Energy & Fuels 1994,8, 88-93
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Relative Activity of Nanoscale Iron Oxide, Iron Carbide, and Iron Sulfide Catalyst Precursors for the Liquefaction of a Subbituminous Coal G. T. Hager,* X. X. Bi, P. C. Eklund, E. N. Givens, and F. J. Derbyshire Center for Applied Energy Research, University of Kentucky, Lexington, Kentucky, 40511 -8433 Received July 12, 1993. Revised Manuscript Received October 26, 199P
Three nanoscale iron-basedcatalyst precursors were tested to determine their effect on the conversion of a subbituminous coal. The results of this study indicate that each of the precursors, superfine iron oxide,Fe 7C3, and Fel,S, exhibited very similar activity and selectivity with only minor variations. However, the hydrogenation activities of the three catalysts were very different. This was related to the particle size and surface area of the precursor, rather than ita phase. The insensitivity to phase was probably due to the rapid transformation of the precursor surface, where the catalytic reaction occurs, to a similar structure in each case. The increase in hydrogen consumption without corresponding increases in conversion indicates that bond cleavage, rather than availability of labile hydrogen, was the rate limiting step in the coal liquefaction reaction.
Introduction Historically,finely divided iron oxide catalysts were first used in the coal liquefaction developments in Germany during WWII to increase the conversion of bituminous and subbituminous coals.lJ Since that time, great efforts have been made to increase the efficiency of the liquefaction process while reducing the severity of the reaction conditions. To this end, a wide variety of techniques have been utilized including the use of multiple processing stages, pretreatment of the coal, and the addition of catalysts. Due to their inexpensive nature, the use of highly dispersed iron-basedcatalysts shows promise for enhancing the process economics, despite their having only moderate activity. Efforts to improve the activity of these catalysts are focused on increasing and maintaining high dispersion during the liquefaction process and on rapidly establishing the presence of an active phase. As already observed, the application of dispersed ironbased catalysts to direct coal liquefaction has been practiced for some time3 and considerable research has also been applied to determining the active phase of the catalyst. It is generally accepted that the sulfided form of the catalyst exhibits a higher activity than the oxide phasea4While the addition of pyrite is known to result in higher conversions and selectivity to oils, studies have shown that, under liquefaction conditions, pyrite decomposes to form pyrrhotite with the evolution of hydrogen sulfide.5.6 Some researchers have proposed that pyrrhotite is the active phase of the catalystM while others have suggested that the hydrogen sulfide evolved during the *Abstract published in Aduance ACS Abstracts, December 1, 1993. (1)Donath, E. E.;Hoering, M. Fuel Process. Technol. 1977,1,13-20. (2) Derbyshire, F. 3. IEACR/08; IEA Coal Research London, 1988;69
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(3)‘Reporton the Petroleum and Synthetic Oil Industry of Germany”, Ministry of Fuel and Power, His Majesty’s Stationary Office, London, 1947. (4)Anderson, R. R.; Bockrath, B. C. Fuel 1984,63,329-33. (5)Montano, P. A,; Brommannavar, A. S.; Shah, V. Fuel 1981, 60, 703-11. (6)Montano, P. A,; Vaishnava, P. P.; King, J. A.; Eisentrout, E. N. Fuel 1981,60,712-6.
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decomposition of pyrite is responsible for the catalytic a c t i ~ i t y Subsequent .~~~ studies have concluded that both hydrogen sulfide and pyrrhotite are necessary for high catalytic activity.gJ0 Still other research has suggested that the active phase of the catalyst is a reduced form of a-Fe which exists as a short-lived intermediate on the surface which is subsequently resulfided.11J2Another report identifies a y-Fe phase formed during the liquefaction process as being the active catalytic species, having a higher activity for the liquefaction of a brown coal than ~yrrh0tite.l~ The active form of the iron-based catalysts is still a matter of considerable debate. Several studies have shown that, for several different forms of the iron precursor, in the presence of sufficient sulfur the iron will transform to pyrrhotite under liquefaction conditions. Similarly, in the presence of sulfur, most molybdenum compounds also tend to sulfide to MoS2. However, recent work has shown that the carbides and nitrides of molybdenum and tungsten possess high catalytic activity while resisting the transformation to the sulfide phase.lP16 This indicates that the sulfide is not the only phase of molybdenum which is catalyticallyactive. The purpose of this research was to compare the catalytic activity of three nanoscale iron catalyst precursors and also to determine if iron carbide would exhibit high activity (7)Baldwin, R. M.; Vinciguerra, S. Fuel 1983, 62,498-501. (8)Lambert, J. M. Jr. Fuel 1982,61,777-8. (9)Sweeny, P. G.; Stenberg, V. I.; Hei, R. D.; Montano, P. A. Fuel 1987, 66,532-41. (10)Wang, L.; Cui, 2.; Lui, S. Fuel 1992, 71,755-9. (11)Kamiya, Y.;Nobusawa, T.; Futamura, S. Fuel Process. Technol. 1988. 18. 1-10. --,--, (12)Cook, P. S.;Cashion, J. D. Fuel 1987,66,669-77. (13)Weng, S.;Wang, 2.; Gao, J.; Cheng, L.; Wu, 2.; Lin, D.; Yu, Y.; Zhao, C. Hyperfine Interactions 1990,58;2635-40. (14)Oyama, S.T.;Schlatter, J. C.; Metcalfe, J. E.; Lambert, J. M. Znd. Eng. Chem. Res. 1988,27,1639-48. (15)Schlatter, J. C.; Oyama, S. T.; Metcalfe, J. E.: Lambert, J. M. Ind. Eng. Chem. Res. 1988,27,1648-53. (16)Sajkowski, D.J.;Oyama, S. T. P r e p . Pap-Am. Chem. Soc., Diu. Fuel Chem. 1990,s (2), 233-36.
0 1994 American Chemical Society
Energy & Fuels, Vol. 8, No. 1, 1994 89
Activity of Nanoscale Catalyst Precursors
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Figure 1. Schematic diagram of laser pyrolysis apparatus.
while remaining in the carbide phase. A laser pyrolysis technique was used to synthesize two of the iron-based particles.
Catalyst Preparation Three nanoscale iron-based catalysts were utilized in this study. Iron carbide and iron sulfide were synthesized using a laser pyrolysis technique first developed by Haggerty and co-workers17for the production of nanoscale ceramic particles. The uniqueness of this technique lies in its rapid heating and cooling rates (-100 000 "C/s), allowing the nonequilibrium production of nanoscale particles. The technique involves the intersection of a tunable C02laser with a gas stream carryinga combination of reactant gases. The energy of the laser is coupled to a strong rotational-vibrational mode in at least one of the reactant gases which in turn creates a small (- 1 mm3) pyrolysis zone at the intersection. The particles produced by this method have been reported to have a narrower size distribution and be less contaminated than those produced by conventional oven-based methods17. Iron carbide particles were generated in the laser pyrolysis apparatus shown in Figure 1.18 The ethylene gas (Air Products, CP grade) flowed, at ambient temperature, through a sintered Pyrex bubbler at the bottom of a container of iron pentacarbonyl (Aldrich) and the flow rate was regulated by a mass flow controller (AGA Gas, Inc.). The reactant gases flowed vertically through the reaction nozzle into the reaction chamber and intersected the laser beam (Laser Photonics Model 150) forming a pyrolysis zone. The pressure in the reaction chamber was maintained at 300-500 Torr by a vacuum pump located downstream of the reaction chamber. A coaxial flow of argon contained the reactant gases in a well-collimated stream forming a "wall-less" reactor. The energy of the laser was coupled to the rotational-vibrational absorption line of ethylene at 950 cm-l by tuning the laser frequency to the P20 line (945 cm-l). The particles formed in the pyrolysis zone were carried out of the reaction chamber and into a Pyrex particle trap where they were separated from the gas stream by a large ferrite magnet placed under the trap. Separation was possible due to the ferromagnetic properties of the iron carbide particles. A 0.2 pm pore Teflon membrane filter was placed at the trap outlet to minimize the number of stray particles entering the vacuum pump.
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(17) Haggerty, J. S. SinterablePowders form Laser-Driven Reactions. I n h e r Induced Chemicaf Processes;Steinfeld,J. I., Ed.;Plenum Press: New York, 1981. (18)Bi, X. X.; Ganguly, B.; Huffman, G.P.; Huggina, F. E.; Endo, M.; Eklund, P. C. J. Mater. Res. 1993,8 (7) 1666-74.
Table 1. Typical Reaction Parameters Used in Law? Pyrolysis a-Fe FesC Fe7C3 Fed 30 50 54 70 laser intensity, W 1 1 0.2 1 beam width, mm 1.7 0.8 0.8 1.8 nozzle diameter, mm 100 300 500 300 chamber pressure, Torr 9 9 25 25 ethylene flow rate, sccm 0 0 0 5 H1S flow rate, sccm Table 2. Catalyst Characterization Data diameter surface phase (XRD) (nm) area (m2/g) (XRD) see text 3 195 SFIO Fel,S 14 42 iron sulfide Fe&3 17 92 iron carbide
The as-formed particles are pyrophoric and were passivated prior to exposure to air. The passivation process involved the slow introductionof a 4-10 76 oxygen in helium gas mixture to the particle trap while carefully monitoring the temperature to detect uncontrolled oxidation. This process required several hours and when complete it allowed the safe handling of the particles in air. The mechanism for the formation of the particles has been reported in detail elsewhere.ls It has been proposed that iron carbides are formed by the catalytic decomposition of ethylene on the iron particles that result from the decomposition of the iron pentacarbonyl. The deposited carbon then diffuses into the particles leading to the formation of carbide phases, provided the particles are sufficiently hot. The departure from the pyrolysis zone causes a rapid quenching of the particles thereby eliminating further particle growth or agglomeration, and arresting the process of carbon diffusion, resulting in the formation of a carbon coating on the surface. By manipulating the reaction parameters, the phase and size of the particlescan be controlled. Typicalreaction parameters for the formation of a-Fe, FesC, and Fe 7C3are shown in Table 1. The identity of each of the phases has been confirmed by X-ray diffraction as well as by Miissbauer spectroscopy.18 The addition of 20% hydrogen sulfide to the reactant gases allowed the production of iron sulfide. The XRD spectrum of the particles confirmed the production of Fe1,S. Table 1shows the reaction parameters used for the production of the pyrrhotite particles. The iron sulfide particles are paramagnetic;therefore, the ferromagnetwas removed and the particles were separated from the gas stream by the Teflon filter. The as-formed particles were less pyrophoric than the iron carbide phase but still required passivation prior to removing from the trap. The third catalyst used in this study, a superfine iron oxide (SFIO), was provided by Mach I Inc. and was reported to be a 30 A diameter a-Fe2Os particle. The properties of the three catalysts used in this study, as determined by XRD and nitrogen BET adsorption measurements, are presented in Table 2.
Experimental Section Liquefactionexperimentewere carried out in 50-mLhorizontal microautoclave reactors, a configuration that reduces mam transfer limitations. The reactors were agitated by vertical oscillation at 400 cycleslmin in a heated fluidized sand bath, allowing the rapid (
