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Flow Microcalorimetric Study of Methanol Adsorption from n-Hexane on Coals 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 June 12, 1997. Revised Manuscript Received June 16, 1997X
The heat evolved by mixing of methanol and a coal was measured by flow microcalorimetry (FMC) to investigate interactions between coal and methanol. Such interactions were classified into weak and strong adsorption. Two adsorption methods were used: continuous flow for measuring combined weak and strong adsorption, and pulse injection to distinguish strong adsorption. Akabira bituminous coal and Yallourn brown coal, having different oxygen contents, were used as adsorbents. Experimental results achieved by the continuous flow method show that both amount and heat of methanol adsorption (2 g of methanol/L in n-hexane) depends on the structure of coal. Methanol uptakes were also measured, off-line, by immersing the coals in methanol-hexane solutions of different concentrations and comparing methanol concentrations in n-hexane before and after adsorption. Methanol adsorption isotherms so obtained have the same shape and the total uptakes of methanol adsorption are almost the same in spite of different coal rank. Strong adsorption of methanol gives a molar heat distribution between 30 and 60 kJ/mol, slightly higher than the hydrogen bond energies in donor-acceptor systems. Strong methanol adsorption may be attributed to formation of new hydrogen bonds between methanol and oxygen-containing functional groups on the coal. The molar heat of adsorption for methanol adsorbed by weak interactions with coal is about 23 kJ/mol. The site for strong methanol adsorption on Yallourn brown coal is more energetically heterogeneous than that on Akabira bituminous coal.
Introduction For coal liquefaction, it is essential to elucidate the interactions of coal with the recycle solvent. Studies on the interaction of coal with solvent also provide information on the chemical and physical structure of coal. Recently, flow microcalorimetry has been employed to investigate the surface chemistry of coal1-8 because of its original ability to determine the heats of desorption and readsorption on the same adsorbent. Using flow microcalorimetry, detailed information on the interaction of the coal surface with various adsorbates such as acids, bases, n-hexane, and water has been obtained. However, in spite of the many studies on coal processing using methanol,9-18calorimetric investigation on the interaction between methanol and coal has not † Present address: Laboratory for 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, PA 16802. § Present address: Universidad Nacional del Litoral, Santiago del Estero 2829, (3000) Santa Fe, Argentina. X Abstract published in Advance ACS Abstracts, August 15, 1997. (1) Fowkes, F. M.; Jones, K. L.; Li, G.; Lloyd, T. B. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1987, 32, 216-225. (2) Fowkes, F. M.; Jones, K. L.; Li, G.; Lloyd, T. B. Energy Fuels 1989, 3, 97-105. (3) Fuerstenau, D. W.; Yang, G. C.; Chander, S. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1987, 32, 209-215. (4) Groszek, A. J. Proc. Symp. Solid-Liq. Interact. Porous Media 1985, 587-597. (5) Groszek, A. J.; Templer, C. E. Fuel 1988, 67, 1658-1661. (6) Ahsan, Tanweer; Wu, J. H.; Arnett, E. M. Fuel 1994, 73, 417422. (7) Rossi, P. F.; Busca, G.; Oliveri, G.; Milana, G. Langmuir 1982, 8, 104-108. (8) Glass, A. S.; Larsen, J. W. Energy Fuels 1994, 8, 629-636.
S0887-0624(97)00085-6 CCC: $14.00
been given sufficient consideration. In our study, we used a commercial flow microcalorimeter (FMC) to measure the adsorption heat of methanol solution flowing onto a coal. Methanol concentrations ranging from 2 to 20 g/L in n-hexane were used to avoid thermal processes besides adsorption, such as solubilization and swelling. Continuous flow and pulse injection methods were used to determine total and strong adsorption heats, respectively. Methanol uptakes were determined directly by an on-line measurement using a refractive index detector set at the outlet of FMC, or indirectly by an off-line measurement to compare methanol concentrations in n-hexane before and after adsorption. On the basis of the heat and amount of adsorption measured, the relationship between methanol adsorption and coal structure is discussed. Experimental Section Coal Sample. Two kinds of coal, having considerably different oxygen contents, were used: Japanese Akabira bituminous coal (81.5% C, 5.6% H, 1.2% N, 0.5% S, 11.2% O, (9) Ross, D. S.; Blessing, J. E. Fuel 1979, 58, 438-442. (10) Ramesh, R.; Francois, M.; Somasundaran, P.; Cases, J. M. Energy Fuels 1992, 6, 239-241. (11) Hornsby, D. T.; Leja, J. Colloids Surf. 1983, 7, 339-349. (12) Yarar, B.; Kaoma, J. Colloids Surf. 1984, 11, 429-436. (13) Marmur, A.; Chen, W.; Zografi, G. J. Colloid Interface Sci. 1986, 113, 114-120. (14) Kelebek, S.; Finch, J. A.; Smith, G. W.; Yoruk, S. Colloids Surf. 1986, 20, 89-100. (15) Kelebek, S.; Smith, G. W.; Finch, J. A. Sep. Sci. Technol. 1987, 22, 1527-1546. (16) Kelebek, S. Colloids Surf. 1987, 28, 219-232. (17) Kelebek, S. J. Colloid Interface Sci. 1988, 124, 504-514. (18) Kelebek, S.; Donini, J.; Smith, G. W.; Finch, J. A. J. Chem. Soc., Faraday Trans. 1 1989, 85, 91-98.
© 1997 American Chemical Society
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Figure 1. Flow microcalorimeter record of continuous flow method. and 3.7% ash), and Australian Yallourn brown coal (63.3% C, 4.6% H, 0.4% N, 1.8% S, 29.9% O, and 2.9% ash). These coal samples were finely ground with pestle and mortar to pass 100 mesh sieve and dried in a vacuum oven at 110 °C for 24 h before calorimetric measurement. The surface areas of ground coal samples were obtained by a BET measurement, where carbon dioxide was employed as the adsorbate. The carbon dioxide surface area represents the total surface of coal.2,3,19,20 Carrier and Adsorbate. n-Hexane was chosen as the carrier and methanol was used as an adsorbate in this study. These reagents were of highest available commercial purities and degassed before use. Methanol solutions for FMC were prepared by dilution in n-hexane to 2-20 g/L. A maximum methanol concentration of 20 g/L was chosen to avoid coal dissolution which was detected at higher concentrations. Swelling. Coal swelling experiments were performed by an improved volumetric method,21 rather than the conventional solvent-swelling technique22 which was reported to be not effective at low hydrogen-bond acceptor concentrations. 50 mg of coal sample was transferred into a volumetric 100 mm long 3 mm Pyrex tube, and 0.8 mL of pure n-hexane or methanol solution (20 g/L in n-hexane) was added into the tube to swell the coal sample. The tube was capped with Sealon film. The contents of the tube was stirred with a stainless rod several times a day to ensure complete mixing. The pure n-hexane and methanol solution were also added into the tube to make up for vaporized solvents, respectively. Swelling ratio was defined as the volume of coal sample swollen by methanol solution to that swollen by n-hexane. FT-IR. FT-IR spectra were taken for the parent coal samples and coal samples swollen by n-hexane or methanol solution 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. Apparatus. A flow microcalorimeter (Microscal FMC-3V, vacuum model) was used to determine adsorption heat. For the FMC, two thermistors were positioned in the coal bed of 0.15 mL capacity, and two were in the block to complete a Wheatstone bridge circuit. This design gives a sensitivity in the microcalorie range.23 At the FMC outlet, a refractive index detector (JASCO RI830) was set to monitor the concentration of methanol passing through coal bed. This RI detector could only give precise determinations of methanol concentration below 2 g/L. Both the temperature of the sample and the concentration of methanol were recorded by a computer interface. (19) Marsh, H.; Siemieniewska, T. Fuel 1965, 44, 355-367. (20) Gan, H.; Nandi S. P.; Walker P. L. Fuel 1972, 51, 272-277. (21) Larsen J. W.; Gurevich I. Energy Fuels 1996, 10, 1269-1272. (22) Green, T. K.; Kovac, J.; Larsen, J. W. Fuel 1984, 63, 935-938. (23) A technical note delivered by Microscal Ltd.
Figure 2. Flow microcalorimeter record of pulse injection method. The carrier and methanol solution were fed smoothly by a twin syringe pump system at a flow rate of 6.0 mL/h. 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 in the coal-carrier system, the carrier was replaced by methanol solution to start the adsorption measurement. Both continuous flow and pulse injection methods were applied for calorimetric measurement, and all the measurements were carried out at 20.5 ( 1.5 °C. Measurement Method of Flow Microcalorimetry. Continuous Flow Method. Methanol was first adsorbed from 2 g/L solution until thermal equilibrium was reached. Subsequently, methanol concentration in the carrier was increased stepwise to avoid dilution heat effects, as shown in Figure 1. Thermal equilibrium was reestablished at each new methanol concentration. To confirm the absence of dilution heat effects, the same concentration profile was run with PTFE (poly(tetrafluoroethylene)) as the adsorbent. No dilution heat effects were observed. Experimentally, it was found that PTFE does not adsorb methanol. Pulse Injection Method. In the case of the pulse injection method, 0.01- 9.0 mL of methanol solution (2 g/L) was injected into the stream of carrier, rather than delivering methanol solution continuously until adsorption heat reached equilibrium. Figure 2 shows a profile of the calorimetric measurement by the pulse injection method. Methanol adsorption gives an exothermic FMC signal. After the methanol pulse has passed through coal bed, weakly adsorbed methanol is washed from coal by the n-hexane carrier and gives an endothermic FMC signal. If exotherm is defined as positive and endotherm as negative, the heat resulting from strong adsorption of methanol is the sum of exothermic adsorption heat and endothermic desorption heat. As compared with the heat for strong adsorption, the heat for weak adsorption is the remainder, achieved by subtracting the heat for strong adsorption from the adsorption heat obtained by the continuous flow method. The amount of methanol adsorbed strongly on the coal can
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Figure 3. Adsorption heat of methanol on the coal.
Figure 4. Amount of adsorbed methanol on the coal.
be calculated by subtracting the amount of discharged methanol from the amount of methanol injected. In this case, the amount of methanol discharged from the coal was determined on-line, with the RI detector set at the outlet of FMC. Finally, the molar heat for strong adsorption of methanol can be calculated from heat and amount of strongly adsorbed methanol. On the other hand, the continuous flow method cannot distinguish between strong and weak adsorption, in either amount of adsorption or heat of adsorption. Methanol Adsorption Isotherm. Due to the limitation of the RI detector we used, the quantity of methanol adsorbed on the coal could not be determined on-line in this part of the experimental work. Therefore, in a separate experiment, about 20 mg of coal sample was accurately weighed and mixed with 10 mL of methanol solution (2-20 g/L in n-hexane). The mixtures were placed in an ultrasonic tub for 10 min and then allowed to stand for about 3 h which corresponds to the taking time of the FMC adsorption heat measurement. Next, the suspensions were divided into solid and liquid by centrifugation. The methanol concentration in the supernatant solution was determined with the RI detector. Finally, amount of methanol adsorbed on the coal was calculated based on the methanol concentration before and after adsorption experiment. Coals are cross-linked macromolecular networks capable of taking up molecules into their bulk. Surface adsorption of methanol on coals is immediately followed by rapid penetration of methanol molecules into coals.24,25 The amount of methanol absorption is expected to be involved in the amount of methanol adsorption obtained by the methods mentioned above.
concentrations. This must be due to the fact that sites for methanol adsorption on coals are almost saturated at 2 g/L. In order to examine other thermal interactions of methanol with coal, swelling experiments and FT-IR analyses were performed. No swelling was observed after 24 h. This means that the presence of methanol, at the maximum concentration used for the calorimetric measurements, does not cause coal swelling beyond swelling caused by n-hexane during the calorimetric measurement. The swelling of coals reached equilibrium after 3 weeks. The equilibrium swelling ratios, defined as the volume of coal sample swollen by methanol solution to that swollen by n-hexane were 1.30 for Akabira bituminous coal and 1.54 for Yallourn brown coal, respectively. FT-IR spectra were taken for the parent coal samples and coal samples swollen by n-hexane or methanol solution. The OH stretching vibration at 3300 cm-1 became broader after methanol adsorption, and no dissolved constituents were detected due to exposure to these solutions. FT-IR spectra for the effluent solutions of FMC showed no dissolved constituents provided the methanol concentration was below 20 g/L. Based on these coal swelling and FT-IR data, adsorption heat measured in this study results from methanol surface adsorption on the coal mainly. Isotherms of Methanol Adsorption on Coals. Figure 4 shows the isotherms for methanol adsorption from n-hexane on the coals, as determined by the offline measurement. Adsorption amount is given in mmol/g of coal base. In spite of the fact that the two coals give significantly different adsorption heats, the two samples have almost the same methanol adsorption capacities. Similar to adsorption heat, methanol uptake reached its maximum value at the lowest concentration and remained constant. This means that once all of the adsorption sites for methanol on the coal have been occupied at the lowest concentration, there will be no more methanol adsorbed by methanol-coal interaction or methanol-methanol interaction. The surface areas of ground coal samples, used as the adsorbents, were obtained by a BET measurement, where carbon dioxide was employed as the adsorbate rather than nitrogen. The carbon dioxide surface areas represent the total surface of coal, while the surface areas obtained by nitrogen adsorption are taken as a measure of the external surface. The carbon dioxide
Results and Discussion Heat of Methanol Adsorption. Figure 3 shows adsorption heat measured by the continuous flow method as a function of methanol concentration. The number on the vertical axis indicates the cumulative heat of adsorption per gram of coal (J/g). For Yallourn brown coal, the equilibrium adsorption heat is about 210 J/g at 20 g/L. This value is much higher than the 70 J/g measured for Akabira bituminous coal. Absolute values of heat were different, though the methanol adsorption profiles have the same shape. The adsorption heat almost reaches the maximum at 2 g/L, the initial methanol concentration, and remains constant at higher (24) Van Krevelen, D. W. Coal, 1st ed.; Elsevier: New York, 1961; pp 127-149. (25) Glass, A. S.; Larsen, J. W. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1992, 37, 1177-1183.
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surface areas are 76.5 m2/g for Akabira bituminous coal and 94.0 m2/g for Yallourn brown coal, respectively. Methanol coverage of the coal surface is defined as the ratio of surface area calculated from methanol uptake to the carbon dioxide BET surface area. Methanol coverages are 81 for Akabira bituminous coal and 70 for Yallourn brown coal, when the methanol uptakes in Figure 4 were used. An area of 17.9 Å2 per adsorbed methanol molecule was used in these calculations as suggested elsewhere.10 These coverages are well above the saturation ones. These results indicate that the offline measurement we used to determine the total uptake of methanol adsorption gives multilayer methanol adsorption on the coals. The multilayer methanol adsorption on the coal must be attributed to coal hydrophilicity. The coal surface is somewhat hydrophilic due to polar functional groups. Methanol is similar to water in structure; one might consider methanol as the methyl derivative of water. For this reason, in a nonpolar solvent such as n-hexane, we propose that methanol forms a diffuse layer around the polar coal surface. The total uptake of methanol in coal, determined by using the off-line measurement, is then the sum of methanol adsorbed physically as well as chemically on the coal and the diffuse methanol layer around the coal particles. However, in case of methanol vapor adsorption, when the surface coverage of methanol exceeds the saturation one, adsorbed methanol molecules condense on coals, liquefying, then penetrating into the bulk of coal and, consequently, swelling coals immediately. Thus, there may exist no such diffuse layers in methanol vapor adsorption. On the other hand, in a flow system, the methanol molecules forming a diffuse layer are flushed from coal while the methanol solution flowing through coal bed. This indicates that the continuous flow method gives a measure combining physisorption and chemisorption, provided methanol concentration is below 20 g/L. The amounts for physisorption and chemisorption of methanol from flowing 2 g/L solution were determined by subtracting the amount of methanol not adsorbed from the total amount of methanol delivered continuously to the coal bed until all the methanol adsorption sites are saturated. In this case, the amount of methanol not adsorbed on the coal was determined on-line, by use of the RI detector set at the outlet of FMC. The values are 1.70 mmol/g coal for Akabira bituminous coal and 6.31 mmol/g coal for Yallourn brown coal. The methanol coverages, corresponding to these uptakes, are 2.39 and 7.23, respectively. It seems that a low-rank coal takes up more methanol because of its higher hydrophilicity, and this evolves more adsorption heat. Similar to the coverages obtained by the off-line measurements, these coverages are also above saturation ones. Sites for Methanol Adsorption on Coals. It is expected that oxygen functional groups on coals are methanol adsorption sites. FT-IR analysis of the parent coal samples reveals that Yallourn brown coal contains more surface oxygen functional groups, such as hydroxyl (3300 cm-1) and carboxyl (1700 cm-1) groups, than Akabira bituminous coal. A portion of these functional groups are likely involved in hydrogen bonding and form
Wang et al.
coal-coal hydrogen bonds.26 Methanol adsorption may form new hydrogen bonds between these surface oxygen functional groups and methanol. Larsen et al. measured the strength of hydrogen bonds in Illinois No. 6 coal and suggested that the formation energy of coal-coal hydrogen bonds range from 5 to 8.5 kcal/mol (21-36 kJ/mol).27 Thus, at least, a 21 kJ/mol molar heat of adsorption is necessary for a methanol molecule to adsorb at the oxygen functional group. Besides oxygen functional groups, hydrophobic sites on the coal can also be adsorption sites.16 It is generally recognized that these hydrophobic sites are composed of aliphatic and aromatic hydrocarbon groups. There may also be other adsorption sites for methanol adsorption on the coal, such as basic aromatic rings. Therefore, methanol adsorption on the coal can be classified as either strong, involving hydrogen bonds with oxygen functional groups, or weak, where physical interactions such as van der Waals’ forces etc. are most important. The amounts of methanol adsorbed both strongly and weakly must be the same as the uptake of physisorption and chemisorption determined by the continuous flow. These values are 1.70 mmol/g coal for Akabira coal and 6.31 mmol/g coal for Yallourn coal. In order to investigate the methanol-coal interaction quantitatively, it is essential to distinguish these two types of adsorption from each other. The continuous flow method contains information on both strong and weak methanol adsorption, while the pulse injection allows direct measurement of strong adsorption. In order to clarify the effect of oxygen functional groups on methanol adsorption, we choose Bio-Beads(S-X8) as an adsorbent. Bio-Beads(S-X8) is a spherical porous styrene-divinylbenzene copolymer with 8% cross-linkage; one can view it as a coal containing no oxygen functional groups. We measured molar heats of strong methanol adsorption by the pulse injection. All of the methanol initially adsorbed on Bio-Beads(SX8) came off, after the methanol pulse passed. No strong methanol adsorption occurred. This supports the possibility that methanol is adsorbed strongly by the interaction with oxygen functional groups on coal. Molar Heats of Methanol Adsorption on Coals. We used the pulse injection method to measure strong adsorption as a function of volume of methanol solution (2 g/L) injected. In each case, a fresh coal sample (adsorbent) was used, and the pulse volume was varied from 0.01 to 9.0 mL. Molar heat of strong methanol adsorption is shown in Figure 5, as a function of the methanol coverage. The molar heat decreases sharply at lower coverages and becomes constant when the strong adsorption sites for methanol are saturated. The decrement of molar heat with coverage indicates that methanol adsorption occurs initially at the strongest adsorption sites, forming strong hydrogen bonds, and then progressively weaker adsorption sites are saturated. Clearly, the sites for methanol adsorption on coal are energetically heterogeneous. We measured the molar heat of strong methanol adsorption on Illinois No. 6 coal. When the strong (26) Green, T.; Kovac, J.; Brenner, D.; Larsen, J. W. Coal Structure; Academic Press: New York, 1982; pp 199-282. (27) Larsen, J. W.; Green, T. K.; Kovac, J. F. J. Org. Chem. 1985, 50, 4729-4735.
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Figure 6. Distribution of molar heat for strong methanol adsorption on coals. Figure 5. Plot of molar heat vs coverage for strong methanol adsorption on coals.
adsorption sites for methanol are saturated, the molar heat is about 33 kJ/mol, almost equal to the highest formation energy of coal-coal hydrogen bonds suggested by Larsen et al.27 This suggests that methanol adsorbed strongly on coal interacts with oxygen functional groups by forming new methanol-coal hydrogen bonds. Besides, the agreement of molar heat of strong methanol adsorption on coal with formation energy of coal-coal hydrogen bonds indicates that the pulse injection method is an effective technique to investigate the strong methanol adsorption on coals. We also applied the pulse injection for the examination of strong methanol adsorption on Akabira and Yallourn coals. The maximum amount of strongly adsorbed methanol is 2.0 and 0.7 mmol/g coal for Yallourn brown coal and Akabira bituminous coal, respectively. The ratio of these two maximum amounts of strong adsorption corresponds well to the ratio of oxygen contents of these two coals. This fact also verifies that oxygen functional groups on the coal are the main sites for strong methanol adsorption. Langmuir pointed out that chemical adsorption cannot extend beyond one molecular layer.28 Studies by other investigators have also shown that the amount of chemisorbed gas required to saturate a surface corresponds to a unimolecular layer.29-31 Thus, at lower coverages than monolayer, where the diffusion of methanol into the coal can be neglected,10 the pulse injection measures chemisorption. But, at higher coverages than (28) Langmuir, I. J. Am. Chem. (29) Roberts, J. K. Proc. R. Soc. (30) Roberts, J. K. Proc. R. Soc. (31) Roberts, J. K. Proc. R. Soc.
Soc. 1916, 38, 2221-2295. (London) 1935, A152, 445-463. (London) 1935, A152, 464-477. (London) 1935, A152, 477-480.
Table 1. Results of Strong and Weak Methanol Adsorption on Coals, Obtained by Using Continuous Flow together with Pulse Injection heat (J/g of coal)
amount (mmol/g of coal)
molar heat (kJ/mol)
coal
weak
strong
weak
strong
weak
strong
Akabira Yallourn
24.4 98.1
29.4 68.1
1.01 4.35
0.69 1.96
24.2 22.5
42.6 34.7
monolayer, we assume that the pulse injection as well as continuous flow gives a measure combining physisorption and chemisorption. Table 1 gives a summary of methanol adsorption on coals from the effluent of 2 g/L solution, achieved by using the continuous flow together with the pulse injection. Strong adsorption is measured directly by pulse injection, whereas weak adsorption is calculated by subtracting the strong adsorption component from the total adsorption determined by continuous flow using 2 g/L methanol solution. As compared with Akabira bituminous coal, Yallourn brown coal takes up more methanol both weakly and strongly and gives higher adsorption heat. The molar heat of weak adsorption is about 23 kJ/mol; however, strong adsorption gives 35 kJ/mol for Akabira bituminous coal and 43 kJ/ mol for Yallourn brown coal. The molar heats of strong adsorption are slightly higher than the hydrogen bond energies in donor-acceptor systems.32 Distribution of Molar Heat of Strong Adsorption. Molar heat of strong adsorption histographs are shown in Figure 6. These are calculated from the data in Figure 5 by differentiating the amount of adsorption at each molar heat. As compared with Akabira bituminous coal, Yallourn brown coal has a wider distribu(32) Singh, S.; Murthy, A. S. N.; Rao, C. N. R. Trans. Farraday Soc. 1966, 62, 1056-1066.
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tion of molar adsorption heats, mostly in the lower energy region. This means that the methanol adsorption sites on Yallourn brown coal are more energetically heterogeneous than those on Akabira bituminous coal.
3. Strong adsorption of methanol gives a distribution of molar heat between 30 and 60 kJ/mol and is thought to involve hydrogen bonding with oxygen-containing functional groups on the coal.
Conclusion
4. The averaged molar heat of adsorption for methanol adsorbed by weak interactions with the coal is about 23 kJ/mol.
In this study, adsorption behavior of methanol on coals and methanol-coal interactions were discussed on the basis of experimental results of heat and amount of adsorption. The following conclusions can be made: 1. The isotherms for methanol adsorption, obtained by the off-line measurement, show that methanol molecules occupy all the adsorption sites on a coal at 2 g/L, the lowest concentration, and adsorb no more even when methanol concentration goes higher. 2. Experimental results achieved by the continuous flow method show that both the amount and heat of methanol adsorption from 2 g/L solution depend on the structure of coal. A low-rank coal takes up more methanol and this gives more heat of adsorption.
5. The adsorption sites for methanol adsorption on Yallourn brown coal are more energetically heterogeneous than those on Akabira bituminous coal.
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 for the measurement of carbon dioxide surface areas of coals. EF970085Y
