Superior catalytic properties in aerobic oxidation of olefins over Au nanoparticles on pyrrolidone-modified SBA-15

Article abstract of DOI:10.1016/j.jcat.2011.03.029

We report on our systematic investigation of Au nanoparticles highly dispersed in the mesopores of (s)-(-)-2-pyrrolidinone-5-carboxylic acid (Py)-modified SBA-15 (Au/SBA-15-Py) by using a series of modern techniques. ¹³C NMR and IR spectroscopies indicate that Py species are successfully grafted on the surface of mesopores in SBA-15; XRD patterns and N₂ adsorption isotherms show that the sample mesostructures are well preserved; TEM images clearly confirm the uniform Au nanoparticles in the mesopores; and XPS suggests an interaction between Au nanoparticles with Py species. Interestingly, Au/SBA-15-Py catalysts always exhibit superior catalytic properties in the oxidation of cyclohexene and styrene by molecular oxygen at atmospheric pressure, compared with the pyrrolidone-free SBA-15 supported Au catalyst (Au/SBA-15-N). This phenomenon is reasonably related to the interaction between Au nanoparticles with Py species, which is consistent with results analyzed from our density-functional theory (DFT) calculations.

Full text of DOI:10.1016/j.jcat.2011.03.029

Journal of Catalysis 281 (2011) 30–39
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Superior catalytic properties in aerobic oxidation of olefins over Au nanoparticles
on pyrrolidone-modified SBA-15
b
c
d
b
b
a
Liang Wang , Hong Wang , Prokop Hapala , Longfeng Zhu , Limin Ren , Xiangju Meng ,
c
a,
⇑
James P. Lewis , Feng-Shou Xiao
a
Key Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University (XiXi Campus), Hangzhou 310028, China
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130012, China
Department of Physics, West Virginia University, Morgantown, WV 26506-6315, USA
b
c
d
Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnická 10, 1862 53 Prague, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
We report on our systematic investigation of Au nanoparticles highly dispersed in the mesopores of (s)-
Received 18 February 2011
Revised 30 March 2011
Accepted 31 March 2011
Available online 10 May 2011
(
–)-2-pyrrolidinone-5-carboxylic acid (Py)-modified SBA-15 (Au/SBA-15-Py) by using a series of modern
13
techniques. C NMR and IR spectroscopies indicate that Py species are successfully grafted on the surface
of mesopores in SBA-15; XRD patterns and N adsorption isotherms show that the sample mesostruc-
2
tures are well preserved; TEM images clearly confirm the uniform Au nanoparticles in the mesopores;
and XPS suggests an interaction between Au nanoparticles with Py species. Interestingly, Au/SBA-15-
Py catalysts always exhibit superior catalytic properties in the oxidation of cyclohexene and styrene
by molecular oxygen at atmospheric pressure, compared with the pyrrolidone-free SBA-15 supported
Au catalyst (Au/SBA-15-N). This phenomenon is reasonably related to the interaction between Au nano-
particles with Py species, which is consistent with results analyzed from our density-functional theory
Keywords:
Au nanoparticles
Pyrrolidone
Cyclohexene and styrene oxidation by
molecular oxygen
(DFT) calculations.
Ó 2011 Elsevier Inc. All rights reserved.
1
. Introduction
Recently, Au nanoparticles (NPs) are widely used in a series of
reactions [16–64]. Particularly, olefin oxidations have attracted
much attention since Haruta’s pioneer work of direct oxidation of
propene over supported Au NPs using molecular oxygen [16]. More
recently, Au NPs stabilized in poly (N-vinyl-2-pyrrolidone) solution
(Au/PVP), a typical homogeneous-like catalyst, shows superior cat-
alytic properties in alcohol oxidations [65,66]. Recently, we have
briefly preformed heterogenization of Au/PVP from supporting
Au NPs in SBA-15 modified by (s)-(–)-2-pyrrolidinone-5-carboxylic
acid (Au/SBA-15-Py), and this catalyst is active in alcohol
oxidations [67]. Herein, we have systemically characterized Au/
SBA-15-Py catalysts. Furthermore, it is found that Au/SBA-15-Py
exhibits excellent catalytic properties in olefin oxidations with
molecular oxygen, and we analyze the origin of its high activity
from the results of density-functional theory (DFT) calculations.
The oxidation of olefins is of great interest and importance in
the production of fine chemicals and chemical intermediates [1–
]. Generally, the olefin oxidations are performed by using organic
or inorganic oxidants (tert-butyl hydroperoxide, TBHP, or KMnO ),
which produce a great deal of environmentally undesirable waste
5–8]. Molecular oxygen, which is widely accepted as a green oxi-
4
4
[
dant with obvious advantages including atom economy and low
cost, is strongly desirable for a long time. However, the use of
molecular oxygen in olefin oxidations is still a challenge [9–11].
It is well known that many metal (Co, Cu and V) compounds are
active homogeneous catalysts for olefin oxidations [12–14], but
these catalysts have difficulty in separation and regeneration.
Heterogeneous catalysts could effectively solve these problems,
but their catalytic activities are much lower than those of homoge-
neous ones. Therefore, heterogenization of homogeneous catalysts
on solid supports could combine the advantages of both homoge-
neous catalysts (high activity) and heterogeneous catalysts (recy-
clability). For example, Knopsgerrits et al. reported that the
complexes of manganese (II) with bipyridine (bpy) entrapped in
zeolites are catalytically active and efficiently recyclable catalysts
in olefin oxidations [15].
2
. Experimental and computational methodology
2.1. Chemicals and regents
All reagents were of analytical grade and used without further
treatment. Nonionic block copolymer surfactant (Pluronic 123,
molecular weight of about 5800) was purchased from Sigma–Aldrich
Co. (s)-(-)-2-Pyrrolidinone-5-carboxylic acid (Py), cyclohexene epox-
ide, 2-cyclohexen-1-ol, 2-cyclohexen-1-one, 1, 2-cyclohexanedione,
and styrene epoxide were obtained from Alfa Aesar. Co. Styrene,
⇑
Corresponding author. Fax: +86 431 85168624.
E-mail address: fsxiao@mail.jlu.edu.cn (F.-S. Xiao).
0
021-9517/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2011.03.029

L. Wang et al. / Journal of Catalysis 281 (2011) 30–39
31
1
3
H
N
O
O
O
4
7
Si
N
H
2
NH
5
6
O
8
9
HN
1
0
1
2
O
11
a
3
,4,5,6
1
1
2
7
b
c
3
,4,5,6
9
2
,12
7
8
11
1
0
4
,5,6
3
2
2
1
7
d
1
11
9
8
10 e
0
50
100
ppm
150
Fig. 1. 13C NMR spectra for solid samples of: (a) SBA-15, (b) SBA-15-N, and (c) Au/SBA-15-Py and liquid samples of (d) DETA and (e) Py.
tetraethyl orthosilicate (TEOS), DMF, cyclohexene, MeCN, and 1,4-
dimethylbenzene were bought from Tianjin Chem. Co. HCl, toluene,
and ethanol were from Beijing Chem. Co. N-[3-(trimethoxysilyl)-pro-
pyl]diethylenetriamine (DETA), 4-dimethylaminopyridine (DMAP),
in the aqueous solution, 2.4 mL of TEOS was introduced. After stir-
ring at 313 K for 20 h, the mixture was transferred into an auto-
clave for condensation at 373 K for 24 h. The as-synthesized
samples (SBA-15) were collected by filtration and treated in etha-
nol containing HCl to remove the surfactants.
HAuCl
4
Á4H
2 3
O, triethylamine (Et N), and 1-ethyl-3-(3-dimethyllami-
nopropyl) carbodiimide hydrochloride (EDCÁHCl) were purchased
from Shanghai Chem. Co.
2.2.2. Preparation of SBA-15-N
2
2
.2. Sample preparation
SBA-15-N was prepared from the interaction between SBA-15
with DETA. As a typical run, 1 g of SBA-15 was dried at 393 K under
vacuum for 3 h, followed by addition of 50 mL of toluene pre-
treated by Na and 1 g of DETA. The mixture was refluxed overnight
and collected by rotary evaporation, followed by washing with a
large amount of ethanol. The sample obtained was designated as
SBA-15-N.
.2.1. Preparation of SBA-15
SBA-15 was synthesized according to literature [68]. As a typi-
cal run, 0.8 g of copolymer surfactant (P123) was dissolved in
5 mL of water, followed by addition of 3 mL HCl (10 M/L). After
stirring at room temperature until complete dissolution of P123
2

3
2
L. Wang et al. / Journal of Catalysis 281 (2011) 30–39
OH
OH
OH
SBA-15
DETAcoating
H
N
O
Auloading
O
O
Si
N
H
NH2
SBA-15-N
Pyrrolidonecoating
Au/SBA-15-N
O
N
N
O
O
O
O
Si
N
Auloading
HN
N
O
O
N
O
O
SBA-15-Py
Au/SBA-15-Py
Scheme 1. Procedures for preparation of Au/SBA-15-N and Au/SBA-15-Py catalysts.
2
.2.3. Preparation of Au/SBA-15-N
Au/SBA-15-N was prepared by addition of SBA-15-N into
nitrogen were measured using a Micromeritics ASAP Tristar. The
samples were outgassed for 10 h at 150 °C before the measure-
ment. Pore-size distribution for mesopores was calculated using
Barrett–Joyner–Halenda (BJH) model. The content of Au was deter-
mined from inductively coupled plasma (ICP) with a Perkin-Elmer
HAuCl
at 373 K under vacuum, reduction by NaBH
and washing with a large amount of ethanol and water.
4
solution, stirring at room temperature for overnight, drying
4
in anhydrous toluene,
1
3
plasma 40 emission spectrometer. The C NMR spectra were re-
corded on a Bruker Avance-400WB spectrometer using CP-TOSS
program with 7.5 mm of MAS probe, 12 kHz of spinning rate, rep-
etition time of 3 s, contact time of 1 s. XPS spectra were performed
by a Thermo ESCALAB 250, and the binding energies were cali-
brated by C1s peak (284.9 eV). IR spectra were recorded using a
Bruker 66 V FTIR spectrometer.
2
.2.4. Preparation of SBA-15-Py
To prepare pyrrolidinone-modified SBA-15 (SBA-15-Py), 3.16 g
of (s)-(–)-2-Pyrrolidinone-5-carboxylic acid (Py) was dissolved in
a mixture of dichloromethane and DMF (molar ratio of dichloro-
methane/DMF at 10), followed by addition of EDCÁHCl and Et
3
N
(
molar ratio of Py/EDCÁHCl/Et
3
N at 1/1.5/1.5). After stirring at room
temperature for 20 min, 1 g of SBA-15-N and a small amount of 4-
dimethylaminopyridine (DMAP, 10 mg) were introduced, stirring
at room temperature for overnight under anhydrous condition.
The sample (SBA-15-Py) was collected by filtration and washing
with a large amount of ethanol and water.
2.4. Catalytic tests
Olefin oxidations were carried out in a 40-ml glass reactor with a
magnetic stirrer. Typically, substrate, solvent, and catalyst were
mixed in the reactor, then the mixture was stirred (900 rpm). After
increasing the temperature (the temperature was measured with a
2.2.5. Preparation of Au/SBA-15-Py
Au/SBA-15-Py was prepared from mixing SBA-15-Py with
2
thermometer in an oil bath), pure O was introduced and sealed in
HAuCl
dryness at 373 K under vacuum, reduction by NaBH
toluene, and washing with a large amount of ethanol and water.
4
solution for overnight at room temperature, followed by
the reaction system at atmosphere pressure by a U tube made by
ourselves. After reaction, the products were analyzed by gas chro-
matography (GC-14C and GC-17A, Shimadzu, FID) with a flexible
quartz capillary column (OV-17 or OV-1). The recyclability of the
catalyst was tested by separating it from the reaction system by cen-
trifugation, washing with large quantity of methanol, and drying at
4
in anhydrous
2
.3. Sample characterization
3
73 K for 6 h, then the catalyst was reused in the next reaction.
Powder X-ray diffraction patterns (XRD) were obtained with a
Siemens D5005 diffractometer and Rigaku D/MAX 2550 diffrac-
tometer with Cu K radiation (k = 0.1542 nm). Transmission elec-
tron microscopy (TEM) experiments were performed on a JEM-
010 electron microscope (JEOL, Japan) with an acceleration volt-
age of 300 kV. Nitrogen isotherms at the temperature of liquid
a
2.5. Computational methodology
3
We performed DFT calculation on a model Au–Py system to bet-
ter interpret the electronic interaction between the Py terminal

L. Wang et al. / Journal of Catalysis 281 (2011) 30–39
33
and Au nanoparticles. We use Au55 (quasicrystalline I
h
structure)
Fig. 2 shows IR spectra of SBA-15-N and Au/SBA-15-Py catalysts.
The band at 1559 cm (Fig. 2b and c) is assigned to CAN bond
À1
with a Py as our model nanoparticle structure to examine the inter-
action between Py and the Au NP; Au55 has been studied both the-
oretically and experimentally [69–71]. For this investigation, we
used the plane-wave computational package VASP [72,73] with
ultrasoft pseudopotential and within both the local-density
approximation (LDA) and the generalized gradient approximation
[75,76]. Particularly, Au/SBA-15-Py has additional bands at 1668
À1
and 1743 cm , indicating the presence of C@O bonds attributed
to pyrrolidone species in Au/SBA-15-Py [75]. These results are well
consistent with successful grafting of DETA and Py species on SBA-
1
3
15 characterized from C NMR spectroscopy.
(
GGA PW91). The unit cell was a cube of 20.0 Å, and the plane-
Fig. 3 shows XRD patterns in small angles of Au/SBA-15-Py and
Au/SBA-15-N. Both show three well-resolved XRD peaks in the re-
gion of 0.6–2° indexed to (1 1 0), (2 0 0), and (2 1 1) reflections of
hexagonal mesoporous arrays, indicating that their hexagonal
mesostructure is well remained. In nitrogen isotherms (Fig. 4),
both samples exhibit typical type-IV curve, giving a hysteresis loop
at relative pressure of 0.52–0.74. Correspondingly, they exhibit
pore-size distribution at 6.3–6.9 nm, which is smaller than that
wave cutoff and convergence criteria were set automatically to
medium precision by keyword (PREC = med), which means that
for the Au55 nanoparticle Ecut = 179.7 eV (Eaug = 303.0 eV for the po-
tential) and for Au55–Py Ecut = 396.0 eV (Eaug = 700.0 eV for the po-
tential as the potentials for hydrogen are very sharp).
A
À2
À3
convergence criteria of 0.1 Â 10 eV (0.1 Â 10 eV) were used
for the ionic motion (electronic SCF).
(
8.4 nm) of SBA-15. In addition, Au/SBA-15-Py and Au/SBA-15-N
2
show relatively low surface area (222 and 286 m /g), compared
with SBA-15 (948 m /g, Table 1). All of these results support that
3
. Results and discussion
2
Au nanoparticles and organic species including DETA and Py exist
in the mesopores [76].
3
.1. Characterization of Au catalysts
Fig. 1 shows 13C NMR spectra of SBA-15, SBA-15-N, Au/SBA-15-
We have also characterized Au NPs by TEM and wide-angle XRD
techniques. TEM images (Fig. 5) give direct observation that Au NPs
with similar sizes of 2–3 nm are highly dispersed in the mesopores
of both Au/SBA-15-Py and Au/SBA-15-N. Furthermore, the sample
wide-angle XRD patterns (Fig. 3) show a very broad diffraction
Py, DETA, and Py species. SBA-15 has not obvious signal associated
with carbon species (Fig. 1a), indicating nearly complete removal
of the copolymer surfactant. SBA-15-N gives peaks at 11.0, 23.0,
4
1.1, 49.5, and 58.0 ppm (Fig. 1b), which are well consistent with
the signals of DETA species (Fig. 1d). Interestingly, Au/SBA-15-Py
exhibits additional peaks at 29.1, 57.1, and 180.0 ppm (Fig. 1c),
in nice agreement with the presence of pyrrolidone species
(Fig. 1e). The appearance of 165 ppm peak (Fig. 1c) indicates the
presence of amide that acts as the linkage of DETA and pyrrolidone
species [74]. These results suggest that both DETA and Py species
are successfully grated on the surface of mesoporous SBA-15, as
proposed in Scheme 1.
(a)
c
(b)
1
2
3
4
5
2Theta (degree)
b
a
Au (111)
(a)
1743
(b)
1668
1559
2
0
30
40
50
60
70
80
90
2000
1800
1600
1400
2
Theta (degree)
-1
Wavenumber CM
Fig. 3. Small (top) and wide (bottom) angle XRD patterns of: (a) Au/SBA-15-N and
Fig. 2. IR spectra of: (a) SBA-15, (b) SBA-15-N, and (c) Au/SBA-15-Py samples.
(b) Au/SBA-15-Py samples.

3
4
L. Wang et al. / Journal of Catalysis 281 (2011) 30–39
8
7
6
5
4
3
2
1
00
00
00
00
00
00
00
00
0
11
(
a)
1
0
9
8
7
6
5
4
3
2
1
0
(a)
(
b)
(
c)
(
b)
c)
(
0
.0 0.2 0.4 0.6 0.8 1.0
0
20
40
Relative Pressure P/P0
Pore Width (nm)
Fig. 4. Nitrogen adsorption–desorption isotherms and pore-size distribution of: (a) SBA-15, (b) Au/SBA-15-N, and (c) Au/SBA-15-Py samples.
ble 2, entry 6) of Au/SBA-15-N. In addition, it is also observed that
the solvent strongly influences the catalytic activity (Table 2),
Table 1
Textural parameters, Au content, and particle size of SBA-15, Au/SBA-15-N, and Au/
SBA-15-Py samples.
which might be associated with their distinguishable polarity.
Notably, in 1,4-dimethylbenzene solvent, the addition of TBHP as
radical initiator is necessary. If TBHP is absent, the Au catalysts
are almost inactive. Similar phenomena have been reported previ-
ously [4,80].
Sample
S
BET
2
Pore width
(nm)
Au content
(wt.%)
Au nanoparticle size
(nm)
(m /g)
Scherrer
Equation
TEM
In cyclohexene oxidation, the major products are cyclohexene
epoxide, 2-cyclohexen-1-ol, 2-cyclohexen-1-one, and 1,2-cyclo-
SBA-15
Au/SBA-
948
286
8.4
6.9
–
2.6
–
–
2.2
2.0–
3.0
2.0–
3.0
hexanedione. The by-products include CO
than C [4]. To understand the reaction route, various products
were used as starting reactants to perform oxidations (Table
). When cyclohexene epoxide was used as a substrate, Au/
2
and the products less
1
5-N
Au/SBA-
5-Py
222
6.3
2.7
2.1
6
1
3
SBA-15-Py was inactive (Table 3, entry 1), which indicates that
the products of 2-cyclohexen-1-ol, 2-cyclohexen-1-one, and
peak at 38.7° associated with (1 1 1) incidence of Au NPs. Accord-
ing to Scherrer equation [77,78], it is estimated that the average
sizes of Au NPs in Au/SBA-15-Py and Au/SBA-15-N are close to
1
,2-cyclohexanedione are from direct oxidation of cyclohexene,
rather than transformation from cyclohexene epoxide intermedi-
ate. When 2-cyclohexen-1-one or 1,2-cyclohexanedione were
used as a substrate, Au/SBA-15-Py was still inactive (Table 3, en-
try 2 and 3), which suggests that the formation of by-products
is not from over-oxidation or decomposition of these com-
pounds. When 2-cyclohexen-1-ol was used as a substrate, 2-
cyclohexen-1-one was selectively formed (Table 3, entry 4). All
of these results suggest that by-products in cyclohexene oxida-
tion are formed from direct over-oxidation of cyclohexene, and
the products of cyclohexene oxide, 2-cyclohexen-1-ol, and 1,2-
cyclohexanedione are resulted from direct oxidation of cyclohex-
ene (Scheme 2).
2
.1 and 2.2 nm (Table 1), confirming similar Au NP size in both
catalysts.
Fig. 6 shows Au4f XPS spectra of Au/SBA-15-N and Au/SBA-15-
Py samples. Au/SBA-15-N gives the binding energy of Au4f7/2 at
4.0 eV, which is typically assigned to the metallic Au [47,79].
Interestingly, Au/SBA-15-Py exhibits Au4f7/2 binding energy at
3.4 eV, with 0.6 eV of downshift from Au/SBA-15-N, indicating
8
8
the presence of an interaction between Au NPs and Py species.
Clearly, the downshift of Au4f7/2 binding energy suggests that the
surface of Au NPs over Au/SBA-15-Py is negatively charged, com-
pared with Au/SBA-15-N [79].
Furthermore, the Au/SBA-15-Py catalyst was treated in toluene
at 100 °C for 8 h. After filtration of Au/SBA-15-Py catalyst, the solu-
tion was used as new catalyst for cyclohexene oxidation. As a re-
sult, no products were found. Additionally, the concentration of
Au in the treated solution was less than 0.01 ppm (ICP analysis).
These results indicate that no Au leaching occurs at Au/SBA-15-
Py catalyst in cyclohexene oxidation.
Moreover, Au/SBA-15-Py catalyst has excellent recyclability in
cyclohexene oxidation (Fig. 7a). For example, after reuse, there is
no loss for catalytic activity (54%); after recycles for four times, Au/S-
BA-15-Py catalyst still shows the conversion at 51%, indicating its
excellent recyclability. Notably, Au4f7/2 XPS spectrum of Au/SBA-
3
.2. Catalytic results
Table 2 presents catalytic data in cyclohexene oxidation by
molecular oxygen under atmospheric pressure using various sol-
vents over Au/SBA-15-Py and Au/SBA-15-N catalysts. Notably, both
catalysts are active, but Au/SBA-15-Py always shows much higher
activity than Au/SBA-15-N. For example, in toluene solvent, Au/
SBA-15-Py shows conversion at 54.0% (Table 2, entry 1), while
Au/SBA-15-N gives low activity (31.2%, Table 2, entry 2); in 1,4-
dimethylbenzene solvent, Au/SBA-15-Py shows conversion at
1
8.7% (Table 2, entry 5), which is much higher than that (5.3%, Ta-

L. Wang et al. / Journal of Catalysis 281 (2011) 30–39
35
(a)
(b)
1
.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2
Particle Size Dimeter (nm)
1
.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4
Particle Size Dimeter (nm)
Fig. 5. TEM images (top) and Au nanoparticle size distribution (bottom) of: (a) Au/SBA-15-Py and (b) Au/SBA-15-N samples (The scale bars are both 50 nm).
1
5-Py recycled for four times shows very similar binding energy
Table 5 presents catalytic data in styrene oxidation by molecu-
lar oxygen under atmospheric pressure using various solvents over
Au/SBA-15-Py and Au/SBA-15-N catalysts. Generally, styrene is not
easy to be oxidized using molecular oxygen on Au catalysts [11,81].
As same as cyclohexene oxidation, Au/SBA-15-Py in styrene oxida-
tion still gives much higher activity than Au/SBA-15-N. For exam-
ple, in toluene solvent, Au/SBA-15-Py exhibits the conversion at
37.4% (Table 5, entry 1), while Au/SBA-15-N gives the conversion
at 15.6% (Table 5, entry 2); in MeCN solvent, Au/SBA-15-Py and
Au/SBA-15-N show the conversion at 22.0% and 11.0%, respectively
(Table 5, entry 3 and 4).
Notably, the use of solvent in styrene oxidation not only influ-
ences the activity but also changes the product selectivity. For
example, in toluene solvent, the selectivity for styrene epoxide is
only 3–4% (Table 5, entry 1 and 2). However, when 1,4-dimethyl-
benzene solvent was used, the selectivity for styrene epoxide
reaches to 10–14% (Table 5, entry 5 and 6). This phenomenon
might be related to that the formation of CAOAC bonds is favor-
able in the presence of non-polar solvent [4].
(
83.5 eV) to that (83.4 eV) of the fresh catalyst, supporting that Au/
SBA-15-Py has good stability during cyclohexene oxidation.
Fig. 8 shows dependences of activities on reaction time at low
conversion (less than 5%) in cyclohexene oxidation at the temper-
ature of 373–393 K over Au/SBA-15-Py and Au/SBA-15-N catalysts.
After fitting, we can estimate initial reaction rate (r
Arrhenius equation, activation energies (E ) of Au/SBA-15-Py and
Au/SBA-15-N catalysts were calculated (Fig. 9), yielding 22.0 and
5.1 kJ/mol, respectively. These results confirm that oxidation of
0
). According to
a
3
cyclohexene over Au/SBA-15-Py is much easier than that over
Au/SBA-15-N catalyst.
It is worth mentioning that Au/SBA-15-Py catalyst is very active
in cyclohexene oxidation under solvent-free condition (Table 4).
À1
Au/SBA-15-Py can give a turnover frequency (TOF) of 12,000 h
(Table 4, entry 1), which is extremely high for the oxidation of ole-
fins on Au catalysts [11]. In contrast, Au/SBA-15-N gives a TOF at
À1
6
400 h (Table 4, entry 2), other Au catalysts including Au/TiO
2
,
Au/MgO and Au/SiO
2
also show relatively low TOF (7020, 4009,
À1
and 4712 h , Table 4, entry 3–5). These results suggest the high
activity of Au sites on Au/SBA-15-Py in solvent-free cyclohexene
oxidation.
Fig. 7b shows the recyclability of Au/SBA-15-Py in styrene
oxidation. After recycles for four times, the catalyst still remains
its activity, indicating its excellent recyclability.

3
6
L. Wang et al. / Journal of Catalysis 281 (2011) 30–39
Table 3
a
Catalytic activities in oxidations of various substrates.
8
3.5
Entry
Substrate
Conversion (%)
Selectivity (%)
1
2
3
4
Cyclohexene epoxide
2-Cyclohexen-1-one
1,2-Cyclohexanedione
2-Cyclohexen-1-ol
<1
<1
<1
26.0
–
–
–
>99.5b
a
Reaction conditions 6 mmol of substrate, 10 ml of toluene solvent, 80 mg of Au/
SBA-15-Py catalyst, time for 8 h, temperature of oil bath at 373 K.
(c)
b
Selectivity for 2-cyclohexen-1-one.
8
3.4
by-products
O
(
b)
8
4.0
OH
O
O
(a)
O
80
84
88
92
by-products
by-products
by-products
Binding energy (eV)
by-products
Fig. 6. Au4f XPS spectra of: (a) Au/SBA-15-N, (b) Au/SBA-15-Py, and (c) Au/SBA-15-
Scheme 2. Proposed routes for the formation of products and by-products in
Py recycled for four times.
cyclohexene oxidation.
3
.3. DFT calculations
obvious differences after the molecule is attached on the Au55
nanoparticle. The frontier orbitals of the Py molecule, which is lar-
gely a contribution from the oxygen atom, have a strong overlap
with the frontier states of an Au atom on the NP surface.
Previously, it was reported that Au55 NPs show strong oxidation
resistance due to the unique geometry and the electronic structure
[11]. In this work, we find that the DOS plot (Fig. 10b) of the iso-
lated Au55 has a small energy gap, which is not promising in oxida-
tion reactions. However, the DOS of the Py molecule attached on
We carried out DFT (both LDA and GGA) calculations on two
model systems of an isolated Au55 nanoparticle and an Au55 NP
with one Py molecule attached (Fig. 10a). The results from both
LDA and GGA are very similar; for clarity, we only present the
LDA results here. The partial density of electronic states (DOS)
for the Au55, Py, and Au55-Py are shown in Fig. 10b. From the
DOS, We find that the electronic properties of Py molecule present
Table 2
a
Catalytic data in cyclohexene oxidation by molecular oxygen over Au/SBA-15-Py and Au/SBA-15-N catalysts.
OH
O
O
O
O , solvent
2
O
Au catalysts
cyclohexene
cyclohexene epoxide 2-cyclohexen-1-ol 2-cyclohexen-1-one 1,2-cyclohexanedione
1
2
3
4
Entry
Solvent
Catalyst
Conversion
Product selectivityb (%)
(
%)
P1
P2
P3
P4
P5
1
2
3
4
5
6
Toluene
Toluene
MeCN
Au/SBA-15-Py
Au/SBA-15-N
Au/SBA-15-Py
Au/SBA-15-N
Au/SBA-15-Py
Au/SBA-15-N
54.0
31.2
40.2
32.9
18.7
5.3
7
3
4
9
1
2
21
33
16
19
7
36
44
38
35
35
21
3
1
7
11
44
50
33
19
35
26
14
18
MeCN
c
1.4-Dimethylbenzene
1.4-Dimethylbenzenec
11
a
b
c
6
mmol of cyclohexene, 10 ml of solvent, 80 mg of catalyst, time for 8 h, temperature of oil bath at 373 K.
P1, cyclohexene epoxide; P2, 2-cyclohexen-1-ol; P3, 2-cyclohexen-1-one; P4, 1,2-cyclohexanedione; P5, CO
t-butyl hydroperoxide (TBHP, 3 mol% based on substrate) is added. The conversion of cyclohexene is very low (<1%) without TBHP in 1,4-dimethylbenzene solvent.
2
and other organic products with less than six C atoms.

L. Wang et al. / Journal of Catalysis 281 (2011) 30–39
37
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0.8
cyclohexene conversion
cyclohexene epoxide selectivity
-cyclohexen-1-ol selectivity
-cyclohexen-1-one selectivity
,2-cyclohexanedione selectivity
(a)
Au/SBA-15-Py
Au/SBA-15-N
2
2
1
0
0
0
0
.6
.4
.2
.0
a
b
-
0.2
2
.52
2.56
2.60
2.64
2.68
2.72
0
1
2
3
4
5
6
-3
-1
1/T 10 K
Run Number
Fig. 9. Experimental points correlating the inverse of the absolute temperature (1/
T) with the initial reaction rates (ln r ) with catalysts of Au/SBA-15-Py and Au/SBA-
5-N. Two straight lines are made with the best fitting of the experimental points.
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
1
(b)
styrene conversion
Table 4
styrene epoxide selectivity
benzaldehyde selectivity
acetophenone selectivity
Solvent-free oxidation of cyclohexene by molecular at atmospheric pressure over Au/
a
SBA-15-Py catalyst. .
À1
Entry
Catalyst
TOF (h
)
1
2
3
4
5
Au/SBA-15-Py
Au/SBA-15-N
12,000
6400
7020
4009
4712
Au/TiO
Au/MgO
Au/SiO
2
2
a
Reaction conditions: 100 mmol of cyclohexene, 15 mg of catalyst, TBHP (3 mol%
based on cyclohexene), O at atmospheric pressure, temperature of oil bath at
2
0
413 K, reaction time for 20 min.
0
1
2
3
4
5
6
Run Number
Fig. 7. Recyclable data in: (a) cyclohexene and (b) styrene oxidations over Au/SBA-
Table 5
1
5-Py catalyst.
Catalytic data in styrene oxidation by molecular oxygen over Au/SBA-15-Py and Au/
SBA-15-N catalysts.
a
O
O
6
6
4
2
0
O
3
3
3
73 K
83 K
93 K
O
3
3
3
73 K
83 K
93 K
O , solvent
2
Au catalysts
mol% TBHP
3
4
2
styrene
Entry Solvent
styrene epoxide
benzaldehyde
acetophenone
phenylacetaldehyde
1
2
3
4
Catalyst
Conversion Product selectivityb
%)
(
(
%)
P1 P2 P3 P4 P5
1
2
3
4
5
6
Toluene
Toluene
MeCN
Au/SBA-15-Py 37.4
Au/SBA-15-N 15.6
Au/SBA-15-Py 22.0
Au/SBA-15-N 11.0
4
3
3
5
79 10
–
1
–
–
–
4
7
4
50
43
30
30
86
45
52
6
2
–
MeCN
1.4-Dimethylbenzene Au/SBA-15-Py 30.2
1.4-Dimethylbenzene Au/SBA-15-N 17.3
14 32 26
10 40 16
0
0
2
4
6
0
2
4
6
a
Reaction time (min)
Reaction time (min)
6 mmol of styrene, 10 ml of solvent, 80 mg of catalyst, time for 8 h, temperature
of oil bath at 373 K; t-butyl hydroperoxide (TBHP, 3 mol% based on substrate) is
added.
Fig. 8. Dependences of activities on reaction time at low conversion in cyclohexene
oxidation at the temperature of 373–393 K over Au/SBA-15-Py and Au/SBA-15-N
catalysts.
b
P1, styrene epoxide; P2, benzaldehyde; P3, acetophenone; P4, phenylacetalde-
hyde; P5, phenylacetic acid, benzoic acid, ester, and some others.
the Au55 NP present quite a different DOS before (Fig. 10c) and
after (Fig. 10d) the attachment of Py on the Au55 nanoparticle.
The discrete molecule orbitals induce specific electronic modifica-
tion to the Au55 NP; therefore, the delocalized electrons distributed
onto the Au NP surface changes after the Py attachment. Because
there is only one Py molecule attached Au55 model, we speculate

3
8
L. Wang et al. / Journal of Catalysis 281 (2011) 30–39
(b)
(
a) model: Au55 and Au Py
55
gap
Au nanoparticle
Au Py complex
55
5
5
(c)
(d)
1
1
1
2
10
8
9
1
3
1
1
1
2
10
9
8
13
Fig. 10. (a) Structures of Au55 nanoparticle and Au55Py complex; (b) The density of states (DOS) of Py alone, Py of Au55Py, Au55 alone, and Au55 after Py attached, the gap
showing in the plots is the HOMO–LUMO gap of Py molecule alone; the projected density of states (PDOS) of (c) all the atoms of Py molecule itself and (d) Py molecule on Au55
nanoparticle.
that more Py attached will serve as a functional group on the NP
surfaces and induce more electronic modification of the entire Au
nanoparticle. This modification induces imbalanced localization
of charge distribution on the entire Au nanoparticle, which will
cause more negative charge on the Au NP surface and assist oxida-
tion reaction occurring. In accordance with the XPS results shown
in Fig. 6, we propose that the Py attachment on Au NP induces
charge redistribution on the surface of the Au NP; therefore, the
catalytic activity is accordingly enhanced.
China (2009CB623501). Dr. H. Wang was supported in part by
the (USA) National Science Foundation (DMR-0903225) and by
the Advanced Energy Initiative of West Virginia University. P.
Hapala was supported by the Czech Republic Grant No. SVV-
2010-263303.
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