Pathways of the Chemical Reaction of Carbon Dioxide with Ionic

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Pathways of the Chemical Reaction of Carbon Dioxide with Ionic Liquids and Amines in Ionic Liquid Solution Pavel V. Kortunov*, Lisa Saunders Baugh, Michael Siskin Corporate Strategic Research Laboratory ExxonMobil Research and Engineering Co. 1545 Rt. 22 East, Annandale, NJ, USA 08801

Abstract This paper focuses specifically on certain ionic liquids which are capable of acting as chemisorbents for CO2 at ambient pressure and temperature. This low pressure approach based on chemical reactivity is more effective than traditional physical absorption / solubility approaches for CO2 capture in ionic liquids for higher pressure carbon capture. We describe a class of imidazolium ionic liquids bearing a relatively acidic hydrogen atom at C2 which upon initial abstraction develops a nucleophilic carbon atom that is carboxylated by CO2. Basicity of the anion plays role in the ability to remove the acidic hydrogen to generate the nucleophilic carbon. The yield of carboxylated ionic liquid is not affected by non-aqueous co-solvents, but changes as a function of CO2 partial pressure, solution temperature, and presence of H2O in solution. CO2 chemisorption by ionic liquids is particularly efficient in the presence of a non-nucleophilic nitrogenous base that serves to promote ionic liquid carboxylation and stabilize the carboxylic acid product as a salt. Selected ionic liquids are able to stabilize the formation of amine carbamic acids in the ionic liquid solution. In this case, each amine captures up to 1 CO2 molecule which is beneficial for the overall CO2 capacity in the solution. Carboxylation of the ionic liquids themselves is lower because the basic anion of the ionic liquid also stabilizes N-carboxylated products. In-situ 13C and 1H NMR spectroscopy using a built-in micro reactor was used to provide real time insights on CO2 – ionic liquid and CO2 – amine reaction pathways and product speciation under various conditions.

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also recyclable, which can be helpful in reducing environmental concerns over their use. Due to the substantial solubility of high pressure carbon dioxide in Room Temperature Ionic Liquids (RTILs) their application for use in flue gas treatment as physical solvents was the initial focus of interest.10-14 Polymerization of the RTILs and subsequent formation of membranes for separation has also been studied.15-18 Admixing traditional amines used in H2S and CO2 capture, such as monoethanolamine (MEA) and methyldiethanolamine (MDEA), into the ionic liquids where the ionic liquid serves as an unreactive solvent for the amine has been reported.19,20 The amine acts as a chemical solvent/absorbent as the driving force for the acidbase carbon capture reaction and thereby requires low pressure. More recently Task Specific Ionic liquids (TSILs) have been synthesized where a reactive amino group was covalently tethered onto the cation or anion of the ionic liquid imparting low pressure reactivity for carbon dioxide absorption by the ionic liquid system.21-25 Numerous systems featuring other functionalized ionic liquids have been reported in the literature.1o Reactions where an ionic liquid is formed during reaction with CO2 are not covered in this paper.26 X-ray crystallography27 and NMR have been used by other researchers to identify final reaction products of the reaction of amine bases with carbon dioxide28-34 and for CO2 transfer in organic synthesis,35-39 but not for in situ, real time reaction pathway and relative kinetic monitoring of CO2 absorption and desorption as a function of time, temperature, CO2 partial pressure and pH. In addition, FTIR and Raman spectroscopy have been utilized.40-42

Introduction and Background Growth of the global economy, industry and population lead to increasing demands for energy. Traditional fossil fuels (e.g., oil, natural gas and coal) will remain the main sources of energy on the planet likely for the next several decades. Thus, the development and utilization of efficient CO2 capture and sequestration/recycling technologies (CCS/CCR) will be critical for enabling the reduction of global emissions of CO2 into the atmosphere. Both liquid-phase and solid-phase sorbents are currently under widespread investigation for CO2 capture and sequestration, and have been extensively reviewed elsewhere.1 All viable CO2 capture options that offer improved process efficiencies, such as increased capacities with lower energy requirements, scale up ease, smaller footprints and reduced costs, are of interest. In particular, liquid sorbent-based technologies are believed to offer short term advantages as drop-in replacements to current technologies vs. less mature solid absorbent technologies. The main chemical and physical solvents used in acid gas removal have been summarized by Nirula and Ashraf.1a In our previous work,2-4 we described the study of carbon capture via chemical reactions with liquid amines in aqueous, non-aqueous and mixed base systems. In situ NMR technique provided detailed real-time monitoring of the reaction pathways, relative kinetics, speciation and quantification of reactants and products. Several recent reviews nicely summarize progress in the use of ionic liquids for carbon capture.5-9 Ionic liquids are liquids that contain essentially only ions rather than uncharged molecular species. Many ionic liquids remain liquid over a wide temperature range, often more than 300ºC. They may have low melting points (as low as -96ºC has been reported), which can be attributed to large asymmetric cations that result in low lattice energies vs. traditional inorganic salts. The term is commonly used for salts whose melting point is relatively low (typically below 100°C) and which typically exhibit no measurable vapor pressure below their thermal decomposition temperature. The properties of ionic liquids result from the composite properties of the wide variety of cations and especially anions which may be present in these liquids. As a class of materials, ionic liquids are highly solvating for both organic and inorganic materials. Many of them are nonflammable, nonexplosive and have high thermal stability. They are

This paper focuses specifically on certain ionic liquids that are capable of acting as chemisorbents for CO2 in cyclic carbon capture separation processes in which the CO2 can be easily desorbed from the ionic liquid which is then recycled to the absorption step. Specific ionic liquids have been found to be effective at low pressures by a process of chemisorption in the liquid thus facilitating exceptionally high overall system CO2 uptake efficiency. This low pressure approach based on chemical reactivity is superior to traditional physical absorption / solubility approaches for CO2 capture in ionic liquids at high pressure. We have studied a class of imidazolium acetate ionic liquids which chemically can react with CO2.27,40,41,43,44,45 The cation of such ionic liquids contains a relatively acidic hydrogen atom bonded


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to a potentially nucleophilic carbon atom at the C-2 position. A very few reports involving CO2 adsorption with 1-butyl-3-methylimidazolium acetate (and related ionic liquids) propose a chemisorptive process.27,46-49 However, these reports are not consistent in their assignment of the exact chemical reaction involved, and therefore invoke complex formation in some cases and interaction of the CO2 with the ionic liquid anion in others. Rogers et al. have crystallographically identified carboxylated imidazolium cations as products of CO2 uptake with imidazolium acetate ionic liquids.27,49 In these systems, CO2 uptake is limited to a theoretical maximum of 33 mol% due to stabilizing interactions between intact acetate anions and the acetic acid generated by their protonation. Dai et al. have reported CO2 capture with imidazolium/strong non-nucleophilic base pairs similar to those described herein, achieving high (>50 mol%) uptakes via imidazolium carboxylate formation.45 In this study, only neat, equimolar mixtures of ionic liquid and base were used. Very limited 23 °C CO2 uptake kinetics, obtained by weight measurements, were reported. Inadvertent use of non-nucleophilic nitrogenous bases in the presence of ionic liquids has also been reported.5,51,52 Imidazolium carboxylates as a source of carbenes in organic synthesis are well known.50

capped 10 mm NMR tube containing the solution to be tested was placed inside the probe (Figure 1). A ® micro pH glass combination Sigma-Aldrich electrode (3.5 mm diameter), connected to an external pH meter, was positioned inside the solution but above the NMR monitoring region in order not to interfere with NMR measurements. In some experiments, a sealed glass capillary tube containing ethylene glycol was added to the solution for accurate temperature and heat release monitoring of the solution during reactions. The tube was sealed with a plastic cap fitted with two thin plastic tubes for CO2 containing gas flow in and out of the solution. The CO2 inlet tube was positioned below the solution surface. The gas outlet tube was connected to laboratory ventilation that set an ambient, e.g. 0 psig (or 1.0 bar absolute) pressure in the NMR tube. The gas flow rate was controlled by calibrated Brooks 5896TM electronic flow regulators in the range of 1.0 – 50.0 cc/min, depending on the CO2 partial pressure in the feed gas. Although the majority of experiments reported here were performed by purging pure CO2 gas (PCO2 = 1.0 bar), special mixtures of gases were also used to study an effect of CO2 partial pressure. For example, 10 mol% / 90 mol% CO2/N2 mixture purchased from Matheson Tri-Gas was used to monitor reaction at CO2 partial pressure 0.1 bar. After bringing the solution to the desired temperature, CO2 flow was initiated. The solution temperature was controlled by a pre-heated N2 purge (either house N2 or liquid N2 vapor) flowing at 1200 L/h through the probe. A thermocouple was mounted 10 mm below the sample. The temperature range for experiments, which was limited by the amine/solvent physical properties (boiling and freezing points), was narrower than the NMR instrument capabilities (-150 to +120 °C).

Experimental Design Using in situ NMR monitoring, we describe the temporal evolution of product formation for the reaction of CO2 with a series of ionic liquids in nonaqueous solution as a function of reaction temperature, ionic liquid concentration, CO2 partial pressure, and effect of the basic solvent. The quantitative results illustrate continuous product formation and decomposition under absorption and desorption conditions, respectively.

Desorption experiments were performed by changing the feed gas to N2, using the same flow rate, and increasing the solution temperature if needed. When applicable, additional 1D and 2D analysis of the solutions carried out in 5 mm NMR tubes was performed using a Bruker Avance IIITM narrow bore 400 MHz spectrometer equipped with a 5 mm QNP probe and Bruker TopSpinTM 2.1 software.

These studies employed a series of imidazolium ionic liquids with various anions such as acetate, thiocyanate, chloride, etc. (Table 1). These ionic liquids were chosen for their commercial availability, generally high solubility in organic solvents, and relatively high boiling points.

General NMR Procedure for CO2 Uptake and Desorption The experimental setup for in-situ monitoring of CO2 uptake by the amine solutions was built inside a wide bore 400 MHz Bruker Avance™ NMR spectrometer equipped with variable temperature capability, a 10 mm Broad-Band liquid NMR probe, and Bruker TopSpinTM 1.3 software. A plastic-

Spectral acquisition For 13C NMR quantitative analysis of the starting solution and final product(s) of CO2 sorption, a standard single-pulse sequence with proton decoupling (zgig pulse sequence) with repetition delay equal or longer than 60 seconds


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was used. At least 64 scans were typically taken to generate the 13C spectrum. In order to observe intermediate reaction products qualitatively on a short time scale, NOE signal enhancement (zgpg or zgpg30) was used with a shorter repetition delay between 2-5 seconds. Further calibration of 13C peak intensities was performed after every reaction on the final reaction products by comparing NMR spectra taken with and without NOE enhancement. For 1H NMR quantitative analysis of the starting solution, intermediate products, and final products of CO2 sorption, a single-pulse zg sequence was used with a repetition delay between 10 and 60 seconds. At least 8 scans were typically taken to generate a 1H spectrum. Manual tuning and matching procedures for the NMR probe were performed between experiments in order to correct impedance changes of 13C and 1H circuits during the reaction caused by the formation of new chemical compounds.

described flow-through system, especially during the desorption cycle at elevated temperatures. When desired, samples were transferred into a 5 mm NMR tube for more accurate ex-situ 1D and 2D NMR analysis on a Bruker Avance IIITM narrow bore 400 MHz spectrometer. It was observed that the =CH- proton of the C-2 carbon of the ionic liquids studied herein appears at 8-12 ppm (confirming the acidic property of this hydrogen atom). The CO2-reacted species showed a new proton resonance at 13-15 ppm (-COOH). The relative concentration of reacted and unreacted ionic liquids in the solution was verified based on quantitative analysis of these two 1H peaks.

Experimental Section RESULTS AND DISCUSSION 1. Chemisorptive Ionic Liquids Many ionic liquids have previously been reported as physical sorbents (solvents) for acid gases, operating under conditions of high pressure,10-14 but have not been previously described as being capable of reaction with CO2 under low pressures as described below.21-25 We have identified a class of ionic liquids which can react with CO2. The cation of such ionic liquids contains a relatively acidic hydrogen atom bonded to a potentially nucleophilic carbon atom. Taking the preferred imidazolium salts as an example, the sorption reaction with CO2 proceeds by a reaction involving carboxylation at the C-2 carbon of the imidazole ring, as follows:28

Spectral analysis The reaction products seen in 13C and 1H NMR spectra were identified and quantified by integration of the 13C NMR carbonyl resonance(s) at 165-155 ppm (representing chemisorbed CO2) and at approximately 125 ppm (physisorbed CO2) versus resonances representing the reacted and unreacted ionic liquid backbones to determine the CO2/ionic liquid mole ratio. 1H NMR spectra were important for monitoring ionic liquid concentration in the solvent (ionic liquid/DMSO ratio when protic solvent was used) as solvent and ionic liquids (to a lesser degree) can gradually evaporate in the

(R1, R2 = alkyl; A = conjugate base) This reaction between the CO2 and the ionic liquid proceeds easily and is quantitatively reversed upon heating to provide a convenient liquid-phase CO2 capture-regeneration process. A limited temperature differential between the sorption and desorption steps makes for an energy efficient cyclic separation process with the potential for a substantially isothermal sorption-desorption cycle.

Such ionic liquids can be highly effective in the CO2 separation process.

Reversible Non-Aqueous CO2 Chemisorption by C-Carboxylation of EMIM-OAc 1-Ethyl-3-methylimidazolium acetate (EMIMOAc) has a simple molecular structure and a low molecular weight (Mw = 170.21 g/mol), which render its beneficial for efficient CO2 capture on a weight


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basis. An approximately 2 molar solution of EMIMOAc in DMSO-d6 was prepared in a 10 mm NMR tube fitted with a plastic cap and capillary dip tube. The NMR tube was placed inside a 10 mm wide-bore Bruker Avance 400 MHz NMR spectrometer with a BBO probe. After confirmation of the ionic liquid purity with 13C/1H NMR (Fig.2, top), the solution was treated with pure CO2 at 1.0 bar at a flow rate of 5.0 cc/min (measured by a Brooks 5896 flow controller) at 30 °C. The 13C NMR spectra showed that new resonance appeared at 155.37 ppm with splitting of the imidazolium structural carbons and protons into two species – presumably reacted and unreacted molecules (Figure 2, bottom). Figure 3 shows the time evolution of the chemical reaction of CO2 with EMIM-OAc in DMSO-d6 monitored by 13C NMR. As pure CO2 at 1.0 bar was introduced into the ionic liquid solution at 30 °C, one product was initially seen in the C=O region of the 13C NMR spectrum at 155.5 ppm (Fig. 3, top). The intensity of this peak increased with time. The backbone carbons of the EMIM-OAc molecule show related splitting in the 13C NMR spectrum, as shown in Figure 3 for the C2 (top) and C4, C5 (olefinic) carbons (bottom). Our study of model amines in non-aqueous solution3 as well as with mixed bases4 showed 13C resonances of carbamate (-NH-COO-), carbamic acid (-NH-COOH), and O-carbonation (-O-COO-) species at approximately 164.5 ppm, 158.0 ppm, and 159.0 ppm, respectively. Carbon dioxide dissolved in the solution showed 13C resonances at approximately 124.5 ppm. Thus, based on the 13C NMR resonance of the equilibrium peak at 155.37 ppm, the EMIM-OAc/CO2 reaction product under the given conditions can be interpreted as a carboxylate (-COO- or -COOH). In contrast to amines, both nitrogen atoms of the EMIM-OAc are not nucleophilic and, therefore, unable to accept a carbon from CO2 molecules. Therefore, the carboxylate group is unlikely attached to a nitrogen atom. Additional 1H-13C DEPT-135 (Distortionless Enhanced Polarization Transfer) NMR experiments showed that the carbons represented by the new peaks at 142.37 and 155.37 ppm, respectively, do not have protons directly attached (Figure S1.1 for next example below). A 1H-13C single bond correlation (HSQC; Heteroatom Single Quantum Correlation, Figure S1.2) experiment showed that the new proton at 15.66 ppm is also not attached to any carbons. These observations suggest a C-carboxylated structure (C-CO2H or C-CO2-) on the C2 carbon of the EMIM-OAc after the reaction with CO2.


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Scheme 1. Carboxylation of EMIM-OAc ionic liquid in non-aqueous solution by CO2 at 1.0 bar and ambient temperature. The nucleophilic C2 carbon of the imidazole ring directly attacks a free CO2 to form a carboxylic acid while the acetate anion participates in secondary hydrogen bonding interaction with a proton. Integration of the carbon spectrum shows 38 mol% formation of the carboxylated product that corresponds to 9.8 wt% CO2 loading per 1-ethyl-3methylimidazolium acetate. Flashing of the solution with an N2 purge at 90 ºC regenerated the original 1-ethyl-3-methylimidazolium acetate. A lower stability of CO2-ionic liquid reaction products in non-aqueous solution suggests low reaction energy and therefore easier CO2 desorption. A similar procedure was carried out with an approximately 3 molar 1-ethyl-3-methylimidazolium acetate in DMSO-d6 solution. Pure CO2 was bubbled through the room temperature solution for approximately 3 hours. Comparison of the 1H NMR spectra before and after carboxylation (Figure S1.1) shows that the imidazolium C2 C-H proton (10.55 ppm) decreases in intensity and a new proton resonance attributed to the carboxylated product appears at 15.76 ppm. The 13C NMR spectra showed that new C2 (CCO2 ) and carboxylate (C-CO2-) resonances appeared at 141.55 and 155.12 ppm, respectively, with splitting of the imidazolium structural carbons and protons into unreacted and carboxylated species (Figure S1.1). Integration of the carbon spectrum shows 40 mol% formation of the carboxylated product that corresponds to 10.3 wt% CO2 loading per 1-ethyl-3methylimidazolium acetate. Additional 1H-13C single bond correlation and DEPT-135 (Figure S1.1, bottom) NMR experiments showed that the carbons represented by the new peaks at 141.55 and 155.12 ppm, respectively, do not have protons directly attached. The new proton at 15.76 ppm is also not attached to any carbons. These observations confirm a C-carboxylated structure (C-CO2H or CCO2-). Qualitatively similar results were obtained by using the neat EMIN-OAc (without DMSO-d6) at 30 0 C and CO2 at 1.0 bar (Figure S1.3). The carboxylate (C-COOH) 13C resonances appeared at 155.20 ppm and 1H NMR showed a new peak at 16.44 (C-

COOH). Approximately 36.9 mol% of the ionic liquid reacted with CO2 under these conditions. Similar CO2 loadings were detected with ionic liquids dissolved in the DMSO-d6 at various concentrations (see above). Therefore, we conclude that the CO2-ionic liquid thermodynamic equilibrium is not affected by the ionic liquid (or DMSO-d6 solvent) concentration. However, the CO2 partial pressure, temperature and type of ionic liquid anion can significantly change CO2 uptake properties of the ionic liquid. 1-butyl-3-methylimidazolium acetate (BMIMOAc) was also successfully carboxylated by the CO2 at ambient conditions (1.0 bar of CO2, 24 0C). After a CO2 purge, the C2 C-H proton (10.77 ppm) in the 1 H NMR spectrum (Figure S1.4, bottom) decreases in intensity and moves upfield, with a new CO2H resonance appearing at 15.04 ppm. New 13C NMR C2 (C-COOH) and carboxylate (C-COOH) resonances appear at 141.71 and 155.24 ppm, respectively, with splitting of the imidazolium structural carbons and protons into unreacted and carboxylated species. Integration of the carbon spectrum shows 35 mol% formation of the carboxylated product. A small peak at 159.9 ppm may represent formation of bicarbonate, possibly due to a trace amount of water in the system.

Effect of CO2 Pressure and Temperature on C-Carboxylation of Ionic Liquids We have shown that the CO2-ionic liquid thermodynamic equilibrium is not affected by the ionic liquid concentration in the solution. However, at low partial pressure of CO2, the reaction driving force is lower and that may significantly affect the equilibrium of reaction products. This effect is especially important for less stable reaction products of CO2 and ionic liquids. To study the effect of CO2 partial pressure on Ccarboxylation of ionic liquids, we prepared a 3M


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solution of EMIM-OAc in DMSO-d6. As shown in Figure 4, top, at the given ionic liquid concentration, the CO2 equilibrium loading changes over the range of 0.12-0.35 at 45 0C, 0.10-0.28 at 65 0 C, and 0.04-0.18 at 90 0C when varying the CO2 partial pressure in the gas stream from 0.01 to 1.0 bar. The CO2 partial pressure significantly changes the CO2 uptake capacity of the ionic liquid. The monitoring results shown in Figure 4 also indicate a strong temperature dependence of CO2 uptake capacity. This suggests a low stability of Ccarboxylated reaction products, which can be very beneficial for the ionic liquid regeneration / CO2 desorption. Figures S1.5 and S1.6 represent the results of TGA (Thermal Gravimetric Analysis) experiments on CO2 uptake/release by the neat EMIM-OAc over the temperature range of 30-200 0C. According to measured vapor-liquid equilibrium (VLE) curves, free CO2 can be regenerated at high pressure from the ionic liquidCO2 adducts. For example, previously isolated CO2 can be used at pressures of 1.0-2.0 bar to regenerate ionic liquid. Alternatively, thermal desorption can also be used for CO2 regeneration. It is worth noting that the chemisorptive ionic liquid is not fully saturated at 1.0 bar of CO2 at 45 0C (CO2/ionic liquid mole ratio < 1.0). Although the detected CO2 loading of 0.35 CO2 per ionic liquid molecule is approximately 20 times more than the expected amount of physisorbed CO2 at identical pressure and temperature (approximately 0.01-0.02 CO2 per ionic liquid molecule), the more effective utilization of the chemisorptive ionic liquids occurs at either lower temperatures (< 30 0C) or at higher CO2 partial pressures (> 10 bar). Chemisorptive ionic liquids can be used for CO2 capture from certain natural gas fields with high CO2 concentration.

(EMIM-SCN) (see below). More importantly, the weakly basic acetate anion of the ionic liquid can be protonated by accepting a proton of water. This neutralizes the anion and its property to promote the carboxylation reaction and its stabilization effect of the carboxylated reaction product.

Attempted Carboxylation of EMIM-OAc by CO2 in Aqueous Solution In order to probe carboxylation of ionic liquids in the presence of water, we chose EMIM-OAc as the most reactive ionic liquid within our study. Approximately 70 wt% of EMIM-OAc was dissolved in de-ionized water with a drop of DMSO-d6 as an NMR reference. In contrast to previous examples in pure non-aqueous solution, no C-carboxylation was observed after a CO2 purge for 1 hour. In water solution, the C2 C-H proton of starting EMIM-OAc appeared at 9.14 ppm versus 10.55 ppm for the ionic liquid in DMSO-d6 solution (see Figures 2, S1.7). This observation suggests a lower acidity of the C2 C-H proton, which is comparable with the acidity of unreactive 1-ethyl-3-methylimidazolium thiocyanate


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Scheme 2. Protonation of EMIM-OAc in aqueous solution inhibits the carboxylation pathway for reaction with CO2. Figure S1.7, bottom, shows the 1H and 13C spectra of the solution after CO2 treatment. The 13C peaks did not split or move. The C-carboxylated product previously observed at around 155 ppm was not detected. A new 13C resonance was observed at 159.98 ppm, possibly representing bicarbonate formation. However, the CO2 loading of the bicarbonate is significantly lower (0.13 CO2 per ionic liquid) than the C-carboxylation yield under similar conditions (~0.38 CO2 per ionic liquid). Therefore, we conclude that an aqueous solution of the ionic liquid is not desirable for CO2 capture.

proton confirmed by 1H NMR) and by the basicity of an anion. To verify the influence of the cation and anion types on the carboxylation reaction, in addition to EMIM-OAc we studied EMIM-SCN, 1methylimidazolium chloride (MIM-Cl) and 1,3bis(2,4,6-trimethylphenyl)imidazolium chloride. In contrast to EMIM-OAc, neat EMIM-SCN showed no carboxylation after CO2 treatment for 1 hour in DMSO-d6. The C2 C-H proton (8.98 ppm) and carbon (136.63 ppm) resonances in the 1H and 13 C spectra did not split or move (Figure S1.8). The 1 H NMR resonance of the C2 carbon at 8.98 ppm suggests lower acidity of the C2 C-H proton of EMIM-SCN relative to the C2 C-H proton of EMIMOAc. Thiocyanate anion appears to be not basic enough to promote the C-carboxylation reaction on the C2 carbon of the ionic liquid.

Unreactive Ionic Liquids As we described in the previous section, the carboxylation of the C2 carbon of the imidazolium ion of an ionic liquid by CO2 is potentially driven by the low proton affinity of the C2 carbon (very acidic


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Scheme 3. Less basic thiocyanate anion does not promote the carboxylation reaction of EMIMThiocyanate ionic liquid with CO2.

One very minor new CO2 resonance was observed at 124.4 ppm, possibly representing the expected property of an ionic liquid to physisorb CO2 molecules. At 1 bar of CO2 at 24 0C, the amount of physisorbed CO2 is very low – approximately 0.017 CO2 per ionic liquid molecule – and approximately 20 times less than CO2 uptake with the chemisorptive ionic liquid. Due to the higher melting point of neat 1methylimidazolium chloride (69 °C), characterization and CO2 saturation were

performed at 80 °C. In contrast to the ionic liquids with acetate anion, no carboxylation of 1methylimidazolium chloride was observed after a CO2 purge for 1 hour. The C2 C-H proton (9.26 ppm), NH proton (14.43 ppm) and carbon (135.62 ppm) resonances in the respective 1H and 13C NMR spectra did not split or move (Figure S1.9). A new CO2 resonance attributable to chemisorbed carbon dioxide products was not observed. Also, CO2 physisorption at 80 0C was below detection limit of the chosen protocol of 13C NMR.


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Scheme 4. Less basic chloride anion does not promote the carboxylation reaction of MIM-Cl ionic liquid with CO2. Similarly to MIM-Cl, no carboxylation of 1,3-bis (2,4,6-trimethylphenyl)imidazolium chloride was observed after a CO2 purge for 1 hour. The imidazolium C2 C-H proton (9.34 ppm) and carbon (139.92 ppm) resonances in the 1H and 13C spectra did not split or move (Figure S1.10, bottom). The 1H NMR resonance of the C2 carbon at 9.34 ppm suggests lower acidity of the C2 C-H proton relative to the C2 C-H proton of EMIM-OAc. One new CO2 resonance was observed at 124.65 ppm, possibly representing a higher concentration of physisorbed CO2 molecules in the ionic liquid solution (CO2:ionic liquid ratio is approximately 0.38:1). The results of this section are tabulated in Table 1.

pressure via chemisorption vs. the traditional physical absorption (solubility) approach. The cation of these ionic liquid compounds contains a relatively acidic hydrogen atom bonded to a potentially nucleophilic carbon atom. Basicity of anion also plays role in removal of the acidic hydrogen to make the C2 carbon nucleophilic. In the following section, we examine the influence of a non-nucleophilic nitrogenous base to enhance or promote the C-carboxylation reaction of the ionic liquid by also facilitating removal of the acidic hydrogen followed by stabilization of the ionic liquid chemisorption reaction products. If the base functions as a Brønsted base, transferring the proton of the C-carboxylation product should push the C-carboxylation equilibrium to the right.45

2. CO2 CHEMISORPTION BY IONIC LIQUIDS PROMOTED BY STRONG BASES We have identified a class of ionic liquids which can effectively capture CO2 at relatively low


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Scheme 5. Anticipated pathway of stabilization of the C-carboxylation product of reaction of the ionic liquid with CO2 in the presence of a strong non-nucleophilic base. structural carbons and protons into unreacted and carboxylated species (Figure 5, bottom).

Enhancement of C-carboxylation of EMIMOAc by Brønsted base TMG To verify the influence of a strong base on ionic liquid C-carboxylation, we first studied the CO2EMIM-OAc reaction in non-aqueous solution. 1,1,3,3-Tetramethylguanidine (TMG, pKa ~15.20) was chosen as a strong non-nucleophilic base. In our previous studies,2-4 we have shown that TMG does not directly react with CO2 in non-aqueous solution. However, we have shown that TMG, as a strong Brønsted base, promoted N-carboxylation, di-Ncarboxylation, and O-carboxylation of amines and alkanolamines by facilitating reaction of the hydrogen atoms on the amine and hydroxyl groups, respectively.4 A DMSO-d6 solution containing approximately 2M of EMIM-AOc and 2M TMG was treated with carbon dioxide at 30 0C for approximately 3 hours while 13C NMR spectra were taken every 85 seconds during the reaction. In TMG solution, the C2 C-H proton of the ionic liquid is effectively more acidic. Its 1H NMR resonance is shifted downfield to 11.09 ppm (10.46 ppm for neat ionic liquid, Table 1, Figure S1.3). Figure 5 shows 13C and 1H NMR spectra of the solution before and after the reaction with CO2, while Figure 6 represents the temporal evolution of the reaction monitored by 13C NMR. Before the reaction, the C2 C-H proton of the ionic and the C=N-H proton of the TMG had a chemical shift 11.09 and 5.82 ppm, respectively (Figure 5, top). After the solution was treated with CO2, these two protons are found as a single broad peak at 9.87 ppm (Figure 5, bottom). This combined peak at 9.87 ppm likely represents the C=NH2+ of the guanidinium salt. A small peak at 10.50 ppm represents the C2 C-H proton of the unreacted ionic liquid. In the 13C NMR spectrum, new C2 (C-CO2-) and carboxylate (C-CO2-) resonances appear at 142.19 and 155.23 ppm, respectively, with splitting of the imidazolium


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Scheme 6. Enhancement/promotion of C-carboxylation of EMIM-OAc by strong base TMG. Basic TMG transfers an acidic C2 C-H proton, which makes the C2 carbon nucleophilic towards CO2. 45 0C, 70.1 mol% of BMIM-OAc was carboxylated. Lower C-carboxylation yield of BMIM-OAc (70.1 mol%) relative to EMIM-OAc (87 and 91 mol%) can be attributed to higher temperature of reaction. It is also worth noting that the ionic liquid BMIM-OAc without the strong base TMG formed only 35 mol% of C-carboxylated products (Figure S1.3). The strong base TMG clearly enhances the yield of C-carboxylation chemical reaction at the C2 carbon of the ionic liquid. To evaluate an effect of other non-nucleophilic bases with lower basicity, we selected bis[2-(N,N-dimethylamino)ethyl] ether (DMAEE) with pKa ~ 9.12. This bifunctional tertiary amine is unable to participate in carbamate/carbamic acid formation with CO2 in non-aqueous solution,3,4 but basic enough to accept a proton from carbonic acid in aqueous solution and form a bicarbonate. 2 A solution containing 50 wt% of an approximately 2:1 molar (1:1 normal) mixture of EMIM-OAc and DMAEE in DMSO-d6 solution was treated with CO2 at 1.0 bar and 24 0C. Carboxylation of EMIM-OAc was confirmed by 13C and 1H NMR (Figure S2.2) with 36.9 mol% formation of the carboxylated product. Since this yield is very similar to that obtained in EMIM-OAc ionic liquid solution in the absence of the amine (Figure 2), we conclude that the tertiary amine DMAEE does not promote nor enhance the Ccarboxylation reaction. The tertiary amine DMAEE is either not basic enough to promote the reaction (pKa 9.12 vs. 15.20 for TMG) or is too sterically hindered to stabilize the C-carboxylated products. It is also possible that polarity differences between DMAEE and TMG may feature in the lack of reactivity observed with DMAEE as compared to TMG.26

The evolution of the chemical reaction of CO2 with EMIM-OAc monitored by 13C NMR (Figure 6) shows that the C=N carbon resonance of TMG gradually shifts upfield from 166.85 ppm before CO2 purge to 162.94 ppm, confirming participation of TMG in the chemical reaction. Integration of the carbon spectrum shows 87.0 mol% formation of the carboxylated product. Additional DEPT-135 NMR experiments showed that the carbons represented by the new peaks at 142.19 and 155.23 ppm do not have protons directly attached. These observations suggest a C-carboxylated structure (C-CO2H or CCO2-) on the ionic liquid, which is stabilized by formation of protonated TMG (TMG-H+). Flashing the solution with an N2 purge at room temperature (~24 ºC) showed significant regeneration of the original EMIM-OAc and CO2 desorption. Integration of the carbon spectrum shows 87 mol% of EMIM-OAc formed the carboxylated product with CO2 in the presence of a strong base TMG (Figure 5). At similar conditions (1.0 bar of CO2, 30 0C) and without strong base in the solution, only 38 mol% of EMIM-OAC was C-carboxylated by CO2 (Figure 2). This result confirms that the strong Brønsted base enhances the yield of the Ccarboxylation reaction of the ionic liquid, presumably by acting as a proton acceptor for the acidic C2 C-H hydrogen. Similar results were obtained for a solution containing 50 wt% of an approximately 1:1 molar mixture of EMIM-OAc and TMG in DMSO-d6. Figure S2.1 shows that 91 mol% of EMIM-OAc formed the carboxylated product with CO2 in the presence of TMG. Structurally and chemically similar 1-butyl-3methylimidazolium acetate showed similar results. When a solution containing 87 wt% of an approximately 1:1 molar mixture of and TMG in DMSO-d6 was treated with pure CO2 at 1.0 bar and


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Scheme 7. Lack of enhancement of EMIM-OAc C-carboxylation by a tertiary amine DMAEE. EMIM-OAc is carboxylated in the presence of acetate anion while DMAEE remains unreactive. proton of the ionic liquid (9.22 ppm) and the N-H proton of TMG (5.25 ppm) after reaction with CO2 presented a single peak at 8.07 ppm in the 1H NMR spectrum. New C2 (C-CO2-) and carboxylate (CCO2-) resonances appeared at 141.43 and 155.10 ppm, respectively, in the 13C NMR spectrum, with splitting of the imidazolium structural carbons and protons into unreacted and carboxylated species. Simultaneously, the C=N carbon resonance of TMG shifts to 161.89 ppm (from 166.65) confirming participation of TMG in the chemical reaction. In contrast to the ionic liquid solution without added TMG (Figure S1.7), integration of the carbon spectrum shows 79.5 mol% formation of the carboxylated product. Additional DEPT-135 NMR experiments show that the carbons represented by the new peaks at 141.43 and 155.10 ppm do not have protons directly attached. These observations suggest a C-carboxylated structure (C-CO2H or CCO2-) on the ionic liquid, stabilized by protonated TMG (TMG-H+).

Promotion of C-carboxylation of EMIM-SCN by strong Brønsted base EMIM-OAc reacts with CO2 and forms carboxylated product in the absence of any strong base in the solution. However, strong Brønsted base enhances C-carboxylation reaction by transferring the acidic C2 C-H proton and making the C2 carbon nucleophilic enough to react with the carbon of CO2. In contrast to EMIM-OAc with weakly basic acetate anion, EMIM-SCN with thiocyanate anion did not form carboxylated species in non-aqueous solution (Figure S1.7) presumably due to the much lower basicity of thiocyanate anion (Table 1). The presence of a strong Brønsted base in the solution promotes a C-carboxylation reaction. To verify this hypothesis, we prepared a DMSOd6 solution containing 50 wt% of an approximately 1:1 molar mixture of EMIM-SCN and TMG. Carbon dioxide was bubbled through the room temperature solution for approximately 13 hours. Both 13C and 1H NMR confirmed the reaction of CO2 with EMIMSCN in the presence of TMG (Figure S2.3). The C-H


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Scheme 8. Promotion of EMIM-SCN carboxylation by strong base TMG. Basic TMG transfers an acidic C2 C-H proton, which makes the C2 carbon nucleophilic towards reaction with CO2.

Flashing the solution with a N2 purge at room temperature (~24 ºC) for 7 hours showed almost complete regeneration of the original EMIM-SCN by CO2 desorption. 1,3-bis(2,4,6-trimethylphenyl)imidazolium chloride also did not react with CO2 without a strong base (Figure S1.9). As confirmed by 13C NMR, CO2 was physisorbed/dissolved in the ionic liquid solution (Figure S2.4, top). After treatment with CO2, TMG was added to the solution (2:1 TMG:ionic liquid molar ratio). The resulting mixture was analyzed after about 20 minutes. The C-H proton of the ionic liquid (9.33 ppm) did not change its

position in the 1H NMR spectrum (Figure S2.4, bottom). New C2 (C-CO2-) and carboxylate (C-CO2-) resonances appeared at 139.34 and 154.65 ppm, respectively, in the 13C NMR spectrum, with splitting of the imidazolium structural carbons into unreacted and carboxylated species (Figure 2.5, bottom). This result confirms the chemical reaction of the physisorbed CO2 with the ionic liquid in the presence of the strong base TMG as a promoter. Simultaneously, the C=N carbon resonance of TMG splits into two peaks at 165.65 ppm and 164.62 ppm; the latter resonance confirms participation of TMG in the chemical reaction.

In contrast to the ionic liquid solution without added TMG, integration of the carbon spectrum shows approximately 45.4 mol % formation of carboxylated product. This confirms a promotion of the chemical reaction between CO2 and 1,3bis(2,4,6-trimethylphenyl)imidazolium chloride ionic liquid by the strong base TMG. Further treatment with CO2 led to precipitation of reaction products (presumably due to further reaction of unreacted ionic liquid with CO2). Analysis of these reaction products was not conducted. The results of this section are tabulated in Table 1.

Effect of Concentration of Strong Base on Ccarboxylation of Ionic Liquid As shown above, presence of one molar equivalent of a strong base in the solution either enhances carboxylation of reactive ionic liquids (such as EMIM-OAc) or promotes carboxylation of unreactive ionic liquids (such as EMIM-SCN). To study the effect of strong base concentration on stabilization of C-carboxylated reaction products between CO2 and ionic liquids, we prepared the following solutions containing EMIM-OAc and TMG in DMSO-d6 at two concentrations and molar ratios:


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strong base at elevated temperature and lower CO2 partial pressure. An approximately 3M solution of EMIM-OAc (51 wt% ) and 3M TMG (34 wt%) in DMSO-d6 was heated to 45°C and then treated with a continuous flow of 1 vol% CO2 in N2 at 1 atm as described in the Experimental Procedure. The solution was next treated with 10 vol% CO2 in N2 at 1 atm and finally 100 vol% CO2 at 1 atm. The equilibrium loading of CO2 under these conditions was 33.7, 60.7, and 70.2 mole %, respectively, and represents the EMIMOAc/TMG/CO2 vapor-liquid equilibrium (VLE) at 10 mbar, 100 mbar and 1 bar of CO2 at 45°C. The same procedure was carried out with fresh 85 wt% solutions of 3M EMIM-OAc / 3M TMG (1:1 molar mixture) at 65 °C and 90 °C. The monitoring results shown in Figure 8 indicate a strong temperature dependence of CO2 uptake capacity. This result confirms the low stability of the reaction product, which is beneficial for achieving lower regeneration energy. According to measured VLE, CO2 can be regenerated at high pressure. For example, previously isolated CO2 can be used at pressures of from 1.0-2.0 bar to regenerate an ionic liquid. Alternatively, thermal desorption can also be used for CO2 regeneration. The monitoring results shown in Figure 8 also indicate a significantly higher CO2 uptake capacity for ionic liquids promoted by strong bases such as TMG as compared to the neat ionic liquids (Figure 4). The CO2 uptake capacity for promoted ionic liquids is comparable to that for alkanolamines, and the strong temperature dependence of the vaporliquid equilibrium for the given system confirms the potential for application of neat or promoted ionic liquids for effective CO2 capture from various sources, such as furnace flue and natural gas.

85 wt% solution of 3 molar EMIM-OAc and 3 molar TMG (1:1 molar mixture) • 82 wt% solution of 2 molar EMIM-OAc and 4 molar TMG (1:2 molar mixture) It was not possible to prepare a more concentrated solution due to the high molecular weight of the ionic liquid and TMG. The solutions were heated to 45°C and then treated with a continuous flow of 1 vol% CO2 in N2 at 1 atm that corresponds to a partial pressure of CO2 of 0.01 bar. After 13C and 1H NMR spectra were collected, the solutions were next treated with 10 vol% CO2 in N2 at 1 atm (0.1 bar of CO2), and finally 100 vol% CO2 at 1 atm (1.0 bar of CO2). The equilibrium loading of CO2 under these conditions represents an EMIMOAc/CO2 vapor-liquid equilibrium in the presence of TMG at 10 mbar, 100 mbar and 1 bar of CO2 at 45°C. Figure 7 shows the loading in the form of moles of CO2 per mole of EMIM-OAc. Over the CO2 partial pressure range 0.01 – 1.0 bar, the mole percent of C-carboxylated products is higher for the solution more concentrated in TMG. Thus, at 1.0 bar of CO2, approximately 90 mol% of the ionic liquid was C-carboxylated in the solution with a 1:2 molar ratio of the ionic liquid to the strong base. Under similar conditions, only 70 mol% of carboxylated products were detected in the solution containing a 1:1 molar ratio of the ionic liquid to TMG (Figure 7). However, the CO2 loading per the entire solution is higher for the solution with the 1:1 molar ratio of ionic liquid and base, due to a higher concentration of the ionic liquid in the solution (Figure S2.5). The same tendency was observed at lower partial pressure of CO2 – the second solution showed higher CO2 capacity per each ionic liquid molecule, but lower total solution capacity due to lower ionic liquid concentration. At constant concentration of the ionic liquid, a higher concentration of the strong base is favorable to promote and stabilize the C-carboxylation reaction between the ionic liquid and CO2. •

3. REACTIVE IONIC LIQUIDS AS SOLVENTS FOR AMINES Ionic liquids can also be considered as nonaqueous solvents for amines.19,20 As we showed in the previous work3, selected ionic liquids are able to stabilize the formation of the carbamic acids on amines over a broad range of amine concentration in the ionic liquid. In this case, each amine captures up to 1 CO2 molecule which is beneficial for the overall CO2 capacity in the solution. Additionally, the ionic liquid itself is able to capture a significant amount of CO2 via C-carboxylation providing an overall increase in CO2 capacity for the solution. In the following section, we give several examples of such increases in CO2 capacity for solutions comprising amines dissolved in the ionic liquids. Enhanced CO2 capacity of amines dissolved in EMIM-OAc

Effect of CO2 Pressure and Temperature on C-carboxylation of Ionic Liquid Promoted by the Strong Base We have already shown that the yield of Ccarboxylation of ionic liquid (without a strong base) is approximately 30-40 mol% at ambient temperature and 1.0 bar of CO2 and further decreases at higher solution temperature and lower CO2 partial pressure (Figure 4). The presence of a strong Brønsted base helps to increase the Ccarboxylation yield to 80-90 mol% at ambient temperature and pressure. We also studied carboxylation of ionic liquids in the presence of a


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EEA/EMIM-OAc. Approximately 15 wt% of 2ethoxyethylamine (EEA, pKa 8.92) was dissolved in EMIM-OAc with a drop of DMSO-d6 (as an NMR reference) in a 5 mm NMR tube. After confirmation of the amine and ionic liquid purity and absence of side reactions with 13C/1H NMR (Figure 9, top), the solution was treated with pure CO2 at 1.0 bar at a flow rate of 5.0 cc/min at 24 °C. The 13C NMR spectra showed that a new resonance appeared at 155.27 ppm with splitting of the imidazolium structural carbons and protons into two species – presumably reacted and unreacted ionic liquid molecules (Figure 9, bottom). Based on our study of CO2 reaction with ionic liquid described above, we conclude the peak at 155.27 ppm reflects C-carboxylation of the ionic liquid by the CO2. Approximately 18 mol% of the ionic liquid molecules were C-carboxylated. Without the amine in the solution, the C-carboxylation yield of the same ionic liquid was approximately 38 mol% (Figure 2). More importantly, amines also reacted with CO2 in high yield. CO2-amine reaction products were detected by 13C NMR as new peaks at 160.22 and 158.05 ppm (Figure 9, bottom). The first peak (160.22 ppm) indicates carbamic acid formation on EEA (possibly, stabilized by the anion of the ionic liquid via H-bonding) and overlaps with a broad undefined peak at 160.0 ppm.


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Scheme 9. Proposed scheme of the reaction of CO2 with amine in ionic liquid solution. Stabilization of carbamic acid by ionic liquid anion via H-bonding interaction.

We interpret the second peak (158.05 ppm) as formation of a dicarbamic acid on EEA, which is stabilized by either one or two molecules of the ionic liquid. This product is shown below in its most extreme dianionic form to conveniently illustrate all of the potential reactions. However, it

is likely that the actual structure lies somewhere along the continuum of neutral dicarbamic acid monoanionic product - dianionic product, depending on the reaction conditions, ratios of reagents present, and extent of loading.


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Scheme 10. Proposed di-N-carboxylation of primary amine in ionic liquid solution. Dicarbamic acid is stabilized by ionic liquid anion via H-bonding interaction. The 13C resonances of the EEA backbone carbons (Figure 9) are consistent with a split of the EEA structure into separate species, assigned as mono- and dicarbamic acid structures.3 The peak at 68.13 ppm shows the predicted -O-CH2CH2-N(COOH)2 carbon integral value, while the area of the peak at 69.60 ppm matches that expected for O-CH2CH2-NH-COOH amine molecules bearing a monocarbamic acid. The CO2 loading per amine in carbamic acid and dicarbamic acid form is at least 1.08 (excluding the undefined peak at 160.0 ppm) or 1.38 (including the undefined peak at 160.0 ppm) CO2 per EEA molecule. Since the ionic liquid molecules stabilize the carbamic acid formation with the CO2 and amine, they lose the unique property to chemisorb the CO2 at the C2 carbon. The basic acetate anion can accept a proton from the carbamic acid (see scheme 10). In this case, the C2 carbon of the ionic liquid becomes less nucleophilic to CO2. Such a pathway can explain the lower C-carboxylation reaction yield on the ionic liquid in the solution with the amine versus neat ionic liquid.

with an amine, a solution containing approximately 20 wt% of EEA dissolved in EMIM-SCN was prepared and treated with CO2 at 24 0C and 1.0 bar of CO2 for 9 hours. Figure S3.7, bottom, shows the 13C and 1H NMR spectra of the CO2 saturated solution. The peak at 160.49 ppm represents carbamic acid formation on EEA (which can be in equilibrium with the carbamate formed between two EEA molecules). The CO2 loading, defined by the integration of the 160.49 ppm peak versus the EEA structural peaks at 70-65 ppm (-CH2OCH2-), is 0.70 CO2 per EEA. Such low CO2 loading suggests that either 30 mol% of amines did not react with CO2 or that they assume the role of carbamate cations. The carbon dioxide loading of EEA dissolved in DMSO-d6 (without EMIM-SCN) at similar concentration was very similar - 0.75 CO2 per EEA3 (Figure 10, S3.2). This confirms that EMIM-SCN ionic liquid does not provide any special stabilization effect for the carbamic acid and, rather, acts as a polar nonaqueous solvent similar to DMSO-d6 or Nmethylpyrrolidinone (NMP). In contrast to the relatively basic acetate anion, which is believed to accept a proton and stabilize the carbamic acid, thiocyanate anion is not basic enough.

Ionic liquids as inert solvent for amines In the previous section we also showed that the ability of the ionic liquid anion to hydrogen bond is important for C-carboxylation of ionic liquid but also play as critical role for stabilization of Ncarboxylation reaction product on the amine. In contrast to EMIM-OAc with basic acetate anion, 1ethyl-3-methylimidazolium thiocyanate does not react with CO2 in the absence of a strong proton acceptor such as TMG. This behavior was explained by weaker H-bonding interactions of its anion, which in not expected to influence CO2-amine reaction. In order to probe the ability of EMIM-SCN to promote and stabilize carbamic acid formation


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Scheme 11. Proposed scheme of the reaction of CO2 with amine in a solution containing unreactive ionic liquid such as EMIM-SCN. The amine reacts with CO2 while ionic liquid plays a role of a polar nonaqueous solvent such as DMSO or NMP.

It’s worth noting that EMIM-SCN in amine solution does not react with CO2 (no resonance at ~155 ppm). This observation is in agreement with previous experiments.

Ionic liquids deactivating CO2 reaction with amines In previous sections we showed how ionic liquids can enhance the CO2 uptake capacity of the amine or act as a regular non-aqueous solvent. In contrast to those examples, 1-methylimidazolium chloride was found to be a deactivating solvent for an amine, preventing its reaction with CO2. The structure of EEA changed in 1methylimidazolium chloride solution as shown by 13 C and 1H NMR spectra for 15 wt% EEA (Figure S3.8, top). Two backbone carbons – the CH2-O-CH2- of EEA, which were typically observed at ~72.5 and ~65.5 ppm in other solvents (see Figures 9, top or S3.1, top), appeared as an overlapping resonance at ~65.8 ppm in 1-methylimidazolium chloride solution (Figure S3.8). The backbone carbons of 1methylimidazolium chloride also split, confirming reaction with an amine before CO2 addition (compare figures S3.8 and S1.8). The basic amino group of EEA is likely exchanging a proton with the unalkylated nitrogen atom of the 1methylimidazolium chloride which results in no effective reaction with CO2.


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Scheme 12. Proposed pathway of amine deactivation by the ionic liquids with unalkylated nitrogen atom. Amine becomes unreactive towards CO2 after accepting a proton from the ionic liquid containing a very basic anion. With an excess of the 1-methylimidazolium chloride molecules in the solution, the majority of the amino groups of EEA are protonated as very stable and acidic hydrochloride salts. As result, EEA is unable to participate in an acid-base reaction with CO2. In order to verify this hypothesis, the solution was treated with pure CO2. Figure S3.8, bottom, confirms that the 13C and 1H NMR spectra of the EEA/1-methylimidazolium chloride solution did not change after CO2 addition. EEA molecules did not react with CO2. Thus, the utilization of monoalkylated imidazolium ionic liquids such as 1methylimidazolium chloride as amine solvents is not effective because of their high acidity. The results of this section are tabulated in Table 2.

the pKa of the anion of the ionic liquid is critical in determining the ionic liquid’s capability to react with CO2. Basic ionic liquid anions, such as acetate enhance chemisorption of CO2 while less basic anions such as thiocyanate and more basic chloride are ineffective. The C-carboxylation reaction between CO2 and the ionic liquid can be enhanced or promoted by the presence of a strong non-nucleophilic nitrogenous base having a pKa (as measured or predicted at 25°C in aqueous solution or as measured in other solvent and converted to an aqueous value) of sufficiently high value. Based on the results described above, it is reasonable to suggest a lower pKa boundary of about 12 (above that of simple aliphatic an aromatic amines such as DMAEE, but below that of TMG and similar heterocyclic/conjugated bases). The base should be strong enough to influence the C-carboxylation product equilibrium effectively, but not so strong that it stabilizes the carboxylated reaction product to the point that the effect becomes irreversible and desorption of the CO2 from the carboxylated reaction product becomes difficult or infeasible (e.g., by an inconveniently high temperature requirement). Additionally, the protonated form of the base must remain quantitatively available to the ionic liquid for deprotonation/regeneration during the CO2 desorption step of the cycle. The base must also lack the propensity to act as a competing nucleophile towards CO2 under the conditions of the sorption process. The ionic liquids described here function to sorb the CO2 by chemisorption. They have not shown themselves to be effective for non-reactive physisorption at low pressures, typically below 1 to 2 bar absolute; although both chemisorption and smaller amounts of physisorption may take place under such conditions, one or the other may be the predominant mode of CO2 uptake depending upon the sorbent medium and operating conditions. These low pressures are typical of those

DISCUSSION We have identified a class of ionic liquids which can effectively capture CO2 at relatively low pressure via chemisorption vs. the traditional physical absorption (solubility) approach expected of ionic liquids. The cation of these ionic liquid compounds contains a relatively acidic hydrogen atom bonded to a potentially nucleophilic carbon atom, as in cations having a C-H bond present as part of a conjugated -NC(H)N- structure or an -NC(H)Sstructure. The carbon referred to as nucleophilic is qualified as “potentially nucleophilic” since the carbon itself does not become a nucleophile until deprotonation occurs by removal of the acidic hydrogen. Thus, the more effective cations are those in which the potentially nucleophilic carbon bears a hydrogen which is sufficiently acidic on a relative basis to be susceptible to deprotonation and subsequent reaction with the electrophilic carbon of CO2. Imidazolium compounds provide optimal CO2 sorption, because the salts derived from the imidazolium cation are almost planar which makes them true amidines, particularly those derived from the 1,3-di(lower alkyl) imidazolium cations. In the absence of a non-nucleophilic nitrogenous Brønsted base promoter, it appears that


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encountered in treating flue gases from hydrocarbon combustion processes. The chemisorptive approach described here thus commends itself well for post combustion flue gas CO2 capture, when CO2 partial pressures are in the range of about 0.03 to 0.15 bar. The ionic liquid and the optional nonnucleophilic nitrogenous base may be used alone or taken up in an aprotic polar non-aqueous solvent. The use of the additional solvent can help to achieve a liquid of appropriate viscosity, since it may diminish the sorption capacity of the system. When the ionic liquids are used as promoter solvents for an amine sorbent, the intermediate carbamic acid species may be stabilized relative to the ammonium carbamate species, resulting in increased molar loading capacity of the sorbent amine above the theoretical maximum of 0.5:1 for carbamate formation. Carbon dioxide:amine ratios approaching the theoretical carbamic acid ratio of .

R2

1:1 are potentially achievable, as was seen for amines in other highly polar, aprotic solvents.3 In the case of primary amines, the highly polar and stabilizing effect of the ionic liquid solvent may lead to CO2 sorption exceeding 100 mole percent (one CO2 per amine group) as a result of double carboxylation onto the amino nitrogen(s); in addition, the ionic liquid has the potential to act as a physical absorbent for the CO2 under appropriate conditions, leading to the sorption of additional amounts of CO2.

Conclusions Ionic liquids containing a cation with a potentially nucleophilic carbon atom bearing a relatively acidic hydrogen atom, here exemplified by the conjugated imidazol(idin)ium -NC(H)Nstructure e.g., as the acetate, are capable of acting as chemical sorbents for CO2

R2

R2

N

CO2

N

O

N

O

N

OH

N

O

H N R1

A

R1 A (R1, R2 = alkyl; A = conjugate base) R1

A

The ionic liquid may be used on its own, mixed with a solvent, preferably an aprotic, polar, nonaqueous solvent (here exemplified by dimethyl sulfoxide) to chemically absorb CO2 vs. traditional high pressure physical absorption (solubility) for CCS. In conjunction with a non-nucleophilic nitrogenous base compound having a sufficiently

R2

high pKa (e.g., tetramethylguanidine) to promote ionization of the ionic liquid at C-2 and carboxylation of the ionic liquid which is then stabilized by the high pKa base as stabilize the carboxylated products.

R2

N

O

N

CO2

+ Base-H

H Base

N R1

H

A

N

O

R1

A


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We have illustrated that some ionic liquids are capable of promoting high levels of CO2 chemisorption with suitable amines. In addition, the desorption of the CO2 from the sorbent solution may take place readily at low temperatures, providing for a low energy CO2 capture process with its attendant economic advantages. The sorption may be carried out at low temperatures, e.g., ambient to 30°C, but if the entering gas stream is at a higher temperature, as with flue gas, the sorption may be carried out at relatively higher temperatures (ca. 90-120 ºC). In contrast to other chemisorptive processes, such potassium carbonate absorption (~175 ºC), high temperatures are unnecessary, and the use of solid sorbents mixed with the liquids is not required. The reaction pathway understanding presented herein of these ionic liquid/amine systems will help facilitate development of more efficient liquid phase CO2 sorbents capable of operating under a wide range of process conditions.

Experimental Section Ionic liquids, amines, and solutions. 1-Ethyl-3methylimidazolium acetate ([143314-17-4], SigmaAldrich), 1-butyl-3-methylimidazolium acetate ([284049-75-8], Fluka), 1-ethyl-3methylimidazolium thiocyanate ([331717-63-6], Fluka), 1-methylimidazolium chloride ([35487-17-3], Aldrich/BASF), and 1,3-bis(2,4,6-trimethylphenyl)imidazolium chloride ([141556-45-8], Aldrich) were used as received. Amine bases were obtained as described in ref. 2-4.

SUPPLEMENTARY INFORMATION 1

H and 13C NMR spectra of unreacted and reacted ionic liquids discussed in the paper as well as CO2 reaction products with primary amines EEA and APN in DMSO, toluene, NMP, sulfolane and EMIMOAc over the range of concentrations are included in the supplementary material. This information is available free of charge via the Internet at http://pubs.acs.org/

AUTHOR INFORMATION Corresponding Author [email protected]


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References 1.

2.

Systems: New Insights on Carbon Capture Reaction Pathways” EnergyFuels, submitted for publication.

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Kerle, D.; Ludwig, R.; Geiger, A.; Paschek, D. “Temperature Dependence of the Solubility of Carbon Dioxide in Imidazolium-Based Ionic Liquids” J. Phys. Chem. B 2009, 113, 12727-12735.

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Sharma, P.; Park, S.D.; Baek, I.H.; Park, K.T.; Yoon, Y.I.; Jeong, S.K. “Effect of Anions on Adsorption Capacity of Carbon Dioxide in Carboxylic Acid Functionalized Ionic Liquids” Fuel Processing Technology, 2012, 100, 55-62.

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Bara, J. E.; Lessmann, S.; Gabriel, C. J.; Hatakeyama, E. S.; Noble, R. D.; Gin, D. L. “Synthesis and performance of polymerizable room-temperature ionic liquids as gas separation membranes.” Ind. Eng. Chem. Res. 2007, 46, 5397-5404.

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Bhavsar, R.S.; Kumbharkar, S.C.; Kharul, U.K. “Polymeric ionic liquids (PILs): Effect of anion variation on their CO2 absorption” J. Membrane Sci. 2012, 389, 305-315.

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Xiong, D.; Wang, H.; Li, Z.; Wang, J.”Recovery of Ionic Liquids with Aqueous Two-Phase System Induced by Carbon Dioxide” ChemSusChem. 2012, 5, 2255-2261. Bates, E. D.; Mayton, R. D.; Ntai, I.; Davis, J. H. “CO2 capture by a task-specific ionic liquid.” J. Am. Chem. Soc. 2002, 124, 926-927.

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Gurkan, B.E.; de la Fuente, J.C.; Mindrup, E.; Ficke, L.E.; Goodrich, B.F.; Price, E.A.; Schneider, W.F.; Brennecke, J.F. "Equimolar CO2 Absorption by Anion-functionalized Ionic Liquids" J. Am. Chem. Soc. 2010, 132, 2116-2117.

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Brennecke, J.F.; Gurkan, B.E. "Ionic Liquids for CO2 Capture and Emission Reduction" J. Phys. Chem. Lett. 2010, 3459-3464.

Gurkan, B.E.; Goodrich, B.F.; Mindrup, E.M.; Ficke, L.E.; Massel, M.; Seo, S.; Senftle, T.P.; Wu, H.; Glaser, M.F.; Shah, J.K.; Maginn, E.J.; Brennecke, J.F Schneider, W.F. "Molecular Design of High Capacity, Low Viscosity, Chemically Tunable Ionic Liquids for CO2 Capture" J. Phys. Chem. Lett. 2010, 1, 3494-3499.

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Goodrich, B.F.; de la Fuente, J.C.; Hurkan, B.E.; Zadigian, D.J.; Price, E.A.;Huang, Y.H.; Brennecke, J.F. "Experimental Measurements of AmineFunctionalized Anion-Tethered Ionic Liquids with Carbon Dioxide" Ind. Eng. Chem. Res. 2011, 50, 111118. Mathias, P.M.; Afshar, K.; Zheng, F.; Bearden, M.D.; Freeman, C.J.; Andrea, T.; Koech, P.K.; Kutnyakov, I.; Zwoster, A.; Smith, A.R.; Jessop, P.G.; Nik, O.S.; Heldebrant, D.J. “Improving the regeneration of CO2-binding organic liquids with a polarity change” Energy & Environ. Sci., 2013, 6, 2233-2242, and references cited therein. Gurau, G.; Rodriguez, H.; Kelley, S.P.; Janiczek, P.; Kalb, R.S.; Rogers, R.D. “Demonstration of Chemisorption of Carbon Dioxide in 1,3Dialkylimidazolium Acetate Ionic Liquids” Angew. Chem. Int. Ed. 2011, 50, 12024-12026.

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Ahmady, A.; Hashim, A.; Aroua, M.K.; “Kinetics of Carbon Dioxide Absorption into aqueous MDEA + [bmim][BF4] solutions from 303 to 333 K” Chem. Eng. J. 2012, 200-202, 317-328.

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Mani, F.; Peruzzini, M.; Stoppioni, P. “CO2 absorption by aqueous NH3 solutions: speciation of ammonium carbamate, bicarbonate and carbonate by a 13C NMR study.” Green. Chem. 2006, 8, 9951000.

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Pereira, F.S.; Ribeiro de Azevedo, E.; da Silva, E.F.; Bonagamba, T.J.; da Silva, A., Deuber L.; Magalhaes, A.; Job, A.E.; Perez G.; Eduardo R. “Study of the carbon dioxide chemical fixationactivation by guanidines.” Tetrahedron 2008, 64, 10097-10106.

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Hazelbaker, E.D.; Budhathoki, S.; Katihar, A.; Shah, J.K.; Maginn, E.J.; Vasenkov, S. “ Combined Application of High Field Diffusion NMR and Molecular dynamics Simulations to Study Dynamics in a Mixture of Carbon Dioxide and Imidazolium-Based Ionic Liquid” J. Phys. Chem. B 2012, 116, 9141-9151.

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Carvalho, P. J.; Alvarez, V. H.; Schröder, B; Gil, A. M.; Marrucho, I. M.; Aznar, M.; Santos, L. M. N. B. F.; Coutinho, J. A. P. “Specific Solvation Interactions of CO2 on Acetate and Trifluoroacetate Imidazolium Based Ionic Liquids at High Pressures” J. Phys. Chem. B 2009, 113, 6803-6812.

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superbase/polyethylene glycol and its subsequent conversion” Energy Environ. Sci. 2011, 4, 3971-3975. 34.

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Wang, C.M.; Luo, X.Y.; Luo, H.M.; Jiang, D.E.; Li, H.R.; Dai, S. “Tuning the basicity of Ionic Liquids for Equimolar CO2 Capture” Angew. Chem. Int. Ed. 2011, 50, 4918-4922. Kayaki, Y.; Suzuki, T.; Ikariya, T. “Utilization of N,N-Dialkylcarbamic Acid Derived from Secondary Amines and Supercritical Carbon Dioxide: Stereoselective Synthesis of Z Alkenyl Carbamates with a CO2-Soluble Ruthenium-P(OC2H5)3 Catalyst” Chem. Asian J. 2008, 3, 1865-1870.

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Dijkstra, Z. J.; Doornbos, A. R.; Weyten, H.; Ernsting, J. M.; Elsevier, C. J.; Keurentjes, J. T. F. “Formation of Carbamic Acid in Organic Solvents and in Supercritical Carbon Dioxide” J. Supercritical Fluids 2007, 41, 109-114.

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Masuda, K.; Ito, Y.; Horiguchi, M.; Fujita, H. “Studies on the Solvent” Fuel Cells Bulletin 2005, Issue 9, 3-10.

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Besnard, M.; Cabaco, M.I.; Chavez, F.V.; Pinaud, N.; Sebastiao, P.J.; Coutinho, A.P.; Danten, Y. “On the spontaneous carboxylation of 1-butyl-3methylimidazolium acetate by carbon dioxide” Chem. Commun. 2012, 48, 1245-1247.

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Besnard, M.; Cabaco, M.I.; Chavez, F.V.; Pinaud, N.; Sebastiao, P.J.; Coutinho, A.P.; Mascetti, J.; Danten, Y. “CO2 in 1-Butyl-3-methylimidazolium Acetate. 2. NMR Investigation of Chemical Reactions” J. Phys. Chem. A 2012, 116, 4890-4901.

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Cabaco, M.I..; Besnard, M.; Danten, Y.; Coutinho, J.A.P. “Carbon Dioxide in 1-Butyl-3methylimidazolium Acetate. I. Unusual Solubility Investigated by Raman Spectroscopy and DFT Calculations” J. Phys. Chem. A 2012, 116, 1605-1620.

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Baugh, L. S.; Kortunov, P.; Siskin, M. "Ionic Liquids as Amine Promoter Solvents for Removal of Carbon Dioxide" USSN 61/381,294, Filed September 9, 2010. US2012-0063978.

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Baugh, L. S.; Kortunov, P; Siskin, M. "Ionic Liquids for Removal of Carbon Dioxide" USSN 61/381,281, Filed September 9, 2010; USSN 13228556 Filed September 9, 2011. US2012-0063977.

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Wang, C.; Luo, H.; Luo, X.; Li, H.; Dai, S. “Equimolar CO2 capture by imidazolium-based ionic liquids and superbase systems” Green Chem. 2010, 12, 2019-2023.

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Maginn, E. J. Design and Evaluation of Ionic Liquids as Novel CO2 Absorbents. Quarterly. Technical Report to DOE. May 31, 2005 (DOE Award Number DE-FG26-04NT42122).

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Yokozeki, A.; Shiflett, M. B.; Junk, C. P.; Grieco, L. M.; Foo, T. Physical and Chemical Absorptions of Carbon Dioxide in Room-Temperature Ionic Liquids. J. Phys. Chem. B 2008, 112, 16654-16663.

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Shiflett, M. B.; Kasprzak, D. J.; Junk, C. P.; Yokozeki, A. Phase Behavior of {Carbon Dioxide + [bmim][Ac]} Mixtures. J. Chem. Thermodynamics 2008, 40, 25-31.

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Barber, P.S.; Griggs, C.S.; Gurau, G.; Liu, Z.; Li, S.; Li, Z.; Lu, X.; Zhang, S.; Rogers, R.D. “Coagulation of Chitin and Cellulose fro, 1-Ethyl-3methylimidazolium Acetate Ionic-Liquid Solutions Using Carbon Dioxide” Angew. Chem. Int. Ed. 2013, 52, 12350-12353.


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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

50.

Imidazol(in)ium carboxylates are used as carbene transfer reagents in organometallic chemistry. Typically, their preparation by reaction with CO2 is facilitated by deprotonation of the imidazole C-2 carbon with a strong base to form a carbene. Such carboxylated imidazoles have been utilized themselves as ionic liquids, and carbene polymers have been briefly reported as substrates for CO2 capture . See, for example: (a) Voutchkova, A. M.; Feliz, M.; Clot, E.; Eisenstein, O.; Crabtree, R. H. “Imidazolium Carboxylates as Versatlie and Selective N-Heteocyclic Carbene Transfer Agents: Synthesis, Mechanism, and Applications” J. Am. Chem. Soc. 2007, 129, 12834-12846. (b) Duong, H. A. Tekavee, T. N.; Arif, A. M.; Louie, J. “Reversible Carboxylation of N-Heterocyclic Carbenes” Chem. Commun. 2004, 112-113. (c) Tudose, A.; Demonceau, A.; Delaude, L. “Imidazol(in)ium-2carboxylates as N-heterocyclic Carbene Precursors in Ruthenium-Arene Catalysts for Olefin Metathesis and Cyclopropanation” J. Organomet. Chem. 2006, 691, 5356-5365. (d) Zhou, H.; Zhang, W.-Z.; Liu, C.-H.; Qu, J.-P.; Lu, X.-B. “CO2 Adducts of N-Heterocyclic Carbenes: Thermal Stability and Catalytic Activity Toward the Coupling of CO2 with Epoxides” J. Org. Chem. 2008, 73, 8039-8044. (e) Tommasi, I.; Sorrentino, F. “Synthesis of 1,3Dialkylimidazolium-2-carboxylates by Direct Carboxylation of 1,3-dialkylimidazolium Chlorides with CO2” Tetrahedron Lett. 2006, 47, 6453-6456. (f) Smiglak, M.; Holbrey, J. D.; Griffin, S. T.; Reichert, W. M.; Swatloski, R. P.; Katritzky, A. R.; Yang, H.; Zhang, D.; Kirichenko, K.; Rogers, R. D. “Ionic Liquids via Reaction of the Zwitterionic 1,3Dimethylimidazolium-2-carboxylate with Protic Acids. Overcoming Synthetic Limitations and Establishing New Halide Free Protocols for the Formation of Ils” Green Chem. 2007, 9, 90-98. (g) Zhou, H.; Zhang, W.-Z.; Wang, Y.-M.; Qu, J.-P.; Lu, X.-B. “N-Heterocyclic Carbene Functionalized Polymer for Reversible Fixation-Release of CO2” Macromolecules 2009, 42, 5419-5421.

51.

Abraham, S.; Weiss, R.G. “Control of pyrene fluorescence intensity by in situ addition of CO2 or to an amidine/amine mixture or CO2 removal from an amidinium carbamate ionic liquid” Photochem. Photobiol. Sci. 2012, 11, 1642-1644.

52.

Shannon, M.S.; Bara, J.E. “Reactive and Reversible Ionic Liquids for CO2 Capture and Acid Gas Removal” Separation Science and Technlogy 2012, 47, 178-188.


Page 26 of 39

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Table 1. Summary of C-carboxylation of ionic liquids by CO2 and enhanced/promoted C-carboxylation of ionic liquid by strong base. pKa of counterion conjugate acid (1)

Ionic liquid

4.75 1-ethyl-3-methylimidazolium acetate (EMIM-OAc)

co-solvent

=C-H ppm 1 shift, H NMR

Mol% Ccarboxylation 0 (at 24-30 C 1.0 bar CO2)

neat

10.47

36.9

DMSO-d6

10.55

40

H2O

9.14

0

TMG (pKa 15.2)

11.09

91.2

DMAEE

10.55

39.0

neat

10.17

35

TMG (pKa 15.2)

10.77

70

neat

8.91

0

DMSO-d6

8.98

0

TMG (pKa 15.2)

9.22

79.5

neat

9.26

0

DMSO-d6

9.34

0

TMG (pKa 15.2)

9.33

>45

4.75 1-butyl-3-methylimidazolium acetate (BMIM-OAc)

-1.85 (2) 1-ethyl-3-methylimidazolium thiocyanate (EMIM-SCN)

-3 1-methylimidazolium (MIM-Cl)

chloride

-3 1,3-bis(2,4,6TMP)-imidazolium chloride (TMPI-Cl)

*TMP = trimethylphenyl (1) pKa values are those reported in MacFarlane et al, Acids and Bases in Ionic Liquids, ACS Symposium Series [2003], 856 [Ionic Liquids as Green Solvents], pp. 264-276, see Table 1, p. 272. (2) ACD / PhysChem Suite™ predicted pKa for thiocyanic acid: 0.93; pKa reported as 4.0 in the Bordwell online pKa database, http://www.chem.wisc.edu/areas/reich/pkatable/index.htm


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Table 2. Summary of the reactions of CO2 with amines in ionic liquid solution. Amine

Conc. Wt%

Mol % N-carboxylation of amine

Mol % C-carboxylation of ionic liquid

15

100*

18

EMIM-OAc

30

100*

4

EMIM-OAc

50

91

0

DMSO-d6

10

88

n/a

DMSO-d6

30

75

n/a

DMSO-d6

50

68

n/a

15

100*

20

EMIM-OAc

30

100*

0

EMIM-OAc

50

86

0

DMSO-d6

15

86

n/a

DMSO-d6

30

75

n/a

DMSO-d6

50

68

n/a

20

70

0

15

73

n/a

15

0

0

Solvent

EMIM-OAc Ethoxyethylamine (EEA, pKa 8.92)

EMIM-OAc Aminopropionitrile (APN, pKa 7.14)

1,5-dimethylamino-3oxapentane (DMAOP, pKa 9.07)

Ethoxyethylamine (EEA, pKa 8.92)

EMIM-SCN DMSO-d6

MIM-Cl

*some amines are di-N-carboxylated. See text for details


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Figure 1. In-situ NMR setup for studying amine-CO2 reaction chemistry and schematic of NMR probe used for in-situ CO2 uptake studies.


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Figure 2. 13C and 1H NMR spectra of 2 molar EMIM-OAc in DMSO-d6 solution before (top) and after CO2 treatment (bottom) at 30 °C and 1.0 bar of CO2. The reaction product is associated with the 13C peak at 155.37 ppm (=C-COOH) and 1H peak at 15.66 ppm; the equilibrium CO2 concentration per ionic liquid is 0.38.


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Energy & Fuels

Figure 3. Evolution of the ionic liquid - CO2 reaction for 2 molar EMIM-OAc in DMSO-d6 solution at 1.0 bar of CO2 at 30 0C monitored by 13C NMR. Formation of reaction product =C-COO(H) on the C-2 carbon and transformation of the C-2 carbon =C-H into =C-COO(H) species (top), evolution of C-4 and C-5 structural carbons of the ionic liquids (bottom).


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Figure 4. CO2 loading capacity of 3 molar EMIM-OAc in DMSO solution as a function of CO2 partial pressure and temperature. CO2 loading is presented in moles of CO2 per mole of ionic liquid units.


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Energy & Fuels

Figure 5. 13C and 1H NMR spectra of a DMSO-d6 solution consisting of 2 molar EMIM-OAc and 2 molar TMG before (top) and after CO2 treatment (bottom) at 30 °C and 1.0 bar of CO2. The reaction product is associated with the 13C peak at 155.23 ppm (=C-COO-) and 1H peak at 9.87 ppm (=NH2+); the equilibrium CO2 / EMIM-OAc mole ration is 0.87.


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Figure 6. Evolution of the ionic liquid - base - CO2 reaction for a DMSO-d6 solution consisting of 2 molar EMIM-OAc and 2 molar TMG at 1.0 bar of CO2 at 30 0C monitored by 13C NMR: formation of reaction product =C-COO- on the C-2 carbon (peak at 155 ppm), transformation of the C-2 carbon =CH- into =C-COO- species (peaks at 138-143 ppm), and transformation of guanidine –C=NH into guanidinium ion –C=NH2+ (168-164 ppm).


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Energy & Fuels

Figure 7. The Vapor-Liquid Equilibrium for a DMSO-d6 solution consisting of EMIM-OAc and TMG at two concentrations as a function of CO2 partial pressure at 45 0C. CO2 loading is presented in moles of CO2 per mole of ionic liquid units.


Energy & Fuels

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Figure 8. The Vapor-Liquid Equilibrium for a DMSO-d6 solution with 3 molar EMIM-OAc and 3 molar TMG as a function of CO2 partial pressure and temperature. CO2 loading is presented in moles of CO2 per mole of ionic liquid units.


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Figure 9. 13C and 1H NMR spectra of 15 wt% EEA in EMIM-OAc solution before (top) and after CO2 treatment at 24 °C and 1.0 bar of CO2 (bottom). CO2 reaction products are associated with the new 13C NMR peaks at 160.22 ppm (carbamic acid on EEA), 158.05 ppm (dicarbamic acid), 155.27 ppm (C-carboxylation of EMIM-OAc), and 1H peaks at 14.46 and 7.33 ppm. The total loading is at least 1.08 CO2 per each EEA and an additional 0.18 CO2 per each EMIM-OAc.


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Figure 10. Comparison of CO2 loading capacities determined by 13C NMR for EEA dissolved in EMIM-OAc ionic liquid, in DMSO-d6, and in toluene-d8 at various amine concentrations at a fixed CO2 partial pressure of 1.0 bar at 30 °C.


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Energy & Fuels

TOC:


Pathways of the Chemical Reaction of Carbon Dioxide with Ionic

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