CO2atmosphere enables efficient catalytic hydration of ethylene oxide by ionic liquids/organic bases at low water/epoxide ratios

Article abstract of DOI:10.1039/d1gc00758k

The development of an efficient and low-cost strategy for the production of monoethylene glycol (MEG) through hydration of ethylene oxide (EO) at low H₂O/EO molar ratios is an important industrial challenge. We have established that by using CO₂as the reaction atmosphere, hydration of EO can be achieved at a low H₂O/EO ratio of 1.5?:?1 along with high yields (88-94%) and selectivities (91-97%) of MEG catalyzed by binary catalysts of ionic liquids and organic bases. The results are significantly better than those of experiments conducted under an atmosphere of N₂. Isotope labeling experiments revealed that CO₂had altered the reaction pathway and participated in the reaction, in which cycloaddition of EO with CO₂occurred first followed by the hydrolysis of ethylene carbonate (EC) to generate MEG and recover CO₂. The ionic liquids and organic bases synergistically catalyzed the one-pot two-step reaction. DFT calculations confirmed that this route is more kinetically favorable compared to the pathway of direct epoxide hydration.

Full text of DOI:10.1039/d1gc00758k

Green Chemistry
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CO atmosphere enables efficient catalytic
2
hydration of ethylene oxide by ionic liquids/
organic bases at low water/epoxide ratios†
Cite this: Green Chem., 2021, 23,
386
3
a
b
a
a
a
b
Tong Ding, Jinyin Zha, Dawei Zhang,* Jingshun Zhang, Huixia Yuan, Fei Xia
*
a
and Guohua Gao
*
The development of an efficient and low-cost strategy for the production of monoethylene glycol (MEG)
through hydration of ethylene oxide (EO) at low H O/EO molar ratios is an important industrial challenge.
We have established that by using CO as the reaction atmosphere, hydration of EO can be achieved at a
low H O/EO ratio of 1.5 : 1 along with high yields (88–94%) and selectivities (91–97%) of MEG catalyzed
by binary catalysts of ionic liquids and organic bases. The results are significantly better than those of
experiments conducted under an atmosphere of N . Isotope labeling experiments revealed that CO had
altered the reaction pathway and participated in the reaction, in which cycloaddition of EO with CO
2
2
2
2
2
2
Received 28th February 2021,
Accepted 1st April 2021
occurred first followed by the hydrolysis of ethylene carbonate (EC) to generate MEG and recover CO2.
The ionic liquids and organic bases synergistically catalyzed the one-pot two-step reaction. DFT calcu-
lations confirmed that this route is more kinetically favorable compared to the pathway of direct epoxide
hydration.
DOI: 10.1039/d1gc00758k
rsc.li/greenchem
Catalytic production of MEG under low H
2
O/EO ratios has also
Introduction
8
been achieved in select cases. Silica-based nanocages encap-
III
Monoethylene glycol (MEG) has many industrial applications, sulating Co (salen) or Lewis acids have shown an excellent
such as for the manufacture of antifreeze and coolants, poly- MEG selectivity with H O/EO ratios approaching the stoichio-
2
1
8a,b
ester fibers and resins, desiccants and fuel cells. The global metric value of the reaction.
production of MEG was 30.5 million metric tons annually heterogeneous catalysts are complicated to prepare, limiting
MMTA) in 2019, which was mainly produced through the their practical applications. The development of an effective
industrial process of liquid-phase thermal hydration of ethyl- and simple catalytic strategy for the hydration of EO with low
Note that these sophisticated
(
2
ene oxide (EO). However, a high H
0–25) was required in such a process in order to achieve a
high conversion of EO and a high selectivity of MEG. The large extensive attention in recent years. A number of reports have
excess of water that had been used resulted in additional documented the use of CO as the raw material to produce
energy consumption and cost for the distillation of the final value-added chemicals, such as carbonates, urea derivatives,
2
O/EO molar ratio (around
H
2
O/EO ratios is still a challenge.
2
CO utilization through chemical methods has received
2
9
2
1
0
MEG product.
carbamates, carboxylic acids, alkanes, and alcohols.
A series of catalysts have thus been developed for the Moreover, the use of CO
2
to promote reactions has been
1
a
hydration of EO to obtain a high selectivity of MEG, such as reported as well, which provides a novel strategy to efficiently
3
4
5
11
sulfuric acid, amines, soluble salts, ion-exchange resins,
control the reactivity and selectivity of reactions. Several
studies have focused on CO -promoted hydration of pro-
6
7
acidic poly(ionic liquid)s, and supported metal oxides.
2
pargylic alcohols in the presence of metal or ionic liquid cata-
lysts, which involves the generation of α-alkylidene cyclic car-
a
bonates by the fixation of CO
produce α-hydroxy ketones and regenerate CO
process, CO was not consumed, but promoted the reaction to
2
followed by in situ hydrolysis to
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of
1
2
Chemistry and Molecular Engineering, East China Normal University, Shanghai,
2
.
In the whole
2
b
00062, China. E-mail: dz302@cam.ac.uk, ghgao@chem.ecnu.edu.cn
2
School of Chemistry and Molecular Engineering, NYU-ECNU Center for
Computational Chemistry at New York University Shanghai, East China Normal
University, Shanghai, 200062, China. E-mail: fxia@chem.ecnu.edu.cn
generate the product effectively and selectively. The role of CO
2
as a promoting reagent (or cocatalyst) to alter reaction path-
ways inspired us to explore this methodology for the hydration
of epoxides.
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1gc00758k
3386 | Green Chem., 2021, 23, 3386–3391
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Herein, we present the strategy of using CO
2
as the reaction ratio of H
2
O/EO as low as 1.5 : 1 (entry 1, Table 1). Other reac-
atmosphere to promote the hydration of EO catalyzed by tion conditions such as the amount of catalyst, CO pressure
2
simple catalysts of ionic liquids and organic bases. A high and the H
yield and selectivity for MEG were achieved at a H O/EO ratio condition was found to give the best performance in terms of
as low as the stoichiometric value of the reaction. CO2 was the yield and selectivity of MEG and the amount of water used.
found to participate in the reaction through the cycloaddition In contrast, an atmosphere of N led to a significantly lower
2
O/EO ratio were also tested (Table S1†), while this
2
2
of EO followed by in situ hydrolysis to generate MEG and yield (21%) and selectivity (32%) for MEG, accompanied by the
recover CO , allowing the reaction to occur efficiently at low formation of considerable amounts of by-products such as
2
H
2
O/EO ratios.
DEG (12%), TEG (7%) and other polymeric ethylene glycols
(
entry 2, Table 1).
Various combinations of VBImBr with other common
organic bases bearing varying basicities such as DBU, DMAP,
DAA, TAA and 4-VP were also tested, and all of them exhibited
significantly better catalytic performances under an atmo-
Results and discussion
CO
2
-promoted catalytic hydration of EO with ionic liquids/
organic bases
2 2 2
sphere of CO than those under N . The reactions under CO
Hydrations of EO catalyzed by various binary ionic liquid/ generally afforded MEG in yields of 88–91% and selectivities of
organic base catalysts under a CO or N atmosphere were per- 94–96%, while only 11–17% in yields and 18–24% in selectiv-
2
2
formed (Table 1). Under the CO
2
2
atmosphere, the hydration of ities were obtained under N (entries 3–12, Table 1). Similarly,
EO catalyzed by VBImBr/VIm (1:1, 2 mol%) gave rise to an the combination of the base VIm with other ionic liquids
excellent yield (89%) and selectivity (97%) of MEG at a molar (BMImBr, BnImBr, BdMImBr, and Bu
NBr) also gave a 4–5
4
a
Table 1 Hydration of EO under an atmosphere of CO
2
or N
2
catalyzed by various ionic liquids/organic bases
b
Catalysts
Yield (%)
Entry
Ionic liquid
VBImBr
Organic base
VIM
Reaction atmosphere
MEG
EC
DEG
TEG
Selectivity of MEG (%)
1
2
3
4
5
6
7
8
9
CO
2
2
2
2
2
2
89
21
91
11
90
13
91
13
90
17
88
13
1
—
2
—
3
—
2
—
3
—
3
1
12
1
9
1
9
1
10
1
11
1
—
7
—
6
—
7
—
7
—
6
—
6
97
32
96
18
95
18
96
19
95
24
94
22
N
2
VBImBr
VBImBr
VBImBr
VBImBr
VBImBr
DBU
DMAP
DAA
TAA
CO
N
2
CO
N
2
CO
N
2
CO
1
1
1
0
1
2
N
2
4-VP
CO
N
2
—
7
13
14
15
16
17
18
19
20
BMImBr
BnBImBr
BdMImBr
VIm
VIm
VIm
VIm
CO
2
2
2
2
93
13
94
15
93
13
90
11
2
—
1
—
3
—
1
1
9
1
10
1
10
4
10
—
6
—
7
—
7
—
7
96
20
97
19
95
19
91
16
N
2
CO
N
2
CO
N
2
Bu
4
NBr
CO
N
2
—
a
Reaction conditions: EO (2.2 g, 50 mmol), H
00 °C, 3 h, 1.5 MPa. Monoethylene glycol (MEG), ethylene carbonate (EC), diethylene glycol (DEG), and triethylene glycol (TEG).
2
O (1.35 g, 75 mmol), and ionic liquid (0.5 mmol, 1 mol%)/organic base (0.5 mmol, 1 mol%),
b
1
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2
times higher yield and selectivity of MEG under CO than
under N (entries 13–20, Table 1). The generation of DEG and
2
2
TEG under the atmosphere of N was owing to the conden-
3
,4
sation of MEG with EO, while no noticeable DEG or TEG
was detected for the reactions conducted under CO2.
Note that all the above-mentioned results were obtained at
a H O/EO ratio of 1.5:1. A yield of 80% and a selectivity of 93%
2
for MEG were also achieved when a stoichiometric H O/EO
2
ratio (1 : 1) was used for the reaction catalyzed by VBImBr/VIm
2
under CO (Table S2†). This performance was even better than
the hydration of EO under N at a H O/EO ratio of 15/1 in the
2
2
presence of the same catalysts (Table S2†). The remarkable per-
Fig. 2 (a) Proposed two-step reaction mechanism for the hydration of
formance of the current catalytic system is comparable to the EO under CO
pioneering heterogeneous catalysts reported previously,
2
. (b) Isotope labeling experiments for the investigation of
18
8
a,b
the EO hydration mechanism using isotope-labeled water (H
2
O).
while the simplicity of the current catalysts and the versatile
combination of various ionic liquids and organic bases are
noteworthy. Furthermore, the excellent catalytic performance
To confirm the proposed reaction pathway, three isotope
1
8
of imidazolium ionic liquids and organic bases with polymer- labeling experiments using isotope-labeled water (H
2
O) were
izable vinyl/allyl groups may provide an opportunity to hetero- conducted and the products were analyzed by GC-MS. The
1
8
genize the homogeneous catalysts.
direct hydration of EO with H
O under N afforded isotope-
2 2
labeled MEG (m/z 64) as the main product (eqn (1) in Fig. 2b
1
8
Mechanism investigation through isotope labeling
experiments and DFT calculations
and Fig. S1†), indicating that the O was transferred from
1
8
18
H2 O to MEG. In contrast, the hydration of EO with H2
under the CO atmosphere predominantly gave unlabeled
MEG (m/z 62), while O was found to be transferred from
H2 O to C OO (m/z 46) (eqn (2) in Fig. 2b and Fig. S2†).
Similarly, the hydrolysis of EC with H2 O also produced
unlabeled MEG (m/z 62) as the main product, while isotope-
labeled C OO (m/z 46) was detected (eqn (3) in Fig. 2b and
Fig. S3†). These results support the proposed two-step mecha-
nism for the catalyzed reaction under CO
addition of EO with CO occurs first, followed by the hydrolysis
O
The detection of trace amounts of ethylene carbonate (EC) for
the reactions under CO2 implied that CO2 may have partici-
pated in the reaction, affording cycloaddition of EO catalyzed
2
1
8
1
8
18
1
8
1
3–16
by ionic liquids and organic bases.
The presence of this
intermediate was further confirmed through monitoring its
amounts at different reaction times. As shown in Fig. 1, a con-
siderable amount of EC (19%) was generated at the beginning,
while its concentration gradually decreased and almost dis-
appeared after 180 min (1%), accompanied by an increase of
the MEG yield. These observations indicate a one-pot two-step
procedure for the synthesis of MEG (Fig. 2a). The first step
1
8
2
, in which the cyclo-
2
1
8
18
of EC with H
2
O to generate MEG and C OO.
The proposed mechanism was further verified by designing
1
8
cascade reactors. Hydration of EO with H2 O under a CO2
2
involves cycloaddition of EO with CO to form EC, and the
second step is the hydrolysis of EC to produce MEG and
atmosphere was designed to occur in the first reactor, and
cycloaddition of EO with the released CO vented from the
2
first reactor took place in the second reactor (Fig. S4†). It was
found that after completion of the cascade reactions, the
regenerate CO . Apart from the binary ionic liquid/organic
2
2
base catalysts, CO has not been consumed either, which can
be treated as a promoting reagent or a co-catalyst for the
hydration of EO.
1
8
18
isotope O of H
to the product in the second reactor, forming O-labeled EC
m/z 90) (Fig. S5†), through the medium of CO
This one-pot two-step reaction mechanism has been investi-
gated by DFT calculations. As shown in the red pathway in
2
O in the first reactor had been transferred
1
8
(
2
.
−
1
Fig. 3, the ring of EO in the complex Int-0 (0.0 kcal mol ) is
first activated by VBImBr through the transition state TS-1 to
generate the ring-opening intermediate Int-2. Then, CO adds
2
to the hydroxyl oxygen atom of Int-2 to yield Int-3, followed by
the formation of the EC ring via TS-4. The oxygen atom of a
H O molecule attacks the carbonyl carbon of EC in Int-5, while
2
another hydrogen atom of the water is abstracted by VIm.
Meanwhile, VBImBr could also stabilize the transition state
TS-6 by forming a H bond with the C–O double bond of EC.
The yielded product Int-7 could complex with another water
molecule by forming H bonds in Int-8. Through a hexa-ring
transition state TS-9, the penta-ring is opened to yield Int-10,
Fig. 1 Yields of MEG and EC at different reaction times in the hydration
of EO. Reaction conditions: EO (2.2 g, 50 mmol), H
CO (1.5 MPa), VBImBr (0.5 mmol, 1 mol%)/VIm (0.5 mmol, 1 mol%),
00 °C.
2
O (1.35 g, 75 mmol),
2
1
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which leads to the generation of the product MEG with the
release of a CO molecule.
2
Based upon the DFT results, the rate-determining step of the
red pathway is the ring-opening step of EO via TS-1, with an
−1
energy barrier of 32.3 kcal mol . The barrier is lower than that
of direct hydration of EO under N
2
via TS-11 (the pathway in Fig. 4 Reactions of the product MEG with EO (a) and EC (b). Reaction
−1
conditions: EO (2.2 g, 50 mmol) (a) or EC (4.4 g, 50 mmol) (b), MEG
black in Fig. 3) by 6.8 kcal mol , which indicates that the CO
involved pathway is more kinetically favorable. In particular, this
2
-
(
1
3.10 g, 50 mmol), and VBImBr (0.5 mmol, 1 mol%)/Vim (0.5 mmol,
mol%), 100 °C, 3 h, 1.5 MPa.
−1
barrier is also 5.7 kcal mol lower than that of the blue conden-
sation pathway of MEG with EO via TS-12, which leads to the
side product DEG. The pathway of hydration of EO involving
for our new strategy using CO as the reaction atmosphere, the
intermediate EC does not react with MEG, allowing for the
2
2
CO is more favorable than the side reaction of EO with MEG,
which highlights the origin of the high selectivity of MEG.
The origin of the MEG selectivity has been further investi-
gated by experiments. Reactions of the product MEG with the
starting material EO and the intermediate EC were respectively
tested (Fig. 4). It was observed that the reaction of EO with
MEG catalyzed by VBImBr/VIm afforded DEG, TEG and other
polymeric ethylene glycols with a total yield of 61%. However,
no reaction of EC with MEG was observed to occur in the pres-
ence of VBImBr/VIm. In fact for the conventional hydration of
2
high selectivity of MEG at low H O/EO ratios.
Synergistic catalysis of ionic liquids and organic bases
Cycloaddition of EO with CO and hydrolysis of EC catalyzed
2
respectively by VBImBr/VIm (1:1), VBImBr or VIm were carried
out, in order to investigate the role of each component of the
1
7–20
binary catalysts.
of EO with CO , VBImBr and VIm gave EC in the yields of 85%
and 22%, respectively, indicating the higher catalytic activity of
As shown in Fig. 5a, for the cycloaddition
2
2
EO without CO , the reactivity of MEG with EO leads to the for-
1
3
the ionic liquid than the organic base. In contrast, for the
hydrolysis of EC to MEG, VIm gave a significantly higher yield
for MEG (79%) than did VBImBr (6%) (Fig. 5b), implying the
higher catalytic activity of the organic base than the ionic
liquid in this case. These results were rationalized by DFT cal-
culations (Fig. S7†). The combination of VBImBr and VIm (1:1)
afforded excellent performance in both the yields of EC (78%)
and MEG (87%) for the two separate reactions.
mation of by-products (DEG and TEG), for which high H O/EO
2
1
,3–7
ratios are required to prevent the side reactions.
Note that
We have also explored the effect of the molar ratio of the
binary catalysts on the hydration of EO under a CO2 atmo-
sphere. As shown in Table 2, when VIm was used alone
(
n(VBImBr)/n(VIm) = 0:1), the reaction gave a 10% yield of DEG
and a 74% selectivity of MEG. If VBImBr was used alone
n(VBImBr)/n(VIm) = 1:0), the reaction afforded a 28% yield of
(
EC and only a 68% selectivity of MEG (entries 1 and 2,
Table 2). These results are consistent with the observations
made above that the organic bases or the ionic liquids indivi-
dually catalyzed a single step of the two-step process efficien-
tly, in which ionic liquids were beneficial for the cycloaddition
2
of CO and organic bases were efficient for the hydrolysis of
Fig. 5 Various catalysts for the cycloaddition of EO with CO
the hydrolysis of EC (b). Reaction conditions: EO (2.2 g, 50 mmol) (a) or
EC (4.4 g, 50 mmol) and H O (1.35 g, 75 mmol) (b), CO (1.5 MPa), and
catalyst (1 mmol, 2 mol%), 100 °C, 3 h.
2
(a) and
2 2
Fig. 3 Calculated free energy profiles of reactions under CO or N , in
the presence ionic liquid/organic base catalysts. Optimized structures of
intermediates and transition states are shown below.
2
2
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Table 2 Effect of the molar ratio of VBImBr and VIm on the hydration dimethyl-imidazolium bromide (BdMImBr) and tetrabutyl-
a
of EO
ammonium bromide (Bu NBr) were purchased from the
4
Centre for Green Chemistry and Catalysis, LICP, CAS. 1-Benzyl-
Yield/%
3
-butylimidazolium bromide (BnBImBr) was synthesized
1
Entry n (VBImBr)/n (VIm) MEG EC DEG Selectivity of MEG/% according to the literature. Ethylene oxide (EO), monoethyl-
ene glycol (MEG), diethylene glycol (DEG) and triethylene
glycol (TEG) were supplied by Sinopharm at CP grade.
1
2
3
4
5
0 : 1
1 : 0
1 : 2
1 : 1
2 : 1
61
61
90
89
87
—
28
1
1
4
10
1
4
2
1
74
68
95
97
94
Hydration of EO
In a typical experiment, EO (2.20 g, 50 mmol), deionized water
a
Reaction conditions: EO (2.2 g, 50 mmol), H
2
O (1.35 g, 75 mmol),
(
(
1.35 g, 75 mmol), VBImBr (0.116 g, 0.5 mmol) and VIm
0.047 g, 0.5 mmol) were added into a 30 mL stainless steel
CO
h.
2
(1.5 MPa), and catalysts: VBImBr + VIm (1 mmol, 2 mol%), 100 °C,
3
Teflon-lined autoclave equipped with a magnetic stirrer and an
autoclave with an automatic temperature control system. The
2 2
autoclave was pressurized with CO or N (1.5 MPa) and heated
to 100 °C. After stirring for 3 h, the autoclave was cooled to
room temperature in an ice-water bath, followed by slow
2
EC in the CO -promoted hydration of EO. The combination of
VBImBr and VIm in the ratio of 1 : 2, 1 : 1 and 2 : 1 gave rise to
both high yields and high selectivities of MEG with trace
amounts of DEG and EC being detected (<4%) (entries 3–5,
Table 2 and Fig. S6†), demonstrating the synergistic effect of
the binary catalysts.
2 2
venting of CO or N . The reaction mixtures were analyzed by
gas chromatography (GC) (GC-2014, Shimadzu, Japan) with a
DM-1701 (30 m × 0.53 mm × 1.0 μm) capillary column and a
flame ionization detector (FID) and biphenyl was used as the
internal standard.
Conclusions
Conflicts of interest
In summary, a new strategy for the selective synthesis of MEG
has been established via CO -promoted hydration of EO cata-
2
There are no conflicts to declare.
lyzed by ionic liquids and organic bases. The presence of CO
2
significantly improved the catalytic performance for EO
hydration, giving excellent yields and selectivities of MEG at a
H O/EO ratio as low as 1 : 1. Mechanism investigations indi-
2
Acknowledgements
cated a one-pot two-step process including the cycloaddition
of EO with CO2 and the hydrolysis of EC, which was fully
demonstrated by isotope labeling experiments and theoretical
This work was supported by the National Key Research and
Development Program of China (2017YFA0403102) and the
National Natural Science Foundation of China (21773068,
2
calculations. During the process, CO played the role of a pro-
21811530273).
moting reagent or a co-catalyst in that it was involved in the
reaction but was not consumed. Importantly, the EC formed
2
through the cycloaddition of EO with CO was found to be a
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