Gold on thiol-functionalized magnetic mesoporous silica sphere catalyst for the aerobic oxidation of olefins

Article abstract of DOI:10.1016/j.molcata.2014.04.032

We have synthesized a new catalyst based on thiol-functionalized silica-coated magnetic mesoporous nanocrystals as support and Au nanoparticles as active sites through a facile and environment-friendly approach and characterized by transmission electron microscopy (TEM), N₂ adsorption-desorption, elemental analysis and inductive coupled plasma atomic emission spectrometer (ICP-AES), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometry (VSM). The synthesized catalyst exhibited high catalytic activity in the oxidations of cyclohexene and styrene by molecular oxygen at atmospheric pressure. Furthermore, this catalyst had an excellent recyclability, evidenced by being extensively reused for five times without any substantial loss of activity, which was mainly attributed to the enough strong combination between the Au nanoparticles and thiol groups onto the surface of support. Meanwhile, the catalyst could be easily separated from the reaction solution by applying an external magnetic field.

Full text of DOI:10.1016/j.molcata.2014.04.032

Accepted Manuscript
Title: Gold on thiol-functionalized magnetic mesoporous
silica sphere catalyst for the aerobic oxidation of olefins
Author: Yiyun Fang Yuanzhe Chen Xinzhe Li Xingchun
Zhou Jing Li Weijie Tang Jingwei Huang Jun Jin Jiantai Ma
PII:
S1381-1169(14)00170-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2014.04.032
MOLCAA 9094
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
8-1-2014
5-4-2014
26-4-2014
Please cite this article as: Y. Fang, Y. Chen, X. Li, X. Zhou, J. Li, W. Tang, J. Huang,
J. Jin, Gold on thiol-functionalized magnetic mesoporous silica sphere catalyst for
the aerobic oxidation of olefins, Journal of Molecular Catalysis A: Chemical (2014),
http://dx.doi.org/10.1016/j.molcata.2014.04.032
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Gold on thiol-functionalized magnetic mesoporous silica sphere
catalyst for the aerobic oxidation of olefins
Yiyun Fang, Yuanzhe Chen, Xinzhe Li, Xingchun Zhou, Jing Li, Weijie Tang,
Jingwei Huang, Jun Jin* and Jiantai Ma*
The Key Laboratory of Catalytic engineering of Gansu Province, College of
Chemistry and Chemical Engineering Lanzhou University, Lanzhou 730000, P. R.
China.
Abstract:
We have synthesized a new catalyst based on thiol-functionalized silica-coated
magnetic mesoporous nanocrystals as support and Au nanoparticles as active sites
through a facile and environment-friendly approach and characterized by transmission
2
electron microscopy (TEM), N adsorption-desorption, elemental analysis and
inductive coupled plasma atomic emission spectrometer (ICP-AES), X-ray powder
diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and vibrating sample
magnetomety (VSM). The synthesized catalyst exhibited high catalytic activity in the
oxidations of cyclohexene and styrene by molecular oxygen at atmospheric pressure.
Furthermore, this catalyst had an excellent recyclability, evidenced by being
extensively reused for five times without any substantial loss of activity, which was
mainly attributed to the enough strong combination between the Au nanoparticles and
thiol groups onto the surface of support. Meanwhile, the catalyst could be easily
seperated from the reaction solution by applying an external magnetic field.
Key words: thiol-functionalized magnetite mesoporous silica; Au nanoparticles;
1
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recyclable catalyst; oxidation of cyclohexene and styrene.
*Corresponding author:
E-mail: jinjun@lzu.edu.cn (J.Jin); majiantai@lzu.edu.cn (J. Ma);
Tel.: +86-931-8912577;
Fax: +86-931-8912582
2
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1. Introduction
It is well known that the oxidation of olefins is of significant importance in modern
chemical transformations [1-4]. Traditionally, the organic or inorganic oxidants
TBHP or KMnO ) are commonly used in the oxidation reaction. However, they often
(
4
generate voluminous amounts of environmentally undesirable waste [5-8]. Hence,
using a safer and cleaner oxidizing agent is of great practical importance. Molecular
oxygen, which is universally accepted as a green, safe, clean and economic oxidant, is
strongly desirable for a long time. However, it is still a challenge that efficient
utilization of molecular oxygen in olefin oxidations at atmospheric pressure [9-11].
Recently, many metal (Cu, Co and V) compounds are active homogeneous
catalysts for olefin oxidations [12-14]. Although the homogeneous catalysts are
remarkably efficient, they share a universal handicap: separation, catalyst recycling
and product contamination. Therefore, the design of efficient and recoverable
catalysts has become an important issue. One approach to such an environmentally
benign process is based on the development of heterogeneous catalysts because these
heterogeneous systems are easy to handle, recover and are “green” processes [15, 16].
Generally, attempts included the use of biphasic systems or the immobilization of
catalysts on solids, have been taken to improve catalyst recovery and recycling
[17, 18]. In such circumstances, systems that use silica-coated magnetic mesoporous
nanoparticles have drawn attention. These systems, which are comprised of
magnetically recoverable solid supports, have unique physical properties including the
3
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ordered pore arrangement, high surface area and superparamagnetism, can be easily
and quickly removed from the reaction medium [19-21]. Nevertheless, a key
challenge is to prevent their agglomeration and enhance the stability. To address these
2
problems, metal affinity groups, such as -NH , -SiH, -SH groups have been
post-grafted onto the surface of catalyst support to stabilize and disperse metal
nanoparticles [22-25].
More recently, Au nanoparticles are widely used in a series of reactions due to their
unique physical and chemical properties [26-30]. Particularly, olefin oxidations over
supported Au nanoparticles using molecular oxygen have attracted much attention.
Inspired by the work of Xiao and co-workers, who showed that Au/SBA-15-Py
catalyst exhibit superior catalytic properties in the oxidation of cyclohexene and
styrene [31], we began a study into the immobilization of Au on a thiol-modified
silica-coated magnetic support with excellent catalytic and separation properties. It
was found that the catalyst exhibited prominent catalytic properties in olefin
oxidations with molecular oxygen at atmospheric pressure.
2. Experimental section
2.1 Synthesis of oleic acid coated magnetic nanoparticles (MNs)
Fe
FeCl
3
O
4
nanoparticles were prepared by co-precipitation method [32]. First, 4.8 g of
3
·6H O (17.7 mmol), 2.0 g of FeCl ·4H O (10.1 mmol) and 1 mL oleic acid were
2
2
2
dissolved in 30 mL deionized water under nitrogen with vigorous stirring at 363 K.
Then, 10 mL of ammonium hydroxide (26 wt%) was added rapidly to adjust the pH
value about 9, and it immediately turned black. After 2.5 h, the black precipitate was
4
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collected with a permanent magnet and resuspended in chloroform with an end
concentration of 10 mg/mL oleic acid-capped Fe
.2 Synthesis of magnetite nanocrystals embedded in mesoporous silica spheres
MSS)
The preparation route of magnetite nanocrystals embedded in mesoporous silica
3
4
O .
2
(
spheres (MSS) was according to reference [33]. Briefly, 100 mg of oleic acid
stabilized MN dispersed in 10.0 mL of chloroform was added to 50 mL of aqueous
solution containing 2.0 g of cetyltrimethylammoniun bromide (CTAB, 5.5 mmol).
The resulting solution was stirred for several hours and a homogeneous oil-in-water
microemulsion was obtained. After removing chloroform using a rotary evaporator,
about 50 mL aqueous phase dispersed nanoparticles was generated. Then the resulting
aqueous solution was diluted with 500 mL of deionized water. And then 20 mL of
ethyl acetate (204.7 mmol), 12 mL of ammonia solution (80.6 mmol), and 2.0 mL of
tetraethoxysilane (TEOS, 9.0 mmol) were successively added to the diluted aqueous
solution containing the magnetite nanoparticles. The mixture was continuously stirred
for 5 h at room temperature, then the resulting products were collected by
centrifugation and then washed with water and ethanol for 5 times and dried under
vacuum at 50 °C overnight. Finally, the Fe
silica spheres (MSS) were obtained by the calcination at 550 °C for 6 hours.
.3 Synthesis of thiol-modified silica-coated magnetic mesoporous nanocrystals
3 4
O nanocrystals embedded in mesoporous
2
(MSS-SH)
The functionalization of the MSS with thiol was performed by adding 1 mL of
5
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3-mercaptopropyltrimethoxysilane (MPS) to 100 mL dry toluene that contained 1 g
MSS nanoparticles. The resulting solution was heated at reflux for 24 h and then
washed with acetone and ethanol. The obtained solid material was dried under
vacuum at 50
ꢀor 24 h.
2.4 Loading of Au on thiol-modified silica-coated magnetic mesoporous nanocrystals
0
(
MSS-SH-Au )
In a typical synthesis, 1 g of above-synthesized MSS-SH were dispersed in 100 mL
of deionized water by sonication for 30 min and then 6.0 mL of chloroauric acid
24mM) was added dropwise with vigorous stirring at room temperature. After
(
stirring for 12 h, the resulting precipitate was collected by an external magnet and
washed 3 times with deionized water and 3 times with ethanol. And then, the product
was dried under vacuum at 50
overnight. Finally, the catalyst was reduced directly
by hydrogen in autoclave for 6 h at 80 ℃. Thereafter, the material was washed twice
with distilled water and acetone and dried under vacuum (Scheme 1). The content of
0
Au in MSS-SH-Au as estimated by inductively coupled plasma atomic emission
spectroscopy (ICP-AES) analysis, was 2.50 wt%.
2.5 Catalytic tests
Olefin oxidations were carried out in a 25 ml glass reactor equipped with a reflux
condenser. Typically, substrate, solvent, and catalyst were mixed in the reactor, then
the system was evacuated by a vacuum pump, and then pure O was introduced and
2
sealed in the reaction system at atmosphere pressure by a three direct links. And then
the mixture was stirred (1000rpm) and the temperature was increased to 373 K (the
6
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temperature was measured with a thermometer in an oil bath). After reaction, the
products were analyzed by GC (P.E. AutoSystem XL) or GC–MS (Agilent 6890
N/5973 N). The recyclability of the catalyst was tested by separating it from the
reaction system with a magnet, and then, washing with large quantity of ethanol, and
drying at 50
ꢀor 6 h, then the catalyst was reused in the next reaction.
2.6 Characterization
The morphology and chemical composition of the catalyst were characterized by
Tecnai G2 F30 transmission electron microscopy (TEM). Au contents of the catalyst
were measured by inductively coupled plasma atomic emission spectroscopy
2
(ICP-AES). The low-temperature N adsorption–desorption experiments were carried
out using a Tristar II 3020. The pore diameter distributions were calculated from
desorption branches using the BJH (Barrett-Joyner-Halenda) methods, and the surface
areas were calculated using the BET (Brunauer-Emmett-Teller) method based on the
desorption isotherms. XRD measurement was performed on a Rigaku D/max-2400
diꢀꢀractometer, using CuKα radiation as the X-ray source in the 2θ range oꢀ 10°-80°.
The magnetic measurement was done using a Quantum Design vibrating sample
magnetometer (VSM). 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.
3. Results and discussion
3.1 Catalysts characterization
The morphologies and structural features of the supported material MSS and
7
Page 7 of 24

0
as-synthesized catalyst MSS-SH-Au could be observed directly through TEM and
HRTEM images (Fig. 1). As was shown in Fig.1a, each magnetic particle was
composed of plentiful nanocrystals. Besides, the catalyst had obvious disordered
mesostructure derived from CTAB-templating. The average diameter of the support
spheres was about 130 nm. The TEM image in Fig. 1b revealed that the MSS-SH-Au
catalyst didn’t change considerably after attachment of the Au onto the surface of the
MSS. In addition, Au NPs with similar sizes of 5 nm were well dispersed in the MSS.
From the Fig. 1c, it could be seen that the resolved lattice fringes of (111) planes (d=
0.235 nm) detected in the HRTEM image were attributed to Au nanocrystals,[34].
0
The elemental composition of the MSS-SH-Au samples was determined by EDX
analysis. The result shown in Fig. 2 reveals that the as-prepared products contain Fe,
Si, Au, S, Cu, C and O. Among these elements, Cu C and O are generally influenced
by the copper network support films and their degree of oxidation, Si, O, Fe, S and Au
signals result from the MSS-SH and Au particles which form the products.
In order to obtain the structural information about the catalyst on a mesoscopic
2
scale, N adsorption-desorption was used to characterize the catalyst. The MSS had a
2
3
0
specific surface area 694.5 m /g and pore volume 0.73 cm /g. The MSS-SH-Au had a
2
3
specific surꢀace area 241.5 m /g and pore volume 0.18 cm /g (Table 1). It was
worthwhile to note that the specifc surface area and pore volume of the parent MSS
underwent a significant decrease after grafting thiol group and Au nanparticles.
The XRD patterns of the prepared MSS and MSS-SH-Au were shown in Fig. 3a-b.
As shown in Fig. 3a, a broad band at around 2θ = 22° (*) was derived from the
8
Page 8 of 24

amorphous mesoporous silica spheres and the characteristic peaks of (220), (311),
400), (511) and (440) were typical of a spinel magnetite structure. As presented in
Fig.3b, the new peaks with 2θ at 38°, 44°, 65° and 78°, relating to the (111), (200),
220) and (311) crystal faces of Au nanoparticles respectively, and mostly
(
(
corresponding to face-centered cubic metallic gold diffraction [35]. No peak was
observed to be connected with gold chloride salt crystalline phase, indicating that the
reduction of chloroauric acid was completed during the preparing process. This result
was confirmed by XPS data.
0
The chemical composition of the obtained MSS-SH-Au nanomaterial was detected
from the XPS spectra (Fig. 4). From the wide-scan XPS spectrum, the existence of the
S2p (172.4 eV) provided an evidence for the successful introduction of thiol groups
onto the surface of MSS. The XPS signature of Au 4f doublet for the Au nanoparticles
supported on MSS was given in the inset figure. The Au 4f7/2 and Au 4f5/2 peaks
appeared at 84.7 and 88.3 eV respectively were the characteristic peaks for the
0
metallic Au [36].
0
The magnetization curves for MSS and MSS-SH-Au were compared in Fig. 5.
There was no hysteresis in the magnetization for the two types of nanoparticles.
Neither coercivity nor remanence was observed, which suggested that these
nanoparticles were superparamagnetic. It could be seen that the magnetic saturation
values of MSS and MSS-SH-Au were 9 and 3 emu/g, respectively. The decrease in
0
saturation magnetization of MSS-SH-Au could be attributed to the increase of
organic matter and Au. Even with this reduction in the saturation magnetization, the
9
Page 9 of 24

solid catalyst could still be efficiently separated from solution with a permanent
magnet. Meanwhile, the inset figure also showed the photograph of the catalyst was
pulled magnetically.
3.2 Catalytic reaction
0
The MSS-SH-Au was used as a catalyst for the oxidantions of styrene and
cyclohexene. Table 2 presented catalytic data in cyclohexene oxidation by oxygen at
atmospheric pressure using various solvents. In toluene solvent, the catalyst showed
conversion at 61% (Table 2, entry 1). In N,N-dimethylformamide solvent, the catalyst
showed conversion at 44% (Table 2, entry 2). In acetonitrile solvent, it gave low
activity (28%, Table 2, entry 3). In addition, it was also observed that the solvent
strongly influences the catalytic activity. When different oxidants were used, TBHP
gave reasonably good performance for the oxidantion of cyclohexene, the conversion
was 87% and the selectivity for cyclohexene epoxide reached to 21% (Table 2, entry
4
), while low conversion was observed when H
Table 3 showed the catalytic performances for the oxidantions of styrene with
different solvents, oxidants, and amount of catalyst. Generally, styrene is not easy to
be oxidized using molecular oxygen on Au catalysts [37]. In the case of O , the
catalyst exhibited the conversion at 35% (Table 3, entry 1). If t-butyl hydroperoxide
TBHP) was present, the catalyst exhibited the conversion at 85% (Table 3, entry 2).
Low conversion was observed when the reaction was carried out using hydrogen
peroxide as oxidants (Table 3, entry 3). The rapid decomposition of H into O over
the catalysts at the initial stage would be the main reason for the lower activity. As
O was used as oxidant (Table 2, entry 5).
2 2
2
(
2
O
2
2
10
Page 10 of 24

same as cyclohexene oxidation, in toluene solvent, the catalyst exhibited the good
performance while in acetonitrile solvent, the catalyst showed conversion at zero
(Table 3, entry 4). There were many factors that may cause the low conversion. We
considered that the active centre was poisoned by coordination of the hydroperoxides, which
may be the most important factor. In order to verify this speculation, the catalyst was separated
from the reaction after the oxidation of styrene in acetonitrile solvent, and washed with acetone
and ethanol thoroughly. And then, the product was dried under vacuum at 50°C overnight. IR
spectroscopy was used to investigate it. Fig. 7 gave the FT-IR spectra of the catalyst before and
-1
after use. The characteristic bands of Fe–O at 590.5 and 456.6 cm were attributed to the Fe–O
stretch bands. The data were consistent with the values reported for Fe in the literature [38].
Two strong bands at about 1084.4 and 802.3 cm were assigned to v (Si–O–Si) and ν (Si–O–Si),
3
O
4
-1
as
s
-1
respectively [39]. Two strong bands at 3428.2 and 1635.2 cm were assigned to water stretches
and bends[40]. 3-mercaptopropyltrimethoxysilane (MPS) displayed characteristic –CH stretching
2
-1
bands at 2935.8 cm . After the oxidation of styrene in acetonitrile solvent, the new band at 2358.8
-1
cm appears which was assigned to the C≡N stretching. This result indicated that acetonitrile
irreversibly adsorb on the surface of catalyst even after careful washing. The catalytic active sites
were occupied by the cyanogroup, which lead to the catalyst was poisoned. Thus, the catalyst gave
unsatisfactory performance in acetonitrile solvent.
Meanwhile, it could be seen, the use of solvent in styrene oxidation not only
influences the activity but also changes the product selectivity. For example, in
toluene solvent, the selectivity for styrene epoxide is 14% (Table 3, entry 2). When
N,N-dimethylformamide solvent was used, the selectivity for styrene epoxide reaches
11
Page 11 of 24

to 16% (Table 3, entry 5).
The effects of the amount of catalyst were shown in Table 3 (entry 6). Increasing
the amount of catalyst from 40 to 70 mg enhanced the conversion from 55% to 87%.
As can be seen, with the increase of the amount from 60 to 70 mg, there was no
obvious increase of the conversion. Therefore, all epoxidation reactions were carried
out using 60 mg of catalyst.
3.3 Magnetically recoverable gold-based catalyst
0
To further investigate the stability of MSS-SH-Au catalyst, a series of catalyst
recycle and reuse experiments for the cyclohexene and styrene oxidation were
conducted, the catalyst could be easily recycled for further use because of its
magnetism. After recycles for five times, the catalyst still remained its activity,
indicating its excellent recyclability (Fig. 6).
4. Conclusion
In conclusion, a novel gold-based catalyst supported on thiol-functionalized
mesoporous silica-coated magnetite nanoparticles was successfully prepared and Au
0
nanoparticles are highly dispersed in the mesopores of MSS-SH-Au . These
ligand-modified Au MNPs are highly efficient and stable heterogeneous catalyst for
oxidantions of cyclohexene and styrene by molecular oxygen at atmospheric pressure.
Moreover, the catalyst could be recovered in a facile manner from the reaction
mixture without any significant loss in activity, which was mainly attributed to the
small size of Au nanoparticles and the thiol groups onto the surface of MSS
nanospheres.
12
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Acknowledgments
This research was supported by the National Science Foundation of China (No. 213
45003), Gansu Province for nancial support, the FundamentalResearch Funds for the
Central Universities (Lzujbky-2013-67).
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Figure Captions
Scheme 1. Synthesis of the catalyst MSS-SH-Au .
0
0
Fig. 1 TEM images of (a) MSS, (b) MSS-SH-Au and (c) HRTEM image of the Au
nanoparticles crystal structure in detail on MSS.
0
Fig. 2 EDX spectrum of the MSS-SH-Au .
0
Fig. 3 XRD patterns of (a) MMS and (b) MSS-SH-Au .
0
Fig. 4 XPS wide-scan spectrum of the MSS-SH-Au (inset: high-resolution spectrum
of Au 4f).
0
Fig. 5 Magnetic curves of MSS (a) and MSS-SH-Au (b).
Fig. 6 Recyclable data in: (a) cyclohexene and (b) styrene oxidations over
0
MSS-SH-Au catalyst.
0
0
Fig. 7 FT-IR spectras of (a) MSS-SH-Au and (b) MSS-SH-Au after use in
acetonitrile solvent.
0
a
Table 1 Textural Parameters of MMS and MSS-SH-Au samples .
Table 2 Catalytic date in cyclohexene oxidation by molecular oxygen over
0
a
MSS-SH-Au catalyst.
0
Table 3 Catalytic date in styrene oxidation by molecular oxygen over MSS-SH-Au
a
catalyst.
17
Page 16 of 24

Scheme 1
Fig. 1
18
Page 17 of 24

Fig. 2
Fig. 3
19
Page 18 of 24

Fig. 4
Fig. 5
20
Page 19 of 24

Fig. 6
Fig. 7
2
3
Sample
S
BET (m / g)
V(cm /g)
D (nm)
4.97
MSS
694.5
241.5
0.73
0.18
0
MSS-SH-Au
3.69
a
SBET the BET surface area, V the total pore volume, D the average
porediameter calculated using the BJH method.
Table 1
21
Page 20 of 24

b
Entry
Solvent
Oxidant
Conversion
%)
Product selectivity (%)
(
P1
14
10
9
P2
4
P3
63
43
35
36
40
P4
19
28
49
28
26
1
2
3
4
Toluene
DMF
O
2
61
44
28
87
46
O2
19
7
MeCN
Toluene
Toluene
O
2
TBHP
H O
21
13
15
21
5
2
2
a
6
mmol of cyclohexene, 10 ml of solvent, 60 mg of catalyst, time for 8 h, temperature of oil
bath at 373 K. Oxidant amount, t-butyl hydroperoxide (TBHP, 3 mol% based on
substrate）; hydrogen peroxide（H O ,3 mol% based on substrate）.
2
2
b
P1, cyclohexene epoxide; P2, 2-cyclohexen-1-ol; P3, 2-cyclohexen-1-one; P4, CO
2
and other
organic products with less than six C atoms.
Table 2
22
Page 21 of 24

b
Entry Solvent Oxidant Amout of catalyst Conversion
Product selectivity (%)
(
mg)
(%)
35
85
23
0
P1
3
P2
71
55
66
-
P3
20
9
P4
6
1
2
3
4
5
6
7
Toluene
O
2
60
60
60
60
60
40
50
70
Toluene TBHP
14
7
22
16
-
Toluene
MeCN
DMF
H
2
O
2
11
-
TBHP
TBHP
-
43
55
71
87
16
9
34
40
49
53
22
10
7
28
41
32
21
Toluene TBHP
Toluene TBHP
Toluene TBHP
12
13
8
13
a
6
mmol of styrene, 10 ml of solvent, time for 8 h, temperature of oil bath at 373 K. Oxidant
amount, t-butyl hydroperoxide (TBHP, 3 mol% based on substrate）; hydrogen peroxide
H O ,3 mol% based on substrate）.
（
2
2
b
P1, styrene epoxide; P2, benzaldehyde; P3, acetophenone; P4, phenylacetic acid, benzoic
acid, ester, and some others.
Table 3
23
Page 22 of 24

24
Page 23 of 24

Highlights
0
(
1) A facile method was put forward to prepare the catalyst MSS-SH-Au .
(2) The catalyst had mesoporous structure and Au NPs were well dispersed in the
MSS.
3) The catalyst showed high activity in oxidations of cyclohexene and styrene by
oxygen.
(
(4) The catalyst was easily recovered with external magnetic field.
(5) The catalytic could be reused for five times without any obvious loss of activity.
25
Page 24 of 24