Dependence of bonding interactions in Layered Double Hydroxides on metal cation chemistry

Accepted Manuscript Dependence of bonding interactions in Layered Double Hydroxides on metal cation chemistry Mostofa Shamim, Kausik Dana PII:

S0022-2860(16)30631-7

DOI:

10.1016/j.molstruc.2016.06.045

Reference:

MOLSTR 22664

To appear in:

Journal of Molecular Structure

Received Date: 13 April 2016 Revised Date:

16 June 2016

Accepted Date: 16 June 2016

Please cite this article as: M. Shamim, K. Dana, Dependence of bonding interactions in Layered Double Hydroxides on metal cation chemistry, Journal of Molecular Structure (2016), doi: 10.1016/ j.molstruc.2016.06.045. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Dependence of bonding interactions in Layered Double

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Hydroxides on metal cation chemistry

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Mostofa Shamim, Kausik Dana*

CSIR-Central Glass and Ceramic Research Institute, Kolkata-700032 INDIA

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Refractory and Traditional Ceramics Division

Abstract:

The evolution of various Infrared bands of Layered Double Hydroxides (LDH) with variable Zn:

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Al ratio was analyzed to correlate it with the changes in octahedral metal cation chemistry,

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interlayer carbonate anion and hydroxyl content of LDH. The synthesized phase-pure LDHs were

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crystallized as hexagonal 2H polytype with a Manasseite structure. The broad and asymmetric

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hydroxyl stretching region (2400-4000 cm-1) can be deconvoluted into four different bands. With

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increase in Zn2+: Al3+ metal ratio, the peak position of stretching frequencies of Al3+-OH and

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carbonate- bridged hydroxyl (water) decrease almost linearly. Individual band’s peak position and

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area under the curve have been successfully correlated with the carbonate and hydroxyl content of

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LDH. Due to lowering of symmetry of the carbonate anion, the IR-inactive peak νC-O, symm at 1064

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cm-1 becomes IR active. The peak position of metal- oxygen bands and carbonate bending modes

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are practically unaffected by the Zn2+: Al3+ ratio but the area under the individual M-O bands

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shows a direct correlation.

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Keywords: LDH, FTIR, Carbonate, Band component analysis.

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*Corresponding author. Tel.: +91 33 24733496 ; fax: +91 33 24730957

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E-mail: [email protected] (K. Dana)

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1. Introduction

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Layered Double Hydroxides (LDHs) belong to the family of layered anionic clays which find

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diverse interesting applications such as catalyst [1-3], adsorbent material [4], supporting matrix for

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sensors [5-7], filler in organic dyes to enhance photo physical properties [8-10], fire resistant

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coating [11] and to modify the mechanical properties of polymer nanocomposite [12-14] etc.

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These spectacular properties are achievable because pure LDH can be easily synthesized with

29

control over its tunable metal cation chemistry, interlayer anion and texture. Further, it can be

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functionalized by intercalation, grafting and controlled calcinations to generate required numerous 1

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functionalities. The chemical formula of LDH is represented as [MII1-x MIIIx (OH)2]Z+ An-Z/n . y

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H2O (MII, MIII represents bi and tri valent metal ion, An- is the exchangeable interlayer anion and

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‘x’ is layer charge). The first half represents the 2-D layer composition and the second half

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interlayer composition. The structure is based on that of brucite type layer where some of the

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bivalent metal ions are replaced by the trivalent metal ions and the anions are intercalated in

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interlayer position to maintain the electro neutrality. Schematic of LDHs showing edge sharing

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octahedrons of bi and tri valent metal ions along with the interlayer water and carbonate anion are

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shown in Figure 1. The chemistry of octahedral layer (i.e. metal cations and its stoichiometry) can

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be easily varied to generate LDH of different layer charge. Apart from possessing different ability

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for intercalation, these LDH can be calcined to generate an interesting “mixed metal oxide”

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(MMO) where the octahedral cations are statistically mixed in atomic level, giving rise to unique

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catalytic activity. Several recent reports discuss the promising application of such MMOs [1, 15-

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17] .

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XRD and vibrational spectroscopy are two major tools to investigate the bonding interaction in

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LDH. XRD is an accurate method to reveal the crystallography, lattice strain, crystallite size and

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detailed atom positions in unit cell. However, due to lower crystallinity (and subsequently low

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scattering intensity) of most sub-micron sized LDH and the presence of its various polytypes,

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analysis of XRD data (e.g. Rietveldt refinement) becomes difficult and inaccurate. Also, XRD

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cannot provide much information about structural changes after grafting reaction and calcinations

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(e.g. MMOs) - both of which have important consequences on its application as catalyst and

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functional material. In this backdrop, IR spectroscopy can provide very useful structural

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information which can be correlated to its properties. The origin of the infrared spectra is a

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mechanical vibration of the molecules, which are governed by the molecular symmetry of that

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molecule and the set of vibrations can be correlated with the specific orientation of the molecules

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in the substrate. In other words, mechanical vibration is a function of crystal structure and infrared

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spectroscopy is a versatile method for analyzing bonding interactions at the molecular level.

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The study of different aspects of structure of hydrotalcites (Ht) has been reported by Raman and

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infrared spectroscopy. Kloprogge et al [18] found a small but interesting change in band positions

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of the modes related to the hydroxyl groups with different Ht. Extensive work on the

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characterization of Ht by the spectroscopic study has been done by Frost et al group [19-26].

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The effect of different composition of LHDs or Ht (by varying bi and tri-valent metal ions,

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interlayer anion, modification with different organic anions) on the Raman and infrared 2

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characterization of LDHs using Raman and infrared spectroscopy have been reported [24, 25, 27-

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29]. These reports mainly concentrate on the characterization of LDHs (or Ht) with different

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combination of M2+ and M3+ or modified LDHs with different organic anions.

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However, the spectroscopic study of LDH as a function of bi and tri-valent metal ratio remains

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less understood. It is expected that the band position of different bending (δ) and stretching (ν)

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modes associated with the different constituent ions (Zn2+, Al3+, OH, CO32- in this case) should

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vary with the cation ratio. As the percentage of Zn2+, Al3+, CO32- varies and so does their

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interactions with each other. The change of band position (if any) with cation ratio may reveal new

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interesting information regarding the complex structure of LDHs. The only report about the

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spectroscopic study of a synthetic Ht with a complex octahedral chemistry as a function of

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different bi and tri valent metal ratio is by Palmer et al [30] .

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In the present study, interesting changes in Fourier transform infrared spectra (FTIR) of different

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synthetic LDH has been identified by band component analysis. Attempt has been made to

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correlate these changes in the different bands (in terms of peak position and area under the curve)

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with bi and tri valent metal ratio- obtained from chemical analysis of LDH. The activation of an

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IR inactive peak in the spectrum has been identified and the effect of metal ratio on this peak is

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also studied in details. To the best of our knowledge, there is no such report which correlates the

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evolution and variation of various IR bands with LDHs composition.

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2. Experimental Details

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2.1 Chemicals

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All the reagents (ZnCl2, AlCl3. 6H2O and Urea - Merck India Pvt. Ltd.) were used without further

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purification and all the experiments were performed at room temperature. Ultrapure membrane

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filtered water (MFW) of conductivity=18.2 mS, was used in all experiments.

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2.2 Synthesis of Zn-Al CO3 LDH

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Zn-Al LDH was synthesized by homogeneous co-precipitation method using urea [31, 32]. 10 ml

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of AlCl3.6H2O and 30 ml ZnCl2 solution (both 1M) were mixed together and filtered. To 7 ml of

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that mixture, 1100 mg urea and 210 ml MFW was added and placed in teflon-coated stainless steel

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autoclave. The mixture was heated without stirring for 22 h at 120 oC. Then it was cooled to room

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temperature and washed with hot water several times until it became chloride free. Finally, the

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powder was dried overnight under vacuum to produce Zn-Al CO3 LDH of layer charge 0.25. By

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using the above procedure the other two layers charged materials were synthesized and the batch

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composition is provided in Table 1.

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2.3 Characterization

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For chemical analysis ~ 10 mg of powdered sample was dissolved in 15 ml (N/10) HCl (pH~1) for

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two hours. Then an aliquot of this solution was taken to determine the concentration of Zn and Al

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ion by inductively coupled plasma atomic emission spectrometer (ICP-AES, Ciros Vision, Spectro

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Analytical Instruments Inc.). Powder X-ray diffraction analysis of synthesized LDH was carried

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out by X’Pert Pro diffraction unit (PANalytical) with Ni filtered Cu-Kα radiation. The diffraction

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pattern was recorded within the angle range 10o <2θ < 70o, where θ=Bragg’s angle. Samples were

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oven dried at 60 oC for 24 h to remove the adsorbed water and stored in a desiccator before FTIR

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experiment. A small amount of the sample was prepared in pellet form with KBr (sample: KBr

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ratio was 1:1000) and FTIR was conducted (Frontier FTIR/FIR Spectrometer, PerkinElmer) in

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4000 to 400 cm-1 range having a spectral resolution of 4 cm-1. Initially FTIR data was acquired

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without any sample for the background correction followed by the sample in KBr pellet form.

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Spectral manipulation such as base line adjustment, smoothing (if any) were performed using

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ORIGIN8 Pro software. Non-linear curve fitting was achieved by the same software using Gauss

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function and the fitting was well reproducible with a very high correlation coefficient (r2) value.

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Particle size analysis was carried out by Dynamic Light Scattering method by dispersing the

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sample in aqueous medium (Nano Practica SZ100, Horiba). Micro-structural characterization was

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done by field emission scanning electron microscopy (FESEM), for which a very dilute dispersion

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(in MFW) of the material was coated with carbon to make the surface conducting. FESEM images

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were captured by Gemini Zeiss SupraTM 35VP model.

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3. Results and Discussion

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XRD patterns of the synthesized LDHs (Figure 2) show sharp and symmetrical reflections at

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lower 2θ values, which are characteristic reflections of Ht -like compounds with high degree of

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crystallinity. Reflections in the diffractogram can be classified into several basal (00l) and non

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basal reflections (hk0) and matches with 2H polytype of LDH (Manasseite, JCPDS 14-525) which

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can be indexed into P -6 2 m space group. The unit cell parameters were calculated using formula-

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a= 2d110 and c= (2d002+3d003+4d004)/3 and it was around a=b= 3.04 Å, c=13.73 Å, α=β= 90o γ=

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120o. It is known that metal hydroxide layers of LDH consist of close packed hydroxide ions with

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octahedral cations separated by distance that corresponds to the 110 reflection which appear

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around 2θ= 60o (i.e., d110 =1.52 Å) and can be stacked in several ways to generate various

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polytypes. The most intense reflection at around 2θ=12.68o, attributed to basal reflection (d002),

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corresponds to successive stacking of two brucite-like sheets in the 2H polytype. Presence of

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higher order of basal reflections (003, 004, 006 and 008) is due to good stacking in the c –

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direction of the LDH. The remaining reflections are attributed to non-basal reflections. The

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position and intensity of these reflections more or less similar with Zn2+: Al3+ ratio, indicating that

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the crystal structure and stacking is maintained throughout the experimental batches.

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The particle size distribution curve of the LDH obtained from dynamic light scattering (Figure 3)

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shows a broad monomodal distribution having the median diameter of 235 nm (mode =256 nm)

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for Zn2+: Al3+=2.82. With increase in Zn2+: Al3+ ratio the diameter increases but not

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monotonically. For Zn2+: Al3+=3.1 and Zn2+: Al3+=3.7 respectively the median diameter of 92 and

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140 nm (corresponding modes 96 and 139 nm) were found. The particle texture was studied with

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FESEM (Figure 4), which showed the synthesized LDH CO32- to be a uniform hexagonal plate-

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like particle with well defined edge.

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Chemical analysis of the synthesized LDH was performed to compare the expected and

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experimental Zn: Al atomic ratio. It was found that although the expected Zn: Al atomic ratios

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were 3, 2.33 and 2, in synthetic LDH the ratio becomes 3.72, 3.10 and 2.82 respectively. When

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these ratios were plotted against each other (Figure 5) – a perfect linear correlation is noticed. This

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clearly indicates that the molar concentration of starting AlCl3 was less than 1 (M) and the purity

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level of the chemical claimed by the manufacturer was flawed. This created above discrepancy in

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metal cation ratio in the synthesized LDH.

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3.1 Fourier Transform-IR spectroscopy

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FTIR spectroscopic data of different LDHs is represented in the Figure 6. The general formula of

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LDH with carbonate as interlayer anion is – M

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chemical composition based on chemical analysis is represented in Table 3. The major bonding

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interactions that can generate an IR band are i) metal cation –oxygen bond in the brucite-like layer

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II (1-x)

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III x(OH)2(CO3)x/2

0.5H2O [33] and its

ACCEPTED MANUSCRIPT ii) hydroxyl interaction with metal cations and water and iii) bands from carbonate anion. All

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these interactions should correlate quantitatively with the number of such interactions.

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Each spectrogram can be analyzed by categorizing into following general bands-

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a) Hydroxyl stretching zone (νΟ−Η) at 2400-4000 cm-1, b) Stretching and bending vibrations of

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carbon-oxygen (C-O) bond of carbonate anion, c) Metal-oxygen stretching frequency (νM-O) at

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400-700 cm-1.

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Close inspection of spectrograms reveals some interesting shifts in the standard peak positions

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which are investigated in details to correlate with octahedral metal cation chemistry of LDH (viz.,

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the effect Zn2+: Al3+ ratio).

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Table 3 revealed some interesting trends regarding hydroxyl: carbonate ratio with variation in

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Zn2+: Al3+ ratio. The chemical analysis shows that with decreasing Zn2+: Al3+ ratio the total

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amount of hydroxyl in synthesized material remains more or less same but the percentage of

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interlayer anion (carbonate) increases. So it is expected that the interaction between constituent

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molecules/ions might vary with the cation ratio and this is explored by band component analysis

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of various IR bands of the synthesized LDHs.

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3.1.1 Hydroxyl stretching zone

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The broad hump observed between 2400 and 4000 cm-1 zone is asymmetric in nature with one

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spike. It is expected that this asymmetric hydroxyl hump is a combination of different hydroxyl

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stretching vibrations which may arise from the interaction with Zn2+, Al3+, CO32- and hydroxyl

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molecules peaks. Band component analysis reveals that the asymmetric

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4000 cm-1) zone can be successfully deconvoluted into four different bands (Figure 7). The

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following assignment of four bands has been done with logical deduction aided by previous

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studies [34, 35].

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The bands are assigned as-

νΟ−Η region (2400 and

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Band-I: CO3-2-H2O bridging band (2950-3050 cm-1)

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Band-II: hydrogen-bonded interlayer water band (around 3300 cm-1)

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Band-III: Al-OH (around 3450 cm-1)

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Band-IV: Zn-OH (around 3555 cm-1)

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Shift of peak position and change in % area under the curve of these four bands have been

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identified and attempt has been made to correlate those variations with Zn2+: Al3+ ratio.

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νΜ−ΟΗ peaks around 3550 and 3450 cm-1 region. The

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Band component analysis results in two

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assignment of different

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corresponding metals. In Gibbsite,

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Zn(OH)2 appears around 3570 cm-1 [36, 37]. However, the interaction between Al and Zn cation

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with the hydroxyl group in LDH is quite complex compared to simple hydroxides and

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consequently the assignment of M-OH peaks is convoluted [25, 26]. Here, the assignment of Al-

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OH (around 3450 cm-1) and Zn-OH (around 3555 cm-1) bands are based on the νΜ−ΟΗ values of

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corresponding simple metal hydroxides with additional consideration of the stronger polarisibility

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of trivalent cation (Al3+) on polar hydroxyl bond. The presence of multiple M-OH band [18, 20]

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and sometimes broad band around 3400-3600 cm-1 [38, 39] have also been reported, but the

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change in peak positions and nature of the peaks is yet to be correlated with the M2+: M3+ ratio.

νΜ−ΟΗ peaks has been done after comparing with simple hydroxides of

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νAl-OH peak appears around 3530 cm-1 and the νZn-OH peak in

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The shift in the peak position of these bands (Band-III and Band-IV) and the area under the curve

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are reported in Table 4 and correlated with Zn2+: Al3+ ratio in following discussion. The key

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parameter to synthesize the higher layer charged LDH is to decrease molar ratio of bivalent and

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trivalent metal (Table 1). Since all the metal cations are bonded to hydroxyls in the brucite-like

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layer, it is expected that this change in cation chemistry will have some interesting effect on the

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characteristics of M-OH bands.

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As the percentage of Al3+ increases (with decreasing Zn2+: Al3+ ratio), the formation of stronger

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Al-OH bond is expected with the available hydroxyl groups. As a result, the peak position of νAl-

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OH

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Al3+=3.72) to 3420 cm-1 (for Zn2+: Al3+=2.82). Opposite effect is observed with the

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The progressive decrease of Zn2+ (with decreasing Zn2+: Al3+ ratio) makes the band weaker and

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broad in nature. The peak position changes from 3544 cm-1 (for Zn2+: Al3+=3.72) to 3560 cm-1 (for

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Zn2+: Al3+=2.82). The shift in band position of Al-OH (30 cm-1) is almost double compared to Zn-

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OH (16 cm-1) (Table 4).

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frequency (band-III) is shifted towards the lower wave number from 3450 cm-1 (for Zn2+:

νZn-OH band.

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The plotting in Figure 8 shows the effect of Zn2+: Al3+ ratio on the CO32- -H2O bridging band

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(Band-I) resulting from the band component analysis of 7

νΟ−Η zone. The peak shows a shift of 22

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units in the peak position towards higher wave number with increasing the Zn2+: Al3+ ratio. Palmer

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et al [30] reports a similar trend with a different hydrotalcite with a complex octahedral tri-valent

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metal chemistry, Mg6(Al,Fe)2(OH)16(CO3)·4H2O. With decreasing Zn2+: Al3+ ratio the % of

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carbonate in the LDH increases (Table 3) from 6.3 % to 7.8 %. So it is expected that the carbonate

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anion will have a propensity to form a stronger bridging with water molecules available in the

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interlayer. As a consequence, the area under the curve decreases (Figure 8) and the band becomes

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sharper (Figure 7) with decreasing Zn2+: Al3+ ratio.

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The second band (Band-II) (Figure 8) around 3270 cm-1 is interpreted as the hydrogen bonded

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interlayer water [34, 35]. A 15 unit shift of peak position to the lower wave number with

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increasing Zn2+: Al3+ ratio is observed. The small shift indicates that Zn2+: Al3+ ratio has very little

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effect on the properties of the band. Detailed analysis allowed us to discern those little effects.

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With decreasing Zn2+: Al3+ ratio the hydrogen bonds are weakened (band position shifts towards a

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higher wave number) a little and become broad in nature (evident from the increase in % area

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from 38 to 46 %). A probable explanation is- with increasing Zn2+: Al3+ ratio the percentage of

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free interlayer water molecules (for hydrogen bonding) decreases due to formation of stronger

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carbonate -water bridging. The remaining free interlayer water molecules (not participating in

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carbonate- water bridging) are quite capable of forming hydrogen bonding among themselves.

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However, due to some decrease in quantity, these bonds become a little weak and broad in nature.

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3.1.2 Stretching and bending vibrations of Carbon-Oxygen (C-O) bond of carbonate anion

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and lowering of its symmetry

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Free carbonate ion is a triangular planar with point symmetry D3h. The free ion, CO32- with D3h

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symmetry exhibits four normal vibrational modes: (i) a symmetric stretching vibration (ν C-O, symm.

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1064 cm-1),

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C-O ,assym. 1415 cm-1),

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carbonate anion the νC-O, symm. 1064 cm-1 mode is IR inactive (Figure 9).

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(ii) non planar bending (δC-O ,880 cm-1), (iii) a doubly degenerate asymmetric stretching (ν

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and (iv) a doubly degenerate bending mode (δC-O ,680 cm-1) [18, 40] . For the free

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A detailed analysis shows that the bending modes are practically unaffected by the Zn2+: Al3+ ratio

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but the same is not true for the stretching modes (Table 5). The shift of νC-O 880 cm-1 band to lower

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wave number (833 cm-1) indicates a possible loss of freedom compared to free CO32− and as a

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consequence a lowering of the carbonate symmetry may occur. Interestingly, a peak at 1064 cm-1 8

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present study the peak becomes active. A possible explanation is distortion or lowering of

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symmetry of carbonate molecule in LDH compare to its free state. Few literatures are available

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[18, 20, 23, 30, 40, 41] on this unexpected behavior of ν C-O, symm. 1064 cm-1 peak, but the effect of

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Zn2+: Al3+ ratio on this peak is still unknown. A probable explanation for lowering of the

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symmetry of carbonate anion based on the charged field theory is proposed here. The substitution

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of the Zn2+ by Al3+ ion in octahedral brucite-like layer generates a positively charged field within

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the gallery of LDH. When negatively charged carbonate anion enters into the gallery (to maintain

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the electro neutrality), the interaction between the layers and the incoming carbonate anion

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becomes prominent. The positively charged gallery field tries to orient the carbonate anion

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according to its field to minimize the electronic repulsion and thereby distorting it, making it IR

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active.

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Close inspection indicates the existence of one or more minuscule peaks in proximity of 1064 cm-1

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band and are deciphered by band component analysis (Figure 10). The 1064 cm-1 peak is actually

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a combination of one or two peaks and is highly dependent on the Zn2+: Al3+ ratio (Table 6). These

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bands are interpreted as the carbonate anion in different chemical environment of synthetic LDH.

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The peak around 1046 cm-1 is assigned to the free carbonate anion, 1059 cm-1 as carbonate bonded

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to interlayer water molecules and 1090 cm-1 as carbonate chemically bonded to LDH hydroxyl

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surface [19, 20, 42]. This complex profile of carbonate anion becomes more convoluted with Zn2+:

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Al3+ ratio. For Zn2+: Al3+=3.72 all the three bands are well observed but with decreasing Zn2+: Al3+

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ratio the peak around 1046 cm-1 (assigned to the free carbonate anion) disappears. These results

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are quite different from those of Palmer et al [30]. One probable explanation is as the Zn2+: Al3+

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ratio decreases the tendency of carbonate anion to form bonds with hydroxyl/or water molecules

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increases. This diminishes the probability of carbonate anion to reside in a free state within the

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LDH structure.

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ACCEPTED MANUSCRIPT was detected and assigned as ν C-O, symm. 1064 cm-1. Originally the peak was IR inactive, but in the

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3.1.3 Metal oxygen bonds

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Band component analysis in the region of 400 to 700 cm-1 has been performed to find any

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significant effect on the M-O band positions with Zn2+: Al3+ ratio (Figure 11). The band around

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473 and 568 cm-1 may be assigned the translational modes of hydroxyl groups mainly influenced

280

by tri (Aluminium) and bi-valent (Zinc) metal ions respectively. It also shows that the Zn2+: Al3+

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ratio has marginal effect on the band positions but greatly affects the area under the curve. The 9

ACCEPTED MANUSCRIPT area under the curve of Zn-OH and Al-OH translational bands shows a variation in accordance

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with the Zn: Al ratio. Figure 12 reveals that with increasing Zn2+: Al3+ ratio the area under the Al-

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OH translational band decreases and the same for the Zn-OH translational band increases. It is

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expected as the Zn2+: Al3+ ratio increases the amount of Al3+ decreases and Zn2+ increases. A

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linear relationship has been found (Figure 13) when the ratio of area of these two translational

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bands compared with Zn2+: Al3+ ratio in the corresponding LDH.

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4. Conclusions

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1. The broad and asymmetric hydroxyl stretching region (2400-4000 cm-1) is actually

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composed of four different bands. With decrease in Zn2+: Al3+ ratio, the peak position of

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MIII-OH and Carbonate-bridged hydroxyl (water) bands decrease almost linearly.

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Individual band’s peak position and area under the curve also successfully correlated with

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the carbonate and hydroxyl content of the synthetic LDH.

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2.

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For each synthetic LDH, due to lowering of symmetry of the carbonate anion, the ν C-O, -1

symm. at 1064 cm becomes IR active. Band component analysis shows that this band is

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composed of multiple peaks and appearance of those component peaks is dependent on

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Zn2+: Al3+ ratio. For LDH with Zn2+: Al3+ ratio=3.72 this band is composed of three peaks

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where as for other LDH the component peak is two.

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3. The carbonate bending modes (δC-O ,833 cm-1 and δC-O ,630 cm-1 ) are practically unaffected by

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the Zn2+: Al3+ ratio, but the stretching mode (ν C-O ,assym. 1430 cm-1) exhibits direct correlation.

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4. The peak of the metal- oxygen bands are practically unaffected by Zn2+: Al3+ ratio.

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However, the area under the individual band shows a linear correlation with Zn2+: Al3+

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ratio. Even the ratio of area under the curve of two M-O bands shows a linear correlation

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with corresponding metal ratio.

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5. Acknowledgement

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The research work was funded by CSIR under EDMISSIBLE project and one of the author (MS)

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acknowledges the “JRF-GATE” research fellowship granted to him by CSIR, New Delhi, India to

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carry out this work. The authors wish to thank Pratiti Mandal, Jadavpur University for

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improvements in English language of the manuscript.

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Reference

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[1] E. Dumitriu, V. Hulea, C. Chelaru, C. Catrinescu, D. Tichit, R. Durand, Influence of the acid-

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base properties of solid catalysts derived from hydrotalcite-like compounds on the condensation of

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formaldehyde and acetaldehyde, Appl. Catal., A 178(2) (1999) 145-157.

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intercalated Mg-Al layered double hydroxide with blue luminescence by hydrothermal synthesis,

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Adv. Funct. Mater. 20(17) (2010) 2848-2856.

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transparent ultrathin films with tunable-color luminescence based on the assembly of photoactive

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organic molecules and layered double hydroxides, Adv. Funct. Mater. 21(13) (2010) 2497-2505.

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resistant coatings for flexible polyurethane foam, Polymer 56 (2014) 284-292.

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ACCEPTED MANUSCRIPT [12] S. Mallakpour, M. Dinari, Facile synthesis of nanocomposite materials by intercalating an

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optically active poly(amide-imide) enclosing (l)-isoleucine moieties and azobenzene side groups

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into a chiral layered double hydroxide, Polymer 54(12) (2013) 2907-2916.

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[13] D.-M. Xu, M.-Y. Guan, Q.-H. Xu, Y. Guo, Multilayer films of layered double

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hydroxide/polyaniline and their ammonia sensing behavior, J. Hazard. Mater. 262 (2013) 64-70.

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lignosulfonated layered double hydroxide/plasticized starch and thermoplastics, Carbohydr.

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Polym. 96(1) (2013) 91-100.

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versatile applications in organic synthesis, Catal. Sci. Technol. 2(6) (2012) 1113-1125.

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zno-based mixed metal oxides by thermal decomposition: Transformation mechanism and uv-

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blocking properties of the product, Chem. Mater. 22(13) (2010) 3933-3942.

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[17] S. Babay, A. Bulou, A.M. Mercier, M. Toumi, The decomposition of the layered double

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hydroxides of Co and Al: Phase segregation of a new single phase spinel oxide, Spectrochim. Acta

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[18] J.T. Kloprogge, R.L. Frost, Fourier transform infrared and raman spectroscopic study of the

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local structure of Mg-, Ni-, and Co-hydrotalcites, J. Solid State Chem. 146(2) (1999) 506-515.

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[19] R.L. Frost, M.L. Weier, J.T. Kloprogge, Raman spectroscopy of some natural hydrotalcites

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with sulphate and carbonate in the interlayer, J. Raman Spectros. 34(10) (2003) 760-768.

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[20] J.T. Kloprogge, L. Hickey, R.L. Frost, Ft-raman and ft-ir spectroscopic study of synthetic

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mg/zn/al-hydrotalcites, J. Raman Spectros. 35(11) (2004) 967-974.

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[21] R.L. Frost, A.W. Musumeci, W.N. Martens, M.O. Adebajo, J. Bouzaid, Raman spectroscopy

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of hydrotalcites with sulphate, molybdate and chromate in the interlayer, J. Raman Spectros.

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36(10) (2005) 925-931.

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[22] S.J. Palmer, R.L. Frost, G. Ayoko, T. Nguyen, Synthesis and raman spectroscopic

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characterisation of hydrotalcite with CO32- and(MoO4)2- anions in the interlayer, J. Raman

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Spectros. 39(3) (2008) 395-401.

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[23] R.L. Frost, A. Soisnard, N. Voyer, S.J. Palmer, W.N. Martens, Thermo-raman spectroscopy

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of selected layered double hydroxides of formula Cu6Al2(OH)16CO3 and Zn6Al2(OH)16CO3, J.

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Raman Spectros. 40(6) (2009) 645-649.

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ACCEPTED MANUSCRIPT [24] R.L. Frost, R. Scholz, A. Lopez, F.L. Theiss, Vibrational spectroscopic study of the natural

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layered double hydroxide manasseite now defined as hydrotalcite-2h- Mg6Al2(OH)16[CO3]c.4H2O,

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Spectrochim. Acta A Mol. Biomol. Spectrosc. 118 (2013) 187-191.

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[25] S.J. Palmer, L.M. Grand, R.L. Frost, The synthesis and spectroscopic characterisation of

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hydrotalcite formed from aluminate solutions, Spectrochim. Acta A Mol. Biomol. Spectrosc. 79(1)

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(2011) 156-160.

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[26] S.J. Palmer, R.L. Frost, L.-M. Grand, Raman spectroscopy of gallium- and zinc-based

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hydrotalcites, J. Raman Spectros. 42(5) (2011) 1168-1173.

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double hydroxides containing magnesium and trivalent metals, Mater. Lett. 120 (2014) 193-195.

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intercalated hydrotalcite, Spectros. Spect. Anal. 33(3) (2013) 704-708.

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microspectroscopy of hydrotalcite-like compounds modified with sulphate and sulphonate organic

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anions, J. Mol. Struct. 1034 (2012) 38-42.

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hydrotalcites with variable cationic ratios, J. Raman Spectros. 40(9) (2009) 1138-1143.

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[31] U. Costantino, F. Marmottini, M. Nocchetti, R. Vivani, New synthetic routes to hydrotalcite-

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like compounds - characterisation and properties of the obtained materials, Eur. J. Inorg. Chem.

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[32] S.M.C. Auerbach, K .A.; Dutta, P. K. , Handbook of layered materials, New York, 2004.

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[33] H.F.W. Taylor, Crystal structures of some double hydroxide minerals, Min. Mag. 39 (1973)

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377-389.

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[34] J.T. Kloprogge, L. Hickey, R.L. Frost, Heating stage raman and infrared emission

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spectroscopic study of the dehydroxylation of synthetic Mg-hydrotalcite, App. Clay. Sci. 18(1-2)

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(2001) 37-49.

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[35] M.K. Titulaer, J.B.H. Jansen, J.W. Geus, The quantity of reduced nickel in synthetic takovite

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- effects of preparation conditions and calcination temperature, Clays Clay Miner. 42(3) (1994)

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249-258.

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[36] J. Gadsden, Infrared spectra of minerals and related inorganic compounds, Butterworths,

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[37] R.L. Frost, J.T. Kloprogge, S.C. Russell, J.L. Szetu, Vibrational spectroscopy and

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dehydroxylation of aluminum (oxo)hydroxides: Gibbsite, Appl. Spectrosc. 53(4) (1999) 423-434.

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ACCEPTED MANUSCRIPT [38] K.T. Ehlsissen, A. Delahayevidal, P. Genin, M. Figlarz, P. Willmann, Preparation and

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characterization of turbostratic ni/al layered double hydroxides for nickel-hydroxide electrode

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applications, J. Mater. Chem. 3(8) (1993) 883-888.

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[39] S. Kannan, C.S. Swamy, Synthesis and physicochemical characterization of cobalt aluminum

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hydrotalcite, J. Mater. Sci. Lett. 11(23) (1992) 1585-1587.

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anions CO32-, NO3-, SO42- and ClO4- in Mg/Al hydrotalcite, Am. Mineral. 87(5-6) (2002) 623-629.

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minerals, J. Raman Spectros. 38(1) (2007) 68-77.

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[42] Y.-H. Lin, M.O. Adebajo, J.T. Kloprogge, W.N. Martens, R.L. Frost, X-ray diffraction and

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raman spectroscopic studies of Zn-substituted carrboydite-like compounds, Mater. Chem. Phys.

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100(1) (2006) 174-186.

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[43] M. Wei, M. Pu, J. Guo, J.B. Han, F. Li, J. He, D.G. Evans, X. Duan, Intercalation of l-dopa

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into layered double hydroxides: Enhancement of both chemical and stereochemical stabilities of a

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drug through host-guest interactions, Chem. Mater. 20(16) (2008) 5169-5180.

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[44] Q. Yuan, M. Wei, D.G. Evans, X. Duan, Preparation and investigation of thermolysis of l-

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aspartic acid-intercalated layered double hydroxide, J. Phys. Chem. B 108(33) (2004) 12381-

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12387.

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List of Figures

429 430

Figure 1: Schematic presentation of LDH showing edge sharing octahedrons of bi and tri-valent

431

metal ions along with interlayer water and exchangeable carbonate anions. The layer

432

thickness for brucite-like hexagonal LDH is assumed to be 4.8 Å [43, 44] Figure 2: X-ray powder diffraction pattern of different synthetic Zn-Al CO32- LDH

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Figure 3: Particle size distribution of different synthetic Zn-Al CO32- LDH

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Figure 4: Electron micrograph of LDH showing uniform hexagonal plate-like particles with well-

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defined edge for Zn2+: Al3+=3.72

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Figure 5: Theoretical and experimental Zn: Al atomic ratio for synthesized LDH

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Figure 6: Infrared spectra of the different synthetic Zn-Al CO32- LDH

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Figure 7: Band component analysis of hydroxyl stretching zone of different synthetic LDH

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Figure 8: Peak position and % area variation with Zn: Al ratio of different bands in hydroxyl

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Figure 9: Infrared spectra of different νC-O and δC-O in synthetic LDH having Zn2+: Al3+=3.10

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Figure 10: Band component analysis of ν C-O, symm. 1064 cm-1 for different synthetic LDH

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Figure 11: Band component analyses of M-O bands

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Figure 12: Variation of area of two M-O bands with Zn2+: Al3+ ratio

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Figure 13: Variation of area ratio of two M-O bands with Zn2+: Al3+ ratio

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Table 1: Batch composition of synthetic LDH

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Table 2: Chemical analysis of synthetic LDH

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Table 3: Compositional analysis of synthetic LDH

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Table 4: Change in peak position and % area of νΟ−Η zone with different Zn2+: Al3+ ratio

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Table 5: Variation of stretching and bending vibrations of Carbon-Oxygen (C-O) bond of carbonate anion with Zn2+: Al3+ ratio

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Table 6: Variation of components of 1064 cm-1 peak and its variation with Zn2+: Al3+ ratio

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Tables: Table 1 Batch composition of synthetic LDH Sample code

Experimental layer

1 (M) ZnCl2 (ml)

1(M) AlCl3. 6H2O (ml)

0.21

30

10

Zn : Al =3.10

0.24

28

12

Zn2+: Al3+=2.82

0.26

26.8

Zn2+: Al3+=3.72 2+

3+

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Table 2 Chemical analysis of synthetic LDH 3+

[Zn ] ppm

[Al ] ppm

Theoretical Zn: Al (atomic ratio)

3+

81.17

9.01

3

2+

3+

72.14

9.60

2.33

2+

3+

76.33

11.16

Zn : Al =3.72 Zn : Al =3.10 Zn : Al =2.82

Experimental Zn: Al

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(atomic ratio)

464

3.72

3.10

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2+

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Table 3 Compositional analysis of synthetic LDH Sample code

CO2-3

OH total

OH total : CO2-3

97.5

6.3

27

5.79

97.6

7.2

27

5.07

97.66

7.8

27

4.68

Formula

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2+

3+

Zn : Al =3.72 Zn2+: Al3+=3.10 2+

3+

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Table 4. Change in peak position and % area of νΟ−Η zone with different Zn2+ :Al3+ ratio Sample code

Band I

Band II

Band III

Band IV

CO3 -H2O

Hydrogen-bonded

Al-OH

Zn-OH

Bridging

interlayer water

2-

Zn2+: Al3+=3.72

% area

cm-1

% area

cm-1

% area

cm-1

% area

2958

29

3265

38

3450

28

3544

4

2+

3+

2940

28

3270

41

3435

23

3556

8

2+

3+

2936

24

3280

46

3420

21

3560

9

Zn : Al =3.10 Zn : Al =2.82

17

cm-1

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Table 5 Variation of stretching and bending vibrations of Carbon-Oxygen (C-O) bond of carbonate anion with Zn2+: Al3+ ratio C-O angular bending

C-O non planar bending

-1

-1

(cm )

(cm )

Zn2+: Al3+=3.72

630

833

2+

3+

632

833

2+

3+

630

833

Zn : Al =3.10

(cm-1) 1428 1422 1415

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C-O anti symmetrical stretching mode

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Sample code

Zn2+: Al3+=3.72

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1066

1096

X

1066

1098

X

1065

1100

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Highlights for review

The consequence of Zn2+: Al3+ ratio of LDH on its structure is explored in detail.

•

Lowering of symmetry of carbonate anion is found to activate ν C-O, symm. 1064 cm-1 peak and it depends on Zn2+: Al3+ ratio.

•

Metal-oxygen translational bands show good correlation with octahedral metal chemistry.

•

This study will help in better structure-property correlation of LDH and LDH-organic hybrids.

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