An effective strategy to obtain highly porous alumina powders having robust and designable extra-large pores

Article abstract of DOI:10.1246/bcsj.20190222

Crystalline alumina is a significant inorganic solid that has been utilized as a high-surface-area catalyst support. However, it has been in fact difficult to obtain alumina having high interior porosity and adequate crystallinity in their powder forms be

Full text of DOI:10.1246/bcsj.20190222

Advance Publication Cover Page
An Effective Strategy to Obtain Highly Porous Alumina Powders Having Robust
and Designable Extra-large Pores
Hirokazu Maruoka and Tatsuo Kimura*
Advance Publication on the web September 6, 2019
doi:10.1246/bcsj.20190222
© 2019 The Chemical Society of Japan
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An Effective Strategy to Obtain Highly Porous Alumina Powders Having Robust
and Designable Extra-large Pores
Hirokazu Maruoka1 and Tatsuo Kimura*1
1
Inorganic Functional Materials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST)
Shimoshidami, Moriyama-ku, Nagoya 463-8560, Japan
E-mail: t-kimura@aist.go.jp
Tatsuo Kimura
Tatsuo Kimura received his PhD from Waseda University, Japan in 1999. After working as a post-doctor, he obtained a
permanent research position in National Industrial Research Institute of Nagoya (NIRIN) in 2000 which has been
integrated as the AIST Chubu of National Institute of Advanced Industrial Science and Technology (AIST) since 2001.
His research mainly focused on the structural design of periodic porous materials in the molecular scale including the
morphological control for utilizing as advanced materials of adsorbents, catalysts, electrodes, etc.
Abstract
Crystalline alumina is one of significant inorganic solids
induced methods that can also afford powders as precipitates by
cooperative organization.27,31-33 Even in the cases for obtaining
mesoporous alumina powders,34 such EISA processes has been
found during complete drying of solvents from homogeneous
precursor solutions.35-37 Unfortunately, elimination rate of the
solvents is changed depending on the size of container/vessel as
well as temperature, revealing its less reproducibility to obtain
exactly same materials as the powder form.
The aerosol-assisted approach by using a spray dryer is
quite helpful for controlling evaporation rate of solvents from
resultant droplets of a precursor solution.38-43 This is because
the drying temperature can be adjusted suitably for formation
of surfactant-assisted mesoporous structures. Actually, ordered
mesoporous particles of silica were prepared in the presence of
alkyltrimethylammonium (CnTMA) halides, poly(oxyethylene
glycol) alkyl ethers (CnEOm) and poly(oxyethylene)-block-poly
that have been utilized as high-surface-area catalyst supports.
However, it has been in fact difficult to obtain alumina having
high interior porosity and adequate crystallinity as their powder
forms because complete crystallization of pore walls (alumina
frameworks) is generally suppressed by the porous structures.
Here, we report an aerosol-assisted synthesis of highly porous
alumina using asymmetric polystyrene-b-poly(ethylene oxide)
(PS-b-PEO) type diblock copolymers, and summarize it as an
effective strategy to achieve quick optimization of the synthetic
conditions. Extra-large pores (~40 nm using PS35000-b-PEO17000
and ~200 nm using PS58500-b-PEO37000) were architected in the
aerosol-assisted particles successfully. The alumina frameworks
can be adequately crystallized to its γ-phase through calcination
at high temperature (e.g., 1000 °C) with the retention of initial
porous structures. In addition, surface property of the alumina
frameworks was changed from hydrophilic to hydrophobic with
the crystallization degree, being a significant insight for tuining
functions through the porous materials design.
(oxypropylene)-block-poly(oxyethylene)
(EOnPOmEOn)
triblock copolymers.44-46 In addition, several mesoporous oxide
(e.g., Al2O3, TiO2 and ZrO2) powders were obtained by spray
drying at appropriate temperatures (e.g., 400 °C) that induced a
rapid evaporation of ethanolic solutions (with a little amount of
water, EtOH/H2O = 10/1) with self-assembly of Pluronic F127
(EO106PO70EO106).39,41 Corresponding previous works showed
experimental data on the treatment conditions (temperature
~900 °C and time) for crystallization of oxide frameworks with
the retention of mesoporous structures, but resultant porosity
did not seem to be enough. Thus, we have proposed a rational
synthetic method to enhance the accessibility into mesopores of
alumina powders in our recent papers.47,48
Keywords: alumina powders, surfactant-assisted
synthesis, quick optimization
1. Introduction
A huge number of supramolecular-mediated mesoporous
materials have been synthesized using a variety of amphiphilic
organic molecules.1-11 The micro-, meso-, and macro-structured
designs are documented very well with their compositional and
morphological variations.12-15 Such interdisciplinary researches
have then opened a new era for exploring potential applications
based on unique properties of ordered mesoporous materials to
drug delivery containers,16-20 size-selective filters,21-25 designed
electrodes,24,26-30 and so on.
Except for a few compositions (e.g., silica and carbon),
reliable synthetic methods of highly porous powders have not
been developed so adequately. This would be often because the
evaporation-induced self-assembly (EISA) process for yielding
ordered mesoporous oxides (not only silica but also alumina
and titania) films seems to be too effective to sol-gel reaction
Synthetic pathways of γ-alumina, one of the most widely
used as catalyst supports in the petroleum industry,49 strongly
influence interface structure of γ-alumina crystallites related to
the surface properties such as Lewis acidity, structural stability,
corrosion resistance, etc., as well as other phases of alumina.
Surfactant-assisted synthesis with the sol-gel chemistry leads to
micelles formation surrounded by oligomeric aluminum oxide
species and further organization to mesostructures.50,51 In this
case, resultant alumina frameworks should be amorphous and
crystallized γ-alumina based materials having enough porosity
have been demanded for designing high-performance catalyst
supports. However, crystallization of oxide frameworks allows
notable deformation or collapse of mesoporous structures with

sudden decrease of the mesoporosity.52-54 To solve this kind of
issue, we have to develop a novel method to fabricate alumina
powders with extra-large pores and thick pore walls.55 Herein,
we utilized asymmetric PS-b-PEO type deblock copolymers as
the structure directing agent because such molecules are useful
for the formation of larger-sized spherical micelles.2,56
25 °C (BELSORP-aqua3, after degassing at 110 °C for 6 hr
under vacuum). The equilibrium was kept for 500 s after the
change of a pressure below 4.0 Pa (3 x 10-2 Torr) at each point.
Table 1. Precursor solutions for porous alumina films prepared
using PS-b-PEO and optimization through the fabrication of
corresponding alumina powders
2. Experimental
Materials. A wide variety of PS-b-PEO can be purchased
1
from Polymer Source Inc. All the analytical data including H
PS-b-PEO
(g)
AlCl3 THF EtOH H2O
NMR spectra of the copolymers are available on the website of
the company. In this study, PS18000-b-PEO7500 (Mw/Mn = 1.05),
PS35000-b-PEO17000 (Mw/Mn = 1.10), PS58500-b-PEO37000 (Mw/Mn
= 1.15), PS60000-b-PEO18000, PS60000-b-PEO61000 (Mw/Mn = 1.18),
and PS100000-b-PEO150000 (Mw/Mn = 1.25) were used as received
for obtaining spin-coated films and aerosol-assisted powders of
sol-gel derivative alumina having extra-large pores. To exhibit
them simply, the PS-b-PEO molecules were expresses as e.g.,
150000-b-100000 by using the molecular weight of each unit.
Polystyrene homopolymers (h-PS) with molecular weights of
500 (h-PS500), 4100 (h-PS4100), 10000 (h-PS10000), and 211600
(h-PS211600) were obtained from Sigma-Aldrich, being possibly
usage as pore expanders showing an affinity to the PS unit of
PS-b-PEO and described as e.g., h-500. Anhydrous aluminum
chloride (AlCl3), concentrated hydrochloric acid (HCl, 37%),
and dehydrated solvents of tetrahydrofran (THF) and ethanol
(EtOH, 99.5%) were obtained from FUJIFILM Wako Pure
Chemical Co.
Fabrication of porous alumina films and powders. Our
research concept towards the development of an effective way
to fabricate highly ordered mesoporous powders was started by
optimizing the synthetic conditions for obtaining corresponding
films. We refereed the contents of a paper on the fabrication of
ordered and controllable PS-b-PEO-templated extra-large pores
in alumina films.57 In a typical preparation,55 concentrated HCl
(37%, 0.4 mL) and H2O (0.6 mL) were added quickly to AlCl3
(0.266 g) and then started to be stirred. After the stirring was
maintained for 10 min, a certain amount of the AlCl3 solution
(0.167 mL) was combined with a PS-b-PEO solution (0.10 g)
of THF (2.7 g) and EtOH (2.4 g). After stirring for another 20
min, the precursor solution was spin-coated at 3000 rpm over a
silicon wafer, put for hours at room temperature, and calcined
at 400 °C for 3 hr (heating rate; 1 °C min-1). The compositions
of starting chemicals were summarized in Table 1. After almost
completing optimization of the synthetic conditions, precursor
solutions were spray-dried at 130 °C and 170 °C (Spray Dryer
ADL311, Yamato Scientific Co., Ltd.) for recovering powder
samples and calcined at temperatures in the range from 400 °C
to 1000 °C for 3 hr (heating rate; 2 °C min-1). In addition, we
investigated a possibility to expand PS-b-PEO-templated pores
with addition of h-PS having different molecular weight (e.g.,
500, 4100, 10000 and 211600).
(g)
(g)
2.7
5.4
2.7
2.7
2.7
2.7
2.7
2.0
(g)
0.8
0.8
2.4
2.4
1.6
2.4
2.4
1.6
(g)
18000-b-7500a
0.10
0.266
0.266
0.067
0.067
0.067
0.067
0.044
0.040
0.90
0.90
0.23
0.23
0.23
0.23
0.15
0.27
100000-b-150000 0.10
35000-b-17000
60000-b-18000
58500-b-37000
60000-b-61000
35000-b-17000b 0.10
58500-b-37000b 0.10
0.10
0.10
0.10
0.10
a Completely same as the composition used in the reference.57
b
Optimized for the formation of extra-large pores through the
aerosol-assisted synthesis of alumina powders.
3. Results and Discussion
Alumina powders containing extra-large pores has hardly
been obtained using any amphiphilic organic molecules. Only
regularly arranged several nanometer-sized pores were formed
not only for thin films58,59 but also for dry-up36-37 and aerosol-
assisted powders39,41,47,48 with self-assembly of EOnPOmEOn.
Several papers on the successful fabrication of large mesopores
using PS-b-PEO and PI-b-PS-b-PEO, polyisoprene-block-poly-
styrene-block-poly(ethylene oxide) have been published.57,60,61
The former PS-b-PEO (Mw, PS = 18000, PEO = 7500) assisted
mesopores can be expanded from 35 nm to 80 nm by changing
the ratio of solvents such as THF and EtOH.59 The latter pores
were also changed from ~36 nm (toluene/n-butanol) to ~14 nm
(THF/chloroform/n-butanol) according to the solubility of PI-b-
PS-b-PEO (Mn = 38.3 kg/mol, 68.4 mass % PI, 18.0 mass % PS
and 13.6 mass % PEO).60,61 However, we have hardly found the
successful synthesis of alumina powders with the formation of
large pores arising from the self-assembly of such asymmetric
block copolymers.47
Only one paper has been found for a dry-up fabrication of
highly ordered mesoporous alumina using PS-b-PEO (roughly
calculated Mw; PS = 19000, PEO = 5000), though diameter of
the spherical pores, around 16~19 nm, was still in the mesopore
region.62 To achieve the preparation of alumina powders having
extra-large pores, we improved an aerosol-assisted synthesis for
fabricating porous alumina powders having robust1 and tunable
large pores using PS-b-PEO. A uniqueness of this study is that
the synthetic conditions for obtaining such alumina powders
are optimized by an effective use of corresponding spin-coating
process. As illustrated in Figure 1, the synthetic conditions (e.g.,
chemical composition of precursor solution and stirring time as
well as kind of starting chemicals) are initially optimized for
the formation of highly porous films. The designated chemical
composition is only applied to the aerosol-assisted synthesis at
a temperature adjusted for fabricating highly porous (e.g., high
surface area and large pore volume) particles. If the porosity
seems to be not adequately, the amount of amphiphilic organic
molecules and/or that of inorganic species are mainly changed
until highly porous structures are fabricated whole the particles
and occasionally feedbacks to the fabrication of corresponding
films.
Characterization. Formation of pores with self-assembly
of PS-b-PEO molecules was initially confirmed by using a field
emission scanning electron microscope (FE-SEM, HITACHI
S-4300) at the top surfaces of films and powders. Porosity and
crystallinity of alumina powders were discussed by evaluating
their nitrogen (N2) adsorption-desorption isotherms at 77 K of
liquid N2 (Quantachrome Autosorb-1, after degassing at 110 °C
for 6 hr under vacuum) and X-ray diffraction (XRD) patterns
(Rigaku RINT 2000 with monochromated Cu Kα radiation). In
addition, the porous structure and surface texture of alumina
frameworks were checked by transmission electron microscopy
(TEM, JEOL JEM-2010, operated at 200 kV). Surface property
of highly porous alumina powders was investigated carefully
by using water (H2O) adsorption-desorption measurements at

Optimizing composition of precursor solution for thin film
In this regards, the molecular weight of the PS and PEO units
should be considered for understanding the resultant pore size.
If PS-b-PEO molecules are successfully assembled as spherical
micelles, size of such micelles mainly depends on the length of
the PS chain like volume of the PS cores. Accordingly, by using
the different PS chain length (35000 and 60000), the pore sizes
were observed as 60~ nm and 100~ nm, respectively.
Amount of surfactant
Stirring time
and/or inorganic species
(Ratio to inorganic species)
Ordering
↑↑↑↑
sec., min., hr, day
Rapid evaporation
even at room temperature
- Preparation in parallel
- Coating in short time
Amount of solvent(s)
(a)
(b)
(Solubility/Concentration/Evaporation rate,etc.)
60~100 nm
100～400 nm
Fine tuning for aerosol-assisted particle
Designated
chemicalcomposition
Amount of solvent(s)
1 μm
1 μm
(c)
(d)
300~1000 nm
100~500 nm
↑↑↑↑
High porosity
Controlled evaporation
with spray-drying temperature
1 μm
1 μm
Figure 1. A research concept for minimizing a period to
optimize synthetic conditions of highly porous particles by the
aerosol-assisted approach.
Figure 2. SEM images of calcined (at 400 °C) alumina films
prepared using (a) 35000-b-17000, (b) 60000-b-18000, (c)
58500-b-37000 and (d) 60000-b-61000.
Spin-coated fabrication of porous alumina films using
PS-b-PEO. we initially checked the reproducibility of an early
work on the fabrication of ordered mesoporous alumina films
using PS-b-PEO (18000-b-7500);54 a chemical composition of
the precursor solution was 18000-b-7500 (0.10 g), anhydrous
aluminium chloride (AlCl3, 0.266 g), tetrahydrofuran (THF, 2.7
g), ethanol (EtOH, 0.8 g), and H2O (0.9 g with HCl). The
scanning electron microscopic (SEM) image of a resultant film
is shown in the Supporting Figure S1, showing the formation of
uniform mesopores (20~30 nm). The mesostructural ordering
did not seemed to be enough in comparison with a periodicity
of the 18000-b-7500 templated pores achieved by the original
work,57 but another fabrication was also conducted by using an
extremely large PS-b-PEO molecule (100000-b-150000). The
SEM images were displayed in the Supporting Figure S2. By
using the same chemical composition, large pores ranging from
30 to 100 nm were formed in a film, but the uniformity was not
satisfactory. As explained in the reference,57 the uniformity of
100000-b-150000 templated pores was relatively improved by
changing the ratio of the solvents (rich-THF and poor-EtOH
against the PS unit in PS-b-PEO). Only after such a process, we
designated chemical compositions for PS-b-PEO molecules and
summarized in Table 1.
After optimizing chemical composition of the precursor
solutions (see the last two lines in Table 1), alumina films were
prepared using PS-b-PEO again. The SEM images are shown in
Figure 3, exhibiting unique reticular type films by networking
thin alumina containing macropores. Compared with a previous
image of the alumina film prepared using 35000-b-17000 (see
Figure 2a), the nanostructure, especially the kind of pores was
drastically changed from spherical to cellular. In the latter case,
we have called as a reticular type pore. Average pore size of the
58500-b-37000 templated pores was much larger that of the
35000-b-17000 templated ones. However, it was quite difficult
to identify diameter and/or distribution of the extra-large pores.
Such reticular nanostructures were previously found in titania
films that were formed by the precise control of remarkable
shrinkage of titania frameworks containing flexible spherical
PS-b-PEO aggregates.27,68,69 If the reticular type structures are
found during SEM observations of porous oxide films, it would
not be able to notice that highly porous powders are obtained
by spray drying of the same precursor solution. This is a quite
significant insight, being possibly supported by an analogous
titania system.
The SEM images of alumina films prepared using several
PS-b-PEO molecules are displayed in Figure 2. Although the
compositions of the precursor solutions were listed in Table 1,
the use of 35000-b-17000 was very helpful for the formation of
pores that were distributed in the macropore region (60 to 100
nm). As explained above, uniformity of PS-b-PEO-templated
pores was able to be improved by changing THF/EtOH ratio
(see the Supporting Information, Figure S2), as well as the
variation of interfacial tension between PS cores and solvents
during its micellization.63 Other PS-b-PEO molecules were also
potential for obtaining alumina films having macropores larger
than 100 nm, though uniformity of the pores was not abundant.
In the case using amphiphilic block copolymers as the organic
template, regardless of the symmetricalness of their molecular
structures, molecular weights of each block critically influence
size of resultant micelles, facilitating the pore size design.64-67
(a)
(b)
25 nm
25 nm
Figure 3. SEM images of calcined (at 400 °C) alumina films
prepared by using compositions of the precursor solutions
suitable for obtaining highly porous alumina powders using (a)
35000-b-17000 and (b) 58500-b-37000.

The SEM images revealed that average pore diameters at
the particle surfaces were estimated to be ~25 nm (actually ~40
nm evaluated by TEM, described later) for 35000-b-17000 and
~200 nm for 58500-b-37000. The pores generated at around the
surfaces were significantly larger than those observed for the
film surfaces (see Figure 3). If a fine tuning of pore diameter is
strongly demanded for researches and applications, we need to
optimize all the synthetic conditions for each PS-b-PEO. Thus,
as in the case of a work on the synthesis of carbon-based
materials using PS-b-PEO (Mn = 297000) with a polystyrene
homopolymer (h-PS, Mn = 5100),72 we also investigated the
possibility to expand PS-b-PEO-templated spherical pores by
the addition of h-PS with several molecular weights (h-500,
h-4100, h-10000 and h-211600). The SEM images of a series of
alumina powders prepared in the presence of 35000-b-17000
with 0.1 mass % of h-PS are shown in Figure 5. Although the
addition of smaller h-500 and h-4100 led to a slight expansion,
average pore diameter was increased from 25 nm up to ~40 nm
with the addition of h-10000. However, further addition of the
h-10000 (from 0.1 mass % to 0.2 and 0.3 mass %) was not
helpful for expansion of pores. Similarly, continuous alumina
networks were deformed and/or collapsed by adding the largest
h-211600.
Aerosol-assisted synthesis of porous alumina powders
using PS-b-PEO. For recovering porous alumina powders, the
relatively optimized precursor solutions for fabricating films in
Figure 2 (see Table 1) were spray-dried at temperatures that can
adjust evaporation rate of the solvents (e.g., THF and EtOH). In
the case using 18000-b-7500 and 100000-b-150000, we cannot
show clear SEM images expressing formation of corresponding
pores larger than 20 nm. This would be due to the thickness of
alumina frameworks compared to the pore diameters, which is
speculated by the ratio (g/g) of AlCl3 to PS-b-PEO = 2.66 (see
the Supporting Information, Figures S1 and S2). According to
this consideration, we can find the possibility to obtain porous
alumina powders with reduction of the weight ratio (g/g) down
to 0.67. The SEM images of aerosol-assisted alumina powders
prepared with the self-assembly of 35000-b-17000, 60000-b-
18000, 58500-b-37000, and 60000-b-61000 are delivered as the
Supporting Information, Figure S3. Large pores were observed
in part for almost all alumina particles. In the case using
60000-b-61000, much larger pores were found for the alumina
powders, being good agreement to our hypothesis that reticular
type nanostructures constructed by slim alumina networks are
suitable for fabricating highly porous alumina powders by the
aerosol-assisted synthesis (see Figures 3 and 4).
(a)
(b)
(a)
(b)
~25 nm
200 nm
200 nm
(c)
(d)
500 nm
200 nm
(c)
(d)
~200 nm
200 nm
200 nm
500 nm
200 nm
Figure 5. SEM images of calcined (at 400 °C) alumina
particles prepared using 35000-b-17000 with (a) h-500, (b)
h-4100, (c) h-10000 and (d) h-211600.
Figure 4. SEM images of calcined (at 400 °C) alumina
particles prepared using (a, b) 35000-b-17000 and (c, d)
58500-b-37000 after further optimization by reduction of the
amount of AlCl3 with (b, d) their enlarged images.
Stability of extra-large pores during crystallization of
alumina frameworks. Stability of PS-b-PEO-templated pores
larger than ~25 nm was investigated through SEM observations
with N2 adsorption measurements. After calcination at different
temperatures from 400 °C to 1000 °C, surface porosity of the
alumina powders prepared using 35000-b-17000 were checked
by SEM. As displayed in Figure 6, the surface structure having
extra-large pores was almost maintained even after calcination
at 1000 °C with a shrinkage of the pores down to around 20 nm.
The BET surface area and total pore volume (about 300 m2 g-1
and 1.05 cm3 g-1 after calcination at 400 °C) were decreased
with the increase in density of alumina frameworks by thermal
condensation and crystallization (70 m2 g-1 and 0.75 cm3 g-1
after calcination at 1000 °C). A tendency during the decrease of
the BET surface area and total pore volume is summarized in
Figure 7. The BET surface area as well as the pore volume was
relatively high after calcination at 400 °C, being related to the
presence of smaller pores inside the alumina frameworks. By
elevating calcination temperature, such small pores are shrunk
and finally diminished by substantial condensation of alumina
frameworks and then the resultant porosity was almost reached
According to the synthetic recipe of our porous alumina
powders after the fabrication of corresponding films showing
high uniformity of PS-b-PEO-templated pores (see Figure 1),
we optimized the synthetic conditions further with reduction of
the AlCl3/PS-b-PEO/H2O ratio. As shown in Figure 4, we then
succeeded in synthesizing highly porous alumina powders by
the aerosol-assisted synthesis, though all the alumina particles
was not spherical. In case of the synthesis using 35000-b-17000,
the shape of the alumina particles seemed to be dented hollows.
Such a morphology is typically observed in case with a high
Péclet number that is a dimensionless number defined as the
evaporation rate of solvents divided by the diffusion rate of
solute.70,71 When an evaporation of solvents is much faster than
a solute diffusion, the solute is enriched at around the particle
surfaces and form a shell-like structure. And thus, such hollow
particles are likely to form with the increase in the spray-drying
temperature up to 170 °C (see the Supporting Information,
Figure S4).

to that due to the 35000-b-17000 templated extra-large pores.
of alumina frameworks.55 When alumina particles having much
much larger pores (~200 nm, very low surface curvature), that
can be obtained with the self-assembly of 58500-b-37000, were
calcined at 1000 °C, alumina frameworks can be crystallized
and transformed from γ- to α-phase smoothly with complete
retention of the large porous structure.55
(a)
(b)
Surface texture and property of the extra-large pores were
investigated by transmission electron microscope (TEM) and
H2O adsorption-desorption. The results are exhibited in Figures
8 and 9, respectively. According to the TEM images of the
alumina particles after calcination at temperatures lower than
700 °C, the pore diameter was evaluated to be ~40 nm and
inner surfaces of the spherical pores were smooth. At higher
temperatures (e.g., 850 °C and 1000 °C), inner surfaces were
deformed with crystallization of alumina frameworks (see also
Figure 7), though the extra-large porous structure was kept
even after calcination at 1000 °C. Observing adsorption blanch
of H2O molecules at a very low region of the relative pressure
(P/P0 < 0.1) possibly revealed that amorphous alumina surfaces
were relatively hydrophilic.
200 nm
200 nm
(c)
(d)
200 nm
200 nm
Figure 6. SEM images of aerosol-assisted alumina particles
prepared using 35000-b-17000 followed by calcination at (a)
550 °C, (b) 700 °C, (c) 850 °C and (d) 1000 °C.
(a)
(b)
(a)
400
300
200
(c)
●
●; Pores largerthan 10nm
●; Others
●; α-Al2O3
●; γ-Al2O3
●
●
●
●
100
0.0
1000 C
850 C
10 nm
10 nm
(c)
(d)
400 500 600 700 800 900 1000
Calcination temperature / C
(b)
1.00
0.75
0.50
700 C
550 C
400 C
0.25
0.00
10 nm
10 nm
15
25
35
45
55
65
75
400 500 600 700 800 900 1000
2θ /  (Cu Kα)
Calcination temperature / C
Figure 8. TEM images of the extra-large mesoporous alumina
particles prepared using 35000-b-17000 followed by
calcination at (a) 550 °C, (b) 700 °C, (c) 850 °C and (d)
1000 °C.
Figure 7. Variations of (a) BET surface area and (b) total pore
volume of the aerosol-assisted alumina particles prepared using
35000-b-17000, with (c) their XRD patterns.
Crystallization behavior of alumina particles containing
35000-b-17000 templated spherical pores was also investigated
by X-ray diffraction (XRD). The XRD patters are displayed in
Figure 7, mainly indicating thermal crystallization of alumina
frameworks to its γ-phase. The alumina frameworks were still
amorphous after calcination at lower temperatures (e.g., 400 °C
and 550 °C). Crystallization to γ-phase was started by heating
at around 700 ᵒC and fully proceeded by calcination at higher
temperatures (e.g., 1000 °C) while keeping the porosity of the
35000-b-17000 templated pores. However, the crystallization
behavior is not governed by calcination temperature only. In
our previous work on mesoporous alumina powders prepared
using Pluronic P123 (EO20PO70EO20),47 the mesopores were
maintained after calcination at around 850 °C with a partial
crystallization to γ-phase of alumina but such a crystallization
hardly started at 700 °C.
(a)
(b)
; adsorption
; desorption
; adsorption
; desorption
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
0.0
0.2
0.4
P/P0
0.6
0.8
1.0
0.0
0.2
0.4
P/P0
0.6
0.8
1.0
Figure 9. Water adsorption-desorption isotherms of the
extra-large mesoporous alumina particles prepared using
35000-b-17000 followed by calcination at (a) 400 °C and (b)
1000 °C.
To understand the difference in crystallization behavior
around mesopores having different diameters (small mesopores,
~5 nm using Pluronic P123 and large mesopores, 40 nm using
35000-b-17000), we discussed the difference by considering
the surface curvature of the spherical pores and wall thickness
Depending on the pore volume (0.89 cm3 g-1) of alumina
particles, adsorption of H2O molecules was increased and its
capacity was increased at P/P0 > 0.8 and reached up to 0.60 g

- Cellular (Highly porous)
- Thin continuous wall
25 nm
200 nm
- Spherical
(Uniform)
- Thick wall
Spray-drying
Haste makes waste?
Spin-coating
Optimizing synthetic conditions
by the aerosol-assisted approach
1 μm
- Spherical (Distributed)
Figure 10. A schematic procedure for quick optimization of the synthetic conditions of highly porous alumina powders by the
aerosol-assisted synthesis by applying the spin-coating process of corresponding precursor solutions the chemical composition of
starting mixture.
g-1 at around at P/P0 = 0.9 by the presence of extra-large pores.
After calcination at 1000 °C, adequately crystallized γ-alumina
surface was hydrophobic and the final adsorbed amount (0.27 g
g-1) would not be enough against its pore volume (0.75 cm3 g-1).
An occupancy by H2O molecules in 35000-b-17000 templated
mesopores was changed from about 60% (almost amorphous
after calcination at 400 °C) to around 35% (partly γ-phase after
calcination at 1000 °C) by reducing the surface hydrophilicity
of alumina frameworks. In this context, interaction of catalyst
components would be managed by changing the property of
alumina surfaces from hydrophilic to hydrophobic.
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