Thermal transformations of aluminium nitrate hydrate

Article abstract of DOI:10.1016/S0040-6031(01)00703-1

By means of the thermogravimetric (TG) method, and differential thermal analysis (DTA) with simultaneous chemical analysis of the gaseous reaction products, X-ray diffraction phase analysis, and infrared spectroscopy, stages of decomposition of Al(NO₃)₃·9H₂O were studied. It has been found that the thermal decomposition of Al(NO₃)₃.9H₂O proceeds in a few stages. First stage is connected with partial dehydration of the compound without decomposition of the nitrate groups. The successive stages comprise the hydrolytic processes, which are connected with further dehydration and loss of the nitrate groups. The process proceeds with the formation of amorphous alumina.

Full text of DOI:10.1016/S0040-6031(01)00703-1

Thermochimica Acta 385 (2002) 73–80
Thermal transformations of aluminium nitrate hydrate
Barbara Pacewska*, Mohamed Keshr
Faculty of Building Engineering, Mechanics, and Petrochemistry, Institute of Chemistry, Warsaw University of Technology,
ul. Łukasiewicza 17, 09-400 Płock, Poland
Received 4 April 2001; received in revised form 31 July 2001; accepted 31 July 2001
Abstract
By means of the thermogravimetric (TG) method, and differential thermal analysis (DTA) with simultaneous chemical
analysis of the gaseous reaction products, X-ray diffraction phase analysis, and infrared spectroscopy, stages of decomposition
of Al(NO3)3ꢀ9H2O were studied. It has been found that the thermal decomposition of Al(NO3)3ꢀ9H2O proceeds in a few
stages. First stage is connected with partial dehydration of the compound without decomposition of the nitrate groups. The
successive stages comprise the hydrolytic processes, which are connected with further dehydration and loss of the nitrate
groups. The process proceeds with the formation of amorphous alumina. # 2002 Elsevier Science B.V. All rights reserved.
Keywords: Aluminium nitrate hydrate; Thermal transformations
1. Introduction
tion of the mechanism of thermal decomposition of the
salt is very important for manufacture of alumina of
precisely specified properties. The properties of the
obtained alumina must be correlated not only with
such properties of the starting material, as its crystal
structure or grain size, but also with the mechanism of
decomposition of the given salt and the recrystalliza-
tion processes that proceed in alumina as a function of
temperature [4].
The aim of this work has been to study the mechan-
ism of thermal decomposition of aluminium nitrate
hydrate. The decomposition of this salt proceeds at
rather low temperatures. This fact gives a possibility to
obtain aluminium oxides of well-developed specific
surface, that can be used in catalysis or in sorption
processes. Combined thermoanalytical methods, com-
parative structural and morphological studies show
different mechanisms of product formation.
With total annual production of more than 6.5
million tonnes, the oxides, oxyhydroxides and hydro-
xides of the elements aluminium, silicon and titanium
are undoubtedly among the most important industrial
inorganic chemicals. The aluminium oxides, oxyhydr-
oxides and hydroxides are the second most important
refined products of the natural mineral bauxite [1,2].
The aluminium oxides are used as catalysts, catalyst
supports, adsorbent and ceramic materials. This spec-
trum of applications requires different properties, such
as particle size, morphology, surface, etc. Among the
three mentioned metals, aluminium offers the richest
variety of Al–O and Al–O–H phases [2,3].
The thermal decomposition of aluminium salt is one
of the essential steps in the technology of Al2O3
production by the acidic method. A precise recogni-
The use of aluminium nitrate as a raw material for
the manufacture of hydrous aluminium oxides by
hydrolysis has the advantage of easy removal of nitrate
* Corresponding author. Fax: þ48-24-262-3691.
E-mail address: ich@zto.pw.plock.pl (B. Pacewska).
0040-6031/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 4 0 - 6 0 3 1 ( 0 1 ) 0 0 7 0 3 - 1

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B. Pacewska, M. Keshr / Thermochimica Acta 385 (2002) 73–80
anion from the reaction mixture and yielding a final
product of high purity [5].
MENTS adapted for simultaneous recording of DTA
and TG curves, coupled on-line with mass spectro-
meter QMS Thermostar produced by BALZERS. The
samples (20 mg) were heated at the rate of 5 8C/min.
The results are shown in Fig. 1.
2. Experimental conditions and results
Irrespective of the thermal analyses, the X-ray
phase analyses, and the IR analyses were performed
both for the initial samples and for the products of their
decomposition for 1 h, in air, at definite temperatures
120, 200, 400, and 550 8C, in a high-temperature
reactor (Czylok).
The material used in the experimental work was
Al(NO3)3ꢀ9H2O, analytical grade, produced by
POCH, Gliwice, Poland.
The mechanism of thermal decomposition was
studied by thermal analysis coupled with mass spec-
trometry of the gaseous decomposition products,
infrared spectroscopy, and X-ray phase analysis.
The initial samples Al(NO3)3ꢀ9H2O polythermal
differential thermal analysis (DTA) and thermogravi-
metric (TG) studies were carried out with simulta-
neous chemical analysis of the gaseous reaction
products with the use of a quadrupole mass spectro-
meter. The instrument was SDT 2960 TA INSTRU-
The thermal analyses involved the TG, differential
thermogravimetry (DTG), and DTA measurements
using Derywatograf C produced by MOM (Hungary).
The samples (100 mg) were heated in air atmosphere
at the rate of 10 8C/min in temperature range 20–
1000 8C. Pure alumina was used as the reference. The
TG, DTG, and DTA curves obtained in the measure-
ments have been shown in Figs. 2–4, respectively.
The IR spectra were taken within 4000–400 cmꢁ1
using Specord 75 IR spectrophotometer produced by
Carl Zeiss, Jena. The results are shown in Fig. 5.
The phase composition of the samples was determi-
ned by qualitative X-ray phase analysis using HZG-4C
diffractometer with Cu cathode (Cu Ka radiation)
with Ni filter. The measurements were carried out
Fig. 1. TG and mass spectrometric analysis of aluminium nitrate
hydrate.
Fig. 2. TG curves of aluminium nitrate hydrate and products of its
decomposition at various temperatures.

B. Pacewska, M. Keshr / Thermochimica Acta 385 (2002) 73–80
75
Fig. 4. DTA curves of aluminium nitrate hydrate and products of
its decomposition at various temperatures.
the curves that represent the ionic currents in the mass
spectrophotometer for the following M/q ratios: 18
(H2O), 32 (O2), 44 (N2O), 30 (NO), 46 (NO2), 63
(HNO3), and 28 (N2) (M: molar mass, q: ionic charge
in electron charge units), and this reflects the quali-
tative composition of the gaseous products of disso-
ciation of the initial compound.
Fig. 3. DTG curves of aluminium nitrate hydrate and products of
its decomposition at various temperatures.
in 0.018 steps. The time of pulse counting was 2 s. The
results have been shown in Fig. 6.
The specific surface (SBET) determined by the volu-
metric method of low-temperature adsorption of nitro-
gen is shown in Fig. 7.
The DTA curve shows four endothermic effects
with the extremes at 70, 80, 110, and 130 8C. All
these effects (except that at 80 8C being due to melting
of the salt) are associated with mass losses.
Analysis of the gaseous reaction products shows
that only dehydration occurs in the first step of decom-
position proceeding below 80 8C. The sample looses
about 11.4% of its mass, which corresponds to the
loss of 2.5 molecules of water from the outer coordi-
nation sphere, without breaking the structure of the
nitrate groups. The successive steps of decomposition
3. Discussion of results
Fig. 1 shows the TG and DTA curves obtained in the
thermal decomposition of Al(NO3)3ꢀ9H2O, along with

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B. Pacewska, M. Keshr / Thermochimica Acta 385 (2002) 73–80
Fig. 5. IR spectra of aluminium nitrate hydrate and products of its decomposition at various temperatures.
Fig. 6. X-ray diffraction spectra of aluminium nitrate hydrate and products of its decomposition at various temperatures.

B. Pacewska, M. Keshr / Thermochimica Acta 385 (2002) 73–80
77
Fig. 7. SBET of aluminium nitrate hydrate and products of its decomposition at various temperatures.
proceeding above 80 8C are connected with further
dehydration of the salt, and simultaneous destruction of
the nitrate groups. The salt melts at 80 8C. The mecha-
nism of the stepwise dehydration may be explained by
assuming the following formula to the initial hydrate
[Al(H2O)6](NO3)3ꢀ3H2O [6].
process. In the case of aluminium cation, having small
atomic radius and large electric charge (3þ), the
neutralization of the charge by the nitrate anions
would require a too high coordination number (assum-
ing the formal charge of oxygen to be equal to 1/3).
Such a number is neither observed in practice nor
possible from the geometrical viewpoint.
Like other hydrous nitrates, mainly of the group ‘d’
of the elements [4,7], aluminium nitrate hydrate can-
not give anhydrous salt in thermal decomposition
The problem of charge neutralization at the small
radius of the Al3þ cation may be resolved by creation

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B. Pacewska, M. Keshr / Thermochimica Acta 385 (2002) 73–80
of the hydrated ion, with the electric charge distributed
on its surface, that can be neutralized by a suitable
number of oxygen atoms proceeding from the nitrate
groups. Thus, the structure of aluminium nitrate
hydrate is featured by the presence of two kinds of
water, which is reflected in the stepwise dehydration
of the compound.
The mechanism of decomposition of the hydrous
salt changes upon changing the experimental condi-
tions: the larger the sample mass, the higher the
heating rate (see Figs. 2–4).
Thermal decomposition of Al(NO3)3ꢀ9H2O is pre-
ceded by melting, which manifests itself by endother-
mic effect on the DTA curve (Fig. 4), with an
extremum at 80 8C, and no change in the sample mass
(see TG curve in Fig. 2).
Thermal dissociation of the hydrate proceeds in a
rather narrow temperature range within 150–400 8C,
practically as a one-stage process. The overall mass
loss of the sample is about 85%, which corresponds
roughly to theoretical contents of volatile components
in Al(NO3)3ꢀ9H2O (see TG and DTG in Figs. 2 and 3,
respectively). The loss in mass is accompanied by a
sharp peak on the DTA curve, with the extremum at
170 8C (Fig. 4).
mass is observed in the second stage, which proceeds
at 220–450 8C. The rate of the mass loss is, however,
much slower in this step and it decreases with increas-
ing temperature. The thermal decomposition is here
not preceded by the melting of the salt. Stoichiometric
calculations show that the composition of the product
of partial dehydration at 120 8C corresponds to the
formula Al(OH)(NO)3ꢀ2H2O.
An analysis of the results of X-ray diffraction
studies shows that in both the initial salt and the
sample heated for 1 h at 120 8C (see Fig. 6) the
crystalline phase, corresponding to Al(NO3)3ꢀ9H2O
(ASTM 24-0004) is present. The products obtained
from partial decomposition of the salt have amorphic
structure. The aluminium nitrate hydrate, occurring in
the both samples, has a different degree of crystal-
linity. The size of crystals present in the sample
calcined at 120 8C is much finer.
The infrared absorption spectra of the hydrate
heated for 1 h at 120 8C do not especially differ from
those obtained for the starting salt (Fig. 5). The IR
absorption spectra show only some change in the
shape of the absorption band within wave numbers
400–800 cmꢁ1, probably indicated for variety of
Al–O–Al bonds [10,11].
An analysis of the TG and DTG curves shows that
the rate of the mass loss is variable (Figs. 2 and 3). The
rate observed at temperatures below 170 8C is much
higher than that in higher temperature range, between
170 and 400 8C. The process of the salt decomposition
ends practically at 400 8C. The initial stage of decom-
position, below 170 8C, comprises probably a series of
simultaneous reactions such as dehydration, hydroly-
sis, and destruction of nitrate groups, whereas such
processes as dehydroxylation of hydroxy salts, formed
during the decomposition as a consequence of hydro-
lytic processes, occur at temperatures above 170 8C.
The hydrolytic process and formation of solid hydroxy
salts can be observed when the process is carried out
under isothermal conditions.
Analysis of thermoanalytical curves of the decom-
position product of aluminium nitrate hydrate carried
out for 1 h at 120 8C shows that the process proceeds
in two stages (Figs. 2–4). In the first step, below
220 8C, the sample loses about 33.5% of its weight.
The process is associated with a peak on the DTG
curve and an endothermic effect on the DTA curve,
with an extremum at 170 8C. A further 42.2% loss in
The thermoanalytical curves of the intermediate
product, obtained by heating the hydrate for 1 h at
200 8C do not substantially differ from those obtained
at 120 8C. Only the height of the effect connected with
the first stage of decomposition is markedly reduced.
The greatest mass loss, amounting to about 40%,
takes place in the temperature range 220–450 8C. It is
probably due to the dehydration of the hydrolysis
products formed during the isothermal heating of
the salt.
On changing the process conditions from non-
isothermal to isothermal ones it is possible to observe
the reactions of hydrolysis associated with the decom-
position. The shape of the DTG and DTA curves of the
intermediate reaction products of aluminium hydrate
decomposition, carried out for 1 h at temperatures
120, 200, 400 and 550 8C, as compared with identical
curves obtained for the starting salt, is shown in Figs. 3
and 4. The DTG curves (Fig. 3) show the decrease
of the effect observed at 170 8C, as the temperature
of heating increases. The weakening of this effect is
accompanied by the appearance of a new effect at
220 8C. The new effect increases and shifts toward

B. Pacewska, M. Keshr / Thermochimica Acta 385 (2002) 73–80
79
higher temperatures as the temperature of isothermal
heating increases. The new effect is probably due to
the dehydroxylation of the hydroxy salts, formed in
the hydrolysis associated with the decomposition of
the hydrate. An endothermic effect at 80 8C, observed
for the initial sample and connected with the melting
of the substance (Fig. 4), is absent on the DTA curves
of the samples heated at higher temperatures, in which
the processes of thermal decomposition and hydro-
lysis had been much more advanced. The course of the
hydrolytic reactions and the decomposition of the
hydrate has been confirmed by the results of spectro-
photometric analyses.
The IR absorption spectra (see Fig. 5) show a
distinct change in the shape of the absorption band
within wave numbers 2800–3600 cmꢁ1. A broad
asymmetric band observed for the sample of the initial
salt may prove the presence of water bound differently
in the hydrate. The band narrows markedly as the
temperature of isothermal heating increases. This fact
points to the presence of hydroxy groups in the sample
compounds. The band at 1370 cmꢁ1, characteristic for
the nitrate anion, decreases steadily with increasing
temperature of isothermal heating [11,12]. The IR
spectra of the samples heated at 400 and 550 8C are
similar to each other and are characteristic of alumi-
nium oxide [8,9].
Analysis of these results shows that the degree of
surface development of the products of thermal decom-
position aluminium nitrate hydrate depends largely on
thehistoryofthesampleused.Whenthethermaldecom-
position of the hydrate is carried out under isothermal
conditions thevalue of specific surface first increases to
95.6 m2/g for the product obtained at 500 8C, then it
decreases rapidly down to 31.8 m2/g for a product
obtained by heating the initial salt at 600 8C.
It is to be pointed out that the product obtained by
thermal decomposition of the hydrate at 500 8C is
amorphous (see Fig. 6).
Changes of different kind are observed on two-stage
calcination of the starting salt. Samples heated for 1 h
at 200 8C, then for 1 h at 550 8C, are characterized by
relatively high specific surface of 87.5 m2/g, similar to
SBET of aluminium oxide obtained in a one-stage
calcination of the salt at 550 8C.
Increasing the temperature of isothermal calcina-
tion in the second stage up to 900 8C does not lead to a
substantial decrease of specific surface of aluminium
oxides obtained by this procedure. The value of SBET
does not fall below 60 m2/g when the calcination
temperature is increased up to 900 8C. The results
of X-ray diffraction show that the samples are amor-
phous. Some amount of crystallinity can only be
detected in samples obtained in two-stage temperature
treatment: first at 550 8C, then at 900 8C.
The thermal decomposition of aluminium nitrate
hydrate proceeds in a rather narrow temperature range
and at low-temperatures. Besides, the decomposition
is associated with a considerable loss in mass, exceed-
ing 80%. The simultaneous processes of dehydration
and of elimination of nitrogen compounds result in
rapid collapse of the hydrate structure and amorphiza-
tion of the obtained product. The samples obtained in
calcination for 1 h in air atmosphere at 200 and 550 8C
are amorphous according to X-ray studies (Fig. 6). The
reactions of decomposition and hydrolysis occurring
under the conditions of the experiment are associated
with destruction of crystal structure of the hydrate.
Owing to the low decomposition temperatures no
recrystallization processes take place, and use can
be made of this fact in obtaining aluminium oxides
of highly developed porous structure. Fig. 7 shows the
values of specific surface SBET of the products of
thermal decomposition of aluminium nitrate hydrate
and of aluminium oxides obtained by calcination of
these products at various temperatures.
The changes in the porous structure of aluminium
oxides, hence, also in the degree of development of the
specific surface, during the heating at various tempera-
tures, can be explained as a result of sintering. In the
temperature range 550–1000 8C, this process may be
duetodehydroxylationofthesurfaceofprimarycrystal-
litesleadingtotheformationofAl–O–Al bondsanddue
tomasstransfer bysurfaceandbulkdiffusionprocesses.
The surface diffusion transfers the mass from free
surface of primary crystallites to the points of their
contact. This process leads to increase of the effective
pore size diameter and closing the contacts between
neighboring particles. As a result, the pore volume and
the specific surface of the sample decrease. Increasing
the calcination temperature accelerates probably the
recrystallization processes and leads to the growth of
crystallites and to their sintering.
The results of the studies on thermal decomposition
of aluminium nitrate hydrate, in particular of its two-
stage decomposition, obtained until now, are not

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B. Pacewska, M. Keshr / Thermochimica Acta 385 (2002) 73–80
convincing proof of these phenomena. A fact of
primary importance is probably, whether the process
proceeds in the liquid phase, after the salt has been
molten, or in the solid phase in the second stage of the
calcination.
Further studies on the kinetics of thermal decom-
position and on the porous structure of aluminium
oxides obtained are necessary for better understanding
of the phenomena occurring in the formation of the
structure of these materials.
A detailed description of the phenomena occurring
will be possible after precise studies of the porous
structure of the oxides and its changes provoked by the
increase of temperature.
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4. Conclusions
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