Efficient transformation of CO2 to cyclic carbonates using bifunctional protic ionic liquids under mild conditions

Article abstract of DOI:10.1039/c9gc01165j

A series of 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU) based bifunctional protic ionic liquids (DBPILs) were easily prepared by acid-base reactions at room temperature. They were used to catalyze the cycloaddition reaction of CO₂ with epoxides u

Full text of DOI:10.1039/c9gc01165j

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Efficient transformation of CO2 to cyclic
carbonates using bifunctional protic ionic liquids
Cite this: DOI: 10.1039/c9gc01165j
under mild conditions†
Xianglei Meng, a,b Zhaoyang Ju,a Suojiang Zhang, a Xiaodong Liang,b
a,c
Nicolas von Solms,b Xiaochun Zhanga and Xiangping Zhang
*
A series of 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU) based bifunctional protic ionic liquids (DBPILs)
were easily prepared by acid–base reactions at room temperature. They were used to catalyze the cyclo-
addition reaction of CO2 with epoxides under mild conditions. As a metal free catalyst, the best DBPIL
showed a 92% yield of products within 6 hours at 30 °C and 1 bar CO2 without any solvents and co-cata-
lysts. It could afford carbonates in good yields with broad epoxide substrate scope and CO2 from simu-
lated flue gas (15% CO2/85% N2). IR spectroscopy and DFT studies were carried out to investigate the
mechanism of the cycloaddition reaction. The results showed that the DBPILs could activate both CO2
and epoxides by alkoxy anions and powerful hydrogen-bonding, which was well consistent with
experiments.
Received 9th April 2019,
Accepted 17th May 2019
DOI: 10.1039/c9gc01165j
rsc.li/greenchem
ponents in lithium batteries,18 useful monomers for acyclic
carbamates19,20 and carbonates,21 intermediates in the pro-
Introduction
Carbon dioxide is one of the main components of flue gas, duction of fine chemicals, etc.22,23
which is responsible for global warming. CO2 is also regarded
To date, various catalysts have been developed for the syn-
as an abundant, nontoxic and renewable C1 resource.1–4 The thesis of useful cyclic carbonates from CO2 and epoxides, such
utilization of CO2 as a raw material for the production of as metal containing catalysts,24–34 organocatalysts,35–44 ionic
energy carriers and chemicals is a promising alternative, liquids etc.45–54 Although most of the catalytic systems show a
which can reduce greenhouse gas and thus alleviate the good yield of carbonates, high temperatures (>100 °C) and/or
impact of climate change, while producing various organic high CO2 pressure are always needed in these studies. Hence,
chemical commodities.5–10 However, applications related to great efforts have been focused on the development of efficient
CO2 are limited because of its thermodynamic stability and and sustainable catalytic systems that can be used for chemi-
therefore the CO2 activation always requires electrolytic cally converting CO2 under relatively mild reaction conditions.
reduction processes or a high-energy input.11 One of the In this respect, many catalysts have been synthesized and they
typical feasible routes is the synthesis of five-membered cyclic exhibited good performance in the synthesis of cyclic carbon-
carbonates by cycloaddition of CO2 with epoxides.12–14 As one ates from CO2. Metal containing catalysts including metal–
of the few successful industrial products that efficiently utilize organic frameworks (MOFs),55,56 metal (salen) complexes57,58
CO2 as a carbon feedstock, cyclic carbonates can be widely and metal-porphyrins59–61 have been reported to provide a
used as solvents in chemical processes,15–17 electrolyte com- good yield of cyclic carbonates under atmospheric pressure
and/or room temperature owing to the active sites of metal
ions and halide ions. For example, North and coworkers
reported Cr(III) salphen complexes for the cycloaddition reac-
aBeijing Key Laboratory of Ionic Liquids Clean Process, Key Laboratory of Green
Process and Engineering, State Key Laboratory of Multiphase Complex Systems,
tion under ambient conditions in the presence of TBAB (tetra-
Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190,
butylammonium bromide).29 The catalyst could catalyze CO2
People’s Republic of China. E-mail: xpzhang@ipe.ac.cn
bDepartment of Chemical and Biochemical Engineering, Center for Energy Resources
Engineering (CERE), Technical University of Denmark, DK2800 Kgs. Lyngby,
Denmark
and epoxides to cyclic carbonates in 57–92% isolated yields
after a reaction time of 24 hours. Organocatalytic systems,
such as boronic acids,62 tetraarylphosphonium salts (TAPS)63
and pyridine-methanol/onium salts,64 could promote the
cycloaddition of CO2 with epoxides under moderate conditions
due to the activation of epoxides by a hydrogen bond
cSchool of Chemical Engineering, University of Chinese Academy of Sciences,
Beijing 100049, China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9gc01165j
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(H-bond). Ionic liquids with a strong hydrogen bond donor catalytic behaviour with various epoxides and simulated flue
(HBD) have been widely reported for this reaction. Gual and gas. DFT studies, IR spectroscopy and experiments were con-
Cano et al.65 reported an imidazolium based iron-containing ducted to study the mechanism of the activation of CO2 and
ionic liquid, and it showed quantitative conversion and 94% H-bond interactions between DBPILs and epoxides.
chemoselectivity for the cycloaddition of CO2 to epoxides
under near ambient conditions. Park et al.66 developed a pyri-
dinium IL-decorated MOF and used it as a solvent-free catalyst
for CO2–oxirane coupling reactions. The best catalyst displayed
Results and discussion
a high catalytic activity at 60 °C and 1.2 MPa CO2 because of To synthesize the protic ionic liquids, electron withdrawing
the synergetic effect of the IL functional sites. In addition, we groups are selected to increase the acidity of alcohols and car-
also demonstrated carboxylic acid-based ionic liquids that boxylic acids through an inductive effect. Halogens are well
could promote the cycloaddition reaction of CO2 at 50 °C and known as electron-withdrawing groups. It has been proven that
0.1 MPa.53 Recently studies have been focused on protic ionic Br− is an excellent nucleophilic ion for the cycloaddition reac-
liquids, which exhibited not only an excellent ability of revers- tion in our previous work.48,52 Herein, halogenated carboxylic
ible CO2 capture but also highly efficient CO2 chemical conver- acids and alcohols with short chain lengths (the closer the
sion even under ambient conditions because of the powerful halides and acid, the stronger the inductive effect) are chosen
H-bonding.67,68 For example, Han et al.12 employed a novel to react with superbase DBU. During the experiments, a series
dual-ionic liquid system for cycloaddition of epoxides with of DBPILs composed of protonic acid and nucleophilic groups
CO2 at temperatures from 30 to 60 °C at 1 atm, and the results was prepared by acid–base neutralizing reactions at room
showed excellent yields without any solvents. However, com- temperature. A DBU based ionic liquid with a phenolic
pared to high temperature and pressure catalysts, unsatisfac- hydroxyl group (1d) was prepared using DBU and 4-bromophe-
tory activities, a long reaction time (>20 h), and the presence of nol due to the weak acidity of 4-bromophenol attributed to the
metals and/or toxic solvents are still drawbacks that need to be conjugative effect and the long chain length between the
overcome. It is a great challenge to design an efficient, green bromide and hydrogen group (details are shown in ESI
and metal-free catalyst toward effective CO2 conversion under Scheme S1†). 4-Dimethylaminopyridine (DMAP) based ILs (1e)
mild conditions.
were synthesized according to the literature69 (Scheme S2†).
Inspired by these studies, we report here the synthesis of DBU-based catalysts 1g with NTF2 were synthesized using an
DBU-based bifunctional protic ionic liquids (DBPILs) by acid– ion-exchange method (Scheme S3†). The catalytic activities of
base reactions at room temperature based on the good acidity various DBPILs were carried out with epichlorohydrin and 1
of halogenated carboxylic acids and alcohols which is achieved bar of CO2 (balloon) in a Schlenk tube at 30 °C. The results are
by the inductive effect of the electron-withdrawing group shown in Table 1.
(Fig. 1). DBPILs that were composed of alkoxy anions, protic
It was found that the common ILs [Bmim]Br (Table 1, entry
acid and nucleophilic groups were successfully used in the 1) showed low activity in catalysing the reaction. Interestingly,
cycloaddition reaction of CO2 with epoxides at 1 bar CO2 and DBU-based ILs without protonic acid (1a) were observed with a
30/50 °C. We found that they were efficient metal-free catalysts similar activity to [Bmim]Br (Table 1, entry 2 vs. 1), which indi-
for this reaction, and displayed high activity in producing car- cated that cations may have little effects on the reaction. Then
bonates without the need for a long reaction time or a co-cata- the activity of DBU-based ILs composed of protic acid and
lyst. We explored the influence of the substrate scope on the bromine ions (1b and 1c) was studied under the same con-
Fig. 1 ILs used in this study.
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Table 1 Reactions of CO2 with epichlorohydrin catalyzed by different ILsa
Entry
Catalyst
Time (h)
Yieldb (%)
Selectivityb (%)
1c
2
3
4
5
[Bmim]Br
1a
1b
1c
1d
1e
1f
1g
6
6
6
6
6
6
6
10
10
12
23
26
70
76
14
47
92
4
>99
>99
99
99
98
99
99
98
99
—
6
7
8
9d
10e
1f
91
84
[TMGH+][−O2MMIm+][Br−] 70
1
a Reaction conditions: epichlorohydrin (0.5 ml), ILs (6 mol%), 1 bar of CO2 (balloon), temperature (30 °C). b Determined by H NMR. c [Bmim]Br
= 1-Butyl-3-methylimidazolium bromide (8 mol%). d Catalyst (4 mol%) and temperature (25 °C). e Epichlorohydrin (2 mmol, 0.1850 g), catalyst
(0.5 mmol), CO2 (0.1 MPa), 30 °C, 12 h.
ditions, and they showed a very high yield of chloropropene Surprisingly, 1f showed a 91% yield of CPC within 10 hours at
carbonate (CPC) (70% and 76%, respectively) as compared to room temperature (Table 1, entry 9), while the reported pincer-
1a without protic acid (Table 1, entries 3 and 4 vs. 2). The prob- type compound catalysts,73 which had the highest TON and/or
able reason may be that the protic acid of DBPILs could acti- TOF for organocatalysts under ambient conditions, took
vate epichlorohydrin efficiently by the powerful H-bond inter- 24 hours to reach 92% yield of CPC. Furthermore, a compari-
action. Besides, as a nucleophilic anion, the alkoxy anion son of DBPILs with the reported dual-ionic liquid is also dis-
might play the same role as Br−, which could increase the played in Table 1. The dual-ionic liquid showed an 84% yield
active sites. To compare the effect of the H-bond interaction, a of CPC within 12 h, and 1f exhibited a higher activity, and
DBU based ionic liquid with a phenolic hydroxyl group (1d) 92% yield of CPC could be obtained after 6 h (Table 1, entry 10
was prepared with DBU and 4-bromophbenol. Even the vs. 7). In view of this, 1f was chosen as a benchmark catalyst to
phenolic hydroxyl group has been reported as
a
good investigate the influence of the reaction parameters.
The effects of the reaction time and temperature are pre-
HBD,71,72 but in this study, 1d only showed a low activity of
14%. 4-Dimethylaminopyridine (DMAP) based IL (1e) with a sented in Fig. 2. The yields of CPC increased rapidly to 82%
hydroxyl group, which had been reported as an efficient cata- within the first 4 h at 30 °C, and then slowly reached 96% in
lyst for this reaction in our previous work,53 was also investi- the next 4 hours. The results indicated an obvious decrease of
gated (Table 1, entry 6). 1e showed a promising yield of CPC reaction rates. As the reaction temperature increased to 50 °C,
(47%) for the hydroxyl group activating epichlorohydrin the yield of CPC increased to 98% sharply in 4 h without an
(Table 1, entry 6 vs. 1, 2), but much lower than DBU based
protic ionic liquids (DBPILs). The possible mechanism specu-
lated was that the hydrogen proton from DBPILs had a stron-
ger H-bond interaction with the epoxide than the –OH group
from 1e, which made the ring opening of the epoxide much
easier, and resulted in the higher activity of DBPILs compared
to 1e (Table 1, entries 3, 4 vs. 6). Then, DBPIL 1f incorporated
with protonic acid and multi-nucleophilic sites was prepared
and used for this reaction. It showed the highest CPC yields of
all the catalysts under the same conditions as in the aforemen-
tioned studies (Table 1, entry 7). To find whether the alkoxy
anion can play the same role with the bromine ion, DBPIL 1g
without the bromine ion was prepared and used in this reac-
tion. Only a 4% yield of CPC was obtained after 10 hours as
shown in Table 1 (entry 8), which demonstrated that the alkoxy
anion had little effect on the catalytic performance of this reac-
tion. However, the selectivity of the DBPILs decreased slightly
Fig. 2 Effects of the reaction time and temperature when catalyzed by
compared to that of conventional ionic liquids (entries 3, 4, 7,
1f. Reaction conditions: epichlorohydrin (0.5 ml), ILs (6 mol%), CO2
8 vs. 1, 2), which may be caused by side reactions.63,65 (balloon).
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Table 2 Reaction of CO2 with substrates catalyzed by 1fa
obvious slow-growth stage. This may be caused by the increas-
ing viscosity of the reaction system with the production of
CPC, which has a negative effect on the CO2 transfer in the
liquid phase. The mechanism mentioned above was confirmed
by the same behavior from the catalyst loading study shown in
Fig. 3, which also showed a slow-growth stage with the increase
of 1f.
CO2 is known as the main component of dry flue gas. More
attention has been paid to the conversion of CO2 from the flue
gas.1,28,34 In order to investigate whether DBPILs could be
used in the flue gas, a 15% CO2/85% N2 system was chosen to
simulate flue gas for the cycloaddition reaction with epoxides
(Scheme 1). However, only 24% of CPC was obtained within
6 hours with catalysis by 1f, which was much lower than that
of pure CO2 under the same conditions (24% vs. 92%). The
yield of CPC can reach up to 90% after 36 hours of reaction.
The results indicated the feasibility for the conversion of CO2
in the flue gas catalyzed by DBPILs, but the comparably low
activity is still a drawback which needs to be overcome.
A range of different substituted terminal epoxides were
examined under a balloon of CO2 condition in the presence of
DBPIL 1f. The results are summarized in Table 2. The terminal
epoxide (2a) with a bromomethyl group could afford the
product 3a in a good yield of 88% under the optimal con- a Yields were determined by NMR. b Isolated yields were determined by
weighing.
ditions. Taking into account the industrial application, the
loading of 1f was reduced, and the reaction temperature
increased to 50 °C. However, the carbonate 3b showed much
lower yield than 3c under the same conditions, which is prob-
Fig. 4 Recycling experiments with catalysis by 1f. a Reaction conditions:
2f (0.5 ml), 1f (6 mol%), 1 bar CO2 (balloon), temperature (50 °C), reac-
tion time (6 h).
Fig. 3 Effects of the catalyst amount when catalyzed by 1f. Reaction
conditions: epichlorohydrin (0.5 ml), CO2 (balloon), temperature (30 °C),
reaction time (8 h).
ably due to the high steric hindrance effect. Furthermore, the
terminal epoxide (2d) with a phenyl group and glycidol deriva-
tives 2c, 2e–2h were examined. All of these epoxides generated
the corresponding cyclic carbonates in good yields (3c–3h).
2f was subsequently chosen as an optimal terminal epoxide
to study the cycling performance of DBPIL 1f. When we tried
to recycle 1f after the reaction, we found that the isolated yield
of 3f decreased obviously from 95% to 77% after four runs
(Fig. 4). Based on the report, the reduction of catalytic activity
Scheme 1 Cycloaddition of epichlorohydrin with simulated flue gas
catalyzed by 1f.
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after use might be caused by the partial loss of the catalyst for
the sublimation property67 or catalyst addition to epoxides.63,65
Then, a series of experiments were designed to study the
reason for the decreased activity (see ESI Scheme S4†). The
experiments and spectral results indicated that the DBPIL 1f
could undergo addition to 2f and further generate 1h with a
hydroxyl group, because of the “super-dissociating” feature of
the protic ionic liquids.74 Furthermore, 1h could not activate
CO2 and had a relatively weak H-bond interaction with the
epoxide compared to the powerful H-bonding of 1f, which led
to the reduction of the yields in the recycling experiments.
FT-IR spectra were employed to identify whether the alkoxy
anion of 1f could activate CO2. As shown in Fig. 5, a new band
at 1795 cm−1 appeared after the reaction of CO2 with 1f, which
corresponded to the new asymmetric (CvO) vibrations of the
carbamate salt, thus implying the activation of CO2 by 1f.
It has been reported that the H-bond interaction between
the catalyst and epoxide can reduce the activation energy of
the ring-opening step.48 Hence, a comparison of 1f and 1h
catalyzed ring-opening steps was performed by a DFT study to
identify whether the hydrogen proton from 1f had a more
powerful H-bond interaction with the epoxide than the –OH
group from 1h. All calculations were carried out with the
B3LYP-D3/6-31+G** level implemented in the Gaussian 09
package. As shown in Fig. 6, the 1h-catalyzed ring-opening
step has an energy barrier of 44.2 kcal mol−1, which is slightly
higher than that of the 1h-catalyzed process (39.6 kcal mol−1).
The hydrogen atom of 1h is coordinated with the oxygen of the
epoxide through a hydrogen bond leading to an increase of
the length of the C–O bond from 1.455 Å to 2.099 Å,
which makes the ring-opening much easier. The same
interaction can be found between 1f and the epoxide. The
hydrogen bond length between 1h and epoxide is 1.601 Å,
while that of 1f and epoxide is 1.502 Å, showing a stronger
hydrogen bond interaction between 1f and epoxide. The
results indicated a higher activity of 1f than 1h due to the
stronger H-bond interaction, which was well consistent with
our experiment results.
Fig. 6 Comparisons of the relative energy of the ring-opening step.
Based on all the results above, a possible mechanism of the
1f-catalyzed process is proposed, and the DFT study was used
to study the mechanism. As shown in Scheme 2 and Fig. 7, the
H-bond interaction between 1f and the epoxide firstly caused
an increase of the length of the C–O bond, which could
improve the ring-opening process of the epoxide (step 1). The
attack of the bromide ion on the epoxide resulted in the ring-
opening of the epoxide and made A convert into intermediate
B (step 2) via transition state TS1 with a barrier of 39.6 kcal
mol−1. When CO2 was added into the reaction system, it could
be activated by the alkoxy anion, and led to the formation of
complex C (step 3). Subsequently, the alkyl carbonate D was
generated by the nucleophilic attack of the intermediate
Fig. 5 FT-IR spectra of the activation of CO2.
Scheme 2 Mechanism of the cycloaddition reaction catalysed by 1f.
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Fig. 7 Potential energy surface profiles of the 1f-catalyzed process and the optimized geometries for the intermediates and transition states. See
the ESI† for computational details.
through TS2 through a low energy barrier of 2.9 kcal mol−1 from experiments, spectroscopy and DFT illustrated the acti-
(step 4). Finally, the cyclic carbonate was obtained by a ring- vation of CO2 of DBPILs and strong H-bond interactions
closure step with an energy barrier of 7.9 kcal mol−1 (TS3). The between DBPILs and epoxides. This study provides a high
study illustrated that the epoxy ring-opening step was a efficiency epoxy ring-opening catalyst for the transformation of
rate-limiting step with the highest energy barrier of 39.6 kcal CO2 with the epoxide. The greatly improved catalytic activity
mol−1 (TS1).
makes DBPILs promising environmentally benign catalysts for
the applications of CO2 conversion under mild conditions.
Conclusions
Conflicts of interest
In summary, this work exhibits a simple way to prepare
DBPILs by introducing electron withdrawing groups to an O–H
acid. The DBPILs with protonic acid and nucleophilic sites
were used as single-component and metal-free catalysts in the
cycloaddition reaction of CO2 with epoxides under very mild
temperature and 1 bar CO2 conditions. DBPILs showed a great
The authors declare no conflicts of interest.
Acknowledgements
improvement of catalytic activity and good substrate compat- This work is financially supported by the National Key R&D
ibility to various epoxides. Furthermore, DBPILs were found Program of China (2018YFB0605802), the National Natural
suitable for the conversion of simulated flue gas. The results Science Fund for Distinguished Young Scholars (21425625)
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and Key Research Program of Frontier Sciences CAS 23 A. Cai, W. Guo, L. Martínez-Rodríguez and A. W. Kleij,
(QYZDY-SSW-JSC011). The support from the Department of J. Am. Chem. Soc., 2016, 138, 14194–14197.
Chemical and Biochemical Engineering, Technical University 24 C. M. Miralda, E. E. Macias, M. Zhu, P. Ratnasamy and
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Dr Dawei Shang and Meng Shi for their kind help.
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