Catalytic properties of Au and Pd nanoparticles decorated on Cu 2O microcubes for aerobic benzyl alcohol oxidation and Suzuki-Miyaura coupling reactions in water

Article abstract of DOI:10.1016/j.apcata.2014.02.020

The catalytic activities of Cu₂O microcubes decorated with Au and Pd nanoparticles (Cu₂O/Au and Cu₂O/Pd) were synthesized by a microwave assisted technique and examined for the specific organic reactions, separately. Cu₂O/Au and Cu₂O/Pd catalysts were characterized and examined as the catalysts for the aerobic oxidation of benzyl alcohol to benzaldehyde and the Suzuki-Miyaura coupling reaction in water, separately. The yields of products were >90% with a selectivity of >99% for both catalytic reactions. The cheap, ligand free, efficient, non-toxic, and water soluble properties make the present catalyst ultimate for Suzuki and aerobic oxidation reaction.

Full text of DOI:10.1016/j.apcata.2014.02.020

Applied Catalysis A: General 476 (2014) 72–77
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Catalytic properties of Au and Pd nanoparticles decorated on Cu2O
microcubes for aerobic benzyl alcohol oxidation and Suzuki–Miyaura
coupling reactions in water
K. Sudhakara Prasad1, Hui-Bog Noh1, S. Subba Reddy, A. Eswar Reddy, Yoon-Bo Shim∗
Department of Chemistry and Institute of Bio Physio Sensor Technology, Pusan National University, Busan 609-735, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
The catalytic activities of Cu2O microcubes decorated with Au and Pd nanoparticles (Cu2O/Au and
Cu2O/Pd) were synthesized by a microwave assisted technique and examined for the specific organic
reactions, separately. Cu2O/Au and Cu2O/Pd catalysts were characterized and examined as the catalysts
for the aerobic oxidation of benzyl alcohol to benzaldehyde and the Suzuki–Miyaura coupling reaction
in water, separately. The yields of products were >90% with a selectivity of >99% for both catalytic reac-
tions. The cheap, ligand free, efficient, non-toxic, and water soluble properties make the present catalyst
ultimate for Suzuki and aerobic oxidation reaction.
Received 13 December 2013
Received in revised form 6 February 2014
Accepted 11 February 2014
Available online 18 February 2014
Keywords:
Cu2O microcubes
Nano-Au
© 2014 Elsevier B.V. All rights reserved.
Nano-Pd
Benzyl alcohol
Suzuki–Miyaura coupling reactions
Water
1. Introduction
solvent for Suzuki reactions [23–26]. Very recently Au–Pd
core–shell structures based hybrid nanoparticles with Pd as the
The transition metal oxide micro/nano catalyst has been
attracted more applications and industrial uses in recent years,
because of their tuneable chemical and physical properties [1–3].
Since the metal oxide particles are very stable physically and chem-
ically, their large surface areas make them frequently as catalysts
in wide range of fields [4–6]. Of these, copper (Cu) based mate-
rials have been studied extensively in the last decade due to their
excellent catalytic activities and stability [7–11]. On the other hand,
gold (Au) nanoparticle with its unique catalytic characteristics also
attracted much attention in many organic syntheses at labora-
tory and industrial applications [12–14]. Au-based catalysts are
found to be very effective for the oxidation of alcohols [15–17].
A recent review has been reported for the effective catalytical
behavior of Au, platinum (Pt), palladium (Pd), and manganese as
active catalyst toward the oxidation of alcohols [18]. In addition
to Au nanoparticles, Pd is also particularly important among noble
metals in the field of catalysis, and Pd-supported inorganic mate-
rials are known catalysts for coupling reactions [19–22]. On the
other hand, considerable effort has been made to use water as
active catalyst [27,28], Pd supported graphite oxide [29–31], and
Pd nanoparticle decorated graphene [32] have been reported as a
catalyst for the Suzuki reaction. Despite noble metals have been
studied for catalytic applications extensively, a simple microwave
method to prepare the Pd/Cu2O and Au/Cu2O catalysts and their
applications in organic synthesis have not been investigated to date.
Recently, the catalytic oxidation of alcohols into corresponding
carbonyl compounds selectively are of great significance in chem-
istry. Additionally, the liquid phase oxidation of benzyl alcohol is of
vital importance for both scientific and industrial fields because of
the production of chlorine-free benzaldehyde [33,34]. Due to much
environmental and industrial friendly property, aerobic catalytic
oxidation in water has been widely appreciated; it helps to reduce
the application of other toxic organic solvents and also leads to min-
imum amount of waste after the reaction. Considering the diverse
application of selective oxidation of benzyl alcohol and Suzuki reac-
tion in academic and industrials, an easily prepared inexpensive
and environmentally benign catalyst is indeed a matter of inter-
est. Nevertheless of having aforementioned catalysts reported for
Suzuki–Miyaura coupling reactions and aerobic oxidation of alco-
hol to corresponding aldehyde, the known catalysts suffers from the
implications of requiring sensitive and costly ligands with tedious
before and after reaction conditions. In addition, the soluble pal-
ladium complexes with phosphine ligands used as catalysts for
∗
Corresponding author. Tel.: +82 5 1510 2244.
E-mail address: ybshim@pusan.ac.kr (Y.-B. Shim).
1
Both authors contributed equally to this work.
http://dx.doi.org/10.1016/j.apcata.2014.02.020
0926-860X/© 2014 Elsevier B.V. All rights reserved.

K.S. Prasad et al. / Applied Catalysis A: General 476 (2014) 72–77
73
the coupling reactions are known for their disadvantage of diffi-
culty in recovering and reusing the expensive catalyst. Although
many catalysts have been studied for the above said reactions, a
selective oxidation and coupling reactions in mild reaction con-
ditions are highly desirable for both economic and environmental
reasons. To overcome these sensitive issues, herein through the ver-
satile catalytic activity of Cu, Au, and Pd, environmentally friendly
and easy usable cheap catalysts were attained for simple aerobic
alcohol oxidation and Suzuki–Miyaura coupling reactions. For this,
we designed a Cu2O microcubes catalysts, decorated with Au and
Pd nanoparticles in a simple microwave assisted technique, which
can promote both benzyl alcohol oxidation and Suzuki reaction in
water.
In the present study, the synthesis of Cu2O/Au and Cu2O/Pd cata-
lysts were tried to have commercially competitive through a simple
production procedure by availing a domestic microwave oven
while comparing to the known catalysts. The prepared Cu2O/Au
and Cu2O/Pd materials were characterized, and used as catalysts
for the selective oxidation of alcohols by using molecular oxygen
as an oxidant in water. Suzuki–Miyaura coupling reactions with
Cu2O/Pd were studied with K2CO3 as base and water as a solvent.
To the best of our knowledge, this is the first report establishes the
application of noble metal (Au and Pd) decorated Cu2O catalysts
for aerobic benzyl alcohol oxidation and Suzuki coupling reaction
in water.
Scheme 1. Illustration of the synthetic pathway for Au and Pd nanoparticles deco-
rated Cu2O microcubes.
2.4. General procedure for Suzuki–Miyaura coupling reactions
At first,
a mixture of phenyl boronic acid (2.4 mM), 4-
iodobenzonitrile (2.0 mM), K2CO3 (4.0 mM) and 8.0 mg of Cu2O/Pd
in 20.0 mL of water taken in a round-bottom flask. The reaction
mixture is then stirred at 80 ◦C for overnight (12 h). Following
the completion of the reaction, the crude reaction mixture is cen-
trifuged to remove the catalyst. The resulting product was then
extracted with dichloromethane and subsequently used for prod-
uct analysis with 1H-NMR and GC–MS.
2. Experimental
2.1. Materials
CuCl2 (99.999%), HAuCl4·3H2O (≥99.9%), H2PdCl4 (99.999%),
polyvinylpyrrolidone (mol wt 40,000), NaOH (≥98.0%), d-(+)-
glucose, benzyl alcohol (99.8%), toluene (99.8%), phenyl boronic
acid (95.0%), 4-iodobenzonitrile (97.0%), K2CO3 (99.995%), HCl
(37%), ethyl acetate (≥99.8%), and MgSO4 (≥99.99%) were pur-
chased from Sigma-Aldrich Chemical Co. (USA). All aqueous
solutions were prepared in doubly distilled water, which was
obtained from a Milli-Q water purifying system (18 Mꢀ cm). All
other reagents were the best commercially available.
3. Results and discussions
3.1. Synthesis of Cu2O/Au and Cu2O/Pd materials
The Cu2O microcubes decorated with nano Au and Pd were syn-
thesized by a simple microwave assisted technique (Scheme 1).
At first, Cu2O microcrystals were synthesized by adding 0.03 g of
polyvinylpyrrolidone to 5.0 mL of a 10.0 mM CuCl2 aqueous solu-
tion followed by the addition of 0.18 mL 1 N NaOH and 0.38 mL
deuterium-depleted water. The solution is mixed well and further
0.0072 g of glucose was added as a reducing agent. A final mixed
solution was placed into a microwave oven. The microwave was
operated for 3 min with 100 W and subsequently 1.0 mL of 10.0 mM
HAuCl4 was added into the brick red color Cu2O colloidal solu-
tion, and followed by reaction in the microwave for another 1 min
with 100 W, and consequently a blackish wine red colloidal solu-
tion of Cu2O/Au was obtained. Similarly, Cu2O/Pd catalyst were
synthesized by employing 1.0 mL of 10.0 mM H2PdCl4 instead of
HAuCl4 and continuing the same method used for the preparation
of Cu2O/Au. The obtained dark greenish brown color Cu2O/Pd col-
loid was further characterized and studied for coupling reaction
synthesis (Fig. 1).
2.2. Instrumental characterization
UV–Visible spectra were recorded using Scinco, S-3100. Surface
morphologies were characterized using VEGA
3 SB analyti-
cal scanning electron microscopy (SEM) from Tescan USA Inc.
Energy dispersive X-ray spectroscopy (EDXS) analysis was per-
formed to determine the composition ratio. Transmission electron
microscopy (TEM) images were recorded with a H7600 TEM,
HITACHI instrument. The X-ray photospectroscopy (XPS) exper-
iments were performed using a VG Scientific ESCALAB 250 XPS
spectrometer with a monochromatic Al Ka source including charge
compensation at KBSI (South Korea). Bruker advance 300 spectrom-
eter was used to record the 1H-NMR spectra in CDCl3.
3.2. Morphology and characterization of catalysts
2.3. General procedure for aerobic benzyl alcohol oxidation
Scanning electron microscopy (SEM), energy dispersive X-ray
spectroscopy (EDXS), and transmission electron microscopy (TEM)
were used to examine the surface morphology and size of the pre-
pared samples. Fig. 2 depicts the SEM images of the Cu2O (edge
length of 0.97–0.85 ␮m) microcrystals with cubical shape, deco-
rated with tiny (dia. 10–15 nm) (A(a)) Au and (B(a)) aggregated
Pd nanoparticles. The compositions of Cu2O/Au and Cu2O/Pd were
further confirmed from the EDXS measurement, which exhibited
peaks for (A(b)) Au and (B(b)) Pd along with Cu and O. The surface
morphology was re-observed with TEM images of (A(c)) Cu2O/Au
Typical aerobic oxidation processes were carried out in a
two-necked round-bottom flask with 10.0 mM of benzyl alcohol
or substituted benzyl alcohol in 100.0 ␮L NaOH (1 N), catalyst
Cu2O/Au (3.0 mg), and water (15.6 mL) with oxygen purging and
stirring at 80 ◦C for 5 h. The reaction mixture was then quenched
with 0.1 M HCl and, extracted with ethyl acetate and the organic
layer was dried over anhydrous MgSO4. The crude product was then
analyzed by 1H-NMR and gas chromatography-mass spectrometry
(GC-MS).

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K.S. Prasad et al. / Applied Catalysis A: General 476 (2014) 72–77
Fig. 1. Digital photographs of reagents involved during the preparation of Cu2O based catalyst.
and (B(c)) Cu2O/Pd. As shown in Fig. 2 (A(c)) inset, the nanoparti-
cles decorated on Cu2O microcube can be clearly seen (scale bar:
0.2 ␮m). It was confirmed from the SEM and TEM that the Au and
Pd nanoparticles were embedded onto Cu2O microstructures. The
deposition of Au and Pd takes place through an in situ reduction of
AuCl4− and PdCl4− on Cu2O. It is believed that once Au and Pd nuclei
are formed, they will deposit onto the facets of the Cu2O crystals
directly in order to reduce the surface energy through the following
reaction [35]:3Cu2O + 2MCl4− + H+ = 6Cu2+ + 2M + 3H2O + 8Cl−
(M = Au and Pd)
assigned to Au4f7/2 and 4f5/2, respectively, which are typical val-
ues for elemental Au (Fig. 3(A(c))). In addition, the splitting energy
of the two doublets is calculated to be 3.6 eV, confirming the for-
mation of zero valent Au nanoparticles on the surfaces of Cu2O
microcubes. The presence of Pd3d doublets shown in Fig. 3(B(c)),
Pd3d3/2 and 3d5/2 at 340.2 and 334.8 eV correspond to the metallic
Pd state. The observed peak position for Au4f and Pd3d confirms
the metallic Au and Pd nanoparticles on Cu2O. The observed result
exhibited good consistency with the EDXS observations.
The prepared samples were also characterized with X-ray pho-
toelectron spectroscopy (XPS) studies and the observed peaks were
analyzed with XPS handbook [36]. XPS spectra, displayed in Fig. 3,
(A) Cu2O/Au and (B) Cu2O/Pd microcrystals illustrated the peaks
derived from (a) survey, (b) Cu2p3/2, 1/2, and (c) Au4f5/2, 7/2 or
Pd3d5/2, 3/2. (A(b)) Cu2O/Au exhibited Cu2p peaks at 932.2 and
952.08 eV corresponding to Cu2p3/2 and Cu2p1/2, respectively,
which were assigned to the peaks for Cu2O. Similarly, the XPS spec-
trum for (B(b)) Cu2O/Pd showed peaks for Cu2p3/2 and Cu2p1/2 at
931.6 and 951.4 eV, which were corresponded to the signals for
Cu2O. In both cases, there is an occurrence of small satellite feature
observed in the 2p region indicates the presence of small amount of
CuO, and this can be due to the exposure of sample to atmosphere.
It is worth noting that the peaks located at 83.9 and 87.5 eV are
3.3. Aerobic oxidation of benzyl alcohol and Suzuki–Miyaura
coupling reaction by Cu2O/Au and Cu2O/Pd catalysts
We first investigated the catalytic activities of Cu2O/Au toward
aerobic oxidation of benzyl alcohol in toluene and water. It is rea-
sonable to consider that the base deprotonates the alcohol and
initiates the catalytic reaction, because the alcohol oxidation over
Au based catalyst needs a base as a promoter generally [16,37–39].
Hence, we used NaOH to promote the benzyl alcohol oxidation and
also the choice of benzyl alcohol as a model reaction was due to
the presence of side reactions, such as over-oxidation of benzalde-
hyde and esterification of benzyl alcohol with benzoic acid. All the
reactions showed a satisfactory yield after 5 h and in the context of
green chemistry, we used water as a solvent for further reactions.
Fig. 2. (a) SEM, (b) EDXS, and (c) TEM images of (A) Au and (B) Pd nanoparticles decorated Cu2O microcubes.

K.S. Prasad et al. / Applied Catalysis A: General 476 (2014) 72–77
75
Fig. 3. XPS spectra of the (a) survey, (b) Cu2p3/2, 1/2, and (c) Au4f7/2, 5/2 or Pd3d5/2, 3/2 peaks for (A) Au and (B) Pd nanoparticles decorated Cu2O microcubes.
The reactions were carried out with benzyl alcohol (10.0 mM)
as a test substrate, NaOH (1 N, 100.0 ␮L), Cu2O/Au (3.0 mg), and
water (15.6 mL) with oxygen purging and stirring at 80 ◦C for 5 h.
It was found that benzaldehyde was obtained with high selectivity
(Scheme 2(A) and Fig. 4(A)). To demonstrate the versatility of the
catalytic activity of Cu2O/Au toward benzaldehyde oxidation was
further studied with electron withdrawing and electron-donating
groups, resulted in the formation of respective aldehyde with excel-
lent selectivity and yields. The catalyst free from Au, i.e. only Cu2O
was also tested for its catalytical activity using benzyl alcohol as a
substrate and then following the similar experimental condition,
resulted only trace conversion of alcohol to aldehyde (13%). The
TON (turn over number) and TOF (turn over frequency) for the cat-
alyst for the alcohol oxidation in entry 1a–d in Scheme 2(A) was
calculated to be 21,241, 21,719, 21,241, and 21,241. And the TOF
values are 4248, 4344, 4248, and 4248 h−1 based on a 5 h reaction
time. The observed values were comparable with the benzyl alcohol
oxidation over Au catalyst [37,38].
test substrates. The Suzuki coupling of between phenyl boronic
acid (2.4 mM) and 4-iodobenzonitrile (2.0 mM) was conducted
overnight in the presence of 8.0 mg Cu2O/Pd colloid with K2CO3
(4.0 mM) at 80 ◦C in water (18.5 mL), and the reaction produced
4-cyanobiphenyl with good selectivity and satisfactory yield.
The result of the catalytic reaction revealed the versatile
nature of Cu2O/Pd catalyst for the coupling reaction of iodoarenes
substituted with electron donating and withdrawing groups
(Scheme 2(B) and Fig. 4(B)). The obtained result shows the suc-
cessful application of Cu2O/Pd for the Suzuki–Miyaura reaction in
water. Overall, the conversion of boronic acid and aryl iodides to
biphenyl was over 85%. For a comparison, we experimented Pd
free Cu2O for the coupling reaction of phenyl boronic acid and 4-
iodobenzonitrile by following the same procedure as mentioned
earlier. It was found that Cu2O showed catalytic activity by pro-
ducing 4-cyanobiphenyl, however the yield was less than that of
Cu2O/Pd. Furthermore, Cu2O/Pd also used to conduct the aerobic
oxidation of benzyl alcohol using water as a solvent. The reac-
tion produced benzaldehyde with satisfactory >90% yields and
with >99% selectivity, the yields and selectivity were determined
with GC–MS and 1H NMR. The mass spectra obtained for the
products showed complete agreement with the known data of
products in national institute of standards and technology (NIST)
database for benzaldehyde and 4-cyanobiphenyl [40]. The TON for
The catalytic effect of Pd based-catalyst was investigated by
conducting the Suzuki–Miyaura coupling reaction, which was a
powerful and convenient synthetic method in organic chemistry
for generating biaryls, conducting polymers, and liquid crystals.
To test the catalytic activity of Cu2O/Pd for the Suzuki reaction,
phenyl boronic acid and 4-iodobenzonitrile were selected as the
Fig. 4. Mass spectra of (A) the product (benzaldehyde) obtained after the aerobic oxidation of benzyl alcohol with Cu2O/Au, and (B) the product (4-cyanobiphenyl) obtained
after the Suzuki coupling of between phenyl boronic acid and 4-iodobenzonitrile with Cu2O/Pd.

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K.S. Prasad et al. / Applied Catalysis A: General 476 (2014) 72–77
Scheme 2. (A) Aerobic oxidation of benzyl alcohol to corresponding aldehyde in the presence of Cu2O/Au. (Reactions were carried out with 10.0 mM of benzyl alcohol or
substituted benzyl alcohol in water with 100.0 ␮L NaOH (1 N), catalyst Cu2O/Au (3.0 mg) with 6.59 × 10−5 mol% Au with oxygen purging and stirring at 80 ◦C for 5 h.). (B)
Suzuki–Miyaura reactions catalyzed by in the presence of Cu2O/Pd. (Reactions were carried out with a mixture of boronic acid (2.4 mM), aryl halide (2.0 mM), K2CO3 (4.0 mM)
and 8.0 mg of Cu2O/Pd with 5.70 × 10−4 mol% Pd with oxygen purging and stirring at 80 ◦C for 12 h.).
the Suzuki–Miyaura reaction for entry 2a–d in Scheme 2(B) was cal-
culated to be 608, 616, 637, and 608. The TOF values are 50.6, 51.3,
53, and 50.6 h−1 based on 12 h reaction time and it is comparable
with the TON obtained for the Pd nanoparticle clay composites and
better than the Pd nanoparticle supported metal organic frame-
works [41,42]. Overall, the catalyst Cu2O/Pd showed the catalytic
activity not only for the coupling reaction but also for the selec-
tive oxidation of benzyl alcohol. Furthermore, the catalyst could
be easily recovered from the reaction mixture by simple centrifu-
gation and it can be used again. The catalyst from the resulting
product was first separated by simple centrifugation at 3500 rpm
and UV–Visible spectra and TEM image were taken (Supplemen-
tary data, Fig. S1). No change of the morphology or UV–Visible
spectra before and after the catalytic reactions could be observed,
indicating that the catalysts were recyclable.
process of metal/metal oxide catalysts. The Cu2O/Au and Cu2O/Pd
functioned as an efficient catalyst for organic reactions including
the aerobic benzyl alcohol oxidation and the Suzuki–Miyaura cou-
pling reactions in water. The ligand free efficient catalyst and cheap
and non-toxic water as a solvent make the present catalysts ulti-
mate for specific organic reactions.
Acknowledgment
This work was supported by the NRF grant funded by the MEST,
South Korea (Grant no. 20100029128).
Appendix A. Supplementary data
Supplementary material related to this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.plantsci.
2004.08.011.
4. Conclusions
In conclusion, noble metal nanoparticles decorated-Cu2O cata-
lyst, Cu2O/Au and Cu2O/Pd−were successfully synthesized by the
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