Effective Heterogeneous MoOx-Modified CeO2 Catalyst for Michael Addition of Dimethyl Malonate to 2-Cyclohexen-1-one

Article abstract of DOI:10.1002/cctc.202100682

Molybdenum oxide-modified cerium oxide (MoO_x?CeO₂) with 1 wt % Mo acted as an effective and reusable heterogeneous catalyst for Michael addition of dimethyl malonate to 2-cyclohexen-1-one, providing high yield of the target product (

Full text of DOI:10.1002/cctc.202100682

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Effective Heterogeneous MoOx-modified CeO2 Catalyst for
Michael Addition of Dimethyl Malonate to 2-Cyclohexen-1-one
Authors: Keiichi Tomishige, Masazumi Tamura, Yamato Doi, Yingai Li,
and Yoshinao Nakagawa
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10.1002/cctc.202100682
ChemCatChem
COMMUNICATION
Effective Heterogeneous MoOx-modified CeO2 Catalyst for
Michael Addition of Dimethyl Malonate to 2-Cyclohexen-1-one
Masazumi Tamura,*[a] Yamato Doi,[b] Yingai Li,[b] Yoshinao Nakagawa[b] and Keiichi Tomishige *[b]
[a]
Prof. Dr. M. Tamura
Research Center for Artificial Photosynthesis, 558-8585, Japan.
Osaka City University
3-3-138, Sugimoto, Sumiyoshi-ku, Osaka 558-8585 Japan
E-mail: mtamura@osaka-cu.ac.jp
[b]
Y. Doi, Y. Li, Prof. Dr. Y. Nakagawa, Prof. Dr. K. Tomishige
Department of Applied Chemistry, School of Engineering
Tohoku University
6-6-07, Aoba, Aramaki, Aoba-ku, Sendai 980-8579 Japan
E-mail: tomi@erec.che.tohoku.ac.jp
Supporting information for this article is given via a link at the end of the document.
Abstract: Molybdenum oxide-modified cerium oxide (MoOx-CeO2)
with 1 wt% Mo acted as an effective and reusable heterogeneous
catalyst for Michael addition of dimethyl malonate to 2-cyclohexen-1-
one, providing high yield of the target product (99%). The catalyst
showed higher activity with high selectivity (>99%) than simple metal
oxides and the other combination of metal oxides and CeO2. Based
on the catalyst characterization and experimental studies, MoOx
species in the MoOx-CeO2 catalyst are isolated Mo6+, and the interface
between the isolated MoOx and CeO2 is the active site. The activity
based on the active site was about 74-fold and 1800-fold higher than
that of CeO2 surface and the typical strong organic base of DBU (1,8-
diazabicyclo[5.4.0]undec-7-ene), respectively.
mixed metal oxides such as SiO2–MgO, ZnxZryOz, TiO2-ZrO2,
CexZr1−xO2 and so on were reported to work as a strong base or
acid-base bifunctional heterogeneous catalyst for nucleophilic
addition reactions.7 The reports on the effective inorganic solid
catalyst systems are limited, and there is room for development
of only inorganic solid catalysts with suitable acid-base property.
Cerium oxide (CeO2) has unique acid-base and redox
properties and can exhibit superior catalysis for various organic
reactions at comparatively low temperature (<473 K).8 CeO2
shows higher activity in nucleophilic reactions with alcohols and
amines as nucleophiles due to the high activation ability of the
polarized functional groups.9 CeO2 also has some activity for the
nucleophilic addition reactions with activated methylene
compounds (C-C bond formation reactions),10 and the acid-base
property is proposed to play an important role in the reaction.
However, the activity is generally much lower than the strong solid
base catalysts such as MgO, CaO and hydrotalcite.10a,c We
envisioned that the modification of CeO2 with some metal oxides
could improve the catalytic activity for the nucleophilic addition
reactions such as aldol and Michael reactions.
Nucleophilic addition reactions such as aldol reaction, Michael
reaction, Mannich reaction and so on are of great importance in
organic synthesis because they can enable the formation of a C-
C or C-X (O, N, S, etc…) bond in organic compounds.
Conventionally, Lewis (or Brønsted) acid or Lewis base catalysts
are used for the reactions, which however suffers from the
formation of salts and separation of the catalysts. To overcome
Herein, we found that MoOx-modified CeO2 (MoOx-CeO2) with
1 wt% Mo was an effective heterogeneous catalyst for the Michael
addition of dimethyl malonate to 2-cyclohexen-1-one, and that the
activity is 74-fold and 1800-fold higher than that of CeO2 surface
and DBU(1,8-diazabicyclo[5.4.0]undec-7-ene), a typical strong
organic base, respectively.
the
problems,
various
effective
homogeneous1
and
heterogeneous catalysts2 have been developed, and
heterogeneous catalysts are preferable to homogenous ones in
terms of the catalyst reusability and separation from reaction
mixtures. Organic bases and acids immobilized on solid materials
are well-known effective heterogeneous catalysts for nucleophilic
addition reactions,3 which is due to the acid-base bifunctional
property. However, the stability and leaching of the organic
modifiers under the reaction conditions are problematic. Strong
solid base catalysts such as MgO, KF/Al2O3 and Mg-Al-O-t-Bu
hydrotalcite are also reported to be effective for the reactions,[4]
however, these catalysts are easily deactivated by H2O, CO2 and
polymerized products, and the handling and reuse of the catalysts
are generally difficult. On the other hand, metal hydroxides 5 such
as hydrotalcite-like materials and serpentine, and metal
phosphates6 such as amorphous aluminophosphate and cerium
phosphate are also reported to be effective acid-base bifunctional
heterogeneous catalysts for aldol condensation and acetalization
reactions. Modification of main metal oxides with other metal
oxides is a typical method for controlling the acid-base and/or
redox properties of metal oxides. Binary metal oxides including
At first, the catalytic performance of simple metal oxides was
tested. Michael addition of dimethyl malonate to 2-cyclohexen-1-
one was conducted as a model reaction by using various simple
metal oxides without pre-heating treatment. (Table S1). CeO2,
La2O3 and Y2O3 showed activity (conversion: 9, 3 and 1%,
respectively) with high selectivity to the target product (>99%),
and the other metal oxides (La2O3, Y2O3, ZrO2, TiO2, -Al2O3, SiO2,
MgO, ZnO, MoO2, MoO3, Mn2O3, Fe2O3, MnO, NiO and Cu2O)
have no activity in the reaction. Among the active metal oxides,
CeO2 showed higher conversion than the other metal oxides. Next,
the conversion of CeO2 and La2O3 under air and Ar was compared.
In the case of CeO2, the conversion under Ar (20%, selectivity:
>99%) was clearly higher than that under air (9%, selectivity:
>99%), while those under air and Ar were similar in the case of
La2O3 (3%, selectivity: >99%). Therefore, we selected CeO2 and
1
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10.1002/cctc.202100682
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COMMUNICATION
60
50
40
30
20
10
0
Table 1. Michael addition of dimethyl malonate to 2-cyclohexen-1-one with
metal oxide-modified CeO2 (MOx-CeO2) catalysts
O
O
O
O
O
MOx-CeO2
+
O
O
O
O
O
1
Selectivitya /%
Conversiona
Entry
M in MOx-CeO2
/%
53
39
35
31
22
22
4
1
Others
<1
<1
<1
<1
1
1
2
Mo
>99
>99
>99
>99
99
Re
3
W
4
V
5
Mn
Nb
0
0.1
0.5
0.75
1
1.5
2
4
6
>99
>99
>99
>99
>99
-
<1
<1
<1
<1
<1
-
Loading amomunt of Mo /wt%
7
Fe
Figure 1. Effect of Mo loading amount in MoOx-CeO2 catalysts in Michael
addition of dimethyl malonate to 2-cyclohexen-1-one
8
Mg
2
9
Na
2
10
11
none (CeO2)
Mo(acac)2
20
<1
Reaction conditions: 2-cyclohexen-1-one 1.0 mmol, dimethyl malonate 1.1
mmol, MoOx-CeO2 30 mg, toluene 1.5 mL, 303 K, 4 h, Ar. Yield is calculated on
2-cyclohexen-1-one basis.
Reaction conditions: 2-cyclohexen-1-one 1.0 mmol, dimethyl malonate
1.1 mmol, MOx-CeO2(M=1 wt%) 30 mg, toluene 1.5 mL, 303 K, 4 h, Ar.
[a]These values are calculated on 2-cyclohexen-1-one basis.
obtaining the high activity, and the interface of CeO2 and MoOx
will be the active site.
Ar as the main metal oxide and reaction atmosphere, respectively
in the following study.
The effect of Mo loading amount in MoOx-CeO2 was
investigated (Figure 1). The conversion increased with increasing
the Mo loading amount up to 1 wt%, in contrast, it decreased with
higher Mo loading amount than 1 wt%. Therefore, MoOx-CeO2
with 1 wt% Mo was the most effective catalyst for the reaction.
MoOx-CeO2 catalysts with different Mo loading amounts were
characterized by XRD, TEM, XPS and Raman (Figure 2). XRD
patterns show only the signals due to CeO2, and the signals due
to MoOx species were not detected (Figure 2(A)). These results
indicate that MoOx species are highly dispersed or amorphous
over CeO2. Next, TEM image of MoOx-CeO2 with 1 wt% Mo was
measured (Figure 2(B)), showing only CeO2 particles and no
Modification of CeO2 catalyst with various metal oxides (MOx)
was studied (Table 1). The selectivity to the target product is high
(≥99%) with all catalysts. Mo, Re, W and V provided higher
conversion than CeO2 (entries 1-4 and 10), which suggests that
these metal oxides are effective additives for CeO2 catalyst. Mn
and Nb (entries 5 and 6) showed similar conversion than CeO2,
indicating that these metal oxides have no effect on the catalysis
of CeO2. Fe, Mg and Na (entries 7-9) provided negative effect on
the reaction, and lower conversion (<5%) was obtained. To check
the activity of a homogenous Mo complex, Mo(acac)2 was applied
to the reaction, showing no conversion. Among these metal
oxides, MoOx provided the highest conversion with high selectivity,
and hence, MoOx was the most effective modifier for CeO2.
The combination of MoOx with other metal oxides was tested in
the same reaction with various MoOx-modified metal oxide
catalysts (Table 2). Modification of MgO and La2O3 with MoOx
showed activity (entries 2 and 3), however, the conversion (<11%)
is much lower than that over CeO2 (53%). Modification of the other
metal oxides showed almost no conversion. Considering that only
MoO3 and MoO2 have no activity in the reaction under air and Ar
(Table S1), the combination of CeO2 and MoOx is essential for
Table 2. Michael addition of dimethyl malonate to 2-cyclohexen-1-one with
MoOx-Metal oxide catalysts
Selectivityb /%
Conversiona
Entry Metal oxide in MoOx-Metal oxide
/%
53
11
3
1
Others
1
2
CeO2
MgO
>99
<1
<1
<1
-
>99
3
La2O3
-Al2O3
-Al2O3
ZrO2
>99
4
<1
<1
<1
<1
<1
<1
<1
-
-
-
-
-
-
-
5
-
6
-
7
TiO2
-
Figure 2. Characterization of MoOx-CeO2 and CeO2. (A) XRD patterns of CeO2
(a) and MoOx-CeO2 catalysts with different Mo loading amounts (b-f). (B) TEM
image of MoOx-CeO2 (Mo: 1 wt%), (C) Mo 3d XPS of MoOx-CeO2 (Mo: 1 wt%)
before (i) and after (ii) the reaction. (D) Raman spectra of CeO2 (a) and MoOx-
CeO2 catalysts with different Mo loading amounts (b-f). (a) 0 wt% (CeO2), (b)
0.1 wt%, (c) 0.5 wt%, (d) 1 wt%, (e) 2 wt%, (f) 4 wt%).
8
SiO2-Al2O3
SiO2
-
9
-
10
La2O3
-
Reaction conditions: 2-cyclohexen-1-one 1.0 mmol, dimethyl malonate
1.1 mmol, MoOx-Metal oxide (Mo=1 wt%) 30 mg, toluene 1.5 mL, 303 K,
4 h, Ar. [ ]These values are on 2-cyclohexen-1-one basis.
a
2
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10.1002/cctc.202100682
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(a)
(b)
particles for MoOx species. This result agrees well with the results
of XRD analyses. In order to clarify the state of MoOx species on
MoOx-CeO2 catalysts, XPS and Raman analyses were conducted.
Mo 3d XPS of MoOx-CeO2 (Mo: 1 wt%) before and after the
reaction showed two main signals at 232 and 235 eV (Figure 2
(C)), which are assigned to Mo6+ species. These signals were not
changed before and after the reaction, suggesting that the
valence of MoOx species was not changed during the reaction.
The Raman spectra of MoOx-CeO2 catalysts with different Mo
loading amounts are shown in Figure 2(D). The bands at 460 and
595 cm-1 can be assigned to F2g vibration mode of fluorite
structure of CeO2 and the oxygen vacancies, respectively and the
intensity ratio was not changed, suggesting that the bulk structure
of CeO2 was not changed by addition of MoOx species. Focusing
on the band due to MoOx species, main signals around 908 and
810 cm-1 were detected below 1 wt% Mo loaded CeO2 catalysts,
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
0
6
12
18
24
0
6
12
18
24
Reaction time /h
Reaction time /h
Figure 3. Time-courses of Michael addition reaction with CeO2 (a) and MoOx-
CeO2 (b) catalysts
Reaction conditions: 2-cyclohexen-1-one 1.0 mmol, dimethyl malonate 1.1
mmol, CeO2 or MoOx/CeO2(Mo: 1 wt%) 30 mg, toluene 1.5 mL, 303 K, Ar. (:
Conversion, : Selectivity). Conversion and selectivity were based on 2-
cyclohexen-1-one.
2-
which can be assigned to the isolated tetrahedral MoO4
(a)
(b)
species.11 The signal was shifted to higher wavenumber with
increasing the Mo loading amount, and the bands around 950,
800 and 676 cm-1 can be assigned to the polymerized MoOx
clusters11. Therefore, MoOx-CeO2 (Mo: 1 wt%) has isolated
tetrahedral MoO42- species on CeO2. Based on the result that the
yield decreased with higher Mo loading amount than 1 wt% in
MoOx-CeO2 catalysts (Figure 1), the isolated Mo6+ species is an
important species and the interface between the isolated Mo
species and CeO2 is the main active site.
4.8
100
90
80
70
60
50
40
30
20
10
0
5
4
3
2
1
0
0.065
0.014
Bulk
0.0027
DBU
Surface Active site
MoOx-CeO2
0
6
12
18
24
Reaction time /h
CeO2
The time-courses of the reactions with MoOx-CeO2 (Mo: 1 wt%)
and CeO2 catalysts are shown in Figure 3. MoOx-CeO2 showed
higher conversion than CeO2. The conversions over MoOx-CeO2
and CeO2 catalysts were levelled off at about 60% and 20%,
respectively. XRD (Figure S1), BET (Figure S1) and XPS (Figure
2(C)) analyses of the used MoOx-CeO2 catalyst showed that the
structure was not changed, and hence the active site on the
surface of MoOx-CeO2 can be poisoned or covered by some
organic compounds during the reaction. TG-DTA analyses of
MoOx-CeO2 and CeO2 after 4 h reaction (Figure S2) showed an
exothermic peak and the corresponding weight loss, and the
weight loss over CeO2 is about three times larger than that over
MoOx-CeO2 catalyst. These results support that the active site
was covered or poisoned by the deposited organic compounds.
The reusability of MoOx-CeO2 was investigated (Figure S3).
When the used catalyst was applied to the next reaction without
calcination (only washing), the yield was drastically decreased by
a factor of about three (53% to 17%). In contrast, the calcination
treatment of the used catalyst at 573 K improved the catalyst
reusability, and more than 35% conversion was maintained at
fourth time reactions. These results also support that the active
site of the catalyst was covered by the deposited organic
compounds.
The MoOx-CeO2 amount was increased from 30 (standard) to
60, 90 and 120 mg to obtain a high yield of the target product
(Figure 4(a), and the details are in Table S2). The conversion
clearly increased with increasing the catalyst amount with high
selectivity of >99%. In the case of 120 mg MoOx/CeO2 catalyst,
92 and 99% yields were obtained at 1 and 24 h, respectively.
To evaluate the reaction rate of MoOx-CeO2 and CeO2 catalysts,
the time-courses at low conversion level were measured with 120
mg MoOx-CeO2 and CeO2 (Figure S4). The reaction rate, V (mmol
min-1 gcat-1) was calculated by the following equation: V (mmol min-
1 gcat-1) = (Formed target product (mmol))/ (Reaction time (min)) /
(Catalyst amount (g)). The V of MoOx-CeO2 and CeO2 was
Catalyst
Figure 4 The catalyst amount effect on the performance of Michael addition
reaction (a) and comparison of the activity of catalysts (b).
Reaction conditions of (a): 2-cyclohexen-1-one 1.0 mmol, dimethyl malonate 1.1
mmol, MoOx-CeO2(Mo: 1 wt%), toluene 1.5 mL, 303 K, Ar. (Black: 30 mg, green:
60 mg, blue: 90 mg, red: 120 mg). Yield was calculated on 2-cyclohexen-1-one
basis. The TOFs (Table S3) were calculated from Figure S4 (Table S2) and
Figure S5.
calculated to be 0.58 and 0.08 mmol min-1 gcat-1, respectively.
Since the specific surface areas of CeO2 (87 m2 g-1) and MoOx-
CeO2 (85 m2 g-1) are similar, the activity contribution of Mo-based
active sites of MoOx-CeO2 catalyst can be estimated to be at least
0.50 mmol min-1 gcat-1 (0.58 – 0.08 mmol min-1 gcat-1). Considering
that the main active site is the interface between isolated Mo
species and CeO2, the turnover frequency (TOF) based on the
active site, namely Mo metal, is calculated to be 4.8 min-1, and
those based on CeO2 and CeO2 surface is estimated to be 0.014
and 0.065 h-1, respectively (Figure 4(b) and Table S3).
The activity of MoOx-CeO2 was compared with that of DBU (1,8-
diazabicyclo[5.4.0]undec-7-ene),
a
strong organic base
(pKaH~24.3 in acetonitrile12, Figure S5), which was reported to be
effective for Michael reactions.13 DBU showed activity with a little
lower selectivity of ~96% than MoOx-CeO2, and the TOF of DBU
is calculated to be 0.0027 min-1 (Figure 4(b) and Table S3). The
TOF of the main active site over MoOx-CeO2 (4.8 min-1) was 1800-
fold higher than that of DBU. Considering that the basicity of the
CeO2 and MoOx is lower than that of DBU, the acid-base
bifunctionality of MoOx-CeO2 will play an important role in the
reaction. CeO2 is generally a basic metal oxide and the Lewis
acidity of the Mo cation in MoOx is higher than Ce cation in CeO2
due to the higher oxidation state of Mo6+. Therefore, CeO2 will act
as the Brønsted base for the activation of the C-H bond in the
dimethyl malonate, and MoOx will do as the Lewis acid site for the
activation of the carbonyl group in 2-cyclohexen-1-one. The
3
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10.1002/cctc.202100682
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COMMUNICATION
[9]
a) M. Tamura, H. Wakasugi, K.-i. Shimizu, A. Satsuma, Chem. Eur. J.
2011, 17, 11428-11431; b) M. Tamura, T. Tonomura, K.-i. Shimizu, A.
Satsuma, Green Chem. 2012, 14, 717-724; c) M. Tamura, T. Tonomura,
K.-i. Shimizu, A. Satsuma, Green Chem. 2012, 14, 984-991. d) M. Honda,
M. Tamura, Y. Nakagawa, S. Sonehara, K. Suzuki, K.-i. Fujimoto, K.
Tomishige, ChemSusChem 2013, 6, 1341-1344; e) M. Tamura, S. M. A.
H. Siddiki, K.-i. Shimizu, Green Chem. 2013, 15, 1641-1646; f) M.
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ACS Catal. 2015, 5, 20–26. h) Y. Wang, F. Wang, Q. Song, Q. Xin, S.
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Tomishige, Sci. Rep. 2016, 6, 24038; l) M. Honda, M. Tamura, Y.
Nakagawa, K. Nakao, K. Suzuki, K. Tomishige, J. Catal. 2014, 318, 95-
107; m) M. Tamura, M. Honda, K. Noro, Y. Nakagawa, K. Tomishige, J.
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details of the reaction mechanism and catalysis are under
investigation.
In conclusion, we found that MoOx-modified CeO2 (MoOx-CeO2)
with 1 wt% Mo was an effective and reusable heterogeneous
catalyst for Michael addition reaction, and the high yield (99%) of
the target product was obtained. Based on the catalyst
characterization results such as XRD, XPS, TEM and Raman,
isolated Mo6+ species were formed on CeO2 in MoOx-CeO2(Mo: 1
wt%) catalyst, and considering the experimental results, the
active site of the catalyst is the interface between the isolated
Mo6+ species and CeO2. The activity based on the active site is
74-fold and 1800-fold higher than that of CeO2 surface and a
typical strong organic base of DBU.
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This work was supported by JSPS KAKENHI Grant Number
JP20H04799.
Keywords: Heterogeneous catalyst • Molybdenum oxide •
Cerium oxide • Michael addition
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10.1002/cctc.202100682
ChemCatChem
COMMUNICATION
O
MoOx-CeO2
(Mo: 1 wt%)
303 K
O
O
O
O
O
+
O
O
O
O
4.8
5
4
3
2
1
0
Image of active site
and reaction mechanism
O
O
O
O
O
O
H
O
0.065
0.014
0.0027
Mo
O
O
Bulk Surface Active site DBU
Ce
Ce
Ce
MoOx-CeO2
Catalyst
CeO2
1wt% Mo loaded CeO2 (MoOx-CeO2) catalyst was an efficient and reusable heterogeneous catalyst for the Michael addition of
dimethyl malonate to 2-cyclohexene-1-one.
5
This article is protected by copyright. All rights reserved.