Ind. Eng. Chem. Res. 2007, 46, 5397-5404
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Synthesis and Performance of Polymerizable Room-Temperature Ionic Liquids as Gas Separation Membranes Jason E. Bara,*,† Sonja Lessmann,‡ Christopher J. Gabriel,‡ Evan S. Hatakeyama,† Richard D. Noble,† and Douglas L. Gin†,‡ Department of Chemical and Biological Engineering, UniVersity of Colorado at Boulder, Boulder, Colorado 80309-0424, Department of Chemistry and Biochemistry, UniVersity of Colorado at Boulder, Boulder, Colorado 80309-0215
Room-temperature ionic liquids (RTILs) with polymerizable groups can be readily converted into solid, dense poly(RTILs) for use as gas separation membranes. A series of RTIL monomers with varying length n-alkyl substituents were synthesized and converted into polymer films. These membranes were tested for their performance in separations involving CO2, N2, and CH4. CO2 permeability was observed to increase in a nonlinear fashion as the n-alkyl substituent was lengthened. CO2/N2 separation performance was relatively unaffected as CO2 permeability increased. Plotting the performance of these membranes on a “Robeson plot” for CO2/N2 shows that first-generation poly(RTILs) “hug” the “upper bound” of the chart, indicating that they perform as well or better than many other polymers for this separation. The CO2/CH4 separation is less impressive when compared to other polymer membranes on a “Robeson plot”, but poly(RTILs) perform as well or better than molten RTILs do in bulk fluid gas absorptions for that gas pair. Furthermore, poly(RTILs) were determined to be able to absorb about twice as much CO2 as their liquid analogues, an important factor which may give them potential use as gas and vapor sorbents. 1. Introduction Room-temperature ionic liquids (RTILs) as a media for gas separation and storage is a promising “green technology”.1-3 RTILs are nonvolatile (under most conditions),4,5 largely inflammable, and thermally stable. These properties make them ideal candidates to replace volatile organic compounds (VOCs) in gas scrubbing, separations, and storage/delivery applications.6-11 There is significant interest in the commercialization of systems utilizing RTILs for the safe handling of hazardous gases (BF3, PH3, etc.).11,12 RTILs may be of even greater utility to applications involving CO2 separations, due to the large solubility of CO2 in RTILs.2,6,9,13-16 However, technology that utilizes RTILs for CO2-based separations has yet to be adopted by industry. Feasibility reports express that CO2 separations with RTILs may become economically viable processes in the future.6,17 To achieve this goal, more research is needed into improving CO2 loading levels, as well as enhancing CO2/N2 and CO2/CH4 separation performance. A large number of studies have focused on determining the solubility of gases such as CO2, N2, CH4, and small hydrocarbons in various 1-alkyl-3-methylimidazolium RTILs, represented herein as [Cnmim][X].7-9,13-16,18 The general structures of these RTILs are illustrated in Figure 1. [C2mim][Tf2N] has been shown to exhibit the highest solubility of CO2 among the family of [Cnmim][X] RTILs.2,13,14 However, [C2mim][dca] exhibits the best CO2/N2 and CO2/CH4 ideal separation performances.2,13,14 There is no “perfect” RTIL, i.e., one that possesses both high CO2 solubility and CO2/N2 or CO2/CH4 selectivity properties. Consequently, the choice of which RTIL is best suited for CO2 separations depends on the requirements of the application. While there are a seemingly * To whom correspondence should be addressed. E-mail:
[email protected]. † Department of Chemical and Biological Engineering. ‡ Department of Chemistry and Biochemistry.
infinite number of RTILs that can be synthesized, gas solubility studies have focused on a small subset. Regular solution theory (RST) has been used to model the solubility of CO2 and CO2/N2 and CO2/CH4 ideal solubility selectivities in [Cnmim][X] RTILs.13,14,18 The behavior of these gases in “simple” [Cnmim][X] RTILs at low pressures and ambient conditions can be accurately predicted as a function of the molar volume of the RTIL.13 Ideal diffusion coefficients using a “semi-infinite volume” approach have also been determined for several [Cnmim][X] RTILs.19 Application of the solution-diffusion (S-D) mechanism to these data allows for the ideal permeability (Pi) of a single gas to be calculated from eq 1 (permeability as the product of solubility and diffusivity).13
Pi ) SiDi Furthermore, the ideal separation selectivity (Ri,j) can be found by dividing the ideal permeabilities of two gases in an RTIL. This selectivity can be separated into separate solubility and diffusion contributions (eq 2; ideal separation selectivity and contributing factors).
Ri,j )
Pi Si Di ) Pj Sj Dj
Permeabilities, solubilities, and diffusivities of gases in RTILs can also be measured directly using a supported ionic liquid membrane (SILM) configuration.20-23 SILMs have shown very promising performance for CO2 separations. SILMs of [C2mim][Tf2N] on a microporous support exhibited ideal CO2 permeabilities in excess of 1000 Barrers with CO2/N2 ) 21.20 However, one of the major drawbacks associated with SILMs is that the liquid is held in the pores of the support via relatively weak capillary forces. When the transmembrane pressure differential is greater than those forces, the liquid will be pushed through the support, destroying the membrane. As a result, the
10.1021/ie0704492 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/11/2007
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Figure 1. Structure of [Cnmim][X] RTILs.
SILMs mentioned above were only tested at pressure differentials of ∼0.2 atm.20 Nanoporous supports have been used to successfully overcome these limitations, and SILMs made using these supports have been reported to be stable at pressures up to 7 atm.22 SILMs of [C4mim][Tf2N] on a nanoporous support yielded H2/CO selectivity as high as 4 for a binary gas feed.22 This demonstration of enhanced stability in SILMs may open new opportunities for gas separations with RTILs. Most recently, SILMs using phosphonium-based RTILs were examined for CO2-based separations, though these SILMs did not perform as well as those utilizing imidazolium-based RTILs.23 Furthermore, the use of SILMs is not limited solely to gases, as these materials are being studied for organic liquid separations24 and in catalysis.25 In a previous study, our research group began to explore methods to overcome the limitations associated with SILMs. Our approach was to cast solutions of cross-linkable poly(ethylene glycol) diamethacrylate (PEGDMA) macromers and RTILs into thin films and photopolymerize the composite into membranes. As PEGDMA and imidazolium-based RTILs are miscible, films with up to 70 wt % RTIL content could be produced. However, at all loadings of RTIL, the cross-linked PEG matrix merely acted as a “sponge”, and under moderate applied pressures of 1-2 atm, the RTILs could be “squeezed” from the matrix. Little advantage over SILMs was realized through this method. The incorporation of RTILs into rubbery, lightly cross-linked polymers without chemical tethers binding the RTIL to the polymer backbone does not appear to be a viable technique for making polymer-RTIL composites. It quickly became apparent that another method was needed to study the properties of imidazolium-based RTILs in gas separation membranes. The potential performances of polymerized RTILs (poly(RTILs)) as gas separation membranes have been modeled in a previous paper from our group.13 Using experimentally determined solubility data for CO2, N2, and CH4, permeabilities of “immobilized” RTILs were calculated by lowering the diffusion coefficients of the gases by subsequent orders of magnitude. Through comparison with “Robeson plots”,26 CO2/N2 membrane separations with poly(RTILs) were predicted to be more viable than CO2/CH4 separations with the same materials.13 The coordinates of ideal CO2 permeability in hypothetical poly(RTILs) and their ideal permselectivity in CO2/N2 separation were shown to be above the “upper bound” of the Robeson plot for more configurations of poly(RTILs) than for CO2/CH4 separations.13 The greater solubility differences between CO2 and N2 than between CO2 and CH4 were assumed to be the only driving force for separating gases in the membranes.13 To simplify the model, solubility differences were assumed to be the only contribution to permeability selectivity and diffusion differences were insignificant, as in SILMs. In reality, light gases can diffuse at very different rates through solid materials, usually proportional to their size. Diffusion selectivity was ignored in our models, as we chose not to speculate if any differences could be expected. As previously expressed in eq 2, permeability selectivity is a product of the solubility and diffusivity differences of the gases in the polymer. As such, any diffusion selectivity that poly(RTILs) can achieve for CO2/N2 and CO2/ CH4 is yet to be experimentally determined. Furthermore, the
Figure 2. General structures of RTIL monomers.
positive or negative effects polymerization might have on gas solubility could also not be predicted.13 Poly(RTILs) are now a reality. Imidazole materials are versatile building blocks with modular or “snap together” chemistry, which allows for the incorporation of a wide variety of functional groups in RTILs.9,27-29 Imidazolium cations have been tailored to form “task-specific” ionic liquids (TSILs) for gas separations and other applications.9,30-32 Polymerizable groups can also be readily added to RTILs, allowing for the conversion, under proper conditions, of the molten salts into solid-state materials.33-35 These poly(RTILs) are attracting interest for use in several fields. They have been considered for use as ion conductive matrixes,33-35 catalyst supports,36 nanoparticle stabilizers,37 and gas separation materials.38-41 Radosz and co-workers have thus far been the only group to report on the use of poly(RTILs) for gas separations, both as sorbents or copolymerized with PEG macromers to form PEGpoly(RTIL) composite gas separation membranes.38-41 Poly(RTIL) sorbents have been reported to have high volumes of CO2 uptake and rapid rates of sorption/desorption.41 CO2 was found to have nearly twice the solubility in the poly(RTILs) when compared to simple [Cnmim][X] RTILs.41 Copolymers of poly(RTILs) and PEG were observed to have CO2 permeabilities as high as 120 Barrers and CO2/N2 selectivities up to 70.40 The films easily surpassed the upper bound of a Robeson plot for CO2/N2. However, as PEG already has excellent properties for CO2-based separations as a neat material,42-44 the contribution of the poly(RTIL) to the separation power of the largely PEG membranes is not obvious.40 None of these papers on gas separations with poly(RTILs) utilized a singlecomponent, polymerizable RTIL monomer to form a thin film gas separation membrane. Thus, the properties of neat poly(RTILs) as gas separation membranes have yet to be determined. Pure poly(RTIL) films were previously reported to be too brittle to make mechanically stable membranes.40 Poly(RTIL) membranes can be strengthened through the addition of a small amount of a non-CO2 interactive polymer cross-linker and the use of a porous support. In this first report on the gas separation performances of single component, poly(RTIL) membranes, five simple RTIL monomers with either acrylate or styrene-based polymerizable groups were tested (Figure 2). Through photopolymerization, these monomers can be converted into dense films of any size. The permeabilities, solubilities, and diffusivities of CO2, N2, and CH4 in these membranes are presented and the transport behaviors of the gases are rationalized from the structures of the poly(RTIL). As the alkyl subsituent R increased in length, the permeabilities of all gases were observed to increase in a nonlinear fashion. The ideal CO2 permeability increased from 7 to 32 Barrers as the alkyl group was lengthened from methyl to n-hexyl in the styrene-based systems. The CO2/N2 ideal separation selectivity
Ind. Eng. Chem. Res., Vol. 46, No. 16, 2007 5399 Scheme 1 . Synthesis of Styrene-Based RTIL Monomers
Scheme 2 . Synthesis of Acrylate-Based RTIL Monomers
was largely unaffected by the change and ranged from 32 to 28. However, the CO2/CH4 separation fared worse as the chain became longer, and the selectivity dropped from 37 to 17 as the chain was lengthened. Acrylate-based systems showed very similar behaviors. Each of the membranes tested are located at the upper bound for a CO2/N2 Robeson plot.26
organic phase was dried over anhydrous MgSO4, and the filtrate reduced by rotary evaporation. The remaining solvent was removed under vacuum lines while stirring at 40 °C overnight. 1-[(4-Ethenylphenyl)methyl]-3-methyl-imidazolium Bis(trifluoromethane)sulfonimide (1a). Yield ) 47.52 g, 79%. The 1H NMR data were consistent with published values.37 1-[(4-Ethenylphenyl)methyl]-3-butyl-imidazolium bis(trifluoromethane)sulfonimide (1b). Yield ) 53.73 g, 82%. The 1H NMR data were consistent with published values.46 1-[(4-Ethenylphenyl)methyl]-3-hexyl-imidazolium Bis(trifluoromethane)sulfonimide (1c). Yield ) 48.45 g, 66%. 1H NMR (400 MHz, DMSO-d6) δ 0.845 (t, 3H), 1.21-1.26 (m, 6H), 1.78 (q, 2H), 4.16 (t, 2H), 5.29 (dd, 1H), 5.39 (s, 1H), 5.87 (dd, 1H), 6.71-6.77 (m, 1H), 7.38 (d, 2H), 7.53 (d, 2H), 7.81 (d, 2H), 9.27 (s, 1H). 2.1.2. Synthesis of Acrylate-Based RTIL Monomers. Acrylate-based RTIL monomers were synthesized according to Scheme 2. 2.1.2.1. Synthesis of 2-Bromoethylacrylate (3). 2-Bromoethanol (100 g, 56.5 mL, 800 mmol) was dissolved in THF (300 mL) in a 1000-mL round-bottom flask. Triethylamine (134 mL, 960 mmol) was added, and the reaction cooled to 0 °C. Acryloyl chloride (87.0 g, 77.7 mL, 960 mmol) in THF (80 mL) was then added dropwise while stirring. The reaction was allowed to warm to room temperature overnight while stirring. After this time, the reaction was stopped and the solids filtered. The filtrate was poured into 1.2 M HCl (500 mL), and the product extracted with hexanes (300 mL). The organic phase was washed with deionized H2O (150 mL), dilute aqueous sodium carbonate (150 mL), and finally deionized H2O (150 mL). The organic phase was dried over anhydrous MgSO4 and then filtered through a plug of silica gel. The solvent was removed via rotary evaporation, and the excess solvent was removed in vacuo. The remaining product was distilled at 40 mTorr and 40 °C to yield a clear, colorless liquid. 1H NMR revealed the product to be a mixture of 2-bromoethylacrylate and 2-chloroethylacrylate (90: 10), as some bromide was displaced by chloride during the acid workup step. Yield ) 106.44 g, 59%. 1H NMR for the bromo product: (400 MHz, CDCl3) δ 3.56 (t, 2H), 4.48 (t, 2H), 5.89 (dd, 1H), 6.16 (dd, 1H), 6.47 (dd, 1H). 2.1.2.2. Synthesis of 1-Alkyl-3-[2-[(1-oxo-2-propenyl)oxy]ethyl]-imidazolium Bis(trifluoromethane)sulfonimides (4ab): Typical Procedure. 1-Methylimidazole (5.18 g, 5.00 mL, 63.0 mmol) was dissolved in CH3CN (50 mL) in a 100-mL round-bottom flask. 2-Bromoethylacrylate (16.9 g, 11.4 mL, 95.0 mmol) was added, and the reaction was heated at reflux (85 °C) overnight. The reaction was then stopped and allowed to cool to room temperature. The solution was poured into Et2O (400 mL) and placed in a freezer to cool for several hours. A separate phase was visible at the bottom of the flask. The Et2O phase was carefully decanted, and the precipitate taken up in deionized H2O (100 mL). The aqueous phase was then washed with EtOAc (3 × 50 mL). LiTf2N (27.1 g, 95.0 mmol) was added to the aqueous phase, and an oily liquid was instantly visible at the bottom of the flask. The reaction was allowed to
2. Experimental Methods 2.1. Materials. Anhydrous THF was prepared by passing it through a column of activated alumina. Lithium bis(trifluoromethane)sulfonimide was purchased from 3M (St. Paul, MN). All other chemicals were used as received from Sigma-Aldrich (Milwaukee, WI) or TCI America (Portland, OR). Gases were obtained from Airgas (Radnor, PA) and were of at least 99.99% purity. 2.1.1. Synthesis of Styrene-Based RTIL Monomers. Styrenebased RTIL monomers with three different length alkyl substituents were synthesized according to Scheme 1. 2.1.1.1. Synthesis of 1-Hexylimidazole (1). NaH (14.7 g, 60 wt % in mineral oil, 368 mmol) was added to a 500-mL round-bottom flask, equipped with a stir bar, reflux condenser, and argon atmosphere. Anhydrous THF (300 mL) was added, and the mixture was stirred to form a slurry. Imidazole (20.00 g, 294.0 mmol) was added slowly to the mixture, and H2 gas was observed to bubble from the reaction. The reaction mixture was allowed to stir until the bubbling ceased. After this time, 1-bromohexane (41.3 mL, 294 mmol) was added, and the vessel was heated at reflux (65 °C) under argon overnight. The reaction was stopped, and the solids were filtered and washed with THF. The filtrate was reduced to an oily liquid, and MeOH was then added. The MeOH solution was filtered through a pad of octadecyl-functionalized silica gel. The solvent was again removed via rotary evaporation, and the product was dried in vacuo overnight. 1-Hexylimidazole was obtained as a yellow oil. Yield ) 31.29 g, 70%. The 1H NMR spectrum was consistent with published values.45 2.1.1.2. Synthesis of 1-[(4-Ethenylphenyl)methyl]-3-alkylimidazolium Bis(trifluoromethane)sulfonimides (2a-c): General Procedure. RTILs with polymerizable styrene groups were synthesized in the following manner: A 1-alkylimidazole (125 mmol) was dissolved in CH3CN (30 mL) in a 100-mL roundbottom flask equipped with a stir bar. 4-Chloromethylstyrene (19.5 mL, 139 mmol) was then added, and the reaction was heated at 50 °C while stirring overnight. The reaction was stopped after this time, and the reaction mixture was poured into Et2O (250 mL). The ionic product precipitated, and the mixture was placed in a freezer for several hours. The Et2O phase was decanted, and the product was dissolved in deionized H2O (125 mL). The aqueous phase was washed with EtOAc (3 × 100 mL). Lithium bis(trifluoromethane)sulfonimide (LiTf2N) (36.17 g, 125.0 mmol) was added to the aqueous phase, and an oily liquid was observed to separate immediately. The mixture was stirred for 1 h. The oily phase was extracted into EtOAc (250 mL) and washed with deionized H2O (3 × 100 mL). The
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Table 1. Densities of Poly(RTILs) styrene
density (g/cm3)
acrylate
density (g/cm3)
methyl butyl hexyl
1.28 1.34 1.31
methyl butyl
1.45 1.47
stir for 1 h; after which time, the oil was taken up in EtOAc (250 mL) and washed with deionized H2O (3 × 100 mL). The organic phase was dried over anhydrous MgSO4 and filtered, and the solvent was removed via rotary evaporation. Excess solvent was removed in vacuo at 40 °C overnight. 1-Methyl-3-[2-[(1-oxo-2-propenyl)oxy]ethyl]-imidazolium Bis(trifluoromethane)sulfonimide (4a). Yield ) 26.63 g, 92%. 1H NMR (400 MHz, DMSO-d6) δ 3.86 (s, 3H), 4.46 (t, 2H), 4.62 (t, 2H), 5.98 (dd, 1H), 6.18 (dd, 1H), 6.36 (dd, 1H), 7.70, (t, 1H), 7.78 (t, 1H), 9.14 (s, 1H). 1-Butyl-3-[2-[(1-oxo-2-propenyl)oxy]ethyl]-imidazolium Bis(trifluoromethane)sulfonimide (4b). Yield ) 30.75 g, 79%. 1H NMR (400 MHz, DMSO-d ) δ 0.89 (t, 3H), 1.10 (m, 2H), 6 1.76 (q, 2H), 4.18 (t, 2H), 4.50 (m, 4H), 6.00 (dd, 1H), 6.16 (dd, 1H), 6.31 (dd, 1H), 7.80 (d, 1H), 7.81 (d, 1H), 9.23 (s, 1H). 2.2. Polymer Membrane Formation and Characterization. RTIL monomers (∼1.5 g) were mixed with 5 mol % of an appropriate, matching cross-linking agent (divinylbenzene or 1,6-hexanediol diacrylate) and 1 wt % of a photoinitiator (2hydroxy-2-methylpropiophenone). A 3 in. × 3 in. square of a porous membrane filter (Supor 200, Pall, Ann Arbor, MI) (porosity, φ ) 0.80) was cut and placed on top of a Rain-X coated quartz plate. Rain-X is a commercially available, siloxane coating used to make glass surfaces hydrophobic. This coating aids in the release of the polymer product from the plates after photopolymerization but is otherwise inert. The monomer was dropped from a pipet onto the support surface, and a second quartz plate was placed on top to spread the monomer evenly across the support surface. Excess monomer was spread beyond the edges of the support. The plates were placed under a 365 nm UV lamp with an intensity of 8.5 mW/cm2 at the sample surface (XX-15A, Spectroline, Westbury, NY) and exposed for 30 min. After this time, the plates were removed and separated by inserting a clean razor blade in the gap. The film was then carefully peeled from the surface with the same razor. The polymerized film and support were transferred to a cutting board, where a 47 mm diameter stainless steel die was used to punch a sample film of exact size for membrane testing. The densities of the films were obtained using the same compositions of the polymerizable mixtures and polymerizing a known mass in a preweighed 10.00-mL volumetric flask. After polymerization, hexanes were added to the calibration line, and the additional mass noted. Hexanes were chosen as the solvent as it is immiscible with the RTIL monomers and was not expected to swell the poly(RTILs) during their brief contact. The volume of hexanes calculated was subtracted from the volume of the flask, and the volume of polymer was taken to be the difference. The mass of poly(RTIL) was divided by the volume of poly(RTIL) to give the density of each crosslinked network. These densities were used to calculate the volume and thickness of the poly(RTIL) in the membranes. Densities of polymer systems are displayed in Table 1. Acrylate-based poly(RTILs) were found to have densities about 10% higher on average than styrene-based poly(RTILs). Thicknesses of polymer films were calculated by subtracting the mass of polymer and support after polymerization from the average mass of a clean, 47 mm diameter circular portion of
Figure 3. Time-lag membrane gas permeability measurement apparatus.
Figure 4. Typical permeation data.
support (∼70 mg). Thicknesses calculated in this manner were similar to the listed thickness of the support, 145 µm. The degrees of conversion of polymerizable groups from RTIL monomer/cross-linker mixtures to poly(RTILs) were measured using Fourier transform infrared (FTIR) analysis (960M0027 Satellite, Mattson, Madison, WI). Thirty minutes of exposure to UV light was sufficient time for >90% of polymerizable groups to be converted to polymer. The films became more rubbery as the alkyl substituent increased in length, but all could undergo brittle fracture. 2.3. Permeability Measurements. The permeabilities of poly(RTIL) membranes were measured using a time-lag measurement apparatus. Details concerning the construction of this apparatus can be found in a previously published work.47 The equipment used for making these measurements is illustrated below (Figure 3). All experiments were performed at ambient temperature (20 °C). Membranes are degassed at pressures below 1 Torr dynamic vacuum overnight between runs, and each experiment begins under vacuum conditions. Three replicates were performed for each gas. A single gas (CO2, N2, or CH4) is added to the upstream “feed” volume at ∼2 atm against the initial vacuum downstream. The gas absorbs into the membrane, where it diffuses through, and finally desorbs from the downstream or “permeate” side. The feed volume is approximately four times that of the permeate volume, so as to keep the driving force at a near constant level throughout the experiment. Typically, data takes the form shown in Figure 4. The ideal permeability of the gas (Pi) is calculated from the membrane thickness (l), driving force (∆pi), and steady-state flux through the membrane (Ji). The steady-state flux can be found from the permeate pressure rise (∆P) with time (t) and the constants: permeate volume (Vperm), membrane area (A),
Ind. Eng. Chem. Res., Vol. 46, No. 16, 2007 5401 Table 2. Permeability, Solubility, and Diffusivity of Light Gases in Styrene-Based Poly(RTILS)a CO2
N2
CH4
styrene
Pb
Sc
Dd
P
S
D
P
S
D
methyl butyl hexyl
9.2 ( 0.5 20 ( 1 32 ( 1
4.0 ( 0.1 4.4 ( 0.3 3.9 ( 0.1
1.7 ( 0.1 3.5 ( 0.4 7.7 ( 0.4
0.29 ( 0.01 0.67 ( 0.02 1.4 ( 0.1
N/A N/A 0.10 ( 0.01
N/A N/A 11 ( 2
0.24 ( 0.01 0.91 ( 0.06 2.3 ( 0.1
0.21 ( 0.05 0.55 ( 0.07 0.57 ( 0.03
0.88 ( 0.16 1.28 ( 0.20 3.10 ( 0.15
a Uncertainty represents one standard deviation. b Permeability in Barrers (10-10 cm3(STP)‚cm/cm2‚s‚cm Hg). c Solubility presented in cubic centimeters gas (STP) per cubic centimeter polymer atmosphere. d Diffusivity values ×108 presented in squared centimeters per second.
Table 3. Permeability, Solubility, and Diffusivity of Light Gases in Acrylate-Based Poly(RTILS)a CO2
N2
CH4
acrylate
Pb
Sc
Dd
P
S
D
P
S
D
methyl butyl
7.0 ( 0.4 22 ( 1
3.6 ( 0.1 4.5 ( 0.4
1.5 ( 0.1 3.6 ( 0.4
0.23 ( 0.02 0.71 ( 0.06
N/A N/A
N/A N/A
0.19 ( 0.02 0.97 ( 0.08
0.17 ( 0.04 0.59 ( 0.09
0.89 ( 0.20 1.27 ( 0.09
a Uncertainty represents one standard deviation. b Permeability in Barrers (10-10 cm3(STP)‚cm/cm2‚s‚cm Hg). c Solubility presented in cubic centimeters gas (STP) per cubic centimeter polymer atmosphere. d Diffusivity values ×108 presented in squared centimeters per second.
gas constant (R), temperature (T), and support porosity (φ) (eq 3; calculation of permeability from measurable variables).
Pi )
Jil Vperm∆P l ) ∆pi AtRTφ ∆pi
The diffusivity (Di) can be found by extrapolating the slope of the steady-state flux back to the time axis. This intercept is the “time lag” (Θi) of the gas through the membrane and is related to Di by eq 4 (calculation of diffusivity from time lag (θ) and membrane thickness (l)).
Di )
l2 6Θ
Knowledge of Pi and Di allows for the calculation of solubility (Si) via eq 1. These values are useful for comparing the various components of separation selectivity shown by eq 5.2. 3. Results and Discussion 3.1. Ideal Gas Properties and Performance. The permeabilities, solubilities, and diffusivities of CO2, N2, and CH4 for the styrene- and acrylate-based poly(RTIL) membranes at 20 °C are shown below (Tables 2 and 3). Data was not obtained for a third acrylate-based monomer because the results from membrane testing indicated almost no difference in performance relating to the different types of polymerizable group. In future work, we plan to use styrenebased RTIL monomers exclusively unless conditions necessitate acrylates. Styrene-based RTIL monomers are superior in that they are more easily prepared from the inexpensive, commercially available 4-chloromethylstyrene. We do not prefer to use 2-bromoethylacrylate, since it is a strong irritant and takes several steps to prepare cleanly. Permeability and diffusion of CO2, N2, and CH4 in both types of poly(RTILs) increased dramatically and in a nonlinear fashion, as the n-alkyl substituent increased in length (Figure 5). In the styrene-based poly(RTILs), CO2 permeability more than doubled between the methyl to n-butyl system and doubled again from the n-butyl to n-hexyl system. The CO2 permeability was lower in the acrylate-based poly(RTIL) with a methyl group than its styrene-based equivalent, but the permeability was roughly equivalent to the styrene-based equivalent when the group was extended to n-butyl. CH4 permeability experienced an even greater increase from the methyl to n-butyl systems, but it only doubled when changing from the n-butyl to n-hexyl
Figure 5. CO2 permeability trends in styrene- and acrylate-based poly(RTILs). Error bars within symbols.
systems. The diffusion rates of CO2 through poly(RTILs) with either type of polymerizable group were found to be roughly equivalent and were in line with expectations for dense polymers. Unfortunately, N2 diffusion coefficients are unavailable for most poly(RTILs) because the time lag was less pronounced during N2 permeation experiments. The solubility of CO2 in both systems was found to be quite high for polymers,48 although not quite to the levels reported for poly(RTIL)-PEG copolymers.40 CO2 solubility increased in both systems when the methyl group was changed to n-butyl. However, CH4 solubility increases dramatically between the methyl and n-butyl poly(RTILs) but appears to be constant from n-butyl to n-hexyl. Unfortunately, most N2 solubility values cannot be reported due to the problems associated with the calculating diffusivities of N2 in four membranes. Figure 6 shows solubility trends for CO2 and CH4 in styrene-based poly(RTILs). Poly(RTILs) are fairly unique among gas separation membranes, as they contain large ions, one of which is tethered to the backbone and one that is free. There is also an alkyl component whose size is controllable. The behavior of these light gases in poly(RTILs) with increasingly longer alkyl substituents may be a consequence of the creation of more free volume within the polymer network due to inefficient packing of the n-alkyl side chains. Additionally, free volume may also have been created due to local repulsions between the immobilized ions and “ionphobic” alkyl chains. Unlike molten, liquid-phase RTILs, which can continuously rearrange themselves to accommodate ion-alkyl repulsions, poly(RTILs) have no such mechanism. Cations are in relatively fixed positions in the polymers, although anions may still move very slowly.
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Figure 6. Ideal solubility of CO2 and CH4 in styrene-based poly(RTILs). Figure 7. Robeson plot annotated to include poly(RTILs) from this paper and polymers reported in the literature (open symbols) (adapted from ref 26).
Table 4. Ideal Gas Separation Properties of Styrene-Based Poly(RTILs) CO2/N2
CO2/CH4
styrene
P
S
D
P
S
D
methyl butyl hexyl
32 30 28
N/A N/A 39
N/A N/A 0.7
39 22 17
19 8.0 6.9
2.0 2.7 2.5
Table 5. Ideal Gas Separation Properties of Acrylate-Based Poly(RTILs) CO2/N2
CO2/CH4
acrylate
P
S
D
P
S
D
Methyl Butyl
31 30
N/A N/A
N/A N/A
37 22
21 7.7
1.6 2.9
Repulsive forces between immobilized alkyl chains and ions within amorphous, solid materials may force local phase separation. This local immiscibility should result in the creation of more free volume within the polymer. Extending the alkyl group increases diffusion rates and, in turn, permeability for all gases. Simultaneously, the extension of the alkyl chain dilutes the concentration of polar groups (i.e., ions). It is this polar component of RTILs which is largely responsible for high CO2 solubility.13-24,48 Conversely, CH4 prefers to dissolve in nonpolar components,48 and as such, its solubility appears unaffected in poly(RTILs) with n-butyl or n-hexyl substituents. However, CH4 solubility suffered greatly when only a methyl group was present on the imidazolium ring. In an attempt to more completely understand the behavior of CO2 and CH4 in poly(RTILs) with even longer alkyl chains, styrene-based poly(RTILs) with n-octyl and n-decyl substituents were synthesized. However, upon polymerization of these larger RTIL monomers, the resultant poly(RTILs) lacked sufficient mechanical stability (even on the porous polymer support) for membrane testing and were easily damaged by the pressure exerted on them by compressed o-rings. The ideal gas separation properties of the poly(RTILs) studied are presented in Tables 4 and 5. As can be seen in Tables 4 and 5, permselectivity for CO2/ N2 in both types of poly(RTILs) remains fairly constant even as the membranes’ permeability increased due to the lengthening
of the n-alkyl substituent. The permselectivity for CO2/N2 in the styrene-based poly(RTIL) with a n-hexyl group can be shown to be favored by a large solubility difference and slightly hindered by faster diffusion of N2. The slower diffusion of CO2 may be a consequence of its large solubility, and its interactions with the polar groups slow its transport.49 CO2/CH4 separation was favored by both solubility and diffusivity differences. These diffusion differences were enhanced in poly(RTILs) that had n-butyl or n-hexyl substituents, as the polymer networks were more open to the smaller CO2, yet only to a point where CH4 transport was still somewhat restricted. Poly(RTILs) with a methyl group also had a favorable difference, but diffusion of CO2 was also retarded due to the more tightly packed polymer chains. CO2/CH4 ideal solubility selectivity was highest when the alkyl substituent was a methyl group. These poly(RTILs) minimized dissolution of CH4, as they had the highest concentration of ions and the least free volume. A tightly packed, highly polar matrix would be expected to severely limit the amount of CH4 that could dissolve.49 The solubility selectivity for CO2/CH4 in poly(RTILs) with a methyl substituent on the imidazolium ring was much greater than that achieved in bulk-fluid separations with RTILs.13 However, CO2/ N2 appears to be the much more viable separation for these first-generation poly(RTIL) membranes, as was predicted in our previous paper.13 3.2. Comparison to Upper Bound Polymers on a CO2/N2 Robeson Plot. A frequently employed metric for polymer gas separation membrane performance is the use of Robeson plots.26 These popular charts gauge progress in the field of membrane science by plotting the ideal separation selectivity for a gas pair (e.g., CO2/N2) in a polymer membrane against the permeability of the more permeable gas (i.e., CO2) at ∼1 atm driving force. The log-log plot also displays an upper bound which has been defined by glassy polymers with large diffusion selectivities.26,49,50 Membranes with performances exceeding the upper bound are deemed noteworthy. The upper right quadrant is more attractive for industrial applications. A Robeson plot for CO2/N2 annotated with data from this report appears below (Figure 7).
Table 6. CO2 Solubility in Two Widely Studied RTILsa
a
RTIL
molar volume (cc/mol)
H (atm) 25 °C
mol fraction CO2 (1 atm)
mol CO2/ L RTIL (1 atm)
cc CO2 (STP)/ cc RTIL (1 atm)
[C2mim][Tf2N] [C6mim][Tf2N]
261 326
39 34
0.026 0.029
0.098 0.090
2.20 2.02
Data taken from ref 13.
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First-generation poly(RTIL) membranes “hug” the upper bound of the chart. The large solubility difference between CO2 and N2 drives the separation in poly(RTILs), which is in contrast to the large diffusion selectivities exhibited by glassy polymers near the upper bound.49 The first-generation poly(RTIL) membranes synthesized in this study show promise and need further research to improve permeability and selectivity. The near constant CO2/N2 ideal separation factor in the poly(RTILs) studied coupled with increasing permeability is another reason why poly(RTILs) with alkyl chains of n-octyl or larger were of interest. If these trends do indeed continue, poly(RTIL) membranes with large CO2 permeabilities and selectivities near the upper bound of the Robeson plot for CO2/N2 appear possible. 3.3. Comparison to Bulk-Fluid Separations in Liquid RTILs. Poly(RTILs) outperform many conventional organic polymers for CO2/N2 separations, and they also appear to perform as well or better than their liquid RTIL counterparts containing [Tf2N] anions.13 While these membrane experiments were performed at 20 °C, the majority of solubility data for RTILs is available at 25 or 40 °C, so the comparisons will not be exact. Temperature has a large effect on ideal solubility and selectivity in bulk fluid separations with liquid RTILs.13 Nonetheless, the available data are sufficient to make meaningful observations about the respective performances of each material. The poly(RTIL) membranes studied can achieve CO2/CH4 separation selectivities that liquid RTILs with [Tf2N] anions cannot,13 because of the added diffusion selectivity the polymers impart. CO2/N2 separation selectivity in poly(RTILs) is on par with liquid RTILs possessing [Tf2N] anions. Solubility of CO2 in poly(RTILs) appears to be larger than in the analogous liquids. Table 6 shows the solubility of CO2 at 1 atm in both [C2mim][Tf2N] and [C6mim][Tf2N] liquid RTILs, expressed as Henry’s constants and as a volume ratio of CO2 (STP) to volume of RTIL. Poly(RTIL) membranes are able to dissolve about twice as much volume of CO2 per cubic centimeter of material than their liquid analogues at similar temperatures. This is consistent with previously published results for other poly(RTIL) systems.38,39,41,46 The increased uptake of CO2 relative to liquid RTILs is a very interesting feature of poly(RTILs). This behavior may be due to larger free volumes in poly(RTILs) than in the molten salts, due to repulsions between ions and alkyl chains. Poly(RTILs) appear to be potentially useful as sorbents for gas storage and delivery as well as membranes. 4. Conclusions and Future Work Poly(RTILs) are very promising materials for CO2 separations, and more research is needed to further their progress and truly understand their capabilities. Membranes composed of poly(RTILs) with systematic structural variations were synthesized and tested for their gas separation performance with CO2, N2, and CH4. These polymers were found to have high CO2 solubility and nearly constant CO2/N2 permselectivity with increasing permeability as the n-alkyl substituent on the cation became longer. First-generation RTILs show promise as a novel membrane platform, as evidenced by their positions on a Robeson plot for CO2/N2. The solubility of CO2 in these poly(RTILs) is nearly double that in analogous liquid RTILs. RTILs present a unique platform for monomer design where functional groups can be easily tailored to an application. As such, there are numerous opportunities to tune the chemistry of these materials to further enhance CO2/N2 and CO2/CH4 separations. Oligo(ethylene glycol) units and perfluoroalkyl chains present interesting functional groups for enhanced CO2
interactions with poly(RTILs). The role of different anions on selectivity in poly(RTIL) membranes also remains to be seen. The addition of liquid RTILs into a poly(RTIL) matrix may also allow for increased diffusion and permeability. Research and design of poly(RTILs) has also been inspired, in part, by the achievements of poly(RTILs) as ion-conductive materials.33-35 The design of poly(RTILs) in that application appears to be ahead of those in gas separations. It is important to monitor the work of those researchers for advances in the design of poly(RTILs) which may aid in the field of gas separations. Literature Cited (1) Zhao, H. Innovative Applications of Ionic Liquids as “Green” Engineering Liquids. Chem. Eng. Commun. 2006, 193, 1660. (2) Camper, D.; Scovazzo, P.; Koval, C.; Noble, R. Gas Solubilities in Room Temperature Ionic Liquids. Ind. Eng. Chem. Res. 2004, 43, 3049. (3) Blancard, L. A.; Hancu, D.; Beckman, E. J.; Brennecke, J. F. Green Processing Using Ionic Liquids and CO2. Nature 1999, 399, 28. (4) Earle, M. J.; Esperanca, J. M. S. S.; Gilea, M. A.; Canongia Lopes, J. N.; Rebelo, L. P. N.; Magee, J. W.; Seddon, K. R.; Widegren, J. A. The distillation and volatility of ionic liquids. Nature 2006, 439, 831. (5) Wasserscheid, P. Chemistry: Volatile Times for Ionic Liquids. Nature 2006, 439, 797. (6) Baltus, R. E.; Counce, R. M.; Culbertson, B. H.; Luo, H.; DePaoli, D. W.; Dai, S.; Duckworth, D. C. Examination of the Potential of Ionic Liquids for Gas Separations. Sep. Sci. Technol. 2005, 40, 525. (7) Huang, J.; Riisager, A.; Wasserscheid, P.; Fehrmann, R. Reversible Physical Absorption of SO2 by Ionic Liquids. Chem. Commun. 2006, 38, 4027. (8) Wu, W.; Han, B.; Gao, H.; Liu, Z.; Jiang, T.; Huang, J. Desulfurization of Flue Gas: SO2 Absorption by an Ionic Liquid. Angew. Chem., Int. Ed. 2004, 43, 2415. (9) Bates, E. D.; Mayton, R. D.; Ntai, I.; Davis, J. H., Jr. CO2 Capture by a Task-Specific Ionic Liquid. J. Am. Chem. Soc. 2002, 124, 926. (10) Yokozeki, A.; Shiflett, M. B. Hydrogen Purification Using RoomTemperature Ionic Liquids. Appl. Energy 2006, 84, 351. (11) Tempel, D. J.; Henderson, P. B.; Brzozowski, J. R.; Pearlstein, R. M.; Garg, D. Ionic Liquid Based Mixtures for Gas Storage and Delivery. U.S. Patent Application Publication; US Patent and Trademark Office: Alexandria, VA, 2006; p 15, continuation in part of U.S. Serial No. 948,277. (12) Holbrey, J. D.; Plechkova, N.V.; Seddon, K. R. Recalling COIL. Green Chem. 2006, 8, 411. (13) Camper, D.; Bara, J.; Koval, C.; Noble, R. Bulk-Fluid Solubility and Membrane Feasibility of Rmim-Based Room-Temperature Ionic Liquids. Ind. Eng. Chem. Res. 2006, 45, 6270. (14) Scovazzo, P.; Camper, D.; Kieft, J.; Poshusta, J.; Koval, C.; Noble, R. Regular Solution Theory and CO2 Gas Solubility in Room-Temperature Ionic Liquids. Ind. Eng. Chem. Res. 2004, 43, 6855. (15) Cadena, C.; Anthony, J. L.; Shah, J. K.; Morrow, T. I.; Brennecke, J. F.; Maginn, E. J. Why Is CO2 So Soluble in Imidazolium-Based Ionic Liquids? J. Am. Chem. Soc. 2004, 126, 5300. (16) Baltus, R. E.; Culbertson, B. H.; Dai, S.; Luo, H.; DePaoli, D. W. Low-Pressure Solubility of Carbon Dioxide in Room-Temperature Ionic Liquids Measured with a Quartz Crystal Microbalance. J. Phys. Chem. B 2004, 108, 721. (17) Anthony, J. L.; Aki, S. N. V. K.; Maginn, E. J.; Brennecke, J. F. Feasibility of Using Ionic Liquids for Carbon Dioxide Capture. Int. J. EnViron. Technol. Manage. 2004, 4, 105. (18) Camper, D.; Becker, C.; Koval, C.; Noble, R. Low Pressure Hydrocarbon Solubility in Room Temperature Ionic Liquids Containing Imidazolium Rings Interpreted Using Regular Solution Theory. Ind. Eng. Chem. Res. 2005, 44, 1928. (19) Camper, D.; Becker, C.; Koval, C.; Noble, R. Diffusion and Solubility Measurements in Room Temperature Ionic Liquids. Ind. Eng. Chem. Res. 2006, 45, 445. (20) Scovazzo, P.; Kieft, J.; Finan, D. A.; Koval, C.; DuBois, D.; Noble, R. Gas Separations Using Non-hexafluorophosphate [PF6-] Anion Supported Ionic Liquid Membranes. J. Membr. Sci. 2004, 238, 57. (21) Morgan, D.; Ferguson, L.; Scovazzo, P. Diffusivities of Gases in Room-Temperature Ionic Liquids: Data and Correlations Obtained Using a Lag-Time Technique. Ind. Eng. Chem. Res. 2005, 44, 4815.
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ReceiVed for reView March 27, 2007 ReVised manuscript receiVed May 7, 2007 Accepted May 22, 2007 IE0704492
