Dialkylpyrazolium ionic liquids as novel catalyst for efficient fixation of CO2 with metal- and solvent-free

Article abstract of DOI:10.1016/j.apcata.2017.09.009

The efficient fixation of CO₂ without co-catalyst and solvent under metal-free condition is still an urgent topic in sustainable chemistry. In this work, a series of dialkylpyrazolium ionic liquids are employed to promote the cycloaddition of CO₂ and PO to produce PC. They would be easily synthesized by a simple one-pot reaction. The effect of alkyl chain length in cation and different anion is explored. Diethylpyrazolium iodide presents the excellent catalytic activity with the product yield of 96% and selectivity of 99% in a benign condition. Moreover, the catalyst could be reused for at least five times without significant loss of catalytic activity. An intensive structure-activity research testifies that the cycloaddition of CO₂ with PO is activated by a synergistic effect from both cation and anion of ILs. To confirm it, the detailed mechanism is investigated by density functional theory associated with the non-covalent interactions and atoms in molecule analysis. Besides the electrostatic interaction between cation of ionic liquid and PO, the noncovalent interaction, especially for hydrogen bond, plays a vital role in promoting the reaction.

Full text of DOI:10.1016/j.apcata.2017.09.009

Accepted Manuscript
Title: Dialkylpyrazolium ionic liquids as novel catalyst for
efficient fixation of CO with metal- and solvent-free
2
Authors: Yuan Ma, Ci Chen, Tengfei Wang, Jingshun Zhang,
JiaJia Wu, Xiangdong Liu, Tiegang Ren, Li Wang, Jinglai
Zhang
PII:
S0926-860X(17)30437-4
DOI:
Reference:
http://dx.doi.org/10.1016/j.apcata.2017.09.009
APCATA 16403
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
19-7-2017
4-9-2017
6-9-2017
Please cite this article as: Yuan Ma, Ci Chen, Tengfei Wang, Jingshun Zhang, JiaJia Wu,
Xiangdong Liu, Tiegang Ren, Li Wang, Jinglai Zhang, Dialkylpyrazolium ionic liquids
as novel catalyst for efficient fixation of CO2 with metal- and solvent-free, Applied
Catalysis A, Generalhttp://dx.doi.org/10.1016/j.apcata.2017.09.009
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Dialkylpyrazolium ionic liquids as novel catalyst for efficient
fixation of CO with metal- and solvent-free
2
a,b
a,b
a,b
a,b
b
Yuan Ma , Ci Chen , Tengfei Wang , Jingshun Zhang , JiaJia Wu , Xiangdong
b
*a,b
*b
* b
Liu , Tiegang Ren , Li Wang , Jinglai Zhang
aEngineering Laboratory for Flame Retardant and Functional Materials of Henan Province, Fine
Chemistry and Engineering Research Institute
bCollege of Chemistry and Chemical Engineering, Henan University,Kaifeng, Henan 475004, PR
China
Graphic Abstract
*
Corresponding author E-mail: rtg@henu.edu.cn
Corresponding author E-mail: chemwangl@henu.edu.cn
Corresponding author E-mail: zhangjinglai@henu.edu.cn
*
*
1

Highlights




Dialkylpyrazolium ILs are firstly used as catalysts for the fixation of CO .
2
Good yields are achieved under a mild condition without solvent or co-catalyst.
The mechanism with a synergistic effect is found to promote the reaction.
They would be reused by five times without decrease of catalytic activity.
Abstract
The efficient fixation of CO
condition is still an urgent topic in sustainable chemistry. In this work, a series of
dialkylpyrazolium ionic liquids are employed to promote the cycloaddition of CO
2
without co-catalyst and solvent under metal-free
2
and PO to produce PC. They would be easily synthesized by a simple one-pot reaction.
The effect of alkyl chain length in cation and different anion is explored.
Diethylpyrazolium iodide presents the excellent catalytic activity with the product
yield of 96% and selectivity of 99% in a benign condition. Moreover, the catalyst
could be reused for at least five times without significant loss of catalytic activity. An
intensive structure-activity research testifies that the cycloaddition of CO with PO is
2
activated by a synergistic effect from both cation and anion of ILs. To confirm it, the
detailed mechanism is investigated by density functional theory associated with the
non-covalent interactions and atoms in molecule analysis. Besides the electrostatic
interaction between cation of ionic liquid and PO, the noncovalent interaction,
especially for hydrogen bond, plays a vital role in promoting the reaction.
Keywords: dialkylpyrazolium ionic liquids; CO
2
;
cycloaddition reaction;
structure-activity relationship; DFT;
1
Introduction
With the rapid development of economy and industrialization process, the fossil
fuels are gradually depletion. Carbon dioxide (CO ) is inexpensive, nontoxic,
abundant, and renewable C1 resource in the world. Many efforts have been made to
convert CO to useful chemical products including methane [1], carboxylic acid [2],
2
2
hydrocarbon [3], and cyclic carbonate [4]. The cyclic carbonate has been widely
applied in numerous regions, such as electrolytes of Li ion batteries [5], aprotic polar
solvents [6], degreasing agents [7], and valuable monomers of polycarbonates and
2

polyurethanes [8]. Up to date, the cycloaddition of CO with epoxides is regarded as
2
one of the most efficient methods to synthesize cyclic carbonate because of the 100%
atom utilization and negligible side product. However, the thermo stability and kinetic
inertness of CO
catalysts have been developed, such as, modified molecular sieves [10], metal oxide
11], transition metal complexes [12] and ionic liquids (ILs) [13], they confront some
2
greatly impede its large scale application [9]. Although numerous
[
common shortcomings including the requirement of co-catalyst and organic solvent,
harsh reaction condition, and others [14,15]. It is still an urgent problem to develop
single-component catalyst to activate the chemical fixation of CO
efficiency.
2
with high
Recently, ionic liquids have become a hotspot in catalytic region because of their
special structure and properties [16]. Moreover, they are also an excellent solvent.
Many ionic liquids have been employed as effective catalysts with metal- and
solvent-free to activate the cycloaddition of CO with epoxides [17]. Imidazolium
2
ionic liquids, as one of the most popular catalysts, have attracted extensive
experimental and theoretical attentions [18]. As their structural isomer, fewer
investigations have been focused on pyrazolium ionic liquids. In 1983,
dimethylpyrazolium iodide (DMPzI) was firstly synthesized by Claramunt et al [19].
Later, Enrique et al. have synthesized dibutylpyrazolium iodide (DBPzI) by
microwave radiation heating [20]. After that, Huynh et al. have synthesized
diethylpyrazolium bromide (DEPzBr) and used it to prepare N-heterocyclic carbene
complexes [21]. However, their catalytic activity for the conversion of CO
been investigated.
2
has never
In this work, they are firstly employed to accelerate the cycloaddition of CO
2
with epoxides. In addition, other twelve pyrazolium ionic liquids are also synthesized
to explore the influence of different alkyl chain length and anions on the catalytic
performance. Moreover, the effect of catalyst loading, initial CO pressure, reaction
2
temperature, and reaction time is studied to confirm the optimal reaction condition.
The reusability and generality of diethylpyrazolium iodide (DEPzI) is also considered.
Finally, the reaction mechanism is elucidated by density functional theory (DFT) to
uncover the difference of various catalysts from the micro viewpoints. It is expected
that pyrazolium ionic liquids would become a new focus in catalytic field after
imidazolium ionic liquids. Combination of theoretical and experimental studies would
open an express pathway to search for efficient and single-component ionic liquids to
3

promote the chemical fixation of CO in a proper condition.
2
2
Experimental section
2
.1 Instruments and Reagents
Fourier transform infrared (FT-IR) spectra were collected on Bruker Vertex 70
spectrometer (KBr disks). HR-MS was determined with Agilent 1290 Infinity LC with
224 TOF MSD. MS was determined by Bruker AmaZon SL (ESI) mass spectrometer.
6
1H NMR (400 MHz) and 13C NMR (100MHz) spectra were recorded on Bruker
Avance III HD spectrometer with the presence of tetramethylsilane as an internal
standard. The melting point was tested by using a differential scanning calorimeter
(
Mettler Toledo DSC851e) in the temperature range from 0°C to 200°C and the
thermal decomposition temperature was analyzed with a thermal gravimetric analyzer
Mettler Toledo TGA/SDTA851e). Elemental analyses were performed on a Vario EL
(
cube elemental analyzer. GC analyses were performed on Agilent GC-7890B using a
flame ionization detector.
The pyrazole and halohydrocarbon were purchased from Macklin Biochemical
Co., Ltd. Propylene oxide (PO), styrene oxide, epoxy chloropropane, and ethylene
oxide were purchased from Aladdin Co. Other commonly used chemical regents were
produced by Sinopharm Chemical Reagent Co., Ltd. CO (99.9% purity) were
2
purchased from Kaifeng Xinri Gas Co. All reactants were used directly as received
without any further purification.
2
.2 Preparation of pyrazolium ionic liquids
These dialkylpyrazolium ionic liquids were prepared according to the literature
[
21] with some revision. The typical synthesis procedure of dialkylpyrazolium ILs is
shown in Scheme 1. 1,2-Diethylpyrazolium Iodide (DEPzI): In a three-necked bottle,
pyrazole (10 mmol) and powdered KOH (10 mmol) were added into 10 mL CH CN
3
and the mixture was stirred at ambient temperature for 1 h. Then ethyl iodide (24
mmol) was added to reactants and the reaction was continued for 24 h under reflux
conditions. After the reaction, the volatiles were removed under reduced pressure.
CH
2
Cl was added to the residue and the resulting suspension was filtered. The
2
solvent of the filtrate was removed by vacuum distillation, and the resulting residue
was subsequently washed with ethyl acetate to give a yellow solid. After removing the
moisture by cyclohexane reflux, the pure DEPzI was obtained: 1.162 g, yield 46%,
1
m.p: 140.3°C. Tm (decomposition temperature of 10% weight loss): 201.1°C. H
NMR (400 MHz, CDCl
3
): δ 8.48-8.40 (d, J = 3.0 Hz, 2H, Py-H), 6.79 (t, J = 3.0 Hz,
4

1
3
1
H, Py-H), 4.86-4.79 (q, J = 7.3 Hz, 4H, -NCH
NMR (100 MHz, D O): δ 135.82 (s, N-CH), 107.43 (s, CH), 45.40 (s, N-CH
s, -CH ). Anal. Calcd for C I: C, 33.33; H, 5.159; N, 11.11. found: C, 33.66; H,
.375; N, 10.80. MS (ESI): m/z 125.14 [C
+]; 127.04 [I-].
Other dialkylpyrazolium iodide and bromide were synthesized with the identical
procedure for DEPzI.
,2-Dimethylpyrazolium Iodide (DMPzI). White solid (51%); m.p: 194.0°C.
2
), 1.69 (t, J = 7.3 Hz, 6H, -CH
3
). C
2
2
), 13.40
(
3
7
H
13
N
2
5
7
H
13
N
2
1
lit[19]: 186-187°C). Tm: 201.4°C. H NMR (400 MHz, D
H, Py-H), 6.60 (td, J = 3.0 Hz, 1H, Py-H), 3.99 (s, J = 1.2 Hz, 6H, N-CH
100 MHz, D O): δ 137.42 (s, N-CH), 107.00 (s, CH), 36.44 (s, -CH ). Anal. Calcd
I: C, 26.79; H, 4.018; N, 12.50. found: C, 26.35; H, 3.848; N, 12.29. MS
+].
1
O): δ 8.02 (d, J = 2.9 Hz,
(
2
13
2
3
). C NMR
(
2
3
for C
5
H
9
N
2
(ESI): m/z 97.30 [C
5
H
9
N
2
1
,2-Dipropylpyrazolium Iodide (DPPzI). Yellow solid (43%); m.p: 83.7°C. Tm:
1
1
99.6°C. H NMR (400 MHz, D
2
O): δ 8.07 (d, J = 3.0 Hz, 2H, Py-H), 6.62 (t, J = 3.0
), 1.79 (m, J = 7.4 Hz, 4H, -CH -), 0.80
). C NMR (100 MHz, D O): δ 136.87 (s, N-CH), 107.28 (s,
), 21.98 (s, -CH -), 9.81 (s, -CH ). Anal. Calcd for C I: C,
8.57; H, 6.071; N, 10.00. found: C, 38.58; H, 6.316; N, 10.03. MS (ESI): m/z 153.07
+].
Hz, 1H, Py-H), 4.27 (t, J = 7.3 Hz, 4H, N-CH
2
2
13
(
t, J = 7.4 Hz, 6H, -CH
3
2
CH), 51.35 (s, N-CH
2
2
3
9
H
17
N
2
3
[
C
9
H
17
N
2
1
,2-Dibutylpyrazolium Iodide (DBPzI). Yellow solid (42%); m.p: 71.4°C. Tm:
1
1
95.8°C. H NMR (400 MHz, D
2
O): δ 8.11 (d, J = 2.9 Hz, 2H, Py-H), 6.67 (t, J = 3.0
), 1.80 (m, J = 7.5 Hz, 4H, -CH -), 1.27
). C NMR (100 MHz, D O):
), 30.27 (s, -CH -), 18.84 (s,
I: C, 42.86; H, 6.818; N, 9.09. found:
C, 42.93; H, 6.807; N, 8.94. MS (ESI): m/z 181.06 [C11
+].
,2-Diamylpyrazolium Iodide (DAmPzI). Yellow solid (33%); m.p: 75.7°C. Tm:
Hz, 1H, Py-H), 4.35 (t, J = 7.4 Hz, 4H, N-CH
m, J = 7.4 Hz, 4H, -CH -), 0.84 (t, J = 7.4 Hz, 6H,-CH
δ 136.73 (s, N-CH), 107.29 (s, CH), 49.76 (s, N-CH
CH -), 12.65 (s, -CH ). Anal. Calcd for C11
2
2
13
(
2
3
2
2
2
-
2
3
H
21
N
2
H
21
N
2
1
1
1
2
1
95.0°C. H NMR (400 MHz, CDCl
.9 Hz, 1H, Py-H), 4.72 (t, J = 7.5 Hz, 4H, N-CH
.41 (m, J = 6.1, 3.3, 2.6 Hz, 8H, -CH -), 0.88 (t, 6H, -CH
I (m/z): 209.2018, found: 209.1985.
,2-Diethylpyrazolium Bromide (DEPzBr). Yellow solid (47%); H NMR (400
MHz, D O): δ 8.10 (d, J = 3.0 Hz, 2H, Py-H), 6.67 (t, J = 3.0 Hz, 1H, Py-H), 4.37 (q,
J = 7.3 Hz, 4H, N-CH
3
): δ 8.54 (d, J = 2.9 Hz, 2H, Py-H), 6.78 (t, J =
), 1.96 (m, J = 7.4 Hz, 4H, -CH -),
). HR-MS (QTOF) calcd.
2
2
2
3
for C13
H
25
N
2
1
1
2
2
), 1.46 (t, J = 7.2 Hz, 6H, -CH
3
). MS (ESI): m/z 125.14
5

[
C
7
H
13
N
2
+], 81.80 [Br-].
,2-Dipropylpyrazolium Bromide (DPPzBr). Yellow solid (43%); H NMR (400
MHz, D O): δ 8.07 (d, J = 3.0 Hz, 2H, Py-H), 6.62 (t, J = 3.0 Hz, 1H, Py-H), 4.27 (t, J
7.3 Hz, 4H, N-CH
MS (ESI): m/z 153.07 [C
1
1
2
=
2
), 1.79 (q, J = 7.4 Hz, 4H, -CH
2
-), 0.80 (t, J = 7.4 Hz, 6H, -CH ).
3
H
17
N
2
+].
9
1
1
,2-Dibutylpyrazolium Bromide (DBPzBr). Yellow solid (42%); H NMR (400
MHz, D
7.4 Hz, 4H, N-CH
.86 (t, J = 7.4 Hz, 6H, -CH
,2-Diamylpyrazolium Bromide (DAmPzBr). Yellow solid (33%); H NMR (400
MHz, D O): δ 8.07 (d, J = 3.0 Hz, 2H, Py-H), 6.63 (t, J = 3.0 Hz, 1H, Py-H), 4.31 (t, J
7.4 Hz, 4H, N-CH ), 1.83-1.73 (m, 4H, -CH -), 1.19 (tt, J = 5.2, 2.5 Hz, 8H, -CH -),
.76-0.70 (t, 6H, -CH ). MS (ESI): m/z 209.08 [C13
+].
,2-Dihexylpyrazolium Bromide (DHPzBr). White solid (25%); m.p: 94.2°C. Tm:
85.8°C. 1H NMR (400 MHz, CDCl
): δ 8.65 (d, J = 2.9 Hz, 2H, Py-H), 6.75 (t, J =
.8 Hz, 1H, Py-H), 4.73 (t, J = 7.5 Hz, 4H, N-CH ), 1.94 (q, J = 7.4 Hz, 4H, -CH -),
.42 (h, J = 6.2, 4.4 Hz, 4H, -CH -), 1.32 (tt, J = 8.8, 5.0 Hz, 8H, -CH -), 0.94-0.84
). HR-MS (QTOF) calcd. for C15 Br (m/z): 237.2331, found:
2
O): δ 8.15 (d, J = 2.9 Hz, 2H, Py-H), 6.70 (t, J = 3.0 Hz, 1H, Py-H), 4.38 (t, J
), 1.87-1.77 (m, 4H, -CH -), 1.29 (m, J = 7.4 Hz, 4H, -CH -),
). MS (ESI): m/z 181.06 [C11
+].
=
2
2
2
0
3
H
21
N
2
1
1
2
=
2
2
2
0
3
H
25
N
2
1
1
2
1
3
2
2
2
2
(
m, 6H, -CH
3
H
29
N
2
2
37.2338.
1
,2-Dioctylpyrazolium Bromide (DOPzBr). White solid (18%); m.p: 112.4°C.
1
Tm: 186.0°C. H NMR (400 MHz, DMSO-d6): δ 8.60 (d, J = 2.9 Hz, 2H, Py-H), 6.93
(
-
(
t, J = 2.9 Hz, 1H, Py-H), 4.47 (t, J = 7.4 Hz, 4H, N-CH
CH -), 1.27 (dd, J = 11.4, 5.7 Hz, 20H, -CH -), 0.89-0.83 (t, 6H, -CH
QTOF) calcd. for C19 Br (m/z): 293.2957, found: 293.2962.
2
), 1.82 (p, J = 7.2 Hz, 4H,
2
2
3
). HR-MS
H
37
N
2
1
,2-Didecylpyrazolium Bromide (DDePzBr). White solid (13%); m.p: 115.5°C.
1
Tm: 184.8°C. H NMR (400 MHz, DMSO-d6): δ 8.57 (d, J = 2.9 Hz, 2H, Py-H), 6.93
(
t, J = 2.9 Hz, 1H, Py-H), 4.45 (t, J = 7.4 Hz, 4H, N-CH
J = 13.5 Hz, 28H, -CH -), 0.88-0.84 (t, 6H, -CH ). HR-MS (QTOF) calcd. for
Br (m/z): 349.3583, found: 349.3679.
2
), 1.80 (s, 4H, -CH -), 1.26 (d,
2
2
3
C
23
H
45
N
2
1
,2-Didodecylpyrazolium Bromide (DDoPzBr). White solid (11%); m.p:
1
1
17.2°C. Tm: 188.1°C. H NMR (400 MHz, CDCl
3
): δ 8.58 (d, J = 2.9 Hz, 2H, Py-H),
), 1.93 (p, J = 7.5 Hz,
-), 0.88 (t, J = 6.8 Hz, 6H, -CH ). HR-MS
6
4
.74 (t, J = 2.9 Hz, 1H, Py-H), 4.73 (t, J = 7.5 Hz, 4H, N-CH
2
H, -CH -), 1.48-1.18 (m, 32H, -CH
2
2
3
(QTOF) calcd. for C27
H
53
N
2
Br (m/z): 405.4209, found: 405.4272.
6

1
,2-Diethylpyrazolium tetrafluoroborate (DEPzBF
mmol NaBF was suspended in 10mL acetone and stirred at 60°C for 6 h. After the
reaction, the mixture was filtered and the solvent of filtrate was removed, then the
product was dried under vacuum. Yellow solid (76%): MS (ESI): m/z 87.83 [BF
-].
,2-Diethylpyrazolium hexafluorophosphate (DEPzPF ). 10 mmol DEPzI were
dissolved in 30mL distilled water, then 10 mmol KPF were added in batches, small
4
). 10 mmol DEPzI and 12
4
4
1
6
6
white crystal has been generated immediately, reaction continues to stir for 5 h and
the mixture was filtered and the residue was washed with water until no precipitation
formation with AgNO
3
, the product was dried under vacuum. Yellow solid (42%): MS
-]. Both DEPzBF
and DEPzPF were verified by silver nitrate
(ESI): m/z 144.99 [PF
6
4
6
solution, there were no apparent precipitations. And almost no iodide ion peak was
appeared in the mass spectrum indicating the high purity (>99%) of two ILs.
2
.3 Cycloaddition reaction of CO
An epoxide/catalyst mixture was introduced in the high pressure cell (100 mL) at
ambient temperature, then CO (1.0-3.0 MPa) was added to the cell and heated up to
2
with epoxides
2
the desired temperature. The reaction was stirred at 90-140°C for 1-5 h. After this
reaction, the reactor was cooled to room temperature in an ice-water bath and the
pressure was released. The autoclave was opened, and the isolated yields were
calculated from the weight of product (combination of cyclic carbonate and starting
material only, or (as indicated) by the weight of isolated product after column
chromatography. Some reactions were repeated and the reproducibility of yields was ±
1
3
%). The structures of cyclic carbonates were characterized by H NMR. In order to
verify the reliability of isolated yield and detect the selectivity, the product yields of
some reactions were analyzed on GC.
3
Results and discussion
3
.1 Synthetic and property of ILs
Fifteen dialkylpyrazolium ILs with different alkyl chain and anions have been
easily synthesized with a one-pot alkylation reaction. DEPzBF
4
and DEPzPF are
6
synthesized by anion exchange reaction on the basis of DEPzI (Scheme 1). Among
them, DMPzI, DEPzBr, and DBPzI have been reported, while others are firstly
synthesized by us. In order to improve the product yield, KOH is added to the mixture
as acid-binding agent. In the post-processing course, the moisture was removed by
cyclohexane reflux, which could effectively purify the product. The structures of all
1
13
ionic liquids have been characterized by H NMR, C NMR, HR-MS, and elemental
7

analysis. The yields of dialkylpyrazolium ILs are reduced with the increase of chain
length, which is attributed to the large steric hindrance of long alkyl chain.
Additionally, the low yield of ionic liquids is aroused by introduction of two alkyl
chains into the pyrazolium cation.
The thermal stability of five representative dialkylpyrazolium ILs are
investigated by thermogravimetric analysis (TGA) (See Fig. 1). All of them present
good thermal stability since the degradation started in a range of 195-201°C. The ILs
have one-stage decomposition behaviors and the decomposition temperature is
gradually reduced with increase of alkyl chain length. Moreover, there is no distinct
mass loss below 200°C, which also justify the excellent purity of synthetic ILs. The
dialkylpyrazolium ILs would be changed absolutely to gaseous products by thermal
decomposition since they are decomposed in higher temperature with nearly zero
weight percentage.
3
.2 Effect of catalyst
The synthesized dialkylpyrazolium ionic liquids are tested as catalyst for the
synthesis of propylene carbonate (PC) via the model reaction of propylene oxide (PO)
with CO under solvent-free condition without adding any co-catalyst. The results
were summarized in Table 1. The formation of PC is not observed without any
catalyst (entry 1) indicating that the cycloaddition of CO with PO is difficult to be
2
2
performed without catalyst even during 8 h reaction time. There is almost no catalytic
activity with the existence of KBr during the same time. Acceptable yields of PC are
observed when dialkylpyrazolium iodides are employed as catalysts (entries 3-7)
during only 4 h. Among them, DEPzI is found to be the most active with the highest
product yield of 91.2% (entry 4). In contrast, DMPzI has the lowest activity with the
lowest product yield (83.3%, entry 3), while DPPzI, DBPzI, and DAmPzI have the
similar catalytic activity with almost the same product yield (around 89%, entries 5-7).
To further investigate the influence of alkyl chain length on catalytic activity, a series
of dialkylpyrazolium bromide are synthesized and used as catalysts (entries 8-15).
When alkyl chain length increases from DEPzBr to DPPzBr, there is a slight decrease
in the product yield (entries 8-9). After that, the product yield is almost not varied
from DPPzBr to DAmPzBr (entries 9-11). However, there is obvious decrease when
the alkyl chain length is up to DHPzBr (entry 12). The product yield fluctuates in a
range of 73.9%-80.3% with further elongation (entries 12-15). In general, the product
yield presents stepped variation with the elongation of alkyl chain length. The larger
8

steric bulk of longer alkyl chain length would obstruct the electrophilic attack
resulting in the less activity. However, the larger bulk of pyrazolium cation would also
weaken the electrostatic interaction between cation and anion, which renders the
counter halide more nucleophilicity. Consequently, there is not obvious reduction of
product yield. The variation order of dialkylpyrazolium bromide is similar with that of
dialkylpyrazolium iodide except that the product yields catalyzed by the latter are
higher, which is attributed to their different anions.
The product yields decrease in the order of DEPzI (entry 4) > DEPzBr (entry 8) >
DEPzBF
4
(entry 16) > DEPzPF
6
(entry 17), which is consistent with the nucleophilic
and PF -
anions would impede
and leaving ability of anions. The larger bulk of BF -
4
6
the nucleophilic attack resulting in the poor activity. In the other word, anion plays an
important role in catalytic process.
The product yields catalyzed by DEPzI (entry 4) and DEPzBr (entry 8) are also
confirmed by the GC method. Although the absolute product yields are improved,
their relative order is still kept. The relative sequence of isolated yield is reliable,
which would be increased if GC analysis method is employed to determine the
product yield.
As compared with DEImI (entry S2) (See Table S1), DEPzI (entry S1) presents
better product yield and TOF under the same reaction condition. Since they have the
same anion, the different product yield should be attributed to their different cation.
Moreover, the catalytic performance of DEPzI is better than that of TBAI (entry S3)
[
22], although the latter reaction is performed under higher temperature and during
longer time. In contrast, the product yield of DEPzI is much less than that of HBetI
entry S4) [23] and HEMImI (entry S5) [24] and comparable to that of other task
specific ionic liquids (entries S6-S8) [25-27]. It is worth to note that the reaction
temperature and initial CO pressure for CEMImBr (entry S6) is even lower than that
(
2
for DEPzI indicating the high catalytic activity for the former. Incorporation of
functional group is an efficient pathway to improve the catalytic performance. It is
reasonable to believe that the higher product yield or milder reaction condition would
be expected if the functional group is incorporated into the pyrazolium cation, which
would be better than DEPzI (entry S1).
3
.3 Effect of reaction conditions
In the following discussion, DEPzI is selected for further investigation because
of its highest catalytic activity for cycloaddition of CO with PO. The effect of
2
9

catalyst loading, initial CO pressure, reaction temperature, and reaction time on the
2
catalytic activity is explored one by one.
First, the influence of catalyst loading on PC yields is investigated under
identical reaction conditions (See Fig. 2). The PC yield is sharply increased from 44%
to 94.4% with the increase of DEPzI loading from 0.2 mol% to 1.0 mol%. After that,
the further increase of DEPzI amount only leads to a slight enhancement of PC yield
indicating that the optimal catalyst amount is 1.0 mol%. DEPzI could exhibit high
activity under a low catalyst amount (1.0 mol%). Moreover, the PC selectivity is
almost impervious (>99%) under catalyst loading from 0.2 mol% to 1.0 mol%. The 99%
selectivity rather than 100% should be attributed to the generation of few possible
by-products including acetone or 1,2-propylene diol [28].
Next, the influence of initial CO
studied (See Fig. 3). The PC yield dramatically increases from 70.1% to 94.4% when
the initial CO pressure increases from 1 MPa to 2 MPa. But there is mild decline by
further incensement of initial CO pressure to 3 MPa. The high initial CO pressure
would aggrandize CO concentration in the liquid phase of reaction system leading to
higher product yield. However, the higher CO pressure puts more PO into the gas
phase. Consequently, the concentration of PO around ILs is reduced, which is not
favorable for the reaction since it is also a substrate [29]. The relative lower CO
pressure presents more potential industrial application [30,31]. Moreover, the PC
selectivity is independent with CO pressure with above 99% throughout.
2
pressure on the yield and selectivity of PC is
2
2
2
2
2
2
2
Third, the influence of reaction temperatures on PC yield and selectivity is
considered (See Fig. 4). The reaction is difficult to be carried out at 90°C with PC
yield less than 50%. The PC yield increases greatly with the temperature increasing
from 90°C to 120°C. Under the experimental conditions, a good yield (91.2%) could
be reached at 120°C. The product yield would be increased to 94.4% at 130°C. When
the temperature is higher than 130°C, there is no significant increase for PC yield. The
selectivity of PC has almost no changes in the whole process. Meanwhile, the
decomposition temperature of DEPzI is higher or close to 200°C in TGA analysis,
which certifies the catalyst is stable at reaction temperature (130°C).
Finally, the effects of reaction time on PC yield and selectivity are shown in Fig.
5
. The PC yield increases rapidly up to 94.4% within the first 4 h. Then, there is only
a mild enhancement when another hour is prolonged. The PC selectivity is >99%
throughout. In general, the optimal reaction condition is under 130°C, 2 MPa initial
10

CO pressure and 4 h with 1 mol% catalyst loading.
2
3
.4 Catalyst recycling
The catalyst reusability is also an important criterion to judge the performance of
homogeneous catalyst. To test the catalyst reusability, the reaction of PO with CO is
2
performed under the optimal reaction conditions. After the catalytic reaction, the
mixture is distilled under reduced pressure to remove large amounts of propylene
carbonate, and the residue is washed by ethyl acetate. The ILs are obtained by
centrifugalization. Subsequently, the catalyst is dried under vacuum to be used in next
return. The results are shown in Fig. 6. The PC yield almost remains constant after
five cycles. There is only a small amount loss of DEPzI suggesting that the DEPzI has
good stability and recyclability in the cycloaddition of CO
.5 Catalytic activity toward other epoxides
The generality of DEPzI is examined to catalyze the cycloadditon of CO
2
with PO.
3
2
with
five epoxides and the corresponding results are summarized in Table 2. DEPzI is an
efficient catalyst for most of epoxides (entries 1-4) with the cyclic carbonate yields
around 90% under the experimental reaction condition. The lower 5b yield should be
attributed to the higher hindrance of 5a, which would greatly impede the nucleophilic
attack from anion. Even the reaction time is extended to 24 h, the product yield of 5b
is still the lowest in five investigated epoxides. The difficulty to activate 5a testifies
again the importance of anion to promote the reaction. Reactions listed in Table 2 are
performed under 120°C, since there is a slight improvement for product yield of
DEPzI when the temperature is increased from 120°C (91.2%) to 130°C (94.4%). To
obtain a high ratio of performance/cost, the milder temperature 120°C is employed to
study other reactions. Moreover, our main goal is to compare the relative product
yields for them rather than obtain their optimal product yields.
3
.6 Reaction mechanism
To deeply understand the different catalytic activity of various dialkylpyrazolium
ILs with different alkyl chain length, the detailed mechanisms are investigated by
DFT method involved by the Gaussian 09 program package [32] associated with the
non-covalent interactions (NCI) [33,34] and atoms in molecule (AIM) analysis [35].
The coupling reaction of CO
2
with PO follows three steps, i.e., ring-opening of
PO, CO insertion, and ring-closure to generate PC (See Scheme 2). The ring-opening
2
of PO is the synergetic result of both electrophilic attack from cation and nucleophilic
attack from anion. The different catalytic activity is aroused by the different
11

electrophilic attack of cation since they have same anions. In the following
-
-
calculations, the I anion is replaced by Br anion to save the computational cost. Not
only the mechanism but also the relative sequence of catalytic activity would not be
varied. The structures of reactants, intermediates, and transition states are optimized
by the B3PW91 [36,37] method combined with 6-31G(d,p) [38] basis set. The
energies are refined at the M06/6-311+G(2d,2p) level [38,39] on the basis of
optimized geometries. The solvent effect is taken into consideration by the polarized
continuum model (PCM) in ethyl ether (Et O) solvent [40,41]. The atomic charge of
2
some transition states is calculated by natural bond orbital (NBO) analysis [42,43].
Normally, the electrophilic attack is completed by one or several hydrogen atoms
in cation. Three possible reaction routes (routes 1-3) are explored to confirm the
optimal route, in which each hydrogen atom is taken as the possible electrophile,
respectively. The corresponding barrier heights of ring-opening step are shown in
Table S2 along with the schematic structures of transition states. The catalytic activity
would be further increased if more hydrogen atoms are included in the electrophilic
attack. Other three possible routes (routes 4-6) are considered (See Table S2), in
which two hydrogen atoms are taken as the electrophile. Owing to the steric hindrance,
no more hydrogen atoms from one IL would be employed to activate the O atom of
PO. Route 5 is the most possible reaction mechanism because of its lowest barrier
heights. Following in the same model, reaction mechanism of other ILs is also
confirmed. As shown in Table S3, the ring-opening step is the rate-determining step
with the higher barrier height for all studied ILs. In the next discussion, only the
barrier height of ring-opening step is mentioned.
The rate-determining barrier height of route A is the lowest suggesting the best
catalytic activity for DEPzBr. There is an increase for rate-determining barrier height
of route B, which is consistent with the decreased product yield in the presence of
DPPzBr. The variation of barrier heights from route B to route D is as small as 0.15
kcal/mol indicating the slightly difference of catalytic activity for DPPzBr, DBPzBr,
and DAmPzBr, which is consistent with the experimental sequence. Further extending
the alkyl chain length results in relatively little improvement of barrier heights, which
is in agreement with their decreased product yield. However, a slightly decreased
barrier height is observed for DDoPzBr corresponding to the slight increase of
product yield. In general, the sequence of calculated rate-determining barrier heights
is totally consistent with the measured experimental product yields. The DEPzBr,
12

DPPzBr, and DHPzBr are three critical turning points in variation of catalytic activity.
The potential energy profiles are shown in Fig. 7 along with the schematic structures
of transition states.
Their AIM analysis is also performed to elucidate the role of weak interaction
played in the ring-opening step (See Table 3). Two interactions, C-H1…O and
C-H2…O, are definitely noncovalent because of the positive Laplacian values of the
2
electron density ( ). Moreover, C-H2…O interaction is stronger than C-H1…O
interaction due to the larger  values. Actually, C-H2…O interaction has both
covalent and hydrogen bond characteristics since the  value of C-H2…O is larger
than the upper limitation for classical hydrogen bond (0.035 a.u.). [44,45] Not only
C-H1…O interaction but also C-H2…O interaction in TSB1 are smaller than those
corresponding values in TSA1 indicating the slightly smaller electrophilic attack of
former. While C…Br interaction is almost the same in TSA1 and TSB1 with the 
values of 0.03400 a.u. and 0.03402 a.u., respectively, suggesting the similar
nucleophilic attack for them. The larger electrophilic ability associated with the
almost equivalent nucleophilic ability finally results in the lower barrier height for
TSA1. As compared with TSA1, TSE1 has the smaller C-H1…O interaction and
larger C-H2…O interaction, which is difficult to make a decision between them.
Moreover, the C…Br interaction in TSE1 is much larger than that in TSA1, which is
favorable to reducing the barrier height. However, the barrier height of TSE1 is higher
than that of TSA1. The contrary result should be attributed that the noncovalent
interaction is not the sole critical factor to determine the catalytic activity. The
electrostatic interaction should also be considered. The natural bond orbital (NBO) of
both TSA1 and TSE1 is calculated at the same level (See Fig. S1). TSE1 has not only
-
smaller positive charge for H1 atom but also less negative charge for Br anion
indicating the weaker electrostatic interaction between DHPzBr and PO, which results
in its higher barrier height. However, the difference of various routes is not much
large since no one has absolute superiority with all favorable items. The
corresponding non-covalent interactions (NCI) plots are presented in Fig. 8 to display
the non-covalent interaction more clearly.
To investigate the hydrogen bond interaction between reactive hydrogen atom of
1
pyrazolium cation and PO, FT-IR and H NMR analysis of DEPzI with and without
13

PO is performed. In the FT-IR spectra (Fig. 9), a new characteristic absorbance
-1
appears in curve b at 3281 cm due to the stretching vibrations of OH groups
indicating the formation of C-O…H bond. Analogous research is also reported by Dai
1
et al [46]. In the H NMR spectra of mixture of DEPzI and PO (Fig. 10), the signal at
δ = 8.56 ppm assigned to the C3/C5 proton of pyrazole ring shifts to δ = 8.59 ppm.
Additionally, the signal for N-CH
2
proton of DEPzI shifts from δ = 4.48 ppm to δ =
4
.55 ppm suggesting the formation of hydrogen bond between DEPzI and PO. The
above analysis indicates that both C3/C5-H and methylene-H can form hydrogen
bonds with oxygen atom of PO, which is favorable for ring-opening of PO [47]. Both
1
the FT-IR and H NMR analysis supports the results obtained by DFT calculation and
NCI analysis.
4
Conclusions
Dialkylpyrazolium ILs are firstly employed as single-component catalysts for the
conversion of CO with PO to produce PC with metal- and solvent-free. Moreover,
2
they would be synthesized by simple and high efficient one-pot method via alkylation
reaction. Among the investigated ILs, DEPzI presents the best catalytic activity with
the highest product yield. When the alkyl chain in cation is increased, the catalytic
activity is decreased with step-shaped character. The optimal reaction condition is
confirmed to be 130°C, 2 MPa initial CO pressure and 4 h with 1 mol% DEPzI
2
loading. Moreover, DEPzI could be reused for five cycles without obvious loss of
catalytic activity. Finally, the detailed mechanism investigated by DFT associated
with NCI and AIM analysis shows that the ring-opening of PO is promoted by both
the hydrogen bond and electrostatic interaction. The dialkylpyrazolium ILs are
potential candidates in the CO fixation with prominent advantages including facile
2
synthesis, good recyclability, and acceptable catalytic activity, which is an important
topic for sustainable development.
Acknowledgment
We thank the National Supercomputing Center in Shenzhen (Shenzhen Cloud
Computing Center) for providing computational resources and softwares. This work
was supported by the National Natural Science Foundation of China (21376063,
2
1476061, 21503069, 21676071), Program for He’nan Innovative Research Team in
University (15IRTSTHN005).
14

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Util. 17 (2017)
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[
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17

Fig. 1 TGA traces for five selected ILs.
18

100
80
60
40
Yield
Sel.
0.2
0.4
0.6
0.8
1.0
1.2
Catalyst loading (mol%)
Fig. 2 Influence of catalyst loading on PC yield and selectivity (reaction condition:
PO 0.1 mol, initial CO pressure 2 MPa, 130 °C, 4 h).
2
19

100
80
60
40
Yield
Sel.
1
.0
1.5
2.0
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3.0
Initial CO pressure (MPa)
2
Fig. 3 Influence of CO
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pressure on PC yield and selectivity (reaction condition: PO
0
.1 mol, DEPzI 1 mol%, 130 ºC, 4 h).
100
80
60
40
Yield
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80
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120
140
Temperature (℃)
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.1 mol, DEPzI 1 mol%, initial CO pressure 2 MPa, 4 h).
0
2
20

100
80
60
40
Yield
Sel.
0
1
2
3
4
5
6
Time (h)
Fig. 5 Influence of reaction time on PC yield and selectivity (reaction condition: PO
0
.1 mol, DEPzI 1 mol%, initial CO
2
pressure 2 MPa, 130 °C).
100
Yield
90
80
70
60
1
2
3
4
5
Reuse
Fig. 6 Reused performance of the catalyst (Reaction condition: PO 0.1 mol, DEPzI
mol%, initial CO pressure 2 MPa,130 ºC, 4 h).
1
2
21

Fig. 7 Energy profiles for the cycloaddition reaction along route A, route B, and route
E calculated at the M06/6-311+G(2d, 2p)//B3PW91/6-31G(d, p) (PCM) level in Et O
2
solvent.
22

Fig. 8 NCI plots for TSA1, TSB1, and TSE1. The corresponding 3D plots are displayed
below with blue regions representing strong electrostatic interactions and green
regions.
23

3
281
a
b
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm )
Fig. 9 FT-IR spectra of DEPzI (a) without and (b) with PO.
24

1
Fig. 10 H NMR spectra ([D
6
]DMSO, 25°C) of DEPzI (0.1 mmol) with and without
PO (0.2 mmol).
25

Scheme 1 Synthetic route and structures of the dialkylpyrazolium
ionic liquids (DAPzILs).
26

Scheme 2 The proposed mechanism of route A.
27

Table 1 Catalytic performance of various catalysts a
b
Entry
Catalysts
None
Time/h
Yield /%
1
2
3
4
5
6
7
8
9
8
8
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
-
KBr
3.3
DMPzI
DEPzI
83.3
91.2(96.1) c
89.1
DPPzI
DBPzI
89.1
DAmPzI
DEPzBr
DPPzBr
DBPzBr
DAmPzBr
DHPzBr
DOPzBr
DDePzBr
DDoPzBr
89.9
89.1(94.2) c
87.4
1
0
87.9
11
87.4
1
1
1
1
1
1
2
3
4
5
6
7
80.3
79.1
73.9
78.1
DEPzBF
4
51.2
DEPzPF
6
5.5
aReaction conditions: PO 0.1 mol, catalyst 1 mol% (mol% of catalyst moieties to
b
c
epoxide), 120 °C, initial CO
2
pressure 2 MPa. Isolated yield. Yield was determined
by GC.
28

Table 2 Cycloaddition of CO
with various epoxides catalyzed by DEPzIa.
2
b
Entry
Epoxide
Cyclic carbonate
Yield /%
1
88.1
1
a
1
b
2
3
91.2
86.8
2
a
2
b
3
a
3
b
4
4
5
90.4
4
a
b
60.2 c
5
a
5
b
aReaction conditions: epoxide 0.1 mol, DEPzI 1 mol% (mol% of pyrazolium salt
moieties to epoxide), 2 MPa CO
h.
2
, 120 °C, 4 h. bIsolated yield. cReaction time: 24
29