Deep eutectic solvents as attractive media for CO2 capture

Article abstract of DOI:10.1039/c5gc02319j

We report a family of deep eutectic solvents (DESs) consisting of various hydrogen bonding donor-acceptor pairs as CO₂ capturing media. These DESs capture CO₂via carbamate formation upon reaction between their hydrogen bonding donor units and CO₂. Among the members tested herein, a DES made up of monoethanolamine hydrochloride-ethylenediamine exhibits an unprecedentedly high gravimetric uptake of 33.7 wt% with good initial kinetics (25.2 wt% uptake within 2.5 min) and recyclability. The given DES also shows sustainable performance in the presence of water, decent tolerance against temperature rise, and a relatively low heat of absorption which is attractive for regeneration. Even with the high gravimetric uptake, the DES has a far more suppressed corrosiveness compared to its pure monoethanolamine and ethylenediamine counterparts due to low oxygen/moisture permeability and the hydrogen bonding network that alleviates the corrosion redox cycle. The observed excellent properties in various key aspects of CO₂ capture suggest that DESs are strong candidates to replace the conventional monoethanolamine-based scrubbing technology and are worth further exploration.

Full text of DOI:10.1039/c5gc02319j

View Article Online
View Journal
Green
Chemistry
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: T. J. Trivedi, J. H.
Lee, H. J. Lee, Y. K. Jeong and J. W. Choi, Green Chem., 2016, DOI: 10.1039/C5GC02319J.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/greenchem

Please do not adjust margins
Green Chemistry
Page 1 of 10
View Article Online
DOI: 10.1039/C5GC02319J
Journal Name
ARTICLE
Deep Eutectic Solvents as Attractive Media for CO Capture
2
Tushar J. Trivedi, Ji Hoon Lee, Hyeon Jeong Lee, You Kyeong Jeong, and Jang Wook
Choi*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
We report a family of deep eutectic solvents (DESs) consisting of various hydrogen bonding donor-acceptor pairs as CO
2
www.rsc.org/
capturing media. These DESs capture CO
2
via carbamate formation upon reaction between their hydrogen bonding donor
units and CO . Among the members tested herein, a DES made of monoethanolamine hydrochloride-ethylenediamine
2
exhibits an unprecedentedly high gravimetric uptake of 33.7 wt% with good initial kinetics (25.2 wt% uptake within 2.5
min) and recyclability. The given DES also shows sustainable performance in the presence of water, decent tolerance
against temperature rise, and relatively low heat of absorption attractive for regeneration. Even with the high gravimetric
uptakes, the DES has far suppressed corrosiveness compared to pure monoethanolamine and ethylenediamine
counterparts due to low oxygen/moisture permeability and the hydrogen bonding network that alleviates the corrosion
2
redox cycle. The observed excellent properties in various key aspects of CO capture suggest that DESs are strong
candidates to replace conventional monoethanolamine-based scrubbing technology and are worth to further explore.
formation were usually limited below 0.5 moles per each mole
1
8
Introduction
of ILs. A viscosity increase of these amine-functionalized ILs
during the course of CO capture is another hurdle that impairs
2
Increasing carbon dioxide (CO ) emission originating from the
2
the process efficiency. More critically, nearly all ILs are too
expensive for their large-scale application.
use of fossil fuels results in various environmental and energy
1
issues on the global scales. Carbon capture and sequestration
It is also noted that most of task specific ILs (TSILs) used for
(
CCS) is a comprehensive measure to resolve these issues, but
depends stringently on materials design, process efficiency,
and cost. In particular, for CO capture, it is desirable to
CO
2
capture were evaluated based on their molar uptake
moles per each mole of IL). Because of
values (CO
2
2
2
disadvantageous high molecular weights of most ILs, the molar
uptake values could appear less attractive once converted to
9, 12, 17
develop new sorbent materials or systems, which are more
advantageous than the conventional amine scrubbing
3
technology in terms of uptake amount, energy consumption,
the gravimetric values (CO
2
weight/IL weight).
The
highest gravimetric uptake values were found from superbase
protic IL (16.4 wt%, 1,3,4,6,7,8-hexahydro-1-methyl-2H-
cost, and recyclability.
Among many candidates, ionic liquids (ILs) have been
pyrimido[1,2-a]
pyrimidinium
trifluoroethanolate,
2
extensively investigated to increase CO uptake mostly by
1
MTBDH][TFE]) and dual amino-functionalized IL (18.5 wt%,
2
[
tuning ILs functionality in both cations and anions. In this
direction, a variety of ILs containing amine-functionalized
4
cations/anions, amino acids, aprotic heterocyclic anions
1
9
aqueous 1,3-bis(3-aminopropyl)imidazolium bromide, DAIL).
-7
7-9
However, the intrinsic drawbacks of ILs, including inferior
kinetics, high cost, and inefficient energy consumption, are still
to be resolved.
1
0, 11
12, 13
14
(AHAs),
super bases,
guanidinium
and pyridinium
based
absorption depending on
binding mechanism and strength. As in conventional
scrubbing processes (~30 wt% aqueous MEA solution), amine-
1
5
anions have been examined. Also, ILs can capture CO
2
1
6
4-15, 17
In an effort to overcome the intrinsic shortcomings of ILs
on both physical and chemical
the CO
for CO
2
capture, deep eutectic solvents (DESs), a class of
2
2
0
sustainable solvents analogous to ILs, have been investigated
4
-7
for the past decade. In general, DESs are prepared by mixing of
two components, namely hydrogen bond acceptors (HBAs)
such as organic halide salts and hydrogen bond donors (HBDs)
such as amines, amides, carboxylic acids, and alcohols, to form
low melting temperature solutions by means of hydrogen
tethered ILs capture CO
2
by forming carbamates. However,
the CO uptake capacities from ILs relying on carbamate
2
2
1-23
bonding interactions.
DESs have conspicuous advantages
over ILs, such as simple synthesis, lower material cost, no by-
product generation, and biodegradable nature, making them
2
more attractive for practical applications. The synthesis of
3
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 2 of 10
View Article Online
DOI: 10.1039/C5GC02319J
ARTICLE
Journal Name
DESs is quite simple, as it requires mainly a mixing step of measurements (SDT Q20 V23.5, TA Instrument, USA): -80 to
o
HBAs and HBDs. Also, ammonium-containing DESs are known 250 C, scan rate=5 C/min for hydrochloride salts and -80~80
o
2
4
to be non-toxic. The studies on the use of DESs for CO
o
°C, scan rate=2.5 C/min for DESs. Thermal stability of DESs
2
capture date back to the original investigations with various was characterized by thermogravimetric analysis (TGA) by
o
HBDs (urea, glycerol, lactic acid, etc.) paired with different heating the samples to 450 C at 5 °C min (TGA 92-18,
−1
2
5-30
HBAs
ammonium/phosphonium halides, etc.).
uptakes of these DESs were all limited in the range of 6000 rheometer with plate-plate geometry (Thermo Scientific).
.02~0.14 mol per each mole of DES with the highest value
(choline
chloride,
other
alkyl Setaram, France). Viscosity analyses were performed in the
3
1
However, CO temperature range of 30~90 °C using a HHAKE Rheostress
2
0
equivalent to 7 wt%.
Preparation of Hydrochloride Salts [MEA.Cl]
Herein, we report a new class of DESs that exhibit far
MEA was reacted with equimolar amounts of HCl in deionized
o
water at 0 °C. The solution was heat-treated to 60 C and
higher CO
These DESs employ ethylenediamine (EDA) as an HBD to derive
2
uptakes compared to those of the existing ones.
stayed at this temperature for 3 hrs. After completion of the
reaction, water was evaporated using a rotary evaporator. The
unreacted MEA and excess HCl were removed by washing with
a mixture of ethyl ether and acetone to acquire a white solid
with >90% yield. The product was finally vacuum dried and
stored inside a desiccator.
the carbamate formation upon the interaction with CO . EDA
2
was paired with various HBAs (Scheme 1), and the DES
exhibiting the highest CO uptakes was identified. Notably, the
2
best DES reaches a gravimetric CO uptake above 30 wt%,
2
which is, to the best of our survey, the highest value among all
the existing CO storage materials including ILs (the highest ~
2
1
8.5 wt%), DESs (the highest ~ 7 wt%) as well as various dry
9
31
1
Syntheses of the deep eutectic mixtures (DESs) [MEA.Cl][EDA]
3
2
sorbents reported to date. Also, the current DESs exhibit
decent tolerance against temperature rise and corrosion due
to stable hydrogen bonding networks. Moreover, the uptake
increases even in the increased content of water because of
viscosity drop that enhances the accessibility of absorption
sites, which is beneficial for actual on-site operations.
DESs were prepared by mixing [MEA.Cl] with EDA in molar
ratios from 1:1 to 1:4. The mixtures were stirred for 30 min
until they became homogeneous. The mixtures were then
dried under vacuum to remove excess moisture. The dried
DESs were monitored for 3 hrs to check any precipitant
formation. Other DESs based on different HBAs were also
synthesized by the same procedure. The chemical structures,
molar compositions and abbreviation of the prepared DESs are
shown in Scheme 1.
Additionally, these DESs hold relatively smaller CO absorption
2
-1
enthalpy (i.e. –63.8 ~ –17.4 kJ mole ) than that of
conventional scrubbing counterparts and are thus
advantageous for regeneration. Together with the inherent
advantages of DESs including low material cost and eco-
friendly nature, the high gravimetric uptakes and robust
recyclability would position this new class of DESs as promising
alternatives to the traditional scrubbing process.
Experimental
Materials
Monoethanolamine (MEA), triethanolamine (TEA), urea (UE),
thioacetamide (TAE), ethylendiamine (EDA), and hydrochloric
acid (HCl, 37%) ethylene glycol (EG) were purchased from
Sigma Aldrich with purities higher than 98%. CO
supplied from Lead Gas Chemicals, Korea.
2
(99.99%) was
Characterization
1
13
H NMR and C NMR spectra were recorded using Agilent
00MHz 54mm NMR DD2 in D O with tetramethylsilane as the
Scheme 1. A synthetic route to prepare deep eutectic solvents (DESs) together with
hydrogen bonding acceptors (HBAs) investigated in this study.
4
2
standard. Electrospray ionization mass spectroscopy (ESI-MS)
characterization was performed using a high resolution hybrid
tandem liquid chromatography (micrOTOF-Q instrument, Characterization of CO absorption
Bruker Daltonics). The peaks in the ESI-MS spectra were
2
For CO absorption measurements of the synthesized DESs,
2
+
+
+
interpreted based on adduct (H , Na , and K ) formation.
Fourier transform infrared (FT-IR) spectra were obtained using
CO gas was flown into a vial (10 mL) containing 2 g of DES at a
2
–1
flow rate of 10 mL min . The weight percent of CO absorbed
2
a
Perkin-Elmer FT–IR spectrometer. Melting points of
was determined by weighing the vial at a regular interval using
an electronic balance with an accuracy of ± 0.1 mg. For
hydrochloride salt and DESs were attained by conducting DSC
2
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 10
Pleas Ge rdeo e nn o Ct ah de j mu si ts mt r ya rgins
View Article Online
DOI: 10.1039/C5GC02319J
Journal Name
ARTICLE
Table 1 Comparative CO
2
uptakes (both in mole and wt%) and molecular weights of
different DESs and ILs
CO
2
CO
2
T
MW
o
b
IL/DESs
-1
uptake
uptake
(wt%)
7.40
( C)
References
(
g mol )
a
(
mole)
0.50
1.13
2.10
c
17
[
APBim][BF
4
]
269.09
253.26
612.07
~22
23
Davis
c
12
[
MTBDH][TFE]
16.4
Dai
c
9
[
N
66614][Lys]
13.1
22
Riisager
[
P
66614][3-OMe
3
-
20
1
5
c
607.97
1.65
1.02
12.0
8.00
Wang
2
-Op]
c
10
[
P
66614][Pyr]
66614][Pro]
aP4443][Gly]
DAIL]
MTPP.Br][EA]
455.15
597.98
334.48
249.15
103.38
98.41
23
22
RT
30
25
25
30
30
30
30
Dai
d
d
7
[P
0.9
~7.70
Brennecke
e
e
8
[
1.00
13.0
Zhang
d
d
19
[
1.05
~18.5
Zhang
f
31
31
[
0.14
0.12
7.10
5.91
Hadj-kali
Hadj-kali
f
[
TBA.Br][EA]
g
h
h
[
MEA.Cl][EDA]
69.46
0.54
33.7
Present work
Present work
Present work
Present work
g
h
h
[TEA.Cl][EDA]
91.49
0.45
24.2
g
h
h
[
UE.Cl][EDA]
69.20
0.26
17.8
g
h
h
[TAE.Cl][EDA]
72.97
0.25
14.6
a
b
c
d
g
mole CO
2
/mole solvent. Absorption temperature. Based on pure ILs or DESs.
e
f
Obtained in aqueous ILs. ILs on porous SiO
2
. DES with HBA:HBD=1:6 (molar).
h
DESs with HBA:HBD=1:3 (molar). Uptakes after 24 h. RT= Room temperature.
-analysis of temperature dependence on the uptake, the vial
kept partly immersed in water bath at constant
a
temperatures of interest. For desorption experiments, the CO2
absorbed vials were located inside an oil bath at 100 °C while
continuously stirred. The release of CO was also monitored by
2
weighing the vials as in the absorption experiment.
The time-dependent CO
2
uptakes of [MEA.Cl][EDA] with
Results and discussion
different HBA-to-HBD ratios were measured at 30 °C and 1 atm
DESs were synthesized as shown in Scheme 1. MEA was first (Fig. 1a). In Figure 1a, the following points are especially
reacted with HCl to form an ammonium hydrochloride salt, noticeable. First, among all of these ratios of [MEA.Cl]:[EDA],
[
MEA.Cl]. This HBA can undergo hydrogen bonding interaction the 1:3 and 1:4 ratios exhibited the highest CO2 uptakes.
with EDA, a HBD, to yield a final DES, [MEA.Cl][EDA]. Other Second, both the 1:3 and 1:4 ratios showed good kinetics in
DESs with varied ammonium hydrochloride salt precursor CO absorption, as the 1:3 and 1:4 ratios reached 25.2 and
2
(
HBAs), such as triethanolamine [TEA.Cl], urea [UE.Cl], and 27.4 wt% only after ~2.5 min, respectively. The uptake values
thioacetamide [TAE.Cl], were also prepared based on the same started to saturate after ~5 min for both ratios and increased
reaction scheme. According to spectroscopic characterizations very slowly thereafter. The slow increase behavior is
1
see ESI) of these DESs including H & C NMR (Fig. S1, ESI), attributed
13
9, 18
(
2
to increased viscosity upon CO absorption that
FTIR, and ESI-MS, the as-synthesized DESs are highly pure and hinders the diffusion and migration of CO . Third and most
2
match well with the designated chemical compositions. DES is importantly, the uptake amounts in both molar ratios reached
generally produced by mixing a quaternary ammonium halide 31.5 wt% and 30.8 wt%, respectively, after 3 hrs. These values
salt with a HBD to form an HBD-halide complex. During this increased a bit further to 33.7 wt% and 34.2 wt% after 24 hrs
complex formation, the interaction between HBD and halide (Fig. S3, ESI). The similar uptake behaviors between both
anion of HBA leads to a significant depression of the melting compositions might be able to be explained by similar basicity
2
0
point. For the case of [MEA.Cl][EDA], different molar ratios and polarities of them, although further investigation is
between [MEA.Cl] and EDA were examined to check the required. In this regard, their exact intermolecular
depression of the melting point via DSC measurements (Fig. configurations need to be elucidated to unveil the
S2, ESI). From these characterizations, it was confirmed that compositional effect. The uptake cross-over between the 1:3
[
MEA.Cl]:[EDA] is liquid at all of the tested molar ratios with and 1:4 at ~15 h is currently under investigation and is
o
the 1:3 ratio exhibiting the largest depression (-16.8 C) from speculated to be associated with viscosity change. Remarkably,
our measurement.
our best DES, [MEA.Cl][EDA]=1:3, also exhibited efficient CO2
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 4 of 10
View Article Online
DOI: 10.1039/C5GC02319J
ARTICLE
Journal Name
uptake (26.98 wt% after 3h) with simulated flue gas (Fig. S4,
ESI). These values are quite remarkable as they are higher than
3
those of any other DESs (the highest ~ 7 wt%) and ILs (the
1
1
highest ~ 18.5 wt%) as well as most of dry sorbents
9
32
reported to date. For reference, the gravimetric uptakes of the
3
conventional MEA scrubbing processes (~30 wt% aqueous
solution) are approximately 12 wt%. The uptakes of
representative ILs and DESs together with those from the
present DESs are summarized in Tables 1 and S1. Notably, the
best performing IL exhibited a high uptake reaching 2.1 mol
per each mole of IL. But, this value becomes inferior to that of
[MEA.Cl]:[EDA]=1:3 once converted based on the gravimetric
evaluation because of the large molecular weight of the IL.
EDA alone also showed similarly high CO uptakes to those of
2
[MEA.Cl]:[EDA]=1:3 (Fig. S5, ESI), but turned gel-like much
earlier than [MEA.Cl][EDA] with all compositions so its uptake
did not increase at all after ~30 min. This limited function in
addition to the intrinsic drawbacks of pure amine in handling
in large-scales, such as oxidative degradation at high
temperatures and corrosion (will be discussed later), implies
that the current DES is preferable over pure EDA for
sustainable operations.
The gravimetric uptakes of 33.7 wt% and 34.2 wt% upon
saturation correspond to 0.537 mol and 0.528 mol per each
mole of DESs. For the calculation of molar uptakes, the
2
9, 31
molecular weight of each DES was obtained
based on the
molar average of the participating components as follows:
MW (DES) = [MW (HBA) + n×MW (HBD)] / (n+1),
n = moles of HBD
The saturated molar uptakes indicate that CO binding with
2
[MEA.Cl][EDA] takes place in a 1:2 molar ratio. This molar ratio
can be explained by the reaction scheme described in Fig. 1b.
The carbamic acid is first generated upon the reaction with
CO (top in Fig. 1b), followed by the deprotonation of the
2
carbamic acid (reaction 1 in Fig. 1b). During this step, the
amine groups of the neighboring [MEA.Cl][EDA] are
protonated (reaction 2 in Fig. 1b), yielding the observed 1:2
molar ratio between [MEA.Cl][EDA] and CO . Once again, the
2
slow uptake increase after saturation can be ascribed to the
6
increased viscosity during the course of CO absorption. The
-9
2
increased viscosity is related to a strong hydrogen-bonding
network between the generated carbamate anions and
7
, 18
ammonium cations (Fig. 1b).
The reaction of [MEA.Cl][EDA] with CO
by diverse analyses before and after CO absorption (Fig. 2).
First, in the FT-IR characterization of [MEA.Cl]:[EDA]=1:3 (Fig.
2
was investigated
2
2
a), the as-made [MEA.Cl] [EDA] showed a broad to that of
3
3
pure MEA because of the hydrogen bonding interaction.
2
Upon CO absorption, the primary amine (–N–H stretching)
–1
peak of the DES at ~2900 cm diminished due to the
7
formation of carbamate groups (–NHCOO). The formation of
carbamate groups was also supported by the appearance of
-1
new peaks at ~1600 and 851 cm assigned to νCOO
7
asymmetric stretching and bending modes, respectively.
2
Similarly, the CO absorption was additionally indicated by
4
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 10
Journal Name
Pleas Ge rdeo e nn o Ct ah de j mu si ts mt r ya rgins
View Article Online
DOI: 10.1039/C5GC02319J
ARTICLE
–
1
enhanced peaks at 1098 and 1190 cm corresponding to the wt%, respectively, in comparison with 31.5 wt% of
C–N stretching and C–O symmetric stretch modes, [MEA.Cl][EDA] after 3 hrs. These values of the three DESs were
1
respectively. Second, NMR analysis also provides a consistent saturated to 24.2 wt%, 17.8 wt% and 14.6 wt% after 24 hrs
3
1
picture regarding the CO
CO absorption (Fig. 2b) exhibited new peaks at 6.51 and 3.06 the observed saturation behaviours are ascribed to the
ppm together with downfield shifting of other peaks, verifying increased viscosity during CO
the conversion of the –NH groups to –NH. In the same line, The distinct uptakes among the DESs can be explained by
absorption (Fig. 2c) exhibited new the basicity and polarity of the DESs which is often quantified
peaks at 164.37 and 41.25 ppm assigned to –NHCOO and – by Kamlet-Taft empirical scale that specifies acidity (α), basicity
2
absorption. H NMR spectrum after (Tables 1 & 2 and Fig. S3, ESI). As in the case of [MEA.Cl][EDA],
2
6
-9, 18
2
absorption.
2
1
3
34
C NMR spectrum after CO
2
9
,
35-37
CH
2
–NH, respectively, reflecting the formation of carbamates.
(β), and polarizability (π*).
Finally, ESI-MS spectra further support the aforementioned with the common HBD of EDA, the polarity and basicity of DES
interactions between [MEA.Cl][EDA] and CO . As displayed in becomes varied after complex formation with the different
In the current series of DESs
1
5
2
Fig. 2d, after CO absorption, a sharp peak at m/z=283.26 HBAs. It is anticipated that the higher uptakes of
2
under the negative-ion mode was observed. This peak [MEA.Cl][EDA] are due to its higher basicity that would
corresponds to an adduct, [Cl(EDA) ]–CO , which originates facilitate the interaction with CO . Interestingly, [TEA.Cl][EDA]
3
2
2
from m/z=255.26 assigned to[Cl(EDA) ] before CO absorption. exhibited lower uptakes because its inter-intra hydrogen
3
2
The NMR and ESI-MS results did not exhibit any signature for bonding from the larger number of hydroxyl groups decreases
3
8
pure EDA, indicating that no residual EDA was left for the dielectric constant and thus the basicity and polarity. A
additional CO absorption. similar switching in the basicity after CO absorption in ILs was
2
2
3
9
also observed. Likewise, the lower uptakes of [UE.Cl][EDA]
and [TAE][EDA] in spite of the larger numbers of amine groups
could be explained by the weak basicity of their HBAs, which
can facilitate the deprotonation of the HBAs and consequently
protonate EDA and deactivate its active sites for CO2
absorption. The slightly higher uptakes of [UE.Cl][EDA] than
those of [TAE.Cl][EDA] might also be understood in the same
context based on the weaker basicity of [TAE.Cl]. Obviously,
the basicity of DESs could be successively varied during CO2
absorption, which affects the subsequent interaction with
3
9
additional CO2. The elucidation of this point requires further
investigation.
In order to simulate CO2 absorption in more realistic
conditions, the CO absorption was examined in the presence
2
of various contents of water, as actual flue gas in industrial
processes inevitably contain moisture. After testing
[
MEA.Cl][EDA]=1:3 containing different water contents(5~20%)
Fig. 4a), it was found the initial uptake kinetics improves with
(
the increased water content, as the uptake after 2.5 min
increased from 25.2 wt% to 26.9 wt% to 27.3 wt% to 27.5 wt%
to 28.1 wt% as the water content increased from 0% to 5% to
[MEA.Cl][EDA] exhibited higher uptakes than the other
7
.5% to 10% to 20%. This improved kinetics can be explained
DESs based on different HBAs (Fig. 3 and Table 2). When
measured at atmospheric pressure for the fixed molar ratio of
HBA:HBD=1:3, [TEA.Cl][EDA], [UE.Cl][EDA], and [TAE.Cl][EDA]
by the fact that water weakens intermolecular hydrogen
bonding interactions between DESs by forming its own
interactions with DES.
2
exhibited smaller CO uptakes of 17.5 wt%, 11.7 wt%, and 10.1
2 m
Table 2 Molecular composition, nomenclature, CO absorption capacity in 3 h (mole and wt %), melting point (T ) and heat of absorption (ΔH) of different prepared DESs.
a
f
b
No
DES
Mole
ratio
1:1
Uptake
(in mole)
0.358
Uptake
T
o
ΔH
-1
(wt%)
20.5
24.4
31.5
30.8
17.5
11.7
10.1
( C)
(kJ mol )
-32.49
-54.88
-61.18
-63.75
-38.72
-22.94
-17.42
DES1
DES2
DES3
DES4
DES5
DES6
DES7
[MEA.Cl][EDA]
[MEA.Cl][EDA]
[MEA.Cl][EDA]
[MEA.Cl][EDA]
[TEA.Cl][EDA]
[UE.Cl][EDA]
-11.5
-15.7
-16.9
-16.1
-16.4
-12.5
-16.3
1:2
0.408
1:3
0.502
1:4
0.476
1:3
0.354
1:3
0.184
[TAE.Cl][EDA]
1:3
0.168
a
b
c
mole CO
2
/mole solvent. Values obtained from DSC. Heat of absorption calculated based on Van’t Hoff equation. (Fig. S7, ESI)
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 6 of 10
View Article Online
DOI: 10.1039/C5GC02319J
ARTICLE
Journal Name
-show an unstable uptake behavior, as the case with 20%
water exhibited the uptake decay immediately after the first
uptake point. The decaying phenomenon is associated with
dominant CO
2
desorption, as described in Fig. 4b. The excess
water can attack the carbamate, yielding carbonic acid as an
42, 43
intermediate compound,
which is eventually dissociated
into water and CO . When tested for all the other DESs, 10%
2
water addition improves the initial kinetics in a consistent
manner (Fig. 4c), reconfirming the role of water towards
decreasing the viscosity. But, it is likely that the saturated
uptakes of all the DESs are pretty invariant for both with and
without the water inclusion, as both uptake profiles are
2
Similar water effect on the CO uptake behavior was also
7
-9, 40
converging after 1 h of CO absorption.
2
observed for ILs.
The weakened intermolecular
It is also worth evaluating the temperature dependence of
interactions decrease the viscosity and consequently enhance
the initial uptake kinetics. By the formation of the water-DES
2
the current DESs in their CO absorption (Fig. 5a and 5b).
4
1
Interestingly, in the beginning of absorption, all of the DESs
showed enhanced kinetics with the temperature rise due to
network, the free volume of DES also is increased, which also
contributes to the enhanced kinetics. For the same reason, the
case with 10% water exhibited the highest uptakes among the
water contents examined in the initial time range up to 1 hr.
However, further increase in the water content started to
4
4
the viscosity decrease at higher temperatures, similar to the
cases with water present in DESs. By contrast, the saturated
uptakes decreased with the temperature rise due to more
2
Table 3 Temperature dependent viscosity values of different DESs before and after 2.5 min of CO loading. For each value, measurement time frame was 30 sec
a
Viscosity (cP) of pure DESs
2
Viscosity (cP) of DESs after CO uptake
No
DESs
o
o
o
o
o
o
o
o
3
0 C
50 C
70 C
90 C
30 C
50 C
70 C
90 C
1
2
3
4
.
.
.
.
[MEA.Cl][EDA]
[TEA.Cl][EDA]
[UE.Cl][EDA]
[TAE.Cl][EDA]
21.6
52.4
29.5
30.7
15.0
25.4
23.3
21.3
10.0
18.3
12.4
11.5
7.5
3995.0
3343.0
3011.0
748.8
1162.0
420.1
820.2
236.5
328.7
99.7
56.2
45.1
70.4
41.9
9.7
4.6
170.8
68.4
3.6
a
DESs with HBA:HBD=1:3 (molar)
6
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 7 of 10
Journal Name
Pleas Ge rdeo e nn o Ct ah de j mu si ts mt r ya rgins
View Article Online
DOI: 10.1039/C5GC02319J
ARTICLE
Fig. 7 Corrosion penetration rates (CPRs) of DES ([MEA.Cl]:[EDA]=1:3), MEA, and EDA in
the presence of 25 wt% water.
main DES was reflected by the large absolute value of ΔH, the
DES is competitive in terms of regeneration.
-
significant desorption (Fig. S6, ESI). For example, in the case of
MEA.Cl]:[EDA]=1:3, the CO uptake decreased from 33.7 wt%
to 30 wt% to 28.3 wt% to 25.4 wt%, as the temperature went
To check the feasibility on recycling CO2 absorption-
desorption with DESs, [MEA.Cl]:[EDA]=1:3 was cycled (Fig. 6).
To facilitate atmospheric regeneration and avoid high
viscosity, the test was conducted in a more realistic condition
that [MEA.Cl]:[EDA]=1:3 is diluted to be 30 wt% in ethylene
[
2
o
o
o
o
from 30 C to 50 C to 70 C to 90 C. The observed trend is
attributed to less favourable CO –amine interaction originating
from 1) the disrupted exothermic acid-base interaction
between CO and DESs and 2) the increased vapor pressure of
the CO -absorbed DES that weakens the penetration of CO
into the DES. The correlation between the viscosity and early
CO uptake was clarified when the viscosity of
MEA.Cl]:[EDA]=1:3 was measured after 1 min of CO
2
4
9, 50
glycol (EG) by benchmarking the recent reports.
The CO2
2
desorption in each cycle was achieved by heating the solution
o
at 100 C for ~2.5 hr until the uptake became near zero. For
o
reference, the given DES was found to be stable up to ~ 115 C
2
2
2
(Fig. S8, ESI). When the absorption cut-off was set to 15 wt%
[
2
based on the mass of both the DES and EG, five consecutive
cycles were observed. Also, the structural stability of the DES
absorption at different temperatures (Fig. 5c). The viscosity
dropped from 3905 cP to 368 cP to 88.7 cP, as the
1
after the repeated cycles was confirmed by C NMR analysis
3
o
o
o
temperature went from 30 C to 50 C to 70 C. At the same
temperature change, however, the CO uptake increased from
1.5 wt% to 22.2 wt% to 23.9 wt%, once again owing to the
(
Fig. S9, ESI). The possibility of recycling is consistent with
1
2
5, 30
previous literature
2
that introduced CO capture by ILs and
2
DESs. In addition, the volume of the DES was preserved
throughout the cycling, indicating that the DES is not volatile,
unlike pure EDA with volatility issue.
enhanced kinetics. More detailed results of the viscosity
measurements for all the DESs at different temperatures are
presented in Table 3.
Since the active species in [MEA.Cl][EDA] for CO
absorption is EDA and aqueous EDA solution is known to be
2
To examine the energy consumption during CO absorption
2
as well as the feasibility of regeneration, the heat of
5
1
corrosive, the corrosiveness of [MEA.Cl]:[EDA]=1:3 was
tested and compared with aqueous EDA and MEA solutions. To
see the trend of the corrosion of these three solvents in a
reasonable time frame, the accelerated corrosion condition,
absorption (ΔH) was obtained by using Van’t Hoff equation
(see detail in ESI). For this, equilibrium constant (K) for the CO2
absorption reaction was obtained at series of temperatures,
and a linear fitting to the ln (K) vs. 1/T plot retrieves ΔH (Fig.
4-46
o
4
such as high concentration (75 wt%) and temperature (90 C),
S7, ESI).
According to this method, it was found that ΔH for
-1
was adopted. The corrosion penetration rate (CPR) was
quantified based on the weight loss of the stainless steel piece
all of the DESs ranges from –63.8 to –17.4 kJ mole (Fig. 5d),
and these values are in a good linear correlation with the CO2
uptake. These values are noticeable because they are smaller
than those of MEA-based conventional scrubbing processes (–
(see details in ESI). After 10 days of the immersion with 20%
CO loading, the CPRs of the DES, MEA, and EDA solutions
2
-
1
o
47
were 0.124, 0.385, and 0.448 mm/yr, respectively (Fig. 7),
indicating that [MEA.Cl]:[EDA]=1:3 is clearly more corrosion-
resistant and is thus expected to be more sustainable in actual
practical conditions. The diminished corrosiveness is ascribed
to the relatively high viscosity of the DES that enables an
adhesive contact to the steel surface and the exposure to
8
5 kJ mol at 40 C) as well as amino-functionalized TSILs
−
−80 kJ mol ). Interestingly, ΔH of [TAE.Cl]:[EDA]=1:1 (−17.4
1 7
(
−1
kJ mol ) is even close to those of CO physisorption of most
2
dry sorbents and the lowest IL, [Bmim][TF N] (−12.5 kJ
2
−
mol ). The ΔH of the main sample, [MEA.Cl]:[EDA]=1:3,
1 48
-1
which exhibited the highest uptake was –61.18 kJ mol . This
5
2
moisture and oxygen. This feature weakens the known
ΔH value implies that while the enhanced CO uptake of the
2
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 8 of 10
View Article Online
DOI: 10.1039/C5GC02319J
ARTICLE
Journal Name
5
3-55
corrosion process
of pure EDA that forms so-called heat- 2. C. Wu, T. P. Senftle and W. F. Schneider, Phys Chem Chem
+
stable salt consisting of protonated amine (R-NH
–
counter anion (R-COO ) through oxidative degradation of EDA 3. G. T. Rochelle, Science, 2009, 325, 1652-1654.
3
) and
Phys, 2012, 14, 13163-13170.
and concurrently mitigates reduction of surrounding proton to 4. J. M. Zhang, S. J. Zhang, K. Dong, Y. Q. Zhang, Y. Q. Shen and
hydrogen gas, which is a counter process to the oxidation
X. M. Lv, Chem-Eur J, 2006, 12, 4021-4026.
(corrosion) of iron in the steel surface. The hydrogen bonding 5. I. Niedermaier, M. Bahlmann, C. Papp, C. Kolbeck, W. Wei, S.
network in the DES may also contribute to the lessened
corrosiveness because the network formation makes the
amine group less available for protonation.
K. Calderon, M. Grabau, P. S. Schulz, P. Wasserscheid, H. P.
Steinruck and F. Maier, J Am Chem Soc, 2014, 136, 436-441.
6. B. F. Goodrich, J. C. de la Fuente, B. E. Gurkan, D. J. Zadigian,
E. A. Price, Y. Huang and J. F. Brennecke, Ind Eng Chem Res,
2
011, 50, 111-118.
Conclusions
7
. B. E. Gurkan, J. C. de la Fuente, E. M. Mindrup, L. E. Ficke, B.
F. Goodrich, E. A. Price, W. F. Schneider and J. F. Brennecke, J
Am Chem Soc, 2010, 132, 2116-2117.
In conclusion, we have found a family of DESs whose
2
gravimetric CO uptakes far surpass those of the previous DESs
8
9
1
1
. Y. Q. Zhang, S. J. Zhang, X. M. Lu, Q. Zhou, W. Fan and X. P.
Zhang, Chem-Eur J, 2009, 15, 3003-3011.
and ILs as well as the conventional scrubbing process. Among
the tested HBA-HBD combinations, [MEA.Cl][EDA] exhibited
o
the highest uptake of 31.5 wt% after 3 hrs at 30 C with good
. S. Saravanamurugan, A. J. Kunov-Kruse, R. Fehrmann and A.
Riisager, Chemsuschem, 2014, 7, 897-902.
initial absorption kinetics (25.2 wt% after 2.5 min) and robust
0. C. M. Wang, X. Y. Luo, H. M. Luo, D. E. Jiang, H. R. Li and S.
Dai, Angew Chem Int Edit, 2011, 50, 4918-4922.
1. S. Seo, M. Quiroz-Guzman, M. A. DeSilva, T. B. Lee, Y. Huang,
B. F. Goodrich, W. F. Schneider and J. F. Brennecke, J Phys
Chem B, 2014, 118, 5740-5751.
recyclability. The CO
2
absorption mechanism was based on
and EDA
carbamate formation upon the reaction between CO
2
in the hydrogen bonding network. The distinct uptake
behaviors among the DESs investigated were explained by
different polarity and basicity of the DESs originating from the
HBA functional groups. Furthermore, the best performing DES,
1
1
1
1
1
1
1
1
2
2. C. M. Wang, H. M. Luo, D. E. Jiang, H. R. Li and S. Dai, Angew
Chem Int Edit, 2010, 49, 5978-5981.
[MEA.Cl]:[EDA]=1:3, exhibited several attractive features:
3. C. M. Wang, H. M. Luo, X. Y. Luo, H. R. Li and S. Dai, Green
Chem, 2010, 12, 2019-2023.
robust performance in the presence of water, decent
tolerance against temperature rise, relatively small heat of
absorption beneficial for regeneration, and good corrosion
resistance, most of which originate from its unique hydrogen
bonding network. Although [MEA.Cl]:[EDA]=1:3 still keeps
some disadvantages of conventional scrubbing processes, such
as moderate desorption kinetics, it is emphasized that the
4. X. X. Lei, Y. J. Xu, L. L. Zhu and X. H. Wang, Rsc Adv, 2014,
052-7067.
4,
7
5. X. Y. Luo, Y. Guo, F. Ding, H. Q. Zhao, G. K. Cui, H. R. Li and C.
M. Wang, Angew Chem Int Edit, 2014, 53, 7053-7057.
6. L. A. Blanchard, D. Hancu, E. J. Beckman and J. F. Brennecke,
Nature, 1999, 399, 28-29.
2
advantages of DESs are significant in the overall CO capture
7. E. D. Bates, R. D. Mayton, I. Ntai and J. H. Davis, J Am Chem
Soc, 2002, 124, 926-927.
perspective, as the advantages include the solution to the
longstanding critical problems, such as severe corrosion and
high regeneration energy penalty. The present study conveys a
clear message that rich chemistry available for a variety of
DESs can contribute significantly to ranking this family of
8. K. E. Gutowski and E. J. Maginn, J Am Chem Soc, 2008, 130
,
1
4690-14704.
9. J. Z. Zhang, C. Jia, H. F. Dong, J. Q. Wang, X. P. Zhang and S. J.
Zhang, Ind Eng Chem Res, 2013, 52, 5835-5841.
2
solvents as strong and viable candidates for CO capture.
0. A. P. Abbott, G. Capper, D. L. Davies, R. K. Rasheed and V.
Tambyrajah,
Chem
Commun,
2003,
DOI:
Doi
Acknowledgements
10.1039/B210714g, 70-71.
2
2
2
2
1. A. P. Abbott, D. Boothby, G. Capper, D. L. Davies and R. K.
Rasheed, J Am Chem Soc, 2004, 126, 9142-9147.
We thank Professor Ali Coskun for helpful feedback. This work
was supported by a National Research Foundation of Korea
2. D. Carriazo, M. C. Serrano, M. C. Gutierrez, M. L. Ferrer and
F. del Monte, Chemical Society reviews, 2012, 41, 4996-5014.
3. Q. H. Zhang, K. D. Vigier, S. Royer and F. Jerome, Chemical
Society reviews, 2012, 41, 7108-7146.
(NRF) grant, funded by the Korea government (MEST) (NRF-
2
012-R1A2A1A01011970, 2012M1A2A2026587, and NRF-
014R1A4A1003712).
2
4. K. Radosevic, M. C. Bubalo, V. G. Srcek, D. Grgas, T. L.
Dragicevic and I. R. Redovnikovic, Ecotox Environ Safe, 2015,
Notes and references
1
12, 46-53.
2
2
2
5. X. Y. Li, M. Q. Hou, B. X. Han, X. L. Wang and L. Z. Zou, J Chem
Eng Data, 2008, 53, 548-550.
1
. B. Metz and Intergovernmental Panel on Climate Change.
Working Group III., IPCC special report on carbon dioxide
capture and storage, Cambridge University Press, for the
Intergovernmental Panel on Climate Change, Cambridge,
6. R. B. Leron, A. Caparanga and M. H. Li, J Taiwan Inst Chem E,
2
013, 44, 879-885.
7. R. B. Leron and M. H. Li, Thermochim Acta, 2013, 551, 14-19.
2
005.
8
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 9 of 10
Journal Name
Pleas Ge rdeo e nn o Ct ah de j mu si ts mt r ya rgins
View Article Online
DOI: 10.1039/C5GC02319J
ARTICLE
2
8. R. B. Leron and M. H. Li, J Chem Thermodyn, 2013, 57, 131-
36.
1
2
3
3
9. M. Francisco, A. van den Bruinhorst, L. F. Zubeir, C. J. Peters
and M. C. Kroon, Fluid Phase Equilibr, 2013, 340, 77-84.
0. L. L. Sze, S. Pandey, S. Ravula, S. Pandey, H. Zhao, G. A. Baker
and S. N. Baker, Acs Sustain Chem Eng, 2014, 2, 2117-2123.
1. E. Ali, M. K. Hadj-Kali, S. Mulyono, I. Alnashef, A. Fakeeha, F.
Mjalli and A. Hayyan, Chem Eng Res Des, 2014, 92, 1898-
1
906.
3
3
2. M. Nandi and H. Uyama, Chem Rec, 2014, 14, 1134-1148.
3. C. P. Li, D. Li, S. S. Zou, Z. Li, J. M. Yin, A. L. Wang, Y. N. Cui, Z.
L. Yao and Q. Zhao, Green Chem, 2013, 15, 2793-2799.
4. A. Pandey, R. Rai, M. Pal and S. Pandey, Phys Chem Chem
Phys, 2014, 16, 1559-1568.
3
3
5. M. A. Ab Rani, A. Brandt, L. Crowhurst, A. Dolan, N. H.
Hassan, J. P. Hallett, P. A. Hunt, M. Lui, H. Niedermeyer, J. M.
Perez-Arlandis, M. Schrems, T. Q. To, T. Welton and R.
Wilding, Phys Chem Chem Phys, 2011, 13, 21653-21653.
6. C. Chiappe, C. S. Pomelli and S. Rajamani, J Phys Chem B,
3
3
3
3
4
2
011, 115, 9653-9661.
7. P. G. Jessop, D. A. Jessop, D. B. Fu and L. Phan, Green Chem,
012, 14, 1245-1259.
2
8. M. M. Huang, Y. P. Jiang, P. Sasisanker, G. W. Driver and H.
Weingartner, J Chem Eng Data, 2011, 56, 1494-1499.
9. W. J. Li, Z. F. Zhang, B. X. Han, S. Q. Hu, J. L. Song, Y. Xie and
X. S. Zhou, Green Chem, 2008, 10, 1142-1145.
0. B. F. Goodrich, J. C. de la Fuente, B. E. Gurkan, Z. K. Lopez, E.
A. Price, Y. Huang and J. F. Brennecke, J Phys Chem B, 2011,
115, 9140-9150.
4
4
1. J. H. Huang and T. Ruther, Aust J Chem, 2009, 62, 298-308.
2. K. Huang, D. N. Cai, Y. L. Chen, Y. T. Wu, X. B. Hu and Z. B.
Zhang, Chempluschem, 2014, 79, 241-249.
4
4
4
4
4
4
4
5
5
5
5
5
5
3. G. Gurau, H. Rodriguez, S. P. Kelley, P. Janiczek, R. S. Kalb and
R. D. Rogers, Angew Chem Int Edit, 2011, 50, 12024-12026.
4. J. Ren, L. B. Wu and B. G. Li, Ind Eng Chem Res, 2013, 52
565-8570.
5. X. Y. Luo, F. Ding, W. J. Lin, Y. Q. Qi, H. R. Li and C. M. Wang, J
Phys Chem Lett, 2014, , 381-386.
6. Q. Huang, Y. Li, X. B. Jin, D. Zhao and G. Z. Chen, Energ
Environ Sci, 2011, , 2125-2133.
7. I. Kim and H. F. Svendsen, Ind Eng Chem Res, 2007, 46, 5803-
809.
,
8
5
4
5
8. J. L. Anthony, J. L. Anderson, E. J. Maginn and J. F. Brennecke,
J Phys Chem B, 2005, 109, 6366-6374.
9. F. Barzagli, M. Di Vaira, F. Mani and M. Peruzzini,
Chemsuschem, 2012,
0. F. Barzagli, S. Lai, F. Mani and P. Stoppioni, Energ Fuel, 2014,
, 1795-1804.
1. S. Zhou, X. Chen, T. Nguyen, A. K. Voice and G. T. Rochelle,
Chemsuschem, 2010, , 913-918.
5, 1724-1731.
63
3
2. A. P. Abbott, E. I. Ahmed, R. C. Harris and K. S. Ryder, Green
Chem, 2014, 16, 4156-4161.
3. H. Lepaumier, D. Picq and P. L. Carrette, Ind Eng Chem Res,
2
009, 48, 9068-9075.
4. W. Tanthapanichakoon, A. Veawab and B. McGarvey, Ind Eng
Chem Res, 2006, 45, 2586-2593.
5. A. Veawab and A. Aroonwilas, Corros Sci, 2002, 44, 967-987.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
Please do not adjust margins

Green Chemistry
Page 10 of 10
View Article Online
DOI: 10.1039/C5GC02319J
Graphical Abstract
A family of deep eutectic solvents with high gravimetric capacities is reported for
CO capture.
2