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Conversion of Lignin with Supercritical Water-Phenol Mixtures Motofumi Saisu,† Takafumi Sato,‡ Masaru Watanabe,§ Tadafumi Adschiri,⊥ and Kunio Arai*,†,‡,§ Department of Chemical Engineering, Tohoku University, 07 Aoba Aramaki, Aoba, Sendai, 980-8579, Japan, Supercritical Fluid Research Center, National Institute of Advanced Industrial Science and Technology, 4-2-1, Nigatake, Miyagino, Sendai, 983-8551, Japan, Research Center of Supercritical Fluid Technology, Tohoku University, 07 Aoba, Aramaki, Aoba, Sendai 980-8579, Japan, and Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1, Katahira, Aoba, Sendai, 980-8577, Japan Received December 13, 2002
The decomposition of lignin was examined in supercritical water with and without phenol at 673 K. In the absence of phenol, the yield of tetrahydrofuran (THF)-insoluble (TIS) products decreased and the molecular weight distribution of THF-soluble (TS) products shifted toward lower molecular weights as the water density increased. The increase in water density enhanced the lignin conversion. In the presence of phenol, the yield of TIS products was lower and the molecular weight distribution of TS products shifted toward lower molecular weights than those in the absence of phenol. Some alkylphenols were obtained only in the presence of phenols, because of the reaction of phenol with the decomposition products. These results show that the reaction of phenol with reactive sites occurred in supercritical water and suppressed cross-linking reactions among reactive sites of large fragments. This promoted the decomposition of lignin to lowermolecular-weight compounds.
Introduction Plant biomass consists of ∼50 wt % cellulose, 20 wt % hemicellulose, and 30 wt % lignin. Lignin has been proposed to be an alternative source of chemicals for fossil resources such as coal and petroleum.1 Lignin is a biopolymer in which hydroxyphenylpropane units such as trans-p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol are connected with ether and carboncarbon linkages1-3 in a helical structure.4 Phenolic chemicals can be obtained from lignin by some chemical processes. Freudenberg et al.5 reported that vaniline was produced from lignin by the alkali-nitrobenzene method. Kashima6 conducted a conversion of Kraft lignin, using an iron oxide catalyst at 673-713 K under a hydrogen atmosphere of 19.6 MPa. They obtained 33.6 wt % light oil and 23.3 wt % heavy oil, as well as 20.2 wt % phenol and 14.4 wt % benzene. * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Department of Chemical Engineering, Tohoku University. ‡ Supercritical Fluid Research Center, National Institute of Advanced Industrial Science and Technology. § Research Center of Supercritical Fluid Technology, Tohoku University. ⊥ Institute of Multidisciplinary Research for Advanced Materials, Tohoku University. (1) Dorrestijn, E.; Laarhoven, L. J. J.; Arends, I. W. C. E.; Mulder, P. J. Anal. Appl. Pyrolysis 2000, 54, 153-192. (2) Lin, S. Y., Dence, C. Y., Eds. Methods in Lignin Chemistry; Springer- Verlag: Berlin, 1992; pp 50-142. (3) Alder, E. Wood Sci. Technol. 1977, 11, 169-218. (4) Faulon, J.; Hatcher, P. G. Energy Fuels 1994, 8, 402-407. (5) Freudenberg, K. Angew. Chem. 1939, 52, 362-363. (6) Kashima, H. Noguchi Kenkyusho Jiho 1960, 9, 23-35.
Supercritical water (Tc ) 647.3 K and Pc ) 22.1 MPa) has received attention as a reaction solvent for biomass, because of its unique properties, as described below. Supercritical water is the so-called dense steam; it is miscible with light gases, hydrocarbons, and aromatics, and its dielectric constant is ) 2-20 near the critical point, which is similar to that of polar organic solvents at room temperature.7,8 Physical properties such as and pH can be changed significantly by manipulating the temperature and pressure with a great change in water density.7,8 Furthermore, various acid or base organic reactions that typically use catalysts proceed in subcritical and supercritical water without a catalyst.7-9 The decomposition of lignin model compounds has been studied in supercritical water. For example, guaiacol is easily hydrolyzed to catechol and methanol in supercritical water without a catalyst.10 The dealkylation of 2-isopropylphenol occurs and the dealkylation rate increases as the water density increases.11 In other words, the ether and carbon-carbon bonds in the lignin structure probably can be cleaved in supercritical water. Noncatalytic processes of reforming biomass have been performed in subcritical and supercritical water.12-17 (7) Savage, P. E. Chem. Rev. 1999, 99, 603-621. (8) Savage, P. E.; Gopalan, S.; Mizan, T. I.; Martino, C. J.; Brock, E. E. AIChE J. 1995, 41, 1723-1778. (9) Katritzky, A. R.; Nichols, D. A.; Siskin, M.; Murugan, R.; Balasubramanian, M. Chem. Rev. 2001, 101, 837-892. (10) Lawson, J. R.; Klein, M. T. Ind. Eng. Chem. Res. 1985, 24, 203208. (11) Sato, T.; Sekiguchi, G.; Saisu, M.; Watanabe, M.; Adschiri, T.; Arai, K. Ind. Eng. Chem. Res. 2002, 41, 3124-3130. (12) Mok, W. S. L.; Antal, M. J., Jr. Ind. Eng. Chem. Res. 1992, 31, 1157-1161.
10.1021/ef0202844 CCC: $25.00 © 2003 American Chemical Society Published on Web 05/15/2003
Conversion of Lignin with Water-Phenol Mixtures
A large portion of hemicellulose in sugar cane can be solubilized in hot compressed water at 463-503 K and 34.5 MPa with a tubular percolating reactor.12 Ando et al.13 reported that >95 wt % of bamboo and chinquapin could be extracted with water at 558 K and 9.8 MPa. Sakaki et al.14 decomposed cellulose with hot compressed water and examined the fermentation of products for the production of alcohol. Antal et al.15 conducted the gasification of biomass with a carbon catalyst in supercritical water at >923 K and achieved almost complete gasification to produce hydrogen, carbon dioxide, and methane as the major products. Sasaki et al.16,17 conducted cellulose decomposition in supercritical water at 623-673 K under noncatalytic conditions and obtained a yield of hydrolysis product of ∼75%. In hightemperature water, especially near the critical temperature, hydrolysis is one of the main factors to promote the decomposition of biomass. Yoshida and Matsumura18 conducted gasification of lignin mixtures with a nickel catalyst in supercritical water at 673 K and revealed the importance of the lignin fraction in gasification characteristics. Johnson et al.19 conducted the liquefaction of organosolve lignin in supercritical water, using a platinum catalyst at 648 K under 27.6 MPa of hydrogen and found that the yield of hydrolysis products such as catechol increased as the reaction time increased. Water density seems to affect the product distribution in lignin conversion in supercritical water. Funazukuri et al.20 reported that the yield of oil increased as the water density in lignin conversion with supercritical water at 673 K increased. Yokoyama et al.21 examined the decomposition of lignin in supercritical water at 623-693 K up to 40 MPa and reported that the yield of oil and that of products containing hydroxyl groups increased and that of char decreased as the water density increased. These results indicate that hydrolysis is an important reaction for lignin decomposition in supercritical water; however, the formation of char due to the condensation also occurs. On the other hand, the use of phenol and waterphenol mixtures is an effective liquefaction technique for lignin. Wayman and Lora22 reported that the addition of phenolic compounds suppressed the condensation among reactive intermediates from the lignin decomposition products during auto autohydrolysis at 448 K. Lin et al.23-25 examined the reaction of guaiacygrycerolβ-guaiacyl ether as lignin model compounds in a water(13) Ando, H.; Sakaki, T.; Kokusho, T.; Shibata, M.; Uemura, Y.; Hatate, Y. Ind. Eng. Chem. Res. 2000, 39, 3688-3693. (14) Sakaki, T.; Shibata, M.; Miki, T.; Hirosue, H. Energy Fuels 1996, 10, 684-688. (15) Antal, M. J., Jr.; Allen, S. G.; Schulman, D.; Xu, X. Ind. Eng. Chem. Res. 2000, 39, 4040-4053. (16) Sasaki, M.; Kabyemela, B. M.; Malaluan, R. M.; Hirose, S.; Takeda, N.; Adschiri, T.; Arai, K. J. Supercrit. Fluids 1998 13, 280286. (17) Sasaki, M.; Fang, Z.; Fukushima, Y.; Adschiri, T.; Arai, K. Ind. Eng. Chem. Res. 2000, 39, 2883-2890. (18) Yoshida, T.; Matsumura, Y. Ind. Eng. Chem. Res. 2001, 40, 5469-5474. (19) Johnson, D. K.; Chum, H. L.; Anzick, R.; Baldwin, R. M. Res. Thermochem. Biomass Convers. [Ed. Rev. Pap. Int. Conf.]1988, 485496. (20) Funazukuri, T.; Wakao, N.; Smith, J. M. Fuel 1990, 69, 349353. (21) Yokoyama, T.; Kazuhiko, H.; Nakajima, A.; Seino, K. Sekiyu Gakkaishi 1998, 41, 243-144. (22) Wayman, M.; Lora, J. H. Tappi 1978, 61, 55-57. (23) Lin, L.; Yao, Y.; Yoshioka, M.; Shiraishi, N. Holzforschung 1997, 51, 316-324.
Energy & Fuels, Vol. 17, No. 4, 2003 923
phenol mixture at 523 K and concluded that the condensation reaction could be suppressed in the presence of phenol. For coal extraction, supercritical waterphenol mixtures act as an effective extraction solvent. Aida et al.26 extracted Taiheiyo coal, which had phenolic structures as well as lignin, with supercritical waterphenol mixtures at 673 K and obtained 70% extraction yield. They noted that phenol reduced retrograde reactions in residual coals. In subcritical and supercritical water, the reaction of phenol with alcohols and aldehydes yields alkylphenols through alkylation without a catalyst.27-30 These phenomena suggest that phenol acts as a capping agent and prevents char formation in supercritical water. The water-phenol mixture can also be an effective solvent for the decomposition of lignin. In this study, we conducted the lignin decomposition in supercritical water with and without phenol at 673 K. We then evaluated the effect of water density and that of phenol on lignin conversion. From the experimental results, we proposed reaction mechanisms for lignin conversion in supercritical water-phenol mixtures. Experimental Section Organosolve lignin (purchased from Aldrich) was used as the lignin. The lignin powder was completely soluble in tetrahydrofuran (THF). THF of HPLC grade and phenol of 99.8% purity was purchased from Wako Chemicals and used without further purification. Experiments were conducted in stainless steel (SUS316) tube bomb reactors (volume of 10 cm3), using a thermocouple that was inserted internally into the reactor. Each reactor had one port for purging. For the reaction of lignin, 0.1 g of lignin, 0-1.5 g of phenol, and up to 5.0 g of water were loaded into the reactor. These amounts corresponded to a phenol/lignin ratio (equal to the weight of phenol loaded dividied by the weight of lignin loaded (g/g)) of 0-15 and a water density of 0-0.5 g/cm3. Air inside the reactor was purged with argon gas. After the reactor was loaded, it was submerged into a sand bath (Takabayashirika, model TH-3) that was maintained at 673 K. For typical experiments, the temperature reached 663 K within 2 min, with another 2 min being required for the reactor to reach 673 K. The reported reaction times include these heat-up times. After a given reaction period, the reactor was taken out of the sand bath, submerged in a water bath, and rapidly cooled to below 373 K. This generally occurred within 1 min. Experiments in an argon atmosphere without water were also conducted. Figure 1 shows the analytical procedure that was used. After the reaction, the reaction products were completely recovered by rinsing the reactor with THF. The products recovered were separated into THF-insoluble (TIS) and THF-soluble (TS) fractions by filtration with a 1 µm membrane filter. The TIS products then were dried at 333 K for 1 day and weighed. The TIS products should be almost stable during the drying process at 333 K, because the products were obtained after the reaction (24) Lin, L.; Yoshioka, M.; Yao, Y.; Shiraishi, N. Holzforschung 1997, 51, 325-332. (25) Lin, L.; Yoshioka, M.; Yao, Y.; Shiraishi, N. Holzforschung 1997, 51, 333-337. (26) Aida, T. M.; Sato, T.; Sekiguchi, G.; Adschiri, T.; Arai, K. Fuel 2002, 81, 1453-1461. (27) Chandler, K.; Deng, F.; Dillow, A. K.; Liotta, C. L.; Eckert, C. A. Ind. Eng. Chem. Res. 1997, 36, 5175-5179. (28) Sato, T.; Sekiguchi, G.; Adschiri, T.; Arai, K. Chem. Commun. 2001, 17, 1566-1567. (29) Sato, T.; Sekiguchi, G.; Adschiri, T.; Arai, K. Ind. Eng. Chem. Res. 2002, 41, 3064-3070. (30) Sato, T.; Sekiguchi, G.; Adschiri, T.; Smith, R. L., Jr.; Arai, K. Green Chem. 2002, 4, 449-451.
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Figure 3. TIS yield, relative to water density, after a reaction time of 60 min and a temperature of 673 K.
Figure 1. Analytical procedure.
Figure 2. TIS yield, relative to reaction time, at a water density of 0.5 g/cm3 and a temperature of 673 K. at 673 K, although the nonvolatile acid present may reach a very high concentration. Thermal analysis of the TIS products was conducted under a nitrogen atmosphere with a thermogravimetric analysis (TGA) instrument (Mettler, model TG50). The temperature program was set over a temperature range of 318-1273 K at a rate of 10 K/min. Analyses of the TS fractions were conducted by gas chromatography-mass spectroscopy (GC-MS) (JEOL, model Automass 20) with a Hewlett-Packard HP-5MS column and by gas chromatography-flame ionization detection (GC-FID) (Hewlett-Packard, model HP-6890) with a Hewlett-Packard HP-5 column. The molecular weight distribution of TS product was measured using a gel permeation chromatography (GPC) system (Waters, GPC150C-plus) that was equipped with a refractive index detector and double Shodex KF-803L columns. For the GPC analysis, the TS product was dried at 333 K, to remove excess water, and then dissolved again in THF. TIS yield and that of the products in the TS fraction were defined on the basis of the weight of loaded lignin:
TIS yield (wt%) )
TIS product (g) × 100 lignin loaded (g)
product yield (wt%) )
products (g) × 100 lignin loaded (g)
(1) (2)
Results Figure 2 shows the TIS yield, relative to reaction time, at 673 K, a water density of 0.5 g/cm3, and a phenol/ lignin ratio of 0-15. For each condition, the TIS yield increased as the reaction time increased. Considering that raw lignin completely dissolves in THF, highermolecular-weight fragments are formed after reaction.
Figure 4. Results of TGA analysis: raw lignin (curve 1); TIS product with a water density of 0 g/cm3, without phenol (curve 2); TIS product with a water density of 0.1 g/cm3, without phenol (curve 3); TIS product with a water density of 0.5 g/cm3, without phenol (curve 4); TIS product with a phenol/lignin ratio of 10 and a water density of 0.1 g/cm3 (curve 5); and TIS product with a phenol/lignin ratio of 10 and a water density of 0.5 g/cm3 (curve 6).
The TIS yield decreased as the phenol/lignin ratio increased. The magnitude of the difference between the TIS yield with phenol and that without phenol decreased as the reaction time increased. The formation of TIS was especially suppressed at the early reaction stage in the presence of phenol. Figure 3 shows the TIS yield, relative to water density, at a phenol/lignin ratio of 0-15, after reaction for 60 min at 673 K. In the absence of phenol, the TIS yield was 90 wt % at a water density of 0 g/cm3, namely, pyrolysis under argon atmosphere. The TIS yield dramatically decreased as the water density increased, to 24.2 wt % at a water density of 0.5 g/cm3. Water inhibited the formation of the TIS material. In the presence of phenol, the TIS yield was always less than that in the absence of phenol. The TIS yield decreased as the phenol/lignin ratio and water density each increased. The results obtained here show that both phenol and water suppressed the formation of TIS product. Figure 4 shows the results of TGA analyses for TIS products under five experimental conditions, along with raw lignin results (denoted as condition 1). These conditions were as follows: (2) TIS products and a water density of 0 g/cm3, without phenol; (3) TIS products and a water density of 0.1 g/cm3, without phenol; (4) TIS products and a water density of 0.5 g/cm3, without phenol; (5) TIS products, with a phenol/lignin ratio of 10 and a water density of 0.1 g/cm3; and (6) TIS products
Conversion of Lignin with Water-Phenol Mixtures
Energy & Fuels, Vol. 17, No. 4, 2003 925 Table 1. Yield of Hydrolysis Products in the TS Fraction at a Water Density of 0.5 g/cm3a time (min)
Figure 5. Results of GPC analysis: raw lignin (curve 1); TS product with a water density of 0 g/cm3, without phenol (curve 2); TS product with a water density of 0.1 g/cm3, without phenol (curve 3); TS product with a water density of 0.5 g/cm3, without phenol (curve 4); TS product with a phenol/lignin ratio of 10 and a water density of 0.1 g/cm3 (curve 5); and TS product with a phenol/lignin ratio of 10 and a water density of 0.5 g/cm3 (curve 6)
with a phenol/lignin ratio of 10 and a water density of 0.5 g/cm3. In all cases, the weight loss of TIS products obtained after the reaction was less than that of raw lignin, which clearly shows that the TIS products became heavier. The final weight of material obtained under conditions 2-6 ranked in the following order: 4 > 3 > 5 > 2 > 6. In the absence of phenol (curves 2, 3, and 4 in Figure 4), the weight of the TIS products increased as the water density increased. In the presence of phenol (curves 5 and 6 in Figure 4), the weight of the TIS products was less than that at the same water density without phenol (curves 3 and 4), and those values decreased as the water density increased, which indicates that the addition of phenol made the TIS products lighter, regardless of the existence of water. Figure 5 shows the results of gel permeation chromatography (GPC) analysis of TS products and raw lignin obtained under the same conditions as the TGA analyses for the TIS products. The numbers of the peaks in Figure 4 correspond to the aforementioned numbered conditions. The peak of the curve of raw lignin (curve
Figure 6. Structures of detected liquid products in TS.
1
2
3
4
yield (wt %) 5 6
7
8
total
0.44 0.40 0.56 0.13
0.44 0.76 0.57 0.73
7.48 4.37 2.16 1.40
Phenol/Lignin Ratio ) 10 0.22 1.09 0.32 0.52 0.15 0.74 0.36 0.33 0.17 0.87 0.28 0.30 0.21 0.85 0.17 0.32
0.31 0.28 0.39 0.42
0.32 0.32 0.62 0.33
4.82 3.54 3.34 2.54
10b 20 30b 64
1.96 0.89 0 0.19
Phenol/Lignin Ratio ) 0 0.79 0.52 0.77 1.05 1.11 0 0 0.56 0.89 0.87 0 0 0.26 0.78 0 0.19 0 0 0 0.17
10 20 30 64
1.43 0.93 0.32 0
0.62 0.43 0.39 0.24
a See Figure 6 for identification of the compounds. b Average of two data.
1) was located at a mean molecular weight of ∼1500. This peak clearly disappeared after the reaction. In the absence of phenol, there was no significant peak at a water density of 0 g/cm3 (curve 2). The broad peak located between mean molecular weights of 200 and 1000 appeared at a water density of 0.1 g/cm3 (curve 3). At a water density of 0.5 g/cm3, the highest peak appeared below a mean molecular weight of 200 (curve 4). In supercritical water without phenol, the peaks of each of the curves were shifted to lower molecular weights as the water density increased. In the presence of phenol, the position of the peak was below a mean molceular weight of 200, regardless of the water density (curves 5 and 6). In the presence of water or phenol, lignin conversion proceeded at lower molecular weights. We analyzed the TS fraction at a water density of 0.5 g/cm3 and detected the products as shown in Figure 6. Syringol (1), methylsyringol (2), ethylsyringol (3), guaiacol (4), ethylguaiacol (5), acetoguaiacol (6), catechol (7), and methoxycatechol (8) were produced in the presence and absence of phenol. On the other hand, 2-cresol (9), 4-cresol (10), 2-ethylphenol (11), 4-ethylphenol (12), xanthene (13), 2,2-dihydroxymethane (14), and 2-hydroxyphenylguaiacol (15) were produced only in the presence of phenol. Table 1 shows the yield of products 1-8, relative to reaction time. The quantity of species detected was very low, because the yield of these
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Table 2. Yield of Products Formed in the Presence of Phenol in the TS Fraction at a Phenol/Lignin Ratio of 10 and a Water Density of 0.5 g/cm3 time (min)
9
10
11
10 20 30 64
3.27 5.96 6.34 7.15
1.33 2.29 2.85 3.34
0.65 1.34 1.79 2.22
yield (wt %) 12 13 0.37 0.75 1.00 1.32
0.88 1.66 2.46 3.25
14
total
0.24 1.23 1.78 1.74
6.74 13.23 16.22 19.02
products was
