Effect of hydrogen bond of hydroxyl-functionalized ammonium ionic liquids on cycloaddition of CO2

Article abstract of DOI:10.1016/j.tetlet.2015.01.174

A synergistic effect of the hydrogen bond on the cycloaddition of CO₂ and epoxides to form cyclic carbonates was investigated through experimental study and characterization. A highly effective homogeneous system of hydroxyl-functionalized quaternary ammonium ionic liquids with a different number of the hydroxyl group in the cation was developed for the fixation of CO₂ to form cyclic carbonates. A mechanism via the hydrogen bond activation for the cycloaddition was proposed based on both experimental data and modeling. This research enhances the understanding of the promotion of reaction via hydrogen bonding and forms the basis for the rational design of catalytic systems for the fixation of CO₂ into organic compounds.

Full text of DOI:10.1016/j.tetlet.2015.01.174

Tetrahedron Letters 56 (2015) 1416–1419
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Effect of hydrogen bond of hydroxyl-functionalized ammonium ionic
liquids on cycloaddition of CO2
b
Weiguo Cheng a, Benneng Xiao a, Jian Sun a, Kun Dong a, Peng Zhang a, Suojiang Zhang a, , Flora T. T. Ng
⇑
a Beijing Key Laboratory of Ionic Liquids Clean Process, Key Laboratory of Green Process and Engineering, Chinese Academy of Sciences, Institute of Process Engineering,
Chinese Academy of Sciences, Beijing 100190, PR China
b Department of Chemical Engineering, University of Waterloo, N2L 3G1 Waterloo, Canada
a r t i c l e i n f o
a b s t r a c t
Article history:
A synergistic effect of the hydrogen bond on the cycloaddition of CO2 and epoxides to form cyclic carbon-
ates was investigated through experimental study and characterization. A highly effective homogeneous
system of hydroxyl-functionalized quaternary ammonium ionic liquids with a different number of the
hydroxyl group in the cation was developed for the fixation of CO2 to form cyclic carbonates. A
mechanism via the hydrogen bond activation for the cycloaddition was proposed based on both
experimental data and modeling. This research enhances the understanding of the promotion of reaction
via hydrogen bonding and forms the basis for the rational design of catalytic systems for the fixation of
CO2 into organic compounds.
Received 23 December 2014
Revised 22 January 2015
Accepted 27 January 2015
Available online 3 February 2015
Keywords:
Hydrogen bond
Ionic liquid
CO2
Ó 2015 Elsevier Ltd. All rights reserved.
Cycloaddition
Carbon dioxide, which has been released annually to the atmo-
sphere at ca. 30,000 Mt,1 is also an attractive C1 feedstock as it is
nontoxic, abundant, and inexpensive. An important application is
the fixation of CO2 with epoxides to yield cyclic carbonates, which
are used to produce a wide range of commercial and industrial
products, including polar aprotic solvents, intermediates, and
electrolytes in lithium-ion batteries. Various catalysts have been
developed for the production of cyclic carbonates including metal
oxides,2 alkali metal salts,3 quaternary onium salts,4 transition
metal complexes,5 functional organic polymers,6 and ionic liquids
(ILs).7 Among these catalysts, ILs are demonstrated to be effective
and environmentally benign. It is worth mentioning that
functionalized ILs are more efficient as a synergistic effect of the
combination of the functional groups of cations and anions
accelerates the cycloaddition reactions of epoxides.
Recently, synergistic effect of ILs catalytic system has received
more and more attention for promoting the cycloaddition reactions
of epoxides. Functional groups in catalysts such as hydroxyl
groups,8 amino groups,9 and carboxylic groups10 have been widely
developed. Hydrogen bond donors (HBD) based on functional
groups promote the reaction. The HBD such as cellulose,11 H2O,12
carboxylic acid imidazolium,13 and hydroxyl-functionalized
imidazolium,14 show a positive influence on the reaction due to
the synergistic effect. Specifically the hydroxyl-functionalized ionic
liquids (HFILs) have a positive effect on the ring-opening of epox-
ide and accelerate the reaction as its energy barrier is much lower
than that without a hydroxyl group. Much effort was put into
investigating the hydroxyl group effect on the reaction. However,
details of the hydrogen bond-promoted mechanism were still
unclear.
As a part of our continuing efforts on understanding the effect of
the hydrogen bond of ILs on promoting the conversion of CO2 into
cyclic carbonates, a series of quaternary ammonium ILs with differ-
ent number of the hydroxyl group in the cation were synthesized15
and their activity and reusability toward the coupling of epoxide
and CO2 without any additional co-catalyst and organic solvent
were investigated. In this letter, a synergistic effect of the combina-
tion of hydroxyl groups of the cation and anion on promoting the
cycloaddition reactions of epoxides was discussed. The mechanis-
tic details of the synergistic effect were elucidated by FT-IR and
DFT.
Catalyst screening experiments were carried out using propyl-
ene oxide (PO) as a model substrate.16 Reactions were reacted with
CO2 using a variety of ILS under certain reaction conditions without
any additional co-catalyst and organic solvent. The effect of hydro-
xyl groups of catalysts on the cycloaddition is characterized in
Table 1. The number of hydroxyl groups in quaternary ammonium
ILs varies from 0 to 4. The reaction catalyzed by the tetraethyl
ammonium bromide (NEt4Br) exhibited low activity. When one
ethyl group was substituted by the hydroxyethyl (HE) group, the
resulting hydroxyethyltriethyl ammonium bromide (NEt3HEBr)
⇑
Corresponding author. Tel./fax: +86 10 82627080.
E-mail address: sjzhang@ipe.ac.cn (S. Zhang).
http://dx.doi.org/10.1016/j.tetlet.2015.01.174
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.

W. Cheng et al. / Tetrahedron Letters 56 (2015) 1416–1419
1417
Table 1
O
Catalyst screen for the synthesis of propylene carbonatea,b
OH
OH
Br
R
N
O
1
Entry
Catalyst
PO conversion (%)
PC yield (%)
HO
R
O
N
O
O
H
H
1
2
3
4
NEt4Br
NEt3(HE)Br
NEt2(HE)2Br
NEt(HE)3Br
N(HE)4Br
NEt(HE)3Br
NEt4Br/Ethanol
NEt4Br/Glycol
NEt4Br/Glycerol
NEt4Br/propanol
NEt4Br/1,2-propanediol
NEt4Br/propanetriol
64
82
89
93
78
98
75
92
94
69
77
95
63
81
88
92
77
97
74
91
93
68
76
94
O
O
N
O
HO
4
Br
R
5
O
6c
7d
8d
9d
10d
11d
12d
O
H
2
O
HO
R
O
H
Br
R
O
H
H
O
3
CO2
N
HO
Br
a
b
c
O
Conditions: PO (0.014 mol), catalyst (1 mol %), 120 °C, 1.5 MPa, 1 h.
Results determined by GC and GC–MS.
Reaction temperature: 130 °C.
Scheme 1. Proposed mechanism for the synthesis of propylene carbonate.
d
The molar ratio = 1:1.
a
showed much higher activity than that of NEt4Br (entries 1 and 2).
With an increase of the number of hydroxyl groups in the cation
from 1 to 3, the activity of the catalyst increased steadily. The reac-
tion catalyzed by NEt(HE)3Br reached 98% PO conversion with 97%
PC yield (entry 6). A HBD of the hydroxyl group group, which could
remarkably reduce the activation energy, showed a synergistic
effect with BrÀ and accelerated the ring opening of epoxides.14
Interestingly, the tetrakis(2-hydroxyethyl)ammonium bromide
(N(HE)4Br) with four hydroxyl groups did not show the highest
activity among the catalysts employed. In fact the corresponding
PO conversion is only 78%, which is lower than that of NEt3(HE)Br
(entry 5, Table 1). It is possible that the additional hydroxyl group
groups in the cation of the catalyst prefer to form the hydrogen
bond with the halide anion which reduces the nucleophilic behav-
ior of the anion rather than to promote the ring opening of epox-
ide.17 Hence, apparently there is an optimal hydrogen bonding
effect on enhancing the ring opening of epoxides. Based on the
results, the activity order of the functionalized ILs is as follows:
NEt(HE)3Br > NEt2(HE)2Br > NEt3(HE)Br while N(HE)4Br > NEt4Br
(entries 1–5). In order to further verify that the hydroxyl group
can promote the cycloaddition reaction, experiments were carried
out with the NEt4Br catalytic system with ethanol, ethylene glycol,
glycerol, propanol, 1,2-propanediol, and propanetriol as co-catalyst
(Table 1, entries 7–12). It could be seen from Table 1 that with the
increase of the number of hydroxyl groups in the co-catalyst mol-
ecule, the activity of catalysts also increased accordingly. The alco-
hol with one or two hydroxyl groups and a shorter alkyl chain
length is more effective in promoting the reaction (Table 1, com-
pare entries 7 and 10; 8 and 11). Thus, a synergistic effect, which
polarizes the oxygen atom by hydrogen-bonding of hydroxyl group
groups and nucleophilic activation of the carbon atom by anion
(BrÀ), enhances the catalytic performance of quaternary ammo-
nium ILs.
b
3500
3000
2500
2000
1500
Wavenumber (cm-1)
Figure 1. The IR spectrum of NEt(HE)3Br with and without PO (a) NEt(HE)3Br with
PO, (b) NEt(HE)3Br without PO.
between the OH group and PO. It is important to note that in the
DFT study, the frequency of OH groups also shifts from
3534 cmÀ1 to 3212 cmÀ1 due to the hydrogen bond between the
N(HE)3Br+ cation and the substrate PO. Meanwhile, the formation
of hydrogen bonds between the OH groups and styrene oxide leads
to a shift of the OH group to 3319 cmÀ1 (Fig. S2). Moreover, in IR
spectra of catalyst with styrene oxide, the signal of the stretching
vibrations of OH groups shifts downfield gradually with an
increase of temperature. That also suggests stronger interaction
between the OH groups and O atom of styrene oxide (Fig. 2). Then,
the epoxide ring opens via a synergistic effect of the combination
of the hydroxyl groups and the Br-, and the CO2 is inserted.
Subsequent cyclization via an intramolecular nucleophilic attack
leads to the cyclic carbonate and the catalyst is regenerated.
a
b
c
d
e
A possible mechanism for the fixation of CO2 with epoxide to
form cyclic carbonate catalyzed by hydroxyl-functionalized qua-
ternary ammonium ILs was proposed (Scheme 1). Firstly, the epox-
ide is activated by hydrogen bond interaction, which facilitates the
ring opening. The hydrogen bond interaction is studied by a DFT
calculation (Fig. S1). The distance of HÁ Á ÁO of N(HE)+3 cation is
0.9702 Å, but the distance of HÁ Á ÁO of N(HE)+3–PO is 1.742 Å due
to the hydrogen bond between N(HE)3Br+ cation and the substrate
PO. The hydrogen bond interaction is also confirmed by IR tech-
nique. It can be seen in the IR spectra (Fig. 1) that the characteristic
sharp absorbance of the hydroxyl groups at 3388 cmÀ1 due to the
stretching vibrations of OH groups of NEt(HE)3Br shifts to
3320 cmÀ1. That suggests the formation of the hydrogen bond
3500
3000
2500
2000
1500
Wavenumber (cm-1)
Figure 2. The IR spectra of NEt(HE)3Br with styrene oxide at different tempera-
tures. (a) 40 °C, (b) 60 °C, (c) 80 °C, (d) 100 °C, (e) 120 °C.

1418
W. Cheng et al. / Tetrahedron Letters 56 (2015) 1416–1419
Table 2
Synthesis of other cyclic carbonates catalyzed by NEt(HE)3Bra
100
80
60
40
20
0
NEt4Br
NEt3(HE)Br
NEt2(HE)2Br
NEt(HE)3Br
N(HE)4Br
Entry Epoxide
Product
T
(h)
Conv.b
(%)
Selec.b
(%)
O
O
O
1a
1
1
1
99
98
99
99
O2a
O
O
1b
O
O
O
2
3
2b
0
100 200 300 400 500 600
O
O
O
O
CH Cl 1c
2
Temperature oC
O
1
6
2
99
98
97
99
99
99
Figure 3. TGA curves of ammonium ionic liquids.
CH Cl2c
2
O
Bu1d
O
O
O
4
5
PO conversion
PC selectivity
Bu
2d
100
80
60
40
20
0
O
Ph1e
O
Ph
2e
O
O
O
O
O
O
6
7
2
99
91
99
99
1f
2f
O
O
O
O
14
1
2
3
4
5
6
1J
Runs
2g
a
Figure 4. Recycling experiments for NEt(HE)3Br. Conditions: PO (0.285 mol),
catalyst (1 mol %), 130 °C, 1.5 MPa, 1 h.
Reaction conditions: epoxides (0.0143 mol), catalyst (1 mol %), CO2 1.5 MPa,
130 °C.
b
Determined by GC.
TGA data (Fig. 3) show that the synthesized hydroxyl-function-
alized ammonium ILs are all stable up to 200 °C, which is much
higher than the reaction temperature of 130 °C. The stability of
the different ILs are dependent on the degree of the substitution
modeling provides an increased understanding of the promotional
effect of hydrogen bonding on the fixation of CO2 into organic com-
pounds in ionic liquids.
of
the
hydroxyl
group:
NEt4Br > NEt3HEBr > NEt2(HE)2-
Br > NEt(HE)3Br, N(HE)4Br.
Acknowledgments
The recycling experiment of the catalyst was carried out by
using PO as the substrate. After each experiment, the catalyst
was separated by distillation under vacuum and then recycled
directly for the next running experiment. The catalysts were quite
stable as no decrease in PC yields was observed even after six recy-
cling runs (Fig. 4).
The highly efficient NEt(HE)3Br catalyst was also found to be
effective for conversion of a variety of terminal epoxides to cyclic
carbonates under the optimal reaction conditions found for PO.
The results are summarized in Table 2. Disubstituted epoxides
(entry 7) showed lower activity than mono-substituted terminal
epoxides (entries 7 vs 1–6), and required a long time (14 h) to
obtain a high yield.
This work was supported by the National Natural Science Foun-
dation of China (91434107), the Chinese Academy of Sciences
for Visiting Professorship for Senior International Scientists
(2013T1G0022) and National Key Technology Research and Devel-
opment Program of the Ministry of Science and Technology of
China (2012BAF03B01).
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.01.
174.
In conclusion, a series of hydroxyl-functionalized quaternary
ammonium ILs with a different number of the hydroxyl group in
the cation were found to be highly efficient for the coupling of
epoxide and CO2 to form cyclic carbonates without any additional
co-catalyst and organic solvent. The catalyst showed reusability
and could be recycled six times without loss in activity and selec-
tivity. A synergistic effect, due to the polarization of the oxygen
atom via the hydrogen-bonding of OH groups and nucleophilic
activation of the carbon atom by the anion, was proposed for
promotion of the cycloaddition reactions of CO2 and epoxides. A
proposed mechanism based on both experimental data and
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1H); 3.76 (t, J = 4.8 Hz, 2H); 3.32 (q, J = 7.2 Hz, 6H); 3.30 (t, J = 5.2 Hz, 2H); 1.17
(t, J = 7.0 Hz, 9H). Anal. Calcd for NEt3(HE)Br: C, 42.49; H, 8.91; N, 6.19.
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were prepared by the same method. N,N-Diethyl-2-hydroxy-N-(2-
hydroxyethyl)ethanaminium bromide (NEt2(HE)2Br), Yield: 78%; white solid;
91.8 °C. 1H NMR (600 MHz, DMSO-d6)
d 5.20 (tm, J = 5.0 Hz, 2H), 3.74
(dt, J = 5.4 Hz, J = 5.0 Hz, 4H), 3.23 (m, 8H), 1.21 (t, J = 7.2 Hz, 6H). Anal.
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5.74. N-Ethyl-2-hydroxy-N,N-bis(2-hydroxyethyl)ethanaminium bromide
(NEt(HE)3Br), yield: 81%, white solid; 62.8 °C. 1H NMR (600 MHz, DMSO-d6) d
5.20 (tm, J = 5.0 Hz, 3H), 3.80 (dt, J = 5.4 Hz, J = 5.0 Hz, 6H), 3.51 (q, J = 7.0 Hz,
2H), 3.48 (t, J = 5.4 Hz, 6H), 1.24 (t, J = 7.0 Hz, 3H). Anal. Calcd for NEt(HE)3Br: C,
37.22; H, 7.81; N, 5.43; found: C, 37.02; H, 7.82; N, 5.44. Tetrakis(2-
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the coupling reactions were conducted in
a 25 ml stainless-steel reactor
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equipped with a magnetic stirrer and automatic temperature control system. A
typical reaction was carried out as follows: in the reactor, an appropriate CO2
(1.0 MPa) was added to a mixture of PO (0.014 mol) and catalyst (1 mol %) at
room temperature. Then, the temperature was raised to 130 °C with the
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addition of CO2 from a reservoir tank to maintain a constant pressure
(1.5 MPa). After the reaction had proceeded for 1.0 h, the reactor was cooled
to room temperature, and the remaining CO2 was removed slowly. The
recycling experiment of the catalyst was carried out in a 100 ml stainless-steel
reactor. The catalyst was separated by distillation under vacuum and then
recycled directly for the next running experiment. The products were analyzed
by Agilent 6890/5973B GC–MS equipped with a FID detector and a DB-wax
using acetophenone as the internal standard.
15. Typical synthesis procedure of hydroxyl-functionalized ionic liquids: For the
synthesis of N,N,N-triethyl-2-hydroxyethanaminium bromide (NEt3(HE)Br), a
mixture of triethylamine (0.03 mol), 2-bromoethanol (0.04 mol) and dry
toluene (10 mL) was heated at 110 °C for 12 h in a 50 ml single flask with
vigorous stirring. Then the mixture was cooled down to room temperature, and
a white solid formed rapidly. The resultant crude solid was filtered off, washed
with acetone (5 Â 20 ml) and dried at 70 °C for 12 h under vacuum. Yield: 90%,
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