Rational Design of a Polyoxometalate Intercalated Layered Double Hydroxide: Highly Efficient Catalytic Epoxidation of Allylic Alcohols under Mild and Solvent-Free Conditions

Article abstract of DOI:10.1002/chem.201604180

Intercalation catalysts, owing to their modular and accessible gallery and unique interlamellar chemical environment, have shown wide application in various catalytic reactions. However, the poor mass transfer between the active components of the intercalated catalysts and organic substrates is one of the challenges that limit their further application. Herein, we have developed a novel heterogeneous catalyst by intercalating the polyoxometalate (POM) of Na₉LaW₁₀O₃₆?32 H₂O (LaW₁₀) into layered double hydroxides (LDHs), which have been covalently modified with ionic liquids (ILs). The intercalation catalyst demonstrates high activity and selectivity for the epoxidation of various allylic alcohols in the presence of H₂O₂. For example, trans-2-hexen-1-ol undergoes up to 96 % conversion and 99 % epoxide selectivity at 25 °C in 2.5 h. To the best of our knowledge, the Mg₃Al?ILs?C₈?LaW₁₀composite material constitutes one of the most efficient heterogeneous catalysts for the epoxidation of allylic alcohols (including the hydrophobic allylic alcohols with long alkyl chains) reported so far.

Full text of DOI:10.1002/chem.201604180

A Journal of
Accepted Article
Title: Rational Design of a Polyoxometalate Intercalated Layered
Double Hydroxides: Highly Efficient Catalytic Epoxidation of
Allylic Alcohols Under Mild and Solvent-free Conditions
Authors: Yu-Fei Song, Tengfei Li, Zelin Wang, Wei Chen, and
Haralampos N Miras
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To be cited as: Chem. Eur. J. 10.1002/chem.201604180
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Rational Design of a Polyoxometalate Intercalated Layered
Double Hydroxides: Highly Efficient Catalytic Epoxidation of
Allylic Alcohols Under Mild and Solvent-free Conditions
[
a]
[a]
[a]
[b]
[a]
Tengfei Li, Zelin Wang, Wei Chen, Haralampos N. Miras and Yu-Fei Song*
Abstract: Intercalation catalysts, owing to the modular and
accessible gallery and unique inter-lamellar chemical environment,
have shown wide application in various catalytic reactions. However,
the poor mass transfer between the active components of the
intercalated catalysts and organic substrates is one of the
challenges that limit their further application. Herein, we fabricate a
novel heterogeneous catalyst by intercalating the polyoxometalate
the recycling process largely restrict their application.[12] The
urgency of environmentally-benign and sustainable approaches
to chemical processes stimulates the requirement for recyclable
heterogeneous catalyst as alternative to unfavorable
homogeneous catalyst. Under such circumstances, some
heterogeneous catalysts including Mo-AMP-CuBTC (AMP = 4-
aminopyridine;
BTC
=
benzene-1,3,5-tricarboxylate),[13]
vanadium-based complexes[
14]
and Co-MCM-41,[14] were
(POM) of Na
9 2
LaW10O36·32H O
(LaW10
)
into layered double
hydroxides (LDHs), which has been covalently modified with ionic
liquids (ILs). The intercalation catalyst demonstrates high activity
and selectivity for epoxidation of various allylic alcohols in the
developed and applied for epoxidation of allylic alcohols. For
these heterogeneous catalytic systems reported so far, 1) toxic
organic solvents or additives such as toluene, dichloromethane
etc were used,[14,16] which are generally highly corrosive and
carcinogenic, and require frequent and laborious work-up
treatments; 2) the accessibility of the substrates and the active
components was limited;[17] 3) these systems exhibit quite often
catalyst leaching and thermal stability issues, thus no scale-up
experiments were carried out.[ Consequently, it is of great
presence of H
2
O
2
. Taking trans-2-hexen-1-ol as an example, it
o
shows up to 96 % conversion and 99 % epoxide selectivity at 25 C
3
in 2.5 h. To the best of our knowledge, the Mg Al-ILs-C8-LaW10
composite material constitutes one of the most efficient
heterogeneous catalysts for epoxidation of allylic alcohols (including
the hydrophobic allylic alcohols with long alkyl chains) reported so
far.
18]
significance
to
develop
efficient,
recyclable,
and
environmentally-benign heterogeneous catalytic systems for
epoxidation of allylic alcohols.
Ionic liquids (ILs) can be used as novel green solvents or
advanced catalytic materials, which are receiving keen interest
Introduction
currently due to their unique properties such as intramolecular
forces, ionicity, polarity, and their inherent nature.[19] As a result,
research involving ILs is expanding to many different areas of
knowledge.[1,20-22] Layered double hydroxides (LDHs) or
Epoxidation of allylic alcohols is of great importance[1-5] since the
resultant epoxides are widely utilized as raw materials for epoxy
resins and as building blocks for synthesis of fine chemicals and
biologically important pharmaceuticals.[6-8] The application of
epoxides is growing rapidly every year in chemical industry,
giving rise to remarkable attentions in scientific community and
industry. To date, a large number of catalysts have been
investigated for expoxidation, in which polyoxometalates (POMs),
hydrotalcite-like compounds are
a large family of two-
dimensional (2D) anionic clay materials which can be
represented by the general formula [M1-x2+M
O.[23,24] The 2D inorganic matrix of LDHs consist of brucite-
mH
2
3+(OH)
x+
n-
x
2
] [Ax/n]
.
like host sheets with edge-sharing octahedra and the lateral
particle sizes range from nanometer to micrometer scale.
2
(ZnW
9 34 2
O )
]n- [M=Mn(II), Ru(III), Fe-(III), Pd(II),
such as [WZnM
Pt(II), Zn(II);
PMo11Mn(H
O)O39]5-[11] etc, have shown to be efficient for
[
9]
3- [10]
n=10-12],
38 2 2
[γ-PW10O V (μ-OH) ] ,
On the basis of previous reports, it can be summarized that
an efficient heterogeneous catalytic system for epoxidation
should take into consideration of the following aspects: a) green
solvents or solvents-free medium should be applied; b) close
proximity of the substrates and the active species; c) structural
stability of the catalytic system. Bearing these in mind, we report
the design and fabrication of a novel heterogeneous catalyst by
[
2
expoxidation reactions. However, the common issues
associated with the homogeneous catalysts, such as the
hydrolytic stability under oxidative conditions and difficulty during
[
[
a]
b]
T. Li, Z. Wang, Dr. W. Chen and Prof. Y.-F.Song*
State Key Laboratory of Chemical Resource Engineering,
Beijing University of Chemical Technology,
Beijing 100029, P. R. China.
Email: songyufei@hotmail.com or songyf@mail.buct.edu.cn
Dr. H. N. Miras
9 2
intercalation of Na LaW10O36·32H O (LaW10) into ionic liquids
(
ILs) modified LDHs. The intercalation catalyst shows highly
efficient epoxidation of various allylic alcohols in the presence of
o
H
2
O
2
at 25 C under organic solvent-free conditions.
WestCHEM, School of Chemistry
University of Glasgow
Glasgow, G12 8QQ, UK
Results and Discussion
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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3 .
Scheme 1. Schematic illustration of the synthetic procedure for the catalysts Mg Al-ILs-Cn-LaW10 (n=4, 8, 12)
Preparation and characterization of Mg
Mg Al-ILs-Cn-LaW10 (n = 4, 8, 12) were prepared according to
the procedures illustrated in Scheme 1. First, the Mg Al-NO was
exfoliated into LDH-nanosheets in formamide solution. Next, the
EtO) Si-ILs-Cn (n = 4, 8, 12) was covalently anchored onto the
LDHs layers via condensation reaction. Further on, the Mg Al-
3
Al-ILs-Cn-LaW10
.
respectively, due to the interactions between host layers and
guest anions. FT-IR spectrum of Mg Al-ILs-Cn-LaW10 (n = 4, 8,
12) also exhibits newly-appeared C-H stretching vibrations at
3
3
3
3
2930 and 2856 cm-1 due to the alkyl chain -CH
and -CH , and
2 3
(
3
the C=N stretching at 1658 cm-1 because of the imidazole of ILs,
respectively. Additionally, the bands in the 1030-1045 cm-1
3
ILs-Cn-LaW10 was obtained by adding LaW10 into the above
mixture. Such exfoliation and assembly strategy has been
applied successfully in a number of POMs/LDHs composites.[25]
region are observed in the Mg
samples, which are assigned to the vibration of Si-O-M bond (M
= Al, Mg).[27] It should be noted that FT-IR spectrum of Mg
Al-
NO
shows stretching bands at 1384 cm-1 is assigned to the N-O
vibration of NO
corresponding FT-IR spectra of Mg
3
Al-ILs-Cn-LaW10 (n = 4, 8, 12)
3
3
-
(Figure S1),[28] which disappears in the
3
3
Al-LaW10 and Mg
3
Al-ILs-Cn-
-
LaW10 (n = 4, 8, 12), indicating the complete exchange of NO
3
by the LaW10. All these facts demonstrate that the ILs have been
grafted onto the layers of LDHs and the guest anions LaW10
have been intercalated into LHDs successfully. FT-IR spectra of
(
EtO)
As shown in Figure 1B, the solid-state 29Si cross-polarization
magic-angle spinning (CP/MAS) NMR spectra of Mg Al-ILs-Cn-
LaW10 (n = 4, 8, 12) samples exhibit resonance peaks centered
at -66.4 ppm (Mg Al-ILs-C4-LaW10), -66.7 ppm (Mg Al-ILs-C8-
3
Si-ILs-Cn (n=4, 8, 12) are shown in Figure S2.
3
3
3
LaW10) and -67.0 ppm (Mg
which correspond to T [Si(OM)
3
Al-ILs-C12-LaW10), respectively,
3
],[29] The presence of T signal
3
3
indicates there are three M-O-Si bonds around the Si atom.
These results confirm that the ILs are tethered onto the layers of
LDHs covalently, resulting in the formation of Mg-O-Si and/or Al-
O-Si in LDHs layers.
Thermogravimetry with differential thermal analysis (TG-
DTA) of Mg
have been carried out in order to investigate the thermal
stabilities. As shown in Figure 1C, the TG-DTA of Mg Al-LaW10
shown for comparison, exhibits a two-stage weight loss. The first
one take place in the range of 25 to 230 oC and is due to the
loss of water molecules adsorbed on the surface and interlayer
space, whereas the second weight loss can be attributed to the
structural compromise of the layered structure of LDHs. Similarly,
3 3
Al-LaW10 and Mg Al-ILs-Cn-LaW10 (n = 4, 8, 12)
Figure 1. A) FT-IR spectra of (a) Na
Mg Al-ILs-C4-LaW10; (d) Mg Al-ILs-C8-LaW10; (e) Mg
Solid-state Si CP/MAS NMR spectra of (a) Mg Al-ILs-C4-LaW10; (b) Mg
Al-ILs-C12-LaW10; inset picture shows the structure
Al-LaW10 (red) and Mg Al-ILs-C8-LaW10
9
LaW10
O
36·32H
2
O, (b) Mg
Al-ILs-C12-LaW10；B)
Al-
3
Al-LaW10; (c)
3
3
3
3
2
9
3
3
ILs-C8-LaW10; (c) Mg
model of T ; C) TG-DTA profile of Mg
3
3
3
3
(blue).
in the case of Mg
can be observed, where the first weight loss of 6.58 % between
5 and 220 oC can be ascribed to the removal of water
molecules absorbed on the surface and interlayer, and the
second weight loss of 16.15 % between 220 and 650 oC
corresponds to the collapse of the layered structure and
decomposition of the ionic liquid. The thermal stabilities of
3
Al-ILs-C8-LaW10, two main weight-loss stages
As shown in Figure 1A, FT-IR spectrum of LaW10 exhibits
-1
characteristic peaks at 941, 842 and 783 cm , which can be
attributed to the vibrations of W-O, W-O -W and W-O -W (t,
terminal; c, corner-sharing; e, edge-sharing), respectively.
These W-O stretching bands can be clearly observed in the FT-
IR spectra of Mg Al-LaW10 and Mg Al-ILs-Cn-LaW10 (n = 4, 8,
2). Taking Mg Al-ILs-C8-LaW10 as an example, its W-O
stretching bonds with slight shifts to 936, 840 and 778 cm ,
2
t
c
e
[
26]
3
3
1
3
3 3
Mg Al-ILs-C4-LaW10 and Mg Al-ILs-C12-LaW10 are similar to
-1
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that of Mg
and elemental analysis results, the composition of Mg
LaW10 can be determined (Table S1). For example, the
composite material of Mg Al-ILs-C8-LaW10 can be identified as
3
Al-ILs-C8-LaW10 (Figure S3). Based on the TG-DTA
splitting of the 4f7/2 and 4f5/2 located at 35.2 eV and 37.3 eV,
3
Al-ILs-Cn-
respectively. The doublets are ascribed to the W in the W-O
bond configuration and are observed typically for W6
+ [32]
.
This
3
result is in accordance with the oxidation state of W of LaW10
The oxidation state of the tungsten centers in the composites
Mg Al-ILs-C4-LaW10 (Figure S4) and Mg Al-ILs-C12-LaW10
.
3 28 2 6 36 0.031
Mg0.75Al0.25(OH)1.88[O SiC14H N ]0.04[PF ]0.011 [LaW10O ] ·
0
7
.58H
.51 wt. %, LaW10=193.85 μmol/g).
XRD data of Mg Al-CO and Mg
spacing of 0.76 nm, 0.89 nm, respectively. For Mg
2
O; (Mg=11.26 wt. %, Al=4.22 wt. %, W=35.67 wt. %, ILs=
3
3
(Figure S5) are also in agreement with the oxidation states of
the starting material (LaW10).
3
3
3
Al-NO
3
revealed basal
Al-Ils-C8-
3
LaW10, the calculated gallery height value of 1.02 nm was
determined by subtracting the thickness of the host layer (0.48
nm) from the value of the d(003) spacing (Table S1),[30] which
are in good agreement with the diameter of the short axis of the
LaW10 cluster.[31] This observation suggests that the LaW10
clusters are intercalated with the short axis perpendicular to the
layers of LDHs.
As shown in Figure 2a, the Mg
LaW10 exhibit a type I adsorption isotherm at relative lower
pressure (P/P < 0.1), and a H3 type N hysteresis loop at relative
higher pressure (P/P > 0.6) according to BDDT (Brunauer,
3 3
Al-LaW10 and Mg Al-ILs-C8-
0
2
0
Deming, Deming and Teller) classification, indicating the
presence of both interlayer micropores and inter-particle
mesopores. Furthermore, the surface areas of Mg
3
Al-LaW10 and
2
Mg
3
Al-IL-C8-LaW10 are 59.56 and 63.16 m /g (Table S1),
3
respectively. The pore diameter distribution of Mg Al-ILs-C8-
LaW10 is calculated by the Barret-Joyner-Halenda (BJH) method
showed a peak centered at around 5.87 nm (Figure 2b).
Figure 3. SEM images of a) Mg
Mg Al-ILs-C12-LaW10; HRTEM images of d) Mg
ILs-C8-LaW10; g) Mg Al-ILs-C12-LaW10; h) EDS of Mg
distributions of Mg Al-ILs-C8-LaW10
3
Al-ILs-C4-LaW10; b) Mg
Al-ILs-C4-LaW10; e-f) Mg
Al-ILs-C8-LaW10; i) size
3
Al-ILs-C8-LaW10; c)
3
3
3
Al-
3
3
3
.
SEM images of the Mg
samples (Figure 3a-c) show the porous stacking of sheet-like
crystallites. Taking Mg Al-ILs-C8-LaW10 as an example, we can
see clearly the particle sizes of these crystallites are ~700 nm.
Figure 3d-g illustrates the HRTEM images of the Mg Al-ILs-Cn-
LaW10 (n=4, 8, 12) samples. The black spots of 1~1.5 nm
denoted by yellow circle in Figure 3f) in diameter are highly
3
Al-ILs-Cn-LaW10 (n=4, 8, 12)
3
3
(
dispersed. From the partial enlarged areas in figure 3f, the size
of the black spots is in good agreement with the size of LaW10
clusters, and these results further suggest that the LaW10 anions
are incorporated into the interlayer of LDHs successfully.
3
Energy-dispersive X-ray spectrometry image of Mg Al-ILs-C8-
LaW10 (Figure 3h) demonstrates the presence of C, La, Mg, Al,
and W et al elements, which is in a good consistent with the
Figure 2. a) The N
Mg Al-ILs-C8-LaW10 ； b) the pore diameter distribution of Mg
LaW10; c) XPS survey spectrum of Mg Al-ILs-C8-LaW10; d) XPS spectrum for
the W4f core level of Mg Al-ILs-C8-LaW10 sample.
2
adsorption-desorption isotherms of Mg
3
Al-LaW10 and
3
3
Al-ILs-C8-
3
structure and composition of Mg
scattering (DLS) measurement of Mg
Al-ILs-C8-LaW10. Dynamic light
Al-ILs-C8-LaW10 shows
3
3
3
that the particle size is narrowly distributed with the average
diameter of ~700 nm (Figure 3i).
To illustrate the interaction between the catalyst and the
allylic alcohols under the experimental conditions, the
amphiphilic properties of the catalysts were investigated. As
shown in Figure 4c, the contact angle of approximately 149o
The X-ray photoelectron spectroscopy (XPS) was used to
characterize the Mg Al-ILs-C8-LaW10 composite material. As
shown in Figure 2c, the photoelectron peaks of C, O, N, Si, La
and W et al elements can be clearly observed. Moreover, the
W(4f) photoemission peak can also be observed (Figure 2d),
which consists of two peaks originating from the spin orbital
3
indicates that the Mg
3
Al-ILs-C8-LaW10 is highly hydrophobic. In
the case of Mg Al-ILs-C12-LaW10, upon increase of the alkyl
3
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o
chain length, it shows an increase of the contact angle to a value
respectively, at 25 C in 2.5h. Therefore, 20 mg of Mg
3
Al-ILs-C8-
3
of 151°. In contrast, the contact angles of Mg Al-LaW10 and
LaW10 will be applied for the following studies.
Mg Al-ILs-C4-LaW10 are 57° and 78°, respectively (Figure 4a-d).
3
These results suggest that after ILs modification of the LDHs
layers, the hydrophobicity increases according to the following
Table 1. Solvent-free epoxidation of trans-2-hexen-1-ol with different
catalysts.[a]
3 3 3
sequence Mg Al-ILs-C4-LaW10 < Mg Al-ILs-C8-LaW10 ≈ Mg Al-
ILs-C12-LaW10, ensuring readily access of the substrate to the
catalytic site due to the hydrophobic-hydrophobic interactions.
Entry
Catalyst
None
Conversion[b] (%)
Selectivity[b] (%)
1
2
3
4
5
0
0
3
Mg Al-LaW10
47.0
87.0
96.0
79.0
99
99
99
99
Mg
3
Al-ILs-C4-LaW10
Al-ILs-C8-LaW10
Mg
3
3
Mg Al-ILs-C12-LaW10
[
(
a] Reaction conditions: 2 mmol of trans-2-hexen-1-ol, 2.4 mmol of H
2 2
O
30 wt.%), 20 mg of the catalyst, T = 25 oC, t = 2.5 h. [b] Conversion and
selectivity were determined by GC analysis using reference standards.
1
Assignment of trans-2-hexen-1-ol epoxide was analysed by H NMR and
13
C NMR in Figure S6.
Figure 4. Schematic diagram of the catalytic process (top); the water contact
3 3 3
angle measurements of (a) Mg Al-LaW10; (b) Mg Al-ILs-C4-LaW10; (c) Mg Al-
ILs-C8-LaW10; (d) Mg Al-ILs-C12-LaW10, respectively.
3
Opimization of catalyst. As shown in Table 1, taking
expoxidation of trans-2-hexen-1-ol as an example, the reaction
hardly takes place without catalyst (entry 1). The catalyst of
o
Mg
). In contrast, the intercalated catalysts of Mg
n = 4, 8, 12) demonstrate better catalytic activity than Mg3Al-
LaW10 (entry 3-5). Notably, the Mg Al-ILs-C8-LaW10 exhibits
3
Al-LaW10 exhibits only 47 % conversion at 25 C, 2.5 h (entry
2
3
Al-ILs-Cn-LaW10,
(
3
better catalytic performance with 96.0 % conversion and 99 %
selectivity (entry 4) under the same experimental conditions.
3
Figure 5. a) Effect of Mg Al-ILs-C8-LaW10 amount on catalytic epoxidation of
trans-2-hexen-1-ol (The data were obtained by carrying out parallel
experiments under the same experimental conditions). B) Effect of
Substrate/H
2
O
2
molar ratio on catalytic epoxidation of allylic alcohols. Reaction
o
conditions: 2 mmol trans-2-hexen-1-ol, 20 mg Mg
3
Al-ILs-C8-LaW10, T = 25 C,
2
.5 h; c) Kinetic profiles of epoxidation in scale-up experiment; Reaction
Scheme 2. Model reaction of the epoxidation of allylic alcohols.
conditions: 20 mmol trans-2-hexen-1-ol, 24 mmol H
Mg
2
O
2
(30 wt. %), 200 mg
o
3
Al-ILs-C8-LaW10
,
T
=
25 C. d) Experiment to prove the nature of
heterogeneous catalyst Mg
3
Al-ILs-C8-LaW10.
Effect of catalyst dosage. On the basis of the results reported
3
in Table 1, we selected the Mg Al-ILs-C8-LaW10 for further
Effect of the substrate/H
5
1
2
O
2
molar ratio. As shown in Figure
b, with the molar ratio of substrate/H varies from 1:0.8, 1:1.0,
:1.2 to 1:1.5, the conversion of epoxidation of trans-2-hexen-1-
investigation of the epoxidation reaction of allylic alcohols. The
influence of the amount of the catalyst on the epoxidation of
allylic alcohols was studied by varying the catalyst dosage from
2 2
O
ol changes from 64 %, 77 %, 96 % to 92.0 %, respectively.
Therefore, the epoxidation reaction has been performed using
1
0 mg to 30 mg while maintaining the H
2 2
O /substrate molar ratio
constant (2.4 mmol mmol substrate). Figure 5a
2 2
H O ; 2
the initial molar ratio of substrate/H
free condition.
Effect of solvents. Investigation of solvents effect on the
epoxidation of trans-2-hexen-1-ol using Mg Al-ILs-C8-LaW10
shows that the conversion of 18.37 %, 19.72 %, 21.96 % and
2 2
O at 1:1.2 under solvent-
illustrates the epoxidation of trans-2-hexen-1-ol as a function of
time at different catalyst dosage. With the increase of the
catalyst dosage from 10, 20 to 30 mg, the conversion of trans-2-
hexen-1-ol increases from 60 %, 96 % to 97 % accordingly,
3
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1
0.82 % in methanol, ethanol, acetonitrile and ethyl acetate,
respectively, can be obtained (Figure S7). Notably, the direct
mixing of trans-2-hexen-1-ol with Mg Al-ILs-C8-LaW10 without
the kinetic parameters of Mg
3 3
Al-LaW10, Mg Al-ILs-C4-LaW10 and
Mg Al-ILs-C12-LaW10 (Figure S8).
3
3
−
dCt
solvent shows remarkable conversion of allylic alcohols to the
corresponding epoxides. We assume that the presence of ILs
anchored on the layers promotes the interactions between the
catalyst and substrate, leading to the increase of catalytic
efficiency. Note that the epoxidation reactions have been
performed on solvent-free condition.
=
kCt (1)
dꢀ
C0
Ct
ln = kꢀ (2)
To confirm that the catalyst of Mg
3
Al-ILs-C8-LaW10 is a truly
Kinetic study of epoxidation of allylic alcohol. To obtain the
kinetic parameters for epoxidation of allylic alcohols, a scaled-up
experiment was performed with 20 mmol trans-2-hexen-1-ol, 24
heterogeneous catalyst, the epoxidation reaction of allylic
alcohol was carried out using Mg
catalyst and H
3
Al-ILs-C8-LaW10 as the
o
2
O
2
as the oxidant at 25 C under solvent-free
mmol of H
LaW10 at 25 C under solvent-free conditions. Conversion and
ln(C /C ) are plotted against reaction time in Figure 5c, in which
and C are initial allylic alcohols concentration and allylic
alcohols concentration at time t, respectively. The Mg Al-ILs-C8-
2
O
2
o
(30 wt. %) and 200 mg catalyst Mg
3
Al-ILs-C8-
conditions. When the conversion reaches about 55 % in 1 h, the
solid catalyst can be filtered by simple filtration. The reaction
mixture is allowed to proceed under the same experimental
conditions. It was observed that no additional trans-2-hexen-1-ol
can be transformed into the corresponding epoxides (Figure 5d).
t
0
C
0
t
3
LaW10 shows excellent catalytic activity and the conversion can
reach 94.76 % in 2 hours. The linear fit of the data reveals that
the catalytic reaction exhibits pseudo-first-order kinetics for the
While the solid catalyst Mg
the reaction mixture, the catalytic reaction continued at the
expected rate. This result suggests that Mg
true heterogeneous catalyst.
3
Al-ILs-C8-LaW10 is added back to
3
Al-ILs-C8-LaW10 is a
2
epoxidation of allylic alcohols reaction (R = 0.995). The rate
constant k for epoxidation reaction was determined to be 1.923
h-1 on the basis of Eqs. (1) and (2). Epoxidation of trans-2-
hexen-1-ol could be completed with the conversion of 98.24 % in
To demonstrate the wide applicability of the catalytic system,
epoxidation of various allylic alcohols were carried out at 25 C
under solvent-free conditions. As shown in Table 2, the Mg Al-
3
o
2
.5 h and 99.2 % in 3 h. Thus, the Mg
3
Al-ILs-C8-LaW10 exhibits
ILs-C8-LaW10 exhibits excellent epoxidation results for different
substrates. For example, epoxidation of 2-buten-1-ol, 3-methyl-
2-buten-1-ol, cis-2-penten-1-ol results in 92 %, 99 %, and 93 %
high catalytic efficiency for epoxidation of allylic alcohols, and
the catalytic reaction strictly obey pseudo-first-order kinetics with
>
compare the kinetic parameters with different intercalation
catalysts for epoxidation of allylic alcohols, we also investigate
-1
99 % selectivity of corresponding epoxides. In order to
conversion and TOF values of 587, 1010, 475 h , respectively
(entry 1-3). In contrast, the Mg Al-LaW10 shows only 78 %, 75 %
and 57 % conversion for the above-mentioned substrates.
3
Table 2. Epoxidation of various allylic alcohols using Mg
3
Al-LaW10 and Mg
3
Al-ILs-C8-LaW10 as catalyst in the presence of H
Mg Al-LaW10 Mg
Selec (%)
2 2
O .
Entry
Substrate
T
Time
(h)
3
3
Al-ILs-C8-LaW10
(oC)
Conv (%)
78
Conv (%)
92
Selec (%)
TOF (h-1)
587
1
2
25
0.8
0.5
99
99
99 (trans)
25
75
99
99
1010
3
4
5
6
7
8
9
25
25
25
50
50
50
50
1.0
2.5
2.5
3.0
3.0
3.0
3.0
57
72
45
50
18
6
99
99
99
97
96
95
98
93
93
96
94
85
82
88
99 (cis)
99 (cis)
475
190
196
157
140
134
148
99 (trans)
97 (trans)
96 (trans)
95 (trans)
98 (cis)
8
10
50
2.0
31
93
92
93
220
2 2
Note: Reaction conditions: 2 mmol substrate, 2.4 mmol H O , 20 mg catalyst; Conversion and selectivity were determined by GC analysis using reference
standards; TOF is based on the epoxides yield after 0.5 h, and given in mmol of epoxide produced per mmol of LaW10 in the catalyst per hour in the catalyst.
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Chemistry - A European Journal
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These values can be explained based on size considerations of
the substrates and their accessibility to the active species in the
restricted region of LDHs.
geraniol.[2,10,33] In contrast, Mg
conversion due to the poor mass transfer ability in the absence
of the grafted ionic liquids. The catalytic activity for epoxidation
3
Al-LaW10 exhibited much lower
Upon increase of the length and hydrophobicity of the alkyl
chains, the difference of the catalytic efficiency becomes more
of various allylic alcohols using Mg
ILs-C12-LaW10 are listed in Table S2-3.
3 3
Al-ILs-C4-LaW10 and Mg Al-
3 3
pronounced in the case of Mg Al-LaW10 and Mg Al-ILs-C8-
LaW10. In the case of cis-2-hexen-1-ol and trans-2-hexen-1-ol,
the obtained conversion values found to be 93 % and 96 %
Comparison with other reported heterogeneous catalyst:
Catalytic epoxidation of trans-2-hexen-1-ol using different
heterogeneous catalysts reported in the literature are
summarized in Table 3. The epoxidation of trans-2-hexen-1-ol
using Mg
3
Al-ILs-C8-LaW10 as catalyst with the selectivity of
Al-LaW10
99 % (entry 4-5), respectively, whereas in the Mg
3
composite, exhibited 72 % and 45 %, respectively. It is worth
noting that a few heterogeneous catalysts demonstrated great
epoxidation activity for long hydrophobic allylic alcohols. Using,
trans-2-hepten-1-ol, trans-2-octen-1-ol, trans-2-nonen-1-ol, cis-
3
using Mg Al-ILs-C8-LaW10 as catalyst exhibits two prominent
features including 1) solvent-free condition and 2) excellent
epoxidation conversion (96 %) and selectivity (99 %) have been
obtained at 2.5 h at 25 oC under environmentally-benign
conditions. In contrast, other heterogeneous catalysts either use
3
2-nonen-1-ol and geraniol as substrates, the Mg Al-ILs-C8-
LaW10 exhibited excellent catalytic performance with upon
increase of the reaction temperature to 50 oC (entry 6-10).
Interestingly, the highly efficient epoxidation results for geraniol
is due to the fact that the oxygen transfer takes place to the
proximal 2,3-allylic double bond rather than to the remote 6,7-
double bond, leading to the regioselective epoxidation of
CH
inferior catalytic activity. For example, the [PO
took much longer reaction time of 4.0 h in CH
reach comparable performance (entry 9); For K-
(O (H O) ], longer reaction time was applied (5 h) in order
to get good catalytic result (entry 12, homogeneous system).
3
CN, toluene or CH
2
Cl
2
solvents (entry 1-11), or exhibit
{WO(O ]-PIILP,
CN in order to
4
2 2 4
) }
3
a
W
2
O
3
2
)
4
2
2
Table 3. Catalytic epoxidation of trans-2-hexen-1-ol using different heterogeneous catalysts.
Time
h)
T
( C)
Sub/oxidant[a]
molar ratio
Entry
Catalyst
Solvent
Conv (%)
Selec (%)
Ref
o
(
1
2
3
4
Mg
3
Al-WZn
3
(ZnW
9
O
34
)
2
1.0
50
1:2
CH
Toluene
CH Cl
3
CN
80
88
75
99
96
34
17
16
MNP11-Si-inic-Mo
MCM-Pr-1
24.0
24.0
80
1:2[b]
1:2[b]
55
2
2
100
Fe O @SiO
3 4 2
24.0
80
1:1.2[b]
CH
3
CN
100
100
35
@APTMS@V-MIL-101
5
6
7
8
9
WO
2
(acac)
2
3.0
3.0
2.0
5.0
4.0
24.0
4.0
5
RT
32
RT
0
1 : 2
1:1
CH
CH
CH
Toluene
CH CN
CH Cl
CN
2
Cl
CN
Cl
2
92
89
95
-
36
(TBA)
2
[SeO
4
{WO(O
2
)
2
}
2
]
3
18
SiO
2
@2-Percarboxyethyl
VO(acac) -BBHA
[PO {WO(O ]-PIILP
MSh-bpy-MoVI
1:2[c]
1:3[b]
1:2
2
2
99
99
99
94
71
99
100
99
99
99
99
37
2
75
14
4
) }
2 2 4
50
55
21
32
50
0
3
99
38
10
11
12
13
14
15
14
1:1.2[b]
1:2
2
2
30
39
2
[ZnWZn (H
2
O)
(O
(ZnW
IL-Nb
[TBA][Nb=O(O-O)(OH)
Mg Al-ILs-C8-LaW10
2
(ZnW
9
O
34
)
2
]
CH
H
3
100
98
40
K-W
O
2 3
2 4
) (H O)
2
2
]
1:1
2
O
41
MgAl-WZn
3
9
O
34
)
2
2.5
8.0
12.0
2.5
1:1.2
1:1.2
1:1
None
None
None
None
90
42
O
2 7
62.4
85
43
1
2
]
0
3
25
1:1.2
96
This work
Note: [a] Unless specified, the oxidant is H
2
O
2
in most case. [b] TBPH = tert-butyl hydroperoxide as oxidants; Inic = isonicotinoyl; Pr-1= 3-
)(CO) (8-aminoquinoline ligand)]. [c] Peracid as oxidants; WO (acac) = tungsten-bishydroxamic acid;
= helical mesostructured silica materials; IL = 1-dodecyl-3-
3
iodopropyltrimethoxysilane spacer-[MoBr(η -C
3
H
5
2
2
2
BBHA = binaphthyl bishydroxamic acid; PIILP = polymer-immobilised ionic liquid phase; MS
h
methylimidazolium hydroxide or conventional tetradecyl trimethyl ammonium hydroxide surfactants.
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Chemistry - A European Journal
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FULL PAPER
For MgAl-WZn
3
(ZnW
9
O
34
)
2
, higher temperature was applied
We propose a possible catalytic mechanism for the current
epoxidation (Figure S9).[2,4,44] Firstly, the catalytic active species
of peroxotungstate are generated by LaW10 located in the
o
(
50 C) in order to achieve good catalytic results (entry 13); In the
2 7 2
case of IL-Nb O (entry 14) and [TBA][Nb=O(O-O)(OH) ] (entry
1
6
5), long reaction time 8.0 h and 12.0 h were used, resulting in
2.4 % and 85 % conversion, respectively, under solvent free
Al-ILs-C8-LaW10 is
2 2
restricted region of LDHs in the presence of H O ; Secondly, the
O-H bond in the allylic alcohols could get access to the active
species by ligating to tungstate atom to form a tungsten-
alcoholate species; Subsequently, the deprotonation of the O-H
bond of an allylic alcohol is accompanied by a proton transfer to
the peroxo ligand; Further on, the peroxide part forms an
conditions. To the best of our knowledge, Mg
one of the most efficient heterogeneous catalysts for epoxidation
of allylic alcohols.
3
It is essential to investigate the activity and stability of the
catalyst for epoxidation of allylic alcohols for practical application. electronegative conjugated system followed by a double bond
Taking the trans-2-hexen-1-ol as an example, the recycling
experiments have been performed using Mg Al-ILs-C8-LaW10
The solid catalyst can be readily separated from the reaction
mixture by centrifugation and the recycling experiments have
formation generating an intermediate state (step 3); Finally, the
epoxides are formed, leading to the original structure of W
coordination (step 4).
3
.
been carried out five times by re-addition substrates and H
2
O
2
o
(
30 wt. %) at 25 C (Figure 6a). There is no obvious loss of the
Conclusions
initial catalytic activity (the conversion for fresh catalyst: 96.0 %;
1st: 93.2; 2 : 94.7, 3 : 93.0 %; 4 : 92.4; 5 : 92.1 %) and the
selectivity remains 99 %. No leaching of tungsten into the
reaction system was detected by inductively coupled plasma
atomic emission analysis (ICP-AES).
nd
rd
th
th
In summary, the POM anion of LaW10 has been successfully
intercalated into the ionic liquids (ILs) modified LDHs for the first
time, leading to the formation of the Mg
3
Al-ILs-Cn-LaW10 (n = 4,
8,
12) employing an exfoliation and assembly synthetic
approach. The resulting intercalation catalysts have been fully
characterized by various techniques. Epoxidation of various
allylic alcohol to the corresponding epoxides using Mg
C8-LaW10 as heterogeneous catalyst was demonstrated in the
presence of H under mild and solvent-free conditions with an
3
Al-ILs-
2
O
2
excellent conversion and selectivity. Additionally, this catalytic
system exhibits remarkable catalytic efficiency for the
epoxidation of long chain hydrophobic allylic alcohols as well.
3
For example, the Mg Al-ILs-C8-LaW10 composite material
catalyzed the epoxidation of trans-2-hexen-1-ol, with 96 %
o
conversion and 99 % epoxide selectivity at 25 C in 2.5 h under
solvent-free condition. Furthermore, the catalyst can be
separated easily and recycled for more than 5 times without
obvious loss of catalytic activity. The structural integrity as well
as the composition of the intercalation catalyst Mg
LaW10 remains unaltered during the course of the catalytic
reactions. Therefore, the cooperative effect between LaW10
3
Al-ILs-C8-
,
LDHs and ILs offers a highly effective, green epoxidation system.
The prepared heterogeneous catalyst shows a great potenial for
further application.
Figure 6. a) Recycling of the Mg
3
Al-ILs-C8-LaW10; b) FT-IR spectra, c) solid-
state Si CP/MAS NMR spectra and d) XPS spectra for the W4f core level of
the fresh and used Mg Al-ILs-C8-LaW10
29
3
.
FT-IR spectra of the fresh and reused catalyst Mg
LaW10 exhibit nearly same peaks at 1658; 936, 840 and 778 cm-1
Figure 6b), which could be assigned to the C=N stretching
3
Al-ILs-C8- Experimental Section
Chemicals and Materials: All chemicals were of analytical grade and
were used as received without any further purification. The 2-buten-1-ol,
3-methyl-2-buten-1-ol, cis-2-penten-1-ol, trans-2-hexen-1-ol, cis-2-hexen-
(
vibrations of ILs and the vibration associated with the LaW10
cluster, respectively. Compared the solid-state 29Si cross-
polarization magic-angle spinning (CP/MAS) NMR spectra of the
1
-ol, trans-2-hepten-1-ol, trans-2-octen-1-ol, geraniol, cis-2-nonen-1-ol,
trans-2-nonen-1-ol, hexamethylenetetramine, Na WO ·2H O, CH CN,
CHCl , CH CH OH, LaCl ·7H O were purchased from Alfa Aesar, NaOH,
·6H O, Al(NO ·9H
purchased from
36·32H
),[46] [Mg0.75Al0.25(OH)
2
4
2
3
fresh and recycled Mg
resonance at -66.7 ppm (Figure 6c). Moreover, the oxidation
state of W in fresh and recycled Mg Al-ILs-C8-LaW10 is +6,
indicating the good stability of the LaW10 active centers (Figure
d). These results reveal that the structure of the recycling
catalyst Mg Al-ILs-C8-LaW10 retained its structural integrity,
3
Al-ILs-C8-LaW10, they both exhibit the
3
3
2
3
2
Mg(NO
3
)
2
2
3
)
3
2
O, HNO
Beijing
3
, NH
3
·H
2
O, H
Company.
](CO
(Mg Al-NO
butyl-3-(3-triethoxy-silylpropyl)-4,5-dihydroimidazolium
hexafluorophosphate (EtO) Si-ILs-C4), 1-octyl-3-(3-triethoxy-silylpropyl)-
2
O
2
, acetic acid,
3
were
Na
Chemical
[Mg0.75Al0.25(OH)
0.25·2H
The
LaW10
Al-CO
O
2
O
(LaW10),[45]
2
3 2
)0.125·2H O
9
(Mg
3
3
2
](NO
3
)
2
O
3
3
),[47] 1-
6
3
3
composition and performance during the series of consecutive
catalytic cycles.
4
3
,5-dihydroimidazolium hexafluorophosphate (EtO)
-(3-triethoxy-silylpropyl)-4,5-dihydroimidazolium
3
Si-ILs-C8), 1-dodelyl-
hexafluorophosphate
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Chemistry - A European Journal
10.1002/chem.201604180
FULL PAPER
(
EtO)
3
Si-ILs-C12),[48] were synthesized and characterized according to
Fundamental Research Funds for the Central Universities
the literature procedure.
(
YS1406).
Instruments: Fourier transform infrared (FT-IR) spectra were recorded
on a Bruker Vector 22 infrared spectrometer by using KBr pellet method.
The solid-state NMR experiments were carried out on a Bruker Avance
Keywords: Layered double hydroxides • Polyoxometalates •
Epoxidation • Ionic liquid • Allylic alcohol
300M solid-state spectrometer equipped with a commercial 5 mm MAS
NMR probe. Thermogravimetric (TG) analysis was done on STA-449C
Jupiter (HCT-2 Corporation, China) with a heating rate of 10 oC min-1
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4734.
2
N
2
adsorption-desorption isotherms were measured using Quantachrome
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chromatography (GC) system using a 30 m 5 % phenylmethyl silicone
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vigorously stirred for 2 days. After that, the mixture was centrifuged for 10
min and the suspension was separated simply by filtration. Then,
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3 3
Al-ILs-Cn-LaW10 (n=4, 8, 12): Mg Al-NO (0.5 g) was
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(
EtO)
1:1, 5 mL) and added dropwise to the above isolated suspension. After
for 24 hours. The
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to the above solution. The resultant precipitate of Mg Al-ILs-Cn-LaW10
~1.0 g) was collected by filtration and washed with ethanol and water,
and dried under vacuum overnight.
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(
[
[
[
[
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O
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[
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Chemistry - A European Journal
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FULL PAPER
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Chemistry - A European Journal
10.1002/chem.201604180
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The POM anion of LaW10 has been
successfully intercalated into the ionic
liquids (ILs) modified LDHs for the first
time, leading to the formation of the
Tengfei Li,[a] Zelin Wang,[a] Wei Chen,[a]
[b]
Haralampos N. Miras and Yu-Fei
Song*[
a]
Mg
3
Al-ILs-Cn-LaW10 (n = 4, 8, 12)
Page No. – Page No.
employing an exfoliation and
assembly synthetic approach.
Application of Mg Al-ILs-C8-LaW10 for
3
epoxidation of various allylic alcohols
to the corresponding epoxides shows
excellent activity in the presence of
Rational Design of a Polyoxometalate
Intercalated Layered Double
Hydroxides: Highly Efficient Catalytic
Epoxidation of Allylic Alcohols Under
Mild and Solvent-free Conditions
H
2
O
2
under mild and solvent-free
conditions.
This article is protected by copyright. All rights reserved.