One-pot synthesis of gold nanoparticles embedded in silica for cyclohexane oxidation

Article abstract of DOI:10.1039/c2cy20692g

Highly-dispersed gold nanoparticles embedded in amorphous silica (Au/M-SiO₂) were prepared by a facile one-pot process utilizing thioether functional groups to anchor AuCl₄^-. A surprisingly high TOF up to 21097 h^-1

Full text of DOI:10.1039/c2cy20692g

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One-pot synthesis of gold nanoparticles embedded in
silica for cyclohexane oxidation
Cite this: Catal. Sci. Technol., 2013,
3, 1123
Cuihua Wang, Lifang Chen* and Zhiwen Qi*
Highly-dispersed gold nanoparticles embedded in amorphous silica (Au/M-SiO2) were prepared by a
facile one-pot process utilizing thioether functional groups to anchor AuCl4À. A surprisingly high TOF
up to 21 097 hÀ1 was obtained over the 0.2% Au/M-SiO2 catalyst, and the 1.5% Au/M-SiO2 catalyst
exhibited a high catalytic activity of 22.7% for cyclohexane conversion and 80.6% selectivity to
cyclohexanol and cyclohexanone under dipolar non hydrogen bond donor (HBD) acetone solvent at
423 K and 1.5 MPa O2 for 3 h.
Received 9th October 2012,
Accepted 18th December 2012
DOI: 10.1039/c2cy20692g
www.rsc.org/catalysis
Thus, high calcination temperatures were needed to remove the
templating agents, resulting in large Au particles.7 In order to
Introduction
The selective oxidation of cyclohexane to cyclohexanone and
cyclohexanol (also known as K/A oil) is significant, which attracts
particular interest in the chemical industry because the two
main products are important intermediates for the manufacture
of nylon 6 and nylon 6,6 polymers.1 In the present industrial
processes, the aerobic cyclohexane oxidation is operated at
415–435 K and 1–2 MPa, in which the conversion is kept at
about 4% to suppress the further formation of by-products,
giving 70%–85% selectivity towards K/A oil.2 The catalysts currently
used, such as Mn and Co naphthenates, are homogeneous,
undesirable to the environment and energy-saving, but they are
still widespread. These disadvantages make it necessary and urgent
to design and cultivate effective heterogeneous catalysts to active
hydrocarbons using an environmentally benign oxidant.
Recently, gold-based catalysts have received increasing inter-
est and demonstrated promising activity for different types of
oxidative reactions3 since gold nanoparticles were discovered to
display a surprisingly high catalytic activity in the oxidation of
CO at low temperatures.4 Moreover, different kinds of gold-
based homogeneous and heterogeneous catalysts in the form of
metal complexes or nanoparticles have been developed.5 In
general, the catalytic properties of the heterogeneous gold
catalysts strongly depend on the size, stability, and amount of
gold nanoparticles. However, Au nanoparticles supported on
inert supports that were reported recently were mostly prepared
directly during the synthesis process by an in situ addition.6
obtain very small gold nanoparticle sizes and highly active gold
catalysts, a chemical grafting and reduction process was reported.
Grafting ligands, such as amine,8 thiol6a and thioether6c have been
applied to anchor gold nanoparticles because of the weak inter-
À
action between AuCl4 and unshared pair electrons of organic
groups, followed by a reduction procedure to achieve gold nano-
particles supported on the surface of pure supports.7 However, gold
nanoparticles are mobile on the surface of most supporting oxides.
One of key problematic issues is the stability of Au nanoparticles
against sintering under reaction conditions.
In this work, we present a novel and facile one-pot synthesis
of highly dispersed gold nanoparticles embedded in amorphous
silica in the absence of any template. A thioether organosilane
[1,4-bis(triethoÀxysily) propane tetrasulfide, TESPTS] was used to
anchor AuCl4 and connect tetraethyl orthosilicate forming
organic–inorganic hybrid linkers via co-condensation. Calcination
of the catalyst precursors removed organic moieties and reduced
the gold salt to gold nanoparticles. The reaction performance
showed that the 1.5% Au/M-SiO2 catalyst displayed high activity in
the selective cyclohexane oxidation to K/A oil with molecular
oxygen. The effects of traditional solvents on the catalytic perfor-
mances of the Au catalysts were also investigated.
Experimental
The Au catalysts were prepared using a facile one-pot synthesis
process. A mixture of 2 mL TEOS and 35 mL TESPTS was dissolved
in 30 mL isopropanol quickly added with vigorous magnetic
stirring at room temperature, followed the pre-designated
State Key Laboratory of Chemical Engineering, School of Chemical Engineering,
East China University of Science and Technology, Shanghai 200237, China.
E-mail: lchen@ecust.edu.cn, zwqi@ecust.edu.cn; Fax: +86 21 64253528;
Tel: +86 21 64250935
À
amounts of aqueous AuCl4 solution were added dropwise.
After about 20 min, 2 mL deionized water was added dropwise
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and kept at BpH 8 by adding a small amount of aqueous
ammonia. The solution was stirred for 30 h at room temperature.
The solid was filtered off, washed with water and ethanol three
times, air-dried at 333 K overnight and calcined at 723 K for 5 h.
X-ray powder diffraction (XRD) patterns were recorded on a
Rigaku D/MAX-2550 X-ray diffractometer with a Cu-Ka radiation of
wavelength l = 1.5418 Å, in the 2y range of 10–901. N2 absorption–
desorption isotherms were measured on a Micrometrics ASAP
2010 spectrometer at 77.35 K after being degassed in a vacuum
at 473 K for more than 4 h prior to the measurement. Ultraviolet-
visible (UV-vis) spectra were performed on a Shimadzu, UV-2450
spectrophotometer. Transmission electron microscopic (TEM)
characterizations of the samples were carried out on a JEM-2010
operated at 200 kV. The samples were prepared by spreading
an ultrasonicated suspension in ethanol onto a grid. The
histogram of the Au particle sizes was established from the
measurement of more than 300 particles. The size limit for
the detection of Au particles on SiO2 is about 1 nm.
Fig. 1 N2 adsorption/desorption isotherms and BJH pore size distribution curve
(inset) of 1.5% Au/M-SiO2 catalyst.
The selective oxidation of cyclohexane was carried out in a were removed, and gold salts were reduced into metallic gold
100 mL Parr batch autoclave with a polytetrafluoroethylene nanoparticles (III). In addition, the colors of the prepared samples
(PTFE) liner. Typically, 2 mL cyclohexane, 5 mg catalyst, change from pink to dark red with increased Au loading.
0.08 g tert-butyl hydroperoxide (TBHP) (as radical initiator)
A number of analytical studies have been carried out to gain
and 20 mL conventional solvent were added into the reactor. information about the Au nanoparticles confined in modified
After purging with O2 5 times to remove residual air in the reactor, amorphous silica. Fig. 1 shows the N2 adsorption/desorption
the mixture was stirred at 600 rpm, 423 K and 1.5 MPa O2 for the isotherms and the BJH pore size distribution curve of the 1.5%
desired time. After cooling to room temperature, the products Au/M-SiO2 catalyst. The isotherm shows an irreversible type IV
were analyzed using a gas chromatogram (Agilent 7890A) with a adsorption isotherm with an H-1 hysteresis loop as defined by
capillary column (PEG-20M) before and after adding an excessive IUPAC, suggesting that the sample is a mesoporous material.
amount of triphenylphosphine to completely reduce cyclohexyl The pore size distribution curve obtained by the BJH method is
hydroperoxide, which is an intermediate in reaction. The used relatively broad, implying unordered mesopores. A relatively
catalyst was collected from the reaction system by centrifugation, high surface area of 230 m2 gÀ1, with a 129 m2 gÀ1 micropore area
washed with acetone, dried at 343 K for 6 h. The catalyst and 101 m2 gÀ1 external surface area, is obtained considering the
reusability was examined using the recovered solid catalyst under removal of organic groups during the synthesis process. The
the optimized reaction conditions.
average pore diameter is 15.9 nm, while the pore size distribution
curve is at B40 nm. It is due to the existence of large amounts
of micropores which can be reached from large micropore area
Results and discussion
(129 m2 À1) as well as the BJH pore size distribution curve
g
The one-pot synthesis process of Au catalysts embedded in being less than 4 nm. The existence of micropores may be due
modified silica is illustrated in Scheme 1. TEOS as a starting to the formation of void defects after removing the organic
silicon source and thioether organosilane as a AuCl4 ion groups during the annealing process.6a,c
À
dispersing and attracting agents were added to an isopropanol
Au nanoparticles have a characteristic absorption band in
solution (I). After stirring for 30 h, TEOS and TESPTS form a the visible light region of the electromagnetic spectra at
B520 nm, which is caused by the surface plasmon resonance
three-dimensional netting structure through hydrolysis and
polycondensation (II). During calcination, the organic groups
(SPR) of colloidal gold.9 Fig. 2 displays the expected UV-vis
Scheme 1 Illustration of the one-pot synthesis process of Au/M-SiO2.
1124 Catal. Sci. Technol., 2013, 3, 1123--1128
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Fig. 2 UV-vis spectra of the catalysts (a) 0.2% Au/M-SiO2, (b) 0.5% Au/M-SiO2,
(c) 1.0% Au/M-SiO2, (d) 1.5% Au/M-SiO2, (e) 2.0% Au/M-SiO2, and (f) 2.5%
Au/M-SiO2.
spectra of supported Au/M-SiO2 with different Au loadings.
Fig. 2a and b exhibit very weak peaks due to the low Au loading.
Samples with a 1.0–2.5% Au loading show evident absorption
bands with UV-vis absorption peaks being slightly shifted to
longer wavelength with increased Au loading, indicating bigger
particle sizes.10
Fig. 3 (a) TEM images of the 1.5% Au/M-SiO2 catalyst, (b) size histogram of the
As shown in Fig. 3a, the TEM images of the 1.5% Au/M-SiO2
catalyst clearly show that the Au particles are successfully
embedded and well-dispersed in the amorphous silica. The
diameters of more than 300 Au particles were measured and
plotted as the histograms in Fig. 3b. It shows that a Au particle
size distribution that mainly ranges from 4–7 nm was obtained
with retained monodispersity when the calcination tempera-
ture was high to 723 K for 5 h. The particle size is relatively
uniformed which may be attributed to the removal of disper-
sing and attracting organic groups from the three-dimensional
netting structure. As a result, the uniformed particle size
plays an important role in the catalytic performance.7 Besides,
large amounts of unordered pores can be seen, owed to the
decomposition of the organic moieties, which is consistent
with the results of N2 adsorption/desorption measurements.
Fig. 4 shows the XRD patterns of the catalysts with different
Au loadings. The characteristic peaks at 38.21, 44.41, 64.71,
77.71 and 81.81 are assigned to the (111), (200), (220), (311) and
(222) reflections of cubic Au particles (JCPDS card no. 4-784).
The catalysts with a Au loading less than 1.0% only display
partial peaks due to the low Au concentration and small Au
particles in the silica support. The peaks become narrower and
Au particles.
Fig. 4 XRD patterns of the catalysts (a) 0.2% Au/M-SiO2, (b) 0.5% Au/M-SiO2,
(c) 1.0% Au/M-SiO2, (d) 1.5% Au/M-SiO2, (e) 2.0% Au/M-SiO2, and (f) 2.5%
Au/M-SiO2.
sharper with increased Au loading, implying relatively larger So a series of traditional solvents were chosen to evaluate the
Au particles. solvent effects, as shown in Table 1. As is known, the polarity of
The catalytic performances of the catalysts were investigated solvents can play a very important role in some catalytic
in the selective oxidation of cyclohexane with molecular oxygen. oxidation reactions during the stabilization of the reaction
In the reaction system, the substrate cyclohexane is a non-polar intermediate states due to solute–solvent interactions.11 There-
organic compound, while products cyclohexanone and cyclo- fore, ethylbenzene, as a non-polar solvent (entry 5, Table 1),
hexanol are polar compounds. Certain solvents can change the clearly exhibits a lower conversion than that of conventional
polarity of the reaction system, providing the possibility polar solvents. However, for most of the polar solvents, the
of improving the conversion and inhibiting deep oxidation. catalytic reactions were faster in dipolar non-hydrogen bond
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Table 1 Effects of solvent on cyclohexane oxidationa
Selectivity (mol%)
Conversion (mol%)
Yieldc (mol%)
TOFd (hÀ1
)
CyH
CyO
CyOH
CyOOH
Othersb
Entry
Solvent
1
2
3
4
5
6
Acetone
Ethanol
Acetic acid
Benzonitrile
Ethylbenzene
Acetonitrile
14.5
0.5
9.0
8.9
8.3
52.4
0
44.6
32.9
40.2
20.1
30.7
75.7
39.4
63.7
19.4
25.7
4.5
0
3.4
2.9
2.0
0.0
12.5
24.3
12.6
0.4
38.5
54.2
12.1
0.4
7.6
8.6
5.0
5.7
7079
234
4392
4353
4051
6018
12.4
a
Reaction conditions: cat. 0.5% Au/M-SiO2 5 mg, CyH 2 mL, solvent 20 mL, TBHP 0.08 g, 423 K, 1.5 MPa, 600 rpm, 3 h, 100 mL PTFE-lined
b
autoclave. CyH, cyclohexane. CyO, cyclohexanone. CyOH, cyclohexanol. CyOOH, cyclohexyl hydroperoxide. By-products are mainly adipic acid,
d-valerolactone, hexanoic acid, valeric acid, etc. Yield is calculated by moles of generated CyO and CyOH per mole of CyH converted. TOF is
c
d
calculated by moles of cyclohexane converted per mole of Au per hour.
donor (HBD) solvents because the reaction intermediate is reaction time. Fig. 6 displays the variation in the selectivity to the
much more solvated by protic solvents than by dipolar non- three products and the yield of CyO and CyOH versus CyH
HBD solvents, and this outweighs any effects due to the conversion from 1 to 5 h. In all cases, the selectivities to CyOOH
solvation of the transition state molecule.12 Moreover, protic and CyOH decrease with increased CyH conversion, while the
solvents have a higher internal pressure and exhibit a negligible selectivity to CyO increases firstly then decreases, exceeding the
solvent dependence of the reaction intermediate decomposi- 15% CyH conversion. The selectivity of the individual product
tion rate. Thus, the catalytic activity is extremely low for the decreases with a high conversion of more than 15% due to the side
HBD solvent ethanol. Besides, the functional groups and the products formed, such as adipic acid, d-valerolactone, hexanoic
steric hindrance of the dipolar non-HBD solvents may have a acid, valeric acid, etc. In addition, cyclohexane oxidation is a
considerable influence on the oxidation reaction.13 Carbonyl- complicated, multistage, free-radical chain reaction with degener-
containing solvents and products can form R–O–HÁ Á ÁOQC ated chains, comprising of different chain initiation, propagation
hydrogen bonds to some extent. All factors taken into account, and termination steps.15 CyOOH is the first oxygenated product
in this work acetone becomes the most appropriate solvent for in the autocatalytic process15,16 and some catalytic processes.17
the oxidation of cyclohexane.
The selectivity of CyOOH decreases with increased conversion,
The effect of acetone was intensively investigated to optimize as shown in Fig. 5 and 6, which indicates that CyOOH could
the selected solvents. Fig. 5 depicts the reaction time effect on also be the first oxygenated product in the oxidation process.
the cyclohexane oxidation and CyO/CyOH selectivities. The Furthermore, CyOOH is decomposed into CyOH and CyO with a
cyclohexane oxidation reaction makes a lot of progress from 1 ratio CyO/CyOH o 1 at low CyH conversion. As time proceeds,
to 5 h, affording a conversion of 14.5% and a selectivity of 83.0% CyOH is consecutively oxidized to CyO with a corresponding
to the two desired oxygenates at 3 h. Different from results increase in CyO selectivity. However, both CyO and CyOH may
reported previously,14 the oxidation reaction does not appear to be oxidized to deep oxides at high CyH conversion, and the
slow down with time, suggesting that there is no loss of catalytic selectivities of CyO and CyOH reduce after 3 h in Fig. 5,
activity. The conversion and the ratio of CyO/CyOH increase, implying an increased rate of over oxidation.18
while the values of CyOH and CyOOH decrease with increased
We herein found that the 0.5% Au/M-SiO2 catalyst was an
exceptionally good catalyst for cyclohexane oxidation using
Fig. 5 Effects of reaction time on the cyclohexane oxidation. Reaction condi-
tions: cat. 0.5% Au/M-SiO2 5 mg, CyH 2 mL, acetone 20 mL, TBHP 0.08 g, 423 K,
1.5 MPa, 600 rpm, 100 mL PTFE-lined autoclave.
Fig. 6 Selectivities of products and yield of CyO and CyOH versus conversion
based on the data of Fig. 5.
c
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Table 2 Effects of Au loading on the cyclohexane oxidation reactiona
Selectivity (mol%)
Conversion (mol%)
CyH
Entry
Catalyst
Yield (mol%)
TOF (hÀ1
)
CyO
CyOH
CyOOH
Others
1
2
3
4
5
6
7
8
0.2% Au/M-SiO2
0.5% Au/M-SiO2
17.3
14.5
15.3
22.7
19.1
18.7
27.1
29.5
31.0
7
56.8
52.4
57.4
45.9
49.8
54.2
45.2
44.2
43.3
36
30.3
30.7
25.1
34.7
34.9
29.8
33.3
33.6
32.8
55
2.4
4.5
4.3
2.5
2.1
2.3
2.7
2.1
1.9
10.5
12.5
13.2
16.9
13.2
13.8
18.8
20.0
22.0
15.1
12.1
12.7
18.3
16.2
15.7
21.3
22.9
23.6
6.4
21 097
7079
3734
3677
2326
1825
4402
4792
5033
3092
2592
1.0% Au/M-SiO2
1.5% Au/M-SiO2-1stb
2.0% Au/M-SiO2
2.5% Au/M-SiO2
1.5% Au/M-SiO2-2ndb
1.5% Au/M-SiO2-3rdb
1.5% Au/M-SiO2-4thb
0.55 Au/ZSM5c
9
10
11
d
1.0 Au/Al2O3
10.9
54
30.3
2.8
9.2
a
b
Reaction conditions: cat. 5 mg, CyH 2 mL, TBHP 0.08 g, acetone 20 mL, 423 K, 1.5 MPa, 3 h, 600 rpm, 100 mL PTFE-lined autoclave. The 1.5%
c
Au/M-SiO2 catalyst was reused. Designations 1st–4th indicates the cycle number of the same catalyst. Reaction conditions of ref. 5: cat. 5 mg, CyH
2 mL, 423 K, 1.0 MPa, 3 h. Reaction conditions of ref. 17: cat. 50 mg, CyH 20 mL, 423 K, 1.0 MPa, 3 h.
d
acetone as the solvent (Table 1). Moreover, the catalytic perfor- activity. However, the recycling test shows that the catalytic
mance of Au based catalysts in cyclohexane oxidation reaction activity of the 1.5% Au/M-SiO2 catalyst keeps increasing even
using molecular oxygen as an oxidant has been widely reported after four cycles of reuse, similar to the results of previous
with high cyclohexane conversions (Table 2, entries 10–11).5,17 reports.6b,20 During our preparation process, gold nanoparticles
However, the amount and sizes of the gold nanoparticles play were embedded in silica and hindered the aggregation of
very important roles in the oxidation process. Table 2 displays neighbouring Au nanoparticles under the reaction conditions.
the effects of gold loading on cyclohexane oxidation. Among the Moreover, the microscopic structure of the catalyst may change
catalysts with different amounts of gold, the 1.5% Au/M-SiO2 to a more favourable direction for mass and heat transfer,
catalyst shows the best performance with respect to cyclohex- feasibly leading to an even higher catalytic activity after every
ane conversion and selectivity to CyO and CyOH. The overall oxidation process.
tendency of the cyclohexane conversion first increases and then
decreases with increased Au loading, reaching the maximum
CyH conversion of 22.7% with the 1.5% Au/M-SiO2 catalyst.
Conclusions
However, the conversion of the oxidation reaction with the
0.2% Au/M-SiO2 catalyst exceeds that of the 0.5% Au/M-SiO2
and 1.0% Au/M-SiO2 catalysts. A surprisingly high TOF of up to
21 097 hÀ1 was obtained over 0.2% Au/M-SiO2 catalyst, which
endows this system with the best activity ever reported for
cyclohexane oxidation in terms of TOF value. The catalytic
properties of the heterogeneous Au catalysts have been found
to strongly depend on the particle sizes and number of Au
nanoparticles. The gold nanoparticle sizes increase with
increased Au loadings according to the XRD results in Fig. 4.
For the 0.2% Au/M-SiO2 catalyst, we could hardly observe any
gold characteristic peaks, except weak the (111) reflection. This
indicates that the catalyst exhibits super highly dispersed and
small gold nanoparticles, and shows the highest TOF value
owing to it having the largest number of active sites per mole of
Au in the catalytic system.19 The lower Au loaded catalysts have
smaller and more dispersed Au nanoparticles with relatively
more active sites, while the higher Au loading catalysts may
have less active sites due to the bigger Au nanoparticles. Thus,
the best catalytic activity of 1.5% Au/M-SiO2 is attributed to the
best combination of Au loading and particle sizes. Moreover,
the TOF value decreases with increased Au loading, which
demonstrates that smaller Au particles have relatively more
active sites per mole of gold. In addition, heterogeneous
catalysts often suffer extensive leaching of the active metal
species during the reaction, accompanied by loss of catalytic
A novel one-pot synthesis method of Au nanoparticles embedded
in amorphous silica was put forward and demonstrated to be a
facile approach to prepare well distributed gold catalysts in
silica. Results showed that the catalysts exhibited high catalytic
activities in selective cyclohexane oxidation with molecular
oxygen, using acetone as the solvent. The functional groups
and dipolar non-HBD solvents played important roles in the
selective oxidation of cyclohexane to cyclohexanol and cyclo-
hexanone. The catalytic activity of the Au catalysts strongly depends
on the particle size and the amount of the Au loading.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21006029, 21076074), National High
Technology Research and Development (863 Program
2012AA061601), Shanghai Natural Science Foundation
(10ZR1407200), and the Fundamental Research Funds for the
Central Universities of China. The financial support by the Max
Planck Partner Group Funding from the Max Planck Society,
Germany, is greatly acknowledged.
Notes and references
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