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Energy & Fuels 1996, 10, 250-253
Characterization of Impregnated Iron Catalysts on Coal Jianmin Zhao, Zhen Feng, Frank E. Huggins, and Gerald P. Huffman* Department of Chemical Engineering and Materials Science, and the Consortium for Fossil Fuel Liquefaction Science, 341 Bowman Hall, University of Kentucky, Lexington, Kentucky 40506 Received July 11, 1995X
The phase, surface adsorption, and dispersion of iron catalysts impregnated on coal were characterized using Mo¨ssbauer spectroscopy. The magnetic relaxation behavior of the catalyst is similar to that obtained from binary ferrihydrite catalysts; the catalyst particles are aggregated rather than dispersed or “impregnated” on coal and the surface of the catalyst is chemisorbed with impurity anions. These impurity anions inhibit particle agglomeration, thus allowing the catalyst to maintain its surface area under reaction conditions.
I. Introduction The effectiveness of impregnation of iron catalysts on coal for direct coal liquefaction (DCL) has been known for many years.1-4 Traditionally, the activity of the catalyst has been attributed to improved catalyst dispersion due to impregnation, as observed for impregnated catalysts on supports, such as SiO2 and Al2O3. To impregnate a catalyst on coal, an iron salt solution was first mixed with fine coal powder and subsequently precipitated by adding ammonium hydroxide.3,4 Without coal, the procedure results in an iron phase known as two-line ferrihydrite (FHYD) because its XRD pattern exhibits two broad diffraction lines (for the sake of simplicity, two-line ferrihydrite is referred to here as ferrihydrite or FHYD).5 Ferrihydrite is a naturally occurring iron oxide that exists widely in ironcontaining soil and water. The material has a large surface area of >100 m2/g with average particle size of 3-5 nm. Recently, we have extensively studied the structure, surface adsorption, and DCL activity of ferrihydrite catalysts. Based on X-ray absorption fine structure (XAFS) analysis, a new surface structure model was proposed.6,7 The model explains the mechanism of particle agglomeration and phase transformation of ferrihydrite to hematite.8 It also suggests that impurity anions can be readily adsorbed at the catalyst surface, and thus block the crystal growth sites and effectively inhibit particle agglomeration at high temperatures.7,9,10 Abstract published in Advance ACS Abstracts, December 1, 1995. (1) Weller, S. W. Energy Fuels 1994, 8, 415-420, and references therein. (2) Garg, D.; Givens, E. N. Fuel Process Technol. 1984, 8, 123-134. (3) Utz, B. R.; Cugini, A. V. U.S. Patent 5,096,570, 1992. (4) Cugini, A. V.; Krastman, D.; Martello, D. V.; Frommell, E. F.; Wells, A. W.; Holder, G. D. Energy Fuels 1994, 8, 83-87. (5) Schwertmann, U.; Cornell, R. M. Iron Oxides in the Laboratory; VCH: Weinheim, Germany, 1991; pp 89-94. (6) Zhao, J.; Huggins, F. E.; Feng, Z.; Lu, F.; Shah, N.; Huffman, G. P. J. Catal. 1993, 143, 499-509. (7) Zhao, J.; Huggins, F. E.; Feng, Z.; Huffman, G. P. Clays Clay Miner. 1994, 42, 737-746. (8) Feng, Z.; Zhao, J.; Huggins, F. E.; Huffman, G. P. J. Catal. 1993, 143, 510-519. (9) Zhao, J.; Feng, Z.; Huggins, F. E.; Huffman, G. P. Energy Fuels 1994, 8, 38-41. X
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Under simulated DCL conditions, ferrihydrite catalysts with chemisorbed impurity anions (binary ferrihydrites) transform to a well-dispersed Fe1-xS (pyrrhotite) phase with increased concentration of Fe vacancies.11 Coal liquefaction tests using Si/ferrihydrite and citric acid treated ferrihydrite have shown improvement of coal liquefaction conversion over that obtained with pure ferrihydrite.9,12 In this paper, we report a comparative Mo¨ssbauer spectroscopy study of an iron catalyst impregnated on coal and a series of binary ferrihydrite catalysts. In addition to phase identification, Mo¨ssbauer spectroscopy is a powerful probe for investigation of magnetic relaxation phenomena in nanoscale particles, from which information concerning particle size distribution, surface chemisorption, and catalyst dispersion can be obtained.13-16 II. Experimental Section 1. Impregnated Iron Catalyst on Coal. Ferric nitrate (2.5 g) (Fe(NO3)3‚H2O) was dissolved in 40 g of water. The solution was then mixed with 50 g of ground bituminous coal (DECS-17). The slurry was subsequently poured into an ammonium hydroxide solution. After washing and pressure filtration, the admixture was vacuum-dried at 65 °C. The weight ratio of Fe to coal is 0.6%. 2. Binary Si/Ferrihydrite Catalysts (Six/FHYD, with x ) 5, 10, and 15%). Fe(NO3)3‚H2O (40.4 g) together with an appropriate amount of Na2SiO3 was added to 1 L of water. Ammonium hydroxide was slowly added to the solution to bring the pH to ∼10 with constant stirring. The precipitate was filtered and washed repeatedly with water. Finally, the (10) Zhao, J.; Feng, Z.; Huggins, F. E.; Shah, N.; Huffman, G. P.; Wender, I. J. Catal. 1994, 148, 194-197. (11) Zhao, J.; Feng, Z.; Huggins, F. E.; Rao, K. R. P. M.; Huffman, G. P. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1995, 40, 351355. (12) Zhao, J.; Feng, Z.; Huggins, F. E.; Huffman, G. P. Energy Fuels 1994, 8, 1152-1153. (13) Mørup, S.; Dumesic, J. A.; Topsøe, H. In Application of Mo¨ ssbauer Spectroscopy; Cohen, R. L., Ed.; Academic Press: New York, 1980; Vol. 1, pp 1-53. (14) Huffman, G. P.; Ganguly, B.; Zhao, J.; Rao, K. R. P. M.; Shah, N.; Feng, Z.; Huggins, F. E.; Taghiei, M. M.; Lu, F.; Wender, I.; Pradhan, V. R.; Tierney, J. W.; Seehra, M. S.; Ibrahim, M. M.; Shabtai, J.; Eyring, E. M. Energy Fuels 1993, 7, 285-296. (15) Ganguly, B.; Huggins, F. E.; Rao, K. R. P. M.; Huffman, G. P. J. Catal. 1993, 142, 552-560. (16) Mørup, S.; Madsen, M. B.; Franck, J.; Villadsen, J.; Koch, C. J. W. J. Magn. Magn. Mater. 1983, 40, 163-174.
© 1996 American Chemical Society
Characterization of Impregnated Iron Catalysts on Coal cake was oven dried at 65 °C and ground into a fine powder (200 mesh). In addition to Si/FHYD, a pure ferrihydrite sample (FHYD) was prepared without added Na2SiO3. X-ray diffraction data of the ferrihydrite samples exhibit a broad twoline pattern, typical of two-line ferrihydrite.5 No other iron oxides phases such as R-Fe2O3 and R-FeOOH were found. These phases would exhibit sharp XRD diffraction lines because of their crystallinity. 3. Ferrihydrite Impregnated on SiO2 (FHYD/SiO2). SiO2 gel (10 g) (Aldrich, BET surface area 500 m2/g, pore volume 0.75 m3/g) was mixed with 25 mL of 1 M ferric nitrate solution and stirred for 20 min. After the excess ferric nitrate solution was filtered out, the slurry was poured into a 2 N solution of ammonium hydroxide. After washing, the sample was dried at 90 °C. Mo¨ssbauer spectra were recorded with a constant acceleration spectrometer. The radioactive source consists of ∼50 mCi of 57Co in a Pd matrix. Temperature-dependent measurements from 12 to 300 K were performed using an Air Products Displex cryogenic system. All spectra were calibrated with respect to metallic iron (R-Fe) run simultaneously at the opposite end of the Mo¨ssbauer drive. Ferrihydrite is fairly stable at room temperature over the time period of the experiments. No special measures were taken for sample handling.
III. Results and Discussion Superparamagnetism and Superferromagnetism: Background. Magnetic nanoscale particles are single domain particles. At sufficiently high temperatures, the thermal energy kT may be comparable to the magnetic energy of the particles. This results in magnetic relaxation, i.e., spontaneous fluctuation of the magnetization direction. For an assembly of particles, the magnetic energy for particle i can be expressed as17
Ei ) KiVi sin2 θ -
∑i KijMiMj
(1)
The first term represents the energy of noninteracting particles, or superparamagnetism (SPM), where Ki is the magnetic anisotropy energy constant, Vi the volume of particle i, and θ the angle between the magnetization direction and an easy direction of magnetization. The second term is for the magnetic energy arising from particle interaction, or superferromagnetism (SFM), where Kij represents the magnetic coupling constant between particle i and its neighboring particle j, and Mi and Mj are the magnetizations of the particles. The summation is taken over all neighboring particles j. Superferromagnetism was first proposed by Mørup et al.16,17 to interpret the asymmetrically broadened magnetic splitting at temperatures below the Ne´el temperature (TN) for fine particle (d ) 10-20 nm) goethite (RFeOOH, TN ) 393 K). The magnetic coupling constant Kij is dependent on interparticle distance and the medium between the particles. The particles in pure ferrihydrite formed by precipitation are heavily aggregated.7,9 Therefore, a strong particle interaction is expected. Chemisorption of species such as SiO44- creates a nonmagnetic medium and compensates the surface unpaired spins, thus reducing the magnetic coupling between the surface atoms from neighboring particles. In the case of FHYD/ SiO2, in which the catalyst particles are well separated/ dispersed by impregnation, the magnetic interaction (17) Mørup, S. Hyperfine Interact. 1994, 90, 171-185.
Energy & Fuels, Vol. 10, No. 1, 1996 251
becomes negligible, and the magnetic energy is expected to be dominated by the SPM term in eq 1. Mo1 ssbauer Spectra. Mo¨ssbauer spectra for FHYD, Si0.05/FHYD, Si0.1/FHYD, and Si0.15/FHYD recorded at temperatures from 12 to 100 K are shown in Figure 1. At 12 K, all four samples exhibit broad magnetic hyperfine sextets. The line broadening indicates a distribution of magnetic hyperfine fields arising from the different atomic environments of the interior and the surface iron ions. For all the samples except Si0.15/ FHYD, as temperature increases, the magnetic hyperfine splitting decreases and the lines broaden, evolving into a complex V-shaped pattern, before finally collapsing to a paramagnetic doublet. The spectra for FHYD/ SiO2 are quite different (Figure 2) in that a well-resolved magnetic sextet is superimposed on a paramagnetic doublet at 12 K. The spectrum is characteristic of sizedependent SPM samples with the sextet representing the larger particles with Ei > Ei(Vave, 12 K) and the doublet representing the smaller particles with Ei < Ei(Vave, 12 K). Due to limitations of the sample refrigerator, we were unable to reach temperatures 100 m2/g, which is mainly contributed by open mesopores (d ) 2-50 nm). During impregnation, because of capillary action effect, these open pores quickly absorb the impregnation solution and subsequent precipitation leads to the formation of nanoscale catalyst particles impregnated/dispersed on the support. Descriptions of the pore structure in coal are still evolving. A dominant model in the past is that coal has a high surface area due to a network of slitlike pores interconnected by narrow capillary constrictions.18 This model appears to validate the practice of catalyst impregnation on coal. However, this model is not consistent with the recent findings by Larsen et al.,19 who employed small-angle X-ray scattering (SAXS) to determine the fractal dimensionality of the pore surface and used an extended series of gases of varying molecular sizes to determine the coal surface area. Larsen et al. conclude that the pores in coal are closed, isolated bubbles in a solid which are reachable only by diffusion through the solid. Accordingly, these pores are unreachable by an impregnation solution and subsequent precipitation would not lead to catalyst impregnation in the pores. On the other hand, while the coal powder is mixed with an iron salt solution, significant amounts of organic and inorganic impurities are inevitably dissolved from the coal and its mineral contents and subsequently adsorbed at the surface of ferrihydrite catalyst. Therefore, it is not surprising that the magnetic relaxation behavior for the impregnated iron catalyst on coal is similar to that for Si/FHYD. IV. Conclusion In a recent review article on catalyst dispersion in coal liquefaction, Weller emphasized that, although impregnation of iron catalysts on coal can significantly improve catalyst activity, it is not economical for industrial-scale coal liquefaction, because it requires pretreatment of coal.1 Weller suggested using Mo as the primary catalyst and recycling the Mo catalyst after reaction. From the Mo¨ssbauer spectroscopic evidence presented here, we have shown that the improved catalyst dispersion in impregnated catalysts on coal is probably not caused by “impregnation”. Instead, it is accomplished by chemisorption of impurities at the ferrihydrite surface. These impurities block the crystal (18) Bond, R. L. Nature 1956, 25, 49-58. (19) Larsen, J. W.; Hall, P.; Wernett, P. C. Energy Fuels 1995, 9, 324-330.
Figure 4. Mo¨ssbauer spectra for the impregnated iron catalyst on coal.
growth sites, thus allowing the catalyst to maintain its dispersion under DCL reaction condition. Similar results may be obtained by using binary ferrihydrite catalysts. The preparation of binary ferrihydrites is simpler and more energy efficient. In addition, the surface of binary ferrihydrites can be controlled by selection of impurity anions and varying the amount of surface adsorption. Acknowledgment. This research is supported by the U.S. Department of Energy under contract No. DEFC22-93PC3035, as part of the cooperative research program of the Consortium for Fossil Fuel Liquefaction Science. EF950136W
