Hydroxyl-functionalized pyrazolium ionic liquids to catalyze chemical fixation of CO2: Further benign reaction condition for the single-component catalyst

Article abstract of DOI:10.1016/j.molliq.2019.111479

Lots of ionic liquids have been utilized as catalyst for the coupling reaction of carbon dioxide with epoxides, however, catalyzed conditions could not be regarded as benign, especially when no co-catalyst and/or organic solvent is involved. A series of hydroxyl-functionalized pyrazolium ionic liquids are firstly synthesized. They would efficiently catalyze the cycloaddition of carbon dioxide and propylene oxide under 110 °C and 1.0 MPa carbon dioxide initial pressure with 1 mol% catalyst during 4 h resulting in the product yield of 91.2%. The catalytic condition is greatly refined as compared with other single-component ionic liquids with simple anion. Simultaneously, the effect of reaction temperature, amount of catalyst, carbon dioxide initial pressure, and reaction time is explored along with the reusability of catalyst. For most of epoxides with terminal substituted group, HEEPzBr presents acceptable catalytic activity. The difference of HEMPzBr, HEEPzBr, and HPEPzBr is also explored by the density functional theory calculations.

Full text of DOI:10.1016/j.molliq.2019.111479

Journal of Molecular Liquids 293 (2019) 111479
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Hydroxyl-functionalized pyrazolium ionic liquids to catalyze chemical
fixation of CO2: Further benign reaction condition for the single-
component catalyst
Tengfei Wang a,b,c,1, Yuan Ma a,b,c,1, Jiamin Jiang a,b,c, Xinrui Zhu a,b,c, Baowan Fan a,b,c, Guanyao Yu d,
a,b,c,
a,b,c,
Ningning Li d, Shasha Wang d, Tiegang Ren a,b,c, , Li Wang
⁎
⁎
, Jinglai Zhang
⁎
a
Institute of Upconversion Nanoscale Materials, PR China
b
Henan Provincial Engineering Research Center of Green Anticorrosion Technology for Magnesium Alloy, PR China
College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan 475004, PR China
Minsheng College, Henan University, Kaifeng, Henan 475004, PR China
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 June 2019
Received in revised form 28 July 2019
Accepted 30 July 2019
Available online 31 July 2019
Lots of ionic liquids have been utilized as catalyst for the coupling reaction of carbon dioxide with epoxides, how-
ever, catalyzed conditions could not be regarded as benign, especially when no co-catalyst and/or organic solvent
is involved. A series of hydroxyl-functionalized pyrazolium ionic liquids are firstly synthesized. They would effi-
ciently catalyze the cycloaddition of carbon dioxide and propylene oxide under 110 °C and 1.0 MPa carbon diox-
ide initial pressure with 1 mol% catalyst during 4 h resulting in the product yield of 91.2%. The catalytic condition
is greatly refined as compared with other single-component ionic liquids with simple anion. Simultaneously, the
effect of reaction temperature, amount of catalyst, carbon dioxide initial pressure, and reaction time is explored
along with the reusability of catalyst. For most of epoxides with terminal substituted group, HEEPzBr presents ac-
ceptable catalytic activity. The difference of HEMPzBr, HEEPzBr, and HPEPzBr is also explored by the density func-
tional theory calculations.
Keywords:
Hydroxyl-functionalized pyrazolium ionic liq-
uids
Epoxides
CO2
Catalysis
Density functional theory
© 2019 Elsevier B.V. All rights reserved.
1. Introduction
Ionic liquids (ILs) [9,10] are one of the most prominent catalysts in
various catalytic systems with distinct advantages including high ther-
The utilization of carbon dioxide (CO2) has attracted continuous at-
tentions from academic and industrial communities [1], since it is an ef-
ficient pathway to control the CO2 concentration in the air. The
utilization of CO2 is not limited to alleviate the environmental problems
but afford the inexpensive, nontoxic, and abundant C1 resources to
moderate the depletion of fossil fuels [2,3]. The coupling reaction of
CO2 with epoxides to produce five-membered cyclic carbonates [4,5]
is one of the most promising routes. It is an atomic efficiency route
resulting in valuable products with wide applications, such as electro-
lytes [6], polar aprotic solvents [7], and chemical intermediates [8]. It
is a challenge to promote reaction under mild conditions due to the
high thermostability and kinetic inertness of CO2. The involvement of
high-efficiency catalyst is a powerful and simple pathway to refine the
reaction conditions.
mostability, negligible vapor pressure, and excellent chemical stability
[11]. The reaction condition catalyzed by single-component ILs is still
not regarded as benign. Direct inclusion of co-catalyst, normally Lewis
acid, in the catalytic system is a facile and simple method to improve
the catalytic activity. However, the reaction condition is harsh due to
the water sensitivity of Lewis acids [12]. Later, an alternative method
is proposed to introduce the solvent containing hydroxyl-group into
the IL-based catalytic system [13]. Inspired by it, the hydroxyl-
funtionalized imidazolium ILs have been synthesized by Sun et al. [14]
Besides the nucleophilic activation, the hydroxyl group would activate
the epoxide leading to the energy-rich intermediate [15]. The carboxylic
acid group is the stronger Brønsted acid along with better hydrogen
bond donor as compared with hydroxyl group. Consequently,
carboxyl-functionalized imidazolium ILs are developed with the better
catalytic activity [16]. Later, carboxyl-functionalized quaternary ammo-
nium salts [17], carboxyl-functionalized pyridinium ILs [16], and others
are synthesized in succession with the critical aim to develop the high-
efficiency single-component ILs. Although some progresses have been
made, there are still some problems required to be resolved. The reac-
tion temperature and required CO2 pressure are still too high, which is
⁎
Corresponding authors at: Institute of Upconversion Nanoscale Materials, PR China.
E-mail addresses: rtgang@126.com (T. Ren), chemwangl@henu.edu.cn (L. Wang),
zhangjinglai@henu.edu.cn (J. Zhang).
1
These authors contributed equally to this work.
https://doi.org/10.1016/j.molliq.2019.111479
0167-7322/© 2019 Elsevier B.V. All rights reserved.

2
T. Wang et al. / Journal of Molecular Liquids 293 (2019) 111479
difficult to be controlled [18]. Or the reaction time is very long resulting
in the lower turnover frequency (TOF) [19]. To equilibrate several reac-
tion conditions is the more difficult in design of catalysts along with
other benefits such as, facile synthesis, economic available, easy purifi-
cation, and others.
= 3.1 Hz, 1H, Pz-H), 5.26 (s, 1H, –OH), 4.61 (t, J = 5.0 Hz, 2H, –CH2-),
4.20 (s, 3H, –CH3), 3.76 (t, J = 5.2 Hz, 2H, –CH2-).13C NMR (101 MHz,
D2O) δ 138.38, 137.98, 107.43, 59.34, 52.34, 37.61. HR-MS (QTOF)
calcd. for [C6H11N2O+] (m/z): 127.0866, found: 127.0868.
Other hydroxyl-functionalized pyrazolium ionic liquids were syn-
thesized by alkylpyrazole and bromhydrin with the identical procedure
for HEMPzBr.
1-ethyl-2-hydroxyethyl pyrazolium bromide (HEEPzBr). White
solid, 1.348 g, yield 61%; m.p: 43–44 °C·1H NMR (400 MHz, DMSO‑d6)
δ 8.67 (m, 1H, Pz-H), 8.58 (m, 1H, Pz-H), 6.93 (t, J = 3.0 Hz, 1H, Pz-H),
5.27 (t, J = 5.4 Hz, 1H, –OH), 4.63 (t, J = 5.0 Hz, 2H, –CH2-), 4.58 (q, J
= 7.2 Hz, 2H, –CH2-), 3.78 (q, J = 5.2 Hz, 2H, –CH2-), 1.45 (t, J =
7.2 Hz, 3H, –CH3). 13C NMR (101 MHz, DMSO‑d6) δ 138.33, 137.00,
107.77, 59.43, 52.40, 45.60, 14.62. HR-MS (QTOF) calcd. for
[C7H13N2O+] (m/z): 141.1022, found: 141.1025.
1-ethyl-2-hydroxypropyl pyrazolium bromide (HPEPzBr). White
solid, 1.363 g, yield 58%; m.p: 54–55 °C·1H NMR (400 MHz, D2O) δ
8.14 (dt, J = 2.6, 1.3 Hz, 2H, Pz-H), 6.70 (td, J = 3.0, 0.9 Hz, 1H, Pz-H),
4.48 (t, J = 7.2 Hz, 2H, –CH2-), 4.40 (q, J = 7.3 Hz, 2H, –CH2-),
3.59–3.54 (m, 2H, –CH2-), 2.11–2.01 (m, 2H, –CH2-), 1.48 (td, J = 7.3,
0.9 Hz, 3H, –CH3). 13C NMR (101 MHz, DMSO‑d6) δ 137.77, 136.88,
107.84, 57.43, 47.37, 45.35, 31.84, 14.50. HR-MS (QTOF) calcd. for
[C8H15N2O+] (m/z): 155.1179, found: 155.1178.
It is found that the task specified imidazolium ILs is one of the most
popular ILs with better catalytic activity as compared with other task
specified ILs, including task-functionalized quaternary ammonium
salts or pyridinium ILs. The special five-member ring of imidazole
would play key role in improving the catalytic activity. Pyrazole has
the similar five-member ring with imidazole, which would perhaps
present the similar or better catalytic activity. It has been testified in
our previous work. Some pyrazolium ILs [20,21] have been employed
as the catalyst for the title reaction presenting the comparable catalytic
activity with imidazolium ILs. However, reaction conditions including
reaction temperature and pressure are not greatly refined. To our best
knowledge, task specified pyrazolium ILs are rarely reported till now.
Or they are rarely applied as catalyst for the coupling reaction of CO2
and propylene oxide (PO).
In this work, five hydroxyl-functionalized pyrazolium ILs including
1-methyl-2-hydroxyethyl pyrazolium bromide (HEMPzBr), 1-ethyl-2-
hydroxyethyl pyrazolium bromide (HEEPzBr), 1-ethyl-2-hydroxypro-
pyl pyrazolium bromide (HPEPzBr), 1-ethyl-2-hydroxyethyl-3-methyl
pyrazolium bromide (HEEMPzBr), and 1-ethyl-2-hydroxyethyl-3,5-
dimethyl pyrazolium bromide (HEEDMPzBr) are synthesized to be cat-
alysts for the title reaction. On the basis of HEEPzBr, the optimal reaction
conditions are further refined. Finally, the reaction mechanism is eluci-
dated by density functional theory (DFT). Our central goal is to find a
high efficient catalyst, which would greatly decrease the reaction tem-
perature but not increase other conditions.
1-ethyl-2-hydroxyethyl-3-methyl
pyrazolium
bromide
(HEEMPzBr). White solid, 1.504 g, yield 64%; m.p: 41–42 °C·1H NMR
(400 MHz, D2O) δ 8.03 (m, 1H, Pz-H), 6.53 (m, 1H, Pz-H), 4.48 (q, J =
4.9 Hz, 2H, –CH2-), 4.38 (tt, J = 11.9, 6.3 Hz, 2H, –CH2-), 3.85 (dt, J =
10.9, 5.1 Hz, 2H, –CH2-), 2.41 (m, 3H, –CH3), 1.33 (m, 3H, –CH3). 13
C
NMR (101 MHz, D2O) δ 147.72, 136.96, 107.95, 59.26, 51.91, 42.26,
13.40, 11.34. HR-MS (QTOF) calcd. for [C8H15N2O+] m/z: 155.1179,
found: 155.1177.
2. Experimental and theoretical details
1-ethyl-2-hydroxyethyl-3,5-dimethyl
pyrazolium
bromide
(HEEDMPzBr). White solid, 1.606 g, yield 65%; m.p: 118–119 °C. 1H
NMR (400 MHz, DMSO‑d6) δ 6.61 (s, 1H, Pz-H), 4.54–4.45 (m, 4H, –
CH2-CH2-), 3.69 (t, J = 5.0 Hz, 2H, –CH2-), 2.46 (d, J = 4.8 Hz, 6H, –
CH3), 1.27 (t, J = 7.2 Hz, 3H, –CH3). 13C NMR (101 MHz, DMSO‑d6) δ
147.57, 146.32, 108.46, 59.58, 49.52, 42.37, 14.49, 12.28, 11.74. HR-MS
(QTOF) calcd. for [C9H17N2O+] m/z: 169.1335, found: 169.1338. 1H
and 13C NMR spectra of five hydroxyl functionalized pyrazolium ionic
liquids are showed in Fig. S1.
2.1. Instruments and materials
High resolution mass spectra (HR-MS) was measured in Agilent
1290 Infinity LC with 6224 TOF MSD. 1H NMR (400 MHz) and 13C
NMR (101 MHz) spectra were obtained via Bruker Avance III HD spec-
trometer with tetramethylsilane (TMS) as the internal standard. The
thermal decomposition temperature was analyzed with a thermal
gravimetric analyzer (Mettler Toledo TGA/SDTA851e). GC analyses
were performed on Agilent GC-7890B with a flame ionization detector.
The 1-methylpyrazole, 1-ethylpyrazole, 3-methylpyrazole, 3,5-
dimethylpyrazole, bromoethanol, and 3-bromo-1-propanol were pur-
chased from Shanghai Macklin Biochemical Co. Ltd.. PO, epoxy
chloropropane, styrene oxide, and other epoxides were purchased
from Aladdin Industrial Co. Other ordinary used organic chemical re-
gents were produced from Sinopharm Chemical Reagent Co. Ltd. All re-
actants were used directly as received without any further purification.
The CO2 (99.9%) was purchased from Kaifeng Xinri Gas Co.
2.3. Coupling reaction of CO2 with epoxides
Epoxides and ionic liquids were firstly added into the stainless steel
autoclave (100 mL) at ambient temperature. Then, CO2 (1.0–3.0 MPa)
was introduced to the reactor vessel and heated up to the designed tem-
perature. The reaction was carried out at 90–140 °C for 1–5 h. After that,
the reactor was cooled to ambient temperature and the remaining CO2
was slowly released. Finally, the products were isolated and yields
were obtained. Some reactions were repeated to ensure the reproduc-
ibility of yields was 3%. The structures of various cyclic carbonates
were characterized by 1H NMR (see SI).
2.2. Preparation of hydroxyl pyrazolium ionic liquids
These hydroxyl-functionalized pyrazolium ionic liquids were syn-
thesized according to the literature [14] with some revision. For exam-
ple, the synthesis procedure of 1-methyl-2-hydroxyethyl pyrazolium
ILs is shown in Scheme 1.
1-methyl-2-hydroxyethyl pyrazolium bromide (HEMPzBr). In a
three-necked bottle, 0.82 g (10 mmol)1-methylpyrazole and 1.25 g
(10 mmol) bromoethanol were added into 10 mL CH3CN. Then, the mix-
tures were stirred at reflux temperature for 48 h in a nitrogen atmo-
sphere. After the reaction, the volatiles were removed under reduced
pressure. The residual was washed several times by ethyl acetate to
give a white solid. After filtration and drying in vacuum, the pure
HEMPzBr was obtained: 1.532 g, yield 74%, white solid. 1H NMR
(400 MHz, DMSO‑d6) δ 8.58 (dd, J = 21.9, 2.8 Hz, 2H, Pz-H), 6.86 (t, J
2.4. Computational details
Geometric optimization and vibration analysis of the reactants, in-
termediates, and transition states were carried out by the Becke's
three parameters exact exchange-functional combined with Perdew
and Wang (B3PW91) [22,23] method along with the 6-31G(d,p) basis
set [24]. On the basis of the optimized transition states, the minimum-
energy path (MEP) was constructed following the intrinsic reaction co-
ordinates (IRC) [25]. The energy was refined at the M06/6-311 + G
(2d,2p) level [26] without variation of the optimized geometry. And
the solvent effect of ethyl ether (Et2O) is considered by the polarized
continuum model (PCM) [27,28]. The non-covalent interactions are
considered by non-covalent interactions (NCI) [29,30] and atoms in

T. Wang et al. / Journal of Molecular Liquids 293 (2019) 111479
3
Scheme 1. Synthetic route and structures of the hydroxyl functionalized pyrazolium ILs.
molecule (AIM) [31]. The atomic charge is calculated by natural bond
orbital (NBO) analysis [32,33] The aforementioned electronic calcula-
tions were performed by the Gaussian 09 program [34].
by HEMPzBr (entry 1) and HPEPzBr (entry 3), which is attributed to
their different alkyl chain length. The ethyl group on N1 atom along
with hydroxyethyl group on the other N atom is the more suitable com-
bination. Except for the alkyl chain length, the catalytic activity would
be also affected by the substituted position of alkyl group. The catalytic
activity of HEEMPzBr (entry 4) and HEEDMPzBr (entry 5) is compara-
ble, which is even larger than that of HEEPzBr (entry 2) suggesting
that the inclusion of methyl group on pyrazole ring would further in-
crease the catalytic activity. The overall performance is not enhanced
with the further increasing functional group number, which is similar
to the hydroxyl-functionalized quaternary ammonium salt [13]. The
NEt(HE)3Br (ethyltrihydroxyethyl ammonium bromide) has the higher
3. Results and discussion
3.1. Effect of catalysts
The catalytic performance of five hydroxyl-functionalized
pyrazolium ILs is tested for the coupling reaction of CO2 and PO. The cor-
responding data are tabulated in Table 1. For entries 1–3, the product
yield catalyzed by HEEPzBr (entry 2) is the larger than items catalyzed
Table 1
Catalytic performance of various catalysts.a
Entry
Structure
Catalyst
Yieldb (/%)
85.3
1
HEMPzBr
2
3
4
HEEPzBr
HPEPzBr
91.2
89.5
92.9
HEEMPzBr
5
6
HEEDMPzBr
DEPzBr [22]
92.2
76.1
a
Reaction conditions: PO 0.1 mol, catalyst loading 1 mol%, temperature 110 °C, initial CO2 pressure 1.0 MPa, time 4 h.
Isolated yield.
b

4
T. Wang et al. / Journal of Molecular Liquids 293 (2019) 111479
catalytic activity than N(HE)4Br (tetrakis(2-hydroxyethyl) ammonium
bromide). Under the same reaction parameters, the catalytic activity
of DEPzBr (entry 6) [21] is much lower than that of HEEPzBr indicating
that the inclusion of hydroxyl group greatly refine the reaction.
As compared with HEMImBr (see entry S3 in Table S1) [14], the
product yield catalyzed by HEEPzBr is the slightly lower (99.2% for
HEMImBr v.s. 91.2% for HEEPzBr). However, the required reaction tem-
perature and CO2 initial pressure are much higher for HEMImBr. When
the reaction temperature is decreased by 15 °C reaching 110 °C, the
product yield catalyzed by HEMImBr is decreased to be 90% (entry S4
in Table S1) [35]. At the same reaction condition (Cat. 1.6 mol%, 125
°C, 2.0 MPa, 1 h), the catalytic activity of HETBAB (entry S5 in
Table S1) is the smaller than that of HEMImBr [14]. If both temperature
and CO2 initial pressure are decreased, the catalytic activity of HETBAB
would be even worse, which is expected to be lower than that of
HEEPzBr. When the temperature and CO2 initial pressure are decreased
to be 120 °C and 1.5 MPa, the product yield is only 81% for NEt3(HE)Br
(entry S6 in Table S1). When carboxyl- and amino-functionalized
imidazolium ILs 3-carboxyethyl-1-methyl imidazolium bromide
(CEMImBr) and 3-aminopropyl-1-methyl imidazolium bromide
(APMImBr) [36] are employed as catalysts, the reaction conditions are
catalyst 1 mol%, temperature 120 °C, CO2 pressure 1.5 MPa, and time
1.5 h leading to the product yields of 84.0% for CEMImBr (entry S7 in
Table S1) and 88.6% for APMImBr (entry S8 in Table S1). Although the
reaction temperature and CO2 initial pressure are decreased, the cata-
lytic activity is also decreased. Only CMMImBr (entry S9 in Table S1)
[37] could be comparable with HEEPzBr. However, not only the catalyst
dosage but also CO2 initial pressure is the higher for CMMImBr. In gen-
eral, HEEPzBr presents the better catalytic activity than aforementioned
ionic liquids. More importantly, the reaction parameters, especially for
reaction temperature and CO2 initial pressure, are greatly decreased,
which is better than most of single component ionic liquid reported
previously.
would not dissolve in the catalytic reaction. The corresponding thermo-
gravimetric analysis (TGA) curves for HEMPzBr, HEEPzBr, HPEPzBr,
HEEMPzBr, and HEEDMPzBr are plotted in Fig. 2.
The CO2 initial pressure also has a significant effect on the catalytic
activity (Fig. 3). The PC yield is increased by 11.9% when the CO2 pres-
sure varies from 0.5 MPa to 1.0 MPa. After that, the PC yield is kept till
that the CO2 pressure is increased to be 2.5 MPa. However, there is slight
decrease when the CO2 pressure reaches 3.0 MPa. Initially, the increased
CO2 pressure would enhance the concentration of reactant, which is
beneficial for promoting the reaction. The concentration of ionic liquids
would be decreased with the more CO2 entering the system, the reac-
tion is suppressed to some extent resulting in the less product yield.
Next, the effect of catalyst loading is also considered. As shown in
Fig. 4, the PC yield increases sharply from 53.8% to 87.9% with catalyst
amount increasing from 0.2 mol% to 0.5 mol%. However, PC yield has a
slight enhancement even up to 1 mol% of catalyst loading.
Finally, the dependence of reaction time on the PC yield is plotted in
Fig. 5. The reaction proceeds rapidly at the initial time and the conver-
sion of PO is almost finished within 4 h. The elongation of reaction
time would not promote the reaction. In general, the optimal reaction
conditions are reaction temperature 110 °C, CO2 initial pressure
1.0 MPa, HEEPzBr 1 mol%, and reaction time 4 h.
Both reaction temperature and CO2 initial pressure are greatly de-
creased, which are two important items to arouse the potential danger-
ous during the reaction. The reaction temperature is 130 °C for alkyl
pyrazolium ILs [38] and protic pyrazolium ILs [20], which is 20 °C higher
than the employed temperature in this work. Although the reaction
temperature for the amino-functionalized pyrazolium ILs [39] is de-
creased to be 110 °C, CO2 initial pressure is still as high as 1.5 MPa.
Even for the hydroxyl-functionalized imidazolium ILs [14], the reaction
temperature is 125 °C along with CO2 initial pressure of 2.0 MPa. The re-
action temperature and CO2 initial pressure for the coupling reaction
catalyzed by HEEPzBr is much better than most of previous reported
single-component ionic liquids with the exception of phenolic
hydroxyl-functionalized imidazolium ILs [40]. However, the cost to syn-
thesize phenolic hydroxyl-functionalized imidazolium ILs is too expen-
sive to be used in large scale. It is expected that it would present the
better result. Perhaps, the heterogeneous catalysts [41,42] have the
higher product yields, however, the reaction temperature is as high as
140 °C, which is even harsh (see Table S2).
3.2. Effect of reaction parameters
The HEEPzBr is chosen as an example to investigate the influence of
reaction parameter on the catalytic activity. Reaction temperature has a
remarkable effect on the catalytic activity (see Fig. 1). The yield of pro-
pylene carbonate (PC) enhanced about 12.4% when the reaction tem-
perature is increased from 90 °C to 110 °C. Afterwards, the yield of PC
has a slight decrease with the further enhancement of temperature,
which is ascribed to the occurrence of side reactions. The selectivity is
almost invariably in the whole process. The HEEPzBr is stable even at
206 °C without obvious weight loss suggesting that the HEEPzBr
3.3. Reusability of catalyst
Under the optimized conditions, the HEEPzBr is employed to cata-
lyze the reaction and recovered by distillation and reused for subse-
quent reactions (see Fig. 6). The PC yield is not significantly decreased
100
HEMPzBr
HEEPzBr
HPEPzBr
HEEMPzBr
HEEDMPzBr
100
80
60
40
20
0
90
80
70
Yield
Sel.
60
90
100
110
120
o
130
140
100
200
300
400
500
600
Temperature ( C)
Temperature (°C)
Fig. 1. Influence of temperature on PC yield and selectivity (reaction condition: PO 0.1 mol,
HEEPzBr 1 mol%, initial CO2 pressure 1.0 MPa, 4 h).
Fig. 2. TGA curves of five hydroxyl pyrazolium ILs.

T. Wang et al. / Journal of Molecular Liquids 293 (2019) 111479
5
100
90
80
70
60
100
90
80
70
Yield
Sel.
60
Yield
Sel.
50
1
2
3
4
5
0.5
1.0
1.5
2.0
2.5
3.0
Time (h)
Initial CO pressure (MPa)
2
Fig. 5. Influence of reaction time on PC yield and selectivity (reaction condition: PO
0.1 mol, HEEPzBr 1 mol%, CO2 initial pressure 1.0 MPa, 110 °C).
Fig. 3. Influence of CO2 initial pressure on PC yield and selectivity (reaction condition: PO
0.1 mol, HEEPzBr 1 mol%, 110 °C, 4 h).
if the HEEPzBr is reused in the second time. The distillation process is
complex, which is not suitable for large scale application. After the sec-
ond run, the catalyst is not separated. The catalyst is reused with addi-
tion of the 0.1 mol PO in the reaction system to ensure the reaction.
The PC yield is decreased for every run, which is attributed to the con-
sume of catalyst. At the sixth run, additional 25% HEEPzBr is involved
in the reaction system leading to the increase of PC yield, which is
even larger than the first run. It is not necessary to separate the catalyst
every time. The addition of slight amount of catalyst would further pro-
mote the reaction, which is easy to be operated.
3.5. Elucidation of different catalytic activity by DFT method
To further understand the difference between various ionic liquids,
the reaction mechanism is simulated by the DFT method. The three-
step reaction mechanism has been thoroughly elucidated in previous
literature along with the rate determining step of ring-opening of PO
[43,44]. In this work, only the rate determining step is studied following
the three-step mechanism. It has been testified that the result obtained
by “Double-IL” model is more reliable than “Single-IL” model [45].
Therefore, only the former model is employed in this work. The cation
is responsible for the electrophilic activation and the anion is in charge
of the nucleophilic activation.
3.4. Applicability of catalyst
HEEPzBr is still taken as the example to explore the mechanism. Pos-
sible routes by “Double-IL” model are considered according to the differ-
ent electrophilic activation. First, both hydroxyl groups are employed to
activate the O atom of PO; second, one hydroxyl group is employed to
activate the O atom of PO and the other is used to stabilize the Br−
anion; Third, one HEEPzBr plays both the electrophile and nucleophile
to activate the C\\O bond and the other one stabilize the system by
weak interactions. The corresponding structures and barrier heights
are plotted in Table S3. If the different hydrogen atoms are used to acti-
vate PO or stabilize the whole system, lots of other possible routes
would be confirmed. However, the deviation among them would not
The suitability of HEEPzBr are tested for other epoxides, the corre-
sponding data are tabulated in Table 2. The product yield is slightly af-
fected by the size of substituted group on C atom with the exception
of cyclohexene oxide. The large bulk of substituted group would block
the activation of C\\O bond of PO. However, the anion of HEEPzBr
would activate the other carbon atom with nonsubstitution. Conse-
quently, HEEPzBr has the least catalytic activity towards cyclohexene
oxide (7a) because of the largest bulk block for both carbon atoms.
100
100
90
25% of the initial amount
of HEEPzBr were added
*
*
80
*
*
80
60
40
20
0
70
60
Yield
Sel.
50
40
1
2
3
4
5
6
0.2
0.4
0.6
0.8
1.0
Reuse
Catalyst loading (mol%)
Fig. 6. Reused performance of the catalyst (Reaction conditions: PO 0.1 mol, HEEPzBr
1 mol%, CO2 initial pressure 1.0 MPa, temperature 110 °C, time 4 h), N99% PC selectivity
is maintained. * indicates another 0.1 mol PO was added into the reaction mixture.
Fig. 4. Influence of HEEPzBr loading on PC yield and selectivity (reaction condition: PO
0.1 mol, CO2 initial pressure 1.0 MPa, 110 °C, 4 h).

6
T. Wang et al. / Journal of Molecular Liquids 293 (2019) 111479
Table 2
Cycloaddition reaction of CO2 and various epoxides catalyzed by HEEPzBr.a
Entry
1
Epoxide
Cyclic carbonate
Yieldb (%)
91.2
1a
1b
2b
3b
2
3
4
89.4
87.4
74.3
2a
3a
4a
4b
5b
5
6
94.4
94.8
5a
6a
7a
6b
7b
7c
63.6
a
Reaction conditions: epoxides 100 mmol, HEEPzBr 1 mol% (mol% of pyrazolium salt moieties to epoxide), CO2 pressure 1.0 MPa, reaction time 4.0 h, temperature 110 °C.
Isolated yield.
Reaction conditions:temperature 130 °C.
b
c
be large since the difference among them is only aroused by the config-
On the basis of the natural bond orbital (NBO) analysis (see Fig. S2),
the charge of H atom in hydroxyl group is almost the same in three tran-
sition states suggesting the similar interactions of static electricity. Be-
sides it, the weak interactions also play a critical role to activate the
ring opening. The difference between them is mainly aroused by the
weak interaction. The non-covalent interaction is analyzed by atoms
in molecule (AIM) and noncovalent interactions (NCI), which is listed
in Table 3 and Fig. 8. There is hydrogen bond between H atom of hydro-
gen group and O atom of PO with the positive value of ∇2ρ. Two
uration rotation. They are not considered one by one. The route 1 is the
most favorable one with the lowest barrier height. Following the similar
pathway, the rate-determining step catalyzed by HEMPzBr and HPEPzBr
are further confirmed. The potential profiles are plotted in Fig. 7. The
barrier heights decrease in the order of D-HEMPzBr (HEMPzBr, 13.85
kcal/mol) N D-HPEPzBr (HPEPzBr, 12.12 kcal/mol) N D-HEEPzBr
(HEEPzBr, 10.43 kcal/mol), which is totally consistent with the experi-
mental product yields.
Fig. 7. Potential energy profiles and sketch structures of transition states for the ring-opening step along routes D-HEMPzBr, D-HEEPzBr, and D-HPEPzBr calculated at the M06/6-311 + G
(2d,2p) (PCM)//B3PW91/6-31G(d,p) level.

T. Wang et al. / Journal of Molecular Liquids 293 (2019) 111479
7
Table 3
Selected topological parameters of the bond critical points (BCP) and the charge of some important atoms in the transition states calculated at the B3PW91/6-31G(d,p) level of theory.
X-Y…Z
Sign(λ2)ρ
ρ
∇
2 ρ
G
V
H
G/︱V︱
D-HEMPzBr
D-HEEPzBr
D-HPEPzBr
O1-H…O3
O2-H…O3
Br1…C1
O1-H…O3
O2-H…O3
Br1…C1
O1-H…O3
O2-H…O3
Br1…C1
−0.04317
−0.04970
−0.03921
−0.03942
−0.05479
−0.03785
−0.04024
−0.05268
−0.03952
0.04317
0.04970
0.03921
0.03942
0.05479
0.03785
0.04024
0.05268
0.03952
0.12694
0.13310
0.07215
0.11578
0.14527
0.06839
0.12159
0.14158
0.06872
0.03259
0.03614
0.02098
0.02958
0.04025
0.01972
0.03061
0.03877
0.02034
−0.03345
−0.03900
−0.02392
−0.03021
−0.04418
−0.02234
−0.03082
−0.04215
−0.02350
−0.00086
−0.00286
−0.00294
−0.00063
−0.00393
−0.00262
−0.00021
−0.00338
−0.00316
0.97434
0.92657
0.87708
0.97907
0.91098
0.88260
0.99313
0.91992
0.86561
hydrogen bonds O1-H…O3 and O2-H…O3 are not equilibrium with the
different ρ value. The hydrogen bond, O1-H…O3, is the weaker than O2-
H…O3 with the smaller ρ value indicating the less important role. In
contrast, the other hydrogen bond plays the more important role to be
Fig. 8. NCI plots of D-HEMPzBr, D-HEEPzBr, and D-HPEPzBr. The corresponding 3D plots are displayed on the right with blue regions representing strong electrostatic interactions and
green regions representing more dispersion attractive interactions.

8
T. Wang et al. / Journal of Molecular Liquids 293 (2019) 111479
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HEEPzBr N D-HPEPzBr N D-HEMPzBr, which is consistent with the vari-
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4. Conclusions
Five hydroxyl-functionalized pyrazolium ionic liquids with different
chain length or substituted group are first synthesized. All of them pres-
ent high catalytic efficiency for the cycloaddition reaction of CO2 and PO
without any solvent and co-catalyst. HEEMPzBr is the best with the
product yield of 92.9% under 110 °C and 1.0 MPa CO2 initial pressure
with 1 mol% catalyst during 4 h. It is even better than most of previous
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We thank the National Supercomputing Center in Shenzhen
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Computing Center) for providing computational resources and soft-
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dation of China (21476061, 21503069, 21676071), Key Scientific
Research Project of Colleges and Universities in Henan Province
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