Multi-wall carbon nanotube supported tungsten hexacarbonyl: An efficient and reusable catalyst for epoxidation of alkenes with hydrogen peroxide

Article abstract of DOI:10.1080/00958972.2011.643788

Highly efficient epoxidation of alkenes with H₂O₂ catalyzed by tungsten hexacarbonyl supported on multi-wall carbon nanotubes (MWCNTs) modified with 1,2-diaminobenzene is reported. The prepared catalyst, [W(CO)₆@DAB-MWCNT], was characterized by elemental analysis, scanning electron microscopy, FT-IR, and diffuse reflectance UV-Vis spectroscopic methods. The prepared catalyst was applied as an efficient catalyst for green epoxidation of alkenes with hydrogen peroxide in CH₃CN. This heterogeneous metal carbonyl catalyst showed high stability and reusability in epoxidation without loss of its catalytic activity. Copyright

Full text of DOI:10.1080/00958972.2011.643788

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Multi-wall carbon nanotube supported
tungsten hexacarbonyl: an efficient
and reusable catalyst for epoxidation of
alkenes with hydrogen peroxide
Mehdi Nooraeipour a , Majid Moghadam b , Shahram
Tangestaninejad b , Valiollah Mirkhani b , Iraj Mohammadpoor-
Baltork b & Nasir Iravani a
a Department of Chemistry , Islamic Azad University, Gachsaran
Branch , Gachsaran , Iran
b Chemistry Department, Catalysis Division , University of
Isfahan , Isfahan 81746-73441 , Iran
Published online: 22 Dec 2011.
To cite this article: Mehdi Nooraeipour , Majid Moghadam , Shahram Tangestaninejad , Valiollah
Mirkhani , Iraj Mohammadpoor-Baltork & Nasir Iravani (2012) Multi-wall carbon nanotube supported
tungsten hexacarbonyl: an efficient and reusable catalyst for epoxidation of alkenes with hydrogen
peroxide, Journal of Coordination Chemistry, 65:2, 226-238, DOI: 10.1080/00958972.2011.643788
To link to this article: http://dx.doi.org/10.1080/00958972.2011.643788
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Journal of Coordination Chemistry
Vol. 65, No. 2, 20 January 2012, 226–238
Multi-wall carbon nanotube supported tungsten
hexacarbonyl: an efficient and reusable catalyst
for epoxidation of alkenes with hydrogen peroxide
MEHDI NOORAEIPOURy, MAJID MOGHADAM*z,
SHAHRAM TANGESTANINEJADz, VALIOLLAH MIRKHANIz,
IRAJ MOHAMMADPOOR-BALTORKz and NASIR IRAVANIy
yDepartment of Chemistry, Islamic Azad University, Gachsaran Branch, Gachsaran, Iran
zChemistry Department, Catalysis Division, University of Isfahan,
Isfahan 81746-73441, Iran
(Received 14 June 2011; in final form 8 November 2011)
Highly efficient epoxidation of alkenes with H2O2 catalyzed by tungsten hexacarbonyl
supported on multi-wall carbon nanotubes (MWCNTs) modified with 1,2-diaminobenzene is
reported. The prepared catalyst, [W(CO)6@DAB-MWCNT], was characterized by elemental
analysis, scanning electron microscopy, FT-IR, and diffuse reflectance UV-Vis spectroscopic
methods. The prepared catalyst was applied as an efficient catalyst for green epoxidation of
alkenes with hydrogen peroxide in CH3CN. This heterogeneous metal carbonyl catalyst showed
high stability and reusability in epoxidation without loss of its catalytic activity.
Keywords: Multi-wall carbon nanotubes; Tungsten hexacarbonyl; Heterogeneous catalyst;
Epoxidation; Hydrogen peroxide
1. Introduction
Epoxides play an important role in industry as intermediates for production of fine
chemicals and pharmaceuticals [1]. Application of green oxidations has received great
interest from economical and environmental points of view. Environmentally friendly
oxygen donors such as molecular oxygen and hydrogen peroxide have attracted
attention. In particular, hydrogen peroxide has been widely used as oxidant due to its
high oxygen content and formation of water as by-product [2, 3]. Catalysts such as iron
and manganese porphyrins [4, 5], phthalocyanines [6], iron amide complexes [7–9],
TAML [10], selenoxides [11], polyoxometalates [12–15], titanium silicalite [16, 17],
tungsten, molybdenum, and vanadium complexes [18], Sn–zeolite beta [19, 20], and
MTO [21–28] are known to activate hydrogen peroxide.
Transition metal complexes are effective catalysts for a variety of organic reactions,
especially epoxidation of alkenes [29]. But the major problem associated with these
homogeneous catalysts is recovery of catalyst from the reaction medium. There have
*Corresponding author. Email: moghadamm@sci.ui.ac.ir
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2011.643788

Carbon nanotube supported tungsten
227
[W(CO)6@DAB-MWCNT]
H2O2, CH3CN
O
Scheme 1. Epoxidation of alkenes with H2O2 catalyzed by [W(CO)6@DAB-MWCNT].
been intense efforts for recovering and reusing homogeneous catalysts [30]. An efficient
and practical way for commercial applications of these catalysts is their immobilization
on solid supports [31–51].
Supported catalysts based on tungsten species have found many applications in
heterogeneous catalysis, widely used in several important industrial reactions, such as
hydrotreatments, hydrocracking of heavy fractions from oil, dehydration of alcohols,
metathesis, and isomerization of olefins [52]. The supports carrying tungsten species are
A12O3 and TiO2 [53–56], SiO2 [57, 58], MgO [59], activated carbon [60], polymers
[51, 61–63], and MgF2 [64]. These catalysts have been used for efficient oxidation
reactions.
Carbon nanotubes (CNTs) have attracted attention in synthesis, characterization,
and other applications because of their unique structural, mechanical, thermal, optical,
and electronic properties [65, 66]. Due to their high surface area, chemical stability,
and insolubility in most solvents, CNTs can be used as catalyst supports. For example,
Pt nanoparticles supported on CNTs have been used for methanol oxidation [67],
palladium nanoparticles supported on CNTs for semihydrogenation of phenylacetylene
[68], chiral vanadyl salen complex supported on single-wall CNTs for enantioselective
cyanosilylation of aldehydes [69, 70], manganese(III) porphyrin and manganese(III)
salophen supported on multi-wall CNTs (MWCNTs) for epoxidation of alkenes with
NaIO4 [71, 72], and molybdenum hexacarbonyl supported on MWCNTs for alkene
epoxidation with tert-BuOOH [73–75].
In this article, preparation, characterization, and investigation of catalytic activity of
W(CO)6 supported on functionalized MWCNTs is reported (scheme 1). The catalyst,
[W(CO)6@DAB-MWCNT], was characterized by elemental analysis, scanning electron
microscopy (SEM), FT-IR, and diffuse reflectance UV-Vis spectroscopic methods.
This heterogeneous catalyst was applied as an efficient catalyst for green epoxidation
of alkenes with H2O2 in CH3CN. The stability and reusability of this catalyst were also
investigated in the epoxidation.
2. Experimental
All materials (SOCl2, Et3N, DMF, 1,2-diaminobenzene, CH3CN, W(CO)6, THF, H2O2,
and Et2O) were of commercial reagent grade, purchased from Merck and Fluka
chemical companies. All alkenes (cyclooctene, cyclohexene, styrene, ꢀ-methylstyrene,
1-octene, 1-dodecene, cis-stilbene, and trans-stilbene) were passed through a column
containing active alumina to remove peroxide impurities. A 400 W Hg lamp was used
for activation of metal carbonyl. FT-IR spectra were obtained as potassium bromide
pellets from 500 to 4000 cmꢀ1 with a Bomen-Hartmann instrument. Scanning electron
1
micrographs of the catalyst were taken on SEM Philips XL 30. H NMR spectra were
recorded on a Bruker-Avance AQS 400 MHz. Gas chromatography (GC) experiments

228
M. Nooraeipour et al.
were performed with a Shimadzu GC-16A instrument using a 2 m column packed
with silicon DC-200 or Carbowax 20M. The ICP analyses were performed on an
ICP-Spectrociros CCD instrument. The products were identified by comparison of their
1
retention times with known samples and also with their H NMR spectra. MWCNTs
containing carboxylic acid groups (MWCNTs with diameters between 20 and 30 nm)
were purchased from Shenzen NTP Factory.
2.1. Chlorination of MWCNT-COOH
In a 100 mL round-bottom flask equipped with a condenser and a magnetic stir bar,
MWCNT-COOH (5 g) and SOCl2 (30 mL) were mixed and refluxed under N2 for 1 h.
Then, the reaction mixture was cooled and SOCl2 evaporated. The resulting precipitate
is chlorinated MWCNTs, MWCNT-COCl [72].
2.2. Modification of MWCNT-COCl with 1,2-diaminobenzene
1,2-Diaminobenzene (DAB) (2 g) and triethylamine (5 mL) were added to a suspension
of MWCNT-COCl (5 g) in DMF (50 mL). The mixture was vigorously stirred at 80ꢁC
for 48 h. After cooling the mixture, black solids were collected by filtration, washed
thoroughly with CH3CN, and dried in vacuum for 5 h to reach the modified support,
DAB-MWCNT [74].
2.3. Preparation of the catalyst, [W(CO)6@DAB-MWCNT]
W(CO)6 (1.5 g, 4.26 mmol) was dissolved in THF (60 mL) and stirred under UV
irradiation for 15 min [76]. Then, DAB-MWCNTs (2 g) was added to this solution and
refluxed for 1 h. At the end of the reaction, the solution was filtered, washed thoroughly
with THF to remove unreacted W(CO)6 and dried in vacuum at room temperature to
remove THF [74].
2.4. General procedure for epoxidation of alkenes with H2O2 catalyzed by
[W(CO)6@DAB-MWCNT]
In a 25 mL round bottom flask equipped with a magnetic stir bar, a mixture of alkene
(1 mmol), H2O2 (0.5 mL), catalyst (200 mg, 0.006 mmol), and CH3CN (5 mL) was
prepared and refluxed with stirring. The reaction progress was monitored by GC. At the
end of the reaction, the mixture was diluted with Et2O (20 mL) and filtered. The catalyst
was thoroughly washed with Et2O and the combined washing and filtrates were purified
on a silica gel plate to obtain the pure product.
2.5. Reusability of the catalyst
The reusability of the catalyst was studied in the repeated epoxidation of cyclooctene.
The reactions were carried out as described above. At the end of each reaction, the
catalyst was filtered, washed thoroughly with Et2O, dried, and reused.

Carbon nanotube supported tungsten
229
3. Results and discussion
3.1. Preparation and characterization of [W(CO)6@DAB-MWCNT]
The specifications of MWCNT-COOH used in this study are presented in table 1.
The preparation route for attachment of W(CO)6 to MWCNTs modified with
1,2-diaminobenzene, DAB-MWCNT, is shown in scheme 2. First, MWCNT-COOH
was chlorinated to activate the MWCNTs. Then, DAB-MWCNT was prepared by
covalent attachment of 1,2-diaminobenzene to MWCNT-COCl via an amide linkage.
Table 1. The specifications of MWCNT-COOH used in this study.
Outside diameter
20–30 nm
Inside diameter
5–10 nm
Length
COOH content
1.5%
Specific surface area
30 mm
4110 m2 gꢀ1
SOCl2
Reflux, 1h
COOH
COCl
DMF, 80°C
Et3N, 48 h
H2N
NH2
O
C
N
H
hv (30 min)
H2N
W(CO)5THF
W(CO)6
+ THF
Reflux, 1h
O
C
O
C
+
NH
CO
W
N
OC
CO
H
NH2
H2N
W
CO
CO
CO
CO
CO
CO
Scheme 2. The preparation route for [W(CO)6@DAB-MWCNT].

230
M. Nooraeipour et al.
Such functionalization allows preparation of a ligand which can coordinate to a metal
center. Then, W(CO)5THF (prepared by activation of W(CO)6 in THF under UV
irradiation) was reacted with DAB-MWCNT to produce [W(CO)6@DAB-MWCNT].
The tungsten species coordinated to amine groups. The DAB-MWCNT can act as
unidentate or bidentate ligand. Both proposed structures could be catalysts, active in
epoxidation. The prepared catalyst, [W(CO)6@DAB-MWCNT], was characterized by
elemental analysis, SEM, FT-IR, and diffuse reflectance UV-Vis spectroscopic
methods. The nitrogen content of DAB-MWCNT was determined by CHN analysis
which showed a value of 2.94%. Based on these values, the nitrogen content of the
support was about 2.1 mmolgꢀ1 of the DAB-MWCNT. The nitrogen content of the
catalyst was about 2.84% (2.02 mmolgꢀ1). The W content of the catalyst was also
measured by ICP, which showed a value of about 0.03 mmolgꢀ1 of [W(CO)6@DAB-
MWCNT].
FT-IR spectra of MWCNTs, DAB-MWCNT, and [W(CO)6@DAB-MWCNT] are
shown in figure 1. The C¼O stretching band of the carboxylic acid is at 1640 cmꢀ1
(figure 1a). Upon attachment of DAB to MWCNTs, the C¼O stretch of amide
appeared at 1632 cmꢀ1 (figure 1b). In the FT-IR spectrum of [W(CO)6@DAB-
MWCNT], bands at 1948 and 1801 cmꢀ1 were assigned to the terminal carbonyl groups
(figure 1c). These observations prove coordination of W(CO)6 species to DAB-
MWCNT. Further evidence for the attachment of W(CO)6 to DAB-MWCNT was
provided by UV-Vis spectroscopy in the diffuse reflectance mode. W(CO)6 showed two
strong absorptions at 246 and 290 nm which were attributed to W!CO charge transfer
bands (figure 2a). These peaks were also observed in the diffuse reflectance UV-Vis of
[W(CO)6@DAB-MWCNT] (figure 2b), while MWCNTs showed no absorption peak
in this region (figure 2c). These observations indicated that tungsten hexacarbonyl
is supported on MWCNTs. The SEM images of the [W(CO)6@DAB-MWCNT]
showed that the nanotubes are aggregated and retain their nanotube nature (figure 3).
3.2. Epoxidation of alkenes with H2O2 catalyzed by [W(CO)6@DAB-MWCNT]
First, the catalytic activity of [W(CO)6@DAB-MWCNT] was studied in epoxidation of
cyclooctene with hydrogen peroxide. Different amounts of [W(CO)6@DAB-MWCNT]
were used to optimize the catalyst amount. The best results were obtained in the
presence of 200 mg (0.006 mmol) of catalyst (table 2). In order to optimize the reaction
media, different solvents such as acetone, tetrahydrofuran, 1,2-dichloroethane, aceto-
nitrile, carbon tetrachloride, chloroform, and dichloromethane were checked in the
epoxidation of cyclooctene with H2O2 under reflux (for obtaining the highest product
yield). The results (table 3) showed that the highest epoxide yield was observed in
CH3CN. The exact role of the solvent is not completely clear. But the higher catalytic
activity of the catalyst in acetonitrile can be attributed to the polarity of the solvent and
better solubility of all substrates in it. CH3CN can facilitate formation of metal oxo
complex which is responsible for oxidation of alkenes. During the reaction,
[W(CO)6@DAB-MWCNT] is suspended in the solvent. This is due to the bundled
agglomerates of MWNT that aggregate slowly (this is due to the presence of functional
groups and also the weak ꢁ–ꢁ interactions) and make the catalyst suspend in the
reaction mixture for a long period of time; other heterogeneous catalysts were
aggregated more rapidly in comparison with CNTs. Control experiments in the absence

Carbon nanotube supported tungsten
231
Figure 1. FT-IR spectrum of (a) MWCNTs, (b) DAB-MWCNT, and (c) [W(CO)6@DAB-MWCNT].

232
M. Nooraeipour et al.
Figure 2. UV-Vis spectrum of (a) homogeneous W(CO)6, (b) [W(CO)6@DAB-MWCNT], and
(c) MWCNT.
Figure 3. SEM image of [W(CO)6@DAB-MWCNT].

Carbon nanotube supported tungsten
233
Table 2. Optimization of catalyst amount in the epoxidation of cyclooctene with H2O2.a
Entry
Time (h)
Catalyst amounts (mmol, mg)
Yield (%)b
1
2
3
4
5
6
7
8
2
2
2
2
2
2
2
2
No catalyst
10
78
90
93
98
98
12
67
50 mg (0.0015 mmol)
100 mg (0.003 mmol)
150 mg (0.0045 mmol)
200 mg (0.006 mmol)
250 mg (0.0075 mmol)
DAB-MWCNT (200 mg)
W(CO)6 (0.006 mmol)
aReaction conditions: cyclooctene (1 mmol), H2O2 (0.5 mL), catalyst, CH3CN (5 mL).
bGC yield based on the starting cyclooctene.
Table 3. Epoxidation of cis-cyclooctene with H2O2 catalyzed by [W(CO)6@DAB-
MWCNT] under reflux in different solvents.a
Solvent
Epoxide yield after 2 h (%)b
T (ꢁC)
(CH3)2CO
THF
CH3CN
ClCH2CH2Cl
CHCl3
9
17
98
29
22
18
14
53
61
77
78
57
72
38
CCl4
CH2Cl2
aReaction conditions: cyclooctene (1 mmol), H2O2 (0.5 mL), catalyst (200 mg, 0.006 mmol), solvent
(5 mL).
bGC yield based on the starting cyclooctene.
of catalyst or using MWCNT-DAB as catalyst were also performed (table 2); the
amount of epoxide was about 10–12%. When the same reaction was carried out using
W(CO)6 (0.006 mmol, which is equal to the amount of W(CO)6 in 200 mg of MWCNTs)
as catalyst, the epoxide yield was 67%. The reaction conditions were also optimized for
epoxidation of 1-octene and the same results were observed.
Under the optimized conditions, [W(CO)6@DAB-MWCNT]/H2O2 catalytic system
was used for epoxidation of a wide range of alkenes in refluxing acetonitrile (table 4).
Both cyclic and linear alkenes were efficiently converted to their corresponding epoxides
with H2O2 in the presence of catalytic amounts of [W(CO)6@DAB-MWCNT].
Cyclooctene, cyclohexene (as cyclic alkenes), 1-octene, and 1-dodecene (as linear
alkenes) were efficiently converted to their corresponding epoxides by [W(CO)6@DAB-
MWCNT]. In the case of styrene and ꢀ-methyl styrene, benzaldehyde and
acetophenone were produced as by-products, respectively. In the case of trans-stilbene,
the reaction was stereospecific and only trans-epoxide was produced while for
cis-stilbene some loss of stereochemistry was observed and afforded 80% cis- and
10% trans-stilbene epoxides (table 4). During the oxidation reactions, no acetamide
due to oxidation of acetonitrile was observed.
The results obtained with [W(CO)6@DAB-MWCNT]/H2O2 are compared with some
previously reported epoxidation catalysts in table 5. These comparisons show that the
present catalytic system is more efficient than the others. Comparison of the catalytic

234
M. Nooraeipour et al.
Table 4. Epoxidation of alkenes with H2O2 catalyzed [W(CO)6@DAB-MWCNT]
in refluxing CH3CN.a
Conversion
(%)b
Entry
1
Alkene
Epoxide (%)b
Time (h)
2
98
98
2
3
4
100
76c
84d
100
73
2
5
3
60
5
6
100
26
100
26
3
5
7
8
84e
84 (trans-epoxide)
10
90e
80 (cis-epoxide) 10 (trans-epoxide) 10
aReaction conditions: alkene (1 mmol), H2O2 (0.5 mL), catalyst (200 mg, 0.006 mmol), CH3CN
(5 mL).
bGC yield based on starting alkene.
cThe by-product was benzaldehyde.
dThe by-product was acetophenone.
eBoth 1H NMR and GC data approved the reported yields.
Table 5. Comparison of the results obtained for epoxidation of cyclooctene catalyzed by [W(CO)6@DAB-
MWCNT] with some of those reported in the literature.
Conditions
(ꢁC)/solvent
Entry
Catalyst
Support
Oxidant
TOF (hꢀ1
)
Ref.
1
2
3
4
5
6
Tungsten hexacarbonyl
Tungsten hexacarbonyl
MWCNT
Polystyrene
H2O2
H2O2
Reflux/CH3CN
Reflux/CH3CN
Molybdenum hexacarbonyl Polystyrene TBHP Reflux/CCl4
81.7
18.8
31.3
78.4
8.0
This work
[51]
[50]
Molybdenum hexacarbonyl MWCNT
[Mn(salophen)Cl]
Mn(TNH2PP)Cl
TBHP Reflux/CCl4
NaIO4 RT/CH3CN–H2O2
NaIO4 RT/CH3CN–H2O2
[74]
[72]
[71]
MWCNT
MWCNT
9.5

Carbon nanotube supported tungsten
235
Table 6. Reusability of [W(CO)6@DAB-MWCNT] in the epoxidation
of cyclooctene with H2O2 under reflux.a
Run
Time (h)
Yield (%)b
W leached (%)c
1
2
3
4
5
6
2
2
2
2
2
2
98
98
98
98
98
98
1.1
0.5
0
0
0
0
aReaction conditions: cyclooctene (1 mmol), H2O2 (0.5 mL), [W(CO)6@DAB-
MWCNT] (200 mg, 0.006 mmol), CH3CN (5 mL).
bGC yield based on starting alkene.
cDetermined by ICP.
OC
CO
MWCNT
MWCNT
N
W
CO
N
CO
OH
O
N
W
O
OH
O
N
H2O2
H2O
O
O
O
O
MWCNT
N
W
OH
MWCNT
N
W
O
N
N
O
O
O
O
MWCNT
N
W
O
N
O
Scheme 3. Proposed mechanism for epoxidation of alkenes with H2O2 catalyzed by [W(CO)6@DAB-
MWCNT].

236
M. Nooraeipour et al.
Figure 4. FT-IR spectrum of recovered [W(CO)6@DAB-MWCNT].
activity of tungsten hexacarbonyl supported on polystyrene [51] and MWCNTs shows
the effect of support in which the TOF increases from 18.8 for polystyrene to 81.7 for
MWCNTs. This can be attributed to the higher surface area of MWCNTs compared
to polystyrene.
3.3. Catalyst reuse and stability
The reusability of [W(CO)6@DAB-MWCNT] was monitored in multiple sequential
epoxidations of cyclooctene with H2O2 (table 6). At the end of each reaction, the
catalyst was filtered and reused in the next run. The catalyst was reused (six consecutive
times were checked) without loss of its initial activity. The amount of tungsten, which
was detected in the filtrates in first two runs, was low and after 3rd 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 H2O2 to the filtrates after each run. Execution of the oxidation
reaction under the same reaction conditions, as with catalyst, showed that the obtained
results were the same as the results of blank experiments.
The recovered catalyst was studied by FT-IR spectroscopy. No band corresponding
to terminal C¼O was observed in the FT-IR spectrum of the recovered catalyst. This is
in accord with the mechanism previously reported [77]. In this mechanism, CO ligands
are eliminated and the W¼O species are produced (scheme 3, figure 4). Therefore,
W(CO)6 can be considered as a catalyst precursor.
4. Conclusion
Tungsten hexacarbonyl supported on MWCNTs modified with 1,2-diaminobenzene
was used as an efficient catalyst in epoxidation of alkenes with H2O2. This supported

Carbon nanotube supported tungsten
237
catalyst, [W(CO)6@DAB-MWCNT], was reactive in the epoxidation of a wide range of
alkenes such as linear and cyclic ones. The catalyst was reusable and recycled six times
without loss of catalytic activity.
Acknowledgments
The support of this work by the Center of Excellence of Chemistry of University of
Isfahan (CECIU) is gratefully acknowledged.
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