Driving dimethyl carbonate synthesis from CO2 and methanol and production of acetylene simultaneously using CaC2

Article abstract of DOI:10.1039/c8cc01005f

The synthesis of dimethyl carbonate (DMC) from CO₂ and methanol is a very interesting reaction, but is thermodynamically limited. In this work, CaC₂ was used to consume the water produced in the reaction to shift the reaction equilibrium, and C₂H₂ was produced at the same time. This is the first work on the combination of driving a thermodynamically unfavorable reaction and producing C₂H₂ using CaC₂.

Full text of DOI:10.1039/c8cc01005f

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Driving dimethyl carbonate synthesis from CO2
and methanol and production of acetylene
simultaneously using CaC2†
Cite this:DOI: 10.1039/c8cc01005f
Received 5th February 2018,
Accepted 5th April 2018
Zhaofu Zhang,*a Shuaishuai Liu,ab Lujun Zhang,ab Shuai Yin,ab Guanying Yangab and
ab
Buxing Han
*
DOI: 10.1039/c8cc01005f
rsc.li/chemcomm
The synthesis of dimethyl carbonate (DMC) from CO2 and methanol enhance the yield of DMC, as can be known from eqn (1). A 3 Å
is a very interesting reaction, but is thermodynamically limited. In molecular sieve has been used in an outline dehydrating system
this work, CaC2 was used to consume the water produced in the of this reaction to promote the yield of DMC,15 but this process
reaction to shift the reaction equilibrium, and C2H2 was produced is highly energy-consuming. Various organic dehydrating
at the same time. This is the first work on the combination of driving regents have been used for this reaction, including dicyclohexyl
a thermodynamically unfavorable reaction and producing C2H2 carbodiimide, trimethyl phosphate,16 orthoester,17 ketals18 and
using CaC2.
various nitriles,19 and this topic has been reviewed.20 These routes
have some disadvantages, such as consumption of dehydrating
Carbon dioxide (CO2) is the major greenhouse gas, it is also a agents, yielding of byproducts, and energy intensive nature. So
renewable, abundant, nontoxic, non-flammable and easily available finding suitable substances to substitute these dehydrating agents
carbon resource, and its utilization has attracted extensive attention. is highly desirable.
Various chemicals have been synthesized using CO2 as a feedstock,1
CO2 + 2CH3OH " (CH3O)2CO + H2O,
DrH0298 = À27.90 kJ molÀ1, DrG0298 = 26.21 kJ molÀ1
such as cyclic carbonates,2 methanol,3 formic acid,4 nitrogen
containing chemicals,5 and polymers.6
(1)
Dimethyl carbonate (DMC) has negligible toxicity and good
biodegradability, and has been used as a green solvent, a
gasoline octane enhancer,7 a precursor of polycarbonate resins,
and a methylating and carbonylating agent to replace some
toxic substances such as dimethyl sulphate, phosgene, and
methyl chloroformate.8 DMC can be produced from the reaction
of phosgene with methanol,9 oxidative carbonylation of methanol,10
oxidative reaction of carbon monoxide and methyl nitrite,11 and
transesterification of methanol with cyclic carbonates.12 However,
obvious shortcomings exist in these routes, such as toxic, corrosive,
and expansive reactants.
DMC can be synthesized from CO2 and methanol as shown
in eqn (1). This reaction is very attractive and has been studied
extensively.13 The standard Gibbs free energy of the reaction is
26.21 kJ molÀ1, indicating that the reaction is thermodynamically
unfavourable.14 Removing water from the reaction system can
Calcium carbide (CaC2) is a fundamental chemical in industry.
The main application of CaC2 is in the production of acetylene via a
reaction with water, in which a large amount of heat is released.21,22
CaC2 has been used in some reactions23,24 as a substitute for
acetylene, with the risk of explosion and requiring complex equip-
ment. CaC2 has also been used in other purposes, such as carbon
sources for preparing carbon nanostructures,23 as a reducing
agent,25 and for absorbing water.26
CaC2 can react with alcohols to produce calcium alkoxide and
acetylene27 at certain temperature as shown in eqn (2). Calcium
alkoxide can react with water and form calcium oxide and the
corresponding alcohol as shown in eqn (3). Combining eqn (1)–(3),
we can get eqn (4), i.e., the reaction of CO2 with methanol and CaC2
can generate DMC, acetylene and calcium oxide. The standard
Gibbs free energy of eqn (4) is À65.2 kJ mol
;
À1 22 this means that
CaC2 can be used as a drying agent to remove water formed in
the DMC producing process and enhance the yield of DMC.
a Beijing National Laboratory for Molecular Sciences,
CAS Key Laboratory of Colloid and Interface and Chemical Thermodynamics,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
E-mail: zhangzf@iccas.ac.cn, hanbx@iccas.ac.cn
CaC2 + 2ROH - HCRCH + Ca(OR)2
Ca(OR)2 + H2O - CaO + 2ROH
(2)
(3)
b School of Chemistry and Chemical Engineering,
University of Chinese Academy of Sciences, Beijing 100049, China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8cc01005f
CO2 + 2CH3OH + CaC2 - (CH3O)2CO + HCRCH + CaO,
DrH0298 = À89.8 kJ molÀ1, DrG0298 = À65.2 kJ molÀ1
(4)
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In this work, CaC2 was used as a dehydrating agent in DMC
The effect of temperature on DMC synthesis is demon-
synthesis from CO2 and methanol, and acetylene was produced at strated in Fig. 1c. It can be known that the yield of DMC was
the same time. In this way, the water generated in the synthesis of very low at lower temperature, and increased significantly with
DMC from methanol and CO2 was consumed by the reaction with the increase of temperature.
CaC2 to produce acetylene, and thus the reaction equilibrium is
The effect of reaction time on DMC synthesis at 180 1C is
enhanced. As far as we know, this is the first work on the illustrated in Fig. 1d. The yield of DMC increased rapidly with
combination of driving a thermodynamically unfavorable reaction time at the beginning, and then increased slowly with increasing
and producing acetylene using CaC2.
reaction time. The reaction reached equilibrium at about 10 h,
The purity of CaC2 was 77%, which was determined in this which was known from the fact that the yield was unchanged
work by the amount of acetylene formed from the reaction of with time over 10 h.
CaC2 with water. The reaction of CaC2 and methanol (eqn (2))
The effect of the CaC2 amount on the yield of DMC was also
was first studied in this work. CaC2 powder reacted with investigated and the results are given in Fig. 2. The yield of
methanol at 60 1C and acetylene was released smoothly, and DMC was only 0.9% without adding CaC2. As discussed above,
the reaction could finish in one hour. The amount of acetylene the direct synthesis of DMC by the reaction of CO2 and methanol
generated from the reaction of CaC2 and methanol was the (eqn (1)) is thermodynamically limited, so the conversion of
same as that obtained from the reaction of CaC2 with water. methanol was very low. The yield increased linearly with the
This indicates that CaC2 could react with methanol completely amount of CaC2 added, and the yield reached 11.3% when
and yield equimolar acetylene and calcium methoxide.
12.3 mol% CaC2 (based on methanol) was added, and the reason
The influence of reaction conditions on DMC synthesis from will be discussed in the following.
CO2 and methanol using calcium methoxide (formed from
As discussed above, CaC2 reacted with alcohols to produce
CaC2 and methanol, eqn (2)) as the drying agent was studied Ca(OCH3)2, which may react with CO2 to form DMC without
in this work. Some simple and commonly used catalysts for the the catalyst considering that Ca(OCH3)2 has stronger nucleo-
synthesis of DMC from CO2 and methanol were tested, including philicity than CH3OH. To clarify this, we also studied the effect
ZnCl2, ZnO, Bu2SnO, K2CO3, SnCl2, CuCl, CuI, KI, and ZnI2.20,28 It of the CaC2 amount on the reaction without a catalyst, and the
can be seen from Fig. 1a that catalysts influenced the formation of results are shown in Fig. 2. Trace amount of DMC was formed
DMC greatly. Bu2SnO, ZnCl2 and CuI showed satisfactory activities, and the yield increased with increasing amount of CaC2, but
and Bu2SnO had the best performance. Therefore, the reaction the yield of DMC was extremely low without the catalyst
conditions were further optimized using Bu2SnO as the catalyst.
(Bu2SnO). This indicates that the efficiency of the reaction of
The effect of the Bu2SnO amount on the reaction is shown in Ca(OCH3)2 and CO2 was very low, and the catalyst was necessary
Fig. 1b. The yield of DMC increased with increasing amount of for the reaction.
the catalyst, and was independent of the Bu2SnO amount after
On the basis of the results above and knowledge in the
5 wt% (methanol basis). Therefore, the reaction conditions literature, we can discuss the possible reaction pathway and the
were further optimized using 5 wt% Bu2SnO as the catalyst.
reason for the enhancement of the equilibrium yield, as shown
in Scheme 1. Initially, CaC2 reacts with methanol and generates
acetylene and Ca(OCH3)2 (eqn (2)). As CO2 reacts with methanol
and generates DMC and water (eqn (1)), Ca(OCH3)2 reacts with
the water to form CaO and methanol (eqn (3)). As water is
consumed, the equilibrium of the reaction (eqn (1)) is shifted
towards the right side and the equilibrium yield is increased.
In summary, the route to simultaneously producing acetylene
and enhancing reaction equilibrium using CaC2 as a dehydrating
agent has been proposed. The method has been used to shift
the reaction equilibrium for the synthesis of DMC from CO2
and methanol efficiently and produce acetylene simultaneously.
Fig. 1 Effect of reaction conditions on DMC synthesis from CO2 and
methanol in the presence of CaC2. Common conditions: methanol 2.0 g,
CaC2 0.22 g, catalyst 0.1 g, CO2 15 MPa, 180 1C, 2 h; DMC yields are based
on methanol. (a) Performance of various catalysts (none: without CaC2 and
catalyst); (b) effect of Bu2SnO amount; (c) effect of temperature; and
(d) effect of reaction time.
Fig. 2 Effects of the CaC2 amount on DMC synthesis from CO2 and
methanol with and without Bu2SnO. Methanol 2.0 g, Bu2SnO 0.1 g, CO2
15 MPa, 180 1C, 10 h.
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Conflicts of interest
There are no conflicts to declare.
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