Co(ii) complexes loaded into metal-organic frameworks as efficient heterogeneous catalysts for aerobic epoxidation of olefins

Article abstract of DOI:10.1039/c5cy01099c

A series of efficient cobalt(ii)-anchored Cr-MOF (Cr-MIL-101-NH₂) catalysts, such as Co(ii)@Cr-MIL-101-Sal, Co(ii)@Cr-MIL-101-P2I and Co(ii)@Cr-MIL-101-P3I, have been successfully synthesized by one-pot modification of the terminal amino group with salicylaldehyde, pyridine-2-aldehyde or pyridine-3-aldehyde and anchoring of Co(ii) ions into the mesoporous Cr-MOF supports. The Co(ii)@Cr-MIL-101-P2I catalyst exhibited high catalytic performance for epoxidation of olefins with air as an oxidant due to the nitrogen atom in the pyridine ring as a strong electron-withdrawing substituent, high dispersion of Co(ii) species and high surface area for sufficient contact between the substrate and active sites. The strong coordination interaction between the Co(ii) ions and chelating groups in the Co(ii)@Cr-MIL-101-P2I catalyst guaranteed the excellent recycling performance. Furthermore, the synthesized Co(ii)@Cr-MIL-101-P2I catalyst realized its general applicability towards various olefins, such as cyclic olefins, tri-substituted olefins, aliphatic olefins and aromatic olefins.

Full text of DOI:10.1039/c5cy01099c

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DOI: 10.1039/C5CY01099C
Journal Name
ARTICLE
Co(II) complexes loaded into metal organic framework as an
efficient heterogeneous catalyst for aerobic epoxidation of olefins
a
a
a
a
a
a
b
Jingjing Wang, Mu Yang, Wenjun Dong, Zhaokui Jin, Jia Tang, Shuang Fan, Yunfeng Lu and Ge
Wang*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A series of efficient cobalt(II) anchored Cr-MOF (Cr-MIL-101-NH
2
) catalysts, such as Co(II)@Cr-MIL-101-Sal, Co(II)@Cr-MIL-
1
01-P2I and Co(II)@Cr-MIL-101-P3I, have been successfully synthesized by one-pot modifying terminal amino group with
www.rsc.org/
salicylaldehyde, pyridine-2-aldehyde or pyridine-3-aldehyde and anchoring Co(II) ions into mesoporous Cr-MOFs support.
The Co(II)@Cr-MIL-101-P2I catalyst exhibited high catalytic performance for epoxidation of olefins with air as oxidant due
to nitrogen atom in pyridine ring as a strong electron-withdrawing substituent, high dispersion of Co(II) species and high
surface area for sufficient contact between substrate and active sites. The strong coordination interaction between Co(II)
ions and chelating groups in Co(II)@Cr-MIL-101-P2I catalyst guaranteed the excellent recycled performance. Furthermore,
the synthesized Co(II)@Cr-MIL-101-P2I catalyst realized general applicability towards varied olefins, such as cyclic olefins,
tri-substituted olefins, aliphatic olefins and aromatic olefins.
common organic solvents.13 Moreover, the reactive amino
Introduction
2
groups existed on the surface of Cr-MIL-101-NH pores can
provide great opportunities to covalently attach functional
species. Tremendous efforts of the PSM strategy of Cr-MIL-101-
Metal organic frameworks (MOFs) are crystalline hybrid
inorganic/organic solids with extended structures which can be
utilized in many fields, such as gas separation and storage,
sensors, drug delivery.1-3 Especially, MOFs used for
heterogeneous catalysis materials have sparked numerous
interesting due to their high crystallinity with well-defined pore
properties, high specific surface area and tailorable structure.4-7
However, established MOFs have relatively low catalytic
activity due to their limited selections of metals and organic
2
NH , such as urea derived MIL-101 by introducing ethyl
isocyanate14,15 and photo-switchable MIL-101 by installing an
azo functionality,12 have been achieved successfully. However,
the PSM strategy of Cr-MIL-101-NH
reaction received much less attention.
2
in catalytic oxidation
Cobalt(II)-based organometallics, as efficient and selective
homogeneous catalysts, have been applied in the epoxidation of
olefin.16,17 However, the main drawback of these catalysts is the
catalyst recycling and product separation. To overcome these
problems, various supports, such as polymer,18,19 zeolite,20 and
silica,21,22 have been used to load the homogeneous complex.
Nevertheless, these heterogeneous catalysts typically involve
some problems, such as trivial preparation, low chemical
stability, non-uniform structure and low efficiency. Therefore,
how to obtain an accessible, reusable and novel heterogeneous
cobalt(II)-based catalyst which shows high activity of
homogeneous catalyst and good re-usability of heterogeneous
catalyst remains a challenge.
8
ligands. In order to further enhance the catalytic activity, the
postsynthetic modification (PSM), defined as chemical
derivatization of MOFs after their formation, becomes a practical
and powerful tool to generate MOF-based heterogeneous
catalysts with improved catalytic performance.9,10
MIL-101 discovered by Férey et al. shows a rigid zeotype
cubic structure, which defines nano cages of about 2.9 and 3.4
nm with windows of 1.2 and 1.6 nm, respectively.11 MIL-101-
NH
2
maintains features of MIL-101 and introduces amino groups
can
to the organic frames of the MOF lattice.12 Cr-MIL-101-NH
2
be a good candidate for PSM due to its large pores, good thermal
stability as well as excellent chemical stability to water and
In this paper, a novel system for aerobic oxidation of olefins
under air atmosphere employing metal organic framework-
anchored cobalt(II) catalyst has been developed. Co(II)-based
a.Beijing Key Laboratory of Function Materials for Molecule & Structure
complex were immobilized into mesoporous Cr-MIL-101-NH
2
Construction, School of Materials Science and Engineering, University of Science
and Technology Beijing, Beijing 100083, PR China
Department of Materials Science and Engineering, University of California, Los
Angeles, CA 90034, USA
Corresponding author. E-mail: gewang@mater.ustb.edu.cn; Tel: +86-10-
in one-pot method using salicylaldehyde, pyridine-2-aldehyde or
pyridine-3-aldehyde as linker agent (Scheme 1). The amino
b.
group of Cr-MIL-101-NH
2
upon condensation with
*
6
†
salicylaldehyde, pyridine-2-aldehyde or pyridine-3-aldehyde
afforded a Schiff base moiety in the porous matrix. The Schiff
base moieties as chelating groups can anchor the Co(II) ions. The
2333765
Electronic Supplementary Information (ESI) available: [FTIR, 1HNMR, diffuse
reflectance UV-vis spectra and TGA the samples. Proposed reaction mechanism.
XRD, FTIR and SEM of the recovered catalyst. Internal standard calibration.] See DOI:
0.1039/x0xx00000x
Cr-MIL-101-NH
2
MOFs with high surface area were used as
1
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support to improve the dispersion of Co(II) ions. The uniform
and big pores of MOFs guarantee sufficient contact between the or pyridine-3-aldehyde and loading cobaDltO(II:I)10o.1n03C9/rC-5MCIYL01-019091C-
catalytic active sites and the substrate. The strong coordination NH was same as that of Co(II)@Cr-MIL-101-Sal but replace
The synthesis procedure of modifying pyridinVeie-w2-AartlicdleehOynldinee
2
interaction between Co(II) ions and chelating groups in salicylaldehyde with pyridine-2-aldehyde or pyridine-3-
Co(II)@Cr-MIL-101-P2I catalyst enhances the re-usability of aldehyde, respectively. The obtained product was accordingly
catalyst after the reaction. The developed cobalt(II)-based solid named as Co(II)@Cr-MIL-101-P2I or Co(II)@Cr-MIL-101-P3I.
catalysts showed high catalytic activity, easy recovery, excellent
Catalytic epoxidation reactions
recycling stability for the epoxidation of olefin in mild reaction
condition.
The catalytic epoxidation reaction was carried out in the
liquid phase in a 25 mL two-neck flask. Olefin (1 mmol), catalyst
(
0.6 mol% cobalt) and isobutyraldehyde (2 mmol) were added
Experimental
into acetonitrile (5 mL). The reaction mixture was stirred at 35
oC for 5 h under air atmosphere. After the reaction, nitrobenzene
Synthesis of Cr-MIL-101-NH
2
(
0.2 mmol) as an internal standard was added into the mixture
hydrothermal for the determination of yield and selectivity. The filtrate was
synthesis method according to the literature.23 In a typical analyzed by Agilent 7890/5975C-GC/MS.
Cr-MIL-101-NH
2
was prepared by a
process, chromic nitrate hydrate (6.40 g, 16 mmol), 2-
For catalyst recycling, the catalyst was filtered from the
aminoterephthalic acid (2.88 g, 16 mmol) and sodium hydroxide reaction solution, washed with acetonitrile and dried for the next
(
1.60 g, 40 mmol) were dissolved into deionized water (120 mL) run. The dried catalyst was reused for the epoxidation of cis-
under stirring, then the solution was transferred to a 200 mL cyclooctene in acetonitrile under the same reaction conditions.
o
Teflon-lined stainless steel autoclave and kept at 150 C for 12 h.
The hot filtration test was performed by separating the
The autoclave was left to cool down to room temperature. The catalyst at 1 h from the reaction mixture, and the filtrate was
green precipitate was collected by centrifugation, and washed stirred for a further 4 h. The reaction progress was monitored by
with deionized water, DMF and methanol, respectively. Finally, GC/MS.
o
the products were dried under vacuum at 40 C for overnight.
Characterization
Preparation of cobalt(II)-based solid catalyst
Field-emission scanning electron microscope (FESEM)
In order to ensure that sufficient Co(II) moiety could be images were observed by a ZEISS SUPRA55 operated at the
immobilized in the MOFs pores, the molar ratio of terminal accelerating voltages of 10 kV. Fourier transform infrared (FTIR)
amino group of MOFs, modifying aldehyde group and Co(II) spectra were obtained by a Nicolet 6700 using the potassium
was set at 1:4:1 during the synthesis of three kinds of catalysts.
To modify Cr-MIL-101-NH with salicylaldehyde and load spectra were recorded on a UV-2550 spectrometer equipped with
Co(II) ions, 0.40 g Cr-MIL-101-NH were first dispersed into 30 a diffuse reflectance integrating sphere coated with BaSO4,
bromide (KBr) pellet technique. Diffuse reflectance UV-vis
2
2
mL methanol at a flask under ultrasond, and 0.62 mL which also served as a standard. Powder X-ray diffraction (XRD)
salicylaldehyde and 0.43 g cobalt nitrate hexahydrate was added patterns were recorded by a M21X diffractometer (MAC Science
o
into the mixture, and then the solution refluxed at 85 C for 10 h. Co. Ltd., Japan) using a Cu Karadiation (k = 1.541 Å) source at
Afterward, the precipitate was washed with abundant ethanol for 40 kV and 200 mA. The chemical compositions were analysed
o
three times and dried under vacuum at 40 C overnight. The with inductively coupled plasma-atomic emission spectrometry
obtained product was labeled as Co(II)@Cr-MIL-101-Sal.
(ICP-AES, Vavian 715-ES) and an X-ray photoelectron
spectrometer (XPS, ESCALAB 250Xi). C 1s line at 284.8 eV
from adventitious carbon was used as the reference for binding
energy. The thermal gravimetric analysis (TGA) was used to
investigate the thermal decomposition behavior of the samples
o
using Netzsch STA449F3 instrument at a heating rate of 10 C
min-1 under an air flow. Nitrogen adsorption-desorption
isotherms were measured at 77 K with a Micromeritics ASAP
2420 adsorption analyzer. The specific surface area was
calculated with the Brunauer-Emmett-Teller (BET) method. The
pore size distributions were derived from the adsorption
branches of isotherms by using the Barrett-Joyner-Halenda (BJH)
model. The catalytic results were analyzed with a gas
chromatography-mass spectrum (GC-MS, Agilent7890/5975C-
GC/MSD), and nitrobenzene was used as an internal standard.
Scheme 1 Schematic illustration of the preparation of Co(II) complexes loaded into Cr-
MIL-101-NH .
2
Results and discussion
2
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5CY01099C
2
Fig. 3 Nitrogen adsorption-desorption isotherms of Cr-MIL-101-NH , Co(II)@Cr-MIL-101-
Sal, Co(II)@Cr-MIL-101-P2I and Co(II)@Cr-MIL-101-P3I.
2
Fig. 1 SEM images of (a) Cr-MIL-101-NH , (b) Co(II)@Cr-MIL-101-Sal, (c) Co(II)@Cr-MIL-
1
01-P2I and (d) Co(II)@Cr-MIL-101-P3I.
In order to evaluate the surface area, pore volume and pore
structure of the samples, nitrogen adsorption-desorption
isotherms were performed and shown in Fig 3. All the samples
featured same type IV isotherm, which further indicated that the
pore structure was well retained after functionalization of the
linker and anchoring of Co(II) ions. The surface area of the
samples was calculated with the Brunauer-Emmett-Teller (BET)
method. The total pore volume was obtained from the volume of
Catalyst characterization
Typical morphologies of Cr-MIL-101-NH
01-Sal, Co(II)@Cr-MIL-101-P2I and Co(II)@Cr-MIL-101-P3I
samples were shown in Fig. 1. SEM images of Cr-MIL-101-NH
Fig. 1a) showed that the as-prepared MOFs was particles and
2
, Co(II)@Cr-MIL-
1
2
(
the particle size was around 50-300 nm, which was similar to the
previous reports.23 After modifying with salicylaldehyde,
pyridine-2-aldehyde or pyridine-3-aldehyde followed by
anchoring of Co(II) ions, the morphology and size of Co(II)@Cr-
MIL-101-Sal (Fig. 1b), Co(II)@Cr-MIL-101-P2I (Fig. 1c) and
Co(II)@Cr-MIL-101-P3I (Fig. 1d) were similar to those of Cr-
MIL-101-NH
MIL-101-NH
and Co(II) ions anchoring procedure.
XRD patterns of Cr-MIL-101-NH
Sal, Co(II)@Cr-MIL-101-P2I and Co(II)@Cr-MIL-101-P3I
samples were shown in Fig. 2. The XRD pattern of Cr-MIL-101-
2
NH sample corresponded well with the literature data, which
indicated that the desired MOFs support was obtained.14,23
Co(II)@Cr-MIL-101-Sal,
Co(II)@Cr-MIL-101-P3I samples showed similar diffraction
patterns to that of Cr-MIL-101-NH , which clearly suggested
that the framework structure of Cr-MIL-101-NH remained
intact during the modifying and loading procedure. Moreover,
comparing with Cr-MIL-101-NH , a decrease at the peak
2
N adsorption at P/P0 = 0.985. The BET surface area and the total
2
-
pore volume of the Cr-MIL-101-NH
2
sample were 1039.2 m g
and 1.56 cm g , respectively. An obvious decrease in the BET
surface area as well as the total pore volume was observed in the
Co(II)@Cr-MIL-101-Sal, Co(II)@Cr-MIL-101-P2I and
Co(II)@Cr-MIL-101-P3I sample, which demonstrated that the
pores of Cr-MIL-101-NH were occupied by Co(II)
complexes.21,24 FTIR (Fig. S1
diffuse reflectance UV-vis spectra (Fig. S3a
confirmed the successful modifying of Cr-MIL-101-NH
1
3
-1
2
, indicating the strong chemical stability of Cr-
support during the aldehyde group modification
2
2
1
†
) and H NMR (Fig. S2
) results also
. The
†) and
†
2
, Co(II)@Cr-MIL-101-
2
Co(II)@Cr-MIL-101-P2I sample had the lowest BET surface
area and total pore volume, which could be ascribed to its high
content of the anchored Co(II) complex. The content of cobalt
measured by ICP-AES in the Co(II)@Cr-MIL-101-Sal,
Co(II)@Cr-MIL-101-P2I and Co(II)@Cr-MIL-101-P3I sample
Co(II)@Cr-MIL-101-P2I
and
-
1
-1
-1
(
Table 1) was 0.12 mmol g , 0.29 mmol g and 0.03 mmol g ,
2
respectively. Under the same synthesis procedure and the same
molar ratio of raw material, the varied loading content of cobalt
in these samples should be due to the different coordination
environment of Co(II) ions.
In order to gain deeper insight into the surface constitution
of the catalysts, the XPS spectrum of the Co(II)@Cr-MIL-101-
P2I sample was investigated. As shown in Fig. 4a, the sample
contained C, O, Cr, N and Co elements. The Cr 2p1/2 and Cr
2
2
intensity 2θ = 3.2 of the Co(II)@Cr-MIL-101-Sal, Co(II)@Cr-
MIL-101-P2I and Co(II)@Cr-MIL-101-P3I samples was
observed, which was attributed to the modifying complexes
inside the pores.24
2
4
p3/2 signals showed two peaks at 586.8 eV and 577.2 eV (Fig.
b), respectively. Both peaks corresponded to typical binding
2
Table 1 The BET surface area, the pore volume and ICP analysis result of Cr-MIL-101-NH ,
Co(II)@Cr-MIL-101-Sal, Co(II)@Cr-MIL-101-P2I and Co(II)@Cr-MIL-101-P3I.
Total Pore
Volume (cm g ) mmol g )
ICP (Co
2
-1
Samples
S
BET (m g )
3
-1
-1
Cr-MIL-101-NH
2
1039.2
702.5
632.2
757.1
1.56
0.73
0.69
0.81
-
Co(II)@Cr-MIL-101-Sal
Co(II)@Cr-MIL-101-P2I
Co(II)@Cr-MIL-101-P3I
0.12
0.29
0.03
2
Fig. 2 Powder XRD patterns of Cr-MIL-101-NH , Co(II)@Cr-MIL-101-Sal, Co(II)@Cr-MIL-
101-P2I and Co(II)@Cr-MIL-101-P3I.
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+ 25
Table 2 Catalyst screening in the epoxidation of cis-cyclooctene
DOI: 10.1039/C5CY01099C
a
energies for Cr . No other oxidation states of chromium were
observed, which indicated the chromium trimers were
chemically stable during functionalization of the linker and
anchoring of cobalt precursor. The peak-fitted N1s core-line
spectrum of Co(II)@Cr-MIL-101-P2I (Fig 4c) showed three
resolved peaks at 400.4 eV, 399.7 eV and 398.9 eV,
View Article Online
2
6
Conv.
Sel.
(%)
TOFepo
(h )
corresponding to imine (-C=N-) groups amino (-NH
2
) groups
Entry
Catalyst
b
b
-1
(%)
2
7
and pyridine N,28 respectively. Additionally, the peak at 406.7
eV was due to the existence of the N in NO3- group.29 The Co
1
No
20
>99
-
2
7
p
3/2 and Co 2p1/2 core level peak was at a binding energy of
2c
Cr-MIL-101-NH
2
21
>99
>99
>99
-
54.45
-
81.7 eV and 796.9 eV, respectively (Fig 4d). The Co 2p1/2 and
3d
4e
Co(NO
Co(NO
)
·6H
·6H
O
>99
47
3
2
2
Co 2p3/2 separation of 15.2 eV for tethered cobalt catalyst was
identical to the value reported for Co(salen)·1/2H
O.30
2
)
3 2
2
O
Additionally, the presence of satellite lines at a binding energy
of 803.4 eV and 786.4 eV was furthermore indicative of a high-
spin Co2+ center in the Co(II)@Cr-MIL-101-P2I sample.31
Co(NO
diaminocyclohexane
Co(NO ·6H O + 2-
3
)
2
·6H
2
O +
5f
48
51
>99
>99
-
-
3
)
2
2
6g
TGA curves of the Cr-MIL-101-NH
01-Sal, Co(II)@Cr-MIL-101-P2I and Co(II)@Cr-MIL-101-P3I
sample were shown in Fig. S4 . The total weight loss of the Cr-
MIL-101-NH sample occurred by two weight-loss steps. The
2
, Co(II)@Cr-MIL-
aminoterephthalic acid
1
4h
Co(II)@Cr-MIL-101-Sal
78
>99
92
>99
>99
>99
>99
>99
>99
25.74
†
5h
6h
7i
Co(II)@Cr-MIL-101-P2I
Co(II)@Cr-MIL-101-P3I
Co(II)@Cr-MIL-101-Sal
Co(II)@Cr-MIL-101-P2I
Co(II)@Cr-MIL-101-P3I
32.67
2
o
initial weight loss of 9.8 % in the range 40-230 C was assigned
to the loss of free guest water in the pores. The subsequent weight
loss of 56.6% in the range 230-330 oC was due to the
decomposition of the organic framework. It was noteworthy that
the weight-loss of Co(II)@Cr-MIL-101-Sal, Co(II)@Cr-MIL-
30.36
76
-
-
-
8i
>99
57
9i
101-P2I and Co(II)@Cr-MIL-101-P3I sample were similar to
a
Reaction conditions: cis-cyclooctene (1.0 mmol), catalyst, isobutyraldehyde (2
that of Cr-MIL-101-NH
2
. The cobalt(II)-based solid catalysts mmol), acetonitrile (5.0 mL), at 35
o
C under air atmosphere stirring for 5 h.
b
Conversion and selectivity were determined by GC/MS using nitrobenzene as
showed a higher decomposed temperature and were more stable
than Cr-MIL-101-NH attributed to the rigid structure of the
2
formed coordinated Co(II) complexes.24
internal standard.
c
Reaction was performed using 20 mg MOF.
d
e
f
Reaction was stirring for 3 h.
.3 mol% Co(II), reaction was stirring for 3 h..
0.3 mol% Co(II) + diaminocyclohexane, reaction was stirring for 3 h.
.3 mol% Co(II) + 2-aminoterephthalic acid, reaction was stirring for 3 h.
Fresh catalyst, 0.6 mol% Co(II) based on the amount of cobalt in sample.
2 cycle catalyst.
0
Catalytic study
g
h
i
0
The catalytic activity of cobalt(II)-based solid catalyst was
studied by catalyzing olefin epoxidation with 1 atm air and
isobutyraldehyde in acetonitrile. Epoxidation of cis-cyclooctene employing Cr-MIL-101-NH as the catalyst (Table 2, entry 2).
2
was carried out as a model reaction to identify the optimal Interestingly, when 0.6 mol% Co(NO ) ·6H O was used as the
3
2
2
reaction conditions. Initially, a blank control experiment was catalyst, the conversion was about 99% (Table 2, entry 3). The
performed, and the conversion of cis-cyclooctene was only 20% conversion of cis-cyclooctene was only 47% when 0.3 mol%
in 5 h (Table 2, entry 1), which agreed with previous report.32 No Co(NO ) •6H O was applied as the catalyst (Entry 4, Table 2).
3
2
2
change of the conversion of cis-cyclooctene was observed by
The conversion of cis-cyclooctene was not improved when the
mixture of Co(NO •6H O and diaminocyclohexane (Entry 5,
Table 2) or the mixture of Co(NO •6H and 2-
3
)
2
2
3
)
2
2
O
aminoterephthalic acid (Entry 6, Table 2) were used as the
homogeneous catalyst, which indicated that the homogeneous
organic aminos did not contribute to the catalytic activity and
Co(II) ions played a significant role in the epoxidation of cis-
cyclooctene. However, the homogeneous system leads to
difficult for product separation and catalyst recycling. When Cr-
MIL-101-NH
Co(II)@Cr-MIL-101-Sal,
2
-anchored
cobalt(II)
Co(II)@Cr-MIL-101-P2I
complex
including
and
Co(II)@Cr-MIL-101-P3I sample were used as the catalyst, the
conversion of cis-cyclooctene was 78% (Table 2, entry 7), 99%
(
Table 2, entry 8) and 92% (Table 2, entry 9), respectively. The
TOFs for cis-cyclooctene epoxidation catalysed by Co(II)@Cr-
MIL-101-Sal, Co(II)@Cr-MIL-101-P2I and Co(II)@Cr-MIL-
Fig. 4 X-ray photoelectron spectrum of (a) the Co(II)@Cr-MIL-101-P2I sample, (b) Cr 2p, 101-P3I catalyst were calculated to be 25.74 h-1, 32.67 h-1 and
(
c) N 1s and (d) Co 2p.
30.36 h-1, respectively, which were higher than the Co6-CP-
4
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catalysed cis-cyclooctene epoxidation with a TOF of 3.72 h at
ARTICLE
-
1
Table 4 Oxidation of cis-cyclooctene in different solvents over Co(II)@Cr-MIL-101-P2I
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catalysta
o
33
DOI: 10.1039/C5CY01099C
1
20 C. Although the TOF of the solid catalysts were lower
-
1
3 2 2
than that of Co(NO ) •6H O with a TOF of 54.45 h , they could
be recollected and reused. The enhanced catalytic efficiency for
the epoxidation of cis-cyclooctene was due to the high dispersion
of active catalytic sites at the pore walls of MOFs. Importantly,
the conversion of cis-cyclooctene with Co(II)@Cr-MIL-101-Sal
catalyst was lower than that of Co(II)@Cr-MIL-101-P2I and
Co(II)@Cr-MIL-101-P3I catalyst, which could be due to the
nitrogen atom in pyridine ring is a π electron acceptor.34,35 It can
be regarded as a strong electron-withdrawing substituent which
could be conducive to electrophilic attack of the Co-peroxy
species and then accelerated the rate of epoxidation (Fig. S5†).
However, the conversion of cis-cyclooctene was significantly
decreased after re-collected Co(II)@Cr-MIL-101-P3I sample
was used as the catalyst again (Table 2, entry 9), which can be
ascribed to the relatively weakly coordination ability of the
chelating groups. The XRD patterns of Co(II)@Cr-MIL-101-Sal,
Entry
Solvent
Conv. (%)b
Sel. (%)b
1
2
3
4
5
acetonitrile
acetone
>99
82
59
48
25
0
>99
>99
>99
>99
>99
-
ethyl acetate
dichloromethane
n-Hexane
6
ethyl alcohol
a
Reaction conditions: cis-cyclooctene (1.0 mmol), Co(II)@Cr-MIL-101-P2I catalyst
0.6 mol% cobalt), isobutyraldehyde (2.0 mmol), solvent (5.0 mL), at 35 C under
o
(
Co(II)@Cr-MIL-101-P2I
and
Co(II)@Cr-MIL-101-P3I air atmosphere stirring for 5 h.
b
Conversion and selectivity were determined by GC/MS using nitrobenzene as
internal standard.
catalysts after 2 cycles reaction were shown in Fig. S6†. No
obvious change can be observed comparing the powder XRD of
the fresh and reused catalyst. Furthermore, the cobalt content of
the reused Co(II)@Cr-MIL-101-Sal and Co(II)@Cr-MIL-101-
P2I catalyst determined by ICP-AES analysis also showed
almost no obvious reduction. However, the cobalt content of the
reused Co(II)@Cr-MIL-101-P3I catalyst was reduced to 0.018
mmol g from 0.03 mmol g , ICP-AES analysis of the reaction
solution after one cycle of epoxidation for Co(II)@Cr-MIL-101-
P3I catalyst showed the presence of 0.34 ppm of the cobalt in
reaction solution, which further indicated that Co(II) ions were
chelated relatively weakly in Co(II)@Cr-MIL-101-P3I sample.
Therefore, among three kinds of heterogeneous cobalt(II)-based
catalysts, Co(II)@Cr-MIL-101-P2I was chosen as optimized
catalyst for the epoxidation of cis-cyclooctene.
Aldehyde acted as a sacrificial co-reductant, could be
transformed in situ to a carboxylperoxo radical which facilitated
oxygen transfer to aerobic epoxidation of olefins.36,37 Several
aldehydes were evaluated as additive for the epoxidation of cis-
cyclooctene in the presence of Co(II)@Cr-MIL-101-P2I catalyst.
The blank control experiment showed no reactivity for the
formation of 1,2-Epoxycyclooctane (Table 3, entry 1), which
indicated aldehyde was an critical additive for aerobic
epoxidation. The yield of 1,2-Epoxycyclooctane was reached up
to 99% when isobutyraldehyde was used as the additive (Table
-
1
-1
3, entry 2), which was higher than that of the cyclic
cyclohexanecarboxaldehyde (86%, Table 3, entry 3), aliphatic
heptaldehyde (71%, Table 3, entry 4) and aromatic benzaldehyde
(
<5%, Table 3, entry 5). The distinctly different yields further
Table 3 Aldehyde screening in the oxidation of cis-cyclooctenea
suggested the steric and electronic effects of the aldehyde were
vital for dioxygen activation in the epoxidation of olefins.
Moreover, it gave compromised yield of 1,2-Epoxycyclooctane
using lower amounts of isobutyraldehyde (Table 3, entry 6-7).
Solvents play a crucial role in epoxidation reactions. The
selected model reaction was performed in different solvents and
the results were summarized in Table 4. The yield of 1,2-
Epoxycyclooctane was 99%, 82%, 59%, 48% and 25% when
acetonitrile, acetone, ethyl acetate, dichloromethane and n-
Hexane were employed as the solvent, respectively (Table 4,
entry 1-5), which revealed that the solvent with greater polarity
was beneficial to the enhancing of the conversion of cis-
cyclooctene. This can be due to that the greater polarity of the
Conv.
Sel.
(%)
Entry
Aldehyde (mmol)
-
b
b
(%)
1
2
-
-
Isobutyraldehyde (2 mmol)
Cyclohexanecarboxaldehyde
>99
86
>99
>99
3
(2 mmol)
4
5
6
7
Heptaldehyde (2 mmol)
Benzaldehyde (2 mmol)
71
<5
44
18
>99
>99
>99
>99
solvent facilitated the dispersion and collision of the catalyst, the
substrate and the oxygen source.38 However, no 1,2-
Epoxycyclooctane was observed when ethyl alcohol was used as
solvent owing to the coordination with Co(II) ions and
competing oxidation reaction.5,27
Isobutyraldehyde (1 mmol)
Isobutyraldehyde (0.5 mmol)
a
Reaction conditions: cis-cyclooctene (1.0 mmol), Co(II)@Cr-MIL-101-P2I catalyst
0.6 mol% cobalt), aldehyde, acetonitrile (5.0 mL), at 35 C under air atmosphere
The evolution of the conversion of cis-cyclooctene as
reaction time using the fresh Co(II)@Cr-MIL-101-P2I catalyst
o
(
stirring for 5 h.
b
Conversion and selectivity were determined by GC/MS using nitrobenzene as was explored and shown in Fig. 5 (curve a). The conversion was
internal standard.
increased linearly as the reaction time and reached 77% at 2 h.
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J. Name., 2013, 00, 1-3 | 5
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Ca Pt al el ya ss ies d So c ni eo nt ca ed j&u s Tt emc ah rng oi nl os gy
Page 6 of 8
ARTICLE
Journal Name
Then, the conversion was increased gradually and cis-
cyclooctene was completely converted to 1,2-Epoxycyclooctane
at 5 h. Additionally, the selectivity of desired product maintained
View Article Online
DOI: 10.1039/C5CY01099C
99% during the whole reaction process. Therefore, the optimized
catalytic condition involved 1 mmol substrate, 0.6 mol%
Co(II)@Cr-MIL-101-P2I catalyst based on cobalt, 2 mmol
isobutyraldehyde as an additive under 1 atm air atmosphere at 35
oC stirring for 5 hours. Such catalytic system is more efficient
and eco-friendly than the cobalt(II)-based zeolites X catalyst at
00 C and cobalt(II)-based MCM-41 catalyst at 110 C.21
o
39
o
1
To confirm that the Co(II)@Cr-MIL-101-P2I catalyst was
heterogeneous for the epoxidation reaction, a hot filtration test
was carried out. The solid catalyst was separated by
centrifugation at 1 h in the first cycle, and the residual liquid
phase was transferred into a clean reactor vessel and stirred for
another 4 h under the same reaction condition. In absence of
catalyst (Fig 5b), GC results showed that the conversion of cis-
cyclooctene was increased to 51% from 44%, which was caused
by isobutyraldehyde (Fig. S7†). Furthermore, ICP-AES analysis
of the filtrate showed almost no presence of the cobalt in the
solution.
In order to evaluate the reusability of Co(II)@Cr-MIL-101-
P2I catalyst, the epoxidation of cis-cyclooctene was performed
using recovered catalyst under the optimized catalytic condition.
After each reaction, the catalyst was recovered by simple
centrifugation and washed with acetonitrile for the next reaction
run. The catalyst remained active during each recycle and
retained 95% of conversion, 93% of selectivity even after 5
Fig. 6 The reusability of the Co(II)@Cr-MIL-101-P2I catalyst for the epoxidation of cis-
cyclooctene.
cyclooctene under the same reaction condition because of more
difficult electrophilic attack of the Co-peroxy species (Table 5,
entry 5). The epoxidation of 1,5-cyclooctadene occurred only at
one double bond with compromised yield and the main product
was 9-oxabicyclo [6.1.0]non-4-ene (Table 5, entry 6). The tri-
substituted olefin, such as α-pinene, was converted to the
corresponding epoxide with excellent conversion (>99%) and
selectivity (>99%) (Table 5, entry 7). Moreover, aliphatic olefin,
generally considered as inactive towards epoxidation, such as 1-
Table 5 Scope in the epoxidation of olefins catalyzed by Co(II)@Cr-MIL-101-P2I catalysta
recycles (Fig. 6). XRD and FTIR results (Fig. S8†) of fresh and
recovered Co(II)@Cr-MIL-101-P2I catalyst showed that the
composition of the catalyst had no obvious change after reaction.
While the SEM images (Fig. S8
†) showed that the structure of
the catalyst was slightly aggregated. ICP-AES analysis indicated
-
1
the cobalt content was reduced from 0.29 mmol g to 0.26 mmol
-
1
g . This may lead to the slight reduction in the conversion and
selectivity of the epoxidation of cis-cyclooctene.
The general applicability of the epoxidation in the presence
of air over Co(II)@Cr-MIL-101-P2I catalyst was demonstrated
in Table 5. Varied cyclic olefins, such as cyclopentene,
cyclohexene, cycloheptene and norbornene, were transformed to
the corresponding epoxides with high conversion and selectivity
(
Table 5, entry 1-4). The relatively bulky cyclic olefin of
cyclododecene showed lower catalytic activity than cis-
a
Reaction conditions: subatrate (1.0 mmol), Co(II)@Cr-MIL-101-P2I catalyst (0.6
o
mol% cobalt), isobutyraldehyde (2.0 mmol), solvent (5.0 mL), at 35 C under air
atmosphere.
b
Conversion and selectivity were determined by GC/MS using nitrobenzene as
Fig. 5 Curve (a): change of conversion over the reaction time for model reaction catalyzed
by fresh Co(II)@Cr-MIL-101-P2I catalyst, curve (b): leaching test.
internal standard.
6
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
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Page 7 of 8
Ca Pt al el ya ss ies d So c ni eo nt ca ed j&u s Tt emc ah rng oi nl os gy
Journal Name
ARTICLE
G. Férey, C. Mellot-Draznieks, C. Serre, F. MillanVgieew,AJr.ticDleuOtnoliuner
DOI: 10.1039/C5CY01099C
S. Bernt, V. Guillerm, C. Serre and N. Stock, Chem. Commun.,
decene, was oxidized to 1,2-epoxydecane in decent conversion
and high selectivity (Table 5, entry 8). The aromatic olefins, such
as styrene, trans-stilbene and cis-stilbene, were also catalytically
epoxidized. Interestingly, the epoxidation of trans-stilbene
proceeded more smoothly than that of styrene and cis-stilbene
due to the steric hindrance effect (Table 5, entry 9-11).40 The
epoxidation of styrene proceeded in low yield along with the
formation of benzaldehyde byproduct. Trans-stilbene showed
remarkable conversion over 91% of epoxide as the sole product
while cis-stilbene only gave 45% yield of epoxide.
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Conclusions
In summary, highly effective and recyclable cobalt(II)-based
solid catalysts have been successfully synthesized and applied
for epoxidation of olefins with air as the oxidant and
isobutyraldehyde as a sacrificial co-reductant. In Co(II)@Cr-
MIL-101-P2I catalyst, nitrogen atom in pyridine ring as a strong
electron-withdrawing substituent ensured high activity, and the
strong coordination interaction between Co(II) ions and
chelating groups guaranteed the excellent recycled performance.
The high dispersion of Co(II) species promoted the sufficient
contact between substrate and active sites. The catalyst showed
good general applicability towards varied olefins, such as cyclic
olefins, tri-substituted olefins, aliphatic olefins and aromatic
olefins. The Co(II) incorporation strategy provides a highly
efficient, low-cost and simple approach for the synthesis of
heterogeneous catalyst and can be extended to the structure
design of other MOF-based nanohybrid catalysts.
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This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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Catalysis Science & Technology
Page 8 of 8
View Article Online
DOI: 10.1039/C5CY01099C
Co(II) complexes loaded into metal organic framework as an efficient
heterogeneous catalyst for aerobic epoxidation of olefins
Efficient
cobalt(II)
anchored
Cr-MOF
(Cr-MIL-101-NH
2
)
catalyst,
Co(II)@Cr-MIL-101-P2I, have been successfully synthesized by one-pot modifying
terminal amino group with pyridine-2-aldehyde and anchoring Co(II) ions into
mesoporous Cr-MOFs support. Nitrogen atom in pyridine ring as a strong
electron-withdrawing substituent ensured high catalytic performance, and strong
coordination interaction between Co(II) ions and chelating groups guaranteed the
good re-usability.