MoO3@SiO2 nanoreactors: Synthesis with a thermal decomposition strategy and catalysis on alkenes epoxidation

Article abstract of DOI:10.1016/j.jssc.2018.05.005

A general thermal decomposition strategy is reported to fabricate MoO₃@SiO₂ nanoreactors, with a mesoporous silica shell and embedded MoO₃ nanoparticles. The novel preparation procedure involves mixing certain mass ratio o

Full text of DOI:10.1016/j.jssc.2018.05.005

Author’s Accepted Manuscript
MoO @SiO nanoreactors: synthesis with a
3
2
thermal decomposition strategy and catalysis on
alkenes epoxidation
Yirui Shen, Pingping Jiang, Yingchun Wang, Gang
Bian, Phyu Thin Wai, Yuming Dong
www.elsevier.com/locate/yjssc
PII:
S0022-4596(18)30183-X
https://doi.org/10.1016/j.jssc.2018.05.005
YJSSC20211
DOI:
Reference:
To appear in: Journal of Solid State Chemistry
Received date: 18 February 2018
Revised date: 17 April 2018
Accepted date: 5 May 2018
Cite this article as: Yirui Shen, Pingping Jiang, Yingchun Wang, Gang Bian,
Phyu Thin Wai and Yuming Dong, MoO @SiO nanoreactors: synthesis with a
3
2
thermal decomposition strategy and catalysis on alkenes epoxidation, Journal of
Solid State Chemistry, https://doi.org/10.1016/j.jssc.2018.05.005
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MoO @SiO nanoreactors: synthesis with a thermal
3
2
decomposition strategy and catalysis on alkenes
epoxidation
Yirui Shen, Pingping Jiang*, Yingchun Wang, Gang Bian, Phyu Thin Wai, Yuming Dong
Key Laboratory of Synthetic and Biological Colloids (Jiangnan University), Ministry of
Education, School of Chemical and Material Engineering, Wuxi 214122, PR China.
*
Corresponding author E-mail addresses: ppjiang@jiangnan.edu.cn.
ABSTRACT:
A general thermal decomposition strategy is reported to fabricate MoO @SiO nanoreactors,
3
2
with a mesoporous silica shell and embedded MoO nanoparticles. The novel preparation
3
procedure involves mixing certain mass ratio of (NH ) Mo O ·4H O (AMM) and hollow
4
6
7
24
2
mesoporous silica spheres (HMSS) by grinding, fusion and thermal decomposition of
NH ) Mo O ·4H O under calcination and removing the residual via filtration. The as-prepared
(
4
6
7
24
2
MoO @SiO nanoreactors were characterized by X-ray diffraction (XRD), scanning electron
3
2
microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis
(
TGA), N adsorption/desorption and X-ray photoelectron spectra (XPS). The nanoreactors were
2
1

utilized in epoxidation of alkenes and displayed high catalytic activity and stability. The mass
ratio of AMM and HMSS greatly affected the properties and catalytic performance of the
nanoreactors. The optimal mass ratio of AMM: HMSS has been confirmed as 1/2. After reacting
for 12 h with H O (50 wt%) as oxidant, conversion and selectivity of optimal MoO @SiO -400-
2
2
3
2
1
/2 almost reached up to 98% and 99%, respectively. Furthermore, the catalyst still had high
conversion (78 %) and selectivity (95 %) at 4 h epoxidation of cyclooctene after recycling for 6
runs. Kinetics study was also carried out and demonstrated the epoxidation of alkenes follows
the first order model.
Graphical abstract
A thermal decomposition strategy is employed to fabricate MoO @SiO nanoreactors with a
3
2
mesoporous silica shell and embedded MoO nanoparticles, showing promising reactivity and
3
stability in epoxidation of alkenes.
Keywords: Thermal decomposition; Molybdenum trioxide; Nanoreactor; Alkenes; Epoxidation
2

1
. Introduction
For a variety of applications in the fields of catalysis [1], delivery [2], lithium-ion batteries
[
3] and biosensors [4] et al., yolk-shell nanomaterials with movable cores and porous shells are
promising platforms, endowed by their tailorability and functionality in both the interior cores
and exterior hollow shells. Yolk-shell nanomaterials with core@void@shell nanostructure are
potential candidates as nanoreactors for various applications, owing to their easy functionalized
cores and hollow shell, the high density of exposed active sites resulting from the movable cores,
rapid transportation of reactants and products, the possibility for controlling chemical reactions
by using shells with different properties, and the protection effect deriving from the shell during
the recycling process [5, 6]. As two major structures of nanoreactors, movable cores and porous
shells make prosperous contribution to reactions. In one respect, a permeable shell isolates each
movable core in the yolk-shell nanomaterials. Thus, each core will locate in a relatively
homogeneous environment, which can greatly promote the contact between reactants [7]. For
another, porous shells of the yolk-shell nanoparticles also play a significant role in efficiently
preventing the catalytic cores from escaping to the exterior, and simultaneously allowing the
easy transportation of reactants and products [8]. To date, yolk-shell materials with noble metal
cores have developed rapidly, such as Au@HSNs [9] and Au@TiO [10]. However, the
2
synthesis of yolk-shell materials with transition metals in the core and shell still remains a
challenge.
In general, many effective routes have been proposed to incorporate catalytic nanoparticles
inside the cavity of the hollow nanoparticles, such as ship-in-bottle means, selective etching or
3

dissolution approaches, Ostwald ripening strategies, bottom-up methods by soft templating
assembly, preparation solutions based on the Kirkendall effect and galvanic replacement routes
[
11, 12]. K. Soni et al. used incipient wetness impregnation method to modify KIT-6 as effective
hydrodesulfurization catalysts. The well-ordered 3-D mesoporous material was successfully
developed as support for Mo, CoMo and NiMo and exhibited good activity [13]. Ship-in-bottle
strategy is the most renowned method for fabricating yolk-shell nanoparticles. Through this
strategy, catalytic cores can grow inside the hollow cavity through self-assembly means or
chemical reactions. Ding and co-workers [14] soaked the hollow mesoporous silicas in the
molten metal salt hydrate to make the active cores generate in the hollow interior of nanoreactors
to construct yolk-shell nanoreactors.
In this work, we developed a MoO @SiO nanoreactor utilized in epoxidation of alkenes
3
2
according to ship-in-bottle strategy using the melting (NH ) Mo O ·4H O to introduce active
4
6
7
24
2
sites MoO into the hollow cavity of hollow mesoporous silica spheres. First, hollow mesoporous
3
silica nanoparticles were fabricated with facial selective etching method. Second, the active sites
MoO were introduced by molten AMM and thermal decomposition at certain temperature to
3
produce MoO by absolutely releasing NH and H O. Third, the products were filtered to remove
3
3
2
the residual MoO . The catalytic activity of MoO @SiO nanoreactor was assessed by
3
3
2
epoxidation of cyclooctene with oxidant H O (50 wt%) as a probe reaction.
2
2
2
. Experimental Section
2
.1. Materials
Tetraethyl orthosilicate (TEOS), ammonia solution (25-28 wt %), cetyltrimethyl ammonium
bromide (C TAB), anhydrous ethanol, sodium carbonate, hexaammonium heptamolybdate
1
6
4

tetrahydrate ((NH ) Mo O ·4H O, AMM) and other solvents were purchased from Sinopharm
4
6
7
24
2
Chemical Reagent Co. (Shanghai, China). Analytical reagent grade olefins were purchased from
Aladdin, China. All the reagents were used without further purification. Deionized water was
used all around the experimental process.
2
.2. Synthesis of MoO @SiO nanoreactors
3 2
The hollow mesoporous silica spheres (HMSS) were obtained after calcination according to
our early study [15]. A thermal decomposition strategy was exploited to introduce MoO3
nanoparticles into shell and cavity referring to Lou’s work [14]. A certain amount of
(
NH ) Mo O ·4H O (AMM) and hollow mesoporous silica spheres (HMSS) were mixed by
4 6 7 24 2
o
grinding. Then the mixture was calcinated at 200 C for 24 h in the air for complete diffusion of
NH ) Mo O ·4H O to hollow mesoporous silica spheres through the pores in the shell. After
(
4
6
7
24
2
that, the excess (NH ) Mo O ·4H O and isolated MoO nanoparticles were removed by washing
4
6
7
24
2
3
with deionized water and filtrating with membrane. Afterwards the products were dried and
denoted as MoO @SiO -200-X, X = AMM: HMSS mass ratio. The products denoted as
3
2
o
o
MoO @SiO -400-X were obtained by another 6 h calcination at 400 C after treating at 200 C
3
2
for 24 h. The other procedure was the same as MoO @SiO -200-X. The synthetic process was
3
2
shown in Scheme 1.
5

Scheme 1. Synthetic process of MoO @SiO nanoreactors.
3
2
2
.3. Alkenes epoxidation
Epoxidation of alkenes was carried out to test the reactivity of catalysts. Alkenes (2.5
mmol), H O (50 wt%, 5.0 mmol), n-octane (internal standard, 1.25 mmol) and catalyst (25 mg)
2
2
were added to acetonitrile (solvent, 10 mL) in a 25 mL two-neck round bottom flask equipped
o
with a reflux condenser at 70 C. Samples were taken at intervals and stored in the refrigerator.
After centrifugation, the supernatant liquid was analyzed by a gas chromatograph (GC, SP-
6
2
890A) equipped with a FID detector and a capillary column (SE-54 30m×0.32mm×0.25μm).
.4.Methods
Scanning electron microscopy (SEM: Hitachi S-4800, 2 kV) and transmission electron
microscopy (TEM: JEOL JEM-2100plus, 200 kV) were utilized to detect the morphologies of
o
samples. Nitrogen adsorption/desorption isotherms were determined at -196 C with nitrogen as
analysis gas (Micromeritics ASAP 2010). Surface area measurements were performed according
to the Brunauer-Emmett-Teller (BET) method. Thermogravimetric analysis (TGA) of
(
NH ) Mo O ·4H O was conducted using TA Instruments (TGA/1100SF). The
4 6 7 24 2
o
-1
thermogravimetric analysis (TGA) was ramped at 20 C·min under a nitrogen flow rate of 50
-1
o
o
mL·min from 25 C to 600 C. X-ray diffraction (XRD) patterns of the materials were carried
out at room temperature using a Rigaku D/MAX-2500PC diffractometer (Rigaku Co., Japan)
equipped with Cu-Kα radiation (λ = 1.5418 Å) and operated at 40 kV and 100 mA. Inductively
coupled plasma-mass spectrometry (ICP-MS: NEXION300) was used to analyze the content of
molybdenum in materials. X-ray photoelectron spectroscopy (XPS: ESCALAB 250 XI with 200
6

W Al Kα probe beam) and Fourier transform infrared spectroscopy (FT-IR: Nicolet 6700) were
used to detect the surface chemical composition of materials.
3
. Results and discussion
3
.1. Characterization of catalysts
1
1
1
20
10
00
1
2
1
9
8
7
0
0
0
2
100
200
300
40o 0
500
600
Temperature ( C)
Figure 1. TG curves of (NH ) Mo O ·4H O in N .
4
6
7
24
2
2
Scheme 2. Decomposition of (NH ) Mo O ·4H O in air.
4
6
7
24
2
Thermal decomposition of (NH ) Mo O ·4H O was studied under the N flow (Figure 1).
4
6
7
24
2
2
Two obvious separated decomposition peaks of the mass loss were observed. The mass loss was
accompanied by an endothermic heat effects [16]. The first decomposition step was observed at
o
approximately 225 C, NH released in parallel with H O and (NH ) Mo O appeared [17]. In
3
2
4 2
4
13
o
the second endothermic step (approximately 293 C), the evolution of NH and H O continued
3
2
7

and the phase transformed into MoO . According to the thermal decomposition process, we
3
designed a novel strategy to construct MoO @SiO nanoreactors. AMM and HMSS were mixed
3
2
o
o
by grinding and kept at 200 C for 24 h (The melting point of AMM is 190 C). Under the
thermal treatment, the molten AMM would diffuse into the hollow cavity and shell via porous
structure in the shell. Meanwhile, NH and H O released and (NH ) Mo O ·4H O transformed
3
2
4 6
7
24
2
o
into (NH ) Mo O . When the temperature continued to increase to 400 C, the residual NH and
4
2
4
13
3
H O was completely removed to obtain active sites MoO . At the same time, the confined MoO
3
2
3
nanoparticles in shell would sinter on the wall. This thermal decomposition strategy was in
accordance with the decomposition of (NH ) Mo O ·4H O under calcination in the air (Scheme
4
6
7
24
2
2
) [18, 19].
(a)
HMSS
MoO @SiO -200-1/3
(b)
HMSS
3
2
MoO @SiO -400-1/3
3
2
MoO @SiO -200-1/2
3
2
MoO @SiO -400-1/2
3
2
MoO @SiO -200-1
MoO @SiO -400-1
3
2
3
2
MoO @SiO -200-2
MoO @SiO -400-2
3
2
3
2
MoO @SiO -200-3
3
2
MoO @SiO -400-3
3
2
JCPDS:80-0757
JCPDS:05-0508
50 60 70
1
0
20
30
40 50
60
10
20
30
40
2 Theta (deg)
2
Theta (deg)
Figure 2. XRD patterns of HMSS, MoO @SiO -200-X (a) and MoO @SiO -400-X (b).
3
2
3
2
XRD measurement was utilized to certify the crystal structure of samples with different
o
calcination conditions. All the samples indicated a broad peak between 20-30 , corresponding to
amorphous silica [20, 21]. With the increasing dosage of AMM, characteristic peaks turned
obvious accordingly. Compared to the standard patterns, the characteristic peaks can be indexed
8

to relevant molybdenum composites, which demonstrated the existence of molybdenum
components. The diffraction peaks of MoO @SiO -200-X samples were indexed to be
3
2
(
NH ) Mo O (JCPDS: 80-0757) [22] (Figure 2a). With the increasing dosage of AMM, more
4 2 4 13
target material entered into the hollow mesoporous silica spheres and typical XRD patterns of
o
o
(
NH ) Mo O appeared. When the materials were calcined at both 200 C and 400 C, NH and
4 2 4 13 3
H O almost released completely, the crystal structure transformed into MoO (JCPDS: 05-0508)
2
3
[
23, 24] (Figure 2b). And characteristic peaks of MoO were observed by the increasing mass
3
ratio of AMM: HMSS, resulting from the bigger MoO nanoparticles.
3
(a)
(b)
7
96
796
HMSS
HMSS
9
42
912
MoO @SiO -200-1/3
3
2
MoO @SiO -400-1/3
3
2
MoO @SiO -200-1/2
3
2
7
39
MoO @SiO -400-1/2
3 2
MoO @SiO -200-1
3
2
MoO @SiO -400-1
3
2
MoO @SiO -200-2
3
2
MoO @SiO -400-2
3
2
MoO @SiO -200-3
3
2
MoO @SiO -400-3
3
2
1630
3
196
1630 1400
057
3411
1
859
635
1057
8
90
979
1000
4
000
3500
3000 2000
1500
1000
500
4000
3500
3000 2000
1500
500
-
1
-1
Wavenumber (cm )
Wavenumber (cm )
Figure 3. FT-IR spectra of HMSS, MoO @SiO -200-X (a) and MoO @SiO -400-X (b).
3
2
3
2
FT-IR spectra of MoO @SiO -200-X and MoO @SiO -400-X were measured in the range
3
2
3
2
-1
of 4000-525 cm (Figure 3). NH and H O released during the calcination process. When the
3
2
o
samples were only treated at 200 C, weak N-H and O-H deformation bands of ammonium ions
-1
and water were observed at 1400 and 1630 cm , respectively. The stretching vibrations of O-H
-1
-1
and N-H appeared at 3196 cm [17]. The vibrational band at 1057 cm was assigned as ν (Si-
as
-1
O-Si) and the band at 796 cm was attributed to ν (Si-O-Si) [25]. With the increasing mass ratio
s
9

-1
of AMM and HMSS, characteristic peaks of Mo-O bands appeared below 1000 cm . When the
o
samples continued to keep at 400 C for another 6 h, NH and crystalliferous water almost
3
released completely. The (NH ) Mo O transformed into MoO . Therefore, N-H bands almost
4
2
4
13
3
disappeared. While adsorbed water and weak stretching vibration of O-H bands at 3411 and
-1
1
630 cm have been observed, respectively. The vibrational bands of ν (Si-O-Si) and ν (Si-O-
as s
-1
-1
Si) were still detected at 1057 and 796 cm , respectively. Meanwhile, the band at 796 cm
weakened and some typical peaks of MoO appeared after modification. The peaks at 979 and
3
-1
9
12 cm were assigned to ν (Mo=O) and ν (Mo-O-Mo) for MoO [26-30]. Moreover, with the
s as 3
-1
increasing dosage of AMM, peaks at 979 and 912 cm strengthened. For a comparison, the
spectrum of MoO @SiO -200-X and MoO @SiO -400-X exhibited different Mo-O vibration
3
2
3
2
-1
regions below 1000 cm due to different crystal structures. The results of FT-IR indicated the
different results after treating with two calcination conditions.
Mo 3d
(a)
(b)
Mo 3d
5/2
O 1s
Mo 3d
3/2
2
33.0 eV
2
36.1 eV
Si 2p
Mo 3d
C 1s
0
200
400
600
800 1000 1200
222 224 226 228 230 232 234 236 238 240
Binding Energy (eV)
Binding Energy (eV)
1
0

O 1s
Si 2p
(c)
(d)
1
03.7 eV
SiO2
MoO3
30.5 eV
533 eV
5
525
528
531
534
537
540
543
96
99
102
105
108
111
Binding Energy (eV)
Binding Energy (eV)
Figure 4. XPS survey spectra (a) and high-resolution spectra of Mo 3d (b), O 1s (c) and Si 2p (d)
present in MoO @SiO -400-1/2.
3
2
The elemental composition of material was further characterized by X-ray photoelectron
spectroscopy (XPS) using the C 1s peak at 284.8 eV as the reference. The XPS spectra of
catalyst MoO @SiO -400-1/2 were shown in Figure 4. The survey spectrum (Figure 4a)
3
2
demonstrated that the sample was composed of O, Si and Mo elements. A representative Mo 3d
spectra was observed in Figure 4b, the Mo 3d3/2 and Mo 3d5/2 peaks located at ca. 236.1 and
6
+
2
33.0 eV, respectively. This result implied the formal oxidation state of Mo in MoO [31, 32].
3
The O 1s spectra (Figure 4c) could be fitted into two peaks, 530.5 and 533 eV, corresponding to
O in trace MoO and abundant SiO , respectively [33-36]. In the Si 2p spectra (Figure 4d), the
3
2
peak at ca. 103.7 eV was attributed to SiO [37].
2
1
1

Figure 5. SEM images of HMSS (a), MoO @SiO -400-1/2 (d), and MoO @SiO -400-1 (g),
3
2
3
2
TEM images of HMSS (b, c), MoO @SiO -400-1/2 (e, f) and MoO @SiO -400-1 (h, i).
3
2
3
2
Morphology of samples were studied by SEM and TEM (Figure 5). Compared with the
SEM images, materials maintained the initial structure after modification with MoO . Obviously,
3
TEM images exhibited remarkable changes. Compared with HMSS, MoO @SiO -400-1/2 and
3
2
MoO @SiO -400-1, dark dots have been observed in MoO @SiO -400-1/2 and MoO @SiO -
3
2
3
2
3
2
4
00-1, especially in MoO @SiO -400-1. These dark dots were supposed to be MoO
3 2 3
nanoparticles, which acted as active sites. MoO @SiO nanoreactors with mesoporous shell
3
2
could prevent aggregation of MoO nanoparticles, which would result in highly dispersed active
3
sites (MoO @SiO -400-1/2 with a size of 4±0.5 nm) and promote the mass diffusion and
3
2
transport of reactants and products [39]. After treating under high temperature, the active
nanoparticles would sinter on the wall of the silica spheres stably. Significantly, since each
MoO nanoparticles in the hollow cavity was isolated and had a relatively homogeneous
3
environment, the active sites could fully contact with reactants and improve the reactivity.
1
2

Owing to a 67±10 nm shell with wormlike structure, the MoO nanoparticles in cavity would not
3
escape from the nanoreactors. However, with the increasing AMM and HMSS ratio, more
molten AMM was introduced into the hollow silica spheres and produced bigger MoO3
nanoparticles with a size above 10 nm.
Table 1. Textural properties of catalysts.
Sample
BET surface area
Pore volume
Peak in pore size
distribution
2
3
(
m /g)
(cm /g)
(nm)
HMSS
990
336
170
97
1.06
0.50
0.19
0.13
0.10
0.20
2.1 and 3.9
MoO @SiO -400-1/3
2.6
2.5
2.8
3.0
2.7
3
2
MoO @SiO -400-1/2
3
2
MoO @SiO -400-1
3
2
MoO @SiO -400-2
75
3
2
MoO @SiO -400-3
126
3
2
1
3

(a)
(b)
A
A
B
C
B
C
D
E
D
E
F
F
0
.0
0.5
1.0
2
3
4
5
6
7
8
9
Pore diameter (nm)
Relative pressure (P/Po)
Figure 6. N adsorption-desorption isotherms (a) and the corresponding pore size distributions
2
(
b) of HMSS(A), MoO @SiO -400-1/3 (B), MoO @SiO -400-1/2 (C), MoO @SiO -400-1 (D),
3 2 3 2 3 2
MoO @SiO -400-2 (E) and MoO @SiO -400-3 (F).
3
2
3
2
Specific surface area and porosity of the materials (HMSS, MoO @SiO -400-X and
3
2
MoO @SiO -200-X) were investigated by N adsorption-desorption measurements (Table 1 and
3
2
2
S1, Figure 6 and S1). Figure 6a revealed N adsorption-desorption curves of HMSS,
2
MoO @SiO -400-1/3, MoO @SiO -400-1/2, MoO @SiO -400-1, MoO @SiO -400-2 and
3
2
3
2
3
2
3
2
MoO @SiO -400-3 with a type IV isotherm and H4 hysteresis loop, indicating the formation of
3
2
mesoporous structure [40]. The corresponding pore size distributions were presented in Figure
6
b and Table 1. All the samples exhibited a pore distribution ranged around 2-3 nm. As shown in
2
Table 1, Brunauer-Emmett-Teller (BET) surface area and pore volume of HMSS was 990 m /g
3
and 1.06 cm /g, respectively. However, BET surface area and pore volume of MoO @SiO
3
2
nanoreactors dramatically decreased with the increasing AMM: HMSS ratio. The molten AMM
diffused into the shell and hollow cavity and transformed to MoO after calcination. These MoO
3
3
blocked in pores leading to the depression in BET surface area and pore volume. When the
1
4

AMM: HMSS ratio increased to 3, abundant MoO nanoparticles appeared and the increasing
3
tendency were supposed to be aggregation of MoO nanoparticles. The peak pore size
3
distribution of HMSS were 2.1 and 3.9 nm. When molybdenum composites introduced, channels
were blocked and made the peak pore size distribution move to ca. 2-3 nm. MoO @SiO -200-X
3
2
exhibited the similar results and regulations compared with MoO @SiO -400-X (shown in Table
3
2
S1 and Figure S1).
3
.2. Catalytic epoxidation activity
1
00
1
00
A
A
B
8
6
4
2
0
0
0
0
0
9
9
8
6
C
D
D
C
B
E
94
92
90
E
(
a)
(b)
0
2
4
6
8
10
12
0
2
4
6
8
10
12
Time (h)
Time (h)
Figure 7. Epoxidation of catalysts with different AMM: HMSS ratio using MoO @SiO -200-X
3
2
as catalysts, (a) conversion, (b) selectivity.
Reaction conditions: catalyst (25 mg), cyclooctene (2.5 mmol), H O (50 wt%, 5.0 mmol),
2
2
o
acetonitrile (10 mL), n-octane (1.25 mmol), temperature 70 C. MoO @SiO -200-1/2 (A),
3
2
MoO @SiO -200-2 (B), MoO @SiO -200-1 (C), MoO @SiO -200-1/3 (D) and MoO @SiO -
3
2
3
2
3
2
3
2
2
00-3 (E).
1
5

1
00
100
98
96
94
92
90
A
A
B
B
8
6
4
2
0
0
0
0
0
E
C
D
D
C
E
(a)
(b)
0
2
4
6
8
10
12
0
2
4
6
8
10
12
Time (h)
Time (h)
Figure 8. Epoxidation of catalysts with different AMM: HMSS ratio using MoO @SiO -400-X
3
2
as catalysts, (a) conversion, (b) selectivity.
Reaction conditions: catalyst (25 mg), cyclooctene (2.5 mmol), H O (50 wt%, 5.0 mmol),
2
2
o
acetonitrile (10 mL), n-octane (1.25 mmol), temperature 70 C. MoO @SiO -400-1/2 (A),
3
2
MoO @SiO -400-2 (B), MoO @SiO -400-1 (C), MoO @SiO -400-1/3 (D) and MoO @SiO -
3
2
3
2
3
2
3
2
4
00-3 (E).
Table 2. Results of epoxidation of cyclooctene using MoO @SiO -400-X as catalysts.
3
2
-1
Entry
Catalyst
Mo (wt%)
1.63
Yield (%)
TOF (h )
1
2
MoO @SiO -400-1/3
33
54
137
3
2
MoO @SiO -400-1/2
3.39
151
3
2
1
6

3
4
5
MoO @SiO -400-1
6.59
44
38
11
64
38
7
3
2
MoO @SiO -400-2
11.90
16.38
3
2
MoO @SiO -400-3
3
2
Reaction conditions: catalyst (25 mg), cyclooctene (2.5 mmol), H O (50 wt%, 5.0 mmol),
2
2
o
acetonitrile (10 mL), n-octane (1.25 mmol), temperature 70 C, reaction time (2 h).
The reactivity of MoO @SiO -200-X and MoO @SiO -400-X was investigated by
3
2
3
2
epoxidation of cyclooctene under neat conditions. Compared with Figure 7 and 8, MoO @SiO -
3
2
4
00-X exhibited better reactivity than MoO @SiO -200-X. It was supposed to be the
3 2
incompletely decomposed molybdenum compounds in hollow mesoporous silica spheres. The
result was accordance with the result of XRD and FT-IR. MoO @SiO -200-X displayed certain
3
2
reactivity in epoxidation of cyclyooctene. After NH and H O releasing, (NH ) Mo O ·4H O in
3
2
4 6
7
24
2
MoO @SiO -200-X transformed into major (NH ) Mo O . MoO was more efficient
3
2
4 2
4
13
3
performance in epoxidation than (NH ) Mo O which made MoO @SiO -400-X display
4
2
4
13,
3
2
superior reactivity than MoO @SiO -200-X. And as shown in Figure 8, MoO @SiO -400-1/2
3
2
3
2
was found to give the optimal reactivity. With the increasing of AMM: HMSS mass ratio,
content of MoO increased. So MoO @SiO -400-1/2 resulted in higher conversion than
3
3
2
MoO @SiO -400-1/3. However, a large AMM: HMSS ratio would cause blockage in
3
2
mesoporous shell, which would weaken the transportation of reactants and products, leading to a
lower conversion. Meanwhile, higher AMM: HMSS ratio would contribute to bigger active sites,
which would also exhibit lower reactivity compared with MoO @SiO -400-1/2 with highly
3
2
dispersed active sites. In conclusion, the optimal AMM: HMSS ratio was observed to be 1/2.
1
7

According to the results from Table 2, it can be seen that MoO @SiO -400-1/2 had the optimal
3
2
-1
TOF value of 151 h .
1
20
00
Yield
Selectivity
1
80
60
40
20
0
0
1
2
3
4
5
6
7
Figure 9. Epoxidation of different alkenes using MoO @SiO -400-1/2 as catalyst.
3
2
Reaction conditions: catalyst (25 mg, MoO @SiO -400-1/2), alkene (2.5 mmol), H O (50 wt%,
3
2
2
2
o
5
.0 mmol), acetonitrile (10 mL), n-octane (1.25 mmol), temperature 70 C, reaction time (4 h).
In order to research the general applicability of catalysts, catalytic epoxidation of different
kinds of olefins was further measured. Epoxidation of cyclic alkenes and terminal linear alkenes
was carried out to study the reactivity of different alkenes (Figure 9). Cyclic alkenes exhibited
higher reactivity than terminal linear alkenes with the increasing number of carbon atoms which
would attribute to more electron donating (CH ) bridge groups connected to the double bond
2
n
[
41]. The conversion increased in the order of cyclooctene > cyclohexene > cyclopentene. For
terminal linear alkenes, the conversion increased in the order of 1- pentene > 1- hexene > 1-
1
8

octene. With the increasing number of chain length, cross-linking and attraction between the
molecules of the linear olefin, interactions including van der Waals forces, hydrogen bond
existed between molecules resulted in forming a so-called ‘‘macromolecule’’ [42]. The
probability of linear alkenes in contact with active sites became smaller and the transportation of
reactants and products in and outside the shell was found to be difficult, leading to the decrease
in reactivity.
-
-
-
3.5
4.0
4.5
1
00
(b)
(a)
8
6
4
2
0
0
0
0
0
1
-octene
Cyclooctene
2
R =0.9900
-
1
-5.0
k= 0.115 h
-5.5
-6.0
-6.5
-7.0
Cyclooctene
2
1
-octene
R =0.9930
-
1
k= 0.329 h
0
2
4
6
8
0
2
4
6
8
Time (h)
Time (h)
Figure 10. (a) Product yield versus time in epoxidation of various olefins using MoO @SiO -
3
2
4
1
00-1/2 catalyst. (b) The kinetic plots of epoxidation of various olefins using MoO @SiO -400-
3 2
/2 catalyst.
Reaction conditions: catalyst (25 mg, MoO @SiO -400-1/2), alkene (2.5 mmol), H O (50 wt%,
3
2
2
2
o
5
.0 mmol), acetonitrile (10 mL), n-octane (1.25 mmol), temperature 70 C, reaction time (4 h).
To further confirm the trend of epoxidation, reactions were run by using the rather
unreactive 1-octene and the highly reactive cyclooctene as substrates. All reactions were carried
out with excess hydrogen peroxide as oxidant. Figure 10a exhibited the concentration of products
against time by using MoO @SiO -400-1/2 catalyst. It was observed that the produced epoxides
3
2
1
9

followed the descending trend of 1, 2- epoxycyclooctane > 1, 2-epoxyoctane. During the first 4
h, all the olefins reacted rapidly to produce epoxides. After 4 h, the reaction rate slowed down
and headed to the equilibrium plateau where the reactions were expected to approach the
optimum yield. The order of this kind of reaction was derived for epoxidation results of various
olefins (Figure 10b). All the epoxidation reactions established similar straight-line plots with
2
regression value (R close to 1). The reactions were deemed to be the first order kinetic reaction
whose reaction ratio solely depended on the reactants [43]. Amongst, cyclooctene displayed
-1
-1
higher rate constant, with a k value of 0.329 h , followed by 1-octene (0.115 h ). The kinetic
behavior was consistent with reports from literatures [44].
1
00
100
80
60
40
20
0
8
6
4
2
0
0
0
0
0
1
st
2nd
3rd
4th
5th
6th
0
4
8
12
Time (h)
16
20
24
Figure 11. Recycle tests of optimal catalysts via epoxidation of cyclooctene in six consecutive
runs.
Reaction conditions: catalyst (25 mg, MoO @SiO -400-1/2), cyclooctene (2.5 mmol), H O (50
3
2
2
2
o
wt%, 5.0 mmol), acetonitrile (10 mL), n-octane (1.25 mmol), temperature 70 C.
2
0

The reusability of catalyst was investigated by using the optimal catalyst (MoO @SiO -
3
2
4
00-1/2) in epoxidation of cyclooctene with 50 wt% H O as oxidant. The used catalyst was
2 2
recovered, washed with ethanol, dried and directly utilized in cycling tests without further
treatment or regeneration. As shown in Figure 11, MoO @SiO -400-1/2 displayed good stability
3
2
both of conversion and selectivity. The reactivity was maintained at the same level after six
cycles. Channels in shell was wormlike, which was difficult for active sites to escape. Major
active sites MoO particles were successfully trapped in the cavity of hollow mesoporous silica
3
spheres. Moreover, minor MoO particles were sintered in channels in shell with wormlike pores
3
o
under the calcination of 400 C. Hence, the MoO particles would retain during and after
3
reaction.
1
00
8
6
4
2
0
0
0
0
0
No catalyst separation
Catalyst filtered
0
2
4
6
8
Time (h)
Figure 12. Leaching tests.
Reaction conditions: catalyst (25 mg, MoO @SiO -400-1/2), cyclooctene (2.5 mmol), H O (50
3
2
2
2
o
wt%, 5.0 mmol), acetonitrile (10 mL), n-octane (1.25 mmol), temperature 70 C.
2
1

To further explain the stability of catalysts, leaching tests were carried out. Hot filtration
was exploited during the reaction after reacting for 2 h to separate catalyst [45]. Afterwards, the
filtrate continued to react under the same conditions. As shown in Figure 12, no further
conversion was observed after separation of catalyst, which demonstrated the stability of
catalyst. We believe that the MoO particles have been successfully trapped in hollow
3
mesoporous spheres, which was stable enough to avoid leaching. Furthermore, no molybdenum
was observed from the supernatant liquid of epoxidation through ICP-MS whose detection limit
of the method is 0.0001 wt%. FT-IR spectra of MoO @SiO -400-1/2 before and after recycling
3
2
-1
test were shown in Figure S2. The remained peaks at 979 and 912 cm which assigned to ν
s
(
Mo=O) and ν (Mo-O-Mo) for MoO indicated the maintained active sites. SEM analysis was
as 3
used to detect the structure after recycling. As shown in Figure S3, MoO @SiO -400-1/2
3
2
remained the same without breakage after recycling. Both FT-IR and SEM certificated the
stability of catalyst.
4
. Conclusion
In summary, the facile and efficient fabrication of MoO @SiO nanoreactors was
3
2
successfully developed via a thermal decomposition strategy. The obtained MoO @SiO
3
2
nanoreactors demonstrated a high catalytic activity for epoxidation of alkenes and performed
well in the durability and recyclability. Obviously, synthesis of yolk-shell nanoparticles still
meets significant conceptual challenges, where most methods involve multi-step approaches and
only work successfully on specific cases. This thermal decomposition strategy is easy to operate
and can provide a possibility for the fabrication of various kinds of MoO @shell composite
3
nanoreactors and search opportunities for usage in other areas such as drug delivery, lithium-ion
batteries and biosensors etc.
2
2

Acknowledgements
This work is supported by the Fundamental Research Funds for the Central Universities
(JUSRP51507), the Innovation Foundation in Jiangsu Province of China (BY2014023-08) and
the Postgraduate Innovation Project of Jiangsu Province (KYLX16_0787).
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(
Highlights



A thermal decomposition strategy applied in fabricating MoO @SiO nanoreactors
3 2
Efficient catalysts in epoxidation of alkenes with high reactivity and stability
Kinetics study on epoxidation of alkenes which follows the first order model
2
7