Spherical V-MCM-48: The synthesis, characterization and catalytic performance in styrene oxidation

Article abstract of DOI:10.1039/c4ra08333d

A series of spherical, vanadium-containing, mesoporous V-MCM-48 catalysts with different V/Si atomic ratios were synthesized by a direct hydrothermal method. The structure and morphology of the samples and the states of vanadium in the materials were syst

Full text of DOI:10.1039/c4ra08333d

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Spherical V-MCM-48: the synthesis,
characterization and catalytic performance in
styrene oxidation
Cite this: RSC Adv., 2014, 4, 50832
a
a
a
b
a
a
Haiqing Wang, Wang Qian, Jin Chen, Yong Wu,* Xiaoying Xu, Jun Wang
a
and Yan Kong*
A series of spherical, vanadium-containing, mesoporous V-MCM-48 catalysts with different V/Si atomic
ratios were synthesized by a direct hydrothermal method. The structure and morphology of the samples
2
and the states of vanadium in the materials were systematically characterized by XRD, N physisorption,
51
FE-SEM, HRTEM, ICP, UV-vis, V MAS-NMR, ESR and XPS. The possible formation mechanism of
spherical V-MCM-48 was proposed based on the St o¨ ber process. Simultaneously, the catalytic activities
2 2
of the samples were evaluated in the oxidation of styrene to benzaldehyde using H O as oxidant. The
results show that the V-MCM-48 samples have regular spherical morphology, homogeneous dispersion
and highly ordered cubic mesostructure. Characterization results about the coordination states of
4+
vanadium species demonstrated that most of vanadium existed as tetracoordinated V species in a
5+
silicate framework and a small amount of vanadium as isolated V species on the surface. The prepared
spherical V-MCM-48 exhibits much higher catalytic activity in the catalytic conversion of styrene to
benzaldehyde. Spherical morphology contributes significantly to the improved catalytic performance of
Received 8th August 2014
Accepted 16th September 2014
5+
4+
materials. V species on the surface and/or produced from the oxidation of some V in the framework
act as active sites. Furthermore, the reaction parameters such as vanadium content in V-MCM-48, molar
DOI: 10.1039/c4ra08333d
www.rsc.org/advances
2 2
ratio of H O /styrene, temperature, time and solvents were optimized.
diffusion of reactants and products in heterogeneous catal-
1
. Introduction
10–15
ysis.
However, many current studies have focused on the
Since the researchers at the Mobil Research and Development synthesis of MCM-41 with different morphologies, while less
1
16
Corporation reported the existence of the M41S family,
signicant effort has been devoted to the synthesis and appli- MCM-48 with special morphology remains difficult because it
attention was paid on MCM-48. The synthesis of high-quality
2
3,4
cation of mesoporous molecular sieves in catalysis, sorption,
can only be produced under delicate experimental conditions.
drug delivery, biosensors and so on during the past few Minor changes in experimental conditions, including pH,
decades due to their ordered pore arrangement, narrow pore- temperature, aging time or concentration of the template, may
5
6
10,17
size distribution, high specic surface area and thermal lead to the absence of cubic mesoporous structure.
stability.
Due to the lack of catalytic sites for pure mesoporous silicate
MCM-41 and MCM-48 are two typical members in the well- materials, the incorporation of transition metals,
known M41S family. Compared with the one-dimensional including Ti, Mn, Fe, Cu and V, into the framework of M41S has
1D) pore system of MCM-41, the three-dimensional (3D) been widely examined. The doping of transition metals can
cubic pore network of MCM-48 is recognized as a more prom- modify the surface acidity/basicity and the redox property of the
7,18–24
(
7
–9
ising system with respect to catalytic applications because it is mesoporous molecular sieves, and as a result, improve their
not only more favorable to the mass transfer in heterogeneous catalytic activity. There are numerous reports concerning the
1
0
catalysis but also more resistant to pore blocking.
It is well known that the morphology and the pore structure the direct hydrothermal (DHT) method,
guest incorporation into mesoporous silicate matrix, including
25,26
the wet impregna-
27
28
of supports play very important roles in determining both the tion method, the graing method and the template ion-
7
exchange (TIE) method. In particular, the DHT method is
considered to be advantageous because of its simplicity of
process and higher efficiency in introducing heteroatoms into
silicate framework, whereas other methods either produce
a
State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University
of Technology, Nanjing 210009, China. E-mail: kongy36@njtech.edu.cn; Fax: +86-25-
8
3587860; Tel: +86-25-83587860
b
more crystalline metal oxide or aggregated metal species
outside of the framework, which result in easy leaching of active
Institute of Chemistry and Environmental Science, Nanjing Normal University,
Nanjing, 210097, China. E-mail: wuyongnjnu@gmail.com
50832 | RSC Adv., 2014, 4, 50832–50839
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
27,29,30
sites in the liquid-phase reactions.
However, the
For comparison, V-MCM-48-M (irregular morphology) and V-
heteroatom-doped mesoporous silica prepared via the DHT MCM-48-I (prepared by impregnation) were synthesized refer-
7,29
method oen exhibit poor morphologies.
The synthesis of ring to the reported literatures ref. 17 and 35, respectively.
mesoporous silicate with both heteroatom doping and regular
morphology has to be prepared under optimal conditions.
Some synthesis parameters should be controlled precisely for
the simultaneous hydrolyzation of heteroatom and silicon
species, which ensured the heteroatoms in the frame-
2
.3 Characterization
The XRD patterns of the samples were collected with a diffrac-
tometer (RigakuD/Max-RAX) equipped with a rotating anode
and Cu Ka radiation (l ¼ 0.154178 nm).
7,20,25,31
work.
Thus, the synthesis of mesoporous silicate with
N
adsorption–desorption isotherms were measured on a
2
heteroatoms in the framework and special morphology is full of
challenge, particularly in the case of MCM-48.
Micromeritics ASAP-2020 analyzer at 77 K. Before the
measurements, calcined samples were degassed in a vacuum at
Vanadium-modied mesoporous molecular sieves exhibit
high catalytic activity in many reactions, particularly in the
ꢀ
300 C for 5 h. Surface areas were calculated using the BET
equation and pore-size distributions were obtained by the Bar-
rett–Joyner–Halenda (BJH) method using desorption branch
data.
High-resolution transmission electron microscopy (HRTEM)
images were recorded on a JEM-2010 EX microscope operated at
an accelerating voltage of 200 kV. The samples were crushed in
A.R. grade ethanol, and the resulting suspension was allowed to
dry on carbon lm supported on copper grids.
Field emission scanning electron microscope (FE-SEM) was
performed on a Hitachi S4800 Field Emission Scanning Elec-
tron Microscopy.
The vanadium content in the samples was determined using
a Jarrell-Ash 1100 Inductively Coupling Plasma emission spec-
trometer. The samples were completely dissolved in hydro-
13,17,32,33
oxidation of aromatics,
including benzene, phenol and
styrene. In this study, V-MCM-48 catalysts with regular spherical
morphologies and different vanadium content were synthesized
by the DHT method and characterized by various techniques.
The catalytic performances of produced catalysts were system-
atically studied in the oxidation of styrene, which is of consid-
erable commercial and academic interest for the synthesis of
important products such as styrene epoxide, benzaldehyde and
7,34
phenylacetaldehyde.
The possible mechanism for the
formation of spherical V-MCM-48 as well as the catalytic process
was further discussed.
2. Experimental section

uoric acid before analysis.
2.1 Materials
Diffuse reectance UV-vis spectra were recorded by a Perkin-
Elmer Lambda 35 spectrophotometer in the range of 200–800
nm with BaSO as reference.
The X-ray photoelectron spectra (XPS) were conducted on a
PHI 5000 Versa Probe X-ray photoelectron spectrometer equip-
Surfactant. Cetyltrimethylammonium bromide (CTAB, 99%)
was purchased from J&K Chemical Ltd. Tetraethylorthosilicate
4
(
(
TEOS, AR), acetone (AR) and ammonium metavanadate
NH VO , AR) were purchased from Sinopharm Chemical
4
3
ped with Al K radiation (1486.6 eV). The C1s peak at 284.6 eV
a
Reagent Co., Ltd. Ammonia solution (25 wt%), ethanol (AR),
methanol (AR), acetic acid (AR), acetonitrile (AR), styrene (CP)
and hydrogen peroxide (H O , 30 wt%) were purchased from
was used as the reference for binding energies.
Magic angle spinning nuclear magnetic resonance (MAS-
NMR) spectra were collected at a Larmor frequency of 105.181
MHz and a magnetic eld strength of 9.4 T using a 4.0 mm MAS
probe with spinning at 14 kHz on a Bruker Avance III
spectrometer.
2
2
Nanjing Chemical Reagent Co., Ltd.
2
.2 Synthesis
Electron spin-resonance spectroscopy (ESR) spectra were
recorded on a Bruker EMX-10/12 ESR spectrometer with X-band
at 110 K.
A series of vanadium-containing MCM-48 samples with spher-
ical morphology were synthesized in the following manner.
1
.0125 g cetyltrimethylammonium bromide (CTAB) was dis-
solved in a mixed solvent composed of 69 mL deionized water,
.3 mL concentrated ammonia solution (25 wt%) and 32.4 mL
2.4 Catalyst test
6
ethanol. The pH value of the solution is about 11.0. 10 mL The catalytic testing was carried out in a two-necked ask
aqueous solution containing the desired amount of NH VO3 placed in a water bath. Typically, 0.1 g catalyst, 20 mL solvent
4
and 2.1 mL tetraethylorthosilicate (TEOS, 0.0093 mol) were then and a desired amount of styrene were mixed. When the
ꢀ
slowly added into the aforementioned template solution under temperature reached 30–70 C, a given amount of H
2
O
2
was
stirring. Aer being stirred for another 2 h, the resulting gel was added dropwise into the reaction mixture under stirring. Aer
transferred into Teon-lined stainless-steel autoclaves and kept reaction for 2–12 h, the catalyst was separated by centrifugation.
ꢀ
at 100 C for 24 h. The sample obtained was washed three times The product distribution was analyzed by a SP-6890 gas chro-
with deionized water and ethanol and then dried at room matograph (Lunan Ruihong Chemical Instrument Co., Ltd.,
temperature. The as-synthesized product was calcinated at 550 China) equipped with a 0.32 mm ꢁ 30 m SE-54 capillary column
ꢀ
C in a ow of air for 6 h to remove the template. The obtained and a ame ionization detector (FID). The content of each
samples are denoted as xV-MCM-48, where x is 100 times of the component was calculated by a revised area normalization
calculated molar ratio of V/Si used in the synthesis gel.
method.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 50832–50839 | 50833

View Article Online
RSC Advances
Paper
3. Results and discussion
3.1 Mesostructure
The small-angle XRD patterns of the calcined V-MCM-48
samples with different vanadium contents are shown in
ꢀ
Fig. 1. The sharp (211) reection peak at 2q z 2.4 demon-
strates the characteristic of a typical mesoporous structure.
Three other weak peaks conrmed the presence of a highly
ordered 3D cubic Ia3d mesophase, which are indexed to the
36,37
(220), (420) and (332) reections.
Therefore, the results
indicate that the 3D cubic Ia3d mesostructure was retained for
the catalysts modied with vanadium. Moreover, the reections
of the 8V-MCM-48 sample become broad and weak, indicating
that the over-doping (molar ratio used in the synthesis gel of V/
Si > 0.06) of the vanadium species resulted in the decrease of the
order degree of cubic mesostructure. In particular, no charac-
teristic peaks of vanadium oxides were found in high-angle XRD
patterns (gures not given), suggesting that the vanadium
species should be incorporated into the framework of MCM-41
Fig. 2 Representative TEM and SAED images of calcined 4V-MCM-48
samples.
25,38
or disperse highly on the wall.
The TEM image of 4V-MCM-48 is depicted in Fig. 2a. A well-
ordered mesostructure is clearly visualized, and the pore size is
found to be in the range of 2.3–2.5 nm, which is in good
agreement with the result calculated from the BJH model using
desorption data (Fig. 3b). A well-resolved cubic diffraction
pattern is observed in the FT pattern (Fig. 2b), which demon-
strates the regular three-dimensional pore structure of V-MCM-
10,17,37
48 sample.
These observations indicate that the moderate
incorporation of vanadium atoms barely inuence the cubic
mesostructure of MCM-48.
Fig. 3 shows the N
2
adsorption–desorption isotherms and
pore-size distributions of all of the samples. The typical IV type
39
isotherms (Fig. 3a) are the characteristic of mesoporous solids.
Due to the capillary condensations of nitrogen, the steep
increases of absorption volume in the P/P range of 0.25–0.35
0
2
Fig. 3 (a) N adsorption–desorption isotherms and (b) pore diameter
distributions of calcined V-MCM-48 samples.
represent the highly ordered mesostructure of the materials and
very narrow pore-size distributions, which are evident in
10,35,37
Fig. 3b.
Some structural parameters of the samples are
listed in Table 1. As illustrated in Table 1, no apparent changes
are observed for the most probable pore diameter of all the
samples, which suggests that the incorporation of vanadium
has little inuence on the pore-size distribution. The BET
surface area and total pore volume decrease with the increase of
vanadium content, which perhaps results from the decreased
order degree of the mesostructure caused by high amounts of
vanadium doping, respectively. The total pore volumes decrease
with the increase of vanadium content, which suggests the
formation of extra-framework vanadium species.
The recorded data as well as the aforementioned analyzed
results demonstrate that we offer a convenient strategy to
produce highly ordered cubic mesoporous materials incorpo-
rated with a moderate amount of vanadium. Although the
overload of vanadium does not inuence on the pore-size
distribution, it will inevitably cause the distortion of meso-
structure of materials.
3.2 Morphology
The representative SEM and TEM images of samples are shown
in Fig. 4. As shown in Fig. 4a, the regularly and homogeneously
Fig. 1 XRD patterns of the calcined V-MCM-48 samples.
50834 | RSC Adv., 2014, 4, 50832–50839
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
Table 1 The structural properties of V-MCM-48 samples
Total pore
volume (cm g
a
b
c
2
ꢂ1
d
e
p
f
t
3
ꢂ1
Sample
V/Si molar ratio /%
V/Si molar ratio /%
S
BET (m g
)
D
211 (nm)
a
0
(nm)
D
(nm)
W
(nm)
)
2V-MCM-48
4V-MCM-48
6V-MCM-48
8V-MCM-48
0.71
1.29
1.95
2.88
—g
0.41
0.59
1.04
1090
969
944
788
3.56
3.59
3.59
3.56
8.73
8.80
8.80
8.73
2.39
2.40
2.42
2.39
1.63
1.65
1.64
1.63
0.73
0.72
0.65
0.56
a
b
c
d
e
Calculated from ICP data. Obtained from XPS data. BET surface area. Unit cell parameter (a
0
) calculated using a
0
¼ d211O6. Pore sizes
f
g
obtained from BJH analysis of desorption data. Wall thickness¼(a
0
/3.0919)ꢂ[(pore size)/2]. Not available.
shown in Scheme 1, the micelles are produced from the
template solution (step I). Both the TEOS and vanadium sources
hydrolyze to form disordered clusters (step II), which grow into
large and packed particles (step III). To minimize the surface
10,13
free energy,
the large-sized particles will further aggregate
into a spherical form.
ꢂ1
Note that the pH value is ca. 11 and the ꢂlog(V/mol L
)
values decrease from 2.6 to 2.1 with the increasing amount of
vanadium. According to the occurrence of various vanadate and
polyvanadate species as a function of pH and total concentra-
tion of vanadium depicted in ref. 42 (page 984), the vanadium
2
ꢂ
4ꢂ 42
species mainly exist as two species, HVO
4
and V
should increase with the
increasing amount of vanadium considering the equation
2
O
7
.
In
4
ꢂ
addition, the proportion of V O
2 7
4
ꢂ 2ꢂ
4ꢂ
42
2ꢂ
2[HVO
]
! [V
2
O
7
]
+ H
2
O. Therefore, HVO
4
is reduced
to V(IV) by ethanol and hydrolyzes simultaneously with TEOS to
4
+
form V-MCM-48 clusters with tetrahedral V ions in the
Fig. 4 SEM images of (a) calcined 4V-MCM-48, (c) 8V-MCM-48 and
d) V-MCM-48-M samples and (b) TEM image of calcined 4V-MCM-
8.
4ꢂ
framework (step II). Simultaneously, V O
species with larger
2
7
(
4
ionic diameters either stay in the solution, which results in
materials of much lower vanadium content than expected, or
5
+
serve as the source of V species in MCM-48.
4
ꢂ
Therefore, in the cases of lower concentrations of V
2
O
7
in
dispersed spherical particles of 4V-MCM-48 sample are clearly
visible. The TEM image (Fig. 4b) conrms the spherical
morphology with a uniform particle size of about 0.5 mm. For
the 8V-MCM-48 sample (Fig. 4c), although there are a few
particles with other morphologies, the spherical particles are
dominant. In addition, Fig. 4d shows the irregular sample
the solution, the MCM-48 clusters may favor the growth into a
4
ꢂ
spherical form. By contrast, at higher concentrations of V
2
O
7
,
the MCM-48 clusters will become the irregular particles due to
electrostatic repulsion.
1
7
3.3 The states of vanadium species
synthesized as previously reported.
40,41
According to the St ¨o ber process,
small particles of silicate
UV-vis. As shown in Fig. 5, the UV-vis spectrum of pure MCM-
are initiated, which aggregate gradually to form a spherical 48 support exhibits little absorption in the range of 200–700
morphology. In this study, we suggested the formation of
spherical V-MCM-48 following the similar procedures. As
Scheme 1 The proposed formation mechanism of the spherical V- Fig. 5 UV-vis spectra of calcined V-MCM-48 and pure MCM-48
MCM-48 based on the St o¨ ber process.
samples.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 50832–50839 | 50835

View Article Online
RSC Advances
Paper
12
nm. All of the V-MCM-48 samples show two distinct absorp-
tion bands, in which an intense broad band between 220 nm
and 320 nm centered at 260 nm is assigned to low-energy
charge-transfer transitions between tetrahedral oxygen ligands
4
+
and central V ion present in the framework, and the intense
5
+
band centered at 370 nm is attributed to the isolated V species
with tetrahedral structure that incorporated on the silicate
surface of MCM-48. The UV-vis absorption band at ca. 450 nm of
5
+
the octahedral structure of the extra-framework V may be
covered by the tails of absorption lines decreasing up to ca. 550
2
0,43
nm.
The band of bulk V O crystallites (ca. 450 to 480 nm) is
2 5
51
17,18
Fig. 7
V MAS-NMR spectra of calcined V-MCM-48 samples.
not observed for all samples,
results.
which accords well with XRD
It is found that the intensity of the two absorbance bands
increases with the vanadium content, which is more evident for
the bands centered at 370 nm indicating the presence of more
5
+
that the V species exist in greater amounts in the sample with
the increasing amount of vanadium, which is consistent with
the results of UV-vis and XPS measurements. Meanwhile, there
are no characteristic peaks around ꢂ300 ppm observed in all
5
+
V
species dispersed on the surface.
XPS. The XPS spectra of obtained materials are given in
17,38
2 5
samples, suggesting the absence of bulk V O crystallites.
ESR. To further evaluate the states of vanadium, the ESR
spectra of samples were acquired. As shown in Fig. 8, all of the
Fig. 6. The broad asymmetrical V2p3/2 spectra can be de-
convoluted into two components at about 516.2 eV and 517.4
4
+
5+
32,44
eV, which are assigned to V and V , respectively.
The peak
4
+
samples exhibit an axially symmetrical signal of V , which
area at 516.2 eV is found to be larger than that of 517.4 eV,
1
4
+
results from the d electron interaction with nuclear spin I ¼ 7/2
n
indicating that the V species is dominant in V-MCM-48
samples. Moreover, the ratio of peak areas at 517.4 eV relative
to 516.2 eV increases with the amount of vanadium species,
5
1
45
of V (natural abundance 99.8%). The spin Hamiltonian
parameters A and g tensors were determined as g|| ¼ 1.94, A||
¼
5
+
197 G, g ¼ 1.98, and A ¼ 64 G, which is typied as tetrahedral
t
t
demonstrating more V species disperse on the surface in the
samples. These ndings further support the conclusion
obtained from UV-vis spectroscopy.
4
+
25
V
species. In addition, the ESR signal intensity of the calcined
V-MCM-48 samples increases linearly with the increasing vana-
dium content. No observation of spin–spin interactions among
ions indicates that V should be highly dispersed in the
silicate matrix.
To summarize the characterization results from UV-vis, XPS,
V MAS-NMR and ESR, we nd that there are two types of
vanadium species existing in spherical V-MCM-48 materials
Furthermore, by the inspection of Table 1, it can be seen that
the bulk vanadium contents in 4V-MCM-48, 6V-MCM-48 and
4
+
4+
V
25,45,46
8V-MCM-48 samples determined by ICP are much higher than
those of surface vanadium content obtained from XPS. This
observation indicates that most of vanadium species are
incorporated into the framework of MCM-48.
5
1
4
+
5
1
51
that we produced, i.e. the tetrahedral V ions in the framework,
V MAS-NMR. V MAS-NMR spectra of 4V-MCM-48, 6V-
5
+
and the V species dispersed on the surface, as illustrated in
Fig. 9.
MCM-48 and 8V-MCM-48 samples were also recorded. As
depicted in Fig. 7, the relatively weak signals at ꢂ585 ppm are
observed in 6V-MCM-48 and 8V-MCM-48 samples, which are
5
+
ascribed to the characteristic of V in a tetrahedral environ-
3
.4 Catalytic activity
17,25,38,45
ment.
Moreover, the signal intensity becomes weaker
Catalytic performance of vanadium-containing MCM-48
Active sites. Table 2 summarizes the catalytic activity of
produced materials in styrene oxidation to benzaldehyde. Pure
with the decrease in the amount of vanadium, and no distin-
guished signal is observed for 4V-MCM-48 sample, possibly due
to its weak signal-to-noise (S/N) ratio. This observation indicates
Fig. 6 XPS V 2p3/2 spectra of calcined V-MCM-48 samples.
Fig. 8 ESR spectra of calcined V-MCM-48 samples.
50836 | RSC Adv., 2014, 4, 50832–50839
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
Scheme 2 The proposed reaction mechanism for the oxidation of
Fig. 9 Proposed structure of V (a) in silicate framework and V (b) styrene over the V-MCM-48 catalyst.
4+
5+
on the surface.
in Table 2, it is found that the catalytic activities of V-MCM-48
g
Table 2 Catalytic performance of vanadium-containing catalysts
samples rst increase and then decrease with the increasing
content of vanadium. In addition, the respective structural
parameters of all samples, such as total pore volume and BET
specic surface area, are observed to be decreased with
increased vanadium content (Table 1). Therefore, the unex-
pected dependence of the catalytic activity of materials on the
vanadium content suggests that morphology plays an essential
role for the remarkable catalytic performance of the materials.
Selectivity (%)
a
b
c
d
e
f
Catalyst
V/Si (%) Conversion (%) TON BZ PhA SE BA
Pure MCM-48 0.00
—
—
—
—
—
—
2
4
6
8
V-MCM-48
V-MCM-48
V-MCM-48
V-MCM-48
0.71
1.29
1.95
2.88
22.0
39.2
44.1
40.1
25.6
10.2
18.4
11.1
224
257
231
166
166
98
95.2 1.7
95.5 1.8
94.5 1.8
94.3 1.7
91.0 1.0
91.7 1.3
90.3 1.2
91.3 1.6
1.1 1.0
1.2
1.1 1.2
—
1.1 1.0 Specically, for the 2V- and 4V-MCM-48 samples with similar
2.4 1.0 spherical morphology, their catalytic activities are highly related
1.9 1.1
2.1 1.0
1.8 1.2
V-MCM-48-M 1.35
4
8
V-MCM-48-I 1.22
V-MCM-48-I 2.98
to the amount of vanadium. By comparison, despite its
improved vanadium content, an 8V-MCM-48 sample of irregular
forms shows a dramatic decrease in catalytic activity. In fact,
comparing with the V-MCM-48-M sample with irregular
morphology, spherical 4V-MCM-48, which contains the same
vanadium content, exhibits markedly improved catalytic
activity. These observations can be explained by the fact that the
regularly spherical morphology can speed up the mass trans-
port involved in a catalytic reaction in heterogeneous catalysis
70
21
V
2
O
5
—
a
c
b
The molar ratio of V/Si determined by ICP. Turnover number (TON).
d
e
f
Benzaldehyde. Phenyl acetaldehyde. Styrene epoxide. Benzoic
g
ꢀ
acid. Reaction condition: temperature, 30 C; styrene, 2.2 mL; H
2 2
O ,
2
mL; catalyst, 0.1 mg; reaction time, 12 h; and acetonitrile, 20 mL.
MCM-48 exhibited no catalytic activity due to the absence of
thus contributing to an improved catalytic performance of
1
2,13
active sites.
Crystalline V
2
O
5
exhibits improved catalytic
11–14
materials.
Next, we explored the optimization of reaction
5
+
activity, which indicates that the V species act as active sites.
Accordingly, V-MCM-48-I prepared by the impregnation method
possesses a larger turnover number (TON) value, compared with
condition on the basis of 4V-MCM-48 catalyst.
Optimization of reaction condition
2 2
Molar ratio of H O /styrene. In the range of the molar ratio of
crystalline V O , which should attribute to the dispersion of the
2
5
H O /styrene from 0.5 to 2, the styrene conversion is observed to
2
2
5
+
5+
V
species. Notably, the degree of dispersion of the V species
be increased signicantly from 23% to 83%. At the higher molar
ratio of H /styrene (>2), the styrene conversion decreases.
4
7
is further improved for V-MCM-48 prepared by DHT method,
2 2
O
which results in remarkably higher catalytic activity and TON
value.
This observation accords well with the reaction mechanism as
totally two H O molecules are involved in the reaction. The
2
2
5
+
Reaction mechanism. Considering that the V acts as active
sites, we propose that the oxidation of styrene over vanadium-
doping the MCM-48 catalyst should involve the following reac-
selectivity to benzaldehyde is observed to be in the range of
9.6–97.7% at different molar ratios of H /styrene. Therefore,
/styrene equal to 2 for the
8
2 2
O
we choose the molar ratio of H
current study.
2 2
O
48
tion processes, similar to that over La-MCM-48. Three main
reaction steps should be involved, as shown in Scheme 2: (1)
Reaction temperature. We nd that the styrene conversion rst
5
+
4+
V ((SiO)
materials react with H
2) styrene molecules react with V-superoxo species to form
styrene epoxide; and (3) styrene epoxide is further oxidized by
3
V]O) and V ((SiO)
3
V–OH) species on the surface of
ꢀ
increases with temperature from 30 to 50 C and then decreases
O
2
to form V-superoxo (VOOH) species;
ꢀ
2
slightly at a higher temperature (70 C), which should involve
(
the rapid decomposition of hydrogen peroxide. Simultaneously,
the selectivity to benzaldehyde decreases at higher tempera-
tures, possibly due to the deep oxidation of products. The
27,48
H
2
O
2
to obtain benzaldehyde.
Effect of spherical morphology. We know that the perfor-
mance of a catalyst is oen related to the amount of active sites,
morphology and structural parameters of the support. As listed
ꢀ
highest yield of benzaldehyde is observed at 50 C.
Reaction time. As observed in Fig. 10, the styrene conversion
increases from 40.7% to 52.1% in a reaction time ranging from
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 50832–50839 | 50837

View Article Online
RSC Advances
Paper
From the aforementioned experimental results, the 4V-
MCM-48 catalyst exhibited the best activity with 83.0% conver-
sion of styrene and 90.5% selectivity to benzaldehyde under the
optimized reaction conditions of the 2 : 1 molar ratio of H O /
2
2
ꢀ
styrene with acetonitrile as solvent at 50 C for 12 h.
4. Conclusions
We have successfully synthesized a series of V-MCM-48 samples
with various contents of vanadium, which exhibit the spherical
morphology and highly ordered 3D cubic pore structures, by a
one-step hydrothermal method. The spherical particles are
formed following the St ¨o ber process, in which small particles
with 3D-cubic mesoporous structure start to form aer the
hydrolysis of silicon and vanadium sources on the pre-
aggregated micelles of the template, and then grow into
spherical morphology.
Fig. 10 Effects of reaction time on conversion of styrene and selec-
2 2
tivity to benzaldehyde. Reaction conditions: styrene, 2.2 mL; H O , 2
ꢀ
mL; catalyst, 0.1 mg; acetonitrile, 20 mL; and temperature, 50 C.
2–12 h. Prolonged reaction time makes the styrene conversion
almost unchanged with a slight decrease of selectivity to benz-
aldehyde. 12 h is the optimal reaction time.
The characterization results reveal the existence of two types
4
+
of vanadium species, including the tetrahedral V in the
Solvents. For most of the liquid-phase reactions, solvent is an
essential factor because it affects the reaction kinetics as well as
the product selectivity. Generally, the aprotic solvent is more
effective in the catalytic oxidation of aromatics than the protic
5
+
framework and surface-dispersed V in the produced mate-
5
+
rials. We identify that the V species on the surface and/or
4
+
produced from the oxidation of some V in the framework by
acting as active sites in the catalytic conversion of styrene
2 2
H O
48
solvent. The solvent of higher polarity will increase the
to benzaldehyde. Although the catalytic performance of mate-
rials is closely related to the number of active vanadium species,
the spherical morphology plays a dominant role due to the
speeding up of mass transport. Under the optimal reaction
condition, the conversion of styrene and the selectivity to
benzaldehyde are 83.0% and 90.5%, respectively. Taken alto-
gether, we offer a facile method to synthesize spherical V-MCM-
49
substrate concentration, which favors the catalytic reaction. As
listed in Table 3, the lower styrene conversion is observed in
methanol and acetic acid (protic) than in acetone and acetoni-
trile (aprotic). Because acetonitrile possesses a higher polarity
than that of acetone, the styrene conversion in acetonitrile is
found to be higher. The acidity of acetic acid is favorable to the
open-ring (C–O–C) reaction in styrene epoxide as mentioned in
the second step of the reaction mechanism, which results in a
higher selectivity to benzaldehyde. On the contrary, methanol is
48 with excellent catalytic activity.
involved in the third step of reaction mechanism, which makes Acknowledgements
48
the selectivity to benzaldehyde much lower.
The authors acknowledge the nancial support of the National
Natural Science Foundations of China (21276125, 20876077 and
Stability. From the test of catalyst stability, we found that the
conversion of styrene decreased from 39.2% (fresh catalyst),
8.5% (rst recycle) to 12.3% (second recycle), owing to the
leaching of a small amount of vanadium ions, and the selec-
tivity to benzaldehyde is in the range of 90.2–95.5%. The result
indicated that 4V-MCM-48 is more resistant to the leaching of
vanadium ions because of the vanadium ions incorporated in
the silicate framework, which is necessary for the stability of the
vanadium-containing silicate catalyst.
2
1476108), the Major Projects of the Natural Science Founda-
tion for the Higher Education Institutions of Jiangsu Province
10KJA530015), the Program for Scientic Innovation Research
2
(
of College Graduates in Jiangsu Province (CXZZ13_0443), the
Key Program of National Natural Science Foundation of China
(no. 21136005) and the Project of Priority Academic Program
Development of Jiangsu Higher Education Institutions (PAPD).
Notes and references
a
Table 3 Effects of solvents on the catalytic performance
1
2
3
C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and
J. S. Beck, Nature, 1992, 359, 710–712.
Selectivity (%)
M. Egi, K. Sugiyama, M. Saneto, R. Hanada, K. Kato and
S. Akai, Angew. Chem., Int. Ed., 2013, 52, 3654–3658.
S. Dib, M. Boufatit, S. Chelouaou, F. Sadi-Hassaine,
J. Croissant, J. Long, L. Raehm, C. Charnay and
J. O. Durand, RSC Adv., 2014, 4, 24838–24841.
Y. Zhou, W. G. Lin, M. M. Wan, J. Yang and J. H. Zhu, J.
Mater. Chem., 2012, 22, 23633–23641.
F. Muharnmad, M. Guo, W. Qi, F. Sun, A. Wang, Y. Guo and
G. Zhu, J. Am. Chem. Soc., 2011, 133, 8778–8781.
Solvent
Conversion (%)
BZ
PhA
SE
BA
Methanol
Acetic acid
Acetone
22.1
39.1
42.9
52.1
37.3
93.8
84.5
96.5
0.8
0.9
0.94
0.8
7.0
1.0
2.3
1.1
23.6
1.9
0.9
0.6
4
5
Acetonitrile
a
ꢀ
Reaction condition: styrene, 2.2 mL; H
2
O
2
, 2 mL; temperature, 50 C;
catalyst, 0.1 mg; reaction time, 12 h; and solvent, 20 mL.
50838 | RSC Adv., 2014, 4, 50832–50839
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
6
7
8
9
S. H. Cheng, C. H. Lee, C. S. Yang, F. G. Tseng, C. Y. Mou and 28 S. Samanta, S. Das, P. K. Samanta, S. Dutta and P. Biswas,
L. W. Lo, J. Mater. Chem., 2009, 19, 1252. RSC Adv., 2013, 3, 19455–19466.
S. Gomez, L. J. Garces, J. Villegas, R. Ghosh, O. Giraldo and 29 G. Zhang, J. Long, X. Wang, Z. Zhang, W. Dai, P. Liu, Z. Li,
S. L. Suib, J. Catal., 2005, 233, 478. L. Wu and X. Fu, Langmuir, 2010, 26, 1362–1371.
T. W. Kim, F. Kleitz, B. Paul and R. Ryoo, J. Am. Chem. Soc., 30 L. L. Lou, S. Jiang, K. Yu, Z. Gu, R. Ji, Y. Dong and S. Liu,
005, 127, 7601–7610. Microporous Mesoporous Mater., 2011, 142, 214–220.
T. Suteewong, H. Sai, R. Cohen, S. T. Wang, M. Bradbury, 31 C. He, J. Li, P. Li, J. Cheng, Z. Hao and Z. P. Xu, Appl. Catal.,
2
B. Baird, S. M. Gruner and U. Wiesner, J. Am. Chem. Soc.,
011, 133, 172–175.
0 T. W. Kim, P. W. Chung and V. S. Y. Lin, Chem. Mater., 2010,
2, 5093–5104.
1 T. M. Suzuki, T. Nakamura, K. Fukumoto, M. Yamamoto,
B, 2010, 96, 466–475.
2
32 B. Guo, L. F. Zhu, X. K. Hu, Q. Zhang, D. M. Tong, G. Y. Li
and C. W. Hu, Catal. Sci. Technol., 2011, 1, 1060–1067.
33 L. Y. Hu, B. Yue, X. Y. Chen and H. Y. He, Catal. Commun.,
2014, 43, 179–183.
1
1
2
Y. Akimoto, Y. Akimoto and K. Yano, J. Mol. Catal. A: 34 Y. Wu, Y. J. Zhang, J. Cheng, Z. Li, H. Q. Wang, Q. L. Sun,
Chem., 2008, 280, 224–232.
2 A. I. Carrillo, E. Serrano, R. Luque and J. Garcia-Martinez,
B. Han and Y. Kong, Microporous Mesoporous Mater., 2012,
162, 51–59.
1
1
1
1
1
1
1
1
2
2
2
2
2
2
Appl. Catal., A, 2013, 453, 383–390.
3 Y. Chen, X. Shi, B. Han, H. Qin, Z. Li, Y. Lu, J. Wang and
Y. Kong, J. Nanosci. Nanotechnol., 2012, 12, 7239–7249.
4 G. C. Bond, Heterogeneous Catalysis: Principles and 37 W. Zhao, Q. Li, L. Wang, J. Chu, J. Qu, S. Li and T. Qi,
Applications, 2nd edn, 1987. Langmuir, 2010, 26, 6982–6988.
5 Z. Yang, Y. Lu and Z. Yang, Chem. Commun., 2009, 2270– 38 M. Piumetti, B. Bonelli, M. Armandi, L. Gaberova, S. Casale,
35 A. Benhamou, M. Baudu, Z. Derriche and J. P. Basly, J.
Hazard. Mater., 2009, 171, 1001–1008.
36 A. Sayari, J. Am. Chem. Soc., 2000, 122, 6504–6505.
2277.
P. Massiani and E. Garrone, Microporous Mesoporous Mater.,
2010, 133, 36–44.
39 W. Zhao, Q. Z. Li, L. N. Wang, J. L. Chu, J. K. Qu, S. H. Li and
T. Qi, Langmuir, 2010, 26, 6982–6988.
40 K. T. Lee, A. N. Sathyagal and A. V. McCormick, Colloids Surf.,
A, 1998, 144, 115–125.
41 K. Schumacher, P. I. Ravikovitch, A. Du Chesne,
A. V. Neimark and K. K. Unger, Langmuir, 2000, 16, 4648–
4654.
6 C. C. Liu, X. M. Wang, S. Lee, L. D. Pfefferle and G. L. Haller,
Microporous Mesoporous Mater., 2012, 147, 242–251.
7 P. Selvam and S. E. Dapurkar, Appl. Catal., A, 2004, 276, 257–
265.
8 S. Q. Hu, D. P. Liu, L. S. Li, A. Borgna and Y. H. Yang, Catal.
Lett., 2009, 129, 478–485.
9 J. Strunk, W. C. Vining and A. T. Bell, J. Phys. Chem. C, 2010,
114, 16937–16945.
0 Y. Kong, R. Zhang, X. J. Xu, Z. L. Wang and J. Wang, Chin. J. 42 N. N. Greenwood and A. Earnshaw, Chemistry of the Elements,
Inorg. Chem., 2008, 24, 1124–1127. 2nd edn, 1997.
1 C. Wu, Y. Kong, F. Gao, Y. Wu, Y. N. Lu, J. Wang and L. Dong, 43 C. Subrahmanyam, B. Louis, B. Viswanathan, A. Renken and
Microporous Mesoporous Mater., 2008, 113, 163–170. T. K. Varadarajan, Appl. Catal., A, 2005, 282, 67–71.
2 Y. Kong, X. J. Xu, Y. Wu, R. Zhang and J. Wang, Chin. J. 44 C. T. Wang, M. T. Chen and D. L. Lai, Appl. Surf. Sci., 2011,
Catal., 2008, 29, 385–390. 257, 5109–5114.
3 F. T. Kuo, S. Y. Chen, T. H. Lin, J. F. Lee and S. Cheng, RSC 45 M. Chatterjee, T. Iwasaki, H. Hayashi, Y. Onodera, T. Ebina
Adv., 2013, 3, 12604–12610. and T. Nagase, Chem. Mater., 1999, 11, 1368–1375.
4 J. Sun, Q. Kan, Z. Li, G. Yu, H. Liu, X. Yang, Q. Huo and 46 A. Held, J. Kowalska-Kus and K. Nowinska, Catal. Commun.,
J. Guan, RSC Adv., 2014, 4, 2310–2317. 2012, 17, 108–113.
5 F. Gao, Y. H. Zhang, H. Q. Wan, Y. Kong, X. C. Wu, L. Dong, 47 M. Piumetti, B. Bonelli, P. Massiani, S. Dzwigaj, I. Rossetti,
B. Q. Li and Y. Chen, Microporous Mesoporous Mater., 2008,
10, 508–516.
S. Casale, M. Armandi, C. Thomas and E. Garrone, Catal.
Today, 2012, 179, 140–148.
1
26 M. A. Betiha, H. M. A. Hassan, A. M. Al-Sabagh, 48 W. C. Zhan, Y. L. Guo, Y. Q. Wang, X. H. Liu, Y. Guo,
A. E. R. S. Khder and E. A. Ahmed, J. Mater. Chem., 2012,
2, 17551–17559.
Y. S. Wang, Z. G. Zhang and G. Z. Lu, J. Phys. Chem. B,
2007, 111, 12103–12110.
2
27 M. Piumetti, M. Armandi, E. Garrone and B. Bonelli, 49 D. Chen, N. Li, P. Sun and Y. Kong, Chin. J. Catal., 2009, 30,
Microporous Mesoporous Mater., 2012, 164, 111–119. 643–648.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 50832–50839 | 50839