Structure and thermal behavior of nanocrystalline boehmite

Article abstract of DOI:10.1016/j.tca.2004.06.009

First, the structural features of nanocrystalline boehmite synthesized by hydrolysis of aluminum sec-butoxide according to the Yoldas method are reported. The nanosized boehmite consists of rectangular platelets averaging 8 by 9 nm and 2-3 nm in thickness

Full text of DOI:10.1016/j.tca.2004.06.009

Thermochimica Acta 425 (2005) 75–89
Structure and thermal behavior of nanocrystalline boehmite
∗
Pierre Alphonse , Matthieu Courty
CIRIMAT, UMR-CNRS 5085, Universit e´ Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex 04, France
Received 8 March 2004; received in revised form 11 June 2004; accepted 21 June 2004
Available online 2 September 2004
Abstract
First, the structural features of nanocrystalline boehmite synthesized by hydrolysis of aluminum sec-butoxide according to the Yoldas
method are reported. The nanosized boehmite consists of rectangular platelets averaging 8 by 9 nm and 2–3 nm in thickness which contain
a large excess of water. Dehydration by heating under vacuum induced an increase in the specific surface area, down to a minimum water
2
content (∼0.2 H
2
O per Al
2
O
3
); values up to 470 m /g can be reached. However this enlargement of specific surface area only results from
water loss, the surface area remaining constant. The particle morphology, the excess of water, as well as the specific surface area, depend
on the amount of acid used for the peptization during the synthesis. Second, a comprehensive investigation of the dehydration kinetics is
presented. The simulations of the non-isothermal experiments at constant heating rates show that thermally stimulated transformation of
nanocrystalline boehmite into alumina can be accurately modeled by a 4-reaction mechanism involving: (I) the loss of physisorbed water, (II)
the loss of chemisorbed water, (III) the conversion of boehmite into transition alumina, (IV) the dehydration of transition alumina (loss of
residual hydroxyl groups). The activation energy of each step is found to be very similar for experiments done in various conditions (heating
rate, atmosphere, kind of sample,. . .).
©
2004 Elsevier B.V. All rights reserved.
Keywords: Boehmite; Transition alumina; Non-isothermal kinetics; Computer simulation
1
. Introduction
of their high surface area, mesoporosity, and surface acid-
ity.
Alumina is a low cost material used in many domains
Transition aluminas are prepared by calcining aluminum
hydroxides. Starting from different hydroxides leads to dif-
ferent forms having different thermal stability, surface acid-
ity and textural properties. Among aluminum hydroxides,
boehmite, aluminum oxyhydroxide (AlOOH) is an important
precursor because the heat treatment of boehmite produces
a series of transition aluminas from ␥-Al2O3 and ␩-Al2O3
to ␦-Al2O3, and ␪-Al2O3, which exhibit high surface areas
like catalysis, ceramics and mechanical ceramics, refractory,
electrotechnology, electronics, bio-medical,. . .. The wide va-
riety of these applications comes from the fact that alu-
mina occurs in two forms, corundum or ␣-alumina with
an hexagonal close-packing of oxygen ions and transition
aluminas with a cubic close packing of oxygen. Transi-
tion aluminas include a series of metastable forms that ex-
ist on an extended temperature range, but all of them lead
to ␣-alumina by calcining at high temperatures. Corundum
has excellent mechanical, electrical, thermal, and optical
properties. Transition aluminas are widely used as adsor-
bents, catalysts, catalyst supports, and membranes because
2
◦
(200–500 m /g) and thermal stability up to 1000 C.
The structures of the transition aluminas all are based on a
face-centered cubic (fcc) array of oxygen anions. The struc-
tural differences between these forms only involve the ar-
rangement of aluminum cations in the interstices of the fcc
array of oxygen anions. ␥-Al2O3 and ␩-Al2O3 have defect
spinel structures [1]. ␦-Al2O3 has a tetragonal superstruc-
ture of the spinel lattice with one unit-cell parameter tripled
[1] and ␪-Al2O3 has a monoclinic structure [2]. ␩-Al2O3 is
∗
Corresponding author. Tel.: +33 5 61 55 62 85; fax: +33 5 61 55 61 63.
E-mail addresses: alphonse@chimie.ups-tlse.fr (P. Alphonse),
courty@chimie.ups-tlse.fr (M. Courty).
0
040-6031/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2004.06.009

7
6
P. Alphonse, M. Courty / Thermochimica Acta 425 (2005) 75–89
produced by dehydration of bayerite Al(OH)3, whereas ␥-
Al2O3 is formed by dehydration of boehmite. Upon heating,
coatings or in catalysts manufacturing, good modeling of its
dehydration kinetics would be very useful for the accurate
design of industrial thermal processes.
␥
-Al2O3 and ␩-Al2O3 are gradually converted in ␪-Al2O3.
In the case of ␥-Al2O3, this transformation occurs via the
formation of ␦-Al2O3 which is not observed for ␩-Al2O3.
Boehmite was thought to exist under two distinct forms,
well-crystallized boehmite and pseudoboehmite (also called
gelatinous boehmite) [3,4], with significantly different mor-
phologies, porosity, and surface areas. However, recent crys-
tallographic studies [5–9] have clearly demonstrated that
pseudoboehmite is simply micro- or rather nano-crystallized
boehmite, the differences observed between the two forms
coming from the difference in crystallite size.
Since the oxygen sublattice of boehmite is cubic pack-
ing, boehmite dehydration to form transition aluminas and
the further transformations through the transition alumina
series only involve short-range rearrangements of atoms in
the crystal structure. These conversions are topotactic and re-
quire only a small energy. Hence, the temperatures at which
they are observed are variable and depend on the crystallinity
of the boehmite precursor as well as on the thermal treatment
conditions. Therefore, the characteristics of the transition alu-
minas, such as specific area, porosity, pore-size distribution,
acidity, are deeply affected by the microscopic morphology
of the boehmite precursor.
2. Experimental
2.1. Synthesis
Three kinds of boehmite samples were synthesized ac-
cording to the well-known Yoldas process [22–25], which
consists of introducing aluminum tri-sec-butoxide (ASB)
Al(OC4H9)3 in an excess of distilled water (H2O/Al = 100)
under vigorous stirring at 85 C. Stirring was maintained for
15 min. The first sample (S1) was obtained by drying the
◦
◦
hydrolyzed slurry at 50 C in air (no peptization). For the
second sample (S2), the hydroxide precipitate was peptized
by adding 0.07 mol of nitric acid per mol of alkoxide and
◦
stirring at 85 C until a clear sol is obtained (∼24 h). For
the third sample (S3), peptization was done with 0.20 mol of
nitric acid per mol of alkoxide.
2.2. Powder X-ray diffraction (PXRD)
The crystal structure was investigated by powder X-ray
diffraction. Data were collected on a Seifert 3003TT θ−θ
diffractometer in Bragg–Brentano geometry, using filtered
Cu K␣ radiation and a graphite secondary-beam monochro-
mator. Diffraction intensities were measured by scanning
The identification and characterization of various transi-
tion aluminas formed by the dehydration of boehmite have
been extensively investigated. Several studies demonstrate
the close relationship between the microstructural features
of boehmite and the structure and texture of the resulting
transition aluminas [1,7,10–15]. The few papers [16–21] pub-
lished on the kinetics of the boehmite dehydration generally
give conflicting results. As pointed out by Tsuchida et al.
◦
◦
from 5 to 90 (2θ) with a step size of 0.02 (2θ).
Whole powder pattern fitting (WPPF) was done by the le
Bail method using the software PowderCell developed by
Nolze and Kraus [26]. The reflection profiles were mod-
eled by pseudo-Voigt functions. The peak positions were
constrained by lattice parameters. The variation of the peak
FWHMs (full-width at half-maximum) across the pattern
were calculated from the classical Caglioti [27] formula.
[20], the discrepancies between them are attributable to the
differences in particle size, specific surface area and crys-
tallinity of the boehmite used for the experiments. Most
of these kinetic studies assume that the dominating reac-
tion process is a single rate-controlling process throughout
all stages of the reaction. Furthermore, it is often accepted
that the associated activation energy remains constant dur-
ing the dehydration process. In this work, we clearly show
that these assumptions are inaccurate for nanocrystalline
boehmite.
The main goal of the present paper is to report a compre-
hensive investigation of the dehydration kinetics of nanocrys-
talline boehmite synthesized by hydrolysis of aluminum
alkoxide. In kinetic studies, a precise knowledge of the struc-
tural and microstructural features of the involved materials is
essential, therefore the first part of this paper reports the thor-
ough characterization of both nanocrystalline boehmite and
its dehydration products. Using a material with a very small
crystal size is critical when dehydration affects the structure
rather than the surface layer. With larger crystals the limiting
step is very often the diffusion of gas through the solid and
the true kinetic parameters cannot be determined. Finally, it
is worth noting that, given the importance of boehmite in
2.3. Specific surface area and density
The specific surface areas were computed from the N2
adsorption isotherms (recorded at 77 K with a Micromerit-
ics ASAP 2010), using the Brunauer–Emmett–Teller (BET)
method.
Skeletal densities of powder were determined using a
gas pycnometer (Micromeritics AccuPyc 1330) and work-
ing with helium. Each experimental value results from the
average of 10 successive measurements on the same sample.
2.4. Electron microscopy
Transmission electron microscopy (TEM) analyses were
done on a JEOL 2010. A small amount of sample powder was
put in ethanol and dispersed with ultrasound during 1 min.
Then the carbon-coated grid was dipped in the suspension
and allowed to dry at room temperature.

P. Alphonse, M. Courty / Thermochimica Acta 425 (2005) 75–89
77
2
.5. Thermal analysis
than finding a solution which describes the state of the system
at all points in time, this software uses a stochastic method
wherechangesinasystemaremodeledbyrandomlyselecting
among probability-weighted reaction steps [30].
Simultaneous thermogravimetric (TG) and differential
thermal (DT) analyses were carried out on a SETARAM TG-
DTA 92 thermobalance using 20 mg of sample; ␣-alumina
was used as reference.
Kinetics studies were performed with a special apparatus
built around a CAHN D200 electrobalance. The balance can
operate in vacuum and detect a mass change of 10 g. Gen-
erally, these experiments were done either under Ar or under
a gas mixture containing 20% O2 in Ar. The concentration of
water in these gases was less than 50 ppm. A vacuum purge
of atmospheric air was done before starting the experiments.
This operation induces a systematic mass loss because, under
vacuum, the samples start to lose water from room tempera-
ture.
3. Results and discussion
−
6
3.1. Characterization of boehmite samples
TEM micrographs of the boehmite samples (Fig. 1) re-
veal the very small size of the crystallites. Nanocrystals of
less than 10 nm can be seen, which have a strong tendency
to organize themselves to build polycrystalline fibers, sheet
or slabs. The main difference between the samples seems to
be the ability of crystallites to undergo auto-organization. S1
gives bidimensional objects (sheets) often folded. S3 essen-
tially contains almost monodimensional particles (fibers). S2
presents an intermediate behavior.
2
.6. Computer simulations of reactions
The X-ray diffraction patterns of the three samples are
reported in Fig. 2. Except for the first reflection they appear
to be very similar. The broad diffraction lines reveal that the
crystallites are very small. Moreover, in the same pattern, the
peak widths are also different, the first reflection being the
widest. The structural elements in boehmite crystals consist
The computer simulations of the chemical kinetic reac-
tions involved in the supposed reaction mechanism have been
done using the classical deterministic model based on the nu-
merical integration of a set of differential equations through
time. The differential equations are the rate laws of each re-
action step. The algorithm is a predictor-corrector algorithm
of double chains of AlO octahedra giving double molecules
6
(
based on Heun’s method [28]) which automatically adjusts
[1,31].
the step size of the integration. Iterative refinement of the
parameters (frequency factors, activation energies and reac-
tion orders) was done by a non-linear least-squares method
based on the simplex algorithm [28] to optimize a fit to the
experimental data.
Moreover, to check the validity of the results given by the
numerical integration procedure, we have also used the IBM
Chemical Kinetics Simulator (CKS) program [29]. Rather
These chains are parallel, forming layers with the OH
groups outside. The double chains are linked by hydro-
gen bonds between hydroxyl ions in neighboring planes.
Boehmite crystals exhibit perfect cleavage perpendicular to
the general direction of the hydrogen bonding [3].
Fig. 1. TEM micrographs of the boehmite samples.

7
8
P. Alphonse, M. Courty / Thermochimica Acta 425 (2005) 75–89
Fig. 4. Pattern decomposition of the experimental pattern recorded with the
sample S1. Each diffraction profile has been modeled using a separated
set of parameters (intensity, FWHM, Lorentzian fraction of pseudo-Voigt
function). Orthorhombic unit cell, space group Cmcm.
Fig. 2. X-ray diffraction patterns of boehmite samples (Cu K␣ radiation).
Orthorhombic unit cell, space group number 63, Cmcm.
This structure corresponds to an orthorhombic unit cell.
The diffraction peaks have been indexed using the space
group Cmcm. Whole pattern fitting of the experimental di-
agrams of Fig. 2 gives an acceptable agreement with the
boehmite structure (Fig. 3) provided that the first line (0 2 0)
wasexcludedbecauseitpresentstoolargeashifttowardsmall
angles from its calculated position (see Table 1). This shift of
the (0 2 0) line, observed for microcrystalline boehmite, has
been the subject of much controversy in the past. It has been
attributed to interlayer water [31,32], or to a smaller attractive
force between layers due to water absorption at the periphery
of layers [33]. More recent works [5–7] have demonstrated
that the (0 2 0) peak shift is essentially due to a particle-size
effect like the one observed for clays [34].
Wholepatternfittingagreementwiththeexperimentalpat-
tern is not very good for two groups of lines, at about 50 and
◦
6
5 (2θ) (Fig. 3). This is because the crystallite shapes are
anisotropic. If the refinement is done using a separate set of
parameters–peak intensity, FWHM, Lorentzian fraction of
pseudo-Voigt function, for modeling each diffraction profile
(
peak positions being fixed), the agreement between experi-
mental and modeled patterns improves (Fig. 4). The widths
of the 0 0 2 and 2 0 0 reflections are smaller than the others
because the crystallite dimensions along the a- and c-axes
are larger than along the b-axis, which means that crystallites
have a platelet shape. Such a shape has often been reported
for microcrystalline boehmite and can be related to the fact
that the cleavage plane is perpendicular to the b-axis. The re-
fined FWHM of the 2 0 0, 0 2 0 and 0 0 2 lines can be used to
estimate, by Scherrer’s equation, the average crystallite size
along the a-, b- and c-axis. The instrumental broadening con-
tribution was evaluated by using ␣-alumina (S1 calcinated at
◦
1
400 C for 2 h) as standard. The results, reported in Table 1,
show that crystallites are rectangular plates averaging 8 by
nm and 2–3 nm in thickness. This size is in good agreement
9
with the size of the elementary particles observed by TEM.
The marked enhancement in intensity of the 0 2 0 diffrac-
tion line indicates that a large fraction of the crystallites have
their b-axis oriented in a direction close to normal to the
sample holder plane. This texture is due to the plate shape
of the crystallites and has already been reported [35–37] for
microcrystalline boehmite.
The values of the cell parameters corresponding to the best
fit are very similar for the three samples (Table 1) and they are
˚
close to the parameters reported in literature (a = 2.868 A; b =
˚ ˚
1
2.214 A; c = 3.694 A) for crystallized boehmite [38]. From
Fig. 3. Whole powder pattern fitting (le Bail method) of the experimental
pattern recorded with the sample S1. Orthorhombic unit cell, space group
number 63, Cmcm.
the cell parameters, we have also calculated the cell volume
and the crystal density (Table 1).

P. Alphonse, M. Courty / Thermochimica Acta 425 (2005) 75–89
79
Table 1
Characteristics of boehmite samples
S1 not peptized
S2 peptized with 0.07 mol HNO3/Al
S3 peptized with 0.21 mol HNO3/Al
◦
2
θ shift ( ) for 0 2 0 line
1.05 ± 0.01
0.287 ± 0.0005
1.222 ± 0.001
0.371 ± 0.0005
0.130 ± 0.0005
3.063 ± 0.012
6.3 ± 0.2
0.74 ± 0.01
0.287 ± 0.0005
1.219 ± 0.001
0.371 ± 0.0005
0.130 ± 0.0005
3.071 ± 0.012
7.3 ± 0.2
0.86 ± 0.01
0.287 ± 0.0005
1.223 ± 0.001
0.371 ± 0.0005
0.130 ± 0.0005
3.059 ± 0.012
8.5 ± 0.2
a (nm)
b (nm)
c (nm)
3
Cell volume/nm
Crystal density/g cm
D200 (nm)
D020 (nm)
D002 (nm)
−3
2.8 ± 0.1
2.9 ± 0.1
2.5 ± 0.1
9.0 ± 0.3
9.0 ± 0.3
8.9 ± 0.3
Specific surface area/m2g
L.O.I. (wt.%)
Al2O3 (%)
−1
352 ± 0.4
341 ± 1.5
256 ± 0.5
30.5 ± 0.5
28.5 ± 0.5
32.0 ± 0.5
69.5 ± 0.5
71.5 ± 0.5
68.0 ± 0.5
AlOOH (%)
81.8 ± 0.5
84.1 ± 0.5
80.0 ± 0.5
Formula
AlO(OH)·0.74H2O
AlO(OH)·0.63H2O
AlO(OH)·0.83H2O
Experimental density (g cm 3)
Calculated density (g cm
−
2.355 ± 0.0004
2.478 ± 0.0005
2.378 ± 0.0007
−3
)
∼2.2
∼2.3
∼2.2
The specific surface area, measured after outgassing 15 h
forms a monolayer on the surface can be done [15,33]. The
maximum adsorption capability for water molecules is ob-
tained by expanding the coordination number of surface alu-
minum to six and of surface oxygen atoms to four (oxide
or hydroxide). The number of adsorption sites for water
molecules on a platelet crystallite (8 nm × 2.5 nm × 9 nm),
worked out following the same model as Nguefack et al. [15],
is given in Table 2. Adsorption on all the sites leads to the
stoichiometry AlOOH·0.5H2O. Experimentally we have be-
tween 0.63 and 0.83 H2O in excess according to the samples
(Table 1), which means that there is multilayer adsorption.
The water excess can also explain the strong discrepancy
between experimental densities measured with a helium pyc-
nometer and the densities calculated from lattice parameters
(Table 1). The crystal density, calculated from the lattice pa-
rameters, is:
◦
2
at 100 C, are close to 350 m /g for S1 and S2, but only
2
2
60 m /g for S3. Yoldas reported that the specific surface
area is affected by the amount of acid used for peptization
22]. He found that a critical amount of acid was needed to
[
peptize the hydroxide to a clear sol (the acid concentration
is about 0.07 mol per mol of Al). Increasing this amount in-
creases the repulsive force between colloidal particles and
enlarges the gelling volume which has a negative influence
on various properties of the oxide resulting from this gel.
By measuring the loss on ignition (LOI), which is the mass
◦
loss after calcination at high temperature (2 h at 1200 C), the
total amount of water contained in the samples is obtained
because they are transformed into ␣-alumina which is pure
Al2O3. The expected mass loss for the formation of anhy-
drous boehmite AlOOH, according to
m
crystal
2
AlOOH → H2O + Al2O3
(1)
d
=
(2)
crystal
V
crystal
would be 15%. Our samples contain about twice this amount
Similarly the experimental density dexp is:
(Table 1). Such values are not uncommon for very small
boehmite crystallites [12,14,15,31,39].
m
+ m
H2O
crystal
dexp =
(3)
As stated above, two models have been reported to explain
the water excess in boehmite; surface water adsorbed on the
particles [33] and inter-layer water in the crystal structure of
boehmite [31,32]. The second hypothesis was proposed to
account for the shift of the 0 2 0 line towards small angles but
we have seen that this shift has been explained by a particle-
size effect [5,6]. Now it is admitted that the excess of water
in boehmite is simply adsorbed on the crystallite surface.
A recent DFT study of the surface properties of boehmite
VH O + V
2
crystal
For 1 g of sample, mH O + m
= 1 and VH O = mH O =
2
crystal
2
2
1
−AlOOH/100, which gives:
ꢀ
ꢀ
ꢁꢁ−1
%
AlOOH/100
%AlOOH
100
dcalc =
+ 1 −
(4)
d
crystal
The results, reported in Table 1, demonstrate that, although
the calculated densities are slightly lower than the experi-
mental values (which could mean that the real volume taken
[40] has shown that the surface energy strongly depends on
3
the crystalline planes. The basal (0 1 0) surface, which forms
about 50% of the total surface, is unreactive. Water is only
physisorbed on it, whereas on the (1 0 0) and (0 0 1) surfaces,
water is chemisorbed, mainly dissociatively.
by water is less than 1 cm /g), the agreement is rather good.
3.2. Thermal decomposition of nanocrystalline boehmite
From the dimensions of crystallites, an estimation of the
amount of adsorbed water (physi- or chemi-sorbed) which
Typical TG–DTG–DTA curves are reported in Fig. 5a. The
dehydration appears to occur in three main steps. The first

8
0
P. Alphonse, M. Courty / Thermochimica Acta 425 (2005) 75–89
Table 2
Determination of the number of adsorption sites for water molecules for a platelet crystallite 8 × 2.5 × 9 nm (the same model as [15] has been used)
Direction
a
b
c
Crystal size (nm)
Cell parameters (nm)
Number of cells
Faces
8
2.5
1.22
2
(0 1 0)
133
9
0.37
25
(0 0 1)
0.287
25
(1 0 0)
45
2
Surface of face (nm )
35
Cell number (on 2 faces)
Atoms (cell)
2 × 2 × 25
2 × 25 × 25
2 × 2 × 25
2 AlIV + 2 OIII + 2 HOIII
800
2 AlV + 2 OIII + 2 HOIII
600
1 HOIII
Adsorption sites
1250
9.4
2
Molecule H2O/nm
13.3
22.8
Formula
AlO(OH)·0.12H2O
AlO(OH)·0.25H2O
AlO(OH)·0.1H2O
◦
profiles seem more-or-less the same, though the shape of the
second peak is different and its maximum is shifted towards
higher temperatures. This discrepancy between the DTG and
DTA curves, which indicates that the change in enthalpy is
not directly proportional to the rate of mass loss, is generally
encountered in complex reactions.
gives a sharp symmetrical endotherm and finishes at 200 C.
It accounts for 5–9% of the mass loss. The second step gives
◦
a broad unsymmetrical endotherm and ends before 500 C.
It represents the major part of the mass loss, about 20–25%.
The last step gives no thermal event but appears as a con-
tinuous mass loss and seems to stop at about 900 C. It only
◦
The shape of the DTG curve strongly depends on the ex-
perimentalconditions, suchassamplemass, staticordynamic
atmosphere, heating rate,. . . Fig. 5b illustrates that in the tem-
corresponds to about 3% of the mass loss. The DTG and DTA
◦
perature range 20–800 C, our samples give rather different
shapes of DTG profiles.
These kind of curves have often been reported in literature
[12,14,15,41]. The first step has been attributed to the des-
orption of physically adsorbed water, the second step to the
conversion of boehmite into ␥-alumina, and the last step to
the elimination of residual hydroxyls. The exotherm, which
appears at the very end of DTA curve, corresponds to the
transformation into ␣-alumina. The shape of these curves has
been found to be closely related to the crystal size of boehmite
[12,14]. The first peak is not observed for crystals larger than
50 nm. The second peak becomes sharper as the crystal size
increases and is shifted towards higher temperatures [14]. In
some papers [12,15,41], to explain the asymmetrical profile
of the second peak, an additional step is considered, attributed
to the removal of chemisorbed water before the conversion
into transition alumina.
For a given sample, the change of specific surface area,
as well as the mass loss induced by heating under vacuum,
have been followed. Because this mass loss essentially results
from the loss of water, to help the interpretation of the experi-
mental data, as well as the comparison between samples, it is
more convenient to convert the sample mass into nH O, where
2
nH O represents the number of water molecules in the sample
2
per Al2O3 formula. Thus for anhydrous boehmite, AlOOH,
nH O = 1 while nH O = 0 for ␣−Al2O3. For example, in the
2
2
case of a sample which had an initial mass m and a current
i
mass m after being heated
nH O
m − m
M
Al2O3
2
Al2O3
nH O =
=
2
n
MH O
m
Al2O3
2
Al2O3
Fig. 5. a. TG–DTA curves recorded with sample S2 (mass = 20 mg, air flow
◦
=
1.5 l/h, heating rate = 120 C/h) b. Comparison of the DTG curves for the
100 − LOI
◦
with mAl O = mi
(5)
different samples (mass = 30 mg, Ar flow = 1.5 l/h, heating rate = 150 C/h).
2
3
1
00

P. Alphonse, M. Courty / Thermochimica Acta 425 (2005) 75–89
81
Table 3
Effect of temperature on specific surface area
◦
SBET (m2g
−1
)
Sample
Outgassed in
T( C)
nH O
2
S1
S1
S1
S1
S1
S1
S1
S1
S1
S1
S1
S2
S2
S2
S2
S2
S2
S2
S2
S2
S2
S2
S2
S2
S3
S3
S3
S3
S3
S3
S3
S3
S3
S3
S3
S3
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
Air
Air
Air
Air
Air
Air
Air
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
50
1.45 ± 0.02
1.32 ± 0.02
1.16 ± 0.01
0.97 ± 0.01
0.76 ± 0.01
0.46 ± 0.01
0.32 ± 0.01
0.19 ± 0.01
0.13 ± 0.01
0.09 ± 0.01
0.08 ± 0.01
1.56 ± 0.02
1.16 ± 0.01
0.97 ± 0.01
0.79 ± 0.01
0.22 ± 0.01
339 ± 0.3
352 ± 0.3
361 ± 0.3
377 ± 0.4
395 ± 0.4
417 ± 0.6
426 ± 0.7
433 ± 0.8
433 ± 0.9
430 ± 0.9
428 ± 0.9
341 ± 1.5
383 ± 1.3
395 ± 1.3
412 ± 1.1
456 ± 2.5
468 ± 2.5
441 ± 1.3
404 ± 1.3
437 ± 1.2
339 ± 2.9
333 ± 2.9
237 ± 1.4
171 ± 0.5
229 ± 0.4
256 ± 0.5
282 ± 0.6
297 ± 0.7
327 ± 0.7
345 ± 0.7
354 ± 0.7
365 ± 0.7
378 ± 0.7
388 ± 0.8
386 ± 1.3
382 ± 1.6
100
150
200
250
300
325
355
390
455
465
100
250
270
300
350
450
350
400
350
500
500
700
800
50
100
150
200
250
265
285
305
320
350
390
440
1.72 ± 0.03
1.58 ± 0.03
1.41 ± 0.02
1.27 ± 0.02
0.97 ± 0.02
0.78 ± 0.01
0.67 ± 0.01
0.54 ± 0.01
0.38 ± 0.01
0.23 ± 0.01
0.12 ± 0.01
0.08 ± 0.01
Fig. 6. a. Effect of water content on the specific surface area. For each
analysis the sample was heated under vacuum (10 Pa) at the selected tem-
−2
perature for 18 h. To avoid a cluttered diagram, the absolute errors have not
been reported, they are given in Table 3. b. Effect of temperature on the water
−2
content. For each analysis the sample was heated under vacuum (10 Pa)
at the selected temperature for 18 h.
where LOI represents the loss on ignition and MH O and
for S1. Thus, under secondary vacuum, the transformation of
nanocrystallized boehmite into transition alumina can begin
from 200 C
2
M
Al2O3
are the molar masses of water and Al2O3, respec-
◦
tively.
For each surface analysis, the sample was heated under
For nH O > 0.2 − 0.15 the specific surface area increases
2
−
2
vacuum (10 Pa) at the selected temperature for 18 h. The
results are reported in Table 3 and Fig. 6. Fig. 6a shows the
steadily along with the water loss (Fig. 6a). Because the spe-
cific surface area is the surface to mass ratio, this means that
the surface area remains constant, that is the loss of water
does not create any additional porosity. Actually it has been
shown by Lippens [31] that the magnitude of the internal sur-
face area formed by dehydration on heating strongly depends
on the particle size of boehmite: the smaller the crystals, the
smallertheincreaseofthesurfacearea. Thisisbecausethede-
hydration is topotatic. If crystallite size of hydroxides is large
(small specific surface area), when water is expelled, space
will be created in the particle, which so becomes porous be-
cause the external volume of particles does not change very
much [10,20,42]. But in the case of nanocrystalline boehmite,
the crystallites size is so small that a large number of them
contain only a few unit cell along b axis so that the collapse of
the layered structure during the expelling of the water leads
to the particle shrinkage but does not create significant inter-
dependence of BET area on nH O, whereas Fig. 6b gives the
2
dependence of nH O on the temperature. The same trends can
2
be observed for the three samples, but, for the same water con-
tent, the specific surface area differs significantly according
to the sample and the difference remains for the whole range
of nH O. Fig. 6b shows that, when temperature increases,
2
there is a linear decrease of nH O down to nH O = 1 which
2
2
corresponds to AlOOH. This zone corresponds to the loss
of adsorbed (physisorbed and chemisorbed) water. Water is
more strongly bonded in S3 than in S1 or S2, because S3
◦
must be heated 100 C above S2 to reach the same value of
nH O. Below nH O = 1, the conversion of boehmite into tran-
2
2
sition alumina begins and there is a steep decrease of nH O.
2
In this zone the plots of the three samples merge into the
◦
same curve. Values of nH O < 1 were attained from 200 C
2

8
2
P. Alphonse, M. Courty / Thermochimica Acta 425 (2005) 75–89
nal porosity. The same kind of behavior has also be observed
in other topotatic dehydrations like those of ␣-FeOOH [43]
or ␥-FeOOH [44]. The larger the particle size of the start-
ing hydroxide, the larger the surface area of the iron oxide
obtained.
The transformation of nanocrystalline boehmite into tran-
sition alumina starts to decrease the surface area only at the
very end of the conversion, when nH O becomes less than
2
0
.15. This zone corresponds to the last step observed in ther-
mal analysis.
The surface, calculated from the mean crystallite dimen-
sions, assuming that every crystallite is a rectangular plate,
is given by the formula:
2
(uv + vw + wu)
S
calc
=
(6)
duvw
where d is the crystal density and u, v, w are the dimensions of
the crystal. Taking d = 3.06, u = 8, v = 2.5 and w = 10 nm (cf.
2
Table 1) gives Scalc ≈ 410 m /g. This value is in a rather good
agreement with experimental surfaces area which confirms
the absence of significant internal porosity.
Fig. 7 reports the changes in BET area with temperature
for the sample S2. When the samples were heated under vac-
uum (filled symbols), the surface increased up to 400 C. But
Fig. 8. Gradual change of the X-ray diffraction patterns during the thermally
stimulated conversion of boehmite (S3 sample) to transition alumina; (Cu
K␣ radiation).
◦
when the calcination was done at atmospheric pressure (open
◦
symbols), the surface decreased from 350 C. Hence dehy-
dration under vacuum allows the maximum surface area to
◦
intermediate compound is formed at any temperature, and
be obtained. Above 350 C, the surface steadily decreased
2
◦
the conversion of boehmite into alumina is gradual. Up to
(
about 60 m per 100 C).
◦
4
50 C, the 0 2 0 line of boehmite can be detected, thus the
In order to follow the crystal structure transformations
◦
whole temperature range of decomposition exceeds 100 C.
of nanocrystalline boehmite that occur upon heating, X-ray
diffraction patterns were recorded, at room temperature, on
samples heated under air for 2 h at specific constant tempera-
tures. ThepatternsobtainedwiththeS3samplearereportedin
◦
From 450 C, whole pattern fitting with a model based
on cubic spinel-type structure (space group number 227,
Fd3m) gives good agreement, if a separate set of parame-
ters is used for each reflection (Fig. 9). As stated above, both
◦
Fig. 8. Thecrystalstructureofboehmiteremainsupto350 C.
␥
-alumina and ␩-alumina have a spinel-type structure, gener-
For higher temperatures, the intensities of the boehmite lines
decrease, while new reflections characteristic of the spinel-
type structure of transition alumina progressively appear. No
ally reported as tetragonally deformed. However ␥-alumina
exhibits a more pronounced tetragonal distortion than ␩-
alumina. Although the intensity of (1 1 1) reflection appears
to be rather low, there is almost no tetragonal deformation,
which suggests that these patterns could correspond to ␩-
alumina. Lippens [31] reported that the thermal decomposi-
tion of well-crystallized boehmite produced ␥-alumina, while
pseudoboehmite gave the ␩ form. Even though ␩-alumina ap-
pears at relatively low temperatures, no sign of the formation
◦
of ␦- or ␪-alumina was detected up to 800 C. The cell pa-
rameters are similar over the whole temperature range (a =
0.792 ± 0.001 nm).
3.3. Kinetic analysis of the transformation of boehmite
into transition alumina
As seen above, the thermal decomposition of nanocrys-
talline boehmite to transition alumina is a complex process
involving four consecutive stages. Using the same formula-
tion as Tsukada et al. [12], the reaction mechanism can be
Fig. 7. Effect of temperature on the specific surface area. The filled symbols
correspondtosamplesheatedundervacuum, whileopensymbolscorrespond
to samples calcined in air at atmospheric pressure.

P. Alphonse, M. Courty / Thermochimica Acta 425 (2005) 75–89
83
◦
Fig. 9. Pattern decomposition of the experimental pattern recorded with the sample S3 heated in air at 450 C. Unit cell cubic, space group number 227, Fd3m.
written:
In order to evaluate the variation of the activation energy
with the extent of the reaction, an isoconversional method has
been used. Isoconversional methods are based on the princi-
ple that the reaction rate at a constant extent of conversion is
only a function of temperature [45]. In practice, several exper-
imentsshouldbecarriedoutunderthesameconditions(atmo-
sphere, pressure, flow rate, sample mass,. . .) only changing
the heating rate (β). Then, for a given fraction transformed
desorption of physisorbed water :
(AlOOH) · (m + n)H2O → nH2O
2
+
(AlOOH) · mH2O
(7)
(8)
2
desorption of chemisorbed water :
(α), a plot of ln(β) versus 1/T should give a straight line. For
(AlOOH) mH2O → mH2O + 2AlOOH
2
a constant heating rate and using the Doyle approximation
[
46], the slope is about −1.052 Ea/R. However, because we
obtained rather low activation energies, we used the correc-
tion for the Doyle approximation introduced by Flynn [47].
As above, the curves mass = f(t) have been converted
into nH O = f(t) where nH O stands for the number of wa-
conversion into transition alumina :
AlOOH → (2 − ν)/2H2O + Al2O3−ν/2(OH)ν
2
(9)
2
2
ter molecule per Al2O3 formula. The reaction coordinate, or
fraction transformed (α), was determined by:
dehydration of transition alumina :
Al2O3−ν/2(OH) → ν/2H2O + Al2O3
(10)
n − n
ν
0
t
α(t) =
(11)
n0 − nf
where m and n are, respectively, the number of physisorbed
and chimisorbed water molecules on the surface of boehmite,
and ν is the number of residual hydroxyl groups remaining
in the transition alumina.
where n0 is the value of nH O at the beginning of the process,
nt the value at the time t and n at the end of the process. The
results obtained using values of β in the range 30–600 C h
2
f
◦
−1
The goal of this study was to determine the kinetics pa-
rameters for each step, i.e., the activation energy Ea, the pre-
exponential factor Ao, and the reaction order, as well as the m,
n and ν values. We can reasonably assume that the kinetics pa-
rameters of each step do not change during the reaction course
because: (i) the reaction proceeds through an almost constant
surface area, except for the last step; (ii) the process only
involves short-range rearrangements of atoms in the crystal
structure; that is, the transformations are topotactic; (iii) the
particle are very small so that the size effects are minimized.
are reported in Fig. 10 and clearly confirm the complexity of
the reaction. Ea is approximately 40 kJ/mol for the first step
(0 < α < 0.2, loss of weakly adsorbed water). It increases
to 130 kJ/mol for the second step (0.2 < α < 0.6, loss of
chemisorbed water) and increases further to 160 kJ/mol for
the third step (0.6 < α < 0.9, conversion of boehmite into
transition alumina). Finally Ea decreases to 90 kJ/mol for the
last step (0.9 < α < 1.0, dehydration of transition alumina).
Standard deviations were very large for α below 0.4. This
reveals that, in this range, Ea depends on the heating rate.

8
4
P. Alphonse, M. Courty / Thermochimica Acta 425 (2005) 75–89
1
1
1
1
60
40
20
00
where [P] and [C] are the concentrations of each species and
k (T) and k (T) are the rate constants for dehydration and hy-
d
h
dration, respectively. The concentration of water is not taken
into account because, since we were working in a flowing at-
mosphere it is assumed to be constant and can be incorporated
in the rate constant. Hence we have three parameters for each
reaction (Ea, Ao and the initial concentration). The simula-
tion process consists of iteratively searching for the values
of the parameters which give the best fit (by a non-linear
least-squares method) between the experimental data and the
cumulative concentration of the H2O species computed from
numerical integration of these equations through time. The
time-temperature dependence is taken from the experimental
curve.
As in any non-linear least square refinement process, rea-
sonable initial values of the fitting parameters are needed
to avoid convergence to a local minimum rather than to the
global minimum. For the activation energy of dehydration we
have taken 45 kJ/mol which is the value found by the Flynn
method. The dehydration being endothermic, the activation
energy of the reverse reaction should be less than the energy
of the direct step and the difference between these values is
equal to the enthalpy of the reaction. We have assumed that
this energy is of the same magnitude as a hydrogen bond.
An average value for the hydrogen bond energy is 30 kJ/mol
8
6
4
2
0
0
0
0
0
0
0.2
0.4
0.6
0.8
1
Fraction transformed (α)
Fig. 10. Activation energies determined by an isoconversional method
Flynn [47]).
(
It has been demonstrated [48] that Ea depends on the heat-
ing rate in reversible reactions and, as we will see below, at
low temperatures the reverse reaction (hydration) cannot be
neglected.
Ourexperimentaldevicewasnotwelldesignedforisother-
malstudiesbecausethereactionratebecomesrapidbeforethe
furnace has reached the target temperature. However, in order
to study the first step separately, some experiments were done
◦
in the low temperature region (up to 100 C). After the short
heating ramp, the sample was maintained at a constant tem-
perature until there was no significant change of the sample
[49], hence the activation energy of hydration was taken as
15 kJ/mol. We have taken P = C = 0.4, as initial values for P
and C, because the starting value for nH O after outgassing at
2
mass with time. The final equilibrium value of n depended
f
room temperature was about 1.8, which means that the sum
P + C should be about 0.8. Moreover, the sum P + C was con-
strained to remain equal to 0.8. Initial values for Ao have been
estimated by trial and error in such a way that the calculated
curve was not too far from the experimental one.
The results of the simulations of some experimental tests,
realized with different heating ramps and plateau tempera-
tures, are reported in Table 4. The quality of fitting is evalu-
ated by the goodness of fit (GoF) defined as:
on temperature. This behavior has already been reported in
kinetic studies of boehmite [18]. This means that, in this tem-
perature range, the sample reached a stable water content for
each temperature. On the other hand, several studies have
shown that increasing the water vapor pressure causes the
rate constant to decrease but has little effect on the activation
energy[17,20]. Thestraightforwardinterpretationisthatther-
modynamic equilibrium between direct (dehydration) and
reverse (hydration) reaction was reached. Because the gas
purge contained a very low amount of water (<50 ppm), the
rate constant of hydration should be considerably larger than
that for dehydration.
ꢂ
ꢃ
2
wi(yi(exp) − yi(calc))
i
GoF =
(15)
(
N − NP)
We have simulated this equilibrium using a set of two
equations:
where yi(exp) are the experimental values, yi(calc) the values
calculated by numerical integration, wi the weight attributed
to each yi(exp), N the number of experimental points and NP
the number of parameters.
dehydration P → H2O + C
hydration H2O + C → P
(12)
(13)
◦
The fit obtained for a plateau temperature of 100 C is
worse than the others because, at this temperature, the sec-
ond step (loss of chemisorbed water) was beginning. In all
cases the best fit was obtained for rather similar Ea values:
about 50 kJ/mol for dehydration and 30 kJ/mol for hydration.
The difference between these energies, that is the enthalpy of
dehydration, is in the range 20–30 kJ/mol. Fig. 11 illustrates
that this model, based on an equilibrium between adsorption
and desorption of water, allows the shape of the experimental
curves, especiallytheinitialportionwherethesampleadsorbs
where P, the compound containing physisorbed
water—(AlOOH)2·(m + n)H2O in Eq. (7)—gives C
which only contains chemisorbed water.
The kinetics of these reactions are described by the differ-
ential equation:
d[P]
d[C]
=
−
= k (T)[P] − k (T)[C]
(14)
d
h
dt
dt
water, to be simulated well. The values of A for hydration
o

P. Alphonse, M. Courty / Thermochimica Acta 425 (2005) 75–89
85
Table 4
Simulations of the first step of the thermal dehydration of boehmite sample S2—mass, 25 mg; atmosphere, Ar
◦
Tplateau ( C)
70
80
90
100
◦
−1
)
␤
( C h
300
150
500
300
GoF
P
C
Ao dehydration
Ao hydration
Ea dehydration (J mol 1)
Ea hydration (J mol 1)
GoF
Ao (s 1)
Ea (J mol 1)
110
270
230
390
0.45 ± 0.01
0.35 ± 0.01
39500 ± 200
13 ± 0.1
0.44 ± 0.01
0.36 ± 0.01
6860 ± 80
2.05 ± 0.03
46200 ± 400
24200 ± 400
240
10500 ± 300
44250 ± 400
1.5 ± 0.10
0.47 ± 0.01
0.33 ± 0.01
199000 ± 1000
3.5 ± 0.2
0.45 ± 0.01
0.35 ± 0.01
81200 ± 2000
26 ± 1
−
52600 ± 100
31000 ± 200
110
58200 ± 200
27200 ± 200
210
54600 ± 1000
33300 ± 1000
−
−
65700 ± 1000
51700 ± 200
1.2 ± 0.05
415000 ± 10000
54600 ± 400
1.9 ± 0.05
−
Order
are far lower than for dehydration but, as stated above, Ao is
the product of the true pre-exponential factor and the concen-
tration of water in the gas flow. Because this concentration is
very low (about 50 ppm) the true pre-exponential factor for
hydration is actually larger than for dehydration.
part of the curves were ignored. They gave similar GoF, ex-
cept for the data of the last column for which agreement was
poor. In all cases, the activation energies are slightly lower
than the activation energies of dehydration.
The last task is the simulation of the whole process by the
set of Eqs. (7)–(10). However, in this mechanism, the first
threereactionsareconsecutive;thisrestrictionisactuallysup-
ported by no experimental evidence and, especially, nothing
requires that conversion into transition alumina should only
take place when boehmite has lost its adsorbed water. Hence
in our simulations, we have replaced Eqs. (7) and (8) by the
Eqs. (16) and (17):
It is clear that the same kind of equilibrium exists for the
second step and indeed a computation using a model based
on two successive reversible reactions (simulated by four re-
actions) gives a good fit with experimental data recorded for
◦
samples heated above 100 C. However, when the whole pro-
cess is considered, this increases the complexity considerably
because six reactions rather than four are then involved and
it would be an illusion to believe that it is possible to evaluate
so many parameters with reliability.
We can reduce the complexity and return to the initial
mechanism (with four reactions) if the first-order reversible
reactions are simulated by only one reaction, but with an
order different from one. Of course the price to pay is that
the first part of the curve (initial adsorption of water at low
temperature) cannot be correctly modeled. The results of the
simulations, using this simplified one-reaction model, are re-
ported at the bottom of Table 4. These computations were
done on the same experimental data as above, but the first
desorption of physisorbed water : P → H2O + B1 (16)
desorption of chemisorbed water : C → H2O + B2 (17)
conversion into transition alumina :
2AlOOH → (2 − ν)/2H2O + Al2O3−ν/2(OH)ν
(18)
dehydration of transition alumina :
Al2O3−ν/2(OH) → ν/2H2O + Al2O3
(19)
ν
where, as above, P is the compound containing physisorbed
water and C contains only chemisorbed water. The species
B1 and B2 are not used in the following steps, thus we have
three parallel reactions.
At this stage, recalling that a correct analysis uses as few
adjustable parameters as possible, it should be emphasized
that even if the peaks corresponding to step 2 and 3 are partly
overlapping, only a set of four (at least) equations allow to
obtain a correct fitting of the experimental curves. Neverthe-
less that does not demonstrate the uniqueness of the kinetic
parameters corresponding to the best fit of a single curve.
Therefore the choice of the starting values for the parameters
is very important, especially the activation energy.
Initial values for activation energies were those found by
the isoconversional method of Flynn. We have taken C = 0.5
Fig. 11. Simulation of the first reaction step (loss of physisorbed water)
with a model based on an equilibrium between adsorption and desorption of
water. For the sake of clarity, the number of experimental points has been
divided by 2. (Sample S2 under Ar flow).
as an initial value for C, and P = nini − C where n was the
ini
starting value for nH O after outgassing at room temperature.
2
The sum P + C was constrained to remain equal to n . As
ini

Table 5
Simulations of the thermal dehydration of boehmite
Sample
S2-f
S2-g
S2-j
S2-k
Ar
S2-e
Ar
S2-r
Ar
S2-m
S2-c
S2-t
S2-b
Ar
S2-u
Ar
S1-b
Ar
S3-b
Ar
Atmosphere
Ar
Ar
Ar
20%O2/Ar Vacuum 100 Pa Ar
◦
−1
)
β ( C h
300
30
1.75
280
0.35
0.40
1/2
300
30
1.72
260
0.34
0.38
1/2
300
30
1.70
280
0.33
0.36
1/2
150
30
1.71
520
0.36
0.35
1/2
60
30
30
30
300
30
150
30
150
30
150
30
2.23
530
0.60
0.63
1/2
150
10
1.75
670
0.42
0.33
1/2
150
30
1.86
890
0.51
150
30
1.86
510
0.50
0.36
1/2
m (mg)
Initial nH O
GoF
P
C
ν
1.72
500
0.36
0.35
1/2
1.70
560
0.38
0.32
1/2
1.77
450
0.42
0.35
1/2
1.70
680
0.35
0.35
1/2
2.00
720
0.61
0.39
1/2
2
0.35
1
Order step 1
Order step 2
Order step 3
Order step 4
Ea step 1 (J mol
Ea step 2 (J mol
Ea step 3 (J mol
Ea step 4 (J mol
2.4
1.0
1.3
3.0
44
60
115
2.3
1.0
1.2
2.8
45
60
115
2.4
1.0
1.2
2.7
45
60
115
2.8
1.0
1.3
2.8
42
60
115
2.9
1.0
1.4
2.9
43
59
114
3.0
1.0
2.0
2.8
39
60
113
2.4
1.1
1.3
2.9
44
59
112
2.9
1.4
1.5
2.6
36
51
105
2.6
0.9
1.4
3.0
42
59
113
2.9
1.7
1.3
2.9
44
62
118
2.5
0.9
1.7
3.0
40
59
113
1.6
1.2
2.1
3.0
44
54
109
3.0
1.0
1.4
3.0
46
61
117
−1
−1
−1
−1
)
)
)
)
63
64
63
63
65
65
65
51
63
68
63
52
68
−1
−1
−1
−1
Ao step 1 (s
Ao step 2 (s
Ao step 3 (s
Ao step 4 (s
)
)
)
)
5100
420
194000
7
4400
410
164000
7
4500
440
168000
7
4800
360
154000
5
3800
140
107000
4
1400
220
95900
17
7700
430
138000
12
3000
110
141000
4
6100
330
147000
6
5200
430
218000
7
6200
410
175000
10
3600
410
418000
9
4300
440
328000
19
For the sake of clarity, errors have not been reported and values have been rounded. Maximum relative errors are less than 5% for order, 1% for Ea and 10% for Ao.

P. Alphonse, M. Courty / Thermochimica Acta 425 (2005) 75–89
87
previously, initial values for Ao have been estimated by trial
and error so that the curve calculated from the initial guesses
was not too far from the experimental one. Moreover, ν was
not refined as the other parameters. Five simulations were
done, taking successively ν = 1, 2/3, 1/2, 2/5 and 1/3.
for the fourth. It is interesting to notice that the Ea of the third
step (conversion of boehmite into transition alumina) is close
to the heat of chemisorption of H2O on ␥-Al2O3 [50]. This
step could be limited by the difficulty of removing strongly
chemisorbed water. The Ea of the last step is very low for a
◦
Theresultsofthesimulations, obtainedwithdifferentsam-
ples, heating ramps, and atmospheres are reported in Table 5.
Though these experiments have been done under various con-
ditions, the values of the parameters giving the best fit with
experimental data are generally remarkably similar, espe-
cially the activation energies. The simulated curves follow
the experimental profiles very closely as shown in Fig. 12.
To check that the numerical integration procedure has given
correct results, we have fed the CKS program (see experi-
mental) with the equation of each step and the parameters
of Table 5. It can be seen in Fig. 13 that the agreement is
perfect. On the other hand, this figure confirms that the first
three reactions are not consecutive but parallel. The loss of
chemisorbed water begins although physisorbed water still
remained. Similarly transition alumina appears even though
boehmite still contains almost 50% of chemisorbed water.
reaction occurring in the temperature range 400–500 C, but
all the attempts to increase this energy lead to worse fits. A
reason for so low an activation energy could be that a diffu-
sion process essentially limits this step. The order of the first
step is generally close to 3 which indicates the reversibil-
ity of this step. Conversely, the second step is found to be
approximately a first-order process, which shows a minor
contribution of the reverse reaction.
The sample S2-m has been analyzed using a flowing gas
mixture of 20%O2 in Ar. Comparison with samples heated
under Ar with the same heating rate (the three first columns)
shows that oxygen plays no role in the dehydration process.
A0, for the step 1, was found to be larger probably because
the O2-containing mixture was drier than the Ar.
The sample S2-c has been analyzed under vacuum. We
have worked under a relatively high pressure (100 Pa) be-
cause, for lower values, the TG signal was too noisy. It is clear
thatworkingundervacuuminducesashifttowardslowertem-
peratures. All the Ea values are found to be lower. The rate of
the second step is also decreased. We have seen above that,
under secondary vacuum, the conversion of boehmite into
Fig. 13 also indicates that transition alumina is formed above
◦
3
00 C, which is in good agreement with our previous find-
ings (Section 1.2).
The three first columns (S2-f, S2-g, S2-j) of the Table 5
demonstrate that three similar samples tested under the same
conditions lead to very similar results. The small differences
◦
transition alumina begins from 200 C. This demonstrates
in the starting value for nH O are due to the difficulty of
achieving the same outgassing.
that the kinetics of the whole process are governed by the
partial pressure of water.
2
The Ea of step 1 is generally very close to the value found
by the Flynn method. However, for the next steps, Ea is signif-
icantly lower than predicted by this method: 60 kJ/mol for the
The samples S2-k, S2-t and S2-b contained different
amounts of adsorbed water but were otherwise analyzed un-
der the same conditions. The initial value of nH O seems to
2
2
nd step, 110–120 kJ/mol for the third and only 60–70 kJ/mol
affect the reaction parameters only when it becomes larger
Fig. 12. Simulation of the whole reaction with a model, based on four reaction steps. For the sake of clarity, the number of experimental points has been divided
◦
by 2. (Sample S2 under Ar flow, β = 300 C/h).

8
8
P. Alphonse, M. Courty / Thermochimica Acta 425 (2005) 75–89
Fig. 13. Comparison between simulation of the whole reaction i) by the numerical integration of the differential equations (symbols) ii) by the stochastic
method used by the IBM Chemical Kinetics Simulator (CKS) program [29] (full curves). The experimental data are the same as in Fig. 12.
than 2.0. This can be linked to the fact that, for nH O ≈ 2.0, the
The kinetic simulations of the non-isothermal experiments
at constant heating rates show that the thermally stimulated
transformation of nanocrystalline boehmite into alumina can
be modeled by a 4-reaction mechanism involving: (i) the loss
of physisorbed water, (ii) the loss of chemisorbed water, (iii)
the conversion of boehmite into transition alumina, (iv) the
dehydration of transition alumina (loss of residual hydroxyl
groups).
2
surface is fully covered. Above that value there is formation
of a multilayer.
For the samples S2 and S3, the best fits were obtained for ν
=
1/2; but for the sample S1, the optimum was found for ν = 1.
Thesehighvalues, forthenumberofresidualhydroxylgroups
remaining in transition alumina, can be explained by the fact
that, as pointed out before, there is still a lot a chemisorbed
water when transition alumina starts to form.
The activation energies of each step are found to be very
similar for experiments done under various conditions (heat-
ing rate, atmosphere, kind of sample,. . .). Values found are
40–45 kJ/mol for the first reaction, 50–60 kJ/mol for the sec-
4
. Conclusions
ond, 105–115 kJ/mol for the third and 50–70 kJ/mol for the
last. The first reaction is actually an equilibrium because, at
low temperature, the hydration rate is larger than the dehy-
dration rate. Formation of transition alumina occurs although
boehmite still contains chemisorbed water, which could ex-
plain the large number of residual hydroxyl groups. The sur-
prisingly low activation energy found for the last reaction
probably reveals that the dehydration of transition alumina is
essentially limited by a diffusion process.
Hydrolysis of ASB according to the Yoldas method yields
nanocrystalline boehmite with crystallites which are rectan-
gular platelets averaging 8 by 9 nm and 2–3 nm in thickness.
This nano-sized boehmite contains a large excess of water.
When water is eliminated by heating under secondary vac-
uum, an increase in the specific surface area is observed down
to a minimum water content (≈0.2 H2O per Al2O3). Values
2
up to 470 m /g can be reached. However, this enlargement
of specific surface area is only due to the loss of water, the
surface area remaining constant. Though the conversion into
transition alumina can begin from 200 C, the temperature at
Acknowledgements
◦
◦
which the maximum surface is observed is about 350 C. The
The authors would like to thank Mr. Lucien Datas (CIRI-
MAT) for the TEM work. This research was done in the frame
of the MINIREF (ENK6-CT-2001-00515) project supported
by the E.C.
2
◦
surface steadily decreased (about 60 m per 100 C) when
the sample was heated beyond this temperature. The parti-
cle morphology, the excess of water, as well as the specific
surface area, depend on the amount of acid used for the pep-
tization during the synthesis. The transition alumina has a
cubic spinel-type structure with almost no tetragonal defor-
mation. No significant change, either in the crystal structure
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