Selective Production of Organic Acids and Depolymerization of Lignin


Jan 7, 2011 - Considering the rigid structure of lignin, we have developed a new method to recover chemicals from lignin under severe conditions. This...
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Energy Fuels 2011, 25, 791–796 Published on Web 01/07/2011

: DOI:10.1021/ef101477d

Selective Production of Organic Acids and Depolymerization of Lignin by Hydrothermal Oxidation with Diluted Hydrogen Peroxide Isao Hasegawa, Yusuke Inoue, Yosuke Muranaka, Toshiya Yasukawa, and Kazuhiro Mae* Department of Chemical Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan Received November 2, 2010. Revised Manuscript Received December 22, 2010

There has been a great interest in converting lignin into chemicals, but lignin is not used as feedstock for chemical production at present. Considering the rigid structure of lignin, we have developed a new method to recover chemicals from lignin under severe conditions. This method is the hydrothermal oxidative degradation using 0.1% hydrogen peroxide solution in the flow reactor at 150-200 °C. When alkali lignin was oxidized for 2 min at 200 °C, the total yield of organic acids was as large as 0.45 g/g-lignin. The organic acids consisted of formic, acetic, and succinic acids, and the high-molecular-weight lignin also remained after the oxidation. On the other hand, when organosolv lignin was oxidized at 160 °C, lignin molecules were depolymerized into the oligomer of MW = ca. 300 and the total yield of organic acids was 0.20 g/g-lignin. The product distribution depended on the difference in the structures between the two lignin samples. It was clarified that the proposed method is valid to produce valuable chemicals from any kind of lignin.

main reason for this is the fact that repeating units in the lignin molecule are more complex than polysaccharides. In addition, nonselective modifications during isolation make lignin molecules more heterogeneous and inactive. Thus, we find it very difficult to convert highly transformed lignin into valuable chemicals. One of the solutions of this problem is to convert lignin fraction simultaneously in the isolation process, that is to say, before being deactivated. Funaoka et al9 developed a method for separating lignocellulosic biomass into lignin and carbohydrate moieties with the conversion to reactive forms at room temperature. This process includes the phase-separation reaction system composed of phenol derivatives and concentrated acid. We have also presented a new process for separating hemicellulose, cellulose, and lignin from lignocellulosic biomass and for converting them into valuable chemicals,10 such as organic acids and reactive lignin11 for gasification. This process consists of hot water pretreatment and a liquid-phase oxidation at 60 °C under ambient pressure for 24 h. Because an excess of hydrogen peroxide is used under mild conditions in the liquid-phase oxidation (H2O2/lignin = 1.5 and more), the unreacted hydrogen peroxide remains after 24 h of oxidation. When this method is applied to inactive alkali lignin with a diluted hydrogen peroxide under severe conditions, it is expected that rigid lignin can be decomposed into small molecules with the consumption of hydrogen peroxide. In this study, we tried to achieve the rapid oxidation of alkali lignin under hydrothermal condition with as little hydrogen peroxide as possible and to obtain organic acids selectively by taking advantage of the stereoregularity of lignin. Previously, we developed another separation pretreatment,12 in which a lignin fraction is extracted in organic

1. Introduction Lignin is an aromatic three-dimensional network polymer and is second to cellulose in natural abundance. Unlike synthetic polymers, lignin is absolutely biodegraded in nature, exerting no undesirable influence on the environment, although it is one of the most durable biopolymers and exists in plant cell walls as one of the major components. Therefore, there is a possibility that lignin would be an ideal biodegradable polymer material for biorefinery conversion. Because of this unique characteristic feature, which synthetic polymers do not have, there has been a great interest in its degradation usage and potential application in the manufacture of various chemicals and other products. However, lignin-based materials have hardly been in practical human life, and the utilization of lignin has not yet been successfully achieved. At the moment, most industrially discharged lignin is burned, and the limited lignin is just utilized for a few purposes, such as dispersants1-3 or pellet binder materials.4,5 Alkali kraft lignin is discharged mainly as a byproduct in the pulping process. Kraft lignin is burned for the recovery of energy for the pulping process, and that is not used as feedstock for valuable chemicals. For the effective utilization of kraft lignin, conversion of that into functional materials has been attempted by researchers.6-8 However, industrial processes for lignin-derived biorefinery has not been realized. The *To whom correspondence should be addressed. Telephone: þ81 75 383 2668. Fax: þ81 75 383 2658. E-mail: [email protected] (1) Zhou, M.; Kong, Q.; Pan, B.; Qiu, X.; Yang, D.; Lou, H. Fuel 2010, 89, 716–723. (2) Matsushita, Y.; Imai, M.; Iwatsuki, A.; Fukushima, K. Bioresour. Technol. 2008, 99, 3024–3028. (3) Cechova, M.; Alince, B.; Ven, T. G. M. Colloids Surf., A 1998, 141, 153–160. (4) Alekseev, O. A.; Shamsutdinov, M. E.; Kutyshev, F. K.; Kostochko, A. V. Combust., Explos. Shock Waves 2001, 37, 67–71. (5) Proudfoot, F. G.; DeWitt, W. F. Poult. Sci. 1976, 55, 629–631. (6) Notley, S. M.; Norgren, M. Langmuir 2010, 26, 5484–5490. (7) Saidane, D.; Barbe, J.-C.; Birot, M.; Deleuze, H. J. Appl. Polym. Sci. 2010, 116, 1184–1189. (8) Binh, N. T. T.; Luong, N. D.; Kim, D. O.; Lee, S. H.; Kim, B. J.; Lee, Y. S.; Nam, J. D. Compos. Interfaces 2009, 16, 923–935. r 2011 American Chemical Society

(9) Funaoka, M.; Abe, I. Tappi J. 1989, 72, 145–149. (10) Mae, K.; Hasegawa, I.; Sakai, N.; Miura, K. Energy Fuels 2000, 14, 1212–1218. (11) Mae, K.; Hasegawa, I.; Kawashita, H.; Miura, K. J. Jpn. Inst. Energy 2001, 80, 436–443. (12) Hasegawa, I.; Tabata, K.; Okuma, O.; Mae, K. Energy Fuels 2004, 18, 755–760.

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Energy Fuels 2011, 25, 791–796

: DOI:10.1021/ef101477d

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Figure 2. Change in molecular weight distributions through the reactions under several conditions.

Figure 1. Schematic apparatus of the flow reaction system.

That is, as shown in Figure 1, ambient water was heated to around 150-300 °C in a heating bath, and then the hot compressed water was sent to the entrance of a quick-heating part (union tee made of Swagelok stainless steel) through a stainless-steel tube with an outer diameter of 1/16 in. and an inner diameter of 1 mm. A stream of an ambient lignin and hydrogen peroxide aqueous solution passing through a tube was struck against a flow of the preheated water in a union tee and could be raised to the fixed temperature of 100200 °C within a very short period; the mixture was then introduced to a flow tube reactor, in which the reaction times were adjusted to be 0-5 min. After the reaction, the solution was quenched rapidly to room temperature to prevent further decomposition. To effect such a rapid heating of the lignin solution and to prevent temperature changes from occurring upon the oxidation of the lignin, the relatively faster flow rate of preheated water was kept at more than 10 times against that of the lignin solution and was then momentarily mixed with it in the quick-heating part. The temperature before the reactor was accurately regulated by adjusting the temperature of preheated water against the ambient lignin solution. The pressure was held at 20 MPa by the back pressure regulator. We use the symbol (reactor type-H2O2 concn-reaction time-reaction temp) to represent reaction conditions in this paper. 2.4. Analyses of Products. Ultimate analysis of the samples was performed using an elemental composition analyzer (BEL Japan, Inc., ECS4010). The pyrolysis of the solid sample was performed using a thermogravimetric analyzer (Shimadzu, TGA-50). The chemical structure, such as a functional group, was analyzed by an FTIR spectrometer (JEOL Ltd., JIRSPX60). The total organic carbon in the aqueous solution was estimated using a TOC analyzer (Shimadzu, TOC-VCSH). The gel permeation chromatography (GPC) was used to estimate the molecular weight distribution of the lignin solution and the product solution after the oxidation. The packed column used was a Shodex OHpak SB-802.5 (SHOWA DENKO), and an eluent of distilled water was supplied at the flow rate of 1.0 mL/ min to the HPLC equipped with an RI detector (Shimadzu, RID-10A). For the organic acid analysis, an aqueous solution containing 951 mg/L of p-toluene sulfonic acid, 4185 mg/L of Bis-Tris, and 29 mg/L of EDTA was used as the eluent, and it was fed at 0.8 mL/min to the HPLC equipped with a sulfonated polystyrene gel column (Shim-pack SCR-102H) and an electric conductivity detector (Shimadzu, CDD-6A). The yield of each component of organic acids estimated from the above measurements was represented on the daf basis.

Table 1. Ultimate Analyses and Ash Contents of Lignin Samples and Products ultimate analysis (wt %, daf) sample

C

H

O (diff)

S

ash (wt %, db)

alkali lignin organosolv lignin soluble product soluble product (exc. acids)

57.2 67.0 54.7 64.5

4.6 5.7 3.8 2.8

35.2 27.3 38.8 28.4

3.0 N.D. 2.7 4.3

9.6 0.1 13.3 27.4

aqueous solution with the less structural change. The delignification processes that use organic solvents are known as organosolv.13,14 Organosolv lignin is expected to have a certain degree of activity for degradation. Thus, in the latter half of this paper, we investigated the difference in the oxidative reactivity and the product distribution for the organosolv and alkali lignin samples, and we explored an availability of lignin as feedstock for biorefinery. 2. Experimental Section 2.1. Samples. Alkali lignin (Nakalai Tesque Co.) was used to examine the oxidative degradation behavior. Because alkali lignin from black liquor is water-soluble, we used an aqueous solution of 1 wt % lignin. For comparison of the reactivity and the products, organosolv lignin (Sigma-Aldrich Co.) was also used as a sample. Organosolv lignin is water-insoluble, so we conducted a preoxidation with H2O2 at 60 °C under ambient pressure for the solubilizaion of lignin into water. These two lignin samples are derived from softwood. The yield of the products was represented on the basis of a dry ash-free original lignin sample. The ultimate analyses of both lignin samples used are listed in Table 1. From the elemental composition of alkali lignin, we found alkali lignin contains considerable ash and sulfur. 2.2. Batch Reaction System. Under the batch operation, the experiments were performed by using a 100 cm3 glass flask or Swagelok stainless-steel batch reactor with an internal volume of 20 cm3. Predetermined amounts of sample and hydrogen peroxide solution were loaded into the reactor under a nitrogen atmosphere. The reactor vessel was immersed in an oil bath. After the desired reaction time had elapsed, the reactor was removed from the oil bath and rapidly quenched in an ice/water bath. 2.3. Flow Reaction System. We performed the hydrothermal oxidation using a flow reactor system as well as a batch reactor system. The flow reaction system (Figure 1) can heat a lignin solution to the hydrothermal state very quickly and can then quench rapidly to sufficiently low temperatures after the reaction.

3. Results and Discussion 3.1. Effect of Oxidative Degradation of Lignin. First, we compared the weight basis molecular weight distributions (MWDs) of product solutions in the batch reactor (BR) and flow reactor (FR), as shown in Figure 2. Through the batch hydrolysis without hydrogen peroxide (BR-0%-1 h-200 °C), the molecular weight of lignin partially increased to some

(13) Kleinert, T. N. Tappi 1974, 57, 99–102. (14) Chum, H. L.; Johnson, D. K.; Black, S.; Baker, J.; Grohmann, K.; Sarkanen, K. V.; Wallance, K.; Schroeder, H. A. Biotechnol. Bioeng. 1988, 31, 643–649.

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extent (MW = 70 000) because of the repolymerization and condensation under hydrothermal condition for a long reaction time. Solid product was also deposited on the reactor after the hydrolysis. Furthermore, the low-molecular-weight organic acids were scarcely produced in this condition. From this result, it is desirable that the lignin solution is heated and quenched rapidly with the flow reactor, in which the reaction time can be rigidly adjusted to such a short residence period. It is difficult to recover the low-molecular-weight valuable chemicals, (e.g., organic acids) by hydrolysis with the hot compressed water only. Thus, we tried an oxidative degradation of lignin with hydrogen peroxide. The oxidation condition was as follows: 60 °C, 24 h, ambient pressure, the weight ratio of lignin to 30 wt % hydrogen peroxide aqueous solution was 1/3 (BR-23%-24 h-60 °C). Through the liquid-phase mild oxidation at 60 °C, lignin molecules were depolymerized slightly, as shown in Figure 2. However, a main peak of the MWD of the lignin solution did not shift so much. Here, the carbon conversion to CO2 was 8% through the mild oxidation, and the total yield of the organic acids produced was 0.04 kg/kg-lignin. In our previous study,10 the total yield of the organic acids through the liquidphase oxidation of the pretreated palm oil shell under the same condition was 0.18 kg/kg-palm shell. Palm oil shell contains 49 wt % lignin fraction. Considering this previous result, it seems that the oxidative effects of hydrogen peroxide on the alkali lignin were lowered despite the same condition. This result means that the activity of alkali lignin of softwood is lower than that of raw lignin in hardwood (palm tree). In other words, the lignin structure of softwood is different from that of hardwood or becomes inactivated through alkali pulping delignification.15 Thus, it was found that the severer condition was required for the oxidative degradation of alkali lignin of softwood. Actually, the liquid-phase oxidation with diluted hydrogen peroxide under hydrothermal condition (FR-0.08%-1.8 min-200 °C) brought about depolymerization of alkali lignin molecules. The main peak of the MWD of lignin shifted from MW = 20 000 to 11 000. Here, the carbon conversion into CO2 was 19%, and the total yield of the organic acids produced was as large as 0.45 kg/kg-lignin, as described later. Therefore, alkali lignin should be oxidized with the diluted hydrogen peroxide under severe conditions in a short reaction time in order to obtain the more organic acids effectively and to avoid the decomposition into CO2 as much as possible. 3.2. Change in the Oxidizer and the Products during the Hydrothermal Oxidation. In this section, we examined the change in the concentrations of the hydrogen peroxide and the products during the oxidative degradation under hydrothermal condition and explored the optimal condition for the recovery of organic acids. First, we investigated the effect of the reaction temperature on the conversion. Figure 3a shows the changes in the hydrogen peroxide concentration with increasing reaction temperature by the initial concentrations. The reaction time was 3.6 min. With the increase of the oxidation temperature, the hydrogen peroxide concentrations decreased. The concentrations of hydrogen peroxide were kept almost constant below 100 °C, and hydrogen peroxide decomposed completely at 200 °C regardless of the initial concentration of that. Because the remaining hydrogen peroxide causes further oxidation of lignin and products gradually even at low temperature, hydrogen peroxide should be consumed completely through the hydrothermal oxidation at 200 °C. On the other hand, Figure 3b shows the

Figure 3. Change in the oxidizer concentration and organic acid yield with the reaction temperature.

Figure 4. Example of a chromatogram for organic acid analysis.

changes in the yields of the organic acid products. The most impressive fact was that three kinds of organic acids were detected at most: formic, acetic, and succinic acids. Figure 4 shows a typical example of an HPLC chromatogram for organic acid analysis, in which only three peaks can be observed. From this result, it was expected that these three organic acids were reflected in the regularity of the lignin structure. The yields of the formic and acetic acids started to increase at 100 °C. This is the same temperature at which hydrogen peroxide started to be consumed. Hydrogen peroxide oxidized lignin into the formic and acetic acids. Succinic acid started to be formed at 150 °C, so succinic acid was produced under a more severe condition than formic and acetic acids were. This result means that a persistent site of lignin was oxidized into succinic acid. Because succinic acid is an especially valuable key component for biorefinery, 200 °C was chosen as an optimum oxidation temperature for alkali lignin. Next, we investigated the effect of the initial concentration of hydrogen peroxide on the oxidation products. We had better minimize the amount of oxidizer to be used because of the cost. Figure 5 shows the relationship between the product distribution and the initial hydrogen peroxide concentration. As expected, we found that succinic acid was produced under the severer condition, with a higher concentration of hydrogen peroxide. Formic and acetic acids were produced with low oxidizer concentration, and their yields did not change very

(15) Bose, S. K.; Omori, S.; Kanungo, D.; Francis, R. C.; Shin, N.-H. J. Wood Chem. Technol. 2009, 29, 214–226.

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Figure 5. Relationship between the product distribution and the initial hydrogen peroxide concentration.

Figure 7. FTIR spectra of alkali lignin and the soluble product.

Figure 6. Changes in concentration of hydrogen peroxide and organic acid yields with the progress of the oxidation.

Figure 8. Weight change curves during pyrolysis of alkali lignin and the soluble products.

much regardless of the initial hydrogen peroxide concentration. The carbon conversion to CO2 increased linearly with the initial hydrogen peroxide concentration. With the initial concentration of 0.15 wt %, the carbon conversion to CO2 reached up to 45%. Considering the succinic yield and the amount of CO2, 0.08 wt % was chosen as the initial concentration of hydrogen peroxide. We have decided on the reaction temperature and the initial concentration of the oxidizer for the effective oxidation of alkali lignin, as mentioned above. From the viewpoint of secondary reactions, reaction time is also important. Figure 6 shows the changes in concentration of hydrogen peroxide and organic acid yields with the progress of the oxidation (FR-0.08%-200 °C). The hydrogen peroxide concentration decreased gradually with the oxidation time and reached almost 0 at 1.8 min. The yields of the formic and succinic acids increased monotonically with oxidation time and reached 0.12 and 0.13 kg/kg, respectively. On the contrary, the acetic acid yield increased until 1.8 min and decreased after that. Acetic acid was judged to be consumed by secondary decomposition. From this result, the oxidation for around 2 min under hydrothermal condition is sufficient to recover a large amount of organic acids more than 0.45 kg/kg-lignin. The advantages of this method are rapid reaction and less oxidizer for the effective recovery of valuable chemicals. 3.3. Change in the Structure of Lignin during the Oxidation. To investigate the change in the structure of alkali lignin, the product solution obtained under the optimum condition was examined by an FTIR spectrometer. By the analysis of the FTIR spectrum ranging from 2400 to 3700 cm-1, we can estimate the strength distribution of hydrogen bonds in the sample.16,17 Those spectra were obtained using a diffuse reflectance IR

Fourier transform (DR) technique with neat, undiluted samples. Before the measurement, the product solution was dried up at 80 °C. The spectrum of the oxidized soluble product shown in Figure 7 was entirely different from that of alkali lignin. This figure clearly shows how the OH stretching bands of lignin change through the hydrothermal oxidation. The absorption bands at around 3400-3700 cm-1 (weaker hydrogen bonds) decreased, whereas those at 2300-3300 cm-1 (stronger hydrogen bonds) increased. This result indicates that oxidative degradation of lignin formed rather strong hydrogen bonds of OH groups, such as carboxyl OH, regardless of the total amount of OH. Furthermore, the formation of carbonyl groups (assigned from 1630 to 1780 cm-1) was observed. These results mean that hydrogen peroxide under hydrothermal condition oxidized lignin effectively, but it is not obvious that the oxygen content of lignin increased. To confirm the above, we examined the elemental composition of the soluble product containing organic acids and calculated that excluding organic acids. The ultimate analyses of both were added in Table 1. This shows that the oxygen content of the soluble product containing organic acids increased naturally through the liquid-phase oxidation. On the other hand, the oxygen content of the product excluding organic acids decreased conversely. Judging from the fact that the aromatic carbon ratio ( fa value) of the alkali lignin is 0.63 and the remaining carbon ratio of the oxidized lignin was 0.40, the oxidation under hydrothermal condition ruptured partially aromatic rings in lignin. The fa value of alkali lignin was estimated from the aromatic carbon band of the 13C NMR spectrum in our previous study.10 It was presumed that the aliphatic side chains existed originally and/or produced by rupturing aromatic rings were oxidized and decomposed into the organic acids with hydrogen peroxide. Figure 8 shows the weight change curves during pyrolysis of alkali lignin and the

(16) Miura, K.; Mae, K.; Li, W.; Kusakawa, T.; Morozumi, F.; Kumano, A. Energy Fuels 2001, 15, 599–610. (17) Miura, K.; Mae, K.; Hasegawa, I.; Chen, H.; Kumano, A.; Tamura, K. Energy Fuels 2002, 16, 23–31.

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Figure 9. Presented mechanism for the oxidative degradation of lignin.

soluble products. Pyrolysis of both samples occurred over a wide temperature range. However, alkali lignin and the oxidative product differed in the degree of decomposition. The oxidative product was easy to decompose, and the solid residual yield of that was lower than that of alkali lignin over the whole temperature range. This result also shows that the collective aromaticity of alkali lignin declined through the oxidative degradation. From the experimental results described above, we would present the oxidation mechanisms of lignin shown in Figure 9. Succinic acid is produced by rupturing the aromatic ring of lignin, whereas formic and acetic acids are produced by the oxidative degradation of the aliphatic side chains. This mechanism is compatible with the fact that succinic acid could be obtained under the severe conditions, such as high temperature and oxidizer concentration. 3.4. Comparison between Alkali Lignin and Organosolv Lignin in the Oxidation Behavior. Finally, we examined the difference in reactivity between alkali lignin and organosolv lignin. It is known that hardwood lignin shows higher reactivity than softwood lignin does. For example, delignification bleaching is less effective for softwood kraft pulp than for hardwood kraft pulp.18 Hardwood is more easily decomposed and liquefied in supercritical methanol than softwood.19 They said that this seems be due to the difference in the structure of lignin. Assor et al.20 specifically reported that softwood lignin had lower β-O-4 (β-arylether) linkages, which can be easily degraded. Thus, the structure of hardwood lignin is different from that of softwood. In this study, both lignin samples are derived from softwood. Therefore, we can consider the effect of the change in lignin structure during the (alkali or organosolv) pretreatment on the reactivity. Figure 10 shows the comparison of the MWD between the oxidative products of alkali and organosolv lignin.

Figure 10. Molecular weight distribution of the products through the oxidation of each lignin sample.

Organosolv lignin has already been depolymerized when that was solubilized through the first-step oxidation. The molecular weight of the water-soluble organosolv lignin was broadly distributed from MW = 800 to 10 000. This suggests that organosolv treatment does not deactivate lignin for the subsequent degradation unlike alkali delignification. Actually, organosolv lignin was completely converted into organic acids under the optimal condition (at around 200 °C) for the alkali lignin, as shown in Figure 10. The MWD of the organosolv lignin oxidized at 190 °C had only organic acids’ peaks in the region of MW = 100. This product solution was a colorless and transparent liquid, whereas the organosolv lignin solution was yellowish brown. When the organosolv lignin was oxidized at the lower temperature of 160 °C, the MWD of its product solution had a distinct peak at MW = 300 and the high-molecular-weight components more than 10 000 in MW disappeared. Because it was found that this low-molecular-weight component of MW = 300 contained aromatic rings from the UV measurement, organosolv lignin was depolymerized into the oligomer under this milder condition. This low-molecular-weight lignin was assumed to consist of syringyl or guaiacyl aromatic nuclei units, so there is a possibility for the low-molecular-weight lignin to be used as materials for resin production. From this figure,

(18) Ikeda, T.; Hosoya, S.; Tomimura., Y.; Magara, K.; Takano, I. J. Wood Sci. 1999, 45, 233–237. (19) Minami, E.; Saka, S. J. Wood Sci. 2003, 49, 73–78. (20) Assor, C.; Placet, V.; Chabbert, B.; Habrant, A.; Lapierre, C.; Pollet, B.; Perre, P. J. Agric. Food Chem. 2009, 57, 6830–6837.

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aromatic condensed structure was partially ruptured into the organic acids but was not depolymerized into a certain degree of oligomers even if alkali lignin was oxidized at 200 °C. On the other hand, organosolv lignin could be depolymerized into a large amount of oligomer at 160 °C because of keeping the original degradable structure. The carbon conversion to the oligomer less than 1000 in MW reached up to 45%, and the total yield of organic acids was 0.2 kg/kg-lignin. A ratio containing succinic acid was lower than that from alkali lignin. Succinic acid is judged to be produced from the persistent structure of lignin as described in section 3.2. When the organosolv lignin solution was hydrothermally oxidized at 190 °C, the water-soluble products were just small molecular organic acids and a certain amount of CO2 was produced too. Thus, the optimal condition for this hydrothermal oxidation method depends on the desired product and the structure of the lignin used. From these findings, we may be able to control the product distribution by modifying the structure of the lignin beforehand through the appropriate pretreatment and by making a choice of biomass species.

Figure 11. Effect of the operating conditions on the product distributions for the oxidation of each lignin sample.

there was no organic acids’ peak detected at around MW = 100 for alkali lignin at 200 °C, though a large amount of organic acids were measured with HPLC for ions. The reason why is as follows. Alkali lignin contains alkali metals as ash. Thus, organic acids from the oxidation of alkali lignin exist as alkali salts, such as sodium acetate. The elution in GPC measurement for the alkali salts of organic acids is delayed compared with the that of free organic acids. This means that the MWD of the alkali salts shifts to the smaller molecules. Therefore, there was no peak detected in the region of 100 < MW < 1000 for the MWD of alkali lignin at 200 °C in Figure 10. Because the peaks of alkali salts of organic acids overlap with that of hydrogen peroxide in MW < 100, we cannot analyze them. On the other hand, in HPLC measurement for ionic compounds, all organic acids are dissociated into their ions. We can analyze organic acids quantitatively whether they are salts or molecules. Figure 11 shows the product distributions on a carbon basis through the oxidation of alkali and organosolv lignin. We found that alkali lignin was converted into a large amount of organic acids and there was still almost the same amount of high-molecular-weight lignin of MW > 1000 (approximately 10 000) through the severe oxidation at 200 °C. These oxidative products of alkali lignin are two extremes of small and large molecules and are free from the aromatic oligomer of lignin. This may be because alkali lignin molecules are condensed, as mentioned above. The

4. Conclusion We have developed a new oxidative degradation process for utilizing lignin as chemical resources, in which lignin was oxidized with 0.1% diluted hydrogen peroxide under hydrothermal condition. Through the hydrothermal oxidation at 200 °C, 0.45 g/g-lignin of three organic acids was recovered from alkali lignin. When organosolv lignin was subjected to the oxidation at 160 °C, lignin was depolymerized into oligomers and 0.20 g/g-lignin of organic acids was successfully recovered. These results suggest that a difference in the structure of the lignin affected the product distribution and the reactivity for the hydrothermal oxidation. Thus, it was clarified that the proposed method is effective for converting lignin into valuable chemicals and can be applied to any kind of lignin. Acknowledgment. This work was financially supported by NEDO Green Sustainable Chemical Process Development (Project No. 09010) and by JSPS through a Grant-in-Aid for Scientific Research (A) (22246100).

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