Magnetically recyclable core-shell Fe3O4@chitosan- Schiff base complexes as efficient catalysts for aerobic oxidation of cyclohexene under mild conditions

Article abstract of DOI:10.1016/j.molcata.2013.12.007

Five magnetic core-shell type Fe₃O₄@chitosan-Schiff base Co(II), Cu(II) and Mn(II) complexes were prepared in a simple way and well characterized by Fourier transform infrared spectrophotometry (FT-IR), X-ray photoelectron spectroscopy (XPS), X-ray diffractometry (XRD), transmission electron microscopy (TEM), dynamic light scattering (DLS) and thermogravimetry (TG). Their abilities to catalyze oxidation of cyclohexene with molecular oxygen in the absence of any solvents or reducing agents were investigated. It has been revealed that the as-prepared samples have high catalytic activities for heterogeneously aerobic oxidation of cyclohexene and 2-cyclohexene-1-one is the main product in all cases. Especially, when the Schiff base Co(II) complex derived from 5-nitrosalicyaldehyde was employed as catalyst, 46.8% of cyclohexene conversion, 77.2% of selectivity for 2-cyclohexene-1-one and as high as 4.7 × 10³ of turnover number were obtained under ambient pressure at 70 C for 12 h of reaction. The catalyst can be magnetically separated easily for reuse and no obvious loss of activity was observed when reused in five consecutive runs.

Full text of DOI:10.1016/j.molcata.2013.12.007

Journal of Molecular Catalysis A: Chemical 383–384 (2014) 217–224
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Magnetically recyclable core–shell Fe3O4@chitosan-Schiff base
complexes as efficient catalysts for aerobic oxidation of cyclohexene
under mild conditions
Xiaodong Caia,b, Haiyan Wanga,b, Qianping Zhanga,b, Jinhui Tonga,b,∗, Ziqiang Leia,b,∗
a Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education, Lanzhou 730070, PR China
b Key Laboratory of Gansu Polymer Materials, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Five magnetic core–shell type Fe3O4@chitosan-Schiff base Co(II), Cu(II) and Mn(II) complexes were pre-
pared in a simple way and well characterized by Fourier transform infrared spectrophotometry (FT-IR),
X-ray photoelectron spectroscopy (XPS), X-ray diffractometry (XRD), transmission electron microscopy
(TEM), dynamic light scattering (DLS) and thermogravimetry (TG). Their abilities to catalyze oxidation of
cyclohexene with molecular oxygen in the absence of any solvents or reducing agents were investigated. It
has been revealed that the as-prepared samples have high catalytic activities for heterogeneously aerobic
oxidation of cyclohexene and 2-cyclohexene-1-one is the main product in all cases. Especially, when the
Schiff base Co(II) complex derived from 5-nitrosalicyaldehyde was employed as catalyst, 46.8% of cyclo-
hexene conversion, 77.2% of selectivity for 2-cyclohexene-1-one and as high as 4.7 × 103 of turnover
number were obtained under ambient pressure at 70 ◦C for 12 h of reaction. The catalyst can be mag-
netically separated easily for reuse and no obvious loss of activity was observed when reused in five
consecutive runs.
Received 4 September 2013
Received in revised form 4 December 2013
Accepted 7 December 2013
Available online 19 December 2013
Keywords:
Magnetic recovery
Chitosan-Schiff base complexes
Molecular oxygen
Cyclohexene oxidation
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Montmorillonite-supported Mn(II) complexes of pyridine Schiff-
base ligands have shown high performances on aerobic epoxide
Allylic oxidation of olefins to ␣,␤-unsaturated ketones, particu-
larly cyclohexene to 2-cyclohexen-1-one is an important and useful
transformation in both chemical and pharmaceutical industries
[1–3]. Many catalysts have been employed in cyclohexene oxida-
tion in recent years [4–8]. Among these, Schiff base complexes
of transition metal ions have shown high catalytic activities and
many of homogeneous systems have been developed. However,
homogeneous catalysis suffers from the problematic separation
of the catalyst from the product for reuse. This problem is of
particular environmental and economic concern in large-scale
syntheses. So great efforts have been made to develop heteroge-
neously analogous catalysts. Several examples of heterogenization
of Schiff base complexes by immobilization of homogeneous
ones onto some supports, such as polymers [9–11], clays [12]
and zeolites [13,14] have been reported. Tridentate Schiff-base
metal complexes supported by chloromethylated polystyrene were
employed as catalysts for aerobic oxidation of cyclohexene and
allylic hydroperoxide was obtained as the main product [9].
of cyclohexene in the presence of an excess of sacrificial aldehyde
[12]. Schiff base copper complex and the one supported on organ-
ically modified silica were also employed to catalyze oxidation of
cyclohexene and 2-cyclohexen-1-one was obtained as the major
product at ambient conditions [15]. However, most of the cata-
lysts reported suffer from complicated preparation processes or
poor selectivities. Awareness of environmental issues has proven
a potent driving force in development of more effective and envi-
ronmentally friendly processes and technologies in the chemical
industry.
Chitosan, the most abundant natural amino polysaccharide, is
produced by the deacetylation of chitin which is one of the key
constituents of the shells of crustaceans and a byproduct of the
fishing industry. The flexibility of the material, insoluble in the vast
majority of solvents, but capable of being cast into films and fibers
from dilute acid, along with its inherent chirality makes chitosan
an excellent candidate as a support for catalysis. In this respect,
several catalytic systems using chitosan as supports have been
developed [16–18]. Functionalization of chitosan to provide co-
ordination sites has also been carried out and has provided catalysts
for oxidation reactions [19–22].
∗
Corresponding authors at: Key Laboratory of Gansu Polymer Materials, College
The increasing demand for environment-conscious chemical
processes has impelled many researchers to develop heteroge-
neously catalysts which can be recovered facilely and reused
of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou
730070, PR China. Tel.: +86 931 7970359; fax: +86 931 7970359.
E-mail addresses: jinhuitong@126.com (J. Tong), leizq@nwnu.edu.cn (Z. Lei).
1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.12.007

218
X. Cai et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 217–224
Table 1
effectively. Recently developed magnetically recyclable catalysts
have proved to be a kind of promising ones combining the advan-
tages of high activities and facile recovery in heterogeneously
catalytic processes [23–25].
The FT-IR date of chitosan, ligands and catalysts.
Sample
Selective IR bands (cm−1
)
VC
VNH
VC
VFe
O
N
C
2
Moreover, from an environmental and cost-effective viewpoint,
catalytic oxidation processes employing molecular oxygen, under
solvent-free conditions and ambient pressure are extremely valu-
able. In this work, we prepared five magnetic core–shell type
Fe3O4@chitosan-Schiff base M(II) complexes (M = Co, Cu and Mn)
derived from chitosan modified by salicylaldehyde with differ-
ent substituents. These samples were employed as catalysts for
selective oxidation of cyclohexene with molecular oxygen under
solvent-free conditions in ambient pressure. Key reaction parame-
ters such as reaction temperature, reaction time, amount of catalyst
were optimized and the reusability of the catalyst was also evalu-
ated.
Chitosan
MG@Sal
MG@NSal
MG@TBSal
MG@Sal-Co
MG@Sal-Cu
MG@Sal-Mn
MG@NSal-Co
MG@TBSal-Co
–
1601
–
–
1632
1638
1623
1624
1618
1621
1604
1614
–
–
–
–
–
–
–
–
1584, 1503, 1458, 1416
1537, 1498, 1436
1576, 1437
1578, 1543, 1417
1586, 1493, 1452, 1416
1592, 1569, 1472
1555, 1451, 1415
1549, 1428, 1413
585
598
597
580
583
578
598
598
derived from salicyaldehyde, 5-nitrosalicyaldehyde and 3,5-di-
tert-butylsalicylaldehtde were prepared and denoted as MG@Sal,
MG@NSal and MG@TBSal respectively (Fig. 1).
The magnetic Fe3O4@chitosan-Schiff base Co(II), Cu(II) and
Mn(II) complexes were prepared by coordination reaction of corre-
sponding ligand with metal acetate in ethanol [27]. Typically, 1.0 g
of ligand was added to 60 mL ethanol solution of metal acetate and
the mixture was then refluxed for 20 h under an argon atmosphere.
After the reaction, the solid was collected by filtration, precondi-
tioned by multiple washings to remove any loose metal species,
and dried at 90 ◦C under vacuum to give the complex. Five samples
MG@Sal-Co, Cu and Mn, MG@NSal-Co and MG@TBSal-Co were
prepared and then used to catalyze the oxidation of cyclohexene
with molecular oxygen. The metal contents of the catalysts deter-
mined by atomic absorption spectroscopy are 4.20%, 4.52%, 2.28%,
3.85% and 4.00%, respectively (Table 3).
2. Experimental
2.1. Materials and equipments
Chitosan was finely purified to a deacetyl degree of 90%.
Other reagents were of analytical grade and used without further
purification. FT-IR spectra were measured on a Nexus 870 FT-IR
spectrophotometer. XPS measurements were performed with a VG
Scientific ESCALAB 210 instrument with Mg K␣ radiation. Atomic
absorption results were obtained on a Hitachi 180-80 polarized
Zeeman atomic absorption spectrophotometer. XRD patterns of
the samples were collected using a PANalytical X’Pert Pro diffrac-
tometer with Cu K␣ radiation. TEM micrographs were obtained
using a Hitachi H-600 microscope. Particle sizes distributions of
the samples were determined on a Nano-ZS Zetasizer ZEN3600
(Malvern Instruments Ltd., Worcestershire, UK) dynamic light scat-
tering instrument with a He–Ne laser beam at 633.8 nm at 25 ◦C in
ethanol suspension. TG analyses were performed on a TGA7 ther-
mogravimetric analyzer. The oxidation products were determined
by an HP 6890/5973 GC/MS instrument and quantified by a Shi-
madzu GC-2010 gas chromatograph.
2.3. Characterization of the samples
Table 1 shows the FT-IR bands of the as-prepared catalysts. The
spectrum of chitosan has a strong absorption around 1601 cm−1
which masks the NH2 stretching band. Diagnostic peaks appeared
in MG@Sal, MG@NSal and MG@TBSal at 1632, 1638 and 1623 cm−1
respectively. These bands are due to the stretching vibrations of
the C N bonds. In chitosan-Schiff base metal complexes, however,
the absorption of C N bonds shifted to lower wave number after
chelated with metal ions as described previously [28]. The bands
1592–1413 cm−1 are assigned to stretching vibrations of benzene
rings. The bands at 578–598 cm−1 are due to Fe O stretching vibra-
tion in magnetic core of Fe3O4 [26].
2.2. Synthesis of the catalysts
2.2.1. Synthesis of magnetic Fe3O4@ chitosan
Solid Fe3O4 (magnetite, abbreviated as MG) nano-powder was
synthesized according to the reported route [26]. 1% (w/v) chi-
tosan stock solution was prepared by dissolving 1.0 g of chitosan in
100 mL of 2% (v/v) acetic acid solution followed by a 5 h sonication
to dissolve the chitosan completely. Magnetic chitosan nanoparti-
cles, Fe3O4@ chitosan, were prepared as follows. 390 mg of freshly
precipitated (not dried) magnetite were dispersed in 78 mL of above
chitosan solution (weight ratio of chitosan to Fe3O4 = 2:1). The
mixture was then diluted to 150 mL, stirred for 1 h at room tem-
perature and neutralized with 10% (w/v) solution of NaHCO3. After
the foam was subsided, the precipitate was collected by centrifu-
gation, washed twice with deionized water and spread on a glass
plate for drying.
XPS was used to further confirm the interaction between metals
and chitosan-Schiff base ligands, the spectra were shown in Fig. 2
and the binding energy data were listed in Table 2. Three peaks
around 529.5, 531.0 and 532.5 eV in the three ligands attribute to
O1s in C OH, C
O C and Ph O bonds (Fig. 2A–C). The binding
energies of O1s, especially O1s in Ph O bonds in the complexes are
0.4–0.7 eV higher compared the corresponding ligands (Fig. 2A–C).
A new peak at 534.4 eV in the ligand MG@NSal attribute to O1s in
NO2 groups (Fig. 2C). The binding energies of N1s in C N bonds
around 399.0 eV in the ligands also shifted 0.5–3.9 eV to higher
binding energies in the corresponding complexes (Fig. 2D and E). A
new peak at 405.6 eV in ligand MG@NSal attribute to N1s in NO2
groups (Fig. 2E). Furthermore, there are three types of N around
N1s binding energies of 403.0, 400.0 and 399.0 eV in the complexes
of MG@Sal-Co, Cu and Mn, and this may indicate that the cata-
lysts contain three different complexes of 1:1, 1:2 and 1:3 of metal
to Shiff base (Fig. 2D). Correspondingly, there are three types of
metal centers in the MG@Sal-Co, Cu and Mn complexes and the
similar situations were also observed for metal centers in the com-
plexes of MG@NSal-Co and MG@TBSal-Co (Fig. 2F–H). On the other
hand, it can be obviously seen that the binding energies of metals
are 0.6–1.1 eV lower than the corresponding acetate (Fig. 2F–H).
2.2.2. Preparation of the magnetic Fe3O4@chitosan-Schiff base
complexes
Fe3O4@chitosan-Schiff base ligands were prepared according to
the following procedures. 1.1 g of the as-prepared Fe3O4@chitosan
nanoparticles (equivalent to 4.2 mmol NH2) and 4.2 mmol of
salicyaldehyde derivative were added to 40 mL of ethanol, and
the mixture was then refluxed for 30 h. After the resultant mix-
ture was cooled, the solid was separated by filtration, washed
thoroughly with ethanol and then dried at 50 ◦C under vacuum
for 12 h to give the product. Three different Schiff base ligands

X. Cai et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 217–224
219
Fig. 1. Procedure for synthesis of catalysts.
Based on the above characterization results of FT-IR and XPS, it can
be concluded that the Fe3O4@chitosan-Schiff base complexes were
formed by coordination of metal centers with N and O in the ligands.
XRD patterns of the as-synthesized Fe3O4 nanoparticles and cat-
alysts were illustrated in Fig. 3. All the samples showed nine peaks
at 18.3, 30.2, 35.4, 38.1, 43.2, 53.1, 57.2, 62.7 and 74.1◦, which can
be ascribed to the reflection of (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0),
(4 2 2), (5 1 1), (4 4 0) and (5 3 3) diffractions of Fe3O4 (JCPDS NO.
79-0418). The results confirm that no phase transition occurred for
Fe3O4 core during the following processes of chitosan coating and
chitosan-Schiff base complexes synthesis.
The morphologies of the samples were also characterized by
TEM. All the samples showed the similar morphologies as repre-
sented by the images of MG@NSal-Co in Fig. 4. It has shown that
the as-prepared samples formed by dark Fe3O4 cores embedded
in membranous light gray chitosan-Schiff base shells. The particle
sizes of Fe3O4 cores range from 13 to 27 nm and most particles
with diameters about 21 nm. The thickness of shells range from 4
to 12 nm and the most is 7 nm. However, the agglomeration can
also be seen.
The size distributions of bare Fe3O4 and the chitosan-schiffs
coated samples in ethanol suspension were measured by DLS
(Fig. 5). As can be seen, the Z-average diameters of MG@Sal-Co,
MG@Sal-Cu, MG@Sal-Mn, MG@NSal-Co and MG@TBSal-Co are
453, 468, 478, 449 and 461 nm respectively, which are larger than
312 nm for bare Fe3O4 sample. The increase in average diameter
might attribute to the formation of a chitosan-schiff layer. This is
thus indicative of formation of core–shell structure nanoparticles.
The particle sizes derived from DLS analysis are much larger than
those obtained from TEM, and this can be attributed to serious mag-
netic aggregation of the nanoparticles in suspension during the DLS
measurement process. It is characterized by high polydispersity
Table 2
The data of X-ray photoelectron spectroscopy (XPS).
Sample
Binding energy (eV)
O 1s
N 1s
Co 2p
Cu 2p
Mn 2p
Co2p1/2
–
Co2p3/2
–
Cu2p1/2
–
Cu2p3/2
–
Mn2p1/2
–
Mn2p3/2
–
MG@Sal
529.6
531.4
532.4
529.7
530.8
532.5
534.3
529.5
530.8
532.5
529.8
531.5
533.0
530.0
531.2
533.1
529.9
531.3
533.1
530.5
531.6
532.9
530.0
532.0
533.1
–
399.3
MG@NSal
399.3,
405.6
–
–
–
–
–
–
–
–
MG@TBSal
398.8
–
–
–
–
–
–
–
–
–
–
MG@Sal-Co
MG@Sal-Cu
MG@Sal-Mn
MG@NSal-Co
MG@TBSal-Co
399.1
399.9
403.0
399.2
400.1
403.2
399.1
399.8
403.0
399.9
797.0
802.4
806.7
–
781.1
784.3
787.4
–
952.2
953.9
932.3
933.8
943.5
–
–
–
–
–
–
–
653.1
657.0
640.5
641.9
645.3
–
797.4
802.9
781.1
782.3
786.1
781.2
785.5
–
–
–
–
–
399.6
797.0
802.4
–
Co(OAc)2·4H2O
Cu(OAc)2·H2O
–
–
798.0
803.2
–
782.2
786.3
–
–
–
–
–
–
952.8
954.5
962.4
933.1
934.8
944.3
940.9
–
Mn(OAc)2·4H2O
–
–
–
–
–
653.8
657.8
641.1
642.5
646.2

220
X. Cai et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 217–224
O
1s
(A)
(B)
O
1s
MG@Sal-Co
MG@Sal-Cu
MG@Sal-Mn
MG@Sal
540
537
534
531
528
525
522
540
538
536
534
532
530
528
526
Binding energy(eV)
Binding energy(eV)
(D)
N
1s
(C)
O
1s
MG@Sal-Co
MG@Sal-Cu
MG@NSal
MG@Sal-Mn
MG@Sal
MG@NSal-Co
MG@TBSal
MG@TBSal-Co
542 540 538 536 534 532 530 528 526 524 410 408 406 404 402 400 398 396 394 392
Binding energy(eV)
Binding energy(eV)
N
1s
(E)
(F)
Co
2p
MG@Sal-Co
MG@NSal
MG@NSal-Co
MG@NSal-Co
MG@TBSal-Co
MG@TBSal
Co(OAc) •4H O
2
2
MG@TBSal-Co
410 408 406 404 402 400 398 396 394 392 815 810 805 800 795 790 785 780 775 770 765
Binding energy(eV)
Binding energy(eV)
(G)
(H)
Mn
2p
Cu
2p
MG@Sal-Cu
MG@Sal-Mn
Mn(OAc) •4H O
2
2
Cu(OAc) •H O
2
2
660 657 654 651 648 645 642 639 636 633
Binding energy(eV)
975 970 965 960 955 950 945 940 935 930 925 920 915
Binding energy(eV)
Fig. 2. XPS spectra of the ligands, complexes and raw metal acetates.

X. Cai et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 217–224
221
Table 3
Oxidation of cyclohexene over different catalysts.a
Entry
Catalyst
Metal content (wt%)
Conversion (%) (%SD) c
TON (×103)b
Selectivity (%)
EXO (%SD) c
OH (%SD) c
C
O (%SD) c
Others (%SD) c
1
2
3
4
5
6
7
8
9
MG@Sal-Co
MG@Sal-Cu
MG@Sal-Mn
MG@NSal-Co
MG@TBSal-Co
NSalophen-Cod
Fe3O4
4.20
4.53
2.28
3.85
4.00
10.61
–
32.8 (3.9)
25.7 (4.3)
14.6 (1.8)
46.8 (3.5)
50.2 (4.5)
48.3 (3.9)
Trace
3.0
2.2
2.3
4.7
4.8
1.8
–
7.1 (2.3)
3.8 (2.9)
7.4 (3.2)
5.3 (2.2)
8.3 (1.2)
4.8 (2.9)
–
7.2 (1.9)
6.7 (1.8)
5.9 (1.1)
8.7 (3.1)
12.4 (2.7)
12.6 (2.1)
–
80.3 (4.8)
70.3 (3.7)
81.2 (1.9)
77.2 (4.2)
67.1 (3.1)
76.4 (2.9)
–
5.4 (2.9)
19.2 (4.5)
5.5 (2.6)
8.8 (1.9)
12.2 (3.7)
5.2 (3.8)
–
MG@NSal
Blank
–
–
Trace
Trace
–
–
–
–
–
–
–
–
–
–
a
Reaction conditions: cyclohexene 19.7 mmol, catalyst 3.0 mg, reaction temperature 70 ◦C, reaction time 12 h.
Moles of substrate converted per mole of coordinated metal in the catalyst.
SD, standard deviation.
b
c
d
Derived from condensation of 5-nitrosalicyaldehyde with o-phenylenediamine according to the procedure described in the literature [27].
The TG data of the catalysts were depicted in Fig. 6. The destruc-
tion temperatures of MG@Sal-Co, Cu and Mn, MG@NSal-Co and
MG@TBSal-Co are 245, 220, 243, 228 and 187 ◦C, respectively. The
weight loss of the catalysts before the destruction is attributed to
desorption of water (below 150 ◦C) and partly cleavage of glycosidic
bonds in the chitosan polymer (around 200 ◦C) [30,31].
Fe3O4
2.4. Oxidation of cyclohexene
MG@Sal-Co
Typical oxidation of cyclohexene was performed according to
the procedure described in the literature [10]: a glass flask was
charged with 3.0 mg catalyst and 2.0 mL cyclohexene (19.7 mmol).
The dry oxygen was filled from the gauge glass and the atmo-
sphere was discharged out of the glass reactor with the gas outlet
tube. The gas outlet tube was closed. The reactor was put into
a bath, which had been heated to the desired temperature, and
stirring was started. After the reaction, the products were iden-
tified by GC–MS and quantified by GC. The main products of
the reaction were cyclohexene oxide (EXO), 2-cyclohexene-1-ol
MG@Sal-Cu
MG@Sal-Mn
MG@NSal-Co
MG@TBSal-Co
10
20
30
40
50
60
70
80
90
2θ(degree)
Fig. 3. XRD patterns of catalysts.
(
OH) and 2-cyclohexene-1-one (C O), the main by-product is
2-cyclohexene-1-hydroperoxide (Scheme 1).
index (PDI) values of 0.294, 0.168, 0.223, 0.113, 0.285 and 0.124
for Fe3O4, MG@Sal-Co, MG@Sal-Cu, MG@Sal-Mn, MG@NSal-Co
and MG@TBSal-Co respectively. PDI = 0.05 is a threshold value for
assessing whether nanoparticles are monodisperse in a given sol-
vent. Generally, size distributions with a PDI of less than 0.1 are
considered to be monodisperse. The higher the PDI, the more easily
the nanoparticles can aggregate together [29]. Hence, the Z-average
values were larger than the TEM diameters.
3. Results and discussion
3.1. Catalysis tests
The results of cyclohexene oxidation catalyzed by magnetic
core–shell Fe3O4@chitosan-Schiff base metal complexes were
Fig. 4. The TEM images of the MG@NSal-Co.

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X. Cai et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 217–224
Fig. 5. The size distributions of Fe3O4 and the catalysts.
105
100
95
than the other two Co(II) complexes of MG@Sal-Co (entry 1)
and MG@NSal-Co (entry 4). As for the distributions of the prod-
ucts, the top two high selectivities above 80.0% for the main
product of 2-cyclohexene-1-one were obtained in the presence
of MG@Sal-Mn and MG@Sal-Co. In view of pursuit of both high
conversion and selectivity, the catalyst MG@NSal-Co is the opti-
mum one. This catalyst also obtained comparable conversion and
selectivity of 2-cyclohexene-1-one with those in the case of anal-
ogously homogeneous Salophen Co(II) complex catalyst (denoted
as NSalophen-Co, entry 6). However, the turnover number over
the supported catalyst is 2.6 times as high as that over the homo-
geneous one. This may be due to the active sites isolating effect
of supporting. Comparatively, almost no reaction took place in the
presence of only Fe3O4, ligand of MG@NSal or free of catalyst (entry
7–9) under the same conditions. The complex MG@NSal-Co was
selected as catalyst to further optimize the reaction conditions. The
recyclable performance of the catalyst was also investigated.
MG@Sal-Co
MG@Sal-Mn
90
MG@NSal-Co
85
80
MG@Sal-Cu
MG@TBSal-Co
75
70
65
60
50
100
150
200
250
300
350
Temperature(oC)
Fig. 6. The TG date of catalysts.
3.2. Effect of reaction time
listed in Table 3. As can be seen, all of the five catalysts have shown
high performances on oxidation of cyclohexene and high turnover
numbers have been obtained. Oxidation reactions occurred in allyl
carbon atoms and 2-cyclohexene-1-one was found to be the major
product in all cases. Central metals and substituents of ligands
have significant influences on catalytic performances of the cat-
alysts. As a result, MG@Sal-Co showed higher catalytic activity
than MG@Sal-Cu and MG@Sal-Mn (entry 1–3) while MG@TBSal-
Co (entry 5) obtained the highest turnover number of 4.8 × 103
The effect of reaction time on the oxidation reaction was inves-
tigated and the results were shown in Fig. 7. With increasing of the
reaction time, cyclohexene conversion and 2-cyclohexene-1-one
yield increased rapidly during the initial 12 h and then reached a
plateau. As for the yields of cyclohexene oxide and 2-cyclohexene-
1-ol, they increased firstly but then decreased with increase of
reaction time, and the maximum values of 2.9 and 4.2% were
obtained for 11 and 9 h reaction respectively. In pursuit of higher
selectivity for the main product 2-cyclohexene-1-one, 12 h was
considered as optimum reaction time.
O
OH
O2
3.3. Effect of reaction temperature
O
Other by-product
Catalyst
As shown in Fig. 8, only about 3.6% of cyclohexene was converted
at 30 ◦C. Expectedly, cyclohexene conversion increased sharply
with increase of reaction temperature and the value of 46.8% was
EXO
C=O
-OH
Scheme 1. Oxidation of cyclohexene.

X. Cai et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 217–224
223
9
8
7
6
5
4
3
2
1
55
50
45
40
35
30
25
20
15
10
5
Conversion
Yield of C=O
Yield of EXO
Yield of -OH
6
8
10
12
14
Reaction time (h)
a
Fig. 9. Reuse of the catalyst.
Reaction conditions: cyclohexene 19.7 mmol,
Fig. 7. The effect of cyclohexene oxidation with different reaction time. Reaction
conditions: cyclohexene 19.7 mmol, MG@NSal-Co 3.0 mg, reaction temperature
70 ◦C.
MG@NSal-Co 3.0 mg, reaction temperature 70 ◦C, reaction time 12 h.
OH
O
55
50
45
40
35
30
25
20
15
10
5
9
8
7
6
5
4
3
2
1
0
+
Conversion
Yield of C=O
Yield of EXO
Yield of -OH
OOH
OH
O2
+
O
Step 2
Step 1
0
30
40
50
60
70
Reaction temperature (ºC)
O
Fig. 8. The effect of cyclohexene oxidation under different reaction temperature.
Reaction conditions: cyclohexene 19.7 mmol, MG@NSal-Co 3.0 mg, reaction time
12 h.
H2O
+
obtained at 70 ◦C. As for the distributions of the products, with
increasing reaction temperature, the yield of cyclohexene oxide
and 2-cyclohexene-1-one increased while that of 2-cyclohexene-
1-ol increased firstly but then decreased, this indicates that high
temperature is favorable for generation of 2-cyclohexene-1-one.
Scheme 2. The formation of the products according to the proposed mechanism.
to the second run under the same conditions. The results of five
runs were shown in Fig. 9. The selectivity for 2-cyclohexene-1-one
changed slightly after five runs, however, the conversion of cyclo-
hexene decreased from 46.8 to 40.2%. The decrease in the activity
could be mainly attributed to unavoidable loss of the catalyst dur-
ing the process of collection. The results confirm that the magnetic
chitosan-Schiff base complexes have good stability and recyclable
applicability for the oxidation of cyclohexene under our experi-
mental conditions.
3.4. The effect of catalyst amount
The catalyst amount also has obvious effect on the oxidation of
cyclohexene (Table 4). Cyclohexene conversion increased from 25.4
to 46.8% when the amount of catalyst increased from 2.0 to 3.0 mg,
However, with further increase of catalyst amount to 7.0 mg, cyclo-
hexene conversion decreased to 31.8% possibly due to adsorption
or chemisorptions of two reactants on separate catalyst particles,
thereby reducing the chance to interact. Similar observations were
noted by Sharma and Pardeshi [11,32]. Based on above results,
3.0 mg is considered as optimum catalyst amount.
3.6. Discussion
According to the literature [7,9], the oxidation of cyclo-
hexene with molecular oxygen initially forms 2-cyclohexene-1-
hydroperoxide as shown in Scheme 2 (step 1). This product is not
stable and can form cyclohexene oxide and 2-cyclohexene-1-ol by
epoxidation of cyclohexene (step 2), decompose to 2-cyclohexene-
1-ol and 2-cyclohexene-1-one in the presence of catalyst (step 3), or
decompose to 2-cyclohexene-1-one and water (step 4). The conver-
sion of cyclohexene is controlled by the rate of step 1 in Scheme 2.
As shown in Figs. 7 and 8, with prolong of reaction time or increase
3.5. Reuse of the catalyst
After the reaction, the catalyst was magnetically fixed at the
bottom of the reactor, then the reaction mixture was decanted,
the reactor was recharged with fresh substrate and then subjected

224
X. Cai et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 217–224
Table 4
The effect of cyclohexene oxidation with different catalyst amounts.a
Entry
Amount of the catalyst (mg)
Conversion (%) (%SD)c
TON (×103)b
Selectivity (%)
EXO (%SD)c
OH (%SD)c
C
O (%SD)c
Others (%SD)c
1
2
3
4
2.0
3.0
5.0
7.0
25.4 (3.5)
46.8 (2.5)
37.5 (3.9)
31.8 (4.2)
3.8
4.2
2.3
1.4
6.3 (2.8)
6.9 (1.9)
6.4 (3.1)
6.7 (2.8)
7.9 (2.1)
8.5 (2.9)
10.7 (3.1)
8.3 (2.1)
78.5 (3.1)
77.3 (3.6)
70.2 (2.8)
73.8 (4.3)
7.5 (2.4)
7.3 (2.4)
12.7 (1.1)
11.2 (3.1)
a
Reaction conditions: cyclohexene 19.7 mmol, reaction temperature 70 ◦C, reaction time 12 h.
Moles of substrate converted per mole of coordinated metal in the catalyst.
SD, standard deviation.
b
c
in reaction temperature, the conversion of cyclohexene and the
[3] R.M. Wang, Z.F. Duan, Y.F. He, Z.Q. Lei, J. Mol. Catal. A 260 (2006) 280–287.
[4] D. Habibi, A.R. Faraji, M. Arshadi, J.L.G. Fierro, J. Mol. Catal. A 372 (2013) 90–99.
[5] K. Leus, G. Vanhaelewyn, T. Bogaerts, Y.Y. Liu, D. Esquivel, F. Callens, G.B. Marin,
V. Van Speybroeck, H. Vrielinck, P. Van Der Voort, Catal. Today 208 (2013)
97–105.
[6] S.J.J. Titinchi, H.S. Abbo, Catal. Today 204 (2013) 114–124.
[7] P. Bujak, P. Bartczak, J. Polanski, J. Catal. 295 (2012) 15–21.
[8] M. Vafaeezadeh, M.M. Hashemi, M. Shakourian-Fard, Catal. Commun. 26 (2012)
54–57.
[9] Y. Chang, Y. Lv, F. Lu, F. Zha, Z. Lei, J. Mol. Catal. A 320 (2010) 56–61.
[10] Y. Chang, X.L. Shi, D.M. Zhu, Y. Liu, Polym. Adv. Technol. 19 (2008) 877–881.
[11] S. Sharma, S. Sinha, S. Chand, Ind. Eng. Chem. Res. 51 (2012) 8806–8814.
[12] Z.Y. Li, R.R. Tang, G.Y. Liu, Catal. Lett. 143 (2013) 592–599.
[13] M. Rivallan, S. Thomas, M. Lepage, N. Takagi, H. Hirata, F. Thibault-Starzyk,
ChemCatChem 2 (2010) 1599–1605.
yield of 2-cyclohexene-1-one increased, which indicates that high
temperature or long reaction time is of advantage to steps 1 and
4 in Scheme 2. Moreover, the yield of 2-cyclohexene-1-one in this
study is much higher than that of 2-cyclohexene-1-ol, which indi-
cates that the rate of step 4 was much faster than that of step
2. Once 2-cyclohexene-1-hydroperoxide formed, it converts to 2-
cyclohexene-1-one and water very quickly.
4. Conclusions
Magnetic core–shell Fe3O4@chitosan-Schiff base Co(II), Cu(II)
and Mn(II) complexes derived from different substituted sali-
cyaldehyde were prepared by a simple method. The complexes
have been proved to be highly active catalysts for cyclohexene oxi-
dation with molecular oxygen in the absence of reducing agents
or solvents. High turnover numbers of 4.8 × 103 and the highest
selectivity of above 80%, to the best of our knowledge, for 2-
cyclohexene-1-one have been obtained. Chitosan is plentiful and
biocompatible, the catalysts can be easily separated magnetically
after the reaction and have shown great reusability.
[14] V.K. Bansal, P.P. Thankachan, R. Prasad, Appl. Catal. A 381 (2010) 8–17.
[15] S. Mukherjee, S. Samanta, B.C. Roy, A. Bhaumik, Appl. Catal. A 301 (2006) 79–88.
[16] Q.Z. Huang, S.M. Wang, J.F. Huang, L.H. Zhuo, Y.C. Guo, Carbohyd. Polym. 68
(2007) 761–765.
[17] S.E.S. Leonhardt, A. Stolle, B. Ondruschka, G. Cravotto, C. De Leo, K.D. Jandt, T.F.
Keller, Appl. Catal. A 379 (2010) 30–37.
[18] D.W. Wei, Y.Z. Ye, X.P. Jia, C. Yuan, W.P. Qian, Carbohyd. Res. 345 (2010)
74–81.
[19] C.C. Guo, G. Huang, X.B. Zhang, D.C. Guo, Appl. Catal. A 247 (2003) 261–267.
[20] G. Huang, C.C. Guo, S.S. Tang, J. Mol. Catal. A 261 (2007) 125–130.
[21] J. Tong, Z. Li, C. Xia, J. Mol. Catal. A 231 (2005) 197–203.
[22] J. Tong, Y. Zhang, Z. Li, C. Xia, J. Mol. Catal. A 249 (2006) 47–52.
[23] Y.Z. Li, F.Y. Ma, X.T. Su, C. Sun, J.C. Liu, Z.Q. Sun, Y.L. Hou, Catal. Commun. 26
(2012) 231–234.
Acknowledgements
[24] C.R. Shen, S.T. Wu, Z.T. Tsai, J.J. Wang, T.C. Yen, J.S. Tsai, M.F. Shih, C.L. Liu, Polym.
Int. 60 (2011) 945–950.
[25] M.M. Mojtahedi, M.S. Abaee, A. Rajabi, P. Mahmoodi, S. Bagherpoor, J. Mol. Catal.
A 361 (2012) 68–71.
[26] V. Belessi, R. Zboril, J. Tucek, M. Mashlan, V. Tzitzios, D. Petridis, Chem. Mater.
20 (2008) 3298–3305.
[27] W. Sun, Chiral Salen Metal Complexes Catalyzed Asymmetric Reactions (PhD
Dissertation), Graduate School of Chinese Academy of Sciences, 2003.
[28] J.J.E. Hardy, S. Hubert, D.J. Macquarrie, A.J. Wilson, Green Chem. 6 (2004) 53–56.
[29] S. Zhang, L.G. Zou, D. Zhang, X. Pang, H. Yang, Y. Xu, J. Nanopart. Res. 13 (2011)
3867–3876.
The authors are grateful to the National Natural Science Foun-
dation of China (51302222, 21363021), Gansu Provincial Natural
Science Foundation (1308RJYA017) and Program for Changjiang
Scholars and Innovative Research Team in University (IRT1177) for
financial support.
References
[30] R. Antony, S. Theodore David Manickam, K. Saravanan, K. Karuppasamy, S.
Balakumar, J. Mol. Struct. 1050 (2013) 53–60.
[31] H. Hu, X. Wang, J. Wang, F. Liu, M. Zhang, C. Xu, Appl. Surf. Sci. 257 (2011)
2637–2642.
[32] S.K. Pardeshi, R.Y. Pawar, J. Mol. Catal. A 334 (2011) 35–43.
[1] S.E. Dapurkar, H. Kawanami, K. Komura, T. Yokoyama, Y. Ikushima, Appl. Catal.
A 346 (2008) 112–116.
[2] D. Jiang, T. Mallat, D.M. Meier, A. Urakawa, A. Baiker, J. Catal. 270 (2010) 26–33.