Ind. Eng. Chem. Res. 2008, 47, 9919–9924
9919
Effect of Anion on Gas Separation Performance of Polymer-Room-Temperature Ionic Liquid Composite Membranes Jason E. Bara,†,‡ Douglas L. Gin,†,‡ and Richard D. Noble*,† Department of Chemical & Biological Engineering, UniVersity of Colorado, Boulder, Colorado 80309-0424, and Department of Chemistry & Biochemistry, UniVersity of Colorado, Boulder, Colorado 80309-0215
Composite gas separation membranes were fabricated by photopolymerization of a room-temperature ionic liquid (RTIL) monomer in the presence of 20 mol % of nonpolymerizable RTILs with various anions. The solid, composite membranes contained polymer-bound cations, “free” cations, and “free” anions. The composite materials exhibit no phase separation between these components. The permeabilities of CO2, N2, and CH4 in these poly(RTIL)-RTIL composites were observed to increase by 2-5 times relative to those in the neat poly(RTIL) without a “free” RTIL component. These largely increased permeabilities resulted in CO2/N2 and CO2/CH4 ideal separation selectivities that were only slightly diminished relative to the poly(RTIL) without a “free” RTIL. When viewed on “Robeson plots”, poly(RTIL)-RTIL composites are shown to be more favorable for CO2/N2 separation than CO2/CH4. Poly(RTIL)-RTIL composites are highly tunable materials with excellent promise as gas separation membranes. 1. Introduction The study of room-temperature ionic liquids (RTILs) as selective gas separation media has progressed significantly in recent years. Research has focused primarily on CO2-based separations,1-11 with SO2 removal also appearing to be a promising pursuit.12-14 RTILs, especially those based on imidazolium cations, exhibit an affinity for CO2 relative to CH4 and N2.1-11 CO2/CH4 separation is of critical importance to natural gas processing and improving fuel quality. CO2/N2 separation from flue gas streams (CO2 capture and sequestration) is an issue currently garnering significant global attention. RTILs have been proposed as alternative “green” solvents to replace the volatile organic compounds (VOCs) typically employed in CO2 scrubbing.15,16 Several different approaches have been employed to exploit the desirable properties of RTILs for gas separation applications. Many papers have focused on measuring the solubility of various gases of interest in RTILs at a range of pressures.1-11 The larger solubility of CO2 compared to CH4 and N2 could perhaps be utilized to achieve separation through pressure swing absorption.15,16 CO2 could be selectively absorbed into the RTIL solvent, while the less soluble gas is swept away, creating a CO2-lean stream. CO2 could then be desorbed from solution to produce a CO2-rich stream. This type of configuration appears more viable in RTILs than in traditional VOCs, as there is little risk of volatilizing RTILs in the desorption step.17 An inherent drawback of such a pressure swing configuration with RTILs is that the volume of solvent required is directly proportional to the volume of gas to be processed and inversely proportional to the concentration (partial pressure) of CO2 in the feed stream. As the largest solubility of CO2 in some common, imidazolium-based RTILs is ca. 0.08 mol L-1 atm-1 (2.2 cm3 (STP) cm-3 atm-1) at 40 °C;2 it becomes apparent that large volumes of RTILs would be required to process large volumes of low pressure CO2 from flue gas streams. Supported ionic liquid membranes (SILMs) have been examined as a means to process CO2 in a selective RTIL * To whom correspondence should be addressed. E-mail: nobler@ colorado.edu. † Department of Chemical & Biological Engineering. ‡ Department of Chemistry & Biochemistry.
medium without the need for large volumes of fluids.8 SILMs can be prepared by “wetting” a porous polymer (or inorganic) support with an RTIL of interest.8 The volume of gas that can be processed is directly proportional to the membrane surface area membrane and the feed pressure. Some SILMs exhibit ideal (i.e., single gas) CO2 permeability approaching 1000 barrers and ideal separation factors for CO2/N2 up to 60 or higher.2,8 When viewed on a “Robeson plot”,18,19 these data indicate that SILMs are highly competitive with polymer membranes and may be an industrially attractive technology for CO2/N2 separations. SILMs do not appear as viable in CO2/CH4 separations when examined on a “Robeson plot” for that separation.2,8,18,19 However, as a gas separation membrane platform, SILMs are not without their own drawbacks. In many supports, weak capillary forces hold the RTIL within the matrix.8 While the lack of strong RTIL-support interactions allows for high gas permeability through the liquid phase, this also negatively impacts the stability of the SILM configuration. The transmembrane pressure differentials that SILMs can withstand appear limited to a few atmospheres, before the RTIL is “squeezed” from the support.8 The long-term integrity of the support, especially those that are polymer-based, is also of concern. One of the most attractive features of imidazolium-based RTILs is their ability to be chemically tailored to improve performance in particular applications.1,9,20-22 The influences of various functional groups on the interactions of RTILs with CO2 and other gases have been explored in detail by our group and others.1,9,20-22 Early work focused on the effect of alkyl chain length and/or anion and in imidazolium-based RTILs.2,10 However, given the range of functional groups known to impact gas solubility in molecular solvents (i.e., CH3CN, MeOH, acetone, etc.),23 it became apparent that the study of RTILs for gas separations should not be limited to strictly alkyl-functionalized systems.1,9,22 The incorporation of polar functional groups such as oligo(ethylene glycol)1 units and nitrile-terminated alkyl chains22 has been reported to increase CO2/N2 and CO2/CH4 solubility selectivity in RTILs, with little detriment to CO2 solubility. RTILs containing fluoroalkyl groups9 have also been shown to provide modest improvements in CO2 solubility relative to their alkyl-functionalized analogues. When tested in
10.1021/ie801019x CCC: $40.75 2008 American Chemical Society Published on Web 11/21/2008
9920 Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008
Figure 1. General structure of styrene-containing, imidazolium-based RTIL monomer.
Figure 2. Components of first poly(RTIL)-RTIL composite gas separation membrane.
an SILM configuration, fluoroalkyl-functionalized RTILs displayed improved CO2/CH4 selectivity relative to alkyl-functionalized RTILs but diminished CO2/N2 selectivity.24 Also, an imidazolium-based RTIL featuring a primary amine tethered to the cation has been shown to be capable of reversible CO2 capture.20,21 Another approach to functional RTILs for gas separations is to incorporate polymerizable groups for the formation of solid, poly(RTILs) under controlled conditions.25-30 Figure 1 depicts some of the reported structures of a styrene-containing, imidazolium-based RTIL monomer.25-27 Poly(RTILs), as powders, have shown affinity for CO2 in uptake experiments.28-30 Recently, we have shown that poly(RTILs) can also be formed as thin films, allowing for the study of these materials as gas separation membranes.25-27 Poly(RTILs) are an attractive alternative platform to SILMs, as the chemistry of RTILs can be utilized for selective CO2 transport, while significantly improving the mechanical stability of the membrane. RTIL monomers can be synthesized to contain a variety of chemical groups (R in Figure 1) in addition to the polymerizable unit.25-27 Imidazolium-based monomers containing n-alkyl chains (1a-c), short oligo(ethylene glycol) linkages (1d,e), and nitrile-terminated n-alkyl chains (1f,g) have been successfully synthesized by our group.25-27 “Gemini” or difunctional RTIL monomers are also possible.26 All of the monomers that we have previously synthesized featured bis(trifluoromethane)sulfonimide (Tf2N) anions.25-27 The chemical nature of the polymerizable RTIL monomer has significant effects on the gas separation performance of the resultant poly(RTIL) gas separation membrane.25-27 We have shown that increasing the length of the nonpolymerizable substituent (R in Figure 1) results in increased permeability for all gases.25,27 For n-alkyl chains, increasing the length of the substituent resulted in decreased ideal selectivity in CO2/N2 and CO2/CH4 separations.25 The ideal permeability of CO2 in poly(RTIL) membranes with oligo(ethylene glycol) substituents was similar to that observed in poly(RTILs) containing n-alkyl substituents.25,27 However, with increasing length of oligo(ethylene glycol) substituent, the ideal selectivity of CO2/N2 was observed to increase, while CO2/CH4 decreased.27 A similar trend in ideal selectivity behavior was observed in poly(RTIL) membranes containing alkyl-terminated nitrile groups, but those poly(RTIL) membrane suffered from CO2 permeabilities that were less than the original alkyl-functionalized systems.25,27 Permeability in poly(RTIL) membranes ranged from 4 to 39 barrers, with CO2/ N2 selectivities ranging from 28 to 44 and CO2/CH4 selectivities of 17-39.25,27 Alkyl-functionalized poly(RTILs) approximate the “upper bound” of the “Robeson plot” for CO2/N2 while those with oligo(ethylene glycol) and nitrile-terminated alkyl groups exceed an “upper bound” for this separation.18,19,25,27 None of the poly(RTIL) membranes have been observed to exceed the “upper bound” for CO2/CH4 separation.18,19,25,27 All types of poly(RTIL) membranes exhibited CO2 permeabilities of only 1-10% of that of the most permeable SILMs,8,25,27 primarily as a result of slower gas diffusion in solids than liquids.31 Modification of the substituent on the imidazolium cation in poly(RTILs) was observed to have a much greater effect on the selectivity of the membrane than on the permeability of
CO2.25,27 While an understanding of how to improve the CO2 selectivity of poly(RTILs) through substituent modification has been achieved, it is also very desirable to improve CO2 permeability through poly(RTIL) membranes to better compete with the high permeability of SILMs and high-performance polymer membranes in general. One possible approach would be to continue increasing the length of the substituent group on the RTIL monomer. This method could increase permeability at the expense of the diminished concentration of ions within the membrane, losing many of the desirable properties of the RTIL platform. The resulting polymers would more resemble the substituent (i.e., PE or PEG) rather than a poly(RTIL). However, an even more straightforward approach to improving CO2 permeability in poly(RTILs) is available and requires little or no new organic synthesis. This method is the formation of poly(RTIL)-RTIL composite membranes.32 In a recent proof-of-concept work, we detailed the performance of such a composite for CO2-based separations.32 RTIL monomer 1a was polymerized in the presence of 20 mol % of 1-ethyl-3-methylimidazolium bis(trifluoromethane)sulfonimide, herein referred to as [C2mim][Tf2N], a common RTIL (Figure 2). The resulting solid poly(RTIL)-RTIL composite was homogeneous and exhibited no evidence of phase separation, even after many months of storage at ambient conditions.32 RTILs within poly(RTIL) matrices should be viewed as very different from traditional additives or plasticizers in conventional polymers. Not only are RTILs nonvolatile, they also experience significant ionic interactions with poly(RTILs). It was proposed that the charged backbone of poly(RTILs) strongly holds RTILs within the matrix and prevents phase separation.32 The permeability of CO2 in 1a was first reported to be 9.2 barrers, while the composite of 1a and [C2mim][Tf2N] was found to have a CO2 permeability of 44 barrers, approximately a 400% increase.25,32 CH4 and N2 exhibited 600% and 300% increases in their respective permeabilities.25,32 The increased permeabilities were attributed to more rapid gas diffusion through a composite with “free” or mobile ions.32 As a result, CO2/N2 selectivity in the composite membrane was found to be greater than that of 1a alone, with values 40 and 32, respectively.25,32 CO2/CH4 selectivity experienced a decline in the composite relative to 1a, falling from a value of 39 in 1a to 28 in the composite.25,32 However, an ideal CO2/CH4 selectivity of 28 still represents an improvement over many poly(RTIL) membranes we have studied.25-27,32 Given the success of this first-generation poly(RTIL)-RTIL composite, we seek to develop a systematic understanding of the factors that influence gas permeability and separation selectivity in these novel membrane materials. Clearly, there are many more factors available to tune in the composite materials than in poly(RTILs) themselves. In each of the poly(RTIL) membranes we have studied thus far, we have exclusively used Tf2N anions, as the parent RTIL monomers with those anions are most convenient to synthesize.25-27 However, RTILs with a wide variety of anions are readily synthesized or are commercially available. While we have already explored the effect of substituent groups on poly(RTIL) gas separation membranes, it is not yet clear how those substituents will interact
Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 9921 Table 1. Physical Properties of RTILs and Densities of Poly(RTIL) Composites cation Figure 3. Structures of monomer 1d and [C2mim][X] salts used to fabricate poly(RTIL)-RTIL composite gas separation membranes in this study.
with “free” RTILs in the composite materials. Thus, the effect of the type of anion in the “free” RTIL, as well as the structure of the cation, is also of interest. Finally, the concentration of “free” RTIL in the composite may have the most dramatic effect on performance, as the permeability differences between poly(RTILs) and RTILs span a range of nearly 3 orders of magnitude.8,25-27 Herein, we present the first systematic study of poly(RTIL)RTIL composite gas separation membranes, examining the effect of anion in the “free” RTIL component on the gas permeability and selectivity of poly(RTIL) matrices fabricated from 1d and a variety of [C2mim][X] salts at 20 mol % (Figure 3). The nature of the anion associated with the “free” RTIL component was observed to have a dramatic effect on gas permeability. A poly(RTIL)-RTIL composite membrane containing exclusively Tf2N anions was observed to have an ideal CO2 permeability of 60 barrers. Poly(RTIL)-RTIL composite membranes containing mixed anion systems with OTf, dca, or SbF6 were all found to have very similar ideal CO2 permeabilities of ca. 40 barrers. The nature of the anion was found to have a very subtle effect on ideal permeability selectivity for CO2/N2 and CO2/CH4 separations, which ranged from 36 to 39 and 24 to 27, respectively. The composite containing only Tf2N anions defined the lower limit of ideal selectivity in both separations. Plotting the performance of these poly(RTIL)-RTIL composites on “Robeson plots” reveals that CO2/N2 separations exceed the “upper bound” of the chart and that these poly(RTIL)-RTIL composites represent a significant improvement to previous poly(RTIL) membranes. When the same is done for a similar chart of CO2/CH4 performance, poly(RTIL)RTIL composites are also shown as an improvement upon previous poly(RTIL) membranes. While poly(RTIL)-RTIL membranes are also below the “upper bound” of this “Robeson plot”, they are closer to that line than previous poly(RTIL) membranes. 2. Experimental Section 2.1. Materials. Monomer 1d and [C2mim][Tf2N] were synthesized as reported in previous works.6,27 [C2mim][dca] was used as received from Fluka. [C2mim][OTf] and [C2mim][SbF6] were synthesized by Dr. Seth Miller of Zettacore and were received as a gift. All other chemicals were purchased from Sigma-Aldrich (Milwaukee, WI) and were used as received. All gases were purchased from AirGas (Radnor, PA) and were of at least 99.99% purity. 2.2. Typical Preparation of Poly(RTIL)-RTIL Composite Membranes. This procedure is identical to that which we have described in a prior work using a different monomer-RTIL solution.32 Monomer 1d (2.50 g, 47.5 mmol) and a [C2mim][X] RTIL of interest were mixed in a 4:1 molar ratio, and the mixture homogenized on a vibrating mixer. Divinylbenzene (0.0325 g, 2.50 mmol) was then added, and the mixture homogenized again. Finally, a photoinitiator, 2-hydroxy-2-methylpropiophenone (0.0250 g, 0.152 mmol), was added and the mixing step repeated a final time. The solution was cast on to a 50 cm × 50 cm piece of porous poly(ether sulfone) support (Supor 200, Pall, Ann Arbor, MI) on top of a Rain-X-coated quartz plate. Rain-X
anion
Tf2N dca OTf SbF6 poly(1d)
[C2mim]
density (g/cm3)
MW (g/mol)
Vm (cm3/mol)
composite density (g/cm3)
1.52 1.08 1.36 1.85 1.29
391.31 177.21 260.23 346.92
257 164 191 188
1.32 1.27 1.33 1.52
is a commercially available, hydrophobic coating for glass surfaces, which aids in the removal of the film after the photopolymerization is completed. A second, identical quartz plate was placed on top to spread the monomer completely through the support with excess flowing beyond the support but remained within the quartz plates. The plates were placed under a 365 nm UV light for 30 min. After this time, the plates were separated with a clean razor blade. The supported area was liberated from the freestanding polymer. The support was cut with a 47 mm diameter stainless steel die. The freestanding section was collected for determining polymer density. Membrane thicknesses ranged from 150 to 190 µm. No phase separation between the poly(RTIL) and “free” RTIL components has been observed after storing these membranes at ambient conditions for several months. 2.3. Determination of Composite Densities. Densities of poly(RTIL)-RTIL composites were determined using a straightforward, volumetric method.25-27,32 Approximately 1.00 g of composite was added to a 10.00 ( 0.02 mL, Class A volumetric flask of known mass. The mass of the polymer was recorded and hexanes (mixture of isomers) were added to the flask, and the mass of hexanes added was noted. The volume of hexanes added was calculated from its density (0.672 g/mL at 25 °C) and that value subtracted from the volume of the empty flask. The difference was taken to be the volume of the composite. The density of the composite was taken as the quotient of its mass and volume. Physical properties for each RTIL used and the densities of neat poly(1d) and the four composites studied are presented below (Table 1). 2.4. Gas Permeation Experiments. Single-gas permeation experiments using CO2, N2, and CH4 were carried out at 295 K using a time-lag apparatus. Construction and operation of this equipment have been detailed in our previous works relating to poly(RTILs) and other polymer gas separation membranes.25-27,32,33 Experiments were performed with a driving force of ∼2 atm upstream against initial vacuum downstream. Membranes were degassed under dynamic vacuum (
