Synthesis of thermally stable micro spherical χ-alumina by thermal decomposition of aluminum isopropoxide in mineral oil

Article abstract of DOI:10.1016/S1387-7003(03)00153-9

Thermal decomposition of aluminum isopropoxide (AIP) in mineral oil at 250-300 °C over a 2 h duration results in χ-alumina powders having high thermal stability. The mechanism of the process involves the formation of amorphous complex before further decomposition takes place. Phase transformation of the obtained products was also investigated. It was found that χ-alumina synthesized by this method transformed directly to α-alumina at temperature higher than 1000 °C.

Full text of DOI:10.1016/S1387-7003(03)00153-9

Inorganic Chemistry Communications 6 (2003) 930–934
www.elsevier.com/locate/inoche
Synthesis of thermally stable micro spherical v-alumina by
thermal decomposition of aluminum isopropoxide in mineral oil
Okorn Mekasuwandumrong , Peter L. Silveston , Piyasan Praserthdam a,*,
a
a
b
Masashi Inoue , Varong Pavarajarn , Waraporn Tanakulrungsank
a
c
a
Research Center on Catalysis and Catalytic Reaction Engineering, Department of Chemical Engineering, Faculty of Engineering,
Chulalongkorn University, Bangkok 10330 Thailand
b
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 606-8077, Japan
Department of Chemical Engineering, Faculty of Engineering, Rajamangala Institute of Technology, Pathumthani 12110, Thailand
c
Received 9 February 2003; accepted 20 April 2003
Abstract
Thermal decomposition of aluminum isopropoxide (AIP) in mineral oil at 250–300 °C over a 2 h duration results in v-alumina
powders having high thermal stability. The mechanism of the process involves the formation of amorphous complex before further
decomposition takes place. Phase transformation of the obtained products was also investigated. It was found that v-alumina
synthesized by this method transformed directly to a-alumina at temperature higher than 1000 °C.
Ó 2003 Elsevier Science B.V. All rights reserved.
Keywords: Thermal decomposition; Direct transformation; Mineral oil; v-Alumina
1
. Introduction
isopropoxide (AIP) in inert organic solvents, such as
toluene and benzene, remains in its original phase up to
temperature around 1150 °C, after which a direct
transformation to a-alumina takes place [6]. Hence, the
v-alumina prepared by this method is considered having
high thermal stability. Moreover, since transformation
from v- to j-alumina is avoided, the loss in surface area
and changes in surface properties of alumina product
due to j-phase transformation lessen.
Alumina is one of the most common crystalline ma-
terials to be used as adsorbents, coatings, soft abrasives,
catalyst and catalyst support [1,2] because of its fine
particle size, high surface area, and surface catalytic
activity. The structure and stability of alumina make it
an important constituent of many protective oxides
formed on the surface of high-temperature metals and
alloys.
v-Alumina is one of the metastable polymorphs of
transition alumina, which is normally obtained by de-
hydration of gibbsite [3–5]. v-Alumina transforms to j-
alumina at temperature in the range of 650–750 °C and
consequently to a-alumina at temperature around 1000
In this work, results from the reaction of AIP in
mineral oil, including morphology and phase transfor-
mation behavior of products are presented.
2. Experiment
°
C [5]. Both transformations lead to undesired loss in
surface area and changes in surface properties. How-
ever, it has been reported that nanosized v-alumina
prepared from thermal decomposition of aluminum
2.1. Sample preparation
Aluminum isopropoxide (25 g) was suspended in 100
ml mineral oil in a test tube, which was then placed in a
3
00 ml autoclave. In the gap between the test tube and
*
the autoclave wall, 30 ml of mineral oil was added. The
autoclave was purged completely by nitrogen before
Corresponding author. Tel.: +66-2-218-6766; fax: +66-2-218-6769.
E-mail address: piyasan.p@chula.ac.th (P. Praserthdam).
1
387-7003/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S1387-7003(03)00153-9

O. Mekasuwandumrong et al. / Inorganic Chemistry Communications 6 (2003) 930–934
931
heating up to the desired temperature, in the range of
50–300 °C at a rate of 2.5 °C/min. Once the prescribed
temperature was reached, the temperature was held
constant for desired period of time (0, 1 and 2 h, re-
spectively). After the system was cooled down, the re-
sulting powders were repeatedly washed with acetone
and dried in air.
A part of the synthesized product was then calcined
in a box furnace by heating up to the desired tempera-
ture, in the range of 1000–1200 °C, at a rate of 10 °C/
min and held at that temperature for 1 h.
3. Results and discussions
2
The XRD patterns of as-synthesized products are
shown in Fig. 1. It can be seen that, when holding time
was 2 h, all obtained products exhibited typical XRD
pattern of v-alumina, even when the reaction tempera-
ture was 250 °C. However, at 250 °C, the products were
amorphous when the reaction period was limited to 1 h
or less. The crystallite size of all samples calculated from
Scherrer equation are summarized in Table 1.
The IR spectra of as-synthesized products are shown
in Fig. 2. The amorphous powder obtained from the
reaction at 250 °C with 0 h holding time shows strong
absorption bands due to the n- and iso-aliphatic hy-
drocarbon groups [7]. These absorption bands decrease
and eventually disappear with an increase in reaction
holding time and temperature. On the other hand, the
characteristic absorption bands of boehmite at 773 and
2
.2. Characterization
Powder X-ray diffraction (XRD) analysis was done
by using a SIEMENS D5000 diffractometer with Cu-Ka
radiation. The crystallite size was calculated from
Scherrer equation by using a-alumina as external stan-
dard. Infrared (IR) spectra were recorded on a NICO-
LET FT-IR Impact 400 spectroscopy. Morphologies of
the products were observed on JEOL JSM6400 Scan-
ning Electron Microscope and JEOL TEM-200cx
Transmission Electron Microscope.
À1
615 cm [8] were observed in the products obtained
from 2 h reaction at 300 and 250 °C. Boehmite was re-
sulted from the presence of small amount of water,
which was by-product from AIP decomposition, in the
reaction system.
Morphologies of as-synthesized and calcined samples
observed by SEM are shown in Fig. 3. Spherical parti-
cles with average diameter around 1.8 lm were observed
for the amorphous product obtained from the reaction
at 250 °C with no holding time (Fig. 3(a)). When the
holding time during the synthesis was prolonged to 2 h,
the product obtained was crystalline. SEM micrograph
Fig. 1. The XRD patterns of powder synthesized by the reaction of
AIP in mineral oil at various reaction conditions.
Fig. 2. IR spectra of powder synthesized by the reaction of AIP in
toluene at various reaction conditions.
Table 1
The crystallite size of the as-synthesized and calcined products calculated by the Scherrer equation
Reaction
temperature (°C)
Holding
time (h)
Crystallite size of alumina (nm)
As-syn
1000
1100
1150
1200
6:2c
9:2v
9:4v
10:8v
16:5 , 31:1a
c
33:7a
–
2
50
0
2
2
2
–
9v
9:6v
12:3 , 36:4
v
v
a
a
a
68a
97:5a
44:7a
270
00
10:2v
9:4v
10:3v
15:6 , 33:5
v
14:2 , 39:5a
v
14:6 , 41:3
3
v c
and a represent crystallite size of v-, c- and a-phase, respectively.
,

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O. Mekasuwandumrong et al. / Inorganic Chemistry Communications 6 (2003) 930–934
Fig. 3. SEM images: (a) as-synthesized product prepared by reaction in mineral oil at 250 °C with 0 h holding time; (b) as-synthesized product
prepared in mineral oil at 250 °C for 2 h; (c) as-synthesized product prepared in mineral oil at 300 °C for 2 h; (d) product (b) calcined at 1150 °C; (e)
product (c) calcined at 1150 °C.
of that product revealed spherical particles as well, but
the average size decreased to 1.2 lm (Fig. 3(b)). With the
reaction temperature increased to 300 °C, particle size
further decreased to 1 lm (Fig. 3(c)).
Transmission electron micrographs of the as-ob-
tained powder and calcined powder are shown in Fig. 4.
It can be seen that the as-synthesized v-alumina prod-
ucts are comprised of agglomerated small particles
having average diameter of 10 nm. This nanosized par-
ticle is referred to as primary particle, while the spherical
agglomerate observed in SEM is called secondary par-
ticle. The crystallite size calculated by Scherrer equation
(9 nm) was in good agreement with size of the primary
particle observed by TEM. This is an indication that
each primary particle is a single crystal of v-alumina.
Fig. 5(a) shows XRD patterns of products from the
calcination of the amorphous powder obtained from the
reaction at 250 °C with no holding time. It can be seen
that the powder remains amorphous even after calci-
The thermogravimetric analysis of the amorphous
product in air revealed weight decrease at temperature
around 300 °C which is attributed to the combustion of
organic moiety on the sample. Since it has been gener-
ally accepted that AIP decomposes prior to the forma-
tion of alumina product, the results suggested that AIP
decomposed during the initial period of the reaction and
formed an intermediate complex which then precipitated
as spherical solid. When the holding time was pro-
longed, this complex further decomposed and conse-
quently resulting in crystalline v-alumina.

O. Mekasuwandumrong et al. / Inorganic Chemistry Communications 6 (2003) 930–934
933
Fig. 4. TEM images: (a) as-synthesized product prepared in mineral oil at 250 °C for 2 h; (b) as-synthesized product prepared in mineral oil at 300 °C
for 2 h; (c) product (a) calcined at 1150 °C; (d) product (a) calcined at 1150 °C; (e) product (b) calcined at 1150 °C; (f) a-alumina obtained from
calcination of v-alumina product at 1200 °C.
nation at 600 °C. However, at higher calcination tem-
perature, crystalline phases were formed. c-Alumina was
detected at 1000 °C, but gradually disappeared when the
temperature was increased. On the other hand, more a-
alumina was detected at higher temperature. After cal-
cination at 1150 °C, only a-alumina was found. Noted
that no v-alumina was not formed by calcination of the
amorphous product. This result suggested that the for-
mation of v-alumina occurs only in the reaction condi-
tion.
Phase transformation of v-alumina product obtained
from the reaction at 250 °C with 2 h holding time was
studied from XRD patterns shown in Fig. 5(b). As
mentioned earlier, v-alumina normally transforms to j-
alumina before further transforms to a-alumina. How-
ever, no j-alumina was detected from the calcination of
a-alumina product obtained by thermal decomposition
of AIP in mineral oil. Majority of v-alumina product
transformed directly to a-alumina at calcination tem-
perature around 1150 °C. The a-phase transformation
completed at 1200 °C. It should be noted that trace h-
alumina found during the calcination is the result from
phase transformation of contaminated boehmite de-
tected earlier by IR spectra [4,5].
Considering uncompleted v–a phase transformation
from the calcination at 1150 °C for 1 h, it was found that
morphology of secondary particles of the calcined
products (Figs. 3(d) and (e)) remained the same as the
as-synthesized products (Fig. 3(b)). However, TEM
micrographs clearly show two groups of primary parti-
cles in products, i.e., spherical and finger-like particles
(see Figs. 4(c)–(e)). It should be noted that the finger-like
particles are much larger than the spherical particles. By
comparing the particles size from TEM micrographs to
the crystallite sizes calculated from XRD line broaden-
ing, it was found that the average size of the finger-like

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O. Mekasuwandumrong et al. / Inorganic Chemistry Communications 6 (2003) 930–934
9
–10 nm and grew up to around 16 nm by the calcina-
tion, before phase transformation to a-alumina took
place, regardless of the temperature which the a-phase
transformation started. The final product, after phase
transformation was complete, was polycrystalline a-
alumina with much larger crystallite size than the initial
crystallite size of v-alumina. Results from our pre-
liminary results also confirmed this observation. v-alu-
mina with crystallite size greater than 16 nm has never
been observed in our work. Nevertheless, further study
is needed to verify mechanism of phase transformation
from v- to a-alumina.
4
. Conclusion
Thermal decomposition of AIP in mineral oil at
temperature in the range of 250–300 °C with holding
time of 2 h resulted in micro-spherical particles, which
were aggregate of nanocrystalline v-alumina. Study of
products obtained at various holding time suggested
that AIP decomposed in mineral oil and formed amor-
phous spherical solid during the initial period of the
reaction. Prolonged holding time resulted in transfor-
mation of the amorphous to v-alumina product.
Fig. 5. The XRD patterns of calcined sample prepare by various re-
action conditions: (a) reaction at 250 °C with 0 h holding time; (b)
reaction at 250 °C for 2 h. (
alumina; (N) represents h-alumina and (
ꢀ
) represents v-alumina; (M) represents c-
Acknowledgements
ꢁ
) represents a-alumina.
The authors would like to thank the Thailand Re-
search Fund (TRF), the Thailand-Japan Technology
Transfer Project (TJTTP) for their financial support.
particles observed by TEM was much larger than the
crystallite size of both v- and a-phase in the product.
This is an indication that the finger-like primary parti-
cles are polycrystalline. Although the results are incon-
clusive whether the finger-like particles formed during
phase transformation are v- or a-phase, only finger-like
primary particles were observed when the phase trans-
formation from v- to a-alumina was completed (Fig.
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