Mesoporous Silica Supported Au Nanoparticles with Controlled Size as Efficient Heterogeneous Catalyst for Aerobic Oxidation of Alcohols

Article abstract of DOI:10.1155/2015/737921

A series of Au catalysts with different sizes were synthesized and employed on amine group functionalized ordered mesoporous silica solid supports as catalyst for the aerobic oxidation of various alcohols. The mesoporous silica of MCM-41 supported Au nano

Full text of DOI:10.1155/2015/737921

Hindawi Publishing Corporation
Journal of Chemistry
Volume 2015, Article ID 737921, 7 pages
http://dx.doi.org/10.1155/2015/737921
Research Article
Mesoporous Silica Supported Au Nanoparticles with
Controlled Size as Efficient Heterogeneous Catalyst for
Aerobic Oxidation of Alcohols
Xuefeng Chu,1,2 Chao Wang,1 Liang Guo,1,2 Yaodan Chi,1
Xiaohong Gao,1 and Xiaotian Yang1
1Jilin Provincial Key Laboratory of Architectural Electricity & Comprehensive Energy Saving,
School of Electrical and Electronic Information Engineering, Jilin Jianzhu University, Changchun 130118, China
2Department of Basic Science, Jilin Jianzhu University, Changchun 130118, China
Correspondence should be addressed to Xiaotian Yang; hanyxt@163.com
Received 24 August 2015; Revised 14 November 2015; Accepted 15 November 2015
Academic Editor: Jean-Luc Blin
Copyright © 2015 Xuefeng Chu et al. is is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
A series of Au catalysts with different sizes were synthesized and employed on amine group functionalized ordered mesoporous
silica solid supports as catalyst for the aerobic oxidation of various alcohols. e mesoporous silica of MCM-41 supported Au
nanoparticles (Au-1) exhibited the smallest particle size at ∼1.8 nm with superior catalytic activities owing to the confinement effect
of the mesoporous channels. Au-1 catalyst is also very stable and reusable under aerobic condition. erefore, this presented work
would obviously provide us a platform for synthesizing more size-controlled metal catalysts to improve the catalytic performances.
1. Introduction
on inert supports is very important for producing an efficient
Au nanocatalyst.
Since the pioneering work of Hutchings and Haruta [1–7], Au
nanoparticles were found to be an active material in catalytic
science for a number of oxidation and hydrogenation reac-
tions. Some recent work suggests that Au nanoparticles can
be efficient catalysts for the selective oxidation of alcohols,
olefins, and alkanes in the presence of molecular oxygen as
oxidant [1, 2, 8–15]. e unique catalytic performance of Au
nanoparticles is strongly depending upon the solid supports,
particle size, shape, and some other factors [1, 2, 5–7, 16–23].
In many cases, Au nanoparticles size and the solid supports
have a crucial role in enhancing the catalytic activity [1, 2, 5–
23]. Furthermore, the junction between Au nanoparticles
and metal oxide support contributes as active site for the
catalytic reactions. Normally, Au nanoparticles supported on
inert metal oxide supports (e.g., silica) result in relatively
poor catalytic performances due to the lack of electronic
interaction between the active sites and support [24, 25].
erefore, the development of highly active Au nanoparticles
e size dependent catalytic performance of Au nanopar-
ticles has been extensively studied by previous reports, where
the Au nanoparticle sizes were controlled by employing
expensive ligand molecules with low loading efficiency on
the solid supports in order to obtain highly active Au
nanoparticles. For example, Wang and coworkers reported
the pyrrolidone modified silica supported small-size Au
nanoparticle (∼4 nm) is highly active for aerobic oxidation
of alcohols [26]; Liu and coworkers found that the sub-
2 nm Au nanoparticles synthesized using mercapto ligand are
more active for the alkane oxidation than the larger ones
[27]. More recently, Zhang and coworkers demonstrated that
nanoporous ionic organic network could stabilize the small-
size Au nanoparticle at ∼2 nm, giving high activity in aerobic
oxidation of alcohols [28].
Afer the discovery of M41S family [29–33], ordered
mesoporous silica (OMS) has emerged as an active material

2
Journal of Chemistry
Table 1: Structural parameters and supported Au nanoparticles size on mesoporous silica with different amounts of Au loading.
Au nanoparticles size (nm)
Sample
Support
Pore diameter (nm)
Weight ratio of Au (%)a
From XRD patternsb
1.4
From TEM images
1–3
Au-1
Au-2
Au-3
Au-4
MCM-41
SBA-15
3.0
8.5
8.5
3.0
2.4
3.0
9.1
2.3
4.2
5.4
2–4
5–9
SBA-15
MCM-41
10.3
5–15
ae ratio of Au calculated from ICP.
be particle grain size calculated from Scherrer’s equation.
to support metal nanoparticles for catalytic applications due
to their high surface area, tunable and confined mesopores,
diverse morphology, and large adsorption capacity. A sig-
nificant amount of work has already been devoted to Au
nanoparticles supported on mesoporous silica using variety
of methods, but limited attention was only paid to the cat-
alytic activity of size-controlled Au nanoparticles on OMS for
aerobic oxidations [34, 35]. In this work, we demonstrate an
easy strategy to synthesize size-controlled Au nanoparticles
as catalysts by using amine functionalized OMS solid support
for aerobic oxidation of alcohols. MCM-41 and SBA-15 with
same mesostructure (p6mm) but different pore sizes were
considered as solid supports. e channel confinements of
mesopores are utilized as template for the size-controlled
synthesis of Au nanoparticles.
D/MAX 2550 diffractometer using CuKꢀ radiation. e
average grain size of gold nanoparticles was calculated by
∘
using half-width of the gold peak at 2ꢁ = 38.3 by apply-
ing Scherrer’s equation. Transmission electron microscopy
(TEM) analysis was performed on a JEM-200CX electron
microscope (JEOL, Japan) with an acceleration voltage of
300 kV. e nitrogen adsorption and desorption isotherms at
the temperature of using liquid nitrogen were measured by
using a Micromeritics ASAP Tristar system. e samples were
outgassed for 10 h at 150∘C before the measurements. e
pore size distribution for mesoporous silica was calculated by
using the Barrett-Joyner-Halenda (BJH) model. e contents
of Au were determined by using inductively coupled plasma
(ICP) with a Perkin-Elmer plasma 40 emission spectrometer.
Diffuse reflectance ultraviolet-visible (UV-Vis) spectra were
recorded on spectrometer of PE Lambda 20, and BaSO was
4
an internal standard sample.
2. Experimental Section
2.1. Synthesis of Au Nanoparticles. MCM-41 and SBA-15 sam-
ples were prepared by using previously reported procedure.
e samples were washed with the mixture of ethanol and
HCl to remove the unreacted surfactants. 1 g of washed
mesoporous silica was dried at 120∘C under vacuum for
3 hours and then followed by the addition of 50 mL of
pretreated toluene containing 1 g of DAPTS. e mixture was
refluxed overnight. e experiments above were carried out
using anhydrous condition to avoid any reaction between
2.3. Catalytic Study. e catalytic study was performed on
a 50 mL of glass reactor and the solution was stirred with a
magnetic stirrer. e substrate and catalyst were mixed in the
reactor and heated up to the given temperature. en, oxygen
was introduced into the reactor and sealed the reaction
system. Afer 24 h reaction, the product was taken out from
the system and analyzed by using gas chromatography (GC-
17A, Shimadzu, using a flame ionization detector) with a
flexible quartz capillary column coated with OV-17. e
turnover frequencies (TOFs) were calculated based on the
total number of Au atoms in the reaction system according
to the following equation:
amine ligands and H O. Finally, the samples were collected
2
by rotary evaporation and washed with a large quantity
of ethanol for several times. 0.5 g of amine functionalized
mesoporous samples was separately added into different
volume of HAuCl solution (as shown in Table 1) and then
Converted mole of substrate
(1)
4
TOF =
.
stirred overnight. e product was collected and dried under
Mole of Au atoms ∗ Reaction time
vacuum for 1 h at 100∘C. en, NaBH was used to reduce
4
the gold(III) into gold nanoparticles as the following: 1 g
of mesoporous silica containing gold(III) was added into
40 mL of anhydrous toluene and followed by the addition of
3. Results and Discussion
3.1. Characterization of Au Nanoparticles Supported on Meso-
porous Silica. XRD patterns of various Au nanoparticles sup-
ported on mesoporous silica samples are shown in Figure 1.
e samples with higher amount of Au loading exhibited
four well-resolved peaks in the range of 20–80∘ as shown in
Figure 1(a), indexed to the Au nanoparticles crystal planes
of (111), (200), (220), and (311). However, the samples with
lower amount of Au loading display a very broad peak at
around 38.5∘, suggesting decreased particle size compared
NaBH under stirring. Afer 10 minutes of stirring, 10 mL of
4
anhydrous ethanol was added into the mixture and stirred
overnight. e products were collected by rotary evaporation
and washed with a large quantity of ethanol and water for
several times.
2.2. Characterization. X-ray diffraction (XRD) patterns were
obtained with a Siemens D5005 diffractometer and Rigaku

Journal of Chemistry
3
90000
60000
50000
40000
30000
20000
10000
0
(110)
(110)
Au-1
Au-2
80000
Au-1
(111)
(200)
(220)
(311)
70000
60000
50000
40000
30000
20000
10000
0
(200)
(211)
Au-2
Au-3
Au-3
Au-4
(110)
2
Au-4
1
3
4
5
6
20
40
60
80
2ꢀ (deg.)
2ꢀ (deg.)
(a)
(b)
Figure 1: (a) Small- and (b) wide-angle XRD patterns of various Au nanoparticles supported mesoporous silica samples.
to the samples with higher Au loading. Additionally, the
mesostructure of mesoporous silica remained stable during
the Au loading process which confirmed with the low angle
XRD patterns in Figure 1(b).
(TOF) was more than 300 h−1 which would be 3 times
higher than that for efficient soluble catalysts of palladium
complex [36]. e catalytic conversion efficiency of Au-2 was
also higher than Au-3 and Au-4 with higher amount of Au
loading. e excellent catalytic performances of Au-1 and Au-
2 should be attributed to the smaller Au nanoparticles size
in the matrix of mesoporous silica. On the contrary, Au-4
showed very poor catalytic performance which might be due
to the Au nanoparticles agglomeration. Even though there
was no obvious aggregation for Au particles on SBA-15(H),
the larger sized Au nanoparticles decreased the catalytic
conversion.
TEM analysis provides direct evidence for the Au
nanoparticles size and the stable structure of the mesoporous
silica (Figure 2). According to the TEM results, it is clear that
the highly ordered structures along (110) or (100) crystal
planes for 2D hexagonal mesoporous silica occurred in all
samples. e well-dispersed Au nanoparticles within the
mesoporous matrix except the sample of Au-4 are shown in
Figure 2(d). e size of Au nanoparticles was measured to be
in the range 1–3 nm for the samples with low amount of Au
loading as shown in Figures 2(a) and 2(b), which shows good
agreement with the XRD patterns in Figure 1. Figure 2(c)
shows TEM image for Au/SBA-15 with higher amount of Au
loading, and the particle size distribution was centered in
the range of 6–8 nm without any obvious aggregation. But an
obvious aggregation occurred for Au nanoparticles supported
on MCM-41 (Au-4) when higher amount of Au was loaded as
shown in Figure 2(d) (Table 1).
Based on the results above, it can be confirmed that
the Au nanoparticles (2-3 nm) were well dispersed in the
mesoporous channels for both MCM-41 and SBA-15 when
low amount of Au was loaded. In the case of higher amount
of Au loading agglomeration occurred for MCM-41 as the
pore diameter was relatively smaller at around 3 nm, while
SBA-15 has enough space to accommodate larger sized Au
nanoparticles isolation with larger pore diameters of around
8.5 nm.
3.3. Catalyst Stability. To investigate the stability of various
Au catalysts, we recycled the samples for 5 times and calcu-
lated the catalytic conversion efficiency. Other than highest
catalytic conversion efficiency, Au-1 catalyst is also highly
stable and reusable as similar catalytic activity was observed
while recycling the same sample for 5 times (Table 2). As
shown in Figure 3, TEM images of Au-1, no changes were
observed in the Au nanoparticles size afer recycling the
same sample for 5 times, which is totally different from the
general phenomenon that the smaller particles would grow
and become larger during the catalytic reactions. is is
probably due to the limitation of mesopores of MCM-41,
which can hinder the Au nanoparticles agglomeration during
the catalytic reaction.
4. Conclusion
In conclusion, a series of Au catalysts with controlled sizes
were synthesized on mesoporous silica solid supports. e
MCM-41 supported Au nanocatalyst with lower amount of
Au loading (Au-1) displayed higher catalytic activities than
other catalysts for the aerobic oxidation of 1-phenylethanol
and benzyl alcohol. e Au-1 catalyst also showed highly
stable and reusable capability while recycling the same sample
3.2. Catalytic Properties of Au Nanoparticles for Aerobic
Oxidation of Alcohols. 1-Phenylethanol and its paraderivates
were chosen as model to investigate the catalytic properties of
various Au catalysts. e obtained results were summarized
in Table 2. Au-1 has showed highest catalytic conversion
efficiency for all compounds and the turnover frequency

4
Journal of Chemistry
40
30
20
10
0
0
2
4
6
8
10 12 14 16
Diameter (nm)
(a)
40
30
20
10
0
0
2
4
6
8
10 12 14 16
Diameter (nm)
(b)
50
40
30
20
10
0
0
2
4
6
8
10 12 14 16
Diameter (nm)
(c)
Figure 2: Continued.

Journal of Chemistry
5
45
40
35
30
25
20
15
10
5
0
0
2
4
6
8
10 12 14 16
Diameter (nm)
(d)
Figure 2: TEM images and size distributions for (a) Au-1, (b) Au-2, (c) Au-3, and (d) Au-4 catalysts.
Table 2: Catalytic activities of various Au/OMS samples for aerobic oxidations of 1-phenylethanol and its paraderivatesa.
Catalysts
Conversion efficiency (%)
TOF (h−1
)
Selectivity (%)
Substrate
Products
Au-1
Au-1
Au-1
Au-2
Au-3
>99.5a
>99.5b
97.8c
342
342
334
241
151
>99.5
>99.5
>99.5
>99.5
>99.5
OH
O
88.0
MeO
MeO
80.5
Au-4
Au-1
Au-2
Au-3
32.1
95.0
83.0
73.0
29
>99.5
>99.5
>99.5
>99.5
325
227
199
OH
O
Cl
F
Cl
F
Au-4
Au-1
Au-2
Au-3
61.5
96.7
88.9
79.0
56
331
243
147
>99.5
>99.5
>99.5
>99.5
OH
O
Au-4
Au-1
Au-2
Au-3
Au-4
46.4
89.3
81.7
70.0
54.2
42
305
224
56
>99.5
>99.5
>99.5
>99.5
>99.5
OH
O
49
aReactions conditions: 60 mg of catalyst, 0.6 mmol of substrates, 0.3 mmol of Na CO , 100 C, 24 h, and 6 mL of toluene as solvent.
∘
2
3
bRecycled results.
cRecycled for 5 times. e sample was dried at 140 C before the 5th recycle.
∘
for 5 times. erefore, this work would definitely provide a
platform for the future about synthesizing more active metal
nanoparticles on inert solid supports for the efficient catalytic
conversion.
Conflict of Interests
e authors declare that there is no conflict of interests
regarding the publication of this paper.

6
Journal of Chemistry
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