sources indicates that both hydroxyl groups and hydrogen bonds are isotropically distributed in coals and this model is, therefore, untenable.20 In fact, there is no reason to expect the parallel/perpendicular swelling ratio to remain constant for a given coal with the three solvents. For convenience, let us consider the simplest treatment of polymer solvent swelling, that due to Flory and Rehner.23 As shown by eq 1, the dependence of M con x and on the degree of solvent swelling, measured by the volume fraction of polymer a t equilibrium swelling (V), is nonlinear. If M c is constant, the extent of swelling (V) will depend in a nonlinear manner on the interaction parameter x,which is different for each coal-solvent pair. x is isotropic, but the expansion of the coal in the different directions will show different and nonlinear dependences on x because Mc is different in the two directions. The observed behavior is thus qualitatively consistent with isotropically distributed hydrogen bonds. The value of Mc will vary with solvent because of varying degrees of coal-coal hydrogen bond breaking by the solvents, the hydrogen bonds being cross-links. Mined coals are in a metastable, strained state, prevented from attaining their equilibrium state by their glassy nature. During their formation, they have been forced into an anisotropic state that is not at equilibrium at atmospheric pressure and that cannot relax from this state because the molecular motion is too slow. The evidence for this is the structural relaxation that occurs when the coals are swollen with pyridine and the pyridine is removed. Pyridine swelling is known to place coals in a rubbery state at room temperature.’ They can now relax and achieve their equilibrium shape. This shape is higher (larger perpendicular to the bedding plane) than the starting coal and narrower in the bedding plane. The coal has been squashed flat during coalification and recovers its most stable shape when pyridine swelling enables the macromolecules to move and achieve their minimum energy conformations. With this background, the swelling kinetics appear less strange. Since the cross-link densities differ directionally with respect to the bedding plane, motion in the two directions should be hindered to different degrees. Naively, it should be easier to expand perpendicular to the bedding plane, in the direction of lowest bond density. Swelling rates are fastest in this direction. The occurrence of the overshoot in the kinetic curves demonstrates the formation of a metastable swollen state containing more than an equilibrium amount of solvent or particularly inefficient macromolecular packing. This relaxes slowly. Our data confirm the observations of Peppas, and we support his interpretation.ls The temporary existence of this nonequilibrium state is due to the slow rate at which the coal macromolecules can self-diffuseto reach their most stable arrangement. The existence of nonequilibrium states in coal swelling by nonpolar solvent vapors at 110 OC has been demonstrated in a very nice study by Hsieh and Duda.” They provide strong evidence for the glassy nature of coals, which they propose as the origin of the observed hysteresis in solvent uptake. They suggest that the exposure of the coal to toluene alters its gel structure, but the altered (23) Flow, P. J. Principles of. Polymer Chemistry; Comell University . Press: Ithaca, NY, 1953.(24) Hsieh, S. T.; Duda, J. L. Fuel 1987, 66, 170-178.
344
Energy & Fuels 1988,2,344-350
structure is not the equilibrium structure. Evidence for the alteration of coal gel structure by methanol and pentane absorption was published 30 years ago.26 Our data make clear that nonequilibrium states due to slow macromolecular relaxation can also be observed with polar solvents. Once the structure has relaxed (initial pyridine exposure), an equilibrium state is reached and further swelling shows no hysteresis. With nonpolar solvents, as observed by Duda, the equilibrium state is apparently never reached so hysteresis continues. Solvent swelling in nonpolar liquids shows no hysteresis.26 Apparently, Soxhlet extraction of the samples by pyridine allowed them (25)Fugaesi, P.;Hudson, R.; Ostapchenko, G. Fuel 1958,37,25-28. (26)Lareen, J. W.;Lee,D.; Shawver, S. E. Fuel Process. Technol. 1986, 12,51-62.
to obtain their equilibrium state. Hsieh and Duda used a similar extraction procedure, which should also have caused the coal structure to relax. We do not understand why the different experimental conditions with nonpolar solvents should yield such different results. The data contained in this paper support Duda’s picture of very complex absorption processes, and we echo his statements that one must be sure equilibrium has been reached before making quantitative use of solvent-swelling data. Coals are extremely complex macromolecular systems. They are clearly anisotropic. In further papers, we will explore the significance of these observations for the behavior of coking coals and further explore the anisotropic structure. Registry No. PhC1, 25167-80-0;tetrahydrofuran, 109-99-9; pyridine, 110-86-1.
Macromolecular Chemistry of Coalification. Molecular Weight Distribution of Pyridine Extracts John W. Larsen* and You-Ching Wei Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015 Received July 1, 1987. Revised Manuscript Received December 28, 1987
A gel permeation chromatograph with a mass sensitive detector has been constructed, interfaced with a computer, calibrated with coal extracts, and tested. Because of the nature of the detector, the molecular weight distributions obtained underestimate the amount of very low molecular weight material present, but the weight-average molecular weights are only slightly underestimated. The molecular weight distributions for a series of lower Kitanning coal extracts were measured. All of the coal pyridine extracts studied contained materials in the 10000 molecular weight range. Both the amount of pyridine extract and its average molecular weight increase with rank up to a carbon content of 86%. Both of these trends are consistent with the coalification process over this rank range being the depolymerization of a cross-linked macromolecular network. Thus,the origin of the extractable portion of high-rank coals is the depolymerization of the network, and the extracts are primarily products of network fragmentation. The ratio of weight-average to number-average molecular weights decreases with increasing rank, and the possible origin of this behavior is discussed.
Introduction The vitrinite portion of coals is thought to originate largely from lignin, a complex three-dimensional macromolecular netw0rk.l The process by which lignin becomes coal is best treated by using the well-developed kinetic and thermodynamic models for macromolecular network systems. We concern ourselves here with the coalification of a set of lower Kitanning bituminous coals and seek to answer the question, “can their coalification be treated as reactions of a three-dimensionally cross-linked macromolecular network?” The approach used is to determine the molecular weight distributions for the pyridine extracts from a set of coals and to determine whether the changes in those distributions are consistent with those expected if the extracts are in equilibrium with a network, that is, constitute a sol. It was necessary to develop a method for determining the molecular weight distributions of coal extracts. A (1)Freudenberg, K.Science (Washington D.C.)1965,148,595.
Table I. Elemental Analyses of Coals Used (dmmf) % mineral
coal %C %H %N %S PSOC 1278 82.5 5.5 1.9 0.8 PSOC 1309 85.8 5.8 1.8 1.6 PSOC 1215 86.3 5.2 1.6 0.8
P S O C 1236 87.4 PSOC 1024 88.9 PSOC 1133 90.7
5.2 5.1 5.0
1.6 1.3 1.4
0.8 0.8 0.4
% O(diff).
9.2 5.0 6.2 4.9 3.8 2.4
matter 15.7 8.6 7.3 7.7 13.7 22.2
number of groups have successfully used gel permeation chromatography for this, but most have been hindered by the lack of a mass-sensitive detector and the choice of the proper calibration standard is also a A (2)Unger, P.E.;Suuberg, E.M. Fuel 1984,63,606-611. ( 3 ) Strachan, M.G.; Johns, R. B. J. Chromatogr. 1985,329,65-80. (4)Reerink, H.; Lijzenga, J. A n d . Chem. 1975,47,2160-2167. (5)Khan, M.M.Fuel 1982,61,553. (6)Wong, J. L.; Gladstone, C. M. Fuel 1983,62,870-872.
0887-0624/88/2502-0344$01.50/00 1988 American Chemical Society
