Catalytic olefin epoxidation over cobalt(ii)-containing mesoporous silica by molecular oxygen in dimethylformamide medium

Article abstract of DOI:10.1039/c4cy00084f

A cobalt(ii) Schiff base complex has been immobilized onto the surface of Si-MCM-41 to prepare a new catalyst. The amine group-containing organic moiety 3-aminopropyl-triethoxysilane had first been anchored on the surface of Si-MCM-41 via a silicon alkoxide route. Upon condensation with salicylaldehyde, the amine group affords a bidentate Schiff-base moiety in the mesoporous matrix, which is subsequently used for anchoring of cobalt(ii) centers. The prepared catalyst has been characterized by UV-vis, infrared (IR), EPR spectroscopic and small angle X-ray diffraction (XRD) analyses, and N₂ sorption studies. The catalytic activity was tested in epoxidation reactions of olefinic compounds, including styrene and allyl alcohol, with molecular oxygen at atmospheric pressure in dimethylformamide medium in the absence of additional sacrificial reductant. The reactions seemed to proceed through a radical formation mechanism. The immobilized catalyst showed good activity and epoxide selectivity in the alkene epoxidation. Notably, the catalyst can be recovered and reused without any loss of activity. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cy00084f

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PAPER
Catalytic olefin epoxidation over cobalt(II)-containing
mesoporous silica by molecular oxygen in
dimethylformamide medium†
Cite this: Catal. Sci. Technol., 2014,
4, 1820
Susmita Bhunia, Sreyashi Jana, Debraj Saha, Buddhadeb Dutta
*
and Subratanath Koner
A cobalt(II) Schiff base complex has been immobilized onto the surface of Si–MCM-41 to prepare a new
catalyst. The amine group-containing organic moiety 3-aminopropyl-triethoxysilane had first been
anchored on the surface of Si–MCM-41 via a silicon alkoxide route. Upon condensation with salicyl-
aldehyde, the amine group affords a bidentate Schiff-base moiety in the mesoporous matrix, which is
subsequently used for anchoring of cobalt(II) centers. The prepared catalyst has been characterized by
UV-vis, infrared (IR), EPR spectroscopic and small angle X-ray diffraction (XRD) analyses, and N2 sorption
studies. The catalytic activity was tested in epoxidation reactions of olefinic compounds, including sty-
rene and allyl alcohol, with molecular oxygen at atmospheric pressure in dimethylformamide medium in
the absence of additional sacrificial reductant. The reactions seemed to proceed through a radical forma-
tion mechanism. The immobilized catalyst showed good activity and epoxide selectivity in the alkene
epoxidation. Notably, the catalyst can be recovered and reused without any loss of activity.
Received 20th January 2014,
Accepted 25th February 2014
DOI: 10.1039/c4cy00084f
www.rsc.org/catalysis
the oxidant can be achieved by using molecular oxygen. Many
efforts have been made to find epoxidation methods using
1. Introduction
The catalytic epoxidation of alkenes is a reaction of great
industrial interest given the numerous applications of epox-
ides as precursors in the production of speciality chemicals
and agrochemicals.1 The conventional procedure for epoxida-
tion of alkenes is oxidation by stoichiometric amounts of
peracids.2 However, peracids are very expensive, hazardous to
handle, and non-selective for epoxide formation, and also
lead to the formation of undesirable products, creating a lot
of waste.3 Besides, the commercial manufacturing method
for epoxides involving chlorohydrin causes serious environ-
mental problems. Therefore, many alternative oxidizing
agents, such as NaIO4, NaOCl, PhIO, ROOH, and H2O2, have
been used for the epoxidation of alkenes. Alkyl hydroperox-
ides are used on a large scale in industrial epoxidation, for
example, in the Halcon-Arco and Sumitomo processes.4
Recycling of the co-product, that is, tert-BuOH, has been real-
ized in the Sumitomo process. In this context, selective oxida-
tion of hydrocarbons with molecular oxygen is an elegant
reaction because of its low cost and environmentally friendly
nature.5 Avoidance of the formation of any co-products from
molecular oxygen.6–10 Amongst them, the Mukaiyama epoxi-
dation system,9,11,12 which uses a metal complex as the
homogeneous catalyst with aldehydes or alcohols as the
co-reductant, is very effective. However, there is only a remote
possibility of recovering and reusing the catalyst in homoge-
neous catalytic processes. Hence, in view of the environmen-
tal and economic considerations, use of molecular oxygen as
an oxidant in heterogeneous epoxidation reaction of alkenes
is of interest.13 Cobalt ions and their coordination complexes
catalyze the selective oxidation of alkanes and alkylbenzenes
with O2.14 Cobalt complexes are also used in the epoxidation
of alkenes with tert-butyl hydroperoxide (TBHP) and iodo-
sylbenzene.15 The catalytic oxidation of terminal olefins,
including styrene, by O2 to the corresponding 2-ketones and
2-alcohols using a cobalt(II) complex has been reported.16,17
Cobalt–salen complexes were reported to show catalytic activ-
ity for the epoxidation of styrene with O2 in the presence of a
co-reductant, isobutyraldehyde.18 Recently, Mallat et al.
described the influence of organic solvents as “sacrificial”
solvents in oxidation reactions.19 They proposed an alterna-
tive explanation of epoxidation on the basis of a solvent co-
oxidation mechanism. Oxidation of mono-terpenes by oxygen
employing a CoCl2 catalyst has been investigated, and in this
case allylic oxidation takes place predominantly.20 Cobalt-
based heterogeneous catalysts have been employed for the
selective oxidation of alkanes, especially cyclohexane, and
Department of Chemistry, Jadavpur University, Kolkata 700 032, India.
E-mail: snkoner@chemistry.jdvu.ac.in; Fax: +91 33 24146414; Tel: +91 33 24572778
† Electronic supplementary information (ESI) available: EPR spectrum of
Co–MCM-41 and pore size distribution plots of Si–MCM-41 and Co–MCM-41.
See DOI: 10.1039/c4cy00084f
1820 | Catal. Sci. Technol., 2014, 4, 1820–1828
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alkenes.21–30 Aerial oxidation of styrene catalyzed by an
immobilized cobalt–HMS (Hexagonal Mesoporous Silica) cat-
alyst has been reported by Pruß et al.31 Though styrene con-
version was 98%, the yield of styrene oxide was 44%. Tang
et al. used CoII-exchanged zeolite as a catalyst for the epoxi-
dation of styrene by molecular oxygen.32 Jasra et al. also
carried out the catalytic epoxidation of styrene33a and cyclo-
alkenes33b to their corresponding epoxides using molecular
oxygen in the presence of Co2+-exchanged zeolite X. It has
been proposed that a DMF–Co3+OO˙ superoxo complex plays
the key role in the oxygen transfer to the CC bond.33 The
DMF solvent has a definite role in this mechanism. It coordi-
nates with cobalt sites to form the active oxygen species
during the reaction. However, examples of epoxidation of
alkenes by molecular oxygen without a co-reductant are
scarce.34 Recently, Beier et al. used the metal–organic frame-
work catalyst STA-12(Co) which shows high activity in the
aerobic epoxidation of E-stilbene in DMF.35
air in order to remove the templates. The mesoporous mate-
rial thus obtained is designated as Si–MCM-41.
2.3. Functionalization of Si–MCM-41 with organosilanes
Organic modification of Si–MCM-41 with 3-aminopropyltriethoxysilane
(3-APTES) was performed by stirring 0.2 g of Si–MCM-41 with
0.016 mL 3-APTES (0.068 mmol) in dry chloroform at room
temperature for 12 h under N2 atmosphere. The white solid
MCM-41–(SiCH2CH2CH2NH2)x thus produced was filtered,
and washed with chloroform and dichloromethane. This solid
was then refluxed with 0.008 g of salicylaldehyde (0.068 mmol)
in methanol (10 cm3) for 3 h at 60 °C to generate the desired
Schiff-base moiety. The resulting yellowish solid was then
collected by filtration and dried in a desiccator (Scheme 1).
The elemental analysis for MCM-41–(SiCH2CH2CH2NH2)x
found: C, 4.99; H, 4.3; N, 1.89%.
2.4. Anchoring of cobalt(II) in functionalized MCM-41
In the course of our continuing investigation of different
organic reactions using ordered mesoporous silica materials
as heterogeneous catalytic supports, we have found that
hybrid materials demonstrate desirable catalytic efficacy.36–41
Herein, we report the anchoring of a cobalt(II) Schiff-base
moiety on the surface of mesoporous silica, Si–MCM-41.
3-Aminopropyl-triethoxysilane was first anchored on the
surface of Si–MCM-41 via a silicon alkoxide route. Upon con-
densation with salicylaldehyde, the amine group affords a
Schiff-base in the mesoporous matrix for anchoring cobalt(II)
ions. The catalyst exhibits excellent activity in the catalytic
epoxidation reaction of olefins with molecular oxygen in
DMF medium.
The cobalt-anchored mesoporous material was prepared by
dissolving 0.01 g of Co(NO3)2·6H2O (0.034 mmol) in 10 mL
dry methanol and stirring with the functionalized
Si–MCM-41 (0.2 g) in suspension at room temperature for
12 h. The greenish yellow solid thus formed was filtered,
washed with methanol using a Soxhlet extractor and dried
under vacuum at 50 °C for 24 h (Scheme 1). The catalyst
thus obtained is designated as Co–MCM-41. Atomic absorp-
tion spectrometric results showed that the cobalt content of
the catalyst is ca. 0.8% (wt). The elemental analysis for
Co–MCM-41 found: C, 3.26; N, 0.40%. Hence, the N : Co
molar ratio ≈ 2.1 and the C : Co molar ratio ≈ 20, which
indicates that the cobalt(II) ions have an N2O2 ligand envi-
ronment as shown in Scheme 1.
2. Experimental
2.1. Materials
2.5. Catalytic epoxidation reactions
All solvents used were of AR grade, and they were distilled
and dried before use. Fumed SiO2, the cationic surfactant
cetyltrimethylammonium bromide (CTAB, 98%), 3-aminopropyl-
triethoxysilane (3-APTES), hexahydrate cobalt nitrate, salicyl-
aldehyde, styrene, cyclooctene, cyclohexene, 1-hexene, 1-octene,
allyl alcohol and tert-butyl-hydroperoxide (70% aqueous) were
purchased from Sigma-Aldrich/Fluka or Merck (India) and were
used as received.
The catalytic epoxidation reactions were carried out under an
oxygen atmosphere in the liquid phase in a batch reactor at
the desired temperature. Typically, a 50 mL three-neck
round-bottomed flask equipped with a water condenser
containing 1 g alkene in 8 mL dry N,N-dimethylformamide
(DMF) solvent and 50 mg of Co–MCM-41 catalyst was kept in
a pre-heated oil bath. Oxygen gas was bubbled through the
reaction mixture at atmospheric pressure at a flow rate of ca.
3.0 cm3 min−1. The reaction mixture was magnetically stirred
continuously for 24 h. The products of the epoxidation reac-
tions were collected at different time intervals and were iden-
tified and quantified by gas chromatography.
2.2. Synthesis of Si–MCM-41
Mesoporous Si–MCM-41 was prepared according to the litera-
ture method,42 with a slight modification as mentioned in
our earlier publication,36 using C16H33N(CH3)3Br (CTMABr)
as the template and tetrabutylammonium bromide (TBABr),
with the molar composition of the reactants: 1.0 SiO2 : 0.48
CTMA+ : 0.96 TBA+ : 0.39 Na2O : 0.29 H2SO4 : 110 H2O. The gel
mixture was stirred for 24 h at room temperature, transferred
into a Teflon-lined autoclave and heated statically at 100 °C
for 4 days. The solid product was obtained by filtration,
washed with distilled water, dried in air at room temperature,
and calcined at 550 °C for 6 h in nitrogen and then for 6 h in
2.6. The reusability of the catalyst
After the reaction, the catalyst was recovered from the reac-
tion mixture by filtration, washed thoroughly with DMF and
dichloromethane, and then dried under vacuum at room
temperature. The used catalyst was reused for the epoxida-
tion of styrene in DMF, maintaining the same reaction
conditions.
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Scheme 1 (a) Modification of Si–MCM-41 channel wall: APTES/CHCl3; (b) condensation with salicylaldehyde in methanol; (c) metal complex
formation: Co(NO3)2·6H2O/MeOH.
2.7. Catalyst characterization
characteristic vibrations of the mesoporous framework (Si–O–Si),
and a broad band at approximately 3450 cm−1, mainly due to
the adsorbed H2O molecules. These absorption peaks were
not significantly affected by organic modification and cobalt
complex immobilization. A comparison of the IR spectra of
Si–MCM-41 (a) and MCM-41–(SiCH2CH2CH2NH2)x (b) showed the
presence of N–H and C–H vibration bands in the 3100–2800 cm−1
and 1550–1250 cm−1 regions in the IR spectrum of
Infrared spectra were recorded on a Shimadzu S-8400 Fourier
transform infrared (FTIR) spectrometer. EPR spectra were
recorded on a JEOL JES-FA 200 EPR spectrometer. To quantify
the catalytic reactions, a Varian CP3800 gas chromatograph
equipped with a flame ionization detector and a CP-Sil 8 CB
capillary column was used. The cobalt content of the sample
was measured by atomic absorption spectrometric analysis on
a Perkin-Elmer Analyst spectrometer. The amounts of carbon
and nitrogen were estimated using a Perkin Elmer 240C ele-
mental analyzer. The powder X-ray diffraction (XRD) patterns
of the samples were recorded with a Scintag XDS-2000 diffrac-
tometer using CuKα radiation. N2 sorption studies were under-
taken using a Quantachrome Autosorb (iQ) automated gas
sorption system. Prior to sorption experiments, samples were
outgassed at 250 °C for ca. 8–9 h under vacuum (10−3 Torr).
Diffuse reflectance (DR) UV-vis-NIR spectra were recorded on a
Perkin-Elmer Lambda 19 UV-vis-NIR spectrometer equipped
with a diffuse reflectance integrating sphere coated with BaSO4,
which also served as a standard. All spectra were recorded at
room temperature under ambient atmosphere. GC-MS analysis
of the reaction products was performed on a Perkin Elmer
CLarus SQ8 GC-MS analyzer, as well as a Waters Xebo G2-S QT
high resolution mass analyzer. Other instruments used in this
study were the same as those reported earlier.36
MCM-41–(SiCH2CH2CH2NH2)x.
In
the
spectrum
of
MCM-41–(SiCH2CH2CH2NH2)x, the IR band at 1546 cm−1
could be assigned to bending vibrations of the amine group.
The appearance of N–H and C–H vibration bands in the
3100–2800 cm−1 and 1550–1250 cm−1 regions upon grafting
of 3-APTES onto Si–MCM-41 confirmed the attachment of
aminopropyl groups on the surface of the support. These
bands were absent in the case of Si–MCM-41. The character-
istic band for the azomethine group (CN) of the cobalt(II)
complex moiety appeared at ca. 1632 cm−1 in the IR spectrum of
Co–MCM-41 (c). The IR band for the azomethine group of the
Schiff-base moiety appeared at 1643 cm−1, and shifted to a lower
frequency upon complex formation with CoII in Co–MCM-41,
indicating coordination of the Schiff-base with cobalt.
The UV-vis spectra of the supported Schiff-base and
Co–MCM-41 in the 300–600 nm range are shown in Fig. 2.
The electronic spectrum of the supported Schiff base showed
two main bands. The band within the 285–370 nm range is
assigned to the π → π* transitions of the CN chromophore,
while the longer wavelength band at 370–470 nm is due to an
intramolecular charge transfer (CT transition) involving the
whole molecule.43 On complex formation, these bands were
shifted to higher wavelength, suggesting coordination of the
3. Results and discussion
3.1. Spectroscopic measurements
The FTIR spectra (Fig. 1) of mesoporous silica showed bands
at approximately 1090, 800, and 470 cm−1, assigned to
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Fig. 1 IR spectra of: (a) Si–MCM-41; (b) MCM-41–(SiCH2CH2CH2NH2)x; (c) Co–MCM-41 catalyst.
Fig. S1). The signals are weak as the metal content in the
sample is very low. Co–MCM-41 clearly shows an axial spec-
trum, in which the g|| and g⊥ values of the catalyst are cal-
culated to be 2.19 and 2.01, respectively. A zeolite-encaged
Co–salen complex also demonstrates a similar type of EPR
spectrum.47 Spectroscopic studies of Co–MCM-41, therefore,
indicate that the cobalt(II) complex formed in the MCM-41
matrix may be depicted as shown in Scheme 1.
3.2. Small angle X-ray diffraction studies
The X-ray diffraction pattern of Si–MCM-41 shows a typical
three-peak pattern,48,49 with a very strong d100 = 40.44 Å
reflection at 2θ ≈ 2.18° and two other weaker reflections at
2θ ≈ 3.75° and 2θ ≈ 4.32° for d110 and d200, respectively, due
to the quasi-regular arrangement of mesopores with hexago-
nal symmetry (Fig. 3). Si–MCM-41 shows an additional peak
at 2θ ≈ 5.71° which can be assigned to the d210 reflection. All
the peaks are well-resolved, indicative of a good quality
material. Co–MCM-41 exhibits a strong d100 reflection with a
spacing of ca. 35.64 Å at 2θ ≈ 2.48° with almost the same
intensity as that of Si–MCM-41, and two other weaker reflec-
tions at 2θ ≈ 4.31° and 2θ ≈ 5.00° for d110 and d200, respec-
tively, while the weakest reflection for d210 has been buried
in the background noise. A comparison of the X-ray powder
diffraction patterns of Co–MCM-41 and Si–MCM-41 shows
that the typical three-peak pattern of MCM-41 has been
retained after the anchoring of the cobalt(II) complex in
Si–MCM-41. However, all the diffraction lines shifted to
higher angles. A similar type of behavior was observed by
Burkett et al. in phenyl-modified mesoporous sieves,50 and
Fig. 2 UV-vis spectra of the supported Schiff-base and Co–MCM-41.
azomethine nitrogen with Co(II). The red shift of the absorp-
tion band in the UV-vis spectrum upon immobilization of a
metal complex in a zeolite cage has been observed in several
previously reported studies.44–46
The powder EPR spectrum of Co–MCM-41 measured at
300 K consists of several nuclear hyperfine lines (see ESI;†
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Fig. 4 N2 adsorption–desorption isotherms of Si–MCM-41 and
Co–MCM-41.
mesoporous MCM-41.53a The N2 adsorption isotherm and the
pore size distribution of Co–MCM-41 were different to those
of Si–MCM-41. Co–MCM-41 displays a considerably lower
N2 uptake (BET surface area 365 m2 g−1) in comparison to
Si–MCM-41. These isotherms are type IV, with well-defined
sharp inflections observed in the p/p0 ranges 0.1–0.3 for
Si–MCM-41 and 0.2–0.4 for Co–MCM-41, which represent
spontaneous filling of the mesopores due to capillary conden-
sation. This indicates that the overall mesoporous structure
of the parent support was retained in the immobilized cata-
lyst. While the pore size distribution curve for Si–MCM-41
shows a remarkably narrow peak with an average pore size of
wBJH = 22.6 Å, indicating uniformity of the pore distribution,
Co–MCM-41 shows a wide variety of pore sizes, with a highest
peak at ~23 Å (see ESI;† Fig. S2 and S3). However, this is not
inconsistent with the fact that anchoring of the cobalt com-
plex might not be uniform throughout the mesoporous silica
matrix. The decrease in BET surface area and the loss of
uniformity of pore size for Co–MCM-41 in comparison with
Si–MCM-41 clearly demonstrate that anchoring of the cobalt
Schiff-base moiety into the mesoporous silica has significant
effects on the pore structure of the catalyst. One possible rea-
son for this remarkable difference in the BET surface areas
of Si–MCM-41 and Co–MCM-41 might be anchoring of a con-
siderable amount of the cobalt Schiff-base moiety into the
nanosized pores of the mesoporous material. However, con-
sidering the amount of cobalt present in Co–MCM-41, the
decrease in surface area is significantly large. This could be
caused by partial amorphization53b of the layered phase of the
mesoporous material during anchoring of the Co complex.
Fig. 3 Small angle XRD patterns of: Si–MCM-41; Co–MCM-41.
by Lim and Stein in directly synthesized thiol–MCM-41.51
Upon post-synthesis grafting of the cobalt Schiff-base com-
plex into Si–MCM-41, an overall decrease in the intensity of
the diffraction lines was noticed. This result could be attrib-
uted to lowering of the local order, such as variations in the
wall thickness, or might be due to reduction of the scattering
contrast between the channel wall of the silicate framework
and the Schiff base ligand present in Co–MCM-41, as previ-
ously mentioned by Lim and Stein.51 Marler et al. reported
that the intensity of the diffraction lines decreases systemati-
cally on increasing the concentration of organic sorbates in
boron-containing MCM-41.52 Therefore, the shift of the dif-
fraction lines to higher angle and the decrease in the inten-
sity of the peaks upon organic modification of Si–MCM-41
and subsequent complex formation with cobalt(II) are not
inconsistent.
3.3. Nitrogen sorption study
The nitrogen sorption experiments showed that Si–MCM-41
has a BET surface area (AsBET) of 1136 m2 g−1 and a primary
mesopore volume (Vp) of 0.61 cm3 g−1 (Fig. 4). The average
pore diameter of Si–MCM-41 is calculated to be 22.6 Å using
the Barrett–Joyner–Halenda (BJH) method. All the calculated
values are in agreement with those reported for good quality
3.4. Epoxidation reactions
The catalytic efficacy of the prepared catalyst was tested in
the epoxidation of both aromatic and aliphatic alkenes,
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including straight chain and cyclic alkenes, with molecular
oxygen. The performance of the catalyst in terms of conver-
sion, selectivity and turnover number (TON) is given in
Table 1. The epoxidation of styrene with molecular oxygen
gives styrene oxide in 45% yield (selectivity ~90%) (Table 1)
under heterogeneous conditions; in addition to this, a small
amount (5%) of benzaldehyde is detected. A turnover number
of ~707 for catalytic epoxide production is attained in this
particular reaction. Epoxidation of styrene with molecular
oxygen over a variety of cobalt catalysts under heterogeneous
conditions has been studied in the recent past (Table 2). A
cobalt–salen catalyst immobilized in silica was applied in the
epoxidation of styrene with molecular oxygen in the presence
of butyraldehyde.54 CoOx–MCM-41, prepared by ultrasonic
deposition–precipitation of cobalt tricarbonyl nitrosyl in
decalin, was reported to catalyze the epoxidation of alkenes
by O2 in the presence of excess isobutyraldehyde, which func-
tioned as the co-reductant.27
Linear aliphatic olefins, such as 1-hexene and 1-octene, can
be directly oxidized by molecular oxygen to afford the corre-
sponding 1,2-epoxy alkanes. As shown in Table 1, the yield of
1,2-epoxy alkane is about 47% for 1-hexene and 30% for
1-octene. This shows that the catalytic activity decreases with
increasing olefin chain length. This may be due to the fact that
the larger hexyl group connected to the double bond of
1-octene sterically hinders its approach to the catalyst metal
center, compared with 1-hexene, in which the double bond
carries a smaller butyl group. Again, there may be some
electronic effect, i.e. higher electron density of the double bond
is expected to result in higher epoxidation reactivity. Therefore,
cyclooctene and cyclohexene with double bonds between two
secondary carbons exhibit higher activities in comparison with
1-hexene and 1-octene, which contain double bonds between
secondary and primary carbons. Cyclohexene shows 54% con-
version to form cyclohexene oxide as the major product with
74% selectivity; in addition, 2-cyclohexen-1-ol was generated,
Table 1 Heterogeneous alkene epoxidation catalyzed by Co–MCM-41e
% Yield of products
Reaction
Conversion
Alkenes
temp. (°C)
(wt%)
Epoxide
45
Other
5a
TONd
707
80
50
75
80
80
54
78
47
40
78
47
14b
—
970
1044
824
—
80
50
35
12
30
5
460
304
12c
—
a
b
c
d
Benzaldehyde. 2-Cyclohexen-1-ol was formed. R-(+)-glycidol. Turnover number = moles converted/moles of active site. The products of
the epoxidation reactions were collected at different time intervals and were identified and quantified using a Varian CP3800 gas
e
chromatograph equipped with an FID detector and a CP-Sil 8 CB capillary column. Reaction conditions: alkenes (1 g); catalyst (50 mg); flow
rate of O2: 3.0 cm3 min−1; DMF (8 mL).
Table 2 Epoxidation of styrene with molecular oxygen catalyzed by a variety of cobalt catalysts
% Yield of products
Conversion
Catalyst
(wt%)
Epoxide
Other
TONa
References
Co–MCM-41
50
45
44.2
45.3
58
91
100
46
45
28
26.5
28.2
37
49
67
16
5
17
17.7
17.1
21
42
33
30
707
169
13.2
13.5
10
94
16.4
54
This study
Co–MCM-41
56
32
32
54
34
33a
30
Co2+–X
Co2+–Y
Immobilized Co–salen complex in silica
Cobalt–hydroxyapatite (CoHAp)
NaCoX96
CoALPO-36
a
Turnover number = moles converted/moles of active site.
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owing to allylic C–H oxidation. Kameyama et al.55 studied the
epoxidation of cyclohexene with molecular oxygen with the co-
reductant isobutyraldehyde catalyzed by cobalt porphyrin com-
plexes immobilized on montmorillonite, which showed ~66%
conversion to 1,2-epoxycyclohexane (yield ~56%). The bulkier
cycloalkene, cyclooctene, has also been effectively and selec-
tively converted to cyclooctene oxide (conversion 78%, selectiv-
ity 100%). Jinka et al. performed epoxidation of cycloalkenes
using molecular oxygen with the NaCoX96 catalyst, and
observed only 26% conversion of cyclohexene and 47% conver-
sion of cyclooctene.33 Tang et al. introduced cobalt metal into
MCM-41 by a template ion exchange (TIE) method to study the
styrene epoxidation reaction.56 However, they did not investi-
gate the possibility of leaching of cobalt metal from the surface
of MCM-41, which is common amongst ion-exchanged cata-
lysts. Notably, cobalt metal does not leach out during the cata-
lytic reaction in the case of Co–MCM-41 (vide infra). In our
case, the metal ions are covalently attached to organically mod-
ified Si–MCM-41.
Solvent plays a crucial role in epoxidation reactions. We
have also performed the epoxidation reaction of styrene in
different solvents, such as acetonitrile, chloroform, hexane,
and ethyl acetate, and even in high boiling alcohols like
n-decyl alcohol; however, no conversion of styrene to its oxide
was detected. Only acylamides, such as DMF, are particularly
efficient in providing both high conversion and high epoxide
selectivity in this case. Notably, Beier et al. suggested that a
radical reaction pathway operates in DMF-promoted epoxida-
tions.35 In the reaction medium, a radical oxygen species is
generated by the activation of molecular oxygen over the
DMF-coordinated Co(II) site. To clarify this point, the influ-
ence of the addition of a radical scavenger, hydroquinone, on
the catalytic performance has been investigated. The results
revealed that styrene conversion decreased to almost zero
after addition of a small amount of hydroquinone (Fig. 5),
confirming the radical nature of the active oxygen species
formed by molecular oxygen with cobalt(II) in the presence of
DMF. However, Opre et al. suggested another pathway, in
which DMF first interacts with molecular oxygen, leading to
its autoxidation to form N-(hydroperoxymethyl)-N-methyl-
formamide, which subsequently acts as an oxygen transfer
agent in the epoxidation of styrene, thereby producing
N-formyl-N-methylformamide in considerable amounts.34 We
have analyzed the reaction solution using a GC-MS analyzer,
as well as a high resolution mass spectrometer, however, we
could not detect the formation of an N-formyl-N-methyl-
formamide-like moiety in the reaction products.
3.5. Separation, heterogeneity test and catalyst reuse
The major advantages of the use of heterogeneous catalysts
are recovery from the reaction mixture and possible reuse. To
test whether metal is leached out from the solid catalyst dur-
ing the reaction, we carried out a hot filtration test. After the
reaction, the solid catalyst was filtered out under hot condi-
tions. Then, the filtrate was treated with aqua regia several
times and the mixture was evaporated to dryness on a hot
plate to remove the organic part. After preparing the aqueous
solution of the filtrate, it was subjected to atomic absorption
spectroscopic analysis. The analysis showed that cobalt was
absent from the solution. In addition, the liquid phase of the
reaction mixture was collected by filtration after ~30% of the
epoxidation reaction was completed, and the residual activity
of the supernatant solution after separation of the catalyst
was studied. The supernatant solution was kept under the cat-
alytic reaction conditions, as stated above, and the composi-
tion of the solution was determined by GC. No reaction
progress was observed during this period, which precludes
the presence of active species in solution.
Fig.
5 Percentage conversion versus time plots for comparison
between reactions with a radical scavenger and a control reaction
without a radical scavenger.
Fig. 6 Reuse of Co–MCM-41 for the epoxidation of styrene with O2:
(■) styrene conversion, (○) epoxide selectivity.
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The Co–MCM-41 catalyst was recovered from the reaction
mixture and reused three times under the same reaction con-
ditions. The solid catalyst was recovered by filtration after
each reaction and was washed thoroughly with DMF. The
recovered catalyst was found to exhibit almost the same cata-
lytic activity for the epoxidation of styrene by molecular
oxygen in every run, as shown in Fig. 6.
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Conclusion
In summary, we succeeded in preparing a new example of a
cobalt-based heterogeneous catalyst by anchoring cobalt(II)
centers into a mesoporous silica matrix via silicon–alkoxide
chemistry. Importantly, the cobalt-based catalyst Co–MCM-41
is active in aerobic epoxidation reactions in DMF. Co–MCM-41
catalyzes the epoxidation reactions of a variety of olefinic
substrates, including allyl alcohol, in
a heterogeneous
medium. Notably, the catalyst can be recovered and reused
without any loss of activity.
Acknowledgements
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Financial support from a grant from CSIR, New Delhi (grant
no. 01(2542)/11/EMR-II) (to S.K.) is gratefully acknowledged.
S.B. thanks CSIR, New Delhi, India (no. 09/096(0671)2k11-
EMR-I) for a Senior Research Fellowship.
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