Energy & Fuels 2001, 15, 487-491
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Effect of Tetrabutylammonium Acetate Addition on the Aggregation of Coal Molecules at Solution and Solid States Hengfu Shui, Koyo Norinaga, and Masashi Iino* Institute for Chemical Reaction Science, Tohoku University, Katahira 2-1-1, Sendai 980-8577, Japan Received October 17, 2000. Revised Manuscript Received December 6, 2000
Effect of tetrabutylammonium acetate (TBAA) addition on the aggregation of coal-derived materials at solution and solid states was investigated. As coal-derived materials, a pyridineinsoluble fraction (PI) of the extract of Upper Freeport coal was used. Two kinds of the solid states with quite different solubility in N-methyl-2-pyrrolidinone (NMP) were obtained for PI by the removal of the solvents from its solution in NMP with and without TBAA, respectively. Similar results were obtained when CS2/NMP mixed solvent was used instead of NMP. Characterizations of these two types of PI by FT-IR, elementary, and XRD analysis suggest that they have different states of molecular aggregation, which seem to be the reflection of their aggregation states in solution.
Introduction Coal-derived materials such as coal extracts and coal tars are known to readily aggregate in bulk or solution state.1 They have various kinds of functional groups and form associates through hydrogen bonds,2,3 ionic attraction,4 interaction between aromatics,5,6 and so forth. These associative interactions are believed to affect greatly the solubility of coal and coal-derived materials in organic solvents and the viscosity of coal-derived liquids. Soaking of bituminous coals in refluxing chlorobenzene or pyridine has been found to bring about conformational rearrangement to let the coal molecules be more strongly associated than before the soaking,7-12 which has been evidenced by the decrease in pyridine extractability,7 solvent-swelling ratio,8 specific heat9 and hydrogen mobility,10 the increase in Einstein temperature,11 and reactivity.12 Recently, Painter et al.13 measured the infrared spectra of a bituminous coal before and after heat treatment in refluxing chlorobenzene. They concluded that the decrease in the extractability of the coal by the heat treatment could be explained by * Author to whom all correspondence should be addressed. Fax: +81-22-217-5655. (1) Stenberg, V. I.; Baltisberger, R. J.; Patel, K. M.; Raman, K.; Woolsey, N. F. Coal Science; Gorbaty, M. L., Larsen, J. W., Wender, I., Eds.; Academic Press: New York, 1983; p 125. (2) Larsen, J. W.; Baskar, A. J. Energy Fuels 1987, 1, 230. (3) Painter, P. C.; Sobokowiak, M.; Youtcheff, J. Fuel 1987,66, 973. (4) Nishioka, M. Fuel 1993, 12, 1725. (5) Quinga, E. M. Y.; Larsen, J. W. Energy Fuels 1987, 1, 300. (6) Miyake, M.; Stock, L. M. Energy Fuels 1988, 2, 815. (7) Nishioka, M.; Larsen, J. W. Energy Fuels 1990, 4, 100. (8) Larsen, J. W.; Mohammadi, M. Energy Fuels 1990, 4, 107. (9) Larsen, J. W.; Flowers, R. A.; Hall, P. J.; Carlson, G. Energy Fuels 1997, 11, 998. (10) Norinaga, K.; Hayashi, J.-i.; Kato, R.; Chiba, T. Energy Fuels 2000, 14, 511 (11) Hall, P. J. Energy Fuels 1997, 11, 1003. (12) Larsen, J. W.; Azik, M.; Korda, A. Energy Fuels 1992, 6, 109. (13) Painter, P. C.; Opaprakasit, P.; Scaroni, A. Energy Fuels 2000, 14, 1115.
the increase in cross-links by the formation of ionic complexes including metal cations in the coal. We have found14,15 that carbon disulfide-N-methyl2-pyrrolidinone (CS2/NMP) mixed solvent (1:1 by volume) is a uniquely powerful solvent for the extraction of some bituminous coals. The extracts obtained were fractionated using acetone and pyridine into acetonesoluble (AS), pyridine-soluble/acetone-insoluble (PS), and pyridine-insoluble (PI) fractions, and the solubility of PI in the CS2/NMP mixed solvent was measured. It was found that a considerable part of PI was insoluble in the mixed solvent, though PI is a part of the mixed solvent extract.16,17 This indicates that once the light fractions, i.e., AS and PS are removed from the CS2/ NMP extracts, the remained PI becomes partly insoluble in the mixed solvent, presumably because of molecular aggregations.18 However, an addition of a small amount of electron acceptors such as tetracyanoethylene (TCNE) makes PI almost completely soluble in the mixed solvent.19,20 TCNE was also found to be effective for the enhancement of coal extraction yield with the mixed solvent.17 Recently, we have found that tetrabutylammonium acetate (TBAA) and halides were as effective as TCNE for enhancing coal extraction yield with the CS2/NMP mixed solvent.21 It was shown that any chemical reactions between these additives and coal are (14) Iino, M.; Takanohashi, T.; Osuga, H.; Toda, K. Fuel 1988, 67, 1639. (15) Iino, M.; Takanohashi, T.; Obara, S.; Tsueta, H.; Sanokawa, Y. Fuel 1989, 68, 1588. (16) Takanohashi, T.; Iino, M. Energy Fuels 1995, 9, 788. (17) Liu, H.-T.; Ishizuka, T.; Takanohashi, T.; Iino. M. Energy Fuels 1993, 7, 1108. (18) Takanohashi, T.; Fengjuan, X.; Saito, I.; Sanokawa, Y.; Iino, M. Fuel 2000, 79, 955. (19) Ishizuka, T.; Takanohashi, T.; Ito, O.; Iino, M. Fuel 1993, 72, 579. (20) Chen C.; Iino, M. Energy Fuels 1999, 13, 1180. (21) Takahashi, K.; Norinaga, K.; Masui, Y.; Iino, M. Energy Fuels (in press).
10.1021/ef000230z CCC: $20.00 © 2001 American Chemical Society Published on Web 02/07/2001
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unlikely to take place, and the suppressions of the associations among coal molecules by the additives may be responsible for the enhancement of coal solubility. However, detailed mechanisms for the aggregation of coal molecules are not yet known. But, the ionic complex formations proposed by Painter et al.13 can be ruled out in the present case, since the extracts contain no metal cations.15 Coal-derived materials such as coal extracts are extraordinarily complex mixtures of molecules with various chemical structures and molecular sizes. Hence, it is expected that coal-derived materials can have numerous metastable states of aggregation because of their complex natures. However, few systematic studies on aggregation states of coal-derived materials and the effect of aggregation states on their physical properties are carried out so far. In this study, aggregation state of coal extract fraction, PI, at solution and solid states was studied in detail using the CS2/NMP mixed solvent or NMP as a solvent. Experimental Section Preparation of PI. An Argonne Premium Upper Freeport coal, with particle sizes finer than 150 µm, was dried in a vacuum oven at 80 °C for 12 h. The dried coal was exhaustively extracted with the CS2/NMP mixed solvent (1:1 by volume) under ultrasonic irradiation (38 kHz) at room temperature. The extraction yield was 60 wt %, determined on a dry ashfree base (daf) from the weight of the extraction residue. The extract was further fractionated using acetone and pyridine to yield acetone-soluble (AS), pyridine-soluble/acetone-insoluble (PS), and pyridine-insoluble (PI) fractions. The yield of PI was 27 wt %. The ash content in the extract is practically zero.15 Detailed extraction and fractionation procedures were described elsewhere.14-16 Solvent Treatment of PI with and without TBAA. A 0.2 g sample of PI was dissolved in 50 mL of the CS2/NMP mixed solvent or NMP with or without the additive, TBAA, under ultrasonic (38 kHz) irradiation for 30 min at room temperature. The amount of TBAA added was 0.25 mmol (0.075 g) per 1 g of PI. The mixture was then centrifuged under 29000 g for 15 min, and immediately filtered through a membrane paper with a pore size of 0.8 µm. After the filtration, CS2 and NMP were removed from the filtrate by a rotary evaporator at around 90 °C. The precipitated PI was washed exhaustively with water/acetone mixed solvent (4/1 by volume) to remove CS2, NMP, and TBAA, then dried under vacuum at 80 °C for 12 h. The washing procedure with the water/acetone mixed solvent includes ultrasonic (38 kHz) irradiation for 15 min, centrifugation (29000g) for 15 min, and filtration. The washing was usually repeated 6 times to remove completely the solvents and TBAA which are strongly retained in PI. In this study, as shown in Figure 1, first the original PI (referred as PI-0) was treated with the mixed solvent-TBAA to give PI-1, and then PI-1 was treated with the mixed solvent and NMP without TBAA to give PI-2 and PI-3, respectively. Solubility Determination of PI. A 0.1 g sample of various PI was dissolved in 50 mL of the mixed solvent or NMP under ultrasonic (38 kHz) irradiation for 30 min at room temperature. After the filtration through a membrane paper and washing with the water/acetone mixed solvent the residue was dried under vacuum at 80 °C for 12 h. The solubility (wt %) was determined from the weight of the residue:
solubility (wt %, db) ) [1-residue (g)/PI feed (g)] × 100 (1) The reproducibility of the solubility determination was within (5%.
Figure 1. Solvent treatment procedure of pyridine-insoluble extract fraction, PI, from Upper Freeport coal with the CS2/ NMP mixed solvent or NMP with or without TBAA. Table 1. Solubilities of Various PIs in the CS2/NMP Mixed Solvent aand NMP solubility (wt %, db) sample
CS2/NMP
CS2/NMP+TBAA
NMP
NMP+TBAA
PI-0 PI-1 PI-2 PI-3
69 98 63
99
53 98
99
48
99
a
99
1:1 by volume.
FTIR and X-ray Measurements. Diffuse reflectance FTIR spectra were measured by JEOL JIR-100 spectrometer at a resolution of 4 cm-1 by co-adding 200 scans. Samples for FTIR measurement were prepared by diluting 5 mg of PI in 200 mg of KBr. Wide angle X-ray diffraction patterns were measured on a SHIMADZU XD-D1W diffractometer using a Cu KR target.
Results and Discussion Mechanism for the Solubility Enhancement of PI by TBAA Addition. The solubilities of various PIs in the CS2/NMP mixed solvent and NMP with and without TBAA are shown in Table 1. The solubility of the raw PI (PI-0) in the CS2/NMP mixed solvent is 69%, though PI-0 is a part of the mixed solvent extract. But, it becomes almost completely (99%) soluble in the CS2/ NMP mixed solvent when a small amount (0.075 g/g-PI) of TBAA was added into the solvent, as shown in Table 1. The solubility of PI-0 in NMP is also increased from 53 to 99% by the addition of the same amount of TBAA. It is clear that TBAA increases the solubility of PI in both the CS2/NMP mixed solvent and NMP. Two explanations are possible for this enhancement of the solubility; (1) TBAA breaks noncovalent associative interactions among PI molecules, and (2) TBAA breaks covalent bonds of PI molecules. The solvent treatments of PI with or without TBAA were carried out to examine which explanation is proper. PI-1 was obtained by removal of CS2, NMP, and TBAA from the
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Figure 2. FT-IR spectra of the original PI (PI-0) (a); the mixture of 1 g of PI-0 and 0.075 g of TBAA (b); PI-1′ obtained by 3 times washing of treated PI-0 with acetone-water mixed solvent (c); PI-1 obtained by 6 times washing of treated PI-0 with acetonewater mixed solvent (d). Table 2. Elemental Compositions of the PIs (wt %, db) sample
C
H
N
S
Oa
PI-0 PI-1 PI-2 PI-3
83.1 82.6 82.0 81.1
5.2 5.3 5.1 5.0
1.8 1.8 1.8 1.5
1.8 2.0 1.8 1.7
8.1 8.3 9.3 10.7
a
By difference.
solution of PI-0 in the mixed solvent containing TBAA. PI-1 was found to be almost completely soluble (98%) in the CS2/NMP mixed solvent and NMP in the absence of TBAA, as shown in Figure 1 and Table 1. This indicates that PI-0 was converted to PI-1 with different solubility by the only 30 min treatment at room temperature. Figure 2 shows the FTIR spectra of PI-0 (a), the physical mixture of PI-0 and TBAA (b), PI-1′ obtained by 3 times washing of treated PI-0 with acetone-water mixed solvent (c); PI-1 obtained by 6 times washing of treated PI-0 with acetone-water mixed solvent (d). A sharp peak at 2950 cm-1 clearly appeared in Figure 2b is ascribed to the aliphatic C-H stretching band of butyl groups of TBAA. The intensity of this peak for PI-1′ is significantly diminished (Figure 2c) and almost disappeared for PI-1 (Figure 2d). Furthermore, the elemental composition of PI-1 is almost the same as that of PI-0 (Table 2). These observations suggest that chemical reactions between TBAA and PI do not take place, since the physical treatment such as solvent washing can remove TBAA almost completely from PI-1 and little chemical compositions changed from PI-0 to PI-1. Therefore, explanation (2) seems dubious, and the difference in the solubilities of PI-0 and PI-1
would be attributed to the difference in their molecular aggregation states, i.e., explanation (1) is reasonable. PI-2 and PI-3 were obtained by the removal of the solvents from PI-1 solution in the CS2/NMP mixed solvent and NMP without TBAA, respectively, and their solubilities were measured. If PI-0 was changed to PI-1 irreversibly by chemical reactions such as covalent bonds breaking by TBAA, the solubility of PI-2 or PI-3 should be as high as PI-1. However, Table 1 shows that the solubilities of PI-2 and PI-3 in the CS2/NMP mixed solvent and NMP are 63 and 48%, respectively. They are quite different from 98% for PI-1, though FTIR spectra (Figure 3) and elemental compositions (Table 2) of PI-2 and PI-3 are similar with PI-1. In addition, Table 1 shows that the solubilities of PI-2 and PI-3 in the mixed solvent and NMP with TBAA (0.25 mmol/ g-PI) are 99%, the same as those of PI-0. This result also suggests that TBAA breaks noncovalent associative interactions among PI molecules, producing soluble PI1, which is then converted to partly insoluble PI-2, 3 by the solvent treatment without TBAA, due to promoted aggregation of PI molecules during the treatment and/ or subsequent solvent removal procedure. The effectiveness of lithium and tetrabutylammonium salts of halides for the enhancement of the coal extraction yields with the CS2/NMP mixed solvent has found21 to be the order F- > Cl- > Br- > I-, and the difference in the cations of the salts hardly affects the extraction yields. We have also reported20 that TCNE-derived anion may be responsible for the enhancement of coal solubility by TCNE addition. Considering these results, a mechanism for the solubility enhancement by TBAA
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Figure 5. Change of the amount of the insolubles in PI1solution in the CS2/NMP solution with time at 25 °C (b) and 50 °C (4) without TBAA, and at 50 °C with TBAA (2).
Figure 3. FT-IR spectra of various PIs.
Figure 4. XRD patterns of PI-0 and PI-1.
seems like that the acetate anion of TBAA breaks noncovalent bonds in PI aggregates through the interaction with acidic sites in coal structures, with the formation of acetate-PI associates. Aggregation State of PI-0 and PI-1. X-ray diffraction (XRD) is the most direct tool for studying the structural regularities of solids. To probe the difference in the molecular aggregation state between PI-0 and PI1, their XRD patterns were measured. Because of the amorphous nature of the coal soluble constituents there is no sharp band in XRD curves, but some wide diffuse bands are observed as shown in Figure 4. A small increase in 002 peak intensity at around 25° (2θ) assigned to face-to-face arrangements of aromatic rings is observed for PI-0 compared to PI-1, suggesting that PI-0 has an increased face-to-face stacking arrangements of aromatic rings. This more oriented structures of PI-0 is likely to be related to its lower solubility. It is also intriguing that a band (2θ ) ∼ 4.5°) which corresponds to a regular structure of about 20 Å in size is observed for PI-0. Larsen et al.9 have reported that an increase in the intensity of a similar peak with the
chlorobenzene treatments of a bituminous coal. We cannot assign this peak at present, but it is possible that the peak is related to associative interactions of coals. It should be noted that other factors such as packing and surface variations of the samples possibly influence the XRD intensity, though the XRD patterns obtained here have a good reproducibility for other samples of PI-0 and PI-1 which were newly prepared. We are now studying on their aggregation state from other methods such as DSC. Effect of Treatment Temperature and Time on the Aggregation of PI-1. The amount of the insolubles formed from PI-1 solution in the CS2/NMP mixed solvent (0.05 g PI-1 in 50 mL of the mixed solvent) at 25 or 50 °C was measured as a function of time. Figure 5 shows that about 30% of PI-1 was precipitated as insolubles for 1 h at 50 °C, but, only 7% insolubles were formed for 5 h at 25 °C. It took 48 h to yield 30% insolubles at 25 °C. While, when a small amount (0.075 g/g-PI-1) of TBAA was added to the PI-1 solution, only 3% of the insolubles were formed after 4 h standing even at 50 °C. Furthermore, the addition of the same amount of TBAA to the PI-1 solution containing 30% insolubles after 1 h at 50 °C decreased the insolubles to only 2%. These observations suggest the reason PI-1, which is almost completely soluble in the mixed solvent or NMP without TBAA, was converted to the partially insoluble PI-2 or PI-3. As described in the Experimental Section, the evaporation of NMP from PI-1 solution in the mixed solvent or NMP was carried out at about 90 °C for 1-2 h, and as seen from Figure 5, 1 h at 90 °C is surely enough to induce the aggregation among PI molecules, and the resultant PI-2 or PI-3 becomes partly insoluble. While, the evaporation of the solvents from PI-0 solution was carried out with TBAA which prevents the aggregation, as the result at 50 °C with TBAA is shown in Figure 5. This less aggregated state can be kept during washing to remove TBAA and drying, since the aggregation rate should be very slow at the solid state, because of the extremely limited freedom of molecular motion of PI molecules.
TBAA and the Aggregation of Coal Molecules
Conclusion The effect of TBAA addition on the aggregation of the pyridine-insoluble extract fraction, PI, from Upper Freeport coal in solution and solid states was investigated. About 30 wt % of original PI (PI-0) was insoluble in the CS2/NMP mixed solvent. While, PI-0 was almost completely soluble in the mixed solvent containing a small amount of TBAA. After the removal of CS2, NMP, and TBAA from the solution, PI was recovered with little mass loss. The recovered PI (PI-1) was found to become almost completely soluble in the mixed solvent and NMP even in the absence of TBAA, although the elemental compositions and IR spectra of PI-0 and PI-1 indicate that their chemical structures were identical. XRD patterns observed for PI-0 and PI-1 suggest that
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PI-0 has more aggregated and oriented structures than PI-1. PI-2, or PI-3, which were obtained by the removal of the solvents from PI-1 solution in the mixed solvent or NMP without TBAA, was found to become again partly insoluble in the both solvents. This can be explained by the increased aggregation of PI during the evaporation of the solvents at about 90 °C without TBAA, unlike the case for PI-0 solution with TBAA, which seems to act as an aggregation inhibitor. Acknowledgment. This work was supported by a “Research for the Future” project of the Japan Society for the Promotion Science (JSPS). EF000230Z
