An improved one-pot synthesis of dimethyl carbonate from propylene oxide, CO2 and methanol

Article abstract of DOI:10.3184/174751911X13192908398731

The direct synthesis of dimethyl carbonate (DMC) and propylene glycol from propylene oxide, CO₂ and methanol has been studied with potassium carbonate as catalyst. This is the first time that K₂CO₃ which is readily available has been used as a catalyst. It is quite simple and cheap to use. This route may be an effective way of producing DMC on the large-scale. DMC is used in polycarbonate synthesis, polyurethane synthesis, carbonylating reagents, alkylating reagents, polar solvents, octane boosters in gasoline, carbon-particle reducing agents for diesel engines It is a safe and environmentally benign alternative for phosgene in some reactions.

Full text of DOI:10.3184/174751911X13192908398731

654 RESEARCH PAPER
NOVEMBER, 654–656
JOURNAL OF CHEMICAL RESEARCH 2011
An improved one-pot synthesis of dimethyl carbonate from propylene
oxide, CO2 and methanol
Bin Fana, Bo Qua, Qingchuan Chenb, Yicun Wenb, Liang Caia and Rui Zhangb*
aState Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of
Sciences, Beijing 100085, P. R. China
bState Key Laboratory of Chemical Engineering, East China University of Science andTechnology, Shanghai 200237, P. R. China
The direct synthesis of dimethyl carbonate (DMC) and propylene glycol from propylene oxide, CO2 and methanol has
been studied with potassium carbonate as catalyst. This is the first time that K2CO3 which is readily available has been
used as a catalyst. It is quite simple and cheap to use. This route may be an effective way of producing DMC on the
large-scale. DMC is used in polycarbonate synthesis, polyurethane synthesis, carbonylating reagents, alkylating
reagents, polar solvents, octane boosters in gasoline, carbon-particle reducing agents for diesel engines It is a safe
and environmentally benign alternative for phosgene in some reactions.
Keywords: one-pot synthesis, dimethyl carbonate, propylene oxide, carbon dioxide, methanol, potassium carbonate, green
chemistry
The development of environmentally benign processes utilis-
ing carbon dioxide, which is an abundant carbon resource
and a greenhouse compound, as a cheap and safe C1 building
block as well as a nontoxic reaction medium has become a
very attractive topic in recent years.1–3 Synthesis of dimethyl
carbonate (DMC) using carbon dioxide is one of the promising
reactions for this purpose. DMC is an important target
molecule derived from CO2. Application of DMC cover poly-
carbonate synthesis, polyurethane synthesis, carbonylating
reagents, alkylating reagents, polar solvents, octane boosters
in gasoline, carbon-particle reducing agents for diesel engines,
and so on.4 Especially, DMC is gaining importance as a safe
and environmentally benign alternative for phosgene in some
reactions5.
Currently the synthetic routes to DMC are based on the
methanolysis of phosgene or oxidative carbonylation of meth-
anol (non-phosgene route).2,6 Both routes use poisonous and/or
corrosive gases like chlorine, phosgene and CO, and there is
the possibility of explosion in the case of methanol carbonyl-
ation. The cleanest synthesis would be the direct reaction of
methanol with CO2,7–9 which uses organometallic complexes,
phosphoric acid on zirconia or inorganic bases as catalysts.
Although this method is very simple, the conversion is
very low because of thermodynamic limitations. The major
problems of this process are decomposition of the catalysts
and hydrolysis of the carbonate. Increasing CO2 pressure,
employing effective dehydrating agents or developing effec-
tive catalysts can resolve these problems. The yield of DMC is
increased when ortho esters10 or acetals11 are used as starting
materials instead of methanol. They work as drying agents in
the synthesis process. The disadvantages of this route are high
cost of the raw materials and the difficulty of catalyst-product
separation due to the homogeneous nature of the reaction
system.
Another non-phosgene method for the synthesis of DMC
starting from CO2 is a two-step reaction in which the formation
of a cyclic carbonate is involved12 (Scheme 1). The first step
is the reaction of CO2 with an epoxide to produce a cyclic
carbonate13. The second step is the carbonate interchange
reaction, e.g., the transesterification of methanol with ethylene
carbonate14 or propylene carbonate (PC).15 Different solid
catalysts have been studied systematically for the two-step
synthesisofDMC, andsatisfactoryresultshavebeenobtained.15
One-pot synthesis of DMC from epoxides, methanol, and CO2
(Scheme 2, first line) simplifies the industrial process signifi-
cantly, and MgO has used as the catalyst for the one-pot
synthesis because it is a very good catalyst for the two-pot syn-
thesis. Unfortunately, this is not successful due to the large
amounts of by-products produced by the inevitable alcoholysis
of the epoxide (Scheme 2, second line).
In recent years, reactions in supercritical fluids (SCFs) have
attracted much attention because of the special properties of
Scheme 1
* Correspondent. E-mail: r.zhang@ecust.edu.cn

JOURNAL OF CHEMICAL RESEARCH 2011 655
Scheme 2
SCFs. For example, reaction yields, rates, and selectivity can
be adjusted by changing pressure; SCFs (such as CO2 or H2O)
can be used as green solvents; mass transfer is improved for
heterogeneous reactions due to the increased solubility; and
simultaneous reaction and separation may be accomplished for
some reactions. Many papers about chemical reactions in SCFs
have been published, including some reviews.16–17
CO2:PO initially, and becomes independent of the ratio as CO2:
PO is larger than 10:1. It may be that the increase in the ratio
or the amount of CO2 shifts the equilibrium and increases
the yield. However, the existing methanol in the system will
enhance the alcoholysis of PC (Scheme 1), so the ideal ratio
of CO2:PO is about 10:1. This ratio was used by chance in
previous experiments.
In this work, we investigate the one-pot synthesis of DMC
using propylene oxide (PO), supercritical CO2, and methanol
with K2CO3 as catalyst. The results indicate that the inorganic
salt K2CO3 is effective catalyst. High conversions of epoxides
and high yields of DMC and glycols (ethylene glycol or pro-
pylene glycol) can be achieved, and the amount of by-products
is about only 10%. Super-critical CO2 acts as both the reactant
and the solvent and this process has special advantages. For
example, comparing with the two-step process, the separation
process after the first reaction step is avoided. The reaction
was conducted in homogenous supercritical conditions, where
non-corrosive CO2 was the main component in the reaction
mixture, and this prolongs the life of the catalyst.
The effect of the molar ratio of methanol to PO
The influence of the molar ratio of methanol to PO on the yield
of DMC is demonstrated in Figure 2. The yield of DMC
increases with the increasing molar ratio of MeOH:PO while
MeOH:PO<10:1. Further increase of the methanol:PO molar
ratio reduces the yield. The reason is that the ratio of MeOH:
PO affects the yield of DMC in two opposite ways. First, the
increase of the molar ratio favours the formation of DMC
because the alcoholysis of PC (Scheme 1) is a reversible
reaction. On the other hand, the alcoholysis of PO to produce
by-products (compounds 5 and 6 in Scheme 2) is also inevita-
bly accelerated as the concentration of methanol is increased.
The competition of the two factors results in Fig. 2.
In the present study, the one-pot synthesis of DMC from
PO, methanol and CO2 has been carried out with potassium
carbonate as catalyst under various conditions and a satisfac-
tory conversion of PO and yield of DMC can be obtained. This
is the first time K2CO3 has been used as a catalyst in this
reaction. This opens up a new way to produce DMC, which is
a versatile green compound, on the large scale.
The effect of temperature
Table 1 shows the dependence of the conversion of PO and the
yield of DMC on the reaction temperature with a reaction time
of 6 hours, with K2CO3 as the catalyst. High conversion of PO
is achieved at 120 °C or higher. The conversions of PO almost
keep constant as temperature increases from 120 °C to 180 °C.
However, the yield of DMC reaches a maximum at 140 °C.
This can be partially ascribed to the direct alcoholysis of PO
because the yields of 5 and 6 are high.2 In the following exper-
iments, the temperature was fixed at 140 °C and we studied the
effect of varying the other experimental conditions.
The effect of the amount of CO2
The influence of the amount of CO2 on the yield of DMC is
illustrated in Fig. 1. The amount of PO in the reactor is 5 mmol.
The yield of DMC increases with the increasing molar ratio of
Table 1 The conversion and yields at different temperatures
Temp.
/ °C
PO Conv/%
Yield/mol%
DMC
PC
PG
5 + 6
100
120
140
160
180
53.9
98.2
98.3
98.9
98.3
1.62
19.3
20.3
19.9
13.1
35.6
32.0
33.8
36.4
23.9
3.65
8.61
10.3
11.9
11.6
28.1
33.6
36.2
33.9
18.97
Fig. 1 The effect of the molar ratio of CO2/PO on the yield of
DMC at 140 °C. Reaction conditions: the molar ratio of PO:
CH3OH:K2CO3 = 1:4: 0.065; the amount of PO in the reactor is
5 mmol
The molar ratio of PO:CH3OH:CO2:K2CO3 = 1:4:10:0.065; the
amount of PO was 5 mmol; reaction time was 6 h.

656 JOURNAL OF CHEMICAL RESEARCH 2011
passing through the cold trap with N, N-dimethylformamide (DMF)
as an absorbent. Experiments showed that the amount of reactants and
products entrained by CO2 was negligible. The liquid chemicals in
the reactor were extracted in situ by super-critical CO2 at 12 MPa.
Experiments showed that all the reactants and products were collected
in the cold trap, and about 80 g CO2 was required to extract all
liquid chemicals. The catalyst was left in the reactor, because it
was not soluble in super-critical CO2. The liquid mixture in the reactor
was analysed by gas chromatography (Agilent 4890D, Agilent
Technologies Inc.) with a FID detector.
This work is supported by National Natural Science Founda-
tion of China (21004021), the Fundamental Research Funds
for the central universities (WA1014007), the State Special
S&T Project on Treatment and Control of Water Pollution
2009ZX07318-006 and the National Key Technology R&D
Program 2009BAC57B04.
Fig. 2 The effect of the molar ratio of MeOH:PO on the yield of
DMC at 140 °C. Reaction conditions: the molar ratio of PO:CO2:
K2CO3 = 1:10:0.065; the amount of PO in the reactor is 5 mmol.
Received 5 August 2011; accepted 13 October 2011
Paper 1100831 doi: 10.3184/174751911X13192908398731
Published online: 22 November 2011
Experimental
References
K2CO3 and anhydrous methanol were produced by the Beijing
Chemical Reagent Factory (A. R. Grade). Propylene oxide was sup-
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Factory. Methanol was dried with 4 Å molecular sieves. All the other
chemicals were used without further purification.
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