Protic ionic liquid-promoted synthesis of dimethyl carbonate from ethylene carbonate and methanol

Article abstract of DOI:10.1016/j.cclet.2019.07.053

In this work, the protic ionic liquid [DBUH][Im] (1,8-diazabicyclo[5.4.0]-7-undeceniumimidazolide) was developed as an efficient catalyst for the transesterification of ethylene carbonate with methanol to produce dimethyl carbonate. At 70 °C, up to 97% conversion of ethylene carbonate and 91% yield of dimethyl carbonate were obtained with 1 mol% [DBUH][Im] (relative to ethylene carbonate) as catalyst in 2 h. Even at room temperature, the conversion of ethylene carbonate can reach 94% and the yield of dimethyl carbonate can approach 81% for 6 h. Catalytic mechanism investigation showed the high catalytic efficiency of this ionic liquid results from the synergistic activation effect, wherein the cation can activate ethylene carbonate and the anion can activate methanol through hydrogen bond formation. Although the reusability of the ionic liquid need to be further improved, high efficiency and commercial availability of [DBUH][Im] render it a promising catalyst for the preparation of dimethyl carbonate.

Full text of DOI:10.1016/j.cclet.2019.07.053

Accepted Manuscript
Title: Protic ionic liquid-promoted synthesis of dimethyl
carbonate from ethylene carbonate and methanol
Authors: Yu Song, Xing He, Bing Yu, Hong-Ru Li,
Liang-Nian He
PII:
DOI:
Reference:
S1001-8417(19)30459-0
https://doi.org/10.1016/j.cclet.2019.07.053
CCLET 5136
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Chinese Chemical Letters
Accepted date:
25 July 2019
Please cite this article as: Song Y, He X, Yu B, Li H-Ru, He L-Nian, Protic ionic
liquid-promotedsynthesisofdimethylcarbonatefromethylenecarbonateandmethanol,
Chinese Chemical Letters (2019), https://doi.org/10.1016/j.cclet.2019.07.053
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Communication
Protic ionic liquid-promoted synthesis of dimethyl carbonate from ethylene carbonate
and methanol
Yu Songa, Xing Hea, Bing Yub, Hong-Ru Lia,c, Liang-Nian Hea*
a State Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai University, Tianjin, 300071, China
b College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China
c College of Pharmacy, Nankai University, Tianjin 300353, China
ARTICLE INFO
ABSTRACT
Article history:
In this work, the protic ionic liquid [DBUH][Im] (1,8-diazabicyclo[5.4.0]-7-undeceniumimidazolide)
was developed as an efficient catalyst for the transesterification of ethylene carbonate with methanol
to produce dimethyl carbonate. At 70 ºC, up to 97% conversion of ethylene carbonate and 91% yield
of dimethyl carbonate were obtained with 1 mol% [DBUH][Im] (relative to ethylene carbonate) as
catalyst in 2 h. Even at room temperature, the conversion of ethylene carbonate can reach 94% and the
yield of dimethyl carbonate can approach 81% for 6 h. Catalytic mechanism investigation showed the
high catalytic efficiency of this ionic liquid results from the synergistic activation effect, where in the
cation can activate ethylene carbonate and the anion can activate methanol through hydrogen bond
formation. Although the reusability of the ionic liquid need to be further improved, high efficiency
and commercial availability of [DBUH][Im] render it a promising catalyst for the preparation of
dimethyl carbonate.
Received 30 June 2019
Received in revised form 17 July 2019
Accepted 25 July 2019
Available online
Keywords:
Dimethyl carbonate
Transesterification
Proticionic liquid
Synergistic effect
Sustainable chemistry
As an important chemical, dimethyl carbonate (DMC) has
unique physical properties and versatile reactivity. It has been
widely used as a solvent, fuel additive and lithium battery
electrolyte [1-3]. Besides, DMC can be regarded as a green
carbonylation reagent and methylation reagent to replace highly
toxic phosgene, methyl chloroformate and dimethyl sulfate [4-6].
Nowadays, the DMC demand is increasing and therefore,
efficient and inexpensive synthetic protocol for this chemical is
highly desirable.
The reported synthetic methods of dimethyl carbonate,
including phosgene route [7,8], methanol oxidative carbonylation
[9,10], urea alcoholysis [11,12], direct synthesis from CO2 and
methanol [13-16], and transesterification [17], were summarized
in Scheme 1. Wherein, the traditional phosgene route has
nowadays been banned due to its toxicity and safety issue. For
the oxidative carbonylation process of CO and methanol, the
expensive gasification processes and the use of explosive
material make the process non-preferred. Currently, employing
CO2 as raw material for DMC synthesis would be more attractive.
However, direct synthesis of DMC from CO2 and methanol needs
harsh reaction conditions. As an alternative, transesterification
between ethylene carbonate (EC) and methanol can be performed
under mild conditions, producing DMC and ethylene glycol (EG)
as the products. In this context, as ethylene carbonate can be
derived from CO2 and ethylene epoxide, this protocol can be
regarded as the indirect use of CO2.
In the transesterification of EC with methanol, both acidic and
basic catalysts have been developed to improve the yield of DMC
by activating EC and methanol respectively. As the strong acidic
catalysts might cause competing ether-forming side reactions and
weak acidic catalysts cannot activate EC efficiently, the catalytic
activity of base catalysts is always superior to the acid ones [18].
Therefore, the catalyst development is mainly focused on base
catalysts. Considering the superior mass transfer performance of
homogeneous alkaline catalysts, sodium methoxide is widely
used in industry to catalyze this transesterification. Similarly, the
molecular catalyst such as alkali metal carbonates [19],
hydroxides [20], and alkoxides [21], as well as tungstate [22] and
quaternary ammonium salt [23] can also effectively promoted the
reaction. However, the above-mentioned catalytic systems have
also problems in product separation and catalyst recovery despite
the high efficiency. To solve this issue, many solid base catalysts
such as alkali metal oxides [24-26], inorganic salts of polybasic
acids, hydrotalcites and montmorillonites [27, 28], basic ion
exchange resins [29], have also been developed for this purpose.
Nevertheless, the relatively low mass transfer performance limits
their industry application.
As an alternative type of catalyst, ionic liquids have many
special features, such as good solubility, thermal stability,
negligible vapor pressure and tunable structure, which make them
promising candidates to address the product separation problems
while keep the excellent mass transfer performance in many
reactions.
———
* Corresponding authors.
E-mail addresses: lihongru@nankai.edu.cn (H.-R. Li); heln@nankai.edu.cn (L.-N. He).

In recent years, several ionic liquids have been verified as
efficient catalysts in the transesterification reaction of EC and
methanol (Scheme 2). For example, Park's group developed the
ionic liquids [EMIm]Cl [30], [n-BuIm]Br-CS [31], [BMIm]Br-
AS [32]and THA-MS41 [33] as catalysts for the
transesterification reaction. For the above-mentioned catalysts,
the nucleophilic halogen anions can activate methanol and the
cation was designed to enhance the dissociation of halogen
anions. Albeit showing acceptable catalytic efficiency, these
catalysts encounter difficulty incommercial application due to
their complicated and costly preparation procedure. Besides,
these catalysts require high reaction temperature (more than 120
ºC) to promote the reaction and thus high pressure CO2 is always
needed to suppress the decomposition of ethylene carbonate.
Scheme 1. Preparation methods of dimethyl carbonate.
envisioned that the PILs are promising candidates for the
transesterification reaction of EC and methanol and a high yield
of DMC can be expected due to the synergistic activation effect.
In this work, we screened different ionic liquids, including PILs
and aprotic basic ionic liquids, in the synthesis of DMC and
found that the protic ionic liquid [DBUH][Im] is a highly
efficient catalyst for the transesterification reaction of EC with
methanol. By optimizing the reaction parameters, up to 91%
yield of DMC can be obtained and the mechanism investigation
showed that the cation and anion of this protic ionic liquid can
activate EC and methanol respectively by forming hydrogen
bond.
The required ionic liquids were synthesized according to the
published procedures [39-43] and characterized by NMR. Then
the resulting ionic liquids were examined for the DMC synthesis
reaction from EC with methanol and the resulting reaction
mixture were analyzed using GC method (for details see Support
information).
Scheme 2. Ionic liquid catalysts reported in the literatures for the
transesterification reaction of ethylene carbonate with methanol.
Later studies showed that increasing the basicity of anion can
increase the catalytic activity of ionic liquid. For example, the
[SmIm]OH-MCF [34] and [C4DABCO]OH [35] can facilitate the
transesterification reaction proceeding at mild temperature.
Recently, the synergistic activation effect of the cations of ionic
liquid has attracted much attention. With this in mind, our group
developed [(TBD)2PEG150]Br2 [36], where the cation can co-
operatively activate methanol with anion (Scheme 2). Bhanage
group also designed the ionic liquid-based catalyst [DABCO–
PDO]I [37] with hydrogen bond donor in its cation. This ionic
liquid can activate methanol by its anion and activate EC by its
cation simultaneously, providing 93% yield of DMC. Although
these ionic liquids present better catalytic activity, the preparation
procedure is still complicated and the strong basicity of these
ionic liquids makes them sensitive to moisture, thus difficult to
store.
In the ionic liquid-catalyzed transesterification reaction, both
the desired products DMC, EG and the intermediate (2-
hydroxyethyl)methyl carbonate (HMEC) were detected as listed
in Table 1. It is inferred that the formation of DMC undergoes
two steps of transesterification and HEMC is the key
intermediate. As different ionic liquid has different activation
capacity for these two steps, the dependence of product
distribution on the catalyst structure was observed. Even in the
absence of any catalyst, the transesterification intermediate i.e.
HMEC was produced with 30% yield (entry 1), which implies
that the first-step transesterification has a really low activation
energy. When employing ionic liquids as catalysts, the yield of
DMC was improved due to the substrate activation (entries 2-11).
Especially, when the DBU-based ionic liquids were used as
catalysts, an increasing conversion of EC with the increasing
basicity of anion can be observed (Table 2). This is consistent
with the principle of alkali-catalyzed transesterification. For the
yield of DMC, increasing the basicity of anion presents a positive
effect with the exception of [DBUH][OAc]. The reason for the
The protic ionic liquids (PILs), which can be easily prepared
by neutralization of Brønsted acid with Brønsted base, represent
the important part in ionic liquid family. A key feature
distinguishing PIL from other ILs is the coexistence of proton
donor and acceptor, both of which can participate in substrate
activation by forming hydrogen bond [38]. In view of this, we

exceptionally low catalytic activity of [DBUH][OAc] for the
second-step transesterification was unclear up to now.
[DBUH][Im]
14.17[51]
a The pKa value in water.
The screening results of anions showed that with [DBUH]+,
the anion [Im]- was suitable for this reaction with 97%
conversion of EC and 91% yield of DMC. Considering the cation
may participate in the substrate activation, we then turned toward
finding proper cation. With [Im]- as anion of the ionic liquid,
when the cation part was changed to a quaternary phosphonium
or a quaternary ammonium, the yields of DMC was reduced to 83%
and 62%, respectively, even the conversion rate of EC was still
above 90% (entries 10 and 11). These results emphasized the
importance of the protic cation in this transesterification reaction.
The control experiments were also performed by using imidazole
or DBU as catalyst (entries 11 and 12). It can be seen that
imidazole cannot promote the reaction as its basicity is not strong
enough to activate MeOH. While for DBU, up to 97% conversion
of EC and 90% yield of DMC were obtained probably owing to
its strong basicity (pKa = 13 (H2O) [44]). Albeit presenting high
efficiency, DBU encounters difficulty in recovery after reaction
due to its low boiling point (Tb = 80-83 ºC/0.6 mmHg) [44].
Considering the catalytic activity and recyclability, the ionic
liquid [DBUH][Im] is selected as the optimal catalyst for this
transesterification reaction.
As above mentioned, both anion and cation of the ionic liquid
can affect the reaction outcome. However, the cation effect is not
obvious. For example, the ionic liquid [DBUH][Im] and [n-
Bu4P][Im] presented quite close results (entries 8 and 9). To find
more details of the protic and aprotic ionic liquids, the amount of
[DBUH][Im] or [n-Bu4P][Im] was reduced from 1 mol% to 0.25
mol% relative to EC (Fig. 1). As for [DBUH][Im], most of the
catalytic capacity was retained when the amount was reduced.
Even only 0.25 mol% of [DBUH][Im] was used, the conversion
of EC was 90% and the yield of DMC was 81%, providing the
largest TON of 324. In contrast, the catalytic ability of [n-
Bu4P][Im] reduced sharply when its usage was reduced to 0.25
mol%, with 80% conversion of EC and only 51% yield of DMC.
It is inferred that the difference is originated from the activation
capacity of the cation, i.e. [DBUH]+ by forming hydrogen bond
with the C=O in EC. The synergistic activation effect of the
cation and anion of [DBUH][Im] make it a highly efficient
catalyst.
Table 1
Screening results for the ionic liquid catalyzed transesterification
reaction of ethylene carbonate with methanol. a
Yield(%)b
DMC
0
Entry Catalyst
ECConv.(%)b
HMEC
30
25
39
33
28
22
10
5
1
—
32
39
44
93
93
95
93
97
94
92
28
97
2
3
4
5
6
7
8
9
[DBUH][TFA]
[DBUH][OAc]
[DBUH][PME]
[DBUH][Bztz]
[DBUH][Pho]
[DBUH][TFE]
[DBUH][Im]
[n-Bu4P][Im]
[n-Bu4N][Im]
Imidazole
15
3
59
65
71
79
91
83
62
3
Fig. 1. Catalytic activity versus the ionic liquid amount for
[DBUH][Im] and [n-Bu4P][Im] (Reaction conditions: EC (10.0
mmol, 0.8806 g), CH3OH (150 mmol, 6 mL), 6 h; 70 ºC)
7
With 1 mol% [DBUH][Im] as catalyst, the other reaction
parameters, including reaction time, mole ratio of methanol and
EC as well as the reaction temperature were further investigated
(Table 3). The results in Table 3 showed that when the mole ratio
of methanol to EC is 15:1, 2 h is enough for the reaction to
approach the equilibrium at 70 ºC (entries 1-5). Then, the molar
ratio of methanol to EC was screened from 5:1 to 20:1. The
results showed that the conversion of EC and yield of DMC
increased with this ratio and when the ratio exceeds 15:1, it has
little effect on the reaction (entries 1 and 6-8).
To verify the catalytic capacity of the protic ionic liquid
[DBUH][Im] at low temperature, the reaction was performed at
26 ºC with the mole ratio of methanol to EC being set at 15:1.
Surprisingly, the catalytic system still has high activity at room
temperature with 94% conversion of EC and 81% yield of DMC
for 6 h (entries 9).This means that the catalytic system has good
temperature tolerance, which provides potential to reduce energy
cost in commercial production.
10
11
28
24
5
12
a
DBU
90
Reaction conditions: EC (10.0 mmol, 0.8806 g), CH3OH (150
mmol, 6 mL), Cat. (1 mol%, 0.1 mmol) relative to EC; 6 h; 70 ºC;
b Determined by GC and using 1,3,5-trimethoxybenzene (200 mg) as
internal standard.
Table 2
pKa of the anion of protic ionic liquids.
pKa of anion
Catalyst
Structure
(H2O)a
[DBUH][TFA]
[DBUH][OAc]
0.52[45]
4.76[46]
5.20[47]
8.57[48]
[DBUH][PME]
[DBUH][Bztz]
Table 3
Effect of reaction parameters on transesterification.a
Yield(%)b
[DBUH][Pho]
[DBUH][TFE]
9.99[49]
nMeOH
:
EC
Entry
Time (h)
12.37[50]
nEC
Conv.(%)b
DMC
HMEC

1
2
3
4
5
6
7
8
9c
6
4
2
1
0.5
6
6
6
6
15:1
15:1
15:1
15:1
15:1
5:1
10:1
20:1
15:1
97
97
97
95
93
73
88
96
94
91
90
90
85
80
63
78
90
81
5
6
5
5
12
8
8
4
11
temperature, the protic ionic liquid still gave the comparable
yield of DMC with other reported ionic liquids. It is inferred that
the high efficiency of [DBUH][Im] is originated from the co-
operative effect of the anion and cation in the ionic liquid for
activation of the starting material. As a hydrogen bonding donor,
the protic cation [DBUH]+ is supposed to activate EC by
interacting with carbonyl while for [Im]-, sufficient basicity
makes it competent to activate MeOH. By contrast, for the
previously reported ionic liquids, either the basicity of anions is
not strong or the cation cannot activate the substrate. Moreover,
compared with the complicated preparation process for other
ionic liquids, the protic ionic liquids can be produced by a simple
acid-base neutralization reaction.
a
Reactions were performed withEC (10.0 mmol, 0.8806 g), CH3OH (150
mmol, 6 mL), [DBUH][Im] (1 mol%, 0.1 mmol) relative to EC;
b Determined by GC and using 1,3,5-trimethoxybenzene (200 mg) as internal
standard;
c 26 ºC
The applicability of [DBUH][Im] for various alcohols
including methanol, ethanol and iso-propanol in the
transesterification reaction with EC was further investigated. The
Table 6
Comparison of different ionic liquid catalytic systems.
i
results listed in Table 4 showed that for EtOH and PrOH, both
T
Time
DMC
Yield(%)
75 [30]
86 [34]
81 [35]
85 [36]
93 [37]
91
Catalyst
Cat.(mol%)
nMeOH:nEC
(ºC) (h)
the EC conversion and yield of dialkyl carbonate were reduced,
which can be rationalized by the decreased acidity of hydroxylic
hydrogen in these alcohols. As a result, the nucleophilic
activation of these alcohols becomes difficult. Actually, in our
previous study, the same trend was also observed [35].
Subsequently, the reusability of [DBUH][Im] was investigated.
To recover the ionic liquid, vacuum distillation was performed
from 50 ºC to 150 ºC to sequentially remove methanol, DMC,
EG, EC and HMEC, leaving the ionic liquid in the flask. Then,
10 mmol EC and 150 mmol methanol were filled into the flask
containing the ionic liquid to begin the next run at 70 ºC. Ideally,
the ionic liquid-based catalysts feature good reusability. However,
in our investigation, the activity of [DBUH][Im] gradually
decreased with the recycle numbers, which has been shown in
Table 5. It is speculated that the partial decomposition of
catalystas well as the formation of the neutral species is the main
reason for the decreased activity in catalyst recycle.
[EMIm]Cl
[SmIm]OH
8
6
1
1
6
1
1
140
65
70
70
80
70
26
6
8:1
5
6
4
4
2
6
10:1
15:1
20:1
15:1
15:1
15:1
[C4DABCO]OH
[(TBD)2PEG150]Br2
[DBACO-PDO]I
[DBUH][Im] a
[DBUH][Im] a
81
a
Reaction conditions in this work: EC (10.0 mmol, 0.8806 g),
CH3OH (150 mmol, 6 mL), cat. (1 mol%, 0.1 mmol) relative to EC;
6 h; 70 ºC; Determined by GC and using 1,3,5-trimethoxybenzene
(200 mg) as internal standard.
To verify the multisite activation capacity of the protic ionic
liquid, the interaction between the substrates including EC and
methanol and the catalyst [DBUH][Im] was characterized using
NMR. Firstly, the 13CNMR of EC and the mixture of EC and
different equivalent of ionic liquid [DBUH][Im] (2, 4, 6 equiv. to
EC) was conducted using CDCl3 as solvent and the results were
showed in Fig. 2. It can be seen that with the increasing quantity
of protic ionic liquid, the chemical shift of carbonyl carbon in EC
moved from the original 155.63 ppm to 154.74 ppm. This can be
regarded as the evidence of the interaction between EC and
[DBUH][Im] by forming the hydrogen bond between the N-H
bond of the cation and C=O of EC.
Then the activation ability of [DBUH][Im] to methanol was
determined by 1H NMR, wherein the 1H NMR spectra of
methanol before and after adding [DBUH][Im] were recorded
respectively. The comparison of the spectra showed that chemical
shift of hydroxyl hydrogen in methanol moved from 4.10 ppm to
5.02 ppm (Fig. 3). This tendency of moving to lower field is a
typical feature of hydrogen bond formation, which corroborates
the fact that methanol can be activated by the protic ionic liquid
[DBUH][Im].
Based on the NMR results, the plausible mechanism is inferred
as Fig. 4. Firstly, the methanol is activated by forming hydrogen
bond with the imidazolium anion. Then the methoxy group of the
activated methanol attacks the carbonyl carbon activated by the
cation [DBUH]+ to realize the first-step transesterification,
generating the intermediate HMEC. Next, the activated methanol
molecule attacks the activated carbonyl carbon to form DMC and
EG, releasing the protic ionic liquidto participate in the next run
of the reaction (Fig. 4). Although the basicity of imidazole anion
is weaker than hydroxyl ion, which was employed in previously
reported ionic liquid catalysts, the improved catalytic activity of
Table 4
Scope of alcohol for the synthesis of dialkyl carbonate. a
Entry Alcohol
EC conv. (%)b
97
55
9
Yield of dialkyl carbonate (%)b
1
2
3
MeOH
EtOH
iPrOH
90
15
2
a
Reaction conditions: EC (10.0 mmol, 0.8806 g), Alcohol (150
mmol); [DBUH][Im] (1 mol%, 0.1 mmol) relative to EC; 2 h; 70 ºC.
b Determined by GC and using 1,3,5-trimethoxybenzene (200 mg) as
internal standard.
Table 5
Reusability of [DBUH][Im].a
Runs
EC conv. (%)b
Yield of DMC (%)b
1
2
3
4
97
92
79
62
46
90
81
65
43
14
5
a
Reactions were performed with EC (10.0 mmol, 0.8806 g), alcohol
(150 mmol) and recycled [DBUH][Im] as catalyst; 2 h; 70 ºC.
b Determined by GC and using 1,3,5-trimethoxybenzene (200 mg) as
internal standard.
The optimized transesterification results catalyzed by
[DBUH][Im] was compared with the results of other ionic
liquids-promoted synthesis of dimethyl carbonate (Table 6). It is
found that the protic ionic liquid [DBUH][Im] has higher
catalytic activity and leads to a faster reaction than most of the
ionic liquids even only 1 mol% of ionic liquid was used. At room

[DBUH][Im] can be explained by the synergistic effect of the
protic cation on EC.
bifunctional catalyst and the possible reaction path was proposed.
Albeit the reusability of this protic ionic liquid need to be further
improved, Its features of commercial availability and storage
stability make it to rival the previously reported ionic liquid-
based catalysts.
Acknowledgments
We thank the Ministry of Science and Technology of China
(National Key Research and Development Program, No.
2016YFA0602900), the National Natural Science Foundation of
China (No. 21672119) for financial support.
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Fig. 4. Proposed mechanism for [DBUH][Im] catalyzed
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the reaction running at 70 ºC can be completed in 2 h with 91%
DMC yield. Even at ambient temperature, 81% yield of DMC
can still be obtained. The mechanism investigation showed that
the cation and anion of this protic ionic liquid can activate EC
and methanol respectively by forming hydrogen bond.
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Graphical Abstract
[DBUH][Im] shows high catalytic efficiency in the synthesis
of dimethyl carbonate. The high efficiency can be rationalized
by the synergistic activation of ethylene carbonate and
methanol in the catalysis.