Epoxidation of styrene over Fe(Cr)-MIL-101 metal-organic frameworks

Article abstract of DOI:10.1039/c4ra05402d

Epoxidation of styrene is one of the key reactions in organic synthesis. In this paper, we investigated the effect of different metal ions in MIL-101 metal-organic framework on styrene epoxidation using various oxidants such as air, H₂O₂

Full text of DOI:10.1039/c4ra05402d

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Epoxidation of styrene over Fe(Cr)-MIL-101 metal–
organic frameworks
Cite this: RSC Adv., 2014, 4, 38048
a
b
b
a
a
*
Jian Sun, Guangli Yu, Qisheng Huo, Qiubin Kan and Jingqi Guan
Epoxidation of styrene is one of the key reactions in organic synthesis. In this paper, we investigated the
effect of different metal ions in MIL-101 metal–organic framework on styrene epoxidation using various
oxidants such as air, H2O2 and TBHP. For the aerobic epoxidation of styrene, Fe-MIL-101 and Cr-MIL-
101 presented good styrene conversion (87.2 vs. 65.5%) and epoxide selectivity (54.4% vs. 37.7%) using air
as the oxidant. The styrene epoxidation activity for the different oxidants over Fe-MIL-101 was as follows:
air > TBHP > H2O2, while that over Cr-MIL-101 showed a different trend: H2O2 > air > TBHP. Moreover,
the selectivity to styrene oxide for the different oxidants was similar over the two catalysts: TBHP > air >
H2O2. The study of the effect of various solvents revealed that CH3CN is the optimal solvent for the
aerobic epoxidation of styrene. Furthermore, the catalysts could be reused three times without any
significant loss in catalytic activity.
Received 6th June 2014
Accepted 4th August 2014
DOI: 10.1039/c4ra05402d
www.rsc.org/advances
comparison with other organic–inorganic hybrid heteroge-
neous catalysts,14–16 MOFs do not require cumbersome multi-
Introduction
step graing procedures to achieve the heterogenization of
homogeneous catalysts. The zeotype crystal structure of Cr-MIL-
101 and Fe-MIL-101 consist of trimeric chromium(III) or iron(III)
octahedral clusters interconnected by 1,4-benzenedicarboxylate
anions. Aer the removal of terminal water molecules from
MIL-101 framework by heating in vacuum,17 the formed active
coordinatively unsaturated sites can provide accessible sites for
guest molecules, playing a role as Lewis acid sites or catalytically
active sites.18,19 On the other hand, the robust MOFs of MIL-101
family are not only easy to synthesize but also present good
resistance to water steam, common solvents, and possess rela-
tively high thermal stability (especially Cr-MIL-101, which is
The catalytic epoxidation of styrene is vitally important in
modern chemical industry because the obtained epoxide is an
indispensable intermediate in the synthesis of both ne
chemicals and pharmaceuticals.1,2 Traditionally, homogeneous
catalytic systems (e.g. metal-salen complexes, and chiral met-
alloporphyrins) or hazardous stoichiometric oxidants (e.g.
organic peracids, PHIO, TBHP) have been employed to accom-
plish the epoxidation reaction.3,4 Unfortunately, these proce-
dures have considerable drawbacks, such as difficulty in catalyst
separation and regenerability, utilization of unsafe and expen-
sive oxidants, resulting in serious environmental pollution and
industrial loss of interest. With the ever-increasing economic
and environmental concerns, the development of heteroge-
neous catalytic system that can operate with air in place of
traditional oxidants has drawn tremendous interest.
even stable up to 300 C).20 Therefore, MIL-101 metal–organic
ꢀ
framework as heterogeneous catalysts have been widely applied
in several organic reactions.
Recently, Kholdeeva and co-workers found that Fe-MIL-101
and Cr-MIL-101 showed efficient catalytic performance in the
selective oxidation of alkenes and cyclohexane to unsaturated
ketones with molecular oxygen as oxidant and TBHP as initi-
Metal–organic frameworks (MOFs) are a rapidly emerging
class of porous organic–inorganic hybrid materials synthesized
by the self-assembly of metal ions with organic ligands.5 They
display outstanding properties, including large specic surface
area, tunable pores, diverse structure and chemical function-
ality, which make them potentially useful for applications in
drug delivery, hydrogen storage, catalysis, separation, nonlinear
optics and chemical sensors.6–11 For catalysis, the most valuable
preponderance of MOF is the large concentration of uniform
accessible metal centers and coordination unsaturation.12,13 In
ator.21,22 Ferey et al. used Cr-MIL-101 as catalyst for the selective
´
sulfoxidation of aryl suldes with H2O2 as oxidant and obtained
satisfactory catalytic results.23 Moghadam and co-workers found
that Cr-MIL-101 was efficient for the direct oxidation of alkenes
to carboxylic acids with H2O2 as oxidant.24 Cr-MIL-101 also
exhibited good catalytic activity and selectivity in the oxidation
of tetralin with TBHP as oxidant.17 However, to best of our
knowledge, there are no detailed reports on styrene epoxidation
over MIL-101 family.
aCollege of Chemistry, Jilin University, Changchun, 130023, P.R. China. E-mail:
guanjq@jlu.edu.cn; Tel: +86 431 88499140
bState Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of
In the present study, we explored the catalytic performance
of Fe-MIL-101 and Cr-MIL-10 in the aerobic epoxidation of
Chemistry, Jilin University, Changchun 130012, P.R. China
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styrene. The structure of M-MIL-101 (M ¼ Fe or Cr) can be seen
in Fig. 1. The effects of different oxidants and solvents have
been investigated. Both the catalysts show high conversion and
epoxide selectivity for aerobic epoxidation of styrene, and the
catalysts can be reused three times without any signicant loss
in catalytic activity.
Synthesis of Cr-MIL-101
Cr-MIL-101 was synthesized based on the procedure described
20
´
by Ferey et al. Typically, a mixture of H2BDC (6 mol),
Cr(NO3)3$9H2O (6 mmol), 1.2 mL of 5 M HF (6 mmol) in 30 mL
H2O was heated at 220 ꢀC for 8 h in a Teon-lined stainless-steel
autoclave. Aer cooling to room temperature, the obtained
green powder was ltered under vacuum to remove organic
species trapped in the pores. The product was activated by
boiling in DMF and methanol several times, and then dried
under vacuum.
Experimental
Materials and methods
Terephthalic acid (H2BDC, 99%), uorhydric acid (HF, 40%),
ferric chloride hexahydrate (FeCl3$6H2O, 99%), chromium(III)
nitrate nonahydrate (Cr(NO3)3$9H2O, 99%), ethanol (99.5%),
N,N-dimethylformamide (DMF, 99.9%), styrene (98%), iso-
butylaldehyde (98%) and acetonitrile (99.8%) were used for
synthesis. All the organic solvents were of analytical grade.
The powder X-ray diffraction (XRD) measurements were
performed on a Rigaku D/MAX 2550 diffractometer with Cu-Ka
Catalytic epoxidation of styrene
The aerobic epoxidation of styrene was performed in a 50 mL
two-necked ask equipped with a liquid condenser and an air
pump. In a typical run, styrene (10 mmol), catalyst (50 mg) and
isobutyraldehyde (25 mmol) as reductant were added to 10 mL
of acetonitrile (CH3CN). Then, the mixture was reuxed at 80 ꢀC
for 8 h, and air was introduced at a stable owing rate of 80 mL
minꢁ1. Aer completion of the reaction, the solid catalyst was
centrifuged, washed with acetonitrile and ethanol, dried in
vacuum and reused without further purication. The products
of the epoxidation reaction were quantied and monitored
using a gas chromatograph (Shimadzu, GC-8A) equipped with a
HP-5 capillary column and a FID detector. The aerobic epoxi-
dation of styrene with other solvents (DMF, toluene or MeOH)
was performed under identical reaction conditions. The epoxi-
dation of styrene with other oxidants (H2O2 or TBHP) was per-
formed in a similar manner, except that 30 mmol of oxidant was
added into the reactor instead of reductant and air.
˚
radiation (l ¼ 1.5418 A) and operating at 50 kV and 200 mA. The
infrared spectra of the materials were recorded on a Nicolet
6700 instrument in the range of 500–2000 cmꢁ1 using the KBr
pellet technique. The morphology of the samples was obtained
with JEOS JSM 6700F led-emission scanning electron micro-
scope. N2 adsorption–desorption experiments were carried out
at 77 K by an Autosorb iQ2 adsorptometer, Quantachrome
Instrument. Prior to analysis, the samples were degassed at 423
K overnight under vacuum. Specic surface areas values of Cr-
MIL-101 were calculated by the Brunauer–Emmett–Teller
(BET) equation; and the pore volumes and pore size distribu-
tions were determined by applying t-plot, and non-local density
functional theory (NL-DFT) methods, respectively.
Results and discussion
Structure and morphology characterization
Synthesis of Fe-MIL-101
Fe-MIL-101 was prepared via a reported solvothermal route.25
Briey, FeCl3$6H2O (4.9 mol) and H2BDC (2.5 mmol) were dis-
solved in DMF (30 mL) under vigorous stirring to form a clear
solution. Then, the resulting solution was transferred into a
To conrm the successful synthesis of Fe-MIL-101 and Cr-MIL-
101, XRD and FT-IR characterization have been used, and the
results are shown in Fig. 2 and 3, respectively. The XRD patterns
of the initial samples are similar to the previously reported
pattern of MIL-101 family.26 FT-IR characterization revealed the
presence of the asymmetrical and symmetrical stretching
modes of the framework O–C–O in the region from 1400 to 1600
ꢀ
Teon-lined stainless-steel autoclave and heated at 110 C for
20 h. Aer cooling to room temperature, the obtained brown
powder was ltered to remove organic species trapped in the
pores. The product was activated by boiling in ethanol overnight
and then dried under vacuum.
Fig. 1 Left: zeotype crystalline structure of M-MIL-101 (M ¼ Fe or Cr).
Right: trimeric Fe or Cr(III)-cluster in MIL-101, the octahedral represent
the Fe or Cr ions (in green), oxygen and carbon atoms are represented
in red and light grey, respectively.
Fig. 2 XRD patterns of (a) Fe-MIL-101 and (b) Cr-MIL-101.
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observed for the initial materials and the reused catalysts. The
specic surface areas and porosity of the fresh and reused Cr-
MIL-101 catalysts were measured by N2 adsorption–desorption
experiments, and the results are presented in Fig. 5. The BET
surface and pore volume of fresh Cr-MIL-101 are 2987 m2 gꢁ1
and 1.68 cm3 gꢁ1, respectively. The pore sizes of MIL-101 mainly
˚
range from 8.1 to 25 A. Aer being used three times in the
aerobic epoxidation of styrene, the BET surface area of Cr-MIL-
101 decreased from 2987 to 1848 m2 gꢁ1. The pore volume of Cr-
MIL-101 reduced from 1.68 to 0.72 cm3 gꢁ1. The pore size
distribution shows no obvious difference. The decrease in the
BET surface and pore volume might be because of the incom-
plete removal of reactant and product in the pores and/or a
partial damage of the structure of Cr-MIL-101 aer reusing
several times.
Catalytic properties
The catalytic activity of Fe-MIL-101 and Cr-MIL-101 in the
epoxidation of styrene with various oxidants under identical
conditions was investigated. The catalytic results are summa-
rized in Table 1. The blank reaction was performed in the
absence of catalyst for the aerobic epoxidation of styrene, sug-
gesting very low styrene conversion (entry 1). When we used air
as the oxidant and isobutyraldehyde as the reductant, Fe-MIL-
101 showed good styrene conversion (87.2%) and epoxide
Fig. 3 FT-IR spectra of (a) Fe-MIL-101 and (b) Cr-MIL-101: fresh
catalyst (A) and the reused catalyst after 3 cycles (B).
cmꢁ1, which are the typical characteristic bands of MIL-101
metal–organic framework.27 We can clearly see characteristic
bands both in the fresh and the recovered catalysts. Further-
more, SEM was performed to compare the morphology of the
fresh and recycled catalysts, as shown in Fig. 4. Both the cata-
lysts are octahedral in shape. No noticeable difference is
Fig. 4 SEM images of (a) the fresh catalyst Fe-MIL-101, (b) the
recovered catalyst Fe-MIL-101 after 3 cycles, (c) the fresh catalyst Cr- Fig. 5 N2 adsorption–desorption isotherms and pore size distribution
MIL-101, and (d) the recovered catalyst Cr-MIL-101 after 3 cycles.
of the fresh and reused Cr-MIL-101.
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Table 1 Styrene epoxidation using different oxidants over the Fe-MIL-101 and Cr-MIL-101 catalyst
Product selectivity (mol%)
Conversion
(%)
Entry
Catalyst
Oxidant
Time (h)
Epoxide
Benzaldehyde
Othersd
References
1a
No catalyst
Fe-MIL-101
FeQ3–Y
Air
Air
Air
Air
8
8
8
8
8
1
4
8
1
4
8
8
1
4
8
1
4
8
4.1
87.2
83.4
76.5
80.9
32.8
36.5
37.3
8.5
37.4
52.1
65.5
73.2
96.3
97.6
3.4
34.3
54.4
24.5
49.8
59.7
1.9
65.7
39.1
58.2
46.4
32.8
96.8
96.5
93.2
6.8
0
Herein
Herein
28
29
30
2a
6.5
17.3
3.8
7.5
1.3
1.4
4.3
0
0.8
1.2
4.3
1.9
2.6
6.1
0
3a
4a
Fe–salen–GO
Fe–salen–SBA
Fe-MIL-101
Fe-MIL-101
Fe-MIL-101
Fe-MIL-101
Fe-MIL-101
Fe-MIL-101
Cr-MIL-101
Cr-MIL-101
Cr-MIL-101
Cr-MIL-101
Cr-MIL-101
Cr-MIL-101
Cr-MIL-101
5a
Air
6b
H2O2
H2O2
H2O2
TBHP
TBHP
TBHP
Air
H2O2
H2O2
H2O2
TBHP
TBHP
TBHP
Herein
Herein
Herein
Herein
Herein
Herein
Herein
Herein
Herein
Herein
Herein
Herein
Herein
7b
2.1
2.5
8b
9c
93.2
91.7
92.2
37.7
2.7
6.3
5.7
92.0
88.7
82.6
10c
11c
12a
13b
14b
15b
16c
17c
18c
7.5
6.6
58.0
95.4
91.1
88.2
8.0
21.4
35.4
9.8
15.7
1.5
1.7
a
Reaction conditions: catalyst 50 mg, styrene 1.14 mL (10 mmol), CH3CN 10 mL, ow of air 80 mL minꢁ1, isobutyraldehyde (25 mmol), temperature
80 ꢀC and time 8 h. H2O2 (30 mmol). c TBHP (30 mmol). d Others: including phenylacetaldehyde and benzoic acid.
b
selectivity (54.4%) aer reaction for 8 h. In comparison to the TBHP is expensive and unfriendly to environment. Thus, air is a
traditional Fe-containing heterogeneous catalysts (e.g. FeQ3–Y, suitable oxidant in the epoxidation of styrene over Fe-MIL-101.
Fe-salen–GO, Fe-salen–SBA),28–30 Fe-MIL-101 exhibited either
The effect of different solvents on the aerobic epoxidation of
higher conversion or epoxide selectivity in the aerobic epoxi- styrene over MIL-101 was studied and the results are shown in
dation of styrene. The satisfactory catalytic results can be Table 2. The styrene conversion for different solvents over Fe-
attributed to the following reason: MIL-101 possesses abundant MIL-101 follows this trend: CH3CN > Toluene > DMF > MeOH,
coordinatively unsaturated sites (CUS) that play a role as cata- while the styrene conversion for different solvents over Cr-MIL-
lytically active sites.31,32 Because of the regular arrangement and 101 follows another trend: CH3CN > DMF > toluene > MeOH. In
well-understood surrounding environments of metal centers in all the investigated cases, CH3CN was observed to be the best
the pore channels, MIL-101 with CUS could be used to induce solvent in terms of styrene conversion for aerobic epoxidation.
regioselectivity and shape or size selectivity toward guest Although the highest epoxide selectivity (64.2% for Fe-MIL-101
molecules or reaction intermediates.33 Furthermore, the large and 57.4% for Cr-MIL-101) is achieved when DMF is used as the
pore windows and pore volumes of MIL-101 are efficient for solvent, styrene conversion is relatively low compared with
guest molecules diffusing in and the product moving out, thus CH3CN as the solvent. Islam et al. proposed that CH3CN is a
improving reaction efficiency.34 Cr-MIL-101 shows lower styrene polar solvent with high dielectric constant and dissolves a wide
conversion (65.5%) and epoxide selectivity (37.7%) under the range of chemical compounds, which are favorable to high
same experimental conditions. When H2O2 is used as an activity and selectivity for styrene oxide.40 These results indicate
oxidant, the Fe-MIL-101 achieves a relatively low styrene that CH3CN is the most suitable solvent for aerobic epoxidation
conversion (37.3%), which may be caused by the rapid decom- styrene.
position of H2O2 into O2 in the initial stage.35 Very high styrene
Based on the in-depth study of experimental results and
conversion (97.6%) is acquired by Cr-MIL-101 using H2O2 as relevant literature,41–43 a tentative reaction mechanism for the
oxidant but the epoxide selectivity is very poor (only 5.7%). A aerobic epoxidation of styrene, in the presence of iso-
similar situation involving benzaldehyde as
product in styrene epoxidation, with H2O2 as oxidant and Cr or Fe) metal–organic framework, is proposed in Scheme 1.
CH3CN as solvent can be found in many earlier reports.36–38 Coordinatively unsaturated sites MIII in M-MIL-101 interacts
a
dominant butyraldehyde as a sacricial co-oxidant over M-MIL-101 (M ¼
A
reasonable explanation for this phenomenon is that a nucleo- with isobutyraldehyde (RCHO) to generate intermediate species
philic attack of H2O2 on epoxide occurs, followed by the (I) and then activates molecular oxygen, resulting in the
cleavage of C]C bond to form benzaldehyde.39 The effect of formation of species (II). MIII is regenerated by species (II) and
TBHP as an oxidant on styrene epoxidation was also investi- the acylperoxy radical is released, following which the reaction
gated. Although epoxide selectivity far exceeded 80% when may proceed through two routes. One is that acylperoxy radical
TBHP was used, the styrene conversion over Fe-MIL-101 is lower directly attacks the C]C double bond of styrene molecule to
compared to that achieved using air as the oxidant. In addition, form styrene epoxide. Another pathway is that metal-peroxy
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Table 2 Aerobic epoxidation of styrene in different solvents over MIL-101a
Product selectivity (%)
Conversion
Entry
Catalyst
Solvent
(%)
Epoxide
Benzaldehyde
Othersb
1
2
3
4
5
6
7
8
Fe-MIL-101
Fe-MIL-101
Fe-MIL-101
Fe-MIL-101
Cr-MIL-101
Cr-MIL-101
Cr-MIL-101
Cr-MIL-101
CH3CN
Toluene
DMF
MeOH
CH3CN
DMF
87.2
63.7
19.7
17.6
65.5
21.9
18.3
1.5
54.4
32.6
64.2
3.5
37.7
57.4
5.3
39.1
63.9
26.7
87.8
58.0
32.1
78.8
78.7
6.5
3.5
9.1
8.7
4.3
10.5
15.9
0
Toluene
MeOH
21.3
a
Reaction conditions: catalyst 50 mg, styrene 1.14 mL (10 mmol), solvent 10 mL, ow of air 80 mL minꢁ1, isobutyraldehyde (25 mmol), temperature
b
80 ꢀC and time 8 h. Others: including phenylacetaldehyde and benzoic acid.
decrease in styrene conversion from 87.2% to 82.4% aer
recycling three times. When we promoted the reaction cycles to
four times, the catalytic activity of both the catalysts showed an
obvious decrease. Fe-MIL-101 and Cr-MIL-101 presented a low
styrene conversion (52.4 vs. 47.5%), which may be caused by
partial structure collapse upon using three times. Thus, the
MIL-101 catalyst can efficiently be reused three times. To
further evaluate the stability of Fe-MIL-101 and Cr-MIL-101
during the catalytic process, the leaching test was carried out
and the results are shown in Fig. 7. The aerobic epoxidation
reaction was suspended aer 3 hours. Subsequently, the
Scheme
1 Tentative reaction mechanism for the epoxidation of
styrene with air in the presence of isobutyraldehyde over M-MIL-101
(M ¼ Cr or Fe).
active species (III) interacts with substrate to form styrene
epoxide, regenerating the catalytic active species MIII. The
formation of phenylacetaldehyde may be caused by the iso-
merisation of styrene oxide.44 Lewis acid sites in the MOF most
likely promote the epoxide-opening and rearrangement.45 In
addition, benzoic acid may be formed by the deep oxidation of
benzaldehyde.
The reusability and stability of the heterogeneous catalysts
are of signicant importance in terms of practical application
and green chemistry. To address the concern, recycling experi-
ments were performed over the Fe-MIL-101 and Cr-MIL-101
using styrene as the substrate, and the results are presented
in Fig. 6. Aer each reaction, the catalyst was centrifuged,
washed thoroughly with acetonitrile and ethanol, dried under
vacuum and reused for the subsequent cycles. In a series of
three consecutive runs, Cr-MIL-101 exhibits basically stable
catalytic activity and selectivity (63.7–65.5% conversion and
36.5–37.7% epoxide selectivity). In addition, the reused Fe-MIL-
101 showed good epoxide selectivity (ca. 53.6%) but a slight Fig. 6 Recycling experiments of (a) Fe-MIL-101 and (b) Cr-MIL-101 for
aerobic epoxidation of styrene.
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be reused three times with insignicant loss in catalytic activity
and epoxide selectivity. The leaching experiments further indi-
cate that the MIL-101 family are truly heterogeneous catalysts.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21303069), Jilin province (201105006).
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clear solution was stirred at 80 ꢀC for another 5 h. However, no
obvious improvement in styrene conversion could be observed
aer catalysts were removed. Aer completion of the reaction,
the solution was analyzed by inductively coupled plasma anal-
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Conclusions
This research demonstrates that Fe-MIL-101 and Cr-MIL-101
are efficient heterogeneous catalysts for the epoxidation of 16 A. R. Silva, K. Wilson, A. C. Whitwood, J. H. Clark and
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good styrene conversion (87.2% vs. 65.5%) and epoxide selec- W. S. Ahn, Chem. Commun., 2009, 3904–3906.
tivity (54.4% vs. 37.7%). The effects of various oxidants for 18 L. Alaerts, E. Seguin, H. Poelman, F. Thibault-Starzyk,
styrene epoxidation have been investigated. Over Fe-MIL-101,
the conversion with air is higher than that with TBHP and
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