Intercalation of Macrocyclic Crown Ether into Well-Crystallized LDH

Publication Date (Web): July 7, 2009 ... vertically-orientated CSAE ions as the secondary host showed high adsorptivity towards transition metal ions,...
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3602 Chem. Mater. 2009, 21, 3602–3610 DOI:10.1021/cm9007393

Intercalation of Macrocyclic Crown Ether into Well-Crystallized LDH: Formation of Staging Structure and Secondary Host-Guest Reaction Shulan Ma, † Cuihong Fan, † Li Du, † Gailing Huang, † Xiaojing Yang,*,† Weiping Tang,‡ Yoji Makita,§ and Kenta Ooi§ †

College of Chemistry, Beijing Normal University, Beijing 100875, China, ‡Research Institute for Solvothermal Technology, 2217-43 Hayashi-cho, Takamatsu 761-0301, Japan, and §National Institute of Advanced Industrial Science and Technology, 2217-14 Hayashi, Takamatsu 761-0395, Japan Received March 17, 2009. Revised Manuscript Received May 5, 2009

A carboxyethyl substituted azacrown ether derivative (CSAE) was intercalated as a second host into a parent host of well-crystallized crystal of Mg-Al layered double hydroxide (MgAl-LDH) by a CSAE/NO3- ion-exchange reaction. The influence of intercalation temperature on the structures and compositions of CSAE-LDH nanocomposites was investigated. The composites obtained at the temperatures below 70 °C had almost the same CASE contents and layered structures with a basal spacing of about 1.6 nm, corresponding to the vertical orientation of CSAE plane to the LDH layer. The chemical analysis showed that a considerable amount of CO32- (with CO32-/CSAE molar ratio of 1.4) was incorporated in the interlayer of LDH. The CSAE content decreased while CO32- content increased with an increase of the intercalation temperature in the region above 70 °C. At 100 °C, a second staging phase of 2.33 nm appeared, attributed to the ordered stacking of the 1.6 nm phase and a 0.77 nm phase produced by the CO32-/CSAE exchange. At higher temperatures, a new phase with a basal spacing of 1.18 nm appeared, which corresponds to the tilt/twisted orientation of CSAE anions in the interlayer. The other second staging phase of 2.08 nm appeared obviously at 150 °C, due to the regular stacking of the 1.18 and 0.77 nm phases. The adsorptive properties for transition metal ions were studied using the 70 and 150 °C reacted composites. The 70 °C reacted one showed higher adsorptivity toward transition metal ions; the adsorptive capacity increased in the sequence of Cu2þ > Ni2þ > Co2þ > Zn2þ, and distribution coefficient for Cu2þ was markedly higher than those for the other ions. However, the 150 °C reacted one showed little adsorptivity toward these ions. The adsorption for transition metal ions was accompanied by the intercalation of nearly equivalent amount of nitrate ions. This shows that the interlayer CSAE ions in the 1.6 nm phase act as a second host, but those in the 1.18 nm phase do not. Introduction From the 1980s, much attention has been paid to a new family of organic/inorganic composite material.1 Macrocyclic compounds, an important class of organic molecules, have been intensively studied in research involving the host-guest chemistry because of their significant applications in structural assembly2 and molecule/ion recognition,3 etc. The intercalation of the macrocyclic compounds into the interlayer of inorganic compounds can give rise to a new *Corresponding author. Fax: 86-10-5880-2075. Tel.: 86-10-5880-2960. E-mail: [email protected].

(1) (a) Pinnavia, T. J. Science 1983, 220, 365. (b) Kijima, T.; Matsui, Y. Nature 1986, 322, 533. (c) Kijima, T.; Tanaka, J.; Goto, M.; Matsui, Y. Nature 1985, 316, 280. (d) Kijima, T.; Tanaka, J.; Goto, M.; Matsui, Y. Nature 1984, 310, 45. (e) Kijima, T. J. Chem. Soc., Dalton, Trans. 1990, 425. (2) (a) Izatt, R. M.; Christensen, J. J. Synthesis of Macrocycles: The Design of Selective Complexing Agents; Wiley-Interscience: New :: York, 1987; Vol. 3. (b) Vogtle, F.; Weber, E. Host Guest Complex Chemistry Macrocycles; Springer-Verlag: Berlin, 1985. (3) (a) Lamb, J. D.; Izatt, R. M.; Christensen, J. J. Progress in Macrocyclic Chemistry; John Wiley & Sons: New York, 1981; Vol. 2. (b) Lehn, J.-M. Science 1985, 227, 849. (c) Lindoy, L. F. The Chemistry of Macrocyclic Ligand Complexes; Cambridge University Press: Cambridge, U.K., 1989.

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solid phase. Because the inserted macrocycles may serve as secondary hosts to recognize guest species, the new solid phase may be used in adsorption separation and heterogeneous catalysis. Macrocyclic compounds mainly include calixarenes, cyclodextrines, crown ethers, etc. Most of the studies are involved in intercalations of the macrocycles, mainly cyclodextrins, into inorganic layered parent hosts with negative charge in the layer, such as montmorillonites and zirconium phosphates.4 Crown ethers, containing oxygen and/or nitrogen atoms with smaller cavities available for ionic fixing or sieving,5 are excellent choices for the separation of particular metal ions.6 The intercalates of crown ethers into layered compounds might impose more rigid conformation for interlayer macrocycles, thus maybe leading to (4) (a) Kijima, T.; Tanaka, J.; Goto, M.; Matsui, Y. Nature 1984, 310, 45. (b) Kijima, T.; Kobayashi, M.; Matsui, Y. J. Inclusion Phenom. 1984, 2, 807. (5) Synthetic Multidentate Macrocyclic Compounds Izatt, R. M., Christensen, J. J., Eds.; Academic Press: New York, 1978. (6) (a) Christensen, J. J.; Hill, J. O.; Izatt, R. M. Science 1971, 174, 459. (b) Aranda, P.; Casal, B.; Fripiat, J. J.; Ruiz-Hitzky, E. Langmuir 1994, 10, 1207.

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more selective ion binding. However, there has been a lack of studies focusing on the intercalation. Among the few reported cases, one method for crown ethers to insert into the interlayers of phyllosilicates,7 chalcogenides (e.g., MoS2) and phosphorus trichalcogenides (e.g., CdPS3),8 is via the complexing reaction of electroneutral crown ethers with alkaline ions that already exist or are preintercalated in the interlayer. Another method is to directly intercalate positively charged azacrowns into the negatively charged layers such as sulfides (TiS2, TaS2), usually under drastic experimental condition (e.g., 200 °C).9 The two processes are dependent on the negative charge of the layers, and in the resultant composites, either the molecular cavities of crown ethers are fully occupied by metal cations7,8 or the molecular planes of crown ethers are parallel to the inorganic layers.9 In both cases, the insertion of guests are blocked, and thus the intercalated crown ethers could not serve as secondary hosts to recognize guests such as metal ions. A precipitation (covalent reaction) method was used to synthesize azacrowns-zirconium phosphate compositse,10 for which the azacrowns were prereacted with H3PO3 forming macrocycle derivatized phosphonic acids and then reacted with zirconium. The obtained composites exhibited remarkable separation efficiency toward transition metal ions. This may stem from the vertical orientation of the crown ethers in the interlayer that favor the import of metal ions. Layered double hydroxides (LDHs) are an important type of layered compounds containing positively charged metal hydroxide layers as well as charge-balancing anions in the interlayers. They can undergo facile anion exchange to introduce a vast of organic species, achieving organic/ inorganic composites as new functional materials.11 In developing the organic/inorganic layered functional materials, it is essential to control the orientation of the incorporated organic anions in the interlayer space.12 In this work, we select an MgAl-LDH as the parent host layered material to intercalate, via a soft-chemical ion exchange reaction, a negatively charged carboxyethyl-substituted azacrown ether derivative (CSAE anion), in order to have nanocomposites with vertically orientated crown ethers in the interlayer. The ionexchange behaviors are investigated at different temperatures. The results show that the intercalated CSAE ions have distinct orientations in the interlayer depending on the intercalation temperature, and the composites obtained at relatively high intercalation temperatures exhibit staging phenomenon. We also investigate their adsorptive properties for metal ions and discuss the ion (7) (a) Herber, R. H.; Cassell, R. A. J. Chem. Phys. 1981, 75, 4669. (b) Herber, R. H.; Cassell, R. A. Inorg. Chem. 1982, 21, 3713. (8) (a) Lara, N.; Ruiz-Hitzky, E. J. Braz. Chem. Soc. 1996, 7, 193. (b) Glueck, D. S.; Brough, A. R.; Mountford, P.; Green, M. L. H. Inorg. Chem. 1993, 32, 1893. (9) Villanueva, A.; Ruiz-Hitzky, E. J. Mater. Chem. 2004, 14, 824. (10) (a) Alberti, G.; Boccali, L.; Dionigi, C.; Kalchenko, V. I.; Vivani, R.; Atamas, L. I. Supramol. Chem. 1996, 7, 129. (b) Zhang, B. L.; Clearfield, A. J. Am. Chem. Soc. 1997, 119, 2751. (11) Ogawa, M.; Kuroda, K. Chem. Rev. 1995, 95, 399. (12) Iyi, N.; Kurashima, K.; Fujita, T. Chem. Mater. 2002, 14, 583.

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Figure 1. XRD patterns of starting samples (a) MgAl-CO32--LDH and (b) MgAl-NO3--LDH, and those of resultant samples reacted with CSAE at 70 °C using (c) MgAl-CO32--LDH and (d) MgAl-NO3-LDH. The d-value in nanometers.

selectivity in terms of the arrangement of the CSAE ions in the interlayer. Results and Discussion Precursors and Their Ion-Exchanges with CSAE. Figure 1 shows the powder X-ray diffraction (XRD) patterns of the CO32-- and NO3--type LDH samples, both of which can be indexed to a hexagonal symmetry, with lattice parameters a=0.30353(3) and c=2.2585(2) nm for CO32--type LDH, and a = 0.30367(10) nm and c=2.6659(9) nm for NO3--type LDH. The lattice parameter a, the average distance between two adjacent M ions (M = Mg or Al), is almost the same, indicating that the ion-exchange does not induce a remarkable change in the LDH layer. The sharp and symmetric features of the diffraction peaks indicate that the sample exists as a single phase and the high crystallinity is well retained during the decarbonation process. The precursors have the basal spacings (dbasal) of 0.76 and 0.89 nm, which are identical to those reported for typical CO32--type13,14 and NO3--type15 LDHs, respectively. (13) Okamoto, K.; Iyi, N.; Sasaki, T. Appl. Clay Sci. 2007, 37, 23. (14) (a) Xu, Z. P.; Zeng, H. C. J. Phys. Chem. B 2001, 105, 1743. (b) Costantino, U.; Marmottini, F.; Nocchetti, M.; Vivani, R. Eur. J. Inorg. Chem. 1998, 1439. (c) Bellotto, M.; Rebours, B.; Clause, O.; Lynch, J.; Bazin, D.; Elkaim, E. J. Phys. Chem. 1996, 100, 8535. (d) Yun, S. K.; Pinnavaia, T. J. Chem. Mater. 1995, 7, 348. (e) Allmann, R. Chimia 1970, 24, 99. (15) (a) del Arco, M.; Gutierrez, S.; Martin, C.; Rives, V.; Rocha, J. J. Solid State Chem. 2000, 151, 272. (b) Gago, S.; Pillinger, M.; Valente, A. A.; Santos, T. M.; Rocha, J.; Goncalves, I. S. Inorg. Chem. 2004, 43, 5422.

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Fourier transformed infrared (FT-IR) spectra confirm this result. The band at 1354 cm-1 (Figure 2a) assigned to the absorption band of CO32- disappeared, and a strong band at 1384 cm-1 (Figure 2b) corresponding to the band of NO3- appeared after treating the sample with a NaNO3 þ HNO3 solution. The scanning electron microscope (SEM) observations (Figure 3a) indicate that the obtained NO3--LDH has a hexagonal prism shape, retaining the morphology of the CO32--LDH precursor. The thermogravimetric and differential thermal analysis (TG-DTA) curves for the NO3--LDH precursor (see the Supporting Information, Figure S1) are very similar to those of the literature.16 A weak endothermic peak at 167 °C corresponds to the water loss, and two strong peaks at 283 and 392 °C show the removal of the interlayer NO3- anions and the dehydroxylation of the layers.

The compositions of the CO32--LDH and NO3--LDH were determined to be Mg0.53Al0.42(OH)2(CO3)0.15 3 0.56H2O and Mg0.56Al0.39(OH)2(NO3)0.29 3 0.37H2O, respectively, on the basis of inductively coupled plasma (ICP) and CHN elemental analysis data, as shown in Table 1. The CO32-LDH has a Mg/Al ratio of 1.3, lower than that of 2.0 prepared in the literature,13 which may be caused by the present lower reacting temperature of 100 °C than that (140 °C) used in the literature.13 A systemic study on the structures of CO32--LDH products obtained at lower temperatures and shorter reacting time would be reported elsewhere. The NO3-/CO32- exchange rarely influences the Mg/Al ratio of LDH. Samples CO32--LDH and NO3--LDH were treated with a CSAE aqueous solution (pH was adjusted to 9.0) at 70 °C. As shown in patterns c and d in Figure 1, the sample from CO32--LDH did not show a dbasal change, whereas that from NO3--LDH had a larger dbasal of 1.61 nm. CSAE was not detected by FT-IR for the former, but did for the latter (Figure 2d, to be discussed below). The CSAE molecules can be exchanged with NO3-, but not with CO32-. The difference of the exchange ability can be explained by the difference of the affinity of anions to the exchange site of the interlayer, which has a decreasing order of CO32- > SO42- > OH> F- > Cl- > Br- > NO3-.17 The affinity of CO32- is too strong to be exchanged with CSAE ions. Note that CSAE can also be exchanged with Cl- in the interlayer of Cl--LDH, which is obtained with a (1.5 M NaCl þ 5 mM HCl) solution by a method similar to that used to prepare NO3--LDH. Structures of CSAE/LDH Composites Prepared at Low Intercalation Temperatures. As shown in Figure 1d, the pattern of the 70 °C-reacted sample, labeled as CSAE/ LDH-70 (the number represents the intercalation temperature), contains a set of basal reflections with d values of 1.61, 0.80, 0.53, 0.40, 0.32, and 0.26 nm that correspond to a minimum periodicity along c axis equal to 1.61 nm. The high intensities of (00l) reflections relative to the 2θ peaks > 35° indicate the high orientation along [00l] direction. The reflections could be indexed to a hexagonal symmetry with lattice parameters of a = 0.3048(3) nm and c = 4.777(3) nm, except for two relatively weak peaks at d = 0.76 and 0.38 nm, respectively assigned to (003) and (006) planes of CO32--type LDH (comparing with Figure 1a) and several small peaks at d= 0.49, 0.47, and 0.44 nm. These small peaks may suggest the presence of a small amount of Al(OH)3 polymorphs, which resulted from the ion-exchange process because in the precursors they are hardly observable. A transmission electron microscope (TEM) observation on the cross section (see the Supporting Information, Figure S2) indicates a lattice image of layered structure with spacing of 1.6 nm, which agrees well with the dbasal in the XRD analysis. The SEM observation (Figure 3b) revealed that the crystals of the obtained composite had the same prism shape as that of the precursor (Figure 3a), but their

(16) Zhao, H.; Vance, G. F. J. Chem. Soc., Dalton Trans. 1997, 1961.

(17) Miyata, S. Clays Clay Miner. 1983, 31, 305.

Figure 2. FI-IR spectra of (a) MgAl-CO32--LDH, (b) MgAl-NO3-LDH, (c) HCl-form CSAE, and (d) CSAE/LDH-70.

Figure 3. SEM images of (a) MgAl-NO3--LDH and (b) CSAE/LDH70.

15.79 (15.64) 16.94 (16.62) 10.95 (10.51) 6.46 6.06

14.56 (14.56) 13.26 (12.81) 9.39 (9.30) 9.10 (8.93) 10.83 (10.39) 10.61 9.74 0.76 0.89 1.6 þ (0.76)a 1.6 þ (0.76)a 2.33 þ 1.6 þ 0.77 (2.08)a þ 1.18 þ 0.77 2.08 þ 1.18 þ 0.77 CO32--LDH NO3--LDH CSAE/LDH-RT CSAE/LDH-70 CSAE/LDH-100 CSAE/LDH-120b CSAE/LDH-150b

Mg0.53Al0.41(OH)2(CO3)0.15 3 0.55H2O Mg0.56Al0.39(OH)2(NO3)0.29 3 0.37H2O Mg0.57Al0.41(OH)2(CSAE)0.06(CO3)0.07 3 1.29H2O Mg0.61Al0.42(OH)2(CSAE)0.07(CO3)0.10 3 1.27H2O Mg0.64Al0.43(OH)2(CSAE)0.03(CO3)0.22 3 1.17H2O

16.51 (16.51) 16.75 (16.19) 11.66 (11.40) 12.00 (11.63) 14.29 (13.54) 15.68 16.60

2.37 (2.37)

4.06 (4.03) 3.42 (3.31) 5.95 (5.89) 5.89 (5.78) 5.19 (4.98) 4.82 3.97

N H C

content, found (calcd) (%)

Al Mg chemical formula (CSAE = C24H40N4O10) dbasal (nm) samples

Table 1. Chemical Composition for Precursors and CSAE/LDH Composites at Different Ion-Exchange Temperatures

a Minor phase in the parentheses. b After water-washing and centrifugation, some gel substance was found in the liquid, but Mg and Al contents in the filtered supernatant solutions were not detected by ICP analysis. The increased Mg/Al ratio may be due to the formation of Al-containing substance, whose particles are so small as to penetrate the filter paper during filtration after water-washing the solid.

Chem. Mater., Vol. 21, No. 15, 2009 5.11 (4.91) 2.94 (2.91) 3.11 (3.05) 1.65 (1.58) 0.18 0.16

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Figure 4. XRD patterns of CSAE/LDH composites obtained at different temperatures: (a) RT and (b) 40, (c, c0 ) 70, (d, d0 ) 100, (e, e0 ) 120, and (f, f0 ) 150 °C. The d-value is in nanometers.

platelets have round and coarse contours, suggesting some surface corrosion may take place in the exchanging process. The XRD patterns of CSAE/LDH composites obtained at room temperature (RT) (Figure 4a) and 40 °C (Figure 4b) are almost the same as that of CSAE/LDH-70 (Figures 1d and 4c), except for small changes of the basal spacing and the relative intensity of the peaks concerning the CO32--LDH. The Mg/Al ratios of these three composites are nearly equal to that of the precursor. All these results indicate that the samples treated at low temperatures have similar structures. Figure 5 is a schematic presentation of the proposed arrangement of CSAE anion in the interlayer of LDH. Because the thickness of the LDH layer is 0.48 nm,18 the gallery height can be calculated as 1.13 (=1.61-0.48) nm. On the basis of the crystallographic data of HCl-form CSAE,19 its larger expanding height, with two adjacent nitrogen atoms on the upper side and other two on the lower side, is calculated to be about 1.1 nm (Figure 5), and the lateral thickness is about 0.7 nm. The former is close to a van der Waals diameter of 1.05 nm, whereas the latter is larger than a flat thickness of 0.45 nm for diazacrown ether,20 which has a similar structure to CSAE but with different nitrogen number and no side-arms, calculated from a Corey-Pauling-Koltun (CPK) model.20 Considering the gallery height of 1.13 nm calculated from the XRD data in the present case, the expanding plane of CSAE may arrange in a vertical orientation to the LDH

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Figure 5. Schematic structure and probable arrangement of CSAE in the interlayer reacting below 70 °C.

layer, differing from the usually observed pseudoplanar arrangement of the azacrowns, which gives an interlayer height of about 0.4 nm.9 A peculiar feature of the crown ether compounds is their high conformational flexibility.19-21 Because of the flexibility of CSAE, the bilayer horizontal/flat arrangement with two parallel flat CSAE molecules in the interlayer can be assumed as another possible model.20 However, since the whole thickness of CSAE caused by its four carboxyl side-arms is ∼0.7 nm, the expected gallery for bilayer flat arrangement of CSAE is ∼1.4 nm, being much larger than the observed value of 1.13 nm. So it is reasonable to consider that the interlayer CSAE is in a monolayer22 and vertical arrangement, rather than a mono- or bilayer flat arrangement. Two of the four carboxyl groups of CSAE may be respectively anchored to each wall of the LDH, thus favoring the vertical orientation. The vertical model can be further explained based on the area-charge matching. The area per unit charge (Scharge) of NO3--LDH calculated from the crystal parameter a of the LDH is (1/0.29)a2sin 60°=0.28 nm2, where 0.29, the charge of the layer in unit cell, can be calculated as 0.56(Mg content)  2 þ 0.39(Al content)  3 - 2(OH content) based on its composition (Table 1). At the same time, the Scharge of CSAE, calculated from the size of the projective plane of CSAE (the horizontal width is ∼0.9 nm and the lateral thickness is ∼0.7 nm),19 is 0.9  0.7/4 = 0.16 nm2. The latter is smaller than the former, so a monolayer arrangement should be formed,23 and there is free space in the interlayer gallery, allowing the entrance of carbonate anion and water molecules. The chemical analysis data (Table 1) show that in the interlayer of the composites CO32- rather than NO3exists, which is also testified by the adsorption band at 1354 cm-1 in the FT-IR spectrum (Figure 2d). The two (18) Miyata, S. Clays Clay Miner. 1975, 23, 369. (19) Xu, M. Q. Master Dissertation. Beijing Normal University, Beijing, China, 2004. (20) Kijima, T.; Sakoh, K.; Machida, M. J. Chem. Soc., Dalton Trans. 1996, 1245. (21) Akutagawa, T.; Sato, D.; Koshinaka, H.; Aonuma, M.; Noro, S.; Takeda, S.; Nakamura, T. Inorg. Chem. 2008, 47, 5951. (22) (a) Fujita, W.; Awaga, K. J. Am. Chem. Soc. 1997, 119, 4563. (b) Takagi, K.; Shichi, T.; Usami, H.; Sawaki, Y. J. Am. Chem. Soc. 1993, 115, 4339. (23) (a) Iyi, N.; Kurashima, K.; Fujita, T. Chem. Mater. 2002, 14, 583. (b) Costantino, U.; Coletti, N.; Nocchetti, M.; Aloisi, G. G.; Elisei, F. Langmuir 1999, 15, 4454.

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small peaks at d=0.76 and 0.38 nm in the XRD patterns (Figure 4a-c) indicate that there exists a separate CO32--LDH phase. However, the peak intensities of the phase are too weak to account for the large amount of CO32- in the composite (the CO32-/CSAE ratio of CSAE/LDH-70 is 1.42). So the CO32- may be also in the 1.6 nm phase with CSAE ions, as shown in Figure 6. The cointercalation of carbonate species into LDH with CSAE ions may be due to its favorable lattice stabilization enthalpy associated with the small size and high charge.24 There have been few studies on LDHs with mixed ion-exchange forms, compared with the fully exchanged products. The coexistence of two phases may result in a mixed structure, with a random25 or ordered stacking26 of two types interlayers. In the present case, the 1.6 nm and 0.76 nm layers are stacked separately or randomly. In the IR spectrum of CSAE/LDH-70 (Figure 2d), two bands at 1736 and 1412 cm-1 attributed to vas and vs vibrations of the carboxyl group (-COOH) are shifted to lower wavenumbers (1556 and 1403 cm-1), indicating that the carboxyl groups are in a deprotonated form (-COOH) after the intercalation. The band at 1130 cm-1 assigned to the C-O-C ring vibrations of CSAE in the HCl-form CSAE (Figure 2c), is shifted to 1120 cm-1, may be due to the formation of H-bonds between the ring oxygen atoms and hydroxyl groups of the LDH layers and/or interlayer waters. The band observed around 2900 cm-1 is assigned to the stretching vibrations of -CH2 groups of CSAE, and those at 685 and 446 cm-1, to the v(M-O) and δ(O-M-O) vibrations,27 respectively. These results exhibit the interactions between CSAE and the LDH layer due to the intercalation of CSAE. Staging Structure of Composites Formed at High Intercalation Temperatures. The phenomenon of interstratification with the ordered stacking is referred to as staging, commonly observed in the graphite system.26 With regard to staging, there have been two proposed models, :: Rudorff28 and Daumas-Herold29 models. The relatively high rigidity of the LDH layers makes it unlikely to adopt the Daumas-Herold model, for which the layers should bend in order to localize region of guest occupancy.30 The XRD patterns for the composites obtained at temperatures between 100 and 150 °C are shown in Figure 4d-f. In the XRD patterns of CSAE/LDH-100, the diffraction peaks corresponding to the 0.77 nm phase (24) Oriakhi, C. O.; Farr, I. V.; Lerner, M. M. J. Mater. Chem. 1996, 6, 103. (25) Reynolds, R. C. In Crystal Structures of Clay Minerals & Their Xray Identification; Brindley, G. W., Brown, G., Eds.; Mineralogical Society: London, 1980; pp 249-303. (26) Bartlett, N.; McQuillan, B. W. In Intercalation Chemistry; Whittingham, M. S., Jacobson, A. J., Eds.; Academic Press: New York, 1982; pp 19-53. (27) (a) Liu, Z.; Ma, R.; Osada, M.; Iyi, N.; Ebina, Y.; Takada, K.; Sasaki, T. J. Am. Chem. Soc. 2006, 128, 4872. (b) Adachi-Pagano, M.; :: Forano, C.; Besse, J.-P. J. Mater. Chem. 2003, 13, 1988. (28) Rudorff, W. Z. Phys. Chem. 1940, 45, 42. (29) Daumas, N.; Herold, A. C. R. Acad. Sci., Ser. C 1969, 268, 373. (30) Williams, G. R.; Fogg, A. M.; Sloan, J.; Taviot-Gueho, C.; O’Hare, D. Dalton Trans. 2007, 3499.

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Figure 6. Arrangements of CSAE in the interlayer and formation of staging structure at different temperatures. The numbers are basal spacings in nanometers.

strengthened, whereas those for the 1.6 nm phase weakened. In addition to these peaks, a new peak with d value of 2.33 nm is observed. This d value is close to the sum of 1.6 and 0.77 nm, the basal spacing of the vertical CSAEcontaining phase and that of CO32--LDH phase, respectively. This shows the formation of a second-staging phase with the 1.6 and 0.76 nm phases stacked alternatively. Because the reflections corresponding to the 1.6 and 0.77 nm phases are significantly strong compared with that to the 2.33 nm phase, these three phases may coexist in CSAE/LDH-100. The chemical analysis shows that CSAE content is a half, while CO32- content is the double of those of CSEA/LDH-70 (Table 1), with a CO32-/CSAE molar ratio of 7.3. A further CO32-/ CSAE4- exchange progresses at 100 °C and the increase of CO32- content may be related to the formation of the second staging structure. The Mg/Al ratio is nearly equal to those of nanocomposites prepared below 70 °C. At higher intercalation temperature, the XRD pattern of CSAE/LDH-120 (Figure 4e) shows that a 1.18 nm phase, instead of the 1.6 nm phase, and two weak peaks around d=2.08 and 1.06 nm appear. The two weak peaks increase in intensity and a shoulder of the 0.77 nm peak occurs when reacted at 150 °C (Figure 4f), whereas the 1.18 nm phase still exists. The 1.18 nm phase corresponds to a gallery height of 0.70 nm (= 1.18-0.48); the CSAE ions may have a tilt or twisted orientation in the interlayer as shown in Figure 6, similar to that reported in the literature.20 The 2.08 nm phase can be identified to another second-staging structure, whose (006) and (009) reflections respectively correspond to the peak at 1.06 nm and the shoulder around 0.77 nm. The staging comes from the regular stacking of the 1.18 and 0.77 nm interlayers although the sum is a little smaller than 2.08. The Mg/Al ratio increases from 1.48 to 1.92 when going from CSAE/LDH-100 to CSAE/LDH-150. This latter value corresponds to a stoichiometric LDH phase (Mg/Al = 2.0). These features suggest that small amount of extraction of aluminum hydroxide takes place from the MgAl sheet. The CO32-/CSAE ratios (determined from the N content for CSAE and the residual C content for CO32- according to Table 1) in these two samples were

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increased up to ∼150, showing that a further CO32-/ CSAE4- exchange progresses above 120 °C. Such a small amount of CSAE makes it have a more stable configuration, i.e. tilt or twisted orientation. Because of the particular flexibility of the azacrown ether molecules, tilt or twisted orientation can be formed besides the horizontal orientation, as shown by broad peaks (Figure 4f) on XRD patterns compared with that of d=0.77 nm. An explanation for the high CO32- content is that some CO32- may react with the layer to form compounds “layered basic salt (LBS)”31 in the hydrothermol condition. Since the formation of second staging is related to the increase of CO32- content, it is important to clarify the origin of interlayer CO32-. Because the hydrothermal reaction progresses in a closed system, the acquisition of this large amount of CO32- is difficult from the atmosphere. Thus it is possibly a result of the decomposition of CSAE at high temperature. We can describe the mechanism during the hydrothermal treatment as (1) the intercalation of CSAE, (2) slow decomposition of CSAE in aqueous phase to produce CO32-, and (3) ion exchange of interlayer CSAE ions with CO32-. A further study is needed to clarify the mechanism of CO32- intercalation. There have been a few studies on the staging phenomena for LDH system.12,30,32 Though the mechanism for the formation of second staging have not yet been clear, its appearance may be associated with the composition and orientation of the guests in the interlayer, which vary depending on the layer charge density, exchange conditions, drying extent (vacuum drying33 or thermal treatment34), aging temperature,12 etc. In the present case, CSAE/LDH-100 has a larger amount of CO32than CSAE/LDH-70 (Table 1). The larger CO32- content increases the fraction of the 0.76 nm phase up to the level same as that of the 1.6 nm phase, which may be advantageous to the formation of second staging phase consisting of the 1.6 and 0.76 nm phases. The compositions of CSAE/LDH-120 and CSAE/LDH-150 are close; CSAE/ LDH-150 obviously shows the staging phase, suggesting a higher amount of the phase than in CSAE/LDH-120. This can be explained by the thermal stabilization effect; at higher temperature the ordered stacking of the 1.18 and 0.77 nm interlayers may become more stable than the random style. A further higher-order staging was not detected besides the second-order staging, the same as observed in other cases of LDH system.12,30,34 This also :: shows the Rudorff model is more appropriate for LDH system.30 We tried to prepare a single second-stage phase through increasing the starting CSAE/LDH ratios but were unsuccessful. (31) (a) Meyn, M.; Beneke, K.; Lagaly, G. Inorg. Chem. 1993, 32, 1209. (b) Arizaga, G. G. C.; Satyanarayana, K. G.; Wypych, F. Solid State Ionics 2007, 178, 1143. (32) Kaneyoshi, M.; Jones, W. Chem. Phys. Lett. 1998, 296, 183. :: (33) Pisson, J.; Taviot-Gueho, C.; Israeli, Y.; Leroux, F.; Munsch, P.; Itie, J.-P.; Briois, V.; Morel-Desrosiers, N.; Besse, J.-P. J. Phys. Chem. B 2003, 107, 9243. (34) Kooli, F.; Chisem, I. C.; Vucelic, M.; Jones, W. Chem. Mater. 1996, 8, 1969.

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Table 2. Adsorptive Capacity for Transition Metals and Distribution Coefficient (Kd) for CSAE/LDH-70 and CSAE/LDH-150 (50 mg) adsorptive capacitya (mmol g-1) composites

ions

ion uptakes

CSAE/LDH-70

Co2þ Ni2þ Cu2þ Zn2þ

0.40 0.49 0.77 0.35

CSAE/LDH-150

Co2þ Ni2þ Cu2þ Zn2þ