Carbon dioxide capture by superbase-derived protic ionic liquids

Article abstract of DOI:10.1002/anie.201002641

CO₂ capture: Protic ionic liquids (PILs) from a Superbase and fluorinated alcohol, imidazole, pyrrolinone, or phenol were designed to capture CO₂ based on the reactivity of their anions to CO₂. These PILs are capable of rapid and reversible capture of about one equivalent of CO₂, which is superior to those sorption systems based on traditional aprotic ILs. (Figure Presented).

Full text of DOI:10.1002/anie.201002641

Communications
DOI: 10.1002/anie.201002641
Carbon Capture
Carbon Dioxide Capture by Superbase-Derived Protic Ionic Liquids**
Congmin Wang, Huimin Luo, De-en Jiang, Haoran Li,* and Sheng Dai*
Emissions of carbon dioxide have received worldwide atten-
tion because of the environmental and economic threats
posed by possible climate change. Accordingly, the develop-
ment of sorbent materials that efficiently, reversibly, and
economically capture CO2 from burning of fossil fuels is
essential in realizing practical carbon capture and sequestra-
tion. The traditional capture of CO2 in industry is through a
chemical adsorption by an aqueous solution of monoethanol-
amine, which has some advantages, such as high reactivity, low
cost, and a gravimetric capacity of about 7%. However, the
use of monoethanolamine and water has the serious inherent
drawbacks, including the solvent loss, corrosion, and high
energy demand for regeneration.[1]
attributed to two factors: 1) the requirement of two amines to
capture one CO2, and 2) formation of solid or highly viscous
gel products. More recently, Brennecke, Schneider, and their
co-workers[6] reported new IL capture systems based on
amino acid anions, which can capture about 0.9 mol CO2 per
mol of IL with a gravimetric capacity of about 6%. However,
this gravimetric capacity is still low, and their carbon-capture
kinetics are also expected to be slow because these IL systems
are extremely viscous. Alternative IL strategies based on
totally different classes of ILs that are able to achieve rapid,
reversible CO2 capture at higher sorption capacities are
highly sought. This research need prompted us to investigate
protic ionic liquids (PILs) for carbon capture.
Ionic liquids (ILs) offer a new opportunity for developing
novel capture systems that are capable of reversibly capturing
CO2 with a high capacity,[2] because of their unique properties,
such as negligible vapor pressures, high thermal stabilities,
excellent CO2 solubilities, and tunable properties.[3] Davis and
co-workers[4] reported the first example of the chemisorption
of CO2 that employs an amino-functionalized task-specific
ionic liquid. Their results show that 0.5 mol CO2 can be
captured per mol of IL with a gravimetric capacity of about
7% for 3 h under ambient pressure. Subsequently, a number
of research groups[5] have studied other amino-functionalized
ILs for carbon capture, including functionalized sulfones and
amino acid anions with imidazolium or phosphonium cations.
Although these investigations have made important improve-
ments, the maximum gravimetric absorption capacity is still
limited to about 9%, and furthermore the sorption kinetic is
very slow. This low capture efficiency associated with the
capture system based on amino-functionalized ILs can be
Herein we describe an efficient carbon-capture system
based on a diverse class of anion-functionalized PILs. The
essence of our strategy to derive CO2-reactive PILs is to use a
very strong base (superbase or proton sponge) to directly
deprotonate weak proton donors, such as fluorinated alco-
hols, imidazoles, pyrrolidinones, or phenols. We show that
these superbase-derived PILs with low melting points are
capable of reversibly capturing CO2 with an extremely high
capacity (more than 1 mol per mol IL). Furthermore, we also
show that both polarities and basicities of these PILs can be
easily tuned using CO2 and N2 as a switch, which opens up
their potential applications in separation and catalysis.
PILs can be easily prepared by combining a Brønsted acid
with a Brønsted base.[7] Although a number of PILs have been
developed using a wide variety of anions, such as tetrafluor-
oborate, choride, carboxylate, and hexafluorophosphate, CO2
capture by PILs has not been reported because of the low
reactivity of the conventional PILs toward CO2. Our
approach was to design novel PILs from strong organic
bases and a wide variety of weak proton donors by making use
of their tunable chemical reactivity toward CO2. Superbases,
which are neutral organic bases with proton affinities so high
that their protonated conjugate acids (BH+) cannot be
deprotonated by a hydroxide ion,[8] play a key role as strong
proton acceptors, thereby providing a thermodynamic driving
force for formation of reactive PILs. We have recently
demonstrated that the PILs derived from organic superbases
behave like aprotic ILs with considerably reduced vapor
pressures.[8b] Scheme 1 shows the structures of two superbases
and six weak proton donors used in our current investigation.
These novel PILs were synthesized in high yield (> 98%)
by neutralization of superbases (MTBD or P2-Et) with
partially fluorinated alcohols (TFE, TFPA, or HFPD),
imidazole (Im), pyrrolidone (Pyrr), or phenol (PhOH),
where the pKa values in DMSO of the proton donors are in
the range of 18–24 (see Scheme 1).[9] The formation of these
superbase-derived PILs was shown by both NMR spectros-
copy and ion conductivity measurements. For example, a
broad band that appeared at 4.07 ppm in the 1H NMR
[*] Dr. C. Wang, Prof. H. Li
Department of Chemistry, Zhejiang University
Hangzhou 310027 (P. R. China)
Fax: (+86)571-8795-1895
E-mail: lihr@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. 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).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201002641.
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superior to the amount of CO2 captured by traditional ILs
(Supporting Information, Table S3).
The effect of different proton donors on the absorption of
CO2 is significant. It was clear that the absorption was almost
complete within 5 min for [MTBDH+][TFEÀ], whereas that
for [MTBDH+][ImÀ] it was about 30 min (Figure 1). In
Scheme 1. Selected superbases, fluorinated alcohols, imidazole (Im),
pyrrolidone (Pyrr), and phenol used as building blocks of superbase-
derived PILs. pKa values in DMSO (values given in brackets are in
H2O).
spectrum of a neat TFE disappeared in a spectrum of
[MTBDH+][TFEÀ], suggesting the absence of free alcohol
OH in [MTBDH+][TFEÀ]. Furthermore, the 1H NMR peaks
of [MTBDH+][ImÀ] agree well with those of [ImÀ] in aprotic
ILs [P66614+][ImÀ] (see Supporting Information, Scheme S1,
for its structure) and are very different with those of
imidazole, clearly indicating the formation of ILs (Supporting
Information, Table S1). The ionic conductivity of
[MTBDH+][TFEÀ] and [MTBDH+][ImÀ] increased to 1.082
and 1.139 mScmÀ1, respectively, also suggesting that MTBD
was the protonated species (Supporting Information,
Table S2). All ten newly synthesized PILs showed low melting
point and moderate ion conductivity. Coulombic interactions
were critical to improving the thermal stability of PILs. For
example, the decomposition temperature increased more
than 808C when the mixture of P2-Et and ethanol was
replaced by [(P2-Et)H+][TFEÀ]. Clearly, the thermal stability
of PILs could be significantly influenced by the combination
of different superbases and proton donors.
The effect of different superbase-derived PILs on the
capture of CO2 was investigated (Table 1). Most of these PILs
showed excellent CO2 capture under atmospheric pressure.
For example, a molar ratio of CO2 to PILs of 1.13 could be
achieved when [MTBDH+][TFEÀ] was used; [MTBDH+]2-
[HFPD2À] showed a higher CO2 capacity of more than 2.0 mol
per mol PIL because of the presence of two CO2-reactive
groups. For these PIL capture systems, a gravimetric capacity
of more than 16% can be achieved, which is significantly
Figure 1. CO2 absorption by typical superbase-derived PILs.
contrast, the chemically unreactive PILs, such as [MTBDH+]-
[Tf2NÀ], had a capacity of only 0.02 mol per mole PILs at
atmospheric pressure. This rapid absorption, in the range of
few minutes to tens of minutes, is related to their low
viscosities. For example, the viscosity of [MTBDH+][TFEÀ] is
8.63 cP, and that of [MTBDH+][ImÀ] is 31.85 cP; these values
are distinctively lower than that of amino-functionalized
ILs.[5c,d]
During the capture of CO2 observed in this study, CO2
reacted with PILs to form a liquid carbonate, carbamate or
phenolate salt. For example, the formation of carbamate is
evident from NMR and IR spectra of [MTBDH+][ImÀ] and
the corresponding carbamate [MTBDH+][ImCÀ]. After CO2
bubbling, a new band for [MTBDH+][ImCÀ] at 1696.4 cmÀ1
produced, which can be assigned to carbamate stretches
(Supporting Information, Figure S1). Based on previous
reports[10] and the observed reaction product, the CO2
absorption by our PILs can be straightforwardly described
(Scheme 2). We computed the gas-phase energetics according
to the reactions in Scheme 2 at the B3LYP/TZVP level of
theory and obtained changes of energy at À116.8, À85.2, and
À41.7 kJmolÀ1 for TFEÀ, ImÀ, and PhOÀ, respectively. This
energetic trend indicates that the relatively weak driving force
Table 1: CO2 absorption by different superbase-derived PILs.[a]
Ionic liquids
Time [min]
CO2 absorption[b]
State
[MTBDH+][TFEÀ]
[(P2-Et)H+][TFEÀ]
[MTBDH+][TFPAÀ]
[MTBDH+]2[HFPD2À
[MTBDH+][ImÀ]
[(P2-Et)H+][ImÀ]
[MTBDH+][PyrrÀ]
[(P2-Et)H+][PyrrÀ]
[MTBDH+][PhOÀ]
[(P2-Et)H+][PhOÀ]
[MTBDH+][Tf2NÀ]
10
10
60
60
30
10
30
30
1.13
1.04
0.93
2.04
1.03
0.96
0.92
0.86
0.49
0.45
0.02
liquid
liquid
liquid
gel
liquid
liquid
liquid
liquid
gel
]
300
300
60
gel
liquid
Scheme 2. CO2 absorption by the anions of superbase-derived protic
[a] The absorption was carried out at 238C. [b] Mol CO2 per mol PILs.
ionic liquids.
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Communications
for reaction of CO2 with PhOÀ leads to a smaller CO2
absorption capacity.
Table 2: Polarities of selected solvents and PILs as indicated by
solvatochromic dyes.[a]
Solvents or PILs
lmax (Nile Red) [nm][b]
The stability of the CO2 captured by these PILs was
studied by thermogravimetric analysis (TGA), as shown in
the Supporting Information, Figure S2. When the temper-
ature reached 1108C, the weight losses for [MTBDH+][ImÀ]
and [MTBDH+][ImCÀ] are 3% and 17%, respectively,
indicating that the release of CO2 was almost complete. The
captured CO2 was easy to strip out by heating or bubbling N2
through the PILs. For example, the release of CO2 by
bubbling N2 at 808C for [MTBDH+][ImCÀ] was fast, and
almost complete in 10 min. The processes of CO2 absorption
by PILs are completely reversible, and the PILs show a slight
loss in capacity after multiple absorption–desorption cycles
(Figure 2). Thus, these PILs are good candidates as switchable
solvents for the chemical reaction and separation.
DMF
541.2
543[c]
549.6
550.8[d]
551[c]
552[c]
565.2
567[c]
[MTBDH+][TFPAÀ]
Methanol
[Bmim+][BF4
]
À
[MTBDH+][ImÀ]
[MTBDH+][TFPACÀ]
Ethylene glycol
[MTBDH+][ImCÀ]
[a] Data were determined using Nile Red at 238C. [b] Data as published
in Ref. [12] except as indicated. [c] This work. [d] Ref. [13].
Figure 3. Photographs of the switchable polarity and basicity of
[MTBDH+][ImÀ] by CO2 and N2 as the triggers. The solutions were
colored with a basicity indicator (2,4-dinitroaniline). Left: the miscibil-
ity and basicity of 2:1 (v/v) mixture of [MTBDH+][ImÀ] and fatty acid
methyl esters in the presence of 2,4-dinitroaniline under N2. Right: the
immiscibility and neutrality of 2:1 (v/v) mixture of [MTBDH+][ImCÀ]
and fatty acid methyl esters in the presence of 2,4-dinitroaniline under
CO2.
Figure 2. Three consecutive cycles of CO2 absorption (1a, 2a, and 3a,
at 238C) and release (1b, 2b, and 3b, at 808C under N2) by
[MTBDH+][ImÀ].
Switchable solvents formed by mixing an amidine with an
alcohol,[10] alkylamine,[10] or amino alcohol[11] have been
reported. However, one key drawback associated with these
switchable solvents is the volatile nature of their molecular
constituents. The development of ILs with switchable chem-
ical properties should be able to mitigate the evaporative loss
of current switchable solvents. The study on switchable
solvents based solely on ILs using CO2 and N2 as the switch
is limited. This deficiency forms the rationale for us to explore
ILs with switchable properties.
The switchable polarity of our superbase-derived PILs
was investigated using [MTBDH+][TFPAÀ] and [MTBDH+]-
[ImÀ] as examples and Nile Red as a solvatochromic probe.
As shown in Table 2, [MTBDH+][HFPAÀ] showed a lmax
increase of about 9 nm for the optical spectrum of Nile Red
upon uptake of CO2 to form the carbonate salt [MTBDH+]-
[TFPACÀ]. On the other hand, the formation of [MTBDH+]-
[ImCÀ] by reaction of [MTBDH+][ImÀ] with CO2 exhibited a
lmax increase of as large as 16 nm.[12] Accordingly, the polarity
change of [MTBDH+][ImÀ] upon uptake of CO2 is more
significant than that of [MTBDH+][TFPAÀ]. These results
further underscore an important point that the degree of the
polarity change associated with our PILs can be finely tuned
through use of different proton donors.
ities of organic species in ILs. Fatty acid methyl ester is
miscible with [MTBDH+][ImÀ], whereas two phases form
upon uptake of CO2. This phase separation was triggered by
the formation of the more polar [MTBDH+][ImCÀ]. The
switchable basicity of our superbase-derived PILs was inves-
tigated using [MTBDH+][ImÀ] as an example and 2,4-
dinitroaniline as a basicity indicator. As seen in Figure 3,
[MTBDH+][ImÀ] is strongly basic and can readily change the
color of 2,4-dinitroaniline from yellow to purple, whereas
[MTBDH+][ImCÀ] derived from uptake of CO2 is neutral.[14]
The switchabilities associated with our PILs in both polarity
and basicity can be exploited in catalyzing the transesterifi-
cation of soybean oil to produce biodiesel. These switchable
PILs provide an ideal reaction medium for efficient recycling
of base catalysts and potential simplification of product
separation (Supporting Information, Figure S3).
In summary, the properties of superbase-based PILs,
which were prepared by the proton-transfer reaction between
an organic superbase and a partially fluorinated alcohol,
imidazole, pyrrolinone, or phenol, are easy to tune because of
the diversity of organic superbases and weak proton donors
that are available. We have shown that these anion-function-
alized PILs are excellent systems for the rapid and reversible
capture of CO2 with equimolar CO2 absorption by the anions
Figure 3 demonstrates a specific application of this switch-
able polarity associated with our PILs in controlling solubil-
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as to develop a potential method for the capture of CO2 by
totally different anion-functionalized ILs in industry.
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Experimental Section
The superbase-derived PILs were synthesized according to methods
detailed in the literature.[9] These PILs samples were dried in vacuum
at 608C for 24 h to reduce possible traces of water. The water contents
of these PILs were measured by Karl Fischer titration and were found
1
to be less than 0.1%. H and 13C NMR spectra were recorded on a
Bruker MSL-400 NMR spectrometer with tetramethylsilane as the
standard. FTIR data were obtained using a Bio-Rad Excalibur FTS-
3000 spectrometer at room temperature. Viscosity was measured on a
Grabner Minivis micro-viscometer. Glass-transition and decomposi-
tion temperatures were measured with a TGA 2950 and DSC Q 100
meter, respectively.
Absorption of CO2: In a typical run, CO2 at atmospheric pressure
was bubbled through about 1.0 g PILs in a glass container with an
inner diameter of 10 mm, and the flow rate was about 60 mLminÀ1
.
The glass container was partly immersed in an oil bath of desirable
temperature. The amount of CO2 absorbed was determined at regular
intervals by the electronic balance with an accuracy of Æ 0.1 mg. The
PILs were regenerated by heating or bubbling nitrogen through PILs.
Computations: Turbomole V5.10 was used for parallel density
functional theory calculations[15] at the B3LYP level. The def2-TZVP
orbital and auxiliary basis sets[16] were used for all atoms for structural
optimization. Convergence criterion for self-consistent-field energy
was 10À6 a.u.; convergence criterion for forces was 10À3 a.u.
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Received: May 2, 2010
Published online: July 15, 2010
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Keywords: anions · carbon capture · carbon dioxide ·
.
ionic liquids · superbases
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