- Email: [email protected]
Synthesis and characterization of layered double hydroxides (LDHs) with intercalated chromate ions
Materials Research Bulletin 42 (2007) 1028–1039 www.elsevier.com/locate/matresbu
Synthesis and characterization of layered double hydroxides (LDHs) with intercalated chromate ions Srinivasa V. Prasanna a, P. Vishnu Kamath a,*, C. Shivakumara b a
Department of Chemistry, Central College, Bangalore University, Bangalore, Karnataka 560001, India b Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India Received 22 June 2006; received in revised form 31 August 2006; accepted 26 September 2006 Available online 1 November 2006
Abstract Chromate intercalated layered double hydroxides (LDHs) having the formula MII6M0 III2(OH)16CrO44H2O (MII = Ca, Mg, Co, Ni, Zn with M0 III = Al and MII = Mg, Co, Ni with M0 III = Fe) have been prepared by coprecipitation. The products obtained are replete with stacking disorders. DIFFaX simulations show that the stacking disorders are of three kinds: (i) turbostratic disorder of an originally single layered hexagonal (1H) crystal, (ii) random intergrowth of polytypes with hexagonal (2H) and rhombohedral (3R) symmetries and (iii) translation of randomly chosen layers by (2/3, 1/3, z) and (1/3, 2/3, z) leading to stacking faults having a local structure of rhombohedral symmetry. IR spectra show that the CrO42 ion is incorporated either in the Td or in the C3v ˚ characteristic of a single atom thick interlayer showing that the CrO42 symmetry. The interlayer spacing in the latter case is 7.3 A ion is grafted to the metal hydroxide slab. On thermal treatment, the CrO42 ion transforms into Cr(III) and is incorporated into the spinel oxide or phase separates as Cr2O3. In the LDH of Mg with Al, Cr(III) remains in the MgO lattice as a defect and promotes the reconstruction of the LDH on soaking in water. In different LDHs, 18–50% of the CrO42 ion is replaceable with carbonate anions showing only partial mineralization of the water-soluble chromate. The extent of replaceable chromates depends upon the solubility of the corresponding LDH, which in turn is determined by the solubility of the MCrO4. These studies have profound implications for the possible use of LDHs for chromate amelioration in green chemistry. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Layered compounds; B. Chemical synthesis; C. X-ray diffraction
1. Introduction Layered materials possessing anion exchange properties are useful host materials to sequester insidious anionic contaminants from solution. Among several layered materials, layered double hydroxides (LDHs) have evoked considerable interest in recent years due to their potential application in environmental remediation of anionic pollutants [1]. LDHs are structurally and functionally the inverse of cationic clays [2]. The structure of LDHs is derived from that of mineral brucite, Mg(OH)2 [3]. Brucite comprises a close packing of hydroxyl ions in which Mg2+ ions occupy alternate layers of octahedral sites, leading to stacking of charge-neutral metal hydroxide slabs of composition [Mg(OH)2]. When a fraction, x, of Mg2+ ions is isomorphously substituted by a trivalent ion such as Al3+ or Fe3+, the positive charge x+ generated on the metal hydroxide slab is compensated by the inclusion of anions, An,
* Corresponding author. Tel.: +91 80 22961354; fax: +91 80 22931310. E-mail address: [email protected] (P.V. Kamath). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.09.021
S.V. Prasanna et al. / Materials Research Bulletin 42 (2007) 1028–1039
1029
in the interlayer region to give LDHs of composition [MII1xM0 IIIx(OH)2](An)x/nmH2O [4], where MII = Mg, Ca, Co, Ni, Zn, etc. and M0 III = Al, Cr, Fe, etc. and 0.2 x 0.33. We refer to the LDHs as [M-M0 -A]. LDHs exhibit anion exchange property and incorporate a wide variety of anions into the interlayer [5–7]. By virtue of this property, the LDHs have the potential to scavenge hazardous anionic contaminants present in waste water [8–10]. In this context, the synthesis and characterization of LDHs, which incorporate the anionic pollutant is important as these LDHs are the final products of sequestration reactions in the application of LDHs as water decontaminating agents. One of the most insidious anionic contaminants in industrial waste water is the chromate ion. However, there are only a few studies of CrO42 containing LDHs in the literature [11,12]. A study of the structural and compositional features of CrO42 containing LDHs is important in the context of remediation of CrO42 ions by LDHs. In this paper, we report the synthesis and characterization of CrO42 containing LDHs of Mg, Ca, Co, Ni and Zn with Al and Fe. The LDHs are characterized using powder X-ray diffraction (PXRD), IR spectroscopy, TGA and wet chemical analysis. 2. Experimental The [M-M0 -CrO4] LDHs were prepared by the drop-wise addition (3 ml min1) of a mixed metal (MII + M0 III) nitrate solution into a reservoir containing ten times the stoichiometric requirement of CrO42 ions taken as its potassium salt. 2N NaOH was dispensed using a Metrohm Model 718 STAT Titrino to maintain a constant pH at precipitation. The pH at precipitation of various LDHs is presented in Table 1. The pH value is chosen based on the formation pH of each individual LDH [13]. N2 gas was bubbled through the solution during precipitation and aging for 18 h at 65 8C. The precipitate was rapidly filtered under suction and washed with deionized (1015 V cm specific resistance), decarbonated water and then dried at 65 8C for 24 h. Hydrothermal treatment of the dried powder sample was carried out in deionized, decarbonated water in a Teflon lined autoclave (150 ml, 50% filling) under autogenous pressure at 160 8C for 18 h. Thermal decomposition was carried out in a silica crucible by heating approximately 350 mg of the sample in a muffle furnace at 800 8C for 18 h. The oxide residues were then furnace cooled and analyzed by PXRD. All samples were characterized by powder X-ray diffraction using a X’pert ProPhilips diffractometer (Cu ˚ ) fitted with a graphite monochromator. IR spectra were recorded using a Nicolet Model Ka source, l = 1.541 A Impact 400D FTIR spectrometer (4000–400 cm1, resolution 4 cm1, KBr pellet). TGA studies were carried out using a Mettler-Toledo 851e TG/SDTA system driven by Stare 7.1 software (heating rate 5 8C min1, N2 gas). The samples were first heated to 100 8C in the TG balance for 0.5 h to drive away the adsorbed water before being ramped up to 800 8C. In the case of the [Mg-Al-CrO4] LDH, pre-weighed quantities of the oxide residue obtained at different temperatures between 500 and 800 8C were soaked in water for 24 h. The solid was then recovered by filtration through a sintered glass crucible and weighed. Wet chemical analysis of the CrO42 content of all the LDHs was done by dissolving a pre-weighed (0.2 g) quantity of the sample in acid (2 ml of conc. H2SO4) and titrating against standard ferrous ammonium sulphate (0.05N) potentiometrically. The CrO42 content is estimated in equivalents of Cr to obviate the need of estimating independently the CrO42 (pH > 6) and Cr2O72 (pH < 6) species. Chromate leaching was carried out by suspending Table 1 Synthesis conditions and wet chemical analysis of the chromate containing LDHs LDH
[Ca-Al-CrO4] [Mg-Al-CrO4] [Zn-Al-CrO4] [Ni-Al-CrO4] [Co-Al-CrO4] [Mg-Fe-CrO4] [Ni-Fe-CrO4] [Co-Fe-CrO4] a b
pH of synthesis 11.5 10.0 7.0 7.0 8.5 9.5 7.0 8.5
Cell parameters
PXRD line width (8 2u)
˚) Interlayer distance (A
˚) a (A
˚) c (A
Before HTa
After HTa
Before HTa
After HTa
5.75 3.056 3.097 3.039 3.059 3.037 3.069 3.116
20.374 24.690 22.033 23.310 23.559 23.809 24.699 22.347
0.3 1.2 0.6 2.2 1.8 0.8 2.3 2.0
– 0.5 – 1.2 – – 1.2 –
10.18 8.20 7.34 7.77 7.85 7.93 8.23 7.44
– 7.9 – 7.33 – – 7.35 –
HT: hydrothermal treatment. Approximate values due to spontaneous chromate leaching.
CrO42 content (mol%)
Replaceable CrO42 content (%)
0.168 0.114 0.208 0.184 0.162 0.060b 0.209 0.166
88.3 43.2 49.8 18.1 27.8 16.3b 32.0 25.1
1030
S.V. Prasanna et al. / Materials Research Bulletin 42 (2007) 1028–1039
pre-weighed (0.20 g) batches of the LDH in 25 ml of water containing five times the stoichiometric requirement of carbonate ions required to affect a complete exchange of the CrO42 ions in the starting LDH. The reaction was carried out for 5 h with stirring after which the solid was separated by centrifugation and washed with deionized water. The chromate leached out in these experiments is referred to as the replaceable chromate content and is compared with the theoretical exchange capacity computed from the molecular formula. 3. DIFFaX studies The nature of structural disorder in the CrO42 containing LDHs was studied by simulation of the PXRD patterns using the FORTRAN based computer program DIFFaX [14]. The DIFFaX program considers a crystalline solid as a stacking of layers of atoms. The diffraction pattern is computed by integrating the diffraction intensities layer by layer. The stacking sequence and the layer composition can be varied to engineer disorder in the lattice. The relative intensities, FWHM values and the positions of the peaks in the observed pattern are compared with those in the simulated patterns to determine the nature of disorder. The details of DIFFaX simulations of LDHs are described in our earlier papers [15,16]. In this paper, structure of the [Zn-Al-SO4] LDH (ICSD No. 91860, space group P-3, ˚ , c = 8.91 A ˚ ) is used as the model to describe the layer. Each layer comprises a metal hydroxide slab and a = 3.063 A the atoms of the interlayer, TO42 (T Cr). In the model structure, the SO42 ion is intercalated with one of the S–O bonds parallel to the c-crystallographic axis. This oxygen is referred to as the apical oxygen, while the other three are called basal oxygen atoms. The basal oxygen atoms and the oxygen of the intercalated water share a single set of sites (6g), while the apical oxygen atom occupies a crystallographically distinct site (2c). When these layers are stacked exactly one above the other using a stacking vector, (0, 0, 1), a 1H polytype is obtained. ‘1’ stands for the one-layer periodicity and ‘H’ stands for the hexagonal symmetry. A stacking vector incorporating a random translation (x, y, 1) (x, y: random) introduces what is known as turbostratic disorder, by destroying the registry between the layers. A systematic translation of successive layers by (1/3, 2/3, z) or (2/3, 1/3, z) results in a three-layered ordered crystal of rhombohedral symmetry, 3R. The simultaneous use of both these stacking vectors results in a crystal with stacking disorders. The local structure of the stacking faults however has a rhombohedral symmetry [16]. For the purpose of comparison with the observed patterns, the calculated reflections were broadened using a suitable Lorentzian profile function [15,16]. 4. Results and discussion In contrast with the cationic clays, the LDHs have aggressive solution chemistry [17]. The thermodynamic stability of the LDH is determined by the solubility of the salt of the divalent cation. Carbonate-containing LDHs are more stable than their chloride and nitrate containing counterparts [17] and therefore do not participate in anion exchange reactions [18]. The chloride and nitrate containing LDHs participate in anion exchange reactions by a dissolution–reprecipitation mechanism [19]. Thereby LDHs can themselves become a source of heavy metal contamination. This severely limits the choice of LDHs in water purification applications to Ca-Al, Ca-Fe, Mg-Al and Mg-Fe LDHs only. The other chromate containing LDHs offer models having different physical properties for comparison. 4.1. LDHs of Ca and Mg Fig. 1 presents the PXRD pattern of the [Ca-Al-CrO4] (x = 0.33) LDH. The PXRD pattern has a low angle reflection ˚ ) followed by another at 2u = 17.58 (5.05 A ˚ ) typical of LDHs. The pattern could be appearing at 2u = 8.68 (10.3 A ˚ ˚ indexed to a hexagonal cell with a = 5.75 A and c = 20.36 A. A Ca-Al LDH with CrO42 as the interlayer anion has been recently reported by Segni et al. [20]. The X-ray pattern is illustrative of a highly ordered structure typical of mineral hydrocalumite [21]. The structure of mineral hydrocalumite Ca2Al(OH)6Cl3H2O differs from that of the mineral hydrotalcite Mg6Al2(OH)16CO34H2O in that the former is cation ordered, while the latter is disordered. ˚ ) compared to the Al3+ ion Cation ordering in hydrocalumite arises due to the large radius of the Ca2+ ion (1.00 A 2+ ˚ (0.535 A). Ca therefore has seven coordination and bonds with one of the oxygen atoms of the interlayer water in addition to the six hydroxyl ions in the metal hydroxide slab. Consequently this structure has a higher a parameter ˚ ) compared to that of hydrotalcite (a = 3.09 A ˚ ). The 1 1 0 reflection in hydrocalumite, therefore appears at a (5.75 A
S.V. Prasanna et al. / Materials Research Bulletin 42 (2007) 1028–1039
1031
Fig. 1. Powder X-ray diffraction pattern of the [Ca-Al-CrO4] (x = 0.33) LDH. Feature marked by the asterisk corresponds to Gibbsite.
lower angle (31.18 2u) compared to that in hydrotalcite (60.68 2u) [21]. Our attempts to prepare [Ca-Fe-CrO4] LDH by coprecipitation yielded a X-ray amorphous phase. The PXRD pattern expected of the 1H polytype of the [Mg-Al-CrO4] (x = 0.25) LDH simulated using the model structure is given in Fig. 2a. The calculated pattern has the following families of Bragg reflections: (1) 0 0 l comprising the 0 0 1 and 0 0 2 reflections; (2) h 0 l comprising the 1 0 l (l = 1–5) reflections; (3) h k 0 comprising the 1 0 0 and 1 1 0 reflections; (4) h k l comprising the 1 1 1 and 1 1 2 reflections. In contrast, the observed pattern of this compound exhibits a PXRD pattern with only four broad and asymmetric peaks ˚ ) and 22.38 2u (4.0 A ˚ ) correspond to the basal reflections 0 0 l (l = 1, (see Fig. 2b). The first two peaks at 10.88 2u (8.2 A 2). The two higher angle reflections are asymmetrically broadened indicative of structural disorder [22]. The complete extinction of the h k l and h 0 l families of reflections points to the loss of three-dimensional periodicity. This can occur by the random orientation of successive layers about the c-crystallographic axis. The resulting loss of registry between layers is known as turbostratic disorder. The PXRD pattern of [Mg-Al-CrO4] (x = 0.25) LDH was simulated by the
Fig. 2. DIFFaX simulated PXRD patterns of (a) model structure of the 1H polytype and (b) (a) with turbostratic disorder compared with the experimental pattern of the as-prepared [Mg-Al-CrO4] (x = 0.25) LDH.
1032
S.V. Prasanna et al. / Materials Research Bulletin 42 (2007) 1028–1039
Fig. 3. DIFFaX simulated PXRD pattern of (a) model structure of 3R polytype and (b) (a) with 50% 2H kind of stacking disorders compared with the observed pattern of the hydrothermally treated [Mg-Al-CrO4] (x = 0.25) LDH.
inclusion of turbostratic disorder in the model structure and the results are shown in Fig. 2b. There is a good agreement between the observed and simulated patterns. From the simulated pattern, the following conclusions may be drawn. (1) The 0 0 l reflections are unaffected as loss of registry between successive layers does not affect the periodicity of electron density along the c-crystallographic axis, which is also the stacking direction. (2) Since the structure within the layers is unaffected, they generate two-dimensional Bragg reflections, which exhibit Warren broadening [23]. They are indexed as 10 and 11 respectively. The [Mg-Al-CrO4] LDH was hydrothermally treated to induce structural order. The results are shown in Fig. 3. The high angle (>608 2u) reflection has two sharp peaks. The mid-2u region also shows three broad features. In Fig. 3a is shown the DIFFaX simulated pattern of a three-layered polytype belonging to the rhombohedral symmetry (3R). The positions of the peaks in the observed pattern (Fig. 3b) match with those in the model pattern, showing that on hydrothermal treatment, there has been a structural transformation. However the peaks in the observed pattern are excessively broadened reflecting the presence of considerable structural disorder. The observed pattern was simulated by the incorporation of 50% of stacking disorders, whose local structure corresponds to that of a two-layered polytype (2H) of the hexagonal symmetry [24]. The stacking disorders arising from the intergrowth of two different polytypes is distinguishable from turbostratic disorder from the line shape of the h k 0 reflections. These represent two distinct models of structural disorder that commonly manifest in layered materials [25]. The PXRD pattern of the [Mg-Fe-CrO4] LDH shows all the features of a turbostratically-disordered phase as in the case of the Al analogue. The origin of disorder in the chromate containing LDHs may be traced to the nature of interlayer sites in the LDHs. The interlayer region of the LDHs incorporates trigonal prismatic sites [26]. The prismatic sites are suited for anions such as the carbonates, having the D3h symmetry. The carbonate ions are intercalated with their plane perpendicular to the c-crystallographic axis. The hydrogen bonding interactions between the interlayer carbonates and the metal hydroxide slabs are maximized in this mode of intercalation. All naturally occurring LDHs therefore incorporate carbonate anions in the interlayer. When anions having other symmetries are intercalated, there is a mismatch between the crystallographic site symmetry and the symmetry of the anion. This results in stacking disorders. Turbostraticity can be viewed as an extreme form of stacking disorder [23]. The mode of intercalation of the CrO42 ions is therefore critical to the structural order of the LDH. The mode of intercalation is evident from the IR spectra. The free (Td) CrO42 ion has only two IR active modes namely, the asymmetric stretch (y3) and the symmetric deformation (y4) [27]. The latter is expected at 330 cm1. Consequently the absorptions observed are due to the y3 mode, which in free chromates appears at 765 cm1 and is triply degenerate. The IR spectra of [Mg-Al-CrO4] LDH and [Ca-Al-CrO4] LDH presented in Fig. 4 exhibit a band at 886 cm1. We
S.V. Prasanna et al. / Materials Research Bulletin 42 (2007) 1028–1039
1033
Fig. 4. IR spectra of (a) [Mg-Al-CrO4] (x = 0.25) LDH and (b) [Ca-Al-CrO4] (x = 0.33) LDH.
assign this to the y3 vibration of the intercalated CrO42 ion. Upon hydrothermal treatment, the IR spectrum of [MgAl-CrO4] LDH remains unchanged (data not shown), as does the interlayer distance as observed in the PXRD pattern, showing that the mode of coordination of the CrO42 ion remains unchanged on hydrothermal treatment. 4.2. LDH of Zn with Al ˚ as In Fig. 5 is shown the PXRD pattern of the [Zn-Al-CrO4] LDH. The interlayer spacing in this LDH is only 7.3 A ˚ compared to 8.2 A observed in the LDH of Mg. The PXRD profile in the mid-2u region in this case differs from that seen in other LDHs, indicative of a different model of structural disorder. This profile could be simulated by the incorporation of stacking faults whose local structure has a rhombohedral symmetry. Such faults have been described in detail elsewhere [16].
Fig. 5. PXRD pattern of [Zn-Al-CrO4] (x = 0.25) LDH compared with the corresponding DIFFaX simulation.
1034
S.V. Prasanna et al. / Materials Research Bulletin 42 (2007) 1028–1039
Fig. 6. IR spectra of (a) [Zn-Al-CrO4] (x = 0.25) LDH and (b) [Ni-Al-CrO4] (x = 0.25) LDH.
The interlayer spacing corresponds to a one-atom thick interlayer showing that the CrO42 ion is directly grafted to the metal hydroxide slab. This altered mode of coordination reflects in the IR spectra. By Halford’s rules, the ccrystallographic axis of the LDH is treated as the main axis of the CrO42 ion [28]. Thus the grafted CrO42 ion behaves as if it belongs to C3v symmetry. The asymmetric stretch splits into two different modes (A1 + E) in C3v symmetry. The difference in the frequencies of these two modes would depend on the strength of the interaction between the intercalated chromate and the metal hydroxide layer. The apical oxygen of the TO42 ion does not participate in any hydrogen bonding with the metal hydroxide slab, where as the basal oxygen atoms do [29]. In the case of [Zn-Al-CrO4] LDH, the two modes are clearly resolved as reflected by the appearance of two bands at 922 and 876 cm1 (see Fig. 6a). This is direct evidence for the lowering of symmetry of the CrO42 ion in the interlayer. 4.3. LDHs of Ni and Co In Fig. 7 are shown the PXRD patterns of the as-prepared and hydrothermally treated [Ni-Al-CrO4] LDHs. Both the patterns are characteristic of materials with turbostratic disorder and have been simulated. The only difference between the two patterns is a narrowing of the peaks after hydrothermal treatment. This is reflected in the reduction of the FWHM value of the Lorentzian used in the simulation from 2.28 2u to 1.28 2u after hydrothermal treatment. Such a narrowing of the line shape is reflective of crystal growth, but the nature of disorder remains unchanged. The interlayer ˚ in the as-prepared sample to 7.3 A ˚ after hydrothermal treatment. The latter value is spacing decreases from 7.8 A reflective of a grafted species. Similar behavior was observed in the [Ni-Fe-CrO4] LDH as well (data not shown). The characteristics of the LDHs of Co with Al and Fe are very similar to those of their Ni counterparts. The interlayer spacings and the line width of the Lorentzians used in the simulation are listed in Table 1. The IR spectra in all these systems show two absorptions at 933 and 876 cm1 respectively, which we attribute to the two asymmetric stretching modes of the CrO42 ion in the C3v symmetry. An illustrative spectrum of the Ni-Al system is shown in Fig. 6b. 4.4. Thermal behavior of the chromate containing LDHs All the LDHs reported here were studied by the TGA technique. In Fig. 8 are shown representative TG data of the LDHs. All the thermograms exhibit the following features: (1) There is a very large mass loss below 100 8C. We attribute this to the loss of adsorbed moisture. However as this mass loss does not appear complete even after a stay at 100 8C (0.5 h), it is likely some of the structural water is
S.V. Prasanna et al. / Materials Research Bulletin 42 (2007) 1028–1039
1035
Fig. 7. PXRD patterns of the [Ni-Al-CrO4] (x = 0.25) LDH (a) before and (b) after hydrothermal treatment compared with the corresponding DIFFaX simulations.
Fig. 8. TG data of chromate containing (a) Mg-Al, (b) Co-Al, (c) Mg-Fe and (d) Ni-Fe LDHs.
1036
S.V. Prasanna et al. / Materials Research Bulletin 42 (2007) 1028–1039
Table 2 Results of thermal analysis of chromate containing LDHs LDH
Expected weight loss (%)
Observed weight loss (%)
Products of decomposition at 800 8C
[Ca-Al-CrO4] (x = 0.33) [Mg-Al-CrO4] (x = 0.25) [Zn-Al-CrO4] (x = 0.25) [Ni-Al-CrO4] (x = 0.25) [Co-Al-CrO4] (x = 0.25) [Mg-Fe-CrO4] (x = 0.25) [Ni-Fe-CrO4] (x = 0.25) [Co-Fe-CrO4] (x = 0.25)
37.3 36.3 26.4 27.7 24.0 33.4 26.48 22.4
38.2 42.2 31.4 29.4 26.0 35.6 31.44 21.5
Ca4Al6O12CrO4 + CaO MgO + MgAl2O4 ZnO + Cr2O3 NiO + NiAl2O4 Co3O4 MgO + MgFe2O4 NiO + NiFe2O4 Co3O4
also lost at these low temperatures. The mass loss observed at this temperature varies between 6 and 8% in different LDHs. (2) Between 500 and 800 8C, there is an extended mass loss of over 5%. There are no signs that this tail is complete even at 800 8C. This kind of an extended loss seen in many layered hydroxides has been attributed to the gradual loss of volatile gas from the micropores of the oxide residue [30]. This mass loss proceeds in proportion to the extent of sintering of the oxide and the consequent collapse of the pores up to high temperatures. However the layered structure of the LDH breaks down below 500 8C itself. (3) In the 100–500 8C range of temperature, two prominent mass losses can be observed in the M-Al (M = Mg, Co, Ni) LDHs. The M-Fe LDHs (M = Mg, Ni) exhibit three mass losses. The low temperature (100–250 8C) mass loss is due to the loss of intercalated water molecules, while the high temperature mass loss (250–800 8C) is due to dehydroxylation of the layers [31,32]. The CrO42 ion undergoes reduction at high temperatures to yield Cr(III) species [33]. As the various steps overlap, the net mass loss in the temperature range 100–800 8C was compared (see Table 2) with the mass loss expected for the reaction: MII 6 M0III 2 ðOHÞ16 CrO4 4H2 O ! 0:5MII Cr2 O4 þ MII M0III 2 O4 þ 4:5MII O Except for the Mg-Al and Zn-Al systems, the match was found to be satisfactory. The products of the decomposition reaction are oxides and the PXRD patterns of representative oxide residues are shown in Fig. 9. The LDHs of Co yield
Fig. 9. PXRD pattern of the products obtained after thermal decomposition of chromate containing (a) Ni-Fe, (b) Zn-Al and (c) Co-Al LDHs. The major phases are NiO, ZnO and Co3O4 respectively. Features marked * correspond to the spinel. Features marked + correspond to Cr2O3.
S.V. Prasanna et al. / Materials Research Bulletin 42 (2007) 1028–1039
1037
Fig. 10. PXRD pattern of (a) freshly calcined (600 8C) oxide residue of the [Mg-Al-CrO4] (x = 0.25) LDH and (b) LDH obtained by soaking (a) in water.
a single-phase spinel having the structure of Co3O4. The LDHs of Ni yield a mixture of NiO and spinel Ni(M0 ,Cr)2O4 (M0 = Al, Fe). The LDH of Zn yields a mixture of wurtzite ZnO and (Al,Cr)2O3. The case of Mg-Al LDH is unique in that it yields a poorly ordered periclase, MgO above 500 8C (see Fig. 10a). There are no reflections associated with any Al-containing phase, showing that Al3+ ions are incorporated in the MgO lattice itself. The peaks are broad due to disorder brought about by the insertion of Al3+ ions into the MgO lattice. This kind of a defect oxide is highly unstable and reconstructs back into the parent LDH on soaking in water. The reconstruction is signified by the reappearance of the basal reflections in the PXRD pattern (see Fig. 10b). The oxide regains 42% of its weight by picking up hydroxyl ions and CO2 from water. The reconstruction phenomenon also known as ‘memory effect’ has been reported earlier [34,35] among the LDHs of Mg. The reversible thermal behavior is associated with the high formation temperature of the spinels of Mg [36]. 4.5. Wet chemical analysis and replaceable chromate content In Table 1 are given the results of wet chemical analysis of the chromate content of the LDHs. With the exception of [Ca-Al-CrO4] LDH and [Mg-Al-CrO4] LDH, the CrO42 present in other LDHs is nearly twice the stoichiometrically expected quantity. This is likely due to the formation of unitary chromates during the precipitation and post-synthetic treatment of the LDHs. The unitary chromates of transition metals such as ZnCrO4, NiCrO4 and CoCrO4 are insoluble and may have poor crystallinity by virtue of which their presence is not detected in the PXRD patterns of the LDHs. The chromate content is consistent with the nominal composition only in the case of Mg-Al (x = 0.25) and Ca-Al (x = 0.33) LDHs. Two factors are important in the application of LDHs as CrO42 scavengers: (1) the chromate carrying capacity of the LDHs and (2) chromate mineralization. Our wet chemical analysis shows that the LDHs take up chromate ions in excess of the theoretical maximum afforded by the chemical formula and the number of crystallographically permitted interlayer sites. To determine the extent of mineralization of the intercalated chromate, the chromate containing LDHs were soaked in a Na2CO3 solution. The choice of Na2CO3 was dictated by the ubiquitous presence of both CO2 and Na+ in the natural environment of the earth and also by the fact that carbonates are the most preferred anions in the interlayer of the LDHs by virtue of their thermodynamic stability [17]. It was found that all the LDHs discharge their chromate ions either wholly or partially in a Na2CO3 solution (see Table 1). In some instances such as the Ca-Al LDH, the final product is calcite CaCO3 showing that this particular LDH reacts by dissolution of the LDH and reprecipitation of calcite. In the process, the entire chromate is discharged into the liquid phase. In the case of the LDHs of Mg and Zn, nearly half the chromate is discharged, while in the LDH of Ni and Co with Al and Fe respectively only 18–25% of the chromate is discharged. While none of the LDHs release chromate ions into plain deionized water, the exception being LDH of Mg with Fe which
1038
S.V. Prasanna et al. / Materials Research Bulletin 42 (2007) 1028–1039
continually discharges chromate ions even in water. Even after several days of washing the as-prepared sample, the wash was not chromate-free. Chromate leaching takes place as a result of carbonate exchange. Anion exchange reactions are governed by many factors, of which the solubilities of the precursor and product LDHs play a critical role [17,19]. These factors are constrained by the pH of the reaction medium. Efficient mineralization of the anion is contingent on the thermodynamic stability of the corresponding LDH as reflected by its lower solubility. The chromate incorporated LDH has a lower solubility product than the chloride and nitrate containing LDHs, making it possible to synthesize the former using mixed metal (MII + M0 III) nitrate or chloride salt solutions. However among the LDHs, those containing carbonate ions are the most stable [17], thus making the intercalated chromate replaceable by carbonate. In general grafted chromates such as those in the LDHs of Ni and Co are less replaceable than intercalated chromates by virtue of their stronger interaction with the metal hydroxide slab. 5. Conclusions In conclusion, chromate containing LDHs prepared by coprecipitation incorporate stacking disorders arising out of a mismatch between the symmetry of the chromate anions and the symmetry of the interlayer site in the LDHs. These LDHs serve as excellent models to study the possible use of LDHs as chromate scavenging agents for green chemistry applications. The mineralization of chromates by the LDH is partial and paves the way for the possible recovery and reuse of the chromate for industrial applications. Acknowledgement The authors thank the Department of Science and Technology, Government of India, for financial support. References [1] R.L. Goswamee, P. Sengupta, K.G. Bhattacharyya, D.K. Dutta, Appl. Clay Sci. 13 (1998) 21. [2] T.J. Pinnavaia, Science 220 (1983) 365. [3] H.R. Oswald, R. Asper, in: R.M.A. Leith (Ed.), Preparation and Crystal Growth of Materials with Layered Structures, D. Riedel Publishing Company, Dordrecht, 1977, p. 71. [4] K.A. Carrado, A. Kostapapas, S.L. Suib, Solid State Ionics 26 (1988) 77. [5] A.I. Khan, Dermot O’Hare, J. Mater. Chem. 12 (2002) 3191. [6] S.P. Newman, W. Jones, New J. Chem. (1998) 105. [7] S. Carlino, Solid State Ionics 98 (1997) 73. [8] Y. Roh, S.Y. Lee, M.P. Elless, J.E. Foss, Clays Clay Miner. 48 (2000) 226. [9] B. Houri, A. Legrouri, A. Barroug, C. Forano, J.P. Besse, Collect. Czech. Chem. Commun. 63 (1998) 732. [10] S. Tezuka, R. Chitrakar, K. Sakane, A. Sonoda, K. Ooi, T. Tomida, Bull. Chem. Soc. Jpn. 77 (2004) 2101. [11] S. Miyata, A. Okada, Clays Clay Miner. 23 (1977) 14. [12] K. El Malki, A. de Roy, J.P. Besse, Eur. J. Solid State Inorg. Chem. 26 (1989) 339. [13] J.W. Boclair, P.S. Braterman, Chem. Mater. 11 (1999) 298. [14] M.M.J. Treacy, M.W. Deem, J.M. Newsam, Computer Code DIFFaX, Version 1.807. [15] T.N. Ramesh, R.S. Jayashree, P.V. Kamath, Clays Clay Miner. 51 (2003) 570. [16] G.S. Thomas, M. Rajamathi, P.V. Kamath, Clays Clay Miner. 52 (2004) 693. [17] R.K. Allada, A. Navrotsky, H.T. Berbeco, W.H. Casey, Science 296 (2002) 721. [18] D.L. Bish, Bull. Mineral. 52 (1980) 1036. [19] A.V. Radha, P.V. Kamath, C. Shivakumara, Solid State Sci. 7 (2005) 1180. [20] R. Segni, L. Vieille, F. Leroux, C. Taviot-Gueho, J. Phys. Chem. Solids 67 (2006) 1037. [21] L. Vieille, I. Rousselot, F. Leroux, J.P. Besse, C. Taviot-Gueho, Chem. Mater. 15 (2003) 4361. [22] T.N. Ramesh, M. Rajamathi, P.V. Kamath, Solid State Sci. 5 (2003) 751. [23] D.R. Hines, G.T. Seidler, M.M.J. Treacy, S.A. Solin, Solid State Commun. 101 (1997) 835. [24] A.V. Radha, P.V. Kamath, C. Shivakumara, Clays Clay Miner. 53 (2005) 521. [25] A. Viani, A.F. Gualtieri, G. Artioli, Am. Mineral. 87 (2002) 966. [26] A.S. Bookin, V.A. Drits, Clays Clay Miner. 41 (1993) 551. [27] K. Nakamoto, Infrared and Raman of Inorganic and Coordination Compounds, Wiley, New York, 1986. [28] S.D. Ross, Inorganic Infrared and Raman Spectra, McGraw-Hill, London, 1972. [29] A.S. Bookin, V.I. Cherkashin, V.A. Drits, Clays Clay Miner. 41 (1993) 558. [30] A. Delahay-Vidal, K. Tekaia-Elhsissen, Genin, M. Figlarz, Eur. J. Solid State Inorg. Chem. 31 (1994) 823.
S.V. Prasanna et al. / Materials Research Bulletin 42 (2007) 1028–1039 [31] [32] [33] [34] [35] [36]
V. Rives, Inorg. Chem. 38 (1999) 406. P. Bera, M. Rajamathi, M.S. Hegde, P.V. Kamath, Bull. Mater. Sci. 23 (2000) 141. D. de Waal, A.M. Heyns, J. Alloys Compd. 187 (1992) 171. T. Sato, K. Kato, T. Endo, M. Shimada, React. Solids 2 (1986) 253. V.R.L. Constantino, T.J. Pinnavia, Inorg. Chem. 34 (1995) 883. V. Rives, Mater. Chem. Phys. 75 (2002) 19.
1039
Synthesis and characterization of layered double hydroxides (LDHs) with intercalated chromate ions Srinivasa V. Prasanna a, P. Vishnu Kamath a,*, C. Shivakumara b a
Department of Chemistry, Central College, Bangalore University, Bangalore, Karnataka 560001, India b Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India Received 22 June 2006; received in revised form 31 August 2006; accepted 26 September 2006 Available online 1 November 2006
Abstract Chromate intercalated layered double hydroxides (LDHs) having the formula MII6M0 III2(OH)16CrO44H2O (MII = Ca, Mg, Co, Ni, Zn with M0 III = Al and MII = Mg, Co, Ni with M0 III = Fe) have been prepared by coprecipitation. The products obtained are replete with stacking disorders. DIFFaX simulations show that the stacking disorders are of three kinds: (i) turbostratic disorder of an originally single layered hexagonal (1H) crystal, (ii) random intergrowth of polytypes with hexagonal (2H) and rhombohedral (3R) symmetries and (iii) translation of randomly chosen layers by (2/3, 1/3, z) and (1/3, 2/3, z) leading to stacking faults having a local structure of rhombohedral symmetry. IR spectra show that the CrO42 ion is incorporated either in the Td or in the C3v ˚ characteristic of a single atom thick interlayer showing that the CrO42 symmetry. The interlayer spacing in the latter case is 7.3 A ion is grafted to the metal hydroxide slab. On thermal treatment, the CrO42 ion transforms into Cr(III) and is incorporated into the spinel oxide or phase separates as Cr2O3. In the LDH of Mg with Al, Cr(III) remains in the MgO lattice as a defect and promotes the reconstruction of the LDH on soaking in water. In different LDHs, 18–50% of the CrO42 ion is replaceable with carbonate anions showing only partial mineralization of the water-soluble chromate. The extent of replaceable chromates depends upon the solubility of the corresponding LDH, which in turn is determined by the solubility of the MCrO4. These studies have profound implications for the possible use of LDHs for chromate amelioration in green chemistry. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Layered compounds; B. Chemical synthesis; C. X-ray diffraction
1. Introduction Layered materials possessing anion exchange properties are useful host materials to sequester insidious anionic contaminants from solution. Among several layered materials, layered double hydroxides (LDHs) have evoked considerable interest in recent years due to their potential application in environmental remediation of anionic pollutants [1]. LDHs are structurally and functionally the inverse of cationic clays [2]. The structure of LDHs is derived from that of mineral brucite, Mg(OH)2 [3]. Brucite comprises a close packing of hydroxyl ions in which Mg2+ ions occupy alternate layers of octahedral sites, leading to stacking of charge-neutral metal hydroxide slabs of composition [Mg(OH)2]. When a fraction, x, of Mg2+ ions is isomorphously substituted by a trivalent ion such as Al3+ or Fe3+, the positive charge x+ generated on the metal hydroxide slab is compensated by the inclusion of anions, An,
* Corresponding author. Tel.: +91 80 22961354; fax: +91 80 22931310. E-mail address: [email protected] (P.V. Kamath). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.09.021
S.V. Prasanna et al. / Materials Research Bulletin 42 (2007) 1028–1039
1029
in the interlayer region to give LDHs of composition [MII1xM0 IIIx(OH)2](An)x/nmH2O [4], where MII = Mg, Ca, Co, Ni, Zn, etc. and M0 III = Al, Cr, Fe, etc. and 0.2 x 0.33. We refer to the LDHs as [M-M0 -A]. LDHs exhibit anion exchange property and incorporate a wide variety of anions into the interlayer [5–7]. By virtue of this property, the LDHs have the potential to scavenge hazardous anionic contaminants present in waste water [8–10]. In this context, the synthesis and characterization of LDHs, which incorporate the anionic pollutant is important as these LDHs are the final products of sequestration reactions in the application of LDHs as water decontaminating agents. One of the most insidious anionic contaminants in industrial waste water is the chromate ion. However, there are only a few studies of CrO42 containing LDHs in the literature [11,12]. A study of the structural and compositional features of CrO42 containing LDHs is important in the context of remediation of CrO42 ions by LDHs. In this paper, we report the synthesis and characterization of CrO42 containing LDHs of Mg, Ca, Co, Ni and Zn with Al and Fe. The LDHs are characterized using powder X-ray diffraction (PXRD), IR spectroscopy, TGA and wet chemical analysis. 2. Experimental The [M-M0 -CrO4] LDHs were prepared by the drop-wise addition (3 ml min1) of a mixed metal (MII + M0 III) nitrate solution into a reservoir containing ten times the stoichiometric requirement of CrO42 ions taken as its potassium salt. 2N NaOH was dispensed using a Metrohm Model 718 STAT Titrino to maintain a constant pH at precipitation. The pH at precipitation of various LDHs is presented in Table 1. The pH value is chosen based on the formation pH of each individual LDH [13]. N2 gas was bubbled through the solution during precipitation and aging for 18 h at 65 8C. The precipitate was rapidly filtered under suction and washed with deionized (1015 V cm specific resistance), decarbonated water and then dried at 65 8C for 24 h. Hydrothermal treatment of the dried powder sample was carried out in deionized, decarbonated water in a Teflon lined autoclave (150 ml, 50% filling) under autogenous pressure at 160 8C for 18 h. Thermal decomposition was carried out in a silica crucible by heating approximately 350 mg of the sample in a muffle furnace at 800 8C for 18 h. The oxide residues were then furnace cooled and analyzed by PXRD. All samples were characterized by powder X-ray diffraction using a X’pert ProPhilips diffractometer (Cu ˚ ) fitted with a graphite monochromator. IR spectra were recorded using a Nicolet Model Ka source, l = 1.541 A Impact 400D FTIR spectrometer (4000–400 cm1, resolution 4 cm1, KBr pellet). TGA studies were carried out using a Mettler-Toledo 851e TG/SDTA system driven by Stare 7.1 software (heating rate 5 8C min1, N2 gas). The samples were first heated to 100 8C in the TG balance for 0.5 h to drive away the adsorbed water before being ramped up to 800 8C. In the case of the [Mg-Al-CrO4] LDH, pre-weighed quantities of the oxide residue obtained at different temperatures between 500 and 800 8C were soaked in water for 24 h. The solid was then recovered by filtration through a sintered glass crucible and weighed. Wet chemical analysis of the CrO42 content of all the LDHs was done by dissolving a pre-weighed (0.2 g) quantity of the sample in acid (2 ml of conc. H2SO4) and titrating against standard ferrous ammonium sulphate (0.05N) potentiometrically. The CrO42 content is estimated in equivalents of Cr to obviate the need of estimating independently the CrO42 (pH > 6) and Cr2O72 (pH < 6) species. Chromate leaching was carried out by suspending Table 1 Synthesis conditions and wet chemical analysis of the chromate containing LDHs LDH
[Ca-Al-CrO4] [Mg-Al-CrO4] [Zn-Al-CrO4] [Ni-Al-CrO4] [Co-Al-CrO4] [Mg-Fe-CrO4] [Ni-Fe-CrO4] [Co-Fe-CrO4] a b
pH of synthesis 11.5 10.0 7.0 7.0 8.5 9.5 7.0 8.5
Cell parameters
PXRD line width (8 2u)
˚) Interlayer distance (A
˚) a (A
˚) c (A
Before HTa
After HTa
Before HTa
After HTa
5.75 3.056 3.097 3.039 3.059 3.037 3.069 3.116
20.374 24.690 22.033 23.310 23.559 23.809 24.699 22.347
0.3 1.2 0.6 2.2 1.8 0.8 2.3 2.0
– 0.5 – 1.2 – – 1.2 –
10.18 8.20 7.34 7.77 7.85 7.93 8.23 7.44
– 7.9 – 7.33 – – 7.35 –
HT: hydrothermal treatment. Approximate values due to spontaneous chromate leaching.
CrO42 content (mol%)
Replaceable CrO42 content (%)
0.168 0.114 0.208 0.184 0.162 0.060b 0.209 0.166
88.3 43.2 49.8 18.1 27.8 16.3b 32.0 25.1
1030
S.V. Prasanna et al. / Materials Research Bulletin 42 (2007) 1028–1039
pre-weighed (0.20 g) batches of the LDH in 25 ml of water containing five times the stoichiometric requirement of carbonate ions required to affect a complete exchange of the CrO42 ions in the starting LDH. The reaction was carried out for 5 h with stirring after which the solid was separated by centrifugation and washed with deionized water. The chromate leached out in these experiments is referred to as the replaceable chromate content and is compared with the theoretical exchange capacity computed from the molecular formula. 3. DIFFaX studies The nature of structural disorder in the CrO42 containing LDHs was studied by simulation of the PXRD patterns using the FORTRAN based computer program DIFFaX [14]. The DIFFaX program considers a crystalline solid as a stacking of layers of atoms. The diffraction pattern is computed by integrating the diffraction intensities layer by layer. The stacking sequence and the layer composition can be varied to engineer disorder in the lattice. The relative intensities, FWHM values and the positions of the peaks in the observed pattern are compared with those in the simulated patterns to determine the nature of disorder. The details of DIFFaX simulations of LDHs are described in our earlier papers [15,16]. In this paper, structure of the [Zn-Al-SO4] LDH (ICSD No. 91860, space group P-3, ˚ , c = 8.91 A ˚ ) is used as the model to describe the layer. Each layer comprises a metal hydroxide slab and a = 3.063 A the atoms of the interlayer, TO42 (T Cr). In the model structure, the SO42 ion is intercalated with one of the S–O bonds parallel to the c-crystallographic axis. This oxygen is referred to as the apical oxygen, while the other three are called basal oxygen atoms. The basal oxygen atoms and the oxygen of the intercalated water share a single set of sites (6g), while the apical oxygen atom occupies a crystallographically distinct site (2c). When these layers are stacked exactly one above the other using a stacking vector, (0, 0, 1), a 1H polytype is obtained. ‘1’ stands for the one-layer periodicity and ‘H’ stands for the hexagonal symmetry. A stacking vector incorporating a random translation (x, y, 1) (x, y: random) introduces what is known as turbostratic disorder, by destroying the registry between the layers. A systematic translation of successive layers by (1/3, 2/3, z) or (2/3, 1/3, z) results in a three-layered ordered crystal of rhombohedral symmetry, 3R. The simultaneous use of both these stacking vectors results in a crystal with stacking disorders. The local structure of the stacking faults however has a rhombohedral symmetry [16]. For the purpose of comparison with the observed patterns, the calculated reflections were broadened using a suitable Lorentzian profile function [15,16]. 4. Results and discussion In contrast with the cationic clays, the LDHs have aggressive solution chemistry [17]. The thermodynamic stability of the LDH is determined by the solubility of the salt of the divalent cation. Carbonate-containing LDHs are more stable than their chloride and nitrate containing counterparts [17] and therefore do not participate in anion exchange reactions [18]. The chloride and nitrate containing LDHs participate in anion exchange reactions by a dissolution–reprecipitation mechanism [19]. Thereby LDHs can themselves become a source of heavy metal contamination. This severely limits the choice of LDHs in water purification applications to Ca-Al, Ca-Fe, Mg-Al and Mg-Fe LDHs only. The other chromate containing LDHs offer models having different physical properties for comparison. 4.1. LDHs of Ca and Mg Fig. 1 presents the PXRD pattern of the [Ca-Al-CrO4] (x = 0.33) LDH. The PXRD pattern has a low angle reflection ˚ ) followed by another at 2u = 17.58 (5.05 A ˚ ) typical of LDHs. The pattern could be appearing at 2u = 8.68 (10.3 A ˚ ˚ indexed to a hexagonal cell with a = 5.75 A and c = 20.36 A. A Ca-Al LDH with CrO42 as the interlayer anion has been recently reported by Segni et al. [20]. The X-ray pattern is illustrative of a highly ordered structure typical of mineral hydrocalumite [21]. The structure of mineral hydrocalumite Ca2Al(OH)6Cl3H2O differs from that of the mineral hydrotalcite Mg6Al2(OH)16CO34H2O in that the former is cation ordered, while the latter is disordered. ˚ ) compared to the Al3+ ion Cation ordering in hydrocalumite arises due to the large radius of the Ca2+ ion (1.00 A 2+ ˚ (0.535 A). Ca therefore has seven coordination and bonds with one of the oxygen atoms of the interlayer water in addition to the six hydroxyl ions in the metal hydroxide slab. Consequently this structure has a higher a parameter ˚ ) compared to that of hydrotalcite (a = 3.09 A ˚ ). The 1 1 0 reflection in hydrocalumite, therefore appears at a (5.75 A
S.V. Prasanna et al. / Materials Research Bulletin 42 (2007) 1028–1039
1031
Fig. 1. Powder X-ray diffraction pattern of the [Ca-Al-CrO4] (x = 0.33) LDH. Feature marked by the asterisk corresponds to Gibbsite.
lower angle (31.18 2u) compared to that in hydrotalcite (60.68 2u) [21]. Our attempts to prepare [Ca-Fe-CrO4] LDH by coprecipitation yielded a X-ray amorphous phase. The PXRD pattern expected of the 1H polytype of the [Mg-Al-CrO4] (x = 0.25) LDH simulated using the model structure is given in Fig. 2a. The calculated pattern has the following families of Bragg reflections: (1) 0 0 l comprising the 0 0 1 and 0 0 2 reflections; (2) h 0 l comprising the 1 0 l (l = 1–5) reflections; (3) h k 0 comprising the 1 0 0 and 1 1 0 reflections; (4) h k l comprising the 1 1 1 and 1 1 2 reflections. In contrast, the observed pattern of this compound exhibits a PXRD pattern with only four broad and asymmetric peaks ˚ ) and 22.38 2u (4.0 A ˚ ) correspond to the basal reflections 0 0 l (l = 1, (see Fig. 2b). The first two peaks at 10.88 2u (8.2 A 2). The two higher angle reflections are asymmetrically broadened indicative of structural disorder [22]. The complete extinction of the h k l and h 0 l families of reflections points to the loss of three-dimensional periodicity. This can occur by the random orientation of successive layers about the c-crystallographic axis. The resulting loss of registry between layers is known as turbostratic disorder. The PXRD pattern of [Mg-Al-CrO4] (x = 0.25) LDH was simulated by the
Fig. 2. DIFFaX simulated PXRD patterns of (a) model structure of the 1H polytype and (b) (a) with turbostratic disorder compared with the experimental pattern of the as-prepared [Mg-Al-CrO4] (x = 0.25) LDH.
1032
S.V. Prasanna et al. / Materials Research Bulletin 42 (2007) 1028–1039
Fig. 3. DIFFaX simulated PXRD pattern of (a) model structure of 3R polytype and (b) (a) with 50% 2H kind of stacking disorders compared with the observed pattern of the hydrothermally treated [Mg-Al-CrO4] (x = 0.25) LDH.
inclusion of turbostratic disorder in the model structure and the results are shown in Fig. 2b. There is a good agreement between the observed and simulated patterns. From the simulated pattern, the following conclusions may be drawn. (1) The 0 0 l reflections are unaffected as loss of registry between successive layers does not affect the periodicity of electron density along the c-crystallographic axis, which is also the stacking direction. (2) Since the structure within the layers is unaffected, they generate two-dimensional Bragg reflections, which exhibit Warren broadening [23]. They are indexed as 10 and 11 respectively. The [Mg-Al-CrO4] LDH was hydrothermally treated to induce structural order. The results are shown in Fig. 3. The high angle (>608 2u) reflection has two sharp peaks. The mid-2u region also shows three broad features. In Fig. 3a is shown the DIFFaX simulated pattern of a three-layered polytype belonging to the rhombohedral symmetry (3R). The positions of the peaks in the observed pattern (Fig. 3b) match with those in the model pattern, showing that on hydrothermal treatment, there has been a structural transformation. However the peaks in the observed pattern are excessively broadened reflecting the presence of considerable structural disorder. The observed pattern was simulated by the incorporation of 50% of stacking disorders, whose local structure corresponds to that of a two-layered polytype (2H) of the hexagonal symmetry [24]. The stacking disorders arising from the intergrowth of two different polytypes is distinguishable from turbostratic disorder from the line shape of the h k 0 reflections. These represent two distinct models of structural disorder that commonly manifest in layered materials [25]. The PXRD pattern of the [Mg-Fe-CrO4] LDH shows all the features of a turbostratically-disordered phase as in the case of the Al analogue. The origin of disorder in the chromate containing LDHs may be traced to the nature of interlayer sites in the LDHs. The interlayer region of the LDHs incorporates trigonal prismatic sites [26]. The prismatic sites are suited for anions such as the carbonates, having the D3h symmetry. The carbonate ions are intercalated with their plane perpendicular to the c-crystallographic axis. The hydrogen bonding interactions between the interlayer carbonates and the metal hydroxide slabs are maximized in this mode of intercalation. All naturally occurring LDHs therefore incorporate carbonate anions in the interlayer. When anions having other symmetries are intercalated, there is a mismatch between the crystallographic site symmetry and the symmetry of the anion. This results in stacking disorders. Turbostraticity can be viewed as an extreme form of stacking disorder [23]. The mode of intercalation of the CrO42 ions is therefore critical to the structural order of the LDH. The mode of intercalation is evident from the IR spectra. The free (Td) CrO42 ion has only two IR active modes namely, the asymmetric stretch (y3) and the symmetric deformation (y4) [27]. The latter is expected at 330 cm1. Consequently the absorptions observed are due to the y3 mode, which in free chromates appears at 765 cm1 and is triply degenerate. The IR spectra of [Mg-Al-CrO4] LDH and [Ca-Al-CrO4] LDH presented in Fig. 4 exhibit a band at 886 cm1. We
S.V. Prasanna et al. / Materials Research Bulletin 42 (2007) 1028–1039
1033
Fig. 4. IR spectra of (a) [Mg-Al-CrO4] (x = 0.25) LDH and (b) [Ca-Al-CrO4] (x = 0.33) LDH.
assign this to the y3 vibration of the intercalated CrO42 ion. Upon hydrothermal treatment, the IR spectrum of [MgAl-CrO4] LDH remains unchanged (data not shown), as does the interlayer distance as observed in the PXRD pattern, showing that the mode of coordination of the CrO42 ion remains unchanged on hydrothermal treatment. 4.2. LDH of Zn with Al ˚ as In Fig. 5 is shown the PXRD pattern of the [Zn-Al-CrO4] LDH. The interlayer spacing in this LDH is only 7.3 A ˚ compared to 8.2 A observed in the LDH of Mg. The PXRD profile in the mid-2u region in this case differs from that seen in other LDHs, indicative of a different model of structural disorder. This profile could be simulated by the incorporation of stacking faults whose local structure has a rhombohedral symmetry. Such faults have been described in detail elsewhere [16].
Fig. 5. PXRD pattern of [Zn-Al-CrO4] (x = 0.25) LDH compared with the corresponding DIFFaX simulation.
1034
S.V. Prasanna et al. / Materials Research Bulletin 42 (2007) 1028–1039
Fig. 6. IR spectra of (a) [Zn-Al-CrO4] (x = 0.25) LDH and (b) [Ni-Al-CrO4] (x = 0.25) LDH.
The interlayer spacing corresponds to a one-atom thick interlayer showing that the CrO42 ion is directly grafted to the metal hydroxide slab. This altered mode of coordination reflects in the IR spectra. By Halford’s rules, the ccrystallographic axis of the LDH is treated as the main axis of the CrO42 ion [28]. Thus the grafted CrO42 ion behaves as if it belongs to C3v symmetry. The asymmetric stretch splits into two different modes (A1 + E) in C3v symmetry. The difference in the frequencies of these two modes would depend on the strength of the interaction between the intercalated chromate and the metal hydroxide layer. The apical oxygen of the TO42 ion does not participate in any hydrogen bonding with the metal hydroxide slab, where as the basal oxygen atoms do [29]. In the case of [Zn-Al-CrO4] LDH, the two modes are clearly resolved as reflected by the appearance of two bands at 922 and 876 cm1 (see Fig. 6a). This is direct evidence for the lowering of symmetry of the CrO42 ion in the interlayer. 4.3. LDHs of Ni and Co In Fig. 7 are shown the PXRD patterns of the as-prepared and hydrothermally treated [Ni-Al-CrO4] LDHs. Both the patterns are characteristic of materials with turbostratic disorder and have been simulated. The only difference between the two patterns is a narrowing of the peaks after hydrothermal treatment. This is reflected in the reduction of the FWHM value of the Lorentzian used in the simulation from 2.28 2u to 1.28 2u after hydrothermal treatment. Such a narrowing of the line shape is reflective of crystal growth, but the nature of disorder remains unchanged. The interlayer ˚ in the as-prepared sample to 7.3 A ˚ after hydrothermal treatment. The latter value is spacing decreases from 7.8 A reflective of a grafted species. Similar behavior was observed in the [Ni-Fe-CrO4] LDH as well (data not shown). The characteristics of the LDHs of Co with Al and Fe are very similar to those of their Ni counterparts. The interlayer spacings and the line width of the Lorentzians used in the simulation are listed in Table 1. The IR spectra in all these systems show two absorptions at 933 and 876 cm1 respectively, which we attribute to the two asymmetric stretching modes of the CrO42 ion in the C3v symmetry. An illustrative spectrum of the Ni-Al system is shown in Fig. 6b. 4.4. Thermal behavior of the chromate containing LDHs All the LDHs reported here were studied by the TGA technique. In Fig. 8 are shown representative TG data of the LDHs. All the thermograms exhibit the following features: (1) There is a very large mass loss below 100 8C. We attribute this to the loss of adsorbed moisture. However as this mass loss does not appear complete even after a stay at 100 8C (0.5 h), it is likely some of the structural water is
S.V. Prasanna et al. / Materials Research Bulletin 42 (2007) 1028–1039
1035
Fig. 7. PXRD patterns of the [Ni-Al-CrO4] (x = 0.25) LDH (a) before and (b) after hydrothermal treatment compared with the corresponding DIFFaX simulations.
Fig. 8. TG data of chromate containing (a) Mg-Al, (b) Co-Al, (c) Mg-Fe and (d) Ni-Fe LDHs.
1036
S.V. Prasanna et al. / Materials Research Bulletin 42 (2007) 1028–1039
Table 2 Results of thermal analysis of chromate containing LDHs LDH
Expected weight loss (%)
Observed weight loss (%)
Products of decomposition at 800 8C
[Ca-Al-CrO4] (x = 0.33) [Mg-Al-CrO4] (x = 0.25) [Zn-Al-CrO4] (x = 0.25) [Ni-Al-CrO4] (x = 0.25) [Co-Al-CrO4] (x = 0.25) [Mg-Fe-CrO4] (x = 0.25) [Ni-Fe-CrO4] (x = 0.25) [Co-Fe-CrO4] (x = 0.25)
37.3 36.3 26.4 27.7 24.0 33.4 26.48 22.4
38.2 42.2 31.4 29.4 26.0 35.6 31.44 21.5
Ca4Al6O12CrO4 + CaO MgO + MgAl2O4 ZnO + Cr2O3 NiO + NiAl2O4 Co3O4 MgO + MgFe2O4 NiO + NiFe2O4 Co3O4
also lost at these low temperatures. The mass loss observed at this temperature varies between 6 and 8% in different LDHs. (2) Between 500 and 800 8C, there is an extended mass loss of over 5%. There are no signs that this tail is complete even at 800 8C. This kind of an extended loss seen in many layered hydroxides has been attributed to the gradual loss of volatile gas from the micropores of the oxide residue [30]. This mass loss proceeds in proportion to the extent of sintering of the oxide and the consequent collapse of the pores up to high temperatures. However the layered structure of the LDH breaks down below 500 8C itself. (3) In the 100–500 8C range of temperature, two prominent mass losses can be observed in the M-Al (M = Mg, Co, Ni) LDHs. The M-Fe LDHs (M = Mg, Ni) exhibit three mass losses. The low temperature (100–250 8C) mass loss is due to the loss of intercalated water molecules, while the high temperature mass loss (250–800 8C) is due to dehydroxylation of the layers [31,32]. The CrO42 ion undergoes reduction at high temperatures to yield Cr(III) species [33]. As the various steps overlap, the net mass loss in the temperature range 100–800 8C was compared (see Table 2) with the mass loss expected for the reaction: MII 6 M0III 2 ðOHÞ16 CrO4 4H2 O ! 0:5MII Cr2 O4 þ MII M0III 2 O4 þ 4:5MII O Except for the Mg-Al and Zn-Al systems, the match was found to be satisfactory. The products of the decomposition reaction are oxides and the PXRD patterns of representative oxide residues are shown in Fig. 9. The LDHs of Co yield
Fig. 9. PXRD pattern of the products obtained after thermal decomposition of chromate containing (a) Ni-Fe, (b) Zn-Al and (c) Co-Al LDHs. The major phases are NiO, ZnO and Co3O4 respectively. Features marked * correspond to the spinel. Features marked + correspond to Cr2O3.
S.V. Prasanna et al. / Materials Research Bulletin 42 (2007) 1028–1039
1037
Fig. 10. PXRD pattern of (a) freshly calcined (600 8C) oxide residue of the [Mg-Al-CrO4] (x = 0.25) LDH and (b) LDH obtained by soaking (a) in water.
a single-phase spinel having the structure of Co3O4. The LDHs of Ni yield a mixture of NiO and spinel Ni(M0 ,Cr)2O4 (M0 = Al, Fe). The LDH of Zn yields a mixture of wurtzite ZnO and (Al,Cr)2O3. The case of Mg-Al LDH is unique in that it yields a poorly ordered periclase, MgO above 500 8C (see Fig. 10a). There are no reflections associated with any Al-containing phase, showing that Al3+ ions are incorporated in the MgO lattice itself. The peaks are broad due to disorder brought about by the insertion of Al3+ ions into the MgO lattice. This kind of a defect oxide is highly unstable and reconstructs back into the parent LDH on soaking in water. The reconstruction is signified by the reappearance of the basal reflections in the PXRD pattern (see Fig. 10b). The oxide regains 42% of its weight by picking up hydroxyl ions and CO2 from water. The reconstruction phenomenon also known as ‘memory effect’ has been reported earlier [34,35] among the LDHs of Mg. The reversible thermal behavior is associated with the high formation temperature of the spinels of Mg [36]. 4.5. Wet chemical analysis and replaceable chromate content In Table 1 are given the results of wet chemical analysis of the chromate content of the LDHs. With the exception of [Ca-Al-CrO4] LDH and [Mg-Al-CrO4] LDH, the CrO42 present in other LDHs is nearly twice the stoichiometrically expected quantity. This is likely due to the formation of unitary chromates during the precipitation and post-synthetic treatment of the LDHs. The unitary chromates of transition metals such as ZnCrO4, NiCrO4 and CoCrO4 are insoluble and may have poor crystallinity by virtue of which their presence is not detected in the PXRD patterns of the LDHs. The chromate content is consistent with the nominal composition only in the case of Mg-Al (x = 0.25) and Ca-Al (x = 0.33) LDHs. Two factors are important in the application of LDHs as CrO42 scavengers: (1) the chromate carrying capacity of the LDHs and (2) chromate mineralization. Our wet chemical analysis shows that the LDHs take up chromate ions in excess of the theoretical maximum afforded by the chemical formula and the number of crystallographically permitted interlayer sites. To determine the extent of mineralization of the intercalated chromate, the chromate containing LDHs were soaked in a Na2CO3 solution. The choice of Na2CO3 was dictated by the ubiquitous presence of both CO2 and Na+ in the natural environment of the earth and also by the fact that carbonates are the most preferred anions in the interlayer of the LDHs by virtue of their thermodynamic stability [17]. It was found that all the LDHs discharge their chromate ions either wholly or partially in a Na2CO3 solution (see Table 1). In some instances such as the Ca-Al LDH, the final product is calcite CaCO3 showing that this particular LDH reacts by dissolution of the LDH and reprecipitation of calcite. In the process, the entire chromate is discharged into the liquid phase. In the case of the LDHs of Mg and Zn, nearly half the chromate is discharged, while in the LDH of Ni and Co with Al and Fe respectively only 18–25% of the chromate is discharged. While none of the LDHs release chromate ions into plain deionized water, the exception being LDH of Mg with Fe which
1038
S.V. Prasanna et al. / Materials Research Bulletin 42 (2007) 1028–1039
continually discharges chromate ions even in water. Even after several days of washing the as-prepared sample, the wash was not chromate-free. Chromate leaching takes place as a result of carbonate exchange. Anion exchange reactions are governed by many factors, of which the solubilities of the precursor and product LDHs play a critical role [17,19]. These factors are constrained by the pH of the reaction medium. Efficient mineralization of the anion is contingent on the thermodynamic stability of the corresponding LDH as reflected by its lower solubility. The chromate incorporated LDH has a lower solubility product than the chloride and nitrate containing LDHs, making it possible to synthesize the former using mixed metal (MII + M0 III) nitrate or chloride salt solutions. However among the LDHs, those containing carbonate ions are the most stable [17], thus making the intercalated chromate replaceable by carbonate. In general grafted chromates such as those in the LDHs of Ni and Co are less replaceable than intercalated chromates by virtue of their stronger interaction with the metal hydroxide slab. 5. Conclusions In conclusion, chromate containing LDHs prepared by coprecipitation incorporate stacking disorders arising out of a mismatch between the symmetry of the chromate anions and the symmetry of the interlayer site in the LDHs. These LDHs serve as excellent models to study the possible use of LDHs as chromate scavenging agents for green chemistry applications. The mineralization of chromates by the LDH is partial and paves the way for the possible recovery and reuse of the chromate for industrial applications. Acknowledgement The authors thank the Department of Science and Technology, Government of India, for financial support. References [1] R.L. Goswamee, P. Sengupta, K.G. Bhattacharyya, D.K. Dutta, Appl. Clay Sci. 13 (1998) 21. [2] T.J. Pinnavaia, Science 220 (1983) 365. [3] H.R. Oswald, R. Asper, in: R.M.A. Leith (Ed.), Preparation and Crystal Growth of Materials with Layered Structures, D. Riedel Publishing Company, Dordrecht, 1977, p. 71. [4] K.A. Carrado, A. Kostapapas, S.L. Suib, Solid State Ionics 26 (1988) 77. [5] A.I. Khan, Dermot O’Hare, J. Mater. Chem. 12 (2002) 3191. [6] S.P. Newman, W. Jones, New J. Chem. (1998) 105. [7] S. Carlino, Solid State Ionics 98 (1997) 73. [8] Y. Roh, S.Y. Lee, M.P. Elless, J.E. Foss, Clays Clay Miner. 48 (2000) 226. [9] B. Houri, A. Legrouri, A. Barroug, C. Forano, J.P. Besse, Collect. Czech. Chem. Commun. 63 (1998) 732. [10] S. Tezuka, R. Chitrakar, K. Sakane, A. Sonoda, K. Ooi, T. Tomida, Bull. Chem. Soc. Jpn. 77 (2004) 2101. [11] S. Miyata, A. Okada, Clays Clay Miner. 23 (1977) 14. [12] K. El Malki, A. de Roy, J.P. Besse, Eur. J. Solid State Inorg. Chem. 26 (1989) 339. [13] J.W. Boclair, P.S. Braterman, Chem. Mater. 11 (1999) 298. [14] M.M.J. Treacy, M.W. Deem, J.M. Newsam, Computer Code DIFFaX, Version 1.807. [15] T.N. Ramesh, R.S. Jayashree, P.V. Kamath, Clays Clay Miner. 51 (2003) 570. [16] G.S. Thomas, M. Rajamathi, P.V. Kamath, Clays Clay Miner. 52 (2004) 693. [17] R.K. Allada, A. Navrotsky, H.T. Berbeco, W.H. Casey, Science 296 (2002) 721. [18] D.L. Bish, Bull. Mineral. 52 (1980) 1036. [19] A.V. Radha, P.V. Kamath, C. Shivakumara, Solid State Sci. 7 (2005) 1180. [20] R. Segni, L. Vieille, F. Leroux, C. Taviot-Gueho, J. Phys. Chem. Solids 67 (2006) 1037. [21] L. Vieille, I. Rousselot, F. Leroux, J.P. Besse, C. Taviot-Gueho, Chem. Mater. 15 (2003) 4361. [22] T.N. Ramesh, M. Rajamathi, P.V. Kamath, Solid State Sci. 5 (2003) 751. [23] D.R. Hines, G.T. Seidler, M.M.J. Treacy, S.A. Solin, Solid State Commun. 101 (1997) 835. [24] A.V. Radha, P.V. Kamath, C. Shivakumara, Clays Clay Miner. 53 (2005) 521. [25] A. Viani, A.F. Gualtieri, G. Artioli, Am. Mineral. 87 (2002) 966. [26] A.S. Bookin, V.A. Drits, Clays Clay Miner. 41 (1993) 551. [27] K. Nakamoto, Infrared and Raman of Inorganic and Coordination Compounds, Wiley, New York, 1986. [28] S.D. Ross, Inorganic Infrared and Raman Spectra, McGraw-Hill, London, 1972. [29] A.S. Bookin, V.I. Cherkashin, V.A. Drits, Clays Clay Miner. 41 (1993) 558. [30] A. Delahay-Vidal, K. Tekaia-Elhsissen, Genin, M. Figlarz, Eur. J. Solid State Inorg. Chem. 31 (1994) 823.
S.V. Prasanna et al. / Materials Research Bulletin 42 (2007) 1028–1039 [31] [32] [33] [34] [35] [36]
V. Rives, Inorg. Chem. 38 (1999) 406. P. Bera, M. Rajamathi, M.S. Hegde, P.V. Kamath, Bull. Mater. Sci. 23 (2000) 141. D. de Waal, A.M. Heyns, J. Alloys Compd. 187 (1992) 171. T. Sato, K. Kato, T. Endo, M. Shimada, React. Solids 2 (1986) 253. V.R.L. Constantino, T.J. Pinnavia, Inorg. Chem. 34 (1995) 883. V. Rives, Mater. Chem. Phys. 75 (2002) 19.
1039