氢氧化镁

Homogeneous Precipitation of Uniform Hydrotalcite Particles Makoto Ogawa*,†,‡,§ and Hiroshi Kaiho§ Department of Earth Sciences, Waseda University, Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan, PRESTO, Japan Science and Technology Corporation, and Graduate School of Science and Engineering, Waseda University, Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan Received November 26, 2001. In Final Form: March 25, 2002 Hydrotalcite was synthesized by a homogeneous precipitation method utilizing urea hydrolysis. When the homogeneous aqueous solutions containing magnesium chloride, aluminum chloride, and urea were heated, hydrotalcite particles were obtained. Scanning electron micrographs revealed that the products were well-defined hydrotalcite particles. The particle sizes have been controlled from ca. 2 to 20 ím by the reaction temperature and the concentration of the reactants. The particle morphology of hydrotalcite was retained even after thermal decomposition, showing the possible application of the present well- defined hydrotalcite particles for the preparation of layered double hydroxide intercalation compounds by the reconstruction method. Introduction Intercalation of organic guest species into layered inorganic solids is a way of producing ordered inorganic- organic assemblies with unique microstructures controlled by host-guest and guest-guest interactions.1,2 Layered double hydroxides (LDHs, general formula of M2+1-x- M3+x(OH)2(An-)n/xâmH2O) are a class of layered materials consisting of positively charged brucite-like layers and the charge compensating interlayer exchangeable anions.3 LDHs with variable chemical compositions have been known as minerals and synthesized materials. The structures and properties of LDHs have extensively been investigated. Due to the variation of the chemical com- positions in both the brucite-like layer and the interlayer anions, the synthesis of LDHs and their intercalation compounds have been conducted for advanced materials applications. The possible applications of LDHs include catalysts and their supports,4 adsorbents,5 ceramic pre- cursors,6 reaction media for controlled photochemical7 and electrochemical reactions,8 and bioactive nanocomposites.9 The synthesis of LDHs with controlled particle size and uniformity is vital for optimum performance of the functional LDHs. However, the synthesis of well-defined LDH particles is yet to be investigated. In this paper, we report the synthesis of monodisperse particles of hydrotalcite, which is a LDH with the ideal formula of Mg6Al2(OH)16CO3â4H2O, by homogeneous precipitation from aqueous solutions in the presence of urea. Urea liberates hydroxide and carbonate ions when its aqueous solution is heated. The hydrolysis of urea has been used to promote the precipitation of metal hydrous oxides and carbonates with uniform size,10-14 upon heating homogeneous aqueous solutions containing soluble metal salts. The homogeneous precipitation from aqueous solu- tions in the presence of urea has been applied to prepare particles of ternary systems15-17 as well as inorganic- organic hybrid materials.18 The urea method has already been applied to synthesize large hydrotalcite particles8,19 for the AFM observation of the adsorbed species19a and the electrode application.8 The urea method is an ideal way for the synthesis of hydrotalcite because both of the ions liberated by the urea hydrolysis, hydroxides and carbonates, are the main components of hydrotalcite. Constantino and co-workers studied the parameters such as temperature and concentration on the composition and structures of the resulting products.19c In the present† Department of Earth Sciences, Waseda University. ‡ PRESTO, Japan Science and Technology Corporation. § Graduate School of Science and Engineering, Waseda Univer- sity. (1) Ogawa, M. Annu. Rep. Prog. Chem., Sect. C: Phys. Chem. 1998, 94, 209. (2) (a) Progress in Intercalation Research; Mu¨ller-Warmuth, W., Scho¨llhorn, R., Eds.; Kluwer Academic Publishers: Dordrecht, 1994. (b) Comprehensive supramolecular chemistry; Alberti, G., Bein, T., Eds.; Pergamon: Oxford, 1996; Vol. 7. (c) Ogawa, M.; Kuroda, K. Chem. Rev. 1995, 95, 399. (d) Ogawa, M.; Kuroda, K. Bull. Chem. Soc. Jpn. 1997, 70, 2593. (3) Trifiro, F.; Vaccari, A. Comprehensive supramolecular chemistry; Alberti, G., Bein, T., Eds.; Pergamon: Oxford, 1996; Vol. 7, p 251. (4) (a) Sels, B.; Vos, D. D.; Buntinx, M.; Pierard, F.; Mesmaeker, A. K.; Jacobs, P. Nature 1999, 400, 855. (b) Suzuki, E.; Ono, Y. Bull. Chem. Soc. Jpn. 1988, 61, 1008. (c) Cavani, F.; Trifiro, E.; Vaccari, A. Catal. Today 1991, 11, 173. (5) Pavan, P. C.; Gomes, G.; Valim, J. B. Microporous Mesoporous Mater. 1998, 21, 659. (6) (a) Hibino, T.; Tsunashima, A. Chem. Mater. 1998, 10, 4055. (b) del Arco, M.; Malet, P.; Trujillano, R.; Rives, V. Chem. Mater. 1999, 11, 624. (c) Alejandre, A.; Medina, F.; Salagre, P.; Correig, X.; Sueiras, J. E. Chem. Mater. 1999, 11, 939. (7) Takagi, K.; Shichi, T.; Usami, H.; Sawaki, Y. J. Am. Chem. Soc. 1993, 115, 4339. (8) Yao, K.; Taniguchi, M.; Nakata, M.; Takahashi, M.; Yamagishi, A. Langmuir 1998, 14, 2410. Yao, Y.; Taniguchi, M.; Nakata, M.; Takahashi, M.; Yamagishi, A. Langmuir 1998, 14, 2890. (9) Choy, J. H.; Kwak, S. Y.; Park, S. J.; Jeong, Y. J.; Portier, J. J. Am. Chem. Soc. 1999, 121, 1399. (10) Willard, H. H.; Tang, N. K. J. Am. Chem. Soc. 1937, 59, 1190. (11) Sordelet, D.; Akinc, M. J. Colloid Interface Sci. 1988, 122, 47. (12) Matijevic, E.; Hsu, W. P. J. Colloid Interface Sci. 1987, 118, 506. (13) Ocana, M.; Morales, M. P.; Serna, C. J. J. Colloid Interface Sci. 1999, 212, 317. (14) Wang, L.; Sondi, I.; Matijevic, E. J. Colloid Interface Sci. 1999, 218, 545. (15) Oren, E. E.; Taspinar, E.; Tas, A. C. J. Am. Ceram. Soc. 1997, 80, 2714. (16) Matsushita, N.; Tsuchiya, N.; Nakatsuka, K.; Yanagitani, T. J. Am. Ceram. Soc. 1999, 82, 1977. (17) Soler-Illia, G. J. de A. A.; Candal, R. J.; Regazzoni, A. E.; Blesa, M. A. Chem. Mater. 1997, 9, 184. (18) Yada, M.; Machida, M.; Kijima, T. Chem. Commun. 1996, 769. (19) (a) Cai, H.; Hiller, A. C.; Franklin, K. R.; Nunn, C. C.; Ward, M. D. Science 1994, 266, 1551. (b) Constantino, U.; Coletti, N.; Nocchetti, M.; Aloisi, G. G.; Elsei, F. Langmuir 1999, 15, 4454. (c) Constantino, U.; Marmottini, F.; Nocchetti, M.; Vivani, R. Eur. J. Inorg. Chem 1998, 1439. 4240 Langmuir 2002, 18, 4240-4242 10.1021/la0117045 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/03/2002 study, we investigated the effects of the synthetic pa- rameters, including temperature and concentration, on the particle size and its distribution of the resulting hydrotalcite. For the use of the monodisperse hydrotalcite particles in host-guest systems, the change in the morphology upon thermal decomposition for the possible application to the reconstruction method was investigated for the first time. Experimental Section Materials. Urea (Extra pure grade, >99.0%; Wako Pure Chemical Industries, Ltd.) and magnesium and aluminum chloride hexahydrates (MgCl2â6H2O, AlCl3â6H2O, Kanto Chemi- cal Co., Inc.) were used without further purification. Sample Preparation. A typical synthetic procedure is as follows: An aqueous stock solution of urea (1.0 M), magnesium chloride (0.1 M), and aluminum chloride (0.1 M) were mixed together at the molar Mg/Al/urea ratio of 4:1:10 with magnetic stirring at room temperature. The concentrations of the com- ponents in the starting solutions were 4 � 10-3, 1 � 10-3, and 1 � 10-2 M for MgCl2, AlCl3, and urea, respectively. Then the homogeneous solution was transferred into a Teflon-lined autoclave (Taiatsu Glass Ind. Co,) and heated at 120 °C for 1 day. After cooling to room temperature, the solid precipitate was collected by centrifugation and washed with deionized water subsequently. The pH of the solution changed from 3.4 at the beginning to 8.4 at the end of the reaction. Effects of the temperature and concentration on the products were investigated employing the synthetic conditions summarized in Table 1. Characterization. X-ray powder diffraction patterns were obtained on a Rigaku Rad IB diffractometer using monochromatic Cu KR radiation operated at 40 kV and 20 mA. Thermogravi- metric-differential thermal analysis (TG-DTA) curves were recorded on a Rigaku TAS 200 instrument at a heating rate of 10 °C min-1 and using R-alumina (R-Al2O3) as the standard material. Infrared spectra of the samples were recorded on a Shimadzu FT-IR 8200PC Fourier transform infrared spectro- photometer by the KBr disk method. Scanning electron micro- graphs were obtained on a Hitachi S-2380N scanning electron microscope. Results and Discussion Figure 1 shows the X-ray diffraction (XRD) pattern of the precipitate prepared by condition A, which was ascribable to hydrotalcite as indexed in a hexagonal lattice.20 The infrared spectrum of the product is shown in Figure 2. The infrared absorption bands ascribable to the brucite-like layer (OH stretching vibration) and the interlayer carbonate ions were observed in Figure 2. The TG-DTA curves of the product (Figure 3) showed the desorption of adsorbed water at around 100-200 °C and the subsequent decomposition of the layered structures due to the liberation of carbonate ions as well as dehy- droxylation of the brucite-like layer at 300-500 °C. These observations confirmed the formation of hydrotalcite by the present urea method. A scanning electron micrograph of the hydrotalcite is shown in Figure 4a. The particles are hexagonal plates as reported previously. The particle size is relatively uniform with the average plate diameter of 2.0 ím. The previously reported coprecipitation method, where base was added into the solution of metal salts to synthesize hydroxides, yielded LDHs with a large particle size distribution.20 In the present synthesis, the controlled supply of carbonate and hydroxide by the decomposition of urea successfully led to the formation of monodisperse hydrotalcite particles. To vary the size of hydrotalcite, experimental conditions (temperature and solution con- centration) were varied as summarized in Table 1. Irrespective of the reaction conditions, the formation of hydrotalcite was confirmed by XRD, IR, and TG-DTA. Scanning electron micrographs of the hydrotalcites prepared by the reactions at 100 and 120 °C (conditions A and B, respectively) are shown in parts a and b of Figure 4, respectively. The particle size distributions, which were obtained by scanning electron microscopy for no less than 80 particles, are shown in Figures 4 and 5. As seen in Figure 4, monodisperse hexagonal plates of hydrotalcite(20) See for example: Reichle, W. T. Solid State Ionics 1986, 22, 135. Table 1. Synthetic Conditions Employed in the Present Study sample [MgCl2] (mol/L) [AlCl3] (mol/L) [urea] (mol/L) reaction temp (°C) product yield (%) A 4 � 10-3 1 � 10-3 1 � 10-2 100 46 B 4 � 10-3 1 � 10-3 1 � 10-2 120 56 C 4 � 10-3 1 � 10-3 1 � 10-2 150 68 D 4 � 10-4 1 � 10-4 1 � 10-3 150 42 E 4 � 10-5 1 � 10-5 1 � 10-4 150 Figure 1. X-ray powder diffraction pattern of the precipitate (condition A). Figure 2. Infrared spectrum of the precipitate (condition A). Figure 3. TG-DTA curves of the precipitate (condition A). Precipitation of Uniform Hydrotalcite Particles Langmuir, Vol. 18, No. 11, 2002 4241 formed at 120 and 100 °C. At the lower reaction temper- atures, the resulting particles are larger (mean particle diameters are 2.4 and 2.9 ím for the products obtained by the reactions at 120 and 100 °C, respectively). Since the decomposition rate of urea in aqueous solutions depends on the temperature,21 the larger particles formed due to the slower particle generation rate at lower temperatures. In addition to the reaction temperature, the concentra- tion of the starting solutions affects the uniformity and the size of the particles. The scanning electron micrographs of the precipitates prepared by conditions D and E are shown in Figure 5. Particle size in this series of experi- ments did not follow a simple trend. Additionally, the particle size distributions are wider if compared with those obtained when relatively concentrated solutions were employed (conditions A-C). However, very large particles of hydrotalcite with the diameter of �20 ím were found when condition D was employed (Figure 5a). To our knowledge, such large crystals of hydrotalcite are difficult to synthesize. The chemical composition of hydrotalcite prepared by condition D was determined by ICP and CHN analysis (C content of 2.4 mass %) to be the formula of Mg0.65Al0.35- (OH)20.13CO3ânH2O. The observed Mg/Al ratio (0.65:0.35) is slightly different from that (4:1) of the starting mixture. Similar results have been obtained previously by Con- stantino and co-workers.19c It is known that the selectivity of carbonate ions to occupy the interlayer space of LDHs is very high, so it is not so easy to substitute the carbonate anions with other anions.22 Therefore, the application of the present urea method for the synthesis of the monodisperse particles of functional LDH intercalation compounds is limited. There are two possible solutions to overcome the limitation; one is the use of other reagents for alkalinization of solutions and the other is the use of the reconstruction method.3 The calcined hydrotalcite-like compounds, which are oxide solid solutions, can be reconstructed to layered structures with guest anions in the interlayer spaces by exposure to an aqueous solution containing appropriate anions. The application of the reconstruction method to the present monodisperse hydrotalcite particles is worth investigating. To check the possibility, the precipitate prepared under condition D was calcined in air at 600 °C. The XRD pattern of the calcined product showed two broad diffraction peaks which are ascribable to the Mg/Al oxide solid solution with a rock salt structure. The scanning electron micrograph of the calcined product showed that the morphology of the as-synthesized hydrotalcite was retained to a large extent even after the calcination. This observation shows the possibility of functionalization of the monodisperse hy- drotalcite synthesized by the urea method. The homogeneous deposition of hydrous oxides on polymer particles as well as ceramic fibers has been reported previously.23,24 The applications of the present synthesis are promising for such heterostructures besides the synthesis of particles with variable chemical composi- tions and the size. Conclusions A layered double hydroxide, hydrotalcite, was synthe- sized by a homogeneous precipitation method utilizing urea hydrolysis. When the homogeneous aqueous solutions containing magnesium chloride, aluminum chloride, and urea were heated, hydrotalcite particles were obtained. Scanning electron micrographs of the products revealed that the products were well-defined hydrotalcite particles. The particle sizes have been controlled by the reaction temperature and the concentration of the reactants from ca. 2 to 20 ím. The particle morphology of hydrotalcite was retained even after thermal decomposition. LA0117045 (21) Shaw, W. H. R.; Bordeaux, J. J. J. Am. Chem. Soc. 1955, 77, 4729. (22) Miyata, S. Clays Clay Miner. 1983, 31, 305. (23) Kawahashi, N.; Matijevic, E. J. Colloid Interface Sci. 1990, 138, 534. (24) Zhao, H.; Draelants, D. J.; Baron, G. V. Catal. Today 2000, 56, 229. Figure 4. Scanning electron micrographs and the correspond- ing particle size distribution of hydrotalcites synthesized by conditions A (a), B (b), and C (c). Figure 5. Scanning electron micrographs and the correspond- ing particle size distribution of hydrotalcites synthesized by conditions D (a) and E (b). 4242 Langmuir, Vol. 18, No. 11, 2002 Ogawa and Kaiho