CL-131180
Received: December 17, 2013 | Accepted: January 8, 2014 | Web Released: January 16, 2014
Chemisorption of Carbon Dioxide in Carboxylate-Functionalized Ionic Liquids:
A Mechanistic Study
Yoshiro Yasaka,* Masakatsu Ueno, and Yoshifumi Kimura
Department of Molecular Chemistry and Biochemistry, Faculty of Science and Engineering, Doshisha University,
-3 Tatara-Miyakodani, Kyotanabe, Kyoto 610-0321
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(
E-mail: yyasaka@mail.doshisha.ac.jp)
Spectroscopic investigations on the CO2 chemisorption by
tetra-n-butylphosphonium formate as an example of carbox-
ylate-functionalized ionic liquids reveal that the formation of
hydrogen-bond complexes such as diformate anion supplies the
driving force of the chemisorption.
acetates the cation in our system is chemically inert. Thus, the
role played by the carboxylate anion in CO2 chemisorption can
be highlighted. We demonstrate a new chemisorption mechanism
that operates in the presence of water via carbonic acid formation
by using in situ NMR and Raman spectroscopy.
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P4444HCOO was synthesized as described in ESI. The
purity (>99%; H O, ca. 0.06%) of P4444HCOO was confirmed
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Ionic liquids (ILs) attract growing attention as efficient CO2
by NMR. The ionic liquid was mixed with water at various
concentrations. The CO2 absorption into the IL was performed
as follows: 2.00 g of the ILwater solution was loaded into a 10-
mL flask. The flask was evacuated for 30 s and then connected
via stop valve to a ground-glass 100-mL syringe filled with CO2
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absorbents.
The negligibly low vapor pressure of ILs is
environmentally preferable and has a potential for an advantage
in cost over conventional amine-based CO2 absorbents such as
monoethanolamine. In addition, ILs are designable solvents.
This means that we can control the ILs’ affinity to various gases
by proper choice of cations and anions to attain high selectivity
in gas-separation processes. Mechanistic studies on CO2
absorption are indispensable for successful design of ILs.
ILs absorb CO2 in two ways; physisorption and/or
chemisorption. In hydrophobic ILs whose anion components
gas. The piston could move freely so that the CO pressure was
2
kept at atmospheric pressure. After the valve was opened, the
volume of CO2 absorbed at each moment was evaluated from
the reading of the syringe. It took 1020 min before absorption
reached equilibrium. No solid products were observed. After 2 h,
the resultant solution was sampled in a NMR tube under CO2
¹
¹
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are fluorinated (e.g., PF6 and NTf2 ), absorption is of physical
origin (“physisorption”) and involves electrostatic and disper-
sion interactions between dissolved molecular CO2 and the
atmosphere to measure C NMR and Raman spectra. For
NMR measurements, JEOL ECA-300W equipped with standard
TH5 probe was used. Raman measurements were performed
at the 90 degree scattering geometry using a green laser
(532 nm, Spectra-Physics, EXLSR-532-150). For the detection,
a Peltier-cooled CCD camera (Princeton Instrument, Spec-
10:400BRXTE) attached to a spectrometer (Jobin Yvon,
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solvent ions. On the other hand, chemisorption involves
chemical reactions between CO and the solvent at dissolution.
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Disclosing the reaction mechanism involved is an active
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research topic.
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The chemisorption of CO2 was first studied in amino-
T64000) was used.
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functionalized ILs. Amino groups can be introduced either on
The P4444HCOOwater system absorbs CO2 mainly through
chemisorption when water is added to the IL. This is evidenced
the cations or the anions or both. The amino groups attack CO2
as a nucleophile and/or a base to form carbamate and/or
bicarbonate salts. Later, 1,3-dialkylimidazolium acetates were
found to chemisorb CO2, and the mechanism behind this is
attracting significant attention.6 In contrast to amino-function-
ality, the carboxylate anions are neither bases nor nucleophiles.
The chemisorption mechanism turned out to be rather complex,
involving both the cations and the anions. Maginn et al. reported
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3
by the C NMR spectra collected after absorption in Figure 1.
One finds only a weak signal for molecular CO at 125 ppm
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and instead a very strong one at 159 ppm. The physisorption
contribution is not significant (less than 70 mM in molar
concentration) under experimental conditions studied here.
Thus, we only discuss the chemisorption part hereafter. The
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using NMR that the absorbed CO reacts with the imidazolium
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cations to form zwitterionic species. A question then arises why
only imidazolium acetates can chemisorb CO2. Rodríguez et al.
reasoned that the weak basicity of the carboxylate anions
facilitate the formation of carbene, which then can be attacked
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a
by CO2. Gurau et al. analyzed the crystal structure of the
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b
solid product that is formed after CO2 chemisorption. They
identified an anionic complex composed of acetate anion and
acetic acid. They pointed out that the stoichiometry for the
complex formation explains the observed maximum chemisorp-
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tion capacity of the imidazolium acetates (ca. 0.5 in mole ratio
to the IL).
Figure 1. 13C NMR spectra of the IL after CO2 absorption
equilibrium is reached at 20 °C and 1 atm. The IL initially
contained water at the mole ratio of 1.0 to the IL. C-Labeled
CO2 was employed in this experiment.
In this communication we report CO2 chemisorption in tetra-
n-butylphosphonium formate (P4444HCOO), a typical quaternary
phosphonium carboxylate IL; in contrast to the imidazolium
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© 2014 The Chemical Society of Japan