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Energy & Fuels 1997, 11, 190-193
Active Site of Iron-Based Catalyst in Coal Liquefaction Takeshi Kotanigawa,* Mitsuyoshi Yamamoto, Masahide Sasaki, Nan Wang, Hiroshi Nagaishi, and Tadashi Yoshida Hokkaido National Industrial Research Institute, AIST, MITI, Toyohira-ku, Sapporo 062, Japan Received March 29, 1996X
Catalytic activity for coal liquefaction of sulfate-promoted iron oxide was investigated by a high-pressure differential thermal analysis technique and an autoclave test. It was found by X-ray photoelectron spectrometry that the main component of the catalyst was iron oxide, but this included 1.1 wt % sulfur. A particular finding from these experiments was that the catalyst showed sufficiently high activity for coal liquefaction without added sulfur and the activity was comparable to the activities of the catalyst with sulfur and FeS2 catalyst in a range of temperatures between 375 and 450 °C. This was very interesting because the sulfate-promoted iron oxide catalyst showed significantly high activity without addition of a promoter. To find possible reasons, temperature-programmed desorption profiles of hydrogen (TPRD) of these catalysts were measured. The sulfate-promoted iron oxide catalyst showed clear TPRD profiles in a wide range of temperatures between 100 and 350 °C. In contrast, FeS did not show any TPRD profile. However, after oxidation of FeS with air at elevated temperatures, it did show TPRD profiles, although these were weak. This suggests that surface-sulfate species can activate hydrogen molecules. It is well-known that sulfur or sulfides are easily oxidized with water or air. It is also well-known that much water is included in the coal liquefaction system. Therefore, the catalysts used in coal liquefaction are always exposed to oxidative agents in the reaction system. Therefore, it was concluded that an active site in working states for coal liquefaction was sulfate species formed during the process on the surface of the iron-based catalysts in the coal liquefaction.
Introduction Since Bergius used slag from a blast furnace as the catalyst for coal liquefaction in 1920,1 iron sulfides with different atomic ratios of S/Fe have been studied as catalysts for coal liquefaction.2 Montano3 tried to determine the working state of iron sulfide catalysts by using Mo¨ssbauer spectroscopy. On the basis of these studies, pyrrhotite has been considered probably to be a working state of the iron sulfide catalyst in coal liquefaction. Since then, Amestica and Wolf4 have proposed that pyrite is a very active catalyst for coal liquefaction, but papers concerning the surface species of iron sulfides have not been found. Tanabe and Hattori5 reported that a super acid of iron oxide prepared by impregnating diluted sulfuric acid on hematite showed fairly high activity for liquefaction of bituminous coal. This is informative because the chemical state of sulfur in the super acid of iron oxide is S6+. It is completely different from the chemical form of sulfur of pyrite, which is S2-. Also, we have found and demonstrated that Fe2O3(SO4)2- prepared by a reaction of ferric sulfate with urea in aqueous solution heated at 96-98°C for over 1 h showed fairly high activity for coal liquefaction. In an additional study, we have analyzed the surface compositions of FeS2 of analytical Abstract published in Advance ACS Abstracts, November 1, 1996. (1) Bergius. Br. Pat. 18232, 1920. (2) Ishii, T.; Sanada, Y.; Takeya, G. Kogyo Kagaku Zasshi 1969, 72, 1269. (3) Montano, P. P. Fuel 1977, 56, 397. (4) Amestica, L. A.; Wolf, E. E. Fuel 1986, 65, 1226. (5) Tanabe, K.; Hattori, H. J. Pet. Soc., Jpn. 1986, 29, 280.
reagent grade by X-ray photoelectroscopy and found that a major portion of its surface changed to sulfate due to oxidation by oxygen and moisture in the atmosphere.6 Subsequently, we analyzed the distribution of sulfur after the reaction of iron sulfide with water under hydrogen atmosphere of initial pressure of 10.1 MPa by using DTA analysis, XRD, XPS, and chemical analysis. As a result, it was found that the major portion of the surface of iron sulfides was already oxidized before it was used and it is very difficult to reduce the sulfate formed by the presence of water to sulfide even under hydrogen of high pressure.6 Through these studies, it was clearly seen that there is much water in the reaction system of coal liquefaction because types of raw coals include much water and some water is produced by hydrodeoxygenation (HDO) of oxygen-containing compounds in coals during the process. Therefore, we believe that the presence of sulfide species should be considered doubtful in the working state of coal liquefaction. This is one reason we tried to prepare the sulfate-promoted iron oxide catalyst by the ferric sulfateurea method for coal liquefaction.7 Recently we have focused on the study of the activation of molecular hydrogen over the sulfate-promoted iron oxide catalyst.8 Therefore, in this work we propose an active site in the working state of iron-based catalysts for coal liquefaction.
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(6) Kotanigawa, T.; Takahashi, H.; Yokoyama, S.; Yamamoto, M.; Maekawa,Y. Fuel 1988, 67, 927. (7) Kotanigawa, T.; Yokoyama, S.; Yamamoto, M.; Maekawa,Y. Fuel 1989, 68, 618. (8) Kotanigawa, T.; Yamamoto, M.; Wang, N.; Sabu, K. R.; Owada, M.; Sugioka, M. J. Phys. Chem. 1996, 100, 5452.
© 1997 American Chemical Society
Iron-Based Catalyst in Coal Liquefaction
Energy & Fuels, Vol. 11, No. 1, 1997 191
Experimental Section Catalysts used were FeS2 and FeS of analytical reagent grade and the sulfate-promoted iron oxide catalyst (Fe-sulfate catalyst) prepared by neutralization with urea illustrated elsewhere.6 Fe-sulfate catalyst was calcined at 500 °C for 3 h in an electric oven, followed by drying at 100 °C overnight after thorough washing with deionized water. Fe-sulfate catalyst was already in a fine powder (ca. 2 µm sphere) without crushing. It was checked by SEM observation. Taiheiyo coal, which is a Japanese subbituminous coal, was used. The coal was pulverized, sieved to yield 100 mesh size particles (
