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- Experimental Section
- Supporting Information
Aiming at full sustainability of CO2 separation processes, a series of supported ionic liquid membranes based on environmentally friendly cholinium carboxylate ionic liquids were successfully prepared. Their gas permeation properties were measured and high permselectivities were obtained for both CO2/CH4 and CO2/N2.
Due to their close connection to global warming, anthropogenic emissions of carbon dioxide (CO2) are one of the most important environmental challenges of our era. Since the elimination of CO2 from power plants and natural gas streams involves important separation processes, there is a continuing effort towards the development of appropriate cost-effective technologies. In this context, membrane separation technology represents a viable option because it offers fundamental engineering and economic advantages compared to classical processes such as adsorption, extraction, or distillation.1
Several types of polymeric membranes for CO2 separation have been developed during the last few years,2 whereas supported liquid membranes have also been widely investigated.3 In particular, supported ionic liquid membranes (SILMs) have recently attracted considerable attention owing to the unique properties of ionic liquids (ILs) such as negligible volatility,4 high thermal stability,5 low flammability,6 and high CO2 solubility and selectivity.7 These properties provide distinctive advantages over organic solvents commonly used in supported liquid membranes.8 Although a broad diversity of ILs has been already tested for developing SILMs, covering most commonly used cations such as imidazolium,9 pyrrolidinium,10 sulphonium,11 phosphonium,12 pyridinium,13 ammonium,14 or thiazolium,15 combined with halogens, fluorinated anions, sulfates, sulfonates and nitrile-containing anions, their ecotoxicity has been until now overlooked. Recent studies showed that many of these ions have remarkable toxicities and are poorly biodegradable.16 The development of new environmental-friendly ionic liquids prepared from renewable materials has received growing attention in the last few years. In particular, cholinium-based ILs have been synthesized through simple and economical procedures.17 Due to their particular features, namely biocompatibility, biodegradability, and low toxicity, ILs combining the cholinium cation with non-hazardous anions have been tested in diverse applications, such as crosslinking agents for collagen-based materials,18 constituents of aqueous biphasic systems,19 or in the pretreatment or dissolution of biomass.20
Aiming at full sustainability of CO2 separation processes, and simultaneously keeping high permselectivities, we propose the use of cholinium carboxylates as liquid phases in supported liquid membranes for CO2 separation. The use of carboxylate anions has been shown to provide high CO2 solubilities,21 but the use of these true biodegradable ionic liquids to prepare SILMs has never been attempted before. Accordingly, we prepared four SILMs based on such ILs by combining the cholinium cation with four different anions belonging to the carboxylic acid family (Figure 1), and evaluated their CO2, methane (CH4) and nitrogen (N2) permeation properties.
Figure 1. Chemical structures of (i) cholinium cation ([Ch]) and (ii) levulinate ([Lev]), (iii) lactate ([Lac]), (iv) glycolate ([Gly]) and (v) malonate ([Mal]) anions.
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Ionic liquids used in this work, namely cholinium levulinate ([Ch][Lev]), cholinium lactate ([Ch][Lac]), cholinium glycolate ([Ch][Gly]) and cholinium malonate ([Ch][Mal]), were prepared by dropwise addition of the corresponding acid (1:1 v/v) to aqueous cholinium bicarbonate, following an established procedure.17c The mixtures were stirred at ambient temperature and pressure for 8 h. The resulting products were washed with diethyl ether to remove unreacted acid. Excess of water and traces of other volatile substances were removed first by rotary evaporation, and then by stirring and heating under vacuum. The water contents after the drying step were below 0.7 wt %. The chemical structures of the cholinium-based ILs were confirmed by 1H- and 13C NMR, electrospray ionization mass spectrometry, and elemental analysis (see Supporting Information for further details). Viscosity and density were also determined in view of the fact that both the viscosity and the molar volume of the IL are key features in the design of SILMs. A detailed description of these data in the temperature range from 293.15–343.15 K is presented in the Supporting Information (Table S2).
The gas permeation properties of the ILs were investigated by using a flat membrane. Porous hydrophilic poly(tetrafluoroethylene) (PTFE) membranes were used to prepare the SILM configurations according to our previously reported procedure.22 The single CO2, CH4, and N2 permeation properties of cholinium-based SILMs were measured by using a time-lag apparatus (details in Supporting Information), allowing the simultaneous determination of gas permeability and diffusivity. It was also possible to calculate gas solubilities given that the gas transport through a dense liquid membrane follows a solution-diffusion mechanism where permeability is the product of solubility and diffusivity.23 Thus, permeability of the SILMs depends on both thermodynamic and kinetic mechanisms, that is, gas absorption/desorption and gas diffusion, respectively.
Permeabilities and diffusivities of the prepared SILMs toward gases, measured at 293 K with a transmembrane pressure differential of 1 bar, are shown in Figure 2 and Figure 3, respectively. The CO2 permeability values vary from 18 to 2 Barrer, while CH4 and N2 permeabilities differ from 0.05 to 0.84 Barrer. For all the measured gases, the [Ch][Lev]-based SILM presents the highest gas permeabilities (Figure 2) and diffusivities (Figure 3). This behavior is most likely due to the lower viscosity of [Ch][Lev] compared to the other cholinium-ILs studied in this work (see Supporting Information, Table S2). The permeability of all gases in the cholinium-based SILMs is related to their respective gas diffusivities, with the series from the highest to the lowest permeabilities (Figure 2) being equal to that of diffusivities (Figure 3): [Ch][Lev]>[Ch][Lac]>[Ch][Gly]>[Ch][Mal].
Figure 2. Gas permeability in the prepared cholinium-based supported ionic liquid membranes (1 Barrer=10−10 cm3STP cm−1 s−1 cmHg−1). Error bars represent standard deviations based on three experimental replicas.
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Figure 3. Gas diffusivity in the prepared cholinium-based supported ionic liquid membranes. Error bars represent standard deviations based on three experimental replicas.
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The cholinium-based SILMs have lower gas permeabilities than imidazolium-, other ammonium- (i.e., different from cholinium), phosphonium-, and sulphonium-based SILMs.11 For example, we showed in a previous study that a SILM made of pure [C2mim][Lac] has CO2, CH4, and N2 permeability values of 55, 3, and 1 Barrer,22 whereas the [Ch][Lac]-based SILM reported here has 7, 0.33, and 0.43 Barrer, respectively. One possible reason for these lower gas permeabilities is the high viscosity of cholinium carboxylates when compared to the corresponding imidazolium ILs. In particular, at 293.15 K, the [Ch][Lac] viscosity is 3661.33 MPa s, while for [C2mim][Lac] the viscosity is 370.41 MPa s.22 Higher viscosity hampers gas diffusion through the SILM and consequently reduces the membrane permeability.11 Our data made apparent that the cholinium carboxylate-based SILMs follow the generally established trend that gas permeability and diffusivity through the SILM decreases as the ionic liquid viscosity increases.
The CO2, CH4, and N2 solubilities calculated in this work are presented in Table 1. For all of the prepared cholinium-based SILMs, the trend obtained for gas permeability (P>P>P; Figure 2) was also observed for solubility (S>S>S); Table 1). The CH4 and N2 solubilities were similar, and significantly lower than that of CO2, amongst the cholinium carboxylate-based SILMs. These data are in agreement with reported results for other SILMs where usually the CO2 separation is largely driven by solubility differences between CO2 and N2 or CH4 in the ionic liquids.11 Conversely, the SILM made of [Ch][Lev] exhibits the highest CO2 solubility followed by the [Ch][Mal], surpassing the [Ch][Lac] and [Ch][Gly]-based ones. The CO2 solubility values of the last two SLIMs are equivalent probably due to similarity of the chemical structures of the [Lac]− and [Gly]− anions (Figure 1). It is well known that gas solubility in SILMs is related to the IL molar volume.24 Accordingly, and despite the narrow molar volume range of the ILs used in this work, the highest CO2 solubilities were obtained in the SILMs made of [Ch][Lev] and [Ch][Mal], which also have the highest molar volumes (Supporting Information, Table S2).
Overall, all of the cholinium-based SILMs tested exhibited larger CO2 solubility values than the prototypical ILs commonly used to prepare SILMs, which are based on fluorinated or nitrile-containing anions.9f, g, 12a, 14, 25 The anion nature seems to have a stronger influence on the gas solubility than the cation.26 This is in agreement with the finding that the CO2 absorption capacity can be drastically enhanced by using ILs combining basic anions, such as acetate27 or aminoacids,28 because the CO2 solvation in those ILs occurs through chemisorption schemes.29
The ideal selectivity (or permselectivity) was obtained by dividing the permeabilities of two different pure gases. The obtained ideal CO2/N2 and CO2/CH4 permselectivities of the cholinium-based SILMs are shown in Figure 4. Because the CH4 permeability is greater than that of N2 along the cholinium-based SILMs (Figure 2), the CO2/CH4 permselectivity is always smaller than CO2/N2 permselectivity (Figure 4). The largest CO2/CH4 and CO2/N2 permselectivities were achieved for the [Ch][Gly]-based SILM. Remarkably, the CO2/CH4 permselectivity herein reported for the [Ch][Gly] represents the highest value reported so far for SILMs. Figure 4 also shows a comparison between the CO2/CH4 and CO2/N2 permselectivities obtained in this work with those of other SILMs taken from literature.9g, 12a, 14, 22, 25 For example, CO2/CH4 and CO2/N2 permselectivity values of 21.7 and 46.2, respectively, were obtained in the [Ch][Lac]-based SILM, which are slightly higher than those of 17.6 and 43.4 found in the SILM made of [C2mim][Lac] (Figure 4).22
Figure 4. Permselectivity of CO2/CH4 and CO2/N2 in the cholinium-based supported ionic liquid membranes. Literature data reported for other supported ionic liquid membranes are also plotted for comparison.9g, 12a, 14, 22, 25
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Actually, in general the CO2/CH4 permselectivity of cholinium-based SILMs not only exceed the performance of imidazolium-ILs, but also those of phosphonium-ILs and other ammonium-ILs different from cholinium.12a, 14 In addition, the CO2/N2 permselectivity is on par or greater than that of other reported SILMs, including those made of ILs combining nitrile-containing anions of proven potential for CO2 separation.9f, g, 25 These results undoubtedly show that the cholinium carboxylate ionic liquids are competitive to the use of hazardous cations carrying similar basic anions.
Although improved CO2 permselectivities and high CO2 solubilities were obtained, the permeability of cholinium-based SILMs toward dry gases was low compared to other families of ILs. The major bottleneck of using cholinium carboxylates in supported liquid membranes for gas separation processes is their high viscosities, which lead to long absorption equilibrium times. Thus, reducing the viscosity of cholinium carboxylates is certainly an important issue for their application as supported ionic liquid membranes. It is well reported that the viscosities of ILs drop when they absorb water.30 Therefore, one possible way for increasing the gas permeability through these SILMs is to reduce the IL viscosity by the addition of water.
In conclusion, we have successfully prepared supported ionic liquid membranes based on cholinium carboxylates which present both high CO2 permselectivity and CO2 solubility at 293 K under dry conditions. The results obtained in this work suggest that many opportunities exist for utilizing cholinium-carboxylates in CO2 separation applications. The high CO2 separation efficiencies herein obtained together with the remarkable environmental-friendly properties and the simple and low production costs of cholinium carboxylates make these fluids a very attractive alternative for sustainable CO2 separation processes.