Gas Permeation Properties of Fluorinated Ionic Liquids - Industrial

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Gas Permeation Properties of Fluorinated Ionic Liquids Ana B. Pereiro, Liliana C. Tomé, Susana Martinho, Luis Paulo N. Rebelo, and Isabel M. Marrucho Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie4002469 • Publication Date (Web): 13 Mar 2013 Downloaded from http://pubs.acs.org on March 19, 2013

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Industrial & Engineering Chemistry Research

Gas Permeation Properties of Fluorinated Ionic Liquids Ana B. Pereiro,†,* Liliana C. Tomé,†,‡ Susana Martinho,† Luís Paulo N. Rebelo,† Isabel M. Marrucho†,‡* †

Instituto de Tecnologia Química e Biológica, www.itqb.unl.pt, Universidade Nova de Lisboa,

Apartado 127, 2781-157, Oeiras, Portugal. ‡

CICECO, Departamento de Química, Universidade de Aveiro, Campus Universitário de

Santiago, 3810-193 Aveiro, Portugal. ABSTRACT: Despite the increasing amount of research in the ionic liquids field, there are still quite unexplored themes. That is the case of the fluorinated ionic liquids (FILs) family, here defined as ionic liquids with fluorine tags longer that four carbon atoms. In this work, gas permeation

properties

of

two

fluorinated

ionic

liquids,

tetrabutylammonium

heptadecafluorooctanesulfonate and 1-ethyl-3-methylpyridinium perfluorobutanesulfonate, were studied. For that purpose, supported liquid membranes of the fluorinated ionic liquids were prepared using a polymeric porous membrane as supporting material and their gas permeation properties for 10 different gases at 294 K were measured using a time-lag apparatus. The results show that the gas solubility of these FILs is of the same order of magnitude as gas solubilities for previously tested fluorinated ionic liquids and that solute size plays a more important role on gas diffusivity than viscosity. The perfluorocarbons and carbon dioxide separation performances were evaluated and the results show that 1-ethyl-3-methylpyridinium perfluorobutanesulfonate is

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a better candidate than tetrabutylammonium heptadecafluorooctanesulfonate for the gas separation processes tested in this work. 1. INTRODUCTION Ionic Liquids (ILs) have recently become a green media for engineering problems due to the combination of their exceptional physicochemical properties1 with their practically non-volatile character.2 Although the number of publications in ionic liquids has witnessed an exponential growth, the large amount of possible combinations of cations and anions still renders a plethora of unexplored families. That is the case of the ionic liquids bearing a fluorous tag in either the cation or the anion. It has been shown that fluorous phases present quite distinct behavior from polar/non polar and hydrophilic/hydrophobic phases.3 The introduction of these unusual but potentially useful fluorous groups into the cation or anion pair can impart ionic liquids with attractive properties not observed in more conventional systems. Usually, literature refers to fluorinated ionic liquids (FILs) if anions such as bis(trifluoromethylsulfonyl)amide ([NTf2]¯), hexafluorophosphate ([PF6]¯) or tetrafluoroborate ([BF4]¯) are used. ILs combining a wide variety of cations with these three anions have been extensively studied from both the fundamental and the applied point of view. The carbon dioxide-phylic behavior of fluorinated compounds in general4,5 and ILs in particular6 is well known. A large amount of gas solubility in the most common ILs is available in the open literature6-8 and it has been recognized by now that the [NTf2]¯ -based ILs have in general greater carbon dioxide solubilities, due to their flexible and free volume features.9 Furthermore, in these studies, it has been shown that gas solubility tends to decrease with an increasing ionic liquid molecular weight, molar volume and free volume. However, the absorption capacity has successfully improved when functionalized groups are included into conventional ionic liquids.


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Nevertheless, the mechanism of dissolution of gases in general, and CO2 in particular, in ionic liquids is still not fully understood.8 This work only details the behavior of FILs with fluorinated chains longer than four carbon atoms, that are distinct from those conventionally called fluorinated ionic liquids ([NTf2]¯, [PF6]¯ or [BF4]¯). The use of ILs with fluorinated pony tails might be of interest in a wide variety of areas where perfluorocarbons (PFCs) are used. For example, in the biomedical field, PFCs have been suggested as artificial blood substitutes10,11 due to their ability to dissolve large volumes of respiratory gases (oxygen and carbon dioxide). This gas transport and other separation processes have been developed based on the high chemical stability and energy of the C‒F bonds. The properties of fluorinated compounds lead not only to high gas solubilities, but also to an easiness regeneration due to the repulsive forces of fluorine atoms.12 However, some drawbacks need to be overcome for developing and implementing FILs-based processes. Among the most important, we highlight the need of lifetime, recyclability, safety, health and environmental studies, and the high price of FILs, which are more expensive than conventional solvents. Although the price would drop for a large scale production of these compounds, similar prices to those of conventional solvents will never be achieved because FILs are complex molecules that require advanced synthesis and purification steps. Most of the studies published to date involving FILs with longer fluorinated chains are only focused on synthesis and characterization, and their application as reaction media and as material.13-20 Moreover, only few works have been devoted to gas permeation properties of ionic liquids bearing fluorous tags.21-26 Those studies are focused on the solubility of carbon dioxide,2125

nitrogen, oxygen, methane,23 and difluoromethane,26 and little attention has been paid to gas

permeability and permselectivity. The experimental methods used in these works were: quartz crystal and gravimetric microbalance,22,25 transient thin liquid film,24 time-lag23 and isochoric saturation.21 Only one of these studies evaluates the carbon dioxide separation performance of ACS Paragon Plus Environment

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three fluoroalkyl imidazolium based cations combined the [NTf2]¯ anion,23 and the results show that ideal selectivities for CO2/N2 in the fluorinated ionic liquids are lower than their alkyl analogues, whereas ideal selectivities for CO2/CH4 were found to be higher in the ionic liquids with fluorinated cations. On the other hand, the solubilities of perfluoromethane, perfluoroethane and

perfluoropropane

in

two

bis(trifluoromethylsulfonyl)amide27

[NTf2]¯-based

ILs,

and

trihexyltetradecylphosphonium 1-hexyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide27,28 were also studied and the results show that the gas solubilities increase with the perfluoroalkane size, but decrease with the temperature. Amongst the different technologies commonly used for gas separation, membrane technology has attracted considerable attention because it provides several advantages over other conventional separation techniques, including high energy efficiency and low capital costs.29 The large development of supported liquid membranes in the last few years can be mainly attributed to the use of ionic liquids. Non-volatile and selective ILs supported into a porous polymer membranes have distinctive advantages over conventional liquid membranes due to their negligible vapor pressure, that minimizes evaporative losses.30 Supported ionic liquid membranes (SILMs) consist on the impregnation of a small amount of the selected ionic liquid into the porous of a solid membrane to perform the desired separation, and the transport of the permeating gases occurs according to a solution-diffusion mass transfer mechanism.31,32 Although the membrane stability and the IL viscosity can be considered as the bottlenecks of this technology,30 SILMs allows for a ready and rapid access to information regarding the gas permeability and the ideal selectivity of ILs. Furthermore, the stability of SILMs is still an open issue because different behaviors have been observed in terms of stability depending on the nature of the ionic liquid used and the hydrophobic/hydrophilic character of the membrane support.33 Therefore, and despite the fact that the use of this methodology could be limited to low transmembrane pressure


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differentials,32 SILM is undoubtedly a valuable research tool for understanding the gas permeation properties of ionic liquids that are liquid at ambient temperature.23,34-40 In this work, the gas permeation properties of two ionic liquids bearing fluorinated chains of variable

size

in

([NBu4][(PFOc)SO3])

the

anion, and

tetrabutylammonium

heptadecafluorooctanesulfonate

1-ethyl-3-methylpyridinium

perfluorobutanesulfonate

([EtMepy][(PFBu)SO3]), were measured using several gases, such as carbon dioxide, nitrogen, oxygen, hydrocarbons gases (methane, ethane, propane and propylene) and

PFCs gases

(perfluoromethane, perfluoroethane and perfluoropropane). Permeabilities, solubilities and diffusivities of the prepared supported fluorinated ionic liquid membranes (SFILMs) towards the different pure gases were determined using a time-lag apparatus. The ideal permselectivities of the measured gases in these two fluorinated ionic liquids were also calculated and discussed. 2. MATHERIALS AND METHODS 2.1. Materials. Tetrabutylammonium perfluorobutanesulfonate (>97% mass fraction purity, halides (IC) < 250 ppm, cation (IC) > 99%, anion (IC) > 99%) and 1-ethyl-3-methylpyridinium perfluorobutanesulfonate (>97% mass fraction purity, halides (IC) < 100 ppm, cation (IC) > 99%, anion (IC) > 99%) were supplied by IoLiTec GmbH and their chemical structures are represented in Figure 1. The fluorinated ionic liquids were dried under vacuum (3 × 10-2 Torr) and vigorous stirring at 323.15 K for at least 2 days, immediately prior to their use. The water content, determined by Karl Fischer titration, was less than 100 ppm. No further purification of the ILs was carried out. The purity of the final products was checked by 1H-NMR. Table 1 lists the relevant physical properties for these fluorinated ionic liquids.41 Nitrogen (N2) (>99.99%), oxygen (O2) (>99.99%), carbon dioxide (CO2) (>99.99%), methane (CH4) (>99.95%), ethane (C2H6) (>99.5%), propane (C3H8) (>99.5%), propylene (C3H6) (>99.5%), perfluoromethane (CF4) (>99.99%), perfluoroethane (C2F6) (>99.99%) and perfluoropropane (C3F8) (>99.99%) were obtained from Air Liquide (France). ACS Paragon Plus Environment

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2.2. Preparation of Supported Fluorinated Ionic Liquid Membranes (SFILMs). Microporous hydrophilic polyethersulfone (PES) membranes, provided by Pall Corporation, were used as the support of the fluorinated ionic liquids. PES membrane filters with a pore size of 0.2 µm and a thickness of 145µm were chosen since they have been successfully tested in the literature for the same purpose.23,39 To prepare SFILMs, a PES membrane filter was placed inside a desiccator and vacuum was applied for 1 hour, in order to remove air from within the pores, therefore, allowing the fluorinated ionic liquid to be easily introduced in the pores of the membrane filter. Subsequently, still under vacuum, drops (1 mL) of the fluorinated ionic liquid were spread on the membrane surface using a syringe that was previously introduced in the desiccator for this purpose. After this impregnation procedure, SFILMs were stored inside the desiccator under vacuum for another 1 hour to degas the sample, which could influence the measurements of gas permeation properties. After degassing, both sides of the membrane surface were carefully wiped out with a soft tissue paper to remove the excess of fluorinated ionic liquid prior to loading the SFILM into the permeation test cell. The amount of fluorinated ionic liquid immobilized was determined gravimetrically by weighing the PES membrane filter before and after immobilization. 2.3. Single Gas Permeation Experiments. Single gas permeability and diffusivity values in the prepared SFILMs were measured using a constant volume-variable pressure or time-lag apparatus. Details on the construction and operation of this experimental setup are described in a previous work of our group.42 Before testing, each membrane was degassed under vacuum for 24 hours inside the permeation cell. The gas permeation experiments were performed at 294 K with an upstream pressure of 750 mbar and vacuum (< 1 mbar) as the initial downstream pressure. A feed pressure of pure gas around 750 mbar was applied to the membrane. This pressure corresponds to the maximum pressure that the SFILM can withstand before small amounts of the fluorinated ionic liquid were expelled and it was determined by increasing the pressure difference ACS Paragon Plus Environment

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between both sides of the SFILM until a strong increase of the flux through the membrane was observed. All permeation data were measured in triplicate (e.g. three separate N2, O2, CO2, CH4, C2H6, C3H8, C3H6, CF4, C2F6 and C3F8 experiments) on each SFILM sample and the experimental error was less than 2.5%. The membranes were degassed under vacuum between each run until the downstream pressure was below 1 mbar. No residual ionic liquid was found inside the permeation cell at the end of the experiments and, similarly, the membrane mass remained constant throughout the experiment. Gas transport through the supported fluorinated ionic liquid membranes was assumed to follow a solution-diffusion mechanism.43 The gas absorbed into the membrane, diffused through it and desorbed on the permeate side. As the testing gas permeates through the membrane, the permeate pressure rises. Under steady-state conditions, the pressure history derivative indicates the permeate flux. Assuming a homogeneous membrane and an ideal gas behavior, the permeability of the prepared SFILMs toward each gas (Pi) can be calculated from the steady-state flux through the membrane (Ji), the membrane thickness ( l ) and the pressure drop across the membrane (∆pi) as shown in Equation (1):44

Pi = Ji

l ∆pi

(1)

The time-lag parameter (θ), which can be obtained before achieving the steady-state flux conditions, relates to gas diffusivity (Di) as follows:45

Di =

l2 6θ

(2)

Since permeability is the product of solubility and diffusivity, it is possible to calculate gas solubility (Si):


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Si =

Pi Di

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(3)

3. RESULTS AND DISCUSSION 3.1 Gas Permeation Properties. The gas permeation properties (permeability, diffusivity and solubility) of each supported fluorinated ionic liquid membrane towards nitrogen, oxygen, carbon dioxide, methane, ethane, propane, propylene, perfluoromethane, perfluoroethane and perfluoropropane are presented in Table S1 (see supporting information). The gas permeability results are plotted in Figure 2 along with literature data for other two supported fluorinated ionic liquid membranes based on the imidazolium cation with fluorous pony tails in the cation combined with the NTf2 anion. Despite the fact that both the FILs studied in this work have different cations, the difference between the CO2 permeability for both SFILMs might be probably attributed to the different lengths of fluoroalkyl chains. A proper comparison between the obtained permeability results with other fluorinated ionic liquids with the same cation or anion would be very important and necessary to clarify this conclusion. However, no literature data is available. As described by equation (3), the gas permeability in a dense membrane can be separated in two contributions, the gas solubility and diffusivity. The Stokes-Einstein equation is usually used to describe the diffusivity of a solute in a solvent as follow:

D≈

T

(4)

ηV −1 3

where D is the diffusivity, η is the viscosity of the solvent, T is the absolute temperature, and V

−1 3

is the solute molar volume.

The increase in the fluoroalkyl chain lengths of the fluorinated ionic liquids leads to an increment of their viscosities, as it can be confirmed in Table 1. Thus, the smaller permeability of [NBu4][(PFOc)SO3] towards gases might be a consequence of the slower gas diffusion, which is ACS Paragon Plus Environment

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directly related to the increase in viscosity. This trend has also been observed for other ionic liquids,23,31,37,46,47 in particular for CO2 permeabilities of imidazolium ionic liquids containing fluorinated

substituents,

such

as

1-methyl-3-(3,3,4,4,5,5,6,6,7,7,8,8,8-

tridecafluorooctyl)imidazolium bis(trifluoromethylsulfonyl)imide, [(EtPFBu)MeIm][NTf2], and 1-methyl-3-(3,3,4,4,5,5,6,6,6-nonafluorohexyl)imidazolium

bis(trifluoromethylsulfonyl)imide,

[(EtFHex)MeIm][NTf2].23 As it can be seen in Figure 2, the SFILMs studied in this work present higher

gas

permeabilities

[(EtFHex)MeIm][NTf2].23

than

However,

those a

observed comparison

for

[(EtPFBu)MeIm][NTf2] between

the

viscosity

and of

[(EtFHex)MeIm][NTf2]21 at 293 K, 1814 mPa·s, and the viscosity of FILs at the same temperature (Table 1) shows that the difference between imidazolium-based fluorinated ionic liquids and our FILs are not controlled by this parameter. This means that not only the diffusivity plays an important role on the permeability but the solubility should also be considered. The permeability results of the two studied SFILMs towards the hydrocarbon gases (CH4, C2H6 and C3H8) and the corresponding perfluorocarbon gases (CF4, C2F6 and C3F8) indicate that the former have a larger permeability. Nevertheless, the permeability values of both SFILMs towards the three fluorinated gases are very similar to each other. In the case of [NBu4][(PFOc)SO3], the solubility of this FIL increases with the number of carbons in the perfluorocarbon gas (see Table S1 in supporting information). Therefore, the product of higher fluorinated gas solubilities through smaller diffusivities out comes in similar gas permeabilities. The experimental gas diffusivities of the two prepared SFILMs, determined at 294 K, are plotted in Figure 3. For the hydrocarbon and the fluorocarbon solutes, the viscosity trend followed by fluorinated ionic liquids is similar to the previously reported viscosity trends for conventional ionic liquids,35 when the viscosity of FILs increases (see Table1) their diffusivity decreases. However, CH4 and CF4 exhibit an anomalous behavior, since similar diffusivities were


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obtained for these gases in both SFILMs. Furthermore, an opposite behavior is observed for respiratory gases (CO2 and O2). Figure 4 contains a log-log plot of the measured gas diffusivities as function of the solute (gas) molar volumes at their normal boiling point, Vb , based on the Stokes-Einstein equation. The molar volumes were calculated using the Tyn and Calus method:48

Vb = 0.285⋅Vc1.048

(5)

where Vc is critical volume and both Vb and Vc are expressed in cubic centimeters per mole. As expected, Figure 4 shows that the gas diffusivity is generally inversely proportional to solute molar volume. Nevertheless, for the smaller gases, O2, N2, CO2, CH4 and CF4, this trend is not observed. As for conventional room temperature ionic liquids, the Stokes-Einstein model cannot accurately describe the diffusivities of small solutes in an environment of large solvent molecules.31,37 Regarding the gas solubilities of [NBu4][(PFOc)SO3] and [EtMepy][(PFBu)SO3], it can be observed that this property increases with the number of carbons in both the hydrocarbons and the fluorinated gases as well as with the number of carbon double bonds for the hydrocarbon gases with the same number of carbons (propylene versus propane). This trend was also found in imidazolium-based and phosphonium-based ionic liquids with different anions.37,49 For gases having weak interactions with ionic liquids, such as O2 and N2, it has been shown that solubility is largely governed by entropic contributions,9 meaning that the creation of cavities (see free volume in Table 1) which can better accommodate the gas molecules might have a primary role on the gas solubility of fluorinated ionic liquids. However, that is not the case for the FILs studied in this work since higher gas solubility was observed for [EtMepy][(PFBu)SO3], which has a smaller free volume. Nevertheless, this trend holds for the gases with the larger molar volume, such as C3H8, C2F6 and C3F8 (see Table S1 in supporting information). ACS Paragon Plus Environment

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The solubility of CO2 in other fluorinated ionic liquids has been reported by other research groups21-25 and these results are illustrated in Figure 5. The literature data suggest that, in general, the CO2 solubility in fluorinated ionic liquids increases when the alkyl chains are substituted by fluorinated chains. However, for imidazolium-based ionic liquids with fluorinated chains only a slight improvement was found.23 In order to compare the results obtained in this work with literature data, Henry’s law constants were calculated from the determined gas solubilities and they are summarized in Table S2 (see supporting information) and represented in Figure 5. Since the correction of the Henry’s constants for a partial solute pressure of 1 bar is very small, calculated Henry´s constants were directly compared with literature data. Although Muldoon et al.25 measured CO2 solubility of [(EtPFBu)MeIm][NTf2], [(EtFHex)MeIm][NTf2] and 1-pentyl3-methylimidazolium tris(nonafluorobutyl)trifluoro-phosphate ([PeMeIm][PF3(C4F9)3]) at 298 K and 13 bar, the results for the two former ionic liquids are very similar to those obtained by other authors21-24 at 298 K and approximately 1 bar. Also in Figure 5, the CO2 solubility of [(EtFHex)MeIm][NTf2] from Baltus et al.22 differs from that of other authors by more than 80% at the same conditions (298 K and 1 bar).21,22 Thus, this value was discarded from our discussion. The Henry's law constants of the FILs studied in this work for CO2 are of the same order of magnitude as those published for [(EtPFBu)MeIm][NTf2], [(EtFHex)MeIm][NTf2], with the exception of [PeMeIm][PF3(C4F9)3].25 Although it is usually acknowledged that the anion plays a primary role in the solvation phenomena and thus on gas solubility, it is interesting to verify that despite the very different nature of the anions, [NTf2]¯, [(PFBu)SO3]¯ and [(PFOc)SO3]¯, the CO2 solubility in these ionic liquids is not very different. Nevertheless, while for [NBu4][(PFOc)SO3] and [EtMepy][(PFBu)SO3] the CO2 solubility increases with the increase of the perfluoroalkyl chain length, the opposite behavior was found in literature for [(EtPFBu)MeIm][NTf2] and [(EtFHex)MeIm][NTf2].


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3.2 Gas Separation. Carbon dioxide and PFCs gases are present in many industrial processes and their separation from other gases is often required. Figure 6 illustrates the ideal permselectivity values of SFILMs obtained for several gas pair separations. The permselectivity (or ideal separation factor), α, for a given gas pair is the ratio of the permeabilities of two individual permeation species (i and j) as described by the Equation (6):

αi j =

Pi Pj

(6)

3.2.1. CO2 separation performance. Nowadays, CO2 separation from flue gases (CO2/N2) in industrial processes is being considered as an option to reduce the so-called greenhouse effect. On the other hand, separation of CO2 from natural gas (CO2/CH4) is of vital importance to maintain and expand the availability of the clean-burning and efficient fuel sources.6 Due to the exceptional CO2 solubility of fluorinated compounds,50 fluorinated ionic liquids should be considered for CO2 separation from gaseous effluents. The results obtained show that the ideal CO2/N2 permselectivity decreases when the molar volume of the fluorinated ionic liquid increases. However, a different behavior is observed for CO2/CH4 separation where the permselectivity is similar for the both SFILMs studied. Methane exhibits an anomalous behavior, which has already been repeatedly shown in the open literature.23,51,52 A comparison for CO2/N2 and CO2/CH4 separations between the results obtained in this work and the published data23 for other SFILMs is plotted in Figure 6. The permselectivities of imidazolium-based fluorinated ionic liquids are higher than those measured in this work. However, if the values of the CO2 permeability are compared, Figure 2 shows that the SFILMs studied in this work present higher CO2 permeabilities than imidazolium-based fluorinated ionic liquids studied in the literature.


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The performance of SILMs is generally evaluated through a “Robeson plot”,53 where the tradeoff between permeability and permselectivity for the separation of two gases can be evaluated. The upper bound was empirically defined using literature experimental data for a wide range of materials that were tested as CO2 separation membranes.53 In Figure 7, the data obtained in this work for both SFILMs is plotted together literature data for other ILs. It can observe that both SFILMs studied in this work fall in the same region of other supported ionic liquid membranes reported in the literature.23,35-37,39,54-56 3.3.2. Fluorinated gases separation performance. The PFCs used in this work are being used as etching/cleaning gases in semiconductor manufacturing processes57 and the emissions of these greenhouse gases from industrial processes must be controlled. Thus, the development of recovery/recycle techniques, such as membrane technology, is of vital importance. Only two studies report the solubility of PFCs (CF4, C2F6 or C3F8) in conventional ionic liquids (common alkylphosphonium and alkylimidazolium-based ionic liquids)27,28 but gas permeabilities and permselectivies have not yet been investigated. In fluorinated ionic liquids, the molecular structures are dominated by very strong C‒F bonds, causing an increase in rigidity and a decrease in polarity.58 This fact leads to high gas solubility, whereas low forces are required for expelling the gas molecules and regenerating the solvent either upon decreasing pressure or increasing temperature.21-26 These unique properties enable the a priori choice of FILs as good solvents for recovery/recycle of PFCs gases from their mixtures with air. Figure 6 shows that in fluorinated gas separations, the increase of the fluorinated domain in [NBu4][(PFOc)SO3] leads to a rise in the permselectivity of these fluorinated gases. These results confirm that FILs are a viable choice in the separation these gases. 4. CONCLUSIONS The gas permeation properties (permeability, diffusivity and solubility) of two fluorinated ionic liquids ([NBu4][(PFOc)SO3] and [EtMepy][(PFBu)SO3]) regarding nitrogen, oxygen, carbon ACS Paragon Plus Environment

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dioxide, methane, ethane, propane, propylene, perfluoromethane, perfluoroethane and perfluoropropane were determined using a supported fluorinated ionic liquid membrane configuration. The gas solubilities of the studied fluorinated ionic liquids are of the same order of magnitude as gas solubilities for previous tested fluorinated ionic liquids. However, fluorinated ionic liquids exhibit slight improvement in the gas solubilities when they are compared with conventional ionic liquids. The diffusivity of fluorinated ionic liquids is less dependent on viscosity but more dependent on solute size than the predicted by the conventional StokesEinstein model. The gas separation performance of the supported fluorinated ionic liquid membranes was also evaluated. The permselectivities were calculated and the results show that 1-ethyl-3ethylpyridinium perfluorobutanesulfonate is the best choice to be used for studied gas separation processes with significantly reduction of the carbon dioxide and perfluorocarbons gases emissions. However, there is an enormous amount of research opportunity available in the design of new gas separation membranes and other separation approaches using fluorinated ionic liquids.


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Tetrabutylammonium perfluorooctanesulfonate: [NBu4][(PFOc)SO3]

1-Ethyl-3-methylpyridinium perfluorobutanesulfonate: [EtMepy][(PFBu)SO3]

Figure 1. Chemical structures of the fluorinated ionic liquids used in this work.


15


1000 30 30

SILM - -[NBu SFILM [NBu [(PFOc)SO 4]4][(PFOc)SO 3]3] SFILM - [EtMepy] [(PFBu)SO3]

Permeability (Barrer)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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800

SFILM - [(EtPFBu)MeIm] [Ntf2] [23] SFILM - [(EtPFHex)MeIm] [Ntf2] [23]

20

600 10

400 0 CF4

200

C2F6

C3F8

0 CO2

O2

N2

CH4

C2H6

C3H8

C3H6

CF4

C2F6

C3F8

Figure 2. Gas permeabilities of [NBu4][(PFOc)SO3] and [EtMepy][(PFBu)SO3] measured in this work and their comparison with other fluorinated ionic liquids taken from literature.23 Error bars represent standard deviations based on three experimental replicas.


16

Page 17 of 32

1200 SILM - [NBu SFILM - [NBu ] [(PFOc)SO 4]4[(PFOc)SO 3]3] SFILM - [EtMepy] [(PFBu)SO3]

900

12

2

Diffusivity x 10 (m /s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


600

300

0 CO2

O2

N2

CH4

C2H6

C3H8 C3H6

CF4

C2F6 C3F8

Figure 3. Gas diffusivities of [NBu4][(PFOc)SO3] and [EtMepy][(PFBu)SO3] at 294 K. Error bars represent standard deviations based on three experimental replicas.


17


10000 SFILM SILM–-[NBu [NBu44]][(PFOc)SO [(PFOc)SO33]]

SILM–-[EtMepy] [EMpy] [(PFBu)SO SFILM [(PFBu)SO 3]3]

N2 CH4 CO2

1000

100

C3F8

10

O2

10

C3H6 C3H8 C2F6

100

CF4 C2H6

12

2

Gas diffusivity x 10 (m /s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 32

3

Liquid Molar Volume at Normal Boiling Point (cm /mol)

Figure 4. Gas diffusivities of [NBu4][(PFOc)SO3] and [EtMepy][(PFBu)SO3] versus solute molar volume. The line is provided as a guide to the eye and is not fitted to the data.


18

Page 19 of 32

35

Henry's law constants (atm)

30

25

This work From [23] From [24] From [25] From [22] From [21]

20

5

)S tP O FB 3] u) M eI m [(E ][N tP tf FH 2] ex )M eI m [P ][N eM tf eI 2] m ][P F 3 (C 4F 9) 3]

3

[(E

M ep y] [(P FB u

[E t

Bu

4

][(

PF O c) SO

]

0

[N

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Henry’s law constants of [NBu4][(PFOc)SO3] and [EtMepy][(PFBu)SO3] for carbon dioxide obtained in this work and taken from literature data.21-25


19


21

SILM SFILM- [NBu - [NBu ] [(PFOc)SO 4]4[(PFOc)SO 3]3] SFILM - [EtMepy] [(PFBu)SO3]

18 Permselectivity

SFILM - [(EtPFBu)MeIm] [Ntf2] [23]

16

SFILM - [(EtPFHex)MeIm] [Ntf2] [23]

13 10 8 5 3 8

/C 2

O

3

F

6

F 2

O

/C

/C F 2

O

2

2

4

8

F 3

/C

2

N

2

N

2

/C

F

/C F

6

4

4

N

CO

2

/C

H

2

2

/O

2

CO

2

/N

0 C O

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 32

Figure 6. Ideal permselectivity of the supported fluorinated ionic liquids membranes obtained in this work and taken from literature data.23


20

Page 21 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. Robeson plot of the CO2 separations studied in the supported fluorinated ionic liquid membranes prepared. Data are plotted on a log-log scale and the upper bound is adapted from Robeson.53 Literature data reported for other supported ionic liquid membranes are also plotted in (a)23,35,37,39 and (b).23,35-37,39,54-56


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 32

Table 1. Values of Dynamic Viscosity, η, Density, ρ, Molar Volume, Vm, and Free Volume, fm, of [NBu4][(PFOc)SO3] and [EtMepy][(PFBu)SO3] at 293 K (Pereiro et al.41). [NBu4][(PFOc)SO3]

[EtMepy][(PFBu)SO3]

η (mPa⋅s)

12159

278.2

ρ (g⋅cm-3)

1.3215

1.5199

Vm (cm3·mol-1)

561.12

277.14

fm (cm3·mol-1)

425.73

206.80


22

Page 23 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


AUTHOR INFORMATION Corresponding Author Corresponding authors. Tel.: +351 214469414; fax: +351 214411277. E-mail address: [email protected] (A. B. Pereiro) and [email protected] (I. M. Marrucho). ACKNOWLEDGMENTS A. B. Pereiro would like to acknowledge Marie Curie Actions Intra-European Fellowships (IEF) for a contract under FP7-PEOPLE-2009-IEF - 252355 – HALOGENILS as well as for financial support of FCT/MCTES (Portugal) by way of Post-Doc grant SFRH/BPD/84433/2012. L. C. Tomé would like to thank the FCT (Fundação para a Ciência e a Tecnologia) for her PhD grant (SFRH/BD/72830/2010). I. M. Marrucho acknowledges FCT/MCTES (Portugal) for a contract under Programa Ciência 2007. The authors also wish to thank FCT (Fundação para a Ciência e Tecnologia) through Project PTDC/EQU-FTT/118800/2010. In addition, the authors are also grateful to Catarina Duarte for supplying the perfluoropropane.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 32

NOMENCLATURE Ji = flux of gas i Pi = gas i permeability ∆pi = pressure drop across the membrane l = membrane thickness

θ = time-lag parameter Di = gas i diffusivity Si = gas i solubility η = viscosity of the solvent V

−1 3

Vb

= molar volume of solute

= volumes at their normal boiling point

Vc = critical volume

α i j = ideal permselectivity for a given gas pair Subscripts i = gas i j = gas j


24

Page 25 of 32

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Gas Permeation Properties of Fluorinated Ionic Liquids - Industrial

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