Molybdenum hexacarbonyl supported on amine modified multi-wall carbon nanotubes: An efficient and highly reusable catalyst for epoxidation of alkenes with tert-butylhydroperoxide wileyonlinelibrary.com

Article abstract of DOI:10.1002/aoc.1671

In the present work, highly efficient epoxidation of alkenes catalyzed by Mo(CO)₆ supported on multi-wall carbon nanotubes modified by 2-aminopyrazine, APyz-MWCNTs, is reported. The prepared catalyst was characterized by elemental analysis, scanning electron microscopy, FT IR and diffuses reflectance UV-vis spectroscopic methods. This new heterogenized catalysts, [Mo(CO)₆@APyz-MWCNT], was used asa highly efficient catalyst for epoxidation of alkenes with tert-BuOOH. This robust catalyst was reused several times without loss of its catalytic activity. Copyright

Full text of DOI:10.1002/aoc.1671

Full Paper
Received: 21 February 2010
Accepted: 17 April 2010
Published online in Wiley Online Library: 28 June 2010
(wileyonlinelibrary.com) DOI 10.1002/aoc.1671
Molybdenum hexacarbonyl supported
on amine modified multi-wall carbon
nanotubes: an efficient and highly reusable
catalyst for epoxidation of alkenes
with tert-butylhydroperoxide
Majid Moghadama,b∗, Shahram Tangestaninejada, Valiollah Mirkhania,
Iraj Mohammadpoor-Baltorka and Naghmeh Sadat Mirbagheria
In the present work, highly efficient epoxidation of alkenes catalyzed by Mo(CO)6 supported on multi-wall carbon nanotubes
modified by 2-aminopyrazine, APyz-MWCNTs, is reported. The prepared catalyst was characterized by elemental analysis,
scanning electron microscopy, FT IR and diffuses reflectance UV–vis spectroscopic methods. This new heterogenized catalysts,
[Mo(CO)6@APyz-MWCNT], wasusedasahighlyefficientcatalystforepoxidationofalkeneswithtert-BuOOH. Thisrobustcatalyst
c
was reused several times without loss of its catalytic activity. Copyright ꢀ 2010 John Wiley & Sons, Ltd.
Keywords: multi-wall carbon nanotubes; molybdenum hexacarbonyl; heterogeneous catalyst; epoxidation; tert-butylhydroperoxide
Introduction
O
[Mo(CO)6@APyz-MWCNT]
tert-BuOOH, CCl4
Epoxidation of alkenes catalyzed by metal complexes is an
important reaction in organic synthesis because these com-
pounds serve as useful intermediates in the synthesis of a
wide variety of valuable compounds such as polyethers, di-
ols and aminoalcohols.[1] Transition metals such as rhenium,[2]
titanium,[3] vanadium,[4] manganese and molybdenum[5–10] have
been used for alkene epoxidation. Among them, Mo (VI) com-
pounds are the most versatile catalysts for the epoxidation of
alkenes.[5–10] An important example is the Halcon process, for
the industrial synthesis of propylene oxide, which is carried out
by liquid-phase epoxidation of propylene with alkyl hydroperox-
ides catalyzed by a homogeneous Mo(VI).[11] The homogeneous
catalysts of transition metals are often more difficult to pre-
pare and expensive to purchase. Some industrial problems such
as deposition on reactor wall, difficulty in recovery and sep-
aration of the catalyst from reaction products are associated
with homogeneous catalysts. One way to overcome these dis-
advantages is immobilization of homogeneous catalysts on solid
supports.
Scheme 1. EpoxidationofalkeneswithTBHPcatalyzedby[Mo(CO)6@APyz-
MWCNT].
support for immobilization of molybdenum compounds. On the
other hands, molybdenum catalysts have been supported on
silica,[24–28] modified MCM-41,[29–37] zeolites[38] and layered dou-
ble hydroxides.[39]
Carbon nanotubes (CNTs) have attracted much attention in
synthesis, characterization and other applications because of their
unique structural, mechanical, thermal, optical and electronical
properties. Since CNTs are insoluble in the most solvents, these
materials can be used as catalysts support.[40–43]
Inthispaper,thepreparation,characterizationandinvestigation
of catalytic activity of Mo(CO)6 supported on multi-wall carbon
nanotubes in the epoxidation of alkenes with tert-BuOOH is
reported (Scheme 1).
Therefore, attention is now being drawn to the synthesis
of heterogeneous catalysts based on these complexes, which
can be easily separated from reaction mixture and recycled.
Different approaches have been used to immobilize molybde-
num on various supports to obtain heterogeneous catalysts.
Sherrington and coworkers have reported efficient epoxidation
of alkenes with tert-butylhydroperoxide catalyzed by reusable
Mo(VI) supported on imidazole containing polymers.[12–15]
Other organic polymers including modified polystyrenes,[16–20]
polyaniline,[21] ion-exchange resins,[22] ethylene-propylene rub-
ber and modified poly(ethylene oxide)[23] have been used as
∗
Correspondence to: Majid Moghadam, Chemistry Department, Catalysis
Division, University of Isfahan, Isfahan 81746-73441, Iran.
E-mail: moghadamm@sci.ui.ac.ir
a
ChemistryDepartment,CatalysisDivision,UniversityofIsfahan,Isfahan81746-
73441, Iran
b
Department of Nanotechnology Engineering, University of Isfahan, Isfahan
81746-73441, Iran
c
Appl. Organometal. Chem. 2010, 24, 708–713
Copyright ꢀ 2010 John Wiley & Sons, Ltd.

Molybdenum hexacarbonyl supported on amine modified multi-wall carbon nanotubes
(A)
(B)
Figure 1. The FT IR spectrum of (A) MWCNT-APyz and (B) [Mo(CO)6@APyz-MWCNT].
Experimental
the range 500–4000 cm−1 with a Bomen–Hartmann instrument.
Scanning electron micrographs of the catalyst were taken on
SEM Philips XL 30. 1H NMR spectra were recorded on a
Bruker–Avance AQS 400 MHz. Gas chromatography experiments
(GC) 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
All materials were commercial reagent grade and obtained from
Merck and 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 metal carbonyl.
FT-IR spectra were obtained as potassium bromide pellets in
c
Appl. Organometal. Chem. 2010, 24, 708–713
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M. Moghadam et al.
50 ml round-bottom flask equipped with a magnetic stirring bar,
MWCNT-COCl (1 g) and Et3N (1 ml) were added to a solution
of 2-aminopyrazine (0.5 g) in dimethyl formamide (DMF) (10 ml)
and heated at 80 ◦C for 72 h. Then, the reaction mixture was
filtered, washed with CH3CN and dried at 60 ◦C. CHN analysis of
MWCNT-APyz: C, 84.83%; H, 1.53%; N, 6.15%.
(A)
Preparation of the catalyst, [Mo(CO)6@APyz-MWCNT]
First, the metal carbonyl was activated by stirring a mixture of
Mo(CO)6 (2 g, 7.5 mmol) in tetrahydrofuran (THF; 60 ml) under UV
irradiation for 15 min.[44] Then, MWCNT-APyz (1 g) was added to
this solution and refluxed for 1 h. At the end of the reaction, the
catalyst was filtered, washed thoroughly with THF and dried in
vacuum. The unreacted Mo(CO)6 was recovered after evaporation
of the solvent
(B)
General Procedure for Epoxidation of Alkenes with tert-
BuOOH Catalyzed by [Mo(CO)6@APyz-MWCNT]
In a 25 ml round bottom flask equipped with a magnetic stirrer
bar and a condenser, alkene (1 mmol), tert-BuOOH (2 mmol, 80%
solution in di-tert-butylperoxide), catalyst (100 mg, 0.042 mmol)
and CCl4 (6 ml) were mixed and refluxed. The reaction progress
was monitored by GC. At the end of the reaction (since different
alkenes has different reactivity toward oxidation, the reactions
were continued until no further progress was observed), the
reaction 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.
(C)
Reusability of the Catalyst
The reusability of the catalyst was studied in the repeated
epoxidationreactionofcis-cyclooctene. Thereactionswerecarried
out as described above. At the end of each reaction, the catalyst
was filtered, washed thoroughly with Et2O, dried and reused.
Results and Discussion
PreparationandCharacterizationofcatalyst,[Mo(CO)6@APyz-
MWCNT]
The specification of MWCNT-COOH used in this study is presented
in Table 1. Scheme 2 shows the preparation procedure of
MWCNT-supported molybdenum catalyst. In this scheme, the
probable modes for coordination of [Mo(CO)6] are shown. The
modified MWCNT, APyz-MWCNT, was prepared by covalent
attachment of 2-aminopyrazine to MWCNT-COCl via an amide
linkage. The [Mo(CO)6@APyz-MWCNT] catalyst was prepared by
the reaction of APyz-MWCNT with a solution of Mo(CO)5THF.
The catalyst was characterized by elemental analysis, scanning
electron microscopy, FT-IR and diffuses reflectance UV–vis
spectroscopic methods. The nitrogen content of support was
6.14% (4.38 mmol/g). The metal loading of [Mo(CO)6@APyz-
MWCNT], measured by ICP, was 0.42 mmol/g. The FT-IR spectra
of MWCNT-APyz and [Mo(CO)6@APyz-MWCNT] are shown in
Fig. 1. The C O stretching band of the amide group had
appeared at 1656 cm−1. The band at 1961 cm−1 was assigned
to terminal carbonyl stretching band. These observations proved
the coordination of [Mo(CO)6] to APyz-MWCNT. Further evidence
Figure 2. The UV-Vis spectrum of: (A) [Mo(CO)6]; (B) [Mo(CO)6@APyz-
MWCNT]; and (C) MWCNT-APyz.
instrument. The products were identified by comparison of their
retention times with known samples and also with their 1H NMR
spectra.
Preparation of Multi-wall Carbon Nanotubes-supported
Molybdenum Hexacarbonyl
Preparation of MWCNT-APyz
The carboxylic acid groups (MWCNT-COOH) in MWCNTs were
converted to acid chloride (MWCNT-COCl) as reported.[43] In a
c
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Appl. Organometal. Chem. 2010, 24, 708–713

Molybdenum hexacarbonyl supported on amine modified multi-wall carbon nanotubes
Table 1. The specification of MWCNT-COOH used in this study
MWCNT-COOH
Outside
Inside
COOH
Specific
diameter
diameter
Length
content
surface area
20–30 nm
5–10 nm
30 µm
1.5%
>110 m2/g
for attachment of [Mo(CO)6] to APyz-MWCNT was obtained by
diffuse reflectance UV–vis spectroscopy. The [Mo(CO)6] showed
absorption peaks at 230 and 280 nm (Fig. 2A). In the supported
catalyst, these peaks had appeared at 226 and 278 nm and were
attributed to Mo → CO charge transfer bands (Fig. 2B), while
MWCNTs showed no absorption peak in this region (Fig. 2C). These
observations indicated that molybdenum hexacarbonyl has been
supported on MWCNTs. The SEM images of the [Mo(CO)6@APyz-
MWCNT] showed that the nanotubes were aggregated and had
retained their nanotube nature (Fig. 3).
Figure 3. SEM image of [Mo(CO)6@APyz-MWCNT].
used to find the reaction media. The results showed that a higher
epoxide yield was observed in CCl4 (Table 2). It seems that non-
coordinate solvents such as chlorinated ones are the best solvents
for oxidation reactions by molybdenum-based catalysts. Different
amounts of catalyst were used to optimize the catalyst amount.
The best results were obtained using 100 mg (0.042 mmol) of
[Mo(CO)6@APyz-MWCNT]. Control experiments in the absence of
catalyst and using MWCNT-Apyz as catalyst were also performed
and the results showed that the amount of epoxide was less
than 5%.
Epoxidation of Alkene with tert-BuOOH Catalyzed by
[Mo(CO)6@APyz-MWCNT]
The [Mo(CO)6@APyz-MWCNT] was used as catalyst for epoxidation
of alkenes with tert-BuOOH. First, the reaction parameters were
optimized in the oxidation of cyclooctene. Different solvents were
SOCl2
COOH
COCl
Reflux, 1 h
N
DMF, 80 °C
H2N
Et3N, 72 h
N
O
C
N
N
H
N
hv (30 min)
Mo(CO)5THF
Mo(CO)6 + THF
Reflux, 1 h
Mo(CO)5
O
O
C
N
C
N
N
HN
H
N
N
CO
OC
Mo
CO
OC
Mo(CO)5
N
O
C
O
C
N
HN
N
H
N
N
OC
OC
Mo
CO
CO
(CO)5Mo
Scheme 2. Preparation of [Mo(CO)6@APyz-MWCNT].
Appl. Organometal. Chem. 2010, 24, 708–713
c
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M. Moghadam et al.
Table 2. Epoxidationofcis-cyclooctenewithtert-BuOOHcatalyzedby
Table 3. Epoxidation of alkenes with tert-BuOOH catalyzed
[Mo(CO)6@APyz-MWCNT] in refluxing CCl4
a
[Mo(CO)6@APyz-MWCNT] underreflux conditions in different solventsa
Conversion
(%)b
Epoxide
(%)b
Time
(min)
Solvent
Epoxide (%)b
Temperature (o C)
Entry
1
Alkene
(CH3)2CO
No reaction
53
61
77
78
57
72
38
–
97
97
25
THF
No reaction
CH3CN
21
57
65
97
48
41c
ClCH2CH2Cl
2
3
4
95
97
95
95
90
60
45
210
90
CHCl3
CCl4
CH2Cl2
Neat reaction (solvent free)
a Reaction conditions: cis-cyclooctene (1 mmol), tert-BuOOH (2 mmol),
catalyst (100 mg, 0.042 mmol), 6 ml solvent.
5
6
7
8
90
75
90
75
200
160
240
150
b GC yield based on the starting cyclooctene.
c 10 mmol of cyclooctene was used.
85
100c
85
99c (trans)
Under the optimized conditions, the [Mo(CO)6@APyz-
MWCNT]/tert-BuOOH catalytic system was used for epoxidation of
a wide range of alkenes (Table 3). In this system, both cyclic and
linear alkenes were efficiently converted to their corresponding
epoxides using tert-BuOOH as oxidant. 1-Octene and 1-dodecene
as linear alkenes were efficiently converted to their corresponding
epoxides by [Mo(CO)6@APyz-MWCNT]. In the case of stilbenes,
the epoxidation of cis isomer was associated with some loss of
stereochemistry and a 7.4 : 1 mixture of cis/trans-epoxides was
produced while epoxidation of trans-stilbene led to trans-epoxide
in 90% yield with complete retention of configuration (Table 3).
9
100c
96 (cis), 3 (trans)c
150
a Reaction conditions: alkene (1 mmol), tert-BuOOH (2 mmol), catalyst
(100 mg, 0.042 mmol), CCl4 (6 ml).
b GC yield based on starting alkene.
c Both 1HNMR and GC data approved the reported yields.
the first two runs was low and after the third run no molybdenum
was detected in the filtrates. These results demonstrate the
strong attachment of molybdenum to the MWCNT. The catalytic
behavior of the separated liquid was also tested by addition of
fresh cyclooctene and tert-BuOOH to the filtrates after each run.
Execution of the oxidation reaction under the same reaction
conditions as with the catalyst showed that the obtained results
were the same as for the blank experiments.
Catalyst Recovery and Reuse
The reusability of a supported catalyst is of great importance from
economic and environmental points of view. Heterogenization
of homogeneous catalysts makes them useful for commercial
applications. The reusability of [Mo(CO)6@APyz-MWCNT] was
investigated in the sequential epoxidation of cyclooctene with
tert-BuOOH (Fig. 4). The catalyst was consecutively reused several
times (20 times was checked) without significant loss of its
initial activity. In the 11 first runs, the initial catalytic activity was
observed, but then the catalytic activity decreased in 25 min.
The elongation of reaction times gave the higher epoxide yield
in which, after 20 consecutive runs, the epoxide yield was 90%
(Fig. 4). The amount of molybdenum detected in the filtrates in
Conclusion
WeimmobilizedmolybdenumhexacarbonylonMWCNTsmodified
by 2-aminopyrazine and found that this supported catalyst
was active in the epoxidation of alkenes with tert-BuOOH. This
Figure 4. Reusability results for [Mo(CO)6@APyz-MWCNT] in the epoxidation of cyclooctene with tert-BuOOH.
c
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Copyright ꢀ 2010 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2010, 24, 708–713

Molybdenum hexacarbonyl supported on amine modified multi-wall carbon nanotubes
supported catalyst is highly reactive in the epoxidation of a wide
range of alkenes such as linear and cyclic ones. The catalyst was
highly reusable and was recycled 20 times without appreciable
decrease in its initial activity.
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