Recyclability of Encapsulated Ionic Liquids for Post-Combustion CO2

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Recyclability of Encapsulated Ionic Liquids for Post-Combustion CO2 Capture Tangqiumei Song, Gabriela M. Avelar Bonilla, Oscar MoralesCollazo, Michael J. Lubben, and Joan Frances Brennecke Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00251 • Publication Date (Web): 04 Mar 2019 Downloaded from http://pubs.acs.org on March 6, 2019

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Industrial & Engineering Chemistry Research

Recyclability of Encapsulated Ionic Liquids for Post-Combustion CO2 Capture Tangqiumei Song a, Gabriela M Avelar Bonilla b, Oscar Morales-Collazo b, Michael J. Lubben b, and Joan F. Brennecke b,* aDepartment

of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana,

46556, USA bMcKetta

Department of Chemical Engineering, University of Texas at Austin, Austin, Texas 78712, USA

______________________________________________________________________________ ABSTRACT: Ionic liquids (ILs) with aprotic heterocyclic anions (AHAs) are promising candidates for postcombustion carbon capture technologies since they react with CO2 stoichiometrically and reversibly. CO2 solubilities in two AHA-ILs, triethyl(octyl)phosphonium 2-cyanopyrrolide ([P2228][2CNPyr]) and triethyl(octyl)phosphonium benzimidazolide ([P2228][BnIm]), are reported for multiple temperatures and a third, triethyl(octyl)phosphonium 6-bromobenzimidazolide ([P2228][6-BrBnIm]), at one temperature. Ionic liquid [P2228][2CNPyr] and phase-change ionic liquid (PCIL) tetraethylphosphonium benzimidazolide ([P2222][BnIm]) were encapsulated in a chemically compatible and CO2-permeable polydimethylsiloxane (PDMS) polymer shell in order to enhance absorption and desorption kinetics. Both the free and encapsulated [P2228][2CNPyr] and [P2222][BnIm] were subjected to thermodynamic testing. The CO2 solubilities in the encapsulated IL and PCIL were in good agreement with the free IL and PCIL, meaning that the encapsulation of IL and PCIL greatly enhanced the kinetics of CO2 absorption while maintaining the high CO2 capture capacity. Recyclability testing was also performed on both the free and encapsulated [P2228][2CNPyr] and [P2222][BnIm]. The IL and PCIL materials, as well as the capsules, were stable upon cycling, with the CO2 capacities for each cycle remaining unchanged. The IL and the PCIL showed no sign of degradation after cycling, which demonstrated excellent performance.

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 INTRODUCTION In pursuit of reducing greenhouse gas emissions, tremendous research efforts have been focused on developing different carbon capture technologies. Although the cause of global warming remains debatable, many researchers believe CO2 emissions from industrial and fossil fuel processes are one of the biggest contributors. There are mainly three CO2 capture systems, which are associated with different combustion processes, namely, pre-combustion, post-combustion, and oxyfuel combustion. In the pre-combustion process, CO2 is captured from the shifted syngas before completing the combustion, and the CO2 concentration in the syngas ranges from 15-50 %.1 In the post-combustion process, CO2 is captured from the flue gas after fuel combustion. Unlike syngas, which is rich in CO2, the flue gas only contains dilute CO2, (~5-15 %).1-3 Although the concentration of CO2 varies, in both systems, successful development of energy and costeffective CO2 separation methods are needed. Common CO2 separation technologies include absorption, adsorption, membrane separation, chemical looping combustion, hydrate-based separation, and cryogenic distillation.1-5 The most mature method is absorption using amines.4-5 However, there are several problems associated with this process: potential amine degradation, corrosion, solvent loss due to evaporation, energy intensity, etc.6-8 In an attempt to find alternative solvents with advantageous properties, ionic liquids (ILs) have been proposed and widely studied for the past two decades as novel media for CO2 capture.2, 9-14 Ionic liquids (ILs) are salts with melting temperatures below 100°C. ILs are often characterized as green solvents with favorable properties, including vanishing vapor pressure, high thermal and chemical stability, excellent chemical tunability, and large capacities and selectivity for CO2.2, 1214

Firstly, ILs have large physical capacities for CO2 absorption, which can be further enhanced by

tuning the molecular structure, such as adding fluorination. Moreover, the IL structure can be designed to chemically react with CO2 and lead to even higher capacities, around one mole of CO2 per mole of IL.9-11, 15 For some cooperative binding ILs, even greater than one mole of CO2 per mole of IL can be achieved.16 In addition, the non-volatility of ILs is beneficial to the process since this allows for regeneration without solvent loss. Last but not least, an IL-based CO2 capture process can require less total energy compared to the traditional processes using monoethanolamine (MEA) or other amines in aqueous solution. This is because 1) the energy


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required to heat the large amount of water present in the aqueous solution is eliminated, and 2) the enthalpy of reaction for ILs can be tuned to an optimal range by molecular structure design17, so that the energy required for regenerating the absorbent is less than that associated with the MEA+CO2 reaction (~-80 kJ/mol).18-20 Among different classes of IL, the aprotic heterocyclic anion ionic liquids (AHA-ILs) have been particularly interesting to us since they are capable of reacting with CO2 stoichiometrically and reversibly without suffering from a viscosity increase after the reaction has taken place.17 The AHA anions were developed based on their amino-functionalized predecessors which suffered from the aforementioned viscosity increase because of the formation of a hydrogen bond network.21 The AHA anions have no free protons so the hydrogen bond network between IL-CO2 complexes is frustrated. Four AHA-ILs were chosen as potential candidates in this study for post-combustion CO2 capture applications. Since flue gas streams are at low pressure and only contain dilute CO21-3, AHA ILs are suitable candidates, as they can react with CO2 to nearly a 1:1 mole ratio at very low partial pressures (~0.1-0.2 bar).17, ([P2222][BnIm]),

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The four ILs are

triethyl(octyl)phosphonium

tetraethylphosphonium benzimidazolide 2-cyanopyrrolide

([P2228][2CNPyr]),

triethyl(octyl)phosphonium benzimidazolide ([P2228][BnIm]), and triethyl(octyl)phosphonium 6bromobenzimidazolide ([P2228][6-BrBnIm]). Besides the favorable properties of AHA-ILs mentioned earlier, the enthalpies of reaction for CO2 binding for these four ILs are in the range of optimal chemical absorption strength;16 specifically, the estimated enthalpies are between 47 and -55 kJ/mol.17 The goal is to have a strong driving force for the CO2 reaction (large enthalpy), yet not too strong since a large enthalpy of reaction results in a higher energy requirement in the regeneration of the neat IL. Moreover, [P2222][BnIm] and [P2228][BnIm] are phase-change ionic liquids (PCILs),22 meaning the solid [P222X][BnIm] reacts with CO2 and forms a liquid [P222X][BnIm]-CO2 complex, whose melting point is sufficiently lower than that of the pure PCIL. This unique phase transition property can be used to further reduce the total energy consumption. During the regeneration process, when CO2 is removed, liquid [P222X][BnIm]-CO2 complex changes phase from liquid to solid [P222X][BnIm]


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and this releases heat, so the added energy required for regeneration can be reduced with the help of the IL’s enthalpy of fusion. A great deal of progress has been made to improve CO2 solubility in ILs; however, the relatively high viscosity of ILs limits their use as liquid absorbents for industrial applications. In order to break through the mass transfer barriers caused by the high viscosity, the concept of encapsulated ionic liquids was proposed in recent years in an effort to improve the absorption and desorption kinetics. In our previous collaboration with Moya et al.23, porous carbon spheres of 350 nm outer diameter and 70 nm shell thickness encapsulating [P66614][2CNPyr] were tested for CO2 absorption. The encapsulated ILs underwent several absorption-desorption cycles, and the capacity of the IL to absorb CO2 remained unchanged compared to the neat IL, while absorption kinetics increased drastically due to the increased gas-liquid contact area. Similar studies were also done using ILs with imidazolium cations and acetate and amino acid anions encapsulated in porous carbon capsules.24 Ionic liquids that do not chemically react with CO2 have also been encapsulated in carbon capsules.25-26 Kaviani et al.27 encapsulated ILs with imidazolium cations and bis(trifluoromethylsulfonyl)imide in much larger fluoropolymeric capsules of 1-2 mm outer diameter, and cycled CO2 pressure many times, resulting in reproducible uptake isotherms. However, these ILs are not very basic, exhibiting only physical CO2 uptake. Wang et al.28 encapsulated imidazolium-amino acid ILs in polymethylmethacrylate capsules of 500 µm diameter, which are more similar to the capsule size in this work, and cycled CO2 pressure, where CO2 capacity decreased slightly after several cycles. Additionally, researchers have attempted to achieve similar results with micron sized droplets of ILs surrounded with silica nanoparticles.29 In this study, an IL and a PCIL were encapsulated in a chemically compatible and CO2-permeable polydimethylsiloxane (PDMS) polymer shell. These encapsulated small particles have diameters between 100 and 600 μm, which means they have extremely high surface area, allowing for enhanced absorption kinetics.30-32 The encapsulated particles used in this work were produced by researchers at the Lawrence Livermore National Laboratory, using a microfluidic assembly.30 The details of the capsule production can be found elsewhere31, 33-34.


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The objective of this research is to demonstrate the performance of the encapsulated IL/PCIL, including chemical stability, recyclability, and reproducibility. We first identified four potential IL candidates that are suitable for the post-combustion CO2 capture application, and subsequently measured the CO2 solubilities at multiple temperatures. Further, one IL ([P2228][2CNPyr]) and one PCIL ([P2222][BnIm]) were encapsulated in the PDMS polymer shells, and their thermodynamic properties were tested. The recyclability of the CO2 uptake of both the free and encapsulated IL and PCIL was investigated.

 MATERIALS AND METHODS Materials. Carbon dioxide (Instrument grade, 99.99% purity, H2O < 10 ppm) was purchased from Airgas and used without further purification. All ionic liquids used in this work were synthesized in our laboratory, and are depicted in Table 1. The purity of the ILs (98-99%) were evaluated using 1H

NMR (Bruker AVANCE III HD 400 MHz spectrometer) spectroscopy after dissolving the sample

in DMSO-d6 (99.9 atom% D, Sigma-Aldrich). The ILs were pre-dried at 60 °C under high vacuum for at least 48 h to remove any volatiles including water. Materials used in the synthesis of the ILs include tetraethylphosphonium bromide (99% purity) purchased form TCI, triethylphosphine (99% purity), octylbromide (99% purity), and 6-bromobenzimidazole (97% purity) purchased from Sigma-Aldrich, and anhydrous toluene (99.8% purity), benzimidazole (99% purity), Amberlite IRN-78 (Nuclear Grade), and pyrrole-2-carbonitrile (99% purity) purchased from Alfa Aesar. Methanol (99.8% purity) was purchased from Baker and used without further purification.

Synthesis of ILs [P2228][Br], triethyl(octyl)phosphonium bromide. Under inert atmosphere, in a flame dried round bottom flask (500 mL) a solution of triethylphosphine (25 g, 212 mmol) in anhydrous toluene (200 mL) was treated with 1-bromooctane (43 g, 222 mmol) via cannula. The reaction was stirred at room temperature for 20 minutes and 70 °C for 5 h. After completion, the reaction was concentrated, dissolved in acetonitrile and recrystallized from acetonitrile/ethyl acetate


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(1:2) to obtain triethyl(octyl)phosphonium bromide ([P2228][Br]) as a white solid, obtaining a yield of 98% (64.5 g, 207 mmol). (1H NMR (600 MHz, DMSO-d6) δ 2.13 (m, 8H), 1.34 (m, 12H), 1.08 (m, 9H), 0.86 (m, 3H). The 1H NMR spectrum is shown in Figure S1, Supporting Information. [P2222][BnIm], tetraethylphosphonium benzimidazolide. In a round bottom flask (500 mL), a solution of tetraethylphosphonium bromide ([P2222][Br]) (7.29 g, 32.1 mmol) in methanol (200 mL) was treated with Amberlite IRN78 (93 g) and mixed until no residual halide precipitation was observed by AgNO3 test to obtain tetraethylphosphonium hydroxide ([P2222][OH]). The mixture was filtered and treated with benzimidazole (3.79 g, 32.1 mmol) and stirred for 2 days. (1H NMR (400 MHz, DMSO-d6) δ 7.62 (s, 1H), 7.30 (dd, J = 5.9, 3.2 Hz, 2H), 6.68 (dd, J = 5.9, 3.2 Hz, 2H), 2.12 (ddd, J = 13.4, 8.1, 3.0 Hz, 8H), 1.06 (dtd, J = 17.5, 7.7, 2.1 Hz, 12H). The 1H NMR spectrum is shown in Figure S2, Supporting Information. [P2228][2CNPyr], triethyl(octyl)phosphonium 2-cyanopyrrolide. In a round bottom flask (500 mL) a solution of triethyloctylphosphonium bromide ([P2228][Br]) (10 g, 32.1 mmol) in methanol (200 mL) was treated with Amberlite IRN78 (93 g) and mixed until no residual halide precipitation was observed by AgNO3 test to obtain triethyloctylphosphonium hydroxide ([P2228][OH]). The mixture was filtered and treated with 1H-pyrrole-2-carbonitrile (2.21 g, 32.1 mmol) and stirred for 2 days. (1H NMR (600 MHz, DMSO-d6) δ 6.65 (d, J = 1.4 Hz, 1H), 6.41 (dt, J = 3.0, 0.8 Hz, 1H), 5.81 (dt, J = 3.6, 1.0 Hz, 1H), 2.13 (m, 8H), 1.34 (m, 12H), 1.08 (m, 9H), 0.86 (m, 3H). The 1H NMR spectrum is shown in Figure S3, Supporting Information. [P2228][BnIm], triethyl(octyl)phosphonium benzimidazolide. In a round bottom flask (500 mL), a solution of triethyloctylphosphonium bromide ([P2228][Br]) (10 g, 32.1 mmol) in methanol (200 mL) was treated with Amberlite IRN78 (93 g) and mixed until no residual halide precipitation was observed by AgNO3 test to obtain triethyloctylphosphonium hydroxide ([P2228][OH]). The mixture was filter and treated with benzimidazole (3.79 g, 32.1 mmol) and stirred for 2 days. ( 1H NMR (600 MHz, DMSO-d6) δ 7.65 (s, 1H), 7.33 (dd, J = 5.9, 3.2 Hz, 2H), 6.73 (dd, J = 5.9, 3.2 Hz, 2H), 2.11 (m, 8H), 1.33 (m, 12H), 1.06 (dt, J = 18.0, 7.7 Hz, 9H), 0.86 (t, , J = 7.2, 3H). The 1H NMR spectrum is shown in Figure S4, the Supporting Information.


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[P2228][6-BrBnIm], triethyl(octyl)phosphonium 6-bromobenzimidazolide. In a round bottom flask (500 mL), a solution of triethyloctylphosphonium bromide ([P2228][Br]) (10 g, 32.1 mmol) in methanol (200 mL) was treated with Amberlite IRN78 (93 g) and mixed until no residual halide precipitation was observed by AgNO3 test to obtain triethyloctylphosphonium hydroxide ([P2228][OH]). The mixture was filtered and treated with 6-bromobenzimidazole (6.32 g, 32.1 mmol) and stirred for 2 days. (1H NMR (600 MHz, DMSO-d6) δ 7.65 (s, 1H), 7.42 (d, J = 2.0 Hz, 1H), 7.25 (d, J = 8.3 Hz, 1H), 6.80 (dd, J = 8.3, 2.0 Hz, 1H), 2.13 (dt, J = 13.2, 7.7 Hz, 8H), 1.34 (m, 12H), 1.08 (dt, J = 18.0, 7.7 Hz, 9H), 0.86 (t, J = 7.2 Hz 3H). The 1H NMR spectrum is shown in Figure S5, Supporting Information. Methanol and other volatiles were removed at 50°C under vacuum after the synthesis. Complete removal of volatiles was confirmed by 1H NMR. Water byproduct was then removed by further drying at 60°C under vacuum for approximately 3 days. The water content of each IL was determined by a Brinkman 831 Karl Fischer coulometer and was less than 0.05% by weight (500 ppm). Since there was no precipitation observed by the AgNO3 test, we estimate the residual halide to be less than 2000 ppm.


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Table 1. Chemical Structures and Nomenclatures of ILs used in this study Cations Abbreviations

Chemical Structures

Nomenclature

[P2222]

tetraethylphosphonium

[P2228]

triethyl(octyl)phosphonium Anions

Abbreviations

Chemical Structures

Nomenclature

[BnIm]

benzimidazolide

[2CNPyr]

2-cyano-pyrrolide

[6-BrBnIm]

6-bromo-benzimidazolide

Viscosity. The viscosity measurements of [P2228][6-BrBnIm] were conducted using an ATS Rheosystems Viscoanalyzer with an ETC-3 Joule-Thomson effect temperature cell, equipped with a 20 mm cone and plate spindle. The viscosities were measured from 15°C to 75°C, under a nitrogen atmosphere to minimize the uptake of water. The viscosities measured have an uncertainty of ± 5% above 100 cP and ± 10% between 50 cP and 100 cP. The viscosity of [P2228][2CNpyr] has been measured previously by Seo et al.35, and the [P2222][BnIm] and [P2228][BnIm] are solids at room temperature so their viscosities could not be measured.


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Density. The density of [P2228][6-BrBnIm] was measured from 15°C to 75°C and the density of [P2228][BnIm] complexed with CO2 was measured from 20°C to 60°C using a DMA 4500 Anton Paar oscillating U-tube densitometer. The instrument uncertainty is ± 0.00005 g/cm3. However, taking the sample impurity into account, we estimate the experimental uncertainty to be ± 0.0001 g/cm3. The density of [P2228][2CNpyr] has been measured previously by Seo et al.35, [P2222][BnIm] and [P2228][BnIm] are solids at room temperature so their densities could not be measured directly. Therefore, we only report the density of [P2228][BnIm] complexed with CO2. These values are shown in supporting information. Additionally, Seo et al.22 estimated the density of [P2222][BnIm] previously, by mixing it with water. Melting Point. The melting points of [P2228][BnIm] and [P2228][6-BrBnIm] were measured with a Mettler-Toledo differential scanning calorimeter DSC (DSC 3). A Mettler-Toledo standard 40 L aluminum crucible with lid was used as sample holder. For [P2228][BnIm] (solid at room temperature), the crucible was first heated from room temperature to 150°C under nitrogen and then cooled to -120°C, held for 3 mins, and heated to 150°C again. For [P2228][6-BrBnIm] (liquid at room temperature), the crucible was cooled from room temperature to -120°C, held for 3 mins, and then heated to 50°C. The cooling/heating rate were 10°C/min. The estimated experimental uncertainty in these measurements is ± 1°C. The melting points of [P2228][CNpyr] and [P2222][BnIm] have been measured previously.22, 35 CO2 solubilities measurements: gravimetric technique. A gravimetric technique was one of the methods used in this work to measure gas uptake, and it involves monitoring the change in the sample weight as it absorbs or desorbs the gas at a fixed temperature and pressure. Specifically, the intelligent gravimetric analyzer (IGA-001) produced by Hiden Analytical was used. This technique is validated by comparing the CO2 solubility in 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([hmim][TFSI]) obtained using this method with the data reported in the literature, and the results are in good agreement within experimental uncertainties. A small sample of IL (about 70 mg) was added to the sample holder and was dried in situ at about 10-6 bar and elevated temperature (around 90°C). Water and volatile impurities were removed, and it is confirmed by constant weight for at least 20 min. The sample was cooled


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to the desired temperature. Then, the chamber was pressurized with CO2. The vapor-liquid equilibrium between gas and IL was obtained and confirmed by constant weight for about 20 min. The pressure in the sample chamber was then increased or decreased to the next set point. After obtaining a full isotherm, which is composed of both absorption and desorption points, the sample was regenerated under heat treatment and vacuum. The procedure was repeated as described above to obtain isotherms at different temperatures. The buoyancy effect was accounted for in the calculation, as described previously36-37. The experimental uncertainty is estimated to be ± 0.02 mol CO2/IL, mostly due to the temperature fluctuation of the control unit, and the purity of the ionic liquid which affects the solubility measurements through the buoyancy correction. CO2 solubilities measurements: volumetric technique. A volumetric technique is another method used in this work to measure the CO2 uptake. It consists of measuring the variations of the pressure inside a reaction vessel, while the volume of the vessel is held constant. This technique was used to measure CO2 solubility isotherms of [P2228][BnIm] and for the recyclability tests of [P2228][2CNPyr] and [P2222][BnIm]. The volumetric apparatus consists of a CO2 reservoir and a glass reaction cell of known volumes: 290 ml and 170 ml, respectively. Using the density of the IL the volume displaced by the liquid sample has been subtracted from the total volume of the vessel (in the case of [P2228][BnIm] the density of the IL complexed with CO2 was used). The temperature in the system is determined using a type K thermocouple (Omega HH501DK, ±0.5 °C uncertainty). The pressure in the reservoir and reaction vessel is monitored using Heise model PM digital pressure indicators (±0.5 mbar uncertainty). The apparatus is kept inside a YAMATO DKN602 oven in order to control the temperature. The experiment consists in loading the IL sample into the reaction cell under a nitrogen atmosphere, afterwards the glass reaction cell is evacuated using a vacuum pump (Welch Gem 8890) to a pressure of less than 4 mbar. The water content of the IL samples was measured before the experiments using a Brinkman 831 Karl Fischer coulometer, and was below 500 ppm. After the reaction vessel and reservoir have reached the desired temperature (25°C, 40°C, 60°C or 80°C), the sample is exposed to different pressures of CO2 ranging from 0.1 to 1 bar, while being stirred continuously with an Autoclaves MagneDrive III stirrer. The pressure and temperature inside the reaction cell were monitored


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until equilibrium was reached (as indicated by no further change in the pressure). The CO2 uptake was calculated from the pressure drop inside the reaction cell at the equilibrium temperature and pressure, and inside the reservoir vessel, using the ideal gas law and Lee-Kesler correlation.38 The uncertainty of the CO2 solubility measurements associated with this apparatus is ± 0.02 mol CO2/IL, determined by calculation of the standard deviation of multiple runs when available. Reaction equilibrium and equilibrium constant. The total CO2 uptake in the system is a combination of the physically dissolved gas in the liquid phase and the chemical formation of the IL-CO2 complex. Therefore, parallel paths were assumed: the physical dissolution of CO2 in the IL (eqn 1), and the chemical reaction between CO2 and the anion of the IL (eqn 2). 𝐶𝑂2(𝑔)↔𝐶𝑂2(𝑙) 𝐾𝑒𝑞

𝐼𝐿(𝑙) + 𝐶𝑂2(𝑔)

𝐼𝐿 ― 𝐶𝑂2(𝑙)

(eqn 1) (eqn 2)

The total IL concentration is a sum of the IL, IL-CO2 complex, and the deactivated IL, which is the amount of IL that does not take part in the binding reaction due to various prohibiting factors. This accounts for the fact that the CO2 capacity does not fully reach 1.0 mole CO2 per mole of IL, even at saturation. [𝐼𝐿0] = [𝐼𝐿] + [𝐼𝐿 ― 𝐶𝑂2] +[𝑑𝑒𝑎𝑐𝑡𝑖𝑣𝑎𝑡𝑒𝑑 𝐼𝐿]

(eqn 3)

Henry’s Law constants are used to model the amount of CO2 absorbed physically in the IL: [𝐶𝑂2(𝑙)]

𝑃𝐶𝑂2 = 𝐻

[𝐼𝐿0]

(eqn 4)

where H is the Henry’s Law constant and has the unit of pressure, PCO2 is the partial pressure of CO2 in the system, and [𝐶𝑂2(𝑙)] is the concentration of CO2 which physically dissolved in the IL, and [IL0] is the IL total concentration. A reaction equilibrium constant is used to describe the thermodynamics of the chemical reaction between CO2 and the IL,


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𝐾𝑒𝑞 =

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[𝐼𝐿 ― 𝐶𝑂2]

(eqn 5)

𝑃𝐶𝑂2 [𝐼𝐿]

The total concentration of CO2 in the IL solution is a combination of the CO2 absorbed physically and the chemically reacted IL-CO2 complex, [𝐶𝑂2𝑡𝑜𝑡𝑎𝑙] = [𝐶𝑂2(𝑙)] +[𝐼𝐿 ― 𝐶𝑂2]

(eqn 6)

Rearranging eqn 3, 4, and 5, and plugging in eqn 6 yields, [𝐶𝑂2𝑡𝑜𝑡𝑎𝑙] [𝐼𝐿0]

=

𝑃𝐶𝑂2 𝐻

𝐾𝑒𝑞 𝑃𝑐𝑜2

+ 1 + 𝐾𝑒𝑞 𝑃𝐶𝑂 ∗

[𝐼𝐿0] ― [𝑑𝑒𝑎𝑐𝑡𝑖𝑣𝑎𝑡𝑒𝑑 𝐼𝐿] [𝐼𝐿0]

2

(eqn 7)

Setting z equal to the mole ratio [CO2total]/[IL0], and c equal to ([IL0]-[deactivated IL])/[IL0], which is the fraction of active IL available for CO2-binding (i.e., the maximum absorption capacity from reaction of CO2 with the IL), the Langmuir model for the reaction between CO2 and the IL can be obtained and used to fit the isotherms. The constants are Keq, H and c. Since the stoichiometry of the reaction between CO2 and the anion is 1:1, the value of c is generally close to 1. 𝑧=

𝑃𝐶𝑂2 𝐻

𝐾𝑒𝑞 𝑃𝐶𝑂2

+𝑐1 + 𝐾𝑒𝑞 𝑃𝐶𝑂

2

(eqn 8)

Standard enthalpy and entropy of reaction. When multiple isotherms are available, the standard enthalpy and standard entropy of absorption can be obtained. The enthalpy of absorption describes the strength of interactions between dissolved gases and the ILs, whereas the entropy of absorption indicates the change in ordering that take place when a molecule of the gas is transferred into the gas/IL mixture. The change in standard Gibbs free energy may be expressed in terms of the equilibrium constant (Keq) or the standard enthalpy and entropy of reaction: ∆𝐺0𝑟𝑥𝑛(𝑇) = ―𝑅𝑇 ln (𝐾𝑒𝑞) ∆𝐺0𝑟𝑥𝑛(𝑇) = ∆𝐻0𝑟𝑥𝑛 ―𝑇∆𝑆0𝑟𝑥𝑛

(eqn 9) (eqn 10)

Setting eqn 9 equal to eqn 10,


12

Page 13 of 32 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


∆𝐺0𝑟𝑥𝑛(𝑇) = ―𝑅𝑇 ln (𝐾𝑒𝑞) = ∆𝐻0𝑟𝑥𝑛 ―𝑇∆𝑆0𝑟𝑥𝑛 ln (𝐾𝑒𝑞) =

―∆𝐻0𝑟𝑥𝑛 𝑅𝑇

+

∆𝑆0𝑟𝑥𝑛

(eqn 11) (eqn 12)

𝑅

The standard enthalpy and entropy of reaction can be obtained from eqn 12 by plotting Rln(Keq) versus T-1, where the slope is –H0rxn, and the intercept is S0rxn. Although G0rxn is a function of temperature, H0rxn and S0rxn are not strong functions of temperature; therefore, H0rxn and

S0rxn are assumed to be temperature independent in this work. The uncertainties associated with the enthalpy and entropy of reaction are calculated using a linear regression method. As mentioned before, a plot of Rln(Keq) vs 1/T will yield a straight line of slope –H0rxn and intercept of S0rxn. Therefore, the uncertainty present in each term is given by:

𝜎H =

𝜎S =

Σ(𝑙𝑛𝐾𝑖 ― H ∗ 1 𝑇𝑖 ― S)

2

𝑁 𝑁Σ(1/𝑇𝑖)2 ― (Σ 1/𝑇𝑖)2

𝑁―2 Σ(𝑙𝑛𝐾𝑖 ― H ∗ 1 𝑇𝑖 ― S) 𝑁―2

2

Σ(1/𝑇𝑖)2

𝑁Σ(1/𝑇𝑖)2 ― (Σ 1/𝑇𝑖)2

Where Ki=Keq, H=H0rxn, ΔS=S0rxn and N is the number of data points.

 RESULTS AND DISCUSSION CO2 solubility in neat [P2228][BnIm]: temperature dependence study and alkyl chain length effect. CO2 solubilities in [P2228][BnIm] were measured at three temperatures, and the standard enthalpy and entropy of reaction were obtained. Isotherms are presented in Figure 1 and the tabular data can be found in the Supporting Information. The phase-change ionic liquid [P2228][BnIm] has a melting point of 103°C (melting point data is summarized in Table S1 in Supporting Information) and undergoes a phase change when exposed to CO2 even at room


13


Page 14 of 32

temperature. When comparing the isotherms at 25°C and 40°C, there is not a significant difference between them, since they both show a high uptake of almost 0.9 mol CO2/ mol PCIL when exposed to less than 0.2 bar of CO2. However, at 60°C and low pressures the ionic liquid showed a slightly lower uptake, of less than 0.8 mol CO2/ mol PCIL. It is also important to notice that the pressure of CO2 at which the [P2228][BnIm] becomes completely liquid is very low at every temperature; ~14 mbar of CO2 at 25°C, ~10 mbar of CO2 at 40°C, and ~29 mbar of CO2 at 60°C. Therefore, it was not possible to fit the data where solid-vapor-liquid equilibrium is present. As a result, only the points in the respective isotherms where the PCIL is completely liquid were used to obtain the enthalpy and entropy of reaction, using the equations above to model vapor liquid equilibrium. The reaction equilibrium constants at three temperatures were obtained from fitting the experimental isotherms using eqn 8, and the standard enthalpy and entropy of reaction were then obtained by plotting Rln(Keq) vs 1/T, as shown in Figure 2. H0rxn is calculated to be -51.3  1.5 kJ/mol, and S0rxn is -125  4 J/mol. All experimentally determined standard enthalpies and entropies of reaction are tabulated in Table 2. We note that for some tetra-alkylphosphonium AHA ILs, reaction with CO2 at temperatures above ambient can include some complexation with the cation through ylide formation.15 Any ylide formation and subsequent reaction with CO2 is neglected herein.


14

Page 15 of 32

1

mole CO2 /mole PCIL

0.9 0.8 0.7 0.6 0.5 0.4

25ºC

0.3

40ºC

0.2

60ºC

0.1 0 0

0.2

0.4 0.6 Pressure (bar)

0.8

1

Figure 1. Solubility of CO2 in [P2228][BnIm] at 25°C, 40°C, and 60°C, where the dashed lines are the fits of each isotherm using eqn 8.

Rln(Keq)

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 45 40 35 30 25 20 15 10 5 0 0.0029

[P [P2228][BnIm] 2228][BnIm] [P [P2228][2CN] 2222][BnIm] Linear ([P 2228][BnIm]) ([P2228][BnIm]) Linear 2228][2CNPyr]) Linear ([P ([P2228][2CN]) 0.003

0.0031

0.0032 0.0033 I/T (K-1)

0.0034

0.0035

Figure 2. ln(Keq) vs T-1 plot for [P2228][BnIm] and [P2228][2CNPyr] For AHA ILs, the anion is primarily responsible for the CO2 capture reaction17; however, the cation also plays an important role. When comparing two ILs with the same [BnIm]- anion, [P2228][BnIm] (with shorter alkyl chains on the cation) exhibited higher CO2 solubility compare to [P66614][BnIm],


15


as shown in Figure 3. This is because the amount of re-ordering required for the IL-CO2 complex to form is greater for cations with longer alkyl chains compared to cations with shorter chains. The anion could be wrapped by the long chains of the cation which might need to relax first before CO2 can reach the anion and react. This is supported by the fact that the entropy of reaction, S0rxn, for [P66614][BnIm] (-130 kJ/mol-K)17 is a larger negative number than for [P2228][BnIm] (-125  4 kJ/mol-K). This finding is consistent with what Seo et al.35 reported for a series of AHA ILs with the [2CNPyr] anion. 1 mole CO2 /mole 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

Page 16 of 32

0.8 0.6 0.4 [P2228][BnIm] at25°C 25°C [P2228][BnIm] [P66614][BnIm] at22°C 22°C [P66614][BnIm]

0.2 0 0

0.2


0.8

1

Figure 3. Comparison of CO2 solubility in [P66614][BnIm]17 and [P2228][BnIm] at room temperature, where the dashed lines are the fits of each isotherm using eqn 8.

CO2 solubility in neat [P2228][2CNPyr]: temperature dependence study. The CO2 solubilities in [P2228][2CNPyr] were measured at 40C and 60C, and plotted with 22C data measured previously35 (tabular data in Supporting Information), and the standard enthalpy and entropy of reaction were obtained (Figure 2). As seen in Figure 4, the CO2 solubility in [P2228][2CNPyr] decreases with increasing temperature, as expected. The enthalpy of reaction for [P2228][2CNPyr] is -46.6 kJ/mol, which is a less negative value compared to [P2222][BnIm] (-52 kJ/mol)17, 22 and [P2228][BnIm] (-51.3 kJ/mol), meaning that the binding energy of the [2CNPyr]- anion is weaker


16

Page 17 of 32

than that of the [BnIm]- anion. This is attributed to the lower basicity of the [2CNPyr]- compared to the [BnIm]-. The equilibrium constants, and standard enthalpies and entropies of reaction for [P2228][2CNPyr], [P2228][BnIm], and [P2222][BnIm] are summarized in Table 2. 1 mole CO2 /mole 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


0.8 0.6 22ºC

0.4

40ºC 0.2

60ºC

0 0

0.2


0.8

1

Figure 4. Solubility of CO2 in [P2228][2CNPyr] at 22C35, 40C, and 60C, where the dashed lines are the fits of each isotherm using eqn 8.

Table 2. Equilibrium constant, and standard enthalpy and entropy of reaction for [P2228][2CNPyr], [P2228][BnIm], and [P2222][BnIm]22 Ionic liquids and phase-change ionic liquids

Melting point (°C)

[P2228][2CNPyr]

Keq (bar-1)

H0rxn (kJ/mol)

S0rxn (J/mol-K)

Reference

25°C

40°C

60°C

N/Aa

41b

14.4

4.7

-46.6  1.1

-127  3

17, and this work

[P2228][BnIm]

103

280

110

32

-51.3  1.5

-125  4

this work

[P2222][BnIm]

-25, 166

-54

-

22

[P2228][ 6-BrBnIm]

N/Aa

-48

-126

17, and this work

-

-

9


17


Page 18 of 32

a N/A a

= a melting point does not exist measured at 22C, c=0.92

CO2 solubility in neat [P2228][6-BrBnIm] at 60C. The solubility of CO2 in [P2228][6-BrBnIm] (tabular data in Table S4, Supporting Information) was only measured at 60C because the viscosities at 22°C and 40°C are too high for the measurements to be finished in a reasonable amount of time. Therefore, in order to estimate the equilibrium constant and standard entropy of reaction, it was assumed that the standard enthalpy of reaction depends mainly on the anion so it should be very similar to the standard enthalpy of reaction of [P66614][6-BrBnIm] with CO2 (-48 kJ/mol) calculated previously by Seo, et al.17 By fixing the standard reaction enthalpy value to -48 kJ/mol when fitting the experimental CO2 uptake values (z) using eqns 8 and 12, we obtain standard reaction entropy and equilibrium constant values of S0= -126 kJ/mol and Keq= 9 bar-1 @ 60 OC (see Table 2). The value obtained for the standard entropy of reaction with CO2 of [P2228][6-BrBnIm] is a smaller negative number than the standard entropy of reaction with CO2 of [P66614][6-BrBnIm] ((S0= 130 kJ/mol),17 which means less reordering for the IL with the shorter alkyl chains. This is consistent with what was observed previously when comparing ILs with cations of different alkyl chain length paired with [2CNPyr]- and [BnIm]- anions. A comparison of the solubility of CO2 in [P2228][2CNPyr], [P2228][BnIm], and [P2228][6-BrBnIm] at 60C is shown in Figure 5, where the dashed lines are fitted isotherms using eqn 8. The CO2 solubility in [P2222][BnIm], which has been reported previously,22 is also plotted for comparison. All four ILs have the ability to react with CO2 stoichiometrically through Lewis acid-base interactions.17 However, the CO2 capacities of [P2228][2CNPyr], [P2228][BnIm], and [P2228][6-BrBnIm] did not reach one mole of CO2 per mole of IL at 1 bar of CO2 pressure, because their intermediate values of the enthalpy of reaction with CO2 (-46.6 to -51.3 kJ/mol) requires a higher CO2 partial pressure to fully saturate the ILs. Additionally, the lower CO2 solubility at 60C (compared to lower temperatures) of all three ILs is a common effect of temperature on gas solubility, since both [P2228][2CNPyr] and[P2228][BnIm], exhibit higher CO2 solubilities, close to unity, at lower temperatures.


18

Page 19 of 32

The CO2 solubility in the four ILs at 60C decreases in the following order: [P2222][BnIm] > [P2228][BnIm] > [P2228][6-BrBnIm] > [P2228][2CNPyr], which agrees with the ILs’ enthalpy of reaction with CO2 values (-54 kJ/mol, -51.3 kJ/ mol, -48 kJ/mol and -46 kJ/mol respectively). From the studied anions [BnIm]- exhibits the strongest binding with CO2. When comparing the [BnIm]anion to its substituted-counterpart, [6-BrBnIm]-, the binding energy of [BnIm]- to CO2 is stronger, due to the electron-withdrawing nature of the bromine atom which delocalizes the electron density in the [6-BrBnIm]-, reducing the interaction strength between the anion and CO2. The IL with the [2CNPyr]- anion has the weakest binding of the three. 1 0.9 0.8 mole CO2 /mole 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


0.7 0.6 0.5

[P2222][BnIm] [P2222][BnIm] [P2228][BnIm] [P2228][BnIm]

0.4 0.3

[P2228][6-BrBnIm] [P2228][6-BrBnIm] [P2228][2CNPyr] [P2228][2CNpyr]

0.2 0.1 0 0

0.2


0.8

1

Figure 5. CO2 solubility in [P2222][BnIm], [P2228][2CNPyr], [P2228][BnIm], and [P2228][6-BrBnIm] at 60C, where the dashed lines for the three [P2228]+ ILs are the fits of each isotherm using eqn 8. *[P2222][BnIm] underwent a phase change from solid to liquid, therefore is not fitted with eqn 8.

Effect of SO2 and NOx. In reality, flue gas consists of a variety of components besides CO2. Of particular concern are SO2 and NOx. Therefore, we investigated the effect of SO2 and NO on the IL and the PCIL. We used a gravimetric microbalance system (magnetic suspension balance manufactured by Rubotherm, now TA Instrument) in a walk-in hood to expose the IL and the PCIL


19


Page 20 of 32

to pure SO2 and NO. More details regarding the balance can be found elsewhere.39-40 Ionic liquid [P2228][2CNPyr] was exposed to 12 bar of pure NO at 20C for a day, and was then subjected to vacuum and removed from the apparatus for analysis. The 1H NMR of the IL after this test indicated a significant degradation of [P2228][2CNPyr] after exposure to NO, leading us to conclude that there was unwanted chemistry taking place. A similar procedure was repeated for SO2, where [P2228][2CNPyr] was exposed to 1 bar of SO2; the 1H NMR suggested an irreversible reaction between [P2228][2CNPyr] and SO2. All 1H NMR spectrum can be found in Figures 6-11 of Supporting Information. Since [P2222][BnIm] is a solid at room temperature, it was dissolved in 1-hexyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide ([hmim][TFSI]) before exposing to NO and SO2. In the literature, Anderson et al.41 showed that [hmim][TFSI] has high SO2 solubility, and the IL showed no change before and after exposure to SO2. We have also put [hmim][TFSI] in contact with 12 bar of NO and no sign of degradation was observed. However, the 1H NMR of the [P2222][BnIm]/[hmim][TFSI] mixture after the test revealed that [P2222][BnIm] decomposed after the absorption experiments of NO and SO2. The effect of NO and SO2 on the [BnIm]- anion has been studied by Taylor et al.42 and Greer et al.43. The authors evaluated the ionic liquid [P66614][BnIm], and observed irreversible binding of the [BnIm]- with those gases. As a result, SO2 and NO would need to be removed from the flue gas prior to CO2 capture with the IL or PCIL investigated here.

CO2 solubilities in the encapsulated [P2228][2CNPyr] and [P2222][BnIm]. All four ILs appear to be viable absorbent candidates for a post-combustion CO2 capture process. Ionic liquid [P2228][2CNPyr] and phase-change ionic liquid [P2222][BnIm] were selected to be encapsulated in a polydimethylsiloxane (PDMS) polymer shell for further testing. IL [P2228][6-BrBnIm] was not chosen for encapsulation because of its high viscosity compared to [P2228][2CNpyr]. The viscosity of [P2228][6-BrBnIm] at 25C is 2665 cP, while the viscosity of [P2228][2CNPyr] is only 198 cP.17 The viscosity and density of [P2228][6-BrBnIm] can be found in Table S5, Supporting Information. The encapsulation process was performed by colleagues at the Lawrence Livermore National


20

Page 21 of 32

Laboratory, and the details of the process can be found elsewhere.30-31 The capsules are approximately 100 to 600 μm in diameter. The CO2 solubility in the polymer shell material was measured first. As seen in Figure 6. CO2 is physically absorbed into the shell material and the CO2 solubility increases with decreasing temperature. The CO2 solubilities at 60C, 70C, and 80C were very close. This physical uptake of CO2 into the shell material was later subtracted from the total uptake in the encapsulated IL or PCIL. 3.5E-07 mol CO2 / 1mg of polymer shell

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


22°C

3.0E-07

40°C

2.5E-07

60°C

2.0E-07

70°C 80°C

1.5E-07 1.0E-07 5.0E-08 0.0E+00 0

1

2 Pressure (bar)

3

4

Figure 6. Solubility of CO2 in empty PDMS capsules at multiple temperatures. The dashed lines are linear fits of each isotherm. CO2 solubilities were measured in encapsulated [P2228][2CNPyr] and [P2222][BnIm], and compared with the free IL and PCIL, respectively. For [P2228][2CNPyr], the weight percent of active [P2228][2CNPyr] in the dried capsules was around 40 wt%, calculated based on the flowrates of the polymer and IL fluids in the microfluidic device used to produce the capsules. As shown in Figure 7, after subtracting the physical solubility of the CO2 in the shell material from the total uptake, the uptake for the encapsulated [P2228][2CNPyr] is in good agreement with the uptake of the free IL. This demonstrates excellent performance, as the encapsulation of IL can enhance the


21


kinetics of CO2 absorption30 while maintaining high CO2 capacity of the IL. The capsules were broken open after the solubility measurement, and the IL showed no sign of degradation, as confirmed by 1H NMR analysis. 1.0 mole CO2/mole 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

Page 22 of 32

0.8 0.6 0.4

Free [P2228][2CNPyr] Free [P2228][2CNPyr]

0.2

Encapsulated [P2228][2CNPyr] Encapsulated [P2228][2CNPyr]

0.0 0

0.2


0.8

1

Figure 7. Solubility of CO2 in encapsulated [P2228][2CNPyr] at 22C. The CO2 uptake has been corrected for physical uptake by the polymer shell material. The dashed lines are the fits of each isotherm using eqn 8.

CO2 solubility in encapsulated [P2222][BnIm] was also measured gravimetrically at 60C, 70C, and 80C, and the weight percent of active [P2222][BnIm] in the dried capsules was 58 wt%, also calculated from the flowrates of the polymer and PCIL fluids in the microfluidic device. (Note that the PCIL was dissolved in water and a ~70 wt% PCIL solution was used to make the capsules. The water was then removed from the capsules when they were dried in situ). The CO2 solubility in the encapsulated [P2222][BnIm] was compared to the free PCIL. As shown in Figure 8, the uptakes for the encapsulated [P2222][BnIm] at all temperatures are in good agreement with the uptakes for the pure material. However, PCIL in the capsules absorbed more CO2 at the lower pressures, where solid-liquid-vapor equilibrium behavior is observed for the bulk free PCIL. The capsules


22

Page 23 of 32

were broken open after the solubility measurement, and the PCIL showed no sign of degradation, as confirmed by 1H NMR analysis. 1 0.9 0.8 mol CO2 / mole PCIL

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


0.7 0.6

60Cfree free[P2222][BnIm] [P2222][BnIm] 60°C

0.5


0.4


0.3

60Cencapsulated encapsulated[P2222][BnIm] [P2222][BnIm] 60°C

0.2


0.1


0 0

0.2


0.8

1

Figure 8. Solubility of CO2 in encapsulated [P2222][BnIm] at 60C, 70C, and 80C. The CO2 uptake has been corrected for physical uptake by the polymer shell material. The dashed lines are the fits of each isotherm (encapsulated PCIL) using eqn 8.

Recyclability of CO2 in neat [P2228][2CNPyr] and [P2222][BnIm] In order to study the regeneration capacity of the IL and PCIL and their suitability for long term use, [P2228][2CNPyr] and [P2222][BnIm] were subjected to recyclability tests using roughly 1 bar CO2 pressure. The recyclability tests of the neat ILs were done in the volumetric CO2 uptake apparatus described previously. The CO2 solubility measurements were conducted at 25C for [P2228][2CNPyr] and at 80C for [P2222][BnIm]. The regeneration process consisted of pulling vacuum at elevated temperatures after each absorption cycle. The IL [P2228][2CNPyr] was regenerated at 60C for 5 hours, and the PCIL [P2222][BnIm] was regenerated at 80C for 3 days. This regeneration process was not meant to mimic the actual process, where the temperature and/or pressure swing chosen would not


23


remove all of the CO2. Rather, it was meant as a “proof of concept” to determine whether or not the IL and PCIL could be reused. 1.2 mol CO2/mol IL

1

0.97

0.94

0.93

0.9

0.91

0.91

0.95

1

2

3

4 Cycle #

5

6

7

0.8 0.6 0.4 0.2 0

Figure 9. Recyclability of the CO2 uptake in [P2228][2CNPyr] at 25C 1

mol CO2/mol PCIL

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

Page 24 of 32

0.8

0.79

0.77

0.77

1

2

3

0.84

0.81

0.80

4

5

6

0.6 0.4 0.2 0 Cycle #

Figure 10. Recyclability of the CO2 uptake in [P2222][BnIm] at 80C The recyclability of CO2 uptake in [P2228][2CNPyr] and [P2222][BnIm] at each cycle are presented in Figures 9 and 10, respectively. Both [P2228][2CNPyr] and [P2222][BnIm] maintain practically the same CO2 uptake capacity each cycle; the variation in the mole ratio are less than 8% (for [P2228][2CNPyr]) and 6% (for [P2222][BnIm]) from the starting capacity and do not show any trend


24

Page 25 of 32 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


with cycle number. This confirms that the reaction of the IL and PCIL with CO2 is completely reversible. The recyclability test results of the neat [P2228][2CNPyr] and [P2222][BnIm] are as expected, since we have previously shown that the reaction of AHA ionic liquids with CO2 is reversible.17, 22 The IL and PCIL showed no sign of degradation, as confirmed by 1H NMR and 13C NMR analysis, results which are shown in Figures S12, S13, S16- 19 of Supporting Information. In addition, these results suggest that [P2228][2CNpyr] and [P2222][BnIm] may be suitable for long term use in CO2 capture technologies.

Recyclability of CO2 in the encapsulated [P2228][2CNPyr] and [P2222][BnIm]. The recyclability of CO2 in the encapsulated [P2228][2CNPyr] and [P2222][BnIm] was determined gravimetrically at 22°C and 80°C, respectively. To regenerate the capsules, following each absorption cycle, the system was evacuated under ultra-high vacuum at either 60C (for [P2228][2CNPyr]) or 90C (for [P2222][BnIm]) until the sample returned to its original mass, which indicated the regeneration was completed. Then the next absorption cycle was performed. As mentioned above, this regeneration process was not meant to mimic the actual process, where the temperature and/or pressure swing chosen would not remove all of the CO2. Rather, it was meant as a “proof of concept” to determine whether or not the encapsulated IL and PCIL could be reused. As shown in Figures 11 and 12, the capacity of encapsulated [P2228][2CNPyr] and [P2222][BnIm] were both stable upon cycling. The fact that the capsules maintain the same sustained level of CO2 uptake after cycling proves their outstanding performance and potential suitability for CO2 capture technologies. Both types of capsules were broken open after the recyclability tests and there was no degradation of the IL or PCIL, as confirmed by 1H NMR analysis, shown in Figures S14 and S15 of Supporting Information. From these experiments we have concluded that the reactions between the encapsulated [P2228][2CNPyr] and [P2222][BnIm] with CO2 are reversible, recyclable, and reproducible.


25


mole CO2/mole PCIL

1 0.8 Free[P2222][BnIm] [P2222][BnIm] Free

0.6

Cycle 1

0.4

Cycle 2 Cycle 3

0.2

Cycle 4 0 0

0.2


0.8

1

Figure 11. Recyclability of the CO2 uptake in the encapsulated [P2222][BnIm] at 80°C (PCIL content of capsule: 58 wt%) 1.0 mole CO2 /mole 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

Page 26 of 32

0.8 Free[P2228][2CNpyr] [P2228][2CNPyr] Free

0.6

Cycle 1

0.4

Cycle 2 Cycle 3

0.2

Cycle 4

0.0 0

0.2


0.8

1

Figure 12. Recyclability of the CO2 uptake in the encapsulated [P2228][2CNPyr] at 22°C (IL content of capsule: 40 wt%)


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 CONCLUSION Four ionic liquids (ILs) and phase-change ionic liquids (PCILs), [P2222][BnIm], [P2228][2CNPyr], [P2228][BnIm], and [P2228][6-BrBnIm], were investigated as potential post-combustion CO2 capture materials. All four compounds were shown to react reversibly at near stoichiometric ratios with CO2. Standard reaction enthalpies, as determined from new (temperature dependent) absorption isotherms, ranged from -46.6 to -51.3 kJ/mol for [P2228][2CNPyr], [P2228][BnIm], and [P2228][6-BrBnIm]. The standard enthalpy of reaction with CO2 can be tuned. For instance, here the reaction was made weaker by the addition of an electron withdrawing group on the anion. The encapsulation of one IL [P2228][2CNPyr], and one PCIL [P2222][BnIm], in PDMS polymer shells, which should enhance absorption and desorption kinetics due to high surface areas of the particles, did not result in any decrease in CO2 capacity. Not only are the free [P2228][2CNPyr] and [P2222][BnIm] stable upon cycling, the encapsulated [P2228][2CNPyr] and [P2222][BnIm] also showed excellent recyclability, retaining their high CO2 capture efficiency. Thus, we have demonstrated excellent performance of an encapsulated ionic liquid and a phase-change ionic liquid for postcombustion CO2 capture applications.

 ASSOCIATED CONTENT Supporting Information The Supporting Information file contains details on the NMR characterization for each IL or PCIL synthesized, melting points, tables of the CO2 uptake values, and densities and viscosities of [P2228][6-BrBnIm].  AUTHOR INFORMATION Corresponding Author *Tel: (512) 471-5092. Fax: (512) 471-1760. E-mail: [email protected].


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Notes The authors declare no competing financial interest.  AKNOWLEDGEMENT This material is based upon work supported by United States Department of Energy under Federal Award No. DE-FE0026465. We also acknowledge financial support from the Robert A. Welch Foundation (Grant F-1945) and stipend support for Tangqiumei Song from the KeatingCrawford Professorship at the University of Notre Dame. Finally, we thank Dr. Han Xia and Zhichao Chen for the melting point measurements.  REFERENCES 1. Figueroa, J. D.; Fout, T.; Plasynski, S.; McIlvried, H.; Srivastava, R. D., Advances in CO 2 capture technology—the US Department of Energy's Carbon Sequestration Program. International journal of greenhouse gas control 2008, 2 (1), 9-20. 2. Kenarsari, S. D.; Yang, D.; Jiang, G.; Zhang, S.; Wang, J.; Russell, A. G.; Wei, Q.; Fan, M., Review of recent advances in carbon dioxide separation and capture. Rsc Advances 2013, 3 (45), 22739-22773. 3. Samanta, A.; Zhao, A.; Shimizu, G. K.; Sarkar, P.; Gupta, R., Post-combustion CO2 capture using solid sorbents: a review. Industrial & Engineering Chemistry Research 2011, 51 (4), 1438-1463. 4. Leung, D. Y.; Caramanna, G.; Maroto-Valer, M. M., An overview of current status of carbon dioxide capture and storage technologies. Renewable and Sustainable Energy Reviews 2014, 39, 426-443. 5. Bhown, A. S.; Freeman, B. C., Analysis and status of post-combustion carbon dioxide capture technologies. Environmental science & technology 2011, 45 (20), 8624-8632. 6. Rochelle, G. T., Thermal degradation of amines for CO 2 capture. Current Opinion in Chemical Engineering 2012, 1 (2), 183-190. 7. Ramdin, M.; de Loos, T. W.; Vlugt, T. J., State-of-the-art of CO2 capture with ionic liquids. Industrial & Engineering Chemistry Research 2012, 51 (24), 8149-8177. 8. Fredriksen, S.; Jens, K.-J., Oxidative degradation of aqueous amine solutions of MEA, AMP, MDEA, PZ: A review. Energy Procedia 2013, 37, 1770-1777. 9. Gurkan, B. E.; de la Fuente, J. C.; Mindrup, E. M.; Ficke, L. E.; Goodrich, B. F.; Price, E. A.; Schneider, W. F.; Brennecke, J. F., Equimolar CO2 absorption by anion-functionalized ionic liquids. Journal of the American Chemical Society 2010, 132 (7), 2116-2117. 10. Gurkan, B.; Goodrich, B.; Mindrup, E.; Ficke, L.; Massel, M.; Seo, S.; Senftle, T.; Wu, H.; Glaser, M.; Shah, J., Molecular design of high capacity, low viscosity, chemically tunable ionic liquids for CO2 capture. The Journal of Physical Chemistry Letters 2010, 1 (24), 3494-3499. 11. Brennecke, J. F.; Gurkan, B. E., Ionic liquids for CO2 capture and emission reduction. The Journal of Physical Chemistry Letters 2010, 1 (24), 3459-3464.


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TOC Graphic:


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Recyclability of Encapsulated Ionic Liquids for Post-Combustion CO2

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