Synthesis, characterization and catalytic activity of nano-spherical MCM-41 modified with vanadium

Article abstract of DOI:10.1166/jnn.2014.8965

Vanadium modified MCM-41 nanospheres were synthesized by the impregnation and sol-gel supermolecular method respectively. Structure and morphology of the obtained materials were characterized using X-ray diffraction (XRD), inductively coupling plasma (ICP

Full text of DOI:10.1166/jnn.2014.8965

Article
Journal of
Nanoscience and Nanotechnology
Vol. 14, 7300–7306, 2014
Copyright © 2014 American Scientific Publishers
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Synthesis, Characterization and Catalytic Activity of
Nano-Spherical MCM-41 Modified with Vanadium
∗
Nan Zhao, Haixia Gao, Haiqing Wang, Huizi He, Yu Chen, and Yan Kong
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering,
Nanjing University of Technology, Nanjing 210009, China
Vanadium modified MCM-41 nanospheres were synthesized by the impregnation and sol–gel super-
molecular method respectively. Structure and morphology of the obtained materials were charac-
terized using X-ray diffraction (XRD), inductively coupling plasma (ICP), Fourier transform infrared
(
FT-IR), scanning electron microscopy (SEM) and N -adsorption–desorption techniques. The influ-
2
ence of particle sizes on their catalytic activities in styrene oxidation and benzene hydroxylation
was investigated. Results revealed that all the samples had regular sphere shape and adjustable
particle size within 40–160 nm, and exhibited high catalytic activities in the oxidation of styrene and
hydroxylation of benzene. The smaller the particle size is, the higher the activity becomes. Mean-
while, the catalysts synthesized by the sol–gel method are more effective than that synthesized via
impregnation method, indicating the vanadium species in the mesoporous framework are probably
the catalytic active sites.
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Keywords: Vanadium, MCM-41, Nanosphere, Catalytic Activity.
Copyright: American Scientific Publishers
1
. INTRODUCTION
The products of styrene oxidation are widely used in
many industrial processes. However, the traditional pro-
duction methods have some drawbacks, e.g., serious pol-
1
Since first reported by Mobil company in 1992, meso-
porous molecular sieves have become one of the focuses
of chemistry and material science because of their high
specific surface area, large pore volume, well-ordered pore
lution and low productivity. Using H O as oxidant is
2
2
the preferred process because of its mild reaction con-
1
0–12
ditions and environmentally benign nature.
with Mo, V, Ti
Catalysts
structure with uniform pore size distribution from 2 to
1
3
14
15ꢀ16
2
as the active elements have been
5
0 nm, etc. However, pure mesoporous silica materials
studied in the reaction and exhibited satisfactory perfor-
mance. So far, how to realize the one-step process for
direct hydroxylation of benzene to phenol is still a big
show poor catalytic activity due to their neutral skele-
ton structure and limited surface lattice defects and acidic
sites.
1
7–19
challenge.
A number of catalysts have been tested
Catalytic performance of a catalyst depends not only on
the nature of its active sites, but also on its physical proper-
ties such as diffusion factor, particle morphology and size,
etc. One of the strategies to improve the catalytic activi-
ties of mesoporous silica materials is to introduce active
heteroatoms to their surface or framework via impregna-
2
0ꢀ21
for the hydroxylation of benzene by H O ,
includ-
metallic
2
2
2
2
23ꢀ24
ing titanium silicalite, supported catalysts,
2
5ꢀ26
27–30
oxides,
and vanadium containing catalysts.
In the present work, vanadium incorporated MCM-41
nanospheres with different diameters were synthesized by
the impregnation (V/MCM-41) and sol–gel (V-MCM-41)
methods respectively. Their catalyst activities in styrene
oxidation and hydroxylation of benzene by H O were
3
–9
tion or direct hydrothermal method. In the later case,
in order to incorporate heteroatoms into the framework, the
synthetic conditions must be finely controlled, and these
conditions also greatly influence the particle morphology
and size. In general, it is a big challenge to synthesize
mesoporous molecular sieves with both regular morphol-
ogy and incorporated heteroatoms in the framework.
2
2
investigated.
2. MATERIALS AND METHODS
2.1. Synthesis of V/MCM-41 and V-MCM-41
Pure MCM-41 nanospheres with various sizes of 40, 60,
80, and 120 nm were first prepared following our previous
∗
Author to whom correspondence should be addressed.
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doi:10.1166/jnn.2014.8965

Zhao et al.
Synthesis, Characterization and Catalytic Activity of Nano-Spherical MCM-41 Modified with Vanadium
method by controlling the NaOH/TEOS ratio at 0.2, 0.3,
3
1
0
.4 and 0.5 respectively. The synthesis of V/MCM-41
nanospheres was carried out as follows: 0.5 g spherical
MCM-41 with various particle sizes was stirred in an
aqueous solution containing certain amount of ammonium
metavanadate at room temperature for 45 minutes, then
ꢀ
dried at 100 C. The amount of NH VO added to the
4
3
solution was well controlled to ensure that the unit surface
area has sufficient active sites. With the spherical diameter
of the support at 40, 60, 80, and 120 nm, the corresponding
samples were marked as a1, a2, a3 and a4 respectively.
The synthesis of V-MCM-41 nanospheres with differ-
ent diameters was carried out as follows: CTAB (0.4 g)
was dissolved in the solution containing H O (192 ml) and
2
certain amount of NaOH with a stirring rate of 300 rpm
ꢀ
at 80 C. When CTAB was dissolved completely, certain
amount of ammonium metavanadate was added. Half an
hour later, TEOS (2.68 ml) was added into the solution and
the reaction mixture was stirred for 2 h. Both the tempera-
ture and the stirring rate were remained constant during the
reaction. The resulting products were collected by washing
with water in a centrifuge machine, and then dried at room
Figure 2. XRD patterns of the samples c1, c2, c3, c4, c5.
ꢀ
temperature. Finally, the samples were calcined at 550 C
For SEM analysis, the samples were shadowed with
gold, and then taken with a Hitachi S4800 Field Emission
Scanning Electron Microscopy.
Fourier Transform Infrared (FT-IR) spectra of the sam-
ples were obtained in the range of 400–4000 cm
for 6 h. When the molar ratios of NaOH/TEOS were set up
to be 0.2, 0.3, 0.4 and 0.5, the corresponding products were
labeled as b1, b2, b3 and b4. V-MCM-41 nanospheres with
V/Si molar ratio of 0.01, 0.02, 0.04, 0.06, and 0.08 were
−1
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synthesized while the radio of NaOH/TEOS was 0.4. The
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samples were denoted as c1, c2, c3, c C4 ,o cp 5 y, r ri ge sh pt :e cA t imv ee l ry i .c an Scientific Publishers
resolution.
The N (77.4 K) adsorption–desorption measurements
were performed with a TriStar 3000 automated gas adsorp-
tion analyzer (Micrometrics) in a relative pressure range
2
2
.2. Analysis of the Catalysts
Powder X-ray diffraction (XRD) patterns were recorded on
a Bruker AXS D8 Advance powder diffractometer using
Ni filtered Cu K radiation (ꢂ = 0ꢃ154178 nm).
P/P from 0.01 to 0.99, and before the measurements, cal-
0
ꢀ
cined samples were outgassed in vacuum at 300 C for
ꢁ
Figure 1. XRD patterns of samples of V/MCM-41 (A) and V-MCM-41 (B).
J. Nanosci. Nanotechnol. 14, 7300–7306, 2014
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Synthesis, Characterization and Catalytic Activity of Nano-Spherical MCM-41 Modified with Vanadium
Zhao et al.
(
10 ml), styrene (1.1 ml) and H O (2 ml) were mixed.
2
2
ꢀ
After reaction for 12 h at 50 C, the catalyst was separated
by centrifugation.
The catalytic activity assessment of the V-MCM-41
and V/MCM-41 on direct hydroxylation of benzene was
performed as follows: 0.1 g catalyst, 2 ml benzene and
6
.89 ml H O were added to 15 ml acetonitrile in a 50 ml
2 2
flask. The reaction mixture was stirred for 10 h with tem-
ꢀ
perature maintained at 40 C, and then the resulting solu-
tion was centrifuged.
The products of the two reactions were analyzed by
a SP-6890 gas chromatograph (Lunan Ruihong Chemical
Instrument Co. Ltd., China) with 0.32 mm×30 m SE-54
capillary column. The conversion and the selectivity of
products were calculated by external standard method.
Figure 3. N ads–des isotherms and pore size distribution (inner) of the
sample a2.
2
3. RESULTS AND DISCUSSION
3.1. Characteristics of V/MCM-41 and V-MCM-41
5
h. The specific surface area and the pore size distribution
The small-angle XRD patterns of V/MCM-41 and
V-MCM-41 samples are shown in Figure 1. It is obvi-
ous that all the samples exhibit (100) reflection peak at
were calculated from the BET and BJH data, respectively.
The vanadium contents were analyzed using Jarrell-Ash
1
ꢀ
100 Inductively Coupling Plasma spectrometer (ICP).
2ꢄ ≈ 2ꢃ4 , and the three other small peaks are indexed
to the (110), (200) and (210) reflections, respectively.
These XRD results confirm that the samples possess highly
ordered two-dimensional hexagonal structure. The intensi-
ties of the diffraction peaks become stronger with increase
2
.3. Catalyst Test
The catalytic activity of V-MCM-41 and V/MCM-41
towards oxidation of styrene was carried out in a two-
necked flask. Typically, catalyst (50 mg), acetonitrile
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of the amount of NaOH, indicating that the structures
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Figure 4. The FE-SEM images of the V/MCM-41 samples a1–a4.
7
302
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Zhao et al.
Synthesis, Characterization and Catalytic Activity of Nano-Spherical MCM-41 Modified with Vanadium
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Figure 5. The SEM images of the V-MCM-41 samples b1–b4.
of the mesoporous materials become more regular along
with the increase of the amount of NaOH. Moreover,
there is scarcely change of the peak positions, indicat-
ing that the amount of NaOH has few effect on the cell
parameters.
Figure 4 shows the FE-SEM images of V/MCM-41 with
different NaOH/TEOS molar ratios, and Figure 5 shows
the SEM images of V-MCM-41 synthesized by sol–gel
supermolecular method with different NaOH/TEOS molar
ratios. All the samples present regular spherical morpholo-
gies and the diameters range from 40 nm to 160 nm
with the increase of NaOH amount. The introduction of
vanadium has little effect on the morphology of MCM-
41. V-MCM-41 nanospheres with fixed diameters can be
synthesized by changing the NaOH/TEOS molar ratios.
The mole ratio of V/Si has a great influence on the mor-
phology and particle size of the V-MCM-41 nanospheres.
As the molar ratio of V/Si is 0.01, the sample presents
regular spherical morphology and a diameter of 120 nm
shown in Figure 5(b3). With the increase of vanadium
content, the nanospheres become irregular which can be
seen from the FE-SEM images shown in Figure 6. Mean-
while, the average diameters increase and change to about
400 nm for the sample with 0.08 molar ratio of V/Si. It is
believed that the variation in the ion strength in the synthe-
sis system due to the change of ammonium metavanadate
amount causes the change in morphology.
Figure 2 shows the small-angle XRD patterns of
V-MCM-41 samples with different vanadium contents. It
can be seen that all the samples possess typical two-
dimensional hexagonal mesoporous structures. The diffrac-
tion peaks shift to small angle with increase of vanadium
content, suggesting the incorporation of vanadium species
7
into the silica framework.
Figure 3 shows the N adsorption–desorption isotherms
2
and pore diameter distribution of sample a2, and simi-
lar results are observed for the other samples. This sam-
ple exhibits a Brunauer type IV isotherm according to
3
2
the IUPAC classification, with hysteresis at the relative
pressure range of 0ꢃ2 ≤ p/p ≤ 0ꢃ4, which is due to the
0
capillary condensation of nitrogen in the pores. The cor-
responding pore size distribution obtained by the BJH
method from the desorption data indicates a uniform and
sharp pattern.
J. Nanosci. Nanotechnol. 14, 7300–7306, 2014
7303

Synthesis, Characterization and Catalytic Activity of Nano-Spherical MCM-41 Modified with Vanadium
Zhao et al.
Figure 6. The FE-SEM images of the V-MCM-41 samples c2–c5.
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The FT-IR spectra of the samples a2 and b2 are shown
in Figure 7. The broad bands at ca. 1090 cm and ca.
phenomenon is also observed in the hydroxylation of ben-
zene as shown in Figure 8(B).
−
1
−
1
8
10 cm are assigned to the asymmetric and symmet-
Considering the fact that the total vanadium content and
number of active sites on unit surface area for each reac-
tion are controlled identical, the differences of catalytic
performance must be caused by the diameter of support.
Small particle size is more beneficial to the diffusion of
the reagent molecules. As a result, the smaller the particle
3
3
ric stretching vibrations of Si O respectively, and the
band at 460 cm to the bending vibration of surface
−1
3
4
−1
Si O groups. A weak peak at 960 cm attributed to
5
Si
O
V band position3 is observable for the two sam-
ples, proving the incorporation of vanadium atoms in the
framework of V-MCM-41. In addition, the spectra of all
−
1
the samples don’t exhibit the bands at 2921 cm and
−
1
2
851 cm , which are attributed to the vibrations of C
H
bands, indicating the completely removal of the template
agent from the as-synthesized samples.
3
.2. Catalytic Activities of the V-MCM-41 and
V/MCM-41 Samples
3
.2.1. Catalytic Activities of
V/MCM-41 Nanospheres
The catalytic activities of spherical V/MCM-41 with dif-
ferent diameters towards styrene oxidation are presented in
Figure 8(A). All the samples exhibit high activity, which
increases with the decrease of MCM-41 diameter. For
example, the styrene conversion and benzaldehyde selec-
tivity are 72.8% and 91.6% respectively when the spherical
MCM-41 with 160 nm diameter is used as the support,
and a conversion of 85.5% and selectivity of 93.5% are
obtained with the 40 nm diameter of support. The same
Figure 7. FT-IR spectra of the samples a2 and b2.
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Zhao et al.
Synthesis, Characterization and Catalytic Activity of Nano-Spherical MCM-41 Modified with Vanadium
Table II. The catalytic properties of samples in the hydroxylation of
benzene.
Sample V/Si (%) Benzene conversion (%) Phenol selectivity (%)
c1 (b3)
c2
c3
c4
c5
1ꢃ07
1ꢃ6
3ꢃ2
5ꢃ0
6ꢃ2
2ꢃ8
4ꢃ4
7ꢃ7
9ꢃ8
10ꢃ1
100
100
100
100
100
Notes: Reaction conditions: catalyst amount 0.1 g, benzene 2 ml, CH CN 15 ml,
3
ꢀ
3
0% H O 6.89 ml, temperature 40 C and duration 10 h.
2
2
diameter of about 40 nm, it gives a styrene conversion of
4.1% and a selectivity of 96.5% for benzaldehyde. Mean-
while, all the samples display higher activity than V O
9
2
5
and V-MCM-41 with irregular morphology. It is also found
that the catalytic activities of V-MCM-41 are better than
that of V/MCM-41 with the same vanadium content and
similar size. These results demonstrate that the vanadium
species in the framework are the main active sites. Sim-
ilar conclusion can be obtained in the hydroxylation of
benzene.
3
.2.3. Catalytic Activities of V-MCM-41 Nanospheres
with Different Vanadium Contents Towards
Hydroxylation of Benzene
In order to improve the conversion of benzene in the reac-
tion at low temperatures, V-MCM-41 nanospheres with
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the catalytic activities of the V-MCM-41 samples with
different vanadium contents. The conversion of benzene
increases with vanadium content while the phenol selec-
tivity remains at 100%, although the morphology of the
catalysts tend to be irregular and the diameter to be larger.
However, as the molar ratio of V/Si increases from 5.0 to
Figure 8. The catalytic properties of V/MCM-41 in the styrene oxida-
tion reaction (A) and hydroxylation of benzene (B).
size is, the faster the reagent and product spreads, and the
better the catalytic performance becomes.
6.2, the conversion of benzene doesn’t change any more,
possibly due to overloaded vanadium species on the sur-
face, which blocked the channels.
3
.2.2. Catalytic Activities of V-MCM-41 Nanospheres
with Different Diameters
The catalytic activities of V-MCM-41 nanospheres with
different diameters towards the oxidation of styrene are
presented in Table I. These catalysts are efficient in the
reaction and the activities increase with the decrease of
the diameter. Particularly for the sample with the spherical
4
. CONCLUSION
V/MCM-41 and V-MCM-41 nanospheres have been suc-
cessfully synthesized by the impregnation and sol–gel
supermolecular method respectively. These nanospheres
have regular shape and tunable sizes. All the samples dis-
play superior catalytic activities in the oxidation of styrene
and hydroxylation of benzene to V O and MCM-41, and
Table I. The catalytic properties of samples in styrene oxidation.
2
5
the catalyst particle has great influence on the catalytic
activity. Better catalytic performance is observed on the
samples with smaller particle sizes. Increase of vanadium
content also results in higher conversion of benzene, but
an overloading of vanadium species has negative influence
on the morphology and catalytic activity of the samples.
Average
V/Si
Styrene
Benzaldehyde
Sample
diameters (nm) (%) conversion (%) selectivity (%)
b1
b2
b3
b4
40
80
120
160
/
1.03
0.98
1.07
1.08
1.06
/
94ꢃ1
90ꢃ5
87ꢃ6
85ꢃ3
75
96ꢃ5
95ꢃ8
94ꢃ3
93ꢃ1
90
V-MCM-41-ir
2
V O5
/
20
88ꢃ5
Acknowledgments: This work was financially sup-
ported by the National Natural Science Foundation of
China (21276125 and 20876077) and the Key Program
Notes: Reaction conditions: catalyst amount 0.1 g, styrene 1.1 mL, CH CN 10 mL,
0% H O2 2 mL, temperature 50 C and duration 6 h.
3
ꢀ
3
2
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Zhao et al.
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