A New Two-Step Oxidative Degradation Method for Producing

Energy & Fuels 2001, 15, 611-617

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A New Two-Step Oxidative Degradation Method for Producing Valuable Chemicals from Low Rank Coals under Mild Conditions Kazuhiro Mae, Hiroyuiki Shindo, and Kouichi Miura* Department of Chemical Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan Received August 7, 2000. Revised Manuscript Received January 15, 2001

Brown coal and lignite are abundant fossil resources, but they have several disadvantages such as low calorific value and high water content. Considering that the coals consist of small aromatic ring structures with many functional groups, utilization methods reflecting the structure should be explored.From this viewpoint we have presented a new method to produce chemicals from brown coal and lignite under mild conditions. That is the oxidation using 30%-H2O2 aqueous solution at 60 °C under atmospheric pressure. When an Australian brown coal (Morwell) was oxidized for 24 h at 60 °C, the yield of water-soluble organics was as large as 0.60 kg/kg-coal, 0.28 kg/kg-coal of which were chemicals such as oxalic acid and acetic acid. On the basis of the examination of the structure of the water-soluble organics, we also presented two methods for upgrading the water-soluble organics. One is the Fenton oxidation of the water-soluble organics by which the yield of small-molecule components reached more than 0.50 kg/kg-coal. The other is the decomposition of the water-soluble organics in a sub-critical water by which 0.12 kg/kgcoal of benzene or 0.236 kg/kg-coal of methanol was produced, depending on the conditions employed for preparing the water-soluble organics. Thus it was clarified that the proposed methods are effective to produce chemicals from brown coals with low energy supply.

Introduction Low rank coals such as brown coal and lignite are abundant fossil resources, but they have several disadvantages, such as low calorific value and high water content. It is therefore desirable to develop efficient conversion processes which overcome such disadvantages for this abundant resource. Low rank coals have a large amount of oxygen functional groups such as -COOH, -OH, etc. This structure peculiar to low rank coals makes their calorific value low, but it is attractive when producing organic chemicals such as smallmolecule carboxylic acids. Liquid-phase oxidation of low rank coal is one of the promising methods to obtain organic compounds. Kamiya et al.,1 Kapo et al.,2 and Bimer et al.3 examined the oxygen oxidation of coal in aqueous NaOH or Na2CO3 solution at 110 to 270 °C under the pressure of 4.0 to 7.5 MPa. They reported that 50% of carbon of coal was converted to water-soluble benzene poly-carboxylic acids, but the rest of the carbon was lost as CO2. Deno et al.4,5 performed the oxidation of coal and aromatic compounds with a much stronger oxidizing agent consisting of a mixture of H2O2 and CF3COOH, then the * Corresponding author. Tel.: +81-75-753-5578. Fax: +81-75-7535909. E-mail: [email protected]. (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) Deno, N. C.; Gregger, B. A.; Messer, L. A.; Meyer, M. P.; Stroud, S. G. Tetrahedron Lett. 1977, 1703-1704. (5) Deno, N. C.; Gregger, B. A.; Stroud, S. G. Fuel 1978, 57, 455459.

sum of the yields of carboxylic acids and methanol reached 5 to 15% when four coals were oxidized at 50 to 70 °C. Several studies have also been performed for utilizing Australian brown coals and U.S. lignites through liquid-phase oxidation.6-9 Hayatsu et al.6 performed the NaOH oxidation of an Australian brown coal and a Wyoming lignite using CuO as a catalyst and examined the product distribution of benzene carboxylic acids in detail. Olson et al.7 performed the oxidation of U.S. lignites using RuO4 as a catalyst at room temperature and reported that the succinic acid was obtained by 2.4% of yield. Verheyen et al.8 oxidized a Victorian brown coal using Deno’s method and successfully recovered succinic, acetic, and malonic acids, but the total yield was 8 wt % at most. Thus, these attempts have not been successful with respect to the production of small-molecule carboxylic acids in high yield from coal, because 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 producing small-molecule carboxylic acids in high yield with high selectivity. We have recently found that several low rank coals were successfully converted into a large amount of small-molecule carboxylic acids through liquid-phase (6) Hayatsu, R.; Botto, R. E.; Scott, R. G.; Mcbeth, R. L.; Winans, R. E. Fuel 1986, 65, 821-826. (7) Olson, E. S.; Diehl, J. W.; Froehlich, M. L.; Miller, D. J. Fuel 1987, 66, 968-972. (8) Verheyen, T. V.; Johns, R. P. Anal. Chem. 1983, 55, 1564-1568. (9) Young J. E.; Yen T. Energy Sources 1976, 3, 49-55.

10.1021/ef000177e CCC: $20.00 © 2001 American Chemical Society Published on Web 03/07/2001

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Table 1. Properties of Coals proximate analysis (wt %)

ultimate analysis (wt %,daf)

coal (abbrev.)

FC

VM

ash

C

H

N

S

O

Morwell (MW) Beulah-Zap (ND)

48.2 45.4

50.3 44.9

1.5 9.7

64.8 72.9

4.5 4.8

0.6 1.2

0.3 0.8

29.8 20.3

oxidation with H2O2 below 80°C.10 When North Dakota lignite (Argonne Premium Coal) was oxidized with 30%H2O2 for 24 h at 60 °C in atmospheric pressure, the conversion of carbon in coal to water-soluble organics was 0.62 in which 0.16 was the conversion to smallmolecule carboxylic acids such as acetic acid and malonic acid. The yield of small-molecule carboxylic acids represented on coal basis reached up to 0.30 kg/kg-coal. The method was attractive to produce chemicals under mild conditions, but a method for upgrading a large amount of water-soluble organics obtained by the method is desired to establish a new efficient conversion process for low rank coals. In addition, the amount of H2O2 required must be reduced to improve the efficiency of the process. In this paper we first examined the effect of the ratio of coal to H2O2 on the product distribution. Then, we examined the unit structure of the water-soluble organics by various analyses. Based on the understanding of the unit structure, two methods for converting the water-soluble organics into chemicals were presented. Experimental Section Coal Samples. Two coals, an Argonne Premium coal (Beulah Zap lignite, ND) and an Australian brown coal (Morwell, MW) were used. The analyses of the coals are listed in Table 1. The Buelah Zap lignite, supplied in ampules (-100 mesh) from the Argonne National Laboratory, was used without further treatment. Morwell coal was ground into fine particles of less than 74 µm, and dried in vacuo at 70 °C for 24 h before use. First-Step Oxidation of Coal with H2O2. The first-step oxidation of the coals was performed as follows: 1 g of coal particles was mixed with 20 or 5 mL of 30% aqueous hydrogen peroxide in a 500 mL flask with a tight plug. The flask with the reaction mixture was kept for 1 to 24 h in a water bath at a constant temperature of 60 °C. All the product gas in the flask was purged by 10 L of nitrogen gas to be collected in a gas bag. Next, 200 mL of cold water was added to the mixture to terminate the oxidation reaction, then the mixture was filtrated to separate it into the solid residue and the aqueous solution of water-soluble organics using a Teflon filter (0.5 µm opening). The aqueous solution still contained H2O2 in rather high concentration. For the analysis of the first-step oxidation products and for the subsequent decomposition of aqueous solution in sub-critical water, the remaining H2O2 was completely decomposed by adding MnO2 just after the filtration to terminate completely the first-step oxidation. For the second-step oxidation of the water-soluble organics, on the other hand, the aqueous solution was directly subjected to the subsequent experiments. The gaseous product was analyzed for CO2, CO, and hydrocarbon gases using a gas chromatograph. Upgrading of Water-Soluble Organics Derived from First-Step Oxidation. Two methods were examined to upgrade the water-soluble organics: one was a second-step oxidation with the Fenton reagent (second-Oxid), and the other (10) Miura, K.; Mae, K.; Okutsu, H.; Mizutani, N. Energy Fuels 1996, 10, 1196-1201.

was the decomposition in a sub-critical water (SCWD). In the second-Oxid experiment 0.4 g of FeSO4 was just added to 100 mL of aqueous solution of water-soluble organics containing unreacted H2O2, then the solution was kept for 3 h at room temperature. The SCWD experiments were performed in a 10.5-mL microreactor filled up with 10 mg of H2O2-free aqueous solution of water-soluble organics. The reactor was plunged into a sand bath preheated at 350 °C or 375 °C, then the reaction pressure rised up to 16 or 18 MPa. After the elapse of 0.5 h the reactor was immediately soaked in a cool water bath to terminate the reaction. Analyses of Products. The solid residue was evacuated at 60 °C for 24 h, and the ultimate analysis (CHN corder, MT3, Yanagimoto Co.), the FTIR analysis (JIR-WINSPEC 50, JEOL Ltd.), and 13C NMR analysis (CMX300, Chemagnetics) were performed. The aqueous solution was served to TOC, GPC, GC, and HPLC analyses. The molecular weight distribution of the water-soluble organics was determined using a liquid chromatograph (LC-10A model, Shimadzu Co.) equipped with a polystyrene gel column (Shimpak 210H, 8.0 mm in diameter and 0.30 m in length). The calibration curve was constructed using six glycol acids of known molecular weights as standards. The HPLC analysis was performed to detect small-molecule carboxylic acids in the aqueous solution using the liquid chromatograph equipped with a sulfonated polystyrene gel column (SCR-102H, Shimadzu Co.) and an electric conductivity detector. To quantify the product through the subcritical water decomposition, the GC analysis was also conducted. A gas chromatograph equipped with Porapak Q and OV-101 columns was used to analyze inorganic gases (H2, CO, and CO2), hydrocarbon gases (C1 to C6 gaseous compounds, benzene, toluene, and xylene), and water-soluble organic products. The yield of each component estimated from these measurements was represented on dry and ash-free coal basis (kg/kgcoal). This yield is important from a practical viewpoint, but it may exceed unity because oxygen was introduced into the product. Therefore, the carbon conversion, the fraction of carbon converted to the product from the carbon in the original coal, was also used to represent the results.

Results and Discussion Product Distribution through First-Step Oxidation. We reported in the previous paper6 that the liquidphase oxidation of low rank coal by H2O2 is an effective method to obtain a large amount of small-molecule carboxylic acids. However, it was rather difficult to determine the exact yield of the carboxylic acids, because the liquid-phase oxidation proceeded while the aqueous solution is stored at room temperature before the analysis. If the aqueous solution was stored several days before the analysis, for example, the yield reached more than 70% under some conditions. This is because unreacted H2O2 still remained in the aqueous solution. Therefore, the reaction times given in the previous paper were not definitely defined. To avoid such ambiguity, the H2O2 remaining in the aqueous solution was decomposed by adding MnO2 just after the filtration as stated in the Experimental Section. Following this procedure, we reexamined the product yields at different reaction times through the first-step oxidation at 60 °C for Morwell brown coal (MW) and Beulah Zap lignite (ND). Figure 1, parts a and b, shows the changes in the carbon distribution in the product with the progress of the oxidation. The small-molecule components in the figures represents the fraction less than 118 in molec-

Producing Valuable Chemicals from Low Rank Coals

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Table 2. Yields of Small-Molecule Carboxylic Acids through First-Step Oxidation for 24 h at 60 °C yield [kg/kg-coal] coal

HCOOH

CH3COOH

CH2(OH)COOH

CH2(COOH)2

(COOH)2

total

MW ND

0.068 0.037

0.039 0.052

0.033 0.057

0.055 0.088

0.076 0.066

0.271 0.300

Figure 1. Change in the carbon conversion through the firststep oxidation.

ular weight. For MW, the carbon conversion to watersoluble components increased gradually with oxidation time, and reached 0.52 at 24 h. The carbon conversion to small-molecular-weight fraction (Mw < 118) reached 0.13. The carbon conversion to CO2 was less than 0.30 even at 24 h. For ND, the carbon conversion to watersoluble components reached 0.71 in only 4 h. The amount of compounds of Mw < 118 significantly increased between 12 and 24 h. Since the amount of carbon in the solid did not decrease significantly during this time period, the small-molecule compounds were judged to be produced in the aqueous phase through the oxidation of larger molecules (Mw > 105). Figure 2, parts a and b, shows the yields of the smallmolecule components (Mw < 118). The yield of this fraction increased monotonically with oxidation time and reached 0.271 kg/kg-coal at 24 h for MW, and reached 0.300 kg/kg-coal for ND coal. This fraction consisted of only 5 components: formic, acetic, succinic, malonic, and oxalic acids. The yield of each component at 24 h of oxidation is shown for both coals in Table 2. These results clearly indicate that the first-step oxidation by H2O2 at around 60 °C is an effective method for

Figure 2. Change in the yield of small-molecule carboxylic acids through the first-step oxidation.

producing oxygen contaning organics from low rank coals under mild conditions. Effect of the Ratio of Coal to H2O2 on the Product Distribution. Figure 1, parts b and c, shows the effect of the mass ratio of coal to 30 wt % H2O2 aqueous solution, R, on the carbon conversion through the first-step oxidation of MW. At R ) 1/5 the degradation rate decreased and the total conversion and the conversion to water-soluble organics at 24 h were 0.71 and 0.5, respectively, which were smaller than those at R ) 1/20. However, the conversion to CO2 was reduced

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Table 3. Properties of Liq.60.24 Produced from MW ultimate analysis (wt %,daf) abbreviation

coal/H2O2

C

H

N

S+O

O/C

H/C

COOH[mol/kg]

Liq.60.24(MW-1/20) Liq.60.24(MW-1/5)

1/20 1/5

46.3 45.5

4.5 4.1

0.9 1.2

48.3 49.2

0.78 0.81

1.16 1.08

11.4 13.5

Figure 3. Molecular weight distributions of the water-soluble organics produced by the first-step oxidation of coals.

at R ) 1/5. Figure 2, parts b and c, compares the yields of small-molecule components obtained at R ) 1/5 and R ) 1/20. The total yield of small-molecule components at R ) 1/5 was 0.210 kg/kg-coal, which was 0.060 kg/ kg-coal smaller than the total yield at R ) 1/20. The difference in the total yield came from the difference in the yields of formic and oxalic acids. Judging from these results, the oxidation at R ) 1/5 is more profitable than the oxidation at R ) 1/20 to suppress the formation of CO2 without significantly reducing the yields of smallmolecule components. Structure of Water-Soluble Large-Molecule Components. A large amount of water-soluble organics was produced through the first-step oxidation even at the high coal/H2O2 ratio of 1/5. Our main concern in this work is to recover chemicals from coal as much as possible. To realize it, a new method is required for degrading the water-soluble large-molecule components produced through the first-step oxidation. The examination of the structure of the large-molecule components should give a clue to find some suitable upgrading methods. To do so, the three water-soluble organic samples were prepared: one was produced from ND at R ) 1/20, and the other two samples were prepared from MW at R ) 1/20 and 1/5. The three samples were abbreviated to Liq.60.24(ND), Liq.60.24(MW-1/20), and Liq.60.24(MW-1/5), respectively. Figure 3 shows the molecular weight distributions of the three samples. The molecular weights of Liq.60.24(ND) and Liq.60.24(MW1/20) ranged from 100 to 40000 and had three peaks at almost same molecular weights. On the other hand, the molecular weight distribution of Liq.60.24(MW-1/5) was different from the other two samples. This suggests that the structure of water-soluble organics is affected by the reaction conditions of the first-step oxidation rather than the coal type. To examine the properties of the water-soluble organics in more detail, Liq.60.24 (MW-1/20) and Liq.60.24(MW-1/5) were dried up to solid at 80 °C. The ultimate

Figure 4. Comparison of FTIR spectra between Liq.60.24 and pure benzene carboxylic acids.

analyses and the carboxylic group contents of the samples are listed in Table 3. The COOH content was calculated from the CO2 yield during the flash pyrolysis of the sample at 920 °C using a Curie-point pyrolyzer. About a half of the sample consisted of oxygen. The COOH contents were 11.4 mol/kg for Liq.60.24(MW-1/ 20) and 13.5 mol/kg for Liq.60.24(MW-1/5), respectively. This shows that most of the oxygen is contained as the form of COOH. Since we have found that the monomer units of the MW coal oxidized with H2O2 for 2 h at 60 °C are one benzene ring compounds,11 the monomer units of Liq.60.24 prepared from MW are well expected to be one-aromatic-ring compounds. From Table 3 the ratios of carbon in COOH to total carbon were about 0.33 for Liq.60.24(MW-1/20) and 0.36 for Liq.60.24(MW1/5), respectively. The aliphatic carbons were judged to exist to some extent in both samples from the H/C ratio values. However, it was difficult to clarify the difference in the aliphatic carbons between the two samples from the H/C ratio, because it was almost impossible to avoid the influence of adsorbed water on the ultimate analysis. On the other hand, it was rather easy to remove the effect of adsorbed water during the FTIR measurement, and the sensitivity and reproducibility of C-H stretching vibrations were excellent. Then we decided to use the FTIR spectra on examining the aliphatic carbons. Figure 4 shows the FTIR spectra of Liq.60.24(MW1/20) and Liq.60.24(MW-1/5) with two pure compounds, benzene hexa-carboxylic acid and benzene tetra-carboxylic acid. A slight difference appeared at around 2900 cm-1 in the spectra between Liq.60.24(MW-1/20) and (11) Mae, K.; Maki, T.; Okutsu, H.; Miura, K. Proceedings of the 9th International Conference on Coal Science; Essen: 1997; Vol. 1, pp 195-198.


Figure 5. Change in the molecular weight distributions of water-soluble organics through the second-step oxidation.

Liq.60.24(MW-1/5): Two peaks of C-H stretching vibration of aliphatic compounds at 2800 to 3000 cm-1 were very small for Liq60.24(MW-1/20), but they were fairly large for Liq.60.24(MW-1/5). This suggests that Liq.60.24(MW-1/5) still retains a fairly large amount of aliphatic portion because of mild reaction conditions of the first-step oxidation. On the other hand, the OH stretching vibration of both Liq.60.24 samples were close to the spectrum of benzene tetra-carboxylic acid, suggesting that the presumed average unit structure of Liq.60.24 was reasonable. From these observations the average unit structure of Liq.60.24 was presumed to be one benzene ring attached with three or four have a large amount of COOH groups and some aliphatic substituents that can be precursors of small-molecule carboxylic acids. Second-Step Oxidation of Liq.60.24 with Fenton Reagent. Liq.60.24 samples contained a large amount of carboxyl groups, so small-molecule carboxyl acids could be obtained from them if the aromatic ring can be ruptured by some means. The oxidation with the Fenton reagent is well-known as a method to rupture aromatic rings in lignin.12,13 The aqueous solution containing Liq.60.24 still contained H2O2 in about 5% of concentration, so the Fenton reaction was expected to proceed just by adding Fe2+. Then, we conducted the second-step oxidation of the three Liq.60.24 samples prepared above for 3 h at room temperature by simply adding 0.4 g of FeSO4 into 100 mL of their aqueous solution. Figures 5 and 6 show the change in the molecular weight distributions and yields of small-molecule components, respectively, through the second-step oxidation of the three Liq.60.24 samples. Since no solid residue (12) Schumb, W. C.; Scatterfield, C. N.; Wentworth, R. L. Hydrogen Peroxide; Reinhold Publishing Co.: New York, 1955; p 458. (13) Tatsumi, K.; Terashima, N. Mokuzai Gakkaishi 1983, 29, 530536.


was obtained on the filter when the product after the second-step oxidation was filtrated, the components of molecular weight more than 1000 were judged to have completely disappeared through the second-step oxidation for all samples as shown in Figure 5. This means that larger-molecule components were converted into a large amount of small-molecule components. The smallmolecule components consisted of three to four main components from the viewpoint of molecular weight, suggesting that specific n-mers were selectivity produced through the second-step oxidation. The yield of oxalic acid significantly increased through the secondstep oxidation as shown in Figure 6, then the yields of small-molecule carboxylic acids reached 0.263 kg/kg-coal for ND and 0.297 kg/kg-coal for MW through the first oxidation at R ) 1/20 and the second-step oxidation. In addition the yield of the components between 118 and 500 of molecular weight reached ca. 0.2 kg/kg-coal through the second-step oxidation. The total yield of small-molecule components less than 500 of molecular weight surprisingly reached up to 0.463 kg/kg-coal for ND and 0.511 kg/kg-coal for MW. Comparing the effect of R on the product distribution through the second-step oxidation, Liq.60.24(MW-1/5) was also decomposed into small components of less than 1000 of molecular weight as shown in the lower graph of Figure 5. The yield of small-molecule components less than 500 of molecular weight was 0.530 kg/kg-coal. The yield of small-molecule carboxylic acids at R ) 1/5 was smaller than that at R ) 1/20 through the first-step oxidation as shown in Figure 2, but the yield reached 0.417 kg/kg-coal after the second-step oxidation. This was because a large amount of precursor of smallmolecule carboxylic acids was still existent in Liq.60.24(MW-1/5). The yields shown here were for the secondstep oxidation of the water-soluble organics containing a large amount of small-molecule carboxylic acids produced through the first-step oxidation. If the smallmolecule components can be separated before the secondstep oxidation, the total yields of small-molecule carboxylic acids through the first- and the second-step oxidations are expected to exceed 0.5 kg/kg-coal. Thus, it was clarified that two-step oxidation is an effective method to recover small-molecule carboxylic acids in high yield and in high selectivity. Decomposition of Liq.60.24 Solution in SubCritical Water. As mentioned above, the average unit structure of Liq.60.24 was supposed to be one benzene ring attached with three or four COOH groups. Judging from the structure, a large amount of benzene could be recovered by decarboxylation of Liq.60.24. So, we tried to decompose three Liq.60.24 samplessLiq.60.24(ND), Liq.60.24(MW-1/20), and Liq.60.24(MW-1/5), in a subcritical water. The decomposition products at 350 °C for Liq.60.24(ND) and Liq.60.24(MW-1/20) and the decomposition product at 375 °C for Liq.60.24(MW-1/5) were all colorless, transparent aqueous solutions. The decomposition time was set to be 0.5 h for all experiments. Figure 7 shows the molecular weight distributions of the product aqueous solutions. A large and distinct peak appeared at ca. 80 of molecular weight for Liq.60.24(ND) and Liq.60.24(MW-1/20). To identify and quantify the components in the main peaks, we performed the

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Figure 6. Change in the yields of small-molecule components through the second-step oxidation.

Figure 7. Change in the molecular weight distributions of water-soluble organics through the sub-critical water decomposition.

GC analyses of the product aqueous solution using two different columns, Porapak Q and OV-101. Each of the chromatograms obtained for the product aqueous solution of Liq.60.24(MW-1/20) by two different columns showed only a single peak as shown in Figure 8, parts a and b. The peaks were exactly identified as benzene from the retention times for pure components shown in the figure. Thus, the peak corresponding to Mw ≈ 80 in Figure 7 was identified to come solely from benzene. The molecular weight distribution of the product solution of Liq.60.24(MW-1/5) was significantly different from that of Liq.60.24(MW-1/20): it had no peaks at ca. 80 but had peaks at ca. 30 and ca. 200 of molecular weight. It also had peaks at around 10000 to 40000 of molecular weight, indicating that a fairly large amount of large-molecule components is still existent in the product solution of Liq.60.24(MW-1/5). The peak at ca. 30 was judged to come from methanol by the GC charts chromatograms of the product solution and the retention times shown in Figure 8.

Figure 8. Chromatograms of aqueous solution produced by the sub-critical water decomposition.

Figure 9 shows the change in product yields through the first-step oxidation and the decomposition in the sub-critical water. The yields of benzene were, respectively, 0.092 kg/kg-coal for Liq.60.24(ND) and 0.121 kg/ kg-coal for Liq.60.24(MW-1/20). The value of 0.10 kg/ kg-coal or so was comparable to a typical benzene yield obtained by the hydropyrolysis of coal at a high temperatures in the presence of high pressure of hydrogen.14 About two-thirds of Liq.60.24(MW-1/20) was converted into benzene and CO2. The molar ratio of benzene to CO2 in the product approximately coincided with the molar ratio of benzene ring to COOH group in the (14) Asaoka, Y.; Tatsumi, M.; Azuma, T.; Seo, T. Proceedings of the 15th Annual International Pittsburgh Coal Conference; Paper No. 304; 1998.



clarified that the first-step oxidation and the subsequent decomposition in sub-critical water are also effective to recover chemicals such as benzene and methanol from low rank coals in high yield and in high selectivity. Conclusion

Figure 9. Change in the yields of chemicals through the subcritical water decomposition.

average unit structure of Liq.60.24(MW-1/20). On the other hand, 0.236 kg/kg-coal of methanol was produced from Liq.60.24(MW-1/5). The drastic difference in the decomposition products between Liq.60.24(MW-1/20) and Liq.60.24(MW-1/5) was judged to come from the difference in their structures. As mentioned above, the average unit structure of Liq.60.24 is presumed to be one aromatic ring attached with COOH and aliphatic substituents. The relative amount of COOH substituent to aliphatic substituent was dependent on the condition of the first-step oxidation. The reason benzene was not produced from Liq.60.24(MW-1/5) is explained by the fact that largemolecular-weight components were existent in the decomposition product as shown in Figure 7. Crosslinking reactions and/or condensation reactions are judged to produce such large-molecule components, and consequently suppressed the formation of benzene. These results clearly show that a difference in the structure of the water-soluble organics produced by the first-step oxidation controlled the product distribution of the sub-critical water decomposition. In other words, the product distribution of the sub-critical water decomposition can be controlled merely by changing the reaction conditions of the first-step oxidation such as coal/ H2O2 ratio, reaction time, and so on. Thus it was

We have developed a new oxidative degradation process for utilizing low rank coal as a chemical resource, in which coal was oxidized with 30%-H2O2 at 60 °C under ambient pressure. To increase the product yield and selectivity, two methods were presented for upgrading the product from the first-step oxidation based on the detailed examination of the structure of the product. One is a second-step oxidation with the Fenton reagent at room temperature, and the other is decomposition in a sub-critical water. Through the firstand the second-step oxidations, 0.26 kg/kg-coal of smallmolecule carboxylic acids and 0.39 kg/kg-coal of benzene carboxylic acids were recovered from Beulah Zap lignite. When Morwell brown coal was subjected to the two-step oxidations, 0.42 kg/kg-coal of small-molecule carboxylic acids were successfully recovered. Through the first-step oxidation at 60 °C under low coal/H2O2 ratio (1/20 in mass ratio) and the subsequent decomposition in a sub-critical water at 350 °C, 0.121 and 0.092 kg/kg-coal of benzene were recovered from Morwell brown coal and Beulah Zap lignite, respectively. By changing the coal/H2O2 ratio of the first-step oxidation to a large value of 1/5, 0.236 kg/kg-coal of methanol was recovered after the decomposition in the sub-critical water for Morwell coal. This result suggests that a slight difference in the structure of the watersoluble organics produced by the first-step oxidation controlled the product distribution of the sub-critical water decomposition. Thus, the proposed method, developed based on the understanding of the structure of low rank coals, showed the possibilities of an alternative and reasonable conversion method of low rank coals. Acknowledgment. This work was financially supported by the Simple Chemistry Program (Research for Next Generation Chemical Process) of The Society of Chemical Engineers, Japan. EF000177E

A New Two-Step Oxidative Degradation Method for Producing

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