Microstructural transformation with heat-treatment of aluminum hydroxide with gibbsite structure

Article abstract of DOI:10.1246/bcsj.82.618

Aluminum hydroxide with gibbsite structure was prepared, and the microstructural transformation of the sample heat-treated at various temperatures was investigated. The sample was characterized by fieldemission scanning electron microscopy (FE-SEM), X-ray

Full text of DOI:10.1246/bcsj.82.618

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618
Bull. Chem. Soc. Jpn. Vol. 82, No. 5, 618–623 (2009)
© 2009 The Chemical Society of Japan
Microstructural Transformation with Heat-Treatment
of Aluminum Hydroxide with Gibbsite Structure
Tomohiro Mitsui,1 Toshiaki Matsui,1 Ryuji Kikuchi,2 and Koichi Eguchi*1
1Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University,
Nishikyo-ku, Kyoto 615-8510
2Department of Chemical System Engineering, Graduate School of Engineering, The University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656
Received January 14, 2009; E-mail: eguchi@scl.kyoto-u.ac.jp
Aluminum hydroxide with gibbsite structure was prepared, and the microstructural transformation of the sample
heat-treated at various temperatures was investigated. The sample was characterized by field emission scanning electron
microscopy (FE-SEM), X-ray diffraction (XRD), thermogravimetry and differential thermal analysis (TG-DTA), and BET
surface area. The shape of the grains in the prepared sample was hexagonal prism-like morphology. The prepared sample
kept a metastable state of alumina phase at higher temperatures than the commercially available gibbsite powders. The
prepared gibbsite grains underwent characteristic structural change depending on the calcination temperature. The
transformation of the surface morphology was initiated at 400 °C, leading to the formation of cracks with the direction
parallel to the basal plane. After calcination at 1200 °C, a large number of grooves were formed on the surface of the
lateral planes. The specific structural change of gibbsite induced by the heat treatment was strongly related to the
topotactic dehydration from gibbsite and subsequent phase transition to aluminum oxides.
Aluminum hydroxide crystallizes in various structures, such
of Pt particles confined in the pores was effectively suppressed
at high temperatures. Although the phase transition from
gibbsite to ¡-Al2O3 has been intensively investigated, the
change of the surface morphology in this process is still
unclear.
In this study, the microstructural change of gibbsite grains
induced by heat treatment was investigated. The gibbsite grains
with hexagonal prism morphology were prepared from sodium
aluminate solution by ordinary procedures and then were heat-
treated at various temperatures in air. Microstructural trans-
formation of the prepared sample was compared to that of
commercially available material, and the effect of the heat
treatment on the morphology and particle size of gibbsite was
investigated.
as gibbsite, boehmite, bayerite, and diaspore.1,2 Among these
crystalline phases, gibbsite-type aluminum hydroxide is the
main component in bauxite. Although gibbsite has been mass-
produced via the crystallization of supersaturated sodium
aluminate solution, this Bayer process has been known as a
time-consuming preparation.1,3,4
Aluminum hydroxide is an important intermediate for the
production of aluminum oxide, which is readily formed via
dehydration with heat treatment. The dehydration process
strongly depends on the crystal structure of aluminum
hydroxide.5-11 In the case of gibbsite, the dehydration proceeds
via formation of a variety of metastable transition alumina
phases, such as »-Al2O3, £-Al2O3, and ª-Al2O3, finally leading
to stable ¡-Al2O3 above 1200 °C.11 These metastable aluminas
have been widely employed as support oxides for the
deposition of precious metal catalysts because of their large
surface area and porous microstructure. On the other hand,
aluminum hydroxide with diaspore structure is directly trans-
formed into ¡-Al2O3 at 600 °C.10 This phase transition greatly
affects the active metals on the support surface when alumina is
used as a catalyst support. Agglomeration and sintering of
alumina supports generally gives rise to sintering of metal
particles at high temperatures, leading to the deterioration of
the catalytic activity.12-14 Accordingly, ¡-Al2O3 with low
surface area is unsuitable as a catalyst support. We have
recently reported that aluminum hydroxide with gibbsite
structure undergoes the phase transition to ¡-Al2O3 leaving
laminated intra-grain gaps after calcination above 1200 °C.15
This ¡-Al2O3 material with a unique texture was useful as a
catalyst support despite its low surface area, since the sintering
Experimental
Sample Preparation. The gibbsite powder was prepared from
sodium aluminate solution. Aluminum metal powder (purity
99.99%, Kishida Chemical Co., Ltd., Japan) was dissolved in an
aqueous 2 M NaOH solution at 70-90 °C in a Teflon vessel. The
solution was stirred at 70 °C for 4 h. The obtained white precipitate
was washed with distilled water three times and was dried at room
temperature for 2 days and then at 140 °C for 6 h. Subsequently,
the resulting powder was calcined at various temperatures for 5 h
in air so as to investigate the microstructural change of gibbsite.
For comparison, commercially available aluminum hydroxide
powders with gibbsite structure (C12S or C301, Sumitomo
Chemical Co., Ltd., Japan) were investigated.
Sample Characterization. The samples were characterized by
using the following equipment. X-ray diffraction (XRD) patterns
were recorded by using Cu K¡ radiation on a RIGAKU Ultima IV
diffractometer for phase identification in the samples. Thermo-
Published on the web May 13, 2009; doi:10.1246/bcsj.82.618

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T. Mitsui et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 5 (2009)
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Gibbsite
(002)
(110)
(200)
10
20
30
40
50
60
c
Al
H
O
2
θ
/ degree
a
Figure 2. XRD pattern of the prepared gibbsite powder.
b
Figure 1. Crystal structure of aluminum hydroxide with
gibbsite structure.
gravimetry (TG) and differential thermal analysis (DTA) were
¹1
hexagonal prism structure
conducted under air flow with a heating rate of 10 °C min (SII
Nanotechnology Inc., EXTRA6000 TG/DTA 6300). Microstruc-
tural observation was carried out using a JEOL JSM-6705F field
emission scanning electron microscope (FE-SEM). BET surface
area was determined by N2 adsorption at liquid nitrogen temper-
ature using a BEL Japan Belsorp-mini II analyzer. Prior to the
measurement, the samples were pretreated under vacuum at 300 °C
for 30 min.
1 µm
Results and Discussion
Figure 3. FE-SEM image of the prepared gibbsite powder.
Crystal Structure of Gibbsite. The gibbsite crystal is
classified to a monoclinic space group (P21/n). The perspective
view of the crystal structure of gibbsite is illustrated in
Figure 1. The reported lattice parameters are a = 8.684 ¡,
b = 5.078 ¡, c = 9.736 ¡, and ¢ = 94.54°.16,17 The pseudo-
hexagonal structure of gibbsite consisted of stacked layers
along the c axis. Each layer is composed of edge-shared AlO6
octahedra. Two-thirds of the octahedral sites are occupied by
aluminum cations in each bi-layer of the close-packed oxygen
atoms of hydroxy groups. Half of the hydroxy groups
contribute to intralayer hydrogen bonds, and the other half to
interlayer hydrogen bonds along the c direction. This layered
crystal structure affected the morphology of aluminum oxide
formed by dehydration from gibbsite as well as that of
aluminum hydroxide.
Characterization of Gibbsite. The XRD pattern of the
prepared white powder is shown in Figure 2. The pattern
agreed with that of gibbsite (JSPDS File Card No. 33-0018).
An anomalously strong peak ascribable to the diffraction from
(002) planes was observed at 2ª = 18.3°. This indicates that
the gibbsite grains in this white particle possess anisotropic
crystal habit with a well-grown stacking along the c direction.
Figure 3 shows an FE-SEM image of the powder sample.
The shape of the grains in the sample was hexagonal prism-like
morphology. Hereafter, the hexagonal faces of each grain are
expressed as basal planes. Generally, the morphology of the
gibbsite grains has been reported to be pseudohexagonal with
(001) basal plane, and (100) and (110) lateral planes.18-21
Therefore, the crystalline grains have grown along the c
direction. The hexagonal crystallographic planes were piled up
to form hexagonal prismatic grain. The formation process in
this experiment can be represented by the following reactions 1
and 2.
2Al þ 2NaOH þ 6H2O ! 2NaAlðOHÞ4 þ 3H2
NaAlðOHÞ4 ! AlðOHÞ3 þ NaOH
ð1Þ
ð2Þ
Phase Transition of Gibbsite. The TG-DTA profiles of the
prepared powder sample are shown in Figure 4. A sharp
endothermic peak at ca. 290 °C was observed. A weight loss in
this temperature range was in good agreement with the
calculated value (23.1%) by assuming the dehydration of
aluminum hydroxide, i.e., Al(OH)3 ¼ AlOOH + H2O. In
addition, a shoulder endothermic peak appeared at ca. 220 °C.
This additional peak corresponds to dehydration from the
surface of gibbsite particles.8,11,22 A broad endothermic peak
appeared at ca. 500 °C. The total weight loss up to 600 °C
was 34.0% in each sample, which agreed with the expected
value (34.6%) for the complete dehydration to alumina:

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Bull. Chem. Soc. Jpn. Vol. 82, No. 5 (2009)
Microstructural Transformation of Gibbsite
0
-5
Gibbsite
Boehmite
χ
γ
κ
θ
α
-Al2O3
-Al2O3
-Al2O3
-10
-15
-20
-25
-30
-Al2O3
-Al2O3
-35
(a)
100 200 300 400 500 600 700 800
Temperature / oC
Figure 4. TG-DTA profiles of the prepared gibbsite
powder.
(b)
(c)
2Al(OH)3 ¼ Al2O3 + 3H2O. Thus, the endothermic peak at ca.
500 °C is ascribable to the dehydration reaction from boehmite
to Al2O3. It was therefore clarified that the dehydration of
gibbsite to form alumina via boehmite was completed below
600 °C.
(d)
(e)
The XRD patterns of the sample calcined at various
temperatures for 5 h are shown in Figure 5. The boehmite
phase appeared at 200 °C in addition to sharp and dominant
diffraction from the gibbsite phase. At the calcination temper-
ature of 400 °C, the boehmite and »-Al2O3 phases became
dominant accompanied with a disappearance of the gibbsite
phase, and the phase transition to aluminum oxide was
completed at 600 °C. In the range of 600-1000 °C, the sample
underwent complicated transition between a variety of alumi-
nas, such as »-Al2O3, £-Al2O3, ¬-Al2O3, and ª-Al2O3, leading
to the formation of ¡-Al2O3 above 1200 °C. Generally, gibbsite
is dehydrated and converted via two pathways below 1200 °C:
(i) the transition to metastable aluminas via boehmite; (ii) the
direct formation to »-Al2O3.7,9 Thus, the transition process of
the prepared sample appears to involve both pathways.
Figure 6 shows BET surface area of the prepared sample
calcined at various temperatures. The surface area decreased
with an increase in the calcination temperature.
Microstructural Change with Heat Treatment. Figure 7
shows FE-SEM images of the gibbsite samples calcined at
various temperatures. The texture was significantly changed
depending on the calcination temperature. In the sample
calcined at 200 °C, the morphology of each grain was
hexagonal prismatic which was almost the same as the
untreated one in Figure 3. After calcination at 400 °C, however,
a number of cracks were observed on the lateral surfaces. The
cracks have grown in parallel direction to the basal plane of the
hexagonal grain and deepened with an increase in calcination
temperature. The original hexagonal shape of the grains was
retained even after heating to 1000 °C. It is suggested that the
formation of the cracks on the surface resulted from the phase
transition from boehmite to £-Al2O3. Aluminum hydroxide
with boehmite structure was formed by the heat treatment of
gibbsite below 400 °C. It is expected from the crystal structure
(f)
10
20
30
40
50
60
70
80
2
θ / degree
Figure 5. XRD patterns of the prepared sample calcined at
various temperatures for 5 h: (a) 200, (b) 400, (c) 600, (d)
800, (e) 1000, and (f) 1200 °C.
300
250
200
150
100
50
0
400
600
800
1000
1200
1400
Temperature / oC
Figure 6. BET surface area of the prepared sample as a
function of calcination temperature.

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Bull. Chem. Soc. Jpn. Vol. 82, No. 5 (2009)
621
(a) 200 °C
(c) 800 °C
(e) 1200 °C
(b) 400 °C
(d) 1000 °C
(f) 1400 °C
composed of stacked layers, which are largely disordered along
the c direction.2,30,31 Thus, the layered structure is maintained
up to 1000 °C, until the transition to ¬-phase proceeded as
shown in Figure 5. The layered crystal structure collapsed
above 1000 °C accompanied by the conversion into ¬-phase.
Thus, it is concluded that the formation of the cracks at low
temperatures is mainly ascribable to the phase transition via
boehmite.
The texture of the sample was significantly changed by heat
treatment above 1200 °C. In the sample calcined at 1200 °C, the
difference in the morphology between the basal and the lateral
planes was obvious. A large number of grooves were
preferentially observed in the lateral plane, which were
propagated from the cracks parallel to the basal plane. The
hexagonal prismatic external geometry of grains was retained.
The laminated microstructure has been developed in the
original hexagonal prismatic grains. After calcination at
1400 °C, the sample exhibited an intricate structure. The
hexagonal basal plane deformed and was porous. As shown in
Figure 5, the phase transition to ¡-Al2O3 proceeded above
1200 °C. Accordingly, the formation of porous structure
originated in the transition to ¡-phase.
As mentioned above, aluminum hydroxide of gibbsite
structure is transformed into ¡-Al2O3 with highly oriented
grooves by calcination at elevated temperatures. We have
reported previously that sintering of Pt particles located inside
the groove was often affected by the restriction in the gap
between ¡-Al2O3 slabs.15 Growth of Pt particles confined in the
gap of the slabs was effectively suppressed. The present
investigation focused on the specific structural change of
gibbsite induced by the heat treatment. The cracks formed at
low temperatures are developed into the grooves between ¡-
Al2O3 slabs after heating at 1200 °C for grains with well-grown
hexagonal prismatic morphology. Porous ¡-Al2O3 with lami-
nated microstructure and high thermal resistance is a suitable
structure to suppress the agglomeration of supported precious
metal particles. Thus, it will be applicable as a catalyst support
for the sintering inhibition of precious metal particles.
1 µm
1 µm
1 µm
1 µm
1 µm
1 µm
Figure 7. FE-SEM images of the prepared sample calcined
at various temperatures for 5 h: (a) 200, (b) 400, (c) 800,
(d) 1000, (e) 1200, and (f) 1400 °C.
of gibbsite that the dehydration proceeds via topotactic phase
transition. From the layered structure of gibbsite in Figure 1,
the elimination of water molecules from the crystal gave rise to
compaction of the layers along the c direction, leaving the
pseudohexagonal packing of oxygen ions. Boehmite belongs to
a layered crystal structure, and the complete transition to £-
Al2O3 with a defective spinel structure proceeded at ca.
500 °C.23 The transformation of boehmite has been studied in
detail.24,25 The collapse of the layered structure of boehmite
proceeds by elimination of water and the displacement of
aluminum atoms, leading to the formation of cracks. A similar
change of surface morphology is confirmed in iron(III)
hydroxide, cobalt(II) hydroxide, magnesium hydroxide, and
cadmium hydroxide, resulting in the formation of a porous
microstructure at high temperatures.26-29 Therefore, the struc-
tural change of gibbsite at low temperatures is expected to be
initiated by the elimination of water from the interlayer gap by
retaining the high packing density of aluminum and oxygen in
the layers. As shown in Figure 5 on the other hand, a part of
gibbsite underwent the phase transition to »-Al2O3 at ca.
600 °C in the other dehydration pathway. »-Alumina phase is
Effect of the Heat Treatment on the Morphology and
Particle Size.
The heat-induced transformation of the
prepared sample was compared with those of commercially
available ones. The effect of the heat treatment on the
morphology and particle size of gibbsite was investigated. X-
ray diffraction patterns of C12S and C301 samples are basically
the same as that in Figure 2; i.e., these samples are also
categorized as gibbsite. The dehydration process of C12S and
C301 was analogous to that of the prepared sample. However,
the transition temperature to ¡-phase was different from the
prepared sample. The XRD patterns of C12S and C301
calcined at 1200 °C for 5 h are shown in Figure 8. Complete
phase transition to ¡-Al2O3 proceeded for each sample, as the
patterns consisted of a single phase of ¡-Al2O3. As shown in
Figure 5 on the other hand, the transition to ¡-phase was
incomplete in the prepared sample. Therefore, it can be
concluded that the phase transition in the prepared sample
was retarded even at 1200 °C. Table 1 summarizes BET surface
area of the prepared sample, C12S, and C301 calcined at 1200
or 1400 °C. The surface area of the prepared sample was larger
than those of other samples after calcination at 1200 °C, though