Synergistic cooperation of bi-active hydrogen atoms in protic carboxyl imidazolium ionic liquids to push cycloaddition of CO2 under benign conditions

Article abstract of DOI:10.1016/j.molliq.2019.111936

Nine protic carboxyl imidazolium ionic liquids are synthesized. Then, they are employed to catalyze the chemical fixation of carbon dioxide (CO₂) and propylene oxide leading to propylene carbonate in the absence of co-catalyst and organic solvent. HCPImBr presents the best catalytic activity with the product yield of 92% under reaction temperature 120 °C, CO₂ initial pressure 1.5 MPa, catalyst amount 0.5 mol%, and reaction time 2.0 h. Even if the reaction temperature and CO₂ initial pressure are decreased to 80 °C and 1.0 MPa, respectively, the 85% of product yield would be kept with the 1.0% catalyst dosage along with 12.0 h. With the exception of the most optimal reaction conditions, generality, and recyclability of HCPImBr are also investigated. More importantly, the reaction mechanism is investigated by the density functional theory, which is the first time to report the mechanism for protic carboxyl imidazolium ionic liquids. The catalytic activity of ionic liquids would be further improved with the reasonable combination of cation and anion.

Full text of DOI:10.1016/j.molliq.2019.111936

MOLLIQ-111936; No of Pages 8
Journal of Molecular Liquids xxx (xxxx) xxx
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Synergistic cooperation of bi-active hydrogen atoms in protic carboxyl
imidazolium ionic liquids to push cycloaddition of CO2 under
benign conditions
∗
∗∗∗
Tengfei Wang, Xinrui Zhu, Lemin Mao, Yi Liu, Tiegang Ren ∗∗, Li Wang , Jinglai Zhang
Institute of Upconversion Nanoscale Materials, Henan Provincial Engineering Research Center of Green Anticorrosion Technology for Magnesium Alloys, College of Chemistry and
Chemical Engineering, Henan University, Kaifeng, Henan, 475004, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 July 2019
Received in revised form 10 October 2019
Accepted 14 October 2019
Available online xxxx
Nine protic carboxyl imidazolium ionic liquids are synthesized. Then, they are employed to catalyze the chemical
fixation of carbon dioxide (CO2) and propylene oxide leading to propylene carbonate in the absence of co-catalyst
and organic solvent. HCPImBr presents the best catalytic activity with the product yield of 92% under reaction
temperature 120 °C, CO2 initial pressure 1.5 MPa, catalyst amount 0.5 mol%, and reaction time 2.0 h. Even if
the reaction temperature and CO2 initial pressure are decreased to 80 °C and 1.0 MPa, respectively, the 85% of
product yield would be kept with the 1.0% catalyst dosage along with 12.0 h. With the exception of the most op-
timal reaction conditions, generality, and recyclability of HCPImBr are also investigated. More importantly, the
reaction mechanism is investigated by the density functional theory, which is the first time to report the mech-
anism for protic carboxyl imidazolium ionic liquids. The catalytic activity of ionic liquids would be further im-
proved with the reasonable combination of cation and anion.
Keywords:
Protic carboxyl imidazolium ionic liquids
Carbon dioxide
Epoxide
Cyclic carbonate
Mechanism
© 2019 Elsevier B.V. All rights reserved.
1. Introduction
them because of their negligible volatility, environmental benign, easy
separation, and others [13,14]. However, traditional ILs [15,16] are
As a plentiful safe, and cheap C1 resources, the utilization of carbon
dioxide (CO2) to produce the useful organic chemicals has attracted
much attention [1]. It is not only beneficial to ameliorating the green-
house effect but also helpful to solve the depletion of fossil fuels [2].
One of the most successful reactions is the coupling reaction of CO2 to
epoxides resulting in cyclic carbonate [3–5] (See Scheme 1) because of
the advantages of atom-efficiency and low-cost. Moreover, the cyclic
carbonate is widely applied as industrial raw materials [6], electrolytes
in lithium-ion secondary batteries [7], polar aprotic solvents [8], precur-
sors for polycarbonates [9], and pharmaceutical intermediates [10].
In past two decades, some catalysts have been reported for the syn-
thesis of cyclic carbonate [11,12]. Ionic liquids (ILs) stands out from
rarely taken as single-component catalyst because of the low catalytic
reactivity and requirement of solvent and metal. The incorporation of
co-catalyst is a facile approach to effectively improve the catalytic activ-
ity [17,18]. The addition of Lewis acids [19] is a typical choice. However,
most of Lewis acids are sensitive to the water resulting in the more com-
plex synthesis operation and higher production cost. An alternative
method is to involve a suitable amount of solvent containing hydroxyl
group [20].
Inspired by it, the hydroxyl functionalized imidazolium ILs are re-
ported by Sun et al. [21]. They present the better catalytic activity than
the corresponding imidazolium ILs without hydroxyl group for the cou-
pling reaction. Later, hydroxyl functionalized quaternary ammonium
[22], hydroxyl functionalized isothiouronium [23], and hydroxyl func-
tionalized guanidinium [24] ILs are developed in succession with the
similar performance. The theoretical studies have uncovered that the
hydrogen bond between hydrogen bond donor and oxygen atom of pro-
pylene oxide (PO) would greatly promote the ring-opening of PO
[25,26]. As the stronger hydrogen bond donor, involvement of carboxyl
∗
Corresponding author.
∗∗ Corresponding author.
∗∗∗ Corresponding author.
E-mail addresses: rtgang@126.com (T. Ren), chemwangl@henu.edu.cn (L. Wang),
zhangjinglai@henu.edu.cn (J. Zhang).
https://doi.org/10.1016/j.molliq.2019.111936
0167-7322/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: T. Wang, X. Zhu, L. Mao, et al., Synergistic cooperation of bi-active hydrogen atoms in protic carboxyl imidazolium ionic
liquids to push cycloaddition of CO2 under benign conditions, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.111936

2
T. Wang et al. / Journal of Molecular Liquids xxx (xxxx) xxx
Scheme 1. Cycloaddition reaction of CO2 and epoxides catalyzed by protic carboxyl imidazolium ILs.
group would lead to the higher catalytic activity. Later, the carboxyl
functionalized imidazolium ILs have been developed [27,28] to testify
the aforementioned viewpoint.
2. Experiment
2.1. Instruments and materials
The other important subgroup of ILs is protic ILs that are formed by
equimolar amounts of Brønsted acid and base with unique properties
including low cost and easy preparation [29]. In protic ILs, there would
be proton-donor and proton-acceptor sites induced by the proton trans-
fer from the acid to the base, which is beneficial to building the hydro-
gen bond. Some protic quaternary ammonium ILs have been
developed by Liu et al. [30]. To catalyze the fixation of CO2. Both the
protic ILs and task functionalized ILs have the better catalytic activity
than the corresponding traditional ILs.
Perhaps protic carboxyl imidazolium ILs are expected to have the
better catalytic performance than both protic ILs and carboxyl function-
alized ILs. To our best knowledge, they have never been reported. In this
work, nine protic carboxyl imidazolium ILs (See Scheme 2) are synthe-
sized by a simple two-step reaction. They would catalyze the coupling
reaction of CO2 and PO without any metal and solvent [31,32]. The
influence of ionic liquids on the catalytic performance is investigated
including the functionalized group, chain length in cation, and different
anions. Moreover, the optimal reaction conditions are determined. The
generality, thermal stability, and reusability of protic carboxyl
imidazolium ILs are also considered. Finally, the reaction mechanism
is explored by density functional theory (DFT) to uncover the mecha-
nism from atomic level. It is expected that an efficient IL with excellent
catalytic activity would be developed for the fixation of CO2 by the sim-
ple synthesized step under mild condition.
The 1H NMR (400 MHz) and 13C NMR (101 MHz) spectra were run
on Bruker AVANCE III HD spectrometer with tetramethylsilane (TMS)
as the internal standard. Mass spectra (MS) were recorded with Agilent
1100LC-MS mass spectrometer. The thermal decomposition tempera-
ture was analyzed with thermal gravimetric analyzer (Mettler Toledo
TGA/SDTA851e). Elemental analyses (EA) were measured with Vario
EL cube elemental analyzer. Single-crystal X-ray diffraction (XRD) mea-
surement was carried out on a Bruker CCD Apex II X-ray single-crystal
diffractometer. GC analyses were performed on Agilent GC–7890 B
using a flame ionization detector.
The imidazole and halogen esters were purchased from Shanghai
Dibo Biotechnology Co. Ltd.. PO was produced by Sinopharm Chemical
Reagent Co. Ltd. Other epoxides were purchased from Shanghai Macklin
Biochemical Co. Ltd.. The CO2 (99.9%) was purchased from Xinri Gas Co.
Ltd.. All of the reagents and solvents were used without further
purification.
2.2. Preparation and characterization of protic carboxyl imidazolium ILs
These protic carboxyl imidazolium ILs were prepared by two steps
including synthesis of carboxylic ester substituted imidazole [33] and
formation of protic carboxyl imidazolium ILs including hydrolytic pro-
cess and protonation process [34] (See Scheme 2). For example, 1-
Scheme 2. Synthesis route and structures of protic carboxyl imidazolium ILs.
Please cite this article as: T. Wang, X. Zhu, L. Mao, et al., Synergistic cooperation of bi-active hydrogen atoms in protic carboxyl imidazolium ionic
liquids to push cycloaddition of CO2 under benign conditions, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.111936

T. Wang et al. / Journal of Molecular Liquids xxx (xxxx) xxx
3
carboxymethyl imidazolium bromide was prepared by the following
method:
1-carboxyethyl imidazolium bromide (HCEImBr): Light yellow
solid (Yield: 62.43%). 1H NMR (400 MHz, D2O) δ 8.82 (s, –CH–, 1H),
7.58 (d, J = 1.7 Hz, –CH–, 1H), 7.49 (d, J = 1.7 Hz, –CH–, 1H), 4.56
(t, J = 6.3 Hz, –CH2-, 2H), 3.05 (t, J = 6.3 Hz, –CH2-, 2H). 13C NMR
(101 MHz, D2O) δ 174.08, 135.02, 121.90, 119.69, 44.73, 34.05. MS
(ESI): [C6H9N2O2]+ m/z 141.20. Anal. Calcd for C6H9N2O2Br: C,
32.60, N, 12.67, H, 4.10. found: C, 32.89, N, 12.86, H, 4.213.
1-carboxypropyl imidazolium bromide (HCPImBr): White solid
(Yield: 74.33%). 1H NMR (400 MHz, D2O) δ 8.69 (s, –CH–, 1H), 7.46 (d,
J = 1.7 Hz, –CH–, 1H), 7.40 (d, J = 1.7 Hz, –CH–, 1H), 4.24 (t, J =
7.1 Hz, –CH2-, 2H), 2.38 (t, J = 7.2 Hz, –CH2-, 2H), 2.13 (m, J = 7.2 Hz,
–CH2-, 2H). 13C NMR (101 MHz, D2O) δ 176.75, 134.55, 121.84,
119.83, 48.42, 30.34, 24.77. MS (ESI): [C7H11N2O2]+ m/z 155.20. Anal.
Calcd for C7H11N2O2Br: C, 35.77, N, 11.92, H, 4.72. found: C, 36.30, N,
11.77, H, 5.012.
1-carboxybutyl imidazolium bromide (HCBImBr): Light yellow solid
(Yield: 78.17%). 1H NMR (400 MHz, D2O) δ 8.78 (s, J = 1.5 Hz, –CH–,
1H), 7.56 (d, J = 1.8 Hz, –CH–, 1H), 7.51 (d, J = 1.7 Hz, –CH–, 1H),
4.31 (t, J = 7.1 Hz, –CH2-, 2H), 2.47 (t, J = 7.3 Hz, –CH2-, 2H), 1.97 (m,
–CH2-, 2H), 1.64 (m, –CH2-, 2H). 13C NMR (101 MHz, D2O) δ 177.90,
134.36, 121.78, 119.73, 48.99, 33.00, 28.66, 20.92. MS (ESI):
[C8H13N2O2]+ m/z 169.07. Anal. Calcd for C8H13N2O2Br: C, 38.57, N,
11.25, H, 5.26. found: C, 38.95, N, 11.01, H, 5.510.
Imidazole (6.8 g, 0.1 mol) was dissolved in CH2Cl2 (75 ml). Then,
KOH (8.4 g, 0.15 mol), K2CO3 (13.8 g, 0.1 mol), and tetrabutyl ammo-
nium bromide (TBAB) (0.7 g, 0.002 mol) were added and stirred at
room temperature for 30 min. Subsequently, the solution was treated
dropwise with ethyl 2-bromoacetate (16.7 g, 0.1 mol) for about
30 min, and the mixture was heated to reflux for 7.0 h. The resulting
precipitate was filtered and washed with CH2Cl2 (40 × 3 ml). Then,
the filtrate was washed with saturated NaCl solution, and the aqueous
layer was separated. The organic layer was evaporated to obtain the
wine red liquid 1-ethoxycarbonyl methyl imidazole, which was used
without any further purification for the next step.
Hydrochloric acid (5.92 g, 60 mmol) was added dropwise to 1-
ethoxycarbonyl methyl imidazole (4.62 g, 30 mmol), and the mixtures
were stirred for 15.0 h at room temperature (25 °C) to ensure that all
of the substrate were reacted. Ethanol was separated by rotary evapora-
tion. Then water was removed by using cyclohexane as water-carrying
agent. Under the heating condition. Finally, the residue was washed re-
peatedly with ethyl acetate in reflux condition and dried in vacuum to
obtain 1-carboxymethyl imidazolium chloride (HCMImCl).
Other protic carboxyl imidazolium ILs were prepared with a similar
procedure to HCMImCl. The structures of protic carboxyl imidazolium
ILs were characterized by NMR, MS (ESI), and EA. Particularly, HCMImBr
was also characterized by XRD, and the result of crystal test further con-
firmed the correctness of structure of HCMImBr. The single crystal spec-
imen of HCMImBr was acquired via slow evaporation of their solutions
(VMeOH∶VCHCl3≈1∶5, the sample was dissolved by a small amount of
methanol, and then chloroform was added) at room temperature, and
the crystallographic data of HCMImBr is listed in Table S1. Furthermore,
1H and 13C NMR spectra are showed in Fig. S1, other data was provided
as follows:
1-carboxypropyl imidazolium iodide (HCPImI): Yellow green
solid (Yield: 83.13%). 1H NMR (400 MHz, D2O) δ 8.78 (s, –CH–, 1H),
7.55 (d, –CH–, 1H), 7.49 (d, –CH–, 1H), 4.33 (t, J = 7.1 Hz, –CH2-,
2H), 2.46 (t, J = 7.2 Hz, –CH2-, 2H), 2.22 (m, J = 7.2 Hz, –CH2-,
2H). 13C NMR (101 MHz, D2O) δ 176.87, 134.53, 121.81, 119.79,
48.40, 30.33, 24.73. MS (ESI): [C7H11N2O2]+ m/z 155.20. Anal. Calcd
for C8H13N2O2I: C, 29.81, N, 9.93, H, 3.93. found: C, 29.83, N, 9.30, H,
3.684.
1-carboxymethyl imidazolium chloride (HCMImCl): Light yellow
solid (Yield: 76.51%). 1H NMR (400 MHz, D2O) δ 8.79 (s, –CH–, 1H),
7.51 (m, –CH_CH-, 2H), 5.12 (s, –CH2-, 2H). 13C NMR (101 MHz, D2O)
δ 169.73, 135.83, 122.99, 119.45, 49.66. MS (ESI): [C5H7N2O2]+ m/z
127.25. Anal. Calcd for C5H7N2O2Cl: C, 36.94, N, 17.23, H, 4.34. found:
C,36.60, N, 17.45, H, 4.068.
1-carboxyethyl imidazolium chloride (HCEImCl): Light yellow solid
(Yield: 73.45%). 1H NMR (400 MHz, D2O) δ 8.77 (s, –CH–, 1H), 7.54 (d,
J = 2.7 Hz, –CH–, 1H), 7.44 (d, –CH–, 1H), 4.52 (t, J = 6.3 Hz, –CH2-,
2H), 3.00 (t, J = 7.4 Hz, –CH2-, 2H). 13C NMR (101 MHz, D2O) δ
174.10, 134.98, 121.83, 119.65, 44.67, 33.98. MS (ESI): [C6H9N2O2]+
m/z 141.22.
1-carboxypropyl imidazolium chloride (HCPImCl): Light yellow
solid (Yield: 80.83%). 1H NMR (400 MHz, D2O) δ 8.76 (s, –CH–, 1H),
7.54 (d, –CH–, 1H), 7.47 (d, –CH–, 1H), 4.31 (t, J = 7.1 Hz, –CH2-, 2H),
2.45 (t, J = 7.2 Hz, –CH2-, 2H), 2.20 (m, J = 7.2 Hz, –CH2-, 2H). 13C
NMR (101 MHz, D2O) δ 176.67, 134.52, 121.81, 119.81, 48.38, 30.31,
24.75. MS (ESI): [C7H11N2O2]+ m/z 155.20. Anal. Calcd for
C7H11N2O2Cl: C, 44.11, N, 14.70, H, 5.82. found: C, 44.39, N, 14.38, H,
5.161.
2.3. Cycloaddition reaction of CO2 and epoxides
The cycloaddition reaction of CO2 and epoxides catalyzed by protic
carboxyl imidazolium ILs was carried out in 100 ml high-pressure reac-
tion autoclave equipped with a magnetic stirrer. For a typical reaction
process: Protic carboxyl imidazolium IL (0.50 mmol) and propylene
oxide (100 mmol) were added into the reactor vessel without any co-
solvent and co-catalyst. CO2 was charged into the reactor to a certain
pressure (0.5–3.0 MPa). Then the reactor was stirred and heated at
90–130 °C for 0.5–2.5 h. After the reaction, the reactor was cooled to
room temperature and the remaining CO2 was slowly released. Finally,
the products were obtained by separation. In order to verify the yield
and selectivity, the products were detected by GC analysis. According
to the aforementioned data, the purity of synthesized ionic liquids is
near to 100%. The influence of impurities is negligible.
3. Results and discussion
3.1. Synthesis and stability of ILs
Nine protic carboxyl imidazolium ILs have been synthesized by a simple two-step re-
action, alkylation reaction to synthesize 1-ethoxycarbonyl imidazole and protonation re-
action to produce protic carboxyl imidazolium ILs (See Scheme 2). In the first step, only
the 1-ethoxycarbonyl imidazole is got, which is not hydrolyzed to 1-carboxyl imidazole.
Then, hydrolysis reaction and protonation reaction are simultaneously completed in the
present of two equivalent corresponding halogenated acid at room temperature resulting
in protic carboxyl imidazolium ILs. The simplified reaction steps would greatly reduce the
synthetic cost and time. Finally, the structures of desired products are confirmed by the
NMR, MS, and EA.
The thermal stability of four protic carboxyl imidazolium ILs, HCMImBr, HCEImBr,
HCPImBr, and HCBImBr are studied by thermogravimetric analysis (TGA) as representa-
tive (See Fig. S2). There is a one-stage decomposition in the TGA curves with the first turn-
ing point above 240 °C. According to previous literature, the reaction temperature is not
higher than 140 °C for coupling reaction of CO2 and PO catalyzed by ILs [35]. Therefore,
the protic carboxyl imidazolium ILs would not decompose in the following catalytic pro-
cess. The decomposed temperatures are 281.3 °C for HCMImBr, 246.4 °C for HCEImBr,
283.0 °C for HCPImBr, and 291.1 °C for HCBImBr with 10% weight loss.
1-carboxybutyl imidazolium chloride (HCBImCl): White solid (Yield:
78.59%). 1H NMR (400 MHz, D2O) δ 8.76 (s, –CH–, 1H), 7.54 (s, –CH–, 1H),
7.49 (s, –CH–, 1H), 4.29 (t, J = 7.1 Hz, –CH2-, 2H), 2.44 (t, J = 7.3 Hz, –
CH2-, 2H), 1.96 (m, J = 7.3 Hz, –CH2-, 3H), 1.62 (m, J = 7.4 Hz, –CH2-,
2H). 13C NMR (101 MHz, D2O) δ 177.98, 134.33, 121.73, 119.70, 48.94,
32.95, 28.62, 20.89. MS (ESI): [C8H13N2O2]+ m/z 169.07. Anal. Calcd for
C8H13N2O2Cl: C, 46.95, N, 13.69, H, 6.40. found: C, 46.76, N, 13.55, H,
6.416.
1-carboxymethyl imidazolium bromide (HCMImBr): Gray solid
(Yield: 83.16%). 1H NMR (400 MHz, D2O) δ 8.45 (s, J = 1.5 Hz, –CH–,
1H), 7.16 (dt, J = 9.3, 1.9 Hz, –CH_CH-, 2H), 4.42 (s, –CH2-, 2H). 13
C
NMR (101 MHz, D2O) δ 169.83, 135.89, 123.06, 119.51, 49.82. MS
(ESI): [C5H7N2O2]+ m/z 127.25. Anal. Calcd for C5H7N2O2Br: C, 29.01,
N, 13.53, H, 3.41. found: C, 29.47, N, 13.68, H, 3.354.
Please cite this article as: T. Wang, X. Zhu, L. Mao, et al., Synergistic cooperation of bi-active hydrogen atoms in protic carboxyl imidazolium ionic
liquids to push cycloaddition of CO2 under benign conditions, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.111936

4
T. Wang et al. / Journal of Molecular Liquids xxx (xxxx) xxx
100
90
80
70
60
Yield
Selectivity
50
Fig. 1. The crystal structure of HCMImBr.
3.2. Single crystal X-ray crystallography
0.2
0.4
0.6
0.8
1.0
Catalyst amount (mol%)
The crystal structure of HCMImBr is shown in Fig. 1. The crystallographic data of
HCMImBr is listed in Table S2 along with the measured bond lengths and angles. The
bond lengths involved in the imidazole ring, including C_C, C–N, and C_N bonds, are nei-
ther typical double bond nor single bond suggesting the conjugation of the imidazole
rings. The C3–N1 and C3–N2 bond lengths are even shorter than traditional C_N bond,
which is attributed to the strong electrophilic ability of H atom on C3. An ordered chain
structure is built by the hydrogen bonds between protic H atom/H atom in carboxyl and
Br− anion. Finally, a one-dimensional chain-like architecture is formed when the hydro-
gen bonds formed by Br− anion with H atom on N2 and carboxyl, respectively (See
Fig. S3).
Fig. 2. Influence of catalyst amount on PC yield and selectivity (reaction conditions: PO
100 mmol, temperature 120 °C, CO2 pressure 1.5 MPa, time 2.0 h).
87% N HCMImCl 74% (entries 1 and 5), HCEImBr 91% N HCEImCl 86% (entries 2 and
6), HCPImI 94% N HCPImBr 92% N HCPImCl 89% (entries 3, 7, and 9), HCBImBr 89%
N HCBImCl 83% (entries 4 and 8). As compared with other reported carboxyl-based
ionic liquids (See Table S3), reaction condition is not greatly refined. However,
there are two highlights in this work including the less catalyst amount and the
higher product yield.
In addition, the catalytic performance of four protic carboxyl imidazloium ILs is ex-
plored under more benign conditions, i.e., 1.0 mol% catalyst under 80 °C and 1.0 MPa
CO2 initial pressure during 12.0 h. The relative sequence among four ILs is kept under
two different reaction conditions except that the PC yield is decreased under the latter re-
action conditions. However, both reaction temperature and CO2 initial pressure are greatly
reduced, which are two important dangerous items in reaction. Moreover, 85% yield is also
acceptable indicating that HCPImBr could keep high catalytic activity under mild reaction
conditions.
3.3. Effect of catalysts
In order to explore the catalytic activity of nine aforementioned protic carboxyl
imidazloium ILs, the model reaction of PO and CO2 to afford propylene carbonate (PC)
catalyzed by them is carried out in two reaction conditions: (1) 0.5 mol% catalyst under
120 °C and 1.5 MPa CO2 pressure during 2.0 h; (2) 1.0 mol% catalyst under 80 °C and
1.0 MPa CO2 pressure during 12.0 h. No co-catalyst and solvent are included in the
whole catalytic process. The corresponding results are listed in Table 1.
The elongated alkyl chain length in cation is helpful to increase the catalytic ac-
tivity. The product yield reaches the maximum when the substituted group is
carboxypropyl. Then, the product yield is decreased with further elongation of alkyl
chain length. The catalytic activity decreases in the order of HCPImCl (89%)
N HCEImCl (86%) N HCBImCl (83%) N HCMImCl (74%) (entries 1–4). When the coun-
terpart anion is the Br− anion, there is the similar sequence, i.e., HCPImBr (92%)
N HCEImBr (91%) N HCBImBr (89%) N HCMImBr (87%) (entries 5–8). The long alkyl
chain length would weaken the interaction between carboxyl and anion leading to
the stronger acidity, which is favorable for the nucleophilic activation. However,
the steric hindrance would also be improved with the elongation of substituted
alkyl chain length leading to the lower product yield. The product yield of HCBImBr
not only is less than that of HCPImBr but also is less than that of HCEImBr. HCBImBr
just displays better catalytic activity than HCMImBr. The suitable alkyl chain length
(HCPImBr) is the more important item to enhance the catalytic activity. When the
alkyl chain length is fixed, the catalytic activity would be increased with the improve-
ment of nucleophilic activation and leaving ability of halogen anions, i.e., HCMImBr
3.4. Effect of reaction parameter
The HCPImBr has the highest catalytic activity, therefore, it is taken as an example to
explore the influence of reaction parameters on catalytic activity including catalyst
amount, reaction temperature, CO2 pressure, and reaction time. Only one reaction param-
eter is varied while other three reaction conditions are fixed.
First, the influence of catalyst amount on synthesis of PC is investigated under 120 °C
and 1.5 MPa CO2 pressure during 2.0 h with 100 mmol PO. The corresponding results are
shown in Fig. 2. In the whole investigated region, the PC yield increases with the enhance
of HCPImBr amount. The sharp improvement of PC yields happens when the HCPImBr
amount increases from 0.2 mol% (53%) to 0.5 mol% (92%). After that, the increased extent
of PC yield is decreased with the further enhancement of HCPImBr amount up to 1.0 mol%.
So catalyst amount of 0.5 mol% is selected for the following investigation with the best
compromise between catalyst amount and PC yield.
Subsequently, the reaction temperature is another important factor in the catalytic
process. The influence of reaction temperature on the PC yield is determined, and the
Table 1
Catalytic activity of various catalystsa.
100
90
Entry
Catalyst
Yield%b
TON
TOF/h−1
1
2
3
4
5
6
7
8
HCMImCl
HCEImCl
HCPImCl
HCBImCl
HCMImBr
HCEImBr
HCPImBr
HCBImBr
HCPImI
HCMImBr
HCEImBr
HCPImBr
HCBImBr
74
86
89
83
87
91
92
89
94
60
82
85
63
148
172
178
166
174
182
184
178
188
60
74
86
89
83
87
91
92
89
94
5
80
70
60
9
10c
11c
12c
13c
Yield
Selectivity
82
85
63
7
7
5
50
90
100
110
120
130
a
Reaction conditions: PO 100 mmol, catalyst 0.5 mol%, temperature 120 °C, CO2 pres-
sure 1.5 MPa, reaction time 2.0 h.
Temperature (°C)
b
Isolated yield.
c
Reaction conditions: PO 50 mmol, catalyst 1.0 mol%, temperature 80 °C, CO2 pressure
Fig. 3. Influence of reaction temperature on PC yield and selectivity (reaction conditions:
1.0 MPa, reaction time 12.0 h.
PO 100 mmol, HCPImBr 0.5 mol%, CO2 pressure 1.5 MPa, time 2.0 h).
Please cite this article as: T. Wang, X. Zhu, L. Mao, et al., Synergistic cooperation of bi-active hydrogen atoms in protic carboxyl imidazolium ionic
liquids to push cycloaddition of CO2 under benign conditions, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.111936

T. Wang et al. / Journal of Molecular Liquids xxx (xxxx) xxx
5
100
95
90
85
80
75
70
Yield
80
60
40
Yield
20
Selectivity
0.5
1.0
1.5
2.0
2.5
3.0
0
Pressure (MPa)
1
2
3
4
5
Run times
Fig. 4. Influence of CO2 pressure on PC yield and selectivity (reaction conditions: PO
100 mmol, HCPImBr 0.5 mol%, temperature 120 °C, time 2.0 h).
Fig. 6. Recyclability of HCPImBr (reaction conditions: PO 100 mmol, HCPImBr 0.5 mol%,
temperature 120 °C, CO2 pressure 1.5 MPa, time 2.0 h).
result is shown in Fig. 3. The PC yield increases rapidly from 53% to 92% with the tem-
perature increasing from 90 °C to 120 °C. When the reaction temperature is further
increased to 130 °C, there is only a slight enhancement for PC yield. Therefore, the
most optimal reaction temperature is 120 °C. It has been testified that the thermal de-
composition temperature of HCPImBr is much higher than 200 °C suggesting that it
would not decompose during the reaction process.
Next, the effect of CO2 pressure on the PC yield is investigated (See Fig. 4). There is a
turning point at the CO2 pressure of 1.5 MPa. In contrast, the PC yield increases with the
improvement of temperature and catalyst amount. Before 1.5 MPa, the PC yield increases
gradually with the CO2 pressure increasing. However, there is a slight decline of PC yield
with the further increase of CO2 pressure in the high-pressure region (1.5–3.0 MPa).
There are two phases including a CO2-rich phase of the top phase and the PO-rich phase
in the bottom phase in the system [36]. The enhanced pressure would push more CO2
into the reaction system to facilitate the reaction. However, the increased CO2 pressure
would also reduce the PO concentration in the bottom phase resulting in the less product
yield. Therefore, CO2 pressure of 1.5 MPa is the optimal pressure for synthesis of PC in the
presence of HCPImBr. When the CO2 pressure is enhanced, more CO2 would dissolve in PO
to form the CO2-PO complex via the strong interaction. It would suppress the interaction
between PO and ionic liquids resulting in the lower activity. Therefore, the product yield
displays the slight decrease with the further enhancement of CO2 pressure.
distillation. HCPImBr is regenerated from the residue, which is washed by ethyl ace-
tate. Subsequently, the catalyst is dried under vacuum to be reused for the next run. It
can be seen that the catalyst could be utilized for five times with only slight loss of PC
yield (See Fig. 6) suggesting that HCPImBr has good recyclability for the cycloaddition
reaction of CO2 and PO.
3.6. Catalytic activity toward other epoxides
The generality of catalyst is another important item to evaluate the performance of
catalyst. HCPImBr is investigated to catalyze the cycloaddition reaction of CO2 with various
epoxides under the above determined optimal reaction conditions and the corresponding
results are summarized in Table 2. HCPImBr exhibits excellent catalytic activity for most of
epoxides (entries 1–7) with acceptable cyclic carbonates yields above 80% associated with
selectivity larger than 99%. The yield of 1 h is the lowest in several investigated epoxides,
which is consistent with previous report catalyzed by other ionic liquids. Even if the reac-
tion temperature is increased to 130 °C and the reaction time is extended to 12.0 h, the
product yield of cyclohexene carbonate is only 53%. The large steric hindrance of cyclohex-
ene oxide greatly impedes the nucleophilic activation from anion resulting in the low
product yield. Meanwhile, it is also testified that the nucleophilic activation is the other
important item to promote the ring-opening of PO.
Finally, the effect of reaction time is explored. As shown in Fig. 5, The PC yield in-
creases rapidly up to 92% within the first 2.0 h. Then, it keeps smooth when reaction
time continues to be prolonged. In general, the optimal reaction conditions is under 120
°C, 1.5 MPa CO2 pressure with 0.5 mol% catalyst amount for 2.0 h. In addition, the PC selec-
tivity is kept above 99% throughout. In the investigated range, the product yield is en-
hanced with the increase of other reaction parameters, although sometimes the
improved extent is not significant. However, the product yield would be decreased if an
unsuitable CO2 pressure is applied.
3.7. Reaction mechanism
3.7.1. Single-IL model
To provide helpful information for the synthesis of ionic liquid in future, the reac-
tion mechanism of CO2 and PO is further explored at the B3PW91/6-31G(d,p) level
[37–39]. All the electronic calculations are performed in the Gaussian 09 program
[40]. The HCPImCl is chosen as the example to confirm the possible reaction routes.
It is well known that there are three steps for the whole process, ring-opening of
PO, CO2 insertion, and ring-closure to produce PC. The ring-opening of PO is pro-
moted by the nucleophilic activation from Cl− anion along with the electrophilic ac-
tivation from the alive H atoms in cation. Therefore, the protic hydrogen atom (H1),
the hydrogen atom in carboxyl group (H2), and hydrogen atom in imidazole ring
(H3) would be taken as the electrophile. One of hydrogen atoms is in charge of the
electrophilic activation while another hydrogen atom is in charge of stabilizing the
Cl− anion by hydrogen bond. Six possible routes are confirmed according to the dif-
ferent ring-opening steps (See Table S4). Since other two steps are not rate-
determining steps, they are not stated out in this work.
The H2 atom activates the O atom of PO and the H1 atom forms hydrogen bond with
Cl− anion (route S-HCPImCl-1, S indicating Single-IL model, 1 indicating the first route in
this model, other routes are named following the same rule). The role of above two hydro-
gen atoms are exchanged resulting in route S-HCPImCl-2. The H1 atom would also be
combined with the H3 atom leading to route S-HCPImCl-3 and route S-HCPImCl-4. And
so on, the H3 atom would be cooperated with the H2 atom leading to route S-HCPImCl-
5 and route S-HCPImCl-6. The corresponding optimized structures for transition states
are plotted in Table S4 along with the barrier height. The barrier heights are corrected at
the M06/6-311+G(2d,2p) level [39,41] on the basis of the optimized geometries along
with polarized continuum model (PCM) [42,43]. It is unfortunate that route S-HCPImCl-
6 is not successfully located. For other five routes, the barrier height of route S-HCPImCl-
2 is the lowest indicating the most favorable route. Following the same activating method,
the transition states of ring-opening step catalyzed by other three ionic liquids, HCMImCl,
HCEImCl, and HCBImCl are determined. The corresponding optimized structures for tran-
sition states are plotted in Table S5 along with the barrier height. The barrier heights de-
crease in the sequence of route S-HCBImCl-2 (17.22 kcal/mol) N route S-HCMImCl-2
3.5. Recyclability of the catalyst
In order to test the catalyst reusability, the coupling reaction of CO2 and PO is per-
formed under the aforementioned optimal reaction conditions. When the reaction is
finished, PC is removed from the reaction mixture under reduced pressure by
100
95
90
85
80
75
70
Yield
Selectivity
65
0.5
1.0
1.5
Time (h)
2.0 2.5
Fig. 5. Influence of reaction time on PC and selectivity (reaction conditions: PO 100 mmol,
HCPImBr 0.5 mol%, temperature 120 °C, CO2 pressure 1.5 MPa).
Please cite this article as: T. Wang, X. Zhu, L. Mao, et al., Synergistic cooperation of bi-active hydrogen atoms in protic carboxyl imidazolium ionic
liquids to push cycloaddition of CO2 under benign conditions, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.111936

6
T. Wang et al. / Journal of Molecular Liquids xxx (xxxx) xxx
Table 2
Cycloaddition reaction of CO2 and various epoxides catalyzed by HCPImBr.
Entry
1
Epoxide
Cyclic carbonate
Selectivity (%)
Yieldb (%)
95
N99
1a
2a
2
N99
92
1b
1c
1d
2b
2c
3
4
N99
N99
84
87
2d
2e
2f
5
6
7
N99
N99
N99
82
83
83
1e
1f
1g
1h
2g
2h
8c
N99
53
a Reaction conditions: epoxides 100 mmol, catalyst HCPImBr 0.5 mol%, CO2 pressure 1.5 MPa, reaction time 2.0 h, temperature 120 °C.
b
Isolated yield.
c
Reaction conditions:temperature 130 °C, reaction time 12.0 h.
(13.28 kcal/mol) N route S-HCPImCl-2 (12.92 kcal/mol) N route S-HCEImCl-2 (12.13 kcal/
mol), which is not consistent with the experimental result listed in Table 1. Moreover, it
does not follow the variation sequence of substituted alkyl chain length. To obtain more
reliable result, the mechanism is studied by Double-IL mode again.
activity is the highest. After that, the catalytic activity is decreased, which is assigned to
the larger steric hindrance. The carboxypropyl chain length is the best choice for protic
carboxyl imidazolium ionic liquids.
4. Conclusions
3.7.2. Double-IL model
HCPImCl is still taken as the example to explore the mechanism. Thirty three routes
are designed according to the different electrophilic activation modes. It is better to em-
ploy as much as possible hydrogen atoms to activate the PO. However, two hydrogen
atoms would simultaneously activate the oxygen atom of PO at most due to the steric hin-
drance. Six routes, route D-HCPImCl-1-route D-HCPImCl-6 (See Table S6, route D-
HCPImCl-1, D indicating Double-IL model, 1 indicating the first route in this model,
other routes are named following the same rule), are finally located by combination of
two hydrogen atoms to activate the PO. Another situation is that only one HCPImCl is uti-
lized to activate the PO, then, the other one is to stabilize the attacking HCPImCl. In the
other word, only one HCPImCl enters the catalytic center region. In the catalytic center re-
gion, three active hydrogen atoms would activate the PO in succession. When the
attacking HCPImCl is fixed, the other HCPImCl would utilize different hydrogen atoms to
form hydrogen bonds with the first one. As a result, eighteen routes (route D-HCPImCl-
7-route D-HCPImCl-24) are designed, which is plotted in Table S7. Finally, two HCPImCl
and PO form an intermediate, in which one HCPImCl is taken as the electrophile, while
the other one is taken as the nucleophile. When the first one is fixed, the other one is
turned over to get the different routes. Nine possible routes are located (route D-
HCPImCl-25-route D-HCPImCl-33, See Table S8). In the third situation, the most favorable
route has the barrier height of 8.38 kcal/mol. Moreover, it is also the most popular route in
all of routes. Following the same mechanism, the mechanism of ring-opening step cata-
lyzed by other catalysts is considered. The corresponding energy profiles are plotted in
Fig. 7 along with the optimized structures of transition states. The barrier heights decrease
in the sequence of route D-HCMImCl (24.74 kcal/mol) → route D-HCBImCl (14.68 kcal/
mol) → route D-HCEImCl (13.38 kcal/mol) → route D-HCPImCl (8.38 kcal/mol), which is
totally consistent with the experimental product yields of HCMImCl (74%) → HCBImCl
(83%) → HCEImCl (86%) → HCPImCl (89%). The catalytic activity enhances with the in-
crease of alkyl chain length. When the substituted group is carboxypropyl, the catalytic
Nine protic carboxyl imidazolium ILs exhibit excellent catalytic activ-
ity for the synthesis of cyclic carbonates with the product yield over 80%
in the absence of co-catalyst and solvent. HCPImBr presents the best cat-
alytic activity among them achieving the product yield of 92%. Even if the
temperature and CO2 pressure are decreased to 80 °C and 1.0 MPa, the
85% product yield would be acquired by HCPImBr. The optimal reaction
conditions are confirmed to be 120 °C, 1.5 MPa CO2 initial pressure with
0.5 mol% HCPImBr amount during 2.0 h. After five runs, the stable prod-
uct yield is kept under the optimal reaction conditions. The determined
catalytic activity on the basis of barrier height is consistent with the var-
iation of product yield. Both the theoretical and experimental results
show that the suitable alkyl chain would promote the ring-opening of PO.
Conflicts of interest
The authors declare no conflict of interest.
Acknowledgments
We thank the National Supercomputing Center in Shenzhen
(Shenzhen Cloud Computing Center) and Changsha (Changsha Cloud
Computing Center) for providing computational resources and
Please cite this article as: T. Wang, X. Zhu, L. Mao, et al., Synergistic cooperation of bi-active hydrogen atoms in protic carboxyl imidazolium ionic
liquids to push cycloaddition of CO2 under benign conditions, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.111936

T. Wang et al. / Journal of Molecular Liquids xxx (xxxx) xxx
7
Fig. 7. The corresponding energy profiles along with the schematic structures of transition states calculated at the M06/6-311+G(2d,2p) (PCM)//B3PW91/6-31G(d,p) level.
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softwares. This work was supported by the National Natural Science
Foundation of China (21476061, 21676071, 21703053, 21975064),
and the Excellent Foreign Experts Project of Henan University.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.molliq.2019.111936.
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Please cite this article as: T. Wang, X. Zhu, L. Mao, et al., Synergistic cooperation of bi-active hydrogen atoms in protic carboxyl imidazolium ionic
liquids to push cycloaddition of CO2 under benign conditions, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.111936