Demonstration of chemisorption of carbon dioxide in 1,3-dialkylimidazolium acetate ionic liquids

Article abstract of DOI:10.1002/anie.201105198

Real chemistry: Spectroscopic and crystallographic analyses confirm the chemical reaction of CO₂ with carbene present in 1,3- dialkylimidazolium acetate ionic liquids and the supporting role of the acetate ion. When CO₂ was bubbled through [C₂mim][OAc], formation of the corresponding imidazolium carboxylate, [C₂mim ⁺-COO^-], could be observed. Copyright

Full text of DOI:10.1002/anie.201105198

Communications
DOI: 10.1002/anie.201105198
Ionic Liquids
Demonstration of Chemisorption of Carbon Dioxide in
,3-Dialkylimidazolium Acetate Ionic Liquids**
1
Gabriela Gurau, Hꢀctor Rodrꢁguez, Steven P. Kelley, Peter Janiczek, Roland S. Kalb, and
Robin D. Rogers*
Driven by increasing environmental concerns about green-
house gas emissions (particularly carbon dioxide) and global
warming, a growing amount of research has been carried out
over the last decade on the use of ionic liquids (ILs), among
other options, as a potential alternative to conventional
superbase, 1,8-diazabicyclo[5.4.0]undec-7-ene, with formation
of the corresponding 1,3-dialkylimidazolium-2-carboxylate.
[7]
We have recently shown that the C(2) proton can be
abstracted to some extent in neat 1,3-dialkylimidazolium
ILs if they are paired with a basic enough anion such as
[8]
processes based on aqueous amine solutions for CO cap-
acetate even in the absence of any external base. For
example, the carbene concentration in 1-ethyl-3-methylimi-
2
[1]
ture. The tethering of an amine functional group to the
cation was one of the initial possibilities investigated, while
[
2]
dazolium acetate ([C mim][OAc]) and 1-butyl-3-methylimi-
2
more recently, absorption of CO in ILs with amine function-
ality in the anion has also been reported. Still, even without
dazolium acetate ([C mim][OAc]) is high enough to enable
2
4
[
3]
formation of imidazole-2-chalcogenones by the direct addi-
tion of elemental chalcogens to these ILs. However, we also
realized that complex anion formation (e.g., acetic acid/
acetate) resulted in stabilization of the volatile acetic acid
thus formed, preventing further decomposition reactions and
allowing these ILs to act as stable reservoirs of carbenes for
direct carbene-based chemistry. Recently reported quantum
amine functionalization, ILs do generally dissolve CO to a
2
certain extent and CO is generally much more soluble than
2
[
4]
other gases such as N or O . In most cases, solubilization of
2
2
CO in the nonfunctionalized IL occurs through physisorp-
2
tion, although chemisorption has been suggested for ILs with
anions of remarkable basicity (e.g., carboxylate-derived
[5]
[9]
anions). The mechanisms proposed have typically involved
chemical calculations support this concept.
an interaction between the acidic CO and the basic anion;
Here, we report direct experimental evidence in the form
of single-crystal X-ray structures of solid-state products
2
the only exception being a grant report by Maginn in 2005
where, to explain the absorption of CO2 in 1-butyl-3-
methylimidazolium acetate, he used NMR results to propose
the abstraction of the proton at the C(2) position of the
imidazolium ring by the basic acetate anion, followed by
resulting from the reaction of CO with acetate ILs, which
2
confirm both the reaction mechanism and the role of complex
anion formation. Since to the best of our knowledge there
were no reported crystal structures of 1,3-dialkylimidazolium
acetate salts, we first investigated the crystal structure of 1,3-
diethylimidazolium acetate ([C C im][OAc]), an off-white
[5a]
reaction of CO with the carbene species thus formed.
2
Interestingly, we could not find any further reference to this
mechanism in the literature and we assume the idea was not
pursued due to concerns about the lack of explanation for the
presence of an a priori unstable N-heterocyclic carbene in a
relatively stable IL.
2
2
crystalline solid with a melting temperature of 308C (Figure 1
The weak acidity of the proton at the C(2) position of 1,3-
dialkylimidazolium rings is one of the major pathways for
reactivity of imidazolium species, in particular of imidazolium
[
6]
ILs. Wang et al. made use of this to achieve an equimolar
CO capture in 1,3-dialkylimidazolium ILs by addition of a
2
[
*] Dr. G. Gurau, S. P. Kelley, Prof. R. D. Rogers
Department of Chemistry and Center for Green Manufacturing
The University of Alabama, Tuscaloosa, AL 35487 (USA)
E-mail: RDRogers@as.ua.edu
[10]
Figure 1. ORTEP diagram and cation environment of [C C im][OAc].
2
2
Thermal ellipsoids set at 50% probability.
Dr. H. Rodrꢀguez
Department of Chemical Engineering, University of Santiago de
Compostela, E-15782, Santiago de Compostela (Spain)
and Supporting Information). As expected, the anion is
strongly hydrogen-bonded to the C(2)-H proton (O2···H
P. Janiczek, R. S. Kalb
Proionic GmbH Parkring 18, 8074 Grambach (Austria)
2
.16 ꢀ), resulting in unsymmetrical CÀO bond lengths in the
anion (C10ÀO1 1.245(2) ꢀ, C10ÀO2 1.258(2) ꢀ). In the solid
state there is no evidence of carbene, as also might be
expected.
[
**] H.R. is grateful to the Spanish Ministry of Science and Innovation
for support through the “Ramꢁn y Cajal” program. We also thank Dr.
Roland Fischer for providing crystallographic data.
To explore the reactivity of the acetate ILs with CO , we
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201105198.
2
bubbled CO through [C mim][OAc], in a glass bubbler at
2
2
1
2024
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 12024 –12026

atmospheric pressure and ambient temperature. After 24 h,
formation of the corresponding imidazolium carboxylate,
namely
1-ethyl-3-methylimidazolium-2-carboxylate
+
À
(
[C mim -COO ]), could be observed by NMR spectroscopy
2
of the clear liquid (Scheme 1). After 36 h, the liquid mixture
+
À
Figure 2. ORTEP diagram of [C mim][H(OAc) ][C mim -COO ]. The
2
2
2
zwitterion and cation are disordered, modeled with the carboxylate
[
10]
group on C(2A) 75% of the time and on C(2B) 25% of the time.
Thermal ellipsoids set at 50% probability.
Scheme 1. Proposed reaction of CO and [C mim][OAc].
2
2
key factor enabling the presence of carbene species in
[8]
equilibrium within stable imidazolium acetate ILs.
became turbid, and then solidified. The smell of acetic acid
was clearly noticeable after opening the bubbler at the end of
the experiment (a total bubbling time of three days).
Conducting this reaction in a sealed reactor led to much
reduced solid yield. When the reaction was carried out at
elevated pressure (20 bar) with purging, solidification was
observed within 2 h, however, if water was present (1–15
wt%) the solid yields decreased with increasing water
content, in agreement with previous results indicating that
water inhibits the interaction of the acetate anion with the
Fast release of the absorbed CO2 was observed upon
addition of water, with stirring, to the [C mim][H(OAc) ]-
2
2
+
À
[C mim -COO ] complex (see videos in the Supporting
2
Information), leading to the formation of [C mim][HCO ],
2
3
which was also structurally characterized (Figure 3 and
[
8]
C(2)-H proton. The inhibitory effect of water should be
considered if a humid CO stream were contacted with the IL,
2
for example in the case of potential applications for CO2
capture.
We also found analogous reactivity of CO with the 2:1:1
2
statistical mixture [C mim][OAc]:1,3-dimethylimidazolium
2
acetate:[C C im][OAc] synthesized through a one-pot proce-
2
2
[
11]
dure (see Supporting Information for details). The ratios of
the imidazolium carboxylates formed corresponded to the
initial ratios of the three different cations in the mixture.
The crystalline solids isolated from these reactions proved
to be remarkably hygroscopic, as well as difficult to purify
from unreacted IL and acetic acid; however, single crystals of
sufficient size and quality for X-ray diffraction analysis could
be isolated from all of the above single IL studies. The crystal
structure obtained when water is not present (Figure 2 and
Supporting Information) clearly demonstrates the formation
of the imidazolium carboxylate and the role of acetate in
complexing acetic acid. The asymmetric unit consists of the
Figure 3. ORTEP diagram of [C mim][HCO ] showing two asymmetric
units. Thermal ellipsoids set at 50% probability.
2
3
[
10]
Supporting Information). This salt was also isolated in CO2
reactions conducted in the presence of water. Previous studies
have shown the dialkylimidazolium carboxylate zwitterion to
react with water and acids in a similar manner; a fact now
[
13]
used to synthesize ILs. Here, one can envision a regener-
ation process where water or acid can be used to both release
and recover the CO while regenerating the IL.
2
+
À
+
neutral zwitterion ([C mim -COO ]), the [C mim] cation,
The mechanism demonstrated here is consistent with the
experimental data reported to date for the “absorption” of
2
2
À
and an anion that can best be described as [H(OAc) ] . The
2
structural features are dominated by a strong interaction
between the C(2)-H proton of the cation and the carboxylate
portion of the zwitterion, and a strong, symmetric interaction
between two acetate anions sharing a single proton. The
interpretation of the anion is supported by the corresponding
CÀO bond lengths which are statistically identical. This close
CO in these ILs. Shiflett et al. determined the solubility of
2
[
14]
CO in [C mim][OAc] and found a “saturation fraction” of
2
4
CO in the IL, at atmospheric pressure, of almost 30 mol%,
2
which is not far (especially taking into account that there was
a substantial amount of water present in the IL sample used)
from the theoretical maximum of 0.33 molar fraction that
corresponds to the reaction described in Scheme 1. The
authors noted a smell of acetic acid evolving from the mixture
interaction between acetic acid and acetate was observed by
[
12]
Johansson et al., and identified in our previous work as a
Angew. Chem. Int. Ed. 2011, 50, 12024 –12026
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www.angewandte.org 12025

Communications
of IL and suggested the possibility of a chemical reaction,
however, just to a minor extent, giving preference to the
hypothesis of formation of a complex. The increased molar
Keywords: carbenes · carbon dioxide · chemisorption ·
imidazolium carboxylates · ionic liquids
.
fraction of CO absorbed at higher pressures is consistent with
2
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[
[
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[
Received: July 24, 2011
Published online: October 13, 2011
1
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Angew. Chem. Int. Ed. 2011, 50, 12024 –12026