N2 separation properties

Journal of Membrane Science 552 (2018) 341–348

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Mixing poly(ionic liquid)s and ionic liquids with different cyano anions: Membrane forming ability and CO2/N2 separation properties

T

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Raquel M. Teodoroa, Liliana C. Toméa,b, , Daniele Mantionec, David Mecerreyesc,d, ⁎ Isabel M. Marruchoa,b, a

Instituto de Tecnologia Química e Biológica António Xavier, Universidade Nova de Lisboa, Av. da República, 2780-157 Oeiras, Portugal Centro de Química Estrutural, Departamento de Engenharia Química, Instituto Superior Técnico, Universidade de Lisboa, Avenida Rovisco Pais, 1049-001 Lisboa, Portugal c POLYMAT, University of the Basque Country UPV/EHU, Joxe Mari Korta Center, Avda. Tolosa 72, 20018 Donostia-San Sebastian, Spain d IKERBASQUE, Basque Foundation for Science, E-48011 Bilbao, Spain b

A R T I C L E I N F O

A B S T R A C T

Keywords: PIL–IL composites Cyano-functionalized anions Anion mixtures Membrane forming ability Gas permeation properties

In this work, poly(ionic liquid)–ionic liquid (PIL–IL) composite membranes were prepared using the solvent casting technique. The studied PILs have pyrrolidinium polycation backbone ([Pyr11]+), while the five ILs display either an imidazolium ([C2mim]+) or a pyrrolidinium ([Pyr14]+) based cation. Both the PIL and IL components comprised cyano-functionalized anions ([N(CN)2]–, [C(CN)3]– or [B(CN)4]–), being the anion for each component different from one another. The use of the [NTf2]– anion was also tested for comparison. Several experimental conditions for the solvent casting procedure were tested in order to prepare homogenous and free standing PIL–IL composite membranes. The CO2 and N2 permeation properties (permeability, diffusivity and solubility) were evaluated at a fixed temperature (293 K) and constant trans-membrane pressure differential (100 kPa) using a time-lag apparatus, so that trends regarding the different anions either on the PIL or IL could be obtained and evaluated. From all 42 PIL–IL combinations tested, 21 were suitable membranes (homogeneous and free standing) for gas permeation experiments and 4 of them were on top or surpassed the 2008 Robeson upper bound for CO2/N2 separation. The best performance membranes contain the [C(CN)3]– and [B(CN)4]– anions, enlightening therefore the promise these anions entail for future high performance membranes for postcombustion CO2 separation.

1. Introduction Ionic liquids (ILs) are salts constituted by organic cations and inorganic or organic anions that possess a set of unique chemical and physical properties, which make them alternative solvents for CO2 separation applications [1–3]. These properties include low volatility [4] and high CO2 solubility and selectivity [5], but most importantly is the ILs’ tunability [6,7]. With the goal to make use of IL properties and membranes’ technology advantages for CO2 separation, different membrane configurations have been explored [8–11], with the simplest approach being the use of supported ionic liquid membranes (SILMs). In this type of membrane configuration, capillary forces immobilize the IL, which was previously impregnated inside the pores of an inert solid membrane support. SILMs usually yield good CO2 separation performances, high permeability and permselectivity values, and often stay on top or above the 2008 Robeson upper bound for CO2/N2 separation [8,9]. Nevertheless, their operation and long term stability can be

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compromised if the pressure differential across the membrane is high enough to push out the IL phase from the pores of the support [12]. Poly(ionic liquid) (PIL) membranes, comprise polymerized IL monomers, and represent an alternative to SILMs. Due to their polymer macrostructure, PIL membranes not only have improved processability and durability but also, enhanced mechanical stability. In the course of 2006–2008, the pioneering work of Noble’s group established the potential of neat PIL membranes for CO2 separation [13–16]. The permselectivity values obtained for the CO2/CH4 and CO2/N2 gas pairs were on par or greater than those reported for SILMs at that point in time. When moving from SILMs to neat PIL membranes, there is an improvement in mechanical stability, but the gas permeability and diffusivity are hindered through the solid polymer matrix. In order to overcome this drawback, PIL–IL composite membranes, a PIL framework incorporating a certain amount of free (non-polymerizable) IL, were proposed in 2008 [17]. These authors prepared composite

Corresponding authors at: Instituto de Tecnologia Química e Biológica António Xavier, Universidade Nova de Lisboa, Av. da República, 2780-157 Oeiras, Portugal. E-mail addresses: [email protected] (L.C. Tomé), [email protected] (I.M. Marrucho).

https://doi.org/10.1016/j.memsci.2018.02.019 Received 21 December 2017; Received in revised form 7 February 2018; Accepted 10 February 2018 Available online 12 February 2018 0376-7388/ © 2018 Elsevier B.V. All rights reserved.

Journal of Membrane Science 552 (2018) 341–348

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membranes by UV polymerization of imidazolium-IL monomers in the presence of 20 wt% of free IL. The CO2 permeability increased by approximately 400% compared to the neat PIL membrane, while the CO2 permeability selectivity barely changed [17]. After that, other researchers have also dedicated their attention to the preparation of PIL–IL composite membranes and thus, several PIL and IL combinations were studied [18–30]. Overall, it is clear that the use of PIL–IL composites holds great promise to obtain membrane materials with good CO2 separation performances, like SILMs, while being mechanically stable, like polymer-based membranes. Amongst the variety of PIL and IL chemical structures, which have been used to prepare PIL–IL membranes, cyano-functionalized anions show over performing CO2 separation membranes that can achieve or surpass the 2008 Robeson upper bound for CO2/N2 separation [21,28,31–33]. In particular, we proposed pyrrolidinium-based PILs with different cyano counter-anions, namely [N(CN)2]–, [C(CN)3]– and [B(CN)4]– [28]. The synthetic route used for pyrrolidinium-PILs is straightforward: a commercially available polymer is used for simple salt metathesis reactions, without need of several organic synthetic and purification steps at the monomer level, as required for imidazoliumPILs. Since these three neat PIL membranes were very brittle and broke easily, the membrane forming ability of the synthesized PILs blended with different amount (20, 40 and 60 wt%) of free ILs containing similar cyano-functionalized anions was evaluated using the solvent casting method. For PIL–IL composites bearing the [N(CN)2]– anion, it was not possible to prepare membranes with more than 20 wt% of free IL. Conversely, stable and homogeneous PIL–IL membranes were successfully prepared, for all the three percentages of free IL, when the [C(CN)3]– anion is used. Regarding the composites comprising the [B (CN)4]– anion, heterogeneous membranes were obtained for all the compositions tested. The results showed that by increasing the free IL content in the composites not only dramatically increased the CO2 permeability, but also boosted the CO2/N2 permselectivity. Thus, the performance of the C(CN)3-based membrane with 60 wt% of IL surpassed the 2008 Robeson upper bound for CO2/N2 separation [28]. The present work also focuses in the preparation of PIL–IL membranes combining cyano-functionalized anions ([N(CN)2]–, [C(CN)3]– and [B(CN)4]–), but the novelty here is that those anions of both components (PIL and IL) are different from each other. The membrane forming ability of diverse PIL and IL combinations was evaluated using the solvent casting technique and the CO2 and N2 permeation proprieties through the formed membranes were measured using the time-lag method, so that relationships between different anion combinations and gas transport could be discussed. In parallel, PIL–IL membranes containing the [NTf2]– anion (in both PIL and/or IL), were also prepared. The composite membranes with cyano-functionalized anions allowed the study of the anion influence on gas transport, while those with [NTf2]– anion enabled the evaluation of the cation influence.

Fig. 1. Chemical structures of the poly(ionic liquid)s (PILs) and the ionic liquids (ILs) used to prepare the composite membranes.

pure), while the potassium tetracyanoborate (KB(CN)4) was synthesized as reported elsewhere [34]. All the other solvents were of analytical grade and used as received. The water used was double distilled. Carbon dioxide (CO2) and nitrogen (N2) with, at least, 99.99% purity were provided by Air Liquide. 2.2. Synthesis of PILs The four poly(ionic liquid)s (PILs), containing pyrrolidinium pendant units and [N(CN)2]–, [C(CN)3]–, [B(CN)4]– or [NTf2]– as counter anions (Fig. 1), were synthesized by anion metathesis reactions from the commercially available polyelectrolyte precursor, poly(diallyldimethylammonium chloride), followed by further purification steps. The detailed procedures are described elsewhere [24,28].

2. Experimental section 2.3. Preparation of PIL–IL membranes 2.1. Materials Diverse composite membranes, combining the synthesized PILs and different amounts of commercial ILs (Fig. 1), were prepared by solvent casting technique. Solutions containing the PIL and the corresponding amount of free IL were prepared using appropriated solvents. The prepared solutions were magnetically stirred until both PIL and IL components were completely dissolved and a homogeneous solution was obtained. The PIL–IL solutions were then poured into glass or PTFE dishes and left for slow evaporation of the solvents. Finally, and in order to ensure that the solvent was completely evaporated, the membranes were dried in a ventilated oven at 318 K until a constant weight was obtained. As a starting point, the initial casting conditions were adapted from our previous studies [24,28], the membranes were prepared in either acetone or acetonitrile, using 6 (w/v)% PIL–IL solutions, and left to

The poly(diallyldimethylammonium) chloride solution (average Mw 400,000–500,000, 20 wt% in water) was purchased from Sigma-Aldrich and used as received. The salts, namely sodium dicyanamide (NaN (CN)2, > 97 wt% pure), sodium tricyanomethane (NaC(CN)3, 98 wt% pure) and lithium bis(trifluoromethylsulfonyl)imide (LiNTf2, 99 wt% pure), as well as the ionic liquids 1-ethyl-3-methylimidazolium dicyanamide ([C2mim][N(CN)2], > 98 wt%), 1-ethyl-3-methylimidazolium tricyanomethane ([C2mim][C(CN)3], > 98 wt% pure), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2], 99 wt% pure) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Pyr14][NTf2], 99 wt% pure) were supplied by IoLiTec GmbH. Merck KGaA (Germany) provided the 1-ethyl3-methylimidazolium tetracyanoborate ([C2mim][B(CN)4], > 98 wt% 342

Journal of Membrane Science 552 (2018) 341–348

Glass PTFE Glass Glass Glass Glass 20 20 20 20

2.4. Gas permeation measurements Single gas CO2 and N2 permeation experiments were performed using a time-lag apparatus, which is fully described elsewhere [24]. In a typical procedure, vacuum was first applied to the whole system for 12 h, including the membrane inside the permeation cell, and after that only the permeate side was vacuumed to ensure that the initial gas concentration on this side was approximately null. The selected gas was introduced at the desired feed pressure (100 kPa) on the feed side, and finally the gas permeation experiment was conducted at 293 K. At least three separate experiments for each gas, on a single membrane sample, were carried out. Before each run, the permeation cell and lines were evacuated until the pressure was below 0.1 kPa. No residual IL was found inside the permeation cell at the end of the experiments. The thicknesses of the PIL–IL composite membranes (120–200 µm) were measured before and after testing using a digital micrometer (Mitutoyo, model MDE-25PJ, Japan). Average thickness was calculated from six measurements taken at different locations for each membrane. Gas transport through a membrane was assumed to follow a solution-diffusion mass transfer mechanism [35]. The gases permeability (P ) values, defined as the thickness normalized steady-state gas flux ( J ) under a trans-membrane pressure difference (∆p = pu − pd ), were determined experimentally using Eq. (1) [36]:

6 6 6 6 Acetone Acetone Acetone Acetone

1 1 2 2

20 6 Acetone

2

Glass

evaporate in Petri dishes between 2 and 4 days at room temperature. Afterwards, and in order to obtain stable and homogenous membranes, different solvent casting conditions were also tested including the use of different solvents, PIL and IL (w/v)% solution concentrations, evaporation times and temperatures, as well as different plate materials. Due to the large number of PIL–IL combinations contemplated in this work (42), the composition descriptions and experimental conditions of the casting procedure used for each membrane are divided into two tables. Table 1 provides that information for the 21 successfully (mechanically stable and homogenous) prepared PIL–IL membranes, while Table S1 (Supporting information) describes all the different conditions tested to obtain the other 21 membranes, which after many trials, were either non-stable, heterogeneous or both.

P=J

[C2mim][N(CN)2] [C2mim][C(CN)3] [C2mim][B(CN)4] [C2mim][NTf2]

J=

(1)

V p ∆pd AtRT

(2)

Vp

where is the permeate volume, ∆pd is the variation of downstream pressure, A is the membrane area, t is time, R is the gas constant and T is the absolute temperature. The gas diffusivity (D ) values were determined using Eq. (3) [37]:

poly([Pyr11][NTf2])

[C2mim][NTf2]

[C2mim][C(CN)3] poly([Pyr11][B(CN)4])

[Pyr14][NTf2]

[C2mim][NTf2]

[C2mim][N(CN)2] poly([Pyr11][C(CN)3])

l ∆p

where l is the membrane thickness and pu and pd are the upstream and downstream pressures, respectively. The thickness normalized steadystate gas flux ( J ) was, in turn, determined using Eq. (2) [36]:

D=

l2 6θ

(3)

where l is the membrane thickness and θ is the time-lag parameter. Plotting the downstream pressure ( pd ) versus time (t ), there is a linear part which can be extrapolated back to the time axis. The intercepted value is the time-lag parameter (θ ). According to the solution-diffusion model, which applies to homogenous dense membranes, the permeability of a gas (P ) is the product of the gas diffusivity (D ) and solubility (S ) across the membrane [35]. Using Eq. (4), the solubility (S ) values of the gases can be determined.

N(CN)2–20 IL C(CN)3 N(CN)2–40 IL C(CN)3 N(CN)2–60 IL C(CN)3 C(CN)3–20 IL N(CN)2 C(CN)3–40 IL N(CN)2 C(CN)3–20 IL B(CN)4 C(CN)3–40 IL B(CN)4 C(CN)3–60 IL B(CN)4 C(CN)3–20 IL [C2mim][NTf2] C(CN)3–40 IL [C2mim][NTf2] C(CN)3–20 IL [Pyr14][NTf2] C(CN)3–40 IL [Pyr14][NTf2] B(CN)4–40 IL C(CN)3 B(CN)4–60 IL C(CN)3 B(CN)4–20 IL [C2mim][NTf2] B(CN)4–40 IL [C2mim][NTf2] NTf2–20 IL N(CN)2 NTf2–20 IL C(CN)3 NTf2–20 IL B(CN)4 NTf2–20 IL [C2mim][NTf2] NTf2–40 IL [C2mim][NTf2]

[C2mim][B(CN)4]

20 40 60 20 40 20 40 60 20 40 20 40 40 60 20 40 20 20 20 20 40 [C2mim][C(CN)3] poly([Pyr11][N(CN)2])

19 39 59 23 45 19 39 59 12 27 11 25 44 64 13 29 36 34 31 21 41

2 6 Acetone

20

Glass 20 6 Acetonitrile

4

PTFE Glass 20 6 Acetonitrile

3

Glass 20 6 Acetonitrile

4 3

Glass 3 3 9 6 Acetonitrile

20

PTFE 6 Ethanol

45 4

PIL-IL Solution (w/v)% Solvent mol% of IL wt% of IL Ionic Liquid (IL) Polymer (PIL)

P=S×D

(4)

The permeability selectivity (αi/j), also known as permselectivity, was determined, through Eq. (5), by dividing the permeability of most permeable gas i (Pi ) by the permeability of the least permeable gas j (Pj ):

PIL PIL PIL PIL PIL PIL PIL PIL PIL PIL PIL PIL PIL PIL PIL PIL PIL PIL PIL PIL PIL

Composite Membrane

Table 1 Composition and experimental conditions of the casting procedure used to prepare mechanically stable and homogenous PIL–IL membranes.

Evaporation Time (days)

Evaporation Temperature (ºC)

Petri Dish Material

R.M. Teodoro et al.

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Table 2 Summary of all the PIL–IL composite materials prepared in this work. Cells with a gray background represent membranes already reported in our mark were stable and homogenous membranes, while those previous works [24,28]. N.S stands for “not synthesized”. The materials denoted with a with the mark were non-stable and/or heterogeneous membranes.

αi / j =

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

anions between the PIL and IL, a topic that has never been studied before, but that deserves to be explored in future works. The obtained membranes of PIL B(CN)4 and free [C2mim][N(CN)2] IL were all heterogeneous (Table 2), probably due to incompatibility of [B(CN)4]– with the [N(CN)2]– anion. Note that the same behavior was also observed for the opposite PIL N(CN)2–IL B(CN)4 membranes, suggesting that the source of the anions (either PIL or IL component) is not relevant for this combination of anions. Surprisingly, it can also be observed from Table 2 that the PIL B(CN)4–20 IL C(CN)3 membrane is heterogeneous, while their analogues with 40 and 60 wt% of free IL are homogenous and stable (Fig. 2– m and n). In fact, it seems that the presence of higher amount of [C(CN)3]– anions from the IL component increases the compatibility with the PIL B(CN)4 and thus homogeneous membranes can be obtained. In what concerns the composites of PIL B (CN)4 and free [C2mim][NTf2] IL, only the incorporation of 20% and 40 %wt of free IL resulted in stable and homogenous membranes (Fig. 2– o and p). The structural flexibility of the [NTf2]– anion is certainly essential for the successful incorporation of 20 wt% of free IL between the polymeric chains of the PIL B(CN)4. Note that this was not possible when using [C2mim][C(CN)3] neither [C2mim][N(CN)2] ILs (Table 2). The first thing to be noted about the composite membranes bearing PIL NTf2 is that, for all the three studied ILs having cyano-functionalized anions, only 20 wt% of each IL can be incorporated (Fig. 4– s, t, and u). Conversely, when the source of [NTf2]– anions is the free IL, it was possible to blend up to 40 wt% of [C2mim][NTf2] IL within the polymer chains of both the PIL C(CN)3 and PIL B(CN)4 (Fig. 2– i, j, o and p). Likewise, the incorporation up to 40 wt% of free [Pyr14][NTf2] IL into the PIL C(CN)3 originated stable and homogeneous membranes (Fig. 2– k and l). Although we have previously reported a composite membrane made of PIL NTf2 and 60 wt% of [Pyr14][NTf2] IL [24], in the present work the IL [C2mim][NTf2] could be incorporated only up to 40 wt% (Fig. 2– q and r). Both the PIL NTf2 and the [Pyr14][NTf2] IL are based on pyrrolidinium moieties, while the [C2mim][NTf2] IL has an imidazolium-based cation. This difference is the key factor regarding the membrane forming ability of these composites. The membrane formation success rate (number of stable and homogenous membranes, 21, by the number of total membranes, 42) was of 50%. From all the tested PIL–IL combinations containing [C(CN)3]– anion (either in PIL or IL), 71% were free standing and homogeneous membranes. Concerning the other studied anions, a 45% success rate was achieved for the composites containing the [NTf2]– anion, 44% for the composites bearing the [B(CN)4]– anion and only 33% for the composites combining [N(CN)2]– anions. In sum, the PIL–IL

(5)

3. Results and discussion 3.1. Membrane forming ability All the PIL–IL composite materials tested in this work, as well as their final stability and homogeneity, are summarized in Table 2. On the other hand, Fig. 4 illustrates the obtained 21 stable and homogenous membranes (marked with V on Table 2). Due to the hydrophilic nature of [N(CN)2]– anion, it was difficult to obtain stable and homogenous PIL–IL composite membranes using PIL N(CN)2 (Table 2), in particular for the membranes with [C2mim][NTf2] IL, since the [NTf2]– is the most hydrophobic of all IL anions used. However, in the case of composite membranes with [C2mim][B(CN)4] IL, it seems that the [B(CN)4]– IL anion does not “fit” with the [N(CN)2]– PIL anion. The membranes containing the PIL N(CN)2 and free [C2mim] [C(CN)3] IL are an exception (Fig. 2 – a, b and c), mainly because of the excellent compatibility revealed by [N(CN)2]– and [C(CN)3]– anions. Regarding the composite membranes bearing PIL C(CN)3, the use of [C(CN)3]– and [B(CN)4]– anions, which differ only in a cyano group, allowed the formation of three stable and homogenous composite membranes with 20, 40 and 60 wt% of free [C2mim][B(CN)4] IL (Fig. 2– d, e and f). The [C(CN)3]– and [N(CN)2]– anions also differ in one cyano group, resulting in two suitable composite membranes with 20 and 40 wt% of free [C2mim][N(CN)2] IL (Fig. 2– g and h), but the hydrophilic nature of the [N(CN)2]– anion compromised the mechanical stability of the PIL C(CN)3–60 IL N(CN)2 membrane. Despite the chemical and structural differences between [NTf2]– and [C(CN)3]– anions, the use of either the [Pyr14][NTf2] or [C2mim][NTf2] ILs allowed the formation of homogeneous composite membranes up to 40 wt% of free IL incorporated (Fig. 2– i, j, k and l). It is worth mentioning that while the PIL N(CN)2–60 IL C(CN)3 membrane (Fig. 2– c) is homogenous and free standing, the opposite PIL C(CN)3–60 IL N(CN)2 is a gel-like material because of the higher amount of free [N(CN)2]– anions from the IL component (Table 2). Albeit the PIL N(CN)2–20 IL C(CN)3 membrane (Fig. 2– a) also has a high amount of [N(CN)2]– anions, which do not come from the IL, but from the PIL, a solid macromolecule that is less flexible than the IL component, and thus the respective [N(CN)2]– anions have lower mobility, aiding in the mechanical stability of the composite membrane. In this discussion, we are considering that there is no interchange of 344

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Fig. 2. Pictures of the successfully prepared PIL–IL membranes that, after the solvent casting process, were stable and homogenous. Circles: red – PIL N(CN)2 composite membranes; green – PIL C(CN)3 composite membranes; blue – PIL B(CN)4 composite membranes; orange – PIL NTf2 composite membranes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

composites containing [C(CN)3]– anions were the most successful in the preparation of stable and homogenous membranes, probably due to the better structural and/or chemical compatibility of [C(CN)3]– with the other anions. At the other end, PIL–IL composites bearing the [N(CN)2]– anion were the most difficult to prepare. The high hydrophilic nature of this anion resulted in phase separated and/or gel-like materials, impossible to be handled as membranes, bringing the success rate to the lowest (33%) amongst the anions studied in this work.

In what concerns the IL cation effect, either for PIL C(CN)3 or PIL NTf2 based composite membranes, those prepared with the [C2mim] [NTf2] IL exhibited improved CO2 and N2 permeabilities than their analogues comprising the [Pyr14][NTf2] IL, as it can be seen in Table S2 and [24]. In this case, not only the high viscosity of the [Pyr14][NTf2] IL, twice that of the [C2mim][NTf2] IL (Table 3), but also the localized charge of the [Pyr14]+ cation, can account for the gas permeability differences observed. Fig. 3(a) display the CO2 and N2 permeabilities through the PIL–IL membranes bearing PIL C(CN)3 and 40 wt% of all the five different ILs used in this work. It can be seen that the composite membrane combining the [C2mim][C(CN)3] IL shown higher permeabilities than those of the membrane having the [C2mim][B(CN)4] IL. However, the composites prepared with [C2mim][N(CN)2] IL revealed higher permeabilities than those of both the formers. This behavior was not initially expected, since the CO2 and N2 permeabilities through SILMs performed with these ILs follows a different trend, as it can be observed from Table 3: [C2mim][B(CN)4] > [C2mim][C(CN)3] > [C2mim] [NTf2] > [C2mim][N(CN)2] > [Pyr14][NTf2]. Nevertheless, the mass basis in which the PIL–IL membranes were prepared appears to support this unexpected behavior. That is, due to differences on the ILs molecular weight, PIL–IL membranes with the same mass of different free ILs have different number of IL moles. For instance, the molecular weight of [C2mim][N(CN)2] is lower than that of [C2mim][B(CN)4] (Table 3), consequently the PIL C(CN)3–20 IL N(CN)2 membrane has more moles of free IL and higher gas permeabilities than to that of PIL C(CN)3–20 IL B(CN)4 (Fig. 3(a)). Moreover, the same trend obtained for gas permeability (Fig. 3(a)) was also observed for gas diffusivity (Fig. 3(b)). The gas solubility in these PIL–IL membranes remain nearly the same (Fig. 3(c)), showing that gas permeability is mainly ruled by a diffusivity controlled mechanism. From all the obtained PIL–IL composite membranes, three can be considered to have high gas permeabilities, specifically: PIL N(CN)2–60 IL C(CN)3, PIL C(CN)3–60 IL B(CN)4 and PIL B(CN)4–60 IL C(CN)3, illustrated in Fig. 4(a). The PIL C(CN)3–60 IL C(CN)3 membrane, previously proposed [28], is also depicted for comparison. The presence of

3.2. Gas permeation properties The CO2 and N2 permeability and diffusivity values, determined using a time-lag apparatus, in the prepared PIL–IL membranes (Fig. 2) are provided in Supporting information, Table S2 and S3, respectively, while the solubility values calculated using Eq. (4) are presented in Table S4. As expected, it was observed that by increasing the amount of free IL incorporated into the PIL–IL composite membranes, both the CO2 and N2 permeabilities significantly increased (Table S2). This increment in gas permeability can be attributed to the increase of gas diffusivity (Table S3) and solubility (Table S4), but the increase in gas diffusivity was definitely more significant. This larger contribution of gas diffusivity indicates that there was an increase of the free volume into the PIL–IL composite membranes by increasing the IL content, enhancing the polymer chain mobility due to the presence of the free IL ions pairs. This trend has also been observed by several authors for other PIL–IL membranes [20,22,24,28–30,38,39]. Within the composite membranes bearing the same PIL, it was found that the incorporation of ILs having cyano-functionalized anions yielded membranes with higher CO2 and N2 permeabilities compared to those of their corresponding membranes containing either the [C2mim] [NTf2] or [Pyr14][NTf2] ILs (Table S2). This behavior is in fine agreement to what was previously observed for SILMs made of the same ILs [31,40]. The difference in ILs viscosity, with the ILs bearing the [NTf2]– anion being more viscous (Table 3), seems to be the main reason for this behavior. 345

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Table 3 Physical properties of the ILs used to prepare PIL–IL membranes, as well as CO2 and N2 permeabilities and CO2/N2 permselectivities through the respective SILMs. Viscosity (η) and molar volume (Vm) were determined at 293 K, while the gas permeation data were measured at 293 K and 100 kPa. Ionic liquid

M (g mol−1)

η (MPa )

Vm (cm3 mol−1)

P CO2 (Barrer)

P N2 (Barrer)

α CO2/N2

[C2mim][N(CN)2]a [C2mim][C(CN)3]a [C2mim][B(CN)4]a [C2mim][NTf2]b [Pyr14][NTf2]c

177.21 201.23 226.05 391.30 422.40

18 17 21 27 60

160.24 185.54 226.89 258.40 303.95

476 667 742 589 340

7 12 15 17 12

68 57 49 36 28

a,b c

Values taken from [31,40], respectively. IL viscosity and molar volume (at 303 K) [41], while gas permeation data were taken from [24].

Fig. 4. Gas permeability (a), diffusivity (b) and solubility (c) through selected PIL–IL composite membranes. The error bars represent standard deviations based on three experimental replicas. The gas permeation properties of the PIL C(CN)3–60 IL C(CN)3 membrane was taken from Tomé et al. [28].

Fig. 3. Gas permeability (a), diffusivity (b) and solubility (c) through the composite membranes bearing the PIL C(CN)3 and 40 wt% of the different ILs studied. The error bars represent standard deviations based on three experimental replicas. The gas permeation properties of the PIL C(CN)3–40 IL C(CN)3 membrane was taken from Tomé et al. [28].

changing either the PIL C(CN)3 or the [C2mim][C(CN)3] IL of the PIL–60 IL membranes by PIL B(CN)4 or [C2mim][B(CN)4] IL, respectively, the CO2 permeability increased (Fig. 4(a)), primarily due to the higher CO2 solubility (Fig. 4(c)) promoted by the [B(CN)4]– anion, which in turn compensates the decrease of CO2 diffusivity (Fig. 4(b)). This result indicates that the source (PIL or IL) of the [B(CN)4]– anion is not so relevant since the CO2 permeability, diffusivity and solubility

[C2mim][C(CN)3] IL in these composite membranes shows the great versatility and compatibility of this IL. The [C(CN)3]– anion is also present in the PIL of two composite membranes, meaning that the presence of this anion, either in the PIL or IL, leads to PIL–IL membranes with high CO2 permeabilities. Also, it is worth noting that by 346

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obtained in this work to those of other reported PIL–IL composite membranes, a Robeson plot is presented in Fig. 5. In this type of plot, the CO2/N2 permselectivity is plotted against CO2 permeability (in a log scale) and the solid line represents the empirical 2008 upper bound for this gas pair. The upper bound is based on a myriad of experimental data and illustrates the tradeoff between permeability and permselectivity [42,43]. Fig. 5 shows that the best performing PIL–IL composite membranes presented in Table 4 fall right above or on top of the 2008 Robeson upper bound, indicating their potential for post-combustion flue gas treatment. In addition, Fig. 5 illustrates the membranes combining the PIL C(CN)3 and 40 wt% of the different ILs studied, and it can be seen that the PIL C(CN)3–40 IL N(CN)2 membrane also falls on top of the upper bound. Interestingly, the distances on the Robeson plot between the point regarding the PIL N(CN)2–60 IL C(CN)3, PIL C(CN)3–60 IL B (CN)4 and PIL B(CN)4–60 IL C(CN)3 membranes, to some extent, mimics the distances between the SILMs of [C2mim][N(CN)2], [C2mim] [C(CN)3] and [C2mim][B(CN)4] ILs. In other words, the CO2/N2 separation performance of the PIL–IL composite membranes falls back to slightly lower permeability and permselectivity values than that of the respective SILMs, due to the presence of polymeric PIL chains (Fig. 5), the fall backs are represented by arrows. Nevertheless, these PIL–IL membranes still have very good CO2/N2 separation performances, in addition to their robustness advantages when compared to SILMs. With further research on their chemical and physical properties, these PIL–IL membranes hold a great promise for gas membrane technology, in particular for CO2/N2 separation.

Table 4 Single CO2 permeability (P) and ideal permselectivities (α) obtained through the four PIL–IL composite membranes with the best CO2/N2 separation performance. The listed uncertainties represent the standard deviations, based on three experiments. Composite membranes PIL PIL PIL PIL a b

N(CN)2–60 IL C(CN)3 C(CN)3–60 IL C(CN)3b C(CN)3–60 IL B(CN)4 B(CN)4c–60 IL C(CN)3

P CO2 (Barrer)a

α CO2/N2

249.0 ± 1.0 439.3 ± 0.1 472.7 ± 3.0 502.1 ± 1.9

61.3 ± 0.8 64.4 ± 0.3 54.4 ± 0.8 43.1 ± 0.4

Barrer (1 Barrer = 10–10 cm3(STP)cm cm–2 s–1 cmHg–1). Values taken from [28].

values of the two PIL–IL membranes bearing this anion are very similar (Fig. 4). 3.3. CO2/N2 separation performance The single CO2 permeability and ideal CO2/N2 permselectivity of the highest performance PIL–IL membranes obtained in this work are provided in Table 4. As it can be observed, the CO2 permeability values increase from 249 to 502 Barrer, while the CO2/N2 permselectivity values decrease from 64 to 43, meaning that these membranes follow the well-known trade-off between permeability and permselectivity (i.e. when one rises the other falls and vice versa) that has been observed for polymeric membranes [42,43]. It should be noted that the variation of CO2/N2 permselectivity can be attributed to a solubility controlled mechanism, since the diffusivity selectivity (D CO2/N2) values fall in the 0.5 – 0.7 range, while the solubility selectivity (S CO2/ N2) values range from 73.0 to 100.2. This is in fine agreement to what was previously observed for PIL C(CN)3-based membranes combining 20, 40 and 60 wt% of free [C2mim][C(CN)3] IL [28]. Therefore, it can be concluded that while the CO2 permeability through the studied PIL–IL membranes is generally controlled by gas diffusivity, the increase in CO2/N2 permselectivity seems to be controlled by gas solubility. In order to compare the CO2/N2 separation performance results

4. Conclusions Several PIL–IL composite membranes having cyano-functionalized anions ([N(CN)2]–, [C(CN)3]– or [B(CN)4]–) or an [NTf2]– anion, which are different in the PIL and IL components, were prepared by solvent casting. The membrane forming ability and the CO2/N2 separation properties of the formed membranes were evaluated and discussed. From all the 42 PIL–IL combinations tested, 21 were obtained as free standing and homogenous membranes. The materials prepared with the [C(CN)3]– anion (either in the PIL or IL) had a membrane formation success rate of 66%, while the [N(CN)2]– anion had the lowest membrane formation success rate of 33%. The versatility, flexibility and chemical compatibility of the [C(CN)3]– anion seems to be the reason why most membranes containing this anion are homogenous and free standing. In contrast, the hydrophilic nature of the [N(CN)2]– anion generally ends up in gel-like materials, which makes them impossible to be handled. The gas permeation study through the formed PIL–IL membranes indicates that the increase/decrease of CO2 and N2 permeabilities is a diffusivity controlled mechanism, while the increase/decrease of CO2/ N2 permselectivity is a solubility controlled mechanism. The membranes bearing the PIL C(CN)3 and the same wt% of different ILs result in a trend of CO2 and N2 permeability values presenting the opposite behavior to that observed in the respective IL counterparts in SILM configurations, certainly because of the mass basis in which the PIL–IL membranes were prepared in this work. Also, it was found that replacing [C(CN)3]– by [B(CN)4]– anion increased gas permeability owing to an increase in gas solubility. The CO2/N2 separation performance of 4 composite membranes, PIL C(CN)3–40 IL N(CN)2, PIL N(CN)2–60 IL C(CN)3, PIL C(CN)3–60 IL B (CN)4 and PIL B(CN)4–60 IL C(CN)3, were on top or even surpassed the Robeson 2008 upper bound for this gas pair. Further research on the evaluation of these PIL–IL membranes’ behavior under different operating conditions, such as pressure, temperature, compositions of binary gas mixtures and humidity contents, is crucial to realize their potential application for post-combustion CO2 separation.

Fig. 5. CO2/N2 separation performance of the best performing PIL–IL membranes obtained, as well as the composite membranes containing PIL C(CN)3 and 40 wt% of the different ILs studied. Data are plotted on a log-log scale and the upper bound is adapted from Robeson [43]. The data of PIL C(CN)3–60 IL C(CN)3 membrane was taken from reference [28].“Literature” stands for other PIL–IL composite membranes previously reported [18–21,24,27,29,30].

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Acknowledgements [20]

Liliana C. Tomé is grateful to FCT (Fundação para a Ciência e a Tecnologia) for her Post-doctoral research grant (SFRH/BPD/101793/ 2014). This work was partially supported by FCT through the project PTDC/CTM-POL/2676/2014 and R&D units UID/Multi/04551/2013 (GreenIT) and UID/QUI/00100/2013 (CQE).

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Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2018.02.019.

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