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N2 separation
Journal of Membrane Science 353 (2010) 177–183
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Performance of nitrile-containing anions in task-specific ionic liquids for improved CO2 /N2 separation Shannon M. Mahurin ∗ , Je Seung Lee, Gary A. Baker, Huimin Luo, Sheng Dai ∗,1 Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, United States
a r t i c l e
i n f o
Article history: Received 4 November 2009 Received in revised form 15 January 2010 Accepted 15 February 2010 Available online 20 February 2010 Keywords: Gas separations Room temperature ionic liquids Carbon dioxide Supported liquid membranes
a b s t r a c t This work explores the performance of a series of ionic liquids that incorporate a nitrile-containing anion paired to 1-alkyl-3-methylimidazolium cations in tailoring the selectivity and permeance of supported ionic liquid membranes for CO2 /N2 separations. The permeance and selectivity of three ionic liquids, each with an increasing number of nitrile groups in the anion (i.e., two, three, and four), were measured using a non-steady-state permeation method. By predictably varying the molar volume and viscosity of the ionic liquids, we show that the solubility, selectivity, and permeance can be optimized for CO2 /N2 separation through controlled introduction of the nitrile functionality into the anion. Of the three nitrile-based ionic liquids studied, 1-ethyl-3-methylimidazolium tetracyanoborate, [emim][B(CN)4 ], showed the highest permeance with a value of 2.55 × 10−9 mol/(m2 Pa s), a magnitude 30% higher than that of the popular ionic liquid [emim][Tf2 N]. This same nitrile-bearing ionic liquid also exhibited a high CO2 /N2 selectivity of approximately 53. Additionally, the carbon dioxide solubility for each ionic liquid was measured at room temperature with [emim][B(CN)4 ] again exhibiting the highest CO2 solubility. Results from our study of the nitrile-based ionic liquids can be rationalized in terms of regular solution theory wherein the selectivity and permeance of a given SILM system are largely determined by the molar volume and viscosity of the corresponding ionic liquid phase. Published by Elsevier B.V.
1. Introduction The separation of gases from complex mixtures is a central process in a number of applications such as the separation of methane from biogas and the removal of hydrogen from product streams in ammonia plants. Given the mounting concern over greenhouse gas emissions and their implications in climate change, the elimination of carbon dioxide from natural gas streams and power plant emissions is a particularly relevant separation process that is generally carried out by amine scrubbing [1,2]. In this process, carbon dioxide is first captured via a reaction with an amine compound such as monoethanolamine followed by transport and subsequent removal of carbon dioxide by heating which regenerates the amine for additional carbon dioxide capture cycles. While this method is relatively effective for CO2 separation, it is energy intensive due to the high temperature necessary to regenerate the amine. In fact, many of the separations processes which also include cryogenic and distillation steps are similarly energy intensive, accounting for a significant portion of the total national energy consumption in the U.S.
∗ Corresponding authors. Tel.: +1 865 241 3417. E-mail addresses: [email protected] (S.M. Mahurin), [email protected] (S. Dai). 1 Tel.: +1 865 576 7307. 0376-7388/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.memsci.2010.02.045
The use of membranes for gas separation processes has grown steadily since their commercial introduction in the early 1980s, primarily because membrane-based separations require less energy than other methods. Traditionally, these membranes have been composed of non-porous organic polymers that are engineered into a variety of functional forms including sheets, rods, and asymmetric hollow fibers. Regardless of the form, non-porous polymeric membranes typically exhibit lower gas throughput than porous materials but higher separating power, or selectivity. In recent years, however, supported liquid membranes have shown particular promise for gas separations. Supported liquid membranes, which are essentially porous supports filled with a liquid solvent or solvent mixture, are attractive alternatives to non-porous polymer membranes due to the increased gas diffusion rate through the liquid compared to the solid. Unfortunately, because volatile solvents are often used to fill the porous membrane, they are susceptible to liquid loss over time through solvent evaporation which limits the long-term stability of the membrane. One promising candidate to replace volatile solvents in supported liquid membranes is room temperature ionic liquids (RTILs). These unique solvents are composed completely of ions and exhibit a broad liquid range with a melting point at or below ambient temperature. The most obvious and compelling advantage of RTILs for gas separation membranes is that as a general class of materials RTILs exhibit extremely low or even negligible vapor pressure at
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near-ambient temperatures [3]. Consequently, a supported ionic liquid membrane (SILM) exhibits improved long-term stability due to minimal solvent loss through vaporization compared to membranes based upon more traditional (volatile) liquids. Moreover, RTILs have an inherent designer nature in which many of their physicochemical properties can be tuned through appropriate variation in the cation and anion pair comprising the solvent. Properties such as melting point, viscosity, hydrophobicity/hydrophilicity, hydrogen bonding, and coordinating ability can potentially be tailored to preferentially absorb and transport specific gases such as CO2 . Finally, because RTILs are generally nonflammable with high thermal stability and essentially null vapor pressure, they are often categorized as “green” solvents which further enhance their appeal in SILMs given the current interest in more environmentally friendly materials in chemical and industrial processes. The performance of a SILM for carbon dioxide separation is principally defined by the permeance and the selectivity. These two parameters are related to the solubility and the diffusivity of CO2 and N2 gases in a specific RTIL. Thus, the innate ability to vary the properties of the RTIL such as gas solubility and viscosity through cation/anion selection enables optimization of the permeance and selectivity of the SILM. Consequently, a number of groups have investigated the carbon dioxide solubility and permeability properties for different RTIL classes including the imidazoliums, pyridiniums, ammonium, and phosphoniums [4–8]. By far, the imidazolium family has dominated the literature to date, partly because of the early commercial availability of this RTIL class and also because of the perceived ease in modifying cation structure through attachment of various functional groups for formation of RTILs specifically designed for carbon dioxide absorption and separation. For example, Bates et al. reported that tethering an amine group to an imidazolium cation resulted in a task-specific ionic liquid with improved CO2 solubility compared to the corresponding non-functionalized cation [9]. In analogy with conventional supercritical carbon dioxide emulsion surfactants [10], fluorination of the cation can also increase carbon dioxide uptake, however, this often leads to RTILs with relatively high viscosity [11]. Bara et al. showed that the addition of polar functional groups such as ether linkages can improve the CO2 /N2 selectivity for separations despite a similar CO2 solubility [12]. In addition to various cations, a number of anions including tetrafluoroborate ([BF4 − ]), hexafluorophosphate ([PF6 − ]), bis(trifluoromethylsulfonyl)imide ([Tf2 N− ]), and dicyanamide ([N(CN)2 − ]) have been explored to enhance separations in SILMs [13–15]. In fact, previous work indicates that the choice of the anion can have a more pronounced effect on the CO2 solubility and CO2 /N2 selectivity than the choice of cation [11,16]. For example, Cadena et al. used both experiment and molecular modeling to show strong organization of carbon dioxide around the [PF6 − ] anion with only minor differences in CO2 structure around the cation. They further showed that the [Tf2 N− ] anion, exhibited a high affinity for CO2 . This is in agreement with other reports where the 1-alkyl-3-methylimidazolium family with the [Tf2 N− ] anion has shown some of the highest CO2 solubilities among RTILs [6,17,18]. One of the key findings to arise is that fluorination, either of the cation or the anion, can improve the CO2 solubility of the RTIL. Because perfluorinated materials can be harmful to the ozone stratosphere, however, non-fluorinated routes for enhancing the performance of SILMs have garnered attention. Of particular relevance, a recent report by Carlisle et al. shows that adding the nitrile functionality to an imidazolium cation coupled to the [Tf2 N− ] anion improves CO2 /N2 selectivity relative to the non-functionalized cation [19]. The challenge for membrane separations of CO2 lies in simultaneously maximizing the CO2 /N2 selectivity and the carbon dioxide permeance. It has been estimated that a SILM with a
CO2 /N2 selectivity of ∼50 at a cross-membrane pressure difference under 2 bar under vacuum conditions would be more efficient than current technology for the removal of CO2 from flue gases [20–22]. In this work, we explore the use of a series of 1-alkyl-3-methylimidazolium-based RTILs that incorporate the nitrile functionality into the anion in an attempt to improve the CO2 /N2 selectivity and CO2 permeance of a SILM. As we will show, variation in the molar volume and viscosity of the nitrile-containing RTILs impacts the solubility, selectivity, and permeance in CO2 /N2 separations, offering a clear pathway for optimization. Specifically, the permeance and selectivity are determined for 1-butyl-3-methylimidazolium dicynamide ([bmim][N(CN)2 ]), 1-butyl-3-methylimidazolium tricyanomethane ([bmim][C(CN)3 ]), 1-ethyl-3-methylimidazolium tetracyanoborate ([emim][B(CN)4 ]), and the representative RTIL 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([emim][Tf2 N]) in SILMs using a non-steady-state permeation method. In addition, the CO2 solubility for each RTIL is measured at room temperature. We show that [emim][B(CN)4 ] has a high permeance with a CO2 /N2 selectivity greater than 50. Indeed, both the permeance and CO2 /N2 selectivity were higher for [emim][B(CN)4 ] compared to the popular [emim][Tf2 N] homolog. Our results are discussed in terms of the molar volume and viscosity of the RTIL using regular solution theory as a model for the experimentally determined permeance and selectivity [13,18,19]. 2. Experimental 2.1. Materials Nitrogen and carbon dioxide gas cylinders (99.99% purity) were obtained from Air Liquide. The molecular structures of the RTILs used in this work ([emim][Tf2 N], [bmim][N(CN)2 ], [bmim][C(CN)3 ] and [emim][B(CN)4 ]) are provided in Fig. 1. The [bmim][C(CN)3 ] and [emim][B(CN)4 ] were purchased from Merck Chemicals and used without further purification. The [emim][Tf2 N] and [bmim][N(CN)2 ] were synthesized, purified, and dried in our laboratories according to published procedures [23,24]. 2.2. Formation of SILMs SILMs were formed by infusing the desired RTIL into a 47-mm Supor-100 porous membrane filter (Pall Corporation). These membrane filters are hydrophilic polyethersulfone (PES) membranes with a pore diameter of 0.1 m and a thickness of approximately 130 m. For the [bmim][C(CN)3 ] RTIL, an anodic alumina membrane (Whatman) with a pore diameter of 0.1 m and a thickness of 60 m was used as the support because this RTIL was found to damage the PES membrane. To achieve stable and reproducible SILMs, care must be taken to ensure that the RTIL completely fills the pores of the membrane with no pinholes or open pores. Loading of the membrane consisted of first placing the membrane in a glass Petri dish that contained 0.5 mL of the desired RTIL. After allowing the RTIL to infuse into the pores of the membrane for approximately 30 min, an additional 0.5 mL of RTIL was deposited into the Petri dish, completely immersing the membrane. After soaking for 10 additional minutes, the RTIL-infused membrane was placed in a vacuum desiccator for 12 h to degas the sample, eliminate residual water which could influence the measured permeance, and remove any entrapped air from the membrane that might prevent complete filling of the pores. After degassing, the Petri dish was taken out of the vacuum desiccator and the immersed membrane was removed from the RTIL. Both sides of the membrane were carefully wiped with filter
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Fig. 1. Structure of the conventional non-nitrile RTIL [emim][Tf2 N] alongside RTILs formed from anions bearing two, three, and four nitrile moieties, respectively.
paper (Whatman) to remove excess RTIL prior to loading into the membrane test chamber. 2.3. Membrane test chamber Permeance measurements were performed on the SILM using a custom test chamber (Fig. 2) built in house. This system, which consists of a feed and permeate chamber separated by the test membrane, measures the transient permeation through the SILM. For membrane loading, the SILM was first placed on a hydrophobic PTFE membrane (Pall Corporation) with a pore diameter of 0.5 m. Because the RTILs do not wet this membrane, an additional level of stability was imparted to the SILM that helped minimize RTIL loss resulting from the high-pressure gas on the feed side forcing RTIL through the pores. The SILM/PTFE membrane stack was then carefully placed on a highly porous stainless steel support that provided mechanical stability and negligible resistance to gases and the entire SILM assembly was installed into the test chamber. Two Viton o-rings were used to separate the feed chamber from the permeate chamber to eliminate leaking. In addition, a Viton gasket was positioned on top of the SILM to provide an additional seal and to prevent the fragile membrane from directly contacting the metal feed chamber. With this arrangement, the only path from the feed chamber to the permeate chamber occurs through the SILM itself. The open area of the membrane was 10.75 cm2 which corresponds to a 37-mm open diameter. After loading the SILM, the chamber was evacuated with a mechanical pump to a base pressure of 40 mTorr. The membrane was allowed to remain in the test chamber for at least 1 h to fully degas the RTIL and reach a steady base pressure. All measurements described here are single-gas permeation values in which either carbon dioxide or nitrogen is individually introduced to the feed side at a pre-determined pressure. A ballast volume of approximately 300 mL was used to minimize pressure loss during the experiment. After setting the gas pressure on the feed side, the valve to the cylinder was closed. The pressure on the permeate side was then measured as a function of time using
a Baratron pressure gauge (MKS Instruments) connected to a data acquisition board (Vernier). The pressure measurement from the Baratron gauge is independent of gas composition and has a range up to 100 Torr. The pressure on the permeate side was typically measured for 30 min. No corrections were made for the tortuosity of the polymer membrane support and all measurements were acquired at room temperature. 2.4. Gas solubility measurement Low-pressure CO2 solubility measurements were acquired using an Intelligent Gravimetric Analyzer (Hiden Analytical Limited, UK). Typically, approximately 80 mg of a particular RTIL was loaded into a quartz bucket cell and evacuated to 0.1 bar for 6 h at 150 ◦ C to degas and dry the sample. All measurements were acquired at room temperature. The mass uptake (corrected for buoyancy) was then measured as a function of pressure up to a final pressure of 10 atm to obtain the absorption isotherm. Desorption isotherms were subsequently acquired by measuring the mass as a function of decreasing pressure to ensure that the solubility behavior was reversible and to test for hysteresis effects. The solubility was then calculated from the slope of the adsorption isotherm in the low carbon dioxide concentration regime. 3. Results and discussion In general, permeation of a gas through a SILM proceeds via the following steps: (i) absorption of the permeant gas into the supported liquid from the high-pressure side of the membrane, (ii) diffusion of the gas across the membrane, and (iii) desorption of the gas out into the low-pressure side of the membrane. The permeability of a membrane thus depends on both a thermodynamic mechanism as the gas absorbs into and desorbs out of the membrane, as well as a kinetic component reflected in gas diffusion through the membrane. More specifically, gas separation in dense membranes such as polymer membranes and SILMs generally follows a solution-diffusion mechanism in which the permeability of a gas (Pi ) is related to the gas solubility and diffusivity according to: Pi = Si Di
(1)
where Si is the solubility and Di is the diffusivity. The ideal selectivity of a membrane (˛ij ) is simply the ratio of the pure gas permeabilities for species i and j: ˛ij =
Fig. 2. Experimental test setup for the permeance measurements where V = valve, P = pressure gauge, and Ba = Baratron pressure gauge.
Pi S D = i i Pj Sj Dj
(2)
where Di /Dj is the diffusivity selectivity and Si /Sj is the solubility selectivity. The ideal selectivity is a function of both the solubility selectivity and the diffusivity selectivity. For SILMs, however, the selectivity is dominated by the solubility selectivity rather than the diffusivity selectivity [6,25]. This can be justified by noting that the diffusivity selectivity is proportional to the ratio of the molecular volumes of the gases considered; in the case of
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Fig. 4. Carbon dioxide and nitrogen permeance values for all the RTILs. Nitrogen permeance values are multiplied by 5× for clarity. Fig. 3. Pressure rise in the permeate chamber as CO2 diffuses through the SILM for (a) [emim][B(CN)4 ] and (b) [emim][Tf2 N].
CO2 /N2 the diffusivity selectivity is approximately one [6,15,25]. As a result, many groups have focused on the measurement of solubility selectivity in RTILs to predict their overall selectivity in SILMs. In this work, we measured the solubility, permeance, and selectivity for a series of RTILs based on polynitrile anions with 1-alkyl-3-methylimidazolium cations for tackling the CO2 /N2 separation problem. 3.1. Permeance of [emim][Tf2 N] Because [emim][Tf2 N] has been extensively investigated and exhibits both good CO2 permeability and CO2 /N2 selectivity, we used this RTIL to validate our system and to provide a point of comparison for gauging the nitrile-containing RTIL results. For our measurements, [emim][Tf2 N] was loaded into the PES membrane according to standard procedures and the CO2 and N2 permeance values measured using the test apparatus detailed in Section 2.3. There was no explicit control over the temperature of the test chamber, so all measurements were performed at ambient temperature (298 ± 1 K). Fig. 3 shows a typical plot of the carbon dioxide pressure rise measured on the permeate side of the separation chamber for permeation experiments employing [emim][Tf2 N] versus [emim][B(CN)4 ] in SILMs. Note that there is an initial lag time as the carbon dioxide diffuses through the SILM followed by a linear rise in the pressure as the carbon dioxide flux through the membrane reaches a steady state [26]. The pressure rise is clearly higher for [emim][B(CN)4 ] which will be discussed in more detail in subsequent sections. Permeance values were calculated using only those data collected in the post-lag linear regime according to the following expression. Permeance =
V dP RTA(P − P ) dt
(3)
Here, V is the permeate volume, R is the ideal gas constant, T is the absolute temperature, A is the membrane area, P is the upstream pressure, P is the downstream pressure, and dP /dt is the rate of gas pressure increase on the permeate side. The selectivity was obtained by calculating the ratio of the carbon dioxide permeance over nitrogen permeance. Triplicate SILMs containing [emim][Tf2 N] were fabricated and the carbon dioxide and nitrogen permeance values determined multiple times for each membrane. The mean permeance and standard deviation for all measurements were then calculated
and found to be 1.85 ± 0.05 × 10−9 mol/(m2 Pa s). In recent work, Scovazzo et al. reported the mixed-gas permeance of CO2 in an [emim][Tf2 N] SILM and presented a statistically equivalent value of 1.87 × 10−9 mol/(m2 Pa s) with an ideal selectivity of 23 [14]. The average N2 permeance in [emim][Tf2 N] was measured at 6.4 × 10−11 mol/(m2 Pa s) in our case with a calculated selectivity of roughly 29, a result slightly higher but similar to values reported in the literature. In general, the measured permeance of N2 exhibited considerable variability leading to higher uncertainty in the selectivity. There is, however, excellent agreement between our single-gas selectivity results and previously published mixed-gas data for [emim][Tf2 N]. It should be pointed out that this comparison is valid since the ideal selectivity is equivalent to the mixed-gas selectivity of a SILM because the selectivity for these membranes is primarily controlled by the solubility selectivity [22]. 3.2. Effect of nitrile anion on permeance and RTIL viscosity After validating the system with [emim][Tf2 N], the permeance of CO2 and N2 were measured for SILMs incorporating [bmim][N(CN)2 ], [bmim][C(CN)3 ] and [emim][B(CN)4 ] as shown in Fig. 4. Because the N2 permeance is generally an order of magnitude lower than the CO2 permeance, each of the N2 values are multiplied by a factor five (as indicated by the 5×) to make them more visible in the figure. Permeance results for [emim][Tf2 N], discussed in the preceding section, are included for ease of comparison. Once again, it should be noted that the error bars correspond to the standard deviation over multiple permeance measurements from three individual SILMs for each RTIL. The CO2 permeance of [bmim][N(CN)2 ] is approximately 0.54 × 10−9 mol/(m2 Pa s). By way of comparison, the RTIL [emim][N(CN)2 ], which only differs in having a shorter ethyl chain at the 1-position of the imidazolium cation in place of n-butyl, has a reported permeance almost 50% higher at 0.79 × 10−9 mol/(m2 Pa s) [14]. The permeance of [bmim][C(CN)3 ] was strikingly higher at 1.88 ± 0.07 × 10−9 mol/(m2 Pa s), a value equivalent to the permeance of [emim][Tf2 N]. Proceeding to [emim][B(CN)4 ], which contains yet another nitrile moiety in the anion structure, the permeance increases again to 2.71 ± 0.05 × 10−9 mol/(m2 Pa s). Not only is the permeance of [emim][B(CN)4 ]-based SILMs higher than those from SILMs based on RTILs composed of anions with fewer pendant nitriles, it is also substantially higher than for [emim][Tf2 N] which is known to have one of the highest permeabilities among RTILs [22].
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Table 1 Abbreviations, viscosities, densities, CO2 /N2 selectivities, molar volumes, and CO2 solubilities for the RTILs discussed.a . RTIL
IL (cP)b
Density (g/mL)
CO2 /N2 selectivity
VIL (cm3 /mol)c
CO2 solubility (mol L−1 atm−1 )
[emim][Tf2 N] [bmim][N(CN)2 ] [bmim][C(CN)3 ] [emim][B(CN)4 ]
27 33 26 17
1.51 1.07 1.04 1.03
29 51 45 53
259 192 221 219
0.103 0.060 0.075 0.131
a b c
Data for 298 K unless otherwise noted. RTIL dynamic viscosity. RTIL molar volume.
While the permeability is related to both the solubility and diffusivity, it is instructive to first consider the permeance values in terms of the diffusivity. A general form for the diffusivity in RTILs has been given as: D=A
VILa bIL Vlc
(4)
where A, a, b, and c are parameters specific to the class of RTIL, bIL is the viscosity of the RTIL, VILa is the molar volume of the IL (cm3 /mol), and Vlc is the molar volume of the gas solute [5,22,26]. For RTILs with 1-alkyl-3-methylimidazolium cations for an alkyl chain length less than four, a is small, which indicates that the CO2 diffusivity is inversely proportional to the viscosity of the RTIL. Consistent with this scenario, from Table 1, it can be seen that [bmim][N(CN)2 ] has the highest viscosity and also the lowest measured permeance (Fig. 4) for both CO2 and N2 . Likewise, [emim][B(CN)4 ] exhibits the lowest viscosity and simultaneously the highest permeance. Furthermore, [bmim][C(CN)3 ] and [emim][Tf2 N] have essentially identical and intermediate viscosities and similar permeance values as well. Clearly, the overwhelming trend established is that gas permeance scales with RTIL fluidity. In a more quantitative sense, Fig. 5 shows the permeance values as a function of the viscosity. Previous literature reports indicate that the viscosity exponent b in Eq. (1) is approximately 0.5, although there is no rigorously developed theory for this viscosity dependence. The fit in Fig. 5 then is a simple fit to 1/bIL where b is equal to 0.65. With the exception of the [bmim][N(CN)2 ], which has a lower-than-expected permeance value, the permeance data roughly follow the trend of varying inversely with viscosity to the 0.65 power. This is in reasonable agreement with literature values, given that the aforementioned 0.5 power law reported is not a theoretical value but an empirical observation.
The trend of the permeance values as a function of viscosity for the nitrile-containing RTILs is in good agreement with recent benchmarks noted by Scovazzo where it was shown that CO2 permeability increases with increasing viscosity [22]. In that review, it was also noted that the highest permeability values should come from RTILs with a molar volume similar to [emim][Tf2 N]. From Table 1, the molar volume of [emim][B(CN)4 ], which exhibited the highest permeance of the RTILs measured in this study, is less than [emim][Tf2 N]. This could indicate a higher dependence of permeability on viscosity and it also reflects that there is no direct functionality between viscosity and molar volume. 3.3. CO2 /N2 selectivity and carbon dioxide solubility As previously mentioned, the selectivity of a SILM is primarily determined by the solubility selectivity, (Si /Sj ). A number of approaches have been proposed to model the gas solubility within an RTIL. There have been two models proposed with direct application to SILMs, both of which have been based on regular solution theory [17,18]. One is a two-component model that includes both the molar volume of the RTIL (VIL ) and the viscosity (IL ) and has been applied to a variety of RTIL classes [27]. The second model is limited to imidazolium-based RTILs and incorporates only VIL to predict the gas solubility and solubility selectivity [6]. Because we focus solely on imidazolium RTILs in this work, we will explore the selectivity and solubility results in terms of the second model, or the Camper Model. According to this approach, the gas solubility is given by:
S=
exp
˛+
4/3
VIL
− 1 VIL
−1 (5)
where S is the solubility (in moles of gas per liter of RTIL), ˛ and ˇ are gas-specific parameters. If the derivative of Eq. (5) with respect to the VIL is set to 0, then we obtain the molar volume at which the maximum gas solubility is attained [6]:
VIL,max =
Fig. 5. Permeance change as a function of viscosity along with a theoretical curve obtained using the diffusivity equation (Eq. (4)).
ˇ
4ˇ 3
3/4 (6)
The selectivity can then be calculated by forming the ratio of the solubilities of CO2 and N2 . The model then predicts that the CO2 solubility (and most importantly the CO2 /N2 selectivity) is directly related to the molar volume of the RTIL, the CO2 /N2 selectivity increasing as VIL decreases [6]. Fig. 6 shows representative CO2 uptake isotherms for all four RTILs while the resulting carbon dioxide solubilities are given in Table 1. Note that the CO2 solubility for [emim][Tf2 N] is 0.103 mol L−1 atm−1 , which is in excellent agreement with previously reported values [15]. From the solubility data, [emim][B(CN)4 ] exhibits the highest CO2 solubility of the RTILs studied (0.131 mol L−1 atm−1 ), surpassing even [emim][Tf2 N]. [bmim][N(CN)2 ], on the other hand, exhibits the lowest solubility of the nitrile-based RTILs, which is not unexpected given that [emim][N(CN)2 ] also has a low CO2 solubility. The [bmim][C(CN)3 ]
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Scovazzo has shown that an upper bound for the permeability and selectivity of SILMs can be predicted using the Camper Model [22]. In that review, it was reported that [emim][Tf2 N] currently exhibits the highest permeability while [emim][dca] has the best selectivity. These two ILs lie on the predicted upper bound of performance. Of the nitrile-based RTILs that we examined, the [emim][B(CN)4 ] exhibited the best performance with a permeance that is higher than [emim][Tf2 N] and excellent selectivity, comparable to [emim][N(CN)2 ]. The improved performance of the [emim][B(CN)4 ] results from the relatively low VIL which leads to high CO2 solubility selectivity compared to [emim][Tf2 N] coupled with a low viscosity that achieves high permeance values. We do not, however, rule out interactions with the nitrile group in the anion for increasing the CO2 selectivity. 4. Conclusions
Fig. 6. Carbon dioxide solubility as a function of pressure for the four different ionic liquids. The solid lines are linear least square fit lines.
exhibits a CO2 solubility of 0.075 mol L−1 atm−1 , which is intermediate between [emim][B(CN)4 ] and [bmim][N(CN)2 ]. Fig. 7 displays the CO2 /N2 selectivity values for the RTILs measured in this study ([emim][Tf2 N], [bmim][N(CN)2 ], [bmim][C(CN)3 ], [emim][B(CN)4 ]) as well as the selectivity for three additional RTILs ([emim][N(CN)2 ], [bmim][Tf2 N], and [bmim][PF6 ]) as reported in the literature [6,22]. Note that the CO2 /N2 selectivity generally increases with decreasing VIL . Fig. 7 also includes the relationship between selectivity and molar volume as predicted by regular solution theory in the form of the Camper model. Clearly, the trend in CO2 /N2 selectivity for the nitrile-based RTILs approximately tracks with the molar volume where RTILs with decreasing VIL yield higher CO2 /N2 selectivities. It is interesting to note that both molten salts and RTILs share a common relationship between surface tension and VIL , wherein surface tension is inversely correlated to VIL [28]. The selectivities of our nitrile-based RTIL series follow reported trends as evidenced by the reasonable agreement between the Camper model predictions and our results, shown in Fig. 7. In a recent review,
In this work, a new class of non-fluorinated RTILs with excellent separation performance has been developed. We show that the judicious choice of the cation and anion pair leads to SILM with improved permeance and selectivity properties. By varying the number of nitrile groups in the anion of an imidazolium-based RTIL, the separation properties of a SILM could be considerably enhanced relative to previously reported performance data. Specifically, the [emim][B(CN)4 ]-based SILM showed a permeance of 2.55 × 10−9 mol/(m2 Pa s), which is 30% higher than the permeance of [emim][Tf2 N], a well-known RTIL with a high CO2 permeance. Furthermore, this RTIL exhibited a high CO2 /N2 selectivity with a value of 53. The results of our nitrile-based RTILs can be rationalized in terms of the regular solution theory where the selectivity and permeance of a SILM system are determined by the molar volume and viscosity of the corresponding RTIL, respectively. We have shown that the simultaneous optimization of both molar volume and viscosity through the use of nitrile-based RTILs can lead to improved separations performance. Based upon these promising trends, future work will seek to explore alternate designer, lowmolar volume RTILs toward further improved CO2 /N2 separations. Acknowledgements This work was fully sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy. References
Fig. 7. Selectivity of SILMs as a function of molar volume. The closed squares correspond to data from this work while the open squares are literature values for [emim][N(CN)2 ] [bmim][PF6 ] [bmim][Tf2 N] (see Refs. [6,22]). The solid line is a plot of the selectivity predicted by the Camper model.
[1] J.E. Bara, T.K. Carlisle, C.J. Gabriel, D. Camper, A. Finotello, D.L. Gin, R.D. Noble, Guide to CO2 separations in imidazolium-based room-temperature ionic liquids, Ind. Eng. Chem. Res. 48 (2009) 2739–2751. [2] R.E. Baltus, R.M. Counce, B.H. Culbertson, H.M. Luo, D.W. Depaoli, S. Dai, D.C. Duckworth, Examination of the potential of ionic liquids for gas separations, Sep. Sci. Tech. 40 (2005) 525–541. [3] H.M. Luo, G.A. Baker, S. Dai, Isothermogravimetric determination of the enthalpies of vaporization of 1-alkyl-3-methylimidazolium ionic liquids, J. Phys. Chem. B 112 (2008) 10077–10081. [4] J.L. Anderson, J.K. Dixon, J.F. Brennecke, Solubility of CO2 , CH4 , C2 H6 , C2 H4 , O2, and N-2 in 1-hexyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide: comparison to other ionic liquids, Acc. Chem. Res. 40 (2007) 1208–1216. [5] L. Ferguson, P. Scovazzo, Solubility, diffusivity, and permeability of gases in phosphonium-based room temperature ionic liquids: data and correlations, Ind. Eng. Chem. Res. 46 (2007) 1369–1374. [6] D. Camper, J. Bara, C. Koval, R. Noble, Bulk-fluid solubility and membrane feasibility of Rmim-based room-temperature ionic liquids, Ind. Eng. Chem. Res. 45 (2006) 6279–6283. [7] M.B. Shiflett, A. Yokozeki, Solubility of CO2 in room temperature ionic liquid [hmim][Tf2N], J. Phys. Chem. B 111 (2007) 2070–2074. [8] R. Condemarin, P. Scovazzo, Gas permeabilities, solubilities, diffusivities, and diffusivity correlations for ammonium-based room temperature ionic liquids with comparison to imidazolium and phosphonium RTIL data, Chem. Eng. J. 147 (2009) 51–57. [9] E.D. Bates, R.D. Mayton, I. Ntai, J.H. Davis, CO2 capture by a task-specific ionic liquid, J. Am. Chem. Soc. 124 (2002) 926–927.
S.M. Mahurin et al. / Journal of Membrane Science 353 (2010) 177–183 [10] S. Pandey, G.A. Baker, M.A. Kane, N.J. Bonzagni, F.V. Bright, O-2 quenching of ruthenium(II) tris(2,2’-bypyridyl)(2+) within the water pool of perfluoropolyether-based reverse micelles formed in supercritical carbon dioxide, Langmuir 16 (2000) 5593–5599. [11] M.J. Muldoon, S. Aki, J.L. Anderson, J.K. Dixon, J.F. Brennecke, Improving carbon dioxide solubility in ionic liquids, J. Phys. Chem. B 111 (2007) 9001–9009. [12] J.E. Bara, C.J. Gabriel, S. Lessmann, T.K. Carlisle, A. Finotello, D.L. Gin, R.D. Noble, Enhanced CO2 separation selectivity in oligo(ethylene glycol) functionalized room-temperature ionic liquids, Ind. Eng. Chem. Res. 46 (2007) 5380–5386. [13] A. Finotello, J.E. Bara, S. Narayan, D. Camper, R.D. Noble, Ideal gas solubilities and solubility selectivities in a binary mixture of room-temperature ionic liquids, J. Phys. Chem. B 112 (2008) 2335–2339. [14] P. Scovazzo, D. Havard, M. Mcshea, S. Mixon, D. Morgan, Long-term, continuous mixed-gas dry fed CO2 /CH4 and CO2 /N-2 separation performance and selectivities for room temperature ionic liquid membranes, J. Membr. Sci. 327 (2009) 41–48. [15] P. Scovazzo, J. Kieft, D.A. Finan, C. Koval, D. Dubois, R. Noble, Gas separations using non-hexafluorophosphate [PF6](−) anion supported ionic liquid membranes, J. Membr. Sci. 238 (2004) 57–63. [16] J.L. Anthony, J.L. Anderson, E.J. Maginn, J.F. Brennecke, Anion effects on gas solubility in ionic liquids, J. Phys. Chem. B 109 (2005) 6366–6374. [17] D. Camper, P. Scovazzo, C. Koval, R. Noble, Gas solubilities in room-temperature ionic liquids, Ind. Eng. Chem. Res. 43 (2004) 3049–3054. [18] P. Scovazzo, D. Camper, J. Kieft, J. Poshusta, C. Koval, R. Noble, Regular solution theory and CO2 gas solubility in room-temperature ionic liquids, Ind. Eng. Chem. Res. 43 (2004) 6855–6860. [19] T.K. Carlisle, J.E. Bara, C.J. Gabriel, R.D. Noble, D.L. Gin, Interpretation of CO2 solubility and selectivity in nitrile-functionalized room-temperature ionic liquids using a group contribution approach, Ind. Eng. Chem. Res. 47 (2008) 7005–7012.
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Performance of nitrile-containing anions in task-specific ionic liquids for improved CO2 /N2 separation Shannon M. Mahurin ∗ , Je Seung Lee, Gary A. Baker, Huimin Luo, Sheng Dai ∗,1 Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, United States
a r t i c l e
i n f o
Article history: Received 4 November 2009 Received in revised form 15 January 2010 Accepted 15 February 2010 Available online 20 February 2010 Keywords: Gas separations Room temperature ionic liquids Carbon dioxide Supported liquid membranes
a b s t r a c t This work explores the performance of a series of ionic liquids that incorporate a nitrile-containing anion paired to 1-alkyl-3-methylimidazolium cations in tailoring the selectivity and permeance of supported ionic liquid membranes for CO2 /N2 separations. The permeance and selectivity of three ionic liquids, each with an increasing number of nitrile groups in the anion (i.e., two, three, and four), were measured using a non-steady-state permeation method. By predictably varying the molar volume and viscosity of the ionic liquids, we show that the solubility, selectivity, and permeance can be optimized for CO2 /N2 separation through controlled introduction of the nitrile functionality into the anion. Of the three nitrile-based ionic liquids studied, 1-ethyl-3-methylimidazolium tetracyanoborate, [emim][B(CN)4 ], showed the highest permeance with a value of 2.55 × 10−9 mol/(m2 Pa s), a magnitude 30% higher than that of the popular ionic liquid [emim][Tf2 N]. This same nitrile-bearing ionic liquid also exhibited a high CO2 /N2 selectivity of approximately 53. Additionally, the carbon dioxide solubility for each ionic liquid was measured at room temperature with [emim][B(CN)4 ] again exhibiting the highest CO2 solubility. Results from our study of the nitrile-based ionic liquids can be rationalized in terms of regular solution theory wherein the selectivity and permeance of a given SILM system are largely determined by the molar volume and viscosity of the corresponding ionic liquid phase. Published by Elsevier B.V.
1. Introduction The separation of gases from complex mixtures is a central process in a number of applications such as the separation of methane from biogas and the removal of hydrogen from product streams in ammonia plants. Given the mounting concern over greenhouse gas emissions and their implications in climate change, the elimination of carbon dioxide from natural gas streams and power plant emissions is a particularly relevant separation process that is generally carried out by amine scrubbing [1,2]. In this process, carbon dioxide is first captured via a reaction with an amine compound such as monoethanolamine followed by transport and subsequent removal of carbon dioxide by heating which regenerates the amine for additional carbon dioxide capture cycles. While this method is relatively effective for CO2 separation, it is energy intensive due to the high temperature necessary to regenerate the amine. In fact, many of the separations processes which also include cryogenic and distillation steps are similarly energy intensive, accounting for a significant portion of the total national energy consumption in the U.S.
∗ Corresponding authors. Tel.: +1 865 241 3417. E-mail addresses: [email protected] (S.M. Mahurin), [email protected] (S. Dai). 1 Tel.: +1 865 576 7307. 0376-7388/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.memsci.2010.02.045
The use of membranes for gas separation processes has grown steadily since their commercial introduction in the early 1980s, primarily because membrane-based separations require less energy than other methods. Traditionally, these membranes have been composed of non-porous organic polymers that are engineered into a variety of functional forms including sheets, rods, and asymmetric hollow fibers. Regardless of the form, non-porous polymeric membranes typically exhibit lower gas throughput than porous materials but higher separating power, or selectivity. In recent years, however, supported liquid membranes have shown particular promise for gas separations. Supported liquid membranes, which are essentially porous supports filled with a liquid solvent or solvent mixture, are attractive alternatives to non-porous polymer membranes due to the increased gas diffusion rate through the liquid compared to the solid. Unfortunately, because volatile solvents are often used to fill the porous membrane, they are susceptible to liquid loss over time through solvent evaporation which limits the long-term stability of the membrane. One promising candidate to replace volatile solvents in supported liquid membranes is room temperature ionic liquids (RTILs). These unique solvents are composed completely of ions and exhibit a broad liquid range with a melting point at or below ambient temperature. The most obvious and compelling advantage of RTILs for gas separation membranes is that as a general class of materials RTILs exhibit extremely low or even negligible vapor pressure at
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near-ambient temperatures [3]. Consequently, a supported ionic liquid membrane (SILM) exhibits improved long-term stability due to minimal solvent loss through vaporization compared to membranes based upon more traditional (volatile) liquids. Moreover, RTILs have an inherent designer nature in which many of their physicochemical properties can be tuned through appropriate variation in the cation and anion pair comprising the solvent. Properties such as melting point, viscosity, hydrophobicity/hydrophilicity, hydrogen bonding, and coordinating ability can potentially be tailored to preferentially absorb and transport specific gases such as CO2 . Finally, because RTILs are generally nonflammable with high thermal stability and essentially null vapor pressure, they are often categorized as “green” solvents which further enhance their appeal in SILMs given the current interest in more environmentally friendly materials in chemical and industrial processes. The performance of a SILM for carbon dioxide separation is principally defined by the permeance and the selectivity. These two parameters are related to the solubility and the diffusivity of CO2 and N2 gases in a specific RTIL. Thus, the innate ability to vary the properties of the RTIL such as gas solubility and viscosity through cation/anion selection enables optimization of the permeance and selectivity of the SILM. Consequently, a number of groups have investigated the carbon dioxide solubility and permeability properties for different RTIL classes including the imidazoliums, pyridiniums, ammonium, and phosphoniums [4–8]. By far, the imidazolium family has dominated the literature to date, partly because of the early commercial availability of this RTIL class and also because of the perceived ease in modifying cation structure through attachment of various functional groups for formation of RTILs specifically designed for carbon dioxide absorption and separation. For example, Bates et al. reported that tethering an amine group to an imidazolium cation resulted in a task-specific ionic liquid with improved CO2 solubility compared to the corresponding non-functionalized cation [9]. In analogy with conventional supercritical carbon dioxide emulsion surfactants [10], fluorination of the cation can also increase carbon dioxide uptake, however, this often leads to RTILs with relatively high viscosity [11]. Bara et al. showed that the addition of polar functional groups such as ether linkages can improve the CO2 /N2 selectivity for separations despite a similar CO2 solubility [12]. In addition to various cations, a number of anions including tetrafluoroborate ([BF4 − ]), hexafluorophosphate ([PF6 − ]), bis(trifluoromethylsulfonyl)imide ([Tf2 N− ]), and dicyanamide ([N(CN)2 − ]) have been explored to enhance separations in SILMs [13–15]. In fact, previous work indicates that the choice of the anion can have a more pronounced effect on the CO2 solubility and CO2 /N2 selectivity than the choice of cation [11,16]. For example, Cadena et al. used both experiment and molecular modeling to show strong organization of carbon dioxide around the [PF6 − ] anion with only minor differences in CO2 structure around the cation. They further showed that the [Tf2 N− ] anion, exhibited a high affinity for CO2 . This is in agreement with other reports where the 1-alkyl-3-methylimidazolium family with the [Tf2 N− ] anion has shown some of the highest CO2 solubilities among RTILs [6,17,18]. One of the key findings to arise is that fluorination, either of the cation or the anion, can improve the CO2 solubility of the RTIL. Because perfluorinated materials can be harmful to the ozone stratosphere, however, non-fluorinated routes for enhancing the performance of SILMs have garnered attention. Of particular relevance, a recent report by Carlisle et al. shows that adding the nitrile functionality to an imidazolium cation coupled to the [Tf2 N− ] anion improves CO2 /N2 selectivity relative to the non-functionalized cation [19]. The challenge for membrane separations of CO2 lies in simultaneously maximizing the CO2 /N2 selectivity and the carbon dioxide permeance. It has been estimated that a SILM with a
CO2 /N2 selectivity of ∼50 at a cross-membrane pressure difference under 2 bar under vacuum conditions would be more efficient than current technology for the removal of CO2 from flue gases [20–22]. In this work, we explore the use of a series of 1-alkyl-3-methylimidazolium-based RTILs that incorporate the nitrile functionality into the anion in an attempt to improve the CO2 /N2 selectivity and CO2 permeance of a SILM. As we will show, variation in the molar volume and viscosity of the nitrile-containing RTILs impacts the solubility, selectivity, and permeance in CO2 /N2 separations, offering a clear pathway for optimization. Specifically, the permeance and selectivity are determined for 1-butyl-3-methylimidazolium dicynamide ([bmim][N(CN)2 ]), 1-butyl-3-methylimidazolium tricyanomethane ([bmim][C(CN)3 ]), 1-ethyl-3-methylimidazolium tetracyanoborate ([emim][B(CN)4 ]), and the representative RTIL 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([emim][Tf2 N]) in SILMs using a non-steady-state permeation method. In addition, the CO2 solubility for each RTIL is measured at room temperature. We show that [emim][B(CN)4 ] has a high permeance with a CO2 /N2 selectivity greater than 50. Indeed, both the permeance and CO2 /N2 selectivity were higher for [emim][B(CN)4 ] compared to the popular [emim][Tf2 N] homolog. Our results are discussed in terms of the molar volume and viscosity of the RTIL using regular solution theory as a model for the experimentally determined permeance and selectivity [13,18,19]. 2. Experimental 2.1. Materials Nitrogen and carbon dioxide gas cylinders (99.99% purity) were obtained from Air Liquide. The molecular structures of the RTILs used in this work ([emim][Tf2 N], [bmim][N(CN)2 ], [bmim][C(CN)3 ] and [emim][B(CN)4 ]) are provided in Fig. 1. The [bmim][C(CN)3 ] and [emim][B(CN)4 ] were purchased from Merck Chemicals and used without further purification. The [emim][Tf2 N] and [bmim][N(CN)2 ] were synthesized, purified, and dried in our laboratories according to published procedures [23,24]. 2.2. Formation of SILMs SILMs were formed by infusing the desired RTIL into a 47-mm Supor-100 porous membrane filter (Pall Corporation). These membrane filters are hydrophilic polyethersulfone (PES) membranes with a pore diameter of 0.1 m and a thickness of approximately 130 m. For the [bmim][C(CN)3 ] RTIL, an anodic alumina membrane (Whatman) with a pore diameter of 0.1 m and a thickness of 60 m was used as the support because this RTIL was found to damage the PES membrane. To achieve stable and reproducible SILMs, care must be taken to ensure that the RTIL completely fills the pores of the membrane with no pinholes or open pores. Loading of the membrane consisted of first placing the membrane in a glass Petri dish that contained 0.5 mL of the desired RTIL. After allowing the RTIL to infuse into the pores of the membrane for approximately 30 min, an additional 0.5 mL of RTIL was deposited into the Petri dish, completely immersing the membrane. After soaking for 10 additional minutes, the RTIL-infused membrane was placed in a vacuum desiccator for 12 h to degas the sample, eliminate residual water which could influence the measured permeance, and remove any entrapped air from the membrane that might prevent complete filling of the pores. After degassing, the Petri dish was taken out of the vacuum desiccator and the immersed membrane was removed from the RTIL. Both sides of the membrane were carefully wiped with filter
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Fig. 1. Structure of the conventional non-nitrile RTIL [emim][Tf2 N] alongside RTILs formed from anions bearing two, three, and four nitrile moieties, respectively.
paper (Whatman) to remove excess RTIL prior to loading into the membrane test chamber. 2.3. Membrane test chamber Permeance measurements were performed on the SILM using a custom test chamber (Fig. 2) built in house. This system, which consists of a feed and permeate chamber separated by the test membrane, measures the transient permeation through the SILM. For membrane loading, the SILM was first placed on a hydrophobic PTFE membrane (Pall Corporation) with a pore diameter of 0.5 m. Because the RTILs do not wet this membrane, an additional level of stability was imparted to the SILM that helped minimize RTIL loss resulting from the high-pressure gas on the feed side forcing RTIL through the pores. The SILM/PTFE membrane stack was then carefully placed on a highly porous stainless steel support that provided mechanical stability and negligible resistance to gases and the entire SILM assembly was installed into the test chamber. Two Viton o-rings were used to separate the feed chamber from the permeate chamber to eliminate leaking. In addition, a Viton gasket was positioned on top of the SILM to provide an additional seal and to prevent the fragile membrane from directly contacting the metal feed chamber. With this arrangement, the only path from the feed chamber to the permeate chamber occurs through the SILM itself. The open area of the membrane was 10.75 cm2 which corresponds to a 37-mm open diameter. After loading the SILM, the chamber was evacuated with a mechanical pump to a base pressure of 40 mTorr. The membrane was allowed to remain in the test chamber for at least 1 h to fully degas the RTIL and reach a steady base pressure. All measurements described here are single-gas permeation values in which either carbon dioxide or nitrogen is individually introduced to the feed side at a pre-determined pressure. A ballast volume of approximately 300 mL was used to minimize pressure loss during the experiment. After setting the gas pressure on the feed side, the valve to the cylinder was closed. The pressure on the permeate side was then measured as a function of time using
a Baratron pressure gauge (MKS Instruments) connected to a data acquisition board (Vernier). The pressure measurement from the Baratron gauge is independent of gas composition and has a range up to 100 Torr. The pressure on the permeate side was typically measured for 30 min. No corrections were made for the tortuosity of the polymer membrane support and all measurements were acquired at room temperature. 2.4. Gas solubility measurement Low-pressure CO2 solubility measurements were acquired using an Intelligent Gravimetric Analyzer (Hiden Analytical Limited, UK). Typically, approximately 80 mg of a particular RTIL was loaded into a quartz bucket cell and evacuated to 0.1 bar for 6 h at 150 ◦ C to degas and dry the sample. All measurements were acquired at room temperature. The mass uptake (corrected for buoyancy) was then measured as a function of pressure up to a final pressure of 10 atm to obtain the absorption isotherm. Desorption isotherms were subsequently acquired by measuring the mass as a function of decreasing pressure to ensure that the solubility behavior was reversible and to test for hysteresis effects. The solubility was then calculated from the slope of the adsorption isotherm in the low carbon dioxide concentration regime. 3. Results and discussion In general, permeation of a gas through a SILM proceeds via the following steps: (i) absorption of the permeant gas into the supported liquid from the high-pressure side of the membrane, (ii) diffusion of the gas across the membrane, and (iii) desorption of the gas out into the low-pressure side of the membrane. The permeability of a membrane thus depends on both a thermodynamic mechanism as the gas absorbs into and desorbs out of the membrane, as well as a kinetic component reflected in gas diffusion through the membrane. More specifically, gas separation in dense membranes such as polymer membranes and SILMs generally follows a solution-diffusion mechanism in which the permeability of a gas (Pi ) is related to the gas solubility and diffusivity according to: Pi = Si Di
(1)
where Si is the solubility and Di is the diffusivity. The ideal selectivity of a membrane (˛ij ) is simply the ratio of the pure gas permeabilities for species i and j: ˛ij =
Fig. 2. Experimental test setup for the permeance measurements where V = valve, P = pressure gauge, and Ba = Baratron pressure gauge.
Pi S D = i i Pj Sj Dj
(2)
where Di /Dj is the diffusivity selectivity and Si /Sj is the solubility selectivity. The ideal selectivity is a function of both the solubility selectivity and the diffusivity selectivity. For SILMs, however, the selectivity is dominated by the solubility selectivity rather than the diffusivity selectivity [6,25]. This can be justified by noting that the diffusivity selectivity is proportional to the ratio of the molecular volumes of the gases considered; in the case of
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Fig. 4. Carbon dioxide and nitrogen permeance values for all the RTILs. Nitrogen permeance values are multiplied by 5× for clarity. Fig. 3. Pressure rise in the permeate chamber as CO2 diffuses through the SILM for (a) [emim][B(CN)4 ] and (b) [emim][Tf2 N].
CO2 /N2 the diffusivity selectivity is approximately one [6,15,25]. As a result, many groups have focused on the measurement of solubility selectivity in RTILs to predict their overall selectivity in SILMs. In this work, we measured the solubility, permeance, and selectivity for a series of RTILs based on polynitrile anions with 1-alkyl-3-methylimidazolium cations for tackling the CO2 /N2 separation problem. 3.1. Permeance of [emim][Tf2 N] Because [emim][Tf2 N] has been extensively investigated and exhibits both good CO2 permeability and CO2 /N2 selectivity, we used this RTIL to validate our system and to provide a point of comparison for gauging the nitrile-containing RTIL results. For our measurements, [emim][Tf2 N] was loaded into the PES membrane according to standard procedures and the CO2 and N2 permeance values measured using the test apparatus detailed in Section 2.3. There was no explicit control over the temperature of the test chamber, so all measurements were performed at ambient temperature (298 ± 1 K). Fig. 3 shows a typical plot of the carbon dioxide pressure rise measured on the permeate side of the separation chamber for permeation experiments employing [emim][Tf2 N] versus [emim][B(CN)4 ] in SILMs. Note that there is an initial lag time as the carbon dioxide diffuses through the SILM followed by a linear rise in the pressure as the carbon dioxide flux through the membrane reaches a steady state [26]. The pressure rise is clearly higher for [emim][B(CN)4 ] which will be discussed in more detail in subsequent sections. Permeance values were calculated using only those data collected in the post-lag linear regime according to the following expression. Permeance =
V dP RTA(P − P ) dt
(3)
Here, V is the permeate volume, R is the ideal gas constant, T is the absolute temperature, A is the membrane area, P is the upstream pressure, P is the downstream pressure, and dP /dt is the rate of gas pressure increase on the permeate side. The selectivity was obtained by calculating the ratio of the carbon dioxide permeance over nitrogen permeance. Triplicate SILMs containing [emim][Tf2 N] were fabricated and the carbon dioxide and nitrogen permeance values determined multiple times for each membrane. The mean permeance and standard deviation for all measurements were then calculated
and found to be 1.85 ± 0.05 × 10−9 mol/(m2 Pa s). In recent work, Scovazzo et al. reported the mixed-gas permeance of CO2 in an [emim][Tf2 N] SILM and presented a statistically equivalent value of 1.87 × 10−9 mol/(m2 Pa s) with an ideal selectivity of 23 [14]. The average N2 permeance in [emim][Tf2 N] was measured at 6.4 × 10−11 mol/(m2 Pa s) in our case with a calculated selectivity of roughly 29, a result slightly higher but similar to values reported in the literature. In general, the measured permeance of N2 exhibited considerable variability leading to higher uncertainty in the selectivity. There is, however, excellent agreement between our single-gas selectivity results and previously published mixed-gas data for [emim][Tf2 N]. It should be pointed out that this comparison is valid since the ideal selectivity is equivalent to the mixed-gas selectivity of a SILM because the selectivity for these membranes is primarily controlled by the solubility selectivity [22]. 3.2. Effect of nitrile anion on permeance and RTIL viscosity After validating the system with [emim][Tf2 N], the permeance of CO2 and N2 were measured for SILMs incorporating [bmim][N(CN)2 ], [bmim][C(CN)3 ] and [emim][B(CN)4 ] as shown in Fig. 4. Because the N2 permeance is generally an order of magnitude lower than the CO2 permeance, each of the N2 values are multiplied by a factor five (as indicated by the 5×) to make them more visible in the figure. Permeance results for [emim][Tf2 N], discussed in the preceding section, are included for ease of comparison. Once again, it should be noted that the error bars correspond to the standard deviation over multiple permeance measurements from three individual SILMs for each RTIL. The CO2 permeance of [bmim][N(CN)2 ] is approximately 0.54 × 10−9 mol/(m2 Pa s). By way of comparison, the RTIL [emim][N(CN)2 ], which only differs in having a shorter ethyl chain at the 1-position of the imidazolium cation in place of n-butyl, has a reported permeance almost 50% higher at 0.79 × 10−9 mol/(m2 Pa s) [14]. The permeance of [bmim][C(CN)3 ] was strikingly higher at 1.88 ± 0.07 × 10−9 mol/(m2 Pa s), a value equivalent to the permeance of [emim][Tf2 N]. Proceeding to [emim][B(CN)4 ], which contains yet another nitrile moiety in the anion structure, the permeance increases again to 2.71 ± 0.05 × 10−9 mol/(m2 Pa s). Not only is the permeance of [emim][B(CN)4 ]-based SILMs higher than those from SILMs based on RTILs composed of anions with fewer pendant nitriles, it is also substantially higher than for [emim][Tf2 N] which is known to have one of the highest permeabilities among RTILs [22].
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Table 1 Abbreviations, viscosities, densities, CO2 /N2 selectivities, molar volumes, and CO2 solubilities for the RTILs discussed.a . RTIL
IL (cP)b
Density (g/mL)
CO2 /N2 selectivity
VIL (cm3 /mol)c
CO2 solubility (mol L−1 atm−1 )
[emim][Tf2 N] [bmim][N(CN)2 ] [bmim][C(CN)3 ] [emim][B(CN)4 ]
27 33 26 17
1.51 1.07 1.04 1.03
29 51 45 53
259 192 221 219
0.103 0.060 0.075 0.131
a b c
Data for 298 K unless otherwise noted. RTIL dynamic viscosity. RTIL molar volume.
While the permeability is related to both the solubility and diffusivity, it is instructive to first consider the permeance values in terms of the diffusivity. A general form for the diffusivity in RTILs has been given as: D=A
VILa bIL Vlc
(4)
where A, a, b, and c are parameters specific to the class of RTIL, bIL is the viscosity of the RTIL, VILa is the molar volume of the IL (cm3 /mol), and Vlc is the molar volume of the gas solute [5,22,26]. For RTILs with 1-alkyl-3-methylimidazolium cations for an alkyl chain length less than four, a is small, which indicates that the CO2 diffusivity is inversely proportional to the viscosity of the RTIL. Consistent with this scenario, from Table 1, it can be seen that [bmim][N(CN)2 ] has the highest viscosity and also the lowest measured permeance (Fig. 4) for both CO2 and N2 . Likewise, [emim][B(CN)4 ] exhibits the lowest viscosity and simultaneously the highest permeance. Furthermore, [bmim][C(CN)3 ] and [emim][Tf2 N] have essentially identical and intermediate viscosities and similar permeance values as well. Clearly, the overwhelming trend established is that gas permeance scales with RTIL fluidity. In a more quantitative sense, Fig. 5 shows the permeance values as a function of the viscosity. Previous literature reports indicate that the viscosity exponent b in Eq. (1) is approximately 0.5, although there is no rigorously developed theory for this viscosity dependence. The fit in Fig. 5 then is a simple fit to 1/bIL where b is equal to 0.65. With the exception of the [bmim][N(CN)2 ], which has a lower-than-expected permeance value, the permeance data roughly follow the trend of varying inversely with viscosity to the 0.65 power. This is in reasonable agreement with literature values, given that the aforementioned 0.5 power law reported is not a theoretical value but an empirical observation.
The trend of the permeance values as a function of viscosity for the nitrile-containing RTILs is in good agreement with recent benchmarks noted by Scovazzo where it was shown that CO2 permeability increases with increasing viscosity [22]. In that review, it was also noted that the highest permeability values should come from RTILs with a molar volume similar to [emim][Tf2 N]. From Table 1, the molar volume of [emim][B(CN)4 ], which exhibited the highest permeance of the RTILs measured in this study, is less than [emim][Tf2 N]. This could indicate a higher dependence of permeability on viscosity and it also reflects that there is no direct functionality between viscosity and molar volume. 3.3. CO2 /N2 selectivity and carbon dioxide solubility As previously mentioned, the selectivity of a SILM is primarily determined by the solubility selectivity, (Si /Sj ). A number of approaches have been proposed to model the gas solubility within an RTIL. There have been two models proposed with direct application to SILMs, both of which have been based on regular solution theory [17,18]. One is a two-component model that includes both the molar volume of the RTIL (VIL ) and the viscosity (IL ) and has been applied to a variety of RTIL classes [27]. The second model is limited to imidazolium-based RTILs and incorporates only VIL to predict the gas solubility and solubility selectivity [6]. Because we focus solely on imidazolium RTILs in this work, we will explore the selectivity and solubility results in terms of the second model, or the Camper Model. According to this approach, the gas solubility is given by:
S=
exp
˛+
4/3
VIL
− 1 VIL
−1 (5)
where S is the solubility (in moles of gas per liter of RTIL), ˛ and ˇ are gas-specific parameters. If the derivative of Eq. (5) with respect to the VIL is set to 0, then we obtain the molar volume at which the maximum gas solubility is attained [6]:
VIL,max =
Fig. 5. Permeance change as a function of viscosity along with a theoretical curve obtained using the diffusivity equation (Eq. (4)).
ˇ
4ˇ 3
3/4 (6)
The selectivity can then be calculated by forming the ratio of the solubilities of CO2 and N2 . The model then predicts that the CO2 solubility (and most importantly the CO2 /N2 selectivity) is directly related to the molar volume of the RTIL, the CO2 /N2 selectivity increasing as VIL decreases [6]. Fig. 6 shows representative CO2 uptake isotherms for all four RTILs while the resulting carbon dioxide solubilities are given in Table 1. Note that the CO2 solubility for [emim][Tf2 N] is 0.103 mol L−1 atm−1 , which is in excellent agreement with previously reported values [15]. From the solubility data, [emim][B(CN)4 ] exhibits the highest CO2 solubility of the RTILs studied (0.131 mol L−1 atm−1 ), surpassing even [emim][Tf2 N]. [bmim][N(CN)2 ], on the other hand, exhibits the lowest solubility of the nitrile-based RTILs, which is not unexpected given that [emim][N(CN)2 ] also has a low CO2 solubility. The [bmim][C(CN)3 ]
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Scovazzo has shown that an upper bound for the permeability and selectivity of SILMs can be predicted using the Camper Model [22]. In that review, it was reported that [emim][Tf2 N] currently exhibits the highest permeability while [emim][dca] has the best selectivity. These two ILs lie on the predicted upper bound of performance. Of the nitrile-based RTILs that we examined, the [emim][B(CN)4 ] exhibited the best performance with a permeance that is higher than [emim][Tf2 N] and excellent selectivity, comparable to [emim][N(CN)2 ]. The improved performance of the [emim][B(CN)4 ] results from the relatively low VIL which leads to high CO2 solubility selectivity compared to [emim][Tf2 N] coupled with a low viscosity that achieves high permeance values. We do not, however, rule out interactions with the nitrile group in the anion for increasing the CO2 selectivity. 4. Conclusions
Fig. 6. Carbon dioxide solubility as a function of pressure for the four different ionic liquids. The solid lines are linear least square fit lines.
exhibits a CO2 solubility of 0.075 mol L−1 atm−1 , which is intermediate between [emim][B(CN)4 ] and [bmim][N(CN)2 ]. Fig. 7 displays the CO2 /N2 selectivity values for the RTILs measured in this study ([emim][Tf2 N], [bmim][N(CN)2 ], [bmim][C(CN)3 ], [emim][B(CN)4 ]) as well as the selectivity for three additional RTILs ([emim][N(CN)2 ], [bmim][Tf2 N], and [bmim][PF6 ]) as reported in the literature [6,22]. Note that the CO2 /N2 selectivity generally increases with decreasing VIL . Fig. 7 also includes the relationship between selectivity and molar volume as predicted by regular solution theory in the form of the Camper model. Clearly, the trend in CO2 /N2 selectivity for the nitrile-based RTILs approximately tracks with the molar volume where RTILs with decreasing VIL yield higher CO2 /N2 selectivities. It is interesting to note that both molten salts and RTILs share a common relationship between surface tension and VIL , wherein surface tension is inversely correlated to VIL [28]. The selectivities of our nitrile-based RTIL series follow reported trends as evidenced by the reasonable agreement between the Camper model predictions and our results, shown in Fig. 7. In a recent review,
In this work, a new class of non-fluorinated RTILs with excellent separation performance has been developed. We show that the judicious choice of the cation and anion pair leads to SILM with improved permeance and selectivity properties. By varying the number of nitrile groups in the anion of an imidazolium-based RTIL, the separation properties of a SILM could be considerably enhanced relative to previously reported performance data. Specifically, the [emim][B(CN)4 ]-based SILM showed a permeance of 2.55 × 10−9 mol/(m2 Pa s), which is 30% higher than the permeance of [emim][Tf2 N], a well-known RTIL with a high CO2 permeance. Furthermore, this RTIL exhibited a high CO2 /N2 selectivity with a value of 53. The results of our nitrile-based RTILs can be rationalized in terms of the regular solution theory where the selectivity and permeance of a SILM system are determined by the molar volume and viscosity of the corresponding RTIL, respectively. We have shown that the simultaneous optimization of both molar volume and viscosity through the use of nitrile-based RTILs can lead to improved separations performance. Based upon these promising trends, future work will seek to explore alternate designer, lowmolar volume RTILs toward further improved CO2 /N2 separations. Acknowledgements This work was fully sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy. References
Fig. 7. Selectivity of SILMs as a function of molar volume. The closed squares correspond to data from this work while the open squares are literature values for [emim][N(CN)2 ] [bmim][PF6 ] [bmim][Tf2 N] (see Refs. [6,22]). The solid line is a plot of the selectivity predicted by the Camper model.
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