Anion-Exchange Reactions of Layered Double Hydroxides: Interplay

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Ind. Eng. Chem. Res. 2009, 48, 6315–6320

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Anion-Exchange Reactions of Layered Double Hydroxides: Interplay between Coulombic and H-Bonding Interactions S. V. Prasanna and P. Vishnu Kamath* Department of Chemistry, Central College, Bangalore UniVersity, Bangalore-560001, Karnataka, India

The NO3-- and Cl--containing layered double hydroxides (LDHs) of Mg with Al interact weakly with anions in solution under equilibrated conditions and yield a type V isotherm. The uptake of anions under nonequilibrated conditions varies monotonically with the layer charge when the outgoing anion is NO3-, whereas it goes through a maximum when the outgoing anion is Cl-. These observations suggest that Coulombic interactions play a dominant role in Cl--LDHs, whereas weak interactions governing the mode of intercalation of NO3- ions influence the exchange reactions of NO3--LDHs. These studies have significant implications for the remediation of insidious anions by LDHs. Introduction Layered double hydroxides (LDHs) are a class of compounds having the general formula [Mg1-xAlx(OH)2](An-)x/n · yH2O (0.2 e x e 0.33; A ) NO3-, Cl-, CO32-, and others; y ) 0.5).1–3 These solids comprise a stacking of positively charged metal hydroxide layers of the composition [Mg1-xAlx(OH)2]x+, with anions included in the interlayer region. The anions are exchangeable.4 In this article, we abbreviate the formula of LDHs as Mg/Al-Ax. The exchange reactions of layered double hydroxides have evoked considerable interest on account of their potential in environmental amelioration.5–9 The anion-exchange properties of Mg/Al-Ax (x ) 0.25) LDHs have been investigated for a variety of monovalent ions.10 These LDHs were shown to exchange anions incompletely, leading to the formation of mixed-anion LDHs. The impetus for anion exchange is largely provided by the electrostatic attraction between the positively charged metal hydroxide layers and the incoming anion. The metal hydroxide layers are characterized by strong iono-covalent bonding, whereas the interlayer bonding is governed by Coulombic and H-bonding interactions.1 The Coulombic and H-bonding interactions affect the structure and reactivity of LDHs in the following ways: (i) The higher the charge on the layer, the stronger the Coulombic attraction with the interlayer ions. For a given anion, the bonding increases with x, the content of the trivalent cation.1 (ii) The higher the charge on the intercalated anion, the stronger the Coulombic attraction with the metal hydroxide layer. For a given value of x, the affinity of the LDH for different anions varies in the order CO32- > SO42- > Cl- > NO3- . I-.10,11 LDHs intercalated with divalent or trivalent anions, therefore, have a limited exchange capacity in comparison with univalent NO3-- or Cl--LDHs. (iii) The stacking sequence of the metal hydroxide layers determines the nature of the interlayer sites. Because H-bonding is directional, specific orientations of the metal hydroxide layers relative to the anions in the interlayer result in additional stabilization. This leads to anion-mediated polytype selectivity.12 (iv) For the case of monatomic anions such as chloride, bromide, and iodide, the selectivity of the interlayer sites is governed by H-bonding. Therefore, in Cl--LDHs, Cl- ions and * To whom correspondence should be addressed. E-mail: [email protected]. Tel./Fax: + 91-80-22961354.

oxygen atoms of intercalated water molecules share a single set of sites (18h) that is relatively well-suited for H-bonding. Br- and I- occupy other sites in the interlayer. (v) The introduction of large organophillic anions into the interlayer significantly weakens Coulombic interactions. This introduces turbostratic disorder and thermodynamic instability. The moderation of Coulombic forces occasionally leads to delamination of the hydroxide layers.13 Purely on the basis of the molecular formula, the sorption capacity is expected to increase with the charge on the metal hydroxide layers. However, in practice, it has been reported that the sorption capacity is a maximum at the optimal value of x ) 0.25. At x ) 0.33, the sorption capacity has been found to fall.3 A similar observation was made on cationic clays as well. Micas having the limiting charge of -2.0 per formula unit do not exchange cations, whereas other smectite-type clays with lower charges do.14 The lower sorption capacity at high values of x is generally attributed to the higher Coulombic attraction between the metal hydroxide layer and the leaving group. At lower values of x, these forces are moderate and facilitate the departure of the leaving group. In contrast to this observation, in the case of NO3--LDHs, the sorption capacity has been reported to be higher when x ) 0.33.5 In this article, we describe a systematic investigation of the sorption of a variety of anions and explain the variation of the sorption capacity of LDHs from a crystal structure and bonding viewpoint. In studying the relative contributions of H-bonding and Coulombic interactions, we also attempt to explain the selectivity of anions during exchange. This study is significant from both scientific and technological points of view, as it aids the design of suitable precursor hydroxides for effective mineralization of insidious anions from solution. Materials and Methods Preparation of LDHs. All reagents were of analytical grade (Merck, Mumbai, India) and were used without further purification. The Mg/Al-Ax LDHs (A ) NO3-, Cl-; x ) 0.25 and 0.33) were prepared by the dropwise addition (3 mL min-1) of a mixed salt solution (MgA2 + AlA3) into a reservoir containing 10 times the stoichiometric requirement of A- ions as the sodium salt. NaOH (2 M) was dispensed using a Metrohm model 718 STAT Titrino titrator to maintain a constant pH of 9. N2 gas was bubbled through the solution during precipitation and aging for 18 h at 65 °C. The precipitate was rapidly filtered

10.1021/ie9004332 CCC: $40.75  2009 American Chemical Society Published on Web 05/28/2009

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under suction and washed with deionized (15 MΩ cm specific resistance) and decarbonated water and then dried at 65 °C for 24 h. All samples were characterized by powder X-ray diffraction using a Bruker D8 Advance diffractometer (Cu KR source, λ ) 1.5405 Å, step size ) 0.02° 2θ, scan rate ) 1° 2θ per minute) operated in reflection geometry. IR spectra in transmission mode were recorded using a Nicolet Impact 400D FTIR spectrometer (4000-400 cm-1, 4 cm-1 resolution, KBr pellet). A background spectrum was recorded before spectrum collection of each sample. Thermogravimetric analysis (TGA) studies were carried out using a Mettler-Toledo 851e TG/SDTA system driven by Stare 7.1 software (heating rate ) 5 °C min-1, N2 gas). The samples were first heated to 100 °C in the TG balance for 0.5 h to drive away the adsorbed water before being ramped to 800 °C. Three types of reactions of LDHs with the incoming anions were carried out, as follows: (a) Anion-Exchange Reactions. Anion-exchange reactions were carried out by suspending preweighed (0.20 g) batches of the LDH in 30 mL of anion solution (Na+/K+ salt) containing 10 times the stoichiometric amount of anions required to effect a complete exchange of the anion in the pristine LDH. The reaction was carried out for 5 h with stirring, after which the solid was separated by centrifugation and washed with deionized and decarbonated water. The anion uptake observed in these experiments is referred to as the anion carrying capacity and is compared to the theoretical exchange capacity computed from the molecular formula. (b) Uptake Studies. Preweighed (0.20 g) batches of the LDH were suspended in 25 mL of decarbonated water and stirred for 30 min to ensure complete wetting. To this slurry was added 25 mL of anion solution (pH 8.5-9.0, concentration ) 0.003125-0.3 M), and the mixture was stirred for 5 h at the ambient temperature (22-26 °C), after which the slurry was centrifuged. The anion concentrations in the centrifugate were determined by wet chemical methods. I- and Br- were estimated by potentiometric titration versus a standard AgNO3 solution. The chromate concentration in the centrifugate was determined by means of potentiometric titration versus standard (0.025-0.1 N) ferrous ammonium sulfate (FAS) solution. The arsenate concentration in the centrifugate was determined by the silver arsenate method as described elsewhere.15 The anion uptake by the LDHs was calculated from the difference in the initial and final anion concentrations and is reported in millimoles of anion exchanged per gram of LDH taken. (c) Uptake under Equilibration Conditions. The uptake was also measured from anion solutions that were made 1 M in KNO3 or KCl to study the effect of relative activities of the anions. Under these conditions, the pristine LDH was in equilibrium with the outgoing ion. Results Given the importance of iodide sorption from wastewaters in environmental amelioration, iodide exchange for nitrate and chloride ions using the corresponding precursors was carried out as a model study. Anion-exchange reactions are generally carried out at high incoming-anion concentrations in the solution phase. These studies showed the successful exchange of the precursor anion by I-. Anion uptake was estimated by the depletion of the free I- concentration in the solution. Upon Iuptake, the interlayer spacing changed from 8.9 Å in the pristine LDH to 8.2 Å in the product LDH (Supporting Information SI.1).

Table 1. Results of Iodide Uptake Studies on Different LDHs

LDH Mg/Al-Cl (x ) 0.25) Mg/Al-Cl (x ) 0.33) Mg/Al-NO3 (x ) 0.25) Mg/Al-NO3 (x ) 0.33)

I- carrying saturation saturation uptake capacity (mmol/g uptake (mmol/g under equilibration of LDH)a of LDH) (mmol/g of LDH) 3.21 (99%) 2.42 (58.6%) 1.3 (43.3%) 3.72 (100%)

3.15 2.39 1.30 3.72

1.23 0.0 0.0 1.89

a Values in the parentheses correspond to percentages of the stoichiometric I- content.

Table 2. Kd (g-1 cm3) Values Obtained in Sorption Studies on Mg/Al-Cl and Mg/Al-NO3 LDHsa LDH

Iuptake

Bruptake

CrO42uptake

AsO43uptake

Mg/Al-Cl (x ) 0.25) Mg/Al-Cl (x ) 0.33) Mg/Al-NO3 (x ) 0.25) Mg/Al-NO3 (x ) 0.33)

3848 3110 0 4622

7221 4996 4626 6685

3531 1738 1216 3618

0 0 0 7139

a

Evaluated at 3 mM anion concentration.

Iodide exchange is complete in Mg/Al-Cl (x ) 0.25) but is substoichiometric in Mg/Al-Cl (x ) 0.33). However, the results are quite the opposite in the Mg/Al-NO3 LDHs, where the x ) 0.33 composition completely exchanges I- but not the x ) 0.25 composition, for which the exchange is substoichiometric (43% of the theoretical exchange capacity). The I- carrying capacity varies in the order Mg/Al-NO3 (x ) 0.33) > Mg/ Al-Cl (x ) 0.25) > Mg/Al-Cl (x ) 0.33) > Mg/Al-NO3 (x ) 0.25) (Table 1). The Kd values evaluated at 3 mM I- concentration (Table 2) also yielded a decreasing order of affinity for I- as Mg/Al-NO3 (x ) 0.33) > Mg/Al-Cl (x ) 0.25) > Mg/Al-Cl (x ) 0.33) > Mg/Al-NO3 (x ) 0.25) ) 0. Having successfully achieved iodide exchange at high Iconcentration, we studied the I- uptake characteristics of Mg/ Al-Cl and Mg/Al-NO3 LDHs for a range of I- concentrations (Figure 1). The uptake of I- increases with concentration of Iin solution and reaches a limiting value for an I- ion concentration of 0.15-0.20 M. The limiting value refers to saturation uptake. It is clear from these data that the Mg/Al-Cl (x ) 0.25) LDH has a higher I- saturation uptake (3.15 mmol per gram of LDH) compared to the Mg/Al-Cl (x ) 0.33) LDH (2.39 mmol per gram of LDH). The saturation uptake equals the theoretical exchange capacity in the case of the Mg/Al-Cl (x ) 0.25) LDH, whereas it is only 58% of the theoretical exchange capacity in Mg/Al-Cl (x ) 0.33) LDH (Table 1), showing that not all of the Cl- is exchanged in the latter. A study of the Mg/Al-NO3 LDHs revealed a completely different set of results. The Mg/Al-NO3 (x ) 0.33) LDH showed a higher I- saturation uptake of 0.33 mol of I- per mole of LDH (3.71 mmol per gram of LDH), equal to the theoretical exchange capacity. This shows quantitative exchange of NO3for I-, a fact borne out by IR spectra (Supporting Information SI.2). The υ3 mode of NO3- (1385 cm-1) is completely extinguished after exchange. Wet chemical analysis of the exchanged product also showed the quantitative uptake of I-. In contrast, the I- saturation uptake of Mg/Al-NO3 (x ) 0.25) LDH is 43% of the theoretical exchange capacity, showing an incomplete anion exchange. The incomplete exchange is evident from wet chemical analysis and IR spectral measurements where the NO3- absorptions are not completely extinguished. The uptake characteristics are also different, showing negligible uptake until a critical I- concentration ([I-] > 0.1 M) is used. A rapid increase is seen in the range 0.1 M < [I-]

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Figure 1. Iodide uptake isotherms of (a) Mg/Al-Cl (x ) 0.25), (b) Mg/Al-Cl (x ) 0.33), (c) Mg/Al-NO3 (x ) 0.25), and (d) Mg/Al-NO3 (x ) 0.33) LDHs. Insets show isotherms obtained under equilibrated conditions.

< 0.2 M. This shows that the nature of the adsorbate-adsorbent interaction in this system is different from those in the others. These experimental conditions do not correspond to equilibrium conditions, and these data cannot be likened to the adsorption isotherms. Therefore, the iodide sorption was carried out for the two most active LDHs, namely, Mg/Al-Cl (x ) 0.25) and Mg/Al-NO3 (x ) 0.33), from mixed I-/A (A ) Cl, NO3) solutions where the concentrations of A- was held close to 1 M. These results now correspond to I- sorption under equilibrium conditions. The resulting isotherms are given as insets in Figure 1. The isotherms are of type V. The type V isotherms suggest a weak interaction between the adsorbent and the adsorbate. The I- uptake remains constant within experimental errors even when the sorption temperature is increased to 65 °C, showing that the sorption is not thermally activated. Analogous results were obtained in the uptake studies of Brby the Cl--LDHs (Supporting Information SI.3). To verify that these characteristics differ when the charge on the incoming ion increases, sorption and anion-exchange reactions were carried out with CrO42- and AsO43- ions. The exchange reactions were expected to be more facile with increasing charge of the incoming anion. However, the chromate and arsenate uptakes by Cl-- and NO3--LDHs were along the lines of the I- and Br- uptake characteristics. The results are given in Figures 2 and 3. The chromate uptake by the Mg/Al-NO3 and Mg/Al-Cl LDHs yielded type I isotherms. The saturation uptake for the Mg/Al-NO3 LDHs is achieved below 0.02 M, whereas a higher concentration (0.05 M) is required for Mg/Al-Cl LDHs. The

saturation uptake follows the order Mg/Al-NO3 (x ) 0.33) > Mg/Al-Cl (x ) 0.25) > Mg/Al-Cl (x ) 0.33) > Mg/Al-NO3 (x ) 0.25). The Kd values calculated at 3 mM anion concentrations are presented in Table 2. The uptake of arsenate increases with concentration of anion in solution and reaches a limiting value for concentrations of 0.05-0.10 M for the Mg/Al-NO3 (x ) 0.33) LDH. The isotherm is of type I. The limiting value is highest for the Mg/Al-NO3 (x ) 0.33) LDH (1.46 mmol/g of LDH) and equals the theoretical exchange capacity showing quantitative exchange of NO3- for arsenate. Incomplete arsenate exchange was observed for Mg/Al-NO3 (x ) 0.25) LDH. The uptake characteristics of Mg/Al-Cl (x ) 0.25) and Mg/ Al-Cl (x ) 0.33) LDHs show a negligible uptake at low arsenate concentrations. The arsenate uptake begins only at an initial concentration of 0.15 M and reaches saturation values of 1.2 and 0.8 mmol/g of LDH, respectively, for the two Cl--LDHs. The lower saturation uptake for Cl--LDHs suggests that under these conditions, Cl- is a poorer leaving group. Sorption capacity is generally correlated with the surface area of the sorbent. However, among LDHs where the sorption is based on anion exchange, the surface involved is structural rather than morphological. Therefore, the sorption capacity does not show any meaningful correlation with the BET (Brunauer-Emmett-Teller) surface area, as the latter measures the area of the morphological surface.5

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Figure 2. Chromate uptake isotherms of Mg/Al-NO3 and Mg/Al-Cl LDHs. Inset shows the isotherm obtained under equilibration for the Mg/Al-Cl (x ) 0.25) LDH.

Figure 3. Arsenate uptake isotherms of Mg/Al-NO3 and Mg/Al-Cl LDHs.

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These observations suggest that factors other than Coulombic forces play a critical role in the exchange of nitrate ions by the LDH. The uptake characteristics give rise to the following questions: (1) Why is the leaving group tendency of the NO3- ion higher when x ) 0.33 than in the LDH with x ) 0.25? Why is this trend reversed in Mg/Al-Cl LDH? (2) Do the leaving group tendencies of Cl- and NO3- ions have a crystal chemical basis? (3) What are the relative roles of the Coulombic and H-bonding interactions in determining the leaving group tendencies and exchange capacities of the LDHs? Discussion Direct thermochemical measurements show that NO3-containing LDHs are thermodynamically less stable than Cl-LDHs.16 These measurements were made for the x ≈ 0.25 composition. The relative stabilities of the x ) 0.33 composition are not known. The major difference between the Cl- and NO3- ions is their ability to form hydrogen bonds. Cl- ions are not known to form H-bonds, whereas NO3- ions form H-bonds with the hydroxyl groups of the metal hydroxide layer. H-bonds are highly directional and are formed only when the NO3- ions are oriented suitably in the interlayer. The anion-exchange reactions are entropy-driven, whereas the selectivity is governed by enthalpy contribution.17 In the Mg/Al-NO3 (x ) 0.25) LDH, the NO3- ion is intercalated with its plane perpendicular to the c crystallographic axis, whereas in Mg/Al-NO3 (x ) 0.33) LDH, the nitrate is intercalated with one of its N-O bonds (C2 axis) parallel to the c crystallographic axis. The coordination symmetries are D3h and C2V, respectively.18 The D3h coordination symmetry of NO3- in the Mg/Al-NO3 (x ) 0.25) LDH matches exactly with the symmetry of the interlayer site and achieves stabilization by H-bonding. Under these circumstances, the nitrate is a poor leaving group, and there is no anion uptake at lower concentrations. The orientation of NO3- ion changes at x ) 0.33 because of the requirement of accommodating a larger number of anions in the interlayer. The variation in the c parameter and structural disorder in Mg/ Al-NO3- with change in x is described elsewhere and needs no repetition here.5 In the Mg/Al-NO3 (x ) 0.33) LDH, the NO3- ion is no longer stabilized by H-bonding. The mismatch in symmetry between the coordination symmetry (C2V) and the symmetry of the interlayer site (D3h) induces turbostratic disorder, thereby weakening the interactions with the metal hydroxide layer. The NO3- ion in C2V symmetry loses most of its degrees of freedom in this configuration. This contributes to the positive entropy change during exchange, resulting in a facile intercalation of incoming anions in the interlayer. Therefore, in the absence of H-bonding, NO3- is a good leaving group and shows stoichiometric exchange of NO3- for other anions. Although the contribution of H-bonding to enthalpy is small, it appears that its contribution to the leaving group tendencies and exchange capacities is significant. The situation is different in the case of Cl--LDHs where the effects of orientation and H-bonding are nonexistent. The Cloccupies the 18h site in the interlayer, which is relatively close to the metal in the hydroxide layer.19 Therefore, the interaction of Cl- with the metal hydroxide layer is largely Coulombic in nature. With an increase in x value from 0.25 to 0.33, there is an increase in the Coulombic interaction, which constrains the

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facile departure of the Cl ion. As a result, Mg/Al-Cl (x ) 0.33) LDHs interact weakly with incoming anions (Br-, I-, CrO42-, and AsO43-), as inferred from the type V isotherms obtained (see the insets of Figures 1 and 2). Conclusions The anion-exchange capacity of LDHs is governed by the relative contributions of H-bonding and Coulombic interactions of the intercalated anions in the pristine LDHs. The H-bonding is the critical factor in exchange reactions of NO3--LDHs, whereas Coulombic interactions dominate in Cl--LDHs. The magnitude of these interactions depends on the orientation of anions and, indeed, has a crystal chemical basis. Acknowledgment The authors thank the Department of Science and Technology (DST), Government of India (GOI), for financial support. P.V.K. is a recipient of the Ramanna Fellowship of the DST. S.V.P. thanks the Council of Scientific and Industrial Research (CSIR), GOI, for the award of Senior Research Fellowship (SRF). Supporting Information Available: PXRD patterns and IR spectra of Mg/Al-NO3 LDH before and after I- exchange and Br- uptake isotherms of Mg/Al-NO3 and Mg/Al-Cl LDHs. This material is available free of charge via the Internet at http:// pubs.acs.org. Literature Cited (1) Cavani, F.; Trifiro`, F.; Vaccari, A. Hydrotalcite-Type Anionic Clays: Preparation, Properties and Applications. Catal. Today 1991, 11, 173. (2) Trifiro`, F.; Vaccari, A. Hydrotalcite-like anionic clays (layered double hydroxides). In ComprehensiVe Supramolecular Chemistry; Alberti, G., Bein, T., Eds.; Pergamon Press: Oxford, U.K., 1997; Vol. 7, Chapter 8, p 251–291. (3) de Roy, A.; Forano, C.; Besse, J. P. Layered Double Hydroxides: Synthesis and Post-Synthesis Modification. In Layered Double Hydroxides: Past and Future; Rives, V., Ed.; Nova Science: New York, 2001; Chapter 1, pp 1-39. (4) Miyata, S. Anion exchange properties of hydrotalcite-like compounds. Clays Clay Miner. 1983, 31, 305. (5) Prasanna, S. V.; Kamath, P. V. Chromate uptake characteristics of pristine layered double hydroxides of Mg and Al. Solid State Sci. 2008, 10, 260. (6) Prasanna, S. V.; Kamath, P. V. Synthesis and characterization of arsenate-intercalated layered double hydroxides (LDHs): Prospects for arsenic mineralization. J. Colloid Interface Sci. 2009, 331, 439. (7) Lv, L.; He, J.; Wei, M.; Duan, X. Kinetic Studies on Fluoride Removal by Calcined Layered Double Hydroxides. Ind. Eng. Chem. Res. 2006, 45, 8623. (8) Yang, L.; Dadwhal, M.; Shahrivari, Z.; Ostwal, M.; Liu, P. K. T.; Sahimi, M.; Tsotsis, T. T. Adsorption of Arsenic on Layered Double Hydroxides: Effect of the Particle Size. Ind. Eng. Chem. Res. 2006, 45, 4742. (9) Biswas, K.; Saha, S. K.; Ghosh, U. C. Adsorption of Fluoride from Aqueous Solution by a Synthetic Iron(III)-Aluminum(III) Mixed Oxide. Ind. Eng. Chem. Res. 2007, 46, 5346. (10) Bontchev, R. P.; Liu, S.; Krumhansl, J. L.; Voigt, J.; Nenoff, T. M. Synthesis, characterization and ion exchange properties of Hydrotalcite Mg6Al2(OH)16(A)x(A′)2-x · 4H2O (A, A′ ) Cl-, Br-, I- and NO3-, 2 g x g 0 derivatives). Chem. Mater. 2003, 15, 3669–3675. (11) Chatelet, L.; Bottero, J. V.; Yvon, J.; Bouchelaghem, A. Competition between monovalent and divalent anions for calcined and uncalcined hydrotalcite: Anion exchange and adsorption sites. Colloids Surf. A 1996, 111, 167. (12) Ramesh, T. N.; Rajamathi, M.; Kamath, P. V. Anion mediated polytype selectivity among the basic salts of Co(II). J. Solid State Chem. 2006, 179, 2386.

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(13) Hibino, T. Delamination of Layered Double Hydroxides Containing Amino Acids. Chem. Mater. 2004, 16, 5482. (14) Pinnavaia, T. J. Intercalated clay catalysts. Science 1983, 220, 365. (15) Bassett, J.; Denney, R. C.; Jeffery, G. H.; Mendham, J. In Vogel’s Textbook of QuantitatiVe Inorganic Analysis, 4th ed.; Longman: London, 1978; pp344-345. (16) Allada, R. K.; Pless, J. D.; Nenoff, T. M.; Navrotsky, A. Thermochemistry of Hydrotalcite-like phases intercalated with CO32-, NO3-, Cl-, I- and ReO4-. Chem. Mater. 2005, 17, 2455–2459. (17) Israeli, Y.; Taviot-Gueho, C.; Besse, J. P.; Morel, J. P.; MorelDesrosiers, N. M. Thermodynamics of anion exchange on a chlorideintercalated zinc-aluminum layered double hydroxide: A microcalorimetric study. J. Chem. Soc., Dalton Trans. 2000, 791.

(18) Wang, S. L.; Wang, P. C. In situ XRD and ATR-FTIR study on the molecular orientation of interlayer nitrate in Mg/Al-layered double hydroxides in water. Colloids Surf. A 2007, 292, 131. (19) Ennadi, A.; Legrouri, A.; de Roy, A.; Besse, J. P. X-ray diffraction pattern simulation for thermally treated [Zn-Al-Cl] layered double hydroxide. J. Solid State Chem. 2000, 152, 568.

ReceiVed for reView March 17, 2009 ReVised manuscript receiVed May 7, 2009 Accepted May 11, 2009 IE9004332