Vapor-phase synthesis of a solid precursor for α-alumina through a catalytic decomposition of aluminum triisopropoxide

Article abstract of DOI:10.1016/j.materresbull.2011.09.011

A new solid precursor, hydrous aluminum oxide, for α-alumina nanoparticles was prepared by thermal decomposition of aluminum triisopropoxide (ATI) vapor in a 500 mL batch reactor at 170-250 °C with HCl as catalyst. The conversion of ATI increased with inc

Full text of DOI:10.1016/j.materresbull.2011.09.011

Materials Research Bulletin 46 (2011) 2199–2203
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Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Vapor-phase synthesis of a solid precursor for
decomposition of aluminum triisopropoxide
a-alumina through a catalytic
a
a,
a
b
Tu Quang Nguyen , Kyun Young Park *, Kyeong Youl Jung , Sung Baek Cho
a
Department of Chemical Engineering, Kongju National University, 275 Budae-dong, Cheonan, Chungnam 330-717, Republic of Korea
Korea Institute of Geoscience & Mineral Resources (KIGAM), 92 Gwahang-no, Yuseong-gu 305-350, Republic of Korea
b
A R T I C L E I N F O
A B S T R A C T
Article history:
A new solid precursor, hydrous aluminum oxide, for a-alumina nanoparticles was prepared by thermal
Received 12 October 2010
Received in revised form 13 May 2011
Accepted 14 September 2011
Available online 21 September 2011
decomposition of aluminum triisopropoxide (ATI) vapor in a 500 mL batch reactor at 170–250 8C with
HCl as catalyst. The conversion of ATI increased with increasing temperature and catalyst content; it was
nearly complete at 250 8C with the catalyst at 10 mol% of the ATI. The obtained precursor particles were
amorphous, spherical and loosely agglomerated. The primary particle size is in the range 50–150 nm.
The ignition loss of the precursor was 24%, considerably lower than 35% of Al(OH)
for alumina particles. Upon calcination of the precursor at 1200 8C in the air with a heating rate of 10 8C/
min and a holding time of 2 h, the phase was completely transformed into . The spherical particles
composing the precursor turned worm-like by the calcination probably due to sintering between
neighboring particles. The surface area equivalent diameter of the resulting -alumina was 75 nm.
3
, the popular precursor
Keywords:
A. Ceramics
a
B. Chemical synthesis
C. Thermogravimetric analysis
C. X-ray diffraction
a
ß 2011 Elsevier Ltd. All rights reserved.
1
. Introduction
-Alumina is one of the most important oxides because of its
anhydrous aluminum chloride vapor was oxidized, decomposed,
or hydrolyzed [8–10].
a
ATI is less hazardous than TMA and higher in aluminum content
than ATBO. ATI has been used for vapor-phase preparation of thin
films [11,12]. To our knowledge, however, there is no report on the
vapor-phase preparation of particles using ATI. The admission of
ATI vapor into a flame would probably yield alumina nanoparticles
similar in structure to the fumed alumina that has been
commercially produced by flame hydrolysis of aluminum chloride
vapor. The alumina particles produced from the flame process are
known to be severely agglomerated due to the high temperature,
high melting point, high wear resistance, high electrical insulation,
good chemical, thermal, and mechanical stability [1]. The unique
properties of a-alumina make it useful in a variety of applications
including catalyst, polishing, electronic, and bioceramics [2,3].
Increasing attention has been paid to the development of nano-
sized
0 nm
oxide (ATI). A solution method of producing 100 nm
particles using a seed crystal has been patented [4]. Recently,
nanosized -alumina particles were produced by neutralization of
aluminum chloride solution with bases at ambient temperature
5,6] or by thermal decomposition of a mixture of sucrose and
aluminum nitrate solution at 400 8C [7]. The calcination of the
alumina particles derived from the thermal decomposition led to
-alumina particles, 44–55 nm in size. In contrast to these liquid-
a
a
-alumina particles. Park et al. [2] reported a preparation of
-alumina by sol–gel processing of aluminum triisoprop-
-alumina
5
a
and the phase of which is a mixture of
g, d, and a. To obtain a-
g
alumina particles based on any vapor-phase method, two stages
may be required; in the first stage amorphous or transient alumina
particles are produced and in the second stage these particles are
[
g
-
calcined for hours to transform the phase into
In the present study, solid particles that can be used as precursor
for nano-sized alumina were prepared by decomposition of ATI
a.
a
a
phase methods that involve a series of time consuming steps such
as filtration, washing and drying, vapor-phase methods capable of
producing dry nanoparticles in one step were reported by which
trimethylaluminum (TMA), aluminum tri-sec-butoxide (ATBO), or
vapor at a temperature of about 200 8C. The use of such a low
temperature is an advantage, compared with the high-temperature
flame process. ATI, which is solid at ambient condition, was
vaporized and decomposed with HCl as catalyst. The particles as-
x y z
produced are not alumina but intermediates, AlO Cl (OH) , which
turned into -alumina upon calcination. The conversion of ATI and
a
the particle size and morphology of the alumina precursor were
*
Corresponding author. Tel.: +82 41 521 9354; fax: +82 41 554 2640.
investigated under varying decomposition conditions. The calcina-
E-mail address: kypark@kongju.ac.kr (K.Y. Park).
tion condition for the transformation into a-alumina was studied.
0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2011.09.011

2
200
T.Q. Nguyen et al. / Materials Research Bulletin 46 (2011) 2199–2203
(TA Instruments, SDT2960). The number-average particle diameter
was determined with an image analyzer (Media Cybernetics, Image-
Pro plus 40) through diameter measurement of about 300 particles.
The propylene content in the product gas was determined by gas
chromatography (Hewlett Packard, Model HP5890) with a packaged
column (Chromosorb 102) and a flame ionization detector. The
conversion of ATI was then calculated from the propylene content
using the stoichiometry of the reaction [13]:
AlðOC
. Results and discussion
.1. Effect of catalyst content
The molar ratio of catalyst to ATI was varied from 0 to 0.1,
3
H
7
Þ
3
! 0:5Al
2
O
3
þ 3C
3
H
6
þ 1:5H
2
O:
3
3
holding the temperature at 200 8C and the reaction residence time
at 30 min. Three runs were made for each condition. Fig. 2 shows
the conversions of ATI at different HCl/Al molar ratios. The
conversion was 38.1% in the absence of catalyst. The conversion
increased to 70.0% at the ratio of 0.01, and to 83.7% as the ratio was
further increased to 1.0. This may be the first discovery that HCl is
effective in catalyzing the vapor-phase decomposition of ATI,
although HCl has been used as catalyst in sol–gel preparation of
metal oxide particles [14].
Fig. 1. Schematic drawing of experimental setup.
2
. Experimental
There is a reportthat a protontransfercould occur in the presence
2.1. Experimental procedure
3
of water vapor for a gas-phase HCl–NH reaction system [15].
Assumingthatthegas-phaseprotontransferispossibleinthepresent
system because water vapor is available from the vaporization of the
aqueousHClprovidedasasourceforthecatalyst,theroleofHClinthe
decomposition of ATI, a weak base, is proposed as follows:
The experimental setup is schematically shown in Fig. 1. A
00 mL batch reactor was used for the decomposition. The reactor
5
is equipped with an electrically driven stirrer, a thermocouple, a
pressure gauge, a precursor feeder, a filter at the reactor outlet and
a gas sampling bag after the filter. The ATI feeder is a stainless steel
pipe, 0.8 cm in inside diameter and 5 cm in length, with two valves
at both ends. The detailed description of the apparatus is available
elsewhere [10].
H
C
H
H
C
O
H
C
H
C
H
H
H
Cl + H
CH3
H
CH3
+ Cl-
(1)
(2)
Al(OC3H7)2
H HO Al(OC3H7)2
+
0
.03 g of ATI (Aldrich, 98%) and 1.3 mL of 36 wt.% HCl (Samchun
Pure Chem. Co. LTD) were loaded in two cylindrical glass boats,
respectively. These boats were then put in the feeder. The feeder
containing the two boats was mounted vertically on the top of the
reactor. By opening the lower valve of the feeder, the ATI-laden
boat was dropped by gravity to the reactor set at a predetermined
temperature under nitrogen atmosphere. The evaporation of the
ATI was confirmed with the pressure rise. Following the ATI
evaporation, the HCl-laden boat was dropped to the reactor for HCl
evaporation. Following the two-stage evaporation, the decompo-
sition reaction was continued for a given residence time. Upon
termination of a reaction, the product gas was sampled by the gas
sampling bag and analyzed with a gas chromatography analyzer.
The produced particles were carried out of the reactor by a flow of
nitrogen supplied from the bottom, and collected in the filter. The
H
C
H
C
H
C
H
H
CH3
H
C
CH
+ HO Al(OC H )
3
3 7 2
H HO Al(OC H )
3
7 2
+
H
C
H
H
C
H
H
H
CH3
C
C
+
H
Cl
(3)
CH3
Cl-
1
00
9
8
7
6
5
4
0
0
0
0
0
0
as-produced particles were calcined in
transformation into -phase alumina.
a box furnace for
a
2
.2. Characterization
The morphology of the particles was examined by scanning
electron microscopy (SEM) (JEOL, JSM-6700), the crystalline struc-
ture by X-ray diffraction with Cu-K radiation (Rigaku, D/MAX-RB)
a
and the molecular structure by FT-IR spectroscopy (Perkin Elmer,
Spectrum GX). The chemical composition of the particles was
determined by energy dispersive X-ray analysis (EDX) and X-ray
photoelectronspectroscopy (XPS)(ESCA 2000). The surface area of the
particles was measured by nitrogen adsorption using a Micromeritics
ASAP 2010 instrument. The mass loss of the particles upon heating in
an alumina boat was measured with a thermogravimetric analyzer
30
2
0
0
.00
0.02
0.04
0.06
0.08
0.10
0.12
Molar ratio of HCl/ATI
Fig. 2. Effect of HCl/ATI molar ratio on the conversion of ATI.

T.Q. Nguyen et al. / Materials Research Bulletin 46 (2011) 2199–2203
2201
Fig. 3 shows the scanning electron microscopic images of the
alumina precursor particles obtained at various HCl/ATI molar
ratios. The particle size decreased, but the sphericity increased
with increasing the ratio from 0 to 0.01 and 0.1. An increase in the
ratio increased the decomposition rate, as shown in Fig. 2, and
successively increased the nucleation rate to have resulted in
smaller primary particles.
3.2. Effect of decomposition temperature
The reactor temperature was varied from 170 to 250 8C, holding
the residence time at 30 min and the HCl/ATI ratio at 0.1. Fig. 4
shows the effect of decomposition temperature on the conversion
of ATI. The conversion was 62.4% at 170 8C and increased to 96.0%
at 250 8C.
The particles obtained at 170 8C were agglomerated and
irregular in shape. It was difficult to identify particle boundaries.
By comparison, the particles obtained at higher temperatures were
less agglomerated, spherical, and narrower in size distribution. The
number-average particle diameter decreased with increasing
temperature: 105, 94 and 81 nm at 200, 230 and 250 8C,
respectively. In the gas-phase synthesis of titania particles, some
investigators [16,17] reported a similar decrease in particle
diameter with increasing temperature, while other studies
[18,19] showed an increase. An increase in temperature would
increase the collision-coalescence rate and the nucleation rate. The
collision-coalescence contributes to particle growth, while the
nucleation increases the number of particles and consequently acts
to make primary particles smaller. At the temperature level of the
present study, the nucleation rate appears to be the dominant
factor to yield the observed temperature dependency.
Fig. 5 shows the effect of decomposition temperature on the
chlorine content in the particles. The chlorine content was 3.0 at.%
at 170 8C and decreased to 1.2 at.% at 250 8C. The source of chlorine
must be the HCl added as catalyst. The catalyst participated in the
reaction appears not to have been recovered completely.
3.3. Thermogravimetric analysis of as-produced particles
The particles produced at the decomposition temperature of
200 8C with the residence time at 30 min and the molar ratio of
HCl/ATI at 0.1 were heated in oxygen atmosphere up to 1200 8C at a
heating rate of 10 8C/min. As shown in Fig. 6, the mass decreased
with temperature increase due to the removal of volatiles. The
mass was 69% of the initial value at 1080 8C. Thereafter, the mass
rather increased to 76% at 1200 8C and leveled off. Such an
3
extraordinary increase in mass was absent with Al(OH) , a
Fig. 3. Scanning electron microscopic images of particles produced at various HCl/
ATI molar ratios (a) 0, (b) 0.01, and (c) 0.1 (reaction time: 30 min, reaction
temperature: 200 8C).
100
9
8
7
0
0
0
The catalyzing acid gives the protonated alkoxide and the conjugate
base of the acid in Eq. (1). In Eq. (2), the protonated alkoxide
undergoes heterolysis to form the carbocation and hydride. Finally,
in Eq. (3), the carbocation loses a proton to the base to yield
propylene. A condensation polymerization follows between result-
ing hydroxide molecules to form a dimer.
60
ðC
3
H
7
OÞ
ðC
2
AlðOHÞ þ ½ðOHÞAlðOC
OÞ AlꢁOꢁAlðOC
3
H
7
Þ
2
ꢀ !
ꢀ þ H O:
5
4
0
½
3
H
7
2
3
H
7
Þ
2
2
(4)
0
By successive hydride formation in Eq. (2) and condensation
polymerization, the dimericspecies inEq. (4) continuesto growwith
its vapor pressure decreasing, and eventually leads to a particle. The
proposed mechanism requires verification in the future.
1
60
180
200
220
240
260
Temperature, ºC
Fig. 4. Effect of temperature on conversion of ATI.

2
202
T.Q. Nguyen et al. / Materials Research Bulletin 46 (2011) 2199–2203
3
3
2
2
1
1
0
.5
.0
.5
.0
.5
.0
.5
(
104) (113)
(
116)
(012)
(
110)
(
024)
(300)
214)
(
(
1010)
(018)
o
1
200 C
o
(220)
(400)
(440)
1
100 C
As-produced particles
10
20
30
40
50
60
70
80
1
60
180
200
220
240
260
2θ (degree)
Temperature, ºC
Fig. 7. XRD patterns of as-produced particles at different calcination temperatures.
Fig. 5. Effect of temperature on chlorine content of as-produced particles.
1
1
10
00
At 1200 8C, all the peaks are assigned to
0-0425]. The approximate crystallite size of the
determined to be 47 nm, using Scherrer’s equation:
a-alumina phase [JCPDS
1
a
-alumina was
Al(OH)
3
0
:9
l
u
D ¼
;
b
cos
9
8
7
6
0
0
0
0
As-produced alumina
where D is the crystallite size,
l
is the wave length of the X-ray, and
1
.68
b
is the full width at half the maximum intensity. The broadening
due to the machine was neglected.
FT-IR spectra of as-produced and calcined particles are shown
in Fig. 8. In the spectra of the as-produced particles, two peaks
O/Al atomic ratio =1.57
indicating the presence of
3 2
nCH(–CH ) and nCH(–CH –) groups
ꢁ
1
were observed at 2920 and 2850 cm , respectively [22]. Those
peaks disappeared in the calcined particles. The peak at 1630 cm
due to the OH bonding vibration mode [23] decreased in strength
after calcination. A sharp peak at 562 cm
alumina [24] appeared in the spectra of the calcined particles.
Fig. 9 shows the scanning electron microscopic images of the
alumina particles obtained by calcination at 1100 and 1200 8C,
respectively. The morphology of the as-produced particles was
maintained at 1100 8C. The particle size, however, decreased
probably due to the removal of volatiles and densification. As the
calcination temperature was increased further to 1200 8C, the
particles turned worm-like due to sintering between neighboring
ꢁ
1
0
200
400
600
800
1000
1200
ꢁ
1
Temperature, ºC
representing a-
Fig. 6. Thermogravimetric analysis of as-produced particles.
conventional precursor for alumina. The ignition loss of the present
alumina precursor amounts to 24%, considerably lower than that
3
of Al(OH) .
By XPS analysis, the atomic ratio of oxygen to aluminum was
.57 at 1080 8C and 1.68 at 1200 8C. The stoichiometric atomic ratio
2
1
particles. The BET surface area was measured to be 20 m /g. The
of Al
2
O
3
is 1.5. The increase in the ratio by increasing the
surface-area equivalent diameter was calculated to be 75 nm using
the alumina density of 3.98 g/cm . By EDX analysis, the calcined
particles are free of residual chlorine.
3
temperature from 1080 to 1200 8C was consistent. This supports
that the mass increased due to oxygen. A formation of aluminum
peroxide, which should give an atomic ratio higher than 1.5, was
reported for the reaction of
a-alumina with platinum [20]. In the
100
present study, however, no source of platinum was present. An
existence of oxygen vacancies was reported for alumina films in
contact with NiAl [21]. Any oxygen vacancy in the aluminum oxide
particles at 1080 8C may be filled with ambient oxygen to increase
the mass. But, it is questionable that the aluminum sub-oxide can
exist under the operating condition. Further studies may be
necessary to investigate the causes of the mass increase.
Calcined particles
8
6
4
2
0
0
0
0
3.4. Calcination of as-produced particles for a-alumina
As-produced particles
The particles produced at the decomposition temperature of
2
00 8C with the residence time at 30 min and the molar ratio of
0
4
HCl/ATI at 0.1 were calcined for 2 h at 1100 and 1200 8C. Fig. 7
shows a comparison in XRD pattern between as-produced and
calcined particles. The as-produced particles are amorphous. At
000 3600 3200 2800 2400 2000 1600 1200 800 400
Wavenumbers, cm-1
1
100 8C, peaks indicative of
g-alumina [JCPDS 81-2267] appeared.
Fig. 8. FT-IR spectra of as-produced particles and calcined particles.

T.Q. Nguyen et al. / Materials Research Bulletin 46 (2011) 2199–2203
2203
Fig. 9. Scanning electron microscopic images of obtained particles after calcination at (a) 1100 8C and (b) 1200 8C.
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. Conclusion
1
[
A solid particulate precursor for
through a thermal decomposition of ATI vapor at 170–250 8C with
HCl as a catalyst. The decomposition was nearly complete at 250 8C
with the molar ratio of HCl to ATI at 0.1.
5 nm in surface area equivalent diameter, were obtained by
calcining the precursor at 1200 8C for 2 h. The ignition loss of the
precursor was 24%, considerably lower than that of Al(OH) , a
conventional alumina precursor. The obtained -alumina exhib-
ited a worm-like structure due to sintering between neighboring
particles. The unique structure of these -alumina particles may
a
-alumina was prepared
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