Aerobic oxidation of alcohols over carbon nanotube-supported Ru catalysts assembled at the interfaces of emulsion droplets

Article abstract of DOI:10.1016/j.apcata.2010.04.046

Carbon nanotube (CNT)-supported ruthenium catalysts, assembled at the interfaces of emulsion droplets, show excellent activity, selectivity, and stability for the selective oxidations of benzyl alcohol to benzaldehyde with oxygen or air as oxidant in the presence of water. The selective oxidation of benzyl alcohol over Ru/CNTs catalysts is greatly enhanced and quickened due to the presence of water. A reaction pathway is proposed, in which the promotion effect of water on the catalytic activity of Ru/CNTs is discussed. The as-made Ru/CNTs catalysts are also active for the aerobic oxidation of a variety of alcohols with a sulfur or nitrogen atom or a carbon-carbon double bond in the multiphase reaction system. Moreover, after the reactions, the catalysts can be easily separated and recycled by sedimentation.

Full text of DOI:10.1016/j.apcata.2010.04.046

Applied Catalysis A: General 382 (2010) 131–137
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Aerobic oxidation of alcohols over carbon nanotube-supported Ru catalysts
assembled at the interfaces of emulsion droplets
Xiaomin Yanga, Xiuna Wanga, Jieshan Qiua,b,∗
a Carbon Research Laboratory, Center for Nano Materials and Science, State Key Lab of Fine Chemicals, School of Chemical Engineering,
Dalian University of Technology, 158 Zhongshan Road, P.O. Box 49, Dalian 116012, China
b Key Laboratory for Micro/Nano Technology and System of Liaoning Province, Dalian University of Technology, Dalian 116012, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Carbon nanotube (CNT)-supported ruthenium catalysts, assembled at the interfaces of emulsion droplets,
show excellent activity, selectivity, and stability for the selective oxidations of benzyl alcohol to benzalde-
hyde with oxygen or air as oxidant in the presence of water. The selective oxidation of benzyl alcohol over
Ru/CNTs catalysts is greatly enhanced and quickened due to the presence of water. A reaction pathway
is proposed, in which the promotion effect of water on the catalytic activity of Ru/CNTs is discussed. The
as-made Ru/CNTs catalysts are also active for the aerobic oxidation of a variety of alcohols with a sulfur
or nitrogen atom or a carbon–carbon double bond in the multiphase reaction system. Moreover, after the
reactions, the catalysts can be easily separated and recycled by sedimentation.
Received 6 January 2010
Received in revised form 4 April 2010
Accepted 26 April 2010
Keywords:
Aerobic oxidation
Alcohols
Carbon nanotubes
Ru
© 2010 Elsevier B.V. All rights reserved.
Emulsion catalysis
1. Introduction
Pd/HAP [4,5], Ru/Al2O3 [6], Au/CeO2 [7], and Au–Pd/TiO2 [8]. Up to
now, oxidation reactions at low temperature in appropriate sol-
Oxidation of alcohols to carbonyl compounds is one of the
most fundamental and important processes for synthesis of organic
chemicals. A variety of methods for the oxidation of alcohols have
been developed; nevertheless, traditional methods such as non-
catalytic methods with stoichiometric amounts of heavy metal
reagents [1] or moisture-sensitive expensive oxidants [2] are still
widely used. These processes are often conducted in environ-
mentally undesirable media such as chlorinated solvents. From
environmental and atom-economical points of view, it would be
ideal to oxidize alcohols using air or molecular oxygen under atmo-
spheric pressure conditions. Obviously, this cheaper, safer, and
more environmentally benign oxidation process requires novel yet
highly efficient heterogeneous catalysts. The research efforts have
led to a number of efficient catalysts for the aerobic oxidation
of alcohols such as hydroxyapatite (HAP) bound Ru/HAP [3] and
vents are still highly demanded for alcohols with high melting
points or low stability at high temperatures, or in cases where
a minimal amount of an alcohol is needed for scientific research
[9–14].
With the discovery of carbon nanotubes (CNTs) and their large
scale production, much attention has been paid to their potential
applications such as in electronic devices, biosensors, and catal-
ysis due to their unique electron conductivity, thermal stability,
and high mechanical strength [15–23]. Compared with traditional
catalyst supports, CNTs with high external surface area and aspect
ratio display unusual behaviors, such as being able to significantly
increase the contact surface between the reactants and active sites
of catalysts, and to greatly minimize the diffusion limitations. These
features make CNTs very attractive as catalyst supports in liquid
phase reactions. Here we report that, with CNT-supported ruthe-
nium as catalysts, several types of alcohols with a sulfur or nitrogen
atom, or a carbon–carbon double bond in the molecule, can be effec-
tively oxidized to their corresponding carbonyl compounds using
oxygen or air as oxidant under mild conditions, as illustrated in
Eq. (1). It is interesting to note that the CNT-supported ruthenium
catalysts assemble and form droplets in the emulsion-like multi-
∗
Corresponding author at: Carbon Research Laboratory, State Key Lab of Fine
Chemicals, School of Chemical Engineering, Dalian University of Technology, 158
Zhongshan Road, P.O. Box 49, Dalian 116012, China. Tel.: +86 411 39893994;
fax: +86 411 88993991.
E-mail address: jqiu@dlut.edu.cn (J. Qiu).
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.04.046

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X. Yang et al. / Applied Catalysis A: General 382 (2010) 131–137
phase reaction system. After the reaction, the catalysts can be easily
separated from the liquid products by sedimentation and can be
recycled.
90 min, followed by evacuation at 450 ◦C for 4 h. Before the mea-
surement, the catalysts were evacuated at 40 ◦C for 2 h.
(1)
2. Experimental
2.1. Materials
2.4. Alcohol oxidation
CNTs with a diameter of 10–20 nm, a length of 1–2 ␮m, and N2
surface area of 151 m2/g were used. Coconut shell based active
carbon (AC) with N2 surface area of 986 m2/g was purchased
from Beijing Broad Activated Carbon Co., Ltd. (Beijing, China). TiO2
(Degussa P25) with a N2 surface area of 59 m2/g was purchased
from Shanghai Haiyi Scientific & Trading Co., Ltd. (Shanghai, China).
RuO2 was purchased from Alfa Aesar Co., Ltd. Oxygen with a purity
of 99.995% was purchased from Dalian Guangming Special Type
Gas Co., Ltd. (Dalian, China). All reagents used in this work were of
analytical grade.
Alcohol oxidation reactions were carried out in a 50 mL three-
neck round-bottom flask under magnetic stirring. For a typical
run, 0.2 g of Ru/CNTs catalyst, 2 mmol of benzyl alcohol, 10 mL of
toluene, and 5 mL of water were placed in a flask that had been
flushed with flowing oxygen or air at atmospheric pressure, and
heated at 85 ◦C for 3 h. The products were analyzed by GC (Agi-
lent 6890N) and GC–MS (HP6890–HP5973). After the reaction, the
Ru/CNTs catalyst was separated from the reaction products by sed-
imentation, and was subsequently washed first with acetone, then
with an aqueous solution of NaOH (0.5 M), and finally with water.
Thereafter it was dried at 100 ◦C for 8 h under vacuum before recy-
cling.
2.2. Catalyst preparation
Before use, pristine CNTs were refluxed in a mixture of HNO3
and H2SO4 (v/v 2:1) at 120 ◦C for 4 h, then filtered and washed with
deionized water until the pH value of the filtrate reached 7, then
dried at 130 ◦C for 12 h under vacuum. Pristine AC was crushed,
washed, and sieved to 200–320 mesh, then treated by the same
procedure for modifying the CNTs as described above.
Ruthenium catalysts supported on CNTs were prepared by a
wetness impregnation method. The modified CNTs were impreg-
nated in an aqueous solution of RuCl3 under ultrasonic conditions
for 30 min, and then the mixture was incubated at room tempera-
ture (RT) for 12 h. The CNT-supported RuCl3 samples were dried at
110 ◦C for 12 h under vacuum, and reduced at 400 ◦C in flowing H2
for 2 h, and then cooled to RT in argon, yielding Ru/CNTs catalysts
with different Ru loadings. For the Ru/AC and Ru/TiO2 catalysts, the
same procedure as described above was adopted.
3. Results and discussion
3.1. Catalyst characterization
The modification of the CNTs aims to remove amorphous carbon
and to increase the number of surface oxygen-containing functional
groups that are useful for the metal deposition and dispersion.
The FT-IR spectra of the CNTs before and after the modification
are shown in Fig. 1, showing that new peaks at 1710 cm−1 and
1190 cm−1 appear after modification, which indicates that the car-
boxylic groups are formed on the CNTs. The BET surface area of the
modified CNTs is 162 m2/g.
The TPR profile of RuCl3/CNTs is shown in Fig. 2. The peak with a
maximum at about 155 ◦C is similar to the reduction temperature of
unsupported RuCl3 [26,27]. Peaks with maxima at about 248 ◦C and
301 ◦C are attributed to the reduction of oxygen-containing func-
tional groups on the surface. The peak with a maximum at about
554 ◦C is due to the methanation of CNTs. The RuCl3/CNTs catalysts
were reduced at 400 ◦C in flowing H2 for 2 h; under these conditions
RuCl3 could be completely converted into metallic Ru.
2.3. Catalyst characterization
FT-IR spectra of pristine and modified CNTs were recorded on
a Jasco-430 spectrometer. The BET surface areas of the modified
CNTs were measured by N2 physisorption at −196 ◦C (Micromerit-
ics ASAP 2020). Ruthenium reducibility was determined by
temperature-programmed reduction (TPR) using a Micrometrics
Chemisorb 2720 instrument equipped with a thermal conductivity
detector. A non-reduced RuCl3/CNTs sample was heated to 750 ◦C
at 10 K min−1 in a gas stream of 10% hydrogen in argon.
The crystalline structure of Ru/CNTs was analyzed by pow-
der X-ray diffraction (XRD, D/MAX-2400) using Cu-K␣ radiation
with a scanning rate of 2◦ min−1. Transmission electron microscopy
(TEM) examination was conducted using a Philips Tecnai G2 20
microscope equipped with a CCD camera operated at 200 kV. The
composition of the catalyst sample was analyzed by Energy Disper-
sive X-ray (EDX) Spectrometry.
The dispersion of exposed Ru atoms on the supported Ru cat-
alysts was analyzed by H2 chemisorption using a Quantachrome
Autosorb-1 instrument at 40 ◦C, assuming the chemisorption sto-
ichiometry of H/Ru = 1/1 [24,25]. Before chemisorption, catalysts
were heated from room temperature to 450 ◦C at 4 ◦C min−1 in flow-
ing H2 (99.999%). The catalysts were reduced in H2 at 450 ◦C for
Fig. 1. FT-IR spectra of pristine CNTs and modified CNTs.

X. Yang et al. / Applied Catalysis A: General 382 (2010) 131–137
133
Fig. 3. XRD patterns of CNTs and Ru/CNTs.
Fig. 2. TPR curve of RuCl3/CNTs.
the TEM images that the Ru particles with an average size of 2.0 nm
are highly dispersed on the surface of CNTs.
For Ru/CNTs samples with different Ru loadings, no diffraction
peaks related to metallic Ru are detected (Fig. 3), which implies
that Ru nanoparticles are of nanometer scale. The XRD pattern of
CNTs is remarkably similar to that of graphite, while the XRD pat-
tern of activated carbon indicates an amorphous structure (Fig. S1).
For CNTs, there exists a symmetrical and sharp 0 0 2 peak at 26◦,
and a broad 1 0 0 peak at 42◦. The average interlayer spacing is cal-
culated to be 0.34 nm [28,29]. It has been noted that the layered
structure of CNT is favorable for the transfer of electrons and for a
close interaction between the metal and support by intercalation
[30]. Typical TEM images of the 6.0 wt% Ru/CNTs sample, along with
its corresponding EDX spectrum, are shown in Fig. 4, which further
evidences the presence of metallic Ru species. It can be seen from
3.2. Alcohol oxidation
The selective oxidation of benzyl alcohol to benzaldehyde was
used to probe the catalytic activities of the Ru-based catalysts. The
effect of Ru species, catalyst support, and Ru loading on the cat-
alytic activity of the supported ruthenium catalyst are shown in
Table 1. Among various Ru species tested (entries 1–3), Ru/CNTs
shows the highest catalytic activity for the oxidation of benzyl alco-
hol. For TiO2 and AC supported Ru catalysts with the same metal
loading(entries 4 and5), the catalytic activityis lower thantheCNT-
Fig. 4. TEM images and corresponding EDX spectrum of 6.0 wt% Ru/CNTs.

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X. Yang et al. / Applied Catalysis A: General 382 (2010) 131–137
Table 1
Table 3
Effects of Ru species, catalyst support, and Ru loading on the activity of the supported
Effect of bi-solvents on the catalytic activity of Ru/CNTs for the selective oxidation
ruthenium catalysts for the selective oxidation of benzyl alcohol.a,b
.
of benzyl alcohol.a.
Entry
Catalyst
Substrate/Ru
Conversion (%)
Toluene (mL)
Water (mL)
Conversion (%)
Selectivity of
benzaldehyde (%)
1
2
3
4
5
6
7
6.0 wt% RuCl3/CNTs
RuO2
11
17
17
17
17
26
50
10
15
98
41
80
77
41
10
10
10
10
10
1
3
5
7
10
67
92
98
96
67
100
100
100
100
100
6.0 wt% Ru/CNTs
6.0 wt% Ru/TiO2
6.0 wt% Ru/AC
4.0 wt% Ru/CNTs
2.0 wt% Ru/CNTs
Reaction conditions: benzyl alcohol (2 mmol), Ru/CNTs (Ru: 5.9 mol%), 85 ◦C,
a
a
Reaction conditions: substrate (2 mmol), solvent (toluene: 10 mL, H2O: 5 mL),
p(O2) = 0.1 MPa, 3 h.
85 ◦C, p(O2) = 0.1 MPa, 3 h.
b
Selectivity of benzaldehyde is 100%.
In addition, water can react with the aldehyde to form an aldehyde
hydrate, which further reacts with oxygen to form carboxylic acid.
The effect of co-solvents on the catalytic activity of Ru/CNTs
catalysts is shown in Table 3. With toluene as solvent, the conver-
sion of benzyl alcohol is 50% and the selectivity to benzaldehyde is
100%. When water is added, a substantial increase in the conver-
sion of benzyl alcohol is obtained. When 5 mL of water and 10 mL of
toluene are used as solvents, the highest conversion of benzyl alco-
hol of 98% is achieved, and the TOF is 178 h−1 after the first 0.5 h
of reaction. Nevertheless, more water is not favored, which is evi-
denced by the fact that the conversion of benzyl alcohol decreases
when more water is added to the system.
In the reaction system adopted here, four phases are involved:
vapor, organic phase, aqueous phase, and solid catalyst. The aque-
ous phase and the organic phase are mixed homogeneously by
vigorous agitation during the reaction. The aqueous phase is
dispersed homogeneously in the organic phase in the form of emul-
sion droplets under stirring. Optical microscopy showed that the
Ru/CNTs catalysts assemble at the surface of the emulsion droplets
(Fig. 5). It is known that lipophilic powder-like solids, such as car-
bon black and graphite powder, can be used as emulsifying agents
for a W/O emulsion, where the size of the solid particulates is
much smaller than that of the dispersed phase. Furthermore, the
supported one, indicating that CNT is a suitable support for alcohol
oxidation reactions. As the Ru loading increases from 2 wt% to 6 wt%
(entries 7, 6, and 4), the conversion of benzyl alcohol increases cor-
respondingly, while the benzaldehyde selectivity remains at 100%,
which is rationalized by the fact that even at higher catalyst loading,
small clusters are still dominant.
The effect of solvents on the activity of Ru/CNTs catalysts is
shown in Table 2. Under identical reaction conditions, the catalytic
activity of Ru/CNTs in water is much higher than in typical polar
and non-polar organic solvents such as ethanol and toluene. When
water is used as solvent, the main product is benzoic acid, imply-
ing that water promotes carboxylic acid formation. With toluene
as solvent, a higher conversion is achieved in comparison to that
obtained with the ethanol solvent, and at the same time, a much
higher selectivity to benzaldehyde is also obtained. With ethanol
as solvent, the main product is ethyl benzoate. It is known that
the solubility of O2 in the organic solvents is much higher than in
water. Obviously, the concentration of O2 dissolved in a solvent is
not the governing factor for the remarkable changes in the oxida-
tion of benzyl alcohol in water. In fact, it is water, a weak base,
which plays an important role during the reaction [31]. Water can
facilitate H abstraction from alcohol, thus promoting the reactivity.
Table 2
Effect of solvents on the catalytic activity of Ru/CNTs for the selective oxidation of benzyl alcohol.a.
Selectivity (%)
Solvent
Conversion (%)
Benzaldehyde
31
100
30
Benzoic acid
Ester
Ethanol
Toluene
Water
7
50
100
0
0
70
69
0
0
a
Reaction conditions: substrate (2 mmol), Ru/CNTs (Ru: 5.9 mol%), solvent (15 mL), 85 ◦C, p(O2) = 0.1 MPa, 3 h.
Fig. 5. Optical micrographs of emulsion system a) during the process of reaction and b) after reaction formed with 0.2 g of Ru/CNTs catalyst, 2 mmol of benzyl alcohol, 10 mL
of toluene, and 5 mL of water. In order to clearly identify the interphases of the water phase and oil phase, we marked water and oil with methylene blue and methyl orange,
respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

X. Yang et al. / Applied Catalysis A: General 382 (2010) 131–137
135
Scheme 1. Schematic model of multiphase reaction system for the selective oxidation of benzyl alcohol to benzaldehyde over Ru/CNTs.
solid particulates with rough surfaces and asymmetric shape form
a firm solid film at the interfaces of the emulsion droplets, which
maintains the stability of the emulsion droplets. In the present
study, Ru/CNTs with the above-mentioned characteristics also act
as emulsifying agents. Here the Ru/CNTs catalysts have two roles:
firstly, to provide stability for the emulsion droplets by forming
a solid film around the dispersed water droplets and by decreas-
ing the interfacial tension; secondly, to provide higher interfacial
surface area where the oxidation of alcohol molecules takes place.
The reactants and products can be transferred between different
phases via dissolution, diffusion, and extraction. The interfaces of
the emulsion droplets also favor the quick mass transfer; as the
Table 4
Aerobic oxidation of alcohols over Ru/CNTs.a.
Entry
Alcohol
Product
Conversion (%)
Selectivity (%)
TOFc (h−1
)
1
2
98
100
100
69
71
100
3
4
100
89
100
100
71
63
5
6
88
92
100
100
62
65
7b
8b
64
72
100
100
45
51
9b
16
24
100
100
11
17
10
11b
85
100
60
a
Reaction conditions: substrate (2 mmol), Ru/CNTs (Ru: 5.9 mol%), solvent (toluene: 10 mL, H2O: 5 mL), 85 ◦C, p(O2) = 0.1 MPa, 3 h.
Reaction conditions: substrate (2 mmol), Ru/CNTs (Ru: 5.9 mol%), solvent (H2O: 15 mL), 85 ◦C, p(O2) = 0.1 MPa, 3 h.
TOF was calculated on the basis of the analysis data after 3 h reaction.
b
c

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X. Yang et al. / Applied Catalysis A: General 382 (2010) 131–137
selective oxidation of benzyl alcohol is greatly enhanced by water,
which is due to the formation of emulsion droplets where the CNT-
supported catalysts assemble at the interfaces. Ru/CNTs catalysts
are of potential for the selective oxidation of a large variety of
alcohols in the presence of water. Furthermore, the catalyst can
be easily separated and recycled by sedimentation.
The results reported here also highlight the unique property
and functions of solid catalysts assembled at the interfaces of two
liquid phases to form special emulsion catalysis systems, which
leads one to anticipate that such emulsion-stabilizing solid cata-
lysts after being further tailored will be of wide use in a broad range
of reactions.
Scheme 2. Possible reaction mechanism of benzyl alcohol oxidation over the sup-
ported ruthenium catalysts.
amount of water increases, more interfaces would be available,
which results in an increase in the conversion of substrate. The
solubility of benzaldehyde in toluene is higher than that of ben-
zyl alcohol, but its solubility is much lower in water than that
of benzyl alcohol. Toluene extracts benzaldehyde to the organic
phase, avoiding further oxidation. As such, a maximum yield can
be obtained at a proper water/toluene ratio. A model is proposed
and shown in Scheme 1. Following the classical dehydrogenation
mechanism of alcohol oxidation, the possible reaction steps of the
oxidation of benzyl alcohol are shown in Scheme 2. The O–H bond
of benzyl alcohol breaks upon adsorption at the surface sites, yield-
ing an adsorbed alkoxide and hydrogen [32]. For the adsorbed
alkoxide, the ␤-C–H bond is weaker than other C–H bonds due
to the electron-withdrawing effect of the oxygen atom, resulting
in the preferential breaking of the ␤-C–H bond that is the rate-
determining step [33], as shown in Scheme 2. Meanwhile, adsorbed
activated oxygen is necessary to oxidize the hydrogen co-product,
which helps to shift the equilibrium toward the carbonyl com-
pound and to accelerate the whole reaction by liberating surface
metallic sites [34], as schematically shown in Scheme 2. A simi-
lar multiphase reaction system was proposed and discussed before
[35,36].
Acknowledgements
This work is partly supported by NSFC (Nos. 20725619,
20836002). We thank Prof. Roel Prins at ETH, Switzerland, and Dr.
Dangsheng Su at Fritz Haber Institute of the Max Planck Society,
Germany, for helpful discussions and suggestions.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.apcata.2010.04.046.
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Effective catalytic oxidation of alcohols under mild conditions
has been realized in an emulsion system, in which CNT-supported
ruthenium composites function both as catalysts and as emulsi-
fying agents. Ru/CNTs catalysts are prepared by the traditional
wetness impregnation method, and show excellent activity, selec-
tivity, and stability for the selective oxidation of benzyl alcohol with
oxygen or air as oxidant. The catalytic activity of Ru/CNTs for the

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