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Near- and mid-infrared spectroscopy study of synthetic hydrocalumites
Solid State Sciences 13 (2011) 101e105
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Near- and mid-infrared spectroscopy study of synthetic hydrocalumites Manuel Mora, María Isabel López, César Jiménez-Sanchidrián, José Rafael Ruiz* Departamento de Química Orgánica, Universidad de Córdoba, Campus de Rabanales, Edificio Marie Curie, Carretera Nacional IV-A, km. 396, 14014 Córdoba, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 July 2010 Received in revised form 26 October 2010 Accepted 27 October 2010 Available online 2 November 2010
Three hydrocalumites containing Ca and Al in a 3:1 ratio, and carbonate as interlayer anion, were synthesized by coprecipitation, homogeneous precipitation in the presence of urea and the solegel method. The three solids thus obtained were for the first time characterized by using near- and midinfrared spectroscopies. The structural similarity of the solids determined by X-ray diffraction was confirmed by the spectroscopic results. Ó 2010 Elsevier Masson SAS. All rights reserved.
Keywords: NIR spectroscopy Hydrocalumite Layered double hydroxides
1. Introduction Layered double hydroxides (LDHs) constitute a family of compounds, many of natural origin, also known as “anionic clays” [1]. Structurally, LDHs are similar to brucite, Mg(OH)2, where some Mg2þ ions are replaced with trivalent cations such as Al3þ, Ga3þ, In3þ, Cr3þ or Fe3þ [1,2]. This substitution causes electron deficiency in brucite-like layers, which acquire positive charge as a result. In order to restore electroneutrality, the region between layers is occupied by anions of diverse nature [3]. The parent LDH, hydrotalcite, is a naturally occurring mineral of formula Mg6Al2(OH)2CO3$4H2O; hence, LDHs are also known as “hydrotalcite-like compounds”. However, not only the trivalent ion can vary in nature; in fact, Mg2þ ions can be replaced with other divalent cations [1,2]. Therefore, the general formula for LDHs is [M(II)1exM (III)x(OH)2]xþ[Ax/m]me$nH2O, where M(II) and M(III) denote a divalent and trivalent metal, respectively, lying at octahedral positions of Mg2þ in brucite-like layers, and A is the interlayer anion dwhich can vary widely in nature and be either inorganic or organic. x in the formula is the ratio M(II)/[M(II) þ M(III)] and usually ranges from 0.17 to 0.33. An LDH containing Ca(II) as divalent cation and Al (III) as trivalent cation is called a hydrocalumite. Hydrocalumite is a natural mineral of general formula [Ca2Al (OH)6]NO3$2H2O the structure of which consists of octahedra of calcium and aluminium hydroxides connected by their edges to form two-dimensional layers. Structurally, hydrocalumite is similar
* Corresponding author. Tel.: þ34 957218638; fax: þ34 957212066. E-mail address: [email protected] (J.R. Ruiz). 1293-2558/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.10.017
to hydrotalcite, which has been much more widely studied; these two compounds differ in that calcium and aluminium octahedra in hydrocalumite form an ordered array, whereas magnesium and aluminium octahedra in hydrotalcite are randomly distributed in the structure [4]. In addition, unlike hydrotalcite, Ca2þ in hydrocalumite completes its coordination sphere with an oxygen atom from an interlayer water molecule [5,6]. Until fairly recently, LDHs (hydrocalumites included) had only aroused interest in their mineralogy. However, evidence that their thermal treatment often gives basic mixed oxides expanded their field of research to catalysis, where they have been used in a number of base-catalysed chemical processes [7,8]. In addition, the ability to obtain LDHs containing noble metals such as Pd, Pt or Ru has opened up new catalytic prospects for these solids [9e11]. The high variety in chemical composition of LDHs has been a result of their easy synthesis. A number of methods are available for this purpose the most widely used of which continues to be coprecipitation [2]. In addition to catalysis, the ability of LDHs to accommodate virtually any type of anion in their interlayer region has promoted their use as sorbents [3]. Near-infrared (NIR) spectroscopy has grown substantially in use for a variety of scientific purposes in recent years [12e15]. In materials science, this technique has been successfully used to determine natural and synthetic minerals [16e18] and LDHs [19e22]. Hydrocalumites and its calcination products have been extensively used as catalysts in different organic synthetic processes, including acidebase reactions [23,24]. In these reactions the characterization of surface OH groups is essential. We think that NIR spectroscopy can be an excellent technique to characterize these
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surface OH groups. In this work, we used near- and mid-infrared spectroscopies to characterize three hydrocalumites with a Ca/Al ratio of 3 and containing carbonate as interlayer anion. The solids were obtained by coprecipitation, homogeneous precipitation in the presence of urea and the solegel method, respectively.
4000 cm1 (800e2500 nm) by the co-addition of 64 scans at a spectral resolution of 8 cm1, using DRIFT technique. All spectral treatments (baseline correction, smoothing, normalization and deconvolution) were done with the Peakfit v. 4.11. software package.
2. Experimental
3. Results and discussion
2.1. Synthesis of hydrocalumites
3.1. Mid-infrared spectroscopy
The three synthetic procedures used are described elsewhere [23]. In the coprecipitation method, two solutions containing 0.03 M Ca(NO3)2$6H2O and 0.015 M Al(NO3)3$9H2O, respectively, in 25 ml of de-ionized water, were used. The mixture was slowly dropped over 75 ml of an Na2CO3 solution at pH 10 at 60 C under vigorous stirring. The pH being kept constant by adding appropriate volumes of 1 M NaOH during precipitation. The suspension thus obtained was kept at 80 C for 24 h, after which the solid was filtered and washed with 2 l of de-ionized water. Homogeneous precipitation with urea involved adding solid urea to a solution containing the respective metal nitrates in a Ca/Al ratio of 2. The transparent solution thus obtained was heated at 100 C to facilitate precipitation of the corresponding hydrocalumite, and the resulting solid washed with distilled water several times and dried at 100 C in a stove. Finally, the solegel method involved dissolving 0.1 mol of calcium propionate in ethanol containing a small amount of HCl (35% in water). Following refluxing under continuous stirring, the solution was supplied with 175 ml of acetone containing 0.05 mol of aluminium acetylacetonate. The mixture was adjusted to pH 10 with ammonia (33% NH3 in water) and refluxed under continuous stirring until a gel was formed. Finally, the gel was isolated by centrifugation, washed with distilled water several times and dried at 100 C in a stove. The hydrocalumites obtained with the three above-described methods were ion-exchanged with carbonate to remove intercalated ions between layers. For this purpose, an amount of 2.5 g of hydrocalumite was dispersed in 15 ml of de-ionized water, supplied with 250 mg of Na2CO3 and refluxed for 2 h, after which the solid was isolated by centrifugation and the water discarded. This operation was repeated twice.
For easier study, the mid-infrared region was split into two zones spanning the wavenumber ranges 3900e2500 and 1800e 1200 cm1. The former region contained the stretching vibrations of OeH bonds present in hydrocalumites, and the latter the stretching bands for carbonate mainly. 3.1.1. Region I Fig. 1 shows the normalized, deconvoluted IR spectra for the three LDHs in the region from 3900 to 2500 cm1. The three solids exhibited two strong bands at ca. 3300 and 3100 cm1 alongside two other, weaker bands at ca. 3500 and 2925 cm1. Table 2 shows the exact wavenumbers for the bands and their assignations. Similarly to previous studies on Mg/Al and Mg/Ga hydrotalcites by our group [25], and to others on Ni/Al hydrotalcites [21], the strong band at 3300 cm1 can be assigned to OeH stretching vibrations of CaeOH groups in hydrocalumite, and so can the weaker band at 3500 cm1 to OeH stretching vibrations in structural AleOH groups. The other two signals observed at smaller
2.2. Characterization of the solids The solids were structurally characterized by X-ray diffraction and their chemical composition established in previous work [23]. Table 1 summarizes their structural and chemical features. 2.2.1. Mid-infrared spectroscopy Fourier-transform infrared (FT-IR) spectra were recorded over the wavenumber range 400e4000 cm1 on a PerkineElmer Spectrum 100 FTIR spectrophotometer by the co-addition of 32 scans with a resolution of 4 cm1. Samples were prepared by mixing appropriate, powdered aliquots of the solids with KBr as reference. 2.2.2. Near-infrared spectroscopy NIR spectra were collected in a PerkineElmer NIR Foss-NIR Systems 6500 spectrometer. Spectra were obtained from 12,500 to Table 1 Designations of the synthetic hydrocalumites and Ca/Al ratios as determined by ICPMS (from reference [23]). Hydrocalumite
Synthetic method
Ca/Al ratio
HC-1 HC-2 HC-3
Coprecipitation Solegel Urea
1.96 2.12 2.09
Fig. 1. MIR spectra of hydrocalumites in the 3900e2500 cm1 region.
M. Mora et al. / Solid State Sciences 13 (2011) 101e105 Table 2 Wavenumbers (in cm1) and assignations of the MIR bands for the LDHs. HC-1
HC-2
HC-3
Assignation
3492 3303 3103 2928
3495 3304 3101 2917
3506 3307 3099 2935
1665 1444 1372
1652 1451 1380
1675 1455 1391
OH stretching (AleOH in brucite-like layers) OH stretching (CaeOH in brucite-like layers) OH stretching (in interlayer water molecules) OH stretching (water bonded to carbonate in interlayer region) dH2 O bending C]O stretching (reduced carbonate symmetry) C]O stretching (n3)
103
n3 ¼ 1415 cm1). Band n3 was clearly observed at ca. 1380 cm1.
The presence of a shoulder at ca. 1450 cm1 has been ascribed by Serna et al. [27] to a lowering in the symmetry of the interlayer carbonate, as well as to the disordered nature of the interlayer space. 3.2. Near-infrared spectroscopy
3.1.2. Region II This spectral region, spanning the wavenumber range 1800e1200 cm1, exhibits a strong, broad band which, as shown in Fig. 2, can be deconvoluted into two signals centred at ca. 1450 and 1380 cm1. In addition, the spectrum contains a broad shoulder at ca. 1650 cm1. This last signal can be assigned to deformation vibrations in water, which appear at a greater wavenumber than in liquid (1625 cm1) and gaseous water (1595 cm1) by effect of the formation of very strong hydrogen bonds with interlayer carbonate ions or surface hydroxyl groups. On the other hand, carbonate ions in a symmetric environment exhibit three absorption bands close to those for the free anion (viz. n4 ¼ 680 cm1, n2 ¼ 880 cm1 and
Following the protocol used by Frost et al. [22] to examine NIR spectra in previous work, we split them into three different regions (see Fig. 4). One spans the wavenumber range 11,000e9000 cm1 (800e1111 nm), and contains the signals for the second stretching overtone of OeH bonds [22,28,29]. The second region, from 9000 to 6000 cm1 (1111e1667 nm) contains the bands for the first overtone of the fundamental stretching vibration in OeH bonds. Finally, the third region spans the range 6000e4000 cm1 (1667e2500 nm) and contains the overtone bands for OeH bonds in the water molecule. Fig. 3 shows the NIR region between 11,000 and 9000 cm1 for the three hydrocalumites together with their deconvoluted components as obtained following smoothing and normalization of the spectra. As can be seen, the three solids give a broad band in the region from 10,700 to 10,000 cm1. As noted earlier, this is the typical NIR region for bands related to the second overtone of the fundamental stretching vibration of OeH bonds [30]. Therefore, the three hydrocalumites exhibit two signals centred at ca. 10,350 and ca. 10,150 cm1, respectively, that can be assigned to the second overtone of OeH stretching vibrations of hydroxyl groups in octahedral layers bonded to metal atoms dwhich appeared at
Fig. 2. MIR spectra of hydrocalumites in the 1800e1200 cm1 region.
Fig. 3. First NIR spectral region (11,000e9000 cm1) for the hydrocalumites.
wavenumbers are due to stretching vibrations of hydroxyl groups in water molecules [26] and, as with hydrotalcites containing carbonate ion in their interlayer region, the band at ca. 2925 cm1 can be assigned to vibrations of the bridging bonds between carbonate ions and water in the interlayer region [10].
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M. Mora et al. / Solid State Sciences 13 (2011) 101e105
Fig. 4. Second NIR spectral region (8000e6000 cm1) for the hydrocalumites. Fig. 5. Third NIR spectral region (5510e4100 cm1) for the hydrocalumites.
3500e3300 cm1 in the mid-IR. There is also a broad shoulder of the signal for HC-1 (9800e9500 cm1) that can be assigned to the third overtone of OeH stretching vibrations of hydroxyl groups in interlayer water, the MIR signal for which appeared at ca. 3100 cm1. Fig. 4 shows the second region of the MIR spectrum (8000e6000 cm1), which contains the bands for the first overtone of OeH stretching vibrations and the strongest NIR signals [16,31]. All contain a broad band that is more asymmetric for HC-2 and HC-3 than it is for HC-1 and can be deconvoluted into four signals centred at ca. 7400, 7100, 6880 and 6500 cm1, respectively. The two strongest signals can be assigned to the first overtone of stretching vibrations of hydroxyl groups bonded to Al and Ca atoms, respectively; on the other hand, the band at ca. 6500 cm1 can be assigned to the first overtone of OeH stretching vibrations of water in the interlayer region. The last signal, which is that at the greatest wavenumber, and also the weakest deven undetectable in the spectrum for HC-3d, results from the addition or subtraction of the deformation mode for the split first overtone of the fundamental stretching vibration of OH groups (i.e. 2n1 Dn) [22]. The third region spans the wavenumber range from 5500 to 4100 cm1 and is shown in Fig. 5. This region contains two signal groups: one from 4500 to 4100 cm1 containing the combination bands for the mid-IR, which cannot be unequivocally assigned [16,30,31]; and the other from 5500 to 4500 cm1 which shows a broad signal due to OeH stretching overtones and combination bands for carbonate ion. The bands for carbonate usually arise from splitting of symmetric and asymmetric bands for this anion, 2 (n1 þ n3).
4. Conclusions Three hydrocalumites containing calcium and aluminium in a Ca/Al ratio of 3 were prepared by coprecipitation, homogeneous precipitation and the solegel method. For the first time, the hydrocalumites were characterized by using mid-infrared spectroscopy. The MIR and NIR spectra for the three solids were very similar. Therefore, the particular synthetic method used has little influence on the MIR or NIR spectrum. The MIR spectral zone from 3900 to 2500 cm1 contains strong signals due to stretching vibrations of various types of hydroxyl groups present in the solids. Such vibrations are largely responsible for the NIR signals, the second overtones of which appear in the 11,000e9000 cm1 zone and the first in the 8000e6000 cm1 zone. In addition, the MIR zone from 1800 to 1200 cm1 contains the signals for stretching vibrations of interlayer carbonate ions in the hydrocalumites. Acknowledgements The authors wish to acknowledge funding of this work by Spain’s Ministry of Science and Education (Project MAT-201018778) and FEDER Funds. References [1] F. Cavani, F. Trifiro, A. Vaccari, Catal. Today 11 (1991) 173e307. [2] C. Forano, T. Hibino, F. Leroux, C. Taviot-Gueho, Layered double hydroxides. in: F. Bergaya, B.K.G. Theng, G. Lagaly (Eds.), Handbook of Clay Science, Developments in Clay Science, vol. I, 2006 Chapter 13.1.
M. Mora et al. / Solid State Sciences 13 (2011) 101e105 [3] V. Rives, Layered Double Hydroxides:Present and Future. Nova Science Publishers, NewYork, 2001. [4] A. Terzis, S. Filippakis, H.J. Kuzel, H. Burzloff, Zeit. Krist 181 (1987) 29e34. [5] J.P. Rapin, G. Renaudin, E. Elkaim, M. Francois, Cem Concr Res 32 (2002) 513e518. [6] G. Renaudin, J.P. Rapin, B. Humbert, M. Francois, Cem. Concr. Res. 30 (2000) 307e312. [7] D.P. Debecker, E.M. Gaigneaux, G. Busca, Chem. Eur. J. 15 (2009) 3920e3935. [8] F. Zhang, X. Xiang, F. Li, X. Duan, Catal. Surv. Asia 12 (2008) 253e265. [9] D. Tichit, M. Martınez Ortiz, D. Francova, C. Gerardin, B. Coq, R. Durand, F. Prinetto, G. Ghiotti, Appl. Catal. Gen. 318 (2007) 170e177. [10] M. Mora, C. Jiménez-Sanchidrián, J.R. Ruiz, J. Colloid Interface Sci. 302 (2006) 568e575. [11] F. Basile, G. Fornasari, M. Gazzano, A. Vaccari, Appl. Clay Sci. 16 (2000) 185e200. [12] B. Czarnik-Matusewicz, Th. Zeegers-Huyskens, J. Phys. Org. Chem. 13 (2000) 237e243. [13] N. Heigl, C.H. Petter, M. Rainer, M. Najam-ul-Haq, R.M. Valiant, R. Bakry, G.K. Bonn, C.W. Huck, J. Near Infrared Spectros 15 (2007) 269e282. [14] Y. Roggo, P. Chalus, L. Maurer, C. Lema-Martinez, A. Edmond, N. Jent, J. Pharm. Biomed. Anal. 44 (2007) 683e700. [15] S. Tsuchikawa, Appl. Spectros. Rev. 42 (2007) 43e71. [16] J. Madejova, H. Palkova, M. Pentrak, P. Komadel, Clays Clay Miner (2009) 392e403. [17] N. Pailhé, A. Wattiaux, M. Gaudon, A. Demourgues, J. Solid State Chem. 181 (2008) 2697e2704.
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[18] J. Lacatusu, N. Badea, R. Nita, M. Giurginca, D. Bojin, I. Iasub, A. Meghea, J. Phys. Org. Chem. 22 (2009) 1015e1021. [19] M. Mora, M.I. López, C. Jiménez-Sanchidrian, J.R. Ruiz, J. Sol-Gel Sci. Technol. 55 (2010) 59e65. [20] R.L. Frost, H.J. Spratt, S.J. Palmer, Spectrochim. Acta A 72 (2009) 984e988. [21] Q. Tao, B.J. Reddy, H. He, R.L. Frost, P. Yuan, J. Zhu, Mat. Chem. Phys. 112 (2008) 869e875. [22] R.L. Frost, K.L. Erickson, Spectrochim. Acta A 61 (2005) 51e56. [23] M. Mora, M.I. López, C. Jiménez-Sanchidrían, J.R. Ruiz, Catal. Lett. 136 (2010) 192e198. [24] M.J. Campos, J. Santamaría, J. Mérida, R. Moreno, M. Albuquerque, S. Bruque, E. Rodriguez, A. Jiménez, A. Mairelles, Energy Fuels 24 (2010) 979e984. [25] M.A. Aramendia, Y. Avilés, J.A. Benítez, V. Borau, C. Jiménez, J.M. Marinas, J.R. Ruiz, F.J. Urbano, Microp. Mesop. Mater 29 (1999) 319e328. [26] K. Kagunya, R. Baddour-Hadjean, F. Kooli, W. Jones, Chem. Phys. 236 (1998) 225e234. [27] M.J. Hernández-Moreno, M.A. Ulibarri, J.L. Rendón, C.J. Serna, Phys. Chem. Miner 12 (1985) 34e39. [28] R. L Frost, S.J. Palmer, B.J. Reddy, Vib. Spectros. 44 (2007) 154e161. [29] S. Palmer, B.J. Reddy, R.L. Frost, Polyhedron 26 (2007) 524e533. [30] R.L. Frost, B.J. Reddy, S. Bahfenue, J. Graham, Spectrochim. Acta A 72 (2009) 597e604. [31] D. Gastaldi, F. Canonico, E. Boccaleri, J. Mater. Sci. 44 (2009) 5788e5794.
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Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Near- and mid-infrared spectroscopy study of synthetic hydrocalumites Manuel Mora, María Isabel López, César Jiménez-Sanchidrián, José Rafael Ruiz* Departamento de Química Orgánica, Universidad de Córdoba, Campus de Rabanales, Edificio Marie Curie, Carretera Nacional IV-A, km. 396, 14014 Córdoba, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 July 2010 Received in revised form 26 October 2010 Accepted 27 October 2010 Available online 2 November 2010
Three hydrocalumites containing Ca and Al in a 3:1 ratio, and carbonate as interlayer anion, were synthesized by coprecipitation, homogeneous precipitation in the presence of urea and the solegel method. The three solids thus obtained were for the first time characterized by using near- and midinfrared spectroscopies. The structural similarity of the solids determined by X-ray diffraction was confirmed by the spectroscopic results. Ó 2010 Elsevier Masson SAS. All rights reserved.
Keywords: NIR spectroscopy Hydrocalumite Layered double hydroxides
1. Introduction Layered double hydroxides (LDHs) constitute a family of compounds, many of natural origin, also known as “anionic clays” [1]. Structurally, LDHs are similar to brucite, Mg(OH)2, where some Mg2þ ions are replaced with trivalent cations such as Al3þ, Ga3þ, In3þ, Cr3þ or Fe3þ [1,2]. This substitution causes electron deficiency in brucite-like layers, which acquire positive charge as a result. In order to restore electroneutrality, the region between layers is occupied by anions of diverse nature [3]. The parent LDH, hydrotalcite, is a naturally occurring mineral of formula Mg6Al2(OH)2CO3$4H2O; hence, LDHs are also known as “hydrotalcite-like compounds”. However, not only the trivalent ion can vary in nature; in fact, Mg2þ ions can be replaced with other divalent cations [1,2]. Therefore, the general formula for LDHs is [M(II)1exM (III)x(OH)2]xþ[Ax/m]me$nH2O, where M(II) and M(III) denote a divalent and trivalent metal, respectively, lying at octahedral positions of Mg2þ in brucite-like layers, and A is the interlayer anion dwhich can vary widely in nature and be either inorganic or organic. x in the formula is the ratio M(II)/[M(II) þ M(III)] and usually ranges from 0.17 to 0.33. An LDH containing Ca(II) as divalent cation and Al (III) as trivalent cation is called a hydrocalumite. Hydrocalumite is a natural mineral of general formula [Ca2Al (OH)6]NO3$2H2O the structure of which consists of octahedra of calcium and aluminium hydroxides connected by their edges to form two-dimensional layers. Structurally, hydrocalumite is similar
* Corresponding author. Tel.: þ34 957218638; fax: þ34 957212066. E-mail address: [email protected] (J.R. Ruiz). 1293-2558/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.10.017
to hydrotalcite, which has been much more widely studied; these two compounds differ in that calcium and aluminium octahedra in hydrocalumite form an ordered array, whereas magnesium and aluminium octahedra in hydrotalcite are randomly distributed in the structure [4]. In addition, unlike hydrotalcite, Ca2þ in hydrocalumite completes its coordination sphere with an oxygen atom from an interlayer water molecule [5,6]. Until fairly recently, LDHs (hydrocalumites included) had only aroused interest in their mineralogy. However, evidence that their thermal treatment often gives basic mixed oxides expanded their field of research to catalysis, where they have been used in a number of base-catalysed chemical processes [7,8]. In addition, the ability to obtain LDHs containing noble metals such as Pd, Pt or Ru has opened up new catalytic prospects for these solids [9e11]. The high variety in chemical composition of LDHs has been a result of their easy synthesis. A number of methods are available for this purpose the most widely used of which continues to be coprecipitation [2]. In addition to catalysis, the ability of LDHs to accommodate virtually any type of anion in their interlayer region has promoted their use as sorbents [3]. Near-infrared (NIR) spectroscopy has grown substantially in use for a variety of scientific purposes in recent years [12e15]. In materials science, this technique has been successfully used to determine natural and synthetic minerals [16e18] and LDHs [19e22]. Hydrocalumites and its calcination products have been extensively used as catalysts in different organic synthetic processes, including acidebase reactions [23,24]. In these reactions the characterization of surface OH groups is essential. We think that NIR spectroscopy can be an excellent technique to characterize these
102
M. Mora et al. / Solid State Sciences 13 (2011) 101e105
surface OH groups. In this work, we used near- and mid-infrared spectroscopies to characterize three hydrocalumites with a Ca/Al ratio of 3 and containing carbonate as interlayer anion. The solids were obtained by coprecipitation, homogeneous precipitation in the presence of urea and the solegel method, respectively.
4000 cm1 (800e2500 nm) by the co-addition of 64 scans at a spectral resolution of 8 cm1, using DRIFT technique. All spectral treatments (baseline correction, smoothing, normalization and deconvolution) were done with the Peakfit v. 4.11. software package.
2. Experimental
3. Results and discussion
2.1. Synthesis of hydrocalumites
3.1. Mid-infrared spectroscopy
The three synthetic procedures used are described elsewhere [23]. In the coprecipitation method, two solutions containing 0.03 M Ca(NO3)2$6H2O and 0.015 M Al(NO3)3$9H2O, respectively, in 25 ml of de-ionized water, were used. The mixture was slowly dropped over 75 ml of an Na2CO3 solution at pH 10 at 60 C under vigorous stirring. The pH being kept constant by adding appropriate volumes of 1 M NaOH during precipitation. The suspension thus obtained was kept at 80 C for 24 h, after which the solid was filtered and washed with 2 l of de-ionized water. Homogeneous precipitation with urea involved adding solid urea to a solution containing the respective metal nitrates in a Ca/Al ratio of 2. The transparent solution thus obtained was heated at 100 C to facilitate precipitation of the corresponding hydrocalumite, and the resulting solid washed with distilled water several times and dried at 100 C in a stove. Finally, the solegel method involved dissolving 0.1 mol of calcium propionate in ethanol containing a small amount of HCl (35% in water). Following refluxing under continuous stirring, the solution was supplied with 175 ml of acetone containing 0.05 mol of aluminium acetylacetonate. The mixture was adjusted to pH 10 with ammonia (33% NH3 in water) and refluxed under continuous stirring until a gel was formed. Finally, the gel was isolated by centrifugation, washed with distilled water several times and dried at 100 C in a stove. The hydrocalumites obtained with the three above-described methods were ion-exchanged with carbonate to remove intercalated ions between layers. For this purpose, an amount of 2.5 g of hydrocalumite was dispersed in 15 ml of de-ionized water, supplied with 250 mg of Na2CO3 and refluxed for 2 h, after which the solid was isolated by centrifugation and the water discarded. This operation was repeated twice.
For easier study, the mid-infrared region was split into two zones spanning the wavenumber ranges 3900e2500 and 1800e 1200 cm1. The former region contained the stretching vibrations of OeH bonds present in hydrocalumites, and the latter the stretching bands for carbonate mainly. 3.1.1. Region I Fig. 1 shows the normalized, deconvoluted IR spectra for the three LDHs in the region from 3900 to 2500 cm1. The three solids exhibited two strong bands at ca. 3300 and 3100 cm1 alongside two other, weaker bands at ca. 3500 and 2925 cm1. Table 2 shows the exact wavenumbers for the bands and their assignations. Similarly to previous studies on Mg/Al and Mg/Ga hydrotalcites by our group [25], and to others on Ni/Al hydrotalcites [21], the strong band at 3300 cm1 can be assigned to OeH stretching vibrations of CaeOH groups in hydrocalumite, and so can the weaker band at 3500 cm1 to OeH stretching vibrations in structural AleOH groups. The other two signals observed at smaller
2.2. Characterization of the solids The solids were structurally characterized by X-ray diffraction and their chemical composition established in previous work [23]. Table 1 summarizes their structural and chemical features. 2.2.1. Mid-infrared spectroscopy Fourier-transform infrared (FT-IR) spectra were recorded over the wavenumber range 400e4000 cm1 on a PerkineElmer Spectrum 100 FTIR spectrophotometer by the co-addition of 32 scans with a resolution of 4 cm1. Samples were prepared by mixing appropriate, powdered aliquots of the solids with KBr as reference. 2.2.2. Near-infrared spectroscopy NIR spectra were collected in a PerkineElmer NIR Foss-NIR Systems 6500 spectrometer. Spectra were obtained from 12,500 to Table 1 Designations of the synthetic hydrocalumites and Ca/Al ratios as determined by ICPMS (from reference [23]). Hydrocalumite
Synthetic method
Ca/Al ratio
HC-1 HC-2 HC-3
Coprecipitation Solegel Urea
1.96 2.12 2.09
Fig. 1. MIR spectra of hydrocalumites in the 3900e2500 cm1 region.
M. Mora et al. / Solid State Sciences 13 (2011) 101e105 Table 2 Wavenumbers (in cm1) and assignations of the MIR bands for the LDHs. HC-1
HC-2
HC-3
Assignation
3492 3303 3103 2928
3495 3304 3101 2917
3506 3307 3099 2935
1665 1444 1372
1652 1451 1380
1675 1455 1391
OH stretching (AleOH in brucite-like layers) OH stretching (CaeOH in brucite-like layers) OH stretching (in interlayer water molecules) OH stretching (water bonded to carbonate in interlayer region) dH2 O bending C]O stretching (reduced carbonate symmetry) C]O stretching (n3)
103
n3 ¼ 1415 cm1). Band n3 was clearly observed at ca. 1380 cm1.
The presence of a shoulder at ca. 1450 cm1 has been ascribed by Serna et al. [27] to a lowering in the symmetry of the interlayer carbonate, as well as to the disordered nature of the interlayer space. 3.2. Near-infrared spectroscopy
3.1.2. Region II This spectral region, spanning the wavenumber range 1800e1200 cm1, exhibits a strong, broad band which, as shown in Fig. 2, can be deconvoluted into two signals centred at ca. 1450 and 1380 cm1. In addition, the spectrum contains a broad shoulder at ca. 1650 cm1. This last signal can be assigned to deformation vibrations in water, which appear at a greater wavenumber than in liquid (1625 cm1) and gaseous water (1595 cm1) by effect of the formation of very strong hydrogen bonds with interlayer carbonate ions or surface hydroxyl groups. On the other hand, carbonate ions in a symmetric environment exhibit three absorption bands close to those for the free anion (viz. n4 ¼ 680 cm1, n2 ¼ 880 cm1 and
Following the protocol used by Frost et al. [22] to examine NIR spectra in previous work, we split them into three different regions (see Fig. 4). One spans the wavenumber range 11,000e9000 cm1 (800e1111 nm), and contains the signals for the second stretching overtone of OeH bonds [22,28,29]. The second region, from 9000 to 6000 cm1 (1111e1667 nm) contains the bands for the first overtone of the fundamental stretching vibration in OeH bonds. Finally, the third region spans the range 6000e4000 cm1 (1667e2500 nm) and contains the overtone bands for OeH bonds in the water molecule. Fig. 3 shows the NIR region between 11,000 and 9000 cm1 for the three hydrocalumites together with their deconvoluted components as obtained following smoothing and normalization of the spectra. As can be seen, the three solids give a broad band in the region from 10,700 to 10,000 cm1. As noted earlier, this is the typical NIR region for bands related to the second overtone of the fundamental stretching vibration of OeH bonds [30]. Therefore, the three hydrocalumites exhibit two signals centred at ca. 10,350 and ca. 10,150 cm1, respectively, that can be assigned to the second overtone of OeH stretching vibrations of hydroxyl groups in octahedral layers bonded to metal atoms dwhich appeared at
Fig. 2. MIR spectra of hydrocalumites in the 1800e1200 cm1 region.
Fig. 3. First NIR spectral region (11,000e9000 cm1) for the hydrocalumites.
wavenumbers are due to stretching vibrations of hydroxyl groups in water molecules [26] and, as with hydrotalcites containing carbonate ion in their interlayer region, the band at ca. 2925 cm1 can be assigned to vibrations of the bridging bonds between carbonate ions and water in the interlayer region [10].
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Fig. 4. Second NIR spectral region (8000e6000 cm1) for the hydrocalumites. Fig. 5. Third NIR spectral region (5510e4100 cm1) for the hydrocalumites.
3500e3300 cm1 in the mid-IR. There is also a broad shoulder of the signal for HC-1 (9800e9500 cm1) that can be assigned to the third overtone of OeH stretching vibrations of hydroxyl groups in interlayer water, the MIR signal for which appeared at ca. 3100 cm1. Fig. 4 shows the second region of the MIR spectrum (8000e6000 cm1), which contains the bands for the first overtone of OeH stretching vibrations and the strongest NIR signals [16,31]. All contain a broad band that is more asymmetric for HC-2 and HC-3 than it is for HC-1 and can be deconvoluted into four signals centred at ca. 7400, 7100, 6880 and 6500 cm1, respectively. The two strongest signals can be assigned to the first overtone of stretching vibrations of hydroxyl groups bonded to Al and Ca atoms, respectively; on the other hand, the band at ca. 6500 cm1 can be assigned to the first overtone of OeH stretching vibrations of water in the interlayer region. The last signal, which is that at the greatest wavenumber, and also the weakest deven undetectable in the spectrum for HC-3d, results from the addition or subtraction of the deformation mode for the split first overtone of the fundamental stretching vibration of OH groups (i.e. 2n1 Dn) [22]. The third region spans the wavenumber range from 5500 to 4100 cm1 and is shown in Fig. 5. This region contains two signal groups: one from 4500 to 4100 cm1 containing the combination bands for the mid-IR, which cannot be unequivocally assigned [16,30,31]; and the other from 5500 to 4500 cm1 which shows a broad signal due to OeH stretching overtones and combination bands for carbonate ion. The bands for carbonate usually arise from splitting of symmetric and asymmetric bands for this anion, 2 (n1 þ n3).
4. Conclusions Three hydrocalumites containing calcium and aluminium in a Ca/Al ratio of 3 were prepared by coprecipitation, homogeneous precipitation and the solegel method. For the first time, the hydrocalumites were characterized by using mid-infrared spectroscopy. The MIR and NIR spectra for the three solids were very similar. Therefore, the particular synthetic method used has little influence on the MIR or NIR spectrum. The MIR spectral zone from 3900 to 2500 cm1 contains strong signals due to stretching vibrations of various types of hydroxyl groups present in the solids. Such vibrations are largely responsible for the NIR signals, the second overtones of which appear in the 11,000e9000 cm1 zone and the first in the 8000e6000 cm1 zone. In addition, the MIR zone from 1800 to 1200 cm1 contains the signals for stretching vibrations of interlayer carbonate ions in the hydrocalumites. Acknowledgements The authors wish to acknowledge funding of this work by Spain’s Ministry of Science and Education (Project MAT-201018778) and FEDER Funds. References [1] F. Cavani, F. Trifiro, A. Vaccari, Catal. Today 11 (1991) 173e307. [2] C. Forano, T. Hibino, F. Leroux, C. Taviot-Gueho, Layered double hydroxides. in: F. Bergaya, B.K.G. Theng, G. Lagaly (Eds.), Handbook of Clay Science, Developments in Clay Science, vol. I, 2006 Chapter 13.1.
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