Poly(ethylene oxide)/Ag ions and nanoparticles/1-hexyl-3-methylimidazolium tetrafluoroborate composite membranes with long-term stability for olefin/paraffin separation

A poly(ethylene oxide)(PEO)/AgBF4/1-hexyl-3-methylimidazolium tetrafluoroborate (HMIM+BF4−) composite membrane that exhibits long-term stability was prepared for olefin/paraffin separation. The membrane was prepared by simply adding AgBF4 and HMIM+BF4− to a solution of PEO. Long-term stability testing showed that the separation performance of the membrane is maintained for ≈100 h owing to the Ag NPs formed in the membrane, which are olefin carriers, being stabilized by HMIM+BF4−. In terms of separation performance, the PEO/AgBF4/HMIM+BF4− composite membrane exhibited a propylene/propane selectivity of 11.8 and a mixed-gas permeance of 11.3 GPU. We also investigated the factors that determine separation performance by comparison with a PEO/AgBF4/1-butyl-3-methylimidazolium tetrafluoroborate (BMIM+BF4−) composite membrane. The PEO/AgBF4/HMIM+BF4− composite membrane was characterized by scanning electron microscopy, FT-IR spectroscopy, ultraviolet-visible spectroscopy, thermogravimetric analysis, and Raman spectroscopy.


I. Introduction
Olens, commonly known as alkenes, are hydrocarbons with carbon-carbon double bonds, while paraffin, known as alkanes, are hydrocarbons having only single bonds. 1 Light olens, such as ethylene and propylene, are key monomers in petrochemical and polymer synthesis. 2 Thus, olens are used to produce many other organic chemicals. In 2011, 141 million tons of ethylene were produced worldwide. 3,4 Most olens are produced by steam cracking of naphtha, propane, and light oil. However, this process produces a mixture of olens and paraffin. 5 Therefore, the separation of olens from olen/paraffin mixtures is very important in the petrochemical industry. [6][7][8] The separation of olen/paraffin mixtures is most commonly performed by cryogenic distillation because of the similar properties of olens and paraffin. [9][10][11] However, this process suffers from the disadvantages of requiring considerable capital input and being energy intensive. 12 In order to overcome these problems, membrane separation has emerged as a simple, lowenergy manufacturing process. 13,14 However, while glassy polymer membranes show high selectivity, they typically exhibit low permeability. Conversely, rubbery polymers exhibit high permeability but poor selectivity. 15 Recently, facilitated transport membranes, which are selectively permeable to only specic substances, have been attracting research attention. 10 Facilitated transport, which exploits the concentration gradient of solutes generated by the reversible reactions of compounds in membrane with specic solutes, promotes the transport of these solutes. 16 For example, a dimethylpropylenediamine ethoxyacetate ([DMAPAH][EOAc]) membrane for CO 2 /N 2 separation using diamine-monocarboxylate protic ionic liquids (PILs) as carriers has been reported to show a selectivity of 151 and a CO 2 permeability of 3028 barrer. 17 In addition, an arginine salt-chitosan membrane using the amino group in the arginine salt as a carrier was reported to show a selectivity of 144 and a CO 2 permeability of 1500 barrer in CO 2 /H 2 separation. 18 Recently, it has been reported that PEBAX-1657, which is a permeable polymer that contains Ag ions as olen carriers, has a propylene/propane selectivity of 8.8 and a permeability of 22.5 GPU. 19 In general, Ag salts are used as olen carriers, but Ag ions generated from Ag salts are easily reduced and consequently lose carrier activity. [20][21][22] Thus, attempts have been made to partially polarize the surface of Ag NPs so that they react reversibly with olens. 21 However, Ag NPs acting as olen carriers have a drawback in that the permeability of the host membrane is lowered due to the barrier action that occurs when the Ag NPs aggregate to form larger NPs. 22 In addition, membranes using ZIF and MOF have also been reported. [23][24][25][26][27][28][29][30][31][32] Recently, we fabricated a poly(ethylene oxide)(PEO)/AgBF 4 /1butyl-3-methylimidazolium tetrauoroborate (BMIM + BF 4 À ) composite membrane in an attempt to restrict the size of the Ag NPs formed therein. We reasoned that BMIM + BF 4 À would stabilize the smaller nascent Ag NPs, inhibiting their aggregation and thus maintaining membrane permeability. The average size of the particles formed in the composite membrane aer the long-term test was 4.55 nm. The prepared PEO/AgBF 4 / BMIM + BF 4 À composite membrane exhibited a propylene/ propane selectivity of z15 and a permeability of 12 GPU. 33 In the current study, we assessed whether stability could be maintained using ionic liquids other than BMIM + BF 4 À in order to investigate the factors that contribute to the long-term stability of such membranes. 1-Hexyl-3-methylimidazolium tetrauoroborate (HMIM + BF 4 À ) was used as the new ionic liquid to investigate the effect of the carbon chain length of ionic liquids, and compared with BMIM + BF 4 À in viewpoint of both stabilizing the nanoparticles and separation performance.

Materials
Acetonitrile (99.8%) was purchased from Aldrich Chemical Co. and PEO (M w 6 Â 10 5 g mol À1 ) was purchased from ACROS Co. Silver tetrauoroborate (AgBF 4 , 98%) was purchased from Tokyo Chemical Industry Co., and HMIM + BF 4 À was purchased from C-TRI. The microporous polysulfone support was provided by Toray Chemical Inc., Korea. All the initial solvents and materials were used without further purication.

Characterization
Scanning electron microscopy (SEM) images were obtained using JEOL JSM-5600LV. The weight loss of the complex was measured using thermogravimetric analysis (TGA, TGA Q50, TA Instruments) under N 2 ow. The ultraviolet-visible (UV-Vis) absorption spectra were recorded using a Beckman Coulter Life Sciences DU 730 Life Science UV-Vis spectrophotometer with 1 nm resolution. The IR measurements were performed on a VERTEX 70 FT-IR spectrometer; 16-32 scans were signal averaged with a resolution of 8 cm À1 . Raman spectra of neat HMIM + BF 4 À were obtained using a BRUKER RAM II instrument at a resolution of 0.5 cm À1 .

Membrane preparation
The PEO/AgBF 4 /HMIM + BF 4 À complex membrane was prepared by adding AgBF 4 and HMIM + BF 4 À to a PEO polymer solution and vacuum drying. The 5 wt% PEO polymer solution was prepared by dissolving PEO in water/acetonitrile (9 : 10 w/w). AgBF 4 was added to the prepared PEO polymer solution at 1 : 1 molar ratio, and HMIM + BF 4 À was added at different mole ratios. The solution was coated onto polysulfone microporous membrane supports using an RK Control Coater (Model K202, Control Coater RK Print-Coat Instruments Ltd., UK). The longterm stability of the composite membrane prepared at a composition of 1/1/0.052 and vacuum dried for 24 h showed the best performance.

Gas separation performance
The permeation test was performed by permeating the PEO/ AgBF 4 /HMIM + BF 4 À composite membrane with a propylene/ propane mixed gas (1 : 1 v/v). The permeance was measured using a bubble ow meter and the selectivity was assessed by gas chromatography (YoungLin 6500 GC system). The ow rate of the mixed gas was controlled by a mass ow controller (MFC). The unit of gas permeance is GPU (1 GPU ¼ 1 Â 10 À6 cm 3 (STP)/ (cm 2 s cmHg)).

III. Results and discussion
3.1. SEM images

UV-Vis absorption spectra
The stabilizing effect of HMIM + BF 4 À on the Ag NPs generated in PEO/AgBF 4 /HMIM + BF 4 À solution was investigated by UV-Vis spectroscopy. PEO/AgBF 4 /HMIM + BF 4 À solutions were heated at 70 C for 10, 20, 30, and 60 min. The peak for Ag NPs is typically observed at 420 nm. 21 Thus, a peak at about 420 nm indicates the formation of Ag NPs, while the intensity of the peak represents the concentration of Ag NPs formed. When the PEO/AgBF 4 /HMIM + BF 4 À solution is heated for 10 min, a peak is observed at 403-423 nm. Upon heating for 20 min, a slight peak shi is observed. Upon heating for 30 and 60 min, no further peak shiing is observed, but the peak intensity increases.
These results indicate that Ag NPs are stabilized by HMIM + BF 4 À . This is because the surface of the Ag NPs is partially polarized by HMIM + BF 4 À , thus inhibiting aggregation and particle growth. As a result, aer a heating time of 20 min, only small Ag NPs are formed. Moreover, the spectrum presents a symmetric peak, indicating that monodisperse AgNPs were formed in the PEO/AgBF 4 /HMIM + BF 4 À solution. From these results, it was conrmed the effect of stabilizing the NPs by ILs since the relatively narrow peak was observed at UV-Vis spectroscopy (Fig. 2).

FT-IR spectra
FT-IR was measured to conrm the coordination interactions of ether group and Ag ion of PEO. Fig. 3 showed the ether group stretching band peak of PEO/AgBF 4 /HMIM + BF 4 À complex.
Generally, the ether group stretching band peak of neat PEO was known to be observed at 1082 cm À1 . When AgBF 4 was incorporated into PEO, the peak shied to about 1070 cm À1 . This was attributable to the interaction between the Ag ion and oxygen in ether group resulting in weaker C-O bonds. 34 Furthermore, when HMIM + BF 4 À was additionally inserted into PEO/AgBF 4 complex, the peak shied to 1014 cm À1 . This could be explained by the interaction of HMIM + with BF 4 À of AgBF 4 , resulting in the weakened bond between Ag ions and BF 4 À , and the strong interaction between Ag + and oxygen in ether group. Therefore, the C-O bond became weakened and free BF 4 À of HMIM + BF 4 À could stabilize the generated Ag NPs.

Thermogravimetric analysis
The thermal stabilities of neat PEO, PEO/AgBF 4 , and PEO/ AgBF 4 /HMIM + BF 4 À membranes were measured by TGA. Fig. 5 shows the weight loss for each membrane at temperatures ranging from room temperature to 700 C. For the neat PEO membrane, a large weight loss occurs between 380 and 450 C owing to the degradation of the polymer. The PEO/AgBF 4 and PEO/AgBF 4 /HMIM + BF 4 À membranes exhibit weight losses at lower temperatures owing to the interaction between the added AgBF 4 and the PEO chains. Furthermore, when HMIM + BF 4 À was added to PEO/AgBF 4 , there was signicant change in the curve

IV. Conclusions
We succeeded in preparing for the PEO/AgBF 4 /HMIM + BF 4 À composite membrane and conrmed the long-term stability of propylene/propane selectivity of 11.8 and permeance of 11.3 GPU for more than 100 h. The long-term stability is owing to the stabilization of the Ag NPs by ionic liquid HMIM + BF 4 À . HMIM + BF 4 À could partially polarize the surfaces of Ag NPs generated under separation process, resulting in the formation of olen carriers. We also compared the performance of the PEO/AgBF 4 /BMIM + BF 4 À composite membrane with that of the current membrane to help elucidate the factors that determine separation performance in such membranes. As a result, the effect of the carbon chain length of ionic liquids was investigated, and both butyl in BMIM + BF 4 À and hexyl groups in HMIM + BF 4 À were found to be effective in stabilizing the nanoparticles. However, by identifying other ionic liquids with longer carbon chains such as 1-methyl-3-octylimidazolium tet-rauoroborate in the future, we intend to identify main factors   in the design of long-term stable facilitated olen transport membranes.

Conflicts of interest
There are no conicts to declare.