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The intercalation of n-alkyldiamines into crystalline layered titanate
Pergamon
Materials Research Bulletin 35 (2000) 2081–2090
The intercalation of n-alkyldiamines into crystalline layered titanate Claudio Airoldia,*, Liliane M. Nunesa, Robson F. de Fariasb a
Instituto de Quı´mica, Universidade Estadual de Campinas, Caixa Postal 6154, 13083-970 Campinas, Sa˜o Paulo, Brazil b Departamento de Quı´mica, Universidade Federal de Roraima, 69310-270, Boa Vista, Roraima, Brazil (Refereed) Received 18 November 1999; accepted 29 February 2000
Abstract The synthesis, characterization, and intercalation of n-alkyldiamines H2N(CH2)nNH2 (n ⫽ 2, 3, 4, 6, 7, and 8) into layered hydrous tetratitanic acid H2Ti4O9䡠nH2O are reported. From the elemental analysis, the following formula were proposed for the intercalated compounds: (C2H8N2)0.3H2Ti4O9䡠0.40H2O, (C3H10N2)0.6H2Ti4O9䡠0.89H2O, (C4H12N2)0.3H2Ti4O9䡠0.40H2O, (C6H16N2)0.6H2Ti4O9䡠0.92H2O, (C7H18N2)0.5H2Ti4O9䡠0.50H2O and (C8H20N2)0.6H2Ti4O9䡠0.60H2O. X-ray diffraction patterns for these matrices showed that the 920 pm interlayer space of the original matrix has the d-spacing values increased to 1118, 1318, 1549, 1662, 1720, and 1766 pm, respectively, for the C2–C8 sequence of intercalated n-alkyldiamine. The infrared data are in agreement with the protonated nitrogen atoms of the diamines located in the cavity space. The hydrous matrix showed two mass loss steps in the thermogravimetric curve, corresponding to the release of physisorbed and lattice water molecules. A third mass loss was observed for the intercalated matrices due to the release of organic moiety. From these values, the intercalated matrices represented by the number of carbons in the organic chains can be ordered in the following sequence of thermal stability: C4 ⬎ C2 ⬎ C3 ⬵ C6 ⬎ C7 ⬵ C8. The lowest amount of intercalated n-alkyldiamines gave the largest thermal stability of the hybrid. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Layered compounds; A. Nanostructures; B. Intercalation reactions; C. Thermogravimetric analysis (TGA); C. X-ray diffraction
* Corresponding author. E-mail address: [email protected] (C. Airoldi). 0025-5408/00/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 0 ) 0 0 4 2 6 - 8
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1. Introduction Alkaline metal titanates have been extensively investigated as ion exchangers due to their high selectivity towards metal cations. In this respect, the layered crystalline compound K2Ti4O9 had been investigated through some semi-empirical calculations. However, the best condition of exchange is based on its transformed acidic form H2Ti4O9, where the proton is ion-exchanged with the chosen partner cation [1]. Several experimental approaches can be used as routes to synthesize metal titanates. Among the reported methods are those related to sol-gel [2,3], hydrothermal [4], or even mechanically induced reactions [5]. High pressure has also been used as an experimental procedure to intercalate amine molecules in layered titanates [6]. The series of ion-exchangers alkaline titanates can be represented by the general formula A2TinO2n⫹1, where A is an alkaline metal and n can assume values in the range 1 ⱕ n ⱕ 8, where any specific value is defined by the respective structure of the exchanger. Thus, features associated with a characteristic layered structure relate to n values in the 2 ⱕ n ⱕ 4 interval, while for n ⱖ 6, the exchanger manifested as tunnel or fiber structures. From the point of view of applications, the fibrous compound is becoming important as a substitute for hazardous asbestos material [7,8]. The crystalline layered tetratitanates are composed of four octahedral TiO6 units, where edges are shared at one level that combine with similar units above and below, to form zigzag strings of octahedra [9,10]. The stability of this polyanion in acidic solutions is restricted by the facility in retaining protons to transform into protonic oxides [11–13]. However, the resulting compound maintains the original layered structure, which is similar to that of the alkaline exchangers. Considerable attention has been devoted to such kinds of compounds due to the prominent abilities of intercalated species, in acting as ion exchanger, displaying photochemical and semiconductor properties as well as with catalytic activity [9,14 –17]. The acidic layered tetratitanate, also called layered hydrous titanium dioxide, exhibits distinct intercalation behavior towards various cations and some organic polar molecules [8,14,18]. This material may be useful not only to remove species from a given solution, but also to immobilize radioactive nuclides such as 137Cs and 90Sr from high-level concentrations of liquid waste [19]. Based on the high utility of such crystalline compounds, a significant number of investigations have been devoted to ion-exchange reactions, with the great majority focused on structural features of these exchangers, mainly exploring their excellent chemical and thermal stabilities and their facility in developing favorable ion-exchange properties [7,14, 18 –21]. The intercalation of organoammonium cations or neutral n-alkylamines into layered solids normally causes an increase in the interlayer distance, which is necessary to accommodate the intercalated species [6]. However, up to now, the intercalation of the n-alkyldiamines into layered hydrous titanium dioxide has not been reported. An interesting feature associated with the intercalation of organoammonium cations is the use of an intermediate, as observed for other similar inorganic crystalline compounds, for some preparations of pillared tetratitanate [22–25], and also for synthesis of inorganic/ organic complexes [26 –28]. The purpose of this publication is to report the synthesis of intercalated compounds involving n-alkyldiamines, H2N(CH2)nNH2 (n ⫽ 2, 3, 4, 6, 7, or 8),
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with hydrous acidic titanium dioxide, H2Ti4O9䡠nH2O. Special attention is focused on the effects caused by the diamine chain length on the structure and the thermal features of the obtained intercalated matrices.
2. Experimental All chemicals employed were of analytical grade, and were used without further purification. The layered starting material, potassium tetratitanate (K2Ti4O9), was prepared by a solid-state reaction between K2CO3 and TiO2 powders in a 1:3.5 molar ratio. The previous mixture was heated at 1073 K in air for 20 h. It was then ground and heated again at the same temperature for a further 20 h [22]. Layered hydrous titanium dioxide, H2Ti4O9, was obtained by ion-exchange of potassium ions with an acidic solution. In this procedure, 1.0 g of potassium tetratitanate was placed in contact, under stirring, with 20.0 cm⫺3 of 1.0 mol dm⫺3 of hydrochloric acid solution for 12 h at 343 K. The solid material was separated by centrifugation and washed with twice distilled water until the filtrated solution presented a pH in the range 5.0 to 6.0. The final product was dried over a saturated sodium chloride solution under 70% of relative humidity, and the solid was characterized. The intercalation of n-alkyldiamines was conducted by a batch method, which consisted of suspending 100 mg of H2Ti4O9 or K2Ti4O9 in 20.0 cm3 of an aqueous solution of 0.10 mol dm⫺3 of each n-alkyldiamine. The suspensions were sealed in glass tubes and shaken periodically for 30 days at 298 ⫾ 1 K. After this time, the solid was separated by centrifugation, washed with acetone, and dried in air. The original and the intercalated matrices were characterized by the following techniques: X-ray diffractometry by using Cu K␣ radiation in a Shimadzu XD3A diffractometer, in the 2 ⫽ 3 to 50° range and with a scan rate of 7.0 ⫻ 10⫺2 deg䡠s⫺1. The thermogravimetric curves were obtained in a DuPont 1090B instrument, with samples of about 5.0 mg, which were heated under argon atmosphere from 298 to 1223 K, at a heating rate of 0.17 Ks⫺1. Infrared spectra were recorded on a BOMEM apparatus, using KBr discs in the 4000 to 400 cm⫺1 range, with a resolution of 4.0 cm⫺1. CHN elemental analysis was carried out with a Perkin-Elmer PE-2400 analyzer. The scanning electron micrographs were obtained using a JEOL JSM T-300, with an accelerating voltage of 20 kV.
3. Results and discussion The general reaction of intercalation consisted of inserting a series of n-alkyldiamines H2N(CH2)nNH2, named Cn (n ⫽ 2, 3, 4, 6, 7, and 8), where n is the number of carbon atoms in the organic chain, which was accommodated into the free cavity space of the crystalline interlayer layer compounds K2Ti4O9 and H2Ti4O9䡠nH2O. However, the former exchanger did not intercalate any base, indicating that the nature of the cation attached to the matrix in the interlayer space can strongly influence the intercalation properties of this class of lamellar materials. On the other hand, the explanation for this distinct behavior for cationic or acidic
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Table 1 Percentages of carbon (C), nitrogen (N), and hydrogen (H) elemental analysis for the n-alkyldiamines H2N(CH2)nNH2 (n ⫽ 2, 3, 4, 6, 7, or 8) intercalated into hydrous titanate matrices Matrix
C (%)
N (%)
H (%)
Proposed formula
C2 C3 C4 C6 C7 C8
2.13 6.28 3.17 10.50 10.24 13.82
2.17 4.24 1.71 3.79 3.41 3.78
1.45 2.44 1.61 3.28 2.98 4.08
(C2H8N2)0.3H2Ti4O9䡠0.4H2O (C3H10N2)0.6H2Ti4O9䡠0.9H2O (C4H12N2)0.3H2Ti4O9䡠0.4H2O (C6H16N2)0.6H2Ti4O9䡠0.9H2O (C7H18N2)0.5H2Ti4O9䡠0.5H2O (C8H20N2)0.6H2Ti4O9䡠0.6H2O
matrices should be associated with the proposed mechanism of intercalation. In such interactions, it has been demonstrated that a strong dependence on the protonation in a first stage occurs with the entrance of the amine in the interlayer space [29]. Furthermore, it has been shown [30] that the intercalation of alkylammonium chlorides to give alkylammoniumtetratitanate intercalation compounds by using K2Ti4O9 as the host matrices is not possible by the conventional ion exchange method [30]. [2.2.2]-cryptand and 18-crown-6 ether must be employed to allow the intercalation reaction to occur [30]. The elemental analysis results are summarized in Table 1. As can be observed, the matrices with a minor amount of intercalated amines (0.3) are related to the lowest degree of physisorbed water (0.4). This fact suggests that the physisorbed water molecules can be related to an interacting effect of association with the organic molecules. On the contrary, for C3, C6, and C8 matrices, the total amounts of intercalated diamines are identical, and correspond to twice the values obtained for the preceding matrices. X-ray diffraction patterns for the original and all intercalated matrices are shown in Fig. 1. The respective interlayer distances for each compound was collected; these results were based on the d values for the 001 reflection. The original matrices presented the characteristic lamellar distances of 875 and 920 pm for potassium tetratitanate and hydrous titanium dioxide, respectively. For the intercalated hydrous matrices with C2, C3, C4, C6, C7, and C8, the interlayer distance has values of 1118, 1318, 1549, 1662, 1720, and 1834 pm, respectively. Based on carbon, nitrogen, and hydrogen covalent radii (whose values are 77, 70, and 30 pm [31]), the calculated organic diamine chain lengths were 354, 431, 505, 662, 739, and 816 pm for the C2, C3, C4, C6, C7, and C8 n-alkyldiamines, respectively. The plot of d-spacing values as a function of the carbon number of the diamine chain is shown in Fig. 2. As can be observed, C2 and C4 matrices exhibited a different behavior compared with the other matrices, with a deviation from the expected linearity. These results suggested that for both matrices, the spatial orientation of the diamine chains is distinct from those observed for the other matrices. In admitting the proposed straight line, the C2 and C4 matrices exhibited a negative and positive deviation from linearity, respectively. These results implied that in both matrices the organic chains form with the titanate silicate, smaller and larger angles, respectively, when compared with the expected normal orientation for other modified matrices. Probably this arrangement of the intercalated diamines ions depends on the layer charge and the alkyl-chain length. This was demonstrated for smectites, where short-chain alkylammonium ions are composed of a monolayer and long chain cations as a
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Fig. 1. X-ray diffraction patterns of the hydrous tetratinanate matrix (a) and the hydrous dialkylmines, H2N(CH2)nNH2, intercalated matrices C2 (b), C3 (c), C4 (d), C6 (e), C7 (f), and C8 (g).
bilayer with their alkyl chains parallel to the silicate sheets [32]. Although the inorganic backbone of the hydrous acidic titanium dioxide has uncharged layers, the intercalated species are identical in interactive nature. Then, it is accepted that the same behavior occurred with this synthetic host self-organized of highly ordered material. These facts are in agreement with the C2 and C4 guest alkylamines are inserted into the interlayer space like a paraffin-type arrangement [32]. The behavior of the experimental points in Fig. 2 suggests an additional interpretation. From these values, the interlayer distance correlates linearly with the carbon content in the organic chain for C2, C3, and C4 matrices. On the other hand, a possible correlation seems to be observed from C6 to C8 with a smaller angle. This fact could be associated with the orientation of the largest organic chains in the lamellar space.
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Fig. 2. Interlayer distance, d, as function of the number of carbon atoms in the n-dialkylmines, H2N(CH2)nNH2, intercalated into a hydrous titanate matrix.
The intercalation process does not affect the morphology of the materials, as shown by the illustrative examples for SEM micrographs of the hydrous and C6 intercalated samples in Fig. 3a and b, respectively. The obtained infrared data indicated that the basic NHK groups of the n-alkyldiamines are protonated by the available proton bonded on a hydrous acidic host matrix. This same behavior is confirmed for all intercalated matrices, which exhibited a characteristic broad and medium band for the NH3⫹ group in the 1620 to 1630 cm⫺1 range [22]. The infrared spectra of the hydrous titanate and those for C3 and C6 intercalated matrices are shown in Fig. 4. As expected, for all synthesized matrices, the CH2 vibration bands remained unchanged after intercalating the guest molecules. The thermogravimetric curves for the hydrous acidic titanium dioxide and n-alkyldiamine intercalated matrices are shown in Figs. 5 and 6, respectively. The host matrix exhibited two distinct mass loss steps, the first being due to the release of physisorbed water molecules, followed by another one, which corresponded to the loss of lattice water molecules. The water content in the proposed formula presented in Table 1 was calculated by using the mass loss percentage of the last step. These last molecules came as a consequence of the condensation of the free hydroxyl groups on the matrix, as has been observed for other surfaces [33]. For the intercalated n-dialkylamine samples, a third mass loss step, located between the two previously described ones, near 500 –750 K, is observed. The new step for all intercalated matrices was attributed to the release of the organic moiety. By using the correspondent
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Fig. 3. SEM photographs for (a) the hydrous lamellar titanate matrix and (b) the C6 intercalated matrix. Scale bar is in micrometers.
percentage of this mass loss, the total amount of organic molecules inserted into the vacancy space of the matrices can be calculated. These obtained values are in very good agreement with the elemental analysis results, with a deviation of ⫾3%. These facts confirm the validity of thermogravimetry as a reliable analytical tool to study inorganic or inorganic– organic hybrid matrices [33,34]. As observed, the organic-modified matrices released lattice water molecules at about 900 K, after the loss of the organic moiety. The total amount of lattice water released is practically the same for the original and the intercalated amine matrices, suggesting that the unprotonated n-dialkylamine left the interlayer spaces on heating. This is a reasonable proposal due to the easier disruption on breaking the hydrogen bonding between the NH2 group of the diamine and the hydroxyl group of the matrix, formed in the intercalation process. By considering the release of organic moiety to be the most important factor related to the behavior of the host matrices on heating, an ordered sequence of stability can be established: C4 ⬎ C2 ⬎ C3 ⬵ C6 ⬎ C7 ⬵ C8. This order correlated with the amount of intercalated
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Fig. 4. Infrared spectra for the hydrous lamellar titanate (a) and the intercalated matrices of C3 (b) and C6 (c).
Fig. 5. Thermogravimetric and derivative curves for the hydrous tetratinanate lamellar matrix.
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Fig. 6. Thermogravimetric and derivative curves for the n-dialkylamines H2N(CH2)nNH2 intercalated matrices: (a) C2, (b) C3, (c) C4, (d) C6, (e) C7, and (f) C8.
organic moiety with the stability of the synthesized hybrid. The lowest degree of insertion provided larger stability in the respective matrix, which is, in fact, associated with the arrangement of the organic chain length into the free cavity space of the host inorganic layer.
4. Conclusions Based on the series of experimental results involving the crystalline compounds K2Ti4O9 and H2Ti4O9䡠nH2O, the obtained data established that the protonation of the guest molecule is a prominent factor to explain the intercalation process in the host matrix. The orientation
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of the intercalated molecules, as well as the thermal stability of the hybrid matrices, is dependent on the total amount of the molecules intercalated into the lamellae of the host. This behavior can be visualized by the variation in the interlayer distance as the number of carbons of the intercalated n-dialkylamine is increased. The largest thermal stability of C2 and C4 matrices not only is associated with the lowest amount intercalated, but is also due to the paraffin-like arrangement of the organic chains.
References [1] C.N.R. Rao, J. Gopalakrishnan, New Directions in Solid State Chemistry, 2nd ed., Cambridge University Press, Cambridge, 1997, p. 129. [2] M. Andrianainarivelo, R.J.P. Corriu, D. Leclerq, P.H. Mutin, A. Vioux, J Mater Chem 7 (1997) 279. [3] E.L. Sham, M.A.G. Aranda, E.M. Farfan-Torres, J.C. Gottifredi, M. Martı´nez-Lara, S. Bruque, J Solid State Chem 139 (1998) 225. [4] I.J. Clark, T. Takeuchi, N. Ohtori, D.C. Sinclair, J Mater Chem 9 (1999) 83. [5] N.J. Welham, J Mater Res 13 (1998) 1607. [6] S. Nakano, T. Sasaki, K. Takemura, M. Watanabe, Chem Mater 10 (1998) 2044. [7] H. Izawa, S. Kikkawa, M. Koizumi, J Solid State Chem 69 (1987) 336. [8] H. Izawa, S. Kikkawa, M. Koizumi, Polyhedron 2 (1983) 741. [9] J.H. Choy, Y.S. Han, N.G. Park, H. Kim, S.W. Kim, Synth Met 71 (1995) 2053. [10] T.P. Feist, S.J. Mocarski, P.K. Davies, A.J. Jacobson, J.T. Lewandowski, Solid State Ionics 28 –30 (1988) 1338. [11] T. Sasaki, F. Izumi, M. Watanabe, Chem Mater 8 (1996) 777. [12] S. Cheng, T.C. Wang, Inorg Chem 28 (1989) 1283. [13] H. Miyata, Y. Sugahara, K. Kuroda, C. Kato, J Chem Soc Faraday Trans 84 (1988) 2677. [14] T. Sasaki, Y. Komatsu, Y. Fujiki, Mater Res Bull 22 (1987) 1321. [15] T. Nakato, Y. Iwata, K. Kuroda, M. Kaneko, C. Kato, J Chem Soc Dalton Trans (1993) 1405. [16] Y. Kim, S.J. Atherton, E.S. Brigham, T.E. Mallouk, J Phys Chem 97 (1993) 11802. [17] T. Sasaki, S. Nakano, S. Yamauchi, M. Watanabe, Chem Mater 9 (1997) 602. [18] T. Sasaki, M. Watanabe, Y. Komatsu, Y. Fujiki, Bull Chem Soc Jpn 58 (1985) 3500. [19] T. Sasaki, Y. Komatsu, Y. Fujiki, Inorg Chem 28 (1989) 2776. [20] H. Izawa, S. Kikkawa, M. Koizumi, J Solid State Chem 60 (1985) 264. [21] W.A. England, J.E. Birkett, J.B. Goodenough, P.J. Wiseman, J Solid State Chem 49 (1983) 300. [22] S. Roca, C. Airoldi, J Mater Chem 6 (1996) 1963. [23] M. Ogawa, Y. Takizawa, Chem Mater 11 (1999) 30. [24] P. Sylvester, R. Cahill, A. Clearfield, Chem Mater 6 (1994) 1890. [25] M.W. Anderson, J. Klinowski, Inorg Chem 29 (1990) 3260. [26] T. Nakato, D. Sakamoto, K. Kuroda, M. Kaneko, C. Kato, Bull Chem Soc Jpn 65 (1992) 332. [27] T. Nakato, K. Kusunoki, K. Yoshizawa, K. Kuroda, M. Kaneko, J Phys Chem 99 (1995) 17896. [28] C.V. Kumar, Z.J. Williams, J Phys Chem 99 (1995) 17632. [29] S. Roca, C. Airoldi, J Chem Soc Dalton Trans (1997) 2517. [30] M. Ogawa, Y. Takizawa, Chem Mater 11 (1999) 30. [31] F.A. Cotton, G. Wilkinson, P.L. Gauss, Basic Inorganic Chemistry, John Wiley & Sons, New York, 1995. [32] M. Ogawa, K. Kuroda, Bull Chem Soc Jpn 70 (1997) 2593. [33] L.M. Nunes, C. Airoldi, R.F. de Farias, J Solid State Chem 145 (1999) 649. [34] R.F. de Farias, C. Airoldi, J Thermal Anal 53 (1998) 751.
Materials Research Bulletin 35 (2000) 2081–2090
The intercalation of n-alkyldiamines into crystalline layered titanate Claudio Airoldia,*, Liliane M. Nunesa, Robson F. de Fariasb a
Instituto de Quı´mica, Universidade Estadual de Campinas, Caixa Postal 6154, 13083-970 Campinas, Sa˜o Paulo, Brazil b Departamento de Quı´mica, Universidade Federal de Roraima, 69310-270, Boa Vista, Roraima, Brazil (Refereed) Received 18 November 1999; accepted 29 February 2000
Abstract The synthesis, characterization, and intercalation of n-alkyldiamines H2N(CH2)nNH2 (n ⫽ 2, 3, 4, 6, 7, and 8) into layered hydrous tetratitanic acid H2Ti4O9䡠nH2O are reported. From the elemental analysis, the following formula were proposed for the intercalated compounds: (C2H8N2)0.3H2Ti4O9䡠0.40H2O, (C3H10N2)0.6H2Ti4O9䡠0.89H2O, (C4H12N2)0.3H2Ti4O9䡠0.40H2O, (C6H16N2)0.6H2Ti4O9䡠0.92H2O, (C7H18N2)0.5H2Ti4O9䡠0.50H2O and (C8H20N2)0.6H2Ti4O9䡠0.60H2O. X-ray diffraction patterns for these matrices showed that the 920 pm interlayer space of the original matrix has the d-spacing values increased to 1118, 1318, 1549, 1662, 1720, and 1766 pm, respectively, for the C2–C8 sequence of intercalated n-alkyldiamine. The infrared data are in agreement with the protonated nitrogen atoms of the diamines located in the cavity space. The hydrous matrix showed two mass loss steps in the thermogravimetric curve, corresponding to the release of physisorbed and lattice water molecules. A third mass loss was observed for the intercalated matrices due to the release of organic moiety. From these values, the intercalated matrices represented by the number of carbons in the organic chains can be ordered in the following sequence of thermal stability: C4 ⬎ C2 ⬎ C3 ⬵ C6 ⬎ C7 ⬵ C8. The lowest amount of intercalated n-alkyldiamines gave the largest thermal stability of the hybrid. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Layered compounds; A. Nanostructures; B. Intercalation reactions; C. Thermogravimetric analysis (TGA); C. X-ray diffraction
* Corresponding author. E-mail address: [email protected] (C. Airoldi). 0025-5408/00/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 0 ) 0 0 4 2 6 - 8
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1. Introduction Alkaline metal titanates have been extensively investigated as ion exchangers due to their high selectivity towards metal cations. In this respect, the layered crystalline compound K2Ti4O9 had been investigated through some semi-empirical calculations. However, the best condition of exchange is based on its transformed acidic form H2Ti4O9, where the proton is ion-exchanged with the chosen partner cation [1]. Several experimental approaches can be used as routes to synthesize metal titanates. Among the reported methods are those related to sol-gel [2,3], hydrothermal [4], or even mechanically induced reactions [5]. High pressure has also been used as an experimental procedure to intercalate amine molecules in layered titanates [6]. The series of ion-exchangers alkaline titanates can be represented by the general formula A2TinO2n⫹1, where A is an alkaline metal and n can assume values in the range 1 ⱕ n ⱕ 8, where any specific value is defined by the respective structure of the exchanger. Thus, features associated with a characteristic layered structure relate to n values in the 2 ⱕ n ⱕ 4 interval, while for n ⱖ 6, the exchanger manifested as tunnel or fiber structures. From the point of view of applications, the fibrous compound is becoming important as a substitute for hazardous asbestos material [7,8]. The crystalline layered tetratitanates are composed of four octahedral TiO6 units, where edges are shared at one level that combine with similar units above and below, to form zigzag strings of octahedra [9,10]. The stability of this polyanion in acidic solutions is restricted by the facility in retaining protons to transform into protonic oxides [11–13]. However, the resulting compound maintains the original layered structure, which is similar to that of the alkaline exchangers. Considerable attention has been devoted to such kinds of compounds due to the prominent abilities of intercalated species, in acting as ion exchanger, displaying photochemical and semiconductor properties as well as with catalytic activity [9,14 –17]. The acidic layered tetratitanate, also called layered hydrous titanium dioxide, exhibits distinct intercalation behavior towards various cations and some organic polar molecules [8,14,18]. This material may be useful not only to remove species from a given solution, but also to immobilize radioactive nuclides such as 137Cs and 90Sr from high-level concentrations of liquid waste [19]. Based on the high utility of such crystalline compounds, a significant number of investigations have been devoted to ion-exchange reactions, with the great majority focused on structural features of these exchangers, mainly exploring their excellent chemical and thermal stabilities and their facility in developing favorable ion-exchange properties [7,14, 18 –21]. The intercalation of organoammonium cations or neutral n-alkylamines into layered solids normally causes an increase in the interlayer distance, which is necessary to accommodate the intercalated species [6]. However, up to now, the intercalation of the n-alkyldiamines into layered hydrous titanium dioxide has not been reported. An interesting feature associated with the intercalation of organoammonium cations is the use of an intermediate, as observed for other similar inorganic crystalline compounds, for some preparations of pillared tetratitanate [22–25], and also for synthesis of inorganic/ organic complexes [26 –28]. The purpose of this publication is to report the synthesis of intercalated compounds involving n-alkyldiamines, H2N(CH2)nNH2 (n ⫽ 2, 3, 4, 6, 7, or 8),
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with hydrous acidic titanium dioxide, H2Ti4O9䡠nH2O. Special attention is focused on the effects caused by the diamine chain length on the structure and the thermal features of the obtained intercalated matrices.
2. Experimental All chemicals employed were of analytical grade, and were used without further purification. The layered starting material, potassium tetratitanate (K2Ti4O9), was prepared by a solid-state reaction between K2CO3 and TiO2 powders in a 1:3.5 molar ratio. The previous mixture was heated at 1073 K in air for 20 h. It was then ground and heated again at the same temperature for a further 20 h [22]. Layered hydrous titanium dioxide, H2Ti4O9, was obtained by ion-exchange of potassium ions with an acidic solution. In this procedure, 1.0 g of potassium tetratitanate was placed in contact, under stirring, with 20.0 cm⫺3 of 1.0 mol dm⫺3 of hydrochloric acid solution for 12 h at 343 K. The solid material was separated by centrifugation and washed with twice distilled water until the filtrated solution presented a pH in the range 5.0 to 6.0. The final product was dried over a saturated sodium chloride solution under 70% of relative humidity, and the solid was characterized. The intercalation of n-alkyldiamines was conducted by a batch method, which consisted of suspending 100 mg of H2Ti4O9 or K2Ti4O9 in 20.0 cm3 of an aqueous solution of 0.10 mol dm⫺3 of each n-alkyldiamine. The suspensions were sealed in glass tubes and shaken periodically for 30 days at 298 ⫾ 1 K. After this time, the solid was separated by centrifugation, washed with acetone, and dried in air. The original and the intercalated matrices were characterized by the following techniques: X-ray diffractometry by using Cu K␣ radiation in a Shimadzu XD3A diffractometer, in the 2 ⫽ 3 to 50° range and with a scan rate of 7.0 ⫻ 10⫺2 deg䡠s⫺1. The thermogravimetric curves were obtained in a DuPont 1090B instrument, with samples of about 5.0 mg, which were heated under argon atmosphere from 298 to 1223 K, at a heating rate of 0.17 Ks⫺1. Infrared spectra were recorded on a BOMEM apparatus, using KBr discs in the 4000 to 400 cm⫺1 range, with a resolution of 4.0 cm⫺1. CHN elemental analysis was carried out with a Perkin-Elmer PE-2400 analyzer. The scanning electron micrographs were obtained using a JEOL JSM T-300, with an accelerating voltage of 20 kV.
3. Results and discussion The general reaction of intercalation consisted of inserting a series of n-alkyldiamines H2N(CH2)nNH2, named Cn (n ⫽ 2, 3, 4, 6, 7, and 8), where n is the number of carbon atoms in the organic chain, which was accommodated into the free cavity space of the crystalline interlayer layer compounds K2Ti4O9 and H2Ti4O9䡠nH2O. However, the former exchanger did not intercalate any base, indicating that the nature of the cation attached to the matrix in the interlayer space can strongly influence the intercalation properties of this class of lamellar materials. On the other hand, the explanation for this distinct behavior for cationic or acidic
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Table 1 Percentages of carbon (C), nitrogen (N), and hydrogen (H) elemental analysis for the n-alkyldiamines H2N(CH2)nNH2 (n ⫽ 2, 3, 4, 6, 7, or 8) intercalated into hydrous titanate matrices Matrix
C (%)
N (%)
H (%)
Proposed formula
C2 C3 C4 C6 C7 C8
2.13 6.28 3.17 10.50 10.24 13.82
2.17 4.24 1.71 3.79 3.41 3.78
1.45 2.44 1.61 3.28 2.98 4.08
(C2H8N2)0.3H2Ti4O9䡠0.4H2O (C3H10N2)0.6H2Ti4O9䡠0.9H2O (C4H12N2)0.3H2Ti4O9䡠0.4H2O (C6H16N2)0.6H2Ti4O9䡠0.9H2O (C7H18N2)0.5H2Ti4O9䡠0.5H2O (C8H20N2)0.6H2Ti4O9䡠0.6H2O
matrices should be associated with the proposed mechanism of intercalation. In such interactions, it has been demonstrated that a strong dependence on the protonation in a first stage occurs with the entrance of the amine in the interlayer space [29]. Furthermore, it has been shown [30] that the intercalation of alkylammonium chlorides to give alkylammoniumtetratitanate intercalation compounds by using K2Ti4O9 as the host matrices is not possible by the conventional ion exchange method [30]. [2.2.2]-cryptand and 18-crown-6 ether must be employed to allow the intercalation reaction to occur [30]. The elemental analysis results are summarized in Table 1. As can be observed, the matrices with a minor amount of intercalated amines (0.3) are related to the lowest degree of physisorbed water (0.4). This fact suggests that the physisorbed water molecules can be related to an interacting effect of association with the organic molecules. On the contrary, for C3, C6, and C8 matrices, the total amounts of intercalated diamines are identical, and correspond to twice the values obtained for the preceding matrices. X-ray diffraction patterns for the original and all intercalated matrices are shown in Fig. 1. The respective interlayer distances for each compound was collected; these results were based on the d values for the 001 reflection. The original matrices presented the characteristic lamellar distances of 875 and 920 pm for potassium tetratitanate and hydrous titanium dioxide, respectively. For the intercalated hydrous matrices with C2, C3, C4, C6, C7, and C8, the interlayer distance has values of 1118, 1318, 1549, 1662, 1720, and 1834 pm, respectively. Based on carbon, nitrogen, and hydrogen covalent radii (whose values are 77, 70, and 30 pm [31]), the calculated organic diamine chain lengths were 354, 431, 505, 662, 739, and 816 pm for the C2, C3, C4, C6, C7, and C8 n-alkyldiamines, respectively. The plot of d-spacing values as a function of the carbon number of the diamine chain is shown in Fig. 2. As can be observed, C2 and C4 matrices exhibited a different behavior compared with the other matrices, with a deviation from the expected linearity. These results suggested that for both matrices, the spatial orientation of the diamine chains is distinct from those observed for the other matrices. In admitting the proposed straight line, the C2 and C4 matrices exhibited a negative and positive deviation from linearity, respectively. These results implied that in both matrices the organic chains form with the titanate silicate, smaller and larger angles, respectively, when compared with the expected normal orientation for other modified matrices. Probably this arrangement of the intercalated diamines ions depends on the layer charge and the alkyl-chain length. This was demonstrated for smectites, where short-chain alkylammonium ions are composed of a monolayer and long chain cations as a
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Fig. 1. X-ray diffraction patterns of the hydrous tetratinanate matrix (a) and the hydrous dialkylmines, H2N(CH2)nNH2, intercalated matrices C2 (b), C3 (c), C4 (d), C6 (e), C7 (f), and C8 (g).
bilayer with their alkyl chains parallel to the silicate sheets [32]. Although the inorganic backbone of the hydrous acidic titanium dioxide has uncharged layers, the intercalated species are identical in interactive nature. Then, it is accepted that the same behavior occurred with this synthetic host self-organized of highly ordered material. These facts are in agreement with the C2 and C4 guest alkylamines are inserted into the interlayer space like a paraffin-type arrangement [32]. The behavior of the experimental points in Fig. 2 suggests an additional interpretation. From these values, the interlayer distance correlates linearly with the carbon content in the organic chain for C2, C3, and C4 matrices. On the other hand, a possible correlation seems to be observed from C6 to C8 with a smaller angle. This fact could be associated with the orientation of the largest organic chains in the lamellar space.
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Fig. 2. Interlayer distance, d, as function of the number of carbon atoms in the n-dialkylmines, H2N(CH2)nNH2, intercalated into a hydrous titanate matrix.
The intercalation process does not affect the morphology of the materials, as shown by the illustrative examples for SEM micrographs of the hydrous and C6 intercalated samples in Fig. 3a and b, respectively. The obtained infrared data indicated that the basic NHK groups of the n-alkyldiamines are protonated by the available proton bonded on a hydrous acidic host matrix. This same behavior is confirmed for all intercalated matrices, which exhibited a characteristic broad and medium band for the NH3⫹ group in the 1620 to 1630 cm⫺1 range [22]. The infrared spectra of the hydrous titanate and those for C3 and C6 intercalated matrices are shown in Fig. 4. As expected, for all synthesized matrices, the CH2 vibration bands remained unchanged after intercalating the guest molecules. The thermogravimetric curves for the hydrous acidic titanium dioxide and n-alkyldiamine intercalated matrices are shown in Figs. 5 and 6, respectively. The host matrix exhibited two distinct mass loss steps, the first being due to the release of physisorbed water molecules, followed by another one, which corresponded to the loss of lattice water molecules. The water content in the proposed formula presented in Table 1 was calculated by using the mass loss percentage of the last step. These last molecules came as a consequence of the condensation of the free hydroxyl groups on the matrix, as has been observed for other surfaces [33]. For the intercalated n-dialkylamine samples, a third mass loss step, located between the two previously described ones, near 500 –750 K, is observed. The new step for all intercalated matrices was attributed to the release of the organic moiety. By using the correspondent
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Fig. 3. SEM photographs for (a) the hydrous lamellar titanate matrix and (b) the C6 intercalated matrix. Scale bar is in micrometers.
percentage of this mass loss, the total amount of organic molecules inserted into the vacancy space of the matrices can be calculated. These obtained values are in very good agreement with the elemental analysis results, with a deviation of ⫾3%. These facts confirm the validity of thermogravimetry as a reliable analytical tool to study inorganic or inorganic– organic hybrid matrices [33,34]. As observed, the organic-modified matrices released lattice water molecules at about 900 K, after the loss of the organic moiety. The total amount of lattice water released is practically the same for the original and the intercalated amine matrices, suggesting that the unprotonated n-dialkylamine left the interlayer spaces on heating. This is a reasonable proposal due to the easier disruption on breaking the hydrogen bonding between the NH2 group of the diamine and the hydroxyl group of the matrix, formed in the intercalation process. By considering the release of organic moiety to be the most important factor related to the behavior of the host matrices on heating, an ordered sequence of stability can be established: C4 ⬎ C2 ⬎ C3 ⬵ C6 ⬎ C7 ⬵ C8. This order correlated with the amount of intercalated
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Fig. 4. Infrared spectra for the hydrous lamellar titanate (a) and the intercalated matrices of C3 (b) and C6 (c).
Fig. 5. Thermogravimetric and derivative curves for the hydrous tetratinanate lamellar matrix.
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Fig. 6. Thermogravimetric and derivative curves for the n-dialkylamines H2N(CH2)nNH2 intercalated matrices: (a) C2, (b) C3, (c) C4, (d) C6, (e) C7, and (f) C8.
organic moiety with the stability of the synthesized hybrid. The lowest degree of insertion provided larger stability in the respective matrix, which is, in fact, associated with the arrangement of the organic chain length into the free cavity space of the host inorganic layer.
4. Conclusions Based on the series of experimental results involving the crystalline compounds K2Ti4O9 and H2Ti4O9䡠nH2O, the obtained data established that the protonation of the guest molecule is a prominent factor to explain the intercalation process in the host matrix. The orientation
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of the intercalated molecules, as well as the thermal stability of the hybrid matrices, is dependent on the total amount of the molecules intercalated into the lamellae of the host. This behavior can be visualized by the variation in the interlayer distance as the number of carbons of the intercalated n-dialkylamine is increased. The largest thermal stability of C2 and C4 matrices not only is associated with the lowest amount intercalated, but is also due to the paraffin-like arrangement of the organic chains.
References [1] C.N.R. Rao, J. Gopalakrishnan, New Directions in Solid State Chemistry, 2nd ed., Cambridge University Press, Cambridge, 1997, p. 129. [2] M. Andrianainarivelo, R.J.P. Corriu, D. Leclerq, P.H. Mutin, A. Vioux, J Mater Chem 7 (1997) 279. [3] E.L. Sham, M.A.G. Aranda, E.M. Farfan-Torres, J.C. Gottifredi, M. Martı´nez-Lara, S. Bruque, J Solid State Chem 139 (1998) 225. [4] I.J. Clark, T. Takeuchi, N. Ohtori, D.C. Sinclair, J Mater Chem 9 (1999) 83. [5] N.J. Welham, J Mater Res 13 (1998) 1607. [6] S. Nakano, T. Sasaki, K. Takemura, M. Watanabe, Chem Mater 10 (1998) 2044. [7] H. Izawa, S. Kikkawa, M. Koizumi, J Solid State Chem 69 (1987) 336. [8] H. Izawa, S. Kikkawa, M. Koizumi, Polyhedron 2 (1983) 741. [9] J.H. Choy, Y.S. Han, N.G. Park, H. Kim, S.W. Kim, Synth Met 71 (1995) 2053. [10] T.P. Feist, S.J. Mocarski, P.K. Davies, A.J. Jacobson, J.T. Lewandowski, Solid State Ionics 28 –30 (1988) 1338. [11] T. Sasaki, F. Izumi, M. Watanabe, Chem Mater 8 (1996) 777. [12] S. Cheng, T.C. Wang, Inorg Chem 28 (1989) 1283. [13] H. Miyata, Y. Sugahara, K. Kuroda, C. Kato, J Chem Soc Faraday Trans 84 (1988) 2677. [14] T. Sasaki, Y. Komatsu, Y. Fujiki, Mater Res Bull 22 (1987) 1321. [15] T. Nakato, Y. Iwata, K. Kuroda, M. Kaneko, C. Kato, J Chem Soc Dalton Trans (1993) 1405. [16] Y. Kim, S.J. Atherton, E.S. Brigham, T.E. Mallouk, J Phys Chem 97 (1993) 11802. [17] T. Sasaki, S. Nakano, S. Yamauchi, M. Watanabe, Chem Mater 9 (1997) 602. [18] T. Sasaki, M. Watanabe, Y. Komatsu, Y. Fujiki, Bull Chem Soc Jpn 58 (1985) 3500. [19] T. Sasaki, Y. Komatsu, Y. Fujiki, Inorg Chem 28 (1989) 2776. [20] H. Izawa, S. Kikkawa, M. Koizumi, J Solid State Chem 60 (1985) 264. [21] W.A. England, J.E. Birkett, J.B. Goodenough, P.J. Wiseman, J Solid State Chem 49 (1983) 300. [22] S. Roca, C. Airoldi, J Mater Chem 6 (1996) 1963. [23] M. Ogawa, Y. Takizawa, Chem Mater 11 (1999) 30. [24] P. Sylvester, R. Cahill, A. Clearfield, Chem Mater 6 (1994) 1890. [25] M.W. Anderson, J. Klinowski, Inorg Chem 29 (1990) 3260. [26] T. Nakato, D. Sakamoto, K. Kuroda, M. Kaneko, C. Kato, Bull Chem Soc Jpn 65 (1992) 332. [27] T. Nakato, K. Kusunoki, K. Yoshizawa, K. Kuroda, M. Kaneko, J Phys Chem 99 (1995) 17896. [28] C.V. Kumar, Z.J. Williams, J Phys Chem 99 (1995) 17632. [29] S. Roca, C. Airoldi, J Chem Soc Dalton Trans (1997) 2517. [30] M. Ogawa, Y. Takizawa, Chem Mater 11 (1999) 30. [31] F.A. Cotton, G. Wilkinson, P.L. Gauss, Basic Inorganic Chemistry, John Wiley & Sons, New York, 1995. [32] M. Ogawa, K. Kuroda, Bull Chem Soc Jpn 70 (1997) 2593. [33] L.M. Nunes, C. Airoldi, R.F. de Farias, J Solid State Chem 145 (1999) 649. [34] R.F. de Farias, C. Airoldi, J Thermal Anal 53 (1998) 751.