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Communications Specific Interactions of Organic Bases with the Illinois No. 6 Coal Surface Are the Same as Their Hydrogen-Bond Strengths with Phenol Amy S. Glass and John W. Larsen* Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015 Received July 1 , 1993. Revised Manuscript Receiued September 27, 1993 Isosteric heats of adsorption for seven organic bases on demineralized Illinois No. 6 coal have been measured by inverse gas chromatography (IGC). These heats can be separated into nonspecific and specific interactions by using a previously established correlation between molecular polarizability and the enthalpy of nonspecific interactions with the coal.' For all of the bases, the specific interaction heats are equal to the heats of hydrogen-bond formation between that base and p-fluorophenol.2 It appears that these bases associate with the coal surface by hydrogen-bondingto pendant phenolic hydroxylgroups. IGC is a dynamic adsorption technique used routinely to determine adsorption enthalpies for polymer and carbon s~rfaces.3*~ The technique is sensitive to interactions at the coal surface and has been used to obtain adsorption heats on coal^.^^^^^ Fowkes et al. and Jones used flow calorimetry, also a dynamic technique, and obtained a range of adsorption heats for pyridines and acetone on bituminous c0als.~18 In Figure 1, isosteric adsorption enthalpies, -qst, for hydrocarbons and bases on demineralized Illinois No. 6 coal are plotted vs adsorbate polarizability, The line is drawn through the points for alkanes and alkenes. The alkane and alkene data were obtained for Illinois No. 6 coal which had been extracted with tetrahydrofuran (THF) and then heated at 150 "C or at 250 "C (to remove interfering volatiles) in helium for 2 weeks. The interactions of these hydrocarbons with the coal are solely nonspecific and therefore correlate with molecular po1arizability.l The hydrocarbon adsorption heats were determined from van't Hoff plots obtained over a temperature range of at least 30 "C and usually 60 "C or more. (1) Glass, A. S.;Larsen, J. W.EnergyFuels, submitted for publication. (2) Arnett, E. M.: Mitchell, E. J.: Murtv. T. S. S. R.J.Am. Chem. SOC. 1974, 96,3875-91. (3) Lloyd, D. R.;Ward, T. C. Inverse Gas Chromatography; ACS Symposium Series 391; American Chemical Societv: Washinpton, DC, 1989: (4) Avgul, N. N.; Kiselev, A. V. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Marcel Dekker: New York, 1970; Vol. 6, pp 1-124. (5) Larsen, J. W.; Kennard, L.; Kuemmerle, E. W. Fuel 1978,57,30913. (6) Arnett, E. M.; Hutchinson, B. J.; Healy, M. H. J.A m . Chem. SOC. 1988,110, 5255-60. Jones, K. L.; Li, G.; Lloyd, T. B. Energy Fuels 1989, (7) Fowkes, F. M.; 3,97-105. (8) Jones, K. L. Coal Prep. 1992,10,47-57. (9) The polarizability, a,was calculated from the formula a = [3M/ plVl[(nz - l)/(nz + 2)1, where M is the molecular mass, p is the density, N is Avogadro's number, and n is the refractive index, using refractive indices and densities from ref 10. (10)Weast, R. C., Ed. Handbook of Chemistry and Physics; CRC: Boca Raton, FL, 1982.
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Figure 1. Isosteric adsorption enthalpy, qet,vs polarizability,CY, for alkanes, alkenes, and basic adsorbates on Illinois No. 6 coal. The alkane and alkene data are for Illinois No. 6 coal extracted in tetrahydrofuran and heated at 150 or 250 "C in helium. The data for the bases were obtained on Illinois No. 6 coal demineralized in HF and HC1 and heated at 250 "C in helium. The identity of the hydrocarbons on the line in order from left to right (increasingexothermicity) is as follows: methane, ethane, cyclopropane,propane, propene, n-butene, 1-butene, and n-hexane.
Each point in the van't Hoff plots was the average of five to nine replicate determinations at a given temperature. Errors in the adsorption heats were 6% of the reported values.' Adsorption heats for the bases were the result of measurements over at least at 40 "C temperature range. Points were acquired every 5-10' and at least three points were acquired at each temperature. The alkane and alkene enthalpies in Figure 1 are similar to those obtained by Avgul and Kiselev on graphitized carbon black.4 The graphitized carbon black surface is a good model for the dispersive component of the coal surface. The adsorption enthalpies (-qst) for bases on demineralized Illinois No. 6 coal" (Figure 1) fall above the hydrocarbon line, demonstrating that there is another force operating in addition to the nonspecific interactions of these adsorbates with the coal surface. These data were obtained on coal which was demineralized in HF and HC1 to remove anion-exchangeable, carbonate, and silicate minerals and then was heated at 250 "C in helium for 2 (11) Six grams of Illinois No. 6 coal was heated for 24 h at 55-60 OC in 40 mL of 49 5% aqueous HF under dry Nz. This solution was diluted to 600 mL with HzO, filtered, and washed with 150 mL of Hz0. The coal was then heated at 55-60 OC with 50 mL of 5N HC1 for 2 days, diluted to 250 mL, filtered, and washed until the wash water had a neutral pH (-600 mL).
0 1994 American Chemical Society
Energy & Fuels, Vol. 8,No. 1, 1994 285
Communications /' / dimethyl w l f o x i d e ~ ~ p y , , d l n /
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Figure 2. Specific adsorption enthalpy, q l t J H c ,on demineralized Illinois No. 6 coal vs heat of hydrogen-bond formation with p-fluorophenol, DJ.lrpF'p, for seven bases.2 The coal data were obtainedon IllinoisNo. 6 coal which had been demineralized in HF and HC1 and heated at 250 O C in helium.
weeks before chromatograms were obtained.11J2 The ash content was decreased from 16.7 to 2.0 % by this treatment. The distance between each point and the hydrocarbon line in Figure 1represents the specific component of the isosteric adsorption enthalpy, -qst,specific,for a given base with the coal surface. The specific interaction enthalpy (-qst,specific)is determined by subtracting the nonspecific component (defined by the line) from the total interaction enthalpy (qat). The nonspecific component of qat was determined from the adsorbate polarizability and the slope and intercept of the hydrocarbon line in Figure 1(method 1)or the enthalpy of adsorption on the graphitized carbon black surface was used (method 2).4 Method 2 compensates for steric effects in adsorbate-coal interactions whereas method 1cannot take these effects into account. Figure 2 is a plot of the specificcomponent of the isosteric adsorption enthalpy (-qst,specific)on demineralized Illinois No. 6 coal against hydrogen-bond strength with p-fluo(12) Larsen, J. W.; Pan, C.-S.; Shawver, S. Energy Fuels 1989,3,55761.
rophenol in solution, -A.Hf,pFP, for the seven bases.2 The dashed line is the one-to-one correlation. As seen in Figure 2, the specific interaction enthalpies with the coal surface are identical to the hydrogen-bond enthalpies with p-fluorophenol for all of the bases. The error in -qst,specificis &lo% of the reported values and the error for A.Hf,PFP is f 2 % . l s 2 Two separate determinations of qat were made for dimethyl sulfoxide. The base-phenol hydrogen bond is a good model for the specific component of the b a s e coal surface interaction under the conditions of the IGC experiment. The order of hydrogen-bond strengths in Figure 2 ia the same as that found by Larsen et al. for the swelling ratios of original to acetylated Illinois No. 6 coals in pyridine, dimethyl sulfoxide, THF, acetone, and diethyl ether, another measure of coal hydrogen-bond strengths.13 Because the coal-base and p-fluorophenol-base specific adsorption heats are identical to each other for seven different bases, it is likely that a phenolic functionality located on the organic portion of the coal is acting as a hydrogen-bond donor. The data fit with a model where basic adsorbates preferentially interact with a low surface concentration hydroxyl functionality on the organic coal surface. The phenolic hydroxyl content of coal (bulk coal) is about 5 % .I4J5 Since the p-fluorophenol hydrogen-bond enthalpies were measured in such a way as to preclude phenol self-association, these data suggest minimal selfassociation of phenolic hydroxyls on the demineralized coal surface.2
Acknowledgment. We acknowledge support of this work by the U S . Department of Energy under grant no. DE-FG22 89PC9757. We thank Shang Li for preparing the demineralized coal. (13) Larsen, J. W.; Green, T. K.; Kovac, J. J. Org. Chem. 1985, 50, 4729-35.
(14) Alemany, L. B.; Grant, D. M.; Pugmire, R. J.; Stock,L. M. Fuel 1984,63,513-20. (15) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3,187-93.
