Modified polyether glycols supported ionic liquids for CO2 adsorption and chemical fixation

Article abstract of DOI:10.1016/j.mcat.2020.111008

Hydroxyl polyether-grafted imidazolium ionic liquids (HPEG_nRIMX) have been synthesized by a chemical stoichiometric method, which were applied into strengthening CO₂ accessibility and conversing CO₂ into cyclic carbonates.

Full text of DOI:10.1016/j.mcat.2020.111008

Molecular Catalysis 492 (2020) 111008
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Modified polyether glycols supported ionic liquids for CO adsorption and
2
chemical fixation
a,
a,b
b
a
Ying Liu *, Yan Song , Jianhua Zhou , Xiangping Zhang
a
CAS Key Laboratory of Green Process and Engineering, State Key Laboratory of Multiphase Complex System, Beijing Key Laboratory of Ionic Liquids Clean Process,
Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
b
School of Chemistry and Pharmaceutical Engineering, Qilu University of Technology, Jinan 250353, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Hydroxyl polyether-grafted imidazolium ionic liquids (HPEG RIMX) have been synthesized by a chemical
n
Monohydroxyl-capped polyether-grafted
imidazolium ionic liquids
stoichiometric method, which were applied into strengthening CO accessibility and conversing CO into cyclic
2
2
carbonates. The structure was determined by FT-IR, NMR, TGA and MS. Under the optimum reaction conditions
2
CO -philicity
of 1.0 mol% HPEG
n
RIMX, 2.0 MPa CO pressure, 130 °C and 1.5 h, it could achieve excellent catalytic activities.
2
Epoxides
The performance was over than that of PEG400DEIMBr with double terminal ending-capped ILs functional groups
due to its multifunctional synergestic roles including the long polyether chains, hydrogen-bonding between
Cycloaddition
hydroxyl groups on HPEG ILs and oxygen atoms on cyclo-epoxides, and the halide anionic nuclephilicity. The
n
mechanism was further investigated by IGA and in-situ NMR spectrum.
1
. Introduction
CO as the aboundant C1 resource from combustion of fossil fuels
Polyether glycols(PEGs)-supported ILs as the good CO
2
soluble
catalysts [30] containing with double ammonium-based ILs [31],
phosphonium-based ILs [32] and others special-tasked ILs catalysts
[33], which not only performed lower vapor pressure and easily being
withdrawn through simple filtration, but also could show good re-
cyclability like those conventional heterogeneous catalysts. Up to this
date, Yang group [34] has reported glycol-based di-cationic ILs, for
2
(
coal, petroleum and natural gas) has drawn much more attraction to
convert into some useful main chemicals such as cyclic carbonate [1–3],
cyclic acids [4], formic acid [5,6], methanol [7], polycarbonate [8,9],
diethyl carbonate [10], dimethyl carbonate [11] and diphenyl carbo-
nate [12]. Cyclic carbonate [13,14] has been considering as the most
versatile, usable compounds or intermediates in the fields of organic
synthesis [15], electrolytes [16,17], polycarbonates [18], and poly-
urethanes [19].
example
XDMImPEG150DMImX,
BrDBUPEG150DBUBr
and
BrTBDPEG150TBDBr, which could catalyze the chemical fixation of CO
2
to cyclic carbonates with 74–99% yields within the longer reaction time
of 3−4 h under the conditions of 80−120 °C, 1.0–3.0 MPa [35].
Homogeneous catalysts including alkali-metal salts [20], salen
metal complexes [8,21], amino acids [22], quaternary ammoniums
PEG6000-supported ammonium salts transformed CO into cyclic car-
2
bonates under the harsh supercritical condition (> 8 MPa CO
2
pres-
[
23], phosphonium salts [24], and the solid heterogeneous catalysts
sure) [36,37].
such as metal oxide [25], organic-inorganic hybrid catalyst [26], in-
soluble ion-exchange resins [27] or polymeric ILs [2,28] have been
Hydrogen-bonding between hydroxyl groups on catalysts and
oxygen atom of cyclo-epoxides could enhance producing cyclic carbo-
developed and successfully applied into cycloaddition of CO
2
with cyclo
nates from CO over task-specific ILs catalysts [38,39]. For example,
2
epoxides for cyclic carbonates. These homogeneous catalysts were dif-
ficultly recovered and recycled, and cyclic carbonates were usually
through some tedious prufication process, for example distillation and
solvent extraction [29]. During the process, much high energy con-
sumption or some organic solvents were often cost and wasted. Het-
erogeneous catalysts could resolve the separation and reusability pro-
blem. However, they usually encountered the loss of catalytic active
species or withdrawal problem from the large-scaled fixed-bed reactor.
hydroxyl-functionalized micro-porous organic polymer (HF-MOP) [40],
1-(2-hydroxylethyl)-imidazolium-based ionic liquid functionalized
graphene oxide (GO-HEIMBr) [41], polystyrene-supported 1-(2-hy-
droxyl-ethyl)-imidazolium bromide (PS-HEIMBr) [42] and polymer
nanoparticles grafted hydroxyl-functionalized phosphonium-based
ionic liquids (PNP-HPILs) [43] generally performed highly efficient
catalytic activities in conversion of CO
2
into cyclic carbonates because
of the hydrogen-bonding’s synergistic facilitation role.
⁎
Corresponding author.
E-mail address: yingliu1013@ipe.ac.cn (Y. Liu).
https://doi.org/10.1016/j.mcat.2020.111008
Received 5 February 2020; Received in revised form 6 May 2020; Accepted 6 May 2020
2468-8231/ © 2020 Elsevier B.V. All rights reserved.

Y. Liu, et al.
Molecular Catalysis 492 (2020) 111008
In this work, a series of hydroxyl polyethylene ether-grafted imi-
successively rotary concentration process, and dried over vacuum for
dazolium ionic liquids (HPEG
n
RIMX) have been synthesized through a
4−6 h. The yollowish HPEG
n
Cl was obtained with the isolated yield of
1
chemical stoichiometric method, and determined their structures by FT-
IR, NMR, TGA and MS (seeing in Figure S1, S5-S7). The conversion of
above 80%, and determined by H NMR (Figure S2-S4), MS spectrum
(Figure S5) and TGA (seeing in Figure S11).
1
CO
2
into cyclic carbonates was used as the model reaction to evaluate
HPEG200Cl. Pale yellow oily liquid; H NMR (D
2
O, 600 MHz) δ
HPEG
n
RIMX’s catalytic activities. Compared with polyethylene ether
3.76−3.74 (m, 4H, CH
3.57−3.55(m, 2H, CH ).
2
CH
2
Cl), 3.67−3.62(m, 19H, CH OH),
2
ending-capped di-imidazolium ILs (PEGsDIMBr), HPEG
n
RIMX with
2
1
mono-hydroxyl group and terminal ending-capped ILs exhibited an
HPEG400Cl. Pale yellow oily liquid; H NMR (D
2
O, 600 MHz) δ
obvious promoted catalysis, which might be resulted from the multi-
3.76−3.74(m, 4H, −CH
3.60−3.55(m, 2H, CH ).
2
Cl), 3.65−3.62(m, 39H, CH OH),
2
functional facilitated roles including the improved CO
2
-solubilities of
2
1
catalysts, hydrogen-bondings between hydroxyl group and oxygen
atom on cyclo-epoxides, and bromine anionic nuclephilicity for at-
HPEG600Cl. Pale yellow oily liquid; H NMR (D
2
O, 600 MHz) δ
3.76−3.74(m, 4H, −CH
2
), 3.67−3.64(m, 61 H, CH
2
OH), 3.57−3.56
tacking the C
2
sites of cycloepoxides. The mechanism formation process
(m, 2H, CH ).
2
on the chemical fixation of CO
2
into cyclic carbonates has been pro-
posed and further investigated by IGA and in-situ NMR spectroscopy.
2
.3. Synthesis of hydroxyl polyethylene ether-grafted imidazole (HPEG
n
IM)
[
46]
2
. Experimental
2.1. Materials
Halogenated hydrocarbons and polyethylene glycols (PEG
n
, M or
w
n = 200, 400, and 600) as the post-modified supports were purchased
from the J&K (Beijing) CHEMICA Ltd. and directly utilized without any
purification process. Other chemicals containing with sulphuric
dichloride, sodium ethylate, pyridine and imidazole were obtained
from the Beijing Chemical Reagent Co., Ltd. and directly used to syn-
Imidazole (50 mmol, 3.40 g) and 75 mL dried ethanol were added
into a 250 mL round bottomed flask with the spherical reflux con-
denser. The obtained solution was magnetically stirred and refluxed for
thesize HPEG ILs. Ethanol, acetonitrile, toluene and ethyl ether were
n
dried over 4A molecular sieve and then refined under N
2
atmosphere by
1
.0 h, and then 20 mL of 21 wt% sodium ethylate in dried ethanol was
using Schlenk technique. 99.5 wt% purity of CO
2
was the commercial
dripped slowly. After refluxing for 8 h, HPEG Cl (0.05 mol) was added
n
product, and all epoxides were purchased from Aldrich Company and
into the mixture solution for reacting another 12 h. Finally, it was
cooled to r.t. and filtrated off precipitations. The obtained filtrate was
washed with anhydrous ether (50 mL × 3), and then concentrated by
using rotary evaporator. After drying over vacuum at 45 °C for 12 h,
directly used as received.
2
.2. Synthesis of monochlorinated polyethylene glycols (HPEG Cl,
n
n = 200,400,600) [44,45]
HPEG IM with the yellowish colour was obtained and determined by
n
using NMR (Figure S8−10), MS (Figure S6) and TGA (Figure S12), and
its yield was 72%.
1
HPEG200IM. Yellowish oil; H NMR (D
2
O, 600 MHz) δ 7.83(s, 1H,
CH), 7.05 (s, 2H, CH), 3.75−3.67(m, 2H, CH
2
N), 3.59−3.42(m, 13H,
CH OH).
2
HPEG400IM. Yellowish oil; 1H NMR (D
O, 600 MHz) δ 7.73(s, 1H,
2
NCHN), 7.07(s, 2H, 2CH), 3.75−3.59(m, 33H, CH
2
OH).
1
Monochloridated HPEG
n
Cl were synthesized through the chemical
HPEG IM. Yellowish oil; H NMR (D O, 600 MHz) δ 7.76(s, 1H,
6
00
2
stoichiometric method as the below procedure: 0.10 mol of PEGnꢀ
NCHN), 7.07(s, 2H, 2CH), 3.74−3.55(m, 65H, CH OH).
2
(
Mw = 200,400,600), 0.20 mol of pyridine and dried toluene (200 mL)
were subsequently added into a 500 mL three-neck round bottle at the
room temperature. After the mixture was stirred for half an hour by
using the magnetic stirrer, it was warmed to the refluxing temperature
2.4. Synthesis of polyethylene glycols grafted imidazolium ionic liquids
(HPEG RIMX) [47,48]
n
and reacted for 1.0 h. Then, 1.00 equiv. of sulphuric dichloride as the
chloride reagent was slowly dripped into the mixture solution within
The solution of HPEG IM (0.02 mol), acetonitrile (50 mL) and 1-
n
4
5 min. The solution was refluxed for 48 h until completing the reac-
bromoethane (0.03 mol, 3.30 g) was added into a 250 mL round bot-
tome flask equipped with the reflux condenser and the magnetic stirrer.
After refluxing for 24 h, the reaction was completed and then the
mixture solution was warmed to the room temperature. Inorganic
precipitation was removed off by filtration, and the filtrate was washed
with diethyl ether (40 mL × 5) for further purifying the crude products.
tion. The precipitated pyridine salts were filtrated off, and the mixture
solution was washed with toluene (100 mL × 3) and CH Cl
50 mL × 3), and dried over anhydrous sodium sulphate. Finally, after
removing off the organic solvents within, the obtained crude HPEG Cl
was further purified by liquid-liquid extraction from diethyl ether and
2
2
(
n
2

Y. Liu, et al.
Molecular Catalysis 492 (2020) 111008
1
Finally, HPEG
n
RIMX with the yellowish colour was obtained with the
1,3-Dioxolan-2-one[49a]. White solid; H NMR (CDCl
3
, 600 MHz)
yields of 71–86% and dried over vacuum atmosphere. Its structure was
δ 4.50 (s, 4H, OCH ).
2
1
determined by NMR spectrum depicted in Figure S15, S16.
4-methyl-1,3-dioxolan-2-one[49b]. White solid; H NMR (CDCl
3
,
1
HPEG200EIMBr. Yellowish oil;
H
NMR (D
2
O, 600 MHz)
δ
600 MHz) δ 4.85−4.81 (m, 1H, OCH), 4.53 (t, J =8.0 Hz, 1H, CH
2
),
8
.80−8.63 (m, 1H, CH), 7.51−7.38 (m, 2H, CH), 4.38−4.18 (m, 2H,
4.01 (t, J =8.01 Hz, 1H, CH
2
), 1.46 (d, J =6.2 Hz, 3 H, CH
3
).
1
3
1
CH
2
), 3.68−3.51 (m, 13H, OCH
2
), 1.45−1.41 (m, 3H, CH
3
); C NMR
4-Chloromethyl-1,3-dioxolan-2-one[49c]. Yellowish solid;
H
(
D
2
O, 151 MHz) δ 134.70, 122.16, 69.87, 60.38, 45.02, 14.76.
NMR (CDCl , 600 MHz) δ 4.99−4.95 (m, 1H, CH), 4.60 (t, J = 12.0,
3
1
HPEG400EIMBr. Yellowish oil;
H
NMR (D
2
O, 600 MHz)
δ
6.0 Hz, 1H, CH), 4.42 (dd, J =6.0 Hz, 1H, CH), 3.75 (m, 2 H, CH
2
).
1
8
.80−8.62 (m, 1H, CH), 7.51−7.37 (m, 2H, CH), 4.38−4.14 (m, 2H,
1,2-cyclohexyl carbonate[49d]. White solid; H NMR (CDCl
3
),
,
1
3
CH
2
), 3.85−3.51 (m, 36H, OCH
2
), 1.43−1.40 (m, 3H, CH
3
); C NMR
600 MHz) δ 4.70−4.67 (m, 2H, 2 × CH), 1.92−1.89 (m, 4H, 2CH
1.66−1.60 (m, 2H, CH ), 1.46−1.40 (m, 2H, CH ).
2
(
D
2
O, 151 MHz) δ 135.58, 120.74, 69.67−69.17, 60.52, 44.68, 14.51.
2
2
1
1
HPEG400PIMBr. Yellowish oil;
H
NMR (D
2
O, 600 MHz)
δ
4-Phenyl-1,3-dioxolan-2-one[49d]. White solid; H NMR (CDCl ,
3
7
.91−7.80 (m, 1H, CH), 7.19−7.02 (m, 2H, CH), 4.70−4.01 (m, 2H,
600 MHz) δ 7.45−7.35 (m, 5H, Ph), 5.67 (t, J =8.0 Hz, 1H, OCH), 4.79
CH
2
), 3.79−3.56 (m, 36H, OCH
2
), 1.83−1.68 (m, 2H, CH
2
),
(t, J =8.4 Hz, 1H, CH), 4.34 (t, J =8.4 Hz, 1H, CH).
1
3
1
1
.24−1.19 (m, 2H, CH
2
), 0.86−0.80 (m, 3H, CH
3
); C NMR (D
2
O,
4-Phenoxy-1,3-dioxolan-2-one[49e]. White solid;
H
NMR
1
51 MHz) δ 135.10, 122.07, 71.44−69.02, 60.01, 49.79, 22.99, 9.75.
(CDCl , 600 MHz) δ 7.30 (t, J =8.0 Hz, 2H, m-Ph), 7.00 (t, J =7.4 Hz,
3
1
HPEG400BIMBr. Yellowish oil;
H
NMR (D
2
O, 600 MHz)
δ
1H, p-Ph), 6.90 (d, J =8.2 Hz, 2H, o-Ph), 5.04−5.00 (m, 1H, OCH),
4.61 (t, J =8.5 Hz, 1H, CH), 4.53 (t, J =5.8 Hz, 1H, CH), 4.23 (dd,
J = 10.5, 4.3 Hz, 1H, =CHO), 4.15 (dd, J = 10.5, 3.2, 1H, =CHO).
8
.75−8.70 (m, 1H, CH), 7.51−7.42 (m, 1H, CH), 7.41−7.29 (m, 1H,
), 3.84−3.56 (m,
), 1.42−1.10 (m, 2H, CH ),
CH), 4.36−4.31 (m, 1H, CH), 4.18−4.10 (m, 2H, CH
2
3
0
6
6H, OCH
2
), 1.83−1.68 (m, 2H, CH
2
2
1
3
.86−0.82 (m, 3H, CH
3
); C NMR (D O, 151 MHz) δ 135.08, 122.35,
2
2.6. Characterization
9.57−68.49, 60.29, 49.20, 30.93, 18.61, 14.59.
1
HPEG400BIMCl. Yellowish oil;
H
NMR (D O, 600 MHz) δ
2
FT-IR was conducted by using KBr as a standard material on the
8
2
1
1
.68−8.62 (m, 1H, CH), 7.51−7.42 (m, 2H, CH−CH), 4.21−4.11 (m,
H, CH), 3.56–3.66 (m, 36H, OCH ), 2.04−1.63 (m, 2H, CH ),
O,
Thermo Corp. Nicolet 380 FT-IR 6700 (thermo, CO. Let.) spectrometer.
Thermogravimetric analysis (TGA) was carried out on Shimadzu Corp.
DTG-60H Thermal Analysis System at a heating rate of 10 °C/min from
2
2
2
1
3
.41−1.11 (m, 2H, CH
2
), 1.10−1.07 (m, 3H, CH
3
); C NMR (D
51 MHz) δ 135.28, 120.99, 69.56−69.22, 60.24, 47.02, 31.82, 12.68.
2
5 to 650 °C. The NMR spectra of samples were recorded on JNM-
1
HPEG400BIMI. Yellowish oil; H NMR (D
2
O, 600 MHz) δ 8.01−7.91
ECA600 spectrometer. The CO solubility in the supported ILs was
2
(
m, 1H, CH), 7.37−7.01 (m, 2H, CH−CH), 4.20−3.42 (m, 3H, OCH
2
),
analyzed by using the intelligent gravimetric analyser (IGA, Haliga 165
3
3
6
.87−3.83 (m, 1H, CH), 3.63−3.56 (m, 55H, OCH
2
), 1.46−1.41 (m,
IGA-001, HIDEN Company, British) after treating the sample at 400 for
1
3
H, CH
3
); C NMR (D O, 151 MHz) δ 135.15, 122.31, 69.45−69.27,
2
1
0 h under 10−4 Pa vacuum environment. GC yields were determined
0.16, 49.19, 31.10, 18.57, 12.63.
by GC-7890A, Agilent Technology using biphenyl as the standard ma-
terial.
1
HPEG600EIMBr. Yellowish oil;
H
NMR (D O, 600 MHz) δ
2
8
.75−8.62 (m, 1H, CH), 7.50−7.40 (m, 1H, CH), 7.39−7.37 (m, 1H,
CH), 4.21−4.19 (dd, J = 6.0, 18.0 Hz, 1H, CH), 4.16−4.12 (dd, J =
3
. Results and discussion
6
3
6
.0, 18.0 Hz, 2H, CH
2
), 3.75−3.50 (m, 67H, OCH
); C NMR (D O, 151 MHz) δ 134.50, 122.03, 69.68−69.46,
0.37, 44.77, 14.59.
2
), 1.45−1.41 (m,
1
3
H, CH
3
2
3.1. FT-IR analysis
FT-IR spectra of PEG400, HPEG400Cl, HPEG400IM and HPEG400EIMBr
−1
were depicted in Fig. 1. The peak at 3361 cm in Fig. 1b belonged to
OH stretching vibrations of HPEG400Cl, and it was narrower and a little
weaker than that of OH stretching vibration on PEG400 in Fig. 1a due to
the substitution of chloro-group on the long chains of polyethylene
2.5. General procedure for cycloaddition
−1
glycols. The peak of 664 cm in the curve of Fig. 1b attributed to the
stretching vibration of C-Cl in HPEG400Cl. The CeH stretching vibra-
tions at 2880 and 1460 cm−1 in Fig. 1a–c didn’t perform any shifts.
All cycloadditions of CO
5 mL stainless steel autoclave. Typical procedure for the reaction was
RIMX
2
with epoxides were conducted within a
2
depicted as below: 0.014 mol of epoxide and 1.00 mol% HPEG
n
were successively added into a 75 mL stainless steel autoclave with a
magnetic stirring. After the autoclave was sealed, CO was charged into
2
the reactor and adjusted to the reaction pressure and reacted at pre-
setted temperature. After completing the reaction, the mixture solution
was cooled to r.t. and CO residue was given off slowly. The mixture
2
was warmed to r.t. and stirred for completing the reaction residue of
CO was given off slowly. The catalyst was recovered by using a simple
2
extraction process and then washed with anhydrous diethyl ether for
the next recycle. Finally, the reaction solution was collected and de-
termined by GC analysis using diphenyl as the internal standard ma-
terial. All crude products were purified by using the silica gel’s flash
chromatograph with EtOAc/Hexane (volume ratio = 1:20), and their
1
Fig. 1. FT-IR spectra of (a) PEG400, (b) HPEG400Cl, (c) HPEG400IM and (d)
chemical structures were characterized by H NMR (data listed in below
HPEG400EIMBr.
and spectrum seeing in Figure S15-S20).
3

Y. Liu, et al.
Molecular Catalysis 492 (2020) 111008
Fig. 2. TGA graph of HPEG400EIMBr.
Fig. 4. Effect of CO
2
pressures on the catalytic activities under reaction con-
While HPEG400EIBr was analyzed by FT-IR, the C–H stretching vibra-
ditions of propyl oxide (14 mmol), 1.0 mol% of HPEG400EIMBr, 130 °C and
−
1
tion at the peak of 1400 cm
resulted into the obvious redshift to
also gave the similar varying tendency
1
.5 h.
lower wavenumber in Fig.1d. In addition, the OeH bond’s stretching
−
1
vibration at above 3600 cm
7
2.8 mg/g under the conditions of 1.5 MPa and 25 °C; HPEG400EIMBr
(
Fig. 1d) because of the interaction between the task-functional imi-
attained 77.6 mg/g; HPEG600EIMBr only had 40.3 mg/g. Among the
dazolium-based ILs moieties and hydroxyl group on HPEG400EIMBr.
Furthermore, their structures were further determined by ESI-MS as
shown in Figure S1, S5-S7, and the molecular weights of HPEG400Cl,
HPEG400IM and HPEG400EIMBr were 434, 456 and 493, respectively.
The results indicated that HPEG400EIMBr could be successfully syn-
thesized by chemical stoichiometric method.
three curves in Fig. 3, HPEG400EIMBr shown the best solubility for CO
2
within the ranges from 0 to 1.5 MPa at r.t., which could be attributed to
the higher content of ILs functional moieties on HPEG400EIMBr and
their CO -philicity of polyether’s long chains.
2
The reaction pressure played an important role for the chemical
fixation of CO with epoxides, and their results were depicted in Fig.4.
Under the lower reaction pressure, the gaseous CO at the upper of
reaction solution was the CO -rich phase. As CO pressure increased to
.5 MPa, the catalysis in the reaction might be improved and easily
benefited from the raised CO concentration at the bottom of the
mixture solution. When cycloadditions between CO and propyl oxide
2
Thermal stability of HPEG400EIMBr was investigated by TGA ana-
lysis as depicted in Fig. 2, and its thermal stable temperature was below
2
2
2
2
00 °C. The curve of HPEG400EIMBr ranging from 200 to 300 °C ap-
1
peared the first weight loss stage for the thermal decomposition of
bromo-ethane moieties on HPEG400EIMBr. The second decomposed
thermal stage was from 300 to 450 °C as the result of the thermal de-
gradation of polyether long chains of the reported catalyst. Thus, the
catalyst could maintain thermal stability of structure below the tem-
perature of 200 °C.
2
2
were conducted under the pressures ranging from 0.5 to 2.0 MPa, the
catalytic activities could attain the most excellent catalytic activity with
the yield of 99%, and reach the highest selectivity of 98%. While the
reaction pressure was increased to 2.5 MPa, HPEG400EIMBr exhibited
the decreased catalytic activity with propyl carbonate’s yield of 96%
and 97% selectivity within the shorter reaction time of 1.5 h. 2.0 MPa
CO
2
adsorption properties of HPEG RIMX with different molecular
n
weights were analyzed by using IGA instrument, and their comparative
curves were listed in Fig. 3. As the increased CO pressures from 0 to
.5 MPa at r.t. (25 °C), the adsorption capacities of HPEG RIMX showed
the obvious increasing trends. Moreover, the lower molecular weights
2
CO
2
pressure was chosen as the best pressure for HPEG400EIBr in the
1
n
cycloaddition of CO
2
with propyl epoxides.
The reaction parameters containing with the types of catalysts,
of HPEG
n
RIMX, the higher CO adsorption capacities might be ap-
2
temperature and reaction time were also very important to evaluate the
proached as the result of higher concentration of functionalized Ils
moieties and lower concentration of Lewis acidic hydroxyl groups on
catalysis of HPEG
n
RIMX in the chemical fixation of CO with epoxides.
2
Under the optimum CO pressure of 2.0 MPa, these parameters were
2
them. When CO adsorption capacity of HPEG200EIMBr reached to
2
screened and the corresponding results were listed in Table 1. When
PEG400 was directly used to the cycloaddition between propyl oxide and
Table 1
Screening reaction conditions for the chemical fixation of CO
2
by using
Sel. (%)
RIMX[
a]
.
HPEG
n
Entry
Cat.
T (°C)
T (h)
Yield (%)[b]
1
2
3
4
5
6
7
8
9
PEG400
100
100
110
120
130
130
130
130
130
130
130
3.0
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
2.0
67
3.5
97
97
96
98
98
85
97
98
83
84
HPEG400EIMBr
HPEG400EIMBr
HPEG400EIMBr
HPEG400EIMBr
HPEG200EIMBr
HPEG600EIMBr
HPEG400PIMBr
HPEG400BIMBr
HPEG400EIMBr
HPEG400EIMBr
81
93
99
≥99
96
99
99
[
[
c]
d]
1
1
0
1
67
83
[
a]
Reaction conditions: propylene oxide (14 mmol), 1.0 mol% of catalyst and
Fig. 3. CO
2
adsorption curves of HPEG200EIMBr, HPEG400EIMBr and
[b]
[c]
CO
2
pressure(2.0 MPa). GC yield with biphenyl as the standard material.
HPEG600EIMBr at r.t..
[d]
0
.25 mol% of HPEG400EIMBr. 0.5 mol% of HPEG400EIMBr.
4

Y. Liu, et al.
Molecular Catalysis 492 (2020) 111008
Table 2
Substrates’ tolerance[a].
Entry
Epoxides
Products
Yield (%)[b]
Sel. (%)
≥99
1
99
2
3
97
97
≥99
97
4
5
42
97
97
≥99
6
99
≥99
[a]Reaction conditions: epoxides (14 mmol), HPEG400EIMBr (1.0 mol%), CO
(2.0 MPa), 130 °C and 1.5 h. [b]Isolated yield.
2
CO
nate was only 2% within 3 h (Entry 1, Table 1). Under the same reaction
conditions of 1.0 mol% of catalyst, 2.0 MPa CO pressure, 100 °C and
2
and without any functionalization, the yield of propylene carbo-
products’ yield in the reaction. When cyclohexyl epoxide as the sub-
strate was applied into the chemical fixation of CO , HPEG400EIMBr
2
2
gave the yield of 42% with 97% selectivity (Entry 4, Table 2). However,
when ethylene epoxide was used as the substrate, the reported catalyst
exhibited the excellent catalytic activity with the yield of 99% and
above 99% selectivity (Entry 1, Table 2). Moreover, it was noteworthy
that the substrates of cyclo-epoxides with the withdrawing or donating
1
.5 h, HPEG400EIMBr was applied into the reaction, the yield was in-
creased to 67% (Entry 2, Table 1). As the reaction temperatures were
increased to 130 °C from 100 °C, the yields of propylene carbonate
reached to 99% and its selectivity attained 98% (Entries 2–5, Table 1).
By contrast, under the same reaction conditions, the both catalysts of
HPEG400EIMBr (Entry 5, Table 1) and HPEG200EIMBr (Entry 6, Table 1)
preferred to exhibit higher catalytic activities over than that of HPE-
electronic group at the C site of cyclo- epoxides were nearly all con-
3
verted into their corresponding cyclic carbonates. When propylene
epoxide was used as the substrate, propyl carbonate achieved 97% yield
and more than 99% selectivity (Entry 2, Table 2). Chloro-cyclopropyl
epoxide gave its corresponding product with the excellent yield of 97%
G
600EIMBr with 96% yield and 85% selectivity (Entry 7, Table 1). From
the experimental results, it was also found that the lower molecular
weights of HPEG RIMX, the higher catalytic activities easily attained. It
n
could be attributed to their higher CO
2
-philicity of HPEG RIMX (seeing
n
in Fig. 3) and the weaker steric hindrance from short alkyl chains on
imidazolium cation units (Entries 8 and 9, Table 1). Under the condi-
tions of 2.0 MPa CO pressure, 130 °C and the reaction time of 1.5 h, the
2
utilized amount of HPEG400EIMBr affected its catalytic activities in
cycloaddition between propyl epoxide and CO . When the amount of
2
HPEG400EIMBr was 0.25 mol%, the yield was only 67% and the se-
lectivity reached to 83% (Entry 10, Table 1). While 0.50 mol% HPE-
G
400EIMBr was used to catalyze the reaction, it gave the yield of 83%
and 84% selectivity (Entry 11, Table 1). As the amount of catalyst was
increased to 1.0 mol%, the yield of cyclic carbonate approached the
highest value of 99% and the selectivity of propyl carbonate was 98%
(
Entry 5, Table 1). 1.0 mol% of HPEG400EIMBr could smoothly catalyze
the transformation of CO
HPEG400EIMBr’s substrate tolerance had been conducted under the
2
into cyclic carbonates.
best optimized reaction conditions of 1.0 mol% catalyst, 2.0 MPa CO
2
pressure, 130 °C and 1.5 h, and the datum were listed in Table 2. The
Fig. 5. Recyclability of HPEG400EIMBr. Reaction conditions: propylene oxide
(
14 mmol), CO (2.0 MPa), 1.0 mol% of catalyst, 130 °C and 1.5 h.
2
steric hindrance of cyclo-epoxides easily resulted into the lower
5

Y. Liu, et al.
Molecular Catalysis 492 (2020) 111008
Fig. 6. Comparative activities of HPEG400EIMBr and PEG400DEIMBr under the
conditions of propylene oxide (14 mmol), CO
and 130 °C.
2
(2.0 MPa), 1.0 mol% of catalyst
Fig. 8. 1H NMR spectra of propylene oxide (PO) with PEG400EIMBr at different
1
3
concentrations of (a) 0.0045 M, (b) 0.0081 M and (c) 0.014 M ( CDCl ,
3
(
Entry 3, Table 2), phenyl ethylene oxide gave 97% yield with above
6
00 MHz). The symbol “ ” indicates the hydroxyl signal.
9
9% selectivity (Entry 5, Table 2), and phenyl ether propylene oxide
shown 99% yield and ≥99% selectivity (Entry 6, Table 2). HPE-
G
400EIMBr easily performed better tolerance for different cyclo-ep-
oxides.
Under the optimum reaction conditions, the recyclability of
HPEG400EIMBr in cycloaddition between CO and propylene epoxide
time, the reported catalyst exhibited the higher catalysis than that of
PEG400DEIMBr, which shown the excellent yield of 98% for 1.5 h. The
improved catalytic activities of HPEG400EIMBr might be ascribed to the
synergism of hydrogen-bonding between hydroxyl group in HPE-
2
was investigated as depicted in Fig. 5. The catalyst was recovered by a
simple extraction with diethyl ether as the solvent, and it could be re-
cycled or reused five times without any loss of catalytic activity and
decreased selectivity during being recycled. The decrease of catalytic
activities might be resulted from the lose weight of HPEG400EIMBr
during its recovering process. The catalyst could be reused for several
times with highly catalytic activities and remained stabilities being
used before and after (seeing in Figure S21). Moreover, when the run
time was decreased to 1.0 h, HPEG400EIMBr could be recycled seven
times without the loss of catalytic activities, and remained cyclic car-
bonate’s yield more than 91.0% and the selectivity of 97% (Figure S22).
By comparison with the catalysis of PEG400DEIMBr in the chemical
G400EIMBr and oxygen atom of cyclo-epoxides like the reported cata-
lysts of HBD/TBAI[50] and [EvimOH][Cl]/DBU [51]. Moreover, HPE-
400EIMBr behaved the properties of metal-free, lower loading, easily
accessibility and recoverability, which could easily chemically absorb
G
much more CO moleculars from the upper gaseous atmosphere and
2
then acted them by nucleophilic attack to efficiently generate the aim
products of cyclic carbonates.
The mechanism for HPEG
n
RIMX in the chemical fixation of CO
2
with cyclo-epoxides was proposed as shown in Fig. 7. Firstly, hydrogen
bonding between hydroxyl group on HPEG RIMX and oxygen atom of
epoxides induced the opening-ring process of cycloepoxide and played
n
the synergistic promotion for the conversion of CO into cyclic carbo-
2
fixation of CO
2
with propylene epoxides, HPEG400EIMBr exhibited ex-
nates, which was similar like the reported hydrogen bonding’s facil-
1
cellent catalytic activities, and the results were depicted in Fig. 6. As the
increased reaction time varying from 0.5 to 2.5 h, PEG400DEIMBr gave
the yields of propyl carbonate with 8.0–82%. Within the same reaction
itation roles[16,43]. According to our experimental on H NMR spectra
of PO with HPEG400EIMBr at the different concentrations within
1
3
CDCl (Fig. 8, S4 and S5), it could be found that the chemical shifts of
3
hydroxyl group on HPEG400EIMBr were easily detected and could be
observed in the lower fields with the increased concentrations of
HPEG400EIMBr. Hydrogen bonds including intermolecular or in-
tramolecular function were possibly enhanced and further facilitated
the nucleophilic attack of the bromide anion with the epoxide ring of
PO through reducing the barriers, which were similar like the reported
literatures of [52,53]. As a result, the cyclo-rings of cyclo-epoxide might
be easily opened and possibly swiftly activated CO
carbonates.
2
to form cyclic
Secondly, bromide anion on HPEG400EIMBr nucleophilic attacked
site on the cyclo-ring of propylene oxide to generate the inter-
C
2
mediate of compound (I), and then opened the cyclo-ring of epoxide to
form the compound (II). Then, compound (II) as Lewis base chemi-
sorpted free CO
cess, HPEG
the active centers because of the CO
2
to produce the new compound (III). During the pro-
n
RIMX probably adsorbed much more CO
-philic property from their long
adsorption capacity of HPEG400EIMBr was
2
molecules around
2
polyether chains. The CO
2
7
7.6 mg/g under the conditions of 25 °C and 1.5 MPa, which was higher
than the CO
2
adsorption ability of PEG400DEIMBr with double imida-
zolium-based ILs ending-capped polyethylene ether (seeing in Table
S1). The result indicated that the diffusion of CO molecules in the
2
upper gaseous phase into the lower phase around the active species of
HPEG
HPEG
n
n
RIMX could be facilitated by the CO
2
-philic property of
Fig. 7. Proposed mechanism for the chemical fixation of CO .
2
RIMX behaving the long polyether chains and imidazolium-
6

Y. Liu, et al.
Molecular Catalysis 492 (2020) 111008
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