Highly efficient conversion of CO2 to cyclic carbonates with a binary catalyst system in a microreactor: Intensification of "electrophile-nucleophile" synergistic effect

Article abstract of DOI:10.1039/c8ra07236a

An intensification of the "electrophile-nucleophile" synergistic effect was achieved in a microreactor for the coupling reaction of CO₂ and epoxides mediated by the binary Al complex/ternary ammonium salt catalyst system. The microreactor techn

Full text of DOI:10.1039/c8ra07236a

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Highly efficient conversion of CO2 to cyclic
carbonates with a binary catalyst system in
a microreactor: intensification of “electrophile–
nucleophile” synergistic effect†
Cite this: RSC Adv., 2018, 8, 39182
*
Ming-Ran Li, Ming-Chao Zhang, Tian-Jun Yue, Xiao-Bing Lu and Wei-Min Ren
Received 30th August 2018
Accepted 16th November 2018
An intensification of the “electrophile–nucleophile” synergistic effect was achieved in a microreactor for the
coupling reaction of CO2 and epoxides mediated by the binary Al complex/ternary ammonium salt catalyst
system. The microreactor technology is proven to be a powerful tool for the preparation of cyclic
carbonates with an improved reaction rate and a wide substrate scope.
DOI: 10.1039/c8ra07236a
rsc.li/rsc-advances
presence of a nucleophilic co-catalyst, such as organic base or
quaternary ammonium salt, is benecial to obtain cyclic
carbonates efficiently for most metal complexes.40 A widely
Introduction
The chemical xation of carbon dioxide (CO2) into economically
competitive products has attracted much attention, as CO2 is
one of the most important greenhouse gases, as well as being an
abundant, inexpensive, nontoxic, and renewable C1 resource.1–6
One of the most studied reactions for the use of CO2 is the
coupling reaction of CO2 with epoxides to afford cyclic
carbonates,7–10 which are widely used as electrolytes in lithium-
ion secondary batteries11,12 and polar aprotic solvents,13,14 as
well as intermediates in organic synthesis.15–19 Successful
industrial processes for the preparation of cyclic carbonates
from the CO2/epoxides coupling reaction have been realized for
more than 50 years, however, efficient transformations usually
required high catalyst loadings, elevated CO2 pressures, and
long reaction time.
In the past decades, various catalysts have been developed
for the coupling reaction of CO2 with epoxides, such as metal
oxides,20,21 alkali metal salts,22,23 organic bases,24,25 ionic
liquids,26–28 metal complexes,29–31 and so on. Prominent among
these are the homogeneous catalyst systems based on well-
dened metal complexes.32–37 These catalyst systems provide
a high activity and product selectivity for the coupling reaction
of CO2 with various epoxides even under mild reaction condi-
tions. For example, Ema et al. developed a bifunctional
magnesium porphyrin catalyst showing a very high turnover
number (TON up to 103 000) for this coupling reaction.38 Kleij
and coworkers also reported an aluminium complex exhibiting
an unprecedented high activity (TOF up to 36 000 hꢀ1) in the
formation of cyclic carbonates.39 It is worth noting that the
accepted mechanism concerning epoxide ring-opening and CO2
activation was shown in Scheme 1 when a binary metal
complex/quaternary ammonium salt system was employed.
According to this mechanism, the activation of epoxide coor-
dinated to the electrophilic metal center and further ring-
opened by the attack of nucleophilic agent is the rate-
determining step during the coupling reaction. In this
context, an intensication of the “electrophile–nucleophile”
synergistic effect might help to further improve the catalytic
activity of the binary systems.
Microreactor technology is regarded as a promising process
intensication strategy for chemical synthesis due to its unique
characteristics, such as high effective surface-to-volume ratio,
enhanced heat- and/or mass-transfer rates, and excellent
process safety.41–45 In 2013, Chen and co-workers studied the
coupling reaction of CO2 with propylene oxide (PO) in a micro-
reactor using a hydroxyl-functionalized quaternary ammonium
State Key Laboratory of Fine Chemicals, Dalian University of Technology, 2 Linggong
Road, Dalian, 116024, China. E-mail: wmren@dlut.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra07236a
Scheme 1 Catalytic cycle for CO2/epoxide coupling reaction medi-
ated by the binary “electrophile–nucleophile” system.
39182 | RSC Adv., 2018, 8, 39182–39186
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salt as catalyst.46 The reaction rate is signicantly improved in the gas–liquid separator. The liquid product sample was
compared to the conventional stirred reactor with the use of the collected and weighted for the calculation of the yield. Addi-
same reaction conditions. Herein, we wise to further explore the tionally, a small amount of the crude product was dissolved in
application of microreactor technology in the coupling reaction methanol, and analyzed by Agilent 7890B gas chromatography
of CO2 with epoxides. The aluminium salen complex in (GC with ame ionization detector, HP-5 column, 30 m ꢁ
conjunction with a quaternary ammonium salt was chosen as 0.32 mm ꢁ 0.25 mm) for determining the cyclic product selec-
a binary catalyst system for this transformation since it is tivity. Each set of experiments was repeated three times to
robust, easily prepared and recyclable. The main purpose of this ensure the accuracy of the data.
study is to verify the intensication of the “electrophile–nucle-
ophile” synergistic effect for epoxides ring-opening, decrease
the reaction time from hours to seconds, and expand the
Results and discussion
substrate scope.
We initially chose PO as a model substrate to test the catalytic
activity of the binary system in the microreactor. Neither the
Experimental
Materials
aluminum complex (0.5% mol) nor TBAB (1% mol) alone can
mediate the PO/CO2 coupling reaction with satisfactory
conversion (0 and 36%, respectively) at 150 ꢂC and 2.0 MPa CO2
PO and tetrabutylammonium bromide (TBAB) purchased from
pressure (Table 1, entries 1 and 2). When the aluminum
Sinopharm Chemical Reagent Co. Ltd. Carbon dioxide (99.99%)
complex (0.5% mol) in combination with TBAB (1% mol) was
was used as received.
employed as a binary catalyst system for the coupling reaction,
the conversion of PO increased dramatically (entry 3). With
a further increase in the loading of TBAB, quantitative conver-
Experimental procedure
The microreactor is consisted of two stainless-steel plates with
microstructures that combine to form a complex sloped struc-
ture channel (Fig. 1). The geometric size of the mixing channel
is 300 mm ꢁ 300 mm ꢁ 1 cm (length). The uid continuously
rises and falls as it ows through the channel, achieving
multiple separation and recombination to complete the ample
mixing process.
The aluminum catalyst and TBAB were dissolved in epoxides
and pumped by a ow pump into the reaction system. On the
other hand, the CO2 high-pressure gas cylinder was connected
to the pipeline via a gas ow meter and the ow rate was
controlled by the gas quality controller. The two-phase mate-
rials were mixed in microreactor and fully reacted via a delay
reactor (1/8 inch 316L stainless steel tube, wall thickness 0.5
mm, 22.83 mL). Both the microreactor and the delay reactor
located in the oil bath were regarded as the residence time unit
for regulating the reaction time. The temperature and pressure
controlled by the back pressure valve of the reaction system
were detected by sensors and transmitted to the data acquisi-
tion system, making the data uctuation observation more
intuitive. Then, the gas–liquid reaction mixture was separated
sion of PO was achieved with a residence time of 48.8 s (entry 5).
In this case, the turnover frequency (TOF) value was up to 14 700
hꢀ1, which is signicantly higher than the value (5160 hꢀ1) in
a conventional stirred reactor (entry 6).47 In contrast, a yield of
90% was observed under the same conditions when TBAB (5%
mol) was used exclusively (entry 7). These results suggest that
the “electrophile–nucleophile” synergistic effect of the binary
catalyst system can be intensied in the microreactor.
In order to further verify the intensication, the effects of the
residence time on the conversion of PO was studied. The resi-
dence time can be dened as the total volume of the reaction
line divided by the gas–liquid mixing ow rate in the micro-
reactor. In this text, changing the ow rate of CO2 and PO in the
Table 1 Coupling reaction of CO2 and POa
Entry
[Al]/[PO]
[TBAB]/[PO]
Yieldb (%)
<1
36
62
85
>99
35
TOFc (hꢀ1
)
1
2
3
4
0.5%
0
0
—
—
1%
1%
3%
5%
5%
5%
0.5%
0.5%
0.5%
0.5%
0
9150
12 540
14 700
5160
—
5
6d
7
90
a
Typical reaction conditions: 150 ꢂC, 2.0 MPa, [CO2]/[PO] ¼ 2 : 1, (CO2
ow rate of 385 mL minꢀ1 under standard conditions and PO ow rate
of 0.6 mL minꢀ1) residence time 48.8 s for entries 1–5. The propylene
b
carbonate selectivity is >99% based on GC. Yield of propylene
c
carbonate. Turnover frequency (TOF) ¼ moles of cyclic carbonate per
d
Fig. 1 The experimental setup and illustration of the microreactor
mole of Al-catalyst per hour. The coupling reaction was performed
in a conventional stirred reactor for 49 s.
inner structure.
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Fig. 3 Effect of the molar ratio of CO2/PO on the yield of propylene
carbonate, with the use of the binary catalyst system at 140 ꢂC and
2.0 MPa CO2 pressure. [Al]/[TBAB]/[PO] ¼ 0.5%/5%/1.
Fig. 2 Plots of PC conversion vs. time, with the use of the binary
catalyst system at 150 ꢂC and 2.0 MPa CO2 pressure. [Al]/[TBAB]/[PO]
¼ 0.5%/5%/1, [PO]/[CO2] ¼ 1/2.
of ethylene carbonate with a high reaction rate of TOF up to
29 000 hꢀ1 was achieved under a residence time of 24.4 s
(entry 8). For further expanding the substrate scope, some
disubstituted or internal epoxides were chosen as reaction
partners. All of the substrates studied were conveniently
same proportion can adjust the residence time of the reaction
while maintaining other conditions unchanged. For example,
the residence time of 48.8 s corresponded to CO2 ow rate of
385 mL minꢀ1 and PO ow rate of 0.6 mL minꢀ1, while the
residence time of 6.1 s can be got by increasing the CO2 ow rate
to 3080 mL minꢀ1 and PO ow rate to 4.8 mL minꢀ1. Thus,
a series of plots of conversion versus residence time was ob-
tained with a reaction condition of 150 ꢂC and 2.0 MPa CO2
pressure (Fig. 2). The results show that the yield of propylene
carbonate reaches >98% when the residence time is 30 s. More
importantly, the reaction rate did not show an obvious decrease
even the yield of propylene carbonate up to 95%. By contrast,
the reaction rate was gradually decreased aer the yield of cyclic
carbonate reaches 60% in a conventional stirred reactor, and it
may take about half of the total reaction time to convert the
remaining ꢃ20% PO.48
Table 2 Coupling reaction of CO2 and various epoxidesa
Another advantage of the microreactor, compared with the
conventional stirred reactor, is that changing the ratio of CO2 to
PO does not affect the reaction pressure.49 Therefore, we
examined the effects of the molar ratio of CO2 to PO on the
reaction rate under the same CO2 pressure. When the coupling
reaction was performed at 140 ꢂC and 2.0 MPa CO2 pressure
with the residence time of 48.8 s or 24.4 s, an increase in the
molar ratio of CO2 to PO from 2/1 to 4/1 does not lead to an
observable change in the yield of propylene carbonate (Fig. 3).
These results imply the variation of the CO2/PO molar ratio has
little effect on the “electrophile–nucleophile” synergistic effect
for epoxides ring-opening. The very slight uctuation in the
yield of propylene carbonate may be due to the small change of
PO concentration in the liquid phase with the different CO2/PO
molar ratios. Unfortunately, we did not obtained the data with
a CO2/PO molar ratio of 1/1, because the consumption of CO2
may lead to a loss of accuracy in the calculation of the residence
time.
Entry
Substrate
Residence time (s)
Yieldb (%)
TOFc (hꢀ1
)
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
1g
1h
1i
48.8
97.6
48.8
97.6
97.6
97.6
97.6
24.4
97.6
97.6
97.6
97.6
97.6
>99
>99
>99
97
96
96
98
>99
97
14 700
7300
14 700
7100
6860
7000
7200
29 000
7100
6700
6800
6400
6000
9
10
11
12
13
1j
1k
1l
91d
92e
87f
81
To explore the wider applicability of the microreactor,
a series of monosubstituted terminal epoxides and ethylene
oxide were tested. All the terminal cyclic carbonates were ob-
tained with excellent yields (>90%) and high selectivity (99%)
1m
a
Typical reaction conditions: 150 ꢂC, 2.0 MPa, [Al]/[TBAB]/[PO] ¼ 0.5%/
5%/1, [CO2]/[PO] ¼ 2 : 1. The cyclic carbonate selectivity is >99% based
b
c
on GC or NMR spectroscopy. Yield of cyclic carbonate. Turnover
ꢂ
under 150 C and 2.0 MPa CO2 pressure within the residence
frequency (TOF) ¼ moles of cyclic carbonate per mole of Al-catalyst
d
e
f
time of less than 100 s (Table 2, entries 1–7). When ethylene
oxide was employed as a substrate, the quantitative formation
per hour. Cis/trans ¼ 86 : 14. Cis/trans ¼ 12 : 88. Cis/trans ¼
1
38 : 62. The cis/trans ratios were determined by H NMR spectroscopy.
39184 | RSC Adv., 2018, 8, 39182–39186
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converted into the corresponding carbonates with high selec- 12 J. Mindemark, L. Imholt, J. Montero and D. Brandell, J.
tivity (>99%) and good to excellent yields, indicating the high
versatility of the microreactor technology.
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Conflicts of interest
21 M. N. Timofeeva, A. E. Kapustin, V. N. Panchenko,
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22 H. Chang, Q. Li, X. Cui, H. Wang, Z. Bu, C. Qiao and T. Lin, J.
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and the ow rate of the mixture mainly depended on the
CO2. Therefore, the same pressure, same residence time
with different molar ratio of PO and CO2 can be realized,
which is more controllable than the conventional stirred
reactor.
39186 | RSC Adv., 2018, 8, 39182–39186
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