Bioethanol Production from Lignocellulosics Using Supercritical Water

Chapter 27

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Bioethanol Production from Lignocellulosics Using Supercritical Water Shiro Saka and Hisasbi Miyafuji Graduate School of Energy Science, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan

Bioethanol production from lignocellulosics with supercritical water (>374°C, >22.1MPa) technology was studied. Lignocellulosics after supercritical water treatment was fractionated into supercritical water insoluble residue and a supercritical water-soluble portion, which was further fractionated after 12 hours into the water-soluble portion, precipitates, methanol-soluble portion.. In the water-soluble portion, not only fermentable monomeric sugars but also oligomers were recovered. The oligomers and precipitates as polysaccharides could be easily hydrolyzed to glucose with cellulose, but not be fermented to ethanol with Saccharomyces cerevisiae due to the inhibitory contaminants. Thus, wood charcoal and overliming treatments were studied. As a result, wood charcoal was found to remove these inhibitors without removing the fermentable sugars. Furthermore, all fermentable sugars in the water-soluble portion could be converted to ethanol in the binary fermentation system of Saccharomyces cerevisiae and Pachysolen tannophilus. Consequently, supercritical water treatment is thought to be a promising pretreatment for ethanol production from lignocellulosics.

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© 2007 American Chemical Society In Materials, Chemicals, and Energy from Forest Biomass; Argyropoulos, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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Introduction Energy and environmental issues such as the exhaustion of fossil resources and global warming are of major concern. Due to its environmental friendliness, increased focus has been on ethanol production from biomass as an alternative to fossil fuels. Among various biomass resources, lignocellulosics such as wood can be used as one of the raw materials for producing ethanol. Various studies have been performed on the pretreatment of wood such as hydrolysis of lignocellulosics by acid catalysis (7, 2) and enzymatic saccharification (3) to obtain the fermentable sugars for ethanol production. Recently, supercritical fluid technology has been applied in the conversion of lignocellulosics to fuels and chemicals (4-24). In this chapter, recent progress in bioethanol production from lignocellulosics by supercritical water technology is presented.

Supercritical Water Treatment of Lignocellulosics Supercritical Water Supercritical water (>374°C, >22.1MPa) behaves very differently than water under normal conditions, exemplified by the following (25): The ionic product of water (Kw) is about three to four order magnitude larger in its supercritical state than that in a normal state. Therefore, supercritical water can act as an acid catalyst. Furthermore, the dielectric constant of supercritical water is in a range of 10 and 20, compared with about 80 in ordinary water. Thus, the hydrophobic substances can be solvated with the supercritical water. For the treatment of lignocellulosics with supercritical water, three different types of systems such as batch-type (11, 17, 18), semiflow-type (19, 20) and flow-type (11, 21-24) system have been developed by several researchers. More detailed information on these systems is described elsewhere (//, 26).

Separation of Treated Lignocellulosics Figure 1 shows the separation scheme of the lignocellulosics treated in supercritical water. The supercritical water-soluble portion is separated by filtration from the supercritical water-insoluble portion, immediately after the treatment. While standing for 12 hours, the supercritical water-soluble portion results in the precipitates and oily substances due to the change of water in dielectric constant from supercritical state to ordinary one. They are filtrated to

In Materials, Chemicals, and Energy from Forest Biomass; Argyropoulos, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

424 Lignocellulosics Supercritical water treatment Treated sample


Filtration

Supercritical water-soluble

Supercritical water-insoluble

12h standing Methanol extraction Filtration

Water-soluble

Water-insoluble ( Precipitates)

Methanol-soluble (Oily substances)

Methanol-insoluble

Figure L Separation of lignocellulosics treated in supercritical water.

separate from the water-soluble portion. The precipitates and oily substances are extracted with methanol, to separate into precipitates and methanol-soluble portion by filtration. Consequently, the water-soluble portion, water-insoluble portion (precipitates), methanol-soluble portion (oily substances) and methanolinsoluble portion are the results from the treated sample. Therefore, the former two and latter two portions mainly consist of carbohydrate-derived and ligninderived products, respectively (5).

Decomposition of Cellulose and Hemicelluloses Cellulose was decomposed by supercritical water and separated to watersoluble portion. This portion contained cello-oligosaccharides (oligomers) and glucose as hydrolyzed products, while levoglucosan, 5-hydroxymethyl furfural (5-HMF) and furfural were found as dehydrated products. In addition, fragmented products such as methylglyoxal, glycolaldehyde, dihydroxyacetone and erythrose were found. Furthermore, low-molecular weight organic acids, such as pyruvic acid, lactic acid, glycolic acid and acetic acid, were identified by H P L C or G P C analysis (77, 17-24). It was also clarified by matrix assisted laser desorption ionization-time of flight mass spectrometric ( M A L D I - T O F M S ) analysis that oligomers composed of cello-oligosaccharides up to cello-dodecaose (Degree of polymerization;


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DP=12), and that reducing terminal glucose unit of some oligosaccharides were decomposed to levoglucosan, erythrose and glycolaldehyde. On the other hand, the precipitates were revealed to be glucan with D P between 13 and 100, which is insoluble in ordinary water but soluble in supercritical water (77). Table I shows the chemical composition of cellulose treated in supercritical water. The total yield of hydrolyzed products, which are polysaccharides

Table I. Chemical Composition of Cellulose Treated in Supercritical Water at 380°C and 40MPa Using Flow-type System (27).

Product name Hydrolyzed products Polysaccharides Oligosaccharides Glucose Fructose (Subtotal) Dehydrated products Levoglucosan 5-HMF Furfural (Subtotal) Fragmented products Erythrose Methylglyoxal Glycolaldehyde Dihydroxyacetone (Subtotal) Organic acids Pyruvic acid Glycolic acid Lactic acid Formic acid Acetic acid (Subtotal) Others* Total

0.12s

Yield (%) 0.24s

0.48s

31.2 41.1 2.8 0.5 (75.6)

12.1 37.6 8.4 3.4 (61.5)

n.d. 7.5 8.9 10.0 (26.4)

0.2 0.1 n.d. (0.3)

2.4 1.9 1.0 (5.3)

3.7 7.3 3.4 (14.4)

0.7 0.6 2.5 0.2 (4.0)

0.9 3.0 6.7 1.6 (12.2)

1.9 6.7 14.6 2.4 (25.6)

n.d. 1.3 0.7 0.3 0.1 (2.4) 17.7 100.0

0.5 2.9 3.2 1.6 0.7 (8.9) 12.1 100.0

0.7 3.9 4.0 2.5 1.0 (12.1) 21.5 100.0

* Others consist of unidentified products, gasified products and water derived from dehydration of saccharides; n.d, not detected.


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(precipitates), oligosaccharides, glucose and fructose, is 75.6% for 0.12s in treatment time, and decreased when the treatment time is prolonged. On the other hand, the yields of dehydrated products, fragmented products and organic acids are increased. Thus, the shorter treatment time is favorable to obtain the hydrolyzed products which can be used for ethanol production. Hemicelluloses were also decomposed by supercritical water into water-soluble portion, in

Table II. Lignin-derived Monomeric Products in the Methanol-soluble Portion from Japanese Cedar and Japanese Beech. Product Guaiacol Methylguaiacol Ethylguaiacol Vinylguaiacol Syringol Vanillin Eugenol Propylguaiacol Methylsyringol Isoeugenol (cis, trans) Homovanillin Acetoguaiacone Propioguaiacone Ethylsyringol Guaiacylacetone Vinylsyringol Propylsyringol 2-Methoxy-4-(l -hydroxypropyl) phenol Allylsyringol Syringaldehyde Propenylsyringol (cis, trans) Homovanillic acid Sinapylalcohol (cis, trans) 2-Methoxy-4-(prop-1 -en-3-one) phenol Sinapylaldehyde (cis, trans) Acetosyringone Coniferylaldehyde (trans) Propiosyringone Syringylacetone Ferulic acid

Structure * G G-C G-C-C G-C=C S G-CHO G-C-C=C G-C-C-C S-C G-C=C-C G-C-CHO G-CO-C G-CO-C-C S-C-C G-C-CO-C S-C=C S-C-C-C G-C-C-C-OH S-C-C=C S-CHO S-C=C-C G-C-COOH S-C=C-C-OH G-CO-C=C S-C=C-CHO S-CO-C G-C=C-CHO S-CO-C-C S-C-CO-C G-C=C-COOH

* G and S represent guaiacyl and syringyl nuclei, respectively.


427 which the hydrolyzed products could be expected such as glucose and mannose from glucomannan, and xylose from xylan. These monomeric sugars can also be utilized for ethanol production (5,19).


Decomposition of Lignin The methanol-soluble portion was obtained only from the sample containing lignin, and its yield was close to the lignin content of the sample when the appropriate condition was selected. Therefore, the methanol-soluble portion mainly consisted of lignin-derived products. The molecular weight of the methanol-soluble portion was found to be less than 2,000 (J). Table II shows lignin-derived monomeric products identified in the methanol-soluble portion from Japanese cedar and Japanese beech (14). The products composed of phenylpropane units (C6-C3) were derived through the cleavage of ether linkages of lignin. In addition to C6-C3 units, C6-C2 and C6C l units of products were also identified.

Ethanol Production Ethanol Production from Cellulose The oligosaccharides in the water-soluble portion or polysaccharides as precipitates obtained by supercritical water treatment of cellulose can be the substrates for ethanol production through their conversion to glucose which can be fermented with microorganisms. To obtain glucose from the water-soluble portion and the precipitates, enzymatic hydrolysis with cellulase and βglucosidase were carried out. Both substrates were found to be hydrolyzed to glucose more easily than to untreated cellulose as shown in Fig. 2. Due to the lower DP and non-crystalline structure of the oligosaccharides and precipitates, higher accessibility of the enzymes must be attained (28, 29). These results clearly show that supercritical water treatment of cellulose can enhance enzymatic activity. However, the glucose concentration obtained after enzymatic hydrolysis was found to be lower than prospected. By the model experiments on the enzymatic hydrolysis of cellohexaose, it was shown that various compounds as in Table I, obtained by supercritical water treatment of cellulose, have inhibitory effects on enzymatic hydrolysis. Organic acids showed much higher inhibition to reduce the yield of glucose in enzymatic hydrolysis, compared to the fragmented and dehydrated products. These results could account for the lower glucose yield after the enzymatic hydrolysis of the water-soluble portion and precipitates. To increase glucose yield, overtiming or wood charcoal


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treatment of the water-soluble portion was found to be effective as shown in Fig. 2. This is probably due to the conversion of the inhibitors to the non-inhibiting compounds seen in overliming treatment. In wood charcoal treatment, however, inhibitors can be adsorbed. These results clearly show that supercritical water treatment coupled with overliming or wood charcoal treatment is effective to achieve ethanol from cellulose.

Ethanol Production from Lignocellulosics In the water-soluble portion from Japanese cedar, various fermentable sugars such as glucose, fructose, mannose, galactose and xylose can be recovered from cellulose and hemicelluloses. However, these sugars could not be converted to ethanol by the fermentation with Saccharomyces cerevisiae. From H P L C analysis on the water-soluble portion, vanillin, acetoguaiacone, guaiacol and coniferylaldehyde, which are lignin-derived products, were quantified as in Table III (described as "Untreated"). These compounds are

Theoretical (water-soluble porotion)

Hydrolysis time (h)

Figure 2. Changes in glucose concentration during enzymatic hydrolysis of the precipitates (L), water-soluble portion (%) and that treated by overliming (O) or wood charcoal (Π). Just comparison, untreated cellulose (M) is also included.


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mainly contained in the methanol-soluble portion as in Table II, but contaminated in the water-soluble portion. In addition, furan compounds such as furfural and 5-HMF were also found as in Table III. It is known that various furan and lignin-derived products can inhibit the fermentation of sugars to ethanol (30). Therefore, the poor fermentability seen was due to the presence of these compounds. Thus, to achieve high ethanol production, some detoxification treatments are required.

Table III. Concentrations of Furans, Lignin-derived Products and Sugars in the Water-soluble Portion and that Treated with Wood Charcoal Prepared at 900°C. Concentration in the water-soluble portion (mg/L) Treated with wood charcoal Untreated 5-HMF 378 n.d. Furfural 240 n.d. Vanillin n.d. 818 Acetoguaiacone n.d. 3 Guaiacol 33 n.d. 94 Coniferylaldehyde n.d. Glucose Other sugars*

1.04X10 0.92X10

3

3

*The total of fructose, mannose, galactose and xylose,

1.04xl0 0.94xl0

3

3

n. d. ; Not detected.

Improvement of Fermentability with Wood Charcoal Various detoxification methods to improve the fermentability of the hydrolysates obtained from lignocellulosics have been studied such as extraction with organic solvents (57), overliming (32-34), evaporation (33), steam stripping (35, 36), sulfite treatment (33, 37), ion-exchange (38), enzyme treatment (39, 40), zeolite treatment (41) and activated carbon treatment (35, 42): In our research, wood charcoal with high adsorption ability has been applied for a detoxification of the water-soluble portion. As in Table III, furan and ligninderived products were adsorbed completely and not detected anymore after wood charcoal treatment. The adsorption ability of these compounds was higher in the wood charcoal prepared at higher temperature (43). On the other hand, sugars were revealed to remain constant in its concentration (Table III). The



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2

4

6

8

Fermentation time (h)

Figure 3. Concentration changes of sugars and ethanol during the fermentation of the water-soluble portion treated with the wood charcoal. (Π) glucose, (M) fructose + mannose + galactose + xylose, (%) ethanol

wood charcoals can, therefore, selectively remove the fiiran and lignin-derived products without removing the fermentable sugars. This absorption behavior of the wood charcoal is preferable to achieve the high fermentability of sugars to ethanol. The water-soluble portion after wood charcoal treatment can be fermented with Saccharomyces cerevisiae to produce ethanol, as shown in Fig. 3. A l l consumed sugars in the water-soluble portion were found to be converted to ethanol. This result indicates that the water-soluble portion treated with the wood charcoal could be fermented effectively. However, xylose which cannot be fermented to ethanol by Saccharomyces cerevisiae still remained in the water-soluble portion after fermentation. To achieve effective ethanol production from lignocellulosics, the remaining xylose should be fermented. Therefore, a binary fermentation system to mix Saccharomyces cerevisiae with Pachysolen tannophilus, with which xylose can be fermented to ethanol, was applied to the water-soluble portion after wood charcoal treatment. A s shown in Fig. 4, after the ethanol production with Saccharomyces cerevisiae, the remaining xylose was found to be fermented to ethanol by Pachysolen tannophilus. In this binary system, therefore, all fermentable sugars can be utilized by both yeasts to produce ethanol.



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20

40

Fermentation time (h)

Figure 4. Changes of ethanol concentration in binary fermentation system of the water-soluble portion after wood charcoal treatment. (Π) Saccharomyces cerevisiae, (M) Saccharomyces cerevisiae and Pachysolen tannophilus

Conclusions Compared to other pretreatments of lignocellulosics for ethanol production, supercritical water treatment is a non-catalytic and rapid reaction. Although the various degradation products of lignocellulosics displayed some inhibitory effects on enzymatic hydrolysis and subsequent fermentation with yeasts, the inhibition can be reduced by the use of wood charcoal and/or overliming. Therefore, the supercritical water treatment followed by enzymatic hydrolysis is thought to be a promising process for bioethanol production from lignocellulosics.

Acknowledgements This research has been carried out under the program of the Research for the Future (RFTF) of The Japan Society o f the Promotion of Science (JSPSRFTF97P01002), the C O E program in the 2 1 Century o f "Establishment o f C O E on Sustainable-Energy System" and by a Grant-in-Aid for Scientific Research (B)(2) (No. 12460144) and a Grant-in-Aid for Young Scientists (B) (No. 17780139) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. st


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References 1. 2.


3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.

Torget, R. W.; K i m , J. S.; Lee, Y. Y. Ind. Eng. Chem. Res. 2000, 39, pp 2817-2825. Iranmahboob, J.; Nadim, F.; Monemi, S. Biomass Bioenergy 2002, 22, pp 401-404. Ortega, N.; Busto, M. D.; Perez-Mateos, M. Int. Biodeter. Biodeg. 2001, 47, pp 7-14. Ueno, T.; Saka, S. Cellulose 1999, 6, pp 177-191. Saka, S.; Konishi, R. In Progress in Thermochemical Biomass Conversion; Bridgwater, A . V . , Ed.; Blackwell Sci.: Oxford, 2001; pp 1338-1348. Ishikawa, Y.; Saka, S. Cellulose 2001, 8, pp 189-195. Ehara, K . ; Saka, S.; Kawamoto, H . J. Wood Sci. 2002, 48, pp 320-325. Tsujino, J.; Kawamoto, H . ; Saka, S. Wood Sci. Technol. 2003, 37, pp 299307. Minami, E.; Saka, S. J. Wood Sci. 2003, 49, pp 73-78. Minami, E.; Kawamoto, H . ; Saka, S. J. Wood Sci. 2003, 49, pp 158-165. Ehara, K . ; Saka, S. Cellulose 2002, 9, pp 301-311. Takada, D.; Ehara, K . ; Saka, S. J. Wood Sci. 2004, 50, pp 253-259. Ehara, K . ; Saka, S. J. Wood Sci. 2005, 51, pp 148-153. Ehara, K . ; Takada, D.; Saka, S. J. Wood Sci. 2005, 51, pp 256-261. Minami, E.; Saka, S. J. Wood Sci. 2005, 51, pp 395-400. Yoshida, K . ; Kusaki, J.; Ehara, K . ; Saka, S. Appl. Biochem. Biotechnol. 2005, 123, pp 795-806. Sakaki, T.; Shibata, M.; M i k i , T.; Hirosue, H . ; Hayashi, N. Bioresour. Technol. 1996, 58, pp 197-202. Sakaki, T.; Shibata, M.; M i k i , T.; Hirosue, H . ; Hayashi, N. Energy Fuels 1996, 10, pp 684-688. Ando, H . ; Sakaki, T.; Kokusho, T.; Shibata, M.; Uemura, Y . ; Hatate, Y. Ind. Eng. Chem. Res. 2000, 39, pp 3688-3639. Sakaki, T.; Shibata, M.; Sumi, T.; Yasuda, S. Ind. Eng. Chem. Res. 2002, 41, pp 661-665. Sasaki, M.; Kabyemela, B . ; Malaluan, R.; Hirose, S.; Takeda, N.; Adschiri, T.; Arai, K . J. Supercrit. Fluids 1998, 13, pp 261-268. Sasaki, M.; Fang, Z.; Fukushima, Y.; Adschiri, T.; Arai, K . Ind. Eng. Chem. Res. 2000, 39, pp 2883-2890. Sasaki, M.; Adschiri, T.; Arai, K . AIChE J. 2004, 50, pp 192-202. Matsunaga, M.; Matsui, H . Mokuzai Gakkaishi (in Japanese) 2004, 50, pp 325-332. Holzapfel, W . B . J. Chem. Phys. 1969, 50, pp 4424-4428. Kusdiana, D.; Minami, E.; Ehara, K ; Saka, S. In 12th European Conference and Technology Exhibition on Biomass for Energy, Industry and Climate Protection: Amsterdam, 2002; pp 789-792.



433 27. Ehara, Κ.; Saka, S. In Lignocellulose Biodegradation; Saha, B.C.; Hayashi, K., Ed.; A C S Symposium Series 889; A C S : Washington, D C , 2005; pp 6983. 28. Sasaki, M.; Iwasaki, K . ; Hamatani, T.; Adschiri, T.; Arai, K . Kobunshi Ronbunshu (in Japanese) 2001, 58, pp 527-532. 29. Nakata, T.; Miyafuji, H . ; Saka, S. Appl. Biochem. Biotechnol. 2006, (in press). 30. Palmqvist, E.; Hahn-Hägerdal, Β. Bioresource Technol. 2000, 74, pp 25-33. 31. Wilson, J.J.; Deschatelets, L.; Nishikawa, N . K . Appl. Microbiol. Biotechnol. 1989, 31, pp 592-596. 32. Palmqvist, E.; Hahn-Hägerdal, Β.; Galbe, M.; Zacchi, G . Enzyme Microb. Technol. 1996, 19, pp 470-476. 33. Larsson, S.; Reinmann, Α.; Nilvebrant, N-O.; Jönsson, L.J. Appl. Biochem. Biotechnol. 1999, 77-79, pp 91-103. 34. Martinez, Α.; Rodriguez, M . E . ; Wells, M . L . ; York, S.W.; Preston, J.F.; Ingram L.O. Biotechnol. Prog. 2001, 17, pp 287-293. 35. Maddox, I.S.; Murray A . E . Biotechnol. Lett. 1983, 5, pp 175-178. 36. Y u , S.; Wayman, M.; Parekh, S.K. Biotechnol Bioeng. 1987, 29, pp 11441150. 37. Parajó, J.C.; Dominguez, H . ; Domínguez, J . M . Enzyme Microb. Technol. 1997, 27, pp 18-24. 38. Nilvebrant, N-O.; Reinmann, Α.; Larsson, S.; Jönsson, L.J. Appl. Biochem. Biotechnol. 2001, 91-93, pp 35-49. 39. Jönsson, L . J . ; Palmqvist, E.; Nilvebrant, N - O . ; Hahn-Hâgerdal, B . Appl. Microbiol Biotechnol. 1998, 49, pp 691-697. 40. Palmqvist, E.; Hahn-Hägerdal, B . ; Szengyel, Z.; Zacchi, G.; Reczey, K . Enzyme Microb. Technol. 1997, 20, pp 286-293. 41. Eken-Saraçoğlu, N.; Arslan, Y. Biotechnol. Lett. 2000, 22, 855-858. 42. Gong, C.S.; Chen, C.S.; Chen, L.F. Appl. Biochem. Biotechnol. 1993, 3940, pp 83-88. 43. Miyafuji, H . ; Nakata, T.; Ehara, K . ; Saka, S. Appl. Biochem. Biotechnol. 2005, 124, pp 963-971.


Bioethanol Production from Lignocellulosics Using Supercritical Water

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