Reaction Pathway for Catalytic Gasification of Lignin in Presence of

The effect of sulfur on the reaction pathway for lignin gasification was studied over a ... In the absence of sulfur, lignin was completely gasified t...
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Energy & Fuels 2007, 21, 1854-1858

Reaction Pathway for Catalytic Gasification of Lignin in Presence of Sulfur in Supercritical Water Mitsumasa Osada,†,‡ Norihito Hiyoshi,‡ 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 April 2, 2007. ReVised Manuscript ReceiVed May 3, 2007

The effect of sulfur on the reaction pathway for lignin gasification was studied over a titania-supported ruthenium catalyst in supercritical water. In the absence of sulfur, lignin was completely gasified to methane, carbon dioxide, and hydrogen over the titania-supported ruthenium catalyst in supercritical water. In the presence of sulfur, the overall gas yield decreased and THF-insoluble products (namely, char) were formed. Also, the content of hydrogen increased in the resulting gas composition. To investigate the poisoning effect of sulfur for lignin gasification, the gasification behavior of 4-propylphenol and formaldehyde, which are low-molecularweight model compounds from lignin decomposition, was studied in supercritical water. 4-Propylphenol was completely gasified in the absence of sulfur; however, it was hardly gasified in the presence of sulfur. On the other hand, the gasification of formaldehyde proceeded smoothly over the ruthenium catalyst regardless of the presence of sulfur. Also, the content of hydrogen in the gaseous products increased in the presence of sulfur. We concluded that sulfur poisoned the active sites for carbon-carbon bond breaking and the methanation reaction; on the other hand, it did not hinder the sites for the gasification of formaldehyde and the water-gas shift reaction.

Introduction The gasification processes of woody biomass have received great 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 gas. Generally, very high temperatures of over 1073 K are needed for steam reformation of the lignin in the gasification process.1 Hence, low-temperature methods for lignin gasification are the most desirable, so that the waste heat from high-temperature processes in industry can be utilized for energy generation. From this point of view, catalytic gasification in supercritical water (Tc ) 647.3 K, Pc ) 22.1 MPa) could be a potential technology that can operate at low temperatures2-7 because of the high solubility and hydrolysis rates of biomass in supercritical water. This would also lead to minimization of the mass-transfer limitations of the reactant8,9 and rinsing effect of supercritical water for * Corresponding author. Tel.: +81-22-237-5219. Fax: +81-22-2375224. E-mail: [email protected]. † Ichinoseki National College of Technology. ‡ AIST. (1) Antal, M. J., Jr. Ind. Eng. Chem. Prod. Res. DeV. 1983, 22, 366375. (2) Osada, M.; Sato, T.; Watanabe, M.; Shirai, M.; Arai, K. Combust. Sci. Technol. 2006, 178, 537-552. (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) Watanabe, M.; Inomata, H.; Osada, M.; Sato, T.; Adschiri, T.; Arai, K. Fuel 2003, 82, 545-552. (5) Park, K. C.; Tomiyasu, H. Chem. Commun. 2003, 6, 694-695. (6) Antal, M. J., Jr.; Allen, S. G.; Schulman, D.; Xu, X.; Divilio, R. J. Ind. Eng. Chem. Res. 2000, 39, 4040-4053. (7) Kruse, A.; Gawlik, A. Ind. Eng. Chem. Res. 2003, 42, 267-279. (8) Savage, P. E. Chem. ReV. 1999, 99, 603-621.

washing out coke precursors from the active catalyst sites.10 These characteristics of supercritical water gasification are different from the steam reformation that the gasification proceeds mainly through pyrolysis.1,2 The decomposition behavior of lignin in supercritical water around 673 K was reported in which lignin was converted to alkylphenols and formaldehyde via hydrolysis,11-15 and then via the decomposition of alkylphenols to gases. Cross-linking reactions between the alkylphenols and formaldehyde15 also take place to produce solid materials, insoluble in water and tetrahydrofuran.13-16 The gasification of lignin in supercritical water around 673 K is enhanced by several metal catalysts, and titania- or carbonsupported ruthenium catalysts were highly active for lignin gasification in supercritical water.17-19 However, not only the initial activity and selectivity but also the catalyst life, with respect to support stability and impurities (such as sulfur (9) Watanabe, M.; Sato, T.; Inomata, H.; Smith, R. L., Jr.; Arai, K.; Kruse, A.; Dinjus, E. Chem. ReV. 2004, 104, 5803-5821. (10) Savage, P. E. Catal. Today 2000, 62, 167-173. (11) Bobleter, O.; Consin, R. Cell. Chem. Technol. 1979, 13, 583-593. (12) Funazukuri, T.; Wakao, N.; Smith, J. M. Fuel 1990, 69, 349-353. (13) Yokoyama, C.; Nishi, K.; Nakajima, A.; Seino, K. Sekiyu Gakkaishi 1998, 41, 243-250. (14) Ehara, K.; Saka, S.; Kawamoto, H. J. Wood Sci. 2002, 48, 320325. (15) Saisu, M.; Sato, T.; Watanabe, M.; Adschiri, T.; Arai, K. Energy Fuels 2003, 17, 922-928. (16) Antal, M. J., Jr. Solar Energy; Plenum Press: New York, 1983; pp 175-255. (17) Osada, M.; Sato, O.; Watanabe, M.; Arai, K.; Shirai, M. Energy Fuels 2006, 20, 930-935. (18) Osada, M.; Sato, O.; Arai, K.; Shirai, M. Energy Fuels 2006, 20, 2337-2343. (19) Osada, M.; Hiyoshi, N.; Sato, O.; Arai, K.; Shirai, M. Energy Fuels 2007, 21, 1400-1405.

10.1021/ef0701642 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/13/2007

Catalytic Gasification of Lignin

Energy & Fuels, Vol. 21, No. 4, 2007 1855 Table 1. Product Yields of Lignin Gasification in Supercritical Watera

amount of catalyst (g) without catalyst Ru/TiO2 S-Ru/TiO2

yield (C%)

moles of sulfur/ moles of surface metal

gas

water-soluble

THF-insoluble

THF-solubleb

3.7

7.9 97.7 21.0

20.6 0.7 18.3

20.9 0.0 3.1

50.6 1.6 57.6

0.375 0.375

Conditions: 0.1 g of lignin, 7.4 × 10-5 mol of metal in catalyst at 673 K, 0.50 g cm-3 of water density, and 180 min reaction time. b THF-soluble (C%) ) 100 - [gas yield (C%) + water-soluble yield (C%) + THF-insoluble(C%)]. a

Table 2. Gas Composition for Lignin Gasification in Supercritical Watera composition of gas products (mol %) without catalyst Ru/TiO2 S-Ru/TiO2

H2

CH4

CO

CO2

C2∼C4 gases

21.5 4.5 14.5

30.6 49.0 24.5

0.0 0.0 0.0

43.8 46.5 57.7

4.1 0.0 3.3

a Conditions: 0.1 g of lignin, 7.4 × 10-5 mol of metal in catalyst at 673 K, 0.50 g cm-3 of water density, and 180 min reaction time.

compounds) present in biomass, for lignin gasification in supercritical water is very important.18-25 We have reported that the presence of sulfur lowered the gasification activities of the titania- or carbon-supported ruthenium catalyst in supercritical water.19 The poisoning of supported metal catalysts by sulfur in supercritical water gasification has already been reported;19,23 however, the gasification behavior has not been resolved. We report here a detailed study on the reaction pathway for lignin gasification in the presence of sulfur in supercritical water. For this purpose, the influence of sulfur on the gasification behavior of lignin and its model compounds, 4-propylphenol and formaldehyde, was examined in the presence of supported ruthenium catalysts in supercritical water. Experimental Section The preparation procedure of catalysts poisoned by sulfur is as follows. A titania-supported ruthenium catalyst (Ru/TiO2; 2 wt % ruthenium on titania) was soaked in an aqueous sulfuric acid solution, followed by its drying by evaporation. The BrunauerEmmett-Teller (BET) surface area values of fresh and used catalysts were determined by a nitrogen adsorption method (Autosorb-C, Quantachrome). The moles of surface ruthenium atoms of the fresh catalysts were determined by a carbon monoxide adsorption method at 323 K (Bel Japan, Inc. BEL-CAT). It was assumed that a single carbon monoxide molecule was adsorbed on the surface of a single ruthenium atom (linear adsorption) for the determination of the number of surface metal atoms in the catalyst. Metal dispersion was defined as the ratio of moles of carbon monoxide adsorbed to the moles of total ruthenium in a catalyst multiplied by 100. 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). Its 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). THF (+97%), paraformaldehyde (+98%) used as source of formaldehyde, and 5 wt % sulfuric acid solution were purchased from Wako Pure Chemicals Industries, Ltd. 4-Propylphenol was purchased from Tokyo Kasei Kogyo Co., Ltd. and had a purity of 99%. All chemicals were used without further purification. Distilled water was obtained from a water distillation apparatus (Yamato Co., model WG-220). Catalytic gasification was conducted in a stainless-steel 316 bomb reactor with an internal volume of 6 cm3. A weighed amount of catalyst, 0.1 g of lignin (or 4-propylphenol or formaldehyde), and 3.0 g of water were loaded into the reactor. This maximal amount of water corresponded to a density of 0.5 g cm-3 for a 37.1 MPa partial pressure of water at 673 K.26 Air inside the reactor was purged with argon gas. The reactor was submerged into a preheated sand bath (Takahashi Rica Co., model TK-3), and it took about 4 min to heat the reactor to 673 K. The reaction times reported here include this heating time. After a given reaction time, the reactor was taken out of the sand bath and submerged into a water bath to cool it rapidly to room temperature. Gaseous products were collected by a syringe, and 1 mL of them was analyzed by a gas chromatograph for analysis using a thermal conductivity detector (Shimadzu, model GC-8A). After the gaseous products were sampled, the other products from the reactor were recovered with pure water and separated as water-soluble and water-insoluble fractions. The water-insoluble fraction was again separated as THFsoluble 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 total solid fraction. The water-soluble and THF-soluble products were analyzed by gas chromatography (Hewlett-Packard, model HP-6980) using a flame ionization detector and gas chromatography mass-spectroscopy (JEOL, Automass 20). The molecular weight distribution of THF-soluble products was measured using the GPC system. The amount of organic carbon in the water-soluble fractions was evaluated using the total organic carbon analyzer (Shimadzu, model TOC-5000A). An ultimate analysis of the solid product was conducted by a CHNS analyzer. The product yields based on total carbon content and gas composition are defined as given below. product yield based on carbon (C%) ) moles of carbon atom in product × 100 (1) moles of carbon atom in reactant loaded gas composition (%) )

moles of gas product × 100 sum of moles of gas product

(2)

Results and Discussion (20) Elliott, D. C.; Sealock, L. J.; Baker, E. G. Ind. Eng. Chem. Res. 1993, 32, 1542-1548. (21) Elliott, D. C.; Sealock, L. J.; Baker, E. G. Ind. Eng. Chem. Res. 1994, 33, 558-565. (22) Elliott, D. C.; Phelps, M. R.; Sealock, L. J.; Baker, E. G. Ind. Eng. Chem. Res. 1994, 33, 566-574. (23) 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. (24) Elliott, D. C.; Hart, T. R.; Neuenschwander, G. G. Ind. Eng. Chem. Res. 2006, 45, 3776-3781. (25) Davda, R. R.; Shabaker, J. W.; Huber, G. W.; Cortright, R. D.; Dumesic, J. A. Appl. Catal., B 2003, 43, 13-26.

Characterization of Supported Sulfided Ruthenium Catalyst. The ruthenium metal dispersion of fresh Ru/TiO2 was 27%, which was determined by a carbon monoxide adsorption method. The molar ratio of sulfur to surface ruthenium was defined by the concentration of aqueous sulfuric acid and the surface ruthenium atoms. A Ru/TiO2 catalyst poisoned by aqueous sulfuric acid using a molar ratio of sulfur atoms to (26) Wagner, W.; Purb, A. J. Phys. Chem. Ref. Data 2002, 31, 387535.

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Table 3. Product Yields of 4-Propylphenol Gasification in Supercritical Watera amount of catalyst (g) without catalyst Ru/TiO2 S-Ru/TiO2 a

0.375 0.375

product yield (C%)

moles of sulfur/ moles of surface metal

gas

propyl-benzene

reactant (C%)

carbon balance (C%)

3.7

0.0 97.9 2.2

0.0 0.0 17.5

99.1 0.0 75.2

99.1 97.9 94.9

Conditions: 0.1 g of 4-propylphenol, 7.4 × 10-5 mol of metal in catalyst at 673 K, 0.5 g cm-3 of water density, and 180 min reaction time.

surface ruthenium atoms of 3.7 gave the molar ratio of sulfur to all ruthenium atoms as 1.0 (this catalyst is designated as S-Ru/TiO2 in this paper). The BET surface area value of the S-Ru/TiO2 catalyst was 24 m2 g-1, the same as that of the fresh Ru/TiO2 for 24 m2 g-1. Effect of Sulfur on Catalytic Gasification of Lignin in Supercritical Water. Table 1 shows the product yields obtained by the gasification of lignin over Ru/TiO2 and S-Ru/TiO2 at 673 K and with 0.5 g cm-3 of water density for 180 min of reaction time. The values in Table 1 show an average of two experimental runs, and the difference between these runs was within (5%. The gas yield was only 7.9 C%, and the THFinsoluble (namely, char) yield was 20.9 C% in the absence of a catalyst. In the presence of the Ru/TiO2 catalyst, lignin was completely gasified, giving a gas yield > 97 C%. However, the gas yield decreased dramatically from 97.7 to 21.0 C%, and the THF-insoluble yield increased from 0 to 3.1 C% over S-Ru/ TiO2. We have reported that lignin was converted to lowmolecular-weight compounds, such as alkylphenols and formaldehyde, via hydrolysis and then decomposition of the lowmolecular-weight compounds to gases over a supported ruthenium catalyst.17 The lowering of the gas yield and the formation of THF-insoluble products over S-Ru/TiO2 would be caused by the decrease in the number of active surface ruthenium sites for the gasification of the low-molecular-weight compounds by sulfur adsorption. Table 2 summarizes the gas composition obtained by the gasification of lignin over Ru/TiO2 and S-Ru/TiO2. The major gas composition was carbon dioxide, methane, hydrogen, and C2-C4 gases (ethane, propane, and butane). In comparison with the results of the Ru/TiO2 catalyst, the composition of methane decreased, and those of carbon dioxide and hydrogen increased over S-Ru/TiO2. It is reported that the breaking of carboncarbon bonds of aromatic compounds needed the ensemble of metal atoms for the adsorption of the aromatic compounds to metal particles.27 It is also reported that the methanation reaction proceeded through both the carbon-oxygen bond breaking of carbon monoxide or carbon dioxide and the hydrogen-hydrogen bond breaking of hydrogen molecules on the metal surface over an ensemble of metal atoms for adsorption and breaking sites for reactant molecules such as carbon monoxide, carbon dioxide, and hydrogen.28 One possible explanation for the decrease of methane composition in the gasification over S-Ru/TiO2 is caused by the decrease of ruthenium ensembles by the adsorption of sulfur atoms on ruthenium metal particles because ruthenium ensembles are indispensable for methanation reactions to proceed. Effect of Sulfur Concentration on Lignin Gasification. The effect of sulfur concentration on the gas yield for lignin gasification was also studied, and the results are shown in Figure 1. The gas yield sharply decreased from 100 to 20 C% with an increase in sulfur content from 0 to 4 in terms of the molar ratio of sulfur to surface ruthenium. The deposition (or poisoning) of sulfur on the active ruthenium particles caused the

decrease of initial gasification activities. On the other hand, the yields of water-soluble, THF-soluble, and THF-insoluble products increased with the increase in sulfur content. Figure 2 shows the effect of sulfur concentration on the gas composition. The major gas composition was carbon dioxide, methane, hydrogen, and C2-C4 gases. With an increase in the concentration of sulfur, the composition of methane decreased while that of hydrogen increased. Effect of Sulfur on Catalytic Gasification of 4-Propylphenol in Supercritical Water. To investigate the poisoning by sulfur for lignin gasification in detail, the gasification of 4-propylphenol, which is a model compound of alkylphenols formed by lignin decomposition in supercritical water,29 was conducted. Table 3 shows the product yields obtained by the gasification of 4-propylphenol over Ru/TiO2 and S-Ru/TiO2 at 673 K with 0.5 g cm-3 of water density for 180 min of reaction time. Similar to lignin, 4-propylphenol also did not undergo gasification in the absence of ruthenium catalysts. 4-Propylphenol was completely gasified over Ru/TiO2 in supercritical water. For the S-Ru/TiO2 catalyst, the gas yield dramatically decreased from 97.9 to 2.2 C%, and only propylbenzene, which would be derived from the dehydroxylation of 4-propylphenol, was produced as a THF-soluble product. These results indicate that the carbon-carbon bond breaking of aromatic compounds hardly occurred over S-Ru/TiO2. One possible explanation for the inhibition of the gasification of 4-propylphenol over S-Ru/TiO2 is the decrease in ruthenium ensembles by the adsorption of sulfur atoms on the ruthenium metal surface. On the other hand, the dehydroxylation probably did not need ruthenium ensembles, and hence it could proceed even on S-Ru/TiO2. Table 4 summarizes the gas composition obtained by the gasification of 4-propylphenol over Ru/TiO2 and S-Ru/TiO2. In comparison with the Ru/TiO2 catalyst, the composition of methane decreased, and those of carbon dioxide and hydrogen increased over S-Ru/TiO2 for the gasification of 4-propylphenol in supercritical water. The extent of the decrease in activity for the gasification of 4-propylphenol due to the presence of sulfur was larger than that in the case of lignin gasification. From these results, it was clear that sulfur on surface ruthenium decreased the catalyst activity for the carbon-carbon bond breaking significantly. Effect of Sulfur on Catalytic Gasification of Formaldehyde in Supercritical Water. Figure 3 shows the gas yields obtained by the gasification of formaldehyde at 673 K with 0.5 g cm-3 of water density. Less than 40 C% of the formaldehyde was gasified in supercritical water without a catalyst. While in the presence of the Ru/TiO2 catalyst, formaldehyde was completely gasified in supercritical water, similar to the gasification of lignin and 4-propylphenol. Although the gas yield over S-Ru/TiO2 was lower than that over Ru/TiO2, the difference was small, which was a different trend from the gasification of lignin and 4-propylphenol. Figures 4-6 show the gas composition obtained by the gasification of formaldehyde at 673 K with 0.5 g cm-3 of water density. In the absence of Ru/TiO2, the gas composition

(27) Sinfelt, J. H. AdV. Catal. 1973, 23, 91-119. (28) Bond, G. C. Catalysis by Metal; Academic Press: London, 1962.

(29) Sato, T.; Osada, M.; Watanabe, M.; Shirai, M.; Arai, K. Ind. Eng. Chem. Res. 2003, 42, 4277-4282.

Catalytic Gasification of Lignin

Figure 1. Effect of sulfur concentration on the carbon yield for lignin gasification over Ru/TiO2 at 673 K for 180 min, and 0.5 g cm-3 of water density: gas (O), water-soluble (4), THF-soluble (]), and THFinsoluble (0). The amounts of Ru/TiO2 and lignin were 0.375 and 0.1 g, respectively.

Figure 2. Effect of sulfur concentration on the gas composition for lignin gasification over Ru/TiO2 at 673 K for 180 min, and 0.5 g cm-3 of water density: H2 (b), CH4 (2), CO2 (9), and C2∼C4 gases (×). The amounts of Ru/TiO2 and lignin were 0.375 and 0.1 g, respectively.

Energy & Fuels, Vol. 21, No. 4, 2007 1857

Figure 4. Gas composition versus reaction time for formaldehyde gasification without catalyst conditions at 673 K, 0.50 g cm-3 of water density, and 0.1 g of formaldehyde: H2 (b), CH4 (2), CO2 (9), CO ([), and C2∼C4 gases (×).

Figure 5. Gas composition versus reaction time for formaldehyde gasification over Ru/TiO2 at 673 K, 0.50 g cm-3 of water density, and 0.1 g of formaldehyde: H2 (b), CH4 (2), CO2 (9), CO ([), and C2∼C4 gases (×).

Figure 3. Gas yield versus reaction time for formaldehyde gasification at 673 K, and 0.50 g cm-3 of water density: S-Ru/TiO2 (O), Ru/TiO2 (4), and without catalyst (]). The amounts of Ru/TiO2 and lignin were 0.375 and 0.1 g, respectively.

Figure 6. Gas composition versus reaction time for formaldehyde gasification over S-Ru/TiO2 at 673 K, 0.50 g cm-3 of water density, and 0.1 g of formaldehyde: H2 (b), CH4 (2), CO2 (9), CO ([), and C2∼C4 gases (×).

was about 60% carbon dioxide, 40% hydrogen, and a small amount of carbon monoxide (Figure 4). It is reported that two molecules of formaldehyde and one of water reacted to give methanol and formic acid through the Cannizzaro reaction in supercritical water, and also we reported that methanol was stable; on the other hand, formic acid decomposed to form mainly hydrogen and carbon dioxide or a slight amount of water and carbon monoxide in supercritical water.30,31 Moreover, methanol was gasified to carbon monoxide and hydrogen over Ru/TiO2 (Figure 5) in supercritical water.32 It is reported that

Table 4. Gas Composition for 4-Propylphenol Gasification in Supercritical Watera

(30) Watanabe, M.; Osada, M.; Inomata, H.; Arai, K.; Kruse, A. Appl. Catal., A 2003, 245, 333-341. (31) Osada, M.; Watanabe, M.; Sue, K.; Adschiri, T.; Arai, K. J. Supercrit. Fluids 2004, 28, 219-224. (32) Osada, M.; Sato, T.; Watanabe, M.; Adschiri, T.; Arai, K. Energy Fuels 2004, 18, 327-333.

composition of gas products (mol %) without catalyst Ru/TiO2 S-Ru/TiO2

H2

CH4

CO

CO2

C2∼C4 gases

6.2 24.7

53.8 12.7

0.0 0.0

37.4 56.2

2.6 6.4

a Conditions: 0.1 g of 4-propylphenol, 7.4 × 10-5 mol of metal in catalyst at 673 K, 0.5 g cm-3 of water density, and 180 min reaction time.

the carbon dioxide and hydrogen reacted to give methane through the methanation reaction over Ru/TiO2.31 For S-Ru/ TiO2 (Figure 6), the formation of methane decreased, and that of carbon dioxide and hydrogen increased in comparison with the Ru/TiO2 catalyst. Although the gas yields obtained over the Ru/TiO2 and S-Ru/TiO2 catalysts are almost the same for

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Figure 7. Reaction pathway of lignin gasification over a supported ruthenium catalyst containing sulfur in supercritical water.

formaldehyde gasification (Figure 3), the gas composition values were totally different. The main gases produced were hydrogen and carbon dioxide over the S-Ru/TiO2 catalyst. Methane and carbon dioxide would be the main gas products in formaldehyde gasification over Ru/TiO2. It is reported that the water-gas shift reaction did not need ensembles of metal atoms on the surface.33 Hence, the water-gas shift reaction could proceed over S-Ru/ TiO2, because carbon monoxide was not observed for the gasification of formaldehyde over S-Ru/TiO2 (Figure 6). One possible explanation for the progress of the formaldehyde decomposition to carbon monoxide and hydrogen over S-Ru/ TiO2 is that this reaction did not need ruthenium ensembles, which is quite different from the reaction pathway operating for the gasification of lignin and 4-propylphenol, which involves carbon-carbon bond breaking and the methanation reaction. Effect of Sulfur on the Reaction Pathway of Lignin Gasification. The effect of sulfur on the reaction pathway of lignin gasification over the titania-supported ruthenium catalyst in supercritical water is summarized in Figure 7. The decomposition behavior of lignin in supercritical water around 673 K was reported to involve the conversion of lignin to alkylphenols and formaldehyde via hydrolysis.2-4,11-15 Without the supported ruthenium catalyst, a cross-linking reaction between the alkylphenols and formaldehyde would take place to produce THFinsolubles, namely, char.14 Therefore, the products of lignin gasification without a catalyst in supercritical water became, in the following order, THF-soluble > THF-insoluble > watersoluble products. Over Ru/TiO2, the gasification of alkylphenols (carboncarbon bond breaking) and formaldehyde decomposition occurred, and hydrogen, carbon monoxide, and carbon dioxide were formed.32 Some of these gases reacted to give methane via the methanation reaction over supported ruthenium catalysts.17 Only gas products were obtained by lignin gasification over Ru/TiO2, which consisted of mainly methane, carbon dioxide, and hydrogen. (33) Grenoble, D. C.; Esradt, M. M.; Ollis, D. F. J. Catal. 1981, 67, 90-102.

From the results of 4-propylphenol and formaldehyde gasification over S-Ru/TiO2, the carbon-carbon bond breaking and methanation reaction were inhibited by the adsorption of sulfur atoms on the ruthenium metal surface. However, the formaldehyde gasification and water-gas shift reaction proceeded even over S-Ru/TiO2. The products from lignin gasification over S-Ru/TiO2 in supercritical water are in the order THF-soluble > gas > water-soluble products. Since formaldehyde, which was a cross-linking agent in lignin gasification,15 could be decomposed to gases, the yield of the THF-insoluble products from the lignin over S-Ru/TiO2 would be lower than that under noncatalytic conditions. Furthermore, the formation of hydrogen in the gasification of lignin over S-Ru/TiO2 became larger than that over Ru/TiO2, because the methanation reaction was inhibited by the adsorption of sulfur on the ruthenium surface. Conclusion The gasification behavior of lignin and its model compounds (4-propylphenol and formaldehyde) was studied over sulfurpoisoned supported ruthenium catalysts in supercritical water at 673 K. For the lignin gasification, the gas yield and the formation of methane in the presence of sulfur were lower than those in the absence of sulfur. The gasification of 4-propylphenol hardly proceeded in the presence of sulfur. The formaldehyde gasification proceeded smoothly both in the presence and absence of sulfur; on the other hand, the formation of methane in the presence of sulfur was lower than that in the absence of sulfur. These results indicate that sulfur inhibited the carboncarbon bond breaking and the methanation reaction and did not affect formaldehyde decomposition and the water-gas shift reaction. Therefore, the gasification of lignin proceeded slightly over the supported ruthenium catalyst poisoned by sulfur. Acknowledgment. This research was partially supported by Japan Science and Technology Agency (JST). EF0701642