Ind. Eng. Chem. Res. 2002, 41, 4837-4840
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SEPARATIONS Ultrafine Iron Powder as an Oxygen Scavenger for Argon Purification. Active Iron from Organic Precursors P. Pranda,*,† V. Hlavacek,† and M. L. Markowski‡,§ Department of Chemical Engineering, SUNY Buffalo, 303 Furnas Hall, Buffalo, New York 14260, and Praxair Technology Center, Tonawanda, New York 14150
Ultrafine iron powder was found to be able to remove oxygen from argon. The affinity of iron to oxygen is very high, and oxygen can be removed to concentrations well below 0.2 ppm. The successive experimental runs indicated that the sorbent matrix is not deactivated and maintains high oxygen loading capacities. The spent material can be easily regenerated in a stream of hydrogen at low temperatures. Iron powder was prepared by the thermal decomposition from organic precursors. The sorbent can be easily packed in an adsorption column. The process described here can be scaled up for industrial use. Introduction In a previous paper,1 we described a high affinity of very fine iron powders toward oxygen. We demonstrated that traces of oxygen could be eliminated well below 0.2 ppm. Ultrafine reactive powders represent a very effective class of materials well-known for at least 70 years2 that feature through their nanoparticle size very high surface area that is capable of absorbing traces of gas impurities. A description of pyrophoric metals appeared nearly 40 years ago in a review published by Bahn.3 Pyrophoric iron preparation like iron precursor decomposition (FeII tartrate) was first described in a paper by Gorie et al.4 Ultrafine iron powder described in this paper has been obtained through two precursor decompositions: the decomposition of iron citrate and iron oxalate. Ironbased catalysts were extensively studied by Matson et al.5 for hydrocracking applications, by Boot et al.6 and Stobbe et al.7,8 for dehydrogenation reactions, and by Snel9 and Rameswaran and Bartholomew10 for FischerTropsch synthesis. Ferric oxyhydroxysulfate and six-line ferrihydrite,11 ammonium ferric citrate,6 ammonium ferric ethylenediaminetetraacetic acid (EDTA),7,8 Fe(CO)5, and Fe(NO3)310 were used as precursors. The activity of the iron-based catalysts depends on the preparation method, pretreatment conditions, and possible interactions with the support. Loading of iron precursors on carriers is mostly carried out by incipientwetness6-8,10 or aqueous impregnation.9 Activation/ decomposition of iron precursors is performed in an inert and/or reducing atmosphere at elevated temperatures. Metal dispersion and the extent of reduction play important roles in catalyst preparation in CO hydrogenation.9 The activity/selectivity properties of Fe/Al2O3 * Corresponding author. † SUNY Buffalo. ‡ Praxair Technology Center. § Present address: MARTECH Solutions, 1029 Escarpment Dr., Lewiston, NY 14092-2061.
prepared from Fe(CO)5 do not change with metal dispersion when the extent of reduction is a closely controlled variable. The CO hydrogenation activity of Fe/Al2O3 material changes by an order of magnitude with dispersion when changes in the dispersion are accompanied by changes in the extent of reduction. Rao and Gandhe11 studied the thermal decomposition of oxalates. Gas chromatograph analysis indicated that the metal oxalates, MC2O4 (M ) Fe, Ni, Cu), decomposed to metal and carbon dioxide. In this paper we will describe a method of the ultrafine iron powder manufacturing via a decomposition route of ferrous oxalate. The synthesized material possesses also a very high affinity toward oxygen, and ppm traces of oxygen can be safely removed from argon. The iron powder surface area was characterized by nitrogen adsorption at -196 °C. The method described here represents an alternative to the milling technology described earlier.1 Experimental Section All experimental details not described here can be found in ref 12. 1. Decomposition of Iron Precursors. For the preparation of a very fine iron powder supported on a porous carrier, two iron precursors, ferric citrate and ferrous oxalate, were considered. Water solutions are usually the most appropriate for alumina carrier impregnation.13,14 Ferric citrate has a good solubility and therefore was selected for further study. An alumina support, F-200, was chosen because of its high surface area (360 m2/g) and cheap cost. Two methods of reactive material preparation were tested. In the first method, the carrier was impregnated by a saturated water solution of the iron precursor. After drying at room temperature, thermal decomposition was carried out in nitrogen at 190 °C for 2 h. The supported iron precursor was reduced by a hydrogen-nitrogen mixture (4% of H2 in nitrogen) at 400 °C. The reduction
10.1021/ie020321e CCC: $22.00 © 2002 American Chemical Society Published on Web 08/09/2002
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Table 1. Performance of Fine Iron Powder from Ferric Citrate and Ferrous Oxalate in Oxygen Trace Removal for GHSV ) 100 h-1 breakthrough capacity [mL of O2/g of Fe] reactor bed temp [°C]
iron from ferric citrate (alumina impregnation)
iron from ferrous oxalate (spread on inert beads)
Activation by (1) Thermal Decomposition and (2) Reduction 24 2 4 150 26 20 24 150
Activation by Reduction Only 9 24
8 38
time was 2 h. In the second method, the thermal decomposition step was omitted. The supported ferric citrate was decomposed in a mixture of 4% of hydrogen in nitrogen at 400 °C for 2 h. The alumina (F-200)-supported ferric citrate resulted, after reduction in hydrogen, in a material containing approximately 8 wt % of metallic Fe deposited on the alumina matrix. A thermogravimetric/differential temperature analysis (TG/DTA) of ferric citrate was performed, and, accordingly, a temperature of decomposition of 190 °C was chosen. 2. Trace Oxygen Measurements. 2.1. Preliminary Measurements. We used a simple stainless steel reactor, of about 300 cm3 volume, to carry out the decomposition/activation experiments. A comparison of both preparation methods along with the results is reported in Table 1. The results revealed that when the ferric citrate/alumina matrix was decomposed in a reducing atmosphere, a slightly higher oxygen capacity was achieved. The ferrous oxalate salt has a low solubility in water, and therefore it was loosely mixed with the inert beads to avoid a pressure drop. The results in Table 1 show that the oxygen capacities were higher when the ferrous oxalate was decomposed in the reducing atmosphere. Adsorption capacities for both iron precursors were similar at 24 °C; at 150 °C ferrous oxalate prepared in a reducing atmosphere showed better results (24 mL of O2/g of Fe vs 38 mL of O2/g of Fe). Therefore, ferrous oxalate, mechanically supported over inert material (beads or wool), became the focus of our research effort and was studied in greater detail. Iron precursors are environmentally friendly. No harmful byproducts are formed during the activation/ decomposition process. The degree of activation can be recognized by a color change; for example, the yellow anhydrous ferrous oxalate salt decomposes directly to a black iron powder and gaseous carbon dioxide. The initial value of the surface area of iron produced in situ is high and reaches almost 60 m2/g. After several adsorption/regeneration cycles, the value stabilized at 10 m2/g. The iron material features a very high affinity for trace oxygen. This process, unlike milling technology, does not introduce any impurities in the sorbent material. The iron material prepared by milling contains impurities introduced because of the solvent cracking. The cracking reaction, driven by a strong stress field in the mill, occurs at room temperature. 2.2. Trace Oxygen in Argon Measurements in a Flow System and in Situ Studies. The previously described experimental setup did not allow us to get transient “in situ” information about the iron material such as, for example, changes of specific surface area and color during the successive activation/adsorption
Figure 1. Oxygen capacity (columns) and specific surface area (solid circles) of iron from ferrous oxalate at different inlet O2 concentrations and GHSV ) 3500 h-1.
runs. We wanted also to decrease the reaction/adsorption time from days to a “minutes to a few hours” scale and increase the gas hourly space velocity (GHSV) to 35 000 h-1 by using a smaller volume. The gas system was restricted to low flows of 0.2 ppm), the regeneration procedure was performed. The columns present information about the breakthrough capacities at 250 °C, and the solid circles correspond to the specific surface areas. Runs 12 and 14 were finished when complete saturation was achieved. The last two runs, 16 and 17, were accomplished at a lower flow rate (GHSV ) 3500 h-1). Table 2 summarizes the breakthrough capacity related to the reactor bed temperature applied. For a complete saturation (cf. cycles 12 and 14), a color change was observed; the material changed from black to red. For the other experimental cycles, no color change was observed. 3. Physical Characterization of Iron Powders. 3.1. Specific Surface Area Measurement. Nitrogen adsorption at -196 °C in a FlowSorb II 2300 unit (Micromeritics Inc.) was used to determine the specific surface area. This was a simple single-point method that employed a gas mixture of 30% of nitrogen in helium. The blank measurements were performed in order to quantify the contribution of inert material to the total nitrogen uptake. No significant increase (less than 5%) was observed. Results and Discussion Figure 1 (bed A) displays the stable, equilibrated value of total oxygen breakthrough; the corresponding specific surface areas did not change (except for a sharp
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Conclusions We have found experimentally that the decomposition of iron precursors in the reducing atmosphere leads to a very fine iron powder with a high specific surface area. After several oxidation/reduction cycles, no significant degradation of the specific surface area was observed. The breakthrough oxygen capacity remains constant over time. The total breakthrough oxygen capacity remained stable when using the regeneration procedure described above. The oxidation/reduction extent was the same. A color change was observed after full saturation and red ferric oxide was formed. The sorbent material described in this paper was found to be of equal or better quality than the material prepared via the milling route.1 A thermal decomposition at low temperatures is an inexpensive route for catalyst manufacturers to produce large batches. This technology eliminates the contamination issues inherently produced by the milling technology. Acknowledgment The authors gratefully acknowledge the research support provided by Praxair, Inc. The project was administered through the State University of New York at Buffalo Foundation Services. Literature Cited (1) Pranda, P.; Hlavacek, V.; Markowski, M. L. Ultrafine iron powder as an oxygen adsorbent for argon purification. Activation of iron by milling. Ind. Eng. Chem. Res. 2001, 40, 3331. (2) Chem. Abstr. 1928, 22, 2299. (3) Bahn, G. S. Pyrophoricity of metals, and of fine metal powders in particular; Marquardt Corp.: Van Nuys, CA, Western States Section, The Combustion Institute; 1964; Paper 64-31. (4) Gorrie, T. M.; Kopf, P. W.; Toby, S. The kinetics of the reaction of some pyrophoric metals with oxygen. J. Phys. Chem. 1967, 71, 3842. (5) Matson, D. W.; Linehan, J. C.; Darab, J. G.; Camaioni, D. M.; Autrey, S. T.; Lui, E. G. New nanophase iron-based catalysts
for hydrocracking applications. Mater. Res. Soc. Symp. Proc. 1995, 368 (Synthesis and Properties of Advanced Catalytic Materials), 243. (6) Boot, L. A.; Van Dillen, A. J.; Geus J. W.; Van Buren, F. R. Iron-based dehydrogenation catalysts supported on zirconia. I. Preparation and characterization. J. Catal. 1996, 163 (1), 186. (7) Stobbe, D. E.; Van Buren, F. R.; Stobbe-Kreemers, A. W.; Van Dillen, A. J.; Geus, J. W. Iron oxide dehydrogenation catalysts supported on magnesium oxide. 2. Preparation and characterization. J. Chem. Soc., Faraday Trans. 1991, 87 (10), 1623. (8) Stobbe, D. E.; Van Buren, F. R.; Stobbe-Kreemers, A. W.; Van Dillen, A. J.; Geus, J. W.; Geus, J. W. Iron oxide dehydrogenation catalysts supported on magnesium oxide. 2. Reduction behavior. J. Chem. Soc., Faraday Trans. 1991, 87 (10), 1631. (9) Snel, R. Supported iron catalysts in Fischer-Tropsch synthesis: influence of the preparation method. Ind. Eng. Chem. Res. 1989, 28 (6), 654. (10) Rameswaran, M.; Bartholomew, C. H. Effects of preparation, dispersion, and extent of reduction on activity/selectivity properties of iron/alumina carbon monoxide hydrogenation catalysts. J. Catal. 1989, 117 (1), 218. (11) Rao, T. S.; Gandhe, B. R. Gas-chromatographic study of the thermal decomposition of oxalates. J. Chromatogr. 1974, 88 (2), 407. (12) Hlavacek, V.; Pranda, P.; Markowski, M. L. Purification of Argon. U.S. Patent Application filed on Nov 16, 2000. (13) Marchetti, S. G.; Alvarez, A. M.; Mercader, R. C.; Yeramian, A. A. Preparation and characterization of iron catalysts precursors on different supports. Appl. Surf. Sci. 1987, 29 (4), 443. (14) Gorrie, T. M.; Kopf, P. W.; Toby, S. The kinetics of the reaction of some pyrophoric metals with oxygen. J. Phys. Chem. 1967, 71, 3842. (15) Shevchenko, V. G.; Kononenko, V. I.; Latosh, I. N.; Chulova, I. A.; Lukin, N. V. Effect of the size factor and alloying on oxidation of aluminum powders. Fiz. Goreniya Vzryva 1994, 30 (5), 68. (16) BASF Catalyst R 3-11. Chemical Intermediates. Tech. Bull. BASF 1996.
Received for review April 29, 2002 Revised manuscript received July 8, 2002 Accepted July 10, 2002 IE020321E
