7484
Ind. Eng. Chem. Res. 2004, 43, 7484-7491
MATERIALS AND INTERFACES Ammonia Absorption into Alkaline Earth Metal Halide Mixtures as an Ammonia Storage Material Chun Yi Liu and Ken-ichi Aika* Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8502, Japan, and CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
The mixing effect of alkaline earth metal halides on ammonia absorption-desorption behavior was studied to develop a new ammonia storage material to be used for the pressure-swing absorption method. The halide mixtures with different cations (having a common anion) did not form the solid solution after drying of the mixed aqueous solution. The ammonia absorption isotherms on those samples were identical with those of the single-phase halide having the higher ammonia affinity among the two. The halide mixtures with a common cation (having different anions) formed a solid solution through the same mixing method. The isotherms on those samples showed an intermediate profile between the components. Among these kinds of combinations, CaCl2-CaBr2 (actually CaClBr) showed a pronounced ammonia storage capacity (16.4 mmol g-1) under the pressure-swing conditions of 60 and 10 kPa at 298 K, which was 12.6 times as high as that of the Na form of the Y-zeolite. Introduction Alkaline earth metal halides are expected as ammonia storage material. New types of ammonia synthesis catalysts containing Ru have been explored and are active under low pressure (such as 1 MPa).1-4 This could provide a small ammonia synthesis plant, workable at any place such as a power plant and disposal furnaces where ammonia is required. One of the key technologies that should be solved for this system is ammonia separation and storage. The partial pressure of synthesized ammonia under low pressure such as 1 MPa is 40-80 kPa. Then effective ammonia storage material workable within this pressure range (40-80 kPa for ammonia absorption) and at an evacuation pressure (10 kPa for ammonia desorption) is required for the pressure-swing absorption (PSA) operation.5 Here, the storage capacity is the difference of the absorbed amount after absorption (at 40-80 kPa) and that after desorption (at 10 kPa). Conventional adsorbents, such as surface-modified active carbons and ion-exchanged Y-zeolites, have been studied for this purpose. However, the ammonia storage capacities are not high, i.e., 5 mmol g-1 for the Cu form of Y-zeolite for the use of the temperature-swing adsorption (TSA) method of a 323-473 K cycle under 40 kPa.6-8 Alkaline earth halides and their hydrated forms, MgClOH, CaCl2, CaBr2, and SrBr2, have been found to have high capacities. For example, the storage capacity of MgClOH was 26 mmol g-1 for the use of the TSA method of a 298-473 K cycle under 40 kPa.9 For * To whom correspondence should be addressed. Tel.: +81-45-924-5416. Fax: +81-45-924-5441. E-mail: kenaika@ chemenv.titech.ac.jp.
economical use, the PSA method is preferable to TSA. However, these samples have a poor capacity for the use of PSA under the condition mentioned above. Ammonia desorption from these halides is difficult at 298 K. Alkaline earth metal halides intrinsically absorb a great amount of ammonia, forming the ammine complexes. Ammonia absorption occurs homogeneously among whole bulk cations through complex formation. Thus, the absorption isotherm of this system makes a stepwise figure. The horizontal line (absorption amount) at a step corresponds to the number of coordinated ammonia molecules. The vertical line at a step corresponds to an ammonia pressure to make the coordination. These step pressures relate to the cations affinity to ammonia.10 Some modification of the alkaline earth metal halides, such as mixing them with each other, might change the character of their ammonia absorption and desorption behavior, which would make them suitable for the PSA method. Several kinds of halide mixtures have been studied from the structural viewpoint11-17 and also for ammonia absorption in a chemical heat pump.18 Two methods of preparing halide mixtures have been applied: the comelting method11-17 and the solution-mixing method.18 The former method requires high temperatures near the melting point of the components, which is not practical for conventional use. The latter method is simple and practical, requiring only solvent water. However, ammonia absorption into and desorption from halide mixtures have not been studied in detail.18 In this work, four kinds of alkaline earth metal halide mixtures (MgCl2-CaCl2, CaCl2-SrCl2, CaCl2-CaBr2, and SrCl2-SrBr2) were prepared with the solution-
10.1021/ie049874a CCC: $27.50 © 2004 American Chemical Society Published on Web 10/12/2004
Ind. Eng. Chem. Res., Vol. 43, No. 23, 2004 7485
Figure 1. Sketch of the in situ XRD cell for hygroscopic samples.
mixing method, analyzed with X-ray powder diffraction (XRD), and studied with regard to ammonia absorption and desorption behavior. The five (pure) halides used here were already studied in our previous report.9 How the mixing affects the structure and absorption behavior is discussed. The solid solution formation, the ammine complex formation, of halide mixtures is discussed together with its applicability to ammonia storage materials using the PSA method. The discrepancy of the isotherm from the thermodynamic equilibrium is discussed.
Figure 2. XRD patterns of MgCl2-CaCl2 halide mixtures: (a) MgCl2(523); (b) MgCl2-CaCl2(523); (c) CaCl2(523).
Experimental Section Sample Preparation. Four kinds mixtures of alkaline earth metal halides with 1:1 molar ratios (MgCl2CaCl2, CaCl2-SrCl2, CaCl2-CaBr2, and SrCl2-SrBr2) were prepared by the aqueous solution-mixing method.18 MgCl2‚6H2O (99.9%; Wako Pure Chemicals), CaCl2‚ 2H2O (99.9%; Wako Pure Chemicals), CaBr2‚2H2O (99.5%; Wako Pure Chemicals), SrCl2‚6H2O (99.9%; Wako Pure Chemicals), and SrBr2‚6H2O (99%; Aldrich) were used as precursors. They were mixed in an aqueous solution, stirred for a few minutes, and then evaporated to slurry by a rotary evaporator at 343 K. The slurry sample was dried in air (for about 2 days) at 383 K until visible wetness disappeared. The dried samples were pressed into a thin plate (20 mm diameter) at 20 MPa for 10 min, then crushed and sieved to grains of about 2 mm in size, and finally stored in a desiccator with silica gel. Before the experiment, the sample grains were evacuated for 2 h at 723 or 523 K. Here, a sample pretreated at 523 K is described as MgCl2-CaCl2(523). XRD Analysis. The sample structure was analyzed by XRD with an in situ XRD cell designed for this measurement, as shown in Figure 1. The XRD cell is made of a Pyrex plate. The upper opening is covered by a polyimide thin film (d ) 75 µm; Kapton 300H, Dupon), and the bottom side is covered by a 1-mm-thick acryl plate. Using this, the XRD pattern of the hygroscopic sample was successfully obtained without decomposition or phase change caused by air moisture during the measurement. The sample powder (1 g) was placed in a Pyrex tube, evacuated for 1 h at either 723 or 523 K, mixed with 4 g of glass powder, and transferred to the in situ XRD cell in an argon atmosphere, and the entrance to the glass tube was sealed off. XRD measurement was carried out with an X-ray diffractometer (MultiFlex, Rigaku) through the 2θ range of 10-90° at a 4° min-1 scan speed. Monochromatic Cu KR1 radiation, a 40-kV generator voltage, and a 40-mA current were employed. Because every obtained XRD pattern included the broad diffraction pattern (2θ ) 10-40°) of the Kapton film, it was subtracted as the background.
Figure 3. XRD patterns of CaCl2-SrCl2 halide mixtures: (a) CaCl2(523); (b) CaCl2-SrCl2(523); (c) SrCl2(523).
Ammonia Absorption and Desorption Measurement. The ammonia absorption and desorption isotherms were obtained by a volumetric method with an automatic gas adsorption apparatus (OMNISORP 100CX, Beckman Coulter). A total of 50 mg of the sample was installed in the cell and pretreated at either 723 or 523 K under evacuation (at CaBr2 > SrBr2 > CaCl2 > SrCl2.9 The material, which has a strong affinity for ammonia (such as MgCl2 and MgClOH), cannot desorb ammonia under an appropriate evacuation condition. On the other hand, the materials that have a weak affinity, such as SrCl2, cannot absorb ammonia at an appropriate pressure, such as 60 kPa. The intermediate halides, CaBr2, SrBr2, and CaCl2, could be applicable to the PSA method. Here, only the CaCl2-CaBr2 mixture was able to form a solid solution among the combinations of CaCl2, CaBr2, and SrBr2. (The CaBr2-SrBr2 halide mixture has not been tested.) Other mixtures are not able to form solid solutions because of the difference of crystal type or solubility. Here, the CaCl2-CaBr2 mixture (solid solution) was found to be the best material for ammonia storage material with the PSA method. The effect of the Cl/Br ratio in the CaCl2-CaBr2 halide mixture on ammonia absorption is disclosed in detail.21 Conclusion In this work, we studied the behavior of ammonia absorption and desorption of four kinds of alkaline earth metal halide mixtures (MgCl2-CaCl2, CaCl2-SrCl2, CaCl2-CaBr2, and SrCl2-SrBr2), as ammonia storage material with the PSA method, prepared by the aqueous solution-mixing method. From the XRD measurement of these samples, halide mixtures having a common anion led to separated phases of component halides, which did not mix as solid solutions, but halide mixtures having a common cation formed solid solutions by this method. This result was explained by the aqueous solubility of the components. The absorption isotherm of the mixtures with a common anion reflected halides with higher ammonia affinity (by preferential ammonia absorption). The isotherm of the mixtures with a common cation showed an intermediate profile between the isotherms of the two components. Among the latter, the CaCl2-CaBr2 mixture gave a suitable profile applicable to ammonia storage material with the PSA method between 60 and 10 kPa at 298 K, which was the standard condition of ammonia storage for a small-scale ammonia synthesis process running at 1 MPa. The ammonia storage capacity (16.4 mmol
Ind. Eng. Chem. Res., Vol. 43, No. 23, 2004 7491
g-1) of the CaCl2-CaBr2 mixture pretreated 523 K was 12.6 times as high as that of the Na form of the Y zeolite. However, other mixtures (MgCl2-CaCl2, CaCl2-SrCl2, and SrCl2-SrBr2) were not applicable to the PSA method for our purpose, because of the inconvenient affinity of ammonia for these halide mixtures. Literature Cited (1) Zeng, H. S.; Inazu, K.; Aika, K. The Working State of the Barium Promoter in Ammonia Synthesis over an Active-CarbonSupported Ruthenium Catalyst Using Barium Nitrate as the Promoter Precursor. J. Catal. 2002, 211, 33-41. (2) Zeng, H. S.; Inazu, K.; Aika, K. Dechlorination Process of Active Carbon-Supported, Barium Nitrate-Promoted Ruthenium Trichloride Catalyst for Ammonia Synthesis. Appl. Catal. A 2001, 219, 235-247. (3) Zhong, Z. H.; Aika, K. Effect of Ruthenium Precursor on Hydrogen-Treated Active Carbon Supported Ruthenium Catalysts for Ammonia Synthesis. Inorg. Chim. Acta 1998, 280, 183-188. (4) Zhong, Z. H.; Aika, K. The Effect of Hydrogen Treatment of Active Carbon on Ru Catalysts for Ammonia Synthesis. J. Catal. 1998, 173, 535-539. (5) Aika, K.; Kakegawa, T. On-site Ammonia Synthesis in DeNOx Process. Catal. Today 1991, 10, 73-80. (6) Liu, C. Y.; Aika, K. Modification of Active Carbon and Zeolite as Ammonia Separation Materials for a New de-NOx Process with Ammonia On-Site Synthesis. Res. Chem. Intermed. 2002, 28, 409417. (7) Liu, C. Y.; Aika, K. Effect of Surface Oxidation of Active Carbon on Ammonia Adsorption. Bull. Chem. Soc. Jpn. 2003, 76, 1463-1468. (8) Liu, C. Y.; Aika, K. Ammonia Adsorption on Ion Exchanged Y-zeolites as Ammonia Storage Material. J. Jpn. Petrol. Inst. 2003, 46, 301-307. (9) Liu, C. Y.; Aika, K. Ammonia Absorption on Alkaline Earth Metal Halides as Ammonia Separation and Storage Procedure. Bull. Chem. Soc. Jpn. 2004, 77, 123-131.
(10) International Critical Tables of Numerical Data, Physics, Chemistry and Technology; McGraw-Hill: New York, 1929; Vol. 7, pp 224-313. (11) Olejak-Chodan, M.; Eick, H. A. Characterization of the CaCl2-EuCl2 and CaCl2-SrCl2 systems by X-ray powder diffraction. J. Solid State Chem. 1987, 69, 274-279. (12) Lasocha, W.; Eick, H. A. The Structure of Ca0.3Sr0.7Cl2 and Ca0.46Sr0.54Cl2 by the X-ray Rietveld Refinement Procedure. J. Solid State Chem. 1988, 75, 175-182. (13) Hodorowicz, S. A.; Eick, H. A. Phase Relationships in the System SrBr2-SrCl2. J. Solid State Chem. 1982, 43, 271-277. (14) Goodyear, J.; Ali, S. A. D. The Crystal Structure of MgClBr‚ 6H2O. Acta Crystallogr. 1969, B25, 2664. (15) Liebich, B. W. Refinement of the PbFCl Types BaFI, BaFBr and CaFCl. Acta Crystallogr. 1977, B33, 2790-2794. (16) Hodorowicz, S. A.; Eick, H. A. An X-ray Diffraction Study of the SrBrxI2-x System. J. Solid State Chem. 1983, 46, 313-320. (17) Evans, H. T., Jr.; Konnert, J. A.; Chou, I.-M.; Romankiw, L. A. A Crystal Chemical Study of the System CsCl-NaCl-H2O. Structure of the CsCl Derivative Compounds Cs1-x(Na‚H2O)xCl, CsNa2Cl3‚2H2O, and Cs2CaCl4‚2H2O. Acta Crystallogr. 1984, B40, 86-92. (18) Saito, T. Jpn. Kokai Tokkyo Koho JP 06136357, 1994. (19) Chemical Society of Japan. Kagaku Binran, 4th ed.; Maruzen: Tokyo 1993; p II-161. (20) Westman, S.; Werner, P. E.; Schuler, T.; Raldow, W. X-ray Investigations of Ammines of Alkaline Earth Metal Halides. I. The Structures of CaCl2(NH3)8, CaCl2(NH3)2 and the Decomposition Produce CaClOH. Acta Chem. Scand. A 1981, 35, 467-472. (21) Liu, C. Y.; Aika, K. Effect of the Cl/Br Molar Ratio of a CaCl2-CaBr2 Mixture as an Ammonia Storage Material. Ind. Eng. Chem. Res. 2004, in press.
Received for review February 16, 2004 Revised manuscript received August 20, 2004 Accepted August 26, 2004 IE049874A
