A rapid and effective synthesis of propylene carbonate using a supercritical CO2-ionic liquid system

Article abstract of DOI:10.1039/b212823c

The synthesis of propylene carbonate from propylene oxide and carbon dioxide under supercritical conditions in the presence of 1-octyl-3-methylimidazolium tetrafluoroborate was achieved in nearly 100% yield and 100% selectivity within 5 minutes, whose TOF value is 77 times larger than those so far reported.

Full text of DOI:10.1039/b212823c

A rapid and effective synthesis of propylene carbonate using a
supercritical CO2–ionic liquid system
Hajime Kawanami,*ab Akiyoshi Sasaki,a Keitaro Matsuia and Yutaka Ikushima*ab
a Supercritical Fluid Research Center, National Institute of Advanced Industrial Science and Technology,
4-2-1 Nigatake, Miyagino-ku, Sendai 983-8551, Japan. E-mail: h-kawanami@aist.go.jp;
Fax: +81-22-237-5215; Tel: +81-22-237-5211
b CREST, Japan Science and Technology Corporation (JST)
Received (in Cambridge, UK) 17th January 2003, Accepted 14th February 2003
First published as an Advance Article on the web 7th March 2003
The synthesis of propylene carbonate from propylene oxide
and carbon dioxide under supercritical conditions in the
presence of 1-octyl-3-methylimidazolium tetrafluoroborate
was achieved in nearly 100% yield and 100% selectivity
within 5 minutes, whose TOF value is 77 times larger than
those so far reported.
batchwise operation under various conditions as shown in Table
1.11 Under supercritical conditions, NO32, CF3SO32, BF42 and
PF62 were examined as counter ions of the 1-ethyl-3-methyli-
midazolium ionic liquid ([C2-mim]+[X]2 salt) for the synthesis
of 1a (runs 1 to 4). Of these, the BF42 was found to be the most
effective at promoting the chemical fixation and to give a 61%
yield with 87% selectivity. Furthermore, this IL can be re-used
at least 2 times more with almost the same yield (run 5);
however, the time taken to obtain a reasonable yield was as long
as 2 h.
A series of the imidazolium ionic liquid with different alkyl
chain length from C2 to C8 was further investigated. The yield
of 2a is increased with lengthening alkyl chain (runs 4, 6 to 8).
It is quite note worthy that the presence of [C8-mim]+[BF4]2
achieves nearly 100% yield and 100% selectivity for propylene
carbonate production even at shorter reaction times around 5
minutes at the same temperature and pressure (runs 8 to 11),
whose TOF value ( = 517 h21) is 77 times larger than that of
previous reported ( = 6.7 h21).10 This could be due to an
increase in the solubilities of CO2 and 1a in IL phase with
lengthening alkyl chain of IL.12 To our surprise, lowering the
temperature from 100 °C to 60 °C can also effect the 100% yield
though time-consuming for a period of 120 minutes (runs 8 and
12). However, further decrease in temperature to 40 °C was not
allowed to proceed under the same pressure and time-
consuming conditions (run 13). Furthermore, decreasing the
pressure up to 6 MPa around a subcritical pressure of 6 MPa at
a constant temperature of 100 °C led to a remarkable decrease
in yield (run 14), and so this chemical fixation process was
favored by the supercritical conditions. We thus demonstrate
From the standpoint of the protection of environment, develop-
ment of green processes based on chemical fixation of carbon
dioxide (CO2) has been drawing much interest in industrial
chemistry, because there are a lot of possibilities that carbon
dioxide can be used as a safe and cheap C1 building block to
produce useful organic compounds.
The chemical fixation of CO2 to cyclic carbonates has been
investigated, since it is one of the most well-known successful
example of CO2 fixation to produce the carbonates which are
widely used as a starting monomer of polymers, and much more
environmentally benign process compared to that using poi-
sonous phosgene.1 To synthesize such cyclic carbonates
effectively, quite numerous homogeneous catalytic processes
have so far been reported;2 however, its production has suffered
from serious disadvantages of the separation of catalysts.2
Hence solid catalysts that possess high thermostability and easy
catalyst–product separation leading to their recycling look
promising, but the time taken to achieve the reactions in the
presence of their catalysts was more than 6 h even at higher
temperatures than 140 °C,2 and so the activity was not so
high.
Recently, we have demonstrated a remarkable promotion of
CO2 chemical fixation in supercritical CO2,3,4,5 especially at the
near-critical pressure in the presence of small amount of DMF
even without any catalysts.3 This is due to the function of DMF
as a scCO2 soluble acid–base catalyst. However, a reaction time
longer than 12 hours was required for the CO2 fixation to obtain
cyclic carbonate, because of the poor catalytic activity of
DMF.
Fig. 1 Ionic liquid for the scCO2–ionic liquid reaction system.
Table 1 Synthesis of propylene carbonate from propylene oxide with
For the sake of further improvement in the activity and
selectivity, we attempted to apply a scCO2–ionic liquid (IL)
biphasic system6 to the CO2 fixation, because IL can be used as
a prominent acid–base catalyst7,8 as well as a suitable reaction
media.9 Although in gaseous CO2 below 5 MPa, the introduc-
tion of IL into propylene carbonate synthesis from propylene
oxide led to high yields, the reaction times longer than 6 hours
were required for the complete fixation even at 110 °C (density
< 0.08 g cm23), and so the rate of reaction remains low.10 Such
a low reaction rate might be attributable to the relatively lower
density of gaseous CO2, since this type of CO2 fixation proceeds
in SN2 reaction.3 In addition, apolar organic compounds such as
epoxides and carbonates are more highly soluble in higher
density scCO2 than in gaseous CO2, whereas their solubilities in
a polar IL are negligibly small. Therefore we first demonstrate
that the application of scCO2 to this CO2 fixation achieves
nearly 100% yield at reaction time shorter than 5 minutes even
at the temperature of 100 °C as that previously reported.10
The carboxylation of propylene oxide 1a with CO2 into
propylene carbonate 2a in the presence of the ionic liquid based
on 1-methylimidazolium salt (Figure 1) was conducted in a
3-methylimidazolium-based ionic liquid in supercritical carbon dioxide
Pressure/ Temp./ Time/ Yield/ Select./
Run Ionic liquid
MPa
°C
min
%
%
1
2
3
4
5
6
7
8
9
[C2-mim]+[NO3]2
14
100
100
100
100
100
100
100
100
100
100
100
60
120
120
120
120
120
120
120
120
30
15
5
120
120
120
31
25
30
61
64a
75
95
97
99
99
98b
99
59
45
37
87
71
100
100
100
100
100
100
100
–
[C2-mim]+[CF3SO3]2 14
[C2-mim]+[PF6]2
[C2-mim]+[BF4]2
[C2-mim]+[BF4]2
[C4-mim]+[BF4]2
[C6-mim]+[BF4]2
[C8-mim]+[BF4]2
[C8-mim]+[BF4]2
14
14
14
14
14
14
14
14
14
14
14
6
10 [C8-mim]+[BF4]2
11 [C8-mim]+[BF4]2
12 [C8-mim]+[BF4]2
13 [C8-mim]+[BF4]2
14 [C8-mim]+[BF4]2
c
40
100
–
71
78
a The ionic liquid, that is used in run 4, was re-used. b TOF ( = 516 h21) is
77 times larger than that of former report (TOF = 6.7 h21, ref. 10). c No
reaction. Propylene oxide was recovered completely.
896
CHEM. COMMUN., 2003, 896–897
This journal is © The Royal Society of Chemistry 2003

Table 2 Synthesis of various carbonate in the presence of [C8-min]+[BF4]2
a
at 100 °C for 2 h in CO2
Pressure/
Run Epoxide
Carbonate
MPa
Yield/% Select./%
Scheme 1
1
14
90
99
1b
that the scCO2–IL reaction system can synthesize propylene
carbonates by chemical fixation of carbon dioxide in sat-
isfactory yields even a lower temperature of 60 °C or shorter
reaction time around 5 minutes.
2b
2c
2
14
14
100
61
100
97
Fig. 2 shows the pressure dependence of the yield of 2a in
CO2 in the presence of [C2-mim]+[BF4]2 or [C8-mim]+[BF4]2.
In the case of [C2-mim]+[BF4]2 (open circles), one can see the
maximum yield of 94 % at a lower pressure of 7 MPa, and then
a significant decrease in yield till 10 MPa. Such a decrease with
increasing pressure was also reported previousely.10 The
epoxide 1a is soluble more highly in scCO2 than in gaseous
CO2, and this decrease in yield in higher-pressure scCO2 region
could be due to less epoxide concentration in IL phase at which
the reaction occurs. The yield of 2a in the presence of [C8-
mim]+[BF4]2 was at best about 70% at lower pressures, but
increasing the pressure resulted in the improvement in yield,
giving 96% at the near critical pressure, and ultimately nearly
100% in the range above 11 MPa. Visual observation of phase
behavior through high-pressure view cell indicates that the
solubility of CO2 in [C8-mim]+[BF4]2 would be significantly
larger compared to that in [C2-mim]+[BF4]2 under the same
pressure and temperature conditions. It is further known that
such an epoxide compound can be dissolved more sufficiently
in [C8-mim]+[BF4]2 than in [C2-mim]+[BF4]2, and therefore a
marked increase in the concentrations of CO2 and epoxide in
[C8-mim]+[BF4]2 phase under scCO2 region above 11 MPa is
considered to achieve nearly 100% yield even at shorter
reaction times.
A series of epoxide substrates was examined for the synthesis
of the corresponding carbonates in the presence of [C8-
mim]+[BF4]2 at 14 MPa and 100 °C (Table 2). Carbonates with
alkyl side chain groups (runs 1, 2) were successfully synthe-
sized from each epoxide in nearly 100% yield and 100%
selectivity at reaction times shorter than 2 h, whereas the
carbonates were obtained in approximately 100% selectivity but
low yields around 62% when the phenyl substituted epoxide
were used. This is due to the low reactivity of the b-carbon atom
that would be activated by the basic site of the ionic liquid
(BF42 anion). Moreover, the yields of carbonate 2d and 2e at a
higher pressure of 14 MPa (runs 3, 5), are much higher than
those at a lower pressure of 7 MPa (runs 4, 6) in the presence of
the ionic liquid. This pressure dependence of the yield was
found to be similar to that of propylene carbonate as shown in
Table 1, in which the yield decreased with decreasing pressure.
1c
3
4
5
7
28
62
98
98
1d
1e
2d
2e
14
6
7
39
99
a Reaction time; 2h. Carbonate yields were determined by GC using
tridecane as an internal standard.
Consequently, there is a strong possibility that this reaction
system composed of scCO2-[C6 or C8-mim]+[BF4]2 can be
applied to the synthesis of various carbonates except for the
phenyl substituted one.
In conclusion, we found that under scCO2, [C8-mim]+[BF4]2
is the most effective IL for CO2 fixation to carbonate. In
particular, this scCO2–IL reaction media not only achieves
nearly 100% yield at 14 MPa and at 100 °C for the propylene
carbonate production at reaction times shorter than 5 minutes,
leading to a 77-fold faster rate of reaction than so far reported,
but also can be applied to the synthesis of various carbonates in
satisfactory yields.
Notes and references
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Y. Ikushima and M. Arai, Appl. Catal. A, 2001, 219, 259.
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5 H. Kawanami and Y. Ikushima, J. Jpn. Petrol. Inst., 2002, 45, 321.
6 L. A. Blanchard, D. Hancu, E. J. Beckman and J. F. Brennecke, Nature,
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10 J. Peng and Y. Deng, New J. Chem., 2001, 25, 639.Reaction conditions
are as follows: reactor volume 90 cm23, 110 °C, ionic liquid 2.5 mmol
(0.48 mL), pressure 2.5 MPa, 1a 100 mmol, 6 h.
11 The typical experimental procedure is as follows: ionic liquid ([C2-
mim]+[BF4]2, 0.51 mmol, 0.1 mL) was charged into a 50 cm3 reactor,
and CO2 was introduced using a high-pressure pump into the reactor at
desired temperatures. Pressure control was achieved by a back-pressure
regulator. 1a (2.0 mL, 30 mmol) is introduced by a high-pressure liquid
pump, and the reaction in CO2 was started with stirring (about 500 rpm).
After the reaction, the reactor was cooled to 0 °C and pressure was
released slowly. The crude product was analyzed compared with
authentic samples by GC, and the yields were determined by GC using
tridecane as an internal standard. The crude product (yield: 61% by GC)
was purified by distillation and 2a was obtained in 48% (1.43 g, 14
mmol) as an isolated yield.
Fig. 2 Pressure dependence of the yield of propylene carbonate in the
presence of [C8-mim]+[BF4]2 and [C2-mim]+[BF4]2 at 100 °C in CO2.
Reaction time is 2 h.
12 H. Oliver-Bourbigou and L. Magna, J. Mol. Cat. A, 2002, 182–183.
CHEM. COMMUN., 2003, 896–897
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