light gases separation

Accepted Manuscript Title: Preparation and characterization of novel Ionic liquid/Pebax membranes for efficient CO2 /light gases separation Authors: Ehsan Ghasemi Estahbanati, Mohammadreza Omidkhah Nasrin, Abtin Ebadi Amooghin PII: DOI: Reference:

S1226-086X(17)30090-4 http://dx.doi.org/doi:10.1016/j.jiec.2017.02.017 JIEC 3298

To appear in: Received date: Revised date: Accepted date:

3-12-2016 2-2-2017 19-2-2017

Please cite this article as: Ehsan Ghasemi Estahbanati, Mohammadreza Omidkhah Nasrin, Abtin Ebadi Amooghin, Preparation and characterization of novel Ionic liquid/Pebax membranes for efficient CO2/light gases separation, Journal of Industrial and Engineering Chemistry http://dx.doi.org/10.1016/j.jiec.2017.02.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Preparation and characterization of novel Ionic liquid/Pebax

membranes for efficient CO2/light gases separation

Ehsan Ghasemi Estahbanati1, Mohammadreza Omidkhah Nasrin,1, Abtin Ebadi Amooghin2

1

Faculty of Chemical Engineering, Tarbiat Modares University, P.O. Box 14115-143, Tehran, Iran

2

Department of Chemical Engineering, Faculty of Engineering, Arak University, Arak 38156-8-8349, Iran



Corresponding author. Tel./Fax: +98 21 82883334. E-mail Address: [email protected].

Highlights     

A novel Ionic liquid/Pebax membranes were fabricated for CO2/light gases separation. CO2 permeability increased about 73% for the membrane containing 50 wt.% IL. PEO segment of the polymer/anion part of the IL have good affinity with polar gases. CO2/N2 selectivity was considerably increased from 78.6 to 105.6 (about 34%). The fabricated membranes pass the CO2/CH4 and CO2/N2 Robeson upper limit.

Abstract In this study, the goal is to incorporate superior features of the Pebax 1657 copolymer, such as high mechanical resistance and exceptional gas permeability especially for polar gases, with the affinity of the 1-Butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) ionic liquid to CO2 gas, which results in increasing the permselectivity of the membranes for CO 2/light gases separation. Generally, the CO2 solubility in ILs increases with pressure increment, and temperature decrement. Therefore, gas permeation results were obtained on prepared membranes by using a gas separation set-up at temperature of 35°C and feed pressure range 2-10 bar. SEM, ATR and DSC analysis were carried out on different composition of the membranes and the results showed that adding the IL to the polymer make membranes more amorphous and less crystalline which lead to increase permeability for all tested gases. In addition, due to the high affinity of CO2 in both polymer and IL, both CO 2 permeability and selectivity increased simultaneously with increasing IL content. This is confirmed by gas permeation results, where at 35°C and 10 bar, the CO2 permeability increased from 110 Barrer for neat Pebax to 190 Barrer in the blended membrane containing 50 wt.% IL (about 73%). The related CO2/CH4 and CO2/N2 selectivities were increased from 20.8 to 24.4 (about 17%) and from 78.6 to 105.6 (about 34%), respectively. Thus, these

types of membranes are promising to be utilized in gas separation processes in industries for CO2 separation in order to postpone the global warming, which is nowadays the biggest threat to the universe.

Key words: Gas separation, CO2 permselectivity, Pebax1657 copolymer, 1-Butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) ionic liquid.

1. Introduction Membranes in separation processes have been intrigued in recent years. The chief reason for high interests in membrane technology is its low relative energy consumption in comparison with common separation processes such as distillation, drying, extraction, adsorption, etc [1-4]. Besides this, the processability and low capital and operating costs of the membranes prompt the researchers to investigate more and more about implementation and extension of the membranes in industrial units in order to facilitate separation processes [5-9]. Permeability is a measure of productivity and selectivity is a measure of separation efficiency in gas separation processes. The gases are separated due to their different diffusivity and solubility in the polymeric membranes. The investigations about selective transports of the gases through polymeric membranes have been become an interesting area for researchers in the last two decades [10-14]. In recent years polymeric membranes have been considered as an attractive and practical way in gas separation applications, especially rubbery polymers which have received considerable attention due to their high chains mobility as a result of flexible chains, which prompts chain segments to rotate around the main chain bonds more easily [5, 15]. Block copolymers are one of the most interesting types of polymers in gas separation applications. These polymers consist of amide and ethylene oxide groups in their structures, which result in two distinct areas in their chains. In other words, amide groups underlie hard segment and ethylene oxide groups underlie

soft segment in block copolymer structures [16]. Among block copolymers, poly (amide-bethylene oxide) or Pebax has been received remarkable attraction in membranes preparation, duo to its mechanical and chemical properties [17-19]. Pebax copolymers consist of two different phases, the amorphous polyethylene oxide (PEO) phase and the crystalline polyamide (PA) phase. In the first part, gas permeability is higher than that of its selectivity, in spite of the second part in which selectivity is more dominant. In another word, block copolymer like Pebax has both soft and rigid segments in its structure. The former enhances gas permeability especially for polar gases, and provides good adhesion between polymer chains and particles, and the latter causes high mechanical resistance [20, 21]. Among several grades of Pebax polymers, which are commercially available, Pebax 1657 is the most attractive one due to its high potential in polar gas separation such as CO2 from nonpolar light gases [22]. Bondar et al. [23] investigated the permeation properties of different gases in Pebax block copolymer, which consists of poly tetramethylene oxide (PTMEO) or polyethylene oxide (PEO) or as the rubbery phase and nylon-6 (PA6) or nylon-12 (PA12) as the hard phase. In their studies, they observed high selectivities for polar/nonpolar and quadrupolar/nonpolar binary gas mixtures in addition to the high permeabilities for CO2 gas. The results also revealed that as the amounts of polyether increases, the gas permeability increases due to existence of less polar parts, such as polytetramethyleneoxide (PTMEO) and nylon-12 (PA12) in the copolymers’ structures, in comparison with those containing the more polar poly(ethylene oxide)(PEO) and nylon-6 (PA6) parts. Kim et al. [22] showed the Pebax copolymer has considerable permselectivity for polar/nonpolar binary gas mixtures. They found that the permeability of small and nonpolar gases, such as H2, N2 and O2 decrease with increasing the molecular volume of the gases.

Moreover, they revealed that Pebax membranes show high permeability for the polarizable and larger gases, like CO2 due to the existence of the polyether segment in the copolymer. Since Pebax block copolymer shows interesting and promising features, such as flexibility and processability besides its superior CO2 affinity, to be utilized in industrial units, researchers have investigated different methods to enhance the Pebax membranes performance for CO2 captures. Ionic liquids (IL) are promising alternatives to enhance polymeric membranes properties, which leads to higher permeability of different gases, such as CO2, N2, O2, CH4, H2 etc [24-26]. Mass transfer generally is much faster in the IL than in polymeric membranes, which results in great separation properties. Furthermore, the separation properties of the IL can be tuned by selecting appreciate anions and cations, which are compatible with functional groups in their structures [27, 28]. According to this, addition of different IL to the polymeric membranes has been intrigued recently [29-31]. The high CO2 solubility in ILs in comparison with other gases like N2 and CH4 has been attracted some researchers to investigate ILs’ capability for CO2 capture [26, 32-34]. The good CO2 solubility in ILs is the result of the asymmetrical combination of the anion and cation. Therefore, as the incompatibility of the ionic parts of the ILs increases, the higher solubility could be obtained in the ILs. Generally, the CO2 solubility in ILs increases with pressure increment, and temperature decrement. The physical adsorption mechanism is a result of the interaction between CO2 molecules and ILs in which the gas molecules occupy the free space in the ILs structure by means of a high quadrupole and Van der Waals forces. In other words, the Lewis acid-base interactions between the anion part of the IL and CO2 play an important role in CO2 solubility in ILs. Since ILs have intrinsic acid-base properties, addition of acidic/basic functions, such as halide and carbonic groups as acidic functions, and also

amino/fluorine groups as basic functions, could enhance these interactions and as a result increase the CO2 solubility [35, 36]. The anion and cation effects on CO2 solubility in ILs have interested some attentions in recent years [28]. Some investigations revealed that on the one hand, CO2-philic groups in the anion part, such as fluorine and carbonyl could increase CO2 capture. on the other hand, they showed that the more alkyl chains exist in the cation part, the high CO2 solubility observes in the IL, especially at high pressures [37]. Imidazolium-based ILs which has high CO2 solubility in comparison with other ones, have been received remarkable attentions for their high diffusivities, solubilities and permeabilities in membrane gas separation [38]. In fact, in these types of ILs, on one side, there is an acidic hydrogen attached to the 2-Carbon of the cation part, which affects the CO2 solubility slightly via hydrogen bonds. On the other side, the CO2 solubility depends more on the nature of the anion part of the IL, due to fluoroalkyl group. Therefore, anion part plays the main role in CO2 solubility in alkylimidazolium-based ILs. The interaction between CO2 and anion part of the IL, is a Lewis acid-base type, where this gas acts as a Lewis base and the anion part acts as a Lewis acid. The cation interaction with CO2 which is shown in Figure 1 has relatively little effect and plays a second role [39]. In other words, addition of the IL to polymers leads to increase the permselectivity of the polymeric membranes due to the interaction and affinity of the anion parts of the IL with CO2 molecules over light gases [39, 40]. Generally, in IL’s both solubility and diffusivity are effective in CO2 permeation; however, the solubility effect is more dominant. Diffusion role depends on viscosity of the IL’s, where based on double film theory, mass transfer rate and as a result gas diffusion increase as the viscosity decrease [41]. Solubility role depends on Henry’s constant of the gases, where higher

Henry’s constant results in less gas solubility. The viscosities and Henry’s constants of four imidazolium based ionic liquids 1-Ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]), 1-Butyl-3-methylimidazolium tetrafluoroborate

([hmim][BF4])

tetrafluoroborate and

([bmim][BF4]),

1-Butyl-3-methylimidazolium

1-Methylimidazolium hexafluorophosphate

([bmim][PF 6]) are reported in Table 1 [40, 42]. A comparison between [bmim][BF4] and [bmim][PF6] reveals that although CO2 is more soluble in [bmim][PF6], but [bmim][BF4] shows higher CO2 permeability, due to high viscosity of [bmim][PF6] and dominant role of diffusion in its permeability. Jindaratsamee et al. [28] investigated the CO2 permeability through six imidazolium based ionic liquid membranes and showed the membrane of [bmim][PF6] exhibit the lowest permeability. In addition, Neves et al. [43] compared the separation performance of [BF4] and [PF6] ions by investigating among five room temperature ionic liquid based on 1-n-alkyl-3-methylimidazolium cation and revealed that [BF4] exhibit much higher CO2 selectivity in comparison with [PF6]. They obtained the highest selectivities for [bmim][BF4], which were 35 and 11, for the cases of CO2/N2 and CO2/H2, respectively. This conclusion was also confirmed by Jiang et al. [25], where the results showed that the selectivities of [bmim][BF4] was 17 for CO2/N2 and 10 for CO2/CH4, which are higher than selectivities of [emim][BF4] and [hmim][BF4] that reported 14 and 13 for CO2/N2, and 7 and 9 for CO2/CH4, respectively. Rabiee et al. [5] investigated the effect of [Emim][BF4] (1-ethyl-3-methylimidazolium tetrafluoroborate) ionic liquid on separation and transport properties of Pebax-1657. The ionic liquid individually showed promising separation factor for CO2/light gases and thereby they added it to the Pebax in order to increase separation properties of the polymeric membrane. The

addition of [Emim][BF4] to Pebax leads to higher permeability of the gases due to amorphous structure of the prepared membranes. The results showed significant increment in permeation of all the gases, especially for CO2. Bernardo et al. [24] studied the effects of a room temperature ionic liquid (RTIL) on two block copolymers for gas separation properties of the membranes, which made of Pebax 1657 and Pebax 2533, containing 1-butyl-3-methylimidazolium trifluoromethanesulfonate [(BMIM)(CF3SO3)] from 20 to 80 wt.%. The analysis revealed that with increasing IL content in Pebax1657, the melting point of the polyamide (PA) phase decreased and the crystallinity of the polyether (PE) phase completely disappeared. On the other hand, in contrast with the good compatibility and mixing of the IL and the Pebax 1657, there was an effective phase separation between the IL and the crystalline phase of the Pebax 2533, which results in no significant changes in the permeability and selectivity of the polymer. In contrary, the gas permeability of Pebax 1657 showed a remarkable increase by adding the IL, also the selectivity was slightly decreased. However, the highest selectivity was seen for CO2/N2 gas mixtures, which decreases from 60 to 40, but on the other hand, CO2 permeability increased fourfold by adding the IL. In this study, the goal is to incorporate the superior features of the Pebax 1657 polymeric membranes, such as flexibility and processability in gas separation, with the affinity of the 1Butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) ionic liquid to CO2 gas. Then, fabricate new blended membranes in order to facilitate gas transport properties of the neat membranes, results in increasing the permselectivity of the membranes for CO2/light gases separation. The effect of addition of IL to the Pebax 1657 membrane on its gas transport properties were investigated by carrying out CO2, CH4 and N2 gases permeation tests in different operating conditions. In addition, the exact diffusion and solubility coefficients were measured.

Finally, the morphological study was inspected by means of characterization tests in order to examine the feasibility of utilizing the high performance membranes.

2. Experimental 2.1. Materials 1-butanol (CAS number=71-36-3) with the purity of more than 99.5% was purchased from Merck, Germany, to use as a solvent. Pebax 1657 (60 wt.% polyethylene oxide (PEO) and 40 wt.% polyamide (PA)) in form of elliptic pellets was supplied from Arkema Inc, France. 1Butyl-3-methylimidazolium

tetrafluoroborate

([BMIM][BF4])

ionic

liquid

(CAS

number:174501-6-6) with the purity of more than 97.0%, was provided from Aldrich. N2, CH4 and CO2 gases with the purity above 99.99% were provided by technical gas services Inc, Iran. All the mentioned materials used as received without any further treatments. In addition, the most important properties of the materials are summarized in Tables 2-5. Some PTFE petri dishes used to prepare the membranes. An oven, a vacuum oven (fuzzy control system) and a heater-stirrer (feedback control) from Wisd laboratory instruments, and a vacuum pump (45-56(liter/min)), were used to fabricate the membranes. A pressure sensor (trafag, 0-.1 bar) was used to measure the permeate pressures. In addition, a micrometer (CMC) was used to measure the membranes thickness.

2.2. Membrane preparation In order to fabricate defect-free membranes all parameters should be considered. Polymer concentration in the polymeric solution is the most important factor, which leads to good dispersion of the IL without any sedimentations in the membranes. On the other hand, the high concentration of the polymer in the solution leads to having a more viscous solution, which

prevents from having a clear homogenous polymeric solution and therefore the membranes become thicker and less permeable. In this study, the polymer pellets were firstly kept in a vacuum oven at 60°C for 6 h, in order to assure that there is not any moisture in the polymer. Then, 3 wt.% polymeric solution was prepared by dissolving Pebax1657 in 1-butanol, heating at 100-110°C and magnetically stirring for 24 h in a closed Duran laboratory bottles with blue PP screw cap and pouring ring to avoid solvent evaporation until a clear solution was obtained. After that, the IL was added to the polymeric solution at 100°C and it dissolved instantaneously. The solution was further stirred at 100°C for another one hour to ensure that the solution was completely homogeneous. Different amounts of the IL were then added to the solution in the range of 0-50 wt.% based on the polymer. Afterwards, the prepared solution was filtered by a steel mesh and poured into the PTFE petri dishes. Self-supported dense membranes were prepared by controlled solvent evaporation in the petri dishes at ambient temperature for 48 h. In order to complete evaporation of the residual solvent, the membranes were dried in the vacuum oven at 60°C overnight. At last, the prepared membranes cooled at ambient temperature and were put in a desiccator to be away from humidity and other vapors until they have been used for gas permeation and characterization tests. The schematic procedure of membranes preparation is reported in Figure 2.

2.3. Membrane characterization Before conducting any morphological and characterization tests, all the membrane samples were dried again at 100°C in a vacuum oven overnight to ensure that there is no moisture in the samples. After that, the samples investigate by means of different analysis as follows:

2.3.1. Fourier transform infrared (FTIR) All prepared membranes and the IL were characterized by Fourier transform infraredattenuated total reflectance (FTIR-ATR) analysis in the range of 400-4000 cm-1 through an apparatus model of Parkin-Elmer Spectrum Frontier(Parkin-Elmer Instruments, 10.03.06 version, Norwalk, CT, USA) to investigate diverse bonds and transitions in the blended and pure polymeric membranes.

2.3.2. Scanning electron microscope (SEM) The cross section morphologies of the membranes were investigated by scanning electron microscopy (SEM) apparatus (KYKY-EMM3200, KYKYTechnology Development Ltd, Beijing, China), after fracturing the membranes in liquid nitrogen and super-coating with gold through a BAL-TEC (SCD005) sputter coater (BAL-TEC AG, Balzers, Liechtenstien) under argon flow.

2.3.3. Differential scanning calorimeter (DSC) Differential scanning calorimeter (DSC) analysis was conducted on all membranes and the IL by a NETZSCH 200F3 apparatus in order to evaluate the glass transition temperatures, melting points and crystallinity of the prepared membranes, and investigate the interaction between the Pebax 1657 polymer and the IL. The temperature range was from -100 to 250 °C with a heating rate of 10°C /min. It should be mentioned that all samples were put in aluminum pans and heated under the nitrogen atmosphere.

2.3.4. Mechanical properties

Mechanical strength of the prepared membranes were investigated by tensile tests by means of a Zwick/Roell Z100 apparatus at room temperature by 50 kgf tension load with speed of 12.5 mm/min and length of 2 cm (grip to grip). The tests for each membrane were repeated three times for reproducibility.

2.4. Gas permeation experiments Gas permeation properties of fabricated membranes were examined by a gas separation set-up in which, pressure and temperature can be controlled. In addition, as it can be seen in Figure 3 the membrane cell type is cross flow with 11.34 cm2 effective areas for membrane surface and made from two detachable parts. Moreover, two concentric O-ring designed in order to seal the cell from gas leakage. Gas permeation tests carried out on the prepared membranes by using the set-up at temperature of 35°C and feed pressures in the range of 2-10 bar. The tests for N2 and CH4 were carried out first due to possible plasticization effect of the CO2 on the membranes. The permeate side pressure was measured in terms of time by a pressure transducer in the constant volume part of the system with a volume of 110 (cm3), and the computer monitored and saved the data. The permeation coefficients (P) of the membranes were measured by means of the constant volume method as follows: P

273.15  10 10 Vl  dp    760AT((P0  76)/14.7)  dt 

(1)

Where V, L, A, T, P0 are the volume of permeate (cm3), membrane thickness (cm), the effective surface area of the membrane (cm2), the operating temperature (K), feed pressure (psia), respectively. (

dp ) is pressure increment with time, which has the unit of (mmHg/s), and as a dt

result the unit of the permeation coefficient (P) obtained from Eq.1, is (1Barrer=1×10-10 cm3 (STP) cm/cm2 s cmHg). After calculation of the permeation coefficients, the diffusion coefficients (D) of the gases were determined by using time lag method as follows: D

l2

(2)

6θ

Where, l and θ are the membrane thickness and the time lag, respectively. Moreover, the solubility coefficients (S) were obtained via the Eq.3 by assuming that the gas transport mechanism is solution-diffusion: S

P D

(3)

In addition, selectivity is a criterion of membranes ability to separate mixed gases. In fact, the permeabilities ratio of two different gases such as A and B is defined as selectivity of A over B, and since permeability is the product of diffusivity and solubility, this can be written as follows: α

A B

(P P )(D D )( S S ) A B A B A B

(4)

3. Results and discussion 3.1. SEM One of the best ways to study the structure and morphology of the blended membrane is taking SEM images from its cross section. As it can be seen in Figures 4a and b, the structural body of the neat membrane is more uniform and regular than the blended membranes. The results show that by adding the IL as a plasticizer into the polymer matrix, the membranes structure become more irregular and convoluted as illustrated in Figures 4c and d. The DCS analysis, which would be discussed with details in the following sections, showed that the IL

addition up to 50 wt.% reduces the crystallinity of the copolymer by eliminating intermolecular hydrogen bonding and make the membrane structure more amorphous, which is now confirmed by SEM images of the blend membranes. In addition, it can be concluded that surface convolutions of the membranes were intensified as the IL content increases (see Figures 4e-g). Less crystalline and more amorphous structure of a membrane, leads to higher diffusion and solubility coefficients, which could enhance its permselectivity. The obtained results in this work are accordant with other researches in which a low molecular weight component was added to polymeric membranes [5, 17, 24, 44-49].

3.2. FTIR The FTIR analysis of the membranes and [BMIM][BF4] ionic liquid in the region of 5004000 cm-1 are depicted in Figure 5. As it can be seen for pure Pebax, there are six sharp peaks at 1097, 1542, 1636, 1732, 2862 and 3297 cm-1. The peaks are attributed to the ether group (-C-OC-), amine group in PA phase, C=O group in PEO phase, carbonyl group in PA phase, C-H functionalities, and N-H bond in PA phase of the copolymer, respectively. These results are in conformity with other similar works [44, 50-55]. For the [BMIM][BF4] ionic liquid, as shown, there are sharp peaks at 755, 850, 1062 and 1468, which are related to B-F stretching, C-N stretching vibration and C-H stretching bands in the IL ring, methyl group and C=N stretching, respectively. The peak at 1574 shows C=C stretching and indicate that the π system of the aromatic imidazole ring interacts with the electron rich oxygen atoms such as ether segment of the copolymer [56]. The alkyl chains such as butyl chain that attached to imidazolium ring, represented by 2876-2963 peak ranges which also show aliphatic asymmetric and symmetric stretching vibration(C-H) [57]. The two peaks at 3115 and 3158 are attributed to C-H vibration of imidazolium ring which is characteristic for

hydrogen bonds in C-H…F interactions and quaternary amine salt formation with tetrafluoroborate [58]. In addition, the results, which discussed above are accordance with literature [53, 59-62]. It can be concluded from the Figure 5 that the peak at 1097, which shows ether group (-C-OC-) moves to the right and its frequency decrease as the IL content increased. The Shifting of this peak shows hydrogen bonding between amine in the IL and oxygen in the polyamide segment of the copolymer [63, 64]. The frequency of the peak at 1542, 1636 and 1732 which shows amine group in PA phase, C=O group in PEO phase and carbonyl group in PA phase, is constant with increasing IL in Pebax structure. However, the peak at 1732 became sharper with increasing IL content, which shows carbonyl group tendency in the copolymer to form hydrogen bonding with the IL. The peak at 2862, which shows C-H functionalities moves left very slightly and its frequency is nearly constant by adding more IL. In addition, the frequency of the peak 3297 that is attributed to N-H band in PA phase of the copolymer is unchanged in different amounts of the IL. The peak at 755 which shows C-H stretching bands in cation ring of the ionic liquid with Fluor atoms in the anion part, move towards higher frequencies in the blended membranes and confirms the disappearance of hydrogen bonding in IL as a result of polymer addition [65]. Also As it can be seen, In All of the frequencies which discussed above, the related peaks become more sharp with increasing IL content which shows Pebax 1657 tendency to form new bonds with [BMIM][BF4].

3.3. DSC The DSC analysis carried out on the membranes to investigate the IL effects on membrane structure and thermal properties such as changing in glass transition tempereature (T g) and

melting temperature (Tm) of the prepared membranes. As observed in Figure 6, for pure Pebax 1657, unlike the glass transition tempereature (Tg) of the hard phase of the copolymer (PA) which can not be detected by the DSC, there is an obvious turning point around -50.9 °C which is related to the soft segment of the copolymer (PEO). Moreover, there are two peaks around 20 and 195 °C, which are the melting points of the PEO and PA phases in the micro-phase copolymer structure, which are in good agreement with introduced values by supplier and the literature [5, 17, 22-24, 45, 46, 66, 67]. For [BMIM][BF4] ionic liquid, the curve shows the Tg and the melting point about -90 and -72, respectively [62, 68]. It can be concluded that there is only one Tg for each blending membrane which is reduced and lied between the T g’s of pure polymer and pure ionic liquid which confirms the good miscibility and homogeneous mixing between them. Therefore, the ionic liquid addition to the polymer matrix reduces the T g of the blended membranes, which can be justified by the mixing rule (Eq.5), Fox equation (Eq.6) and Jenckel and Heusch’s derivation (Eq.7) as follows [69, 70]: Tg  w 1.Tg  w 2 .Tg 1 2

(5)

1 Tg  w1 Tg1  w 2 Tg 2

(6)



Tg  w 1.Tg  w 2 .Tg  0.27 w 1 w 2 Tg 2  Tg1 1 2



(7)

where w1 and w2 are mass contents of the polymer and IL, respectively. These equations estimate the reduction trend for Tg of the blended membranes which is in good agreement with the experimental results as shown in Figure 7. The reduction in T g’s shows that the IL as a plasticizer increases chain mobility of the membranes by locating between polymer chains, which leads to higher fractional free volume (FFV) of the membranes. As a result, the membranes become more amorphous and less

crystalline. Therefore, the crystallinity of the membranes is an important parameter and must be calculated as follows: X

crystallinity



ΔH m  100 ΔH m0

(8)

In this equation, ΔH m is heat of melting of crystals that can be obtained by integration of the area under the melting peaks and ΔH m0 is heat of melting of the polymer when it is 100% crystalline. The value of ΔH m0 of the PEO and PA segments of the Pebax 1657 is reported 166.4 j/g and 230 j/g, respectively [24, 30, 46, 66]. ΔH m is calculated by dividing heat of melting obtained from peak areas to weight of each segment in Pebax1657 polymeric membranes. By using Eq.8, the crystallinities of the two phases in prepared membranes were calculated and reported in Table 6. The total crystallinity of the membranes calculated as follows: X

total, crystallinity

 0 .6  X

PEO, crystallinity

 0 .4  X

PA, crystallinity

(9)

It can be concluded from the table that as the IL content increases, the crystallinity of the membranes decreases due to the positive interactions between polymer and IL, which prevents from forming hydrogen bonds in the polyamide segment. Hydrogen bonding forces are responsible for intermolecular bonding forces in polymers, where crystallinity depends on magnitude of such intermolecular forces beside the structural features of the polymers [71]. Therefore, shifting C-H peak of blended membranes to lower frequencies in comparison with neat membrane in FTIR-ATR analysis, proves the disappearance of hydrogen bonding, which also results in less crystallinity in the membranes [17, 44]. Crystals in the membranes structure act like crosslinking and therefore the less crystallinity in the membranes, results in more FFV and amorphous structure in the polymeric matrix, which leads to higher diffusion coefficients of penetrants. Moreover, as shown in Figure 6 and Table 6, on one hand, the melting point of the

PA phase of the Pebax reduces with increasing of IL amount in the membranes, which confirms the reduction in crystallinity of the membranes. On the other hand, the melting point peak of the PEO phase are weakening as the IL content increase due to miscibility of the soft segment of the copolymer with [BMIM][BF4]. In fact, the IL acts as a solvent for PEO phase and results in having weaker and smaller crystals [5, 24, 46].

3.4. Tensile test Generally, IL as a low molecular weight additive could decrease mechanical properties of the prepared membranes which is reported in similar works [5, 24]. The results of five different membranes compositions, which used in this study were reported in Table 7, and are in accordance with previous analyses, which revealed that IL addition, decrease crystallinity and make the membranes more amorphous and soft, which could enhanced gas transport properties of the membranes. It can be seen that by adding [BMIM][BF4], yield stress of the blend membranes decrease from 530 KPa(for neat membrane) to 153.8 for Pebax/50%IL membrane. In addition break strain and Young’s modulus of the membrane encountered considerable decrement compared to the neat membrane, which shows that membranes with higher IL contents may have not enough strength at high operating pressures. However, adding more IL may improve gas permeability of the membranes.

3.5. Gas permeation As discussed before, gas transport properties through dense film membranes can be evaluated by the solution-diffusion mechanism. Diffusion and solubility coefficients of CO2, CH4 and N2 gases in the fabricated membranes were calculated, using Eqs. 1-3 at the temperature of 35°C

and pressures from 2 to 10 bar. The solubility in the blended membranes depends on the chemical interaction between gases and polar parts of the copolymer. Also, Pebax grades are known as copolymers in which solubility is controlled the transport properties [46, 72]. The results of the solubility coefficients of the gases are shown in Figure 8. As it can be seen in Figure 8a, the solubility coefficients of CO2 increase with pressure. This increment as mentioned before is duo to the affinity of the PEO part of the Pebax 1657 to polar/quadrupolar gases especially CO2 [16, 18, 21, 73]. On the other hand, the solubility coefficients for CH4 and N2 gases are nearly unchanged in comparison with CO2 (see Figure 8b and c). However, since the condensability of N2 is not very significant, its solubility coefficients slightly decrease with pressure, due to higher vapor pressure and less interaction of N2 especially with the PEO part of the copolymer, because of no difference in electronegativity between nitrogen’s bonds, compared to CH4 and CO2. For the case of CH4, although its condensability is ignorable like N2, its solubility coefficients remain constant with pressure, because this gas is more soluble than N2. Unlike the CH4 and N2, in which the solubility coefficients slightly decrease with the IL increase, the membranes with higher IL content have higher CO2 solubility coefficients. Table 8 presents the dissolved mass of the gases in different amounts of IL content. As can be seen, CO2 dissolved mass in membranes are increased as the IL content increased. Indeed, as the IL content increased to 50 wt.%, the CO2 dissolved mass was significantly increased (about 187%). This is due to the affinity between [BMIM][BF4] and CO2 [74, 75]. Furthermore, the solubility selectivities of CO2 over CH4 and N2 are shown in Figure 9a and b, which reveals that with increasing in IL content, the solubility selectivity increases for CO2/CH4 and especially for CO2/N2 binary gases.

The results of the DSC and SEM confirm that by adding the [BMIM][BF4] ionic liquid as a low molecular weight component to the Pebax, the IL located between polymeric chains, reduces crystallinity and increases FFV of the membranes which result in more amorphous and flexible membranes with higher chain mobility and lead to higher diffusion for all tested gases. As it can be seen in Figure 10a, the calculated diffusion coefficients for each three gases increase with IL increment [5, 24]. In addition, as it is shown in Figure 10b, diffusion selectivity of CO2/CH4 decreases, unlike for CO2/N2 in which diffusion selectivity is nearly unchanged with the increment of the IL weight ratio. A comparison between Figures 9b and 10b revealed that in the case of CO2/CH4, as the IL content increases, the diffusion selectivity decreases, while the solubility selectivity increases. In the case of CO2/N2, as the IL content increases, the solubility selectivity increases, while the diffusion selectivity is nearly unchanged. These results confirms that by adding the IL to the polymer, in one hand the blended membranes become less size selective for the gases, and in the other hand the effect of solubility selectivity becomes more dominant as the IL content increase.

3.6. Effect of pressure Effect of operating pressure on gas permeation properties through prepared membranes was investigated from 2 to 10 bar at 35°C. Generally, there are two driving forces in gas permeation through membranes. Operating pressure and penetrant concentration both could affect on gas transport properties of the membranes. On one side, pressure increment acts as a driving force for gas diffusion and enhances gas permeation properties through the membranes. On the other side, it can compact chains of the polymer and therefore FFV of the membrane will reduce which results reduction in gas diffusion through it. Low penetrant concentration at the back of the

membrane could enhance gas permeation through it, while in the front of the membrane it could make reverse effect. In practice, concentration of gases in the membranes increases with pressure increment, which leads to CO2-induced plasticization effects on the polymeric membranes, and would has bad effects on transport properties of other gases and takes down the selectivity. Some investigations show increasing trends for gas permeation with pressure in polymeric membranes for condensable gases such as CO2, and steady or decreasing trend for non-soluble or other light gases like N2 and CH4 [5, 45, 76]. In this work, Pebax1657 shows relatively high permeability for CO2, which is due to its high condensability and quadrupole-dipole interactions between the gas molecules and PEO part of the polymer. The permeabilities of CO2, CH4 and N2 were measured by Eq.1, in different operating pressures and the results are reported in Figure 11, which are an average value of gas permeation in both crystalline and amorphous phases of the membranes. In addition, the selectivities of CO2 over CH4 and N2 which calculated by Eq.4, are reported in Figure 12. The results in Figures 11 and 12 show that pressure increment increases the permeability of CO2 through the membranes, while it has no considerable effect on the permeability of N2 and CH4, which results in considerable increasing in selectivity. Moreover, at the highest IL content CO2 permeability is about two times and both CO2/CH4 and CO2/N2 selectivities are about 1.5 times higher than the neat Pebax1657. It can be concluded from Figure 11a that in one hand, the higher slope of the CO2 permeability with pressure at less IL contents, reveals that in these membranes driving force and plasticization effects as results of pressure increment are more dominant on gas transport properties than chain compactness and FFV reduction. On the other hand, for membranes with higher IL content, however, the crystallinity of these membranes, decrease and they become more amorphous, but

the effect of chain compactness is more effective and the permeability goes up more slightly with pressure increment in the membranes with higher IL content. Additionally, as explained before according to the Figures 9a and 10a both solubility and diffusion coefficients of CO2 increase with IL ratio increment. But in the cases of N2 and CH4 although solubility reduces slightly as the IL content increase, but diffusion of both gases goes up significantly with IL increment. As a result based on Eq.3, the permeabilities of three gases must increase with increasing of IL ratio in the membranes, which can be seen in Figure 11. The results of this study show that both permeability and selectivity of CO2 increase with increasing the IL content. The permeability of the blended membrane which containing 50 wt.% of the IL at 35°C and 10 bar, is 190 Barrer, while for the pure membrane in the same conditions is about 110 Barrer and therefore the permeability encountered by 73% increase. The related CO2/CH4 and CO2/N2 selectivities were increased from 20.8 to 24.4 (about 17%) and from 78.6 to 105.6 (about 34%), respectively. Some investigations show that there is a vice versa trend between permeability and selectivity for gas separation in polymeric membranes, and it is optional to use the privilege of permeability or selectivity in consideration of the applications that we need to be practical in gas separation processes units [77]. Moreover, it can be optimized to achieve an appropriate trade of between permeability and selectivity in applications in which both permeability and selectivity are important and in favor. But in the present work, as shown and discussed, both permeability and selectivity of CO2 have increased remarkably by incorporating [BMIM][BF4] ionic liquid into Pebax 1657. Therefore, as it can be seen in Figure 13, these types of membranes pass the Robeson upper bound for the case of CO2/N2, and are in threshold of passing it for the CO2/CH4 case. As a result, these membranes are so promising to be utilized in gas separation processes in

petrochemical and refinery industries to separate CO2 from flue gas, natural gas, biogas and atmosphere in an efficient way, because CO2 capture is a vital issue in order to postpone the global warming that is nowadays the biggest threat to the universe.

Conclusion The Pebax 1657 copolymer which consists of the amorphous PEO as a soft segment and the crystalline PA segment as a hard segment was incorporated with an imidazolium-based room temperature ionic liquid 1-Butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) to utilize the affinity of the PEO segment of the polymer and anion part of the IL to polar gases. Therefore, new types of membranes were fabricated in order to separate CO2 from N2 and CH4. SEM, ATR and DSC analysis carried out on different composition of the membranes and the analyses revealed that adding the IL to the polymer make the membranes more amorphous and less crystalline by increasing polymer tendency to form new bonds and homogeneous mixture with the IL, which leads to increase permeability for all tested gases. Moreover, due to the high affinity of CO2 in both polymer and IL in comparison with N2 and CH4, both permeability and selectivity of the CO2 increase simultaneously as the IL content increase. The results were confirmed that these types of membranes are good candidates to be practical in industries and are promising subject for further investigations. The results show that the permeability of CO2 in the blended membrane with highest IL content encountered 73 % increase at pressure of 10 bar and temperature of 35°C, compared to the neat Pebax. In addition, CO2/CH4 and CO2/N2 selectivities had 17% and 34% increase, respectively, in the same conditions and therefore the prepared membranes pass the Robeson upper bound.

Acknowledgment This study was supported financially by National Iranian Gas Company (Tehran, Iran). Also, we would like to thank Chemical Engineering Department of Tarbiat Modares Unversity (Tehran, Iran) for their financial and facility supplies.

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Figure 1: Chemical interaction of CO 2 with 1-Butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) ionic liquid.

Figure 2. Schematic diagram of the membranes preparation procedure.

Figure 3. Schematic diagram of gas permeation set-up.

Figure 4. SEM images of (a) Pebax 1657 Cross-section, (b) Pebax 1657 Cross-section (higher magnification), (c) Pebax/IL (20 wt.% ) Cross-section, (d and e) Pebax/IL (20 wt.% ) Cross-section (higher magnifications), (f) Pebax/IL (50 wt.% ) Cross-section, (g and h) Pebax/IL (50 wt.% ) Cross-section (higher magnifications).

Figure 5. FTIR-ATR analysis of the neat polymer, pure ionic liquid and blended membranes.

Figure 6. DSC analysis of the neat polymer, pure ionic liquid and blended membranes.

Figure 7. Comparison between estimated Tg’s of the membranes with experimental values.

Figure 8. Solubility coefficients of the neat and blended membranes in different operating pressures for (a) CO2, (b) N2 and (c) CH4 gases.

Figure 9. (a) Solubility coefficients of the gases in different amounts of IL content (b) Solubility selectivity of CO2 over light gases (P=10 bar).

Figure 10. (a) Diffusion coefficients of the gases in different amount of IL content (b) Diffusion selectivity of CO2 over light gases (P=10 bar).

Figure 11. Effect of operating pressure on gas permeabilities for (a) CO2, (b) N2 and (c) CH4 gases.

Figure 12. Effect of operating pressure on selectivities of CO2 over (a) N2 and (b) CH4.

Figure 13. (a) CO 2/N2 selectivity and ( b) CO 2/CH4 selectivity versus CO2 permeability of the neat and blended membranes in comparison with Robeson upper bound at pressure of 10 bar and temperature of 35°C [75].

Table 1. Properties of four imidazolium based ionic liquids. Molecular weight

Viscosity

(g/mol)

(mPa.s)

[emim][BF4 ]

197.97

43

67.2

[bmim][BF4]

226.02

219

58.8

[hmim][BF4]

254.08

246

51.5

[bmim][PF6]

284.19

450

53.4

Ionic liquid

Henry’s constant for CO2 (*10-2/kPa)

Table 2. 1-butanol properties (Merck). properties

content

unit

Purity(GC)

>99.5

%

Boiling point(1013hPa)

116-119

°C

Density(20°C)

0.8090-0.8120

g/cm3

Molar mass

74.12

g/mol

Water

 0.1

%

Melting point

-89

°C

Flash point

34

°C

Ignition temperature

340

°C

Table 3. Pebax 1657 properties (Arkema). properties

content

unit

Density(20°C)

1.14

g/cm3

Melting point(10°C/min)

204

°C

Flexural modulus

80

MPa

Glass transition temperature(10°C/min)

-40

°C

Water absorption

120

%

Humidity absorption

4.5

%

Table 4. ([BMIM][BF4]) properties (Aldrich). properties

content

unit

Purity(GC)

 97.0

%

Density(20°C)

1.21

g/cm3

Molar mass

226.02

g/mol

Water

1

%

Melting point

-71.0

°C

Flash point

288

°C

Table 5. Structure and specifications of the materials. Name

Chemical formula

Abbreviation

1-butanol

CH3(CH2)3OH

1BTOH

Chemical structure

Molar mass (g/mol)

74.12

X=PA: polyamide 6 Y=PEO: poly(amide-b-ethylene oxide)

Pebax 1657

poly(ethylene oxide) x/y=40/60

1-butyl3methylimidazolium tetrafluoroborate

C8H15BF4N2

[bemim][BF4]

226.02

Table 6. Thermal properties of neat and blended membranes (DSC analysis). Sample

weight (mg)

Tg (°C)

Tm,PA (°C)

∆Hf,PEO (j/g)

∆Hf,PA (j/g)

XPEO (%)

XPEA (%)

Xtatal (%)

neat Pebax

5.5

-50

195.61

22.64

22.99

22.67

24.98

23.60

Pebax/5%IL

5.7

-51.8

184.09

22.4

22.93

22.43

24.92

23.43

Pebax/20%IL

5.4

-56

171.92

18.98

18.06

19.01

19.63

19.25

Pebax/35%IL

5.8

-64.9

159.27

7.4

7.813

7.41

8.49

7.84

Pebax/50%IL

5.2

-73.6

147.15

0.49

0.91

0.49

0.98

0.69

55

Table 7. Tensile test results of the neat and blend membranes. Sample

Yield Stress (KPa)

Break Strain (%)

Young Modulus (MPa)

neat Pebax

530

394.4

9

Pebax/5%IL

421.4

115.6

7

Pebax/20%IL

261.1

45.5

3

Pebax/35%IL

227.2

40.1

3

Pebax/50%IL

153.8

31.6

1

56

Table 8. The dissolved mass of the gases in different amounts of IL content (P=10 bar). Gas

IL content

solubility coefficient

dissolved gas (cm3)

dissolved gas (gr)

CO2

0

0.021333

1.0048

0.001851

5

0.022

1.45068

0.002672

20

0.022667

1.92168

0.00354

35

0.023

2.38326

0.00439

50

0.0235

2.87781

0.005301

0

0.00313

0.14758

0.00009

5

0.00287

0.18903

0.000126

20

0.0026

0.22043

0.000147

35

0.0023

0.23833

0.000159

50

0.0021

0.25717

0.000171

0

0.00042

0.01978

0.000023

5

0.00039

0.0255

0.000029

20

0.00037

0.03137

0.000036

35

0.00035

0.03627

0.000042

50

0.00033

0.04041

0.000047

CH4

N2

57