Efficient conversion of CO2into cyclic carbonates at room temperature catalyzed by Al-salen and imidazolium hydrogen carbonate ionic liquids

Article abstract of DOI:10.1039/d0gc00458h

A novel process for the efficient synthesis of cyclic carbonates from CO2 and epoxides at room temperature in the absence of a solvent has been achieved by using Al-salen complexes as catalysts and imidazolium hydrogen carbonate ionic liquids ([CnCmIm][HCO3]) as cocatalysts. As a halide ion-free cocatalyst, [CnCmIm][HCO3] showed higher catalytic reactivity compared to traditional halogen-containing quaternary ammonium salts (such as (nBu)4NBr) and organic bases. The catalytic system can be used for the cycloaddition of a series of substrates with good to excellent yields at room temperature in the absence of a solvent. Besides, the catalytic system can be easily recycled at least four times without significant loss of catalytic activity. A possible mechanism was proposed, in which Al-salen and carbene activate the epoxides and CO2 respectively.

Full text of DOI:10.1039/d0gc00458h

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DOI: 10.1039/D0GC00458H
ARTICLE
Efficient conversion of CO2 into cyclic carbonates at room
temperature catalyzed by Al-salen and imidazolium hydrogen
carbonate ionic liquid
Received 00th January 20xx,
Accepted 00th January 20xx
Jia Liu†, Guoqiang Yang†, Ying Liu, Dejin Zhang, Xingbang Hu* and Zhibing Zhang*
DOI: 10.1039/x0xx00000x
A novel process for the efficient synthesis of cyclic carbonates from CO2 and epoxides at room temperature in the absence
of solvent has been achieved by using Al-salen complexes as catalysts and imidazolium hydrogen carbonate ionic liquids
([CnCmIm][HCO3]) as cocatalysts. As a halide ions free cocatalyst, [CnCmIm][HCO3] showed higher catalytic reactivity
compared to traditional halogen-containing quaternary ammonium salts (such as (nBu)4NBr) and organic bases.
The catalytic system can be used for the cycloaddition of a series of substrates with good to excellent yields at
room temperature in the absence of solvent. Besides, the catalytic system can be easily recycled at least four
times without significant loss in catalytic activity. A possible mechanism was proposed, in which the Al-salen and
carbene activate the epoxides and CO2 respectively.
many research interesting in both academic and industrial
communities. 6b,9, 11, 12
Introduction
Carbon dioxide (CO2) is a primary greenhouse gas. On the
other hand, more and more researches showed that it can
become a low cost, non-toxic, widely available and renewable
carbon feedstock.1 Cyclic carbonates are widely applied as
organic synthesis intermediates, polar aprotic solvents,
(a) Previously reported cocatalysts
+
R2
R2
N
-
R1
N
R4
X
R1
R3
R3
electrolytes in lithium-ion batteries and monomers of
2
polymers.1,
A typical route for the synthesis of cyclic
X=Br or Cl
ammonium salt
ammonium salt: corrosion induced by X-
carbonates is the cycloaddition of epoxides with CO2. In recent
decades, many catalytic systems have been developed for the
cycloaddition of epoxides with CO2, such as metal halides,3
organic bases
organic bases: high temperature & low conversion
1c,
4
ionic liquids with halogens as anions,
organic-metal complexes etc.1,6,7
organocatalysts,5
(b) Cocatalyst reported here
O
O
To make the cycloaddition of epoxides with CO2 more
efficient, cocatalysts are necessary (Fig. 1). 1, 2, 6-8 In most cases,
C
-
HCO3
R2
R1
halogenated quaternary ammonium salts are used as the
R1
R2
N
N
N
N
1, 2, 6-8
cocatalysts, such as (nBu)4NBr.
It has been well
[CnCmIm][HCO3]
established that the role of (nBu)4NBr in the cycloaddition of
NHC-CO2
no corrosion, low temperature and high conversion
Fig. 1 (a) Previous cocataleysts and (b) these reported here.
epoxides with CO2 is to assist the epoxide ring opening to form
5c, 6a, 6e, 6j, 6m, 9
bromo-alkoxide.
Though the halide ions can
facilitate the reactions, they are also well-known for their
corrosivity to the metal which is a big problem in the industrial
applications.10 Hence, developing halide ions free cocatalysts
for the cycloaddition of epoxides with CO2 have attracted
Organic bases are the widely used halide ions free
cocatalysts. However, the catalytic reactivity of these systems
using organic bases are far lower than these using halide ions.
For example, when (nBu)4NBr was used as cocatalyst, the
cycloaddition of epoxides with CO2 perform quite well even at
6e, 6h, 6j, 6n, 7i, 11a
room temperature.
The same reaction with
dimethylaminopyridine (DMAP) as cocatalyst almost cannot
happen at room temperature (2% conversion in 24 hours). 11a
When organic bases are used as cocatalyst, to obtain a
satisfying conversion, high reaction temperature still are
necessary, e.g. 120 oC for DMAP/Co(III)-porphyrin systeme, N-
School of Chemistry and Chemical Engineering, Nanjing University, Nanjing
210093, PR China. Fax: +86 2583 336599; Tel: +86 2583 596665; E-mail:
huxb@nju.edu.cn, zbzhang@nju.edu.cn;
†
Author contributed equally
Electronic Supplementary Information (ESI) available: experiment details and
characterization of all compounds. See DOI: 10.1039/x0xx00000x
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As shown in Fig. 2, we synthesized a series of simple Al-salen
DOI: 10.1039/D0GC00458H
catalysts by changing the bridging group R and the axial group
benzyl-4-(dimethylamino)pyridinium Chloride (BDAPC)/Al-
Salen or DMAP/cobalt complexes,
View Article Online
9, 11b, 12a
100 oC for
DMAP/Co(II)-PAM-Py, DMAP/Ru(II)-complexes, DMAP/Cu(II)-
Salen or triethylamine (TEA) /Zn-hydroxyapatite, N-methyl-
homopiperazine/Cu-salphen. 6b, 12b-12f
X. Though there has chlorine in four of the catalysts, it binds
with Al tightly and the equilibrium concentration of chloridion
is too small to induce remarkable corrosivity.
1,
2
Until now, conversion of CO2 into cyclic carbonates under
mild condition using halide ions free cocatalysts is still a
challenge. Herein, we reported the use of imidazolium hydrogen
carbonate ionic liquid ([CnCmIm][HCO3]) as halide ions free
cocatalyst for cycloaddition of epoxides with CO2 at room
temperature in the absence of solvent.
[CnCmIm][HCO3] with different chain length have also been
synthesized. 13
Table 1. Catalyst screening for the cycloaddition of styrene
[a]
oxide with CO2
O
O
Al-salen
O
+
CO2
O
[C1C4Im][HCO3]
Ph
Ph
Results and discussion
DMAP is the most widely used halide ions free cocatalyst for the
cycloaddition of epoxides with CO2.11,
research showed that DMAP can bind with CO2 to form a
DMAP-CO2 adduct which can assist the cycloaddition of CO2. 12a
Our previous researches revealed that [CnCmIm][HCO3] can
Entry
Catalyst
Yield[b]
30
TON[c]
121
TOF[d]/[h-1]
12.1
12
1
2
SH1-Al(Cl)
SH2-Al(Cl)
The mechanism
19
76
7.6
3
4
5
6
7
8
SH3-Al(Cl)
SH4-Al(Cl)
SH3-Al(CH3)
SH4-Al(CH3)
None
43
58
42
47
0
174
235
165
191
0
17.4
23.5
16.5
19.1
0
transform into the carbene-CO2 adducts (NHC-CO2)
13
spontaneously.
The NHC-CO2 adducts have exhibited the
ability to activate CO2 in the cycloaddition reaction of
14
epoxides. However, when NHC-CO2 is used alone without
any other catalyst, the reaction condition is quite harsh, such
as high reaction temperature (120 or 100 C), 14a-14e and high
amount of catalyst loading (20 mol%) 14f, suggesting that NHC-
CO2 is not active enough for the whole catalytic cycle of
cycloaddition of epoxides with CO2. Herein, we ratiocinate that
SH4
0
0
0
o
[a]Reaction conditions: catalyst 0.25 mol%, [C1C4Im][HCO3] 0.25
mol%, styrene oxide 20 mmol, CO2 pressure 1.0MPa, 25 oC, 10h.
[b]Yield was determined by GC. [c]TON = (mol of styrene oxide
consumed)/(mol of catalyst used). [d]TOF = TON/h.
a
tandem catalytic system including both Al-salen and
[CnCmIm][HCO3] may be highly efficient for the conversion of
CO2 into cyclic carbonates. In this system, Al-salen acts as a
Lewis acid to coordinate with the epoxide and activate it for ring-
opening. The NHC-CO2 in [CnCmIm][HCO3] can provide activated
CO2 for the nucleopilic addition. To verify this idea, the
cycloaddition of styrene oxide with CO2 was first investigated as
a model reaction.
Table 2. Cocatalyst screening for the cycloaddition of styrene
oxide with CO2
[a]
TOF[d]/[h-1]
Entry
Cocatalyst
Yield[b]
34
58
TON[c]
138
235
1
2
[C1C2Im][HCO3]
[C1C4Im][HCO3]
13.8
23.5
3
4
5
6
7
8
9
[C1C6Im][HCO3]
[C1C6Im][Br]
(nBu)4NCl
(nBu)4NBr
Et3N
67
20
11
26
5
265
78
44
105
20
32
0
26.5
7.8
4.4
10.5
2.0
3.2
0
DBU
8
None
0
[a]Reaction conditions:
0.25mol%, styrene oxide 20mmol, CO2 pressure 1.0MPa, 25 C,
10h. [b]Yield was determined by GC. [c]TON = (mol of styrene
oxide consumed)/(mol of catalyst used). [d]TOF = TON/h.
SH4-Al(Cl) 0.25mol%, cocatalyst
o
Al-salen with a geometry close to square pyramid is
considered to be a good Lewis acid catalyst. Conversely, with a
geometry close to trigonal bipyramid is considered to be a
poor Lewis acid catalyst. The geometry of Al-salen can be
controlled by the rigidity of the bridging group R of the salen
Fig. 2 Catalytic systems containing Al-salen, [CnCmIm][HCO3]
and NHC-CO2 used in this study.
Catalyst and cocatalyst screening for the model reaction between
styrene oxide and CO2
15
2 | J. Name., 2012, 00, 1-3
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ligand. Flexible bridging group R such as cyclohexyl and -
CH2CH2- prefer trigonal bipyramid geometries, whereas rigid
and 5.0mol %, the yield of styrene carbonate was increased
View Article Online
from 7% to 37 and 62 % respectivelyD(OeIn: 1t0ri.1e0s396/-D80)G. CF0u0r4t5h8eHr
extending the reaction time to 24 h, the yield of styrene
carbonate reached 94% at room temperature and 1 bar CO2
(entry 9).
bridging group R such as naphthalene and phenylene tend to
15, 16
form square pyramid geometries.
As expected, the TON
value increases in the sequence of SH2-Al(Cl) (76) < SH1-Al(Cl)
(121) < SH3-Al(Cl) (174) < SH4-Al(Cl) (235) with increasing
rigidity of salen ligands (SH2 < SH1 < SH3 < SH4) (Table 1, entries
1-4). Though the catalytic activity of SH4-Al(CH3) (TON=191) is
slightly lower than that of SH4-Al(Cl) (TON=235) under the
same reaction conditions (Table 1, entries 4, 6), it suggests that
the reaction can perform quite well when there is absolutely
Table 3. The cycloaddition of CO2 with styrene oxide catalyzed
by SH4-Al(Cl)/[C1C6Im][HCO3] system[a]
no halide ion. There has NHC-CO2 adduct in [CnCmIm][HCO3] 13
.
Entry SH4-Al(Cl) [C1C6Im][HCO3]
CO2
Temp. Yield[b]
No product was obtained in the absence of Al-salen, indicating
that the NHC-CO2 adduct in [C1C4Im][HCO3] cannot catalyze
the cycloaddition reaction alone at room temperature (Table
1, entry 7). Furthermore, when there has no [C1C4Im][HCO3],
the reaction also cannot happen at room temperature in the
presence of SH4-Al(Cl) alone (Table 2, entry 9). It reveals that the
synergistic effect between SH4-Al(Cl) and [CnCmIm][HCO3] is
crucial for improving the catalytic reactivity of the systems.
[CnCmIm][HCO3] with different length of the carbon chain
were used as cocatalysts in the cycloaddition of CO2 and
styrene oxide (Table 2). In previous research, we have
reported that the length of the carbon chain could affect the
equilibrium between [CnCmIm][HCO3] and NHC-CO2 adduct.
The content of NHC-CO2 adduct in [C1C2Im][HCO3],
(mol%)
0.25
(mol%)
(MPa)
1.0
(℃)
100
(%)
21
1
2
0
0
0.25
0.25
0.25
0.25
0.25
2.5
1.0
100
25
25
60
25
25
25
25
12
67
98
99
7
3
4[c]
0.25
0.25
0.25
0.25
0.25
0.5
1.0
1.0
5
1.0
6
0.1[d]
0.1[d]
0.1[d]
0.1[d]
7
37
62
94
8
9[c]
5.0
0.5
5.0
[a]Reaction conditions: SH4-Al(Cl) 0.25mol%, [C1C6Im][HCO3]
o
0.25mol%, styrene oxide 20mmol, CO2 pressure 1.0MPa, 25 C,
10h. [b]Yield was determined by GC. [c]Reaction time 24h. [d]CO2
ballon.
[C1C4Im][HCO3] and [C1C6Im][HCO3] are 18, 34, and 46%
13a
respectively.
Correspondingly, the TON values increase in
the sequence of [C1C2Im][HCO3] (138) < [C1C4Im][HCO3] (235) <
[C1C6Im][HCO3] (265) (Table 2, entries 1-3). [C1C6Im][HCO3] was
the most active cocatalyst. Interestingly, cocatalyst
[C1C6Im][HCO3] gave higher TON value than [C1C6Im][Br],
(nBu)4NCl and (nBu)4NBr, indicating that providing activated
CO2 is quite important for enhancing the catalytic ability.
Obviously, the CO2 activation capacity of NHC is much better
than these of organic bases (Table 2, entries 3, 7 and 8).
Table 4. Study on the active sites of Al-salen/[CnCmIm][HCO3]
catalytic system[a]
Entry
Catalyst
Cocatalyst
Yield[b] (%)
1
2
SH4-Al(Cl)
SH4-Al(Cl)
[C1C2Im][HCO3]
[C1C2ImCH3][HCO3]
61
7
Cycloaddition of CO2 and styrene oxide catalyzed by SH4-
Al(Cl)/[C1C6Im][HCO3] system
3
4
SH4-Al(Cl)
None
IMes-CO2
IMes-CO2
92
0
After obtaining the optimal catalytic system SH4-
Al(Cl)/[C1C6Im][HCO3], we investigated the influences of
cocatalyst concentration, CO2 pressure, reaction temperature
and reaction time on the cycloaddition of CO2 and styrene
oxide. 67% styrene carbonate yield was obtained using
0.25mol % SH4-Al(Cl) and [C1C6Im][HCO3] at room temperature
in 10 h (Table 3, entry 3). When the reaction time was
extended to 24 h, styrene oxide was almost completely
converted and 98% styrene carbonate yield was obtained
(entry 4). The yield of styrene carbonate sharply decreased
from 67% to 7% when the CO2 pressure decreased from 1.0
MPa to 0.1 MPa (entry 6), which is the result of lower CO2
concentration in the solution under lower CO2 pressure. In this
case, increasing the [C1C6Im][HCO3] loading provides a chance
to enhance the NHC-CO2 concentration. As results, when the
[C1C6Im][HCO3] loading was increased from 0.25mol % to 2.5
[a]Reaction conditions:
SH4-Al(Cl) 0.25mol%, cocatalyst
o
0.25mol%, styrene oxide 20mmol, CO2 pressure 1.0MPa, 25 C,
24h. [b]Yield was determined by GC.
Study on the active sites of Al-salen/[CnCmIm][HCO3] catalytic
system
Since HCO3 and NHC-CO2 adduct coexist in [CnCmIm][HCO3],13
to find out which one play the role to assist the reaction, a
series of control experiments were carried out (Table 4). Being
different from [C1C2Im][HCO3], [C1C2ImCH3][HCO3] cannot
generate NHC-CO2 adduct because the N-CH-N hydrogen atom
was replaced by methyl group (see NMR spectrum of
[C1C2ImCH3][HCO3] in ESI). As expected, the performance of
-
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J. Name., 2013, 00, 1-3 | 3
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[a] Reaction conditions: SH4-Al(Cl) 0.25mol%, [C1C6Im][VHieCwOA3r]ticle Online
[C1C2ImCH3][HCO3] was quite poor comparing with
[C1C2Im][HCO3] (Table 4, entries 1, 2). Besides, the cocatalytic
ability of [C1C2ImCH3][HCO3] is obviously lower than these of
[C1C6Im][Br] and (nBu)4NCl (Table 2, entries 4, 5), suggesting
o
DOI: 10.1039/D0GC00458H
0.25mol%, epoxide 20mmol, CO2 pressure 1.0MPa, 25 C, 24h.
[b] GC yield (isolated yield in parentheses) of cyclic carbonates. [c]
80℃.
-
weak ability of HCO3 to assist the epoxide ring opening. In
Cycloaddition of various epoxides catalyzed by Al-
salen/[C1C6- Im][HCO3] catalytic system
addition, we synthesized a pure NHC-CO2 adduct IMes-CO2 and
used it as a cocatalyst. 92% yield of styrene carbonate was
obtained (entry 3), clearly revealing the role of NHC-CO2 in
assisting the reaction. No styrene carbonate was obtained
when IMes-CO2 was used alone (entry 4), indicating that NHC-
CO2 adduct must work synergistically with SH4-Al(Cl) to
catalyze the cycloaddition reaction at room temperature.
Using Al-salen/[C1C6-Im][HCO3] as catalyst, we further explore
the substrate scope at room temperature in the absence of
solvent. Thirteen terminal epoxides (including aromatic and
non-aromatic epoxides) and three internal epoxides were
investigated (Table 5). The SH4-Al(Cl)/[C1C6Im][HCO3] system is
highly reactive for various terminal epoxides with good to
excellent yields of cyclic carbonate products. The highest yield
is 99% and the lowest one is 87% and (entries 1-13). All the
products were indentified by 1H-NMR and 13C-NMR (seeing
ESI). Due to the higher steric hindrance of the internal
epoxides, the cycloaddition reaction of the internal epoxides is
more difficult and higher temperature is required to obtain
satisfying conversion (entries 14-16).
Table 5. Reactions of various epoxides with CO2 catalyzed by
SH4-Al(Cl)/[C1C6Im][HCO3] system[a]
Entry
Substrate
Product
Yield[b] (%)
99 (94)
1
2
3
97 (92)
89 (92)
Recycling of the SH4-Al(Cl)/[C1C6-Im][HCO3] catalytic system
The recyclability of catalyst are important for large-scale
applications. Hence, the recycling of the novel SH4-Al(Cl) and
[C1C6Im][HCO3] system was investigated in the cycloaddition of
styrene oxide and CO2. After the reaction, the catalyst and
cocatalyst can be separated from the solution by adding
diethyl ether to extract the cyclic carbonate out. The loss
amount of Al-salen and [CnCm-Im][HCO3] during the separation
were further determined by UV and NMR. The results
indicated that only 1.5% of SH4-Al(Cl) was lost during the
separation (see Fig. S4 in ESI).The loss of [C1C6Im][HCO3] was
too little to be detected (see Fig. S3 in ESI). After the
extraction, the SH4-Al(Cl)/[C1C6Im][HCO3] mixture was dried
4
95 (89)
5
6
99 (91)
92 (87)
7
8
87 (73)
98 (88)
o
under vacuum at 60 C and reused for the next cycle without
further treatment. The results showed that the SH4-Al(Cl) and
[C1C6Im][HCO3] system can be used for at least five cycles
without significant loss in catalytic activity (Fig. 3).
9
87 (77)
10
11
95 (83)
92 (85)
12
13
92 (79)
94 (88)
5
14
15
16
85 (73)[c]
17
94 (81)[c]
12
Fig. 3 Recyclability of SH4-Al(Cl) and [C1C6Im][HCO3] for the
reaction of CO2 and styrene oxide. Reaction conditions: SH4-
95 (84)[c]
4 | J. Name., 2012, 00, 1-3
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Al(Cl) 0.25mol%, [C1C6Im][HCO3] 0.25mol%, styrene oxide Scheme 1 A plausible mechanism of the cycloaddition of CO2 and
View Article Online
20mmol, 25oC, CO2 pressure 1.0MPa, 24h. Yield was epoxide catalyzed by the Al-salen/[CnCmIm][HCO3] system.
DOI: 10.1039/D0GC00458H
determined by GC.
Possible mechanism of Al-salen/[CnCmIm][HCO3] catalytic process
Conclusions
Based on the experimental results of this study and reported
mechanism for cycloaddition of epoxides with CO2, 6a, 6e, 6j, 9,18
We have developed a process for the efficient synthesis of
a
cyclic carbonates from CO2 and epoxides at room temperature
in the absence of solvent. The catalytic system include Al-salen
and [CnCmIm][HCO3]. [CnCmIm][HCO3] was used as halide ions
free cocatalyst for the cycloaddition. As a novel cocatalyst,
[CnCmIm][HCO3] showed higher catalytic reactivity compared
to traditional halogen-containing quaternary ammonium salts
(such as (nBu)4NBr) and organic bases. The catalytic system
can be used for the cycloaddition of a series of substrates with
good to excellent yields at room temperature in the absence of
solvent. Besides, the catalytic system can be easily recycled at
least four times without significant loss in catalytic activity.
Halide ions and solvent free, reaction at room temperature, high
yield of cyclic carbonates and good recyclability make this new
catalytic system a good candidate for industrial cycloaddition of
epoxide and CO2.
plausible Lewis acid/NHC-CO2 synergistic activation
mechanism is proposed (scheme 1). First, the oxygen atom of
the epoxide coordinates to the Lewis acidic aluminum center
of the Al-salen, and a six-coordinated intermediate (salen)Al-
6a, 6e, 6j,
9
epoxide is formed.
The coordination between
1
(salen)Al and epoxide can be revealed by the H-NMR spectra
of SH4-Al(CH3) with and without propylene epoxide (see Fig. S1
and S2 in ESI). [CnCmIm][HCO3] can convert into NHC-CO2
adduct spontaneously.13 Then, the alkoxide intermediate is
generated by the nucleophilic addition between NHC-CO2 and
(salen)Al-epoxide. After that, intramolecular cyclization of
alkoxide intermediate forms the cyclic carbonate. At the same
time, Al-salen and N-heterocyclic carbene are regenerated.
The N-heterocyclic carbene can bind with new CO2 to
reproduce NHC-CO2 adduct.14 Stoichiometric reaction between
[IMes][HCO3] and propylene epoxide showed that propylene
carbonate can be produced in the absence of CO2 (see Fig. S7
in ESI), demonstrating that the CO2 in NHC-CO2 adduct is active
enough to react with epoxides. Isotope labeling experiments
were further performed to verify whether the carbon dioxide
in NHC-CO2 is the carbon source of the cyclic carbonate. 99%
13C-labeled NaHCO3 was used to synthesize [C1C6Im][H13CO3]
and [IMes][H13CO3]. In the 13C NMR spectra of fresh
[C1C6Im][H13CO3] in D2O, a peak at 161ppm assigned as [H13CO3]-
was observed. No signal was found at 161ppm after the
cycloaddition of epoxides with CO2 (see Fig. S5 in ESI). This
indicated that 13C of [C1C6Im][H13CO3] was replaced by 12C
(from CO2) during the cycloaddition. In addition, the molecular
weight of propylene carbonate (PC) synthesized by the
reaction between [IMes][H13CO3] and propylene epoxide is
103.1 (see Fig. S6 in ESI), indicating that 13C was transferred
from [IMes][H13CO3] to PC.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by National Natural Science
Foundation of China (No. 21776122 and 21878141), National
Key Research and Development Project (2018ZX07208010) and
Jiangsu Province NSF (BE2019095).
Notes and references
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Green Chemistry
View Article Online
DOI: 10.1039/D0GC00458H
O
O
CO2
O
O
R
R
Halide ions free cocatalyst
Reaction at room temperature
Solvent free
Recyclable catalytic system
Good to excellent yields