Energy & Fuels 2005, 19, 1761-1762
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Production of Hydrogen by Steam Reforming of Glycerin on Ruthenium Catalyst Toshihide Hirai, Na-oki Ikenaga, Takanori Miyake, and Toshimitsu Suzuki* Department of Chemical Engineering, Faculty of Engineering, Kansai University, Suita, Osaka, 564-8680 Japan Received April 26, 2005. Revised Manuscript Received May 27, 2005 Biomass has received much attention as a new energy source, because it is renewable and carbon neutral. Considerable amounts of waste vegetable oils are formed from food industries and home cooking. Portions of the waste vegetable oils react with methanol to give fatty acid methyl esters, which can then be used as a diesel fuel oil. Soot in exhaust gas from vehicles fueled by diesel oil is currently one of the most serious environmental issues in urban areas. To overcome this problem, the use of longalkyl-chain, sulfur-free fuel is recommended. From this perspective, fatty acid methyl esterssthe socalled bio-diesel fuelsare of current interest. In converting vegetable oils into their methyl esters, ∼10 wt % of glycerin is produced as a byproduct. Although glycerin is used in medicines, cosmetics, and sweetening agents, world demand is limited. As such, when mass production of the bio-diesel is realized, novel processes that utilize glycerin must be developed. One possibility is to use glycerin as a source of hydrogen, and, in this regard, steam reforming of glycerin would be a suitable reaction. In the steam reforming of glycerin, synthesis gas that contains both carbon monoxide (CO) and hydrogen (H2) is produced. From syngas, methanol, which is used for methyl esterification of vegetable oils, can be produced using conventional technology and, as a result, 100% biomass-based bio-diesel fuel could be produced. The first report concerning the reforming of glycerin and some biomass-derived oxygenated hydrocarbons in the aqueous phase appeared in the pioneering work of Dumesic and co-workers.1-9 Although the advantages of smaller catalyst deactivation have been reported in the aqueous phase, it is inevitable that the reaction must be conducted under high pressure. On the other hand, reforming in the gas phase can be conducted under atmospheric pressure with a conventional fixed-bed flow reactor. Steam reforming of crude glycerin in the gas phase has been reported by Chornet and co-workers.10 * Author to whom correspondence should be addressed. Telephone: 81-6-6388-8869. E-mail address:
[email protected]. (2) Cortright, R. D.; Davda, R. R.; Dumesic, J. A. Nature 2002, 418, 964. (3) Davda, R. R.; Shabaker, J. W.; Huber, G. W.; Cortright, R. D.; Dumesic, J. A. Appl. Catal. B 2002, 43, 13. (4) Shabaker, J. W.; Huber, G. W.; Davda, R. R.; Cortright, R. D.; Dumesic, J. A. Catal. Lett. 2003, 88, 1. (5) Davda, R. R.; Dumesic, J. A. Angew. Chem., Int. Ed. 2003, 42, 4068. (6) Huber, G. W.; Shabaker, J. W.; Dumesic, J. A. Science 2003, 300, 2075. (7) Shabaker, J. W.; Huber, G. W.; Dumesic, J. A. J. Catal. 2004, 222, 180. (8) Shabaker, J. W.; Davda, R. R.; Huber, G. W.; Cortright, R. D.; Dumesic, J. A. J. Catal. 2003, 215, 344. (9) Davda, R. R.; Dumesic, J. A. Chem. Commun. 2004, 36. (10) Shabaker, J. W.; Dumesic, J. A. Ind. Eng. Chem. Res. 2004, 43, 3105.
However, they have reported only the results of two runs with a commercial nickel catalyst; therefore, an efficient catalyst for the steam reforming of glycerin remains to be developed. This communication involves the development of a novel efficient catalyst for glycerin steam reforming in the gas phase, and we report herein that a Ru/Y2O3 catalyst afforded very high activity in a prolonged run. Catalysts loaded with Group 8-10 metals were prepared via a conventional impregnation method, using Y2O3, ZrO2, CeO2, La2O3, SiO2, MgO, and Al2O3 as supports. The activity test was performed with a fixedbed, flow-type reactor that was made of stainless steel (inner diameter of 4.4 mm) and operated at atmospheric pressure. An aqueous solution of glycerin (special grade, from Wako Chemicals) was fed by a micropump for highperformance liquid chromatography (HPLC). To preheat the aqueous solution of glycerin, alumina balls (1 mm in diameter) were placed above a catalyst bed. One hundred milligrams of catalyst was used for each run. Prior to the reaction, the catalyst was reduced with H2 at 600 °C for 1 h. Argon was used as a carrier gas to sweep out the produced gases. The reforming reaction was conducted at a temperature of 500-600 °C, a steam-to-carbon molar ratio of S/C1 ) 3.3, and a W/F (contact time) of glycerin of 13.4 g-cat h/mol. We have previously reported that Group 8-10 metals supported on La2O3 catalysts exhibit high activity for the reforming of methane and dodecane.11,12 As such, steam reforming of glycerin was first performed on La2O3 catalysts that were loaded with Group 8-10 metals. The results for runs 1-9 are provided in the Supporting Information. The conversions were calculated based on the glycerin that was converted to gaseous products (CO, CO2, CH4), and small amounts of unidentified nongaseous byproducts were neglected.
Steam reforming of glycerin: H2O
C3H8O3 98 3CO + 4H2 (1) CO + H2O a CO2 + H2 (2)
Water-gas shift reaction: Overall reaction:
C3H8O3 + 3H2O f 3CO2 + 7H2 (3)
Methanation reaction:
CO + 3H2 f CH4 + H2O (4)
Without a catalyst, the glycerin conversion to gaseous products was only 1.6% (run 1), which is far less than that observed with catalysts. (11) Czernik, S.; French, R.; Feik, C.; Chornet, E. Ind. Eng. Chem. Res. 2002, 41, 4209.
10.1021/ef050121q CCC: $30.25 © 2005 American Chemical Society Published on Web 06/16/2005
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Figure 1. Life test of Ru(3 wt %)/Y2O3 for 24 h in the steam reforming of glycerin, at a temperature of 600 °C and a sweep gas gas-space velocity of 80 000 mL g-cat-1 h-1.
The order of the activity was as follows: Ru ≈ Rh > Ni > Ir > Co > Pt > Pd > Fe. In the steam reforming of methane, Kikuchi et al.13 have reported that the order of the activity of Group 8-10 metal catalysts on silica was as follows: Ru ≈ Rh > Ni > Ir > Pt ≈ Pd . Co ≈ Fe. Therefore, it can be said that active metals in the steam reforming of methane also afford high activity in the steam reforming of glycerin. Differences in the selectivities of the gaseous products were small among catalysts, and all the catalysts showed high CO2 selectivity. These results can be explained by the high steam/carbon molar ratio under the reaction conditions used here and the use of basic support materials, causing a promotion of the water-gas shift reaction (reaction 2). However, because of the abundance of hydrogen through reaction 1, the CO2/ CO ratio of 3.3 for run 10 was below the equilibrium value of ca. 8.6 for reaction 2 at 600 °C. Because ruthenium afforded the highest H2 yield, the effects of supports for ruthenium on the glycerin steam reforming were examined at 600 °C (runs 10-16). Y2O3and ZrO2-supported catalysts exhibited high glycerin conversion and high H2 yield. However, ruthenium on basic MgO (run 15) showed very low conversion of glycerin, compared to Y2O3- and ZrO2-supported catalysts. This is consistent with the previous result in the CO2 reforming of CH4 with ruthenium on MgO, where ruthe-
Communications
nium oxide is hard to be reduced to metallic ruthenium, and number of active centers would decrease.14 Although Al2O3 is used as a favorable support for the steam reforming of hydrocarbons, ruthenium on Al2O3 (run 16) showed the lowest conversion in the steam reforming of glycerin. Higher selectivity to CH4 and low selectivity to CO2 are reasons for the low H2 yield. In regard to the influence of the catalyst surface area, no simple trend was deduced. Among the catalysts studied thus far, ruthenium on Y2O3 (Ru/Y2O3) gave the best results. Here, optimization of the loading level of ruthenium was performed at 500 °C, to compare the catalytic activity at a lower conversion level (runs 17-20). The H2 yield increased as the ruthenium loading increased (up to 3 wt %), and a further increase in ruthenium to 5 wt % did not affect the results. Therefore, loading of 3 wt % is optimal for the steam reforming of glycerin. Figure 1 shows the results of the life test of Ru(3 wt %)/Y2O3. In this run, the carrier gas flow rate was increased from 30 mL/min to 100 mL/min, to sweep out the gaseous products as fast as possible. As shown in Figure 1, no decrease in the conversion and H2 yield was observed for 24 h. With increases in the gas-space velocity of the sweep gas from 38 000 mL g-cat-1 h-1 to 80 000 mL g-cat-1 h-1, only a slight decrease in the glycerin conversion was observed. To examine carbon deposition onto the catalyst during the reaction, thermogravimetric analysis (TGA) of the used catalyst was conducted in flowing air. TGA of the catalysts after reaction for 6 and 24 h revealed weight losses of 0.66 and 0.42 wt %, respectively. Because the weight loss was not dependent on the reaction time, little carbon was deposited on the catalyst during the prolonged run. Therefore, Ru(3 wt %)/ Y2O3 is considered to be durable for the deactivation caused by carbon deposition. These results indicate that the performance of the Ru/ Y2O3 catalyst in the steam reforming of glycerin is very high. Acknowledgment. This work was supported in part by a Grant from the Sumitomo Foundation. Supporting Information Available: Table of results showing the effect of metal species and support materials on the reforming of glycerin (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
EF050121Q (11) Ando, T.; Ikenaga, N.; Nakagawa, K.; Suzuki, T. J. Jpn. Pet. Inst. 2002, 45, 409. (12) Matsui, N.; Anzai, K.; Akamatsu, N.; Nakagawa, K.; Ikenaga, N.; Suzuki, T. Appl. Catal. A 1999, 179, 247. (13) Kikuchi, E.; Tanaka, S.; Yamazaki, Y.; Morita, Y. Bull. Jpn. Pet. Inst. 1974, 16, 95.
(14) Nakagawa, K.; Hideshima, S.; Akamatsu, N.; Matsui, N.; Ikenaga, N.; Suzuki, T. CO2 Reforming of Methane over Ru-Loaded Lanthanoid Oxide Catalysts. In CO2 Conversion and Utilization; Song, C., Gaffney, A. F., Fujimoto, K., Eds.; ACS Symposium Series 809; American Chemical Society: Washington, DC, 2002; p 205.
