Synthesis and characterization of Co3O4 immobilized on dipeptide-functionalized silica-coated magnetite nanoparticles as a catalyst for the selective aerobic oxidation of alcohols

Article abstract of DOI:10.1039/c8nj00781k

Synthesis and characterization of a new silica-coated magnetite nanocatalyst are described. This catalyst was prepared through a multistep procedure consisting of surface modification, functionalization with the product of an Ugi multicomponent reaction, and immobilization of Co₃O₄ on silica-coated magnetite nanoparticles. The prepared nanocatalyst was characterized using various techniques such as Fourier-transform infrared and energy-dispersive X-ray spectroscopies, thermal and elemental analyses, X-ray diffraction, and field-emission scanning and transmission electron microscopies. The catalyst showed high catalytic activity for the aerobic oxidation of alcohols in acetonitrile as the solvent at mild temperatures and reusability for five repeated runs without loss of its activity.

Full text of DOI:10.1039/c8nj00781k

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Synthesis and characterization of Co O immobilized
3
4
on dipeptide-functionalized silica-coated magnetite
nanoparticles as a catalyst for the selective aerobic
oxidation of alcohols†
Cite this:DOI: 10.1039/c8nj00781k
ab
b
Mohammad Mehdi Khodaei
*
and Mahsa Dehghan
Synthesis and characterization of a new silica-coated magnetite nanocatalyst are described. This catalyst
was prepared through a multistep procedure consisting of surface modification, functionalization with
3 4
the product of an Ugi multicomponent reaction, and immobilization of Co O on silica-coated
Received 15th February 2018,
Accepted 30th May 2018
magnetite nanoparticles. The prepared nanocatalyst was characterized using various techniques such as
Fourier-transform infrared and energy-dispersive X-ray spectroscopies, thermal and elemental analyses,
X-ray diffraction, and field-emission scanning and transmission electron microscopies. The catalyst
showed high catalytic activity for the aerobic oxidation of alcohols in acetonitrile as the solvent at mild
temperatures and reusability for five repeated runs without loss of its activity.
DOI: 10.1039/c8nj00781k
rsc.li/njc
7b
In most of these reports, amino acids and amine-containing
rings such as imidazoles, triazoles and pyridines have been
Introduction
11
12
13
Magnetite or Fe O nanoparticles (NPs) have been widely studied used as ligands to produce ionic functions on the NPs surface and
3
4
1
as catalysts in organic reactions due to several interesting proper- these are denoted as ionic liquid (IL) functionalized silica-coated
2
3
ties of these NPs such as high surface area, no toxicity, ease of magnetite NPs.
4
modification and functionalization, and magnetic recyclability.
Oxidation of alcohols to the corresponding carbonyl
The limitations in using magnetite NPs as catalysts are instability compounds (e.g., aldehydes, ketones and acids) is one of the
even in dilute acidic media and oxidation to maghemite under important organic transformations both for fundamental
5
14
mild conditions. Magnetite NPs can be protected from decom- research and industrial manufacturing. The main challenge
position by coating with a layer of organic or inorganic materials in the oxidation of alcohols is selectivity of the methods to
6
7
15
such as biopolymers, ionic liquids, and inorganic polymers. One produce a single product. The selective oxidation of alcohols
of the most important magnetite containing materials is silica- is one of the most effective approaches to produce these
coated magnetite, which is widely used as a catalyst. Silica-coated compounds, and has been abundantly used in recent decades.
8
16
magnetite NPs can be modified through a sol–gel process using In the conventional methods for the selective oxidation of alcohols,
organosilanes. The modified NPs are potentially advantageous for stoichiometric amounts of hazardous reagents such as nitric acid,
9
organic functionalization by reaction with active species. The potassium dichromate, potassium permanganate, sodium hypo-
17
chemically organic functionalization of these inorganic NPs can chlorite and organic peroxides have been used. These methods
significantly improve their dispersion properties in organic or have a number of environmental issues such as production of
18
aqueous media and increase the ability of the NPs surface to abundant wastes. One of the most important oxidation methods
interact with reagents in organic reactions using p–p interactions, of alcohols is aerobic oxidation using molecular oxygen or air as an
19
hydrogen bonding and dipole–dipole interactions between the oxidant in the presence of transition metals or free-radical
1
0
20
organic function of the surface and substrates. Many types initiators. Aerobic oxidation of alcohols has been reported using
21
of modified and functionalized silica-coated magnetite or silica many nanocatalyst systems in recent years.
NPs have been reported as catalysts in organic transformations.
In continuation of our interest in designing a nanocatalyst
2
2
and oxidation of organic substrates, we have focused on the
design and preparation of novel highly dispersed silica-coated
magnetite NPs for the immobilization of Co O NPs as an
a
Department of Organic Chemistry, Razi University, Kermanshah 67149-67346,
3
4
Iran. E-mail: mmkhoda@razi.ac.ir
b
efficient heterogeneous catalyst for oxidation of alcohols. For
this reason, silica-coated magnetite NPs were prepared and
Nanoscience & Nanotechnology Research Center (NNRC), Razi University,
Kermanshah 67149-67346, Iran
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nj00781k modified using chloride-containing organosilane (CPTES), and
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3 4 3 4
Scheme 1 Preparation of Co O NPs supported on Ugi-based ionic liquid-functionalized silica-coated magnetite NPs (Co O @SCM-UIL).
then we functionalized the modified NPs. According to the Varian Vista PRO Radial inductively coupled plasma optical
many reports about using dipeptides with a high tendency emission spectrometer (ICP-OES).
2
3
to chelate with metal ions (e.g., Co, Zn, Cr and Cu), and the
synergic effect of ionic liquid functionalized NPs on their Preparation of Fe
O NPs@SiO -Cl (SCM-Cl)
3 4 2
1
3
catalytic activities, a pyridine-substituted dipeptide synthe-
sized by an Ugi multicomponent reaction was chosen to produce
Fe
3 4
O NPs were prepared using the previously reported
method (co-precipitation) and coated with CPTES in 2-propanol
pyridinium ionic liquid on their surfaces. Next, Co
successfully immobilized on these NPs, and the final catalyst
Co O @SCM-UIL) was characterized using FT-IR, TGA, ICP-OES,
3
O
4
NPs were
7b
(
sol–gel method). Briefly, FeCl Á6H O (1.00 g, 3.7 mmol) and
3
2
FeCl Á4H O (0.37 g, 1.80 mmol) were dissolved in 20 mL of
2
2
(
3
4
deionized water under nitrogen gas with vigorous stirring. The
solution was then basified to pH = 10 by the addition of NH
25%) and stirring was continued for 1 h at 60 1C. The color of the
EDX, FE-SEM, TEM and XRD techniques (Scheme 1).
3
(
bulk solution changed from orange to black immediately. The
precipitated magnetite was separated from the solution using a
magnet, washed several times with deionized water and ethanol,
Experimental
General
All of the materials and solvents were purchased from Merck and dried at ambient temperature. The obtained magnetic NPs
and used without any additional purification.
powder (30 mg) was dispersed in 50 mL of 2-propanol by
sonication for 30 min, and then, 10 mL of deionized water,
Characterization techniques
0.15 mL of TEOS and 1.70 mL of NH (25%) were added into the
3
À1 mixture at ambient temperature under stirring. After 12 h, the
FT-IR spectra were recorded on a BRUKER ALPHA 400–4000 cm
FT-IR spectrophotometer. Field-emission scanning electron
microscopy (FE-SEM, Mapping) and energy-dispersive X-ray
collected product was washed with ethanol and deionized water
and dried at 50 1C. The SCM NPs were functionalized with CPTES
by refluxing in anhydrous toluene at 110 1C. After stirring for
(
EDX) were performed on a ZEISS SIGMA VP-500 instrument.
Transmission electron microscopy (TEM) analysis was performed
using a ZEISS-EM 10C microscope at an acceleration voltage of
00 kV. The sample was sputtered by gold to avoid undesirable
electron charging. The products were analyzed using a Varian
900 GC using undecane as an internal standard. Thermogravi-
24 h, the suspended substance was separated from the solution
using a magnet and washed three times with ethanol. The
precipitated product (SCM-Cl) was dried at room temperature.
1
Synthesis of the pyridine-containing dipeptide (N-benzyl-N-(2-
3
(cyclohexylamino)-2-oxo-1-(pyridin-3-yl)ethyl)benzamide)
metric analysis (TGA) was carried out on a STA PT-1000 instru-
À1
ment at a heating rate of 10 1C min in air. Vibrating Sample 3-Pyridinecarbaldehyde (0.11 g, 1 mmol) was added to a
Magnetometer (VSM) analysis of the functionalized magnetic solution of benzyl amine (0.11 g, 1 mmol) in CH OH (10 mL)
3
nanoparticles was performed by the Iran Nanotechnology at room temperature. After 1 h, benzoic acid (0.12 g, 1 mmol)
Initiative Council (INIC). X-ray powder diffraction (XRD) patterns and cyclohexyl isocyanide (0.11 g, 1 mmol) were added to the
were recorded on an X’PertPro diffractometer using Cu Ka mixture. The reaction mixture was stirred for 24 h at ambient
radiation. The concentration of cobalt was estimated using a temperature. After completion of the reaction [indicated by thin
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layer chromatography, EtOAc : n-hexane (1 : 1)], the resulting Typical procedure for the selective oxidation of benzyl alcohol
solid was recrystallized from EtOAc : n-hexane (1 : 1), and then to benzaldehyde
the yellow needle-like crystals were collected (0.33 g, 78%);
The catalyst (0.05 g, 3.5 mmol%) was added to a solution of
benzyl alcohol (0.108 g, 1 mmol) in dry acetonitrile (5 mL) and
1
mp: 162.2–169.2 1C; H NMR (300 MHz, [D ]DMSO, 25 1C,
6
TMS): d (ppm) = 0.99–1.24 (m, 5H, cyclohexyl), 1.50–1.90
the mixture was stirred at 70 1C for 8 h. The progress of the
(
(
m, 5H, cyclohexyl), 3.92–4.02 (m, 1H, NHCH), 4.38–4.51
reaction was monitored by thin layer chromatography (TLC).
At the end of the reaction, the catalyst was separated by an
external magnet and the remaining solution was analyzed by
the GC method. In this analysis undecane was used as an
internal standard. Oxidation of benzyl alcohol can produce a
few by-products including benzyl benzoate, benzoic acid and
acetal. In our procedure, the main product was benzaldehyde
2
m, 2H, CH ), 5.71 (s, 1H, CH), 6.89–7.42 (m, 12H), 7.92
(
d, J = 12 Hz, 1H), 8.14–8.44 (m, 1H), 8.34 (d, J = 6 Hz, 1H), 8.59
13
(
2
1
s, 1H); C NMR (75 MHz, [D
4.9, 25.6, 32.4, 48.2, 49.6, 78.9, 123.6, 126.7, 128.2, 128.5, 129.0,
29.6, 129.9, 132.2, 137.0, 137.1, 149.2, 150.8, 167.9, 172.5; IR (KBr):
6
]DMSO, 25 1C, TMS): d (ppm) = 24.8,
À1
n = 3315, 3295, 2929, 2851, 1732, 1681, 1523, 1350, 1267 cm
.
(76%) and 14% benzoic acid was produced. In addition, 10% of
the substrate was not reacted. The results of GC-mass analysis
for this oxidation are: benzaldehyde: m/z 106 (93), 77 (100),
Preparation of Ugi-based ionic liquid-functionalized magnetic
NPs (SCM-UIL)
51 (39), 39 (7); benzoic acid: m/z 122 (95), 105 (100), 77 (63),
50 (21), 39 (10); benzyl alcohol: m/z 108 (100), 91 (46), 79 (98),
77 (49) (ESI,† Fig. S2).
The SCM-Cl NPs (0.20 g) were dispersed in a 100 mL round
bottom flask containing dipeptide (0.427 g, 1 mmol) and NaPF
0.202 g, 1.2 mmol) in deionized water under magnetic stirring
6
(
for 24 h at 60 1C. After completion of the reaction, the ionic
liquid-functionalized NPs were separated using an external
magnet and washed five times with ethanol and eventually
dried at 60 1C for 24 h.
Results and discussion
The stages of the catalyst preparation were identified by FTIR
spectroscopy. Fig. 1 shows the FTIR spectra of Fe O , SCM-Cl,
3
4
SCM-UIL and Co O @SCM-UIL. The peaks at 630, 578 and
3
4
Preparation of ionic liquid functionalized magnetic supported
À1
4
46 cm are attributed to the stretching vibrations of Fe–O
Co O NPs (Co O @SCM-UIL)
3
4
3
4
À1
bonds, and the broad absorption peak at 3253 cm is ascribed to
the OH stretching vibration, indicating the presence of hydroxyl
groups which are located on the surface of the iron oxide NPs
and hydrogen-bonded surface water molecules in the atmo-
sphere. This broad peak may also be attributed to the presence
Cobalt nanoparticles were synthesized according to the following
26
procedure. The ionic liquid functionalized NPs (1 g) were
dispersed in 50 mL of n-hexanol under ultrasonication for
3
0 min. Then, a mixture of CoCl Á6H O (0.238 g, 1 mmol) in
2
2
7
a
n-hexanol (20 mL) was added to it and stirred for 30 min.
Subsequently, the mixture was heated at 140 1C for 10 h. After
of some amount of ferric hydroxide in the Fe O4 NPs.
3
The FTIR spectrum of the SCM–Cl shows the corresponding
À1
3 4
cooling to room temperature, the Co O @SCM-UIL NPs were
peak for the Fe–O–Si bonds at 589 cm , resulting from the
separated using an external magnet, washed with ethanol
anchoring of silica onto the surface of magnetite, while this
repeatedly to remove any impurities and dried at 60 1C. The
loading of Co was measured to be 0.70 mmol g by inductively
coupled plasma-optical emission spectrometry (ICP-OES).
band is overlapped with the Fe–O vibration. The sharp band at
À1
À1
1
085 cm is attributed to the Si–O–Si antisymmetric stretching
À1
and the peak at 805 cm
corresponds to the symmetric
3 4 3 4
Fig. 1 FTIR spectra of Fe O (a), SCM-Cl (b), SCM-UIL (c) and Co O @SCM-UIL (d).
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stretching vibration of Si–O–Si. The peak at 460 cm is due to
NJC
À1
7
b
the bending vibration of the Si–O–Si bond. The peaks at 3450,
À1
1
120 and 967 cm correspond to the asymmetric and sym-
metric stretching, and bending vibrations of the Si–OH groups
on the silica surface, respectively. The peaks at 3600–3400 and
À1
8
00 cm correspond to the adsorbed water (H–OH stretching
and H–OH bending vibrations) in the NPs structure. In addition,
À1
the spectrum of the SCM–Cl shows peaks at 2920–2840 cm due
to the C–H bonds of the aliphatic chain stretching vibrations.
After functionalization with the ionic liquid, the new peaks
À1
at 3000–2740 cm are attributed to the stretching vibration
of the CH of the benzylic position and aromatic rings. The
stretching vibrations of the CQC and CQN bonds with
unreacted bending absorption of intermolecular water appeared
À1 13
À1
at 1635–1690 cm
.
A new peak appeared at 884 cm which is
attributed to the bending vibration of phenyl CH. Furthermore,
À1
the carbonyl bonds appeared at 1740–1700 cm as a weak broad
peak. Due to the immobilization of Co
3
O
4
NPs onto the particles,
À1
the intensity of the peak at around 1600 cm was increased and
À1
the peaks at 1300–1060 cm were changed to broad peaks that
24
are in good agreement with the literature.
The XRD patterns of Fe , SCM-UIL, Co
recovered Co @SCM-UIL after five cycles of use in benzyl
3 4
alcohol oxidation are shown in Fig. 2. The appearance of Fe O
3
O
4
3 4
O @SCM-UIL and
3 4
O
peaks at 18.4, 30.2, 35.3, 43.3, 53.7, 57.0, 63.1 and 75 in the
spectra of Fig. 2a and b indicates the spinel magnetite–
2
5b
maghemite system.
It should be mentioned that because of
our typical procedure for the synthesis of magnetite (a mole
ratio of 1 : 2 for Fe(II) : Fe(III)), the conversion of small amounts
of magnetite to maghemite during the precipitation occurred,
25c,d
which has been well established by a number of researchers.
Moreover, the XRD pattern is exactly the same as that in the
published articles that include a mixture of magnetite and
2
5b,e–g
maghemite.
These results are in good agreement with
the IR results because in the IR spectrum for Fe O (Fig. 1a), a
3
4
À1
relatively weak peak at 446 cm has appeared that is assigned
to the T1u component of maghemite (Fe O ), while this peak Fig. 2 The XRD patterns of the Fe
O (a), SCM-UIL (b), Co O @SCM-UIL (c)
3 4 3 4
2
5h
2
3
2
5i
3 4
and recovered Co O @SCM-UIL (d).
can not be observed for pure Fe
3
O
4
.
Based on the obtained
results, there is a possibility of the presence of small amounts
of maghemite in the magnetite core. In the cases of SCM-UIL
and Co
Fe are presented, which shows retention of the structure XRD (see the ESI,† Fig. S1). The recovered catalyst after five
of Fe O through the functionalization and immobilization. cycles has the same pattern, which confirms the recoverability
3 4
O @SCM-UIL NPs, all of the peaks corresponding to confirmed by TEM (lower than 5 nm) are hardly visible by
3 4
O
3
4
In the XRD pattern of the Co O @SCM-UIL NPs (Fig. 2c), weak of the catalyst and the stability of the Co O NPs during the
3
4
3 4
reflections at 2y = 33.4, 36.89, 41.8, 59 and 62 were also catalytic reaction (Fig. 2d). The XRD results confirmed that the
2
4b,25a,26
observed that correspond to small Co
3
O
4
particles.
magnetite structure was retained after the modification and
NPs were success-
4.161 and 51.421 and the characteristic peaks for CoO at fully immobilized onto the surface of the functionalized NPs.
The two sharp characteristic peaks for metallic Co at 2y = functionalization processes, and the Co
3 4
O
4
3
6.351, 42.471and 61.521 were not observed, whereas the peak
The EDX analysis indicates the presence of cobalt and
at 36.891 and the feature at around 591 pointing to small Co
3
O
4
oxygen in the particles (Fig. S1f, ESI†). Thus, it can be concluded
particles were observed. Another characteristic peak of Co O at that the particles contain CoO and/or Co O . It was reported that
3
4
3 4
27
6
5.51 was not seen since it was just above the detection limit of CoO on the surface is not stable and thus oxidized to Co O .
3 4
2
5a
XRD.
In addition to the two observed peaks at 2y = 36.891 Therefore, it can be concluded that the surfaces of all
3 4
and 591, three other peaks appeared at 2y = 33.4, 41.8 and 621 Co-containing particles in the catalyst consist of Co O .
(
Fig. 2c), which are exactly the same as the peaks that appeared
The thermogravimetric analysis of Fe
3 4
O , SCM-UIL and
2
6
for Co
3
O
4
in the literature.
These small size particles Co O @SCM-UIL NPs are shown in Fig. 3. The curves show
3 4
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3 4 3 4
Fig. 3 TGA analysis of Fe O (a), SCM-UIL (b) and Co O @SCM-UIL (c).
the initial weight loss before 200 1C due to the elimination
of physically and chemically adsorbed water and volatiles.
In the cases of SCM-UIL and Co
3 4
O @SCM-UIL, after modifi-
cation, functionalization and immobilization of the Co O
3
4
NPs, a new step of weight loss at 425 1C 1 occurs due to the
decomposition of their organic contents. Comparison of the
remaining weights of Fe
5
1
3
O
4
(98.7%) and SCM-UIL (82.5%) at
50 1C indicates that the percentage of organic content is
6.2% after functionalization. In the case of Co @SCM-UIL
3
O
4
NPs, the remaining weight (88.4%) was increased, which
indicates the existence of the more inorganic content corres-
ponding to the immobilization of Co O NPs (5.9%).
3
4
The FE-SEM images of SCM-Cl and Co O @SCM-UIL NPs
3
4
are presented in Fig. S1 (ESI†). The modified silica-coated
magnetite shows the aggregated semi-spherical particles with
Fig. 4 Magnetization curves for SCM-Cl (a) and SCM-UIL (b).
3 4
average diameters of 35–57 nm. In the case of Co O @SCM-UIL,
the particles have diameters the same as the diameters of SCM-Cl,
but they show a low degree of aggregation between the spherical
particles. The elemental analysis with the mapping technique of
the final catalyst confirms the presence of desirable elements and
a uniform dispersion of the Co O NPs onto the functionalized
3
4
magnetic NPs. In addition, the TEM images of the SCM-Cl
confirm the core–shell structure of the NPs with Fe cores in
3 4
O
the size range of 8 to 10 nm. In the case of SCM-UIL NPs, the
same structure as seen in the SCM-Cl was observed, but in the
case of Co O @SCM-UIL NPs, images were observed with two
3 4
types of small particles in their structures. The average sizes of
these particles measured were 8–10 nm (darker black spots)
and lower than 5 nm (brighter black spots), corresponding to
Fe
3 4 3 4
O and Co O NPs, respectively. This result agrees very well
3 4
Fig. 5 Dispersity properties of SCM-Cl and Co O @SCM-UIL NPs in
with the XRD pattern, showing only very weak signals of Co
3 4
O
acetonitrile.
2
5a
(see Fig. 2c).
The magnetic behaviors of the SCM-Cl and SCM-UIL NPs are
shown in Fig. 4. As shown in the figure, the saturation magne-
À1
the low degree of aggregation in the catalyst which is in good
agreement with the TEM results (Fig. S1h in comparison to
Fig. S1k, ESI†).
tizations for SCM-Cl and SCM-UIL are 38.16 and 34.38 emu g
,
respectively. The decrease in the saturation magnetization of
the SCM-UIL is due to the functionalization of the SCM-Cl NPs
with a dipeptide ligand.
Catalytic activity
The characterized catalyst has a good dispersity in aceto-
nitrile compared to the SCM-Cl NPs (Fig. 5), due to the dipole– Recently, Co-based oxides immobilized on different supports
dipole interaction of the dipeptide ligand with the solvent and have been reported for the oxidation of benzyl alcohol derivatives.
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Table 1 Comparison of cobalt based catalysts for oxidation of benzyl the product was produced at 100 1C in moderate yield.
a
alcohol to benzaldehyde
The best yield of the product was obtained in acetonitrile at
7
0 1C, and thus acetonitrile was chosen as the best solvent.
Entry Catalyst
Solvent
DMF
Conversion (%) Selectivity (%) Ref.
The reaction was also studied under solvent-free conditions
and the product was obtained in low yield at 100 1C. When the
amount of the catalyst was increased, the yield of the reaction
was slightly raised. As a complementary study of the reaction
optimization, the model reaction was repeated in the presence
of the same amounts of SCM-UIL, CoCl and Co O NPs
2 3 4
individually and the results are presented in Table 2. The
results confirmed the synergic effect of the ionic liquid-
1
2
3
4
5
6
7
8
9
Co-NG-750
94.8
82.1
97.4
97.7
72.9
87.3
100
97.6
97.3
28
29
30
31
32
33
33
34
35
Co
3
Co
3
Co
3
O
O
O
4
4
4
/CMK-3 DMF
/SBA-15 Acetonitrile 31
/AC
Toluene
99
98
50
89.5
19.5
18.0
Co/C–N700
Co-ZIF-67/GO DMF
MCG-700
PFMCH
YCo(dmgH)
H
2
O
DMF
DMF
DMF
100
—
2
a
Reaction conditions: catalyst (5 mg), benzyl alcohol (1 mmol), DMF
functionalized silica NPs on the catalytic activity of Co O4
NPs in the aerobic oxidation of primary alcohols. Conse-
3
À1
À1
2
(10 mL), flow of O (20 mL min , 53.6 mmol h ), reaction time 5 h
and temperature 130 1C.
quently, the optimized conditions were Co
0.05 g, 3.5 mmol%) as the catalyst, acetonitrile as a solvent
and 70 1C as the reaction temperature.
The scope and limitations of this catalyst in the selective
3 4
O @SCM-UIL
(
A number of these Co-based catalysts and their results
are presented in Table 1. Thus, the catalytic activity of the aerobic oxidation of a variety of alcohols were investigated
Co @SCM-UIL NPs in the selective aerobic oxidation of under the optimized reaction conditions. The results are
3 4
O
primary alcohols to aldehydes was investigated. In order to collected in Table 3. All products were obtained in good to
optimize the reaction conditions, oxidation of benzyl alcohol excellent yields.
was chosen as a model reaction, and the reaction was carried
In view of the success of the above mentioned reactions, we
out under different conditions using various amounts of decided to extend the study to secondary alcohol oxidation
catalyst and different solvents and temperatures. Initially, under these conditions. Thus, the oxidation of 1-phenylethanol
water and several organic solvents such as n-hexane, toluene, was tested and the result shows that acetophenone was
xylene, acetonitrile and dichloromethane were examined in successfully obtained in shorter reaction time at 70 1C when
the presence of the same amount of catalyst under O
Table 2). It was observed that no product was detected in procedure was applied to other secondary alcohols and the
water at different temperatures. However, the desired product results are presented in Table 4.
benzaldehyde) was obtained in the organic solvents. In the
The results show that this nanocatalyst is comparable with
2
blowing acetonitrile was used and no by-product was seen. Next, this
(
(
case of dichloromethane, the desired product was obtained in the other catalysts mentioned in Table 1 with respect to the
low yield at both 25 1C and 70 1C and in toluene and xylene, reaction conditions.
a
Table 2 Optimization of the reaction conditions for the oxidation of benzyl alcohol to benzaldehyde
b
Entry
Catalyst (g, mol%)
Solvent
Temperature (1C)
t (h)
Conversion (%)
Yield (%)
1
2
3
4
5
6
7
8
9
Co
Co
Co
Co
Co
Co
Co
Co
Co
Co
Co
Co
Co
Co
Co
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
@SCM-UIL (0.05, 3.5)
@SCM-UIL (0.05, 3.5)
@SCM-UIL (0.05, 3.5)
@SCM-UIL (0.05, 3.5)
@SCM-UIL (0.05, 3.5)
@SCM-UIL (0.05, 3.5)
@SCM-UIL (0.05, 3.5)
@SCM-UIL (0.05, 3.5)
@SCM-UIL (0.05, 3.5)
@SCM-UIL (0.05, 3.5)
@SCM-UIL (0.04, 2.8)
@SCM-UIL (0.06, 4.2)
@SCM-UIL (0.07, 4.9)
@SCM-UIL (0.05, 3.5)
@SCM-UIL (0.05, 3.5)
Water
Water
n-Hexane
Toluene
Toluene
Xylene
Acetonitrile
Acetonitrile
Dichloromethane
Dichloromethane
Acetonitrile
Acetonitrile
Acetonitrile
Solvent-free
Solvent-free
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
25
90
25
24
24
24
24
12
12
24
8
24
8
10
8
—
—
—
—
72
78
18
90
17
35
54
90
90
3
—
—
—
—
25
100
100
25
70
25
70
70
70
70
25
100
70
70
70
61
67
18
76
17
35
46
76
77
Trace
58
—
10
11
12
13
14
15
16
17
18
19
8
12
10
8
12
12
8
67
—
14
47
41
SCM-UIL (0.05)
CoCl
Co
Fe
2
O
(0.005, 3.5)
NPs (0.008, 3.5)
NPs (0.05)
14
43
39
3
4
3
O
4
70
a
b
2
Benzyl alcohol (1 mmol), catalyst (0.05 g, 3.5 mmol%) and solvent (5 mL) under O blowing. GC yield.
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a
Table 3 Aerobic oxidation of primary alcohols
b
Yield (%)
76
2.72
8.0
78
3.19
7.0
99
4.73
6.0
89
8.74
3.0
87
5.32
4.7
80
3.14
7.3
84
7.26
3.3
À1
TOF (h
Time (h)
)
Conversion (%)
Selectivity (%)
90
84.44
85.2
91.5
96.9
97.9
95.4
93.3
87.6
99.3
86.2
92.8
90.3
93
b
Yield (%)
79
2.90
7.8
67
9.22
2.1
70
8.25
2.4
À1
TOF (h
Time (h)
)
Conversion (%)
Selectivity (%)
84.5
93.5
77.6
86.3
78.9
88.7
a
b
Alcohol (1 mmol), catalyst (0.05 g, 3.5 mmol%) and acetonitrile (5 mL) under O
2
blowing at 70 1C. GC yield.
a
Table 4 Aerobic oxidation of secondary alcohols
b
Yields (%)
81
9.25
2.5
80
7.35
3.1
93
11.02
2.4
76
10.52
2.1
84
8.68
0.4
À1
TOF (h
Time (h)
)
Conversion (%)
Selectivity (%)
87.8
92.2
86.4
92.6
89.1
95.8
91.7
82.9
94.5
88.9
b
Yields (%)
73
5.56
3.7
84.3
86.6
78
6.84
3.3
86.1
90.6
75
76
6.61
3.3
82.2
92.4
77
64
6.30
3.0
71.8
89.1
58
À1
TOF (h
Time (h)
Conversion (%)
Selectivity (%)
)
8.42
2.5
87
8.01
2.7
85
3.86
4.2
67.4
86
86.2
90.6
a
b
2
Alcohol (1 mmol), catalyst (0.05 g, 3.5 mmol%), acetonitrile (5 mL) under O blowing at 70 1C. GC yield.
after 5 runs, although a slight decrease in the yield of the
product was observed during the last run (Fig. 6); this slight
Reusability of the catalyst
The reusability of the catalyst was examined during the aerobic decrease of more than 10% yield in the repeatability study can
oxidation of benzyl alcohol. After completion of the reaction, be attributed to two factors:
the catalyst was easily removed using an external magnet and
(a) Trivial loss of the catalyst in the process of washing and
washed once with ethyl acetate and twice with ethanol and separation by the magnet,
then, dried at 80 1C for 10 h. The catalyst was tested for five
(b) The water generated during each oxidation reaction,
consecutive runs without the addition of any fresh catalyst. The which can lead to the Co O NPs leaching from the surface of
3
4
results show that the activity of the catalyst was maintained the catalyst.
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NJC
5
6
I. Mitov, Z. Cherkezova-Zheleva and V. Mitrov, Phys. Status
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(a) C. Zhang, H. Wang, F. Liu, L. Wang and H. He, Cellulose,
2013, 20, 127–134; (b) L. Zhou, Y. Wang, Z. Liu and
Q. Huang, Acta Phys.-Chim. Sin., 2006, 22, 1342–1346.
(a) M. Shao, F. Ning, J. Zhao, M. Wei, D. G. Evans and X. Duan,
J. Am. Chem. Soc., 2012, 134, 1071–1077; (b) A. Shaabani,
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Fig. 6 Recyclability of the catalyst in the synthesis of benzaldehyde.
Conclusions
1
1
1
1
0 C. Chen, R. S. Justice, D. W. Schaefer and J. W. Baur,
Polymer, 2008, 49, 3805–3815.
A novel and efficient catalyst consisting of Co O immobilized
3
4
1 B. Lin, B. Qiu, L. Qiu, Z. Si, F. Chu, X. Chen and F. Yan, Fuel
Cells, 2013, 13, 72–78.
2 D. Zhao, Z. Fei, R. Scopelliti and P. J. Dyson, Inorg. Chem.,
on dipeptide-functionalized silica-coated magnetite nano-
particles (Co @SCM-UIL) was synthesized, characterized
3 4
O
and applied for the selective aerobic oxidation of various
primary and secondary alcohols. This nanocatalyst has high
thermal and chemical stabilities. The heterogeneous catalyst is
recoverable and reusable and can be handled easily. It is
believed that the pyridinium groups with amide functions
of the ligand on the surface of the silica-coated magnetite
provided the paths for the generation, stabilization, and uniform
distribution of the Co O nanoparticles and also they can afford a
3 4
means of facilitating mass transfer of the starting materials to the
active sites, which would improve the performance of the catalyst.
2004, 43, 2197–2205.
3 E. Vessally, M. Ghasemisarabbadeih, Z. Ekhteyari,
R. Hosseinzadeh-Khanmiri, E. Ghorbani-Kalhor and L. Ejlali,
RSC Adv., 2016, 6, 106769–106777.
1
1
1
4 J. Jover, M. Garc ´ı a-Rat ´e s and N. Lopez, ACS Catal., 2016, 6,
4135–4143.
5 S. Verma, R. N. Baig, M. N. Nadagouda and R. S. Varma, ACS
Sustainable Chem. Eng., 2016, 4, 1094–1098.
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Conflicts of interest
18, 1061–1069; (c) P. Liu, V. Degirmenci and E. J. Hensen,
J. Catal., 2014, 313, 80–91.
There are no conflicts to declare.
1
7 (a) S. M. Sarathy, P. Oßwald, N. Hansen and K. Kohse-
H o¨ inghaus, Prog. Energy Combust. Sci., 2014, 44, 40–102; (b) W. J.
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Acknowledgements
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The authors gratefully acknowledge Razi University for finan-
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¨
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˘
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