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Synthesis of new compounds involving layered titanates and niobates with copper(II)
Journal of Alloys and Compounds 319 (2001) 94–99
L
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Synthesis of new compounds involving layered titanates and niobates with copper(II) ˜ Nunes a , *, Antonio Gouveia de Souza a , Robson Fernandes de Farias b Liliane Magalhaes a
˜ Pessoa, Paraıba ´ ´ , 58059 -900 Joao ´ , Brazil Departamento de Quımica , CCEN, Universidade Federal da Paraıba b ´ ´ , 69310 -270 Boa Vista, Roraıma ´ , Brazil , Universidade Federal de Roraıma Departamento de Quımica Received 24 July 2000; received in revised form 20 October 2000; accepted 1 November 2000
Abstract A series of titanates and niobates, K 2 Ti 4 O 9 , Na 2 Ti 3 O 7 , KNb 3 O 8 , K 4 Nb 6 O 17 , and their respective acid forms were prepared and characterized by X-ray diffraction, thermogravimetry and scanning electron microscopy. The ion exchange capacity of the alkaline and acid forms towards Cu(II) was studied and it is concluded that, as general behavior, the alkaline titanates and niobates have a higher ion exchange capacity towards copper cations than their respective acid forms. Furthermore, the sodium matrix Na 2 Ti 3 O 7 exhibits the highest ion exchange capacity of 3.00 mmol g 21 . Based on the thermogravimetric results, the Cu(II) cation is coordinated to two water molecules when adsorbed on the titanate and niobate matrices. 2001 Elsevier Science B.V. All rights reserved. Keywords: Titanates; Niobates; Ion exchange; Copper
1. Introduction The inorganic layered oxides have attracted considerable attention due to their unique structural properties and applications, such as in intercalation reactions, ionic exchange processes, their photochemical and semiconductor properties as well as catalytic activities [1–5]. On the
Fig. 1. Schematic representation of interlayers I and II in the K 4 Nb 6 O 17 matrix.
*Corresponding author. ˜ E-mail address: [email protected] (L. Magalhaes Nunes).
other hand, titanates and niobates can combine with organic and inorganic species producing hybrid matrices, which have recently been investigated [6–9]. The layered titanates (K 2 Ti 4 O 9 and Na 2 Ti 3 O 7 ) and niobates (KNb 3 O 8 and K 4 Nb 6 O 17 ) consist of TiO 6 or NbO 6 octahedral units sharing edges on one level which combine with similar units above and below to form zig-zag strings of octahedra [1,10]. In this case, the charge balance is maintained by the occupancy of alkali-metal ions between the layers. The K 4 Nb 6 O 17 matrix is structurally distinguishable from other related layered niobates or titanates since it has two different types of interlayer spaces (interlayers I and II) alternating between [Nb 6 O 17 ] 42 layers, as shown in Fig. 1. Interlayer I contains potassium ions with water molecules, while interlayer II contains unhydrated potassium ions under ambient conditions [11–13]. Kinomura et al. [14,15] concluded that monovalent cations were intercalated into both interlayers and di and trivalent cations were exchanged with potassium ions in interlayer I only. Other reports on intercalation into K 4 Nb 6 O 17 do not disagree with these results; alkylammonium ions seem to be intercalated into both interlayers [16], while methylviologen dication is taken up into interlayer I only [17,18]. Layered oxide quadrivalent metals are frequently employed in the removal / separation of several radio / bio-
0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 00 )01414-6
˜ Nunes et al. / Journal of Alloys and Compounds 319 (2001) 94 – 99 L. Magalhaes
toxic metal ions from aqueous solutions [19–22] since they are quite effective for cation as well as anion separations. Furthermore, they behave like a cation exchanger in alkaline solutions, depending upon the basicity of the central atom and the strength of the M–O bond relative to that of the O–H bond in the hydroxyl group [19–22]. The stability in acidic solutions is restricted by transformation into protonic oxides [23–25]. However, the resulting compound retains a layered structure similar to that of the alkaline materials. The acidic layered oxide, which is also called the layered hydrous titanium or niobium dioxide, exhibits distinct intercalation behavior towards several cations and some organic compounds [2,26,27]. This property can be exploited in the use of this material to remove and immobilize radioactive nuclides such as 137 Cs and 90 Sr from high-level liquid waters [28]. The aim of the present work was to investigate the behavior of the ion exchange process involving copper chloride with the matrices potassium tetratitanate (K 2 Ti 4 O 9 ), sodium trititanate (Na 2 Ti 3 O 7 ), potassium triniobate (KNb 3 O 8 ), and potassium hexaniobate (K 4 Nb 6 O 17 ) and their respective acid forms.
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The interlamellar ions (Na 1 , K 1 or H 1 ) were exchanged by copper, by suspending 0.5 g of matrix in 50.0 cm 3 of a 0.15 mol dm 23 copper chloride solution at 25618C for 6 h. After this time, the solid samples were separated by centrifugation and dried at 408C for 24 h. The concentration of exchangeable copper cations in solution was determined by complexometric titration with a 0.02 mol dm 23 EDTA solution [31]. The amount of exchanged cations (n f ) was determined through the relationship (n i 2 n s ) /m, where n i is the initial number of moles of cation in solution, n s is the number of moles of cations in equilibrium with the solid phase after the exchange process and m is the mass of the exchanger used. The alkaline, acid and copper exchanged matrices were characterized by: X-ray diffraction using Cu Ka radiation in the 2u range from 3 to 50 with a Shimadzu diffractometer Model XD3A; thermogravimetry (TG), using a Shimadzu TGA-50 instrument, by heating samples (|5.0 mg) from 25 to 9508C, with a heating rate of 108C min 21 , under nitrogen atmosphere. The scanning electron micrographs were obtained in a JEOL microscope, JSM T-300, with an accelerating voltage of 20 kV.
3. Results and discussion 2. Experimental All chemicals used, TiO 2 (Aldrich), Nb 2 O 5 (Companhia ˜ Brasileira de Metalurgia e Minerac¸ao), K 2 CO 3 , Na 2 CO 3 (Merck) and CuCl 2 (Aldrich), were of analytical grade and were employed without further purification. Potassium tetratitanate (K 2 Ti 4 O 9 ) and sodium trititanate (Na 2 Ti 3 O 7 ) were prepared by solid-state reaction by the heating of a stoichiometric mixture of K 2 CO 3 or Na 2 CO 3 and TiO 2 powders in a platinum crucible. The mixture was heated at 1073 K for 20 h at room atmosphere. It was then ground and heated again at the same temperature for a further 20 h [29,30]. Potassium triniobate (KNb 3 O 8 ) and hexaniobate (K 4 Nb 6 O 17 ) were prepared by solid-state reaction by the heating of stoichiometric mixtures of K 2 CO 3 and Nb 2 O 5 powders in a platinum crucible. For the KNb 3 O 8 matrix, the stoichiometric mixture was heated at 873 K for 2 h and then ground and heated at 1173 K for a further 3 h [7]. For the K 4 Nb 6 O 17 matrix, the stoichiometric mixture was heated at 1173 K for 44 h. The H 1 -exchanged matrices were prepared by suspending the alkaline matrices in HCl (for titanium matrices) or HNO 3 (for niobium matrices) solution. In a typical procedure, 1.0 g of matrix was placed in contact, under stirring, with 20.0 cm 3 of a 1.0 mol dm 23 solution of hydrochloric or nitric acid at 343 K for 3 days. The solid material was separated by centrifugation, and washed with bidistilled water until pH 5.0 to 6.0. The final products were dried over a saturated NaCl solution under 70% relative humidity and characterized.
The interlayer distance as calculated by X-ray diffraction data as well as the total amount of exchanged copper for all alkaline and acid matrices are shown in Table 1.
3.1. K2 Ti4 O9 compounds As can be observed, the ion exchange capacity of the alkaline matrix is about three times larger than that for the acid matrix. Furthermore, the interlayer distance is increased when K 1 or H 1 are exchanged for Cu 21 . The Dd values are 50 pm for both matrices. The X-ray diffraction patterns for the matrices are shown in Fig. 2. The K 2 Ti 4 O 9 matrix exhibits a mass loss of 3% associated with the release of 0.7 mol of hydration water. The respective acid matrix, H 2 Ti 4 O 9 , shows two distinct mass loss steps. The first (3%) is associated with the Table 1 Interlayer distance and total amount of copper exchanged (n f ) for titanate and niobate matrices. The interlayer distance values between parentheses are for the non-exchanged matrices Matrix
d (pm)
nf (mmol g 21 )
K 2 Ti 4 O 9 H 2 Ti 4 O 9 Na 2 Ti 3 O 7 H 2 Ti 3 O 7 KNb 3 O 8 HNb 3 O 8 K 4 Nb 6 O 17 H 4 Nb 6 O 17
920 (870) 970 (920) 796 (840) 901 (901) 1064 (1064) 1226 / 1104 (1244 / 1104) 940 (940 / 1830) 950 (970)
2.00 0.63 3.00 0.29 0.39 0.50 1.22 0.33
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Fig. 2. X-ray diffraction patterns for the matrices K 2 Ti 4 O 9 ?0.7H 2 O (a), Cu 0.85 K 0.3 Ti 4 O 9 ?0.3H 2 O (b), H 2 Ti 4 O 9 ?0.8H 2 O (c) and Cu 0.21 H 1.58 Ti 4 O 9 ?0.5H 2 O (d).
release of hydration water (0.8 mol) and the second (5%, between 110 and 4708C) is due to the condensation of OH groups, producing lattice water. The K 1 / Cu 21 exchanged matrix exhibits two mass loss steps. The first is due to the release of hydration water (1.7%). The second (7%, 130–4758C) cannot be attributed to the release of lattice water, since the matrix has no OH groups. However, it can be assumed that some water molecules are in the coordination sphere of copper, and are released in the observed temperature range. The total amount of water in the second mass loss step (1.58 mol) is twice the total amount of copper in the matrix (0.85 mol), suggesting that each copper cation is coordinated to two water molecules, as shown schematically in Fig. 3. The copper exchanged acid matrix releases hydration water (2.6%) and lattice water (4.3%). Based on the EDTA titration and thermogravimetric data, the following formulas can be proposed for the matrices: K 2 Ti 4 O 9 ?0.7H 2 O, Cu 0.85 K 0.3 Ti 4 O 9 ?0.3H 2 O,
Fig. 3. Schematic representation of the copper ion exchange process for the potassium titanate matrix.
H 2 Ti 4 O 9 ?0.8H 2 O and Cu 0.21 H 1.58 Ti 4 O 9 ?0.5H 2 O. Is worth noting that, for the copper exchanged acid matrices, the total amount of released lattice water, as calculated from TG data, is in very good agreement with the expected values, taking into account the total amount of copper (EDTA titration), and the electrical neutrality of the compounds.
3.2. Na2 Ti3 O7 compounds In contrast to the behavior of potassium titanate, the copper exchanged sodium matrix exhibits a reduced interlayer distance (Table 1), from 840 to 796 pm, when compared with the pure matrix. On the other hand, the ion exchange process does not produce any interlayer distance change for the acid matrix. The total amount of adsorbed copper is about 10 times higher for the sodium matrix in comparison with the respective acid matrix. The sodium matrix had an absorption capacity 50% higher when compared with the potassium matrix. Furthermore, all sodium ions are exchanged by copper. The sodium matrix does not exhibit any mass loss step, that is, it is anhydrous. On the other hand, the respective acid matrix shows a single mass loss step (6%, 220– 4608C) due to condensation of the OH groups. Taking into account the amount of condensed water (1.0 mol per mol of compound), it can be inferred that the exchange of Na 1 for H 1 to produce the acid matrix was total. The thermogravimetric curve of the Na 1 / Cu 21 ex-
˜ Nunes et al. / Journal of Alloys and Compounds 319 (2001) 94 – 99 L. Magalhaes
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Fig. 4. Thermogravimetric curve for the matrices Na 2 Ti 3 O 7 (a) and Cu 1.0 Ti 3 O 7 ?0.2H 2 O (b).
changed matrix (Fig. 4) exhibits three mass loss steps: 1.3% (60–1808C), 6% (180–3808C) and 6% (720–9308C). The second and third mass losses can be attributed to the release of water molecules coordinated to copper. This mass loss corresponds to 2.12 mol of water, which is about two times the total amount of copper in the matrix (1.0 mol). This fact reinforces the hypothesis proposed for the potassium matrix, that copper is coordinated to two water molecules in the titanate matrices. The H 1 / Cu 21 exchanged matrix shows a single mass loss step (5.6%, 240–3808C) due to the release of lattice water molecules (condensation of OH groups), indicating that the ion exchange process was not complete, as verified by the EDTA titration results. Based on the EDTA titration and thermogravimetric data, the following formulas can be proposed for the synthesized compounds: Na 2 Ti 3 O 7 , Cu 1.0 Ti 3 O 7 ?0.2H 2 O, H 2 Ti 3 O 7 and Cu 0.07 H 1.86 Ti 3 O 7 . The scanning electron micrographs of the matrices Na 2 Ti 3 O 7 and Cu 1.0 Ti 3 O 7 ?0.2H 2 O (Fig. 5) demonstrate 1 21 that ion exchange (Na / Cu ) causes modifications to the morphology of the grains. The mapping image obtained for copper (not shown) demonstrates that this element is homogeneously distributed in the grain surface.
matrix shows another diffraction peak at 1104 pm, suggesting the presence of two distinct structural phases which could be a consequence of an incomplete ion exchange reaction. The ion exchange process with copper does not modify the interlayer distance for the alkaline matrix. However, for the acid matrix this distance is decreased from 1244 to 1226 pm. The ion exchange capacity of the niobate acid matrix is higher than that for the alkaline matrix. This behavior was not observed for any other matrix. In the TG curve of the alkaline matrix, no mass loss step is observed. On the other hand, the acid matrix exhibits two mass loss steps, 0.5% (30–1008C) and 0.7% (240– 3908C), due to the release of hydration and lattice water, respectively. The TG curves show that the ion exchanged matrix (K 1 / Cu 21 ) is anhydrous, whereas the H 1 / Cu 21 matrix exhibits two mass loss steps (0.6%, 30–1008C and 1.7%, 100–4408C) due to the release of hydration and lattice water, respectively. Based on the EDTA titration and thermogravimetric data, the following formulas can be proposed for the synthesized compounds: KNb 3 O 8 , H 0.4 K 0.6 Nb 3 O 8 ? 0.12H 2 O and Cu 0.18 K 0.64 Nb 3 O 8 . Since the acid matrix contains some potassium cations, a stoichiometric formula for the copper exchanged matrix cannot be proposed.
3.3. KNb3 O8 compounds 3.4. K4 Nb6 O17 compounds Based on the X-ray data, it can be verified that the alkaline and acid matrices exhibit different interlayer distances of 1064 and 1244 pm, respectively. The acid
The X-ray diffraction pattern of the alkaline matrix exhibits two diffraction peaks corresponding to interlayer
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loss steps are attributed to the release of water molecules coordinated to the copper cation. The acid matrix exhibits a 1.75% mass loss step (30–1108C) due to the release of hydration water. A second mass loss of 1.92% (110– 3008C) is associated with the release of lattice water. The total amount of released water is in agreement with the supposition that the ion exchange process involves only the potassium ions and not the protons. Based on the EDTA titration and thermogravimetric data, the following formulas can be proposed for the synthesized compounds: K 4 Nb 6 O 17 ?3H 2 O, H 2 K 2 Nb 6 O 17 ? H 2 O, Cu 1.27 K 1.46 Nb 6 O 17 ?0.26H 2 O and Cu 0.27 K 1.41 H 2 Nb 6 O 17 ?0.9H 2 O.
4. Conclusion
Fig. 5. Scanning electron micrographs for the matrices Na 2 Ti 3 O 7 (a) and Cu 1.0 Ti 3 O 7 ?0.2H 2 O (b).
distances of 940 and 1830 pm. The first peak is associated with the anhydrous form and the second with the hydrated structure [7–9]. The acid matrix exhibits an interlayer distance of 970 pm, and the second peak is not observed. According to Nakato et al. [32] the absence of the second diffraction peak can be attributed to the indistinguishability of the two interlayer regions. The alkaline and acid copper exchanged matrices have interlayer distances of 940 and 950 pm, respectively. The alkaline matrix exhibits an ion exchange capacity towards copper about four times larger than the acid matrix, giving values of 1.22 and 0.33 mmol g 21 , respectively. The thermogravimetric curve for H 4 Nb 6 O 17 exhibits two distinct mass loss steps, 2.1% (30–1108C) and 1.9% (110–3558C), corresponding to the release of hydration and lattice water, respectively. The total amount of lattice water suggests that the exchange K 1 / H 1 was only 50%. The K 1 / Cu 21 exchanged matrix shows three mass loss steps: 0.5% (30–1008C, hydration water), 2.5% (100– 2208C) and 2.6% (220–3808C). The second and third mass
For most of the studied matrices, the ion exchange (K 1 or Na 1 / H 1 ; K 1 , Na 1 or H 1 / Cu 21 ) process has, as a consequence, an alteration of the interlayer space, i.e. a change in the nanostructure of the matrix. On the other hand, with the exception of the copper exchanged sodium titanate matrix, it can be verified by scanning electron microscopy that the ion exchange process does not change the microstructure of the matrices, i.e. the morphology of the grains. As general behavior, the alkaline titanates and niobates have a higher ion exchange capacity towards copper cations than their respective acid forms. Furthermore, the sodium matrix Na 2 Ti 3 O 7 exhibits the highest ion exchange capacity of 3.00 mmol g 21 . Based on the thermogravimetric results, it can be concluded that, when adsorbed on the titanate and niobate matrices, the Cu(II) cation is coordinated to two water molecules.
References [1] R. Abe, S. Ikeda, J.N. Kondo, M. Hara, K. Domen, Thin Solid Films 343 (1999) 156. [2] M.A. Bizeto, D.L.A. de Faria, V.R.L. Constantino, J. Mater. Sci. Lett. 18 (1999) 643. [3] V.R.L. Constantino, C.A.S. Barbosa, M.A. Bizeto, P.M. Dias, An. Acad. Bras. Cienc. 72 (2000) 45. [4] L.M. Nunes, R.F. de Farias, C. Airoldi, Mater. Res. Bull. (in press). [5] J.H. Choy, Y.S. Han, N.G. Park, H. Kim, S.W. Kim, Synth. Met. 71 (1995) 2053. [6] T.P. Feist, S.J. Mocarski, P.K. Davies, A.J. Jacobson, J.T. Lewandowski, Solid State Ionics 28–30 (1988) 1338. [7] R. Abe, K. Shinohara, A. Tanaka, M. Hara, J.N. Kondo, K. Domen, J. Mater. Res. 13 (1998) 861. [8] V.R.L. Constantino, M.A. Bizeto, H.A. Brito, J. Alloys Comp. 278 (1998) 142. [9] T. Nakato, D. Sakamoto, K. Kuroda, C. Kato, Bull. Chem. Soc. Jpn. 65 (1992) 322. [10] N. Nikomura, K. Zasshi, J. Miner. Soc. Jpn. 16 (1984) 447. [11] N. Nikomura, N. Kumada, F. Muto, J. Chem. Soc., Dalton Trans. (1985) 2349.
˜ Nunes et al. / Journal of Alloys and Compounds 319 (2001) 94 – 99 L. Magalhaes [12] G. Lagaly, K. Beneke, J. Inorg. Nucl. Chem. 38 (1976) 1513. [13] T. Nakato, K. Kuroda, C. Kato, J. Chem. Soc., Chem. Commun. (1989) 1144. [14] T. Nakato, Y. Sugahara, K. Kuroda, C. Kato, Mater. Res. Soc. Symp. Proc. 233 (1991) 169. [15] S.P. Mishra, V.K. Singh, D. Tiwari, Appl. Radiat. Isot. 49 (1998) 1467. [16] S.P. Mishra, V.K. Singh, D. Tiwari, Appl. Radiat. Isot. 48 (1997) 435. [17] S.P. Mishra, S. Upadhyaya, J. Radional. Nucl. Chem. 189 (1995) 247. [18] S.P. Mishra, N. Shinivasu, Radiochim. Acta 61 (1993) 47. [19] T. Sasaki, F. Izumi, M. Watanabe, Chem. Mater. 8 (1996) 777. [20] S. Cheng, T.C. Wang, Inorg. Chem. 28 (1989) 1283. [21] H. Miyata, Y. Sugahara, K. Kuroda, C. Kato, J. Chem. Soc., Faraday Trans. 84 (1988) 2677.
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[22] T. Sasaki, Y. Komatsu, Y. Fujiki, Mater. Res. Bull. 22 (1987) 1321. [23] T. Nakato, Y. Iwata, K. Kuroda, M. Kaneko, C. Kato, J. Chem. Soc., Dalton Trans. (1993) 1405. [24] Y. Kim, S.J. Atherton, E.S. Brigham, T.E. Mallouk, J. Phys. Chem. 97 (1993) 11802. [25] T. Sasaki, S. Nakano, S. Yamauchi, M. Watanabe, Chem. Mater. 9 (1997) 602. [26] H. Izawa, S. Kikkawa, M. Koizumi, Polyhedron 2 (1983) 741. [27] T. Sasaki, M. Watanabe, Y. Komatsu, Y. Fujiki, Bull. Chem. Soc. Jpn. 58 (1985) 3500. [28] T. Sasaki, Y. Komatsu, Y. Fujiki, Inorg. Chem. 28 (1989) 2776. [29] W. Hou, Q. Yan, B. Peng, X. Fu, J. Mater. Chem. 5 (1995) 109. [30] H. Izawa, S. Kikawa, M. Koizumi, Polyhedron 2 (1983) 741. [31] H.A. Flaschka, EDTA Tritation and Introduction to Theory and Practice, 2nd Edition, Pergamon Press, New York, 1967. [32] T. Nakato, K. Kuroda, C. Kato, Chem. Mater. 4 (1992) 128.
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Synthesis of new compounds involving layered titanates and niobates with copper(II) ˜ Nunes a , *, Antonio Gouveia de Souza a , Robson Fernandes de Farias b Liliane Magalhaes a
˜ Pessoa, Paraıba ´ ´ , 58059 -900 Joao ´ , Brazil Departamento de Quımica , CCEN, Universidade Federal da Paraıba b ´ ´ , 69310 -270 Boa Vista, Roraıma ´ , Brazil , Universidade Federal de Roraıma Departamento de Quımica Received 24 July 2000; received in revised form 20 October 2000; accepted 1 November 2000
Abstract A series of titanates and niobates, K 2 Ti 4 O 9 , Na 2 Ti 3 O 7 , KNb 3 O 8 , K 4 Nb 6 O 17 , and their respective acid forms were prepared and characterized by X-ray diffraction, thermogravimetry and scanning electron microscopy. The ion exchange capacity of the alkaline and acid forms towards Cu(II) was studied and it is concluded that, as general behavior, the alkaline titanates and niobates have a higher ion exchange capacity towards copper cations than their respective acid forms. Furthermore, the sodium matrix Na 2 Ti 3 O 7 exhibits the highest ion exchange capacity of 3.00 mmol g 21 . Based on the thermogravimetric results, the Cu(II) cation is coordinated to two water molecules when adsorbed on the titanate and niobate matrices. 2001 Elsevier Science B.V. All rights reserved. Keywords: Titanates; Niobates; Ion exchange; Copper
1. Introduction The inorganic layered oxides have attracted considerable attention due to their unique structural properties and applications, such as in intercalation reactions, ionic exchange processes, their photochemical and semiconductor properties as well as catalytic activities [1–5]. On the
Fig. 1. Schematic representation of interlayers I and II in the K 4 Nb 6 O 17 matrix.
*Corresponding author. ˜ E-mail address: [email protected] (L. Magalhaes Nunes).
other hand, titanates and niobates can combine with organic and inorganic species producing hybrid matrices, which have recently been investigated [6–9]. The layered titanates (K 2 Ti 4 O 9 and Na 2 Ti 3 O 7 ) and niobates (KNb 3 O 8 and K 4 Nb 6 O 17 ) consist of TiO 6 or NbO 6 octahedral units sharing edges on one level which combine with similar units above and below to form zig-zag strings of octahedra [1,10]. In this case, the charge balance is maintained by the occupancy of alkali-metal ions between the layers. The K 4 Nb 6 O 17 matrix is structurally distinguishable from other related layered niobates or titanates since it has two different types of interlayer spaces (interlayers I and II) alternating between [Nb 6 O 17 ] 42 layers, as shown in Fig. 1. Interlayer I contains potassium ions with water molecules, while interlayer II contains unhydrated potassium ions under ambient conditions [11–13]. Kinomura et al. [14,15] concluded that monovalent cations were intercalated into both interlayers and di and trivalent cations were exchanged with potassium ions in interlayer I only. Other reports on intercalation into K 4 Nb 6 O 17 do not disagree with these results; alkylammonium ions seem to be intercalated into both interlayers [16], while methylviologen dication is taken up into interlayer I only [17,18]. Layered oxide quadrivalent metals are frequently employed in the removal / separation of several radio / bio-
0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 00 )01414-6
˜ Nunes et al. / Journal of Alloys and Compounds 319 (2001) 94 – 99 L. Magalhaes
toxic metal ions from aqueous solutions [19–22] since they are quite effective for cation as well as anion separations. Furthermore, they behave like a cation exchanger in alkaline solutions, depending upon the basicity of the central atom and the strength of the M–O bond relative to that of the O–H bond in the hydroxyl group [19–22]. The stability in acidic solutions is restricted by transformation into protonic oxides [23–25]. However, the resulting compound retains a layered structure similar to that of the alkaline materials. The acidic layered oxide, which is also called the layered hydrous titanium or niobium dioxide, exhibits distinct intercalation behavior towards several cations and some organic compounds [2,26,27]. This property can be exploited in the use of this material to remove and immobilize radioactive nuclides such as 137 Cs and 90 Sr from high-level liquid waters [28]. The aim of the present work was to investigate the behavior of the ion exchange process involving copper chloride with the matrices potassium tetratitanate (K 2 Ti 4 O 9 ), sodium trititanate (Na 2 Ti 3 O 7 ), potassium triniobate (KNb 3 O 8 ), and potassium hexaniobate (K 4 Nb 6 O 17 ) and their respective acid forms.
95
The interlamellar ions (Na 1 , K 1 or H 1 ) were exchanged by copper, by suspending 0.5 g of matrix in 50.0 cm 3 of a 0.15 mol dm 23 copper chloride solution at 25618C for 6 h. After this time, the solid samples were separated by centrifugation and dried at 408C for 24 h. The concentration of exchangeable copper cations in solution was determined by complexometric titration with a 0.02 mol dm 23 EDTA solution [31]. The amount of exchanged cations (n f ) was determined through the relationship (n i 2 n s ) /m, where n i is the initial number of moles of cation in solution, n s is the number of moles of cations in equilibrium with the solid phase after the exchange process and m is the mass of the exchanger used. The alkaline, acid and copper exchanged matrices were characterized by: X-ray diffraction using Cu Ka radiation in the 2u range from 3 to 50 with a Shimadzu diffractometer Model XD3A; thermogravimetry (TG), using a Shimadzu TGA-50 instrument, by heating samples (|5.0 mg) from 25 to 9508C, with a heating rate of 108C min 21 , under nitrogen atmosphere. The scanning electron micrographs were obtained in a JEOL microscope, JSM T-300, with an accelerating voltage of 20 kV.
3. Results and discussion 2. Experimental All chemicals used, TiO 2 (Aldrich), Nb 2 O 5 (Companhia ˜ Brasileira de Metalurgia e Minerac¸ao), K 2 CO 3 , Na 2 CO 3 (Merck) and CuCl 2 (Aldrich), were of analytical grade and were employed without further purification. Potassium tetratitanate (K 2 Ti 4 O 9 ) and sodium trititanate (Na 2 Ti 3 O 7 ) were prepared by solid-state reaction by the heating of a stoichiometric mixture of K 2 CO 3 or Na 2 CO 3 and TiO 2 powders in a platinum crucible. The mixture was heated at 1073 K for 20 h at room atmosphere. It was then ground and heated again at the same temperature for a further 20 h [29,30]. Potassium triniobate (KNb 3 O 8 ) and hexaniobate (K 4 Nb 6 O 17 ) were prepared by solid-state reaction by the heating of stoichiometric mixtures of K 2 CO 3 and Nb 2 O 5 powders in a platinum crucible. For the KNb 3 O 8 matrix, the stoichiometric mixture was heated at 873 K for 2 h and then ground and heated at 1173 K for a further 3 h [7]. For the K 4 Nb 6 O 17 matrix, the stoichiometric mixture was heated at 1173 K for 44 h. The H 1 -exchanged matrices were prepared by suspending the alkaline matrices in HCl (for titanium matrices) or HNO 3 (for niobium matrices) solution. In a typical procedure, 1.0 g of matrix was placed in contact, under stirring, with 20.0 cm 3 of a 1.0 mol dm 23 solution of hydrochloric or nitric acid at 343 K for 3 days. The solid material was separated by centrifugation, and washed with bidistilled water until pH 5.0 to 6.0. The final products were dried over a saturated NaCl solution under 70% relative humidity and characterized.
The interlayer distance as calculated by X-ray diffraction data as well as the total amount of exchanged copper for all alkaline and acid matrices are shown in Table 1.
3.1. K2 Ti4 O9 compounds As can be observed, the ion exchange capacity of the alkaline matrix is about three times larger than that for the acid matrix. Furthermore, the interlayer distance is increased when K 1 or H 1 are exchanged for Cu 21 . The Dd values are 50 pm for both matrices. The X-ray diffraction patterns for the matrices are shown in Fig. 2. The K 2 Ti 4 O 9 matrix exhibits a mass loss of 3% associated with the release of 0.7 mol of hydration water. The respective acid matrix, H 2 Ti 4 O 9 , shows two distinct mass loss steps. The first (3%) is associated with the Table 1 Interlayer distance and total amount of copper exchanged (n f ) for titanate and niobate matrices. The interlayer distance values between parentheses are for the non-exchanged matrices Matrix
d (pm)
nf (mmol g 21 )
K 2 Ti 4 O 9 H 2 Ti 4 O 9 Na 2 Ti 3 O 7 H 2 Ti 3 O 7 KNb 3 O 8 HNb 3 O 8 K 4 Nb 6 O 17 H 4 Nb 6 O 17
920 (870) 970 (920) 796 (840) 901 (901) 1064 (1064) 1226 / 1104 (1244 / 1104) 940 (940 / 1830) 950 (970)
2.00 0.63 3.00 0.29 0.39 0.50 1.22 0.33
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Fig. 2. X-ray diffraction patterns for the matrices K 2 Ti 4 O 9 ?0.7H 2 O (a), Cu 0.85 K 0.3 Ti 4 O 9 ?0.3H 2 O (b), H 2 Ti 4 O 9 ?0.8H 2 O (c) and Cu 0.21 H 1.58 Ti 4 O 9 ?0.5H 2 O (d).
release of hydration water (0.8 mol) and the second (5%, between 110 and 4708C) is due to the condensation of OH groups, producing lattice water. The K 1 / Cu 21 exchanged matrix exhibits two mass loss steps. The first is due to the release of hydration water (1.7%). The second (7%, 130–4758C) cannot be attributed to the release of lattice water, since the matrix has no OH groups. However, it can be assumed that some water molecules are in the coordination sphere of copper, and are released in the observed temperature range. The total amount of water in the second mass loss step (1.58 mol) is twice the total amount of copper in the matrix (0.85 mol), suggesting that each copper cation is coordinated to two water molecules, as shown schematically in Fig. 3. The copper exchanged acid matrix releases hydration water (2.6%) and lattice water (4.3%). Based on the EDTA titration and thermogravimetric data, the following formulas can be proposed for the matrices: K 2 Ti 4 O 9 ?0.7H 2 O, Cu 0.85 K 0.3 Ti 4 O 9 ?0.3H 2 O,
Fig. 3. Schematic representation of the copper ion exchange process for the potassium titanate matrix.
H 2 Ti 4 O 9 ?0.8H 2 O and Cu 0.21 H 1.58 Ti 4 O 9 ?0.5H 2 O. Is worth noting that, for the copper exchanged acid matrices, the total amount of released lattice water, as calculated from TG data, is in very good agreement with the expected values, taking into account the total amount of copper (EDTA titration), and the electrical neutrality of the compounds.
3.2. Na2 Ti3 O7 compounds In contrast to the behavior of potassium titanate, the copper exchanged sodium matrix exhibits a reduced interlayer distance (Table 1), from 840 to 796 pm, when compared with the pure matrix. On the other hand, the ion exchange process does not produce any interlayer distance change for the acid matrix. The total amount of adsorbed copper is about 10 times higher for the sodium matrix in comparison with the respective acid matrix. The sodium matrix had an absorption capacity 50% higher when compared with the potassium matrix. Furthermore, all sodium ions are exchanged by copper. The sodium matrix does not exhibit any mass loss step, that is, it is anhydrous. On the other hand, the respective acid matrix shows a single mass loss step (6%, 220– 4608C) due to condensation of the OH groups. Taking into account the amount of condensed water (1.0 mol per mol of compound), it can be inferred that the exchange of Na 1 for H 1 to produce the acid matrix was total. The thermogravimetric curve of the Na 1 / Cu 21 ex-
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Fig. 4. Thermogravimetric curve for the matrices Na 2 Ti 3 O 7 (a) and Cu 1.0 Ti 3 O 7 ?0.2H 2 O (b).
changed matrix (Fig. 4) exhibits three mass loss steps: 1.3% (60–1808C), 6% (180–3808C) and 6% (720–9308C). The second and third mass losses can be attributed to the release of water molecules coordinated to copper. This mass loss corresponds to 2.12 mol of water, which is about two times the total amount of copper in the matrix (1.0 mol). This fact reinforces the hypothesis proposed for the potassium matrix, that copper is coordinated to two water molecules in the titanate matrices. The H 1 / Cu 21 exchanged matrix shows a single mass loss step (5.6%, 240–3808C) due to the release of lattice water molecules (condensation of OH groups), indicating that the ion exchange process was not complete, as verified by the EDTA titration results. Based on the EDTA titration and thermogravimetric data, the following formulas can be proposed for the synthesized compounds: Na 2 Ti 3 O 7 , Cu 1.0 Ti 3 O 7 ?0.2H 2 O, H 2 Ti 3 O 7 and Cu 0.07 H 1.86 Ti 3 O 7 . The scanning electron micrographs of the matrices Na 2 Ti 3 O 7 and Cu 1.0 Ti 3 O 7 ?0.2H 2 O (Fig. 5) demonstrate 1 21 that ion exchange (Na / Cu ) causes modifications to the morphology of the grains. The mapping image obtained for copper (not shown) demonstrates that this element is homogeneously distributed in the grain surface.
matrix shows another diffraction peak at 1104 pm, suggesting the presence of two distinct structural phases which could be a consequence of an incomplete ion exchange reaction. The ion exchange process with copper does not modify the interlayer distance for the alkaline matrix. However, for the acid matrix this distance is decreased from 1244 to 1226 pm. The ion exchange capacity of the niobate acid matrix is higher than that for the alkaline matrix. This behavior was not observed for any other matrix. In the TG curve of the alkaline matrix, no mass loss step is observed. On the other hand, the acid matrix exhibits two mass loss steps, 0.5% (30–1008C) and 0.7% (240– 3908C), due to the release of hydration and lattice water, respectively. The TG curves show that the ion exchanged matrix (K 1 / Cu 21 ) is anhydrous, whereas the H 1 / Cu 21 matrix exhibits two mass loss steps (0.6%, 30–1008C and 1.7%, 100–4408C) due to the release of hydration and lattice water, respectively. Based on the EDTA titration and thermogravimetric data, the following formulas can be proposed for the synthesized compounds: KNb 3 O 8 , H 0.4 K 0.6 Nb 3 O 8 ? 0.12H 2 O and Cu 0.18 K 0.64 Nb 3 O 8 . Since the acid matrix contains some potassium cations, a stoichiometric formula for the copper exchanged matrix cannot be proposed.
3.3. KNb3 O8 compounds 3.4. K4 Nb6 O17 compounds Based on the X-ray data, it can be verified that the alkaline and acid matrices exhibit different interlayer distances of 1064 and 1244 pm, respectively. The acid
The X-ray diffraction pattern of the alkaline matrix exhibits two diffraction peaks corresponding to interlayer
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loss steps are attributed to the release of water molecules coordinated to the copper cation. The acid matrix exhibits a 1.75% mass loss step (30–1108C) due to the release of hydration water. A second mass loss of 1.92% (110– 3008C) is associated with the release of lattice water. The total amount of released water is in agreement with the supposition that the ion exchange process involves only the potassium ions and not the protons. Based on the EDTA titration and thermogravimetric data, the following formulas can be proposed for the synthesized compounds: K 4 Nb 6 O 17 ?3H 2 O, H 2 K 2 Nb 6 O 17 ? H 2 O, Cu 1.27 K 1.46 Nb 6 O 17 ?0.26H 2 O and Cu 0.27 K 1.41 H 2 Nb 6 O 17 ?0.9H 2 O.
4. Conclusion
Fig. 5. Scanning electron micrographs for the matrices Na 2 Ti 3 O 7 (a) and Cu 1.0 Ti 3 O 7 ?0.2H 2 O (b).
distances of 940 and 1830 pm. The first peak is associated with the anhydrous form and the second with the hydrated structure [7–9]. The acid matrix exhibits an interlayer distance of 970 pm, and the second peak is not observed. According to Nakato et al. [32] the absence of the second diffraction peak can be attributed to the indistinguishability of the two interlayer regions. The alkaline and acid copper exchanged matrices have interlayer distances of 940 and 950 pm, respectively. The alkaline matrix exhibits an ion exchange capacity towards copper about four times larger than the acid matrix, giving values of 1.22 and 0.33 mmol g 21 , respectively. The thermogravimetric curve for H 4 Nb 6 O 17 exhibits two distinct mass loss steps, 2.1% (30–1108C) and 1.9% (110–3558C), corresponding to the release of hydration and lattice water, respectively. The total amount of lattice water suggests that the exchange K 1 / H 1 was only 50%. The K 1 / Cu 21 exchanged matrix shows three mass loss steps: 0.5% (30–1008C, hydration water), 2.5% (100– 2208C) and 2.6% (220–3808C). The second and third mass
For most of the studied matrices, the ion exchange (K 1 or Na 1 / H 1 ; K 1 , Na 1 or H 1 / Cu 21 ) process has, as a consequence, an alteration of the interlayer space, i.e. a change in the nanostructure of the matrix. On the other hand, with the exception of the copper exchanged sodium titanate matrix, it can be verified by scanning electron microscopy that the ion exchange process does not change the microstructure of the matrices, i.e. the morphology of the grains. As general behavior, the alkaline titanates and niobates have a higher ion exchange capacity towards copper cations than their respective acid forms. Furthermore, the sodium matrix Na 2 Ti 3 O 7 exhibits the highest ion exchange capacity of 3.00 mmol g 21 . Based on the thermogravimetric results, it can be concluded that, when adsorbed on the titanate and niobate matrices, the Cu(II) cation is coordinated to two water molecules.
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