Uniform Fast Growth of Hydrotalcite-like Compounds - Crystal Growth

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CRYSTAL GROWTH & DESIGN

Uniform Fast Growth of Hydrotalcite-like Compounds

2006 VOL. 6, NO. 8 1961-1966

Patricia Benito, Francisco M. Labajos, and Vicente Rives* Departamento de Quı´mica Inorga´ nica, UniVersidad de Salamanca, 37008-Salamanca, Spain ReceiVed NoVember 23, 2005; ReVised Manuscript ReceiVed June 7, 2006

ABSTRACT: Small size, monodispersed hexagonal particles of Mg,Al-CO3 hydrotalcite-like compounds have been synthesized in short periods of time by the microwave-hydrothermal method. An in-depth study of the influence of the irradiation time and the process temperature has been carried out. Well-crystalized materials with relatively high specific surface area values have been prepared in 30 min at 150 °C. Introduction Much research has been carried out in recent years on the synthesis of materials by methods that permit control of both their chemical and their physical properties; additionally, it would be even better if the method employed saves time and money and it is environmentally friendly. Hydrotalcite-like (HT-l) compounds are materials not very commonly found in nature, but they are easily synthesized. They consist of stacked layers with a brucite structure, positively charged as a consequence of a partial substitution of divalent cations by trivalent ones, anionic species being located in the interlamellar region to balance the net charge; water molecules occupy part of the free interlayer room. They are also known as layered double hydroxides (LDHs) or anionic clays. A large amount of hydrotalcite compounds have been prepared by modifying both the layer composition1 and the anionic interlayer species, so it is possible to control the chemical composition (and properties) of the material and to prepare tailored materials for specific applications. These materials have been synthesized by a variety of methods,2,3 but most of them do not provide fine control of the physical properties of the final solids. Urea is an agent used for homogeneous precipitation from a solution that allows morphological control of crystallite size, a factor that plays a critical role in industrial applications. On heating of the sample, decomposition of urea releases ammonia and carbonate ions into the solution in a very uniform fashion, resulting in a gradual and uniform increase in the pH of the solution.4 A pH about 7-9, depending on the temperature, is suitable for precipitating solids with the hydrotalcite phase; therefore, urea hydrolysis has been applied successfully as a new synthetic route to the preparation of well-crystallized LDHs.5-18 By controlling the urea concentration and the temperature, hydrolysis proceeds very slowly, leading to a low degree of supersaturation during precipitation and resulting in a decrease in the nucleation rate. As a result, the materials develop a better degree of crystallinity, forming larger particle than those of hydrothermally treated samples but with a narrower particle size distribution,13 thus permitting control of particle size. On the contrary, in the commonly applied coprecipitation method, precipitation of the hydroxide crystallites starts immediately once the first drops of the basic solution are added to the mixed solution of the cations (or vice versa), and thus nucleation and particle growth overlap, resulting in particles with a broad distribution of particle size.16 The crystallinity of the HT-l compounds thus obtained depends on the total metal * To whom correspondence should be addressed. Tel. +34923294400. Fax +34923294574. E-mail: [email protected].

cation concentration, the M3+/(M2+ + M3+) molar fraction, the urea concentration, the precipitation temperature, and aging time,9,12,13 but this method presents a drawback, as the time required for carrying out the precipitation is quite large, ranging from 1 month5,6 to 6 h for Mg,Al-LDHs.13 However, the combination of hydrothermal conditions with urea decomposition has been very recently reported, allowing a shortening of the reaction time to 3 h.18 Since the discovery of the heating ability of microwaves in 1946, microwave ovens have been used as an alternative source of heating versus conventional ovens, first in domestic applications and later in organic synthesis,19 analytical chemistry,20 ceramics processing,21 catalysis,22 and inorganic synthesis both in the solid state and in liquid media.23 All these applications have a very important common aspect, that is, the reduction of the process time, which implies a savings in energy as well. On the other hand, it must be taken into account that dielectric heating, due to the interaction of microwaves with dipoles, and ionic conduction (if charged species able to be driven under the microwave field exist) produce uniform heating, without thermal gradients, known as volumetric heating. Previous works reported the use of microwave radiation in the synthesis of hydrotalcite-like compounds during the aging treatment coupled to the coprecipitation process.24-26 However, in the present work, for the first time, the precipitation of Mg,Al-CO3 hydrotalcite-like compounds has been carried out using urea as precipitation agent and simultaneously heating the solutions under the influence of a microwave field at autogenous pressure, the so-called microwave-hydrothermal treatment (MW-HT).27 The advantages of using both methods simultaneously are reported. Experimental Section Synthesis Procedure. A method similar to that proposed by Costantino et al. has been followed.11 All chemicals were from Fluka (Switzerland), and the gases were from L’Air Liquide (Spain); all were used without any further purification. The mixture solution (containing 0.5 M in MgCl2‚6H2O and AlCl3‚9H2O, with a Mg2+/Al3+ molar ratio equal to 2, and urea, added to reach a urea/metal ions molar ratio of 3.3) was placed in a reaction vessel of a microwave oven (Milestone Ethos Plus). The microwave oven applies the power necessary to reach the programmed temperature in each moment. An initial ramp program from ambient temperature to 100, 125, or 150 °C was established for all samples, and then the solutions were maintained at the selected temperature for 5, 10, 20, 30, 60, and 120 min. After the sample was cooled at room temperature, the precipitate was centrifuged and washed with distilled water until the washings were completely free of chloride anions and products formed during urea decomposition. Finally, the solids were dried in an oven at 40 °C in air. The solids prepared are

10.1021/cg0506222 CCC: $33.50 © 2006 American Chemical Society Published on Web 07/08/2006

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Table 1. Mg2+/Al3+ Molar Ratios, Lattice Parameters (c and a), Crystallite Size (D), and Specific Surface Areas (SBET) for Samples Irradiated at 150 °C, MAUt sample

Mg2+/ Al3+

c/Å

a/Å

D(003) Å

D(006) Å

SBET m2 g-1

MAU5-2 MAU10-2 MAU20-2 MAU30-2 MAU60-2 MAU120-2

0.657 0.763 1.109 2.125 2.33 2.33

22.57 22.53 22.56 22.44 22.65 22.62

3.034 3.032 3.036 3.036 3.038 3.039

224 203 219 215 256 268

232 215 239 233 256 263

117 106 73 40 24 21

Table 2. Mg2+/Al3+ Molar Ratios, Lattice Parameters (c and a), Crystallite Size (D), and Specific Surface Areas (SBET) for Samples Prepared by the Conventional Method at 150 °C sample

Mg2+/ Al3+

c/Å

a/Å

D(003) Å

D(006) Å

SBET m2 g-1

MAUHT3 MAUHT5 MAUHT12 MAUHT24

0.977 1.710 1.860 1.864

22.39 22.64 22.58 22.65

3.030 3.039 3.041 3.042

423 346 346 616

416 313 385 696

15 8

named MAUt-a, where t stands for the heating time in minutes and a ) 1 when the reaction is carried out at 125 °C and a ) 2 when it is performed at 150 °C; the samples treated under conventional hydrothermal conditions are named MAUHTt, where t represents the time in hours. Techniques. Element chemical analysis for Mg and Al was carried out by atomic absoprtion in a Mark 2 ELL-240 apparatus, in Servicio General de Ana´lisis Quı´mico Aplicado (University of Salamanca, Spain). The phase composition of the samples was determined by powder X-ray diffraction (PXRD); the patterns were recorded in a Siemens D-500 instrument using Cu-KR radiation (λ ) 1.54050 Å) equipped with Diffrac AT software. Identification of the crystalline phases was made by comparison with the JCPDS files.28 The FT-IR spectra were recorded in a Perkin-Elmer FT1730 instrument, using KBr pellets; 100 spectra (recorded with a nominal resolution of 4 cm-1) were averaged to improve the signal-to-noise-ratio. Specific surface area assessment was carried out in a Gemini instrument from Micromeritics. The sample (ca. 80-100 mg) was previously degassed in flowing nitrogen at 150 °C for 2 h in a FlowPrep 060 apparatus, also from Micromeritics, to remove physisorbed water, and the data were analyzed using published software.29 Thermal decomposition of the solids to the calcined products was studied by thermogravimetric (TG) and differential thermal analyses (DTA) carried out in TG-7 and DTA-7 instruments, respectively, from Perkin-Elmer, in flowing oxygen and/ or nitrogen (from L’Air Liquide, Spain), at a heating rate of 10 °C min-1. Scanning electron microscopy (SEM) images were recorded by a JEOL 6300 instrument at 25 kV.

Results and Discussion Element Chemical Analysis. The amount of Mg2+ ions incorporated in the solid depended on both the reaction temperature and the irradiation time; remarkable differences were found between the samples prepared by the MW-HT and conventional hydrothermal treatments. Regarding the samples microwave-irradiated at 100 °C, the Mg/Al ratio was quite far from the expected one (2.0), the amount of Mg in the solid being negligible. When the reaction was carried out at 125 °C, a value of 0.782 for Mg2+/Al3+ was reached after 120 min. However, this ratio reached a value very close to the expected one in 30 min when the treatment was performed at 150 °C, and slightly deviations were observed if the irradiation treatment was further prolonged (Table 1). On the other hand, the values measured for the samples prepared by the conventional hydrothermal treatment were always lower than 2, (Table 2). Powder X-ray Diffraction. The key role of the synthesis temperature, and its great influence on the preparation of solids with the HT-l structure, can be clearly concluded from the evolution of the PXRD patterns. At 100 °C (results not shown),

Figure 1. PXRD patterns of the samples prepared at 125 °C for 30, 60, and 120 min under MW-HT conditions. Legend: MAUt-a, where t stands for the heating time in minutes and a ) 1 when the reaction is carried out at 125 °C.

Figure 2. PXRD patterns of the samples prepared at 150 °C for 5, 10, 20, and 30 min under MW-HT conditions. Legend: MAUt-a, where t stands for the heating time in minutes and a ) 2 when the reaction is carried out at 150 °C.

even after 180 min treatment, no hydrotalcite phase was formed and merely some broad peaks attributed to the pseudo-boehmite structure were recorded in the PXRD pattern. At 125 °C, Figure 1, it was necessary to extend the MW-HT treatment for at least 60 min to detect some traces of the hydrotalcite phase, which coexist with broad peaks of pseudo-boehmite; even after exposure to microwaves for 120 min, no single-hydrotalcite diffraction lines were recorded. When the temperature was only 25 °C higher, 150 °C, Figure 2, the lamellar structure formed in just 5 min, and a crystallographically single phase was obtained in 30 min; an increase in the treatment time led to well-crystallized solids. The patterns recorded show symmetric and sharp reflections due to well-crystallized compounds without appreciable turbostratic disorder. The results shown by the PXRD patterns support the conclusions reached from the chemical analyses, i.e., the low Mg2+/Al3+ ratios obtained when the reaction is carried out at 100-125 °C for short periods of time are due to the presence of a large content of an alumina phase.

Fast Growth of Hydrotalcite-like Compounds

Figure 3. pH variation during the microwave and conventional hydrothermal treatments.

The cell parameters (c and a) were calculated from the (003) and (006) diffraction lines according to c ) 3[0.5(d(003) + 2d(006))] and (110), a ) 2d(110).30 The rather low c values (in comparison to those of HTlcs synthesized by a coprecipitation method with a c value close to 22.8-22.9 Å),30 Tables 1 and 2, suggest a strong interaction between the layers and the interlayer species, probably as a consequence of a well-arranged interlamellar region. The a parameter increases slightly with the irradiation time, probably due to a steady release of Al3+ ions to diminish the distortions related to its presence in the structure31 (parameter a is related to the average cation-cation distance within the layers, and the ionic radius of Al3+ is smaller than that of Mg2+ in an octahedral coordination). The crystallite size (Table 1) was calculated by using the Debye-Scherrer equation.32 The crystallinity increased as the aging time was increased. However, the thickness of the crystallites does not change once the HT-l phase is formed.16 As shown in Tables 1 and 2, the crystallite size was determined from the broadening at half-height of the diffraction peaks due to planes (003) and (006). Although the values calculated should coincide, whichever diffraction peak is used to determine them, it should be noted that for samples treated at low temperature or for short irradiation times, the presence of an amorphous phase responsible for the broad background of the diagram for low diffraction angles might give rise to unreal broadening of the peak due to diffraction by planes (003); so, in this case, the values calculated from the peak due to planes (006) should be more realistic than those determined from the peak due to planes (003). This fact is more evident for samples prepared following conventional heating methods (Table 2) than for those prepared following the HT-MW method (Table 1). pH Variation. From the above results, it is evident that the reaction temperature is the key factor of the process. For each of the conventional and microwave hydrothermal reactions, the pH was measured at room temperature once the reaction had finished. The variations in pH are shown in Figure 3. As it can be expected, the pH evolution depends on the temperature at which the sample is treated, but it should be remarked that it is also dependent on the specific treatment to which the samples are submitted, microwave or conventional hydrothermal. At 125 °C under MW-HT conditions, the pH increased with the irradiation time, and at 60 min, despite the low pH value reached (6.5), the hydrotalcite phase was already formed, although it coexisted with the aluminum hydroxide phase. Upon increasing

Crystal Growth & Design, Vol. 6, No. 8, 2006 1963

the temperature to 150 °C, the same pH value was attained in only 5 min; it remained almost constant for the first 30 min, then increased until a final value close to 7.5 was reached. In these two examples, the curves indicate that if the irradiation time is further prolonged, the pH would increase as well. When the synthesis was carried out at 150 °C under conventional hydrothermal conditions, urea was not decomposed up to 1 h reaction time, so the pH of the solution was not modified; the value reached was barely one-half of that obtained when the solution was heated at the same temperature under MW-HT conditions. After 60 min reaction, urea decomposition started and the pH increased rather rapidly, reaching a value similar to that reached at 150 °C under MW-HT conditions in just 5 min. However, PXRD results (not shown) above indicated that under these conditions the HT-l structure was not even formed, and the reaction should be prolonged 1 h further. After 180 min, some traces of this secondary phase were still observed and only after 300 min well-crystallized phases were obtained, but the presence of pseudo-boehmite cannot be completely ruled out. Prolonging the treatment to 12 or even 24 h led to the crystallization of secondary phases, and a slight decrease in pH, after reaching a maximum close to 9, was observed. Taking into account these results, it can be concluded that the microwave radiation plays a critical role on the synthesis process, even more important than a single effect of temperature. Thermal Analysis. The DTA curves of the samples synthesized at 150 °C both in the microwave oven and in the conventional furnace are displayed in Figure 4. The thermal stability of the samples is also dependent on the crystallinity degree, which itself depends on the irradiation time. In the DTA curve of the sample aged under microwave radiation for 5 min, two endothermic effects (I1 and I2) at low temperatures were recorded, with two shoulders (I3 and I4) at higher temperatures. Increasing the treatment time led to the progressive disappearance of the first peak, while the shoulders strengthen giving rise to two well-resolved peaks in the MAU120-2 sample, where the first endothermic effect is completely absent. From comparison of the positions of these effects with other studies where analysis of evolved gases was carried out,33 the I1 and I2 peaks should correspond to removal of physisorbed and interlamellar water molecules, respectively, and layers collapse giving rise to the high-temperature effects, I3 and I4. The symmetry and sharpness of peak I2 points to the presence of interlayer water molecules linked with similar strength to interlayer carbonates and to the layers. The PXRD pattern for sample MAU30-2 strongly suggests a good crystallinity of the sample, but the DTA curve indicates that the better interlamellar order is achieved in samples MAU60-2 and MAU120-2 since in the latter I3 and I4 are better resolved. For the samples submitted to the conventional hydrothermal treatment, Figure 4 right panel, the same behavior is observed, but it should be remarked that for samples aged during 12 and 24 h the DTA curves show an additional effect centered at around 650 °C, which may be related to the secondary phase registered in the PXRD patterns (not shown), due to the presence of pseudo-boehmite together with the pursued hydrotalcite-like phase, which is not detected for the samples prepared using microwave radiation. Fourier Transform-Infrared (FT-IR) Spectroscopy. FTIR spectroscopy was used to follow the changes undergone by the samples synthesized by the MW-HT method at 125 °C, as the HT-l phase was already formed at the very first stages of reaction when this was carried out at 150 °C. As described above, the solid formed after short reaction times is mainly

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Figure 4. DTA curves of the samples synthesized at 150 °C by the microwave (left) and the conventional treatment (right). Legend: (left) MAUta, where t stands for the heating time in minutes and a ) 2 when the reaction is carried out at 150 °C; (right) MAUHTt, where t represents the time in hours.

Figure 5. FT-IR spectra of the samples synthesized at 125 °C under MW-HT conditions. Legend: MAUt, where t stands for the heating time in minutes.

constituted by an aluminum hydroxide phase, and as the reaction proceeds the HT-l phase was formed; we have followed these changes from the FT-IR spectrum in the region corresponding to mode ν3 of carbonate. The spectrum recorded for sample MAU20-1, Figure 5, is characteristic of aluminum hydroxide. As the reaction time under MW-HT treatment at 125 °C was prolonged, formation of the HT-l phase, with intercalated carbonate anions, proceeded, as shown by the development of a band close to 1360 cm-1, due to the ν3 mode of carbonate; in addition, other characteristic bands of carbonate and M-O vibrations34 are recorded in the low wavenumbers region. The spectrum recorded after 2 h reaction is characteristic of a wellcrystallized hydrotalcite-like phase.35,36 The same behavior is observed when the reaction was carried out at 150 °C; however, in this case the time necessary to form the HT-l structure was

much shorter, and after 10 min the characteristic carbonate band is already recorded. This behavior confirms that formation of the hydrotalcite phase is facilitated both by the temperature and the microwave radiation, the latter one giving rise to a selective redisolution of the alumina phase and formation of the LDH. An increase in the reaction time leads to an enhancement in the crystallinity of the solids, as shown by the evolution of the shoulder close to 2900 cm-1 (not shown) due to the OH stretching of hydroxyl groups and water molecules hydrogen bonded to carbonate ions in the interlayer, a decrease in the bandwidth, as well as from the better resolution of the bands in the low wavenumber region.37 SEM Images. The SEM micrographs of the samples aged at 150 °C are shown in Figure 6. Particles with randomly distributed size and shape formed after 10 min reaction at this temperature; platelets with rounded edges are observed after 20 min, with sizes in the range 1.5-2 µm, although some smaller particles are still seen. After 30 min reaction the particles show a uniform size close to 1 µm, and no significant changes are observed for samples treated for 60 or 120 min. Textural Properties. The nitrogen adsorption-desorption isotherms recorded belong to type II of the IUPAC classification, with narrow H3 hysteresis loops attributed to aggregates of platelike particles leading to slit-shaped pores.38 The hysteresis loop is rather narrow, suggesting the presence of small and homogeneous interparticle pores. Specific surface areas data for all samples are reported in Table 1. Sample MAU5-2 shows the largest value, close to 120 m2 g-1, but it sharply decreases to 40 m2 g-1 after 30 min irradiation, and then, while still decreasing, it is rather smooth, reaching a value of 21 m2 g-1 for sample MAU60-2. The large specific surface area values determined for the samples treated for a short period of time are attributed to the presence of pseudo-boehmite, which might be a mostly amorphous gel (it is not fully identified by PXRD), displaying a large specific surface area. On the other hand, the specific surface area of sample MAU30-2, wherein hydrotalcite

Fast Growth of Hydrotalcite-like Compounds

Crystal Growth & Design, Vol. 6, No. 8, 2006 1965

Figure 6. SEM images of the samples MAU10-2, MAU20-2, MAU30-2, and MAU60-2. Legend: MAUt-a, where t stands for the heating time in minutes and a ) 2 when the reaction is carried out at 150 °C.

is the only crystalline phase identified, is also larger than the specific surface area of the samples obtained by the conventional hydrothermal treatment and those previously reported for similar compounds also prepared by the urea method.9 This behavior may be a consequence of the rapid heating of the solution under the microwave field; since the time has been shortened to 30 min, the main process that takes place is nucleation. Upon increasing the treatment time to 60 and 120 min, the sinterization process becomes more important, and a decrease in the specific surface area is also detected. These data correlate fairly well with the crystallite size data (Table 1). Final Remarks. The result above confirms the formation mechanism stated by Adachi-Pagano et al. for the precipitation of layered double hydroxides using urea and also confirms the mechanism previously proposed by Boclair et al.39 and recently stated by Xu and co-workers.40 When the pH values are low, the trivalent metal hydroxide precipitates and formation of the LDH takes place at higher pH values. This mechanism also explains the high aluminum content in the samples aged for short periods of time, formation of the pseudo-boehmite phase identified in the PXRD pattern, and the high specific surface areas determined for these samples, mostly due to the presence of this secondary phase forming an amorphous gel. The rate enhancement, using microwave radiation as a heating source, can be consider as a purely thermal/kinetic effect, that is, a consequence of the high reaction temperatures that can be rapidly attained when irradiating polar materials in a microwave field; however, the existence of nonthermal effects is still a matter of controversy.41 In this case, the introduction of microwaves in the synthesis modifies the nucleation process. Making use of urea as a precipitating agent provides slow release of the base, leading to a slow nucleation process and favoring the crystal growth. However, using microwave radiation must lead to an increase in the rate of urea hydrolysis and consequently to rapid precipitation of hydrotalcite compounds;

furthermore, an uniform growth environment is attained, since on microwave irradiation of the liquid sample, temperature and concentration gradients can be avoided, providing an uniform environment for nucleation. In conclusion, the microwave radiation plays a key role in the process, as it favors a fast nucleation, giving rise to a large number of nuclei, which produces a big number of small crystals of uniform size (see the SEM images); consequently, the specific surface area values are larger than those previously reported for samples prepared by the urea method9 and than one of MAUHT5 samples, prepared by conventional hydrothermal heating. On the other hand, we have previously reported42 that hydrotalcite-like compounds can be considered as good microwave absorbers, so that once the solid begins to be formed it could act as microwave absorber, giving rise not only to a better layer stacking but also to an ordered interlamellar region, which can explain the splitting of the second endothermic effect in samples MAU60-2 and MAU120-2. Conclusions Some Mg,Al-CO3 hydrotalcite-like compounds have been synthesized by the urea method assisted by microwave radiation at different temperatures and periods of time. The elemental chemical analyses, PXRD patterns, FT-IR spectra, and SBET data confirm the mechanism of formation of the LDH previously proposed, with previous formation of an aluminum hydroxide and the incorporation of magnesium to the structure as the pH is increased. The use of the microwave radiation as heating source allows a considerable reduction of the synthesis time to yield pure hydrotalcite-phase and well-crystallized compounds, in comparison to the conventional hydrothermal treatment. In addition, the SEM micrographs show well-defined and very uniformly sized hexagonal platelets that display considerably large specific surface areas.

1966 Crystal Growth & Design, Vol. 6, No. 8, 2006

Preliminary results have demonstrated the ability to prepare Ni,Al-LDHs by microwave-asssited homogeneous precipitation by the urea method.43 Current studies are in progress to provide insight into suitable synthesis conditions and to complete the characterization of the solids obtained. Acknowledgment. The authors thank to Dr. Cristobalina Barriga for her assistance in obtaining some of the experimental results. Finantial support from MCyT (Grant MAT2003-06605C02-01) and JCyL (Grant SA030/03) and a grant from JCyL to P.B. are acknowledged. References (1) Rives, V., Ed. In Layered Double Hydroxides: Present and Futrure; Nova Science Publishers: New York, 2001. (2) de Roy, A., Forano, C., Besse, J. P. In Layered Double Hydroxides: Present and Future; Rives, V., Ed.; Nova Science Publishers: New York, 2001; Chapter 1, pp 2-37. (3) Braterman, P. S., Xu, Z. P., Yarberry, F. In Handbook of Layered Materials; Auerbach, S. M., Carrado, K. A., Dutta P. K., Eds; Marcel Dekker: New York, 2004; Chapter 8, pp 373-474. (4) Shaw, W. H. R.; Bordeaux, J. J. J. Am. Chem. Soc. 1955, 77, 47294733. (5) Cai, H.; Hiller, A. C.; Franklin, K. R.; Nunn, C. C.; Ward, M. D. Science 1994, 266, 1551-1555. (6) Yao, K.; Taniguchi, M.; Nakata, M.; Takahashi, M.; Yamagishi, A. Langmuir 1998, 14, 2410-2414. (7) Yao, K.; Taniguchi, M.; Nakata, M.; Yamagishi, A. J. Electroanal. Chem. 1998, 458, 249-252. (8) Yao, K.; Taniguchi, M.; Nakata, M.; Takahashi, M.; Yamagishi, A. Langmuir 1998, 14, 2890-2895. (9) Costantino, U.; Marmottini, F.; Nocchetti, M.; Vivani, R. Eur. J. Inorg. Chem. 1998, 1439-1446. (10) Ramajamathi, M.; Kamath, P. V. Int. J. Inorg. Mater. 2001, 3, 901906. (11) Murcia-Mascaro´s, S.; Navarro, R. M.; Go´mez-Sainero, L.; Costantino, U.; Nocchetti, N.; Fierro, J. L. J. Catal. 2001, 198, 338-347. (12) Ogawa, M.; Kaiho, H. Langmuir 2002, 18, 4240-4242. (13) Oh, J.-M.; Hwang, S.-H.; Choy, J.-H. Solid State Ionics 2002, 151, 285-291. (14) Costantino, U.; Curini, M.; Montanari, F.; Nocchetti, M.; Rosati, O. J. Mol. Catal. A: Chem. 2003, 195, 245-252. (15) Aramendı´a, M. A.; Borau V.; Jime´nez, C.; Marinas, J. M.; Ruiz, J. R.; Urbano, F. Appl. Catal. A 2003, 249, 1-9. (16) Adachi-Pagano, M.; Forano, C.; Besse, J.-P. J. Mater. Chem. 2003, 13, 1988-1993. (17) Mavis, B.; Akinc, M. J. Power Source 2004, 143, 308-317. (18) Mohan Rao, M.; Ramachandra Reddy, B.; Jayalakshimi, M.; Swarna Jaya, V.; Sridhar, B. Mater. Res. Bull. 2005, 40, 347-359.

Benito et al. (19) Varma, R. S. In AdVances in Green Chemistry: Chemical Syntheses Using MicrowaVe Irradiation; Astra Zeneca Research Foundation: India, 2002. (20) Jin, Q.; Liang, F.; Zhang, H.; Zhao, L.; Huan, Y.; Song, D. Trends Anal. Chem. 1999, 18, 479. (21) Sutton, W. H. Am. Ceram. Soc. Bull. 1989, 68, 376-386. (22) Zhang, X.; Hayward, D. O.; Lee, C.; Mingos, M. P. Appl. Catal. B 2001, 33, 137(23) Rao, K. J.; Vaidhyanathan, M.; Ganguli, M.; Ramaskrishnan, P. A. Chem. Mater. 1999, 11, 882-895. (24) Kannan, S.; Jasra, R. V. J. Mater. Chem. 2000, 10, 2311-2314. (25) Tichit, D.; Rolland, A.; Prinetto, F.; Fetter, G.; Martı´nez-Ortiz, M. J.; Valenzuela, M. A.; Bosch, P. J. Mater. Chem. 2002, 12, 38323838. (26) Climent, M. J.; Corma, A.; Iborra, S.; Epping, K.; Velty, A. J. Catal. 2004, 225, 316-326. (27) Komarneni, S.; Roy, R.; Li, Q. H. Mater. Res. Bull. 1992, 27, 13931405. (28) JCPDS: Joint Committee on Powder Diffraction Standards, International Centre for Diffraction Data, Swarthmore, PA, 1977. (29) Rives, V. Adsorpt. Sci. Technol. 1991, 8, 95-104. (30) Cavani, F.; Trifiro`, F.; Vaccari, A. Catal. Today 1991, 11, 173301. (31) Labajos, F. M.; Rives, V.; Ulibarri, M. A. J. Mater. Sci. 1992, 27, 1546-1552. (32) West, A. R. Solid State Chemistry and Its Applications; John Wiley and Sons: New York, 1997. (33) Rives, V. Inorg. Chem. 1999, 38, 406-407. (34) Nakamoto, K. In Infrared and Raman Spectra of Inorganic Compounds, 4th ed.; J. Wiley & Sons: New York, 1986. (35) Kloprogge, T.; Frost, R. L. In Layered Double Hydroxides: Present and Future; Rives, V. Ed.; Nova Sci. Pub. Inc.: New York, 2001; pp 155-192. (36) Kloprogge, J. T. In The application of Vibrational Spectroscopy to Clay Minerals and Layered Double Hydroxides; Kloprogge, J. T., Ed.; CMS Workshop Lectures; The Clay Mineral Society: Aurora, CO, 2005; Vol. 13, pp 203-238. (37) Hickey, L.; Kloprogge, J. T.; Frost, R. L. J. Mater. Sci. 2000, 35, 4347-4355. (38) Rouquerol, F.; Rouquerol, J.; Sing, K. In Adsorption by Powders and Porous Solids. Principles, Methodology and Applications; Academic Press: London, 1999. (39) Boclair, J. W.; Braterman, P. S. Chem. Mater. 1999, 11, 298-302. (40) Xu, Z. P.; Lu, G. Q. Chem. Mater. 2005, 17, 1055-1082. (41) De la Hoz, A.; Dı´az-Ortiz, A.; Moreno, Andres, Chem. Soc. ReV. 2005, 34, 164-178. (42) Benito, P.; Labajos, F. M.; Rives, V. Bol. Soc. Esp. Ceram. Vidrio 2004, 43, 56-58. (43) Benito, P.; Herrero, M.; Labajos, F. M.; Rives, V. Proceedings of the 10th International Conference on Microwave and High Frequency Heating, Modena, Italy, September 12-15, 2005; Leonelli, C., Veronesi, P., Eds.; Microwave Application Group: Atlanta; 52a.

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