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Silver-based deep eutectic solvents as separation media: supported liquid membranes for facilitated olefin transport Bin Jiang, Haozhen Dou, Baoyu Wang, Yongli Sun, Zhaohe Huang, Hanrong Bi, Luhong Zhang, and Huawei Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.7b01092 • Publication Date (Web): 05 Jul 2017 Downloaded from http://pubs.acs.org on July 10, 2017

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ACS Sustainable Chemistry & Engineering

Silver-based deep eutectic solvents as separation media: supported liquid membranes for facilitated olefin transport Bin Jiang, Haozhen Dou, Baoyu Wang, Yongli Sun, Zhaohe Huang, Hanrong Bi, Luhong Zhang*, Huawei Yang* School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China

Corresponding author at: School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People's Republic of China. Tel./fax: +86 2227400199. E-mail address: [email protected] (L.Zhang). E-mail address: [email protected] (H.Yang).

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Abstract Supported liquid membranes (SLMs) have exhibited great potential as interesting materials for the separation of olefin/paraffin mixtures. To further improve the performance of SLMs, novel and sustainable silver-based DESs, which constructed from trifluoromethanesulfonate (AgCF3SO3) and acetamide (CH3CONH2), were first synthetized as the membrane liquids. Their formation mechanism and structure were investigated intensively, confirming that multiple coordination and hydrogen bonding interactions yielded the homogeneous and stable liquid, which contained free silver ions and silver-containing cationic complexes as carriers for the facilitated transport of C2H4. The as-prepared DESs-SLMs were characterized by SEM and ATR-FTIR, while their separation performances were investigated by C2H4/C2H6 separation experiments. The effects of the composition of silver-based DESs, the operation temperature and the trans-membrane pressure were also investigated systemically. The permeability selectivity, solubility selectivity and diffusivity selectivity were also quantitatively analyzed. Compared with the reported results, the as-prepared DESs-SLMs possessed the excellent permeability of C2H4 and comparable selectivity of C2H4/C2H6, thus exceeding the upper bound. This investigation may provide alternative for development of high performance SLMs for olefin/paraffin separation, and the insights into the formation mechanism of silver-based DESs is useful for further design and strengthening. Keywords: Deep eutectic solvents, Supported liquid membrane, Olefin/paraffin, Facilitated transport Graphical abstract

Synopsis Sustainable supported liquid membrane prepared with Ag-based deep eutectic solvents is first developed, which can separate C2H4/C2H6 efficiently 2


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Introduction The olefins are of significance in the petroleum and chemical industry because they are principal building blocks for many chemicals and products.1 Olefins are often separated or recovered from their paraffinic counterparts by distillation and adsorption processes.2 However, these conventional technologies for the olefin/paraffin separation are the most expensive and most energy-consuming separation processes.3 Therefore, it is highly desirable to develop more efficient and sustainable processes for the separation of olefin and paraffin mixtures. SLMs have special technical advantages, such as integration of extraction and stripping processes, small scale of the equipment, relative environmental safety, low energy consumption and operating cost. Compared with the solid polymeric membranes, the SLMs are intrinsically highly permeable as the diffusion in liquid is much faster than that in solid. The selectivity of SLMs is dominated by the solubility selectivity,4-5 which can be further tailored by carriers based on broader material chemistries and more specific interactions.6 The conventional membrane liquid typically used in olefin/paraffin separation SLMs is water.7-8 However, its high volatilization results in additional investments and energy consumption for its subsequent removal and the SLMs suffering from severe loss of membrane liquid. Consequently, some research groups have investigated the potential of utilizing high boiling point solvents as the membrane liquids, such as N-methyl pyrrolidone,9 glycerol and triethylene glycol.10,11 Recently, ionic liquids (ILs) have attracted great interests of researchers and been proposed as green alternatives to organic solvents due to their unique properties such as extremely low volatility, high thermal stability and structural design-ability, which lead to more sustainable separation with less energy consumption.12,13 Therefore, different families of pure ILs have been deeply explored as suitable separation media to carry out olefin/paraffin separation.14-16 Nevertheless, and despite their promising and green advantages, the selectivity for olefin to paraffin in pure ILs is very low. In order to enhance the olefin/paraffin selectivity, the carrier-facilitated transport conception has been introduced to designed novel SLMs. Silver salts have the strengths of effective facilitated transport of olefin, thus obtaining high olefin/paraffin selectivity. Ortiz et al. used ionic liquid to dissolve AgBF4 for the facilitated transport of olefin.17-18 Unfortunately, low solubilities of silver salts in ionic liquids resulted in limited improvement of olefin/paraffin selectivity.19 Moreover, high loading of silver salts resulted in the high viscosity of membrane liquid, thus slowing the mass transport.20 3



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Deep eutectic solvents (DES) have recently emerged as potential alternatives to ILs, since their physicochemical properties are similar to those of ILs.21 DESs, as a new generation of tunable and sustainable solvents,22 are the mixture of two or more components with a melting point lower than either of its individual components.23-24 Compared with ILs, DESs provide more interesting advantages such as biodegradability, biocompatibility, 100% atom economy, ease of preparation, and favorable for large-scale applications. DES have been widely used as green solvents in a plethora of different chemical applications, such as electrochemistry,24 metal extraction,25 nanotechnology,25 stabilization of DNA,26 polymer materials,27 catalysis,28 organic synthesis,29 and gas mixtures separation.30-31 DESs based on cuprous chloride and 1-butyl-3-methylimidazolium chloride, were already reported by our group as promising membrane liquid in the separation of olefin/paraffin mixtures, since remarkably high carrier concentration and permeability were obtained.32 The greatest strength of metal based DESs is that carriers are not introduced by the dissolution but instead as the intrinsic structural unit of membrane liquid. However, the low olefin/paraffin selectivity and poor stability of copper (I) based DESs have created enough incentives for us to develop novel DESs for olefin/paraffin separation. Considering the strengths of silver salt and copper (I) based DESs, the development of silver based DES is highly desirable and may lead to a qualitative leap in the olefin/paraffin separation compared to current research. Inspired by the formation mechanism of DES33-34, we designed a new family of sustainable

silver-based

DESs,

which

were

constructed

from

trifluoromethanesulfonate

(AgCF3SO3)-acetamide (CH3CONH2) with different compositions. Then, the as-prepared silver based DESs were used as membrane liquids for fabricating olefin/paraffin separation SLMs. To the best of our knowledge, this was the first report on utilization of silver-based DESs as the membrane liquids for SLMs. More specifically, The DESs-SLMs was prepared by impregnating the silver-based DESs into the microporous polyvinylidene fluoride (PVDF) support and their separation performance was evaluated by ethylene and ethane mixture gases separation experiments. The structure of silver-based DESs and the transport mechanism of the DESs-SLMs were intensively investigated by various characterizations. The effects of the silver-based DESs composition, the operation temperature and the trans-membrane pressure were investigated systemically. In addition, the long-term stability of the DESs-SLMs was also evaluated. Finally, we compared the separation performance of DESs-SLMs with the latest plot of upper bound, confirming that the DESs-SLMs exhibited the outstanding C2H4 permeability and reasonable C2H4/C2H6 selectivity. 4


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Experimental Materials and Reagents. Ethylene and ethane (≥99.9%) were purchased from Tianjin Tang Dynasty Gas Co., Ltd (China). Acetamide (≥99.5 wt %) was purchased from Shanghai Titan Technology Co., Ltd. (China), and was recrystallized with chloroform prior to use. Silver trifluoromethanesulfonate (AgCF3SO3) (≥99.8 wt %) was supplied by Beijing J&K Chemical Technology Co., Ltd. (China). All chemicals were used as received unless otherwise stated. The microporous PVDF membrane was obtained from Haining Zhongli Filtering equipment Corporation (China). PVDF membrane is a 75% porous hydrophilic PVDF membrane with a nominal pore size of 0.1 µm and 75 mm in diameter. The thickness of PVDF membrane support was determined using a digital micrometer, being the average value of 125 µm. The PVDF membrane was selected as support due to its well-known thermal, chemical and mechanical stability. More important, the contact angle of DES on the PVDF membrane was about 15°, and thus PVDF membrane could be easily wetted by the silver-based DESs.

DESs Synthesis. AgCF3SO3 was dried under light-protected vacuum conditions at 60 oC for 48 h. CH3CONH2 was dried at 65 oC for 72 h. The silver-based DESs was prepared by simply mixing AgCF3SO3 and acetamide. Homogeneous transparent liquids with molar ratios of AgCF3SO3 to CH3CONH2 from 1:2.5 to 1:4 were obtained after stirring the mixtures at room temperature for 2 h. The water content of the silver-based DESs was determined to be less than 5000 ppm by Karl–Fischer titration method (DL37 KF Coulometer, Mettler Toledo).

Membrane Preparation. All the DESs-SLMs were prepared by the pressure-based technique reported by Fernández et al.35. Initially, the microporous PVDF membrane was introduced into a vacuum desiccator for 2 h in order to remove the air and trace water from the pores, thereby allowing an easier immersion of DESs into membrane pores. Subsequently, 3 mL DESs were spread out at the membrane surface using a syringe. Then DESs were impregnated into the membrane pores by the 1 bar of trans-membrane pressure in a circular membrane cell. The pressure was released until a thin layer of DESs was clearly visible on the low surface of the membrane. This procedure was repeated three times in order to ensure the membrane pores were completely filled with the silver-based DESs. Finally, the excessive DESs were wiped from the surface of the membrane with paper tissue. The success fabrication of DESs-SLMs was confirmed by the characterizations of SEM and ATR-FTIR (Figure S1-S2).

Characterizations of DESs-SLMs. The microscopic morphology of the PVDF membrane and the 5



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distribution of DESs within the DESs-SLMs were observed by the Hitachi S-4800 field emission scanning electron microscopy (SEM). Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was employed to investigate the surface chemistry of DESs-SLMs (Bio-Rad, FTS 6000). Each spectrum was recorded from 400 cm-1 to 4000 cm-1 at the resolution of 4 cm-1.

Gas absorption tests. The absorption capacity of C2H4 and C2H6 were determined based on the pressure drop method described in the previous publications. The experimental setup (Figure S3) and specific measurement procedure were illustrated in Supporting Information.36 The absorption capacity of pure gas in DESs was determined by Eq. (1):37 P V

P1 Vs -

0

Z1 RT

ni =ninitial -nfinal = Z 0RTr =

P V

- Z 2RTr

(1)

2

(2)

Where P0 is the pressure of the total gas introduced into the gas storage vessel while P1 and P2 are the pressure of equilibration vessel and gas storage vessel at equilibrium. V1, V2, and VDESs are the total volume of the equilibration vessel, gas storage vessel, and the absorbents in equilibration vessel. Z0, Z1, and Z2 are the compressibility factors for the pure gas at different equilibrium states.

Mixed gas separation.

Mixed-gas permeation experiments were performed as reported by our

group previously and the set-up was shown in Supporting Information (Figure S4).38 The readers should see supporting information for more details, if needed. The gas permeation experiments were performed at desired conditions. Three parallel experiments were carried out to obtain the stable values. For mixed gas experiments, the permeability coefficient ( ) of a particular gas is defined as the permeation flux (J ) normalized to the pressure difference across the membrane ( ∆ ), as well as the membrane thickness ( ), which is described in the Eq. (3):

= J

∆

(3)

The gas transport through the prepared membranes can be interpreted by the solution-diffusion theory. Therefore, the permeability can further be represented as the product of a thermodynamic solubility coefficient (S), and a kinetic effective diffusion coefficient (D).

= ∙

(4)

The gas solubility coefficient can be determined by the gas absorption capacity ( ) . Once determined the gas permeability through the membrane and gas solubility in the DESs, the effective diffusion coefficient can be can be derived from Eq. (4). The permeability selectivity ( , ) is obtained by dividing the permeability of the more permeable specie i to the permeability of the less permeable 6


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specie j for the case where the downstream pressure is negligible relative to the upstream feed pressure. As shown in Eq. (5), the , can be also expressed as the product of the diffusivity selectivity and the solubility selectivity.

, = = ∙

(5)

3. Results and discussion Physical chemical properties of membrane liquid. It is very interesting to notify that AgCF3SO3 and acetamide, whose melting points are 286 oC and 81 oC, respectively, can form transparent homogeneous liquid at room temperature within an appropriate molar ratio range. The silver salt and acetamide became wet immediately after contacting with each other and liquid drops could be observed on the wall of container. DSC curves of the AgCF3SO3-acetamide based DESs with the temperature ranging from -80 oC to 100 oC are shown in Figure S5 and the eutectic points are summarized in the Table 1. Only one endothermic peak in each DSC curve was obtained when the molar ratio of AgCF3SO3 to acetamide varies from 1: 2.5 to 1:4, which verified the typical eutectic characters of the AgCF3SO3-acetamide binary systems.

Table 1. The melting points and viscosities of silver-based DESs Molar ratio

Melting point

Viscosity

(oC)

(cp)

Abbreviation (AgCF3SO3:Acetamide)

Appearance

1:2.5

DES1

-51.1

172.4

Colorless liquid

1:3

DES2

-55.9

80.73

Colorless liquid

1:3.5

DES3

-57.8

41.90

Colorless liquid

1:4

DES4

-59.7

24.42

Colorless liquid

The low viscosity of membrane liquid increases the transport rate within SLMs, which explains why the viscosity is an important parameter for studies of membrane separation performance.39 As shown in Table 1, the viscosities of silver-based DESs increased with the decrease of molar ratio of AgCF3SO3 to acetamide at 298.15 K, which seemed to be a direct consequence of the multiple interactions between the different species in the eutectic mixtures.40 The viscosities of silver-based DESs obtained in our study were similar to that of the DESs based on N-methylacetamide and lithium salt reported by Warda Zaidi et al.,41 while was significantly lower than that of choline chloride-glycerol based DESs reported by Andrew P. Abbott et al.42 7



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The complexation between Ag+ and acetamide. The interactions between Ag+ and acetamide were characterized by FTIR and FT-Raman spectroscopy. As shown in Figure 1a, the peak at 1672 cm-1 was ascribed to the C=O stretching modes of pure acetamide.43 Upon mixing AgCF3SO3 with acetamide, the stretching peak of C=O split into two peaks. A new peak appeared at 1616 cm-1, the other peak exhibited red shifts from 1672 to 1664 cm-1 at AgCF3SO3 : acetamide (1:2.5), and from 1672 to 1667 cm-1 at AgCF3SO3 : acetamide (1:3 and 1:4), respectively. These results suggested the occurrence of strong interactions between Ag+ ions and acetamide, due to the fact that Ag+ ions were likely to coordinate with the O atoms of C=O donor groups in the DESs resulted from the high negative charge density of the O atom of C=O. It should be noted that the strongest interactions between Ag+ and acetamide occurred at the molar ratio of AgCF3SO3 to acetamide of 1:2.5. In addition, the interactions between Ag+ ions and acetamide also induced the significant changes of the C-C and N-C=O stretching vibrations of acetamide in the DESs, which can be revealed by the FTIR spectra in Figure S6. As shown in the Figure 1b, the band at 1405 cm-1 was assigned to the C-N stretching mode. A new band appeared at 1435 cm-1 upon mixing AgCF3SO3 with acetamide and possessed strongest intensity at the molar ratio of AgCF3SO3 to acetamide of 1:2.5. The Ag+ ions strongly coordinated with the C=O group of acetamide, possibly induced the occurrence of the resonance structure of acetamide,44-45 thus C-N bond exhibited the partial double bond characters. In conclusion, the variation of the C-N band was in good agreement with that of C=O band as shown in Figure 1a.

8


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Figure 1. The dissociation of AgCF3SO3 and complexation between Ag+ and acetamide were characterized by the FTIR, FT-Raman, MS. (a) FIIR spectra of the C=O stretching mode in the DESs (b) FT-Raman spectra of C-N stretching mode in the DESs. (c) FT-Raman spectra of the symmetric stretching vibration of SO3- for DESs. (d) The positive model of MS of DESs with different molar ratio of AgCF3SO3 to acetamide at 1:2.5 and 1:3.

Dissociation of AgCF3SO3. Raman spectroscopy is a powerful tool to provide quantitative information on the dissociation and association behaviors of salts. According to the previous research,46-47 the bands for the νs (SO3-) stretching mode at 1032, 1038, and 1048 cm-1 in the complexes were due to free ions (fully dissociated ions or /and solvent separated ion pairs), contact ion pairs, and higher-order ionic aggregates, respectively. As shown in the Figure 1c, the symmetric stretching vibration peaks of SO3- appeared at 1032 cm-1 in DESs with different molar ratios, suggesting that the CF3SO3- anions did not interact with Ag+ directly in DESs, and the molar ratios of DESs had scarcely any effect on the dissociation of AgCF3SO3 due to the low Lattice energy of AgCF3SO31. It is impossible to distinguish between fully dissociated ions and solvent separated ion pairs. However, in the AgCF3SO3-acetamide based DESs, the free anions preferred to be solvent separated ion pairs due to the strong interaction between Ag+ and acetamide, as described above. The relative affinity of Ag+ for 9



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acetamide in DESs has been evaluated by the MS,48 as shown in Figure 1d. Two intense peaks at m/z 226.07 and 462.29 in the DESs (1:2.5), corresponded to clusters of [Ag (acetamide)2]+ and [Ag(acetamide)6]+, respectively. When the molar ratios of AgCF3SO3 to acetamide were 1:3 and 1:4, all the spectra verified the presence of the free Ag+ at m/z 107.82, of [Ag(acetamide)4]+ clusters at m/z 344.15, in addition to the clusters of [Ag(acetamide)2]+ and [Ag(acetamide)6]+. The presence of free Ag+ in the DESs at molar ratios of 1:3 and 1:4 demonstrated the interactions between Ag+ and acetamide were less than that in the DESs at molar ratios of 1:2.5. This change trend agreed well with the results of FTIR, which favored the facilitated transport of C2H4 through DESs-SLMs.

The Destruction and Reconstruction of hydrogen-bond networks in DESs. The characterizations of FT-Raman and FIIR have been used to gain insight into the destruction hydrogen-bond networks in DESs.49-50 As seen from Figure 2a, the spectrum of pure acetamide consisted of a relatively sharp and strong band at 3156 cm-1, and a very weak and broad band at roughly 3345 cm-1, which were ascribed to N-H symmetric stretching and asymmetric stretching vibrations.51 Upon mixing AgCF3SO3 with acetamide, the stretching peak of N-H exhibited the significant blue-shifts, demonstrating the breaking of the hydrogen bonds among the acetamide molecules in the DESs. More importantly, in DESs, the asymmetric stretching vibrations of N-H bond intensified compared with that in pure acetamide, further confirming the breaking of the hydrogen bonds.52 Among the three DESs, the N-H vibrations exhibited the biggest blue-shifts from 3156 to 3233 cm-1 and from 3345 to 3356 cm-1 at the molar ratio of AgCF3SO3 to acetamide of 1:2.5, respectively, which meant the destruction of hydrogen bonds was the most serious. The results of FTIR spectra also demonstrated the destruction of hydrogen-bond networks in DESs. As shown in the Figure 2b, for the pure acetamide, two peaks at 3193 cm-1 and 3378 cm-1 were assigned to the symmetric and asymmetric stretching vibrations of N-H bond. Both peaks showed significant blue shifts to 3209 cm-1 and 3440 cm-1 upon mixing with the AgCF3SO3, which were well in agreement with the measurement results of FT-Raman.

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Figure 2. The destruction and reconstruction of hydrogen-bond networks in DESs were characterized by the FT-Raman, FTIR, 1H NMR and MS. (a) FT-Raman spectra of DESs with different molar ratios in the NH2 stretching modes region. (b) FTIR spectra of DESs with different molar ratios in the NH2 stretching modes region. (c) 1HNMR spectra of NH2 chemical shift in the DESs. (d) The negative model of MS of DESs with molar ratio of AgCF3SO3 to acetamide at 1:2.5. It should be also noted that a new band appeared at 3365 cm-1, 3362 cm-1, 3359 cm-1 for DESs at the molar ratio of AgCF3SO3 to acetamide of 1:2.5, 1:3, 1:4, respectively, as shown in the Figure 2b. The new band probably originated from the hydrogen bonding interactions between the NH2 groups of acetamide and the SO3 groups of CF3SO3- anions. It was understandable that when the Ag+ ions coordinate strongly with the C=O group of acetamide, part of the SO3 group in CF3SO3- anions dissociated from coordination with the Ag+ ions and possessed negative charges. These SO3- groups had a tendency to interact with the partially positive-charged NH2 groups of acetamide. The combination of 1HNMR and ESI-MS provided more evidences of the reconstruction of hydrogen-bond networks in DESs. As shown in Figure 2c, the NH2 groups of acetamide split into two peaks at 6.711 and 7.305 ppm, respectively. The N-H of acetamide shifted to downfield gradually as 11



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the composition changed with addition acetamide, which agreed well with the above-mentioned characterization of FTIR. These results demonstrated that hydrogen bonds between CF3SO3- and acetamide were formed and the strength of the hydrogen bonds became stronger with the decrease of the molar ratio of AgCF3SO3 to acetamide. The downfield shift in 1HNMR also indicated a lower electron density and stronger interionic interaction through forming the hydrogen-bond networks.53 Figure 2d shows the negative MS model of DESs with different molar ratios. The most intensive peak at m/z 503.49 was assigned to the [(acetamide)6·CF3SO3]-. The second most intense peak corresponded to the [CF3SO3]- (m/z 149.45). The peaks of [(acetamide)3·CF3SO3]- and [(acetamide)4·CF3SO3]- were also detected at 326.62 and 385.49, respectively. The forming clusters between acetamide and CF3SO3further corroborated the reconstruction of hydrogen-bond networks in DESs. The intensity of bigger clusters such as [(acetamide)4·CF3SO3]- and [(acetamide)6·CF3SO3]- increased, whereas the [CF3SO3]decreased with the decrease of the molar ratio of AgCF3SO3 to acetamide, which is shown in the Figure S8. In summary, the results of FTIR, FT-Raman, ESI-MS, 1HNMR demonstrated that acetamide worked as a complexing agent for both Ag+ and CF3SO3- via its C=O group and NH2 group (Figure S9). Specifically, Ag+ ions coordinated with the C=O group of acetamide, which resulted in the breaking of the hydrogen bonds among acetamide molecules and the Coulombic interaction between the anions and the cations of AgCF3SO3, eventually contributing to the formation of the stable DESs. Moreover, the CF3SO3- anions interacted with NH2 group of acetamide via hydrogen bonds. The hydrogen bonding between anions and the neutral hydrogen-bond donors made charge delocalization, which was responsible for the decrease in the melting point of the mixture relative to the melting points of the individual components.

Gas Absorption Capacity. As seen from Figure 3, it was apparent that the absorption capacity of C2H4 was much higher than that of C2H6 in the studied DESs, which was due to the chemical interactions between the silver ion and the double bond of ethylene. The C2H6 absorption isotherms in the DESs showed a linear increase with the pressure, which indicated the physical absorption and could be described by the Henry’s law (Figure 3b, 3d). In contrast, the C2H4 equilibrium data gave eloquent proof of the combination of chemical and physical effects (Figure 3a, 3c). As expected, the absorption capacity of the C2H4 and C2H6 in the DESs decreased along with the temperature, and the temperature had much more remarkable effect on the absorption capacity of C2H4 that that of C2H6, which could be 12


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explained by that the higher temperature weakened the complexation interactions between silver ions and C2H4 (Figure 3a, 3b). As also seen from Figure 3d, the C2H6 equilibrium data in DESs with different molar ratios showed the negative influence of the molar ratio of AgCF3SO3 to acetamide on the absorption capacity of C2H6; therefore, a salting out effect was observed. In addition, it was worthy of noting that the molar ratio of AgCF3SO3 to acetamide in the DESs had a complicated effect on the absorption capacity of C2H4. The absorption capacity of C2H4 in the DESs increased with the molar ratio ranging from 1:2.5 to 1:3.5, and then slightly decreased at the molar ratio of 1:4, which suggested that the available Ag+ for the complexation with C2H4 could be tuned by the different molar ratios of AgCF3SO3 to acetamide in DESs (Figure 3c). The available Ag+ for the complexation rested with the silver-acetamide interactions and the silver salt concentration in the DESs.

Figure 3. The absorption capacity of C2H4 (a) and C2H6 (b) in the silver based DESs (1:3) at different temperatures. The absorption capacity of C2H4 (c) and C2H6 (d) at 298.15 K in the silver based DESs at different temperatures molar ratios.

Effect of the DESs composition. Figure 4 shows the gas separation performance of the DESs-SLMs with different molar ratios. As seen from Figure 4a, the gas permeability increased with the molar ratio of AgCF3SO3 to acetamide varying from 1:2.5 to 1:4, which could be clearly attributed to the increase of the gas solubility and diffusivity. Moreover, C2H4 permeability was always almost two orders of magnitude higher than that of C2H6 at any molar ratios from 1:2.5 to 1:4 due to different transport 13



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mechanism (Figure 4a). In addition to the normal Fick transport, C2H4 could reversibly coordinate with the carriers containing silver ions and silver complexes inside the DESs-SLMs, and thus facilitated the transport of C2H4. The same trend was observed for gas solubility (SC2H4 > SC2H6), which further confirmed that only C2H4 formed a complex with the carrier providing higher solubility of the C2H4 in the membrane (Figure 4b). The C2H4 and C2H4 gas diffusivity increased with the variation of the molar ratio, which was in good agreement with the change of above-mentioned viscosities of DESs (Figure 4c). It should be also noted that the C2H4 diffusivity was smaller than that of C2H6 at molar ratio of 1:2.5, which was probably due to the stronger interactions between silver ions with acetamide slowed the transport of the silver-olefin complex (Figure 4c). As also seen from Figure 4d, C2H4/C2H6 selectivity of DESs-SLMs increased initially and then decreased with the molar ratio ranging from 1:2.5 to 1:4, obtaining maximum at the molar ration of 1:3. Which deeply depended on the solubility selectivity and diffusivity selectivity. Comparing the solubility selectivity with diffusivity selectivity, it could be concluded that the C2H4/C2H6 selectivity of DESs-SLMs was essentially dominated by the solubility selectivity (Figure 4d).

Figure 4. The gas permeability (a), solubility (b), diffusivity (c) and C2H4/C2H6 selectivity (d) in the DESs with different molar ratios at the transmembrane pressure of 0.1 bar, at the temperature at 298.15 K and at the sweep gas of 20 mL/min.

Effect of temperature. Increasing temperature increases the rate of diffusion so that permeability increases, but selectivity decreases.54 As shown in the Figure 5, the increased diffusivity and decreased 14


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solubility counteracted with each other, resulting in the changes of C2H4 and C2H6 permeabilities. Both gas permeabilities increased with the temperature, which was mainly due to the faster gas diffusivity in the DESs-SLMs at higher temperature (Figure 5a, 5c). The higher temperature meant lower viscosities of DESs, which favored the transport of gases. It was also evident that the permeability of C2H6 increased more significantly and resulted in lower selectivity at elevated temperatures, which could be explained that the slight decrease of C2H6 solubility with the increase of temperature (Figure 5a, 5b). Compared with the C2H6, the C2H4 solubility experienced a sharp decrease with temperature, which could be explained by that the increased temperature weakened the coordination between the carriers and C2H4 as the reversible complexation reaction was exothermic (Figure 5b). The data of permeability selectivity, solubility selectivity and diffusivity selectivity were also summarized in the Figure 5d, which illustrated the quantity of contribution percent of solubility and diffusivity on the total permeability.

Figure 5. The gas permeability (a), solubility (b), diffusivity (c) and C2H4/C2H6 selectivity (d) in the DESs (1:3) at different temperatures at the transmembrane pressure of 0.1 bar and at the sweep gas of 20 mL/min.

Optimization of Separation Conditions. The effects of feed ratio of C2H4 to C2H6, the flow rate of sweep gas and the transmembrane pressure have been investigated systematically and the results are collected in the Figure 6a-6c. As seen from Figure 6a, the C2H4 permeability increased from 260 Barrer to 1337 Barrer with the feed ratio of C2H4 to C2H6 ranging from 1:5 to 5:1, while the C2H6 permeability 15



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exhibited an opposite trend. Therefore, the C2H4/C2H6 selectivity exhibited a sharp increase up to 303 at the feed ratio of 5:1. As shown in Figure 6b, the C2H4 permeability initially increased as the flow rate of sweep gas increased, then almost remained unchanged. This is because higher flow rates of sweep gas removed more efficiently the C2H4 molecules which arrived to the permeation side, allowing to operate at the higher driving forces. In contrast, the C2H6 permeability was almost unaffected by the sweep gas. As seen from Figure 6c, the C2H4 permeability through DESs-SLMs significantly decreased with the increase of trans-membrane pressure, indicating that most of carriers were saturated even at low pressure and the C2H4 permeation was a diffusion controlled process. However, the C2H6 permeability almost remained unchanged over the investigated trans-membrane pressure differentials, suggesting the C2H6 permeability through the DESs-SLMs could be described by the solution-diffusion mechanism. Therefore, the C2H4/C2H6 selectivity decreased with the increased trans-membrane pressures.

Figure 6. The effects of feed ratio (a), sweep gas (b), and trans-membrane pressure (c) on separation performance and long-term stability of DESs-SLMs (d).

Long term stability of the membranes. The long term stability is crucial for the application of DESs-SLMs. The membrane was carried out for 130 h to evaluate the stability of the membrane at the molar ratio of AgCF3SO3 to acetamide of 1:3, at the temperature of 298 K, at the flow rate of sweep gas of 20 ml/min and at the trans-membrane pressure of 0.1 bar. As presented in Figure 6d, the C2H4 and 16


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C2H6 permeability remained almost unchanged during long-time running. The C2H4/C2H6 selectivity varied slightly in the initial stage of the test experiment, and then remained stable with the test time ranging from 30 h to 130 h. The results clearly showed that the prepared membrane possessed great potential for long-time operation.

Comparison with other membranes. It is well known that the upper bound is commonly used to evaluate the gas separation performance of materials illustrating the process in membrane science for gas separation and displays the tradeoff line between permeability and selectivity. However, the upper bound of liquids membranes for C2H4/C2H6 separation was not involved in the open literature up to now due to the limited data. Koros et al. firstly proposed the C2H4/C2H6 upper bound for polymeric membranes in 2013.55 Therefore we compared the results obtained from this study with the upper bound of polymeric membrane proposed by Koros.55 As seen from Figure 7, our DESs-SLMs exhibited high permeability for C2H4 and reasonable separation selectivity, thus exceeding the upper bound line. The separation performance of DESs-SLMs was superior to the most polymeric membranes. Moreover, compared with the recent reported supported ionic liquids membranes,15,17-18 our membrane still exhibited better performance for the C2H4/C2H6 separation. All the above results suggested DESs-SLMs was a potential alternative to the existing separation technology for separation of C2H4/C2H6.

Figure 7. C2H4/C2H6 membrane separation performance in recent studies. Data are plotted on a log-log scale and the upper bound is adapted from Koros.55

Conclusions 17



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Novel silver-based DESs, consisting of AgCF3SO3 and CH3CONH2, were firstly prepared as the membrane liquid. Base on the above-mentioned DESs, DESs-SLMs were fabricated and their performance of was investigated by gas separation experiments. The C2H4/C2H6 selectivity of DESs-SLMs initially increased with the molar ratio ranging from 1:2.5 to 1:3, and then decreased with the molar ratio ranging from 1:3 to 1:4. The increase of trans-membrane pressure decreased the C2H4/C2H6 selectivity mainly because of the decrease in permeability of C2H4. The increase in temperature increased the permeability of C2H4 and C2H6, but decreased the selectivity of C2H4/C2H6. As is clearly evidenced by the solubility and permeability experiments, the C2H4/C2H6 selectivity of DESs-SLMs was essentially dominated by the solubility selectivity. Compared with other studies, the DESs-SLMs possessed the comparable selectivity for C2H4/C2H6 and excellent permeability of C2H4, thus exceeding the upper bound. In summary, our study provided a promising alternative to the conventional technology for the olefin/paraffin separation and the further design and strengthening of DESs will bring the potential for more excellent performance.

Supporting Information Characterizations of silver-based DESs, characterization results of DESs-SLMs, absorption capacity tests and gas permeability measurements, characterization results of DESs, the test conditions.

Note The authors declare no competing financial interest.

Acknowledgements We are grateful for the financial support from National Key R&D Program of China (No. 2016YFC0400406).

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