Properties of acid sites generated on sol-gel derived Zn 2SiO4

Article abstract of DOI:10.1246/bcsj.78.2142

ZnO-SiO₂ binary oxides prepared by the sol-gel method were calcined at various temperatures in order to reveal the relations between the surface properties as an acid-base catalyst and the bulk structures measured by X-ray diffraction. It was found that the acid amount and strength as well as the catalytic activity increased remarkably when the Zn₂SiO₄ phase was formed exclusively by the reactions between ZnO and SiO₂ particles at 1173 K. Changes in the bulk structures were suggested to affect the surface properties of the oxide particles.

Full text of DOI:10.1246/bcsj.78.2142

2142 Bull. Chem. Soc. Jpn., 78, 2142–2145 (2005)
ꢀ 2005 The Chemical Society of Japan
Properties of Acid Sites Generated on Sol–Gel Derived Zn2SiO4
Ã
Hirotoshi Nakabayashi, Yoritsugu Shino, and Junya Kobayashi1
Department of Materials Science and Engineering, Kochi National College of Technology,
Monobe, Nankoku, Kochi 783-8508
1Department of Material and Environmental Engineering, Hakodate National College of Technology,
Tokura, Hakodate, Hokkaido 042-8501
Received April 22, 2005; E-mail: nakaba@ms.kochi-ct.ac.jp
ZnO–SiO2 binary oxides prepared by the sol–gel method were calcined at various temperatures in order to reveal
the relations between the surface properties as an acid–base catalyst and the bulk structures measured by X-ray diffrac-
tion. It was found that the acid amount and strength as well as the catalytic activity increased remarkably when the
Zn2SiO4 phase was formed exclusively by the reactions between ZnO and SiO2 particles at 1173 K. Changes in the bulk
structures were suggested to affect the surface properties of the oxide particles.
Then, the solution was refluxed for hydrolysis at 353 K while
stirring until the whole solution became viscous and gel-like.
The gel-like substance obtained was evacuated under reduced
pressure and then dried in an oven at 383 K for 12 h, followed
by calcinations at 773–1373 K in air for 3 h. The molar ratio of
ZnO/SiO2 in the binary oxides prepared was set to be 2/1.
The structural differences in the binary oxides calcined at var-
ious temperatures were observed by X-ray diffraction (Rigaku
Denki Co., RINT-1200), operated at 30 kV with a filament current
of 20 mA for Cu Kꢀ radiations. The specific surface areas were
measured by the BET method using nitrogen at 77 K.
Measurements of Acidity and Basicity. The acid amounts on
the binary oxides were estimated from the amounts of ammonia
gas adsorbed at 423 K, using 0.50–1.05 g of sample powders treat-
ed by flowing He gas at 673 K for 1 h.5 A given amount of ammo-
nia was iteratively injected into the He carrier gas, which was
introduced to the sample at 423 K until adsorption of ammonia
was no longer observed. The temperature programmed desorption
(TPD) of ammonia on the binary oxides was carried out using a
He purge with a flow rate of 100 mL minÀ1 and a temperature ris-
ing rate of 10 K minÀ1. The desorbed gas was detected by the TCD
detector of a gas chromatograph. In order to observe the exact pro-
file of the ammonia gas desorbed from the sample surface, the dif-
ferential spectrum was obtained by removing the blank spectrum
from the desorption spectrum.7 Measurements of the base amount
and a TPD were also performed using carbon dioxide instead of
ammonia for the acidity measurements in the same way.
Isomerization of 1-Butene. The catalytic activities of the
binary oxides for the isomerization of 1-butene were measured
in order to characterize the acid sites generated on the binary
oxides. The catalytic reactions were conducted in a closed circu-
lating reactor made of glass, in which 0.1 g of the binary oxides
were packed, at temperatures ranging from 483 to 583 K. The
binary oxides as the catalysts were recalcined at 673 K in air for
0.5 h, followed by treatment under vacuum for 1 h at the same
temperature prior to the reactions. The initial pressure of 1-butene
introduced to the catalyst was about 110 Torr (14:7 Â 103 Pa) and
the products were cis- and trans-2-butene, as analyzed by a gas
chromatograph (Hitachi Co., GC-163) with an FID detector using
Binary oxides, such as SiO2–Al2O3 and TiO2–SiO2, have
been widely applied as acid catalysts for some catalytic reac-
tions because of their strong acid sites on the surfaces.1,2
The ZnO–SiO2 binary oxides have also been reported to have
Lewis acid sites, though ZnO or SiO2 have shown very little
acidity.3 The binary oxides with such acid sites have been
observed to be almost non-crystalline or micro-crystalline
structures by X-ray diffraction measurements.2 Therefore, it
has been accepted that the acid sites on the binary oxides are
attributed to the charge imbalance locally formed by structural
imperfections on the surfaces.4 The authors have also reported
that the acid properties of pure metal oxides such as TiO2 and
ZrO2 depend on the crystalline sizes.5–7 The amount of surface
imperfections increases with a decrease of particle sizes.
The charge imbalances attributed to the imperfections of
the surface structures appear even when the bulk structures of
the binary oxides are transformed by the solid-phase reactions
through calcination at high temperatures. Notably, finely divid-
ed particles are readily prepared by sol–gel processes so that
the solid reactions occur with the result that the complex
oxides are formed at much lower calcination temperatures than
those formed by traditional solid-phase reaction processes.8,9
It is already known that the binary oxides composed of ZnO
and SiO2 are provided with the Zn2SiO4 complex oxide by cal-
cination at about 1173 K when the binary oxide is prepared by
the sol–gel method.8 The purpose of the present work is to
evaluate the effects of Zn2SiO4 formation on the surface acid-
ities and the catalytic properties of the binary oxides.
Experimental
Preparation of Binary Oxides. Binary oxides consisting of
ZnO and SiO2 were prepared by the sol–gel method, where zinc
nitrate (Zn(NO3)2 6H2O, Wako Chemical Co.) and ethyl ortho-
Á
silicate (Si(OC2H5)4, Kishida Chemical Co.) were used as raw
materials. 0.2 mol of zinc nitrate and 0.1 mol of ethyl orthosilicate
were dissolved in 180 cm3 of distilled water and drops of nitric
acid were added to the solution until the pH was adjusted to 4.
Published on the web December 9, 2005; DOI 10.1246/bcsj.78.2142

H. Nakabayashi et al.
Bull. Chem. Soc. Jpn., 78, No. 12 (2005) 2143
10.0
8.0
6.0
4.0
2.0
1.0
0.8
0.6
0.4
0.2
0.0
1373 K, 4.9 m2·g-1
1273 K, 12.8 m2·g-1
1173 K, 16.0 m2·g-1
1073 K, 43.8 m2·g-1
973 K, 54.2 m2·g-1
873 K, 59.2 m2·g-1
773
873
973 1073 1173 1273 1373
Calcination temperature / K
Fig. 2. Changes in the acid and base amounts of ZnO–SiO2
with calcination temperature. : Acid amount, : base
amount.
773 K, 50.6 m2·g-1
20
30
40
50
60
70
1.0
0.8
0.6
0.4
0.2
0.0
70
60
50
40
30
20
10
0
2
θ
/ degree
Fig. 1. X-ray diffraction spectra and surface areas of ZnO–
SiO2 calcined at various temperatures. : Zn2SiO4 (wille-
mite), : ꢁ-Zn2SiO4, : ZnO.
a column packed with Unicarbon A-400. The catalytic activities
were calculated by the isomerization rates per unit weight or sur-
face area of the catalysts.
Results and Discussion
773 873 973 1073 1173 1273 1373
Calcination temperature / K
The X-ray diffraction spectra of the ZnO–SiO2 binary ox-
ides prepared by calcination at various temperatures are shown
in Fig. 1, with the specific surface areas measured by the BET
method. The diffraction patterns assigned to the ZnO phase
were observed only at temperatures lower than 1073 K, indi-
cating that the binary oxides were composed of ZnO crystal-
line phase and non-crystalline SiO2 phase. ꢁ-Zn2SiO4 crystal-
line phase patterns appeared slightly in the binary oxides cal-
cined at 1073 K, the Zn2SiO4 (willemite) phase dominating
above 1173 K. The sudden decrease in specific surface areas
was also recognized at the calcination temperature of 1173
K. These results indicate that the binary oxides changed from
a mixture of ZnO and SiO2 particles to Zn2SiO4 complex
oxides at about 1173 K.
In general, the phase diagram10 shows that the Zn2SiO4
phase is formed by the solid-phase reaction, which occurs at
very high calcining temperatures of around 1573 K. However,
sol–gel derived Zn2SiO4 is proliferated by calcination at lower
temperatures as shown in Fig. 1. Because sol–gel technique
readily offers finely divided particles as well as homogeneous-
ly dispersed particles, the Zn2SiO4 phase is expected to be
formed at temperatures lower than 1573 K.8
Fig. 3. Changes in the catalytic activities per unit surface
areas ( ) and unit weights ( ) of ZnO–SiO2 with calcina-
tion temperature. The catalytic reactions were carried out
at 523 K.
with an increase in calcination temperature. However, only
small base amounts less than 1/10 of the acid amount were
measured on all the samples. That means a relatively small
number of base sites are present on the binary oxides. These
results indicate that a lot of acid sites are generated on the sur-
face of the ZnO–SiO2 sample calcined at around 1173 K, at
which temperature the Zn2SiO4 phase is clearly formed from
the ZnO and SiO2 particles as shown in Fig. 1, but the base
sites are scarcely created even if the samples are calcined at
various temperatures.
The effect of calcination temperature on the catalytic activ-
ities of the binary oxides is shown in Fig. 3. The activity for 1-
butene isomerization is known to be increased by the existence
of acid sites and/or base sites on the catalysts.11 Therefore, the
catalytic reaction has often been utilized for the estimation
of surface acidity or basicity. As shown in Fig. 3, noticeably
higher catalytic activities were observed when the binary ox-
ides were calcined in the range of about 1123 to 1223 K, and
the maximum activity was also obtained at a calcination tem-
perature of 1173 K. This tendency is in accordance with the
trend for change in acid amounts with calcination temperature
in Fig. 2. It seems that the acid sites generated on the Zn2SiO4
surface mainly serve as the active sites for the reaction.
Figure 2 shows the amounts of acid sites and base sites on
the unit surface areas of ZnO–SiO2 binary oxides obtained by
calcination at various temperatures. Very large amounts of
acid sites were observed on the binary oxides calcined in the
range of about 1123 to 1223 K, and the maximum acid amount
peak was located at a calcination temperature of 1173 K. On
the other hand, the base amounts tended to increase slightly

2144 Bull. Chem. Soc. Jpn., 78, No. 12 (2005)
Acidic Properties of Sol–Gel Derived Zn2SiO4
-2.0
-2.5
-3.0
-3.5
-4.0
-4.5
-5.0
-5.5
-6.0
2.0
1.8
1.6
1.4
1.2
1.0
773
873
973 1073 1173 1273 1373
Calcination temperature / K
Fig. 4. The ratios of cis-2-butene to trans-2-butene from 1-
butene on ZnO–SiO2 calcined at various temperatures.
1.6
1.7
1.8
1.9
2.0
2.1
2.2
T
−1/ 10−3 K−1
Fig. 5. Arrhenius plots for 1-butene isomerizations on
ZnO–SiO2 calcined at various temperatures. : 773 K,
: 973 K, and : 1173 K.
Table 1. Activation Energies of 1-Butene Isomerizations on ZnO–SiO2 Calcined at Various Temperatures,
Catalytic Activities, and the Surface Areas
Calcination temperature
/K
XRD pattern
Surface area
/m2 gÀ1
Catalytic activity
/mmol minÀ1 mÀ2
Activation energy
/kJ molÀ1
773
973
1173
ZnO
ZnO
Zn2SiO4
50.6
54.2
16.0
3.57
2.17
58.3
70.2
62.9
27.1
It is also known that the ratios of cis-2-butene to trans-2-
butene of the reaction products from 1-butene are affected
by the properties of the active sites on the catalysts. In Fig. 4,
the cis/trans ratios of the 1-butene isomerizations on the bina-
ry oxides calcined at various temperatures are summarized.
The cis/trans ratios were estimated by extrapolation to 0%
conversion in the 1-butene isomarization. The ratios were also
identified to depend slightly upon the calcination temperatures;
the cis/trans ratios were 1.2–1.4 at temperatures lower than
1073 K, but were 1.7–2.0 above 1123 K. It is suggested that
the properties of the acid sites, which act as the active sites
on the Zn2SiO4 particles, differ between those of the mixture
of ZnO and SiO2 particles and Zn2SiO4 particles. Note that
few basic sites were on the surface of the samples since the
cis/trans ratios were always between 1 and 2.12
Moreover, the activation energies of the isomerizations
were obtained by Arrhenius plots in order to inspect variations
in the acidic properties of binary oxides having different struc-
tures. Figure 5 shows Arrhenius plots for the 1-butene isomer-
izations catalyzed by the binary oxides. The binary oxides cal-
cined at 773, 973, and 1173 K were used for the reactions as
catalysts. The activation energies calculated from the plots
are listed in Table 1 along with the catalytic activities and
the surface areas. Although the activation energies of the iso-
merizations on the binary oxides calcined at 773 and 973 K
were 70.2 and 62.9 kJ molÀ1, respectively, a lower value of
27.1 kJ molÀ1 was observed when the binary oxide was cal-
cined at 1173 K. The activation energies are affected by the
properties of acid sites acting as active sites on the catalysts.
Consequently, the lowering of activation energy indicates that
0.6
0.5
0.4
0.3
0.2
0.1
0.0
(c)
(a)
(b)
373
473
573
673
773
873
973
Temperature / K
Fig. 6. NH3–TPD spectra of ZnO–SiO2 calcined at various
temperatures. The amount of samples was adjusted to
make the same surface area in all samples. (a): 773 K,
(b): 973 K, and (c): 1173 K.
the strength of the newly generated acid sites on the Zn2SiO4
complex oxide obtained at 1173 K is comparatively strong.
The acid strength of the binary oxides was also confirmed
by the measurements of the ammonia TPD spectra. Figure 6
shows the TPD spectra of ammonia adsorbed on typical sam-
ples calcined at the three different temperatures of 773, 973,
and 1173 K. The quantity of each sample used for the TPD
measurement was adjusted to be the same surface area in order
to compensate for the differences in surface area. On the bina-
ry oxide consisting of Zn2SiO4 particles obtained by calcina-
tion at 1173 K, two large desorption peaks were observed at

H. Nakabayashi et al.
Bull. Chem. Soc. Jpn., 78, No. 12 (2005) 2145
around 673 and 513 K, indicating that two types of acid sites
mainly exist on the surface of Zn2SiO4. The peak observed
at the higher temperature of 673 K results from the presence
of many strong acid sites on the binary oxide sample. The oth-
er peak around 513 K is caused by the existence of weaker acid
sites. However, the binary oxides calcined at 773 K as well as
at 973 K have major peaks at lower temperature; most of the
acid sites have only weak strengths on the binary oxides con-
sisting of ZnO and SiO2 particles. These results also indicate
that the appreciable amount of strong acid sites is generated
on the surface when the sol–gel derived Zn2SiO4 is formed
by calcination at around 1173 K.
face imperfections acting as strong Lewis acid sites and in
turn, an improvement in catalytic activity.
Incidentally, in the case of calcination at above 1273 K, the
considerable acid sites disappeared from the surface of the
Zn2SiO4 complex oxides. The particles grow to be larger ac-
cording to the increase in calcination temperature. As shown
in Fig. 1, the diffraction peaks assigned to the Zn2SiO4 phase
are observed to strengthen with a rise in calcination tempera-
ture, indicating that the particles grow. It can be regarded that
the surface imperfections, which contribute to the acid sites,
rapidly decrease with the crystalline growth of Zn2SiO4 at
higher temperatures.
Sol–gel derived Zn2SiO4 has been widely used as a host ma-
terial for a green phosphor in electroluminescence devices.13,14
The structures of the ZnO–SiO2 binary oxide prepared by the
sol–gel method have already been reported in earlier literature
to be transformed from the mixed phase of ZnO and SiO2 to
the Zn2SiO4 phase by the solid-state reactions at around
1173 K.8 The transformation by the solid-state reaction has
also been confirmed in this work (Fig. 1). As mentioned previ-
ously, the finely divided particles and the homogeneously dis-
persed particles are readily prepared by sol–gel methods, so
that the solid-state reactions between the component oxides
can occur at low temperatures. The Zn2SiO4 particles thus
formed at low temperatures have also been reported to be
nano-sized particles.15,16
We have already reported that Lewis acid sites are generat-
ed on the surface of nano-sized metal oxide particles because
the amount of structural imperfections on the surface increases
with a decrease in particle size.5 Since the sol–gel derived
Zn2SiO4, which is formed at low temperatures, consists of
very small-sized particles, strong acid sites may be generated
on the surface. However, it is also expected that the surface
imperfections are created together with the solid reactions
between different metal oxide particles. Thus, conformational
changes of the oxide particles are bound to have some influ-
ence on the surface properties, such as acidity or basicity. As
shown in Figs. 2, 3, and 6, when the sol–gel derived Zn2SiO4
was formed solely by the solid reaction of ZnO and SiO2 par-
ticles at around 1173 K, very high catalytic activities and large
amounts of strong acid sites were observed. These indicate that
the change of bulk structures results in an increase in the sur-
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