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Ship-in-bottle synthesis of the mixed-layered compounds of clay silicate/zirconium phosphate† Hiroshi Kawagoe, Naoya Imo-oka, Hiroyasu Shinohara and Shoji Yamanaka* An attempt was made to synthesize artificial mixed-layered compounds between montmorillonite silicate and α-zirconium phosphate using a ship-in-bottle approach. The interlayer cations of montmorillonite were first exchanged with hydroxy-zirconium oligomeric cations, which were then subjected to a reaction with phosphoric acid or phenylphosphoric acid to develop α-zirconium phosphate layers (Zr(R–OPO3)2; R = H, C6H5) between the clay silicate layers. The attempt of the reaction with phosphoric acid failed; hydroxy-zirconium cations were removed out of the interlayer space, forming α-zirconium phosphate outside of montmorillonite. The phenylphosphate derivative, montmorillonite/Zr(C6H5OPO3)2, with a regular mixed-layered structure has been successfully obtained, showing a basal spacing corresponding to the sum of the thicknesses of the individual layers. When a polyvinyl alcohol (PVA) aqueous solution
Received 17th April 2014, Accepted 12th May 2014
was used in the preparation of the hydroxy-zirconium exchanged montmorillonite, PVA was incorporated with hydroxy-zirconium complex cations between the silicate layers. The resulting compound can adsorb
DOI: 10.1039/c4dt01126k
phosphate ions. Although this is not a mixed-layered compound in the context of this study, the selective
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and reversible phosphate ion exchange properties are worth noting for future study.
Introduction Layered structured compounds show variously different structural aspects depending on the interest of researchers. Materials scientists are interested in the interlayer spaces of layered compounds.1,2 They use the layered compounds as intercalation hosts to develop electrodes for secondary batteries,3 ion-exchangers,4 pillared materials for adsorbents and catalysts,5,6 superconductors,7 and functional nanocomposites with various kinds of functional organic intercalants.8,9 Other researchers regard layered materials as multilayered stacks of two-dimensional inorganic polymer sheets. The inorganic polymer sheets can be used as building units for thin films if the crystals can be exfoliated into a stable dispersion. Naturally occurring smectite clays such as montmorillonite and hectorite (see the next section for the detailed structure) infinitely swell in water, and exfoliate into individual thin silicate sheets with a thickness of ∼1 nm in water dispersions. Alkylammonium intercalated montmorillonite with an expanded interlayer spacing is organophilic, and can be dispersed into a nylon6 matrix as thin lamellar fillers.10 Multilayered nanostructural
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan. E-mail: [email protected]; Tel: +81-824-24-7740 † Electronic supplementary information (ESI) available: The CIF file for the mixed-layered compound montmorillonite/Zr(C6H5OPO3)2 is available as a supplement. See DOI: 10.1039/c4dt01126k
10642 | Dalton Trans., 2014, 43, 10642–10650
films were prepared by a stepwise layer-by-layer stacking of hectorite clay sheets with a cationic polyelectrolyte such as polydiallyldimethylammonium chloride (PDDA) by Kleinfeld and Ferguson.11 A similar type of multilayered film was prepared using a Langmuir Blodgett deposition by Ogata et al.12 The techniques to prepare functional clay nanofilms from aqueous smectite suspensions have been reviewed by Schoonheydt.13 The thickness of the film increases linearly with the number of stacking operations. Each time the thickness of the film increases by the sum of the thickness of the dispersed silicate layer and the thickness of the polymer or organic cations. α-Zirconium phosphate, Zr(HPO4)2·H2O, and some acid-exchanged layered metal oxide semiconductors such as K4Nb6O17 and CsTi2NbO7 can be exfoliated with the help of tetrabutylammonium (TBA+).14,15 Fang et al. reported the synthesis of layer-by-layer assemblies of these oxide layers with organic and inorganic cations.16 The procedure is very similar to the preparation of clay multilayered films. One of the goals of the study of layered nanocomposites is to prepare regularly ordered mixed-layered compounds by a simple mixing of the two suspensions in a solvent (solv) containing different kinds of exfoliated compounds as shown in Fig. 1. The reaction can be given by the following equation: AAAAA þ BBBBB ! ½A A A A Asolv þ ½B B B B Bsolv ! ABABABABAB
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Fig. 1 Schematic image of a possible preparation route for the 1 : 1 regular mixed-layered compound A/B from the two kinds of layered compounds, A and B.
In order to realize the reaction, the interaction between the A and B layers should be larger than the interactions between the same kinds of the layers A–A and B–B. Otherwise, the two compounds A and B will precipitate separately from the mixed suspension. It should be very difficult to find suitable combinations of layered compounds to realize a nanocomposite with a regular mixed-layered structure. Layered double hydroxides (LDH)s, called anionic clays, have the general formula [MII1−xMIIIx(OH)2] (An−)x/n·mH2O, where MII is a divalent cation such as Mg2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, and Mn2+; MIII is a trivalent cation such as Al3+, Ga3+, Fe3+, Co3+, Mn3+, and Cr3+, and An− is an exchangeable n-valent anion such as Cl−, NO3−, SO42−, and CO32−. In hydrotalcite Mg2−xAlx(OH)2(CO3)x/2, for example, the Mg2+ sites are partially substituted with Al3+, and the net positive charge is balanced by the exchangeable interlayer anions CO32−. Liu et al. developed the exfoliation method for LDHs using formamide.17 The CO32−–LDHs were converted into monovalent NO3−–LDHs, followed by exfoliation into a large excess amount of formamide. They succeeded in preparation of inorganic
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sandwich layered materials as flocculates by a direct mixing of the exfoliated LDH suspension and exfoliated oxide nanosheets as shown in Fig. 1.18 Regular mixed-layered compounds, Mg2/3Al1/3(HO)2/Ti0.91O2 and Mg2/3Al1/3(HO)2/Ca2Nb3O10, were obtained, where the driving force of the alternate heterodeposition is an electrostatic attraction between the cationic and the anionic sheets. Since swellable anion exchangeable compounds with cationic sheets like LDHs are very rare, it is very difficult to prepare a mixed-layered compound A/B by the method illustrated in Fig. 1. The LDH based mixed-layered compounds described above are the first and only one example. The purpose of this study is to develop a new route for the preparation of mixed-layered compounds using a ship-in-bottle approach. In a previous study,19 we introduced nickel hydroxide layers into the interlayer space of montmorillonite to derive a chlorite type mixed-layered compound. The nickel hydroxide layers with the Mg(OH)2 (brucite) structure are sandwiched by clay silicate layers. Chlorite is a group of mixed-layered clay minerals of 2 : 1 silicate sheets with hydroxide layers.20 It can be seen as mixed-layered compounds of 2 : 1 clay silicate and LDH. When a nickel salt solution is titrated alone with a NaOH solution, Ni(OH)2 precipitates immediately. However, if a similar titration is carried out in the presence of ionexchangeable montmorillonite, Ni(OH)2 is preferentially deposited between the silicate layers until the hydroxide layers are fully developed. The negative charge of the silicate layers can be balanced by the substitution of the hydroxyl groups with water molecules, Ni(OH)2−x(H2O)x. This titration is an example of ship-in-bottle synthesis of a mixed-layered compound; in the first step of the reaction, Ni2+ ions are introduced by ion-exchange, and then hydrolyzed with NaOH. In this study, more complicated zirconium phosphate layers are developed between the silicate layers of montmorillonite by the ship-in-bottle method. We will first introduce hydroxyzirconium complex cations between the silicate layers, which are then subjected to reaction with phosphoric acid or phenylphosphoric acid to form zirconium phosphate layers.
Structural relationship between montmorillonite and zirconium phosphate Montmorillonite is a typical ion-exchangeable layered structured silicate mineral belonging to the smectite group.20 As shown in Fig. 2, each silicate crystalline layer consists of a 2 : 1 layer with two tetrahedral silicate sheets (2 T) and one octahedral gibbsite (Al(OH)3) sheet (O). The SiO4 tetrahedral units are arranged to form a hexagonal network with the apical oxygen directing toward the center of the sheet. The octahedral gibbsite sheet (O) at the center of the 2 : 1 (TOT) layer is linked with the two tetrahedral sheets (2 T) from both the sides through the apical oxygen. The Al ions are octahedrally coordinated by oxygen and hydroxyl ions. In montmorillonite, negative charge arises from a partial substitution of Al(III) with
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Fig. 2 Structures of (a) montmorillonite and (b) α-zirconium phosphate. Both structures consist of tetrahedral (T) and octahedral (O) coordination polyhedral. The ab projections of the two kinds of layered structures with similar lattice parameters are shown in the upper frames.
lower valent cations such as Mg(II), which is balanced by the presence of the interlayer exchangeable cations such as Na+, K+, and Ca2+. A typical composition of montmorillonite is Na1/3(H2O)n[Si4Al2−1/3Mg1/3O10(OH)2] with an ion-exchange capacity in the range of 80–100 meq per 100 g. Hectorite has brucite Mg(OH)2 layers for the octahedral sheet, and a typical composition Na1/3(H2O)n[Si4Mg3−1/3Li1/3O10(OH)2] with the Mg sites being partially substituted by Li ions. The unit cell of montmorillonite is approximately 9.1 × 5.2 Å within the ab plane; the dimension of the c axis varies in the range of 9.6 to 22 Å depending on the degree of the hydration of interlayer cations. In a water dispersion, Na-montmorillonite can be swelled infinitely, and completely exfoliated to molecular layers. The thickness of the molecular silicate layer including the van der Waals radii of the oxygen ions on the interlayer surface is estimated to be ∼9.6 Å. There are several types of zirconium phosphates. Typical compounds have a composition P/Zr = 2/1. There are two kinds of structures, the so-called α- and γ-type structures with compositions of Zr(HPO4)2·H2O and Zr(PO4)(H2PO4)·2H2O, respectively.4,21,22 It is interesting to note that the α-type structure is analogous to the 2 : 1 (TOT) silicate structure as shown in Fig. 2.23 The zirconium phosphate layer is also composed of two phosphate (PO4) tetrahedral (2 T) sheets and one zirconia (ZrO2) octahedral (O) sheet. The ZrO6 octahedra are arranged in a hexagonal sheet, and PO4 tetrahedral units are linked with the ZrO6 octahedra from both the sides of the sheet. Three
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oxygen atoms of each PO4 unit are bonded to three different zirconium atoms. In the α-zirconium phosphate, the apical oxygen atoms of the PO4 units bear hydrogen ions which are ion-exchangeable. Zirconium phosphate is a typical inorganic ion exchanger with a cation exchange capacity of 6 meq g−1. The OH groups at the tips of the tetrahedral units are directing toward the outside of the layers unlike the apical oxygen of montmorillonite. One mole of water molecules reside between the layers forming hydrogen bonds with the OH groups of the phosphate tips. The ab lattice parameter of the α-type zirconium phosphate is 9.1 × 5.3 Å, very similar to that of montmorillonite (Fig. 2). The γ-type has a different layered structure, as expected from the different chemical formula Zr(PO4)(H2PO4)·2H2O. The γ-type compounds form interesting organic derivatives, maintaining the two-dimensional layered structures.24 α-Zirconium phosphate is prepared by precipitation from a mixture of a soluble zirconium salt such as ZrOCl2·8H2O and phosphoric acid. The organic derivatives with compositions Zr(R-OPO3)2 and Zr(R-PO3)2 with R = organic radicals can be prepared using phosphate esters or phosphonic acids instead of phosphoric acid.25 The organic derivatives have essentially the same zirconium phosphate layers as that of α-zirconium phosphate.26 In this study, an attempt has been made to prepare a regular mixed-layered compound of montmorillonite silicate with zirconium phosphate. The lattice parameters of the two kinds of layered compounds can match very well within the layer planes.
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Experimental Materials: montmorillonite used was sodium montmorillonite (Kunipia G from Kunimine Industry) with the structural formula Na0.35K0.01Ca0.02(Si3.89Al0.11)(Al1.60Mg0.32Fe0.08)O10(OH)2· nH2O, and the cation-exchange capacity (CEC) of 100 meq per 100 g.27 An aqueous suspension (about 1 wt%) of the montmorillonite was prepared and kept for 1 day before the reaction with a hydroxy-zirconium aqueous solution, which was prepared by aging 0.1 M ZrOCl2·8H2O aqueous solution at 60 °C for 4 h. The aqueous solution contains tetrameric hydroxy complexes of the type [Zr4(OH)16−n(H2O)n+8]n+.28 The charge n was estimated to be about 2, i.e., the effective charge per Zr atom is near +0.5.29 Phenylphosphoric acid (Tokyo Kasei) was used as received. The degree of polymerization of polyvinyl alcohol (PVA) was 500 (Sigma Aldrich). Synthesis of hydroxy zirconium pillared montmorillonite (Zr-mont): the montmorillonite suspension in water (1 wt%) and 0.1 M ZrOCl2·8H2O solution were mixed in a ratio [Zr4(OH)14(H2O)10]2+/CEC = 2.5, and kept for 3 h under stirring.29 After the reaction, the montmorillonite was separated by centrifugation and washed repeatedly with water to remove excess zirconium and chloride ions. In this reaction the exchangeable cations of montmorillonite were exchanged with zirconium hydroxy complex cations, and the basal spacing was expanded to 21.6 Å on the air-dry sample (Fig. 3c). For the succeeding reaction with phosphoric acid and phenylphosphoric acid, the wet sample was used without drying. Zirconium phenylphosphate Zr(C6H5OPO3)2 (ZrPP) was prepared by mixing a 1 M ZrOCl2·8H2O solution with phenylphosphoric acid in a molar ratio [P]/[Zr] = 10, followed by aging at 70 °C for 3 h. The basal spacing of the obtained precipitate was determined to be 14.6 Å by XRD measurements (Fig. 3g). The basal spacing is in good agreement with the value (14.4 Å) reported for the same compound prepared by Kim et al.30 Zr-mont was also prepared in the presence of PVA. PVA was added to a 1% montmorillonite suspension in a ratio PVA/ montmorillonite = 0.5 (wt/wt). The montmorillonite separated after the dispersion in the PVA aqueous solution showed an expanded basal spacing of 25 Å, implying that PVA can be intercalated from the aqueous solution. When the ratios PVA/montmorillonite = 1.0 and 2.0 (wt/wt) were used, the spacing increased to 31 and 33 Å, respectively. In this study, the ratio PVA/mont = 0.5 (wt/wt) is used. The detailed procedure for the synthesis is the following; 1 g of montmorillonite was dispersed in 100 ml of a mixed solution of water and acetone in a ratio of 2 : 1, and a 10 wt% PVA aqueous solution was added in a ratio PVA/mont = 0.5 (wt/wt). After adding a 0.1 M ZrOCl2·8H2O aqueous solution in a molar ratio [Zr4(OH)14(H2O)10]2+/CEC = 2.5, the dispersion was kept for 2 h under stirring at room temperature. The addition of acetone was essential in the above procedure. The montmorillonite was dispersed so finely in the ZrOCl2·8H2O/PVA solution without acetone that it could not be separated by centrifugation. The obtained solid is referred to as Zr(PVA)-mont. Characterization: before air-drying, a small portion of wet product was spread on a glass slide in order to achieve the pre-
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Fig. 3 XRD patterns of montmorillonite variously treated; (a) starting montmorillonite, (b) simulation pattern for α-zirconium phosphate, (c) Zrmont, (d) Zr-mont treated with a H3PO4 aqueous solution, (e) the sample (d) after washing with water once, (f) the sample (e) after washing with water several times, (g) Zr(C6H5OPO3)2, (h) simulation pattern for the mixed-layered compound, montmorillonite/Zr(C6H5OPO3)2, (i) montmorillonite/Zr(C6H5OPO3)2 prepared by the ship-in-bottle method.
ferred orientation of the samples. X-ray powder diffraction (XRD) patterns were obtained for the orientation samples using a diffractometer (Brucker D8) with CuKα radiation. The composition of the samples was determined by ICP analysis of the solution prepared using a LiBO2 fusion method29 for the clay component, and a dilute HF solution for the Zr content. Differential thermal analysis and thermogravimetric analysis (DTA-TGA) was carried out using the apparatus Shimazu TA 50WS at a heating rate of 5 °C min−1. XRD simulation patterns were calculated using a software package Crystallographica.31
Results and discussion 1. An attempt to prepare a montmorillonite/zirconium phosphate mixed-layered compound by a ship-in-bottle method The wet Zr-mont was dispersed in a 10% phosphoric acid solution. After standing for 3 h under stirring, the solid product
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Fig. 4 DTA-TGA curves of the mixed-layered compound montmorillonite/Zr(C6H5OPO3)2 (a, c), and the TGA curve of Zr(C6H5OPO3)2 (b).
was separated by centrifugation, and the XRD pattern was measured before washing with water. The basal spacing of Zrmont is 21.6 Å (Fig. 3c). The second higher order reflection for the basal spacing was observed at 11.1 Å. After the reaction with phosphoric acid, the XRD peak becomes broad and the center position of the peak is shifted to 19.4 Å as shown in Fig. 3d. An additional reflection (shadowed in Fig. 3d) for a spacing of 7.6 Å is observed. The sample was rinsed with water once, and separated by centrifugation. The peaks due to the 19.4 Å phase became diffused, while the peak at 7.6 Å increased as shown in Fig. 4e. The intensity of the reflection at 7.6 Å decreased on washing the sample repeatedly. After several washing, the reflection peak at 7.6 Å disappeared (Fig. 3f ), and the XRD pattern reverted to that of the starting montmorillonite (Fig. 3a). The reflection with a spacing 7.6 Å is the characteristic basal reflection of α-zirconium phosphate. The XRD simulation pattern of α-zirconium phosphate based on the single crystal data21 is compared in Fig. 4b, where the basal reflection appears at 7.54 Å. It is likely that zirconium phosphate is formed on the outer surface of montmorillonite. The particles of α-zirconium phosphate should be so fine that the crystals cannot be recovered by washing and centrifugation. The spacing of 19.4 Å observed in the product with phosphoric acid before washing appears to be attributable to a hydrated phase of montmorillonite. It is also likely that even zirconium phosphate is formed between the layers of montmorillonite, the compound swells in water, and the zirconium phosphate layers are removed from the interlayer space. 2. Ship-in-bottle synthesis of a mixed-layered compound montmorillonite/zirconium phenylphosphate (ZrPP) In order to obtain a mixed-layered compound with a limited swelling with water, phenylphosphoric acid was used instead of phosphoric acid. The wet Zr-mont sample was added to excess phenyl phosphoric acid aqueous solution (6%), and after stirring for 1 h at room temperature, the sample was separated by centrifugation and washed with water repeatedly.
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The XRD patterns of the sample were measured before and after washing. The XRD pattern remained unchanged even after washing with water, suggesting that the sample is stable in water. Fig. 3i shows the XRD pattern of the sample after washing. Note that the spacing of the compound (24.6 Å) is very close to the sum of the thicknesses of montmorillonite (9.6 Å) and ZrPP (14.6 Å, Fig. 3g). DTA-TGA analysis was carried out on the sample with the basal spacing of 24.6 Å, and the result is shown in Fig. 4 in comparison with the TGA curve of ZrPP (Fig. 4b). ZrPP is decomposed in two steps. The first decomposition starts at about 200 °C, and the second one from about 450 °C. The total weight loss observed in the two steps was 39.0%, in good agreement with the weight loss (39.1%) expected in the decomposition of Zr(C6H5OPO3)2 to ZrP2O7. The ICP analysis of the residue after the TG analysis confirmed that the atomic ratio P/Zr = 1.98. The washed and air-dried sample with a basal spacing of 24.6 Å has an endothermic peak at about 70 °C due to the dehydration of adsorbed water, followed by broad exothermic peaks up to 600 °C. The exothermic peaks are accompanied by weight losses in two steps corresponding to the weight losses observed in the TGA curve of ZrPP. The broad exothermic peaks starting from about 250 °C can be ascribed to the oxidation of the phenyl groups of the phosphate layers. After heating to 900 °C, the composition of the montmorillonite/ZrPP was determined by ICP analysis. Table 1 shows the analytical result. The silicate layer and the phosphate layer compositions can be separately determined, and can be attributed to the compositions in atomic ratios on the O10(OH)2 anion basis of the clay. The Al content is slightly smaller compared with the starting montmorillonite. This is probably due to a partial dissolution during the reaction with a ZrOCl2 solution. A ZrOCl2 solution is very acidic like a HCl solution with a similar concentration. It should be noted that the P/Zr atomic ratio is almost 2/1, implying that a stoichiometric Zr(C6H5OPO3)2 layer is formed between the silicate layers. However, the content is only 67% compared to the fully developed layer composition. The weight loss observed for the sample in the temperature range of 190–900 °C was 20.7% on
Table 1 Chemical analyses of montmorillonite/ZrPP for the composition after calcination at 850 °C
Composition in atomic ratio Oxides
Found (wt%)
SiO2 Al2O3 MgO Na2O P2O5 ZrO2 Total
47.9 15.6 2.8 0.0 17.3 16.9 100.5
Si Al Mg Na P Zr
Montmorillonitea
Found
Expectedb
3.89 0.11 (T)c 1.60 (O)d 0.32 0.35 — —
3.89 0.11 (T) 1.38 (O) 0.33 0 1.19 0.67
3.89 0.11 (T) 1.60 (O) 0.32 0 2.00 1.00
a The montmorillonite composition was determined elsewhere on the O10(OH)2 anion basis.26 b Expected for the fully developed zirconium phenylphosphate, [Si3.89Al1.71Mg0.32O10(OH)2][Zr(C6H5OPO3)2]. c The Al content in the tetrahedral sheet. d The Al content in the octahedral sheet.
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the basis of the sample dehydrated at 190 °C in the TGA curve of Fig. 4c. If we assume the composition of the residue to be [ZrP2O7]0.67[Si4Al2O9], the weight loss corresponds to 2.0 mol of [C6H5O0.5]/mol of [ZrP2O7]. This result also supports that Zr(C6H5OPO3)2 layers with a stoichiometric composition are intercalated between the silicate layers by the ship-in-bottle synthesis. The XRD pattern of the ideal 1 : 1 mixed-layered compound, montmorillonite/ZrPP, was simulated using the fractional coordinates estimated from the structures of montmorillonite20 and α-zirconium phosphate.21 Phenyl groups are tentatively placed between the layers as shown in Fig. 5. The CIF file used for the simulation is given in ESI.† The simulation pattern for the basal reflections is shown in Fig. 3h. Note that the 001 reflection is very weak in intensity. This is due to the intercalation of the α-zirconium phosphate layer at the center of the silicate layers. The X-ray structure factors from the heavy elements cancel the contribution of the silicate layers to the first reflection peak. This is also one piece of evidence that zirconium phosphate layers are grown between the silicate layers. In the simulation of the pattern, the contribution of the phenyl groups was found to be very weak, and can be ignored to expect the general trend of the XRD pattern. The analytical result indicated that zirconium phosphate layers are deficient, and this is the reason for the appearance of the 001 reflection peak in the pattern of the obtained sample. The deficiency of the zirconium phosphate layers is due to the shortage of Zr ion supply. In a previous study it was found that each Zr atom has an effective charge near +0.5 as hydroxy zirconium cations [Zr4(OH)14·nH2O]2+ in the formation of Zr-mont by ion-exchange.29 The amount of Zr introduced is limited by the CEC of the clay, and not sufficient enough for
Fig. 5 Schematic structural model of the 1 : 1 regular mixed-layered compound, montmorillonite/Zr(C6H5OPO3)2. Montmorillonite layers are composed of oxygen (red balls), silicon (green balls), and aluminum (yellow balls).
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the fully developed zirconium phosphate layers. The expected amount of Zr taken up is 0.70 mol in the O10(OH)2 basis formula unit of montmorillonite, in good agreement with the Zr content found in the mixed-layered compound in Table 1. The ZrPP layers may develop as large domains, and water molecules are adsorbed in the space between the domains. The compounds are prepared under acidic conditions, and the charge compensation problems can be solved by introducing hydrogen ions, or protonation as discussed by Hardin et al.32 in the intercalation of partially hydrolyzed aluminum cations in a layered calcium niobate perovskite. 3. Zr-mont prepared in the presence of polyvinyl alcohol (PVA) In order to keep the hydroxy-zirconium cations of Zr-mont between the silicate layers in the reaction with phosphoric acid, a different type of Zr-mont was prepared using a PVA aqueous solution. It was reported that the use of PVA aqueous solutions of cationic hydroxy-aluminum oligomers gave alumina pillared clays with basal spacings much larger than those prepared without PVA.33,34 Fig. 6a and 6b show the XRD patterns of Zr(PVA)-mont after washing with water, followed by drying at room temperature and at 120 °C for 1 h, respectively.
Fig. 6 XRD patterns of (a) Zr(PVA)-mont, (b) Zr(PVA)-mont (120 °C), and (c) the sample (b) after washing with a H3PO4 aqueous solution, (d) the sample (c) after washing with water, (e) the sample (d) washed with a NaOH aqueous solution, (f ) the sample (e) washed with a H3PO4 aqueous solution in the second cycle.
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The spacing (∼25 Å) is larger than 21.6 Å for Zr-mont (Fig. 3c). This is due to the incorporation of PVA between the layers. The Zr(PVA)-mont dried at 120 °C, hereafter called Zr(PVA)mont (120 °C), was used for the reaction with phosphoric acid. The sample was dispersed in a 0.1 M H3PO4 aqueous solution for 2 h, and washed with water to remove excess phosphoric acid, followed by drying in air. As shown in Fig. 6c, Zr(PVA)mont (120 °C) is stable against the reaction with phosphoric acid, and the basal spacing increased from 21.7 to 26 Å. The expanded spacing was maintained even after washing with water (Fig. 6d). 4. Phosphate ion adsorption in Zr(PVA)-mont (120 °C) The sample in Fig. 6d was re-dispersed in a 0.1 M NaOH solution. After standing in the solution for 2 h, the sample was washed with water, and dried in air. Fig. 6e shows the XRD pattern of the sample with a shrinkage of the spacing to 22 Å. The ICP analysis of the NaOH solution revealed that phosphate ions were released into the solution. The separated sample was used again for the reaction with 0.1 M phosphoric acid solution. Fig. 6f shows the XRD pattern of the sample separated after standing for 2 h in the solution, followed by washing with water, and air-dried. The compound appears to be stable, maintaining the expanded spacing. The above sequence of the treatments was repeated to test the durability of Zr(PVA)-mont (120 °C) against the adsorption–desorption of phosphate ions. The amount of phosphate adsorption was estimated from the change of the concentration of the phosphoric acid solution in each sequence. The results are shown in Fig. 7 as a function of the repeated number of operations. Zr(PVA)-mont (120 °C) contains 0.85 mmol of Zr per g of sample, adsorbing 1.10 mmol of phosphate ions per g of sample, i.e. the atomic ratio P/Zr = 1.29. Although the ratio is smaller than 2, this value suggests that Zr ions incorporated by PVA in the interlayer space of the silicate layers are efficiently working as the adsorption sites for phosphate ions.
Fig. 7 Amount of phosphate ions adsorbed by Zr(PVA)-mont (120 °C) in a number of adsorption–desorption cycles.
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Fig. 8 shows a schematic image of the layered nanocomposite of Zr(PVA)-mont for the adsorption of phosphate ions. As discussed above on the XRD pattern of montmorillonite/ZrPP, the weak first order reflection for the basal spacing (Fig. 3i) is due to the formation of the ordered zirconium phosphate layer at the center of the interlayer space. The broad and intense first order reflection of Zr(PVA)-mont (Fig. 6) suggests that heavy zirconium ions are uniformly dispersed in the interlayer space with PVA. Zirconium ions are strongly bound in the PVA matrix, and can stay in the interlayer space to react with phosphoric acid. The small atomic ratio P/Zr = 1.29 for the interlayer composition of Zr(PVA)-mont with phosphoric acid may suggest that the formation of α-zirconium phosphate Zr(HPO4)2 (P/Zr = 2.0) is prohibited due to the difficulty of the rearrangement of the hydroxy-Zr ions in the PVA matrix. By the treatment with NaOH, the Zr–O–P bonds formed between the hydroxy-Zr groups and the phosphate ions are hydrolyzed, and the adsorbed phosphate ions are released into the solution. The durability against the repeated adsorption–desorption treatments is caused by the strong binding of zirconium ions by PVA. The cross-linking between hydroxy-zirconium complex ions and PVA hydroxy groups should take place in the thermal treatment of Zr(PVA)-mont at 120 °C. Although the adsorption capacity decreases gradually by the repetition of the adsorption–desorption operation as shown in Fig. 7, further study may improve the durability. The aim of this study is to develop an artificial mixedlayered compound A/B by a ship-in-bottle approach. The layered components A and B should separately exist as layered compounds. In this context, Zr(PVA)-mont is not a mixedlayered compound, since the component Zr-PVA between montmorillonite layers is not a layered compound, which cannot exist as a layered compound without montmorillonite. Zr(PVA)-mont is an intercalation compound, or a composite based on montmorillonite.
Fig. 8 Schematic illustration of the interlayer arrangement of PVA and hydroxy-zirconium complex ions, [Zrn(O,OH)m] between the silicate layers of montmorillonite.
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In order to understand the nanocomposite effect of Zr(PVA)-mont for the phosphate adsorption properties, a PVA/ ZrOCl2·8H2O aqueous solution was prepared without using montmorillonite, and the polymer solution was spread and dried on a nonwoven cloth of the Vinylon fiber. It was found that after a suitable thermal treatment of the cloth, the film showed a similar adsorption property for phosphate ions even without silicate layers. It is evident that the phosphate ion adsorption is not a specific property of the layered nanocomposite structures with montmorillonite, but the Zr-PVA polymer is solely responsible for the phosphate adsorption. Soluble phosphate ions are usually recovered by precipitation methods as hydroxyapatite (HAP) or magnesium ammonium phosphate (MAP, struvite) from discharged water systems. If a selective and regenerable phosphate ion adsorbent is developed, the phosphate recovery and purification processes can be much simplified. Only a few kinds of such phosphate adsorbents have been reported such as hydrotalcite,35–37 zirconium ferrite,38 and ion-oxide impregnated anion-exchange resin.39 Zr-PVA and Zr(PVA)-mont can be characterized as a new type of phosphate ion adsorbent. In our preliminary study, Zr-PVA can selectively adsorb phosphate ions from artificial seawater containing phosphoric acid. The high selectivity appears to be ascribed to the characteristic adsorption mechanism; the phosphate ions are adsorbed by the reaction with hydroxyzirconium groups. The detailed adsorption properties will be reported elsewhere.
Conclusions We have focused on the lattice matching of the two kinds of layered compounds, montmorillonite and α-zirconium phosphate, in the ab planes, and attempted to prepare the regular 1 : 1 mixed-layered compound by ship-in-bottle synthesis. The hydroxy-zirconium montmorillonite (Zr-mont) was subjected to a reaction with phosphoric acid and phenylphosphoric acid. Phosphoric acid removes zirconium complex cations from the interlayer space of montmorillonite, and forms α-zirconium phosphate on the outside of montmorillonite. Phenylphosphoric acid forms the regular mixed-layered compound, montmorillonite/Zr(C6H5OPO3)2 with a basal spacing corresponding to the sum of the individual thicknesses of the two kinds of layers. The PVA incorporation into Zr-mont can keep the hydroxy-zirconium ions with phosphate ions between the silicate layers. Zr(PVA)-mont possesses reversible adsorption– desorption properties for phosphate ions.
Acknowledgements This work was supported by the Japan Society for the Promotion of Science (JSPS) through its “Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST) Program.”
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References 1 Intercalation Chemistry, ed. M. S. Whittingham and A. J. Jacobson, Academic Press, New York, 1982. 2 D. O’Hare, Inorganic materials, ed. D. W. Bruce and D. O’Hare, Wiley, Chichester, UK, 2nd edn, 1997, ch. 4. 3 Lithium Batteries, Science and Technology, ed. G.-A. Nazri and G. Pistoia, Springer, 2009. 4 Inorganic Ion Exchange Materials, ed. A. Clearfield, CRC Press, Inc., Boca Raton, FL, 1982. 5 J. T. Kloprogge, J. Porous Mater., 1998, 5, 5. 6 Z. Ding, J. T. Kloprogge and R. L. Frost, J. Porous Mater., 2001, 8, 273. 7 S. Yamanaka, J. Mater. Chem., 2010, 20, 2897. 8 Handbook of Layered Materials, ed. S. M. Auerbach, K. A. Carrado and P. K. Dutta, Marcel Dekker, Inc., New York, 2004. 9 M. Ogawa and K. Kuroda, Chem. Rev., 1995, 95, 399. 10 A. Usuki, Y. Kojima, M. Kawasumi, A. Okada, Y. Fukushima, T. Kurauchi and O. Kamigaito, J. Mater. Res., 1993, 8, 1179. 11 E. R. Kleinfeld and G. S. Ferguson, Science, 1994, 265, 370. 12 Y. Ogata, J. Kawamata, C.-H. Chong, A. Yamagishi and G. Saito, Clays Clay Miner., 2003, 51, 181. 13 R. A. Schoonheydt, Clays Clay Miner., 2002, 50, 411. 14 S. W. Keller, H.-H. Kim and T. E. Mallouk, J. Am. Chem. Soc., 1994, 116, 8817. 15 R. E. Schaak and T. E. Mallouk, Chem. Commun., 2002, 706. 16 M. Fang, C. H. Kim, G. B. Saupe, H.-N. Kim, C. C. Waraksa, T. Miwa, A. Fujishima and T. E. Mallouk, Chem. Mater., 1999, 11, 1526. 17 Z. Liu, R. Ma, Y. Ebina, N. Iyi, K. Takada and T. Sasaki, Langmuir, 2007, 23, 861. 18 L. Li, R. Ma, Y. Ebina, K. Fukuda, K. Takada and T. Sasaki, J. Am. Chem. Soc., 2007, 129, 8000. 19 S. Yamanaka and G. W. Brindley, Clays Clay Miner., 1978, 26, 21. 20 G. W. Brindley and G. Brown, Crystal Structures of Clay Minerals and Their X-ray Identification, Mineralogical Society, London, 1980. 21 A. Clearfield and G. D. Smith, Inorg. Chem., 1969, 8, 431. 22 S. Yamanaka and M. Tanaka, J. Inorg. Nucl. Chem., 1979, 41, 45. 23 S. Yamanaka and M. Koizumi, Clays Clay Miner., 1975, 23, 477. 24 S. Yamanaka, Inorg. Chem., 1976, 15, 2811. 25 G. Alberti and U. Costantino, J. Inorg. Nucl. Chem., 1978, 40, 1113. 26 M. D. Poojary, H.-L. Hu, F. L. Campbell III and A. Clearfield, Acta Crystallogr., Sect. B: Struct. Sci., 1993, 49, 996. 27 S. Yamanaka, T. Doi, S. Sako and M. Hattori, Mater. Res. Bull., 1984, 19, 161. 28 A. Clearfield and A. Ph. Vaughan, Acta Crystallogr., 1956, 9, 555. 29 S. Yamanaka and G. W. Brindley, Clays Clay Miner., 1979, 27, 119.
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34 K. Suzuki, T. Mori, K. Kawase, H. Sakami and S. Iida, Clays Clay Miner., 1988, 36, 147. 35 A. Ookubo, K. Ooi and H. Hayashi, Langmuir, 1993, 9, 1418. 36 K. Kuzawa, Y.-J. Jung, Y. Kiso, T. Yamada, M. Nagai and T.-G. Lee, Chemosphere, 2006, 62, 45. 37 P. A. Terry, Environ. Eng. Sci., 2009, 26, 691. 38 D. Ito, K. Nishimura and O. Miura, J. Phys. Conf. Ser., 2009, 156, 012033. 39 S. Sengupta and A. Pandit, Water Res., 2011, 45, 3318.
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30 A. Moon-Sung Kim, B. Lee-Jin Ghil and C. Hee-Woo Rhee, Proceedings of 18th International Conf. Composite Matter., 2011, P2-06. 31 Crystallographica, ver 3.1, Oxford Cryosystems, (www. crystallographica.co.uk). 32 S. Hardin, D. Hay, M. Milikan, J. V. Sanders and T. W. Turney, Chem. Mater., 1991, 3, 977. 33 K. A. Carrado, P. Thiyagarajan and D. L. Elder, Clays Clay Miner., 1996, 44, 506.
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Ship-in-bottle synthesis of the mixed-layered compounds of clay silicate/zirconium phosphate† Hiroshi Kawagoe, Naoya Imo-oka, Hiroyasu Shinohara and Shoji Yamanaka* An attempt was made to synthesize artificial mixed-layered compounds between montmorillonite silicate and α-zirconium phosphate using a ship-in-bottle approach. The interlayer cations of montmorillonite were first exchanged with hydroxy-zirconium oligomeric cations, which were then subjected to a reaction with phosphoric acid or phenylphosphoric acid to develop α-zirconium phosphate layers (Zr(R–OPO3)2; R = H, C6H5) between the clay silicate layers. The attempt of the reaction with phosphoric acid failed; hydroxy-zirconium cations were removed out of the interlayer space, forming α-zirconium phosphate outside of montmorillonite. The phenylphosphate derivative, montmorillonite/Zr(C6H5OPO3)2, with a regular mixed-layered structure has been successfully obtained, showing a basal spacing corresponding to the sum of the thicknesses of the individual layers. When a polyvinyl alcohol (PVA) aqueous solution
Received 17th April 2014, Accepted 12th May 2014
was used in the preparation of the hydroxy-zirconium exchanged montmorillonite, PVA was incorporated with hydroxy-zirconium complex cations between the silicate layers. The resulting compound can adsorb
DOI: 10.1039/c4dt01126k
phosphate ions. Although this is not a mixed-layered compound in the context of this study, the selective
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and reversible phosphate ion exchange properties are worth noting for future study.
Introduction Layered structured compounds show variously different structural aspects depending on the interest of researchers. Materials scientists are interested in the interlayer spaces of layered compounds.1,2 They use the layered compounds as intercalation hosts to develop electrodes for secondary batteries,3 ion-exchangers,4 pillared materials for adsorbents and catalysts,5,6 superconductors,7 and functional nanocomposites with various kinds of functional organic intercalants.8,9 Other researchers regard layered materials as multilayered stacks of two-dimensional inorganic polymer sheets. The inorganic polymer sheets can be used as building units for thin films if the crystals can be exfoliated into a stable dispersion. Naturally occurring smectite clays such as montmorillonite and hectorite (see the next section for the detailed structure) infinitely swell in water, and exfoliate into individual thin silicate sheets with a thickness of ∼1 nm in water dispersions. Alkylammonium intercalated montmorillonite with an expanded interlayer spacing is organophilic, and can be dispersed into a nylon6 matrix as thin lamellar fillers.10 Multilayered nanostructural
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan. E-mail: [email protected]; Tel: +81-824-24-7740 † Electronic supplementary information (ESI) available: The CIF file for the mixed-layered compound montmorillonite/Zr(C6H5OPO3)2 is available as a supplement. See DOI: 10.1039/c4dt01126k
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films were prepared by a stepwise layer-by-layer stacking of hectorite clay sheets with a cationic polyelectrolyte such as polydiallyldimethylammonium chloride (PDDA) by Kleinfeld and Ferguson.11 A similar type of multilayered film was prepared using a Langmuir Blodgett deposition by Ogata et al.12 The techniques to prepare functional clay nanofilms from aqueous smectite suspensions have been reviewed by Schoonheydt.13 The thickness of the film increases linearly with the number of stacking operations. Each time the thickness of the film increases by the sum of the thickness of the dispersed silicate layer and the thickness of the polymer or organic cations. α-Zirconium phosphate, Zr(HPO4)2·H2O, and some acid-exchanged layered metal oxide semiconductors such as K4Nb6O17 and CsTi2NbO7 can be exfoliated with the help of tetrabutylammonium (TBA+).14,15 Fang et al. reported the synthesis of layer-by-layer assemblies of these oxide layers with organic and inorganic cations.16 The procedure is very similar to the preparation of clay multilayered films. One of the goals of the study of layered nanocomposites is to prepare regularly ordered mixed-layered compounds by a simple mixing of the two suspensions in a solvent (solv) containing different kinds of exfoliated compounds as shown in Fig. 1. The reaction can be given by the following equation: AAAAA þ BBBBB ! ½A A A A Asolv þ ½B B B B Bsolv ! ABABABABAB
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Fig. 1 Schematic image of a possible preparation route for the 1 : 1 regular mixed-layered compound A/B from the two kinds of layered compounds, A and B.
In order to realize the reaction, the interaction between the A and B layers should be larger than the interactions between the same kinds of the layers A–A and B–B. Otherwise, the two compounds A and B will precipitate separately from the mixed suspension. It should be very difficult to find suitable combinations of layered compounds to realize a nanocomposite with a regular mixed-layered structure. Layered double hydroxides (LDH)s, called anionic clays, have the general formula [MII1−xMIIIx(OH)2] (An−)x/n·mH2O, where MII is a divalent cation such as Mg2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, and Mn2+; MIII is a trivalent cation such as Al3+, Ga3+, Fe3+, Co3+, Mn3+, and Cr3+, and An− is an exchangeable n-valent anion such as Cl−, NO3−, SO42−, and CO32−. In hydrotalcite Mg2−xAlx(OH)2(CO3)x/2, for example, the Mg2+ sites are partially substituted with Al3+, and the net positive charge is balanced by the exchangeable interlayer anions CO32−. Liu et al. developed the exfoliation method for LDHs using formamide.17 The CO32−–LDHs were converted into monovalent NO3−–LDHs, followed by exfoliation into a large excess amount of formamide. They succeeded in preparation of inorganic
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sandwich layered materials as flocculates by a direct mixing of the exfoliated LDH suspension and exfoliated oxide nanosheets as shown in Fig. 1.18 Regular mixed-layered compounds, Mg2/3Al1/3(HO)2/Ti0.91O2 and Mg2/3Al1/3(HO)2/Ca2Nb3O10, were obtained, where the driving force of the alternate heterodeposition is an electrostatic attraction between the cationic and the anionic sheets. Since swellable anion exchangeable compounds with cationic sheets like LDHs are very rare, it is very difficult to prepare a mixed-layered compound A/B by the method illustrated in Fig. 1. The LDH based mixed-layered compounds described above are the first and only one example. The purpose of this study is to develop a new route for the preparation of mixed-layered compounds using a ship-in-bottle approach. In a previous study,19 we introduced nickel hydroxide layers into the interlayer space of montmorillonite to derive a chlorite type mixed-layered compound. The nickel hydroxide layers with the Mg(OH)2 (brucite) structure are sandwiched by clay silicate layers. Chlorite is a group of mixed-layered clay minerals of 2 : 1 silicate sheets with hydroxide layers.20 It can be seen as mixed-layered compounds of 2 : 1 clay silicate and LDH. When a nickel salt solution is titrated alone with a NaOH solution, Ni(OH)2 precipitates immediately. However, if a similar titration is carried out in the presence of ionexchangeable montmorillonite, Ni(OH)2 is preferentially deposited between the silicate layers until the hydroxide layers are fully developed. The negative charge of the silicate layers can be balanced by the substitution of the hydroxyl groups with water molecules, Ni(OH)2−x(H2O)x. This titration is an example of ship-in-bottle synthesis of a mixed-layered compound; in the first step of the reaction, Ni2+ ions are introduced by ion-exchange, and then hydrolyzed with NaOH. In this study, more complicated zirconium phosphate layers are developed between the silicate layers of montmorillonite by the ship-in-bottle method. We will first introduce hydroxyzirconium complex cations between the silicate layers, which are then subjected to reaction with phosphoric acid or phenylphosphoric acid to form zirconium phosphate layers.
Structural relationship between montmorillonite and zirconium phosphate Montmorillonite is a typical ion-exchangeable layered structured silicate mineral belonging to the smectite group.20 As shown in Fig. 2, each silicate crystalline layer consists of a 2 : 1 layer with two tetrahedral silicate sheets (2 T) and one octahedral gibbsite (Al(OH)3) sheet (O). The SiO4 tetrahedral units are arranged to form a hexagonal network with the apical oxygen directing toward the center of the sheet. The octahedral gibbsite sheet (O) at the center of the 2 : 1 (TOT) layer is linked with the two tetrahedral sheets (2 T) from both the sides through the apical oxygen. The Al ions are octahedrally coordinated by oxygen and hydroxyl ions. In montmorillonite, negative charge arises from a partial substitution of Al(III) with
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Fig. 2 Structures of (a) montmorillonite and (b) α-zirconium phosphate. Both structures consist of tetrahedral (T) and octahedral (O) coordination polyhedral. The ab projections of the two kinds of layered structures with similar lattice parameters are shown in the upper frames.
lower valent cations such as Mg(II), which is balanced by the presence of the interlayer exchangeable cations such as Na+, K+, and Ca2+. A typical composition of montmorillonite is Na1/3(H2O)n[Si4Al2−1/3Mg1/3O10(OH)2] with an ion-exchange capacity in the range of 80–100 meq per 100 g. Hectorite has brucite Mg(OH)2 layers for the octahedral sheet, and a typical composition Na1/3(H2O)n[Si4Mg3−1/3Li1/3O10(OH)2] with the Mg sites being partially substituted by Li ions. The unit cell of montmorillonite is approximately 9.1 × 5.2 Å within the ab plane; the dimension of the c axis varies in the range of 9.6 to 22 Å depending on the degree of the hydration of interlayer cations. In a water dispersion, Na-montmorillonite can be swelled infinitely, and completely exfoliated to molecular layers. The thickness of the molecular silicate layer including the van der Waals radii of the oxygen ions on the interlayer surface is estimated to be ∼9.6 Å. There are several types of zirconium phosphates. Typical compounds have a composition P/Zr = 2/1. There are two kinds of structures, the so-called α- and γ-type structures with compositions of Zr(HPO4)2·H2O and Zr(PO4)(H2PO4)·2H2O, respectively.4,21,22 It is interesting to note that the α-type structure is analogous to the 2 : 1 (TOT) silicate structure as shown in Fig. 2.23 The zirconium phosphate layer is also composed of two phosphate (PO4) tetrahedral (2 T) sheets and one zirconia (ZrO2) octahedral (O) sheet. The ZrO6 octahedra are arranged in a hexagonal sheet, and PO4 tetrahedral units are linked with the ZrO6 octahedra from both the sides of the sheet. Three
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oxygen atoms of each PO4 unit are bonded to three different zirconium atoms. In the α-zirconium phosphate, the apical oxygen atoms of the PO4 units bear hydrogen ions which are ion-exchangeable. Zirconium phosphate is a typical inorganic ion exchanger with a cation exchange capacity of 6 meq g−1. The OH groups at the tips of the tetrahedral units are directing toward the outside of the layers unlike the apical oxygen of montmorillonite. One mole of water molecules reside between the layers forming hydrogen bonds with the OH groups of the phosphate tips. The ab lattice parameter of the α-type zirconium phosphate is 9.1 × 5.3 Å, very similar to that of montmorillonite (Fig. 2). The γ-type has a different layered structure, as expected from the different chemical formula Zr(PO4)(H2PO4)·2H2O. The γ-type compounds form interesting organic derivatives, maintaining the two-dimensional layered structures.24 α-Zirconium phosphate is prepared by precipitation from a mixture of a soluble zirconium salt such as ZrOCl2·8H2O and phosphoric acid. The organic derivatives with compositions Zr(R-OPO3)2 and Zr(R-PO3)2 with R = organic radicals can be prepared using phosphate esters or phosphonic acids instead of phosphoric acid.25 The organic derivatives have essentially the same zirconium phosphate layers as that of α-zirconium phosphate.26 In this study, an attempt has been made to prepare a regular mixed-layered compound of montmorillonite silicate with zirconium phosphate. The lattice parameters of the two kinds of layered compounds can match very well within the layer planes.
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Experimental Materials: montmorillonite used was sodium montmorillonite (Kunipia G from Kunimine Industry) with the structural formula Na0.35K0.01Ca0.02(Si3.89Al0.11)(Al1.60Mg0.32Fe0.08)O10(OH)2· nH2O, and the cation-exchange capacity (CEC) of 100 meq per 100 g.27 An aqueous suspension (about 1 wt%) of the montmorillonite was prepared and kept for 1 day before the reaction with a hydroxy-zirconium aqueous solution, which was prepared by aging 0.1 M ZrOCl2·8H2O aqueous solution at 60 °C for 4 h. The aqueous solution contains tetrameric hydroxy complexes of the type [Zr4(OH)16−n(H2O)n+8]n+.28 The charge n was estimated to be about 2, i.e., the effective charge per Zr atom is near +0.5.29 Phenylphosphoric acid (Tokyo Kasei) was used as received. The degree of polymerization of polyvinyl alcohol (PVA) was 500 (Sigma Aldrich). Synthesis of hydroxy zirconium pillared montmorillonite (Zr-mont): the montmorillonite suspension in water (1 wt%) and 0.1 M ZrOCl2·8H2O solution were mixed in a ratio [Zr4(OH)14(H2O)10]2+/CEC = 2.5, and kept for 3 h under stirring.29 After the reaction, the montmorillonite was separated by centrifugation and washed repeatedly with water to remove excess zirconium and chloride ions. In this reaction the exchangeable cations of montmorillonite were exchanged with zirconium hydroxy complex cations, and the basal spacing was expanded to 21.6 Å on the air-dry sample (Fig. 3c). For the succeeding reaction with phosphoric acid and phenylphosphoric acid, the wet sample was used without drying. Zirconium phenylphosphate Zr(C6H5OPO3)2 (ZrPP) was prepared by mixing a 1 M ZrOCl2·8H2O solution with phenylphosphoric acid in a molar ratio [P]/[Zr] = 10, followed by aging at 70 °C for 3 h. The basal spacing of the obtained precipitate was determined to be 14.6 Å by XRD measurements (Fig. 3g). The basal spacing is in good agreement with the value (14.4 Å) reported for the same compound prepared by Kim et al.30 Zr-mont was also prepared in the presence of PVA. PVA was added to a 1% montmorillonite suspension in a ratio PVA/ montmorillonite = 0.5 (wt/wt). The montmorillonite separated after the dispersion in the PVA aqueous solution showed an expanded basal spacing of 25 Å, implying that PVA can be intercalated from the aqueous solution. When the ratios PVA/montmorillonite = 1.0 and 2.0 (wt/wt) were used, the spacing increased to 31 and 33 Å, respectively. In this study, the ratio PVA/mont = 0.5 (wt/wt) is used. The detailed procedure for the synthesis is the following; 1 g of montmorillonite was dispersed in 100 ml of a mixed solution of water and acetone in a ratio of 2 : 1, and a 10 wt% PVA aqueous solution was added in a ratio PVA/mont = 0.5 (wt/wt). After adding a 0.1 M ZrOCl2·8H2O aqueous solution in a molar ratio [Zr4(OH)14(H2O)10]2+/CEC = 2.5, the dispersion was kept for 2 h under stirring at room temperature. The addition of acetone was essential in the above procedure. The montmorillonite was dispersed so finely in the ZrOCl2·8H2O/PVA solution without acetone that it could not be separated by centrifugation. The obtained solid is referred to as Zr(PVA)-mont. Characterization: before air-drying, a small portion of wet product was spread on a glass slide in order to achieve the pre-
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Fig. 3 XRD patterns of montmorillonite variously treated; (a) starting montmorillonite, (b) simulation pattern for α-zirconium phosphate, (c) Zrmont, (d) Zr-mont treated with a H3PO4 aqueous solution, (e) the sample (d) after washing with water once, (f) the sample (e) after washing with water several times, (g) Zr(C6H5OPO3)2, (h) simulation pattern for the mixed-layered compound, montmorillonite/Zr(C6H5OPO3)2, (i) montmorillonite/Zr(C6H5OPO3)2 prepared by the ship-in-bottle method.
ferred orientation of the samples. X-ray powder diffraction (XRD) patterns were obtained for the orientation samples using a diffractometer (Brucker D8) with CuKα radiation. The composition of the samples was determined by ICP analysis of the solution prepared using a LiBO2 fusion method29 for the clay component, and a dilute HF solution for the Zr content. Differential thermal analysis and thermogravimetric analysis (DTA-TGA) was carried out using the apparatus Shimazu TA 50WS at a heating rate of 5 °C min−1. XRD simulation patterns were calculated using a software package Crystallographica.31
Results and discussion 1. An attempt to prepare a montmorillonite/zirconium phosphate mixed-layered compound by a ship-in-bottle method The wet Zr-mont was dispersed in a 10% phosphoric acid solution. After standing for 3 h under stirring, the solid product
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Fig. 4 DTA-TGA curves of the mixed-layered compound montmorillonite/Zr(C6H5OPO3)2 (a, c), and the TGA curve of Zr(C6H5OPO3)2 (b).
was separated by centrifugation, and the XRD pattern was measured before washing with water. The basal spacing of Zrmont is 21.6 Å (Fig. 3c). The second higher order reflection for the basal spacing was observed at 11.1 Å. After the reaction with phosphoric acid, the XRD peak becomes broad and the center position of the peak is shifted to 19.4 Å as shown in Fig. 3d. An additional reflection (shadowed in Fig. 3d) for a spacing of 7.6 Å is observed. The sample was rinsed with water once, and separated by centrifugation. The peaks due to the 19.4 Å phase became diffused, while the peak at 7.6 Å increased as shown in Fig. 4e. The intensity of the reflection at 7.6 Å decreased on washing the sample repeatedly. After several washing, the reflection peak at 7.6 Å disappeared (Fig. 3f ), and the XRD pattern reverted to that of the starting montmorillonite (Fig. 3a). The reflection with a spacing 7.6 Å is the characteristic basal reflection of α-zirconium phosphate. The XRD simulation pattern of α-zirconium phosphate based on the single crystal data21 is compared in Fig. 4b, where the basal reflection appears at 7.54 Å. It is likely that zirconium phosphate is formed on the outer surface of montmorillonite. The particles of α-zirconium phosphate should be so fine that the crystals cannot be recovered by washing and centrifugation. The spacing of 19.4 Å observed in the product with phosphoric acid before washing appears to be attributable to a hydrated phase of montmorillonite. It is also likely that even zirconium phosphate is formed between the layers of montmorillonite, the compound swells in water, and the zirconium phosphate layers are removed from the interlayer space. 2. Ship-in-bottle synthesis of a mixed-layered compound montmorillonite/zirconium phenylphosphate (ZrPP) In order to obtain a mixed-layered compound with a limited swelling with water, phenylphosphoric acid was used instead of phosphoric acid. The wet Zr-mont sample was added to excess phenyl phosphoric acid aqueous solution (6%), and after stirring for 1 h at room temperature, the sample was separated by centrifugation and washed with water repeatedly.
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The XRD patterns of the sample were measured before and after washing. The XRD pattern remained unchanged even after washing with water, suggesting that the sample is stable in water. Fig. 3i shows the XRD pattern of the sample after washing. Note that the spacing of the compound (24.6 Å) is very close to the sum of the thicknesses of montmorillonite (9.6 Å) and ZrPP (14.6 Å, Fig. 3g). DTA-TGA analysis was carried out on the sample with the basal spacing of 24.6 Å, and the result is shown in Fig. 4 in comparison with the TGA curve of ZrPP (Fig. 4b). ZrPP is decomposed in two steps. The first decomposition starts at about 200 °C, and the second one from about 450 °C. The total weight loss observed in the two steps was 39.0%, in good agreement with the weight loss (39.1%) expected in the decomposition of Zr(C6H5OPO3)2 to ZrP2O7. The ICP analysis of the residue after the TG analysis confirmed that the atomic ratio P/Zr = 1.98. The washed and air-dried sample with a basal spacing of 24.6 Å has an endothermic peak at about 70 °C due to the dehydration of adsorbed water, followed by broad exothermic peaks up to 600 °C. The exothermic peaks are accompanied by weight losses in two steps corresponding to the weight losses observed in the TGA curve of ZrPP. The broad exothermic peaks starting from about 250 °C can be ascribed to the oxidation of the phenyl groups of the phosphate layers. After heating to 900 °C, the composition of the montmorillonite/ZrPP was determined by ICP analysis. Table 1 shows the analytical result. The silicate layer and the phosphate layer compositions can be separately determined, and can be attributed to the compositions in atomic ratios on the O10(OH)2 anion basis of the clay. The Al content is slightly smaller compared with the starting montmorillonite. This is probably due to a partial dissolution during the reaction with a ZrOCl2 solution. A ZrOCl2 solution is very acidic like a HCl solution with a similar concentration. It should be noted that the P/Zr atomic ratio is almost 2/1, implying that a stoichiometric Zr(C6H5OPO3)2 layer is formed between the silicate layers. However, the content is only 67% compared to the fully developed layer composition. The weight loss observed for the sample in the temperature range of 190–900 °C was 20.7% on
Table 1 Chemical analyses of montmorillonite/ZrPP for the composition after calcination at 850 °C
Composition in atomic ratio Oxides
Found (wt%)
SiO2 Al2O3 MgO Na2O P2O5 ZrO2 Total
47.9 15.6 2.8 0.0 17.3 16.9 100.5
Si Al Mg Na P Zr
Montmorillonitea
Found
Expectedb
3.89 0.11 (T)c 1.60 (O)d 0.32 0.35 — —
3.89 0.11 (T) 1.38 (O) 0.33 0 1.19 0.67
3.89 0.11 (T) 1.60 (O) 0.32 0 2.00 1.00
a The montmorillonite composition was determined elsewhere on the O10(OH)2 anion basis.26 b Expected for the fully developed zirconium phenylphosphate, [Si3.89Al1.71Mg0.32O10(OH)2][Zr(C6H5OPO3)2]. c The Al content in the tetrahedral sheet. d The Al content in the octahedral sheet.
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the basis of the sample dehydrated at 190 °C in the TGA curve of Fig. 4c. If we assume the composition of the residue to be [ZrP2O7]0.67[Si4Al2O9], the weight loss corresponds to 2.0 mol of [C6H5O0.5]/mol of [ZrP2O7]. This result also supports that Zr(C6H5OPO3)2 layers with a stoichiometric composition are intercalated between the silicate layers by the ship-in-bottle synthesis. The XRD pattern of the ideal 1 : 1 mixed-layered compound, montmorillonite/ZrPP, was simulated using the fractional coordinates estimated from the structures of montmorillonite20 and α-zirconium phosphate.21 Phenyl groups are tentatively placed between the layers as shown in Fig. 5. The CIF file used for the simulation is given in ESI.† The simulation pattern for the basal reflections is shown in Fig. 3h. Note that the 001 reflection is very weak in intensity. This is due to the intercalation of the α-zirconium phosphate layer at the center of the silicate layers. The X-ray structure factors from the heavy elements cancel the contribution of the silicate layers to the first reflection peak. This is also one piece of evidence that zirconium phosphate layers are grown between the silicate layers. In the simulation of the pattern, the contribution of the phenyl groups was found to be very weak, and can be ignored to expect the general trend of the XRD pattern. The analytical result indicated that zirconium phosphate layers are deficient, and this is the reason for the appearance of the 001 reflection peak in the pattern of the obtained sample. The deficiency of the zirconium phosphate layers is due to the shortage of Zr ion supply. In a previous study it was found that each Zr atom has an effective charge near +0.5 as hydroxy zirconium cations [Zr4(OH)14·nH2O]2+ in the formation of Zr-mont by ion-exchange.29 The amount of Zr introduced is limited by the CEC of the clay, and not sufficient enough for
Fig. 5 Schematic structural model of the 1 : 1 regular mixed-layered compound, montmorillonite/Zr(C6H5OPO3)2. Montmorillonite layers are composed of oxygen (red balls), silicon (green balls), and aluminum (yellow balls).
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the fully developed zirconium phosphate layers. The expected amount of Zr taken up is 0.70 mol in the O10(OH)2 basis formula unit of montmorillonite, in good agreement with the Zr content found in the mixed-layered compound in Table 1. The ZrPP layers may develop as large domains, and water molecules are adsorbed in the space between the domains. The compounds are prepared under acidic conditions, and the charge compensation problems can be solved by introducing hydrogen ions, or protonation as discussed by Hardin et al.32 in the intercalation of partially hydrolyzed aluminum cations in a layered calcium niobate perovskite. 3. Zr-mont prepared in the presence of polyvinyl alcohol (PVA) In order to keep the hydroxy-zirconium cations of Zr-mont between the silicate layers in the reaction with phosphoric acid, a different type of Zr-mont was prepared using a PVA aqueous solution. It was reported that the use of PVA aqueous solutions of cationic hydroxy-aluminum oligomers gave alumina pillared clays with basal spacings much larger than those prepared without PVA.33,34 Fig. 6a and 6b show the XRD patterns of Zr(PVA)-mont after washing with water, followed by drying at room temperature and at 120 °C for 1 h, respectively.
Fig. 6 XRD patterns of (a) Zr(PVA)-mont, (b) Zr(PVA)-mont (120 °C), and (c) the sample (b) after washing with a H3PO4 aqueous solution, (d) the sample (c) after washing with water, (e) the sample (d) washed with a NaOH aqueous solution, (f ) the sample (e) washed with a H3PO4 aqueous solution in the second cycle.
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The spacing (∼25 Å) is larger than 21.6 Å for Zr-mont (Fig. 3c). This is due to the incorporation of PVA between the layers. The Zr(PVA)-mont dried at 120 °C, hereafter called Zr(PVA)mont (120 °C), was used for the reaction with phosphoric acid. The sample was dispersed in a 0.1 M H3PO4 aqueous solution for 2 h, and washed with water to remove excess phosphoric acid, followed by drying in air. As shown in Fig. 6c, Zr(PVA)mont (120 °C) is stable against the reaction with phosphoric acid, and the basal spacing increased from 21.7 to 26 Å. The expanded spacing was maintained even after washing with water (Fig. 6d). 4. Phosphate ion adsorption in Zr(PVA)-mont (120 °C) The sample in Fig. 6d was re-dispersed in a 0.1 M NaOH solution. After standing in the solution for 2 h, the sample was washed with water, and dried in air. Fig. 6e shows the XRD pattern of the sample with a shrinkage of the spacing to 22 Å. The ICP analysis of the NaOH solution revealed that phosphate ions were released into the solution. The separated sample was used again for the reaction with 0.1 M phosphoric acid solution. Fig. 6f shows the XRD pattern of the sample separated after standing for 2 h in the solution, followed by washing with water, and air-dried. The compound appears to be stable, maintaining the expanded spacing. The above sequence of the treatments was repeated to test the durability of Zr(PVA)-mont (120 °C) against the adsorption–desorption of phosphate ions. The amount of phosphate adsorption was estimated from the change of the concentration of the phosphoric acid solution in each sequence. The results are shown in Fig. 7 as a function of the repeated number of operations. Zr(PVA)-mont (120 °C) contains 0.85 mmol of Zr per g of sample, adsorbing 1.10 mmol of phosphate ions per g of sample, i.e. the atomic ratio P/Zr = 1.29. Although the ratio is smaller than 2, this value suggests that Zr ions incorporated by PVA in the interlayer space of the silicate layers are efficiently working as the adsorption sites for phosphate ions.
Fig. 7 Amount of phosphate ions adsorbed by Zr(PVA)-mont (120 °C) in a number of adsorption–desorption cycles.
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Fig. 8 shows a schematic image of the layered nanocomposite of Zr(PVA)-mont for the adsorption of phosphate ions. As discussed above on the XRD pattern of montmorillonite/ZrPP, the weak first order reflection for the basal spacing (Fig. 3i) is due to the formation of the ordered zirconium phosphate layer at the center of the interlayer space. The broad and intense first order reflection of Zr(PVA)-mont (Fig. 6) suggests that heavy zirconium ions are uniformly dispersed in the interlayer space with PVA. Zirconium ions are strongly bound in the PVA matrix, and can stay in the interlayer space to react with phosphoric acid. The small atomic ratio P/Zr = 1.29 for the interlayer composition of Zr(PVA)-mont with phosphoric acid may suggest that the formation of α-zirconium phosphate Zr(HPO4)2 (P/Zr = 2.0) is prohibited due to the difficulty of the rearrangement of the hydroxy-Zr ions in the PVA matrix. By the treatment with NaOH, the Zr–O–P bonds formed between the hydroxy-Zr groups and the phosphate ions are hydrolyzed, and the adsorbed phosphate ions are released into the solution. The durability against the repeated adsorption–desorption treatments is caused by the strong binding of zirconium ions by PVA. The cross-linking between hydroxy-zirconium complex ions and PVA hydroxy groups should take place in the thermal treatment of Zr(PVA)-mont at 120 °C. Although the adsorption capacity decreases gradually by the repetition of the adsorption–desorption operation as shown in Fig. 7, further study may improve the durability. The aim of this study is to develop an artificial mixedlayered compound A/B by a ship-in-bottle approach. The layered components A and B should separately exist as layered compounds. In this context, Zr(PVA)-mont is not a mixedlayered compound, since the component Zr-PVA between montmorillonite layers is not a layered compound, which cannot exist as a layered compound without montmorillonite. Zr(PVA)-mont is an intercalation compound, or a composite based on montmorillonite.
Fig. 8 Schematic illustration of the interlayer arrangement of PVA and hydroxy-zirconium complex ions, [Zrn(O,OH)m] between the silicate layers of montmorillonite.
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In order to understand the nanocomposite effect of Zr(PVA)-mont for the phosphate adsorption properties, a PVA/ ZrOCl2·8H2O aqueous solution was prepared without using montmorillonite, and the polymer solution was spread and dried on a nonwoven cloth of the Vinylon fiber. It was found that after a suitable thermal treatment of the cloth, the film showed a similar adsorption property for phosphate ions even without silicate layers. It is evident that the phosphate ion adsorption is not a specific property of the layered nanocomposite structures with montmorillonite, but the Zr-PVA polymer is solely responsible for the phosphate adsorption. Soluble phosphate ions are usually recovered by precipitation methods as hydroxyapatite (HAP) or magnesium ammonium phosphate (MAP, struvite) from discharged water systems. If a selective and regenerable phosphate ion adsorbent is developed, the phosphate recovery and purification processes can be much simplified. Only a few kinds of such phosphate adsorbents have been reported such as hydrotalcite,35–37 zirconium ferrite,38 and ion-oxide impregnated anion-exchange resin.39 Zr-PVA and Zr(PVA)-mont can be characterized as a new type of phosphate ion adsorbent. In our preliminary study, Zr-PVA can selectively adsorb phosphate ions from artificial seawater containing phosphoric acid. The high selectivity appears to be ascribed to the characteristic adsorption mechanism; the phosphate ions are adsorbed by the reaction with hydroxyzirconium groups. The detailed adsorption properties will be reported elsewhere.
Conclusions We have focused on the lattice matching of the two kinds of layered compounds, montmorillonite and α-zirconium phosphate, in the ab planes, and attempted to prepare the regular 1 : 1 mixed-layered compound by ship-in-bottle synthesis. The hydroxy-zirconium montmorillonite (Zr-mont) was subjected to a reaction with phosphoric acid and phenylphosphoric acid. Phosphoric acid removes zirconium complex cations from the interlayer space of montmorillonite, and forms α-zirconium phosphate on the outside of montmorillonite. Phenylphosphoric acid forms the regular mixed-layered compound, montmorillonite/Zr(C6H5OPO3)2 with a basal spacing corresponding to the sum of the individual thicknesses of the two kinds of layers. The PVA incorporation into Zr-mont can keep the hydroxy-zirconium ions with phosphate ions between the silicate layers. Zr(PVA)-mont possesses reversible adsorption– desorption properties for phosphate ions.
Acknowledgements This work was supported by the Japan Society for the Promotion of Science (JSPS) through its “Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST) Program.”
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References 1 Intercalation Chemistry, ed. M. S. Whittingham and A. J. Jacobson, Academic Press, New York, 1982. 2 D. O’Hare, Inorganic materials, ed. D. W. Bruce and D. O’Hare, Wiley, Chichester, UK, 2nd edn, 1997, ch. 4. 3 Lithium Batteries, Science and Technology, ed. G.-A. Nazri and G. Pistoia, Springer, 2009. 4 Inorganic Ion Exchange Materials, ed. A. Clearfield, CRC Press, Inc., Boca Raton, FL, 1982. 5 J. T. Kloprogge, J. Porous Mater., 1998, 5, 5. 6 Z. Ding, J. T. Kloprogge and R. L. Frost, J. Porous Mater., 2001, 8, 273. 7 S. Yamanaka, J. Mater. Chem., 2010, 20, 2897. 8 Handbook of Layered Materials, ed. S. M. Auerbach, K. A. Carrado and P. K. Dutta, Marcel Dekker, Inc., New York, 2004. 9 M. Ogawa and K. Kuroda, Chem. Rev., 1995, 95, 399. 10 A. Usuki, Y. Kojima, M. Kawasumi, A. Okada, Y. Fukushima, T. Kurauchi and O. Kamigaito, J. Mater. Res., 1993, 8, 1179. 11 E. R. Kleinfeld and G. S. Ferguson, Science, 1994, 265, 370. 12 Y. Ogata, J. Kawamata, C.-H. Chong, A. Yamagishi and G. Saito, Clays Clay Miner., 2003, 51, 181. 13 R. A. Schoonheydt, Clays Clay Miner., 2002, 50, 411. 14 S. W. Keller, H.-H. Kim and T. E. Mallouk, J. Am. Chem. Soc., 1994, 116, 8817. 15 R. E. Schaak and T. E. Mallouk, Chem. Commun., 2002, 706. 16 M. Fang, C. H. Kim, G. B. Saupe, H.-N. Kim, C. C. Waraksa, T. Miwa, A. Fujishima and T. E. Mallouk, Chem. Mater., 1999, 11, 1526. 17 Z. Liu, R. Ma, Y. Ebina, N. Iyi, K. Takada and T. Sasaki, Langmuir, 2007, 23, 861. 18 L. Li, R. Ma, Y. Ebina, K. Fukuda, K. Takada and T. Sasaki, J. Am. Chem. Soc., 2007, 129, 8000. 19 S. Yamanaka and G. W. Brindley, Clays Clay Miner., 1978, 26, 21. 20 G. W. Brindley and G. Brown, Crystal Structures of Clay Minerals and Their X-ray Identification, Mineralogical Society, London, 1980. 21 A. Clearfield and G. D. Smith, Inorg. Chem., 1969, 8, 431. 22 S. Yamanaka and M. Tanaka, J. Inorg. Nucl. Chem., 1979, 41, 45. 23 S. Yamanaka and M. Koizumi, Clays Clay Miner., 1975, 23, 477. 24 S. Yamanaka, Inorg. Chem., 1976, 15, 2811. 25 G. Alberti and U. Costantino, J. Inorg. Nucl. Chem., 1978, 40, 1113. 26 M. D. Poojary, H.-L. Hu, F. L. Campbell III and A. Clearfield, Acta Crystallogr., Sect. B: Struct. Sci., 1993, 49, 996. 27 S. Yamanaka, T. Doi, S. Sako and M. Hattori, Mater. Res. Bull., 1984, 19, 161. 28 A. Clearfield and A. Ph. Vaughan, Acta Crystallogr., 1956, 9, 555. 29 S. Yamanaka and G. W. Brindley, Clays Clay Miner., 1979, 27, 119.
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34 K. Suzuki, T. Mori, K. Kawase, H. Sakami and S. Iida, Clays Clay Miner., 1988, 36, 147. 35 A. Ookubo, K. Ooi and H. Hayashi, Langmuir, 1993, 9, 1418. 36 K. Kuzawa, Y.-J. Jung, Y. Kiso, T. Yamada, M. Nagai and T.-G. Lee, Chemosphere, 2006, 62, 45. 37 P. A. Terry, Environ. Eng. Sci., 2009, 26, 691. 38 D. Ito, K. Nishimura and O. Miura, J. Phys. Conf. Ser., 2009, 156, 012033. 39 S. Sengupta and A. Pandit, Water Res., 2011, 45, 3318.
Published on 13 May 2014. Downloaded by Carleton University on 30/06/2014 13:56:27.
30 A. Moon-Sung Kim, B. Lee-Jin Ghil and C. Hee-Woo Rhee, Proceedings of 18th International Conf. Composite Matter., 2011, P2-06. 31 Crystallographica, ver 3.1, Oxford Cryosystems, (www. crystallographica.co.uk). 32 S. Hardin, D. Hay, M. Milikan, J. V. Sanders and T. W. Turney, Chem. Mater., 1991, 3, 977. 33 K. A. Carrado, P. Thiyagarajan and D. L. Elder, Clays Clay Miner., 1996, 44, 506.
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