Heterogeneous epoxidation of alkenes with H2O2 catalyzed by a recyclable organic-inorganic polyoxometalate-based framework catalyst

Article abstract of DOI:10.1039/c5nj01661d

Four organic-inorganic polyoxometalate-based frameworks (HSiW-MOF, PMo-MOF, HPMo-MOF and PW-MOF) were synthesised and characterized by elemental analysis, UV-vis and FT-IR spectroscopy, cyclic voltammetry and XRD analysis. Organic-inorganic polyoxometalate-based frameworks could act as effective and reusable heterogeneous catalysts for selective epoxidation of alkenes with aqueous hydrogen peroxide in acetonitrile. In the presence of PW-MOF, various kinds of alkenes could selectively be converted into the corresponding epoxides with high conversion and selectivity. PW-MOF was stable to leaching, behaved as true heterogeneous catalysts, easily recovered by filtration, and reused four times with maintenance of the catalytic performance.

Full text of DOI:10.1039/c5nj01661d

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Heterogeneous epoxidation of alkenes with H2O2
catalyzed by a recyclable organic–inorganic
polyoxometalate-based framework catalyst†
Cite this:DOI: 10.1039/c5nj01661d
Hedayat Haddadi,a Shahrbanou Moradpour Hafshejani,bc Mostafa Riahi Farsani*c
and Ali Kazemi Babahydaric
Four organic–inorganic polyoxometalate-based frameworks (HSiW-MOF, PMo-MOF, HPMo-MOF and
PW-MOF) were synthesised and characterized by elemental analysis, UV-vis and FT-IR spectroscopy,
cyclic voltammetry and XRD analysis. Organic–inorganic polyoxometalate-based frameworks could act
as effective and reusable heterogeneous catalysts for selective epoxidation of alkenes with aqueous
hydrogen peroxide in acetonitrile. In the presence of PW-MOF, various kinds of alkenes could selectively
be converted into the corresponding epoxides with high conversion and selectivity. PW-MOF was stable
to leaching, behaved as true heterogeneous catalysts, easily recovered by filtration, and reused four
times with maintenance of the catalytic performance.
Received (in Victoria, Australia)
27th June 2015,
Accepted 13th October 2015
DOI: 10.1039/c5nj01661d
www.rsc.org/njc
[g-SiW10O34(H2O)2]4ꢀ
,
[g-PW10O38V2(m-OH)2]3ꢀ
,
H2Na14[FeIII
-
,
2
Introduction
(NaOH2)2(P2W15O56)2], ([TMIIFeIII2(P2W15O56)(P2TMII2W13O52)]16ꢀ
Selective epoxidation of alkenes is of high interest in TM = Cu, Co), [WZnM2(H2O)2(ZnW9O34)2]12ꢀ (M = MnII, ZnII),
industrial organic synthesis. Different epoxides have been used and so on.8–14 Kholdeeva et al. have demonstrated the catalytic
widely in paints, epoxy resins, surfactants, and intermediates performance of (TBA)4[PTi(OH)W11O39] in the oxidation of
in various organic syntheses and industries (for example, alkenes.15 Furthermore, the immobilization of polyoxometalates
4.5 million tons and 70 000 tons of propylene and butene on solid supports such as silica has attracted great research
oxides, respectively, are produced each year).1,2 H2O2 is an interest because solid catalysts have the advantages of being
attractive and clean oxidant for different oxidation reactions easier to recover and recycle.16–23 However, a few examples of
because it has high content of active oxygen species and can using heterogeneous catalysts with polyoxometalates in alkene
be safely stored and used.3 Polyoxometalates (POMs) are epoxidation with H2O2 were also reported.24 Mizuno reported
appealing compounds whose versatility and accessibility have that the epoxidation hardly proceeded with [g-H2PV2W10O40]3ꢀ
led to the use of these compounds for various applications of or SiW12 alone but the [g-H2PV2W10O40]–[SiW12] composite
science.4,5
and [g-H2PV2W10O40]–[SiW12] supported on different com-
Heteropoly compounds of Keggin structure are widely used pounds showed much higher catalytic activity than those of
3ꢀ
as catalysts for oxidative type reactions due to their unique [g-H2PV2W10O40
properties, such as Brønsted acids, stronger than the conven-
tional solid acids and fast and reversible multielectron redox been reported by different groups.26 Radical chain processes,
transformations under rather mild conditions.6,7
including those associated with O2 and H2O2, and non-radical
]
and SiW12 used alone.25
The mechanistic pathway for the epoxidation of alkenes has
In recent decades, many catalytic systems with various types mechanisms and peroxometal intermediates are more commer-
of POMs as effective catalysts for the epoxidation of alkenes cially significant.
have been used, for example, [{g-SiTi2W10O36(OH)2}2(m-O)2]8ꢀ
,
Metal–organic frameworks (MOFs) have been shown to be
efficient supports to incorporate polyoxometalates for the for-
mation of efficient heterogeneous catalysts. In recent years,
MOFs have attracted considerable attention because of their
unique combination of properties such as high surface area,
crystalline open structures, tunable pore size, functionality, and
so forth.27,28 All these properties allow considering MOFs as
prospective catalytic materials and supports for immobilization
a Department of Chemistry, Faculty of Sciences, Shahrekord University,
P. O. Box 115, Shahrekord, Iran
b Chemical Nanoscience Laboratory, School of Chemistry, Bedson Building,
Newcastle University, Newcastle upon Tyne, NE1 7RU, UK
c Department of Chemistry, Islamic Azad University, Shahrekord Branch,
Shahrekord, Iran. E-mail: Riahim91@gmail.com, Riahim91@yahoo.com;
Tel: +98 9133831017
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nj01661d of homogeneous catalysts.29–31
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under autogenous pressure at 160 1C for 5 days and then left to
cool to room temperature. Dark block crystals could be isolated
in about 61% yield (based on Mo). Elemental analysis (%) calcd:
Mo, 44.54; P, 1.20; Cu, 7.38; C, 16.26; H, 1.21; N, 4.34. Found: Mo,
43.75; P, 1.02; Cu, 7.14; C, 16.7; N, 4.25; H: 1.18.
Scheme 1 Epoxidation of alkenes catalyzed by POM-MOF.
In addition, different polyoxometalate structures catalyzed
various oxidation reactions.32–34 Nevertheless, such reactions are
still rare and lack generality.
Preparation of HPMo-MOF
HPMo-MOF was synthesized hydrothermally from a mixture
of H3PMo12O40ꢁxH2O (FW E 1825.25, 0.5 g, 0.274 mmol),
C6H5NO2 (2-picolinic acid) (0.16, 1.357 mmol), CuCl2ꢁ2H2O
(0.333 g, 1.953 mmol), 4,40-bpy (0.20 g, 1.04 mmol), C5H6N2
(2-aminopyridine) (0.10 g, 1.062 mmol) and distilled water
(25 ml). The pH of the mixture was necessarily adjusted to
6 with NH3ꢁH2O solution. The mixture was heated under
autogenous pressure at 160 1C for 5 days and then left to cool
to room temperature. Dark block crystals could be isolated in
about 58% yield (based on Mo). Elemental analysis (%) calcd:
Mo, 48.85; P, 1.31; Cu, 5.39; C, 12.74; H, 0.98; N, 3.57. Found:
Mo, 48.70; P, 1.18; Cu, 5.11; C, 12.87; N, 3.54; H, 0.89.
The present work on the development of novel oxidation
methods for the oxidation of organic compounds35 reports a
heterogeneous epoxidation with hydrogen peroxide catalyzed
by POM hybrid materials based on Keggin polyanion building
blocks and copper organic frameworks with pyrazine derivatives36
(Scheme 1), [Cu3(4,40-bpy)3] [HSiW12O40]ꢁ(C3H4N2) (abbreviation:
HSiW-MOF), [Cu3(4,40-bpy)3] [PMo12O40]ꢁ(C5H6N2)ꢁ0.5H2O (abbre-
viation: PMo-MOF), [Cu2(4,40-bpy)2][HPMo12O40]ꢁ(C5H6N2) (abbre-
viation: HPMo-MOF) and [Cu(Phen)(4,40-bpy)(H2O)]2[PW12O40]ꢁ
(4,40-bpy), (abbreviation: PW-MOF); C3H4N2 = imidazole, C5H6N2 =
2-aminopyridine, bpy = bipyridine, Phen = 1,10-phenanthroline.
Moreover, the catalyst was reused in the procedure four times
without any loss of activity.
Preparation of PW-MOF
PW-MOF was synthesized hydrothermally from a mixture of
Na2WO4ꢁ2H2O (0.5 g, 1.516 mmol), H3PO4 (0.20 ml, 85%),
C6H5NO2 (isonicotinic acid) (0.1 g, 0.812 mmol), CuCl2ꢁ2H2O
(0.201 g, 1.179 mmol), Phen (0.133 g, 0.671 mmol), 4,40-bpy
(0.052 g, 0.271 mmol) and distilled water (25 ml). The pH of the
mixture was necessarily adjusted to 5 with NH3ꢁH2O solution.
The mixture was heated under autogenous pressure at 160 1C
for 5 days and then left to cool to room temperature. Blue block
crystals could be isolated in about 45% yield (based on W).
Elemental analysis (%) calcd: W, 57.02; P, 0.80; Cu, 3.28; C,
16.76; H, 1.15; N, 3.62. Found: W, 56.12; P, 0.81; Cu, 3.51; C,
16.47; N, 3.37; H, 0.99.
Experimental section
All common laboratory chemicals were of reagent grade, pur-
chased from commercial sources, and used without further
purification.
Preparation of the catalysts
HSiW-MOF, PMo-MOF, HPMo-MOF and PW-MOF were prepared
and fully characterized according to the procedure already
published in the literature (see ESI,† Fig. S1–S10).36
Preparation of HSiW-MOF
A typical procedure for the catalytic oxidation of alkenes
HSiW-MOF was synthesized hydrothermally from a mixture of
H4SiW12O40ꢁxH2O (FW E 2878.17, 0.4 g, 0.14 mmol), CuCl2ꢁ
2H2O (0.23 g, 1.33 mmol), H2C2O4ꢁ2H2O (0.41 g, 3.25 mmol),
im (im = imidazole, 0.027 g, 0.4 mmol), 4,40-bpy (0.101 g,
0.65 mmol) and distilled water (20 ml). The pH of the mixture
was necessarily adjusted to 4 with NH3ꢁH2O solution. The
mixture was heated under autogenous pressure at 160 1C for
5 days and then left to cool to room temperature. Dark red
block crystals could be isolated in about 51% yield (based on
W). Elemental analysis (%) calcd: W, 61.24; Si, 0.78; Cu, 5.29;
C, 11.00; H, 0.81; N, 3.11. Found: W, 62.2; Si, 0.87; Cu, 5.04;
C, 9.85; N, 2.94, H, 0.75.
The catalytic reactions were performed in a 10 ml round bottom
flask equipped with a magnetic stirring bar and a reflux con-
denser. The desired catalyst (HSiW-MOF, PMo-MOF, HPMo-MOF
and PW-MOF), alkenes and 1.25 mmol of H2O2 in acetonitrile
were charged in the reaction flask at the set constant temperature
for the whole duration of reactions. The same reaction was
carried out by varying different parameters such as the effect
of molar ratio of H2O2 to substrate, the amount of catalyst,
reaction time, and reaction temperature. After completion of
the reaction, the catalyst was removed by filtration. The progress
of the reaction was detected by gas chromatography (GC).
Conversion = 100% ꢂ (mol epoxide product + mol byproducts)/
mol initial H2O2. Selectivity = 100% ꢂ mol epoxide product/
Preparation of PMo-MOF
PMo-MOF was synthesized hydrothermally from a mixture (mol epoxide product + mol byproducts). The concentration of
of H3PMo12O40ꢁxH2O (FW E 1825.25, 0.5 g, 0.274 mmol), H2O2 in the reaction mixture was determined by the titration with
NH4VO3 (0.234 g, 2.0 mmol), C8H6O4 (isophthalic acid) (0.30 g, sodium thiosulfate (with starch as the indicator) in the presence
1.81 mmol), CuCl2ꢁ2H2O (0.333 g, 1.953 mmol), 4,40-bpy (0.166 g, of potassium iodide, sulfuric acid and ammonium molybdate. No
0.864 mmol), C5H6N2 (2-aminopyridine) (0.10 g, 1.062 mmol) and remaining H2O2 was detected due to the self-decomposition of
distilled water (25 ml). The pH of the mixture was necessarily H2O2 accompanying the oxidation of alkene. In this case, the
adjusted to 6.5 with NH3ꢁH2O solution. The mixture was heated conversion of the substrate based on H2O2 calculated using the
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above formula also reflects the H2O2 utilization efficiency. Other used as a catalyst for the epoxidation of cyclooctene and no
alkene substrates were tested accordingly.
conversion was found. Epoxidation of cyclooctene was carried
out without a catalyst. It was identified that no oxidation occurs
and among all catalysts PW-MOF produced a heterogeneous
epoxidation system and showed an 83.5% conversion with 99%
selectivity (entry 7). In addition, pure H3PW12O40, H4SiW12O40,
and H3PMo12O40 (entries 1–3), despite being homogeneous
catalysts, gave only 5.1–37% conversion with desirable selectivity.
Regarding PW-MOF as a more effective catalyst, detailed reaction
parameters such as reaction temperature, the amount of catalyst
and H2O2 concentration were taken into account in the epoxida-
tion of cyclooctene to optimize the conditions.
Results and discussion
Synthesis and structure
In this study, HSiW-MOF, PMo-MOF, HPMo-MOF and PW-MOF
were obtained by hydrothermal reactions according to the pub-
lished procedure of Prof. Cui.36 He reported that all compounds
are synthesised by mixing polyoxometalates or polyoxometalate
precursors, CuCl2 and an organic linker with different molar
ratios in water solution. Also, the pH of the mixture was necessa-
rily adjusted to specific pH with NH3ꢁH2O solution. Furthermore,
he reported that H2C2O4ꢁ2H2O plays an important role in the
synthesis of HSiW-MOF as a reducing agent. Interestingly, Cui
et al. reported that preparation of the PMo-MOF without adding
isophthalic acid or NH4VO3 resulted in only amorphous powders.
The roles of NH4VO3 and isophthalic acid are still elusive.36 After
synthesizing the four compounds using the above method, the
purity and structure of all compounds were confirmed by infrared
spectroscopy, UV-vis spectroscopy, cyclic voltammetry, elemental
analysis and XRD analysis (see ESI,† Fig. S1–S10). They are in
complete agreement with the observations of Cui et al. for
HSiW-MOF, PMo-MOF, HPMo-MOF and PW-MOF.36
Effect of temperature
Temperature can also be important for achieving the optimum
results. To study the relevant effect, the reaction was studied at
four different temperatures of 25, 50, 60, and 85 1C with 39 mg
of PW-MOF containing 0.01 mmol of PW12, 1 mmol of cyclo-
octene, 1.25 mmol of hydrogen peroxide, and 3 ml of CH3CN.
The results of the same are presented in Fig. 1. The outcomes
indicated that the conversion of cyclooctene increased with the
increase of temperature while selectivity to epoxide decreased.
Although the increase of the reaction temperature improved
the conversion of cyclooctene at higher temperatures selectivity
to epoxide decreased.
Effect of the nature of catalyst
Therefore, the temperature of 50 1C was found optimal for
the higher conversion of cyclooctene to the corresponding
epoxide.
At first, the catalytic activity of CuCl2, H3PW12O40, H4SiW12O40,
H3PMo12O40, HSiW-MOF, PMo-MOF, HPMo-MOF and PW-MOF
was evaluated for the epoxidation of cyclooctene using 1.25 mmol
of H2O2 as an oxidant at 50 1C in acetonitrile. With the results Effect of the amount of catalyst
shown in entries 1–9 (Table 1) it could be observed that HSiW-
The effect of the amount of catalyst on the epoxidation of
MOF, PMo-MOF, HPMo-MOF and PW-MOF could be applied to
the epoxidation of cyclooctene using H2O2, and as a result of this
in all cases beneficial to excellent activities and selectivities were
obtained. However, H3PW12O40, H4SiW12O40, and H3PMo12O40
were inactive or exhibited extremely low catalytic activities for
cyclooctene in homogeneous systems, which further confirm the
promotional role of MOF structure in the high epoxidation
activity of POM-MOF in heterogeneous systems. CuCl2 was also
cyclooctene is presented in Fig. 2.
The reaction was performed with various amounts of PW-MOF
with 1 mmol of cyclooctene and 1.25 mmol of H2O2 for 5 h at
50 1C. The conversion and selectivity for the epoxidation of
cyclooctene are reported in Fig. 2. It is observed from Fig. 2 that
the conversion increases with an increase in the amount of
Table 1 Epoxidation of cyclooctene with H2O2 in the presence of various
catalystsa
Entry Catalyst
Weight of catalyst (mg) Conv.b (%) Sel.c (%)
1
2
3
4
5
6
7
8
9
H3PW12O40
29
29
18
36
26
24
39
0.2
—
37
5.1
17
44.7
68.5
66.4
83.5
4.5
1
92
71
73
86
95.5
93.7
99
12
—
H4SiW12O40
H3PMo12O40
HSiW-MOF
PMo-MOF
HPMo-MOF
PW-MOF
CuCl2
Without catalyst
a
Reaction conditions: catalyst (0.01 mmol POM), substrate (1 mmol),
30% H2O2 (1.25 mmol), acetonitrile (3 ml), 50 1C for 5 h for cyclooctene.
b
Conversion and selectivities were determined by gas chromatography
Fig. 1 Epoxidation of cyclooctene catalyzed by PW-MOF at various
temperatures.
using an internal standard technique and are based on alkenes.
Selectivity for the epoxide product.
c
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1.25 equimolar amount of H2O2 in acetonitrile (Table 2).
Various cyclic alkenes such as cyclohexene, 1-methyl-cyclohexene,
1-phenyl cyclohexene and cyclooctene were epoxidized with good
conversion and selectivity (entries 1–4). The applied method
resulted in 73% conversion of styrene (entry 5) within 6 h.
Bulky cyclic olefins (norbornene) were epoxidized with
77.5% conversion and 91% epoxide selectivity (entry 6). In
addition, non-activated terminal alkenes such as 1-hexene,
Table 2 The selective epoxidation of various substrates with aqueous
H2O2 catalyzed by PW-MOFa
Time Conversionb Selectivityc
Entry Substrate
1
(h)
(%)
(%)
Fig. 2 Epoxidation of cyclooctene with different amounts of catalyst.
5
83.5
99
PW-MOF from 0.001 mmol to 0.1 mmol but higher selectivity
was noted with 0.01 mmol of catalyst. Furthermore, with
0.01 mmol of PW-MOF, the conversion was 83.5%. Thus, a
further optimization of the conditions was performed with
0.01 mmol of PW-MOF.
2
3
4
5
6
80
94
5.5
6
61.4
50.5
73
95.5
97
Effect of the amount of H2O2
In order to determine the role of H2O2 in the epoxidation of
cyclooctene, this epoxidation was studied with different concen-
trations of H2O2 from 0.5 to 5 mmol maintaining a constant
amount of catalyst of 39 mg (0.01 mmol), temperature (50 1C) and
reaction time (5 h). The results are shown in Fig. 3. The molar
ratios of H2O2 to cyclooctene 1 : 10 and 1.25 : 1 resulted in 7 and
83.5% conversions, respectively, and when the molar ratios of
H2O2 to cyclooctene were changed to 1.4 : 1, 1.6 : 1, 1.8 : 1 and 2 : 1
then the conversion increased to 88–95.5%. However, selectivity
to epoxide decreased with increasing concentration of H2O2.
6
83
6
5.5
77.5
91
Epoxidation of different alkenes
7
8
9
4.5
6.5
8
67
86
93
92
In the presence of PW-MOF, different terminal, internal, and
cyclic alkenes were epoxidized with high selectivity using
71.5
70
10
5
79.1
97
81
85
11
7.5
7
74
12
71.4
a
Reaction conditions: catalyst 39 mg (0.01 mmol POM), substrate
(1 mmol), 30% H2O2 (1.25 mmol), acetonitrile (3 ml); reaction temperature:
b
50 1C. Conversion and selectivities were determined by gas chromato-
graphy using an internal standard technique and are based on alkenes.
Selectivity for the epoxide product.
c
Fig. 3 Epoxidation of cyclooctene with different amounts of H2O2.
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1-heptene, and 1-octene could be converted into the corres- alkenes over polyoxometalates using H2O2 as an oxidant was exten-
´
ponding epoxides with 67–70% conversion and 86–92% selectivity sively investigated by several groups. For example, Bregeaul et al.
to epoxide (entries 7–9). For the epoxidation of cis-2-heptene discovered that several salts of general formula Q2[HPO4{WO(O2)2}2]
(entry 10), the configuration around the CQC bond was were obtained by the reaction of H3PW12O40 with hydrogen per-
retained in the corresponding epoxide (cis-2,3-epoxyheptane oxide.40 Hill et al.39 and Griffith et al.41 reported that [PW12O40]3ꢀ
was obtained with 97% selectivity). The method possesses and its lacunary derivative [PW11O39]7ꢀ can also form smaller active
novelty regarding chemoselectivity. The hydroxyl group which species [(PO4){WO(O2)2}4]3ꢀ
, ,
[(PO4){WO(O2)2}2{WO(O2)2(H2O)}]3ꢀ
is sensitive to oxidation remained completely intact under the and [(PO3(OH)){WO(O2)2}2]2ꢀ in the presence of hydrogen
influence of the catalyst, producing the pertinent epoxides in peroxide.
good selectivity (entries 11 and 12). It could be perceived that
Recycling of the catalyst
the present catalyst could be applied to the epoxidation of
different alkenes using H2O2. In all cases, beneficial to excellent
activities and selectivities were achieved.
The stability of polyoxometalates in the oxidation reaction by
using H2O2 as an oxidant was extensively investigated by several
groups. Previous work suggested that PW12 partially transformed
into lower nuclearity peroxotungstates, such as PW4 and PW2.
PW12 and PW4 could be transformed into each other under the
reaction conditions, and most of PW4 transformed into PW12
after the reaction.33,42–46 Epoxidation of cyclooctene was per-
formed with the recycled catalyst under the optimized condi-
tions. It can be observed that the performance of PW-MOF was
considerably stable during three runs, but the catalytic activity
of PW-MOF slightly decreased after the third run (Table 4). The
IR spectrum and the XRD pattern of the PW-MOF run after four
cycles of cyclooctene epoxidation (Fig. 4 and 5) indicated the
same IR spectrum and XRD pattern for the new and reused
catalysts.
The recycled catalyst PW-MOF was characterized by FT-IR
and XRD analyses. The FT-IR spectra of the fresh as well as
regenerated catalysts (four cycles) are presented in Fig. 4. The
IR spectra of fresh and reused PW-MOF show similar peaks at
807–1100 cmꢀ1 corresponding to (W–Ot), (W–Ob), (W–Oc), and
(P–O).36 Furthermore, XRD patterns of the fresh catalyst show
an impressive similarity to the XRD patterns of the reused
catalyst (Fig. 5). Hot catalyst filtration tests were performed and
no further substrate conversion was found in the filtrate after
the removal of PW-MOF. This proves the true heterogeneous
nature of oxidation catalysis over the PW-MOF catalyst. Addition-
ally, the ICP analysis data confirmed that no evident tungsten or
copper leaching occurred under the conditions employed. After
the hot filtration experiment, the separated catalysts were washed
with acetonitrile and dried at 100 1C. A minor reduction was
observed in the catalytic activity of PW-MOF (83% conversion and
99% selectivity to epoxide). We found no further substrate con-
version in the filtrate after the removal of PW-MOF (Fig. 6).
Comparison with other catalysts
Comparative data on the cyclooctene epoxidation with previously-
reported catalysts and present catalysts are given in Table 3.
Leng et al. identified that the epoxidation of alkenes was
catalyzed by heteropolyanion-based amino-containing cross-linked
ionic copolymers.37 This group of researchers performed the
oxidation reaction using acetonitrile as a solvent, and 98.5%
conversion and 100% selectivity of epoxide were reported
(Table 3, entry 1). In addition, Kholdeeva et al. carried out the
epoxidation of alkenes catalyzed by PW12 and hybrid poly-
oxotungstate/MIL-101 using CH3CN as a solvent with H2O2 as
an oxidant (Table 3, entries 2 and 3).33 Furthermore, Kholdeeva
et al. reported that PW12/MIL-101 materials behave as true hetero-
geneous catalysts and can be recycled several times without loss
of activity and selectivity with the retention of both POM and
MIL-101 structures.33 Moreover, Pac et al. found out that hetero-
geneous catalysts prepared by the immobilization of polyoxo-
metalates on chemically-modified hydrophobic mesoporous silica
gel have been successfully applied to the selective epoxidation of
olefins with 15% aqueous H2O2 without the use of an organic
solvent (Table 3, entry 4).38 In the present work, the PW-MOF gives
83.5% conversion and 99% selectivity toward the epoxide (Table 3,
entry 5). Hill et al. performed the epoxidation of alkenes catalyzed
by phosphotungstic acid using H2O/CHCl3 as a solvent with H2O2
as an oxidant, and 78% conversion with 98% selectivity of epoxide
were observed39 (Table 3, entry 6).
Therefore, the present catalyst (PW-MOF) provides desirable
catalytic activity for the epoxidation of alkenes compared to PW12.
Proposed mechanism
In this work, the mechanism of the oxidation reaction was not
studied. However, the reaction mechanism of the epoxidation of
Table 4 Catalytic activity of PW-MOF in different runsa
Run
Conversionb (%)
Selectivityc (%)
Table 3 Catalytic activity of various catalysts for the epoxidation of
1
2
3
4
5
83.5
83.5
83
83
80
99
99
99
98
98
alkenes with H2O2
Entry Catalyst
Substrate
Solvent Conv. (%) Sel. (%) Ref.
1
2
3
4
5
6
AM-BM-PW
PW12/MIL-101
PW12
PW12/modified SiO2 Cyclooctene —
PW-MOF
PW12
Cyclooctene CH3CN 98.5
Cyclooctene CH3CN 76
Cyclooctene CH3CN 78
100
99
95
100
99
98
37
33
33
38
—
39
a
Reaction conditions: catalyst (0.01 mmol), substrate (1 mmol), 30%
97
H2O2 (1.25 mmol), acetonitrile (3 ml), 50 1C for 5 h for cyclooctene.
b
c
Cyclooctene CH3CN 83.5
Cyclooctene CHCl3 78
Conversion of the substrate based on alkenes. Selectivity for the
epoxide product.
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These results demonstrate that the framework structures of
PW-MOF can still be retained after the catalytic performance.36
Conclusion
In this work, the catalytic performance of organic–inorganic
polyoxometalate-based frameworks in the epoxidation of different
olefins using H2O2 as an oxidant in acetonitrile was demonstrated.
A comparison between the catalytic activity of heterogeneous and
homogeneous catalysts indicated that heterogeneous catalysts
exercised higher catalytic activities in the epoxidation of alkenes
with hydrogen peroxide compared to the corresponding homo-
geneous catalysts. Different aromatic and aliphatic alkenes were
oxidized to the corresponding epoxide with excellent selectivity
(81–99%) and often high conversion. Moreover, the epoxidation
method showed excellent chemoselectivity. The hydroxyl group
which is sensitive to oxidation remained completely intact under
the influence of the catalyst. The catalyst is recyclable and stable
and can be reused at least four times in the epoxidation of
cyclooctene without significant loss in its catalytic activity, which
indicates the high stability of the catalyst.
Fig. 4 FT-IR spectrum of PW-MOF recovered in runs 1 (up) and 4 (down).
Acknowledgements
The authors gratefully acknowledge the support of the Research
Council of Shahrekord University for this study.
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