Study on Gas Permeation and CO2 Separation through Ionic Liquid

Article pubs.acs.org/IECR

Study on Gas Permeation and CO2 Separation through Ionic LiquidBased Membranes with Siloxane-Functionalized Cations Liliana C. Tomé,†,∥ Andreia S. L. Gouveia,†,∥ Mohd A. Ab Ranii,‡ Paul D. Lickiss,‡ Tom Welton,‡ and Isabel M. Marrucho*,†,§ †

Instituto de Tecnologia Química e Biológica António Xavier, Universidade Nova de Lisboa, Av. da República, 2780-157 Oeiras, Portugal ‡ Department of Chemistry, Imperial College London, London SW7 2AZ, United Kingdom § Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Avenida Rovisco Pais, 1049-001 Lisboa, Portugal S Supporting Information *

ABSTRACT: This work explores ionic liquid-based membranes with siloxane functionalized cations using two different approaches: supported ionic liquid membranes (SILMs) and poly(ionic liquid)−ionic liquid (PIL−IL) composite membranes. Their CO2, CH4, and N2 permeation properties were measured at T = 293 K with a trans-membrane pressure differential of 100 kPa. The thermophysical properties of the synthesized siloxane-functionalized ILs, namely viscosity and density (data in the Supporting Information), were also determined. Contrary to what was expected, the gas permeation results show that the SILMs containing siloxane-functionalized cations have CO2 permeabilities that are lower than those of their analogues without the siloxane functionality. The addition of siloxane-based ILs into PILs increases both CO2 permeability and CO2/N2 permselectivity, although it does not significantly change the CO2/CH4 permselectivity. The prepared membranes present very diverse CO2 permeabilities, between 57 and 568 Barrer, while they show permselectivities varying from 16.8 to 36.8 for CO2/N2 and from 9.8 to 11.5 for CO2/CH4. As observed for other ILs, superior CO2 separation performances were obtained when the IL containing [C(CN)3]− is used compared to that having the [NTf2]− anion. of ILs,14−16 maintaining in the polymer several of the unique IL features, especially the tunability of their properties. Bearing in mind that the relatively high viscosity of ILs is a key barrier to their use in a huge number of different applications, including the development of CO2 separation membranes, significant efforts have been made to reduce the viscosity of ILs by combining specific functional groups or adding different cations and/or anions.17−20 In particular, and concerning the use of specific functional groups, Niedermeyer et al.21 focused on the effect on the viscosity of introducing a siloxane group in the imidazolium cation. It was found that in spite of the increment in the cation’s mass, the viscosity of [(SiOSi)C1mim][NTf2] did not show a substantial increase compared to that of [C4mim][NTf2]. Shirota et al.22 also studied the effects on viscosity and intermolecular interactions of having silicon-functionalized imidazolium cations, such as [(Si)mim]+, [(PhSi)mim]+, and [(SiOSi)mim]+, in the IL structure. The authors observed that the low viscosity of [(SiOSi)mim][NTf2] compared to that of [(Si)mim][NTf2]

1. INTRODUCTION During the last 15 years, ionic liquids (ILs) have blossomed as alternative solvents for CO2 separation applications,1−5 not only because of their high CO2 solubility and selectivity but also because of the exceptional tunability of their properties. Taking into account the attractive properties of ILs for CO2 separation, different engineered membrane approaches have been considered,6−8 while gas solubility studies support the importance of the design of task-specific IL-based materials.9−13 Within the several membrane configurations explored to date,6 the simplest approach is the use of supported ionic liquid membranes (SILMs), in which the selected IL is immobilized into the pores of an inert solid membrane support. The most important advantage of SILMs is perhaps the negligible displacement of the liquid phase from the membrane pores through solvent evaporation, guaranteed by the negligible vapor pressure of ILs, thus overcoming one of the problems associated with traditional liquid membranes. Alternatively, poly(ionic liquid)−ionic liquid (PIL−IL) composite membranes have also been intensively studied as a way to prepare membranes that can combine the adequate gas permeability and selectivity of SILMs with the robustness of polymers.6−8 Actually, PILs are functional materials that merge the macromolecular architecture of polymers with the chemistry © 2017 American Chemical Society

Received: Revised: Accepted: Published: 2229

December 1, 2016 January 30, 2017 February 2, 2017 February 2, 2017 DOI: 10.1021/acs.iecr.6b04661 Ind. Eng. Chem. Res. 2017, 56, 2229−2239

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Industrial & Engineering Chemistry Research and [(PhSi)mim][NTf2] was linked to the flexible properties of the Si−O−Si functional group, which lead to a smaller cation− anion interaction, despite its higher molar volume. Within the context of IL-based membranes for CO2 separation, and despite the large array of different SILM systems developed during the past decade,6,7,23 the gas permeation properties of SILMs with siloxane-functionalized cations have still not been entirely studied and discussed. In contrast, Bara et al.24 explored the CO2 separation performance of PIL−IL composite membranes comprising an etherfunctionalized imidazolium PIL and alkyl, ether, nitrile, fluoroalkyl, and siloxane-functionalized cations in the IL. The authors observed that PIL−IL composite membranes containing 20% of [(SiOSi)C1mim][NTf2] IL exhibited the greatest gas permeabilities but presented the lowest selectivities.24 Later, Carlisle et al.25 studied the CO2/N2 and CO2/CH4 membrane separation performances of composite membranes made of PILs containing different functional cationic groups, such as alkyl, fluoroalkyl, oligo(ethylene glycol), and disiloxane, and 20% of [C2mim][NTf2] IL. The disiloxane-functionalized vinylimidazolium PIL showed the highest CO2 permeability but low CO2/N2 and CO2/CH4 selectivities.25 Furthermore, polydimethylsiloxane (PDMS) has been commonly used for several gas separation applications not only because of its chemical and thermal stabilities and low price but also because of its high permeability to a wide range of gas species, justified by its chain flexibility and large free volume.26−29 In the present work, the gas permeation properties and CO2 separation performances of IL-based membranes with siloxanefunctionalized cations were evaluated using two different approaches: SILMs and PIL−IL composite membranes. First, two ILs containing the same siloxane-functionalized cation, [(SiOSi)C1mim]+, and different anions such as [NTf2]− and [C(CN)3]− were synthesized and used to prepare SILMs. The [NTf2]− anion was selected owing to its high thermal stability and CO2 permeability, while the [C(CN)3]− anion was chosen because of its low viscosity17 and recognized CO2 separation efficiency.30 Afterward, and considering that pyrrolidiniumbased PILs can be simply prepared by metathesis reactions from a commercially available polyectrolyte,31,32 PIL−IL composite membranes based on two pyrrolidinium-based PILs, such as poly([Pyr11][NTf2]) and poly([Pyr11][C(CN)3]), with 40 and 60 wt % of free siloxane-functionalized ILs were also prepared and evaluated. The use of 40 and 60 wt % of free IL not only allows the preparation of homogeneous and stable PIL−IL composite membranes but also improves their CO2 separation performance when compared to those with only 20 wt % of free IL, as we have previously observed in our studies.31,32

2.2. Synthesis of Siloxane-Functionalized Ionic Liquids. The synthetic route of ILs focused on first preparing the imidazolium halide salt followed by anion metathesis reaction. 33 The 1-methyl-3-pentamethyldisiloxymethylimidazolium chloride, [(SiOSi)C1mim]Cl, was initially synthesized according to a previously described procedure.21,34 The preparation of 1-methyl-3-pentamethyldisiloxymethylimidazolium bis(trifluoromethylsulfonyl)imide, [(SiOSi)C1mim][NTf2], was then carried out via an anion exchange reaction involving the [(SiOSi)C1mim]Cl (1.0 equiv) and a slight excess of LiNTf2 (1.1 equiv). In particular, a solution of LiNTf2 (11.3 g, 39.45 mmol) in water (10 mL) was added to a solution of [(SiOSi)C1mim]Cl (10 g, 35.86 mmol) in water (10 mL). The mixture was magnetically stirred for 24 h, extracted with dichloromethane (3 × 20 mL), and washed with aliquots of water (5 mL) until halide free, as indicated by the AgNO3 test of the water washings. The solvents were afterward eliminated by rotary evaporation, affording [(SiOSi)C1mim][NTf2]. A similar procedure was used to prepare the 1-methyl3-pentamethyldisiloxymethylimidazolium tricyanomethanide, [(SiOSi)C1mim][C(CN)3], except that sodium tricyanomethanide (5.35 g, 47.33 mmol) and [(SiOSi)C1mim]Cl (12 g, 43.03 mmol) were used. The chemical structures of the synthesized siloxane-functionalized ILs are shown in Figure 1. Finally, the

Figure 1. Chemical structure of the siloxane-functionalized ionic liquids (ILs) and the pyrrolidinium-based poly(ionic liquid)s (PILs).

synthesized ILs were dried at approximately 1 Pa and 318 K for at least 4 days. The water contents of the siloxane-functionalized ILs were determined by Karl Fischer titration using a 831 KF Coulometer (Metrohm), and the results are shown in Table 1. 2.3. Density and Viscosity Determination. The measurements of density and viscosity of the pure siloxanefunctionalized ILs, [(SiOSi)C1mim][NTf2] and [(SiOSi)C1mim][C(CN)3], were performed in the temperature range between 293.15 and 353.15 K at atmospheric pressure using an SVM 3000 Anton Paar rotational Stabinger viscometerdensimeter, where the standard uncertainty for the temperature is 0.02 K. The repeatability of density and dynamic viscosity of this equipment is 0.0005 g·cm−3 and 0.35%, respectively. Three measurements of each sample were performed, and the data reported are average values. The highest relative standard uncertainties registered for the dynamic viscosity and density measurements were 0.03 and 2 × 10−4, respectively. 2.4. Preparation of Supported Ionic Liquid Membranes. The pure [(SiOSi)C1mim][NTf2] was supported in a porous hydrophobic poly(vinylidene fluoride) (PVDF) membrane (Millipore Corporation, United States), with an average thickness of 125 μm and a pore size of 0.22 μm. Although these membrane filters are known by their chemical resistance and

2. EXPERIMENTAL SECTION 2.1. Materials. Acetone (99.8%), acetonitrile (99.8%), dichloromethane (99.8%), and poly(diallyldimethylammonium) chloride solution (average 400−500 kDa, 20 wt % in water) were supplied by Sigma-Aldrich. Sodium tricyanomethanide (NaC(CN)3, 98 wt % pure) and lithium bis(trifluoromethylsulfonyl)imide (LiNTf2, 99 wt % pure) were purchased from IoLiTec GmbH. The chemicals utilized in the synthesis of ILs and PILs were used as received, and the water was double-distilled. Carbon dioxide (CO2), methane (CH4), and nitrogen (N2) were provided by Air Liquide with a minimum purity of 99.99%. 2230

DOI: 10.1021/acs.iecr.6b04661 Ind. Eng. Chem. Res. 2017, 56, 2229−2239

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Industrial & Engineering Chemistry Research Table 1. Water Contents and Physical Properties of the ILs Used to Perform the SILMs ionic liquid

wt % of water

M (g·mol−1)

η (mPa·s)a

ρ (g·cm−3)a

Vm (cm3·mol−1)b

[(SiOSi)C1mim][NTf2] [(SiOSi)C1mim][C(CN)3] [C2mim][NTf2]c [C2mim][C(CN)3]d

0.04 0.11 0.02 0.01

523.6 333.6 391.3 201.2

100.8 60.21 39.09 16.62

1.321 1.046 1.524 1.085

396.5 318.8 256.8 185.5

a

Density (ρ) and viscosity (η) measured at 293.15 K. bMolar volume (Vm) obtained for 293.15 K. cValues of [C2mim][NTf2] taken from Tomé et al.35 dValues of [C2mim][C(CN)3] taken from Tomé et al.30

stability,36 the impregnation of [(SiOSi)C1mim][C(CN)3] into porous hydrophobic PVDF resulted in an unstable SILM. To circumvent this problem, the pure [(SiOSi)C1mim][C(CN)3] was supported in a porous hydrophilic poly(tetrafluoroethylene) (PTFE) membrane (Merck Millipore), with an average thickness of 65 μm and a pore size of 0.2 μm. SILM configurations of the siloxane-functionalized ILs were prepared by the vacuum method.35 The thickness of the SILMs was assumed to be equivalent to the membrane filter thickness. 2.5. Synthesis of Poly(ionic liquid)s. PILs containing a pyrrolidinium polycation and [NTf2]− or [C(CN)3]− as counter anions (Figure 1) were prepared by anion metathesis reactions, following established procedures described elsewhere.32,37 The obtained white solids, poly([Pyr11][NTf2]) and poly([Pyr11][C(CN)3]), termed PIL NTf2 and PIL C(CN)3, respectively, were washed with water, filtered, and dried at 318 K. 2.6. Preparation of PIL−IL Membranes. The solvent casting method was used to prepare free-standing composite membranes (Figure 2) based on the synthesized PILs and

different contents of siloxane-functionalized ILs having the same anion. At first, 6 (w/v) % solutions of each PIL with 40 and 60 wt % of free IL were prepared and stirred until complete dissolution of PIL and IL components. After that, the solutions were poured into Petri dishes and left for slow evaporation for at least for 2 days. Additional details of the experimental conditions used are represented in Table 2. Membrane thicknesses (245−330 μm) were determined using a digital micrometer (Mitutoyo, model MDE-25PJ, Japan). 2.7. Gas Permeation Experiments. Ideal CO2, CH4, and N2 permeabilities and diffusivities were measured using a timelag apparatus.31 In the present work, each membrane was degassed under vacuum inside the permeation cell for 12 h immediately prior to test. The experiments were conducted at T = 293 K with a trans-membrane pressure differential of 100 kPa. The permeation data presented is the average result of three separate CO2, CH4, and N2 experiments on each membrane sample. The permeation cell and lines were always evacuated until the pressure was below 0.1 kPa before each run. Note that no residual IL was found inside the permeation cell at the end of the experiments. Gas transport through the prepared membranes was assumed to track a solution-diffusion mass-transfer mechanism,38 where permeability (P) relates to diffusivity (D) and solubility (S) as follows: (1)

P=D×S

The permeate flux of each gas (Ji) was determined experimentally using eq 2:39 Ji =

V pΔpd (2)

AtRT

where V is the permeate volume, Δpd the variation of downstream pressure, A the effective membrane surface area, t the experimental time, R the gas constant, and T the temperature. Ideal gas permeability (Pi) was then calculated from the pressure driving force (Δpi) and membrane thickness (S ) as shown in eq 3. p

Pi =

Figure 2. Pictures of the prepared composite membranes based on PILs and different amounts of free IL containing siloxane-functionalized cations.

Ji Δpi /S

(3)

Gas diffusivity (Di) was determined according eq 4. The time-lag parameter (θ) was deduced by extrapolating the slope

Table 2. Composition Descriptions and Experimental Conditions of the Casting Procedure Used to Prepare the Composite Membranes composite membrane PIL PIL PIL PIL

NTf2−40 IL Si NTf2 NTf2−60 IL Si NTf2 C(CN)3−40 IL Si C(CN)3 C(CN)3−60 IL Si C(CN)3

polymer (PIL)

ionic liquid (IL)

poly([Pyr11][NTf2])

[(SiOSi)C1mim][NTf2]

poly([Pyr11][C(CN)3])

[(SiOSi)C1mim][C(CN)3]

2231

wt % of IL

solvent

T (K)

40 60 40 60

acetone

298

acetonitrile

313


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Industrial & Engineering Chemistry Research

Figure 3. Gas (a) permeability, (b) diffusivity, and (c) solubility through the prepared SILMs. Error bars represent standard deviations based on three experimental replicas. The gas permeation properties of [C2mim][NTf2] and [C2mim][C(CN)3] SILMs were taken from Tomé et al.30,35

of the linear portion of the pd versus t curve back to the time axis, where the intercept is equal to θ.40 Di =

S2 6θ

product of the diffusivity selectivity and the solubility selectivity: αi/j =

(4)

After both Pi and Di were known, the gas solubility (Si) was calculated using the relationship shown in eq 1. The ideal permeability selectivity (or permselectivity), αi/j, was determined by dividing the permeability of the more permeable species i to the permeability of the less permeable species j. The permselectivity can also be expressed as the

⎛D ⎞ ⎛S ⎞ Pi = ⎜⎜ i ⎟⎟ × ⎜⎜ i ⎟⎟ Pj ⎝ Dj ⎠ ⎝ Sj ⎠

(5)

3. RESULTS AND DISCUSSION 3.1. Gas Permeation through SILMs. The chemical structures and physical properties of the siloxane-functionalized ILs used in the preparation of SILMs are presented in Figure 1 and Table 1, respectively. Note that the density (ρ) and 2232


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Figure 4. Relationship between the experimental CO2 diffusivities determined through the prepared SILMs and the respective IL viscosity measured at 293.15 K. The values of the [C2mim][NTf2] and [C2mim][C(CN)3] SILMs were taken from Tomé et al.30,35

the fact that the siloxane-functionalized ILs have higher viscosities (Table 1), the CH4 and N2 permeabilities were found to significantly increase when compared to those previously determined through the SILMs without the siloxane functionality (Figure 3a). This means that the gas permeability through the studied SILMs containing siloxane-functionalized cations does not entirely correlate directly with the IL viscosity. From the experimental gas diffusivities measured in this work (Figure 3b), it can be seen that the presence of the siloxane functionality does not significantly affect the gas diffusivities through SILMs having the [C(CN)3]− anion, whereas when the [C2mim]+ is replaced by the [(SiOSi)C1mim]+ in the SILMs containing the [NTf2]− anion, the CO2, CH4, and N2 diffusivities increased by 44%, 39%, and 56%, respectively. Several works have reported an inversely proportional relationship between IL viscosity and gas diffusivity.23,45−47 For instance, Scovazzo proposed a general correlation,23 which has been used as follows:

viscosity (η) values of the siloxane-functionalized ILs were measured in the temperature range from 293.15 to 353.15 K. A detailed description of these data is presented in the Supporting Information. The highest viscosity values and molar volumes were obtained for [(SiOSi)C1mim][NTf2] IL. These data will be used in the following section in the understanding of the gas permeation results obtained through the prepared SILMs. The gas permeabilities and diffusivities through the prepared SILMs are illustrated in panels a and b of Figure 3, respectively, while the gas solubility values estimated from eq 1 are plotted in Figure 3c. To the best of our knowledge, the CO2, CH4, and N2 gas permeabilities of [(SiOSi)C1mim][NTf2] SILM were determined only by Bara et al.,3 at 295 K using a poly(ether sulfone) (PES) membrane as support, while the [(SiOSi)C1mim][C(CN)3] SILM is reported here for the first time. The gas permeation properties of both [C2mim][NTf2] and [C2mim][C(CN)3] SILMs, which were previously determined using the same experimental conditions,30,35 are also included in Figure 3 for comparison. In Figure 3a, it can be observed that a similar trend in gas permeability is valid for all the studied SILMs: P CO2 ≫ P CH4 > P N2. The obtained CO2 permeability values differ from 545 to 667 Barrer, while for CH4 and N2 they vary from 12 to 54 Barrer. Regarding the chemical nature of the anions, SILMs based on the [NTf2]− anion present CO2 permeabilities that are slightly lower than the SILMs with the [C(CN)3]− anion. This behavior is usually linked to the lower viscosities of [C(CN)3]− anion when compared to the [NTf2]− anion (Table 1). Additionally, the SILMs containing siloxane-functionalized cations have CO2 permeabilities that are lower than those of their analogues without the siloxane functionality (Figure 3a), which can also be related to the ILs’ relative viscosities. At 293.15 K, the viscosity values for [(SiOSi)C1mim][NTf2] and [(SiOSi)C1mim][C(CN)3] are 100.8 and 60.2 mPa s, respectively, while for their nonfunctionalized analogues the measured viscosities are 39.1 and 16.6 mPa s, respectively. This trend, where ILs with higher viscosities generally produce SILMs having lower gas permeabilities, has been recognized by different authors.23,41−44 Nonetheless, an unexpected behavior was found for the CH4 and N2 permeabilities. In contrast with

D=A

VIL a ηIL bVgas c

(6)

where A, a, b, and c are IL-class-specific parameters; ηIL is the IL viscosity, VIL the IL molar volume; and Vgas the solute gas molar volume. In the case of ILs with 1-alkyl-3-methylimidazolium cations having an alkyl chain length smaller than four carbon atoms, a is equal to zero, and consequently, the gas diffusivity should be linked to the IL viscosity alone. Following this line, the CO2 diffusivity and IL viscosity of the studied SILMs is depicted in Figure 4. For the SILMs without the siloxane functionality, a decrease in the IL viscosity owing to the presence of the [C(CN)3]− anion corresponds to an increase in CO2 diffusivity, which is in agreement with what was previously observed for other SILMs.23,45−47 Nevertheless, an odd behavior can be observed for the SILMs containing siloxanefunctionalized cations prepared in this work. Contrasting to what was expected from their respective IL viscosities, the [(SiOSi)C 1 mim][NTf 2 ] and [(SiOSi)C 1 mim][C(CN) 3 ] SILMs have nearly the same CO2 diffusivities (Figure 4). Additionally, these SILMs containing siloxane-functionalized 2233


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Figure 5. Gas (a) permeability, (b) diffusivity, and (c) solubility through the prepared PIL−IL composite membranes. Error bars represent standard deviations based on three experimental replicas. The gas permeation properties of the PIL C(CN)3−IL C(CN)3 composites and [C2mim][C(CN)3] SILM were taken from Tomé et al.30,32

diffusivity simply in terms of IL viscosity does not account for all the behaviors obtained in this work, as it is possible to have approximately similar CO2 diffusivities in SILMs that have a

cations have similar CO2 diffusivities when compared to those of [C2mim][C(CN)3] SILM, which presents lower IL viscosity (16.6 mPa s). This means that the interpretation of gas 2234


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Industrial & Engineering Chemistry Research Table 3. Single CO2 Permeability (P)a and Ideal Permselectivities (α) of the Studied Membranesb membrane sample PIL NTf2−40 IL Si NTf2 PIL NTf2−60 IL Si NTf2 [(SiOSi)C1mim][NTf2] PIL C(CN)3−40 IL Si C(CN)3 PIL C(CN)3−60 IL Si C(CN)3 [(SiOSi)C1mim][C(CN)3] a

α CO2/N2

P CO2 181 426 545 57 238 568

± ± ± ± ± ±

0.4 0.3 3.9 0.6 0.4 0.7

16.8 24.7 23.5 29.3 35.2 36.8

± ± ± ± ± ±

0.1 0.1 0.7 0.4 0.1 1.7

α CO2/CH4 9.8 11.5 10.1 10.7 11.5 11.1

± ± ± ± ± ±

0.5 0.1 0.2 0.2 0.1 0.3

Barrer (1 Barrer = 10−10 cm3(STP) cm cm−2 s−1 cmHg−1) bThe listed uncertainties represent the standard deviations based on three experiments.

tions,32 are also included in Figure 5. It can be observed in Figure 5c that although PIL C(CN) 3 −IL Si C(CN) 3 membranes present similar or slightly higher CO2 solubilities than that of PIL C(CN)3−IL C(CN)3, Figure 5a,b shows that the PIL C(CN)3−IL Si C(CN)3 membranes generally have CO2 permeabilities and diffusivities that are lower than those of their corresponding composites without the siloxane functionality (PIL C(CN)3−IL C(CN)3). Again, these are unexpected results because of the higher molar volume and flexibility of [(SiOSi)C1mim][C(CN)3] IL when compared to [C2mim][C(CN)3]. It was expected that the presence of [(SiOSi)C1mim]+ cations in the composites might afford less packed membrane structures with higher gas permeabilities. According to Figure 5a, the gas permeabilities significantly increase from 40 to 60 wt % of free siloxane-functionalized IL incorporated into the PIL−IL membranes. This behavior is in fine agreement to what has been noticed by several authors.55−59 In fact, the incorporation of IL decreases the glass transition temperature and increases the free volume within the polymer chains, which allows for higher polymer chain mobility and thus enhanced gas permeabilities. Nevertheless, it can be observed from Figure 5a that the increment of the permeability toward the studied gases is much higher when the free [(SiOSi)C1mim][C(CN)3] IL is incorporated into PIL C(CN)3 than when [(SiOSi)C1mim][NTf2] is added into the PIL NTf2. That is, the gas permeability increments among PIL C(CN)3−40 IL Si C(CN)3 and PIL C(CN)3−60 IL Si C(CN)3, in which the CO2, CH4, and N2 permeabilities increased by 317%, 320%, and 250%, respectively, are superior to those between PIL NTf2−40 IL Si NTf2 and PIL NTf2−60 IL Si NTf2, where permeability increments of only 135%, 95%, and 54% occurred, respectively. Moreover, Figure 5a shows that the composites containing the [C(CN)3]− anion display CO2, CH4, and N2 permeabilities that are much lower than those obtained for the composites with the [NTf2]− anion, though the [(SiOSi)C1mim][C(CN)3] SILM presents gas permeabilities slightly lower than those of the [(SiOSi)C1mim][NTf2] SILM. These results clearly show that both constituents, PIL and IL, play important roles in the gas transport through PIL− IL composite membranes. Continuing the discussion of the anion effect, it is important to note that the higher gas permeabilities through the PIL NTf2−IL Si NTf2 composites compared to those of PIL C(CN)3−IL Si C(CN)3 are connected to their gas diffusivities, with the order from the lowest to the highest permeabilities (Figure 5a) being the same as their diffusivities (Figure 5b). Because the [NTf2]− anion has a molar volume that is greater than that of the [C(CN)3]− anion, the presence of [NTf2]− in both components of the composite probably promotes a less packed membrane structure with lower resistance to gas diffusion. Concerning gas solubility, it can be seen from Figure 5c that the PIL NTf2−IL Si NTf2 composites also exhibit CO2,

wide range of different IL viscosities (from 16.6 up to 110.8 mPa s). Similar deviations were also recently reported in other works.30,35,48 Moreover, this type of mismatch between gas diffusivity and IL viscosity has been addressed through microviscosity,46,49 which can substantially differ from macroviscosity, because the latter depends on movement of the entire solvent molecule, whereas the former addresses movement of segments of solvent molecules thus affecting the free volume distribution in the IL,50,51 which is important for ILs with longer chains or flexible groups, such as the Si−O−Si group. The CO2, CH4, and N2 solubility values calculated using eq 1 are depicted in Figure 3c. The CH4 and N2 solubilities are always significantly lower than that of CO2 among the SILMs studied, which is in agreement with reported results for other SILMs, in which the CO2 separation is mainly governed by gas solubility differences in the IL. From Figure 3c, it can also be observed that both the [(SiOSi)C1mim][NTf2] and [(SiOSi)C1mim][C(CN)3] SILMs exhibited the lowest CO2 solubilities, while they present the highest CH4 solubilities, compared to those of [C2mim][NTf2] and [C2mim][C(CN)3] SILMs, respectively. This means that the existence of a siloxane group (Si−O−Si) in the cation instead of only a methyl group (−CH3) leads to SILMs with lower CO2 and higher CH4 solubilities, while those of N2 do not significantly change. In order to understand the links between CO2 solubility and the intrinsic properties of ILs, several different correlations have been proposed over the past few years,52−54 and in brief all the models recognized that CO2 solubility increases by increasing the IL molecular weight, molar volume, and free volume.1 Looking at the CO2 solubility plotted in Figure 3c, and bearing in mind the wide range of molar volume (from 185.5 to 396.5 cm3 mol−1) and molecular weight (from 201.2 to 523.6 g mol−1) of the ILs studied, deviations from the aforementioned trends can be clearly observed for the SILMs based on the siloxane-functionalized ILs. For instance, the outlier [(SiOSi)C1mim][NTf2] SILM exhibits the lowest CO2 solubility (16 × 10−6 m3(STP) m−3 Pa−1), albeit it has the highest molecular weight (523.6 g mol−1) and molar volume (396.5 cm3 mol−1) among the IL phases considered. These results indicate that the gas solubilities of siloxane-functionalized ILs cannot be fully described by IL molecular weight or molar volume. 3.2. Gas Permeation through PIL−IL Membranes. The gas permeability and diffusivity values determined in the prepared PIL−IL composite membranes (Figure 2) are shown in panels a and b of Figure 5, respectively, while the gas solubility values are plotted in Figure 5c. Note that the composite membranes prepared in this work are blends that combine the properties of their components (PIL and IL). For comparison purposes, the gas permeation properties of PIL C(CN)3−IL C(CN)3 composites (using [C2mim][C(CN)3] IL instead of [(SiOSi)C1mim][C(CN)3]), which were previously determined using the same experimental condi2235


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Industrial & Engineering Chemistry Research CH4, and N2 solubilities that are greater than those obtained for PIL C(CN)3−IL Si C(CN)3. However, upon a comparison between the gas diffusivity (Figure 5b) and solubility (Figure 5c) data, it is clear that gas diffusivity is the key parameter distinguishing the gas permeability among the prepared PIL−IL composite membranes. 3.3. CO2 Separation Performance. The CO2 permeabilities and both the CO2/N2 and CO2/CH4 permselectivities of the studied SILMs and PIL−IL composites are summarized in Table 3. The CO2/CH4 permselectivity is permanently smaller than the CO2/N2 permselectivity, because the CH4 permeability is larger than that of N2 for all the membranes studied (Figure 5). This is in agreement with the results previously reported for other SILMs23,30,35,43 and PIL−IL composite membranes.24,31,60−62 From Table 3, it can also be observed that, among all the membranes studied, those containing the [C(CN)3]− anion generally display CO2/N2 and CO2/CH4 permselectivities that are higher than those of the membranes having the [NTf2]− anion. This behavior can be probably attributed to higher diffusivity selectivities (0.48− 1.34) and solubility selectivities (8.9−61.3) of the membranes comprising the [C(CN)3]− anion when compared to those obtained (diffusivity selectivities of 0.47−1.05 and solubility selectivities of 96.6−35.6) when the [NTf2]− anion is used. The comparison of CO2/N2 and CO2/CH4 separation efficiencies between the results of this work and the previously reported data for other SILMs and PIL−IL composite membranes can be appreciated on the Robeson plots63 in panels a and b of Figures 6, respectively, where the permselectivity between CO2 and N2 (or CH4) is plotted against the CO2 permeability. In fact, Figure 6b shows that the separation performances obtained for CO2/CH4 are below the upper bound, near to those already published in the literature for other PIL−IL membranes. Conversely, from Figure 6a, the CO2/N2 separation performance of the [(SiOSi)C1mim][C(CN)3] SILM is very close to the upper bound. These results are in close agreement with other works on SILMs published by different authors,30,41,64 in which the implementation of cyanofunctionalized anions results in high CO2 separation performances when compared to those previously obtained for ILs containing the [NTf2]− anion. Although the CO2 separation performances of the prepared PIL−IL composite membranes drop within the range of other reported data, it can be observed from Figure 6 that the use of different amounts (40 or 60%) of free siloxane-functionalized ILs impacts both CO2/N2 and CO2/CH4 efficiencies of the prepared composites. For example, the addition of free IL, either [(SiOSi)C1mim][C(CN)3] or [(SiOSi)C1mim][NTf2], promotes increased CO2 permeability but does not significantly change the CO2/CH4 permselectivity (Table 3). In view of that, the CO2/CH4 Robeson plot (Figure 6b) shows a shift of the results for PIL−IL membranes along the x-axis without noticeably sacrificing the CO2/CH4 permselectivity. On the other hand, and in addition to the CO2 permeability increments, the CO2/N2 permselectivity also increases from PIL NTf2−40 IL Si NTf2 (16.8) to PIL NTf2−60 IL Si NTf2 (24.7) and from PIL C(CN)3−40 IL Si C(CN)3 (29.3) to PIL C(CN)3−60 IL Si C(CN)3 (35.2); consequently, the CO2/N2 separation performance nearly approaches the respective upper bound (Figure 6a). These results demonstrate that the addition of siloxane-functionalized ILs increases the CO2/N2 permselectivity, though it does not influence that of CO2/CH4,

Figure 6. CO2 separation performance of the studied membranes plotted on (a) CO2/N2 and (b) CO2/CH4 Robeson plots. The upper bound for each gas pair is adapted from Robeson,63 while the experimental error is within the data points. For comparison, several literature data points of other reported (□) SILMs30,35,43,46,65−68 and (○) PIL−IL membranes24,31,32,59,61,69,70 are also illustrated.

primarily because of the inherent permselectivity of the different ILs for each gas pair (Table 3).

4. CONCLUSIONS Ionic liquid-based membranes containing siloxane-functionalized cations were prepared using two different membrane configurations, SILMs and PIL−IL composites, and their single CO2, CH4, and N2 permeation properties were discussed. The results indicate that the presence of a siloxane group in the cation results in SILMs with lower CO2 permeabilities and solubilities. Conversely to what would be expected from the IL viscosities, the SILMs made of the siloxane-functionalized ILs displayed considerably higher CH4 and N2 permeabilities when compared to those of SILMs without the siloxane functionality. In fact, these results indicate that the interpretation of gas permeability and diffusivity merely in terms of IL viscosity does not give a full explanation of the behaviors obtained in this work for siloxane-functionalized ILs. Also, it was found that the PIL−IL membranes containing the siloxane-functionalized cations generally have CO 2 permeabilities and diffusivities that are lower than those of their corresponding composites without the siloxane function2236


Article

Industrial & Engineering Chemistry Research ρ = Density θ = Time-lag parameter η = Viscosity αi/j = Permselectivity Vp = Permeate Volume T = Temperature t = Time S = Solubility R = Gas constant P = Permeability N2 = Nitrogen Mw = Molecular Weight S = Membrane thickness Ji = Steady-state gas flux D = Diffusivity CO2 = Carbon dioxide CH4 = Methane A = Effective membrane surface area Δpi = Pressure driving force Δpd = Variation of downstream pressure Vgas = Solute gas molar volume VIL = IL molar volume (Si−O−Si) = Siloxane group (−CH3) = Methyl group

ality. This in contrast to what was initially expected, since the presence of the siloxane-functionalized cations in the composites might afford less packed membrane structures with higher gas permeabilities because of the higher molar volume and flexibility of the siloxane group. Conversely, the addition of siloxane-functionalized ILs into PILs increased the CO2/N2 permselectivity of the composites but did not change the CO2/CH4, mainly because of the inherent permselectivity of the different ILs for each gas pair. The Robeson plots showed that the performances of the prepared membranes are still below the upper bounds for both separations CO2/N2 and CO2/CH4. The best performance was attained with the [(SiOSi)C1mim][C(CN)3] SILM, which nearly approach the 2008 upper bound for CO2/N2 separation, with CO 2 permeability of 568 Barrer and CO 2 /N 2 permselectivity of 36.8.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b04661. Experimental viscosity and density values; calculated thermal expansion coefficients and molar volumes; comparison of the density and viscosity data gathered in this work with the literature data available; density values fitted as a function of temperature by the method of least-squares; viscosity values fitted in function of temperature using the Vogel−Fulcher−Tammann model; permeability, diffusivit,y and solubility values of the studied SILMs and PIL−IL composite membranes (PDF)

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Cations

[C2mim]+ = 1-Ethyl-3-methylimidazolium [C4mim]+ = 1-Butyl-3-methylimidazolium [(SiOSi)C1mim]+ = 1-Methyl-3-pentamethyldisiloxymethylimidazolium [(Si)mim]+ = 1-Methyl-3-trimethylsilylmethylimidazolium [(PhSi)mim]+ = 1-Dimethylphenylsilylmethyl-3-methylimidazolium

AUTHOR INFORMATION

Anions

Corresponding Author

*Tel: +351-21-8413385. Fax: +351-21-8499242. E-mail: isabel. [email protected].

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ORCID

Andreia S. L. Gouveia: 0000-0003-2809-7863 Isabel M. Marrucho: 0000-0002-8733-1958

REFERENCES

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Author Contributions ∥

[NTf2]− = Bis(trifluoromethylsulfonyl)imide [C(CN)3]− = Tricyanomethanide [Cl]− = Chloride

LC.T. and A.S.L.G. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L.C.T. is grateful to FCT (Fundação para a Ciência e a Tecnologia) for her Postdoctoral research grant (SFRH/BPD/ 101793/2014). I.M.M. acknowledges FCT/MCTES (Portugal) for a contract under Investigador FCT 2012 (IF/363/2012). This work was partially supported by FCT through the project PTDC/CTM-POL/2676/2014 and R&D unit UID/Multi/ 04551/2013 (GreenIT).

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ABBREVIATIONS ILs = Ionic liquids PDMS = Polydimethylsiloxane PILs = Poly(ionic liquid)s PTFE = Poly(tetrafluoroethylene) PVDF = Poly(vinylidene fluoride) SILMs = Supported ionic liquid membranes A, a, b, and c = IL class specific parameters 2237


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