Dehydrating agent effect on the synthesis of dimethyl carbonate (DMC) directly from methanol and carbon dioxide

Article abstract of DOI:10.1039/d0ra06034h

CO₂emissions and global warming have increased with the growth of the world economy and industrialization. Direct synthesis of dimethyl carbonate (DMC) from CO₂and methanol (CH₃OH) has been considered a promising route from a green chemistry point of view due to global warming mitigation by CO₂emission reduction. However, DMC yield, when obtained by direct synthesis, is limited due to unfavorable thermodynamics and catalyst deactivation by water formation in the reaction process. This problem motivated us to investigate the effect of dehydration on DMC production by direct synthesis. Herein, different dehydrating agents (2,2-dimethoxypropane, sodium sulfate, magnesium oxide and butylene oxide) were combined with molecular sieves to remove the water and minimize the reverse reaction. A new reactor presenting a compartment to accommodate molecular sieves in the gas phase was developed as well. The chemical/product analysis was carried out by gas chromatography and the results were used to calculate methanol conversion and DMC selectivity. The highest methanol conversion value was found for the combination of molecular sieves in the gas phase with 2,2-dimethoxypropane in the reaction liquid phase (methanol conversion = 48.6% and 88% selectivity). The results showed that dehydration systems may promote increased yield in direct DMC synthesis under mild conditions. The dehydration systems tested in this work exhibited excellent conversion and yield as compared to other reported studies.

Full text of DOI:10.1039/d0ra06034h

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Dehydrating agent effect on the synthesis of
dimethyl carbonate (DMC) directly from methanol
and carbon dioxide
Cite this: RSC Adv., 2020, 10, 34895
Douglas Jose Faria,a Leonardo Moreira dos Santos,b Franciele Longaray Bernard,b
´
Ingrid Selbacch Pinto,b Maria Angelica Carmona da Motta Resendec
´
ab
*
and Sandra Einloft
CO2 emissions and global warming have increased with the growth of the world economy and
industrialization. Direct synthesis of dimethyl carbonate (DMC) from CO2 and methanol (CH3OH) has
been considered a promising route from a green chemistry point of view due to global warming
mitigation by CO2 emission reduction. However, DMC yield, when obtained by direct synthesis, is limited
due to unfavorable thermodynamics and catalyst deactivation by water formation in the reaction
process. This problem motivated us to investigate the effect of dehydration on DMC production by
direct synthesis. Herein, different dehydrating agents (2,2-dimethoxypropane, sodium sulfate,
magnesium oxide and butylene oxide) were combined with molecular sieves to remove the water and
minimize the reverse reaction. A new reactor presenting a compartment to accommodate molecular
sieves in the gas phase was developed as well. The chemical/product analysis was carried out by gas
chromatography and the results were used to calculate methanol conversion and DMC selectivity. The
highest methanol conversion value was found for the combination of molecular sieves in the gas phase
with 2,2-dimethoxypropane in the reaction liquid phase (methanol conversion ¼ 48.6% and 88%
selectivity). The results showed that dehydration systems may promote increased yield in direct DMC
synthesis under mild conditions. The dehydration systems tested in this work exhibited excellent
conversion and yield as compared to other reported studies.
Received 10th July 2020
Accepted 15th September 2020
DOI: 10.1039/d0ra06034h
rsc.li/rsc-advances
DMC is an important carbonate being used in several niches,
such as intermediate in polycarbonate production for building
1. Introduction
utilization, medical devices, automobiles and electrolytic
solvent in lithium batteries due to the high dielectric constant.
Yet, due to the high oxygen composition (53%) and octane
number (105) dimethyl carbonate has been described as an
excellent diesel additive reducing the emission of soot particles
and, consequently, the environmental impact.6 Methyl tert-butyl
ether (MTBE) is used as additive in fuels, however, several
countries are banning its use due to the generated environ-
mental impacts. Therefore, DMC comes as a substitute, since,
in the concentration of 40% (v/v) in relation to the MTBE
concentration achieves the same effectiveness as MTBE
reducing waste generation by 50%.7–10
Industrial production of DMC is environmentally friendly,
however the Gibbs energy for DMC production by direct reac-
tion is greater than zero. Reaction equilibrium constant around
10ꢀ5 indicates that the formation reaction does not occur
spontaneously. DMC synthesis has traditionally occurred
through three routes. In the rst, DMC is synthesized from
phosgene and methanol (phosgene method) having a high
environmental impact.11 The second route uses methanol,
oxygen and carbon monoxide (oxidative carboxylation)
The growth of the world economy allied to human and indus-
trial activities is the main origin of the increase in CO2 emission
in the atmosphere.1 CO2 emission reached 24 Gt in 1995,
growing about 1% a year from 1990 to 1999 and arriving at 35 Gt
in 2015. The CO2 level is expected to reach 40 Gt in 2020,
growing 2.0% by year over the decade.2 This scenario corrobo-
rates the need for exploring new ways of using CO2 as a raw
material in innovative industrial processes obtaining high
added value products and reducing the environmental impact.3
Direct synthesis reactions from CO2 and alcohol producing
organic carbonates, such as dimethylcarbonate (DMC) are
being investigated.4 Alkyl carbonate production allows the use
of excess CO2 generating low toxicity, biodegradable and non-
corrosive products.5
aPost-Graduation Program in Materials Engineering and Technology, Pontical
Catholic University of Rio Grande do Sul – PUCRS, Brazil. E-mail: einlo@pucrs.br
bSchool of Technology, Pontical Catholic University of Rio Grande do Sul PUCRS,
Brazil
cCENPES/Petrobras, Brazil
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 34895–34902 | 34895

View Article Online
RSC Advances
Paper
Fig. 2). Temperature was controlled by means of a thermo-
couple connected to a temperature controller and kept constant
using a resistive thermal band in both reactors. A reactor was
equipped with a compartment metallic support in the gas phase
lled with molecular sieves (see Fig. 2A). For a typical reaction
213 mmol of methanol, 10 mmol of CH3OK, 20 mmol of CH3I,
the desired dehydrating agent and 40 bar of CO2 were used.
Reactions were carried out using different dehydrating agents
(molecular sieve, 2,2-dimethoxypropane, sodium sulfate,
magnesium oxide and butylene oxide) added direct to the liquid
phase or by combining a molecular sieve placed in the gas
phase with dehydrating agent added in the liquid phase. The
reactor was pressurized with CO2 at 40 bar and heated at 80 ꢁC.
Reaction time ranged from 6 h to 30 h. At the end of the reac-
tion, the reactor was cooled to room temperature and slowly
depressurized. The new reactor designed in this work (Fig. 2)
contains an apparatus in the upper part to supporting the
molecular sieves (A) capturing water in the gaseous phase. As
a way of comparison of the results for the DMC synthesis using
the new reactor and the already described process the sieves
were also tested in the liquid part (B). Yet, the use of molecular
sieves in the gaseous phase combined with different dehydrat-
ing agents in the liquid phase in the DMC obtainment reaction
by direct synthesis will also be tested using the system
described in Fig. 2A.
Fig. 1 DMC direct synthesis.
presenting an inherent health and safety risk by using CO as
reagent in addition to the high process and feedstock cost.12–14
The third route uses dimethyl sulfate and sodium carbonate
(transesterication method), products from the oil renery,
presenting the highest total cost per DMC produced.13,15 In
addition to the traditional methods, DMC can be obtained by
direct synthesis using carbon dioxide and methanol. This route
provides the lowest total cost per DMC produced and safer
chemicals besides a favorable carbon balance. The main
drawback of DMC direct synthesis is the low yield and selec-
tivity.13,14 Therefore, to improve yield and selectivity in DMC
direct synthesis modelled reactors, changes in pressure and
temperature and the use of techniques for directing the equi-
librium constant for product formation must be evaluated.16 In
addition, DMC direct synthesis from methanol and CO2
produces water as a by-product (see Fig. 1), decreasing yield by
directing the reaction balance towards reagents.17,18
This scenario evidences the need of assessing efficient
dehydrating agents for the removal of water produced during
reaction, consequently improving DMC yield. Several dehy-
drating agents have been investigated for DMC synthesis.11,12
Among investigated dehydrating agents molecular sieves can be
highlighted due to their large surface area, thermal stability and
excellent adsorption.13 Organic and inorganic dehydrating
agents, such as 2,2-dimethoxypropane (DMP) and magnesium
oxide, are also described as efficient water withdrawn agents
during DMC synthesis increasing product yield.14,15
In order to compare the results a reaction without dehy-
drating agent was also performed. All tests were performed in
triplicate. The products of catalytic tests were analyzed by gas
chromatography (GC) in order to determine yield, conversion
and selectivity. Gas Chromatograph Shimadzu GC-2014 equip-
ped with SH-Rtx-5 coꢀlu1mn using a heating ramp of 31 ꢁC for 0.5
minutes, 10 ꢁC min to 50 ꢁC for ꢀ11 minute, 20 ꢁC minꢀ1 to
100 ꢁC for 2 minutes and 50 ꢁC min to 220 ꢁC for 2 minutes.
Samples were diluted with a concentration of 4% (v/v) in ethyl
The main goal of this work is to evaluate a new dehydration
system for direct DMC synthesis using CH3I as promoter,
CH3OK as catalyst. In this new designed system, molecular
sieves were tested in the reaction liquid phase and in the gas
phase. Yet, a combination of organic and inorganic dehydrating
agents (2,2-dimethoxypropane, sodium sulfate, magnesium
oxide and butylene oxide) in liquid phase and molecular sieve in
the gas phase were tested as well.
2. Methodology
2.1 Materials
Methanol (>99.9% – EMSURE®), potassium methoxide (>95% –
ALDRICH), iodomethane (>99.5% – ALDRICH), diethyl ether
(>99.9% – EMSURE®), magnesium oxide (98% – ALDRICH), 2,2-
dimethoxypropane – DMP (98% – ALDRICH), sodium sulfate
(>98% – ALDRICH), dimethylcarbonate – DMC (>99.5% –
ALDRICH), pearl-shaped molecular sieves (3A – ALDRICH),
butylene oxide (99% – ALDRICH) and CO2 (99,8% – White
Martins).
Fig. 2 Scheme of experimental apparatus: (A) reactor 1 equipped with
a compartment to accommodate molecular sieves in the gas phase;
(B) reactor 2 used to add molecular sieve and/or dehydrating agent in
2.2 DMC synthesis
Two 120 ml reactors (made of titanium alloy) with constant
magnetic stirring were used to perform the experiments (see the liquid phase.
34896 | RSC Adv., 2020, 10, 34895–34902
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
ether and the peak area of DMC (2.4–2.7 minutes) was used to reaction unfavorable resulting in low DMC yield.5,24,25 Aiming to
dene the concentration through the calibration curve of pure evaluate the effect of dehydrating agents (chemical or physical)
DMC following procedures described by Valente, Riedo and on the yield of DMC obtained by direct synthesis reaction
Augusto (2003).23 Conversion and selectivity were performed as CH3OK was used as catalyst and CH3I as promoter. Table 1
described by Chen et al. (2012).20
presents methanol conversion, DMC selectivity and DMC yield
values, reaction parameters and Tukey test results used to
evaluate statistical equality between samples (see letters a to g
in conversion column, Table 1).
Methanol conversion was calculated using eqn (1).
Methanol conversion (%) ¼ ((Methanol reacted)/
(Methanol total)) ꢂ 100
(1)
One of the acceptable mechanisms for DMC direct synthesis
from CO2 and methanol (CH3OH) using CH3OK as catalyst and
iodomethane CH3I (Table 1, entry 1) is shown in Fig. 3. CH3I can
act as reaction promoter or as a reagent.26 In this mechanism
the hydrogen of the methanol hydroxyl is abstracted by the
catalyst producing a methoxy anion, which reacts with CH3I or
CO2, forming dimethyl ether or methyl carbonate anion which
successively reacts with CH3I to form DMC. The iodine anion
formed during the reaction steps reacts with the proton recy-
cling the catalyst. Produced HI reacts with methanol recovering
the CH3I.26
where methanol reacted (mol) ¼ 2 ꢂ DMC produced (mol).
DMC selectivity was obtained by eqn (2).
DMC selectivity (%) ¼ ((DMC)/(DMC + (by-products))) ꢂ
100
(2)
DMC yield was determined using eqn (3).
DMC yield (%) ¼ ((Methanol conversion (%))
ꢂ (DMC selectivity (%)))/100
(3)
Results obtained in this work suggest that CH3I may be
In addition, water content was determined (in triplicate) acting as a promoter as no selectivity reduction was observed by
using the Karl Fischer test by digital automatic titration gas chromatography when only CH3I and CH3OK were used
equipment with the Karl Fischer solution.
(Table 1, entry 1).
The catalytic system employed by Cai et al. 2005 (ref. 27)
using CH3OK (catalyst) and CH3I (promoter) in the direct
synthesis of DMC from CO2 was selected to evaluate the effi-
ciency of dehydrating agents. In this work, catalyst content and
reactor size were different from that used by Cai et al. (2005).27
The reaction described in literature obtained a yield of ꢃ9.7%
2.3 Statistical analysis
Minitab 18 Statistical Soware-ANOVA was used to perform
statistical analysis in order to assess the standard deviation of
tests performed in triplicate and analyze the Tukey test with
95% reliability, where equal letters show statistical equality
between the samples averages.
ꢁ
with 100% of selectivity using reaction conditions of 80 C, 40
bar and 6 h of reaction time. Different catalyst content and
promoter combined with the use of a smaller reactor may
explain the lower yield (see entry 1, Table 1) when using the
same catalyst system reported by Cai et al. (2005).27 The use of
different amounts (1.0–3.0 g) of molecular sieves in the liquid
phase demonstrated to be effective when compared to the
reaction carried out without the presence of dehydrating agent
3. Results and discussion
3.1 Direct synthesis of DMC using molecular sieve (3A) and
combination of dehydrating agents
Thermodynamic limitations and/or catalyst deactivation due to
water formation during DMC direct synthesis makes this
Table 1 Methanol conversion, DMC selectivity, DMC yield and parameters for reactions using sieves in the liquid and gaseous part with standard
deviation valuesa
Sieve
Dehydrating
agent
Time
(h)
Temperature
(ꢁC)
Pressure
(bar)
DMC selectivity
(%)*
Methanol
conversion (%)**
DMC yield
(%)***
Water
(%)
Entry
Phase
(g)
1
2
3
4
5
6
7
8
—
0
1
2
3
1
2
3
2
2
2
2
—
24
24
24
24
24
24
24
6
80
80
80
80
80
80
80
80
80
80
80
40
40
40
40
40
40
40
40
40
40
40
100
100
100
100
100
100
100
100
100
100
100
7.0 ꢄ 1.6f,g
13.6 ꢄ 1.9d
17.2 ꢄ 1.1c,e
9.3 ꢄ 0.7f
7.0 ꢄ 1.6
13.6 ꢄ 1.9
17.2 ꢄ 1.1
9.3 ꢄ 0.7
7.0 ꢄ 0.7
30.5 ꢄ 2.3
8.7 ꢄ 0.5
9.7 ꢄ 1.2
14.3 ꢄ 0.7
16.1 ꢄ 0.4
26.2 ꢄ 0.8
7.1 ꢄ 0.1
3.7 ꢄ 0.1
1.6 ꢄ 0.1
1.5 ꢄ 0.3
3.3 ꢄ 0.2
0.5 ꢄ 0.1
4.5 ꢄ 0.1
2.5 ꢄ 0.1
2.6 ꢄ 0.2
3.9 ꢄ 0.8
6.1 ꢄ 1.2
Liquid
Liquid
Liquid
Gas
Gas
Gas
Gas
Gas
Gas
Gas
Sieve
Sieve
Sieve
Sieve
Sieve
Sieve
Sieve
Sieve
Sieve
Sieve
7.0 ꢄ 0.7g
30.5 ꢄ 2.3a
8.7 ꢄ 0.5f
9.7 ꢄ 1.2f
9
10
11
12
18
30
14.3 ꢄ 0.7d
16.1 ꢄ 0.4c
26.2 ꢄ 0.8b
a
Fixed parameters / methanol: 213 mmol; catalyst (CH3OK): 10 mmol; promoter (CH3I): 20 mmol. *: eqn (1); **: eqn (2); ***eqn (3) described in
Methodology section.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 34895–34902 | 34897

View Article Online
RSC Advances
Paper
Fig. 3 Proposed mechanism for CH3I reaction in DMC synthesis.
(entry 1, Table 1: yield ¼ 7.0%), as statistically proved by the
Surprisingly tests performed with molecular sieve in the gas
Tukey test represented in Table 1 by unequal letters. The phase showed an increase of more than 300% when using 2.0 g
highest DMC yield was obtained using 2.0 g of molecular sieves of molecular sieve (entry 6, Table 1: yield ¼ 30.5%), 22%, 3.0 g
(entry 3, Table 1: yield ¼ 17.2%) followed by reaction yields (entry 7, Table 1: yield ¼ 8.7%) and 0%, 1.0 g (entry 5, Table 1:
obtained with 1.0 g (entry 2, Table 1: yield ¼ 13.6%) and 3.0 g yield ¼ 7.0%) when compared to the reaction without molecular
(entry 4, Table 1: yield ¼ 9.3%) of molecular sieve respectively. sieve addition (entry 1, Table 1, yield ¼ 7.0%). Increasing
The use of molecular sieve reduced the water content from 7.1% molecular sieve content from 1.0 g (entry 5, Table 1: water ¼
to 3.7% (entry 2, Table 1: 1.0 g of molecular sieve), 1.6% (entry 3, 3.3%) to 2.0 g (entry 6, Table 1: water ¼ 0.5%) signicantly
Table 1: 2.0 g of molecular sieve) and 1.5% (entry 4, Table 1: reduced the water content present in the reaction medium.
3.0 g of molecular sieve). These results indicate that increasing However, increasing molecular sieve content from 2.0 g (entry 6,
the amount of molecular sieve in the liquid phase from 2.0 to Table 1: water ¼ 0.5%) to 3.0 g (entry 6, Table 1: water ¼ 4.5%)
3.0 g can hinder the interaction of reagents with the catalyst increased the water content present in the reaction medium.
active sites reducing catalytic activity. The use of 1.0 g of This result indicates that the increase in molecular sieve
molecular sieve may not be sufficient to remove the formed content is probably reducing the contact surface of molecular
water and achieve optimal yield values.
sieves with water vapor due to the sieves agglomeration in the
reactor metallic support. These results are corroborated by the
Table 2 Methanol conversion, DMC selectivity, DMC yield, water content and reactions parameters using molecular sieves in the gas phase and
dehydrating agents in the liquid phase, with standard deviation valuesa
Sieve
Dehydrating
(g) agent
Temperature Pressure DMC selectivity Methanol conversion DMC yield
Entry Phase
(ꢁC)
(bar)
(%)*
(%)**
(%)***
Water (%)
1
2
3
4
5
6
7
8
—
—
—
—
—
—
—
—
2
2
2
2
DMP
Na2SO4
Butylene oxide
MgO
Sieve/DMP
Sieve/Na2SO4
80
80
80
80
80
80
40
40
40
40
40
40
40
40
55
6.4 ꢄ 0.9c
5.0 ꢄ 1.4c,d
NDb
3.5 ꢄ 0.9
5.0 ꢄ 1.4
ND
0.6 ꢄ 0.1
1.1 ꢄ 0.3
Traces
100
ND
ND
88
100
15
NDb
ND
Traces
Combined-gas
Combined-gas
Combined-gas
Combined-gas
48.6 ꢄ 1.9a
14.8 ꢄ 2.0e
2.5 ꢄ 1.4c,d
4.5 ꢄ 0.9c
42.8 ꢄ 1.9 0.6 ꢄ 0.1
14.8 ꢄ 2.0 2.3 ꢄ 0.2
Sieve/butylene oxide 80
Sieve/MgO 80
0.3 ꢄ 1.4
4.5 ꢄ 0.9
2.3 ꢄ 0.8
0.7 ꢄ 0.1
100
a
Fixed parameters / methanol: 213 mmol; catalyst (CH3OK): 10 mmol; promoter (CH3I): 20 mmol; dehydrating agent: 10 mmol; time: 24 hours.
ND ¼ not detected. *: eqn (1); **: eqn (2); ***: eqn (3) described in Methodology section.
34898 | RSC Adv., 2020, 10, 34895–34902
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
Fig. 4 Gradual dehydration reaction between DMP and water during DMC synthesis.
Fig. 5 Direct synthesis of DMC in the presence of butylene oxide as a dehydrating agent.
Tukey test. It should also be emphasized that the use of and 0.3% butylene oxide (entry 3, Table 2)). Although the
molecular sieves in the gas phase facilitates the dehydrating addition of solid dehydrating agent (Na2SO4 or MgO) in the
agent separating process from the reaction mixture reducing liquid phase promotes the reduction of the amount of water
cost and steps in DMC production process.
formed in the reaction medium the presence of these
Effect of reaction time on DMC yield when molecular sieves compounds may be reducing the interaction between the
are used in the gaseous phase was also evaluated. In 6 hours of reactants and the catalyst active sites. Yet, the use of DMP
reaction time a yield of around 9.7% (entry 8, Table 1) was decreases selectivity due to the ketone production in the reac-
achieved. This result is similar to Cai et al. (2005)27 founds using tion medium, consequently decreasing yield. However, even
the same reaction conditions. However, aer 6 hours an with this setback the yield is superior when using this system.
increase in DMC production was observed. Yield of 14.3% (entry Ketone removal step, by distillation, can be added to the process
9, Table 1) was achieved in 12 h, 16.1% (entry 10, Table 1) 18 in order to improve selectivity increasing it to 100%.19,32
hours and the highest yield value of 30.5% (entry 6, Table 1) in
Butylene oxide decreased DMC selectivity by 15%, reaching
24 hours. Aer the 24 hour period, a small decrease in the a methanol conversion of 2.5% being inefficient. So, it is clear
amount of DMC yield was observed reaching 26.2% (entry 11, from results that the best performance was obtained by
Table 1). According to literature,28 water withdrawal from the combining the molecular sieve and DMP resulting in a yield
reaction medium has great interference in the DMC synthesis increase of almost 50% when compared to the reaction per-
optimization time since smaller amount of water allows greater formed using only molecular sieve in the gaseous phase (entry
catalytic activity and catalyst durability. From these results one 6, Table 1). The dehydrating agents DMP (entry 1, Table 2),
can infer that the molecular sieve porosity improved catalytic Na2SO4 (entry 2, Table 2), butylene oxide (entry 3, Table 2) and
activity and increased DMC production by withdrawing the MgO (entry 4, Table 2) were also evaluated. Obtained yield
water produced during the reaction. Therefore, probably aer values were lower/similar when compared to the reaction
24 hours of reaction time the molecular sieve become less carried without dehydrating agent (entry 1, Table 1: yield ¼
effective decreasing reaction yield. The use of 2.0 g of molecular 7.0%).
sieve in the gas phase for 24 hours was the best result for the
DMP (entry 1, Table 2) showed a methanol conversion value
experimental conditions tested. Therefore, it was used in statistically equal when compared to the reaction carried
combination with other dehydrating agents in the liquid phase without dehydrating agent (entry 1, Table 1). However, a reduc-
(DMP, Na2SO4, MgO and butylene oxide) aiming further yield tion in selectivity of 55% was observed and attributed to acetone
improvement as shown in Table 2.
formation as a by-product. When using sodium sulfate a lower
DMP was evaluated as dehydrating agent (Table 2, entry 1) yield was achieved when comparing with the reaction without
due to its quick action by chemical hydrolysis making it easy to dehydrating agent addition. Sodium sulfate is an effective
use in dehydration processes. In addition DMP is stable during dehydrating agent for use at room temperature and is consid-
storage.29 As seen in Fig. 4 acetone is produced as by-product ered a slow-acting dehydrating agent. These features can
decreasing reaction selectivity.21,22
Butylene oxide is an effective dehydrating agent promoting medium.33,34
inuence the process of water removing from the reaction
an increase in product conversion due to the excellent dehy-
When using butylene oxide (entry 3, Table 2) and MgO (entry
dration capacity into the reaction medium.30 When reacting 4, Table 2) as dehydrating agent no methanol conversion was
with water, produces ethylene glycol decreasing DMC selectivity detected under the tested reaction conditions. When using
due to by-product formation,31 as show in Fig. 5.
butylene oxide as dehydrating agent and more drastic reaction
ꢁ
When combining molecular sieve with dehydrating agents conditions (150 C and pressure of 90 bar) a methanol conver-
different yield values were obtained (DMP 42.8% (entry 5, Table sion of 11.7% was reached as described elsewhere.31 Magne-
2), Na2SO4 14.8% (entry 2, Table 2), MgO, 4.5% (entry 4, Table 2) sium oxide suffers a large dilation when retaining water. This
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 34895–34902 | 34899

View Article Online
RSC Advances
Paper
Table 3 Catalyst, promoter/dehydrating agent, temperature, pressure and DMC yield of different works
Temperature
(ꢁC)
DMC yield
(%)
Entry
Catalyst
Promoter/dehydrating agent
Pressure (bar)
Literature
1
2
CH3OK
CH3OK
CH3I/sieves (gas phase)
CH3I/sieves (gas phase)
80
80
40
40
30.5
42.8
This work
This work
+ DMP (liquid phase)
—
—
—
CH3I
CH3I
CH3I, DMP
3
4
5
6
7
8
Cu/Ce
140
100
110
80
80
140
50
75
30
73
73
200
1.6
Marciniak et al. (2019)38
Tamboli et al. (2016)39
Deerattraku et al. (2020)40
Kabra et al. (2016)41
Fang and Fujimoto (1996)37
O'Neil, Clayton and
Chitosan/IL
Cu–Ni/graphene
K2CO3 + EmimBr
CH3OK
16.7
13.0
5.7
16.2
12.0
K2CO3
Mayeda (1969)42
9
CeO2
Molecular sieves (4A)
Molecular sieves (3A)
120
180
6
300
3.2
31
Zhang et al. (2015)43
Choi et al. (2002)25
10
Dibutyltin
dimethoxide
Dibutyltin
dimethoxide
Cu/Ce
11
—
180
300
3
Choi et al. (2002)25
12
13
2-Cyanopyridine
Methyl trichloroacetate
140
140
50
50
5.0
12
Marciniak et al. (2019)38
Marciniak et al. (2019)38
Cu/Ce
feature can reduce the contact of reagents and catalyst dis- system for dehydration under mild conditions was proposed.
favoring DMC synthesis.35,36
The best results were obtained when 2.0 g of molecular sieve
Temperature and pressure range used for DMC synthesis was placed in the gas phase (yield ¼ 30.5%) or when 2.0 g of
ꢁ
described in literature (see Table 3) were varied from 80 C to molecular sieve was combined in the gas phase with 10 mmol
ꢁ
140 C and 6 bar up to 200 bar respectively. Our results, using DMP in the liquid phase (yield ¼ 42.8%). In the dehydrating
ꢁ
mild conditions (80 C, 40 bar) and molecular sieves placed in system proposed in this work the sieves can be easily separated,
the gas phase or the combination of molecular sieve placed in since they are deposited in the gas phase and the acetone
the gas phase/DMP placed in the liquid phase were superior to formed from the DMP can be easily separated from the reaction
results described in literature (see Table 3, entries 1 and 2). medium using the distillation technique. This catalytic system
Molecular sieves effect on DMC yield obtained by direct also achieved higher yield values when compared to other
synthesis was described elsewhere obtaining a yield up to 31% systems reported in literature for direct synthesis of DMC. Thus,
using dibutyltin dimethoxide (catalyst), high pressure (300 bar) the proposed dehydration system in this work may be an
and 180 ꢁC of temperature (see Table 3, entry 10) using a facility effective way to promote increase in DMC direct synthesis yield
that allow to place the dehydrating agent at room temperature whereas can be used with different catalysts.
and circulate the reaction mixture through the dehydration
tube by a high-pressure circulation pump.25 DMC synthesis
Conflicts of interest
using CH3OK and CH3I, without the presence of dehydrating
ꢁ
There are no conicts of interest to declare.
agents, at a pressure of 73 bar and 80 C of temperature was
described achieving a conversion of 16.2% (ref. 37) (see Table 3,
entry 7). Our results evidenced that the use of molecular sieves
in the gas phase allied to DMP in the liquid phase increases
DMC yield up to 42.8%. This yield value is superior when
compared to current literature as seen in Table 3. Yet, a DMC
yield of 30.5% was obtained when placing molecular sieves in
the gas phase using mild temperature and pressure conditions
(80 ꢁC and 40 bar) as seen in Table 3, entry 1. The placement of
the molecular sieves in the gas phase is somehow facilitating
the water withdrawing besides being very important in terms of
easiness of molecular sieve separation.
Acknowledgements
Douglas Faria thanks CAPES; Sandra Einlo thanks CNPq for
research scholarship. This study was nanced in party by the
˜
´
Coordenaçao de Aperfeiçoamento de Pessoal de Nıvel Superior –
Brasil (CAPES) – Finance Code 001. The authors thank Petrobras
for nancial support.
References
1 M. K. Kim and J.-H. Choi, Can increased outdoor CO2
concentrations impact on the ventilation and energy in
buildings?
Environ.,
A
case study in Shanghai, China, Atmos.
4. Conclusions
2019, 210, 220–230, DOI: 10.1016/
Different dehydrating agents (molecular sieve, 2,2-dimethox-
ypropane, sodium sulfate, magnesium oxide and butylene
oxide) were evaluated in the DMC direct synthesis. Yet, a new
j.atmosenv.2019.04.015.
2 I. Chang, M. Lee and G. C. Cho, Global CO2 emission-related
geotechnical engineering hazards and the mission for
34900 | RSC Adv., 2020, 10, 34895–34902
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
sustainable geotechnical engineering, Energies, 2019, 12(13), 15 H. Li, G. Zhang, Y. Wang and S. Zhang, Transesterication of
2567, DOI: 10.3390/en12132567.
3 A. Bansode and A. Urakawa, Towards full one-pass
conversion of carbon dioxide to methanol and methanol-
ethylene carbonate with methanol over Zn-La mixed oxide
catalysts, J. Fuel Chem. Technol., 2018, 46(8), 977–984, DOI:
10.1016/s1872-5813(18)30041-0.
derived products, J. Catal., 2014, 309, 66–70, DOI: 10.1016/ 16 B. Y. Yu, M. K. Chen and I. L. Chien, Assessment on CO2
j.jcat.2013.09.005.
4 Z. Fu, Y. Zhong, Y. Yu, et al., TiO2-Doped CeO2 nanorod
catalyst for direct conversion of CO2 and CH3OH to
Utilization through Rigorous Simulation: Converting CO2
to Dimethyl Carbonate, Ind. Eng. Chem. Res., 2018, 57(2),
639–652, DOI: 10.1021/acs.iecr.7b02923.
dimethyl carbonate: catalytic performance and kinetic 17 S. Chaemchuen, O. V. Semyonov, J. Dingemans, et al.,
study, ACS Omega, 2018, 3(1), 198–207, DOI: 10.1021/
acsomega.7b01475.
5 X. Hu, H. Cheng, X. Kang, L. Chen, X. Yuan and Z. Qi,
Progress on Catalyst Development for Direct Synthesis of
Dimethyl Carbonate from CO2 and Methanol, Chem. Afr.,
2019, 2(4), 533–549, DOI: 10.1007/s42250-019-00082-x.
Analysis of direct synthesis of dimethyl carbonate from 18 P. Unnikrishnan, P. Varhadi and D. Srinivas, Efficient, direct
methanol and CO2 intensied by in-situ hydration-assisted
reactive distillation with side reactor, Chem. Eng. Process.,
2018, 129, 109–117, DOI: 10.1016/j.cep.2018.05.007.
synthesis of dimethyl carbonate from CO2 using a solid,
calcined zirconium phenylphosphonate phosphite catalyst,
RSC Adv., 2013, 3(46), 23993–23996, DOI: 10.1039/
c3ra44368j.
´
´
6 A. Sanchez, L. M. Gil and M. Martın, Sustainable DMC
production from CO2 and renewable ammonia and 19 J.-C. Choi, K. Kohno, Y. Ohshima, H. Yasuda and
methanol, J. CO2 Util., 2019, 33, 521–531, DOI: 10.1016/
j.jcou.2019.08.010.
7 G. Fiorani, A. Perosa and M. Selva, Dimethyl carbonate:
T. Sakakura, Tin- or titanium-catalyzed dimethyl carbonate
synthesis from carbon dioxide and methanol: large
promotion by a small amount of triate salts, Catal.
Commun., 2008, 9(7), 1630–1633, DOI: 10.1016/
j.catcom.2008.01.013.
a
versatile reagent for a sustainable valorization of
renewables, Green Chem., 2018, 20(2), 288–322, DOI:
10.1039/c7gc02118f.
8 H.-Z. Tan, Z.-Q. Wang, Z.-N. Xu, et al., Review on the
synthesis of dimethyl carbonate, Catal. Today, 2018, 316,
2–12, DOI: 10.1016/j.cattod.2018.02.021.
20 H. Chen, S. Wang, M. Xiao, D. Han, Y. Lu and Y. Meng,
Direct Synthesis of Dimethyl Carbonate from CO2 and
CH3OH Using 0.4 nm Molecular Sieve Supported Cu-Ni
Bimetal Catalyst, Chin. J. Chem. Eng., 2012, 20(5), 906–913,
DOI: 10.1016/s1004-9541(12)60417-0.
9 R. Porcar, P. Lozano, M. Isabel Burguete and S. V. L. Eduardo
Garcia-Verdugo, Dimethyl carbonate as a non-innocent 21 D.-H. Lee, J. You, J.-M. Woo, et al., Inuence of dehydrating
benign solvent for the multistep continuous ow synthesis
of amino alcohols, React. Chem. Eng., 2018, 4, 572, DOI:
10.1039/c8re00097b.
agents on the oxidative carbonylation of methanol for
dimethyl carbonate synthesis over a Cu/Y-zeolite catalyst,
Chin. J. Chem. Eng., 2018, 26(5), 1059–1063, DOI: 10.1016/
j.cjche.2018.02.018.
10 S. K. Kabra, M. Huuhtanen, R. L. Keiski and G. D. Yadav,
Selectivity
engineering
of
O-methylation
of 22 K. Tomishige and K. Kunimori, Catalytic and direct
synthesis of dimethyl carbonate starting from carbon
dioxide using CeO2-ZrO2 solid solution heterogeneous
catalyst: effect of H2O removal from the reaction system,
Appl. Catal., A, 2002, 237(1–2), 103–109, DOI: 10.1016/
hydroxybenzenes with dimethyl carbonate using ionic
liquid as catalyst, React. Chem. Eng., 2016, (3), 330–339,
DOI: 10.1039/c6re00016a.
11 C.-M. Hsu, S.-J. Wang, Y.-T. Chen and D. S.-H. Wong, Novel
separation
process
design
for
non-phosgene
s0926-860x(02)00322-8.
´
dimethylhexane-1,6-dicarbamate synthesis by reacting 23 A. L. P. Valente, C. R. F. Riedo and F. Augusto, Analise
dimethyl carbonate with 1,6-hexanediamine, J. Taiwan Inst.
Chem. Eng., 2019, 97, 54–65, DOI: 10.1016/
j.jtice.2019.02.004.
quantitativa por cromatograa, Revista Chemkeys, 2018, 10,
1–10, DOI: 10.20396/chemkeys.v0i10.9602.
24 Y. Wen, R. Zhang, Y. Cang, et al., Direct synthesis of
dimethyl carbonate and propylene glycol using potassium
bicarbonate as catalyst in supercritical CO2, Pol. J. Chem.
Technol., 2015, 17(1), 62–65, DOI: 10.1515/pjct-2015-0010.
12 J.-M. Woo, J. Y. Seo, H. Kim, et al., CuY zeolite catalysts
prepared by ultrasonication-assisted ion-exchange for
oxidative carbonylation of methanol to dimethyl carbonate,
Ultrason. Sonochem., 2018, 44, 146–151, DOI: 10.1016/ 25 J. C. Choi, L. N. He, H. Yasuda and T. Sakakura, Selective and
j.ultsonch.2018.02.013.
high yield synthesis of dimethyl carbonate directly from
carbon dioxide and methanol, Green Chem., 2002, 4(3),
230–234, DOI: 10.1039/b200623p.
´
13 R. Guil-Lopez, N. Mota, J. Llorente, et al., Methanol Synthesis
from CO2: Review of the Latest Developments in
A
Heterogeneous Catalysis, Materials, 2019, 12(23), 3902, 26 S. Fujita, B. M. Bhanage, M. Arai and Y. Ikushima, Synthesis
DOI: 10.3390/ma12233902.
14 M. Zhang, Y. Xu, B. L. Williams, et al., Catalytic Materials for
Direct Synthesis of Dimethyl Carbonate (DMC) from CO2, J.
of dimethyl carbonate from carbon dioxide and methanol in
the presence of methyl iodide and base catalysts under mild
conditions: effect of reaction conditions and reaction
mechanism, Green Chem., 2001, 3(2), 87–91, DOI: 10.1039/
b100363l.
Cleaner
Prod.,
2020,
123344,
DOI:
10.1016/
j.jclepro.2020.123344.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 34895–34902 | 34901

View Article Online
RSC Advances
Paper
27 Q. Cai, C. Jin, B. Lu, H. Tangbo and Y. Shan, Synthesis of
Dimethyl Carbonate from Methanol and Carbon dioxide
sinter hydration-dehydration behavior in refractory
ˆ
castables), Ceramica, 2008, 54, 145–151.
using Potassium Methoxide as Catalyst under Mild 36 M. V. Dover and J. W. Marden, A Comparison of the
Conditions, Catal. Lett., 2005, 103(3–4), 225–228, DOI:
10.1007/s10562-005-7158-2.
Efficiency of Some Common Desiccants, J. Am. Chem. Soc.,
1917, 39(8), 1609–1614, DOI: 10.1021/ja02253a009.
28 Y. Chen, Y. Yang, S. Tian, et al., Highly effective synthesis of 37 S. Fang and K. Fujimoto, Direct synthesis of dimethyl
dimethyl carbonate over CuNi alloy nanoparticles@porous
organic polymers composite, Appl. Catal., A, 2019, 587,
117275, DOI: 10.1016/j.apcata.2019.117275.
carbonate from carbon dioxide and methanol catalyzed by
base, Appl. Catal., A, 1996, 142(1), L1–L3, DOI: 10.1016/
0926-860x(96)00081-6.
29 J. R. Thorpe and D. M. R. Harvey, Optimization and 38 A. A. Marciniak, O. C. Alves, L. G. Appel and C. J. A. Mota,
investigation of the use of 2,2-dimethoxypropane as
a dehydration agent for plant tissues in transmission
electron microscopy, J. Ultrastruct. Res., 1979, 68(2), 186–
194, DOI: 10.1016/s0022-5320(79)90153-9.
Synthesis of dimethyl carbonate from CO2 and methanol
over CeO2: role of copper as dopant and the use of methyl
trichloroacetate as dehydrating agent, J. Catal., 2019, 371,
88–95, DOI: 10.1016/j.jcat.2019.01.035.
¨
30 E. Leino, P. Maki-Arvela, V. Eta, et al., The inuence of 39 A. H. Tamboli, A. A. Chaugule and H. Kim, Highly selective
various synthesis methods on the catalytic activity of
cerium oxide in one-pot synthesis of diethyl carbonate
starting from CO2, ethanol and butylene oxide, Catal.
Today, 2013, 210, 47–54, DOI: 10.1016/j.cattod.2013.02.011.
and multifunctional chitosan/ionic liquids catalyst for
conversion of CO2 and methanol to dimethyl carbonates at
mild reaction conditions, Fuel, 2016, 166, 495–501, DOI:
10.1016/j.fuel.2015.11.023.
¨
¨
31 V. Eta, P. Maki-Arvela, J. Warn, T. Salmi, J. P. Mikkola and 40 V. Deerattrakul, A. Panitprasert, P. Puengampholsrisook and
D. Y. Murzin, Kinetics of dimethyl carbonate synthesis
from methanol and carbon dioxide over ZrO2-MgO catalyst
in the presence of butylene oxide as additive, Appl. Catal.,
A, 2011, 404(1–2), 39–46, DOI: 10.1016/j.apcata.2011.07.004.
32 K. Kohno, J.-C. Choi, Y. Ohshima, A. Yili, H. Yasuda and
P. Kongkachuichay, Enhancing the Dispersion of Cu-Ni
Metals on the Graphene Aerogel Support for Use as
a Catalyst in the Direct Synthesis of Dimethyl Carbonate
from Carbon Dioxide and Methanol, ACS Omega, 2020,
5(21), 12391–12397, DOI: 10.1021/acsomega.0c01143.
T. Sakakura, Reaction of dibutyltin oxide with methanol 41 S. K. Kabra, E. Turpeinen, R. L. Keiski and G. D. Yadav,
under CO2 pressure relevant to catalytic dimethyl
carbonate synthesis, J. Organomet. Chem., 2008, 693(7),
1389–1392, DOI: 10.1016/j.jorganchem.2008.01.028.
Direct synthesis of dimethyl carbonate from methanol and
carbon dioxide: a thermodynamic and experimental study,
J. Supercrit. Fluids, 2016, 117, 98–107, DOI: 10.1016/
j.supu.2016.05.039.
33 D. B. G. Williams and M. Lawton, Drying of organic solvents:
Quantitative evaluation of the efficiency of several 42 J. R. O'Neil, R. N. Clayton and T. K. Mayeda, Oxygen Isotope
desiccants, J. Org. Chem., 2010, 75(24), 8351–8354, DOI:
10.1021/jo101589h.
Fractionation in Divalent Metal Carbonates, J. Chem. Phys.,
1969, 51(12), 5547–5558, DOI: 10.1063/1.1671982.
34 D. R. Bureld, K.-H. Lee and R. H. Smithers, Desiccant 43 M. Zhang, M. Xiao, S. Wang, D. Han, Y. Lu and Y. Meng,
efficiency in solvent drying. A reappraisal by application of
a novel method for solvent water assay, J. Org. Chem., 1977,
42(18), 3060–3065, DOI: 10.1021/jo00438a024.
Cerium oxide-based catalysts made by template-
precipitation for the dimethyl carbonate synthesis from
carbon dioxide and methanol, J. Cleaner Prod., 2015, 103,
847–853, DOI: 10.1016/j.jclepro.2014.09.024.
˜
˜
˜
35 R. Salomao and V. C. Pandolfelli, Hidrataçao e desidrataçao
´
´
´
de oxido de magnesio em concretos refratarios (Magnesia
34902 | RSC Adv., 2020, 10, 34895–34902
This journal is © The Royal Society of Chemistry 2020