Highly efficient CO2 fixation into cyclic carbonate by hydroxyl-functionalized protic ionic liquids at atmospheric pressure

Article abstract of DOI:10.1016/j.mcat.2021.111756

We report an efficient strategy for CO₂ fixation into cyclic carbonates catalyzed by hydroxyl-functionalized protic ionic liquids (PILs) without any solvent and additive. Excellent yields of cyclic carbonates are obtained for the varied substrates under mild conditions (1 bar and 60°C). Mechanism studies have shown that hydroxyl promoted the activation of epoxide molecules through hydrogen bonding interaction, so that CO₂ can be easily converted into the desired product of cyclic carbonates. In addition, the hydroxyl-functionalized PILs can also be recycled repeatedly. This hydroxyl-functionalized PILs-catalyzed process is proved to be a promising scheme for the chemical conversion of CO₂.

Full text of DOI:10.1016/j.mcat.2021.111756

Molecular Catalysis 511 (2021) 111756
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Highly efficient CO2 fixation into cyclic carbonate by
hydroxyl-functionalized protic ionic liquids at atmospheric pressure
,
,
,
,*
Cheng Lia 1, Fei Liua 1, Tianxiang Zhaoa *, Jiarui Gua, Peng Chena, Teng Chenb
a Key Laboratory of Green Chemical and Clean Energy Technology, School of Chemistry and Chemical Engineering, Guizhou University, Guiyang 550025, China
b Air Force Logistics College, Xuzhou 221000, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
We report an efficient strategy for CO2 fixation into cyclic carbonates catalyzed by hydroxyl-functionalized protic
ionic liquids (PILs) without any solvent and additive. Excellent yields of cyclic carbonates are obtained for the
varied substrates under mild conditions (1 bar and 60◦C). Mechanism studies have shown that hydroxyl pro-
moted the activation of epoxide molecules through hydrogen bonding interaction, so that CO2 can be easily
converted into the desired product of cyclic carbonates. In addition, the hydroxyl-functionalized PILs can also be
recycled repeatedly. This hydroxyl-functionalized PILs-catalyzed process is proved to be a promising scheme for
the chemical conversion of CO2.
CO2
Protic ionic liquids
Cyclic carbonate
Hydroxyl-functionalized
Hydrogen bonding
1. Introduction
cyclic carbonates from epoxides and CO2 catalyzed by a bifunctional
phosphonium salt catalyst. Whereafter, Mujmule and coworkers re-
Carbon dioxide (CO2) is a naturally abundant, cheaper and generally
non-toxic C1 resource that can be used as building block for the syn-
thesis of value-added chemicals. However, it is difficult to utilize CO2
efficiently due to kinetic inertness and thermodynamic stability of CO2,
which limits the utilization of CO2 in chemical industry [1]. Despite the
difficulties, CO2 has been successfully converted into valuable chemicals
such as formic acid [2], methane [3], methanol [4], amide [5], car-
bonate [6] etc. Particularly, synthesis of cyclic carbonate by cycload-
dition of CO2 and epoxides is considered to be one of the most promising
way of utilization [7-16]. Up to now, a multitude of homogeneous and
heterogeneous catalysts have been explored for the synthesis of cyclic
carbonates, such as metal oxide [7], alkali metal salt [8], metal organic
complex [9], modified molecular sieve [10], and ionic liquids (ILs)
[11-17]. Thereinto, ILs have high thermal stability, low vapour pressure,
as well as their unique tunable structure, making as very attractive
catalysts for the cycloaddition of CO2 and epoxides. In this background,
various functionalized ILs are widely investigated, such as hydroxyl
functionalized ILs [11-13], carboxy functionalized ILs [14-15], urea
derivative-based ILs [16], and porous ionic polymers [18]. Among these
ionic liquid catalysts, hydroxyl functionalized ILs can efficiently fix CO2
into cyclic carbonates through synergistic catalysis [11-13]. For
instance, Shirakawa et al. [11] demonstrated an efficient synthesis of
ported binary catalytic system of [EvimOH][Cl] and base for the CO2
fixation into cyclic carbonate at 100◦C and 4.0 MPa, good to excellent
catalytic activity with 99.8% conversion and selectivity of ≥99% was
obtained [19]. Recently, hydroxyl-functionalized ILs modified melem
oligomers composite served as catalyst in cycloaddition of CO2 and
epoxides, affording cyclic carbonates in 31-97% yields at 140◦C and 2
MPa [20]. Although hydroxyl functionalized ILs show high catalytic
activity, the reaction temperature and pressure are still high, especially
these ILs need to undergo tedious preparation processes [11-22].
Therefore, the development of efficient methods for the fixation of CO2
into cyclic carbonate with simple ionic liquid catalysts under mild
conditions is highly desired.
Protic ionic liquids (PILs) are an essential type of ILs, distinguished
from other ILs by their critical ease of preparation and low price. For the
past few years, many PILs have been employed as catalysts for the
cycloaddition of CO2 and epoxides (Scheme 1) [23-28]. These PILs show
good catalytic activity, but the most reactions also need to undergo high
reaction temperature and pressure in order to achieve satisfactory
conversion [23,25]. Particularly, it cannot be ignored that the cycle
performance of the PILs is significantly reduced due to poor stability
[24].
Inspired by the catalytic mechanism of hydroxyl-functionalized ILs
* Corresponding authors.
E-mail addresses: txzhao3@gzu.edu.cn (T. Zhao), chentyp@yeah.net (T. Chen).
1
These authors contributed equally to this work.
https://doi.org/10.1016/j.mcat.2021.111756
Received 12 May 2021; Received in revised form 30 June 2021; Accepted 6 July 2021
Available online 16 July 2021
2468-8231/© 2021 Elsevier B.V. All rights reserved.

C. Li et al.
Molecular Catalysis 511 (2021) 111756
in synthesis of cyclic carbonate and the favorable reactivity of iodine-
based ILs under neat and mild conditions [11-13,29], we speculate
that if hydroxyl groups are introduced into PILs, it is expected to pro-
mote the activation of epoxide molecules, thereby realizing the con-
3. Results and discussion
3.1. Structural characterization of PILs
version of CO2 under mild conditions. Herein,
a
series of
We started our experiment to prepare a series of PILs through acid-
base neutralization reaction, their structures were characterized by
NMR (Fig. S1-S7). These PILs were vacuum dried for 48 h before use to
remove water as much as possible. The structures of PILs are shown in
Scheme 2. Subsequently, we studied the thermal stability of several
representative PILs and found that the decomposition temperature of
these PILs was around 100◦C (IL-1, IL-2, and IL-6), which means that the
structure of the PILs can be maintained at low reaction temperatures
(Fig. 1).
hydroxyl-functionalized PILs formed by 2-morpholinoethanol and hal-
oid acid are prepared and used as catalysts for the cycloaddition of CO2
and epoxides; meanwhile, some well-defined PILs have also been used as
catalysts for comparative studies.
2. Experimental Section
2.1. Materials
Chemicals of 2-morpholinoethanol (NHEM) (>98.0%), DBU
(>99.0%), triethylamine (TEA) (>99.0%), 1-ethylimidazole (1-EI)
(>98.0%), N-ethylmorpholine (NEM) (>98%), hydriodic acid (55% w/
w aq. soln.), hydrogen bromide (40%), epoxides were purchased from
Energy Chemical Technology (Shanghai) Co., Ltd, China. CO2
(>99.99%) were supplied by Guiyang Sanhe Special Gas Centre, China.
All chemicals were used directly without further purification. FTIR
spectra were recorded on a Nicolet iS50 FTIR spectrometer. 13C NMR
and 1H NMR spectra were recorded on a JNM-ECZ-400 spectrometer.
3.2. Synthesis of cyclic carbonates
Then reaction conditions were optimized by evaluating the activity
of PILs for the cycloaddition of CO2 and styrene oxide (SO), and the
results are shown in Table 1.
As shown in Table 1, we found that IL1-3 are effective and give the
styrene carbonate (SC) in 83-92% yields (Entry 1-3). The hydroxyl-
functionalized PILs formed by 2-morpholinoethanol and haloid acid
were also tested (Entry 4-6). The catalytic activity is strongly dependent
on the nucleophilicity of the anion (Iꢀ > Brꢀ > Clꢀ ) [31-33]. The highest
SC yield of 95% was obtained for IL-6 with Iꢀ as the nucleophilic anion.
Further increase the yield of SC to 97% by enhancing temperature of the
reaction to 80◦C (Entry 7). Reducing the reaction temperature and the
loading of the IL-6 resulted in a significant decrease in the yields of the
SC (Entry 8 and 9). Shortening the reaction time led to a reduction in the
yield of the SC to 83% (Entry 10). Importantly, water was an effective
hydrogen bond donor, it can promote the cycloaddition reaction. In
order to rule out the influence of water in our works, we quantified the
water content of the PILs, as shown in Table S1. It was found that the
water contents of the PILs are all low after water removal. Therefore, the
effect of water could be ignored compared to literature studies [34].
Interestingly, it was found that the catalytic activity of the IL-7 slightly
reduced (Entry 11), indicating that the hydroxyl group may be involved
in the activation of the epoxide. In addition, the reaction cannot occur in
the absence of a catalyst (Entry 12), meaning that PILs catalysts are
critical to the reaction. Above results indicate that IL-6 is the best
candidate catalyst at atmospheric pressure, and it is used in subsequent
research.
2.2. Synthesis and characterization of PILs
All PILs were prepared by acid-base neutralization reaction at
ambient environment for 24 h in a molar ratio of 1:1, and their struc-
tures were characterized by NMR. All PILs were vacuum dried for 48 h
before use to remove water as much as possible. TGA was recorded on a
PerkinElmer Pyris
1 thermal analyser with a scanning rate of
10◦C•minꢀ 1 under N2 atmosphere. Details on NMR information of ILs
can be found in the Supporting Information. The water contents in PILs
were determined by Carl Fischer method of moisture analysis (TKF-
1Titanwaters).
2.3. Synthesis of cyclic carbonate
In a typical procedure, a Schlenk flask (10 ml) was bubbled with CO2
to replace air. Then styrene oxide (10 mmol) and IL-6 (10 mol%) were
added successively. The flask was heated at 60◦C for 24 h under a CO2
atmosphere (balloon). After being cooled to room temperature, the
yields of cyclic carbonate are determined by GC/MS using dodecane as
internal standard, refered to our previous work [30]. For catalyst
recycle, SC and unreacted SO were extracted with hexane three times
(IL-6 is insoluble in hexane), residue IL-6 was dried in vacuum for 1 h to
remove residual hexane and directly used for the next run under the
identical conditions.
With the optimized conditions in hand, the substrate scope of
cycloaddition with various epoxides were evaluated, and the results are
listed in Table 2. This protocol can tolerate the terminal epoxides (Entry
1-8) and give cyclic carbonates in 78-96% yields under mild conditions
(1 bar of CO2, 60◦C) without any solvent and additive. In contrast, in-
ternal epoxides show poor activity (Entry 9-11), affording a maximum
yield of only 32%. It may be that the steric hindrance at the β-carbon
atom limited the nucleophilic attack of the iodide ion, therefore
reducing the reactivity [35-37]. Nonetheless, it was worth mentioning
that IL-6 reflected excellent generality under solvent- and additive-free
conditions. Compared with the catalytic activity of PILs reported in
Scheme 1. Structures of PILs reported in the literature.
2

C. Li et al.
Molecular Catalysis 511 (2021) 111756
Scheme 2. Structures of PILs studied in this work.
the literatures, PILs studied in this work show comparable or better
catalytic activity for the cycloaddition of CO2 to SC (Table 3), which
provide a promising alternative for CO2 conversion catalyzed by PILs.
To evaluate the utility of the PILs, we scale up the reaction to 50
mmol. The results show that the yield of SC is still 86% (Scheme 3),
meaning that the IL6 can be used for the scale-up synthesis of SC. In
addition, the cycle performance of the IL-6 was also investigated. After
five successive catalytic cycles, it was found that the activity of the IL-6
decreased gradually, but still maintained above 85% (Fig. 2).
Table 1
Optimization of the reaction conditions.
Entry
Cat.
T (◦C)
Yields of SC (%)a
1
IL-1 (10 mol%)
IL-2 (10 mol%)
IL-3 (10 mol%)
IL-4 (10 mol%)
IL-5 (10 mol%)
IL-6 (10 mol%)
IL-6 (10 mol%)
IL-6 (10 mol%)
IL-6 (5 mol%)
IL-6 (10 mol%)
IL-7 (10 mol%)
None
60
60
60
60
60
60
80
30
60
60
60
60
92
87
83
13
60
95
97
27
66
83
90
0
2
3
4
5
6
7
8
3.3. Mechanism research
9
10b
11
12
To gain insight into the reaction mechanism, the interaction between
IL-6 and SO was studied by FTIR. As shown in Fig. 3, 3407 cmꢀ 1 is the
stretching vibration of the OH in IL-6. This absorption peak shifts from
3407 to 3382 cmꢀ 1 with increasing of concentration of SO. This fact is
due to the hydrogen bond interaction between IL-6 and SO, which can
promote the activation of SO [38,39]. To further confirm the interaction
between IL-6 and SO, 1H NMR analysis was performed. As shown in
Reaction conditions: SO (10 mmol), CO2 (1 bar), 24 h. aThe yields of SC are
determined by GC/MS using dodecane as internal standard. b10 h.
Table 2
Synthesis of cyclic carbonates catalyzed by IL-6.
Fig. 4, the chemical shifts of proton on the N-linked α-carbon move from
3.21 to 2.56 ppm with increasing of concentration of SO, indicating that
there is a hydrogen bond between the N-H of IL-6 and SO. It is worth
noting that either N-H, O-H, and C2-H of aromatic ring of imidazole
(IL-2) all works as a proton donor and facilitates the formation of
intermolecular hydrogen bonding that can promote the activation of the
epoxide [40-46].
Entry
Substrate
Products
Yields (%)
90
1
2
3
92
85
In accordance with the experimental results and literatures [47-50],
a plausible mechanism for the catalytic cycloaddition of CO2 and
epoxide catalyzed by IL-6 is proposed and shown in Scheme 4. First,
intermolecular hydrogen bonding results in polarization of C-O bond in
epoxide, which promotes iodine ion to attack epoxide and form alkoxide
intermediate. Then, alkoxide intermediate is stabilize by hydrogen
bonding, which can further react with CO2 to generate a halogenated
carbonate. Eventually, the cyclic carbonate is synthesized by internal
4
5
78
95
6
90
Table 3
7
8
95
96
Comparison of various PILs for the cycloaddition of CO2 to SC.
Entry
Cat.
T(◦C)/t(h)/CO2
Yield of SC
(%)
Ref.
(bar)
1
2
3
4
[HMim][Br](1 mol
%)
120/2/15
35/24/1
120/4/1
70/4/1
77.1
21
22
23
24
9
28
32
17
[Et3NH][Br](10 mol
%)
20
10
11
[DMAPH][Br](1 mol
%)
95
[DBUH][Br](10 mol
%)
77
Reaction conditions: IL-6 (10 mol%), substrate (10 mmol), CO2 (1 bar), 60◦C, 24
h, solvent-free, the yields are determined by GC/MS using dodecane as internal
standard.
5
6
DBUPILs(6 mol%)
IL-6(10 mol%)
50/6/1
87
95
25
60/24/1
This
work
3

C. Li et al.
Molecular Catalysis 511 (2021) 111756
Scheme 3. Scale-up synthesis of SC
cyclisation of the halogenated carbonate and the Iꢀ is removed to
complete the catalytic process.
4. Conclusions
In summary, we demonstrated that the hydroxyl-functionalized PILs
are efficient catalysts for the cycloaddition of CO2 and epoxides. The
targeted products of cyclic carbonates are obtained in excellent yields.
Mechanism studies have revealed that the intermolecular hydrogen
bonding between epoxide and hydroxyl-functionalized PILs is the key
reason to promote the activation of epoxide. This PILs-catalyzed reac-
tion is proved to be a promising scheme for conversion of CO2 into cyclic
carbonates at mild conditions in the absence of additive, metal and
solvent.
Fig. 1. TGA of selected PILs.
Notes
The authors declare no competing financial interest.
Credit Author Statement
Cheng Li: Investigation, Writing - Original Draft. Fei Liu: Writing -
Review & Editing. Tianxiang Zhao: Conceptualization, Supervision,
Writing - Review & Editing. Jiarui Gu: Data Curation. Peng Chen:
Visualization. Teng Chen: Conceptualization, Funding acquisition.
Appendix A. Supplementary data
The Supporting Information is available free of charge via the
Internet at http://pubs.acs.org.
Fig. 2. The cycle performance of IL-6
Scheme 4. Proposed mechanism for cycloaddition of CO2 and epoxide catalyzed by IL-6.
4

C. Li et al.
Molecular Catalysis 511 (2021) 111756
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