Materials Sciences and Applicatio ns, 2011, 2, 684-691
doi:10.4236/msa.2011.26094 Published Online June 2011 (
Copyright © 2011 SciRes. MSA
Using Kaolinitic Clay for Preparation of a
Hydrotalcite-Like Compound
Khaled Hosni*, Ezzeddine Srasra
Centre National des recherches en Sciences des Matériaux, Pôle technologique de Borj Cedria, Nabeul, Tunisia.
Received December 24th, 2010; revised March 21st, 2011; accepted May 18th, 2011.
In this study, Mg-Al-CO3 hydrotalcite was synthesized from a kaolinite as natural source aluminium using two simple
methods. The first method uses the kaolinite in natural solid state, the second method use the filtrate of the kaolinite
after dissolution by acidic solutions. The structure of the materials was characterized by X-ray diffraction, Fourier
transform infrared spectroscopy, differential scanning calorimetry (DSC) and Brunauer, Emmett, and Teller (BET)
Keywords: Hydrotalcite, Kaolinite, Layered Compound, XRD, Memory Effect
1. Introduction
A wide range of compositions are possible for synthetic
hydrotalcites based on the general formula
[ML(OH)][X][H O]
1-yy2y/n2z , where MII and MIII are
the divalent and trivalent cations in the octahedral posi-
tions within the hydroxide layers. The value y can have a
range between 0.17 and 0.33, while Xn– is an interlayer
anion with a negative charge n [1,2], and z is the number
of water molecules. Many anions or anionic complexes,
both organic and inorganic, can be incorporated into the
hydrotalcite structure. The hydrotalcite can be abbrevi-
ated as [MII – MIII – X], where, MII = Mg2+, Zn2+, Cu2+,
Ca2+, Mn2+; MIII = Al3+, Cr3+, Fe3+, ; and Xn– = Cl,
NO3–, CO3–, , etc.
Hydrotalcite-like compounds, also called layered dou-
ble hydroxides (LDHs), have received considerable at-
tention in recent years owing to their layered structure
and high anion-exchange capacity [3], which makes them
potential materials for technical applications in various
domains [4]. These materials have been investigated as
solid ionic conductors [5] and sensors [6]. Layered dou-
ble-metal hydroxides are, or may be used as, catalysts
[4,7], photo-catalysts, catalyst supports [8], adsorbents
[9,10], anion exchangers [11], medicines [12,13] and
bonding materials. The reason for the potential applica-
tion of hydrotalcites as catalysts rests with their ability to
make mixed metal oxides at the atomic level, rather than
the particle level. Such mixed metal oxides are formed
through the thermal decomposition of the hydrotalcite
[14]. Also, LDH materials have been extensively studied
in terms of their thermal evolution [15,16], textural
properties [16], and the formation of nanosized metal
particles [17] as well as for environmental purposes [18].
The present study reports simplified method to synthe-
size the carbonate forms of layered double hydroxides
from a natural source of trivalent cations. Mg-Al-CO3
hydrotalcite-like layered compounds have been synthe-
sized with Mg/Al ratios of 3 by a mechanochemical
method using kaolinite in solid state and by coprecipita-
tion using the under product of the acidic dissolution of
the kaolinite.
2. Experimental
2.1. Material Preparation
The Mg-Al-CO3 hydrotalcite-like layered compounds has
been synthesized by two simple methods using kaolinite
as a natural source of trivalent cations and an aqueous
solution of Na2CO3 as the precipitant.
Method 1
The sample was synthesized by crushing kaolinite with
the magnesium nitrate hexahydrate (their amounts were
so as to have the desired Mg2+/Al3+ molar ratio), fol-
lowed by heating at 500˚C for 4 hours. The product ob-
tained was dispersed under constant stirring in an aque-
ous solution containing Na2CO3 (100 mL). The pH of the
dispersion was maintained constant at 10 ± 0.1 by adding
NaOH or HNO3 when necessary. The slurry was subse-
Using Kaolinitic Clay for Preparation of a Hydrotalcite-Like Compound685
quently agitated at room temperature for 24 h, and then
aged at 150˚C for 24 h. The resulting products were col-
lected by centrifugal separation and washed thoroughly
with deionised water to eliminate excess Na+ followed by
drying overnight at room temperature.
The samples were identified as [Ksd3P10-T150], where
Ksd represents the trivalent cation source (kaolinite) used
to prepare the materials and “sd” represents the solid
state. For example, Ksd3P10-T150 stands for the product
prepared with kaolinite in solid state, a Mg/Al ratio of 3,
an aging temperature of 150˚C, and a pH of synthesis of
Method 2
The sample was synthesized using as aqueous solution
of Na2CO3 as precipitant. The solutions containing
Mg(NO3)2·6H2O and Al3+, cation resulting from dissolu-
tion of the purified kaolinite by acidic attack, (their con-
centration varied so as to have the Mg/Al molar ratio of 3)
were added dropwise to the aqueous solutions of Na2CO3
with vigorous stirring. The pH of the dispersion was
maintained constant at 10 ± 0.1 by adding NaOH (10%).
The slurry was subsequently agitated at room tempera-
ture for tree day. The resulting products were collected
by centrifugal separation and washed thoroughly with
deionised water to eliminate excess Na+ followed by
drying overnight at room temperature.
The samples were identified as [Kliq3P10], where Kliq
represents the trivalent cation source (kaolinite) used to
prepare the materials and “liq” represents its liquid state.
For example, Kliq3P10 stands for the product prepared
with kaolinite in liquid state, an Mg/Al ratio of 3 and a
pH of synthesis of 10.
2.2. Characterization of Materials
The dried precipitates were characterized by X-ray dif-
fraction (XRD) in order to determine the species present
and their degree of crystallinity. Diffractograms were
obtained by using a ‘PANalytical X’Pert HighScore Plus’
diffractometer using monochromated CuKα radiation.
Nitrogen adsorption measurements were performed at
–196˚C with an Autosorb-1 unit (Quantachrome, USA)
for the determination of sample textural properties using
the multipoint Brunaner-Emmet-Teller (BET) method.
The samples were out gassed at 120˚C under a vacuum at
10–3 mmHg for 3.5 h. Fourier-transformed infrared (FT-IR)
spectra were recorded as KBr pellets using a Perkin-
Elmer FT-IR (model 783) instrument. KBr pellets were
prepared by mixing 5 wt% anionic clay with 95 wt%
KBr and pressing. Differential scanning calorimeter
(DSC) experiments were performed with Mettler Tole-
deo DSC-823 type thermal analyzer at heating rates of
3. Results and Discussion
3.1. Characterization of the Clay
The sample selected for this study is Tabarka clay (Tuni-
sian clay).
3.1.1. X-Ray Diffrac ti on
The nature of the impurities was determined by exam-
ining the crude samples. Quartz (reflection at 3.35 Å) is
the major impurity. The diffractogramme of purified
sample (Figure 1(a)) show the reflections at d = 7.21 Å
and 10.05 Å characteristic of the kaolinite and illite re-
spectively [19].
In Figure 1(b) is shown the powder XRD pattern of
the mixture of kaolinitic clay and the magnesium nitrate
after heating at 500˚C. Owing to the fact of the absence
of the peak corresponding of the d-spacing of 7.21 Å and
attributed of the kaolin in the XRD pattern of the product
shows that the structure of the original clay is completely
destroyed and indicates metal oxide peaks, suggesting an
almost total decomposition of the original clay. This ob-
servation is consistent with the result given by the study
of the thermal stability for the clay sample. Indeed, heat-
ing the sample clay in air above 500˚C (Figure 2(b)), the
peak at 7.21 Å disappears while the peak characteristic of
illite (d = 10.05 Å) persists. The treatment with ethylene
glycol does not have any effect (Figure 2(c)).
3.1.2. Infrared Spectra
Figure 3(a) shows the Infrared spectra of purified clay
over the frequency range of 4000 - 400 cm–1. The figure
shows that purified sample contain quartz (800 cm–1).
The spectrum exhibited the characteristic band at 3697
cm–1 confirming the dominant presence of kaolinite. The
band, at 1637 cm–1 and 3450 cm–1 corresponds to the
bonding modes of absorbed and zeolitic water.
3.1.3. Chemi c al Comp osition
The purified clay sample was attacked by a mixture of
three acids (HCl, H2SO4, and HNO3). All elements were
dissolved into solution expect for the Si which was de-
termined by gravimetric method analysis. The Al and Fe
were assayed by atomic absorption spectrophotometer
(AAS Vario 6). The chemical composition data (Table 1)
indicates that the percentage of Al2O3 is 30 % confirming
that sample is kaolinitic clay
3.2. Characterization of the Kliq3P10-LDH and
3.2.1. Powder X-Ray Diffraction
Figure 1 shows the XRD patterns for the precipitates
obtained by method 1 and method 2. It is shown that the
Ksd3P10-LDH and Kliq3P10-LDH samples patterns (Fig-
ures 1(c) and 1(d)) were comparable to that pattern of
Copyright © 2011 SciRes. MSA
Using Kaolinitic Clay for Preparation of a Hydrotalcite-Like Compound
Copyright © 2011 SciRes. MSA
Figure 1. X-ray patterns of clay sample: (a) purified clay; (b) Clay + Mg(NO3)·6H2O and heated at 500˚C; (c) Ksd3P10- T150;
(d) Kliq3P10. [(K) Kaolinite, (I) Illite, (Q) Quartz, (H) hydrotalcite.
Table 1. The chemical composition of the purified clay.
% SiO2 Al2O3 Fe2O3 CaO Na2O MgO K2O Ignition loss
Clay 48.75 30 3.39 0.33 1.45 0.07 1.95 13.93
the sample prepared by the conventional method. The
Kliq3P10-LDH sample showed a layered structure as ob-
served from the peaks at 7.82, 3.89 and 2.61 Å, corre-
sponding to planes (003), (006) and (009) for a layered
hydrotalcite-like material, respectively [20]. The
Ksd3P10-LDH sample display very weak and broad peaks
at a 2θ value of 11˚ compared to the sample prepared by
coprecipitation at the same conditions (pH = 10 and
Mg2+/Al3+ = 3). Ksd3P10-LDH shows a structure different
from the previous samples; it was an ill-defined hydro-
talcite contaminated with argillaceous phase (d = 4.5Å).
3.2.2. IR Spectroscopy
The FT-IR spectra of the Ksd3P10-LDH and Kliq3P10-
LDH hydrotalcite are presented in Figures 3(b) and 3(c).
It shows a broad band around 3470 cm–1 due to the
stretching mode of the structural –OH groups in the
metal hydroxide However, a small shoulder at 2900 -
3000 cm–1 suggests the presence of a second type of –OH
stretching vibration (possibl due to hydrogen bonding
Figure 2. X-ray patterns of: (a) oriented crude simple; (b),
oriented heated sample; (c) oriented sample treated with
glycol. y
Using Kaolinitic Clay for Preparation of a Hydrotalcite-Like Compound 687
Figure 3. Infrared spectra of: (a) purified clay; (b) Ksd3P10-T150; (c) Kliq3P10.
with carbonate in the interlayer spacing [21].
The two spectra show:
1) A shoulder at 1638 cm–1 is ascribed to the bending
mode of the interlayer water molecules [22].
2) The three characteristic bands of carbonate in hy-
drotalcite at around 1384 cm–1 (ν3), 877 cm–1 (ν2) and
~1020 cm–1 (ν1) [23,24].
3) The bands around 420 and 668 cm–1, which are as-
cribed to the bending mode Al-O and Mg-O.
The infrared spectrum of the Kliq3P10-LDH shows ad-
ditional bands appearing at 1193 and 1100 cm–1 which
could not be identified.
3.2.3. Surface Area and N2 Adsorption-Desorption
The N2 adsorption-desorption isotherm is of type II for
all samples, which is typical of mesoporous materials
(Figure 4) [25]. All of the materials possessed zero mi-
cropore volume. Adsorption isotherms of this type are
represented by mesoporous materials with no micropores
and strong interactions between adsorbent and adsorbate
molecules. This type of hysteresis loop is formed when
the adsorption and desorption curves do not coincide and
is caused physically by the phenomenon of capillary
condensation in the mesopores.
From Figure 4, it was determined that all samples
shows a horizontal course of the hysteresis branch over
an appreciable range of gas uptake (p/p0 0.6), while it
is vertical above this ratio. This type of hysteresis loop is
often observed with aggregates of plate-like particles that
give rise to slit-shaped pores.
The textural properties for samples prepared by dif-
ferent method are gathered in Table 2. Starting from
these results we can conclude that Ksd3P10-LDH presents
significant textural properties. Indeed, the value of spe-
cific surface area (80 mg·g–1) is near that obtained for the
sample prepared by conventional method. While,
Kliq3P10-LDH prepared by method 2 has a very weak
value of specific surface area.
3.2.4. DSC St udy
The DSC curves of the Kliq3P10 and Ksd3P10 hydrotal-
cite prepared by mechanochemical synthesis method and
co-precipitation method usning kaolinite clay as aluminum
Copyright © 2011 SciRes. MSA
Using Kaolinitic Clay for Preparation of a Hydrotalcite-Like Compound
Figure 4. N2 adsorption-desorption isotherms: (a) Ksd3P10-
LDH; (b) Kliq3P10-LDH.
Table 2. Textural properties for various sorbent samples.
KsdP10 KLiqP10
(cm3/g) SBET
calcination 80 0.2044 78 0.6052
calcination 178 -------- 165 ------
source are shown in Figure 5. Both the DSC profiles
exhibited two apparent endothermic events during the
thermal decomposition. The first event is at 200˚C for
Ksd3P10 and at 198˚C for Kliq3P10, the second thermal
event is at 410˚C for Ksd3P10 and at 361˚C for Kliq3P10.
In the first decomposition stage, the crystal water re-
leased. And in the second decomposition stage, the hy-
droxyl (OH) octahedral structure is destroyed; the hy-
droxyl (OH) and gas of H2O and CO anion release,
and CO2, MgO and Al2O3 are formed. The second ther-
mal event for Ksd3P10 (410˚C) which more larger than
for Kliq3P10 (361˚C) indicated that the sample prepared
by kaolinite clay in natural solid state possessed high
thermal stability, this observation can be explained by the
presence of SO2 in the sample.
3.2.5. Memory Effect of Calcined Kliq3P10 and
Ksd3P10 Hydrotalcite
The most important characteristic of layered double hy-
droxides is their ability to reconstruct themselves to their
original structure. It has already been reported that
Mg-Al hydrotalcites can be reconstructed when the sam-
ples are calcined below the temperature at which spinel
formation does not take place. Figures 6 and 7 show the
XRD pattern of samples calcined at 500˚C. From the
pattern it was observed that the layered structure was
Figure 5. DSC curves of: (a) Ksd3P10-LDH; (b) Kliq3P10-
Figure 6. Powder XRD patterns of Ksd3P10-LDH (a) cal-
cined in air at 500˚C; (b) calcined at 500˚C and treated in
water for 2 days for reconstruction.
completely destroyed, which is clearly indicated by the
disappearance of the (003) and (006) peaks at lower 2θ
values in the original material (Figures 1(c) and 1(d)).
For Ksd3P10-LDH, the Figure 6(a) shows the pres-
ence of a peak at 4.5Å due argillaceous phase, which is
present as an impure phase, along with peaks due to
MgO. To find out whether the hydrotalcite synthesized
from the natural clay could be reconstructed to its origi-
nal structure, the Ksd3P10-LDH and Kliq3P10-LDH sam-
ples, which were previously calcined at 500˚C, was put
in water for 2 days. As shown in Figure 6(b) and Figure
7(b), the materials were completely reconstructed to its
Copyright © 2011 SciRes. MSA
Using Kaolinitic Clay for Preparation of a Hydrotalcite-Like Compound689
Figure 7. Powder XRD patterns of Kliq3P10-LDH (a) cal-
cined in air at 500˚C; (b) calcined at 500˚C and treated in
water for 2 days for reconstruction.
original layered structure, as indicated by the appearance
of the (003) and (006) peaks.
3.3. The Optimisation of the Operating
Conditions of Synthesis
3.3.1. The Effect of Mg2+/Al3+ Molar Ra tio
The XRD patterns of samples prepared by method 1 at
different Mg2+/Al3+ molar ratio show that no hydrotalcite
structure was formed when R < 3. From Figure 8,
well-crystalline LDH was obtained for Mg2+/Al3+ = 3.0.
From Figure 9, it was determined that as the Mg2+/
Al3+ molar ratio increases, from 0.5 to 4, the intensity of
003 and 006 reflections increases, are corresponding to
an increase of the formation of the LDH structure.
Well-crystalline LDH was obtained for Mg2+/Al3+ =1.0
(Figure 9) belong R = 1, the intensity of 003 and 006
reflections decreases corresponding to the decreasing in
3.3.2. The Effect of of pH of Synthesis
The pH of preparation is important in the formation of all
hydrotalcite-type materials and the optimum pH depends
on the cations used. From Figure 10, it was determined
that samples prepared by method 1 and at pH = 8 dis-
played very weak, broad reflections at 2θ value of 11˚
compared to Ksd3P10-LDH and Ksd3P12-LDH prepared
at pH = 10 and 12 respectively (Figure 10). The XRD
patterns of Ksd3P8 (pH = 8) show that no hydrotalcite
structure was formed under these synthesis conditions.
The broadness of the reflections indicates that the sample
was poorly crystalline; brucite was formed instead of hy-
drotalcite. Samples prepared within the pH range of 10 -
12 show patterns similar to that of hydrotalcite. The dif-
ference between these samples is in the intensity of the
Figure.8. X-ray patterns of clay sample synthesized by
method 1 at different Mg2+/Al3+ molar ratio.
Figure 9. X-ray patterns of clay sample synthesized by
method 2 at different R (R = Mg2+/Al3+ molar ratio).
Figure 10. X-ray patterns of clay sample synthesized by
method 1 at different pH.
Copyright © 2011 SciRes. MSA
Using Kaolinitic Clay for Preparation of a Hydrotalcite-Like Compound
Figure 11. X-ray patterns of clay sample synthesized by
method 2 at different pH.
(00l) reflections. Ksd3P10 have the most intense and
sharpest reflections, and thus are the most crystalline
Samples prepared by method 2 and at pH = 9 dis-
played very weak, broad reflections at 2θ value of 11˚
The broadness of the reflections indicates that the sample
was poorly crystalline. Samples prepared within the pH
range of 10 - 12 show patterns similar to that of hydro-
talcite. The difference between these samples is in the
intensity of the (00l) reflections. The Kliq3P10 has the
most intense and sharpest reflections. Kliq3P12 (pH = 12)
shows a structure different from the previous samples; it
was contaminated with Al(OH)2.
4. Conclusions
Pure and well-crystalline phases of Mg-Al-CO3 LDH can
be prepared by coprecipitation and by mechano-chemical
synthesis method from the cationic clay (the kaolinite)
using an aqueous solution of Na2CO3 as a precipitant.
The pH of preparation was an important factor. At
pH = 10 well-crystalline LDH was formed. Below this
value, the crystallinity of the LDH decreased. Strong
alkaline conditions seem no favourable for the synthesis.
The optimum values of Mg2+/Al3+ molar ratio depend of
the method of synthesize. It was about 3 for synthesize
by the method 1 and about 1 for the method 2. Below
these ratios poorly crystalline products were obtained.
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