Influence of aluminium precursor on physico-chemical properties of aluminium hydroxides and oxides: Part I. AlCl3?6H2O

Article abstract of DOI:10.1007/s10973-005-9987-z

An attempt to obtain aluminium hydroxide that could give aluminium oxides of increased thermal stability was made. Aluminium hydroxide was precipitated during a hydrolysis of aluminium chloride in ammonia medium. The influence of preparative conditions, such as a dosing rate of aluminium precursor, pH, duration of the precipitate refluxing and temperature of calcination, on the properties of obtained hydroxides and oxides was investigated. The materials were studied with the following methods: thermal analysis, IR spectroscopy, low-temperature nitrogen adsorption and adsorption-desorption of benzene vapours. Precipitated boehmites had high values of S _BET determined from nitrogen adsorption (220-300 m²g^-1), good sorption capacity for benzene vapours, developed mesoporous structure and hydrophilic character. It has been proved that a high pH value during the precipitation of aluminium hydroxide favoured better crystallisation of boehmite structure, higher temperature of its dehydroxylation into γ-Al₂O ₃, and delayed transformation of γ phase into α-Al ₂O₃. Aluminium oxides derived from the hydroxides precipitated at a high pH were the most stable at high temperatures, and were characterised with the best surface properties.

Full text of DOI:10.1007/s10973-005-9987-z

Journal of Thermal Analysis and Calorimetry, Vol. 85 (2006) 2, 351–359
INFLUENCE OF ALUMINIUM PRECURSOR ON PHYSICO-CHEMICAL
PROPERTIES OF ALUMINIUM HYDROXIDES AND OXIDES
Part I. AlCl 6H O
3
2
*
Barbara Pacewska , Olga Kluk-PÓoskoÕska and D. Szychowski
Warsaw University of Technology, Faculty of Building Engineering, Mechanics and Petrochemistry, Instytute of Chemistry,
˜
ukasiewicza 17 St., 09-400 PÓock, Poland
An attempt to obtain aluminium hydroxide that could give aluminium oxides of increased thermal stability was made. Aluminium
hydroxide was precipitated during a hydrolysis of aluminium chloride in ammonia medium. The influence of preparative condi-
tions, such as a dosing rate of aluminium precursor, pH, duration of the precipitate refluxing and temperature of calcination, on the
properties of obtained hydroxides and oxides was investigated. The materials were studied with the following methods: thermal
analysis, IR spectroscopy, low-temperature nitrogen adsorption and adsorption–desorption of benzene vapours.
2
–1
Precipitated boehmites had high values of SBET determined from nitrogen adsorption (220–300 m g ), good sorption capacity
for benzene vapours, developed mesoporous structure and hydrophilic character. It has been proved that a high pH value during the
precipitation of aluminium hydroxide favoured better crystallisation of boehmite structure, higher temperature of its
dehydroxylation into γ-Al
precipitated at a high pH were the most stable at high temperatures, and were characterised with the best surface properties.
2
O
3
, and delayed transformation of γ phase into α-Al
2 3
O . Aluminium oxides derived from the hydroxides
Keywords: boehmite, metastable aluminium oxides, thermal decomposition
Introduction
ture from the range 600–1300°C on the thermal trans-
formation of aluminium hydrates was investigated,
too. All precipitates converted into α-Al O via γ-, δ-
The aluminium oxo-hydroxide is a two-component
–
system built up from Al, O atoms, OH groups and
2
3
and θ-Al O , however process of dehydration to
2
3
water molecules. The network may consist of rib-
bonlike fibres, as well as monodisperse spherical par-
ticles; it can be microcrystalline and partially or to-
tally amorphous depending on methods and condi-
tions of preparation [1]. Among popular methods in-
vestigated recently there are: synthesis from metal-or-
ganic precursors (so-called sol–gel method), co-pre-
cipitation and precipitation from homogenous solu-
tions [2]. The heterogeneous precipitation is more
conventional technique and it is still powerful and
widely used; however many research works must be
undertaken to estimate the influence of preparative
conditions (such as a dosing rate of reagents, pH, du-
ration of digestion and temperature of calcination) on
the form of obtained precipitates and properties of the
products of its calcination.
γ-Al O , as well as transformation to corundum con-
2
3
ducted at lower temperatures in powders precipitated
under low pH value. Musiº et al. revealed [4] that
lower pH values result in forming boehmite, whereas
higher pH values lead to bayerite formation, and a
slight change of pH in alkaline medium has a signifi-
cant effect on the ratio of bayerite to boehmite at the
beginning of the precipitation process. The authors of
[5] evidenced the importance of pH and temperature
values on the final product morphology (fibres or po-
rous aggregates). Boehmite particles were synthesized
with using Al(NO ) ·9H O and NaOH in conditions
3
3
2
of controlled pH and temperature. The product ob-
tained at 80°C and pH=8 (the conjunction of a suffi-
ciently high temperature and pH) constituted of
polycrystalline fibres of a diameter from the range
3–8 nm and length equal approximately 100 nm. This
was a porous material with high specific surface area
Hwang et al. [3] precipitated aluminium hydrates
using AlCl ·6H O and NH ·H O at pH equal 7, 8, 9
3 2 3 2
2
(~352 m g ) and porous volume (~0.8 cm g ). The
–1
3
–1
or 10. As the pH value increased from 7 to 10 phases of
obtained aluminium hydrates changed from amor-
phous to boehmite, bayerite, nordstrandite, which was
precipitate obtained at 20°C and pH=5 was amorphous
2
with the specific surface area of about 72 m g and a
–1
–
attributed to an increased amount of OH ions from
3 –1
pore volume 0.4 cm g . On the basis of the investiga-
added NH ·H O. The influence of calcination tempera-
3 2
tions conducted for the samples prepared at pH=9 and
*
Author for correspondence: ich@zto.pw.plock.pl
1
388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
©
2006 Akadémiai Kiadó, Budapest

PACEWSKA et al.
at varying temperature, as well as obtained at 60°C and
boehmite powder, the smallest specific surface area
2
–1
varying pH, it was concluded that higher temperature
leads to better crystallinity, higher specific surface and
higher porous volume, whereas the higher pH the
better crystallinity (however at pH≥10 a bayerite con-
sisted of large crystallites was formed simultaneously
to the boehmite phase). The way in which the pH value
influences thermal stability of aluminium oxides ob-
tained from boehmite through calcination process was
the purpose of investigations described in [6].
Boehmite was prepared in a precipitation process from
(91 m g ) was measured. According to authors, the
increase in crystallite size and crystallinity improve-
ment decreased the internal, as well as external sur-
faces of the particles, which was reflected in the val-
ues of the specific surface area of boehmite powders.
The conditions of hydrothermal process on the
microstructural properties of boehmite formed in an
alkaline medium were the subject of an interest of the
authors of [4]. The main product of the precipitation
from Al(NO ) ·9H O and NaOH solutions was
3 3 2
Al(NO
3
)
3
·9H O with NH ·H O at pH=6 or 7, and then
2 3 2
bayerite, which after digestion at 160°C for 5 h com-
pletely converted into boehmite via the dissolu-
tion/re-precipitation mechanism. Boehmite crystal-
lities were in the nanosize range and possessed an ani-
sotropic shape. A tendency to increased crystallite
size with prolonged heating time was evident. The
particles of boehmite produced by autoclaving for
10 h were in the form of thin foils of varying size.
Their morphology was different in relation to those
usually obtained for boehmite (rods, fibres and very
small plates). Since the crystallite sizes of this
boehmite sample were in nanosize range, it was con-
cluded that the particles consisted of nanosized
boehmite crystallites.
subjected to ageing in the mother liquor for 264 h at el-
evated temperature. The authors revealed that alu-
minium oxides obtained during calcination at 1200°C
of aluminium hydroxides precipitated at lower pH=6
(undigested, as well as digested for 264 h) contained
more α phase than the respective samples prepared at a
higher pH and were characterised by lower values of
specific surface.
As the process of digestion used by the au-
thors [6] is known to improve thermal stability of
γ-Al
O through the removal of defect sites in the
2 3
crystallites and inhibition of the surface diffusion
necessary for crystallite growth [7], it is a subject of
extended research works. Chuagh et al. [7] made an
attempt to evaluate the surface properties of alu-
minium hydroxides prepared at pH=6 from
The main goal of the present study was to evalu-
ate the preparative conditions (pH, dosing rate of alu-
minium precursor and duration of precipitate diges-
tion) for preparing aluminium oxo-hydroxide, as a ma-
terial which could, in turn, be used for obtaining a
low-temperature transition alumina (γ-Al O ) of en-
Al(NO
ing them to the process of digestion for 24, 48, 96,
92 and 384 h, as well as thermal stability of γ-Al O
) ·9H O and NH ·H O solutions after subject-
3 3 2 3 2
1
2
3
2
3
obtained from these hydroxides. It was shown that the
hanced stability at high temperatures. The presented
paper is a fragment of extensive research works con-
cerning the influence of different aluminium precur-
sors on physicochemical properties of aluminium hy-
droxides and oxides. In this paper, aluminium chloride
is used as the precursor of precipitated hydroxides.
freshly precipitated gel (pseudoboehmite) had a very
2 –1
low surface area (~35 m g ), however the surface
area increased after digestion of the hydroxide in the
2 –1
mother liquor (307 m g after 384 h of ageing). Af-
ter calcination at 500°C the resulting alumina had a
2
–1
surface area of 230–310 m g depending on the
length of digestion. The aged aluminas were charac-
terised with higher porosity, higher surface area and
better stability than the undigested sample. These
properties showed correlation with digestion time of
hydroxides. Aluminas prepared from aged precursors
were able to withstand calcination to 1200°C main-
Experimental
The products of hydrolysis of hydrous aluminium
chloride AlCl ·6H O anal. grade produced by POCh
3 2
Gliwice, were the main objects of the studies. The hy-
drolysis process was carried out in ammonia medium
as follows:
2 –1
taining a surface area of about 68 m g . Moreover,
the transformation into α-Al O was delayed in case
2
3
of these samples. Musiº et al. [8] investigated the
The substrates (the 0.5 M solution of AlCl
and
3
product of precipitation at pH~8 from Al(NO ) ·9H O
3 3 2
0.75 M solution of NH ·H O) were dosed with a peri-
3 2
with NH ·H O after its autoclaving for varying time
3 2
staltic pump according to a fixed dosing rate to a
3
1000 cm beaker placed in a thermostat. The precipi-
intervals (1, 6, 24, 72 and 86 h). The process of pre-
cipitation resulted in boehmite that after 1 h of
tation was led at 100°C with continuous stirring of re-
agents. The following parameters of the process were
kept:
2
–1
autoclaving showed surface area of 164 m g . The
2
–1
value of specific surface area increased to 246 m g
after 6 h of ageing, and then decreased with an in-
crease in crystallinity. For the best-crystallized
st
• for the I series: pH=7 and a dosing rate of AlCl3
3
equal 7 cm min
–1
352
J. Therm. Anal. Cal., 85, 2006

INFLUENCE OF ALUMINIUM PRECURSOR ON ALUMINIUM HYDROXIDES AND OXIDES
nd
for the II series: pH=7 and a dosing rate of AlCl
•
•
3
3
ture nitrogen adsorption and by the adsorption and
desorption of benzene vapours. On the basis of ob-
tained adsorption/desorption isotherms, by means of
the programme [9], the following parameters of the
porous structure were calculated: specific surface ac-
cording to BET method (SBET), the surface of
mesopores (SMEZ) from the adsorption and desorption
part of the isotherms using Kisielev method, and the
distribution of mesopore volume and surface by
Dollimore–Hill method.
3
equal 1 cm min
–1
rd
for the III series: pH=8 and a dosing rate of AlCl
3
equal 7 cm min
–1
In each case, the pumping rate of NH ·H O was
3 2
regulated in a way securing a demanded pH value in
the solution. A part of the milky-white colloidal pre-
cipitate obtained was filtered off, washed with dis-
tilled water, dried at 60°C in a drier and finally pow-
dered in a mortar. The sample thus obtained was used
for further studies. The other part of the reaction mix-
ture was transferred to a round-bottomed flask and
heated at 100°C under a reflux condenser. The heat-
ing was stopped for several night hours, during which
the mixture was kept at the temperature of 60°C in a
drier. The precipitates were digested for 59 h; how-
ever a total time of keeping them in mother liquor, in-
cluding night-time spent in a drier at 60°C, was 192 h.
Results and discussion
Thermal analysis
TG and DTA curves of the starting aluminium hy-
st
rd
droxides from the I –III series both undigested and
digested at 100°C for varying time intervals are pre-
sented in Figs 1a and b; whereas Table 1 reveals the
values of mass loss for each step of thermal decompo-
sition calculated for all hydroxide samples.
st
For the I series precipitates were additionally di-
gested for 20 and 39 h, which corresponded to a total
time equal 48 and 96 h. When the required time was
over, the precipitate was filtered off, washed with dis-
tilled water, dried at 60°C in a drier and powdered in a
mortar. The obtained in such a way powder samples
were used for further studies.
The thermal analysis results show that undi-
gested hydroxides, as well as digested at elevated
temperature for varying time intervals decompose in a
similar way.
Conditions under which the process of hydroly-
sis of aluminium chloride was conducted, were cho-
sen on the basis of previous research, as well as of the
analysis of literature works quoted in this paper.
The thermal decomposition of the products of
hydrolysis of hydrous aluminium chloride in ammo-
nia medium was carried out both under isothermal
conditions and under dynamic conditions i.e. under
permanent temperature increase.
The TG curve evidences two-step decomposition
of the samples (Fig. 1a). In the first step ranging from 20
to 170°C, the samples lose humidity water. This endo-
thermic effect is reflected as a peak on the DTA curve
with an extremum at 60–80°C (Fig. 1b). The mass loss
values calculated for digested hydroxides of all series
are lower than these for the samples not subjected to the
process of ageing (Table 1), which points at smaller
amount of physically adsorbed water in them.
The partial thermal dissociation of the products of
hydrolysis was carried out by their heating in a
high-temperature flow reactor (CZYLOK, Poland),
leading the calcination process at 550°C for 2 h in air at-
mosphere. Aluminium oxides obtained at 550°C were
subjected to a further calcination at 900 or 1200°C.
The TG, DTA and DTG curves, IR spectra and
sorption studies were conducted for the starting prod-
uct of hydrolysis, as well as for the products of their
thermal decomposition under isothermal conditions.
The thermoanalytical curves TG, DTG and DTA
were recorded using a thermoanalyser TA Instruments
SDT 2960. The measurements were performed for
In the second step, corresponding to the tempera-
ture range 170–550°C, the samples lose a further
5–10 mg samples in air atmosphere and in a temperature
–1
range of 20–1000°C, with a heating rate of 10°C min .
The IR spectra were recorded with a FTIR
Matson Spectrophotometer in a wave number range
st
Fig. 1a TG curves for aluminium hydroxides from the I –III
rd
–
1
of 4000–400 cm . Powders were dispersed in KBr
matrix and pressed into thin, transparent pellets.
The degree of specific surface development was
studied by volumetric determination of low-tempera-
series both undigested and digested for varying time
3 3
intervals; 1 – Al(OH) \I\0 h, 2 – Al(OH) \I\20 h,
3 – Al(OH) \I\39 h, 4 – Al(OH) \I\59 h,
3
3
3
3
5 – Al(OH)
– Al(OH)
\II\0 h, 6 – Al(OH)
3
\II\59 h,
7
\III\0 h, 8 – Al(OH)
3
\III\59 h
J. Therm. Anal. Cal., 85, 2006
353

PACEWSKA et al.
III\59 h series precipitated at higher pH. It confirms
the conclusion made by the authors of [3] that higher
pH value favours higher temperature of boehmite
dehydroxylation into γ-Al
O .
2 3
FTIR spectroscopy
Figure 2a represents the IR spectra recorded for the
product of hydrolysis of aluminium chloride in am-
monia medium from I\0 h series and the products of
its calcination for 2 h at 550, 900 or 1200°C; Fig. 2b
shows the IR spectra of aluminium hydroxides from
I\0 h, II\0 h and III\0 h series, as well as the hydrox-
ides from I\59 h, II\59 h and III\59 h series, whereas
Fig. 2c demonstrates the IR spectra of aluminium ox-
ides prepared from these hydroxides through calcina-
tion at 1200°C.
st
rd
Fig. 1b DTA curves for aluminium hydroxides from the I –III
series both undigested and digested for varying time in-
tervals; 1 – Al(OH) \I\0 h, 2 – Al(OH) \I\20 h,
\I\39 h, 4 – Al(OH) \I\59 h,
\II\0 h, 6 – Al(OH) \II\59 h,
\III\0 h, 8 – Al(OH) \III\59 h
3
3
3
5
7
– Al(OH)
– Al(OH)
– Al(OH)
3
3
3
3
3
3
The IR spectra of aluminium hydroxides and the
products of their calcination at 550, 900 or 1200°C re-
corded for undigested samples from I\0 h, II\0 h and
III\0 h series are similar to the respective IR spectra
obtained for analogous samples digested for varying
time intervals.
1
7–18, 16–18 and 14–16% of their masses, respec-
st
rd
tively, for the hydroxides from the I –III series (Ta-
ble 1). These values correspond to the loss of 1.4–1.5;
.3–1.6 and 1.1–1.3 mol of water per 1 mol of Al O .
1
2
3
Taking into account that decomposition of boehmite
takes place at 410°C [10] according to the reaction:
In the spectra recorded for all hydroxides, there is a
broad band at the wave number range of
–1
3
700–2900 cm ascribed to stretching vibrations of OH
groups in the hydroxide structure, as well as in water
physically adsorbed [11]. Within this band, another
2
AlO(OH)→γ-Al O +H O
2
3
2
and that this process is accompanied by the loss
of 1 particle of water, which corresponds to the theo-
retical mass loss of 15%, one may suppose that exam-
ined samples contained mainly boehmite that decom-
posed according to the above reaction. This stage of
decomposition is reflected on the DTA curve as an
endothermic effect with an extremum ranging from
–1
weaker one is observed at about 3029 cm characteris-
tic for stretching vibrations of OH groups in boehmite
12, 13]. A weakly visible band for deformation vibra-
[
–1
tions of this group is identified at 1155 cm [14]. The
–1
band at 900 cm is due to vibrational modes localized
in the surface layer and most likely involving the defor-
mation of surface OH groups in γ-AlOOH [15]. The for-
mation of boehmite structure in the samples is also con-
3
80 to 420°C. The value of this extremum is shifted to
higher temperatures for the hydroxides of III\0 h and
Table 1 The mass loss values for individual steps of thermal decomposition calculated for hydroxide samples of the
st
rd
I ––III series, both undigested and digested for 20, 39 or 59 h
Temperature range/°C
170–550
Mass loss/%
Sample
20–170
Mass loss/% Al
20–1000
2 3 2
Mass loss/% Al O ·nH O
2
O
3
·nH
2
O
Al
2
O
3
·nH
2
O
st
I series
Al(OH)
Al(OH)
Al(OH)
Al(OH)
3
3
3
3
\0 h
13.8
9.2
1.2
0.7
0.9
0.9
17.8
16.9
17.9
17.2
1.5
1.4
1.5
1.4
34.5
29.3
31.0
31.2
3.0
2.4
2.6
2.6
\20 h
\39 h
\59 h
10.5
11.4
nd
II series
Al(OH)
3
3
\0 h
14.5
10.9
1.2
0.9
18.3
15.9
1.6
1.3
33.9
28.8
2.9
2.3
Al(OH)
\59 h
rd
III series
Al(OH)
3
\0 h
12.0
8.5
1.0
0.7
13.8
16.4
1.1
1.3
29.7
27.3
2.4
2.1
3
Al(OH) \59 h
354
J. Therm. Anal. Cal., 85, 2006

INFLUENCE OF ALUMINIUM PRECURSOR ON ALUMINIUM HYDROXIDES AND OXIDES
–1
firmed by a very intensive peak ~1070 cm [15] as-
cribed to an Al–O vibrational mode [4, 16], as well as a
–1
broad band at the wave number range of 480–750 cm ,
–1
consisted of three peaks: 741, 610 and 490 cm , attrib-
uted to ‘condensed’ AlO octahedra. The intensity of
6
these peaks is stronger for the samples aged in mother li-
quor at elevated temperature with respect to undigested
ones (compare the spectra in Fig. 2b). This may evi-
dence that the process of digestion of aluminium hy-
droxide probably favours the formation of boehmite
structure. In the IR spectra of hydroxides from I\20 h,
I\39 h, I\59 h series any distinct differences in the inten-
sity of peaks characteristic for boehmite are visible,
which points at slight influence of digestion time on the
formation of boehmite structure.
Fig. 2a IR spectra for the samples from the I\0 h series;
1
3
4
– Al(OH)
3
\I\0 h, 2 – Al
\I\0 h\550°C\900°C,
\I\0 h\550°C\1200°C
2 3
O \I\0 h\550°C,
– Al
– Al
2
2
O
3
O
3
Except above mentioned bands in the IR spectra
st
of all hdyroxides from the I –III series, there are an-
rd
–
other ones such as at 1640 cm ascribed to bending
1
vibrations of OH groups in structural water [17] and
–
1
slightly visible bands at 800–920 cm that most
likely involve aluminate anions or Al–O–Al coupling
formed through condensation.
The process of calcination at 550°C for 2 h for
all starting hydroxides led to creation γ-Al O . It is
2 3
confirmed by a strong broad absorption band in the
–
region 750–900 cm , due to stretching vibrations of a
1
lattice of interlinked tetrahedra AlO [15]. This is in
4
agreement with thermal analysis results. An increase
of calcination temperature up to 900°C probably re-
sults in high-temperature transition oxides such as δ
or θ of higher crystalline order. This is reflected in the
st
Fig. 2b IR spectra for aluminium hydroxides from the I –III
rd
series both undigested and digested for 59 h;
–
1
character change of the band at 750–900 cm , which
becomes broader, stronger and partly resolved into
several sharp components of small or medium inten-
sity [15]. Simultaneously, the band in the range of
1
3
5
– Al(OH)
– Al(OH)
– Al(OH)
3
3
3
\I\0 h, 2 – Al(OH)
\III\0 h, 4 – Al(OH)
\II\59 h, 6 – Al(OH)
3
\II\0 h,
\I\59 h,
\III
3
3
–
1
700–2900 cm attributed to stretching vibrations of
3
OH groups in the hydroxide structure, as well as in
physically adsorbed water, is becoming weaker and
weaker and its intensity is the smallest in the IR spec-
tra of the samples calcined at 1200°C (compare in
Fig. 2a). The analysis of the IR spectra recorded for
st
the aluminium oxides from the I series calcined
1
200°C shows the character change of the band at
–
50–900 cm corresponding to Al–O modes in the
1
7
lattice. In the IR spectra, new peaks at 465, 610 and
–
1
45 cm appear, which are due to vibrations of ‘con-
6
densed’ AlO octahedra in α-Al O [15]. Their inten-
6
2
3
sity is weaker for the oxides prepared from hydrox-
ides from I\20 h, I\39 h, I\59 h series in comparison
with the sample obtained from the hydroxide I\0 h,
however it hardly changes with lengthening of diges-
tion time. The peaks for α-Al O also appear in the
Fig. 2c IR spectra for the products of calcination at 1200°C
st rd
for 2 h of aluminium hydroxides from the I –III se-
ries both undigested and digested for 59 h;
1
2
3
4
5
6
– Al
– Al
– Al
– Al
– Al
– Al
2
2
2
2
2
2
O
3
O
3
O
3
O
3
O
3
O
3
\I\0 h\550°C\1200°C;
\II\0 h\550°C\1200°C;
\III\0 h\550°C\1200°C;
\I\59 h\550°C\1200°C;
\II\59 h\550°C\1200°C;
\III\59 h\550°C\1200°C
2
3
spectra of aluminium oxides from II\0 h and II\59 h
series, however of much weaker intensity and they are
J. Therm. Anal. Cal., 85, 2006
355

PACEWSKA et al.
accompanied by another ones at 675, 715, 770, 810
–
ture is not formed completely, which is confirmed by
increasing values of specific surface for the hydroxides
from I\20 h, I\39 h and I\59 h series.
1
and 835 cm typical for θ-Al O [18].
2
3
The character of the IR spectra recorded for anal-
ogous samples from III\0 h and III\59 h series is con-
genial with the respective spectra of oxides from
II\0 h and II\59 h series, with such a small difference
that the bands characteristic for α phase are much
weaker, while the bands typical for θ-Al O – much
st
rd
All hydroxides from the I –III series decrease
their specific surface after calcination at 550, 900 or
1200°C. However, aluminium oxides obtained by
heating at 550 or 900°C maintain relatively high val-
2
ues of SBET, from 210 to 261 m g for the samples
–1
2
3
2
–1
stronger (Fig. 2c). It may proof that a higher pH value
at which aluminium hydroxides from III\0 h and
III\59 h series were precipitated favours delayed
calcined at 550°C, and from 118 to 169 m g for the
samples calcined at 900°C. Such high values of spe-
cific surface are typical for metastable aluminium ox-
ides [15], whose formation is proved by the results of
IR spectroscopy.
transformation of γ-Al
O obtained from this hydrox-
2 3
ide into α phase. This is in agreement with the results
of the researchers [3]. On the basis of the investiga-
tions described in this paper, it may be concluded that
the decrease of precipitation rate is likely to lead to
similar results.
The analysis of the data collected in Table 2
shows that regardless of the number of series, SBET
measured for the oxides prepared from digested hy-
droxides are very similar to the values for the respec-
tive samples obtained without the digestion step. An
increase of calcination temperature up to 1200°C re-
sults in a prominent decrease of specific surface val-
ues. The IR spectra denoted that at this temperature
high-temperature transition aluminium oxides such as
Low-temperature nitrogen adsorption
Table 2 presents the values of specific surface deter-
mined by the method of low-temperature nitrogen ad-
sorption for the hydroxides of all series, both undi-
gested and digested for 59 h (and for I\20 h, I\39 h
series), as well as the products of their calcination at
θ- or α-Al O are formed (Fig. 2c), which is accompa-
2 3
nied by the sintering of powder particles, as well as
devastation of their internal porosity. The highest val-
rd
5
50, 900 or 1200°C for 2 h.
An analysis of the data collected in Table 2 shows
that freshly precipitated aluminium hydroxides are char-
acterized with relatively high values of specific surface
ues of SBET are retained for the oxides from the III
2
–1
series (about 50 m g ) containing the most θ phase
and at the same time – the least α-Al . Further-
O
3
2
more, all aluminium oxides of individual series ob-
tained through heating at 1200°C are characterised by
congenial values of SBET, regardless of the fact
whether the hydroxides, from which the oxides were
derived, were digested or not. Prolonging digestion
time from 20 to 59 h causes hardly any effect on the
values of specific surface. On the base of these two
notices, one may come to conclusion that these are the
precipitation conditions, but not a digestion process
or its duration, that influence a degree of surface de-
velopment of aluminium oxides calcined at 1200°C.
2 –1
220–300 m g ). All hydroxides, except of this from
(
III\59 h, have higher values of specific surface in com-
parison with the respective hydroxides prepared without
the digestion step. During the process of ageing, the hy-
drolysis of basic aluminium salts takes place together
with the removal of surface OH groups resulting in de-
veloping of crystal structure [7, 11]. When water is re-
moved from the interlayer space pores develop, the for-
mation of bigger and more ordered primary particles of
aluminium hydroxide takes place which consequently
leads to the increase of specific surface for the samples
of digested hydroxides. It explains higher values of spe-
cific surface for digested hydroxides from I\20 h, I\39 h,
I\59 h and II\59 h series in comparison with the samples
not subjected to ageing (I\0 h and II\59 h).
Adsorption and desorption of benzene vapours
The exemplary adsorption and desorption isotherms
of benzene vapours are shown in Figs 3a and b,
whereas parameters of the porous structure calculated
from them are collected in Table 2.
Musiº et al. [8] investigating an influence of di-
gestion time on the crystallinity of boehmite and its
surface development noticed the decrease of the spe-
The shape of isotherms obtained for the hydrox-
ides and oxides from I\0 h, II\0 h, II\59 h and III\0 h
series (Fig. 3a) may be classified according to IUPAC
nomenclature as the H3 type [19]. For the materials of
this kind, a hysteresis loop is qualified by capillary
condensation between two layers. Thus, in these sam-
ples bottle shape pores predominate. The isotherms
obtained for the hydroxide and oxides (except from
the aluminium oxide calcined at 1200°C) from I\59 h
2
–1
cific surface value from 164 to 146 m g with length-
ening time of digestion from 1 to 6 h. Samples aged for
longer time intervals (24, 72 or 86 h) had better and
better formed crystal structure, however lower and
2
–1
lower value of specific surface (SBET=91 m g for the
sample digested for 86 h). Taking this observation into
consideration, one may suppose that for the samples
st
from the I series, a fully crystallized boehmite struc-
356
J. Therm. Anal. Cal., 85, 2006

INFLUENCE OF ALUMINIUM PRECURSOR ON ALUMINIUM HYDROXIDES AND OXIDES
Table 2 Porous structure parameters for selected samples determined by low-temperature nitrogen adsorption and adsorp-
tion\desorption of benzene vapours
Specific
surface
Specific surface (benzene)
Surface of mesopores (Kisielev method)
Sample
2
BET(vertical)/m g
–1
2
BET(planar)/m g
–1
2
MEZ(adsorption)/m g
–1
2
MEZ(desorption)/m g
–1
S
S
S
S
(
nitrogen)
st
I series
Al(OH)
3
\0 h
244
214
131
21
179
132
87
13
–
286
211
140
21
–
155
195
142
*
241
270
194
*
Al
Al
Al
2
2
2
O
3
O
3
O
3
\0 h\550°C
\0 h\550°C\900°C
\0 h\550°C\1200°C
Al(OH)
3
\20 h
260
217
118
21
–
–
Al
Al
2
2
O
3
3
\20 h\550°C
–
–
–
–
O
\20 h\550°C\900°C
\20 h\550°C\1200°C
–
–
–
–
Al O
2
3
–
–
–
–
Al(OH)
3
\39 h
261
220
143
31
–
–
–
–
Al
Al
2
2
O
3
3
\39 h\550°C
–
–
–
–
O
\39 h\550°C\900°C
\39 h\550°C\1200°C
–
–
–
–
Al O
2
3
–
–
–
–
Al(OH)
3
\59 h
267
215
145
23
215
157
93
16
344
251
149
26
222
207
187
*
322
295
250
*
Al
Al
2
2
O
3
3
\59 h\550°C
O
\59 h\550°C\900°C
\59 h\550°C\1200°C
Al O
2
3
nd
II series
Al(OH)
3
\0 h
222
211
146
28
171
138
115
18
274
221
184
29
349
279
273
*
504
351
348
*
Al
Al
Al
2
2
2
O
3
O
3
O
3
\0 h\550°C
\0 h\550°C\900°C
\0 h\550°C\1200°C
Al(OH)
3
\59 h
256
210
138
38
209
130
95
334
207
153
47
269
255
252
*
408
338
297
*
Al
Al
2
2
O
3
3
\59 h\550°C
O
\59 h\550°C\900°C\
\59 h\550°C\1200°C
Al O
2
3
30
rd
III series
Al(OH)
3
\0 h
297
261
169
53
201
184
100
35
321
295
160
56
385
556
375
*
584
728
375
*
Al
Al
Al
2
2
2
O
3
O
3
O
3
\0 h\550°C
\0 h\550°C\900°C
\0 h\550°C\1200°C
Al(OH)
3
\59 h
252
235
147
48
182
143
84
291
228
134
65
433
476
378
*
531
736
773
*
Al
Al
2
2
O
3
3
\59 h\550°C
O
\59 h\550°C\900°C
\59 h\550°C\1200°C
Al O
2
3
41
*
lack of hysteresis loop
series (Fig. 3b) are characterised with the shape con-
genial with the H2 type. Such a hysteresis loop corre-
sponds to pores with shape resembling an ‘inkstand’
and to spherical pores open from both ends with sig-
nificant inside necks.
zene vapours. SBET values calculated supposing pla-
nar orientation of benzene molecules in adsorption
2
–1
monolayer are close to 300 m g . So high values of
nd
mesopore surface, in case of the samples from II and
rd
III series – much higher than SBET(planar), as well as
The values of specific surface presented in Ta-
broad hysteresis loops evidence their developed
mesoporous structure.
ble 2, show that the aluminium hydroxides from
st rd
I –III series have good adsorption capacity for ben-
J. Therm. Anal. Cal., 85, 2006
357

PACEWSKA et al.
Fig. 3a The adsorption/desorption isotherms of benzene va-
pours for the samples from the I\0 h series;
Fig. 3d The distribution of mesopore surface area as a function of
the effective radii for the samples from the III\59 h series;
1 – Al(OH) \III\59 h, 2 – Al \III\59 h\550°C,
3 – Al \III\59 h\550°C\900°C
1
3
4
– Al(OH)
3
\I\0 h, 2 – Al
\I\0 h\550°C\900°C,
\I\0 h\550°C\1200°C
2
O
3
\I\0 h\550°C,
3
2 3
O
– Al
– Al
2
2
O
3
2 3
O
O
3
capacity for benzene vapours, which is confirmed by
vanishing or emphatic narrowing of hysteresis loop,
the lowest placed adsorption isotherms, and in conse-
quence, low values of specific surface. Within all ox-
ides calcined at 1200°C, the samples from III\0 h and
III\59 h series have the highest SBET values (56 and
2 –1
5 m g , respectively).
6
A comparative analysis of nitrogen and benzene
adsorption results gives the possibility of estimating
the hydrophilic-hydrophobic properties of aluminium
hydroxides and oxides [20, 21]. These properties de-
pend on the surface functional groups that are for the
different orientation of benzene molecules sorbed and
for the degree of their packing in the adsorption layer.
The nitrogen molecule (inert gas) has a small seating
Fig. 3b The adsorption/desorption isotherms of benzene va-
pours for the samples from the I\59 h series;
1
3
4
– Al(OH)
3
\I\59 h, 2 – Al
\I\59 h\550°C\900°C,
\I\59 h\550°C\1200°C
2 3
O \I\59 h\550°C,
2
surface (0.16 nm ) as compared with that of benzene
2
molecule (0.25 nm in the vertical orientation and
– Al
– Al
2
2
O
3
O
3
2
0
.40 nm in the planar orientation). For this reason, if
the surface structure is favourable for the planar ori-
entation of benzene molecule, the specific surface
should be smaller or equal to the SBET value
S
BET(planar)
determined from the adsorption of nitrogen. Exces-
sive values of SBET(planar) with respect to the specific
surface values determined from nitrogen adsorption
may be accounted for the decrease of the surface ac-
cessible for seating of benzene molecules, which can
be a result of the change in orientation of its mole-
cules in adsorption layer. The deviation of benzene
molecules on the surface of hydroxides and oxides is
likely to be caused by surface hydroxyl groups.
Regardless of the series, SBET(planar) values calcu-
lated for starting hydroxides are much higher than
those of SBET determined from nitrogen adsorption. It
may be the evidence of a prominent concentration of
hydroxyl groups on the surface of studied materials
enabling deviation of adsorbed benzene molecules
from planar orientation. Thus, the hydroxides are of
hydrophilic character.
Fig. 3c The distribution of mesopore surface area as a func-
tion of the effective radii for the samples from the
I\59 h series; 1 – Al(OH) \I\59 h,
2 3
– Al \I\59 h\550°C, 3 – Al O \I\59 h\550°C\900°C
3
2
2
O
3
The process of calcination at 550–900°C leads to
decreasing values of SBET, however, regardless of the
series number and digestion time, the materials main-
tain a mesoporous character. Aluminium oxides cal-
cined at 1200°C have significantly lower adsorption
358
J. Therm. Anal. Cal., 85, 2006

INFLUENCE OF ALUMINIUM PRECURSOR ON ALUMINIUM HYDROXIDES AND OXIDES
The process of calcination at 550–1200°C results
in decreasing a discrepancy between the values of SBET
determined from benzene and nitrogen adsorption,
which may prove surface dehydroxylation of the sam-
ples, and in consequence, an increasing degree of
hydrophobicity. For the samples calcined at 1200°C, it
is possible to assume that benzene molecules have pla-
nar orientation in monomolecular adsorption layer.
The distribution of mesopore surface (Figs 3c
and d) as a function of effective radii was determined
by the Dollimore–Hill’s method with assumption of
the model of cylindrical pores, open from both sides,
for quartz as the adsorption layer.
value (pH=8) are the most stable at high tempera-
tures and are characterized with the best surface
properties (SBET values determined from low-tem-
perature adsorption of nitrogen and adsorption of
2 –1
benzene vapours are about 50 m g ).
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•
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1
•
•
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2
3
2
3
2
2
value during the precipitation favours higher tem-
(
perature of boehmite dehydroxylation into γ-Al O3
and delayed transformation of γ-Al O into α-Al O .
2
2
3
2
3
8
•
•
An increase of calcination temperature from 550 to
1200°C leads to decreasing of SBET values deter-
Received: March 25, 2005
Accepted: May 24, 2005
OnlineFirst: January 11, 2006
mined from low-temperature adsorption of nitrogen
and adsorption capacity of benzene vapours, and to
increasing of the degree of sample hydrophobicity.
The samples of aluminium oxides derived from
aluminium hydroxides precipitated at a high pH
DOI: 10.1007/s10973-005-7016-x
J. Therm. Anal. Cal., 85, 2006
359