Energy & Fuels 2000, 14, 915-919
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Adsorption and Diffusion of Alcohol Vapors by Argonne Premium Coals Toshimasa Takanohashi* National Institute for Resources and Environment, Tsukuba 305-8569, Japan
Yuki Terao, Takahiro Yoshida, and Masashi Iino Institute for Chemical Reaction Science, Tohoku University, Sendai 980-8577, Japan Received February 4, 2000. Revised Manuscript Received April 26, 2000
Adsorption of methanol, ethanol, n-propanol, and n-butanol by Pocahontas No. 3, Upper Freeport, Illinois No. 6, and Beulah-Zap Argonne premium coals was investigated to clarify the effect of alkyl group bulk on adsorption and to evaluate the micropore and cross-linked structure of coals. A strong steric effect was found for all coals. The sorption isotherms could be fitted to the Langmuir-Henry dual-mode equation, and the total sorption could be divided into adsorption and diffusion (dissolution) components, which were separately estimated. For all coals, the Henry’s law constant kD decreased as the size of substituted group increased in the order methyl < ethyl < propyl, and the kD for n-butanol was similar to that for n-propanol. There was a significant decrease for the high-rank Pocahontas No. 3 and Upper Freeport coals. In contrast, the kD for the Upper Freeport extraction residue was quite large for all alcohols, compared to those of the raw coal, although the pore saturation constant was not greatly affected. The increase in kD for the residue is mainly the result of an increase in the physical diffusion sites (voids) formed by extraction.
Introduction Absorption of a reagent by a coal is a very complicated process. Coals are thought to have a large surface area with an interconnected network of slitlike pores.1 However, Larsen et al.2 measured the adsorption of various gases on five Argonne Premium coals and, except for Beulah-Zap lignite, found that the very steep dependence of BET surface area was related to the molecular volume of the gas. They concluded that pores in Argonne coals are isolated and can be reached only by diffusion through solid.2 Hsieh and Duda reported3 that solvent diffusion into coals can involve several complex phenomena such as structural relaxation. Green and Selby reported4,5 that pyridine sorption isotherms can be explained by a dual-mode model that has been widely applied to absorption by glassy polymers. This model is represented by two phenomena: adsorption on the surface, which is predominant, represented by the initial curved portion of a Langmuir isotherm, and diffusion (dissolution or absorption) into the bulk structure, which is described by Henry’s law, the linear portion of the plot. Shimizu et al. found6 that * To whom correspondence should be addressed. (1) Sharkey, A. G., Jr.; McCartney, J. T. In Chemistry of Coal Utilization; Wiley and Sons: New York, 1981. (2) Larsen, J. W.; Hall, P.; Wernett, P. C. Energy Fuels 1995, 9, 324. (3) Hsieh, S. T.; Duda, J. L. Fuel 1987, 66, 170. (4) Green, T. K.; Ball, J. E.; Conkright, K. Energy Fuels 1991, 5, 609. (5) Green, T. K.; Selby, T. D. Energy Fuels 1994, 8, 213. (6) Shimizu, K.; Takanohashi, T.; Iino, M. Energy Fuels 1998, 12, 891.
sorption data for Illinois No. 6 coal could be treated by the Langmuir-Henry equation regardless of the organic vapors used, i.e., methanol, benzene, pyridine, and cyclohexane. Takanohashi et al. reported7 that methanol sorption by extraction residues obtained in high extraction yields greatly increased compared to the corresponding raw coals, suggesting that extensive microporosity has been developed by the extraction. In contrast, the sorption behavior for extraction residues obtained in low yields was similar to that of the raw coals, regardless of coal rank. Solvent swelling of coal involves diffusion of solvent molecules into the macromolecular structure of coals. Larsen et al. measured8 the equilibrium swelling ratios of coals in various alkyl-substituted pyridines and anilines and reported that the steric effect of alkyl groups on the swelling ratio was much greater for the bituminous coals than for the lower rank coals. Aida et al.9 determined the initial swelling rates and the equilibrium swelling values of Illinois No. 6 coal in various solvents and showed that the rate of diffusion of alkylsubstituted amines decreased greatly by changing the group from n-butylamine to tert-butylamine. However, swelling of coal in the liquid phase causes other structural changes such as extraction and relaxation of the macromolecular structure that can influence swelling behavior. (7) Takanohashi, T.; Terao, Y.; Iino, M. Fuel 2000, 79, 349. (8) Larsen, J. W.; Lee, D. Y. Fuel 1985, 64, 981. (9) Aida, T.; Fuku, K.; Fujii, M.; Yoshihara, M.; Maeshima, T.; Squires, T. G. Energy Fuels 1991, 5, 79.
10.1021/ef000014q CCC: $19.00 © 2000 American Chemical Society Published on Web 06/17/2000
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Table 1. Ultimate and Proximate Analyses of Samples ultimate analysis, wt % (daf)
proximate analysis, wt % (db)
coal
C
H
N
S
Oa
Pocahontas No. 3 Upper Freeport raw Upper Freeport residue Illinois No. 6 Beulah-Zap
89.7 86.2 81.7 76.9 71.6
4.5 5.1 4.7 5.5 4.8
1.1 1.9 1.8 1.9 1.0
0.7 2.2 5.5 5.6 0.9
4.0 4.6 6.3 10.1 21.7
VM
ash
FC
17.6 28.2 18.1 38.6 42.2
4.8 13.1 26.0 15.0 9.6
77.6 58.7 53.5 46.4 48.2
In the present paper, the sorption isotherms of alcohols with different alkyl groups were measured for four Argonne premium coal samples in order to investigate the steric effect on alcohol vapor sorption. The steric effect for extraction residues obtained from Upper Freeport coal was also investigated. Data were analyzed using the Langmuir-Henry dual-mode sorption equation. On the basis of the adsorption and diffusion parameters from the model equation, micropore and bulk (cross-linked) structures of the coals are discussed. Experimental Section Sample Preparation. Pocahontas No. 3, Upper Freeport, Illinois No. 6, and Beulah-Zap Argonne premium coal samples10 were obtained in ampules (5 g of -150 µm). The ultimate analyses of the coals are listed in Table 1. Coal samples were dried at 80 °C for 12 h in vacuo. Methanol, ethanol, n-propanol, and n-butanol (Special grade, KANTO CHEMICALS, CO., INC.) were used without further purification as the sorbate; the purity values of reagents were 99.8%, 99.5%, 99.5%, and 99.0%, respectively. Solvent Extraction. Upper Freeport coal was extracted exhaustively with a carbon disulfide/N-methyl-2-pyrrolidinone (CS2/NMP) mixed solvent under ultrasonic irradiation (38 kHz) at room temperature.11 The residues obtained were washed with neat acetone under ultrasonication and dried in a vacuum oven at 80 °C for 12 h. The extraction yield of 67 wt % was calculated from the weight of the residue (dry-ash free basis). Sorption Experiments. Sorption isotherms were measured with an automatic vapor adsorption apparatus (BELSORP18, BEL Japan, INC.) at 30 °C. Approximately 200 mg of coal sample was placed in the sample tube and weighed. The deaeration treatment of alcohol in the solvent tank was carried out with liquid nitrogen three times through freezethaw cycles. Samples were pretreated under vacuum (0.3. Shimizu et al. reported6 that C′H values for pyridine, benzene, and cyclohexane for Illinois No. 6 coal were similar, even though the sorbates have quite different chemical properties. Their molecular sizes are similar, however, which indicates that the C′H may be determined by the micropore volume in the coal. Thus, by using the dual mode equation, sorption behaviors of various coal-vapor systems may be analyzed. Raw Coal. Isotherms for absorption of four alcohols by Pocahontas No. 3 coal are shown in Figure 2. At relative pressures ethyl > propyl ∼ butyl. Sorption isotherms for Illinois No. 6 and Beulah-Zap coals are shown in Figures 4 and 5, respectively. The total amount of sorption for both coals was much larger than for high-rank Pocahontas No. 3 and Upper Freeport coals; the shapes of the isotherms were similar, however. Because the surface of coals has many interacting sites such as functional groups, adsorption preferentially occurs at low vapor pressure. The steep increase of sorption observed for all coals at low vapor pressures, especially for low-rank coals, can be caused by adsorption on oxygen functional groups. The steric effect for alcohol adsorption was also observed for the low-rank coals. The rates of increase in the sorptions of methanol and ethanol at vapor pressures >0.2 were similar. Constants of the Langmuir-Henry equation obtained by successive fitting are listed in Table 2. Because a discontinuity point exists at a vapor pressure of 0.20.4 for the Beulah-Zap-ethanol, -n-propanol, and -nbutanol systems, the adsorption parameter was obtained by fitting in the vapor pressure range 0.4. For Illinois No. 6 coal, the difference in C′H (mmol/g of sample) between methanol and n-butyl alcohol was small compared to other coals, showing that the steric effect on the adsorption was
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Table 2. Adsorption and Dissolution Parameters by Langmuir-Henry Sorption Equation for All Samples coal Pocahontas No. 3
Upper Freeport Raw Coal
Upper Freeport residue
Illinois No. 6
Beulah-Zap
sorbate
C′Ha
bb
kDc
MeOH EtOH n-PrOH n-BuOH MeOH EtOH n-PrOH n-BuOH MeOH EtOH n-PrOH n-BuOH MeOH EtOH n-PrOH n-BuOH MeOH EtOH n-PrOH n-BuOH
0.98 0.46 0.31 0.19 0.81 0.30 0.18 0.20 0.58 0.33 0.35 0.37 0.97 0.66 0.58 0.61 2.3 1.7 1.3 0.21
10 26 23 27 35 30 9 7 55 69 35 13 58 108 32 10 29 39 6 1
0.61 0.49 0.16 0.22 0.79 0.29 0.01 0.05 3.1 2.3 1.6 1.6 2.9 1.9 1.1 1.3 5.0 4.8 3.7 1.3
a Pore saturation constant. b Pore affinity constant. c Dissolution constant.
relatively small for the coal. In contrast, the difference in C′H for high-rank Upper Freeport and Pocahontas No. 3 coals was relatively large, although the C′H for methanol was similar to that for Illinois No. 6 coal. This result suggests that the steric bulk of the alkyl groups greatly affected adsorption on the surface of the highrank coals. Smith and Williams measured the adsorption of methane and 2,2-dimethylpropane on twelve coals at low pressures ( ethanol > n-propanol ∼ n-butanol, which shows that the difference in the size of the alcohol alkyl group greatly influenced their diffusibility into the bulk. These results are consistent with the swelling data by Larsen et al. 8 that showed that the steric effect of alkyl groups on the swelling ratio was much greater for the bituminous coals than for the lower rank coals. Consequently, the difference in both C′H and kD among the alcohols was observed for Pocahontas No. 3 and Upper Freeport coals, showing that the difference in sorption of the alcohols is the result of the alkyl group steric effects. This result suggests that the high rank coals have more micropores, the diffusion into which would be blocked by steric hindrance. The SAXS data by Larsen et al.2 showed the similar size dependence of the adsorbate diffusion rates through Argonne Premium coals by using cyclopropane, cyclobutene, cyclopentane, and cyclohexane as adsor(12) Smith, D. M.; Williams, F. L. In Coal Science and Chemistry; Elsevier: Amsterdam, 1987; p. 381
Figure 6. Sorption isotherm of various alcohols at 30 °C for the extraction residue from Upper Freeport coal, together with curve-fitting lines by the dual-mode sorption equation.
bate. Takanohashi et al. investigated the interaction between high-rank coals and alcohols with different straight chain alkyl groups using an inverse liquid chromatography (ILC) technique13 and suggested that the high rank coals have more micropores than the lower rank coals. From an ILC study of n-alkanes and Pocahontas No. 3 coal, Hayashi et al.14 concluded that there were few pores with diameters of 10-100 Å diameter in the coal, in agreement with the above results. On the other hand, we reported11,15 that of all Argonne coals Upper Freeport gave the highest yield (60 wt %) when extracted with carbon disulfide/N-methyl-2-pyrrolidinone mixed solvent (1:1 by volume) under mild conditions. This result indicates that micropores can be broken by solvent even at room temperature. Therefore, the structure around the micropores may consist of an aggregation structure of coal molecules, not a covalently cross-linked structure. In addition, Li et al. recently found16 that the dissolution yield for Pocahontas No. 3 coal with NMP/hexahydroanthracene (2:3 by weight) mixed solvent at 300 °C for 3 h was 86 wt % (daf). This result shows that the wall around the micropores for Pocahontas No. 3 coal can be broken under mild conditions, where no significant covalent bond breaking occurs for the high-rank coals. Thus, Pocahontas No. 3 may also consist primarily of an aggregated structure, although the strength of interactions forming the structure is larger than that for Upper Freeport coal. Effect of Extraction. The sorption isotherms for alcohols in the Upper Freeport extraction residue and the isotherm for methanol with the corresponding raw coal are shown in Figure 6. We have reported7 that methanol sorption by extracts obtained in high yields increased greatly compared to the raw coals, regardless of coal rank, and suggested that more microporosity was developed by extraction. The order methanol > ethanol > n-propanol ∼ n-butanol for the residue (Figure 6) was similar to that for the corresponding raw coal (Figure 3). It is noted that the sorptions of all alcohols were much larger than that of methanol for the raw coal; sorption in the middle vapor pressure range increased (13) Takanohashi, T.; Nakano, K.; Yamada, O.; Kaiho, M.; Ishitsuka, A.; Mashimo, K. Energy Fuels 2000, 14, 720. (14) Hayashi, J.-i.; Amamoto, S.; Kusakabe, K.; Morooka, S. Energy Fuels 1995, 9, 1035. (15) Takanohashi, T.; Iino, M. Energy Fuels 1990, 4, 452. (16) Li, C.-Q.; Ashida, S.; Iino, M.; Takanohashi, T. Energy Fuels 2000, 14, 190.
Adsorption of Alcohol Vapors by Coals
greatly. This result indicates that even alcohols with bulky alkyl groups such as n-propanol and n-butanol could diffuse easily into the bulk of the extract residue. Lines curve-fitted with the dual-mode equation for each alcohol are also shown in Figure 6. For all alcohols, the agreement between the lines and experimental points seems to be good. Constants of the LangmuirHenry equation obtained by fitting are listed in Table 2. The C′H values of each alcohol for the raw coal and the extraction residue were similar in magnitude, although the methanol value for the residue was smaller than that for the raw coal. For the residue, the values of b increased for all alcohols, showing that adsorption occurred very easily. This may be the result of the formation of more stable conformations between the residue and alcohols in the pores expanded by the extraction. The kD values for the residue were much larger than those for the raw coal in all cases. High kD values were obtained even for n-propanol and n-butanol. These results suggest that the adsorption sites on the surface were not greatly affected by extraction, while a considerable amount of large void was formed in the bulk. Because the oxygen content of the Upper Freeport residue, 6.3 wt %, is much lower than that for lower rank Illinois No. 6 (10.1 wt %) and Beulah-Zap (21.7 wt %) coals, the increase in the kD for the residue may be primarily the result of increased physical diffusion into the formed voids and not the result of an increase in polar sites.
Energy & Fuels, Vol. 14, No. 4, 2000 919
Conclusions Almost all sorption isotherms of various alcohols (methanol, ethanol, n-propanol, and n-butanol) could be explained by the Langmuir-Henry dual-mode sorption equation, and adsorption and diffusion parameters could be estimated. Both pore saturation and dissolution constants were smallest for Upper Freeport coal. As the size of the alkyl group increased (methyl < ethyl < propyl ∼ butyl), their constants decreased. For highrank Pocahontas No. 3 and Upper Freeport coals, the dissolution constant greatly decreased. Pocahontas No. 3 and Upper Freeport coals seem to have more micropores into which the relatively bulky n-propanol and n-butanol could diffuse only marginally. On the other hand, while the pore saturation constant did not change greatly, the dissolution constant for the Upper Freeport extraction residue was quite large for all alcohols compared to those of the raw coal. A considerable number of micropores (voids) with relatively large sizes formed by extraction may be responsible for the increased diffusion. Acknowledgment. This work has been carried out as one of “Research for the Future” project of the Japan Society for the Promotion of Science (JSPS) through the 148 committee on coal utilization technology of JSPS. EF000014Q
