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Energy & Fuels 1995, 9, 192-193
Electrically Conducting Coals Abhimanyu 0. Patil* and Simon R. Kelemen Corporate Research Laboratory, Exxon Research & Engineering Company, Route 22 East, Clinton Township, Annandale, New Jersey 08801 Received July 21, 1994. Revised Manuscript Received October 3, 1994
A stable complex of FeC13 with Illinois No. 6 coal was made. The novel iron complex is evenly dispersed throughout the coal matrix. On exposure to pyrrole monomer, a. film of polypyrrole is formed. Incorporation of as little as 5 wt % pyrrole makes the coal electrically conducting. Coal is perhaps the most abundant fossil fuel, and large deposits of it are distributed throughout the wor1d.l Future applications of coal may extend beyond the present major use for power generation, metal processing, and chemical production. Coal is a complex heterogeneous material which exhibits properties similar to a cross-linked polymeric gel network. Coals are highly porous materials of high surface area. Coal is known t o contain abundant heteroatom functionality and is rich in both aromatic and aliphatic carbons. However, much of coal's specific chemistry remains to be explored. Impregnation of coal with FeC13 has been examined. Shabtai and co-workers2recently reported on a depolymerization procedure in which the first step involves mild hydrotreatment of an FeCl3 impregnated coal. There is also an interest in the role of iron in electrochemical oxidation of coals.3 Complex formation with coal depends on several factors. It is known that coal forms a charge-transfer complex with electron acceptor molecules such as TCNQ or benz~quinone.~ Aromatic functionality in coal might form this type of complex with FeC13. Low-rank coals are known to contain phenolic oxygen functional groups, and it is well-known that Lewis acids form complexes with phenols. It is reasonable to suspect that FeCl3 could form a complex with coal through its phenolic oxygens. Electrically conducting polymers are an interesting class of new polymeric materials. They exhibit conductivities comparable to metals while retaining the advantages of polymer^.^ Conducting polymers have been prepared in a variety of forms in which the electrical conductivity can be systematically varied over 10 orders of magnitude. Blends of conducting polymers with conventional insulating polymers have been studied (1)Gorbaty, M. L.; et al. Science 1979, 206, 1029.Larsen, J. W.; Gorbaty, M. L. Encycl. Phys. Sei. Technol. 1987, 62.Davidson, R. M. In Coal Science; Gorbaty, M. L., Larsen, J. W., Wender, I., Eds., Academic Press, New York, 1982,Vol. I, p 83. (2)Wang, H. P.;Sommerfeld, D. A.; Huai, H.; Pugmire, R. J.; Shabtai, J.; Eyring, E. M. Fuel 1992, 71, 723.Shabtai, J. S.;Saito, I. U. S. Patent. 1988, 4,728,418. (3)Tomat, R.;Salmaso, R.; Zecchin, S. Fuel 1992, 71, 459,463. (4)Larsen, J. W.; Flowers, R. A.; Hall, P.; Silbernagel, B. G.; Gebhard, L. A. Proc. Int. Confi Coal Sci. 1991, 1. ( 5 )Handbook of Conducting Polymers; Skotheim, T. J., Ed.; Marcel Dekker: New York, 1986. Conjugated Polymeric Materials: Opportunities in Electronics, Optoelectronics a n d Molecular Electronm; Bredas, J. L., Chance, R. R., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1990.Patil, A. 0.; Heeger, A. J.; Wudl, F. Chem Rev. 1988, 88, 183.
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recently.6 The formation of electrically conducting coal could provide novel substrates for electrochemical or spectroscopic characterization studies, both facilitated by conducting samples. In this paper we report the formation of a coal.FeCl3 complex and its use to polymerize pyrrole with resultant formation of electrically conducting coal. The coals used in these experiments were obtained from the Argonne Premium Coal Sample Program.' A typical experimental procedure is as follows: 0.5 g of FeCl3 was dissolved in 10 mL of acetonitrile and the solution was allowed to stir at room temperature. To this solution 1.0 g of Illinois No. 6 coal was then added. After stirring for 3 h, the solid was filtered and washed with 20 mL of acetonitrile, and the product was dried under vacuum. To make electrically conducting coal, the coal*FeCls product obtained above was exposed t o pyrrole vapor for 5 days a t room temperature. The procedure was found effective with different rank coals, from lignite to low-volatilebituminous coals.8 Interestingly, all coals swelled upon complex formation. The complex prepared from coal and FeCl3 in acetonitrile was Soxhlet extracted in acetonitrile for 24 h. The initial acetonitrile extract was colored, suggesting removal of some FeCls. After 24 h, however, the acetonitrile solution was colorless. The complex obtained upon drying had 5.33 wt % chlorine compared to 8.46 wt % for the starting material, as judged by bulk elemental analysis. This suggests that a substantial portion of the FeC13 is so tightly bound that it cannot be extracted into hot solvent. The distribution of iron in the coal matrix was studied using X-ray photoelectron spectroscopy (XPS).Figure l a shows the XPS plot of photoemission intensity versus binding energy for 11linois No. 6 coal and Figure l b shows the plot for Illinois No. 6 coal after formation of the complex with FeC13. The chloride peak is clearly seen in the complexed coal, along with peaks due to coal itself. Quantitative analysis of the amount of chlorine relative to carbon gave 5.2 chlorines per 100 carbons. This compares to 4.6 chlorines per 100 carbons by bulk analysis. Since XPS is a surface-sensitive technique and samples roughly the first 50 A of the sample, the close correspondence of two of these results indicates that the iron catalyst is evenly distributed throughout the coal matrix. Figure ICshows the coal*FeClscomplex after exposure to pyrrole. On exposure to pyrrole monomer, a large peak due to polypyrrole nitrogen is evident and is attributed to a polymerized film of polypyrrole on the (6)Andreatta, A.; Heeger, A. J.; Smith, P. Polym. Commun 1990, 31,275.Wessling, B.Adu. Muter. 1991,3,507. Moulton, J.; Ihn, K. J.; Smith, P. Polym. Muter. Sci. Eng. 1991, 64, 256. (7)Vorres, K. S.Energy Fuels 1990, 4, 420. Kelemen, S. R., unpublished data. (8)Patil, A. 0.;
0 1995 American Chemical Society
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Energy &Fuels, Vol. 9, No. 1, 1995 193
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FeCIJCoal
FeCIJ /Pyrrole/ Coal
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Figure 1. (a) XPS spectrum of Illinois No. 6 coal. (b) XPS spectrum of Illinois No. 6 coal after formation of the FeCls complex. (c) XPS spectrum after exposure of the coal.FeCl3 complex to pyrrole.
coal surface. The other peaks due to the original coal also show decreased intensity. This is also consistent with a layer of polypyrrole on the coal surface. Conductivity measurements were done on pressed pellets of polypyrrole-coal composite. The conductivity was found t o be 0.3 x Slcm. The formation of the atomically thin polymer film makes the coal electrically conducting. Note that the original untreated coal, as well as the FeCl3-treated coal, is nonconducting.
There are a number of observations that support the view that polypyrrole has penetrated the coal structure at the sub microscopic level and in effect produced a conducting coal. When conducting coal is analyzed by secondary ion mass spectrometry (SIMS),a stable image can be produced and maintained, a characteristic of a conducting substrate. During the SIMS experiment the surface of the material is sputtered causing the removal of many layers of material. If the polypyrrole did not penetrate the microstructure of coal the stable SIMS image would be expected to degrade after a short time. An attempt to make a conducting material by simply blending coal with 5 w t % polypyrrole failed. When other substrates such as silica gel were used, it was not possible to form a stable complex with FeC13 and generate a conductingmaterial upon exposure to pyrrole vapor. The electronic modifications a t an atomic level to the organic portion of the coal are clearly not addressed by these experiments; however, the observations indicate that the induced changes in the electrical properties cannot be simply disrupted once formed or reproduced with other materials. Generally, in FeCla-doped polypyrrole, for every two to three pyrrole rings there is one FeC14- ioh. In a control experiment on polypyrrole, 31 chlorines per 100 carbon atoms were found by XPS, suggesting 2 pyrrole rings per dopant anion. Interestingly, in pyrrole polymerized on the FeClycoal complex, there are 7-8 pyrrole rings for every dopant anion. This suggests that the dopant FeC13 may be more effectivelyutilized in this situation for polymer formation. This behavior is currently being explored. In conclusion, we have made a novel complex of FeCl3 with coal. XPS provides information about the distribution of the iron-based catalyst in coal. The data indicate that the iron catalyst is evenly dispersed throughout the coal matrix. On exposure to pyrrole monomer, a film of polypyrrole is formed on the coal surface which makes the coal electrically conducting. The phenomenon is general and occurs with different ranks of coals.8 The materials will enable future electrochemical and spectroscopic studies of coal. EF9401482
