Energy & Fuels 1997, 11, 433-438
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Solubilization of Mesocarbon Microbeads by Potassiumor Dibutylzinc-Promoted Butylation and Structural Analysis of the Butylated Products Yan Zhang, Koh Kidena, Takeshi Muratani, Satoru Murata, and Masakatsu Nomura* Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565, Japan
Yoshiharu Yoneyama and Tsutomu Kato Department of Chemical and Biochemical Engineering, Faculty of Engineering, Toyama University, 3190 Gofuku, Toyama 930, Japan
Chiharu Yamaguchi Research & Development Center, Osaka Gas Co. Ltd., 19-9, 6-Chome, Torishima, Konohana-Ku, Osaka 554, Japan Received July 10, 1996. Revised Manuscript Received December 4, 1996X
In order to obtain an insight into structural features of mesocarbon microbeads (MCMB), we examined its butylation reaction promoted by potassium or dibutylzinc. Both reactions gave THF-soluble products in 86-95% yield. This is the first example for effective solubilization of these carbonaceous materials. The resulting products were analyzed by 13C NMR and gel permeation chromatography. The results of structural analyses indicate that MCMB molecules consist of highly condensed aromatic hydrocarbons (average ring size is calculated to be 15-16) with very minor amounts of alkyl substituents and oxygen-functional groups.
Introduction Mesocarbon materials are generally prepared by heat treatment of heavy hydrocarbons like coal tar pitches, petroleum pitches, or bitumens.1-3 These classes of compounds are expected to be made into advanced materials such as carbon fiber, electrode, and so on. Mesocarbon microbeads (MCMB) are known to be one of the most important carbonaceous materials, a calcined product of which is applicable to electrodes of a lithium ion battery.4-10 However, information concerning the structure of MCMB is limited because of their low solubility toward conventional organic solvents. Abstract published in Advance ACS Abstracts, February 1, 1997. (1) Taylor, G. H. Fuel 1961, 40, 465. (2) Honda, H.; Yamada, Y.; Oi, S.; Fukuda, K. Tanso 1973, 7, 3. (3) Yamada, Y.; Imamura, T.; Kakiyama, H.; Honda, H.; Oi, S.; Fukuda, K. Carbon 1974, 12, 307. (4) Kodama, M.; Fujiura, T.; Ikawa, E.; Esumi, K.; Meguro, K.; Honda, H. Carbon 1991, 29, 43. (5) Takami, N.; Satoh, A.; Ohsaki, T. The Electrochemical Society Extended Abstracts, 92-2, Toronto, Canada, 1992; p 38. (6) Yamamura, J.; Ozaki, Y.; Morita, A.; Ohta, A. Extended Abstracts of 6th International Meeting on Lithium Batteries, Munster, Germany, 1992, THU-05. (7) Yamamura, J.; Ozaki, Y.; Morita, A.; Ohta, A. J. Power Sources 1993, 43-44, 233. (8) Tatsumi, K.; Mabuchi, A.; Iwashita, N.; Sakaebe, H.; Sioyama, H.; Fujimoto, H.; Higuchi, S. The Electrochemical Society Extended Abstracts, 93-1, Honolulu, Hawaii, 1993; p 8. (9) Mabuchi, A.; Tokumitsu, K.; Fujimoto, H.; Kashuh, T. Extended Abstracts of 7th International Meeting on Lithium Batteries, Boston, MA, 1994; I-A-11. (10) Hara, M.; Satoh, A.; Takami, N.; Ohsaki, T. Tanso 1994, 165, 261.
Until now, many studies have employed nondestructive analytical methods including NMR, ESR, and so on.11-14 Yamaguchi et al.15 referred to chemical structural analyses of MCMB samples and their precursors by using both 13C NMR and ESR spectrometers and investigated relationships between the chemical structure of MCMB and their precursors. In a previous paper,16 we conducted dibutylzinc-promoted butylation of MCMB with butyl iodide and found that this pretreatment could convert MCMB to tetrahydrofuran (THF)-soluble materials in ca. 90% yield. In the present paper, we also conducted butylation of MCMB with potassium-butyl iodide in THF under ultrasonic irradiation17-19 (the conditions were similar to Sternberg’s alkylation procedure20,21) and detailed analyses of the
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(11) Honda, H.; Kimura, H.; Sanada, Y.; Sugawara, S.; Turuta, T. Carbon 1970, 8, 181. (12) Imamura, T.; Yamada, Y.; Oi, S.; Honda, H. Carbon 1978, 16, 481. (13) Yamada, Y.; Imamura, T.; Kakiyama, H.; Oi, S.; Honda, H.; Fukuda, K. Carbon 1974, 12, 307. (14) Yamada, Y.; Kobayashi, K.; Honda, H.; Tsuchitani, M.; Matsushita, Y. Tanso 1976, 86, 101. (15) Yamaguchi, C.; Matsuyoshi, H.; Tokumitsu, K.; Baba, S.; Kumagai, H.; Sanada, Y. Tanso 1994, 164, 193. (16) Zhang, Y.; Kidena, K.; Murata, S.; Nomura, M.; Yoneyama, Y.; Kato, T. Chem. Lett. 1996, 491. (17) Miyake, M.; Sukigara, M.; Nomura, M.; Kikkawa, S. Fuel 1980, 59, 638. (18) Miyake, M.; Uematsu, R.; Nomura, M. Chem. Lett. 1984, 535. (19) Miyake, M.; Yamamoto, S.; Nomura, M. Fuel Process. Technol. 1986, 14, 201. (20) Sternberg, H. W.; Dell Donne, C. L.; Pantages, P.; Moroni, E. C.; Markby, R. E. Fuel 1971, 50, 432. (21) Sternberg, H. W.; Dell Donne, C. L. Fuel 1974, 53, 172.
© 1997 American Chemical Society
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Zhang et al.
Table 1. Elemental Analyses of MCMB and Its Butylated Products elemental analyses (wt %) sample
C
H
N
O + Sa
H/Cb
fa
coal tar pitch-A coal tar pitch-B MCMB-A original KT BZ MCMB-B original KT BZ
93.70 91.80
4.70 4.70
1.20 1.20
0.40 2.30
0.60 0.61
0.97 0.96
94.20 87.95 88.39
3.20 6.48 8.45
1.10 0.94 0.61
1.50 4.63 2.55
0.40 0.88 1.14
0.98
94.12 90.57 90.06
3.25 6.60 8.61
0.96 0.77 0.54
1.67 2.06 0.79
0.41 0.87 1.14
0.97
a
By difference. b Atomic ratio.
butylated products. On the basis of these results, we discussed the structural features of MCMB molecules. Experimental Section Samples and Reagents. Two MCMB samples employed in this study, MCMB-A and -B, were prepared according to the following method:15 Two different kinds of coal tar were submitted to vacuum distillation to have appropriate softening points (around 80 °C). Then, each resulting pitch (500 g) was put in a 1 L autoclave and heated at 430 °C for 8 h under 4 kg/cm2 of nitrogen. When the aliquot of each product (1 g) was heated in quinoline (20 mL) at 75 °C for 30 min, precipitates appeared from each mixture, these being filtered, washed with quinoline (10 mL, 4 times) and acetone (10 mL, 5 times), and dried. The resulting MCMB samples have spherical structures, the mean diameter of which was about 20 µm. Their elemental analyses are shown in Table 1 along with those of the original pitches. CP/MAS 13C NMR spectra of MCMB were recorded on a JEOL FX-100 spectrometer.15 The parameters employed were as follows: 6.5 ms pulse width, 2 ms contact time, pulse delay 5 s, scan number 5000, and MAS frequency 3 kHz. Carbon aromaticities determined based on the NMR spectra were 0.98 for MCMB-A and 0.97 for MCMB-B. The other reagents used in this study were commercially available and purified according to the conventional methods before use. Butylation of MCMB by Potassium/Butyl Iodide/THF Reagents under Ultrasonic Irradiation. The details of the procedure employed here were described elsewhere.17-19 MCMB (0.5 g), potassium (0.7 g), and THF (10 mL) were placed in a 100 mL three-necked flask and then heated at 60 °C for 5 h under ultrasonic irradiation. To the resulting mixture, 9 mL of butyl iodide was added at 0 °C; then, the whole mixture was stirred at room temperature for a night. After the end of the reaction, ethanol was introduced to the apparatus to deactivate any remaining potassium. Ethanol, THF, and butyl iodide were distilled off using a rotary evaporator, the remaining solids being washed with water-ethanol mixture (50/50, v/v). A part of each resulting butylated MCMB sample (ca. 0.5 g) was extracted with three portions (50 mL each) of THF, successively, under ultrasonic irradiation at room temperature. The resulting THF extract was evaporated and dried at 60 °C for 40 h in vacuo after removing a trace amount of THF insoluble. Butylation of MCMB by Dibutylzinc/Butyl Iodide Reagents. The details of the procedure employed here were described elsewhere.22-24 A mixture of MCMB (0.5 g), dibutylzinc (10.5 g), and butyl iodide (5 mL) was heated in a 100 (22) Yoneyama, Y.; Akaki, Y.; Kato, T. Bull. Chem. Soc. Jpn. 1989, 62, 3959. (23) Yoneyama, Y.; Yamamura, Y.; Hasegawa, K.; Kato, T. Bull. Chem. Soc. Jpn. 1991, 64, 1669. (24) Yoneyama, Y.; Gonda, K.; Kato, T. Chem. Lett. 1992, 843.
mL flask at 180 °C for 4 h. After the end of the reaction, the remaining dibutylzinc was decomposed at 0 °C by addition of ethanol; then butyl iodide and ethanol were distilled off. The remaining solids were washed with dilute hydrochloric acid (5 wt %, 100 mL) to remove the remaining zinc salts. THF extraction of the products was conducted according to the method described above. Measurement of 13C NMR of the Butylated MCMB Samples. 13C NMR spectra of the butylated products were recorded on a Bruker AC-600 spectrometer as a CDCl3 solution under the following conditions: gated decoupling method, 45° pulse, 700 scans, 200 s pulse delay, tetramethylsilane as internal standard, without relaxation reagents like chromium acetylacetonate. Processing (FT, phase correction, and integration) of the FID data was conducted on an Apple Macintosh personal computer with a commercial NMR data processing software MacAlice (JEOL Datum). Measurement of Molecular Size Distribution of the THF-Soluble Portion of Butylated Products. 20 µL of a DMF solution of the butylated MCMB (0.7 g/L) was introduced into the GPC system consisting of a Shimadzu LC-10AS pump, a Shimadzu SPD-10A ultraviolet detector (λ ) 270 nm), and a Shodex KF-80M GPC column. Either pure DMF or DMF solution containing 10 mM of lithium bromide was used as an eluant. Calibration of relationships between molecular weight and retention time was conducted by using a series of standard polystyrene samples, the sets of which, SM-105 and SL-105, were purchased from Showa Denko Co. Ltd., Japan: these contain 12 samples with number-average molecular weight of 5.8 × 102, 9.5 × 102, 1.26 × 103, 2.83 × 103, 1.14 × 104, 2.76 × 104, 6.46 × 104, 1.52 × 105, 5.23 × 105, 1.02 × 106, 2.23 × 106, and 2.98 × 106 (Mw/Mn ) 1.03-1.15). X-ray Diffraction Measurement of MCMB Samples. Measurements of X-ray diffraction of MCMB samples were conducted using a MacScience Co. Ltd. M18XHF-SRA apparatus. The measurement conditions employed here were as follows: Cu KR radiation (λ ) 1.5405 Å), scan range of angle 12-50°, scan rate 10°/min.
Results and Discussion Solubilization of MCMB Samples. We investigated methods to increase solubility of MCMB toward conventional organic solvents. In the field of coal chemistry, there are several effective methods for this purpose: for example, (a) heating at 350-450 °C in polar and nondonor solvents like pyridine, quinoline, or phenol, (b) heating at 350-450 °C in hydrogen-donor solvents such as tetralin or 9,10-dihydroanthracene, and (c) alkylation of aromatic nuclei and oxygen-containing functional groups. We tried several methods for solubilization of MCMB samples. At first, we conducted a pyrolytic depolymerization (method a) of MCMB-A sample in quinoline at 350 °C for 1 h under 50 kg/cm2 of nitrogen. When this method was applied to coals with subbituminous or bituminous ranges, yields of quinoline extracts reached 70-95% (daf coal basis);25 while this treatment of the MCMB sample afforded the extract in only 14% yield, this being less than the yield of quinoline-soluble materials in the original MCMB sample (actually the extraction of MCMB-A sample with quinoline at room temperature afforded a soluble portion in 18% yield). Then, the heat treatment of MCMB-A (0.5 g) in the presence of 9,10dihydroanthracene (0.5 g) was conducted at 420 °C for 1 h, this affording quinoline extracts in 15% yield. The above two methods are believed to cleave weaker (25) Murata, S.; Kawakami, E.; Nomura, M. Energy Fuels 1996, 10, 220.
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Table 2. Results of Butylation of MCMB sample
yield of butylated product (wt %, MCMB base)
KT-A KT-B BZ-A BZ-B
132.5 127.9 167.5 166.9
no. of Bu groups incorporated (/100 C) carbon balance atomic ratio wt change 5.9 5.8 14.3 14.9
8.6 8.3 16.5 16.4
7.2 6.1 14.9 14.7
yield of THF-soluble products (wt %) butylated MCMB basea MCMB baseb 91.9 89.4 97.1 93.9
89.3 86.4 95.1 89.8
a This yield is based on soluble products, showing the upper limit for the conversion. b This yield is on the basis of the insoluble residues, showing the lower limit for the conversion.
linkages in carbonaceous materials (e.g., ether bonds or dimethylene bridges) so that these results might suggest that MCMB originally do not have these kinds of linkage and certain condensation reaction of aromatic moieties took place. As to the alkylation method for solubilization of carbonaceous materials, various studies have been examined to solubilize coal so far, for example, Sternberg’s reductive alkylation,20,21 its modified method proposed by us,17-19 Liotta’s O-alkylation,26,27 nonreductive alkylation by Stock et al.,28-30 and alkylzincpromoted alkylation.22-24 The origin of the higher solubility of the products is believed to be reduction of hydrogen-bonding interactions, cleavage of ether bonds, and reduction of aromatic π-π stacking interaction by the introduction of alkyl groups on aromatic rings. MCMB is believed to consist of highly stacked polycondensed aromatic sheets.1 Therefore, we tried to solubilize MCMB samples with alkylation methods. We examined two alkylation methods: anionization of MCMB with potassium in THF under ultrasonic irradiation followed by butylation with butyl iodide (KT method) and dibutylzinc-promoted butylation with butyl iodide (BZ method). Elemental analyses, yield of butylated products, number of butyl groups introduced, and THF solubility of butylated products are summarized in Tables 1 and 2. These methods afforded 8997% of soluble materials based on the butylated MCMB and 5-14% of original MCMB was found to remain still insoluble. These results indicate that alkylation of MCMB resulted in solubilization of major parts of it, at least 86-95% (calculated on the basis of the insoluble residues). This is the first example, to our knowledge, for effective solubilization of this kind of carbonaceous materials. When comparing these two methods, we found that the BZ method is more effective than the KT method. The number of butyl groups introduced was calculated to be 14-16 per 100 carbons of MCMB for BZ method and 6-9 for KT method. This might be one of the origins of higher solubility toward THF of the alkylated products. It should be noted that the MCMB-A sample afforded slightly more amounts of soluble products than MCMB-B sample. Characterization of the Butylated MCMB Samples by 13C NMR. To obtain an insight into the chemical structure of the butylated products, we measured the 13C NMR spectra of two butylated MCMB samples, BZ-A and KT-A, each of which means the butylated MCMB-A sample by BZ and KT methods, (26) Liotta, R. Fuel 1979, 58, 724. (27) Liotta, R.; Rose, K.; Hippo, E. J. Org. Chem. 1981, 46, 277. (28) Miyake, M.; Stock, L. M. Energy Fuels 1988, 2, 815. (29) Chatterjee, K.; Miyake, M.; Stock, L. M. Prep. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1990, 35, 46. (30) Chatterjee, K.; Miyake, M.; Stock, L. M. Energy Fuels 1990, 4, 242.
Figure 1. 13C NMR spectra for the butylated MCMB samples, KT-A (top) and BZ-A (bottom).
Figure 2. The aliphatic region of the spectrum for BZ-A sample and assignment of each peak.
respectively. Since we found that the THF-soluble portions of the butylated samples (92-97% based on the butylated MCMB samples) could be completely soluble in CHCl3, we measured the spectra in CDCl3. The resulting spectra are shown in Figure 1. From these spectra, carbon aromaticities for two samples were calculated to be 0.55 for BZ-A and 0.75 for KT-A. Two types of alkylation reactions are reported to take place in alkylation of aromatic nuclei, i.e., substitution (nonreductive alkylation, type I in Figure 2) and addition (reductive alkylation, type II).22 The alkyl groups introduced are thought to affect the solubility of the products, because these act as spacer between stacked aromatic sheets. Stock et al. had reported chemical shifts for compounds containing butyl groups.31 In their paper, chemical shifts of butyl groups in 9,10-dibutylanthracene were 14, 24, 28, and 34 ppm and those for 9,10-dibutyl-9,10-dihydroanthracene were 14, 23, 31, and 42 ppm. Chemical shifts for δ- and γ-carbons for both compounds were similar to each other, while (31) Alemany, L. B.; King, S. R.; Stock, L. M. Fuel 1978, 57, 738.
436 Energy & Fuels, Vol. 11, No. 2, 1997
Zhang et al. Table 3. Crystalline Parametersa MCMB-A MCMB-B a
La (Å)
Lc (Å)
d (Å)
nb
31.2 29.7
37.7 40.3
3.45 3.45
11.9 12.7
Determined by X-ray diffraction. b n ) Lc/d + 1.
Figure 3. Gel permeation chromatograms for butylated MCMB samples in the presence or absence of LiBr.
differences between β-carbons of two compounds were rather large. In the aliphatic region of the spectrum of BZ-A, there appeared five peaks centered at 14, 23, 27, 30, and 34 ppm (Figure 2). According to the Stock’s report, the two peaks at higher field could be assigned as δ- and γ-carbons, respectively, the broad peak at the lowest field being assigned as R-carbons of butyl groups introduced and, reduced and substituted bridgehead carbons in type II products (Cred in Figure 2). The remaining two peaks, centered at 27 and 30 ppm, should be β-carbons in type I and II products, respectively. Consequently, the ratio of intensity of the peak at 30 ppm to that at 27 ppm could give us information concerning fashion of introduction of the butyl groups. The relative intensity of two peaks at 27 (integrated from 25.0 to 28.6 ppm) and 30 ppm (integrated from 28.6 to 32.0 ppm) was found to be 47:53 for KT-A sample and 38:62 for BZ-A sample. These indicate that reductive butylation is dominant in the butylation reaction with the dibutylzinc-butyl iodide system, while, in the system consisting of potassium, butyl iodide, and THF, both reductive and nonreductive butylation took place to a similar extent. This might be one of the origins of the slightly higher solubility of the BZ-A sample than that of the KT-A sample. Molecular Size Distribution of Butylated Samples. To obtain an insight into molecular size distribution of the butylated MCMB samples, gel permeation chromatographic analyses were conducted. Figure 3 shows molecular size distribution of four kinds of the butylated MCMB samples, BZ-A, KT-A, BZ-B, and KTB, using N,N-dimethylformamide (DMF) as eluant with or without lithium bromide dissolved, where the X-axis means polystyrene equivalent molecular weights. In the case of using DMF without lithium bromide as eluant, there appeared two peaks at 103 and 106-107, while, the peak at upper region of the chromatograms disappeared when lithium ion-containing DMF was employed. This might suggest that MCMB strongly tends to associate each other. These phenomena were also observed for gel permeation chromatographic analyses of coal extracts and were interpreted by assuming that lithium bromide could cleave some noncovalent interactions between coal molecules.25 Maximum value of dW/d[log(MW)] was observed at about 900 for all samples. On the basis of this value and yield of the butylated products, the average molecular weight of the original MCMB could be calculated to range from 500 (900/167.5 × 100) to 700 (900/127.9 × 100).
Figure 4. Relationships between the number of aromatic carbons, H/C atomic ratio, and ring size of typical aromatic hydrocarbons with circular orientation (b) and linear orientation (9). The numbers correspond to the compounds as shown.
It is generally known that gel permeation chromatographic analysis overestimates the molecular weights of these carbon-rich molecules. Therefore, it should be noted that the molecular weights estimated in this study were polystyrene equivalent molecular weights. We are now trying to measure the molecular weights of butylated MCMB by the other methods like FDMS or VPO. XRD Measurements. We also measured XRD pattern of the original MCMB samples. There appeared two broad peaks at 26° and 40° in their diffractograms, these corresponding to (002) and (10) bands of aromatic planes (graphite-like structure). We calculated crystalline parameters, La, Lc, and d according to the method reported so far,32,33 the resulting parameters being shown in Table 3. From this table, the diameters of aromatic sheets of MCMB molecules and layer spacing were found to be around 37-40 and 3.45 Å, respectively. The latter value is rather near the distance between graphitic planes, 3.35 Å. On the other hand, we observed that Lc and d were 10-20 and 3.6 Å, respectively, for bituminous coals.34 This indicates that MCMB molecules have highly ordered aromatic sheets. As to the parameter, La, the values were found to be 29.7-31.2 Å for MCMB samples. Construction of Structural Model for MCMB-A and Its Butylated Products. In the recent studies,35 we had proposed a methodology for the construction of a chemical structural model for heavy hydrocarbons like coal on the basis of the data from solid state 13C NMR and pyrolysis GC/MS. According to this approach, we had measured CP/MAS 13C NMR.15 There appeared one broad peak at around 120-130 ppm of CP/MAS 13C NMR along with very minor amount of aliphatic peak centered at ca. 20 ppm, carbon aromaticity being found to be 0.97-0.98. The latter peak corresponds to methyl (32) Pollack, S. S.; Alexander, L. E. J. Chem. Eng. Data 1960, 5, 88. (33) Pollack, S. S.; Yen, T. F.; Erdewan, G. J. Anal. Chem. 1961, 33, 158. (34) Kidena, K.; Murata, S.; Nomura, M. Energy Fuels 1996, 10, 678. (35) Nomura, M.; Matsubayashi, K.; Ida, T.; Murata, S. Fuel Process. Technol. 1992, 31, 169.
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Energy & Fuels, Vol. 11, No. 2, 1997 437
Figure 5. Proposed structural models for MCMB-A (left), KT-A (center), and BZ-A (right). Table 4. Elemental Analyses, Carbon Aromaticity, and Molecular Weight of MCMB and Its Model Structure sample MCMB-A KT-A BZ-A
model observed model observed model observed
formula
C
C47H18O
94.30 94.20 91.49 87.95 90.31 88.39
C63H54O C75H80O
elemental analyses (wt %) H N O 3.03 3.20 6.58 6.48 8.08 8.45
0.00 1.10 0.00 0.94 0.00 0.61
groups attached to aromatic rings. Then, we tried to deconvolute the spectra; however, we could not apply this deconvolution method to the spectra of MCMB, since the chemical shifts of each carbon functional group were very similar to each other. We also tried to conduct Curie-point pyrolytic analysis of MCMB samples at 670 °C for 3 s (our standard conditions for pyrolysis of coal); however, this afforded the volatile fraction in only 10% yield. Therefore, we tried another approach for the construction of chemical structural model for MCMB molecules. The data employed for this approach were listed as follows: (a) elemental analyses of the original and butylated MCMB-A samples (KT-A and BZ-A) shown in Table 1, (b) their carbon aromaticity (from solid and solution state 13C NMR), and (c) molecular sizes of MCMB-A, KT-A, and BZ-A were assumed to be 600, 800, and 1000, respectively (on the basis of the data from GPC analyses). At first, we calculated the tentative molecular formula of MCMB-A, these being C47H19O (MW ) 597). From the value of carbon aromaticity and the above molecular formula of MCMB-A, the numbers of aromatic and aliphatic carbons in the assumed molecule should be 46 and 1, respectively. We assumed that this aliphatic carbon is methyl group (according to 13C NMR spectrum of MCMB-A). The molecular formula of the remaining aromatic moieties should be C46H17O ()C47H19O - CH2), whose H/C atomic ratio was 0.37. Using these data, we estimated average ring size of MCMB molecules. There are two major categories for typical aromatic hydrocarbons, linear and circular orientations. The former has a general molecular formula, C4n+2H2n+4 (where n means number of aromatic rings), while as to the latter, there is no general formula. So, we investigated relationships between number of aromatic carbons, H/C atomic ratio, and aromatic ring size by using nine compounds with a circular orientation (Figure 4). As a result, we obtained the following two empirical equations: n ) 0.39616C - 2.7585 and n ) 146.70 × 10(-2.6251(H/C)), where n, C, and H mean number of aromatic rings, aromatic carbon atoms, and hydrogen atoms contained in the compounds, respectively. Using C (46) and H/C (0.37) of MCMB molecule assumed, the
2.67 1.50 1.93 4.63 1.60 2.55
H/C
fa
MW
fashion of butyln % addition
0.38 0.41 0.86 0.88 1.07 1.14
0.98 0.98 0.67 0.75 0.53 0.55
598 500-700 826 800-1000 996 800-1000
50 53 57 62
average ring size is calculated to be 15.5. On the other hand, if MCMB has a linear orientation structure, ring size and H/C atomic ratio should be 11 and 0.56 (C46H26), respectively, the latter value being very far from the experimental data, 0.37. Consequently, we have judged that MCMB have a circular orientation. As to oxygen-functional groups, we have no information, but there is a possibility that oxygen atoms are existing in heterocyclic compounds by considering the fact that these samples experienced heat history. Therefore, we inserted an oxygen atom into heterocyclic compounds. On the basis of the above discussion, we constructed the structural model for MCMB-A shown in Figure 5. As for the butylated samples, KT-A and BZ-A, at first, number of butyl groups introduced to one molecule of MCMB was calculated to be from 2.8 ()5.9 × 47/100) to 4.0 ()8.6 × 47/100) for KT-A and from 6.9 ()14.9 × 47/100) to 7.8 ()16.4 × 47/100) for BZ-A based on the data shown in Table 2. Consequently, we decided to employ four butyl groups for KT-A and seven groups for BZ-A. The fashion of butyl groups introduced into MCMB molecule, i.e., addition or substitution, could be calculated on the basis of the ratio of intensity of the peak at 27 ppm to that at 30 ppm of their 13C NMR spectra. The resulting models are also shown in Figure 5. Elemental compositions, carbon aromaticities, molecular weights, and % addition are summarized in Table 4, this indicating that the models well represent the experimental observation of these samples. The diameter of the model proposed here was about 12-15 Å, while the mean diameter of MCMB molecules determined by XRD was about 30 Å. These large differences between diameters for the model and existing MCMB molecules could be rationalized by assuming that MCMB molecules have an associated structure shown in Figure 6. Now we are conducting more detailed analyses of the butylated products including determination of real molecular weight by FDMS and simulation of an associative structure of MCMB molecules.
438 Energy & Fuels, Vol. 11, No. 2, 1997
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Although both addition and substitution reactions took place in the above reactions, the ratio of addition to substitution was varied depending on the nature of the reagents employed: with the dibutylzinc-butyl iodide system, reductive butylation was dominant, while with the system consisting of potassium-butyl iodide and THF both reactions took place to a similar extent. 2. Gel permeation chromatographic analyses of the butylated samples indicated that some nonbonding interactions between MCMB molecules would be important even in the polar solvents like N,N-dimethylformamide. On the basis of these analyses, molecular weights of MCMB were found to be around 102-104. Figure 6. Schematic figure for the MCMB molecules.
Summary The results obtained in this study are summarized as follows. 1. Butylation reaction is one of the most effective methods for solubilization of carbonaceous compounds like MCMB. The reaction with dibutylzinc and butyl iodide was slightly more effective than the reaction with potassium, tetrahydrofuran, and butyl iodide systems.
3. On the basis of elemental analyses, molecular weight distribution, and NMR analysis, we constructed average molecular structures for the original and butylated MCMB molecules. These suggest that MCMB molecules have highly condensed aromatic moieties (average ring size is 15-16 along with circular orientation) with very minor amounts of alkyl and oxygen functional groups. EF960108T
