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Energy & Fuels 2004, 18, 450-454
A Study of the Interaction of Carbon Disulfide/ N-Methyl-2-pyrrolidinone Mixed Solvents with Argonne Premium Coals Using an Inverse Liquid Chromatography Technique Toshimasa Takanohashi,* Kaoru Nakano, Mamoru Kaiho, Osamu Yamada, and Ikuo Saito Institute for Energy Utilization, National Institute of Advanced Industrial Science and Technology, 16-1, Onogawa, Tsukuba 3058569, Japan Received June 10, 2003. Revised Manuscript Received November 20, 2003
The interactions of four Argonne Premium coals (Pocahontas No. 3, Upper Freeport, Pittsburgh No. 8, and Illinois No. 6) with carbon disulfide (CS2), N-methyl-2-pyrrolidinone (NMP), and their mixtures (CS2/NMP) were studied using an inverse liquid chromatography technique, in which the coals provided the stationary phase. The capacity factor CF (the increment of the elution volume of the probe relative to the elution volume of a carrier solvent) of each reagent was determined using acetonitrile as the carrier solvent. For the Illinois No. 6 coal, the elution curve of NMP showed an extended tail, which is a behavior that is not observed with the other higherrank coals. In contrast, CS2 gave a pronounced tail with all high-rank coals, reflecting the strong interaction of CS2 with all available interaction sites in these coals. Furthermore, the CF of CS2, measured in both the Pocahontas No. 3 and Upper Freeport coals, became significantly increased when injected along with NMP, which demonstrates the considerable enhancement that occurs in the interaction of CS2 with these high-rank coals in the presence of NMP. Dimethylformamide and pyridine produced a similar increase in the CF of CS2 in the presence of NMP; however, benzene did not. The increase in the CF values found among the mixed solvents parallels the enhancement that these same mixed solvents bring to the extraction yields of bituminous coals.
Introduction Solvent swelling is used widely to explore coal-solvent interactions, because of the simplicity of the experimental methodology.1-3 However, several problems are inherent in the use of strong solvents such as pyridine and N-methyl-2-pyrrolidinone (NMP); these include the dissolution of soluble portions of the coal, which causes changes in both the chemical potential of the solvent and in the volume of the coal.4,5 Larsen et al. established a relationship between the swelling ratios and solubility parameters for nonpolar solvents;2 however, significant scatter in the data obtained from polar solvents was observed. Furthermore, it has been found necessary to consider physical factors (such as microporosity and cross-link density) in any evaluation of coal-solvent interactions.6-8 For example, the kinetics of solvent swelling of coals are affected by steric * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Hombach, H.-P. Fuel 1980, 59, 465. (2) Larsen, J. W.; Green, T. K.; Kovac, J. J. Org. Chem. 1985, 50, 4729. (3) Green, T. K.; Larsen, J. W. Fuel 1984, 63, 1538. (4) Larsen, J. W.; Cheng, J. C.; Pan, C.-S. Energy Fuels 1991, 5, 57. (5) Takanohashi, T.; Iino, M.; Nishioka, M. Energy Fuels 1995, 9, 788. (6) Aida, T.; Fuku, K.; Fujii, M.; Yoshihara, M.; Maeshima, T.; Squires, T. G. Energy Fuels 1991, 5, 79. (7) Larsen, J. W.; Lee, D. Fuel 1985, 64, 981. (8) Takanohashi, T.; Terao, Y.; Yoshida, T.; Iino, M. Energy Fuels 2000, 14, 915.
factors, and Larsen et al. described the effects that various alkyl-substituted pyridines and anilines exert on the equilibrium swelling ratio of coals.7 Aida et al. determined the initial swelling rates of Illinois No. 6 coal in various solvents and showed that the diffusion rate of alkyl-substituted amines decreases almost 1000fold between n-butylamine and tert-butylamine.6 In a subsequent examination of the sorption of various alcohols, aromatics, and pyridine on coals, to investigate their penetration into the coal structure,8 we were able to establish that significant steric hindrance is involved in the sorption of alkyl-substituted alcohols by coals, especially high-rank coals. In the case of sorption of strong reagents such as pyridine, equilibrium is not attained, even after two weeks.9 Hsieh and Duda were able to establish that solvent diffusion into coals involves several complex phenomena and demonstrated that models based on the diffusion behavior into synthetic polymers is not directly applicable to coals.10 Consequently, direct evaluation of the interactions of coal and strong solvents is not straightforward. Inverse gas chromatography and inverse liquid chromatography (ILC) techniques that utilize coals as the stationary phase have been used to study the interaction between organic compounds and coals. Materials that (9) Shimizu, K.; Takanohashi, T.; Iino, M. Energy Fuels 1998, 12, 891. (10) Hsieh, S. T.; Duda, J. L. Fuel 1987, 66, 170.
10.1021/ef0340211 CCC: $27.50 © 2004 American Chemical Society Published on Web 01/16/2004
Interaction of CS2/NMP with Argonne Premium Coals
Energy & Fuels, Vol. 18, No. 2, 2004 451
Table 1. Ultimate and Proximate Analyses of Coals Ultimate Analysis (wt % (daf))
Proximate Analysis (wt % (db))
coal
C
H
N
S
Oa
Pocahontas No. 3 Upper Freeport Pittsburgh No. 8 Illinois No. 6
89.7 86.2 82.6 76.9
4.5 5.1 5.5 5.5
1.1 1.9 2.1 1.9
0.7 2.2 2.4 5.6
4.0 4.6 7.4 10.1
a
volatile matter
ash
fixed carbon
17.6 28.2 38.3 38.6
4.8 13.1 8.7 15.0
77.6 58.7 53.0 46.4
By difference.
interact strongly can be used, and it is possible to evaluate physical effects, such as those related to sizeexclusion mechanisms, simultaneously. Winans et al. used an ILC column packed with methanol-extracted coal. They found that a linear relationship existed between the carbon number of polyaromatic hydrocarbons (PAHs) and the logarithm of their capacity factor CF (the increment of the elution volume of the probe, relative to the elution volume of a carrier solvent).11 These authors suggested that the effect was the result of π-π interactions between the coal and PAHs. Hayashi et al. used ILC to study the thermodynamics of the interactions of various aromatic compounds with coals and suggested that the OH-π interaction had an important role in low-rank coals.12 Kaneko et al. used columns packed with the extraction residues and extract fractions of coals and reported similar elution behaviors among both groups of samples, which suggested that interactions between organic compounds and bulk coal, via diffusion into the coal mass, are important.13 A novel ILC technique that uses standard polystyrenes, with a wide range of molecular weights, was proposed by Morino et al.14 as a probe to study the porosity and cross-linked structure of coals. We have also noted that the CFs of isomeric butyl alcohols are influenced by steric effects and have suggested that the steric effect has two components: a steric hindrance that affects penetration into coal micropores and the effect of bulk that comes from a more unstable conformation caused by the bulky substituted group between interacting sites in coals and the isomers.15 Iino et al. found that a carbon disulfide (CS2)/NMP mixed solvent gave high extraction yields of 40%-65% daf at room temperature for several bituminous coals.16,17 The results of ultimate analysis indicated that no significant chemical reaction had occurred during the extraction.16 Dyrkacz investigated the solvent properties of a CS2/NMP mixed solvent, using density, viscosity, solvatochromic, and Fourier transform infrared (FTIR) spectroscopy measurements, and argued that some type of NMP chain oligomer was an important synergistic factor in the extraction of coal in such a mixed solvent.18 Dyrkacz et al. also noted that, in mixed solvent extraction, the shape of the solvent (i.e., flatness) is an important factor when their basicities are similar.19 However, the precise mechanism of the extraction process, which might explain why mixed solvents can give high extraction yields, has yet to be clarified. In the present study, the ILC behavior of organic compounds such as CS2, NMP, and CS2/NMP was examined in columns packed with four Argonne Premium coals, to make direct measurements of the interactions between the coal and the strongly interacting solvents. Of particular interest were those involving interaction between the coals and the CS2/NMP mixtures. Each retention behavior of the mixtures could be
Table 2. Amount of Soluble Materials from Coals in Acetonitrile coal
extractables (wt % (daf))
Pocahontas No. 3 Upper Freeport Pittsburgh No. 8 Illinois No. 6
0.1 2.2 0.7 0.6
observed with a photodiode-array-type UV detector. The results allowed a relationship to be established between the interaction behavior and the extraction yield obtained in coal-CS2/NMP systems. Experimental Section Coal Samples. Four 5-g ampules of Argonne Premium coal samples20sPocahontas No. 3, Upper Freeport, Pittsburgh No. 8, and Illinois No. 6 coals, each ground to -150 µmswere used. Their ultimate and proximate analyses are given in Table 1. Inverse Liquid Chromatography (ILC). Each coal sample was sieved to obtain particles with a particle size of 74-150 µm, and ∼3.3 g of each separate was mixed with 40 mL of acetonitrile and allowed to stand for three days. The amount of soluble matter extracted from the coals by the acetonitrile is given in Table 2. The yields were all notably low (0.1 mol/L). For the other three high-rank coals, the CF was small and failed to show any concentration dependency (see Figure 3). We have described elsewhere the numerous micropores present in high-rank coals, into which relatively bulky reagents may diffuse, to only a limited extent.15 Furthermore, the swelling ratios of (22) Yokokawa, C. Fuel 1969, 48, 29. (23) Russell, G. A. J. Am. Chem. Soc. 1985, 80, 4987.
Interaction of CS2/NMP with Argonne Premium Coals
Figure 4. Chromatograms at different wavenumbers of a CS2/ NMP mixture (0.03 mol/L of CS2, 0.3 mol/L of NMP) for the Upper Freeport coal.
high-rank raw coals are relatively low in pure solvents such as NMP, pyridine, tetrahydrofuran (THF), methanol, and benzene.24 Hence, little of the injected NMP may be able to diffuse through some high-rank coals, with the result that they yield only small CF values. Recently, we reported25 that two weeks of soaking in NMP or treatment at 100 °C in NMP before roomtemperature extraction with NMP greatly increased the extraction yield (by 10-30 wt %), and we concluded that the increase in yield can be the diffusibility effect of NMP into the coal inside. Capacity Factors of Mixed Samples. Figure 4 shows typical chromatograms obtained from the Upper Freeport coal with a CS2/NMP mixed sample (0.03 mol/L of CS2, 0.3 mol/L of NMP). The shape of the chromatograms varies, depending upon the measured wavenumber (196, 206, and 216 nm). The absorbances of CS2 and NMP at each wavenumber are known; therefore, the eluted curve for each can be calculated and their CFs can be determined. When the CS2/NMP sample was injected into the Illinois No. 6 coal, CS2 eluted earlier than NMP, whereas the opposite result was obtained for both the Upper Freeport and Pocahontas No. 3 coals. Furthermore, the latter two coals both showed two peaks for CS2; one peak corresponded, in regard to elution time, to that of NMP, whereas the other peak occurred later. This second, delayed peak presumably reflects interaction with the coals. It was used to determine the CF. For the Pittsburgh coal, the two peaks that arose from CS2 and NMP overlapped and no separation of these peaks could be achieved. The two contrasting effectss that of the addition of CS2 on the NMP elution in the Illinois No. 6 coal, and that of the effect of NMP addition on the CS2 elution in the Upper Freeport and Pocahontas No. 3 coalsswere further investigated, particularly the remarkable correspondence between the behavior of the individual solvents in these coals with the lateelution response of the mixed solvents. Figure 5 shows the CF of NMP admixed in different proportions with CS2 for the Illinois No. 6 coal. The CF of NMP increased where it co-existed with CS2, with the tendency being more marked for small fractions of NMP in the presence of larger proportions of CS2. The effect was not observed where the concentra(24) Fujiwara, M.; Ohsuga, H.; Takanohashi, T.; Iino, M. Energy Fuels 1992, 6, 859. (25) Takanohashi, T.; Xiao, F.; Yoshida, T.; Saito, I. Energy Fuels 2003, 17, 255.
Energy & Fuels, Vol. 18, No. 2, 2004 453
Figure 5. Effect of the CS2 concentration on the capacity factor (CF) of NMP for the Illinois No. 6 coal.
Figure 6. Effect of NMP (0.3 mol/L) on the capacity factor (CF) of CS2 for high-rank coals.
Figure 7. Effect of different solvents (0.3 mol/L) on the capacity factor (CF) of CS2 for the Pocahontas coal.
tion of CS2 was only 0.02 mol/L. These results indicate that, where NMP co-exists with relatively high concentrations of CS2, it can interact with a greater amount of coal. CS2/NMP mixtures are known to give higher swelling ratios for bituminous coals than either CS2 or NMP alone.16 Hence, NMP seems to be able to diffuse more readily inside coal where CS2 is present. For the high-rank Upper Freeport and Pocahontas No. 3 coals, the changes in the CF of CS2 when 0.3 mol/L of NMP was admixed are shown in Figure 6. For both coals, the CF of CS2 failed to change significantly at concentrations of DMF > pyridine . benzene and was in agreement with the order of increase in extraction yield obtained for binary mixtures of these solvents with CS2.16 A similar behavior was also obtained with the high-rank Upper Freeport coal. These results indicate that CS2 interacts more strongly with high-rank coals in the presence of NMP, DMF, or pyridine and is paralleled by the similar synergistic effect observed for extraction yields obtained with the same binary mixtures of these solvents. The elution curves of NMP, both with and without CS2, are shown for the Upper Freeport coal in Figure 8. Both curves have been normalized to display their differences. The effect of the presence of CS2 on the elution of NMP is small, with an increase of only ∼10% in the eluted peak area of the chromatogram when CS2 is present. A contrasting difference in the chromatograms for NMP is shown in Figure 9, where the difference peak of NMP (the elution curve of NMP admixed with CS2 minus the curve for NMP without CS2) was calculated from the two curves shown in Figure 8, and it is compared with the chromatogram of CS2 when
Takanohashi et al.
a CS2/NMP mixture is injected into Upper Freeport coal. As noted previously, the CS2 chromatogram has two peaks: an early-eluted one that is almost similar to the elution time of NMP, and another that occurs later. The chromatogram of NMP also exhibits two responses, with the later peak eluting earlier than that of CS2. A similar behavior was obtained with the Pocahontas No. 3 coal. Furthermore, a very large tail occurred with CS2 elution, extending over the range of 30-60 min. These results indicate that, after elution of NMP, a significant amount of CS2 continues to be eluted over a prolonged time later and, hence, significant interaction must occur between CS2 and high-rank coals in the presence of NMP. It is unlikely that CS2 and NMP form any complex in the course of their admixture diffusing within the high-rank coals. Rather, the degree of interaction of CS2 with the coals seems to reflect the general pattern of interaction obtained with binary mixtures of solvents with these coals. However, the precise cause of the great enhancement of the interaction of CS2 and coal in the presence of NMP remains unclear. Conclusions The interaction between four Argonne Premium coals with carbon disulfide (CS2), N-methyl-2-pyrrolidinone (NMP), and mixtures of the two solvents was investigated using inverse liquid chromatography techniques. For the Illinois No. 6 coal, the elution curve of NMP showed a pronounced tail, with a similar response being obtained with CS2 and the other three coals, which indicates that CS2 must interact extremely strongly at any and all available interacting sites in these highrank coals. Furthermore, the capacity factor (CF) of CS2 in the Pocahontas No. 3 and Upper Freeport coals increased markedly when the CS2 was injected admixed with NMP, which indicates a synergistic enhancement in the interaction between high-rank coals and CS2 where it co-exists with NMP. Dimethylformamide and pyridine showed an increase in their CFs that was similar to that for NMP; however, benzene did not. This mixed solvent dependency parallels the increase in extraction yields obtained with mixed solvents among high-rank coals. Acknowledgment. This work was conducted as a “Research for the Future” project of the Japan Society for the Promotion of Science (JSPS), funded by the 148 Committee on Coal Utilization Technology. EF0340211
