Thermal decomposition of the layered double hydroxides of formula Cu 6Al2(OH)16CO3 and Zn 6Al2(OH)16CO3

Article abstract of DOI:10.1007/s10973-008-9169-x

Zn-Al hydrotalcites and Cu-Al hydrotalcites were synthesised by coprecipitation method and analysed by X-ray diffraction (XRD) and thermal analysis coupled with mass spectroscopy. These methods provide a measure of the thermal stability of the hydrotalcit

Full text of DOI:10.1007/s10973-008-9169-x

Journal of Thermal Analysis and Calorimetry, Vol. 96 (2009) 2, 481–485
THERMAL DECOMPOSITION OF THE LAYERED DOUBLE
HYDROXIDES OF FORMULA Cu Al (OH) CO AND Zn Al (OH) CO
3
6
2
16
3
6
2
16
1
,2
1,2
1
1
N. Voyer , A. Soisnard , Sara J. Palmer , W. N. Martens and R. L. Frost
1*
1
Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology
2
George Street, Brisbane, GPO Box 2434, Queensland 4001, Australia
2
ENSICEN, Ecole Nationale Supérieure d’Ingénieurs et Centre de Recherche, 6 Boulevard Maréchal Juin, 14050 Caen Cedex, France
Zn–Al hydrotalcites and Cu–Al hydrotalcites were synthesised by coprecipitation method and analysed by X-ray diffraction (XRD)
and thermal analysis coupled with mass spectroscopy. These methods provide a measure of the thermal stability of the hydrotalcite.
The XRD patterns demonstrate similar patterns to that of the reference patterns but present impurities attributed to Zn(OH)
2
and
Cu(OH) . The analysis shows that the d003 peak for the Zn–Al hydrotalcite gives a spacing in the interlayer of 7.59  and the esti-
2
mation of the particle size by using the Debye–Scherrer equation and the width of the d003 peak is 590 . In the case of the Cu–Al
hydrotalcite, the d003 spacing is 7.57  and the size of the diffracting particles was determined to be 225 .
The thermal decomposition steps can be broken down into 4 sections for both of these hydrotalcites. The first step decompo-
sition below 100°C is caused by the dehydration of some water absorbed. The second stage shows two major steps attributed to
the dehydroxylation of the hydrotalcite. In the next stage, the gas CO
actions occur over 400°C and involved CO evolution in the decomposition of the compounds produced during the
dehydroxylation of the hydrotalcite.
2
is liberated over a temperature range of 150°C. The last re-
2
Keywords: dehydration, dehydroxylation, hydrotalcite, thermal stability, thermogravimetric analysis
Introduction
Thermal analysis using thermogravimetric tech-
niques (TG) enables the mass loss steps, the tempera-
ture of the mass loss steps and the mechanism for the
mass loss to be determined [4, 5]. Thermoanalytical
methods provide a measure of the thermal stability of
the hydrotalcite [6–8]. The hydrotalcite series involv-
In nature a group of minerals is found, based upon the
brucite structure (Mg(OH) ), in which the divalent cat-
2
2
+
3+
ion Mg is replaced by a trivalent cation (Al , Fe or
3+
3+
Cr ), resulting in a positive charge on the brucite-like
surface. This positive charge is counterbalanced by an-
ions held within the brucite interlayer. These minerals
are known as hydrotalcites and layered double hydrox-
ides (LDHs) and are fundamentally anionic clays. The
structure of hydrotalcite is derived from the brucite
2
+
2+
ing Zn and Cu mineral series is of interest because
such minerals may have photocatalytic potential
[9, 10]. Interest in the study of these hydrotalcites re-
sults from their potential use as catalysts, adsorbents
and anion exchangers [11–15]. The reason for the po-
tential application of hydrotalcites as catalysts rests
with the ability to make mixed metal oxides at the
atomic level, rather than at a particle level. Such
mixed metal oxides are formed through the thermal
decomposition of the hydrotalcite [16, 17]. One
would expect that the potential application of
hydrotalcites as catalysts will rest on reactions occur-
ring on their surfaces.
3+
3+
structure (Mg(OH) ) in which e.g. Al or Fe
2
2+
(pyroaurite-sjögrenite) substitutes for part of the Mg .
When LDHs are synthesised any appropriate anion can
be placed in the interlayer. Included anions may be car-
bonate, chloride, sulphate, nitrate or any mixture of an-
ions. This substitution creates a positive layer charge on
the hydroxide layers, which is compensated by
interlayer anions or anionic complexes [1, 2]. The
hydrotalcite may be considered as a gigantic cation,
which is counterbalanced by anions in the interlayer. In
hydrotalcites a broad range of compositions are possible
Thermal analysis is a technique for the measure-
ment of the thermal stability of LDH’s [4–6, 18–24].
In this work we report the stability and thermal de-
2
+
3+
of the type [M1–x M (OH) [A ] ·yH
x+
n
2+ 2+
composition of Zn and Cu based LDH’s with car-
O, where
x
2]
x/ n
2
2+
3+
M and M are the di- and trivalent cations in the octa-
bonate in the interlayer.
hedral positions within the hydroxide layers with x nor-
n–
mally between 0.17 and 0.33. A is an exchangeable
interlayer anion [3].
*
Author for correspondence: r.frost@qut.edu.au
1
388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
©
2009 Akadémiai Kiadó, Budapest

VOYER et al.
Experimental
Synthesis of hydrotalcite minerals
Hydrotalcites with a composition of Cu Al (OH) CO
6 2 16 3
and Zn Al (OH) CO were synthesised by the
6 2 16 3
coprecipitation method. Two solutions were prepared
using boiled ultra pure water: Solution 1 contained 2M
NaOH and 0.125M Na CO while solution 2 contained
.75M Cu (Cu(NO ) · 2.5H O) (or 0.75M Zn
2
3
2+
2+
0
3
2
2
3+
(
Zn(NO ) ·6H O)) and 0.25M Al (Al(NO ) ·6H O).
3 2 2 3 3 2
Solution 2 in the appropriate ratio was added to solu-
–1
3
tion 1 using a peristaltic pump at a rate of 25 cm min ,
under vigorous stirring. The precipitated minerals were
washed via vacuum filtration at ambient temperature
with a solution of 0.1M Na
nitrate. The hydrotalcites were then centrifuged for
0 min, before being washed again with boiled ultra
2 3
CO to remove any residual
1
pure water.
X-ray diffraction
X-ray diffraction patterns were collected using a
Philips X’pert wide angle X-ray diffractometer, oper-
ating in step scan mode, with CoK
1.78897 ). Patterns were collected in the range 3 to
5° 2q with a step size of 0.02° and a rate of 1.2 s per
a
radiation
(
7
Fig. 1 X-ray diffraction patterns of the synthesized
step. Samples were prepared in ethanol and placed on
glass slides as thin films.
hydrotalcites
patterns are not quantitative. The Zn(OH) is more
2
Thermal analysis
crystalline as evidenced by the width of the peaks and
thus will give a more intense pattern. It is unlikely
that ZnO is present in the prepared mineral without
heating. The d003 peak for the Zn–Al hydrotalcite
gives a spacing of 7.59 . By using the
Debye–Scherrer equation and the width of the d003
peak, an estimate of the particle size of the Zn–Al
hydrotalcite can be made. It was found to be 590 .
The pattern for the Cu–Al hydrotalcite shows an
identical pattern to that of the reference patterns for
the Cu–Al hydrotalcite (PDF 46–99). The broad fea-
ture at about 15 degrees two theta may be attributed to
Thermal decompositions of the LDH’s were carried
®
out in a TA Instruments incorporated high-resolu-
tion thermogravimetric analyser (series Q500) in a
3 –1
flowing nitrogen atmosphere (80 cm min ). Approx-
imately 50 mg of sample was heated in an open plati-
1
–
num crucible at a rate of 2.0°C min up to 1000°C.
The TG instrument was coupled to a Balzers (Pfeif-
fer) mass spectrometer for gas analysis. Several gases
(
and isotopic analogues) and their ionic fragments
were analysed including: Cl , CO, CO , HCl and H O.
2 2 2
an impurity of Cu(OH) . The d003 spacing for the
2
Cu–Al hydrotalcite is 7.57 . The size of the diffract-
ing particles of the Cu–Al hydrotalcite was deter-
mined to be 221 .
Results and discussion
X-ray diffraction
The XRD patterns of the synthesised hydrotalcites to-
gether with the reference patterns are shown in Fig. 1.
In the synthesis of hydrotalcites, it is not unexpected
to have traces of impurities present either of the start-
ing materials, by-products of the reaction or as syn-
thesised impurities. In the case of the Zn–Al
hydrotalcite the pattern matched that of the standard
reference pattern together with the additional pattern
Thermogravimetric analysis
Zn–Al hydrotalcite
The thermogravimetric analysis of the Zn–Al hydro-
talcite is shown in Fig. 2a and the ion current curves
in Fig. 2b. The thermal decomposition steps in this
analysis can be broken down into sections (a) below
1
00°C (b) in the 100 to 130°C temperature range
of Zn(OH) . It should be remembered that the XRD
2
4
82
J. Therm. Anal. Cal., 96, 2009

6 2 3 6 2 3
Cu Al (OH)16CO AND Zn Al (OH)16CO
Zn Al (OH)16CO ®
6 2 3
yAl (CO ) +(1–y)Al O +(6–x)ZnO+
2 3 3 2 3
+
xZnCO +8H O
3 2
It is noted that the m/z=17 and 18 match the DTG
curves. Thus the thermal decomposition at 106
and 127°C is attributed to the dehydroxylation of
the Zn–Al hydrotalcite. No loss of CO
served at these temperatures. The total mass loss
for the decomposition steps at 106 and 127°C is
3.9+6.9)%. It is proposed that adsorbed water and
2
is ob-
(
Fig. 2a TG and DTG of Zn–Al hydrotalcite
intercalated water is lost at these temperatures.
The 10.4% mass loss is observed in the tempera-
ture range between 150 and 450°C. It is suggested
that dehydroxylation of the hydrotalcite takes
place over this temperature range. Simultaneosuly
a small amount of CO
metal carbonates occurs. The theoretical mass loss
based upon the formula Zn Al (OH) CO is
2
is lost and the formation of
6
2
16
3
1
8.51%. Thus the experimental mass loss closely
approaches that of the theoretical mass loss based
upon the above formula. This data suggests that
the OH units are lost before the decomposition of
2
the (CO ) units.
–
3
(
c) The ion current curve for m/z=44 (CO ) indicates
2
that the evolved gas CO is liberated over a tem-
2
perature range centred upon 215°C. The experi-
mental mass loss over this temperature range is
3
.6%. The theoretical mass loss at 158 and 215°C
is 5.65%. The difference in values may be attrib-
uted to the presence of some impurities and also
some CO may be liberated simultaneously with
the OH units.
d) The two higher temperature mass loss steps at 554
and 735°C also involve CO evolution. It is proba-
2
(
2
ble that the reactions are as follows:
xZnCO ®xZnO+xCO at 554°C
3 2
and
Fig. 2b Ion current curves of evolved gases of Zn–Al
yAl (CO ) ®yAl O +3yCO at 735°C.
2 3 3 2 3 2
hydrotalcite
Most of the CO is lost at these temperatures.
2
(
c) in the 130 to 300°C temperature range and (d) in
the 500 to 800°C temperature range.
(a) Below 100°C, dehydration occurs as is observed in
the ion current curves where for m/z=18 a peak at
58°C is observed. This reaction may be written as:
Cu–Al hydrotalcite
The thermogravimetric analysis of the Cu–Al hydro-
talcite is shown in Fig. 3a and the ion current curves
in Fig. 3b. The thermal decomposition steps in this
analysis can be broken down into sections (a) below
~
Zn Al (OH)16CO ·xH O®Zn Al (OH)16CO +xH O
6 2 3 2 6 2 3 2
1
00°C (b) in the 100 to 150°C temperature range
c) in the 150 to 350°C temperature range and (d) in
the 425 to 820°C temperature range.
a) Below 100°C, dehydration occurs as is observed in
the ion current curves where for m/z=18 a peak at
The very small peak in the DTG curve shows only
a small amount of water is lost. The calculated
mass loss is <0.7%. This is attributed to the mass
loss of some adsorbed water.
(
(
(
b) Two major thermal decomposition steps occur at
~50°C is observed. This reaction may be written as:
1
06 and 127°C with mass losses of 6.9 and 10.4%.
A possible reaction is as follows:
J. Therm. Anal. Cal., 96, 2009
483

VOYER et al.
Cu
Al (OH)16CO ®
6 2 3
yAl (CO ) +(1–y)Al O +(6–x)CuO
2 3 3 2 3
+
xCuCO +CO +8H O
3 2 2
It is noted that the m/z=17 and 18 match the DTG
curves. Thus the thermal decomposition at 110
and 140°C is attributed to the dehydroxylation of
the Cu–Al hydrotalcite. Only some small amounts
of CO
loss for the decomposition steps at 110 and 145°C
is (6.4+6.2)%=12.6%. The theoretical mass loss
based upon the formula Cu Al (OH)16CO is
8.77%. Thus the experimental mass is less than
the theoretical mass loss based upon the formula
Cu Al (OH) CO . This data suggests that some
2
are lost at 140°C (Fig. 3b). The total mass
6
2
3
Fig. 3a TG and DTG of Cu–Al hydrotalcite
1
6
2
16
3
CO is evolved simultaneously with the water
2
vapour.
(
c) The ion current curve for m/z=44 (CO ) indicates
2
that the evolved gas CO is liberated over a tem-
2
perature range centred upon 513°C. The experi-
mental mass loss over this temperature range is
3
.3%. The theoretical mass loss at 158 and 215°C
is 5.70%. The difference in values is attributed to
the mass of CO at the same time as the evolution
of water vapour.
2
(
d) A number of thermal decomposition steps are ob-
served at 354, 479, 517°C. The shape of the DTG
curve resembles that of carbonate decomposition.
It is probable that the reactions are as follows:
xCuCO ®xCuO+xCO at ~479°C
3 2
and
yAl (CO ) ®yAl O +3yCO at 517°C.
2 3 3 2 3 2
Conclusions
Using XRD combined with thermal analysis, the de-
composition of the layered double hydroxides of for-
mula Cu Al (OH) CO and Zn Al (OH) CO were
6
2
16
3
6
2
16
3
studied. The XRD patterns showed that the Zn–Al
hydrotalcites and Cu–Al hydrotalcites synthesised
have similar patterns to that of the reference patterns
of the Zn–Al hydrotalcites (PDF 48-100220) and
Cu–Al hydrotalcites (PDF 46-99), but they present
Fig. 3b Ion current curves of evolved gases of Cu–Al
hydrotalcite
Cu Al (OH) CO ·xH O®
6 2 16 3 2
Cu Al (OH) CO +xH O
6 2 16 3 2
impurities attributed to Zn(OH) and Cu(OH) . Both
2 2
The very small peak in the DTG curve shows only
a small amount of water is lost. The calculated
mass loss is <0.1%. This is attributed to the loss of
some adsorbed water.
of these hydrotalcites have a close spacing respec-
tively 7.59 and 7.57 , but the use of Cu reduced the
particle size from 590 to 221 .
During the thermal decomposition of the hydro-
talcites, four steps are identified, (a) below 100°C, de-
hydration occurs with the loss of some adsorbed wa-
ter. (b) Beyond 100°C, two major thermal decomposi-
tion steps are attributed to the dehydroxylation of
(
b) Two major thermal decomposition steps occur at
1
10 and 145°C with mass losses of 6.4 and 6.2%.
A possible reaction is as follows:
hydrotalcite. No loss of CO
2
is observed at these tem-
4
84
J. Therm. Anal. Cal., 96, 2009

6 2 3 6 2 3
Cu Al (OH)16CO AND Zn Al (OH)16CO
peratures. (c) The gas CO is liberated over a tempera-
2
11 J. T. Kloprogge and R. L. Frost, Appl. Catal.,
A: General, 184 (1999) 61.
ture of 150°C. (d) The last reactions occur over 400°C
1
2 A. Alejandre, F. Medina, X. Rodriguez, P. Salagre,
Y. Cesteros and J. E. Sueiras, Appl. Catal.,
B 30 (2001) 195.
and involved CO evolution in the decomposition of
2
the products produced during the dehydroxylation of
the hydrotalcite. Both hydrotalcites showed similar
thermal stability.
1
1
3 J. Das and K. Parida, React. Kinet. Catal. Lett.,
6
9 (2000) 223.
4 S. H. Patel, M. Xanthos, J. Grenci and P. B. Klepak,
J. Vinyl Addit. Technol., 1 (1995) 201.
Acknowledgements
15 V. Rives, F. M. Labajos, R. Trujillano, E. Romeo,
C. Royo and A. Monzon, Appl. Clay Sci., 13 (1998) 363.
16 F. Rey, V. Fornes and J. M. Rojo, J. Chem. Soc., Faraday
Trans., 88 (1992) 2233.
The financial and infra-structure support of the Queensland
University of Technology Inorganic Materials Research Pro-
gram of the School of Physical and Chemical Sciences is
gratefully acknowledged. The Australian Research Council
17 M. Valcheva-Traykova, N. Davidova and A. Weiss,
J. Mater. Sci., 28 (1993) 2157.
(
ARC) is thanked for funding the thermal analysis facility.
18 R. L. Frost, J. M. Bouzaid, A. W. Musumeci,
J. T. Kloprogge and W. N. Martens, J. Therm. Anal. Cal.,
8
6 (2006) 437.
9 R. L. Frost and Z. Ding, Thermochim. Acta,
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0 R. L. Frost and K. L. Erickson, Thermochim. Acta,
21 (2004) 51.
1
2
2
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Received: April 8, 2008
Accepted: July 15, 2008
Online First: January 12, 2009
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DOI: 10.1007/s10973-008-9169-x
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J. Therm. Anal. Cal., 96, 2009
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