Synthesis and catalytic activity of organic-inorganic hybrid catalysts coordinated with cobalt(II) ions for aerobic epoxidation of styrene

Article abstract of DOI:10.1016/j.catcom.2014.12.007

New organic-inorganic hybrid materials (HM) containing 3-mercaptopropyl groups (-(CH₂)₃-SH) have been synthesized through a dry gel conversion (DGC) route. The complex catalyst Co-HM was prepared through a simple coordination of -SH with cobalt(II) ions, which was firstly applied in the aerobic epoxidation of alkenes to obtain good results. Co-HM-50 exhibited the highest activity for the epoxidation of styrene with air to achieve 95.8 mol% of conversion with the epoxide selectivity of 89.2%. Recycling and control tests showed high durability and heterogeneity of Co-HM-50 as a heterogeneous catalyst.

Full text of DOI:10.1016/j.catcom.2014.12.007

Catalysis Communications 61 (2015) 48–52
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Synthesis and catalytic activity of organic–inorganic hybrid catalysts
coordinated with cobalt(II) ions for aerobic epoxidation of styrene
b
X.-L. Wei a,c, X.-H. Lu a,c, , X.-T. Ma , C. Peng a,c, H.-Z. Jiang a,c, D. Zhou a,c, Q.-H. Xia a,c,
⁎
⁎
a
Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei University, Wuhan 430062, PR China
Hubei Collaborative Innovation Center of Targeted Antitumor Drug, Jingchu University of Technology, Jingmen 448000, PR China
Ministry-of-Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules, Hubei University, Wuhan 430062, PR China
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 October 2014
Received in revised form 3 December 2014
Accepted 7 December 2014
Available online 10 December 2014
New organic–inorganic hybrid materials (HM) containing 3-mercaptopropyl groups (–(CH2)3–SH) have been
synthesized through a dry gel conversion (DGC) route. The complex catalyst Co–HM was prepared through a
simple coordination of –SH with cobalt(II) ions, which was firstly applied in the aerobic epoxidation of alkenes
to obtain good results. Co–HM-50 exhibited the highest activity for the epoxidation of styrene with air to achieve
95.8 mol% of conversion with the epoxide selectivity of 89.2%. Recycling and control tests showed high durability
and heterogeneity of Co–HM-50 as a heterogeneous catalyst.
Keywords:
Organic–inorganic hybrid material
© 2014 Elsevier B.V. All rights reserved.
Co–HM-50
Styrene
Epoxidation
Air
1. Introduction
catalytic activity of transition-metal-coordinated organic–inorganic
hybrid materials for the epoxidation of alkenes. The present work firstly
The catalytic epoxidation of alkenes is very important in the industry
of chemicals and pharmaceuticals, since the epoxides are widely used in
the manufacture of drugs, perfumes, vitamins and optical materials
[1–3]. Many alternative methodologies have been developed for the
epoxidation of alkenes using single oxygen donor reagents, such as
H2O2 [4–9], tert-butyl hydroperoxide (TBHP) [10–12], and PhIO [13,
14]. Because of low cost and environmentally friendly nature, the epox-
idation of olefins using molecular oxygen or air as the oxidant has been
more attractive and challenging [15–18].
In the last decade, some researchers have worked hard in the devel-
opment of efficient heterogeneous catalysts [19,20]. Nonetheless, there
is one key limitation in the use of heterogeneous catalysts, due to low
reaction rates as compared to homogeneous ones. An interesting ap-
proach to obtain heterogeneous catalysts is to prepare metal containing
organic–inorganic hybrid materials, which can simulate the catalytic
properties of homogeneous complexes. This type of hybrid materials
has attracted great interest in the field of catalysis, such as the oxidation
of anthracene [21], sulfides and olefins [22], alcohols [23], thioethers
and thiophenes [24], hydrolysis of oligosaccharides [25], and acid-/
base-catalyzed reactions [26]. To date, no report has approached the
reports the synthesis and catalytic activity of organic–inorganic hybrid
catalysts coordinated with cobalt(II) ions for aerobic epoxidation of
styrene to obtain encouraging results.
2. Experimental
2.1. Materials
The reagents inclusive of NaAlO2 (84.2%), NaOH (AR, N99.0%),
3-mercaptopropyl triethoxysilane (MPTES) and fumed silica were
purchased from China National Medicines Corporation Ltd. TBHP,
DMF and all alkenes were purchased from Aladdin Industrial Corpo-
ration. Olefins were directly used without further purification, but
DMF underwent redistillation under reduced pressure to obtain a purity
of 99.99%.
2.2. Synthesis of HM-x and Co–HM-x
The organic–inorganic hybrid material (HM) was synthesized by a
dry gel conversion (DGC) method, which used a mixture of MPTES
and fumed silica as SiO2 source. As the molar ratio of MPTES to silica
was x: (100-x), the resulting sample was designated as HM-x. Typically,
0.385 g NaAlO2 (84.2 wt.%) and 0.743 g NaOH (96.0 wt.%) were dis-
solved in 30 g of deionized water, followed by the addition of 4.956 g
of MPTES. After the mixture was vigorously stirred at room temperature
⁎
Corresponding authors at: Hubei Collaborative Innovation Center for Advanced
Organic Chemical Materials, Hubei University, Wuhan 430062, PR China.
E-mail addresses: xinhuan003@aliyun.com (X.-H. Lu), xiaqh518@aliyun.com
(Q.-H. Xia).
http://dx.doi.org/10.1016/j.catcom.2014.12.007
1566-7367/© 2014 Elsevier B.V. All rights reserved.

X.-L. Wei et al. / Catalysis Communications 61 (2015) 48–52
49
for 2–3 h, 1.247 g of fumed silica was added into the solution and the
stirring was kept for 2 h. Then, the mixture solution was heated to 80 °C
in a water bath until it turned into dry gel, which was ground into fine
powder and placed in a 30-ml Teflon beaker. The crystallization was per-
formed in a sealed 130-ml stainless steel autoclave at 170 °C for 6 days
under autogenous pressure, where 5 ml of deionized water was poured
into the bottom of the autoclave as the steam source and physically sep-
arated from the Teflon beaker. The solid product was recovered by filtra-
tion, washed several times with hot distilled water until no basic residue
was detected, and followed by drying at 100 °C for 5 h. In order to remove
the excess MPTES, the recovered solids were subjected to the Soxhlet ex-
traction in ethanol under refluxing until no free MPTES was detected from
the extract.
Co–HM-x was prepared by a simple coordination of –SH in HM-x
with Co2+ cations. For example, Co–HM-50 was prepared by dispersing
0.5 g of HM-50 powder into a solution of Co(Ac)2·4H2O (0.106 g) in
25 ml of deionized water, in which the stirring was kept at 90 °C for
12 h. The brown solid was recovered by filtration, washed with deion-
ized water, and dried at 100 °C for 4 h. The whole synthesis process of
the catalyst is shown in Scheme 1. Meanwhile, other transition metal
coordinated catalysts were prepared by the same method.
Fig. 1. XRD patterns of (1) HM-30, (2) HM-40, (3) HM-50, (4) Co–HM-50, (5) Co–HM-50
after being used 5 times, and (6) HM-60.
2.3. Characterization of catalysts
X-ray diffraction (XRD) patterns of samples were recorded on a
Rigaku D/MAX-IIIC diffractometer with CuKα radiation (λ = 1.54184 Å)
from 2θ = 5 to 60° with a scanning speed of 2.0°/min. The morphology
and size of crystals were imaged on a JEOL JSM-6510A scanning electron
microscope (SEM). X-ray fluorescence (XRF) analyses (Co, Si, Al and
S) were conducted on a Shimadzu XRF-1800 spectrometer. The X-ray
photoelectron spectroscopy (XPS) spectra were recorded on a VG
ESCALAB MK II spectrometer, using a monochromatic Mg–Kα radiation,
operating at a constant transmission energy pass (80 eV). UV–Vis spec-
tra of samples were determined on a Shimadzu UV-2550 spectrometer.
Laser Raman spectra (LRS) were measured on a Renishaw InVia Confo-
cal Raman microscope.
2.4. Typical procedure for epoxidation reaction
In a typical run, an appropriate amount of catalyst and styrene
(3 mmol), 10 g of solvent and a small amount of TBHP (0.3 mmol) were
mixed in a 50-ml two-necked flask equipped with a cryogenic-liquid
condenser and with an air pump under magnetic stirring and heated
SH
SH
HS
SH
HS
dry gel
power
SH
SH
SH
170oC, 6 d
HS
HS
HS
S
H2O
HS
HS
S
HS
HM-x
Ac
S
Ac
Co
S
S
S
.
SH
SH
SH
HS
(1)Co(Ac)2 4H2O,
HS
(2)ethanol, reflux
HS
HS
HS
S
S
HS
Co-HM-x
Scheme 1. The synthesis process and structure of HM-x and Co–HM-x.

50
X.-L. Wei et al. / Catalysis Communications 61 (2015) 48–52
Table 1
moles of styrene oxide formed
moles of styrene reacted
Epoxide selectivityð%Þ ¼
ꢀ 100:
Physicochemical data of hybrid HM and Co–HM samples.
Catalyst
S content
(wt.%)
Co content
(wt.%)
SiO2/Al2O3
S/Co
Molar
ratio
Molar
ratio
3. Results and discussion
3.1. Catalyst characterizations
HM-30
Co–HM-30
HM-40
Co–HM-40
HM-50
Co–HM-50
Co–HM-50 (5 recycles)
HM-60
13.27
13.09
14.72
14.62
15.45
15.18
15.06
15.59
15.27
0
26.33
26.46
24.57
24.61
23.37
23.38
24.67
22.17
22.20
0
1.489
0
1.679
0
1.780
1.719
0
16.21
0
16.06
0
15.73
16.15
0
The XRD patterns of catalysts are shown in Fig. 1, from which the
analcime (ANA) structure is observed. This ANA structure belongs to
the cubic crystal system, for which the positions and relative intensities
of all diffraction peaks match well with those from the Joint Committee
on Powder Diffraction Standards (JCPDS, card number 41-1478). In
addition, the coordination treatment by Co2+ cations does not change
the structure of the parent materials. The compositions of the parent
HM and Co–HM samples are determined by XRF and listed in Table 1.
The molar ratio of S/Co ranges from 15.27 to 16.21, indicative of the
abundance of thiol ligands.
UV–Vis spectra of HM-50 and Co–HM-50 are shown in Supplemen-
tary Fig. S1. Clearly, there are recognizable two new bands at 367 and
678 nm for Co–HM-50. X-ray photoelectron spectroscopy (XPS) spectra
were measured to characterize chemical state of Co species. As shown in
Supplementary Fig. S2, the peaks of 781.28 and 796.98 eV can be
attributed to Co2p, indicating that the majority of Co species in Co–
HM-50 exists in the form of Co2+ cations with different coordination
Co–HM-60
1.843
15.27
to desired temperatures. Then dry air of 40 ml/min was introduced by
bubbling into the bottom of reactor at atmospheric pressure. Several
hours later, the reaction was terminated and the solid catalyst was fil-
tered off. The liquid filtrate was quantitatively analyzed by GC-9800,
and the catalyst was washed with deionized water and ethanol, and
dried at 80 °C for 3 h, the catalyst could be used for the next catalytic
recycle.
moles of styrene reacted
total moles of styrene
Styrene conversionðmol%Þ ¼
ꢀ 100:
Fig. 2. SEM images of (a) HM-30, (b) HM-40, (c) HM-50, (d) Co–HM-50, (e) Co–HM-50 after being used 5 times, and (f) HM-60.

X.-L. Wei et al. / Catalysis Communications 61 (2015) 48–52
51
Table 3
environments [27]. As shown in Supplementary Fig. S3, the shoulder
line at 650 cm−1 in the LRS spectrum corresponds to internal vibrations
of C\S bonds. For Co–HM-50, the intensity of this shoulder peak is
significantly weaker than that of HM-50, further revealing the coordina-
tion of organic –SH groups with cobalt ions. The scanning electron
micrographs (SEM) of samples in Fig. 2 indicate the irregular morphol-
ogies of samples composed of the particles with the size of 300 nm–
1 μm. TG curves of HM and Co–HM materials are illustrated in Supple-
mentary Fig. S4, in which the weight loss between room temperature
and 350 °C was due to the evaporation of water and the decomposition
of organics in the materials, and that between 350 and 700 °C was at-
tributable to the combustion of organics occluded in the molecular
sieve.
Effect of different metals on the epoxidation of styrene.
Reaction conditions: styrene (3 mmol), DMF (10 g), catalyst (0.1 g), initiator (TBHP,
0.3 mmol), time (5 h), temperature (90 °C), and flow rate of air (40 ml/min).
Catalyst
Conv. (mol%)
Selectivity (%)
Epoxide
Phen.
Ben.
Others
Co–HM-50
Cr–HM-50
Mn–HM-50
Fe–HM-50
Ni–HM-50
Cu–HM-50
Zn–HM-50
95.8
10.0
7.9
16.8
9.4
89.2
75.8
79.2
73.5
69.3
80.0
72.6
3.4
1.8
0.9
0
0
0.7
1.2
4.7
16.5
18.9
20.1
24.8
16.5
19.9
2.7
5.9
1.0
6.4
5.9
2.8
6.3
14.9
7.5
3.2. Catalytic epoxidation of styrene with air
indicative of the initiating effect of TBHP on the titled reaction, as has
been reported by other works [28,29].
3.2.1. Effect of different catalysts and reaction conditions
The effect of various solvents on the epoxidation of styrene with air
is shown in Table S2 (Supplementary data). Obviously, the solvent plays
an important role in the epoxidation reactions. The conversion of
styrene decreases in the sequence of DMF (95.8 mol%) N 1,4-dioxane
(45.6 mol%) N MIBK (41.6 mol%) N acetylacetone (12.8 mol%) N toluene
(10.4 mol%) N pyridine (0). After the reaction was completed, we quan-
titatively detected that only 0.1–0.2% of DMF was oxidized, indicating
that DMF was not a real reductant in this reaction. Our previous studies
by means of UV spectrum and cyclic voltammetry (CV) analyses, had re-
vealed the coordination of DMF to Co(II) [31]. Such a coordination could
affect the binding ability of O2 to Co(II) as well as their redox potential,
further leading to the activation of O2 in the epoxidation.
The influence of reaction temperature on the epoxidation of styrene
over Co–HM-50 is depicted in Supplementary Fig. S5. The conversion
and selectivity gradually increase with the rise of temperature from 50
to 90 °C. At 90 °C, the optimal activity is achieved with 95.8 mol% con-
version of styrene and 89.2% selectivity of epoxide. However, at 100 °C
the selectivity of epoxide drops to 85.2%. This means that too high tem-
perature will accelerate the oxidation of benzaldehyde to benzoic acid,
leading to the drop of epoxide selectivity.
The catalytic activity of different Co–HM-x samples for the epoxida-
tion of styrene is compared in Table 2. These catalysts are considerably
active for the titled epoxidation, achieving more than 80.6 mol% conver-
sion of styrene, notably higher than those of the parent HM (7.8 mol%)
and the blank test (7.2 mol%). The conversion of styrene decreases in the
order of Co–HM-50 (95.8 mol%) N Co–HM-60 (91.4 mol%) N Co–HM-40
(87.2 mol%) N Co–HM-30 (80.6 mol%), similar to the decreasing se-
quence of the epoxide selectivity: Co–HM-60 (89.4%) ≈ Co–HM-50
(89.2%) N Co–HM-40 (88.9%) N Co–HM-30 (84.5%). This arrangement
can be correlated with the cobalt content (in Table 1) and the high crys-
tallinity of HM-50 (in Fig. 1). As shown in Table 3, among all the transi-
tion metal catalysts Co–HM-50 exhibits the best activity with 95.8 mol%
conversion and 89.2% selectivity of epoxide. Note that even if pure
Co(Ac)2·4H2O is used as the catalyst, only 16.0 mol% conversion is
achieved. When comparing the UV–Vis spectra of HM-50, Co–HM-50
and Co(Ac)2·4H2O, we can observe that after the coordination, the
d–d transition absorption peak of Co shifts from 509 nm to 367 and
678 nm, due to the change of Co coordination environment. Thus, elec-
tronic transition of cobalt(II) ions becomes more active in the oxidation
reaction to result in an outstanding performance in the epoxidation of
styrene. Other transition metal-containing HM-50 catalysts merely
achieved low conversions of b16.8 mol%, which could be explained by
the difference of the ability to activate molecular oxygen in agreement
with our previous reports [28–30].
Table S1 (Supplementary data) presents the effect of various
oxidants on the epoxidation of styrene with dry air over Co–HM-50 at
90 °C. Almost no oxidation occurs with NaClO or under inert N2 atmo-
sphere without any oxidant. The oxidants like H2O2, TBHP and NaIO4
show very low activities for the epoxidation of styrene, for which
16.8–20.4 mol% conversions and 60.6–82.8% selectivities are achieved.
However, when only dry air is used as unique oxidant, 65.9 mol% con-
version of styrene and 79.6% selectivity of epoxide are obtained, higher
than those obtained by usual oxidants. When air is used as the oxidant
together with a small amounts of TBHP as the initiator, 95.8 mol% con-
version of styrene and 89.2% selectivity of epoxide can be harvested,
3.2.2. Recycling studies of Co–HM-50
Catalyst recycling experiments were performed with the repeated
uses of Co–HM-50 at 90°C. It can be seen from Fig. 3 that when
Co–HM-50 is reused five times, the conversion of styrene is kept almost
unchanged, while the selectivity of epoxide maintains a small fluctua-
tion of 86.9–89.2%. XRD patterns testified that the recovered catalyst
that had been used 5 recycles still retained the structure of fresh sample
(Fig. 1), but the intensity of peaks decreased, probably due to the
removal of partial Al species from the framework of zeolite during
recycling, further leading to the reduction of Co content from 1.780 to
1.719% a little bit.
100
80
60
40
20
0
100
80
60
40
20
0
Conv.
Sele.
Table 2
Effect of different materials on the epoxidation of styrene.
Reaction conditions: styrene (3 mmol), DMF (10 g), catalyst (0.1 g), initiator (TBHP,
0.3 mmol), time (5 h), temperature (90 °C), and flow rate of air (40 ml/min).
Catalyst
Conv. (mol%)
Selectivity (%)
Epoxide
Phen.
Ben.
Others
No catalyst
Co(Ac)2·4H2O
HM-50
Co–HM-30
Co–HM-40
Co–HM-50
Co–HM-60
7.2
16.0
7.8
80.6
87.2
95.8
91.4
75.8
83.4
79.2
84.5
88.9
89.2
89.4
2.4
4.8
1.7
4.6
3.6
3.4
3.2
13.6
6.8
10.4
6.0
5.0
4.7
8.2
5.0
8.7
4.9
2.5
2.7
2.9
0
1
2
3
4
5
Number of recycle
4.5
Fig. 3. Recycling studies of catalysts.

52
X.-L. Wei et al. / Catalysis Communications 61 (2015) 48–52
Table 4
Catalytic epoxidation of various alkenes over Co–HM-50.
Reaction conditions: alkene (3 mmol), solvent (10 g), catalyst (0.1 g), initiator (TBHP, 0.3 mmol), time (5 h), temperature (90 °C), and flow rate of air (40 ml/min).
Run
Alkene
Conv. (mol%)
Selectivity (%)
Epoxide
89.2
Others
10.8
1
2
3
95.8
85.5
93.4
78.3
88.0
21.7
12.0
4
22.0
96.0
4.0
5
6
0
–
–
4.0
100
0
In order to further understand the heterogeneity of catalyst, the
control experiments were conducted over Co–HM-50 as shown in
Supplementary Fig. S6 [20]. After aerobic epoxidation of styrene was
initially conducted for 1 h, the obtained conversion of styrene was
16.3 mol%; then solid catalyst was filtered off and the reaction with
the resulting filtrate was continued for another 4 h. Finally, the conver-
sion was analyzed by GC to be 25.0 mol% with only a small increment of
8.7 mol%. However, in the normal reaction of 5 h the conversion and
selectivity could reach 95.8 mol% and 89.2%. Above studies reveal the re-
cyclable stability and heterogeneity of organic–inorganic hybrid catalyst
Co–HM-50.
and the 2014 Sci-tech Support Project of the Science and Technology
Department of Hubei Province of China (No. 2014BAA098) for the finan-
cial support.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2014.12.007.
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The authors thank the National Natural Science Foundation of China
(Nos. 21173073, 21273064), the key project of the Education Depart-
ment of Hubei Province (D20141004), the Natural Science Fund for
Creative Research Group of Hubei Province of China (2014CFA015),