J. Phys. Chem. B 2007, 111, 3411-3418
3411
Conservation of Order, Disorder, and “Crystallinity” during Anion-Exchange Reactions among Layered Double Hydroxides (LDHs) of Zn with Al A. V. Radha,† P. Vishnu Kamath,*,† and C. Shivakumara‡ Department of Chemistry, Central College, Bangalore UniVersity, Bangalore 560 001, India, and Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India ReceiVed: December 7, 2006; In Final Form: January 18, 2007
Carbonate and chloride ions mediate an ordered stacking of metal hydroxide slabs to yield ordered layered double hydroxides (LDHs) of Zn with Al, by virtue of their ability to occupy crystallographically welldefined interlayer sites. Other anions such as ClO4- (Td), BrO3- (C3V), and NO3- (coordination symmetry C2V) whose symmetry does not match the symmetry of the interlayer sites (D3h or Oh) introduce a significant number of stacking faults, leading to turbostratic disorder. SO42- ions (coordination symmetry C3V) alter the long-range stacking of the metal hydroxide slabs to nucleate a different polytype. The degree of disorder is also affected by the method of synthesis. Anion-exchange reactions yield a solid with a greater degree of order if the incoming ion is a CO32- or Cl-. Incoming NO3- ions yield an interstratified phase, whereas incoming SO42- ions generate turbostratic disorder. Conservation or its converse, elimination, of stacking disorders during anion exchange is the net result of several competing factors such as (i) the orientation of the hydroxyl groups in the interlayer region, (ii) the symmetry of the interlayer sites, (iii) the symmetry of the incoming ion, and (iv) the configuration of the anion. These short-range interactions ultimately affect the long-range stacking order or “crystallinity” of the LDH.
Introduction In a layered solid such as graphite, strong covalent bonding is confined to only two dimensions, whereas in the third, the regular stacking of layers is mediated by weak van der Waals interactions. Consequently, the layers stack in two different sequences to give rhombohedral and hexagonal polytypes.1 The phenomenon of polytypism becomes complex when (1) the layers are more than one atom thick and (2) the layers acquire a charge, the compensation of which causes guest species to be intercalated in the interlayer sites. Consider, for instance, the divalent metal hydroxides having the formula MII(OH)2 (M ) Mg, Fe, Co, Ni).2 These comprise a hexagonal close packing of hydroxyl ions in which every alternate layer of octahedral sites is occupied by M2+ ions. The layers are three atoms thick and can be represented as AbC or simply as AC, where “b”, the lowercase symbol, represents the cation positions. A simple stacking sequence AC AC AC... yields the structure of mineral brucite, Mg(OH)2.3 Bookin and Drits4 refer to this as the 1H polytype, “1” indicating the single-layer periodicity and “H” the hexagonal symmetry. Periodic insertion of other layers, AB, CA, CB, etc., into the primary stacking sequence of 1H yield different polytypes, of which the most important are the two-layered polytypes of hexagonal symmetry (2H1, 2H2, 2H3) and triple-layered polytypes of rhombohedral symmetry (3R1, 3R2) (where R represents rhombohedral). These polytypes differ from one another in the nature of the interlayer sites that they generate. For instance, the 3R1 and 2H1 polytypes generate exclusively trigonal-prismatic interlayer sites, * Corresponding author. Phone 91-80-22961354. E-mail: vishnukamath8@ hotmail.com. † Bangalore University. ‡ Indian Institute of Science.
whereas 3R2 and 2H2 polytypes generate exclusively octahedral sites. Other polytypes generate mixtures of octahedral and prismatic interlayer sites. When there are no atoms in the interlayer region, packing fraction considerations favor the 1H, 2H2, and 3R2 polytypes or their random intergrowths.5 On the other hand, when the interlayer is occupied, as in the layered double hydroxides (LDHs), for instance, other stacking sequences are preferred based on (1) crystal chemical considerations that demand a match between the symmetry of the guest moieties and the symmetry of the interlayer site and (2) thermodynamic considerations that dictate the strength of bonding between the atoms of the layer and those in the interlayer. Thus, CO32--containing LDHs crystallize in the structure of the 3R1 or 2H1 polytypes, having prismatic interlayer sites. These not only provide crystallographically well-defined sites for the C and O atoms of the CO32- ions but also facilitate hydrogen bonding between the oxygen atoms of the carbonate ions and the hydroxyl ions of the layer.6 Evidently, anions of other symmetries are expected to select for other polytypes. LDHs participate extensively in anion-exchange reactions, by virtue of which they are used as sorbents and anionic scavengers.7,8 Anion-exchange reactions among LDHs are widely thought to occur by a topotactic mechanism that is expected to largely preserve the structure of the parent phase while bringing about compositional changes in the interlayer region.9-10 The objectives of this article are two-fold: (i) to examine the polytype selectivity of simple univalent and divalent inorganic anions having D3h, C3V, C2V, and Td symmetries and (ii) to examine whether polytypism, order/disorder, and “crystallinity” of the precursor LDH is conserved during anion exchange. We take the LDH of Zn with Al having the formula Zn2Al(OH)6(An-)1/n‚mH2O as an illustrative system for our studies.
10.1021/jp0684170 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/09/2007
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TABLE 1: Precipitation Conditions Used for the Synthesis of Different LDH Samples cell parameters LDH system Zn-Al-CO32Zn-Al-ClZn-Al-NO3Zn-Al-SO42Zn-Al-BrO3Zn-Al-ClO4-
pH
T (°C)
post-precipitation treatment
a (Å)
c (Å)
10.0 8.0 8.0 8.0 8.0 8.0
60 60 60 60 60 40
60°C, 72 h 60°C, 18 h 60°C, 18 h 60°C, 18 h 80°C, 18 h 60° C, 18 h
3.076 3.08 3.077 3.079 3.073 3.076
22.78 23.33 26.67 11.16 10.82 9.01
Experimental Section Zn-Al LDHs with various anions were prepared by employing both coprecipitation and anion-exchange methods. Coprecipitation Method. In a typical preparation, 50 mL of the mixed-metal (Zn2+ + Al3+) salt solution in a 2:1 mole ratio was added to a solution (100 mL) containing 10 times the stoichiometric requirement of the desired anion taken as its Na+/ K+ salt. The Zn-Al-ClO4- and Zn-Al-BrO3- LDHs were prepared using a mixed-metal nitrate solution. A constant pH of 8 was maintained during precipitation by the simultaneous addition of 1 N NaOH using a Metrohm model 718 STAT titrino instrument operating in the pH stat mode. All precipitations were carried out at constant temperature (60 °C); N2 gas was bubbled through the solution continuously. In each case, the slurry thus obtained was aged at 60 °C for 18 h under N2 and then filtered rapidly under suction. The precipitate was washed with warm decarbonated water several times and finally with acetone before being dried at 80 °C. Zn-Al-CO32- LDH was prepared at pH 10 using a Na2CO3 solution (100 mL) containing 3 times the stoichiometric requirement of CO32- ions without the use of N2 gas. The slurry was aged for 72 h (60 °C), washed, and dried at 80 °C. Anion-Exchange Reactions. Zn-Al-An- (A ) Cl-, CO32-, NO3-, SO42-) LDHs were also prepared by anion exchange using Zn-Al-NO3-, Zn-Al-Cl-, or Zn-Al-ClO4- precursors. In a typical exchange reaction, 0.5 g of the precursor LDH was stirred in a 30-mL volume of a previously deaerated solution containing a 5- or 10-times excess of the sodium salt of the anion for 5 h in a screw-cap bottle to exclude atmospheric CO2. The pH of the solution at the beginning and the end of the reaction was noted. After 5 h, the solution was filtered using a sintered glass crucible, and the precipitate was washed with copious amounts of decarbonated water and then with acetone. All precipitates were dried at 65 °C for 24 h in an air oven. The complete descriptions of conditions used for different coprecipitation and anion-exchange reactions are listed in Tables 1 and 2. Characterization. All samples were characterized by powder X-ray diffraction (PXRD) using a Phillips X’pert X-ray diffractometer (graphite secondary monochromator) operated in reflection geometry. Data were collected with Cu KR radiation (λ ) 1.541 Å) using a continuous scan rate of 2° (2θ) min-1 and were then rebinned into steps of 0.05° (2θ). Data for some of the samples were collected using a Siemens D5005 diffractometer operated under the same conditions without a monochromator. The instrumental broadening of the Bragg peaks is estimated to be 0.15-0.2° (2θ) in the range 77-28° (2θ) for the Si standard. Any broadening of the peaks beyond this value is attributed to the sample. The samples were further characterized by infrared spectroscopy (Nicolet model Impact 400D FT IR spectrometer, KBr pellets, 4 cm-1 resolution) to verify the presence of intercalated anions. For thermogravimetry (Mettler Toledo TGA/SDTA 851e,
Figure 1. (a) Rietveld fit of the PXRD pattern of the coprecipitated Zn-Al-CO32- LDH. PXRD patterns of the Zn-Al-ClO4- LDH (b) before and (c) after anion-exchange reaction with carbonate anions. The corresponding DIFFaX simulations and difference profiles are also shown.
Stare 7.01) studies, the samples were dried to constant weight at 100 °C in the TG balance to expel adsorbed water before the temperature was ramped (100-800 °C, 5 °C min-1 heating rate, flowing air). Computational Studies Rietveld refinement and DIFFaX simulation studies were employed for structural analysis of the various LDH samples. Crystalline samples that exhibited uniform broadening of all Bragg peaks in their PXRD patterns were treated by the Rietveld procedure. Rietveld refinements were carried out using the FullProf.2k code (version 3.3 June 2005-LLB JRC).11 In all refinements, the modified pseudo-Voigt line-shape function with five variables (U, V, W, X, and η) was used to fit the experimental profiles. The PXRD pattern of Zn-Al-CO32- LDH was fit using the structure of the Mg-Al-CO32- LDH (CC no. 81963, space group R3hm, a ) 3.046 Å, c ) 22.772 Å) as the model. The PXRD pattern of the Zn-Al-Cl- LDH was fit using the published structure of the Zn-Al-Cl- LDH as the model (CC no. 91155, space group R3hm, a ) 3.084 Å, c ) 23.47 Å). Any nonuniform broadening of peaks in the PXRD pattern is indicative of structural disorder. In such cases, the PXRD patterns were simulated using the Fortran-based computer program DIFFaX (version 1.807).12-13 Details of the DIFFaX simulations of the PXRD patterns of LDHs can be found in our earlier reports.14,15 DIFFaX simulations not only facilitate polytype identification but also help in the classification and quantification of structural disorder in layered materials. (See Supporting Information SI-I).
Anion-Exchange Reactions among LDHs of Zn with Al
J. Phys. Chem. B, Vol. 111, No. 13, 2007 3413
TABLE 2: Conditions Used and Products Obtained in the Anion-Exchange Reactions pH
cell parameters
precursor LDH
anion
initial
final
anion source
product LDH
A (Å)
c (Å)
Zn-Al-ClO4Zn-Al-ClZn-Al-NO3Zn-Al-NO3Zn-Al-ClZn-Al-ClO4Zn-Al-ClZn-Al-Cl-
CO32CO32CO32ClNO3NO3SO42BrO3-
12 10.4 10.8 10.4 10.2 7.4 9.6 10.2
12 8.2 10.8 8.0 8.0 8.6 8.7 10.0
Na2CO3 Na2CO3 Na2CO3 NaCl NaNO3 NaNO3 Na2SO4 KBrO3
Zn-Al-CO32Zn-Al-CO32Zn-Al-CO32Zn-Al-ClZn-Al-NO3Zn-Al-NO3Zn-Al-SO42Zn-Al-BrO3-
3.075 3.076 3.074 3.075 3.076 3.077 3.078 3.073
22.88 23.85 23.85 23.29 25.5 26.48 8.76 9.46
TABLE 3: Results of the Rietveld Refinements of the Zn2Al(OH)6(An-)1/n‚mH2O LDHs An-
atom
site
x
y
z
Biso (Å2)
CO32-
Zn Al O1 C O2 Zn Al O1 Cl O2
3(a) 3(a) 6(c) 6(c) 18(h) 3(a) 3(a) 6(c) 18(h) 18(h)
0.000 0.000 0.000 0.3333 0.117(2) 0.000 0.000 0.000 0.161(3) 0.161(3)
0.000 0.000 0.000 0.6667 -0.117(2) 0.000 0.000 0.000 -0.161(3) -0.161(3)
0.0000 0.0000 0.3773(3) 0.5000 0.5000 0.0000 0.0000 0.3771(4) 0.5000 0.5000
1.000 1.000 1.000 1.000 1.000 0.0000 0.0000 0.13918 0.0000 0.0000
Cl-
shape and width parameters 2-
CO3 Cl-
space group
U
V
W
X
η
R3hm R3hm
0.406907 0.027092
-0.054403 -0.031688
0.069189 0.087776
0.001625 0.017284
0.56307 0.41758
cell parameters 2-
CO3 Cl-
goodness of fit
a (Å)
c (Å)
Rwp
RB
Rf
Rp
χ2
3.0737(1) 3.0813(3)
22.743(2) 23.351(5)
15.8 39.6
4.16 9.75
4.31 8.65
14.4 31.0
3.85 2.89
Results and Discussion 2-
Zn-Al-CO3 LDH. In Figure 1a are shown the results of structure refinement using the PXRD data obtained for the coprecipitated Zn-Al-CO32- LDH. There are 20 refinable parameters. The results of the refinement, including the atom positions and goodness-of-fit parameters, are reported in Table 3. The fit is satisfactory. In Zn-Al-CO32-, the CO32- ion is intercalated and has its plane perpendicular to the c crystallographic axis so that this interlayer is one atom thick. The oxygen atoms of CO32- and intercalated water occupy a single set of sites (18h), and C occupies the (6c) sites.16 The z coordinate of the hydroxyl oxygen (O1) and the x and y (-x) coordinates of the interlayer oxygen (O2) are the only two refinable atomic displacement factors in the crystal structure. The refined values for O1 [z ) 0.3773(3)] and O2 [x ) 0.117(2)] are slightly higher than those reported for the Mg-AlCO32- LDH17,18 [O1, z ) 0.37634(9); O2, x ) 0.1058(7)] in the literature. Table 4 lists interatomic distances and angles calculated for Zn-Al-CO32- from the Rietveld refined data. The higher O1O1 distance (2.665 Å) of shared octahedral edges compared to that in the Mg-Al-CO32- LDH17,18 (2.632 Å) indicates the elongation of the hydroxyl octahedra in the Zn-Al-CO32- LDH because of the larger ionic radius of the Zn2+ ions. The layerinterlayer (O1-O2) distance (2.861 Å) is similar to that in the Mg-Al-CO32- LDH (2.871 Å,17 2.857 Å18). This shows that the layer-interlayer distance is not sensitive to the ionic radius of the cation. Often more important than the exact structure of the LDH is the stacking sequence of the layers, as the latter provides an
TABLE 4: Interatomic Distances and Angles from the Rietveld Refinement of Zn2Al(OH)6(An-)1/n‚mH2O LDHs CO32(Zn,Al)-O1 O1-O1 O1-O2 C-O2 O1-(Cl,O2)
Distances (Å) 2.034 2.665 2.861 1.170 -
O1-(Zn,Al)-O1 O2-C-O2
Angles (deg) 81.86 120.01
Cl2.054 2.715 2.992 82.78
overall view of the structure and enables the direct determination of the polytype. Toward this end, DIFFaX simulations are more useful than the Rietveld refinement method. DIFFaX simulations indicate that Zn-Al-CO32- LDH crystallizes in the structure of the 3R1 polytype (see Supporting Information SI-II). To understand the role of precursor structure in ion-exchange reactions, the Zn-Al-CO32- LDH was also synthesized by anion exchange using the Zn-Al-ClO4- LDH as a precursor. In Figure 1b,c are shown the PXRD patterns of the Zn-AlClO4- LDH before and after anion-exchange reaction with carbonate anion. The PXRD pattern of Zn-Al-ClO4- comprises only four broad peaks. The first two peaks appearing at 9.01 Å [9.79° (2θ)] and 4.5 Å [19.75° (2θ)] correspond to the two basal reflections. The extinction of all hkl reflections is indicative of the loss of three-dimensional order. The asymmetric broadening of the other reflections, also called Warren broadening, is typical of materials with turbostratic disorder.19,20 This pattern could be simulated by mixing 25% 1H motifs + 30%
3414 J. Phys. Chem. B, Vol. 111, No. 13, 2007
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Figure 2. PXRD patterns of the Zn-Al-CO32- LDH prepared by anion exchange in sodium carbonate solution from (a) Zn-Al-Cland (b) Zn-Al-NO3- LDH precursors. The corresponding DIFFaX simulations and difference profiles are also shown.
turbostraticity in the primary 3R1 polytype. The introduction of extensive stacking disorder on the intercalation of the ClO4ions is a result of the mismatch between the symmetry of the interlayer sites (D3h) in the 3R1 polytype and the symmetry of the ClO4- ions (Td). Figure 1c shows the PXRD pattern of the same LDH after anion exchange with carbonate ions. The interlayer distance decreased from 9.03 to 7.7 Å, corresponding to that of intercalated CO32- ions. IR spectra (data not shown) also confirm the formation of carbonate LDH. The pattern of the carbonate-exchanged LDH shows reasonably sharp reflections compared to the ClO4--containing LDH. However, the peaks corresponding to the different reflections are nonuniformly broadened. For instance, the fwhm value of the peak corresponding to the 012 reflection is 0.46° (2θ), whereas the 015 reflection is much broader at 0.78° (2θ). This kind of nonuniform broadening is on account of stacking disorders14,15 and could be simulated by introducing 25% 2H1 motifs in the 3R1 polytype. To explore these effects further, the Zn-Al-CO32- LDH was also synthesized by anion exchange using Zn-Al-Cl- and Zn-Al-NO3- precursors (Figure 2). The PXRD patterns of the two precursors are shown in Figures 3a and 4a, respectively. The Zn-Al-Cl- LDH is highly ordered, and its structure could be refined by the Rietveld method, as reported in Table 3. The Cl and O atoms of the intercalated water molecules occupy a single set of sites (18h) in this structure.21 The refined z value of the hydroxyl oxygen (O1), 0.3771(4), matches that reported in the literature for the same LDH, 0.3775(2).21 The interatomic distances and angles obtained from the refined data are listed in Table 4. The O1-O1 distance (2.715 Å) calculated for the shared octahedral edges matches that of the Zn-Al-CO32LDH reported here. Further, the observed Bragg angles of the Zn-Al-Cl- LDH also match the Bragg angles calculated for the 3R1 polytype (see Table S1 of the Supporting Information). The Zn-Al-CO32- LDH obtained by ion exchange from this precursor is reasonably ordered (Figure 2a) and contains only 10% stacking disorders of the 2H1 variety. The nitrate ion has a planar structure with D3h symmetry. However, it differs from other planar anions in its mode of coordination in the interlayer. The nitrate-containing LDH has an interlayer spacing of 8.8 Å (Figure 4a), which is much larger
Figure 3. (a) Rietveld fit of the PXRD pattern of coprecipitated ZnAl-Cl- LDH compared to (b) the PXRD pattern of the Zn-Al-ClLDH obtained by anion exchange from a Zn-Al-NO3- LDH precursor. The corresponding DIFFaX simulations and difference profiles are also shown.
Figure 4. PXRD patterns of the Zn-Al-NO3- LDHs prepared by (a) coprecipitation and (b) anion exchange from a Zn-Al-Cl- LDH precursor. The corresponding DIFFaX simulations and difference profiles are also shown.
than what is expected of a single-atom-thick interlayer. The nitrate ion is known to intercalate with one of its NO bonds collinear with the c crystallographic axis when the M(II)/M(III) ratio is 2, as in the present instance.22 The PXRD pattern shows nonuniformly broadened peaks in the mid-2θ region, which can be indexed to the 01l (l ) 2, 5, 8) reflections of the 3R1 polytype. The excessive broadening of the 018 reflection and the wellresolved sharp 113 reflection in the pattern point toward the
Anion-Exchange Reactions among LDHs of Zn with Al existence of stacking faults rather than turbostratic disorder in this sample.23 This pattern was satisfactorily simulated by a combination of 70% 3R1 + 30% 2H1 motifs using 27 stacked layers (thickness ) 24 nm). When this precursor was subjected to anion exchange with carbonate anions, the resulting ZnAl-CO32- LDH was found to have a greater degree of structural order. The PXRD pattern could be simulated by incorporating just 10% 2H1 motifs into the primary 3R1 polytype (see Figure 2b). In conclusion, carbonate ions seem to have an overwhelming tendency to produce ordered structures in LDHs. Other anions such as ClO4- and NO3-, which intercalate with a symmetry different from that of the interlayer site, tend to produce stacking disorders. On exchanging the ClO4- and NO3- ions for CO32ions, the symmetry of the interlayer site is matched by the anion symmetry, as the CO32- ions also exhibit D3h symmetry. This symmetry compatibility reestablishes a certain degree of structural order, as shown by the emergence of the hkl reflections. The removal of turbostratic disorder occurs by the translation of successive layers with respect to each other, a process aided by the diffusion of the anions during exchange. The mutual translations result in a stacking sequence that yields the best hydrogen bonding between the hydroxide sheets and the incoming carbonate ions. Hydrogen bonding is maximized in trigonal-prismatic interlayer sites. However, two different stacking sequences, 3R1 and 2H1, generate trigonal-prismatic interlayer sites. Consequently, the Zn-Al-CO32- LDH prepared by anion exchange comprises a mix of both 3R1 and 2H1 stacking motifs, in contrast to the coprecipitated LDH, which crystallizes in the structure of an ordered 3R1 polytype. Zn-Al-X- (X- ) Cl-, NO3-) LDHs. We attribute the structural disorder observed in the Zn-Al-NO3- LDH compared to its chloride-containing analogue to the mismatch between the symmetry of the interlayer site (D3h) and the coordination symmetry (C2V) of the NO3- ion. The Zn-AlCl- LDH, on the other hand, crystallizes in an ordered structure, as the Cl- ions occupy the 18h site of the ordered 3R1 polytype. To further explore ideas concerning the preservation of structural order during anion exchange, a Zn-Al-Cl- LDH was prepared by anion exchange from the nitrate precursor. The PXRD pattern is given in Figure 3b. The interlayer spacing has decreased from 8.8 to 7.76 Å, characteristic of the chloride-containing LDH. The IR spectra also show the disappearance of the nitrate-related vibrations (data not shown), indicating that exchange is complete. Unlike its coprecipitated counterpart, the Zn-Al-ClLDH prepared by anion exchange displays broad peaks in the mid-2θ region similarly to the coprecipitated nitrate LDH. The DIFFaX solution for this pattern can be obtained by mixing of 30% 2H1 motifs in 3R1 polytype. This is exactly the same kind of disorder as observed in the nitrate precursor, clearly showing that the stacking disorders of the precursor are preserved in the product LDH. To verify whether the mode of coordination of the nitrate ion can be altered by changing the synthesis procedure, we prepared the Zn-Al-NO3- LDH by anion exchange from the highly ordered Zn-Al-Cl- LDH precursor. Because the chloride-containing precursor is highly ordered and has a singleatom-thick interlayer, it was our expectation that the nitrate ion would get intercalated with its plane perpendicular to the c crystallographic axis as in the carbonate-containing LDHs. Figure 4b shows the PXRD pattern of the LDH obtained by anion exchange. The peak positions of all of the reflections in this pattern match well with those of the pattern for the coprecipitated nitrate LDH, with the exception of 00l. An
J. Phys. Chem. B, Vol. 111, No. 13, 2007 3415
Figure 5. DIFFaX simulations of the 3R1 polytype of the Zn-AlNO3- LDH interstratified with (a) 0%, (b) 10%, and (c) 33% ZnAl-Cl- LDH.
additional feature of this pattern is the excessive broadening of the 006 reflection [fwhm 1.5° (2θ)], whereas the peaks corresponding to the 003 and 012 reflections are sharp [fwhm 0.8° and 0.3° (2θ), respectively]. The interlayer distance of this sample corresponds to 8.5 Å, which is between those of the Cl-- and nitrate-containing LDHs. To explain these features, model DIFFaX simulations were carried out for an interstratified phase containing both intercalated nitrate and chloride ions.24 Model simulations (Figure. 5) show that this kind of selective broadening is on account of interstratification arising from incomplete anion exchange. A DIFFaX simulation of the 3R1 polytype comprising equal amounts of interstratified Zn-Al-NO3- and Zn-Al-Cl- layers produced an exact match with the experimental pattern (Figure 4b). The interstratification model is supported by wet chemical analysis, which shows the presence of residual chloride in the interlayer. The observation of a single-phase LDH with interstratification stands in contrast to the phase segregation observed by other authors on partial anion exchange.25 Whereas the intercalated nitrate could be fully replaced by chloride, the opposite was not true, showing that nitrate is a better leaving group than chloride. Zn-Al LDHs with Nonplanar Oxoanions. In Figure 6 are shown the PXRD patterns of the Zn-Al-SO42- LDHs obtained by coprecipitation and ion exchange. The two methods yielded products with different interlayer spacings. The coprecipitated sample (Figure 6a) has an enhanced interlayer spacing of 11.2 Å [8.2° (2θ)] and shows three broad basal reflections in the low-angle region of the PXRD pattern. A similar phase with an enhanced c parameter was reported earlier for Zn-Cr and Zn-Ga LDH systems with intercalated SO42- ions.26,27 The increase in the c parameter is attributed to the presence of interlayer sulfate and water in the bilayer structure. The ZnAl-SO42- LDH is known to crystallize in the 1H, 2H1, and 3R1 polytypes.28 From a comparison of the observed pattern with those expected for the different polytypes (see Figure S2 and Table S2 of Supporting Information SI-III), it is concluded that the LDH prepared by us has the structure of the 1H polytype and is indexed to a cell with a ) 3.09 Å and c ) 11.16 Å. There is nevertheless some degree of nonuniform broadening of the reflections in the mid-2θ region, which could be simulated by the incorporation of 35% turbostratic disorder. When the same
3416 J. Phys. Chem. B, Vol. 111, No. 13, 2007
Figure 6. PXRD patterns of the Zn-Al-SO42- LDHs prepared by (a) coprecipitation and (b) anion exchange from a Zn-Al-Cl- LDH precursor. The corresponding DIFFaX simulations and difference profiles are also shown. The feature marked * is due to an impurity.
Figure 7. IR spectra of the Zn-Al-SO42- LDHs prepared by (a) coprecipitation and (b) anion exchange from a Zn-Al-Cl- LDH precursor.
LDH is prepared by anion exchange starting from an ordered Zn-Al-Cl- LDH, however, the product obtained has a considerable degree of disorder very similar to that in the ZnAl-ClO4- LDH (see Figure 6b). It is evident that the observed pattern does not correspond to any of the polytypes. However, the position of the first peak in the mid-2θ region in the pattern matches that of the 1H polytype. Incorporation of 35% 3R1 motifs and 30% turbostratic disorder in the parent 1H polytype (disk diameter ) 1500 Å) produced a good match to the experimental pattern. The Zn-Al-SO42- LDHs prepared by the two methods can also be distinguished by their IR spectra (Figure 7). The tetrahedral SO42- ion has two IR-allowed vibrations, namely, the antisymmetric stretch (ν3) and the antisymmetric deformation mode (ν4), which appear at 1120 and 620 cm-1, respectively. In the coprecipitated sample having the structure of an ordered 1H polytype, the triply degenerate ν3 mode (F2) splits into two peaks (1155 and 1114 cm-1) corresponding to the A1 + E modes of C3V symmetry. Characteristic of this reduced symmetry is the ν1 mode, which appears at 959 cm-1 (Figure 7a). These observations are in keeping with Halford’s rules,29 which state
Radha et al.
Figure 8. PXRD patterns of the Zn-Al-BrO3- LDHs prepared by (a) coprecipitation and (b) anion exchange from a Zn-Al-Cl- LDH precursor. The corresponding DIFFaX simulations are also shown.
that, for a “molecule in a solid”, the crystallographic axis is defined as the principal axis. In the LDH obtained by anion exchange, extensive stacking disorder ensures that the c crystallographic axis is ill-defined, so that the SO42- ions behave as though they are of Td symmetry and exhibit only the ν3 and ν4 modes (Figure 7b). This clearly shows that the method of preparation has a profound influence on the phase formation, crystallinity, and mode of incorporation of anions in the interlayer. Direct precipitation yields the 1H polytype, whereas an exchange reaction, though starting from an ordered precursor, yields a turbostratically disordered phase. There is a likelihood that these differences arise from the difference in Zn/Al ratio in the LDHs prepared by different techniques. Chemical analysis of LDHs is not always easy because of the formation of X-ray-amorphous binary hydroxides.30 Consequently, the most reliable way to estimate the M(II)/M′(III) ratio is by determining the a parameter of the LDH and applying Vegard’s law. In Tables 1 and 2, we list the a parameters of the LDHs obtained by both coprecipitation and anion exchange. It is clear that the a parameters of pairs of LDHs prepared by different methods are comparable, showing that the Zn/Al ratio is conserved. Another oxoanion of related symmetry is BrO3-. The bromate-containing LDH was prepared both by coprecipitation and by ion exchange. The two methods yielded products with different interlayer spacings similarly to the SO42--intercalated LDHs. The PXRD patterns are given in Figure. 8. The coprecipitated sample has an enhanced interlayer spacing of 10.7 Å [8.26° (2θ)] and shows three broad basal reflections in the low-angle region of the PXRD pattern. The increase in the c parameter can be attributed to the presence of interlayer of bromate and water species in the bilayer structure. With this knowledge, we attempted the simulation of this pattern. Figure 8a shows the DIFFaX simulation of this pattern obtained by introducing 50% turbostratic disorder into the 1H polytype. In this simulation, the interlayer water content was increased to 2, and the number of layers restricted to 15 to achieve a good match of the basal reflections. The ion-exchanged sample showed an interlayer distance of 9.2 Å [9.61° (2θ)], and this pattern was simulated using a combination of the 1H polytype with 35% turbostratic disorder and particle size effects (Figure 8b). In these LDHs, the BrO3- ions are placed in the positions occupied by the sulfate ions in the model structure, with the apical oxygen site of the sulfate left vacant. This vacant site provides space for the lone pair on the Br atom. In this case,
Anion-Exchange Reactions among LDHs of Zn with Al
J. Phys. Chem. B, Vol. 111, No. 13, 2007 3417
TABLE 5: Results of DIFFaX Simulations of the Different LDHs
a
LDH
method
1H (%)
2H1 (%)
3R1 (%)
turboa (%)
Lorenb (deg 2θ)
disk diam (Å)
no. of layers
Zn-Al-ClO4Zn-Al-CO32Zn-Al-CO32Zn-Al-CO32Zn-Al-ClZn-Al-NO3Zn-Al-NO3Zn-Al-NO3Zn-Al-SO42Zn-Al-SO42Zn-Al-BrO3Zn-Al-BrO3-
Cp Ex(ClO4-) Ex(Cl-) Ex(NO3-) Ex(NO3-) Cp Ex(Cl-) Ex(ClO4-) Cp Ex(Cl-) Cp Ex(Cl-)
25 65 35 50 65
25 10 10 30 30 30 -
45 75 90 90 70 70 50c 70 35 -
30 35 30 50 35
0.5 0.4 0.3 0.3 0.4 0.4 0.4 0.4 0.3 0.3 0.5 0.4
∞ ∞ ∞ ∞ ∞ ∞ ∞ 3000 ∞ 1500 ∞ 1500
∞ ∞ ∞ ∞ ∞ 27 40 ∞ ∞ ∞ 15 15
Turbostratic disorder. b Lorentzian line-shape width. c Interstratified with 50% 3R1 polytype of Zn-Al-Cl- LDH.
the method of preparation affects the interlayer spacing but not the nature of structural disorder. The model is approximate, however, and fails to produce a satisfactory match with the experimental patterns. The results of all of the DIFFaX simulations are summarized in Table 5. Discussion Among the different interactions of LDHs, the iono-covalent interactions within the metal hydroxide layer are responsible for product formation. The interlayer interactions dictate the stacking order between the layers and are mediated by the species present in the interlayer region. The anionic species in the interlayer balance the positive charge on the brucite-like layers. The water in the interlayer plays an important role in the stability of the structure by forming an efficiently packed interlayer. The following factors contribute to the interlayer interactions: (1) ionic interactions between the positively charged brucite-like layer and the negatively charged interlayer and (2) hydrogen bonding between the brucite-like layer and the interlayer anions and water molecules. In LDHs, the strength of the ionic interactions between the brucite-like layer and the interlayer primarily depends on the trivalent cation content in the hydroxide layer. The trivalent metal content in LDHs varies between 0.15 and 0.33, and hence, the maximum charge a layer can acquire is 33% of the total cationic content. These charges on the layers are not localized but are randomly distributed over the entire layer. Further, this ionic interaction is diffuse in nature, as anions do not directly participate in the octahedral coordination around the metal atom. This ionic interaction, also being nondirectional, is not expected to play a role in the configuration of the interlayer ions with respect to the hydroxide layer. The hydrogen bonding, on the other hand, is directional and more localized. H-bonds are generally linear with a small variation in the bond angle and play an important role in determining the stacking sequence of the metal hydroxide layers. The interlayer of LDHs comprises both water and anions, and these participate in H-bonding with the hydroxide layer. The following factors influence H-bond formation in LDHs: (1) Orientation of hydroxyl ions: The directional nature of H-bonding requires a specific orientation of hydroxyl ions and the interlayer species with respect to each other to optimize the hydrogen-bonding contacts. This is a short-range interaction. (2) Interlayer sites: The interlayer sites in the LDH depend on the stacking sequence of the brucite-like sheets. This reflects the effect of long-range order on the local symmetry of the interlayer sites. Often, as was pointed out earlier, more than
one stacking sequence can yield interlayer sites of a given symmetry, thereby introducing stacking disorders. (3) Anion configuration: The symmetry, size, charge, and polarizability of the anions and their orientation in the interlayer region also mediate the interlayer interactions. The three factors listed above act cooperatively to maximize the bonding interactions between the layers. Factors 1 and 3 are predetermined by the anion structure and the symmetry of its coordination. A monatomic anion or one with high symmetry has considerable orientational degrees of freedom to participate in hydrogen bonding with the brucite-like layers. When the anions are nonplanar and/or have low symmetry, a reorganization of the stacking sequence of the brucite-like layers is needed to maximize the hydrogen-bonding interaction. A demonstration of this phenomenon is found in the ZnAl-SO42- LDH obtained by coprecipitation. Although free sulfate belongs to the high-symmetry point group Td, the operation of Halford’s rules reduces the symmetry of the intercalated sulfate to C3V. The intercalated sulfate is then able to suitably mediate the long-range ordering of the metal hydroxide slabs in such a way as to generate octahedral interlayer sites, thereby generating a new structure, that corresponding to the 1H polytype. When the energy required for altering the long-range ordering of the metal hydroxide slabs is more than what is afforded by hydrogen-bond formation, the crystal retains turbostratic disorder, owing to the incompatibility of the symmetry of the anion with that of the interlayer site. This is found in the Zn-AlSO42- LDH obtained by anion exchange, wherein the longrange stacking of the metal hydroxide slabs is already predetermined by the structure of the precursor LDH. Another question arises: Why is the LDH prepared by coprecipitation at a certain pH more ordered than the LDH prepared by anion exchange? Different synthetic routes affect the kinetics of a reaction. A low rate of the reaction favors the growth of ordered crystals. The formation pH of LDHs of Zn with Al is around 6.31 The precipitation reactions reported in this work were carried out at pH 8 in all cases, except for that of the carbonate-containing LDH, which was performed at pH 10. As the precipitation pH is close to the formation pH, the rate of crystal growth is very low. Thereby, even the nitratecontaining LDH precipitated under pH stat conditions is obtained without turbostratic disorder, although it does incorporate a small number of stacking faults. Anion-exchange reactions have a very high rate. The rates of anion-exchange reactions are measured by P-jump and T-jump techniques.32 This explains why the products of anion-exchange reactions nucleate with higher degrees of structural disorder.
3418 J. Phys. Chem. B, Vol. 111, No. 13, 2007 Conclusions In conclusion, several factors determine the order, disorder, and crystallinity of LDHs. When the symmetry of the intercalated anion matches the local symmetry of the interlayer site, ordered LDHs are obtained in a phenomenon reminiscent of molecular recognition. Anions of other symmetries mediate the nucleation of crystals with stacking disorders and/or turbostratic disorder. Crystallinity of the LDHs is also affected by the kinetics of crystallization, offering kinetic control over the order, disorder, and crystallinity of LDHs. Supporting Information Available: Explanation of the DIFFaX simulations, figures showing DIFFaX-simulated PXRD patterns, and tables listing Bragg angles of reflections. This material is available free of charge via the Internet at http:// pubs.acs.org. Acknowledgment. The authors thank the Department of Science and Technology (DST), Government of India for financial support. References and Notes (1) Okino, F.; Touhara, H. In ComprehensiVe Supramolecular Chemistry; Atwood, J. L., Macnicol, D. D., Davies, J. E. D., Vogtle, F. Eds. Pergamon: Oxford, 1996; Vol. 7, pp 25-76. (2) Wells, A. F. Structural Inorganic Chemistry, 4th ed.; The English Language Book Society and Oxford University Press: London, 1979. (3) Oswald, H. R.; Asper, R. In Preparation and Crystal Growth of Materials with Layered Structures; Lieth, R. M. A., Ed.; D. Reidel Publishing Company: Dordrecht, The Netherlands, 1977; Vol. 1, pp 71140. (4) Bookin, A. S.; Drits, V. A. Clays Clay Miner. 1993, 41, 551. (5) Ramesh, T. N.; Kamath, P. V.; Shivakumara, C. J. Electrochem. Soc. 2005, 152, A806. (6) Cavani, F.; Trifiro, F.; Vaccari, A. Catal. Today 1991, 11, 173. (7) Miyata, S.; Hirose, T. Clays Clay Miner. 1978, 26, 441. (8) Newman, S. P.; Jones, W. New J. Chem. 1998, 105.
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