Highly efficient oxidation of cyclopentene catalyzed by magnetically recoverable Ca–Co ferrite spinels with high solvent selectivity

Article abstract of DOI:10.1016/j.poly.2018.07.037

The calcium doped CoFe₂O₄ spinels Ca_xCo_1?xFe₂O₄ (x = 0, 0.1, 0.3, 0.5 and 0.7) were prepared by simple sol–gel auto-combustion method. The samples were characterized by X-ray diffractometry, Raman spectrometry, scanning and transmission electron microscopy, N₂-physisorption and inductive coupled plasma-atomic emission spectroscopy. The samples have shown prominent activities in oxidation of cyclopentene using 30% hydrogen peroxide as oxidant. When compared to the pristine CoFe₂O₄, the Ca-doped ones were more efficient having a conversion ranging from 72.1% to 100% and a H₂O₂ utilization efficiency range of 80.0–84.7%. When the calcium content was 0.3 and above, the cyclopentene was quantitatively converted to cyclopentenone at 60 °C for 8 h reaction. More interestingly, the catalyst has shown great solvent selectivity with respect to the product distribution. Finally, the catalyst can be easily separated magnetically for reuse and no obvious loss of activity was observed after five consecutive runs.

Full text of DOI:10.1016/j.poly.2018.07.037

Accepted Manuscript
Highly efficient oxidation of cyclopentene catalyzed by magnetically recover-
able Ca-Co ferrite spinels with high solvent selectivity
Lili Bo, Yusen Hu, Zhixia Zhang, Mannix Balanay, Haiyan Fan, Jinhui Tong
PII:
S0277-5387(18)30433-9
https://doi.org/10.1016/j.poly.2018.07.037
POLY 13302
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
18 April 2018
10 July 2018
22 July 2018
Please cite this article as: L. Bo, Y. Hu, Z. Zhang, M. Balanay, H. Fan, J. Tong, Highly efficient oxidation of
cyclopentene catalyzed by magnetically recoverable Ca-Co ferrite spinels with high solvent selectivity,
Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.07.037
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Highly efficient oxidation of cyclopentene catalyzed by
magnetically recoverable Ca-Co ferrite spinels with high
solvent selectivity
Lili Bo a,*, Yusen Hu a, Zhixia Zhang a, Mannix Balanay b, Haiyan Fan b, Jinhui Tong c,*
a Institute of Agricultural Resource Chemistry and Applications, College of Science, Gansu
Agricultural University, Lanzhou, Gansu 730070, China.
b Department of Chemistry, School of Science and Technology , Nazarbayev University, Astana
010000, Kazakhstan.
cKey Laboratory of Polymer Materials of Gansu Province, Key Laboratory of
Eco-Environment-Related Polymer Materials Ministry of Education, College of Chemistry and
Chemical Engineering, Northwest Normal University, Lanzhou, Gansu 730070, China.
* Corresponding Authors: boxl6666@163.com (Dr. Lili Bo)
jinhuitong@126.com (Dr. Jinhui Tong)
Abstract
The calcium doped CoFe2O4 spinels CaxCo1-xFe2O4 (x = 0, 0.1, 0.3, 0.5 and 0.7)
were prepared by simple sol-gel auto-combustion method. The samples were
characterized by X-ray diffractometry, Raman spectrometry, scanning and
transmission electron microscopy, N2-physisorption and inductive coupled plasma–
atomic emission spectroscopy. The samples have shown prominent activities in
oxidation of cyclopentene using 30 % hydrogen peroxide as oxidant. When compared
to the pristine CoFe2O4, the Ca-doped were more efficient having a conversion
ranging from 72.1 to 100 % and a H2O2 utilization efficiency range of 80.0 to 84.7 %.
When the calcium content was 0.3 and above, the cyclopentene was quantitatively
converted to cyclopentenone at 60 oC for 8 hours reaction. More interestingly, the
catalyst has shown great solvent selectivity with respect to the product distribution.
1

Finally, the catalyst can be easily separated magnetically for reuse and no obvious
loss of activity was observed after five consecutive runs.
Keywords: Sol-gel auto-combustion; Magnetic nanocrystals; Ca-Co composite
ferrites; Cyclopentene oxidation; Solvent selectivity
1. Introduction
The increased industrial pyrolysis processes raise the problem of comprehensive
use of their products, in particular, cyclopentene[1]. The ineluctable amount of
cyclopentene has been obtained as a main by-product of the C5 fraction in the
petrochemical or coking industry[2, 3]. Thus, various researches have been introduced
to be able to convert cyclopentene to more useful functionalized products, in which
mostly employed the selective oxidation process to produce cyclopentene oxide,
cyclopentanone, glutaraldehyde and cyclopentenone. These products are widely
utilized in the production of macromolecular compounds, plasticizers, fragrances,
pharmaceuticals, bactericides, and agricultural chemicals[1, 4]. In past decade, most
of the efforts were on the oxidation of cyclopentene to glutaraldehyde. So far,
majority of the reported catalysts are tungsten based, such as supported WO3 [3, 5, 6]
tungsten-substituted molybdophosphoric acids[4], phosphotungstic quaternary
,
ammonium salt[7], tungsten-containing mesoporous material[8] and others. To our
best knowledge, little attention has been paid in the allylic oxidation of cyclopentene.
From the “green chemistry” point of view, it is critical to develop a highly active,
selective and recyclable catalysts for heterogeneous processes to replace those
homogeneous reactions which introduces serious environmental problems[9]. One
example is the use of magnetically-recyclable catalysts that was able to combine the
advantages of high activity and easy recycling in heterogeneous catalytic
processes[10]. Among the magnetically-recyclable catalysts, the magnetic spinel
ferrites have attracted more attention due to their controllable structure, magnetism
2

and catalytic properties[11-14]. These ferrite composite oxides have been widely used
in oxidation reactions, such as oxidative conversion of methane[15], oxidation of
alcohols[16], toluene[17], cyclohexane[18], styrene[19], and monoterpenic alkenes[20]
among others.
It is well known that the properties of the ferrites strongly depend on its
compositions[21-23] which can be easily manipulated, in which the development of
new ferrites is focused on the doping with various metals[24-29] that is commonly
prepared using sol–gel auto-combustion method which combines the advantages of
chemical sol–gel and combustion processes through a thermally induced anionic
redox reaction. The process takes the advantages of inexpensive precursors, simple
process, and produces highly reactive nanosized powder[30-32].
In our previous work[18, 19], Cobalt and Mg–Cu ferrite oxides nanocrystals were
synthesized through sol–gel auto-combustion method and the samples were proven to
be the most active and easily recoverable catalysts in the oxidation of cyclohexane
and styrene. As part of our interest in hydrocarbon oxidation catalyzed by spinel
ferrites, we are reporting here alkaline earth metal Ca-doped cobalt ferrites that can
efficiently catalyze the oxidation of cyclopentene. Interestingly, the catalysts showed
high solvent selectivity, wherein the product distribution can be tuned by simply using
different solvents.
2. Experimental
2.1. Materials and equipments
All reagents are of analytical grade and were used as received. Visible Raman
spectra were recorded on a Jobin–Yvon U1000 scanning double monochromator with
a spectral resolution of 4 cm-1. The line at 532 nm from a DPSS 532 Model 200 532
nm single-frequency laser was used as the excitation source. X-ray powder diffraction
(XRD) patterns of the samples were collected using a PANalytical X’Pert Pro
diffractometer with Cu Kα radiation. Scanning (SEM) and transmission (TEM)
electron microscopy images were obtained using a JSM-5600LV and Hitachi H-600
3

microscope, respectively. The Brunauer–Emmett–Teller (BET) surface area
measurements were performed on a Micromeritics ASAP 2010 instrument at liquid
nitrogen temperature. The metal content was determined by inductive coupled plasma
–atomic emission spectrometer (ICP-AES) on a PerkinElmer ICP/6500. The oxidation
products were determined by an HP 6890/5973 GC/MS instrument and quantified by
a Shimadzu GC-2010 gas chromatograph using toluene as internal standard.
2.2. Preparation of the catalysts
The alkaline earth metal Ca doped cobalt ferrite samples CaxCo1-xFe2O4 (x = 0,
0.1, 0.3, 0.5, 0.7 and 1.0) were prepared by sol-gel auto-combustion route under
optimized conditions reported in our previous work[18] and were designated as
CFO-Ca0.1, CFO-Ca0.3, CFO-Ca0.5, CFO-Ca0.7 and CaFe2O4, respectively.
2.3. Oxidation of cyclopentene
The selective oxidation of cyclopentene was carried out in a 25 mL Schlenk tube.
In a typical procedure, 0.06 mmol of catalyst, 2.0 mL (22.6 mmol) of cyclopentene,
10 mL of solvent and 3.5 mL of 30 % hydrogen peroxide, molar ratio of
cyclopentene/H2O2 = 2:3, were added successively into the flask. The flask was then
immersed in an oil bath at desired temperature for desired reaction time under stirring.
The products were identified by gas chromatrography-mass spectrometry (GC-MS)
and quantified by GC using toluene as internal standard. Four products of
cyclopentene oxide (CPEO), cyclopentanone (CPTA), cyclopentenone (CPTE) and
glutaraldehyde (GTA) was detected.
3. Results and discussion
3.1. Characterization of the catalysts
Figure 1 shows the XRD patterns of the samples, wherein only the spinel phase
was found in low Ca-content samples such as CFO-Ca0.1 and CFO-Ca0.3. This
indicates that Ca was doped in the lattices of CoFe2O4 spinel. The diffraction peak
around 2θ value of 18.2°, 30.3°, 35.4°, 37.3°, 43.2°, 53.4°, 57.2°, 62.1° and 74.2° can be
ascribed to the diffractions of (111), (220), (311), (222), (400), (422), (511), (440) and
4

(533) of the spinel CoFe2O4 (International Centre for Diffraction Data (ICDD)
Number 221086). However, in the samples of CFO-Ca0.5 and CFO-Ca0.7 having high
Ca-content, CaFe2O4 phase (ICDD No. 72-1199) was also found aside from the spinel
phase, which suggests that at this high calcium concentration resulted to the
replacement of some cobalt with calcium in the spinel phase. The mean particle sizes
of the samples based on the Scherrer equation are 27, 30, 28, 26 and 29 nm for the
sample CoFe2O4, CFO-Ca0.1, CFO-Ca0.3, CFO-Ca0.5 and CFO-Ca0.7, respectively.
(311)
CaFe2O4
*
(440)
(220)
(511)
(422)
CoFe2O4
(400)
(222)
(533)
(111)
CFO-Ca0.1
CFO-Ca0.3
CFO-Ca0.5
CFO-Ca0.7
*
*
*
*
*
*
10
20
30
40
50
60
70
80
2(degree)
Figure 1. The XRD patterns of the as-prepared samples.
The Raman spectra of as-prepared samples along with those of CoFe2O4 are
displayed in Figure 2. CoFe2O4 crystallizes in cubic spinel structures which yield 6
Raman active modes (A1g (A1g*) + Eg + 3T1g), wherein the highest frequency was split
into two modes due to cation inversion. The six modes are clearly seen in Figure 2
consistent with the literature[33, 34]. These Raman active modes are divided into two
configuration sites, the tetrahedral (> 600 cm–1) and the octahedral (< 600 cm–1)
metal-oxygen bonding sites[35]. When calcium was added to the spinel structure,
5

three main Raman active modes at 304 cm–1 (Eg), 464 cm–1 (T1g), and 688 cm–1 (A1g)
can still be seen on the as-prepared samples. As the concentration of Ca increases, the
A1g is broaden and its position shifts to a lower frequency. The frequency red shift of
A1g peak implies that there was an elongation of Fe-O bond, which was induced by
doping of Ca. The peak broadening is due to the decrease in the grain size caused by
the doping. This is consistent with the XRD results which is also reflected in Zn2+
doped spinel ferrites (ZnxCo1-xFe2O4), wherein the octahedral sites were enlarged
when Co2+ were replaced by Fe2+ ions, due to the exchange reaction of Co2+ + Fe3+
Co3+ + Fe2+, while Zn is located at the tetrahedral sites[36]. Thus, it is reasonable
to assume in the present work that Co2+ and Fe2+ are distributed at the octahedral sites,
and Ca2+ and Fe3+ are distributed at the tetrahedral sites.
Octahedral sublattice
Tetrahedral sublattice
A1g
T1g
A1g*
T1g
E1g
T1g
CoFe2O4
CFO-Ca0.1
CFO-Ca0.3
CFO-Ca0.5
CFO-Ca0.7
100
200
300
400
500
600
700
800
900
Raman shift (cm-1)
Figure 2. The Raman spectra of the as-prepared samples and CoF2O4 nanoparticles.
6

Figure 3. The SEM and TEM images of the sample CFO-Ca0.3.
The SEM and TEM images of the sample CFO-Ca0.3 are shown in Figure 3. The
sample CFO-Ca0.3 features nanoparticles with an irregular morphology and a broad
particle size distribution ranging from 10-20 to 40-60 nm, with a high percentage of
small particles (30-40 nm). Other samples showed similar morphologies to the sample
CFO-Ca0.3, although their particle sizes are different based on the calculation with the
Scherrer equation using the XRD patterns.
N2-physisorption measurements were also performed for the as-prepared samples
and the N2 adsorption-desorption isotherm of CFO-Ca0.3 was shown in Figure 4 as
representative. As can be seen, the sample exhibited type III isotherm with H3
hysteresis loop, which indicates existence of piled macropores and slips in the sample.
The BET surface area of the sample CFO-Ca0.1, CFO-Ca0.3, CFO-Ca0.5, CFO-Ca0.7
and CaFe2O4 is 36.1, 35.3, 34.8, 37.2 and 38.1 m2/g respectively. For comparison
purpose, the pristine CoF2O4 was also prepared under the same conditions and the
sample exhibited BET surface area of 35.8 m2/g. The metal content of the samples
determined by ICP-AES are listed in Table 1. As can be seen, the determined metal
contents are consistent with the given formula.
7

16
14
12
10
8
CFO-Ca0.3
6
4
2
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
Figure 4. The N2 adsorption-desorption isotherm of CFO-Ca0.3.
Table 1. Metal content of the samples
Catalyst
Ca content (wt %)
Co content (wt %)
Fe content (wt %)
47.63
CoFe2O4
CFO-Ca0.1
CFO-Ca0.3
CFO-Ca0.5
CFO-Ca0.7
CaFe2O4
–
25.16
22.72
18.06
13.01
8.02
–
1.68
5.30
8.87
12.71
18.55
47.93
48.70
49.50
50.36
51.80
3.2. Catalysis tests
The results of cyclopentene oxidation over the as-prepared samples are listed in
Table 2. As shown in the table, all six samples can efficiently catalyze the oxidation of
cyclopentene and CPTE was found to be the major product in all the Ca contained
samples. It was observed that increasing the Ca content favors the production of
CPTE and the selectivity of CPTE reached 100% when the calcium content is above
8

0.3. As for cyclopentene conversion, it increased firstly but then decreased with
increase of Ca content, and the 100% conversion was obtained with CFO-Ca0.3. The
selectivity of both CPEO and CPTA decreased with the further increase in Ca content
and no CPTA was detected for any Ca contained samples. With respect of GTA
selectivity, only very low GTA has been detected with CFO-Ca0.1 and CFO-Ca0.3
samples. It was hypothesized that the activity of spinels depends on the ability of the
metallic ions to migrate between the sublattices without altering the crystal
structure[22]. We hypothesized that the migration of iron ions was made easier as the
cobalt ions were partially substituted by larger Ca ions. Considering both
cyclopentene conversion and the selectivity for CPTE, the sample CFO-Ca0.3 was
chosen as catalyst to optimize the reaction conditions and investigate the recyclable
performance of the catalyst. As can be seen, the sample CFO-Ca0.3 also obtained the
highest H2O2 utilization efficiency and turnover number (TON) of 84.7% and 377
respectively.
Table 2. Oxidation of cyclopentene over the as-prepared samples a
H2O2 utilization
Efficiency
(%)
Selectivity (%)c
Conversion
(%)
Entry Catalyst
TONb
CPEO CPTA CPTE GTA
1
2
3
4
5
6
CoF2O4
58.1
76.3
219
332
377
321
272
246
15.6
12.9
−
5.7
−
78.7
85.9
99.5
100
100
100
−
CFO-Ca0.1 88.2
CFO-Ca0.3 100
CFO-Ca0.5 85.2
CFO-Ca0.7 72.1
83.1
1.2
0.5
−
84.7
−
82.6
−
−
80.0
−
−
−
CaF2O4
65.3
78.4
−
−
−
a Reaction conditions: cyclopentene 2.0 mL (22.6 mmol); catalyst 0.06 mmol; DMF
10 mL; 30 % hydrogen peroxide 3.5 mL ( molar ratio of cyclopentene/H2O2 = 2:3); 60
oC, 8h; b TON = molar of substrate converted per mol catalyst; c CPEO = cyclopentene
epoxide, CPTA = cyclopentanone, CPTE = cyclopentenone, GTA = glutaraldehyde.
9

3.3. Effect of the solvent
Table 3 shows the effects of different solvents on cyclopentene conversion and
products distribution over the sample CFO-Ca0.3. It is clear that the solvent has great
influence on cyclopentene conversion and the products distribution. The highest
cyclopentene conversion and TON of 100% and 377 was achieved in
dimethylformamide (DMF) respectively. Interestingly, the selectivity of products
exhibited strong dependence on the solvents. As shown in Table 3, as high as 99.5%
of selectivity for CPTE was obtained when DMF was used as solvent. Up to 86.7% of
selectivity for CPEO in 1,4-dioxane and 100% of selectivity for CPTA in toluene were
observed. Such high selectivity for CPTE, CPEO and CPTA when using different
solvents has rarely been observed especially with this high conversion rate. However
when acetonitrile (CH3CN) and tert-butanol were used as solvents, the conversion
decreased to below 50% and the selectivity of each product was below 45%.
Table 3. Effect of solvent on cyclopentene oxidation a
H2O2
Selectivity (%)c
Conversio utilizatio
TON
Entry Solvent
n
n
CPE
O
CPT
A
CPT
E
GT
A
b
(%)
efficiency
(%)
1
2
3
4
−
16.2
100
30.1
84.7
72.9
83.0
61
78.5
−
3.3
−
–
8.2
0.5
11.3
−
DMF
CH3CN
Toluene
1,4-dioxan
e
377
183
330
99.5
44.1
−
48.5
87.6
28.8
−
15.8
100
5
6
76.7
81.0
66.5
289
135
86.7
25.3
5.2
8.1
−
tert-butanol 35.8
15.3
42.2
17.2
10

a Reaction conditions: cyclopentene 2.0 mL (22.6 mmol); catalyst 0.06 mmol; solvent
10 mL; 30 % hydrogen peroxide 3.5 mL ( molar ratio of cyclopentene/H2O2 = 2:3); 60
oC, 8h; b TON = molar of substrate converted per mol catalyst; c CPEO = cyclopentene
epoxide, CPTA = cyclopentanone, CPTE = cyclopentenone, GTA = glutaraldehyde.
Pathway II
H2O2
DMF
catalyst
OOH
O
T
o
l
u
e
CPTE
GTA
n
Pathway I
e
catalyst
C5H8
/
1
,
4
-
d
i
o
catalyst-OOH
x
a
n
e
catalyst-H
catalyst
H2O2
O
O
O
O
CPTA
CPEO
Scheme 1. Proposed pathways for the catalytic oxidation of cyclopentene.
The solvent effect observed here is rather interesting. The oxidation of
cyclopentene by H2O2 generally undergoes in two distinctive pathways (Scheme 1). It
can occur either on the carbon-carbon double bond to form epoxide or on the allylic
C-H bond resulting in 3-hydroperoxycyclopent-1-ene[37-39]. As presented in Table 3,
although with a very low conversion (16.2%), the dominant product for the
solvent-free reaction is an epoxide which is shown as pathway I in Scheme 1.
It is apparent that in toluene and 1, 4-dioxane, the oxidation is also via
carbon-carbon double bond. The epoxide can be further converted to ketone in the
presence of the catalyst. The cobalt catalyst was proposed to form a cationic
intermediate with epoxide[40] and nonpolar solvent like toluene accelerate the
conversion from the cationic intermediate to ketone product[41]. The source of the
hydrogen needed to convert the epoxide to a ketone might be coming from the toluene
11

via the Yamase/Hill mechanism that will result in the formation of a dimer[42].
However, no trace of toluene dimer was observed under the present condition. For the
case of oxidation of cyclopentene in DMF, it would probably follow pathway II
(Scheme 1). DMF would likely form a strong complex with the catalyst as observed
in synthesis of cobalt oxide/N-doped grapheme nanohybrids (CNG) in DMF wherein
it forms very strong coordination complexes[43]. The oxidation reduction reaction
onset potential of CNG-DMF was 0.919 V, close to that of Pt/C (1.005V). The
oxidation of cyclopentene in DMF is likely through the allylic C-H bond interacting
with the H2O2 to form cyclopentene hydroperoxide and further oxidation to
cyclopentenone[37].
Before convincing evidences are obtained, as for the increasing Ca ions doping
causing higher selectivity to cyclopentenone in DMF solvent, a possible explanation
is that DMF has tendency to coordinate with cobalt ions in the catalyst through its
C=O group at the reaction temperature to form Co-DMF complex [43]. The
coordination lead to introduction of nitrogen in the Co-DMF complex and therefore
offering active centers where 3-hydroperoxycyclopent-1-ene can be adsorbed by
Co-DMF complex to form an intermediate. On the other hand, partially substitution of
Co ions in the spinel by larger Ca ions will make Co ions more accessible to DMF
molecule and therefore supply more active centers for 3-hydroperoxycyclopent-1-ene
absorption to form more intermediates. Adsorbed by Co-DMF complex may also
facilitate rearrangement of 3-hydroperoxycyclopent-1-ene in the intermediates to form
cyclopentenone [44-45]. The above possible reasons can account for increasing
cyclopentenone selectivity with increase in Ca doping. We are trying our best
investigating the reaction mechanism and more useful results will be reported in our
future works.
Based on Scheme 1, the cyclopentene hydroperoxide has also a tendency to form
an epoxide but was not observed in our results. This is probably due to the full
conversion of the epoxide to the glutaraldehyde. In order to confirm this hypothesis,
12

we are currently undertaking a computational study to be published as a separate
paper. Based on the above facts, DMF was selected as the proper solvent for
following investigations in pursuit of high cyclopentene conversion and CPTE
selectivity. The highest H2O2 utilization efficiency of 84.7% was also obtained in the
case of DMF.
3.4. Effect of reaction temperature
The temperature dependence of the cyclopentene oxidation was tested in the range
of 50 to 90 °C for 8 h reaction and the results are listed in Table 4. The conversion
increases with the increasing temperature and reaches 100% at 60 oC. However, the
H2O2 utilization efficiency decreases with the temperature. The selectivity for CPTE
reaches the highest at 60 oC and decreases at higher temperature. Although the
selectivity for GTA is highest at 90 oC, the utilization efficiency and the selectivity for
CPTE are both lowest at this temperature. In order to obtain high selectivity of CPTE,
60 °C is considered as proper reaction temperature.
13

Table 4. Effect of reaction temperature on cyclopentene oxidation a
Reaction
H2O2
Selectivity (%) b
Conversion
(%)
temperature
utilization efficiency
CPEO CPTA CPTE GTA
(oC)
(%)
50
95.3
100
100
100
100
85.8
84.7
78.7
76.0
72.7
12.2
−
−
87.8
99.5
95.4
88.3
76.6
−
60
−
0.5
2.7
5.5
8.7
70
−
1.9
6.2
14.7
80
−
90
−
a Reaction conditions: cyclopentene 2.0 mL (22.6 mmol); catalyst 0.06 mmol; DMF
10 mL; 30% hydrogen peroxide 3.5 mL ( molar ratio of cyclopentene/H2O2 = 2:3); 8h;
b CPEO = cyclopentene epoxide, CPTA = cyclopentanone, CPTE = cyclopentenone,
GTA = glutaraldehyde.
3.5. Effect of cyclopentene /H2O2 molar ratio
The effects of molar ratio cyclopentene/H2O2 were investigated and the results are
shown in Table 5. The increase in the amount of H2O2 is in favor of conversion of
cyclopentene and selectivity of GTA. As the molar ratio of cyclopentene/H2O2
increased from 2:1 to 2:3, the conversion of cyclopentene increased 64.4 %. Although
the selectivity of GTA increased as the molar fraction of H2O2 increases, the
selectivity of CPTE is highest at the molar ratio cyclopentene/H2O2 of 2:3, and the
utilization efficiency of H2O2 reaches maximum at the molar ratio cyclopentene/H2O2
of 1:1. Finally, to ensure a high conversion and high selectivity for CPTE, but not to
sacrifice the H2O2 utilization efficiency, the molar ratio cyclopentene/H2O2 of 2:3 was
selected as the optimum molar ratio of cyclopentene to H2O2.
14

Table 5. Effect of H2O2 amount on cyclopentene oxidation a
Cyclopentene/
H2O2
Selectivity (%) b
Conversion
(%)
H2O2
(mol/mol)
2:1
utilization efficiency
CPEO CPTA CPTE GTA
(%)
35.6
87.1
100
100
100
87.6
88.0
84.7
76.9
69.0
13.8
8.7
−
7.8
4.6
−
78.4
86.7
99.5
94.0
84.1
−
1:1
−
2:3
0.5
6.0
15.9
1:2
−
−
2:5
−
−
a Reaction conditions: cyclopentene 2.0 mL (22.6 mmol); catalyst 0.06 mmol; DMF
10 mL; 60 oC, 8h; b CPEO = cyclopentene epoxide, CPTA = cyclopentanone, CPTE =
cyclopentenone, GTA = glutaraldehyde.
3.6. Effect of reaction time
The effect of reaction time on cyclopentene oxidation was investigated at 60 oC in
the range of 2-10 hours at the cyclopentene/H2O2 molar ratio of 2:3. As shown in
Table 6, the conversion of cyclopentene increases with the increase in the reaction
time. The utilization efficiency of H2O2 does not exhibit strong reaction time
dependence. The selectivity of CPTE is highest at the reaction time of 8 hours. Within
the short reaction time (2 hours), there is significant amount of CPEO (16.5%) and
CPTA (15.1%) other than CPTE (68.4%). As the reaction time prolongs, the
selectivity of CPTE increases and reaches the highest at the reaction time of 8 hours.
Further extending the reaction time from 8 hours leads to the conversion from CPTE
to GTA. Therefore, 8 hours was selected as optimum reaction time.
15

Table 6. Effect of reaction time on cyclopentene oxidation a
H2O2
Selectivity (%) b
Reaction
time (h)
Conversion
(%)
utilization efficiency
CPEO CPTA CPTE GTA
(%)
2
56.2
71.8
87.1
100
80.4
81.7
82.9
84.7
81.6
16.5
13.7
6.7
−
15.1
10.6
4.9
−
68.4
75.7
88.4
99.5
93.5
−
4
−
6
−
8
0.5
6.5
10
100
−
−
a Reaction conditions: cyclopentene 2.0 mL (22.6 mmol); catalyst 0.06 mmol; DMF
10 mL; 30 % hydrogen peroxide 3.5 mL ( molar ratio of cyclopentene/H2O2 = 2:3); 60
oC; b CPEO = cyclopentene epoxide, CPTA = cyclopentanone, CPTE =
cyclopentenone, GTA = glutaraldehyde.
3.7. Reuse of the catalyst
After the reaction, the catalyst can be adsorbed on the magnet. The catalyst
attached to the magnet can be easily separated using simple decantation by applying a
magnetic field on the surface of the tube, and then subjected to the second run under
the same conditions. The results of these experiments are shown in Figure 5. Both the
selectivity of CPEO and the conversion of cyclopentene barely changed after five runs.
The slight decrease in cyclopentene conversion can be mainly attributed to
unavoidable loss of the catalyst during the process of collection. The average value of
cyclopentene conversion and CPTE selectivity was 99.4 % and 99.7 %, respectively.
The results confirm that the sample CFO-Ca0.3 possesses good stability and recyclable
applicability for the oxidation of cyclopentene under our present laboratory
conditions.
16

Conversion
Selectivity
100
80
60
40
20
0
1
2
3
4
5
Reaction run
Figure 5 Reuse of the catalyst CFO-Ca0.3.
Reaction conditions: cyclopentene 2.0 mL (22.6 mmol); catalyst 0.06 mmol; DMF 10
mL; 30 % hydrogen peroxide 3.5 mL ( molar ratio of cyclopentene/H2O2 = 2:3); 60 oC,
8h;
4. Conclusions
Nanosized Ca-Co ferrite oxides were prepared through a simple and effective
route of sol-gel auto-combustion using less expensive precursors. The ferrite catalysts
are more active and easily reusable catalysts for cyclopentene oxidation using
hydroperoxide as green oxidant. The catalytic performances of the samples are highly
related to their components. The Ca contained samples have shown much higher
cyclopentene conversion and cyclopentenone selectivity, and the cyclopentenone
selectivity increased with increasing Ca content. Interestingly, the products
distribution showed strong dependence on the solvents, that is, special product can be
obtained by selecting special solvent. The highest cyclopentene conversion of 100%
and cyclopentenone selectivity of 99.5% was achieved with the catalyst sample
Ca0.3Co0.7Fe2O4 in DMF solvent. The catalyst has shown prominent recyclability and
no drop in activity was observed in five consecutive runs. This work, along with other
17

published ones, indicates that ferrite complex oxides have a considerable potential for
becoming a kind of tunable catalysts with respect of catalytic performances and
components. Based on this purpose, more efforts have been and will be focused on
investigating structure-activity relationships of the ferrite complex oxides in catalytic
oxidation reactions in our present and future work.
Acknowledgements
The authors are grateful to the National Natural Science Foundation of China
(21363021, 51302222), Natural Science Foundation of Gansu Province
(1308RJYA017), and Program for Changjiang Scholars and Innovative Research
Team in University (IRT15R56) for financial support.
Supporting information
1H NMR data of the reaction products were shown in supporting
information.
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Graphical abstract
O
(86.7%, In 1,4-dioxane)
O
CaxCo1-xFe2O4
by-products
+
(99.5%, In DMF)
H2O2
x=0.1, 0.3, 0.5, 0.7 and 1
H2O
O
(100%, In toluene)
In this work, Ca-doped spinel Co ferrites were prepared by a facile, mild and
environment-friendly route. The samples exhibit remarkably high activities in the
oxidation of cyclopentene using H2O2 as green oxidant. The Ca-doped samples have
shown greatly enhanced performances than the pristine CoFe2O4. More interestingly,
23

the products distribution showed strong dependence on the solvents, that is, special
product can be obtained by selecting special solvent. The highest cyclopentene
conversion of 100% and cyclopentenone selectivity of 99.5% was achieved with the
catalyst sample Ca0.3Co0.7Fe2O4 in DMF solvent.
24