CO2 Capture Efficiency, Corrosion Properties, and Ecotoxicity

Article pubs.acs.org/IECR

CO2 Capture Efficiency, Corrosion Properties, and Ecotoxicity Evaluation of Amine Solutions Involving Newly Synthesized Ionic Liquids Xenophon L. Papatryfon,†,‡ Nikolaos S. Heliopoulos,‡ Igor S. Molchan,⊥ Lawien F. Zubeir,¶ Nathan D. Bezemer,¶ Michalis K. Arfanis,‡ Athanassios G. Kontos,‡ Vlassis Likodimos,‡,∥ Boyan Iliev,# George Em. Romanos,*,‡ Polycarpos Falaras,‡ Kostas Stamatakis,§ Konstantinos G. Beltsios,† Maaike C. Kroon,¶ George E. Thompson,⊥ Jessica Klöckner,# and Thomas J. S. Schubert# †

Department of Materials Science and Engineering, University of Ioannina, Ioannina 45110, Greece Department of Physical Chemistry, Institute for Advanced Materials, Physicochemical Processes, Nanotechnology, and Microsystems (IAMPPNM) and §Institute of Biosciences and Applications, NCSR “Demokritos”, Aghia Paraskevi Attikis, Athens 153 10, Greece ⊥ School of Materials, The University of Manchester, Manchester M13 9PL, U.K. ¶ Separation Technology Group, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, Den Dolech 2, Eindhoven 5600 MB, The Netherlands ∥ Solid State Physics Department, Faculty of Physics, University of Athens, Panepistimioupoli, Athens 157 84, Greece # IOLITEC, Ionic Liquids Technologies GmbH, Salzstraße 184, Heilbronn D-74076, Germany ‡

S Supporting Information *

ABSTRACT: The CO2 capture efficiency of nine newly synthesized ionic liquids (ILs), both in their pure states as well as in binary and ternary systems with water and amines, was investigated. The study encompassed ILs with fluorinated and tricyanomethanide anions as well as ILs that interact chemically with CO2 such as those with amino acid and acetate anions. Compared to amines, some of the novel ILs exhibited a majority of important advantages for CO2 capture such as enhanced chemical and thermal stabilities and negligible vapor pressure; the previous features counterbalance the disadvantages of lower CO2 absorption capacity and rate, making these ILs promising CO2 absorbents that could partially or totally replace amines in industrial scale processes. In addition to their ability to capture CO2, important issues including corrosivity and ecotoxicity were also examined. A thorough investigation of the capture efficiency and corrosion properties of several solvent formulations proved that some of the new ILs encourage future commercial-scale applications for appropriate conditions. ILs with a tricyanomethanide anion confirmed a beneficial effect of water addition on the CO2 absorption rate (ca. 10-fold) and capacity (ca. 4-fold) and high efficiency for corrosion inhibition, in contrast with the negative effect of water on the CO2 absorption capacity of ILs with the acetate anion. ILs with a fluorinated anion showed high corrosivity and an almost neutral effect of water on their efficiency as CO2 absorbents. ILs having amino acid anions presented a reduced toxicity and high potential to completely replace amines in solutions with water but, in parallel, showed thermal instability and degradation during CO2 capture. Tricyanomethanide anion-based ILs had a beneficial effect on the capture efficiency, toxicity, and corrosiveness of the standard amine solutions. As a consolidated output, we propose solvent formulations containing the tricyanomethanide anion-based ILs and less than 10 vol % of primary or secondary amines. These solvents exhibited the same CO2 capture performance as the 20−25 vol % standard amine solutions. The synergetic mechanisms in the capture efficiency, induced by the presence of the examined ILs, were elucidated, and the results obtained can be used as guidance for the design and development of new ILs for more efficient CO2 capture.

1. INTRODUCTION

solvent volumes induce a variety of problems related to the size and capital cost of the capture plant, and the concomitant parasitic energy loss to heat cools inert construction materials. Furthermore, the toxicity of the used chemicals defines their maximum permissible concentration limits in the gas stream that escapes from the top of the scrubbing column.2 Low limits

In the design of an efficient CO2 capture process that employs mass transfer columns, the CO2 absorption capacity of the chemicals involved as absorbents is the definitive factor for the estimation of the flow rate of the solvent, which, in turn, is the basis for all further calculations related to the column dimensioning and packing.1 However, there are additional important properties of the absorbent that must be considered. For instance, high corrosiveness is usually confronted by high dilution in water. In that way, huge solvent amounts are necessary in order to achieve efficient CO2 capture. Enhanced © 2014 American Chemical Society

Received: Revised: Accepted: Published: 12083

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amines were monoethanolamine (MEA), diethanolamine (DEA), and methyl-diethanolamine (MDEA). The investigated ILs included the following: four members from the homologous series of 1-alkyl-3-methylimidazolium tricyanomethanides ([RMIM][TCM], with R = ethyl, butyl, hexyl, octyl); two amino acid anion-based ILs, 1-ethyl-3-methylimidazolium lysinate and serinate ([EMIM][LYS] and [EMIM][SER]); two lactam cation-based ILs, pyrrolidium-2-one bis(trifluoromethylsulfonyl)imide and pyrrolidium-2-one trifluoroacetate ([BHC][BTA] and [BHC][TFA]); and one IL with an acetate anion, 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]). The CO2 absorption capacity and kinetics of the dry and aqueous ILs at several water contents, temperatures, and pressures up to 1 bar, were measured with the gravimetric technique. The density and viscosity of the ILs and their mixtures with water and the excess molar volumes of the mixtures were also defined. The CO2 capture tests were performed in a lab-scale device that involved a mass transfer column with dumped packing and a specially designed regenerator, where the solvent was heated via a Ni-brazed, plate heat exchanger and thermal fluid. To explore the corrosion properties of the applied solvents, mild steel (MS) specimens were laid on the bottom of the regenerator vessel and were left immersed into the hot fluid under aggressive conditions (high temperature, high O2 and CO2 content) during the entire experimental period. Because of the low cost, the use of MS might be the preferable choice for building CO2 capture installations provided that the corrosion processes could be effectively alleviated. In a previous study by the authors,9 the corrosion mechanism of MS in [RMIM][TCM]s and [RMIM][TCM]−H2O mixtures was elucidated by immersion tests in autoclavable plastic bottles at 80 °C for periods ranging from 1 h to 10 days. In this work, the corrosion tests on MS were conducted under real conditions that encompassed the involvement of amines, solvent flow, higher temperatures, and the presence of CO2 and O2 in the reaction medium. Finally, the ecotoxicity of the tested solvent formulations was evaluated by monitoring the culture growth of the green algae Chlamydomonas reinhardtii in terms of chlorophyll concentration. While ILs pose little threat of airborne toxicity, they would likely enter aquatic ecosystems in the case of an accidental release, and because many of them are water-soluble, they are likely to transport readily through the environment and contact the biota. Typically phytoplankton, a dominant primary producer in aquatic food chains, is more sensitive toward pollution than is zooplankton.10,11 Thus, the algal growth inhibition test has already been widely used to determine the effects of several pollutants on algal species including polycyclic aromatic hydrocarbons (PAHs),12 microcystins,13 phenols,14 glutaraldehydes,15 and heavy metals.16 However, little work has been reported on the toxic effects that are induced on algal primary producers by ILs17−19 and other chemicals (amines) or mixtures of ILs with amines that were developed with the target of CO2 capture applications.

in combination with high fugacity increase the complexity of the entrainment separation and gas washing processes as well as the required amounts of water for cooling. Another important issue relates to the effects of leached substances on the CO2 capture efficiency and stability of the solvents. Corrosion results in the transfer and dissolution of material from the metallic parts of the capture units into the solvent. For instance, leached ferrous ions catalyze the oxidation of monoethanolamine (MEA) in the presence of oxygen and carbon dioxide.3,4 On the other hand, ionic liquids (ILs) exhibit very low vapor pressure, which offers the potential to meet most of the challenges related to evaporative losses and slippage into the atmosphere. It is also known that most ILs are capable of physical CO2 absorption with excellent CO2−N2 separation. As a consequence, it is highly probable that solvent degradation can be avoided. (For IL degradation considerations see, for example, the work by LeFrate et al.5) Additional advantages are the thermal and chemical stabilities of ILs in an oxidative or acidic gas environment, which minimize the need for continuous solvent replenishment and the use of antioxidant additives that in most cases impair the capture efficiency. The enhanced chemical stability of several ILs under the conditions of a CO2 capture process might be regarded as an indirect way to confront the major problem that is their high costs. Contrary to aqueous solutions of amines, solvent formulations containing certain types of ILs would not need to be replenished frequently. In this way, the operation cost is reduced and the process undergoes extra time of operation without the need for frequent shut downs. Moreover, because of our capacity to perform the synthesis and purification of ILs via a continuous flow microreactor technology, most of the ILs examined in this work were produced at a cost well below the limit of 500 €/kg. In particular, the ILs with the tricyanomethanide anion, which showed very promising CO2 capture performances, are currently produced with a cost of 125€/kg at the 100 kg scale. Further, an increasing number of publications report on the inhibiting properties of ILs in aggressive environments.6,7 In one case,7a it was shown that the addition of relatively small amounts of selected ILs to hydrochloric or sulfuric acid solutions, typically of the order of tens of mmol/L or hundreds of ppm, results in an increased inhibition efficiency of mild steel against corrosion to a value above 90% that is due to the chemisorption or physisorption of the ILs on the alloy surface and the blocking of active sites. Presently, the gas−liquid phase equilibrium properties as well as the thermodynamics and kinetics of absorption are being extensively studied for more than 120 ILs.8 Most of these studies suggest that the CO2 absorption rate and capacity of ILs are not as high as those of conventional amine solvents. Therefore, the practical application of ILs as efficient CO2 solvents may be feasible by either the use of high pressure in the scrubbing columns to enhance the absorption capacity or mixing with water to decrease the viscosity and increase the absorption rate. In this context, thorough investigations on the effect of water content on the CO2 solubility as well as on the corrosion behavior of mixed IL−H2O solvents are clearly necessary. Moreover, a toxicity evaluation should accompany any effort toward the definition of the most appropriate solvents based on ILs. In this work, nine newly synthesized ILs were evaluated as solvents for CO2 capture. The experimental campaign was conducted using the ILs in their pure forms as well as in binary and tertiary mixtures with water and amines. The involved

2. EXPERIMENTAL SECTION 2.1. Synthesis of Ionic Liquids. All ionic liquids used in this study were synthesized by Iolitec upon request and were used as such. The syntheses of the 1-alkyl-3-methylimidazolium tricyanomethanides were performed in analogy to the previously described procedure20 at yields exceeding 95% in all cases. 12084

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Figure 1. Complete lab-scale setup simulating the CO2 capture process with a mass transfer column and a still column (regenerator).

1.95−2.01 (2H, m), 2.26 (2H, t, J = 8.1), 3.29 (2H, t, J = 7.1, 8.52 (1H, br)), 13.09 (1H, br). To determine the impurities consisting of inorganic salts, a combination of NMR and ion chromatography (IC)-spectra had to be measured whenever possible. The purity of all synthesized ILs was above 98% and was determined by IC or NMR. For the TCM compounds, the purity was always above 99% (IC). The structure of the synthesized ILs and their purity, water content, and halide content were all provided in the Supporting Information (Table S1, Table S2, Figure S1). 2.2. CO2 Absorption Capacity and KineticsCapture Performance. The CO2 absorption isotherms were obtained with the gravimetric technique using a force restoration, full beam electronic microbalance (CI MK2-M5 recording microbalance, CI Precision) with a high capacity (5 g load), a 0.1 μg readability, and a wide dynamic range (±1 g). The complete gravimetric setup operating from a high vacuum up to 1.5 bar as well as the procedures followed for acquiring the CO2 absorption capacity of the pure ILs and for evaluating the effect of water content on the CO2 absorption rate and capacity are thoroughly described in a previous publication of the authors.25 To investigate absorption kinetics, mass uptake data versus time were acquired from the sorption microbalance for each equilibrium pressure and temperature and were corrected for buoyancy. For the definition of the diffusion coefficient, D (m2/ s), of dissolved CO2 gas in the examined ILs, the transient curves of absorption (mass uptake vs time) were fitted to the appropriate solution for the transient absorption curve, holding for diffusion rate-controlled gas absorption in liquid films exhibiting one gas−liquid interface. The solution was derived from the transient diffusion equation according to a number of assumptions that are included in the Supporting Information (paragraph 1). The solution for the transient sorption curve is:

1-Alkyl-3-methylimidazolium tricyanomethanides ([RMIM][TCM], with R = ethyl, butyl, hexyl, octyl): An amount of 1.00 mol of the corresponding imidazolium-based IL chloride (>98%, Iolitec) was dissolved in 1.5 L of dry dichloromethane. An amount of 1.05 mol of sodium tricyanomethanide was added at once, and the reaction mixture was stirred for 48 h at room temperature (RT). The mixture was then filtered through Celite, with the mother liquor evaporated under reduced pressure, and the product was dried under high vacuum for 24 h at 40 °C. 1-Ethyl-3-methylimidazolium lysinate and 1-Ethyl-3-methylimidazolium serinate ([EMIM][LYS], [EMIM][SER]): To a solution of 1-ethyl-3-methylimidazolium hydroxide ([EMIM][OH]) in water (10%, purchased from Sigma-Aldrich) was added 1.00 equiv of lysine or serine, and the mixture was stirred at RT. After the water was evaporated, the product was dried under high vacuum for several days at 30 °C. 1-Ethyl-3-methylimidazolium acetate ([EMIM][OAc]): To 1 kg of 1-ethyl-3-methylimidazolium methylcarbonate [EMIM][MeCO3] (30% in methanol, 1.61 mol) was added dropwise acetic acid (96.8 g, 1.61 mol) under constant stirring at RT. After the CO2 evolution had stopped, the solvent was removed under reduced pressure and the final product, a yellow to brown oil, was dried under high vacuum for several days at 30 °C. Pyrrolidium-2-one bis(trifluoromethylsulfonyl)imide ([BHC][BTA]) and pyrrolidium-2-one trifluoroacetate ([BHC][TFA]): One equivalent of pyrrolidin-2-one was dissolved in toluene, and one equivalent of the corresponding acid (trifluoromethanesulfonimide and trifluoromethanesulfonic acid, respectively) was added dropwise. After the phase separation charcoal was added and filtered off. Finally the solvent was evaporated, and the product was dried under high vacuum for 72 h at 30 °C. All analytical data corresponded to the previously published data21−24 except for [BHC][BTA]: 1H NMR (DMSO-d6): 12085

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∞

∑ n=0

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⎡ D(2n + 1)2 π 2t ⎤ 1 exp ⎢− ⎥ ⎣ ⎦ (2n + 1)2 4S2

and temperature difference of the fluid streams at the input and output of HE2. The evaporation rate of water for variable heat duties was also defined experimentally and was taken into account in the overall energy balance. Except for some cases in which the effect of several process parameters such as the gas to liquid ratio (G/L) and the temperature of the lean solvent (Ttop) were examined in detail, the capture performance and stability tests for several solvent formulations were conducted with a total gas flow rate of 2500 L/min (18.4 vol % CO2 in N2 or 18.4 vol % CO2 in air) and were measured at 25 °C and 1 bar, with a solvent flow rate of 370 mL/min, a solvent temperature of 32 °C at the top of the scrubber (Ttop), and varying heat duties (heat flow from the thermal fluid to the solvent) from 0.4 to 1.12 kJ/s that corresponded to solvent regeneration temperatures (TREGEN), which varied from 72 to 104 °C. 2.3. Corrosion Tests. Mild steel (0.13−0.18% C, max; 0.40% Si, 0.7−0.9% Mn, max; 0.05% S, max; 0.05% P and Fe balance) was supplied by RS Components (UK) as a rod with a 10 mm diameter. The rod was cut into disks approximately 4 mm thick, which were mechanically polished normally to the rolling direction using, consecutively, P240, P600, P1200, and P4000 emery papers followed by 3 and 1 μm diamond pastes on a velvet pad with the addition of a polyethylene glycol-based lubricant. The resultant roughness, Ra, measured by a Contour GT (Veeco) white light interferometer, was approximately 8 nm. Corrosion tests were performed by placing the MS specimens on the bottom Teflon-flange in the SS-316 vessel of the regeneration system (Figure 1) and leaving them immersed in the heated solvent for the entire period of testing under varying conditions. A Zeiss Ultra-55 scanning electron microscope (SEM) was employed for the examination of the surface and local chemistry of the MS specimens after the immersion tests. The microscope was equipped with SE1, SE2, BSE, and EDX detectors. Both secondary and backscattered electron images were recorded at an acceleration voltage of 2 kV with a high Zcontrast. The acceleration voltage was increased to 10−15 kV for EDX characterization. Micro-Raman spectra were recorded in a backscattering configuration on a Renishaw inVia Reflex microscope using an Ar+ ion laser excitation source (λ = 514.5 nm). The laser beam was focused on the MS specimens by means of a 100× (NA = 0.85) objective at a laser power below 0.5 mW in order to avoid local heating and phase transformation of the iron oxides. Analysis of the scattered beam was performed on a 250 mm focal length spectrometer along with a 1800 lines/mm diffraction grating and a high-sensitivity charge coupled device (CCD) detector. Raman mapping was implemented on a motorized feedback-controlled XYZ mapping specimen stage that allowed area mapping with a step size of 0.1 μm in the point-by-point mode. 2.4. Toxicity Tests. The ecotoxicity was determined using the green algae Chlamydomonas reinhardtii. Chlamydomonas reinhardtii were cultured in tris-acetate-phosphate (TAP) medium.26 The culture growth was monitored in terms of chlorophyll (Chl) concentration, which was determined using N,N-dimethylformamide (DMF).27 For reference, the algae were also grown in TAP without a test substance. The cultures were provided with white fluorescent light (100 μE m−2/s) in an orbital shaker incubator (Galenkamp INR-400) at 25 °C.

(1)

where mt is the total amount of gas that has diffused into the liquid film at time t, m∞ is the total amount of gas that has diffused into the liquid after an infinite time, and S (cm) is the liquid film thickness. The diffusion coefficients for the multiple pressure steps were calculated according to eq 1 for a liquid film thickness, S , of about 7 × 10−4 m, as calculated by the liquid mass and density and the diameter of the sample container used in the microbalance setup. Furthermore, the CO2 capture performance of several solvent formulations consisting of ILs, amines, and water were investigated in the laboratory scale device illustrated in Figure 1. The process streams were circulated through a 1/4 in. Teflon tube while the used fittings were made either of stainless steel (SS-316) or nickel-plated brass. The scrubber was a SS-316 mass transfer column with a 12 cm internal diameter and 1 m height. The column was filled with dumped packing (polypropylene pall rings, 16 mm) up to a height of 0.8 m. The Teflon-made bottom and the top vessels of the column accommodated the rich solvent and the liquid distribution system, respectively. The solvent regeneration system (stripping set up, Figure 1) consisted of a Ni-brazed, SS-316 plate-type heat exchanger (HE2) and a closed recirculation loop of thermal fluid (silicone oil) that was heated in a thermally insulated tank (Heating Unit 1) by immersed resistance and a proportional, integral, derivative temperature controller (PID). A SS-316 vessel with top and bottom Teflon flanges was installed at the outlet of the HE2 and received the hot stream of lean solvent, which was further transferred to a solvent collection vessel made of glass and equipped with a dip tube. From there, pump 2 drove the stream of the lean solvent to the top of the scrubbing column via the cross heat exchanger (HE1-cross) and the cooling heat exchanger (HE3). The SS-316 vessel was equipped with a condenser and a fluid level indicator (level switch) and served three functions. The first function was to allow CO2 to be efficiently separated from the solvent vapor. The second function was to control the relative output of pumps 1 and 2 in a way that the glass collection vessel and the bottom vessel of the scrubber were never emptied of lean and rich solvent, respectively. The third function was to accommodate the pieces of MS for monitoring the corrosion strength of the solvent mixtures. Parameters of the process that can be controlled were the gas flow rates of CO2 (20−1000 mL/min), N2 (40−2000 mL/ min), and O2 (8−400 mL/min); the liquid flow rate of the solvent (100−1400 mL/min); the temperature of the heating fluid (Tx1) at the inlet of HE2 (35−150 °C); and the temperature (Ttop) of the solvent at the top of the scrubber (30−50 °C). The CO2 concentration at the output of the scrubber and the temperature of the fluid phase at six different points of the apparatus were continuously recorded during the experiments. These were the temperatures of the heating fluid and the solvent (Tx1, Tx2), at the inlet (Tin) and outlet (TREGEN) of the HE2, respectively, and the temperature of the solvent at the top (Ttop) and bottom (Tbottom) of the scrubber. Thermal losses at the regenerator were also calculated with known variables including the volumetric flow, density, specific heat constant, 12086

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Figure 2. (a) Comparison between the CO2 capture efficiency of the physisorbing ILs. (b) Comparison between the CO2 capture efficiency of the chemisorbing ILs. (c) CO2 isotherms of physisorbing ILs including the desorption branch. (d) CO2 isotherms of chemisorbing ILs including the desorption branch. (e) Correlation of the CO2 capture efficiency of four of the examined ILs with their molar volumes, for several CO2 pressures. 12087

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Figure 3. (a) Binary CO2/IL diffusion coefficients at three temperatures and a pressure of 1 bar. (b) Binary CO2/IL diffusion coefficients at 309 K and several pressures up to 1 bar.

At very low pressures, up to 0.2 bar, [BHC][BTA] exhibited the highest CO2 absorption capacity among all of the examined physisorbing ILs. At higher pressures (0.6−1 bar) its absorption efficiency lay between the efficiencies of [OMIM][TCM] and [BMIM][TCM]. The ranking of the CO2 absorption capacity at higher pressures (0.6−1 bar) is in agreement with the theory of the free volume effect as the determining mechanism of the CO2 solubility in ILs. Indeed, as shown in Figure 2e, at the pressure of 1 bar and at 309 K, the CO2 solubility tended to increase as the molar volumes of the four ILs also increased, demonstrating the rationality of the free volume effect. On the other hand, in the low pressure region (0.1 and 0.25 bar), the fluorinated [BHC][BTA] deviates radically from the linear correlation. The specific interaction between the CO 2 molecules and the fluorinated anions, an issue already studied and confirmed both experimentally31 and theoretically,32 is the reason behind this behavior. It can be concluded that both mechanisms contribute to the dissolution of CO2 in ILs; however, the interaction of the CO2 molecules with the fluorinated anion was stronger than that with the tricyanomethanide anions giving rise to enhanced CO2 solubility in [BHC][BTA] at low pressures. Up to a certain CO2 pressure, the specific absorption sites in [BHC][BTA] were occupied, and from this point on the solubility correlated well with the molar volume. A similar shape of absorption isotherm, with a characteristic steep uptake at low pressures but higher amounts of loaded CO2, was also observed for the chemisorbing ILs [EMIM][LYS] and [EMIM][OAc] (Figure 2b). The major difference is that at the pressure of 0.2 bar, [EMIM][LYS] and [EMIM][OAc] had already absorbed 65% and 75% of the total CO2 amount absorbed at 1 bar, while the corresponding percentages for the fluorinated [BHC][BTA] and [RMIM][TCM]s were 47% and 25%, respectively. The other difference between the chemisorbing and physisorbing ILs was the shape of the desorption branches of their CO2 isotherms (Figure 2c,d). For the former, enhanced hysteresis was observed and the desorption loop was a straight line that was almost vertical to the ordinate (amount of absorbed CO2), while for the latter, the desorption loop almost coincided with that of the absorption. Significant differences were also observed between

Toxicity tests were performed in three replicate experiments using at least five geometrically scaled dilutions for each compound concentration. The algae was inoculated in each test solution in the exponential growth phase at algal concentrations of approximately 1 μg Chl/mL. The toxicity of the ILs was evaluated as (1) the effective concentrations (g/L) of the test substance inhibiting algal growth by 50% (IC-50) relative to the control cultures; in this test, growth rates were calculated by nonlinear regression analyses of the log of the algal growth curves as well as the area under the growth curves (biomass) for each concentration of the chemical and for references; and (2) the minimum effective concentration (g/L) of the test substance inhibiting algal growth by 100% (MDC).

3. RESULTS AND DISCUSSION 3.1. CO2 Absorption Capacity and Rate in ILs. A substantial volume of solubility data of CO2 in ILs has been published in the past decade. The solubility of CO2 in ILs can be expressed on the basis of a dimensionless mole fraction, molality (units: mol CO2·kg−1 IL), volume concentration (units: mol CO2·L−1 IL), and the Henry constant (MPa−1). The mole fraction-based description has been adopted in most of the references relevant to gas solubility in ILs as a reflection of the molecular interaction between gas and ILs; we follow the same approach in the present work. Different solubility expressions lead to different structure−property apparent trends. For instance, CO2 solubility expressed in molality is believed to be independent of the kind of IL,28 but this conclusion is valid only for a limited number of ILs.29 Besides, it was observed that for ILs with alkylmethyl imidazolium as the cation and tetrafluoroborate as the anion, the CO2 solubility expressed in volume concentration decreases unexpectedly with the length of the alkyl chain.30 In Figure 2a, it is shown that when the CO2 absorption capacity of the [RMIM][TCM]s is expressed as mole fraction it scales with the alkyl chain length on the alkyl-methylimidazolium cation following the order [OMIM] > [BMIM] > [EMIM]. The exothermic character of CO2 absorption in [BMIM][TCM] was also concluded by comparing the CO2 isotherms at three different temperatures (Figure 2a). 12088

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Figure 4. Photographs showing the physical state of [EMIM][LYS] after the CO2 absorption experiments performed with a Rubotherm magnetic suspension balance at high pressures.

Figure 5. Effect of water on the CO2 absorption capacity of (a) [BMIM][TCM], (b) [OMIM][TCM], (c) [BHC][BTA], and [EMIM][OAc]. (d) Effect of water content on the binary D1,2 CO2/solvent diffusivity constants. The temperature in all cases was 309 K.

ILs, the reported diffusivity values can be regarded as effective diffusivities that account for both mass transfer and reaction. Especially for [EMIM][LYS], fast kinetics were observed solely during the first pressure step, whereas the D1,2 values strongly declined as the pressure increased (Figure 3b). The major reason behind this behavior was a significant increase of the

the physisorbing and chemisorbing ILs in regard to their binary CO2(1)/IL(2) diffusion coefficients (D1,2 and m2/s). The D1,2 values of the physisorbing ILs at 309 K were between 2.6 × 10−11 and 7.5 × 10−11 m2/s, while the chemisorbing ILs (Figure 3a) absorbed CO2 much faster with D1,2 values in the range of 1.8 × 10−10−4.5 × 10−10 m2/s. In the case of the chemisorbing 12089

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Figure 6. Viscosity deviations of the binary systems (a) [BMIM][TCM]/H2O, (b) [OMIM][TCM]/H2O, (c) [EMIM][OAc]/H2O, and (d) [BHC][BTA]/H2O versus the mole fraction of the IL at the indicated temperatures. The symbols represent the experimental values and the solid lines represent correlations using the Redlich−Kister expression.

Figure 7. Excess molar volume of the binary systems (a) [BMIM][TCM]/H2O, (b) [OMIM][TCM]/H2O, (c) [EMIM][OAc]/H2O, and (d) [BHC][BTA]/H2O versus the mole fraction of the IL at the indicated temperatures. The symbols represent the experimental values and the solid lines represent correlations using the Redlich−Kister expression.

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viscosity of [EMIM][LYS] upon CO2 absorption due to the formation of carbamates. Indeed, when the sample was removed from the gravimetric system it looked like a highly viscous jelly paste (Figure 4). Signs of viscosity change and possibly degradation of the [EMIM][LYS] are also given in Figure 2b. It can be seen that, in contrast to the expected behavior for exothermic absorption, the obtained absorption capacities at 309 K and during a duplicate experiment at 333 K were lower than that obtained during the first experiment, performed at 333 K. 3.2. Effect of Water on the CO2 Absorption Capacity and Rate. The effect of the water content on the CO2 absorption capacity was also a parameter differing significantly between the ILs examined in this work. Specifically the [RMIM][TCM] ILs (Figure 5a,b) exhibited a continuous enhancement in their CO2 absorption capacities as the water content increased. This dependency stands over the entire range of the applied water molar fractions from 0.038 to 0.73. At a very low water concentration (molar fraction, 0.7 kJ/sec), which corresponds to solvent regeneration temperatures above 90 °C, DEA exhibited the highest efficiency

unveil the reasons for the inconsistency observed in the CO2 absorption efficiency of the [BHC][BTA]/H2O system (first decreases and then increases with water content). Nonetheless, regarding the [RMIM][TCM] ILs, the almost 4-fold increase of the CO2 absorption capacity along with the 10-fold increase of the CO2 absorption rate when mixed with water makes them very promising materials for CO2 capture. For that reason they were thoroughly investigated as potential solvents in the laboratory-scale device involving a mass transfer column. 3.3. CO2 Capture Efficiency of IL and Amine Solutions in Water. In Figure 8a, the CO2 capture efficiencies of several 12092

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followed by MEA and MDEA. The concluded ranking is quite reasonable if we take into account the CO2 absorption capacity of the three amines.35,36 On the contrary, at low heat duties the CO2 capture efficiency follows the order of MDEA ≈ DEA > MEA, which is indicative of the very high heat of reaction between MEA37 and CO2. Indeed, at low temperatures (T < 85 °C) and at the same partial pressure of CO2 (180 mbar), MEA is less efficiently regenerated than are DEA and MDEA and therefore exhibits the worst capture performance among the three examined amines. At higher regeneration temperatures (Figure 8b, 99 > Treg > 96 °C, 1.1 > Q > 0.8 kJ/sec), just below the boiling point of the solvent (∼102 °C), the % CO2 capture efficiency of all amine solutions (20 vol %) appears to level off. DEA reached a value of 99% followed by MEA (93%). MDEA exhibited a modest performance (60%), and the % CO2 capture efficiency depended on its concentration in the aqueous solution. The performance results (Figure 8a) indicate that ILs cannot compete with amines with the only exception being the pure [EMIM][OAc] at very low regeneration temperatures. It is also important to mention that the effect of water on the CO2 absorption performance of most ILs, as concluded by the gravimetric measurements (Figure 5), is in good agreement with the performances of the respective aqueous solutions as obtained via the scrubbing−stripping prototype. The chemisorbing [EMIM][OAc], with a 10-fold higher CO2 absorption capacity than that of pure [EMIM][TCM] (Figure 2), lost a significant portion of its efficiency upon being mixed with water (20 vol %), exhibiting a 2-fold lower capture capacity than that of the respective aqueous solution of [BMIM][TCM]. The capture efficiency of the 20 vol % [EMIM][OAc] solution was also examined at pH = 9, but the results did not show any improvement. Similarly to what happened with [EMIM][OAc], the aqueous solutions of [BHC][TFA] exhibited moderate CO2 capture performances. Apart from their low CO2 absorption capacities compared to amines, an additional and probably the most important reason for the poor performance of ILs in the CO2 scrubbing process is the slower rate of gas absorption. It is a fact that despite the high dilution of ILs in the tested aqueous solutions (20 vol %), which should have eliminated the adverse effect of viscosity on the absorption kinetics (the viscosity of the 20% [BMIM][TCM] solvent is below 3 mPa s), CO2 physisorption cannot compete with chemisorption in terms of capture rates. What is missing is the enhancement factor that is induced by chemisorption. Chemisorption results in the rapid abstraction of the CO2 molecules from the liquid phase near the gas−liquid interface, thus maintaining a high concentration gradient (driving force) that increases the mass transfer rate of the gas into the liquid phase. The results presented in Figure 9 are indicative of this effect. Indeed, with the liquid flow rate kept constant (L = 0.37 L/min) and a gradual reduction of the gas to liquid flow ratio (G/L) down to 1.2, the efficiency of the 20 vol % [BMIM][TCM] solvent became significantly enhanced in contrast to the 20 vol % MDEA solvent. In the case of the physisorbing ILs, the significant resistance to mass transfer is in the liquid phase and the process is controlled by the rate of the mean transfer through the liquid film. Thus, lower superficial gas flow rates prolong the contact time leading to a higher capture efficiency. Concerning MDEA, it is evident that the specific rate of CO2 absorption into the aqueous solution remains essentially unchanged or, more precisely, it declines with the variation in contact time, that is,

Figure 9. Effect of G/L on the capture performance of aqueous MDEA 20 vol % and [BMIM][TCM] 20 vol % solvents. Inset: effect of the temperature of the lean solvent (top of scrubbing column) on the capture performance of the [BMIM][TCM] 20 vol % solvent.

with the variation in the liquid−film mass transfer coefficient. This observation signifies that under lower superficial gas flow rate conditions, the enhancement factor declines because of poor replenishment of the CO2 phase at the gas−liquid interface. The inset of Figure 9 shows the dependence of the [BMIM][TCM] capture efficiency on the temperature at the top of the scrubbing column (Ttop) and provides further evidence for the dominance of the liquid phase mass transfer in the capture process with pure IL solvents. The capture of CO2 by [BMIM][TCM] is a physisorption process, and as expected, the absorption capacity declines with the temperature of the solvent (Figure 2). On the other hand, the absorption rate, expressed by the binary diffusivity, is enhanced significantly with temperature (Figure 3). The combined effect of these two phenomena produced the maximum on the plot of the CO2 capture efficiency versus Ttop, as presented in the inset of Figure 9. It should be noted that the points included in the plot correspond to experiments performed at the same regeneration temperature (94 °C) and identical L/G ratios (1.2), and the only variable was the temperature of the solvent at the top of the scrubbing column. On the basis of these results, it can be concluded that in most cases water has a positive effect on the CO2 absorption rate of physisorbing ILs. However, the 10-fold increase in the binary CO2−solvent diffusion coefficient is not sufficient for the IL− water system to compete with the aqueous solutions of amines. To this aim, very low L/G ratios are required, thereby making the process unfeasible for an industrial application. To this end, ILs were subsequently examined as additives to the standard amine solutions with the purpose to enhance their capture efficiency, while reducing the amine content. 3.4. CO2 Capture Efficiency of IL−Amine Mixtures in Water. 3.4.1. MDEA and [RMIM][TCM]s. The first tests with aqueous amine−IL solutions started with mixtures of MDEA and [BMIM][TCM]. It can be seen (Figure 10) that a reduction of the concentration of MDEA from 20 to 10 or to 5 vol % and the removed amount of amine supplemented with the respective percentage (vol %) of [BMIM][TCM] resulted in the generation of solvents that could not compete with the CO2 capture performance of the 20 vol % MDEA solution. Conversely, the use of a solvent formulation containing two ILs, [HMIM][TCM] and [OMIM][TCM], with higher 12093

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Figure 10. Positive effect of CO2 sorption by ILs on the capture performance of the mixed amine−IL solvent.

Figure 11. CO2 capture performance of a solvent containing low amounts of amines and physisorbing [RMIM][TCM] ILs.

absorption capacities than that of [BMIM][TCM] (Figure 2) produced capture performances that were higher than that of the 20 vol % MDEA solution. This is consistent with the description of the CO2 kinetics with aqueous tertiary amine solutions. In most of the literature it is assumed that the reaction of CO2 with MDEA (bicarbonate formation) is a pseudo-first-order reaction with respect to CO2,38−41 thereby the increased concentration of CO2 in the liquid phase, due to the presence of the ILs, enhances the rate of the bicarbonate formation. Although MDEA is much less corrosive than the primary and secondary alkanolamines and has a lower heat of regeneration and much higher maximum loading capacity of 1.0 (mol of CO2)/(mol of amine), the obtained 70% CO2 capture efficiency is not sufficient to render the application of MDEA−IL mixtures feasible. Despite the significant enhancement of the bicarbonate formation rate, bicarbonate formation still remains slow when compared to the very fast formation of the zwitterion when primary and secondary amines react with CO2. In this context, a strategy of mixing a secondary amine with MDEA−IL mixtures was undertaken. 3.4.2. DEA/MDEA−ILs. In most of the studies, primary alkanolamines such as MEA were used to replace a small fraction of MDEA, in 30 wt % MDEA−H2O solutions, with the purpose of enhancing the rate of absorption of CO2 to a large extent without appreciably changing the stripping characteristics and corrosiveness of the solvent.42,43 The usual formulations contain primary amines at concentrations from 3 to 7 wt %, with a balance of MDEA up to a total amine content of 30 wt %. Through variation of the relative concentrations of the amines, an optimum absorption system can be designed for a specific application. In this study, a secondary alkanolamine, DEA at 7 vol %, was used as a reaction rate promoter instead of MEA because of its lower corrosion rate for carbon steel 1020 (3 kmol/m3 DEA is at 89 μm per year (MPY) compared to the 126 MPY of a solution of similar concentration44) and its lower heat of reaction with CO2 (1500 kJ/kg compared to 1900 kJ/kg for MEA). Moreover, the target was to reduce the total percentage of amines from 20 vol % to 14 vol %, involving equal volumes of MDEA and DEA and supplementing the missing amine content with the mixture of [RMIM][TCM] ILs that had already showed a promising performance in the formulations containing solely MDEA and ILs. The performance results (Figure 11) indicated that, except from the area of low heat

duties, where the CO2 loading of lean DEA remains high, the applied amount of 7 vol % DEA readily enhanced the capture efficiency of a solvent containing 6.9% of MDEA. However, the performance of this hybrid solvent was still moderate compared to the 99% CO2 capture efficiency achieved with a 20 vol % solution of DEA. Involving again a mixture of [RMIM][TCM]s of up to 7.5 vol %, we have concluded the generation of a solvent that contains low amounts of two of the less corrosive amines and exhibits a similar CO2 capture efficiency (96%) to the 20 vol % solution of DEA. On the basis of the expression of the overall reaction rate for CO2 with a mixture of primary (or secondary) and tertiary amines,41 we can attribute the high capture performance of the amine−ILs containing solvent to the enhanced CO 2 dissolution, which was caused by the presence of ILs. Moreover, the strong basic character and high proton conductivity of the involved ILs assisted in the deprotonation of the zwitterion (DEA) and the protonation of MDEA, respectively. The overall expression for reaction rate rov is as follows: rov =

⎛ ⎜1 + ⎝

kRNH2[CO2 ][RNH 2] k −RNH2

⎞ ⎟

k H2O[H2O] + k OH−[OH−] + kRNH2[RNH2] + kRR ′ R ″ N[RR ′ R ″ N] ⎠

* −[CO2 ][OH−] + kRR ′ R ″ N[CO2 ][RR′R″N] + k OH

(7)

where the reaction rates in the denominator of the second term correspond to the proton abstraction from the zwitterion by the bases of the system. The presence of the ILs, as much stronger bases than H2O and OH−, limits the strong contribution of MDEA and DEA to the deprotonation of the zwitterion and leaves more amine molecules to react with CO2. Furthermore, two additional efforts were conducted toward the definition of the best solvent formulation that would contain significantly reduced amounts of amines. In one of the experiments (Figure 12), both MDEA and DEA were completely replaced by the chemisorbing ILs [EMIM][LYS] and [EMIM][SER], but the generated solvent could not compete with the 20 vol % DEA solution. In addition, the performance of the solvent containing [EMIM][LYS] and [EMIM][SER] was continuously degraded (see histogram in Figure 12) because of the thermally labile character of the ILs with the amino acid anion (see the TGA analysis in Figures S2 and S3 of the Supporting Information). In the second tested formulation, MDEA was the amine 12094

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Figure 12. Comparison of the capture efficiency between solvents containing mixtures of amines with physisorbing ILs and mixtures of chemisorbing and physisorbing ILs. The histogram reveals a continuous degradation of a solvent containing solely chemisorbing and physisorbing ILs.

Figure 13. Life cycle assessment, under oxidative conditions, of the best solvent formulations defined in this work.

The solvent containing the mixture of [RMIM][TCM]s and amines (Figure 13b) exhibited a similar behavior, with a characteristic decline and stabilization of the CO2 capture performance at low heat duties. It is important that at high heat duties the performance of this hybrid solvent was stable and exceeded that of the solvent containing solely the mixture of amines. The CO2 capture performance was stable for 9 days on stream, which according to the technical characteristics and flow conditions involved in the lab scale scrubbing and stripping device corresponds to about 600 repeated cycles of absorption and regeneration. 3.5. Corrosion of MS in ILs, Amines, and Aqueous IL− Amine Solutions. The corrosion behavior of MS in [RMIM][TCM] ILs was already investigated by the authors.9 Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX) and Raman analysis on the surface of immersed specimens revealed that the corrosion of MS in the [RMIM][TCM]s is driven by two processes, specifically (1) corrosion initiation at the manganese sulfide (MnS) inclusion sites due to the interaction with dissolved oxygen in the presence of residual water accompanied by the dissolution of MnS and (2) the inhibition of corrosion by adsorption of the IL molecules at the active sites on the metal surface. The corrosion inhibition efficiency was found to be significantly

replaced by [EMIM][LYS] and the total [RMIM][TCM] content was replaced with [BHC][BTA]. The results indicated a good performance only at the high heat duty region. It is interesting to note (Figure 12) that the involvement of the amino acid anion ILs resulted in satisfactory performances of the solvents only at high heat duties. This fact reveals that the heat of the reaction with CO2 must be higher than the respective one between the alkanolamines and CO2. The best solvent formulation (e.g., DEA, 7 vol %; MDEA, 6.9 vol %; [RMIM][TCM]s, 7.5 vol %), as concluded from the results of the capture performance, was also evaluated with regard to the long-term stability in a gas stream containing significant amounts of oxygen (16.4 vol % O2, 18.4 vol % CO2, 65.2 vol % N2). For reasons of comparison, a solution containing DEA and MDEA at concentrations of 7 vol % and 6.9 vol % was also tested under similar conditions and for the same duration. The results, presented in Figure 13a,b, indicate that even in the presence of oxygen, a period of 8−9 days on stream was not sufficient for the mixture of amines to present any sign of degradation. The performance of the amine containing solvent (Figure 13a) at high heat duties (0.98 kJ/s) was remarkably stable. At lower heat duties (0.66 kJ/s), the CO2 capture performance declined during the first 4 days on stream and then remained stable for another 4 days. 12095

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Table 1. Solvent Composition and Respective Conditions for Examining the Corrosion of Mild Steel time on stream (h) solvent composition (vol %), balance: water

abbreviation

O2 vol %

70 °C 4 2 2.5 2.9 4 2

>80 °C

>96 °C

1 1.5

13 4 19 14.5 13 9 24 44 44

1 1.5 3 2 35 35

oxidized significantly. Few inclusions of different compositions, for example, presumably containing a mixture of oxides of Mg, Al, Si, Ca, and Ti, can also be found on the surface; no copper was detected in the spectra of these inclusions. Thus, the deposition of copper takes place at the active sites of MnS inclusions. Generally, little or no corrosion was revealed by SEM/EDX examination for MIXD, which consisted of the ILs with amino acid anions (Figure 14d). Etching of the material over the macroscopic surface probably at the grain boundaries took place (shown by arrows). Local EDX analysis of the inclusion revealed the presence of the S and Cu peaks in the related spectrum shown in the inset. Mn-related peaks are not present in the spectrum. Testing in MIXE resulted in the generation of numerous seed-shaped features of approximately 1.5 × 3 μm2 dimensions that were distributed over the entire macroscopic surface of the MS (Figure 14e). The surface was significantly rougher than those after testing in MIXB or the as-polished alloy, which suggests that the entire surface of the alloy was chemically etched. The undissolved inclusions containing sulfur and copper were identified by EDX analysis (see inset). An oxygen peak was also present in the spectrum associated with the inclusion. Visually, no corrosion products were observed around the inclusion sites; thus, it may be assumed that the oxygen peak originated from the oxidation of copper. The seedshaped features marked are probably composed of iron oxide because of the relatively intense O and Fe peaks in the relevant EDX spectra. The typical surface appearance of MS after immersion in DEA is shown in the micrograph in Figure 14f, which reveals nonuniformly shaped features of dimensions up to several micrometers that are distributed nearly uniformly over the macroscopic surface. Interestingly, a Zn or Cu peak was present in the EDX spectra of the individual features (see insets), whereas no zinc or copper was detected in the metallic area. An oxygen peak is also present in the spectra, which may be attributed to the oxidation of zinc or copper since no corrosion products are evident in the micrographs. Further, an intense S peak is present in the spectrum with the Cu peak, whereas the spectra with Zn peaks does not reveal sulfur yields. The features with Cu are generated at the active MnS sites, as confirmed by the presence of the S peak in the related spectrum. Most likely, deposition of Zn also occurs at the active MnS sites accompanied by the dissolution of MnS inclusion since no Mn or S peaks were revealed in the related spectra. The surface of the alloy was not modified markedly after testing in MDEA; scratches in the polishing direction and MnS inclusions were revealed (Figure 14g). Peaks of Cu and S are

enhanced for the [RMIM][TCM]s with the longer length of alkyl chain in the cation (R = hexyl, octyl). In this work we examined the corrosion behavior under a real environment, for example, the regeneration of rich solvent in the stripping device in the presence or absence of oxygen. The solvents examined and conditions applied are summarized in Table 1. 3.5.1. Short Time Tests in the Absence of O2. MIXA was tested first in the series in the laboratory scale scrubbing−stripping device. After testing the specimen in MIXA, a visual observation revealed a black appearance of the entire surface of the specimen compared to a mirror-like appearance of the aspolished alloy. Further, the surface also changed significantly microscopically when compared to the as-polished alloy that is shown in the micrographs of Figure 14a,b. The surface of the as-polished alloy is featureless, with shallow scratches evident in the polishing direction (Figure 14a). In contrast, the surface is modified markedly after testing in MIXA, which leads to a significantly rougher appearance with brighter, nearly circular features of around 10 μm dimensions distributed randomly on the surface (Figure 14b). A typical local EDX spectrum revealed an intense Cu Lα peak at 0.93 kV and a weak Cu Kα peak at 8.04 kV, which indicate the presence of copper on the surface of the alloy (inset to Figure 14b). The black color of the surface and the oxygen peak at 0.52 eV suggest that the surface is covered by copper oxide(II), CuO. The absence of Fe peaks in the EDX spectrum suggests that the penetration depth of the electron beam is smaller than the thickness of the CuO layer. The penetration depth of the electron beam in CuO at 10 kV, calculated using a Casino software based on the Monte Carlo simulation, is 400 nm. Thus, the thickness of the CuO layer covering the steel surface is greater than 400 nm. The carbon and nitrogen peaks may originate from the traces of the components of MIXA present on the surface. The surface appearance of the MS tested in MIXB, which was the most prominent solvent in regard to the capture performance, was featureless. Scratches in the polishing direction and MnS inclusions, with dimensions varying from several hundred nanometers to a couple of micrometers, were observed on the surface. The population density of the MnS inclusions determined from SEM observation is approximately 1 × 106 cm−2. In the micrograph of Figure 14c, an individual inclusion of approximately 400 × 900 nm dimensions is revealed. Intense S and Mn peaks (see EDX spectrum in the inset) confirm that the inclusion is composed of MnS. Copper is also detected at the inclusion site, whereas it was not found in the metallic matrix area. Oxygen was not detected at the MnS inclusion sites, which indicates that insignificant amounts of corrosion products were generated and copper was not 12096

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Figure 14. Secondary electron micrographs showing the surface appearance of (a) as-polished MS and MS after testing in (b) MIXA, (c) MIXB, (d) MIXD, (e) MIXE, (f) DEA 20, and (g) MDEA 20. Local EDX spectra recorded in the areas pointed to by arrows are shown in insets.

present in the EDX spectra recorded at the MnS inclusion sites (see insets). The absence of oxygen peaks in the spectra suggests that that copper was not oxidized significantly. Similar to the testing in DEA, zinc and oxygen were detected locally, probably due to the deposition of zinc at the selected MnS

inclusions sites with further oxidation, accompanied by the dissolution of MnS inclusions. In summary, the results of the corrosion testing for short times and in the absence of oxygen in the reactor reveal insignificant or no corrosion attack occurred during testing in 12097

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Figure 15. Micro-Raman spectra acquired from selected spots (see inset photos) of the immersed mild steel in (a) EMIM−OAc−H2O; (b) 7% DEA, 6.9% MDEA; and (c) MIX-B.

sites. Dissolved from the fitting, copper ions may react with aqueous sulfur species that are produced during the dissolution of MnS inclusions with the formation of insoluble Cu2S and prevent the formation of corrosion products.47 Zinc probably produces zinc oxide at the inclusion sites because of the presence of the intense oxygen peak in the related spectra; however, the mechanism of formation of the zinc compound and the selective formation of copper sulfide or the zinc compound at the MnS sites require further investigation. 3.5.2. Long Time Tests in the Presence of O2. The corrosiveness of MIXB was further compared to that of an amine solvent consisting of 6.90 vol % MDEA and 7 vol % DEA under very aggressive conditions (16.4 vol % O2) for 6 days on stream. A solvent consisting of 20 vol % [EMIM][OAc] in water was also examined in the absence of O2. A macroscopic view of the three immersed MS specimens is given in Figure S4 (Supporting Information). At first sight, MIXB had a less severe effect than did the other formulations on the surface of the MS. To investigate the corrosion products, their phase compositions and distributions across the surface of the MS, micro-Raman spectroscopy was applied by the selection of a representative area of the specimens and the performance of Raman mapping at different spots. Spectra were acquired from points inside or in the region around the craters (inclusions) as well as from parts that had different colored appearances in the optical images and from reference areas that appeared unaffected by corrosion.

all solutions except MIXE containing the IL with the fluorinated anion, where the metal was etched over the macroscopic surface and seed-shaped features, possibly composed of iron oxide, were generated. To ascertain the origin of the zinc and copper, the vessel in which the MS specimens were accommodated during testing was carefully inspected. It was revealed that the nickel coating on the brass fitting mounted inside the vessel was damaged locally. It was deduced that the specimen that was tested first in the series, namely employing MIXA, came in physical contact with the fitting and removed the nickel coating locally, thus causing the generation of copper oxide over the entire surface of the specimen. Garza-Montes-de-Oca et al.45 studied the formation of ZnO and Cu2O at the nickel−brass interface when the nickel coating on brass for plumbing applications was damaged. Giridhar and van Ooij46 reported that during testing of brasscoated steel cord in sodium chloride, the release of zinc (dezinfication) was followed by the release of copper from a brass coating and iron from a steel cord and, finally, the surface was covered by “red rust” that is presumably cuprous(I) oxide, Cu2O. Similar processes probably occurred in the testing of MIXA. Steel that was in contact with the brass fitting with a damaged nickel coating was covered by Cu2O that was further oxidized to black CuO. During testing in other solutions, additional measures were taken to avoid contact of the specimens with the fitting; however, the formation of copper or zinc-containing compounds was not avoided at the inclusion 12098

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In summary, Raman results confirmed the different corrosion routes of the amines and IL solvents. In both cases, corrosion was initiated by the dissolution of MnS inclusions. However, in the case of [EMIM][OAc], corrosion proceeded with the formation of elemental sulfur and MnO2, while in the case of the amines DEA and MDEA, the leaching of ferrous ions took place that were rapidly oxidized to ferric ions in the presence of oxygen and both act as catalysts for the conversion of amines to imines and their further oxidation to ammonia and aldehydes.54 MIXB was very efficient in preventing the extensive corrosion of MS; nevertheless, a few cavities were also produced in this case. Even though they are very small in size, they accumulate various kinds of iron oxides and oxyhydroxides as well as metal particles (they were evidenced by EDX taken from the MS immersed in a similar solvent and conditions, but they have to be verified by EDX on the particular samples) that may act as SERS nucleation sites. The surrounded steel remained intact from the corrosion process with no signal from corrosion product precipitates. 3.6. Toxicity of IL−Amine Mixtures in Water. Figure 16a shows the growth of Chlamydomonas reinhardtii cells in the presence of the solvent that consisted of DEA 8 vol %,

The specimen treated in [EMIM][OAc] presented very rough and inhomogeneous surface characteristics under the optical microscope. Extensive areas appeared with various red color tones upon partially hidden craters, which suggests the formation of a variety of oxides. The Raman spectra obtained from the color-dominant areas indicated the presence of very broad, strong modes at 530 and 640 cm−1 (Figure 15a). The strongest 640 cm−1 mode was identified due to MnO2, while the 530 broad cm−1 shoulder can be formed from the superposition of the other two, also broad, MnO2 modes at 510 and 580 cm−1, which can broaden because of confinement in the MnO2 nanocrystals.48 Furthermore, the Raman scattering activity in the 500−700 cm−1 range and the peak shoulder at 688 cm−1 corroborate the presence of iron oxides, most probably mixtures of maghemite and magntetite phases as found in a previous work.9 MnO2 comes along with elemental sulfur, which is identified as a product of MnS dissolution from the sharp Raman modes at 146, 215, and 472 cm−1, which is in accordance with previous corrosion studies.49,50 Regions that are shown with a white color under the optical microscope do not give any Raman signal, which suggests that they remain unaffected by corrosion (Figure 15a). The alloy immersed in 7 vol % DEA, 6.9 vol % MDEA appeared less affected by corrosion compared to the specimen immersed in [EMIM][OAc]−H2O. The primary corrosion product was identified as α-FeOOH (goethite) with high accuracy (Figure 15b) by the coincidence of the detected Raman modes (295, 390, and 685 cm−1) with results in the literature;51 however, the presence of other iron oxides as minor phases cannot be excluded since many of them give Raman activity in the same broad frequency range. Moreover, the low frequency band at 140 cm−1 indicates the presence of extra species that might be corrosion products like elemental sulfur52 or inhibition layers formed on the surface. The MS specimen immersed in MIXB presented a relatively “clean” surface with craters of relatively small dimensions. The metal matrix mainly remained uncontaminated as shown by the featureless reference spectra obtained from this region (Figure 15c, no. 5, reproduced in two more spots from the maps in other samples areas). However, by focusing the laser beam in the craters’ area, either featureless Raman spectra were recorded (Figure 15c, no. 4, reproduced in two extra spots), signals from iron oxides (Figure 15c, no. 3, reproduced in one extra spot), or instead a very strong broad signal in the low frequency range emerged (Figure 15c, no. 1, 2, reproduced in four extra spots). Thus, a variety of Raman spectra that were observed reflect the mixed nature of the solvent, which contains both ILs and amines. The main iron oxide phase in Figure 15c, no. 4, is attributed to maghemite,51 but both the low signal level and its presence in very few craters suggest that it is a corrosion product in a very low amount. The very broad Raman band at 240 cm−1, centered very close to the strongest band (250 cm−1) of the γ-FeOOH (lepidocrocide), was also observed in previous studies.53 It was attributed to γ-FeOOH formed by hydrolyzed γ-Fe2O3 particles in the acidic environment and was shown with high Raman intensity due to surface enhanced Raman scattering (SERS) on substrates with the deposition of gold nanoparticles. In our case, SERS conditions may be reached by the accommodation of metallic nanoparticles inside the MS craters. Such metallic nanoparticles (Cu) were evident in the EDX analysis taken from the MS immersed in similar solvents and conditions.

Figure 16. (a) Growth curves of Chlamydomonas reinhardtii in the presence of MIXE. (b) Optical confirmation of the lower toxicity of MIXE compared to that of the other solvent formulations. The concentration of the solutions (MIXA, MIXB, MIXC, MIXD, and MIXE) is 2.914 g/L. 12099

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consequences of the toxicity of ILs, especially of IL mixtures with amines, to algae toward the development of new solvent formulations for CO2 capture that will reduce the risk of harm to aquatic ecosystems from unintended release to the environment.

[EMIM][LYS] 3 vol %, and [BHC][BTA] 10 vol % (MIXE), while Figure 16b gives optical confirmation on the lower toxicity of MIXE compared to that of the other mixtures, all applied with the same concentration of 2.914 g/L. Growth curves clearly show that algal growth was inhibited while the MIXE solvent concentration increased. Similar growth curves were used in order to estimate the MDC and MIC of all of the solvent formulations examined in this work. Table 2 shows the IC-50 and MDC values of the solvents

4. CONCLUSIONS This study unveiled important issues pertaining to the applicability of ILs for CO2 capture. None of the nine examined ILs can compete alone with amine solutions. Although dilution in water helped to overcome the problem of high IL viscosity and the concomitant slow CO2 diffusivity, the capture performance remained below 30% with the exception of amino acid anion IL, [EMIM][LYS], which has a chemical binding capacity for CO2 but poses stability problems. In the case of ILs with fluorinated [BHC][BTA], [BHC][TFA], and acetate [EMIM][OAc] anions, the addition of water led to a reduction of CO2 absorption capacity and corrosivity larger than those of amines. Surprisingly enough, a solvent composed of DEA, [EMIM][LYS], and [BHC][BTA] was the least ecotoxic among the examined solvent formulations, but its corrosiveness and capture performance did not support its applicability for CO2 capture. The only real option suggested by this work is to proceed beyond simple formulations and to use a category of ILs with specific characteristics in order to replace a significant fraction of the amine content in the final solvent formulation. The involved ILs must interact physically with CO2 and enhance their CO2 absorption capacity in the presence of water. In this way a beneficial effect on the performance of amines is expected because of the increased concentration of CO2 in the solvent and its availability for the formation of carbamates. These ILs should also act as corrosion inhibitors for the amines and suppress their toxicity. The [RMIM][TCM] ILs examined in this work appear to fulfill all of the aforementioned characteristics. A solvent with volumetric composition of MDEA 6.90%, DEA 7%, [BMIM][TCM] 2.2%, [HMIM][TCM] 2.8%, and [OMIM][TCM] 2.5% exhibited similar capture performance as did DEA 20%, reduced corrosiveness, exhibited far less toxicity, and is proposed as a very promising solution for industrial-scale CO2 capture.

Table 2. MDC and IC-50 Values of Tested Substances DEA MDEA MIXA MIXB MIXC MIXE

MDC (g/L)

IC-50 (g/L)

1.577 2.234 2.016 5.704 2.809 9.470

1.315 1.875 1.082 4.447 0.214 5.253

compared to the frequently used aqueous solutions of DEA and MDEA. In the comparison of the minimum effective concentrations inhibiting algal growth by 100% (MDC) it can be stated that MIXE was the least toxic among the solvents while MIXA was as highly toxic as was MDEA. Moreover, the MDC value shows that the tested solvents are classified in ascending order of toxicity as follows: MIXE < MIXB < MIXC < MIXA < MDEA < DEA. We also examined the effective concentration of test substance inhibiting algal growth by 50% (IC-50 or MIC), which shows the ability of cells to proliferate under the toxic effect of the tested substances. Table 2 confirms that MIXE and MIXB are the least toxic solvent formulations, with respective IC-50 values that are 2.8 and 2.4 times greater than the IC-50 value of MDEA and 4.0 and 3.4 times greater than the IC-50 value of DEA, respectively. Contrary to what was concluded by comparing the MDC values, the IC-50 values showed that MIXA and MIXC are more toxic than are DEA and MDEA. It should be noted that only recently have phytotoxicity data started to play an important role in the classification of hazardous chemicals. This is because many plants, especially algal species, have been found to be as sensitive as invertebrates and fish to a wide variety of chemicals.55 In the up to date investigations related to the toxicity of ILs on algae,19,56−58 solely ILs composed of a first generation cation (RMIM+) and first and second generation anions such as halides and PF6−, BF4− were examined, and the results indicated an increase of toxicity with the increase of the alkyl chain length of the imidazolium ring. However, as elucidated from the CO2 capture performances (section 3.4), the commercial application of the examined ILs in this work necessitates their use in mixtures with water and amines. In this sense, it was essential to compare the toxicity of ready to use solvent formulations rather than to focus on the toxicity of each of the newly synthesized ILs. The results indicate that although DEA is more toxic than MDEA, it generates less toxic formulations when mixed with ILs (compare MIXE and MIXB with MIXA). Another unexpected finding was that MIXC, composed of ILs with biocompatible anions ([LYS], [SER]) and ILs with the shortest alkyl lengths, [EMIM], exhibited 6 times greater toxicity than did DEA. Given that algae represent the base of the food chain of aquatic ecosystems and its photosynthetic apparatus is the same as in higher plants, it is essential to continue examining the

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ASSOCIATED CONTENT

S Supporting Information *

The structures and purity of the synthesized ILs, their TGA analyses, and macroscopic pictures of the specimens after exposure to the solvents. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research leading to these results received funding from the European Community’s Seventh Framework Program FP7/ 2011−2014 under Grant Agreement No. 283077 (IOLICAP). 12100

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