Stability of Supported Ruthenium Catalysts for Lignin Gasification in

Energy & Fuels 2006, 20, 2337-2343

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Stability of Supported Ruthenium Catalysts for Lignin Gasification in Supercritical Water Mitsumasa Osada,†,‡ Osamu Sato,‡ Kunio Arai,‡ and Masayuki Shirai*,‡ Department of Chemical Engineering, Ichinoseki National College of Technology, Takanashi, Hagisho, Ichinoseki, Iwate 021-8511, Japan, and Research Center for Compact Chemical Process, National Institute of AdVanced Industrial Science and Technology (AIST), 4-2-1, Nigatake, Miyagino, Sendai 983-8551, Japan ReceiVed August 2, 2006. ReVised Manuscript ReceiVed September 2, 2006

We evaluated the stability of several supported ruthenium catalysts, on titania (Ru/TiO2), on γ-alumina (Ru/γ-A2O3), and on activated carbon (Ru/C), for lignin gasification in supercritical water. The initial activity of the catalysts was in the order of Ru/TiO2 > Ru/γ-A2O3 > Ru/C. The Ru/TiO2 catalyst maintained high gasification activities for three subseqent uses in supercritical water. The Ru/C catalyst showed the high gasification activity for the first run; however, the activity decreased gradually after repetitive use because of its decreasing surface area during the gasification. The Ru/γ-A2O3 catalyst also showed a high activity at the initial stage; however, it lost its activity fast because of the change in its structure from gamma- to alphaphase and dissolution of active ruthenium speacies into supercritical water.

Gasification processes of wood biomass have received attention as an efficient technology for the conversion of biomass into energy. The gas obtained by biomass gasification has wide ranging applications, for example, in gas turbines, in fuel cells, and for the synthesis of chemicals. Generally, very high temperatures of over 1073 K are needed for steam reforming of lignin in the gasification process. However, low-temperature methods for lignin gasification are desirable, because waste heat available from high-temperature processes in industry can be utilized for energy generation.1 Gasification in supercritical water (Tc ) 647.3 K, Pc ) 22.1 MPa) could be a potential technology to reduce the biomass gasification temperatures1-9 because of high solubility and hydrolysis rates of biomass in supercritical water, leading to minimized mass-transfer limitations of the reactant10-12 and its rinsing effect by washing coke precursors on the active catalyst

sites.13 The decomposition behavior of lignin in supercritical water around 673 K was reported in which lignin was converted to alkylphenols and formaldehyde via hydrolysis14-20 and then the decomposition of alkylphenols to gases. Cross-linking reactions between the alkylphenols and formaldehyde18-21 also take place to produce solid materials, insoluble in water and tetrahydrofuran.16-20 The gasification of biomass in supercritical water around 673 K is enhanced by metal catalysts.1-6,22-24 We have reported the order of activity of various catalysts for lignin gasification in supercritical water at 673 K as ruthenium > rhodium > platinum > palladium > nickel.23 Sato et al. conducted the gasification of alkylphenols as model compounds for lignin over several supported metal catalysts in supercritical water at 673 K and reported ruthenium as the most effective catalyst.24 However, not only initial activity and selectivity but also the stability of a catalyst in supercritical water is very important.25-31 The stability of the supports for metal particles is also a significant

* Corresponding author. E-mail: [email protected]. Tel.: +81-22-2375219. Fax: +81-22-237-5224. † Ichinoseki National College of Technology. ‡ National Institute of Advanced Industrial Science and Technology. (1) Osada, M.; Sato, T.; Watanabe, M.; Shirai, M.; Arai, K. Combust. Sci. Technol. 2006, 178, 537-552. (2) Matsumura, Y.; Sasaki, M.; Okuda, K.; Takami, S.; Ohara, S.; Umetsu, M.; Adschiri, T. Combust. Sci. Technol. 2006, 178, 509-536. (3) Matsumura, Y.; Minowa, T.; Potic, B.; Kersten, S. R. A.; Prins, W.; van Swaaij, W. P. M.; van de Beld, B.; Elliott, D. C.; Neuenschwander, G. G.; Kruse, A.; Antal, M. J. Biomass Bioenergy 2005, 29, 225-302. (4) Yoshida, T.; Oshima, Y. Ind. Eng. Chem. Res. 2004, 43, 4097-4104. (5) Park, K. C.; Tomiyasu, H. Chem. Commun. 2003, 6, 694-695. (6) Watanabe, M.; Inomata, H.; Osada, M.; Sato, T.; Adschiri, T.; Arai, K. Fuel 2003, 82, 545-552. (7) Antal, M. J., Jr.; Allen, S. G.; Schulman, D.; Xu, X.; Divilio, R. J. Ind. Eng. Chem. Res. 2000, 39, 4040-4053. (8) Kruse, A.; Gawlik, A. Ind. Eng. Chem. Res. 2003, 42, 267-279. (9) Sinagˇ, A.; Kruse, A.; Rathert, J. Ind. Eng. Chem. Res. 2004, 43, 502508. (10) Savage, P. E. Chem. ReV. 1999, 99, 603-621. (11) Akiya, N.; Savage, P. E. Chem. ReV. 2002, 102, 2725-2750. (12) Watanabe, M.; Sato, T.; Inomata, H.; Smith, R. L., Jr.; Arai, K.; Kruse, A.; Dinjus, E. Chem. ReV. 2004, 104, 5803-5821.

(13) Savage, P. E. Catal. Today 2000, 62, 167-173. (14) Bobleter, O.; Consin, R. Cell. Chem. Technol. 1979, 13, 583-593. (15) Funazukuri, T.; Wakao, N.; Smith, J. M. Fuel 1990, 69, 349-353. (16) Yokoyama, C.; Nishi, K.; Nakajima, A.; Seino, K. Sekiyu Gakkaishi 1998, 41, 243-250. (17) Ehara, K.; Saka, S.; Kawamoto, H. J. Wood. Sci. 2002, 48, 320325. (18) Saisu, M.; Sato, T.; Watanabe, M.; Adschiri, T.; Arai, K. Energy Fuels 2003, 17, 922-928. (19) Okuda, K.; Umetsu, M.; Takami, S.; Adschiri, T. Fuel Process. Technol. 2004, 85, 803-813. (20) Okuda, K.; Man, X.; Umetsu, M.; Takami, S.; Adschiri, T. J. Phys.: Condens. Matter 2004, 16, 1325-1330. (21) Antal, M. J., Jr. Solar Energy; Plenum Press: New York, 1983; pp 175-255. (22) Osada, M.; Sato, T.; Watanabe, M.; Adschiri, T.; Arai, K. Energy Fuels 2004, 18, 327-333. (23) Osada, M.; Sato, O.; Watanabe, M.; Arai, K.; Shirai, M. Energy Fuels 2006, 20, 930-935. (24) Sato, T.; Osada, M.; Watanabe, M.; Shirai, M.; Arai, K. Ind. Eng. Chem. Res. 2003, 42, 4277-4282. (25) Elliott, D. C.; Sealock, L. J.; Baker, E. G. Ind. Eng. Chem. Res. 1993, 32, 1542-1548. (26) Elliott, D. C.; Sealock, L. J.; Baker, E. G. Ind. Eng. Chem. Res. 1994, 33, 558-565.

Introduction

10.1021/ef060356h CCC: $33.50 © 2006 American Chemical Society Published on Web 09/29/2006

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Osada et al.

Table 1. Supported Ruthenium Catalysts Used in This Study catalyst

manufacturer

metal loading (wt %)

metal dispersiona (%)

surface area (m2·g-1)

Ru/TiO2 Ru/C Ru/γ-Al2O3

Osaka Gas Co., Ltd. Wako Pure Chemical Industries, Ltd. N. E. CHEMCAT Co., Ltd.

2 5 5

27 51 37

24 768 95

a All catalysts were pretreated under flowing hydrogen at 573 K for 10 min before measurement. The metal dispersion was measured by a carbon monoxide adsorption at 323 K. Metal dispersion ) (mol of carbon monoxide adsorbed)/(mol of total metal atom in a catalyst) × 100.

factor. Elliott et al.25 investigated the stability of supports for metal catalysts in subcritical water at 623 K and reported that R-alumina, zirconia, and activated carbon were very stable supports. They pointed out that γ-alumina was not a useful catalyst support for more than a few hours in aqueous systems because γ-alumina hydrolyzed in the presence of water. Silica has high surface area and is commonly used as a support for metal particles; however, it gets dissolved in water at high temperature and, therefore, cannot be used as a support for metal catalysts in lignin gasification in supercritical water.25 In this paper, we studied the stability of several supported ruthenium catalysts in supercritical water. Experimental Section Organosolv-lignin powder (lignin) was purchased from Aldrich, and its molecular formula was C42.39H45.46O12.15, as determined by an ultimate CHNS analyzer (Perkin-Elmer, model 2400). The molecular weight was between 1000 and 1500, as determined by a gel permeation chromatography (GPC) system (Waters, GPC150C-plus). The lignin powder was completely soluble in tetrahydrofuran (THF), which was purchased from Wako Pure Chemicals Industries, Ltd. All chemicals were used without further purification. Distilled water was obtained from a water distillation apparatus (Yamato Co., model WG-220). All catalysts tested in this study are summarized in Table 1. All catalysts were used without the pretreatment of reduction. Moles of surface ruthenium atoms of the fresh and used catalysts were determined by a carbon monoxide adsorption method at 323 K (Bel Japan, Inc., BEL-CAT). It was assumed that a carbon monoxide molecule adsorbed on a surface ruthenium atom (linear adsorption) for determining the number of surface ruthenium atoms in a catalyst. Metal dispersion was defined as the ratio of moles of carbon monoxide adsorbed to the moles of total ruthenium in a catalyst × 100. The Brunauer-EmmettTeller (BET) surface areas of fresh and used catalysts were determined by a nitrogen adsorption method (Autosorb-C, Quantachrome). The size characterization and morphology of the supported metal particles were carried out by transmission electron microscopy (TEM) (FEI Co., modelTECNAI-G2, 200 kV) and scanning electron microscopy (SEM) (Hitachi, S-800, 25 kV) analyses. Particle-size distribution and average particle size were determined based on the diameter of about 200 particles measured from TEM results. The size of metal particles on the supports and the crystalline structure of the supports were also estimated by X-ray diffraction method (XRD) (Mac Science, model MAC1800SRA). A source of X-ray was Mo (27) Elliott, D. C.; Phelps, M. R.; Sealock, L. J.; Baker, E. G. Ind. Eng. Chem. Res. 1994, 33, 566-574. (28) Elliott, D. C.; Neuenschwander, G. G.; Hart, T. R.; Scott Burner, R.; Zacher, A. H.; Engelhard, M. H.; Young, J. S.; McCready, D. E. Ind. Eng. Chem. Res. 2004, 43, 1999-2004. (29) Elliott, D. C.; Hart, T. R.; Neuenschwander, G. G. Ind. Eng. Chem. Res. 2006, 45, 3776-3781. (30) Davda, R. R.; Shabaker, J. W.; Huber, G. W.; Cortright, R. D.; Dumesic, J. A. Appl. Catal., B 2003, 43, 13-26. (31) Davda, R. R.; Shabaker, J. W.; Huber, G. W.; Cortright, R. D.; Dumesic, J. A. Appl. Catal., B 2005, 56, 171-186.

Figure 1. Carbon yield and gas composition for lignin gasification over Ru/TiO2 in supercritical water at 673 K and 0.5 g‚cm-3 of water density: (a) gas yield (first use (O), second use (4), and third use (])); gas composition of (b) first use, (c) second use, and (d) third use (H2 (b), CH4 (2), CO (×), CO2 (9), and C2-C4 gases ([)). The amounts of Ru/TiO2 and lignin were 0.375 and 0.1 g, respectively.

KR (λ ) 0.0709 nm), and the tube voltage and current were 40 kV and 30 mA, respectively. Catalytic gasification was conducted in a stainless-steel 316 tube bomb reactor having an internal volume of 6 cm3. A weighed amount of catalyst, 0.1 g of lignin, and 3.0 g of water were loaded into the reactor. This amount of water corresponded to a density of 0.5 g‚cm-3, and the partial pressure of water at 673 K was 37.1 MPa.32 Air inside the reactor was purged with argon gas. The reactor was submerged into a sand bath (Takahashi Rica Co., model TK-3) and was maintained at the reaction temperature. The reactor was heated to 673 K within 4 min. Reaction time reported here includes this heat-up time. After a given reaction time, the reactor was taken out of the sand bath and submerged in a water bath for rapid cooling to room temperature. Gaseous products were collected by a syringe through sampling loops attached to gas chromatography for analysis by a thermal conductivity detector (GC-TCD) (Shimadzu, model GC-8A). After sampling the gaseous products, the other products in the reactor were recovered with pure water and separated into water-soluble and water-insoluble fractions. The water-insoluble fraction was separated into THF-soluble and solid fractions. Because the solid fraction contained the supported metal catalysts used, we evaluated the amount of THF-insoluble products by subtracting the weight of the catalyst loaded from the solid fraction. (32) Wagner, W.; Purb, A. J. Phys. Chem. Ref. Data 2002, 31, 387535.

Supported Ruthenium Catalysts for Lignin Gasification

Energy & Fuels, Vol. 20, No. 6, 2006 2339

Figure 2. Carbon yield and gas composition for lignin gasification over Ru/C in supercritical water at 673 K and 0.5 g‚cm-3 of water density: (a) gas yield (first use (O), second use (4), and third use (])); gas composition of (b) first use, (c) second use, and (d) third use (H2 (b), CH4 (2), CO (×), CO2 (9), and C2-C4 gases ([)). The amounts of Ru/carbon and lignin were 0.150 and 0.1 g, respectively.

Figure 3. Carbon yield and gas composition for lignin gasification over Ru/γ-Al2O3 in supercritical water at 673 K and 0.5 g‚cm-3 of water density: (a) gas yield (first use (O), second use (4), and third use (])); gas composition of (b) first use, (c) second use, and (d) third use (H2 (b), CH4 (2), CO (×), CO2 (9), and C2-C4 gases ([)). The amounts of Ru/γ-Al2O3 and lignin were 0.150 and 0.1 g, respectively.

For studying the activity of catalyst used once, the following procedure was conducted. After gasification for 180 min, the catalyst used was recovered by filtration and dried at 333 K for 24 h. The recovery of the catalyst used was >90%. The subsequent gasification was carried out using the catalyst recovered from the first run, by charging fresh 0.1 g of lignin and 3.0 g of water into the reactor. The gasification activity of the same catalyst was also checked for the third time as described above. Water-soluble and THF-soluble products were analyzed by gas chromatography-flame ionization detection (GC-FID) (Hewlett-Packard, model HP-6980) and gas chromatographymass spectroscopy (GC-MS) (JEOL, Automass 20) analyses. The molecular weight distribution of THF-soluble products was measured using GPC. The amount of organic carbon in the water-soluble fractions was evaluated using the total organic carbon analyzer (Shimadzu, model TOC-5000A). The amount of ruthenium in the water-soluble fractions was evaluated using the inductively coupled plasma emission spectroscopy (ICPSPS 7800, Seiko). Ultimate analysis of the solid product was conducted by the CHNS analyzer. The product yield based on carbon, gas composition, and turnover number (TON) of the catalyst are defined as given below.

product yield based on carbon (C%) ) mol of carbon atom in product × 100 (1) mol of carbon atom in lignin loaded gas composition (%) ) mol of gas product × 100 (2) sum of mol of gas product TON )

mol of carbon atom in gaseous product mol of surface metal atom

(3)

Results and Discussion Gasification over Supported Ruthenium Catalysts. The stability of Ru/TiO2, Ru/C, and Ru/γ-Al2O3 was studied by

Figure 4. TEM images of Ru/TiO2: (a) before reaction (fresh catalyst), (b) after reaction (first use catalyst of 180 min), and (c) after reaction (third use catalyst of 180 min × 3).


Osada et al.

Figure 6. SEM images of Ru/TiO2: (a) before reaction (fresh catalyst) and (b) after reaction (first use catalyst of 180 min).

Figure 7. XRD patterns of Ru/TiO2: TiO2 (anatase type) (a) before reaction (fresh catalyst) and (b) after reaction (first use catalyst of 180 min). Table 2. Surface Area of the Ruthenium Catalyst surface area (m2·g-1) treatment

Ru/TiO2

Ru/C

Ru/γ-Al2O3

fresh after supercritical water treatment a after lignin gasification (first use)b after lignin gasification (third use)c

24 22

768 779

95 6 5

21

272

a

Figure 5. Particle-size distributions of Ru/TiO2: (a) before reaction (fresh catalyst), (b) after reaction (first use catalyst of 180 min), and (c) after reaction (third use catalyst of 180 min × 3).

Conditions: 673 K, 0.50 g/cm3 water density, and 180 min reaction time. b Conditions: 0.1 g of lignin, 673 K, 0.50 g/cm3 water density, and 180 min reaction time. c Conditions: 0.1 g of lignin, 673 K, 0.50 g/cm3 water density, and 180 min × 3 reaction time.

evaluating the activities of catalysts for several times at 673 K and 0.5 g‚cm-3 water density (Figures 1-3). In the absence of catalysts, the gas yield at 180 min was only 8 C%, and THFinsoluble products were formed. The TON values of the catalysts for 15 min for the first use were in the order of Ru/TiO2 (166) > Ru/γ-Al2O3 (117) > Ru/C (70). The gasification of lignin proceeded in supercritical water, and all lignin was gasified completely over Ru/TiO2 after 180 min during the first use. The gas yield after the initial 15 min decreased from 60 C% to 50 C% with its subsequent use; however, the gas yield after 180 min was similar to that of the first gasification run. THF-insoluble products were not formed in the gasification over Ru/TiO2. The composition of gaseous

products of lignin gasification is shown in Figure 1b. At 15 min, the composition was methane 40%, carbon dioxide 45%, hydrogen 13%, and C2-C4 (ethane, propane, and butane) gases 2%. Methane composition increased at 60 min, whereas hydrogen composition decreased. After 120 min, the composition became methane 50%, carbon dioxide 45%, and hydrogen 5%. One possible explanation for the increase in the methane composition is that carbon dioxide and hydrogen were formed at an early stage of lignin gasification and they reacted to produce methane. For repetitive use of catalysts, methane composition decreased, and hydrogen and C2-C4 gases compositions increased (Figure 1 parts c and d), indicating the deterioration of the methanation sites.



Figure 8. TEM images of Ru/C: (a) before reaction (fresh catalyst), (b) after reaction (first use catalyst of 180 min), and (c) after reaction (third use catalyst of 180 min × 3).

The initial activity of Ru/C for the first run was similar to that of Ru/TiO2. However, the activity of Ru/C decreased for repetitive uses. The gas yield at 15 min decreased from 50 C% for the first run to 20 C% for the second run. The gasification activity decreased; however, the gas yields after 180 min were 100 C% in all the runs. THF-insoluble products were also not formed in the gasification over Ru/C. For the first run, the composition was methane 48%, carbon dioxide 40%, hydrogen 10%, and C2-C4 gases 2% at 15 min. In comparison with Ru/ TiO2, methane composition increased and carbon dioxide composition decreased slightly over Ru/C. The trend of an increase in methane composition and a decrease in hydrogen composition with time over Ru/C was the same as that for Ru/TiO2 catalyst. The gasification activity (the ability of C-C bond cleavage) of Ru/C greatly deteriorated in comparison with Ru/TiO2. However, the deterioration of the methanation activity of Ru/C is the same as that of Ru/TiO2. For the case of Ru/γ-A2O3, the gas yield was saturated to 80 C% after 180 min for the first use of catalyst. It should be noted that the gasification activity greatly reduced for the second use of this catalyst, and 14 C% of THF-insoluble products were formed after 180 min. The gas composition for the first use for 180 min was methane 50%, carbon dioxide 45%, and hydrogen 5%, which was almost the same as those over Ru/TiO2 catalyst. For the second use of Ru/γ-A2O3, hydrogen composition increased and methane composition decreased, indicating that the methanation activity of the catalyst decreased. For the case of Ru/γ-A2O3 catalyst, the gasification activity (the C-C bond cleavage) was rapidly lost in the initial stage of reaction and

Figure 9. Particle-size distributions of Ru/C: (a) before reaction (fresh catalyst), (b) after reaction (first use catalyst of 180 min), and (c) after reaction (third use catalyst of 180 min × 3).

the gasification activity was not observed beyond the second use of the catalyst. Structure Change after Gasification. Figures 4 and 5 show the TEM images and the metal particle-size distribution of Ru/ TiO2 before and after gasification. The mean diameter of ruthenium metal particles of fresh Ru/TiO2, after its first use and after three recycles uses, were 1.4, 1.7, and 1.8 nm, respectively, indicating ruthenium particle size remained almost the same during the gasification reaction. The solution recovered after the gasification was analyzed by an ICP analysis. The ruthenium was not detected in the solution, indicating that the ruthenium metal did not leach out into water during gasification.


Figure 10. SEM images of Ru/C: (a) before reaction (fresh catalyst) and (b) after reaction (first use catalyst of 180 min).

Osada et al.

Figure 12. TEM images of Ru/γ-Al2O3: (a) before reaction (fresh catalyst) and (b) after reaction (first use catalyst of 180 min).

Figure 11. XRD patterns of Ru/C: (a) before reaction (fresh catalyst) and (b) after reaction (first use catalyst of 180 min).

The morphology and surface structure of titania support was observed by SEM (Figure 6), and a large difference could not be observed before and after gasification. The surface area of the Ru/TiO2 catalyst was almost the same before and after gasification (Table 2). XRD analysis showed the structure of titania having anatase-type structure did not change after the gasification in supercritical water (Figure 7). Elliott et al.29 reported that the phase of support of a Ru/TiO2 catalyst was changed from anatase to rutile after the gasification of phenol in subcritical water at 623 K. The crystal form and shape of the Ru/TiO2 catalyst used in this study remained stable under the reaction conditions in the present work. These results clearly indicate that Ru/TiO2 is an appropriate catalyst for the lignin gasification in supercritical water. Figures 8 and 9 show the TEM images and the metal particlesize distribution of Ru/C before and after gasification. The mean diameter of fresh Ru/C was 2.3 nm, which increased to 3.2 nm after subsequent gasification and again to 4.2 nm after three recycles of the catalyst. The ruthenium was not detected in the solution recovered after gasification by ICP analysis, indicating that ruthenium metal did not leach out into water during the gasification reaction. SEM (Figure 10) and XRD (Figure 11) patterns of the activated carbon support before and after gasification did not reveal any major changes. However, the surface area of Ru/C decreased after three recycles of the catalyst; its surface area decreased significantly (Table 2). All of the lignin

Figure 13. Particle-size distributions of Ru/γ-Al2O3: (a) before reaction (fresh catalyst).

introduced was gasified over Ru/C, and the weight of Ru/C catalyst recovered after gasification was almost the same as that before gasification. Elliott et al.25 reported that the activated carbon support was gasified by metal particles slightly; however, we confirmed that the formation of methane and carbon dioxide from activated carbon was only 0.1% (based on C%) of the activated carbon loaded. One possible explanation for the decrease of surface area values after gasification is the pore structure change of the carbon support; however, we cannot completely deny the blockage of pores by a small amount of carbonaceous products during gasification. Figure 12 shows the TEM images of Ru/γ-A2O3 before and after gasification. The size of ruthenium particles on the supports before gasification was 2.1 nm (Figure 13). However, we could not find ruthenium particles in TEM images after gasification. ICP analysis showed the presence of ruthenium species in the solution recovered after the reaction time of 15 min, indicating that the ruthenium metal particles leached out into water during the gasification reaction. The concentration of ruthenium detected was 2-3 ppm, corresponding to 1% of ruthenium on the Ru/γ-A2O3 catalyst. Also, the ruthenium was not detected in the solution after the reaction of 180 min; however, we confirmed that the fresh Ru/γ-A2O3 catalyst in the powder form



in water at 673 K. Ruthenium species were detected by ICP analysis in the solution recovered after only 15 min. All the above results show that the gasification activity of Ru/γ-A2O3 catalyst stopped at an early stage of gasification by leaching of active species during the structural change of support from gamma- to alpha-phase in supercritical water. Conclusion

Figure 14. XRD patterns of Ru/γ-Al2O3: R-Al2O3 (a) before reaction (fresh catalyst) and (b) after reaction (first use catalyst of 180 min).

aggregated to a large lump of alumina particles in the reactor after the gasification. The surface area of the Ru/γ-A2O3 catalyst decreased significantly after the supercritical water gasification (Table 2). XRD analysis showed that the alumina support used in this study changed from gamma- (fresh) to alpha-phase after the supercritical water gasification carried out for 180 min (Figure 14). We also confirmed separately that the structure of Ru/γ-A2O3 catalyst was changed to Ru/R-A2O3 by treatment

Lignin gasification in supercritical water was studied over various supported ruthenium catalysts. The following results were obtained. 1. Ru/TiO2 showed stable activities for repetitive uses in lignin gasification in supercritical water. 2. Ru/C showed high lignin gasification; however, the activities decreased gradually with repetitive use because of the decrease in its surface area. 3. Ru/γ-A2O3 showed high initial activities for the first run; however, the sturucture of alumina easily changed from gammato alpha-phase in supercritical water, resulting in a dramatic loss of its surface area and gasification activities. Acknowledgment. The authors would like to thank Osaka Gas Company Ltd. for supplying the catalysts. EF060356H

Stability of Supported Ruthenium Catalysts for Lignin Gasification in

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