Coppercoresilvershell nanoparticle-yolk/shell Fe 3O4@chitosan-derived carbon nanoparticle composite as an efficient catalyst for catalytic epoxidation in water

Article abstract of DOI:10.1039/c4ra05283h

The fabrication of yolk/shell spheres consisting of a movable magnetic core and a chitosan-derived porous carbon shell, and plenty of tiny copper _coresilver_shell nanoparticles confined within the porous shell, is reported. Fe₃

Full text of DOI:10.1039/c4ra05283h

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Graphical Abstract
CoppercoreSilvershell Nanoparticles-Yolk/Shell Fe3O4@Chitosan-
Derived Carbon Nanoparticles Composite as an Efficient Catalyst
for Catalytic Epoxidation on Water
The fabrication of yolk/shell spheres consisting of a movable magnetic core, a chitosan-
derived porous carbon shell, and plenty of tiny coppercoresilvershell nanoparticles,
confined within the porous shell, is reported. The ensuing catalyst was employed for the
epoxidation reaction in aqueous media at ambient temperature. The products were
obtained in high yields with low amounts of the catalyst.

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ARTICLE
CoppercoreSilvershell NanoparticlesꢀYolk/Shell
Fe3O4@ChitosanꢀDerived Carbon Nanoparticles
Composite as an Efficient Catalyst for Catalytic
Epoxidation on Water
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
Mohammad Reza Nabid*, Yasamin Bide, and Maryam Abuali
DOI: 10.1039/x0xx00000x
The fabrication of yolk/shell spheres consisting of a movable magnetic core, a chitosanꢀderived porous
carbon shell, and plenty of tiny coppercoresilvershell nanoparticles, confined within the porous shell, is
reported. Fe3O4 nanoclusters were first synthesized through solvothermal method and then SiO2 shell was
coated on the Fe3O4 surface via a solꢀgel process. Subsequently, chitosan was used to modify the surface
of Fe3O4@SiO2 and then, the selective etching of the middle SiO2 layer was accomplished to obtain the
yolk/shell Fe3O4@chitosan composites. The coppercoresilvershell nanoparticles immobilized on yolk/shell
Fe3O4@chitosanꢀderived carbon nanoparticle could be synthesized starting with coppercoresilvershell
nanoparticlesꢀyolk/shell Fe3O4@chitosan composite; which is due to the wellꢀknown strong affinity of
chitosan for transition metals. The ensuing catalyst was employed for the epoxidation reaction in aqueous
media at ambient temperature. The products were obtained in high yields with low amounts of the
catalyst.
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interesting family of nanoꢀarchitectures. These systems have
attracted considerable attention over the past few years in a
variety of applications including catalysis, storage, and drug
delivery due to their unique properties, such as large surface
area, multiꢀfunctionality and excellent loading capacity.15ꢀ18
Among them, YSNs with a magnetic core, because of the
Introduction
Due to many different advantages of nanoparticles (NPs) such
as the large surfaceꢀtoꢀvolume ratio and combining the
characteristics of homogeneous (relatively low catalyst loadings
and good selectivity) and heterogeneous catalysis (recovery and
recyclability),1 employing metal nanoparticles in catalysis has
significantly increased recently.2 However, because of the high
surface area of metal clusters, they tend to aggregate into larger
particles which lead to the loss of catalytic activity. Solving this
problem and enhancing environmental consciousness and also
promoting the efficiency of chemical reactions under benign
conditions with recyclability of catalysts are the essential parts
of chemical researches.3 One of the best approaches to improve
the efficiency and recyclability of the catalysts is their
immobilization on the solid supports.4ꢀ6 Among the
biopolymers, chitosan (CTS) has received growing attention as
a functional, nonꢀtoxic, renewable, and biocompatible polymer,
which has many applications, such as metal extraction,7ꢀ9 and in
some recent examples as catalytic support.10,11 Recent reports
propose that carbon nanoparticles (CNPs), because of their
stability, sizes, ease of dispersion in liquid medium and facile
formation of composites, especially with catalytic metal
nanoparticles, could provide better options.12ꢀ14 Chitosanꢀ
derived carbon nanoparticles, because of the convenient method
of synthesis, being bioꢀcompatible, inexpensive and thermally
stable, are really good candidates for stabilizing noble metals
and heterogeneous catalysis
improved stability, biocompatibility,
and significant
magnetization strength of iron oxides are desirable in
biomedical applications.19,20 Also YSNs with a magnetic core
and a functional shell are attractive because of their potential
applications as catalyst supports and also elimination of timeꢀ
consuming separation procedures such as filtration or
centrifugation. On the other hand, the outer shells of yolkꢀshell
composites usually need to have both high surface areas and
good dispersion characteristics to be the catalyst support, which
chitosanꢀderived carbon nanoparticles are excellent choices.
Based on our interest in the development of sustainable benign
pathways for organic transformations, and nanoꢀcatalysis,21ꢀ26
we decided to synthesis coppercoresilvershell nanoparticles
immobilized on the yolk/shell Fe3O4@chitosanꢀderived carbon
nanoparticle and to explore their performance for catalytic
epoxidation reactions “on water”. The main difficulty in
utilization of nonꢀnoble metals such as Co, Ni, Fe and Cu arises
from their tendency toward oxidation at ambient conditions,
particularly as their size gets smaller.27,28 However, very high
cost of silver limits its wide industrial applications as a catalyst.
Since copper is much cheaper than silver but possesses a very
high conductivity (only 6% less than that of Ag), Cu NPs can
be considered as a replacement for silver NPs. Employing
copper–silver core–shell (CucoreAgshell) nanoparticles leads to
Yolk/shell nanostructures (YSNs) consisting
encapsulated in a hollow capsule with a porous shell are
a
core
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VJieowuArrtnicalelONnlainme e
DOI: 10.1039/C4RA05283H
have the optical and electrical properties of both metals, and products were separated with magnet and washed with
also prevent the oxidation of copper core.29 Moreover, Cu–Ag deionized water for the nextꢀstep.
bimetallic catalyst should be more selective than pure silver for
epoxidation.30
Preparation of the yolkꢀshell Fe3O4@chitosan
To perform chitosan coating, 0.4 g of asꢀprepared Fe3O4@SiO2
coreꢀshell NPs was mixed with 0.4 g of chitosan in 60 mL
water for 3 h. The obtained Fe3O4@SiO2@chitosan coreꢀshell
NPs were washed with ethanol and water for several times and
dried in vacuum oven. The synthesized Fe3O4@SiO2@chitosan
composites were soaked in 1.0 M NaOH solution under
mechanical stirring for 12.0 h. The SiO2 layer between the
chitosan shell and the Fe3O4 core was selectively etched. The
black product was washed with deionized water and ethanol
several times and dried in oven.
Experimental
Materials
Chitosan, copper nitrate trihydrate (Cu(NO3)2.3H2O) and silver
nitrate (AgNO3) were purchased from Aldrich Company. Ferric
chloride hexahydrate (FeCl3.6H2O), trisodium citrate dehydrate,
sodium acetate (NaAc), ethylene glycol (EG), tetraethyl
orthosilicate (TEOS), and sodium hydroxide (NaOH) were
purchased from Merck Chem. Co. All other chemicals were
purchased from Aldrich or Merck companies and used as
received without any further purification.
Preparation of the yolk/shell Fe3O4@chitosan/CucoreAgshell
composites
Instruments and characterization
For immobilization of CucoreAgshell on the yolkꢀshell
Fe3O4@chitosan, in
a
typical procedure, 0.072
g
of
Fourier transform infrared (FTꢀIR) spectra were recorded on a
Bomem MBꢀSeries FTꢀIR spectrophotometer. Transmission
electron microscopy (TEM) was performed by LEO 912AB
electron microscope. Ultrasonic bath (EUROSONIC® 4D
Cu(NO3)2.3H2O (0.3 mmol) was dissolved in 5 mL water and
then 0.15 g of yolkꢀshell Fe3O4@chitosan was added to the
solution. The mixture was stirred for 8 h and subsequently,
0.194 ml hydrazine hydrate (4 mmol) in 5 mL water was added.
ultrasound cleaner with a frequency of 50 kHz and an output After 1 h of stirring, the solution was diluted with 100 ml of
triple distilled water followed by dropwise addition of 0.1 ml
(2.5 mmol) acetaldehyde. After 5 min of stirring, 0.006 g
(0.036 mmol) silver nitrate was dissolved in 10 mL water and
added with stirring to the above mixture. After 2 h, the product
was excluded with an external magnetic field, washed with
deionized water and ethanol and dried for 24 h in vacuum oven
at 50° C. The amounts of metals in the synthesized catalyst
were measured with AAS to be 4.4% (w/w) Ag and 19.2%
(w/w) Cu.
power of 350 W) was used to disperse materials in solvents.
Thermogravimetric (TG) and differential thermal analysis
(DTA) were carried out using STA 1500 instrument at a heating
rate of 10 ºC minꢀ1 in air. Xꢀray powder diffraction (XRD) data
were collected on an XDꢀ3A diffractometer using Cu Kα
radiation. Xꢀray photoelectron spectroscopy (XPS) was
performed using a VG multilab 2000 spectrometer (ThermoVG
scientific) in an ultrahigh vacuum. Ultraviolet–Visible (UVꢀ
Vis) spectra were obtained using a Shimadzu UVꢀ2100
spectrophotometer. Scanning electron microscope (SEM) and
energyꢀdispersive xꢀray spectroscopy (EDS) were performed on
a Zeiss Supra 55 VP SEM instrument. Catalysis products were
analyzed using a Varian 3900 gas chromatograph (GC)
(conversions were obtained using toluene as an internal
standard). AAꢀ680 Shimadzu (Kyoto, Japan) flame atomic
absorption spectrometer (AAS) with a deuterium background
corrector was used for determination of the metals.
Preparation of coppercoresilvershell nanoparticles
immobilized on the yolk/shell Fe3O4@chitosanꢀderived
carbon nanoparticle
0.4 g of the CucoreAgshell/yolk/shell Fe3O4@chitosan composites
was dissolved in 12 mL 1% acetic acid solution and then the
mixture was sealed into a Teflon equipped stainless steel
autoclave, which was then placed in a muffle furnace followed
by hydrothermal treatment at 180oC for 12 h. A programmed
temperature profile was adopted to control the heating process.
o
The heating rate was set at 5 C.minꢀ1. After the reaction, the
Synthesis and coating of the Fe3O4 with silica
autoclave was cooled down naturally. The product was
magnetically separated and washed successively with deionized
water, ethanol and acetone and dried at 40 C for 12 h before
Fe3O4 nanoclusters were synthesized according to the
procedure described in literature.31 Typically, FeCl3 (0.65 g, 4.0
mmol) and trisodium citrate (0.20 g, 0.68 mmol) were first
dissolved in ethylene glycol (20 mL). Afterward, NaAc (1.20 g)
was added with stirring. The mixture was stirred vigorously for
30 min and then sealed in a Teflonꢀlined stainlessꢀsteel
o
further usage.
General procedure for the catalytic epoxidation of alkenes
Catalytic experiments were initiated with the oxidation of
styrene (0.5 mmol) with O2 (1 atm) in the presence of 0.3 mol
% Cu and 0.04 mol% Ag in distilled water at 25°C. It led to 17,
36, 49 and >99% conversion (obtained with GC based on
toluene as internal standard) after 20, 40, 60 and 120 min,
respectively. After the reaction completion, the catalyst was
removed via an external magnetic field and then reused in the
next cycle. The catalytic epoxidation reactions of other alkenes
were conducted under the same conditions of styrene.
autoclave. The autoclave was heated at 200
°C and maintained
for 10 h, and then allowed to cool to room temperature. The
obtained Fe3O4 particle suspension was washed with H2O and
EtOH three times and then dried in a vacuum oven for 12 h at
room temperature. For coating of Fe3O4 with silica, 50.0 mL asꢀ
prepared Fe3O4 nanocluster solution (containing 0.4 g Fe3O4)
was dispersed into the mixture of deionized water (10.0 mL),
ethanol (100.0 mL) and NH3.H2O (50.0 mL). After imposing to
ultrasonic radiation for 10 min, a certain amount of TEOS was
quickly added into the system. The reaction was allowed to
proceed with stirring for 8 h at room temperature. Finally the
Results and discussion
2 | J. Name., 2012, 00, 1-3
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Journal Name
DOI: 10.1039/C4RA05283H
vibrating and OꢀH vibrating, respectively. In the spectrum of
Fe3O4@SiO2 composites (Fig. 1b), in addition to the absorption
Synthesis and recognition of the catalyst
The practical procedure for the synthesis of coppercoresilvershell
nanoparticles immobilized on the yolk/shell Fe3O4@chitosanꢀ
derived carbon nanoparticle can be divided to four steps which
is schematically shown in Scheme 1: (i) The magnetite particles
were synthesized via a modified solvothermal reaction of FeCl3
in the presence of trisodium citrate (Na3Cit) and sodium acetate
ꢀ1
bands of Fe–O at 598 and 462 cmꢀ1, the band at 1096 cm
related to Si–O–Si stretching vibrations of SiO2 shell is also
seen. From Fig. 1c, for Fe3O4@SiO2@chitosan, the
characteristic peaks of chitosan have been overlapped with the
SiO2 peaks. In the spectrum of yolkꢀshell Fe3O4@chitosan (Fig.
1d), the absorption peak of Si–O–Si at 1096 cmꢀ1 has been
disappeared, indicating the middle SiO2 shell is completely
o
at 200 C according to procedure described by Liu et al.31 Due
to the strong coordination affinity of carboxylate groups of
Na3Cit to Fe(III) ions, they prevents Fe(III) ions from
aggregating into large particles.32 Then, SiO2 shell was coated
on the Fe3O4 nanoparticle surface through a sol–gel process to
prevent coagulation of the particles and to overcome the
solubility problems in organic media. Also SiO2 shell is
required to obtain the desired yolkꢀshell structure; (ii) A
functional, nonꢀtoxic, and biocompatible polymer, chitosan,
etched. Also, the C–H bending vibrations of the pyranose ring
of chitosan appear at 1026ꢀ1155cmꢀ1, which are similar to that
of the pure chitosan sample confirming that the chitosan shell is
successfully synthesized. The XRD spectra of Fe3O4,
Fe3O4@SiO2,
Fe3O4@SiO2@chitosan,
yolkꢀshell
Fe3O4@chitosan, and CucoreAgshell/YSꢀFe3O4@(Chit.der)CNPs
are presented in Fig. 2aꢀe. In the XRD pattern of Fe3O4, six
characteristic diffraction peaks at 2θ =30.27°, 35.61°, 43.29°,
57.24° , 62.84°, and 73.94° are observed which are related to
(220), (311), (400), (511), (440) and (622) Bragg diffractions of
face centered cubic structure of Fe3O4 (Fig. 1a). The XRD
pattern of Fe3O4@SiO2 shows an additional broad peak at
around 2θ=23.35° because of the amorphous silica shell (Fig.
2b). The pattern of Fe3O4@SiO2@chitosan presented in Fig. 2c
shows two peaks of chitosan; the main reflection at 20.10° and
another smaller and broader peak at 2θ = 10.52°. However, the
diffraction peak of chitosan at 20.10° has been overlapped with
the characteristic peak of silica shell in that region. In the XRD
pattern of the yolkꢀshell Fe3O4@chitosan (Fig. 2d), the peak of
silica at 20.10° is eliminated, as is expected due to the
removing of silica intermediate shell, and therefore the
diffraction peak at 19.98° is attributed to chitosan. As can be
was used to modify the surfaces of Fe3O4@SiO2 core–shell
NPs. Then, the resulting Fe3O4@chitosan NPs was treated with
NaOH solution which the SiO2 layer was selectively etched and
the yolk/shell structure with a movable Fe3O4 nanoparticle
inside the hollow chitosan shell was formed; (iii) The yolk/shell
Fe3O4@chitosan stabilized CucoreAgshell NPs was synthesized by
a
transmetalation reaction on the surface of copper
nanoparticles immobilized on the yolk/shell Fe3O4@chitosan,
where the copper atoms on the particles’ surface are used as the
reducing agent for the silver. This process leads to formation of
only CucoreAgshell NPs on the yolk/shell Fe3O4@chitosan, with
no individual silver particles;29 (iv) In the next step, carbon
nanoparticles were obtained by hydrothermal carbonization of
chitosan at a mild temperature to obtain the CucoreAgshell NPs
immobilized on the yolk/shell Fe3O4@chitosanꢀderived carbon
nanoparticle which the method is very cheap and absolutely
green.33 FTꢀIR spectra of Fe3O4 magnetic particles,
seen
in
the
XRD
pattern
of
CucoreAgshell/YSꢀ
Fe3O4@(Chit.der)CNPs presented in Fig. 2e, after hydrothermal
carbonization, the crystallinity of chitosan is reduced and the
CNPs show a broader peak at 2θ =21.02, exhibiting an
amorphous carbon phase. Moreover, the pattern shows the main
diffraction peaks of both copper and silver with fcc crystal
structures at 2θ values of 44.58° and 37.17°, respectively.
Fe3O4@SiO2,
Fe3O4@SiO2@chitosan,
yolkꢀshell
Fe3O4@chitosan, and CucoreAgshell/YSꢀFe3O4@(Chit.der)CNPs
are shown in Fig. 1 aꢀe, respectively. FTꢀIR spectrum of the
magnetite particles (Fig. 1a) shows absorption bands at 1630
and 1405 cmꢀ1 which are attributed to carboxylate, indicating
the presence of carboxyl groups which is attributed to Na3Cit.
The bands at 580 and 3424 cmꢀ1 are ascribed to the FeꢀO
Scheme 1 Schematic diagram of the synthesis of CucoreAgshell/YSꢀFe3O4@(Chit.der)CNPs.
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90
60
30
0
ꢀ30
ꢀ60
ꢀ90
a
b
ꢀ120
0
100 200 300 400 500 600 700 800
Temperature [oC]
Fig. 4 DTA curves of Fe3O4 (a), and Fe3O4@SiO2@chitosan (b).
Fig. 3b shows the TGA curve of Fe3O4@SiO2@chitosan where
o
twoꢀstage weight loss is observed. The first stage below 250 C
resulted from the elimination of water on the surface of the
Fig. 1 FTꢀIR spectra of Fe3O4 (a), Fe3O4@SiO2 (b), polymer shell. The second stage is related to the decomposition
o
Fe3O4@SiO2@chitosan (c), yolkꢀshell Fe3O4@chitosan (d), and
CucoreAgshell/YSꢀFe3O4@(Chit.der)CNPs (e).
of chitosan which started at 332 C and completely burned out
o
at 598 C. Also, TGA curve indicates that the chitosan content
of Fe3O4@SiO2@chitosan is
∼30.2%. Fig. 4 shows the DTA
curves of Fe3O4, and Fe3O4@SiO2@chitosan, which clearly
shows the steps.
The SEM and TEM image of coppercoresilvershell nanoparticles
immobilized on the yolk/shell Fe3O4@chitosanꢀderived carbon
nanoparticle are shown in Fig. 5. The SEM images show
uniform spherical particles (Fig. 5A and B). The TEM image
clearly shows that Fe3O4 was indeed coated by a layer of
carbon nanoparticles (Fig. 5C). Three distinct components, the
black cores of Fe3O4, the grey shells of carbon nanoparticle and
the small black spheres of coppercoresilvershell nanoparticles, can
be observed. The TEM image also shows that the metal
nanoparticles with uniform size are densely distributed not only
on the outer surface but also on the inner surface of carboneous
shell. Moreover, the core/shell structure of coppercoresilvershell
nanoparticles is clearly observable in Fig. 5D.
Fig.
2
XRD patterns of Fe3O4 (a), Fe3O4@SiO2 (b),
Fe3O4@SiO2@chitosan (c), yolkꢀshell Fe3O4@chitosan (d), and
CucoreAgshell/YSꢀFe3O4@(Chit.der)CNPs (e).
TGA of magnetite particles (Fig. 3a) shows a weight loss of
about 6.8 wt% in the range of 100–800oC, suggesting the
presence of organic species.
Fig. 5 SEM (A, B) and TEM (C, D) images of CucoreAgshell/YSꢀ
Fe3O4@(Chit.der)CNPs.
XPS has often been employed for the surface characterization
of various materials. So, to further analyze the CucoreAgshell/YSꢀ
Fig. 3 TGA curves of Fe3O4 (a), and Fe3O4@SiO2@chitosan (b).
4 | J. Name., 2012, 00, 1-3
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Fe3O4@(Chit.der)CNPs system, XPS was used to examine the are really coated by a thin layer of silver. The UVꢀVis
composition of its surface (Fig. 6). The binding energy at 710.3 absorption spectra of Fe3O4@SiO2 and CucoreAgshell/YSꢀ
eV for Fe 2p cannot be observed, which further confirms that Fe3O4@(Chit.der)CNPs in water are illustrated in Fig. 6a and b,
all the Fe3O4 cores in the composite are confined within a shell respectively. A broad featureless peak appears in the spectrum
of chitosan derived carbon nanoparticles/CucoreAgshell (inset of of coreꢀshell Fe3O4@SiO2 at a wavelength of about 395 nm. In
Fig. 6). As shown in Fig. 6, the labelled peaks C 1s, N 1s, O 1s, the
absorption
spectrum
of
CucoreAgshell/YSꢀ
Ag 3p, Ag 3d, and Cu 2p offer the accurate elemental Fe3O4@(Chit.der)CNPs, the peak of Ag component appears at
composition of the prepared catalyst, which is in good about 410 nm.
accordance with the designation of the experiment. The absence
of silicon band at around 104 eV confirms the selective etching
of silica shell. Moreover, the XPS pattern shows the
characteristic peaks of Cu and Ag with no indications for the
presence of copper oxides. So, according to the results, Cu NPs
Fig. 6 XPS full spectrum of CucoreAgshell/YSꢀFe3O4@(Chit.der)CNPs, XPS highꢀresolution spectra of the Fe 2p of the asꢀprepared
CucoreAgshell/YSꢀFe3O4@(Chit.der)CNPs (inset).
EDS was used to measure the components of CucoreAgshell/YSꢀ
Fe3O4@(Chit.der)CNPs as shown in Figure 8. It is clear that the
CucoreAgshell/YSꢀFe3O4@(Chit.der)CNPs is composed of Fe, C,
N, O, Ag and Cu elements. Appearance of Au element was due
to the material coating by a layer of Au for EDS and SEM
characterizations.
The copper nanoclusters show no absorption band at greater
than 300 nm but exhibit a monotonic spectrum rising
exponentially toward shorter wavelengths.34 The absence of
observable absorption band at about 800 nm, which is attributed
to the formation of copper oxide particles, demonstrates that the
CucoreAgshell nanoparticles were highly stable.
Fig. 7 UVꢀVis spectra of Fe3O4@SiO2 (a) and CucoreAgshell/YSꢀ
Fe3O4@(Chit.der)CNPs (b).
Fig.
8
Xꢀray energy dispersive spectrum (EDS) of
CucoreAgshell/YSꢀFe3O4@(Chit.der)CNPs.
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catalytic epoxidation reaction of styrene on water was
accomplished in 5 h with 88% yield.
The Catalytic activity of the CucoreAgshell/YSꢀ
Fe3O4@(Chit.der)CNPs for epoxidation reaction of alkene
s
To investigate the effect of solvent, the epoxidation reaction of
styrene as a model reaction was accomplished with different
solvents and the conversion yields were measured by GC which
the results are summarized in Table 1. According to these
results, the best solvent was water which its effect was
mentioned previously. Then, to evaluate the general
applicability of the catalyst, different alkenes were tested for
oxidation reaction using O2 (1 atm) and 0.0005 g catalyst
containing 0.3 mol % Cu and 0.04 mol% Ag at 25oC (Table 2).
Recently, reactions of waterꢀinsoluble organic compounds in
aqueous suspensions (“on water”) have received a great deal of
attention,35,36 which is due to their high efficiency and facile
synthetic protocols. However, such reactions are still relatively
unexplored. On the other hand, the development of the selective
catalytic epoxidation pathways, without any radical initiators or
reducing reagents, is highly desirable. So, in this work, the
prepared catalyst was employed for the epoxidation reaction of
alkenes on water with molecular oxygen as terminal oxidants.
Our investigation on the catalyst loading showed a high atom
efficiency of title CucoreAgshell/YSꢀFe3O4@(Chit.der)CNPs.
Only after 2 h, styrene converted completely using a low
amount of catalyst (0.3 mol % Cu and 0.04 mol% Ag) and
styrene oxide was obtained in 95% yield by an easy isolation
with ethyl acetate as a safe solvent after removing the catalyst
with an external magnetic field. More interestingly, the use of
excess amount of styrene (1 mmol), rather than the catalyst
(1×10ꢀ5 mmol, 0.001 mol%) gave 51% epoxide yield after 28 h,
Table 1 Influence of the different solvents on epoxidation reaction of
styrene.a
Conversion
(%)
Entry
1
Solvent
CH3CN
60
2
3
4
5
6
DMF
THF
73
54
indicating
a high turnover number of 51000 for this
Toluene
H2O
44
nanostructured catalyst. Actually, due to the hydrophobic
character of carbon nanoparticles in water medium the reactants
tend to collect around carbon nanoparticles where the metal
catalysts exist, so as a result of the existence of reactants and
metal catalyst in the same region, an efficient catalytic reaction
occurs. It is notable that when the catalyst was replaced by
yolk/shell Fe3O4@chitosan/CucoreAgshell composites, the
>99
81
EtOH
a Reaction conditions: 2 mL of solvent, 0.5 mmol alkene, O2 (1 atm),
and 0.0005 g catalyst at 25oC.
.
Table 2 The epoxidation reaction of alkenes with CucoreAgshell/YSꢀFe3O4@(Chit.der)CNPs as catalyst.a
Product
Entry
Compound
Time (h)
Yield (%)b
Selectivity (%)
98
1
2
95
2
3
4
5
6
7
2.5
1.5
1.8
3
96
98
97
83
79
72
99
>99
>99
>99
>99
>99
4.5
4
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Journal Name
View ArticAle ROnTliInCeLE
DOI: 10.1039/C4RA05283H
8
4
4
61
92
>99
9
>99
a Reaction conditions: 2 mL of deionized water, 0.5 mmol alkene, O2 (1 atm), and 0.0005 g catalyst at 25oC.
As can be seen, various alkenes can generate the related
epoxides in good to excellent yields. The selectivities are given Table 3 Effect of recycling on the catalytic activity and productivity of
in Table 2 which showed that most of the alkenes only give the CucoreAgshell/YS Fe3O4@(Chit.der)CNPs after 30 min (a) and after 2 h
corresponding epoxide and only very low amount of (b).a
benzaldehyde was detected due to ring opening reaction of
styrene oxide (entry 1). Actually, the addition of copper in the
bimetallic CuꢀAg catalyst increases the epoxidation selectivity
with raising the relative difference between the barriers for the
oxametallocycle ringꢀclosure VS Hꢀshift pathways, resulting in
more epoxide formation.37 It should be noted that that the
No. of
recycling steps
1
Conversion after 30
Conversion after 2 hb
minb (%)
(%)
99
31
2
3
4
5
6
7
8
30
30
30
30
29
29
29
99
99
98
98
97
96
96
catalyst was able to oxidize the tribustitutedꢀ,
Eꢀalkenes and
also electronꢀdeficient alkenes to the relevant epoxides in good
yields (entries 5ꢀ8). In heterogeneous catalysis, considering the
leaching of active metal species into solution is an important
issue. So, to investigate it, in a separate experiment the catalyst
was removed by the magnetic extraction after ~50% conversion
under the reaction conditions. Then, the residue mixture was
allowed to further treat under similar reaction conditions and
the conversion yield was obtained by GC which showed the
reaction did not proceed. Moreover, atomic absorption
spectroscopy indicates that the metal content in the solution of
the residue mixture is below the detection limit (0.1 ppm).
According to these results, we conclude that metal leaching in
the catalyst is negligible.
aReaction conditions: 2 mL of deionized water, 0.5 mmol alkene, O2 (1
atm), and 0.0005 g catalyst at 25oC.
Conclusion
Coppercoresilvershell nanoparticles immobilized on the yolk/shell
Fe3O4@chitosanꢀderived carbon nanoparticle were successfully
synthesized and characterized. In this report, we have achieved
several important objectives: i) Having a heterogeneous catalyst
bearing the advantage of being magnetically separable and
therefore removing the need of the catalyst filtration or
centrifugation after completion of the reaction; ii) Employing
hollow structure which has larger vacant space, lower density
and higher surface area compared to the corresponding coreꢀ
shell; iii) Using chitosanꢀderived carbon nanoparticles as really
good candidates for stabilizing noble metals and heterogeneous
catalysis due to the convenient method of synthesis, being bioꢀ
compatible and inexpensive; iv) Due to the hydrophobic
character of carbon nanoparticles, in water medium the
reactants tend to collect around carbon nanoparticles where the
metal catalyst exist, so because the reactants and metal catalyst
are in the same region, efficient catalytic reaction occurrs; v)
Providing the catalyst contained CucoreAgshell nanoparticles to
have the optical and electric properties of both metals and also
preventing the oxidation of the copper core. Moreover, Cu–Ag
bimetallic catalyst is more selective than the pure silver toward
epoxidation; and vi) Developing an efficient and green
synthetic process for the facile and selective catalytic
epoxidation of styrene.
Recycling CucoreAgshell/YSꢀFe3O4@(Chit.der)CNPs
To investigate the productivity of the prepared catalyst, we
examined the epoxidation reaction of styrene as a model
reaction up to eight cycles after completion of the reaction. By
design, the magnetic extraction eliminates the need for filtration
or centrifugation and the workup of the final reaction mixture to
recover the catalyst and consequently allows for easy recycling
of the catalyst. The results are presented in Table 3 which
indicate that the yield of product after every run does not
change significantly. But, catalysis is
a totally kinetic
phenomenon and if a single data point was taken in each run
after long reaction time, one might wrongly conclude that the
catalyst is completely stable and recyclable. So, investigating
the initial rates obtained from kinetic plots is a good approach
to examine the recyclability and deactivation of the catalyst.38
Therefore, the conversion yields for every run after 30 min for
epoxidation reaction of styrene were measured (Table 3). As
the conversion yields after 30 min for eight runs did not
decrease significantly, we may conclude the good stability and
recyclability of the synthesized catalyst.
Acknowledgements
The amounts of CucoreAgshell after eight runs were measured
with AAS to be 4.3% (w/w) Ag and 19.2% (w/w) Cu.
We are grateful to Shahid Beheshti University Research
Council for partial financial support of this work.
Notes and references
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 7

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ARTICLE
RSC Advances
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DOI: 10.1039/C4RA05283H
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