Solvothermal synthesis of spherical zirconia particles having meso-macro bimodal pore structures

Article abstract of DOI:10.1246/cl.200038

By the thermal reaction of tetrakis(acetylacetonato)zirconium in 1,4-butanediol at 300 °C for 2 h, spherical zirconia particles with large surface areas and large pore volumes both in the meso and macro pore regions were directly obtained. The thermal reactions at lower temperatures afforded intermediate compounds having a layered-structure, attributed to the formation of the unique pore system of the products obtained in 1,4-butanediol.

Full text of DOI:10.1246/cl.200038

Received: January 16, 2020 | Accepted: February 26, 2020 | Web Released: March 3, 2020
CL-200038
Solvothermal Synthesis of Spherical Zirconia Particles Having
Meso-Macro Bimodal Pore Structures
Fuya Sugiyama and Shinji Iwamoto*
Division of Molecular Science, Graduate School of Science and Technology, Gunma University,
Kiryu, Gunma, 376-8515, Japan
E-mail: siwamoto@gunma-u.ac.jp
By the thermal reaction of tetrakis(acetylacetonato)zirco-
nium in 1,4-butanediol at 300 °C for 2 h, spherical zirconia
particles with large surface areas and large pore volumes both in
the meso and macro pore regions were directly obtained. The
thermal reactions at lower temperatures afforded intermediate
compounds having a layered-structure, attributed to the forma-
tion of the unique pore system of the products obtained in 1,4-
butanediol.
replaced with nitrogen, the mixture was heated to 300 °C at a
rate of 2.5 °C/min, and kept at that temperature for 2 h. After the
solvothermal reaction for 2 h, the valve of the autoclave was
slightly opened to remove organic vapor from the autoclave by
flashing evaporation while keeping the temperature at 300 °C.
After cooling, dry powders were obtained directly. The obtained
products were calcined at 500 °C for 1 h in a box furnace. For
comparison, synthesis of zirconias was also carried out using
ZNP (10, 20, or 60 mmol in 100 mL 1,4-BG). To examine the
formation process of the pore system, the Zr sources in 1,4-BG
were also heated at lower temperatures, and the products were
recovered by centrifugation, washed with acetone repeatedly,
and then air-dried overnight.
Keywords: ZrO2
| Solvothermal method |
Bimodal pore structure
Zirconias and zirconia-based oxides have been used as
catalysts and catalyst supports for various reactions,1-5 and
control of the pore structure of these materials is greatly
important. Besides catalytic applications, zirconias have been
also attracting much attention as stationary phase supports for
liquid chromatography,6-8 and synthesis of spherical zirconia
particles with homogeneous size and shapes has been inves-
tigated.9,10 Sol-gel method is known as a preferable method for
this purpose; however, the obtained products are amorphous or
oxides with low crystallinity, and therefore, heat treatments are
necessary to obtain well-crystallized products.11,12 Preparation of
aerogel crystalline ziconias with large surface area and controlled
pore structures via sol-gel process and subsequent supercritical
drying has been also reported.13,14 On the contrary, Inoue et al.
previously reported that microcrystalline zirconias with high
surface areas were directly obtained by the solvothermal method
using several zirconium sources and organic solvents.15 Influence
of synthesis conditions, such as concentration of zirconium(IV)-
n-propoxide (ZNP) as the Zr source and reaction temperature, on
the physical properties of the zirconia powders were examined
using 1,4-butanediol (1,4-BG) and 1,5-pentanediol, and it was
found that spherical zirconia particles with a large specific
surface area and large pore volume in the mesopore region were
directly obtained in 1,4-BG.16 However, the pore sizes of the
thus-obtained products were relatively small (<10 nm), and an
expansion of the pore size distribution has been sought to
improve their performance. In this study, solvothermal syntheses
of zirconias in 1,4-BG were examined using tetrakis(acetyl-
acetonato)zirconium (Zr(acac)4) and the pore structures of the
products were investigated. Furthermore, to elucidate the reasons
for the difference of the pore structures of these products, thermal
treatments at low temperatures were conducted.
The thus-obtained products and those calcined at 500 °C
were characterized by several methods. Powder X-ray diffraction
(XRD) patterns were collected on a Rigaku RINT 2200VF using
CuKα radiation. Scanning electron microscopy (SEM) meas-
urements were performed on a JEOL JSM-6510AS. Nitrogen
adsorption experiments were carried out with a Quantachrome
Instruments NOVA 2200e for the samples calcined at 500 °C.
Specific surface areas were calculated using the BET multipoint
method and pore size distributions were calculated on the basis
of N2 adsorption isotherms using the BJH method.
Figure 1 shows XRD patterns of as-synthesized products
obtained by the solvothermal treatment of ZNP or Zr(acac)4 in
1,4-BG at 300 °C and the samples after calcination at 500 °C.
For the product obtained from 10 mmol of ZNP in 100 mL of
1,4-BG, diffraction peaks solely for the tetragonal zirconia were
observed. The crystallite size of the product calculated from the
half-height width of the XRD peak was ca. 5 nm, which is in
good agreement with the crystallite sizes reported in the previous
study.16 For the product synthesized from 60 mmol of ZNP in
1,4-BG, large peaks for the tetragonal phase and small peaks for
the monoclinic zirconia were observed. After the calcination
at 500 °C, phase transformation to the monoclinic ZrO2 was
observed. In the case of Zr(acac)4, the product obtained from
10 mmol of Zr(acac)4 in 1,4-BG also showed diffraction peaks
only for the tetragonal phase, which is consistent with the
previous result.15 On the contrary, small peaks for the mono-
clinic zirconia were observed for the product obtained from the
20 mmol Zr(acac)4 solution, and the monoclinic zirconia phase
was mainly observed for the product from the 48 mmol Zr(acac)4
in 80 mL 1,4-BG solution, which suggests the products using
Zr(acac)4 were formed via an altered crystallization process.
In Figure 2, SEM images of the products are depicted. In the
case of using ZNP, spherical particles were observed regardless
of the ZNP concentration. The particle size of the product
obtained from the 10 mmol ZNP solution was some hundred
nanometers (a), and the particle size increased significantly as
the amount of ZNP in 1,4-BG increased to 20 and 60 mmol
In a test tube, appropriate amounts of Zr(acac)4 and 1,4-BG
(10, 20, or 40 mmol of Zr(acac)4 in 100 mL of 1,4-BG, or 48
mmol of Zr(acac)4 in 80 mL of 1,4-BG) were added and the
mixture was set in a 300-mL autoclave. Into the gap between the
test tube and the autoclave wall, 30 mL of 1,4-BG was poured
additionally. After the atmosphere inside the autoclave was
Chem. Lett. 2020, 49, 505–508 | doi:10.1246/cl.200038
© 2020 The Chemical Society of Japan | 505

tetragonal ZrO2
monoclinic ZrO2
tetragonal ZrO2
monoclinic ZrO2
(a)
(b)
(c)
ZNP
ZNP
10 mmol
10 mmol
20 mmol
60 mmol
20 mmol
60 mmol
3 μm
3 μm
3 μm
(d)
(e)
(f)
20
40
60
Zr(acac)4
20
40
60
Zr(acac)4
10 mmol
20 mmol
48 mmol*
60
10 mmol
20 mmol
48 mmol*
60
3 μm
3 μm
3 μm
Figure 2. SEM images of the products ZNP or Zr(acac)4 in
1,4-BG by the solvothermal treatment at 300 °C; 10 (a), 20 (b),
and 60 mmol (c) of ZNP; 10 (d) and 20 mmol (e) of Zr(acac)4
in 100 mL of 1,4-BG, and 48 mmol of Zr(acac)4 in 80 mL of
1,4-BG (f).
20
40
20
40
2θ (degree)
2θ (degree)
(b) 500 °C
(a) as-syn
Figure 4 shows the N2 adsorption isotherms for the ZrO2
samples synthesized using Zr(acac)4. The ZrO2 samples obtained
by using Zr(acac)4 had much larger pore volumes. The pore size
distribution of the ZrO2 sample obtained from the 10 mmol
Zr(acac)4 solution had a noticeable peak in the mesopore region
of 2-10 nm. Quite interestingly, the pore size distribution of the
sample obtained from the 20 mmol Zr(acac)4 solution had two
peaks, one at around 7 nm and the other at around 50 nm. The
pore size distribution of the sample from the 40 mmol Zr(acac)4
solution also showed two peaks (Figure S2). For the sample
from the 48 mmol Zr(acac)4 in 80 mL 1,4-BG solution, however,
a broad pore size distribution from the mesopore to macropore
region was obtained. The physical properties of the samples
calcined at 500 °C are summarized in Table 1. The sample
obtained from the 60 mmol ZNP solution had a large surface
area of 111 m2/g and a pore volume of 0.16 cm3/g. However,
for those obtained from the 10 and 20 mmol ZNP solutions, the
BET surface areas and pore volumes decreased significantly.
On the contrary, using the Zr(acac)4 solutions, the ZrO2 samples
with large BET surface areas were obtained. In addition, these
samples had much larger pore volumes compared with those
obtained using ZNP. These results also suggest the texture of the
Figure 1. XRD patterns of the products obtained by the
solvothermal treatment of ZNP or Zr(acac)4 in 1,4-BG at 300 °C
and the samples after calcination in air at 500 °C. *48 mmol
Zr(acac)4 in 80 mL of 1,4-BG.
(b, c). On the other hand, the products obtained from the
10 mmol Zr(acac)4 solution were spherical particles with particle
size of ca. 1-2 ¯m (d), which were much larger compared with
the product from the 10 mmol ZNP solution (a). The products
from the 20 mmol Zr(acac)4 solution were mainly spherical
particles with diameters of ca. 3 ¯m and small particles were
also observed (e). On the contrary, the products from the
48 mmol Zr(acac)4 in 80 mL 1,4-BG solution were composed of
aggregation of fine particles and no spherical particles were
observed (f).
In Figure 3, the N2 adsorption isotherms of the ZrO2
samples synthesized using ZNP are shown. The effect of the
calcination temperature on the pore structure is shown in
Figure S1. The sample from the 60 mmol solution had meso-
pores in the range of 2-10 nm; however, for those from the lower
concentrations, the samples had much smaller pore volumes.
240
240
(c)
(a)
(b)
0.6
0.4
0.2
0
160
80
0
160
80
0
0
0.5
Relative Pressure P/P0
1
1
5
10 20 50 100
d (nm)
0
1
2
de Boer Statistical Thickness (nm)
Figure 3. N2 adsorption isotherm (a), pore size distribution (b), and V-t plot (c) of the ZrO2 samples obtained by heating ZNP in 1,4-
BG at 300 °C, followed by calcination in air at 500 °C;
adsorption, open symbols: desorption.
,
: 10 mmol,
,
: 20 mmol, and
,
: 60 mmol, closed symbols:
506 | Chem. Lett. 2020, 49, 505–508 | doi:10.1246/cl.200038
© 2020 The Chemical Society of Japan

240
160
80
240
160
80
(a)
(b)
(c)
0.6
0.4
0.2
0
0
0
0
0.5
Relative Pressure P/P0
1
1
5
10 20 50 100
d (nm)
0
1
2
de Boer Statistical Thickness (nm)
Figure 4. N2 adsorption isotherm (a), pore size distribution (b), and V-t plot (c) of the ZrO2 samples obtained by heating Zr(acac)4 in
1,4-BG at 300 °C followed by calcination in air at 500 °C; : 10 mmol and : 20 mmol in 100 mL of 1,4-BG, and
48 mmol of Zr(acac)4 in 80 mL of 1,4-BG, closed symbols: adsorption, open symbols: desorption.
,
,
,
:
thermal reactions are presumed to proceed via a replacement of
alkoxide groups with 1,4-BG to form glycoxide intermediates
and a subsequent heterolytic cleavage of the O-C bonds of the
glycoxides to form tetrahydrofuran (THF).17,18 In the case of
ZNP, similar reactions plausibly take place at the early stage
of the reaction; however, as shown in Figure 5, intermediates
having layered-structures were formed, suggesting the crystal-
lization into ZrO2 nanocrystals proceeded in a stepwise fashion.
As mentioned above, in the case of the thermal treatments of ZNP
in 1,4-BG at 300 °C, spherical particles were directly obtained.
For the formation of products having such a spherical morphol-
ogy, a droplet formation process would be indispensable. We
suppose that the layered compounds melt to form droplets at
elevated temperatures, and the crystallization to ZrO2 nano-
crystals takes place in the droplets with releasing THF molecules.
For the products obtained from the 20 mmol Zr(acac)4 solution,
two sharp peaks were also observed at 7.02° and 8.08°. These
peaks correspond to the lattice spacings of 12.6 and 10.9 ¡,
and these values are slightly larger than those of the products
obtained from the 20 and 60 mmol ZNP solutions. In addition,
shoulder peaks were recognized for the product from the
Zr(acac)4 solution. These results suggest that the layered structure
obtained from the Zr(acac)4 solution was slightly expanded
and partly disordered. When the 20 mmol Zr(acac)4 solution
was heated at 250 °C, the product showed no peaks for the
layered compound but the tetragonal ZrO2, indicating the layered
structure is not stable enough at this temperature. These changes
in the intermediates affect the texture of the products and hence
spherical particles having meso and macro bimodal pore structure
were attained.
Table 1. Physical properties of the zirconia samples synthe-
sized by the solvothermal method
Surface area
(m2/g)
Pore volume
(cm3/g)
1,4-BG
(mL)
Zr source (mmol)
BET External* Total Mesopore*
ZNP
10
20
60
100
100
100 111
10.8
13.4
0.05
0.02
0.16
6.1
4.9
0.01
0.15
Zr(acac)4
10
20
48
100 101
100 122
80 114
13.2
54.7
0.13
0.34
0.35
0.10
0.21
*Calculated from the V-t plot.
products using Zr(acac)4 and ZNP were formed via slightly
different processes.
To investigate the formation mechanism of the pore system,
the solvothermal treatments of ZNP or Zr(acac)4 in 1,4-BG were
carried out at lower temperatures. When the mixture of 20 mmol
ZNP in 100 mL of 1,4-BG was heated in a glass vessel, it became
a transparent solution at ca. 140 °C, and then white precipitates
were formed at ca. 160 °C (Figure S3(a)). For the 60 mmol ZNP
solution, a yellow transparent solution was obtained at ca. 90 °C
and precipitates were formed at ca. 160 °C (Figure S3(b)). In the
case of the 20 mmol Zr(acac)4 in 1,4-BG, a transparent solution
was obtained at ca. 125 °C, and white precipitates were also
formed at ca. 170 °C (Figure S3(c)). On the contrary, in the case
of the 48 mmol Zr(acac)4 solution, no precipitations were formed
up to 190 °C (Figure S3(d)). In Figure 5, the XRD patterns of the
precipitates obtained by the solvothermal treatments of ZNP and
Zr(acac)4 at 200 and 250 °C are shown. The product obtained by
heating the 60 mmol of ZNP solution at 200 °C showed two sharp
diffraction peaks at 7.40° and 8.48°, which correspond to lattice
spacings of 11.9 and 10.4 ¡, respectively. When the same
solution was heated at 250 °C, similar peaks were observed.
Thermal analysis revealed that the products contained consid-
erably large amounts of organic moieties (Figure S4). From
these results, we speculate the products obtained at lower
temperatures have a layered-structure as shown in Figure 6. For
the solvothermal reactions of metal alkoxides in 1,4-BG, the
In summary, by heating a 1,4-BG solution containing
20 mmol of Zr(acac)4 at 300 °C for 2 h, spherical ZrO2 particles
with mesopores and macropores were directly obtained. By
heating the Zr(acac)4 solution at 200 °C, an intermediate with a
layered-structure was obtained. The intermediate compound had
slightly larger lattice spacings and lower stability compared with
those obtained by the thermal treatment of ZNP in 1,4-BG at
200 °C. These changes affect the reactivity of the intermediates
and the pore structure of the spherical products composed of
nanocrystalline zirconias. Using the obtained ZrO2 particles,
superior properties in various applications, especially in catal-
ysis and chromatography, are expected.
Chem. Lett. 2020, 49, 505–508 | doi:10.1246/cl.200038
© 2020 The Chemical Society of Japan | 507

tetragonal ZrO2
7.22° (12.2 Å)
8.32°
250 °C
(10.6 Å)
250 °C
20
40
60
6
8
10
20
40
60
7.02° (12.6 Å)
7.40° (11.9 Å)
8.48°
8.08°
(10.4 Å)
(10.9 Å)
200 °C
200 °C
20
40
60
6
8
10
20
40
60
6
8
10
2θ (degree)
2θ (degree)
2θ (degree)
2θ (degree)
(a) ZNP, 60 mmol
(b) Zr(acac)4, 20 mmol
Figure 5. XRD patterns of the products obtained by heating 60 mmol ZNP (a) and 20 mmol Zr(acac)4 (b) in 100 mL 1,4-BG at 200
and 250 °C.
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ca.10Å
3
4
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This work was partly supported by a research grant from
Suzuki Foundation and the promotion of university-industry
collaborative research based on Regulatory Sciences, Gunma
University. The XRD mesurement was performed with a Rigaku
RINT 2200VF at the Center for Instrumental Analysis of Gunma
University. We thank Prof. S. Shiraishi (Gunma University) for
use of SEM, a JEOL JSM-6510AS.
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Supporting Information is available on https://doi.org/
10.1246/cl.200038.
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508 | Chem. Lett. 2020, 49, 505–508 | doi:10.1246/cl.200038
© 2020 The Chemical Society of Japan