Energy & Fuels 1998, 12, 531-535
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Characterization of Different Possible Solvent-Coal Interaction Mechanisms by the Relationship between the Volumetric Swelling of Coals and Heat Release in Swelling Solvent Nan Wang,*,† Masahide Sasaki,‡ Tadashi Yoshida, and Takeshi Kotanigawa§ Hokkaido National Industrial Research Institute, 2-17-2-1 Tsukisamu-Higashi, Toyohira-ku, Sapporo 062, Japan Received August 14, 1997. Revised Manuscript Received February 24, 1998
The heat of mixing a polar or nonpolar swelling solvent with coal was measured by flow microcalorimetry (FMC). Eight coals, having different oxygen contents, were used as the adsorbent, and methanol, THF, tetralin were used as the adsorbate. We examined a range of swelling solvent concentrations in n-hexane, 2-20 g/L, and found that there are large differences in the adsorption heats for the three solvents. Methanol adsorption heat reaches the maximum at low methanol concentrations and remains constant at higher concentrations, while heats for THF and tetralin adsorption increase with swelling solvent concentrations. The methanol adsorption heat maximum is about 2000 mJ/m2, THF is 70 mJ/m2, and tetralin is 40 mJ/m2 over the concentration range 0-20 g/L. Uptake for swelling solvent was determined by comparing the concentration of swelling solvent in n-hexane before and after adsorption. Methanol uptake reaches about 80 mmol/g coal, while THF only reaches 0.3 mmol/g coal. Tetralin uptake is too small to be determined by such a technique. Similar to adsorption heat, methanol uptake reaches its maximum value at a lower concentration and then remains constant, and THF uptake increases with its concentration over the concentration range. We also investigated swelling of the eight coals in such swelling solvents and found that in contrast to methanol, THF has relatively low adsorption heats but large swelling ratios. This suggests that methanol likely interacts with specific sites on coal without disrupting coal-coal cross-links between macromolecular chains in coal, while THF may break such coal-coal cross-links. The low swelling ratios for tetralin suggest no disruption of coal-coal cross-links, although this solvent does interact with coal as seen from its moderate adsorption heat values. For coals swollen in a solvent, the heat for swelling solvent adsorption on coal correlates strongly with coal swelling ratio in the solvent.
Introduction It has been recognized for some time that coals have a macromolecular structure, which allows swelling in appropriate solvents and has a significant effect on the reaction behavior of coals.1-4 Studies on coal swelling by solvent have helped the understanding of coal macromolecular network structure.5-16 An improved knowledge of coal swelling benefits elucidation of mac† Present address: Laboratory of Organic Resources Chemistry, Institute for Chemical Reaction Science, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-77, Japan. ‡ Present address: Laboratory for Hydrocarbon Process Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802. § Present address: Universidad Nacional del Litoral, Santiago del Estero 2829, (3000) Santa Fe, Argentina. (1) Van Krevelen, D. W. Coal, 1st ed.; Elsevier: New York, 1961; pp 440-444. (2) Lucht, L. M.; Peppas, N. A. Chemistry and Physics of Coal Utilization; American Institute of Physics: New York, 1981; p 28. (3) Green, T.; Kovac, J.; Brenner, D.; Larsen, J. W. Coal Structure; Academic Press: New York, 1982; p 192-282. (4) Larsen, J. W. Clean Utilization of Coal-Coal Structure and Reactivity, Cleaning and Environmental Aspects; Kluwer Academic Publishers: Dordrecht, 1991; pp 1-14. (5) Larsen, J. W.; Green, T. K.; Kovac, J. J. Org. Chem. 1985, 50, 4729-4735. (6) Nishioka, M.; Larsen J. W. Energy Fuels 1990, 4, 100-106.
romolecular network structure of coals and consequently leads to new and more effective coal-utilization processes. Recently, attention has been focused on correlation of swellability with solvent properties. Szeliga et al. swelled a coal in various solvents, which are characterized by electron-donor and electron-acceptor number, and found that coal swelling and solvent electron-donor number show reasonably strong correlation.17 Suuberg et al. reviewed literature on such correlation and found that there exists a correlation (7) Painter, P. C.; Park, Y.; Sobkowiak, M.; Coleman, M. M. Energy Fuels 1990, 4, 384-393. (8) Aida, T.; Fuku, K.; Fujii, M.; Yoshihara, M.; Maeshima, T.; Squites, T. G. Energy Fuels 1991, 5, 79-83. (9) Yun, Y.; Suuberg, E. M. Energy Fuels 1992, 6, 328-330. (10) Suuberg, E. M.; Otake, Y.; Yun, Y.; Deevi, S. C. Energy Fuels 1993, 7, 384-392. (11) Yang, X.; Silbernagel, B. G.; Larsen, J. W. Energy Fuels 1994, 8, 266-275. (12) Larsen, J. W.; Li, S. Energy Fuels 1994, 8, 932-936. (13) Faulon, J.-L. Energy Fuels 1994, 8, 1020-1023. (14) Suuberg, E. M.; Otake, Y.; Langner, M. J.; Leung, K. T.; Milosavljevic, I. Energy Fuels 1994, 8, 1247-1262. (15) Larsen, J. W.; Gurevich, I.; Glass, A. S.; Stevenson, D. S. Energy Fuels 1996, 10, 1269-1272. (16) Milligan, J. B.; Thomas, K. M.; Crelling, J. C. Energy Fuels 1997, 11, 364-341. (17) Szeliga, J.; Marzec, A. Fuel 1983, 62, 1229-1231.
S0887-0624(97)00146-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/07/1998
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Table 1. Analyses of Coal Adsorbents ultimate analysis (daf %) no.
coal
C
H
N
S
O
ash (mf %)
CO2 surface area (m2/g)
1 2 3 4 5 6 7 8
Akabira (Japan) Battle River (Canada) Datong (China) Illinois (U.S.) Taiheiyo (Japan) Wandoan (Australia) Wyoming (U.S.) Yallourn (Australia)
81.5 66.8 81.5 74.4 74.3 74.1 68.1 63.3
5.6 4.5 4.5 5.2 6.1 5.6 4.8 4.6
1.2 1.1 0.5 1.0 0.8 0.6 0.6 0.4
0.5 2.2 1.2 2.3 0.0 1.2 0.2 1.8
11.2 25.4 12.3 17.1 18.8 18.5 26.3 29.9
3.7 11.2 8.7 11.7 12.6 8.1 5.3 2.9
76.5 107.8 87.8 70.8 78.7 82.0 97.2 94.0
between solvent swelling of coal and the enthalpy of formation of hydrogen bonds or other electron donoracceptor interactions between the solvent and the coal.14 However, although some relationship between various physicochemical parameters of solvents and their coal swelling has been reported, there remains a lack of understanding of the mechanism for coal swelling. In addition, the various physicochemical parameters plotted often do not show direct correlation with coal swelling ratio. In our previous study, we found that molar heat of strong methanol adsorption is slightly higher than the hydrogen bond energies in donor-acceptor systems.18 Thus, adsorption heat for solvent on coal must correlate solvent swelling of coal. However, there are few reports on such correlation. Accordingly, we measured adsorption heat of polar and nonpolar swelling solvents on coals as well as the extent of coal swelling in such solvents. On the basis of experimental results, the relationship between adsorption heats and coal swelling is discussed. Experimental Section Coal Sample. Eight kinds of coal, having considerably different surface chemical composition, were used. The analyses for the coals are presented in Table 1. These coal samples were finely ground with pestle and mortar to pass a 100 mesh sieve and dried in a vacuum oven at 110 °C for 24 h before calorimetric measurements. The surface areas of ground coal samples, given in Table 1, were obtained by a BET measurement at 25 °C, where carbon dioxide was employed as an adsorbate. Compared with the nitrogen surface area, the carbon dioxide surface area represents the total surface of coal.19-22 Flow Microcalorimetry. Heats, generated by adsorption of swelling solvent on coals, were determined in a flow microcalorimeter (Microscal FMC-3V, vacuum model), which gives a sensitivity in the microcalorie range.23 n-Hexane was chosen as the carrier, and methanol, tetrahydrofuran (THF), and tetralin were chosen as the adsorbate in this study. These reagents were of highest available commercial purities and degassed before use. Solutions for flow microcalorimetry (FMC) were prepared by dilution in n-hexane to 2-20 g/L. A maximum swelling solvent concentration of 20 g/L was chosen to avoid coal dissolution, which was detected at higher concentrations. The carrier and solvent solution were fed smoothly by a twin syringe pump system at a constant flow rate. (18) Wang, N.; Sasaki, M.; Yoshida, T.; Kotanigawa, T. Energy Fuels 1997, 11, 1293-1298. (19) Fowkes, F. M.; Jones, K. L.; Li, G.; Lloyd, T. B. Energy Fuels 1989, 3, 97-105. (20) Fuerstenau, D. W.; Yang, G. C.; Chander, S. Prepr. Pap.sAm. Chem. Soc. Div. Fuel Chem. . 1987, 32, 209-215. (21) Marsh, H.; Siemieniewska, T. Fuel 1965, 44, 355-367. (22) Gan, H.; Nandi, S. P.; Walker, P. L. Fuel 1972, 51, 272-277. (23) A technical note delivered by Microscal Ltd.
Heat for adsorption of swelling solvent on coals was determined by a continuous flow method, which has been described previously.18 Initially, about 50 mg of coal sample was loaded into the sample cell of the FMC and preevacuated for 1 h. Then n-hexane was introduced into the sample cell. After thermal equilibrium was achieved between coal and pure n-hexane carrier solvent, the carrier was replaced by solutions, containing different amounts of swelling solvent, to start the adsorption measurement. The polar or nonpolar swelling solvent was first adsorbed from 2 g/L solution until thermal equilibrium was reached. Subsequently, swelling solvent concentration in the carrier was increased stepwise to avoid dilution heat effects. Thermal equilibrium was reestablished at each new swelling solvent concentration. To confirm the absence of dilution heat effects, the same concentration profile was run with PTFE (polytetrafluoroethylene) as the adsorbent, which was not found experimentally to adsorb any swelling solvents used for calorimetric measurement. No dilution heat effects were observed for polar adsorbates such as methanol and THF. But since negative dilution heat effects were detected for tetralin, measuring results for tetralin adsorption heat were corrected by use of the values of such dilution heat effect. Swelling. Coal swelling experiments were performed by an improved volumetric method24 for a mixture of n-hexane and swelling solvent, and the conventional solvent-swelling technique25 was used for pure swelling solvents. The latter was reported to be ineffective at low hydrogen-bond acceptor concentrations.24 The swelling ratio was defined as the volume of coal sample swollen by the mixture of swelling solvent and n-hexane or by pure swelling solvent to that swollen by pure n-hexane. FT-IR. FT-IR spectra were taken for the parent coal samples, for coal samples swollen by n-hexane or a mixture of n-hexane and swelling solvent, and for the effluent solutions of FMC, with a SHIMAZU FTIR spectrometer (FTIR-8100M). Transmission FT-IR spectra were obtained for the effluent solutions of FMC and diffuse reflectance spectra for coal samples. n-Hexane and KBr were employed as blanks for solutions and coal samples, respectively. Adsorption Isotherm. Uptake of swelling solvent on a coal was determined by comparing its concentration before and after adsorption. About 20-50 mg of coal sample was accurately weighed and mixed with 10 mL of solution containing the swelling solvent (2-20 g/L in n-hexane). The mixtures were placed in an ultrasonic tub for 10 min, then allowed to stand for about 3 h, which corresponds to the time of the FMC adsorption heat measurement. Next, the suspensions were divided into solid and liquid by centrifugation. The concentration of swelling solvent in the supernant solution was determined by a refractive index detector (JASCO RI-830). Finally, the amount of methanol adsorbed on the coal was calculated on the basis of the concentration of swelling solvent before and after the adsorption experiment. Coals are cross-linked macromolecular networks capable of taking up molecules into their bulk. Surface adsorption of swelling solvent on coals must be immediately followed by (24) Larsen, J. W.; Gurevich, I. Energy Fuels 1996, 10, 1269-1272. (25) Green, T. K.; Kovac, J.; Larsen, J. W. Fuel 1984, 63, 935-938.
Solvent-Coal Interaction
Energy & Fuels, Vol. 12, No. 3, 1998 533
Figure 2. Amount of adsorbed swelling solvent.
Figure 1. Heat for swelling solvent adsorption on coal. rapid penetration of solvent molecules into coals.26,27 The amount of swelling solvent absorption is expected to be involved in the amount of adsorption obtained by the methods mentioned above. All the measurements were carried out at 20.5 ( 1.5 °C.
Results and Discussion Adsorption Heat. Figure 1 shows the heats for methanol, THF, and tetralin adsorption on coals as a function of swelling solvent concentration. Akabira bituminous coal and Yallourn brown coal, having considerably different oxygen content, were used as adsorbents for the data in these figures. The number on the vertical axis of Figure 1 indicates the cumulative heat of adsorption per unit surface area of coal. For both coals, the methanol adsorption heat reaches the maximum at low methanol concentrations and remains constant at higher concentrations, while heats for THF and tetralin adsorption increase with swelling solvent concentrations. Methanol adsorption heat maximum is about 2000 mJ/m2, THF is 70 mJ/m2, and tetralin is 40 mJ/m2 over the concentration range 0-20 g/L. Although the adsorption profiles have the same shape, absolute values of adsorption heat for polar adsorbates were significantly different. Adsorption of polar swell(26) Van Krevelen, D. W. Coal, 1st ed.; Elsevier: New York, 1961; pp 127-149. (27) Glass, A. S.; Larsen, J. W. Prepr. Pap.sAm. Chem. Soc. Div. Fuel Chem. . 1992, 37, 1177-1183.
ing solvent on Yallourn brown coal evolves higher heat compared with Akabira bituminous coal. On the other hand, low-rank coals swell more than high-rank coals in an identical solvent.6 To examine other thinkable thermal interactions between swelling solvent and coal, swelling experiments and FT-IR analyses were performed. No coal was found to swell in the mixture of the swelling solvent and n-hexane in the first 24 h by using the improved volumetric method. The concentration of swelling solvent in the mixture is 20 g/L. This means that the presence of swelling solvent, at the maximum concentration (20 g/L) used for the calorimetric measurements, does not cause coal swelling beyond swelling caused by n-hexane during the calorimetric measurement. FT-IR spectra were taken for the parent coal samples, and coal samples were swollen by n-hexane or swelling solvent solution (20 g/L). The OH stretching vibration at 3400 cm-1 became broader after methanol adsorption or shifted to a lower position after THF adsorption but was unchanged after tetralin adsorption. In any case, no dissolved constituents were detected owing to exposure to these solutions. FT-IR spectra for the effluent solutions of FMC also showed no dissolved constituents provided the swelling solvent concentration was below 20 g/L. These coal swelling and FT-IR data suggest that heat measured in this study mainly results from surface adsorption of swelling solvent on the coal. We tried to determine heats for phenol and pyridine adsorption on coals. However, low-rank coal was found to swell slightly when it was immersed in a phenol solution of 2 g/L concentration, and dissolution of naphthalene was detected in the effluent of FMC by GC when pyridine was employed as adsorbate and low-rank coal as adsorbent. Thus, even at low concentrations, surface adsorption may not dominate the interaction of phenol or pyridine with coal. Adsorption Isotherm. The amount of adsorbed polar swelling solvent, obtained by comparing its concentration before and after adsorption, is given on a basis of mmol/g of coal in Figure 2. But no reproducible uptake for the nonpolar swelling solvent, tetralin, was obtained by the same method. Similar to adsorption heat, methanol uptake reaches its maximum value at a lower concentration and then remains constant. This means that once all of the adsorption sites for methanol on coal have been occupied at a lower concentration, there will be no more methanol adsorbed by methanol-coal interaction or methanolmethanol interaction. Methanol may hydrogen-bond strongly with oxygen functional groups or weakly with basic aromatic rings on coal.18
534 Energy & Fuels, Vol. 12, No. 3, 1998
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Figure 3. FT-IR spectra for THF and Akabira bituminous coal.
In contrast to methanol, the amount of adsorbed THF increases linearly with its concentration, provided the THF concentration is below 20 g/L in n-hexane. This type of adsorption isotherm is characterized by better penetrating power of adsorbate28 and suggests that THF may break the original hydrogen-bond cross-links between macromolecular chains in coal. Figure 3 shows FT-IR spectra for THF sorbed on KBr, untreated Akabira bituminous coal, and Akabira coal where THF was sorbed in advance and then evacuated. The OH stretching vibration at 3400 cm-1 shifted to a lower position after exposure to THF, and the coal spectrum remains unchanged after a 6 h evacuation. This suggests that THF adsorption forms a strong hydrogen bond to hydroxyl groups in coal. This result corresponds well to other spectroscopic studies on sorption of basic solvent in coal.29 Thus, interaction of THF with coal must be followed by the fact that THF molecules break original coal-coal hydrogen bonds and replace them with new coal-THF hydrogen bonds. Although adsorption heats for polar swelling solvent were significantly different, the two coal samples have almost the same capacities for polar swelling solvent. On the other hand, methanol uptake reaches about 80 mmol/g coal, while THF only reaches 0.3 mmol/g coal. In all cases, methanol uptake is much larger than THF. However, this does not mean that methanol has the greater penetrating power into coal bulk than THF because methanol is expected to form a diffuse layer around the polar coal surface.18 Compared with polar swelling solvent, nonpolar swelling solvent, such as tetralin, was expected to adsorb on coals by forming weak hydrogen bonds to free hydroxyl groups without disrupting coal-coal cross-links or by π-π interactions with aromatic rings without affecting hydrogen bonds in coal. However, the amount of tetralin adsorbed on coal is so small that no reproducible uptake for tetralin could be obtained by comparing its concentration in n-hexane before and after adsorption. Correlation between Adsorption Heats and Swelling Ratios. As mentioned above, the heat, obtained by the FMC technique when adsorbate molecules contacts the coal, is a measure of the energy of the bonds formed between the adsorbed species and the (28) Giles, C. H.; MacEwan, T. H.; Nakhwa, S. N.; Smith, D. J. Chem. Soc. 1960, 3973-3993. (29) Larsen, J. W.; Baskar, A. J. Energy Fuels 1987, 1, 230-232.
Figure 4. Relationship between heat for swelling solvent adsorption on coal and coal swelling.
coal adsorbent. Thus, there must exist a quantitative relationship between the adsorption heat and the coal swelling. In Figure 4, the initial heat for swelling solvent adsorption on coal is plotted against the extent of coal swelling in such solvents. Experimental data for the eight coals are used in Figure 4, and each point in the plots is for a single coal. Adsorption heats are given for 2 g/L solutions on the basis of surface area of coal adsorbent. Equilibrium swelling ratio, defined as the volume of coal sample swollen by pure swelling solvent to that swollen by pure n-hexane, was achieved by the conventional volumetric method. In all cases, adsorption heat and swelling shows a strong correlation. Compared with methanol, THF generates a smaller heat for adsorption on coal but has a better ability to swell coal. This must be attributed to the special interaction of THF with coal. The correlation of adsorption heat with swelling ratio, shown in Figure 4, supports a significant difference between methanol and THF. For methanol, the adsorption heat, corresponding to a swelling ratio of 1.00, is about 400 mJ/m2, which can be obtained by extrapolating the correlation. This means that methanol adsorption on coal begins from exothermic formation of coal-methanol hydrogen bonds without breaking original coal-coal cross-links. How-
Solvent-Coal Interaction
ever, for THF, the value is about 0 mJ/m2. This suggests that coal swelling likely occurs simultaneously with THF adsorption. Furthermore, the swelling ratio, corresponding to an adsorption heat of 0 mJ/m2, is about 1.7 for THF, which was obtained from the correlation for coals whose swelling ratios are over 1.7. This indicates that THF-coal interactions start from endothermic disruption of coal-coal hydrogen bonds, which is followed by exothermic formation of THF-coal hydrogen bonds; i.e., THF selectively disrupts hydrogenbond cross-links in the coal. This selective interaction of THF with cross-linking hydroxyl groups is thought to be due to the much more favorable entropy change caused by selective association between THF and such hydroxyl groups in coal.15 For THF, the adsorption heat is a net exothermic heat evolved by both endothermic dissociation of coal-coal interactions and exothermic formation of new associative coal-solvent interactions. For this reason, the heat for THF adsorption on coal must be small. The low swelling ratios for nonpolar tetralin suggests no disruption of coal-coal cross-links, although this solvent does interact with coal as seen from its moderate adsorption heat values. Conclusion Adsorption heats for methanol, THF, and tetralin were determined by flow microcalorimetry (FMC) over the concentration range 2-20 g/L. Methanol adsorption heat reaches a maximum at low methanol concentrations and remains constant at higher concentrations, while heats for THF and tetralin adsorption heat increase with swelling solvent concentrations. The methanol adsorption heat maximum is about 2000 mJ/ m2, THF is 70 mJ/m2, and tetralin is 40 mJ/m2 over the concentration range 0-20 g/L.
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Uptake for swelling solvent was also determined by comparing the concentration of the swelling solvent in n-hexane before and after adsorption. Methanol uptake reaches about 80 mmol/g coal, while THF only reaches 0.3 mmol/g coal. Tetralin uptake is too small to be determined by such a technique. Similar to adsorption heat, methanol uptake reaches its maximum value at a lower concentration and then remains constant, while THF uptake increases with its concentration over the concentration range. The three solvents show large differences in the behaviors of adsorption heat and uptake as a function of solvent concentration, and in the swelling behaviors. This indicates that the mechanism of interactions between the solvents used and coal is significantly different. Methanol likely interacts with specific sites on coal without disrupting many coal-coal cross-links between macromolecular chains in coal, while THF may break such coal-coal cross-links. The low swelling ratios for tetralin suggests no disruption of coal-coal cross-links, although this solvent does interact with coal as seen from its moderate adsorption heat values. Heat for polar and nonpolar swelling solvent adsorption on coal, obtained by the continuous flow method, is used to correlate the coal swelling in such solvents. It is conclusive that adsorption heat for swelling solvent on coal correlates strongly with coal swelling in such solvents. Acknowledgment. The authors thank Dr. Zhan-guo Zhang and Dr. Andrew D. Schmitz for helpful discussion and valuable comments on the manuscript. The authors are also grateful to Mr. Mitsuyoshi Yamamoto and Miss Eriko Murata for CO2 surface area measurements. EF970146S
