Organic-inorganic hybrid SiO2 supported gold nanoparticles: Facile preparation and catalytic hydrogenation of aromatic nitro compounds

Article abstract of DOI:10.1007/s10562-012-0821-5

Highly dispersed gold nanoparticles supported on organic-inorganic hybrid silica have been successfully prepared through a novel and facile approach. In the process, 3-aminopropyltriethoxysilane was hydrolyzed in HCHO aqueous solution to prepare silica with organic functional groups (-SiCH ₂CH₂CH₂NHCH₂OH) formed through the reaction between -NH₂ and HCHO, then the silica reacted with HAuCl4 in aqueous solution. Due to the reducibility of -SiCH₂CH ₂CH₂NHCH₂OH, the gold precursor was in situ reduced on the silica. The materials were characterized by powder X-ray diffraction, transmission electron microscopy, Fourier-transform infrared spectroscopy, solid-state nuclear magnetic resonance spectroscopy, and X-ray photoelectron spectroscopy techniques. The results indicated Au nanoparticles were highly dispersed on silica with an average particles size 1.8 ± 0.5 nm. The asobtained Au/SiO₂-org exhibited good catalytic activity and stability for liquid phase catalytic hydrogenation of aromatic nitro compounds with H₂.

Full text of DOI:10.1007/s10562-012-0821-5

Catal Lett (2012) 142:788–793
DOI 10.1007/s10562-012-0821-5
Organic–Inorganic Hybrid SiO Supported Gold Nanoparticles:
2
Facile Preparation and Catalytic Hydrogenation of Aromatic
Nitro Compounds
Xiaoying Tan
Qiang Xu Changhai Liang
•
•
Zhixin Zhang
•
•
Zihui Xiao
Xinkui Wang
•
Received: 18 January 2012 / Accepted: 5 April 2012 / Published online: 27 April 2012
Ó Springer Science+Business Media, LLC 2012
Abstract Highly dispersed gold nanoparticles supported
on organic–inorganic hybrid silica have been successfully
prepared through a novel and facile approach. In the pro-
cess, 3-aminopropyltriethoxysilane was hydrolyzed in
HCHO aqueous solution to prepare silica with organic
functional groups (–SiCH CH CH NHCH OH) formed
in catalysis by supported gold nanoparticles has dramatically
expanded [1–5]. It is now generally accepted that the unex-
pected activity of supported gold catalysts is related to the
particle size of gold, the nature of the support, and the
preparation method [6–8]. At present, the main supports of
gold catalysts are inorganic materials such as zeolite,
amorphous SiO or Al O and activated carbon [9, 10].
2
2
2
2
through the reaction between –NH and HCHO, then the
2
2
2 3
silica reacted with HAuCl in aqueous solution. Due to the
4
However, due to the weak Van der Waals force between Au
and carrier, leaching or aggregation of gold nanoparticles in
high-temperature liquid reactions becomes one of the most
important and challenging problems. In order to overcome
the disadvantage of inorganic supports on which the active
component is easy to leach into the solution, somecomplexes
that can combine with the active component are introduced,
forming organic–inorganic hybrid materials [11].
reducibility of –SiCH CH CH NHCH OH, the gold pre-
2
2
2
2
cursor was in situ reduced on the silica. The materials were
characterized by powder X-ray diffraction, transmission
electron microscopy, Fourier-transform infrared spectros-
copy, solid-state nuclear magnetic resonance spectroscopy,
and X-ray photoelectron spectroscopy techniques. The
results indicated Au nanoparticles were highly dispersed on
silica with an average particles size 1.8 ± 0.5 nm. The as-
3-Aminopropyltriethoxysilane (APTES) is a common
coupling agent that contains both hydrolysable –OC H
2 5
obtained Au/SiO -org exhibited good catalytic activity and
2
stability for liquid phase catalytic hydrogenation of aro-
matic nitro compounds with H2.
and non-hydrolysable organic hydrophobic groups
–CH CH CH NH , which was widely used to prepare
2
2
2
2
organic–inorganic NH –SiO gel. Recently, a chemical
2
2
Keywords In situ reduction Á Organic–inorganic hybrid
silica Á Gold nanoparticles Á Hydrogenation
grafting and reduction process was reported for preparation
of this kind of gold catalysts, in which organosilanes with
?
basic –NH or –R N moieties were introduced into the
2
4
internal pore surfaces of pre-fabricated mesoporous silica.
Gold ions were then introduced via a neutralization reac-
tion between HAuCl and the basic groups, followed by H
1
Introduction
4
2
Following the discoveries by Haruta and Hutchings in the use
of gold nanoparticles as catalysts for CO oxidation and
ethyne hydrochlorination respectively in the 1980s, interest
reduction to produce the supported gold nanocatalysts [12,
13]. Chen et al. [14] reported a novel synthetic method
where the amino groups of APTES modified on SiO can
2
be used to anchor formaldehyde to form reductive species,
which could in situ reduce AgNO to form Ag nanoparti-
3
1
3
X. Tan Á Z. Zhang Á Z. Xiao Á Q. Xu Á C. Liang Á X. Wang (&)
Laboratory of Advanced Materials and Catalytic Engineering,
School of Chemistry, Dalian University of Technology,
Dalian 116024, China
cles. In addition, Sun et al. [15] utilized C CP/MAS NMR
spectroscopy to prove that the reductive species were
–
SiCH CH CH NHCH OH groups, and they fabricated
2 2 2 2
e-mail: xinkuiwang@sina.com
monodispersed Ag nanoparticles in channels of mesoporous
1
23

Organic–Inorganic Hybrid SiO
2
Supported Gold Nanoparticles
789
silica using a post chemical grafting method. However, for
the above-mentioned methods, the preparation of silica and
their surface modification with –NH and then –NHCH OH
(ICP-AES). The materials were examined using transmis-
2
sion electron microscopy (TEM, Philips Tecnai G 20
equipped with a CCD camera operated at 200 kV). X-ray
diffraction (XRD) patterns were recorded on a PW3040/60
X’ Pert PRO (PANalytical) diffractometer (CuKa X-ray
source, operated at 40 kV and 50 mA). The Fourier-
transform infrared (FT-IR) spectra were acquired with a
spectrometer (Bruker tensor27) equipped with a DLATGS
2
2
are completely separated, requiring a long period of time,
usually a few days, while the amount of –NHCH OH on
2
silica is also limited for reducibility of metal ions.
Herein, we report a simple and convenient route to pre-
pare supported gold nanoparticles on the surface of silica-
based materials. During the process, HCHO not only plays
the role of hydrolysis catalyst of APTES but also reacts with
-
1
detector and operated at a resolution of 4 cm . X-ray
photoelectron spectroscopy (XPS) spectra were obtained
with a VG ESCALAB 250 equipped with a monochro-
mated Al–Ka radiation source (1486.6 eV) under a residual
the amino of APTES to form an –NHCH OH group, which
2
can reduce HAuCl . In this case, the –NHCH OH groups
2
4
-
9
anchored on the silica network would act as linkage groups
between inorganic material and the active component Au,
and prevent Au nanoparticles from leaching when they are
used in solutions. In comparison to similar Au catalysts
prepared through the chemical grafting method in the liter-
atures [11, 14, 15], more –NH or –NHCH OH groups are on
pressure of 10 Torr. The binding energy scale was ref-
1
3
erenced to C1s at 284.6 eV. Solid-state C CP/magic
angle spinning nuclear magnetic resonance (MAS NMR)
experiments were conducted in a Bruker Avance III
500 MHz NMR spectrometer equipped with a 4 mm
standard bore probe head, using a magnetic field of 11.7 T
2
2
the silica surface and more gold ions would be reduced in situ
because APTES has been adopted as the silicon source.
Furthermore the organic groups around the Au nanoparticles
may tune the electronic nature of Au, leading to favorable
catalysis for some organic reactions. In fact, the present
study of the catalytic hydrogenation of aromatic nitro com-
pounds proves that the as-prepared Au-loaded catalysts
exhibit enhanced activity and recycling stability.
at 296 K. The samples were packed in the ZrO rotor
2
closed with Kel-F cap, which were then spun at 5 kHz rate.
A total of 512 scans were recorded with 5 s recycle delay
1
for each sample. All C MAS chemical shifts are refer-
3
enced to the resonances of C H NO standard (d =
2
5
2
-176.00 ppm).
2.3 Catalytic Test
The catalytic hydrogenation of aromatic nitro compounds
was performed in a 100 mL stainless steel autoclave with
mechanical stirrer under the following reaction conditions:
2
Experimental
2
.1 Catalyst Synthesis
140 °C, 4.0 MPa H , 0.1 g of catalyst, 0.5 g of substrates,
2
and 40 mL of ethanol. The products were analyzed by a
7890F gas chromatograph equipped with a SE-54 capillary
column and a flame ionization detector.
The organic–inorganic hybrid silica supported gold nano-
particles were prepared as follows. First, 4.7 mL of APTES
was added into 150 mL of deionized water. Then, 1.4 mL
of HCHO (37 %) was added into the mixture under stir-
ring, where immediately a white precipitate appeared.
After stirring at room temperature for 2 h, the obtained
solid was filtered and washed with an excess amount of
deionized water. The solid was then dried in vacuum at
3 Results and Discussion
The Au/SiO -org catalyst was prepared via two major
2
steps: (1) one-pot preparation of SiO -org with
2
1
20 °C for 12 h, which yielded a solid white powder
–SiCH CH CH NHCH OH groups; (2) in situ reduction of
2
2
2
2
(
denoted as SiO -org). Then 1.0 g of SiO -org was sus-
gold ions in aqueous solution to form gold nanoparticles on
2
2
-
pended in 30 mL of 1.62 9 10 mol L HAuCl aque-
3
-1
SiO -org. The method is easy to perform and no organic
2
4
ous solution. After stirring for 2 h at 100 °C, the mixture
was filtered and washed with an excess amount of deion-
ized water until no chlorine ions were detected in the fil-
trate. Following drying in vacuum at 120 °C for 12 h, a
solvent such as toluene is used. This contrasts with the
chemical grafting method, which has usually adopted a
toluene solution of organic silane to anchor organic groups,
and a much longer processing time. The ICP-AES analysis
solid powder of Au/SiO -org material was obtained.
showed that the gold loading for Au/SiO -org was 0.99 %,
2
2
essentially identical to the designed 1.0 %, proving a high
efficiency of gold utilization. This is an advantage of this
method considering that only a fraction of the gold ions
used were deposited onto supports by using other reported
preparation methods [16–18].
2
.2 Characterizations
The actual gold loading of the catalysts was determined by
inductively coupled plasma atomic emission spectroscopy
123

7
90
X. Tan et al.
(Fig. 3a), the binding energy of Au4f7/2 is 83.4 eV, which
is lower than that of bulk metallic gold (84.0 eV). Such a
negative shift has also been observed with gold catalysts by
other groups [19–21]. Arrii et al. [19] observed the
dependence of the Au4f7/2 binding energies on different
kinds of supports, and concluded that the gold is in the
metallic state, because they did not observe any peaks of
oxidized gold species (located around 85.5 and 86.3 eV).
They attributed such a negative shift to the possible elec-
tron transfer from the support to the particle. Hence, based
on the above reports [19–21], we propose that Au in Au/
Au(111)
(
d)
(
c)
(
b)
a)
(
SiO -org is of a metallic nature, and the 0.6 eV difference
2
is due to the interaction between the organic groups on
1
0
20
30
40
50
60
70
80
SiO -org and the Au nanoparticles, This in turn may pro-
2
2
Theta(deg.)
vide the Au/SiO -org with high stability for catalysis.
2
Figure 4 shows the FT-IR spectra of SiO -org and 1.0 %
2
Fig. 1 XRD patterns of 1.0 % Au/SiO
c 3.0 % Au/SiO -org, and d 1.0 % Au/SiO -org prepared by Chen
2
-org a fresh, b used and
-
Au/SiO -org between 4,000 and 400 cm . The two
1
2
2
2
method
spectra are similar in terms of peak profile other than peak
-
1
intensities. Two bands at 2,935 and 2,872 cm
are
Figure 1a shows the XRD pattern of fresh 1.0 % Au/
observed, which conventionally are assigned to character-
istic asymmetric and symmetric vibrations of the –CH2
groups in the propyl chain of the APTES. In addition, the
SiO -org. No peaks due to Au were detected besides a
2
diffraction peak of amorphous silica, implying that the gold
particles are highly dispersed on the support. In order to
clearly examine the metal particle size and size distribution
of the gold catalysts, a high-resolution TEM technique was
-
1
bands at 1,470 and 688 cm are attributed to the scissor
bending vibrations of the –CH –CH –CH – groups and the
2
2
2
–CH rocking vibration of Si–CH R, respectively [22, 23].
2
2
-
1
employed. From the TEM images of 1.0 % Au/SiO -org
2
The band at 917 cm is assigned to d (N–H). The strong
-
1
(
Fig. 2a), the homogeneously dispersed gold nanoparticles
peaks are noted at 1,040 and 1,124 cm
due to the
with an average size 1.8 ± 0.5 nm were observed, which is
well in agreement with the XRD result.
siloxane vibrations (Si–O–Si and Si–O–C) [11]. The band
-
at 925 cm is assigned to Si–OH stretching, while the two
1
-
1
In order to clarify the surface state of Au nanoparticles,
bands at 786 and 452 cm are due to Si–O–Si [22]. The
results of FT-IR characterization confirmed the existence
XPS was performed. For the fresh 1.0 % Au/SiO -org
2
a
b
c
d
2
2
1
1
5
35
30
35
35
D=1.8±0.5 nm
30
25
20
15
10
5
D=1.9±0.5 nm
30
25
20
15
10
5
0
5
0
5
0
D=1.9±0.5nm
D=1.8±0.5nm
2
2
1
1
5
0
5
0
5
0
0
0
0
.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6
Particle Size (nm)
0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6
Particle Size (nm)
0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6
Particle Size (nm)
0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6
Particle Size (nm)
Fig. 2 TEM images and the corresponding particle size distributions of 1.0 % Au/SiO
Au/SiO -org prepared by Chen method
2 2
-org a fresh, b used, c 3.0 % Au/SiO -org, and d 1.0 %
2
1
23

Organic–Inorganic Hybrid SiO
2
Supported Gold Nanoparticles
791
C1
1
2
3
4
C2
Si-CH -CH -CH -NH-CH -OH
2
2
2
2
C3
C4
b
a
b
a
92
90
88
86
84
82
80
78
Binding Energy (eV)
1
40
120
100
80
60
40
20
0
-20
-40
Fig. 3 XPS spectra of 1.0 % Au/SiO -org a fresh, b used
2
ppm
1
3
Fig. 5
2 2
C CP/MAS NMR spectra of a SiO -org, b Au/SiO -org
(b)
100
a
(a)
8
6
4
2
0
0
0
0
b
4
000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumbers(cm )
0
0
20
40
60
80
100
120
140
160
180
Fig. 4 FT-IR spectra of a 1.0 % Au/SiO
2
-org, b SiO
2
-org
t (min)
Fig. 6 Reaction time dependence of p-CNB conversion over a our
Au/SiO -org and b Au/SiO -org prepared by Chen method
of aminopropyl groups and the successful preparation of
organic–inorganic hybrid silica [14, 15]. In addition, the
results demonstrated that the reductive groups on the SiO2-
org can be observed even though 1.0 % Au was loaded on
the support, and the gold nanoparticles had no influence on
the FT-IR activities of organic groups surrounding them.
To investigate the formation mechanism of metal
2
2
report [15], the signal at 76 ppm doesn’t disappear after the
sample reacts with HAuCl (Fig. 5b). We deduced that the
4
-
quantity of -NHCH OH was too much relative to AuCl
2
4
ions for the 1.0 % Au/SiO -org. Thus only a minority of
2
-NHCH OH played a reductive role, which is in line with
2
1
3
nanoparticles on the SiO -org support, C CP/MAS NMR
2
the FT-IR result. In order to prove this possibility, an Au/
1
spectroscopy was employed. Figure 5a shows the C CP/
3
SiO -org with nominal 3.0 % gold loading has been pre-
2
MAS NMR spectrum of as-synthesized SiO -org. The three
2
pared. The ICP-AES analysis confirms an actual Au load-
ing of 2.9 %, and the XRD (Fig. 1c) and TEM (Fig. 2c)
results show that the gold particles are still highly dispersed
with an average particle size 1.9 ± 0.5 nm.
strong signals at 12, 20, and 55 ppm can be ascribed to the
resonances of the three methylene groups in the grafted
aminopropyl moieties [24, 25]. The appearance of a peak at
ca.76 ppm suggests that formaldehyde did react with the
anchored amino groups, resulting in a new methylene
The hydrogenation of p-chloronitrobenzene (p-CNB)
with H was employed as a probe reaction to examine the
2
group in –NHCH OH. However, compared with Sun’s
2
catalytic performance of the as-synthesized Au/SiO -org.
2
123

7
92
X. Tan et al.
For comparison, SiO -org supported Au catalyst was also
2
selectivity of p-chloroaniline was 67 %, and the byproduct
was chlorine-containing nitroso compound, a kind of
reaction intermediate for the hydrogenation of nitro aro-
matic compounds. Further increasing the reaction time to
120 min, the conversion increased significantly to reach
99 %, and the selectivity of p-chloroaniline was improved
to 80 %, proving that chlorine-containing nitroso com-
pound could be further hydrogenated to p-chloroaniline.
To uncover the underlying reason for the difference of
catalytic activity, we examined the gold particle size dis-
tribution on the two catalysts by means of XRD and TEM.
From the TEM images (Fig. 2a, d), it can be seen that the
prepared by the method of Chen et al. [14] by just changing
Ag to Au. Our catalyst exhibited higher catalytic activity
than that obtained by Chen et al. method (Fig. 6). For the
catalyst by Chen et al. method, after 40 min of reaction at
140 °C and 4.0 MPa, the conversion was only 58 %, and it
took 180 min to reach near 100 %; however, in our case
the conversion reached 99 % after 40 min and 100 % after
7
0 min. In order to investigate the influence of reaction
temperature on the conversion and the selectivity, the
hydrogenation of p-CNB was performed at 100 °C. The
conversion of p-CNB was only 33 % for 40 min, the
2
Table 1 Catalytic hydrogenation of various aromatic nitro compounds over Au/SiO -org
Entry
1
Substrate
Product
T (°C)
Time (min)
40
Conv. (%)
33
Sel. (%)
a
67
100
Cl
NO2
NO2
NO2
NO2
NO2
NO2
Cl
NH2
NH2
NH2
NH2
NH2
NH2
a
2
3
100
140
140
140
140
140
120
40
[99
99
80
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
99
98
90
96
98
b
4
40
99
c
5
40
95
6
7
120
120
[99
[99
NO2
NH2
Cl
Cl
NO2
8
9
140
140
140
120
120
360
[99
[99
[99
100
97
NH2
H C
NO2
H C
NH2
3
3
1
0
95
NO2
NH2
NO2
NH2
Reaction conditions: 4.0 MPa H
a
The byproduct was hydrogenation intermediate (chlorine-containing nitroso compound)
2
, 0.1 g of catalyst, 0.5 g of substrates, and 40 mL of ethanol
b
c
Catalyst was filtered off after the 1st reaction cycle (entry 3) and the substrate was added for the second reaction cycle
Catalyst after the 2nd reaction cycle (entry 4) was adopted
1
23

Organic–Inorganic Hybrid SiO
2
Supported Gold Nanoparticles
793
sizes of gold particles are 1.8 ± 0.5 nm, which are well in
agreement with the XRD results (Fig 1a, d), implying that
the dispersions of our catalyst and the catalyst by Chen et al.
method are almost the same. So the enhanced activity of our
sample could be attributed to the more organic groups around
the Au nanoparticles, which may tune the electronic nature of
Au, leading to favorable catalysis for organic reactions.
When HAuCl and SiO -org were separately used as cata-
mainly due to the more organic groups around the Au
nanoparticles. This new route provides a useful platform for
the fabrication of metal nanoparticles (such as Pd and Pt)
based on SiO grafted with –NH by altering the metal pre-
2
2
cursors. This methodology should therefore prove to be
useful in catalysis and other fields.
Acknowledgments This study was supported by the National Natural
Science Foundation of China (20906008 and 21176037), the Science
Foundation of Dalian University of technology (DUTSF200805) and
the Fundamental Research Funds for the Central Universities
(DUT09RC(3)158 and DUT12LK30).
4
2
lysts for the reaction, no reduction products were detected.
This confirms that itis the gold nanoparticles on Au/SiO -org
2
that function as real active sites. The catalytic reductions of
various aromatic nitro compounds over Au/SiO -org were
2
also examined. From Table 1, it can be concluded that the
conversion of several substrates and selectivity of products
could reach nearly 100 % and [95 %, respectively. How-
ever, the formation of nitroso compound intermediate cannot
be totally avoided.
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