Facilitated transport of CO2 through Co(II)-S-EPDM ionomer membrane

Journal of Membrane Science 469 (2014) 151–161

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Facilitated transport of CO2 through Co(II)-S-EPDM ionomer membrane Omolbanin Hosseinkhani, Ali Kargari n, Hamidreza Sanaeepur Membrane Processes Research Laboratory (MPRL), Department of Petrochemical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Mahshahr Campus, Mahshahr, P.O. Box 415, Iran

art ic l e i nf o

a b s t r a c t

Article history: Received 18 January 2014 Received in revised form 31 May 2014 Accepted 12 June 2014 Available online 19 June 2014

A new facilitated transport ionomer membrane was prepared by incorporating Co(II) ions, as fixed carrier, into the sulfonated ethylene-propylene-diene terpolymer, Co(II)-S-EPDM, and was applied for preferential transport of carbon dioxide rather than nitrogen from CO2/N2 gas streams. The membranes were also characterized by using the FTIR, SEM and TG/DSC analyses. Furthermore, single gas permeation experiments at different membrane thicknesses (30 and 200 μm), sulfonic group contents (0.1 and 0.2 mmol of sulfonic acid (SO3H)/g EPDM), feed gas pressures (2–10 bar) and temperatures (30– 50 1C) were carried out. The permeation results of the dense Co(II)-S-EPDM ionomer membranes show that the incorporation of Co(II) ions into the sulfonated EPDM ionomers increases the permeance of both CO2 and N2 gases. Moreover, due to facilitated CO2 transport by the carriers, the CO2/N2 selectivity is also increased. These are mainly attributed to the formation of polymer coordinate complexes in the ionomer. & 2014 Elsevier B.V. All rights reserved.

Keywords: Facilitated transport membrane CO2 separation Fixed carrier Co(II)-S-EPDM Ionomer membrane

1. Introduction Carbon dioxide capture and sequestration (CCS) technologies have been extensively developed. Capturing CO2 – with the most contribution in greenhouse gas effects – from large stationary sources has been frequently observed in the main international legal documents, United Nations Framework Convention on Climate Change (UNFCCC) and its attachment, the Kyoto Protocol. The captured CO2 from the flue gas streams of the power plants not only causes the concerns of greenhouse effects to reduce, but has also founded more attention as an economic source for direct use in enhanced oil recovery (EOR) and water gas shift reaction (WGS) of producing syngas (H2 and CO) in the petrochemical industries, and thus indirect use as an origin of carbon in its related reactions [1–6]. Polymeric membrane gas separation has attained great consideration from both the operational and economical points of view [7–13]. In the case of CO2 capture, some existing synthetic polymeric membranes such as cellulose esters have been successfully used and a series of newly developed polymers are also being studied [14–24]. Unfortunately, this coincided with the great engineering challenges of low CO2 concentration in industrial flow gas streams or its low partial pressures as the driving force of the operation. Luckily, affinity-based separation via the newly proposed facilitated transport membranes (FTMs) has achieved more success in overcoming these drawbacks. These kinds of

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Corresponding author. Tel./fax: þ 98 65223 43645. E-mail addresses: [email protected], [email protected] (A. Kargari).

http://dx.doi.org/10.1016/j.memsci.2014.06.021 0376-7388/& 2014 Elsevier B.V. All rights reserved.

membranes have the potential of achieving simultaneous high permeability and selectivity at low feed pressures [25–38]. In these types of membranes, CO2 molecules can permeate selectively by a reversible reaction of CO2 with carrier in the membrane, while other gases such as N2 can permeate only by the solution– diffusion mechanism [39]. The fixed-site-carrier facilitated transport membranes have a complexing agent (carrier), which covalently bonded to the polymer backbone. There are a number of published studies investigating the facilitated transport of CO2 [40-42]. Kim et al. [39] prepared the fixed-site-carrier polyvinylamine (PVAm) membrane for CO2 capturing on various microporous supports such as poly (ethersulfone) (PES), polyacrylonitrile (PAN), cellulose acetate (CA), and polysulfone (PSf). The PVAm casted on PES exhibited the highest selectivity of CO2 over CH4 (41000). Zhang et al. [43] synthesized the facilitated membrane by the hydrolysis of polyvinylpyrrolidone (PVP) through radical polymerization and applied it in CO2/CH4 separation. Sulfonation of polymers is a viable method of making different membranes with various applications such as ion exchange membranes, proton conducting membrane fuel cells, membranes for reverse osmosis, nanofiltration and microfiltration [44,45]. It results in further hydrophilicity of the polymer and therefore a more significant increase in its solubility. As an example of the sulfonated membrane, sulfonated polystyrene (SPS) supported on poly tetrafluoroethylene (PTFE) was synthesized by fine-tuning the styrene/divinylbenzene (DVB) ratio in the reaction solution that was impregnated in the PTFE support [46]. Ethylene-propylene-diene terpolymer (EPDM), whose chemical structure is shown in Fig. 1, is the most popular material which has

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Fig. 1. Reaction patterns for the sulfonation of EPDM (Keltans 2340A, containing 53 wt% ethylene and 6 wt% of ethylidene norbornene (ENB)).

an ethylene content of at least 45 wt% in order to attain improved physical properties. Enriching the ethylene content in the structure of the EPDM polymer creates better physical properties. The EPDM can be sulfonated at widespread degrees by controlling the amount of sulfonation agent, reaction time and temperature. The sulfonation of vulcanized EPDM membranes in a swelling solvent with acetyl sulfate was developed by Barroso-Bujans et al. [47]. This technique prevents the need for pre-dissolution of the raw polymer. The results revealed that a direct relationship exists between the swelling degree and the concentration of sulfonic groups. The mechanical resistance, tensile strength and Young's modulus of sulfonated EPDM were 5–6 times more than EPDM. Co(II) coordinated polymers can be prepared by using (for example, soap-free emulsion) copolymerization with the simultaneous loading of different Co(II) contents [48] and/or by Co(II) complex formation in a solution of polymer and Co(II) ions [49]. In addition, Co(II)-based Schiff base metal–organic complexes can be synthesized separately from the polymer and then incorporated into the polymer matrix. This has been extensively used to produce the fixed carrier membranes for facilitating oxygen transport through the research polymeric membranes such as octyl methacrylate and vinyl imidazole copolymers [50-52], poly(styrene-block-polyisoprene-block-polystyrene)-graft-poly(1vinylimidazole) (SIS-g-VIm) [53], and the commercial membranes such as polycarbonate [54], ethyl cellulose [55,56] and EPDM [57]. For example, Yang and Huang [55] successfully synthesized five Co (II) meso-tetrakis (substituted phenyl) porphyrin complexes and used them as oxygen carriers in the polymeric matrix of ethyl cellulose (EC) membranes for O2/N2 separation. They studied the effects of fifth ligands, the imidazole and pyridine respectively as electron-accepting and -donating substituents in the Co(II) porphyrins, and showed the significant roles of imidazole. The imidazole contained Co(II) porphyrin complexes as an oxygen carrier cause the values of 9.68 and 3.55 to increase for O2 permeability and O2/N2 selectivity of pure EC membrane at the pressure of 8 kPa up to 12.39 and 4.44 for the facilitated transport membrane. It was prepared by incorporating only 1.0 wt% cobalt (II)meso-tetrakis(2-chlorophenyl)porphyrin complexes containing imidazole as the fifth ligand into the EC matrix (which is named EC/Co(T(2-Cl)PP)Im membrane). Moreover, they investigated the effect of upstream pressure up to 200 kPa and stated that due to the pressure-indecency of N2 permeability, the synthesized fixed oxygen carriers in the EC membrane can reversibly interact with O2 and facilitate its transport in the membranes [56]. Composite membranes were synthesized from incorporating the Co(II)-N,N0 -disalicylideneethylenediamine (Cosalen) into Co (II)-neutralized sulfonated EPDM (Co(II)-S-EPDM) by Zhang and Lin [57]. In comparison with the neat EPDM, the Co(II)-S-EPDM/ Cosalen membrane with 15 wt% Co-Salen loading led to an increase in O2 permeability from 11 to 37 barrer and the O2/N2 selectivity from 4.38 to 9.6.

In this work, the EPDM terpolymer was initially sulfonated to S-EPDM ionomer membranes. Then, different loads of cobalt ions – equivalent to different amounts of sulfonic groups in the polymer – as the fixed carriers for CO2 transport were introduced into the S-EPDM to form Co(II)-S-EPDM ionomer membranes. The membrane characterizations were evaluated by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and thermogravimetry (TG) analyses. Furthermore, the gas permeances and ideal selectivities of the Co(II)-S-EPDM ionomer membrane were explored and compared with those of the neat EPDM membrane under the same circumstances. Moreover, the effects of different membrane thicknesses, sulfonic group contents, feed gas pressures and temperatures as well as the long-term stability of the membranes were also examined. One important trait of this work is the selection of EPDM owing to low cost, commercial availability, ease of processing and its distinct rubbery properties at room temperature. Additionally, the CO2/N2 separation was investigated for the first time for these membranes prepared by a special grade of EPDM, Keltans 2340A.

2. Experimental 2.1. Materials Ethylene-propylene-diene terpolymer (EPDM), Keltans 2340A (DSM Elastomers B.V., the Netherlands) containing 53 wt% ethylene, 41 wt% propylene and 6 wt% ethylidene norbornene (ENB) was used as membrane polymer in this study. The number-average (Mn ¼41.204 kg/mol) and weight-average (Mw ¼ 45.308 kg/mol) molecular weights of EPDM were determined using gel permeation chromatography (GPC, Shimadzu 6A instrument, Kyoto, Japan) based on a calibration with polystyrene standards. The test was carried out using a Waters (Milford, MA) HPLC pump equipped with Ultrastyragel 103 Å column, tetrahydrofuran (THF) as mobile phase, column temperature of 40 1C, a UV detector, and 1 mL/min flow rate. All reagents and solvents including toluene, p-xylene, methanol, cobalt(II) acetate tetrahydrate, sulfuric acid, hydrochloric acid, acetic anhydride, potassium hydroxide, phenolphthalein and antioxidant 2246 [2-20 -methylene-bis(4-methyl-6-tert butyl phenol)] were purchased from Merck (Darmstadt, Germany) and used without further purification. Deionized water was used throughout the experiments. CO2 and N2 with the purity of 99.9% were supplied from Oxygen Yaran Company, Mahshahr, Iran. 2.2. Preparation of Co(II)-S-EPDM ionomer Cobalt-neutralized sulfonated ethylene-propylene-diene terpolymer (Co(II)-S-EPDM) ionomer was synthesized via developing a solvent-based procedure. In this regard, the EPDM polymer was dissolved in p-xylene at a concentration of 50–100 g/L. Concentrated sulfuric acid, as a sulfonating agent, was slowly introduced to the polymer solution drop-wise at a concentration of 0.42 mmol/g EPDM at ambient temperature. This can decrease the solvent evaporation and consequently the problems associated with an increase of solution viscosity are significantly reduced. Sulfuric acid was chosen because it functionalizes polymers without any degradation. Some of the sulfonating agents cause polymer chain degradation during this reaction due to their high reactivity and toxicity levels. However, a certain amount of acetic anhydride (1.6 mmole anhydride for each mmole sulfuric acid) was slowly added drop-wise to the solution. Afterwards, the solution was gently stirred at room temperature for 0.5 h. The sulfonation was terminated through the addition of methanol.

O. Hosseinkhani et al. / Journal of Membrane Science 469 (2014) 151–161

A methanolic solution from dissolving cobalt(II) acetate tetrahydrate in methanol (4 equiv cobalt(II)/g EPDM) coinciding with a mixture consisting of 0.03 g antioxidant and 5 ml toluene was introduced simultaneously to the resultant polymer solution. The solution was further agitated for 2 h and then filtered to remove insoluble or un-reacted species. The polymer was washed several times with large quantities of boiling deionized water to extract the solvent. The residual solvent was removed in a vacuum oven at 60 1C for 24 h [58]. Among the metallic bases, metal acetates are solubilized easily in water and methanol/water mixtures. Moreover, the polymeric sulfonic acid in the presence of water forms an unfavorable gellike solution which reduces the metallic base-induced neutralization. On the other hand, in the absence of water, the metal sulfonates are dissolved in an aliphatic hydrocarbon and an alcohol mixture. Therefore, as the cobalt(II) acetate tetrahydrate is very highly soluble in methanol, it is represented here as an excellent candidate for neutralization. 2.3. Degree of sulfonation

where W (g) is the weight of polymer, V1 and V0 (ml) denote the volume of alcoholic potassium hydroxide solution used for titration of the sample and blank, respectively. M refers to the molar concentration of alcoholic potassium hydroxide solution. According to the results of titration, the average degree of sulfonation for the EPDM which was used here was 60%. Bearing this in mind, for the two considered polymeric samples which

A schematic diagram of preparation methodology used to synthesize the Co(II)-S-EPDM membranes is shown in Fig. 2. The Co(II)-S-EPDM ionomer was dissolved in a (1:20 volume ratio) toluene–methanol mixture to produce a 2 wt% homogenous solution at room temperature. The solution was filtered to remove the insoluble species. The resulting homogeneous solution was degassed and then casted on a clean, flat glass plate. The resultant film was dried slowly in an atmospheric environment for 24 h and then placed in a vacuum oven for further drying for 24 h at 30 1C. As it was demonstrated, the polymer's sulfonic acids having SO3H acidic groups can be neutralized with metallic bases to form a crosslinked polymer [59]. The metallic sulfonates in a binary mixture of non-polar aliphatic hydrocarbons and polar alcohol are easily dissolved, and therefore, the final solution is relatively non-viscous. Thus, in this study, the Co(II)-S-EPDM ionomer was dissolved at a low concentration of 2 wt%, in a toluene–methanol mixture. The abovementioned procedure was essentially used for that of the unmodified polymer samples. The only difference was in the solvent, wherein the pure EPDM was dissolved in p-xylene. Both of the pure EPDM and modified membrane samples were prepared with two different thickness values of 33 and 200 mm. Preparation of 50-100 g/L EPDM solution Addition of concentrated sulfuric acid Addition of acetic anhydride (1.6 mmol/mmol sulfuric acid) Alcoholic termination of sulfonation

Degree of sulfonation

Co(II)-S-EPDM ionomer Polymer phase

Polymer solution

Dissolve in p-xylene

Add methanol

pH=7

Co(II)

ð1Þ

2.4. Preparation of Co(II)-S-EPDM membrane

Antioxidant

MðV 1  V 0 Þ DS ðmmol=100g rubberÞ ¼ 100 W

consumed 0.4 and 0.5 mmol H2SO4/g EPDM, the related values of sulfonic groups (mmol of sulfonic acid (SO3H)/g EPDM) were 0.1 and 0.2, respectively.

YES

The degree of sulfonation (DS) of the EPDM polymer was defined as the percentage of repeating units of EPDM that were sulfonated. The ion-exchange capacity (IEC) of the EPDM polymer (e.g., mequiv of SO3H per g of EPDM), which is a measure of the number of ions exchanged in EPDM, was determined by the titration technique at room temperature. Though attempts have been made for the accurate preparation of acetyl sulfate, the conversion was not satisfactory when the preliminary EPDM has impurities, whereby the titration was taken into account for measuring the conversion of sulfuric acid to acidic (sulfonic) groups within the network [59]. The cobalt-free solution as described in Section 2.2 (i.e., the mixture of methanolic solution of Co(II) and antioxidant which is not introduced to the resultant polymer solution) was prepared to measure the DS. The sulfonated polymer was separated from the liquid phase. Subsequently, the p-xylene was added to the polymer and stirred for 10 min. Methanol was then added to the solution and this mixture was further stirred for another 10 min. The dissolving and precipitation of the polymer were frequently repeated until the liquid solution was neutralized and the excess acid, which is not involved in the reaction of sulfonic groups, was completely removed from the liquid solution. Thereafter, the polymer was filtered to remove the methanol and then dissolved in the p-xylene. A specific volume of the sulfonated polymer solution was titrated with a 0.1 N solution of potassium hydroxide in methanol at the presence of phenolphthalein, which is a commonly used acid–base indicator. To determine the endpoint of the titration and therefore the precise quantity of reactant in the titration, back titration was carried out, in which the sulfonated polymer solution was titrated again (or titrated back) with a mixture of 0.1 N solution of hydrochloric acid in acetic anhydride to find the volume of the un-reacted reactant. The free sulfuric acid solution of titration which is called the “blank solution” of the polymer was prepared to measure the conversion percentage. The degree of sulfonation was calculated as follows [60]:

153

NO

Filtration

Filtration Washing Dissolve in p-xylene Titration

Vacuum oven

Fig. 2. Schematic diagram of preparation methodology used to synthesize the Co(II)-S-EPDM membranes.

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2.5. Membrane characterizations FTIR spectroscopy is an experimental method, which was used in this study to obtain information about the chemical structure within EPDM and its sulfonated ionomers to figure out the potential structural changes that occur as a result of the membrane chemical treatment or degradation. FTIR spectra of polymers were recorded using a Galaxy Series FTIR 5000 Spectrometer. The morphological investigations of the membranes were carried out by a Camescan MV2300 SEM instrument. For this reason, the membrane samples were initially fractured in liquid nitrogen and then coated with gold. Thermal analyses of membranes were carried using DSC and TGA. The DSC tests were conducted with a Universal V3.8B TA instrument. Data were collected from the samples having an average mass of 5 mg at a scan rate of 101 min  1 from an equilibrated temperature of  55 to 150 1C in the nitrogen atmosphere. The TGA measurements were carried out by using a TA instruments model SDT 2960. TGA runs were recorded at a scan rate of 101 min  1 up to 700 1C. The sample compartment was flushed with dried, ultrahigh pure nitrogen at all times. 2.6. Gas permeation measurements A circular stainless steel membrane holder was used for conducting the permeation measurements. In this case, by a precise pressure regulator, the trans-membrane pressure was maintained constant during each of the experiments. At steady-state conditions, gas permeance was calculated using the following equation: P¼

1 p0 273:15 dV A Δp 76 T 0 dt

ð2Þ

where P is the gas permeance in GPU (gas permeation unit, 1 GPU¼10  6 cm3 (STP)/cm2 cmHg s), A is the effective membrane area (cm2) with the value of 15.90 cm2, Δp is the trans-membrane pressure (cmHg) which ranges from 152 to 760 cmHg (2–10 bar), p0 is the average ambient pressure (cmHg) with a value of 79.04 cmHg, T0 is the feed gas temperature (K) which ranges from 303.15 to 323.15 K (30–50 1C), and dV/dt is the volumetric flow rate of the penetrant gas across the membrane (cm3/s).

3. Results and discussion 3.1. The chemical, thermal and morphological analyses of the membranes Sulfonation of the EPDM polymer occurs merely in its diene monomer. FTIR spectra of the EPDM polymer and its sulfonated ionomers are given in Fig. 3. As can be observed, the presence of sulfonic groups in the polymer is confirmed. Two absorptions at 1039 and 1152 cm  1 are referred to as the SO3 stretching of the sulfonic groups. The broad band at 720–1463 cm  1 can be assigned to the C–H vibration of ethylene groups while for the propylene groups this is observed at 1376 cm  1. Moreover, another C–H stretching vibration of 2980 cm  1 is attributed to methyl groups in the polymer structure. In general, no significant shifts in the characteristic bands of the virgin EPDM and its sulfonated one are observed which show that the group vibrations are not changed whether the components are covalently linked or not. However, the main changes in the FTIR spectra are due to the SO3H groups (3424 cm  1). Various sulfur–oxygen vibrations are also observed which can be assigned to asymmetric OQSQO stretch (1161 cm  1), symmetric OQSQO stretch (1080 cm  1), SQO stretch (1024 cm  1), and S–O stretch (610 and 709 cm  1). DSC/TGA spectra of all three samples, pure EPDM, S-EPDM and Co(II)-S-EPDM, are shown in Figs. 4 and 5. Since the S-EPDM was

only swelled in the solvent and it was not capable of forming an upstanding membrane, then we considered the cobaltus S-EPDM after neutralization, i.e. Co(II)-S-EPDM, as the material for membrane fabrication. However, the DSC and TG data of S-EPDM were also presented here to obtain better insight into the results. In the case of DSC spectra (Fig. 4), all three samples exhibit a distinct Tg at negative temperatures of 38.89,  41.50 and  41.06 1C, respectively for pure EPDM, S-EPDM and Co(II)-S-EPDM samples. Lower Tgs for S-EPDM and Co(II)-S-EPDM can be also observed in comparison to the EPDM. The changes occurring in the polymer intersegmental chain movements due to inserting new interactions through the sulfonation of EPDM and also by incorporating the Co(II) ions influence the issue. By further heating the S-EPDM and Co(II)-SEPDM samples, the other two exothermic transitions are observed at a low positive temperature Tc1 (26.16 and 23.87 1C respectively for S-EPDM and Co(II)-S-EPDM) and a high positive temperature Tc2 (59.14 and 53.67 1C respectively for S-EPDM and Co(II)-S-EPDM) indicating micro phase separated structures consisting of micro domains of these two rubbery samples. In these two samples, two main constituent blocks of EPDM, i.e. the ethylene and propylene, will gain enough energy to move into very ordered arrangements, or in other words, reaching the crystallization temperature. The crystallization peak becomes narrower for Co(II)-S-EPDM, possibly because the Co(II) ions introduced may act as heterogeneous nucleating centers for the polymer block crystallizations. Finally, it is notified that a remarkable decline in the slope of S-EPDM at higher temperatures compared to the other two samples can be attributed to the thermal changes occurring due to the remaining solvents trapped in this swelled sample after the drying procedure. In the case of TGA analyses (Fig. 5), TG mass loss curve of pure EPDM shows only one step with no degradation residue, indicating that the overall decomposition takes place in a single step,  420– 480 1C. On the other hand, a two-step decomposition takes place for S-EPDM. Decomposition of sulfonic acid functional groups contributes to the first remarkable decline in the curve at  330–380 1C [61]. The second weight loss in the curve corresponds to the overall decomposition of S-EPDM, 425–485 1C, which shows a gradual shift to higher temperatures as compared to pure EPDM. This is also repeated for CoS-EPDM sample with approximately broader intervals, respectively at 350–390 1C and 428–497 1C. In the case of temperature, a temperature at which a 25% weight loss occurs could be a measure of the relative thermal stability [62], and also a temperature at which 50% degradation occurs is generally considered as an index of thermal stability [63]. In the case of time, the induction time before the degradation reaction begins is taken as a measure of thermal stability [64]. Therefore, the relative thermal stability, thermal stability index, and induction time of degradation (50% weight loss) for EPDM increase respectively from 453 1C, 476 1C and 53 min to 476 1C, 488 1C and 57 min for Co(II)-SEPDM. As observed, the thermal stability is enhanced by incorporation of Co(II) ions as these hinder the diffusion of the volatile decomposition products [65]. Fig. 6 shows the SEM images of pure and Co(II)-S-EPDM over the cross sections and top surfaces of the membranes. As it is observed in Fig. 6, compared to neat EPDM, the Co(II)-S-EPDM has a rather rough surface with a more accessible effective surface area, which will be more capable of trapping gas molecules and the consequent ease of facilitation in a molecular transport through the membrane. Furthermore, the pictures indicate that not only are all membrane defects free, but they also have uniform structures with the homogeneous pattern. 3.2. The effect of membrane thickness on the membrane performance Fig. 7a, b and c depicts the effect of membrane thicknesses, with two different low (33 μm) and high (200 μm) values,

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Fig. 3. FTIR spectra of (a) pure EPDM and (b) S-EPDM.

Fig. 4. DSC spectra of pure EPDM, S-EPDM and Co(II)-S-EPDM.

permeation rate of CO2 with increasing the FTM thicknesses was reported by Teramoto et al. [66]. Deng et al. have encouraged explaining the loss of the CO2 transport phenomenon through the competition of CO2-carrier-mediated transport by the carriers and the CO2 Fickian diffusion through the membrane matrix as the rate-determining step [67]. Yu et al. concluded that in the formation of thin film composite FTMs through the interfacial polymerization reaction, with an increase in film thickness to a certain extent, the reaction stopped due to difficulties occurring in diffusion of the reactant into the very thick layer, which is reported as a “self-limiting” phenomena in CO2 transport [68]. In contrast, as the case here, in the solution cast symmetric fixed-site FTMs, the increase in CO2 permeability and selectivity with increasing thickness is due to the increase in the (amounts and) density of CO2 carriers which can result in more facilitation of CO2 transport through the membrane (Fig. 7a). However, the CO2 permeance values, both for pure EPDM and Co(II)-S-EPDM membranes, decrease with increasing thickness (Fig. 7b). It is obvious that the gas flux and/or permeance are directly related to the membrane thickness. A thinner membrane is more desirable to obtain the higher permeance. This is while the N2 transport does not show considerable changes with the variation of membrane thickness (Fig. 7c) [69]. 3.3. The effect of feed gas temperature on the membrane performance

Fig. 5. TGA spectra of pure EPDM, S-EPDM and Co(II)-S-EPDM.

respectively on the permeability, permeance and selectivity of pure EPDM and Co(II)-S-EPDM membranes. The membrane with a thickness of 200 μm shows much higher CO2 permeability and selectivity values than the membrane with a thickness of 33 μm, both for pure EPDM and Co(II)-S-EPDM membranes. The sensible changes of CO2 permeability with the film thicknesses of FTMs have been previously concluded by some authors. A decrease in

Fig. 8a and b shows the temperature dependence (from 30 to 50 1C) of CO2 permeance and selectivity. As expected, a wellknown increase in the CO2 permeance with an increase in temperature is observed both for pure EPDM and Co(II)-S-EPDM membranes (Fig. 8a). The temperature-induced increase in gas diffusivity as well as a probable increase in the free volume of the polymer can lead to permeance increase. The selectivity of pure and Co(II)-S-EPDM membranes show different behavior with the temperature changes (Fig. 8b). As the pure EPDM approximately conserves its selectivity with the temperature increase, Co(II)-SEPDM bends further downward at temperatures from 30 to 50 1C. This can be due to a negative effect of temperature on the exothermic, weak acid–base complexation reaction between CO2 and Co(II) cations for CO2 facilitated transport at higher temperatures which is rather competitive against the free diffusion of CO2 through the free volume of the polymer. Furthermore, the growth of crystalline microphases of the polymer and limitation upon the molecule diffusion at these elevated temperatures may be another reason for this issue (DSC data in Fig. 4). This coincides with a

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Fig. 6. SEM images of cross sections (at left) and surfaces (at right) of the (a) pure EPDM and (b) Co(II)-S-EPDM.

significant increase in non-polar non-interacting N2 molecule transport across the membrane, and hence, the CO2/N2 selectivity decrease at high temperatures. 3.4. The effect of trans-membrane pressure on the membrane performance The behavior of membrane permeance and selectivity as a function of trans-membrane pressure at 30 1C is presented in Fig. 9a and b, respectively. The effect of cobalt ions in sulfonated EPDM membranes is clearly observed. Based on the pure gas permeation results at 2 bar, in the case of neat EPDM, the CO2 permeance is 0.11 GPU and the related CO2/N2 selectivity is 3.18. By introducing Co(II) ions into the sulfonated EPDM, its CO2 permeance is distinctively enhanced to 0.77 GPU while the CO2/N2 selectivity shows a lower increase up to 7.85. The N2 permeance is also increased from 0.035 to 0.098 GPU. The main reason for such behavior can be the formation of polymer coordinate complexes in the ionomer. The cobalt complexes in the sulfonated EPDM membranes can absorb and transport CO2 selectively. They can interact with CO2 molecules which have higher quadruple moments than N2 molecules (nitrogen molecules have no distinct moments due to their stiff triple bands). As can be seen, the permeance of CO2 through Co(II)-S-EPDM decreases with enhancing pressure. Indeed, this phenomenon is

directly related to the reversible reaction between Co(II) ions of the polymer coordinate complexes (Co(II)-S-EPDM) and CO2 molecules; and is considered a square complex for the cobaltous ions in this case. This is not unobtainable since the four- or five-coordinates and square complex species were also known [58]. In this case, one or two vacant coordinate sites can bind to the CO2 molecule and dissociate from it in the four- or five-coordinate complexes (Fig. 10). Therefore, the CO2 transport is facilitated through the reversible absorption and dissociation to and from the polymer cobalt complex. This reaction leads to increasing the retention time in this membrane as compared to that of the free carrier membrane. On the other hand, by increasing the CO2 concentration at the feed side, the saturation of carriers in the membrane increases significantly. This is a consequent effect of the low ENB content in the neat polymer or the low sulfonic acid groups and equivalently cobaltous carriers in the ionomer membranes. Accordingly, it is concluded that with the increase in feed pressure, the fraction of bounded CO2 to dissociated ones is increased here and this results in a decrease in the CO2 diffusion coefficient. The permeance of N2 is also increased which is related to enhancement in the free volume associated with incorporation of the large sized carrier. When the pressure increases, the N2 permeance obeys Henry's law and shows an independent behavior on pressure. Consequently, as the increase of CO2 permeances is higher than

O. Hosseinkhani et al. / Journal of Membrane Science 469 (2014) 151–161

157

Fig. 7. Effect of membrane thicknesses (33 and 200 μm) on the CO2 permeability (a) CO2 permeance (b) and the related CO2/N2 selectivity (c) of pure EPDM and Co(II)-SEPDM membranes; Gas "permeability" through membranes is most commonly measured in Barrer, where 1 Barrer = 10  10cm3 (STP) cm/cm2 s cmHg. In addition, the gas "permeance" or "pressure-normalized flux" through membranes is often measured in terms of gas permeation units (gpu), where 1 gpu is defined as 10  6 cm3 (STP)/cm2 s cmHg. Therefore, 1 gpu = 1 Barrer/μ.

that of N2, the selectivities of the Co(II)-S-EPDM ionomer membranes increase remarkably.

The descriptions used for the pressure effect on the permselectivities in the previous section can also be valid in this case. 3.6. Comparing with other membranes

3.5. The effect of sulfonation on the membrane performance Here, the effect of sulfonation on the permselectivities for the samples with two different values of 0.1 and 0.2 mmol sulfonic acid groups/g EPDM, and also the same equivalents of cobaltous carriers, are presented in Fig. 11a and b. Indeed, with an increase in the sulfonic group contents of EPDM, the presence of cobalt(II) cations in the membrane increases as well. This results in more facilitation of CO2 transport for more sulfonic group content at a constant pressure. Due to the restricted amounts of double bounds (ENB content of only 6 wt%) in the pristine EPDM, the degree of sulfonation is suppressed only up to 60% which is not able to play a significant preservative role in increasing N2 transport and consequently leads to insignificant changes in the CO2/N2 selectivities.

Table 1 presents the performance of the pure EPDM (Keltans 2340A) and its ionomer (Co(II)-S-EPDM) membranes prepared by using solution casting in this work and the performance of other EPDM membranes [70-72] and also a fixed carrier facilitated transport membrane with a high CO2 separation performance reported elsewhere [73]. Compared with the other EPDM grades, Keltans 2340A showed both the lower CO2 permeability and CO2/ N2 selectivity, but the CO2 permeability of its ionomer forms are capable of competing with the others. In addition, another hint as a future direction to reach further enhancement in its separation performance may be the simultaneous use of EPDM crosslinkers such as dicumyl peroxide (DCP) with the sulfonating agents [70,72]. This is conducted in high separation performance FTMs,

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Fig. 8. Temperature dependence of the CO2 permeance and the related CO2/N2 selectivity (trend lines: solid, dot and dash lines correspond respectively for pressures of 2, 6, and 10 bar. Moreover, gray and black colors stand for EPDM and Co(II)-S-EPDM membranes, respectively).

Fig. 9. CO2 permeance and the related CO2/N2 selectivity of pure EPDM and Co(II)S-EPDM membranes as a function of pressure.

such as the poly(vinyl alcohol)/diethanolamine (PVA/DEA) blend, by moderate hydrogen bond crosslinking which causes densification of the polymer matrix [73]. Therefore, besides the facilitated transport of CO2 by DEA, solution–diffusion selectivity enhances the CO2 diffusion and selectivity. 3.7. The stability tests The experiments were done to verify the stability of the prepared membranes. The Co(II)-S-EPDM membrane containing 0.1 mmol of sulfonic acid/g EPDM (4 equiv cobalt(II)/g EPDM) was considered here at 2 bar and 30 1C. The data of experiments over a three-day period of operation are presented in Fig. 12a and b. As it is expected, in the early hours of this period, the permeances of both gases (CO2 and N2) show a sharp descending behavior while the related selectivities are ascending. Then, both the permeances and selectivities tend to form a plateau. The saturation of carriers with the penetrant species can cause an early decrease in the permeances of CO2. This coincides with the plasticization phenomenon in the polymer which is originated from the CO2 molecules. It can result in decreasing the available free volume for N2 molecules to transport through the membrane. Therefore, N2 permeance also decreases (Fig. 12a). With the passage of time,

Fig. 10. The schematic structure of Co(II)-S-EPDM ionomer coordinated to CO2 in the fixed site facilitated CO2 transport membranes.

and the reversible reaction between the polymer coordinate complexes and CO2 molecules, all the active sites of the membrane reach a quasi-equilibrium state in their operation. On the other hand, the polymer reaches a completely relaxed status and thus an approximately steady behavior in both the permeance and selectivity are achieved (Fig. 12a and b).

4. Conclusions Co(II) ions as a fixed carrier were introduced into the sulfonated EPDM polymer to synthesize a Co(II)-S-EPDM facilitated transport membrane for CO2/N2 separation. FTIR confirmed that the sulfonation reaction successfully took place in the EPDM

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159

polymer. DSC spectra of the ionomer membrane showed that a microphase separation might be occurring in this sample which could result in more permeation of molecular species through the membrane. TGA analysis revealed more thermal stability of the

ionomer membranes compared to the pristine EPDM. SEM images showed a rougher surface of the sulfonated EPDM compared to the neat EPDM which assists the facilitation transport through the membrane. Furthermore, a defect free uniform structure was attained from these images. Incorporating Co(II) ions into the polymer backbone increased the free volume. Permeation performance of the Co(II)-S-EPDM membranes were evaluated by means of a laboratory-scale setup. The CO2 transport was facilitated by the fixed Co(II) carriers. The results indicated that the CO2

Fig. 11. The effect of sulfonation (0.1 and 0.2 mmol of sulfonic acid/g EPDM) on the CO2 permeance and the related CO2/N2 selectivity.

Fig. 12. The long-term stability experiment of the Co(II)-S-EPDM membrane (containing 0.1 mmol of sulfonic acid/g EPDM) at 2 bar and 30 1C.

Table 1 Comparing the membrane obtained in this work with other membranes. Membrane

Ethylene content (wt%)

ENB content (wt%)

T (1C)

p (bar)

P CO2 , GPU (barrer)

αCO2 =N2

Dense film thickness (μm)

Ref.

EPDMa EPDM/CBb Keltans 578 Keltans 578c Keltans 778 c Keltans 4778 c Keltans 712 c Keltans 2340A Keltans 2340A Keltans 2340A Keltans 2340A Co(II)-S-Keltans Co(II)-S-Keltans Co(II)-S-Keltans Co(II)-S-Keltans PVA/DEAd

53 53 65 65 65 70 53 53 53 53 53 53 53 53 53 –

5–6 5–6 5 5 5 4.8 4.5 6 6 6 6 6 6 6 6 –

20 20 24 30 30 30 30 30 30 40 50 30 30 40 50 23

2.6 2.6 5.3 3.5 3.5 3.5 3.5 2 2 2 2 2 2 2 2 1.7

(53 7 10) (357 8.5) (81) (110) (135) (122) (107) 0.55 (18.18) 0.11 (22.07) 0.21 (41.01) 0.35 (71.01) 4.03 (133.12) 0.77 (153.66) 0.76 (151.87) 1.21 (241.84) 7.70 (0.03)

– – 15.2 14 10 14 11 3.05 3.16 3.70 2.70 6.87 7.85 5.91 4.71 95

670 800 50–200 120 120 120 120 30 200 200 200 30 200 200 200 268

[70] [70] [71] [72] [72] [72] [72] This This This This This This This This [73]

2340A 2340A 2340A 2340A

work work work work work work work work

a 79 wt% EPDM; R&S Processing Co. (Curing and crosslinking agents are zinc oxide powder (5 phr, ph r ¼gram per hundred gram of rubber), dicumyl peroxide (DCP) (12 phr), and trimethylolpropane trimethacrylate (10 phr).) b 52 wt% EPDM filled with 34 wt% carbon black (CB); RD Rubber Technology Co. (Curing and crosslinking agents are zinc oxide powder (5 phr, ph r¼ gram per hundred gram of rubber), dicumyl peroxide (DCP) (12 phr), and trimethylolpropane trimethacrylate (10 phr).) c Crosslinked with 5 phr DCP for 1 h at 150 1C d Poly(vinyl alcohol)/diethanolamine.

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permeances and the related CO2/N2 selectivities of the Co(II)-SEPDM membranes are higher than those of the neat EPDM membrane. Moreover, both the permeability and selectivity values of the Co(II)-S-EPDM membranes increased by an increase in the dense film thicknesses. However, the permeance values increased with decreasing membrane thickness. Generally, CO2 permeances of Co(II)-S-EPDM membranes increased while their related CO2/N2 selectivities decreased with temperature. Pressure had approximately a negative effect on ionomer membrane performances, especially at pressures higher than 4 bar. Furthermore, it was shown that with an increase in sulfonic acid groups of the membranes, the CO2 permeance increased while CO2/N2 selectivity was not considerably changed. According to the stability tests, after a day of operation, a quasi-equilibrium state in the membrane performance was achieved. Finally, it was shown that the synthesized membranes could be projected as promising candidates for commercialization in the future.

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