Structural Changes of Alcohol-Solubilized Yallourn Coal in the

Energy & Fuels 1998, 12, 503-511

503

Structural Changes of Alcohol-Solubilized Yallourn Coal in the Hydrogenation over a Ru/Al2O3 Catalyst Takaaki Isoda, Hideyuki Takagi, Katsuki Kusakabe, and Shigeharu Morooka* Department of Materials Physics and Chemistry, Graduate School of Engineering, Kyushu University, Fukuoka 812-8581, Japan Received August 11, 1997

Yallourn coal was oxidized in aqueous H2O2 in the presence of 1-propanol at 70 °C for 6 h under atmospheric pressure. The coal was partially decomposed by the OH radicals formed from H2O2, and carboxy and methylene groups originating from H2O2 and 1-propanol were added to the coal structure. As a result, the solubility of the coal in ethanol was increased to 80 kg per 100 kg of the raw coal. The H/C ratio, which was 0.95 for the raw coal, was increased to 1.07 by the oxidization in the presence of 1-propanol. The ethanol-soluble fraction of the oxidized coal was then hydrogenated using a Ru/Al2O3 catalyst in a mixed solvent of ethanol and acetic acid at 120 °C for 12-72 h at a hydrogen pressure of 10 MPa. After the hydrogenation, a yellowish white solid (hereafter referred to as hydrogenated white coal) was obtained. Major gaseous products were CO and CO2, and the yield of these gases produced by the hydrogenation in the presence of acetic acid was less than 3 kg per 100 kg of raw coal. The H/C ratio of the hydrogenated coal increased with increasing hydrogenation time and amount of acetic acid and was 1.23 after a 72 h reaction. The hydrogenation also decreased the total nitrogen content in the raw coal. The denitrogenation increased with increasing hydrogenation time and reached 60% after 72 h. It is noteworthy that this conversion was attained by a reaction at 120 °C. The IR peaks at 3000 and 2900 cm-1, assigned to methylene group, were increased by the hydrogenation. Further structural analyses of the hydrogenated white coal indicated that aromatic rings were changed to saturated rings, i.e., that the sp2 bonding structure of the coal was transformed to the sp3 bonding structure. In order to better understand the hydrogenation mechanism, benzyl alcohol, benzoic acid, phenol, and toluene were hydrogenated as model compounds, representing the structure of the alcohol-soluble fraction of the oxidized coal. The addition of carboxylic acids greatly enhanced the hydrogenation of aromatic rings in the model compounds over the Ru/Al2O3 catalyst. The molecular orbital calculation, based on the WinMOPAC program, suggested that dipole moments and charges on the oxygen atoms of carboxylic acids played important roles in the above transformation of the coal structure.

Introduction al.1

Mae et demonstrated that a mild oxidation with H2O2 substantially increased the yields of soluble products from Yallourn coal. They also reported that oxidation of Yallourn coal with H2O2 at 60 °C produced low molecular weight fatty acids at yields in excess of 50 wt %. Coal molecules are composed of covalent bonds, such as carbon-carbon and carbon-oxygen bonds, and, in addition, contain noncovalent interactions. Previous investigators2-4 have reported that noncovalent bonds cumulatively influence the physical * Author to whom correspondence should be addressed. Telephone: +81-92-642-3551. Fax: +81-92-651-5606. E-mail: smorotcf@ mbox.nc.kyushu-u.ac.jp. (1) Mae, K.; Inoue, S.; Miura, K. Energy Fuels 1996, 10, 364. (2) Hayashi, J.-i.; Matsuo, Y.; Kusakabe, K.; Morooka, S. Energy Fuels 1997, 11, 227. (3) Hayashi, J.-i.; Mizuta, H.; Kusakabe, K.; Morooka, S. Energy Fuels 1994, 8, 1353. (4) Hayashi, J.-i.; Matsuo, Y.; Kusakabe, K.; Morooka, S. Energy Fuels 1995, 9, 284. (5) Isoda, T.; Tomita, H.; Kusakabe, K.; Morooka, S. Int. Conf. Coal Sci. 1997, 581. (6) Sato, Y.; Kamo, T.; Yamamoto, K.; Inaba, T.; Miki, K. J. Jpn. Pet. Inst. 1991, 34, 327.

and chemical structure and, as a result, the pyrolysis reactivity of coal. Thus, the disruption of these bonds prior to coal conversion is an effective pathway to high yields. Mae et al.1 found that lower rank coals after oxidation with an aqueous solution of H2O2 at 40-80 °C were converted to alcohol- and water-solubles in a yield of 70 wt %. The oxidized coals exhibited a higher pyrolysis reactivity than the raw coals. Hayashi et al.2 proposed a liquid phase oxidation of lower rank coals. They suspended coal particles in a weakly alkaline aqueous solution through which atmospheric oxygen was bubbled at 20-85 °C. The oxidation dramatically increased the yield of extractables, via the introduction of carboxylic and phenolic hydroxy groups into coal fragments and by decomposing aliphatic C-H and ether (7) Ueda, K.; Matsui, H.; Song, C.; Xu, W. J. Jpn. Pet. Inst. 1990, 33, 413. (8) Wiser, W. H.; Singh, S.; Qader, S. A.; Hill, G. R. W. Ind. Eng. Chem. Prod. Res. Dev. 1970, 9, 350. (9) Nishimura, S. Bull. Chem. Soc. Jpn. 1959, 32, 1155. (10) Freifelder, M.; Stone, G. R. J. Org. Chem. 1962, 27, 3568. (11) Smith, H. A.; Fuzek, J. F. J. Am. Chem. Soc. 1949, 71, 415. (12) Takanohashi, T.; Iino, M.; Nakamura, K. Energy Fuels 1994, 8, 395.

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504 Energy & Fuels, Vol. 12, No. 3, 1998

bonds. By treatment with molecular oxygen in the presence of peracetic acid at 40 °C for 96 h, approximately 80 wt % of a bituminous coal and 77 wt % of a lignite were converted to methanol-soluble acids and CO2. In an earlier study,5 we found that the addition of lower alcohols, such as methanol, ethanol, propanol, or butanol, considerably increased the yield of methanolsoluble materials during H2O2 oxidation. Yallourn coal was suspended in an aqueous solution of H2O2 and lower alcohols and was then subjected to oxidization at 70 °C for 6-12 h. During the oxidation, 5-6 mol % of carbon was consumed, mainly as CO2, and carboxy groups and alcohol-derived alkyl groups were introduced into the coal. The swelling of the coal by alcohols was assumed to contribute to the efficiency of the oxidation. Aromatic rings of the coal were cleaved, to some extent, by the oxidation, but the results of pyrolysis suggested that this was not a major factor for pyrolysis reactivity. Hayashi et al.2 characterized the extracts derived from O2-oxidized Yallourn coal. The O2 oxidation caused depolymerization of the coal, breaking C-C and C-O linkages, and introducing oxygen-containing functional groups. The covalent cross-link density and the molecular weight of the coal were decreased, and the hydrogen bond concentration was increased, as the result of O2 oxidation. The average ring number of aromatic clusters produced was two at most. In the cracking of polyaromatics, such as naphthalene,6 phenanthrene,7 and anthracene,8 the reactivity of the substrates is greatly enhanced when they are hydrogenated prior to the reaction. The large bonding energy of aromatic rings is lessened by reducing CdC bonds to C-C bonds. This suggests that the cracking reactivity of coal is also enhanced by the prior hydrogenation of aromatics in the coal structure. Alcohol extracts from mildly oxidized coals can be readily hydrogenated using noble metal catalysts such as ruthenium, palladium, and platinum. Since these catalysts possess high hydrogenation activity for polyaromatics, the reaction temperature can be reduced to 70-150 °C.9-11 In the present study, Yallourn coal was oxidized under mild conditions in aqueous H2O2 in the presence of 1-propanol and then extracted with ethanol. Using this procedure, the coal was solubilized without major changes in the unit structure, as has been reported previously.2,5 The ethanol-soluble fraction of the oxidized coal was then hydrogenated at 120 °C over a Ru/ Al2O3 catalyst, assuming that the sp2 bonds in the coal structure would be largely transformed to sp3 bonds. The change in coal structure before and after the hydrogenation was analyzed by elemental analysis, diffuse reflectance Fourier transform infrared spectroscopy (DRIFT), gel permeation chromatography (GPC), and Raman spectroscopy. Structural changes were also evaluated based on products from the flash pyrolysis of the raw and modified coals using a Curie-point pyrolyzer (CPP). Model compounds were also hydrogenated, and the mechanism of the hydrogenation was discussed based on molecular orbital calculation. The objectives of the present study are to develop a procedure for the catalytic hydrogenation of the alcohol-solubilized coal and to investigate the effects of hydrogenation in altering the structure of the coal. Optimization of

Isoda et al.

Figure 1. Oxidation and fractionation procedure.

reaction conditions as well as catalyst and process design for hydrogenation of coal are our future targets. Experimental Section H2O2 Oxidation and Alcohol Extraction. Figure 1 illustrates the procedure used for the oxidation and ethanol extraction of the coal. Yallourn coal (C ) 62 wt %, H ) 4.9 wt %, N ) 0.5 wt %, daf) was pulverized to give particles 74-125 µm in size, and 1-4 g of the coal was oxidized in a mixture of 30 wt % H2O2 (2-8 mL) and 1-propanol (5-20 mL) at 70 °C for 6 h with stirring under nitrogen at atmospheric pressure. The H2O2 oxidation procedure is based on that developed by Mae et al.1 The coexistence of alcohols, as developed by the previous study,1 was effective in increasing the solubility of oxidized coal.5 In the present study, ethanol was used as the solvent since methanol slightly inhibited the Ru/Al2O3 catalyst used in the subsequent hydrogenation step. The product (hereafter, referred to as oxidized coal) was extracted with ethanol under ultrasonic irradiation at room temperature, and the mixture was separated into ethanolsoluble (ES) and -insoluble (EI) fractions by centrifugation. The ES fraction was only lightly evaporated, since exhaustive evaporation gave a material which could not be completely redissolved in ethanol. The EI fraction was exhaustively dried at 70 °C for 6 h under vacuum and weighed. Hydrogenation of Ethanol Extract. Figure 2 shows the procedure used for hydrogenation of the ES fraction from the oxidized coal. A ruthenium-supported alumina powder catalyst (Ru/Al2O3, Wako Chemical Ind., metal content ) 5 wt %) was used as the hydrogenation catalyst. A Pt/Al2O3 catalyst was strongly inhibited by alcohols. The hydrogenation of the ES fraction, recovered from the oxidized coal, was carried out in a 25 mL autoclave equipped with a magnetic stirrer. One gram of the ES fraction was dissolved in a mixture of 0-8 mL of acetic acid and 6 mL of ethanol and was hydrogenated using 0.5 or 1.5 g of the catalyst at 120 °C for 12-72 h under a hydrogen pressure of 10 MPa. The product was recovered using a mixture of THF and methanol, and the catalyst was separated by centrifugation. After the removal of the solvent, the product (hereafter referred to as hydrogenated coal) was extracted with water under ultrasonic irradiation to give a water-soluble and a water-insoluble fraction. The former fraction is referred to as water-soluble organics (WSO), and

Structural Changes of Alcohol-Solubilized Yallourn Coal

Energy & Fuels, Vol. 12, No. 3, 1998 505 Windows 95 (Fujitsu Inc.), Molecular Orbital Package, and Parametric Method 3.14

Results

Figure 2. Hydrogenation and fractionation procedure. the latter fraction is referred to as the hydrogenated ethanolsoluble fraction (H-ES). Each fraction was dried at 70 °C for 6 h under vacuum and then weighed. Analysis of Products and Coal Structure. Gaseous products evolved in the oxidation and hydrogenation reactions were qualitatively analyzed using a GC-FID equipped with a methanizer, by which CO and CO2 were converted to methane.3,4 The H/C atomic ratio and nitrogen content in the raw coal, oxidized coal (i.e., ES + EI), and hydrogenated coal (i.e., H-ES + WSO + EI) were determined by elemental analysis. Liquid and solid products as well as the raw coal were characterized by DRIFT, GPC, and Raman spectroscopy. In the case of GPC analysis of ES and H-ES fractions, the stationary phase was a styrene-divinylbenzene gel (Showa Denko K. K., Shodex KD-806M), and the mobile phase was N,N-dimethylformamide mixed with 0.01 mol/L of LiBr, flowing at a rate of 0.5 mL/min. Molecular weight was calibrated using polystyrene standards at 50 °C. Takanohashi et al.12 reported that the molecular weights of coal extracts were decreased by the addition of LiBr in the mobile phase. The strong electrolyte would be expected to disintegrate the aggregated coal molecules as well as prevent deactivation of the column. In the case of Raman spectroscopy analysis, a tablet, which was prepared by pressing each coal sample at 30 MPa, was measured 20 times by changing locations for 5 s each, using a Laser Raman spectrometer (JASCO, NRS-2000) equipped with an argon laser of 514.2 nm. Changes in the coal structure, induced by oxidation and hydrogenation, were evaluated based on products of flash pyrolysis using a Curiepoint pyrolyzer (Japan Analytical Ind., JHP-22) at 764 °C. Details of the CPP have been reported previously.13 Hydrogenation of Model Compounds. Toluene, phenol, benzyl alcohol, and benzoic acid (Wako Chemical Ind.) were used as the model compounds, representing the coal structure. A 3 g portion of each substrate was mixed with 0-6 mL of alcohol (methanol, ethanol, or 1-propanol) and 0.5 g of the Ru/ Al2O3 catalyst and was hydrogenated in a 50 mL autoclave at 120 °C for 0.5 h under a hydrogen pressure of 6 MPa. In order to investigate the effect of additives on hydrogenation of the ES fraction, 1.5 g of formic acid, acetic acid, buthanoic acid, or dodecanoic acid (Wako Chemical Ind.) was added to the reaction system. Products were qualitatively and quantitatively analyzed by GC-FID and GC-MS, equipped with a capillary column. Computation. Dipole moment and charge on oxygen atoms in carboxylic acids were estimated by molecular orbital calculation, using the WinMOPAC program Version 1 for (13) Hayashi, J.-i.; Kawakami, T.; Kusakabe, K.; Morooka, S. Energy Fuels 1993, 7, 1118. (14) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 221.

Product Distributions in Oxidation and Hydrogenation Reactions. Figure 3 shows photographs of the raw coal (A), oxidized coal (B), and H-ES fractions (C-E). The oxidation was performed using 1 g of the raw coal. The H-ES fractions were prepared by hydrogenating the ES fraction for (C) 12 h, (D) 24 h, and (E) 72 h, using 6 mL of ethanol, 8 mL of acetic acid, and 1.5 g of the Ru/Al2O3 catalyst. The hydrogenated coal was a yellowish white powder, and the color changed with increasing hydrogenation time. The H-ES fraction is shown in the photograph does not contain the WSO and EI fractions. Table 1 shows product distributions in the oxidized and hydrogenated coals. The oxidation produced ES, EI, and OG fractions, and the hydrogenation provided the H-ES, HG, and WSO fractions. The total yield of hydrogenation was defined as the sum of the EI and OG yields from the oxidation and the H-ES, HG, and WSO yields from the hydrogenation. The major purpose of this study is to establish a method for evaluating the structural changes which occur in coal during catalytic hydrogenation. Hence, the H2O2 oxidation was carried out under fixed conditions, at 70 °C for 6 h, as employed by Mae et al.,1 and the conditions of the hydrogenation step were varied over a wide range. The amount of the ES fraction, based on the mass of the raw coal, was dramatically increased by H2O2 oxidation in the presence of 1-propanol, from 2.9 wt % of the raw coal to 80.6 wt % of the oxidized coal. The total yield of the oxidized coal reached 126.3 wt %, which exceeded 100% of the raw coal mass, since alkyl and ester groups derived from 1-propanol, as well as carboxy groups oxidized by H2O2, were introduced into the coal fragments.5 However, the total material balance for the oxidation step is not intensively discussed in this study since the ES fraction was not exhaustively dried as described in the experimental section. The yield of the EI fraction was in the range of 23-48 wt %, although the oxidation was carried out under identical conditions. This probably arises from uncertainties in the extraction step, due to the small amount of coal used in the experiment. The yield of the OG fraction was 4-17 kg per 100 kg of raw coal under representative conditions used. The major product was CO2 in a yield of 4-16 kg/100 kg of raw coal, along with CO in a yield of 0.2-0.7 kg/100 kg of raw coal. Mae et al.1 reported that lower rank coals were decomposed by oxidation in an aqueous H2O2 solution, and that the THF/methanol-soluble yield reached 80-90 wt % after oxidation at 70 °C for 7-12 h. The total yield of gases as the result of H2O2 oxidation exceeded 20 wt %. The yield of the OG fraction in the present study is 18-22 wt %, which is consistent with that reported by Mae et al.1 When the ES fraction, which was produced by oxidizing 4 g of raw coal, was hydrogenated using 0.5 g of the Ru/Al2O3 catalyst in 6 mL of ethanol, the yield of H-ES fraction was 70-80 wt %, regardless of hydrogenation time and the amount of acetic acid. When the ES fraction obtained from 1 g of raw coal was hydrogenated


Isoda et al.

Figure 3. Photographs of raw, oxidized, and hydrogenated coals: (A) raw coal; (B) oxidized coal; (C) hydrogenated coal (reaction time 12 h); (D) hydrogenated coal (reaction time 24 h); (E) hydrogenation coal (reaction time 72 h). Table 1. Product Distributions from Oxidation and Hydrogenation of Coal hydrogenation conditions (120 °C, H2 10 MPa) sample raw coal oxidized coal hydrogenated coal hydrogenated coal hydrogenated coal hydrogenated coal hydrogenated coal hydrogenated coal hydrogenated coal hydrogenated coal hydrogenated coal hydrogenated coal hydrogenated coal hydrogenated coal hydrogenated coal hydrogenated coal hydrogenated coal

reaction time (h)

12 24 48 72 12 24 48 72 24 48 72 12 24 48 72

Ru/Al2O3 (g)

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 1.5 1.5 1.5 1.5

yields (kg/100 kg of raw coal)

acetic acid (mL)

coal (g)

H-ESf

EIb

CO2

OGc CO total

2 2 2 2 4 4 4 4 0 0 0 8 8 8 8

4 4 4 4 4 4 4 4 4 4 4 4 4 1 1 1 1

2.9 80.6 73.5 59.4 63.9 73.1 71.5 74.0 70.3 73.5 79.7 75.4 80.8 45.8 37.3 38.9 34.9

96.6 27.9 32.7 48.6 47.4 33.9 32.6 32.5 34.1 34.2 28.0 31.6 33.7 22.0 30.8 23.4 25.9

16.8 4.1 9.1 4.0 3.7 6.7 5.4 9.6 4.0 12.4 16.3 7.1 6.2 5.5 5.1 5.1

1.0 0.2 0.5 0.2 0.2 0.4 0.3 0.6 0.2 0.7 0.9 0.4 0.4 0.3 0.4 0.4

ESa

17.8 4.3 9.6 4.2 3.9 7.1 5.7 10.2 4.2 13.1 17.2 7.5 6.6 5.8 5.5 5.5

CO2

0.3 0.4 0.3 0.7 0.4 0.4 0.5 0.6 0.8 0.8 0.9 0.5 0.4 0.7 0.7

HGd CO total

0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.1 0.1 0.0 0.1 0.2 0.2 0.3

0.3 0.4 0.4 0.7 0.4 0.4 0.6 0.6 0.9 0.9 0.9 0.6 0.6 0.9 1.0

WSOe

total

6.1 4.1 3.1 4.1 8.2 6.8 5.6 3.9 3.4 4.1 1.8 16.1 17.7 17.7 16.4

99.5 126.3 116.9 122.1 119.0 115.7 119.8 119.4 120.8 116.4 125.1 129.2 124.7 91.1 92.2 86.4 83.7

a Ethanol-soluble fraction from oxidation. b Ethanol-insoluble fraction from oxidation. c Gaseous products from oxidation. products from hydrogenation. e Water-soluble organics from hydrogenation. f Ethanol-soluble fraction from hydrogenation.

using 1.5 g of the catalyst in a mixture of 6 mL of ethanol and 8 mL of acetic acid, however, the H-ES yield decreased to 35-46 wt %. A considerable amount of the WSO fraction was produced in the latter case. The major components of the WSO have been reported to be oxalic acid, acetic acid, formic acid, and malonic acid.2 Characterization of Hydrogenated Coal. Figure 4 shows H/C atomic ratios of the hydrogenated white coals, calculated by elemental analysis, as a function of hydrogenation time. H2O2 Oxidation in the presence of 1-propanol resulted in an increase in H/C atomic ratio of the raw coal to 1.07 from 0.95. The H/C ratio was further increased to 1.18 after 12 h of hydrogenation, 1.22 for 48 h, and 1.23 for 72 h, when the ES fraction

d

Gaseous

produced from 1 g of the raw coal was hydrogenated using 1.5 g of the Ru/Al2O3 catalyst in a mixture of 6 mL of ethanol and 8 mL of acetic acid. The addition of acetic acid promoted the hydrogenation reaction of the oxidized coal, as is shown in Figure 4. The solubility of the oxidized coal in ethanol does not change unless acetic acid participates in the reaction. We confirmed that the oxidized coal was insoluble in acetic acid. Figure 5 illustrates GPC profiles of the ES fraction from the oxidized coal and the H-ES and WSO fractions from the hydrogenated white coal. The hydrogenation was performed for 72 h, using 1.5 g of the Ru/Al2O3 catalyst in a mixture of 6 mL of ethanol and 8 mL of acetic acid. The ES fraction was recovered from the


Energy & Fuels, Vol. 12, No. 3, 1998 507

Figure 7. Nitrogen distribution in oxidized and hydrogenated coals.

Figure 4. H/C atomic ratio of hydrogenated coal as a function of hydrogenation time.

Figure 8. Oxygen content as a function of H/C atomic ratio in raw, oxidized, and hydrogenated coals.

Figure 5. Molecular weight distribution of ES, H-ES, and WSO fractions.

Figure 6. Nitrogen content as a function of H/C atomic ratio in raw, oxidized, and hydrogenated coals.

oxidized coal which was produced from 1.0 g of the raw coal. The other conditions were the same as described in the Experimental Section. The molecular weight at the peak of H-ES and WSO was approximately 5 000, which was smaller than that at the peak of the ES, 10 000. This indicates that the ES fraction was slightly cracked as a result of hydrogenation over the Ru/Al2O3. Figure 6 shows the nitrogen content in the oxidized and hydrogenated coals as a function of the H/C atomic ratio. The nitrogen content of the oxidized coal is the sum of the contents in the ES and EI fractions, whereas

the nitrogen content of the hydrogenated coal is the sum of the contents in the H-ES and WSO fractions produced by the hydrogenation and the EI fraction produced by the oxidation. The hydrogenation was performed under the same conditions as in Figure 5 except for the reaction time, which was varied in the range of 24-72 h in this case. Denitrogenation slightly proceeded as the result of H2O2 oxidation in the presence of 1-propanol, and the nitrogen content was 0.5 wt % for the oxidized coal with an H/C ratio ) 1.1. However, the hydrogenation increased denitrogenation, and the nitrogen content was 0.45 and 0.3 wt % for the hydrogenated coals with H/C ratios ) 1.25 and 1.3, respectively. These results suggest that C-N bonds in coal are effectively cleaved by hydrogenation at 120 °C. Figure 7 illustrates the distribution of nitrogen in the oxidized and hydrogenated products. The hydrogenation conditions are the same as in Figure 6. Nitrogen in the oxidized coal was decreased to 88% of the raw coal and was distributed in the ES and EI fractions at 65% and 23%, respectively. Hydrogenation for longer times decreased the nitrogen content in the H-ES fraction and increased that in the WSO fraction. The total yield of the hydrogenated coal, which had a nitrogen content of 41 mol per 100 mol of nitrogen in the raw coal, was 86 kg per 100 kg of raw coal. The variation of the nitrogen content in the EI fraction can be attributed to experimental scattering. Figure 8 shows the oxygen content as a function of H/C atomic ratio in the raw, oxidized, and hydrogenated coals determined by elemental analysis. The raw coal contained 32 wt % oxygen on the daf basis. Oxidation slightly increased the oxygen content to 34 wt % on the daf basis, and the oxygen content was not further altered by hydrogenation. In contrast, H/C atomic ratio was increased as the result of hydrogenation. This


Isoda et al.

Figure 9. Raman spectra of raw (a), oxidized (b), and hydrogenated (c) coals.

Figure 10. DRIFT spectra of raw (a), oxidized (b), and hydrogenated (c) coals.

indicates that the deoxygenation reaction of oxygen functionalities in coal did not occur during the hydrogenation reaction. Figure 9 shows Raman spectra of the raw, oxidized, and hydrogenated coals. The hydrogenation was performed under the same conditions as in Figure 4. Two peaks appear for the raw coal at 1400 cm-1 and 16001620 cm-1, which are assigned to sp2 and sp3 carbons in the coal structure, respectively. The hydrogenation of the oxidized coal reduced the intensity of the peak at 1400 cm-1, while oxidation reduced that at 1600-1620 cm-1. This suggests that aromatic sp2 bonding was decreased by the hydrogenation. Figure 10 illustrates the DRIFT spectra of the raw, oxidized, and hydrogenated coals. The hydrogenation was performed under the same conditions as in Figure 4. A sharp peak at 3000 cm-1, assigned to methylene bonds in the coal structure, was increased by the H2O2 oxidation in the presence of 1-propanol. This suggests that alkyl groups originating from 1-propanol were introduced to coal fragments.5 The intensity of the methylene peak was increased by hydrogenation, suggesting the conversion of CdC bonds to C-C bonds in coal. Mae et al.1 reported that carboxy groups were introduced into the coal structure by a liquid-phase H2O2 oxidation. A sharp peak at 1700 cm-1, which is

Figure 11. Product yields from pyrolysis of raw, oxidized, and hydrogenated coals vs H/C atomic ratio: (a) char, (b) tar, (c) CO2, (d) CO, (e) H2O, (f) H2, and (g) BTX.

assigned to CdO bond of carboxy groups in the hydrogenated coal, indicates that no carboxy groups on aromatic rings were hydrogenated or hydrocracked over the Ru/Al2O3 catalyst under the present conditions. Small peaks at 1500 and 1550 cm-1, assigned to the cycloalkane structure, are observed, especially for the case of hydrogenated white coal. Figure 11 shows the product yields from flash pyrolysis of the raw, oxidized, and hydrogenated coals as a function of H/C ratio. The yields are expressed based on the mass of the raw coal. Hayashi et al.4 reported that changes in coal structure, especially changes in the



Table 2. Hydrogenation Conversion of Model Compounds in Alcohols over Ru/Al2O3 Catalysta substrate

methanol

ethanol

1-propanol

toluene (1)b phenol (2)b benzyl alcohol (3)b benzoic acid (4)b

95 100 87 100

100 100 100 100

100 100 100 100

a 120 °C, 0.5 h, Ru/Al O :substrate ) 0.5:3, solvent 6 mL. b See 2 3 Chart 1 for structure.

Chart 1 OH 1

OH 2

O CH2

OH

C H

3

O C H O C H 4

extent of hydrogen bonding of oxygen functionalities, affected the yields of pyrolysis products such as char, tar, CO2, CO, and H2O. Since secondary cracking reactions of volatiles are suppressed in the flash pyrolysis using the Curie-point pyrolyzer, product distributions are related to the changes in the coal structure. As shown in Figure 11a, the char yield based on 100 kg of raw coal was 48 kg for the raw coal (H/C ) 0.95) and 35 kg for the oxidized coal (H/C ) 1.07). After hydrogenation, the char yield was 17 kg, and the H/C ratio was 1.28. The tar yield was significantly increased by the hydrogenation as shown in Figure 11b, and the major pyrolysis product was changed from char for the raw coal to tar for the hydrogenated coal. This suggests that aromatic rings were effectively hydrogenated by this treatment. Figure 11c-g shows the yield of CO2, CO, H2O, H2, and BTX from pyrolysis of the raw, oxidized, and hydrogenated coals. It is known that CO2 and CO are formed by decomposition of carboxy and ketone groups and that H2O is produced from hydroxy groups in the coal structure. Hayashi et al.4,13 reported that yields of CO2 and H2O could be sensitively varied by preheating4 as well as O-alkylation13 of coals. In the present study, the raw coal produced 10 kg of CO2, 7 kg of CO, and 12 kg of H2O. The oxidation increased the CO2 and CO yields to 20 and 11 kg, respectively. This suggests that the oxygen functionalities were introduced by the oxidation using H2O2, as reported by Mae et al.1 Hydrogenation, however, suppressed yields of CO2, CO, and H2O to 8, 3, and 7 kg, respectively. The reduction of these gases suggests the suppression of char formation as the result of a decrease in oxygen functionalities, which lead to the formation of linkages in the pyrolysis. Yields of H2 and BTX remained unchanged before and after hydrogenation, indicating no dehydrogenation occurred in the flash pyrolysis. In this study, oxidized coal solubilized in ethanol was hydrogenated over a Ru/Al2O3 catalyst in the presence of acetic acid. The fact that the hydrogenation did not increase the yields of CO2, CO, and hydrocarbon gases

Figure 12. Effect of additives on hydrogenation of benzyl alcohol over Ru/Al2O3 catalyst in ethanol.

in the flash pyrolysis reaction suggests that neither acetic acid nor ethanol was incorporated into the coal structure. Hydrogenation of Model Compounds in Alcohol. Table 2 shows hydrogenation conversions of toluene, phenol, benzyl alcohol, and benzoic acid, used as the model components of the oxidized coal, over the Ru/ Al2O3 catalyst. The hydrogenation reaction was performed using 3 g of each substrate and 0.5 g of catalyst in 6 mL of methanol, ethanol, or 1-propanol. The other conditions are the same as described in the experimental section. Toluene, phenol, benzyl alcohol, and benzoic acid carry methyl, hydroxy, hydroxymethyl, and carboxy groups, respectively, and they are thought to be components of the coal structure. Hydrogenation reaction of these compounds under the same conditions as with the oxidized coal may provide information on the reactivity of coal peripherals. Chart 1 shows the hydrogenation pathways of the model compounds. Under the conditions employed in the present study, the aromatics with hydroxy and carboxy groups were hydrogenated by the following two steps: (1) hydrogenation of hydroxy, hydroxymethyl, and carboxy groups to methyl group, followed by (2) hydrogenation of aromatic rings. The Ru/Al2O3 catalyst exhibited an excellent activity for hydrogenating these aromatic structures. Figure 12 illustrates the effect of carboxylic acids on the hydrogenation reactivity and product distributions of benzyl alcohol over the Ru/Al2O3 catalyst. The reaction was performed using 0.5 g of catalyst, 3 mL of benzyl alcohol, and 1.5 mL of each additive. Methylcyclohexane and cyclohexylaldehyde were found as the major products. Thus the hydrogenation is assumed to proceed as (1) hydrogenation of benzene ring and dehydrogenation of the hydroxymethyl group via an intermediate; and (2) dehydrogenation of the hydroxymethyl group, hydrodeoxygenation of the carboxy group, and hydrogenation of benzene ring in series, as shown in Chart 1. The addition of carboxylic acids enhanced the hydrogenation reactivity. The yield of methylcyclohexane was 40 and 50% in the presence of acetic acid and butanoic acid, respectively, but was only 10% in the absence of these acids. Acetone had essentially no effect on the hydrogenation reaction of benzyl alcohol, while formic acid inhibited the reaction. Added carboxylic acids were not decomposed under the conditions used, whereas the benzyl alcohol was selectively hydrogenated by the Ru/Al2O3 catalyst. The oxidized and ethanolsolubilized coal, which carry aromatic structures, may be hydrogenated via similar pathways as was found for the model compounds. The role of added acetic acid in


Figure 13. Calculated dipole moment and charge on the oxygen atoms in carboxy group of carboxylic acid.

the reaction is to enhance the hydrogenation of the benzene ring to a cyclohexane ring in the coal structure. Simulation of the Effect of Additives on Hydrogenation. Figure 13 shows the dipole moment and charge on oxygen atoms of carboxylic acids, calculated using the WinMOPAC program, as a function of the number of carbon atoms in the alkyl group of the carboxylic acids. The normalized dipole moment is 1.5 for formic acid (carbon number in alkyl chain ) zero) and 4.3 for acetic acid (carbon number in alkyl chain ) 1). It remains unchanged for acids with larger carbon numbers. The charge on the oxygen of a ketone and a hydroxy group is increased to -0.28 and -0.33 eV, respectively, for carboxylic acids having carbon numbers larger than 2. These results indicate that the addition of carboxylic acids increases the polarity of the solvent. Discussion Structure of Hydrogenated White Coal. Mae et al.1 reported that lower rank coals, oxidized with aqueous H2O2, were composed of 1-2-ring aromatics, substituted with carboxy and hydroxy groups and connected by methylene groups. The molecular weight was estimated to be 2000-5000, and the linkages between clusters in the coal structure were cleaved by OH radicals originating from H2O2 at the initial stage of the oxidation. On the other hand, the present findings show that alkyl groups are introduced into coal clusters when lower alcohols are added in the H2O2 oxidation step.5 The unique features of the hydrogenation developed in the present study are (1) substantially no gaseous products, such as CO, CO2, and C1-C4 hydrocarbons, are produced; (2) the IR peak at 1700 cm-1, assigned to CdO bond in a carboxy group, remains unchanged before and after the hydrogenation; (3) the hydrogen content in the coal is increased as a result of an increase in methylene and cycloalkane components; and (4) sp2 carbon in the raw coal is reduced by the hydrogenation. It should also be noted that product distributions from flash pyrolysis were greatly changed after the hydrogenation. The catalytic hydrogenation of the oxidized coal markedly suppresses char formation and enhances tar formation, although yields of CO, CO2, and H2O are slightly reduced. Acetic acid enhances the hydrogenation of aromatic rings. It is known that less energy is required to crack C-C bonds than CdC bonds. The catalytic hydrogenation transforms sp2 bonds to sp3 bonds in the coal structure, as indicated by the changes in Raman spectra. A longer hydrogenation time de-

Isoda et al.

creases the molecular weight of the coal cluster as well as the nitrogen content in the coal. This suggests that C-C and C-N bonds are also cleaved by the hydrocracking or hydrodenitrogenation reaction, while CdO bonds in carboxy or ketone groups are largely unaffected. The change of color before and after hydrogenation of the oxidized coal, from brown to yellowish white, reflects the above-mentioned transformation of the coal structure. Mechanism of Hydrogenation of Oxidized Coal in Alcohol. Ruthenium is a typical noble metal catalyst used for hydrogenation of aromatics.9-11 The ES fraction and the model compounds are both effectively hydrogenated over the Ru/Al2O3 catalyst in ethanol. This shows that the Ru catalyst is excellent for the hydrogenation of aromatic rings which are soluble in ethanol. However, hydrogenation of the model compounds, i.e., toluene, phenol, benzyl alcohol, and benzoic acid, over the Pt/Al2O3 was inhibited by coexisting alcohols. This suggests that active sites of the Pt catalyst are occupied by strongly adsorbed alcohols rather than aromatics. As shown in Figure 4, hydrogenation of the ES fraction proceeds only slowly without the addition of acetic acid, even over the Ru/Al2O3 catalyst. However, the addition of acetic acid enhances the hydrogenation of both the ES fraction and model compounds over the Ru catalyst as shown in Figure 12, and the additives remain unchanged during the hydrogenation reaction. The result based on the use of model compounds shows that aromatic rings are hydrogenated in the presence of the additives. The same trends are found for the hydrogenation of the ES fraction. A molecular orbital calculation indicates that carboxylic acids which possess alkyl chains of C2-C10 carry large dipole moments and charges. This suggests that the role of the additives in the hydrogenation reaction is to increase polarity of the solvent. Hydrogenation and hydrocracking reaction rates of polyaromatics and olefins over noble metal catalysts are influenced by the polarity of solvents. Added carboxylic acids change the environments of the hydrogenation reaction, and charged transition intermediate complexes may be easily formed in a highly polar solvent and, as a result, hydrogenation is enhanced. Conclusions 1. Yallourn coal was oxidized with aqueous H2O2 in the presence of 1-propanol. The ethanol extract of the oxidized coal was then subjected to hydrogenation over a Ru/Al2O3 catalyst. After hydrogenation, the color was changed from brown to yellowish white. Thus, we propose to call the product hydrogenated white coal. Aromatic rings in the ethanol extract of the oxidized coal were hydrogenated, and sp2 bonds in the coal structure were transformed into sp3 bonds. However, CdO bonds in carboxy and ketone groups were largely unaffected. 2. Ruthenium was effective for hydrogenation of the oxidized coal which was solubilized in ethanol. The hydrogenation of aromatic compounds, carrying oxygencontaining functional groups and representing partial structures of coal clusters, over the Ru/Al2O3 catalyst was not hindered by coexisting alcohols. Hydrogenation of the ES fraction over the Ru/Al2O3 catalyst in ethanol was estimated from these model reactions.



3. The addition of acetic acid enhanced the hydrogenation reaction, both of the ES fraction and the model compounds. The additive was not affected by the hydrogenation reaction. Carboxylic acids with C2-C10 chains carry large dipole moments and charge potentials, based on the computer calculation. Thus, the role of the additives in the hydrogenation reaction appears to increase the polarity of the solvent. This solvent effect may enhance the formation of transition complexes for the hydrogenation reaction.

Coal Utilization (BRAIN-C) Program and the International Joint Research Program, sponsored by New Energy and Industrial Technology Development Organization, Japan (NEDO). We also thank the Center of Coal Utilization, Japan, and the Japan Institute of Energy. Useful discussion with Professor Masakatsu Nomura of Osaka University is deeply acknowledged. The computer software used in this study was supported by Dr. Jerzy M. Rudziski and Ms. Mayumi Matsushita of Fujitsu Kyushu System Engineering Ltd., Japan, and Dr. Keiichiro Samejima of Fujitsu Ltd., Japan.

Acknowledgment. This study was financially supported by the Basic Research Association for Innovated

EF9701394

Structural Changes of Alcohol-Solubilized Yallourn Coal in the

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