Phenolic hydroxyl-functionalized imidazolium ionic liquids: Highly efficient catalysts for the fixation of CO2 to cyclic carbonates

Article abstract of DOI:10.1016/j.molcata.2016.03.002

A series of imidazolium ionic liquids were designed and synthesized by the reaction of imidazole derivatives with alkyl bromides. These ionic liquids were applied to catalyze the reaction of CO₂ and epoxides. A detailed investigation was carried out on the relationship between catalytic activities and catalyst structures. The result showed that phenolic hydroxyl-functionalized imidazolium ionic liquids containing both phenolic hydroxyl and C2-H were highly efficient catalysts. Meanwhile, the spatial positions of the phenolic hydroxyl and C2-H in the imidazolium also exhibited great effect on the catalytic efficiencies, and the order of catalytic activity was meta-isomer > ortho-isomer > para-isomer. NMR titration and DFT calculations showed a synergetic effect, which C2-H and phenolic hydroxyl cooperatively activated epoxides by hydrogen bonds, was crucial for the reaction to proceed smoothly under mild conditions. In addition, the TOF of the most active catalyst 3-(3-phenolic hydroxyl)-1-butyl-imidazolium bromide (IL-2) reached to 900 h^-1 in the reaction of CO₂ and epichlorohydrin under the reaction conditions of 0.1 mol% catalyst, 1 MPa CO₂, 120 °C and 1 h.

Full text of DOI:10.1016/j.molcata.2016.03.002

Accepted Manuscript
Title: Phenolic hydroxyl-functionalized imidazolium ionic
liquids: highly efficient catalysts for the fixation of CO2 to
cyclic carbonates
Author: Shi Wu Binshen Wang Yongya Zhang Elnazeer H.M.
Elageed Haihong Wu Guohua Gao
PII:
S1381-1169(16)30069-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.03.002
MOLCAA 9801
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Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
17-12-2015
1-3-2016
1-3-2016
Please cite this article as: Shi Wu, Binshen Wang, Yongya Zhang, Elnazeer
H.M.Elageed, Haihong Wu, Guohua Gao, Phenolic hydroxyl-functionalized
imidazolium ionic liquids: highly efficient catalysts for the fixation of
CO2 to cyclic carbonates, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2016.03.002
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Phenolic hydroxyl-functionalized imidazolium ionic liquids: highly
efficient catalysts for the fixation of CO2 to cyclic carbonates
Shi Wu, Binshen Wang, Yongya Zhang, Elnazeer H. M. Elageed,
Haihong Wu*, Guohua Gao*
Shanghai Key Laboratory of Green Chemistry and Chemical Processes,
Department of Chemistry, East China Normal University, North Zhongshan
Road 3663, Shanghai 200062, China
1

Graphical Abstract
Bu
OH
Br
N
N
O
O
O
High yields
Mild reaction conditions
CO
R+
2
R
O
1 mol %,1MPa,2h,80 oC
2

Highlights
(1) A series of novel phenolic hydroxyl-functionalized imidazolium ionic liquids were
designed and synthesized by the reaction of imidazole derivatives and alkyl bromides.
(2) The high catalytic activity of phenolic hydroxyl-functionalized imidazolium ionic liquids
for the reaction of CO2 and epoxides was attributed to the synergistic effect of phenolic
hydroxyl and C2-H.
(3) The TOF of IL-2 was 900 h-1 in the reaction of CO2 and epichlorohydrin (in 90% yield)
under mild reaction conditions. To the best of our knowledge, this is the highest TOF
among the ionic liquids catalyzed the reaction of CO2 and epoxides reported in the
literatures.
Abstract
A series of imidazolium ionic liquids were designed and synthesized by
the reaction of imidazole derivatives with alkyl bromides. These ionic
liquids were applied to catalyze the reaction of CO2 and epoxides. A
detailed investigation was carried out on the relationship between
catalytic activities and catalyst structures. The result showed that phenolic
hydroxyl-functionalized imidazolium ionic liquids containing both
phenolic hydroxyl and C2-H were highly efficient catalysts. Meanwhile,
the spatial positions of the phenolic hydroxyl and C2-H in the
imidazolium also exhibited great effect on the catalytic efficiencies, and
the order of catalytic activity was meta-isomer > ortho-isomer > para-
isomer. NMR titration and DFT calculations showed a synergetic effect,
which C2-H and phenolic hydroxyl cooperatively activated epoxides by
hydrogen bonds, was crucial for the reaction to proceed smoothly under
mild conditions. In addition, the TOF of the most active catalyst 3-(3-
3

phenolic hydroxyl)-1-butyl-imidazolium bromide (IL-2) reached to 900
h-1 in the reaction of CO2 and epichlorohydrin under the reaction
conditions of 0.1mol% catalyst, 1MPa CO2, 120 ºC and 1 h.
Keywords: phenolic hydroxyl; ionic liquids; CO2 fixation; synergistic
effect.
1. Introduction
The amount of CO2 has increased continuously since the industrial
revolution because of the combustion of huge amounts of fossil fuels.
CO2 is a main greenhouse gas that causes climate change, and also an
abundant, nontoxic, nonflammable, easily available, and renewable
carbon source[1-3]. Hence, the fixation of CO2 to high value-added
chemicals and energy products has significance for the environmental
protection and economic development. In the last decade, a growing
number of studies have been conducted on the fixation of CO2 to cyclic
carbonates via the coupling reactions of CO2 and epoxides. This is
partially because cyclic carbonates are very useful organic compounds
and can be utilized in many areas, such as green solvents, valuable
synthetic intermediates, feed stocks for engineering plastics and
pharmaceutical[4-6]. Many catalysts have been developed for the
production of cyclic carbonates by the coupling of CO2 with epoxides
illustrated by alkali metal salts[7-11], metal oxides[12-16], metal
complexes[17-22], Schiff bases[23-25] and functional polymers[26-29].
Ionic liquids regarded as highly efficient green organic catalysts have
been also extensively studied in various organic synthesis [30-32].
Especially, ionic liquids with functional groups such as amino[33],
4

carboxyl[34,35], hydroxyl[36-39] can improve catalytic activities
obviously in reaction of CO2 and epoxides. Mechanism studies indicated
that those functional groups could accelerate the ring-opening reaction of
epoxides through a hydrogen bond which resulted in the polarization of
C-O bonds of epoxides. Furthermore, the phenolic compounds in
combination with organic bases[22,40], nBu4NI[41] or nBu4NBr[42] were
found to catalyze cyclic addition reaction of CO2 and epoxides in
relatively mild reaction conditions. Very recently, Wu and co-workers
[43, 44] found FDU-type mesoporous polymer, which had plenteous
phenolic hydroxyl groups supported imidazolium-based ionic liquids,
were highly efficient and recyclable catalysts for the cycloaddition of
CO2 and epoxides. The superior efficiency of phenolic compounds on the
reaction was attributed to the formation of strong hydrogen-bond between
the phenolic hydroxyl group and epoxides.
It has been well-known that C2-H of imidazolium ionic liquids are
efficient hydrogen-bond donors[45-47]. We and others found that C2-H
of imidazolium could activate the C-O bond of epoxide or carbonyl group
of carbonates by the formation of hydrogen-bond between C2-H and
oxygen [48-53]. However, although few of the imidazolium ionic liquids
with phenolic hydroxyl functional group have been synthesized and used
as precursors for the synthesis or catalysts[54-56], the synergistic effect
of phenolic hydroxyl and C2-H groups in the reaction of CO2 and
epoxides have not been studied yet. Herein, in continuation of our studies
for the reaction of carbonates catalyzed by ionic liquids [57-59], a series
of phenolic hydroxyl-functionalized imidazolium ionic liquids were
designed, synthesized and applied to catalyze the reaction of CO2 and
epoxides (Scheme 1). A cooperative activation of epoxides by phenolic
5

hydroxyl and C2-H of the imidazolium in the reaction of CO2 and
epoxides was demonstrated.
2. Experimental
2.1. General
CO2 with a purity of 99.995% was commercially available. Epoxy
chloropropane and other epoxides were supplied by TCI. 3-(2-Phenolic
hydroxyl) imidazole, 3-(4-phenolic hydroxyl) imidazole and 3-
bromomethyl-phenol were synthesized according to the literature with
minor modification [60, 61]. NMR spectra were recorded on Bruker
Ascend400 instruments with tetramethylsilane as the internal standard.
DFT calculations were carried out by using the ωB97XD functional with
the 6-31G basis set as implemented in Gaussian09 program package.
2.2. Synthesis of ionic liquids
3-(2-Phenolic hydroxyl)-1-butyl-imidazolium bromide (IL-1):
3-(2-Phenolic hydroxyl) imidazole (2.61 g, 15 mmol) and n-butyl
bromide (20 mL) were mixed and reacted at 120 ºC for 12 h. The
suspension was allowed to cool to room temperature and washed with
Et2O (3*30 mL). A yellow viscous oil was obtained after vacuum dried at
1
70 ºC in the yield of 80%. H NMR (400HMz, DMSO-d6, TMS) δ 10.05
(s, 1H), 9.25 (s, 1H), 7.79 (s, 1H ), 7.71 (s, 1H ), 7.32 (d, J = 8.0 Hz, 1H),
7.23 (t, J = 8.0 Hz, 1H), 6.92 (d, J = 8.0 Hz, 1H), 6.84 (t, J = 8.0 Hz, 1H),
5.31 (s, 2H), 4.18 (t, J = 8.0 Hz, 2H), 1.79-1.72(m, 2H), 1.28-1.21 (m,
13
2H), 0.88 (t, J = 8.0 Hz, 3H). C NMR (100HMz, DMSO-d6, TMS) δ
155.66, 136.12, 130.50, 130.48, 122.61, 122.36, 120.64, 119.25, 115.46,
48.49, 48.12, 31.35, 18.72, 13.23.
6

Other ionic liquids were synthesized by the reaction of corresponding
imidazole derivatives and halides with the similar procedure.
3-(3-Phenolic hydroxyl)-1-butyl-imidazolium bromide (IL-2): yield: 75%.
1
Light yellow oil. H NMR (400HMz, DMSO-d6, TMS) δ 9.64 (s, 1H),
9.48 (s, 1H), 7.88 (d, J = 8.0 Hz, 2H), 7.20 (t, J = 8.0 Hz, 1H), 6.84-6.80
(m, 3H), 5.39 (s, 2H), 4.21 (t, J = 6.0 Hz, 2H), 1.80-1.76(m, 2H), 1.28-
13
1.22 (m, 2H), 0.89 (t, J = 8.0 Hz, 3H). C NMR (100HMz, DMSO-d6,
TMS) δ 157.78, 136.11, 130.00, 122.70, 122.61, 118.51, 115.61, 114.94,
51.84, 48.65, 31.25, 18.76, 13.23.
3-(4-Phenolic hydroxyl)-1-butyl-imidazolium bromide (IL-3): Yield:
1
85%. Yellow oil. H NMR (400HMz, DMSO-d6, TMS) δ 9.64 (s, 1H),
9.39 (s, 1H), 7.83-7.81 (m, 2H), 7.30 (d, J = 8.0 Hz, 2H), 6.81 (d, J = 8.0
Hz, 2H), 5.32 (s, 2H), 4.19 (t, J = 8.0 Hz, 2H), 1.81-1.73(m, 2H), 1.25-
13
1.22 (m, 2H), 0.89 (t, J = 8.0 Hz, 3H). C NMR (100HMz, DMSO-d6,
TMS) δ 157.84, 135.72, 130.07, 124.89, 122.63, 122.25, 115.63, 51.61,
48.59, 31.25, 18.76, 13.23.
3-(2-Phenolic hydroxyl)-1-butyl-2-methyl-imidazolium bromide (IL-4):
1
Yield: 78%. Yellow solid. H NMR (400HMz, DMSO-d6, TMS) δ 10.05
(s, 1H), 7.74 (s, 1H ), 7.57 (s, 1H), 7.20 (d, J = 8.0 Hz, 2H), 6.93 (d, J =
8.0 Hz, 1H), 6.82 (t, J = 8.0 Hz, 1H), 5.27 (s, 2H), 4.13 (t, J = 6.0 Hz,
2H), 2.68 (s, 3H), 1.72-1.67 (m, 2H), 1.29-1.26 (m, 2H), 0.90 (t, J = 6.0
Hz, 3H). 13C NMR (100HMz, DMSO-d6, TMS) δ 155.40, 143.88, 130.13,
130.07, 121.61, 121.20, 120.42, 119.25, 115.47, 47.32, 46.81, 31.08,
18.88, 13.38, 9.51.
3-(3-Phenolic hydroxyl)-1-butyl-2-methyl-imidazolium bromide (IL-5):
1
Yield: 75%. Yellow oil. H NMR (400HMz, DMSO-d6, TMS) δ 9.63 (s,
1H), 7.79 (d, J = 4.0 Hz, 2H), 7.19 (t, J = 8.0 Hz, 1H), 6.77 (d, J = 8.0 Hz,
7

1H), 6.70 (d, J = 8.0 Hz, 2H), 5.36 (s, 2H), 4.14 (t, J = 8.0 Hz, 2H), 2.62
(s, 3H), 1.71 (t, J = 4.0 Hz, 2H), 1.28 (d, J = 8.0 Hz, 2H), 0.90 (t, J = 8.0
Hz, 3H). 13C NMR (100HMz, DMSO-d6, TMS) δ 157.83, 143.99, 135.79,
130.01, 121.78, 121.51, 117.95, 115.37, 114.33, 50.55, 47.43, 30.99,
18.91, 13.38, 9.53.
3-(4-Phenolic hydroxyl)-1-butyl-2-methyl-imidazolium bromide (IL-6):
Yield: 85%. Yellow solid. 1H NMR (400HMz, DMSO-d6, TMS) δ 9.62 (s,
1H), 7.72 (d, J = 8.0 Hz, 2H), 7.19 (d, J = 8.0 Hz, 2H), 6.79 (d, J = 8.0
Hz, 2H), 5.27 (s, 2H), 4.11 (t, J = 8.0 Hz, 2H), 2.64 (s, 3H), 1.73-1.65 (m,
13
2H), 1.30-1.24 (m, 2H), 0.89 (t, J = 8.0 Hz, 3H). C NMR (100HMz,
DMSO-d6, TMS) δ 157.63, 143.66, 129.59, 124.52, 121.41, 121.34,
115.64, 50.36, 47.34, 30.98, 18.90, 13.37, 9.52.
3-Benzyl-1-butyl-imidazoliumbromide (IL-7): Yield: 82%. Yellow oil.
1H NMR (400HMz, CDCl3, TMS) δ 10.61 (s, 1H), 7.49-7.47 (m, 2H),
7.43(s, 1H), 7.39(s, 1H), 7.34-7.32 (m, 3H), 5.59 (s, 2H), 4.27 (t, J = 6.0
Hz, 2H), 1.90-1.83 (m, 2H), 1.38-1.29 (m, 2H), 0.91(t, J = 6.0 Hz, 3H).
13C NMR (100HMz, CDCl3; TMS) δ 136.12, 134.49, 128.94, 128.68,
128.30, 122.78, 122.50, 51.79, 48.65, 31.23, 18.76, 13.24.
3-(4-Methoxy-benzyl)-1-butyl-imidazolium bromide (IL-8): Yield: 82%.
Yellow oil. 1H NMR (400HMz, CDCl3, TMS) δ 10.53 (s, 1H), 7.44 (d, J
= 8.0 Hz, 2H), 7.39(d, J = 8.0 Hz, 2H ), 6.84(d, J = 8.0 Hz, 2H), 5.51 (s,
2H), 4.32-4.23 (m, 2H), 3.74(s, 3H), 1.89-1.79 (m, 2H), 1.34-1.28 (m,
2H), 0.90(t, J = 8.0 Hz, 3H). 13C NMR (100HMz, CDCl3, TMS) δ 159.53,
135.84, 130.09, 126.73, 122.69, 122.28, 114.31, 55.22, 51.39, 48.61,
31.25, 18.77, 13.23.
1-(2,6-Dimethyl-phenyl)-3-(3-phenolic hydroxyl)-imidazolium bromide
1
(IL-9): Yield: 91%. Yellow solid. H NMR (400HMz, DMSO-d6, TMS)
δ 9.70 (s, 1H), 9.69 (s, 1H), 8.11 (s, 1H), 8.02 (s, 1H), 7.46 (t, J = 8.0 Hz,
8

1H), 7.35 (d, J = 8.0 Hz, 2H), 7.25 (t, J = 8.0 Hz, 1H), 6.83 (t, J = 8.0 Hz,
3H), 5.48 (s, 2H), 2.07 (s, 6H). 13C NMR (100HMz, DMSO-d6, TMS) δ
157.83, 137.57, 136.03, 134.59, 133.56, 130.56, 130.14, 128.80, 124.07,
123.37, 118.25, 115.65, 114.69, 52.26, 16.98.
3-(3-Phenolic hydroxyl)-1-(2-phenolic hydroxyl)-imidazolium bromide
(IL-10): Yield: 89%. Yellow solid. 1H NMR (400HMz, DMSO-d6, TMS)
δ 10.09 (s, 1H), 9.64 (s, 1H), 9.38 (s, 1H), 7.79 (s, 1H), 7.73 (s, 1H),
7.35-7.33 (m, 1H), 7.25-7.17 (m, 2H), 6.94 (d, J = 8.0 Hz, 1H), 6.84 (t, J
13
= 8.0 Hz, 1H), 6.81-6.78 (m, 3H), 5.37 (s, 2H), 5.34(s, 2H). C NMR
(100HMz, DMSO-d6, TMS) δ 157.72, 155.70, 136.19, 136.18, 130.60,
130.52, 130.02, 122.90, 122.51, 120.54, 119.27, 118.51, 115.59, 115.48,
114.96, 51.75, 48.22.
3-Butyl-1-(3-hydroxy-benzyl)-imidazolium chloride (IL-11): Yield: 80%.
1H NMR (400HMz, DMSO-d6, TMS) δ 9.85 (s, 1H), 9.46 (s, 1H), 7.75 (d,
J = 8.0 Hz, 2H), 7.05 (t, J = 8.0 Hz, 1H), 6.73-6.67 (m, 3H), 5.25 (s, 2H),
4.08 (t, J = 6.0 Hz, 2H), 1.68-1.61(m, 2H), 1.16-1.07 (m, 2H), 0.76 (t, J =
13
6.0 Hz, 3H). C NMR (100HMz, DMSO-d6, TMS) δ 157.98, 136.20,
136.09, 129.86, 122.69, 122.58, 118.36, 115.66, 115.01, 51.80, 48.60,
31.26, 18.77, 13.24.
3-Butyl-1-(3-hydroxy-benzyl)-imidazolium acetate (IL-12):
In an IL-2 (1.555 g, 5 mmol) aqueous (10 mL) solution, silver acetate
(0.919 g, 5.5 mmol) was added and stirred for 2h under dark condition.
The liquid was collected by filtration and the water was removed by
vaccum rotary evaporation. Then a yellow viscous oil was obtained after
1
vacuum dried at 70 ºC in the yield of 87%. H NMR (400HMz, CDCl3,
TMS) δ 10.13 (s, 1H), 7.20 (s, 1H), 7.11 (s, 1H), 7.00 (t, J = 8.0 Hz, 1H),
6.82 (d, J = 8.0 Hz, 2H), 6.58 (d, J = 8.0 Hz, 1H), 5.10 (s, 2H), 4.09 (t, J
= 6.0 Hz, 2H), 1.96 (s, 3H), 1.76-1.68(m, 2H), 1.29-1.20 (m, 2H), 0.86 (t,
13
J = 8.0 Hz, 3H). C NMR (100HMz, CDCl3, TMS) δ 178.03, 159.54,
9

137.70, 134.29, 130.16, 121.85, 121.82, 118.18, 116.96, 115.95, 53.05,
49.56, 31.97, 24.85, 19.43, 13.38.
1-(2-Hydroxy-ethyl)-3-methyl-imidazolium bromide (IL-13): Yield: 90%.
White solid. 1H NMR (400HMz, DMSO-d6, TMS) δ 9.20 (s, 1H), 7.77 (s,
1H), 7.74 (s, 1H), 5.15(s, 1H), 4.23 (t, J = 4.0 Hz, 2H), 3.87 (s, 3H), 3.71
(s, 2H). 13C NMR (100HMz, DMSO-d6, TMS) δ 136.76, 123.27, 122.61,
59.25, 51.53, 35.68.
2.3. Typical procedures for the reaction of CO2 and epoxides
In a typical experiment, epichlorohydrin (1.84g, 20mmol) and IL-2
(0.062g, 0.2mmol) were added to a stainless steel autoclave reactor with
an inner volume of 50 mL. The reactor was pressurized with approximate
1 MPa of CO2 at ambient temperature. Subsequently, the reactor was
heated to 80 ºC for 2 h. After the completion of the reaction, the
autoclave was cooled to room temperature, followed by slowly venting of
1
the remaining CO2. The yield of product was analyzed by H NMR with
1,1,2,2-tetrachloroethane as an internal standard. The pure product was
obtained by chromatography on silica gel and structurally characterized
by NMR spectroscopy.
3-Chloro-1,2-propylene carbonate. Colourless liquid, 1H NMR (400HMz,
CDCl3, TMS) δ 5.01-4.95 (m, 1H), 4.57 (t, J = 8.0 Hz, 1H), 4.39-4.35 (m,
13
1H), 3.81-3.77 (m, 1H), 3.72-3.68 (m, 1H). C NMR (100HMz, CDCl3,
TMS) δ 154.24, 74.33, 66.99, 43.74.
Ethylene carbonate. Colourless liquid, 1H NMR (400HMz, CDCl3, TMS)
δ 4.54 (s, 4H). 13C NMR (100HMz, CDCl3, TMS) δ 155.53, 64.69.
Propylene carbonate. Colourless liquid, 1H NMR (400HMz, CDCl3, TMS)
δ 4.86-4.78 (m, 1H), 4.52 (t, J = 8.0 Hz, 1H), 3.98 (t, J = 8.0 Hz, 1H),
13
1.44 (d, J = 8.0 Hz, 3H). C NMR (100HMz, CDCl3, TMS) δ 155.17,
73.70, 70.76, 19.42.
10

n-Butylethylene carbonate. Colourless liquid, 1H NMR (400HMz, CDCl3,
TMS) δ 4.72-4.67 (m, 1H), 4.50 (t, J = 8.0 Hz, 1H), 4.04 (t, J = 8.0 Hz,
1H), 1.82-1.74 (m, 1H), 1.71-1.62 (m, 1H), 1.47-1.29 (m, 4H), 0.9 (t, J =
13
8.0 Hz, 3H). C NMR (100HMz, CDCl3, TMS) δ 155.17, 77.14, 69.46,
33.58, 26.47, 22.28, 13.81.
1
1-Phenyl-1,2-ethylene carbonate. Yellow solid, H NMR (400HMz,
CDCl3, TMS) δ 7.46-7.40 (m, 3H), 7.35 (d, J = 8.0 Hz, 2H), 5.67 (t, J =
8.0 Hz, 1H), 4.79 (t, J = 8.0 Hz, 1H), 4.32 (t, J = 8.0 Hz, 1H). 13C NMR
(100HMz, CDCl3, TMS) δ 154.93, 135.87, 129.72, 129.23, 125.90, 78.04,
71.20.
1
3-Phenoxy-1,2-propylene carbonate. White solid, H NMR (400HMz,
CDCl3, TMS) δ 7.30 (t, J = 8.0 Hz, 2H), 7.01 (t, J = 8.0 Hz, 1H), 6.91 (d,
J = 8.0 Hz, 2H), 5.05-4.99 (m, 1H), 4.60 (t, J = 8.0 Hz, 1H), 4.52 (t, J =
13
8.0 Hz, 1H), 4.25-4.21 (m, 1H), 4.15-4.12 (m, 1H). C NMR (100HMz,
CDCl3, TMS) δ 157.82, 154.78, 129.71, 121.98, 114.66, 74.25, 66.94,
66.24.
1
Cyclohexene carbonate. Colourless liquid, H NMR (400HMz, CDCl3,
TMS) δ 4.67-4.63 (m, 2H), 1.89-1.79 (m, 4H), 1.60-1.52 (m, 2H), 1.42-
1.33 (m, 2H). 13C NMR (100HMz, CDCl3, TMS) δ 155.40, 75.78, 26.69,
19.08.
3. Results and discussion
3.1 Design and synthesis of phenolic hydroxyl-functionalized
imidazolium ionic liquids
To investigate the influence of the phenolic hydroxyl and C2-H of
imidazolium in ionic liquid on the reaction of CO2 and epoxides, a series
of novel imidazolium ionic liquids were designed as shown in Fig. 1. ILs
1-3 contain both phenolic hydroxyl and C2-H with different spatial
positions, including ortho-isomer (IL-1), meta-isomer (IL-2) and para-
11

isomer (IL-3). ILs 4-6 have similar structures with ILs 1-3, in which
there are methyl groups in the C-2 of imidazolium rings. ILs 7-8 have not
any phenolic hydroxyl groups. Compared to the structure of IL-2, the
second nitrogen of the imidazole ring of ILs 9-10 are replaced by aryl
group and phenolic hydroxyl, and the anions of ILs 11-12 are replaced by
Cl and OAc respectively. In addition, IL-13 contains both hydroxyl and
C2-H. All these ionic liquids were synthesized by the reaction of
corresponding imidazole derivatives with halides (Scheme S1).
3.2 The reaction of CO2 and epoxides catalyzed by various ionic liquids
3.2.1. The synergistic effect of phenolic hydroxyl and C2-H in ionic
liquids
ILs 1-13 were applied to catalyze the reaction of CO2 and
epichlorohydrin at 70 ºC. The results were shown in Table 1. When ILs
1-3 were used as catalysts, the yields of 3-chloro-1,2-propylene carbonate
reached to 92, 96 and 84% respectively (Entries 1-3). This indicated that
ionic liquids containing both phenolic hydroxyl and C2-H had high
catalytic activity. Moreover, the different spatial positions of the phenolic
hydroxyl and C2-H in the imidazolium also exhibited great effect on the
catalytic efficiencies, and the order of catalytic activity was meta-isomer
(IL-2) > ortho-isomer (IL-1) > para-isomer (IL-3). The higher activity of
the meta-isomer (IL-2) may attribute to the spatial structure of -OH and
C2-H in which the reactant molecules could interact in a suitable position.
Yoshihiro Tsutsumi et al [62] also reported that the spatial structure of
catalysts had strong influence on catalytic activities in the reaction of CO2
and epoxides. ILs 4-6, in which the C2-H was replaced by the C2-Me,
gave lower activities than that of the corresponding spatial structure of
ILs 1-3. The yields of 3-chloro-1,2-propylene carbonate decreased to 85,
84 and 75% respectively (Entries 4-6). These results indicated that C2-H
12

of the imidazolium also played an important role in accelerating the
activation of epichlorohydrin. The catalytic activities of ILs 7-8 which
contain only C2-H were lower than that of ILs 1-6, and 3-chloro-1,2-
propylene carbonate was obtained in the yields of 68 and 66% (Entries 7-
8). This proved that phenolic hydroxyl as stronger hydrogen bond donor
could activate epichlorohydrin more efficiently than C2-H. These results
showed both phenolic hydroxyl and C2-H in the ionic liquids could
activate epichlorohydrin cooperatively. The synergistic effect of phenolic
hydroxyl and C2-H made the polarization of the C-O bonds more easily
than the effect in which there is only one of phenolic hydroxyl or C2-H.
Therefore, the ring-opening reaction of epichlorohydrin was accelerated,
and consequently, the catalytic activity was improved. Subsequently, the
effects of steric hindrance of imidazolium ring and anions on the catalytic
activities were also investigated. When ILs 9-10 (Entries 9-10) were used
as catalysts, 3-chloro-1,2-propylene carbonate was obtained in the yields
of 91 and 90%. The slight decrease of the yields in comparison with IL-2
(Entry 2) indicated that the steric hindrance of substituent in the second
nitrogen of the imidazolium ring of ionic liquids influenced the catalytic
activity. Meanwhile, the multi-phenolic hydroxyl in ionic liquid (IL-10)
did not improve the catalytic activity obviously. When ILs 11-12 (Entries
11-12) were used as catalysts, the yields of 3-chloro-1,2-propylene
carbonate decreased to 78 and 73%. These results showed that the anions
of ionic liquids also influenced the catalytic activities according to the
following order: Br > Cl > OAc. In addition, the yield of 3-chloro-1,2-
propylene carbonate reached to 76% when IL-13 (Entry 13) was used as
catalyst. The decrease of catalytic activity indicated that the phenolic
hydroxyl could activate epichlorohydrin more effectively than hydroxyl.
As supplementaries, ILs 1-13 were also applied to catalyze the reaction at
60 ºC. The similar results were found and shown in supporting
13

information (Table S1). In addition, the TOF of IL-2 reached to 900 h-1
in the reaction of CO2 and epichlorohydrin (in 90% yield) under the
reaction conditions of 0.1mol% catalyst, 1MPa CO2, 120 ºC, 1 h. As
shown in Table 2, IL-2 exhibited the highest TOF among the ionic
liquids catalyzed the reaction of CO2 and epoxides reported in the
literatures.
3.2.2 Optimization of reaction conditions
The influence of reaction parameters, including reaction temperature,
reaction time, catalyst amount and CO2 pressure, on the reaction of CO2
and epoxides were investigated using the most active IL-2.
3.2.2.1 Effect of temperature and time
The effect of time from 0.5 to 3h and temperature from 60 to 90 ºC on the
reaction was summarized in Fig. 2. The yield increased with rising in
temperature and the prolonging of the reaction time. It was noted that the
yield reached to 99% at 80 ºC for 2 h or 90 ºC for 1h.
3.2.2.2 Effect of catalyst amount
The influence of the catalyst amount on the reaction was shown in Fig. 3.
With the increase of the amount of IL-2 from 0.25 to 1.5 mol%, the yield
improved dramatically. When the amount of IL-2 was higher than 1mol%,
the yield reached to nearly 100%.
The effect of CO2 pressure from 0.25 to 1.5MPa on the reaction was
listed in Fig. 4. The yield increased gradually when the CO2 pressure
increased from 0.25 to 1MPa. When the pressure was higher than 1MPa,
the yield reached to 99% and did not change anymore.
14

3.2.3 Reaction of CO2 and other epoxides catalyzed by IL-2
To investigate the generality of the procedure, the reaction of various
epoxides and CO2 catalyzed by IL-2 was carried out (as shown in Table
3). Ethylene oxide, propylene oxide, 1,2-epoxyhexane, styrene oxide and
phenyl glycidyl ether (Entries 1-5) afforded to the corresponding cyclic
carbonates in 92-97% of isolated yields. This indicated that IL-2 was a
highly effective catalyst for the reactions of various terminal epoxides
with CO2. The mild reaction conditions and high yields benefited from
the synergistic effect of phenolic hydroxyl and C2-H in IL-2.
Furthermore, 1,2-cyclohexene oxide (Entry 6) gave the cyclic carbonates
of 75% isolated yield at 130 ºC for 9 h. The relative lower yield was
presumably due to the steric hindrance of 1,2-cyclohexene oxide[65].
3.2.4 Reaction mechanism studies
To investigate the reaction mechanism, NMR spectroscopy and DFT
calculations were used to determine the interaction between IL-2 and
1
epichlorohydrin. H NMR titration based on the addition of aliquots
epichlorohydrin to IL-2 was carried out. As shown in Fig. 5, when
epichlorohydrin was added gradually, the chemical shifts of both C2-H
and phenolic hydroxyl moved to upfield step by step. After adding 4
equivalents of epichlorohydrin, the chemical shift of C2-H moved from
9.484 to 9.360 ppm and the chemical shift of phenolic hydroxyl moved
from 9.637 to 9.628 ppm. It is broadly accepted that hydrogen-bond
formation generally results in a downfield shift of the hydrogen-bonded
hydrogen nucleus in NMR spectra. However, the present result showed
the chemical shifts of both C2-H and phenolic hydroxyl moved to upfield.
This indicate that the strength of hydrogen-bonding decreases upon
addition of epichlorohydrin. Indeed, the C2-H and phenolic hydroxyl of
15

cation is initially involved in hydrogen bonding with Br-. Addition of
epichlorohydrin results in competition between hydrogen-bond acceptors
(epichlorohydrin and Br-) for hydrogen-bond donors (phenolic hydroxyl
and C2-H in ionic liquid cation), forming a new hydrogen-bonded species
with C2-H and phenolic hydroxyl involved in weaker hydrogen bonds.
This indicate that the C2-H and phenolic hydroxyl participate together in
the formation of hydrogen bonding between cation of IL-2 and
epichlorohydrin. These results imply that epichlorohydrin can be
cooperatively activated by the C2-H and phenolic hydroxyl in IL-2.
To further prove the synergistic effect of phenolic hydroxyl and C2-H in
IL-2, DFT calculations were carried out. The optimized geometrical
structures of epichlorohydrin, the complex of epichlorohydrin with the
cation of IL-8, the complex of epichlorohydrin with the cation of IL-5,
and the complex of epichlorohydrin with the cation of IL-2 were
simulated at the ωB97XD/ 6-31G level. The results were shown in
Figure 6. The bond length of the C1--O1 of epichlorohydrin was 1.424 Å
(a). The bond length of the C1--O1 of epichlorohydrin was elongated to
1.433 Å after complexation with the cation of IL-8, and hydrogen bond
of O1--H1 was formed with a bond length of 1.954 Å (b). The bond length
of the C1--O1 of epichlorohydrin was elongated to 1.434 Å after
complexation with the cation of IL-5, and hydrogen bond of O1--H2 was
formed with a bond length of 1.807 Å (c). These results show that
epichlorohydrin is activated by a hydrogen bonding interaction between
oxygen of epichlorohydrin and the hydrogen-bond donor (either phenolic
hydroxyl or C2-H). Meanwhile, the phenolic hydroxyl can form stronger
hydrogen-bond with epichlorohydrin than C2-H. The bond length of the
C1--O1 of epichlorohydrin was elongated to 1.442 Å after complexation
with the cation of IL-2, and hydrogen bonds of both O1--H1 and O1--H2
16

were formed with a bond length of 2.453 Å and 1.937 Å, respectively (d).
This result indicates that epichlorohydrin can be cooperatively activated
by phenolic hydroxyl and C2-H. Compared to the effect of phenolic
hydroxyl or C2-H on epichlorohydrin, the synergistic effect makes the C-
O bond of epichlorohydrin stretched longer. This means the cleavage of
C-O bond is easier, and consequently the ring-opening reaction of
epichlorohydrin is accelerated. Therefore, IL-2 has higher catalytic
activity than IL-8 and IL-5. DFT calculations also indicate that phenolic
hydroxyl and C2-H have a synergistic effect to activate the epoxides
cooperatively via the formation of hydrogen bonds.
Based on literatures [34, 48, 57] and the above analysis, the reaction
mechanism of IL-2 was proposed in Scheme 2. Firstly, phenolic
hydroxyl and C2-H in the cation of IL-2 cooperatively activated
epichlorohydrin through hydrogen bond and resulted in the polarization
of C-O bonds, followed by the nucleophilic attack by anion of ionic
liquid (Br-) which opened up the epoxide ring to form an oxy-anion
species (step 1). Then, the oxy-anion species further reacted with CO2 to
give the alkylcarbonate anion (step 2). Lastly, cyclization of
alkylcarbonate anion via an intramolecular nucleophilic attack leaded to
the formation of 3-chloro-1,2-propylene carbonate and the regeneration
of the catalyst (step 3). The intermediates of both II and III were also
studied by DFT calculations. As shown in supporting information (Fig.
S1), phenolic hydroxyl and C2-H cooperatively stabilized oxy-anion
species (or alkylcarbonate anion) via the formation of hydrogen bond.
4. Conclusions
In summary, phenolic hydroxyl-functionalized imidazolium ionic liquids
were synthesized and applied as catalysts for fixation of CO2 to cyclic
17

carbonates under the mild reaction conditions. ILs 1-3 showed high
catalytic activities due to the synergistic effect of phenolic hydroxyl and
C2-H to activate epoxides through formation of hydrogen-bonding. The
spatial positions of the phenolic hydroxyl and C2-H in imidazolium
played an important role on the reaction, and the order of catalytic
activity was meta-isomer (IL-2)> ortho-isomer (IL-1)> para-isomer (IL-
3). Among ILs 1-13, 3-(3-phenolic hydroxyl)-1-butyl-imidazolium
bromide (IL-2) exhibited the highest catalytic activity, and the TOF of
IL-2 reached to 900 h-1 under the reaction condition of 0.1 mol % catalyst,
1 MPa CO2, 120 ºC and 1 h.
Acknowledgments
We thank the National Natural Science Foundation of China (21273078,
21573072, 21573073) and Shanghai Leading Academic Discipline
Project (project number B409) for financial support.
18

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23

OH
OH
OH
Br
OH
Bu
N
Br
Br
Br
N
N
Bu
N
N
Bu
N
N
N
Bu
IL-2
OH
IL-1
IL-3
IL-4
OMe
OH
Br
Bu
Br
Bu
N
Br
Br
N
N
N
N
N
N
Bu
N
Bu
Bu
IL-6
IL-5
Br
IL-8
IL-7
HO
OH
OH
Bu
N
OAc
N
OH
Cl
N
OH
Br
N
N
N
N
N
IL-9
Br
IL-12
IL-10
IL-11
OH
N
N
IL-13
Fig. 1. The structures of phenolic hydroxyl-functionalized
imidazolium ionic liquids
90oC
100
90
80
70
60
50
40
30
20
80oC
70oC
60oC
0.5
1.0
1.5
2.0
2.5
3.0
Time/h
Fig. 2. Effect of temperature and time
Reaction conditions: epichlorohydrin (20 mmol). IL-2 (0.2 mmol), CO2
1MPa.
24

100
90
80
70
60
50
0.0
0.5
1.0
1.5
Amount of catalyst/mol%
Fig. 3. Effect of amount of IL-2
Reaction conditions: epichlorohydrin (20 mmol). CO2: 1MPa. 80 ºC,2h.
3.2.2.3 Effect of CO2 pressure
100
95
90
85
80
75
70
65
0.5
1.0
1.5
Pressure/MPa
Fig. 4. Effect of CO2 pressure
Reaction conditions: epichlorohydrin (20 mmol), IL-2 (1 mol %,0.2
mmol), 80 ºC, 2h.
25

Figure 5. Chemical shift in the 1H NMR of IL-2. Solvent: DMSO, (a)
IL-2 , (b) epichlorohydrin: IL-2 = 1:2, (c) 1:1, (d) 2:1, (e) 4:1 (mol),
respectively.
26

Figure 6. DFT optimized geometry of (a) epichlorohydrin, (b) the
complex of epichlorohydrin with the cation of IL-8, (c) the complex of
epichlorohydrin with the cation of IL-5, (d) the complex of
epichlorohydrin with the cation of IL-2.
27

Bu
OH
Br
N
O
N
O
R
O
CO2
O
R
R= -H, -CH3, -Bu, -Ph, -OCH2Ph, -CH2Cl
Scheme 1. Phenolic hydroxyl-functionalized imidazolium ionic liquids
catalyzed the reaction of CO2 and epoxides
Cl
O
Br
N
N
H
Bu
N
OH
N
H
Bu
O
O
H
O
O
O
step3
Br
Cl
Cl
(I)
H O
step1
O
H
O
O
N
N
Bu
(III)
step2
CO2
Cl
N
Br
N
H
Bu
O
H
O
Br
(II)
Cl
Scheme 2. Proposed reaction mechanism
28

Table 1. Reaction of CO2 and epichlorohydrin catalyzed by ionic liquids
O
Cl
O
Cl
O
CO2
O
Entry
Yield/%
Catalyst
1
2
3
4
IL-1
IL-2
IL-3
IL-4
92
96
84
85
84
75
68
66
5
6
IL-5
IL-6
7
8
IL-7
IL-8
9
91
90
78
IL-9
10
11
IL-10
IL-11
12
13
73
76
IL-12
IL-13
Reaction conditions: Epichlorohydrin (20 mmol), IL (0.2 mmol), CO2 1.0
MPa, 70 ºC, 3h.
29