Energy & Fuels 2004, 18, 755-760
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New Pretreatment Methods Combining a Hot Water Treatment and Water/Acetone Extraction for Thermo-Chemical Conversion of Biomass Isao Hasegawa,† Kazuhide Tabata,† Osamu Okuma,‡ and Kazuhiro Mae*,† Department of Chemical Engineering, International Innovation Center, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan Received July 28, 2003
New pretreatment methods were developed for separating hemicellulose, cellulose, and lignin from biomass for their efficient use in the thermo-chemical conversion of each component. One method is basically a two-step process. Biomass treated in hot water at 180 °C was extracted in a flowing stream of water/acetone mixture under 10 MPa at 230 °C. Through the hot water treatment, hemicellulose in biomass was successfully recovered as saccharides, leaving lignin and cellulose as a solid. Through the sequential extraction by the water/acetone solvent, lignin was depolymerized into the water/acetone-soluble compounds and the residual cellulose was partly dehydrated. The other method is a one-step process, in which biomass was directly extracted in 50% water/acetone solution at 200 °C using a batch reactor, and the residue was pure cellulose. The proposed methods were expected to be new routes for converting low-grade resources into valuable chemicals.
1. Introduction Recently, biomass resources are receiving much attention because of their carbon neutral characteristics. Biomass consists of three major componentsshemicellulose, cellulose, and ligninswhich have quite different reactivities. If each component is individually converted into some materials, the selectivity of their products is expected to become higher, which is favorable for the subsequent process. For example, a high yield of levoglucosan can be recovered by the pyrolysis of cellulose. On the contrary, only a wide-ranging lowcaloric tar is produced by the pyrolysis of biomass due to the lignin decomposition. The lignin fraction also inhibits alcoholic fermentation of saccharides. In addition, the quality of biomass resources can be maintained stably for the subsequent process. Therefore, to utilize biomass efficiently as supplementary energy sources or chemicals, a proper separation method of the three components should be developed. In general, lignin is derived from kraft,1 hydrolysis,2 steam-explosion,3 and organosolv.4 Among these delignification methods, kraft is a major pulping process and produces large amounts of highly polluting wastewater, particularly when it contains sulfur components. Lignin and hemicellulose cannot be recovered due to the alkali cooking reaction, and cellulose is bleached. On the other * Corresponding author. Tel: +81-75-383-2668. Fax: +81-75-3832658. E-mail:
[email protected]. † Department of Chemical Engineering. ‡ International Innovation Center. (1) Gierer, J. Wood Sci. Technol. 1985, 19, 289-312. (2) Sivers, M.; Zacchi, G. Biores. Technol. 1995, 51, 43-52. (3) Grous, W. R.; Converse, A. O.; Grethlein, H. E. Enzyme Microb. Technol. 1986, 8, 274-280. (4) Kleinert, T. N. Tappi 1974, 57 (8), 99-102.
hand, hydrolysis is a representative method of pretreatment for fermentation. The yield from polysaccharide to monosaccharide is 85%. In this process, lignin is highly condensed by the acid and leaves the solid residue. There is increasing interest in the study of alternative pulping processes that achieve both high performance and protection to the environment. The delignification processes that use organic solvents are known as organosolv, which can reduce environmental pollution. In the organosolv process, an organic or aqueous organic solvent mixture is used to extract the lignin fraction through breaking the internal lignin and hemicellulose bonds. The organic solvents used in the process include methanol, ethanol, ethylene glycol, and tetrahydrofurfuryl alcohol.5-7 Paszner et al. found the salts of alkaline earth metals, such as calcium and magnesium chlorides, to be effective catalysts in organosolv liquors with high methanol and ethanol contents.8 Organic acids such as oxalic, acetylsalicylic, and salicylic acid can also be used as catalysts in the organosolv process.9 Usually, a high yield of xylose can be obtained with the addition of acids. Solvents used in the process need to be drained from the reactor, evaporated, condensed, and recycled to reduce the cost. Removal of solvents from the system is necessary because the solvents may be inhibitory to the growth of organisms, enzymatic hydrolysis, and fermentation. To overcome these demands, it is desirable for the solvent (5) Chum, H. L.; Johnsoon, D. K.; Black, S.; Baker, J.; Grohmann, K.; Sarkanen, K. V.; Wallance, K.; Schroeder, H. A. Biotechnol. Bioeng. 1988, 31, 643-649. (6) Roberts, R. S.; Muzzy, J. D.; Faass, G. S. U.S. Patent 4,746,401, 1988. (7) Sano, Y. Kami Pa Gikyoshi 1991, 45 (5), 525-539. (8) Paszner, L.; Cho, H. J. Tappi J. 1989, 72, 135-142. (9) Sarkanen, K. V. Prog. Biomass Convers. 1980, 2, 127-144.
10.1021/ef030148e CCC: $27.50 © 2004 American Chemical Society Published on Web 03/20/2004
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Table 1. Ultimate Analyses and Proximate Analyses of Samples ultimate anaysis (d.a.f)
proximate anaysis (d.b.)
sample
C
H
O
ash
FC
VM
oil palm shell Japanese apricot tree Residue for shell (flow, 230°C) residue for apricot tree (1-step batch, 200°C) residue for apricot tree (flow, 230°C) residue for apricot tree (flow, 200°C, 5%-acetic acid) W/A-soluble for shell (flow, 230°C) W/A-soluble for apricot tree (1-step batch, 200°C) W/A-soluble for apricot tree (flow, 230°C)
51.1 49.6 53.3 46.3 51.2 47.9 61.8 54.2 58.7
6.0 5.9 6.7 6.0 5.9 6.0 5.8 5.9 5.7
42.9 44.5 41.0 47.7 42.9 46.1 32.4 39.9 35.6
6.34 0.08 2.57 0.05 0.04 0.07 0.58 0.11 0.05
15.75 12.20
77.91 87.72
to be easily recoverable. Acetone is one solvent that has a lower boiling point than water, which leads to the easy separation from the mixed aqueous solvent. Moreover, acetone can be recovered from biomass wastes,10 so the process in which only biomass is utilized as raw material would be realized even if acetone is consumed. Jimenez et al.11 conducted acetone organosolv pulping; however, their work was limited to the influence of the conditions on the physical properties of the resulting paper sheets such as stretch, tear index, and brightness. Thus, the organosolv has a potential to degrade biomass into each component under mild conditions, so the study from the viewpoint of a pretreatment method is required for the effective biomass conversion. In this paper, we present new pretreatment methods combining a conventional hot-water treatment and the extraction with acetone/water mixture using a batch or a flow reactor for separating biomass into hemicellulose, cellulose, and lignin fractions. We examine the effect of the extraction conditions on the product distribution, and confirm the validity of the method to separate into each fraction for the subsequent conversion. 2. Experimental Section Samples and Solvents. Oil palm (Elaeis quineensis) shell wastes derived from an oil palm mill in Malaysia were used because of a large amount of production and high lignin content. Fruit bunch of the oil palm is treated with steaming. Oil is squeezed from the fruit, and its solid residual wastes are fiber and shell. Japanese apricot (Prunus mume) tree, the heartwood part of the trunk, was used as a representative woody biomass. A commercial microcrystalline cellulose (Nakalai Tesque Co.) was also used to examine the pyrolysis behavior of pure cellulose. Oil palm shell and Japanese apricot tree were ground into particles under 250 µm. All the samples were dried in vacuo for 24 h at 70 °C prior to use. The analyses of these samples are listed in Table 1. Water, acetone, and water/acetone binary solvents (1 to 4, 1 to 1, and 4 to 1 mixing ratio on weight basis) were used as solvents. Experimental Procedure for Extraction. A quantity of 500 mg of oil palm shells were placed on filters in a reactor made of Swagelok. Solvent was supplied continuously at the flow rate of 1 mL/min under 10 MPa. The reactor was heated at a rate of 10 K/min for the prevention against overshooting to a final temperature where it was kept for 0.5-48 h, then it was dipped into a sufficient amount of water to be cooled immediately in order to terminate the dissolution of the samples. The fraction extracted at the reactor temperature, which are dissolved in the solvent, came out from the reactor with the flowing solvent. Product gas was collected in a gasbag, (10) Masuda, T.; Kondo, Y.; Miwa, M.; Shimotori, T.; Mukai, S. R.; Hashimoto, K.; Takano, M.; Kawasaki, S.; Yoshida, S. Chem. Eng. Sci. 2001, 56, 897-904. (11) Jimenez, L.; Garcia, J. C.; Perez, I.; Ferrer, J. L.; Chica, A. Biores. Technol. 2001, 79, 23-27.
and analyzed using a gas chromatograph for CO, CO2, and the hydrocarbon gases (CH4, C2H4, C2H6, and C3H8). Thus each shell sample was separated into solid residue, soluble, and gaseous components. On the other hand, Japanese apricot tree was also extracted using a small batch reactor (5 mL in volume). A 500 mg quantity of apricot particles and 4 mL of binary solvents were filled in the reactor. After purging by N2 gas, the reactor was plunged into a sand bath preheated to a temperature of 200 °C, then the reaction pressure increased rapidly up to the saturated vapor pressure at the temperature. After an elapse of 1 h, the reactor was immediately soaked in a cool water bath. Gaseous products were collected in a gasbag. Analyses of Products. Ultimate analysis of the residue was performed using a CHN corder (Yanaco, CHN-500). The pyrolysis of the residue and the soluble was also performed using a thermogravimetric analyzer (Shimadzu, TGA-50) and an FTIR spectrometer (JEOL JIR-WINSPEC50). The in-situ FTIR spectra of the residues were analyzed with use of the method proposed by Miura et al.12 The matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDITOFMS; Shimadzu/Kratos KOMPACT-MALDI-II) was also used to estimate the molecular weight distributions of the solubles. Representation of Product Yield. The residue recovered was washed by acetone and dried in vacuo at 70 °C before measuring its weight. The yield of soluble was estimated by difference. The carbon conversion, the fraction of carbon converted to the product from the carbon in the raw biomass samples, was used to represent the results.
3. Results and Discussion 3.1. Exploration of Optimum Delignification Condition of Oil Palm Shell. First, we examined the effects of water/acetone mixing ratio on the extraction. The original oil palm shell was extracted for 0.5 h at 180 °C, since it was found that the optimum hot water treatment condition was 180 °C from our previous study.13 Through the hot water treatment under this condition, hemicellulose in the shell was successfully recovered as 19%-saccharides and 5%-organic acids on the carbon basis, leaving lignin and cellulose, and ash was removed from the loose solid structure. Figure 1 shows the changes in the carbon conversion and the carbon content of the soluble through a water/acetone extraction of the oil palm shell. The carbon content of soluble was measured by the ultimate analysis of the extract dried up. Judging from the observation that the carbon content of lignin is higher than that of cellulose or hemicellulose, it seems that an increase in carbon content of the soluble was due to the increase in lignin component in the extracts. The yields of soluble com(12) Miura, K.; Mae, K.; Li, W.; Kusakawa, T.; Morozumi, F.; Kumano, A. Energy Fuels 2001, 15, 599-610. (13) Mae, K.; Hasegawa, I.; Sakai, N.; Miura, K. Energy Fuels, 2000, 14, 1212-1218.
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Figure 1. Changes in the production distributions through water/acetone extraction of the oil palm shell and the carbon contents of solubles.
ponents reached a maximum value of 25% when the concentration of acetone was 50%. The carbon content of soluble, however, increased with the increase of acetone concentration. These results indicate that the solubility of lignin in pure water or pure acetone is low (the solubility parameter δ-value is 28.0 (MPa)1/2 for Alcell lignin,14 47.9 (MPa)1/2 for pure water and 20.1 (MPa)1/2 for pure acetone15) and that lignin cannot be extracted until hemicellulose is removed from the shell by hot water treatment. Thus hemicellulose and lignin fractions were not completely recovered even in the best condition in the flow reactor system, and hot water treatment of the shell was found to be essential first of all. From this viewpoint, the shell treated in a hot water at 180 °C was extracted for several kinds of extraction time at 180 °C in an aqueous solution of 50% acetone. Figure 2(a) shows the change in the production distributions with the extraction time during the water/ acetone extraction of the sample pretreated by the hot water at 180 °C. The change in the carbon content of water/acetone-soluble is also shown in Figure 2(b). The carbon conversion to the water/acetone-soluble increased with the increase of the extraction time, and finally reached 25%. After 8 h have elapsed, however, CO2 was slightly produced and the carbon content of the water/acetone-soluble significantly decreased without the increase in the extraction yield. Since the lignin content in oil palm shell was 36.2% on carbon basis, the extraction temperature of 180 °C was found to be not enough to extract the whole lignin fractions. Next, we tried to explore an optimum water/acetone extraction temperature for the complete delignification. Figure 3 shows the effect of the water/acetone extraction temperature on the product distribution. With the increase in the extraction temperature, the carbon conversion to the solid residue dramatically decreased above 200 °C. The carbon distributions to the respective products after two-step extractions, which consist of a (14) Ni, Y.; Hu, Q. J. Appl. Polym. Sci. 1995, 57, 1441-1446. (15) Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd ed.; Wiley: New York, 1989; pp 519-557.
Figure 2. Changes in the production distributions through the extraction and the carbon content of water/acetone-solubles with the extraction time at 180 °C for the oil palm shell.
Figure 3. Change in the production distributions through water/acetone extraction with the extraction temperature for the oil palm shell.
hot water treatment at 180 °C and a water/acetone extraction at 230 °C, are 25% for the water-soluble, 35% for the water/acetone-soluble, and 40% for the solid residue. These values almost coincided well with the contents of three major components, that is to say hemicellulose, lignin, and cellulose in the oil palm shell, respectively. At 250 °C the carbon distribution to the water/acetone-soluble reached up to more than 45% and CO2 gas was formed by the decomposition of the shell. This seems to indicate that cellulose also is hydrolyzed at a high temperature above 250 °C.16 From these results, it was found that an optimum condition for water/acetone extraction was 230 °C of temperature and 1 h of extraction time. 3.2. Comparison of Extraction in Flowing Solvent and Batch Extraction. Thus, it was shown that the two-step extraction was effective to separate biom(16) Minowa, T.; Zhen, F.; Ogi, T. J. Supercrit. Fluids 1998, 13, 253259.
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Figure 4. Comparison of the carbon conversion during the extraction using a flow reactor and those obtained using a batch reactor for Japanese apricot tree.
ass into each component. When the method is applied to an actual process, a continuous stirred tank reactor (CSTR) will be adopted. In that case, the extracts are present in the extraction solvent. So, we examined the effect of extracts on the extraction yield by water/ acetone using a batch reactor system. Figure 4 compares the carbon distributions extracted at 200 and 230 °C using a flow reactor and that extracted at 200 °C using a batch reactor for the Japanese apricot tree. The onestep extraction with water/acetone was also examined. For the flowing system, the carbon conversions to the water-soluble, the water/acetone-soluble, and the residue were 19, 35, and 46%, respectively, at 230 °C. The value of the carbon conversion to the water/acetonesoluble coincided well with the content of lignin in the Japanese apricot tree, which was determined in accordance with TAPPI Standard T13m. Although 200 °C was not high enough to extract the whole lignin fraction, 230 °C was found to be effective in the complete delignification not only for the oil palm shell but also for the woody biomass such as an apricot tree. On the other hand, for the batch system the amount of water/ acetone-soluble reached up to 57% even at 200 °C by one-step extraction. Thus, the extraction yields at 200 °C between two reaction systems were quite different. The reason for that difference was caused by the reaction condition. The extracts were immediately swept out of the reactor in the flowing system; on the contrary, all the extracts remained in the batch reactor. So, the increase in the extraction yield by the one-step extraction using a batch reactor seems to be brought about by reaction between biomass and extracts. To clarify which components affect the extraction, we compared the extraction performance between one-step and twostep extractions at 200 °C using the batch reactor. As shown in Figure 4, the extraction yield by two-step extraction was lower than that by one-step extraction and was almost the same for the two-step extraction using a flow reactor system. This result clearly shows that the component extracted during a hot water treatment strongly affected the extraction of lignin. It is known that acetic acid was produced by the deacetylation of hemicellulose in biomass.13,17 It was also reported that organic acids could be used as catalysts
and reduce the delignification temperature.9 We measured pH of the extract solutions by the pH meter; the values of the water/acetone-soluble extracted at 200 °C from Japanese apricot tree were 4.2 for the flow reactor, and 3.6 for the batch reactor, respectively. Strong acidity for the soluble in batch system was caused by the presence of acidic compounds such as acetic acid. To clarify the above speculation, the two-step extraction at 200 °C with the water/acetone solution including 5 wt % of acetic acid was also performed. As shown in Figure 4, the extraction with water/acetone containing acetic acid at 200 °C achieved the almost equivalent performance to the extraction at 230 °C. This suggests that organic acids can be used as catalysts, and a stirred tank reactor has an advantage of holding the extractives as catalysts for autohydrolysis.18 To summarize the discussion, biomass can be delignified effectively by the one-step extraction using a batch reactor at 200 °C as well as by the two-step extraction using a flow reactor at 230 °C. If we desire only delignification to obtain pure cellulose, we conduct the one-step extraction under a high biomass concentration using a continuous stirred tank reactor. In that case, we can recycle and add the organic acids. On the other hand, if the complete separation into hemicellulose, cellulose, and lignin is desired, we conduct the twostep extraction. The solid-liquid separation process is necessary between both extraction processes. 3.3. Characterization of Products. To confirm the separation of lignin and cellulose, we compared the weight changes during the pyrolysis of the residues and pure cellulose. Figure 5 shows the weight changes during pyrolysis for the residues by the two different extraction methods, pure cellulose and the raw biomass samples. At 900 °C, char yield of each sample was given as follows: 21.9% for the shell residue, 12.4% for the raw shell, 6.6% for pure cellulose, 7.9% for the apricot residue, and 10.5% for the raw apricot. When the raw biomass samples were heated from room temperature, the weights started to decrease gradually at a lower
(17) Bouchard, J.; Nguyen, T. S.; Chornet, E.; Overend, R. P. Biores. Technol. 1991, 36, 121-131.
(18) Garrote, G.; Dominguez, H.; Parajo, J. C. Proc. Biochem. 2001, 36, 571-578.
Figure 5. Weight changes of the raw biomass, pure cellulose, and the residue during their pyrolysis.
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Figure 6. FTIR spectra of the residues and pure cellulose.
temperature of 200 °C. On the contrary, pure cellulose and the residues were pyrolyzed rapidly at around 350 °C. These results show that lignin was completely removed from the raw biomass by means of both flow and batch water/acetone extraction systems. On the other hand, the final char yield above 500 °C of the residue produced at 230 °C using a flow reactor system became much higher than that produced at 200 °C using a batch reactor system. This suggests that the water/ acetone extraction at 230 °C caused the appreciable increase in the char yield. Earlier work on the treatment with the pressurized hot water at 200 °C for 50 h was done by Sugimoto et al.19 as one of the confirmatory tests to trace the coalification behavior of cellulose into brown coal. According to their 13C NMR analysis, the spectra of hydrous residues from cellulose after 20 h were similar to that of Morwell brown coal. We examined the structures of the residues in the two different extraction conditions and pure cellulose by FTIR. The spectrum of the residue at 200 °C shown in Figure 6 was almost the same as that of pure cellulose. The spectrum of the residue at 230 °C was different. It was found that the amount of hydroxyl groups (assigned from 2400 to 3700 cm-1) decreased to approximately one-quarter that of pure cellulose by the Miura’s method.12 We can estimate the strength distribution of hydrogen bonds in the solid sample by this method. While hydroxyl groups disappeared, the formation of carbonyl groups (assigned from 1630 to 1780 cm-1) was observed. These results mean that the water/acetone extraction at 230 °C promoted the cross-linked structure through the dehydration reaction of hydroxyl groups in cellulose coincidentally during delignification. This led to the increase in char yield of residual cellulose. To confirm the above explanation, we examined the elemental balance from the data in Table 1. The average molecular formula for the residue at 200 °C was C6H9.3O4.6, which was close to that of pure cellulose (C6H10O5), and that for the residue at 230 °C was C6H7.7O3.5, which was richer in carbon. The char yield and the carbon content became higher for the residue of apricot at 230 °C, too. From these results it was clarified that the temperature of a water/acetone extraction was a crucial factor to control the properties of the cellulose products. Utilizing this feature, a water/ acetone extraction would be an effective method to control the O/C atomic ratio in the cellulose product. Next we examined how lignin was dissolved in the solvents through the water/acetone extraction. The molecular weight distributions (MWDs) of the solubles (19) Sugimoto, Y.; Miki, Y.; Hayamizu, K.; Umeda, S.; Komano, T.; Mashimo, K.; Wainai, T. J. Jpn. Inst. Energy 1996, 75, 829-838.
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Figure 7. Number basis MWDs of the solubles.
were measured by use of the MALDI-TOFMS, which is very convenient not only for solubilized samples but also for solid samples. Figure 7 shows the number basis MWDs of the water/acetone-solubles prepared with the two different extraction methods. No matrix was required for the measurement of these MWDs. The MWD of the extraction-soluble obtained by a flowing system was quite different from that by a batch system. The MWD of the extraction-soluble obtained by a flowing system had a single peak at Mw ) 900. On the other hand, the MWD of the extraction-soluble obtained by a batch system had several peaks: one sharp peak at Mw ) ca. 300, and two broad peaks at Mw ) 1800 and 7200. From the previous study,13 the peak at Mw ) 300 was found to be consisting of saccharides such as xylobiose which came from hemicellulose hydrolysis using a batch reactor. It is probable that the differences in the MWDs were caused by the extraction temperatures and types; the soluble at 230 °C was lower molecular weights than that at 200 °C since its higher extraction temperature accelerated depolymerization of lignin, and the MWD had two broad peaks for the batch system due to the secondary self-condensation of solvolysis fragments. Sano et al.20 demonstrated the following delignification mechanisms; ether linkages in lignin were cleavaged by homolysis and its radical products were attacked nucleophilically by the solvent molecules during organosolve delignification. They also reported that the solubilized lignin was found to be lower molecular weight than MWL (Milled Wood Lignin) from the GPC measurement.21 Considering that the peak positions of our soluble MWD were at Mw ) 900, 1800, and 7200, it was judged that lignin was depolymerized into the soluble components, of which molecular weights were in multiples of 900, and extracted by the solvent. This result suggests that the water/acetone-soluble components consist of oligomer (in the range from several to several dozen degrees of polymerization) of syringyl-aromatic nuclei units. 3.4. Subsequent Conversion for the Separated Products. Lignin and cellulose in biomass were thoroughly separated into the soluble and the residue as mentioned above. Next, feasible methods to convert appropriately them into valuable chemicals were explored according to their respective characteristics. Pure cellulose, that is to say the residue at 200 °C through the batch extraction system, can be converted into glucose by the acid saccharification22 or the hot com(20) Sano, Y. Mokuzai Gakkaishi 1989, 35, 813-819. (21) Sano, Y.; Maeda, H.; Sakashita, Y. Mokuzai Gakkaishi 1989, 35, 991-995. (22) Saeman, J. F. Ind. Eng. Chem. 1945, 37, 43-52.
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pressed water hydrolysis,23 followed by the alcohol fermentation. In addition to this, pyrolysis is one method to recover the high-yield volatile matters by taking advantage of the easy decomposability of cellulose. Contrary to these conversions, carbonization is applicable to the cross-linked cellulose, the residue at 230 °C through the flow extraction system. Although a limited yield has been a problem for the production of activated carbon from biomass so far, the increase in the char yield brought about by the cross-linked structure meets our requirements when we fix the activated carbon. As just described, at low temperature a control of the reaction concerned with the functional groups is found to be highly important for the solid biomass conversion. Now let us shift to the extracted lignin. The extractives can be recovered as soluble lignin when water/ acetone mixed solvent is evaporated. Since lignin has a high heating value and low decomposability, the suitable conversions for the solubilized lignin are judged to be combustion or gasification, especially the catalytic hydrothermal gasification. We have already confirmed the formation of fuel gases such as H2 and CH4 by using the catalyst and technique of Nakagawa et al.24 The yields of both gases reached up to ca. 0.4 mol/mol-feedcarbon, respectively, at 350 °C, LHSV (Liquid Hourly Space Velocity) ) 50 h-1. This method has the merits of rapid reaction and nondischarging of the organic compounds. Conclusion New pretreatment methods for completely separating biomass components were presented. The methods consist of a hot water treatment and a water/acetone extraction, in other words two-step extraction, or onestep water/acetone extraction. The optimum extraction temperatures are as follows: 180 °C for a hot water treatment, 230 °C for a water/acetone extraction (twostep), and 200 °C for one-step water/acetone extraction, respectively. The optimum acetone concentration is 50% for the water/acetone extractions. Figure 8 summarizes the pretreatment processes newly proposed. In a two-step extraction, hemicellulose was recovered as saccharides through the hot water treatment at 180 °C, leaving lignin and cellulose in the (23) Mok, W. S-L.; Antal, M. J., Jr. Ind. Eng. Chem. Res. 1992, 31, 1157-1161. (24) Nakagawa, H.; Nanba, A.; Boehlmann, M.; Miura, K. Prepr. PapsAm. Chem. Soc, Div. Fuel Chem. 2003, 48 (1), 464-465.
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Figure 8. The schematic diagram of the proposed pretreatment processes.
solid. After the water/acetone extraction of the solid with 50%-acetone at 230 °C, cross-linked cellulose and lower molecular weight solubilized lignin were recovered as the residue and the soluble, respectively. In a one-step extraction, saccharides and lignin were recovered together at 200 °C because of being in the presence of hydrolyzed organic acids. The residue was pure cellulose. In addition, the amount of the CO2 emission at the stage of delignification decreased considerably due to being free from the oxidative decomposition. Furthermore the newly proposed processes have the advantage of recovery of the solvent. Because acetone is far more volatile than water, we can easily recover 90% of the acetone utilized during delignification by means of a flash distillation at 75 °C, in which the required heat source is judged to be available from the waste heat. Thus, the proposed pretreatment processes were found to realize mild separations of biomass into main three components. Acknowledgment. This work was financially supported by the NIRO (New Industry Research Organization) as one of Regional Science Promoter Programs conducted by JST, and by the Ministry of Education, Culture, Sports, Science and Technology of Japan, through the Grant-in-Aid on Scientific Research B(2) (Grant No. 15360429) and for Encouragement of Young Scientists B (Grant No. 15760577). EF030148E
