Why Do Carbonate Anions Have Extremely High Stability in the

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Why Do Carbonate Anions Have Extremely High Stability in the Interlayer Space of Layered Double Hydroxides? Case Study of Layered Double Hydroxide Consisting of Mg and Al (Mg/Al = 2)

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Ryo Sasai,*,† Hiroaki Sato,† Mako Sugata,† Takuya Fujimura,† Shinsuke Ishihara,‡ Kenzo Deguchi,§ Shinobu Ohki,§ Masataka Tansho,§ Tadashi Shimizu,§ Naoto Oita,∥ Mako Numoto,⊥ Yasuhiro Fujii,*,∥,⊥ Shogo Kawaguchi,# Yoshiki Matsuoka,○ Koki Hagura,#,○ Tomohiro Abe,○ and Chikako Moriyoshi*,○ †

Department of Materials Chemistry, Graduate School of Natural Science and Technology, Shimane University, 1060 Nishi-Kawatsu-cho, Matsue, Shimane 690-8504, Japan ‡ World Premier International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan § High Field NMR Group, National Institute for Materials Science (NIMS), 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japan ∥ Department of Physics and Mathematics, College of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252-5258, Japan ⊥ Department of Physical Sciences, College of Science and Engineering, Ritsumeikan University, 1-1-1 Noji-higashi, Kusatsu, Shiga 525-8577, Japan # Japan Synchrotron Radiation Research Institute (JASRI), 1-1-1 Kouto, Sayo-Cho, Sayo-Gun, Hyogo 679-5198, Japan ○ Department of Physical Science, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan S Supporting Information *

ABSTRACT: Layered double hydroxides (LDHs) are promising compounds in a wide range of fields. However, exchange of CO32− anions with other anions is necessary, because the CO32− anions are strongly affixed in the LDH interlayer space. To elucidate the reason for the extremely high stability of CO32− anions intercalated in LDHs, we investigated in detail the chemical states of CO32− anions and hydrated water molecules in the LDH interlayer space by synchrotron radiation X-ray diffraction, solid-state NMR spectroscopy, and Raman spectroscopy. We found the rigidity of the network structure formed between the CO32− anions, hydrated water molecules, and the hydroxyl groups on the metal hydroxide layer surface to be a crucial factor underlying the stability of CO32− anions in the LDH interlayer space.

NO3̅-MgAl(1/3)LDH, which readily exchange various organic and inorganic anionic species, are frequently used as starting materials. However, it is very difficult to synthesize either of them directly, because CO32−-MgAl(1/3)LDH is preferentially obtained as the main product. It is especially difficult to exchange CO32− anions intercalated in the LDH interlayer with other anions under ambient conditions. Thus, various methods for exchanging the CO32− anions with other anions have been reported.34−40 The intercalated CO32− anions in LDHs are known to be extremely stable, which cannot be explained by the electrostatic interactions between CO32− and the positive charges originating from the isomorphous replacement of Mg2+

1. INTRODUCTION Layered double hydroxides (LDHs) are well-known, charged, inorganic layered compounds with exchangeable interlayer anions. In particular, LDHs based on Mg and Al have long shown promise for applications in materials science, catalysis, environmental science, polymer science, medicine, and pharmaceutical science.1−30 Recently, LDH nanosheets, which can be prepared by exfoliation of bulk LDH crystals with formamide, have attracted attention as components of nanostructured functional materials.30−33 One of the typical LDHs has the formula of [Mg2/3Al1/3(OH)2](An−)1/3n·mH2O, where A is an exchangeable anion such as halide, nitrate, sulfate, or carbonate, n is the anion charge, and m is the hydration number. The abbreviated formula may be written as An−-MgAl(1/3)LDH. LDHs such as Cl−-MgAl(1/3)LDH and © XXXX American Chemical Society

Received: May 10, 2019

A

DOI: 10.1021/acs.inorgchem.9b01365 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

analysis of the properties of CO32− and Cl− in both LDHs, structural models with nominal compositions were determined as best-fit models. The Rietveld profile fitting result and structural parameters of CO32−-MgAl(1/3)LDH are shown in Figure S1 and Table S1, respectively. The samples for 17O NMR experiments were prepared by immersing 100 mg of CO32−- or Cl−-MgAl(1/3)LDH in 0.25 mL of 17O-labeled water (90% 17O, RWE NUKEM, Ltd.). The suspensions were stirred for 1 week to promote the exchange of interlayer water. Samples were then dried under reduced pressure at room temperature overnight. High-resolution solid-state NMR experiments were carried out at 67.8 MHz for 17O using a JEOL ECA500 spectrometer (JEOL, Tokyo, Japan). The instrument was equipped with a high-power amplifier for proton decoupling and a cross-polarization/magic angle spinning (CP/MAS) probe. Samples were packed as powders in a ZrO2 rotor (φ = 4 mm). 17O NMR spectra were externally referenced to H2O at 0 ppm. The spinning sideband was identified by altering the MAS frequency from 10 to 15 kHz. For 3QMAS 17O NMR measurements, the spectral range was 80 kHz, and the duration t1 was set at 66.7 μs for rotor synchronizing. The pulse delay was 100 ms. NMR signals were accumulated 48000 times. Confocal micro-Raman measurements were performed using an inhouse microscope and a Jobin-Yvon HR-320 spectrograph (HORIBA Scientific, Kyoto, Japan) equipped with an Andor iDus420 CCD camera (Oxford Instruments, Abingdon, U.K.), for which a singlecrystalline silicon wafer and a 50:50-acetonitrile/toluene mixture were employed as Raman frequency standards.46 An LCX-532S-300 solidstate laser (Oxxius, Lannion, France) was used for excitation at 532 nm. A 20× objective lens was used to focus the light, which was attenuated to 5 mW to prevent local heating of the samples. The polarized scattered light collected by the same lens was analyzed using the spectrograph. The elastic stray component was suppressed using three ultranarrow band holographic notch filters (OptiGrate, Oviedo, FL).47

with Al3+. It is therefore imperative to understand the properties responsible for the extreme stability of CO32− anions incorporated in the LDH interlayer. In this study, we investigated the crystal structure features and the chemical states of hydrated water and carbonate anions in CO32−MgAl(1/3)LDH in detail and elucidated the reason for the exceptional stability of interlayer CO32− anions.

2. EXPERIMENTAL SECTION 2.1. Synthesis of LDH. A 30 mL aqueous solution containing 100 mM Mg(NO3)2·6H2O (Guaranteed Reagent, FUJIFILM Wako Pure Chemical Corporation), 50 mM Al(NO3)3·9H2O (Guaranteed Reagent, FUJIFILM Wako Pure Chemical Corporation), and 175 mM hexamethylenetetramine (Guaranteed Reagent, FUJIFILM Wako Pure Chemical Corporation) was sealed in a pressure vessel lined with fluorocarbon resin. The pressure vessel was subjected to hydrothermal treatment in a dry oven for 24 h at 140 °C. After cooling under ambient conditions, CO32−-MgAl(1/3)LDH was collected by vacuum filtration at room temperature. The CO32−-MgAl(1/3)LDH powder was rinsed with a 1:1 (v/v%) water/ethanol solution and then dried at room temperature under vacuum. Cl−-MgAl(1/3)LDH was prepared from CO32−-MgAl(1/3)LDH powder by a previously reported sodium chloride-acetate buffer decarbonation method.35−37 2.2. Anion-Exchange Experiment. The anion-exchange experiments were performed by implementing the procedures used by Miyata.41 First, 0.1 g of Cl−-MgAl(1/3)LDH or CO32−-MgAl(1/ 3)LDH powder was suspended in 10 mL of an aqueous solution containing both CO32− and Cl− at a total anion concentration of 0.1 mequiv/L. The molar ratio of 2 × [CO32−] and [Cl−] (2 × [CO32−]: [Cl−]) ranged from 0 to 1.0. The suspension was then shaken at 30 °C for 24 h under dried N2 gas to prevent influx of atmospheric CO2. The exchanged solids were collected via filtration, and the residual concentration of Cl− anion in the filtrate was quantified by ion chromatography (IC, Prominence Series, Shimadzu Corporation, Kyoto, Japan). 2.3. Characterization. X-ray diffraction (XRD) analysis of the LDHs was carried out on a MiniFlex II powder X-ray diffractometer (RIGAKU, Tokyo, Japan) equipped with a Ni-filtered Cu Kα radiation source (30 kV, 15 mA; scan rate: 1°/min; sampling step: 0.02°). Infrared spectroscopy of the LDHs was performed using the KBr pellet method on an FT/IR 6100 spectrometer (JASCO, Tokyo, Japan). Determination of the elemental Mg and Al composition of the LDHs was carried out with inductively coupled plasma atomic emission spectroscopy (ICP-AES) on an Optima-2000 ICP (PerkinElmer, Waltham, MA) following dissolution of the LDHs in 0.1 M HNO3 and/or 0.1 M H2SO4. The water content of the LDHs was calculated from the weight lost during thermogravimetric analysis (TGA) on a ThermoPlus TG8120 (RIGAKU, Tokyo, Japan). Quantitative CHN analysis was performed to measure the amount of intercalated CO32− anions in the LDHs using a CHN elemental analyzer (Yanaco, Kyoto, Japan). Quantitation of intercalated chloride anions was performed by IC. As LDH particles are planar, synchrotron-radiation powder XRD (SXRD) with transmission geometry was used to avoid preferred orientation and determine the crystal structures of CO32−- and Cl−MgAl(1/3)LDHs. For these measurements, CO32−- or Cl−-MgAl(1/ 3)LDH powders with homogeneous particle size were sealed in 0.3 mm-inner-diameter borosilicate capillary tubes, and data were recorded using a large Debye−Scherrer camera and an imaging plate42 at the BL02B2 beamline of the SPring-8 synchrotron radiation facility in Hyo̅ go Prefecture, Japan. The wavelength of the synchrotron radiation X-ray was 0.8 Å. The obtained SXRD profiles were analyzed by maximum entropy (MEM) and Rietveld methods.43,44 No superstructure reflections were observed in both patterns, which suggested disordered arrangements of CO32− and Cl− anions. Therefore, the disordered R3m model45 was adopted as an initial space group for determining the fine crystal structures of CO32−- and Cl−-MgAl(1/3)LDHs. The coexistence of CO32− and Cl−-MgAl(1/3)LDH was not observed in Rietveld analysis. After

3. RESULTS AND DISCUSSION 3.1. Characterization of CO32−- and Cl−-MgAl(1/ 3)LDHs. The XRD patterns and FT-IR spectra of the LDHs (Figure S2 and S3) confirmed the successful synthesis of the desired CO32−- and Cl−-MgAl(1/3)LDHs without any crystal impurities. The chemical compositions of the CO32−- and Cl−MgAl(1/3)LDHs were estimated to be [Mg0.67Al0.33(OH)2](CO3)0.17·0.5H2O and [Mg0.67Al0.33(OH)2](CO3)0.01Cl0.31· 0.4H2O, respectively. The content of CO32− in Cl−-MgAl(1/ 3)LDH was assumed to equal half the difference between Al and Cl contents. 3.2. Anion-Exchange Experiments. The anion-exchange isotherms of CO32−-MgAl(1/3)LDH from CO32− to Cl− and Cl−-MgAl(1/3)LDH from Cl− to CO32− shown in Figure 1 were obtained by Miyata’s method.41 In the figure, SAn− on the x-axis represents the equivalent fraction of the anion (An−, either CO32− or Cl−) based on the total anion concentration in an aqueous solution at equilibrium, and (LDH)An− on the ordinate axis represents the equivalent fraction of the respective anion present in the LDH interlayer following ion exchange. The dependence of (LDH)An− on SAn− was monitored; that is, the anion-exchange isotherm curve was employed to assess the progress of the anion-exchange reaction. As expected, the anion-exchange reaction of Cl−MgAl(1/3)LDH from Cl− to CO32− occurred readily, whereas the anion-exchange reaction of CO32−-MgAl(1/3)LDH from CO32− to Cl− scarcely proceeded. The logarithms of the equilibrium constant (log Ke) for the anion-exchange reactions of CO32−-MgAl(1/3)LDH from CO32− to Cl− and Cl−MgAl(1/3)LDH from Cl− to CO32− were calculated as B

DOI: 10.1021/acs.inorgchem.9b01365 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

anions possessed a large isotropic thermal vibration parameter, as we reported previously.49 Furthermore, it was found that the hydrated water molecules in Cl−-MgAl(1/3)LDH were delocalized in the interlayer space. From these structural features, we predicted that both interlayer Cl− anions and hydrated water molecules in Cl−-MgAl(1/3)LDH would have relatively high mobilities. Thus, relatively high mobility could be the reason for the facile exchange of the Cl− anions with other anionic species, such as CO32−. The electron density distribution in the z = 1/2 plane of a 4a × 4b area of CO32−-MgAl(1/3)LDH is shown in Figure 2(b). The carbon, oxygen, and hydrogen atoms belonging to the intercalated CO32− anions and interlayer hydrated water molecules in CO32−-MgAl(1/3)LDH were localized, meaning that both CO32− anions and hydrated water molecules were strongly fixed in the interlayer space of this LDH. The interlayer distance (Figure S4) in CO32−-MgAl(1/3)LDH was smaller than that in Cl−-MgAl(1/3)LDH, although the brucite layer has the same charge (e/3). Taylor et al. reported that a hydrogen-bonded network containing CO32− anions, hydrated water, and −OH groups can be formed in the interlayer space of CO32−-MgAl(1/3)LDH.50,51 Thus, the present electron density distribution data is the first-time experimental evidence of the existence of such a hydrogen-bonded network in the interlayer space of CO32−-MgAl(1/3)LDH. It was thus conceivable that both CO32− anions and hydrated water molecules could not migrate in the interlayer space freely. This would make it very difficult for the CO32− anions incorporated in the MgAl(1/3)LDH to exchange with other anionic species in aqueous media, such as Cl−. Although many examples of 1D or 2D water network within confined space of nanomaterials are reported,52−55 the formation of 2D network comprised of carbonate and water is quite characteristic to layered double hydroxides. A comparison of the crystal structure features of the CO32−and Cl−-MgAl(1/3)LDHs revealed that the chemical state of the hydrated water molecules and/or the interactions between the anionic species, hydrated water molecules, and surface hydroxyl groups in the interlayer space of MgAl(1/3)LDH was an important factor for the thermodynamic stability of the intercalated anion species interlayer, regardless of whether the intercalated anionic species could be exchanged. The 17O MAS NMR spectra of 17O-enriched CO32−- and Cl−-MgAl(1/ 3)LDHs are shown in Figure 3. We anticipated that the hydrated interlayer water molecules of MgAl(1/3)LDH would be exchanged with NMR-active 17O-labeled water but that the oxygen atoms contained in the layered hydroxide framework

Figure 1. Anion exchange isotherms of MgAl(1/3)LDH. (○) Cl−MgAl(1/3)LDH from Cl− to CO32−, (□) CO32−-MgAl(1/3)LDH from CO32− to Cl−.

reported by Gaines and Thomas48 and equaled −0.84 (CO32−MgAl(1/3)LDH from CO32− to Cl−) and 0.99 (Cl−-MgAl(1/ 3)LDH from Cl− to CO32−). These results clearly demonstrated that the CO32− anions incorporated in the MgAl(1/ 3)LDH interlayer were strongly held and exhibited higher thermodynamic stability than Cl− anions. 3.3. Investigation of the High Stability of CO32− in MgAl(1/3)LDH. The fine crystal structures derived by the MEM/Rietveld method from SXRD data42−45 revealed that both hydrated water molecules and anions were located in the central-most position (z = 1/2) of the interlayer space in MgAl(1/3)LDH. Figure 2(a) shows the electron density

Figure 2. Electron density distribution in the z = 1/2 plane of a 4a × 4b area of (a) Cl−-MgAl(1/3)LDH and (b) CO32−-MgAl(1/3)LDH. A possible configuration of Cl−, H2O, and CO32− is shown schematically under the assumption that the molecular arrangement is disordered.

distribution in the z = 1/2 plane of a 4a × 4b area of Cl−MgAl(1/3)LDH. In this LDH, the number of Cl− anions was twice that of the CO32− anions, whereas the number of the H2O molecules (m ∼ 0.5) in both MgAl(1/3)LDHs was about the same. The Cl− anions appeared loosely bound in the interlayer space, which suggested that the intercalated Cl−

Figure 3. 17O NMR spectra of 17O-enriched (a) Cl−- and (b) CO32−MgAl(1/3)LDHs. MAS speed was 15 kHz. The NMR signal at around 380 ppm originated from the ZrO2 rotor. (a) Reproduced with permission from (60). Copyright (2014, ACS). C

DOI: 10.1021/acs.inorgchem.9b01365 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

slightly larger than that observed for Cl−-MgAl(1/3)LDH. This indicated that hydrogen bonding between the −OH groups on Al3+ and hydrated water molecules in the CO32−MgAl(1/3)LDH interlayer was stronger than that in Cl−MgAl(1/3)LDH. Results from the 3QMAS 17O NMR measurements were consistent with this interpretation. The broad 17O NMR signals at around −55 and −20 ppm in the 3QMAS NMR spectra64 of CO32−-MgAl(1/3)LDH shown in Figure 5

would not. Hydrated water molecules with free molecular motion usually generate a sharp 17O MAS NMR signal at around 0 ppm. Indeed, the NMR spectrum of Cl−-MgAl(1/ 3)LDH in Figure 3(a) contained a sharp signal around 0 ppm.56−58 Additionally, Cl−-MgAl(1/3)LDH yielded a weak, broad signal around −80 ppm, which is a typical signal of water molecules with reduced dynamics and symmetry of molecular motion.56−58 In contrast, the 17O MAS NMR spectrum of CO32−-MgAl(1/3)LDH shown in Figure 3(b) did not contain a sharp NMR signal at 0 ppm, but two resolved signals were observed at around −50 and 120 ppm. An NMR signal at around −50 ppm is typical of water molecules with reduced dynamics and symmetry of molecular motion,56−58 which may be the consequence of a network of hydrated water molecules, CO32− anions, and −OH groups on the layered surface participating in hydrogen bonds.59 The results of the analysis by Raman microscopy were consistent with our NMR findings. The presence of hydrated water molecules with free molecular motion was indicated by a sharp peak at 3654 cm−1 in the Raman spectrum of Cl−MgAl(1/3)LDH, shown in Figure 4(a).46,47,61 The presence of

Figure 5. 17O 3QMAS NMR spectrum of 17O-enriched CO32−MgAl(1/3)LDHs. MAS speed was 15 kHz. Reproduced with permission from ref 60. Copyright (2014 ACS).

involved at least two signals. Reduced molecular motion of water was indicated, which was presumably due to the different chemical environments in which water existed in the LDH. These results indicated that the water molecules in CO32−MgAl(1/3)LDH were strongly fixed by the hydrogen-bonded network between the −OH groups on Al3+ and hydrated water molecules and the bridging interaction between CO32− and hydrated water molecules. The other 17O NMR signal at around 120 ppm observed only for CO32−-MgAl(1/3)LDH was ascribed to CO32− anions.57 From these reports, we concluded an equilibrium must exist between the interlayer CO32− anions and the 17O-labeled hydrated water molecules, leading to the transfer of 17O atoms from water to CO32− anions.60 VT 17O MAS NMR spectra of the CO32−-MgAl(1/3)LDH measured at various temperatures are shown in Figure 6. NMR signals from two kinds of hydrated water molecules were observed at −20 and −55 ppm in the spectra obtained at all temperatures. The NMR signal at −55 ppm was ascribed to icelike hydrated water with a low mobility due to the formation of a hydrogen bonding network.59 In contrast, the signal at −20 ppm was attributed to loosely bound hydrated water with relatively free motion. The signal originating from hydrated water molecules with free molecular motion was not observed, even when the measured temperature was 80 °C. Moreover, the relative signal intensity from icelike hydrated water against loosely bound hydrated water gradually increased with decreasing temperature from 80 to −80 °C. In the Raman spectra of CO32−-MgAl(1/3)LDH measured at various temperatures shown in Figure 7, broadening of the Raman band at around 3500 cm−1 attributable to water molecules with reduced dynamics and symmetry of molecular motion could be observed. This phenomenon could be caused by the loose hydrogen-bonded network of the hydrated water molecules in the interlayer space with increasing temperature. However, the Raman spectra of CO32−-MgAl(1/3)LDH measured at various temperatures contained no band corresponding to free hydrated water molecules in the

Figure 4. Raman spectra of (a) Cl−- and (b) CO32−-MgAl(1/3)LDHs at RT.

water molecules with reduced dynamics and symmetry of molecular motion was also indicated by the broad peak at around 3400 cm−1.62 Moreover, a sharp peak attributed to a translation mode of metal hydroxide affected by hydrogen bonding between the −OH group on Al3+ and hydrated interlayer water molecules was observed at about 550 cm−1.63 Thus, these results confirm the existence of such bridging, which was previously predicted by Taylor et al.50,51 The reduced dynamics and symmetry of molecular motion among hydrated water molecules in the interlayer space in this LDH could be a consequence of the formation of a hydrogenbonded network between the −OH groups on Al3+ and the hydrated water molecules. The Raman spectrum of CO32−-MgAl(1/3)LDH shown in Figure 4(b) did not contain a sharp peak at 3654 cm−1. Instead, a broad peak around 3500 cm−1 and a weak shoulder around 3050 cm−1 were observed. As was the case with the Cl−-MgAl(1/3)LDH, the broad peak around 3500 cm−1 was attributed to water molecules with restricted dynamics and symmetry of molecular motion.62 The shoulder around 3050 cm−1 originated from the bridging water species between CO32− and −OH groups on the hydroxide layer.62 Meanwhile, the Raman shift of the sharp peak observed at around 550 cm−1 in the spectrum of the CO32−-MgAl(1/3)LDH was D

DOI: 10.1021/acs.inorgchem.9b01365 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

hydrated water molecules present in the interlayer space of CO32−-MgAl(1/3)LDH. The TG-DTA profiles of both Cl−and CO32−-MgAl(1/3)LDH featured an endothermic peak ascribed to the loss of hydrated water at 100 and 210 °C, respectively (Figure S5). This large difference of dehydration temperatures clearly indicated that the hydrated water existing in the interlayer space of CO32−-MgAl(1/3)LDH has higher thermal stability than that in the interlayer space of Cl−MgAl(1/3)LDH. Therefore, for the first time, we established that a unique hydrogen-bonded network with extremely high thermal stability can form between hydrated water, CO32−, and the −OH groups on the hydroxide layer in the interlayer nanospace of CO32−-MgAl(1/3)LDH shown as Figure 8, which is not observed in usual aqueous solutions.

Figure 6. 17O MAS spectra of 17O-enriched CO32−-MgAl(1/3)LDH at various temperatures. Pulse width and MAS speed were 1 s and 10 kHz, respectively. BG and SSB denote the background signal from the ZrO2 rotor and the spinning sideband, respectively.

Figure 8. Schematic image model of hydrogen-bonded network among CO32−, hydrated water, and −OH group on the hydroxide layer formed in the CO32−-MgAl(1/3)LDH.

4. CONCLUSIONS To elucidate the reason for the extremely high stability of CO32− anions intercalated in MgAl(1/3)LDH, we investigated the details of the chemical states of CO32− anions and hydrated water molecules in the LDH interlayer by SXRD measurement, solid-state NMR spectroscopy, and Raman spectroscopy. As results, the following experimental findings were obtained. (1) The CO32− anions are strongly fixed in the interlayer of CO32−MgAl(1/3)LDH, because the intercalated CO32− anions have extremely low isotropic thermal vibration parameters estimated from the SXRD pattern. (2) The hydrated water molecules form the rigidly hydrogen-bonding network from solid-state NMR and Raman spectroscopies. (3) This rigidly hydrogenbonding network formed by the hydrated water molecules in the interlayer has extremely high thermodynamic stability from the results in the temperature dependence of solid-state NMR and Raman spectroscopies. Based on the findings of this study, we have concluded that CO32−-MgAl(1/3)LDH does not undergo efficient anion exchange reactions because of the formation of an extremely strong hydrogen-bonded network between the hydrated water molecules, CO32−, and the −OH groups on the hydroxide layer (Mg2+ and/or Al3+) in the interlayer space. Based on the facts concluded in the present study, many researchers will be interested in whether the relationship between the stability of anion species and the strength of the hydrogen-bonded network between the hydrated water, anion species, and −OH groups on the hydroxide layer is a general property or not for LDH with various chemical compositions and anion species. From the

Figure 7. Raman spectra of CO32−-MgAl(1/3)LDH at various temperatures.

interlayer space. Thus, these results revealed that the dynamics of the hydrated water molecules in the interlayer space of CO32−-MgAl(1/3)LDH were strongly restricted. Further, we found that hydrated water molecules with an icelike hydrogenbonded network remained even at 80 °C. Moreover, the weak Raman shoulder observed at around 3050 cm−1, which originated from the bridging water species between the CO32− and − OH groups on the hydroxide layer, gradually decreased with an increase in temperature. This corresponded with the shift at around 120 ppm in the 17O NMR signal from CO32−, in which the signal gradually shifted downfield with an increase in temperature. These results indicated that the bridging water species remained even at 80 °C along with other hydrogen-bonded networks. However, the bonding strength between the bridging water species and CO32− species appeared to be weaker than that of the other hydrogen-bonded networks consisting of E

DOI: 10.1021/acs.inorgchem.9b01365 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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present data, it can be predicted that the highly selective anion species may form the rigid hydrogen-bonded network with the hydrated water and −OH group on the hydroxide layer in the interlayer space of the LDH. However, it is still unclear, because we have no data such as fine-crystal structure information, anion selectivity, Raman spectra, solid-state NMR, and so on for LDHs with various chemical compositions and anion species. We can report on LDHs with various chemical compositions and anion species in the future.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01365.

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Rietveld profile fitting result of CO32−-MgAl(1/3)LDH (Figure S1), XRD patterns (Figure S2), and FT-IR spectra (Figure S3) of (a) CO32−- and (b) Cl−-MgAl(1/ 3)LDHs, comparison of several lengths between CO32−and Cl− -MgAl(1/3)LDHs (Figure S4), TG-DTA profiles of CO32−- and Cl−-MgAl(1/3)LDHs (Figure S5), and atomic positions and thermal parameters of CO32−-MgAl(1/3)LDH (Table S1) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mails: [email protected] (R. S.). *E-mails: [email protected] (Y. F.). *E-mails: [email protected] (C. M.). ORCID

Ryo Sasai: 0000-0002-0345-3094 Takuya Fujimura: 0000-0003-1726-8372 Shinsuke Ishihara: 0000-0001-6854-6032 Masataka Tansho: 0000-0001-7986-3199 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partly supported by KAKENHI Grant Nos. 22350093, 17H03129, and 18K18310 from the Japan Society for the Promotion of Science (JSPS) and by two programs (2014 and 2015−2017) funded by the Shimane Prefecture. Synchrotron radiation X-ray diffraction experiments were performed at BL02B2 at SPring-8 with the approval of Japan Synchrotron-Radiation Research Institute (JASRI) (Program Nos.: 2010A1287, 2010B1279, 2011B1703, 2012B1770, 2013B1677, 2014A1684, 2016A0074, 2017A1483, 2017A0074, 2017B1196, 2017B0074, and 2018A1004). The authors are grateful to Prof. K. Yamada (Kochi University) for fruitful discussions and helpful comments on the analysis of NMR signals from 17O-enriched water molecules in the interlayer space of MgAl(1/3)LDH.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.9b01365 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.9b01365 Inorg. Chem. XXXX, XXX, XXX−XXX