Highly efficient synthesis of dimethyl carbonate from methanol and carbon dioxide using IL/DBU/SmOCl as a novel ternary catalytic system

Article abstract of DOI:10.1016/j.catcom.2015.12.009

Excellent yield of dimethyl carbonate (DMC) was obtained by direct physical or chemical adsorption of carbon dioxide on [EmimOH][NTf₂] ionic liquid (IL) in the presence of samarium oxychloride (SmOCl) and 1,8-diazabicyclo[2.2.2]undec-7-ene (DBU) as a super base. The novel ternary catalyst system consisting of [EmimOH][NTf₂], DBU, and SmOCl was found to appreciably convert methanol (13.01%) to DMC with excellent selectivity (99.13%). The adsorption of CO₂ on IL in the presence of DBU was analyzed by ¹³C experiment. Moreover, catalytic reactivity of SmOCl and OH-functional group was proved by a predictable mechanism. Various parameters such as reaction temperature, pressure, reaction time, and reusability of catalyst were investigated to maximize DMC yield.

Full text of DOI:10.1016/j.catcom.2015.12.009

Highly efficient synthesis of dimethyl carbonate from methanol and carbon
dioxide using IL/DBU/SmOCl as a novel ternary catalytic system
Avinash A. Chaugule, Harshad A. Bandhal, Ashif H. Tamboli, Wook-
Jin Chung, Hern Kim
PII:
DOI:
Reference:
S1566-7367(15)30163-1
doi: 10.1016/j.catcom.2015.12.009
CATCOM 4526
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
24 September 2015
1 December 2015
12 December 2015
Please cite this article as: Avinash A. Chaugule, Harshad A. Bandhal, Ashif H. Tam-
boli, Wook-Jin Chung, Hern Kim, Highly efficient synthesis of dimethyl carbonate from
methanol and carbon dioxide using IL/DBU/SmOCl as a novel ternary catalytic system,
Catalysis Communications (2015), doi: 10.1016/j.catcom.2015.12.009
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ACCEPTED MANUSCRIPT
Highly efficient synthesis of dimethyl carbonate from methanol and carbon dioxide using
IL/DBU/SmOCl as a novel ternary catalytic system
Avinash A. Chaugule, Harshad A. Bandhal, Ashif H. Tamboli, Wook-Jin Chung, Hern Kim
Department of Energy Science and Technology, Energy and Environment Fusion Technology
Center, Myongji University, Yongin, Gyeonggi-do 17058, South Korea
Corresponding author:
Hern Kim
Department of Energy Science and Technology
Energy and Environment Fusion Technology Center
Myongji University
Yongin
Gyeonggi-do 17058
South Korea
Tel. +82 31 330 6688
Fax: +82 31 336 6336
E-mail: hernkim@mju.ac.kr
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Highlights
IL with DBU and SmOCl is used as ternary catalyst system for the synthesis of DMC.
2 and methanol by IL/DBU and SmOCl, respectively.
rnary catalyst system is tested by recycling.
13C NMR experiment.
Abstract
Excellent yield of dimethyl carbonate (DMC) was obtained by direct physical or chemical
adsorption of carbon dioxide on [EmimOH][NTf2] ionic liquid (IL) in the presence of samarium
oxychloride (SmOCl) and 1,8-diazabicyclo[2.2.2]undec-7-ene (DBU) as a super base. The novel
ternary catalyst system consisting of [EmimOH][NTf2], DBU, and SmOCl was found to
appreciably convert methanol (13.01%) to DMC with excellent selectivity (99.13%). The
13
adsorption of CO2 on IL in the presence of DBU was analyzed by C experiment. Moreover,
catalytic reactivity of SmOCl and OH-functional group was proved by a predictable mechanism.
Various parameters such as reaction temperature, pressure, reaction time, and reusability of
catalyst were investigated to maximize DMC yield.
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Keywords: Ionic liquid; Carbon dioxide; Samarium oxychloride; Dimethyl carbonate
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1. Introduction
Carbon dioxide (CO2) is a primary greenhouse gas produced by human activities.
Because of its enormous emission, CO2 contributes >60% to global warming.1 On the contrary,
CO2 is an abundant nontoxic and recyclable carbon source that can substitute conventional toxic
chemicals such as phosgene, isocyanides, and carbon monoxide.2 Synthesis of carbonic acid
esters has attracted considerable research attention recently. Dimethyl carbonate (DMC) is the
lowest homology of this family, which has several industrial applications, including use as
methylating3 and carbonylating agent4 instead of toxic dimethyl sulfate and phosgene, food-
flavoring agent,5 solvent for secondary lithium-ion batteries,6 and reagent for biodiesel
production.7
Phosgenation was the first commercial method for the production of DMC, but due to the
extreme toxicity of phosgene, this process has been replaced by processes such as oxidative
carbonylation and transesterification. Major drawbacks of oxidative carbonylation are the use of
poisonous and corrosive gases and explosion risk associated with the use of CO and O2 mixture.
On the contrary, direct synthesis of DMC from CO2 and methanol could be an ideal alternative
for DMC production at commercial level. The use of cheap, abundant, and noncorrosive raw
materials (CO2 and MeOH) makes it an environmentally benign process.8 Although a number of
catalysts such as Ni(acetate)2, ZrO2, polyphosphoric acid–ZrO2, ZrO2–MgO, CeO2, and Ce–ZrO2
solid solutions have been investigated for this reaction, DMC yield still remains low.9,10 The use
of stoichiometric amounts of dehydrating agents can help increase DMC yield considerably.
However, most of these agents are irrecoverable or require a regeneration process. Therefore,
their use has some limitations in terms of production cost. In addition, DMC reaction also
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proceeded smoothly with high yield using catalyst including tin alkoxy compounds such as
Bu2Sn (OEt)2, Sn(OMe)4, and Sn(OBu)4.10
In recent years, ionic liquids (ILs) have attracted increasing attention as catalyst due to
their broad liquid range and excellent thermal and chemical stability.11,12 Concentration of CO2
in solution increased significantly by adding IL into the reaction mixture. In addition to being a
CO2 sorbent, ILs can act as a water scavenger. Therefore, their use as a solvent, promoter, or
catalyst has been extensively investigated by several groups. In this study, basic IL is synthesized,
whose combination with an organic super base, 1,8-diazabicyclo[2.2.2]undec-7-ene (DBU), can
reversibly capture CO2. The effect of addition of metal salts is also investigated. This ternary
system consisting of an IL, organic base, and metal oxychloride was found to be highly efficient
for catalyzing direct DMC synthesis.
2. Experimental
2.1. Preparation of IL
2.1.1 Preparation of 1-(2-hydroxyethyl)-3-methyl imidazolium chloride [EmimOH][Cl] IL
1-Methyl imidazole (10.0 g 0.12 mol) and 2-chloroethanol (14.7 g, 0.12 mol) were taken
in a 100-ml round-bottomed flask and refluxed with 40-ml acetonitrile under continuous stirring
for 28 h. Then, the reaction mixture was slowly cooled to −4°C to afford a white crystalline solid
as 1-(2-hydroxyethyl)-3-methyl imidazolium chloride IL.
2.1.2 Preparation of 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)
imide [EmimOH][NTf2] IL
1-(2-Hydroxyethyl)-3-methyl imidazolium chloride (6 g, 0.018 mol) was reacted with
bis(trifluoromethanesulfonyl)imide lithium salt (10.6 g, 0.018 mol) and 40 ml of acetone under
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stirring at room temperature for 28 h. Then, the mixture was filtered to remove LiCl salt formed
during the reaction. The filtrate was then evaporated under reduced pressure to obtain [HEMIm]
[NTf2] IL, which was subjected to silver nitrate test to ensure complete removal of chloride.
2.2. Characterization
All synthesized ILs were characterized by 1H nuclear magnetic resonance (NMR) and 13C
NMR spectroscopy using Bruker 500 MHz and 125 MHz spectrometers, respectively, in CDCl3.
Infrared spectra of the prepared ILs were observed by a Fourier transform infrared spectrum
(FTIR, Varian 2000). Thermal decomposition temperature was measured using
thermogravimetric analysis (TGA, PerkinElmer TGA-7 instrument). Differential scanning
calorimetry (DSC) data were obtained in a sealed aluminum pan with a cooling and heating rate
of 10°C/min on Mettler DSC822. The general structure of the compounds was examined by X-
ray diffraction (XRD) using an analytical X’Pert MPD diffractometer with Bragg’s angle ranging
from 20° to 80°.
2.3. DMC synthesis from methanol and CO2
Methanol (25 ml, 0.625 mol) and [HEMIm][NTf2] IL (4 g, 0.07 mol) with DBU (0.007
mol) and samarium oxychloride (SmOCl) (0.005 mol) were charged into the reactor. The
reaction was carried out at 140°C under continuous stirring at 700 rpm for 4 h. After completion
of reaction, the reaction mixture was distilled to separate the IL. The DMC in liquid phase was
analyzed by gas chromatography (GC-17A, Shimadzu Corporation) using capillary column
(Stabilwax, 30-m-long, 0.53-mm inner diameter, and 1-µm film thickness) equipped with a flame
ionization detector. The amount of the product was determined using the external standard
1
method. Furthermore, DMC was identified by H NMR, FTIR, and gas chromatography–mass
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spectrometry (GC–MS).
3. Results and discussion
3.1. Catalyst characterization
3.1.1. TGA of [EmimOH][NTf2] IL
TGA is very important for estimating the stability of the IL. The NTf2-containing room
temperature ionic liquid have a wider liquid range than other organic compounds.13 Thermal
decomposition profiles of the prepared IL are shown in Figure S1a. It was observed that
[EmimOH][NTf2] IL is stable up to 350°C without any significant weight loss. Moreover, DSC
curve of the IL was observed in two cycles that shows no crystallization on cooling from 340 to
−30°C and preserves good stability on heating from −30 to 340°C. This might be due to the
presence of high molecular weight NTf2 anion conjugate with imidazolium containing ring and
strong intermolecular interaction between them.
3.1.2. XRD characterization of the Cocatalyst
Figure S1b illustrates the XRD patterns of Sm2O3, SmOCl, and SmCl3, respectively. The
XRD pattern of Sm2O3 shows diffraction peaks at 2θ = 19.97, 28.26, 32.76, 47.06, and 55.90
with crystal planes (JCPDS card no. 00-015-0813). When SmCl3 was calcined at 600°C, its
monoclinic structure was completely converted to a tetragonal structure of SmOCl that is
confirmed by XRD patterns with peaks at 13.14, 27.95, 31.98, 34.93, 39.42, 41.78, 45.56, 47.94,
52.21, 53.40, and 58.66 with crystal planes (JCPDS card no. 00-012-0790). This, structural
change of SmCl3 to SmOCl might change Lewis acidic and basic nature in crystalline structure.
Therefore, acidic and basic properties of SmOCl in combination with IL and DBU probably
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demonstrate activation of MeOH.
3.2. Catalytic performance of DMC synthesis
The Lewis basic ILs catalyzed the conversion of carbon dioxide to various cyclic
carbonates, such as IL derived from 1,4-diazabicyclo[2.2.2]octane (DABCO) and DBU.14 Easy
availability, low cost, and air/water stability have attracted increased attention toward this salt. In
this context, DMC was synthesized from MeOH and CO2 by IL/DBU/SmOCl catalytic system.
−
For comparison, [EmimOH]+ cation with Cl− and NTf2 anion IL base was tested in this study. In
fact, no reaction occurred without catalyst (Table 1). [EmimOH][NTf2] ILs were found to have
high activity with 4.8% yield and 60% selectivity of DMC, whereas the yield of [EmimOH][Cl]
IL was low (Table 1), because anion containing fluorine alkyl (NTf2) group possess lower
binding energy of anion–CO2 complex than anion containing Cl.15 The DBU was also found to
be inactive, which might be due to loss of activity by the formation amidinium alkyl carbonate
16
(Scheme 1) species with MeOH and CO2 (Table 1). Subsequently, both DBU and
[EmimOH][NTf2] IL had a considerable effect on catalytic activity (Table 1). This increase of
DMC yield upon addition of DBU and IL could be due to their ability to capture CO2 reversibly.
This immobilization of CO2 enhances the reaction by not only converting CO2 in more reactive
molecule, but also increasing the concentration of CO2 in the reaction mixture. Hence,
[EmimOH][NTf2]/DBU catalytic system was considered for future catalytic studies.
Metal oxy salts are associated with Lewis acidic and basic sites. Hence, their use as a
cocatalyst can improve methanol conversion without affecting DMC selectivity. In addition, both
Lewis basic (Sm2O3) + Lewis acidic (SmCl3, Sm (NO3)3) and Lewis acidic + Lewis basic
(SmOCl, FeOCl, LaOCl) metal salts with IL/DBU have a considerable effect on DMC yield.
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These results reveal that catalytic efficiency of metal salts decreases in the following order:
SmOCl>LaOCl>FeOCl>SmCl3>SmONO>Sm2O3>SmNO3 (Table 2). This clearly shows that
metal salts with both Lewis acidic and Lewis basic sites exhibit superior activity to those with
either acidic or basic properties. Moreover, turnover number of the IL + DBU catalytic system
was increased from 6.93 to 11.77 with addition of SmOCl (Table 2). On the contrary, only
SmOCl was found to be completely inactive for DMC (Table 2). The presence of IL/DBU
system has a potential to form the carbonate species with CO2, which can be considered as an
activated form of CO2. This is interesting because IL cation and anion moieties stabilized the
intermediate and DBU can help absorb the CO2, thereby serving as excellent promoters for this
reaction. Moreover, SmOCl mainly activate methanol through coordination, and the generated
methoxide anion can favor the attack on CO2-absorb intermediate 2 (Scheme 1) formed by DBU
and IL. Hence, the IL/DBU/SmOCl catalytic system has shown potential to obtain high yield of
DMC (Table 2).
3.2.1. Effect of temperature, pressure, and time on DMC synthesis
In order to investigate the effect of temperature on DMC yield, the reaction was carried
out at different temperatures, and the results are shown in Figure 1a. Both DMC production and
selectivity increased with an increase of temperature. When the temperature was increased from
80 to 140°C, the amount of DMC produced increased from 0.09 to 0.11 mol. On the contrary,
when the temperature was further increased to 170°C, the amount of DMC produced and
selectivity toward DMC sharply decreased. Moreover, this temperature (170°C) accelerated other
side reactions, such as formation of dimethoxymethane (DMM) and decomposition of DMC to
other gaseous by-products. However, 140°C was determined as the optimal temperature for the
reaction, and all further reactions were carried out at this temperature.
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Figure 1b shows the effect of pressure on the synthesis of DMC. It is evident from the
figure that the production of DMC increases sharply with the increase of pressure up to 6.5 MPa.
Further increase of pressure did not have any pronounced effect on DMC production. Hence, all
further experiments were conducted at the optimal pressure of 6.5 MPa. Time profile of
IL/DBU/SmOCl-catalyzed reaction of DMC is shown in Figure 1c. It is evident from the figure
that DMC yield increases linearly with time and reaches its maximum value after 4 h. Further
progress of the reaction decreased DMC yield from 0.11 to 0.09 mol after 8 h.17 These results
indicate that DMC yield rapidly increased with time up to 4 h, and decomposition of DMC to
other gaseous products occurred on further progression of the reaction, which might be due to
side reactions.
3.2.2. Effect of amount of IL and SmOCl
Figure 2a shows the effect of different amounts of IL on the synthesis of DMC. DMC
yield depends on the composition of IL and the presence or absence of cocatalyst. When both
cocatalyst and IL are absent, the yield of DMC was zero (Fig. 2). Addition of IL increases both
yield and selectivity of DMC. Moreover, addition of 4 g of IL increased DMC yield and
selectivity to 0.11 mol and 99.12%, respectively. Subsequently, addition of 0.5 g of SmOCl
resulted in an excellent yield of DMC (Fig. 2b). In conclusion, addition of IL catalyst (4 g) and
SmOCl (0.5 g) as cocatalyst exhibited high performance at 140°C and 6.4 MPa.
3.3. Reaction mechanism
In this study, absorption of CO2 can occur by reaction between either DBU, methanol,
18
and CO2 or DBU, IL, and CO2.19 Thus, DBU and [EmimOH][NTf2] react first to form
zwitterionic intermediate 1 (Scheme 1). This intermediate then reacts with CO2 to form
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amidinium alkyl carbonate salt 2.20 Formation of both 1 and 2 was verified by 13C NMR
spectroscopy (Fig. S2). The chemical shift of C7 carbon of pure DBU was observed at 162.5 ppm.
After addition of [EmimOH][NTf2] IL in DBU, the chemical shift changes to 163.84 ppm due to
development of partial positive charge on C7 carbon, which indicates formation of intermediate 1
(Fig. S2). In addition, when CO2 gas is bubbled through this mixture, a new peak appears at 160
ppm, which could be attributed to carbonate carbon of intermediate 2.
SmOCl provides a synergistic combination of Lewis acidic chloride and Lewis basic
oxide20 that can activate methanol. First, methanol coordinates with Lewis acidic samarium and
makes hydroxyl hydrogen acidic. Subsequent abstraction of this hydrogen (H) by Lewis basic
oxygen SmOCl will generate methoxide ion. Nucleophilic attack of this methoxide on
intermediate 2 results in the formation of intermediate 3, which undergoes attack from another
methoxide ion to form intermediate 4. Finally, intermediate 4 undergoes an intramolecular
decomposition to give products and regenerate the zwitterionic intermediate 1.
3.4. Reusability
Finally, methanol and DMC were completely removed under reduced pressure. Then,
fresh methanol was added to the remaining mixture of IL, DBU, and SmOCl and reused for the
next cycle under the same reaction conditions. The reusability results are shown in Figure 3,
which demonstrates that the activity of catalysts was almost constant even at third cycle. Thus,
the catalyst system (IL/DBU/SmOCl) can be used thrice without any predominant change of
DMC selectivity. Moreover, after the third cycle, XRD analysis of SmOCl showed that, except
(001) and (003), all peaks repeated exactly. Hence, there was no significant change in the crystal
plane of SmOCl and, therefore, its stability was satisfied (Fig. S5).
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Conclusions
In summary, a new catalyst system for direct DMC synthesis from methanol and CO2 was
developed. The ternary catalytic system [EmimOH][NTf2]/DBU/SmOCl, which facilitates the
reaction, proceeds through CO2 absorption by OH-functional group and results in high yield of
DMC. SmOCl also has a synergetic effect on catalyzing the direct DMC synthesis reaction.
Moreover, catalysts (IL/DBU/SmOCl) can be easily recovered and reused without pronounced
loss in catalytic activity.
Acknowledgments
This study was supported by National Research Foundation of Korea (NRF) – Grants funded by
the Ministry of Science, ICT and Future Planning (2014R1A2A2A01004352) and the Ministry
of Education (2009-0093816), Republic of Korea.
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Scheme captions
Scheme 1. Predicted mechanism for synthesis of DMC in the presence of IL, DBU, and SmOCl.
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Table captions
Table 1. Catalytic performance of various ILs for synthesis of DMC
Table 2. Effect of catalytic performance on synthesis of DMC using IL + DBU with various
metal oxy salts.
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Figure captions
Figure 1. Effect of temperature (a), pressure (b), and reaction time (c) on synthesis of DMC
using 617 mmol of methanol, 6.4 MPa CO2, 0.5 g SmOCl, 4 g IL, 1 g DBU, 140°C,
and 4 h
Figure 2. Effect of ionic liquid (IL) loading (a) and SmOCl loading (b) on synthesis of DMC
under optimized conditions.
Figure 3. Reusability of catalytic system (IL/DBU/SmOCl) for synthesis of DMC under
optimized conditions.
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Table 1
Methanol
Conversion
[%]
DMC
Selectivity
[%]
Entry
Catalyst/additive
Yield
[%]
a
1
2
3
-
-
-
-
DBU
1
2
0
14.3
33.7
4.8
[EmimOH][Cl]
8.1
9.5
60.0
72.2
4.8
6.9
[EmimOH][NTF2]
4
5
[EmimOH][NTF2] +DBU
[EmimOH][NTF2] + DBU +
SmOCl
6
13.0
99.1
12.8
a; Reaction carried out without any catalyst. Reaction conditions: 783 mol of methanol, 6.4 MPa CO2, 4 g IL, 1 g DBU, 4 h.
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Table 2
DMC
Selectivity
Catalyst system
Turnover
number
Methanol
Conversion
[%]
[%]
34.1
59.6
40.4
38.0
99.1
88.0
86.1
1.5
[TON]
1
2
3
4
5
6
7
8
[EmimOH][NTF2]/DBU/
SmNO3
8.6
11.9
18.1
9.0
2.1
[EmimOH][NTF2]/DBU/
SmONO
7.4
7.0
1.5
11.7
9.4
8.5
0.4
[EmimOH][NTF2]/DBU/
SmCl3
[EmimOH][NTF2]/DBU/
Sm2O3
[EmimOH][NTF2]/DBU/
SmOCl
13.0
11.4
10.3
1.2
[EmimOH][NTF2]/DBU/
LaOCl
[EmimOH][NTF2]/DBU/
FeOCl
SmOCl
Reaction conditions: 783 mol of methanol, 6.4 MPa CO2, 4 g IL, 4 h.
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Scheme 1
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Figure 1
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Figure 2
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Figure 3
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Graphical abstract
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