Structural Characterization of Synthetic Hydrotalcite-like [Mg1-xGax

4748

Langmuir 1997, 13, 4748-4753

Structural Characterization of Synthetic Hydrotalcite-like [Mg1-xGax(OH)2](CO3)x/2‚mH2O E. Lo´pez-Salinas,*,†,‡ M. Garcı´a-Sa´nchez,† J. A. Montoya,† D. R. Acosta,§ J. A. Abasolo,† and I. Schifter† Subdireccio´ n de Transformacio´ n Industrial, Instituto Mexicano del Petro´ leo, Eje Central La´ zaro Ca´ rdenas 152, 07730 Me´ xico, D.F., Me´ xico, and Instituto de Fı´sica, Universidad Auto´ noma Nacional de Me´ xico, Cd. Universitaria, A.P. 20-364 Me´ xico, D.F., Me´ xico Received February 21, 1997X A series of Ga-substituted hydrotalcite-like compounds, [Mg1-xGax(OH)2] (CO3)x/2‚mH2O (where 0.07 e x e 0.36; GaHTs), were obtained in order to characterize their structural and crystal properties by means of X-ray diffraction and conventional/high-resolution electron microscopy. A linear relationship between Mg/Ga ratios and the a parameter (3.123-3.086 Å) was found for all x values, indicating that Ga3+ cations incorporate into the layered structure. The interlayer spacing, i.e. the c parameter, shrinks (8.173-7.574 Å) as the Ga content increases, due to a greater electrostatic attraction between layers and interlayers. However, for Ga-poor materials (Mg/Ga ) 7.7, 12.9) the c parameter remains practically constant (8.173 and 8.169 Å), probably due to the high dilution of Ga in the brucite-like layers. GaHTs are made up of hexagonal crystallites. High-resolution observations of the layered stacking structure point out a defective configuration containing dislocations, stacking faults, weaving planes, and disruptions in planes. Outermost layers in a crystal plate present interlayer distances (3.0-3.8 Å) greater than those in the bulk (2.4 Å), suggesting that peripheral layers are probably more loosely bonded to each other than the later ones.

Introduction Clays can be broken down into two broad groups: cationic clays, which nature prefers, and anionic clays, which are rare in nature but are relatively easy and inexpensive to prepare in the laboratory. Hydrotalcite, [Mg6Al2(OH)16]CO3‚4H2O, is one of the naturally occurring anionic clays.1 The structure of hydrotalcite resembles that of brucite, Mg(OH)2, in which magnesium is octahedrally surrounded by hydroxyls. By sharing edges, Mg(OH)2 octahedra form infinite layers. Replacing some of the Mg2+ divalent cations by Al3+ trivalent cations results in hydrotalcite, causing the layered array to be positively charged. These positively charged Mg-Al doublehydroxide layers are electrically compensated for by carbonate anions which are located in the interlayer region. The interlayer anion is ion-exchangeable in aqueous2,3 or organic media.4 A wide variety of synthetic hydrotalcite-like materials can be prepared and are represented by the general formula [MII1-x MIIIx(OH)2](An-)n/x‚mH2O, where MII ) Mg2+, Ni2+, Zn2+, etc., MIII ) Al3+, Fe3+, Ga3+, etc., and A ) [CO3]2-, Cl-, [V10O28]6-, etc. These materials show X-ray diffraction patterns which are very similar to that of hydrotalcite. The mixed-metal oxides obtained from the calcination of hydrotalcites, or double-layered hydroxides as they are also referred to, exhibit catalytic activity in several types of reactions which have been reviewed recently.5 Noteworthy of mention are those chemical reactions such as aldol condensations of aldehydes and ketones6-8 and polymerization of propylene oxide9 catalyzed by solid bases †

Instituto Mexicano del Petro´leo. E-mail: [email protected]. § Universidad Auto ´ noma Nacional de Me´xico. X Abstract published in Advance ACS Abstracts, July 15, 1997. ‡

(1) Frondel, C. Am. Mineral. 1941, 26, 295. (2) Bish, D. L. Bull. Mineral. 1980, 103, 170. (3) Miyata, S. Clays Clay Miner. 1983, 31, 305. (4) Lo´pez-Salinas, E.; Ono, Y. Microporous Mater. 1993, 1, 33. (5) Cavani, F.; Trifiro`, F.; Vaccari, A. Catal. Today 1991, 11, 173. (6) Reichle, W. T. U.S. Patent 4458026, 1984. (7) Reichle, W. T. J. Catal. 1985, 94, 547. (8) Suzuki, E.; Ono, Y. Bull. Chem. Soc. Jpn. 1988, 61, 1008.

S0743-7463(97)00192-3 CCC: $14.00

in which calcined hydrotalcites have shown high activities as catalysts. The use of the pristine hydrotalcites (i.e. without destruction of the layered structure) as catalysts in the halide-exchange reaction between alkyl halides has also been reported.10,11 Aluminum-substituted MgO, obtained via calcination of a hydrotalcite precursor, showed much higher surface area, basicity, and hydrothermal stability than that of single MgO.12 The interest in the development of Ga-substituted hydrotalcite-like materials stems from their potential application, once calcined, as catalysts in the dehydrogenation and aromatization reactions of linear paraffins. Using hydrotalcite-like materials as precursors provides an effective method to obtain well-dispersed two-component catalyst supports and/or catalysts. The residual electropositive charge of the trivalent cation keeps them as far apart as possible, due to electrostatic repulsion, among electrically neutral MIIOH2 units. Synthetic hydrotalcite-like compounds crystallize in either the R3m (rhombohedral phase) or the P6/mmc space group (hexagonal phase). The a and c parameters of both phases depend on (i) the nature of the cations, (ii) their relative amounts (MII/MIII molar ratio), (iii) the degree of hydration, and (iv) the size and orientation of the interlamellar anions present in the structure. The first factor affects only the a parameter, the second one has an influence on both a and c, and the last two factors affect exclusively the c parameter. In natural anionic clays, the x value in the above formula lies between 0.22 and 0.33. These values have been reported to be the limit of x, in natural or synthetic hydrotalcite-like compounds, on the basis of geometric considerations,13 due to the formation of aluminum oxyhydroxide or magnesium hydroxide in most syntheses (9) Kohjiya, S.; Sato, T.; Nakayama, T.; Yamashita, S. Makromol. Chem. Rapid Commun. 1981, 2, 231. (10) Suzuki, E.; Okamoto, M.; Ono, Y. J. Mol. Catal. 1990, 61, 283. (11) Lo´pez-Salinas, E.; Tomita, N.; Matsui, T.; Suzuki, E.; Ono, Y. J. Mol. Catal. 1993, 81, 397. (12) Schaper, H.; Berg-Slot, J. J.; Stork, W. H. J. Appl. Catal. 1989, 54, 79. (13) Brindley, G. W.; Kikkawa, S. Am. Mineral. 1979, 64, 836.

© 1997 American Chemical Society

Hydrotalcite-like [Mg1-xGax(OH)2](CO3)x-2‚mH2O

when starting materials contained x g 0.33 or x e 0.22, respectively. Nevertheless, Schaper et al. have reported pure Mg-Al hydrotalcite phases with x as low as 0.16 (Mg/Al ) 5) using a carefully controlled coprecipitation procedure.12 Recently, we reported that synthetic Gasubstituted hydrotalcite-like materials, [Mg1-xGax(OH)2](CO3)x/2‚mH2O, can be obtained within a wider range of x, typically 0.07 e x e 0.36 (i.e. 12.9 g Mg/Ga g 1.8).14 The replacement of Mg2+ by Ga3+ cations up to near-doping levels (Mg/Ga ) 12.9) has been ascribed to the smaller ionic radii difference of the cations in Mg-Ga (0.10 Å15 ) than in Mg-Al (0.19 Å15) hydrotalcites.14 The present study is concerned with the influence of the Mg/Ga molar ratios in a series of Ga-substituted hydrotalcite-like materials on the double-hydroxide-layer thickness (a parameter) and its interlayer spacing (c parameter). The morphology and laminar structure of these materials have been examined thoroughly by electron microscopy. Experimental Section

Langmuir, Vol. 13, No. 17, 1997 4749 Table 1. Chemical Composition of GaHTs Mg/Ga Mg/Ga nominal found Ga/CO2 MgO Ga2O3 CO2 H2O totalb (M) (M) (M) (wt %) (wt %) (wt %) (wt %) 0.5 3.0 4.5 6.0 7.5 12.0

1.8 2.9 4.3 5.3 7.7 12.9

0.6 0.5 0.6 n.a 0.6 0.5

30.4 38.6 45.1 48.3 53.3 58.7

39.3 30.0 24.4 21.2 16.1 10.6

10.8 6.9 6.8 n.a. 4.5 2.5

27.2 35.1 33.2 n.a. 38.5 39.5

a n.a. ) not analyzed. b The amount of water was calculated by subtracting the CO2 content (0.5 mol) from the total weight loss in each sample.

lines in the samples. The cell parameters were determined using the (110) and (006) reflections, which were recorded at a scanning rate of 0.02° s-1 in 2θ. For the measurement of the a parameter a prominent reflection at about 1.5 Å indexed as d110 was selected. Considering that hydrotalcites crystallize into a hexagonal system where the interplanar distances are given by dhkl ) [(4/(3a2))(h2 + k2 + hk) + (l2/c2)]-1/2 and since the d110 reflection was used, then a ) 2(d110). The c′ parameter was taken directly from the (006) reflection, which appears between 22 and 24° in 2θ, and divided by 3. Conventional and high-resolution electron microscopy (CTEM and HREM, respectively) observations were carried out on gallium-substituted hydrotalcite-like materials. Samples were ground in an agate mortar and dispersed in isopropyl alcohol by means of an ultrasonic bath for several minutes, and then some drops of the solution were deposited on 200 mesh copper grids covered with perforated carbon film. CTEM studies were done with a JEOL 100CX electron microscope, and HREM observations were carried out with a JEOL 4000 EX instrument furnished with a pole piece with Cs ) 1.00 mm. Bright field and selected area electron diffraction (SAED) images were obtained when possible from individual or isolated groups of crystallites. A gold standard film was used to calibrate the electron microscope diffraction parameters.

Preparation of [Mg1-xGax(OH)2](CO3)x/2‚mH2O. The preparation of Mg-Ga hydrotalcites, hereinafter referred to as yGaHTs, where y ) Mg/Ga molar ratio, was carried out using a coprecipitation method in which only the amount of the reactants was varied and all other conditions were the same for all the preparations, except where indicated. Deionized water was used throughout all the experiments without any further treatment. For instance, the synthesis of a hydrotalcite with Mg/Ga ) 3.0 was obtained as follows: first an aqueous solution containing 11.17 g (26.7 mmol) of Ga(NO3)3‚9H2O and 20.48 g (79.9 mmol) of Mg(NO3)2‚6H2O in 100 cm3 of water was prepared. After this, 2.76 g (20 mmol) of K2CO3 and 15.19 g (270.72 mmol) of KOH in 300 cm3 of water were dissolved to make a second solution. These two solutions were added dropwise into a flask containing 200 cm3 of water at 313 K upon vigorous stirring. The rate of addition of the two solutions was controlled in order to keep a constant pH (11-12), which was monitored continuously throughout the coprecipitation procedure by means of a pH meter. After the addition of the solutions was completed, the white gel obtained was immediately washed several times and separated in a centrifuge. After this, the white paste was dried in an oven in static air at 353 K for 24 h. Finally, a white solid was obtained (yield 76%). In a separate experiment after the above gel was obtained and before washing, the mixture was transferred to a stainless-steel autoclave and aged at 448 K with autogeneous pressure (ca. 10 kg cm-2) for 18 h. Then, the material was washed and dried as above. This sample was used exclusively in the electron microscopy observations. Characterization Techniques. The metal content in the solid materials was determined by means of inductively coupled atomic emission spectroscopy (AES) in an SPS 1500VR plasma spectrometer from Seiko Instruments. The solid samples were easily dissolved in aqueous HNO3 at appropriate concentrations in order to carry out the analysis by AES. The CO2 content in the solid Ga-hydrotalcites was determined in a LECO CR-12 apparatus by calcining the samples at 1045 K, and the gases generated were passed through a series of traps to remove fine particles and humidity. Finally, CO2 was quantified by means of a solid-state infrared detector. Powder X-ray diffraction (XRD) patterns were recorded on a D 500 instrument from Siemens using monochromatic Cu KR1 (1.5406 Å) radiation at 35 kV and 25 mA. The diffraction patterns were compared with those included in the JCPDS data base (Joint Committee of Powder Diffraction Standards). For the determination of the unit cell parameters a and c′ the samples were previously water-vapor-saturated in a closed vessel at room temperature for 24 h. After this, the samples were mixed with a certified powder Silicon 640B as standard (National Bureau of Standards) in order to correct the positions of the diffraction

Chemical Composition of GaHTs. The metal contents in the resulting Ga-hydrotalcites are shown in Table 1. Most of the solid materials contain Mg and Ga concentrations very close to the expected values, indicating that the incorporation of the cations into the solid phase is complete, in spite of the fact that Ga(OH)3 species are highly soluble under the coprecipitation conditions used, i.e. alkaline aqueous media. The incorporation of Ga into the brucite-like layers to yield pure Ga-hydrotalcite materials occurs in a very wide range of Mg/Ga molar ratios without or with minimum segregation of other solid phases.3 Structure of GaHTs. The XRD patterns of hydrotalcite-like materials are made up of some general features typical of layered materials: sharp, symmetric, and intense lines at low 2θ values and less intense, generally asymmetric lines at higher 2θ angular values. The XRD patterns of four GaHTs with Mg/Ga ) 1.8, 3.0, 5.3, and 12.9 are shown in Figure 1. A comparison of any of these patterns with that of the hydrotalcite, [Mg6Al2(OH)16](CO3)‚4H2O, indicates clearly that GaHTs have the same layered structure as hydrotalcite. Considering that MgAl hydrotalcite and Mg-Fe pyroaurite are isostructural three-layer polytypes,5,16,17 we assume that the two intense lines at low 2θ values, 7.75 and 3.91 Å, can be taken as (003) and (006), respectively. The d spacing of the first basal reflection yields the combined thickness of one layer and one interlayer. Since Mg(OH)2 octahedral units are always more numerous in GaHTs than [Ga(OH)2]+

(14) Lo´pez-Salinas, E.; Garcı´a-Sanchez, M.; Ramo´n-Garcı´a, M. L.; Schifter, I. J. Porous Mater. 1996, 3, 169. (15) Shannon, R. D.; Prewitt, C. T. Acta Crystallogr. 1969, B25, 925.

(16) Allmann, R. Chimia 1970, 24, 99. (17) Gastuche, M. C.; Brown, G.; Mortland, M. Clay Miner. 1967, 7, 177.

Results and Discussion

4750 Langmuir, Vol. 13, No. 17, 1997

Lo´ pez-Salinas et al.

Figure 2. Unit cell parameters a and c as a function of Ga content in GaHTs.

Figure 1. Powder X-ray diffraction patterns of GaHTs with Mg/Ga (M): (a) 1.8; (b) 2.9; (c) 5.3; (d) 12.9.

octahedral units, and because the Mg2+ ionic radius is larger than that of Ga3+, it can be assumed that the layer thickness in GaHTs is the same as that in Mg-Al hydrotalcites. The thickness of the hydroxide layer in hydrotalcite is 4.8 Å,18 giving a 2.95 Å interlamellar spacing. This value indicates that planar [CO3]2- anions in the interlayer region are accommodated with their tricoordinated planes parallel to the sheets of GaHTs, in agreement with that reported for carbonate-containing Mg-Al hydrotalcites.19 Higher Mg/Ga ratios yield less ordered GaHTs (see pattern d in Figure 1). All of the XRD patterns of the GaHTs synthesized showed XRD patterns similar to those in Figure 1, except for the sample with Mg/Ga ) 12.9. Here, three reflections at about 20, 38 (overlapped with that of hydrotalcite) and 50° (in 2θ) indicated the presence of a small amount of brucite (JCPDS card: 7-0239). The XRD pattern of the most Ga-rich material, Mg/Ga ) 1.8, showed no evidence for the presence of Ga(OH)3 or Ga2O3. However, the presence of amorphous Ga(OH)3 phases (not detectable by powder XRD analysis), (18) Allmann, R. Acta Crystallogr. 1968, B24, 972. (19) Bish, D. L.; Brindley, G. W. Am. Mineral. 1977, 62, 458. (20) Pausch, I.; Lohse, H. H.; Schu¨rmann, K.; Allmann, R. Clays Clay Miner. 1986, 34, 507. (21) Sato, T.; Fujita, H.; Endo, T.; Shimada, M.; Tsunashima, A. React. Solids 1988, 5, 219.

as in the case of amorphous Al(OH)3 in Mg-Al hydrotalcites,20 cannot be ruled out. Cell Parameters as a Function of Mg/Ga. The basal (00l) reflections in hydrotalcite-like materials correspond to succesive orders of the basal spacing c′. The true c parameter is a multiple of c′ and depends on the layer stacking sequence. For hydrotalcite, c ) 3c′.16 In Figure 2, the c′ parameter is almost unaffected for hydrotalcites with x ) 0.072-0.115 (i.e. Mg/Ga ) 12.9-7.7), but for x > 0.115, it decreases linearly with an increase in Ga concentration up to x ) 0.357 (Mg/Ga ) 1.8). An increase in Ga concentration in the double-hydroxide sheets brings about an increase in the overall electropositive charge density, since each [Ga(OH)2]+ unit bears an extra charge. The gradual diminishing of the c′ parameter is related to a shrinking in the interlayer spacing, which is caused by the increasing electrostatic attraction between adjacent positive layers and negative interlayers. A similar behavior in Mg-Al, Ni-Al,13,21 and Mg-Fe21 hydrotalcites within x ) 0.17-0.35 has been reported elsewhere. Clearly, when Ga is very diluted in the layers (x ) 0.0720.115), its contribution to the electrostatic attraction is almost negligible from the c′ parameter viewpoint. The unit cell parameter a can be used as an indication of the nonstoichiometry with respect to the formation of pure hydrotalcite-like compounds. In Figure 2, the a parameter decreases linearly as the Ga content increases. This behavior is related to the fact that Ga3+ cations have a smaller ionic radius than Mg2+ (0.76 and 0.86 Å, respectively). As proposed in earlier studies on Mg-Al and Ni-Al hydrotalcite-like materials,13 the variation of the a parameter as a function of Ga content x ) Ga/(Mg + Ga), that is, ∆a/∆x, can be considered as follows. For a layer made up of ideal octahedral units, a ) 21/2(M-O), where M-O is the bond distance between the metal cation, Mg2+ or Ga3+, and oxygen. The mean radius of the metal ions in Mg-Ga hydrotalcites, r, will be given by

r ) (1 - x)[r(Mg)] + x[r(Ga)] ) r(Mg) - x[r(Mg) - r(Ga)]

Hydrotalcite-like [Mg1-xGax(OH)2](CO3)x-2‚mH2O

Langmuir, Vol. 13, No. 17, 1997 4751

The negative slope of a as a function of x can be expressed as follows:

∆a/∆x ) -21/2[r(Mg) - r(Ga)] Using the ionic radii reported by Shannon and Prewitt, Mg2+ ) 0.86 Å and Ga3+ ) 0.76 Å,15 the calculated value (∆a/∆x)calcd ) -0.1414. The experimental value obtained from the slope of the data in Figure 2 is (∆a/∆x)exptl ) -0.1310, which is very close to the theoretical value. An extrapolation of the straight line in Figure 2 to x ) 0 (i.e. zero Ga3+ substitution) gives a ) 3.132 Å, which is very close to the a parameter reported for Mg(OH)2, 3.147 Å (JCPDS card: 7-0239). It is clear from these results and the linear relationship between parameter a and x in Figure 2, which obey Vegard’s law, that Ga3+ indeed incorporates into the layers of Mg-Ga hydrotalcites for a wide range of x, typically 0.072 e x e 0.357. We have found that it is not possible to synthesize MgGa hydrotalcites with x > 0.357 (at the conditions reported in the experimental section), since the obtained solid will invariably contain compositions near x ) 0.357 (Mg/Ga ) 1.8), even with x values as low as 0.5 in the starting materials.14 Different from the case of Mg-Al hydrotalcites, which at x > 0.33 (Mg/Al ) 2) contain Al(OH)3 impurities,13, 20 in Mg-Ga hydrotalcites undesired Ga(OH)3 phases were not detected at x > 0.357, in spite of the fact that Ga3+ cations did not incorporate completely into the hydrotalcite structure. These results can be explained on the basis of the high solubility of Ga(OH)3 in alkaline media, which may have been withdrawn from the solid hydrotalcite in the washing step. Since [Ga(OH)2]+ units in the layers provide one positive charge, it is likely that because of electrostatic repulsion they are positioned as far apart as possible from each other among electrically neutral Mg(OH)2 units. x values greater than 0.357 (Mg/Ga ) 1.8) could bring about the accommodation of adjacent [Ga(OH)2]+ units which it is not likely to occur, resulting in the formation of non-frame Ga(OH)3. Brindley and Kikkawa detected Al(OH)3 impurities in Mg-Al hydrotalcites with Mg/Al < 2.13 Additionally, interlamellar carbonate anions, which are incorporated with their tricoordinate plane parallel to the layers and are associated with Ga3+ cations (Ga/[CO3] ) 2, stoichiometric molar ratio), set forth a severe steric hindrance since a high amount of [Ga(OH)2]+ units would bring [CO3]2- anions prohibitively close to each other. On the other hand, it has been reported by several researchers that for x < 0.17-0.20 in Mg-Al hydrotalcites, i.e. Mg-rich hydrotalcites, the high population of neighboring octahedral Mg(OH)2 units brings about the formation of Mg(OH)2, brucite, impurities.13,20 In contrast, pure Mg-Ga hydrotalcites in this study can be obtained up to x ) 0.072 (Mg/Ga ) 7.7) without any indication of undesired phases. These results could be attributed to the smaller ionic radii difference in Mg-Ga (0.10 Å15) than in Mg-Al hydrotalcites (0.19 Å15). A similar conclusion was derived from the results of Sato et al. on a series of Mg-Fe pyroaurites (structurally similar to hydrotalcites), [Mg1-xFex(OH)2](CO3)x/2‚mH2O, in which a and c′ parameters vary linearly in all the ranges of x studied, showing no deviation from Vegard’s law.21 In other words, the smaller the ionic radii difference between M2+ and M3+, the lower the x value at which pure hydrotalcite-like materials can be obtained. In this study, however, small amounts of brucite were detected in 12.9GaHT (Figure 1d). Notwithstanding, no deviations from the linear relationship between cell parameters and composition were detected, probably due to the small content of phase impurities.

Figure 3. Bright field electron micrograph of a single crystal of GaHT (Mg/Ga ) 3), showing a typical hexagonal crystal.

Figure 4. Electron diffraction pattern taken at [0001] direction from a well-defined GaHT hexagonal crystal.

Figure 5. Electron diffraction pattern from a faulted crystal. Streaks and diffuse configuration around diffraction spots are visible and indicate the presence of structural defects on GaHT.

Electron Microscopy. From bright field images, very agglomerated laminar or platelet configurations were frequently found and their corresponding SAED patterns revealed crystalline and polycrystalline structures, as can be observed in Figures 3-5. The shape of the crystals are regular and irregular hexagonal; i.e., the lengths of their sides are not the same. The size of the crystals ranged from 260 to 1120 nm. Accordingly, Mg-Al hydrotalcites (22) Reichle, W. T. Solid State Ionics 1986, 22, 135.

4752 Langmuir, Vol. 13, No. 17, 1997

Lo´ pez-Salinas et al.

Figure 6. High-resolution electron micrograph from a Ga-substituted hydrotalcite-like material. Defective configurations such as stacking faults, dislocations, weaving, and bent planes are observed in the image.

aged at 573 K show well-defined hexagonal crystallites.22 From electron diffraction patterns, the hydrotalcite structure was confirmed by comparing our data with those reported in the ICDD 22-700 card. Some SAED patterns (see Figure 5) showed asymmetries in the diffraction pattern configuration or streaks around diffraction spots, indicating the presence of many structural defects in crystallites. Figures 6 and 7 are high-resolution electron micrographs where the layered structure and in most cases very faulty configurations can be observed. Direct observations by HREM of the layered structure of hydrotalcite-like materials has been reported only on a triborateintercalated Mg-Al hydrotalcite-like clay.23 In Figure 7 a partial view of a 18.5 nm wide plate is shown. Zones with plane resolution marked with A and lattice resolution marked with C can be distinguished. In the zone around the one marked with B an apparent amorphous configuration is observed; close to this zone, a lattice configuration with interlayer distance dB1 ) 2.4 Å is observed. Interplanar distance values in zone A fluctuate around dA1 ) 3.0 Å and planes show a bent configuration, stacking faults, plane disruptions, and incomplete layers. In the zone identified with letter C, lattice resolution in several directions can be observed and the most common interplanar distances measured correspond to dC1 ) 3.8 Å and dC2 ) 2.4 Å. Also in this zone, dislocations, stacking faults, weaving planes, and disruptions in planes can be observed. Noteworthy of mention is the fact that the interlayer spacing of the layers around zone A (near the edge of the laminate) is approximately 1.3 times greater than that of (23) Bhattacharyya, A.; Hall, D. B. Inorg. Chem. 1992, 31, 3869.

the interlayer spacing in zone B. These results strongly suggest that layers located near the outermost edges of the crystallite are more loosely bound to each other than those found toward the bulk of the crystallite, where electrostatic attraction among layers and interlayers brings about a smaller interlayer spacing. This could be attributed to (i) a smaller Ga concentration in these peripheral layers, i.e. fewer electropositive charges, or/ and (ii) a different orientation of the interlayer chargecompensating anions and water molecules. To our knowledge, there have been no reports at all about this phenomenon on hydrotalcite or hydrotalcite-like clays. The disorder observed in the hydrotalcite-like layered structure cannot be attributed to electron irradiation damages when samples are in a hostile and aggressive environment inside the microscope column. According to our experiments, configuration of electron diffraction spots in the microscope screen and lattice resolution details observed on a TV monitor, over 1-5 min, did not show any significant variations that might be associated with radiation damage. In Figure 7 another high-resolution electron micrograph of GaHT is shown. The image corresponds to laminates less faulted than that observed in Figure 6. Here, it can be noted that defective zones (with stacking faults, bent planes, etc.), are frequently found close to the edges of the laminate. In the lower right corner we observe a zone with thin small laminates showing a more defined geometry, i.e. toothed edges. The row denoted with arrows suggests a finishing layer thicker than the neighboring layers. Also clearly evident in this zone is the fact that the direction at which layers stack upon the others changes considerably. The interlamellar

Hydrotalcite-like [Mg1-xGax(OH)2](CO3)x-2‚mH2O

Langmuir, Vol. 13, No. 17, 1997 4753

Figure 7. High-resolution electron micrograph from GaHT crystallites less faulted than the ones shown in Figure 6. Configuration variations in the crystallite borders (arrows) in the lower right corner suggest an overgrowth phenomenon.

spacing near the arrowed zones is greater than that of the bulk, as explained above. Conclusions The linear variation of the c and a parameters with different Mg/Ga molar ratios in GaHTs clearly indicate that Ga3+ cations incorporate into a hydrotalcite-like compound. The a parameter decreases when Ga content increases due to its smaller ionic radius in comparison with that of Mg2+ cations. Concomitantly, the c parameter decreases upon greater Ga content, indicating that a higher electrostatic attraction between positively charged layers and compensating negatively charged interlayers shrinks the interlamellar spacing. However, at Mg/Ga ratios between 7.7 and 12.9, the c parameter remains

constant, due probably to a dilution effect of Ga3+ cations in the brucite-like layers. Ga-substituted hydrotalcitelike crystallites have hexagonal symmetry. The lamellar structure is made up of very faulty sheets: stacking faults, plane disruptions, and incomplete layers. Interlayer spacing of peripheral layers is considerably greater than that of layers in the bulk of the crystallite, suggesting that the former layers are more loosely bound to each other in comparison with the latter layers. Acknowledgment. The technical assistance of Mr. P. Mexia and Ms. M. Chavez is highly appreciated. Part of this work was done with the financial support of the IN-106295 project from DGAPA, UNAM. LA970192K