Palladium nanoparticles supported on Fe3O4/amino acid nanocomposite: Highly active magnetic catalyst for solvent-free aerobic oxidation of alcohols

Article abstract of DOI:10.1016/j.catcom.2013.09.029

In this paper, Fe₃O₄ nanoparticles were coated by a number of amino acids, e.g. cysteine, serine, glycine and β-alanine, via a simple method. Because of the surface modification of the magnetic nanoparticles with amino acid, the obtained magnetic nanocomposite is able to trap palladium nanoparticles through a strong interaction between the metal nanoparticles and the functional groups of amino acids. Among the synthesized nanocomposites, Fe₃O₄/cysteine-Pd exhibited the highest catalytic performance and excellent selectivity in the solvent-free aerobic oxidation of various alcohols, along with high level of reusability.

Full text of DOI:10.1016/j.catcom.2013.09.029

Catalysis Communications 43 (2014) 164–168
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Palladium nanoparticles supported on Fe3O4/amino acid nanocomposite:
Highly active magnetic catalyst for solvent-free aerobic oxidation
of alcohols
a
Farzad Zamani a,b, , Seyed Mohsen Hosseini
⁎
a
Department of Chemistry, Shahreza Branch, Islamic Azad University, Shahreza 31186145, Isfahan, Iran
Laboratory of Applied Chemistry, Central Laboratory Complex, Isfahan Science and Technology Town, Isfahan University of Technology, Isfahan 8415683111, Iran
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 August 2013
Received in revised form 26 September 2013
Accepted 27 September 2013
Available online 12 October 2013
In this paper, Fe3O4 nanoparticles were coated by a number of amino acids, e.g. cysteine, serine, glycine and
β-alanine, via a simple method. Because of the surface modification of the magnetic nanoparticles with amino acid,
the obtained magnetic nanocomposite is able to trap palladium nanoparticles through a strong interaction between
the metal nanoparticles and the functional groups of amino acids. Among the synthesized nanocomposites, Fe3O4/
cysteine-Pd exhibited the highest catalytic performance and excellent selectivity in the solvent-free aerobic
oxidation of various alcohols, along with high level of reusability.
Keywords:
© 2013 Elsevier B.V. All rights reserved.
Heterogeneous magnetic catalyst
Palladium nanoparticle
Cysteine amino acid
Solvent-free aerobic oxidation of alcohol
High selectivity
1. Introduction
heterogeneous and homogenous catalytic reactions. Most importantly,
the magnetic-supported catalysts show not only high catalytic activity
The selective oxidation of alcohols to the corresponding carbonyl
compounds is a fundamental organic transformation in both laboratory
and industrial synthetic chemistry because the resulting carbonyl
compounds are widely used in the preparation of pharmaceutical,
agricultural and fragrance chemicals [1]. In general, the oxidation
process based on the use of traditional stoichiometric or excess amount
of oxidants has caused serious environmental problems [2]. Therefore,
the development of efficient catalysts for selective oxidation of alcohols
using molecular oxygen or air to replace the stoichiometric oxidizing
reagents has become urgent from the demand for establishing both
environmentally benign and economically practical synthetic routes.
In this regard, the solvent-free aerobic oxidation of alcohols using
molecular oxygen or air as the oxidant has drawn much attention
recently [3–5], due to positive effects in terms of cost, safety and
environmental impact. In this study, it is highly attractive to develop
the efficient catalytic system for the aerobic oxidation of various
alcohols under solvent-free condition.
but also high degree of chemical stability and they do not swell in
organic solvents [6–8]. They can also be recovered with an external
magnetic field and their catalytic efficiency remains after many
repeated reactions [9]. In recent years, several heterogeneous noble
metal catalysts based on magnetic nanoparticles have been applied
for the aerobic oxidation of alcohols [10–12]. However, the application
of noble metal nanoparticle catalysts based on organic–inorganic
magnetic nanocomposites as heterogeneous catalysts in the oxidation
reaction of alcohols has received no considerable attention.
In recent years, modified magnetic nanoparticles have received a lot
of attention as support for preparation of noble metal nanoparticles.
However, most of these techniques require many reaction steps to
introduce functional groups to the magnetic surface and they use
organosilica precursors as organic shell in order to prepare a suitable
support for trapping metal nanoparticles [13–18]. The organosilane
precursors not only involve complicated synthesis and purification
method, but also are very expensive and toxic. Therefore, from both
environmental and economic points of view, preparation of the
modified magnetic nanoparticles via a simple method and without
using organoalkoxysilane compounds is highly desirable. In this study,
amino acids, which are inexpensive and nontoxic materials, were used
as good candidates for incorporation of palladium nanoparticles.
In continuing our efforts towards the development of efficient and
environmentally benign heterogeneous catalysts [19,20], herein, we will
report a simple preparation of palladium nanoparticles incorporated
Over the last decade, organic–inorganic magnetic nanocomposites
have become interesting as magnetic catalysts in both academic and
industrial fields. The use of these magnetic nanoparticle catalysts can
address the isolation and recycling problem encountered in many
⁎
Corresponding author. Tel.: +98 311 3865355; fax: +98 311 3869714.
E-mail address: fzamani@iaush.ac.ir (F. Zamani).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.09.029

F. Zamani, S.M. Hosseini / Catalysis Communications 43 (2014) 164–168
165
into Fe3O4/amino acid nanocomposite as a new magnetically recoverable
heterogeneous catalyst. The catalytic activity of this magnetic catalyst
was tested in the aerobic oxidation reaction of different alcohols under
solvent-free condition.
2. Experimental
2.1. Catalyst preparation
Fe3O4/amino acid-Pd magnetic nanocomposite was synthesized via
a simple and in-situ method as the following:
In the first step, magnetic nanoparticles were prepared in the
presence of several amino acids, e.g. L-cysteine (Cys), β-alanine
(Ala), serine (Ser) and glycine (Gly), via a co-precipitation route in
an early study [21]. FeCl3·6H2O (13 g, 0.048 mol), FeCl2·4H2O (4.8 g,
0.024 mol) and amino acid (Cys, Ala, Ser or Gly, 0.096 mol) were
dissolved in 100 mL deionized water. Then, the solution pH was
adjusted to 11 with NaOH solution (2 M) to form a black suspension.
Afterwards, the suspension was reflux for 12 h under vigorous
stirring and Ar atmosphere. Finally, the obtained nanocomposite
was separated from the aqueous solution by magnetic decantation,
washed several times with deionized water and dried in an oven
overnight. The resulting powders were denoted as Fe3O4/Cys,
Fe3O4/Ala, Fe3O4/Ser and Fe3O4/Gly.
In the second step, Pd nanoparticles were immobilized on Fe3O4/
amino acid through the reduction of PdCl2 by hydrazine hydrate in
ethanol. Briefly, the as-synthesized Fe3O4/amino acid (1.0 g) and
PdCl2 (0.1 g) were dispersed in an ethanol solution (30 mL) and
reflux for 10 h. Then, the reaction mixture was cooled to room
temperature and hydrazine hydrate solution (catalyst: hydrazine
hydrate = 1:5) was slowly dropped into the mixture and refluxed
further for 2 h. The resultant product was collected by an external
magnetic field, washed several times with ethanol and deionized
water and dried in a vacuum oven at 60 °C. The as-synthesized
magnetic nanocomposites were denoted as Fe3O4/Cys-Pd, Fe3O4/
Ala-Pd, Fe3O4/Ser-Pd and Fe3O4/Gly-Pd. The amount of Pd in the
obtained catalysts, based on ICP analysis, was found to be 6.27, 3.62,
5.28 and 3.14 wt.% for Fe3O4/Cys-Pd, Fe3O4/Ala-Pd, Fe3O4/Ser-Pd
and Fe3O4/Gly-Pd respectively.
Fig. 1. X-ray powder diffraction patterns of (a) Fe3O4/Cys and (b) Fe3O4/Cys-Pd
nanocomposite.
diffraction of (220), (311), (400), (422), (511) and (440) of the Fe3O4
(JCPDS 89-3854), which were found in both samples (Fig. 1a,b). This
result means that the nanocomposite has been successfully synthesized
without damaging the crystal structure of Fe3O4 core. Moreover, apart
from the original peaks related to Fe3O4, the new peaks at 2θ of 40°,
46.7° and 67.9° were observed, which correspond to (111), (200) and
(220) crystalline planes of Pd, indicating that Pd element exists in the
form of Pd(0) (Fig. 1b). The crystallite size of Pd nanoparticles was
evaluated using Scherrer equation for the (111) peak and was found
to be approximately 4 nm in size. The average grain size of the Pd
nanoparticles is approximately the same as the average particle size of
the Pd nanoparticles determined by the TEM observations, which
indicates that the Pd nanoparticles are single crystals. It seems that the
size of the metal nanoparticles, determined by TEM analysis, is more
reliable than using Scherrer formula in XRD analysis.
The FT-IR spectra of Fe3O4/Cys and Fe3O4/Cys-Pd nanocomposites
were recorded to confirm the modification of the magnetite surface
with the amino acid and Pd nanoparticles (Fig. 1) (measured with a
Perkin Elmer 65 spectrometer). The presence of magnetite nanoparticles
is observable by the strong adsorption band at 587cm−1, corresponding
to the Fe\O vibrations (Fig. 2a,b). As can be seen, the adsorption peaks
at 1586, 1418 and 3286 cm−1 are due to asymmetric and symmetric
2.2. General procedure for solvent-free aerobic oxidation of alcohols
In a typical reaction, benzyl alcohol (0.2 mL, 2 mmol) and magnetic
catalyst (0.06 g) were loaded in a two-neck round bottom flask. The
mixture was then immersed in a 50 °C oil bath and the oxygen flow
was bubbled into the mixture to initiate the reaction. The reaction
mixture was then stirred under solvent-free condition. The progress
of reaction was monitored by thin layer chromatography (TLC). On
completion of the reaction, the catalyst was removed by an external
magnet and the liquid organic product was analyzed by an Agilent gas
chromatograph 6890 equipped with a HP-5 capillary column. Dodecane
was the internal standard to calculate benzyl alcohol conversion and
benzaldehyde selectivity.
3. Results and discussion
3.1. Catalyst characterization
The high angle XRD patterns of the Fe3O4/Cys and Fe3O4/Cys-Pd
nanocomposites were shown in Fig. 1 (evaluated by Bruker D8 Advance
diffractometer). It could be seen that the strong characteristic diffraction
peaks at 2θ of 30.1°, 35.4°, 43.2°, 53.7°, 56.9° and 62.9° belong to the
Fig. 2. FT-IR spectra of (a) Fe3O4/Cys and (b) Fe3O4/Cys-Pd nanocomposite.

166
F. Zamani, S.M. Hosseini / Catalysis Communications 43 (2014) 164–168
stretching of the COO− and N\H stretching, respectively, which
indicate the presence of bound amino acid on the magnetite
nanoparticle surface (Fig. 2a,b). Furthermore, it has been reported that
the wavenumber separation between the COO−as and COO−s IR bands
can be used to distinguish the type of the interaction between the
carboxylate head and the metal atom [22]. Since the wavenumber
separation between the COO−as and COO−s bands is 168cm−1 (1586–
1418 = 168 cm−1) (Fig. 2a,b), it can be concluded that the interaction
between the COO− group and the Fe atom was covalent and bridging
bidentate [22]. In addition, the bands in the 2900–3000 cm−1 are
attributed to the stretching of C-H bonds of the amino acid (Fig. 2a,b).
Moreover, the band at around 682 cm−1, which corresponds to C\S
bond of the supported cysteine (Fig. 2a), is shifted to lower
wavenumbers (658 сm−1) (Fig 2.b), suggesting a strong interaction
between sulfur group and metal particles [23]. The bending vibration
absorption band of N\H at 1449 сm−1 (Fig. 2a) is also shifted to
lower wavenumbers (1449 → 1384), which is possibly due to the
interaction of amine group with metal particles (Fig. 2b) [24]. Taking
the above observations into consideration, it can be concluded that
the Fe3O4/Cys-Pd nanocomposite was successfully obtained along
with a strong interaction between sulfur and amine groups of the
supported cysteine and the Pd nanoparticles.
Fig. 4. XPS spectrum of Pd 3d of Fe3O4/Cys-Pd nanocomposite.
The morphology of the nanocomposite was observed on transmission
electron microscopy (Phillips CM10 microscope). Fig. 3 shows TEM
image of the Fe3O4/Cys-Pd nanocomposite catalyst. It can be seen that
the particle diameter of the catalyst is about 20 nm and they are nearly
uniform in size. Moreover, the Pd nanoparticles were found to be highly
dispersed on the surface of the Fe3O4/Cys nanoparticles with the average
diameter size of ~3nm. Li et al. have reported that the metal nanoparticle
catalysts with a medium mean size of about 3 nm exhibit the highest
conversion and selectivity for the oxidation of alcohols, compared
with those with larger or smaller metal particles [25]. As a result, it
could be expected that the synthesized magnetic catalyst has high
activity/selectivity performance, which will be investigated in further
steps.
nanocomposite structure. In comparison to the standard binding energy
of Pd0, with Pd 3d5/2 of about 335 eV and Pd3/2 of about 340 eV [26], it
can be concluded that the Pd peaks in the Fe3O4/Cys-Pd shifted to
lower binding energy than Pd0 standard binding energy. It has been
reported that the position of Pd 3d peak is usually influenced by
the local chemical/physical environment around Pd species besides
the formal oxidation state, and shifts to lower binding energy when
the charge density around it increases [26]. Therefore, the peaks at
332.5 and 337.8 could be due to Pd0 species bound directly to oxygen
and sulfur groups in the Fe3O4/Cys, which is in agreement with IR
results.
In order to obtain information on the structural features of the
nanocomposite, the XPS study was carried out on the Fe3O4/Cys-Pd
catalyst (recorded by Shimadzu ESCA SSX-100). The XPS result of Pd
nanoparticles dispersed in the Fe3O4/Cys nanocomposite for Pd 3d
spectrum with the binding energies of Pd 3d5/2 and Pd 3d3/2 lying
at about 332.5 and 337.8 eV, respectively (Fig. 4). It means that
Pd nanoparticles are stable as metallic state in the Fe3O4/Cys
3.2. Catalytic activity
In order to evaluate catalytic activity of the Fe3O4/amino acid-Pd
nanocomposites and to optimize the reaction conditions, the aerobic
oxidation of benzyl alcohol was chosen as a model reaction. Before
optimizing the reaction conditions, the catalytic activity of synthesized
nanocomposites was compared in the solvent-free aerobic oxidation
of benzyl alcohol. Then, after choosing the best nanocomposite as
catalyst, the aerobic oxidation reaction was conducted in different
reaction parameters such as solvent and amount of catalyst.
In an effort to compare the catalytic activity of synthesized magnetic
nanocomposites for the oxidation of benzyl alcohol under O2 atmosphere
and solvent-free condition, it was found that these nanocomposites
demonstrated the different activity for the alcohol oxidation. As displayed
in Table 1, Fe3O4/Gly-Pd catalyst afforded the lowest reaction yield, while
the Fe3O4/Cys-Pd catalyst exhibited the highest catalytic performance in
Table 1
Solvent-free aerobic oxidation of benzyl alcohol catalyzed by different synthesized
magnetic nanocomposites.a
Catalyst
Selectivity to benzaldehyde (%)b
Yield (%)c
Fe3O4/Cys-Pd
Fe3O4/Ala-Pd
Fe3O4/Ser-Pd
Fe3O4/Gly-Pd
N99
N99
N99
N99
48
23
35
16
a
Reaction conditions: benzyl alcohol (2 mmol), catalyst (0.03 g), reaction time = 1.5 h,
temperature = 50 °C, O2 bubbling.
b
Determined by GC using dodecane as internal standard.
GC yield.
c
Fig. 3. TEM micrograph of Fe3O4/Cys-Pd nanocomposite.

F. Zamani, S.M. Hosseini / Catalysis Communications 43 (2014) 164–168
167
Scheme 1. Structures of the amino acids used in the preparation of Fe3O4/amino acid-Pd nanocomposite.
the solvent-free aerobic oxidation of benzyl alcohol. These results
indicate that the ligand structure could play an important role in the
catalytic activity of nanocomposites. It has been reported that sulfur-
containing ligands are highly efficient stabilizers for the preparation of
palladium nanoparticles [27]. Among the used amino acids, sulfur groups
in the structure of cysteine make it the best candidate for the preparation
of metal nanoparticles, which is in agreement with the metal content of
the catalysts (determined by ICP in the experimental section). The
structures of used amino acids were represented in Scheme 1.
As far as the effect of solvent is concerned, the model reaction was
carried out in several solvents as well as solvent-free condition to
investigate the efficiency of the catalyst (Fe3O4/Cys-Pd). As can be seen
in Table 2, the best yields were obtained when the reactions were
performed in water and solvent-free condition at 50 °C (52% and 48%
respectively). Since the solvent-free condition is more environmentally
benign, it was chosen as the best reaction medium for the aerobic
oxidation of benzyl alcohol in the presence of Fe3O4/Cys-Pd. Moreover,
the effect of catalyst amount on the alcohol reaction was also carried
out by different amounts of the catalyst (Table 2). It was observed that
while the amount of catalyst increased from 0.03 to 0.06 g, the product
yield raised significantly from 48% to 85% with excellent selectivity to
benzaldehyde (N99%). Since then, the percentage of yield remained
stable and selectivity to benzaldehyde decreased by 6% between 0.06 g
and 0.08 g. According to the data, 0.06 g was chosen as the best amount
of catalyst for further steps.
that this catalyst can be reused seven times without any significant
loss in activity/selectivity performance in the solvent-free aerobic
oxidation of benzyl alcohol. This high catalytic activity can be possibly
related to the strong interaction of Pd nanoparticles with active sites
of cysteine, because sulfur-containing ligands are highly efficient
stabilizers for the metal nanoparticles [27].
The Fe3O4/Cys-Pd catalyst also represented a good activity and
selectivity in the solvent-free aerobic oxidation of different aromatic
and aliphatic alcohols (Table 4). All the investigated alcohols were
converted to the corresponding aldehydes in high yields with excellent
selectivity.
4. Conclusion
In conclusion, F3O4 nanoparticles were coated with several amino
acids as organic shell via a co-precipitation method. The Fe3O4/amino
acid nanocomposites were effectively employed as scaffold for in-situ
generation of Pd nanoparticles. This new heterogeneous catalyst
showed the following advantages: (a) simple preparation; (b) easy
separation after reaction by an external magnet; and (c) reusability
for several times without any significant loss in the yield and selec-
tivity of the products in the solvent-free aerobic oxidation of alco-
hols. Interestingly, Fe3O4/cysteine-Pd nanoparticles (Fe3O4/Cys-Pd)
exhibited the highest activity in the oxidation reaction among the
other synthesized nanocomposites, which is possibly due to the
existence of sulfur groups in the cysteine structure. These unique results
open new perspectives for the application of these types of magnetic
catalysts in other organic reactions.
It should be mentioned that the oxidation of benzyl alcohol was
performed under air instead of oxygen, based on the optimized reaction
conditions. It was observed that when the solvent-free oxidation of
benzyl alcohol was carried out under air, the product yield was not
satisfying (55% in 235min).
In the view of industrial purposes, reusability of the catalyst was
tested by carrying out repeated runs of the reaction on the same batch
of the catalyst in the case of the model reaction (Table 3). In order to
regenerate the catalyst, after each cycle, it was separated by a magnet,
and washed several times with deionized water and ethanol. Then, it
was dried in oven at 60 °C and used in the next run. The results show
Acknowledgments
The authors thank the Isfahan Science and Technology Town
(Isfahan University of Technology) for the support in this work. The
authors also appreciate professors R.J. Kalbasi and A.R. Massah for
their helpful comments.
Table 2
Effects of different solvents and amount of catalyst on the oxidation of benzyl alcohol in
Table 3
the presence of Fe3O4/Cys-Pd.a
The catalyst reusability for the solvent-free aerobic oxidation of benzyl alcohol.a
Solvent
Catalyst (g)
Selectivity to benzaldehyde (%)b
Yield (%)d
Yield (%)c
Selectivity to benzaldehyde (%)b
Cycle
Water
0.03
0.03
0.03
0.03
0.03
0.06
0.08
N99
N99
N99
N99
N99
N99
94c
52
42
32
36
48
85
85
85
85
85
83
83
82
80
80
N99
N99
N99
N99
N99
N99
N99
N99
Fresh
Acetonitrile
n-Hexane
Toluene
Solvent-free
Solvent-free
Solvent-free
1
2
3
4
5
6
7
a
Reaction conditions: benzyl alcohol (2 mmol), reaction time = 1.5 h, temperature =
50 °C, O2 bubbling.
a
Reaction conditions: benzyl alcohol (2mmol), catalyst (0.06g, Fe3O4/Cys-Pd), reaction
time = 1.5 h, temperature = 50 °C, O2 bubbling.
b
Determined by GC using dodecane as internal standard.
The main by-product was toluene.
GC yield.
c
b
Determined by GC using dodecane as internal standard.
GC yield.
d
c

168
F. Zamani, S.M. Hosseini / Catalysis Communications 43 (2014) 164–168
Table 4
Solvent-free aerobic oxidation of various aromatic and aliphatic alcohols over Fe3O4/Cys-Pd.a
Entry
1
Substrate
Product
Time (h)
1.5
Yield (%)b
85
2
3
4
5
1
83
82
58
70
1
3
1.5
6
7
1.5
1
82
87
8
9
4
4
74
75
a
Reaction conditions: benzyl alcohol (2 mmol), catalyst (0.06 g, Fe3O4/Cys-Pd), temperature = 50 °C, O2 bubbling, selectivity = N99.
GC yield
b
[13] B. Sahoo, S.K. Sahu, D. Bhattacharya, D. Dhara, P. Pramanika, Colloids Surf. B
Biointerfaces 101 (2013) 280–289.
References
[14] W. Li, B. Zhang, X. Li, H. Zhang, Q. Zhang, Appl. Catal. A Gen. 459 (2013) 65–72.
[15] E. Karaoglu, M.M. Summak, A. Baykal, H. Sozeri, M.S. Toprak, J. Inorg. Organomet.
Mater. 23 (2013) 409–417.
[16] L.M. Rossi, I.M. Nangoi, N.J.S. Costa, Inorg. Chem. 48 (2009) 4640–4642.
[17] D.C. Li, Z.L. Zhang, S.L. Zhang, Adv. Mater. Res. 177 (2011) 415–417.
[18] Z. Lou, X. Zhang, H. Zhang, J. Nanosci. Nanotechnol. 12 (2012) 3142–3149.
[19] R.J. Kalbasi, M. Kolahdoozan, K. Shahabian, F. Zamani, Catal. Commun. 11 (2010)
1109.
[20] R.J. Kalbasi, A.R. Massah, F. Zamani, A.D. Bain, B. Berno, J. Porous. Mater. 18 (2011)
475.
[21] B. Unal, M.S. Toprak, Z. Durmus, H. Sözeri, A. Baykal, J. Nanopart. Res. 12 (2010)
3039–3048.
[22] L. Zhang, R. He, H.C. Gu, Appl. Surf. Sci. 253 (2006) 2611–2617.
[23] M.J.M. Campbell, R. Grzeskowiak, J. Chem. Soc. A (1967) 396–401.
[24] R.J. Kalbasi, A.A. Nourbakhsh, F. Babaknezhad, Catal. Commun. 12 (2011)
955–960.
[25] F. Li, Q. Zhang, Y. Wang, Appl. Catal. A Gen. 334 (2008) 217–226.
[26] T. Teranishi, M. Miyake, Chem. Mater. 10 (1998) 594–600.
[27] J. Cookson, Platinum Met. Rev. 56 (2012) 83–98.
[1] R.A. Sheldon, I.W.C.E. Arends, A. Dijksman, Catal. Today 57 (2000) 157.
[2] J. Muzart, Tetrahedron 59 (2003) 5789–5816.
[3] X.M. Wang, G.J. Wu, N.J. Guan, L.D. Li, Appl. Catal. B Environ. 115–116 (2012) 7–15.
[4] Y. Su, L.C. Wang, Y.M. Liu, Y. Cao, H.Y. He, K.N. Fan, Catal. Commun. 8 (2007)
2181–2185.
[5] Y. Chen, H. Wang, C.J. Liu, Z. Zeng, H. Zhang, C. Zhou, X. Jia, Y. Yang, J. Catal. 289
(2012) 105–117.
[6] K.K. Senapati, S. Roy, C. Borgohain, P. Phukan, J. Mol. Catal. A Chem. 352 (2012)
128–134.
[7] D. Lee, J. Lee, H. Lee, S. Jin, T. Hyeon, B.M. Kim, Adv. Synth. Catal. 348 (2006) 41–46.
[8] R. Abu-Rezig, H. Alper, D. Wang, M.L. Post, J. Am. Chem. Soc. 128 (2006) 5279–5282.
[9] E. Karaoğlu, A. Baykal, H. Erdemi, L. Alpsoy, H. Sözeri, J. Alloys Compd. 509 (2011)
9218–9225.
[10] V.V. Costa, M.J. Jacint, L.M. Rossi, R. Landers, E.V. Gusevskaya, J. Catal. 282 (2011)
209–214.
[11] M. Zhang, Q. Sun, Z. Yan, J. Jing, W. Wei, D. Jiang, J. Xie, M. Chen, Aust. J. Chem. 66
(2013) 564–571.
[12] M.B. Gawande, A. Rathi, I.D. Nogueira, C.A.A. Ghumman, N. Bundaleski, O.M.N.D.
Teodoro, P.S. Branco, Chem. Plus Chem. 77 (2012) 865–871.