Physical Properties and CO2 Reaction Pathway of 1-Ethyl-3

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Physical Properties and CO Reaction Pathway of 1-Ethyl-3Methylimidazolium 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 Downloaded from http://pubs.acs.org on November 29, 2014

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The Journal of Physical Chemistry

Physical Properties and CO2 Reaction Pathway of 1-Ethyl-3Methylimidazolium Ionic Liquids with Aprotic Heterocyclic Anions Samuel Seo, M. Aruni DeSilva and Joan F. Brennecke* 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 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.

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INTRODUCTION 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 nonflammability, low-volatility, and high thermal stability, make ILs a promising replacement for aqueous amine absorbents used in post-combustion CO2 capture from coal-fired 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 ( , enthalpy change during the binding reaction under standard conditions) with

CO2, which makes the regeneration process quite energy-intensive.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) 3 with tunability over a reasonable range of . By minimizing the number of free hydrogens

that are amenable for hydrogen-bonding network, the viscosity of these AHA ILs did not increase upon reaction with CO2,4 which is in contrast to the previously reported task-specific ILs that react chemically with CO2.5-9 While high CO2 loading capacity, adjustable , 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 alkyl-chains 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 IL-CO2 reaction is often considered diffusion-limited.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 1-ethyl-3-methylimidazolium ([emim]+) cation with AHAs of varying basicity. Shi et al. previously investigated the possibility of using [emim][AHA] ILs as electrolytes in Li-ion 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

CO2-[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 hydrogen-bond interaction with an anion.15-19 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 N-heterocyclic 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 1-butyl-3-methylimidazolium 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 studies25-28 on the binary system of dialkylimidazolium acetate and CO2 verified the formation of imidazolium-2-carboxylate. 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 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 [2-CNPyr]-, [4-Triaz]- and [3-Triaz]- anions ( = 14.8 and 13.9 for 4-triazole and 3-triazole, 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 carbene-CO2 reaction. On the other hand, [Tetz]- is a weak base ( = 8.2)31 and is not expected to prompt such intermolecular carbene formation. Previously, dialkylimidazolium ILs paired

with

weakly

basic

anions

such

as

trifluoroacetate

([TFA]-),

bis(trifluoromethylsulfonyl)imide ([Tf2N]-), tetrafluoroborate ([BF4]-) or hexafluorophosphate ([PF6]-) only exhibited physical dissolution of CO2,32-34 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 1-ethyl-3-methylimidazolium-2-carboxylate upon exposure to CO2 under mild conditions (22 °C, 1 bar) has been confirmed in the liquid phase using 1H and

13

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 1-ethyl-3-methylimidazolium 2-cyanopyrrolide, [emim][2-CNPyr], 20 g (0.105 moles) of 1-ethyl-3-methylimidazolium bromide (99%, Iolitec) was converted to 1-ethyl-3-


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methylimidazolium hydroxide by stepwise ion exchange using Amberlite IRN78 hydroxide form anion exchange resin (Sigma Aldrich). An equimolar amount of pyrrole-2-carbonitrile (99% Alfa-Aeser) was added to the 1-ethyl-3-methylimidazolium 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 1,2,4-triazole (98%, Sigma Aldrich), 1,2,3-triazole (97%, Sigma Aldrich), or tetrazole (0.45 M solution in acetonitrile, Sigma Aldrich) as the anion precursor instead of pyrrole-2-carbonitrile. 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.

Table 1. Chemical Structures and Nomenclatures of Imidazolium AHA ILs. chemical structure abbreviation

ion name

[emim]+

1-ethyl-3-methylimidazolium

[2-CNPyr]-

2-cyano-pyrrolide

N

[4-Triaz]-

4-triazolide

N

[3-Triaz]-

3-triazolide

[Tetz]-

Tetrazolide

N

N

N

C

N

N

N

N N 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 DMSO-d6 (99.9 atom% D, Sigma Aldrich). 1H NMR was obtained using a Varian INOVA 500 MHz spectrometer at room temperature. A total of 8 transients were averaged and a relaxation delay of 150 s was used for quantification. AVANCE III HD 400 MHz spectrometer.

13

13

C NMRs were performed on a Bruker

C NMR spectra were collected with inverse gated

decoupling, recycle delay of 2 seconds and 1024 transients. For the detection of bicarbonate, CO2-saturated [emim][2-CNPyr] containing 7 wt % deionized water was dissolved in deuterium oxide (99 atom% D, Sigma Aldrich), and anhydrous 1,4-dioxane (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 µg 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 U-tube 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 10-3 g cm-3. The viscosity measurements were performed with an ATS Rheosystems Viscoanalyzer equipped with a cone-and-plate spindle. The sample was kept under either a N2 or CO2 environment (neat and CO2-saturated 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/Gain-


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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%. CO2 Solubility Measurements. The amount of CO2 absorbed by each IL was obtained using a custom-built 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 vapor-liquid-equilibrium 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 LeeKesler correlation.36 The uncertainty in the CO2 uptake measurement is approximately 0.02 mole


7


CO2/IL, and the effect on CO2 absorption capacity of volume expansion of the IL upon reaction with CO2 is within this uncertainty.

RESULTS AND DISCUSSIONS 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]- > [4-Triaz]- ≥ [3-Triaz]- > [2CNPyr]-. [emim][3-Triaz] and [emim][4-Triaz] 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][2-CNPyr] is 0.901 g·cm-3 at 25 °C).10 This is presumably due to better stacking and increased π-π interaction resulting from the planar structure of both cation and anion.37 1.3

Density (g cm3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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[emim][2-CNPyr]ᵃ [emim][4-Triaz]ᵃ [emim][3-Triaz] [emim][Tetz]

1.2

1.1

1.0 0

20

40 60 Tempearture (°C)

80

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 density a b ηo α Tga ILs -3 -4 -1 -1 [g cm ] [10 g cm ·°C ] [cP] [K] [K] [emim][2-CNPyr] 1.102 [emim][4-Triaz] 1.155 [emim][3-Triaz] 1.148 1.191 [emim][Tetz] a Experimentally measured values.

-6.1 -6.1 -6.0 -6.2

0.417 0.415 0.637 0.549

438 489 457 502

212.15 205.15 201.15 194.15

conductivity σo B Tga -1 [mS cm ] [K] [K] 679 776 607 676

410 463 448 485

212.15 205.15 201.15 194.15

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][4-Triaz] 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 mol-1) is only about a quarter of [P66614]+ (479.8 g mol-1). 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 VogelFulcher-Tammann (VFT) model has been frequently employed to describe the temperature dependence of viscosity:

=


(2)

9


where is viscosity in cP, T is absolute temperature, is glass transition temperature in absolute units, and and 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. 1000 [emim][2-CNPyr]ᵃ [emim][4-Triaz]ᵃ [emim][3-Triaz] [emim][Tetz]

Viscosity (cP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100

10 0

20


80

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 Li-ion 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:

σ = σ


(3)

10

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where σ is conductivity in mS cm-1, T is absolute temperature, is glass transition temperature in absolute units, and σ and B are adjustable parameters in mS cm-1 and K, respectively. 100

Conductivity (cP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[emim][2-CNPyr]ᵃ [emim][4-Triaz]ᵃ [emim][3-Triaz] [emim][Tetz]

10

1 0

20


80

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][2-CNPyr], [emim][4-Triaz] and [emim][3-Triaz], 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 IL-CO2 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 imidazolium-2-carboxylate (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 carbene-CO2 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 imidazolium-based 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. 1.0

1.0

(b) Mole ratio (CO2/IL)

Mole ratio (CO2/IL)

(a) 0.8 0.6

[emim][2-CNPyr] [P₆₆₆₁₄][2-CNPyr] a

0.4 0.2 0.0

0.8 0.6 0.4

[emim][4-Triaz] [P₆₆₆₁₄][4-Triaz]a

0.2 0.0

0

0.2

0.4 0.6 Pressure (bar)

0.8

1

0

0.2 0.4 0.6 Pressure (bar)

0.8

1

1.0

1.0 (c)

(d) [emim][3-Triaz]

0.8

Mole ratio (CO2/IL)

Mole ratio (CO2/IL)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[P₆₆₆₁₄][3-Triaz]a

0.6 0.4 0.2

[emim][Tetz]

0.8

[P₆₆₆₁₄][Tetz]a

0.6 0.4 0.2 0.0

0.0 0

0.2

0.4

0.6

0.8

1

0

Pressure (bar)

0.2

0.4

0.6

0.8

1

Pressure (bar)

Figure 4. CO2 absorption capacity in (a) [emim][2-CNPyr], (b) [emim][4-Triaz], (c) [emim][3Triaz], and (d) [emim][Tetz] at 22 °C. aThe CO2 solubility in [P66614]+ counterparts from Seo et al.10 are also shown for comparison.

(a)

(b)

Figure 5. Concerted two parallel equilibrium reaction pathways between CO2 and [emim][2CNPyr]: (a) anion-CO2 reaction pathway and (b) cation-CO2 reaction pathway.


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In order to provide support for the proposed reaction pathway, we analyzed the CO2saturated IL solutions using 1H and 13C NMR spectroscopy. In Figure 6, the 13C spectrum of the neat [emim][2-CNPyr] 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: 1-ethyl-3metyhlimidazolium-2-carboxylate. Furthermore, the reaction of [emim][2-CNPyr] 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 Similarly, the 1H NMR of CO2-saturated [emim][2-CNPyr] 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 2-cyanopyrrole (2-CNPyrH) was observed due to fast equilibrating species between the neutral species and [2-CNPyr]-. The proportion of the imidazolium-carboxylate species estimated from the integrated intensity of the secondary resonance lines in 1H 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 cation-CO2 binding rather than the anion directly reacting with a CO2 molecule. In addition to the cationCO2 reaction, the evidence for the direct anion-CO2 reaction is provided by the peaks 10, 11, and 12 in Figure 7b, where the protons belonging to the [2-CNPyr]- moved downfield upon complexation with CO2. This trend is consistent with what was observed in [P66614][2-CNPyr] ILs, where the anion-CO2 reaction pathway is apparent.10


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10

12

7 5

(a) Pure IL 5, 13

8

4

6 13

11

2

DMSO

9

(b) IL + CO2

N

COO(on imidazolium)

C12

C9

C13

N

C11 C10

Figure 6. Comparison of the 13C NMR Spectra of [emim][2-CNPyr] (a) before and (b) after the reaction with CO2 at 22 °C. 8

7

(a) Pure IL 10 11 2

12 6

45 DMSO

(b) IL + CO2

(c) After 30 min desorption

(d) After 5 h desorption

Figure 7. 1H NMR spectra for [emim][2-CNPyr]: (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 imidazolium-2-carboxylate. 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][2-CNPyr] or [emim][4-Triaz] at 1 bar are slightly lower than [P66614][2-CNPyr] or [P66614][4-Triaz], 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 (2-CNPyrH, 4-TriazH, or 3-TriazH) 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 hydrogen-bonding between the NH group of the neutral species that are formed and the nucleophilic nitrogen on the free anion. Such hydrogen-bonding 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 one-step 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 acid-anion pairs.29


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Figure 8. Schematic illustration of the interaction between [2-CNPyr]- and 2-CNPyrH. As a result of multiple CO2 reaction pathways, with a major one being the proposed transient carbene-CO2 reaction, [emim][AHA] ILs will have different apparent 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 . NHCs are generally known as strong bases,42-44 and they are reported to have large values

with CO2.45-47 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 of approximately -69 kJ mol-1 according to differential scanning calorimetry.45 This value is substantially more exothermic than the for the

[P66614][AHA] ILs (-45, -42, and -37 kJ mol-1 for [2-CNPyr]-, [4-Triaz]-, and [3-Triaz]-, 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 anion-CO2 reaction as well as the prohibitive effect mentioned above. Qualitatively, the slopes of [emim][2CNPyr] and [emim][4-Triaz] isotherms at lower pressures are comparable to that of tetra-


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Page 18 of 27

alkylphosphonium 6-bromo-benzimidazolide ([P66614][6-BrBnIm]) IL, which has an enthalpy of reaction with CO2 of -52 kJ mol-1.10 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][2-CNPyr] 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 for 1H NMR analysis. A full set of 1H 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 1-ethyl-3-methylimidazolium-2-carboxylate) gradually diminishes upon the start of the desorption step (see Figure 7c). After 5 h, the duplicate peaks have completely disappeared (Figure 7d), and the 1H NMR spectrum is identical to that of the pure [emim][2-CNPyr] shown in Figure 7a. This verifies that both C(2)-carboxylation and the anion-CO2 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][2-CNPyr]-CO2 reactions. This agrees with a claim by Shifflet et al., where the reaction of 1,3-diethylimidazolium acetate ([eeim][OAc])23 or 1butyl-3-methylimidazolium acetate ([bmim][OAc])22 with CO2 was reversible at reduced pressure with mild heating. In the [emim][2-CNPyr]-CO2 system, the reversibility was not necessarily expected since the neutral 2-cyanopyrrole that may be formed could evaporate. However, it has a very low vapor pressure (2.7 × 10-4 bar at 25 °C and Tb = 252.3 °C) and likely has a high affinity for the IL due to strong hydrogen-bond interaction with the [2-CNPyr]- anion. As mentioned in the previous section, the complex anions formed from anion and protonated


18

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anion pairs are particularly stable, and this would effectively accommodate stabilization of the liberated anion precursor.

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 imidazolium-carboxylate 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][2-CNPyr] 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][2-CNPyr] increased nearly six-fold 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][2-CNPyr] also became turbid and thicker upon reaction with CO2; this may be attributed to the emergence of the imidazolium-carboxylate 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. ILs are generally hydroscopic, and the presence of water can significantly alter the physical or thermodynamic properties.49-51 The effect of water on the physical properties and CO2 solubility was studied using [emim][2-CNPyr]. As shown in Figure 9, the viscosity of [emim][2-CNPyr] 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][2-CNPyr] sample containing 7 wt % water was saturated with CO2, the viscosity became even higher than what was obtained from dry [emim][2-CNPyr]-CO2 complex; this trend is in line with the [P66614][AHA]+water+CO2 systems,10 where the formation of the intermolecular hydrogen-bond


19


network between the carbamate on the anion and the water molecules increased the viscosity significantly. 10000

Viscosity (cP)

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Page 20 of 27

ILa IL + CO₂ IL + H₂O IL + H₂O + CO₂

1000

100

10 0

20

40 60 Temperature (°C)

80

Figure 9. Viscosity of [emim][2-CNPyr] (afrom Seo et al.10), CO2-saturated (saturated at 22 °C) IL, IL containing 7 wt % water, and CO2-saturated IL containing 7 wt % water. As shown in Figure 10, the addition of water also influenced the CO2 absorption capacity in [emim][2-CNPyr]. 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][2-CNPyr],3 implying that the carbene formation is suppressed when water is present and that the anion-CO2 reaction is responsible for the majority of uptake. The water molecules can act as a hydrogen-bond 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 cation-CO2 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


20

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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 cation-CO2 reaction, as supported by the observation of secondary resonance lines in the 1H NMR (see Figure S4 in the Supporting Information). Nevertheless, the integration of peaks pertaining to the imidazoliumcarboxylate species significantly decreased in the presence of water. 1.0 0.8 Mole ratio (CO2/IL)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6 IL

0.4

IL + H₂O

0.2 0.0 0

0.2

0.4

0.6

0.8

1

Pressure (bar)

Figure 10: CO2 solubility in [emim][2-CNPyr] and [emim][2-CNPyr] with 7 wt % water. In addition, the tertiary system of IL-H2O-CO2 can lead to the formation of bicarbonate. To verify the formation of bicarbonate in the [emim][2-CNPyr]-H2O-CO2 system, the fully CO2saturated mixture was taken for 13C NMR with D2O as a solvent and 1,4-dioxane as a reference. A sharp peak at 161 ppm on the

13

C 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 imidazolium-2-carboxylate species.26


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CONCLUSION In an attempt to develop ILs with high CO2 solubility and low viscosity, four novel ILs that pair 1-ethyl-3-methylimidazolium ([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 tetra-alkylphosphonium 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 anion-CO2 reaction. The effective reaction enthalpies for [emim][2-CNPyr] and [emim][4-Triaz] are in a range appropriate for post-combustion 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 imidazolium-2-carboxylate along with protonated anions, in addition to the simple anion-CO2 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 re-absorption 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 Supporting Information 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.

AUTHOR INFORMATION Corresponding Author *Tel: (574) 631-5847. Fax: (574) 631-8366. E-mail: [email protected]. Notes The authors declare no competing financial interest.

AKNOWLEDGEMENT This material is based upon work supported by the Department of Energy under Award Number DE-FC26-07NT43091 (National Energy Technology Laboratory) and AR0000094 (Advanced Research Projects Agency – Energy.

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TABLE OF CONTENTS (TOC) IMAGE

+ CO2 Cation-CO2 Anion-CO2


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Physical Properties and CO2 Reaction Pathway of 1-Ethyl-3

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