Energy & Fuels 1997, 11, 227-235
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Depolymerization of Lower Rank Coals by Low-Temperature O2 Oxidation Jun-ichiro Hayashi Center for Advanced Research of Energy Technology (CARET), Hokkaido University, N13, W8, Sapporo 060, Japan
Yoshihiro Matsuo, Katsuki Kusakabe, and Shigeharu Morooka* Department of Chemical Science and Technology, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-81, Japan Received June 27, 1996. Revised Manuscript Received October 28, 1996X
Four lower rank coals were oxidized in 0.5 N Na2CO3 aqueous solution, into which atmospheric oxygen gas was bubbled. The reaction was carried out at 20-85 °C, and, after cooling to ambient temperature, the slurry was acidified (pH 1.3) with a 5 N HCl solution. The coals oxidized at 85 °C for 6-24 h were extracted with methanol/tetrahydrofuran mixtures of 25/75 and 50/50 in volume ratio at 30 °C under ultrasonic irradiation. The O2 oxidation dramatically enhanced the extractability of the coals, and extraction yields reached 80-90 wt % daf. Dimethyl sulfoxide and a mixture of benzene and methanol gave similar extraction yields for each oxidized coal. Pyridine showed lower extraction yields in spite of its higher hydrogen-bond-breaking ability. Extractability was, thus, better correlated with the solubility parameter of solvents than with the heat of hydrogen bond complexation. Diffuse reflectance FTIR analysis revealed that the O2 oxidation introduced carboxylic, phenolic, and alcoholic hydroxyls into the coals with a decrease in alkyl groups and aryl-alkyl ethers. This structural change was due to the oxidation of aliphatic carbon-hydrogen bonds to peroxides and subsequent dissociation by acid-catalyzed hydrolysis. The O2 oxidation also converted a portion of the coals to water-soluble acids such as oxalic, formic, acetic, and malonic acids and carbon dioxide. The carbon conversion to CO2 was negligible for the O2 oxidation at 20-50 °C and was 5-9 wt % daf for that at 85 °C.
Introduction Solubilization by Alkylation and Low-Temperature Coal Degradation. Complete solubilization of coal in conventional solvents such as methanol and aromatic liquids at ambient temperature is the ultimate target of coal utilization technology. Coal molecules are bound by carbon-carbon and carbon-oxygen covalent bonds and are physically associated via noncovalent intermolecular forces. Major noncovalent bonds are hydrogen bonding, dipole-dipole interaction, ion pair association, charge-transfer complexation, and π-π bonding.1-3 Although these interactions are much weaker than covalent bonds,4 their cumulative effect is a major determinant of the physical and thermochemical properties of coal. Alkylation, a well-known technique, can solubilize coal by disrupting noncovalent bonds without breaking covalent linkages. O-alkylation, which replaces protons of acidic OH groups by alkyl groups,5 increases pyridine solubility of higher rank bituminous coals (C ) 85-89 wt %) to 50-70 wt % daf.6 Hereafter, “wt % daf” is referred to as “%” unless otherwise noted. The increase * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, December 15, 1996. (1) Liotta, R.; Rose, K.; Hippo, E. J. Org. Chem. 1981, 46, 277. (2) Nishioka, M.; Larsen, J. W. Energy Fuels 1990, 4, 100. (3) Nishioka, M. Fuel 1991, 70, 1413. (4) Arnett, E. M.; Joris, L.; Mitchell, E.; Murty, T. S. S. R.; Gorrie, T. M.; Schleyer, P. v. R. J. Am. Chem. Soc. 1970, 92, 2365. (5) Liotta, R. Fuel 1979, 58, 724. (6) Mallya, M.; Stock, L. M. Fuel 1986, 65, 736.
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in solubility is due to the steric effect of the introduced alkyl groups, which suppress the polarization force between aromatic clusters and eliminate hydrogen bonds. Nonreductive C-alkylation7-9 converts acidic -CH2- groups into -CHR- or -CR2- (R ) alkyl group) groups and thereby increases the pyridine solubility of higher rank coals up to 90% or higher. Reductive C-alkylation introduces alkyl groups directly onto aromatic clusters in coal10-13 and increases the benzene solubility up to 60-95%. Several investigators have claimed that ether bonds are cleaved during reductive alkylation,14 but the steric effect of introduced alkyl chains disrupts noncovalent interactions between aromatic clusters and contributes to the breakdown of the associated macromolecular structure.10 A mixed solvent of carbon disulfide (CS2) and Nmethylpyrrolidinone (NMP) is capable of solubilizing up to 60% of the weight of higher lank bituminous coals.15-18 Sanokawa et al.18 extracted 60% of Upper Freeport coal (7) Miyake, M.; Stock, L. M. Energy Fuels 1988, 2, 815. (8) Cry, N.; Gawlak, M.; Carsin, D.; Ignasiak, B. S. Fuel 1983, 62, 412. (9) Wachowska, H.; Ignasiak, T.; Strauz, O. P.; Carson, D.; Ignasiak, B. S. Fuel 1986, 65, 1081. (10) Sternberg, H. W.; Delle Donne, C. L.; Pantages, P.; Moroni, C. E.; Markby, R. E. Fuel 1970, 50, 432. (11) Sternberg, H. W.; Delle Donne, C. L. Fuel 1974, 53, 172. (12) Ignasiak, B. S.; Gawlak, M. Fuel 1977, 56, 216. (13) Wachowska, H. Fuel 1979, 58, 99. (14) Alemany, L. B.; Stock, L. M. Fuel 1982, 61, 1088. (15) Iino, M.; Takanohashi, T.; Ohsuga, H.; Toda, K. Fuel 1988, 67, 1639.
© 1997 American Chemical Society
228 Energy & Fuels, Vol. 11, No. 1, 1997
(APCS-1) with an equivolume mixture of CS2 and NMP at ambient temperature and were able to increase the extraction yield to 85% by adding a small amount of an electron acceptor, tetracyanoethylene (TCNE). The CS2/ NMP mixed solvent was, however, not effective for lower rank coals, although it was more efficient than pyridine. The following findings10 are also noteworthy: (1) introduction of alkyl groups is not effective for the solubilization of lower rank coals, subbituminous coals, and lignites, which are composed of one or two aromatic clusters; (2) solubility of raw and alkylated coals, except reductively alkylated coals, in benzene, alcohols, and tetrahydrofuran (THF) is much lower than in pyridine or the CS2/NMP mixed solvent. Thus, the solubilization of subbituminous coals and lignites in solvents such as methanol and benzene is nearly impossible without disrupting covalent bonds. Hereby and Neuworth19,20 showed that the -ArCH2Ar- methylene linkage of coal served as the alkylating agent and reacted with phenol in the presence of boron trifluoride, which acts as an acid catalyst. Phenol was alkylated by ArCH2+, and the coal was depolymerized. Ouchi and co-workers21,22 employed a combination of p-toluenesulfonic acid (PTS) and phenol and depolymerized coals ranging from lignite to anthracite at 180 °C. Coals with a carbon content of 5 kcal/ mol or DN > 20. The coal extractability and its solvent dependence were dramatically changed by the oxidation proposed in this study. As described later, the oxidation actually depolymerized the coals by breaking C-C and C-O linkages and simultaneously introduced oxygen functionalities such as carboxylic and phenolic groups. Thus, the covalent cross-link density and molecular weight of the coals were decreased, and the hydrogen bond concentration was increased. The former change (53) Suuberg, E. M.; Otake, Y.; Langner, M. J.; Leung, K. T.; Milosavljevic, I. Energy Fuels 1994, 8, 1274. (54) Szeliga, J.; Marzec, A. Fuel 1983, 63, 1229. (55) van Krevelen, D. W. In Coal, Typology-Physics-ChemistryConstitution, 3rd ed.; Elsevier: Amsterdam, 1993.
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Figure 4. Extract yields from oxidized coal (SB-85-6) as a function of Hildebrand solubility parameter of single and mixed solvents. Solvents: (1) THF; (2) methanol/THF (0.75/ 0.25); (3) methanol/THF (0.5/0.5); (4) methanol/THF (0.75/ 0.25); (5) methanol; (6) pyridine; (7) DMSO; (8) methanol/ benzene (0.5/0.5).
may enhance the extractability, and the latter change may increase the resistivity of the coals to solvent extraction. Mae et al.56 oxidized brown and subbituminous coals in an aqueous H2O2 solution at 25 °C. They found that the oxidation introduced alcoholic OH and carboxylic groups, broke covalent cross-links to some extent, and increased the swellability of the coals in THF, dioxane, and even benzene. This result suggests that the increase in hydrogen bond concentration, accompanied by depolymerization, does not always suppress the swellability. Figure 5 shows the effect of oxidation conditions on extraction yields with the methanol/THF mixed solvent (0.25/0.75 in volume ratio), which gave the maximum extraction values for the coals oxidized at 85 °C for 6 h. Table 2 shows the yields of coal based on the raw coal mass. The extractability increased with increasing oxidation time and temperature. Even an oxidation at 20 °C improved the extractability. For all coals, the oxidation at 85 °C for a period longer than 6 h increased the extraction yield to 70% or more. The extraction yield of the TH-85-24 was 72%, which was less than that of the YL-85-24, SB-85-24, and WY-85-24 samples. Oxidation-Induced Structural Changes in Coals. Figure 6 shows DRIFT spectra of the dried raw coals as well as difference spectra between the raw and oxidized coals. The difference spectra were obtained according to the following procedure: A spectrum of 500-4000 cm-1 was obtained for a raw coal (spectrum A) and the oxidized one (spectrum B) with the same coal concentration, 1 mg of coal/100 mg of KBr. The linearity of absorbance with coal concentration was confirmed in the range of 0.5-1.5 mg/100 mg of KBr. The absorbance in spectrum B was multiplied by the mass yield of the oxidized coal, normalized by the raw coal mass. Spectrum B was then subtracted from spectrum A. The absorption bands at 1720-1730 and 1650-1750 cm-1 assigned to carboxyl and carbonyl groups, respectively, were significantly increased by the oxidation. The absorption bands attributed to phenolic or alcoholic OH (centered at 1200 cm-1) and aldehydes (1390 cm-1) were (56) Mae, K.; Inoue, S.; Miura, K. Energy Fuels 1996, 10, 364.
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Hayashi et al.
Figure 5. Extraction yields from raw and oxidized coals using methanol/THF and methanol/benzene mixed solvents. Table 2. Yield of Oxidized Coals sample
yield, kg/100 kg of daf raw coal
sample
yield, kg/100 kg of daf raw coal
YL-20-6 YL-50-6 YL-85-2 YL-85-6 YL-85-24
104.1 103.0 97.4 90.6 82.1
WY-20-6 WY-50-6 WY-85-2 WY-85-6 WY-85-24
104.5 106.5 103.7 104.6 95.2
SB-20-6 SB-50-6 SB-85-2 SB-85-6
102.7 104.3 99.1 94.8
TC-85-24
96.1
also positive, while the absorption at 2900-3000 cm-1 due to aliphatic C-H was negative. The difference spectrum shows a loss of aliphatic hydrogen and the production of oxygen-containing functionalities such as carboxyls, phenols, and aldehydes. This result is consistent with the oxidation mechanisms shown in Figure 1. The absorption peak at 1280 cm-1, assigned to arylalkyl ethers, appeared only in the raw coal spectra. This confirms that ether linkages were cleaved by the O2 oxidation at temperatures below 100 °C. The broad absorption band of 2700-3600 cm-1 was also changed by the oxidation. The absorbance at 3300-3600 cm-1 in the difference spectrum was negative, while that at 2700-3300 cm-1 was positive. In this range, the wavenumber of absorption is influenced by the strength of hydrogen bonds, as reported by Painter et al.57 The lower wavenumber portion is usually due to carboxyl groups associated with neighboring OH groups via hydrogen bonds and, hence, the increase is due to a net production of carboxyl groups. The decrease in the higher wavenumber portion is appar(57) Painter, P. C.; Sobkowiak, M.; Youtcheff, J. Fuel 1987, 66, 973.
Figure 6. DRIFT spectra of raw coals and difference spectra between oxidized coals and raw coals.
ently related to a decrease in weak noncovalent bonds formed between OH groups and aromatic planes (OHp) and between OH groups and ethers. The difference spectrum also indicates the loss of aromatic CdC bonds and CsH bonds appearing at 1500-1600 and 800-900 cm-1, respectively, although the reaction mechanisms shown in Figure 1 suggest that direct O2 oxidation of aromatic rings is not feasible at temperatures below 100 °C. Figure 7 shows the carbonbased conversion of the coals to water-soluble products. The oxidation at 85 °C for 24 h converted 15, 8.6, and 8.5% of the carbon initially contained in the YL, SB, and WY coals, respectively. Oxalic acid, acetic acid, formic acid, and malonic acid were major components identified by the HPLC analysis, and the sum of their yields was 30-70% of the total water-soluble products on a carbon basis. The formation of acetic and formic acids can be explained by the oxidation of secondary C-H, ethers, and acyl groups, as shown in Figure 1. The formation of oxalic acid and malonic acid would be expected to be related to the reaction routes described below.58-60 Kusakabe et al.59 decomposed humic acids (58) Hoigne´, J. In Process Technologies for Water Treatment; Plenum Press: New York, 1988; p 121. (59) Kusakabe, K.; Aso, S.; Hayashi, J.-i.; Morooka, S.; Isomura, K. Water Res. 1990, 24, 781. (60) Rice, R. G. AIChE Symp. Ser. 1981, 77 (209), 79.
Depolymerization of Lower Rank Coals
Figure 7. Yields of water-soluble products on a carbon basis.
Figure 8. Atomic hydrogen/carbon and oxygen/carbon ratios of oxidized coals, methanol/THF (0.25/0.75) extracts, and residues.
dissolved in water with O3 and found that more than 50% of the total residual carbon was oxalic acid and acetic acid. Rice60 reported that phenol and catecol were converted to diacids such as oxalic acid, maleic acid, and formic acid by O3 oxidation, which formed radical oxidants such as OH, HO2, and HO3 as a result of reactions among OH-, O3, and organic substances. These oxidants are possibly generated in the O2 oxidation from primary peroxides in the condensed matrix of coals and are involved in radical reaction mechanisms, through which aromatic rings are decomposed. Figure 8 shows the H/C and O/C atomic ratios in extracts and residues derived with a methanol/THF (0.25/0.75 in volume ratio) mixed solvent and those in the whole oxidized coals. The O/C ratios in the whole oxidized coals increased with increasing oxidation severity, but the H/C ratios are dependent on the coal samples. The H/C ratio in the WY and SB coals remained unchanged by the oxidation, while that of the YL coal increased. On the other hand, the effect of the severity of the oxidation on elemental composition was different between extracts and corresponding residues.
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The H/C ratio of the extracts decreased, and that of the residues increased, with increasing severity. The O/C ratios of the extracts and residues changed in a reverse manner with increasing severity. The DRIFT spectra shown in Figure 9 indicate that the residues of the YL85-6 and YL-85-24 samples contain more aliphatics and less aromatics and hydroxyls than their extracts and raw coals. Aliphatic C-H is difficult to introduce by the O2 oxidation under the present experimental conditions. The result shown in Figure 9 then suggests that aliphatics refractory to oxidation originally existed in the raw YL coal. The extracts from the oxidized coals contained less aliphatic hydrogen and possessed a larger H/C ratio than those of the raw coal. Thus, O2 oxidation introduced hydroxyl and carboxyl hydrogens at the expense of initial aliphatic hydrogens. Figure 10 shows the distributions of carbon and hydrogen in oxidation products. As indicated in Figure 10a, carbon in the raw coals was converted to extract, residue, water-soluble acids, and CO2. The yield of CO2 was not directly determined, because the oxidation was performed in a Na2CO3 solution, and it was calculated from the carbon balance. No gaseous products except CO2 were detected by gas chromatography. The formation of CO2, the final product of hydrocarbon oxidation, suggests that the radical mechanisms discussed above occurred to some extent. The loss of carbon as CO2 was negligible in the oxidation at 20-50 °C for 2-6 h, while 5-9% of the total initial carbon was converted to CO2 by the oxidation at 85 °C for 6-24 h. In any case, however, the carbon conversion to CO2 was much less than in conventional O2 oxidation processes at elevated temperatures and pressures.32-35 The cumulative hydrogen contents shown in Figure 10b were calculated from the amount of hydrogen in oxalic, formic, maleic, and acetic acids and by assuming that the H/C ratio of unidentified water-soluble components was unity. Hydrogen loss as H2O was not considered. Elution volumes of several peaks detected by the HPLC did not agree with those of postulated compounds: propionic, maleic, tartaric, succinic, glycolic, and pyruvilic acids. Benzenecarboxylic acids [C6H6-n(COOH)n, n ) 1-4) were not detected. The total amount of hydrogen in the oxidized products from the WY and SB coals was nearly equal to that in the raw coals. For the YL coal, however, the total amount of hydrogen after the oxidation exceeded the value of the raw coal. This cannot be explained by the oxidation mechanisms discussed above. The increase in hydrogen content suggests that hydrogen was incorporated from the reaction medium into the coal during peroxide formation or the hydrolysis period, but no experimental data have been collected relevant to this at this time. Oxygen Consumption. The amount of oxygen consumed in the O2 oxidation process is an important measure of the extent of covalent bond breaking. Figure 11 presents the relationship between oxygen consumption and total carbon conversion to extract, water solubles, and CO2. The amount of oxygen consumed to form H2O and unidentified acids was neglected. The data for the YL, SB, and WY coals are correlated by a sigmoid curve. The efficiency of oxidation can be calculated from the amount of converted carbon per unit amount of consumed oxygen on molar basis and is evaluated from the slope of the line. When the amount of oxygen consumed was
