Direct immobilization of self-assembled polyoxometalate catalyst in layered double hydroxide for heterogeneous epoxidation of olefins

Article abstract of DOI:10.1016/j.jcat.2008.03.022

We present a novel method for the fabrication of immobilized polyoxometalate catalysts that directly use a self-assembled polyoxometalate solution without isolation and purification. Specifically, self-assembled [WZn₃(ZnW₉O₃₄)₂]^12- catalyst can be selectively immobilized into layered double hydroxides in the presence of undesired anions. The material thus obtained demonstrates high dispersion and good hydrothermal stability. The heterogenized catalyst exhibits excellent activity (turnover frequency up to 18,000 h^-1 at 50 °C) in the epoxidation of allylic alcohols with aqueous H₂O₂ and can be readily recycled with no apparent loss of performance.

Full text of DOI:10.1016/j.jcat.2008.03.022

Journal of Catalysis 256 (2008) 345–348
Contents lists available at ScienceDirect
Journal of Catalysis
www.elsevier.com/locate/jcat
Research Note
Direct immobilization of self-assembled polyoxometalate catalyst in layered
double hydroxide for heterogeneous epoxidation of olefins
∗
Peng Liu, Hong Wang, Zhaochi Feng, Pinliang Ying, Can Li
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
a r t i c l e i n f o
a b s t r a c t
Article history:
We present a novel method for the fabrication of immobilized polyoxometalate catalysts that directly use
a self-assembled polyoxometalate solution without isolation and purification. Specifically, self-assembled
[WZn3(ZnW9O34)2]12− catalyst can be selectively immobilized into layered double hydroxides in the
presence of undesired anions. The material thus obtained demonstrates high dispersion and good hy-
drothermal stability. The heterogenized catalyst exhibits excellent activity (turnover frequency up to
Received 5 March 2008
Revised 22 March 2008
Accepted 25 March 2008
Available online 23 April 2008
−1
◦
at 50 C) in the epoxidation of allylic alcohols with aqueous H2O2 and can be readily re-
18,000 h
cycled with no apparent loss of performance.
Keywords:
Heterogeneous catalysis
Epoxidation
© 2008 Elsevier Inc. All rights reserved.
Self-assembled polyoxometalates
Layered double hydroxides
Allylic alcohols
H2O2
1. Introduction
high positive charge density of LDH host [15–17], which may bene-
fit the selective immobilization of ZnWO polyanion in the presence
of other anions. The material thus obtained, LDH-ZnWO, exhibits
high dispersion and good hydrothermal stability and can be used
as a highly active and recyclable catalyst for the epoxidation of al-
lylic alcohols with aqueous H2O2.
Recently, considerable attention has been devoted to the im-
mobilization of olefin epoxidation catalysts onto solid supports
to increase catalyst stability and allow for catalyst recovery and
product separation [1,2]. Among the various homogeneous epox-
idation catalysts, polyoxometalates (POM) are promising due to
their oxidative stability and high efficiency [3–5]. However, tedious
procedures are often needed to prepare such heterogenized POM
catalysts [6–8], and common problems include slow reaction rates
and reduced catalytic activity/selectivity upon reuse. The develop-
ment of a facile, efficient method for the immobilization of POM
catalysts remains a challenge.
Interest is growing in the applications of self-assembled POM
solution for homogeneous oxidation [9–14]. Compared with the
purified POM salts commonly used as catalysts, self-assembled
POM has the advantage of high availability and cost efficiency, be-
cause tedious POM isolation and purification are not required. This
offers the possibility of immobilizing POM in a facile way without
the typically long synthesis time. However, direct immobilization
of self-assembled POM has not been achieved, because the state of
most POMs in solution is generally complicated, particularly at dif-
ferent pH values [3]. Herein we present the first example of the
immobilization of self-assembled [WZn3(ZnW9O34)2]12− (ZnWO)
catalyst into layered double hydroxides (LDHs). The efficiency of
this novel approach is based on the anion-exchange capability and
2. Experimental
The self-assembled ZnWO solution was prepared by gradual ad-
dition of aqueous Zn(NO3)2 to an aqueous solution of Na2WO4
and nitric acid with a molar ratio of Zn(NO3)2/Na2WO4/HNO3
=
5/19/16 [10]. After purification, Na-ZnWO salt also was obtained
as needle-like crystals [18]. The LDH-NO3 support was prepared
by co-precipitation of mixed metal nitrates (Mg2+/Al3+ = 3/1)
with base (1 M NaOH) from aqueous solutions at pH ≈ 10 under
◦
N2 [15]. After aging at 80 C for 12 h, the precipitate was cen-
trifugated and washed several times with water; a portion of the
precipitate was dried and used for characterization, whereas the
remaining portion was dispersed in water and maintained under
N2 as LDH slurry. The immobilization of self-assembled ZnWO was
done by directly adding a two-fold excess of self-assembled ZnWO
solution to the LDH slurry (see Supporting information for details).
◦
The mixed slurry (pH ≈ 7.5) was then stirred for 3 h at 60 C un-
der N2. Finally, the LDH-ZnWO catalyst was obtained as a white
solid after centrifugation, washing, and drying.
FTIR spectra were collected on a Thermo-Nicolet Nexus 470
infrared spectrometer using KBr pallet method. X-ray diffraction
(XRD) patterns were recorded on a Rigaku D/Max 3400 powder
diffraction system (CuKα, 40 kV, 100 mA). TEM micrographs were
Corresponding author. Fax: +86 411 84694447.
E-mail address: canli@dicp.ac.cn (C. Li).
*
0021-9517/$ – see front matter © 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2008.03.022

346
P. Liu et al. / Journal of Catalysis 256 (2008) 345–348
−
−1
obtained with a JEOL JEM-2000EX transmission electron micro-
scope at an acceleration voltage of 120 kV. Inductively coupled
plasma-atomic emission spectrometry (ICP-AES) analysis was per-
formed on Leeman Plasma-spec-II spectrometer. Thermogravimet-
ric analysis (TGA) was carried out in air on a Perkin Elmer Pyris
strong peak of NO3 at 1385 cm
is attributed to the high amount
−1
of nitrate, and the peak at 835 cm
is attributed to the vibra-
tion of the WO24−. Thus, the self-assembled ZnWO solution is a
real mixture of nitrate coproduct, unreacted tungstate, and desired
ZnWO. To our delight, the characteristic peaks of ZnWO could be
clearly observed in the IR spectrum of LDH-ZnWO, whereas the
◦
Diamond TG instrument in a range of 20–800 C at a heating rate
◦
−
peaks of NO3 and WO24− were very weak, indicating the preva-
of 5 C/min.
lence of the ZnWO anion, but not of NO3 or WO24−, in the LDH-
−
Epoxidation reactions were carried out in acetonitrile (CH3CN)
◦
at 50 C with aqueous 30% H2O2 as the oxidant and n-dodecane
ZnWO. These results suggest that (a) the self-assembled ZnWO
anion was selectively immobilized into LDH in the presence of un-
desired anions, and (b) the structure of ZnWO was retained after
the anion-exchange process.
as the internal standard. Qualitative analysis of the reaction prod-
ucts was performed on an Agilent 6890N GC/5973 MS detector.
Quantitive analysis was performed on an Agilent 6890N GC with
an HP-19091G-B213 capillary column. The LDH-ZnWO catalyst was
readily recovered by filtration. After being washed with ethyl ac-
etate, the recovered catalyst was used in the recycling experiments.
Details of the epoxidation tests are given in Supporting informa-
tion.
The powder XRD patterns show that the basal spacing of LDH
increased from 0.82 nm for the LDH-NO3 support to 1.49 nm for
the LDH-ZnWO, indicating a LDH-ZnWO gallery height of 1.02 nm
(see Fig. S1 in Supporting information). This gallery height is in
agreement with the diameter of the short axis of sandwich-type
ZnWO anion (dimensions of ∼1.0 to ∼1.5 nm) [6,18]. The absence
of the reflections of LDH-NO3 and Na-ZnWO in the XRD pattern of
LDH-ZnWO indicates that the mixture of LDH-NO3 and Na-ZnWO
did not exist in the LDH-ZnWO sample. These results demonstrate
that the ZnWO anion was successfully incorporated into the inter-
layer of LDH by the anion-exchange process [19] and had a uniform
dispersion in the LDH host. An additional XRD peak with d-spacing
of ∼1.2 nm also appeared, which can be ascribed to the ZnWO im-
mobilized on the external surface of the LDH layers [20].
3. Results and discussion
FTIR spectra of the various samples are shown in Fig. 1. The
−1
LDH-NO3 support exhibited a very strong peak at 1385 cm
due
−
to the vibration of the NO3 anion in the interlayer region [15].
In the IR spectrum of the dried sample from the self-assembled
ZnWO solution, three peaks at approximately 930, 875 and 770
−1
cm
are assigned to the ZnWO species [6,9], consistent with the
The TEM images of the LDH-ZnWO are given in Fig. 2. The
bright-field TEM image shows well-developed hexagonal plates for
LDH host, and the dark-field TEM image exhibits a uniform disper-
sion of ZnWO nanoclusters (shown as small white dots) through-
out the LDH, is in agreement with the XRD findings. ICP-AES of
LDH-ZnWO revealed a W content of 41.9% and a Zn content of
3.8 wt%. The W:Zn molar ratio of 3.9 is consistent with the the-
oretical value of 3.8 for ZnWO. According to the ICP-AES and TGA
results, the chemical formula of LDH-ZnWO was estimated to be
spectrum of the purified Na-ZnWO salt (Fig. 1d). Furthermore, the
Mg0.73Al0.27(OH)2(ZnWO)0.023·1.1H2O, with a ZnWO loading of ca.
−1
0.12 mmol g
.
The foregoing characterization results suggest that the LDH sup-
port enables the direct immobilization of self-assembled ZnWO
anion in the presence of undesired anions. The success of this se-
lective anion-exchange process may be attributed to two factors:
(a) The ZnWO anion has higher negative charge than other an-
ions, and (b) the LDH layer has high positive charge density and
thus a stronger interaction with the ZnWO anion. We applied a hy-
drothermal treatment to verify the LDH-ZnWO’s structural stability.
After the LDH-ZnWO was treated with water in an autoclave at
◦
120 C for 7 days, FTIR and ICP-AES detected no apparent changes
in structure or components. This finding confirms the LDH-ZnWO’s
good hydrothermal stability.
Fig. 1. FT-IR spectra of (a) LDH-NO3 support, (b) dried sample from self-assembled
ZnWO solution, (c) pure Na2WO4, (d) purified Na-ZnWO salt, and (e) LDH-ZnWO.
Fig. 2. TEM images of LDH-ZnWO: (a) bright field image, (b) dark field image, exposing ZnWO nanoclusters.

P. Liu et al. / Journal of Catalysis 256 (2008) 345–348
347
Table 1
Epoxidation of (E)-2-hexen-1-ol with aqueous H2O2 catalyzed by LDH-ZnWOa
Entry
Catalyst
Solvent
Yield (%)b
Selectivity (%)b
1
2
3
4
5
6
7
8
9
10
LDH-ZnWO
LDH-ZnWO
LDH-ZnWO
LDH-ZnWO
LDH-ZnWO
Reuse 1c
Hexane
THF
H2O
Dioxane
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
39
53
58
72
80
87
93
95
92
93
98
97
99
98
99
99
99
99
99
95
Reuse 2c
Reuse 3c
Reuse 6c
Na-ZnWOd
a
◦
Reaction conditions: 1 mmol (E)-2-hexen-1-ol, 2 mmol of aqueous H2O2, 0.1 mol% of LDH-ZnWO, 0.2 mmol n-dodecane, 0.2 mL of solvent, 50 C, 1 h.
Determined by GC with an internal standard technique.
Recycling experiment.
b
c
d
Homogeneous epoxidation.
We investigated the activity of the LDH-ZnWO catalyst for the
Table 2
◦
Heterogeneous epoxidation of various allylic alcohols with aqueous H2O2 catalyzed
epoxidation of (E)-2-hexen-1-ol with aqueous H2O2 at 50 C. We
evaluated various solvents to identify the appropriate conditions
for the reaction with 0.1 mol% of LDH-ZnWO (Table 1, entries 1–5).
The yield of this heterogeneous catalytic reaction proved to be
solvent-dependent, with CH3CN giving the better results (80% yield
and 99% selectivity after 1 h). In addition, the LDH-NO3 support
alone demonstrated negligible activity (<2% conversion after 1 h
with CH3CN solvent).
by LDH-ZnWO catalysta
Entry
Substrate
Time
(h)
Yield
(%)b
Selectivity
(%)b
TOF
(h
−1
c
)
1
2
0.5
96
93
98
99
1920
1160
We also evaluated the reusability of the LDH-ZnWO catalyst,
which was successfully recycled seven times in the epoxidation
of (E)-2-hexen-1-ol with no apparent loss of activity or selec-
tivity (Table 1, entries 5–9). Significantly, the LDH-ZnWO catalyst
exhibited comparable reactivity to and slightly higher selectivity
than the homogeneous Na-ZnWO catalyst (entry 10) under identi-
cal conditions. LDH-ZnWO’s higher selectivity may be due to the
benefits of the basic LDH host, which may suppress the acid-
catalyzed epoxide hydrolysis [17]. The LDH-ZnWO catalyst could
be readily separated from the reaction mixture by filtration. The
FTIR spectrum of the recovered LDH-ZnWO catalyst comfirmed re-
tention of the original ZnWO structure, and ICP-AES analysis of the
filtrate indicated no leaching of W and Zn species during epoxi-
dation. After the catalyst was removed at about 50% conversion of
the (E)-2-hexen-1-ol, no further epoxidation was detected in the
filtrate after 3 h with additional aqueous H2O2. These results fur-
ther demonstrate the strong interaction between the ZnWO and
the LDH host and show that LDH-ZnWO acts as a real heteroge-
neous catalyst in aqueous epoxidation reactions. Interestingly, an
increase in catalytic activity was seen in the first three recyclings
of the LDH-ZnWO catalyst; this enhanced activity may be due to
the increased number of active sites available in the recycled cata-
lyst.
To extend the scope of the substrates for this heterogeneous
catalytic epoxidation, we carried out the epoxidation of various al-
lylic alcohols with aqueous H2O2 under the optimal conditions. As
shown in Table 2, all allylic alcohols were converted to the corre-
sponding epoxides with high yields and selectivities (up to 99%).
The LDH-ZnWO catalyst demonstrated high turnover frequencies
(TOF > 1000 h−1) for the epoxidation of various allylic alcohols;
in particular, for geraniol (entry 6), the regioselective epoxida-
tion proceeded smoothly to produce 2,3-epoxy alcohol with a TOF
of 1720 h−1. Significantly, excellent catalytic activity (entries 3
and 4) was achieved even at a very low catalyst loading of only
1
3d
4d
0.5
90
89
99
99
18,000
2300
6
5
1.5
0.5
90
86
99
98
1030
6
1720
a
Reaction conditions: 1 mmol substrate, 2 mmol of aqueous 30% H2O2, 0.1 mol%
◦
of LDH-ZnWO, 0.2 mmol n-dodecane, 0.2 mL of CH3CN, 50 C.
b
Determined by GC or GC–MS with an internal standard technique.
Based on the epoxide yield after 0.5 h, and given in mmol epoxide produced
c
per mmol of ZnWO in the catalyst per hour.
d
10 mmol substrate, 20 mmol of aqueous 30% H2O2, 0.01 mol% of LDH-ZnWO,
◦
1 mL of CH3CN, 50 C.
[2,6–8,21], these are among the highest values reported for the
selective epoxidation of allylic alcohols. It is noteworthy that the
excellent activity and recyclability of this type of catalyst can meet
the requirements for continuous operation with a fixed-bed reac-
tor.
4. Conclusion
Here we have reported the first example of direct immobiliza-
tion of a self-assembled POM catalyst in LDH for use as a robust
heterogeneous epoxidation catalyst. The catalyst is readily prepared
through a concise route using inexpensive starting materials and
exhibits high dispersion and good hydrothermal stability. The het-
erogeneous epoxidation of a range of allylic alcohols proceeds in
high yields. Of significant practical importance are the findings of
excellent activity (TOF up to 18,000 h−1) and the ability to read-
ily recover and reuse the catalyst with no apparent loss of activ-
ity.
0.01 mol%, unequivocally demonstrating the high efficiency (TOF
−1
up to 18,000 h
for entry 3) of the LDH-ZnWO catalyst. To the
best of our knowledge, compared with the typically reported pro-
−1
ductivities of heterogenized POM catalysts (TOF = 1–500 h
)

348
P. Liu et al. / Journal of Catalysis 256 (2008) 345–348
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Financial support from the Natural Science Foundation of China
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