5452
J. Phys. Chem. 1996, 100, 5452-5456
Activation of Hydrogen over Sulfate-Promoted Iron Oxide Takeshi Kotanigawa,* Mitsuyoshi Yamamoto, Nan Wang, and Kuzhunellili R. Sabu† Hokkaido National Industrial Research Institute, AIST, MITI, Toyohira-ku, Sapporo, Japan 062
Masahiko Owada and Masatoshi Sugioka Muroran Institute of Technology, Mizumoto-chou, Muroran, Japan 050 ReceiVed: September 25, 1995; In Final Form: January 2, 1996X
The mechanism for activation of hydrogen over sulfate-promoted iron oxide has been investigated by means of hydrogen TPD and FTIR spectrometry of adsorbed pyridine. The catalyst activated isomerization of 1-butene to 2-butene and hydrogenation of 1,3-butadiene to form 1-butene, trans- and cis-2-butene, and small amounts of n-butane, but the isomerization activity decreased when the catalyst was prereduced and disappeared when the catalyst was reduced at 723 K although the hydrogenation activity still remained. In the section of the isomerization of 1-butene when the reaction was carried out in the absence of hydrogen, it is clear that the sulfate itself in the catalyst catalyzed the isomerization of 1-butene in the absence of hydrogen. However, in the presence of hydrogen, the bifunctional mechanism was observed. It should be, therefore, considered that the isomerization was promoted by the protonic acids formed on the Fe-sulfate catalyst by the dissociation of hydrogen molecules. In TPD experiments, the hydrogen TPD spectra appeared in a wide range of temperatures between 373 and 623 K or above. In FTIR spectrometry of the adsorbed pyridine on the catalyst, an increase in intensity of protonic acid sites as well as a decrease of Lewis acid sites was observed by heating the pyridine-covered catalyst in the presence of hydrogen at 423 K or over. This indicates the conversion of hydrogen molecules into protonic acids. Also, we tried to find hydride-like species on the surface of the catalyst but could not find them. In conclusion, the presence of sulfate species in the iron oxide catalyst is essential for the activation of hydrogen, promoting the bifunctional activity for the hydrogenation and the isomerization of olefins. The overall expression for the mechanism of the activation of hydrogen over the Fe-sulfate catalyst is explained by the following equation (Fe2+ + H2 ) Fe0 + 2H+) followed by another equation ([Fe2+-O2- ] + H+ + H- ) Fe+-H + H+ -O-).
Introduction Activation of molecular hydrogen over catalysts other than metals is an interesting subject not only for basic studies but also in chemical industries for processes on catalytic hydrogenation. A number of papers concerning infrared spectroscopic studies on dissociation of hydrogen on metal oxides, such as formation of hydride species on ZnO,1 R-Ga2O3,2 and ZrO2,3 have been published. Also, a theoretical study on dissociation of hydrogen on ZrO24 has been reported. On the other hand, protonic acid formation by dissociation of molecular hydrogen over sulfate-promoted metal oxides such as ZrO2,5 TiO2,6 and Fe2O37 and promotion of acid-catalyzed reaction over these catalysts have been studied in many papers. Our previous work showed the sulfate-promoted Fe2O3 catalyst prepared by neutralization of ferric sulfate with urea-catalyzed hydroliquefaction of coal8,9 at 723 K under hydrogen atmosphere at 10.1 MPa of initial pressure. Hydrogenation activity over sulfate-promoted iron oxide catalysts has not been reported in the literature. Therefore, this is an interesting result with respect to the activation of hydrogen over the sulfate-promoted metal oxides. In this paper, the mechanism of the dissociation of hydrogen molecules over the sulfate-promoted Fe2O3 and the hydrogenation activity of the catalyst will be discussed. Experimental Section The sulfate-promoted Fe2O3 catalyst (Fe-sulfate) and a Fe2O3 catalyst were prepared by the neutralization of ferric sulfate with † Present address: Department of Clay and Clay Minerals, Regional Research Lab. Trivandrum, Trivandrum-695019, Kerala, India. X Abstract published in AdVance ACS Abstracts, March 1, 1996.
0022-3654/96/20100-5452$12.00/0
urea by heating at 369-371 K for 2 h in a hot bath9 and aqueous ammonium solution at room temperature, respectively. The catalysts were calcined at 773 K for 3 h in an electric oven followed by drying at 373 K overnight after significant washing with deionized water. The Fe-sulfate catalyst was already in a fine powder (ca. 2 µm sphere)10 checked by SEM observation without crushing, but the iron oxide catalyst was not. So the iron oxide catalyst was crushed and sieved to yield particles smaller than 100 mesh-size (99.9% purity were supplied by Sumitomo Chemical Co. The reactants were purified by distillation several times before © 1996 American Chemical Society
Activation of Hydrogen over Sulfate-Promoted Fe2O3
Figure 1. XPS spectrum of Fe-sulfate catalyst.
using them for the reactions. The isomerization was carried out under 40 Torr of 1-butene at 373 K in the presence of 0.10 g of the catalysts, and the hydrogenation of 1,3-butadiene was done under 50 Torr of 1,3-butadiene and 100 Torr of hydrogen at 573 K in the presence of 0.10 g of the catalysts. Also, the influence of prereduction of the catalyst for both the reactions was investigated. The catalysts were evacuated at 773 K for 2 h before any reaction. Reaction products were analyzed by a gas chromatograph connected to the reactor system. Refractive FTIR spectrometry was performed in a Spectra tech Co. Ltd. vacuum cell attached to a Shimazu FTIR spectrometer 8100M, and all spectra were integrated over 1000 scans (1.2 s for 1 scan). The vacuum cell was connected to a vacuum system at a vacuum of 10-4 Torr for in gas and out gas. Catalysts were softly packed in a sample pan (7 mm in diameter, 0.15 cc) with a sieve in the bottom. Pyridine used as a probe molecule to determine the type of acid sites was purified by repeated freeze-pump-thraw degassing cycles after distillation. To examine the conversion of Lewis acid sites to protonic acid sites11 by FTIR spectrometry, the Fe-sulfate catalyst was first evacuated at 773 K for 3 h and prereduced with 600 Torr of hydrogen at 623 K for 30 min. After the pretreatment, the catalyst was exposed to 2 Torr of pyridine at 423 K for 30 min and evacuated at the same temperature for 30 min. Finally the catalyst covered with pyridine was heated at 423, 493, 523, and 623 K for 30 min in the presence of 600 Torr of hydrogen gas. FTIR spectra were measured at room temperature in each step. For the determination of the fractions of Lewis acid sites and protonic acid sites on the surface, the absorbances of the bands at 1448 cm-1 (due to pyridine chemisorbed on Lewis acid sites) and 1540 cm-1 (due to pyridine chemisorbed on protonic acid sites)12 were determined. Results and Discussion Figure 1 shows the XPS spectrum of the Fe-sulfate catalyst. The main component of the Fe-sulfate catalyst was concluded to be R-Fe2O3 from binding energies of Fe 2p and O 1s that were determined to be 710.9 and 529.6 eV, respectively. A small amount of sulfur was detected in the catalyst. The amount of sulfur in the catalyst was quantitatively analyzed by elemental analysis to be 1.1 wt %. The binding energy of S 2p of the catalyst was determined to be 168.04 eV, which is a little lower
J. Phys. Chem., Vol. 100, No. 13, 1996 5453
Figure 2. TPRD spectra over Fe2O3 catalyst as a function of hydrogen treatment temperature.
Figure 3. TPRD spectra over Fe-sulfate catalyst as a function of hydrogen treatment temperature.
than the binding energy of ferric sulfate (169.1 eV) but is very close to the binding energy of ferrous sulfate (168.8 eV). Our previous paper6 showed that the Fe-sulfate catalyst has the SdO asymmetric stretching vibration at 1380 cm-1 in the FTIR spectrum and that the band appears at approximately 180 cm-1 higher wave number than that of ferrous and ferric sulfates. Therefore, it can be expected that this type of immobilization of the sulfate in a catalyst creates a quite different kind of active species. In order to demonstrate an essential role of the sulfate species in the Fe-sulfate catalyst, TPRD spectra over catalysts were measured. Figures 2 and 3 show the TPRD spectra over two catalysts. The iron oxide catalyst did not show any peaks on the TPRD measurements, as shown in Figure 2, but in contrast the Fe-sulfate catalyst showed completely different results, as seen in Figure 3. A main component of both catalysts is iron oxide, but the TPRD spectra appeared in a wide range of temperatures between 373 and 623 K or above over the Fesulfate catalyst, which is attributed to the activation of hydrogen molecules. It is very interesting that the only difference of the Fe-sulfate catalyst from the iron oxide catalyst is the content of small amounts of sulfate species. It can, therefore, be
5454 J. Phys. Chem., Vol. 100, No. 13, 1996
Kotanigawa et al.
Figure 4. Changes of composition of products in hydrogenation of 1,3-butadiene.
Figure 6. The changes of FTIR spectra of pyridine adsorbed on Fesulfate catalyst by heating in the presence of hydrogen.
Figure 5. Effect of prereduction of Fe-sulfate catalyst for hydrogenation of 1,3-butadiene.
considered that the presence of sulfate in the catalyst can create active sites for the activation of hydrogen molecules. In order to confirm the activation of hydrogen over the catalyst, hydrogenation of 1,3-butadiene was carried out. The iron oxide catalyst showed very low conversion which is below 1.0%, but the Fe-sulfate catalyst showed significantly high conversion. Figures 4 and 5 demonstrate the results of hydrogenation of 1,3-butadiene over the Fe-sulfate catalyst. Figure 4 shows the changes in the composition of reaction products with reaction time at 573 K, and Figure 5 shows the influence of the prereduction of the catalyst on the hydrogenation of 1,3-butadiene. The unreduced catalyst showed a quite high hydrogenation activity of 1,3-butadiene at 573 K to form 1-butene, trans- and cis-2-butene, and trace amounts of n-butane. As the reaction was carried out at 573 K in the hydrogen atmosphere, the working state of the unreduced catalyst can be regarded as being reductive. Therefore, the influence of the prereduction of catalyst on catalytic activity was investigated, as shown in Figure 5. The catalyst gave the highest activity when it was prereduced at 573 K, and the activity gradually dropped with a rise of the prereduction temperature. It has already been explained above that the Fe-sulfate catalyst includes small amounts of sulfate. So, the isomerization activity
of the catalyst was investigated. In these experiments, selectivity for 2-butene varied from 58% to 46% and to 38% after prereducing the catalyst at 573, 623, and 723 K, respectively. This suggests that activity of isomerization of the catalysts varies depending on the prereduction. For examination purposes, the isomerization of 1-butene to 2-butene was tested at 373 K. The iron oxide catalyst showed no activity, but the Fe-sulfate catalyst showed significant activity. For example, the unreduced catalyst gave 59.3% conversion of 1-butene to 2-butene at 373 K, but the conversion decreased to 52.8% after reducing it with hydrogen at 573 K for 1 h. No big change was observed even after reducing it at 573 K. But if the catalyst was reduced at 625 K or above, the activity for isomerization was no longer present. The fact that water and small amounts of hydrogen sulfide were detected during the reduction at 623 K or above means that the decomposition of the sulfate species on the catalyst occurred. In other words, this clearly suggests that the sulfate species is still present on the catalyst even after reducing it at 573 K and that it is able to promote the isomerization of 1-butene. If the hydrogenation of 1,3-butadiene was carried out at 573 K, as mentioned above, the catalyst gave the highest conversion. At this temperature, the catalyst included as much of the sulfate species as the unreduced catalyst, but the conversion gradually decreased with an increase of the prereduction temperatures, corresponding to a decrease of the amounts of sulfate species. This is extremely interesting as compared with the results for the iron oxide catalyst. Through these results, it should be recognized that the presence of the sulfate species in the iron oxide is essential for promotion of the hydrogenation in addition to the progress of the isomerization. Figure 6 shows the variations of FTIR spectra of the adsorbed pyridine on the Fe-sulfate catalyst in the presence of hydrogen
Activation of Hydrogen over Sulfate-Promoted Fe2O3
J. Phys. Chem., Vol. 100, No. 13, 1996 5455
Figure 8. Mechanism for hydride-like species formation on the surface of Fe-sulfate catalyst.
Figure 7. Dissociation of molecular hydrogen on Fe-sulfate catalyst.
TABLE 1: Changes of Intensity Ratios of Protonic Acid Sites to Lewis Acid Sites by Heating in the Presence of Hydrogen
evac at 298 K heated at 423 K (H2) heated at 523 K (H2) heated at 623 K (H2)
protonic acid sites
lewis acid sites
intensity ratios
0.40 0.47 0.55 0.47
0.96 0.91 0.55 0.22
0.42 0.52 1.00 2.14
and under heating. With a rise of heating temperature in the presence of hydrogen, the intensity of the band at 1448 cm-1 (Lewis acid sites) decreased and, in contrast, the band at 1540 cm-1 (protonic acid sites) increased. It seems that the pyridine adsorbed on the Lewis acid sites was converted to pyridinium ions by heating in the presence of hydrogen. However, this is not so. The protonic acid sites were generated from hydrogen molecules with a decrease of Lewis acid sites. In other words, hydrogen molecules adsorbed on the Lewis acid sites and then the hydrogen molecules on the Lewis acid sites were converted to protonic acid sites, as illustrated in Figure 7. These results are similar to those obtained on Pt/SO42--ZrO2 by Ebitani et al.11 They speculate that a H atom formed by dissociation of a molecular hydrogen on the Pt of the Pt/SO42--ZrO2 releases an electron on Lewis acid sites to form H+ and that the H+ acts as an active site for acid-catalyzed reaction. Also, they state that the hydrogenation activity of Pt for olefins is greatly suppressed when it is supported on SO42--ZrO2. However, our results indicate rather contrarily the promotion of hydrogenation of the olefin in the presence of protonic acid sites. Table 1 shows the variations of the intensity ratios of the protonic acid sites compared to those of the Lewis acid sites. It can be seen through the intensity ratios in Table 1 that the dissociation of hydrogen started at a temperature near 423 K. This phenomenon is concerned with the results of the TPRD spectra in Figure 3, because the activation of hydrogen molecules over the Fe-sulfate catalyst occurs at fairly low temperature. As already described above, the present Fe-sulfate catalyst showed a bifunctional activity for hydrogenation and isomerization. In general, for the hydrogenation of olefins, hydrogen molecules must be activated by heterolytic dissociation. Therefore, the formation of hydridelike species over the catalyst must be expected for hydrogen molecules. Recently dissociation of hydrogen molecules over metal oxides has been studied by means of infrared spectrometry. In early studies, formation of ZnH species on ZnO by heterolytic dissociation of hydrogen molecules was reported.1 Also, in the study of heterolytic dissociation of hydrogen molecules on R-Ga2O3,2 a Ga-H species and a newly formed OH group were found at 573 K at 2020 and 3640 cm-1, respectively, in the infrared spectrum. Also, a theoretical study on dissociation of hydrogen on ZrO24 has been reported and a dissociative Zr-H species and an OH group have been calculated to appear at 1810
and 3310 cm-1, respectively. With respect to heterolytic dissociation of hydrogen molecules, Kondo et al.3 have stated in the infrared study of hydrogen adsorbed on ZrO2 that hydride species are not considered to be stable above 373 K at which temperature more stable OH species are produced rapidly. In this work, FTIR spectrometry was performed to try to find heterolytic hydride-like species on the surface of the Fe-sulfate catalyst in a range of temperatures between 423 K and 673 K, but unfortunately we were not able to observe the hydride-like species although a newly formed OH group was detected at nearly 3700 cm-1 at 523 K. In the case of Pt/SO42--ZrO2, Pt dissociates hydrogen molecules, but in the Fe-sulfate catalyst, no such metal is present. Therefore, a different mechanism for the activation of hydrogen molecules over the catalyst can be expected from that for over Pt/SO42--ZrO2. Baba et al.13 reported that the protonic acid sites are generated by the reduction of Ag+ with hydrogen (Ag+ + 1/2H2 ) Ag0 + H+). This was done to study the promotion effects of molecular hydrogen on the acid-catalyzed reaction. In Figure 1, the binding energy of S 2p measured was determined to be rather closer to that of ferrous sulfate than to that of ferric sulfate. So, the surface composition of the Fesulfate catalyst is considered to be Fe2+-SO42-/R-Fe2O3. As described above, the catalyst gave a bifunctional activity for hydrogenation and isomerization after low prereduction temperatures but only for hydrogenation after high prereduction temperature. Therefore, the mechanism of the activation of hydrogen for the bifunctional activity of the present catalyst must remain speculative in the two cases. Conclusions In summary, hydrogen molecules reduce the ferrous cations to form Fe0 species on the surface. The hydrogen molecules in reducing the ferrous cations convert to protonic acids and are localized on oxygen ions near the Lewis acid sites of R-Fe2O3, as shown in Figure 8. The overall expression for the mechanism of the activation of hydrogen over the Fe-sulfate catalyst is explained by eq 1.
Fe2+ + H2 ) Fe0 + 2H+
(1)
However, before proceeding with the process of eq 1, a heterolytic dissociation of hydrogen molecules into H+ and Hmust be carried out on the ferrous species. Eq 2 could be recognized as an intermediate step of the eq 1.
[Fe2+-O2-] + H+ + H- ) Fe+-H + H+-O-
(2)
The Fe0 species has the ability to promote hydrogenation, and the H+ species act as the protonic acids for the isomerization. So, the catalyst can promote the bifunctional reaction. This is the mechanism when the catalyst was not significantly reduced, but if the catalyst has been significantly reduced, the ferrous species on the surface are lost and the surface species become only the Fe0 species. So, the reaction must be carried out on the surface without any protonic acid sites. In the section of the isomerization of 1-butene when the reaction was carried
5456 J. Phys. Chem., Vol. 100, No. 13, 1996 out in the absence of hydrogen, it is clear that the sulfate itself in the catalyst catalyzed the isomerization of 1-butene. However, in the presence of hydrogen, the bifunctional mechanism was observed. It should be, therefore, considered that the isomerization was promoted by the protonic acids formed on the Fe-sulfate catalyst by the dissociation of hydrogen molecules. Finally we would like to conclude by saying that the presence of sulfate species in the iron oxide catalyst is essential for the activation of hydrogen promoting the bifunctional activity for hydrogenation and isomerization of olefins. References and Notes (1) Eischens, R. P.; Pliskins, W. A.; Low, J. D. J. Catal. 1962, 1, 180. Kokes, R. J. In Catalysis; Hightower, J. W., Ed.; Elsevier: Amsterdam, 1973; Vol. 1, p 1. Saussey, J.; Lavelley, J. C.; Rais, T.; Chakor-Alami, A.; Hindermann, J. P.; Kiennemann, A. J. Mol. Catal. 1984, 26, 159.
Kotanigawa et al. (2) Meriaudeau, P.; Primet, M. J. Mol. Catal. 1990, 61, 227. (3) Kondo, J.; Sakata, Y.; Domen, K.; Maruya, K.; Onishi, T. J. Chem. Soc. Faraday, Trans. 1990, 86(2), 397. (4) Nakatsuji, H.; Hada, M.; Ogawa, H.; Domen, K. J. Phys. Chem. 1994, 98, 11845. (5) Hino, M.; Arata, K. J. Am. Chem. Soc. 1979, 101, 6439. (6) Hino, M.; Arata, K. J. Chem. Soc., Chem. Commun. 1981, 1148. (7) Lee, J. S.; Yeom, M. H.; Park, D. S. J. Catal. 1990, 126, 361. (8) Kotanigawa, T.; Takahashi, H.; Yokoyama, S.; Yamamoto, M.; Maekawa, Y. Fuel 1988, 67, 927. (9) Kotanigawa, T.; Yokoyama, S.; Yamamoto, M.; Maekawa, Y. Fuel 1989, 68, 618. (10) Kotanigawa, T. J. Fuel Soc. Jpn. 1991, 7 (5), 431. (11) Ebitani, K.; Konishi, J.; Hattori, H. J. Catal. 1991, 130, 257. (12) Little, L. H. Infrared Spectra of Adsorbed Species; Academic Press: London, 1966. (13) Baba, T.; Seo, S. G.; Ono, Y. In InnoVation in Zeolite Materials Science; Grobet, P. J., et al., Eds.; Elsevier: Amsterdam, 1988; p 443.
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