Energy & Fuels 1992,6, 863-864
863
Communications Some Comments Concerning the Solvent Swelling of Coal and Coal Extracts Paul Painter Polymers Program, 321 Steidle Building, Penn State University, University Park, Penmylvania 16802 Received June 22, 1992. Revised Manuscript Received August 20, 1992
There is now an extensive body of published work that is concerned with the solvent swelling of coal. Iino and co-workers in the preceding paper and previous work1v2 have made an important contribution to this field with their observation of an enhancement of extraction yields and swelling through the use of mixed solvents. The interpretation of some of this data involves an assumption concerning the nature of junction points and the role of hydrogen bonds in coal swelling which I believe to be erroneous, however, and it is the purpose of this note to raise this issue. In discussing this problem we must first distinguish between covalent networks, where the connections between chains are provided by covalent bonds, and physical networks, which are held together by noncovalent interactions through Yunction zones” formed by one of several mechanisms (e.g., formation of microcrystalline regions). It is now generally accepted that a solvent-extracted coal is a covalently cross-linked network (although it probably contains some trapped soluble material that is only extracted with difficulty). It has become common to consider hydrogen bonds to be additional “cross-links”in these systems, however, so that the molecular weight between junction points is greater in coals swollen with solvents that have hydrogen bond acceptor sites, such as pyridine, or in coals whose hydroxyl groups have been derivatized (see ref 3-7 and citations therein). Similarly, the swelling of (for example) pyridine-soluble extracts with a non-hydrogen-bonding solvent (such as benzene) has been treated as a problem of network swelling, where the connectivity or cross-links between chains is provided by hydrogen bonds. The fundamental question is whether it is appropriate to regard specific secondary interactions such as hydrogen bonds in this manner. The problem is more than one of semantics in that it determines how we apply thermodynamic models to the swelling of coal. For example, if hydrogen bonds are regarded as cross-links, then an elastic energy term for covalent chain extension between the sites of hydrogen-bonding functional groups would have to be included.
There are a number of examples of systems where there is a densely interconnected network of hydrogen bonds that are clearly not gels or elastic networks. Water is the classic example, as each molecule can hydrogen bond to up to four neighbors. If hydrogen bonds were cross-links, then water would not be a liquid, but a rigid thermoset. Similarly, nylon 11has approximately 75% of ita amide groups hydrogen bonded to one another in the melte but flows in a viscoelasticmanner. This polymer has an amide group in every chemical repeat unit, so that at any instant of time there is again a densely interconnected network of hydrogen-bonded chains. These materials are liquids, because hydrogen bonds are dynamic, constantly breaking and forming at the urgings of thermal motion. Kohler? for example, cites hydrogen bond lifetimes that range between lo-” and lo+ s. The former corresponds to interactions between small, approximately spherical molecules such as water, while the longer lifetimes correspond to the formation of cyclic dimers in more strongly hydrogen-bonded species, such as the carboxylic acids. Hydrogen bonds between the phenolic hydroxyl groups of coal in the liquid state presumably lie somewhere between these limita (because the enthalpy of phenolic OH-OH hydrogen bonds is less than that of carbosylic acid pairs). The emphasis on the liquid state is important. If the material under consideration crystallizes or forms a glass, then the hydrogen bonds become frozen in position, as do other types of intermolecular contacts, but this does not make them cross-links. On the basis of these observationsI believe that hydrogen bonds of the type found in coal should be treatad as specific interactions and not as cross-links (unusually strong hydrogen bonds, such as those found in acid salts, would be exceptions to this rule). Indeed, citing a study of w a d e modified polybutadiene melta by Stadler and de Lucca Freitas,lo Burchard and Ross-Murphy” concluded that point cross-links such as hydrogen bonds are not sufficient by themselves to produce a physical gel, but must be accompanied by some other mechanism (see following paragraph). This is not to say that properties such as viscosity are not affected by hydrogen bondina. but the
(1) Fujiwara, M.; Ohsuga, H.; Takanohashi, T.;Iino, M. Energy and Fuels, preceding paper in thii iwue. (2) Iino, M.;Takahonashi, T.; Osugu, H.; Toda, K. Fuel 1988,67,1639. (3) Brenner, D.Fuel 1984, 63, 1324. (4) Brenner, D.Fuel 1986, 64, 167. ( 5 ) Larsen, J. W.; Green, T. K.; Kovac, J. J. Org. Chem. 1986,50,4729. (6)Lucht, L. M.; Peppas, N. A. J . Appl. Polym. Sci. 1987,33, 2777. (7) Hall,P. J.; Marsh, H.;Thomas, K. M. Fuel 1988,67, 863.
(8) Kohler, F. The Liquid State; Verlag Chemie: Berlin, 1972. (9) Skrovanek, D. J.; Painter,P.C.; and Coleman, M. M. Macromolecules 1986, 19, 699. (10) Stadler, R.; deLucca Freitas, L. In Physical Network; Burchard, W., Roes-Murphy, S. B., Eds.; Elsevier Applied Science: London, ISSO. (11) Burchard, W.; Ross-Murphy, S. B. In Physical Network; Burchard, W . ,Ross-Murphy, s. B., Eds.;Elaevier Applied Science: London, 1990.
0887-0624/92/2506-0863$03.00/0 0 1992 American Chemical Society
864 Energy & Fuels, Vol. 6, No. 6,1992
Communications
Calculated binodals
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Figure 1. Plot of the binodals calculated for mixtures of the extract of an Illinois No. 6 coal and the acetylated extract of this coal with benzene. dynamic nature of the interaction is such that they do not by themselves act as cross-links. In the preceding paper,l it is suggested that in coal some sort of undefined aggregation of chains, promoted by cooperativesecondary interactions, provides the necessary junctions, as in physicalgels. But, in such systems junction zones are usually formed through a specific identifiable mechanism, such as the formation of triple helical regions, as in gelatin, small crystalline regions, as in poly(viny1 chloride) gels, or by the intersection of a liquid-liquid phase separation and a glass transition, as in polystyrene gels formed upon cooling certain solutions (see the other papers included in the book cited as ref 11). It is this latter type of behavior that in my view explains various aspects of coal swelling behavior, from the increased swelling of certain samples when acetylated, to the swelling of extracts. This can be illustrated by some simple considerations of phase behavior. In previous work we have described an association model that has been very successful in describing hydrogen bonding interactions in polymer mixtures.12 This approach uses a Flory-type lattice model and separates terms describing strong specific interactions from those describing weak dispersion and polar forces. The contributions to the free energy of the former are modeled by a set of equations that use equilibrium constants determined from infrared spectroscopic measurements, while the latter are described by solubility parameters calculated from a set of group contributions. These group contributions are constructed, as far as possible, from molecules that do not self-associate or hydrogen bond strongly, for reasons we have discussed in detail elsewhere.12 In order to apply this approach to coal we have adapted van Krevelen's atomic group contributions approach, also described in previous work.13J4 Here, in order to illustrate our point, we will consider the phase behavior of a simple extract, taking as an example the material obtained from an Illinois No. 6 coal. A calculated phase diagram for the extract/ benzene system is shown in Figure 1. In calculating the binodal it was assumed that the extract has a relatively low molecular weight, consisting of about 20 'aromatic clusters" linked by methylene and ether bridges (we (12) Coleman,M. M.; Graf, J.; Painter,P. C. Specific Interactions and the Miscibility of Polymer Blends; Technomic Publishing: Lancaster, PA, 1991. (13) Painter,P. C.; Graf,J.;Coleman,M.M.EnergyFuekr1990,4,379. (14) Painter, P. C.; Park, Y.; Sobkowiak, M.; Coleman, M. M. Energy Fuels 1990, 4, 384.
actually define a "specificrepeat unit", see refs 12and 14). The solubility parameters of the extract coal and benzene were given values of 11.9and 9.5 ( c a l / ~ m ~ )respectively. l/~, The value for the extract was chosen on the basis of previous c a l c u l e t i o n ~ ~and ~J~ the experimental value of 12 ( ~ a l / c m ~ )obtained '/~ by Green et al.16 for the extract of an Illinos No. 6 coal (the exact values of the solubility parameters are not critical for these illustrative calculations; the crucial factor is the difference between the value for the coal extract and benzene and how this changes upon acetylattion). It was also assumed that the only hydrogen bonding that occurs is self-association between coal phenolic OH groups and these were described using equilibrium constants determined previ0us1y.l~ In other words, we assumed that benzene does not hydrogen bond to the coal. This does not mean that hydrogen bonds are not "broken" in the swollen extract. An equilibrium distribution of hydrogen-bonded species that depends upon concentration is established (calculations of such distributions for hydrogen-bonding and non-hydrogenbonding solvents were presented in ref 14). The binodal calculated for the extractlbenzene mixture has an inverted U-shaped appearancewith an upper critical solution temperature near 125 O C , so that the system is phase separated at room temperature. The two phases consist of a solvent-rich component that contains very little extract (the phase boundary is close to the composition limit) and an extract rich phase that comprises the solvent-swollencoal material. The precise position of this latter phase boundary depends upon molecular weight, the solubility parameter difference, and so on. If the solvent content of the extractlsolvent mixture is not too high, the material will still be a glass and thus maintain its mechanical integrity. In other words, the extract does not dissolve in benzene, but does swell to some extent, as observed experimentally by Fujiwara et al.' Upon acetylation the solubility parameter of the coal would be expected to change somewhat. The calculated solubility parameter of the acetyl group is about 11.3 ( c a l / ~ m ~ ) ' /If~ we . assume that the solubility parameter of the extract changes from about 11.9 to 11.7 and that there is no longer any hydrogen-bonding term in the expression for the free energy, then the binodal also shown in Figure 1 is Calculated. It can be seen that the phase boundary changes position and the extract rich phase now contains less coal and more solvent. In other words, it is more swollen than the unacetylated coal. Clearly, such effects could also occur in the cross-linked, insoluble network, but in order to calculate the phase behavior of this system we need to introduce an elastic term in the expression for the free energy. We are presently pursuing this, but the crucial point that I wish to make here is that hydrogen bonds (and other secondary forces) should be treated as an interaction and not in the same way as covalent cross-links, so that in amorphous systems such as coal, where there is no possibility of local order (of the type found in gelatin, for example), swellingand solubility need to be understood on the basis of a knowledge of the phase behavior of the system. (15) Green, T. K.; Chamberlin, J. M.; Lopez-Froedge, L. Prepr. Pap-Am. Chem. Soc., Diu. Fuel Chem. 1989, 34 (3), 759.
