Highly efficient epoxidation of alkenes with hydrogen peroxide catalyzed by tungsten hexacarbonyl supported on multi-wall carbon nanotubes

Article abstract of DOI:10.1007/s11243-011-9541-8

The preparation, characterization, and catalytic activity of W(CO) ₆ supported on multi-wall carbon nanotubes modified with 4-aminopyridine is reported. The catalyst, [W(CO)₅@Apy-MWCNT], was characterized by physico-chemical and spectroscopic methods and found to be an efficient heterogeneous catalyst for green epoxidation of alkenes with hydrogen peroxide in MeCN solvent. The catalyst showed good stability and reusability properties in the epoxidation reactions. Springer Science+Business Media B.V. 2011.

Full text of DOI:10.1007/s11243-011-9541-8

Transition Met Chem (2011) 36:861–866
DOI 10.1007/s11243-011-9541-8
Highly efficient epoxidation of alkenes with hydrogen peroxide
catalyzed by tungsten hexacarbonyl supported on multi-wall
carbon nanotubes
Mehdi Nooraeipour
Shahram Tangestaninejad
•
Majid Moghadam
•
•
Valiollah Mirkhani
•
Iraj Mohammadpoor-Baltork S. Masoud Nabavizadeh
•
Received: 13 June 2011 / Accepted: 15 September 2011 / Published online: 28 September 2011
Ó Springer Science+Business Media B.V. 2011
Abstract The preparation, characterization, and catalytic
activity of W(CO) supported on multi-wall carbon nano-
alkenes [4]. However, the major problem associated with
these homogeneous catalysts is the recovery of catalyst
from the reaction medium. In recent years, there have been
intense efforts to develop the recovery and reuse of
homogeneous catalysts [5]. An efficient and practical way
to achieve commercial applications of these expensive
catalysts is their immobilization on solid supports. These
immobilized catalysts can offer numerous advantages over
their homogeneous counterparts, including easy separation
of the catalyst from the reaction media; reduced toxicity;
simple recycling of expensive catalysts; and minimization
of catalyst deactivation through site isolation. The use of
polymer-supported catalysts in organic transformations has
been receiving a great deal of attention [6, 7], and the
design of functionalized polymers carrying catalytically
active metal species has attracted considerable interest
[8–16].
6
tubes modified with 4-aminopyridine is reported. The cat-
alyst, [W(CO) @Apy-MWCNT], was characterized by
5
physico-chemical and spectroscopic methods and found to
be an efficient heterogeneous catalyst for green epoxidation
of alkenes with hydrogen peroxide in MeCN solvent. The
catalyst showed good stability and reusability properties in
the epoxidation reactions.
Introduction
Catalytic epoxidation of alkenes is an important industrial
reaction and also is a useful synthetic method for the
production of a wide variety of fine chemicals [1, 2].
Different single oxygen donors such as NaIO , NaOCl,
4
PhIO, ROOH, O , and H O have been used for alkene
2 2 2
Supported catalysts based on tungsten species show
many applications in heterogeneous catalysis. Thus, they
are widely used in several industrially important reactions
such as hydrotreatments, hydrocracking of heavy fractions
from oil, dehydration of alcohols, metathesis, oxidations,
and isomerization of olefins [17]. The supports used with
these catalysts are A1 O and TiO [18, 19], SiO [20],
epoxidation in the presence of a catalyst. Among these,
hydrogen peroxide is an efficient and attractive oxidant,
particularly due to its low cost and green nature in which
water is produced as the by-product [3].
Transition metal complexes are efficient catalysts for a
wide variety of organic reactions such as the epoxidation of
2
3
2
2
MgO [21], activated carbon [22], polymers [8, 23], and
MgF [24].
2
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-011-9541-8) contains supplementary
material, which is available to authorized users.
Carbon nanotubes (CNTs) have attracted much attention
because of their unique structural, mechanical, thermal,
optical, and electronic properties [25]. Since CNTs are
insoluble in most solvents and have high surface areas,
they can be used as catalyst supports. For example, Pt
nanoparticles supported on CNTs have been used for
methanol oxidation [26], palladium nanoparticles sup-
ported on CNTs for hydrogenation of phenylacetylene
[27], and a chiral vanadyl salen complex supported on
M. Nooraeipour Á M. Moghadam (&) Á S. Tangestaninejad Á
V. Mirkhani Á I. Mohammadpoor-Baltork
Department of Chemistry, Catalysis Division,
University of Isfahan, Isfahan 81746-73441, Iran
e-mail: moghadamm@sci.ui.ac.ir
S. Masoud Nabavizadeh
Department of Chemistry, Shiraz University, Shiraz 71454, Iran
123

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Transition Met Chem (2011) 36:861–866
[
W(CO) @APy-MWCNT]
O
5
H O , CH CN
2
2
3
Scheme 1 Epoxidation of alkenes with
W(CO) @APy-MWCNT]
2
H O
2
catalyzed by
[
5
single-wall CNTs has been used for enantioselective
cyanosilylation of aldehydes [28].
Previously, we reported manganese(III) porphyrin and
manganese(III) salophen supported on MWCNTs for
epoxidation of alkenes with NaIO [29, 30] and molybde-
4
num hexacarbonyl supported on MWCNTs for alkene
epoxidation with tert-BuOOH [31–33].
In this article, the preparation, characterization, and
catalytic activity of W(CO) supported on amine-modified
6
multi-wall carbon nanotubes is reported (Scheme 1).
Results and discussion
Fig. 1 UV-vis spectrum of: A Homogeneous W(CO)
APy-MWCNT]; C Homogeneous W(CO) (Py) and D MWCNT
6 5
; B [W(CO) @
Preparation and characterization of
5
[
W(CO) @APy-MWCNT]
5
Scheme 2 shows the proposed structure for [W(CO) @
5
APy-MWCNT]. First, the APy-MWCNT was prepared by
covalent attachment of 4-aminopyridine to MWCNT–COCl
via an amide linkage. Then, W(CO) was activated in THF
6
under UV irradiation to produce W(CO) THF. The
5
[
W(CO) @APy-MWCNT] catalyst was then synthesized
5
by the reaction of APy-MWCNT with W(CO) THF. The
5
[
W(CO) @APy-MWCNT] was characterized by elemental
5
analysis, scanning electron microscopy, and FTIR and dif-
fuse reflectance UV–Vis spectroscopic methods. The
nitrogen content of the catalyst was 1.17% (0.83 mmol/g).
According to this value, the total amount of nitrogen, which
was available for the attachment of W, was 0.41 mmol/g of
5
Fig. 2 SEM image of [W(CO) @APy-MWCNT]
support. The metal loading of [W(CO) @APy-MWCNT],
5
obtained by UV–Vis spectroscopy in the diffuse reflectance
measured by ICP, was 0.058 mmol/g. Based on these values,
mode. The starting material W(CO) showed two strong
6
1
4% of the available nitrogen sites were coordinated to
absorption peaks at 246 and 290 nm, attributed to W ? CO
charge transfer bands (Fig. 1A). These peaks were also
observed in the diffuse reflectance UV–Vis of
W(CO) . Figure S1 (see the Supplementary Data) shows
6
the FTIR spectra of APy-MWCNT and [W(CO) @
5
APy-MWCNT]. The C=O stretching band of the amide
[
W(CO) @APy-MWCNT] (Fig. 1B) and homogeneous
5
-
group is present at 1654 cm . Bands at 1,908, 1,857, and
1
W(CO) (Py) (Fig. 1C), while MWCNTs showed no
5
-
1
1
,801 cm are assigned to the C v symmetry pattern of
4
absorption peak in this region (Fig. 1D). These observations
indicate that tungsten carbonyl has been supported on the
W(CO) [34]. These observations confirm the coordination
5
of W(CO) to Apy-MWCNT. Further evidence for this was
5
MWCNTs. The SEM images of the [W(CO) @APy-
5
MWCNT] showed that the nanotubes are aggregated and
retain their nanotube nature (Fig. 2).
O
CO
CO
CO
C
Catalyzed epoxidation of alkenes
N
N
W
H
OC
CO
The activity of the prepared catalyst, [W(CO) @APy-
5
Scheme 2 The proposed structure for [W(CO)
5
@APy-MWCNT]
MWCNT], was initially investigated in the epoxidation of
1
23

Transition Met Chem (2011) 36:861–866
863
Table 1 Optimization of catalyst amount in the epoxidation of
cyclooctene with H O after 3 h
2 2
high cis/trans ratio (9.5/1) was detected (Table 3). During
the oxidation reactions, no acetamide due to the oxidation
of acetonitrile was observed.
a
Entry
Catalyst amounts (mg, mmol)
Yield (%)
The results obtained with [W(CO) @APy-MWCNT]/
5
1
2
3
4
5
6
0
10
78
90
93
98
98
H O are compared with some previously reported epoxi-
2
2
50 mg (0.003 mmol)
100 mg (0.006 mmol)
150 mg (0.009 mmol)
200 mg (0.012 mmol)
250 mg (0.015 mmol)
dation catalysts in Table 4. These comparisons show that
the present catalytic system is more efficient than most of
the others, except for molybdenum hexacarbonyl supported
on polystyrene and MWCNTs. This is consistent with the
generally higher catalytic activity of molybdenum com-
pared to tungsten. Comparison of the catalytic activity of
tungsten hexacarbonyl supported on polystyrene [35] and
MWCNTs shows the effect of support in which the TOF
increases from 18.8 for polystyrene to 27.2 for MWCNTs.
This can be attributed to the higher surface area of
MWCNTs compared to polystyrene.
Reaction conditions: cyclooctene (1 mmol), H
MeCN (5 mL)
2 2
O (0.5 mL), catalyst,
a
GC yield based on the starting cyclooctene
Table 2 Epoxidation of cis-cyclooctene with H
5
[W(CO) @APy-MWCNT] under reflux conditions in different
solvents
2 2
O catalyzed by
a
Catalyst recovery and reuse
Solvent
Epoxide yield after 3 h (%)
T (°C)
(
CH
THF
CH CN
3
)
2
CO
7
11
98
33
27
18
24
53
61
77
78
57
72
38
Since transition metal complexes are often expensive and
toxic, the reusability of a supported catalyst is of great
importance from economical and environmental points of
3
ClCH CH
2
2
Cl
view. Therefore, the reusability of [W(CO) @APy-
5
CHCl
3
MWCNT] was monitored by means of multiple sequential
epoxidations of cyclooctene with H O (Table 5). The
CCl
4
2
2
CHCl
2
catalyst was reused for eight consecutive experiments
without loss of activity. The amount of tungsten detected in
the filtrates from the first two runs was low, and after the
third run, no tungsten was detected in the filtrates. These
results demonstrate the strong attachment of W to the
MWCNT. The catalytic behavior of the separated liquid
was also tested by addition of fresh cyclooctene and H O to
2 2
Reaction conditions: cyclooctene (1 mmol), H O (0.5 mL), catalyst
(
200 mg, 0.012 mmol), solvent (5 mL)
a
GC yield based on the starting cyclooctene
cyclooctene with hydrogen peroxide. Different amounts of
W(CO) @APy-MWCNT] were investigated and the best
[
5
2 2
results were obtained in the presence of 200 mg
0.012 mmol with respect to W) of the catalyst (Table 1).
Different solvents were investigated and the results
Table 2) showed that the best epoxide yield was observed
the filtrates after each run. Execution of the oxidation
reaction under the same reaction conditions as for the cat-
alyst showed the same as obtained for blank experiments.
The nature of the recovered catalyst was studied by FTIR
spectroscopy. No band corresponding to terminal C=O was
observed in the FTIR spectrum of the recovered catalyst.
This is in accordance with the mechanism previously sug-
gested [47]. In this mechanism, C=O ligands are eliminated
and W=O species are produced (Fig. S2). Therefore,
(
(
in acetonitrile. During the reaction, the [W(CO) @APy-
5
MWCNT] is suspended in the solvent. This is due to the
bundled agglomerates of MWCNT that aggregate slowly
and allow the catalyst to remain suspended in the reaction
mixture for long periods of time. Control experiments in
the absence of catalyst or using MWCNT-Apy as catalyst
were also performed and the results showed that the
amount of epoxide was about 10%.
W(CO) can be considered as a catalyst precursor.
6
Under the optimized conditions, this catalytic system
was applied for epoxidation of a range of alkenes
Conclusion
(
Table 3). Both cyclic and linear alkenes were efficiently
converted to their corresponding epoxides. Thus, cyclooc-
tene and cyclohexene as representative cyclic alkenes and
In conclusion, tungsten hexacarbonyl supported on
MWCNTs modified with 4-aminopyridine was found to be
an efficient catalyst in the epoxidation of alkenes with
H O . This supported catalyst, [W(CO) @APy-MWCNT]
1
-heptene and 1-dodecene as linear alkenes were efficiently
converted to their corresponding epoxides. In the case of
trans-stilbene, only the trans-epoxide was produced, while
in the case of cis-stilbene, a mixture of epoxides with a
2
2
5
was reactive in the epoxidation of a wide range of linear
and cyclic alkenes. The catalyst was highly reusable and
123

8
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Transition Met Chem (2011) 36:861–866
Table 3 Epoxidation of alkenes with H
O
2 2
catalyzed [W(CO)
5
@APy-MWCNT] in refluxing CH
3
CN
a
a
Entry
1
Alkene
Conversion (%)
Epoxide (%)
Time (h)
98
b
98
72
3
3
6
3
2
100
100
3
c
3
4
77
74
57
4
3
d
77
5
6
7
100
25
100
25
3
4
e
90
90 (trans-epoxide)
10
e
8
95
86 (cis-epoxide)
(trans-epoxide)
10
9
2 2
Reaction conditions: alkene (1 mmol), H O (0.5 mL), catalyst (200 mg, 0.012 mmol), MeCN (5 mL)
a
GC yield based on starting alkene
b
The reaction was carried out in the presence of W(CO)
The by-product was benzaldehyde
5
(py) (0.012 mmol)
c
d
e
The by-product was acetophenone
1
Both HNMR and GC data approved the reported yields
1
could be recycled eight consecutive times without loss of
activity.
known samples and also with their H NMR spectra.
MWCNTs containing carboxylic acid groups (the COOH
content, 1.5%) (multi-wall carbon nanotubes with inside
diameters between 20 and 30 nm, outside diameters
between 5 and 10 nm, length 30 lm, and specific surface
Experimental
2
-1
area more than 110 m g ) were purchased from Shenzen
NTP Factory.
All materials were commercial reagent grade and pur-
chased from Merck or Fluka. All alkenes were passed
through a column containing active alumina to remove
peroxide impurities. A 400-W Hg lamp was used for
activation of the metal carbonyl. FTIR spectra were
Chlorination of MWCNT-COOH
MWCNT-COOH (5 g) and SOCl (30 mL) were mixed
2
-
1
obtained as KBr pellets in the range 500–4,000 cm with
a Bomen–Hartmann instrument. Scanning electron micro-
graphs of the catalyst were taken on a SEM Philips XL 30
and refluxed under N for 1 h. The reaction mixture was
2
cooled, and the SOCl was evaporated. The resulting pre-
2
cipitate is designated as chlorinated multi-wall carbon
nanotubes, MWCNT-COCl (yield 92%, 4.7 g).
1
instrument. H NMR spectra were recorded on a Bruker-
Avance AQS 400 MHz spectrometer. Gas chromatography
experiments were performed with a Shimadzu GC-16A
instrument using a 2 m column packed with silicon DC-200
or Carbowax 20 M. ICP analyzes were performed on an
ICP-Spectrociros CCD instrument. The products were
identified by comparison of their retention times with
Preparation of MWCNT-APy
MWCNT-COCl (1 g) and Et N (1 mL) were added to a
3
solution of 4-aminopyridine (0.5 g) in DMF (10 mL) and
heated at 80 °C for 72 h. The mixture was then filtered and
1
23

Transition Met Chem (2011) 36:861–866
865
Table 4 Comparison of the results obtained for epoxidation of cyclooctene catalyzed by [W(CO)
in the literature
5
@APy-MWCNT] with some of those reported
-
1
)
Entry
Catalyst
Support
Oxidant
Conditions (°C)/solvent
TOF (h
References
1
2
3
4
5
Tungsten hexacarbonyl
Tungsten hexacarbonyl
Molybdenum hexacarbonyl
Molybdenum hexacarbonyl
MTO
MWCNT
Polystyrene
Polystyrene
MWCNT
–
H
H
2
O
O
2
2
Reflux/CH
Reflux/CH
3
CN
CN
27.2
18.8
31.3
78.4
5.0
This work
[35]
2
3
TBHP
TBHP
Reflux/CCl
Reflux/CCl
4
[36]
4
[32]
H
H
H
2
O
2
O
2
O
2
2
2
RT/CH
RT/–
2
Cl
2
[37]
3.3
6
7
8
9
WO
2
Cl
2
(OPPh
2
CH
2
OH)
2
–
70/Ethanol
13.8
7.4
[38]
[39]
[40]
[41]
[42]
[43]
[44]
[45]
[30]
[29]
[46]
[Mn(salophen)Cl]
[Mn(salophen)Cl]
[Mn(salen)]
Silica
NaIO
NaIO
NaIO
4
4
4
RT/CH
RT/CH
RT/CH
3
3
3
2 2
CN–H O
Zeolite
Dowex MSC1
Silica
CN-H
2
O
2
2.1
CN–H
2
O
2
41.9
16.0
14.7
4.7
10
11
12
13
14
15
16
MoO
2
(acac)
(acac)
2
2
TBHP
TBHP
Reflux/DCE
Reflux/DCE
MoO
2
MCM-41
Polystyrene
Silica
[Ru(salophen)Cl]
[Ru(salophen)Cl]
[Mn(salophen)Cl]
NaIO
NaIO
NaIO
NaIO
NaIO
4
4
4
4
4
RT/CH
RT/CH
RT/CH
RT/CH
RT/CH
3
CN–H
3
CN–H
3
CN–H
3
CN–H
3
CN–H
2
2
2
2
2
O
O
O
O
O
2
2
2
2
2
4.6
MWCNT
MWCNT
Silica
8.0
Mn(TNH
Mn(TPP)Cl
Mn(Br TPP)Cl
2
PP)Cl
9.5
8.3
8
4.8
Table 5 Reusability of [W(CO)
of cyclooctene with H
5
@Apy-MWCNT] in the epoxidation
under reflux conditions after 3 h
with THF and dried under vacuum (yield 94%, 0.99 g).
O
2 2
The unreacted W(CO) was recovered from the filtrate
6
a
b
after evaporation of the solvent.
Run
Yield (%)
W leached (%)
1
2
3
4
5
6
7
8
96
96
96
96
96
96
96
96
1.5
0.7
0
General procedure for epoxidation of alkenes
The appropriate alkene (1 mmol), H O (0.5 mL) mmol),
2
2
0
catalyst (200 mg, 0.012 mmol), and MeCN (5 mL) were
mixed together and refluxed. Since different alkenes have
different reactivities toward oxidation, the reactions were
continued until no further progress was observed by GC.
After completion of the reaction, the reaction mixture was
0
0
0
0
Reaction conditions: cyclooctene (1 mmol),
W(CO) @Apy-MWCNT (200 mg), MeCN (5 mL)
H
2
O
2
(0.5 mL),
diluted with Et O (20 mL) and filtered. The catalyst was
2
5
thoroughly washed with Et O, and the combined washings
2
a
GLC yield based on starting alkene
Determined by ICP
and filtrate were purified on TLC on silica gel using
b
n-hexane/Et O (3/1) to obtain the pure product.
2
the solid washed with MeCN and dried at 60 °C (yield
0%, 0.95 g). CHN analysis of MWCNT-APy: C: 93.1%,
H: 0.3%, and N: 1.8%.
Reusability of the catalyst
9
The reusability of the catalyst was studied by multiple
sequential epoxidations of cyclooctene. The reactions were
carried out as described earlier. At the end of each reaction,
Preparation of [W(CO) @APy-MWCNT]
5
the catalyst was filtered off, washed thoroughly with Et O,
2
To activate the W(CO) , a mixture of W(CO) (1.5 g,
6
6
dried, and reused.
4
.26 mmol) in THF (50 mL) was stirred under UV irradi-
ation for 15 min [48]. Then, MWCNT-APy (1 g) was
added to the solution and refluxed for 1 h. At the end of the
reaction, the catalyst was filtered off, washed thoroughly
Acknowledgment The support of this work by the Center of
Excellence of Chemistry of University of Isfahan (CECIU) is grate-
fully acknowledged.
123

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