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Highly efficient and excellent reusable catalysts of molybdenum(VI) complexes grafted on ZPS-PVPA for epoxidation of olefins with tert-BuOOH.

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Full Paper
Received: 20 June 2010
Revised: 12 August 2010
Accepted: 13 August 2010
Published online in Wiley Online Library: 20 October 2010
(wileyonlinelibrary.com) DOI 10.1002/aoc.1727
Highly efficient and excellent reusable
catalysts of molybdenum(VI) complexes
grafted on ZPS-PVPA for epoxidation of olefins
with tert-BuOOH
Zhongkai Hua,b, Xiangkai Fua,b,c∗ , Yuedong Lia and Xiaobo Tua
Four new kinds of heterogeneous catalysts for olefins epoxidation were obtained by grafting diamines on organic polymer–inorganic hybrid material, zirconium poly (styrene-phenylvinylphosphonate)-phosphate (ZPS-PVPA), and subsequently
coordinating with Schiff base Mo(VI) complexes. The catalysts were characterized by IR, XPS, SEM and TEM. All catalysts were
evaluated through the epoxidation of olefins using tert-BuOOH as oxidant. The heterogeneous catalysts possess the advantages
of high conversion, selectivity and excellent reusability. The catalysts were easily separated from the reaction systems and
c 2010 John Wiley & Sons, Ltd.
could be reused 13 times without significant loss of catalytic activity. Copyright Keywords: zirconium poly(styrene-phenylvinylphosphonate)-phosphate; Schiff base Mo(VI) complexes; heterogeneous catalysts;
epoxidation; reusability
Introduction
128
Epoxides are well known as one of the most valuable building
blocks to produce specialty and fine chemicals. They can be
formed from corresponding olefins by oxidation with various
oxygen sources. Considerable progress has been made using
transition metal complexes bearing Schiff base ligands in the
processes.[1] Homogeneous catalysts Mo(VI) compounds are very
versatile for epoxidation of olefins.[2 – 4] However, separation
of the homogeneous catalysts from the reaction mixture and
their reuse are still problematic. To address this issue, several
groups have been used to immobilize molybdenum on various
supports to obtain heterogeneous catalysts. Encapsulation or
immobilization of homogeneous catalysts in solid supports such
as zeolites and covalent grafting of homogeneous catalysts
on reactive polymer surfaces or inorganic solids has been
used.[5 – 9] A major drawback encountered in this regard is
the instability of the catalysts due to leaching during reaction
course.
Zirconium phosphates and zirconium phosphonates are types
of layered multi-functional materials with high thermal and chemical stability. Based on our previous works in hybrid zirconium
(phosphate-phosphonate), we have explored and reported the
synthesis of a series of polystyrene–inorganic zirconium phosphates hybrid materials.[10 – 17] Meanwhile, excellent results have
been obtained using these organic polystyrene–inorganic zirconium phosphate hybrid materials as support and used to prepare
immobilized chiral salen Mn(III) catalysts in our group.[12 – 15] We
have also anchored polyamines and polyethylene glycols Mo(VI)
complexes onto ZCMSPP.[17] This paper describes a catalytic
system based on ZPS-PVPA grafted with diamines which was
used as a support for Schiff base Mo(VI) complexes (as seen in
Scheme 1).
Appl. Organometal. Chem. 2011, 25, 128–132
Experimental
Material and Instruments
Cyclooctene, cyclohexene, styrene and N-nonane were purchased
from Alfa Aesar. MoO2 (acac)2 was prepared according to the
literature method.[18] Other commercially available chemicals were
laboratory-grade reagents from local suppliers.
FT-IR spectra were recorded from KBr pellets using a Bruker
RFS100/S spectrophotometer (USA) and diffuse reflectance UV–vis
spectra of the solid samples were recorded in the spectrophotometer with an integrating sphere using BaSO4 as standard. The Mo
content of the catalyst was determined by a TAS-986G (Pgeneral, China) atomic absorption spectroscopy. X-ray photoelectron
spectra (XPS) were performed on an ESCALAB250 apparatus. Scanning electron microscopy (SEM) analyses were performed using
a KYKY-EM3200 (KYKY, China) microscope. Transmission electron
microscopy (TEM) analysis was recorded on a TECNAI10 (Philips,
The Netherlands) apparatus. Nitrogen adsorption isotherms were
measured at 77 K on a 3H-2000I (Huihaihong, China) volumetric
adsorption analyzer using BET method. The conversions (with Nnonane as internal standard) and the values were analyzed by gas
chromatography (GC) with a Shimadzu GC2010 (Japan) instrument
∗
Correspondence to: Xiangkai Fu, College of Chemistry and Chemical
Engineering, Southwest University, China. E-mail: fxk@swu.edu.cn
a College of Chemistry and Chemical Engineering, Southwest University,
Chongqing, China
b The Key Laboratory of Applied Chemistry of Chongqing Municipality,
Chongqing, China
c Key Laboratory of Eco-environments in Three Gorges Reservoir Region, Ministry
of Education Chongqing, China
c 2010 John Wiley & Sons, Ltd.
Copyright Highly efficient and excellent reusable catalysts of molybdenum(VI)
Scheme 1. Synthesis route of the catalysts.
equipped with a chiral column (HP19 091 GB213, 30 m × 30 m ×
0.32 mm × 0.25 µm) and FID detector, injector 230 ◦ C, detector
230 ◦ C.
Synthesis of the Immobilized Catalysts
Chloromethyl ZPS-PVPA (ZCMPS-PVPA) was synthesized according
to the method reported earlier.[13] The proper amount of diamines
[such as (a) 1, 2-ethanediamine, (b) 1,3-propanediamine, (c) 1,4butanediamine and (d) 1,6-diaminohexane] were separately mixed
with ZCMPS-PVPA (1.3 g), K2 CO3 (0.8 g) and toluene (5 ml), heated
at 70 ◦ C for 12 h, cooled, filtered, washed with water and dried in
vacuum at 80 ◦ C overnight. The obtained aminomethyl-modified
ZAMPS-PVPAs were named as 1a– 1d.
The various supported Schiff base ligands, 2a–2d, were
prepared according to the standard method.[19 – 22] As a general
procedure, to a mixture of 1a (1.2 g) in 25 ml absolute ethanol was
added 3 mmol of salicylaldehyde. The mixture was refluxed for
24 h under a dry nitrogen atmosphere to afford supported Schiff
base ligands 2a. The solid was filtered, washed with ethanol and
dried under vacuum at 80 ◦ C.
The catalysts 3a–3d were prepared using a ligand exchange
procedure. A solution of MoO2 (acac)2 (0.3 g) in absolute ethanol
(30 ml) was added to the immobilized Schiff base ligand 2a (0.6 g)
and heated at 60 ◦ C for 24 h.[23 – 25] The catalyst was separated by
filtration, and Soxhlet-extracted with a mixture of dichloromethane
and ethanol (1 : 1) to remove the unreacted MoO2 (acac)2 , then
dried in vacuum at 80 ◦ C.
Catalytic Reactions
Appl. Organometal. Chem. 2011, 25, 128–132
out in a 25 ml round-bottomed flask equipped with a condenser
and a magnetic stirrer. tert-BuOOH was used as the oxidant. In
a typical procedure, to a mixture of catalyst (0.02 mmol Mo; Mo
contents were estimated by atomic absorption sepectrometry)
and olefins (5 mmol) in 1,2-dichloroethane (6 ml) was added TBHP
(2 mmol) under nitrogen atmosphere and the mixture was refluxed
for the appropriate time.
c 2010 John Wiley & Sons, Ltd.
Copyright wileyonlinelibrary.com/journal/aoc
129
All catalytic reactions were performed under laboratory atmosphere and the course of the product formation was monitored by
GC analysis.[26] A working model of supported Schiff base Mo(VI)
complexes is shown in Fig. 1. Epoxidation of olefins were carried
Figure 1. A working model of supported Schiff base Mo(VI) complexes.
Z. Hu et al.
Table 1. The BET surface area and Co content of supports and
immobilized catalysts
Supports and immobilized
catalysts
BET surface area
(m2 g−1 )
Content of Co (mmol
g−1 )
137
72
80
89
101
–
0.71
0.76
0.81
0.84
ZPS-PVPA
3a
3b
3c
3d
Figure 2. IR spectra of (a) ZCMPS-PVPA, (b) ZAMPS-PVPA 1d, (c) Schiff base
2d and (d) immobilized catalyst 3d.
Figure 4. SEM and TEM of immobilized catalyst 3d.
Table 2. The effect of solvent on the epoxidation of cyclohexene
with tert-BuOOH catalyzed by heterogeneous catalysts 3c under reflux
conditionsa
Solvent
CCl4
CHCl3
CH2 Cl2
(CH3 )2 O
CH3 OH
CH3 CN
ClCH2 CH2 Cl
Figure 3. XPS of immobilized catalyst 3d.
Reuse of the Immobilized Catalyst
Conversion (%)b
Selectivity (%)c
76.8
89.6
91.9
96.9
100
80.8
97.2
29.2
75.3
43.8
3.1
6.8
6.3
98.3
a
Recycling experiments were carried out with the catalyst 3d. In a
typical recycling experiment,[27 – 29] the catalyst was precipitated
from the solution by adding an equal volume of hexane after
each experiment. Then the organic phase was separated, and the
catalyst was washed with hexane to remove tert-BuOH, and dried
invacuo at 80 ◦ C. The recovered catalyst was weighed and reused in
the next run. In every run the same ratio of the substrate-to-catalyst
and solvent-to-catalyst was kept.
and 952 cm−1 ), attributable to terminal Mo O stretching modes,
indicating the presence of a supported oxomolybdenum center.[30]
X-ray photoelectron spectroscopy
Results and Discussion
Characterizations of the Immobilized Molybdenum Catalysts
Spectral analysis
130
As shown in Fig. 2, the FT-IR spectra of the prepared 1d, the sharp
C–Cl peak (due to –CH2 Cl groups) at 706 cm−1 in the ZCMPSPVPA practically disappeared or were seen as a weak band after
introduction of amines. The IR spectra of 2d showed that the
C N stretching vibrations were in the range of 1604–1635 cm−1 .
After immobilization of MoO2 (acac)2 , these C N bands upon
complexation with MoO2 (acac)2 shifted to lower frequencies,
the appearance of one band in the region 890–980 cm−1 (905
wileyonlinelibrary.com/journal/aoc
Reaction conditions: cyclooctene, 5.0 mmol; tert-BuOOH, 2.0 mmol;
catalyst, 0.02 mmol Mo (Mo contents were estimated by atomic
absorption sepectrometer); solvent, 6 ml; time, 3 h.
b
GC yield based on the starting cyclooctene.
c Selectivity toward the formation of cyclooctene epoxide.
Figure 3 shows the Mo XPS spectra of the immobilized catalyst
3d. The major peaks around 232.5 and 235.9 eV correspond to Mo
3d5/2 and Mo 3d3/2 , respectively, which indicate Mo atoms with
an oxidation state of 6+ for the catalyst.[17] . There is no indication
of the presence of the reduced Mo ions.
Nitrogen sorption studies and Mo content of the immobilized
molybdenum catalysts
The specific surface area of the materials and Mo content of
the immobilized molybdenum catalysts are shown in Table 1.The
amounts of the immobilized molybdenum catalysts based on the
c 2010 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2011, 25, 128–132
Highly efficient and excellent reusable catalysts of molybdenum(VI)
Table 3. Results of epoxidation of olefins with different catalysts under reflux conditionsa
Catalyst
Molar ratiob
Time (h)
Conversion (%)c
Selectivity (%)d
TOFe
1
MoO2 (acac)2
1 : 250
1
90.3
91.6
198.5
2
3
4
5
6
7
3a
3b
3c
3c
3d
MoO2 (acac)2
1 : 250
1 : 250
1 : 250
1 : 500
1 : 250
1 : 250
3
3
3
3
3
1
92.1
90.9
97.2
78.5
90.3
97.0
95.7
100
98.3
79.8
99.5
94.5
70.5
73
76.5
50.2
71.5
220.6
8
9
10
11
12
13
3a
3b
3c
3d
3d
MoO2 (acac)2
1 : 250
1 : 250
1 : 250
1 : 250
1 : 500
1 : 250
3
3
3
3
3
3
100
99.2
100
100
82.2
80.5
96.6
98.1
96.3
99.5
80.7
73.6
77.5
78
77.5
80
53.3
71
14
15
16
17
18
3a
3b
3c
3c
3d
1 : 250
1 : 250
1 : 250
1 : 500
1 : 250
6
6
6
6
6
86.1
59.8
89.4
60.5
65.7
75.6
73.2
79.5
64.6
80.6
26
15.5
28.5
18.3
21
Entry
Substrate
a
Reaction conditions: substrate, 5.0–10.0 mmol; tert-BuOOH, 2.0 mmol; catalyst, 0.02 mmol Mo; solvent, 1,2-dichloroethane (6 ml).
Molar ratio: catalyst : substrate.
Same as in Table 2.
d Same as in Table 2.
e Turnover frequency: mmol of product per millimole metal per hour.
b
c
Mo element for 3a, 3b, 3c and 3d calculated by atomic absorption
spectroscopy were 0.71–0.84 mmol g−1 . The introduction of
homogeneous catalysts or metal complexes onto supports leads
to a decrease in the specific surface area of the supporting
material.[13,14] Compared with ZPS-PVPA, the immobilized Schiff
base molybdenum catalysts exhibited decreased BET surface area.
It can be concluded that MoO2 (acac)2 was successfully located on
the Schiff base ligands surface.
1,2-dichloroethane was chosen as the reaction media because
the highest epoxide yield was observed (Table 2).
Catalytic Epoxidation
Effect of Solvent
The Recycling of the Immobilized Catalyst
In order to optimize the reaction conditions, the effect of different
solvents on the catalytic activity of 3c was investigated in the
epoxidation of cyclohexene in the presence of tert-BuOOH.[9]
Among carbon tetrachloride, chloroform, dichloromethane,
acetone, methanol, acetonitrile and 1,2-dichloroethane, the
To assess the long-term stability and reusability of the supported
catalysts, cyclooctene was used as a mode substrate, and recycling
experiments were carried out with catalyst 3d.The results of the
recovered catalyst 3d after 16 recyclings are listed in Table 4. Reuse
of the catalyst 3d 13 times decreased slightly both the conversion
Analysis of surface morphology
Appl. Organometal. Chem. 2011, 25, 128–132
c 2010 John Wiley & Sons, Ltd.
Copyright wileyonlinelibrary.com/journal/aoc
131
SEM and TEM morphology of the catalyst 3d are shown in Fig. 4.
It is also shown that the catalyst is loose, and various caves,
holes, pores and channels with different shapes and sizes exist
in every particle. The diameters of their sizes are on the scale of
20–40 nm.[12 – 15] Therefore, the catalysts could provide enough
space for epoxidation of olefins, which may be one of the main
factors in the excellent catalytic activities and reusability.
The catalytic activities of the prepared catalysts were evaluated
through the epoxidation of olefins with tert-BuOOH in 1,2dichloroethane. The resultants are given in Table 3. It is evident
that, with the chain length increasing, the conversion and
selectivity of olefins did not have a determinate rule. The
epoxidation of cyclohexene and cyclooctene with the different
linkage lengths catalysts’ conversion and selectivity was more
than 90%, and did not show obvious differences within the
limits of experimental errors. The effect of cyclooctene’s effect
on epoxidation was better than that of cyclohexene and styrene
with the different catalysts. The catalytic activities might be related
to their surface areas, structures and the number of active sites of
the catalysts.
Z. Hu et al.
Table 4. The recycles of catalyst 3d in the epoxidation of cyclooctenea
Entry
Cycle
Conversion (%)b
Selectivity (%)c
TOFd
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Fresh
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
100
100
100
99.5
98.3
99.6
98.2
96.1
94.2
94.1
92.4
92.1
90.8
88.1
87.2
81.6
>99
>99
>99
>98
>98
>98
97.7
95.3
94.9
93.8
94.3
93.6
92.1
89.8
89.7
87.4
80
80
80
79
78
79
77.5
74
72
71.5
70.5
69.5
67.5
64
63
57.5
a Reaction conditions: cyclooctene, 5.0 mmol; tert-BuOOH, 2.0 mmol;
3d, 0.02 mmol Mo; solvent, 1,2-dichloroethane, 6 ml; time, 3 h.
b Same as in Table 2.
c Same as in Table 2.
d Same as in Table 3.
and the selectivity. As compared with other heterogeneous
catalysts,[9,31,32] this one was characterized by endsville reusability.
Conclusion
We have shown that a new type of easily prepared heterogeneous
epoxidation catalyst, Schiff base Mo(VI) complexes grafted on
diamines modifying ZPS-PVPA with different linkage lengths
have high conversion, selectivity and excellent reusability for
epoxidation of olefins. The supported catalysts have no obvious
mass loss and activity decrease for 13 continuous uses.
Acknowledgment
Authors are grateful to Southwest University of China for financial
support.
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