Modular Polyoxometalate–Layered Double Hydroxides as Efficient Heterogeneous Sulfoxidation and Epoxidation Catalysts

Article abstract of DOI:10.1002/cctc.201701056

The selective sulfoxidation of sulfides and the epoxidation of olefins are two types of important organic reactions, and the corresponding products of sulfoxides, sulfones and epoxides are used widely as raw materials in industrial processes. The fabrication of one efficient catalyst for both reactions remains a challenging task. We report the preparation of a highly efficient heterogeneous catalyst Mg₃Al-ILs-La(PW₁₁)₂ using an exfoliation/assembly approach. The catalyst was characterised by FT-IR spectroscopy, XRD, thermogravimetric and differential thermal analysis, BET measurements, X-ray photoelectron spectroscopy, ²⁹Si cross-polarisation magic-angle spinning NMR spectroscopy, ²⁷Al magic-angle spinning NMR spectroscopy, SEM, high-resolution TEM, and energy-dispersive X-ray spectroscopy. The designed catalyst showed a high efficiency and selectivity for the sulfoxidation of sulfides and the epoxidation of olefins under mild conditions at a production rate of 208 and 31 mmol g^?1 h^?1, respectively. Moreover, Mg₃Al-ILs-La(PW₁₁)₂ can be recycled and re-used at least five times without a clear decrease of its catalytic activity. The scaled-up experiments revealed that the catalyst retained its efficiency and robustness, which demonstrates the great potential of the catalyst for industrial applications.

Full text of DOI:10.1002/cctc.201701056

Accepted Article
Title: Modular polyoxometalate-layered double hydroxides as efficient
heterogeneous sulfoxidation and epoxidation catalysts
Authors: Haralampos N. Miras, Tengfei Li, Wei Zhang, Wei Chen, and
Yu-Fei Song
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10.1002/cctc.201701056
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Modular polyoxometalate-layered double hydroxides as efficient
heterogeneous sulfoxidation and epoxidation catalysts
Tengfei Li,a† Wei Zhang,a† Wei Chen,a Haralampos N. Miras*b and Yu-Fei Song*ac
Abstract: Selective sulfoxidation of sulfides and epoxidation of
olefins are two types of important organic reactions and the
corresponding products of sulfoxides, sulfones and epoxides are
widely used as raw materials in industrial processes. The fabrication
of one efficient catalyst for both reactions, remains a challenging
task. In this paper, we report the preparation of a highly efficient
heterogeneous catalyst of Mg3Al-ILs-La(PW11)2 using an
exfoliation/assembly approach. The catalyst was characterized by
FT-IR, XRD, TG/DTA, BET, XPS, 29Si CP/MAS NMR, the 27Al-MAS
NMR, SEM, HRTEM, EDX etc. The designed catalyst showed high
efficiency and selectivity for sulfoxidation of sulphides and
epoxidation of olefins under mild conditions at a production rate of
208 mmol g-1 h-1 and 31 mmol g-1 h-1, respectively. Moreover, the
Mg3Al-ILs-La(PW11)2 can be recycled and reused at least 5 times
without obvious decrease of its catalytic activity. The scaled-up
experiments revealed that the catalyst retained its efficiency and
robustness, demonstrating the catalyts’ great potential for industrial
applications.
techniques that have been widely adopted for POM-based
heterogeneous catalysts[9-10] such as the incorporation of POMs
[14]
into porous inorganic supports (silica,[11-12] zeolite,[13] Al2O3
etc.). However, the immobilization on the aforementioned
inorganic supports is usually accompanied by a series of issues
which compromise the performance of the designed catalytic
systems such as hydrolytic and thermal stability under oxidative
conditions; poor mass transfer and accessibility of the active
centres; catalyst leaching issues.[10,15] These inherent limitations
prevent the development of efficient catalytic systems with
desirable functionality and specifications. Thus, it is of
fundamental importance to explore appropriate preparation
methodologies that will allow the design of efficient, recyclable,
stable, and environmentally benign heterogeneous catalytic
systems for various industrial applications.
Layered double hydroxides (LDHs) or hydrotalcite-like
compounds are a large family of two-dimensional (2D) anionic
clays with the general formula [M1-x2+Mx3+(OH)2]x+[Ax/n]n-·mH2O,
where M2+ and M3+ are di- and trivalent metal cations and [A] are
inorganic or organic charge-compensating anions located in the
interlayer galleries.[16-17] Each hydroxyl group in the LDH layers is
oriented towards the interlayer region forming a network of
hydrogen bonds to the interlayer anions and water molecules.[18]
LDHs provide a flexible and modular confined space which can
be modified by varying the size and the structure of guest
molecules. Intercalation of catalytically active species into the
interlayer galleries to generate host-guest supramolecular
intercalation structures with desirable physical and chemical
properties has been proven to be an elegant approach for
improving the stability and recyclability of the catalytic system.
However, the poor mass transfer of the organic substrates within
the interlayer galleries and effective interaction with the active
centres of the intercalated catalysts is a major challenge that
needs to be taken into careful consideration.
Ionic liquids (ILs) have been extensively investigated as
novel green solvents or alternative catalytic materials, which are
currently receiving keen interest owing to their unique properties,
such as negligible volatility and remarkable solubility.[19-21]
Based on the above considerations, we demonstrate a
successful preparation of a novel heterogeneous catalyst of
Mg3Al-ILs-La(PW11)2 by intercalation the designed
catalytically active POMs into the ILs covalently modified
LDHs. Catalytic tests indicate that the designed catalyst
Mg3Al-ILs-La(PW11)2 exhibits excellent activity for selective
sulfoxidation of sulfides and epoxidation of olefins under
mild conditions, coupled with easy recovery and steady
reuse.
Introduction
Polyoxometalates (POMs) are a class of molecular metal oxide
anions, often incorporating V, Mo, W, or Nb etc., in their highest
oxidation states.[1-4] The remarkable properties of POMs such as
redox potential, electron-transfer properties, thermal and
oxidative stability, acidity, etc., can be finely tuned by
incorporation of appropriate addenda metal ions and/or
counterions, which endow superior catalytic performance in
numerous industrial processes.[5-8] However, the industrial
application of POM-based homogeneous catalytic systems has
been restricted mainly due to poor separation ability,
decomposition issues, environmental concerns, and its high
operational cost. Consequently, design of POM-based advanced
heterogeneous catalysts has long been and will continue to be
the focus for a potentially alternative way to overcome the above
limitations and to tune the POM-based material’s functionality.
The solidification and immobilization are the two main
[a] T. Li,† W. Zhang,† 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
[b] Dr. H. N. Miras*
WestCHEM, School of Chemistry
University of Glasgow
Glasgow, G12 8QQ, UK
Email: harism@chem.gla.ac.uk
[c] Prof. Y.-F.Song*
Beijing Advanced Innovation Center for Soft Matter Science and
Engineering, Beijing University of Chemical Technology, Beijing, P. R. China.
[†] These authors contributed equally to this work
Supporting information for this article is available on the WWW under
http://dx.doi,org/10.xxxxxxxxxxx
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Results and Discussion
FT-IR spectra of PW12, P2W18, Mg3Al-ILs-PW12, Mg3Al-ILs-P2W18
are shown in Fig. S1.
Additional confirmation of the catalysts’ composition and
structural features was provided by the (CP/MAS) NMR studies.
As shown in Fig. 1b, the solid-state 29Si cross-polarization
magic-angle spinning (CP/MAS) NMR spectrum of Mg3Al-ILs-
La(PW11)2 exhibits a resonance peak centred at -66.4 ppm,
which corresponds to T3 [Si(OM)3].[26] The presence of T3 signal
indicates there are M-O-Si bonds around Si atom (M = Mg or Al).
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 within the interlayer space.
As shown in Fig. 1c, the XRD pattern of Mg3Al-NO3 exhibit a
reflection peak centered at 2θ = 9.88, which is attributed to the
characteristic reflection of basal plane (003) and corresponds to
a spacing value of d003 = 0.89 nm. After the intercalation of the
La(PW11)2 anions, a shift of the (003) reflection to lower angles
was observed (2θ = 6.28). This change corresponds to a basal
spacing value of d003=1.42 nm, indicating an increase of the
interlayer region due to the intercalation of the La(PW11)2 species
in Mg3Al-ILs-La(PW11)2. The height of the interlayer region value
of 0.94 nm was calculated by subtracting the dimensions of the
host layer (0.48 nm) from the value of the d003 spacing (1.42 nm)
of the Mg3Al-ILs-La(PW11)2 composite. The obtained value is in
good agreement with the width of the POM cluster (with
Structural characterization
As shown in Fig. 1a, FT-IR spectrum of La(PW11)2 exhibits
characteristic peaks centred at 1093, 950, 882 and 769 cm-
1, which can be attributed to the asymmetric and symmetric
vibrations of W-Ot, asymmetric vibration of W-Oc-W and
asymmetric vibration of W-Oe-W (t, terminal; c, corner-
sharing; e, edge-sharing),[22] respectively. These W-O
stretching bands can be clearly observed in the
corresponding FT-IR spectra of Mg3Al-ILs-La(PW11)2 and
Mg3Al-La(PW11)2. Taking Mg3Al-ILs-La(PW11)2 as an
example, the W-O stretching vibrations gave peaks located
at 1089, 954, 889 and 775 cm-1, respectively, which are
slightly shifted due to the strong electrostatic interactions
and hydrogen bonding between the host layers and
intercalated POMs.[23] Moreover, the peak at 1051 cm-1 can
be assigned to the stretching vibration of the P-O bond,
which appears at 1046 cm-1 in the spectrum of La(PW11)2.
The FT-IR spectrum of Mg3Al-ILs-La(PW11)2 also exhibits
new C-H stretching vibrations at 2930 and 2856 cm-1 due to
the alkyl chain -CH2 and -CH3 groups, while the C=N
stretching appears at 1658 cm-1 originating from the
imidazole ring of ILs, respectively. Additionally, the peak
located at 448 cm-1 is assigned to the O-M-O (M = Mg or Al)
vibration of the brucite-like layers of the LDHs.[24]
dimensions of just below
1 nm). This indicates that the
La(PW11)2 anion’s “long axis” is oriented parallel in the interlayer
region of the LDHs .
The TG-DTA of Mg3Al-La(PW11)2 (Fig. 1d) exhibits three-
stage weight losses, where the first one takes place within the
range of 25 to 250 oC and is attributed to the loss of water
molecules adsorbed on the surface and interlayer region; the
second weight loss can be attributed to the collapse of the
layered structure of LDHs and the third weight loss corresponds
to the decomposition of La(PW11)2. In the case of Mg3Al-ILs-
La(PW11)2, three main weight-loss stages can be observed, in
which the first weight loss of 6.46 % between 25 and 240 oC can
be ascribed to the removal of water molecules; the second
weight loss of 13.10 % takes place between 240 and 650 oC
which corresponds to the collapse of the layered structure and
decomposition of the ionic liquid and the third weight loss of
o
1.87 % between 650 and 900 C is due to the decomposition of
La(PW11)2.
As shown in Fig. 2a, the photoelectron peaks of Mg, Al, C, O,
N, P, La and
W can be clearly identified from X-ray
Figure 1. a) FT-IR spectra of (i) La(PW11)2, (ii) Mg3Al-ILs-La(PW11)2, (iii)
Mg3Al-La(PW11)2, (iv) Mg3Al-NO3; b) The 29Si CP/MAS NMR spectrum of
Mg3Al-ILs-La(PW11)2 and inset picture shows the structure model of T3; c) XRD
patterns of (i) Mg3Al-NO3; (ii) Mg3Al-La(PW11)2, (iii) Mg3Al-ILs-La(PW11)2; d)
TG-DTA profiles of (i) Mg3Al-ILs-La(PW11)2 and (ii) Mg3Al-La(PW11)2.
photoelectron spectroscopy (XPS). The XPS spectrum of W(4f)
photoemission (Fig. 2b) consists of two peaks originating from
the spin orbital splitting of the W4f7/2 and W4f5/2 at 35.1 eV and
37.1 eV, respectively, which can be ascribed to the W in the W-
O bond configuration and typically observed for W6+.[28] As
shown in Fig. 2c, Mg3Al-ILs-La(PW11)2 exhibits typical type-IV
adsorption isotherms, and H3 type hysteresis loops at higher
relative pressure (P/P0>0.5), indicating the presence of both
interlayer and interparticle mesopores structure. The pore size
distribution calculated by the Barrett-Joyner-Halenda (BJH)
method shows a peak centred at around 3.8 nm (Inset Fig. 2c).
It should be noted that the stretching band at 1384 cm-1 for
-
Mg3Al-NO3 due to the N-O vibration[25] of NO3 appears to be
weak in the corresponding FT-IR spectra of Mg3Al-ILs-La(PW11)2
and Mg3Al-La(PW11)2, indicating almost quantitative exchange of
-
NO3 by POM clusters. The above observations demonstrate
that the ILs are grafted onto the layers of LDHs and the
La(PW11)2 clusters are intercalated into LDHs successfully. The
The Mg3Al-ILs-La(PW11)2 exhibits
a Brunauer-Emmett-Teller
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10.1002/cctc.201701056
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(BET) surface area of 43.99 m2/g with a total pore volume of
0.18 cm3/g. The N2 adsorption-desorption isotherm and the pore
size distribution of Mg3Al-La(PW11)2 are shown in Fig. S5 while a
comparison of their physicochemical properties is shown in
Table S1.
architecture of LDHs. In LDHs, the divalent (Mg2+) and trivalent
(Al3+) cations can be highly dispersed within the brucite-like
layers.[33] Moreover, the POMs are dispersed and orderly
arranged in the two-dimensional (2D) confined gallery space.
HRTEM images (Fig. 3e-g) and EDX results revealed the
presence of highly dispersed black spots of 1.5 ~ 2 nm in
diameter, which are in good agreement with the size of
La(PW11)2 clusters.
Figure 2. a) XPS survey spectrum of Mg3Al-ILs-La(PW11)2; b) XPS spectrum
for the W4f core level of Mg3Al-ILs-La(PW11)2; c) The N2 adsorption-desorption
isotherm and the pore size distribution (inset picture) of Mg3Al-ILs-La(PW11)2;
d) The 27Al-MAS-NMR spectra of (i) Mg3Al-NO3, (ii) Mg3Al-La(PW11)2, and (iii)
Mg3Al-ILs-La(PW11)2.
Figure 3. SEM images of a-c) Mg3Al-ILs-La(PW11)2, inset picture in image b
shows the water contact angle measurements of Mg3Al-ILs-La(PW11)2, d)
compositional EDX mapping of the Mg3Al-ILs-La(PW11)2; HRTEM images of e-
f) Mg3Al-ILs-La(PW11)2 and g) its corresponding EDX.
The 27Al resonance line positions are very sensitive to the
coordination number and are expected to appear within the
range of -5 to 15 ppm for the octahedral geometry of AlO6 sites.
[26] As shown in Fig. 2d, the solid state 27Al-MAS-NMR spectrum
of Mg3Al-NO3 exhibits a strong single signal at ~10.9 ppm,
indicating the presence of octahedral geometry of AlO6 sites in
the brucite-like layers.[29-31] In the case of Mg3Al-La(PW11)2 and
Mg3Al-ILs-La(PW11)2, the 27Al-MAS-NMR signals are located
almost at the same position (with a slight shift to low field
chemical shift) in comparison to the Mg3Al-NO3. This observation
indicates that the brucite-like layered structure retains its
integrity upon intercalation of the POMs. Based on the TG-DTA,
ICP and elemental analysis considerations, the formula of the
composite material of Mg3Al-ILs-La(PW11)2 was identified as
Mg0.75Al0.25(OH)1.87[O3SiC14H28N2]0.042[PF6]0.036 [La(PW11O39)2]
0.0225·0.72H2O (the POM loading, La(PW11)2, found to be 104.56
μmol/g, W=42.3 wt. %, ILs= 5.9 wt.%).
In order to gain better understanding of the interactions
between the catalyst and substrates under the employed
experimental conditions, the amphiphilic property of the
intercalated catalyst was investigated (inset picture in Fig. 3b).
The LDH layers are hydrophilic due to the presence of
numerous hydroxyl groups and water molecules in the interlayer
galleries. However, when a water droplet contacts the sheet of
Mg3Al-ILs-La(PW11)2, it forms a contact angle of 136°, indicating
a moderate hydrophobicity and excellent surface wettability for
the organic substrate. This observation suggests that the
hydrophobicity of the intercalated catalyst is increased due to
the modification of the LDHs layers with ionic liquids, leading
finally to easier access and more effective interaction of the
substrate with the catalytic site. Additional, SEM, HRTEM, EDX
spectra of the Mg3Al-ILs-PW12 and Mg3Al-ILs-P2W18 composites
are reported in the ESI (Fig. S6).
Scanning Electron Microscopy studies of the composite
material was used in order to obtain further information
regarding the morphology of the catalyst in solid state. The SEM
images of the as-prepared Mg3Al-ILs-La(PW11)2 (Fig. 3) showed
uniform porous stacking of sheet-like crystallites with an average
size of ~1 μm (Fig. 3a-c). The porous sheet-like morphology may
be due to the high percentage of active coordinatively
unsaturated atoms located at the edges on the crystallites.[32]
Energy-dispersive spectroscopy (EDX) mapping (Fig. 3d)
revealed the uniform dispersion of the composition element Mg,
Al, C, O, W, which is closely related to the unique structural
Selective sulfoxidation of thioanisole
Scheme 1. Model reaction of selective oxidation of thioanisole.
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Oxidative transformations of organic compounds under mild
conditions are of great importance in industrial processes and
synthetic organic chemistry.[34-35] The selective oxidation of
sulfides to sulfoxides and sulfones is a pivotal reaction due to its
use in chemical industry, medicinal chemistry and biology.[14] In
this work, we aim at exploring the efficiency of different catalysts
for selective oxidative of thioanisole in the presence of different
substrates and H2O2 (Scheme 1). In order to evaluate the
feasibility, we used thioanisole as a model substrate in the
presence of H2O2 (30 %) as the oxidant for the model reaction.
increase as a function of the electron deficiency on the catalytic
active center.[15] Our previous work have demonstrated that
grafting of ionic liquids onto LDHs nanosheets enhances the
mass transfer in solid-liquid interface by improving the
accessibility of the substrates to the active species.[21] Therefore,
the above experimental results account for the observed
difference of the catalytic activity when we use different catalysts.
In addition, these results strongly support the importance of the
synergistic effect between the components of the heterogeneous
catalyst Mg3Al-ILs-La(PW11)2, which is reflected in the
enhancement of the overall catalytic performance.
Table 1. Selective oxidation of thioanisole in methanol with H2O2 by different
catalysts.[a]
Entry
Catalyst
Conv[b] (%)
80.5
Selec[b] (%)
91.3
1
2
Mg3Al-La(PW11)
2
Mg3Al-ILs-La(PW11
Mg3Al-NO3
ILs
)
2
99.9
97.6
3
25.4
98.6
4
73.6
98.2
5
Mg3Al-NO3+ILs+ La(PW11
)
79.4
95.8
2
6
Mg(OH)2
18.6
82.5
Figure 4. Effect of solvents on sulfoxidation of thioanisole by Mg3Al-ILs-
La(PW11)2. Reaction conditions: 1 mmol of substrate, 1 mmol of H2O2 (30
wt. %), 20 mg catalyst, 200 μL of solvent, 25 oC, 1 h.
7
Al(OH)3
17.8
83.4
8
None
17.3
82.5
9
Mg3Al-ILs-PW12
Mg3Al- ILs-P2W18
83.8
88.7
A series of solvents have been tried to investigate their effect
on the catalytic activity, and the results showed a conversion of
99.9%, 93.7%, 91.6%, 87.1%, 83.8%, 90.6%, 87.3%, 79.5%,
77.4%, 82.3% for methanol, ethanol, i-propanol, 1-butanol,
acetonitrile, acetone, water, dichloromethane, trichloromethane
and solvent-free conditions, respectively (Fig. 4). The observed
difference can be attributed to solvation effects and different
interactions between thioanisole and the solvents. As a result,
the optimized solvent for such a sulfoxidation reaction is
methanol.
10
81.7
91.7
[a] Reaction conditions: 1 mmol of thioanisole, 1 mmol of H2O2 (30 wt.%), 20
mg catalyst, T = 25 oC, t = 1 h. [b] Conversion and selectivity were
determined by GC analysis using reference standards. Assignment of
product was analysed by 1H NMR and 13C NMR in Fig. S7.
As shown in Table 1, the Mg3Al-La(PW11)2 shows 80.5 %
conversion and 91.3 % selectivity (entry 1). Notably, the Mg3Al-
ILs-La(PW11)2 exhibits excellent catalytic performance with 99.9 %
conversion and 97.6 % selectivity (entry 2) under the same
experimental conditions. However, only 17.3 % conversion of
sulfoxidation is obtained in the absence of catalyst (entry 8).
Moreover, the reaction proceeds sluggishly when physical
mixture of the individual components of the composite material,
or the parent LDH (Mg3Al-NO3), and/or ILs (entries 3-5) was
used as catalysts. To prove that the catalytic activity is not the
effect of potential impurities, we studied the efficiency of
Mg(OH)2 and Al(OH)3 (entries 6 and 7) which revealed only 18.6
and 17.8 % conversion, respectively. The control experiments
(entries 9-10) exhibits inferior catalytic activity in the case where
other classical POMs such as Keggin (Mg3Al-ILs-PW12) and
Well-Dawson (Mg3Al-ILs-P2W18) were incorporated in the LDH.
The results suggest that the Mg3Al-ILs-La(PW11)2 is one of the
most efficient catalysts for sulfoxidation reactions.
Kinetic study of sulfoxidation of thioanisole
In an effort to obtain the kinetic parameters for the model
reaction, parallel experiments have been carried out under the
optimized conditions. Taking the Mg3Al-ILs-La(PW11)2 as an
example, the reaction proceeds smoothly at room temperature.
Conversion and ln(Ct/C0) are plotted against reaction time in Fig.
5, where C0 and Ct are the initial thioanisole concentration and
the concentration of thioanisole at time t, respectively. The
Mg3Al-ILs-La(PW11)2 exhibits excellent catalytic activity and the
conversion can reach 99 % with above 97 % selectivity of the
corresponding sulfoxide in 1 h. The linear fit of the data reveals
that the catalytic reaction follows pseudo-first-order kinetics for
selective oxidation of thioanisole reaction (R2 = 0.992). The
observed rate constant k for the selective oxidation reaction was
determined to be 0.069 min-1 on the basis of Eqs. (1) and (2).
Thus, the catalyst exhibits high catalytic efficiency for the
sulfoxidation reaction, and the catalytic reaction obeys pseudo-
first-order kinetics with > 97 % selectivity of the corresponding
The oxidation of organosulfur compounds involves an
electrophilic attack of an activated peroxo species to the sulfide.
[36]
The catalytic activity for such sulfoxidation is supposed to
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10.1002/cctc.201701056
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sulfoxide at a production rate of 208 mmol g-1 h-1 (mmol product
produced per gram of catalyst per hour).
exclusive oxidation of sulfoxide at room temperature at a very
short time.[37] However, it should be noted that some of these
reported catalytic systems suffer from low activity, hydrolytic and
thermal stability under oxidative conditions, catalyst leaching,
recycling issues, etc.
−dCt
= kCt (1)
dt
Table 2. Selective oxidation of various sulfides to sulfoxides catalyzed by
Mg3Al-ILs-La(PW11)2 in methanol.[a]
C0
ln = kt
(2)
Ct
Entry
Substrate
Product
T (h)
Conv [b] (%)
99
Selec[b] (%)
98
1
1
2
1
1
95
99
95
99
3
4
5
6
2
2
2
95
94
96
97
95
99
Figure 5. Kinetic profiles of oxidation of thioanisole under the optimized
conditions; Reaction conditions: 1 mmol of thioanisole, 1 mmol of H2O2 (30
wt.%), 20 mg catalyst, 200 μL methanol, T = 25 oC.
2
6
93
87
94
99
7
8
In order to evaluate the applicability of the heterogeneous
3
83
85
9
catalyst Mg3Al-ILs-La(PW11)2,
a number of thioether-based
compounds were used as substrates under the optimized
reaction conditions (Table 2). The results demonstrate that we
can achieve excellent conversion and selectivity for most
thioethers (entries 1-9, 11). However, the benzyl phenyl sulfide
showed relatively low conversion and poor selectivity, which
might be attributed to the increased steric hindrance (entry 10).
When the substrate/H2O2 molar ratio is 1:2, all the substrates
can be selectively oxidized to the corresponding sulfone with
excellent selectivity (Table S2).
4
4
30
90
35
91
10
11
[a] Reaction conditions: 1 mmol substrate, 1 mmol of H2O2 (30 wt.%), 20 mg
catalyst, T = 25 oC. [b] Conversion and selectivity were determined by GC
analysis using reference standards.
Another important factor is the high cost of some catalytic
systems which renders them unsuitable candidates for large
scale experiments or industrial applications. In contrast, the
sulfoxidation of thioanisole using Mg3Al-ILs-La(PW11)2 exhibits
high efficiency and selectivity, along with excellent stability,
recyclability and very efficient use of H2O2 (Substrate:H2O2 = 1:1)
during the catalytic cycles (Table 2 and Table 3, entry 13), which
renders it an ideal candidate for large scale experiments.
Efficiency for selective oxidation of thioanisole to
sulfoxides
The catalytic selective oxidation of sulfides using various
catalysts that have been reported in the literature is summarized
in Table 3. For example, Wang et al. reported a novel ionic
liquid-based polyoxometalate (POM) catalytic system that shows
Table 3. Oxidation of thioanisole to sulfoxides by different catalysts with H2O2 as oxidant.
H2O2
(equiv.)
Conv.
(%)
Sel.of
sulfoxide
Catalyst
T.(oC)
Time (h)
Ref
Entry
1
2
3
4
5
6
7
8
WO3/MCM-48
TiO2
1.1
1.2
1
rt
80
20
32
rt
4
3
95
100
99
97
0.5
90
99
95
100
-
38
39
40
41
42
43
44
45
(TBA)2[SeO4{WO(O2)2}2]
(TBA)4[γ-SiW10O34(H2O)2]
Fe3O4@SiO2APTES
2
0.2
3
1
98
1.5
1.5
1.5
6
96
WO42-/silica-NH3
3
rt
82
+
BisILs-C8H17-[W2O3(O2)4]
1.1
1
rt
98.6
91
DA-La(PW11
)
25
99
2
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9
[C4mim]2HPM
1.1
2.5
1
25
rt
0.5
0.25
2
98.8
95
98.4
96
37
46
10
11
12
13
[PO4[WO(O2)2]4]@PIILP
Ti-IEZ-MWW
rt
99
94
47
Mg3Al-P2W17Zn
1
rt
6
96
100
98
15
Mg3Al-ILs-La(PW11
)
2
1
25
1
99
This work
Moreover, the catalyst can be applied to the epoxidation of allylic
alcohols (trans-2-hexene-1-ol) with 93 % conversion and 95 %
selectivity (entry 4). The catalytic efficiency of previously
reported systems for the epoxidation of olefin substrates are
summarized in Table S3 (see the Supporting Information). The
results confirm that the Mg3Al-ILs-La(PW11)2 is an excellent
heterogeneous catalyst for the epoxidation of olefins.
Epoxidation of olefin
The efficiency of the Mg3Al-ILs-La(PW11)2 composite was further
evaluated in olefin epoxidation reactions (Fig. 6). In an effort to
identify the optimized epoxidation reaction conditions, a series of
solvents, concentration and the substrate/H2O2 molar ratios
were carefully explored (Fig. S 9-10). The optimum conditions
are: cis-cyclooctene 1 mmol, H2O2 3 mmol, 1 ml CH3CN, 40 mg
catalyst. We investigated the effect of reaction time on the
epoxidation of cis-cyclooctene by Mg3Al-ILs-La(PW11)2. The
reaction conversion increases as a function of the reaction time.
Table 4. The epoxidation of various substrates over catalyst Mg3Al-ILs-
La(PW11)2.[a]
Entry
Substrates
cis-cyclooctene
cyclohexene
Time (h)
Conv[b] (%)
Selec[b] (%)
o
However, when the reaction temperature reaches to 70 C, the
1
2
3
4
5
5
97
96
82
93
99
99
99
95
reaction conversion does not increase anymore. Thus, the
experiments to determine the kinetic parameters for the
epoxidation of cis-cyclooctene were carried out at 70 oC.
cyclododecene
trans-2-hexene-1-ol
12
2
[a]Epoxidation of different substrates with H2O2 catalyzed with Mg3Al-ILs-
La(PW11)2. Reaction conditions: 1 mmol of substrate, 40 mg catalyst, 3 mmol
H2O2 (30 wt.%), 1 ml CH3CN; T = 70 oC. [b] Conversion and selectivity were
determined by GC analysis using reference standards.
Figure 6. Top: schematic diagram of the catalytic epoxidation of cis-
cyclooctene. a) Effect of reaction time on epoxidation of cis-cyclooctene by
Mg3Al-ILs-La(PW11
) under different temperatures b) Kinetic profiles of the
2
epoxidation reaction. Reaction conditions: 1 mmol of substrate, 3 mmol H2O2
(30 wt. %), 40 mg catalyst, 1 ml CH3CN; T = 70 oC.
Conversion and ln(Ct/C0) are plotted against reaction time in Fig.
6b, where C0 and Ct are the initial cis-cyclooctene concentration
and cis-cyclooctene concentration at time t, respectively. The
linear fit of the data reveals that the catalytic reaction shows
Figure 7. a) Catalytic reusability of Mg3Al-ILs-La(PW11
) for sulfoxidation of
2
thioanisole; b) Experiment to prove the nature of heterogeneous catalyst; c)
FT-IR spectra and d) solid-state 29Si CP/MAS NMR spectra of the fresh and
used Mg3Al-ILs-La(PW11)2.
pseudo-first-order kinetics for the epoxidation reaction (R2
=
0.984). The observed rate constant k was determined to be
0.7647 h-1 on the basis of Eqs. (1) and (2). The Mg3Al-ILs-
La(PW11)2 shows pretty good catalytic activity producing 31
mmol g-1 h-1 (mmol product produced per gram of catalyst per
hour) of oxidation product while the conversion and selectivity
reached a value of 99 % in 5 h.
The Mg3Al-ILs-La(PW11)2 exhibited high conversion and
selectivity (Table 4) in the epoxidation of other well-known
olefins such as cyclohexene and cyclododecene (entries 2-3).
The catalyst’s stability and recyclability are essential
requirements for larger scale applications.[48-49] In order to
evaluate the recyclability of the catalytic system, we repeated
the selective oxidation of thioanisole using Mg3Al-ILs-La(PW11)2.
The catalyst was separated from the reaction mixture by
centrifugation, washed with acetone, dried and re-used by
addition of fresh substrate and H2O2 (30 wt. %) at 25 oC (Fig. 7a).
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During the five-run recycling test, the conversion found to be
99.9 %, 98.2 %, 98.4 %, 97.8 %, 97.5 %, 97.1 %, while the
selectivity remains above 98 %. Inductively coupled plasma
atomic emission analysis (ICP-AES) of the recycled catalyst
confirms that the polyoxometalate is stable by intercalating into
the 2D confined space of LDHs. Moreover, the tungsten content
of 41.2 wt.% for the recycled catalyst is close to that of the fresh
catalyst (42.3 wt.%), indicating the absence of leaching issues.
When the epoxidation reaction was half-way through (after
1h the conversion reached a value of 52 %), the Mg3Al-ILs-
La(PW11)2 was removed by filtration. The remaining reaction
mixture kept under the same experimental conditions in the
absence of the catalyst for 4h. Analysis of the reaction mixture
revealed that no additional oxidation product was detected (Fig.
7b, red line). As soon as the solid Mg3Al-ILs-La(PW11)2 was
added back to the reaction mixture, the catalytic reaction
resumed as expected.(Fig. 7b, black line). The experiment
demonstrates that the Mg3Al-ILs-La(PW11)2 is a truly efficient
and robust heterogeneous catalyst. FT-IR (Fig. 7c), solid-state
29Si cross-polarization magic-angle spinning (CP/MAS) NMR
(Fig. 7d) and XPS spectra for the W4f core level (Fig. S12), as
well as the SEM, HRTEM images and EDX of the recycled
Mg3Al-ILs-La(PW11)2 (Fig. S13) suggest that the catalyst retains
its morphology and structural integrity.
g-1 h-1 of epoxides. Moreover, the structural stability of the Mg3Al-
ILs-La(PW11)2 composite is retained during the re-cycling
process due to the multiple interactions between the three
components (host layers, ILs and POMs). The Mg3Al-ILs-
La(PW11)2 composite can be separated easily from the reaction
mixture by simple filtration and recycled at least five times
without obvious decrease of its catalytic activity. Finally, the
scaled-up experiment demonstrated the potential of the
heterogeneous catalyst for industrial applications.
Experimental Section
Chemical Materials
All chemicals were of analytical grade and were used as
received
without
any
further
purification.
The
K11[La(PW11O39)2]∙13H2O
Na3PW12O40∙15H2O
[La(PW11)2],[50]
(PW12),[51]
K6[P2W18O62]∙14H2O
(P2W18),[52]
[Mg0.75Al0.25(OH)2](CO3)0.125∙2H2O
(Mg3Al-
CO3),[53] [Mg0.75Al0.25(OH)2](NO3)0.25·2H2O (Mg3Al-NO3),[54] 1-
octyl-3-(3-triethoxy-silylpropyl)-4,5-dihydro-imidazolium
hexafluorophosphate
(ILs),[55]
Mg3Al-ILs[21]
were
synthesized according to previously reported literature
procedures.
In order to evaluate the potential of the heterogeneous
catalyst, we have focused our attention on the gram-scale
synthesis of sulfoxide and epoxides. We have performed the
selective oxidation of thioanisole and epoxidation of cis-
cyclooctene reaction by scaling up the amount of substrate by a
factor of 50. The scaled-up experiment reactions proceeded
smoothly under the optimized experimental conditions. Using the
selective oxidation of thioanisole as an example, we obtained
excellent yields of the corresponding sulfoxides (96 %) with 99%
selectivity under the optimized experimental conditions of 50
mmol thioanisole, 50 mmol H2O2, 10 ml CH3OH, 1 g Mg3Al-ILs-
La(PW11)2, at 25 oC and 1h reaction time. The results
demonstrate the potential of the designed heterogeneous
catalyst for industrial applications.
Synthesis of Mg3Al-ILs-La(PW11)2
Mg3Al-NO3 (0.3 g) was added into 300 mL of formamide in a
three-necked flask purged with N2. The mixture was vigorously
stirred for 2 days. Then, the mixture was centrifuged for 10 min
and the suspension was separated simply by filtration. Then, ILs
(2.6 mmol) was dissolved in CH3CN/CH2Cl2 (1:1, 5 mL) and
added dropwise to the above isolated suspension. The reaction
mixture was stirred under N2 for 24 hours. The
K11[La(PW11O39)2]∙13H2O (1.68 g, La(PW11)2) was dissolved in
30 mL formamide and added dropwise to the above solution.
The precipitate of Mg3Al-ILs-La(PW11)2 (1.0 g) was collected by
filtration and washed with ethanol, water and dried under
vacuum overnight. Mg3Al-ILs-PW12 and Mg3Al-ILs-P2W18 were
obtained using similar procedure.
Catalytic test
Conclusions
In a typical sulfide oxidation experiment, a mixture of 1
mmol thioanisole, 1 mmol H2O2 (30 wt. %), 200 μL CH3OH
and 20 mg catalyst Mg3Al-ILs-La(PW11)2 (2.1 μmol based
on La(PW11)2) were added into a 10 mL glass bottle and the
reaction mixture was kept under stirring at 25 oC. In a
typical olefin epoxidation experiment, a mixture of 1 mmol
cis-cyclooctene, 3 mmol H2O2 (30 wt. %), 1 mL CH3CN and
40 mg catalyst Mg3Al-ILs-La(PW11)2 (4.2 μmol based on
La(PW11)2) were added into a 10 mL glass bottle and the
In summary, a multicomponent heterogeneous catalyst Mg3Al-
ILs-La(PW11)2
was
synthesized
by
adopting
an
exfoliation/assembly approach. The grafting of ILs on the LDH
not only induces flexibility to the catalyst but also allows easy
access of the substrates to the active centre. The resultant
catalyst shows high efficiency in the catalysis of various
substrates such as the selective sulfoxidation of sulfides to
sulfoxide, sulfones and epoxidation of olefins under mild
conditions. The catalyst of Mg3Al-ILs-La(PW11)2 can promote
efficiently sulfoxidation reactions at 25 oC in 60 min with the
conversion and selectivity remaining above 99 and 98 %
respectively, producing 208 mmol g-1 h-1 of oxidation product.
Additionally, it also can promote efficiently the epoxidation of cis-
o
reaction mixture was kept under vigorous stirring at 70 C.
The resulting products were extracted with diethyl ether,
1
analysed by GC and identified by H-NMR and 13C-NMR in
order to determine the selectivity and conversion.
o
cyclooctene at 70 C in 5 h with the conversion and selectivity
remaining above 97 and 99 % respectively, generating 31 mmol
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Acknowledgements
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This research was supported by the National Basic Research
Program of China (973 program, 2014CB932104), National
Nature Science Foundation of China (U1407127, U1507102,
21521005, 21625101), International Joint Graduate-Training
Program of Beijing University of Chemical Technology, and
Fundamental Research Funds for the Central Universities
(ZY1709). H. N. M. acknowledges the financial support from the
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Entry for the Table of Contents
FULL PAPER
Exploitation of an exfoliation/assembly
design approach led to the fabrication
of an efficient heterogeneous catalyst
for the selective sulfoxidation of
Tengfei Li, a† Wei Zhang, a†, Wei Chen, a
Haralampos N. Miras*band Yu-Fei
Song*ac
sulfides and epoxidation of olefins.
Synergistic effects between the
catalyst’s components led to high
efficiency and selectivity for both types
of reactions under mild conditions.
Page No. – Page No.
Modular polyoxometalate-layered
double hydroxides as efficient
heterogeneous sulfoxidation and
epoxidation catalysts
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