Highly efficient synthesis of cyclic ureas from CO2 and diamines by a pure CeO2 catalyst using a 2-propanol solvent

Article abstract of DOI:10.1039/c3gc40495a

Pure cerium oxide (CeO₂) acts as an effective and reusable heterogeneous catalyst for direct synthesis of cyclic ureas from CO₂ and diamines even at a low CO₂ pressure of 0.3 MPa. 2-Propanol is the most preferable solvent to provide good selectivity. The system composed of a CeO₂ catalyst and a 2-propanol solvent is applied to various diamines to provide the corresponding cyclic ureas in high yields (78-98%), including six-membered-ring ureas that are difficult to be synthesized from CO ₂. Based on the kinetic studies on the effect of CO₂ pressure and amine concentration and FTIR studies on adsorption of ethylenediamine and CO₂ onto CeO₂, the following mechanism for the synthesis of cyclic urea is proposed: (1) formation of carbamic acid and carbamate species on CeO₂, (2) decomposition of carbamic acid to a free amino group and CO₂, (3) nucleophilic attack of the amino group on the carbamate on CeO₂ to produce the cyclic urea and (4) desorption of the product and regeneration of CeO₂.

Full text of DOI:10.1039/c3gc40495a

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Highly efficient synthesis of cyclic ureas from CO2 and
diamines by a pure CeO2 catalyst using a 2-propanol
solvent†
Cite this: DOI: 10.1039/c3gc40495a
Masazumi Tamura, Kensuke Noro, Masayoshi Honda, Yoshinao Nakagawa and
Keiichi Tomishige*
Pure cerium oxide (CeO2) acts as an effective and reusable heterogeneous catalyst for direct synthesis of
cyclic ureas from CO2 and diamines even at a low CO2 pressure of 0.3 MPa. 2-Propanol is the most prefer-
able solvent to provide good selectivity. The system composed of a CeO2 catalyst and a 2-propanol
solvent is applied to various diamines to provide the corresponding cyclic ureas in high yields (78–98%),
including six-membered-ring ureas that are difficult to be synthesized from CO2. Based on the kinetic
studies on the effect of CO2 pressure and amine concentration and FTIR studies on adsorption of ethylene-
diamine and CO2 onto CeO2, the following mechanism for the synthesis of cyclic urea is proposed: (1)
formation of carbamic acid and carbamate species on CeO2, (2) decomposition of carbamic acid to a free
amino group and CO2, (3) nucleophilic attack of the amino group on the carbamate on CeO2 to produce
the cyclic urea and (4) desorption of the product and regeneration of CeO2.
Received 15th March 2013,
Accepted 11th April 2013
DOI: 10.1039/c3gc40495a
www.rsc.org/greenchem
synthetic method of cyclic ureas from CO2 and diamines is
required (eqn(1)).
Introduction
Carbon dioxide is one of the greenhouse gases and its conver-
sion to useful compounds will be an essential technology for
the future sustainable society without the anxiety of global
warming issues.1 The reductive CO2 conversion is one of the
transformation methods of CO2, but in general, the energy
level of CO2 is so low that the reductive conversion needs a
larger energy input using highly reactive reducing agents such
as H2.2–4 As an alternative method, non-reductive conversion is
promising because the reaction can proceed with a low energy
input. The compounds given by non-reductive conversion
include carbonates, carbamates, ureas and so on.1c,5 Among
these compounds, ureas are an important class of carbonyl
compounds and useful chemical intermediates in the syn-
thesis of pharmaceuticals, agricultural chemicals and dyes as
well as antioxidants in gasoline and additives in plastics.
Cyclic ureas are heterocyclic motifs frequently observed in bio-
logically active molecules, and in particular six-membered ring
ureas are important6–9 because of their antineoplastic,7 anti-
viral8 and anti-arrhythmic9 activity. Therefore, an effective
ð1Þ
Conventionally cyclic ureas are synthesized by the reaction
of diamines with several reagents such as phosgene, urea, car-
bonyldiimidazole,10 trichloromethyl chloroformate,11 organic
carbonates,6,12 dithiocarbonate,13 carbonyl selenide14 and
CO.15 However, these reagents are intrinsically very toxic or
prepared from hazardous chemicals such as phosgene, CO
and CS2. From the environmental viewpoint, reaction of CO2
with diamines will provide an attractive alternative method for
the direct synthesis of cyclic ureas. Direct reaction of CO2 with
diamines to cyclic ureas has been conducted using various
catalyst systems.16–24 Effective homogeneous systems for this
reaction such as Ph3SbO/P4S10,16 PhTMG/DPPA17 and Hünig’s
base18 were reported, and recently Mizuno19 reported that
TBA2[WO4] is an efficient homogeneous catalyst for the syn-
thesis of cyclic ureas from diamines and CO2 of atmospheric
pressure. From the viewpoints of catalyst separation and re-
cycling, heterogeneous catalysts or non-catalytic systems are
preferable to homogeneous catalysts.20–23 Arai20 and Zhao21
developed non-catalytic systems for the direct CO2 conversion
to cyclic ureas. However, these systems suffer from harsh reac-
tion conditions such as high pressure (≥6.0 MPa) and high
Graduate School of Engineering, Tohoku University, Aoba 6-6-07, Aramaki, Aoba-ku,
Sendai, 980-8579 Japan. E-mail: tomi@erec.che.tohoku.ac.jp; Fax: +81-22-795-7214;
Tel: +81-22-795-7214
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3gc40495a
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temperature (≥473 K), and cannot achieve a satisfactory yield room temperature and the gas was collected. Methanol and
of six-membered-ring urea in spite of such harsh conditions 1-hexanol were added to the liquid phase as a solvent and
and the yields are below 86%. As for heterogeneous catalysts, an internal standard substance for a quantitative analysis,
only two effective catalysts were reported. Polyethylene-glycol- respectively. The reactor was washed with methanol and water,
supported potassium hydroxide (KOH/PEG1000)23 was an and the liquids used in washing were added to the reaction
efficient catalyst at 8 MPa CO2 pressure and 423 K, which mixture. The products in the liquid and the gas phases were
suffered from high pressure, narrow scope of substrates and analyzed by a gas chromatograph equipped with an FID or
22
low yields of cyclic ureas (≤82%). Nanoparticulate CeO2 pre- quadrupole mass spectrometer (GC-MS) using an InertCap for
pared by a biopolymer-template process using mesoporous amines 5 MS/Sil capillary column (GL science Inc., length =
alginate aerogel was an efficient catalyst at 0.7 MPa CO2 30 m, I.D. = 0.25 mm).
pressure and 433 K, however, this catalyst has drawbacks of
Conversion and selectivity to each product in the reaction
narrow scope of substrates and low yields of cyclic ureas of CO2 + diamine were calculated on the basis of diamines as
(≤37%). As above, there is no green system that can achieve below.
satisfactory yields of cyclic ureas, particularly six-membered-
ꢀ
ꢁ
Amount of diamine after the reaction
Amount of diamine before the reaction
ring ureas, at low pressure (<1.0 MPa) and moderate tempera-
ture (<453 K). Therefore, effective reaction systems are desired
to be developed from the industrial and academic viewpoints.
Our research group has reported that pure CeO2 activates
CO2 effectively and acts as an efficient heterogeneous catalyst
for the direct synthesis of organic carbonates and carbamates
from CO2.24 It has been also found that the solvent is clarified
to be one of the most important factors to obtain high yield
and selectivity.25 In this paper, we demonstrate that the cata-
lyst system composed of pure CeO2 and 2-propanol solvent is
effective for the direct synthesis of cyclic ureas from CO2 and
diamines under mild reaction conditions. This system can be
applied to various diamines to afford the corresponding cyclic
ureas in excellent yields, including six-membered-ring ureas.
The reaction mechanism over CeO2 is also discussed on the
basis of the kinetics and FTIR studies.
Conversion=% ¼ 1 ꢀ
ꢁ 100
Amount of each product
Amount of reacted diamine
Selectivity=% ¼
ꢁ 100
The products which are not identified by GC-MS and NMR
are denoted as “Others”. The amount of “Others” was calcu-
lated as below.
Selectivity of others=% ¼
Amount of reacted diamine ꢀ Amount of identified products
Amount of reacted diamine
ꢁ 100
The reaction rate (V/mmol h−1 g−1) was calculated as below.
V ðmmol hꢀ1 gꢀ1Þ ¼
Experimental
General
Amount of diamine before the reaction ꢁ Conversion=100ꢁ
Selectivity of product=100
Preparation of a pure CeO2 catalyst was carried out by calcina-
tion of cerium oxide HS (Daiichi Kigenso, Japan) for 3 hours
under air at 873 K, which is an optimized calcination tempera-
ture of CeO2 according to the result of carbonate synthesis.24a
The specific surface area (BET method) of pure CeO2 was
80 m2 g−1. The purity of pure CeO2 is 99.97%. All the chemi-
cals for organic reactions were commercially available and
were used without further purification.
Time ꢁ Amount of catalyst
The test of reusability of CeO2 is conducted as follows: the
used catalysts were collected by the decantation. The collected
catalysts were washed with methanol and dried at 383 K for
2 h, followed by calcination in air at 873 K for 3 h. After this
treatment, the recovered catalyst is applied to the successive
reaction. Multiple runs were conducted at the same time
under the same conditions to collect enough amount of the
Catalytic reaction
All the reactions were carried out in an autoclave reactor with used catalyst, and the loss of the catalyst was compensated by
an inner volume of 190 mL. The standard procedure for the reducing the number of runs.
cyclic urea synthesis from CO2 and diamine (CO2 + diamine)
Catalyst characterization. Diffuse reflectance infrared
was as follows: CeO2 catalyst (0.34 g, 2 mmol), diamine Fourier transform (DRIFT) spectra were recorded with a
(10 mmol) and solvent (200 mmol) were put into the autoclave NICOLET 6700 spectrometer (Thermo Scientific) equipped
together with a spinner, and then the reactor was purged with with a liquid nitrogen-cooled MCT(HgCdTe) detector (resolu-
CO2 (Shimakyu Co. Ltd., >99.5%). After that, it was pressurized tion 4 cm−1), using an in situ cell with ZnSe windows. An
with CO2 to the desired pressure (typically 0.5 MPa). The gas in situ cell was used for high-temperature and high-pressure
line was closed and then the reactor was heated to the reaction gas flow systems. The measurement of the adspecies of ethylene-
temperature. The mixture was constantly stirred during the diamine and CO2 is carried out in the following method;
reaction. After the reaction time, the reactor was cooled to ethylenediamine (ethylenediamine/metal oxides = 2 wt%) is
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added to the metal oxide (SiO2 or pure CeO2) and the ad- of high CO2 pressure. Therefore, addition of an effective cata-
mixture was stirred vigorously. About 30 mg of the mixture was lyst is indispensable to make the reaction conditions much
transferred to the ceramic pan in an in situ FTIR cell connected milder. For various metal oxides, turnover frequency (TOF) was
to the flow system. The loaded sample was heated up to the measured under the conditions in which the conversions were
desired temperature (303 K or 433 K) under N2 flow (50 mL below 40%. TOF is calculated from the following equation:
min−1), and the spectrum of adsorbed ethylenediamine was TOF = (amount of produced 1 (mol)/total amount of the metal
measured. Next, CO2 was introduced into ethylenediamine- atom (mol)). ZnO and CeO2 gave higher conversion and TOF
treated CeO2 in the flow, and then pressurized up to 5 MPa than the other metal oxides (Table 1, entries 1 and 3). As
while the sample temperature was kept at the desired tempera- for selectivity, CeO2 shows a higher value of 93% (Table 1,
ture. All the spectra were obtained 5 min after the introduction entry 1), however, the value shown by ZnO is very low (Table 1,
of CO2. The spectra were obtained by subtraction of the entry 3, 68%). The results of the reaction at longer time were
spectrum derived from air and the apparatus.
also compared between CeO2 and ZnO catalysts (Table 1,
The surface area of CeO2 was measured with the BET entries 2 and 4). The reaction over pure CeO2 smoothly pro-
method (N2 adsorption) using Gemini (Micromeritics). X-ray ceeded to afford 94% yield in 12 h. On the other hand, the
diffraction (XRD) patterns were recorded using a Rigaku conversion over ZnO was saturated and the yield was
Ultima IV with Cu Kα (40 kV, 40 mA) radiation.
decreased. In the case of ZnO, insoluble material was observed
in the reaction mixture, particularly on ZnO, which implies
that the polymerization reaction occurred. The produced
polymer may cover the surface of ZnO, leading to deactivation
of ZnO. From the above results, pure CeO2 is the most effective
catalyst for the direct synthesis of 1 from CO2 and ethylenedi-
amine among the various metal oxides tested and the yield of
1 was highest for the reported heterogeneous catalysts (KOH/
PEG1000:23 82%, nanoparticulate CeO2:22 37%). Low yield of
nanoparticulate CeO2 might be due to impurities such as Na
because the nanoparticulate CeO2 in the literature was pre-
pared using sodium alginate.
In order to verify whether the observed catalysis is derived
from solid CeO2 or leached Ce species in the solution, the
direct synthesis of 1 from CO2 and ethylenediamine was
carried out under the above conditions for 1 h to reach 21%
yield of the product, and then CeO2 was removed from the
reaction mixture by filtration (Fig. S1†). Then, the reaction was
tested with the filtrate under the same conditions again. After
12 h, the conversion slightly increased and the increasing
amount of 1 was 5.2% in 11 h (∼0.5%/h), which is derived
from a non-catalyzed reaction considering that 1 is produced
at 0.8% yield in 1 h in the absence of the catalyst (Table 1,
entry 12). This means that the catalytically active species are
not due to the leached species from CeO2. Therefore the
observed catalysis is essentially heterogeneous. A further
advantage of this catalytic system is its reusability. We studied
the reuse of CeO2 for the reaction of CO2 + ethylenediamine
(Fig. 1). The catalyst can be easily retrieved from the reaction
mixture by decantation. CeO2 was reused at least three times
without marked loss of its catalytic activity and selectivity.
Moreover, the specific surface area of CeO2 hardly changed
after the first and second uses (fresh: 80 m2 g−1, first use:
83 m2 g−1, second use: 83 m2 g−1), and XRD (Fig. S2†) and TG
profiles (Fig. S3†) did not change after the first use, either.
These results indicate that the structure of CeO2 maintained
through the recycle tests.
Results and discussion
Catalytic performance in the reaction of CO2 +
ethylenediamine
First, various metal oxides were tested as catalysts for the for-
mation of 2-imidazolidinone (1) from 0.5 MPa CO2 and ethylene-
diamine at 433 K (Table 1). Formation of 1 hardly proceeded
without a catalyst (Table 1, entry 12) under these conditions
(yield 0.8% in 1 h). In the reported non-catalytic systems,20,21
high CO2 pressure (above 5 MPa) and high temperature (above
423 K) are required. For example, 96.9% yield of 1 was
obtained in 24 h at 10 MPa CO2 pressure and 453 K.21 The
effect of CO2 pressure is very large, which means the necessity
Table 1 The results of the reaction of ethylenediamine and CO2 in methanol
over various metal oxidesa
Selectivity (%)
SA
Conversion
(%)
TOFb
Entry Catalyst
(m2 g−1
)
1
Others (h−1
)
1
Pure CeO2
ZnO
78
22
98
40
42
3.1
5.0
2.0
2.6
2.1
5.5
0.8
0.8
93
96
68
7
32
16
95
23
90
13
88
87
7
4
32
93
68
84
5
77
10
87
12
13
4.0
—
5.4
—
0.20
0.16
0.38
0.12
0.38
0.14
0.14
—
2c
3
5.8
4d
5
6
7
8
CaO
La2O3
TiO2
MgO
ZrO2
Pr6O11
Al2O3
None
12
7.0
48
37
91
33
182
—
9
10
11
12
The reaction conditions for the synthesis of 1 from CO2 and
ethylenediamine on CeO2 as a model reaction were optimized.
The effect of reaction temperature was examined in a tempera-
ture range of 413–453 K (Fig. 2). Increase in the temperature
a Reaction conditions: ethylenediamine (10 mmol), methanol (6.4 g),
metal oxide (metal: 0.2 mmol), PCO = 0.5 MPa, T = 433 K, t = 1 h.
b Molar ratio of the converted subs2trate to the total amount of the
metal atom per hour. c 12 h. d 4 h.
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Table 2 Effect of the concentration of ethylenediamine in the reaction of ethy-
lenediamine and CO2 over CeO2 in methanola
Selectivity (%)
Diamine
(mmol)
Conversion
(%)
Reaction rateb
(mmol h−1 g−1
)
Entry
1
Others
1
2
3
4
5
6
2.5
5
10
15
20
25
88
44
22
19
15
12
86
95
93
93
83
83
14
5
7
8
16
16
—
3.19
3.17
3.41
3.37
3.37
a Reaction conditions: ethylenediamine (2.5–25 mmol), methanol
(6.4 g), CeO2 (0.086 g), PCO = 0.5 MPa, T = 433 K, t = 1 h. b Reaction
rate was calculated und2er the reaction conditions where the
conversion is below 50%.
of CO2 pressure, where the selectivity is decreased owing to
increase of byproducts including N-alkylated products. At 12 h
reaction time (Fig. 3b), ethylenediamine is converted almost
completely to 1 at more than 0.3 MPa CO2 pressure with very
high selectivity (>92%). However, at 0.2 MPa CO2 pressure the
conversion is 54% and the selectivity is also low. Note that
CeO2 achieved the high conversion and selectivity even at a
very low CO2 pressure of 0.3 MPa, where neither non-catalytic
systems nor reported heterogeneous catalysts worked effec-
tively: reported non-catalytic systems and reported hetero-
geneous catalysts required ≥8 MPa and ≥0.7 MPa CO2
pressure, respectively. Ultimately, we selected 0.5 MPa CO2
pressure in the following experiments.
Fig. 1 Recycle test of CeO2 for the synthesis of 1 from CO2 and ethylenedi-
amine. Reaction conditions: ethylenediamine (10 mmol), methanol (6.4 g), CeO2
(0.085 g), PCO = 0.5 MPa, T = 433 K, 1 h.
2
Solvent effect and substrate scope of CeO2 catalysts
The solvent effect for the synthesis of 1 from CO2 and ethylene-
diamine was investigated and the results are listed in Table 3.
Under the low conversion conditions, the catalytic perform-
ance is not influenced so significantly by the solvents (Table 3,
entries 1, 3, 4, 6, 8 and 9), which indicates that the interaction
of solvents with CeO2 is weak. On comparing the details of the
solvent effect, 1-propanol and 2-propanol provided slightly
higher conversion and selectivity (Table 3, entries 4 and 6).
Even under the high conversion conditions, 1-propanol and
2-propanol provide higher selectivity than methanol (Table 3,
entries 2, 5 and 7), and these selectivities are almost 100%. To
ascertain that these alcohols are truly efficient for improve-
ment of selectivity, 2-propanol which is a cheap and fairly non-
toxic solvent was applied to various cyclic urea syntheses.
Cyclic urea syntheses from CO2 and various diamines were
examined (Table 4). The effect of the solvent on selectivity was
also examined using both methanol and 2-propanol. In the
case of methanol as a solvent, ethylenediamine, 1,2-propane-
diamine and 2-methyl-1,2-propanediamine (Table 4, entries 1,
3 and 5) reacted to afford the corresponding cyclic ureas in
high yields (86–94%). N-Alkyl ethylenediamines (Table 4,
entries 7, 9 and 11) were also converted selectively. 1,3-Propa-
nediamine and N-methyl-1,3-propanediamine were applied to
provide the six-membered-ring ureas (Table 4, entries 15 and
17). However, N,N′-dimethylethylenediamine (Table 4, entry
Fig. 2 Effect of temperature on formation of 1 (white bar; 1, black bar;
others). Reaction conditions: ethylenediamine (10 mmol), methanol (6.4 g),
CeO2 (0.085 g), PCO = 0.5 MPa, T = 413–433 K, 1 h.
2
from 413 to 453 K raised the conversion from 5 to 39%. On the
other hand, the selectivity was maintained to be very high up
to 443 K, but was decreased at 453 K, accompanying that the
N-alkylated product, a main byproduct, was formed. The effect
of the concentration of ethylenediamine was examined by
changing the amount of the diamine in a range 2.5–25 mmol
(Table 2). The selectivity was high between 5 and 15 mmol
(Table 2, entries 2–4), however, the selectivity became low
when the diamine concentration is out of the range (Table 2,
entries 5and 6). Ultimately, 10 mmol of the diamine to 6.4 g
methanol was adopted. The effect of CO2 pressure was exami-
ned by changing CO2 pressure from 0.2 to 3.0 MPa (Fig. 3). At
1 h reaction time (Fig. 3a), the conversion was almost constant
in the range of 1.0–3.0 MPa, but increased slightly as CO2
pressure decreased from 1.0 to 0.2 MPa. On the other hand,
the selectivity was almost constant except for 0.2 and 3.0 MPa
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Fig. 3 Effect of CO2 pressure on formation of 1 after (a) 1 h and (b) 12 h (white bar; 1, black bar; others). Reaction conditions: ethylenediamine (10 mmol), metha-
nol (6.4 g), CeO2 (0.085 g), PCO = 0.2–3.0 MPa, T = 433 K, 1 h or 12 h.
2
a
amines with alcohols will be considered in the following two
possible pathways (Scheme 1); (1) dehydrogenation of alcohol,
nucleophilic addition of amine to a carbonyl group producing
imine and hydrogenation of imine26,27 and (2) N-alkylation
with dialkylcarbonate.28 The pathway (1) has been widely
reported by transition-metal complexes such as Ir, Ru, Pd,
etc.,26 and strong bases27 such as alkaline hydroxide or alka-
line alkoxide are also known to catalyze the reaction pathway.
However, these strong base catalysts require harsh reaction
conditions such as high temperature (>473 K). Considering
that the basicity of CeO2 is lower than such strong bases and
the temperature of the present reaction system is compara-
tively low (433 K), the contribution of the pathway (1) must be
small. On the other hand, as described in the introduction,
CeO2 is an effective catalyst for the dialkylcarbonate synthesis
from CO2 and alcohols.24a–d It is reported that N-alkylation of
amines by dialkylcarbonates proceeds at a low temperature of
363 K using base catalysts such as alkaline alkoxide.28c The
reported results suggest that the pathway (2) will be valid as a
side reaction in our catalyst system; dialkylcarbonate will be
in situ produced on CeO2 and the produced carbonate will play
the role in the alkylation agent. The yields of N-alkylated pro-
ducts in methanol and in 2-propanol were compared (Table 4).
The yields of N-alkylation products by methanol in entries 7,
11, 13 and 17 are 0.3%, 3%, 9% and 11%, respectively, and the
yields of N-alkylation products by 2-propanol in entries 8, 12,
14 and 18 are almost 0%. N-Alkylation of amines was remark-
ably suppressed using 2-propanol as a solvent. Taking into
consideration that N-alkylation of amines proceeds via the
pathway (2) in Scheme 1, the bulky alkyl group of 2-propanol
will inhibit the insertion of alcohol into CO2 owing to the
steric hindrances of the alkyl chain in the alcohol, which
coincides with the previous result that secondary alcohol is
less reactive for the synthesis of dialkylcarbonate than metha-
nol on CeO2.24d Therefore, the structure of an alcohol solvent
is the most important factor to suppress the side reaction and
achieve high yields of cyclic ureas.
Table 3 Solvent effect for the reaction of ethylenediamine and CO2 over CeO2
Selectivity (%)
Conversion
Entry
Solvent
(%)
1
Others
1
Methanol
22
98
21
26
97
26
97
20
17
93
96
95
96
98
96
99
70
88
7
4
5
4
2
4
1
30
12
2b
3
Ethanol
1-Propanol
4
5b
6
2-Propanol
7b
8
Acetonitrile
1-Methyl-2-pyrrolidone
9
a Reaction conditions: ethylenediamine (10 mmol), solvent
(200 mmol), CeO2 (0.086 g), PCO = 0.5 MPa, T = 433 K, t = 1 h. b CeO2
2
(0.34 g), t = 12 h.
13) was converted in low yield (43%). Using 2-propanol as a
solvent instead of methanol provided high yield (78%) with
high selectivity (94%) (Table 4, entry 14). Other diamines
(Table 4, entries 2, 4, 6, 8, 10, 12, 16 and 18) were also trans-
formed using 2-propanol to afford the corresponding products
in high yields (88–96%). These results indicate that 2-propanol
is a more preferable solvent for the synthesis of various cyclic
ureas. Note that 1,3-propanediamines (Table 4, entries 15–18),
which are known to be more difficult substrates to give
the cyclic urea, can be converted to the corresponding six-
membered-ring ureas, which are core structures of important
medicines for antineoplastic, HIV-1 and anti-arrhythmic.6–9
These yields are higher than the previously reported non-
catalyst systems (max yield: 86%) and there are no reports on
heterogeneous catalysts. Therefore, this catalytic system com-
posed of CeO2 and 2-propanol solvent can be promising in the
aspect of practical use.
To reveal the effect of the solvent, we focused on the bypro-
duct in the synthesis of cyclic ureas. In this reaction system,
the main side reaction is N-alkylation of amines with alcohols
(Table 4), and CeO2 will catalyze this reaction. N-Alkylation of
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Table 4 Scope of diamines for the cyclic urea synthesisa
Yield
Time Conv. (%) Selectivity to cyclic urea (%) Cyclic urea (%) N-Alkylated amine (%)
Entry Diamine
Cyclic urea Solvent
Methanol
1
2
12
98
97
96
99
94
96
Trace
Trace
2-Propanol 12
3
4
Methanol
2-Propanol 12
12
97
98
97
98
94
96
n.a.b
n.a.b
5
6
Methanol
2-Propanol 12
12
90
92
96
98
86
90
n.a.b
n.a.b
7
8
Methanol
2-Propanol 24
24
86
99
94
99
81
98
0.3
Trace
9
10
Methanol
2-Propanol 24
24
83
98
94
98
78
96
n.a.b
n.a.b
11
12
Methanol
2-Propanol 24
24
81
97
93
99
75
96
3
Trace
13
14
Methanol
2-Propanol 24
24
55
83
78
94
43
78
9
Trace
15
16
Methanol
2-Propanol 24
24
86
93
81
97
70
90
n.a.b
n.a.b
17
18
Methanol
2-Propanol 36
24
81
90
85
98
69
88
11
Trace
a Reaction conditions: substrate (10 mmol), solvent (200 mmol), CeO2 (0.34 g), PCO = 0.5 MPa, T = 433 K. b Not analyzed.
2
Scheme 1 Possible reaction pathways for N-alkylation of amine with alcohol on CeO2.
FTIR study and kinetic study
an autoclave. At first, adspecies formed from the treatment of
ethylenediamine with CeO2 and SiO2 at a low temperature
To elucidate the reaction mechanism of cyclic urea synthesis of 303 K were observed by FTIR. Fig. 4 shows the spectra of
from CO2 and diamines over CeO2, kinetic and FTIR studies ethylenediamine adspecies on SiO2 and CeO2. The main bands
were carried out. The adsorption order of the substrates in at 1574 cm−1 for CeO2 and 1601 cm−1 for SiO2 were observed
FTIR experiment obeyed the actual synthesis of cyclic ureas in and were assigned to N–H bending vibrations of adsorbed
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Fig. 4 FTIR spectra of adspecies of ethylenediamine on CeO2 and SiO2 at
303 K in the N–H bending vibration region.
Fig. 5 FTIR spectra of adspecies on CeO2. (a) CeO2 treated with ethylenedi-
amine at 303 K and after introduction of CO2 of (b) atmospheric pressure and
(c) 5 MPa at 303 K to (a).
ethylenediamine species. According to the previous report,29
the NH2 group produced by coordinatively bonded n-butyl-
amine on Lewis acid sites shows bands around
1600–1565 cm−1. In addition, the band for CeO2 was at a lower
wavenumber than that for SiO2. These results indicate that
ethylenediamine is coordinatively adsorbed on the Lewis acid
sites on CeO2. The observed large broad bands on SiO2 below
1550 cm−1 are derived from the absorption of SiO2 itself due to
the stretching vibrations of Si–O–Si bridges.30 Next, introduc-
tion of CO2 into the ethylenediamine-treated CeO2 was con-
ducted at 303 K. Fig. 5 shows IR spectra change after
introductions of atmospheric pressure (Fig. 5b) and 5 MPa
CO2 (Fig. 5c). New bands at 1603 and 1633 cm−1 were observed
after CO2 introduction and these bands increased with increas-
ing CO2 pressure. The band at 1603 cm−1 will be due to ν(CO3)
of hydrogen carbonate, which is derived from CO2 adsorption
on CeO2 (Fig. S4†).31 According to the literature,32 the band
around 1630 cm−1 can be assigned to carbamic acid or carba-
mate adspecies. Taking the basicity of CeO2 into account, the
carbamate species on CeO2 will be formed by the reaction of
diamine adsorbed on CeO2 with CO2. As shown in Scheme 2
route 1, diamine is adsorbed on CeO2 and the diamine adspe-
cies on CeO2 reacts with CO2 to give the carbamate adspecies
on CeO2 and a free carbamic acid. On the other hand, it is
reported that CO2 reacts with diamine at room temperature to
readily produce dicarbamic acid without a catalyst.33 There-
fore, under the reaction conditions diamine may be firstly con-
verted to dicarbamic acid before contacting CeO2, and then
the dicarbamic acid may be adsorbed on CeO2 to provide the
carbamate adspecies on CeO2 (Scheme 2, route 2). Next, ethyl-
enediamine-treated CeO2 was measured at the reaction temp-
erature of 433 K, followed by the introduction of atmospheric
pressure and 5 MPa CO2 on FTIR. As shown in Fig. 6a, ethyl-
enediamine was not desorbed even at 433 K, which indicates
that ethylenediamine is strongly adsorbed onto the CeO2
surface at the reaction temperature. After CO2 introduction,
the main two bands at 1668 and 1572 cm−1 were increased
with increase of CO2 pressure (Fig. 6b and c). These bands
were located at almost the same positions as those of 1
adsorbed on CeO2 (Fig. S5†) and were assignable to the stretch-
ing vibrations of CvO and CO–N–H, respectively.34 Therefore,
the carbamate species on CeO2 is easily converted to 1 at
433 K. A small band observed by treatment of ethylenediamine
Scheme 2 Formation route of carbamate species on CeO2.
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Fig. 7 Effect of amine concentration on the formation rate of 1 from CO2 and
ethylenediamine. Reaction conditions: ethylenediamine (5.0–25 mmol), metha-
Fig. 6 FTIR spectra of adspecies on CeO2. (a) CeO2 treated with ethylenedi-
amine at 433 K and after introduction of CO2 of (b) atmospheric pressure and
(c) 5 MPa at 433 K to (a).
nol (6.4 g), CeO2 (0.085 g), PCO = 0.5 MPa, T = 433 K, 1 h.
2
in Fig. 6a will be produced by the reaction of ethylenediamine
with CO2 adspecies on the CeO2 surface. Moreover, CvO
stretching vibration (1668 cm−1) of 1 adsorbed on CeO2 had a
lower wavenumber than that of liquid phase 1 (around
1686–1670 cm−1), which indicates that 1 was adsorbed on
CeO2 at the carbonyl group (CvO) of 1. The carbamate species
on CeO2 can be transformed into 1 not at 303 K but at 433 K,
which suggests that cyclic urea formation from carbamate
species on CeO2 is a slower step in the reaction cycle, in that
the step will be the rate-determining step.
Next, kinetics for the synthesis of 1 from CO2 and ethylene-
diamine as a model reaction were studied. The dependence of
the reaction rate on the ethylenediamine concentration or CO2
pressure was examined using the results of Table 2 and Fig. 3.
The reaction rate (V/mmol h−1 g−1) for the effect of diamine
concentration and CO2 pressure was calculated under the reac-
tion conditions where the conversion was below 50%. The
reaction rate was plotted as a function of ethylenediamine con-
Fig. 8 Effect of CO2 pressure on the formation rate of 1 from CO2 and ethyle-
nediamine. Reaction conditions: ethylenediamine (10 mmol), methanol (6.4 g),
centration as shown in Fig. 7. The reaction order of ethylene- CeO2 (0.085 g), PCO = 0.2–3.0 MPa, T = 433 K, 1 h.
2
diamine concentration is estimated to be +0.16, which
indicates that free ethylenediamine is not involved in the rate-
determining step or strongly adsorbed onto the CeO2 surface.
Table 5 Comparison of the initial reaction rate of N-alkyl diamines in 2-propa-
Fig. 8 shows the effect of CO2 pressure on the reaction rate.
The reaction order of −0.16 was obtained, which indicates that
CO2 does not affect the rate-determining step or slightly sup-
presses the reaction proceeding. The reactivity of N-alkyl di-
amines was also compared under the conditions where the
conversion is below 30% (Table 5). The order of reactivity is as
follows: ethylenediamine > N-methyl ethylenediamine ≈ N-iso-
propyl ethylenediamine. The amines having some alkyl groups
at the N atom decrease the reactivity, which will be attributed
to the steric hindrance of N-alkyl chains and/or basicity of
amines. The alkyl chain of amines will inhibit the nucleophilic
attack of amines, leading to the decrease of the reactivity.
Basicity of amines is increased by the electron inductive effect
nol solventa
Entry
1
Diamine
Conv. (%)
26
Selectivity to cyclic urea (%)
96
2
3
15
17
93
94
a Reaction conditions: substrate (10 mmol), 2-propanol (12 g), CeO2
(0.086 g), PCO = 0.5 MPa, T = 433 K, 1 h.
2
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of an N-alkyl group, which will stabilize the carbamic acid. erature of 433 K by introduction of CO2 into ethylenediamine-
Stabilization of carbamic acid reduces the nucleophilicity of treated CeO2 on FTIR (Fig. 6b and 6c). Considering that at
the N atom because nucleophilicity of carbamic acid is weaker 303 K the carbamate species was formed but 1 was not formed
than the free amine, leading to the decrease of the reactivity. on CeO2, 1 should be produced by the reaction of the carba-
This result shows that nucleophilic addition of amines is an mate species on CeO2. In general, carbamic acid is decom-
important step in the reaction, which suggests that the nucleo- posed to CO2 and amine at above 383 K, which is much lower
philic attack of an amino group on the carbamate species on than the reaction temperature (433 K). Since the nucleophili-
CeO2 is the rate-determining step.
city of the N atom of carbamic acid is lower than that of the
amine, the reaction proceeds through nucleophilic addition of
a free amino group that is produced by the decomposition of
carbamic acid, which is the rate-determining step. The com-
parison of reactivity for N-alkylated diamines presents that
reactivity of N-alkylated substrates is lower than ethylenedi-
amine (Table 5), which will be derived from steric hindrance
and/or basicity of the N-alkylated amino group. This result
shows that reactivity of an amino group is important for
the reaction, which supports that nucleophilic addition of a
free amino group to a carbamate moiety on CeO2 is the rate-
determining step. In addition, the reaction rate shows a little
negative dependence on CO2 pressure (Fig. 8). At high CO2
pressure, carbamic acid will be more stable than at low CO2
pressure because decomposition of carbamic acid is a rever-
sible reaction, leading to the suppression of decomposition of
carbamic acid. This result also supports that nucleophilic
addition of a free amino group to a carbamate moiety on CeO2
is the rate-determining step.
In summary, the proposed reaction mechanism for the syn-
thesis of 1 from CO2 and ethylenediamine as a model reaction
is shown in Scheme 3: (1) adsorption of amine and CO2 to
afford carbamic acid and carbamate species on CeO2, (2)
decomposition of the carbamic acid to amine and CO2, (3)
nucleophilic addition of the amino group to the carbamate
moiety on CeO2, providing the cyclic urea, (4) desorption of
product 1 and regeneration of CeO2. The third step is the rate-
determining step.
Reaction mechanism
On the basis of the above experimental results, we discuss the
reaction mechanism on CeO2 for the direct cyclic urea syn-
thesis from CO2 and diamine. From FTIR studies on adsorp-
tion of ethylenediamine at 303 and 433 K (Fig. 4 and 6a),
ethylenediamine is adsorbed on Lewis acid sites of CeO2 and
did not desorb from CeO2 even at 433 K, which indicates that
ethylenediamine is strongly adsorbed on the Lewis acid sites
of CeO2 under the reaction conditions. Introduction of CO2
into ethylenediamine-treated CeO2 at a low temperature of
303 K demonstrates that ethylenediamine reacted with CO2 on
CeO2 to afford carbamates species on CeO2 at 303 K (Fig. 5)
and the other amino group forms the carbamic acid. On the
other hand, in general, ethylenediamine easily reacts with CO2
at room temperature to provide dicarbamic acid. An adsorp-
tion of the carbamic acid on CeO2 probably forms the carba-
mate species on CeO2, too (Scheme 2). As shown in Table 3,
the effect of solvents was hardly observed, which indicates that
the interaction of alcohol with the CeO2 surface is weaker than
diamines and carbamates. In addition, dependence of the
reaction rate on the ethylenediamine concentration was small
(Fig. 7), which also supports that diamine is strongly adsorbed
on CeO2. Taking into account that diamine and CO2 easily
formed carbamate species on CeO2 at 303 K, the carbamate
species will cover the CeO2 surface and prevent the adsorption
of alcohol. Production of 1 was observed at the reaction temp-
Scheme 3 Proposed reaction mechanism of cyclic urea from CO2 and ethylenediamine by CeO2.
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and D. Kempf, Bioorg. Med. Chem., 2006, 14, 6695;
(b) H. L. Sham, D. A. Betebenner, W. Rosenbrook,
T. Herrin, A. Saldivar, S. Vasavanonda, J. J. Plattner and
D. W. Norbeck, Bioorg. Med. Chem. Lett., 2004, 14, 2643.
Conclusion
We developed an effective and reusable catalyst system com-
posed of pure CeO2 and 2-propanol solvent for the direct syn-
thesis of cyclic ureas from CO2 and diamines. 2-Propanol is 11 N. Alouane, A. Boutier, C. Baron, E. Vrancken and
the best solvent to achieve high yields of cyclic ureas and the P. Mangeney, Synthesis, 2006, 885.
bulky alkyl group of 2-propanol will play an important role in 12 (a) S.-i. Fujita, H. Kanamaru, H. Senboku and M. Arai,
the suppression of N-alkylation of amine, a main side reaction.
The catalyst system can be applied to various diamines
to afford the corresponding cyclic ureas, particularly six-
Int. J. Mol. Sci., 2006, 7, 438; (b) S. R. Jagtap, Y. P. Pati,
S.-i. Fujita, M. Arai and B. M. Bhanage, Appl. Catal., A,
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the proposed reaction mechanism is as follows: (1) adsorption
of amine and CO2 to afford carbamic acid and carbamate
species on CeO2, (2) decomposition of the carbamate to a free
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