Thermolysis of acidic aluminum chloride solution and its products

Abstract

A saturated acidic aluminum chloride solution with a total composition of AlCl3·HCl·12H2O was obtained, and its behavior under thermal treatments was studied using thermogravimetry, differential scanning calorimetry and mass spectrometry techniques. The thermolysis solid products were characterized with XRD and SEM. Four stages of the thermolysis could be distinguished. Initially, the solution lost free water molecules, and an amorphous precipitate with an approximate composition AlCl3·HCl·12 H2O was obtained as a product. The precipitate released eight water molecules in the temperature range 390–425 K. Then, all chlorine atoms in the form of HCl and two water molecules were outgassed at 425–485 K. The product completely lost water up to 650 K. The crystallization of the solid begins with appearance of the phase γ-Al2O3 at 1073 K, and the final product, α-Al2O3, is observed at 1323 K. The application of the saturated trichloride solutions as a binder and a promoter for activated sintering of composite ceramics on the base of alumina was examined.

Full text

CERAMICS
INTERNATIONAL
Available online at www.sciencedirect.com
Ceramics International 39 (2013) 3843–3848
Thermolysis of acidic aluminum chloride solution and its products
Victor V. Ivanov
a
, Sergei D. Kirik
b
, Alexander A. Shubin
a,
n
, Irina A. Blokhina
a
,
Victor M. Denisov
a
, Lilya A. Irtugo
a
a
Siberian Federal University, 79 Svobodny, Krasnoyarsk 660041, Russia
b
Institute of Chemistry and Chemical Technology SB RAS, 42K. Marx Street, Krasnoyarsk 660049, Russia
Received 12 May 2012; received in revised form 26 September 2012; accepted 18 October 2012
Available online 26 October 2012
Abstract
A saturated acidic aluminum chloride solution with a total composition of AlCl
3
HCl 12H
2
O was obtained, and its behavior under
thermal treatments was studied using thermogravimetry, differential scanning calorimetry and mass spectrometry techniques. The
thermolysis solid products were characterized with XRD and SEM. Four stages of the thermolysis could be distinguished. Initially, the
solution lost free water molecules, and an amorphous precipitate with an approximate composition AlCl
3
HCl 12 H
2
O was obtained as
a product. The precipitate released eight water molecules in the temperature range 390–425 K. Then, all chlorine atoms in the form of
HCl and two water molecules were outgassed at 425–485 K. The product completely lost water up to 650 K. The crystallization of the
solid begins with appearance of the phase g-Al
2
O
3
at 1073 K, and the final product, a-Al
2
O
3
, is observed at 1323 K. The application of
the saturated trichloride solutions as a binder and a promoter for activated sintering of composite ceramics on the base of alumina was
examined.
&2012 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Aluminum chloride; Thermolysis; Thermal analysis; Bonding material
1. Introduction
Aluminum trichloride, AlCl
3
6H
2
O, and its decomposi-
tion products are used as binding activating agents for
sintering when preparing widely used ceramics based on
aluminum oxide [1]. Salt application is used because it
makes it possible to prepare a saturated solution with a
high content of aluminum. The solution efficiently pene-
trates into the pores and covers the surface of the ceramic
ingredients, and under thermal treatment, it gives rise to
the aluminum oxide, which serves as a binding component.
The technical application of the salt is reasonable because
of its quite simple synthesis that employs inexpensive
reagents. The effective, practical application of the salt
requires an understanding of its solutions properties, the
processes of thermal decomposition and the characteristics
of the thermolysis solid products considered for this
purpose. There are some data in the literature on the
thermolysis of aluminum trichloride (AlCl
3
6H
2
O) [1–3].
The aluminum trichloride solution is a weak acid medium.
The acidity changes the viscosity and the surface properties
of the solution, which eventually influences its penetrability
for wetting the microcracks and pores. These properties
are very significant for both preparing ceramics and the
quality of the resulting ceramics. The adhesion properties
and thermal behavior of aluminum trichloride solutions
with a high acid content have not been investigated.
Therefore, it is believed that these solutions will behave
differently. It is well known that the aluminum atoms are
coordinated by oxygen in crystals of AlCl
3
6H
2
O[4]. The
chlorine ions are located in the external coordination
sphere. In salts of the apparent superacid HAlCl
4
[5], such
as LiAlCl
4
[6] and NaAlCl
4
[7], the aluminum atoms are
chemically connected through the chlorine atoms.
Because there is a practical interest in chloride aluminum
salt solutions, particularly for manufacturing aluminum wet-
table TiB
2
–Al
2
O
3
ceramics for cathodes of aluminum electro-
lyzers, the thermal behavior of saturated aluminum trichloride
acid solutions was investigated in our work.
www.elsevier.com/locate/ceramint
0272-8842/$ - see front matter &2012 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
http://dx.doi.org/10.1016/j.ceramint.2012.10.226
n
Corresponding author. Tel.: þ79050878981.
E-mail address: Ashubin@sfu-kras.ru (A.A. Shubin).

2. Experimental
The aluminum trichloride (ATC) solution was prepared
by boiling Al(OH)
3
(technical aluminum hydroxide, 48–
0114–65–91 specification) in a HCl solution ( 36%). The
solution was evaporated to a density of 1.33 g/cm
3
. Sedi-
ment appears after the density exceeds this value.
The pH was measured using an InoLAb pH-730 instru-
ment. The thermal decomposition processes were investi-
gated using thermogravimetry (TG), differential scanning
calorimetry (DSC) and gas emission mass spectrometry
(EMS) during the thermolysis using the synchronous
thermal analyzer Netzsch STA 449C combined with an
A¨
eolos QMS 403C mass spectrometer. All processes were
performed in a platinum crucible with an argon turning
atmosphere (the velocity of the stream was approximately
10–25 ml/min), and the sample heating rate was 5 K/min.
The Al:Cl ratio in the samples was evaluated using
X-ray fluorescence with an ARL Advant’X wave dispersive
spectrometer. The thermolysis products were characterized
using X-ray powder diffraction (XRD) on an X’Pert-Pro
(PANalytical) diffractometer with Cu Karadiation
(l
1
¼0.15406 ˚
A, l
2
¼0.15444 ˚
A). Diffraction patterns were
recorded over an angular range of 51–8012ywith a step
size of 0.0261. Scanning electronic microscopy (SEM) was
used to observe the morphology of the decomposition
products. The SEM images were obtained using a
JEOL JSM-7001 F instrument (with an accelerating vol-
tage of 5 kV).
IR-analyses were performed on a FTIR spectrometer
Nicolet 6700 equipped with DTGS detector and usage of
Smart Orbit single bounce diamond ATR accessory
(SibFU CEJU). The spectrum of each sample was recorded
by accumulating 32 scans at 4 sm
1
resolution between
400 and 4000 cm
1
. Investigation was held for initial ATC
and samples, which were annealed at 375, 430, 470, 600 K.
Choice of temperatures is based on TGA result (Fig. 4a).
Concern to this, mass loss during the annealing time of
each sample corresponds to mass loss results of TGA.
3. Results and discussion
Under normal conditions, the obtained aluminum chlor-
ide acid solution is a limpid glutinous yellowy liquid.
The solution density is 1.33 g/cm
3
, which is close to the
published data for a saturated aluminum chloride solution
that has a density of 1.35 g/cm
3
with an AlCl
3
content of
approximately 41 wt% [8]. The Al:Cl weight ratio (in the
dried solid sample) obtained with XRD is approximately
16:84, and the atomic ratio is 1:4 (drying occurred under
300 K). Simultaneously, the Al:Cl weight ratio for AlCl
3
is
20:80 (atomic ratio is 1:3). This result demonstrates the
excess of hydrochloric acid in the solution. One mol of
trichloride has already been found to correspond to
approximately 1 mol of hydrochloric acid. The pH¼
0.72, and all of the results described above support the
existence of chloride complexes in the solution, which are
typical for HAlCl
4
super acids derived from Lewis acids
(AlCl
3
) and Bronsted protonic acids (HCl) [9,10]:
AlCl3þHCl-HAlCl4ð1Þ
As a result of the isothermal heat treatments of alumi-
num trichloride in open air at temperatures of 423, 448,
473, 573, 873, 1073, 1223, 1323 and 1423 K for 20 h, it was
observed that the mass of the sample stabilizes at 32% of
the initial mass under 423 K. The solid residue was yellow
plate-like particles with hygroscopic properties. Further
thermal treatment at temperatures less than 573 K leads to
the residual matter of 13.5% in the form of a white, but
not hygroscopic, powder. The following stages of the heat
treatment slightly changed the mass of the sample – 13.1%
(Fig. 1). Thus, the general processes of water evaporation
and thermal decomposition with a release of volatile
products during the long-term isothermal treatment prac-
tically stops up to the temperature of 570 K.
Isothermal drying of the aluminum trichloride solution
under a temperature of 373 K leads to an approximate
52% weight loss in comparison with 5–7%, as the
thermogram showed in Fig. 4. Drying occurs with the fast
formation of a solid, dense film of salt on the surface of the
solution, and the evaporation practically stops. It takes
5–7 days to obtain yellow crystals when the film cracked
regularly. XRF analysis revealed that the Al:Cl atomic
ratio in this product is approximately 1:3. Furthermore,
the XRD analysis identified the general phase, which is
aluminum trichloride hydrate (AlCl
3
6H
2
O). As the tem-
perature increases to 423 K, the formation of a salt film
does not occur. After 20 h of heat treatment, the sample
loses approximately 68% of weight (Fig. 1) in comparison
with 43%, as the thermogram showed (Fig. 4). Once the
Al:Cl atomic ratio in the product becomes equal to 1:1.65,
its total composition can be evaluated as A1
2
O
3
3,
3HC1 H
2
O. When the conditions are close to equilibrium,
the substance loses more than one-half of chlorine, even
under 423 K, whereas the thermogram only shows the
beginning of this process under the same conditions, and it
reaches the same level when the sample is overheated
by 40–50 K. Similar results are observed for higher
temperatures: 448 K – Al:Cl (atomic ratio) ¼1:0.7 (A1
2
O
3

0
20
40
60
80
100
250 450 650 850 1050
Annealing temperature, К
weight loss, %
Fig. 1. Aluminum trichloride sample weight loss under isothermal annealing.
V.V. Ivanov et al. / Ceramics International 39 (2013) 3843–38483844

1.4HC1 0.2 6H
2
O), 473 K – Al:Cl (atomic ratio)¼1:0.5
(A1
2
O
3
HC1 0.1H
2
O). The water quantity in the total
composition expressions can be calculated as a large
number difference, and therefore, it can be substantially
underestimated when the Al:Cl ratio is determined, even
for small error (the error is no more than 5%).
The X-ray powder diffraction patterns of the samples
after each stage of annealing are shown in Fig. 2. The
typical SEM images of the powder microstructure are
presented in Fig. 3. The matter is amorphous at tempera-
ture less than 1073 K, despite the data in the literature
on the crystallization of Al(OH)
3
amorphous gels during
aging or weak heating [1]. Weak peaks appear at 1073 K,
and the set of peaks that appeared at 1223 K allows us to
reliably identify the g-Al
2
O
3
. The sample crystallizes at
1423 K to form corundum, which is in agreement with the
data reported in [1].
The powder morphology is identical at all of the
annealing temperatures. The powders are agglomerates of
lamellar structures (Fig. 3a). The thicknesses of the plate-
layers and the pore dimensions are nanoscale. As can be
observed, the plate-like particles are the product of the
quasi-liquefied state. This process results in a particle that
has a sufficiently smooth surface covered by pores. The
initiation of the pores is likely caused by water rise onto
the surface, which occurred because of substance decom-
position into the particle volume. On the particle fracture,
we can observe the morphology of the internal pores. The
pores are of an extended nature that gives a fibrous type to
the internal structure. It is obvious that the specific area of
such powders is large. This result can be used when large
dimensions of this characteristic are necessary.
Experiments, including long-term isothermal heat treat-
ments, resulted in a practically equilibrium product. The
results of such experiments are substantially different from
the thermal study data. The set of aluminum trichloride
solution thermal decomposition characteristic properties
were revealed during the kinetic experiments in the low-
temperature range (Fig. 4). General dehydration and
decomposition processes occur up to 550–600 K. The TG
curves have a complicated form with some specific sections
of sample weight decrease. The DSC curves with a set
of consistent endothermic peaks correspond to these TG
dependences. The increased rate of heating up to 366 K
10 15 20 25 30 35 40 45 50 55 60 65 70
2Theta (°)
0
1000
2000
3000
4000
5000
6000
7000
Intensity (counts)
573K
873K
1073K
1223K
1323K
1423K
*
*
**
*
*
*
*
*
*
*
*
*
C
C
C
C
C
C
C
CC
Fig. 2. X-ray powder patterns of thermolysis products at different temperatures; n– peaks of g-Al
2
O
3
,C–a-Al
2
O
3
.
Fig. 3. SEM images of the final thermolysis products: (a) a-Al
2
O
3
after sintering at 1423 K, (b) – g-Al
2
O
3
on TiB
2
as a substrate after 2 h at 1123 K.
V.V. Ivanov et al. / Ceramics International 39 (2013) 3843–3848 3845

(Fig. 4a) results in a weight decrease of approximately
3–5%, which is accompanied by both a broad endothermic
effect and an EMS-peak that arose from the free water
evolving. The stepped weight change of 37.6% is accom-
panied by a substantial endothermic effect, a peak result-
ing from the free water evolving (peak max at 417 K), and
the starting hydrogen chloride evolving between 390 and
440 K. A weight decrease of approximately 34% with
substantial heat absorption and H
2
O and HCl release
occurs between 440 and 470 K. However, the DSC peaks
are caused by heat absorption, and the peaks of H
2
O and
HCl release are related to the temperature of approxi-
mately 460 K. The water continues evolving at tempe-
ratures higher than 470 K up to 550 K. Substantially
intensive chlorine hydride evolving occurs between 425
and 485 K. The track of ions with weight corresponded to
molecular chlorine, which can also be observed in this
temperature range. Upon reaching 600–650 K, the sample
weight was 17% of the initial weight. The weight is
appropriately less than that during the isothermal anneal-
ing. The thermogram obtained up to 1100 K shows a good
agreement between the solid residue and the isothermal
annealing products (13.07%).
Dependencies obtained for the different heating rates are
the same, and they have characteristic points that are
substantially shifted to lower temperatures for heating
rates of 1 K/min. Both the low-temperature range of these
curves and the DSC peak splitting for 459 K are different
for the described thermogram.
The chemical composition of the close to saturation
ATC solution can be expressed in terms of the conditional
balance formula AlCl
3
HCl 12H
2
O. This formula was
deduced from the XRF analysis data, pH of the solution
and the weight of the dried solid data, which was obtained
from the long-time high-temperature annealing. The
obtained substance thermally decomposes under very slow
heating in accord with the total chemical equation:
AlCl3UHClU12H2O!
H370 K AlCl3U6H2OþHClm
þ6H2Om!
H570 K 1
2A12O3þ6HClmþ9H2OmðÞð2Þ
When the rate of heating is comparatively large (5 K/
min), the AlCl
3
HCl 12 H
2
O compound loses eight mole-
cules of water during the first stage of decomposition
(390–425 K):
AlCl3UHClU12H2O)
390425 K AlCl3UHClU4H2Oþ8H2Om
ð3Þ
The weight loss and the composition of the gases before
HCl began releasing can be concluded from the data.
The calculations from the TG and EMS curves indicate
that the consequent heating for the temperature range of
425–485 K leads to the elimination of 3 HCl molecules and
2H
2
O molecules with formation of Al(OH)
2
Cl:
AlCl3UHClU4H2O)
425485 K AlðOHÞ2Clþ3HClmþ2H2Om
ð4Þ
When the heating temperature is increased to high
values, the Al(OH)
2
Cl decomposition occurs and is accom-
panied by the slow elimination of water and HCl. The
concentrations of H
2
O and HCl in the gas phase are likely
too low to be detected using our equipment.
Inherently proposed reactions schemes of thermolysis rely
on results of TGA and EMS investigations. The structural
peculiarity of the formed compounds is not considered in
Fig. 4. TG, DSC and EMS curves of ATC heating for 5 (a) and 1 (b) K/min rates.
V.V. Ivanov et al. / Ceramics International 39 (2013) 3843–38483846

this paper and need separate researches with involvement of
complementary methods. Nevertheless, we can match the
fact that with accordance to IR-spectroscopy investigation
during annealing process, we can observe Al–Cl and
Al–O modes.
The appearance of 500, 560 and 815 cm
1
modes is
observed at 375 and 430 K (Fig. 5). These lines don’t take
place in initial ATC IR-spectra because of a substantial
water presence. The mode 500 cm
1
concerns to Al–Cl [11]
and 560 cm
1
to Al–OH mode according to [12]. The
mode at 815 cm
1
may be define as stretching (AlO
6
)or
(AlO
4
)[13]. To the extent of the annealing temperature
enhancement up to 470 K and higher intensity of modes,
which were noted before, are reduced. The same situation
takes place for 1630 cm
1
mode, which one corresponds
to water. Appearance of reflexes and reducing of their
intensity can be an evidence for processes (3) and (4).
It is obvious that both the described sequence of stages
and the intermediate products have a very relative nature
that is only approximately satisfied by the obtained TG and
DSC curves. Moreover, it is necessary to take into account
that these measurements do not relate with the equilibrium
state and that the transformation temperature ranges depend
on the increasing temperature rate. Indeed, the actual pro-
cess is more complicated; at the different temperatures, the
products could be mixtures with different basicity and
watering. Nevertheless, such data are very interesting for
the thermolysis process analysis of this binding material
during the synthesis of ceramics and composite materials.
The thermolysis process of AlCl
3
6H
2
O leads to the
formation of intermediate (Al
2
(OH)
5
Cl, Al(OH)
2
Cl etc.)
compounds [2]. The decomposition products have an Al:Cl
ratio of approximately 1.1–2.3. The mixture of hydro-
chlorides was identified during the isothermal treatment of
AlCl
3
6H
2
O at 438 K [2]:
AlCl3U6H2O)
438KA12O3UxHC1UyH2Oð5Þ
where: 1oxo2, yE2. The slow heating of AlCl
3
6H
2
O
up to 543 K [3] leads to the formation of the water
soluble basic chloride Al
2
O
3
2HCl 2H
2
O (or Al(OH)
2
Cl 
0,5H
2
O). These coefficients appropriately exceed both the
data from [2] and our results from the isothermal heat
treatments.
The thermal transformations of the hydroxychlorides also
are presented by means of the scheme where the hydroxy-
chlorides are appropriately more stable with greater basicity
at low temperatures:
Al2ðOHÞ5Cl )
543 K AlðOHÞ2Cl þAlOðOHÞþH2Oð6Þ
2AlðOHÞ2Cl )
723 K Al2O3þ2 HClþH2Oð7Þ
Here, the temperature range of 473–723 K is presented
for total decomposition equation of full AlCl
3
6H
2
Oto
aluminum oxide.
The complicated thermolysis process of aluminum
trichloride through the formation of intermediate hydro-
xychlorides is described in strikingly different ways. These
processes are likely caused by the differences in the heating
rates of the samples, samples prehistory, and the amount
and nature of the impurities. The results of the ATC
decomposition for the kinetic experimental conditions
during the thermal analysis are obtained in the present
work, and there is good agreement between our results and
the results from other works for preparative trichloride
AlCl
3
6H
2
O thermolysis.
By comparing the data obtained from ATC, it is
necessary to conclude that the processes that take place
under the studied solution heating substantially depend on
the heating rate, and these processes can be presented
as the set of transformations that included free water
evaporation, step-by-step crystalline water dehydration,
trichloride hydrolysis due to crystalline water, HCl elim-
ination, residual water and HCl gradual elimination up to
1300 K with chemical crystal transformation.
4. ATC application
It is well known that solutions of aluminum trichloride and
hydroxychlorides as well as aluminum trichloride thermolysis
intermediate products have the properties of low-temperature
bands and high-temperature cements for engineering applica-
tions during the production of ceramics and mineral compo-
sites. An ATC solution in the described nature was tested for
composite materials on basis of titanium diboride TiB
2
/
Al
2
O
3
.TiB
2
–Al
2
O
3
–ATC compositions with various TiB
2
and Al
2
O
3
powder ratios and ATC contents within 2–
10 wt% were mixed thoroughly. The samples were molded
by pressing. After air drying (473 K) and sintering (1123 K,
for 2 h in a closed container under the carbonic fill), the
materials had a compression strength of 20–100 MPa with a
relative density of 0.58–0.62. In all cases, the technological
stability of ‘‘young’’ stock materials was demonstrated. This
result confirms the adhesive capacity of ATC in contrast to
aluminum trichloride solutions. In Fig. 3(b), the SEM image
shows the microstructure of aluminum oxide obtained from
Fig. 5. IR spectra of ATC which was annealed at different temperatures.
V.V. Ivanov et al. / Ceramics International 39 (2013) 3843–3848 3847

ATC on the TiB
2
substrate. Evenly deposited aluminum
oxide created a surface coating with needle-shaped dendrites
that had branch lengths less than 100 nm. The large specific
surface is responsible for the high binding property. The
solutions of hydroxychlorides, which are the products of
Al(OH)
3
and HCl reaction or half-way AlCl
3
6H
2
Other-
molysis, also have the adhesive capacity of the low-
temperature band. However, such bands are poorly applic-
able because of substantial viscosity, low aluminum content,
and short time stability. The ATC solution is free from all
shortcomings described above, and it has unlimited time
stability because four years of storage did not reveal any
change.
5. Conclusions
The close to saturation acidic solution of aluminum
trichloride with the general formula AlCl
3
HCl 12H
2
O
has both low-temperature bands and high-temperature
cement properties during the preparation of some compo-
site powdered materials, specifically based on titanium
diboride and aluminum oxide. The thermolysis processes
of such solutions substantially depend on the heating rate.
These processes occur through the formation of intermedi-
ate hydroxychlorides and stop at approximately 570 K,
which is similar to that for the AlCl
3
6H
2
O salt decom-
position. Isothermal drying at 370 K results in the
formation of AlCl
3
6H
2
O. Crystals of g-Al
2
O
3
begin to
appear at 1073 K, and then, the a-Al
2
O
3
final product
forms at 1323 K. Moreover, g-Al
2
O
3
can be promising as
catalyst supports for the active phase in heterogeneous
catalysis because of the high specific surface area in
g-Al
2
O
3
that was prepared by heat treatments at approxi-
mately 1220 K.
Acknowledgments
This work has been performed under 2.1.2/780 project
of analytic departmental special-purpose programme ‘‘High
school scientific potential development (2009–2010) and
State contract no. 02.740.11.0269 (the Ministry of Educa-
tion and Science Russian Federation)’’
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