Post-synthetic Modification of DUT-5-based Metal Organic Frameworks for the Generation of Single-site Catalysts and their Application in Selective Epoxidation Reactions

Article abstract of DOI:10.1002/cctc.201901434

New single-site catalysts based on mixed-linker metal-organic frameworks with DUT-5 structure, which contain immobilized Co²⁺, Mn²⁺ and Mn³⁺ complexes, have successfully been synthesized via post-synthetic modification. 2,

Full text of DOI:10.1002/cctc.201901434

Accepted Article
Title: Post-synthetic modification of DUT 5-based metal-organic
frameworks for the generation of single-site catalysts and their
application in selective epoxidation reactions
Authors: Ceylan Yildiz, Ksenia Kutonova, Simon Oßwald, Alba Titze-
Alonso, Johannes Bitzer, Stefan Bräse, and Wolfgang Kleist
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ChemCatChem
10.1002/cctc.201901434
FULL PAPER
Post-synthetic modification of DUT-5-based metal-organic frame-
works for the generation of single-site catalysts and their applica-
tion in selective epoxidation reactions
[
a]
[b]
[b,d]
[a]
[a]
Ceylan Yildiz , Ksenia Kutonova , Simon Oßwald , Alba Titze-Alonso , Johannes Bitzer , Stefan
Bräse[b,c] and Wolfgang Kleist*[a]
Abstract: New single-site catalysts based on mixed-linker metal-or-
catalysts in liquid phase reactions.[2] Over the last decades, vari-
ous MOF materials have been applied in oxidation reactions, for
ganic frameworks with DUT-5 structure, which contain immobilized
2+
2+
3+
[3]
[4]
Co , Mn and Mn complexes, have successfully been synthesized
via post-synthetic modification. 2,2’-Bipyridine-5,5’-dicarboxylate link-
ers were directly metalated, while 2-amino-4,4’-biphenyldicarboxylate
linkers were post-synthetically modified by their conversion to Schiff-
example, alcohol oxidation , epoxidation , or phenol hydroxyla-
[5]
tion . Beside a variety of other oxidants, molecular oxygen is par-
ticularly interesting due to its environmentally friendly character,
its abundance and its atom efficiency. However, highly active cat-
base ligands and a subsequent immobilization of the metal complexes. alysts based on transition metals are necessary for the activation
The resulting materials were used as catalysts in the selective epoxi-
dation of trans-stilbene and the activities and selectivities of the differ-
ent catalysts were compared. The influence of various reaction pa-
rameters on conversion, yield and selectivity were investigated. Very
low catalyst amounts of 0.02 mol% were sufficient to obtain high con-
version of trans-stilbene using molecular oxygen from air as the oxi-
dant. For cobalt-containing MOF catalysts, conversions up to 90%
were observed and, thus, they were more active than their manga-
nese-containing counterparts. Recycling experiments and hot filtra-
tion tests proved that the reactions were mainly catalyzed via hetero-
geneous pathways.
of molecular oxygen. Such molecular systems have been widely
investigated in homogeneous catalysis.[ Therefore, metal-or-
ganic frameworks are perfectly suited to transfer these highly ac-
tive single-site metal centers to solid structures, thus, combining
the advantages of homogeneous and heterogeneous catalysis in
one material. To elucidate the structure of active centers and the
catalytic mechanisms, a detailed characterization of the metal
centers is necessary. This is also relevant to get information about
undesired metallic or oxidic nanoparticles inside the pores. For
this purpose, many spectroscopic methods have already been
used to identify the chemical environment, redox properties and
reactivity of the single-site metal centers. Chen et al. confirmed
the coordination interaction between the linkers and palladium via
X-ray photoelectron spectroscopy (XPS) analysis.[ The group of
Martín-Matute reported the synthesis of UiO-67 functionalized
6]
7]
Introduction
with iridium, palladium and rhodium complexes, which were char-
acterized using X-ray absorption spectroscopy (XAS).[8] Further-
more, Braglia et al. investigated a Cu-functionalized UiO-67-bpy
MOF via XAS and FT-IR spectroscopy to prove the presence of
the well-defined copper complex in the framework.[9]
In recent years, metal-organic frameworks (MOFs) have gained
high importance as highly crystalline and porous materials.[
Their chemical and structural diversity makes them potential can-
didates for specific applications, especially as heterogeneous
1]
Various methods have been reported to introduce catalytically
active centers into MOFs. Post-synthetic modification (PSM)[
,
10]
[
a]
C. Yildiz, A. Titze-Alonso, J. Bitzer, Prof. Dr. W. Kleist
Faculty of Chemistry and Biochemistry, Industrial Chemistry –
Nanostructured Catalyst Materials
Ruhr University Bochum
Universitätsstraße. 150, D-44801 Bochum
E-mail: wolfgang.kleist@rub.de
Dr. K. Kutonova, S. Oßwald, Prof. Dr. S. Bräse
Institute of Organic Chemistry
Karlsruhe Institute of Technology
Fritz-Haber-Weg 6, D-76131 Karlsruhe
Prof. Dr. S. Bräse
Institute of Toxicology and Genetics
Karlsruhe Institute of Technology
Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopold-
shafen
S. Oßwald
i.e. the chemical modification of the material after framework syn-
thesis, has received great interest and plays an important role for
the design of well-defined single-site catalysts. Two different strat-
egies are known for the preparation of catalysts, either the post-
synthetic introduction of chelating side groups into the framework
or the utilization of chelating linker molecules. Highly active metal
ions can be introduced into the MOF structure by modifying amine
functionalities with anhydrides or aldehydes. Tanabe and Cohen
performed a PSM reaction of IRMOF-3 with succinic anhydride
resulting in a carboxylate-bearing MOF, which was subsequently
used to incorporate Cu2+ and Fe3+ ions. The iron-containing ma-
terials showed a high catalytic activity in Mukaiyama-type aldol
reactions.[11] In previous works, our group presented a mixed-
[
b]
c]
[
[
d]
Institute for Chemical Technology and Polymer Chemistry
Karlsruhe Institute of Technology
Engesserstraße 20, D-76131 Karlsruhe
2
linker MIL-53(Al)-NH , which was post-synthetically modified with
maleic anhydride. The resulting materials were transformed into
+
3+
Supporting information for this article is given via a link at the end of
the document.
single-site catalysts via the immobilization of Pd2 and Mn and
used in C-C coupling reactions[12,13] or the selective oxidation of
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ChemCatChem
10.1002/cctc.201901434
FULL PAPER
α-pinene[14], respectively. Apart from post-synthetic modifications
with anhydrides, reactions of primary amines and aldehydes re-
sulting in Schiff-bases are well-suited for the incorporation of well-
defined and highly active metal complexes into the MOF struc-
dicarboxylic acid (H
(H
BPDC), 2,2’-bipyridine-5,5’-dicarboxylic acid
BPyDC) or 2-amino-4,4’-biphenyldicarboxylic acid (H ABPDC)
2
2
2
have been used as linker molecules. The BPyDC linkers were
used to facilitate a direct and easy immobilization of cobalt and
manganese ions, which resulted in metal complexes close to the
pore walls (Figure 1a). The amine-functionalized DUT-5 materials
were post-synthetically modified with salicylaldehyde to generate
chelating side groups via Schiff-base reactions, which were sub-
sequently used for the immobilization of cobalt and manganese
ions resulting in metal complexes located farther inside the pores
(Figure 1b).
ture.[15] Doonan et al. modified a UMCM-1-NH
framework with
-pyridinecarboxaldehyde, which was used for a subsequent pal-
2
2
[16]
ladium immobilization.
There are many examples of amine-
functionalized UiO-66 and UiO-67 materials, which have been
modified via PSM reactions with aldehydes and a subsequent
metal immobilization to utilize them in catalytic applications.[17]
Apart from that, the direct incorporation of chelating linker mol-
ecules during the framework synthesis allows the direct immobili-
zation of metal ions.[18] The well-known bidentate linker molecule
a)
b)
2
,2’-bipyridine-5,5’-dicarboxylate has been used in various exam-
ples. For instance, MOF-253 containing 2,2’-bipyridine-5,5’-dicar-
boxylate linkers was used for post-synthetic metalation reactions
with palladium, copper and ruthenium salts and applied in various
liquid phase reactions.[19] The group of Cohen synthesized a
mixed-linker UiO-67 containing equal amounts of bipyridine- and
biphenyldicarboxylate linkers, which was used for palladium im-
mobilization and showed excellent catalytic activity in Suzuki-
Miyaura cross-coupling reactions.[20]
N
N
ML
x
N
O
M
L
x
DUT-5-NH
2
(10)-Sal-M
DUT-5-BPyDC(10)-M
Figure 1. Schematic representation of metal containing a) DUT-5-BPyDC(10)
and b) DUT-5-NH (10)-Sal materials obtained via post-synthetic modification.
2
M = Co , Mn or Mn3+. Green: Al; blue: N; red: O; grey: C; purple: Co , Mn
2+
2+
2+
2+
In the present work, post-synthetic modifications have been
or Mn3+.
applied using DUT-5, which consists of [Al(OH)]
n
chains that are
[
21]
connected by 4,4’-biphenyldicarboxylate linker molecules. The
[
22]
mixed-linker concept (MIXMOFs, MTV-MOFs)
was applied,
Powder X-ray diffraction (PXRD) measurements showed that
the DUT-5 structure was retained during the catalyst preparation
for all materials. Thus, the insertion of the chelating side groups
and/or the metal ions did not affect the framework structure (Fig-
ure 2).
which guaranteed a better framework porosity and a good distri-
bution of catalytically active sites within the framework structure.
Previously, functionalized DUT-5 with sulfone, amine and nitro
groups have been published, which were used in the gas phase
adsorption of alkanes, alkenes and aromatics.[ Furthermore,
DUT-5 and mixed-linker DUT-5 bearing amine, alkyne, azide and
nitro groups with various degree of functionalization have been
synthesized by our group.[24] These amine-functionalized
MIXDUT-5 materials were used for post-synthetic modification re-
actions with maleic anhydride, salicylaldehyde and 2-pyridinecar-
boxaldehyde. However, none of these materials has been used
for metal immobilization and subsequent heterogeneous catalytic
applications yet.
23]
3+
DUT-5-NH (10)-Sal-Mn
2
2+
DUT-5-NH (10)-Sal-Mn
2
DUT-5-NH (10)-Sal-Co
2
Therefore, in the present work, mixed-linker DUT-5 catalyst
DUT-5-BPyDC(10)-Mn3+
materials containing Co , Mn and Mn3+ complexes have been
synthesized via post-synthetic modification and subsequent metal
immobilization. The obtained materials were thoroughly charac-
terized and tested in the aerobic epoxidation of trans-stilbene us-
ing air as oxidant. Different reaction parameters were optimized
and the activities and selectivities of the different catalysts were
compared. Hot filtration and recycling tests were performed to in-
vestigate the heterogeneity of the reaction.
2+
2+
DUT-5-BPyDC(10)-Mn2+
DUT-5-BPyDC(10)-Co
simulated DUT-5
5
10
15
20
25
2
30
Θ [°]
35
40
45
50
Figure 2. PXRD patterns of mixed-linker DUT-5 materials after PSM with salic-
ylaldehyde and metal immobilization.
Results and Discussion
Attenuated total reflectance (ATR) IR spectra proved that no
residual linker (characteristic absorption bands corresponding to
The starting materials for this study were mixed-linker DUT-5 ma-
terials containing a defined percentage (10%) of functionalized
linker molecules. Based on literature procedures[21,24], the frame-
work syntheses were performed in a closed vessel under sol-
vothermal conditions (see ESI). In addition to 4,4’-biphenyl-
-1
the C=O stretching expected at around 1685 cm ) or solvent mol-
ecules (DMF; characteristic absorption bands corresponding to
-1
the C=O stretching expected at around 1650 cm ) were found
within the pores of all frameworks (Figure S2). Thus, the pores of
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ChemCatChem
10.1002/cctc.201901434
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the functionalized frameworks were not blocked and an easy ac-
cess for the post-synthetic modifications and the subsequent cat-
alytic reaction was provided.
X-ray absorption spectroscopy (XAS) was used to character-
ize the introduced metal complexes and to differentiate them from
undesired metal clusters or particles inside the pore systems,
which might have been additionally formed. Previous studies on
palladium containing MIL-53(Al) structure showed that the intro-
duced palladium species in the framework strongly depended on
the amount of the palladium precursor used during the modifica-
tion process. To facilitate the desired formation of the metal com-
plexes, only 0.9 equivalents of the metal precursor relative to the
number of chelating groups were used, since larger amounts of
the metal precursor led to the formation of undesired metal na-
noparticles.[12,13] Hence, the metal immobilization in this work was
also performed with 0.9 equivalents of metal precursor with re-
gard to the chelating group. The Co K-edge XANES spectrum of
DUT-5-BPyDC(10)-Co (Figure 3a) and the Mn K-edge XANES
spectra of the manganese-containing DUT-5 materials (Fig-
ure 3b) revealed that the oxidation states of the incorporated met-
als in the MOF structure were the same as in the precursor salts,
which were used for metal immobilization.
2
N physisorption measurements revealed that all materials
were obtained with large specific surface areas and accessible
micropore volumes. The surface areas of the post-synthetically
modified DUT-5 materials did not change significantly due to the
low modification degree (Table 1 and Table S3). Note that these
obtained results provide only qualitative information about the
successful formation of the framework structure.
However, by using 1H NMR spectroscopy, the quantitative
amount of the functionalized linker molecules could be deter-
mined (Figure S3 – S4). For both, the bipyridine- and the amine-
containing DUT-5 materials, 8% of functionalized linkers have
been incorporated in the materials. Further NMR studies were
employed to determine the modification degree with salicylalde-
hyde relative to amine functionalization. A modification degree of
3
0% could be calculated from the integration of the peaks in the
spectra (Figure S5).
The metal contents of the DUT-5 catalysts were determined
using inductively coupled plasma optical emission spectroscopy
a)
(
ICP-OES) and the values are listed in Table 1. The obtained re-
1
1
1
1
0
.6
.4
.2
.0
.8
sults showed that the chelating moieties were partially occupied
by the metal complexes. Electron microscopy images in combina-
tion with EDX mapping taking the DUT-5-NH (10)-Sal-Co material
2
as an example showed that the active metal is homogeneously
distributed within the framework structure (Figure S6). Since only
metal loading of 0.1 wt% could be achieved with Co and Mn2+
precursors (Table 1, entries 2, 3, 5-7), we also tried Mn3+ precur-
sor and get higher metal loading of 0.47 wt% (Table 1, entry 4).
0.6
0
.4
.2
0
DUT-5-BPyDC(10)-Co
Table 1. Specific surface areas (SBET), total pore volumes (tpv) and mi-
0.0
cropore volumes (mpv) derived from N
metal contents determined via ICP-OES of metal containing DUT-5 materi-
als.
2
physisorption measurements and
7680 7700 7720 7740 7760 7780 7800 7820 7840
Energy [eV]
b)
1.6
Metal
S
BET
2
tpv
[cm /g]
mpv
[cm /g]
Entry
Sample
DUT-5
Loading
wt%]
3
3
1.4
[m /g]
[
1.2
1.0
0.8
0.6
1
2
1880
1870
0.81
0.61
0.68
-
DUT-5-
BPyDC(10)-Co
0.80
0.10
DUT-5-BPyDC(10)-Mn2+
DUT-5-
BPyDC(10)-Mn
3
4
5
6
7
1810
1530
1480
1760
1750
0.82
0.67
0.75
0.79
0.80
0.71
0.55
0.53
0.65
0.63
0.10
0.47
0.10
0.10
0.14
0.4
.2
2
+
+
2+
DUT-5-NH (10)-Sal-Mn
2
DUT-5-BPyDC(10)-Mn3+
0
DUT-5-
BPyDC(10)-Mn
(10)-Sal-Mn3+
DUT-5-NH
3
2
0.0
6
500 6520 6540 6560 6580 6600 6620 6640 6660
Energy [eV]
DUT-5-NH
Sal-Co
2
2
2
(10)-
Figure 3. XANES spectra of the metal-containing DUT-5 catalysts at the Co K-
DUT-5-NH
(10)-
(10)-
2
+
edge (a) and the Mn K-edge (b).
Sal-Mn
DUT-5-NH
3
+
Sal-Mn
Figure 4 shows the Fourier-transformed spectra for DUT-5-
BPyDC(10)-Co, DUT-5-BPyDC(10)-Mn and DUT-5-BPyDC(10)-
Mn . The spectra revealed that the first complex shell showed a
2+
3+
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ChemCatChem
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3
+
high intensity in contrast to the higher shells indicating that the
metals are exclusively present in the form of the desired com-
plexes. The corresponding fitting parameters and results of the
EXAFS analysis are listed in Table S5. In the first shell, two dif-
ferent distances with two and four neighbor atoms, respectively,
could be fitted, which are assigned to two nitrogen atoms of the
bipyridine linker and four oxygen neighbor atoms belonging to the
ligands of the complex and, thus, these values are in good agree-
ment with the expected structure. To exclude the presence of ox-
idic phases in the material, fits of the metal complexes with an
additional metal shell from the corresponding metal oxides were
performed (Figure S6). These fits did not result in a significant
contribution of metal backscatters from an oxide. Consequently,
other metal containing species could be excluded.
2
and DUT-5-NH (10)-Sal-Mn ) were then tested under optimized
reaction conditions and the activities of the different catalyst sys-
tems were compared. Conversions, yields and selectivities were
evaluated by variation of one reaction parameter at a time and
quantified by GC analysis using biphenyl as an internal standard.
air
O
DUT-5 catalyst
DMF
Scheme 1. Aerobic epoxidation of trans-stilbene catalyzed by metal containing
DUT-5 resulting in trans-stilbene oxide (main product).
a)
100
90
80
70
60
50
40
30
20
10
0
Conversion
Yield
Selectivity
0
2
4
6
8
10 12 14 16 18 20 22 24
Reaction time [h]
b)
100
90
80
70
60
50
40
30
20
10
0
Conversion
Yield
Selectivity
Figure 4. Fourier-transformed experimental EXAFS spectra and the fitting re-
0
2
4
6
8
10 12 14 16 18 20 22 24
Reaction time [h]
sults of the first complex shell of DUT-5-BPyDC(10)-Co (a), DUT-5-BPyDC(10)-
2+
3+
Mn (b) and DUT-5-BPyDC(10)-Mn (c).
Figure 5. Conversion, yield and selectivity for the epoxidation of trans-stilbene
catalyzed by DUT-5-BPyDC(10)-Co (a) and DUT-5-BPyDC(10)-Mn2+ (b). Reac-
tion conditions: 24 h, 150 °C, catalyst amount (0.02 mol% of metals), air flow
The resulting catalysts were subsequently used in the aerobic
epoxidation of trans-stilbene. In a typical reaction, trans-stilbene
oxide was obtained as the main product and benzaldehyde as a
side product in the oxidation of trans-stilbene using air as the ox-
idant in N,N-dimethylformamide (DMF) (Scheme 1). DMF was
chosen as the solvent for the epoxidation reactions due to a sol-
vent co-oxidation mechanism, which has been reported be-
(200 ml/min), trans-stilbene (1 mmol), DMF (60 ml).
Blank tests at 150 °C for 24 h with continuous air flow through
the solution (200 ml/min) were carried out to ensure that the cat-
alytic activity originated from the metal-containing DUT-5 materi-
als and not from the framework itself. The blank test without MOF
catalyst after 24 hours resulted in 12% conversion with only 10%
product formation (Table S6, Entry 1). Also a blank test using a
bipyridine-containing DUT-5 material without further metal immo-
bilization resulted in only 9% conversion and 6% yield (Table S6,
[
25,26]
fore.
Various reaction parameters such as temperature, oxi-
dant flow rate, catalyst concentration and substrate concentration
were screened by using the catalysts DUT-5-BPyDC(10)-Co and
2+
DUT-5-BPyDC(10)-Mn . The remaining catalysts (DUT-5-
3+
2+
2 2
BPyDC(10)-Mn , DUT-5-NH (10)-Sal-Co, DUT-5-NH (10)-Sal-Mn
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Entry 2), whereas much higher conversion of 93% with DUT-5-
BPyDC(10)-Co and 32% with DUT-5-BPyDC(10)-Mn2+ was ob-
tained. A time-dependent study of conversion and yield of trans-
stilbene oxide with DUT-5-BPyDC(10)-Co and DUT-5-
BPyDC(10)-Mn as catalyst under the same conditions is shown
in Figure 5. These results proved that the successful formation of
trans-stilbene oxide was truly induced by the cobalt- or manga-
nese-containing DUT-5.
increased conversion from 31 to 50% and to an increased yield
from 28 to 44%. However, an increase of the flow rate from 200
to 300 ml/min did not further enhance the conversion as well as
the yield. For DUT-5-BPyDC(10)-Mn , only a slight difference in
conversion and yield was observed after 24 hours. The best result
was also obtained at 200 ml/min with 32% conversion and 28%
yield (Figure 6b).
2
+
2+
Next, we have studied the influence of different reaction pa-
rameters on the performance of the oxidation. First of all, the in-
fluence of the reaction temperature was investigated. For this pur-
pose, temperatures of 90, 110, 130 and 150 °C were applied. The
conversion of trans-stilbene increased at higher temperatures due
to an increased reaction rate (Table 2 and 3). Higher tempera-
tures using DUT-5-BPyDC(10)-Co led to the additional formation
of benzaldehyde (up to 10%) as a side product resulting in a de-
a)
100
Conversion
Yield
Selectivity
90
80
70
60
50
2
+
40
creased selectivity. For the catalyst DUT-5-BPyDC(10)-Mn , the
formation of benzaldehyde was not observed in detectable
amounts.
30
20
10
0
1
00
200
300
Table 2. Influence of the temperature on the epoxidation of trans-stilbene
catalyzed by DUT-5-BPyDC(10)-Co. Reaction conditions: 24 h, catalyst
amount (0.02 mol% of cobalt), air flow (200 ml/min), trans-stilbene (1 mmol),
DMF (60 ml).
Air flow [ml/min]
1
00
b)
Conversion
Yield
Selectivity
9
8
7
0
0
0
Temperature
°C]
Conversion
[%]
Selectivity
[%]
Entry
Yield [%]
[
1
2
3
4
90
51
80
89
93
46
71
72
70
89
88
81
75
60
50
40
30
20
10
0
110
130
150
100
200
300
Air flow [ml/min]
Table 3. Influence of the temperature on the epoxidation of trans-stilbene
catalyzed by DUT-5-BPyDC(10)-Mn . Reaction conditions: 24 h, catalyst
2+
amount (0.02 mol% of manganese), air flow (200 ml/min), trans-stilbene
Figure 6. Influence of the air flow rate on the epoxidation of trans-stilbene cat-
(1 mmol), DMF (60 ml).
2+
alyzed by DUT-5-BPyDC(10)-Co after 6 h (a) and DUT-5-BPyDC(10)-Mn after
4 h (b). Reaction conditions: 150 °C, catalyst amount (0.02 mol% of metals),
2
Temperature
Conversion
[%]
Selectivity
[%]
Entry
Yield [%]
trans-stilbene (1 mmol), DMF (60 ml).
[°C]
1
2
3
4
90
5
4
76
78
90
87
In the following, the dependency of conversion and yield on
the catalyst amount was investigated. The catalyst concentration
was varied from 0.05 to 0.01 mol% based on the determined
metal content (cf. Table 1). The results were compared after
6 hours for DUT-5-BPyDC(10)-Co (Table 4) and after 24 hours for
DUT-5-BPyDC(10)-Mn2+ (Table 5). Decreasing the amount of
DUT-5-BPyDC(10)-Co from 0.05 to 0.02 mol% caused a drop of
the trans-stilbene conversion from 73 to 58%. Further decrease
of the cobalt concentration to 0.01 mol% led to a decrease of the
conversion to 34%. The highest selectivity (91%) was achieved
using 0.02 mol% cobalt. When the amount of DUT-5-BPyDC(10)-
110
130
150
11
16
32
9
14
28
The air flow was varied using 100, 200 and 300 ml/min. Con-
version of trans-stilbene with the catalyst DUT-5-BPyDC(10)-Co
was evaluated after 6 hours reaction time, since the conversion
was almost complete after 24 hours (Figure 6a). The analysis
showed that the increase from 100 to 200 ml/min led to an
2+
Mn was reduced, the conversion of trans-stilbene decreased
from 54 to 19% and the selectivity increased from 83 to 90%.
Turnover numbers (TON) were calculated to compare the activity
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ChemCatChem
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FULL PAPER
of the catalysts. Despite the small catalyst amount, high yields
were obtained resulting in very high turnover numbers. For
DUT-5-BPyDC(10)-Co, an increase from 1300 for 0.05 mol% to
turnover frequencies using the manganese-containing catalyst
were similar.
3
000 for 0.01 mol% was observed. The lowest TON was achieved
by using 0.05 mol% of DUT-5-BPyDC(10)-Mn2+ (TON ≈ 500).
Nevertheless, these values are significantly higher than for other
MOF-based catalysts previously used for epoxidation reac-
tions.[14,27]
100
a)
90
80
70
60
50
40
Table 4. Influence of the catalyst concentration on the epoxidation of trans-
stilbene catalyzed by DUT-5-BPyDC(10)-Co. Reaction conditions: 6 h,
1 mmol
2
3
4
mmol
mmol
mmol
1
50 °C, air flow (200 ml/min), trans-stilbene (1 mmol), DMF (60 ml).
30
2
0
0
0
Catalyst
Conversion
[%]
Yield
[%]
Selectivity
[%]
Entry
TON
5 mmol
0 mmol
1
amount [mol%]
1
0
2
4
6
8
10 12 14 16 18 20 22 24
Reaction time [h]
1
2
3
0.05
0.02
0.01
73
59
34
64
54
29
87
91
85
1300
2700
3000
b)
4.0
1
2
3
4
5
mmol
mmol
mmol
mmol
mmol
3.5
3.0
2.5
2.0
1.5
Table 5. Influence of the catalyst concentration on the epoxidation of trans-
stilbene catalyzed by DUT-5-BPyDC(10)-Mn . Reaction conditions: 24 h,
2+
150 °C, air flow (200 ml/min), trans-stilbene (1 mmol), DMF (60 ml).
Catalyst
amount [mol%]
Conversion
[%]
Yield
[%]
Selectivity
[%]
Entry
TON
1.0
0
.5
.0
1
2
3
0.05
0.02
0.01
54
32
19
45
28
17
83
87
90
500
0
0
2
4
6
8
10 12 14 16 18 20 22 24
Reaction time [h]
1400
1700
Figure 7. Conversion of trans-stilbene (a) and molar amount of stilbene oxide
b) with increasing molar amount of trans-stilbene by using DUT-5-BPyDC(10)-
(
Co. Reaction conditions: 24 h, 150 °C, DUT-5-BPyDC(10)-Co (6.4 mg), air flow
(200 ml/min), DMF (60 ml).
The influence of the substrate concentration was also tested.
The molar amount of trans-stilbene was increased from 1 to
1
0 mmol, whereas the amounts of the catalyst and solvent were
not varied. With increasing amount of trans-stilbene, the cata-
lyst/substrate ratio decreased from 0.02 mol% to 0.002 mol% of
cobalt and manganese, respectively. For DUT-5-BPyDC(10)-Co,
nearly full conversion was found after 24 hours regardless of the
substrate concentration. After 6 hours, a decrease of the conver-
sion from 55% for 1 mmol trans-stilbene amount to 22% for
Finally, the optimized reaction conditions (24 h, 150 °C,
0.02 mol% catalyst amount, 200 ml/min air flow and 1 mmol
trans-stilbene) were applied to test the catalytic performance of
3+
2+
DUT-5-BPyDC(10)-Mn , DUT-5-NH
2
(10)-Sal-Mn , DUT-5-NH
2
(10)-
3+
2
Sal-Mn and DUT-5-NH (10)-Sal-Co. The cobalt-containing DUT-5
catalysts showed a significantly higher catalytic activity than their
manganese counterparts (Figure 8), which is in accordance with
earlier studies in our group on liquid phase oxidation reactions
1
0 mmol trans-stilbene amount was observed (Table 6). Consid-
ering the time-dependent progress in Figure 7a, the conversion
increased slower with increased substrate concentration. Despite
the marginally decreasing conversion, the absolute amount of
formed product increased for DUT-5-BPyDC(10)-Co from 0.50 to
using cobalt[ - and manganese -based single-site catalysts un-
26]
[28]
der similar conditions. Conversions, yields and selectivities that
were obtained with DUT-5-BPyDC(10)-Co and DUT-5-NH (10)-
2
3
.84 mmol (Figure 7b). Consequently, an increase of the turnover
Sal-Co after 24 h were comparable. Note, however, that DUT-5-
2+
2+
numbers was observed. Using DUT-5-BPyDC(10)-Mn , the con-
version dropped from 32 to 13%, whereas the productivity of the
catalyst increased from a TON of 1400 to 2800 (Table 7). The de-
termined turnover frequencies (TOF) after 6 hours showed in-
creased values from 420 to 1770 h- with higher substrate con-
centrations for the cobalt-containing catalyst, whereas the
2
NH (10)-Sal-Mn gave a higher conversion (44%) than DUT-5-
2+
BPyDC(10)-Mn (32%), while both reached the same selectivity
(86%). DUT-5-BPyDC(10)-Mn3 and DUT-5-NH
+
(10)-Sal-Mn3+ did
2
not differ significantly from the Mn2+ materials.
1
Heterogeneity tests were performed using a hot filtration ex-
periment, which was carried out for DUT--5-BPyDC(10)-Co. The
catalyst was removed after 2.5 h reaction time and the reaction
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ChemCatChem
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Table 6. Influence of the substrate concentration on the epoxidation of trans-stilbene catalyzed by DUT-5-BPyDC(10)-Co. Reaction conditions: 6 h, 150 °C,
catalyst (6.4 mg), air flow (200 ml/min), DMF (60 ml).
-
1
Entry
Molar amount of trans-stilbene [mmol]
Catalyst amount [mol%]
Conversion [%]
Yield [%]
Selectivity [%]
TON
2700
5200
7400
8100
11000
TOF [h ]
1
2
3
4
5
1
2
3
4
5
0.02
59
57
58
48
45
54
52
49
41
42
91
91
85
85
94
420
0.01
870
0.0067
0.005
0.004
1230
1350
1770
Table 7. Influence of the substrate concentration on the epoxidation of trans-stilbene catalyzed by DUT-5-BPyDC(10)-Mn2+. Reaction conditions: 24 h, 150 °C,
catalyst (6.4 mg), air flow (200 ml/min), DMF (60 ml).
-
1 [a]
Entry
Molar amount of trans-stilbene [mmol]
Catalyst amount [mol%]
Conversion [%]
Yield [%]
Selectivity [%]
TON
1400
2300
2200
3400
2800
TOF [h ]
1
2
3
4
5
1
2
3
4
5
0.02
32
25
17
20
13
28
23
15
17
11
87
92
86
86
87
125
0.01
120
0.0067
0.005
0.004
130
230
200
[a] Determined after 6 hours.
was continued without the solid catalyst. At the same time, a con-
trol experiment was performed, in which the catalyst remained in
the reaction mixture. After catalyst removal, the results showed
that the conversion and the yield of trans-stilbene oxide increased
only slightly (Figure 9a). The reaction rate was much lower than
the reaction rate of the control experiment, which indicated that
the reaction was mainly catalyzed via a heterogeneous pathway.
A conversion of 49% and a yield of 45% were obtained via the hot
filtration experiment, whereas the control experiment provided a
conversion of 90% and a yield of 69%. Additionally, it should be
noted that the particle size of the DUT-5 catalysts after the syn-
thesis was very small (< 1.6 µm) and the particles had to be gran-
ulated before use to obtain particles sizes of 200 – 500 µm. Even
though the hot filtration test and the control experiment were con-
ducted without stirring, a grinding of the DUT-5 particles during
the catalytic tests could not be excluded. Thus, the observed ac-
tivity after filtration might also originate from DUT-5-BPyDC(10)-
Co particles, which were too small to be removed from the reac-
tion mixture.
100
90
Conversion
Yield
Selectivity
80
70
60
50
40
30
20
10
0
2
+
2+
3+
n
3+
o
)
-
Co
l
-C
M
n
l-Mn
M
l-Mn
a
0
a
)-
a
)-
0
1
0
)-S
1
1
0
)-S
)-S
0
DC(
2(1
0
DC(
2(1
DC(
2(1
-BPy
-NH
-
5
-5
-BPy
5
-NH
-BPy
-5
-NH
-
-5
-5
DUT
DUT
DUT
DUT
DUT
DUT
Figure 8. Comparison of conversion, yield and selectivity for the different DUT-5
catalysts. Reaction conditions: 24 h, 150 °C, catalyst (0.02 mol% of metals), air
flow (200 ml/min), trans-stilbene (1.0 mmol), DMF (60 ml).
Furthermore, the reusability of DUT-5-BPyDC(10)-Co cata-
lysts was studied. After the first catalytic reaction, the catalyst was
filtered off, washed and dried at room temperature and reused in
a second epoxidation reaction under the same conditions. Alt-
hough granulated powder material (200 – 500 µm) was used to
facilitate an easy recovery of the catalyst, a grinding of the
granules was observed and, thus, the amount of recovered
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ChemCatChem
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FULL PAPER
catalyst was not sufficient for a third run. Nevertheless, the results
shown in Figure 9b proved that the catalyst was active for more
than one reaction cycle without a significant loss of activity. In
summary, the epoxidation reaction of trans-stilbene proceeded
mainly via a heterogeneous pathway using the obtained catalyst
materials, although minor amounts of catalytically active homoge-
neous species could not be ruled out completely.
bipyridine and the salicylideneimine, were modified by immobiliz-
ing Co , Mn and Mn3+ ions. The resulting materials showed high
crystallinity and were obtained with high surface areas throughout
the post-synthetic modification process. X-ray absorption spec-
troscopy analysis proved the successful immobilization of cobalt
and manganese ions in the form of well-defined complexes for all
materials and the absence of other undesired metal containing
species like clusters or nanoparticles.
2+
2+
The selective epoxidation of trans-stilbene in liquid phase was
chosen as the catalytic test reaction. Different reaction parame-
ters were varied to test their effect on conversion, yield and selec-
tivity. The cobalt-containing catalysts featured significantly higher
activity than their manganese counterparts. Under optimized con-
ditions, 93% conversion of trans-stilbene and 75% yield of trans-
stilbene oxide was achieved using DUT-5-BPyDC(10)-Co and
1
00
a)
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
2
+
3
2% conversion and 28% yield using DUT-5-BPyDC(10)-Mn ,
both with a low catalyst loading of 0.02 mol% using molecular ox-
ygen as the oxidant. Concerning on the different positioning of the
Conversion (control experiment)
Yield (control experiment)
Conversion (hot filtration experiment)
Yield (hot filtration experiment)
3+
metal ions, the cobalt- and Mn -containing catalysts showed no
2+
effect on the catalytic activity. Only differences for the Mn -con-
taining catalysts were observed, where DUT-5-NH
2
(10)-Sal-Mn2+
0
2
4
6
8
10 12 14 16 18 20 22 24
Reaction time [h]
gave a significantly higher conversion than its counterpart DUT-5-
2+
BPyDC(10)-Mn . Hot filtration and recycling tests revealed that
the reaction proceeded mainly via a heterogeneous pathway. The
results obtained in this study evidenced that the synthesized cat-
alysts can be considered as promising solid catalyst materials for
other fine chemical syntheses in the liquid phase.
b)
1
00
Conversion
Yield
Selectivity
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
Acknowledgements
The authors thank the synchrotron radiation facility DESY
(
Deutsches Elektronen-Synchrotron) in Hamburg (Germany) for
providing beamtime at the XAS beamline P65 and Dr. Edmund
Welter for his support. This research was funded by the Deutsche
Forschungsgemeinschaft (DFG, German Research Foundation)
within the TRR 247 (project number 388390466). Prof. Ute
Krämer and Petra Düchting are acknowledged for the ICP-OES
analysis. J. Bitzer thanks the Evangelisches Studienwerk Villigst
e.V. for financial support.
1st run
2nd run
Figure 9. a) Comparison of conversion and yield of the trans-stilbene epoxida-
tion with and without catalyst removal after 2.5 h (as indicated by vertical line).
b) Conversion and yield in two cycles of trans-stilbene epoxidation. Reaction
conditions: 24 h, 150 °C, granulated DUT-5-BPyDC(10)-Co (0.02 mol% of co-
balt), air flow (200 ml/min), trans-stilbene (1.0 mmol), DMF(60 ml).
Keywords: aerobic epoxidation • cobalt • manganese • metal-or-
ganic frameworks • post-synthetic modification
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Entry for the Table of Contents
FULL PAPER
Ceylan Yildiz, Ksenia Kutonova, Simon
Oßwald, Alba Titze-Alonso, Johannes
Bitzer, Stefan Bräse, Wolfgang Kleist*
Page No. – Page No.
Post-synthetic modification of DUT-5-
based metal-organic frameworks for
the generation of single-site catalysts
and their application in selective
epoxidation reactions
In this study, new cobalt- and manganese-containing single-site catalysts based on
metal-organic frameworks with DUT 5 structure have been synthesized via post-
synthetic modification routes. The heterogeneous catalysts were utilized in the
selective epoxidation of trans-stilbene in liquid phase where they showed
remarkable activities (up to 90% conversion) using very low catalyst concentrations
(
0.02 mol%) and molecular oxygen from air as the oxidant.
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