1

Study on gas permeation and CO2 separation

through ionic liquid-based membranes with

siloxane-functionalized cations

Liliana C. Tomé,a‡

Andreia S.L. Gouveia,a‡

Mohd A. Ab Ranii,b Paul D. Lickiss,

b Tom

Welton,b and Isabel M. Marrucho

a,c*

a Instituto de Tecnologia Química e Biológica António Xavier, Universidade Nova de Lisboa, Av. da

República, 2780-157 Oeiras, Portugal.

b Department of Chemistry, Imperial College London, London, SW7 2AZ, UK.

c Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Avenida Rovisco Pais,

1049-001 Lisboa, Portugal.

Corresponding Author

*Tel: +351-21-8413385; fax: +351-21-8499242;

E-mail address: isabel.marrucho@tecnico.ulisboa.pt

Author Contributions

‡ Liliana C. Tomé and Andreia S.L. Gouveia contributed equally to this work.

2

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 Supporting Information), were also determined. Contrary to

what was expected, the gas permeation results show that the SILMs containing siloxane-

functionalized cations have lower CO2 permeabilities 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, whilst 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.

KEYWORDS: Siloxane-based ionic liquids, PIL–IL composite membranes, supported

ionic liquid membranes (SILMs), CO2 separation.

3

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 due to 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 so far,6 the simplest approach is the use of supported ionic liquid membranes

(SILMs), where 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 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

4

of introducing a siloxane group in the imidazolium cation on the viscosity. 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 IL structure. The authors observed that the low viscosity of [(SiOSi)mim][NTf2]

compared to that of [(Si)mim][NTf2] 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 last 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 ether-functionalized 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 due its chemical

5

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 siloxane-functionalized 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

Afterwards, and considering that pyrrolidinium-based 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 allow the preparation

of homogeneous and stable PIL–IL composite membranes, but also improve 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. 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

6

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%.

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(tri-fluoromethylsulfonyl)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, then extracted with dichloromethane (3x20

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 afterwards eliminated by rotary evaporation,

affording [(SiOSi)C1mim][NTf2]. A similar procedure was used to prepare the 1-methyl-3-

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 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 are shown

in Table 1.

7

Figure 1 Chemical structure of the siloxane-functionalized ionic liquids (ILs) and the

pyrrolidinium-based poly(ionic liquid)s (PILs).

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] 0.04 523.6 100.8 1.321 396.5

[(SiOSi)C1mim][C(CN)3] 0.11 333.6 60.21 1.046 318.8

[C2mim][NTf2]c

0.02 391.3 39.09 1.524 256.8

[C2mim][C(CN)3]d 0.01 201.2 16.62 1.085 185.5

a Density (ρ) and viscosity (η) measured at 293.15 K. b Molar volume (Vm) obtained for 293.15 K. c Values of [C2mim][NTf2] taken from Tomé et al.

35

d Values of [C2mim][C(CN)3] taken from Tomé et al.30

2.3. Density and Viscosity Determination. The measurements of density and viscosity of

the pure siloxane-functionalized ILs, [(SiOSi)C1mim][NTf2] and [(SiOSi)C1mim][C(CN)3],

were performed in the temperature range between 293.15 K and 353.15 K at atmospheric

pressure using an SVM 3000 Anton Paar rotational Stabinger viscometer-densimeter,

where the standard uncertainty for the temperature is 0.02 K. The repeatability of density

8

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 uncertainty registered for the dynamic viscosity and density

measurements was 0.03 and 2·10−4

, respectively.

2.4. Preparation of Supported Ionic Liquid Membranes (SILMs). The pure

[(SiOSi)C1mim][NTf2] was supported in a porous hydrophobic poly(vinylidene fluoride)

(PVDF) membrane (Millipore Corporation, USA), 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 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]), respectively PIL NTf2 and PIL

C(CN)3, 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

9

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 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).

Figure 2 Pictures of the prepared composite membranes based on PILs and different amounts of

free IL containing siloxane-functionalized cations.

10

Table 2 Composition descriptions and experimental conditions of the casting procedure used to

prepare the composite membranes.

Composite Membrane Polymer (PIL) Ionic Liquid (IL) wt% of IL Solvent T (K)

PIL NTf2 – 40 IL Si NTf2 Poly([Pyr11][NTf2]) [(SiOSi)C1mim][NTf2]

40 Acetone 298

PIL NTf2 – 60 IL Si NTf2 60

PIL C(CN)3 – 40 IL Si C(CN)3 Poly([Pyr11][C(CN)3]) [(SiOSi)C1mim][C(CN)3]

40 Acetonitrile 313

PIL C(CN)3 – 60 IL Si C(CN)3 60

2.7. Gas Permeation Experiments. Ideal CO2, CH4 and N2 permeabilities and

diffusivities were measured using a time-lag 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:

SDP (1)

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

AtRT

pVJ d

p

i

(2)

where Vp is the permeate volume, ∆pd is the variation of downstream pressure, A is the

effective membrane surface area, t is the experimental time, R is the Gas Constant and T is

11

the temperature. Ideal gas permeability (Pi) was then calculated from the pressure driving

force (∆pi) and membrane thickness ( ) as shown in Eq. (3).

/

i

ii

p

JP

(3)

Gas diffusivity (Di) was determined according Eq. (4). The time-lag parameter (θ) was

deduced by extrapolating the slope of the linear portion of the pd vs. t curve back to the

time axis, where the intercept is equal to θ.40

6

2iD (4)

After knowing both Pi and Di, 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 specie i to the permeability of the less permeable

specie j. The permselectivity can also be expressed as the product of the diffusivity

selectivity and the solubility selectivity:

j

i

j

i

j

iji

S

S

D

D

P

P / (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 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 Supporting Information. The highest

12

viscosity values and molar volumes were obtained for [(SiOSi)C1mim][NTf2] IL. These

data will be used ahead 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

Figures 3(a) and 3(b), respectively, while the gas solubility values estimated from Eq. (1)

are plotted in Figure 3(c). To the best of our knowledge, the CO2, CH4 and N2 gas

permeabilities of [(SiOSi)C1mim][NTf2] SILM were only determined by Bara et al.,3 at 295

K using a polyethersulfone (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 3(a), 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, whilst 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

slightly lower CO2 permeabilities 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

lower CO2 permeabilities than their analogues without the siloxane functionality (Figure

3(a)), 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 mPa s and 60.2,

respectively, while for their non-functionalized analogues the measured viscosities are,

respectively, 39.1 mPa s and 16.6 mPa s. This trend, where ILs with higher viscosities

13

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 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 3(a)). 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 and co-authors proposed a general correlation,

23 which has

been used as follows:

c

gas

b

IL

a

IL

V

VAD

(6)

where 𝐴, 𝑎, 𝑏 and 𝑐 are IL-class specific parameters, 𝜂𝐼𝐿 is the IL viscosity, 𝑉𝐼𝐿 is the IL

molar volume and 𝑉𝑔𝑎𝑠 is 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

14

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 to what was previously observed for other SILMs.23, 45-47

Nevertheless, an odd behavior can be observed for the SILMs containing siloxane-

functionalized cations prepared in this work. Contrasting to what was expected from their

respective IL viscosities, the [(SiOSi)C1mim][NTf2] and [(SiOSi)C1mim][C(CN)3] SILMs

have nearly the same CO2 diffusivities (Figure 4). Additionally, these SILMs containing

siloxane-functionalized 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 diffusivity simply in terms of IL viscosity does account for all the

behaviors obtained in this work, as it is possible to have approximately similar CO2

diffusivities in SILMs that have a 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,

since the latter depends on movement of the entire solvent molecule, whereas the former

one 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

3(c). The CH4 and N2 solubilities are always significantly lower than that of CO2 amongst

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

15

3(c), it can also be observed that both the [(SiOSi)C1mim][NTf2] and

[(SiOSi)C1mim][C(CN)3] SILMs exhibited lowest CO2 solubilities, whilst they present

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 3(c), and bearing

in mind the wide range of molar volume (from 185.5 up to 396.5 cm3

mol-1

) and molecular

weight (from 201.2 up 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 that 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 point out that the gas solubilities of siloxane-functionalized ILs

cannot be fully described by IL molecular weight or molar volume.

16

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

17

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

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

Figures 5(a) and 5(b), respectively, whilst the gas solubility values are plotted in Figure

5(c). 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 conditions,32

are also included in

Figure 5. It can be observed in Figure 5(c) 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

18

C(CN)3, Figures 5(a) and 5(b) show that that the PIL C(CN)3–IL Si C(CN)3 membranes

generally have lower CO2 permeabilities and diffusivities than those of their corresponding

composites without the siloxane functionality (PIL C(CN)3–IL C(CN)3). Again, these are

unexpected results due to 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 5(a), 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 5(a) that the

increment of the permeability towards 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%, are superior than 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 5(a)

shows that the composites containing the [C(CN)3]– anion display much lower CO2, CH4

and N2 permeabilities than those obtained for the composites with the [NTf2]– anion, though

the [(SiOSi)C1mim][C(CN)3] SILM present slightly lower gas permeabilities than those of

19

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.

Still about 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 5(a)) being the same as their diffusivities (Figure 5(b)). Since the

[NTf2]–

anion has higher molar volume 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 5(c) that the PIL NTf2–IL Si NTf2 composites also exhibit higher CO2, CH4

and N2 solubilities than those obtained for PIL C(CN)3–IL Si C(CN)3. However, and upon a

comparison between the gas diffusivity (Figure 5(b)) and solubility (Figure 5(c)) data, it is

clear that gas diffusivity is the key parameter distinguishing the gas permeability among the

prepared PIL–IL composite membranes.

20

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

21

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 compared to that of N2 for all the

membranes studied (Figure 5). This is in agreement with the results previously reported for

other SILMs,23, 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 higher CO2/N2 and CO2/CH4 permselectivities than 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 plots,63

respectively, in Figures 6(a) and

6(b), where the permselectivity between CO2 and N2 (or CH4) is plotted against the CO2

permeability. As a matter of fact, Figure 6(b) 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-based membranes. Conversely, from Figure 6(a), 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

where the implementation of cyano-functionalized anions results in

22

high CO2 separation performances when compared to those previously obtained for ILs

combining the [NTf2]– anion.

Albeit 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 6(b)) shows a shift of the results for PIL–

IL membranes along the x-axis without noticeably sacrificing of the CO2/CH4

permselectivity. On the other hand, and besides 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), and consequently the CO2/N2 separation performance nearly approaches the

respective upper bound (Figure 6(a)). These results demonstrate that the addition of

siloxane-functionalized ILs increases the CO2/N2 permselectivity, though it does not

influence that of CO2/CH4, primarily due to the inherent permselectivity of the different ILs

for each gas pair (Table 3).

23

Table 3 Single CO2 permeability (P)a and ideal permselectivities (α) of the studied membranes.

b

Membrane sample P CO2 α CO2/N2 α CO2/CH4

PIL NTf2 – 40 IL Si NTf2 181 ± 0.4 16.8 ± 0.1 9.8 ± 0.5

PIL NTf2 – 60 IL Si NTf2 426 ± 0.3 24.7 ± 0.1 11.5 ± 0.1

[(SiOSi)C1mim][NTf2] 545 ± 3.9 23.5 ± 0.7 10.1 ± 0.2

PIL C(CN)3 – 40 IL Si C(CN)3 57 ± 0.6 29.3 ± 0.4 10.7 ± 0.2

PIL C(CN)3 – 60 IL Si C(CN)3 238 ± 0.4 35.2 ± 0.1 11.5 ± 0.1

[(SiOSi)C1mim][C(CN)3] 568 ± 0.7 36.8 ± 1.7 11.1 ± 0.3

a Barrer (1 Barrer = 10–10 cm3(STP)cm cm–2 s–1 cmHg–1) b The listed uncertainties represent the standard deviations based on three experiments.

24

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 purposes, several literature data points

of other reported (□) SILMs,30, 35, 43, 46, 65-68

and (○) PIL–IL membranes,24, 31, 32, 59, 61, 69, 70

are also

illustrated.

25

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 outcomes 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 point out 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 lower CO2 permeabilities and diffusivities than those of their

corresponding composites without the siloxane functionality. 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

due to 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 changed 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

26

upper bound for CO2/N2 separation, with CO2 permeability of 568 Barrer and CO2/N2

permselectivity of 36.8.

SUPPORTING INFORMATION

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

the least squares; Viscosity values fitted in function of temperature using the

Vogel−Fulcher−Tammann (VFT) model; Permeability, diffusivity and solubility values of

the studied SILMs and PIL-IL composite membranes. This material is available free of

charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author

*Tel: +351-21-8413385; fax: +351-21-8499242;

E-mail address: isabel.marrucho@tecnico.ulisboa.pt

Author Contributions

The manuscript was written through contributions of all authors. All authors have given

approval to the final version of the manuscript. ‡ Liliana C. Tomé and Andreia S.L.

Gouveia contributed equally to this work.

Notes

The authors declare no competing financial interest.

27

ACKNOWLEDGMENTS

Liliana C. Tomé is grateful to FCT (Fundação para a Ciência e a Tecnologia) for her Post-

doctoral research grant (SFRH/BPD/101793/2014). Isabel M. Marrucho 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).

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

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

28

ℓ Membrane thickness

Ji Steady-state gas flux

D Diffusivity

CO2 Carbon Dioxide

CH4 Methane

A Effective membrane surface area

∆pi Pressure driving force

∆pd Variation of downstream pressure

𝑉𝑔𝑎𝑠 Solute gas molar volume

𝑉𝐼𝐿 IL molar volume

(Si–O–Si) Siloxane group

(–CH3) Methyl group

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

Anions

[NTf2]– Bis(tri-fluoromethylsulfonyl)imide

[C(CN)3]– Tricyanomethanide

[Cl]– Chloride

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36

Study on gas permeation and CO2 separation through ionic liquid-based

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*

“For Table of Contents use only”