Ionic liquids for CO2 capture—Development and progress

Chemical Engineering and Processing 49 (2010) 313–322

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Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep

Review

Ionic liquids for CO2 capture—Development and progress M. Hasib-ur-Rahman a , M. Siaj b , F. Larachi a,∗ a b

Department of Chemical Engineering, Laval University, Québec, QC G1V 0A6, Canada Department of Chemistry, Université du Québec à Montréal, Montréal, QC H3C 3P8, Canada

a r t i c l e

i n f o

Article history: Received 1 June 2009 Received in revised form 5 March 2010 Accepted 8 March 2010 Available online 15 March 2010 Keywords: Ionic liquids Flue gas Carbon dioxide capture Regeneration Solubility Toxicity

a b s t r a c t Innovative off-the-shelf CO2 capture approaches are burgeoning in the literature, among which, ionic liquids seem to have been omitted in the recent Intergovernmental Panel on Climate Change (IPCC) survey. Ionic liquids (ILs), because of their tunable properties, wide liquid range, reasonable thermal stability, and negligible vapor pressure, are emerging as promising candidates rivaling with conventional amine scrubbing. Due to substantial solubility, room-temperature ionic liquids (RTILs) are quite useful for CO2 separation from flue gases. Their absorption capacity can be greatly enhanced by functionalization with an amine moiety but with concurrent increase in viscosity making process handling difficult. However this downside can be overcome by making use of supported ionic-liquid membranes (SILMs), especially where high pressures and temperatures are involved. Moreover, due to negligible loss of ionic liquids during recycling, these technologies will also decrease the CO2 capture cost to a reasonable extent when employed on industrial scale. There is also need to look deeply into the noxious behavior of these unique species. Nevertheless, the flexibility in synthetic structure of ionic liquids may make them opportunistic in CO2 capture scenarios. © 2010 Elsevier B.V. All rights reserved.

Contents 1. 2. 3. 4. 5. 6. 7.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CO2 capture by room-temperature ionic liquids (RTILs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CO2 capture by task-specific ionic liquids (TSILs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CO2 capture by supported ionic-liquid membranes (SILMs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CO2 capture by polymerized ionic liquids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Toxicity of ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Current and future developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Likelihood of global-warming-driven adverse atmospheric phenomena and their causal/casual correlation to anthropogenic activities enjoin to abide by the precautionary principle so that sufficient scientific data and understanding would become available to allow efficient parries to be devised to mitigate greenhouse gas effects. Carbon dioxide is widely accepted as the main greenhouse gas. With a lion share amounting to 78.6%, fossil fuel based power plants are the major contributors among large stationary CO2 sources [1]. The Intergovernmental Panel on Climate Change

∗ Corresponding author at: Department of Chemical Engineering, Laval University, Cite Universitaire, Pouliot Building, Quebec, QC G1V 0A6, Canada. 0255-2701/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2010.03.008

313 314 317 318 319 321 321 321 321

(IPCC) perceives that by the year 2100 there may be a rise of 1.9 ◦ C in the global temperature with many anticipated (and out of the blue) distresses [2]. This has turned carbon dioxide capture and sequestration into an extensively investigated topic nowadays. As recently surveyed by IPCC, commercially available CO2 scrubbing processes have hitherto relied upon enabling technologies and knowhow dating back to World War II, necessitating painstaking process retrofitting and considerable investments befitting CO2 capture proper constraints. To prevent global warming and subsequent unwanted climate changes there is a greater concern to look for better skills for carbon dioxide capture and sequestration. Carbon dioxide capture is energy demanding and thus entails a cost which, if not driven by profit, is to be coerced by legislation or by some forms of carbon tax incentives. Therefore, research endeavors to develop the most economical and efficient

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Fig. 1. Some cations and anions constituting ionic liquids (ILs).

technologies in this regard. A number of CO2 capture technologies are already being practiced on laboratory scale or industrially demonstrated that require various processes involving physisorption/chemisorption [3,4], membrane separation [5] or molecular sieves [6], carbamation [7], amine physical absorption [8], amine dry scrubbing [9], mineral carbonation [10,11], etc. Mostly amines like monoethanolamine, diethanolamine and methyldiethanolamine based technologies are being used for CO2 capture through carbamate/carbonate formation. Such capture technologies belong to the classical arsenal of CO2 mitigation techniques. Recurrent drawbacks being reproached to all these systems, along with insufficient carbon dioxide capture capacity, are evaporation, degradation of costly reagents and thermal stability, equipment corrosion and high energy consumption during regeneration [12]. The regeneration step may increase the total operating costs of the capture plant up to 70% [1]. All these discrepancies have to be overcome and addressed on a wider scale for more efficiency but less cost. Recent concept of using ionic liquids (Fig. 1) for CO2 capture is gaining interest due to their unique characteristics, i.e., wide liquid range, thermal stability, negligible vapor pressure, tunable physicochemical character and high CO2 solubility. An important drawback much discussed in the case of ILs is their high viscosity. However, by choosing an appropriate combination of cation and anion, the viscosities can be adjusted over an acceptable range of <50 cP to >10,000 cP. For CO2 capture at high temperatures and high pressures, such as in integrated gasification with combined cycle (IGCC) pre-combustion capture, IL viscosity is less of a concern for its sharp decrease at elevated temperatures, though thermodynamics of CO2 absorption untowardly dictates poor abatement performances. Therefore, among the paths pursued in recent research works are on the use of ionic liquids for carbon dioxide capture involving roomtemperature ionic liquids (RTILs), task-specific ionic liquids (TSILs) or supported ionic-liquid membranes (SILMs) [13–16]. The main goal of this short review is to summarize the work, emanating from the patent as well as scientific literatures, going on the use of ionic liquids for capturing carbon dioxide.

ethanol and acetone (see Fig. 2 and Table 1). Dissolution enthalpy and entropy values suggest stronger interaction of CO2 with the IL. The relatively higher solubility of CO2 may be attributed to its quadrapole moment and dispersion forces. Owing to their negligible volatility and thermal stability under the explicit conditions, ILs are unlikely to contaminate the gas stream to be cleaned up on top of the RTILs greater thermal stability. Mass transfer of the gas is of much importance especially where gas is to undergo a chemical interaction. Hence, during fabrication of an appropriate ionic liquid, drawbacks posed by high viscosity must be addressed [19]. The experimental and simulation studies have shown that CO2 is more soluble in alkylimidazolium-based ILs. The origin of this high solubility could be related more to the anion moiety that enhances interactions by favoring particular distributions of CO2 molecules around the cation [20]. Raeissi and Peters verified the thermal stability of 1-n-butyl-3-methylimidazolium bis[trifluoromethylsulfonyl]imide, [bmim][Tf2 N], by conducting the gas capture experiments in the temperature range of 40–177 ◦ C and pressures up to 140 bar. Even after keeping at 177 ◦ C for more than 10 h, the ionic-liquid conferred reproducible results for CO2 solubility [21]. Alkyl-side chain length of the imidazolium-based cation of the ILs also affects CO2 solubility to a certain extent; however, the outcome is not as prominent as that by switching of anion (Table 2). Fluorine substituted side chains greatly augment the uptake of CO2 with respect to the corresponding non-substituted side chains but at the expense of an increase in viscosity [22–25]. The nature of anion seems to have stronger influence on gas solubility than that of the cation. Ionic liquids possessing [Tf2 N] anion show higher CO2 solubility among imidazolium-based RTILs (Table 3). A number of factors like free volume, size of the counter ions, and strength of cation–anion interactions within the ionicliquid structure seem to govern CO2 solubility in RTILs. Higher

2. CO2 capture by room-temperature ionic liquids (RTILs) Considerable research work is being done showing high carbon dioxide solubility in certain RTILs, especially in those having imidazolium-based cations. Depending on their selectivity, ILs are stronger candidates for CO2 capture [17]. RTILs portray a typical behavior of a physical solvent [18]. The solubility of carbon dioxide, ethylene, ethane, methane, argon, oxygen, carbon monoxide, hydrogen and nitrogen in 1-n-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6 ] in the temperature range between 10 and 50 ◦ C and pressures up to 13 bar proves superiority of IL over various organic solvents like heptane, cyclohexane, benzene,

Fig. 2. Solubilities of CO2 , C2 H4 , C2 H6 , CH4 , Ar and O2 in [bmim][PF6 ] at 25 ◦ C (adapted with permission from [19]).

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315

Table 1 Henry’s constants (bar, at 25 ◦ C) for gases in different organic solventsa .

CO2 C2 H4 C2 H6 CH4 O2 Ar CO N2 H2 a b

[bmim][PF6 ]

Heptane

Cyclohexane

Benzene

Ethanol

Acetone

53.4 173 355 1690 8000 8000 Nondetect Nondetect Nondetect

84.3 44.2b 31.7 293.4 467.8 407.4 587.7 748.3 1477.3

133.3 – 43.0 309.4 811.9 684.6 1022.5 1331.5 2446.3

104.1 82.2 68.1 487.8 1241.0 1149.5 1516.8 2271.4 3927.3

159.2 166.0 148.2 791.6 1734.7 1626.1 2092.2 2820.1 4902.0

54.7 92.9 105.2 552.2 1208.7 1117.5 1312.7 1878.1 3382.0

Adapted with permission from [19]. For ethylene in hexane.

Table 2 Henry’s constants for CO2 in different ionic liquidsa . Ionic liquid

HCO2 (bar)

C3 mimTf2 N C3 mimTf2 N with constant-density gas C3 mimPF6 C4 mimTf2 N C4 mimTf2 N with 2.7 wt% polyethylenimine C6 mimTf2 N C8 mimTf2 N C8 mimTf2 N with 20% relative humidity C8 mimTf2 N with 40% relative humidity C8 F13 mimTf2 N C8 mimTf2 N (58 mol%)/C8 F13 mimTf2 N (42 mol%) 1,4-Dibutyl-3-phenylimidazolium bis(trifluoromethylsulfonyl)imide 1-Butyl-3-phenylimidazolium bis(trifluoromethylsulfonyl)imide

37 39 52 37 38 35 30 30 27 4.5 15 63

a

± ± ± ± ± ± ± ± ± ± ± ±

7 1 5 3 3 5 1 2 4 1 1 7

180 ± 17

Adapted with permission from [25]. Fig. 3. CO2 solubility in [emim][Tf2 N] and [emim][PF6 ] (adapted from [28]).

gas solubility with increase in alkyl-side chain may be the result of increased free volume available for CO2 with corresponding decrease in cation–anion interactions [26,27]. The thermal stability and negligible volatility make RTILs quite imposing. Hou and Baltus found that even after regenerating the ionic-liquid six times, by purging N2 followed by evacuation at 70 ◦ C, there was practically no change in the gas capture capacities [26]. The equilibrium pressure not only depends on temperature but also on CO2 concentration. At 60 bar, CO2 solubility in 1-ethyl-3-methylimidazolium bis[trifluoromethylsulfonyl]imide, [emim][Tf2 N], is found to be 60 mol% which proves the higher efficiency of IL for CO2 capture. When compared with 1-ethyl3-methylimidazolium hexafluorophosphate, [emim][PF6 ], the gas is found more soluble in IL with [Tf2 N]− anion. The difference is further pronounced at higher CO2 mole fraction (Fig. 3). Such data is confirming the effect of anion on CO2 interaction with IL [28]. Fluoroalkyl group enhances the CO2 solubility, thus making [emim][Tf2 N] more efficient for CO2 capture. Room-temperature ionic liquids can be effectively used for hydrogen purification with high selectivity for CO2 /H2 separation. Selectivity of the ionic liquid, [bmim][PF6 ], for

CO2 /H2 mixtures constituting 45–50 wt% H2 is in the range of 30–300. Selectivity drops at higher temperature but enhances with pressure increase [29–31]. Hence this setup may be employed in CO2 capture from pre-combustion power plants. A pressure-swing adsorption/desorption method can be employed for H2 purification by RTILs. CO2 showed maximum solubility in 1-ethyl-3-methylimidazolium 2-(2methoxyethoxy)ethylsulfate, [emim][MDEGSO4 ] at 30 ◦ C in the pressure range of 8.54–67 bar, and expectably increasing with pressure rise (Fig. 4, Table 4). Pyrrolidinium and ammonium based RTILs like 1-n-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([bmpy][Tf2 N]) and trimethyl(butyl)ammonium bis(trifluoromethyl)sulfonyl)imide ([N(4)111 + ][Tf2 N− ]) have also been investigated for H2 purification showing CO2 absorption capacity comparable to imidazolium-based RTILs in the temperature range of 20–140 ◦ C [32,33]. Regarding H2 S/CO2 selectivity, H2 S was found almost three times more soluble than CO2 in 1-(2-hydroxyethyl)-3methylimidazolium tetrafluoroborate ([hemim][BF4 ]). However, owing to the greater concentration of CO2 in the flue gases, higher partial pressure of CO2 diminishes this advantage. This observance

Table 3 Henry’s law constants of CO2 in ionic liquidsa . Ionic liquid

HCO2 (bar) 10 ◦ C

[bmim][Tf2 N] [pmmim][Tf2 N] [bmpy][Tf2 N] [perfluoro-hmim][Tf2 N] [bmim][BF4 ] a

Adapted from [26].

28 29.6 26 25.5 41.9

20 ◦ C ± ± ± ± ±

2 0.6 1 0.2 0.2

30.7 34 31.2 29.2 52

25 ◦ C ± ± ± ± ±

0.3 3 0.1 0.4 2

34.3 38.5 33 31 56

30 ◦ C ± ± ± ± ±

0.8 0.9 1 2 2

42 40.4 35 32 63

40 ◦ C ± ± ± ± ±

2 0.6 2 2 2

45 46 41 36 73

± ± ± ± ±

50 ◦ C 3 3 4 4 1

51 53 46 42 84

± ± ± ± ±

2 2 1 2 4

316

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Fig. 4. Thermogravimetric microbalance setup (adapted from [30]).

illustrates that RTILs can be efficiently tailored to remove H2 S and CO2 concurrently [34,35]. The viscosity of common RTILs is quite high, [bmim][BF4 ] (79.5 cP) is found to be 40 times more viscous as compared to 30% MEA (monoethanolamine) solution at the same temperature (33 ◦ C) [36]. To cope with the viscosity constraints, RTILs may be mixed with some common organic solvents or water. Addition of water (IL aqueous solutions) helped to overcome viscosity problems during processing as shown in Table 5 [37]. However, inclusion of such liquids will accompany their drawbacks as well or this may be accomplished at the expense of decrease in gas capture ability. This is evident from the behavior of an ionic liquid [Choline][Pro] (Scheme 1) examined in pure form as well as after mixing with polyethylene glycol (PEG 200) at temperatures 35–80 ◦ C and ambient pressure [38]. Gas solubility decreased with increasing amount of PEG 200, under constant temperature and pressure conditions. This is explicable because of the low CO2 solu-

Scheme 1. Proposed mechanism for chemical absorption of CO2 by the TSIL (adapted from [38]).

bility in PEG 200. However, to enhance the rate of both absorption and desorption, addition of an appropriate amount of PEG 200 has been found favorable. This may be due to decrease in viscosity and/or solvent role of PEG 200. Another, more workable, option may be the replacement of aqueous media of alkanolamine systems with some stable and non-volatile room-temperature ionic liquids in order to combine the advantages of both, i.e., negligible vapor pressure, higher thermal stability and lower heat capacity of ionic liquids, and fast capture kinetics and low viscosity of certain alkanolamines [39]. Switching the CO2 capture product (carbamate in this case) into a foreign phase would pull the equilibrium-limited CO2 absorption towards higher CO2 conversion values unlike in conventional aqueous amine solutions with soluble carbamate salt (Fig. 5). Thus, it can be inferred that to take advantage of useful properties of ILs, amine-IL solutions need to be investigated more deeply as potential replacement solvents for aqueous amine scrubbing systems. Regarding natural gas purification, certain hygroscopic imidazolium-based ionic liquids like [bmim][PF6 ], [C8 mim][BF4 ] and [C8 mim][PF6 ] have ability of dehydration as well [40–42]. Also, the presence of water, along with acetate ion, in some ionic liquids akin to [hmim][acetate] and [bmim][acetate] may facilitate the capture phenomenon through weak bonding with CO2 [43]. Diminished corrosion of the equipment, almost one-third the heat capacity of (especially imidazolium-based) RTILs compared to aqueous systems may have profound effect in extenuating the high price outcome [39,44–47]. In short, room-temperature ionic liquids especially imidazolium-based RTILs may be employed in natural gas/hydrogen purification or in CO2 capture from fossil fuel

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317

Table 4 CO2 solubility data in [emim][MDEGSO4 ]a . P (bar)

mCO2 b (molCO2 · kg−1 ) IL

◦

30 C 8.540 14.72 28.67 42.30 55.21 62.30

0.3850 0.6654 1.3239 2.0404 2.7357 3.0936

40 ◦ C 8.650 14.97 28.88 42.81 56.62 63.50

0.3301 0.5713 1.1162 1.7053 2.2899 2.5606

50 ◦ C 8.420 15.12 29.38 43.59 57.32 65.20

0.2743 0.4911 0.9587 1.4509 1.9205 2.1710

60 ◦ C 8.470 15.21 29.61 43.95 57.70 66.36

0.2380 0.4257 0.8235 1.2359 1.6254 1.8551

70 ◦ C 8.560 15.22 29.87 44.27 58.68 67.10

0.2110 0.3737 0.7171 1.0655 1.4097 1.6008

a b

Fig. 5. [hmim][Tf2 N]-MEA solutions: (a) no CO2 exposure; (b) on CO2 exposure; showing precipitated MEA-carbamate (reprinted with permission from [39]).

Adapted from [30]. With buoyancy correction.

based power plants. Regarding regeneration, room-temperature ionic liquid based materials may be easily recovered either by pressure sweep process coupled with vacuum treatment, by applying heat or by bubbling nitrogen through the absorbent [36,38]. However, task-specific ionic liquids or RTILs mixed with amine bearing species require temperature sweep regeneration involving vacuum heating [25]. 3. CO2 capture by task-specific ionic liquids (TSILs) As discussed earlier, CO2 is sufficiently soluble in roomtemperature ionic liquids (RTILs). However, the CO2 capture ability can be enhanced by introducing basic character in the ILs. FunctionTable 5 Viscosity values for different compositions of tri-iso-butyl(methyl)phosphonium tosylate/water mixturesa . Mass fraction IL ± 0.0001 (w/w)

 ±  b (cP)

0.0000 0.1250 0.2500 0.3750 0.5000 0.6250 0.7500 0.8720 1.0000

0.89 1.65 ± 0.08 2.6 ± 0.1 4.0 ± 0.2 6.9 ± 0.3 11.6 ± 0.5 23.0 ± 0.7 68.0 ± 2.0 1320± 13

a b

Adapted from [37]. Standard deviations.

alization of ionic liquids with a suitable moiety (like amine) may be implied in this regard [48,49]. CO2 absorption ability of TSILs can reach up to threefold of the corresponding RTILs. The enhanced effect of pressure in case of TSILs was observed by the fact that there was a steady increase in gas load with rise in pressure, providing evidence both for chemical as well as physical sorption. The effect is not so apparent in case of aqueous amine solutions which possess stoichiometric limitations [36]. Reversible sequestration of CO2 is achieved by attaching primary amine moiety to an imidazolium cation, without any dwindle in the ionic-liquid stability. For five consecutive cycles of gas absorption, the regenerated TSIL ([pabim][BF4 ]) did not show any loss of efficiency. [pabim][BF4 ] exhibits better capture of CO2 compared to [hmim][PF6 ], owing to chemical capture phenomenon in the former. The TSIL when exposed to CO2 for 3 h at room temperature and pressure, the mass gain was 7.4% which corresponds to 0.5 molar uptake of CO2 (maximum theoretical value for CO2 capture as amine carbamate). The TSIL remains stable even after five gas sorption/desorption cycles without any detectable loss in efficiency. The proposed mechanism of interaction between CO2 and [pabim][BF4 ] is shown in Scheme 2. The inclusion of water in the ionic liquid was found to increase the CO2 holding capacity which might be due to the formation of bicarbonate species as well [50,51]. In spite of the tunable approach towards TSILs, these functionalized species exhibit much higher viscosities as compared to the corresponding RTILs or other commercially available CO2 scrubbing solutions, posing too serious complications to be applicable on industrial scale. CO2 capture results in sharp increase of viscosity of the TSILs, resulting into a gel-like material [52]. This drawback may be avoided by utilizing mixtures of TSILs and RTILs or TSILs may be adsorbed onto porous membranes.

Scheme 2. Proposed mechanism for CO2 capture by [pabim][BF4 ] (adapted from [50]).

318

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Fig. 6. Molar CO2 loads in solvent volume (for MEA/MDEA consider aqueous solution volume): data for ionic liquids at 30 ◦ C [36]; data for MEA and MDEA at 40 ◦ C [53].

Comparison of CO2 capture by ionic liquids with that by conventional aqueous amine solutions (30 wt% MEA/MDEA) illustrates that the absorption activities of ionic liquids resembles to that of common physical solvents (Fig. 6). Nonetheless, CO2 absorption ability increases significantly on functionalization of ionic liquid with primary amine moiety. Task-specific ionic liquids, [Amim][BF4 ] and [Am-im][DCA], perform like chemical solvents at low pressures (≤1 bar), however, at higher pressures they pursue the performance of room-temperature ionic liquid, [bmim][BF4 ]. On the other hand, aqueous amine solutions accomplished the maximum capacity at about 2 bar and any further increase in pressure does not seem feasible. Whereas functionalized ionic liquids (TSILs) carry on steady CO2 absorption with ascending pressure [36,53]. This behavior depicts that TSILs possess both chemical as well as physical tools for gas capture. 4. CO2 capture by supported ionic-liquid membranes (SILMs) A number of studies have been performed to explore the prospects of supported ionic-liquid membranes involving RTILs or TSILs or both in CO2 capture. To take advantage of thermal/chemical stability and essentially no volatility; and to deal with the limitations due to viscosity and also to increase the contact area between gas and ionic liquid, supported ionic liquids may prove a better choice in CO2 separation from flue gases. RTIL, [bmim][Tf2 N], supported on porous alumina membrane revealed

Fig. 7. Proposed setup for CO2 separation by SILM in a coal-fired power plant (adapted from [54]).

very optimistic results in favor of CO2 capture ability [54]. The SILM with [bmim][Tf2 N] shows higher selectivity of 127 (CO2 /N2 ) than 72 with [C8 F13 mim][Tf2 N]. Also, the fluorinated ionic liquid is much more viscous than [bmim][Tf2 N] causing a decrease in CO2 diffusivity. A proposed process diagram regarding the application of SILM in a coal-fired power plant is shown in Fig. 7. SILMs may compete economically with commercial amine scrubbing provided permeance and selectivity are optimized. Ionic liquids like [bmim][PF6 ] may be adsorbed to a porous (ceramic or zeolite) material for CO2 separation by introducing pressurized gas on one side and collecting the CO2 -depleted gas stream downstream of the porous medium [55]. In another study [56,57], [bmim][BF4 ] was adsorbed onto poly vinylidene fluoride (PVDF) polymeric membrane. The mass ratio of IL/membrane in SILMs was kept 0.5–2.0. With the increase of IL content, the permeability coefficient was seen to increase abruptly. Rise in temperature resulted in a corresponding increase in membrane free volumes caused by increased mobility of polymeric chains. This development stimulated simultaneous increase in permeability. However, the selectivity for CO2 decreased when compared with CH4 . This is because CH4 show more diffusion selective property than solubility selective property and so its solubility is more affected by membrane structure. The rise in pressure demonstrates a positive effect on selectivity. Through optimization of operating conditions, 25–45 CO2 /CH4 selectivity was achieved. The solubility behavior of CO2 , H2 , CO and CH4 in two ionic liquids, [bmim][Tf2 N] and [emim][Tf2 N] makes them feasible to be used in separation membranes [58]. The solubility of CO2 in the two ionic liquids reached up to 60 mol% in contrast to that of H2 that reached

Scheme 3. Proposed mechanisms of CO2 capture: (a and b) without water; (c) with water (reproduced with permission from [59]).

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Table 6 Viscosities and water content of the ionic liquids, at 25 ◦ Ca . Abbreviation

Molecular structure

Water content (%)

Viscosity (cP)

11.4

3760

[C3 NH2 mim][Tf2 N]

5.7

2180

[C4 mim][Tf2 N]

1.8

70

[C3 NH2 mim][CF3 SO3 ]

a

Adapted from [16].

Table 7 Summary of gas absorption capacities (at 592.3 mmHg and 22 ◦ C) and glass transition temperatures of poly(ionic liquid)sa . Poly(ionic liquid)s or ionic liquid

Tg (◦ C)

CO2 absorption capacity (mol%)

P[VBBI][PF6 ] P[VBBI][BF4 ] P[VBBI][Sac] P[VBBI][Tf2 N] P[VBMI][BF4 ] P[MABI][BF4 ] P[EIBO][BF4 ] [bmim][BF4 ]

85 78 40 3 110 54 33 –

2.8 2.27 1.55 2.23 3.05 1.78 1.06 1.34

a

Adapted from [67].

up to 7 mol% at 90 bar. The increase of pressure had a little effect on H2 , CO and CH4 solubility as compared to CO2 . However taking into account the economics of the capture process, optimum conditions of temperature and pressure need to be set. CO2 absorption by amino-acid based ionic liquids supported on porous silica showed higher efficiency as compared when bulk of neat ionic liquids were employed for the purpose. Supported TSIL experiments revealed 50 mol% CO2 capture capacity through carbamate formation with reference to ionic-liquid amount. However, in presence of small amounts of water (1 mass%), the capture capacity reached equimolar ratio as shown in Scheme 3(a)–(c). In the latter case, the capture resulted into carbonate formation [59]. The imidazolium, pyridinium, pyrrolidinium, phosphonium, ammonium and guanidinium based ionic liquids can be adsorbed to polymeric materials for gas separation, especially for CO2 , NOx and SOx [60]. Supported ionic-liquid membranes (especially bearing aminefunctionalized TSILs) possess high selectivity and stability, also diminishing negative impact due to high viscosity of TSILs (Table 6).

In a porous polytetrafluoroethylene (PTFE) membrane facilitated by non-functionalized ionic liquids like [C4 mim][Tf2 N], the permeation of gas is by solution-diffusion mechanism whereas SILMs with adsorbed TSILs like [C3 NH2 mim][CF3 SO3 ] or [C3 NH2 mim][Tf2 N] demonstrate much higher CO2 permeation, mediated by chemical interaction with amine moiety. Nevertheless increase in temperature has negative effect on permeation of CO2 as high temperature prevents the interaction between CO2 and amine moiety. The studied SILMs possess high stability, confirmed by continuous use for 260 days without detectable loss in performance [16,61]. Temperature rise above 85 ◦ C results in corresponding decrease in carbamate stability as well as solubility, and diffusion phenomenon starts to dominate [62]. Combining SILMs with TSILs may possibly be a better choice for CO2 capture at elevated temperatures and pressures [63]. In case of hydrophilic composite membranes, presence of moisture in flue gas affects the CO2 separation performance. Moist feed seems to increase permeability up to 35-fold without any detectable loss in CO2 /H2 or CO2 /N2 selectivity as compared to dry feed [61]. The capabilities of amine-functionalized TSILs based on beta-hydroxy amines, aryl amines and tertiary amines may prove greatly supportive in this regard for proficient reversible CO2 uptake [64]. Development of more efficient and cost-effective SILMs requires in-depth approach into the role of anion/cation in optimization of molar volume of constituent ionic liquids that should lead to the fabrication of more stable, more permeable but thin membranes [65]. 5. CO2 capture by polymerized ionic liquids One of the negative aspects of SILMs is the leaching of the liquid through membrane pores as the pressure drop surpasses

Table 8 Permeability, solubility and diffusivity values in: (a) styrene-based poly(ionic liquid)s; (b) acrylate-based poly(ionic liquid)s, at 20 ◦ Ca . Styrene

CO2

N2

Pb

Sc

(a) Styrene-based poly(ionic liquid)s Methyl 9.2 ± 0.5 4.0 ± 0.1 Butyl 20 ± 1 4.4 ± 0.3 Hexyl 32 ± 1 3.9 ± 0.1 Acrylate

a

c d

S

D

P

S

D

1.7 ± 0.1 3.5 ± 0.4 7.7 ± 0.4

0.29 ± 0.01 0.67 ± 0.02 1.4 ± 0.1

N/A N/A 0.1 ± 0.01

N/A N/A 11 ± 2

0.24 ± 0.01 0.91 ± 0.06 2.3 ± 0.1

0.21 ± 0.05 0.55 ± 0.07 0.57 ± 0.03

0.88 ± 0.16 1.28 ± 0.20 3.10 ± 0.15

N2 S

(b) Acrylate-based poly(ionic liquid)s, at 20 ◦ C Methyl 7.0 ± 0.4 3.6 ± 0.1 Butyl 22 ± 1 4.5 ± 0.4 b

P

CO2 P

CH4

Dd

CH4

D

P

S

D

P

S

D

1.5 ± 0.1 3.6 ± 0.4

0.23 ± 0.02 0.71 ± 0.06

N/A N/A

N/A N/A

0.19 ± 0.02 0.97 ± 0.08

0.17 ± 0.04 0.59 ± 0.09

0.89 ± 0.20 1.27 ± 0.09

Adapted with permission from [71]. Permeability in Barrers. Solubility in cubic centimeters gas (STP) per cubic centimeter polymer atmosphere. Diffusivity in squared centimeters per second × 108 .

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Table 9 Lethal concentrations (LC50 ) of different ionic liquids to fresh water snail (Physa acuta) in 96-h acute toxicity exposuresa . Ionic liquid

Alkyl chain length (carbon atoms)

LC50 b (mg/dm3 )

[omp]Br [omim]Br [hmim]Br [bmim]PF6 Tetrabutyl phosphonium Br [hmp]Br [bmim]Br [bmp]Br Tetrabutyl ammonium Br

8 8 6 4 4 6 4 4 4

1.0 8.2 56.2 123.3 208.0 226.7 229.0 325.2 580.2

a

Adapted from [78]. LC50 is the concentration large enough to kill 50% of a sample of animals under test. b

the liquid stabilizing forces within the matrix. Membranes made up of polymerizable ionic liquids may be a better option for CO2 separation [66]. CO2 absorption experiments with ionic-liquid polymers demonstrate their superiority over RTILs [67]. 1-[2(Methacryloyloxy)ethyl]-3-butyl-imidazolium tetrafluoroborate, [MABI][BF4 ]; 1-(p-vinylbenzyl)-3-butyl-imidazolium tetrafluoroborate, [VBBI][BF4 ]; 1-(p-vinylbenzyl)-3-butyl-imidazolium hexafluorophosphate, [VBBI][PF6 ]; 1-(p-vinylbenzyl)-3-butylimidazolium o-benzoicsulphimide, [VBBI][Sac]; 1-(p-vinylbenzyl)3-butyl-imidazolium trifluoromethane sulfonamide, [VBBI][Tf2 N]; 1-(p-vinylbenzyl)-3-methyl-imidazolium tetrafluoroborate, [VBMI][BF4 ] ionic-liquid polymers were found remarkably fit for CO2 capture (Table 7).

The structures with polystyrene backbone exhibit higher absorption capacity as the larger substituent on imidazolium cation seems to pose hindrance in absorption. In contrast to RTILs, the poly(ionic liquids) with PF6 − show higher efficiency as compared to BF4 − or Tf2 N− anions. Moreover sorption/desorption rates of the polymerized ionic liquid is quite fast as compared to RTILs. The bulk absorption phenomenon appears to govern the capture progress [68–70]. RTILs containing polymerizable entities show higher permeability, solubility and diffusivity values for CO2 , N2 and CH4 as given in Table 8(a) and (b) [71]. As the length of the alkyl chain increases, gas permeability and diffusivity increases considerably. However, styrene-based polymer with methyl group shows higher CO2 permeability than the corresponding acrylate-based polymer. The CO2 solubility was found quite high in both types of poly(RTILs) but lower than that for poly(RTILs)-PEG copolymers [72]. Polymerizable ionic liquids exhibit high CO2 capture capacity and selectivity with respect to N2 , O2 or CH4 [73]. These polymerized structures can capture almost double the amount of CO2 compared to the corresponding RTILs. The efficiency of these polymeric structures can be enhanced further by modifying monomers with appropriate entities like oligo(ethylene glycol) or nitrile-containing alkyl groups [74]. By incorporating an appropriate amount of RTIL and consequently introducing free ion pairs into the poly(RTIL) membranes, CO2 permeability and CO2 /N2 selectivity may be increased up to about 300–600% and 25%, respectively [75,76]. Presence of longer alkyl chains on the cations of poly(ionic liquids) may pose steric hindrance between CO2 –cation interaction. This feature may shrink the microvoid volume resulting from plasticization and rigidity due to cross-linking, thus decreasing CO2 sorption capacity [77].

Table 10 Summary of CO2 capture by ionic liquids. Type

Examples

Equilibrium time

Advantages

Drawbacks

References

Bulk RTILs

[hmpy][Tf2 N] (32.8a ); [hmim][Tf2 N] (31.6a ); [bmim][Tf2 N] (33.0a ); [bmim][PF6 ] (53.4a ); [bmim][BF4 ] (59.0a ); [C6 H4 F9 mim][Tf2 N] (28.4a )

>90 min, depending upon viscosity

High viscosity, so mass transfer a major concern; Longer equilibrium time

[17]

Bulk TSILs

[Amim][BF4 ]; [Pabim][BF4 ]; [Am-im][DCA]; [Am-im]þ[BF4 ]

≥180 min

Negligible vapor pressure; thermally stable (even after multiple absorption/desorption experiments, no detectable loss in mass occurred [17]); highly CO2 -philic; CO2 capture >90% Functionalization increases the CO2 load almost three fold; CO2 loading continue to increase with rise in CO2 pressure; gas load reached up to 0.5, comparable to standard amine scrubber

[32–35]

RTILs based SILMs

[bmim][BF4 ] + PVDF

–

Extremely high viscosity ≥2000 cP, undergo further increase by CO2 complexation; much longer equilibrium time; exceptionally long regeneration time ≥24 h Higher temperatures result in decrease of selectivity

TSILs based SILMs

[C3 NH2 mim][CF3 SO3 ] + PTFE; [C3 NH2 mim][Tf2 N] + PTFE; [H2 NC3 H6 mim][Tf2 N] + crosslinked Nylon 66 P[VBBI][BF4 ] (26.0a ); P[MABI][BF4 ] (37.7a ); P[VBBI][Tf2 N]; P[VBTMA][BF4 ] (3.7a : 22 ◦ C); P[MATMA][BF4] (5.4a : 22 ◦ C)

–

Selectivity increased till 85 ◦ C and then decreased with rise in temperature

[43]

–

[46,47]

Poly(ionic liquid)s

a

<60 min

Henry’s law constant (bar) for CO2 (at 25 ◦ C except where otherwise stated).

Extremely low volatility prevents solvent loss; CO2 /CH4 selectivity 25–45; CO2 /N2 selectivity ≥127; CO2 /H2 selectivity <10; better at low temperatures CO2 /CH4 selectivity reached 100–120; CO2 /H2 selectivity >15

Highly selective CO2 absorption, compared to N2 and O2 (both showed negligible absorption); much faster CO2 sorption; poly(RTIL)s captured twice the CO2 compared to their liquid counterparts

[39]

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6. Toxicity of ILs

Acknowledgements

Due to negligible volatility, ionic liquids are not supposed to contaminate air, yet most of these, being water soluble, may pollute the hydrosphere via industrial effluents or accidental leakages. Though considerable work is being done regarding the physical, thermodynamic, kinetic or engineering aspects, comparatively much less data is available about toxicology of ionic liquids. Bernot et al. [78] investigated the toxic behavior of certain imidazolium- and pyridinium-based ionic liquids. ILs with longer alkyl chains showed higher toxicity (Table 9). Cation type did not show any pattern with respect to toxicity [78]. Similar behavior was found by Wells and Coombe about the role of alkyl-side chain length [79]. However, in order to fully evaluate the effect of cation/anion towards toxicity more sturdy analysis is needed [80]. The studies about the toxic nature of ionic liquids reveal that it is not affirmative to take them as green media. Prior to large scale employment, this very aspect need more extensive knowhow and investigations so that truly green ionic liquids could evolve for the purpose by taking advantage of their tunable nature [81].

Financial support from FL Canada Research Chair “Green processes for cleaner and sustainable energy”, funding from the Centre Québécois sur les Matériaux Fonctionnels (CQMF), and FL and SM Discovery Grants from the Natural Sciences and Engineering Research Council (NSERC) are gratefully acknowledged.

7. Current and future developments This brief survey on the current trends on the ionic-liquid mediated CO2 capture suggests that CO2 capture by ionic liquids is a feasible practice. A variety of ionic-liquid techniques involving RTILs, TSILs or SILMs can be employed for CO2 capture, extending from low to high temperature applications. Some of the advantages/drawbacks discussed in this review are provided in Table 10. At present, the lack of availability of inexpensive and diverse ionic liquids is the major cause of hesitation in employing ionicliquid systems for CO2 capture on large scales. As currently ionic liquids are being used on laboratory scale, mass production of ionic liquids, and also increased stability, low corrosion of equipment may decrease the ionic liquids based CO2 capture system price. Also, in spite of numerous studies on CO2 solubility and its selectivity in systems mimicking industrial effluents where the presence of water or other foreign molecules can affect CO2 transfer yet requires in depth investigations before industrial-scale implementation of ionic liquids is sought. Selection of an appropriate combination of the constituent ion pair (cation + anion) of ionic liquids, particularly in the context of viscosity, slower kinetics and product solubility needs to be further scrutinized. Aspects related to the gas absorption at higher temperatures and higher pressures, and subsequent regeneration without any appreciable loss and/or degradation as well as toxicological side call for intense analysis to take advantage of long-lasting cyclic use of IL-based scrubbers. An appropriate balance between cost and performance is crucial in order for these approaches to take any helm as commercially viable CO2 capture technologies. Though few pilot projects for evaluating the ability of ionic liquids in a wider scope are in progression, gas capture data is not available. Ion Engineering Company, founded by scientists of Colorado University, possesses demonstration facility and intended to use the knowhow of ionic liquids for industrial-scale sweetening of natural gas and flue gas CO2 separation [82–84]. AE&E (Austrian Energy & Environment) group also has plans to work on a pilot plant scale comparative study between MEA and task-specific ionic liquids for post-combustion CO2 capture [85]. Nevertheless, a sturdy boost up is needed as there is much greater flexibility in synthetic scenario of ionic liquids that may make them opportune to come up to the expectations for superior CO2 capture systems.

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