Ionic liquid membranes for carbon dioxide–methane separation

Journal of Membrane Science 383 (2011) 262–271

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Ionic liquid membranes for carbon dioxide–methane separation P. Uchytil a,∗ , J. Schauer b , R. Petrychkovych a , K. Setnickova a , S.Y. Suen c a

Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, Czech Republic Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Czech Republic c National Chung Hsing University, Taichung, Taiwan b

a r t i c l e

i n f o

Article history: Received 7 April 2011 Received in revised form 18 August 2011 Accepted 28 August 2011 Available online 2 September 2011 Keywords: Ionic liquid membrane Gas separation Gas transport

a b s t r a c t The transport of carbon dioxide and methane in polymer containing ionic liquid was studied by a dynamic gas permeation method. Poly(vinylidene fluoride-co-hexafluoropropylene) polymeric membrane (Viton) and two ionic liquids 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([hmim][Tf2 N]) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf2 N]) were used for preparation of ionic liquid membranes (ILM) with different ionic liquid amounts (from 0 to 80 wt%). For the better understanding of transport mechanism through ionic liquid membranes it is very useful to know also transport properties of pure ionic liquids and pure polymeric material. The gas transport through pure ionic liquid was determined indirectly, by means of the gas transport measurement through two special types of membranes: “sandwich arrangement” and “support arrangement”. The dependencies of the separation factor and gas fluxes on the ionic liquid amounts in ionic liquid membranes were determined. The gas permeability increased with the ionic liquid [emim][Tf2 N] content in the membrane, the permeability for the membranes with [hmim][Tf2 N] exhibits the maximum for IL concentration of 70 wt%. Ideal separation factor CO2 /CH4 was low for small contents of ILs in membranes (0–15 wt%), around 7 for [emim][Tf2 N] and 8 for [hmim][Tf2 N] but for higher contents (30–75 wt%) it was approximately constant, 15 for [emim][Tf2 N] and 12 for [hmim][Tf2 N]. Very interesting is the comparison of carbon dioxide permeability, it increases in the series: polymer, ionic liquid and ionic liquid membrane. Although the transport properties values of ILM were expected to be in the middle of the ionic liquid and the polymer from which were formed, surprisingly the obtained transport properties of ILM are much better than those for the pure components. For example, the carbon dioxide permeability for the ionic liquid membrane with 70 wt% of [hmim][Tf2 N] is almost thousand times higher than for the pure polymer, and hundred times higher than for the ionic liquid. This fact indicates that the mechanism of the transport in ionic liquid membranes has to be different from the transport mechanisms in an ionic liquid. An explanation could be that “new transport pores” are created between polymer chains and an ionic liquid. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The unique properties of ionic liquids such as negligible vapor pressure, a wide range of viscosity and thermal and chemical stability are the reason why they have been increasingly investigated in various fields including material engineering [1], chemical reaction [2,3], catalysis [4,5], biotechnology [6,7], electrochemical applications [8,9], separation technology [10–12] or environmental studies [13]. The use of ionic liquids (ILs) in membrane separation processes is one of the most widely researched and fast growing separation

∗ Corresponding author. Tel.: +420 220390268; fax: +420 220920661. E-mail address: [email protected] (P. Uchytil). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.08.061

techniques in last years due to their advantages and ecological aspect compared to classical processes (absorption, extraction, distillation, etc.). The special properties of ionic liquids, namely tunable physicochemical properties and high solubility of different gases in ionic liquids [14,15], predict them as a suitable candidates for use in liquid membranes instead of organic solvents. Often investigated applications of liquid membranes based on IL are the separation/concentration of ions [16–18], the separation of liquid feeds [19] and the separation of gases or vapors. Table 1 contains the names, abbreviations and application of the ionic liquids that are most commonly used and studied in membrane separation processes for gas and vapor separation [20–35]. Also the review by Krull et al. brings detailed information about developments of liquid membranes for gas and vapor separation covering the last 16 years [36]. In comprehensive survey Krull and colleagues reported

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Table 1 Applications of ILs in membrane gas separation processes. Name of ionic liquids

Abbreviation

Separate

Ref.

Polyethylene glycol-400 Tetramethylammonium fluoride tetrahydrate Tetraethylammonium acetate tetrahydrate Diglycolamine, triethylene glycol 1-Ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide 1-Ethyl-3-methyl imidazolium triflate 1-Ethyl-3-methyl imidazolium dicyandiamide Trihexyl(tetradecyl)-phosphonium chloride 1-Butyl-3-methyl imidazolium bis[trifluoromethylsulfonyl]amide 1,3-Dimethylimidazolium bis(trifluoromethylsulfonyl)imide Methyltrioctylammonium bis(trifluoromethylsulfonyl)imide N-octylpyridinium bis(trifluoromethylsulfonyl)imide 1-(3-Aminopropyl)-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide N-aminopropyil-3-ethylimidazolium bis(trifluoromethyl-sulfonyl)imide N-aminopropyil-3-methyl imidazolium triflate 1-Methyl-3-(3,3,4,4,4-pentylfluorohexyl) imidazolium bis(trifluoromethylsulfonyl)imide 1-Methyl-3-(3,3,4,4,5,5,6,6,6-nonafluorohexyl) imidazolium bis(trifluoromethylsulfonyl)imide 1-Methyl-3-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecyl fluorohexyl)imidazolium bis(trifluoromethylsulfonyl)imide 1-Butyl-3-methylimidazolium tetrafluoroborate 1-Ethyl-3-methylimidazolium tetrafluoroborate 1-Hexyl-3-methylimidazolium tetrafluoroborate 1-Ethyl-3-methylimidazolium triflate 1-Butylimidazolim acetate 1-Methylimidazolium acetate Trihexyl(tetradecyl) phosphonium bromide Trihexyl(tetradecyl) phosphonium decanoate Trihexyl(tetradecyl) phosphonium bis(2,4,4-trimethylpentyl) phosphinate Triisobutylmethyl phosphonium tosilate Tetrabutyl phosphonium bromide Triethylsulfonium bis(trifluoromethylsulfonyl)imide 1,3-Dimethylimidazolium dimethylphosphate 1-Decyl-3-methylimidazolium tetrafluoroborate 1-Butyl-3-methylimidazolium hexafluorophosphate 1-Butyl-3-methylimidazolium tetrafluoroborate 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide 1-Octyl-3-methylimidazolium hexafluorophosphate 1-Ethyl-3-methylimidazolim tetrafluoroborate 1-Ethyl-3-methylimidazolim dicyanamide 1-Ethyl-3-methylimidazolim trifluoromethanesulfone 1-Ethyl-3-methylimidazolim bis(trifluoromethanesulfonyl)amide 1-Buthyl-3-methylimidazolim bis(perfluoroethylsulfonyl)imide 1-Hexyl-3-methylimidazolium bis(trifluoromethansulfonyl)amide 1-Butyl-3-methylimidazolium hexafluorophosphate 1-Butyl-1-methylpyrrolidium bis(trifluoromethylsulfonyl)imide 1-Hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide Methyltrioctylammonuim bis(trifluoromethylsulfonyl)imide 1-Butyl-4-methylpyridinium tetrafluoroborate

PEG-400 [(CH3 )(4)N]F·4H2 O [(C2 H5 )(4)N]CH3 CO2 ·4H2 O DGA, TEG [emim][Tf2 N] [emim][CF3 SO3 ] [emim][dca] [thtdp][Cl] [C4 mim][Tf2 N] [dmim][Tf2 N], [N(1)888 +][Tf2 N] [C8 Py][Tf2 N] [H2 NC3 H6 mim][Tf2 N] [C3 NH2 mim][Tf2 N] [C3 NH2 mim][CF3 SO3 ] [MpFHim][Tf2 N] [MnFHim][Tf2 N] [MtdFHim][Tf2 N] [bmim][BF4 ] [emim][BF4 ] [hmim][BF4 ] [emim][CF3 SO3 ] [bim][ace] [mim][ace] Cyphos 102 Cyphos 103 Cyphos 104 Cyphos 106 Cyphos 163 [Set3 ][Tf2 N] ECOENGTM 1111P [dmim][BF4 ] [bmim][PF6 ] [bmim][BF4 ] [bmim][Tf2 N] [omim][PF6 ] [emim][BF4 ] [emim][dca] [emim][CF3 SO3 ] [emim][Tf2 N] [bmim][BETI] [C6 mim][Tf2 N] [bmim][PF6 ] [bmpy][Tf2 N] [hmim][Tf2 N] [mtoa][Tf2 N] [bmpy][BF4 ]

CO2 , CH4 CO2 , CH4 , H2

[20] [21]

CO2 , CH4 N2 , CO2 , CH4

[22] [23]

N2 , CO2 N2 , H2 , O2 , CO

[24] [25]

H2 , CO2 CO2 , CH4

[26] [27]

N2 , O2 , CO2 , CH4

[28]

N2 , CO2 , CH4 , SO2

[29]

CO2 , SO2

[30]

N2 , H2 , CH4 , CO2

[31]

N2 , CH4 , CO2

[32]

CO2 , CH4 , N2

[33]

CO2 , CH4 CO2 , CH4

[34] [35]

possible LM (liquid membrane) configurations, the mass transfer mechanism in an IL, liquids and carriers presented together with their specific separation tasks. The performance of different LMs in terms of permeability and selectivity is compared and discussed as well and finally, different preparation methods of LMs are illustrated. Generally, liquid membranes with and without supports can be differentiated. For those not employing supports, ionic liquids can be used as the so-called bulk liquid membranes (BLM) and emulsion liquid membranes (ELM). The liquid membranes employing a support can be subdivided into immobilized liquid membranes (ILM), supported liquid membranes (SLM) and contained liquid membranes (CLM) [36]. A relatively new idea in membrane separation processes is the use of membrane based on polymer/IL system – especially polymerized ionic liquid membrane (PILM) [37,38] or gelled ionic liquid membrane [39]. These types of membrane seem to be perspective for the excellent properties of ionic liquids (tunable physicochemical properties, high thermal and chemical stability, and high selectivity) and the advantages of dense membrane (low membrane thickness, high stability and high permeability).

Carbon dioxide is the most important greenhouse gas (produced primarily by the combustion of fossil fuels); therefore its removal from gas streams containing air or methane falls within the most encountered separation task in practical industries (Table 1). A number of investigations have shown remarkable carbon dioxide solubility in a commonly used ILs [40,41] It was found that both anion and cation of the IL have influence on the CO2 solubility in IL, nevertheless the anion frequently plays a key [16,17,42–44]. Mostly examined types of membranes used in the process of CO2 capture were SILMs. Supported ionic-liquid membranes are preferred because of their high selectivity and permeability [27,36]; however the negative aspect of this configuration is the leaching of the IL from the support because of the pressure gradient across the membrane. Polymer/IL membrane systems (PILMs and gelled ionic liquid membranes) have improved mechanical properties and eliminate the problem with the liquid displacement [44]. Polymerizable ionic liquid membranes (PILM) have been intensively studied, especially by the group of J.E. Bara and R.D. Noble [37,38,44–48]. They have investigated PILM containing styrene or acrylate, which show high permeability, solubility and diffusivity

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for CO2 , N2 and CH4 [37]. In paper [45] improving CO2 selectivity in PILMs by incorporating substituent like oligo(ethylene glycol) or nitrile-containing alkyl groups was presented. The efficiency of PILMs can be considerably enhanced through the formation of a solid composite membranes containing IL [46,47]. In this work use of room temperature ionic liquids (RTILs) based on the 1-n-alkyl-3-methylimidazolium cation was studied in order to utilize them as membrane material for separation and enrichment of carbon dioxide from a gaseous mixture containing methane, carbon dioxide, nitrogen and other gases. Two RTILS with different chain length of alkyl substituent on imidazolium cation, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [hmim][Tf2 N] and 1-ethylbis(trifluoromethylsulfonyl)imide, 3-methylimidazolium [emim][Tf2 N] were incorporated in different polymers, poly(vinylidene fluoride-co-hexafluoropropene and cellulose acetate, respectively, in order to study their effect on the permeabilities of carbon dioxide and methane and CO2 /CH4 separation. The presented paper is focused on investigation of pure CO2 and CH4 permeabilities in order to determine ideal separation factor of both gases through PILMs containing from 0 to 80 wt% of the ILs and through pure ILs.

2. Experimental

Fig. 1. Scheme of gas transport through individual layers of sandwich arrangement (a), scheme of gas transport through individual layers of support arrangement (b).

2.1. Preparation of ionic liquid membranes The following chemicals were used for preparation of polymerized ionic liquid membranes (PILM). Poly(vinylidene fluoride-co-hexafluoropropene) (fluoroelastomer) of nominal Mn 130,000 (Aldrich), cellulose acetate (Mn = 30,000, Aldrich), 1-hexyl3-methylimidazolium bis(trifluoromethylsulfonyl)imide (99%, Iolitec, Germany, [hmim][Tf2 N]), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (99%, Iolitec, Germany, [emim][Tf2 N]) and acetone p.a. (Lachner, Czech Republic) were used as received. Carbon dioxide and methane were purchased from Linde Technoplyn in purity higher than 99.9%. Chemical compositions of chosen ionic liquids (IL) follow:

N

+ N

CF3SO2NSO2CF3 CH3

CH3CH2CH2CH2CH2CH2

[hmim][Tf2N]

N CH3CH2

+ N

CF3SO2NSO2CF3 CH3

[emim][Tf2N] The ionic liquid membranes, which have gel character, were prepared by casting the mixture of an ionic liquid [hmim][Tf2 N] or [emim][Tf2 N] and 10% acetone solution of fluoroelastomer on a Nalgen dish and evaporating acetone at room temperature (24 h) and then at 70 ◦ C (4 h). The maximum concentrations of [hmim][Tf2 N] and [emim][Tf2 N] in the membrane allowing the preparation of membrane with sufficient mechanical strength were 80 and 75 wt%, respectively. Membrane thickness of pure fluoroelastomer was 70 ␮m, the thickness of gel ILM was in the interval from 120 to 330 ␮m, higher thickness was in the case of higher concentrations of IL in membranes.

2.2. Preparation of samples for determination of ionic liquids permeability The knowledge of the gas transport in pure ionic liquids belongs to the fundamental data is indispensable for the understanding of the transport mechanism in ionic liquid membranes. There exist a huge number of new ionic liquids used for different separation tasks but data about the transport properties of ionic liquids could very seldom be found in literature. Especially the data of ionic liquids which were used in this work or very similar IL (the same cation) are very rare and the published values differ, see Table 2. The main reason of these discrepancies could be the method of ionic liquid samples preparation – for instance in the case of supported IL membranes the IL is under vacuum soaked by capillary forces in porous support and so prepared samples are used for permeation measurements. Using this method you can only very roughly estimate the thickness of the IL layer. But more important is the fact that it could easily happen that some pores are not filled by the IL. Also the removal of the excess liquid from the surface of porous support by some tissue during the sample preparation could cause the same undesirable problem – all pores are not filled by the IL. The empty pores without the ionic liquid lead to higher evaluated permeability and lower separation factor of the tested ionic liquid. In this paper a new type of measurement of ionic liquids gas permeability was proposed to overcome the uncertainty with filling of pores with an IL. Two types of samples – “sandwich arrangement” and “support arrangement” were prepared for the determination of gas permeability in ionic liquids (Fig. 1).

2.2.1. Sandwich arrangement At first an unbroken ionic liquid thin layer was created by spreading the ionic liquid that was deposited on the surface of the assistant material. Then the free surface of the ionic liquid was covered by another sample of the same material and so called “sandwich arrangement” – three-layer sample, in which the ionic

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Table 2 Support for ionic liquid membranes – separation CO2 /CH4 . Ionic liquids

Support

Permeability (mol m m−2 s−1 Pa−1 )

Selectivity CO2 /CH4

Configuration

Ref.

[emim][Tf2 N] [emim][CF3 SO3 ] [bmim][BF4 ] [bmim][BF4 ] [emim][CF3 SO3 ] [emim][Tf2 N] [emim][BF4 ] [emim][CF3 SO3 ] [hmim][Tf2 N] [bmim][PF6] [bmim][BF4 ] [hmim][Tf2 N] [bmim][Tf2 N]

Hydrophilic PES Hydrophilic PES Hydrophilic PES Hydrophobic PVDF Hydrophobic PVDF Hydrophilic PES Hydrophilic PES Hydrophilic PES Hydrophilic PES Alumina ␣-Alumina ␣-Alumina PTFE support for ILM is sandwiched between two hydrophilic PTFE supports

3.52 × 10−13 2.79 × 10−13 1.44 × 10−13 3.14 × 10−14 1.63 × 10−13 5.70 × 10−13 3.24 × 10−13 3.92 × 10−13 3.80 × 10−13 8.10 × 10−14 3.60 × 10−13 4.60 × 10−13 1.67 × 10−13

11 17 10 13 23 12 22 19 9 38 17 9 10

SILM SILM SILM SILM SILM SILM SILM SILM SILM SILM SILM SILM SILM

[23] [29] [29] [31] [31] [33] [33] [33] [33] [34] [35] [35] [36]

6.70 × 10−15 1.07 × 10−14 7.37 × 10−15 1.77 × 10−15 3.08 × 10−15 2.25 × 10−14

22 17 22 20 39 11

PILM PILM PILM PILM PILM PILM

[37] [37] [37] [38] [49] [49]

[bmim][Tf2 N] – styrene based PILM [hmim][Tf2 N] – styrene based PILM [bmim][Tf2 N] – acrylate based PILM [hmim][Tf2 N] [emim][Tf2 N] + styrene – 100% poly(RTIL) [emim][Tf2 N] + vinyl – 100% poly(RTIL)

liquid is closed between two enveloping layers, was used for gas permeation experiments (Fig. 1a). 2.2.2. Support arrangement The second procedure was based on the creation of an ionic liquid thin layer on the polymeric assistant material surface without a closing layer – a two-layer sample (Fig. 1b). This “support arrangement” requires very careful manipulation with the sample in the permeation cell. The support samples were prepared by fixing the assistant material in a permeation cell and depositing a given amount of an ionic liquid on its surface with a syringe. The advantage of this method is smaller transport resistance in an assistant material due to only one layer but the disadvantage is that the thickness of the ionic liquid layer prepared on support has to be higher than in the case of sandwich preparation to be sure that all surface is covered by an IL. 2.2.3. Material for preparation of sandwich and support samples In order to find a suitable material for preparation of a membrane sample (sandwich sample or support sample) for testing of ionic liquid transport properties is a basic requirement. The ionic liquid has to wet the material (high contact angle) and very advantageous is also its very low resistance to the gas transport. Many permeation experiments were made for selection of an appropriate assistant material on the basis of polyethersulfone filled with nanoparticles of sodium montmorillonite (PES + MMT) to prepare samples for the gas transport measurements in ionic liquid [hmim][Tf2 N]. Ideally it is to find a material with negligible transport resistance for carbon dioxide and methane in the comparison with ionic liquid layer. The samples with PES + MMT, which were tested, had different contents of nanoparticles (20 and 30 wt%). The samples with higher concentration of MMT nanoparticles had higher gas permeability but they exhibit very different transport properties. The presence of defects created during the material preparation was the reason of this behaviour. PES membrane with lower content of nanoparticles (20 wt%) was chosen for the preparation of sandwich and supported samples to avoid the problems with defects – if the material with very small resistance to gas transport will be used and then the filling of these defects by IL during the permeation measurements could dramatically change the flux through this assistant material and as the result the incorrect values of IL permeability will be evaluated. The transport resistance of selected assistant material PES with 20 wt%

was not negligible in the comparison with the ionic liquid layer. It was therefore necessary to characterize the material, the permeabilities of carbon dioxide and methane were measured and these values were used for calculation of ionic liquid permeability. This assistant material for [hmim][Tf2 N] ionic liquid was prepared by following procedure. Na+ -MMT (20 wt%) was mixed with the solvent for 1 h at room temperature. Then, PES was added to the solution and mixed for 24 h as the casting solution. The casting solution was then spread over a smooth glass plate using a doctor blade to form a film. The film with the glass plate was placed in a 60 ◦ C oven for 50 min. After exposure in the air for 1 min, the film with the glass plate was immersed in 4 ◦ C water bath for 24 h. Finally, the prepared membrane was dried at room temperature for another 24 h. The DMF was used as the solvent. The thickness of the polymer film was 25 ␮m. Cellulose acetate films (thickness 30 ␮m) were used as an assistant material for the preparation of samples with a [emim][Tf2 N] layer as cellulose acetate shows better wettability towards [emim][Tf2 N] than polyethersulfone filled with sodium montmorillonite. Cellulose acetate homogeneous film was prepared by casting a 15 wt% solution of cellulose acetate in acetone on a glass plate and evaporating the solvent at 30 ◦ C (24 h) and then at 70 ◦ C (4 h). The thickness of the ionic liquid in sandwich samples was estimated from the weight and density of the used ionic liquid – [hmim][Tf2 N] layer 13 ␮m, [emim][Tf2 N] layer 35 ␮m. In the case of support samples the thickness of the [hmim][Tf2 N] layer on a polyethersulfone/montmorillonite film was 60 ␮m, the thickness of the [emim][Tf2 N] layer on a cellulose acetate film was 50 ␮m. 2.3. Permeation apparatus and permeability determination Gas permeation measurements of pure gases (carbon dioxide and methane) through tested material were conducted using a semi-open cell which is divided into two parts by the membrane with the effective diameter of 52 mm (effective membrane area A = 21.2 cm2 ). Schema of a permeation apparatus could be seen in Fig. 2. Constant pressure p1 (2 bar) was maintained at the feed side, while vacuum was at the permeate side (volume V2 ) at the start of an experiment. After evacuation the permeate side compartment was closed. Increase in the permeate pressure p2 with time was measured using a pressure transducer connected to a personal computer. Pressure values were registered at time intervals from 1 to 120 s. The gas flux was evaluated from pressure p2 dependence

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Fig. 2. Scheme of permeation apparatus.

on time, see Eq. (1); the gas accumulation in the membrane was neglected. Example of the dependence was shown in Fig. 3. V2 dp2 J= ART dt

(1)

The permeability P was calculated from the permeate flux: P=

V2 dp2 JL L = p1 − p2 p1 − p2 ART dt

(2)

The quality of sealing was tested by an impermeable membrane of stainless steel. The increase in pressure p2 at the permeate side was negligible. All permeation experiments were carried out at the temperature 25 ◦ C. 2.3.1. Evaluation of gas permeability in ionic liquid from the transport in multilayer samples In steady state the gas flux in every layer of multilayer samples has to be the same Jin = J1 = J2 = J3 = Jout = J. From the steady state flux through sandwich (three-layers, see Fig. 1a) or support arrangement (two-layers, see Fig. 1b) samples J and from already known values of the permeability in a material used for sample preparation (determined by separate permeation experiments), it is possible according Eq. (3) to evaluate the pressure drop in individual layers of a sample and the permeability of gas in the ionic liquid. Symbol p∗i is used for the pressure between individual layers. p1 − p∗1 =

JL P

(3)

0.04

0.03

p2 (bar)

Fig. 4. Pressure dependence of carbon dioxide and methane permeability in a pure polymer Viton.

By this procedure the pressure drops in individual layers were determined and then gas permeability in an ionic liquid was evaluated from the pressure drop in an ionic liquid layer and from the measured flux through the multilayer sample. 3. Results and discussion Gas transport through three types of material was determined: fluoroelastomer membrane without any ionic liquid, ionic liquids and fluoroelastomer membranes containing an ionic liquid (ionic liquid membranes). 3.1. Gas permeability in pure fluoroelastomer membrane Methane and carbon dioxide fluxes through the dense fluoroelastomer Viton membrane with the thickness 70 ␮m were measured. The gas fluxes through the dense membrane of pure fluoroelastomer (U0) were low, especially for methane. The determined permeability of carbon dioxide was – 1.30 × 10−16 mol m m−2 s−1 Pa−1 (see Tables 3 and 4). In the case of methane the flux determination was very problematic because the achieved values were very close to the measurement limit. Particularly it took 60 h for methane to reach the (small) pressure of 0.032 bar in the permeate side of the cell, so its calculated permeability value of 1.69 × 10−17 mol m m−2 s−1 Pa−1 (see Tables 3 and 4, Fig. 4) may be influenced by a large experimental error. Even very small leak in the volume V2 in the permeate side of the cell could lead to the overestimation of methane flux. Separation factor ˛(CO2 /CH4 ) calculated from ratio of observed permeabilities was 7.7 (see Tables 3 and 4). But in reality the separation factor is probably much higher with regards to overestimated methane flux.

0.02

3.2. Gas permeability in ionic liquids

0.01

0 0.0E+00

5.0E+04

1.0E+05

1.5E+05

2.0E+05

2.5E+05

time (s) Fig. 3. Increase of the methane pressure p2 below the Viton membrane with time.

Measurement of the gas transport in an ionic liquid was the most complicated procedure. Since it is not possible to measure the pure ionic liquid permeability directly, two types of arrangement of ionic liquids samples were used. The first one was the preparation of an ionic liquid layer between two assistant polymer layers so called “sandwich arrangement”; the second one was creation of an unbroken ionic liquid layer on the surface of an appropriate polymeric support – “support arrangement”. To evaluate the gas permeability in ionic liquids it was necessary to measure also the

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Table 3 The determined performance of carbon dioxide and methane in ILM containing [hmim][Tf2 N]. Sample

U0 U21 U22 U23 U24 U25 U25A U26 U27 U27A Pure IL

wt% [hmim][Tf2 N]

0 15 30 45 60 70 70 75 80 80 100

Permeability × 1016 (mol m m−2 s−1 Pa−1 ) Methane

Carbon dioxide

0.17 2.41 10.9 29.4 78.1 95.5 84.8 76.3 76.2 77.6 0.34

1.3 19.9 145 370 933 1090 974 807 839 877 10.8

Ideal separation factor

7.7 8.3 13.3 12.6 11.9 11.4 11.5 10.6 11.0 11.3 32.0

Table 4 The determined performance of carbon dioxide and methane in ILM containing [emim][Tf2 N]. Sample

wt% [emim][Tf2 N]

U0 U11 U12 U13 U14 U15 U16 Pure IL

0 15 30 45 60 70 75 100

Permeability × 1016 (mol m m−2 s−1 Pa−1 ) Methane

Carbon dioxide

0.17 3.97 8.61 33.1 59.5 86.7 106 0.24

1.3 24.2 172 508 942 1370 1620 14

gas transport through an assistant material with which the samples were prepared.

3.2.1. Gas permeability in assistant material used for sandwich and support arrangements As it was already mentioned in chapter 2.2. polyethersulfone filled with 20 wt% of sodium montmorillonite (PES + MMT) and cellulose acetate (CA) were chosen as the appropriate materials for the preparation of sandwich and support samples for the evaluation of the gas transport in ionic liquid [hmim][Tf2 N] and [emim][Tf2 N], respectively. In order to verify that the transport properties of the assistant materials were not changed during the permeation measurements in sandwich or support arrangements the permeation experiments with this material were repeated after removing – wiping of an IL layer from an assistant material. The gas permeability was around 15% lower than permeability with IL layer. This decreasing could be caused by a residue of an IL on the sample which was not perfectly removed. Permeability P of both gases in PES + MMT was independent of pressure in used pressure range (see Fig. 5) and its value for methane is P = 7.93 × 10−15 mol m m−2 s−1 Pa−1 and 6.52 × 10−15 mol m m−2 s−1 Pa−1 for carbon dioxide. Also the gas permeability through cellulose acetate was independent of pressure for both gases and it was 5.3 × 10−16 mol m m−2 s−1 Pa−1 for methane and 1.69 × 10−15 mol m m−2 s−1 Pa−1 for carbon dioxide. Comparison of assistant material and IL permeability could be seen in Tables 3 and 4.

3.2.2. Sandwich arrangement The prepared sandwich sample consists of two pieces of an assistant material and an IL closed between them (Fig. 1a). The thickness of the ionic liquid was estimated from the weight and density of the used ionic liquid – the thickness of [hmim][Tf2 N] was 13 ␮m and of [emim][Tf2 N] 35 ␮m.

Ideal separation factor

7.7 6.1 20.0 15.3 15.8 15.8 15.3 59.0

Evaluation of IL permeability is based on the determination of pressure drop in the IL p∗1 − p∗2 as described in paragraph 2.3.1. Eq. (3) and on the measured flux through the whole multilayer sample. The calculated permeability of carbon dioxide in [hmim][Tf2 N] is 9.5 × 10−16 and of methane 2.9 × 10−17 mol m m−2 s−1 Pa−1 . The permeability of carbon dioxide in [emim][Tf2 N] is 1.6 × 10−15 and 3.1 × 10−17 mol m m−2 s−1 Pa− 1 for methane. The obtained results (Table 5) show that the ionic liquids exhibit good separation properties for system CO2 /CH4 (ideal separation factor ˛ ≈ 33 for [hmim][Tf2 N] and 52 [emim][Tf2 N]) but the permeability even of preferentially transported gas – carbon dioxide (9.5 × 10−16 , 1.6 × 10−15 mol m m−2 s−1 Pa−1 , respectively) is quite low.

Fig. 5. Permeability of carbon dioxide and methane in polymeric membrane (PES filled with 20 wt% of sodium montmorillonite) with thickness 25 ␮m which was used for sandwich sample of IL [hmim][Tf2 N].

268

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1.E+03

[emim][Tf2N] - [23] [emim][CF3SO3] - [29] [bmim][BF4] - [29] [bmim][BF4] - [31]

Separation factor CO2/CH 4

Robeson Upper Bound (2008)

[emim][CF3SO3] - [31] [emim][Tf2N] - [33] [emim][BF4] - [33]

1.E+02

[emim][CF3SO3] - [33] [C6mim][Tf2N] - [33] [bmim][PF6] - [34] [bmim][BF4] - [35] [hmim][Tf2N] - [35] [bmim][Tf2N] - [36] [bmim][Tf2N] - [37]

PILM

1.E+01

[hmim][Tf2N] - [37] [bmim][Tf2N] - [37]

SILM

[C6mim][Tf2N] - [38] [emim][Tf2N] - [48] [emim][Tf2N] - [48] [hmim][Tf2N] this work [emim][Tf2N] this work

1.E+00 1E-16

1E-15

1E-14

1E-13

1E-12 -2

-1

1E-11

-1

Permeability CO2 (mol m m s Pa ) Fig. 6. The comparison between published data and results obtained in this paper. Robeson’s upper bond 2008 for CO2 /CH4 is depicted. Besides our experimental data, two groups of data are depicted in the figure. One group represents SILM, the second one PILM.

3.2.3. Support arrangement Another possibility how to evaluate transport properties of an IL is based on the deposition of an ionic liquid thin layer on the polymeric material surface without a closing polymeric layer; the scheme of this arrangement could be seen in Fig. 1b. The support was fixed in a permeation cell and the vacuum below the membrane was adjusted. On its surface the weighted amount of IL was deposited by injection. The layer of the IL with thickness (around 60 ␮m [hmim][Tf2 N] and 35 ␮m [emim][Tf2 N]) was prepared. The permeability of methane in ionic liquid [hmim][Tf2 N] was 3.8 × 10−17 mol m m−2 s−1 Pa−1 , of carbon dioxide 1.2 × 10−15 mol m m−2 s−1 Pa−1 (Table 5). Ideal separation factor is ˛ ≈ 31 for system CO2 /CH4 . The evaluated permeability by this method for both gases are higher (around 29% for carbon dioxide and 13% for methane) than the values obtained by the measurements in sandwich arrangements. The calculated permeability of ionic liquid [emim][Tf2 N] for carbon dioxide was 1.2 × 10−15 mol m m−2 s−1 Pa−1 and 1.8 × 10−17 mol m m−2 s−1 Pa−1 for methane. Ideal separation factor is ˛ ≈ 66 for system CO2 /CH4 . 3.2.4. Summary of gas permeability determination in ionic liquids The measurement of transport properties of ionic liquids is very difficult because the gas fluxes, especially the flux of methane, are low. The evaluated permeability of methane in ionic liquids is overestimated in both experimental arrangements. The methane flux is

on the limit of measurements. The increase of pressure p2 below the samples during the methane permeation experiments was very low – around 0.01 bar/10 h therefore some not negligible part of pressure p2 increase could be caused by non-ideal sealing of the permeation cell. It could be supposed that the ideal separation factor CO2 /CH4 is in reality higher than 70. Also the gas permeability in samples prepared for testing was not exactly independent of the pressure conditions (this assumption was used for evaluation of experimental data). The difference between the permeability in the pressure range used for the measurements was below 20%. Moreover the membrane samples had not uniform thickness therefore also the thickness of the ionic liquid layer is unequal. All these difficulties cannot be easily overcome. Therefore the obtained results are considered as good. The obtained difference between sandwich and support arrangements and comparison of transport properties of both ionic liquids could be seen in Table 5. Ionic liquid [emim][Tf2 N] has a little bit better transport properties – higher separation factor and higher permeability. The comparison between already published data and results obtained in this paper could be seen in Fig. 6. Robeson’s upper bond (from 2008) [49] for CO2 /CH4 is depicted there. Besides our experimental data, two groups of data are depicted in the figure. One group represents SILM (supported ionic liquid membranes), the second one PILM (polymerizable ionic liquid membranes). It could be seen the relation between selectivity and permeability for both ionic liquids corresponds to the predicted Robeson plot.

Table 5 The calculated permeability of carbon dioxide and methane in ILs.

Carbon dioxide Methane

Permeability × 1017 (mol m m−2 s−1 Pa−1 )

Ideal separation factor

[hmim][Tf2 N]

[hmim][Tf2 N]

[emim][Tf2 N]]

Sandwich

Support

Sandwich

Support

Sandwich

95 2.9

120 3.8

160 3.1

120 1.8

33

[emim][Tf2 N] Support 31

Sandwich

Support

52

66

P. Uchytil et al. / Journal of Membrane Science 383 (2011) 262–271

35 30

10

CO2 25

8

20 6 15 4

10

2

5

CH4 0 0

20

40

60

80

0 100

[hmim][Tf2N] concentration in the membrane (wt.%) Fig. 7. The dependence of carbon dioxide and methane permeability and ideal separation factor carbon dioxide/methane in ionic liquid membranes on content of ionic liquid [hmim][Tf2 N].

3.2.7. Comparison of permeability pure polymer, pure ionic liquid and ionic liquid membrane Very interesting is the comparison of carbon dioxide permeability for pure polymer, pure ionic liquid and ionic liquid membrane. It follows from the results that the permeability increases in the row: polymer, ionic liquid and ionic liquid membrane. For example, the carbon dioxide permeability for the ionic liquid membrane with 70 wt% of [hmim][Tf2 N] is almost thousand times higher than for the pure polymer, and hundred times higher than that for the ionic liquid. Reason why the gas flux (permeability) in ionic liquid membranes is much higher than in the pure IL and pure polymer is unknown. The creation of a structural nano-scale organization of ionic liquids, which could influence their transport properties, is described in literature [50,51]. The existence of this structure is more probable for longer length of alkyl imidazolium cations and lower temperature (below room temperature). The crystallization was not found in the case of short alkyl imidazolium cations as ethyl, which was 70 60 16

12

40

Permeability x 10

CO2

8

30 20

Separation factor (-)

50

14

(mol m m -2 s-1 Pa -1)

20

3.2.6. Permeability of ionic liquid membranes – [emim][Tf2 N]/fluoroelastomer Prepared ionic liquid membrane with different amount of [emim][Tf2 N] in fluoroelastomer were measured like in previous cases. The dependencies of the permeabilities and of the ideal separation factor of [emim][Tf2 N]/fluoroelastomer gel membranes on the [emim][Tf2 N] concentration in the membrane are shown in Fig. 8 and Table 4. In contrast to the permeabilities of membranes with [hmim][Tf2 N], those with [emim][Tf2 N] increased with the growing ionic liquid concentration without showing any maxima up to the concentration of 75 wt% of the ionic liquid in the membrane. The dependence of ideal separation factor for the CO2 /CH4 system on concentration of IL [emim][Tf2 N] is very similar as in the case of [hmim][Tf2 N]. The dependence has an indistinct maximum at concentration of 30 wt% of [emim][Tf2 N] ˛ = 20, then for higher concentration of IL ˛ was approximately constant, a little bit higher than 15. It could be concluded that permeabilities of methane, carbon dioxide and the ideal separation factor for the CO2 /CH4 system are higher in the fluoroelastomer membranes containing [emim][Tf2 N] than [hmim][Tf2 N].

Separation factor (-)

14

(mol m m -2 s-1 Pa -1)

12

Permeability x 10

3.2.5. Permeability of ionic liquid membranes – [hmim][Tf2 N]/fluoroelastomer Prepared ILM with different amount of [hmim][Tf2 N] in fluoroelastomer were measured in a permeation apparatus similarly as other samples and floroelastomer. The gas permeability and the ideal separation factor dependencies in [hmim][Tf2 N]/fluoroelastomer gel membranes on the [hmim][Tf2 N] concentration in the membrane are shown in Fig. 6 and the values are given in Table 3. The connection between the last point ILM and the pure IL vanishes, because it is not possible to estimate the results in this region. The permeabilities of both gases were higher in gel membranes than in a pure fluoroelastomer. The considerable increase of gas permeability was beyond expectation, it reached the maximum at about 70 wt% of [hmim][Tf2 N] in the membrane; carbon dioxide and methane permeabilities were 1 × 10−13 mol m m−2 s−1 Pa−1 and 9 × 10−15 mol m m−2 s−1 Pa−1 , respectively (average values for samples U25 and U25A). Ideal separation factor ␣ for CO2 /CH4 was increasing with addition of ionic liquid in membranes (˛ = 7.7 for the pure fluoroelastomer membrane), reached a small maximum for 30 wt% of [hmim][Tf2 N] in the membrane ˛ = 13.3 and then for higher concentration of the IL in the membranes ˛ was approximately constant, a little bit higher than 11, separation factor in a pure IL is more than three times higher. The reproducibility of permeability experiments were tested by repetition of the measurements on the same membranes samples. In the case of the membranes with concentration of IL 60 wt% and lower; the difference between the results was smaller than 5%. The increase of permeability was observed for repeated permeation measurements on the same membranes with 70 wt% of IL and higher. The increase of permeability (more than 30%) was caused by the decrease of IL in membranes; IL was expelled by high pressure difference across the membrane. Therefore the measurements for higher contents of IL were made under pressure p1 = 1 bar but still the small increasing of permeability was observed with time, around 10% for repeated experiments on the same sample. The more detailed study of the membrane stability was not pursued. To verify unexpected high increase of gas permeability in the membranes with high contents of IL [hmim][Tf2 N] in comparison with a pure fluoroelastomer and an IL; new membranes samples U25A (70 wt% of IL) and U 27A (80 wt% of IL) were prepared and tested. Very good agreement of permeability with the older samples was obtained (around 10%), see Table 3 and Fig. 7.

269

4 10

CH4 0

0

20

40

60

80

0 100

[emim][Tf2N] concentration in the membrane (wt.%) Fig. 8. The dependence of carbon dioxide and methane permeability and ideal separation factor carbon dioxide/methane in ionic liquid membranes on content of ionic liquid [emim][Tf2 N].

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used in this work. Moreover, the transport properties of both ILMs (with ethyl- and hexyl-) were very similar. Another very simple and preliminary explanation could be that in ionic liquid membranes the new “transport” pores are created by interactions between polymer chains and an ionic liquid. The gas molecules diffuse in these pores; it is possible that there is a great contribution of the surface diffusion on ionic liquid surface. Obtained results support this conception – with the increase of the IL in a membrane the gas fluxes increase while the ratio of both fluxes (separation factor) is approximately constant. It corresponds to the situation with the increase of the number of pores for gas transport for higher contents of an IL in an ILM. Also the great difference between estimated gas permeability in pure ionic liquids extrapolated from the dependence of gas permeability in ionic liquid membranes (see Figs. 7 and 8) on content of ionic liquid and experimentally obtained permeability in a pure IL attests that the transport mechanisms in IL and ILM are different. To better understand and describe the transport mechanism correctly it will be necessary to propose and made some additional experiments.

4. Conclusion The transport of carbon dioxide and methane in two ionic liquid membranes was studied. To obtain more information about the gas transport through ionic liquid membranes the gas transport through three types of materials was measured: a pure polymer, a pure ionic liquid and finally through ionic liquid membranes. Permeability of carbon dioxide and methane in two different ionic liquids [hmim][Tf2 N] and [emim][Tf2 N] was evaluated. Two different experimental sample arrangements – sandwich and support arrangements – were adopted to better estimate these values. The gas permeability in both ionic liquids is quite low especially methane permeability is on the limit of the measurement reliability. Interesting and unexpected results were obtained. Carbon dioxide permeability in ionic liquid Viton membranes for the samples with content of ionic liquid [hmim][Tf2 N] or [emim][Tf2 N] between 60 and 75 wt% is almost two orders higher than in a pure ionic liquid and three orders higher than in pure polymer Viton. Even if the concentration of the IL in membranes is very high (more than 75 wt%), the gas flux in ILM is much higher than in a pure ionic liquid; it means that the gas molecules do not permeate through the layer of the liquid. Therefore the mechanism of the transport in ionic liquid membranes has to be different from the transport mechanisms in an ionic liquid. One of the possibilities could be that the gas molecules move mainly on the surface of the IL which filled the space between the polymer chains and the main gas flow contribution is caused by surface diffusion. Transport properties of ionic liquid membranes are very interesting and the supplement measurements have to be done to better understand the mechanisms of the gas transport. The obtained results had shown that the ionic liquids exhibit good separation properties for system CO2 /CH4 . Ideal separation factor was ˛ ≈ 32 for [hmim][Tf2 N] and around 59 for [emim][Tf2 N]. The permeability of carbon dioxide is also higher for IL [emim][Tf2 N] around 1.4 × 10−15 mol m m−2 s−1 Pa−1 (average value) but still is quite low. From these results it follows that the gas flux through the supported liquid membranes is strongly limited. Conversely by appropriate preparation of ionic liquid membranes it possible to achieve the gas fluxes which many times overtop the fluxes through the pure IL and the pure polymer and at the same time conserve relatively good separation properties.

Acknowledgements The financial support of the Ministry of Education, Youth and Sports (ME 889) of the Czech Republic, of The Czech Science Foundation (Grant Nos. GA104/09/1165 and P106/10/J038) are gratefully acknowledged.

Nomenclature A J L p p∗i P R t T V2

surface area of membrane (m2 ) flux (mol m−2 s−1 ) membrane thickness (m) pressure (Pa) pressure between individual layers (Pa) permeability (mol m m−2 s−1 Pa−1 ) gas constant (J K−1 mol−1 ) time (s) temperature (K) volume of apparatus permeate side (m3 )

Greek letter ideal separation factor ˛ Subscripts 1, 2, 3 membrane layer in input out output

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