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Solid State Sciences 13 (2011) 101e105

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Solid State Sciences

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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

Article history:Received 6 July 2010Received in revised form26 October 2010Accepted 27 October 2010Available online 2 November 2010

Keywords:NIR spectroscopyHydrocalumiteLayered double hydroxides

* Corresponding author. Tel.: þ34 957218638; fax:E-mail address: qo1ruarj@uco.es (J.R. Ruiz).

1293-2558/$ e see front matter � 2010 Elsevier Masdoi:10.1016/j.solidstatesciences.2010.10.017

a b s t r a c t

Three hydrocalumites containing Ca and Al in a 3:1 ratio, and carbonate as interlayer anion, weresynthesized by coprecipitation, homogeneous precipitation in the presence of urea and the solegelmethod. The three solids thus obtained were for the first time characterized by using near- and mid-infrared spectroscopies. The structural similarity of the solids determined by X-ray diffraction wasconfirmed by the spectroscopic results.

� 2010 Elsevier Masson SAS. All rights reserved.

1. Introduction

Layered double hydroxides (LDHs) constitute a family ofcompounds, many of natural origin, also known as “anionic clays”[1]. Structurally, LDHs are similar to brucite, Mg(OH)2, wheresome Mg2þ ions are replaced with trivalent cations such as Al3þ,Ga3þ, In3þ, Cr3þ or Fe3þ [1,2]. This substitution causes electrondeficiency in brucite-like layers, which acquire positive charge asa result. In order to restore electroneutrality, the region betweenlayers is occupied by anions of diverse nature [3]. The parent LDH,hydrotalcite, is a naturally occurring mineral of formula Mg6A-l2(OH)2CO3$4H2O; hence, LDHs are also known as “hydrotalcite-likecompounds”. However, not only the trivalent ion can vary in nature;in fact, Mg2þ ions can be replaced with other divalent cations[1,2]. Therefore, the general formula for LDHs is [M(II)1exM(III)x(OH)2]xþ[Ax/m]me$nH2O, where M(II) and M(III) denote a diva-lent and trivalent metal, respectively, lying at octahedral positionsof Mg2þ in brucite-like layers, and A is the interlayer aniondwhichcan vary widely in nature and be either inorganic or organic. x inthe formula is the ratio M(II)/[M(II) þ M(III)] and usually rangesfrom 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 ofcalcium and aluminium hydroxides connected by their edges toform two-dimensional layers. Structurally, hydrocalumite is similar

þ34 957212066.

son SAS. All rights reserved.

to hydrotalcite, which has been much more widely studied; thesetwo compounds differ in that calcium and aluminium octahedra inhydrocalumite form an ordered array, whereas magnesium andaluminium octahedra in hydrotalcite are randomly distributed inthe structure [4]. In addition, unlike hydrotalcite, Ca2þ in hydro-calumite completes its coordination sphere with an oxygen atomfrom an interlayer water molecule [5,6].

Until fairly recently, LDHs (hydrocalumites included) had onlyaroused interest in their mineralogy. However, evidence that theirthermal treatment often gives basic mixed oxides expandedtheir field of research to catalysis, where they have been used ina number of base-catalysed chemical processes [7,8]. In addition,the ability to obtain LDHs containing noble metals such as Pd, Pt orRu has opened up new catalytic prospects for these solids [9e11].The high variety in chemical composition of LDHs has been a resultof their easy synthesis. A number of methods are available forthis purpose the most widely used of which continues to becoprecipitation [2]. In addition to catalysis, the ability of LDHs toaccommodate virtually any type of anion in their interlayer regionhas promoted their use as sorbents [3].

Near-infrared (NIR) spectroscopy has grown substantially inuse for a variety of scientific purposes in recent years [12e15]. Inmaterials science, this technique has been successfully used todetermine natural and synthetic minerals [16e18] and LDHs[19e22].

Hydrocalumites and its calcination products have been exten-sively used as catalysts in different organic synthetic processes,including acidebase reactions [23,24]. In these reactions the char-acterization of surface OH groups is essential. We think that NIRspectroscopy can be an excellent technique to characterize these

M. Mora et al. / Solid State Sciences 13 (2011) 101e105102

surface OH groups. In this work, we used near- and mid-infraredspectroscopies to characterize three hydrocalumites with a Ca/Alratio of 3 and containing carbonate as interlayer anion. The solidswere obtained by coprecipitation, homogeneous precipitation inthe presence of urea and the solegel method, respectively.

2. Experimental

2.1. Synthesis of hydrocalumites

The three synthetic procedures used are described elsewhere[23]. In the coprecipitation method, two solutions containing0.03M Ca(NO3)2$6H2O and 0.015MAl(NO3)3$9H2O, respectively, in25 ml of de-ionized water, were used. The mixture was slowlydropped over 75 ml of an Na2CO3 solution at pH 10 at 60 �C undervigorous stirring. The pH being kept constant by adding appropriatevolumes of 1 M NaOH during precipitation. The suspension thusobtained was kept at 80 �C for 24 h, after which the solid wasfiltered and washed with 2 l of de-ionized water. Homogeneousprecipitation with urea involved adding solid urea to a solutioncontaining the respective metal nitrates in a Ca/Al ratio of 2. Thetransparent solution thus obtained was heated at 100 �C to facili-tate precipitation of the corresponding hydrocalumite, and theresulting solid washed with distilled water several times anddried at 100 �C in a stove. Finally, the solegel method involveddissolving 0.1 mol of calcium propionate in ethanol containinga small amount of HCl (35% in water). Following refluxing undercontinuous stirring, the solution was supplied with 175 ml ofacetone containing 0.05 mol of aluminium acetylacetonate. Themixture 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, washedwith distilledwater several times and dried at 100 �C in a stove.

The hydrocalumites obtained with the three above-describedmethods were ion-exchanged with carbonate to remove interca-lated ions between layers. For this purpose, an amount of 2.5 g ofhydrocalumitewas dispersed in 15ml of de-ionized water, suppliedwith 250 mg of Na2CO3 and refluxed for 2 h, after which the solidwas isolated by centrifugation and the water discarded. Thisoperation was repeated twice.

2.2. Characterization of the solids

The solids were structurally characterized by X-ray diffractionand their chemical composition established in previous work [23].Table 1 summarizes their structural and chemical features.

2.2.1. Mid-infrared spectroscopyFourier-transform infrared (FT-IR) spectra were recorded over

the wavenumber range 400e4000 cm�1 on a PerkineElmer Spec-trum 100 FTIR spectrophotometer by the co-addition of 32 scanswith a resolution of 4 cm�1. Samples were prepared by mixingappropriate, powdered aliquots of the solids with KBr as reference.

2.2.2. Near-infrared spectroscopyNIR spectra were collected in a PerkineElmer NIR Foss-NIR

Systems 6500 spectrometer. Spectra were obtained from 12,500 to

Table 1Designations of the synthetic hydrocalumites and Ca/Al ratios as determined by ICP-MS (from reference [23]).

Hydrocalumite Synthetic method Ca/Al ratio

HC-1 Coprecipitation 1.96HC-2 Solegel 2.12HC-3 Urea 2.09

4000 cm�1 (800e2500 nm) by the co-addition of 64 scans ata spectral resolution of 8 cm�1, using DRIFT technique.

All spectral treatments (baseline correction, smoothing, normal-ization and deconvolution) were done with the Peakfit v. 4.11. soft-ware package.

3. Results and discussion

3.1. Mid-infrared spectroscopy

For easier study, the mid-infrared region was split into twozones spanning the wavenumber ranges 3900e2500 and 1800e1200 cm�1. The former region contained the stretching vibrations ofOeH bonds present in hydrocalumites, and the latter the stretchingbands for carbonate mainly.

3.1.1. Region IFig. 1 shows the normalized, deconvoluted IR spectra for the

three LDHs in the region from 3900 to 2500 cm�1. The three solidsexhibited two strong bands at ca. 3300 and 3100 cm�1 alongsidetwo other, weaker bands at ca. 3500 and 2925 cm�1. Table 2 showsthe exact wavenumbers for the bands and their assignations.Similarly to previous studies on Mg/Al and Mg/Ga hydrotalcitesby our group [25], and to others on Ni/Al hydrotalcites [21], thestrong band at 3300 cm�1 can be assigned to OeH stretchingvibrations of CaeOH groups in hydrocalumite, and so can theweaker band at 3500 cm�1 to OeH stretching vibrations in struc-tural AleOH groups. The other two signals observed at smaller

Fig. 1. MIR spectra of hydrocalumites in the 3900e2500 cm�1 region.

Table 2Wavenumbers (in cm�1) and assignations of the MIR bands for the LDHs.

HC-1 HC-2 HC-3 Assignation

3492 3495 3506 OH stretching (AleOH in brucite-like layers)3303 3304 3307 OH stretching (CaeOH in brucite-like layers)3103 3101 3099 OH stretching (in interlayer water molecules)2928 2917 2935 OH stretching (water bonded to carbonate in

interlayer region)1665 1652 1675 dH2O bending1444 1451 1455 C]O stretching (reduced carbonate symmetry)1372 1380 1391 C]O stretching (n3)

M. Mora et al. / Solid State Sciences 13 (2011) 101e105 103

wavenumbers are due to stretching vibrations of hydroxyl groupsin water molecules [26] and, as with hydrotalcites containingcarbonate ion in their interlayer region, the band at ca. 2925 cm�1

can be assigned to vibrations of the bridging bonds betweencarbonate ions and water in the interlayer region [10].

3.1.2. Region IIThis spectral region, spanning the wavenumber range

1800e1200 cm�1, exhibits a strong, broad band which, as shown inFig. 2, can be deconvoluted into two signals centred at ca. 1450and 1380 cm�1. In addition, the spectrum contains a broad shoulderat ca. 1650 cm�1. This last signal can be assigned to deformationvibrations in water, which appear at a greater wavenumber than inliquid (1625 cm�1) and gaseous water (1595 cm�1) by effect of theformation of very strong hydrogen bonds with interlayer carbonateions or surface hydroxyl groups. On the other hand, carbonate ionsin a symmetric environment exhibit three absorption bands closeto those for the free anion (viz. n4 ¼ 680 cm�1, n2 ¼ 880 cm�1 and

Fig. 2. MIR spectra of hydrocalumites in the 1800e1200 cm�1 region.

n3 ¼ 1415 cm�1). Band n3 was clearly observed at ca. 1380 cm�1.The presence of a shoulder at ca. 1450 cm�1 has been ascribed bySerna et al. [27] to a lowering in the symmetry of the interlayercarbonate, as well as to the disordered nature of the interlayerspace.

3.2. Near-infrared spectroscopy

Following the protocol used by Frost et al. [22] to examine NIRspectra in previous work, we split them into three different regions(see Fig. 4). One spans the wavenumber range 11,000e9000 cm�1

(800e1111 nm), and contains the signals for the second stretchingovertone of OeH bonds [22,28,29]. The second region, from 9000to 6000 cm�1 (1111e1667 nm) contains the bands for the firstovertone of the fundamental stretching vibration in OeH bonds.Finally, the third region spans the range 6000e4000 cm�1

(1667e2500 nm) and contains the overtone bands for OeH bondsin the water molecule.

Fig. 3 shows the NIR region between 11,000 and 9000 cm�1 forthe three hydrocalumites together with their deconvolutedcomponents as obtained following smoothing and normalization ofthe spectra. As can be seen, the three solids give a broad band inthe region from 10,700 to 10,000 cm�1. As noted earlier, this is thetypical NIR region for bands related to the second overtone of thefundamental stretching vibration of OeH bonds [30]. Therefore, thethree hydrocalumites exhibit two signals centred at ca. 10,350and ca. 10,150 cm�1, respectively, that can be assigned to the secondovertone of OeH stretching vibrations of hydroxyl groups inoctahedral layers bonded to metal atoms dwhich appeared at

Fig. 3. First NIR spectral region (11,000e9000 cm�1) for the hydrocalumites.

Fig. 4. Second NIR spectral region (8000e6000 cm�1) for the hydrocalumites.

Fig. 5. Third NIR spectral region (5510e4100 cm�1) for the hydrocalumites.

M. Mora et al. / Solid State Sciences 13 (2011) 101e105104

3500e3300 cm�1 in the mid-IR. There is also a broad shoulder ofthe signal for HC-1 (9800e9500 cm�1) that can be assigned to thethird overtone of OeH stretching vibrations of hydroxyl groups ininterlayer water, the MIR signal for which appeared at ca.3100 cm�1.

Fig. 4 shows the second region of the MIR spectrum(8000e6000 cm�1), which contains the bands for the first overtoneof OeH stretching vibrations and the strongest NIR signals [16,31].All contain a broad band that is more asymmetric for HC-2 andHC-3 than it is for HC-1 and can be deconvoluted into four signalscentred at ca. 7400, 7100, 6880 and 6500 cm�1, respectively. Thetwo strongest signals can be assigned to the first overtone ofstretching vibrations of hydroxyl groups bonded to Al and Caatoms, respectively; on the other hand, the band at ca. 6500 cm�1

can be assigned to the first overtone of OeH stretching vibrations ofwater in the interlayer region. The last signal, which is that at thegreatest wavenumber, and also the weakestdeven undetectable inthe spectrum for HC-3d, results from the addition or subtraction ofthe deformation mode for the split first overtone of the funda-mental stretching vibration of OH groups (i.e. 2n1 � Dn) [22].

The third region spans the wavenumber range from 5500 to4100 cm�1 and is shown in Fig. 5. This region contains two signalgroups: one from 4500 to 4100 cm�1 containing the combinationbands for the mid-IR, which cannot be unequivocally assigned[16,30,31]; and the other from 5500 to 4500 cm�1 which showsa broad signal due to OeH stretching overtones and combinationbands for carbonate ion. The bands for carbonate usually arise fromsplitting of symmetric and asymmetric bands for this anion, 2(n1 þ n3).

4. Conclusions

Three hydrocalumites containing calcium and aluminium ina Ca/Al ratio of 3 were prepared by coprecipitation, homogeneousprecipitation and the solegel method. For the first time, thehydrocalumites were characterized by using mid-infrared spec-troscopy. The MIR and NIR spectra for the three solids were verysimilar. Therefore, the particular synthetic method used has littleinfluence on the MIR or NIR spectrum. The MIR spectral zone from3900 to 2500 cm�1 contains strong signals due to stretchingvibrations of various types of hydroxyl groups present in the solids.Such vibrations are largely responsible for the NIR signals, thesecond overtones of which appear in the 11,000e9000 cm�1 zoneand the first in the 8000e6000 cm�1 zone. In addition, the MIRzone from 1800 to 1200 cm�1 contains the signals for stretchingvibrations of interlayer carbonate ions in the hydrocalumites.

Acknowledgements

The authors wish to acknowledge funding of this work bySpain’s Ministry of Science and Education (Project MAT-2010-18778) and FEDER Funds.

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