Selective oxidation of saturated hydrocarbons using Au-Pd alloy nanoparticles supported on metal-organic frameworks

Article abstract of DOI:10.1021/cs300754k

Gold (Au) and palladium (Pd) nanoparticles dispersed on a zeolite-type metal-organic framework (i.e., MIL-101) were prepared via a simple colloidal method. The catalysts were characterized by powder X-ray diffraction, N ₂ physical adsorption, atomic absorption spectroscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. Au and Pd were mostly in the form of bimetallic alloys on the MIL-101 support. The Au-Pd/MIL-101 was active and selective in the oxidation of a variety of saturated (including primary, secondary, and tertiary) C-H bonds with molecular oxygen. For the liquid-phase oxidation of cyclohexane, cyclohexane conversion exceeding 40% was achieved (TOF: 19 000 h^-1) with >80% selectivity to cyclohexanone and cyclohexanol under mild solvent-free conditions. Moreover, the Au-Pd alloy catalyst exhibited higher reactivity than their pure metal counterparts and an Au + Pd physical mixture. The high activity and selectivity of Au-Pd/MIL-101 in cyclohexane aerobic oxidation may be correlated to the synergistic alloying effect of bimetallic Au-Pd nanoparticles.

Full text of DOI:10.1021/cs300754k

Research Article
pubs.acs.org/acscatalysis
Selective Oxidation of Saturated Hydrocarbons Using Au−Pd Alloy
Nanoparticles Supported on Metal−Organic Frameworks
Jilan Long, Hongli Liu, Shijian Wu, Shijun Liao, and Yingwei Li*
Key Laboratory of Fuel Cell Technology of Guangdong Province, School of Chemistry and Chemical Engineering,
South China University of Technology, Guangzhou 510640, China
ABSTRACT: Gold (Au) and palladium (Pd) nanoparticles dispersed on a zeolite-type
metal−organic framework (i.e., MIL-101) were prepared via a simple colloidal method. The
catalysts were characterized by powder X-ray diffraction, N2 physical adsorption, atomic
absorption spectroscopy, transmission electron microscopy, energy-dispersive X-ray spectros-
copy, and X-ray photoelectron spectroscopy. Au and Pd were mostly in the form of bimetallic
alloys on the MIL-101 support. The Au−Pd/MIL-101 was active and selective in the oxidation
of a variety of saturated (including primary, secondary, and tertiary) C−H bonds with
molecular oxygen. For the liquid-phase oxidation of cyclohexane, cyclohexane conversion
exceeding 40% was achieved (TOF: 19 000 h−1) with >80% selectivity to cyclohexanone and
cyclohexanol under mild solvent-free conditions. Moreover, the Au−Pd alloy catalyst exhibited
higher reactivity than their pure metal counterparts and an Au + Pd physical mixture. The high activity and selectivity of Au−Pd/
MIL-101 in cyclohexane aerobic oxidation may be correlated to the synergistic alloying effect of bimetallic Au−Pd nanoparticles.
KEYWORDS: oxidation, saturated C−H bonds, metal−organic frameworks, gold, palladium, bimetallic catalysts
(>80%) at elevated conversion (>30%) still remains a great
challenge. More recently, carbon-based materials were attempted
as metal-free catalysts for this transformation, but they also
furnished a low yield of KA-oil in the liquid phase oxidation of
cyclohexane.28,29
Metal−organic frameworks (MOFs) are a new class of hybrid
porous materials assembled with metal cations and organic
ligands.30 Owing to their high surface area, porosity, and chemical
tunability, the uses of MOFs as heterogeneous catalysts have
recently received tremendous attention, especially for using MOFs
as supports for metal (e.g., Pd, Au, Ru, and Pt) nanoparticles.31−44
Very recently, MOF-supported Au nanoparticles have also been
employed as catalysts for the aerobic oxidation of cyclohexane to
cyclohexanone and cyclohexanol.15
It has been long known that metal alloying can promote
catalytic activity or selectivity of the monometal species.45 In
recent years, Au−Pd bimetallic nanoparticles have attracted
considerable interest owing to their high activities and
selectivities in a number of oxidation reactions;43,46−50 however,
to the best of our knowledge, there is currently no report of
using Au−Pd bimetallic nanoparticles as catalysts for the
oxidation of cyclohexane. In this work, we report, for the first
time, a highly active, selective, and reusable heterogeneous
Au−Pd alloying catalyst, which was deposited on a zeolite-type
MOF (i.e., MIL-101), in the liquid-phase aerobic oxidation of
cyclohexane. It is shown that the synergy effect between Au and
Pd leads to an enhanced performance of cyclohexane aerobic
oxidation.
1. INTRODUCTION
Selective hydrocarbon oxidation is of academic as well as
industrial significance for the production of valuable chemicals
and intermediates, such as ketones and alcohols.1 Among various
oxidation transformations, selective oxidation of saturated
hydrocarbons using environmentally benign molecular oxygen
is believed to be one of the most problematic processes to control
because of the inertness of saturated C−H bonds and the
overoxidation issues derived from the increasing reactivity of
products, as compared with starting materials. As an example,
industrial production of cyclohexanone and cyclohexanol (also
known as KA-oil) from cyclohexane oxidation using molecular
oxygen as oxidant has to be carefully controlled (at conversions
<5% with 75−80% selectivity for KA-oil) to avoid the formation
of excessive amounts of byproducts owing to overoxidation.2−5
As is well-known, cyclohexanone and cyclohexanol are important
chemical intermediates for the bulk production of polyamide and
plastics, such as Nylon 6 and Nylon 66.6
Currently, the commercial processes for the production of
KA-oil from cyclohexane oxidation normally employ homoge-
neous transition-metal salts as catalysts, which have suffered from
serious problems, such as corrosion and pollution. Therefore,
over the past few decades, a great deal of research effort has been
devoted to finding alternative and more environmentally sound
methodologies to achieve a selective oxidation of cyclohexane.
Heterogeneous catalysis can play a key role in the development
of sustainable processes for these oxidation transformations, and
a number of heterogeneous metal catalysts have been explored
in this regard.7−27 Among these heterogeneous catalyst systems,
supported Au catalysts have received the most attention in recent
years because of the unique selectivity in cyclohexane aerobic
oxidation.7−15 However, the attainment of high selectivities
Received: November 22, 2012
Revised: January 21, 2013
© XXXX American Chemical Society
647
dx.doi.org/10.1021/cs300754k | ACS Catal. 2013, 3, 647−654

ACS Catalysis
Research Article
numbers of these parameters were used to calculate the metal
dispersion considering the molar ratio of Au to Pd in the sample.
2.3. Catalytic Testing. The oxidation of cyclohexane was
carried out in a 30 mL Parr batch reactor with a polytetrafluoro-
ethylene (PTFE) liner. Typically, 10 mL of cyclohexane, 50 mg
of solid catalyst, and anisole (as an internal standard) were added
into the reactor. The autoclave was sealed and purged several
times with pure O2 to remove the air. Then the reactor was
heated to 150 °C, and the O2 pressure was adjusted to 1.0−1.5
MPa. After 4 h of reaction, the reactor was cooled to 0 °C. The
gas phase composition was analyzed by a GC equipped with
a TCD detector and a packed column (carbon molecular
sieve TDX-01), which indicated that no gaseous product was
produced in the reactions. The liquid products were analyzed
using a GC (HP 6890 series) with a mass spectrometer detector
(HP 5973 mass selective detector) and a capillary column
(HP 5MS).
2. EXPERIMENTAL SECTION
2.1. Chemicals. All chemicals were purchased from
commercial sources and used without further treatments. All
solvents were analytical grade and distilled prior to use.
2.2. Catalyst Preparation and Characterization. MIL-
101 was synthesized and purified according to the reported
procedures.51 MIL-101-supported Au−Pd catalysts were pre-
pared by a sol−gel method. In a typical synthesis, an aqueous
solution of HAuCl4 and PdCl2 (1 × 10−3 M) was first prepared
with PVP (polyvinylpyrrolidone) as a protecting agent (PVP
monomer/(Au + Pd) = 10:1, molar ratio). The mixture was
vigorously stirred for 1 h in an ice bath of 0 °C. Then a freshly
prepared solution of NaBH4 (0.1 M, NaBH4/(Au + Pd) = 5:1,
molar ratio) was injected rapidly to the solution to obtain a dark
brown sol. Within a few minutes of sol generation, the activated
MIL-101 was added to the colloidal solution and stirred for
6 h, followed by washing thoroughly with deionized water. The
sample was dried under vacuum at 100 °C for 2 h and then
heated at 200 °C in H2 for 2 h. The metal loadings (Au + Pd) for
all the samples were around 1 wt %, on the basis of atomic
absorption spectroscopy (AAS) analysis (Table 1).
For the recyclability test, after reaction, the Au−Pd/MIL-101
catalyst was separated from the reaction mixture by centrifuga-
tion, thoroughly washed with acetonitrile and ethyl acetate, dried
at 80 °C, and then reused as catalyst for the next run.
3. RESULTS AND DISCUSSION
The powder XRD patterns (Figure 1) of the Au−Pd/MIL-101
samples all match with those of the parent MIL-101, indicating
Table 1. Surface Areas, Pore Volumes, and Metal Loadings of
the MIL-101 Samples
Au + Pd
Au/Pd
SBET
(m2 g−1
3251
3051
2974
2907
2887
2872
2500
2851
2767
2623
SLangmuir
Vpore
(cm3 g−1
1.60
sample
MIL-101
(wt%) molar ratio
)
(m2 g−1
4494
4397
4274
4202
4156
4108
3510
4076
3964
3761
)
)
Pd/MIL-101
1.03
0.99
0.97
0.97
1.05
0.98
0.99
0.99
0.97
0:1
1.55
Au/MIL-101
1:0
1.53
Au−Pd/MIL-101
Au−Pd/MIL-101
Au−Pd/MIL-101
Au−Pd/MIL-101
Au−Pd/MIL-101
Au−Pd/MIL-101
Au−Pd/MIL-101
1:1.9
1:1.6
1:1.4
1:1
1.49
1.48
1.48
1.32
1.4:1
1.8:1
2.6:1
1.47
1.42
1.36
Powder X-ray diffraction patterns of the samples were
obtained on a Rigaku diffractometer (D/MAX-IIIA, 3 kW)
using Cu Kα radiation (40 kV, 30 mA, 0.1543 nm). BET surface
area and pore size measurements were performed with N2
adsorption/desorption isotherms at 77 K on a Micromeritics
ASAP 2020 instrument. Before the analysis, the samples were
degassed at 150 °C overnight. The metal loadings of the samples
were measured quantitatively by atomic AAS on a Hitachi
Z-2300 instrument. The size and morphology of the samples
were investigated by using a transmission electron microscope
(TEM, JEOL, JEM-2010HR) with energy-dispersive X-ray
spectroscopy (EDS) analysis. Prior to analysis, solids were
suspended in ethanol and deposited straight away on a copper
grid. X-ray photoelectron spectroscopy (XPS) measurements
were performed on a Kratos Axis Ultra DLD system with a base
pressure of 10−9 Torr. Infrared spectra were recorded on a
Thermo Scientific Nicolet iS10 spectrometer use Smart OMNI-
Transmission Accessory.
Figure 1. Powder XRD patterns of MIL-101 samples: (a) MIL-101
(as-synthesized); (b) Au/MIL-101; (c) Pd/MIL-101; and Au−Pd/
MIL-101 (Au/Pd molar ratio = 1.4:1) (d) before and (e) after catalytic
reaction.
that the structure of MIL-101 was mostly maintained by using
the developed preparation recipes. The stability of the porous
structure of MIL-101 after metal doping could be further
confirmed by the N2 adsorption results at 77 K (Figure 2 and
Table 1). The appreciable decrease in N2 adsorption amount can
be attributed to the blockage of cavities of MIL-101 by the metal
nanoparticles located in the pore or at the surface of the MIL-101
framework. As evident from the pore size distribution curves
(Figure 2), the introduction of metal leads to a slight decrease
in the pore sizes of MIL-101. The window diameters of the
two large cages of MIL-101 are reported to be ∼12 and 16 Å
respectively,51 which make the cages accessible to very large
molecules.
The metal dispersion was calculated by using the equation
DM = (6nsM)/(ρNdp), where ns is the number of atoms at the
surface per unit area (1.15 × 1019 m−2 for Au, and 1.06 × 1019 m−2
for Pd), M is the molecular weight (196.97 g mol−1 for Au, and
106.42 g mol−1 for Pd), ρ is the density (19.5 g cm−3 for Au, and
12.02 g cm−3 for Pd), N is 6.023 × 1023 mol−1, and dp is the
average particle size measured by HRTEM. The average
TEM characterization revealed that the Au−Pd nanoparticles
were homogeneously dispersed in the materials, with particle
sizes typically between 2 and 3 nm (average size: 2.40 0.63 nm)
(Figure 3). No significant formation of aggregates was observed.
648
dx.doi.org/10.1021/cs300754k | ACS Catal. 2013, 3, 647−654

ACS Catalysis
Research Article
Figure 2. Nitrogen adsorption/desorption isotherms at 77 K (top) and
Horvath−Kawazoe pore-size distribution curves (bottom) of the as-
Figure 3. (a−c) TEM images of Au−Pd/MIL-101 (Au/Pd molar ratio =
1.4:1), (d) corresponding size distribution of Au−Pd nanoparticles, and
(e) the EDS pattern.
⧫
■
●
synthesized MIL-101 ( ), Pd/MIL-101 ( ), Au/MIL-101 ( ), and
▲
Au−Pd/MIL-101 (Au/Pd = 1.4:1) ( ).
The high-resolution images of Au−Pd/MIL-101 (Figure 3c)
showed that the particles were highly faceted and twinned in
character. The interplanar spacings of the particle lattice were
0.224 or 0.236 nm, which agree well with the (111) lattice
spacing of face-centered cubic Pd and cubic Au, respectively. The
TEM results indicated that Au and Pd formed mostly bimetallic
alloys on the MIL-101 support by using the present deposition
method. The metal dispersion was calculated to be ∼49%, on the
basis of the average particle size of Au−Pd, assuming spherical
particles.
XPS data indicated that the Au(0) 4f and Pd(0) 3d peaks for
the Au−Pd/MIL-101 (Au/Pd molar ratio = 1.4:1) were shifted
to lower binding energies by ∼0.4 and 0.3 eV, respectively, with
respect to the monometallic Au/MIL-101 and Pd/MIL-101
samples (Figure 4). Such an observed negative shift for both Au
4f and Pd 3d binding energies was previously reported and
reflected a modification of the electronic structure of the surface
Au and Pd atoms, which could be indicative of the formation
of Au−Pd bimetallic alloys.50,52 In Au−Pd alloy, Pd is generally
more electronegative and gains d electrons from Au, which is
supposed to be compensated by the depletion of core s or p
electron count (i.e., net charge flowing into Au).53,54
improved with the addition of Pd, and the Au−Pd/MIL-101
catalysts with different Au/Pd ratios were subsequently screened
in the aerobic oxidation of cyclohexane (Table 2, entries 4−11).
The results of the cyclohexane oxidation pointed to an optimized
performance of Au−Pd/MIL-101 with a 1.4:1 Au/Pd molar ratio
(Table 2, entry 6), which provided the highest KA-oil yield and
TOF value with a high selectivity for KA-oil (∼95%). Further
increasing the content of Pd in the catalysts resulted in lower
conversions and yields of KA-oil (Table 2, entries 7−11).
The use of monometallic Pd/MIL-101 as catalyst gave a very low
conversion of cyclohexane under the investigated conditions
(Table 2, entry 11). We also examined the reactivity of the
physical mixture of pure Au and Pd catalysts (Au/Pd molar
ratio = 1.4:1). The physical mixture exhibited a much lower
activity as compared with the bimetallic alloying Au−Pd/
MIL-101 catalyst under identical conditions (Table 2, entries 6
and 12). These results demonstrated a clear synergistic effect
for the Au−Pd alloying catalysts, as compared with the mono-
metallic Au or Pd species.
Using the optimized Au−Pd catalyst, a further optimization
of the reaction parameters was subsequently conducted. The
O2 pressure and reaction temperature were found to strongly
influence the reaction. The conversion of cyclohexane was
remarkably enhanced with increasing the pressure and temper-
ature (Table 2, entries 6, 13, and 14). Nevertheless, an increase in
pressure and temperature also led to a decrease in the selectivity
to KA-oil. This reduction in selectivity was attributed to the
enhanced overoxidation of cyclohexanol and cyclohexanone at
elevated temperatures or pressures over the Au−Pd catalyst.
The solvent-free oxidation of cyclohexane was carried out
at 150 °C and 1.0 MPa of O2. Blank runs gave essentially no
activity in the systems (even with the parent MIL-101) after 4 h
of reaction (Table 2, entries 1 and 2). The introduction of a
small amount of Au onto MIL-101 remarkably improved the
conversion of cyclohexane with >99.9% selectivity for KA-oil
(Table 2, entry 3). Interestingly, the activity could be further
649
dx.doi.org/10.1021/cs300754k | ACS Catal. 2013, 3, 647−654

ACS Catalysis
Research Article
Figure 4. XPS spectra of Au/MIL-101, Pd/MIL-101, and Au−Pd/MIL-101 (Au/Pd molar ratio = 1.4:1): (a) Au 4f, (b) Pd 3d, and (c) survey spectra of
Au−Pd/MIL-101.
(0.0017 mol % Au + Pd). The reaction also proceeded smoothly,
furnishing a conversion of 41.5% with a good selectivity (∼85%)
to KA-oil after 4 h of reaction (Table 2, entry 15). The turnover
frequency (TOF: moles of KA-oil produced on per mole of
surface Au−Pd per hour) was calculated to be ∼19 000 h−1 at 2 h
of reaction (Table 2, entry 16), which is comparable to the
highest TOF values reported to date on liquid-phase cyclohexane
aerobic oxidation under similar reaction conditions.13
The reaction profile over an initial 4 h reaction period clearly
showed an initiation period of ∼20 min in which almost no
conversion was observed (Figure 5). The presence of such an
induction time was also observed in previous studies on liquid-
phase cyclohexane oxidation.7 It is widely suggested that the
liquid-phase aerobic oxidation of cyclohexane involves radical
intermediates, and the product cyclohexanone could also
catalyze the initiation of the autoxidation process.28 To test the
possible involvement of radical species in our system, we added a
radical scavenger (i.e., p-benzoquinone) to the reaction solution
under the standard conditions. The reaction was almost fully
suppressed to give a conversion of ∼2.3% after 4 h of reaction
(Table 2, entry 17). This result strongly suggests the involvement
of radical species in the present reaction system.
Table 2. Results of the Oxidation of Cyclohexane in the
Absence of Solvent
a
b
sel (%)
Au/Pd molar
ratio
concn
(%)
yield of KA-oil
(%)
entry
OH
CO
c
1
trace
<2.0
16.2
16.5
20.0
28.4
21.3
10.4
9.4
d
2
36.1
18.2
19.0
15.8
16.1
9.8
52.9
81.8
80.6
81.0
78.3
80.8
92.3
93.8
>99.9
>99.9
78.6
57.0
50.2
53.1
55.3
19.0
<2.0
16.2
16.5
19.4
26.8
19.3
10.4
9.3
3
1:0
4
2.6:1
1.8:1
1.4:1
1:1
5
6
7
8
1:1.4
1:1.6
1:1.9
0:1
7.7
9
5.5
10
11
4.2
4.2
<3.0
10.0
45.4
50.8
41.5
37.6
2.3
<3.0
9.6
e
12
1.4:1
1.4:1
1.4:1
1.4:1
1.4:1
1.4:1
17.0
27.2
27.7
31.3
28.7
f
13
38.2
39.6
35.0
31.6
0.4
f g
14 ,
In view of these findings, we have proposed a possible reaction
pathway for the liquid-phase aerobic oxidation of cyclohexane
into cyclohexanol and cyclohexanone over the Au−Pd/MIL-101
catalysts (Figure 6). Au nanoparticles have been widely reported
as efficient catalysts for a variety of aerobic oxidation reactions,
and the activation of O2 could be promoted over Au clusters with
a high electron density.55,56 Therefore, the enhanced surface
electron density of the bimetallic Au−Pd alloying catalyst as
compared with monometallic Au (see XPS data) would be
favorable for O2 adsorption and activation to form active
oxygen species, which is probably in a superoxo-like form
f h
15 ,
f h,i
16 ,
j
17
a
Reaction conditions: cyclohexane (10 mL), catalyst (50 mg), O2
b
(1.0 MPa), 150 °C, 4 h. CO indicates cyclohexanone, and OH
denotes cyclohexanol. Main byproducts: organic acids (e.g., adipic
c
acid), formic acid cyclohexyl ester, and cyclohexyl hydroperoxide. No
d
e
catalyst used. Catalyst: MIL-101. Au/MIL-101 and Pd/MIL-101
f
g
h
physical mixture. 1.5 MPa of O2. 160 °C. 20 mL cyclohexane.
t = 2 h. p-Benzoquinone (1 wt %) was added.
i
j
38,55−57
−
−
(•O2 ).
Subsequently, the as-formed •O2 reacts with
adsorbed cyclohexane to produce the intermediate cyclohexyl
hydroperoxide (CHHP) (Figure 6).29 CHHP could be considered
Under 150 °C and 1.5 MPa of O2, we investigated the
oxidation of cyclohexane at a higher substrate/metal molar ratio
650
dx.doi.org/10.1021/cs300754k | ACS Catal. 2013, 3, 647−654

ACS Catalysis
Research Article
Figure 5. Cyclohexane conversion and product selectivity as a function of time: cyclohexane (10 mL), catalyst (50 mg), O2 (1.5 MPa), 150 °C. Symbols:
⧫
◊
■
●
, conversion; , selectivity to KA-oil; , selectivity to byproducts; , cyclohexane conversion after removing the catalyst at 1.5 h.
Figure 7. IR spectrum of the solution after 4 h of reaction of cyclohexane
with oxygen using Au−Pd/MIL-101 as catalyst. The peaks at 1365,
1349, and 1308 cm−1 can be assigned to the vibrational modes of
cyclohexyl hydroperoxide.
Figure 6. Schematic representation of the mechanism for cyclohexane
aerobic oxidation over Au−Pd/MIL-101.
MIL-101-supported Au or Pd nanoparticles were highly active
and selective for the aerobic oxidation of cyclohexanol to
cyclohexanone under mild conditions.38,62
as the first oxygenated product in this reation,58,59 which has been
detected in cyclohexane aerobic oxidation by using in situ FT-IR
characterization.60,61 The presence of such a species in our system
was also confirmed by FT-IR (Figure 7) as well as GC/MS analysis
(Table 2). The IR spectrum of the reaction mixture after reaction
showed three peaks at 1308, 1349, and 1365 cm−1, respectively,
that can be assigned to the vibrational modes of cyclohexyl
hydroperoxide.60,61
The formed CHHP then undergoes decomposition to produce
cyclohexanone and cyclohexanol products. It is noteworthy that
the ratios of cyclohexanone to cyclohexanol in the products of
the Au−Pd catalyst system were all higher than 2:1 (Table 2),
suggesting a possible catalytic mechanism of CHHP conversion
to KA-oil over the Au−Pd catalysts, because a noncatalytic
decomposition of CHHP generally results in a much lower ratio
of cyclohexanone to cyclohexanol.1,7,28 Moreover, the formed
cyclohexanol could be further oxidized to cyclohexanone over the
Au−Pd catalysts, leading to higher selectivity of cyclohexanone
than cyclohexanol. Our previous results have demonstrated that
To demonstrate the general applicability of the Au−Pd/MIL-
101 catalyst, we examined the efficacy of oxidation trans-
formations of a variety of saturated alkanes under similar reaction
conditions. Various secondary C−H bonds were oxidized
smoothly with excellent selectivities to their corresponding oxida-
tion products at moderate conversions (Table 3, entries 1−4).
Under the standard conditions, Au−Pd/MIL-101 gave only a
trace of conversion of n-hexane (Table 3, entry 5). This could be
due to the weak adsorption of n-hexane on the catalyst, as also
suggested in the literature reports.29 Tertiary C−H bonds could
also be oxidized to corresponding alcohols (Table 3, entry 6). It is
interesting to note that Au−Pd/MIL-101 was also active in the
oxidation of primary C−H bonds (e.g., toluene) with >95%
selectivity to benzaldehyde (Table 3, entry 7), further confirming
the wider applicability of our catalysts for the selective oxidation
of saturated C−H bonds.
In a final set of experiments, we investigated the reusability of
the Au−Pd catalyst because it is of importance to confirm that
the highly active catalyst is stable and can be reused. After the
651
dx.doi.org/10.1021/cs300754k | ACS Catal. 2013, 3, 647−654

ACS Catalysis
Research Article
b
Table 3. Aerobic Oxidation of Various Alkanes Using Au−Pd/MIL-101
a
b
CH3CN (5 mL). Reaction conditions: Substrate/metal = 10 000:1 (molar ratio), Au−Pd/MIL-101 (1.4:1 Au/Pd molar ratio), 50 mg, 150 °C,
1.5 MPa of O2, 4 h.
Figure 9. TEM image (a) of Au−Pd/MIL-101 (Au/Pd molar ratio =
1.4:1) after catalytic reaction and (b) the corresponding size distribution
of Au−Pd nanoparticles.
Figure 8. Reuses of the Au−Pd/MIL-101 catalyst in the liquid-phase
oxidation of cyclohexane. Reaction conditions: cyclohexane (10 mL),
Au−Pd/MIL-101 (1.4:1 Au/Pd molar ratio, 50 mg), O2 (1.5 MPa),
150 °C, 4 h.
that no appreciable loss of conversion and selectivity was
observed in the oxidation of cyclohexane up to four runs. Powder
XRD characterization showed that the crystalline structure of
MIL-101 was mostly retained (Figure 1). AAS experiments on
the reaction mixtures indicated that only traces of metal (<1% of
the total gold or palladium) was detected to have leached into
the liquid phase during reaction. Moreover, the reaction after
removing the catalyst from the solution at 1.5 h essentially
stopped, strongly suggesting that the reaction proceeded mostly
on the heterogeneous surface (Figure 5). TEM images of the
cyclohexane oxidation reaction, Au−Pd/MIL-101 (Au/Pd =
1.4:1) was separated from the reaction mixture by centrifugation,
thoroughly washed with acetonitrile and ethyl acetate, dried at
80 °C, and then reused as the catalyst in subsequent runs under
identical reaction conditions. Results included in Figure 8 reveal
652
dx.doi.org/10.1021/cs300754k | ACS Catal. 2013, 3, 647−654

ACS Catalysis
Research Article
(13) Wu, P.; Bai, P.; Lei, Z.; Loh, K. P.; Zhao, X. S. Microporous
Mesoporous Mater. 2011, 141, 222.
(14) Li, L.; Jin, C.; Wang, X.; Ji, W.; Pan, Y.; Knaap, T. V. D.; Stoel, R.V.
catalyst after the fourth run revealed a minimum aggregation of
particles (average size: 2.59 0.51 nm) in the material (Figure 9).
These results demonstrated that the highly active Au−Pd/
MIL-101 catalyst was stable and reusable under the investigated
conditions.
D.; Au, C. T. Catal. Lett. 2009, 129, 303.
(15) Sun, Z.; Li, G.; Liu, L.; Liu, H. Catal. Commun. 2012, 27, 200.
(16) Li, J.; Shi, Y.; Xu, L.; Lu, G. Ind. Eng. Chem. Res. 2010, 49, 5392.
(17) Raja, R.; Sankar, G.; Thomas, J. M. J. Am. Chem. Soc. 1999, 121,
11926.
4. CONCLUSIONS
(18) Reddy, S. S.; Raju, B. D.; Padmasri, A. H.; Sai Prakash, P. K.; Rama
Rao, K. S. Catal. Today 2009, 141, 61.
(19) Chen, C.; Xu, J.; Zhang, Q.; Ma, Y.; Zhou, L.; Wang, M. Chem.
Commun. 2011, 47, 1336.
(20) Yuan, H.-X.; Xia, Q.-H.; Zhan, H.-J.; Lu, X.-H.; Su, K.-X. Appl.
Catal., A 2006, 304, 178.
(21) Guo, X.; Shen, D.-H.; Li, Y.-Y.; Tian, M.; Liu, Q.; Guo, C.-C.; Liu,
Z.-G. J. Mol. Catal. A: Chem. 2011, 351, 174.
(22) Qian, G.; Ji, D.; Lu, G.; Zhao, R.; Qi, Y.; Suo, J. J. Catal. 2005, 232,
378.
(23) Wang, H.; Li, R.; Zheng, Y.; Chen, H.; Wang, F.; Ma, J. Catal. Lett.
2008, 122, 330.
(24) Yang, X.; Yu, H.; Peng, F.; Wang, H. ChemSusChem 2012, 7, 1213.
(25) Gu, J.; Huang, Y.; Elangovan, S. P.; Li, Y.; Zhao, W.; Toshio, I.;
Yamazaki, Y.; Shi, J. J. Phys. Chem. C 2011, 115, 21211.
(26) Zhao, H.; Zhou, J.; Luo, H.; Zeng, C.; Li, D.; Liu, Y. Catal. Lett.
2006, 108, 49.
(27) Zhao, R.; Wang, Y.; Guo, Y.; Guo, Y.; Liu, X.; Zhang, Z.; Wang, Y.;
Zhan, W.; Lu, G. Green Chem. 2006, 8, 459.
(28) Yu, H.; Peng, F.; Tan, J.; Hu, X.; Wang, H.; Yang, J.; Zheng, W.
Angew. Chem., Int. Ed. 2011, 50, 3978.
In conclusion, we have developed a highly efficient heteroge-
neous catalyst system for liquid-phase aerobic oxidation of a
variety of saturated hydrocarbons using MIL-101-supported
Au−Pd nanoparticles as catalyst without the addition of any
promoters. The Au−Pd bimetallic catalysts exhibited a marked
superiority over their pure metal counterparts and a Au + Pd
physical mixture in terms of cyclohexane conversion, suggesting
a strong molecular-scale synergy of Au−Pd alloys. Radical
intermediates were demonstrated to likely be involved in these
transformations. Moreover, the Au−Pd catalyst was highly
stabilized against metal agglomeration and leaching, maintaining
the high activity and selectivity during a number of recycles under
the investigated conditions. The combination of high activity and
selectivity as well as good stability enables Au−Pd/MIL-101 a
potential material for practical applications in liquid-phase aerobic
oxidation of cyclohexane to cyclohexanone and cyclohexanol.
Work is underway to investigate the reaction mechanism and
further improve and optimize the catalytic system in our
laboratories.
(29) Li, X. H.; Chen, J. S.; Wang, X.; Sun, J.; Antonietti, M. J. Am. Chem.
Soc. 2011, 133, 8074.
AUTHOR INFORMATION
■
(30) Long, J. R.; Yaghi, O. M. Chem. Soc. Rev. 2009, 38, 1213.
(31) Farrusseng, D.; Aguado, S.; Pinel, C. Angew. Chem., Int. Ed. 2009,
48, 7502.
(32) Yuan, B. Z.; Pan, Y. Y.; Li, Y. W.; Yin, B. L.; Jiang, H. F. Angew.
Chem., Int. Ed. 2010, 49, 4054.
Corresponding Author
*E-mail: liyw@scut.edu.cn.
Notes
The authors declare no competing financial interest.
(33) Pan, Y. Y.; Yuan, B. Z.; Li, Y. W.; He, D. H. Chem. Commun. 2010,
46, 2280.
(34) Zhao, Y.; Zhang, J.; Song, J.; Li, J.; Liu, J.; Wu, T.; Zhang, P.; Han,
ACKNOWLEDGMENTS
■
This work was supported by the NSF of China (20936001 and
21073065), the NSF of Guangdong (S2011020002397 and
10351064101000000), the Doctoral Fund of Ministry of
Education of China (20120172110012), and the Fundamental
Research Funds for the Central Universities (2011ZG0009).
B. Green Chem. 2011, 13, 2078.
(35) El-Shall, M. S.; Abdelsayed, V.; Khder, A. S.; Hassan, H. A.; El-
Kaderi, H. M.; Reich, T. E. J. Mater. Chem. 2009, 19, 7625.
(36) Li, H.; Zhu, Z.; Zhang, F.; Xie, S.; Li, H.; Li, P.; Zhou, X. ACS
Catal. 2011, 1, 1604.
(37) Cheon, Y. E.; Suh, M. P. Angew. Chem., Int. Ed. 2009, 48, 2899.
(38) Liu, H. L.; Liu, Y. L.; Li, Y. W.; Tang, Z. Y.; Jiang, H. F. J. Phys.
Chem. C 2010, 114, 13362.
(39) Jiang, H. L.; Akita, T.; Ishida, T.; Haruta, M.; Xu, Q. J. Am. Chem.
Soc. 2011, 133, 1304.
(40) Jiang, H.; Liu, B.; Akita, T.; Haruta, M.; Sakurai, H.; Xu, Q. J. Am.
Chem. Soc. 2009, 131, 11302.
REFERENCES
■
(1) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oxidations of Organic
Compounds; Academic Press: New York, 1981.
(2) Kamata, K.; Yonehara, K.; Nakagawa, Y.; Uehara, K.; Mizuno, N.
Nat. Chem. 2010, 2, 478.
(3) Bergman, R. G. Nature 2007, 446, 391.
(41) Schroder, F.; Esken, D.; Cokoja, M.; van den Berg, M. W. E.;
̈
(4) Bartholome,
́
E. Cyclohexanol and cyclohexanone. In Ullmanns
Lebedev, O. I.; Van Tendeloo, G.; Walaszek, B.; Buntkowsky, G.;
Limbach, H. H.; Chaudret, B.; Fischer, R. A. J. Am. Chem. Soc. 2008, 130,
6119.
Encyklopadie der Technischen Chemie; Verlag Chemie: Weinheim, 1972,
̈
Vol. 4.
(5) Weissermel, K.; Horpe, H. J. Industrial Organic Chemistry, 2nd ed.;
(42) Park, Y. K.; Choi, S. B.; Nam, H. J.; Jung, D. Y.; Ahn, H. C.; Choi,
K.; Furukawad, H.; Kim, J. Chem. Commun. 2010, 46, 3086.
(43) Liu, H.; Li, Y. W.; Jiang, H.; Vargas, C.; Luque, R. Chem. Commun.
2012, 48, 8431.
(44) Dhakshinamoorthy, A.; Alvaro, M.; Garcia, H. Catal. Sci. Technol.
2011, 1, 856.
VCH Press: Weinheim, 1993.
(6) Schuchardt, U.; Calvarlho, W. A.; Spinace, E. V. Synlett 1993, 10,
713.
(7) Hereijgers, B. P. C.; Weckhuysen, B. M. J. Catal. 2010, 270, 16.
(8) Zhao, R.; Ji, D.; Lv, G.; Qian, G.; Yan, L.; Wang, X.; Suo, J. Chem.
Commun. 2004, 904.
(45) Ponec, V.; Bond, G. C. Catalysis by Metals and Alloys; Studies in
Surface Science and Catalysis; Elsevier: Amsterdam, 1995.
(46) Balcha, T.; Strobl, J. R.; Fowler, C.; Dash, P.; Scott, R. W. J. ACS
Catal. 2011, 1, 425.
(9) Lu, G.; Zhao, R.; Qian, G.; Qi, Y.; Wang, X.; Suo, J. Catal. Lett.
2004, 97, 115.
(10) Xu, L.-X.; He, C.-H.; Zhu, M.-Q.; Fang, S. Catal. Lett. 2007, 114,
202.
̈
(47) Wang, D.; Villa, A.; Porta, F.; Su, D.; Prati, L. Chem. Commun.
(11) Xu, L.-X.; He, C.-H.; Zhu, M.-Q.; Wu, K.-J.; Lai, Y.-L. Catal.
2006, 1956.
Commun. 2008, 9, 816.
(48) Kesavan, L.; Tiruvalam, R.; Ab Rahim, M. H.; bin Saiman, M. I.;
Enache, D. I.; Jenkins, R. L.; Dimitratos, N.; Lopez-Sanchez, J. A.;
(12) Wu, P.; Bai, P.; Loh, K. P.; Zhao, X. S. Catal. Today 2010, 158,
220.
653
dx.doi.org/10.1021/cs300754k | ACS Catal. 2013, 3, 647−654

ACS Catalysis
Research Article
Taylor, S. H.; Knight, D. W.; Kiely, C. J.; Hutchings, G. J. Science 2011,
331, 195.
(49) Hermans, S.; Deffernez, A.; Devillers, M. Catal. Today 2010, 157,
77.
(50) Liu, H.; Chen, G.; Jiang, H. F.; Li, Y. W.; Luque, R. ChemSusChem
2012, 5, 1892.
(51) Fer
́
ey, G.; Mellot-Draznieks, C.; Serre, C.; Millange, F.; Dutour, J.;
Surble, S.; Margiolaki, I. Science 2005, 309, 2040.
́
(52) Xu, J.; White, T.; Li, P.; He, C.; Yu, J.; Yuan, W.; Han, Y. F. J. Am.
Chem. Soc. 2010, 132, 10398.
(53) Chou, T. S.; Perlman, M. L.; Watson, R. E. Phys. Rev. B 1976, 14,
3248.
(54) Nascente, P. A. P.; de Castro, S. G. C.; Landers, R.; Kleiman, G. G.
Phys. Rev. B 1991, 43, 4659.
́
(55) Chowdhury, B.; Bravo-Suarez, J. J.; Mimura, N.; Lu, J.; Bando, K.
K.; Tsubota, S.; Haruta, M. J. Phys. Chem. B 2006, 110, 22995.
(56) Stiehl, J. D.; Kim, T. S.; McClure, S. M.; Mullins, C. B. J. Am.
Chem. Soc. 2004, 126, 1606.
(57) Ishida, T.; Nagaoka, M.; Akita, T.; Haruta, M. Chem.Eur. J.
2008, 14, 8456.
(58) Lu, G.; Ji, D.; Qian, G.; Qi, Y.; Wang, X.; Suo, J. Appl. Catal., A
̈
2005, 280, 175.
(59) Zhu, K.; Hu, J.; Richards, R. Catal. Lett. 2005, 100, 195.
(60) Larsen, R. G.; Saladino, A. C.; Hunt, T. A.; Mann, J. E.; Xu, M.;
Grassian, V. H.; Larsen, S. C. J. Catal. 2001, 204, 440.
(61) Sun, H.; Blatter, F.; Frei, H. J. Am. Chem. Soc. 1996, 118, 6873.
(62) Chen, G. Z.; Wu, S. J.; Liu, H. L.; Jiang, H. F.; Li, Y. W. Green
Chem. 2013, 15, 230.
654
dx.doi.org/10.1021/cs300754k | ACS Catal. 2013, 3, 647−654