Silver-Based Deep Eutectic Solvents as Separation Media: Supported

Research Article pubs.acs.org/journal/ascecg

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* School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of China S Supporting Information *

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 deep eutectic solvents (DESs) constructed from trifluoromethanesulfonate (AgCF3SO3) and acetamide (CH3CONH2) were synthesized as membrane liquids for the first time. Their formation mechanism and structure were investigated intensively, confirming that multiple coordination and hydrogen-bonding interactions yielded homogeneous and stable liquids that contained free silver ions and silver-containing cationic complexes as carriers for the facilitated transport of C2H4. The asprepared DES-SLMs were characterized by SEM and ATR-FTIR and their separation performances were investigated by C2H4/C2H6 separation experiments. The effects of the composition of the silver-based DESs, the operating temperature, and the transmembrane pressure were also investigated systemically. The permeability selectivity, solubility selectivity, and diffusivity selectivity were also quantitatively analyzed. Compared with previously reported results, the as-prepared DES-SLMs exhibited excellent permeabilities of C2H4 and comparable selectivities of C2H4/C2H6, thus exceeding the upper bound. This investigation might provide alternatives for the development of high-performance SLMs for olefin/paraffin separations and insights into the formation mechanism of silver-based DESs for further design and strengthening. KEYWORDS: Deep eutectic solvents, Supported liquid membrane, Olefin/paraffin, Facilitated transport

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INTRODUCTION Olefins are significant in the petroleum and chemical industries because they are the 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 olefin/paraffin separations 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. Supported liquid membranes (SLMs) have special technical advantages, such as integration of the extraction and stripping processes, small-sized equipment, relative environmental safety, low energy consumption, and low operating costs. Compared with solid polymeric membranes, SLMs are intrinsically highly permeable, as diffusion in liquids is much faster than that in solids. The selectivities of SLMs are 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 SLMs for olefin/paraffin separations is water.7,8 However, the high volatilization of water results in additional investments and energy consumption for its subsequent removal, and the SLMs © 2017 American Chemical Society

suffer from severe loss of membrane liquid. Consequently, some research groups have investigated the potential of utilizing high-boiling-point solvents as membrane liquids, such as N-methylpyrrolidone,9 glycerol, and triethylene glycol.10,11 Recently, ionic liquids (ILs) have attracted great interest from researchers and have been proposed as green alternatives to organic solvents because of their unique properties such as extremely low volatility, high thermal stability, and structural designability, which lead to more sustainable separations with lower 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, despite their promising and green advantages, the olefin-to-paraffin selectivity in pure ILs is very low. To enhance the olefin/paraffin selectivity, the carrier-facilitated transport concept was introduced for the design of novel SLMs. Silver salts have the strength of effective facilitated transport of olefins, thus obtaining high olefin/ paraffin selectivities. Ortiz and co-workers used ionic liquids to dissolve AgBF4 for the facilitated transport of olefins.17,18 Received: April 10, 2017 Revised: June 22, 2017 Published: July 5, 2017 6873

DOI: 10.1021/acssuschemeng.7b01092 ACS Sustainable Chem. Eng. 2017, 5, 6873−6882

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ACS Sustainable Chemistry & Engineering

Ltd. (Beijing, China). All chemicals were used as received unless otherwise stated. Microporous PVDF membranes were obtained from Haining Zhongli Filtering Equipment Corporation (Haining, China). The membranes were 75% porous hydrophilic PVDF membranes with a nominal pore size of 0.1 μm and a diameter of 75 mm. The thickness of the PVDF membrane supports was determined using a digital micrometer and had an average value of 125 μm. PVDF membrane was selected as the support because of its well-known thermal, chemical, and mechanical stability. More important, the contact angles of DESs on PVDF membranes are about 15°, and thus the PVDF membrane could be easily wetted by the silver-based DESs. DES Synthesis. AgCF3SO3 was dried under light-protected vacuum conditions at 60 °C for 48 h. CH3CONH2 was dried at 65 °C for 72 h. The silver-based DESs were prepared by simply mixing AgCF3SO3 and acetamide. Homogeneous transparent liquids with AgCF3SO3/CH3CONH2 molar ratios ranging from 1:2.5 to 1:4 were obtained after the mixtures had been stirred at room temperature for 2 h. The water content of the silver-based DESs was determined to be less than 5000 ppm by the Karl Fischer titration method (DL37 KF Coulometer, Mettler-Toledo). Membrane Preparation. All of the DES-SLMs were prepared by the pressure-based technique reported by Hernández-Fernández et al.35 Initially, the microporous PVDF membranes were introduced into a vacuum desiccator for 2 h to remove air and trace water from the pores, thereby allowing easier immersion of the DESs into the membrane pores. Subsequently, for each DES, 3 mL of DES was spread on the membrane surface using a syringe. Then, the DES was impregnated into the membrane pores at a transmembrane pressure of 1 bar in a circular membrane cell. The pressure was released until a thin layer of DES was clearly visible on the lower surface of the membrane. This procedure was repeated three times to ensure that the membrane pores were completely filled with the silver-based DES. Finally, the excess DES was wiped from the surface of the membrane with paper tissue. The successful fabrication of DES-SLMs was confirmed by SEM and attenuated-total-reflectance Fourier transform infrared (ATR-FTIR) spectroscopy characterizations (Figures S1 and S2). Characterization of DES-SLMs. The microscopic morphology of the PVDF membrane and the distribution of DESs within the DESSLMs were observed by the field-emission scanning electron microscopy (SEM; Hitachi S-4800). ATR-FTIR spectroscopy was employed to investigate the surface chemistry of the DES-SLMs (BioRad, FTS 6000). Each spectrum was recorded from 400 to 4000 cm−1 at a resolution of 4 cm−1. Gas Absorption Tests. The absorption capacities of C2H4 and C2H6 were determined based on the pressure drop method described in previous publications. The experimental setup (Figure S3) and specific measurement procedure are described in the Supporting Information.36 The absorption capacity of pure gas i in each DES was determined as37

Unfortunately, the low solubilities of silver salts in ionic liquids resulted in limited improvement in the olefin/paraffin selectivity.19 Moreover, high loadings of silver salts resulted in high-viscosity membrane liquids, thus slowing the mass transport.20 Deep eutectic solvents (DESs) have recently emerged as potential alternatives to ILs, because their physicochemical properties are similar to those of ILs.21 DESs, as a new generation of tunable and sustainable solvents,22 are mixtures of two or more components with melting points lower than those of either of the individual components.23,24 Compared with ILs, DESs provide more interesting advantages such as biodegradability, biocompatibility, 100% atom economy, ease of preparation, and favorability for large-scale applications. DESs have been widely used as green solvents in a variety 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-mixture separations.30,31 DESs based on cuprous chloride and 1-butyl-3methylimidazolium chloride were already reported by our group as promising membrane liquids for the separation of olefin/paraffin mixtures, because markedly high carrier concentrations and permeabilities were obtained.32 The greatest strength of metal-based DESs is that the carriers are introduced not by dissolution but instead as the intrinsic structural unit of the membrane liquid. However, the low olefin/paraffin selectivities and poor stabilities of copper(I)based DESs have created sufficient incentives for the development of novel DESs for olefin/paraffin separations. Considering the strengths of silver salts and copper(I)-based DESs, the development of silver-based DES is highly desirable and could lead to a qualitative leap in olefin/paraffin separations compared to current research. Inspired by the formation mechanism of DESs,33,34 we designed a new family of sustainable silver-based DESs constructed from trifluoromethanesulfonate (AgCF3SO3) and 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 is the first report on the utilization of silver-based DESs as membrane liquids for SLMs. More specifically, DES-SLMs were prepared by impregnating silver-based DESs into microporous poly(vinylidene fluoride) (PVDF) supports, and their separation performances were evaluated by experiments on ethylene and ethane gas-mixture separations. The structures of the silverbased DESs and the transport mechanism of the DES-SLMs were intensively investigated by various characterization techniques. The effects of the silver-based DES composition, the operating temperature, and the transmembrane pressure were investigated systemically. In addition, the long-term stability of the DES-SLMs was also evaluated. Finally, we compared the separation performance of DES-SLMs with the latest plot of the upper bound, confirming that the DES-SLMs exhibited outstanding C2H4 permeabilities and reasonable C2H4/C2H6 selectivities.

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ni = n initial − n final =

Ci =

ni VDES

P0Vr P (V − VDES) PV − 1 s − 2 r Z0RT Z1RT Z 2RT

(1)

(2)

where P0 is the pressure of the total gas introduced into the gas storage vessel, weheras P1 and P2 are the pressures of the equilibration vessel and the gas storage vessel, respeectively, at equilibrium. V1, V2, and VDES are the total volumes of the equilibration vessel, the gas storage vessel, and the absorbent in the equilibration vessel, respectively. Z0, Z1, and Z2 are the compressibility factors of the pure gas at different equilibrium states. Mixed-Gas Separations. Mixed-gas permeation experiments were performed as reported by our group previously, and the setup is shown in the Supporting Information (Figure S4).38 The interested reader should see the Supporting Information for more details, if needed. Gas permeation experiments were performed at the desired conditions. Three parallel experiments were carried out to obtain

EXPERIMENTAL SECTION

Materials and Reagents. Ethylene and ethane (≥99.9%) were purchased from Tianjin Tang Dynasty Gas Co., Ltd. (Tianjin, China). Acetamide (≥99.5 wt %) was purchased from Shanghai Titan Technology Co., Ltd. (Shanghai, China) and was recrystallized with chloroform prior to use. Silver trifluoromethanesulfonate (AgCF3SO3) (≥99.8 wt %) was supplied by Beijing J&K Chemical Technology Co., 6874


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ACS Sustainable Chemistry & Engineering stable values. For mixed-gas experiments, the permeability coefficient (Pi) of a particular gas is defined as the permeation flux (Ji) normalized by the pressure difference across the membrane (ΔPi) and the membrane thickness (δ), as expressed in the equation

Pi = Ji

δ ΔPi

silver-based DESs obtained in our study were similar to those of the DESs based on N-methylacetamide and lithium salt reported by Zaidi et al.41 but significantly lower than those of choline chloride−glycerol based DESs reported by Abbott et al.42 Complexation between Ag+ and Acetamide. The interactions between Ag+ and acetamide were characterized by FTIR and FT-Raman spectroscopies. As shown in Figure 1a, the peak at 1672 cm−1 was ascribed to the CO stretching modes of pure acetamide.43 Upon the mixing of AgCF3SO3 with acetamide, the stretching peak of CO split into two peaks. A new peak appeared at 1616 cm−1, and the other peak exhibited a red shift from 1672 to 1664 cm−1 at a AgCF3SO3/ acetamide molar ratio of 1:2.5 and a red shift from 1672 to 1667 cm−1 at AgCF3SO3/acetamide molar ratios of 1:3 and 1:4. 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 the CO donor groups in the DESs because of the high negative charge density on the O atom of CO. It should be noted that the strongest interactions between Ag+ and acetamide occurred at a AgCF3SO3-to-acetamide molar ratio of 1:2.5. In addition, the interactions between Ag+ ions and acetamide also induced significant changes in the CC and NCO stretching vibrations of acetamide in the DESs, as revealed by the FTIR spectra in Figure S6. As shown in 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 the mixing of AgCF3SO3 with acetamide and exhibited the strongest intensity at a 3SO3-toacetamide molar ratio of 1:2.5. The Ag+ ions strongly coordinated with the CO group of acetamide possibly induced the appearance of the resonance structure of acetamide;44,45 thus, the CN bond exhibited partial doublebond character. In conclusion, the variation of the CN band was in good agreement with that of the CO band, as shown in Figure 1a. Dissociation of AgCF3SO3. Raman spectroscopy is a powerful tool that provides quantitative information on the dissociation and association behaviors of salts. According to 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 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 the DESs and that the molar ratios of the DESs had scarcely any effect on the dissociation of AgCF3SO3 because of the low lattice energy of AgCF3SO3.1 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 solventseparated ion pairs because of the strong interaction between Ag+ and acetamide, as described above. The relative affinity of Ag+ for acetamide in the DESs was evaluated by mass spectrometry (MS),48 as shown in Figure 1d. Two intense peaks at m/z 226.07 and 462.29 in the DESs (1:2.5), correspond 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 of the spectra indicated the presence of free Ag+ at m/z 107.82 and of [Ag(acetamide)4]+ clusters at m/z 344.15, in addition to the clusters of [Ag(acetamide)2]+ and [Ag(acetamide)6]+. The

(3)

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

(4)

The gas solubility coefficient can be determined from the gas absorption capacity (ni). Once the gas permeability through the membrane and the gas solubility in the DES have been determined, the effective diffusion coefficient can be derived from eq 4. The permeability selectivity (Si,j) can be obtained by dividing the permeability of the more permeable species I by the permeability of the less permeable species j for the case in which the downstream pressure is negligible relative to the upstream feed pressure. As shown in eq 5, Si,j can be also expressed as the product of the diffusivity selectivity and the solubility selectivity

αi , j =

⎛ S ⎞⎛ D ⎞ Pi = ⎜⎜ i ⎟⎟⎜⎜ i ⎟⎟ Pj ⎝ Sj ⎠⎝ Dj ⎠

(5)

3. RESULTS AND DISCUSSION Physicochemical Properties of Membrane Liquid. It is very interesting to note that AgCF3SO3 and acetamide, whose melting points are 286 and 81 °C, respectively, can form a transparent homogeneous liquid at room temperature within an appropriate molar ratio range. The silver salt and acetamide became wet immediately after contacting each other, and liquid drops could be observed on the walls of the container. Differential scanning calorimetry (DSC) curves of the AgCF3SO3−acetamide-based DESs at temperatures ranging from −80 to 100 °C are shown in Figure S5, and the eutectic points are summarized in Table 1. Only one endothermic peak Table 1. Melting Points and Viscosities of Silver-Based DESs AgCF3SO3/acetamide molar ratio

abbreviation

melting point (°C)

viscosity (cP)

1:2.5

DES1

−51.1

172.4

1:3

DES2

−55.9

80.73

1:3.5

DES3

−57.8

41.90

1:4

DES4

−59.7

24.42

appearance colorless liquid colorless liquid colorless liquid colorless liquid

in each DSC curve was obtained when the molar ratio of AgCF3SO3 to acetamide was varied from 1:2.5 to 1:4, which verified the typical eutectic character of the AgCF3SO3− acetamide binary system. A low viscosity of the 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 the silver-based DESs increased with decreasing 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 mixture.40 The viscosities of the 6875


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Figure 1. Dissociation of AgCF3SO3 and complexation between Ag+ and acetamide characterized by FTIR spectroscopy, FT-Raman spectroscopy, and mass spectrometry (MS). (a) FTIR 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− in the DESs. (d) Positive model of the mass spectra of DESs with molar ratios of AgCF3SO3 to acetamide of 1:2.5 and 1:3.

presence of free Ag+ in the DESs at molar ratios of 1:3 and 1:4 demonstrated that the interactions between Ag+ and acetamide were less than those in the DES with a molar ratio of 1:2.5. This change trend agreed well with the results of FTIR analysis, which favored the facilitated transport of C2H4 through the DES-SLMs. Destruction and Reconstruction of Hydrogen-Bond Networks in DESs. FT-Raman and FTIR spectroscopies were used to gain insight into the destruction of the hydrogen-bond networks in the 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 the mixing of AgCF3SO3 with acetamide, the stretching peak of NH exhibited a significant blue shift, demonstrating the breaking of the hydrogen bonds among the acetamide molecules in the DESs. More importantly, in the DESs, the asymmetric stretching vibrations of NH bonds intensified compared with those in pure acetamide, further confirming the breaking of the hydrogen bonds.52 Among the three DESs, that with a AgCF3SO3 to acetamide molar ratio of 1:2.5 exhibited the largest the blue shifts of the NH vibrations, from 3156 to 3233 cm−1 and from 3345 to 3356 cm−1, which means that the destruction of the hydrogen bonds was most serious for this DES. The FTIR spectra also demonstrated the destruction of the hydrogen-bond networks in the DESs. As shown in Figure 2b, for the pure acetamide, two peaks at 3193 and 3378 cm−1 were assigned to the symmetric and asymmetric stretching vibrations of NH bonds. Both peaks showed significant blue shifts to 3209 and 3440 cm−1 upon mixing with AgCF3SO3, in

good agreement with the measurement results of the FTRaman spectroscopy. It should be also noted that a new band appeared at 3365, 3362, and 3359 cm−1 for DESs with molar ratios of AgCF3SO3 to acetamide of 1:2.5, 1:3, and 1:4, respectively, as shown in Figure 2b. The new band probably originated from the hydrogen-bonding interactions between the NH2 groups of acetamide and the SO3 groups of the CF3SO3− anions. It is understandable that, when the Ag+ ions coordinate strongly with the CO group of acetamide, part of the SO3 group in the CF3SO3− anions would dissociate from coordination with the Ag+ ions and take a negative charge. These SO3− groups had a tendency to interact with the partially positively charged NH2 groups of acetamide. The combination of 1H NMR speectroscopy and electrospray ionization-mass spectrometry (ESI-MS) provided more evidence for the reconstruction of hydrogen-bond networks in the DESs. As shown in Figure 2c, the NH2 groups of acetamide split into two peaks at 6.711 and 7.305 ppm. The NH group of acetamide shifted downfield gradually as the composition changed with the addition of acetamide, in good agreement with the above-mentioned characterization by FTIR spectroscopy. These results demonstrated that hydrogen bonds were formed between CF3SO3− and acetamide and the strengths of the hydrogen bonds increased with the decrease of the molar ratio of AgCF3SO3 to acetamide. The downfield shift in the 1H NMR peaks also indicated a lower electron density and stronger interionic interaction through the formation of hydrogen-bond networks.53 Figure 2d shows the negative MS model of DESs with different molar ratios. The most intense peak at m/z 503.49 was assigned to [(acetamide)6·CF3SO3]−. 6876


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Figure 2. Destruction and reconstruction of hydrogen-bond networks in DESs characterized by FT-Raman spectroscopy, FTIR spectroscopy, 1H NMR spectroscopy, and MS. (a) FT-Raman spectra of DESs with different molar ratios in the NH2 stretching mode region. (b) FTIR spectra of DESs with different molar ratios in the NH2 stretching mode region. (c) 1H NMR spectra of the NH2 chemical shift in the DESs. (d) Negative model of the mass spectra of DESs with a molar ratio of AgCF3SO3 to acetamide of 1:2.5.

Figure 3. Absorption capacities of (a,c) C2H4 and (b,d) C2H6 in silver-based DESs (a,b) with a molar ratio of 1:3 at different temperatures and (c,d) with different molar ratios at 298.15 K.

The second most intense peak corresponds to [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 formation of clusters between acetamide and CF3SO3− further corroborated the reconstruction of the hydrogen-bond networks in the DESs. The intensities of larger 6877


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Figure 4. (a) Gas permeabilities, (b) solubilities, (c) diffusivities, and (d) C2H4/C2H6 selectivities of the DESs with different molar ratios at a transmembrane pressure of 0.1 bar, a temperature of 298.15 K, and a sweep gas flow rate of 20 mL/min.

clusters such as [(acetamide)4·CF3SO3]− and [(acetamide)6· CF3SO3]− increased, whereas that of [CF3SO3]− decreased with decreasing molar ratio of AgCF3SO3 to acetamide, as shown in Figure S8. In summary, the results of FTIR, FT-Raman, ESI-MS, and 1 H NMR analyses demonstrated that acetamide worked as a complexing agent for both Ag+ and CF3SO3− through its CO and NH2 groups (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 stable DESs. Moreover, the CF3SO3− anions interacted with the NH2 group of acetamide through hydrogen bonds. The hydrogen bonding between the anions and the neutral hydrogen-bond donors gave rise to 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 is apparent from Figure 3, 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 ions and the double bond of ethylene. The C2H6 absorption isotherms in the DESs showed a linear increase with pressure, indicating physical absorption, that could be described by Henry’s law (Figure 3b,d). In contrast, the C2H4 equilibrium data clearly demonstrated the combination of chemical and physical effects (Figure 3a,c). As expected, the absorption capacities of C2H4 and C2H6 in the DESs decreased with increasing temperature, and the temperature had a much greater effect on the absorption capacity of C2H4 than on that of C2H6, which can be explained by the fact that the higher temperature weakened the complexation interactions between silver ions and C2H4 (Figure 3a,b). As can also seen from Figure 3d, the C2H6 equilibrium data in DESs with different molar ratios exhibited a 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 is worth 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 as the molar ratio was increased from 1:2.5 to 1:3.5 and then slightly decreased at the molar ratio of 1:4, which suggests that the Ag+ ions available for complexation with C2H4 could be tuned by the different molar ratios of AgCF3SO3 to acetamide in the DESs (Figure 3c). The Ag+ ions available for complexation depended on the silver−acetamide interactions and the silver salt concentration in the DESs. Effects of the DES Composition. Figure 4 shows the gas separation performance of the DES-SLMs with different molar ratios. As can be seen from Figure 4a, the gas permeability increased as the molar ratio of AgCF3SO3 to acetamide was increased from 1:2.5 to 1:4, which can be clearly attributed to an increase in the gas solubility and diffusivity. Moreover, the C2H4 permeability was always almost 2 orders of magnitude higher than that of C2H6 at any molar ratio from 1:2.5 to 1:4 because of the different transport mechanism (Figure 4a). In addition to normal Fickian transport, C2H4 could reversibly coordinate with the carriers containing silver ions and silver complexes inside the DES-SLMs, thus facilitating 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 carriers, providing a higher solubility of C2H4 in the membrane (Figure 4b). The C2H4 and C2H4 gas diffusivities increased with increasing molar ratio, in good agreement with the above-mentioned changes in the viscosities of the DESs (Figure 4c). It should be also noted that the C2H4 diffusivity was smaller than the C2H6 diffusivity at a molar ratio of 1:2.5, which was probably due to the stronger interactions between silver ions and acetamide slowing the transport of the silver−−lefin complex (Figure 4c). As can also be seen from Figure 4d, the C2H4/C2H6 selectivities of the DES-SLMs increased initially and then decreased as the molar ratio was increased from 1:2.5 to 1:4, obtaining a maximum at the molar ratio of 1:3. This strongly depended on the solubility selectivity 6878


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Figure 5. (a) Gas permeabilities, (b) solubilities, (c) diffusivities, and (d) C2H4/C2H6 selectivities of the DESs (1:3) at different temperatures at a transmembrane pressure of 0.1 bar and a sweep gas flow rate of 20 mL/min.

Figure 6. Effects of (a) feed ratio, (b) sweep gas flow rate, and (c) transmembrane pressure on the (a−c) separation performance and (d) long-term stability of DES-SLMs.

and the diffusivity selectivity. Comparing the solubility selectivity with the diffusivity selectivity, it can be concluded that the C2H4/C2H6 selectivities of the DES-SLMs were essentially dominated by the solubility selectivity (Figure 4d). Effects of Temperature. Increasing temperature increases the rate of diffusion so that permeability increases, but selectivity decreases.54 As shown in Figure 5, the increased diffusivity and decreased solubility counteracted each other, resulting in changes in the C2H4 and C2H6 permeabilities. Both gas permeabilities increased with the temperature, which was mainly due to the faster gas diffusivity in the DES-SLMs at higher temperature (Figure 5a,c). Higher temperatures also meant lower viscosities of the DESs, which favored the

transport of the gases. It is also evident that the permeability of C2H6 increased more significantly and resulted in a lower selectivity at elevated temperatures, which can be explained that the slight decrease in C2H6 solubility with increasing temperature (Figure 5a,b). Compared with the C2H6 solubility, the C2H4 solubility experienced a sharp decrease with temperature, which can be explained by the fact that the increased temperature weakened the coordination between the carriers and C2H4, as the reversible complexation reaction was exothermic (Figure 5b). The data on permeability selectivity, solubility selectivity, and diffusivity selectivity are also summarized in Figure 5d, which illustrates the quantitative 6879


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Figure 7. C2H4/C2H6 membrane separation performance in recent studies. Data are plotted on a log−log scale, and the upper bound was adopted from Koros and co-workers.55

membrane science for gas separation and displaying the tradeoff line between permeability and selectivity. However, the upper bound of liquid membranes for C2H4/C2H6 separation has not been reported in the open literature up to now because of the limited data. Koros and co-workers first proposed the C2H4/C2H6 upper bound for polymeric membranes in 2013.55 Therefore, we compared the results obtained in this study with the upper bound for polymeric membranes proposed by Koros and co-workers.55 As can be seen in Figure 7, our DES-SLMs exhibited high permeability for C2H4 and reasonable separation selectivity, thus exceeding the upper bound line. The separation performance of the DES-SLMs was found to be superior to those of most polymeric membranes. Moreover, compared with the recently reported supported ionic liquid membranes,15,17,18 our membrane still exhibited better performance for the C2H4/ C2H6 separation. All of the above results suggest that DESSLMs are a potential alternative to the existing separation technology for the separation of C2H4/C2H6.

contribution percentages of solubility and diffusivity to the total permeability. Optimization of Separation Conditions. The effects of the feed ratio of C2H4 to C2H6, the flow rate of sweep gas, and the transmembrane pressure were investigated systematically, and the results are collected in Figure 6a−c. As can be seen from Figure 6a, the C2H4 permeability increased from 260 to 1337 Barrer as the feed ratio of C2H4 to C2H6 changed from 1:5 to 5:1, whereas the C2H6 permeability exhibited the opposite trend. Therefore, the C2H4/C2H6 selectivity exhibited a sharp increase 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, and then it remained almost unchanged. This is because higher flow rates of sweep gas more efficiently removed the C2H4 molecules that arrived on the permeation side, allowing for operation at higher driving forces. In contrast, the C2H6 permeability was almost unaffected by the sweep gas. As can be seen from Figure 6c, the C 2H 4 permeability through the DES-SLMs decreased significantly with increasing transmembrane pressure, indicating that most of the carriers were saturated even at low pressure and that C2H4 permeation was a diffusion-controlled process. However, the C2H6 permeability remained almost unchanged over the investigated transmembrane pressure differentials, suggesting that the C2H6 permeability through the DES-SLMs can be described by the solution−diffusion mechanism. Therefore, the C2H4/C2H6 selectivity decreased with increasing transmembrane pressure. Long-Term Stability of the Membranes. Long-term stability is crucial for the application of DES-SLMs. The membranes were held for 130 h to evaluate the stability of the membranes at a molar ratio of AgCF3SO3 to acetamide of 1:3, a temperature of 298 K, a flow rate of sweep gas of 20 mL/min, and a transmembrane pressure of 0.1 bar. As presented in Figure 6d, the C2H4 and C2H6 permeability remained almost unchanged during long-time run. The C2H4/C2H6 selectivity varied slightly in the initial stage of the test experiment and then remained stable for test times ranging from 30 to 130 h. The results clearly show that the prepared membrane has 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

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CONCLUSIONS Novel silver-based DESs, consisting of AgCF3SO3 and CH3CONH2, were first prepared as the membrane liquid. Based on these DESs, DES-SLMs were fabricated, and their performance in gas separation experiments was investigated. The C2H4/C2H6 selectivity of the DES-SLMs initially increased as the molar ratio was increased from 1:2.5 to 1:3 and then decreased with a further increase in molar ratio from 1:3 to 1:4. An increase in transmembrane pressure decreased the C2H4/ C2H6 selectivity mainly because of a decrease in the permeability of C2H4. An increase in temperature increased the permeability of C2H4 and C2H6, but decreased the selectivity of C2H4/C2H6. As can be clearly seen from the solubility and permeability experiments, the C2H4/C2H6 selectivities of the DES-SLMs were essentially dominated by the solubility selectivity. Compared with membranes considered in other studies, the DES-SLMs exhibited comparable selectivities for C2H4/C2H6 and excellent permeabilities of C2H4, thus exceeding the upper bound. In summary, our study has provided a promising alternative to conventional technology for olefin/paraffin separations, and the further design and strengthening of DESs will bring the potential for more excellent separation performance. 6880


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01092. Characterization methods for silver-based DESs, characterization results of DES-SLMs, absorption capacity tests, gas permeability measurements, characterization results of DESs, and test conditions (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel./Fax: +86 2227400199. E-mail: [email protected]. *E-mail: [email protected]. ORCID

Luhong Zhang: 0000-0002-7073-4793 Huawei Yang: 0000-0002-3510-9407 Notes

The authors declare no competing financial interest.

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

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Silver-Based Deep Eutectic Solvents as Separation Media: Supported

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