Epoxidation of olefins with hydrogen peroxide catalyzed by a reusable lacunary-type phosphotungstate catalyst

Article abstract of DOI:10.1007/s11426-011-4251-9

Olefins and allylic alcohols have been epoxidized with commercially available hydrogen peroxide (30% H₂O₂) using a phase transfer catalyst, composed of cetyltrimethylammonium cations and a lacunary-type phosphotungstate anion [PW₁₁O₃₉] ^7- or the complete Keggin-type heteropolyanion [PW₁₂O ₄₀]^3-, under two-phase conditions using ethyl acetate as the solvent. It was found that the lacunary-type catalyst showed higher activity and better recyclability than the complete Keggin-type catalyst under the same reaction conditions. ³¹P NMR spectroscopy and solubility measurements for the two catalysts revealed that the [PW₁₁O₃₉] ^7- anion had a much faster degradation rate than the [PW ₁₂O₄₀]^3- anion in an excess of H ₂O₂, which resulted in the formation of more catalytically active species. As a result, the lacunary-type phosphotungstate anion-based catalyst gave a better catalytic performance than the complete Keggin-type anion in ethyl acetate. Science China Press and Springer-Verlag Berlin Heidelberg 2011.

Full text of DOI:10.1007/s11426-011-4251-9

SCIENCE CHINA
Chemistry
•
ARTICLES •
May 2011 Vol.54 No.5: 769–773
doi: 10.1007/s11426-011-4251-9
Epoxidation of olefins with hydrogen peroxide catalyzed by
a reusable lacunary-type phosphotungstate catalyst
HUA Li, QIAO YunXiang, LI Huan, FENG Bo, PAN ZhenYan, YU YinYin,
*
ZHU WenWen & HOU ZhenShan
Key Laboratory for Advanced Materials; Research Institute of Industrial Catalysis, East China University of Science
and Technology, Shanghai 200237, China
Received June 26, 2010; accepted August 26, 2010
Olefins and allylic alcohols have been epoxidized with commercially available hydrogen peroxide (30% H
2 2
O ) using a phase
7
transfer catalyst, composed of cetyltrimethylammonium cations and a lacunary-type phosphotungstate anion [PW11
O
39
]
or
40] , under two-phase conditions using ethyl acetate as the solvent. It was
found that the lacunary-type catalyst showed higher activity and better recyclability than the complete Keggin-type catalyst
3

the complete Keggin-type heteropolyanion [PW12
O
3
1
under the same reaction conditions. P NMR spectroscopy and solubility measurements for the two catalysts revealed that the
7

3
[PW11
O
39
]
anion had a much faster degradation rate than the [PW12
O
40
]
2 2
anion in an excess of H O , which resulted in the
formation of more catalytically active species. As a result, the lacunary-type phosphotungstate anion-based catalyst gave a bet-
ter catalytic performance than the complete Keggin-type anion in ethyl acetate.
heteropolyphosphotungstate, lacunary, epoxidation, degradation rate
1
Introduction
high selectivity to epoxides [2].
There have been a large number of reports of the use of
3
[
PW12
O
40
]
as a catalyst for the epoxidation of olefins with
. The most significant developments were made in the
980s by the groups of Venturello [13] and Ishii [14], who
The epoxidation of olefins is important from the viewpoint
of the industrial production and synthetic organic chemistry,
since epoxides are widely used as raw materials for epoxy
resins, paints and surfactants, and are intermediates in many
organic syntheses [1, 2]. In liquid-phase reactions, hydrogen
peroxide is an attractive oxygen donor as it has a high con-
tent of active oxygen species, and water is only byproduct
2 2
H O
1
found that such heteropolyoxotungstophosphates showed
high activity and selectivity for the olefin epoxidation, and
the reaction mechanisms for these catalysts were later in-
vestigated by many groups. Up to now, however, there have
been few reports focusing on the lacunary derivative
[3]. In the area of oxidation of olefins to epoxides with
7
[
PW11
O
39] . Duncan et al. [15] showed that both H
3 40
PW12O
H
2 2
O
as the oxidant, many homogeneous and heterogeneous
and Na 39 were much more effective than other
7
PW11O
catalytic systems have been disclosed [4–8]. Among them,
polyoxometalates as homogeneous catalysts have attracted
much attention, particularly in the last two decades [9–12],
because polyoxometalates—especially tungsten-based cata-
polyoxometalates in the epoxidation of 1-octene with a
phase transfer agent. Gao et al. [16] demonstrated that both
the Keggin-type polyoxometalate [PW12
cunary-type polyoxometalate [PW O ] gave reaction-
3
O
40

]
and the la-
7
lytic systems—show high efficiency of H
2
O
2
utilization and
11 39
controlled phase transfer properties in the epoxidation of
cyclohexene with 35% H O in CHCl , and both of them
2
2
3
*
Corresponding author (email: houzhenshan@ecust.edu.cn)
2 2
could form smaller active species by reaction with H O .
©
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770
Hua L, et al. Sci China Chem May (2011) Vol.54 No.5
Recently, Wang et al. [17] reported the epoxidation of cis-
1 h until no more precipitate was formed. The white solid
was collected, washed several times with distilled water and
cold alcohol and then dried under vacuum. The white pow-
der finally obtained is denoted PW11. Anal. Calcd for
7

cyclooctene catalyzed by [PW O ] with H O in an am-
1
1
39
2
2
phipathic ionic liquid and obtained a modest activity. Weng
et al. [18] utilized [C N(CH 39] to oxidize
benzyl alcohol, and found the only active species formed by
7
H
7
3 3 7
) ] [PW11O
76 168 4 3
C H N Na PW11O39: C, 23.50; H, 4.36; N, 1.44; Na, 1.78;
7

P, 0.80; W, 52.06. Found: C, 23.10; H, 4.40; N, 1.43; Na,
degradation of [PW11
O
39
]
during the reaction was [PO
4
-
3
1
.72; P, 0.78; W, 52.1.
{
W(O)(O ) } ] . Despite the reported high activity and
2
2 4
3 3
The quaternary ammonium salt [n-C16H33N(CH ) ]-
PW O (PW ) was prepared by the same procedure as
12 40 12
excellent selectivity obtained in epoxidation of alkenes us-
3
7
ing [PW12
O
40
]
39
and [PW11O ]
catalysts, difficulty in re-
reported previously [14] and obtained as white powder.
Anal. Calcd for C57 40: C, 18.35; H, 3.40; N,
.13; P, 0.83; W, 59.13. Found: C, 18.95; H, 3.70; N, 1.53;
P, 0.86; W, 61.00.
covery and reuse of those homogeneous metal species has
restricted their wide application in industrial and laboratory
synthesis. Furthermore, most catalytic systems require toxic
organic solvents (e.g. chloroform, 1,2-dichloroethane, ben-
zene, acetonitrile) as reaction media [14, 19–21].
126 3
H N PW12O
1
7

Although [PW11
O
39
]
has been previously reported as a
2.3 Typical procedure for epoxidation reaction
catalyst for the epoxidation of olefins, the reaction mecha-
nism is not clear. In this paper, we describe the synthesis of
The reaction was carried out in a Schlenk flask equipped
with a reflux condenser and thermometer. To 2 mL of ethyl
acetate was added PW11 (56 mg, 0.0144 mmol), cis-cyclooc-
the reaction-controlled phase transfer catalyst [n-C16
H33N-
(
CH Na 39 and its use for the epoxidation of ole-
3
)
3
]
4
3
PW11O
2 2
tene (0.2 mL, 1.54 mmol), and 30% H O (0.088 g, 0.77
fins with aqueous hydrogen peroxide using ethyl acetate—
which has a relatively low toxicity—as the reaction medium.
The recyclability of the catalysts has also been illustrated.
mmol). The reaction mixture was stirred at 60 °C for 1 h.
Then the reaction mixture was cooled to 0 °C, when the
catalyst precipitated and was then separated. After washing
twice with cold ethyl acetate and drying at 50 °C for 2 h
under reduced pressure, the catalyst was used for the next
catalytic recycle.
3
The difference in catalytic performance of [PW12
O
40
]
and
7
[
PW O ] anions has also been investigated.
11 39
2
Experimental
2
.4 Measurement of the solubilities of PW11 and PW12
2
.1 Reagents and instruments
in ethyl acetate
All solvents and chemicals were of analytical grade, com-
mercially available and used without further purification,
unless otherwise stated. FT-IR spectra were recorded at
room temperature on a Nicolet Fourier transform infrared
spectrometer (Magna 550). Elemental analysis for C, H, N
was performed on an Elementar vario EI III instrument and
analysis for W was carried out by inductively coupled
plasma-atomic emission spectroscopy (ICP–AES) on a TJA
IRIS 1000 instrument. All NMR spectra were recorded on a
To 12 mL of ethyl acetate was added PW11 (0.167 g, 0.043
mmol) and H O (0.264 g, 2.3 mmol). The resulting mixture
2 2
was stirred at 60 °C for either 10 min or 30 min, followed
by hot filtration immediately. 2 mL of the filtrate was added
to 0.1 mL of cis-cyclooctene and reacted at 60 °C for 1 h;
and the other 10 mL was used to determine the amount of
dissolved material by evaporation and gravimetric analysis.
A similar procedure was adopted to measure the solubility
31
of PW12
.
Bruker Avance III 400 instrument (161.9 MHz for P) us-
ing CDCl or DMSO-d . The products were analyzed by GC
with 1-octane as internal standard and the conversion was
calculated on the basis of H . The amount of H re-
In control experiment, PW11 (0.167 g, 0.043 mmol) or
PW12 (0.161 g, 0.043 mmol) was added to 10 mL of ethyl
3
6
acetate without H
2 2
O . The mixture was stirred at 60 °C for
2
O
2
2 2
O
1
h and then cooled to 0 °C. After filtration, ethyl acetate
maining after the reaction was analyzed by a standard io-
dometric method.
was removed by evaporation to determine the solubility of
corresponding catalysts.
3 3 4 3
2.2 Preparation of catalysts [n-C16H33N(CH ) ] Na PW11
O
39 (PW11) and [n-C16
H
33N(CH
3
)
3
]
3
PW12 40 (PW12)
O
3
Results and discussion
The Na PW O was prepared according to the method
7
11 39
3
.1 Catalyst characterization
The starting material Na PW11O39 was characterized by P
7
reported by Brevard et al. [22]. 2.00 g (0.7 mmol) of
Na 39 was then dissolved in 1 mL of distilled water
31
7
PW11
O
at 40 °C with stirring, followed by addition of 0.77 g (2.11
mmol) of cetyltrimethylammonium bromide dissolved in
NMR, and the peak observed at a chemical shift of 10.7
ppm was in accordance with that reported by other research
groups [18, 23]. As shown in Figure 1(1), the IR spectrum
of fresh PW11 exhibited characteristic peaks at 1081 cm
3
mL of distilled water. A white precipitate immediately
1
appeared and the resulting mixture was stirred for another

Hua L, et al. Sci China Chem May (2011) Vol.54 No.5
771
Figure 1 FT-IR spectra of catalysts. 1, Fresh PW11; 2, recovered PW11; 3,
fresh PW12; 4, recovered PW12
.
Figure 2 Time profile of epoxidation of cis-cyclooctene catalyzed by
) and PW12 (●). Reaction conditions : 0.0144 mmol catalyst, 1.54
mmol cis-cyclooctene, 0.77 mmol H , 2 mL ethyl acetate, 60 °C. The
PW11
(■
2
O
2

1
1
1
selectivity to cis-cyclooctene oxide was always over 99%.
and 1047 cm (PO), 952 cm (WO
d
), 894 cm and 857
1
1
1
cm (WO W), 803 cm and 730 cm (WO W), and
was identical to that of [PW O ] salts reported previ-
b
c
7

1
1
39
tion of 1-octene, while Schwegler et al. [27] indicated that
the vacancy of the lacunary derivative activated the sur-
rounding tungsten atoms for transformation into peroxo
ously [17, 24]. Elemental analysis data of PW11 were also
consistent with the expected structure. The characteristic IR
vibrations of fresh PW12 are shown in Figure 1(3). The
characteristic peaks at 1079, 979, 895, and 815 cm are
perfectly in accordance with the structure of complete Keggin-
type phosphotungstates [25].
2 2
structures by reaction with H O .
1
This suggests that the different degradation rates may be
the main reason why the two catalysts gave different activi-
31
ties in the epoxidation reactions. P NMR spectra support
After the catalytic reaction, the recovered PW11 showed
the same characteristic peaks as fresh PW11 in its FT-IR
spectrum (compare Figures 1(1) and (2)), although the split
peaks at 1080 cm were not very clear in the spectrum of
the recovered PW11; thus it can be concluded that the struc-
ture of PW11 was retained after the reaction. FT-IR spectra
2 2
this hypothesis. When PW11 was treated with H O for only
3
0 min, it was rapidly degraded into the Venturello complex
3
[
PO
4
{W(O)(O
2
)
2
}
4
] , as shown by the appearance of a peak
1
31
at 2.7 ppm in the P NMR spectrum [15, 28], while the
original chemical shift at 10.7 ppm assigned to parent la-
cunary-type phosphotungstate anions [17, 18, 24] disap-
peared (Figures 3(1) and 3(2)). Furthermore, a new peak
appeared around 15.3 ppm, which is characteristic of the
Keggin-type structure (Figure 3(3)). However, when PW12
(
compare Figures 1(3) and (4)) also showed that the struc-
3
ture of [PW12
O
40
]
was retained after the catalytic reaction.
31
3
.2 The catalytic performance of PW and PW
2 2
was treated with H O for as long as 1 h, no new P NMR
1
1
12
3

Although [PW12
O
40
]
salts have been widely employed in
the epoxidation of various olefins, only a few reports have
7

indicated that [PW11
O
39
]
salts also have considerable ac-
tivity in epoxidation reactions or compared the different
7

3
catalytic activities of [PW11
O
39
]
and [PW12
O
40
]
salts.
Herein, we examined the reaction kinetics for salts of
7
3
[
PW11
O
39
]
40
and [PW12O ] anions. As shown in Figure 2,
the reaction rate with PW11 in ethyl acetate was much
higher than that with PW12 in the same medium. The rea-
sons why the two heteropolyanions showed significant dif-
ferences in catalytic performance have been identified using
3
1
P NMR spectroscopy and solubility measurements.
The mechanism of the epoxidation of olefins catalyzed
2 2
by tungsten-containing heteropolyanions with H O has
3
1
been reported by several research groups [14, 15, 26]. Dun-
Figure 3
7 2
P NMR spectra of catalysts. (1) Na PW11O39 (in D O); (2)
3

7
PW11 after treatment with H O2 for 30 min (in a mixed solvent of
CN/CDCl = 1:10); (3) fresh PW12 (in DMSO-d ); (4) PW12 after
3 3 6
treatment with H O for 1 h (in a mixed solvent of CH CN/CDCl =1:10); (5)
2
can et al. [15] indicated that [PW12
O
40
]
and [PW11
O
39] ,
CH
3

which both rapidly formed [PO {W(O)(O ) } ] with H O2
4
2 2
4
2
2
2
3
3
under the reaction conditions, were active for the epoxida-
2 2 3
PW12 treated with H O for 1 h (in a mixed solvent of DMF/CDCl =1:1).

772
Hua L, et al. Sci China Chem May (2011) Vol.54 No.5
signal appeared (Figure 3(4)). The PW12 was partly de-
graded only after treatment with H in DMF for 1 h (Fig-
ure 3(5)), and the new peak at 0.6 ppm can be assigned to
the degradation product. These results clearly demonstrate
that PW12 is very difficult to degrade to the active species,
which is in agreement with a previous study [25].
We examined the solubilities of the fresh PW11 and PW12
in ethyl acetate. As shown in Table 1, PW12 had a slightly
higher solubility in ethyl acetate at 0 °C, with PW11 being
almost insoluble (entry 1, Table 1). When PW11 and PW12
2 2
reaction. Therefore, in the PW11/H O /ethyl acetate system,
2
O
2
excellent conversion and selectivity were obtained for ep-
oxidation of cis-cyclooctene (entry 1, Table 2), a moderate
conversion for epoxidation of cyclohexene and styrene (en-
tries 2 and 3, Table 2) and a poor conversion for epoxida-
tion of 1-hexene (entry 4, Table 2). The catalytic system
was also used for epoxidation of allylic alcohols (entries 5
and 6, Table 2). The results obtained were perfectly in ac-
cordance with the electrophilic mechanism.
The utilization efficiency of H
2 2
O was also determined
were degraded by H
O
2 2
for 10 min and 30 min, respectively,
by an iodometric method with cis-cyclooctene as the sub-
strate. It was found that the amount of residual aqueous hy-
drogen peroxide was negligible after the epoxidation of
cis-cyclooctene, showing that the utilization efficiency of
although the solubility of the degraded PW12 was higher
than that of the degraded PW11 (entries 2 and 3, Table 1),
the corresponding catalytic activities for cis-cyclooctene
epoxidation showed the opposite trend. This might explain
why cis-cyclooctene oxide was not detected in any signifi-
cant amounts until the actual active species were formed at
appreciable concentration, as suggested by Duncan et al.
H
2
O
2
in this catalytic system was very high (>99%).
The recyclability of PW11 and PW12 was also examined.
Like other reaction-controlled phase transfer catalysts, PW11
or PW12 can be transferred to the organic phase by reaction
[
15].
Therefore, we can deduce that the concentration of the
2 2 2 2
with H O and then precipitated after H O was consumed.
As shown in Figure 4, PW11 could be employed at least 10
times in the epoxidation of cis-cyclooctene without any de-
crease in the catalytic activity. This reveals that the active
species can be regenerated during catalytic recycling, which
is consistent with the FT-IR spectrum of the recovered PW11
(Figures 1(1) and (2)). The main reason for the decreased
activity of PW12 may be the leaching of the catalyst in ethyl
acetate, since the recovered PW12 also maintained its struc-
ture well (Figure 1(4)).
actual active species formed by degradation of PW12 was
not as high as that formed by degradation of PW11, and the
3
1
P NMR signal of the degradation product of PW12 was too
weak to be detected.
3
1
Taken together, the P NMR spectra and the solubility
measurements suggest that the degradation rate was the de-
cisive factor affecting the catalytic activity of PW11 and
PW12
.
3
.3 Substrate generality of PW in olefin epoxidation
11
4
Conclusion
The possible range of the olefin substrates was also exam-
ined. Epoxidation is an electrophilic reaction and electron-
donating substituents on the C=C bond will facilitate the
A reaction-controlled phase transfer system composed of
the lacunary-type catalyst [n-C16
H
3 3 4 3 39
33N(CH ) ] Na PW11O ,
a)
b)
Table 1 The relationship between catalyst solubility in ethyl acetate and catalytic activity
PW11
PW12
Entry
Degradation time (min)
Solubility (mg/10 mL)
Conversion (%)
Solubility (mg/10mL)
Conversion (%)
c)
1
2
3
0
not detectable

28.3
34.4
5

1.5
10
30
13
25
23
30
11.3
a) The solubility was measured by a gravimetric method. b) After degradation, 2 mL of filtrate was added to 0.7 mmol of cis-cyclooctene and the mixture
reacted at 60 °C for 1 h. The conversion was obtained based on the substrate. The selectivity to cis-cyclooctene oxide was always over 99%. c) This row
denotes the solubility of the fresh PW11 or PW12 in ethyl acetate in the absence of H O at 0 °C.
2 2
a)
Table 2 The epoxidation of various substrates over PW11
Entry
Substrates
cis-cyclooctene
cyclohexene
Reaction time (h)
Conversion (%)
Selectivity (%)
1
2
3
4
5
6
0.2
1
100
85.8
51.8
38.1
93.7
27.7
100
97
b)
styrene
2
73
1-hexene
12
2
100
84
c)
geraniol
allyl alcohol
12
100
2 2 2 2
a) 0.0144 mmol catalyst, 1.54 mmol cis-cyclooctene, 0.77 mmol H O , 2 mL ethyl acetate, reaction temperature 60 °C. The conversion was based on H O . b)
Benzaldehyde was the main by-product. c) Selectivity to 2,3-epoxygeraniol.

Hua L, et al. Sci China Chem May (2011) Vol.54 No.5
773
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
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
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