and 1H Polytypes of Sulfate-Intercalated Layered Double Hydroxides

Feb 1, 2012 - ... Group, Department of Chemistry, St. Joseph,s College, 36 Langford Road, Bangalore 560027, India ... photochemistry, and medicine to ...
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Synthesis of 3R1and 1H Polytypes of Sulfate-Intercalated Layered Double Hydroxides (LDHs) by Postintracrystalline Oxidation and Simultaneous Intercalation-Oxidation of Thiosulfate Nygil Thomas* Materials Research Group, Department of Chemistry, St. Joseph’s College, 36 Langford Road, Bangalore 560027, India S Supporting Information *

ABSTRACT: Well ordered 3R1 polytypes of sulfate-intercalated Zn−Al and Zn− Cr LDHs could be synthesized at room temperature by intracrystalline oxidation of thiosulfate-intercalated LDHs by hydrogen peroxide. When the intercalationoxidation sequence was slightly modified in such a way that the intercalation and oxidation of thiosulfate took place simultaneously, 1H polytypes were formed. The formation of these polytypes is confirmed by simulation of powder X-ray diffraction patterns and phase transformation on heating. Interestingly, the two polytypes show different anion exchange behavior.

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polytypes in LDHs.9,10 They followed a nomenclature based on the stacking sequence of layers. According to them, hydrotalcite can be represented by the symbol 3R1 and manasseite by 2H1. The rhombohedral (R) cell comprises three metal hydroxide layers and the hexagonal (H) cell comprises two. The identity of the polytypic form of the given LDH sample could be verified by the careful examination of the PXRD pattern.10 Especially the positions of (01S ) and (10S ) reflections at mid 2θ values are useful in determining the stacking sequence of the layers.9,10 While it is easy to arrive at the polytypic structures of natural minerals, it is difficult, and at times impossible, to assign polytypic structure to synthetic LDHs, which present various kinds of stacking disorders.10 It is possible to synthesize certain polytypes in the case of a few LDH systems by varying the reaction conditions. In the case of the Li−Al−Cl LDH system, starting from different Al(OH)3 precursors, rhombohedral and hexagonal polytypes could be obtained.11 These two polymorphs were shown to differ in their intercalation behavior.12 Hydrothermal synthesis of an uncommon 3R2 polytype of hydrotalcite mineral has also been reported.13 It has been suggested by Radha et al. that anions mediate the polytype selectivity in LDHs.14 According to them, anions select polytypes whose interlayer site symmetry best matches the symmetry of the anion. A polytype can be converted to another by changing temperature and pressure. There are many examples wherein a polytype could be converted into another by heating to a higher temperature.9,10 For example, Ni−Co−CO3 LDH, having the 3R1 structure, could

INTRODUCTION Layered double hydroxides (LDHs), also known as hydrotalcite-like compounds (HTLCs) or anionic clays are layered compounds that consist of positively charged metal hydroxide sheets with intercalated anions and water molecules in the interlayer region.1,2 The general formula of LDHs is [MII(1−x)MIIIx(OH)2]An−x/n·mH2O, where MII is a divalent metal such as Mg, Co, Ni, Cu, Zn, or Ca; MIII is a trivalent metal such as Al, Cr, Fe, or Ga; An− is an anion with a valency n; and x, defined as [MIII]/([MIII] + [MII]), is usually between 0.25 and 0.33. These compounds derive their structure from that of brucite, Mg(OH)2. In brucite-like hydroxides, OH− ions are hexagonally close-packed and the M2+ ions occupy alternate layers of octahedral sites. Thus, the structure can be described as a stacking of charge neutral M(OH)2 layers. In LDHs a part, x, of the M2+ ions are isomorphously substituted by M3+ ions leading to positively charged layers having the composition [M2+(1−x)M3+x(OH)2]x+. To compensate for the positive charge on the layers, anions, An− are intercalated in the interlayer region. LDHs show interesting properties such as anion mobility, anion exchange, surface basicity, and reconstruction behavior. Becuase of these properties, LDHs find applications in varied fields such as sorption, catalysis, polymer stabilization, electrochemistry, photochemistry, and medicine to cite a few.3−8 The metal hydroxide (M(OH)2) layers that consist of closepacked hydroxide ions with cations occupying octahedral sites can be stacked in a number of different ways to obtain different polytypic structures.9 The minerals, hydrotalcite and manasseite, having the general composition [Mg2+(1−x)Al3+x(OH)2]x+[CO32−x/2·nH2O]x− occur in two polymorphic modifications. The former crystallizes in a rhombohedral form, while the latter crystallizes in hexagonal form.10 Bookin and Drits derived all the theoretically possible © 2012 American Chemical Society

Received: November 3, 2011 Revised: January 5, 2012 Published: February 1, 2012 1378

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be converted to a 1H polytype by heating at 200 °C.15 A synthetic hydrotalcite was shown to transform irreversibly to manassite-like phase when compressed to a pressure of 1.5 GPa.16 Many sulfate intercalated LDHs, especially Zn−Al and Zn− Cr LDHs, have been studied extensively due to their flexible polytypic behavior. 3R1 and 1H are the commonly observed polytypes in Zn−Al and Zn−Cr LDH systems.17,18 Different polytypes may have different physical as well as chemical properties. Hence, a synthetic method for preparing different polytypes of LDH would be of considerable interest. In the recent past, we have been studying the intracrystalline reactions of the intercalated inorganic anions in the interlayer of anionic clays.19,20 While studying the intracrystalline oxidation of thiosulphate intercalated LDHs to sulfate intercalated LDHs, we found that 3R1 and 1H polytypes of the products could be obtained when the conditions of the intracrystalline reactions were slightly modified.

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Anion Exchange Reactions of the Polytypes of LDHs. About 0.2 g of each of ZnAl−SO4-I, ZnCr−SO4-I, ZnAl−SO4-II, and ZnCr− SO4-II was stirred with an aqueous solution of sodium oxalate (0.12 g in 50 mL) in an airtight container overnight. The resultant solid was filtered, washed with decarbonated water followed by acetone, and dried at room temperature. Characterization. Powder X-ray diffraction (PXRD) measurements were performed on a PANalytical Xpert Pro X-ray Diffractometer using CuKα radiation (λ = 0.154 nm) at 40 kV, at a scanning rate of 2 deg/min−1. The infrared (IR) spectra of samples were collected using a Nicolet IR200 FTIR spectrometer using KBr pellets, in the range 4000 to 400 cm−1 with 4 cm−1 resolution. The interlayer anion (chloride/ thiosulfate/sulfate/oxalate) contents were obtained by ion chromatography (IC) using a Metrohm 861 Advanced Compact ion chromatograph with Metrosep A Supp5 250 anion column and conductivity detector. The samples were dissolved in 1 N acetic acid or hydrochloric acid and diluted suitably for this purpose. Thiosulfate reacts slowly with dilute hydrochloric acid to give SO2 and sulfur. To minimize the error due to this reaction, in the case of thiosulfate estimation, the standard thiosulfate solutions were also treated with hydrochloric acid before being injected into the chromatograph. The metal contents of the samples were estimated by atomic absorption spectroscopy (Varion AA240). The approximate formulas of the compounds were calculated by setting [OH] = 2([Zn] + [Al/Cr]); [carbonate] = [Al/Cr] − n[An−], An− = Cl−, S2O32−, or SO42−]; and the unaccounted mass to water. PXRD Simulations. PXRD patterns of different polytypes were calculated using a FORTRAN based computer program, DIFFaX (version 1.807).22,23 In this program, a crystalline solid is treated as a stacking of layers of atoms. The scattering contributions are computed layer by layer and integrated over a number of layers. This allows us to stack the LDH layers with different stacking sequences to generate the PXRD patterns of different polytypes.24 The generated patterns are compared with the experimental diffraction pattern of the synthesized LDH to arrive at the polytype. For the structural model of 1H and 3R polytype, the published models (polytype 1H CC No. 75542 and 3R1 CC No. 91859) were used.

EXPERIMENTAL SECTION

Preparation of Zn2Al(OH)6Cl·mH2O and Zn2Cr(OH)6Cl·mH2O. LDH of the composition Zn2Al(OH)6Cl·mH2O, hereafter referred to as ZnAl-Cl, was prepared by the procedure of Bonnet et al.21 An aqueous solution (100 mL) containing ZnCl2 and AlCl3 in the molar ratio 2:1 and 2 M NaOH solution were added simultaneously to a flask containing 100 mL of deionized water. The rates of addition of the solutions were such that the pH was maintained at 7.0 using a pH stat. Rapid stirring and nitrogen atmosphere was maintained throughout the addition. After the addition was complete, the resultant slurry was aged at 65 °C overnight. The solid product obtained was washed free of ions with decarbonated water followed by acetone and dried at 65 °C in an air oven to constant mass. Another LDH of the composition Zn2Cr(OH)6Cl·mH2O, hereafter referred to as ZnCr−Cl, was also synthesized in a similar manner except that the starting solutions were of ZnCl2 and CrCl3 and pH was maintained at 5.0. Preparation of Zn 2 Al(OH) 6 (S 2 O 3 ) 0.5·mH 2 O and Zn 2 Cr(OH)6(S2O3)0.5·mH2O. The thiosulfate-intercalated LDHs were obtained through anion exchange reaction of the chloride-intercalated LDHs. About 1 g of the chloride LDH was stirred with an aqueous solution of sodium thiosulfate (2 g in 50 mL) in an airtight container for two days. The resultant solid was filtered, washed with decarbonated water followed by acetone, and dried at 65 °C to constant mass. The samples obtained are referred to as ZnAl−S2O3 and ZnCr−S2O3. Formation of Polytypes of Zn2Al(OH)6(SO4)0.5·mH2O and Zn2Cr(OH)6(SO4)0.5·mH2O LDHs. Two methods were employed to form the SO42− intercalated LDHs. In the first method, which we call intercalation-oxidation method, 2 mL of 30% H2O2 solution was added to a suspension of ZnAl−S2O3 or ZnCr−S2O3 LDH (0.1 g in 10 mL of decarbonated water). The mixture was stirred under nitrogen atmosphere for 6 h. The products were isolated by centrifugation washed with decarbonated water and dried in air at room temperature. The samples obtained are called ZnAl−SO4-I and ZnCr−SO4-I. In the second method, called simultaneous intercalation and oxidation method, 1 mL of 30% H2O2 and 0.25 g of Na2S2O3 were simultaneously added to a suspension of ZnAl−Cl or ZnCr−Cl (0.1 g in 10 mL decarbonated water), and the mixture was stirred in a sealed vessel for 6 h. The products were isolated by centrifugation washed with decarbonated water and dried at room temperature. These samples are called ZnAl−SO4-II and ZnCr−SO4-II.

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RESULTS AND DISCUSSION The compositional analysis data of the precursors LDHs is given in Table 1. The nominal chemical formulas obtained indicate that the LDHs were formed with the expected Zn/MIII ratios close to 2. The PXRD patterns of ZnAl−Cl and ZnAl−S2O3 LDHs are shown in Figure 1. ZnAl−Cl LDH (Figure 1a) has a basal spacing of 7.7 Å (calculated from the 00S reflections), as expected for a chloride-intercalated LDH. The pattern can be indexed to a 3R1 polytype. ZnAl−S2O3 (Figure 1b) obtained on anion exchange shows a basal spacing of 8.9 Å. This interlayer spacing matches with what has been reported for thiosulfateintercalated Mg−Al LDH.25 The PXRD patterns of ZnCr−Cl and ZnCr−S2O3 LDHs are shown in Figure 1. The typical features are nearly the same as that of Zn−Al LDHs. ZnCr−Cl (Figure 1c) can be indexed to a 3R1 polytype as in case of ZnAl−Cl. ZnCr−S2O3 LDH (Figure 1d) shows a basal spacing of 8.8 Å. The IR spectra of ZnAl−Cl and ZnAl−S2O3 LDHs are shown in Figure 2. The broad absorption at 3500 cm−1 in both the samples is due to O−H stretching vibration of the hydroxyl

Table 1. Chemical Composition of the LDHs Used in This Study mass percentage sample

Zn

Al

ZnAl−Cl ZnCr−Cl

36.7 33.2

9.7

Cr

An−

nominal formula

15.9

10.7 8.7

Zn2Al1.3(OH)6.6Cl1.1(CO3)0.1·2H2O Zn2Cr1.2(OH)6.4Cl0.98(CO3)0.1·2.8H2O

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are observed at 995 and 1137 cm−1. These values match well with what has been reported for thiosulfate-intercalated Mg−Al LDH.25 The IR spectra of ZnCr−Cl and ZnCr−S2O3 LDHs are shown in Figure 3. In ZnCr−S2O3 LDH (Figure 3b), the

Figure 3. IR spectra of ZnCr−Cl (a), ZnCr−S2O3 (b), ZnCr−SO4 obtained after oxidizing ZnCr−S2O3 (c), and ZnCr−SO4 obtained after oxidizing ZnCr−Cl in the presence of Na2S2O3 (d).

Figure 1. PXRD patterns of ZnAl−Cl (a), ZnAl−S2O3 (b), ZnCr−Cl (c), and ZnCr−S2O3 (d).

symmetric and antisymmetric stretching vibrations of thiosulfate are observed at 1004 and 1147 cm−1. There is a small peak at 1353 cm−1 in both thiosulphate intercalated Zn-Al and Zn-Cr samples, indicating slight carbonate contamination. Sulfate LDHs were prepared by two different oxidation methods using H2O2. In the first method, H2O2 was added to a suspension of ZnAl−S2O3 or ZnCr−S2O3 in water to effect a topotactic interlayer oxidation of the thiosulfate ions to sulfate ions. In the second method, H2O2 and Na2S2O3 were simultaneously added to a suspension of ZnAl−Cl or ZnCr−Cl in water such that intercalation and oxidation occurred simultaneously. The sulfate intercalated LDHs formed by these two methods were found to be different polymorphs. The PXRD pattern of the product obtained on interlayer oxidation of ZnAl−S2O3 is given in Figure 4a. A careful examination of the peaks in the mid 2θ region reveals that it is a 3R1 polytype. The simulated pattern for a 3R1 polytype of Zn− Al LDH is given in Figure 4b. The inset shows the overlay of the experimental and simulated pattern in the region of 2θ = 30−70. The basal reflections have been omitted for clarity. The experimental patterns matches well with the simulated pattern for the 3R1 polytype. The peaks are considerably broadened in the experimental sample possibly due to various stacking faults. The PXRD pattern of the product obtained on simultaneous intercalation and oxidation of thiosulphate in Zn−Al LDH is given in Figure 4c. The product obtained was a phase pure and ordered sulfate intercalated LDH. In this case, even though the positions of the basal reflections are same as those in the product obtained on interlayer oxidation (Figure 4a), the positions of the peaks in the mid 2θ region are different. These peak positions indicate the formation of 1H polytype. The simulated DIFFaX pattern and the overlay of the simulated and the experimental patterns are shown in Figure 4d.

Figure 2. IR spectra of ZnAl−Cl (a), ZnAl−S2O3 (b), ZnAl−SO4 obtained after oxidizing ZnAl−S2O3 (c), and ZnAl−SO4 obtained after oxidizing ZnAl−Cl in the presence of Na2S2O3 (d).

groups of the brucite-like sheets and water in the interlayer space. The O−H bending vibration of the interlayer water is observed at 1630 cm−1. The IR spectrum of ZnAl−Cl LDH (Figure 2a) has no absorption in the range of 900−1500 cm−1 as expected of a chloride-intercalated LDH. There is no absorption at 1356 cm−1, confirming the complete absence of carbonate ions in the LDH. In ZnAl−S2O3 LDH (Figure 2b), the symmetric and antisymmetric stretching vibrations of thiosulfate 1380

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The insets show the overlay of the experimental and simulated patterns in the region of 2θ = 30−70°. The IR spectrum of the ZnAl−S2O3 oxidized by H2O2 is given in Figure 2c. We observe a strong absorption at 1117 cm−1 due to the sulfate ions. There is no peak at 1356 cm−1 confirming the absence of carbonate ions in these samples. The IR spectrum (Figure 2d) of the sample obtained by the oxidation of ZnAl−Cl in the presence of thosulphate has similar IR characteristics. This confirms that the symmetry of sulfate anions was not changed when the oxidation conditions were altered. The IR spectrum of the ZnCr−S2O3 oxidized by H2O2 is given in Figure 3c. Here, once again only one peak due to the SO42− anion was observed. Oxidation of ZnCr−Cl in the presence of thosulphate has only one characteristics peak of sulfate (Figure 3d). The peak is slightly broad compared to the Zn−Al system. Anion exchange behavior of the two (3R1 and 1H) polytypes of ZnAl and ZnCr was investigated. While carbonate exchange was total in both the polytypes, the oxalate exchange took place only partially. The percentage anion exchange is given in Table 2. Interestingly, the anion exchange behavior of 3R1 and 1H polytypes are considerably different as seen from Table 2.

Figure 4. PXRD patterns of ZnAl−SO4 obtained after oxidizing ZnAl−S2O3 (a); corresponding simulated pattern for 3R1 polytype (b); ZnAl−SO4 obtained oxidizing ZnAl−Cl in presence of Na2S2O3 (c); corresponding simulated for 1H polytype (d); insets show the overlay of the experimental and simulated pattern in the region of 2θ = 30−70.

Table 2. Mole Percentage of Anions after Anion Exchange Reactions of 3R1 and 1H Polytypes with Oxalate

The formation of polytypes by the oxidation of thiosulfate after or during intercalation is not limited to the Zn−Al LDH system alone. We observed the formation of 3R1 and 1H polytypes in the two different oxidation methods even in the case of Zn−Cr LDH. Oxidation of ZnCr−S2O3 by hydrogen peroxide yielded a 3R1 polytype (Figure 5a), while simulta-

mole percentage of interlayer anions sample

sulfate

oxalate

ZnAl−SO4-3R1 ZnAl−SO4-1H ZnCr−SO4-3R1 ZnCr−SO4-1H

29 40 29 47

71 60 71 53

In both cases, 3R1 samples showed a higher percentage of anion exchange compared to 1H samples. This was further verified by the IR spectra of these samples. The ratio of intensities of CO stretching of oxalate to S−O stretching of sulfate in 3R1 sample (Figure 6a,b) is considerably high compared to that of the 1H samples (Figure 6c,d). While there is a report on different anion exchange behaviors of polytypes of Li−Al LDHs,13 this is the first observation of such a behavior in II−III LDHs. Possibly the 1H orientation is unsuitable to fit the oxalate ions in the interlayer. In order to understand the thermal behavior of above polytypes, the samples were heated at elevated temperatures. The ZnCr−SO4 sample with 3R1 structure was heated to 30 °C, resulting in the formation of a small amount of a 8.9 Å phase along with the parent phase (Figure S1, Supporting Information). The contraction of basal spacing is due to the removal of an additional layer of water molecules from the interlayer. The 012 and 015 reflections were still present in the sample indicating that the phase retains its 3R1 structure. Upon heating the sample further to 45 °C, resulted in the formation of a 3R1 phase with basal spacing of 9.0 Å (Figure S1, Supporting Information). The ZnCr−SO4 sample with 1H pattern (Figure S2, Supporting Information) also was subjected to a similar thermal treatment as that of 3R1. Upon heating the sample from room temperature to 40 °C, the structure remained as 1H (Figure S2, Supporting Information). The 100, 101, and 102 reflections were still present in the sample. An interesting point observed was that even the basal spacing was not changed during heating up to 100 °C. This shows that the

Figure 5. PXRD patterns of ZnCr−SO4 obtained after oxidizing ZnCr−S2O3 (a); corresponding simulated pattern for 3R1 polytype (b); ZnCr−SO4 obtained after oxidizing ZnCr−Cl in presence of Na2S2O3 (c); corresponding simulated pattern for 1H polytype (d); insets show the overlay of the experimental and simulated pattern in the region of 2θ = 30−70.

neous intercalation and oxidation of thiosulphate in ZnCr−Cl produced a 1H polytype (Figure 5c). The simulated DIFFaX patterns for 3R1 and 1H polytypes are shown in Figure 5b,d. 1381

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(3) Miyata, S. Clays Clay Miner. 1983, 31, 305. (4) Khan, A. I.; O’Hare, D. J. Mater. Chem. 2002, 12, 3191. (5) Laguna, H.; Loera, S.; Ibarra, I. A.; Lima, E.; Vera, M. A.; Lara, V. Microporous. Mesoporous. Mater. 2007, 98, 234. (6) Wang, Y.; Qu, J.; Liu, H.; Hu, C. Catal. Today. 2007, 126, 476. (7) Kim, S. J. Polym. Sci., Part B 2003, 41, 936. (8) Vaccari, A. Appl. Clay Sci. 1999, 14, 161. (9) Drits, V. A.; Bookin, A. S. Crystal Structure and X-ray Identification of Layered Double Hydroxides. In Layered Double Hydroxide: Present and Future; Rives, V., Ed.; Nova Science: New York, 2001; pp 39−92. (10) Bookin, A. S.; Drits, V. A. Clays Clay Miner. 1993, 41, 551. (11) Fogg, A. M.; Freij, A. J.; Parkinson, G. M. Chem. Mater. 2002, 14, 232. (12) Besserguenev, A. V.; Fogg, F. M.; Francis, R. J.; Price, S. J.; O’Hare, D.; Isupov, V. P.; Topochko, B. P. Chem. Mater. 1997, 9, 241. (13) Newman, S. P.; Jones, W.; O’Connor, P.; Stamires, D. N. J. Mater. Chem. 2002, 12, 153. (14) Radha, A. V.; Kamath, P. V.; Shivakumara, C. J. Phys. Chem. B. 2007, 111, 3411. (15) Pausch, I.; Lohse, H. H.; Schurmann, K.; Allmann, R. Clays Clay Miner. 1986, 34, 507. (16) Parthasarathy, G.; Kantam, M. L.; Choudary, B. M.; Reddy, C. V. Microporous Mesoporous Mater. 2002, 56, 147. (17) Khaldi, M.; De Roy, A.; Chaouch, M.; Besse, J. P. J. Solid State Chem. 1997, 130, 66. (18) Radha, S.; Kamath, P. V. Cryst. Growth Des. 2009, 9, 3197− 3203. (19) Thomas, N.; Kumar, G. P.; Rajamathi., M. J. Solid State Chem. 2009, 182, 592. (20) Thomas, N.; Rajamathi., M. Langmuir. 2009, 25, 2212. (21) Bonnet, S.; Forano, C.; de Roy, A.; Besse, J. P.; Maillard, P.; Momenteau, M. Chem. Mater. 1996, 8, 1962. (22) Treacy, M. M. J.; Newsam, J. M.; Deem, M. W. Proc. R. Soc. London. 1991, 433, 499−520. (23) Treacy, M. M. J.; Deem, M. W.; Newsam, J. M. Computer Code DIFFaX, version 1.807; NEC Research Institute, Inc.: Princeton, NJ, 2000. (24) Thomas, G. S.; Rajamathi, M.; Kamath, P. V. Clays Clay Miner. 2004, 52, 693. (25) Meng, W.; Li, F.; Evans, D. G.; Duan, X. Mater. Res. Bull. 2004, 39, 1185.

Figure 6. IR spectra of 3R1 ZnAl−SO4 exchanged with C2O42− (a), 3R1 ZnCr−SO4 exchanged with C2O42− (b), 1H ZnAl−SO4 exchanged with C2O42− (c), and 1H ZnCr−SO4 exchanged with C2O42− (d).

1H phase was thermally stable and retains the additional layer of water molecules in the interlayer. The Zn−Al system was also subjected to a similar kind of thermal treatment. Two types of changes were observed. The ZnAl−SO4 3R1 sample with basal spacing 11.0 Å (Figure S3, Supporting Information) was intact heating to 30 °C and converted to a 1H polytype with reduced basal spacing (8.9 Å) upon heating to 45−70 °C (Figure S3, Supporting Information). The ZnAl-1H sample exhibited a progressive change in basal spacing, but the structure remained as 1H upon heating to 100 °C (Figure S4, Supporting Information).

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CONCLUSIONS We could selectively synthesize 3R1 and 1H polytypes of ZnAl and ZnCr LDH systems by altering the conditions of intracrystalline oxidation. The anion exchange behavior of these polytypes was considerably different, though in both polytypes, oxalate exchange is partial, the 3R1 polytype shows a higher extent of exchange.

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ASSOCIATED CONTENT

S Supporting Information *

PXRD patterns of the 3R1 and 1H samples of ZnAl and ZnCr LDHs heated to higher temperatures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 91-80-2221-1429. Fax: 91-80-2224-5831. E-mail: nygill@ gmail.com.

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ACKNOWLEDGMENTS This work was supported by DST, New Delhi. N.T. thanks the Council of Scientific and Industrial Research, government of India, for the award of a Senior Research Fellowship.

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REFERENCES

(1) Allmann, R. Acta Crystallogr. 1968, B24, 972. (2) Cavani, F.; Trifiro, F.; Vaccari, A. Catal. Today 1991, 11, 173. 1382

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