Cobalt nanoparticles in hollow mesoporous spheres as a highly efficient and rapid magnetically separable catalyst for selective epoxidation of styrene with molecular oxygen

Article abstract of DOI:10.1039/c3ra45383a

Uniform rattle-type hollow mesoporous spheres (HMS) with large cavities have been successfully prepared by the colloidal carbon spheres as templates. The spheres are well monodispersed and nearly uniform in dimension with particle size of ca. 150 nm. Co nanoparticles composed of HMS were found to be a highly efficient catalyst for selective epoxidation of styrene under mild reaction conditions. The catalysts can be easily separated from the reaction mixtures by an external magnet, and reused five times without significant loss of catalytic activity.

Full text of DOI:10.1039/c3ra45383a

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Cobalt nanoparticles in hollow mesoporous
spheres as a highly efficient and rapid magnetically
separable catalyst for selective epoxidation of
styrene with molecular oxygen
Cite this: RSC Adv., 2014, 4, 47
Zhi-Qiang Shi,ab Li-Xin Jiao,ab Jian Sun,ab Zi-Bao Chen,a Yuan-Zhe Chen,ab
ab
Xiao-Hang Zhu,ab Jing-Hui Zhou,ab Xin-Chun Zhou,ab Xin-Zhe Liab and Rong Li
*
Uniform rattle-type hollow mesoporous spheres (HMS) with large cavities have been successfully prepared
by the colloidal carbon spheres as templates. The spheres are well monodispersed and nearly uniform in
dimension with particle size of ca. 150 nm. Co nanoparticles composed of HMS were found to be a
highly efficient catalyst for selective epoxidation of styrene under mild reaction conditions. The catalysts
can be easily separated from the reaction mixtures by an external magnet, and reused five times without
significant loss of catalytic activity.
Received 26th September 2013
Accepted 4th November 2013
DOI: 10.1039/c3ra45383a
www.rsc.org/advances
showed ten times higher activity than commercial Pd/C catalyst
and six times higher activity than Pd/MHSS catalyst.18 Although
1. Introduction
many catalysts have been found to be reliable, the current
chemical transformation emphasizes from environmental and
economical concerns require low cost, high efficiency, easy
separation and the production of minimum waste. Therefore,
more and more researchers are trying to design easily separated
and recyclable, environmental and economical catalyst.19–23
Epoxide compounds are usually synthesized by epoxidation
of alkenes. Therefore, the epoxidation of alkenes is a highly
signicant chemical reaction. Especially the epoxidation of
styrene become increasingly attractive because the epoxide is
useful intermediate in the production of many pharmaceuticals
and ne chemicals.24 Activation of the carbon–hydrogen bonds
of an alkane is considerable difficult due to its stability.25,26
Conventionally, the epoxidation of alkenes was usually carried
out with organic peracid as an oxidant or by a chlorohydrin
process. A lot of waste was formed in many cases.27,28 Now,
organic peroxide and hydrogen peroxide are used as oxidants for
the epoxidation of alkenes under the condition of heteroge-
Ordered mesoporous silica materials were rst synthesized in
1992.1 Mesoporous spherical self-assembly of nanocarriers have
become a promising approach toward the synthesis of ordered
mesoporous materials.2–10 As nanocarriers, hollow mesoporous
silica materials with unique properties and structures (such as
low toxicity, good compatibilities with metal particles, adjust-
able pore diameter, low density, and very high specic surface
area with abundant Si–OH bonds10–12) have attracted much
attention as candidates in adsorption and drug delivery
systems.2,13–15 The studies showed that the hollow mesoporous
silica spheres could provide much higher drug loading capacity,
compared to the conventional mesoporous materials such as
MCM-41, SBA-15 and MCF. Consequently, the hollow meso-
porous spheres have received considerable attention because
they have potential applications. For instance, Wang et al.
synthesized intact and stable hollow silica spheres with radially
oriented amino-functionalized mesochannels, and these hollow
spheres showed a much higher release rate and higher urbi-
profen capacity (>1000 mg gꢀ1) compared with ake-like mes-
oporous SBA-15 particles.16 Zhu et al. demonstrated that hollow
mesoporous silica spheres could charge ibuprofen about three
times as large as conventional MCM-41 (ref. 17). Yang et al.
reported recently gold-promoted palladium catalyst with mes-
oporous hollow silica spheres as support, and the catalyst
´
neous catalyst. For example, Kuzniarska-Biernacka and
co-workers found that manganese(III)-Schiff base complex
encapsulated in an aluminium pillared clay could efficiently
catalyze the epoxidation of styrene by iodosylbenzene (PhIO).29
N.S. Patil and co-workers researched the epoxidation of styrene
by anhydrous t-butyl hydroperoxide over Au/Al2O3, Ga2O3, In2O3
and Tl2O3 catalysis.30 Although many current studies about the
epoxidation of styrene have been reported,31–34 plenty of diffi-
culties appeared during the reaction, such as low catalytic
activity, more byproduct and high reaction temperature. In
order to overcome these limitations, researchers exploited
environmentally friendly and reusable heterogeneous catalysts
aState Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou,
730000, China. E-mail: liyirong@lzu.edu.cn; Fax: +86 0931 891 2582; Tel: +86
0931 891 2311
bThe Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of
Gansu Province, College of Chemistry and Chemical Engineering Lanzhou
University, Lanzhou 730000, China
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than once with distilled water and ethanol. Finally, the products
ꢁ
were oven-dried at 50 C for the following work.
2.2.2. Loading cobalt nanoparticles onto the carbon
nanosphere (Co/C). 150 mg carbon nanosphere was dispersed
in 50 mL distilled water and stirred for 20 minutes as part A.
0.15 g SnCl2 was dissolved in 30 mL 0.02 M HCl solution as part
B. Parts A and B were mixed together under ultrasonicating for
1 hour. The suspension was separated from the solution by
lter, washed several times with distilled water and dispersed in
80 mL distilled water. Then 50 mg Co(NO3)2$6H2O was added
into it. 10 minutes later, 20 mL of 0.15 M sodium formate
solution was added, followed by stirring for 7 hours. The
suspension was separated from the solution by a centrifugation,
washed with ethanol and distilled water for several times and
Scheme 1 Scheme of the synthetic procedure for the preparation of
Co/hollow mesoporous spheres.
ꢁ
dried at 60 C for 12 hours.
2.2.3. Production of Co@mesoporous SiO2 nanoreactor
(Co/HMS). The Co/C composite obtained from last step was
dispersed in the solution containing 80 mL H2O, 60 mL
ethanol, 0.25 g CTAB and 1.2 mL NH3$H2O with ultrasonic
for 20 minutes. The 200 mL TEOS was added into the mixture,
and erce stirred for 8 hours. The suspension was separated
from the solution by a centrifugation, washed many times
with ethanol and distilled water, respectively, then oven-
dried at 60 ꢁC. Then the product was calcined at 400 ꢁC in air
atmosphere for 7 hours to remove carbon sphere, CTAB
template and other organic species. The amount of Co in the
HMS nanoreactor was found to be 2.0 wt% based on ICP
analysis.
for the epoxidation of styrene, such as BaO/Ga2O3,24 Co/Ts-1,34V-
MCM-41 (ref. 35) or gold supported on MgO and other alkaline
earth oxides.33 Although styrene conversion is quite high when
H2O2 or O2 is used as an oxidizing agent, the selectivity for
styrene oxide is poor.
Compared with organic peroxides and hydrogen peroxide,
molecular oxygen is more environmental and economical
oxidant for the epoxidation of styrene. At the same time,
developing an efficient catalyst, especially transition metal
heterogeneous catalyst for the epoxidation of styrene with O2
is preferable. Herein, we developed a facile route to synthesize
HMS nanoreactors with Cobalt nanoparticles resided inside
the mesoporous spheres by using the colloidal carbon spheres
as the templates. The fabrication procedure involved
four steps as shown in Scheme 1. The Co/HMS nanoreactors
possess large magnetization, highly open and ordered
mesopores, and stably conned. The multicomponent nano-
structured materials showed excellent activity for the selective
epoxidation of olens by O2, and could be easily recycled
multiple times without visible decrease in catalytic perfor-
mance. Furthermore, the method is also extended to intro-
duce other transition metal nanoparticles inside the HMS to
prepare other new catalysts.
2.3. Measurement of catalytic performance
The epoxidation of styrene with O2 was performed in a 25 mL
two-necked round bottom ask equipped with a reux
condenser. Typically, 5 mL N,N0-dimethylformamide (DMF)
solution containing a given substrate (0.1 mL) and 30 mg
catalyst was taken, which ultrasonic treatment for 15 min,
Then the mixture was vigorously stirred by a magnetic stirrer
and kept in a constant temperature oil bath with the
ꢁ
temperature maintained at 100 C. Thereaer O2 at a certain
stable ow rate controlled by a mass ow controller was
introduced into the liquid by bubbling at atmospheric pres-
sure. The mixture was vigorously stirred for a specied period
of time. Samples were withdrawn by syringe and were analyzed
by off-line gas chromatography equipped with a capillary
column using ame ionization detector (FID). The catalyst was
recovered from the reaction mixture by an external magnetic
2. Experimental
2.1. Materials and reagents
Glucose, ethanol, tetraethoxysilane (TEOS), ammonia solution
(28 wt%), SnCl2$2H2O, N,N0-dimethylformamide (DMF),
Co(NO3)2$6H2O. olens and cetyltrimethy-lammonium
bromide (CTAB) Were bought from Alfa Aesar. All Chemicals
were of analytical grade and were used as received without
further purication.
ꢁ
force, washed with acetone, and dried at 60 C for 12 hours,
and then it was used in the next experiment under the same
reaction conditions. We employed the catalyst for other
olens by the same method.
2.2. Preparation of catalysts
2.4. Characterization of catalysts
2.2.1. Synthesis of carbon nanospheres.36,37 Glucose (6 g) Powder X-ray diffraction (XRD) patterns were obtained on a
was dissolved in 40 mL distilled water to form a clear solution Rigaku D/max-2400 diffractometer using Cu-Ka radiation as the
and then transferred into a 100 mL Teon-sealed autoclave. The X-ray source in the 2q range of 10–80ꢁ. The morphology of the
autoclave was maintained at 180 ꢁC for 4.5 hours. The products catalyst was observed by a Tecnai G2 F30 transmission electron
were separated by centrifugation, followed by washing more microscopy and samples were obtained by placing a drop of a
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colloidal solution onto a copper grid and evaporating the
solvent in air at room temperature. The conversion was esti-
mated by GC (P.E. Autosystem XL) and GC-MS (Agilent 6890N/
5973N). Co content of the catalyst was measured by induc-
tively coupled plasma (ICP) on IRIS Advantage analyzer. X-ray
photoelectron spectroscopy (XPS) was recorded on a PHI-5702
instrument and the C1s line at 297.8 eV was used as the
binding energy reference. Magnetic measurement of
Co@mesoporous SiO2 nanoreactor was investigated with a
Quantum Design vibrating sample magnetometer (VSM) at
room temperature in an applied magnetic eld sweeping from
ꢀ15 to 15 kOe.
3. Results and discussion
3.1. Characterization of catalysts
Fig. 1 shows the XRD patterns between 10 and 90ꢁ. The XRD
pattern of Co/HMS exhibits four different peaks at 2q ¼ 27.1ꢁ,
35.2ꢁ, 52.4ꢁand 61.5ꢁ, corresponding to the reections of (110),
(111), (200) and (220) crystal planes of cobalt, respectively. The
Fig. 2 (a) TEM of carbon nanospheres; (b) TEM of Co/C nanospheres;
(c) TEM of final composite nanoreactors (Co/HMS) in low calcined
pattern shows that the CoO phase is formed by air oxidation temperature and (d) TEM of final composite nanoreactors in high
calcined temperature.
during the recovery and storage procedure.
The morphologies and structures of the products at
different synthetic steps were observed by TEM. Fig. 2(a)
obvious contrast between the core and the shell of the
sphere. A large number of carbon nanospheres’ cores began
to shrink, but the shell surface is still smooth, no hole
formed at the same time. Fig. 2(d) shows the TEM micro-
graphs of the calcined HMS in 430 ꢁC. In contrast to Fig. 2(c),
the foam-like masses in the cores have disappeared, which
implies that the foam-like masses are organic compounds,
and the calcined HMSs has a hollow structure. At the same
time, numerous cobalt nanoparticles evenly dispersed on the
inner wall of the shell.
shows the TEM micrographs of the carbon nanospheres. It
can be clearly observed that the morphology of carbon
nanospheres is predominated with spheres; they are quite
uniform in size and the average size of the spheres is around
100–150 nm. These carbon nanospheres possess functional
groups (–OH, C]O) on the surface, which offer an important
chemical environment to adsorb metal ion-nanoparticles by
electrostatic attraction in acidic solution. Fig. 2(b) is a TEM
image of the Co/C composite. The particles with an average
size of about 4–6 nm are uniformly distributed on the
surfaces of carbon nanospheres. On the basis of Co/HMS
using TEOS as the silica source and CTAB as a so template,
a thin layer of mesoporous silica is coated onto the Co/HMS
spheres. Then, the Co/HMS nanoreactors were obtained aer
the calcination aiming at removing the carbon templates and
the organic groups of CTAB. Fig. 2(c) shows the TEM
micrographs of the calcined HMS in 330 ꢁC. There is a more
The magnetization curve measured at room temperature for
Co/HMS is shown in Fig. 3. The curve presents a thin hysteresis
loop, which indicates the ferromagnetic behavior of Co/HMS
spheres. The magnetization saturation (Ms) of the mesoporous
Fig. 1 XRD patterns of Co/HMS.
Fig. 3 Room temperature magnetization curves of Co/HMS.
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spheres reaches 5.4 emu gꢀ1, and it suggests that cobalt nano- nanoparticles exist inner the hollow mesoporous spheres from
particles become encapsulated in hollow mesoporous spheres. the TEM micrographs. Since the analysis of XPS is only about
This is consistent with the TEM patterns (Fig. 2). Although it is outer surface of particles, the Co 2p signals in Fig. 5 are very
small hysteresis loop, the brown Co/HMS spheres can still be small. On the basis of above analysis, it can be concluded that
efficiently attracted toward the magnet within a certain period the XPS data further identied the hollow mesoporous structure
(inset of Fig. 3).
of the synthesized Co/HMS.
Fig. 4 shows the XPS spectrum of the synthesized Co/HMS
nanoreactor. Peaks corresponding to oxygen, carbon, silicon, tin
and cobalt are observed. Both the peaks of Si 2s (156.3 eV) and Si
2P (105.5 eV) are quite clear and this means that SiO2 is the main
3.2. Epoxidation of styrene reaction catalyzed by Co/HMS
nanoreactor
component is the outer surface. However, typical peak of Co Table 1 compares the effect of solvent on the epoxidation of
styrene with O2 over Co/HMS at 100 ꢁC. It can be seen that solvent
played an important role on the epoxidation reaction. For example,
DMF was especially efficient in obtaining both high styrene
conversion and high epoxide selectivity. The conversion date of
styrene (Table 1) was obtained in dimethyl sulfoxide (DMSO),
toluene and cyclohexanone, but the epoxide selectivity was low.
The inuence of reaction time on the conversion and
selectivity for the epoxidation was showed in Fig. 6. Along with
time extension, the conversion of styrene increased rapidly
and the epoxide selectivity also increased very markedly
(3 hours), and the conversion and selectivity attained over
90%. However as time went by, a growing number of
elements is not obviously found. To ascertain the oxidation state of
the Co, X-ray photoelectron spectroscopy (XPS) studies
were carried out. Co 2p photoemission is shown in Fig. 5 for
UHV-cleaved CoO (1 0 0) with binding energies of 780.8 eV for 2p3/2
and 795.7 eV for 2p1/2 photoemission maxima, and are in agree-
ment with literature values.38 The majority of cobalt
Table 1 Effect of reaction solvent on epoxidation of styrene with O2
over Co/HMSa
Product selectivity (%)
Styrene
Solvent
conversion (%) Benab Epoxide Pheac Other
DMSO
Chlorobenzene
Cyclohexanone 49
Toluene
DMF
63
0
8
—
9
28
0
51
—
19
14
94
23
—
24
6
18
—
48
52
0
d
34
98
6
a
Reaction condition: styrene (0.1 mL), catalyst (30 mg), temperature (100
b
ꢁC), DMF (5 mL), time (3 hours), ow of O2. Benzaldehyde.
Fig. 4 XPS spectrum of the elemental survey scan of Co@mesopo-
rous SiO2.
c
d
Phenylacetaldehyde. “—” means none.
Fig. 6 Influence of reaction time on the epoxidation of styrene by
DMF over the Co/HMS (in) catalyst: (a) styrene conversion, (b) epoxi-
Fig. 5 XPS spectrum of the Co/HMS showing Co 2p binding energies. dation selectivity.
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Fig. 7 1H-NMR spectra on 400 MHz spectrometer.
by-products were generated, so the selectivity decreased aer 3 JXA ¼ 4 Hz, JXB ¼ 2.4 Hz), 3.122 (d, 1H, JAB ¼ 5.6 Hz, JAX ¼ 4 Hz),
hours reaction.
2.871(d, 1H, JBA ¼ 5.6 Hz, JBX ¼ 2.4 Hz) shown in Fig. 7. 13C-
The compounds were characterized by GC-MS, 1H-NMR NMR (100 MHz, CDCl3) d 128.4, 128.1, 52.2, 51.0 shown in
spectra of 400 MHz and 13C-HMR spectra of 100 MHz spec- Fig. 8.
trometer and used CDCl3 as the solvent. The chemical shis (d)
We compared the results achieved in this work with the
are reported in parts per million (ppm) relative to the residual earlier reported heterogeneous catalysis for epoxidation of
CHCl3 peak (7.26 ppm for 1H-NMR and 77.0 ppm for 13C- styrene yield and selectivity (Table 2). Obviously, in our work
HMR). The coupling constants ( J) are reported in Hertz (Hz). product yield is the highest. Although the reaction over other
Yields refer to isolated material judged to be $95% pure by 1H catalysts also can receive high yield, they needed more time
NMR spectroscopy following silica gel chromatography. All the or higher temperature (Table 2, entry 2–4 and 7–10).
chemicals were used as received unless otherwise stated. Furthermore, some reactions temperature in other works is
1H-NMR (400 MHz, CDCl3) d 7.357–7.255 (m, 5H), 3.841 (d, 1H, lower, but the reactions take longer time and obtain lower
Fig. 8 13C-HMR spectra on 100 MHz spectrometer.
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Table 2 Comparison of Co/HMS catalysts with the earlier reported heterogeneous catalysts for their performance in the styrene-to-styrene
oxide epoxidation
Entry
Catalyst
Solvent
Temp/ꢁC
Time/h
Select/%
Yield/%
Ref.
1
2
3
4
5
6
7
8
Co-ZSM-5(L)
V-MCM-41
Co–Y–ZrO2
Cu-MCM-41
CuO/Ga2O3
Au/Yb2O3
Co-SBA-15x
Co3O4
TBHP
Acetonitrile
DMF
TBHP
TBHP
TBHP
DMF
DMF
DMF
DMF
DMF
90
rt
120
80
95
82
100
70
100
100
100
4
8
3
24
3
3
8
5
3.5
8
3
89
93
90
80
78
73
66
84
74
78
94
87
77
33
75
58
54
62
71
70
37
92
39
35
40
41
42
43
44
45
9
10
11
Co/Ts-1
Co-SSZ-51
Co/HMS
34
46
This work
a
Table 3 Epoxidation of various alkenes with O2
Entry
1
Alkene
Time/h
Solvent
T/ꢁC
Conv.b/%
94
Selec./%
92
3
3
5
DMF
DMF
DMF
50
2
3
100
100
99
96
93
92
4
5
5
5
CH3CN
CH3CN
60
80
85
90
89
91
6
7
6
6
8
DMF
DMF
DMF
100
110
110
88
91
93
92
95
93
8
a
Reaction condition: alkene (0.1 mL), ow of O2. b Determined by GC.
yield (Table 2, entry 1, 4, 7, 10). By contrast, Co/HMS shows As seen from the table there was no distinct change observed
higher conversion and selectivity than entry 3, 5 and 6 in the in conversion, but a slight decrease in yield was observed
same time.
which presents that the catalysts are stable and ve
The catalytic epoxidation of various olens is summarized consecutive cycles without any signicant loss of catalytic
in Table 3. Reactions were performed at the optimized reac- activity.
tion temperature under air in different solvent containing
alkene, oxidant and catalyst. This catalyst efficiently converts
Table 4 Recyclability of catalysta
olens to their corresponding epoxides. Styrene was
converted to styrene oxide in high yield. From entry 1, 2 in
Table 3, it can be found that carbocyclic alkenes are
converted to the corresponding epoxides in high yield, as
well. Although oxidation of the linear alkenes requires longer
reaction times, the conversion and selectivity are high. All in
all, this catalytic system shows a good activity in the case of
different olens.
Catalyst
Co/HMS
Cycle
Conversion (%)
Yieldb (%)
1
2
3
4
5
98
96
95
95
95
92
92
90
88
88
a
Reaction conditions: styrene (0.1 mL), catalyst (30 mg), temperature
b
(100 ꢁC), DMF (5 mL), time (3 hours), ow of O2. Yield based on GC
The catalyst was recycled in order to test its stability as
well as activity. The obtained results are showed in Table 4.
analysis.
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4. Conclusion
We have developed a facile method for the manufacture of
uniform Co/HMS nanoreactor with desired hollow core/meso-
porous shell structures, high catalytic activity and magnetic
recyclability properties. The cobalt nanoparticles are free of
protection ligands and are well-dispersed in mesoporous spheres
with little aggregation. The nanoreactors show prominent cata-
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Acknowledgements
The authors are grateful to Projects in Gansu Province Science
and Technology Pillar Program (1204GKCA047), and the Key
Laboratory of Catalytic engineering of Gansu Province, China
Gansu Province for nancial support.
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