1,3-Dimethylimidazolium-2-carboxylate: A zwitterionic salt for the efficient synthesis of vicinal diols from cyclic carbonates

Article abstract of DOI:10.1039/c4gc00279b

The development of efficient, cheap and recyclable catalysts for reactions under mild reaction conditions is a very attractive topic in green chemistry. Herein, a series of basic ionic liquids (ILs) were investigated as catalysts for the synthesis of vicinal diols via the hydrolysis of cyclic carbonates in order to improve this kind of synthetic process. The effects of the IL structure, the molar ratio of cyclic carbonate to water, and various reaction parameters on the catalytic performance were investigated in detail. It was found that 1,3-dimethylimidazolium-2-carboxylate, a simple halogen-free zwitterionic catalyst, showed high activity (a space-time yield of 1086 h^-1) and excellent selectivity for the preparation of ethylene glycol via the hydrolysis of ethylene carbonate. The catalyst could be reused over six times without obvious loss of catalytic activity. Also, it was applicable to a variety of cyclic carbonates for the production of their corresponding vicinal diols with high yields and selectivities. A possible catalytic cycle for this kind of catalytic process was proposed based on the experimental results, NMR spectroscopy and theoretical calculations. This reaction protocol opens a new possibility for chemical synthesis as a substitution for traditional base or basic ILs. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00279b

Green Chemistry
View Article Online
View Journal | View Issue
PAPER
1,3-Dimethylimidazolium-2-carboxylate:
a zwitterionic salt for the efficient synthesis
of vicinal diols from cyclic carbonates†
Cite this: Green Chem., 2014, 16,
3297
Jian Sun, Xiaoqian Yao, Weiguo Cheng and Suojiang Zhang*
The development of efficient, cheap and recyclable catalysts for reactions under mild reaction conditions
is a very attractive topic in green chemistry. Herein, a series of basic ionic liquids (ILs) were investigated as
catalysts for the synthesis of vicinal diols via the hydrolysis of cyclic carbonates in order to improve this
kind of synthetic process. The effects of the IL structure, the molar ratio of cyclic carbonate to water, and
various reaction parameters on the catalytic performance were investigated in detail. It was found that
1,3-dimethylimidazolium-2-carboxylate, a simple halogen-free zwitterionic catalyst, showed high activity
(a space-time yield of 1086 h−1) and excellent selectivity for the preparation of ethylene glycol via the
hydrolysis of ethylene carbonate. The catalyst could be reused over six times without obvious loss of cata-
lytic activity. Also, it was applicable to a variety of cyclic carbonates for the production of their corres-
ponding vicinal diols with high yields and selectivities. A possible catalytic cycle for this kind of catalytic
process was proposed based on the experimental results, NMR spectroscopy and theoretical calculations.
This reaction protocol opens a new possibility for chemical synthesis as a substitution for traditional base
or basic ILs.
Received 17th February 2014,
Accepted 12th March 2014
DOI: 10.1039/c4gc00279b
www.rsc.org/greenchem
On the other hand, vicinal diols (VDs) such as ethylene
glycol (EG) and propylene glycol (PG), are platform chemicals
Introduction
The need for environmental sustainability and energy of commercial importance in a large number of industrial pro-
efficiency has propelled an interest in the development of cesses including energy, plastics, automobiles, and
green and efficient catalysis.1 In this context, ionic liquids chemicals.9–12 Due to their unique properties and versatile
(ILs) have been explored recently as novel catalysts and addi- commercial applications, a variety of approaches have been
tives due to their unique and tunable physico-chemical charac- explored for the preparation of VDs in order to improve syn-
teristics.2 In addition, it is delighting to note that some thetic processes, enhance catalytic performance, and reduce
advances have been made regarding the issue of halogen-free cost of investment; such as conversion of cellulose,13 hydro-
ILs, towards the further reduction of the potential environ- genation of cyclic di-esters or cyclic carbonates,14 oxidation of
mental and biological problems of ILs themselves.3 As one of olefins,15 hydration of epoxides,16 addition of ethyl diazoace-
the precursors suitable for this objective, 1-alkyl-3-methylimi- tate to aldehydes,17 and hydrolysis of cyclic carbonates.18
dazolium-2-carboxylates have received considerable interest.4–8 Among these methods, at present, the technology of epoxide
They can be versatilely used as halogen-free ILs directly, as key hydration (Scheme 1, dashed line) is still dominant in industry
precursors of NHC-metal complexes4 and NHC catalysts.5 and accounts for the major market share.9,19 However, in
More importantly, this kind of zwitterionic salt can be simply
and directly obtained from 1-alkylimidazoles and dimethyl car-
bonate,6 in addition to being obtained as the NHC-CO2 adduct
by the condensation of N-heterocyclic carbene (NHC) with
CO2,7 and can also be prepared through the Kolbe–Smith reac-
tion of 1,3-dialkylimidazolium salt with Na2CO3 and CO2.8
State Key Laboratory of Multiphase Complex Systems, Beijing Key Laboratory of Ionic
Liquids Clean Process, Institute of Process Engineering, Chinese Academy of Sciences,
Beijing 100190, PR China. E-mail: sjzhang@home.ipe.ac.cn; Tel: +86-10-8262-7080
†Electronic supplementary information (ESI) available. See DOI:
Scheme 1 An integrated process for the synthesis of vicinal diols
via CO2.
10.1039/c4gc00279b
This journal is © The Royal Society of Chemistry 2014
Green Chem., 2014, 16, 3297–3304 | 3297

View Article Online
Paper
Green Chemistry
order to obtain high VD selectivity (e.g., EG, 88–89%), a large
excess amount of water (20–25 molar ratio of water–ethylene
oxide (EO)) must be used.20 The low EG concentration in the
system leads to a high capital investment for unit operations
but a low reaction atom economy.9b As a promising indirect
synthetic process, an integrated technology based on the cyclo-
addition of EO and CO2 to ethylene carbonate (EC) and sub-
sequent hydrolysis to EG has been developed (Scheme 1, R = H),
which is advantageous because of the economic water ratio,
high product selectivity and low construction costs,21 and thus
has been developed in the “omega process” competing with the
hydration method for the unborn manufacture of EG.20a
As noted in our previous work,22 many IL based catalysts
have been explored for the synthesis of cyclic carbonates
(Scheme 1, step a). Unfortunately, less effort has been devoted
to the development of ILs for the hydrolysis process (step b).
To our best knowledge, until now, the reported hydrolysis cata-
lysts are normally traditional acid/base,18 and enzyme.23
Although inorganic bases have been commercialized for this
Scheme 2 Structures of the ILs used in this work.
Results and discussion
reaction,20,21 the disadvantages of low efficiency, poor compati- The usage of an appropriate catalyst is an alternative and
bility,22c corrosion and/or skin irritation require improvement. simple way to attain high selectivity of vicinal diols. In a repre-
As a promising substitute for inorganic bases, basic ILs have sentative reaction, a series of ILs based on imidazolium+,
recently shown to be noteworthy,24–28 and have demonstrated phosphonium+, ammonium+ and HDBU+ cations countered
good performances in carbon capture,24d Michael addition,25 with Cl−, Br−, I−, BF4
, PF6 , ,
Tf2N−, HSO4 HCOO−,
−
−
−
Heck reaction,26 Markovnikov addition,27 and hydrogenation C2H5COO−, PhCOO−, TFA− and OAc− anions were examined
of CO2.28 Xiao et al. reported that the hydrolysis of propylene in the synthesis of ethylene glycol (EG) via the hydrolysis of
carbonate to PG could be performed using polymer supported ethylene carbonate (EC) under identical reaction conditions.
− 29a
basic IL countered with OH− or HCO3
.
However, a long The results including the space-time yield (STY), which was
reaction time (4 h) and a high amount of catalyst (∼3 wt%) are defined as a gram of EG produced per gram of catalyst per
required for those catalysts. Afterwards, we found that [BMIm] hour, are summarized in Table 1, and revealed that both the
OH is an unstable basic catalyst for the hydrolysis of EC and cations and anions of the ILs employed affect the catalytic
its activity decreased quickly from 90 to 20% after being twice activities (entries 2–30).
reused. Additionally, it is difficult to prepare and easily dis-
The effects of ILs with different anions (Cl−, Br−, I−, BF4
,
−
coloured even at r.t., after which its activity is lower than that PF6−, Tf2N−, HSO4−, HCOO−, OAc−, C2H5COO−, PhCOO−, and
of the fresh batch.29b The low stability of [BMIm]OH might be TFA−) on the reaction were also investigated. In the case of imi-
caused by the reactive C(2)–H in the imidazolium ring, dazolium, phosphonium and ammonium based ILs (Table 1,
especially under alkaline conditions.30 Interestingly, a mixture entries 2–23, 30), the activity order of the anions follows the
of [BMIm]BF4 and [BMIm]OH was much more efficient and sequence: CO2 > OAc− > TFA−, Cl− > Br− > I− > Tf2N−, BF4
,
−
−
stable than [BMIm]OH in the reaction.29b Although the above PF6−, implying that the basicity of anions is an important
advances have been made, the employed basic ILs still have factor in determining the catalytic activity of ILs. Poorly
some problems such as being halogen-containing, high cost, nucleophilic anions such as Tf2N−, BF4−, and PF6 were found
−
complicated to prepare, and of low activity and/or stability. Con- to be inefficient for this hydrolysis reaction (entries 2–6,
sequently, the development of environmentally benign, cheap Table 1), and the corresponding results were the same as that
and stable basic ILs still remains a major scientific challenge.
without catalyst (entry 1). For 1,8-diazabicyclo[5.4.0]-undec-7-
Herein, we demonstrate a first example of using 1,3- ene (DBU) based protic ILs, the corresponding anion activity
dimethylimidazolium-2-carboxylate (DMImC) as a basic zwit- variation was found to follow the order: PhCOO− > C2H5COO−
terionic salt that promotes the hydrolysis of cyclic carbonates > Cl−, OAc−, HCOO− > HSO4− (entries 24–29).
to form vicinal diols. As comparisons, 28 ILs (Scheme 2) with
In the case of the TFA− anion, the length of the alkyl chain in
features of Lewis basicity, acidity or neutrality, and countered the cation might account for a slightly decreased catalytic activity
with different anions such as Cl−, Br−, I−, BF4−, PF6−, Tf2N−, in the order of MMIm+ > EMIm+, VEMIm+ > PMIm+ (Table 1,
HSO4−, HCOO−, OAc−, C2H5COO−, PhCOO−, and TFA− were entries 16–19). The same trend was found for the OAc− anion,
synthesized and tested. The corresponding reaction para- where the activity of the cations in ILs varied in the order of
meters were systematically screened, and a possible reaction MMIm+ > EEIm+, EMIm+ > BMIm+ (entries 20–23). The results
process in the presence of DMImC was proposed based on indicated that a small bulk of cation was beneficial for improving
NMR, in situ NMR spectroscopic characterizations and theore- the catalytic activity, which is in accordance with the result in
tical calculations.
our previous work.22d Among the catalysts employed, DMImC
3298 | Green Chem., 2014, 16, 3297–3304
This journal is © The Royal Society of Chemistry 2014

View Article Online
Green Chemistry
Paper
Table 1 Effect of ILs on the synthesis of ethylene glycola
conversion of EC, while the selectivity of EG remained almost
constant. As a result, the product yield increased quickly as the
ratio was varied from 1 to 1.5, and so did the corresponding
STY values (entries 1–6). Further increase in the amount of
water gives but very little increase in the reaction efficiency
(entries 7–8). The slightly higher H2O : EC ratio used than the
stoichiometric ratio was possibly ascribed to the loss of water
by CO2 release during the reaction, which could be detected by
GC-MS. Nevertheless, the water ratio is still competitive com-
pared to those in the non-catalytic hydration and catalytic
hydration reactions.9b As a result, the molar ratio of 1.5 : 1
(H2O to EC) is considered a satisfactory result.
For a homogeneous catalytic system, the intrinsic drawback
of separation results in an additional energy consumption in
the recycling of the catalyst. In our lab, a series of supported
ILs,22b–d and the corresponding fixed-bed process for the syn-
thesis of cyclic carbonates have been developed. If we can
sufficiently reduce the amount of hydrolysis catalyst, a result-
ing whole integrated process (Scheme 1, steps a and b) pro-
ceeding with high efficiency but low catalyst separation cost is
expected from a viewpoint of industrial application. As shown
in Table 3, the presence of catalyst is quite necessary and its
amount had a positive effect on the hydrolysis efficiency. EC
conversion and EG yield uniformly increased with increasing
amount of the catalyst, concurrently with the decline of the
STY value from 1753 to 808 h−1 (entries 2–8). To our delight, a
very low catalyst amount of 2.5 mg (∼1 wt‰), a still high
selectivity (>99%) and yield (96%) of EG could be obtained
with a surprising STY value of 1086 h−1 (entry 6). Therefore,
the optimum amount of DMImC was 2.5 mg.
Entry ILs
Conversionb (%) Yieldb (%) STY (h−1
)
1
2
3
4
5
6
7
8
—
7
8
8
7
8
7
8
8
7
8
3
3
3
3
3
6
32
37
33
32
29
21
36
29
24
37
35
[EMIm]BF4
[BMIm]BF4
[BMIm]PF6
[BMIm]Tf2N
[EMIm]Tf2N
EMImBr
BMImCl
BMImBr
PPh3BuCl
PPh3BuBr
PPh3BuI
17
84
97
87
85
79
55
95
77
65
99
93
91
80
89
82
81
67
17
68
65
66
17
84
97
87
84
78
55
95
77
65
99
93
91
80
88
82
81
66
17
68
65
66
75
87
96
90
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
Bu4NCl
Bu4NBr
Bu4NI
[MMIm]TFA
[EMIm]TFA
[VEMIm]TFA
[PMIm]TFA
[MMIm]OAc
[EEIm]OAc
[EMIm]OAc
[BMIm]OAc
[HDBU]HSO4
[HDBU]Cl
[HDBU]HCOO
[HDBU]OAc
34
30
999
922
914
751
189
772
731
745
849
985
1086
1018
[HDBU]C2H5COO 75
[HDBU]PhCOO
DMImC
87
96
90
K2CO3
a Reaction conditions: EC (2 g, 22.7 mmol), H2O (0.62 g, 34.4 mmol),
140 °C, 0.4 MPa; entries 1–19, catalyst amount/time (25 mg/1.5 h),
entries 20–31, catalyst amount/time (2.5 mg/0.5 h). b Determined by
GC and GC-MS.
Fig. 1a depicts the influence of reaction temperature on the
reaction. With the temperature being increased from 90 to
140 °C, the EG yield could be remarkably enhanced from 10 to
96%. Thereafter, further increase of the temperature from 140
to 170 °C caused a slight increase in the yield of EG from 96 to
100%. The main reason for this is that hydrolysis reaction is
thermodynamically favourable at elevated temperatures.
However, actual industrial operation data demonstrated that
EG could undergo oxidation or coking over a temperature
range of 160 °C, which drastically reduces product quality. As a
result, a suitable reaction temperature of 140 °C would be
acceptable.
exhibited the highest activity (STY = 1086 h−1) under the con-
ditions of 0.5 h and 0.1 wt% catalytic amount (entry 30),
whose activity was even higher than that of K2CO3 (entry 31).
Due to its facile preparation from commercially available and
relatively inexpensive starting materials as well its high activity,
DMImC was chosen for further investigation.
Subsequently, the influence of the amount of water on the
EG synthesis was investigated under identical reaction con-
ditions. The amount of EC was kept constant. Table 2 shows
that the change of water amount played an obvious role in the
Table 2 Effect of water amount on EG synthesisa
Table 3 Effect of catalyst amount on EG synthesisa
Entry
Ratio
Conversionb (%)
Yieldb (%)
STY (h−1
)
Entry Amount (mg) Concentration (wt‰) Yieldb (%) STY (h−1
)
1
2
3
4
5
6
7
8
1
67
77
81
87
91
96
99
100
67
77
81
87
91
96
99
100
758
871
916
1
2
3
4
5
6
7
8
0
0.5
1
1.5
2
2.5
3
0
tr.
31
55
71
90
96
—
1.1
1.2
1.3
1.4
1.5
1.6
1.7
0.19
0.38
0.57
0.76
0.95
1.14
1.33
1753
1555
1338
1272
1086
914
984
1029
1086
1120
1131
97
100
3.5
808
a Reaction conditions: EC (2 g), DMImC (2.5 mg), 140 °C, 0.4 MPa, a Reaction conditions: EC (2 g), H2O (0.62 g), DMImC, 140 °C, 0.4 MPa,
0.5 h. b Determined by GC and GC-MS.
0.5 h. b Determined by GC and GC-MS.
This journal is © The Royal Society of Chemistry 2014
Green Chem., 2014, 16, 3297–3304 | 3299

View Article Online
Paper
Green Chemistry
Fig. 2 The performance for reuse of the catalyst. Reaction conditions:
EC : H2O = 1/1.5 (molar ratio), DMImC (1 wt‰), 140 °C, 0.4 MPa, 0.5 h.
Fig. 1 Effect of reaction conditions on EG synthesis. Reaction con-
ditions: EC : H2O = 1/1.5 (molar ratio), DMImC (1 wt‰), (a) 0.4 MPa,
0.5 h; (b) 140 °C, 0.5 h; (c) 140 °C, 0.4 MPa.
Table 4 Synthesis of vicinal diols from a series of terminal cyclic car-
bonates catalyzed by DMImCa
Tailored by a backpressure valve connected to the reactor,
the effect of reaction pressure was investigated. As is well
known, the hydrolysis reaction is a reversible reaction with gas
release, and the increase in pressure would have an obvious
negative effect on the reaction equilibrium to produce EG. The
results shown in Fig. 1b confirmed this judgment and EG
yield decreased quickly at 140 °C in the range of 0.4–1.6 MPa.
However, at low pressures below 0.4 MPa, the EG yield was
also reduced, possibly as a result of an increasing loss of water
with CO2 release (saturated vapour pressure of water is circa
0.36 MPa at 140 °C). The competition of these opposite factors
resulted in a maximum in the pressure versus yield curve.
During the investigation, EG selectivity was kept as a constant.
For better results, 0.4 MPa was chosen.
Conversion
(%)
Yieldb
(%)
Entry
1
Substrate
Time/h
0.5
Product
96
96
2
3
1
6
96
96
65
64
The effect of reaction time on EC conversion and EG selecti-
vity is also shown in Fig. 1c. The conversion of EC was remark-
ably increased during the first 0.5 h, at which a 96% EG yield
with a 100% EG selectivity was obtained. After 0.5 h, the con-
version of EC was almost a constant. Thus, 0.5 h was chosen.
The reusability of the catalyst is one important factor to
identify whether it finds potentially practical application in
industry. A number of catalytic cycles were run in order to
investigate the constancy of the catalyst recyclability. After reac-
tion, the catalyst was separated from the reaction mixture by
vacuum distillation, and used directly for the next run. As
shown in Fig. 2, under the reaction conditions of 140 °C, 0.4 MPa,
0.5 h, EC : H2O = 1/1.5 and 1 wt‰ catalyst concentration,
DMImC could be recycled six times with good performance.
The stability of DMImC might be due to the replacement of
reactive C(2)–H in the imidazolium ring,30 and the covalent
bond between the anion and cation of DMImC.
4
5
24
1
91
97
90
97
6c
1
99
98
a Reaction conditions: cyclic carbonate : H2O = 1/1.5 (molar ratio),
DMImC (1 wt‰), 140 °C, 0.4 MPa. b Determined by GC and GC-MS.
c DMImC (6 wt‰).
Under similar testing conditions, a series of terminal cyclic give a 96% yield and 100% selectivity of EG (entry 1). The reac-
carbonates were also tested for the synthesis of the corres- tivity decreased when the substrate changed from EC to propy-
ponding vicinal diols through the present process in the pres- lene carbonate (PC, 1b), while the selectivity remained high
ence of DMImC. As shown in Table 4, the conversions and (entry 1 vs. 2). This is due to the steric effect of the bulky
yields of diols decreased with increasing alkyl chain length methyl group in PC as compared with symmetrical EC.
and steric hindrance of cyclic carbonates (entries 1–4). As dis- Additionally, the reactions with 4-butyl-1,3-dioxolan-2-one (1c)
cussed, the reaction of EC (1a) with water within 0.5 h could and 4-phenyl-1,3-dioxolan-2-one (1d) appeared to be more
3300 | Green Chem., 2014, 16, 3297–3304
This journal is © The Royal Society of Chemistry 2014

View Article Online
Green Chemistry
Paper
sluggish (entry 1 vs. 3–4), which might also be due to the high
hydrophobicity. On the other hand, the reaction of 4-vinyl-1,3-
dioxolan-2-one (1e), and 4-(phenoxymethyl)-1,3-dioxolan-2-one
(1f) with water could proceed smoothly providing the corres-
ponding vicinal diols in high yields and selectivities, probably
because of the electronic effect (entries 5–6). Nevertheless, the
above results indicated a good applicability of DMImC to the
terminal cyclic carbonate substrates.
As reported,31a–d the addition of organic alcohol (e.g. ethy-
lene glycol or poly (ethylene glycol)) to a superbase (e.g.
amidine or guanidine) or an IL, could be used for CO2 chemi-
sorption by forming the corresponding liquid ammonium
alkylcarbonate salt, and this process was reversible because
the organic alcohol/superbase system could be regenerated by
heating. Dai and co-authors reported that an integrated sorp-
tion system consisting of 1 : 1 mixtures of an alcohol-functio-
nalized IL and a superbase was also effective for CO2 capture
at atmospheric pressure.31e Inspired by the above findings on
CO2 capture,30 a DMImC and EG or PG binary system can also
be proposed for reversible CO2 capture. In this context, the
switchable amidinium carbonate salt formed from CO2 and
DMImC/vicinal diol might be an intermediate during the
hydrolysis process, successfully affording the target product of
vicinal diol.
In order to get insight into the mechanistic aspects of this
catalytic process, experiments were conducted using DMImC
in conjunction with EG or PG for CO2 absorption by the
method reported.31b The mixtures before and after absorption
were detected by 13C NMR (D2O) and 1H NMR (CDCl3) spectra.
Scheme 3 shows the 13C NMR results of DMImC-EG-CO2 (a),
DMImC-PG-CO2 (b) and DMImC (c) in the range from 155 to
Scheme 4 1HNMR of DMImC-EG-CO2 (top) and DMImC-PG-CO2
(bottom).
165 ppm. A COO− group in DMImC results in a signal at about
158.33 ppm (c).4 An additional signal at about 160 ppm (car-
boxyl group) in the 13C NMR spectrum (a, b) supports the for-
mation of the alkylcarbonate salt, in agreement with previous
reports,31 and the in situ 13C NMR detection of the hydrolysis
of EC (50 °C, 600 MHz, D2O) (ESI, Fig. 1S†).
Scheme 4 shows the 1H NMR results of DMImC-EG-CO2
and DMImC-PG-CO2 in the range from 7.3 to 9.2 ppm. New H
signals centered at about 9.07 and 9.18 ppm, correspond to a
carboxyl group formed in DMImC-EG-CO2 (top) and in
DMImC-PG-CO2 (bottom), respectively. The results are in
agreement with the in situ 1H NMR detection on the hydrolysis
of EC (50 °C, 600 MHz, D2O) (ESI, Fig. 2S†), also supporting
the formation of alkylcarbonate salts.
Next, in order to further elucidate the possible reaction
mechanism, density functional theory calculations were
carried out using PC as a model substrate because of its struc-
tural asymmetry. In addition, OH− was considered as a poss-
ible nucleophile in the reaction system since DMImC is a basic
salt. Based on the calculations, we found that PC would
undergo two possible reaction pathways in the presence of
OH−, which are breakage of the αC–O bond far from –CH3
(path X) and breakage of the αC–O bond near –CH3 (path Y)
(Scheme 5).
The routes along breakage of the αC–O bond were com-
puted first, and energy diagrams of the two possible pathways
Scheme 3 13CNMR of DMImC-EG-CO2 (a), DMImC-PG-CO2 (b) and
Scheme 5 Two main pathways of the ring opening of PC followed by
DMImC (c).
intermolecular hydrogen transfer.
This journal is © The Royal Society of Chemistry 2014
Green Chem., 2014, 16, 3297–3304 | 3301

View Article Online
Paper
Green Chemistry
Scheme 6 Energy diagrams for breakage of αC–O bond, and the
corresponding geometries of the transition state and intermediate
(
path X,
path Y).
(X and Y) are shown in Scheme 6. Following the LST/QST pro-
cedure, the transition state labeled TS (a) connecting the initi-
ating reactant R and the product P (a) in path X was located
and is shown in Scheme 6 (blue dashed line). The C⋯O dis-
tance in the TS (a) is 2.639 Å. The activation energy of this
elementary step is 35.5 kJ mol−1, and the reaction is endergo-
nic by 31.6 kJ mol−1. For the other possible pathway Y, the
optimized structures of the involved species with energies are
shown in Scheme 6 (red dashed line). The corresponding tran-
sition state TS (b) was located, and the C⋯O distance in TS (b)
is 2.664 Å. Compared to path X, this path has a higher acti-
vation energy barrier of 47.9 kJ mol−1 and the reaction is
endergonic by 32.7 kJ mol−1. Thus, path X is the most likely
ring opening mode. Also, it was found by our calculations that
the H of OH is very easy to transfer, and the formed isomer B
or B′ (Scheme 5) has quite a low energy (−157 kJ mol−1) and is
much more stable than A (P (a)) or A′ (P (b)), respectively.
Next, the possible structures of the intermediate, imidazo-
lium alkylcarbonate salt, which was formed by a conjugated
acid of DMImCH and B, were optimized and described in
Scheme 7. There are four possible energy minimum structures
found. Among the structures of (1)–(4), (1) is most stable due
to its lowest energy, and will therefore most likely exist.
Scheme 8 A possible reaction mechanism.
Based on the discussions above, a hydrolysis cycle explain-
ing how DMImC catalyzes the hydrolysis of cyclic carbonate
was proposed using propylene carbonate as the substrate, and
it is shown schematically in Scheme 8. In the reaction cycle,
the catalyst DMImC destroyed the ionization balance of water
and generated its conjugated acid DMImCH. Thereafter, the
OH− anion underwent nucleophilic attack on the carbonyl
carbon of the cyclic carbonate resulting in 1-(carboxyl)alkyl-2-
olate (A). A could be reversibly transformed into 2-hydroxyalkyl
carbonate (B) by intermolecular hydrogen transfer. B reacted
with DMImCH to produce a possible imidazolium alkylcarbo-
nate salt C or D (Scheme 8), which has been discussed with
respect to the corresponding NMR spectra (Schemes 3 and 4).
C could be reversibly changed into D (1, Scheme 7), also by
intermolecular hydrogen transfer. Thereafter, E was produced
from D by the generation of DMImC. Finally, according to the
interactions of OH− and DMImCH, vicinal diols and CO2 were
produced from E, generating the catalyst for the next recycle.
During the process, the synergetic catalytic role of DMImCH
and OH− was important for the reaction to proceed smoothly.
Conclusions
We firstly demonstrated that the preparations of vicinal diols
from terminal cyclic carbonates and water could proceed
smoothly in the presence of zwitterionic 1,3-dimethylimidazo-
lium-2-carboxylate (DMImC) under relatively mild conditions.
As a result, a highest STY value of 1086 h−1 could be realized
using ethylene carbonate as the reactant, which is much
higher than those obtained by other employed catalysts. This
easily prepared catalyst with a simple structure was stable, and
could be reused six times without appreciable loss of catalytic
Scheme 7 Four possible energy minimum structures of imidazolium
alkylcarbonate salt.
3302 | Green Chem., 2014, 16, 3297–3304
This journal is © The Royal Society of Chemistry 2014

View Article Online
Green Chemistry
Paper
activity. Based on the results, a possible reaction mechanism 125.83, 129.20, 129.71, 135.77, 154.78 (CvO). 4-(Phenoxy-
was proposed, implying that DMImC might firstly undergo a methyl)-1,3-dioxolan-2-one(1f): 1H NMR (400 MHz, CDCl3), δ
configuration change after an interaction with water, and the (ppm): 4.15 (dd, J = 4.4 Hz, J = 10.8 Hz, 1H), 4.24 (dd, J = 3.6
generated DMImCH and OH− thereafter exerted a crucial Hz, J = 10.8 Hz, 1H), 4.55 (dd, J = 8.4 Hz, J = 6 Hz, 1H), 4.62 (t,
synergetic catalysis effect on the reaction, by which the reac- J = 8.4 Hz, 1H), 5.03 (m, 1H), 6.91 (d, J = 8.0 Hz, 2H), 7.02 (t, J =
tion was promoted. Also, a switchable amidinium carbonate 7.4 Hz, 2H), 7.31 (t, J = 8.0 Hz, 2H). 13C NMR (100.4 MHz,
salt formed from CO2 and DMImC/vicinal diol might be an CDCl3), δ (ppm): 66.17, 68.84, 74.11, 114.57, 121.92, 129.62,
intermediate during the hydrolysis process. Although details 154.65 (CvO), 157.71.
of the exact process are still unclear, the present example
directly acting as a halogen-free ionic liquid extends the appli-
cations of 1-alkyl-3-methylimidazolium-2-carboxylates as
useful precursors.
Acknowledgements
The authors are grateful to the support from the National
Basic Research Program of China (2009CB219901), the Knowl-
edge Innovation Program of the Chinese Academy of Sciences
(KGCX2-YW-321), and the National Science Fund of China
(21006117 and 21276255).
Experimental section
Chemicals information
Ionic liquids such as BmimCl, EmimCl, BMImBr, EMImBr,
[Bmim]BF4, and [Bmim]PF6 were received from Henan Lihua
Pharmaceutical Co., Ltd. Other ionic liquids were synthesized
by the method described in the ESI.† Ethylene carbonate, pro-
Notes and references
pylene carbonate, and vinyl ethylene carbonate were of indus-
trial grade and commercially available. Other cyclic carbonates
were synthesized in our laboratory. Carbon dioxide was sup-
plied by Beijing Analytical Instrument Factory with a purity of
99.95%. Deionized water was used in the experiments. Other
reagents were of analytical grade and were used as received.
The products were analyzed by gas chromatography/mass
spectrometry.
1 (a) J. Clark, R. Sheldon, C. Raston, M. Poliakoff and
W. Leitner, Green Chem., 2014, 16, 18; (b) R. A. Sheldon,
Chem. Soc. Rev., 2012, 41, 1437.
2 N. V. Plechkovaa and K. R. Seddon, Chem. Soc. Rev., 2008,
37, 123.
3 (a) M. Smiglak, J. D. Holbrey, S. T. Griffin, W. M. Reichert,
R. P. Swatloski, A. R. Katritzky, H. Yang, D. Zhang,
K. Kirichenko and R. D. Rogers, Green Chem., 2007, 9, 90;
(b) N. J. Bridges, C. C. Hines, M. Smiglak and R. D. Rogers,
Chem. – Eur. J., 2007, 13, 5207; (c) B. Oelkers and
J. Sundermeyer, Green Chem., 2011, 13, 608.
Typical procedure for ethylene glycol synthesis from hydrolysis
of ethylene carbonate
All the hydrolysis reactions were carried out in a 25 mL stain-
less steel reactor equipped with a magnetic stirrer and an auto-
matic temperature control system. In a typical procedure,
catalyst (2.5 mg), ethylene carbonate (2 g), and deionized water
(0.62 g) were added into the autoclave. After being sealed, the
reactor was heated to the desired temperature and the reaction
pressure was held constant by a backpressure valve. After the
reaction was completed, the autoclave was cooled to ambient
temperature, and the remaining CO2 was removed. Then, the
catalyst was separated from the reaction mixture by vacuum
distillation, and the analyses of products were performed
using an Agilent 6820 GC and 6890/5973B GC-MS. Catalyst
recycling tests were performed with a large scale of 30× in a
100 mL stainless steel reactor.
4 (a) J. Y. Li, J. J. Peng, Y. Bai, G. Q. Lai and X. N. Li, J. Orga-
nomet. Chem., 2011, 696, 2116; (b) A. Voutchkova, M. Feliz,
E. Clot, E. Eisentein and R. H. Crabtree, J. Am. Chem. Soc.,
2007, 129, 12834.
5 (a) H. Zhou, Y. M. Wang, W. Z. Zhang, J. P. Qu and X. B. Lu,
Green Chem., 2011, 13, 644; (b) F. Huang, G. Lu, L. L. Zhao,
H. X. Li and Z. X. Wang, J. Am. Chem. Soc., 2010, 132,
12388; (c) S. N. Riduan, Y. Zhang and J. Y. Ying, Angew.
Chem., Int. Ed., 2009, 48, 3322.
6 J. D. Holbrey, W. M. Reichert, I. Tkatchenko, E. Bouajila,
O. Walter, I. Tommasi and R. D. Rogers, Chem. Commun.,
2003, 28.
7 H. Zhou, W. Z. Zhang, C. H. Liu, J. P. Qu and X. B. Lu,
J. Org. Chem., 2008, 73, 8039.
Spectral characteristics of the cyclic carbonates 1c, 1d and
1f synthesized in Table 4 were provided as follows: 4-butyl-1,3-
dioxolan-2-one (1c): H NMR (400 MHz, CDCl3), δ (ppm): 0.84
8 T. F. Sorrentino, Tetrahedron Lett., 2006, 47, 6453.
9 (a) M. Tursky, L. L. R. Lorentz-Petersen, L. B. Olsen and
R. Madsen, Org. Biomol. Chem., 2010, 8, 5576; (b) H. R. Yue,
Y. J. Zhao, X. B. Ma and J. L. Gong, Chem. Soc. Rev., 2012,
41, 4218.
1
(t, J = 7.1 Hz, 3H); 1.29–1.41 (m, 2H); 1.62–1.76 (m, 2H);
3.93–4.02 (m, 2H); 4.47 (d, J = 8.3 Hz, 2H); 4.61–4.68 (m, 1H).
13C NMR (75.5 MHz, CDCl3), δ (ppm): 13.38, 21.85, 26.05, 10 (a) P. Hoyos, J. V. Sinisterra, F. Molinari, A. Alcantara and
33.09, 69.12, 76.89, 154.86 (CvO). 4-Phenyl-1,3-dioxolan-2-one
(1d): H NMR (400 MHz, CDCl3), δ (ppm): 4.34 (t, J = 8.4 Hz,
P. D. Maria, Acc. Chem. Res., 2010, 43, 288; (b) T. Maki,
S. Iikawa, G. Mogami, H. Harasawa, Y. Matsumura and
1
1H), 4.80 (t, J = 8.4 Hz, 1H), 5.68 (t, 1H, J = 8.0 Hz), 7.35–7.44
(m, 5H); 13C NMR (100.4 MHz, CDCl3), δ (ppm): 71.13, 77.94,
O. Onomura, Chem.
(c) A. C. Schmidt and C. B. W. Stark, Org. Lett., 2011, 13,
–
Eur. J., 2009, 15, 5364;
This journal is © The Royal Society of Chemistry 2014
Green Chem., 2014, 16, 3297–3304 | 3303

View Article Online
Paper
Green Chemistry
5788; (d) S. C. Dakdouki, D. Villemin and N. Bar,
Eur. J. Org. Chem., 2011, 4448.
K. Dong, X. P. Zhang and S. J. Zhang, Catal. Sci. Technol.,
2012, 2, 600.
11 (a) E. Arceo, J. A. Ellman and R. G. Bergman, J. Am. Chem. 23 (a) L. Chang, L. M. Ouyang, Y. Xu, J. Pan and J. H. Xu,
Soc., 2010, 132, 11408; (b) S. Vkuturi, G. Chapman,
I. Ahmad and K. M. Nicholas, Inorg. Chem., 2010, 49, 4744;
(c) J. E. Ziegler, M. J. Zdilla, A. J. Evans and M. Abu-Omar,
Inorg. Chem., 2009, 48, 9998.
J. Mol. Catal. B: Enzym., 2010, 66, 95; (b) M. Shimojo,
K. Matsumoto and M. Hatanaka, Tetrahedron, 2000,
56, 9281; (c) K. Matsumoto, Y. Sato, M. Shimojo
and M. Hatanaka, Tetrahedron: Asymmetry, 2000, 11,
1965.
12 S. Bahn, A. Tillack, S. Imm, K. Mevius, D. Michalik,
D. Hollmann, L. Neubert and M. Beller, ChemSusChem, 24 (a) F. Jutz, J. M. Andanson and A. Baiker, Chem. Rev., 2011,
2009, 2, 551.
111, 322; (b) J. P. Hallett and T. Welton, Chem. Rev., 2011,
111, 3508; (c) W. Leitner, Nature, 2003, 423, 930;
(d) X. P. Zhang, X. C. Zhang, H. F. Dong, Z. J. Zhao,
S. J. Zhang and Y. Huang, Energy Environ. Sci., 2012, 5,
6668.
13 Y. Liu, C. Luo and H. C. Liu, Angew. Chem., Int. Ed., 2012,
51, 3249.
14 (a) E. Balaraman, E. Fogler and D. Milstein, Chem.
Commun., 2012, 48, 1111; (b) Z. B. Han, L. C. Rong, J. Wu,
L. Zhang, Z. Wang and K. L. Ding, Angew. Chem., Int. Ed., 25 (a) B. C. Ranu and S. Banerjee, Org. Lett., 2005, 7, 3049;
2012, 51, 13041.
15 A. Wang and H. F. Jiang, J. Org. Chem., 2010, 75, 2321.
(b) S. Z. Luo, X. L. Mi, L. Zhang, S. Liu, H. Xu and
J. P. Cheng, Angew. Chem., Int. Ed., 2006, 45, 3093.
16 (a) Y. Xu, X. Jia, S. Panke and Z. Li, Chem. Commun., 2009, 26 Y. Liu, Y. Lu, G. Gao, Q. Yang and M. He, Catal. Commun.,
1481; (b) Z. J. Yang, Y. F. Li, Q. B. Wu, N. Ren, Y. H. Zhang,
Z. P. Liu and Y. Tang, J. Catal., 2011, 280, 247.
17 B. M. Trost, S. Malhotra, P. Koschker and P. Ellerbrock,
J. Am. Chem. Soc., 2012, 134, 2075.
18 E. M. G. A. Van Kruchten, A. A. Smaardijk and H. Stichter,
US Patent, 8,110,712, 2012.
19 (a) L. F. Chen, P. J. Guo, M. H. Qiao, S. R. Yan, H. X. Li,
2006, 7, 985.
27 J. M. Xu, B. K. Liu, W. B. Wu, C. Qian, Q. Wu and X. F. Lin,
J. Org. Chem., 2006, 71, 3991.
28 (a) Z. F. Zhang, S. Q. Hu, J. L. Song, W. J. Li, G. Y. Yang and
B. X. Han, ChemSusChem, 2009, 2, 234; (b) Z. F. Zhang,
Y. Xie, W. J. Li, S. Q. Hu, J. L. Song, T. Jiang and B. X. Han,
Angew. Chem., Int. Ed., 2008, 47, 1127.
W. Shen, H. L. Xu and K. N. Fan, J. Catal., 2008, 257, 172; 29 (a) L. F. Xiao, Q. F. Yue, C. G. Xia and L. W. Xu, J. Mol.
(b) F. E. Celik, H. Lawrence and A. T. Bell, J. Mol. Catal. A:
Chem., 2008, 288, 87.
20 (a) K. Kawabe, Catal. Surv. Asia, 2010, 14, 111; (b) Y. C. Li,
Catal. A: Chem., 2008, 279, 230; (b) Z. Y. Meng, J. Sun,
J. Q. Wang, J. X. Zhang, Z. Z. Fu, W. G. Cheng and
X. P. Zhang, Chin. J. Chem. Eng., 2010, 18, 962.
S. R. Yan, L. P. Qian, W. M. Yang, Z. K. Xie, Q. L. Chen, 30 (a) J. Q. Wang, W. G. Cheng, J. Sun, T. Y. Shi, X. P. Zhang
B. Yue and H. Y. He, J. Catal., 2006, 241, 173.
21 (a) H. W. Scheeline and Y. Watanabe, SRI International,
Review No. 79-2-1, 1980; (b) S. Naqvi, Process Economics
Program, Report No. 2I-5, 2009.
and S. J. Zhang, RSC Adv., 2014, 4, 2360; (b) P. J. Scammells,
J. L. Scott and R. D. Singer, Aust. J. Chem., 2005, 58, 155;
(c) W. H. Awad, J. W. Gilman, M. Nyden, R. H. Harris Jr.,
T. E. Sutto, J. H. Callahan, D. M. Fox, H. C. De Long and
P. C. Trulove, Thermochim. Acta, 2004, 409, 3.
22 (a) J. Q. Wang, J. Sun, C. Y. Shi, W. G. Cheng, X. P. Zhang
and S. J. Zhang, Green Chem., 2011, 13, 3213; (b) J. Sun, 31 (a) D. J. Heldebrant, C. R. Yonker, P. G. Jessopand and
J. Q. Wang, W. G. Cheng, J. X. Zhang, X. H. Li, S. J. Zhang
and Y. B. She, Green Chem., 2012, 14, 654; (c) J. Sun,
W. G. Cheng, W. Fan, Y. H. Wang, Z. Y. Meng and
S. J. Zhang, Catal. Today, 2009, 148, 361; (d) W. G. Cheng,
X. Chen, J. Sun, J. Q. Wang and S. J. Zhang, Catal. Today,
2013, 200, 117; (e) J. Sun, L. J. Han, W. G. Cheng,
J. Q. Wang, X. P. Zhang and S. J. Zhang, ChemSusChem,
2011, 4, 502; (f) J. Q. Wang, J. Sun, W. G. Cheng, C. Y. Shi,
L. Phan, Energy Environ. Sci., 2008, 1, 487; (b) Z. Z. Yang,
L. N. He, Y. N. Zhao, B. Liand and B. Yu, Energy Environ.
Sci., 2011, 4, 3971; (c) Z. Z. Yang, L. N. He, C. X. Miao and
S. Chanfreau, Adv. Synth. Catal., 2010, 352, 2233;
(d) X. Y. Li, M. Q. Hou, Z. F. Zhang, B. X. Han, G. Y. Yang,
X. L. Wang and L. Z. Zou, Green Chem., 2008, 10, 879;
(e) C. Wang, H. Luo, X. Luo, H. Li and S. Dai, Green Chem.,
2010, 12, 870.
3304 | Green Chem., 2014, 16, 3297–3304
This journal is © The Royal Society of Chemistry 2014