Preparation and Characterization of PEBAX-5513/AgBF4/BMIMBF4 Membranes for Olefin/Paraffin Separation

In this study, we investigated a poly(ether-block-amide)-5513 (PEBAX-5513)/AgBF4/1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4) composite membrane, which is expected to have a high stabilizing effect on the Ag+ ions functioning as olefin carriers in the amide group. Poly(ethylene oxide) (PEO) only consists of ether regions, whereas the PEBAX-5513 copolymer contains both ether and amide regions. However, given the brittle nature of the amide, the penetration of BMIMBF4 remains challenging. The nanoparticles did not stabilize after their formation in the long-term test, thereby resulting in a poor performance compared to previous experiments using PEO as the polymer (selectivity 3; permeance 12.3 GPU). The properties of the functional groups in the polymers were assessed using Fourier transform infrared spectroscopy, scanning electron microscopy, and thermogravimetric analysis, which confirmed that the properties endowed during the production of the film using the ionic liquid can impact the performance.


Introduction
Olefins, otherwise known as alkenes, are hydrocarbons with double bonds, whereas paraffins are hydrocarbons containing single bonds. Paraffins are typically used as fuels, whereas olefins can be utilized as polymer monomers because they have double bonds. However, high-purity olefins are required in order to be used in polymerization [1]. Olefin/paraffin separation is an essential process because olefins are produced with the corresponding paraffins. At present, olefin/paraffin separation has primarily been achieved through cryogenic column distillation because olefin and paraffin have extremely similar characteristics [2,3]. However, cryogenic distillation is an expensive and energy-intensive process requiring the use of expensive equipment [4]. Therefore, to address this limitation, considerable research is being conducted on olefin/paraffin separation using a membrane [5,6]. Membrane separation is more advantageous than cryogenic distillation in terms of the energy consumption, eco-friendliness, and equipment. In addition, membrane separation is not only used in olefin/paraffin separation, but can also be applied to gas mixtures such as CO 2 /N 2 , CO 2 /CH 4 , O 2 /N 2 , N 2 /SF 6 , and H 2 /O 2 [7][8][9][10][11]. However, it is difficult to simultaneously increase the selectivity and permeability in membrane separation due to the trade-off phenomenon [12,13]. In addition to the trade-off phenomenon, as a method for solving aging and plasticization, mixed matrix membranes (MMM) containing an organic inorganic material, such as zeolite carbon nanotubes, have been studied [14,15].
The concept of facilitated transportation was applied as a solution to the trade-off phenomenon [16]. Facilitated transportation refers to general dissolution and diffusion, as well as the concept of separation Polymers 2020, 12, 1550 2 of 9 using a carrier. Transition metal cations can generally form double bonds and complexes [17,18]. Silver ions are representative olefin carriers that are applicable in this capacity due to their ability to reversibly form complexes with olefins [19,20]. Paraffin only permeates the membrane by dissolution and diffusion; however, the carriers in olefins simultaneously increase the selectivity and permeance.
However, the sole use of silver ions as carriers becomes a disadvantage when the ions are reduced to silver nanoparticles due to their inability to continue functioning as carriers, thus exhibiting poor performance. Consequently, several studies have been conducted to improve the long-term stability of silver ions by adding an oxidizer, stabilizer, or a similar substance to prevent performance deterioration due to the reduction of silver ions [19,[21][22][23][24]. Moreover, a method involving the use of metal nanoparticles with polarized surfaces as carriers has also been studied [25,26]. Electron acceptors, such as tetracyanoquinodimethane (TCNQ), p-benzoquinone (p-BQ), and ionic liquids (ILs), are required to polarize the silver nanoparticles [27,28].
We conducted experiments to obtain a hybrid effect of ions and silver nanoparticles as carriers. Instead of synthesizing polarized silver nanoparticles during membrane production, the nanoparticles, which are functional as carriers, eventually formed as a membrane in which silver ions were selected as carriers. The utility of high-mobility ILs as electron acceptors enabled the formation of nanoparticles over time. These nanoparticles were wrapped to polarize the surfaces, thereby enabling them to function as carriers. As a result, the PEO/AgBF 4 /BMIMBF 4 membrane demonstrated selectivity and permeance values of 15 and 12 GPU, respectively. In contrast, the PEO/AgBF 4 /MOIMBF 4 membrane exhibited selectivity and permeance values of 17 and 5.3 GPU, respectively [29,30].
In this study, an IL is applied to the copolymer. A poly(ether-block-amide)-5513 (PEBAX-5513)/ AgBF 4 /1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF 4 ) composite membrane is prepared. PEBAX-5513 consists of amide (60%) and ether (40%) groups [31]. The amide group helps to stabilize the carrier, whereas the ether group improves the permeance with respect to the flexibility. Therefore, PEBAX-5513 is expected to fabricate a composite membrane with enhanced selectivity when compared to PEO with only ether groups. The prepared composite membrane was analyzed by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and thermogravimetric analysis (TGA).

Preparation of Membrane
The PEBAX-5513/AgBF 4 /BMIMBF 4 composite membrane was prepared as follows. A PEBAX-5513 solution was prepared using an ethanol-water cosolvent in a weight ratio of 7:3. The AgBF 4 was dissolved in the solution in a weight ratio of 1:9 compared to the case of PEBAX-5513. The BMIMBF 4 was added at a molar ratio of 1:0.03 compared to the case of AgBF 4 . The solution was stirred for 20 min. The resulting solution was coated on a polysulfone support (Toray Chemical Korea Inc., Seoul, Korea) using an RK Control Coater (Control Coater RK Print-Coat Instruments Ltd., Litlington, UK). The prepared composite membrane was dried in a vacuum oven at room temperature for ≥ 15 h.

Gas Separation Experiments
The PEBAX-5513/Ag salt/IL composite membrane was subjected to a propane/propylene (50:50 vol%) mixed gas separation experiment. The gas permeance was measured with a bubble flow meter. The permeance was measured in terms of the gas permeation unit (1 GPU = 1 × 10 −6 cm 3 (STP)/(s·cm 2 ·cm Hg)). The flow rate of the mixed gas was controlled using a mass flow controller Polymers 2020, 12, 1550 3 of 9 (MFC). Gas chromatography (Young Lin 6500 GC system, Young Lin, Gyeonggi, Korea) was used to confirm the propane/propylene selectivity.

Characterization
The cross section of the composite membrane was confirmed using SEM analyses (JEOL JSM-5600LV, JEOL, Tokyo, Japan). The IR measurements were performed using an FTIR spectrometer (VERTEX 70, BRUKER, Billerica, MA, USA); 16 scans were signal averaged with a resolution of 4 cm −1 . The thermal stability of the membranes was measured using TGA (Universal V4.5 A, TA Instruments, New Castle, DE, USA). Figure 1 depicts the results of the permeation test for the propane/propylene gas mixture. The permeation test was conducted for a duration of ≥100 h. The initial performance of the composite membrane demonstrated a high permeance with selectivity and permeance values of 4.9 and 45.3 GPU, respectively. The final selectivity and permeance of the PEBAX-5513/Ag salt/IL composite membrane at 116 h were 3.0 and 12.3 GPU, respectively. In contrast with the previous PEO/AgBF 4 /BMIMBF 4 and PEO/AgBF 4 /1-hexyl-3-methyl imidazolium tetrafluoroborate (HMIMBF 4 ) membranes, the PEBAX-5513/Ag salt/IL composite membranes were unstable in long-term permeation experiments. The remaining solvent in the membrane was removed, thereby resulting in a decreased flexibility of the polymer and a rapid decrease in the initial permeance. Ag ions were reduced to Ag nanoparticles (AgNPs) over time. These formed AgNPs functioned as a barrier, resulting in a continued decrease in the permeance. Furthermore, the selectivity also decreased because the Ag ions that functioned as olefin carriers became inactive as time progressed. It was confirmed that the formed AgNPs were not properly polarized. Moreover, the ether group was more flexible than the amide group. However, the amide group in PEBAX-5513 occupied a larger proportion (60%), thus making PEBAX-5513 more brittle than PEO, suggesting that BMIMBF 4 was less adequately dispersed in the PEBAX-5513 matrix than in the PEO matrix.

Separation Performance
Polymers 2020, 12, x FOR PEER REVIEW 3 of 10 (MFC). Gas chromatography (Young Lin 6500 GC system, Young Lin, Gyeonggi, Korea) was used to confirm the propane/propylene selectivity.

Characterization
The cross section of the composite membrane was confirmed using SEM analyses (JEOL JSM-5600LV, JEOL, Tokyo, Japan). The IR measurements were performed using an FTIR spectrometer (VERTEX 70, BRUKER, Billerica, MA, USA); 16 scans were signal averaged with a resolution of 4 cm −1 . The thermal stability of the membranes was measured using TGA (Universal V4.5 A, TA Instruments, New Castle, DE, USA). Figure 1 depicts the results of the permeation test for the propane/propylene gas mixture. The permeation test was conducted for a duration of ≥100 h. The initial performance of the composite membrane demonstrated a high permeance with selectivity and permeance values of 4.9 and 45.3 GPU, respectively. The final selectivity and permeance of the PEBAX-5513/Ag salt/IL composite membrane at 116 h were 3.0 and 12.3 GPU, respectively. In contrast with the previous PEO/AgBF4/BMIMBF4 and PEO/AgBF4/1-hexyl-3-methyl imidazolium tetrafluoroborate (HMIMBF4) membranes, the PEBAX-5513/Ag salt/IL composite membranes were unstable in long-term permeation experiments. The remaining solvent in the membrane was removed, thereby resulting in a decreased flexibility of the polymer and a rapid decrease in the initial permeance. Ag ions were reduced to Ag nanoparticles (AgNPs) over time. These formed AgNPs functioned as a barrier, resulting in a continued decrease in the permeance. Furthermore, the selectivity also decreased because the Ag ions that functioned as olefin carriers became inactive as time progressed. It was confirmed that the formed AgNPs were not properly polarized. Moreover, the ether group was more flexible than the amide group. However, the amide group in PEBAX-5513 occupied a larger proportion (60%), thus making PEBAX-5513 more brittle than PEO, suggesting that BMIMBF4 was less adequately dispersed in the PEBAX-5513 matrix than in the PEO matrix.

SEM
The SEM image illustrates the state of the selective layer. Figure 2 shows that the PEBAX-5513/ AgBF 4 /BMIMBF 4 solution was evenly coated on the polysulfone support. The structure of the support resembled a sponge, exhibiting a selection layer of approximately 3.3 µm. The SEM image illustrates the state of the selective layer. Figure 2 shows that the PEBAX-5513/AgBF4/BMIMBF4 solution was evenly coated on the polysulfone support. The structure of the support resembled a sponge, exhibiting a selection layer of approximately 3.3 μm.

FT-IR
The interactions between PEBAX-5513, AgBF4, and BMIMBF4 were confirmed using infrared spectroscopy. Figure 3a depicts the change in the C-O stretching bond of PEBAX-5513. The ether peak of neat PEBAX-5513 appeared at 1107 cm −1 . When AgBF4 was added, the ether peak shifted from 1107 to 1009 cm −1 . The C-O bond was weakened due to the interaction between the Ag ion and the oxygen of the ether group. After BMIMBF4 was added, the C-O stretching bond further decreased from 1009 to 1005 cm −1 . The C-O bond weakened as the BMIM + interacted with the BF4 -in AgBF4. Figure 4 shows the deconvoluted FTIR spectrum that accounted for the degree of peak shift. The peak analysis of ether revealed that the intensity of the 994-1002 cm −1 region increased from 68.3% to 79.5% as shown in Table 1. This indicated that BMIMBF4 was properly dispersed and BMIM + interacted with BF4 -, resulting in a peak shift after the addition of BMIMBF4. Therefore, it was confirmed that BMIMBF4 can efficiently penetrate the PE region. The peak deconvolution resulted in a carbonyl group that differed from the peak shift. The area of the 1589-1592 cm −1 region increased from 29.2 to 31.2% as shown in Table 2. However, the peak shift and decomposition results were negligible. These results confirmed that the PA region of BMIMBF4 was more difficult to penetrate than the flexible PE region. Therefore, long-term stability was not observed because the surface of the particles could not be properly stabilized to induce polarization due to the poor dispersion of the BMIMBF4.
As shown in Figure 5, the N-H peak of neat PEBAX5513 appeared at 3300 cm −1 . When AgBF4 and BMIMBF4 were added, the N-H peak increased to 3390 cm −1 and became broadened, confirming that the N-H bond was strengthened. It seemed that some Ag ions interact with the non-covalent electron pair of N, resulting in the weaker C-N bonds and stronger N-H bonds. It was also thought that the intermolecular bond of PEBAX5513 caused the hydrogen bond of N-H bond to be weakened due to Ag ions.

FT-IR
The interactions between PEBAX-5513, AgBF 4 , and BMIMBF 4 were confirmed using infrared spectroscopy. Figure Figure 4 shows the deconvoluted FTIR spectrum that accounted for the degree of peak shift. The peak analysis of ether revealed that the intensity of the 994-1002 cm −1 region increased from 68.3% to 79.5% as shown in Table 1. This indicated that BMIMBF 4 was properly dispersed and BMIM + interacted with BF 4 − , resulting in a peak shift after the addition of BMIMBF 4 . Therefore, it was confirmed that BMIMBF 4 can efficiently penetrate the PE region. The peak deconvolution resulted in a carbonyl group that differed from the peak shift. The area of the 1589-1592 cm −1 region increased from 29.2 to 31.2% as shown in Table 2. However, the peak shift and decomposition results were negligible. These results confirmed that the PA region of BMIMBF 4 was more difficult to penetrate than the flexible PE region. Therefore, long-term stability was not observed because the surface of the particles could not be properly stabilized to induce polarization due to the poor dispersion of the BMIMBF 4 .
Polymers 2020, 12, x FOR PEER REVIEW 5 of 10      As shown in Figure 5, the N-H peak of neat PEBAX5513 appeared at 3300 cm −1 . When AgBF 4 and BMIMBF 4 were added, the N-H peak increased to 3390 cm −1 and became broadened, confirming that the N-H bond was strengthened. It seemed that some Ag ions interact with the non-covalent electron pair of N, resulting in the weaker C-N bonds and stronger N-H bonds. It was also thought that the intermolecular bond of PEBAX5513 caused the hydrogen bond of N-H bond to be weakened due to Ag ions.

TGA
The TGA results confirmed a change in the thermal stability of the PEBAX-5513/AgBF4/BMIMBF4 complex membrane. As shown in Figure 6, the neat PEBAX-5513 experienced a single weight loss between 350 and 450 °C. The PEBAX-5513 polymer containing an amide group exhibited high thermal stability due to intermolecular hydrogen bonding. Following the addition of AgBF4, a weight reduction was observed at 200 °C. The reduction in the thermal stability may be attributed to the interaction of Ag ions with both the ether and the amide groups in the polymer, which hindered intermolecular hydrogen bonding. Furthermore, it was suggested that the addition of BMIMBF4 disturbed the intermolecular interaction and reduced the thermal stability by the plasticizing effect. However, Figure 6 depicts that the thermal stability was enhanced after 200 °C. These results were attributable to the transient crosslinking effect of Ag ions and the generated Ag nanoparticles.

TGA
The TGA results confirmed a change in the thermal stability of the PEBAX-5513/AgBF 4 /BMIMBF 4 complex membrane. As shown in Figure 6, the neat PEBAX-5513 experienced a single weight loss between 350 and 450 • C. The PEBAX-5513 polymer containing an amide group exhibited high thermal stability due to intermolecular hydrogen bonding. Following the addition of AgBF 4 , a weight reduction was observed at 200 • C. The reduction in the thermal stability may be attributed to the interaction of Ag ions with both the ether and the amide groups in the polymer, which hindered intermolecular hydrogen bonding. Furthermore, it was suggested that the addition of BMIMBF 4 disturbed the intermolecular interaction and reduced the thermal stability by the plasticizing effect. However, Figure 6 depicts that the thermal stability was enhanced after 200 • C. These results were attributable to the transient crosslinking effect of Ag ions and the generated Ag nanoparticles.

Conclusions
In this study, a PEBAX-5513/AgBF4/BMIMBF4 composite membrane was prepared. The PEBAX-5513 membrane was expected to exhibit a higher stabilizing effect of the amide group on the olefin carriers than the previous PEO/AgBF4/BMIMBF4 membrane. However, the separation performance of the membrane was not improved. We identified that the BMIMBF4 did not easily penetrate the brittle amide region. The FTIR analysis confirmed that the BMIMMBF4 exhibited no interaction with