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Energy & Fuels 1996, 10, 1196-1201
New Oxidative Degradation Method for Producing Fatty Acids in High Yields and High Selectivity from Low-Rank Coals Kouichi Miura,* Kazuhiro Mae, Hajime Okutsu, and Nori-aki Mizutani Department of Chemical Engineering, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan Received March 26, 1996. Revised Manuscript Received August 27, 1996X
Oxidation of low-rank coals using hydrogen peroxide at low temperature under ambient pressure is a promising method of producing small molecule fatty acids in high yield and in high selectivity. Five low-rank coals (C% ) 65.1-74.7 on daf basis) served to test the validity of the method. When Argonne Premium Beulah Zap lignite was oxidized with 20 volumes of 30% H2O2 aqueous solution for 24 h at 60 °C, the carbon conversion to water-soluble organics reached 0.71. About a half of the water-soluble organics were small molecules: methanol, formic acid, acetic acid, glycolic acid, and malonic acid. The high yields of these compounds are closely related to the structure of low-rank coals. Obtaining these products in such high yield and high selectivity under mild reaction conditions may give us a new route for utilizing low-rank coals.
Introduction Low-rank coals such as brown coal and lignite are abundant fossil resources and have been utilized for electric power generation. They have some disadvantages such as low calorific value and high water content, so it is desirable to develop a conversion method that is able to utilize this abundant resource effectively. On the basis of their structure, production of organic chemicals such as small molecule fatty acids via liquid phase oxidation may be a possible way to utilize these coals. Many attempts have been made to obtain organic acids from coal through oxidation in alkali or acid media. Of the attempts, oxidation by oxygen in aqueous NaOH or Na2CO3 solution at 110-270 °C under a pressure of 4.0-7.5 MPa is most promising.1-3 Carbon conversion to water-soluble benzene polycarboxylic acids reached 0.50 under the operating conditions. The rest of the carbon was converted to CO2.1-3 Oxidation of coal or humic acid using H2O2 as the oxidizing agent has also been performed by Montgomery and Bozer,4 but the purpose was the examination of the coal structure. Small molecule fatty acids were not obtained in high yield by the oxidation because higher rank coals were used and/or high temperature (150 °C) was employed. A much stronger oxidizing agent consisting of a mixture of H2O2 and CF3COOH was used by Deno et al.5,6 The sum of the yields of fatty acids and methanol was only 5-15% when four coals were oxidized by the reagent at 50-70 °C. Thus, previous attempts to obtain small molecule fatty acids from coal through liquid phase oxidation X Abstract published in Advance ACS Abstracts, October 1, 1996. (1) Kamiya, Y. Fuel 1963, 42, 353-358. (2) Kapo, G.; Calvert, S. Ind. Eng. Chem. Des. Dev. 1966, 5, 97104. (3) Bimer, J.; Salbut, P. D.; Berlozecki, S. Fuel 1993, 72, 1063-1068. (4) Montgomery, R. S.; Bozer, K. B. Fuel 1959, 38, 400-402. (5) Deno, N. C.; Gregger, B. A.; Messer, L. A.; Meyer, M. P.; Stroud, S. G. Tetrahedron Lett. 1977, 1703. (6) Deno, N. C.; Gregger, B. A.; Stroud, S. G. Fuel 1978, 57, 455459.
have not been successful. A large amount of CO2 was produced when the decomposition rate was increased by employing severe oxidation conditions such as high temperature, high pressure, and strong acids. A much milder oxidation method seemed promising for the production of small molecule fatty acids in high yield with high selectivity. Selection of a suitable coal that is easily oxidized seemed also to be important. We have recently found that 84% (wt) of an Australian brown coal oxidized by H2O2 was extracted in a mixed solvent of methanol and 1-methylnaphthalene at room temperature7 and that 40% of carbon in the oxidized coal was aliphatic.8 This result suggested that a large amount of small molecule fatty acids could be recovered if the coal is further oxidized in the liquid phase by H2O2. In this paper we show that the liquid phase oxidation of brown coal with H2O2 under mild conditions produces small molecule fatty acids in high yields. We also examine the mechanism by which the organic acids are produced from coal through a detailed study on the product distribution and the coal properties during the H2O2 oxidation. Experimental Section Five coals, an Argonne Premium coal (Beulah Zap lignite, ND), an Australian brown coal (Morwell, MW), a Canadian brown coal (Highvale, HV), an American subbituminous coal (Wyoming, WY), and a Japanese subbituminous coal (Taiheiyo, TC), were used. The analyses of the coals are listed in Table 1. The Buelah Zap lignite supplied from the Argonne National Laboratory in ampules (-100 mesh) was used without further treatment. The other coals were ground into fine particles of 400. The former fraction was quantified by
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Figure 6. Change in the yields of the smallest molecule components through the H2O2 oxidation of the dried MW coal and ND coal.
Figure 4. Change in the molecular weight distributions of the water-soluble organics produced through the H2O2 oxidation of the dried MW coal and ND coal.
Figure 5. Change in the product distributions through the H2O2 oxidation of the dried MW coal and ND coal.
subtracting the amount of the components of Mw < 104, which were quantified by HPLC, from the fraction of Mw < 400. The total organic carbon in the water-soluble product was measured separately. CO2 was the only gaseous product and was quantified by gas chromatography. Using these measurements, we established the carbon balance during the oxidation. Figure 5 shows the changes in the carbon distribution in the product with the progress of oxidation. For MW(dry), the carbon conversion to water-soluble compounds increased gradually with oxidation time and reached
0.52 at 24 h. Carbon conversion to lower molecular weight fraction (Mw < 104) reached 0.28. The carbon conversion to CO2 was 105). The change in the color of the filtrate shown in Figure 2 probably reflects the change in the product distribution. Figure 6 shows the yield of the smallest molecule fraction (Mw < 104) on coal basis for both coals. The yield of this fraction increased monotonically with oxidation time and reached 0.56 at 24 h for MW(dry) and up to 0.78 for ND coal. This fraction consisted of only six components. At 24 h the yields were 0.032 for CH3OH, 0.193 for HCOOH, 0.065 for CH3COOH, 0.078 for CH2(OH)COOH, 0.131 for CH2(COOH)2, and 0.027 for the unknown carboxylic acid for MW(dry) and 0.064 for CH3OH, 0.127 for HCOOH, 0.117 for CH3COOH, 0.188 for CH2(OH)COOH, 0.250 for CH2(COOH)2, and 0.012 for the unknown carboxylic acid for ND. The selectivities for HCOOH, CH3COOH, CH2(OH)COOH, and CH2(COOH)2 were very high. These results clearly indicate that the oxidation of both coals by H2O2 at around 60 °C is a promising method for producing small molecule acids and methanol in high yield and in high selectivity. Test of the Validity of the Proposed Method for Other Coals. It was shown that the proposed oxidation method was very effective at producing small molecule acids and methanol from MW(dry) and ND under mild conditions. Then the utility of the method was tested using several low-rank coals and the wet MW coal [coal as received, abbreviated MW(wet)]. The coals were oxidized for 24 h at 40 and 60 °C and for 4 h at 80 °C. Figure 7a shows the carbon distribution in the product for the coals. All of the coals were decomposed rapidly, and large amounts of water-soluble organics were produced at 40 and 60°C. For MW(dry), the fraction of carbon remaining in the solid was 0.19, and the carbon conversion to water-soluble compounds reached 0.52 at 60°C as stated above. At 80 °C the carbon conversions to water-soluble compounds and CO2 were, respectively,
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Figure 8. Molecular weight distributions of the water-soluble organics produced through the H2O2 oxidation for low-rank coals.
Figure 7. Effect of coal type on the product distributions through the H2O2 oxidation.
0.32 and 0.50 at 4 h. This indicates that oxidation at 80 °C is too severe to produce the small molecule acids in high yield. The product distributions for the MW(wet) were similar to the distributions for the MW(dry), although the decomposition rate of the wet coal was slightly less than that of the dried coal. This result shows that drying the low-rank coal, which consumes a large amount of energy, can be eliminated when the coal is oxidized according to this method. For HV, the fraction of carbon remaining in the solid was smaller than that of the dried MW coal, and a larger amount of CO2, 0.40 of carbon conversion, was produced at 60 °C. At 40 °C, however, the carbon conversion to watersoluble organics was the highest of all the coals. This indicates that HV is the most reactive of the coals and that 60 °C is too high a temperature for HV to produce small molecule fatty acids. For TC coal, the fraction of carbon remaining in the solid was large and the carbon conversion to water-soluble organics was small. This is probably because TC coal (subbituminous coal) consists of more condensed aromatic components9 as compared with MW coal or ND coal. Figure 7b shows the yields of the smallest molecule fraction on a coal basis. Only five kinds of small molecule fatty acids and methanol were present in the fraction for all of the coals. The carbon conversion to the small molecule fatty acids and methanol was 0.100.20 at 40 °C for all of the coals, and it increased to 0.30-0.50 at 60 °C except for TC coal. The largest carbon conversion was obtained for ND coal at 60 °C. It reached 0.78 as stated earlier. Such high selectivities for the small molecule fatty acids were judged to derive from the structure of coal. The H2O2-TFA-H2SO4 oxidation of model compounds such as toluene and dibenzofuran produced one or two kinds of specific small (9) Kato, T.; Ouchi, K. J. Jpn. Inst. Energy 1992, 71, 1193-1198.
Figure 9. Elemental balances for hydrogen and oxygen during the H2O2 oxidation of ND coal.
molecule fatty acids which reflected the structure of the model compound.6 The structure of brown coal consists of one or two aromatic rings connected by short chain linkages such as ether, methoxy, methylene, and ethylene bridges. The production of five small molecule fatty acids and methanol in high yield is closely related to such structure in the brown coal. Figure 8 shows the MWDs (Mw > 60) of the watersoluble compounds for the coals oxidized at 60 °C. They all showed bimodal distributions having peaks at around 200 and 1000. The components of the smaller molecular weight region probably consisted of aromatics of one or two rings. This suggests that a much larger amount of small molecule fatty acids can be recovered under proper oxidation conditions. This oxidation method was effective in producing small molecule fatty acids and methanol in high yield and in high selectivity from brown coals and lignites. Mechanism of the H2O2 Oxidation of Coal. It is our main concern now to clarify how the small molecule fatty acids were produced. The components are prob-
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Figure 10. Change in the FTIR spectra through the H2O2 oxidation of ND coal.
ably produced by the introduction of oxygen, but the possibility of the addition of hydrogen is also expected because H2O2 contains both O and H. To clarify if hydrogen was added, the elemental balance during oxidation was examined for ND coal. The carbon balance could be established and was shown earlier in Figure 5. For oxygen and hydrogen, only the amounts in the solid residue, in the smallest molecule fraction (Mw < 104) of the water-soluble compounds, and in the gaseous product (CO2) could be measured. They are shown in Figure 9. It is clear that a large amount of oxygen was introduced from H2O2 to the coal to produce CO2 and the smallest molecule fraction as shown in Figure 9b. The
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amount of hydrogen in the smallest molecule fraction increased with the progress of oxidation, and at 24 h the sum of the amounts in the smaller molecule fraction and in the solid residue exceeded the hydrogen amount in the raw coal. This clearly shows that a fairly large amount of hydrogen was introduced to water-soluble compounds from H2O2, because a certain amount of hydrogen is also involved in the larger molecule fraction (Mw > 105) of the water-soluble compounds. The ratio of hydrogen to oxygen in the smallest molecule fraction was almost constant at 1.4-1.5 as can be seen from Figure 9. This suggests that fairly large amounts of hydrogen and oxygen, mainly as hydroxyl radicals, were introduced from H2O2 to the larger molecule fraction to produce the smallest molecule fraction. Figure 10 shows the changes in the FTIR spectra of ND coal during the oxidation. The intensities of the peaks at 1710 and 1020 cm-1, which are assigned to carboxyl groups and alcoholic OH groups, respectively, increased with the progress of the oxidation. The intensity of the peak at 2890 cm-1, which is assigned to the aliphatic C-H stretching vibration, increased significantly at 24 h. The elemental balance shown in Figure 9 indicated that the solid residue became richer in hydrogen with the progress of oxidation. The change in the FTIR spectra is consistent with the change in the elemental distribution. The atomic ratio of the aliphatic carbon to the total carbon of the raw coal, 1 - fa, was 0.31.10 The carbon balance in Figure 5 shows that the ratio of the sum of the amounts of carbon in the smallest molecule fraction and in the produced CO2 to the amount of carbon in the raw coal exceeds the value of 1 - fa at an oxidation time >8 h. This shows that some of the aromatic rings of the coal are ruptured by the H2O2 oxidation even at 60 °C. Summarizing the above, we can depict the mechanism of the oxidation of MW with H2O2 as shown in Figure 11. The structure of the coal oxidized for 2 h, as shown
Figure 11. Presumed mechanism for the H2O2 oxidation of the dried MW coal.
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in the top left, was established on the basis of spectroscopic measurements of the coal solubilized in DMF up to 90%.8 First the weak -C-O- linkages are broken, producing a large amount of water-soluble large molecule compounds and CO2. Then, the large molecules will gradually decompose with the progress of the oxidation to produce small molecule fatty acids. Some of the aromatic rings will also rupture to produce small molecule fatty acids. Thus, the high yield and the high selectivity of small molecule fatty acids were judged to arise mainly from the structure of the coal itself. We do not know the mechanism of the rupture of aromatic rings. Proton donation through the Fenton reaction11 may be one possibility. We are now studying (10) Silbernagel, B. G.; Botto, R. E. Advanced Magnetic Resonance Techniques Applied to Argonne Premium Coals; Advances in Chemistry Series 229; American Chemical Socoiety, Washington, DC, 1993; pp 629-643. (11) Schumb, W. C.; Satterfield, C. N. Hydrogen Peroxide; Reinhold: New York, 1955.
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the mechanism in more detail and taking into account the catalytic effect of Fe. Conclusion A new and effective method for utilizing low-rank coal as a chemical resource was presented. Coal was oxidized with 30% H2O2 at below 80 °C under ambient pressure. Oxidizing an Argonne Premium coal, Beulah Zap lignite, and an Australian brown coal, Morwell, for 24 h at 60 °C gave five kinds of small molecule fatty acids and methanol in 0.78 and 0.52 kg/kg-daf coal, respectively. The proposed method may be a new route for the conversion of brown coal. Acknowledgment. This work was financially supported by a NEDO International Joint Research (Development of Oil Alternative Energy). EF960051A
