Controllable synthesis of hollow mesoporous silica spheres and application as support of nano-gold

Article abstract of DOI:10.1016/j.jssc.2014.03.003

Hollow silica spheres with mesoporous structure were synthesized by sol-gel/emulsion method. In the process, the surfactant, cetyltrimethylammonium bromide (CTAB) was used to stabilize the oil droplet and also used as structure direct agent. The diameter of the hollow silica spheres, ranging from 895 nm to 157 nm, can be controlled by changing the ratio of ethanol to water and the concentration of the surfactant as well. The shell thickness of the spheres decreased when the ratio of ethanol to water decreased. The proposed mechanism of the formation of silica spheres could elucidate the experimental results well. Furthermore, the resultant hollow mesoporous silica spheres were then employed as support of nano-gold which was used to catalyze the isomerization reaction of propylene oxide to produce allyl alcohol.

Full text of DOI:10.1016/j.jssc.2014.03.003

Journal of Solid State Chemistry 215 (2014) 67–73
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Controllable synthesis of hollow mesoporous silica spheres
and application as support of nano-gold
n
Tao Wang, Weihua Ma , Junnan Shangguan, Wei Jiang, Qin Zhong
Department of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 December 2013
Received in revised form
Hollow silica spheres with mesoporous structure were synthesized by sol–gel/emulsion method. In the
process, the surfactant, cetyltrimethylammonium bromide (CTAB) was used to stabilize the oil droplet
and also used as structure direct agent. The diameter of the hollow silica spheres, ranging from 895 nm
to 157 nm, can be controlled by changing the ratio of ethanol to water and the concentration of the
surfactant as well. The shell thickness of the spheres decreased when the ratio of ethanol to water
decreased. The proposed mechanism of the formation of silica spheres could elucidate the experimental
results well. Furthermore, the resultant hollow mesoporous silica spheres were then employed as
support of nano-gold which was used to catalyze the isomerization reaction of propylene oxide to
produce allyl alcohol.
22 February 2014
Accepted 1 March 2014
Available online 12 March 2014
Keywords:
Silica
Hollow sphere
Mesoporous
Nano-gold
Propylene oxide
&
2014 Elsevier Inc. All rights reserved.
1
. Introduction
researchers have proposed the formation mechanism of the hollow
sphere. Pan [6] synthesized silica hollow spheres by a CTAB-stabilized
water/oil emulsion system mediated hydrothermal method. They
considered negatively charged silicates and the ammonium cation
of CTAB steadily self-assemble at the interface to form a silica shell.
Many researchers concluded that ethanol played an important role in
the formation of hollow structure [7,8]. As far as the dissolvement of
the core and how the ethanol affect the formation are concerned, it is
not very clear. With a better understanding of the potential mechan-
ism, the synthetic parameters can be optimized to synthesize hollow
silica spheres with satisfied uniformity.
Furthermore, the synthesized hollow mesoporous silica spheres
were used as the support of nano-gold(Au) because of their large BET
surface areas and unique pore structures. Nano-gold is an effective
catalyst for many reactions, such as selective hydrogenation, oxidation
reactions, C–C coupling and other reactions because of its unique size
Hollow mesoporous silica spheres have drawn much attention due
to the wide application in the field of drug delivery, separation and
catalysis. Many different methods have been adopted to synthesize
hollow silica spheres with meso-structure [1–4]. The general method
to achieve hollow silica spheres is template method. The templates
could be removed by chemical etching or by calcination. In the
sol–gel/emulsion method, the oil droplets serve as templates in the
formation process of silica spheres. In former investigations, it has
been proved that the introduction of another co-solvent, ethanol, into
the system can be helpful to improve the stability of the oil droplets.
The size distribution of silica spheres can be affected by the volume
ratio of ethanol-to-water [5]. In our work, we discussed how the
amount of ethanol and how the concentration of surfactant that
introduced into the system affected the size of silica spheres and the
extent of hollowness. In the experiments we found that at too low or
too high ethanol-to-water ratio, the hollow silica spheres cannot be
achieved. And also most of the spheres we obtained were not
completely hollow, many were just like the oranges that were gotten
rid of the flesh. We elucidated the phenomenon by the two effects of
ethanol as cosolvent and as one of the hydrolysis products. Teng et al.
effect and catalytic activity [9]. Here we used the Au/SiO
the isomerization of propylene oxide to produce allyl alcohol. Raptis
et al. [10] used Au/TiO to catalyze the isomerization of epoxides with
2
to catalyze
2
various functional groups into the corresponding allylic alcohols with
82–98% yields. But for the lower epoxides, such as propylene oxide,
there has been no report on their isomerization using nanogold as
[5] also obtained the solid spheres at too low or too high ethanol-to-
2
catalyst. We have tried Au/TiO as the heterogeneous catalyst for the
water ratio and proposed the formation mechanism, but they didin’t
explain the effect of ethanol. For the soft template method, some
isomerization of propylene oxide, but the activity was not good. For
isomerization of simple epoxides, the gas–solid fixed bed reactor is
commonly used because of their boiling point is low. Only some
oxides [11], zeolite [12] and Li
in this heterogeneous reaction. And it was reported that SiO
good support for Li PO for isomerization of propylene oxide [15]. But
3
PO
4
[13,14] have been used as catalysts
n
2
was a
Corresponding author. Tel./fax: +86 25 84315517.
E-mail addresses: maweiuan@163.com, mwh@njust.edu.cn (W. Ma).
3
4
http://dx.doi.org/10.1016/j.jssc.2014.03.003
0
022-4596/& 2014 Elsevier Inc. All rights reserved.

68
T. Wang et al. / Journal of Solid State Chemistry 215 (2014) 67–73
ꢁ
1
before SiO
the material with amine functional group because its isoelectic point
is too low to support nanogold. For Au/TiO , the treatment is
unnecessary.
2
is used as the support of nanogold it must be treated by
a weight hourly space velocity value of about 9 h were isomer-
ized with temperature-programmed heating from 30 1C to 290 1C,
then to 330 1C, 370 1C, to 400 1C finally (10 1C/min, each process
was kept for 20 min).
2
The products were condensed below 0 1C and analyzed chro-
matographically by a 1102 Gas Chromatography (GC) equipped
with a 30 m capillary column inner-coated with carbowax 20M-2-
nitroterephthalic acid.
2
. Experimental
2.1. Materials
2
.5. Characterization
Analytical reagents of anhydrous ethanol and propylene oxide
PO) were provided by Nanjing Ningshi Chemical Co., Ltd. (Nanj-
(
Transmission Electron Microscopy (TEM) images were taken on
ing, China). Concentrated ammonia aqueous solution (25%) was
purchased from Shanghai Lingfeng Chemical Co., Ltd. (Shanghai,
China). And analytical reagents of propylene oxide, cetyltrimethy-
lammonium bromide (CTAB) and Tetraethoxysilane(TEOS) were
provided by Chengdu Kelong Chemical Co. Ltd.(Chengdu, China).
3-aminopropyl)triethoxysilane (APTS; 98%) and gold(III) chloride
trihydrate were supplied by Aladdin Reagent (Shanghai) Inc.. All
the reagents were used as received.
a Philip Tecnai 12 electron microscope at 120 kV. High-resolution
TEM(HR-TEM) images and STEM-HAADF-EDX were obtained by
Tecnai G2 F30 S-TWIN at 300 kV. Scanning Electron Microscopy
(SEM) images were taken on a Hitachi S-4800IIFESEM field
emission scanning electron microscope.
(
The average diameters of the spheres and the size of Au
particles were analyzed by the Nano Measure 1.2, an SEM diameter
computation software.
N
2
adsorption–desorption isotherms were measured on a
2
.2. Preparation of hollow mesoporous silica spheres
Micromeritics ASAP 2020 M Porosity Analyzer. Prior to each
adsorption experiment, the samples were outgassed at 573 K for
at least 2 h under vacuum. BET and BJH methods were used to
determine the pore volume, pore size and the surface areas of the
samples.
Hollow mesoporous silica spheres were synthesized via
sol–gel/emulsion method. Normally, 202.5 ml ethanol aqueous
solution with a certain ethanol to water ratio was prepared first.
Then 2.5 ml TEOS was added into the solution. The CTAB, with
different concentrations, was then dissolved into the mixture.
XRD patterns were recorded on a Bruker D8 X-ray Powder
Diffractometer, using CuK
α
radiation (
0 kV and a current of 30 mA. These samples were scanned in the
range from 0 to 101.
λ¼1.5406̊), with a voltage of
2
.5 ml ammonia aqueous solution, which was used to catalyze
4
2
the hydrolysis of TEOS, was added into the system lastly. The
reaction was undergoing at the stirring speed of 700 rpm at 25 1C.
After 3 h the resultant product was separated centrifugally for
θ
3
6
8
times, washed with water, dried at 200 1C for 6 h, and calcined at
00 1C for 6 h. When the concentration of CTAB was 5.4 mM,
kinds of ethanol water mixtures were used with ethanol to water
3
. Results and discussion
3.1. Effects of the ethanol on the formation of the hollow spheres
ratio of 0.72, 0.62, 0.59, 0.53, 0.50, 0.47, 0.42 and 0.37. And when
the ratio of ethanol to water was 0.59, 6 different concentrations of
CTAB were employed to discuss the effect of CTAB concentration,
that is, 10 mM, 5.4 mM, 4.9 mM, 4.3 mM, 3.8 mM and 3.25 mM.
Fig. 1 shows the SEM images of the silica spheres synthesized at
different ethanol to water volume ratios (in short, E/W ratio) while
under the same CTAB concentration. The SEM images show that
there is no aggregation of small particles at the surface. The
surfaces of the spheres in Fig. 1(a)–(c) are a little coarse while
the spheres synthesized at the ratio of larger than 0.47 remain
smooth after calcinations. The average diameter of the spheres
ranges from 895 nm (at the ratio of 0.72) to 157 nm (at the ratio of
0.37), as shown in Table 1. And it is clear that the average diameter
of the silica spheres decreases as the ratio of ethanol to water is
reduced, which indicates that the diameter of the silica spheres
can be controlled by changing the amount of the ethanol in the
solvent.
2.3. Supporting nanoparticles of gold
Hollow mesoporous silica spheres were grafted by amine
functional groups. First, mesoporous silica (0.5 g) was added to
0 ml of toluene and stirred, 0.5 ml of (3-aminopropyl)triethox-
5
ysilane was then added and the mixture was refluxed for 24 h. The
product was filtered and washed several times with a large
quantity of ethanol and toluene, and then vacuum-dried at 80 1C
for 6 h.
To introduce the gold precursor onto the hollow mesoporous
silica spheres, 0.1 g of functionalized mesoporous silica and 10 ml of
Fig. 2 is the TEM images of the hollow silica spheres prepared at
different ethanol-to-water ratios. Hollow silica spheres can be
achieved at the ratios from 0.62 to 0.47. It is obvious that with a
proper amount of ethanol added into the system, the hollow silica
spheres can be achieved. A certain amount of ethanol can stabilize
the oil droplets in the system. But when too much or too little
ethanol was added in, the synthesized spheres were no longer
hollow inside. The wall thickness of the hollow silica spheres
decreases with less ethanol introduced into the system. Fig. 3 is
the XRD patterns of silica spheres at different E/W ratios and the
HR-TEM image of the silica spheres synthesized under the ethanol
ꢁ
4
5
ꢀ 10 M gold(III) chloride trihydrate was dissolved in water and
then the mixture was sonicated for 30 min. The monoamine groups
on silica surfaces are expected to interact with the gold species, and
the ultrasonic treatment is assumed to facilitate the homogeneous
uptake of gold species. The resultant materials were then filtered
and vacuum-dried at room temperature overnight, and the
obtained dry powder was heated to 200 1C in a muffle furnace at
a rate of 1 1C/min and then calcined at 200 1C for 1 h. The Au/SiO
also written as Au/APTS-SiO ) catalyst was obtained.
2
(
2
to water ratio of 0.59. The XRD pattern exhibits one peak at 2
θ
2
.4. Isomerization of propylene oxide catalyzed by Au/SiO
2
value 2.71 which indicating a mesoporous silica structure lacks
long-range ordering in the arrangement of mesopores. This is
consistent with the wormlike pores structure observed in the HR-
TEM image. It shows clearly the mesoporous structure of the
spheres. The mesopores are radiated throughout the spheres.
Catalytic isomerization reactions of propylene oxide were
conducted in a 10 mm fixed-bed tubular reactor filled with
Au/SiO catalysts. The vaporized propylene oxide (PO) stream with
φ
2

T. Wang et al. / Journal of Solid State Chemistry 215 (2014) 67–73
69
Fig. 1. SEM images of silica spheres at different E/W ratios (CTAB 5.4 mM). (a) 0.37; (b) 0.42; (c) 0.47; (d) 0.50; (e) 0.53; (f) 0.59; (g) 0.62; (h) 0.72.
Table 1
Structures of silica spheres synthesized at different E/W ratios and CTAB concentrations.
2
E/W
CTAB (mM)
Particle size (nm)
Shell thickness (nm)
Surface area (m /g)
Pore diameter (nm)
Structure (hollow/solid)
0.72
0.62
0.59
0.53
0.50
0.47
0.42
0.37
0.59
0.59
0.59
0.59
0.59
5.4
5.4
5.4
5.4
5.4
5.4
5.4
5.4
10
4.9
4.3
3.8
3.25
894.87
453.08
378.82
352.11
249.30
230.83
197.86
156.91
298.19
387.87
391.72
421.49
425.65
–
Solid
73.91
70.42
38.27
30.75
23.34
–
Hollow
Hollow
Hollow
Hollow
Partly hollow
Solid
Solid
Solid
Hollow
Hollow
Hollow
Hollow
907
794
2.2
2.3
–
777
690
2.3
–
52.81
50.53
66.87
43.50
Fig. 2. TEM images of silica spheres at different E/W ratios (CTAB 5.4 mM). (a) 0.37; (b) 0.42; (c) 0.47; (d) 0.50; (e) 0.53; (f) 0.59; (g) 0.62; (h) 0.72.
In the reaction system, ethanol was served as cosolvent to increase
the amount of TEOS dissolved in water, it can be used to control the
size of the micelles or the TEOS droplets to obtain different sizes of
silica spheres. Moreover, it can inhibit the hydrolysis reaction of TEOS
as one resultant of it. Thus, it could explain why at too low or too high
ethanol-to-water ratio, the hollow silica spheres cannot be achieved.

70
T. Wang et al. / Journal of Solid State Chemistry 215 (2014) 67–73
diameters of silica spheres synthesized under different CTAB
concentration were also obtained.
When more CTAB was introduced into the system, the inter-
facial energy was reduced. Thus, the surface area of the oil droplets
was reduced spontaneously which led to the formation of smaller
oil droplets at higher CTAB concentration. With less TEOS con-
tained in the oil droplets for further reaction, the silica spheres
were small in size. But they were well-distributed. It was noted
that when more CTAB was added into the system, the white
precipitate appeared faster after the addition of ammonia aqueous
solution. It can be deduced that the increasing amount of CTAB
could accelerate the hydrolysis reaction of TEOS. Therefore, when
10 mM CTAB was added, the hydrolysis reaction took place so fast
that the TEOS droplets cannot be formed. Thus, the silica spheres
obtained at 10 mM was solid.
3.3. Proposed mechanism of the formation of silica spheres
Fig. 3. XRD patterns of silica spheres at different E/W ratios and HRTEM image at
E/W 0.59 (CTAB concentration 5.4 mM for all the samples).
The proposed mechanism of the formation of silica spheres is
shown in Fig. 6. An ethanol water mixture and a certain amount
of CTAB should be employed to stabilize the oil droplets of TEOS.
The oil droplets enclosed by CTAB micelles serve as the tem-
plates in the formation of hollow silica spheres. When ammonia
aqueous solution is added into the system, the TEOS on the
interface is hydrolyzed and silica is deposited on the interface.
Since the concentration of TEOS decreases on the interface, the
concentration difference makes the TEOS in the oil droplets
diffuse from the inside to the interface. Thus, the hollow silica
sphere can be achieved after calcination. If the TEOS inside does
not all diffuse to the interface, the spheres will be partly hollow.
This results in the unique morphology that is like an orange
gotten rid of its flesh. CTAB surfactant directs the formation of
meso-structures of hollow silica spheres. The solid spheres are
also mesoporous in the structure due to the existence of CTAB
micelles in the system. If too little ethanol is introduced into the
system, the hydrolysis of TEOS is so fast that near the surface of
the droplets the ethanol concentration is much higher, and also
the TEOS inside the oil droplets cannot be diffused so fast
outwards. So the synthesized silica spheres are all solid. In the
experiments of E/W 0.37 and 0.42 the reactions took placed
much faster. When the E/W ratio was between 0.47 and 0.72, as
soon as the ammonium was added into the system, the TEOS of
the surface was hydrolyzed, and the inner TEOS was diffused
towards the surface, so the shell wall got thicker. The TEOS near
the center of the droplet was difficult to move out. But if the
ethanol in the system is redundant, the suppressing effect of
ethanol make the TEOS near the surface of the droplets remain
the same and the inner TEOS could not be diffused outwards,
after calcinations the spheres became solid.
Fig. 4. N
2
adsorption and desorption isotherm and pore size distribution curves of
the sample at E/W ratio 0.59 and CTAB concentration 5.4 mM.
2
Fig. 4 shows the N adsorption and desorption isotherm curves
of hollow silica spheres of the sample at E/W ratio 0.59 and CTAB
concentration 5.4 mM. The isotherm curves show typically type IV
curves based on the definition by IUPAC. The hysteresis loops are
of H category in the range of 0.4–1.0p/p , which definitely proves
2 0
the silica spheres are of mesoporous structures, no matter they are
hollow or not. At the ratio of 0.59, the BET surface area is
2
determined to be 907 m /g. The BJH pore size distribution curve
Such a mechanism can explain why the prepared hollow silica
spheres were not perfectly hollow, as shown in Fig. 1. That is
because the TEOS inside was not all hydrolyzed to form the shell.
The TEOS remained in the middle of the oil droplets was decom-
posed and nucleated inside after calcination.
which is calculated by adsorption branch shows a peak at 2.2 nm.
The measured surface area and pore size of the other materials
were listed in Table 1.
3.2. Effects of the CTAB concentration
3.4. Support of nano-gold
The results showed that not only the ethanol-to-water ratio
had impact on the synthetic process of hollow silica spheres, the
CTAB concentration also had effect on the formation of the
spheres. Fig. 5 shows the TEM images of the silica spheres
synthesized at different CTAB concentrations when the ethanol
to water ratio was fixed at 0.59. When 10 mM CTAB was added, the
average diameter was 298 nm and the spheres were solid. As the
CTAB concentration in the system decreased from 5.4 mM to
The infrared spectra of functionalized and Au supported hollow
mesoporous silica spheres are shown in Fig. 7. The peak at
ꢁ
1
1560 cm , is assigned to the deformation vibration of –NH
addition, the asymmetric stretching vibration of –CH near
after functionalized is stronger, because APTS has the
2
. In
2
ꢁ
1
2930 cm
group of –CH
ꢁ
1
2
2 2 2
CH CH NH . The weak peaks at 1300–1500 cm
can be caused by the support of Au. As observed from TEM image
in Figs. 8 and 9, we confirmed that the gold nanoparticle was
supported successfully onto the silica spheres because many spots
3
4
.25 mM, the average diameter increased from 378 nm to
25 nm and they were all hollow in character. The average

T. Wang et al. / Journal of Solid State Chemistry 215 (2014) 67–73
71
Fig. 5. TEM images of silica spheres at different CTAB concentrations (E/W 0.59). (a) 3.25; (b) 3.8; (c) 4.3; (d) 4.9; (e) 5.4; (f) 10.
Fig. 6. Schematic illustrations for the controllable synthesis of hollow sphere.
2
Fig. 8. TEM image of Au/APTS-SiO .
can be seen on the TEM image in Fig. 8 (some black spots are
circled as shown) and the STEM-HAADF images in Fig. 9(a) and (b),
which shows that they are well-dispersed in the silica spheres.
From the EDX analysis (Fig. 9(c)) we can conclude that these spots
are gold nanoparticles supported on silica spheres which are about
2–10 nm in size. The size distribution based on the STEM image is
shown in Table 2. The mean particle size is 5.1 nm. Most particles
are of 3–7 nm. In Fig. 8 the Au density is much lower than in Fig. 9
(a). From the many images we have taken, the Au density is not so
high as in Fig. 9. From the HAADF images (Fig. 9(b)) and the
mapping of gold (Fig. 9(c)), we can conclude that the Au particles
2 2
Fig. 7. FTIR spectra of APTS-SiO and Au/APTS-SiO .

72
T. Wang et al. / Journal of Solid State Chemistry 215 (2014) 67–73
2
Fig. 9. STEM-HAADF-EDX of Au/APTS-SiO .
Table 2
The size distribution of Au particles.
Diameter (nm)
Amount (%)
2–2.9
5.5
2.9–23.8
12.8
3.8–24.7
26.8
4.7–25.6
24.4
5.6–26.5
14
6.5–27.4
9.1
7.4–211
7.4
Table 3
Catalytic performance of nano-gold/SiO
2
.
Temp (1C)
PO conversion (%)
Selectivity, %
Propanal
Acetone
Isopropanol
Propanol
Allyl alcohol
3
3
3
0–290
30
70
0
0
21.0
24.1
0
6.7
3.5
0
0.2
0.8
0
0.6
1.5
0
71.5
70.1
11.2
52.72
does exist on the surface of the silica spheres, but we can’t judge if
they exist inside the sphere. The location of Au needs to be further
investigated. These results show clearly that the hollow silica
spheres were successfully modified and functionalized and the
nanoparticles of gold were supported by these functionalized
hollow silica spheres.
effect on the isomerization reaction. When the temperature
reached 330 1C, a small amount of propylene oxide was converted,
and the selectivity of main product allyl alcohol could reach 71.5%.
As the temperature continued to increase, the conversion of
propylene oxide was also improved, and the selectivity of allyl
alcohol and other by-products remained the same. Although it is a
novel catalyst to catalyze the isomerization reaction of propylene
oxide, we found that Au catalyst showed relatively good catalytic
3.5. Catalytic performance
performance at relatively high temperature. We also used Au/TiO
2
The catalytic performance of the gold nanoparticles which
as the catalyst for this reaction, the conversions were 21%, 33%,
44%, 42% at the corresponding temperatures of 275 1C, 300 1C,
325 1C, 350 1C. And the highest selectivity was 19% at 325 1C.
were supported on hollow mesoporous silica spheres was tested
by the isomerization reaction of propylene oxide. Catalytic perfor-
mance of Au catalyst was shown in Table 3. When the reaction
temperature ranged from 30 to 290 1C, the catalyst almost had no
2
Au/TiO was not good for the preparation of allyl alcohol by the
isomerization of PO.

T. Wang et al. / Journal of Solid State Chemistry 215 (2014) 67–73
73
4
. Conclusions
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Acknowledgment
[
1
This work is financed by the Natural Science Foundation
(
No. 21276127).