Physical properties and CO2 reaction pathway of 1-ethyl-3-methylimidazolium ionic liquids with aprotic heterocyclic anions

Article abstract of DOI:10.1021/jp509583c

Ionic liquids (ILs) with aprotic heterocyclic anions (AHA) are attractive candidates for CO₂ capture technologies. In this study, a series of AHA ILs with 1-ethyl-3-methylimidazolium ([emim]⁺) cations were synthesized, and their physical properties (density, viscosity, and ionic conductivity) were measured. In addition, CO₂ solubility in each IL was determined at room temperature using a volumetric method at pressures between 0 and 1 bar. The AHAs are basic anions that are capable of reacting stoichiometrically with CO₂ to form carbamate species. An interesting CO₂ uptake isotherm behavior was observed, and this may be attributed to a parallel, equilibrium proton exchange process between the imidazolium cation and the basic AHA in the presence of CO₂, followed by the formation of "transient" carbene species that react rapidly with CO₂. The presence of the imidazolium-carboxylate species and carbamate anion species was verified using ¹H and ¹³C NMR spectroscopy. While the reaction between CO₂ and the proposed transient carbene resulted in cation-CO₂ binding that is stronger than the anion-CO₂ reaction, the reactions of the imidazolium AHA ILs were fully reversible upon regeneration at 80 °C with nitrogen purging. The presence of water decreased the CO₂ uptake due to the inhibiting effect of the neutral species (protonated form of AHA) that is formed.

Full text of DOI:10.1021/jp509583c

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Article
2
Physical Properties and CO Reaction Pathway of 1-Ethyl-3-
Methylimidazolium Ionic Liquids with Aprotic Heterocyclic Anions
Samuel Seo, M. Aruni DeSilva, and Joan Frances Brennecke
J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp509583c • Publication Date (Web): 28 Nov 2014
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Physical Properties and CO2 Reaction Pathway of 1-Ethyl-3-
Methylimidazolium Ionic Liquids with Aprotic Heterocyclic
Anions
Samuel Seo, M. Aruni DeSilva and Joan F. Brennecke*
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Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre
Dame, Indiana, 46556, USA
ABSTRACT
Ionic liquids (ILs) with aprotic heterocyclic anions (AHA) are attractive candidates for
CO2 capture technologies. In this study, a series of AHA ILs with 1ꢀethylꢀ3ꢀmethylimidazolium
([emim]+) cations were synthesized and their physical properties (density, viscosity, and ionic
conductivity) were measured. In addition, CO2 solubility in each IL was determined at room
temperature using a volumetric method at pressures between 0 and 1 bar. The AHAs are basic
anions that are capable of reacting stoichiometrically with CO2 to form carbamate species. An
interesting CO2 uptake isotherm behavior was observed, and this may be attributed to a parallel,
equilibrium proton exchange process between the imidazolium cation and the basic AHA in the
presence of CO2, followed by the formation of ‘transient’ carbene species that react rapidly with
CO2. The presence of the imidazoliumꢀcarboxylate species and carbamate anion species was
verified using 1H and 13C NMR spectroscopy. While the reaction between CO2 and the proposed
transient carbene resulted in cationꢀCO2 binding that is stronger than the anionꢀCO2 reaction, the
reactions of the imidazolium AHA ILs were fully reversible upon regeneration at 80 °C with
nitrogen purging. The presence of water decreased the CO2 uptake due to the inhibiting effect of
the neutral species (protonated form of AHA) that is formed.
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ꢀ INTRODUCTION
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Ionic Liquids (ILs) are defined as molten salts composed entirely of ions with a melting
temperature below the boiling point of water. Their unique characteristics such as nonꢀ
flammability, lowꢀvolatility, and high thermal stability, make ILs a promising replacement for
aqueous amine absorbents used in postꢀcombustion CO2 capture from coalꢀfired power plants.
Strong chemical absorption is required for efficient capture of CO2 from exhaust streams due to
relatively low partial pressures of CO2 (e.g. 0.1 to 0.15 bar), and the technology of choice for
CO2 scrubbing has been aqueous amine solvents.1 However, one of the critical problems with
current amine solvents, primarily monoethanolamine (MEA), is the high standard enthalpy of
reaction (ꢀꢁꢂꢅꢃꢄ, enthalpy change during the binding reaction under standard conditions) with
CO2, which makes the regeneration process quite energyꢀintensive.2 Recently, we proposed
trihexyl(tetradecyl)phosphonium cation ([P66614]+) with aprotic heterocyclic anions ([AHA]ꢀ),
which can stoichiometrically and reversibly react with CO2 at relatively low pressures (< 1 bar)
with tunability over a reasonable range of ꢀꢁꢂꢅꢃꢄ.3 By minimizing the number of free hydrogens
that are amenable for hydrogenꢀbonding network, the viscosity of these AHA ILs did not
increase upon reaction with CO2,4 which is in contrast to the previously reported taskꢀspecific
ILs that react chemically with CO2.5ꢀ9
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While high CO2 loading capacity, adjustable ꢀꢁꢂꢅꢃꢄ, and negligible viscosity change upon
reaction with CO2 are appealing, the intrinsic high viscosities of [P66614][AHA] ILs (ranging from
400 to 1300 cP at 25 °C)10 seem unfavorable. Such high viscosities are primarily due to the
presence of long alkylꢀchains on the phosphonium cation that yield substantial van der Waals
interaction. From the industrial point of view, a lower viscosity is desired as the viscosity of a
system fluid is directly related to the operating costs (e.g. pumping cost) as well as the kinetics of
the CO2 absorption process since the ILꢀCO2 reaction is often considered diffusionꢀlimited.11
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Dialkylimidazolium is one of the most widely studied cations, and it continuously attracts
attention due to its diverse and tunable chemistry and relatively low viscosities and melting
points.12,13 In an attempt to develop low viscosity and potentially robust ILs for CO2 capture, we
have synthesized four novel ILs that pair 1ꢀethylꢀ3ꢀmethylimidazolium ([emim]+) cation with
AHAs of varying basicity. Shi et al. previously investigated the possibility of using
[emim][AHA] ILs as electrolytes in Liꢀion batteries.14 In this study, their physical properties,
such as viscosity, density, and conductivity, were measured as a function of temperature, and
empirical models were used to describe the temperature dependency. Then, CO2 solubility in
each IL was determined using a volumetric method at pressures between 0 and 1 bar at 22 °C.
The CO2ꢀ[emim][AHA] equilibrium isotherms exhibited an unusual behavior
characterized by a stronger CO2 binding compared to that expected from the previously reported
[P66614][AHA] ILs.10 In general, the proton in the C(2) position of the imidazolium ring is known
to be acidic and is prone to a fairly strong hydrogenꢀbond interaction with an anion.15ꢀ19
Introduction of an increasingly basic anion will cause progressively stronger interactions with the
C(2) proton on the imidazolium; an anion with high enough basicity has the potential to abstract
the proton to produce an Nꢀheterocyclic carbene (NHC) along with a neutralized form of the
anion.20,21 One widely studied system that demonstrates such an intermolecular proton exchange
process in the presence of CO2 is dialkylimidazolium acetate, [CnCmim][OAc]. Shiflett et al.
calculated the thermodynamic excess properties such as Gibbs free energy, enthalpy, and entropy
for dialkylimidazolium acetate and CO2 to be largely negative, thus implying the possibility of
intermolecular chemical reactions.22,23 Our group was the first to propose a chemical reaction
between 1ꢀbutylꢀ3ꢀmethylimidazolium acetate ([bmim][OAc]) and CO2 via transient carbene
formation24: the NMR analysis suggested that the acetate anion abstracts the proton on the C(2)
position of the imidazolium ring, followed by the reaction of CO2 with the NHC species to
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produce acetic acid and a stable adduct of imidazolium carboxylate. Furthermore, subsequent
analytical studies25ꢀ28 on the binary system of dialkylimidazolium acetate and CO2 verified the
formation of imidazoliumꢀ2ꢀcarboxylate. We hypothesize that the equilibrium between an IL and
carbene that is formed in the presence of CO2 can be effectively tuned by appropriate choice of
counter anions with varying basicity.29 For instance, such carbene formation was possible with
the presence of acetate, a relatively basic anion with a ꢆꢇꢈꢉꢊꢋꢌ of 12.6.30 Most of AHAs in this
study are anions derived from weak acids, and they are considered strong bases.10 Considering
the relatively high basicities of [2ꢀCNPyr]ꢀ, [4ꢀTriaz]ꢀ and [3ꢀTriaz]ꢀ anions (ꢆꢇꢈꢉꢊꢋꢌ = 14.8
and 13.9 for 4ꢀtriazole and 3ꢀtriazole, respectively),31 we expect these anions to not only react
directly with CO2, but they may also abstract the C(2) proton of the imidazolium cation to trigger
the carbeneꢀCO2 reaction. On the other hand, [Tetz]ꢀ is a weak base (ꢆꢇꢈꢉꢊꢋꢌ = 8.2)31 and is not
expected to prompt such intermolecular carbene formation. Previously, dialkylimidazolium ILs
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paired
with
weakly
basic
anions
such
as
trifluoroacetate
([TFA]ꢀ),
bis(trifluoromethylsulfonyl)imide ([Tf2N]ꢀ), tetrafluoroborate ([BF4]ꢀ) or hexafluorophosphate
([PF6]ꢀ) only exhibited physical dissolution of CO2,32ꢀ34 likely because these anions are bases that
are too weak to abstract the C(2) proton on the imidazolium ring. In this study, the formation of
1ꢀethylꢀ3ꢀmethylimidazoliumꢀ2ꢀcarboxylate upon exposure to CO2 under mild conditions (22 °C,
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1 bar) has been confirmed in the liquid phase using H and C NMR spectroscopy. Additional
experiments have examined the effect of water on viscosity and CO2 solubility of [emim][AHA].
ꢀ MATERIALS AND METHODS
Chemicals and Synthesis. All IL samples were synthesized in our laboratory (see Table 1). For
the synthesis of 1ꢀethylꢀ3ꢀmethylimidazolium 2ꢀcyanopyrrolide, [emim][2ꢀCNPyr], 20 g (0.105
moles) of 1ꢀethylꢀ3ꢀmethylimidazolium bromide (99%, Iolitec) was converted to 1ꢀethylꢀ3ꢀ
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methylimidazolium hydroxide by stepwise ion exchange using Amberlite IRN78 hydroxide form
anion exchange resin (Sigma Aldrich). An equimolar amount of pyrroleꢀ2ꢀcarbonitrile (99%
AlfaꢀAeser) was added to the 1ꢀethylꢀ3ꢀmethylimidazolium hydroxide in methanol and stirred
overnight. Methanol was removed under reduced pressure, and the product was further dried at
55 °C under vacuum for several days to remove any residual water. The rest of the ILs in this
study were synthesized according to the procedure described above, using 1,2,4ꢀtriazole (98%,
Sigma Aldrich), 1,2,3ꢀtriazole (97%, Sigma Aldrich), or tetrazole (0.45 M solution in
acetonitrile, Sigma Aldrich) as the anion precursor instead of pyrroleꢀ2ꢀcarbonitrile. The final
product of [emim][Tetz] occasionally contained a small amount of white precipitate, which was
removed by filtration. The purity of each IL is approximately 98%, as determined by integrating
the peaks in the 1H NMR spectrum, obtained with a long relaxation delay time.
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Table 1. Chemical Structures and Nomenclatures of Imidazolium AHA ILs.
chemical structure
abbreviation
ion name
[emim]+
1ꢀethylꢀ3ꢀmethylimidazolium
N
N
N
N
N
C
[2ꢀCNPyr]ꢀ
[4ꢀTriaz]ꢀ
2ꢀcyanoꢀpyrrolide
4ꢀtriazolide
N
N
N
N
N
[3ꢀTriaz]ꢀ
[Tetz]ꢀ
3ꢀtriazolide
Tetrazolide
N
N
N
N
Characterization Methods. The molecular structure of each IL was verified using NMR
spectroscopy. Samples for NMR characterization were prepared by dissolving anhydrous IL in
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deuterated DMSOꢀd6 (99.9 atom% D, Sigma Aldrich). H NMR was obtained using a Varian
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INOVA 500 MHz spectrometer at room temperature. A total of 8 transients were averaged and a
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relaxation delay of 150 s was used for quantification. C NMRs were performed on a Bruker
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AVANCE III HD 400 MHz spectrometer. C NMR spectra were collected with inverse gated
decoupling, recycle delay of 2 seconds and 1024 transients. For the detection of bicarbonate,
CO2ꢀsaturated [emim][2ꢀCNPyr] containing 7 wt % deionized water was dissolved in deuterium
oxide (99 atom% D, Sigma Aldrich), and anhydrous 1,4ꢀdioxane (99.8%, Sigma Aldrich) was
added as a reference. All IL samples were dried under reduced pressure at 55 °C for at least 48 h
before use. Water contents of ILs were determined using a Metrohm 831 Karl Fischer
Coulometer with ±3 ꢁg water resolution. For samples with added water, a Mettler Toledo V20
Volumetric Karl Fischer (VKF) Titrator was used instead. The VKF has an uncertainty of
±0.05%. All ILs contained less than 1000 ppm water except for the cases when water was
intentionally added to the samples.
The density of each IL was measured under atmospheric pressure with a DMA 4500
Anton Paar oscillating Uꢀtube densitometer. The temperature was controlled with a precision of
±0.01 °C. After taking the purity of the ILs (~98%) into account, the uncertainty in the density
measurements is approximately ±1 x 10ꢀ3 g cmꢀ3.
The viscosity measurements were performed with an ATS Rheosystems Viscoanalyzer
equipped with a coneꢀandꢀplate spindle. The sample was kept under either a N2 or CO2
environment (neat and CO2ꢀsaturated ILs, respectively) by constantly purging the respective gas
over the sample. The uncertainty in the viscosity measurements is ±5% above 100 cP and ±10%
below 100 cP.
The ionic conductivities of ILs were determined with an electrochemical impedance
spectroscopy (EIS) system. The EIS system consists of a Solatron SI 1260 Impedance/Gainꢀ
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phase analyzer connected to a Solartron 1287 electrochemical interface. The cell constant for
each sample cell (with two Pt electrodes, from Materials Mates) was calibrated with diluted KCl
standard solutions with known conductivities at room temperature, as described by Barthel et
al.35 The IL samples were loaded into the cell in the glove box filled with nitrogen to prevent
their exposure to moisture and CO2 in the atmosphere. The temperature was controlled at 10 °C
intervals from 10 to 70 °C (plus 22 and 25 °C) using a Binder Refrigerated Incubator KB53
(E3.1). At least 45 min were allowed for the sample to equilibrate at each set temperature before
measuring the conductivities. The sample size is about 1 mL, and the uncertainty of the
conductivities is approximately ±3%.
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CO2 Solubility Measurements. The amount of CO2 absorbed by each IL was obtained using a
customꢀbuilt instrument using a methodology described in our previous work.10 The CO2
absorbed is primarily determined by reaction of CO2 with the anion of the IL and, as will be
shown here, with the [emim]+ cation. However, a small amount of CO2 (< 2%) can also be
physically absorbed under atmospheric pressure. The apparatus is primarily comprised of two
systems, a CO2 reservoir and a reaction vessel, with known pressure, temperature, and volume.
Once the reaction vessel containing the IL sample (approximately 2 g) is evacuated, a known
amount of CO2 is fed from the reservoir into the reaction cell. Once a magnetic stir bar is
activated, the rapid pressure drop in the reaction cell confirms the absorption of CO2 into the IL
phase. The pressure in the reaction vessel is logged until vaporꢀliquidꢀequilibrium is reached
(when the pressure no longer changes significantly), and this procedure is repeated until the
equilibrium pressure reached is near 1 bar. The total amount of CO2 absorbed by the IL sample at
each equilibrium point is calculated from the pressure drop using an ideal gas law with Leeꢀ
Kesler correlation.36 The uncertainty in the CO2 uptake measurement is approximately 0.02 mole
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CO2/IL, and the effect on CO2 absorption capacity of volume expansion of the IL upon reaction
with CO2 is within this uncertainty.
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ꢀ RESULTS AND DISCUSSIONS
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Density. The density of each IL as a function of temperature is summarized in Figure 1. Tables
of the data can be found in the Supporting Information. As expected, the density decreases
linearly with increasing temperature, and the experimental data can be well represented by a
linear equation:
ρ = a + bT
(1)
where T is temperature in °C and a, b are fitting parameters summarized in Table 2. The density
of [emim] [AHA] ILs deceased in the following order: [Tetz]ꢀ > [4ꢀTriaz]ꢀ ≥ [3ꢀTriaz]ꢀ > [2ꢀ
CNPyr]ꢀ. [emim][3ꢀTriaz] and [emim][4ꢀTriaz] have similar densities, which can be attributed to
their structural similarity. In general, the densities of [emim][AHA] ILs are about 20% higher
than the [P66614][AHA] counterparts (e.g. the density of [P66614][2ꢀCNPyr] is 0.901 g·cmꢀ3 at 25
°C).10 This is presumably due to better stacking and increased πꢀπ interaction resulting from the
planar structure of both cation and anion.37
1.3
[emim][2ꢀCNPyr]ᵃ
[emim][4ꢀTriaz]ᵃ
[emim][3ꢀTriaz]
[emim][Tetz]
1.2
1.1
1.0
0
20
40
60
80
Tempearture (°C)
Figure 1. Density of [emim][AHA] ILs as a function of temperature. aFrom Shi et al.14
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Table 2. Fitting Parameters for Physical Property Models
viscosity
conductivity
B
density
a
a
a
b
ηo
α
Tg
[K]
σo
Tg
[K]
ILs
[g cmꢀ3]
[10ꢀ4 g cmꢀ1·°Cꢀ1]
[mS cmꢀ1]
[cP] [K]
[K]
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0.417 438 212.15
0.415 489 205.15
0.637 457 201.15
0.549 502 194.15
679
776
607
676
410 212.15
463 205.15
448 201.15
485 194.15
[emim][2ꢀCNPyr] 1.102
ꢀ6.1
ꢀ6.1
ꢀ6.0
ꢀ6.2
[emim][4ꢀTriaz]
[emim][3ꢀTriaz]
[emim][Tetz]
1.155
1.148
1.191
aExperimentally measured values.
Viscosity. The viscosity of [emim][AHA] ILs was measured between 10 and 70 °C, and the
results are shown on a logarithmic scale in Figure 2. The full collection of data is given in
Supporting Information. While there is not much difference in the viscosities of the four ILs,
[emim][4ꢀTriaz] had the highest viscosity and [emim][Tetz] the lowest. The viscosity of
[emim][AHA] ILs is substantially lower than that of [P66614]+ counterparts reported previously.10
The reason for such high viscosities in [P66614][AHA] ILs is primarily due to the presence of long
alkyl chains on the cation, which can induce strong van der Waals interactions and subsequently
increase the friction between the ions. On the other hand, the two alkyl moieties on the
imidazolium cation are considerably shorter compared to the ones on the [P66614]+ cation, and the
molecular weight of [emim]+ (111.7 g molꢀ1) is only about a quarter of [P66614]+ (479.8 g molꢀ1). In
addition, the planar structure of imidazolium is expected to promote the diffusion of ions,
particularly when it is paired with another planar anion counterpart (e.g., AHA).37 The Vogelꢀ
FulcherꢀTammann (VFT) model has been frequently employed to describe the temperature
dependence of viscosity:
ꢑ
ꢍ = ꢍꢅꢎꢏꢆ ꢐꢒꢓꢒ
ꢕ
(2)
ꢔ
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where ꢍ is viscosity in cP, T is absolute temperature, ꢖ is glass transition temperature in absolute
ꢗ
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units, and ꢍꢅ and ꢘ are adjustable parameters in cP and K, respectively. The dotted lines in Figure
2 were obtained using the VFT model, and the fitting parameters for each IL are available in
Table 2.
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1000
[emim][2ꢀCNPyr]ᵃ
[emim][4ꢀTriaz]ᵃ
[emim][3ꢀTriaz]
[emim][Tetz]
100
10
0
20
40
60
80
Tempearture (°C)
Figure 2. Viscosity of [emim][AHA] ILs as a function of temperature. aFrom Shi et al.14
Conductivity. The conductivities of [emim][AHA] ILs were measured in order to examine their
potentials in electrochemical applications (e.g. electrolytes in Liꢀion batteries). Similar to the
viscosity trend described in the previous section, all [emim][AHA] studied fall within a similar
conductivity range (see Figure 3). In general, the conductivity of a fluid is inversely proportional
to the viscosity. Owing to low viscosity exhibited by [emim][AHA] ILs, the conductivities of
+
[emim][AHA] ILs are about two orders of magnitude higher than the AHA ILs with a [P66614
]
cation.10 The enhanced conductivities of [emim][AHA] may also be attributable to rapid ion
diffusion resulting from their planar ion structures. Once again, the VFT model was used to
describe the exponentially increasing conductivity behavior with increasing temperature:
ꢓꢙ
σ = σꢅꢎꢏꢆ ꢐꢒꢓꢒ
ꢕ
(3)
ꢔ
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where σ is conductivity in mS cmꢀ1, T is absolute temperature, ꢖ is glass transition temperature
ꢗ
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6
in absolute units, and σꢅ and B are adjustable parameters in mS cmꢀ1 and K, respectively.
7
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100
[emim][2ꢀCNPyr]ᵃ
9
[emim][4ꢀTriaz]ᵃ
[emim][3ꢀTriaz]
[emim][Tetz]
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1
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60
80
Tempearture (°C)
Figure 3. Conductivity of [emim][AHA] ILs as a function of temperature. aFrom Shi et al.14
A Walden plot, which describes how conductive an IL is for a given viscosity, is
available in the Supporting information along with those of similar ILs from the literature. Some
researchers have interpreted the proximity of the plotted values to the reference line (constructed
using dilute KCl solutions) as a qualitative measure of the ‘ionicity’ of an IL,38,39 arguing that
cations and anions must be less associated in systems that exhibit higher conductivity for a
particular viscosity. As shown in Figure S1, the [emim][AHA] ILs not only have low viscosities,
but they also have relatively high conductivities for their viscosities as the plotted values fall in
proximity to the reference line. All four [emim][AHA] ILs have noticeably larger degree of
ionicity compared to AHA ILs with bulky phosphonium cations, where substantial van der
Waals force and steric hindrance may contribute to the formation of ion pairing or aggregations.
On the other hand, the choice of anion seems trivial as the values on the Walden plot for all
[emim]+ꢀbased ILs including [emim][Tf2N] and [emim][OAc] fall within the same range.
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CO2 Solubility and Reaction Pathway. The CO2 solubility in each [emim][AHA] IL was
measured at 22 °C up to 1 bar, and comparisons with isotherms for each of the corresponding
[P66614][AHA] ILs are presented in Figure 4. For ILs where the anion is inherently basic and is
capable of reacting with CO2, one would expect the anion to dominate the CO2 reaction pathway
(and, thus, the CO2 solubility), while the cation should only play a minor role in CO2 absorption
(physical). Within this context, all [emim][AHA] ILs were initially envisioned to have a similar
reaction pathway with CO2 (thus, a similar CO2 uptake isotherm) compared to the [P66614][AHA]
ILs, as depicted in Figure 5a where CO2 reacts directly with the anion. The CO2 isotherms of
[emim][2ꢀCNPyr], [emim][4ꢀTriaz] and [emim][3ꢀTriaz], however, exhibited distinctively
different behavior compared to their phosphonium counterparts. At low CO2 partial pressures,
the imidazolium ILs absorbed more CO2, reflecting a stronger complex formation with CO2,
while each CO2 isotherm tended to level out with increasing CO2 partial pressure. Within the
pressure range investigated in this study, the steepness of the isotherm is expected to correlate
well with the strength of the ILꢀCO2 interaction, as proven in our previous study.10 We propose
that this unexpected behavior is a result of an additional reaction pathway that is not feasible in
phosphonium AHA ILs. Analogous to the reaction pathway proposed for imidazolium acetate
ILs,24 the acidic proton on the C(2) position of the imidazolium cation may be abstracted by an
AHA to form a ‘transient’ carbene, which then reacts rapidly with a CO2 molecule to produce
imidazoliumꢀ2ꢀcarboxylate (see Figure 5b). Thus, we are proposing that CO2 reacts both with the
anion and the cation. In contrast, [emim][Tetz], a weak base, has a very similar isotherm
compared to [P66614][Tetz], implying an absence of the proposed carbeneꢀCO2 reaction pathway
shown in Figure 5b. This further confirms that tuning the basicity of the anion can systematically
control the reaction pathway and isotherm behavior of imidazoliumꢀbased ILs. The small
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difference in the CO2 uptake by [emim][Tetz] and [P66614][Tetz] is likely due to the difference
between physical absorption capacities resulting from different free volumes.
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1.0
0.8
0.6
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(a)
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[emim][2ꢀCNPyr]
a
[P₆₆₆₁₄][2ꢀCNPyr]
[emim][4ꢀTriaz]
a
[P₆₆₆₁₄][4ꢀTriaz]
0
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0
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1
Pressure (bar)
Pressure (bar)
1.0
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1.0
0.8
0.6
0.4
0.2
0.0
(c)
(d)
[emim][3ꢀTriaz]
[emim][Tetz]
a
[P₆₆₆₁₄][3ꢀTriaz]
a
[P₆₆₆₁₄][Tetz]
0
0.2
0.4
0.6
0.8
1
0
0.2
0.4
0.6
0.8
1
Pressure (bar)
Pressure (bar)
Figure 4. CO2 absorption capacity in (a) [emim][2ꢀCNPyr], (b) [emim][4ꢀTriaz], (c) [emim][3ꢀ
Triaz], and (d) [emim][Tetz] at 22 °C. The CO2 solubility in [P66614]+ counterparts from Seo et
a
al.10 are also shown for comparison.
(a)
(b)
Figure 5. Concerted two parallel equilibrium reaction pathways between CO2 and [emim][2ꢀ
CNPyr]: (a) anionꢀCO2 reaction pathway and (b) cationꢀCO2 reaction pathway.
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In order to provide support for the proposed reaction pathway, we analyzed the CO2ꢀ
saturated IL solutions using 1H and 13C NMR spectroscopy. In Figure 6, the 13C spectrum of the
neat [emim][2ꢀCNPyr] is compared with the spectrum obtained after full saturation with CO2 at 1
bar. First, two distinct sets of carbons belonging to the imidazolium cation appeared upon
reaction with CO2, which is definitive evidence of formation of a new species having spectral
signatures close to those of the [emim]+ cation but with different origin: 1ꢀethylꢀ3ꢀ
metyhlimidazoliumꢀ2ꢀcarboxylate. Furthermore, the reaction of [emim][2ꢀCNPyr] with CO2 at
room temperature introduced a new peak around δ=155 ppm, which can be associated with the
carbon atom of the newly formed carboxylate group on C(2) of the imidazolium ring.23,26,28
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Similarly, the H NMR of CO2ꢀsaturated [emim][2ꢀCNPyr] also showed resonance lines of the
[emim]+ accompanied by secondary lines of greater intensity (see Figure 7). The labeling of the
hydrogen atom is displayed according to the convention described in Figure 6. No additional
peak for the proton on the neutral 2ꢀcyanopyrrole (2ꢀCNPyrH) was observed due to fast
equilibrating species between the neutral species and [2ꢀCNPyr]ꢀ. The proportion of the
imidazoliumꢀcarboxylate species estimated from the integrated intensity of the secondary
1
resonance lines in H NMR spectra is about 0.6 mole fraction (integrals available in the
Supporting Information), indicating that a majority of the CO2 uptake is due to the cationꢀCO2
binding rather than the anion directly reacting with a CO2 molecule. In addition to the cationꢀ
CO2 reaction, the evidence for the direct anionꢀCO2 reaction is provided by the peaks 10, 11, and
12 in Figure 7b, where the protons belonging to the [2ꢀCNPyr]ꢀ moved downfield upon
complexation with CO2. This trend is consistent with what was observed in [P66614][2ꢀCNPyr]
ILs, where the anionꢀCO2 reaction pathway is apparent.10
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(a) Pure IL
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2
DMSO
9
(b) IL + CO2
N
N
COO-
(on
imidazolium)
C13
C9
C10
C12
C11
Figure 6. Comparison of the 13C NMR Spectra of [emim][2ꢀCNPyr] (a) before and (b) after the
reaction with CO2 at 22 °C.
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(a) Pure IL
10
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11
6
4
5
2
DMSO
(b) IL + CO2
(c) After 30 min desorption
(d) After 5 h desorption
Figure 7. 1H NMR spectra for [emim][2ꢀCNPyr]: (a) Pure IL, (b) fully saturated with CO2, (c)
after 30 min desorption, (d) after 5 h desorption at 80 °C with N2 gas purging. Peaks in the
highlighted region indicate protons associated with imidazoliumꢀ2ꢀcarboxylate. Detailed data
including integrals and exact chemical shifts are available in the SI.
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While both cation and anion have the potential to bind with CO2, the total CO2 capacities
of [emim][2ꢀCNPyr] or [emim][4ꢀTriaz] at 1 bar are slightly lower than [P66614][2ꢀCNPyr] or
[P66614][4ꢀTriaz], respectively. In addition, the isotherm for each [emim][AHA] ILs except for
[emim][Tetz] (where no chemical reaction is anticipated) has lower slope than the equivalent
phosphonium at higher pressures, which will cause even larger deviation between the
[emim][AHA] and [P66614][AHA] ILs CO2 uptake as the CO2 partial pressure is increased above
1 bar. As a consequence of the carboxylation on the cation, it is likely that protonated forms of
the anions (2ꢀCNPyrH, 4ꢀTriazH, or 3ꢀTriazH) are produced: the neutral species that would be
formed are considered acidic (conjugate acid) and they could readily interact with the anion of
neighboring ion pairs of the IL. This would tie up the anion acceptor though hydrogenꢀbonding
between the NH group of the neutral species that are formed and the nucleophilic nitrogen on the
free anion. Such hydrogenꢀbonding interactions could lead to a reduction in nucleophilicity of
the free anions, inhibiting further chemical reactions including both C(2) proton abstraction as
well as the oneꢀstep reaction between a free anion and CO2. This proposed concerted pathway is
illustrated in Figure 8. The complex anions formed from acid and anion pairs have been
previously noted in imidazolium acetate IL systems, and those involving carboxylic acid and
carboxylate conjugate species are known to be highly stable.40 Rodríguez et al. reported that
addition of excess acetic acid to pure [emim][OAc] hampers the interaction between free anions
and the C(2) proton, effectively preventing the carbene formation.41 Cabaço et al. used Raman
spectroscopy to infer that the carboxylation reaction between imidazolium and CO2 has been
strongly moderated by the interaction between acetate and the acetic acid that is formed.25
Quantum chemical calculations by Hollóczki also support the buffering effect of acidꢀanion
pairs.29
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Figure 8. Schematic illustration of the interaction between [2ꢀCNPyr]ꢀ and 2ꢀCNPyrH.
As a result of multiple CO2 reaction pathways, with a major one being the proposed
transient carbeneꢀCO2 reaction, [emim][AHA] ILs will have different apparent ꢀꢁꢂꢅꢃꢄ values
compared to [P66614][AHA] ILs, where CO2 only binds to the anion to form carbamate species.
According to the isotherms in Figure 4, [emim][AHA] ILs exhibited steeper isotherms at lower
pressures compared to the phosphonium counterparts. We have previously shown that steeper
10
isotherms for different ILs under equal conditions lead to larger magnitude value of ꢀꢁꢂꢅꢃꢄ
.
NHCs are generally known as strong bases,42ꢀ44 and they are reported to have large ꢀꢁꢂꢅꢃꢄ values
with CO2.45ꢀ47 In fact, NHCs have gained considerable attention due to their ability in liquid
solutions to stoichiometrically and rapidly combine with CO2 to form carboxylate adducts.48
Recently, Vogt et al. demonstrated that solid, stable NHC can react stoichiometrically with CO2
at very low partial pressures, with ꢀꢁꢂꢅꢃꢄ of approximately ꢀ69 kJ molꢀ1 according to differential
scanning calorimetry.45 This value is substantially more exothermic than the ꢀꢁꢂꢅꢃꢄ for the
[P66614][AHA] ILs (ꢀ45, ꢀ42, and ꢀ37 kJ molꢀ1 for [2ꢀCNPyr]ꢀ, [4ꢀTriaz]ꢀ, and [3ꢀTriaz]ꢀ,
respectively).10 A comparison of the initial slopes of CO2 isotherms at lower pressures, the NHC
by Vogt et al.45 has even stronger binding with CO2 (nearly saturated under 1 mbar CO2) than the
[emim][AHA] ILs in this study, and this can be attributable to the presence of anionꢀCO2
reaction as well as the prohibitive effect mentioned above. Qualitatively, the slopes of [emim][2ꢀ
CNPyr] and [emim][4ꢀTriaz] isotherms at lower pressures are comparable to that of tetraꢀ
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alkylphosphonium 6ꢀbromoꢀbenzimidazolide ([P66614][6ꢀBrBnIm]) IL, which has an enthalpy of
reaction with CO2 of ꢀ52 kJ molꢀ1.10
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The simple reaction between CO2 and AHA is previously confirmed to be fully reversible,
but the reversibility of the proposed [emim]ꢀCO2 reaction pathway described in Figure 5b is in
question. In this regard, the reversibility of the [emim][AHA] ILs and CO2 reaction was
investigated by desorbing the fully saturated [emim][2ꢀCNPyr] at 80 °C along with N2 gas
purging. Samples were taken 30 min, 1 h, 2 h, and 5 h after the start of the desorption procedure
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for H NMR analysis. A full set of H NMR results is available in the Supporting Information.
As shown in Figure 7, the size of the resonance peaks in the highlighted region (associated with
protons on 1ꢀethylꢀ3ꢀmethylimidazoliumꢀ2ꢀcarboxylate) gradually diminishes upon the start of
the desorption step (see Figure 7c). After 5 h, the duplicate peaks have completely disappeared
1
(Figure 7d), and the H NMR spectrum is identical to that of the pure [emim][2ꢀCNPyr] shown
in Figure 7a. This verifies that both C(2)ꢀcarboxylation and the anionꢀCO2 reaction are fully
reversible. The fully desorbed sample was recovered and used for the second CO2 absorption
cycle, and the results are summarized in Figure S3 in the Supporting Information. The isotherm
obtained using the desorbed sample was identical to that of the initial sample, further
corroborating the reversibility of both [emim][2ꢀCNPyr]ꢀCO2 reactions. This agrees with a claim
by Shifflet et al., where the reaction of 1,3ꢀdiethylimidazolium acetate ([eeim][OAc])23 or 1ꢀ
butylꢀ3ꢀmethylimidazolium acetate ([bmim][OAc])22 with CO2 was reversible at reduced
pressure with mild heating. In the [emim][2ꢀCNPyr]ꢀCO2 system, the reversibility was not
necessarily expected since the neutral 2ꢀcyanopyrrole that may be formed could evaporate.
However, it has a very low vapor pressure (2.7 × 10ꢀ4 bar at 25 °C and Tb = 252.3 °C) and likely
has a high affinity for the IL due to strong hydrogenꢀbond interaction with the [2ꢀCNPyr]ꢀ anion.
As mentioned in the previous section, the complex anions formed from anion and protonated
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anion pairs are particularly stable, and this would effectively accommodate stabilization of the
liberated anion precursor.
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Viscosity Trend and Effect of Water on CO2 Absorption. We have previously shown that the
viscosities of [P66614][AHA] ILs remain the same upon reaction with CO2.10 However, the
formation of imidazoliumꢀcarboxylate and protonated anion adducts may lead to changed in
viscosities of [emim][AHA]ꢀCO2 complex. To determine the effect of CO2 absorption on
viscosity, [emim][2ꢀCNPyr] was saturated with CO2 at 1 bar and its viscosity was measured
while keeping the sample under CO2 gas flow. As shown in Figure 9, the viscosity of dry
[emim][2ꢀCNPyr] increased nearly sixꢀfold upon reaction with CO2 at 1 bar. This phenomenon is
consistent with the thickening of the sample23 or solidification27 observed for the
dialkylimidazolium acetate IL + CO2 system. [emim][2ꢀCNPyr] also became turbid and thicker
upon reaction with CO2; this may be attributed to the emergence of the imidazoliumꢀcarboxylate
species along with protonated anions. In addition, the hydrogen bonds between complex anions
as depicted in Figure 8 may also have contributed to the increased viscosity.
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ILs are generally hydroscopic, and the presence of water can significantly alter the
physical or thermodynamic properties.49ꢀ51 The effect of water on the physical properties and
CO2 solubility was studied using [emim][2ꢀCNPyr]. As shown in Figure 9, the viscosity of
[emim][2ꢀCNPyr] decreased slightly when 7 wt % (0.46 mole fraction) of water was added.
Considering the amount of water in the mixture, such small reduction in viscosity is
unprecedented and is worthy of further investigation. When the [emim][2ꢀCNPyr] sample
containing 7 wt % water was saturated with CO2, the viscosity became even higher than what
was obtained from dry [emim][2ꢀCNPyr]ꢀCO2 complex; this trend is in line with the
[P66614][AHA]+water+CO2 systems,10 where the formation of the intermolecular hydrogenꢀbond
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network between the carbamate on the anion and the water molecules increased the viscosity
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significantly.
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ILa
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IL + CO₂
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IL + H₂O
IL + H₂O + CO₂
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Temperature (°C)
Figure 9. Viscosity of [emim][2ꢀCNPyr] (afrom Seo et al.10), CO2ꢀsaturated (saturated at
22 °C) IL, IL containing 7 wt % water, and CO2ꢀsaturated IL containing 7 wt % water.
As shown in Figure 10, the addition of water also influenced the CO2 absorption capacity
in [emim][2ꢀCNPyr]. The sample containing 7 wt % water not only exhibited a reduced CO2
uptake at lower pressures between 0 and 0.2 bar, but also had noticeably lower total CO2 uptake
when the sample was fully saturated at 1 bar. In fact, the initial slope of the CO2 isotherm is
similar to that of [P66614][2ꢀCNPyr],3 implying that the carbene formation is suppressed when
water is present and that the anionꢀCO2 reaction is responsible for the majority of uptake. The
water molecules can act as a hydrogenꢀbond donors (better proton donors than [emim]+),41
largely interrupting the interaction between the nucleophilic anion and the acidic proton of the
imidazolium cation; this would reduce the probability of the NHC formation that we propose is
prerequisite for the cationꢀCO2 reaction. The inhibitory effect of water is in agreement with what
Rodríguez et al. experimentally observed in a [emim][OAc]+H2O+CO2 system,41 and also with a
subsequent study by Gurau et al.27 and Stevanovic et al.52 In addition, an ab initio molecular
dynamics study of the same mixture by Brehm et al. concluded that the presence of water
molecules modifies and largely disturbs the hydrogen bond network of the pure IL.53
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Furthermore, Shiflett et al. reported that the CO2 solubility in [eeim][OAc] decreased with the
addition of water and that the decrease is in approximate proportion with the amount of water
added.23 However, the presence of water does not completely hamper the cationꢀCO2 reaction, as
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supported by the observation of secondary resonance lines in the H NMR (see Figure S4 in the
Supporting Information). Nevertheless, the integration of peaks pertaining to the imidazoliumꢀ
carboxylate species significantly decreased in the presence of water.
1.0
0.8
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IL
IL + H₂O
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Pressure (bar)
Figure 10: CO2 solubility in [emim][2ꢀCNPyr] and [emim][2ꢀCNPyr] with 7 wt % water.
In addition, the tertiary system of ILꢀH2OꢀCO2 can lead to the formation of bicarbonate.
To verify the formation of bicarbonate in the [emim][2ꢀCNPyr]ꢀH2OꢀCO2 system, the fully CO2ꢀ
saturated mixture was taken for 13C NMR with D2O as a solvent and 1,4ꢀdioxane as a reference.
A sharp peak at 161 ppm on the 13C NMR spectrum (see Figure S5) clearly represents the
formation of bicarbonate.54 This observation is in agreement with previously established results,
which reported that the presence of water in [bmim][OAc] leads to the formation of bicarbonate
at the expense of the imidazoliumꢀ2ꢀcarboxylate species.26
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ꢀ CONCLUSION
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In an attempt to develop ILs with high CO2 solubility and low viscosity, four novel ILs
that pair 1ꢀethylꢀ3ꢀmethylimidazolium ([emim]+) with aprotic heterocyclic anions (AHAs) were
investigated. The viscosities of [emim][AHA] ILs were all below 100 cP at room temperature,
which is a significant improvement over their tetraꢀalkylphosphonium counterparts. While these
ILs are potentially attractive in terms of industrial application due to their low viscosities, the
[emim][AHA] ILs induced more complicated chemistry than a simple anionꢀCO2 reaction. The
effective reaction enthalpies for [emim][2ꢀCNPyr] and [emim][4ꢀTriaz] are in a range
appropriate for postꢀcombustion CO2 capture, but the formation of neutral species means that
there would likely be loss of the absorbent over time in an industrial process. We propose that
the [emim][AHA] ILs act as both the base and carbene sources, and this leads to two separate
equilibrium reaction pathways. Through a combination of CO2 absorption experiment and NMR
spectroscopic investigation, this work has shown that [emim][AHA] ILs can be induced to
undergo intermolecular proton exchange in the presence of CO2 that leads to the formation of
imidazoliumꢀ2ꢀcarboxylate along with protonated anions, in addition to the simple anionꢀCO2
reaction. While the proposed reaction between the formed carbene and CO2 leads to a stronger
binding than CO2 with the anions, the CO2 absorption reaction remains reversible at 80 °C with
nitrogen purge, as confirmed by 1H NMR spectroscopy and CO2 reꢀabsorption experiments. The
presence of water decreases the CO2 solubility due to an inhibiting effect, and it also results in
the formation of bicarbonate. This inhibitory effect of water must be considered when
developing [emim][AHA] ILs for CO2 capture from humid flue gas.
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ꢀ ASSOCIATED CONTENT
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Supporting Information
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The Supporting Information file contains details on the NMR characterization for each IL
synthesized. All density, viscosity, Walden plot, CO2 uptake, and water uptake data are available
in tabular form.
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ꢀ AUTHOR INFORMATION
Corresponding Author
*Tel: (574) 631ꢀ5847. Fax: (574) 631ꢀ8366. Eꢀmail: jfb@nd.edu.
Notes
The authors declare no competing financial interest.
ꢀ AKNOWLEDGEMENT
This material is based upon work supported by the Department of Energy under Award Number
DEꢀFC26ꢀ07NT43091 (National Energy Technology Laboratory) and AR0000094 (Advanced
Research Projects Agency – Energy.
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Anion-CO2
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