Communications
DOI: 10.1002/anie.201008151
Carbon Capture
Tuning the Basicity of Ionic Liquids for Equimolar CO2 Capture**
Congmin Wang,* Xiaoyan Luo, Huimin Luo, De-en Jiang, Haoran Li, and Sheng Dai*
The emission of carbon dioxide (CO2) from fossil fuels has
received worldwide attention because it has been implicated
in climate change, which threatens economies and environ-
ments. Accordingly, new materials that can capture CO2 from
the burning of fossil fuels efficiently, economically, and with
potential energy savings must be developed. The traditional
technology for the capture of CO2 in industry is chemical
adsorption by an aqueous solution of amine, which has some
advantages, such as low cost, good reactivity, and high
capacity.[1] However, this process for the capture of CO2 is
highly energy intensive owing to the thermodynamic proper-
ties of water and high enthalpy of absorption.[2] It is estimated
that the output of energy would drop by about 30% when this
capture technology was applied at coal-fired power plants,
which significantly increases the cost of energy.[3] Currently,
the goal is to design industrial attractive sorbent materials
with high capacity and energy-saving for CO2 capture.
is based on the chemisorption for CO2 capture by task-specific
ILs. Davis and co-workers[7] reported the first example of CO2
chemisorption that employs an amino-functionalized IL; in
their work, 0.5 mol CO2 was captured per mole of IL under
ambient pressure. Subsequently, some other amino-function-
alized ILs, including sulfone anions with ammonium cations
and amino acid anions with imidazolium or phosphonium
cations, were reported for the capture of CO2.[8] Recently, a
novel method for the capture of CO2 in a 1:1 manner by
superbase-derived protic ILs and substituted aprotic ILs using
the reactivity of anion was reported.[9] Normally, the chem-
isorption has high absorption capacity for CO2 along with
high energy requirement for regeneration.[10] One commonly
used parameter to access the regeneration energy require-
ment is the enthalpy of CO2 absorption. We need reduce the
enthalpy of absorption to design the energy-saving ILs for
CO2 capture. Then, how can we design suitable chemical
structures of ILs to reduce the enthalpy of CO2 chemisorp-
tion? Can we prepare highly stable ILs for energy-saving and
equimolar CO2 capture?
Herein, we present a strategy to tune the enthalpy of CO2
absorption by tunable basic ionic liquids, which were pre-
pared by neutralizing weak proton donors with different pKa
values with phosphonium hydroxide. We show that the
stability of ILs, the enthalpy of absorption, and absorption
capacity can be easily tuned by the basicity of ILs. Based on
the relationship between the stability, the enthalpy of
absorption, absorption capacity, and the basicity of ILs,
highly stable ILs for CO2 capture with desirable absorption
enthalpy and high capacity were achieved, which opens the
door to develop industrial attractive ILs for energy-saving
and equimolar CO2 capture.
Most studies on the chemisorption for CO2 capture are
based on the reactivity of two equivalents of amine per CO2
using amino-functionalized ILs. Our approach is to make use
of the reactivity of the anions in basic ILs to CO2, thus
equimolar CO2 capture can be achieved. These basic ILs were
prepared from the deprotonation of a variety of weak proton
donors with phosphonium hydroxide, which was prepared by
the anion-exchange method.[11] A complete CO2 separation
cycle should involve both the capture and the release of CO2.
Our method balances two processes by tuning the basicity of
ILs. Considering both the effect of the basicity of the anion on
the stability and the reactivity of the anion toward CO2, eight
kinds of weak proton donors with different pKa values were
selected, for which the pKa values in DMSO range from 19.8
to 8.2 (Scheme 1).
Ionic liquids (ILs) offer a new opportunity for addressing
this challenge to develop novel CO2 capture systems because
of their unique properties, including negligible vapor pres-
sures, high thermal stabilities, and tunable properties.[4]
A
great deal of effort has focused on the experimental and
theoretical studies on the physical absorption of CO2 in ILs.[5]
The enthalpy of CO2 physical absorption by ILs is about
20 kJmolÀ1, indicating that only a quarter energy is required
to release the physical absorbed CO2 from ILs in the
regeneration step relative to amine solution method.[6] How-
ever, the absorption capacity of CO2 by these ILs is up to
about 3 mol% under atmospheric pressure. Another strategy
[*] Dr. C. Wang, X. Luo, Prof. H. Li
Department of Chemistry, Zhejiang University
Hangzhou 310027 (P. R. China)
Fax: (+86)571-8795-1895
E-mail: chewcm@zju.edu.cn
Dr. C. Wang, Dr. D. Jiang, Dr. S. Dai
Chemical Sciences Division, Oak Ridge National Laboratory
Oak Ridge, TN 37831 (USA)
Fax: (+1)865-576-5235
E-mail: dais@ornl.gov
Dr. S. Dai
Department of Chemistry, University of Tennessee
Knoxville, TN 37966 (USA)
Dr. H. Luo
Nuclear Science and Technology Division
Oak Ridge National Laboratory
Oak Ridge, TN 37831 (USA)
[**] This work was supported by the Division of Chemical Sciences,
Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S.
Department of Energy. The authors also gratefully acknowledge the
support of the National Natural Science Foundation of China (No.
20976151, No. 20704035, No. 20773109, and No. 20990221).
The structures of these basic ILs were confirmed by NMR
and IR spectroscopy (see Supporting Information). Thermal
gravimetric analysis (TGA) revealed that the stability was
significantly influenced by the basicity of these ILs. As seen in
Table 1, the stability of ILs increased with the decrease of pKa
Supporting information for this article is available on the WWW
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 4918 –4922