Highly synergistic effect of ionic liquids and Zn-based catalysts for synthesis of cyclic carbonates from urea and diols

Article abstract of DOI:10.1016/j.molliq.2020.113883

The development of stable and efficient catalysts is an attractive topic for green chemistry reactions under mild reaction conditions. In order to improve solvent-free synthesis of cyclic carbonates from urea and diols, a binary catalyst systems of Zn-based and different ionic liquids (ILs) were developed and examined in this study. The yield of ethylene carbonate (EC) could reach to 92.2% in the presence of C₁₆mimCl/ZnCl₂ catalyst. Through exploring the structure-activity relationships of cation and anion, it was confirmed that a synergistic effect of cation and anion of catalyst had important influences on urea alcoholysis. Additionally, the controlling step of EC synthesis reaction involving the elimination of an ammonia molecule from intermediates had been revealed by in situ FT-IR. This could afford a guided insight for synthesizing cyclic carbonates with high yield. Furthermore, a possible mechanism for the catalytic process was proposed based on DFT and the experimental results via FT-IR, ¹H-NMR and ¹³C NMR analysis, which revealed that not only a probable synergistic effects of cation-anion matters, but also C(2)-H of ILs and Zn²⁺ played a key role in accelerating the reaction of urea alcoholysis. This catalytic mechanism study is to provide a preliminary basis to develop novel catalysts for cyclic carbonates from urea and diols through a green synthetic pathway.

Full text of DOI:10.1016/j.molliq.2020.113883

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Highly synergistic effect of ionic liquids and Zn-based catalysts
for synthesis of cyclic carbonates from urea and diols
Lili Deng, Wenzhong Sun, Zijie Shi, Wei Qian, Qian Su, Li Dong,
Hongyan He, Zengxi Li, Weiguo Cheng
PII:
S0167-7322(20)31525-7
DOI:
https://doi.org/10.1016/j.molliq.2020.113883
MOLLIQ 113883
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
12 March 2020
23 May 2020
19 July 2020
Please cite this article as: L. Deng, W. Sun, Z. Shi, et al., Highly synergistic effect of
ionic liquids and Zn-based catalysts for synthesis of cyclic carbonates from urea and diols,
Journal of Molecular Liquids (2020), https://doi.org/10.1016/j.molliq.2020.113883
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2020 Published by Elsevier.

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Highly synergistic effect of ionic liquids and Zn-based
catalysts for synthesis of cyclic carbonates from urea
and diols
a,b
b,c
a,b
a,b
b,d
b,d
Lili Deng, Wenzhong Sun, Zijie Shi, Wei Qian, Qian Su, Li Dong ,
b
a,b
b,d
Hongyan He , Zengxi Li* and Weiguo Cheng*
a
University of Chinese Academy of Sciences, School of Chemical Sciences, Beijing
00049, China
CAS Key Laboratory of Green Process and Engineering, State Key Laboratory of
1
b
Multiphase Complex Systems, Beijing Key Laboratory of Ionic Liquids Clean Process,
Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China.
c
Dalian Polytechnic University, School of Light Industry Chemical Engineering, Dalian
1
16034, China.
d
Innovation Academy for Green Manufacture, Chinese Academy of Sciences, Beijing
00190, China.
1
E-mail: lizengxi@ucas.edu.cn
wgcheng@ipe.ac.cn
Abstract
The development of stable and efficient catalysts is an attractive topic for green
chemistry reactions under mild reaction conditions. In order to improve solvent-free
synthesis of cyclic carbonates from urea and diols, a binary catalyst systems of Zn-based
and different ionic liquids (ILs) were developed and examined in this study. The yield of
ethylene carbonate (EC) could reach to 92.2% in the presence of C mimCl/ZnCl
2
16
catalyst. Through exploring the structure-activity relationships of cation and anion, it was
confirmed that a synergistic effect of cation and anion of catalyst had important
influences on urea alcoholysis. Additionally, the controlling step of EC synthesis reaction
involving the elimination of an ammonia molecule from intermediates had been revealed
by in situ FT-IR. This could afford a guided insight for synthesizing cyclic carbonates
with high yield. Furthermore, a possible mechanism for the catalytic process was
1
13
proposed based on DFT and the experimental results via FT-IR, H-NMRand C-NMR
analysis, which revealed that not only a probable synergistic effects of cation-anion
2
+
matters, but also C(2)-H of ILs and Zn played a key role in accelerating the reaction of
urea alcoholysis. This catalytic mechanism study is to provide a preliminary basis to
1

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develop novel catalysts for cyclic carbonates from urea and diols through a green
synthetic pathway.
Keywords
Ionic liquids; Catalysts; Synergistic effect; Urea; Cyclic carbonates
1
. Introduction
Organic carbonates are crucial commercial chemicals and can be widely used as
intermediates in various synthetic, industrial applications and also as solvents [1-3].
Cyclic carbonates, as a kind of organic carbonates, can be considered as reactants and
intermediates in chemical reaction for alkoxylation [4, 5], carbamate synthesis [5], diols
formation [6] and preparation of dimethyl carbonate (DMC) [7-9]. They are not only vital
organic chemicals, but also as solvents with high polarity and boiling point, attracting
extensive attention in organic synthesis, electrochemistry, gas separation and other fields
[
2, 5, 10-12]. The routes for the synthesis of cyclic carbonates include reaction of
phosgene method [2], carbon dioxide (CO ) with diols [13, 14], cycloaddition of epoxides
2
with CO [15-18] and urea alcoholysis (Scheme 1) [4, 19, 20]. Among all these routes, the
2
reaction of urea alcoholysis shows a great deal of superiorities, i.e., mild reaction
conditions, inexpensive and accessible raw materials, low production cost and safe
reaction process [4, 19-23]. In addition, it is of great significance that by-product diols
(PG and EG) in the preparation of DMC by transesterification, can be as the reactant to
synthesize cyclic carbonates successfully in this reaction [7-9]. Besides that, the released
ammonia in the reaction of urea and diols can further react with CO to produce urea to
2
realize recycling economy, which should be regarded as an effective and indirect way for
CO utilization [19-23]. Furthermore, this will improve the utilization ratio of the raw
2
materials for downstream DMC synthesis by transesterification reaction. Among them,
the first step is the key for this route and the development of catalysts is the most
important.
Scheme 1 The synthesis of cyclic carbonates from urea and diols
2

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In recent years, numerous catalysts including tin-based [5], metal salts [24, 25] and
mixed) metal oxides [19-23, 26-28] etc. have been reported in the reaction of diols with
(
urea to cyclic carbonates. However, these metal based catalysts are generally faced with
the problems of environment unfriendly, low activity and loss of active components [25,
2
6]. Nowadays, ionic liquids (ILs) are receiving tremendous attention in various chemical
engineering and scientific fields because of their special properties such as green,
adjustable structure and higher thermal stability [29-33]. Kuznetsov et al. [34] proposed a
new method by forming homogeneous catalytic system of ZnCl -urea-glycol in organic
2
solvent-IL under mild conditions, but it still required a long reaction time (24 h) and
additional solvents. In this study, these ILs did not be used as catalyst to realize the
catalytic effect because of the structure of these used ILs were not be designed with
active sites to catalyze reactive molecules. Consequently, it is important to understand the
reaction process and design the structure of ILs for the development of highly active
catalytic systems.
It was found that metal-based catalysts mainly activated the urea molecule by
combining the metal empty orbit with the carbonyl oxygen atom or nitrogen atom in the
reaction of urea alcoholysis from previous studies and the main function of metal salt
anion was to stabilize metal salt [22-25]. And these reported catalysts for this reaction
were mainly zinc-based catalysts, as reported by Arai et al. [35] who firstly established
suitable zinc oxide catalyst for the reaction of urea and EG, which had been found ZnO
with weak basicity showed highly catalytic performance, the formation of these products
under different reaction conditions were illustrated and most plausible reaction pathways
were also proposed. This first study on weakly basic catalyst not only provided a good
research idea for the development and design of weakly basic and amphoteric zinc-based
catalyst by adjusting the charge density of metal active center for urea alcoholysis
reaction, but also made an important foundation about synergetic catalytic mechanism of
metal anions and cations. Based on other catalytic reaction system, it was also verified
the catalytic activity may be improved by controlling the charge density of the metal
holes [36]. In addition, considering the weak compatibility between inorganic salts and
organic reaction molecules, the ILs with excellent solubility can be used as the
connection bridge which is supposed to achieve effective contact between reaction
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molecules and active sites. At the same time, the chain length of ILs have an important
influence on the construction of reaction space to regulate molecular diffusion, for
example, imidazolium ILs with lengths of alkyl side chains for the cycloaddition of CO2
and epoxides showed higher catalytic activity [37-39] and the effective absorption
properties of CO was realized by governing the amino-alkyl chain length of IL through
2
DFT caculation [40]. Therefore, it is expected to realize the construction of binary
catalysts system with high catalysis activity by effectively adjusting the structure of ILs
to control the charge density of metal active center and improving the mass transfer of
reaction molecules.
In this work, the binary catalyst systems of Zn-based and other metal salts (ZnCl2,
ZnBr , FeCl , etc.) with different ILs were examined for solvent-free to synthesize EC
2
3
from urea and EG. Among the catalysts investigated, ILs featuring different lengths of
-
alkyl side chains, different functional groups and different counter anions such as AC ,
-
-
-
-
HSO , PF , NTF and OH were tested (Scheme 2). The effects of reaction parameters
4
6
2
such as temperature, pressure, reaction time, amount of catalysts and molar ratio of
urea/EG had been investigated. Additionally, the catalyst was also proved to be applicable
to other dihydric alcohols and the catalyst could be reused for five times. Moreover, in
situ FT-IR was used for detection of the reaction process and gained the controlling step
of EC synthesis reaction involving the elimination of ammonia from intermediates. A
possible mechanism of this catalytic process was proposed based on DFT and the
1
13
experimental results of FT-IR, H-NMR, C-NMR spectroscopy.
Scheme 2. Structures of the ILs used in this work
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2
Experimental
2
.1 Reagents
Zinc chloride (ZnCl , 98.0%), zinc oxide(ZnO, 98.0%), zinc nitrate (Zn(NO ) ,
2
3 2
9
8.0%), zinc acetate (Zn(AC) , 98.0%)，zinc bromide(ZnBr , 98.0%)，copper chloride
2 2
(CuCl , 98.0%)，ferric chloride (FeCl , 98.0%)，urea (99.0%), EG (99.5%) and PG
2 3
(99.5%) were received from Sinopharm Chemical Reagent Co., Ltd. (China). In this work,
all ILs were purchased from Shanghai Chengjie Chemical Reagent Co., Ltd. (China).
Other diols were obtained from Aladdin Reagent Co., Ltd. All materials were used
without further purification.
2
.2 Catalysts characterization
The fresh and used IL (C mimCl) were characterized by fourier transform infrared
16
spectroscopy (FT-IR, Thermo Nicolet 380) and anhydrous KBr was used as the standard.
Thermogravimetric analysis was analyzed with a DTG-60H thermal analyzer (TGA,
Shimadzu, Japan) under the nitrogen atmosphere at a heating rate of 10 °C/min and an
empty crucible was used as the reference. In situ IR spectra were measured by the
ReactIR 10 with a MCT Detector using HappGenzel apodization, DiComp (Diamond)
probe connected via K6 Conduit (16 mm probe) and sampling 1900 to 1400 at 16
1
wavenumber resolution. H-NMR spectra were recorded by Bruker AVANCE III 600
MHz nuclear magnetic resonance spectrometer with deuterated dimethyl sulfoxide
(
DMSO-d ). High resolution mass spectra (HRMS (ESI-MS)) were collected on a
6
Brucker high resolution mass spectrometer (micrOTOF-QII). Then the purity of EC was
determined by 6820GC-TCD gas chromatography of Agilent Technologies.
2
.3 Typical procedure for the synthesis of EC from EG and urea
The urea alcoholysis reaction was operated in a 50 mL three-necked flask with a
reflux condenser, magnetic stirrer and an absorption device. A series of ILs (Scheme 2)
were tested for comparisons. In a typical procedure, urea (1.8 g), EG (5 mL), C mimCl
1
6
(
0.41 g) and ZnCl (0.33 g) were added into the three-necked flask. Then it was heated to
2
o
1
60 C reaction temperature gradually and adjusted to the initial value under reduced
pressure at the same time, because a great deal of ammonia could be produced in this
reaction and it must be released through an absorption device from the reaction system.
At the end of this reaction, the mixtures were cooled at room temperature, and they were
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analyzed on a gas chromatograph equipped with 6820GC-TCD gas chromatography of
Agilent Technologies. The alike procedure was also carried out for the urea with other
diols. The tests of catalyst recycling were performed in a 200 mL three-necked-flask with
a scale-up experiment of 10 times amount of urea, EG and catalyst. Since IL has high
boiling points compared with organic compounds (EG and EC), IL and ZnCl could be
2
separated from products by distillation under vacuum and can be reused for the next run
directly.
3
Results and discussion
3
.1 Catalytic activity of ILs
The development of an appropriate catalyst was an efficient and simple way to attain
high yield of EC. In this representative reaction, ILs with different lengths of alkyl side
chains were further characterized by TGA and found that they thermally stable up to 200
o
C (Figure S1), which could avoid the degradation of ILs to bring out the contamination
of products. After that, a series of ILs (Scheme 2) based on imidazolium cations
countered with different lengths of alkyl side chains, different functional groups and
different anions were examined for the synthesis of EC from urea and EG under identical
reaction conditions. The results were summarized in Table 1, and revealed the synergistic
effect of anion and cation on catalytic activities (entries 2-17).
−
In the case of the Cl anion, the yield and selectivity of EC increased with the
+
+
lengths of alkyl chains in the cation and obeyed the order of C mim > C mim >
1
6
12
+
+
+
Omim > Bmim > Emim (Table 1, entries 2-6). The results indicated long-chain cation
was beneficial for improving the catalytic activity, in accordance with the result in
previous work [38, 39]. The main reason was that with the increase of side-chain length,
the longer carbon chain prolonged the distance between cation and anion, which could
weaken the electrostatic interaction between them [38, 39]. Then cation C(2)-H had
stronger activation ability and was easier to react with urea. Under the same scenario of
anion, the effects of ILs with different functional groups on the reaction were investigated
(entries 7-9), but these catalytic performances were not significantly improved. The main
reason was not only anions of ILs could form hydrogen bonds with the hydroxyl group of
EG but also imidazole cations of ILs also formed hydrogen bonds with EG [41], and the
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former one was not quite conducive to the formation of hydrogen bonds between cations
and urea.
-
-
-
-
-
The effects of different anions (AC , HSO , PF , NTF , OH ) on the reaction were
4
6
2
interrogated by the same imidazole cation (entries 10-14). The experimental results
-
showed the catalytic effect of NTF was the best. The experimental results were similar
2
-
to those in the literature [42]. The main reason was that NTF was easier to form
2
hydrogen bonds with ethylene glycol, which weakened the interaction between anions
and cations in IL and promoted the interaction of cations with urea, thus increasing the
yield and selectivity of EC. The other three ILs had no obvious catalytic effect on the
reaction (entries 15-17). Besides, white precipitate, which reduced the yield of reaction,
was formed in the reaction of protonic IL and urea (entries 8 and 11). Although single IL
catalysts had no excellent catalytic activity, the binary catalysts showed superior activity,
confirming that a synergistic effect between cation and anion had indispensable results on
urea alcoholysis with diols obviously.
Table 1. Effects of different IL catalyst on the reaction of urea and EG
Entry
Catalyst
None
EC Y. %
1.9
EC Sel. %
4.7
1
2
3
4
5
6
7
8
9
EmimCl
BmimCl
OmimCl
C mimCl
10.5
15.5
21.8
23.0
26.2
12.3
7.3
18.9
20.1
29.3
31.4
33.1
24.3
14.5
22.9
16.1
19.6
27.2
41.0
33.6
12
C mimCl
16
HemimCl
CvmimCl
AemimCl
BmimAC
BmimHSO4
BmimPF6
BmimNTF2
BmimOH
11.5
11.9
10.3
15.6
30.4
22.2
10
11
1
1
1
2
3
4
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1
1
1
5
6
7
N(Bu) Br
15.3
15.0
15.6
29.0
21.4
22.7
4
P(Bu) Br
4
[TMG]HCl
o
Reaction conditions: temperature, 140 C; pressure, 15KPa; reaction time, 3 h; urea:EG (mol:mol)=1:3; IL,
%mol(urea)
4
3
3
.2 Catalytic activity of binary catalysts
.2.1 Catalytic activity of ILs and metal salts catalysts
To investigate the effects of different ILs and metal salts on the formation of EC,
first of all, we need to ensure the structure of ILs with different lengths of alkyl side
chains would not be changed after metal salt was added through FT-IR (Figure S2). Then
Zn-based and other metal salts (ZnCl , ZnBr , FeCl , etc.) with ILs (Scheme 2) were
2
2
3
selected and explored, and the corresponding results were summarized in Table 2.
According to Table 1, it could be concluded that the catalytic effect of single IL was not
good and ZnCl exhibited a 31.6% EC yield (entry 1, Table 2). However, the binary
2
catalysts of ILs and ZnCl realized a further enhancement in catalytic activity compared
2
to that achieved by every single one, indicating the necessary synergistic effect of ILs and
ZnCl on accelerating the reaction by which the EC yield approached 75.8% with a 88.5%
2
selectivity (entry 6). This could also be proved by the FT-IR (Figure 6) and DFT
calculations below. Interestingly, the catalytic performance of binary IL catalysts with
different lengths of alkyl side chains (entries 2-6) and different functional groups (entries
-
7
-9) was similar to Table 1. However, the anion with NTF did not exhibit excellent
2
catalytic effect in the study of binary IL catalysts (entry 13). That possibly because
-
adding ZnCl decreased the H-bond between the anion (NTF ) of IL and EG. Thus, the
2
2
strengthened H-bonding role between anion and cation of IL was able to be
counterbalanced by the weakened interaction of imidazole cation and urea simultaneously.
Therefore, the catalytic performance of the catalytic system was not greatly enhanced.
In order to investigate the effect of the different metal salts (ZnCl , ZnBr , FeCl ,
2
2
3
etc.) on the catalytic activity, the other catalysts including C mimCl/Zn(NO ) ,
1
6
3 2
C mimCl/Zn(AC) , C mimCl/ZnBr , C mimCl/CuCl and C mimCl/FeCl were also
16
2
16
2
16
2
16
3
studied and the results were presented in Table 2 (entries 18-23). It was obvious that
C mimCl/ZnBr catalyst could also show high catalytic activity under the mild
16
2
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conditions, while the others did not. Among the catalysts employed, C mimCl/ZnCl
2
1
6
exhibited the highest activity (yield=75.8% and selectivity=88.5%) under the conditions
◦
of 140 C, 15 KPa, 3 h, urea:EG (mol:mol)=1:3, IL, 4%mol(urea), ZnCl , 8%mol(urea)
2
(
entry 6). Due to its stable and high catalytic activity, C mimCl/ZnCl was chosen for
16 2
further investigation.
Table 2. Effects of different binary catalytic systems on the reaction of urea and EG
Entry
Catalyst
EC Y. %
EC Sel. %
1
2
3
4
5
6
7
8
9
ZnCl2
31.6
39.2
58.6
67.3
73.4
75.8
36.2
32.0
43.1
40.9
37.7
34.3
42.4
45.4
37.3
35.3
33.5
39.3
36.3
35.8
74.3
61.7
67.3
58.9
76.7
86.5
88.3
87.6
88.5
55.6
51.4
83.6
69.9
49.3
64.7
64.4
77.1
60.8
73.4
57.9
53.1
72.6
70.1
87.3
80.1
84.3
EmimCl/ZnCl2
BmimCl/ZnCl2
OmimCl/ZnCl2
C mimCl/ZnCl
12
2
2
C mimCl/ZnCl
16
HemimCl//ZnCl2
CvmimCl/ZnCl2
AemimCl/ZnCl2
BmimAC/ZnCl2
1
0
11
BmimHSO /ZnCl
4
2
1
1
1
1
1
1
1
1
2
2
2
2
2
3
4
5
6
7
8
9
0
1
2
3
BmimPF /ZnCl2
6
BmimNTF /ZnCl2
2
BmimOH//ZnCl2
N(Bu) Br/ZnCl2
4
P(Bu) Br/ZnCl2
4
[TMG]HCl/ZnCl2
C mimCl/ZnO
16
C mimCl/Zn(NO )
3 2
16
C mimCl/Zn(AC)
2
16
C mimCl/ZnBr
16
2
2
C mimCl/CuCl
16
C mimCl/FeCl
3
16
o
Reaction conditions: temperature, 140 C; pressure, 15KPa; reaction time, 3 h; urea:EG (mol:mol)=1:3; IL,
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4
%mol(urea); metal salts, 8%mol(urea)
3
.2.2 Effect of ILs and Zn-based catalysts
The influence of the binary catalytic systems on the urea alcoholysis reaction was
demonstrated and the result was shown in Table 3. From the result, it could be concluded
that the catalytic activity strongly depended on the ratio of C mimCl/ZnCl . When the
1
6
2
molar amount of ZnCl was fixed at 8 mol% of urea, the catalyst activity increased
2
apparently with the mounting molar ratio of C mimCl to ZnCl from 0.1:1-0.5:1, and the
1
6
2
EC yield increased from 31.6% to 75.8% (entries 1-4), whereas further improvement of
molar ratio to 6:1 resulted in an obvious drop for the yield of EC (entries 5-8). This may
be due to the fact that with the increase of C mimCl concentration, the IL was hard to
1
6
flow smoothly, which caused the elevated resistance of mass transfer between active sites
and reagents and the low dispersion of excess C mimCl in this homogeneous catalytic
16
process [43-45]. Thus, the optimum C mimCl/ZnCl molar ratio was 1:2 and the ZnCl
2
16
2
was fixed at 8 mol% of urea.
Table 3. Effects of the molar ratio of C mimCl/ZnCl on the reaction of urea and EG
16
2
C mimCl:ZnCl
2
16
Entry
EC Y. %
EC Sel. %
(mol:mol)
1
2
3
4
5
6
7
8
0:1
31.6
43.3
54.2
75.8
74.8
63.2
56.6
41.7
58.9
64.7
76.0
88.5
87.1
81.6
77.3
69.8
0.1:1
0.2:1
0.5:1
1:1
2:1
4:1
6:1
o
Reaction conditions: temperature, 140 C; pressure, 15KPa; reaction time, 3 h；urea:EG (mol:mol)=1:3; ZnCl2,
8
%mol(urea)
3
.3 Effect of reaction conditions
To find the optimum reaction conditions in this binary catalytic system, the reaction
parameters of temperature, pressure, reaction time, amount of catalysts and molar ratio of
urea/EG were investigated. And yield and selectivity on different parameters catalyzed by
C mimCl/ZnCl were shown in Figure 1. In Figure 1(a), the yield and selectivity of EC
16
2
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0

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o
o
enhanced firstly along with increasing temperature from 130 C to 170 C, and the yield
and selectivity of EC would reach the highest value of 87.8% and 94.8% when reaction
o
temperature rose to 160 C, but the yield and selectivity would decrease at higher reaction
o
temperature of 170 C. The thermodynamic calculation results indicated the elevation of
reaction temperature was advantageous for EC synthesis because it was an endothermal
reaction [46]. However, the reason that EC yield and selectivity reduced with a further
increasing temperature was thought to be the acceleration of side reactions and the
decomposition of urea [24, 47]. Therefore, the optimized reaction temperature for EC
synthesis was 160 °C.
As is well known, a great deal of ammonia was released from the reaction during the
experiment and that would be an obvious negative impact on the reaction equilibrium
with the pressure increased. Therefore, it was very essential to explore the effect of
pressure on the reaction. The result (Figure 1(b)) showed as the pressure of the reaction
system increased, the yield and selectivity of EC decreased in the range of 15 KPa-30
KPa. When the pressure of the system was higher, the ammonia produced by the reaction
was not easily brought out of the reaction system in time, which made the equilibrium
conversion of EC decrease, therefore, 15 KPa was selected as the appropriate pressure on
this reaction.
Figure 1(c) showed the yield and selectivity of EC could increase remarkably with
the added reaction time, and then the EC yield and selectivity decreased with the
extension of the reaction time. That might be caused by the side reactions that would
occur in accordance with the major reaction such as the polymerization of EC when the
concentration of EC was high after 3 h [4, 24, 47]. The optimum reaction time was
determined to be 3 h for the EC synthesis.
To illuminate the effect of catalyst amount on the properties of EC, EC was
synthesized by C mimCl/ZnCl in different amounts. As shown in Figure 1(d), the
1
6
2
catalyst amount also had obvious effection EC yield and selectivity. When the amount of
the catalyst was increased from 1-4 mol% (the molar ration of C mimCl to urea), the
16
yield and selectivity of EC increased drastically. However, the yield and selectivity of EC
decreased with an increased catalyst amount, while the same phenomenon had also been
reported in other literature [4, 24, 47]. Thus, the good yield could be obtained at a catalyst
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amount of 4 mol% (C mimCl) of urea.
1
6
Figure 1(e) presented the effect of urea/EG molar ratio on the reaction of EC
synthesis, the yield and selectivity of EC increased at first and then decreased with the
rising molar ratio of urea/EG, and when the molar ratio reached at 1:4, the highest yield
and selectivity of EC could be obtained, which were 92.2% and 97.9%, respectively. EG
was not only as a reactant but also a solvent for this reaction process. The decrease of EC
yield and selectivity as urea/EG molar ratio continued to rise at 1:5, which might be due
to low concentration of urea reactant and low reaction rate [4, 24]. Therefore, the
appropriate molar ratio of urea/EG was 1:4.
1
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Figure 1 Effect of reaction conditions on EC synthesis
Reaction conditions: (a) 15 Kpa, 3 h, urea:EG (mol:mol)=1:3; IL, 4%mol(urea); ZnCl2, 8%mol(urea); (b) 15Kpa, 3 h, urea:EG (mol:mol)=1:3;
o
o
IL, 4%mol(urea); ZnCl2, 8% mol(urea); (c) 160 C, 15 Kpa, urea:EG (mol:mol)=1:3; IL, 4%mol(urea); ZnCl2, 8%mol(urea); (d) 160 C,
o
1
5Kpa, 3 h, urea:EG (mol:mol)=1:3, IL:ZnCl2 (mol:mol)=1:2; (e) 160 C, 15Kpa, 3 h, IL, 4%mol(urea); ZnCl2, 8%mol(urea)
3
.4 Catalyst recycling and catalytic activity towards other diols
In order to test the catalyst recycling with the dependence of the yield and selectivity
of EC to confirm the stability of catalyst, the C mimCl/ZnCl catalyst was chosen for
1
6
2
recycling reaction. After every cycle, the separation between products and
C mimCl/ZnCl catalyst was achieved by vacuum distillation, and the obtained catalyst
16
2
was reused directly for the next run. The relevant result was shown in Figure 2. The
decline in yield and selectivity of EC was minimal after 5 cycles, showing the catalyst
was stable and suitable for reuse.
Yield(%)
Selectivity(%)
1
00
8
6
4
2
0
0
0
0
0
1
2
3
4
5
Recycle times
Figure 2 The performance for recycling of catalyst
o
Reaction conditions: temperature, 160 C; pressure, 15KPa; reaction time, 3 h; urea:EG (mol:mol)=1:4; IL, 4%mol(urea); ZnCl2, 8%mol(urea)
1
3

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To further survey the scope of substrates, we investigated the synthesis of cyclic
carbonates from urea and various diols (1,2-propylene glycol, 1,2-butylene glycol,
3
-chloro-1,2-diol, 1-phenylethane-1,2-diol, 1,3-Propanediol, 1,3-Butanediol) over
C mimCl/ZnCl (Table 4). All the diols could convert, and the high yield of
16
2
five-membered cyclic carbonates could be obtained from urea. Without a doubt, the yield
of cyclic carbonate was affected by the polarity of vicinal glycols, and the greater polarity
of the vicinal glycol had an adverse effect on the yield of cyclic carbonate [20]. Although
1
-phenylethane-1,2-diol can turn into cyclic carbonates successfully, only 81.2% of
product yielded because of steric hindrance [37]. Nonetheless, the yield of six-membered
cyclic carbonates were lower than that of five-membered cyclic carbonates, which was
due to the reaction influenced by the difference of three-dimensional structure between
1
,2-diols and 1,3-diols [20]. Therefore, the C mimCl/ZnCl had good suitability for urea
16 2
with various diols to transform into corresponding products.
Through the above results analysis, it was found that the catalytic activity of binary
catalyst in the reaction of urea and EG was lower than the ZnO catalyst reported by Arai
et al. [35], but that was better than other explored metal-based catalysts [4, 20, 21, 25].
And it displayed good recycling and universality. Based on the study of binary catalyst
and ZnO catalyst [35] the reaction pathway for EC synthesis, and the synergistic
mechanism of ionic liquid and zinc-based catalyst were further studied.
Table 4 Synthesis of cyclic carbonates from urea and various diols
Entry
Diols
Product
Y. %
94.1
1
2
3
4
94.3
87.7
81.2
1
4

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5
6
72.3
75.6
o
Reaction conditions: temperature, 160 C; pressure, 15KPa; reaction time, 3 h; urea:EG (mol:mol)=1:4; IL, 4%mol(urea); ZnCl2, 8%mol(urea)
3
3
.5 Study on catalytic mechanism
.5.1 Reaction pathway for EC synthesis
To gain more insight for the urea alcoholysis process, the reaction between urea and
EG catalyzed by C mimCl/ZnCl was investigated using in situ IR monitoring under
1
6
2
certain conditions (Figure 3(a) and 3(b)). In the first stage, the characteristic peaks of urea
decrease rapidly while the characteristic peak of hydroxyethyl carbamate (HEC) rose
quickly. This indicated the synthesis of HEC from EG and urea was a fast reaction. Then
the characteristic peak of HEC lessened and the characteristic peaks of EC ascended
gradually in the second stage. However, when the reaction of EC synthesis was near
equilibrium, the characteristic peaks of HEC were also evident. It suggested the
controlling step of EC synthesis was the reaction involving the elimination of ammonia
from HEC to give EC [4]. Therefore, it was significant for the catalyst to activate HEC to
form cyclic carbonates. In situ IR spectra of EC synthesis at different reaction times were
−
1
shown in Figure 3(c). The two bands centered at 1679 and 1620 cm were attributed to
−
1
the C=O and N-H absorption peak of urea [4, 20, 21, 48] and the band at 1714 cm
corresponded to the C=O absorption peak of HEC [4, 5]. In addition, the bands at 1804
−
1
−1
cm and 1775 cm corresponded to the C=O absorption peak of EC and 2-oxazolidone
2-OX), respectively [5]. The curve represented the urea which dissolved totally in EG at
min and the spectrum showed HEC, EC and 2-OX had existed in this reaction at 65 min.
(
5
At this point, the characteristic peaks of urea decreased obviously and a certain amount of
EC was formed while there was still some HEC left in the reaction system at 155 min, but
the characteristic peak of HEC almost disappeared at 265 min, which suggested that the
reaction of EC synthesis was completed essentially.
Arai et al.[35] revealed the side products of this reaction could increase with
increasing the pressure and proposed the plausible reaction pathways (Figure S3). In
order to verify whether side products were produced at higher pressure with
1
5

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C mimCl/ZnCl , the ESI-MS analysis was also implemented. These substances of
16
2
products reported in the literature [35] all existed when the catalyst of ZnO was used at
00 KPa (Figure S4 (a)). Based on the results of ZnO (Figure S4 (a)), the products gained
from binary catalyst (C mimCl/ZnCl ) were also analyzed by ESI-MS under different
1
1
6
2
pressures (15 KPa, 50 KPa and 100 KPa). The result showed there were only three of six
substances in products including 1 (EC), 2 (HEC) and 3 (2-OX) at 15 KPa (Figure S4 (b))
which was consistent with the results of in situ IR (Figure 3) and some reported
literatures [4, 5]. Therefore, the main product was EC under lower pressure and the
selectivity calculated in literatures was mainly based on these three substances [20, 35].
According to the above analysis, it could be found that there were two steps for the
synthesis of EC process. The first step was the forming of HEC by urea and EG, and then
EC was produced by elimination of ammonia from HEC molecule. Thus, the reaction
pathway for EC synthesis was proposed as follows (Scheme 3). When the reaction
pressure was increased to 50 KPa, there were other substances
N-(2-hydroxyethyl)-2-oxazolidone) and 5 (ethyleneurea) apart from the previous three
substances 1, 2 and 3 (Figure S4 (c)), and all substances were also detected at 100 KPa
Figure S4 (d)). More side products would be generated if ammonia was not driven out
4
(
(
properly quite for this reaction, therefore, the removal of ammonia is essential to enhance
the yield and selectivity of EC.
Scheme 3 Possible reaction pathway for EC synthesis
Figure 3 In situ IR monitoring of urea alcoholysis with EG and subsequent analysis
o
Reaction conditions: temperature, 140 C; pressure, 15KPa; reaction time, 5 h; urea:EG (mol:mol)=1:4; IL, 4% mol(urea); ZnCl2, 8%
mol(urea)
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6

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3
.5.2 Determination of action sites of ILs C(2)-H
For further investigation of the interaction between C(2)-H of ILs and urea (HEC),
1
13
the H-NMR and C-NMR analyses were also implemented. The proton signal of C(2)-H
C mimCl) shifted from 9.19 ppm to 9.13 ppm, and other proton signals of C mimCl
(
1
6
16
almost unchanged (Figure 4). As expected, the chemical shifts of the cation atom of
imidazole IL(C mimCl) were all downfield shifts [32, 37]. This was explained by that
1
6
urea and HEC formed H-bond with the C mimCl and caused a substantial change in the
1
6
proton signal of C(2)-H (C mimCl). Upon introduction of the IL, the C signal of the
16
C(2)-H (C mimCl) shifted from 137.08 ppm to 137.01 and 137.03 ppm (Figure 5),
1
6
implying that the C(2)-H of the C mimCl was activated by forming an H-bond with urea
1
6
and HEC. These results were in agreement with the description of the experiment results.
Furthermore, urea and HEC were activated by IL, which could be deduced from the
1
13
H-NMR and C-NMR spectroscopy of the mixture of IL-urea and IL-HEC (Figure S5
and S6 in the supporting information).
1
Figure 4 H-NMR of C mimCl (a), C mimCl-Urea(b), C mimCl-HEC (c)
1
6
16
16
1
7

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1
3
Figure 5 C-NMR of C mimCl (a), C mimCl-Urea(b), C mimCl-HEC (c)
1
6
16
16
3
.5.3 Study on synergistic effect of ILs and Zn-based catalysts
This deduction was verified by the FT-IR spectroscopy (130 C, 30min) of the
mixture of catalyst-urea and catalyst-HEC. The interaction between urea and catalysts
ZnCl , C mimCl and C mimCl/ZnCl ) was shown in Figure 6 (a). Also, it was showed
o
(
2
16
16
2
-1
the absorption peak of the C=O bond of urea moved from 1679 cm to 1633, 1675 and
-
1
-1
1
1
628 cm obviously and the N-H bond of urea shifted from 1618 cm to 1572, 1613 and
-
1
563 cm , respectively. The main reason was that the -NH of urea became active in
2
order to produce isonitrile acid under high temperature and the N-H band shifted clearly
Figure S5 (a) in the supporting information) [43, 48]. However, the C-N bond hardly
(
shifted. These results could show these catalysts interacted with O atom instead of N
atom of urea. Similarly, the catalysts of ZnCl , C mimCl and C mimCl/ZnCl had
2
16
16
2
interacted with HEC shown in Figure 6(b), and the absorption peak of the C=O bond of
-1
-1
HEC moved from 1726 cm to 1711, 1719 and 1709 cm . Although the N-H bond
shifted obviously from the result; the C-N bond did not move evidently. Thus, the C=O
bond of urea and HEC shifted more when ZnCl interacted with the C=O bond than that
2
when IL interacted with the C=O bond (Figure 6 (a) and Figure 6 (b)) [22, 25]. This was
the reason why the catalyst activity increased with ZnCl in IL catalyst system. The FT-IR
2
spectroscopy results were in consistent with the experiment results (Figure S7 in the
1
8

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supporting information).
Figure 6 FT-IR spectra of urea and HEC interacted with different catalysts at 130℃
To verify this deduction of the detailed interactions between catalysts (C mimCl,
16
ZnCl and C mimCl/ZnCl ) and urea (HEC), DFT calculations were also performed by
2
,
16
2
Gaussian 09 program. According to the crystal structure data of urea and HEC, we
calculated the geometry and energy optimizations at the B3LYP/6-31+G(d,p) level. The
energetic minimum of urea and HEC was shown in Figure 7(a) and Figure 7(e),
respectively.
As shown in Figure 7, the complex energy of the C=O group of urea interacted with
2
+
2+
the cation C(2)-H of C mimCl and Zn of ZnCl were higher than C(2)-H and Zn of
1
6
2
C mimCl/ZnCl from the original 0.0 Kcal/mol to -23.8, -37.5 and -67.1 Kcal/mol. And
16
2
the complex energy of the C=O (HEC) group interaction showed the same rule from the
original 0.0 Kcal/mol to -15.7, -34.2 and -53.0 Kcal/mol. That was to say, the cooperative
2
+
C(2)-H of ILs and Zn played a key role in accelerating the reaction of urea alcoholysis.
Additionally, the C=O bond lengths of the C=O group of urea/HEC after interacting with
2
+
2+
C(2)-H, Zn and C(2)-H/Zn became longer than before the interaction (original 1.225
Å to 1.249 Å, 1.261 Å and 1.263 Å/ original 1.219 Å to 1.230 Å, 1.253 Å and 1.255 Å),
2
+
which illustrated C(2)-H/Zn had stronger complex role. These consisted with the
abovementioned experimental results. The complex interaction between urea/HEC and
2
+
2+
catalyst (C(2)-H, Zn and C(2)-H/Zn ) could change the charge of the C=O in urea
HEC, and the electrophilicity of the C of C=O group in urea/HEC was further changed
/
from the original 0.787/0.914 to 0.795/0.933, 0.828/0.970 and 0.928/0.999, respectively.
It made the electrophilicity of the C of C=O in urea/HEC stronger, causing the carbon
1
9

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atom of C=O to be attacked more easily (See Table 5 and S8 in Supporting information)
[
31, 32].
ΔE=0.0 Kcal/mol
ΔE= -37.5 Kcal/mol
ΔE= 0.0 Kcal/mol
ΔE=-23.8 Kcal/mol
ΔE= -67.1 Kcal/mol
ΔE= -15.7 Kcal/mol
2
0

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ΔE= -34.2 Kcal/mol
ΔE= -53.0 Kcal/mol
Figure 7 Optimized structure of (a) urea; (b) urea C=O interaction with C mimCl; (c) urea C=O interaction with ZnCl
2
1
6
(
d) urea C=O interaction with C mimCl/ZnCl ; (e) urea; (f) HEC C=O interaction with C mimCl; (g) HEC C=O
1
6
2
1
6
interaction with ZnCl (h) HEC C=O interaction with C mimCl/ZnCl . C atoms (dark grey), H atoms (light grey), O
2
16
2
atoms (red), N atoms (blue), chlorine atoms (green)
Table 5 Charges in the calculated NPA (natural population analysis) atomic charge of C=O in Urea and
HEC in catalysts
Entry
Charge of C of C=O in Urea
Charge of C of C=O in HEC
None catalyst
C mimCl
0.787
0.795
0.828
0.982
0.914
0.933
0.970
0.999
1
6
ZnCl2
C mimCl/ZnCl
2
1
6
3
.5.4 Possible reaction mechanism
From the experimental results, theoretical calculation results and published reports
22, 25, 43, 48], a possible mechanism was proposed (Scheme 4). The cation C(2)-H of
imidazole IL formed hydrogen bond with urea (C=O) and cooperated with the cation of
[
2
+
metal salts (Zn ) to react with C=O of urea, thus C=O of urea was activated, then one
molecule ammonia was released and the formation of isonitrile acid was promoted. The
anions of imidazole IL and metal salt formed hydrogen bond with one hydroxyl hydrogen
of EG, and the oxygen atom of EG attacked the electrophilic carbonyl carbon on
isonitrile acid to promote the formation of intermediate product (HEC). Similarly, the
cation C(2)-H of imidazole IL formed hydrogen bond with HEC (C=O) and cooperated
2
+
with the cation of metal salts (Zn ) to react with C=O of HEC, thus C=O of HEC was
active, and another hydroxyl atom of EG attacked the carbonyl carbon of intermediate
2
1

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product, which promoted the formation of cyclic carbonate after intramolecular addition.
The composite catalyst of metal salts and imidazole ILs had high selectivity catalytic
performance for the synthesis of cyclic carbonates, since the composite catalysts could
activate not only urea carbonyl but also one hydroxyl hydrogen of EG to accelerate the
formation of intermediate in this catalytic system which was more inclined to form stable
cyclic carbonate through amide alcoholysis reaction.
Scheme 4 A possible reaction mechanism
4
. Conclusion
In this study, we demonstrated the successful binary catalyst systems of Zn-based
and different ionic liquids (ILs) for synthesis of ethylene carbonate (EC) from urea and
ethylene glycol (EG). The lengths of alkyl side chains of imidazolium ILs could improve
catalytic performance in the reaction, and the yield and selectivity of EC could reach 92.2%
and 97.9%, respectively. Furthermore, according to the DFT and experimental results, a
possible mechanism for this kind of catalytic process was proposed, which confirmed that
2
+
a probable synergy cation-anion of catalyst, C(2)-H of ILs and Zn playing key role in
accelerating the reaction, thereby conveniently providing a useful catalyst for urea
alcoholysis to produce cyclic carbonates through a green synthetic pathway. Besides, this
study concerning catalytic mechanism can provide a preliminary basis for the
development of catalysts for cyclic carbonates from urea and diols.
Acknowledgements
We sincerely appreciate Prof. Suojiang Zhang (IPE, CAS) for his careful academic
2
2

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guidance and great support. This work is financially supported by the Innovation
Academy for Green Manufacture, Chinese Academy of Sciences (No. IAGM-2019-A01);
"Transformational Technologies for Clean Energy and Demonstration", Strategic Priority
Research Program of the Chinese Academy of Sciences, Grant No. XDA 21030500;
National Natural Science Foundation of China (No. 21890763, No. 21908226);.
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Author Statement
Lili Deng: Conceptualization, Methodology, Validation, Formal analysis, Investigation,
Data Curation, Writing - Original Draft, Writing - Review & Editing, Visualization
Wenzhong Sun: Software
Zijie Shi: Formal analysis, Writing - Review & Editing
Wei Qian: Formal analysis, Writing- Reviewing & Editing
Qian Su: Writing- Reviewing & Editing, Funding acquisition
Li Dong: Funding acquisition, Project administration
Hongyan He: Software
Zengxi Li: Conceptualization, Methodology, Resources
Weiguo Cheng: Conceptualization, Methodology, Resources, Funding acquisition
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Declaration of interests
☒
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
☐
The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests:
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