Epoxidation of cyclohexene with O2 over the composite catalysts of Mn-montmorillonite coordinated with novel Schiff-base ligands

Article abstract of DOI:10.1039/c3ra42749h

The catalytic activity of the heterogeneous catalysts of Mn-mont (where mont represents montmorillonite) coordinated with Schiff-base ligands for epoxidation of cyclohexene by O₂ under Mukaiyama conditions was studied. The catalysts were characterized by IR, UV-vis DRS, XRD, SEM and ICP. The heterogeneous catalysts were proved to promote the epoxidation of cyclohexene efficiently, the effects of various reaction conditions on the catalytic reaction were optimized and obtained as high as 100% conversion with 90.0% selectivity of epoxycyclohexane at 40 °C in 5 h with molecular oxygen as oxidant in acetonitrile. Repeated runs indicated that the catalysts were stable for at least 3 cycles. Thus, the heterogeneous catalysts of Mn-mont coordinated with Schiff-base ligands were proved to be environmentally friendly and economical for the epoxidation of cyclohexene. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra42749h

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Epoxidation of cyclohexene with O2 over the composite
catalysts of Mn-montmorillonite coordinated with novel
Schiff-base ligands
Cite this: RSC Adv., 2013, 3, 19965
a
b
b
*
Yu peng, Zhongying Li and Ruiren Tang
The catalytic activity of the heterogeneous catalysts of Mn-mont (where mont represents montmorillonite)
coordinated with Schiff-base ligands for epoxidation of cyclohexene by O2 under Mukaiyama conditions
was studied. The catalysts were characterized by IR, UV-vis DRS, XRD, SEM and ICP. The heterogeneous
catalysts were proved to promote the epoxidation of cyclohexene efficiently, the effects of various
reaction conditions on the catalytic reaction were optimized and obtained as high as 100% conversion
with 90.0% selectivity of epoxycyclohexane at 40 ꢀC in 5 h with molecular oxygen as oxidant in
acetonitrile. Repeated runs indicated that the catalysts were stable for at least 3 cycles. Thus, the
heterogeneous catalysts of Mn-mont coordinated with Schiff-base ligands were proved to be
environmentally friendly and economical for the epoxidation of cyclohexene.
Received 4th June 2013
Accepted 12th August 2013
DOI: 10.1039/c3ra42749h
www.rsc.org/advances
silica layers.18 Montmorillonite is a cheaper support compared
with the others, therefore, using montmorillonite-based materials
as catalysts is more economical for industry.
1 Introduction
The epoxidation of cyclohexene has attracted much attention since
its epoxide compound is a key intermediate in organic synthesis of
ne chemicals.1–3 In the recent years, many efforts had been
dedicated to the development of routes for the preparation of
epoxides using efficient catalysts under environmental friendly
conditions. Several active metallo-porphyrins, phthalocyanines
and other metal complexes such as transition metaldiketonate,
Schiff base transition metal complexes4–6 etc., can be used as
catalysts for oxidation of cyclohexene with PhIO, t-BuOOH, H2O2
and molecular oxygen.7,8 The exploitation of these metal
complexes in homogeneous catalytic conditions is inconvenient
because they can not be reused and their activities are limited to a
few catalytic turnovers. Compared with homogeneous catalysts,
the heterogenization of the metal complexes by chemical bonding
to a supporter leads to an increase of their stability and makes it
easier for the separation of the catalysts from the reaction mixture
and recycling. For this purpose, a variety of supporters have been
tested, such as: silica,9 zeolites,10 polymers,11 nanoporous carbon,12
resins13 and clays.14–17 Clays, such as montmorillonite, due to their
large surface area and cation exchange capacity, have considerable
potential supporters for the immobilization of metal complexes in
recent years. Chemically, montmorillonite is hydrated alumino-
silicate of idealized formulation Al2Si4O10(OH)2 (mineral pyro-
phyllite) and its crystalline structure is generated by the
encapsulation of a gibbsite layer (polymorph of Al(OH)3) with two
Employing oxidant reagents which maximize atom efficiency
is a key goal in oxidation reactions. Molecular oxygen is a
cheaper, greener and more readily convenient oxidant for the
epoxidation of cyclohexene and its derivatives, but it is relatively
unreactive toward the strong bond of C–H unless activated by
highly efficient catalysts19 or severe reaction conditions (high
reaction temperature and pressure) are required.14 So it is more
interesting from an academic and industrial point of view to
ameliorate the reaction conditions (mild reaction conditions)
maintaining high catalytic activity through a clean and environ-
mentally friendly process.
Recently,
montmorillonite-based
Schiff-base
metal
composite solid materials as catalysts have been widely applied
to catalyze the oxidation of alkenes.15–18,20 For example, Guru
et al.21 reported that lanthanum salen complex heterogenized in
iron pillared montmorillonite was identied to promote the
cyclohexene epoxidation reaction very efficiently with tert-
butylhydroperoxide (TBHP) as the oxidant, affording the
desired product with a yield up to 95%. Jian Jiang et al.7
reported that the Co(II) salophen complexes immobilized into
montmorillonite as catalysts for the epoxidation of cyclohexene,
the heterogeneous catalysts were found to be efficient catalysts
for the oxidation of cyclohexene (94% conversion and 82%
selectivity of epoxycyclohexane). Unfortunately, these methods
consumed a large amount of TBHP or isobutyraldehyde, which
did not accord with the request of green industrialization.
The aim of the present work is to obtain a better catalyst for
the epoxidation of cyclohexene in friendly process. To pursue this
aScience College of Hunan Agricultural University, Hunan, 410128, China
bSchool of Chemistry and Chemical Engineering, Central South University, Hunan,
410083, China. E-mail: trr@mail.csu.edu.cn; Fax: +86-731-88879616; Tel: +86-731-
88836961
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Scheme 1 Route for the preparation of the catalysts.
goal, three Schiff-base ligands N,N-(2,6-pyridinediyldimethy- 2450 spectrophotometer. The metal loading and leaching of the
lidyne) bis(2-hydroxy-4-methylbenzenamine) (L1), 1,3-bis(2-hy- reaction solution were determined by ICP with ICP-AES 5300D.
droxyphenyl)naphthaldiimine (L2) and 1,2-bis(2-hydroxyphenyl)- ESI-MS were measured with Micromassꢀ ZQꢁ4000.
naphthadiimine (L3) were synthesized, as depicted in Scheme 1.
Subsequently, three Schiff-base ligands were coordinated with
Mn-mont to yield several montmorillonite composite catalysts,
2.2 Synthetic procedure
2.2.1 Preparation of Schiff-base ligands. As shown in
Scheme 1, all ligands were prepared in a similar manner
according to the reported procedures22–24 and here the synthesis
of L1 was given as follow. About 50 mL absolute ethanol solution
of 2,6-pyridinedicarbaldehyde (7.4 mmol, 1.0 g) was added
dropwise over 1.5 h into 45 mL warm absolute ethanol solution
of 2-amino-5-methylphenol (14.8 mmol, 1.8 g) while stirring.
which were applied to catalyze the epoxidation of cyclohexene
under Mukaiyama conditions using molecular oxygen as the
oxidant. And then the stability and catalytic performance of these
catalysts were investigated in the autoxidation of cyclohexene and
the reaction parameters in the system were optimized. It was
observed that Mn-mont (L1) showed an excellent catalytic activity
for the epoxidation of cyclohexene with molecular oxygen under
Mukaiyama conditions.
ꢀ
Then the resulting mixture was reuxed at 80 C for 4 h. Aer
cooling the yellow crystal L1 was ltered from the ethanol
solution, washed with cooled absolute ethanol, recrystallized
from ethanol and then dried overnight in a vacuum desiccator.
L2 and L3 were synthesized in a similar process using the cor-
2 Experimental
2.1 Materials and methods
All the reagents were purchased in analytical grade in China. responding aldehydes and amines. These Schiff-base ligands
1
Melting points were determined on a XR-4 apparatus (ther- were characterized by IR and H NMR.
mometer uncorrected). Infrared spectra were recorded on a
L1 N,N-(2,6-pyridinediyldimethylidyne)bis(2-hydroxy-4-methyl-
Nicolet NEXUS 670 FT-IR spectrophotometer using KBr discs in benzenamine). Yield: yellow solid (2.09 g, 82%). mp 214–216 ꢀC;
the 400–4000 cmꢁ1 region. 1H-NMR spectra were measured with IR (KBr, cmꢁ1): 3373 (O–H), 1624 (C]N); H NMR (400 MHz,
1
a Bruker-400 MHz nuclear magnetic resonance spectrometer CDCl3): d ¼ 8.88(s, 1H), 8.27 (d, J ¼ 8.0 Hz, 1H), 7.97–7.94 (m,
with CDCl3 as solvent and TMS as internal reference. Elemental 1H), 7.36 (s, 2H), 7.26–6.75 (m, 6H), 2.36 (s, 6H).
analysis was carried out by a Perkin Elmer 2400 elemental
L2 1,3-bis(2-hydroxyphenyl)naphthaldiimine. Yield: yellow
analyzer. Powder X-ray diffraction (XRD) patterns of the solid (2.0 g, 77%). mp 183–185 ꢀC; IR (KBr, cmꢁ1): 3340 (O–H),
heterogeneous samples were obtained on Rigaku D/Max III VC 1629 (C]N); 1H NMR (400 MHz, CDCl3): d ¼ 8.780 (s, 2H), 8.405
diffractometer with Cu KR radiation at 40 kV and 40 mA in the (s, 1H), 8.120 (m, 1H), 8.069–8.050 (m, 2H), 7.701–6.752 (m, 6H),
range of 2q ¼ 2–9ꢀ. The UV-vis spectra were recorded on an UV- 2.370 (s, 6H).
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L3 1,2-bis(2-hydroxyphenyl)naphthaldiimine. Yield: yellow
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solid (1.0 g, 40%). mp 240–244 ꢀC; IR (KBr, cmꢁ1): 3287 (O–H),
1950 (C]N); 1H NMR (400 MHz, CDCl3): d ¼ 8.271 (s, 1H), 8.149
(s, 1H), 7.776 (d, J ¼ 7.6 Hz, 2H), 7.648 (s, 2H), 7.417–6.725 (m,
6H), 2.53 (s, 3H), 2.35 (s, 3H).
2.2.2 Synthesis of montmorillonite composite catalysts. To
prepare the composite catalysts, 1.5 g montmorillonite was
added into a 50 mL water solution containing appropriate
amount of MnCl2$4H2O producing as lurry that was stirred
under reuxed for 24 h. The obtained solid (marked by Mn-
mont) was collected by ltration, washed with water, then dried
ꢀ
in vacuum at 40 C for 24 h.
Then 1.0 g Mn-mont was added into a hot methanol solution
of the Schiff-base L1. The moles of the Schiff-base L1 in the
ethanol solution were equal to the moles of Mn2+ in the water
solution used for preparing the Mn-mont. The mixture was
Fig. 1 Characteristic X-ray diffraction (XRD) (5–70ꢀ) of (a) mont, (b) Mn-mont
ꢀ
and (c) Mn-mont (L1).
reuxed at 80 C for 10 h, then the solid was ltered, washed
with 1 : 1 ethanol : water mixture, and dried in vacuum at 50 ꢀC
to form the catalyst Mn-mont (Lx; where x represents 1, 2, 3).
The route for preparation of the catalyst is shown in Scheme 1.
The other montmorillonite composite catalysts were prepared
in a similar process using the corresponding Schiff-bases. They
were characterized by IR, diffuse reectance ultraviolet visible
spectra, powder-X-ray-diffraction and scanning electron
microscopy measurements.
characteristics to that of the montmorillonite. The corre-
sponding basal spacing value of Mn-mont (L1) is slightly lower
than that of montmorillonite which suggests that, more likely,
the exchange of Na ions with Mn ions leading to the slight
swelling in the interlayer of the montmorillonite, the Schiff-
base covered preferentially the outer surface and in a very low
amount the interlayer space.
3.1.2 FT-IR spectra. The FT-IR spectra of L1, Mn-mont, Mn-
mont (L1) and montmorillonite are given in Fig. 2. The band
emerging at about 3373 cmꢁ1 and 1624 cmꢁ1 in the FT-IR
spectrum of Schiff-base (L1) can be assigned to the vibration of
n(O–H) and n(C]N). The montmorillonite and Mn-mont mainly
exhibit Al–O–H and Si–O–Si stretching vibration bands at 3594
cmꢁ1 and 1034 cmꢁ1, respectively, and H–O–H stretching and
bending vibration bands at 3416 cmꢁ1 and 1636 cmꢁ1. As can be
seen in Fig. 2, the FT-IR spectra of Mn-mont (L1) both display
the typical absorption bands of the Schiff-base (L1) and Mn-
mont, showing the direct evidence of the Schiff-base ligand was
coordinated on the surface of Mn-mont.
2.3 Catalytic epoxidation reaction
A typical reaction was performed in a 50 mL three-necked bottle
with a magnetic stirrer. A mixture of cyclohexene (10 mmol),
isobutylaldehyde (10 mmol) and Mn-mont (L1) (20 mg, 2.0 mmol
%) in acetonitrile (20 mL) was placed into three-necked bottle
and heated to 40 ꢀC under stirring. The reaction started by
bubbling molecular oxygen into the stirred reaction mixture at
atmospheric pressure with a rate of 10 mL minꢁ1. Aer
completion of the reaction, the reactor was cooled to the ambient
temperature. The mixture was dried by anhydrous Na2SO4 and
then analyzed by GC-MS with DB-5 MS column (30 m ꢂ 0.25 mm
ꢂ 0.25 um). The conversion and selectivity were determined by
GC equipped with OV-1701 column (50 m ꢂ 0.25 mm ꢂ 0.25
um), a programmed oven (temperature increased from 60 ꢀC to
ꢁ1
ꢀ
ꢀ
ꢀ
200 C at a rate of 10 C min , and then kept at 200 C for 2
min), with N2 as the carrier gas. n-Heptane was used as an
internal standard. The selectivity of the epoxide (cyclohexene
oxide) was a measure of the reactivity of the catalyst.
3 Results and discussion
3.1 Characterization of the heterogeneous catalyst
All the characterization of the heterogeneous catalyst are
similar to each other, herein, the characterization of montmo-
rillonite and Mn-mont (L1) is given.
3.1.1. Powder X-ray diffraction analysis. The characteristic
X-ray diffraction (XRD) (5–70ꢀ) of mont, Mn-mont and Mn-mont
(L1) are given in Fig. 1. The X-ray powder diffraction (XRD)
patterns of the montmorillonite, Mn-mont and Mn-mont (L1)
Fig. 2 Characteristic IR bands (4000–400 cmꢁ1) of (a) mont, (b) Mn-mont, (c)
reveal that Mn-mont and Mn-mont (L1) have identical structural Mn-mont-L1 and (d) L1.
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Table 1 Effect of different Schiff-base catalysts on epoxidation of cyclohexenea
Epoxide selectivity
(%)
Run
Catalyst
Conversion (%)
A
B
C
1
2
3
4
5
6
7
None
Mont
MnCl2
Mn-mont
Mn-mont (L1)
Mn-mont (L2)
Mn-mont (L3)
6.0
7.2
7.3
20.2
100
72.6
98.0
68.1
71.2
69.2
80.0
90.0
89.7
86.0
9.5
9.2
9.7
5.8
2.0
2.8
1.6
21.5
20.0
20.2
13.2
8.6
Fig. 3 Characteristic UV-vis DRS bands (200–700 cmꢁ1) of (a) mont, (b) Mn-
7.1
12.5
mont, (c) Mn-mont (L1) and (d) L1.
a
Reaction conditions: 10 mmol cyclohexene; 0.02 mol % (20 mg) catalyst;
20 mL acetonitrile, 10 mmol isobutyraldehyde, temperature, 40 ꢀC;
3.1.3 UV-vis DRS. The diffuse reectance UV-vis spectra
characterization of mont, Mn-mont, Mn-mont (L1) and L1
samples were measured. The spectrum of the Schiff base ligand
(L1) shows a p–p* band at 310–365 nm due to the absorption of
pyridyl and imine (Fig. 3). The spectrum of the mont and Mn-
mont was nearly same to each other. Compared with the spec-
trum of the mont, the spectrum of Mn-mont (L1) exhibits an
additional broad absorption from 300 nm to 600 nm. The broad
band is from the absorption of pyridyl and imine of ligand and
also from the absorption of metal–ligand and ligand–metal
charge transfer processes. This characterization further
provided the evidence that the Schiff-base ligand was coordi-
nated on the surface of Mn-mont.
3.1.4 SEM. Fig. 4 shows the scanning electron micrograph
(SEM) of the montmorillonite and Mn-mont (L1). A clear change
in the morphology of Mn-mont (L1) compared with the mont-
morillonite observed by SEM conrms that the Schiff-base
ligand was coordinated on the surface of Mn-mont.
3.1.5 ICP analysis. The efficiency of Mn introduction (EMI)
was calculated as the following equation (moles of manganese
loaded/moles of manganese in the solution used for loading ꢂ
100). As revealed by ICP analysis, the efficiency of Mn intro-
duction was 82%.
molecular oxygen rate, 10 mL minꢁ1
.
3.2 Analysis of catalytic epoxidation reaction
3.2.1 Catalytic study. The effect of different Schiff-base
catalysts on epoxidation of cyclohexene was investigated. These
results were summarized in Table 1, the conversion was only
6.0% in the blank reaction when the reaction time was 5 h. The
result which was presented in Entry 3 indicated that Mn2+ ion
did not act in the epoxidation reaction. Mn-mont played a small
role in the epoxidation reaction (Entry 4). When only catalytic
amount of the Mn-mont (Lx) was added in reaction, the
conversion increased rapidly. This result conrms that the
Schiff-base ligands coordinated on the surface of Mn-mont
played an important role in the catalytic reaction. The different
activity of the composite catalysts with different Schiff-base
ligands could be due to the different stabilities of the inter-
mediates formed in the catalytic process. The result (Entries
5–7) conrmed that Mn-Mont (L1) displayed the highest cata-
lytic activity and selectivity (100% conversion with 90.0%
epoxide selectivity) when applied to the epoxidation of
Fig. 4 SEM images of the samples (a) Montmorillonite and (b) Mn-mont (L1).
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Table 2 Epoxidation of cyclohexene catalyzed by the Mn-mont (L1) under different conditionsa
Catalyst
weight (g)
Epoxide
selectivity (%)
Entry
Parameters
Temperature (ꢀC)
Time (h)
Molar ratio
Conversion (%)
1
2
3
4
5
6
7
8
Temperature
25
30
35
40
45
40
40
40
40
40
40
40
40
40
40
40
0.02
0.02
0.02
0.02
0.02
0.01
0.02
0.03
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
5
5
5
5
5
5
5
5
1
3
5
7
5
5
5
5
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.1
0.5
1.0
1.5
38.5
64.8
81.3
100
94.4
83
80.6
82.5
86.6
90.0
88.0
87.2
90.0
89.2
85.6
87.3
90.0
89.0
55.2
78.2
90.0
89.2
Catalyst
Time
100
98.2
14.6
70.8
100
9
10
11
12
13
14
15
16
100
Molar ratio
30.7
75.8
100
99.7
a
Reaction conditions: 10 mmol cyclohexene; 20 mL solvent, molecular oxygen rate, 10 mL minꢁ1
.
cyclohexene, because of the pridine ring in Schiff-base ligands. The conversion increased from 14.6% to 70.8% between 1 and
The coordination ability of metal Mn2+ and Schiff-base were 3 h and then the conversion and epoxide selectivity reached the
enhanced, as with the ability of activating oxygen.
highest at 5 h (Entries 9–12). The molar ratio of iso-
The reaction conditions including reaction temperature, butyraldehyde to cyclohexene plays a key role in the epoxidation
catalyst amount, reaction time, and molar ratio of iso- of cyclohexene by molecular oxygen because the conversion was
butyraldehyde to cyclohexene were studied. As can be seen from only 30.7% at the low molar ratio (isobutyraldehyde to cyclo-
Table 2, 40 ꢀC might be the suitable reaction temperature, high hexene) of 0.1. The conversion reached the maximum at the
temperature may accelerate the rate of chemical reaction, but molar ratio of 1.0 (Entries 13–16).
the conversion decreased a little when the temperature excee-
3.2.2 Reuse of catalyst. The results for the recycling test of
ded 40 ꢀC because isobutyraldehyde with low boiling point Mn-mont (L1) were given in Fig. 5. In this test the cyclohexene
(64 ꢀC) may evaporate more at the relatively higher temperature conversion and epoxide selectivity kept almost constant, which
(Entries 1–5). The variation in catalyst amount from 0.01 to indicated that Mn-mont (L1) was rather stable in the catalytic
ꢀ
0.02 g at 40 C showed an increase of 17% yield (Entries 6–8). reaction media. The Mn content of the catalyst aer two runs
and three runs was determined by ICP, with almost no
manganese leaching in the recycling test, which further
conrmed that the Mn-mont (L1) catalyst was stable for the
epoxidation of cyclohexene.
4 Conclusion
In summary, three economical heterogenized catalysts Mn-
mont (Lx) were prepared successfully. The coordination of
organic ligands and the formation of new bonds on the surface
of Mn-mont were conrmed by XRD, FT-IR, UV-vis DRS, SEM
and ICP. The pyridine Schiff base Mn(II) complex-montmoril-
lonite catalysts Mn-mont (L1) exhibited high activity and
epoxide selectivity for epoxidation of cyclohexene under
Mukaiyama conditions. The catalyst can be reused three times
and its high activity and epoxide selectivity almost did not
change.
Acknowledgements
Fig. 5 Recycling study of the catalyst. Reaction conditions: cyclohexene, 0.01
mol; catalyst, Mn-mont (L1), 0.02 mol% (20 mg) catalyst; acetonitrile, 20 mL;
temperature, 40 ꢀC; isobutyraldehyde, 0.01 mol; reaction time, 5 h. Yield refers
Financial support from the National Natural Science Founda-
tion of China (no. 21071152) is gratefully acknowledged.
to GC.
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