Propylene oxide as a dehydrating agent: Potassium carbonate-catalyzed carboxylative cyclization of propylene glycol with CO2 in a polyethylene glycol/CO2 biphasic system

Article abstract of DOI:10.1039/c6ra04422k

The synthesis of propylene carbonate (PC) from 1,2-propylene glycol (PG) and CO₂ was smoothly performed in a PEG₈₀₀ (polyethylene glycol)/CO₂ biphasic system with K₂CO₃ as a catalyst and propylene oxide (PO) as a dehydrating agent. In the reaction of PG with CO₂, PO presumably removes the water produced, and simultaneously generates more PG, both of which shift the thermodynamic control process and thus accelerate the PC synthesis. The PC yield directly from PG and CO₂ reached 78% under relatively mild reaction conditions (4 MPa, 120 °C, 10 h). Notably, no additional by-product was detected in this process, resulting in economic benefits and the ease of workup procedure.

Full text of DOI:10.1039/c6ra04422k

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Propylene oxide as dehydrating agent: Potassium carbonate-
catalyzed carboxylative cyclization of propylene glycol with CO2
in polyethylene glycol/CO biphasic system
Received 00th January 20xx,
Accepted 00th January 20xx
2
a
a
a
Zhen-Feng Diao , Zhi-Hua Zhou , Chun-Xiang Guo , Bing Yu , Liang-Nian He*
a
ab
DOI: 10.1039/x0xx00000x
The synthesis of propylene carbonate (PC) from 1,2-propylene glycol (PG) and CO
PEG800(polyethylene glycol)/CO biphasic system with K CO as catalyst and propylene oxide (PO) as dehydrating agent. In
the reaction of PG with CO , PO presumably removes the water produced, and simultaneously generates more PG, both of
2
was smoothly performed in
www.rsc.org/
2
2
3
2
which shift the thermodynamic control process and thus accelerates the PC synthesis. The PC yield directly from PG and
o
reached 78% under relatively mild reaction conditions (4 MPa, 120 C, 10 h). Notably, no additional by-product was
CO
2
detected in this process, resulting in economic benefits and the ease of workup procedure.
Introduction
Ethylene carbonate (EC), propylene carbonate (PC) and
dimethyl carbonate (DMC) are important industrial organic
1
carbonates, which have been widely applied as polar aprotic
2
solvents, electrolytes for lithium ion batteries, raw materials
-4
5
for polycarbonates/polyurethanes,
green carbonylation
6
7
reagents and fuel additives. Generally, PC and EC have also
Scheme 1 DMC synthetic methodologies.
been used to prepare DMC through the transesterification with
8
methanol.
Currently, there are two conventional accesses to produce
9
DMC: a) phosgenation of methanol; b) oxidative carbonylation
1
0
of methanol with carbon monoxide (Scheme 1, a, b).
Corrosive and toxic reagents are commonly required in those
procedures. As the most straightforward and challenging
method for direct DMC synthesis, the dehydrating condensation
of methanol with CO
2
is highly attractive (Scheme 1, c).
and methanol
Although DMC synthesis directly from CO
2
catalyzed by transition metals has been developed, the process Scheme 2 Two-step process for DMC production.
has not been yet applied for industrial application because of
(
Scheme 2, a), which has widely been applied in industrial
1
DMC production.
low productivity originating from catalyst deactivation and
DMC hydrolysis caused by co-produced H O. Therefore,
considerable efforts have been devoted to overcoming such
4
2
In this process, however, an equivalent of propylene glycol
1
5
(PG) is inevitably released as co-product. Therefore, if PC can
1
1-13
limitations.
practicable strategies is the two-step process consisting of
cycloaddition of epoxides with CO and subsequent
transesterification of cyclic carbonate e.g. PC with methanol
On the other hand, one of the most efficient and
be synthesized directly from PG and CO , the process can be
2
regarded as DMC synthesis indirectly from MeOH and CO2
2
,
(
Scheme 2, b). In this context, the synthesis of cyclic
carbonates from CO2 with glycol could be promising but
remains in its infancy. We have already developed efficient
metal catalysis for PC synthesis from PG and CO , such as
2
a.
State Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai
University, Tianjin 300071, China. E-mail: heln@nankai.edu.cn; Tel.: (+86) 22-
1
Bu SnO or Bu Sn(OMe) , magnesium and its oxide. Despite
6
9
2
2
2
2
3503878; fax: (+86) 22-23503878
high selectivity, PG conversion and PC yield were
unsatisfactory [e.g. 3.4% yield with Bu Sn(OMe) ] even under
o
harsh reaction conditions (180 C, 15 MPa). Such low
b.
Collaborative Innovation Center of Chemical Science and Engineering, Tianjin
00071, China
2
2
3
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
conversion and yield are likely ascribed to the thermodynamic
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limitations and/or deactivation of the catalysts by the co- GC (entry 7, 8), suggesting that PG and PO are inevitable for
product water as shown in Scheme 3. Therefore, removing PC formation. Either PG or PO alone could not react with CO
2
water to shifting the equilibrium to PC is the key to to give PC under otherwise identical reaction conditions in this
accomplishing high conversion. Various dehydrating agents study. To distinguish PC produced from PG or PO, ethylene
have been introduced for this purpose. Inorganic dehydrating epoxide (EO) was chosen as a dehydrating agent because EC
2
agents such as molecular sieve, CaCl are easily recyclable and and PC can be unequivocally identified by GC technique. As a
do not form by-products. However, PC yields were still low result, 24% of PC and 4% of EC were detected, indicating that
because of the dehydration reversibility under the reaction the PC came mainly from PG rather than PO (entry 9). Herein,
conditions. In other words, it is difficult to physically absorb each PC yield in this study was obtained from two control
water at high temperature.
experiments: a normal one to get the total PC yield from both
As a complement, irreversible hydration with organic PG and PO, another without PG to calculate the PC yield
compounds is more effective at high temperature. For example, directly from PO, then PC yield from PG was obtained by
the hydration of nitriles to the amides can conveniently and subtracting such amount of PC from PO.
1
7-20
selectively consume water.
Therefore, acetonitrile has been
a
Table 1 PC synthesis from PG and CO
2
using epoxide as dehydrating agent
applied as dehydrating agent to enhance the reaction between
2
1-22
PG and CO
2
with catalysts CeO
), modified zinc oxide (i.e. ZnO/KI), zinc acetate,
and organic base i.e. 1,5,7-triazabicyclo[4.4.0]dec-5-ene,
2
-ZrO
2
, alkali carbonate (i.e.
2
3
24
25
2 3
Cs CO
(
2
6
b
b
TBD) respectively. The drawbacks associated with the use of
chemical dehydrating agents are the high cost and the formation
of hydrolytic products, which are hard to separate and recycle.
The acetamide generated via hydrolysis of acetonitrile could
further react with water to afford acetic acid which then reacts
with PG to produce propylene glycol-2-acetate and thereby
reducing the selectivity of PC.
Entry
Catal.
Dehydrating
Selectivity
Yield (%)
agent (2 eq)
(%)
1
2
3
4
5
6
Cs
2
CO
CO
SnO
3
PO
PO
PO
PO
PO
PO
PO
/
>99
19
12
<1
3
K
2
3
>99
Bu
3
>99
2
KF/Al O
3
>99
CsOH
KOH
>99
3
>99
<1
<1
<1
Herein, we performed the K
2
CO
3
-catalyzed PC synthesis
c
7
Cs
Cs
Cs
2
2
2
CO
CO
CO
3
3
3
>99
from PG and CO in PEG/CO biphasic system (Scheme 3). In
2
2
8
9
>99
this work, propylene oxide (PO) works as a dehydrating agent.
The hydration of PO consumes water formed, and generates
more PG, thus allows the equilibrium to shift to PC formation.
The high efficiency may be explained by PEG coordinating
with the potassium cation to enhance the basicity and
d
24 (4 )
EO
>99
a
Reaction conditions: PG (2.5 mmol, 0.1902 g), catalyst (5 mol%), dehydrating
o
agent (200 mol%, 5 mmol), reaction pressure (8 MPa), 120 C, 12 h. Determined
c
by GC using biphenyl as internal standard. Without PG. GC yield of EC.
b
d
2
7,28
nucleophilicity of carbonate anion.
solubility of CO and K CO
such promotive effect.
biphasic system by employing PO as a dehydrant could physical properties, such as lowered melting points and
In addition, good
As an inexpensive, non-toxic, nearly non-volatile and
2
9-31
2
2
2,33
3
in PEG may also contribute to environmentally benign reaction medium,
PEG is
3
32,33
As a consequence, this PEG/CO
2
expandable with CO2,
thus leading to changes in its
3
4
3
5
efficiently prepare PC with no by-product from the use of PO.
lowered viscosity. When PEG was selected as solvent, the PC
yield directly from PG distinctly increased to 55% after
substracting the PC amount formed from PO under the same
conditions (entry 2, Table 2), indicating the introduction of
2
PEG is beneficial to the carboxylation of PG and CO to
produce PC when using PO as dehydrating agent. Typically,
EO as a dehydrant afforded 44% yield of PC accompanying
with 0.70 mmol of EC (entry 3). Accordingly, an epoxide could
act as an efficient dehydrating agent to promote the
2
Scheme 3 PC synthesis from PG and CO using PO as dehydrating agent.
carboxylation of PG with CO
2 2
to produce PC in PEG/CO
Results and discussion
biphasic system. Notably, no additional by-product was
In order to directly and efficiently prepare PC from PG and detected in this process. Subsequently, the influence of PEG
CO2, various bases including Cs CO , K CO , Bu SnO, with different molecular weight ranging from 400 to 6000 was
2
3
2
3
2
KF/Al O , CsOH and KOH were screened as the catalysts at explored. When the reaction was conducted in PEG400, PO was
2
3
o
20 C, 8 MPa. To our delight, when Cs CO and K CO used quantitatively converted to PC but only 7% yield of PC came
1
2 3 2 3
as catalysts with PO as dehydrating agent, PC was obtained in from PG (entry 3, Table 2). As PEG molecular weight
9% and 12% yield, respectively, being consistent with the increased to 800, PC from PG was increased to 66% (entry 4,
1
basicity (entry 1, 2, Table 1), while other catalysts gave only 5). However, further increasing PEG molecular weight to 6000
trace amounts of PC (entries 3-6). Control experiments were leaded to a decrease in PC yield (entry 6), presumably being
conducted to gain deeper insight into the role of PO in PC ascribed to the decreasing in coordination effect and CO2
synthesis. Without PG or PO, almost no PC was detected by
2
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a
Table 2 PC synthesis from PG in PEG/CO
2
with epoxide as dehydrant
to 3 mol% dramatically leaded to decrease in PC yield (entry 2
vs. 1). However, when increased the catalyst amount to 10
mol%, no beneficial effect was shown (entry 3).
Furthermore, the dependence of PC yield on reaction
pressure was carefully examined. PC yield gradually increased
when raising reaction pressure from 2 to 4 MPa (entry 4, 5),
being ascribed to CO2 amount increase. However, too high
PC Yield
PC Amount
b
Entry
PEG
PEG (g)
from PG
(
mmol)
c
(
%)
--
2
8
d
reaction pressure would dilute the reactants concentration,
1
PEG1000
PEG1000
PEG400
PEG600
PEG800
PEG6000
0.5
0.5
0.2
0.3
0.4
0.5
0.44
1.86
0.10
1.59
1.27
1.08
leading to a decrease in PC yield (entry 6). The effect of
reaction time on PC synthesis was also explored. 78% and 70%
yield of PC were obtained when the reactions stopped at 10 h
and 6 h, respectively (entry 7, 8). Consequently, the most
suitable reaction conditions for the carboxylation of PG with
2
3
4
5
6
55
7
54
66
44
a
CO
PEG800 under 4 MPa pressure at 120 C for 10 h.
The proposed mechanism for the present PC synthesis from
PG and CO in PEG/CO biphasic system catalyzed by K CO
with PO as dehydrating agent was depicted in Scheme 4. The
solubility of CO and K CO in PEG is excellent, and PEG can
coordinate with K to enhance the basicity and nucleophilicity
2 2 3
using PO as dehydrate agent were 5 mol% K CO in 0.4g
2 3
Reaction conditions: PG (2.5 mmol, 0.1902 g), Cs CO (5 mol%, 0.0407 g),
o
o
PO (5 mmol, 0.2904 g), reaction pressure (8 MPa), 120 C, 12 h. GC yield
c
with biphenyl as internal standard. Obtained by substracting the PC yield from
d
PO under the same conditions. Without PG.
b
2
2
2
3
solubility, as well as the increase in viscosity as PEG molecular
weight increasing. Therefore, PEG800 was chosen as the
appropriate solvent for further investigation.
2
2
3
+
27,28,32,33
of the carbonate anion.
CO and subsequent insertion of CO
intermediate.
The deprotonation of PG by
Then, the influence of different basic catalysts was
examined in PEG800, as shown in Table 3. Without any catalyst,
no PC directly from PG was detected (entry 1). Alkali
K
2
3
2
gives the carbonate
a
carbonates e.g. Cs
entry 2, 3); whereas Na
particular, Cs CO showed lower activity that Cs
favourable coordination between PEG800 and K . Others,
such as CaCO , MgO, CaO did not work at all under the given
conditions (entries 5-7).Therefore, K CO was proved to be the
2
CO
3
and K
2
CO
was less effective (entry 4). In
CO due to
3
gave relatively higher yields Table 4 Influence of catalyst loading, pressure and reaction time
(
2
CO
3
K
2
CO
3
Pressure
Yield (%)
b,c
Entry
Time (h)
2
3
2
3
(mol %)
(MPa)
+
1
2
3
4
5
6
7
8
3
5
8
8
12
12
12
12
12
12
10
6
55
74
68
49
79
66
78
70
3
2
3
10
5
8
most appropriate catalyst for PC synthesis. Furthermore,
increasing PEG800 amount gave no significant rise in PC yield
2
5
4
(
entries 8-10). Hence, 0.4 g of PEG800 was chosen as reaction
5
10
4
5
medium. In addition, changing the reaction temperature to 100
o
o
C or 140 C resulted in adverse effect (entry 9, 10),
5
4
a
presumably being ascribed to intermolecular dehydrating
2
condensation of PG and PEG.
Reaction conditions: PG (2.5 mmol, 0.1902 g), PO (2 equiv. to PG, 0.2904 g),
b
8
o
2 2 3
in 0.4 g PEG800 with CO and K CO , 120 C. Determined by GC using
c
biphenyl as internal standard. Yield of PC relative to PG.
With those results in hand, we then investigated the effect
of catalyst loading, reaction pressure and time on the reaction
outcome as listed in Table 4. Reducing K
2
CO
3
loading from 5
a
Table 3 Influence of catalyst and PEG amount for PC synthesis from PG and CO
2
b, c
Entry
Catalyst
/
PEG800 (g)
0.4
Yield (%)
1
2
3
4
5
6
7
8
<1
66
74
15
<1
<1
<1
75
38
<1
Cs
2
CO
CO
CO
3
0.4
K
2
3
0.4
Na
2
3
0.4
CaCO
MgO
CaO
3
0.4
0.4
0.4
2
Scheme 4 Proposed mechanism for PC formation from PG and CO by using PO
as dehydrate agent.
K CO
2
K CO
2
K CO
2
3
3
3
0.5
d
9
0.4
e
1
0
0.4
Then PC is generated along with the formation of an equivalent
water. The water further reacts with PO, regenerating more PG;
and thus both shifts the equilibrium to PC production. After the
a
Reaction conditions: PG (2.5 mmol, 0.1902 g), PO (2 equiv. relative to PG,
o
.2904 g), catalyst (5 mol%), in PEG800 under 8 MPa pressure, 120 C for 12 h.
c
Determined by GC using biphenyl as internal standard. PC Yield relative to
0
b
completion of the carboxylation of PG with CO , PC could be
2
d
o
e
o
PG. 100 C. 140 C.
separated or subsequently reacts with methanol to produce
DMC through transesterification reaction. Another reaction in
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this system is the direct cyclization of PO and CO
PC.
2
to afford (CH), 70.7 (CH
2
), 19.5 (CH
3
). EI-MS, m/z (%): 103.14 (26)
+
[M] , 57.03 (100).
1
,3-dioxolan-2-one (EC): Colorless crystal; H NMR (400
1
1
3
MHz, CDCl ): δ = 4.50(s, 4H, CH ); C NMR (101 MHz,
3
2
Conclusions
CDCl ): δ = 155.5 (C), 64.6 (CH ).
3
2
In conclusion, we have developed a simple, ecologically safe,
cost-effective and industrially feasible process for PC synthesis
Acknowledgements
2 2 3 2
from PG with CO using K CO as catalyst in PEG800/CO
This work was financially supported by the National Natural
Sciences Foundation of China, the “111” Project of the
Ministry of Education of China (No. B06005), Specialized
Research Fund for the Doctoral Program of Higher Education
biphasic system. 78% yield of PC from PG can be attained
o
under mild reaction conditions (120 C, 4 MPa). In this process,
PO is utilized as dehydrating agent to react with co-produced
water along with regenerating PG, both are favorable to shift
the equilibrium to PC formation. Compared with common
organic dehydrating agents, no additional by-product was
generated in this process, resulting in workup procedure and
economic benefits.
(
20130031110013), MOE Innovation Team (IRT13022) of
China.
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), 4.02 (t, = 7.8 Hz, 1H, CH ), 1.48 (d, = 6.2 Hz,
3 3
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,
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2
3
J
2
J
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2
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| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C6RA04422K
Graphic Abstract
Propylene carbonate was directly prepared in 78% yield from 1,2-propylene glycol
and CO in polyethylene glycol/CO biphasic system promoted by K CO with
propylene oxide as the dehydrating agent.
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