Phase-Change Ionic Liquids for Postcombustion CO2 Capture

Aug 5, 2014 - (5) Thus, development of innovative CO2 capture technologies is critical for maintaining fossil fuel as an affordable and relatively env...
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Phase-Change Ionic Liquids for Postcombustion CO2 Capture Samuel Seo, Luke D. Simoni, Mengting Ma, M. Aruni DeSilva, Yong Huang, Mark A. Stadtherr, and Joan F. Brennecke* Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: Phase-change ionic liquids, or PCILs, are salts that are solids at normal flue gas processing temperatures (e.g., 40− 80 °C) and that react stoichiometrically and reversibly with CO2 (one mole of CO2 for every mole of salt at typical postcombustion flue gas conditions) to form a liquid. Thus, the melting point of the PCIL−CO2 complex is below that of the pure PCIL. A new concept for CO2 separation technology that uses this key property of PCILs offers the potential to significantly reduce parasitic energy losses incurred from postcombustion CO2 capture by utilizing the heat of fusion (ΔHfus) to provide part of the heat needed to release CO2 from the absorbent. In addition, the phase transition yields almost a step-change absorption isotherm, so only a small pressure or temperature swing is required between the absorber and the stripper. Utilizing aprotic heterocyclic anions (AHAs), the enthalpy of reaction with CO2 can be readily tuned, and the physical properties, such as melting point, can be adjusted by modifying the alkyl chain length of the tetra-alkylphosphonium cation. Here, we present data for four tetrabutylphosphonium salts that exhibit PCIL behavior, as well as detailed measurements of the CO2 solubility, physical properties, phase transition behavior, and water uptake for tetraethylphosphonium benzimidazolide ([P2222][BnIm]). The process based on [P2222][BnIm] has the potential to reduce the amount of energy required for the CO2 capture process substantially compared to the current technology that employs aqueous monoethanolamine (MEA) solvents.

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INTRODUCTION Emission of CO2 is certainly one of the most significant anthropogenic sources of greenhouse gases, and concerns about its effect on climate change are ever increasing. In 2011, global emission of CO2 reached 34 billion tons, which is 3% higher than 2010 and the highest level on record.1 This clearly has an effect on the concentration of CO2 in the atmosphere: the concentration has increased from a preindustrial value of ∼280 ppm to 384 ppm in 2007,2 and finally, the level crept above 400 ppm in 2013 for the first time.3 A recent study by Huber et al. revealed that most (at least three-quarters) of the observed global warming is almost certainly due to human activity.4 While the search for renewable, clean sources of energy is increasing, fossil fuels and natural gas will continue to be the primary fuel and chemical feedstock sources for the next several decades owing to their high energy density, well-established infrastructure, and proven resource base.5 Thus, development of innovative CO2 capture technologies is critical for maintaining fossil fuel as an affordable and relatively environmentally benign energy resource. One of the most viable options for significantly reducing the level of CO2 emissions from existing coal-burning power plants is CO2 mitigation by postcombustion carbon capture. This technology is attractive because it can be retrofitted to existing coal-fired power plants without requiring substantial change in the combustion process. In postcombustion capture, the CO2 is removed after combustion of the fossil fuel with air, typically using a solvent or adsorbent that reversibly reacts with CO2. The material used in the postcombustion CO2 capture is directly linked to the performance and economics of the entire capture process. Although aqueous amine solutions, primarily monoethanol amine (MEA), show high reactivity, low cost, and relatively good absorption capacity (1 mol CO2 per 2 mol © XXXX American Chemical Society

amine), researchers continue to work on developing a replacement to overcome the drawbacks of aqueous amines, such as corrosion, oxidative degradation, and solvent loss due to evaporation, which eventually contribute to increased environmental pollution as well as extra cost of operation through solvent replacement.6−9 Another challenge is the large energy requirements associated with aqueous amine technology. Due to the large enthalpy associated with the MEA−CO2 reaction, as well as the presence of large amounts of water, approximately 30% of the energy generated from the power plant would have to be diverted to the CO2 capture process; this translates into a substantial increase in the cost of electricity.10−12 In an attempt to reduce the greenhouse gas emissions (CO2 being the major contributor), the U.S. Department of Energy (DOE) issued a carbon capture roadmap: to achieve at least 90% CO2 removal from an existing coal-fired power plant with no more than a 35% increase in the cost of electricity.13 In a typical postcombustion CO2 capture process utilizing aqueous MEA solutions, about two-third of the parasitic energy loss is solely due to the regeneration of the CO2 absorbent;14 thus, the development of better absorbents is essential to meet the goal of DOE. Within this context, ionic liquids (ILs) have been proposed as a potential candidate for replacing aqueous amine solutions used in postcombustion CO2 capture. ILs have many favorable characteristics over the current aqueous amine solutions, such as negligible volatility, high thermal stability, and nonflammability. In addition, the chemical tunability of ILs provides innumerable opportunities to design ILs with desirable Received: June 20, 2014 Revised: August 3, 2014

A

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characteristics suitable for CO2 capture.15−19 Recently, Gurkan et al. proposed ILs with aprotic heterocyclic anions (AHAs) that can react stoichiometrically and reversibly with CO2.20 Unlike other previously reported task-specific ILs for CO2 capture application, the viscosities of AHA ILs do not increase upon reaction with CO2. A subsequent study on AHA ILs with a trihexyl(tetradecyl)phosphonium ([P66614]+) cation demonstrated that the heterocyclic platform can provide countless possibilities for chemical functionalization for CO2 binding: the enthalpy of reaction with CO2 can be readily tuned to reduce the parasitic energy loss in the solvent regeneration step.21 In the course of developing AHA-based ILs, we serendipitously discovered some salts whose melting points when they were reacted with CO2 were significantly below the melting points of the unreacted salts. We call salts that exhibit this property phase-change ionic liquids (PCILs). Solid PCIL reacts with CO2 to form a PCIL−CO2 complex, whose melting point is sufficiently lower than that of the pure PCIL that a liquid is formed. The PCIL−CO2 complexes described here can be considered regular ILs, since their melting points are well below 100 °C. For a reaction temperature T, the reaction between PCIL and CO2 can be described by

equilibrium, thermophysical properties, and transport properties of the PCIL materials that we have developed. Specifically, we report on the synthesis and preliminary characterization (melting point (Tm), glass transition temperature (Tg), enthalpy of fusion (ΔHfus), and decomposition temperature (Tdec) in nitrogen and in air, and CO2 solubility as a function of pressure at 60 °C) of four different AHA PCILs containing tetrabutylphosphonium cations. Based on thermodynamic modeling (see below) and process modeling,28 it was determined that the ideal material for a postcombustion flue gas process needed to have a much higher melting point. An extensive synthesis and testing effort produced a very promising candidate: tetraethylphosphonium benzimidazolide ([P2222][BnIm]). Here, we report the following information about [P2222][BnIm]: (1) Tm, ΔHfus, and Tdec in nitrogen and air, (2) CO2 solubility as a function of temperature and pressure, (3) a thermodynamic model for the CO2 uptake (“uptake” is used interchangeably with “solubility”), (4) the ability to fully regenerate the PCIL, (5) the density and viscosity of the CO2 complexed PCIL liquid as a function of temperature, and (6) the water uptake of the solid and liquid PCIL. This last property is important because PCILs will absorb water vapor that is present in the flue gas during the postcombustion CO2 capture process. This can not only affect the physical properties of ILs29 but also interfere with the CO2 absorption chemistry.23 While the CO2 capacity of AHA-based ILs may not decrease when water is present,21 high contents of water in the PCIL could hinder the phase transition from liquid back to solid in the regeneration step, negating the energy reduction benefits of the PCIL process. Process modeling28 confirmed that a predryer unit that partially dehumidifies the incoming flue gas stream is not energy- or cost-prohibitive. Measurements of water uptake by [P2222][BnIm] were required to determine the maximum allowable partial pressure of water that still allowed resolidification of the PCIL in the regenerator when the CO2 was removed. The process models assembled from experimental phase equilibrium and thermophysical property data for [P2222][BnIm] confirmed that the PCIL process utilizing [P2222][BnIm] and a [P2222][BnIm]−CO2 slurry mixture (1-to-2 weight ratio) has the potential to reduce the amount of parasitic energy down to 23%, compared to 30% for the benchmark aqueous MEA system. This leads to a 28% reduction in the increase in the cost of electricity, which is the additional cost to the consumer for electricity for operation of a CO2 capture process on a conventional coal fired power plant. Detailed results may be found in the companion article.28

PCIL (Tm ,1 > T ) + CO2 1

2

⇄PCIL − CO2 (Tm ,3 < T ) 3

Tm ,3 < Tm ,1

(1)

Utilizing this unique phase transition ability, one can envision a postcombustion CO2 capture process involving PCILs that has the potential to require less parasitic energy compared to one using aqueous amines or ILs that remain liquid throughout the process. The idea, which is described in more detail below, is that the liquid complex would change back to a solid when the CO2 is removed in the regenerator. That phase-change releases heat, so the amount of energy that would have to be added to the regenerator would be reduced by the heat of fusion. The idea of using the heat of fusion to reduce the added energy in the regenerator for a CO2 absorption process is not new. Meljac et al. proposed adding a phase-change material to a CO2 absorbent solution to provide this function.22 However, to our knowledge, this is the first example of the absorbent itself being the phase-change material. Materials where the melting point of the material complexed with CO2 is higher than the original material are relatively common. For instance, we have observed that many amino acid−based ILs solidify upon reaction with CO2, presumably due to the formation of pervasive hydrogen-bond networks.23−26 GE has developed a CO2 capture process based on aminosilicone sorbents27 that undergo a phase transition from a low viscosity liquid to a friable solid upon CO2 absorption. The idea is that the solid could be easily compressed and the CO2 released at elevated pressure, thus reducing energy costs for compression to pipeline pressure. However, use of CO2 absorbents that phase-change from solid to liquid upon exposure to CO2 and that utilize the heat of fusion in the regeneration step is a novel idea that has not been explored previously. In this paper, we present a basic process concept for CO2 capture from postcombustion flue gas that takes advantage of the unique properties of PCILs. More details of the process design and results of testing in a laboratory-scale apparatus are presented in a companion paper.28 Here, we focus on the phase

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MATERIALS AND METHODS

Chemicals and Synthesis. All chemicals were purchased from commercially available sources and used without further purification. Tetrabutylphosphonium hydroxide (40% aqueous solution) was purchased from the Hikoko chemical group in Japan. Benzimidazole (99% purity) and pyrrole-2-carbonitrile (99%) were purchased from Alfa Aeser, and 6-bromo-1H-benzimidazole (97%) and 3-trifluoromethylpyrazole (99%) were purchased from Sigma-Aldrich. Tetrabutylphosphonium-based ILs ([P4444][AHA]) were prepared by mixing an equimolar amount of tetrabutylphosphonium hydroxide and the neutral AHA-H precursor (e.g., benzimidazole) and stirring for 24 h. The ratio of [P4444]+ to [AHA]− was confirmed to be 1 to 1 by integrating the peaks in the 1H NMR spectrum, obtained with a long relaxation delay time. Once the reaction was completed, the water byproduct was removed under reduced pressure while heating at 60 °C. Based on NMR spectroscopy, we estimate that the purity of the B

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Table 1. Structure, Abbreviation, and Full Name of Cations and Anions Studied

four [P4444]+ PCILs are greater than 98 mol %. The [P4444]+ cation and the four [AHA]− anions are shown in Table 1. Detailed NMR data are given in the Supporting Information. Tetraethylphosphonium benzimidazolide, or [P2222][BnIm], was prepared using either tetraethylphosphonium bromide ([P2222][Br]) purchased from Sigma-Aldrich (99%, now discontinued) or synthesized in our laboratory. [P2222][Br] was synthesized by reacting triethylphosphine (99%, VWR) and bromoethane (98%, SigmaAldrich) using the following procedure. Triethylphosphine (25 g; 0.211 mol) (which is pyrophoric and corrosive) was transferred to a round-bottom flask containing dry acetonitrile in a properly ventilated glovebox. Approximately 26 g (0.24 mol, in excess) of bromoethane was added and the mixture was refluxed under nitrogen for 36 h to produce [P2222][Br]. After completion of the reaction, acetonitrile and excess bromoethane were removed under reduced pressure. The resulting white solid was washed three times with hexane and the [P 2222 ][Br] was dried under vacuum while heating. Then, tetraethylphosphonium hydroxide (25 g; 0.11 mol) ([P2222][OH]) was prepared by the following procedure. [P2222][Br] (25 g; 0.11 mol) was dissolved in methanol (ACS grade, Fischer Scientific). Pretreated 300 to 500 g of Amberlite IRN-78(OH form) ion-exchange resin (Dow Chemical Company) was added to the solution of [P2222][Br], and the mixture was gently agitated for several hours. The resin was separated by filtration; then, a small amount of the solution was tested for residual bromide by addition of several drops of a 5% w/v aqueous silver nitrate solution. If the solution was positive for bromide, more Amberlite IRN-78(OH form) resin was added and the procedure described above was repeated until the solution did not show any precipitation upon addition of silver nitrate. An equimolar amount (0.11 mol) of benzimidazole was then added to the [P2222][OH] solution in methanol and stirred overnight. The equimolar ratio of [P2222]+ to [BnIm]− was confirmed by integrating the peaks in the 1H NMR spectrum, obtained with a long relaxation delay time. The product was dried under reduced pressure at approximately 60 °C. Based on NMR spectroscopy, we estimate that the purity of the [P2222][BnIm] is greater than 98 mol %. The [P2222]+ cation and [BnIm]− anion are shown in Table 1. NMR data for the neat and CO2saturated sample are given in the Supporting Information.

All gases were obtained from Praxair. For the measurements of CO2 solubilities, CO2 had a purity of 99.995%, while CO2 with 99.99% purity was used for the viscosity measurements of the CO2-saturated sample of [P2222][BnIm]. Nitrogen gas used in the water uptake apparatus was Praxaire grade 4.8, 99.998%, H2O < 3 ppm. High-purity, deionized water from a Milli-Q Integral Water Purification System, or Millipore (>18 MΩ·cm resistivity, < 5 ppb total organic carbon), was used for all the experiments. Characterization. Prior to study all PCILs were dried under vacuum at 60 °C for at least 48 h. Water contents of the liquid PCIL− CO2 complexes were determined using a Metrohm 831 Karl Fischer coulometer with ±3 μg water resolution. For solid samples or liquid samples containing more than 1 wt % water, water contents were determined via a Mettler Toledo V20 Volumetric Karl Fischer (VKF) Titrator. The VKF has an uncertainty of ±0.05%, and it has the best performance when the water content in the sample is 5 to 10 mg. Phase transition properties such as melting points (Tm), glasstransition temperatures (Tg), and enthalpy of fusion (ΔHfus) were measured by a Mettler-Toledo differential scanning calorimeter (DSC), model DSC822e. The uncertainties in the DSC temperature measurements are about ±0.3 °C when a standard sample is evaluated, but the presence of trace impurities increases the estimated reproducibility to ±1 °C. The estimated uncertainty in the ΔHfus measurements is approximately ±4%. The density measurements were performed at atmospheric pressure with a DMA 4500 Anton Paar oscillating U-tube densitometer. The temperature was controlled by two integrated Pt 100 platinum thermometers with a precision of ±0.01 °C. The uncertainty in the density measurements is ±5 × 10−5 g cm−3, but it is approximately ±1 × 10−3 g cm−3 when the purity of the PCILs is taken into account. The viscosity of the CO2-saturated PCIL was measured with an ATS Rheosystems Viscoanalyzer equipped with a cone-and-plate spindle. The parallel sample plate that holds the PCIL sample was constantly purged with dry CO2 gas to prevent water uptake from the atmosphere and to keep the sample saturated with CO2. Measured viscosities have an uncertainty of ±5% above 100 cP and ±10% between 50 cP and 100 cP. CO2 Solubility Measurements. The CO2 solubility in the PCIL was determined using a volumetric method. The system is primarily C

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temperature was adjusted with a Binder APTline ED (EZ) 115 μL heating oven that contains unit B. The saturated water vapor pressure was determined using the Yaws correlation:31

composed of two parts: a CO2 reservoir (290 mL) that holds a known amount of CO2 and a reaction vessel (235 mL) that contains the PCIL sample. Both parts of the apparatus are kept inside a YAMATO DKN602 oven that controls the temperature at ±0.1 °C from 22 to 100 °C. Prior to the CO2 solubility measurements in the PCILs, the solid samples were crushed into fine particles in the glovebox filled with nitrogen gas, in order to enhance the kinetic rate of absorption. Once the crushed samples were loaded into the reaction vessel, the oven was set to a desired temperature at which CO2 solubility was measured. Once the temperature reached the set temperature, the cell was evacuated to a pressure of less than 5 × 10−3 bar. Then, CO2 was introduced into the reaction vessel by briefly opening the valve connected to the reservoir. Vigorous stirring was achieved with an Autoclaves MagneDrive III stirrer. When mechanical stirring was activated to mix the PCIL, the pressure in the cell started to drop, confirming absorption of CO2 by the sample. The pressure in the stirred cell was recorded periodically until equilibrium was reached (confirmed by constant pressure in the vessel). Then, the amount of CO2 absorbed by the sample was determined from the total pressure drop in the stirred cell after the addition of CO2 using the idea gas law with the Lee−Kesler correlation.30 This procedure was repeated until the equilibrium pressure was near 1 bar. Water Uptake Measurements. The solid−vapor equilibrium (SVE) and vapor−liquid equilibrium (VLE) for [P2222][BnIm] and water vapor were obtained using a custom-made apparatus depicted in Figure 1. This system consists of a nitrogen saturation unit (A) that

ln(P) = A −

B 2 − Cln(T̆ ) + DT̆ + ET̆ T̆

(2)

where T̆ = T/1000 (T in Kelvin), P in bar, and the constants for water are A = 11.684, B = 7.2582, C = 7.3037, D = 0, and E = 4.1654. To begin the experiment, the temperature of the bath surrounding the diffuser (0.5 to 60 °C) was allowed to equilibrate along with the temperature of the PCIL sample (0.5 g) in the oven (40 to 100 °C). After the equilibrium temperatures were reached, nitrogen gas was diffused through deionized water at a flow rate of 0.1 L min−1, which we determined is sufficiently slow to reach full saturation. Once the nitrogen gas was fully saturated with water at the respective saturated water vapor pressure, the gas was directed to the double-neck flask containing the PCIL and flowed over the PCIL for 24 h to ensure that equilibrium was reached. The water vapor saturated PCIL sample in the flask was then transferred to the Volumetric Karl Fischer (VKF), where the water content was determined based on total sample mass titrated. The measurement was conducted with a glovebag filled with nitrogen gas around the VKF to prevent any additional water or CO2 absorption from the atmosphere, which could affect the mass and overall water content. The temperature in the water bath is ±0.2 °C, which leads to an uncertainty of ±2 mbar for the saturated water vapor pressures. The oven temperature fluctuates ±2 °C, which results in a repeatability of the experimental water uptake values of approximately 0.056 wt % (or 560 ppm).

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RESULTS AND DISCUSSION Process Concept. The postcombustion CO2 capture process utilizing ILs that involve phase-change has two main operation units, as outlined in Figure 2. The final design28 uses solid PCIL slurried with some of the PCIL−CO2 liquid complex flowing into the top of the absorber. Upon reaction with CO2 in the flue gas as it moves down the column, the PCIL would become a clear liquid, coalescing in the bottom of the column. During this step, the enthalpy of reaction (ΔHrxn, exothermic heat generated due to chemical reaction between the PCIL and CO2) typically increases the temperature in the absorber, so that the unit requires cooling in a typical CO2 capture process to maintain the temperature. However, the process involving PCILs would require a smaller amount heat to be removed because part of the heat released from the ΔHrxn has to be used to liquefy the PCIL. The liquid PCIL−CO2 complex is then pumped to a spray tower after passing through

Figure 1. Schematic of water uptake apparatus. generates a nitrogen gas stream saturated with water and a doubleneck flask (B) that contains the PCIL sample. The vapor pressure of water in the nitrogen gas stream was controlled by adjusting the temperature of the water bath around unit A, and the water absorption

Figure 2. General process concept for CO2 capture with PCIL. D

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Table 2. Thermal Transition Properties and Thermal Stability of PCILs Tdec (°C) PCILs

Tm (°C)

[P4444][BnIm] [P4444][6-BrBnIm] [P4444][2-CNPyr] [P4444][3-CF3Pyra] [P2222][BnIm]

71 69 −14/24/51 21/45 166

Tg (°C) −50 −68

N2

air

−10.4 −13.1 −6.8/−7.2/−11.2 −3.8/−9.2 −19.9

318 325 308 278 314

310 308 293 273 311

measured by TGA, which is consistent with the trend shown by imidazolium salts.34 While all four [P4444][AHA] PCILs readily changed phase from solid to liquid upon reaction with CO2 (as will be discussed further in the following section), the melting points in their pure states are not high enough to offer a practical operating temperature range during the proposed PCIL process. Thermodynamic modeling (discussed below) also shows that a higher PCIL melting point will provide a larger operating pressure range for the stripper. In an attempt to develop ILs with higher melting point, we synthesized [P2222][BnIm]: the smaller, more symmetric [P2222]+ cation contributes to higher melting points, and the [BnIm]− anion offers a significant CO2 solubility even at relatively high absorber temperatures. Several key physicochemical properties of [P2222][BnIm] including melting point, fusion enthalpy, and decomposition temperature are also summarized in Table 2. [P2222][BnIm] has a melting point of 166 °C, which is nearly 100 °C higher than the [P4444]+ counterpart. The magnitude of the measured ΔHfus is approximately 40% of the estimated ΔHrxn. Since the CO2 is reacting with the anion, we anticipate that the ΔHrxn for different phosphonium salts with the same anion will be similar. In previous work,21 we determined that the ΔHrxn of [P66614][BnIm] with CO2 is approximately −52 kJ mol−1, so we expect a similar value for [P2222][BnIm]. Assuming this value for ΔHrxn and CO2 solubility of close to 1 mol CO2 per mole of PCIL, one would anticipate a substantial reduction in the heat duty in the stripper because of the ΔHfus. Similar to other AHA ILs with phosphonium cations such as [P66614]+ or [P4444]+, [P2222][BnIm] is thermally stable under both N2 and air (Tdec > 300 °C), which allows regeneration at higher temperatures than can be used with aqueous amines, if needed. CO2 Solubility and Phase Transitions. CO2 solubility in all [P4444]+ PCILs was measured at 60 °C, and the CO2 solubility isotherms expressed in mole ratio of CO2 per IL as a function of CO2 pressure are presented in Figure 3. The data can be found in tabular form in the Supporting Information. At 60 °C, both [P4444][BnIm] and [P4444][6-BrBnIm] were solid, but they quickly turned to liquid upon reaction with CO2. Once fully saturated with CO2, none of these PCILs turned back to a solid state even after they were cooled down to room temperature, indicating a notable temperature difference between the melting point of the pure PCIL (Tm = 71 and 69 °C, respectively) and the PCIL−CO2 complex. Each PCIL is paired with an AHA that is capable of reacting stoichiometrically with CO2 as described in Figure 4, while some variations in CO2 capacity are expected depending on the basicity of the anion. It is clearly seen that the CO2 capacity, as well as the sharpness of the isotherms, correlate well with the ΔHrxn of the anion with CO2. The decreasing order in CO2 solubility ([P4444][BnIm] > [P4444][6-BrBnIm] > [P4444][2-CNPyr] > [P4444][3-CF3Pyra]) is consistent with the decreasing magni-

a heat exchanger. In the regenerator (essentially a spray tower stripper), heat is added to drive off the CO2 as the liquid PCIL−CO2 complex is sprayed from the top. As the PCIL− CO2 complex releases CO2, the PCIL (now minus the CO2) solidifies, releasing heat equivalent to the enthalpy of fusion (ΔHfus). Thus, part of the energy required to overcome the enthalpy of the PCIL−CO2 reaction (i.e., to reverse the PCIL− CO2 binding reaction) is supplied by the ΔHfus released by the PCIL, as it solidifies in the regenerator: Heat load for regeneration ∝ (ΔHrxn − ΔHfus)

ΔHfus (kJ mol−1)

(3)

In general, the solvent regeneration is the most energyintensive step in any CO2 capture process, and significantly less external heat will be required for the PCIL process by taking advantage of the ΔHfus. The partially regenerated PCIL (only partially regenerated in order to make the slurry required for introduction into the top of the absorber) is collected in the bottom of the spray tower, then cooled through a heat exchanger where the sensible heat can be used to preheat the incoming PCIL−CO2 complex liquid stream from the absorber. More details of the process concept and modeling can be found in a companion article.28 Characterization of PCILs. Understanding basic thermophysical properties of absorbents is essential for design and evaluation of the PCIL CO2 capture process. Especially, one must know melting points or glass transition temperatures to set the feasible operating temperature range for the PCIL. Four tetrabutylphosphonium-based ([P4444]+) ILs were synthesized specifically for CO2 capture involving phase-change. Each PCIL has a basic AHA that can reversibly react with CO2. The cation does not contribute significantly to the CO2 absorption capacity, but it is primarily responsible for physical properties such as melting point, viscosity, and thermal stability. Phosphonium ILs are preferred over ILs with ammonium or imidazolium cations due to their superior thermal stability and higher stability in strongly basic conditions.32 Additionally, their production on the multiton scale has been proven to be both technologically and economically feasible.33 In contrast to the [P66614][AHA] ILs reported recently,21 all [P4444][AHA] PCILs were solid at room temperature, owing to the smaller and more symmetric nature of the cation, which leads to increased lattice energies and melting points. Detailed physical properties including melting points are summarized in Table 2. [P4444][2-CNPyr] and [P4444][3-CF3Pyra] exhibit multiple melting transitions, which we attribute to multiple crystal structures. All four [P4444][AHA] PCILs have Tdec above 300 °C except for [P4444][3-CF3Pyra]. This is consistent with previous studies, which reported that the phosphonium ILs with pyrazolide-based anions are less stable compared to those with other AHAs.21 The presence of an oxidizing atmosphere (O2) in air, rather than an inert one (N2), did not have a significant impact on the decomposition temperature, as E

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Figure 3. CO2 solubility in [P4444][AHA] PCILs at 60 °C. Figure 5. CO2 solubility in [P2222][BnIm] at 60, 70, and 80 °C.

Figure 4. Schematic of equimolar [P2222][BnIm]−CO2 reaction.

tude of the ΔHrxn reported for each of these anions in our previous study (−52 > −48 > −45 > −44 kJ mol−1, respectively).21 The CO2 solubility in [P2222][BnIm] was measured at three different temperatures: 60, 70, and 80 °C. At temperatures of 50 °C or below, [P2222][BnIm] absorbed a significant amount of CO2 but did not fully liquefy upon exposure to pressures up to 1 bar of CO2. These experiments have to be scrutinized carefully because the sample did exhibit some reversible reaction with CO2, presumably on the solid surface. Since the reaction is exothermic, this can cause local heating, which can make the sample appear to be partially liquid. The isotherms at 50 °C and below are not reported here since slow heat and mass-transfer in the solid samples prevented any guarantee that true phase equilibrium had been reached. Upon reaction with CO2 at 60 °C or higher, however, the sample readily became a clear liquid, and the melting point was replaced by a glass transition temperature of −38 °C (as determined by DSC) when the PCIL was fully saturated with 1 bar CO2 at 60 °C. The CO2 solubility isotherms for 60, 70, and 80 °C are shown in Figure 5 and available in Table S2 of Supporting Information. At 70 °C, as CO2 is introduced into the reaction cell that contains the [P2222][BnIm] sample, the sample initially forms a slurry (Figure 6b); this corresponds to a gradual increase in the isotherm at lower pressures ( P*.

solid phase is consumed and the liquid phase grows. This condition corresponds to the green X in Figure 7, which will move from the pure PCIL axis to the left until it reaches the phase boundary (blue dot). At the point, the CO2 solubility due to the chemical reaction, Ychem (moles CO2 per mole total PCIL, complexed and uncomplexed) is Ychem = x*3 (T) = 1 − x1*(T). Once the solid is consumed, the pressure may increase and we have VLE (blue X in Figure 7). As the pressure is increased, there will be additional CO2 uptake due to both chemical reaction and physical absorption. An expression for P* follows directly from the reaction equilibrium relation (expressing the reaction in terms of liquid phase PCIL and PCIL−CO2) and P* is determined from this equation: P*(T ) =

x3*(T ) x1*(T )K rxn(T )

=

1 − x1*(T ) x1*(T )K rxn(T )

Figure 8. Typical PCIL isotherms for idealized model, for the parameter values ΔHfus = −20 kJ mol−1, Tm,1 = 100 °C, ΔHrxn = −50 kJ mol−1, ΔSrxn = −130 J mol−1 K−1, ΔHphys = −13 kJ mol−1, ΔSphys = −73 J mol−1 K−1.

mol−1 K−1, ΔHphys = −13 kJ mol−1, ΔSphys = −73 J mol−1 K−1. This is an illustrative example; the parameters do not correspond to any particular PCIL but are within the range of typical experimental values. Note that the P*(T) locus (solid black curve) passes through a maximum as temperature increases. This maximum corresponds to the maximum operating pressure in the spray dryer. The experimental observations and CO2 solubility results for the [P4444]-based PCILs (Figure 4) suggest a very low phase transition pressure P*, with uptake measurements corresponding to the VLE regime. This would require a very low and impractical operating pressure in the spray dryer to regenerate the solid. To increase P*, eqs 4 and 5 indicate that one should aim to increase the PCIL melting point Tm,1 and/or to make ΔHfus more negative. The choice of [P2222][BnIm] satisfies both goals, and indeed, the experimental observations and CO2 solubility for [P2222][BnIm] (Figures 5 and 6) indicate a transition to VLE at a higher and more practical pressure. The CO2 solubility results for [P2222][BnIm] show that SVLE exists over a range of pressure, not just at a single pressure P* as in the idealized model. The additional degree of freedom may be due to the presence of an additional component, a “deactivated” form of the PCIL (present in the solid and liquid phases) or impurity (the [P2222][BnIm] is estimated as 98% pure) that does not react with CO2, as also suggested in previous studies.21,23,24 On this basis, we have developed a semiempirical model, as described in the Supporting Information. The model curves in Figure 5 are based on this semiempirical model, with the slope discontinuity corresponding to the transition from SVLE to VLE. It is clear that this model can estimate the conditions at which the transition to VLE occurs and can also provide a good fit to the solubility data in both the SVLE and VLE regions. However,

(4)

where x1*(T) is obtained from the equifugacity relationship for PCIL in the solid and liquid phases ⎡ −ΔH ⎛ ⎤ 1 1⎞ fus ⎜⎜ x1*(T ) = exp⎢ − ⎟⎟⎥ ⎢⎣ R ⎝ Tm,1 T ⎠⎥⎦

(5)

with Tm,1 indicating the melting point of pure PCIL. Krxn(T) is obtained from ⎡ ΔS 0 ⎤ ⎡ −ΔH 0 ⎤ rxn ⎥ exp⎢ rxn ⎥ K rxn(T ) = exp⎢ ⎣ R ⎦ ⎣ RT ⎦

ΔH0rxn

(6)

ΔS0rxn

where and are the standard molar enthalpy and entropy changes of reaction (assumed to be temperature independent). For the VLE regime, a Langmuir−Henry-type isotherm model such as used previously20,21,23,24 is adopted, with the main exception that the composition scale used for the Henry’s Law term (physical absorption) is the mole ratio of CO2 to total PCIL (complexed and uncomplexed), not the mole fraction CO2. This introduces an equilibrium constant (inverse of Henry’s Law constant) for physical absorption: ⎡ −ΔH 0 ⎤ ⎡ ΔS 0 ⎤ phys ⎥ exp⎢ phys ⎥ K phys(T ) = exp⎢ ⎢⎣ RT ⎥⎦ ⎢⎣ R ⎥⎦

(7)

The end result for this simple, idealized model is an expression for CO2 solubility given by G

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because the model is semiempirical, we do not ascribe any physical significance to the model parameter values (see Supporting Information). Regeneration of the PCIL. CO2 was fully desorbed from [P2222][BnIm] at 70 °C using vacuum, confirming the reversibility of the [P2222][BnIm]−CO2 reaction described in Figure 4. First, the sample was fully saturated with CO2 at 1 bar and 70 °C; then, CO2 was desorbed from the [P2222][BnIm]− CO2 complex by reducing the pressure in the reaction vessel using a vacuum pump (Welch Gem 8890). In order to determine the pressure at which the first solid particles appeared, the pressure in the vessel was reduced in steps of approximately 0.1 bar, allowing a minimum of 2 h to reach equilibrium between the sample and the gaseous CO2. There was no apparent physical change until the partial pressure of CO2 was reduced to 0.1 bar (Figure 9a). The first clear sign of

Figure 10. Density of CO2-saturated [P2222][BnIm] as a function of temperature.

ρmixture =

1 1 = ∑ xiVi Vmixture

(9)

where xi = mole fraction and Vi = specific molar volume in mol cm−3. As shown in Figure 10, the density of CO2-saturated [P2222][BnIm] shows a linear relationship with temperature. A linear fit was used to project the density data to higher temperatures, which are indicated by open circles. The smaller size of [P2222]+ caused enhanced packing and increased density relative to [P66614][BnIm], which has a bulkier and more flexible cation (ρ = 0.924 g mL−1 at 22 °C, pure IL).21 Viscosity. The viscosity of the CO2 absorbent is an important physical property that can directly impact the pumping cost; thus, the dynamic viscosity of the CO2complexed sample was determined as a function of temperature. The sample was prepared by saturating [P2222][BnIm] with CO2 at 70 °C, and the viscosity was measured under one bar of CO2 pressure to keep the sample saturated with CO2 and to prevent absorption of water from the atmosphere. The CO2complexed sample remained a liquid during the measurements, and the results are shown in Figure 11 (circles). The sample

Figure 9. Reverse phase-change during the desorption process for [P2222][BnIm] at 70 °C under reduced pressure. (a) At 0.1 bar CO2. (b) At 0.03 bar CO2. (c) After full desorption and cooled to room temperature.

precipitation was observed when the pressure was reduced to about 0.03 bar (Figure 9b). The sample was desorbed further at 70 to 80 °C) significant amounts of water can be tolerated in the system and still allow the desired phase-change (from liquid to solid) to occur when the CO2 is desorbed from the [P2222][BnIm]−CO2 complex. In fact, the limit on the partial pressure of water in the spray dryer ended up not being the determining factor in the design of the flue gas predryer system. Another effect of the flue gas predryer is that it increases the pressure of the flue gas, thus increasing the partial pressure of CO2 in the absorber. This turned out to be a more important criterion in determining the operating conditions for the flue gas predryer.28

Experimental Tg value from DSC.

In the proposed PCIL process, the PCIL will actually be fed to the absorber in the form of a slurry, consisting of a physical mixture of CO2-saturated PCIL (liquid) and PCIL (solid). As a result, the CO2-saturated PCIL (liquid) only has to be partially desorbed in the spray dryer to produce a slurry mixture, which is then recycled to the absorber. This scheme was chosen to ease the circulation of the absorbent down the column as well as accelerate the diffusion of CO2 into the absorbent in the absorber. Slurries were prepared by mixing liquid CO2− [P2222][BnIm] complex with pure [P2222][BnIm] particles in a 2-to-1 weight ratio. The viscosity of this mixture was measured from 60 to 90 °C (squares in Figure 11). The mixture became solid-like around 50 °C; therefore, the viscosity measurements were not possible at or below this temperature. The viscosity of the 2-to-1 mixture decreases rapidly with increasing temperature and is comparable to the CO2-saturated [P2222][BnIm] liquid at temperatures above 70 °C: the viscosity of the mixture is expected to be below 100 cP at or above 70 °C, which should provide good operability in the laboratory scale unit and the full-scale system. Effect of Water. ILs are generally hygroscopic and expected to easily and rapidly absorb water from flue gas. This is certainly true for [P2222][BnIm], as mentioned above in the section on viscosity, where the water content of the sample increased during the experiments even when being purged with dry CO2 gas. Here, we quantify the water absorption as a function of temperature and water partial pressure. Of particular concern for the PCIL process is that the absorbed water will prevent solidification of the PCIL in the spray dryer when the amount

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CONCLUSIONS A new concept for postcombustion CO2 capture technology that uses phase-change ionic liquids (PCILs) has been explored for the first time. Here, we present results for five different PCILs, all of which have tetra-alkylphosphonium cations and aprotic heterocyclic anions (AHA), which react stoichiometriI

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cally with CO2. Of the PCILs developed, [P2222][BnIm] is a promising candidate for the proposed CO2 capture process as it readily phase changes with addition of CO2 at temperatures at or above 60 °C, which is more than 100 °C lower than its melting temperature. In addition to its suitable enthalpy of reaction value and high CO2 capacity, utilizing its enthalpy of fusion provides the potential to substantially reduce the parasitic energy load in the absorbent regeneration step. In addition to measurements of PCIL melting points and enthalpies of fusion, we have also presented CO2 solubility data and a thermodynamic model for the CO2 take isotherms. Density, viscosity, and water uptake information have also been presented for [P2222][BnIm]. Process modeling based on the experimental phase equilibrium and thermophysical property data presented in this manuscript is presented in a companion paper. It verifies that the novel PCIL CO2 capture process is significantly more energy-efficient than the conventional aqueous MEA process.

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ASSOCIATED CONTENT

* Supporting Information S

Details on the NMR characterization for each IL synthesized. All density, viscosity, water uptake, and CO2 solubility data in tabular form. Details about the semiempirical PCIL thermodynamic model for CO2 uptake isotherms. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (574) 631-5847. Fax: (574) 631-8366. Email: jfb@nd. edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the Department of Energy ARPAe under Award No. AR0000094. REFERENCES

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