Efficient heterogeneous epoxidation of alkenes by a supported tungsten oxide catalyst

Article abstract of DOI:10.1002/anie.201106064

Oxidation optimization: A combination of tungsten and zinc oxides on a SnO₂ support (W-Zn/SnO₂) is a heterogeneous and reusable solid catalyst for selective oxidation with aqueous H₂O₂. With it, various substrates, such as alkenes, amines, silanes, and sulfides, were oxidized into the corresponding products in high yields (see scheme). The catalyst can be reused several times without an appreciable loss in catalytic performance. Copyright

Full text of DOI:10.1002/anie.201106064

Communications
DOI: 10.1002/anie.201106064
Heterogeneous Catalysis
Efficient Heterogeneous Epoxidation of Alkenes by a Supported
Tungsten Oxide Catalyst**
Keigo Kamata, Koji Yonehara, Yasutaka Sumida, Kazuhisa Hirata, Susumu Nojima, and
Noritaka Mizuno*
Epoxides are important chemical intermediates in the labo-
ratory as well as the chemical industry.[1] Catalytic epoxida-
tion of alkenes with H2O2 has attracted much attention
because of the sustainable nature of the reaction, for example
the use of catalysts, the high content of active oxygen species
in H2O2, and the formation of water as the sole by-product. A
number of efficient methods for the epoxidation of alkenes
with transition-metal catalysts, such as Ti, V, Mn, Fe, W, Re,
Pt, have been reported.[1] However, most of these catalysts are
homogeneous, and require the separation and recycling of the
expensive catalyst.[2] Therefore, immobilization of homoge-
neous catalysts has been attempted through ion exchange,
adsorption, covalent linkage, encapsulation, and substitution,
depending on the nature of the metals and supports.[3] Some
of these immobilized systems have disadvantages, such as
significant decreases in activity after immobilization, leaching
of the catalytically active species, poor thermal stability, and
low recycling rates. Although titanium-containing zeolites are
highly efficient heterogeneous catalysts, their application to
the epoxidation of bulky substrates is limited, owing to the
steric constraints imposed by the micropores.[3] In this
context, developing an easily recycled, highly active, and
widely applicable heterogeneous catalyst for epoxidation is
desirable.
the presence of water inhibits the reaction. Therefore, > 95%
H2O2 is required to attain a high selectivity for 1,2-epoxy-
propane, and to minimize the formation of by-products.[3]
Tungsten-based catalysts, including polyoxometalates, utilize
H2O2 in a highly efficient manner and are highly selective for
epoxides.[4] Such catalysts have been immobilized by ion
exchange or covalent methods on surface-modified supports,
or by simple adsorption on silica or alumina. However, the
catalysts produced by using these methods are not so
effective.[3–5] Supported tungsten oxide catalysts have been
used for various chemical processes, including the isomer-
ization of alkanes,[6] alkene metathesis,[7] and the selective
reduction of NOx.[8] These catalysts often contain additives,
such as promoters and passivating agents,[6–9] but have not yet
been applied to liquid-phase oxidation with H2O2. Herein, we
report the efficient, heterogeneous, and selective oxidation of
various alkenes, amines, silanes, and sulfides with H2O2,
catalyzed by tungsten and zinc oxides supported on SnO2 (W–
Zn/SnO2). To our knowledge, the epoxidation of a wide range
of alkenes, including propene, with aqueous H2O2 and a
reusable, supported catalyst is unprecedented.
Supported tungsten oxide catalysts with various additives
(W–X/SnO2, X = Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Y, La, Ce,
Pr, Sm, Eu, Dy, Yb, Ni, Cu, Ag, Zn, Cd, Ga, and In) were
prepared in two steps (see the Supporting Information for
details). Firstly, the metal X was loaded onto the SnO2 support
(X/SnO2), then the X/SnO2 was impregnated with an aqueous
Heterogeneous epoxidation of alkenes with H2O2 and a
supported catalyst is one of the simplest and most reliable
solutions to the problems described above. A combination of
ethylbenzene hydroperoxide and a heterogeneous Ti/SiO2
catalyst has been used for the industrial production of 1,2-
epoxypropane. However, the Ti/SiO2 catalyst is not effective
for the epoxidation of propene with aqueous H2O2 because
[*] Dr. K. Kamata, S. Nojima, Prof. Dr. N. Mizuno
Department of Applied Chemistry, School of Engineering
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
E-mail: tmizuno@mail.ecc.u-tokyo.ac.jp
Homepage: http://park.itc.u-tokyo.ac.jp/mizuno/
Dr. K. Yonehara, Y. Sumida, K. Hirata
Advanced Materials Research Center
Nippon Shokubai Co., Ltd.
5-8 Nishi Otabi-cho, Suita, Osaka 564-8512 (Japan)
[**] This work was supported by the Global COE Program (Chemistry
Innovation through Cooperation of Science and Engineering), the
Research and Development in a New Interdisciplinary Field Based
on Nanotechnology and Materials Science Program, and a Grant-in-
Aid for Scientific Research from the Ministry of Education, Culture,
Science, Sports, and Technology of Japan.
Figure 1. Plots of the yield of 2a ^ and selectivity for 2a & against
the tungsten loading of W–Zn/SnO2 (W: 0–4.8 wt%; Zn: 0.8 wt%).
Reaction conditions: W–Zn/SnO2 (W: 3.3 mol% with respect to H2O2),
1a (5 mmol), 60% aqueous H2O2 (1 mmol), DMC (3 mL), 333 K, 4 h.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201106064.
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solution of H2WO4 (the pH value was adjusted to 7.0 by using
28% aqueous NH3). The epoxidation of 1-hexene (1a) with
60% aqueous H2O2 was carried out in the presence of W–X/
SnO2 (see Table S1 in the Supporting Information). The
catalytic activity of W–X/SnO2 was strongly dependent on the
additives. W–Zn/SnO2 gave the highest yield (93%), with a
95% selectivity for 1,2-epoxyhexane (2a).[10] Catalysts con-
taining Group 10–14 elements, such as Ni, Cd, In, and Ga
were moderately active, and catalysts with Group 1–3 ele-
ments or lanthanides were not active. The yield of 2a reached
a maximum at a tungsten loading of 4.1 wt%, whereas the
selectivity for 2a did not depend significantly on the tungsten
loading (Figure 1).
The effect of different solvents on the epoxidation of 1a
with H2O2 and W–Zn/SnO2 was investigated (see Table S2 in
the Supporting Information). Dimethyl carbonate (DMC)
was the best solvent for the reaction. Propylene carbonate
was also effective; however, the yield of isolated 2a was lower
than that obtained from the reaction with DMC. The
epoxidation reaction in DMC proceeded efficiently even at
305 K or with 30% aqueous H2O2,
supports, such as Al2O3, SiO2, TiO2, ZrO2, and ZnO, were not
effective (see Table S4 in the Supporting Information).
The scope of the W–Zn/SnO2 system for the oxidation of
various alkenes, amines, silanes, and sulfides with H2O2 was
also investigated (Table 1). Non-activated C3–C12 terminal
alkenes 1a–1d were selectively transformed into the corre-
sponding epoxides 2a–2d in high yields (Table 1, entries 1–5).
=
The configurations around the C C moiety of cis- and trans-2-
octenes (1e and 1 f) were retained in the corresponding
epoxides 2e and 2 f (Table 1, entries 6 and 7). These results
suggest that free-radical intermediates are not involved in the
mechanism of the W–Zn/SnO2-mediated epoxidation. Under
stoichiometric conditions (substrate:H2O2 = 1:1), the bulky
cyclic alkenes cyclooctene (1g) and cyclododecene (1h) were
also epoxidized in high yields, (Table 1, entries 8 and 9). The
epoxidation of dicyclopentadiene (1i) with two equivalents of
H2O2 gave the corresponding di-epoxide 2i as the main
product, with an 87% selectivity (Table 1, entry 10). Pyridine
(1j) and di-n-butyl amine (1k) were oxidized to the corre-
sponding N-oxide 2j and nitrone 2k in 92% and 93% yields,
without significantly affecting the
Table 1: Oxidation of various substrates with H2O2 catalyzed by W-Zn/SnO2.[a]
yield of the product or the selectiv-
ity of the reaction. Although ethyl
acetate (EtOAc) was also a good
solvent, EtOAc hydrolyzed under
the reaction conditions used. On the
other hand, the yield of methanol in
the reaction with DMC remained
constant (2%), regardless of the
presence of the catalyst. This find-
ing suggests that DMC was intrinsi-
cally stable during the reaction.
Other polar aprotic solvents, such
as methyl ethyl ketone, acetonitrile,
and tetrahydrofuran, as well as
polar protic solvents, such as tert-
butanol, 1-butanol, and ethanol,
gave 2a in moderate yields. Non-
polar toluene and n-hexane were
poor solvents for this reaction. The
epoxidation reaction did not pro-
ceed in N,N-dimethylformamide
Entry Substrate
t [h] Yield [%] Product (Selectivity [%])
1[b]
2[c]
3
4
5
R=CH3 1b
R=CH3 1b
R=n-C4H9 1a
R=n-C6H13 1c
R=n-C10H21 1d
5
6
4
4
4
84
75
91
81
80
R=CH3 2b (89)
R=CH3 2b (93)
R=n-C4H9 2a (95)
R=n-C6H13 2c (93)
R=n-C10H21 2d (93)
6
7
8
1e
1 f
1g
2
4
4
85
89
99
2e (96)
2 f (99)
2g (>99)
9
1h
1i
4
6
7
96
76
92
2h (>99)
2i (87)
10
11
1j
2j (99)
12
1k
1l
0.5 99
6.5 92
3.3 94
2k (93)
2l (99)
2m (84)
(DMF), probably because of
a
strong coordination between the
DMF and the tungsten center. The
catalytic activity of W–Zn/SnO2 was
compared with other tungsten and
zinc catalysts (see Table S3 in the
Supporting Information). The pre-
cursor of W–Zn/SnO2, H2WO4, had
a low catalytic activity and low
selectivity for 2a as a result of the
hydrolysis of 2a to the correspond-
ing diol. The epoxidation reaction
barely proceeded in the absence of
W–Zn/SnO2, or in the presence of
13[d]
14[d]
1m
15[e]
16[e]
1n
1o
1.5 99
2n (81)
2o (84)
1
>99
[a] Reaction conditions: W(3.5 wt%)-Zn(0.8 wt%)/SnO2 (W: 3.3 mol% with respect to H2O2), substrate
(entries 3–5, 5 mmol; entries 6 and 7, 2 mmol; entries 8, 9, 11, and 13–16, 1 mmol; entries 10 and 12,
0.5 mmol), DMC (3 mL), 60% aqueous H2O2 (1 mmol), 333 K. The catalyst was placed under vacuum for
2 h at ambient temperature before the reaction. The yields of product and selectivity were determined by
GC. Yield [%]=(moles of products/initial moles of H2O2)x100. [b] 1b (6 atm). [c] 1b (6 atm), DMC (6 mL),
30% aqueous H2O2 (1 mmol). [d] 315 K. [e] W(3.5 wt%)-Zn(0.8 wt%)/SnO2 (W: 0.5 mol% with respect to
Zn(NO3)2·6H2O,
SnO2,
WO3,
ZnWO4, ZnO, or Zn/SnO2. Other H2O2), DMC (1 mL), 293 K.
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Communications
respectively (Table 1, entries 11 and 12). Triethylsilane (1l)
The yield of 2b after reusing the catalyst for the first time was
85%, followed by 94% for the second reuse, 91% for the
third reuse, and 85% for the fourth reuse (Figure 2). In
addition, the catalyst used for the epoxidation of bulky 1g was
also reused successfully (Figure 2). Although titanium-based
materials with large porosities, such as Ti-b zeolites, Ti–
MCM-41 molecular sieves, and titania–silica aerogels, can
catalyze the epoxidation of bulky alkenes,[3,12a–c] these systems
often require organic hydroperoxides, such as tert-butyl
hydroperoxide or cumene hydroperoxide. The delaminated
Ti–MCM-22 zeolite has high turnover numbers (TONs) for
the epoxidation of bulky cycloalkenes, including cyclodode-
cene, with H2O2; however, the conversions are still low.[12d] To
our knowledge, a high-yielding epoxidation of both small and
bulky alkenes with aqueous H2O2 and a reusable, supported
catalyst has not been reported.
was efficiently oxidized to the corresponding silanol 2l,
without the formation of a disiloxane by self-condensation of
2l (Table 1, entry 13). In addition, the more sterically exposed
dimethylphenylsilane (1m) was obtained with a high selec-
tivity for dimethylphenylsilanol (2m), which readily self-
condenses to form a disiloxane (Table 1, entry 14). Thioani-
sole (1n) and methyl-n-octyl sulfide (1o) were converted into
the corresponding sulfoxides 2n and 2o in high yields, under
stoichiometric conditions (substrate/H2O2 = 1:1; Table 1,
entries 15 and 16).
To verify that the active catalyst was the solid W–Zn/SnO2
and not tungsten or zinc species which had been leached into
the reaction mixture, the epoxidation of 1b was carried out
under the conditions described in Figure 2. The W–Zn/SnO2
catalyst was then removed from the reaction mixture by
filtration, in approximately 40% yield. After the removal of
the W–Zn/SnO2 catalyst, 1b (6 atm) was introduced into the
filtrate, and the solution was heated again to 333 K for
4 hours. In this case, no reaction occurred. Negligible leaching
of tungsten and zinc species into the reaction mixture was
confirmed by inductively coupled plasma atomic emission
spectroscopy (W< 0.01% and Zn < 0.02% with respect to
fresh W-Zn/SnO2). These results ruled out leached tungsten
or zinc species from contributing to the catalysis, and
confirmed that the nature of the catalysis was truly hetero-
geneous.[11]
The treatment of W–Zn/SnO2 with H2O2 resulted in new
absorption bands at 860 and 550 cmÀ1 in the Raman spectrum.
These bands were assigned as v(O-O) and v(W(O2))asym
,
=
respectively. In contrast, the intensity of the v(W O) band
changed very little (see Figure S1(b) in the Supporting
Information). These results demonstrated that peroxo species
were formed without significant changes in the W–Zn/SnO2
structure. The Raman spectra of the catalyst before use and
after recovery from a reaction were not significantly different
(see Figure S1(c) in the Supporting Information). In addition,
the rate of the epoxidation of 1g with recovered W–Zn/SnO2
((4.55 Æ 0.17) mm minÀ1) was approximately the same as the
rate of reaction recorded with a fresh sample of catalyst
((4.60 Æ 0.40) mm minÀ1; see Figure S2 in the Supporting
Information). All these results suggest that the W–Zn/SnO2
catalyst is intrinsically stable.
The epoxidation of 1b with W–Zn/SnO2 and H2O2
proceeded efficiently to give 2b in 84% yield (Figure 2).
This reaction had an 89% selectivity for 2b, which increased
to 93% in the presence of 30% aqueous H2O2 (Table 1,
entry 3). The catalyst recovered from this reaction was reused
four times, without significant loss of activity or selectivity.
To demonstrate the W–Zn/SnO2 system on a larger scale,
the epoxidation reaction was performed on 20 mmol of 1g,
with one equivalent of H2O2 (with respect to 1g). In this case,
the yield of isolated 2g was 88% [Eq. (1)]. The TON of this
reaction was 650 and the turnover frequency (TOF) was
108 hÀ1.
Under
approximately
stoichiometric
conditions
(1g:H2O2 = 1:1–1.5),[5,13,14] the TON and TOF values were
larger than the corresponding values for other heterogeneous
tungsten catalysts (TON: 6–148, TOF: 1–14 hÀ1; see Table S5
in the Supporting Information) as well as the homogeneous
H3PW12O40/cetylpyridinium chloride (TON: 20, TOF: 1 hÀ1),
and [(n-C4H9)4N]2[SeO4{WO(O2)2}2] systems (TON: 47, TOF:
9 hÀ1). The TON and TOF values for W–Zn/SnO2 were also
comparable with those for the homogeneous Na2WO4·2H2O/
NH2CH2PO3H2/[CH3(n-C8H17)3N]HSO4 system (TON: 490,
TOF: 123 hÀ1).
Figure 2. Recycling of W–Zn/SnO2 for the epoxidation of 1b (black
bar) and 1g (white bar) with H2O2. Reaction conditions: W–Zn/SnO2
(W: 3.5 wt%, 3.3 mol% with respect to H2O2; Zn: 0.8 wt%), substrate
(1b (6 atm) or 1g (1 mmol)), 60% aqueous H2O2 (1 mmol), DMC
(3 mL), 333 K, reaction time 5 h (1b) or 4 h (1g). The values in
parentheses are the selectivity for 2b or 2g. The yields of 2b and 2g
are the average values of three reactions with 1b and five reactions
with 1g, and the errors are Æ5% and Æ1%, respectively.
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The molecular structure of W–Zn/SnO2 (Zn: 0.8 wt%, W:
0.7–4.8 wt%) was investigated by using Raman and IR
spectroscopy. In addition to the absorption bands for the
SnO2 support,[15] the Raman spectrum of W(0.7 wt%)–Zn/
SnO2 had a broad absorption band at 936 cmÀ1. This band was
bands have been assigned as monotungstates, polytungstates
with dioxo groups ([W7O24]6À and [H2W12O42]10À (paratung-
state)), and polytungstates with monoxo groups ([W10O32]4À
and [H2W12O40]6À (metatungstate)), respectively (see Fig-
ure S4 in the Supporting Information).[9,16] Therefore, the
absorbance bands at 936, 959, and 980 cmÀ1 in the Raman
spectrum of W–Zn/SnO2 were assigned as monotungstates,
polytungstates with dioxo groups, and polytungstates with
monoxo groups, respectively.
=
=
assigned as the W O stretching mode (v(W O); Figure 3a),
When the areas of each deconvoluted absorbance band in
the Raman spectrum were normalized (with respect to the
tungsten content) and plotted against the loading of tungsten
(see Figure S5 in the Supporting Information), the area
corresponding to polytungstates with dioxo groups reached a
maximum at 4.1 wt% of tungsten. Similarly, the yields of
epoxide products reached a maximum at 4.1 wt% of tungsten
(Figure 1). In a separate series of experiments, various
tungstates ([WO4]2À, [W7O24]6À, [H2W12O42]10À (paratung-
state), and [H2W12O40]6À (metatungstate)) were loaded onto
a Zn/SnO2 support (Wn–Zn/SnO2 (n = 1, 7, 12(para), and
12(meta); see the Supporting Information for details) by the
ion exchange method. The W7–Zn/SnO2 and W12(para)–Zn/
SnO2 catalysts had higher catalytic activities for the epox-
idation of 1a than the W1–Zn/SnO2 and W12(meta)–Zn/SnO2
catalysts [Eq. (2)]. These results support the idea that
polytungstates with dioxo groups play an important role in
the present epoxidation.[19]
Figure 3. Raman spectra (200–1100 cmÀ1) of W–Zn/SnO2 (Zn:
0.8 wt%; W: a) 0.7 wt%, b) 1.4 wt%, c) 2.1 wt%, d) 2.8 wt%,
e) 3.5 wt%, f) 4.1 wt%, and g) 4.8 wt%). The SnO2 support produced
strong absorption bands at 780 (B2g), 635 (A1g), and 480 cmÀ1 (Eg).[15]
The spectra in the region of 880–1020 cmÀ1 were closely reproduced by
the sum (dotted lines) of the deconvoluted bands (solid lines).
and was close to that observed in the spectrum of the aqueous
monotungstate [WO4]2À (932 cmÀ1).[9] Upon increasing the
In summary, a combination of tungsten and zinc oxides on
a SnO2 support was an effective and reusable solid catalyst for
selective oxidation with aqueous H2O2. Various organic
substrates such as alkenes, amines, silanes, and sulfides
could be converted into the corresponding epoxides, N-
oxides, silanols, and sulfoxides, in high yields. The catalysis
was heterogeneous and the catalyst could be recovered from
reactions and reused several times, without an appreciable
loss of its high catalytic performance. Experimental evidence
suggests that polytungstates with dioxo groups in W–Zn/SnO2
play an important role in the oxidation process.
=
loading of tungsten, new v(W O) absorption bands appeared
at higher wavenumbers. The intensities of the absorbance
bands at 230 cmÀ1 in the Raman spectrum and at 880 cmÀ1 in
the IR spectrum also gradually increased with increases in the
loading of tungsten. These bands were assigned to the W-O-W
bending and the W-O stretching modes of polytungstates,
[4,9,16,17]
respectively.
At a tungsten loading of 4.8 wt%, weak
absorbance bands appeared at 815, 715, and 270 cmÀ1 in the
Raman spectrum, which were assigned to crystalline WO3.[18]
These results suggest that the W–Zn/SnO2 catalyst consists of
three possible molecular structures, which are dependent on
the loading of tungsten: monotungstates at 0.7 wt% or less,
polytungstates at 0.7–4.1 wt%, and polytungstates/crystalline
WO3 at 4.8 wt%.
Received: August 26, 2011
Revised: September 21, 2011
Published online: October 25, 2011
The broad absorbance band in the Raman spectrum,
=
which was assigned as v(W O), could be reproduced by
combining the three deconvoluted bands at 936, 959, and
980 cmÀ1 (Figure 3). It has been reported that the v(W O)
=
absorptions of supported tungsten oxides on SiO2, Al2O3, and
Keywords: epoxidation · heterogeneous catalysis ·
supported catalysts · tungsten oxides · zinc oxides
.
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
TiO2 occur at 932–951, 950–973, and 981–984 cmÀ1. These
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Communications
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