Preparation of High-Grade Carbonaceous Materials Having Similar

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Preparation of High-Grade Carbonaceous Materials Having Similar Chemical and Physical Properties from Various Low-Rank Coals by Degradative Solvent Extraction Xian Li, Ryuichi Ashida, and Kouichi Miura* Department of Chemical Engineering, Kyoto University, Kyotodaigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ABSTRACT: Eight kinds of low-rank coals including lignites and sub-bituminous coals were subjected to a degradative solvent extraction method that treats carbonaceous resources in a non-hydrogen donor at around 350 °C. The low-rank coals were separated into the residue which cannot be extracted by solvent at 350 °C (termed residue), the fraction which can be extracted at 350 °C but precipitates from solvent at room temperature (deposit), the fraction which is solvent soluble even at room temperature (soluble), and liquid fraction mainly consisting of water and gaseous products mainly consisting of CO2. The soluble fraction was finally recovered as solid by removing solvent. The moisture of the coals was completely removed without phase change, and the ash was almost completely concentrated in the residue. The carbon based yields of the three solid fractions were 19.4−31.2% as solubles, 4.2−16.8% as deposits, and 54.7−69.2% as residues when 1-methylnaphthalene was used as the nonhydrogen donor solvent. Overall, more than 94.4% of carbon was recovered as solid fractions. Meanwhile 30.5−54.9% of oxygen was removed as either H2O or CO2. The interesting findings were that the solubles and deposits obtained from all of the coals were respectively very close to each other in elemental composition, chemical structure, molecular weight distribution, thermal decomposition behavior, and thermoplastic behavior. Elemental compositions of solubles were C = 81.8−84.8 wt %, H = 7.5−8.1 wt %, and O = 6.5−10.2 wt %, which were rather close to the elemental composition of bituminous coal. Thus, the degradative solvent extraction method was found to be effective in converting various types of low-rank coals into residues and compounds having very similar chemical and physical properties without losing heating values. Detailed characterization of the solid fractions showed potential utility of the fractions as solid fuel or precursors of chemicals and carbon materials.

1. INTRODUCTION It is without question that coal is a valuable resource used not only as fuels but also as chemical feedstock in this century. On the other hand, the minable reserve of high grade coal, bituminous coal, has been depleting very rapidly due to the rapid increase of worldwide coal consumption. This inevitably requests us to utilize low-rank coals, brown coal/lignite, and sub-bituminous coal, instead of the high grade coal, because the minable reserve of the low-rank coals is as large as that of the high grade coal. However, the low-rank coals are currently used just for power generation near coal mines. Brown coals, for example, contain a large amount of water (∼60%) and oxygen functional groups in general, resulting in low calorific value and causing various problems for storage and transportation. Various methods have been proposed for dewatering and/or upgrading low-rank coals by many researchers as reviewed by Katalambula.1 However, most of the works on low-rank coal dewatering and upgrading are aiming at using the treated coals as solid fuel. If we think of the low-rank coals as the feedstock of chemicals and carbon materials, we might have to develop methods that enable to effectively extract precursors of chemicals and/or carbon materials from low-rank coals in addition to develop the methods for dewatering and/or upgrading. To do so, it may be a key to develop a method that recovers the precursors in high yield from the low-rank coals under mild conditions for their effective utilization. Solvent extraction of coal may be one of the promising methods for the purpose. However, solvent extraction cannot upgrade but just extract low-molecular-weight compounds from © 2012 American Chemical Society

the coals. Furthermore, the extraction yield is very low at room temperature when using nonpolar solvents from practical viewpoint. Using strong polar solvents increases the extraction yield, but it makes the separation of the extract from solvent difficult.2,3 Overcoming these drawbacks, the authors have recently proposed a degradative solvent extraction method which treats carbonaceous resources in a non-hydrogen donor at around 350 °C, under pressure, using a batch autoclave to dewater without phase change, to remove oxygen functional groups, and to produce low-molecular-weight compounds.4−6 The core concept underlying this method involves exposing the entire sample to thermal reactions in a nonpolar solvent at around 350 °C. The anticipated thermal reactions under these conditions include deoxygenation reactions consisting of dehydration and decarboxylation without primary decomposition reactions accompanying the disruption of C−C bonds. The products formed during the thermal reactions at around 350 °C are then filtrated at the same temperature to recover the extract and residue (the latter is termed residue in this work). The extract is further separated into two fractions at room temperature: the fraction that precipitates as a solid (deposit) and the soluble fraction (soluble). The soluble fraction is also finally recovered as a solid by removing the solvent. When the method was applied to an Australian brown coal using 1Received: August 16, 2012 Revised: September 30, 2012 Published: October 1, 2012 6897

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methylnaphthalene, the yields of soluble, deposit, and residue were respectively 23.5, 13.0, and 49.7 wt % on d.a.f. coal basis. Both the soluble and deposit were free from water and contained little ash. Furthermore, the carbon content of soluble was as high as 81.8 wt % (d.a.f.) and the oxygen content was as low as 10.2 wt %. In addition, the soluble fraction consisted of low-molecular-weight compounds with a peak molecular weight at around 300, and the fraction softened and melted below 100 °C.4,7 This method was also successfully applied to various biomass wastes to recover soluble fractions at as high as 36.8− 71.6 wt % of carbon-based yields. The soluble fractions had elemental compositions in the range of C = 81.0−83.3 wt %, H = 6.1−7.3 wt %, and O = 7.3−11.1 wt %. The solubles consisted of low-molecular-weight compounds with a peak molecular weight at around 300, and the fractions softened and melted below 90 °C.5 In this work, the proposed degradative solvent extraction method was applied to eight kinds of low-rank coals including lignites and sub-bituminous coals to examine the possibility of converting the low-rank coals to parent coal-independent soluble and deposit fractions in high yields. The extraction products obtained were characterized in detail to examine their potential utility as raw materials for production of carbon materials and high quality fuels.

Table 1. Ultimate and Proximate Analyses of Three Solid Fractions and Raw Coals ultimate analysis [wt %, d.a.f.]

MM

LY

WA

BB

MB

2. EXPERIMENTAL SECTION 2.1. Coal Sample and Solvent Used. Eight low-rank coals including lignites from Thailand (abbreviated to MM) and the Philippines (PH), brown coals from Indonesia (WR), Australia (LY), and Malaysia (BB and MB), and two sub-bituminous coals from Indonesia (AD and TH) were used in this work. The properties of the coal samples were shown in Table 1. A nonpolar solvent, 1methylnaphthalene (1-MN), was used for the degradative solvent extraction. 2.2. Experimental Procedure. The degradative solvent extraction was performed using a batch reactor (autoclave) at 350 °C. The schematic diagram of the apparatus is shown in Figure 1. A stainless steel autoclave (350 cm3 in volume, 55 mm I.D.) was charged with around 13 g (on dry basis) of as-received coal and 300 cm3 of 1-MN. A stainless filter (65 mm O.D., 0.5 μm opening) was equipped at the bottom of the autoclave. After sufficiently purging the autoclave with N2, the autoclave sealed with 0.5 MPa of N2 was heated up to 150 °C at a heating rate of 5 K/min, where the inherent water was liberated from the coal as liquid water into the solvent. The reactor was cooled to room temperature, and the solvent with the removed water was recovered by opening the valve connecting the autoclave and the reservoir. Then, the autoclave again charged with 300 cm3 of 1-MN and sealed with 0.5 MPa of N2 was heated up to 350 °C at a heating rate of 5 K/min, where it was kept for 60 min. The pressure inside the autoclave was as low as 3 MPa even at 350 °C. Above 300 °C the oxygen functional groups of the coals were expected to be decomposed during the treatment, producing gaseous products, water, and low-molecular-weight compounds (extract) that dissolve in the solvent. The low-molecular-weight compounds along with the solvent were separated from the residue at the extraction temperature by opening the valve connecting the autoclave and a stainless steel reservoir. The extract with the solvent was collected in the reservoir which was cooled by circulating cooling water of room temperature. A part of the extract precipitated as solid at room temperature. This is called deposit in this work. The extract still soluble in solvent even at room temperature (soluble) along with solvent was filtrated using a PTFE membrane filter (0.5 μm opening) to be separated from the deposit. The solvent dissolving the soluble was evaporated at around 140 °C under reduced pressure to recover the soluble as solid. The yields of residue, deposit, and soluble were determined by measuring their weights. The gaseous products collected in a gas bag were analyzed by a gas chromatograph to be quantified. The rest of the

PH

AD

TH

a

raw coal soluble deposit residue raw coal soluble deposit residue raw coal soluble deposit residue raw coal soluble deposit residue raw coal soluble deposit residue raw coal soluble deposit residue raw coal soluble deposit residue raw coal soluble deposit residue

proximate analysis [wt %, d.b.]

C

H

N

O [diff.]

VMa

FCb

ash

66.4 82.4 76.4 68.8 66.7 81.8 77.5 77.4 67.1 81.9 75.6 77.2 71.0 82.3 77.2 77.2 71.7 82.4 76.4 78.0 72.2 82.9 78.1 75.1 72.9 81.8 76.9 76.4 80.7 84.8 80.0 79.4

3.9 7.4 5.1 3.8 4.7 7.5 5.0 4.0 5.1 7.8 5.7 3.8 4.9 7.8 5.5 4.0 4.8 7.6 5.7 4.2 4.6 8.1 5.4 4.2 5.1 7.7 5.6 4.1 5.0 7.7 5.6 4.8

1.9 2.3 3.8 3.4 0.9 0.5 1.0 1.0 1.0 1.0 1.4 1.6 1.3 1.1 1.9 1.7 1.7 1.8 4.2 2.0 0.9 0.9 1.9 1.2 1.0 0.7 1.2 1.3 2.0 1.1 2.0 2.3

27.8 7.8 14.7 24.0 27.7 10.2 16.9 17.6 26.9 9.2 17.9 17.4 22.8 8.8 15.3 17.2 21.9 8.2 13.7 15.8 22.3 8.1 14.6 19.5 21.0 9.8 16.3 18.2 12.3 6.5 12.4 13.6

50.2 78.5 39.3 27.1 51.5 83.4 39.4 31.8 50.5 85.6 43.4 32.1 43.4 80.6 47.1 29.8 42.8 81.5 47.4 29.4 51.5 85.5 38.7 27.8 51.7 79.0 53.7 32.1 41.6 77.2 45.2 29.3

24.0 21.3 59.3 34.4 47.0 16.3 59.9 66.0 47.9 17.4 55.9 65.0 52.5 18.5 51.9 64.3 52.6 18.1 52.1 64.7 36.7 14.1 60.4 49.3 46.5 20.6 45.6 64.3 50.2 21.7 53.6 61.3

25.8 0.2 1.4 38.5 1.5 0.3 0.7 2.3 1.5 0.0 0.7 2.9 4.1 0.9 1.0 6.0 4.6 0.4 0.4 5.9 11.8 0.4 0.8 22.9 1.8 0.4 0.7 3.6 8.2 1.1 1.2 9.4

Volatile matter. bFixed carbon.

Figure 1. Schematic diagram of degradative solvent extraction system.

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products recovered with the solvent, consisting mainly of water, were called liquid. 2.3. Product Analyses. The solid products were characterized by various analyses. The proximate analysis and examination of thermal decomposition behavior were performed by using a thermogravimetric analyzer (Shimadzu, TGA50). The elemental analysis was performed on a CHN corder (Yanaco, CHN MT-6M). The molecular weight distribution (MWD) was estimated by the laser desorption/ionization time-of-flight mass spectrometry (Shimadzu/Kratos, KOMPACTMALDI-II). Determination of type and distribution of hydrogen in solubles and deposits was performed by 1H NMR analyses (JEOL, ECX-400). Functional groups remaining in the solid fractions were estimated by a FTIR spectrometer (JEOL, JIR-WINSPEC 50). The FTIR spectra ranging from 4000 to 600 cm−1 were obtained using a few milligrams of neat sample on KBr disk at 4 cm−1 resolution and 64 scans. The softening/melting behavior was examined by using a thermomechanical analyzer (Shimadzu, TMA50). The TMA estimates the relative displacement of the sample when 2 mg of sample in a platinum pan (6 mm I.D. and 3 mm high) was heated under 10 g of load in a nitrogen atmosphere.

3.1.2. Element Distributions to Products. The carbon contents of solubles were surprisingly as high as 81.8−84.8 wt %, and the oxygen contents of them were as small as 6.5−10.2 wt % on d.a.f. sample basis. The rational formulas of the soluble were CH1.083−1.119O0.058−0.094 which was rather close to those of bituminous coals. The carbon and oxygen contents of deposits and residues were also respectively higher and lower than those of the raw coals, indicating that the solid products were indeed upgraded coals. The elemental compositions of the solid products were plotted on a H/C vs O/C diagram in Figure 3.

3. RESULTS AND DISCUSSION 3.1. Yields and Element Distributions to Products. 3.1.1. Extraction Yield on Weight Basis. First, the inherent water of the coal was completely removed without phase change while the coal was heated up to 150 °C in 1-MN as reported in a previous work.8 Table 1 shows the ultimate and proximate analyses of the solid products. The ash contents of both solubles and deposits were very small indicating that almost entire ash was concentrated in residues. Figure 2 shows

Figure 3. Elemental compositions of solid products and raw coals on H/C vs O/C diagram.

The elemental compositions of solubles and deposits were respectively rather close to each other. All of the data of solubles converged on the values of H/C ≈ 1.1 and O/C = 0.07−0.09, and all of the data of deposits converged on the values of H/C = 0.77−0.91 and O/C = 0.12−0.18. Thus, the proposed degradative solvent extraction method could convert the wide range of low-rank coals into solubles and deposits which respectively have rather similar elemental compositions. To examine the changes during the degradative solvent extraction of the low-rank coals in more detail, the molar basis yields are more informative than the weight basis yields. Figure 4 shows the molar distributions of elements (C, H, and O) to the products on the basis of 1 kg of d.a.f. coal sample. They were calculated using the weight basis yields and the elemental compositions of the products. The element distributions to liquid were again calculated by difference. The calculated distributions of carbon to liquid were almost null, and the molar ratios of hydrogen to oxygen in liquid were roughly 2 for all of the samples. This means that liquid can be regarded as H2O as expected. The gaseous product consisted almost solely of CO2 as stated above. Then, Figure 4 shows that 30.5−54.9% of oxygen of the coals, which were distributed to the gaseous products and liquid, were removed as either H2O or CO2. This indicates that significant deoxygenation reactions consisting of dehydration and decarboxylation occurred during the treatment. The losses of carbons to gaseous products were mainly due to the formation of CO2. The carbons were distributed to solubles, deposits, and residues, respectively, by 19.1−34.3%, 4.3−16.8%, and 54.2−69.0%, indicating that 96.4−99.1% of carbons were retained in the solid fractions. These numbers are much larger than, for example, those for the char yields obtained by pyrolysis. Thus, the proposed degradative solvent extraction method could efficiently deoxygenate low-rank coals.

Figure 2. Product yields obtained by degradative solvent extraction.

the yields of extraction products on the dry and ash free (d.a.f.) weight basis. The yields of solubles, deposits, and residues, respectively, ranged from 16.9 to 27.2 wt %, from 3.5 to 16.9 wt %, and from 49.7 to 63.6 wt % on d.a.f. coal basis. This means that 21.8−40.7 wt % of coals were extracted by 1-MN at 350 °C. The total extraction yields, sums of soluble and deposit yields, of BB, MB, and TH, which have relatively low VM contents (50%). The gaseous products ranging from 2.8 to 8.7 wt % consisted almost solely of CO2. The liquid yields reached to 10.6 wt % on d.a.f. coal basis. Overall, the lower grade coals (lower carbon content) have relatively higher liquid yields. 6899

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Figure 4. Element distribution to the products.

3.2. Characterization of Solid Products. The next concern is the characteristics of the solid fractions. Then, the solid fractions were characterized from several aspects. 3.2.1. Heating Value of Solid Fractions. If the solid fractions are to be utilized as fuel, the heating value is an essential factor to be considered. The efficiency of heating value recovery is also an important factor to determine the feasibility of practical utilization of the degradative solvent extraction method. The higher heating value (HHV) of each solid fraction was estimated by the following Dulong equation.9

HHV of bituminous coal on a d.a.f. basis. If the facts that solubles and deposits are almost completely free from ash and moisture were taken into account, solubles and deposits are judged to be much better solid fuel than bituminous coal. This indicates that the proposed degradative solvent extraction method is effective to produce high quality solid fuel. Next, the heating value recovered in the solid products was calculated using the HHV and the yield of each fraction, and it was compared with the HHV of the raw coal for each sample in Figure 5b. The heating values recovered were higher than 90.1% of the heating values of the raw coals. Thus, the proposed degradative extraction method was found to be effective not only to produce clean and high heating value product, but to transfer the heating value of the raw coals to the solid products very effectively. This shows that the method is effective even when the solid product is simply utilized as fuel. 3.2.2. Molecular Weight Distributions. Figure 6 shows the molecular weight distributions (MWDs) of solubles, deposits, and the raw coals. It is immediately apparent that all of the solubles have similar MWDs, even though the MWDs of the raw coals were rather different to each other. Solubles were compounds less than 500 in molecular weight with a peak molecular weight at around 300. The MWDs of deposits ranged from 400 to 1000 in molecular weight with a peak molecular weight at around 500. Thus, the low-rank coals were separated into three fractions having different MWDs. The results suggest that the solubles have potential to be utilized as raw materials in the production of value added products. 3.2.3. 1H NMR Analyses. Figure 7 shows the 1H NMR spectra for solubles and deposits prepared from the eight kinds of coals. It is immediately apparent that the spectra of both the solubles and deposits are very similar to each other. The 1H NMR spectra had main peaks attributed to the hydrogen attached to aromatic carbon (Har at δ = 6−10 ppm), the hydrogen attached to α carbon (Hα, δ = 2−5 ppm), the hydrogen attached to β carbon (Hβ, δ = 1.1−2 ppm), and the hydrogen attached to γ carbon (Hγ, δ = 0.2−1.1 ppm). Distributions of the four types of hydrogens in solubles and

HHV[MJ/kg, d. a. f. ] = (338.1C + 1441.8H − 180.2O)/1000

(1)

where C, H, and O respectively represent weight percent of carbon, hydrogen, and oxygen. Figure 5a shows the HHVs of the three solid fractions. The HHVs of solubles and deposits were respectively 38.2−39.6 and 32.5−34.8 MJ/kg. These values corresponded well to the

Figure 5. HHVs of the three solid fractions and raw coals: (a) HHV of fractions and raw coals; (b) total HHVs of the three solid fractions. 6900

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Figure 6. MWDs of solubles, deposits, and raw coals.

given in Figure 8. The fa values of Solubles ranged from 0.59 to 0.64, which were lower than those of Deposits ranging from 0.67 to 0.74. The fa values of solubles and deposits were respectively rather close to each other and independent of the raw coals. The rather close hydrogen distributions and fa values of solubles and deposits clearly show that the both fractions respectively have rather similar chemical structure. 3.2.4. FTIR Analyses. To estimate the chemical structure of solubles and deposits in more detail and to examine the mechanism of the degradative solvent extraction, FTIR analyses for solubles, deposits, and raw coals were performed, and the results are shown in Figure 9. The spectra of the raw coals have

Figure 7. 1H NMR spectra of deposits and solubles.

deposits were calculated by convoluting the spectra by the Gaussian functions, and they are shown in Figure 8. The hydrogen distributions in solubles were Har = 0.22−0.31, Hα = 0.20−0.28, Hβ = 0.32−0.39, and Hγ = 0.09−0.17. The hydrogen distributions in deposits were Har = 0.25−0.39, Hα = 0.32−0.39, Hβ = 0.24−0.31, and Hγ = 0.05−0.08. Solubles had lower Har and Hα contents but higher Hβ and Hγ contents than deposits. The aromaticity indexes, fa values, of solubles and deposits calculated by the Brown−Ladner method10,11 are also

Figure 9. FTIR spectra of solubles, deposits, and raw coals.

Figure 8. Hydrogen distribution in deposits and solubles. 6901

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Figure 10. TG curves of the three fractions and raw coals.

Figure 11. TMA profiles (solid lines) and TG curves (broken lines) of the three fractions and raw coals.

sharp and distinct peaks attributed to aliphatic C−H, C−H2, and C−H3 stretching at 2850 to 2960 cm−1, and appearance of other several peaks attributed to aromatic moieties: C−H stretching at 3050 cm−1 and aromatic out-of plane C−H bending at 770 cm−1. Furthermore, the figure clearly shows that

rather broad absorption bands assigned to O−H stretching bands (3100−3600 cm−1). The spectra of solubles were significantly different from the spectra of the raw coals. Main changes in the spectra from the raw coals to solubles are weakening of broad OH stretching bands, appearance of very 6902

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4. CONCLUSIONS The degradative solvent extraction method proposed by the authors was successfully applied to dewater, upgrade, and fractionate various types of low-rank coals including lignites and sub-bituminous coals. The carbon based yields of the three solid fractions obtained were 19.4−31.2% as solubles, 4.2− 16.8% as deposits, and 54.7−69.2% as residues when using 1methylaphthalene as a solvent at 350 °C. Solubles and deposits were completely free from water, and almost completely free from ash. The heating values of the raw coals were almost completely transferred into the three solid fractions. The solubles and deposits obtained from all of the low-rank coals were respectively rather close to each other in elemental composition, chemical structure, molecular weight distribution, and thermal behavior. Elemental compositions of solubles, for example, were C = 81.8−84.8 wt %, H = 7.5−8.1 wt %, and O = 6.5−10.2 wt %, which were rather close to the elemental composition of bituminous coal. The solubles were composed of low-molecular weight compounds having a molecular peak at ca. 300. Complete melting of the solubles occurred below 100 °C, and 60−70% of the solubles were devolatilized below 400 °C. Thus, the proposed degradative solvent extraction method was found to be very effective in converting various types of low-rank coals into extracts having rather similar chemical and physical properties without heating value loss. Detailed characterization of solid products showed promising utilizations of the products as solid fuel or precursors of chemicals and carbon materials.

the spectra of solubles and deposits are respectively rather similar to each other. On the basis of the results shown in Figures 7−9, we can say that the proposed degradative solvent extraction method can be used to obtain compounds having rather similar chemical structure in high yield from various types of low-rank coals, although more detailed examinations are necessary to identify the chemical structures of solubles and deposits. 3.2.5. Thermal Properties. The potential utility of the solid products as raw materials for carbon materials was evaluated using thermal analyses. Figure 10 shows a comparison of the thermogravimetric (TG) curves of the three solid fractions and the raw coals, and a comparison of the thermomechanical analysis (TMA) profiles of the three solid fractions and the raw coals is shown in Figure 11. The TG curves were also added as broken lines in Figure 11 for comparison purpose. Both assays were carried out under nitrogen atmosphere at a heating rate of 10 K/min. The TMA profile, showing the relative displacement of the sample, reaches −1.0 when the sample melts completely. The thermal properties of the samples can be discussed by comparing the TG and TMA results. Both the TG curves and the TMA profiles of the solubles were exceedingly similar to each other, as clearly shown in both of the figures, indicating that the thermal properties of the solubles are highly similar irrespective of the coal type. Both the TG curves and the TMA profiles of the deposits were also rather similar to each other. The TG curves and TMA profiles of the raw coals were significantly different from those of the solubles/deposits. The TMA profiles of the raw coals are apparently different to each other at first glance. Taking into account the facts that the positions of final displacements are affected by the ash contents of the raw coals, and that the displacement temperature ranges coincide with the temperature ranges in which the weight decreases, the displacements of the raw coals are proposed to be due to the weight decrease caused by the decomposition reactions. On the other hand, the temperature ranges for displacements of the solubles and deposits are respectively as low as 80−100 °C and around 250 °C and are lower than the temperatures at which the weight starts to decrease. These results clearly show that the solubles and deposits respectively undergo complete melting at temperatures of as low as 80−100 °C and around 250 °C. The weight decreases that occur below 350 °C for the solubles were ascribed to the devolatilization of the low-molecularweight compounds shown in the MWDs in Figure 6. These low-molecular-weight compounds contributed to the melting of the entire solubles at temperatures as low as 80−100 °C. The volatile matter contents of solubles and deposits were respectively as high as 78−85% and 55−60% as shown in Table 1. From the viewpoint of utilizing the solubles and/or deposits as solid fuels, the relatively high volatile matter contents should positively enhance the combustion efficiency.12 Moreover, the unique softening/melting characteristics of the solubles and deposits should facilitate the possibility of using them as raw materials for production of carbon materials. The authors have already succeeded to prepare carbon fiber from solubles by conventional method, and we also found that the solubles can be directly liquefied under mild conditions with much less H2 consumption and CO2 emission, compared with the direct liquefaction of raw coals. The works related to the utilization of solubles will be published soon.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +81-75-383-2663. Fax: +81-75-383-2653. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The assistance of Kobe Steel Co. for construction of the extraction apparatus is gratefully acknowledged.

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REFERENCES

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