Synthesis and characterization of Co-SBA-15 via rapid ultrasonic technique and their catalytic properties

Article abstract of DOI:10.1166/jnn.2014.8925

The rapid preparation of SBA-15 with cobalt (Co) introduction was performed via the ultrasonic irradiation in combination with "pH-adjusting" method. The catalytic properties of the synthesized Co-SBA-15 were investigated by examining the oxidation of sty

Full text of DOI:10.1166/jnn.2014.8925

Article
Journal of
Nanoscience and Nanotechnology
Vol. 14, 7234–7241, 2014
Copyright © 2014 American Scientific Publishers
All rights reserved
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Printed in the United States of America
Synthesis and Characterization of Co-SBA-15 via
Rapid Ultrasonic Technique and Their
Catalytic Properties
Xiaojing Jin, Xueyan Qian, and Baitao Li∗
Key Laboratory of Fuel Cell Technology of Guangdong Province, School of Chemistry and Chemical Engineering,
South China University of Technology, Guangzhou 510640, China
The rapid preparation of SBA-15 with cobalt (Co) introduction was performed via the ultrasonic irra-
diation in combination with “pH-adjusting” method. The catalytic properties of the synthesized Co-
SBA-15 were investigated by examining the oxidation of styrene with hydrogen peroxide. The effect
of pH values on the textural properties were extensively investigated using small-angle X-ray Diffrac-
tion (XRD), N2 adsorption–desorption isotherms and Transmission Electron Microscipy (TEM). The
characterization results showed that the incorporation of Co by ultrasonic method did not destroy the
mesoporous structure of SBA-15. The Co-SBA-15 catalyst with Co/Si (the molar ratio of 0.03) at pH
of 7.5 exhibited a well-ordered hexagonal mesoporous structure with higher surface area and pore
volume. This catalyst had excellent styrene conversion and selectivity to benzaldehyde of 21.8%
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and 92.3%, respectively. The physicochemical properties and catalytic activity of the Co-SBA-15
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catalysts via ultrasonic technique possessed the comparable characteristics as those prepared via
Copyright: American Scientific Publishers
conventional hydrothermal method.
Keywords: Ultrasonic, pH Adjustment, Incorporation, Co-SBA-15, Styrene.
1. INTRODUCTION
“two solvents” method.18 and “pH-adjusting” method.19ꢀ20
Compared with these approaches, “pH adjusting” method
appears to be an effective and convenient method to obtain
high contents of metal species in SBA-15, because metal
species exist only in the cationic form and can not be
introduced into the mesoporous walls via a condensa-
tion process.19 However, in the “pH-adjusting” method,
the heteroatom source was added into the initial reac-
tion mixture in strongly acidic media (pH < 0); when
the mesostructure was basically formed, the pH value of
the system was adjusted from a strong acid (pH < 0) to
neutral (pH ≈ 7.5).19 Several feasible research protocols
have been developed to incorporate Al,21 Co,22 Fe23 or
Ag24 heteroatoms by “pH-adjusting” methods. Hydrother-
mal synthesis, chemical reactions carried out in heated
aqueous solution, is widely applied to produce different
kinds of nanomaterials.25 The aforementioned heteroatoms
were incorporated into the silica matrix unexceptionally
by hydrothermal method under static conditions for a few
days (more than 48 hrs). It has been reported that by “pH-
adjusting” method, it usually required a long time more
than 96 hrs to incorporate Al atoms into the framework
Mesoporous materials have attained considerable attention
in the recent years because of their desirable features (e.g.,
high surface areas, uniform mesopore size and easily con-
trolled pore size).1–6 Mesoporous silica of Santa Barbara
Amorphous (SBA) type material, SBA-15 with high spe-
cific surface area (up to 1160 m2/g), high hydrothermal sta-
bility, well-ordered two-dimensional hexagonal structure
with tunable pore size (47–300 Å), thick wall (31–64 Å)
and large pore volume (up to 2.5 cm3/g),7 has numerous
potential applications in catalysis,8 gas sensor,9 separation
and material science.10 However, pure silica mesoporous
materials lack sufficient intrinsic activities for chemical
reactions.11
In order to create the active sites and generate the
catalytic activity, SBA-15 has been modified with a
wide diversity of metal ions, transition metal oxides.12–14
and organic functional groups.15 Many efforts have been
made to incorporate heteroatoms into the framework of
SBA-15, by post-synthesis grafting,16 direct synthesis,17
∗Author to whom correspondence should be addressed.
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1533-4880/2014/14/7234/008
doi:10.1166/jnn.2014.8925

Jin et al.
Synthesis and Characterization of Co-SBA-15 via Rapid Ultrasonic Technique and Their Catalytic Properties
of SBA-15.21 Therefore, much attention has been paid to
reduce the synthesis time for economic benefit without
sacrificing the quality of the products.
distilled water to remove the weakly adsorbed ions, and
ꢀ
then dried overnight at 110 C in an oven at static condi-
ꢀ
tion. Finally, the product was calcined at 550 C for 6 hrs
ꢀ
to remove the surfactants at a heating rate of 2 C/min.
Ultrasound irradiation has emerged as a powerful
technique for the preparation of novel nanostructure
materials.26ꢀ27 The acoustic cavitation effect from the
ultrasound irradiation can produce high temperatures,
promoting the rates of chemical reactions. The ultra-
sonic technique has been applied in the synthesis of silica
MCM-41.28 and SBA-15,29 as well as in the incorpora-
The obtained product was denoted as xCo-SBA-15a, where
“x” is the Co/Si (molar ratio) in the initial gel and “a” is
the adjusted pH value. Catalysts containing various metel
ions M-SBA-15 (M = Nd, Ni, Ce, Y) were synthesized
through the same procedure under pH = 7.5, using the cor-
responding nitrate salts. SBA-15 was synthesized via the
same procedure under ultrasonic irritation for 1 hr with-
out pH adjusting. The sample obtained was referred to
SBA-15-ultra.
tion of MnO2 or ruthenium31 in the pore structure of
30
SBA-15.
This study targeted the challenge of a rapid and feasi-
ble preparation strategy to synthesize the metal-containing
SBA-15 for the oxidation of styrene with higher styrene
conversion and benzaldehyde selectivity using hydrogen
peroxide as oxidant. Hydrogen peroxide is the most desir-
able oxidant for the oxidation of alkenes with respect to
environmental and economic considerations. Cobalt ion
and complexes are well-known catalysts for the selective
oxidation of alkanes and alkylbenzenes.32ꢀ33 The incorpo-
ration of cobalt (Co) into the framework of SBA-15 was
carried out by the ultrasonic irradiation in combination
with “pH-adjusting” method. The effect of pH values
during the preparation process on the textural proper-
ties were extensively investigated using small-angle X-ray
diffraction (XRD), N2 adsorption–desorption isotherms
Another series of Co-substituted SBA-15 were synthe-
sized using the conventional hydrothermal method. The
preparation and calcination procedures were similar with
those of xCo-SBA-15a, but the difference was the long
preparation time. After the addition of Co(NO3ꢁ2 · 6H2O,
the mixture waꢀs transferred to an autoclave for crystal-
lization at 100 C for 24 hrs. Then the pH was adjusted
by adding ammonia. The mixture was then crystallized
ꢀ
at 100 C for another 24 hrs. The obtained powder was
denoted as xCo-SBA-15Ha , where “x” is the Co/Si (molar
ratio) in the initial gel and “a” is the adjusted pH value.
2.2. Characterization Methods
The actual element compositions were determined by
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an inductively coupled plasma (ICP) spectrophotometer
and Transmission Electron Microscopy (TEM). The cat-
IP: 80.222.140.19 On: Mon, 09 Nov 2015 07:33:19
(Spectro Ciros, Germany). The calcined solid samples
alytic properties of synthesized Co-SBA-15 were further
evaluated by examining the oxidation of styrene. In addi-
tion, the physicochemical properties and catalytic activity
of the synthesized catalysts were compared with those pre-
pared by conventional hydrothermal method.
Copyright: American Scientific Publishers
were dissolved by acidic digestion.
The crystalline phases of the catalysts were measured
by X-ray diffraction (XRD) with a CuKꢂ (ꢃ = 0ꢄ154 nm)
radiation source (D8 Advance, Bruker, Germany). The
tests were operated at 40 kV and 40 mA over the scatter-
ing angle of 2ꢅ from 0.6ꢀ to 5ꢀ with a step size of 0.02ꢀ
and a step time of 0.4 s.
2. EXPERIMENTAL DETAILS
2.1. Catalysts Preparation
The textural properties of the samples were derived
from N2 adsorption/desorption measurement at 77 K on
Micromeritics TriStar II 3020. Prior to the measurement,
Cobalt (Co)-substituted SBA-15 was synthesized via ultra-
sonic technique with the modified pH-adjusting method.34
Triblock copolymer Pluronic P123 (EO20PO70EO20,
molecular weight 5800, Aldrich) was used as the template
and tetraethylorthosilicate (TEOS) as the silica source.
2 g Pluronic P123 was dispersed in 100 g 2 M HCl
under vigorous stirring for 2 hrs to obtain a homoge-
neous solution. 4 g TEOS was added into the solution
dropwise and stirred for another 1 hr. A requisite amount
of Co(NO3ꢁ2 · 6H2O was then added to the solution. The
mixture was immediately subjected to sonication at room
temperature irradiated by an ultrasonic generator (Numer-
ical control ultrasonic cleaning machine, KH3200DE,
KunShanHeChang ultrasonic instrument Co., Ltd.) with
the ultrasonic waves (40 kHz) at the output power of
150 W. After 1 hr of irradiation, the pH of the mixture
was adjusted by adding the concentrated ammonia, and the
mixture was then sonicated for 1 hr. The synthesized prod-
uct was thoroughly filtered and extensively washed with
ꢀ
the samples were outgassed at 200 C to a residual pres-
sure below 1×10−4 Torr for 5 hrs. The pore-size distribu-
tion was measured from the desorption branch using the
Barrett–Joyner–Halenda (BJH) method.
Surface morphology of the samples was examined by
transmission electron microscopy (TEM) (JEM-2100HR,
JEOL) equipped with a lanthanum hexaboride electron gun
and operated at an accelerating voltage of 200 kV. The
samples were dispersed ultrasonically in ethanol and then
deposited a TEM copper grid before the measurement.
2.3. Catalytic Activity Measurements
The catalytic reaction was carried out in a 50 ml two-
necked round-bottomed glass flask connected to a reflux
condenser and a thermometer. The temperature of the reac-
tion vessel was maintained using a water bath. Hydro-
gen peroxide (aqueous solution 30 wt%) was used as
J. Nanosci. Nanotechnol. 14, 7234–7241, 2014
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Synthesis and Characterization of Co-SBA-15 via Rapid Ultrasonic Technique and Their Catalytic Properties
Jin et al.
Table II. Oxidation of styrene with H2O2 catalyzed by different cobalt-
base catalysts.
the oxygen-donor and acetonitrile as the solvent. In a
typical oxidation reaction, 0.05 g catalyst was mixed
with 10 ml acetonitrile solvent, 10 mmol styrene and
10 mmol 30 wt% H2O2. The mixture was heated with
Selectivity (%)
Styrene
Styrene
ꢀ
stirring under reflux at 70 C for 6 hrs. After the reac-
Catalyst
conversion (%)a Benzaldehyde epoxide Hyacinthin
tion, the catalyst was removed by hydrophobic membrane.
The organic compounds were analyzed quantitatively by
gas chromatograph, equipped with a 5% diphenyl-95%
dimethylpolysiloxane benzyl siloxane capillary column,
and a flame ionization detector (FID). The products iden-
tification was achieved from the retention time of the
pure compound. Quantitative analyses were performed by
taking into account the FID response factors for each
compound.
Blank test
SBA-15-ultra
3ꢄ3
4ꢄ7
4ꢄ7
18ꢄ3
21ꢄ8
25ꢄ6
12ꢄ8
16.0
77.4
76.5
84.5
92.3
86.9
83.0
84ꢄ0
21ꢄ7
22ꢄ6
15ꢄ2
7ꢄ1
0.0
0.9
0.9
0.3
0.6
0.8
0.5
0.03Co-SBA-15b
0.03Co-SBA-156ꢄ0
0.03Co-SBA-157ꢄ5
0.03Co-SBA-159ꢄ0
Co3O4/SBA-15c
12ꢄ3
16ꢄ4
Notes: aReaction condition: styrene, 10 mmol; H2O2, 10 mmol; acetonitrile, 10 ml;
catalyst, 0.05 g; temperature, 70 ꢀC; reaction time, 6 hrs; bSynthesized without
pH adjustment; cPrepared by conventional impregnation method with Co/Si (molar
ratio) of 0.03. SBA-15 was synthesized by ultrasonic technique.
3. RESULTS AND DISCUSSION
3.1. Catalytic Performance Over
doped in the framework of SBA-1537 or by the coordi-
nation of the solvent to Co2+ which affects the ability of
H2O2 to bind the metal ions.38
Various Metal Species Catalysts
The results showed that different heterogeneous catalysts
had significant effects on the catalytic performance in the
oxidation of styrene with H2O2. The catalytic performance
in terms of the styrene conversion and selectivity was sum-
marized in Table I, which was obtained by using sev-
eral metal element-containing SBA-15 catalysts prepared
by ultrasonic waves at the pH of 7.5 during the prepa-
ration process. Except for La-SBA-15, the main product
3.2. Effect of pH During the Preparation Process on
the Oxidation of Styrene with H2O2
The catalytic performance of various Co-based catalysts
prepared under different pH value was shown in Table II.
A blank test was performed in the identical reaction con-
dition for comparison, and the conversion of styrene was
only 3.3%. Pure siliceous SBA-15 gave a similar conver-
sion. It should be addressed that if the pH was not adjusted
during the preparation process (pH < 0 in the parent solu-
tion), the 0.03 Co-SAB-15 catalyst displayed negligible
catalytic activity that was nearly the same as that of the
blank, which means that there was no active cobalt sites
in the catalyst.
Interestingly, the styrene conversion was significantly
affected by pH values. It substantially increased to 25.6%
when pH was adjusted to 9.0. The cobalt contents in the
solid product increased with the pH values (Table III).
This implies that the higher pH condition favored the
incorporation of metal ions. The highest selectivity to ben-
zaldehyde (92.3%) was obtained at the pH of 7.5. It is
probably that the excess OH− (pH = 9.0) reacted with
Co2+ to form Co(OH)2 precipitation, which prevented the
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was benzaldehyde for other catalysts, resulting from the
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nucleophilic attack of H2O2 to styrene oxide followed by a
Copyright: American Scientific Publishers
cleavage of the intermediate hydroxy–hydroperoxystyrene
and also from the direct oxidative cleavage of the styrene
side chain double bond.35ꢀ36 Using different metal ele-
ments doped SBA-15 mesoporous materials, the obtained
styrene conversions varied substantially. Compared with
the Ni-, Nd-, Ce-, Y- and La-containing SBA-15, the Co-
SBA-15 converted 22% of styrene and exhibited higher
styrene conversion, thus leading to the greatest quantity of
benzaldehyde in the products. Other metal species were
not as good as Co catalyst for the reaction. The different
catalytic performance of M-SBA-15 mesoporous materials
was caused either by the different nature of metal element
Table I. Catalytic performance of M-SBA-15 catalysts with various
metal species in the oxidation of styrene with H2O2.
Selectivity (%)
Table III. Textural properties of cobalt catalysts prepared under differ-
ent pH conditions.
Styrene
Styrene
Catalysta
conversion (%)b Benzaldehyde epoxide Hyacinthin
Co/Si (molar ratio)
Ni-SBA-15
Nd-SBA-15
Ce-SBA-15
Y-SBA-15
La-SBA-15
Co-SBA-15
6ꢄ4
7ꢄ5
5ꢄ1
6ꢄ1
4ꢄ3
72.7
80.2
88.1
76.5
47.8
92.3
26ꢄ4
18ꢄ8
11ꢄ3
22ꢄ4
51ꢄ6
7ꢄ1
1.0
1.0
0.7
1.2
0.5
0.6
Initial
gel
d100 a0
SBET
Vp
Pore size
Materials
Product
(nm) (nm) (m2/g) (cm3/g)a (nm)b
SBA-15-ultra
–
–
9ꢄ3 10.7 501
10ꢄ0 11.5 465
10ꢄ0 11.5 488
9ꢄ7 11.2 336
0.61
0.89
0.97
0.88
4.58
7.44
7.13
9.25
Co-SBA-156ꢄ0 0.03
Co-SBA-157ꢄ5 0.03
Co-SBA-159ꢄ0 0.03
0.006
0.007
0.014
21ꢄ8
Notes: aM/Si molar ratio: 0.03; adjusted pH value: 7.5; bReaction condition:
styrene, 10 mmol; H2O2, 10 mmol; acetonitrile, 10 ml; catalyst, 0.05 g; tempera-
ture, 70ꢀC; reaction time, 6 hrs.
Notes: aTotal pore volume; bAverage pore diameter by the desorption branch of the
isotherm calculated by BJH method.
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Jin et al.
Synthesis and Characterization of Co-SBA-15 via Rapid Ultrasonic Technique and Their Catalytic Properties
attacking of H2O2 to styrene oxide and resulted in a
lower benzaldehyde selectivity.22 Therefore in the follow-
ing experiments, the pH of 7.5 was chosen as the optimal
condition. The quantum calculation on the cobalt growth
mechanism under the alkaline condition is currently car-
ried out to fully understand the influence of the pH on this
reaction system.
Different synthetic methods lead to different location
of the active species. The Co3O4/SBA-15 catalyst, syn-
thesized using impregnation with an equivalent cobalt
loading, was compared with the catalysts prepared by
ultrasonic technique at the pH of 7.5. For the impregnation
method, the conversion of styrene was 12.8% (Table II),
which was much lower than that of the 0.03Co-SBA-
157ꢄ5. The oxidation of styrene with hydrogen peroxide
mainly occurred on the internal surface of mesoporous
pores when the metal active sites are incorporated inside
the framework.34 The Co3O4 particles outside the frame-
work of SBA-15 were obviously inactive for the conver-
sion of styrene. The results in this study demonstrated the
feasibility of the introduction of Co2+ into the mesoporous
pores of SBA-15 via rapid ultrasonic technique and pH
adjustment.
decreased from 20% to 7%, which is attributed to the
formation of benzaldehyde via the further oxidation of the
epoxide.40 The optimal reaction time was 6 hrs in our
experiments, in which the highest benzaldehyde selectivity
was reached.
3.4. Structure and Morphology of Various Cobalt
Catalysts Prepared via Ultrasonic Method
Structure and morphology were studied using XRD,
nitrogen-physisorption analyses and TEM. The small-
angle XRD patterns of 0.03Co-SBA-15 catalysts prepared
by ultrasonic irradiation at different pH values were shown
in Figure 2(A). An intense main diffraction peak and two
small peaks were well-resolved in the 2ꢅ range of 0.7ꢀ to
2ꢀ, corresponding to the (1 0 0), (1 1 0) and (2 0 0) planes,
which were the characteristics of the hexagonal pore
mesostructure of the synthesized materials.41 This sug-
gested that all the materials had a well-ordered 2D hexag-
onal mesoporous structure (p6 mm) and the introduction
(A)
(100)
3.3. Catalytic Performance versus Reaction Time
Over 0.03Co-SBA-157ꢄ5 Catalyst
The conversion of styrene and the selectivity to benzalde-
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alyzed by 0.03Co-SBA-157ꢄ5 was shown in Figure 1. The
(110)
hyde as well as styrene epoxide versus reaction time cat-
Copyright: American Scientific Publishers(200)
a
b
styrene conversion increased with the prolonging of reac-
tion time,39 with 12% in 1 hr to 38% in 10 hrs. The reac-
tion was fast at the initial period, because the total amount
of reactants was added in one batch at the beginning of
the reaction. The selectivity of benzaldehyde reached a
maximum at 92% in 6 hrs and remained constant for
up to 10 hrs. The selectivity of styrene oxide gradually
c
d
1
2
3
4
5
2 Theta (degree)
(B)
50
40
30
20
10
0
100
80
60
40
20
0
♦
Co3O4
♦
♦
♦
♦
♦
a1
a
b
c
d
0
1
2
3
4
5
6
7
8
9
10 11
Reaction time (h)
10
20
30
40
50
60
70
2 Theta (degree)
Figure 1. Effect of reaction time on oxidation of styrene with H2O2
over 0.03Co-SBA-157ꢄ5. (ꢀ) styrene conversion, (ꢁ) benzaldehyde selec-
tivity, (ꢆꢁ styrene epoxide selectivity. Reaction condition: styrene,
10 mmol; H2O2, 10 mmol; acetonitrile, 10 ml; catalyst, 0.05 g; temper-
ature, 70 ꢀC.
Figure 2. (A) Small-angle XRD patterns of (a) SBA-15-ultra,
(b) 0.03Co-SBA-159ꢄ0ꢀ (c) 0.03Co-SBA-157ꢄ5, (d) 0.03Co-SBA-156ꢄ0
(B) Large-angle XRD patterns of (a1) Co3O4/SBA-15, (a) SBA-15-ultra,
(b) 0.03Co-SBA-159ꢄ0ꢀ (c) 0.03Co-SBA-157ꢄ5, (d) 0.03Co-SBA-156ꢄ0
.
.
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Synthesis and Characterization of Co-SBA-15 via Rapid Ultrasonic Technique and Their Catalytic Properties
Jin et al.
of Co via rapid ultrasonic method did not destroy the
original mesoporous structure of SBA-15-ultra. In addi-
tion, the 0.03Co-SBA-156ꢄ0 presented the weak reflection
peaks of (1 1 0) and (2 0 0) planes, indicating the slight
loss of long-range ordering at lower pH condition. This
was the possible for its lower catalytic performance than
0.03Co-SBA-159ꢄ0 and 0.03Co-SBA-157ꢄ5 in the oxidation
of styrene (as indicated in Table II).
According to the wide-angle XRD patterns (Fig. 2(B)),
xCo-SBA-15a showed only a broad peak of the amor-
phous feature of SBA-15 and no peaks of Co3O4 were
Figure 3. TEM images of SBA-15-ultra (A) and 0.03Co-SBA-157ꢄ5
(B) viewed down the direction parallel to pore.
observed. However, the diffraction of 2ꢅ at 31.4ꢀ, 38.7ꢀ,
45.0ꢀ, 59.6ꢀ and 65.5ꢀ attributed to the crystalline Co3O4
(JCPDS 65-3103) were observed over Co3O4/SBA-15 cat-
alyst (Fig. 2(B)-a1). The lack of Co3O4 phase in xCo-
SBA-15a catalysts indicates that cobalt ions introduced
by ultrasonic technique have been incorporated into the
framework of SBA-15.
rapid ultrasonic method. It should be addressed that among
three Co-containing catalysts, Co-SBA-157ꢄ5 showed the
highest pore volume with the similar surface area to the
parent SBA-15-ultra, which indicated the effective and
higher cobalt introduction without sacrificing its high sur-
face area. This is in favor of its higher catalytic activity in
the oxidation of styrene.
Transmission electron microscopy (TEM) observation
clearly showed the well-ordered channels over the SBA-15
catalysts prepared via rapid ultrasonic method (Fig. 3(A)).
Moreover, the representative TEM images (Fig. 3(B)) of
0.03Co-SBA-157ꢄ5 confirmed that the highly ordered chan-
nel was retained when cobalt was introduced. No Co3O4
The incorporation of cobalt into the framework of
SBA-15 affected the position of the diffraction peak in
XRD patterns (Fig. 2(A)). All the peaks for cobalt-
containing catalysts shifted towards lower-angle direction,
which represented an increase in the lattice parameter
(d100ꢁ values. Through the calculation from the peak with
√
hkl = 100 using the equation a0 = 2d100/ 3, the (d100ꢁ val-
ues were obtained along with the corresponding unit cell
parameter (a0ꢁ of different SBA-15 samples (Table III).
The increase of the d-values and a0 indicated the pres-
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particles were observed for 0.03Co-SBA-157ꢄ5 sample,
IP: 80.222.140.19 On: Mon, 09 Nov 2015 07:33:19
indicating that cobalt species were homogenously incorpo-
ence of Co atoms in the framework, which could be
explained by the structure expansion caused by the larger
atomic radius of Co2+ (Pauling radius: 72 pm) than Si4+
(Pauling radius: 41 pm). Previous studies showed differ-
ent metal elements incorporated into molecular sieves.42ꢀ43
The results in this study suggested that the Co2+ cations
could be easily incorporated into the mesoporous frame-
work of SBA-15 to replace Si4+ via rapid ultrasonic
technique, which substantially saved time compared with
conventional methods.
Copyright: American Scientific Publishers
rated into the framework of SBA-15.47 These TEM results
indicated the use of ultrasonic waves does not damage the
ordered structure of SBA-15.
3.5. Comparison of the Catalysts Prepared by
Rapid Ultrasonic and Conventional
Hydrothermal Methods
3.5.1. Catalytic Performance During the
Oxidation of Styrene
Hydrothermal procedure is a common and effective
method for the synthesis of ordered metal element-
containing mesoporous silicas, however it usually takes
1–4 days for the crystallization. The comparison of the
catalytic activity and the textural properties for the cat-
alysts (xCo-SBA-157ꢄ5 and xCo-SBA-15H7ꢄ5ꢁ prepared by
rapid ultrasonic and hydrothermal methods were exten-
sively investigated (Fig. 4).
The pH adjustment under ultrasonic condition obviously
changed the textural properties, including surface area,
pore volume and pore size of the catalysts. Compared
with pure silica SBA-15-ultra, the pH-adjusted catalysts
had lower BET surface area (Table III), however, their
pore volume and average pore diameters increased. The
decrease in surface area can be attributed to the blocking of
the support porosity by the cobalt species that made them
inaccessible for nitrogen adsorption.44 This effect was also
observed on the mesoporous matrix filled with La.45 The
increase in pore volumes was in accordance with XRD
results and confirmed the expansion of the mesopores on
the introduction of the Co(+2) in the Si framework.46
The increase in pore size with cobalt incorporation was
due to the longer bond length of Co O than that of
Si O.22ꢀ43 The N2 adsorption results well corresponded
with the XRD results and clearly showed the incorporation
of Co2+ into the mesoporous framework of SBA-15 via
The Co content in xCo-SBA-157ꢄ5 and xCo-SBA-157Hꢄ5
clearly affected the catalytic performance. At the Co/Si
ratio of 0.03, the selectivity to benzaldehyde and the con-
version of styrene increased remarkably. It should be noted
that xCo-SBA-157ꢄ5 catalysts exhibited excellent benzalde-
hyde selectivity, comparable with the conventional synthe-
sis method, although the latter presented a slight higher
conversion of styrene. When the Co/Si ratio increased to
0.06, the conversion of styrene kept nearly unchanged,
but the selectivity to benzaldehyde decreased. The results
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Jin et al.
Synthesis and Characterization of Co-SBA-15 via Rapid Ultrasonic Technique and Their Catalytic Properties
50
100
the peaks were shifted to the lower 2ꢅ values. The changes
in peak intensity and low angle shift suggest that cobalt
was present on the internal pore wall of SBA-15.48
The advantage in the higher metal content incorpora-
40
30
20
10
0
90
tion (Co/Si = 0.10) by the ultrasonic method was observed
80
using XRD method (Fig. 5). For the xCo-SBA-157ꢄ5 (x =
0ꢄ10), a highly ordered mesostruction was observed, which
70
was proved by the three well-resolved peaks. On the other
hand, for the xCo-SBA-15H7ꢄ5 (x = 0ꢄ10), the diffraction
60
peaks of (1 1 0) and (2 0 0) were relatively weak and
broad, implying a decrease in the framework order by the
higher content of Co2+ 34
This was a clear indication that
.
50
0.00
0.02
0.04
0.06
0.08
0.10
although the ordered mesoporous structure was retained
at the Co/Si molar ratio up to 0.10, higher cobalt con-
tent caused a distortion of hexagonal ordering structure to
some extents. The reason for this phenomenon was that
when the system was adjusted to the neutral condition,
the metal ions transformed to the oxo form and condensed
with adjacent silanols.19 As a consequence, more metal
ions in interstitial regions probably affected the interac-
tion of silica and organic template during the crystalliza-
tion procedure, and resulted in the disordered structure.49
This disordered structure led to an inferior styrene con-
version in the oxidation of styrene to the equivalent cata-
lyst (in terms of Co content) synthesized by the ultrasonic
method, (as indicated in Fig. 4 when Co/Si molar ration
Co/Si
Figure 4. Effect of cobalt contents on oxidation of styrene with
H2O2 over xCo-SBA-157ꢄ5 (solid line) and xCo-SBA-15H7ꢄ5 (dotted line).
(ꢀ) styrene conversion, (ꢁꢁ benzaldehyde selectivity. Reaction condi-
tion: styrene, 10 mmol; H2O2, 10 mmol; acetonitrile, 10 ml; catalyst,
0.05 g; temperature, 70 ꢀC; reaction time, 6 h.
indicate that xCo-SBA-157ꢄ5 catalysts exhibit the com-
parable catalytic performance with xCo-SBA-15H7ꢄ5 prod-
ucts. When the Co/Si molar ratio was further increased to
0.10, xCo-SBA-157ꢄ5 showed 25% of styrene conversion,
higher than xCo-SBA-15H7ꢄ5 catalyst prepared by hydrother-
mal method. This suggested that the ultrasonic method is
beneficial when the metal species are introduced in large
was 0.10).
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amount.
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The different preparation method for Co incorporation
Copyright: American Scientific Publishers
into the framework of SBA-15 had a significant influence
3.5.2. Textural Characteristics
on the pore size distribution. The nitrogen adsorption–
desorption isotherms and the corresponding pore size
distribution for xCo-SBA-157ꢄ5 and xCo-SBA-15H7ꢄ5 were
found to be type IV in nature and exhibited a type-H1 hys-
teresis loop (Fig. 6), which was typical of the meso-
porous SBA-15 with one-dimensional cylindrical channels
according to the IUPAC (International Union of Pure
and Applied Chemistry) classification.50 For xCo-SBA-
157ꢄ5 catalysts, the adsorption branch of each isotherm
showed a sharp inflection at p/po = 0ꢄ6–0.7, which indi-
cated the capillary condensation within uniform pores.51–53
The capillary condensation step suggested a relatively nar-
row and sharp pore size distribution centered in the range
of 7.1–7.3 nm. On the other hand, in the case of xCo-
SBA-15H7ꢄ5 catalysts, the inflection point appeared at higher
p/po = 0ꢄ8, compared with that of xCo-SBA-157ꢄ5 cata-
lysts, indicating a relatively wider pore size distribution
with maximum in the range of 7.9–10.2 nm. The broad
hysteresis loop extending to p/po = 1ꢄ0 was caused by
the presence of long mesopores connected by smaller
micropores.54 Moreover, the sharpness of the inflection
step decreased with an increasing in cobalt loadings,
and the pore size distribution was slightly shifted toward
wide regions, which indicated the larger pore diameter in
the xCo-SBA-15H7ꢄ5 catalysts prepared by the conventional
method.
The comparable structural patterns of xCo-SBA-157ꢄ5 with
that of xCo-SBA-15H7ꢄ5 were observed (Fig. 5). For all xCo-
SBA-157ꢄ5 catalysts, three XRD diffraction peaks were
detected in the region of 2ꢅ = 0.85–2ꢀ, which was indexed
to the (100), (110) and (200) diffractions typical of the
hexagonal lattice for the ordered mesoporous silica. The
results indicated that the structural integrity were retained
after the incorporation of Co under sonication. At higher
Co loadings, the three XRD peaks had lower intensity and
(100)
(100)
xCo-SBA-15H
xCo-SBA-15
(110)
(110)
(200)
(200)
(0.01)
(0.01)
(0.02)
(0.03)
(0.06)
(0.10)
(0.02)
(0.03)
(0.06)
(0.10)
1
2
3
4
5
1
2
3
4
5
2 Theta (degree)
2 Theta (degree)
Figure 5. Small-angel XRD pattern of xCo-SBA-15 and xCo-SBA-15H
catalysts.
J. Nanosci. Nanotechnol. 14, 7234–7241, 2014
7239

Synthesis and Characterization of Co-SBA-15 via Rapid Ultrasonic Technique and Their Catalytic Properties
Jin et al.
600
400
200
600
400
200
H
xCo-SBA-157.5
xCo-SBA-157.5
4
8
12
16
20
4
8
12
16
20
Pore size (nm)
Pore size (nm)
x = 0.01
x = 0.01
x = 0.02
x = 0.03
0
0
600
600
400
200
400
200
4
8
12
16
20
4
8
12
16
20
Pore size (nm)
Pore size (nm)
x = 0.02
0
0
600
600
400
200
400
200
nm
nm
20
4
8
12
16
20
4
8
12
16
Pore size (nm)
Pore size (nm)
x = 0.03
x = 0.06
x = 0.10
0
600
400
200
0
600
400
200
4
8
12
16
20
4
8
12
16
20
Pore size (nm)
Pore size (nm)
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x = 0.06
IP: 80.222.140.19 On: Mon, 009 Nov 2015 07:33:19
0
800
Copyright: American Scientific Publishers
800
600
400
200
0
600
400
4
8
12
16
20
4
8
12
16
20
Pore size (nm)
Pore size (nm)
200
0
x = 0.10
0.8 1.0
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
P/P
P/Po
o
Figure 6. N2 adsorption–desorption isotherms of xCo-SBA-157ꢄ5 (left) and xCo-SBA-15H7ꢄ5 (right). The pore size distribution curve for each isotherm
is shown as insert.
4. CONCLUSION
potentially expanded to the incorporation of other het-
eroatoms into porous materials to modify their catalytic
properties.
Second, pH value and Co/Si molar ratio affected the
surface area, pore volume and ordering structure. The xCo-
SBA-15 with Co/Si of 0.03 obtained at pH 7.5 exhibited
well-ordered hexagonal mesoporous structure with higher
surface area and pore volume, giving excellent styrene
conversion and selectivity to benzaldehyde.
Third, xCo-SBA-157ꢄ5 catalysts have relatively uniform
narrow pore size distribution and exhibited the comparable
catalytic performance with xCo-SBA-15H7ꢄ5 products.
Forth, the catalytic performance in the oxidation of
styrene with H2O2 was strongly dependent on the metal
species. Compared with the Ni-, Nd-, Ce-, Y- and
Cobalt incorporated within the framework of SBA-15,
xCo-SBA-15 (x: Co/Si molar ratio ranging from 0.01–
0.10) was successfully synthesized by the rapid ultrasonic
irradiation in combination with “pH-adjusting” method
in this study. Their catalytic properties were extensively
investigated in the oxidation of styrene with hydrogen
peroxide, and compared with the catalysts prepared by
conventional methods. There were four major conclusions
drawn:
First, the highly ordered hexagonal structure was well-
retained after the incorporation of cobalt via ultrasonic
technique. The rapid preparation strategy appeared to be
feasible and effective for the grafting of various metal
species to mesoporous silica materials. This route can be
7240
J. Nanosci. Nanotechnol. 14, 7234–7241, 2014

Jin et al.
Synthesis and Characterization of Co-SBA-15 via Rapid Ultrasonic Technique and Their Catalytic Properties
La-containing SBA-15, Co-SBA-15 converted 22% of
styrene and achieved the highest benzaldehyde selectivity.
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Acknowledgment: This work was supported by the
National Natural Science Foundation of China (No.
21173086).
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