Mo Schiff base-tungstate ionic hybrid with enhanced heterogeneous catalytic activity for epoxidation reactions

Article abstract of DOI:10.1016/j.apcata.2015.09.011

A novel metal Schiff base-tungstate ionic hybrid (Mo-MimAM-WO₄) was prepared by anion-exchange of Mo Schiff base functionalized imidazole ionic liquid with sodium tungstate. The resulting hybrid catalyst was fully characterized by ¹H NMR, FT-IR, XRD, SEM, TGA, and XPS, and its catalytic activity was studied for the epoxidation of alkenes using aqueous H₂O₂ as the oxidant. The catalyst was found to be highly efficient and showed higher catalytic activity than its corresponding homogeneous and heterogeneous analogues Mo-MimAM and Na₂WO₄. After reaction, the catalyst can be easily recovered by filtration and reused for the next run. The synergistic effect between Mo Schiff base complex and tungstate is revealed to be responsible for the catalyst's excellent performance in epoxidation.

Full text of DOI:10.1016/j.apcata.2015.09.011

Applied Catalysis A: General 506 (2015) 173–179
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Mo Schiff base-tungstate ionic hybrid with enhanced heterogeneous
catalytic activity for epoxidation reactions
Weizheng Fanb, Yan Lenga,∗, Jian Liua, Pingping Jianga, Jiwei Zhaoa
a The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi
214122, China
b School of Pharmaceutical Science, Jiangnan University, Wuxi 214122, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 May 2015
Received in revised form 7 September 2015
Accepted 9 September 2015
Available online 12 September 2015
A novel metal Schiff base-tungstate ionic hybrid (Mo–MimAM–WO4) was prepared by anion-exchange of
Mo Schiff base functionalized imidazole ionic liquid with sodium tungstate. The resulting hybrid catalyst
was fully characterized by 1H NMR, FT-IR, XRD, SEM, TGA, and XPS, and its catalytic activity was studied
for the epoxidation of alkenes using aqueous H2O2 as the oxidant. The catalyst was found to be highly
efficient and showed higher catalytic activity than its corresponding homogeneous and heterogeneous
analogues Mo–MimAM and Na2WO4. After reaction, the catalyst can be easily recovered by filtration and
reused for the next run. The synergistic effect between Mo Schiff base complex and tungstate is revealed
to be responsible for the catalyst’s excellent performance in epoxidation.
Keywords:
Mo Schiff base
Epoxidation
Heterogeneous catalysis
Tungstate
© 2015 Elsevier B.V. All rights reserved.
Ionic liquid
1. Introduction
However, most of these catalytic systems are homogeneous, suffer-
ing from problems associated with the separation and recovery of
Epoxides are well known as one of the most valuable build-
ing blocks that can be used as intermediates and precursors for
chemical production [1,2]. The oxidation of alkenes with aqueous
hydrogen peroxide (H2O2) is very attractive from the viewpoint of
industrial production and synthetic organic chemistry, since aque-
ous H2O2 is cheap, environmentally clean and easy to handle [3–5].
Over the past decades, several transition metals such as molyb-
denum [6–11], tungsten [12–16], vanadium [17,18], manganese
[19,20], titanium [21–23], iron [24,25], etc. have been widely used
as homogenous catalysts in oxidation systems based on aqueous
H2O2. Among them, molybdenum and tungsten-based catalyst sys-
tems are promising for epoxidation of alkenes owing to their high
efficiency and selectivity.
Transition metal Schiff base complexes are an important class
of redox catalysts because of that the metal-organic framework is
suitable for the construction of a highly functionalized coordina-
tion site, which may provide an opportunity to further enhance the
catalytic activity [26,27]. Some of metal Schiff base complexes [6,7],
particularly molybdenum Schiff base complexes have shown very
interesting catalytic properties in epoxidation of alkenes [28,29].
the catalysts and inevitable metal contamination of the products.
To overcome these problems, heterogeneous catalysts have been
prepared by immobilizing the metal Schiff base complexes onto
organic polymers [8–10] or inorganic materials such as grapheme
[23], alumina [21], silica [17] and so on. For example, Jia [30]
have synthesized mesoporous MCM-41 materials modified with
oxodiperoxo molybdenum complexes and found that these mate-
rials are active in the epoxidation of cyclooctene. We recently
prepared a novel kind of POSS-bridged Mo Schiff base hybrids
and used it as efficient heterogeneous catalysts for epoxidation of
alkenes with tert-butyl hydroperoxide (TBHP) [10]. Though het-
erogeneous catalysis was achieved in those cases, most of them
still bear some drawbacks like the slow reaction rate, leaching of
active species, or need expensive oxidant. Therefore, it is necessary
to develop new strategies to generate heterogeneous metal Schiff
base catalysts.
Task specific ionic liquids (TSILs), in which functional groups
are covalently tethered to the cation or anion (or both) of the ILs,
have increased attention over the last few years as it is possible
to form any specific composition depending upon user’s need of
the desired physical, chemical and catalytic properties [31–33].
An interesting example of TSILs is focused on the incorporation of
organometallic groups into a branch appended to the cation of ILs
[34,35]. For example, Tan and co-workers [36,37] have shown that
∗
Corresponding author. Fax: +86 510 85917763.
E-mail address: lengyan1114@126.com (Y. Leng).
http://dx.doi.org/10.1016/j.apcata.2015.09.011
0926-860X/© 2015 Elsevier B.V. All rights reserved.

174
W. Fan et al. / Applied Catalysis A: General 506 (2015) 173–179
ILs complexes functionalized by chiral salen Mn(III) were efficient
and recyclable catalysts for enantioselective epoxidation of styrene.
Further, taking into account that the combination of IL cations
with metal-containing inorganic anions can cause the formation
of ionic hybrids with high melting point and insolubility that are
emerging to be very important in catalysis [38,39]. Accordingly, we
think that it is rational to prepare a composite catalyst by pairing
metal Schiff base functionalized IL cations with metal-containing
inorganic anions. The resulting composite may be a novel hetero-
geneous catalyst for epoxidation of alkenes with H2O2.
2.2.2. Synthesis of [MoO2(acac)2]
Molybdenyl acetylacetonate [MoO2(acac)2] was prepared
according to previous literature [40] with about 51% yield.
2.2.3. Synthesis of Mo–MimAM
Mo–MimAM was prepared according to previous literature [41].
In detail, MimAM (1.21 g, 6 mmol) dissolved in anhydrous ethanol
(10 mL) was added dropwise into the ethanol (10 mL) solution of
[MoO2(acac)2] (0.98 g, 3 mmol) with vigorous stirring, the result-
ing mixture was refluxed at room temperature for 24 h under a
nitrogen atmosphere. The formed light-blue solid precipitate was
filtered off and washed thoroughly by H2O and ethanol, and dried
under vacuum. CHN elemental analysis for Mo–MimAM (by the
mass percentage): C 43.87%; N 13.95%; H 5.02%. Found: C 44.11 %;
N 13.37 %; H 5.94 %.
Herein, we demonstrate a novel metal Schiff base-tungstate
ionic hybrid whereby molybdenum (Mo) Schiff base and tungstate
(WO42−) were attached to IL cation and anion, respectively. It was
fully characterized by 1H NMR, FT-IR, XRD, SEM, TGA, and XPS, and
used as a highly efficient heterogeneous catalyst in the epoxidation
of alkenes with H2O2, showing a better catalytic activity than the
2−
2.2.4. Synthesis of Mo–MimAM–WO4
Schiff base metal complex or WO4 along.
The obtained Mo-MimAM (1.5 g, 2 mmol) was dissolved in 30 mL
ethanol in a 100 mL round bottom flask. Then, an aqueous solu-
tion of Na2WO4·2H2O (1.0 g, 3 mmol) was added to the solution
of Mo–MimAM, followed by the stirring of the mixture for 24 h
at 80 ◦C. The white precipitate Mo–MimAM–WO4 was filtered and
washed thoroughly with H2O and ethanol, and dried under vac-
uum. CHN elemental analysis for Mo–MimAM–WO4 (by the mass
percentage): C 26.93%; N 7.85%; H 3.95%. Found: C 26.65 %; N 7.8
%; H 4.04 %.
2. Experimental
2.1. Reagents and analysis
All chemicals were analytical grade (purity > 99%) and used as
received. FT-IR spectra were recorded on a Nicolet 360 FT-IR instru-
ment (KBr discs) in the 4,000–400 cm−1 region. 1H NMR spectra
were measured with a Bruker DPX 400 spectrometer at ambient
temperature in D2O using TMS as internal reference. TG analysis
was carried out with a STA409 instrument in dry air at a heating
rate of 10 ◦C/min. SEM image was performed on a HITACHI S-4800
field-emission scanning electron microscope. XRD patterns were
collected on the Bruker D8 Advance powder diffractometer using
Ni-filtered Cu/K␣ radiation source at 40 kV and 20 mA, from 5–80◦
with a scan rate of 4◦/min. The metal species were detected by
X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250 Xi).
The spectra were recorded with Al K␣ line as the excitation source
(hꢀ = 1486.6 eV). The binding energy (B.E.) values were referenced
to the C 1s peak of contaminant carbon at 284.8 eV. The amount
of leached Mo species in the filtrate was measured using a Jarrell-
Ash 1100 ICP-AES spectrometer. The CHN elemental analysis was
performed on an elemental analyzer Vario EL cube.
2.3. Catalytic tests
Cyclooctene
(1 mmol),
CH3CN
(5 mL),
and
catalyst
Mo–MimAM–WO4 (10 mg) were added into
a
25 mL round
bottom flask. The reaction system was heated to 60 ◦C with
vigorous stirring, then, and the aqueous H2O2 (30 wt.%, 2.5 mmol)
was added dropwise into the reaction mixture within 10 min.
After that, the reaction was stirred for another 4 h at 60 ◦C. After
the reaction, the catalyst was filtered off, washed with ethanol
and dried at 80 ◦C for another run. The product mixture was
analyzed by gas chromatography (GC) (SP-6890A) equipped with
a FID detector and a capillary column (SE-54 30 m × 0.32 mm × 0.3
m). Calibration area normalization method was used to quantify
the products, and each reaction mixture sample was detected
five times to take the average. Notably, the deviation of the five
measurements is less than 2%.
3. Results and discussion
2.2. Preparation of catalysts
3.1. Catalyst preparation and characterization
2.2.1. Synthesis of [3-aminoethyl-1-methylimidazolium]Br
(MimAM)
The synthetic route for Mo–MimAM–WO4 is shown in
Scheme 1. The organic-molybdenum ionic complex abbreviated as
MimAM
detail, N-methylimidazole
was
prepared
(8.2 g,
as
following:
0.1 mol) and
In
2-
Mo–MimAM was prepared by covalent binding amino function-
2−
bromoethylaminehydrobromide (20.5 g, 0.1 mol) were dissolved in
50 mL absolute ethanol at 80 ◦C for 48 h under nitrogen atmosphere
with reflux and stirring. On completion, solvent was removed by
distillation, and the residue was washed with ethyl acetate for 3
times to afford the IL precursor MimAM·HBr. KOH was added into
the aqueous solution of the above solid for neutralization, followed
by the evaporation under vacuum. Methanol (20 mL) and CHCl3
(2 mL) were added into the resulting mixture with the appearance
of precipitated salts. After filtration, the filtrate was evaporated to
give the MimAM product as yellow oil. 1H NMR (400 MHz, D2O,
TMS) ı(ppm) = 3.47 (m, 2H, CH2), 4.58 (m, 2H, CH2), 5.40 (dd,
1H, CH), 5.81 (dd, 1H, CH), 7.13 (m, 1H, CH), 7.66 (s, 1H, CH),
7.81 (s, 1H, CH), 9.18 (s, 1H, CH). CHN elemental analysis for
MimAM (by the mass percentage): C 34.97%; N 20.39%; H 5.87%.
Found: C 34.58%; N 20.25%; H 5.96%.
alized IL and MoO2(acac)2. Then, WO4
species was introduced
by anion-exchange of Mo–MimAM with Na2WO4 to give the
final catalyst Mo–MimAM–WO4, which was fully characterized
by FT-IR, XRD, SEM, TG, XPS, and CHN elemental analysis. The
CHN elemental analysis for Mo–MimAM–WO4 showed 26.65%
for the weight percentage of C, 7.8% for N, and 4.04% for H,
which is very near to the theoretical values of C: 26.93%, N:
7.85%, and H: 3.95%. Moreover, the TG profile (Fig. 1) indicates
that Mo-MimAM-WO4 was quite stable up to 290 ◦C, the total
weight loss of about 45% was somewhat corresponding to the
decomposition of both organic and inorganic moieties of the cat-
alyst into individual oxides (MoO3 and WO3) (calculated weight
loss: 42%).
Fig. 2 compares the FT-IR spectrum of Mo–MimAM–WO4 with
those of [MoO2(acac)2], Na2WO4, and Mo–MimAM. [MoO2(acac)2]

W. Fan et al. / Applied Catalysis A: General 506 (2015) 173–179
175
N
N
N
O
Mo
O
O
O
O
O
N
Br
MoO2(acac)2
O
O
Mo
O
O
N
N
O
O
N
N
O
Na2WO4
C2H5OH
2-
Mo
O
WO4
+
80 oC 24 h
H2N
N
N
Br
N
Br
N
MimAM
N
N
Mo-MimAM
Mo-MimAM-WO4
Scheme 1. Preparation of Mo–MimAM–WO4 ionic hybrid catalyst.
100
80
60
40
20
0
(a)
(b)
(a)
(b)
10
20
30
40
50
60
70
80
100 200 300 400 500 600 700 800 900
2 Theta (o)
Temperature (oC)
Fig. 3. XRD patterns of (a) Na2WO4 and (b) Mo–MimAM–WO4.
Fig. 1. TG curves of catalysts (a) Mo–MimAM–WO4, and (b) Mo–MimAM.
presents two strong characteristic bands at 935 and 904 cm−1
that are attributed to the stretching vibration of (MoO2)asym and
(MoO2)sym, respectively (Fig. 2, curve a), and Na2WO4 shows two
strong bands at 834 and 857 cm−1 that are attributed to the
stretching vibration of the W O and W O, respectively (Fig. 2,
curve b). The IR spectrum of Mo–MimAM shows the characteris-
tic peaks at 947, 917, and 843 cm−1, assignable to the vibrations
of (MoO2)asym, (MoO2)sym, and Mo O, respectively (Fig. 2, curve
c). For the developed hybrid catalyst Mo–MimAM–WO4 (Fig. 2,
curve d), the above peaks appeared distinctively, and the slight
Thus, the observed phenomenon implies the formation of a new
organometallic-tungstate ionic hybrid via ionic linkage between
the organic cation and the WO4 anion.
2−
The XRD pattern in Fig. 3, curve
a displays the fea-
tured diffraction peaks for the orthorhombic phase of
Na2WO4. However, the intensity of all these peaks declined
in the case of Mo–MimAM–WO4. It is thus suggested that
the developed ionic hybrid catalyst is almost amorphous
in nature. Indeed, the SEM image for Mo–MimAM–WO4
(Fig. 4) shows an amorphous structure with rough
surface.
shifts of the above bands for Mo–MimAM–WO4 result from the
2−
strong ionic interactions between IL cations and WO4
anions.
3.2. Catalytic activity in the epoxidation reaction
The catalytic performance of the catalyst Mo–MimAM–WO4 and
comparison with various reference catalysts were first assessed
in the epoxidation of cyclooctene using H2O2 as oxidant at 60 ◦C
in CH3CN, the results are listed in Table 1. It can be seen that
the reaction did not proceed at all in the absence of catalyst
(Table 1, Entry 1). The pure [MoO2(acac)2] (Table 1, Entry 2)
showed a conversion of 83% with 100% selectivity, but it dis-
solved in the reaction medium to give homogeneous systems.
After the combination of MoO2(acac)2 and IL MimAM, the result-
ing Mo–MimAM caused a heterogeneous reaction system, with a
significant decrease of conversion down to 54% (Table 1, Entry 3).
Additionally, Na2WO4 was partially soluble in the reaction media,
but showed very low conversion of 2% (Table 1, Entry 4). In con-
trast, the hybrid catalyst Mo–MimAM–WO4 obtained by combining
Mo–MimAM and Na2WO4 caused a liquid-solid heterogeneous
epoxidation system, and offered a much higher conversion of 99%
than its precursors did (Table 1, entry 5). The above comparison
results imply that there might generate some synergistic effects
(a)
(b)
904
935
(c)
(d)
857
834
843
947
917
942
837
880
2000
1000
Wavenumber (cm-1)
Fig. 2. FT-IR spectra of catalysts (a) MoO2(acac)2, (b) Na2WO4, (c) Mo–MimAM, and
(d) Mo–MimAM–WO4.

176
W. Fan et al. / Applied Catalysis A: General 506 (2015) 173–179
Fig. 4. SEM images of (A) Mo–MimAM, (B) Mo–MimAM–WO4, and (C) Na2WO4.
Table 1
3.4. Epoxidation of various substrates over catalysts
Mo–MimAM–WO4 and Mo-MimAM
Catalytic performances of various catalysts for the epoxidation of cyclooctene using
H2O2.a
Entry
Catalyst
Solubility in reaction
Xb (%)
Sc (%)
We assessed the catalytic activity of Mo–MimAM–WO4 and
Mo–MimAM under the described reaction conditions, using a series
of various types of alkenes. The results are summarized in Table 2. It
can be seen that heterogeneous catalyst Mo–MimAM–WO4 shows
much higher catalytic activity than that of Mo–MimAM in the
majority of cases. These results further confirm the positive syn-
ergistic effect between Mo–MimAM and WO42− for epoxidation
reactions. Moreover, cyclooctene, cyclohexene, and 3-hexan-1-ol
with inner double bonds exhibit higher activities in comparison
with 1-hexene and 1-octene which contain terminal double bonds.
1d
2
3
4
5
None
–
n.d.
83
54
2
n.d.
100
100
100
100
MoO2(acac)2
Mo–MimAM
Na2WO4
Soluble
Insoluble
Partially soluble
Insoluble
Mo–MimAM–WO4
99
a
Reaction conditions: catalyst (0.01 g), cyclooctene (1 mmol), H2O2 (30%,
2.5 mmol), CH3CN (5 mL), 60 ◦C, 4 h.
b
Conversion.
Selectivity were determined by GC.
The reaction was performed in the absence of catalyst, and the conversion was
c
d
too low to be determined (abbreviated as “n.d.”).
3.5. Understanding of the catalytic behavior of Mo–MimAM–WO4
2−
between Mo–MimAM and WO4
that endows the catalyst with
Fig. 6(A) and (B) display the XPS spectra of the Na2WO4,
Mo–MimAM, and Mo–MimAM–WO4. As can be seen, the W 4f
spectrum of Na2WO4 shows a spin-doublet peak with the binding
energy of 4f7/2 and 4f5/2 at 35.2 and 37.3 eV, respectively, corre-
sponding to the W6+ oxidation state, and the doublet Mo 3d5/2 and
3d3/2 peak at 231.3 and 234.5 eV is attributed to typical Mo6+ oxi-
dation state. In the case of Mo–MimAM–WO4, both W 4f and Mo
3d peaks appeared in the XPS spectra, suggesting the existence of
two types of metal centres on the catalyst Mo–MimAM–WO4. Fur-
thermore, the position of the bands for W 4f and Mo 3d shifted
to higher binding energy and the intensity of which decreased.
This observation indicates the formation of new chemical microen-
superior catalytic activity for epoxidation reaction. The catalytic
activity and selectivity of Mo–MimAM–WO4 for the epoxidation of
cyclooctene are higher or comparable to those of the previously
reported heterogeneous catalysts, such as supported tungsten
oxide [42], peroxometalate-based polymer-mobilized ionic liquid
phase (PIILP) catalyst [43] and CuO nanoclusters coated with meso-
porous SiO2 [44], as well as the homogeneous vanadium complex
[45].
3.3. Influences of reaction conditions
vironment around Mo and W species by the synergistic effects
The influences of the reaction conditions on the epoxidation of
cyclooctene with H2O2 over Mo–MimAM–WO4 are shown in Fig. 5,
including the reaction temperature, catalyst amount, reaction time,
and molar ratio of H2O2 to cyclooctene. In all cases, the selectiv-
ity of epoxidation product was always 100%. It can be seen that
the increase of the reaction temperature from 40 ◦C to 70 ◦C led
to a remarkable increase in the conversion of cyclooctene. How-
ever, at a higher temperature 80 ◦C, the drastic decrease of the
conversion was observed (Fig. 5A), mostly due to the more drastic
self-decomposition of H2O2. Fig. 5B displays the effect of the cata-
lyst amount on the conversion of cyclooctene. When the amount of
catalyst was increased up to 0.01 g, the maximum conversion of 99%
was achieved, a further increase of the catalyst amount reversely
caused a slight decrease of the cyclooctene conversion, which may
result from the deep oxidation of epoxidation product with more
catalyst. The effect of the reaction time (Fig. 5C) and the molar ratio
of H2O2 to cyclooctene (Fig. 5D) on the epoxidation of cyclooctene
were also investigated, and the best experimental condition is 4 h
of reaction time and 2.5:1 of H2O2/cyclooctene molar ratio, pro-
viding a conversion of 99% and a selectivity of 100%. Theoretically,
the molar ratio of H2O2 to cyclooctene for the epoxidation reaction
is 1:1, the H2O2 needed for the highest conversion was about 2.5
times as high as its stoichiometry, which could be attributed to the
self-decomposition of H2O2 [46–48].
2−
between Mo–MimAM and WO4
.
Further insights into Mo–MimAM–WO4 catalyzed epoxidation
was gained by the IR measurement of the Mo–MimAM–WO4 after
reacting with 30% H2O2 in CH3CN at 60 ◦C for 10 min, which mim-
ics the practical reaction but in the absence of alkene substrates,
and the FT-IR spectra of H2O2 treated and fresh Mo–MimAM–WO4
are illustrated in Fig. 7. On treatment with H2O2, the v(Mo O) and
v(W O) bands at 942, and 880 cm−1 shifted to lower wavenumber
in the spectrum for Mo–MimAM–WO4, and two new distinct peaks
at 620 and 540 cm−1 occurred. The new vibrations can be attributed
to the peroxo-oxygen band ␯(O O) in Mo(O2) and WO4 species,
respectively, which have been known as the active species for
H2O2-based epoxidations [41,49,50]. The recovered catalyst from
the reacted mixture returned to its original structure (Fig. 7, curve
c). Therefore, the above results indicate that both peroxo–W and
peroxo–Mo species are the possible intermediate active species,
which may directly associate with the high activity and selectivity
of Mo–MimAM–WO4 for the epoxidation of alkenes.
3.6. Catalyst reusability
Further experiments were conducted to examine the recycla-
bility and reusability of the Mo–MimAM–WO4 in the epoxidation

W. Fan et al. / Applied Catalysis A: General 506 (2015) 173–179
177
100
95
90
85
80
75
70
100
(B)
(A)
95
90
85
80
75
40
50
60
70
80
0.005
0.010
0.015
0.020
0.025
Temperature (oC)
The amount of catalyst (g)
100
95
90
85
80
75
100
90
80
70
60
50
40
(C)
(D)
1.0
1.5
2.0
2.5
3.0
3.5
4.0
1
2
3
4
5
6
H2O2/cyclooctene ratio
Reaction time (h)
Fig. 5. Influences of reaction conditions on epoxidation of cyclooctene with H2O2 over catalyst Mo–MimAM–WO4: (A) reaction temperature, (B) the amount of catalyst, (C)
reaction time, and (D) the molar ratio of H2O2 to cyclooctene. For each figure there is a specific parameter changed, and the selectivity in all cases is 100%.
Table 2
Epoxidation of various substrates over catalysts Mo–MimAM–WO4 and Mo–MimAM.a
Entry
Substrate
Product
t (h)
Xb (%)
Sc (%)
Xb (%)
Sc (%)
Mo–MimAM–WO4
Mo–MimAM
1
4
99
100
54
100
2
3
12
12
65
49
62
61
53
40
30
49
4
12
80
100
28
100
5
6
12
12
27
19
100
100
10
6
100
100
3d5/2
4f7/2
(A)
(B)
W 4f
Mo 3d
4f5/2
3d3/2
(a)
(c)
(b)
(c)
240
238
236
234
232
230
228
226
48
46
44
42
40 38
36
34
32
30
Binding Energy (eV)
Binding Energy (eV)
Fig. 6. XPS spectra of (a) Na2WO4, (b) Mo–MimAM, and (c) Mo–MimAM–WO4.

178
W. Fan et al. / Applied Catalysis A: General 506 (2015) 173–179
(a)
(b)
942
837
880
(c)
620
932
540
840
868
1200 1100 1000 900 800 700 600 500
Wavenumber (cm-1)
Fig. 7. FT-IR spectra of (a) fresh Mo–MimAM–WO4, (b) H2O2 treated Mo–MimAM–WO4, and (c) three times reused Mo–MimAM–WO4.
100
80
60
40
20
0
100
80
60
40
20
0
(A)
(B)
1st run
2nd run
3rd run
1st run
2nd run
3rd run
0
1
2
3
4
5
6
0
1
2
3
4
5
6
Reaction time (h)
Reaction time (h)
Fig. 8. (A) Catalytic reusability of Mo–MimAM–WO4 for the epoxidation of cyclooctene with H2O2. (B) Catalytic reusability of Mo–MimAM–WO4 for the epoxidation of
cyclooctene with H2O2 with the addition of fresh catalyst to resume the constant catalyst amount of 0.01 g. Reaction conditions: cyclooctene (1 mmol), H2O2 (30%, 2.5 mmol),
CH3CN (5 mL), 60 ◦C.
of alkenes by using cyclooctene as a model substrate. After the
reaction, the catalyst was recovered from the reaction mixture by
filtration, washed with ethanol, dried and reused for the next run
without adding any fresh catalyst. Fig. 8(A) gives the three-run cat-
alyst recycling results for epoxidation of cyclooctene with H2O2. A
selectivity of 100% could always be obtained, and high conversions
above 95% were achieved in the second and third run with a slightly
longer reaction time of 6 h. However, the reaction rate decreased
gradually over the three run tests. The result of ICP-AES elemental
analysis for the reacted filtrate shows that about 4.9 wt.% Mo and
2.3 wt.% W in the catalyst had leached into the reaction media. Con-
sequently, a hot-filtration experiment was carried out. Initially, the
mixture of cyclooctene, catalyst Mo–MimAM–WO4, and H2O2 in
acetonitrile was stirred at 60 ◦C for 1 h. The cyclooctene conversion
of 76.3% was obtained at the reaction time of 1 h. Then, the cata-
lyst was removed and the reaction proceeded for another 4 h with
the homogeneous filtrate, the conversion was increased slightly to
80.9%. This result suggests that such a small amount leaching of Mo
and W species did not play a key role in the decrease of catalytic
activities. Notably, the weight loss in the operation for recovering
the catalyst is unavoidable (about 77 wt% of the catalyst was recov-
ered after the first run), when a small amount of the fresh catalyst
was added into the recovered one to resume the constant catalyst
amount of 0.01 g as in the first run, the catalyst showed a relative
steady conversion of cyclooctene (Fig. 8B). The above results indi-
cate that the decrease of the catalytic activity is mostly attributed
to the weight loss in the operation for recovering the catalyst and
the catalyst’s leaching during the reaction.
4. Conclusions
In summary, we have successfully prepared a novel metal Schiff
base-tungstate ionic hybrid Mo–MimAM–WO4 by anion-exchange
of Mo Schiff base functionalized IL Mo–MimAM with Na2WO4. The
resulting ionic hybrid containing two types of metal active centers
Mo and W was proved to be an effective and recyclable catalyst
for heterogeneous epoxidation of alkenes with H2O2, exhibiting
higher catalytic activity than those of its precursors Mo–MimAM
and Na2WO4. The overall superior catalytic activity of the catalyst
associates with the synergistic effect between Mo Schiff base and
2−
WO4 species that result in the improved redox property of the
hybrid catalyst.
Acknowledgements
The authors thank the National Natural Science Foundation of
China (No. 21206052) and MOE & SAFEA for the 111 Project (No.
B13025).
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