Novel Protic Ionic Liquid Composite Membranes with Fast and

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Applications of Polymer, Composite, and Coating Materials

Novel protic ionic liquid composite membranes with fast and selective gas transport nanochannels for ethylene/ethane separation Haozhen Dou, Bin Jiang, Xiaoming Xiao, Mi Xu, Xiaowei Tantai, Baoyu Wang, Yongli Sun, and Luhong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00123 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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ACS Applied Materials & Interfaces

Novel protic ionic liquid composite membranes with fast and selective gas transport nanochannels for ethylene/ethane separation Haozhen Dou a, Bin Jiang a, Xiaoming Xiao a, Mi Xu a, Xiaowei Tantai a, Baoyu Wang b, Yongli Sun a, Luhong Zhang a,* a

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China

b

School of Chemical Engineering and Food Science, Zhengzhou Institute of Technology, Zhengzhou

450000, China ABSTRACT: Protic ionic liquids (PILs) were utilized for the fabrication of composite membranes containing silver salt as the C2H4 transport carrier to perform C2H4/C2H6 separation for the first time. The intrinsic nanostructures of PILs were adopted to construct fast and selective C2H4 transport nanochannels. The investigation of structure-performance relationships of composite membranes suggested that transport nanochannels (polar domains of PILs) could be tuned by the sizes of cations, which greatly manipulated activity of the carrier and determined the separation performances of membranes. The role of different carriers in the facilitated transport was studied, which revealed that the PILs were good solvents for dissolution and activation of the carrier due to their hydrogen bond networks and water-like properties. The operating conditions of separation process were investigated systemically and optimized, confirming C2H4/C2H6 selectivity was enhanced with the increase of silver salt concentration, the flow rate of sweep gas and the feed ratio of C2H4 to C2H6 as well as the decrease of the transmembrane pressure and operating temperature. Furthermore, the composite membranes exhibited long-term stability and obtained very competitive separation performances compared with other results. In summary, PIL composite membranes, which possess good long-term stability, high C2H4/C2H6 selectivity and excellent C2H4 permeability, may have a good perspective in industrial C2H4/C2H6 separation. KEYWORDS: Protic ionic liquids; Composite membranes; C2H4/C2H6 separation; Nanostructure; Facilitated transport

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1. INTRODUCTION As one of the most important petrochemicals, ethylene is widely used in the chemical industry and its worldwide production exceeds that of any other organic compound.1, 2 Ethylene and ethane are usually obtained as a mixture, and thus ethylene must be separated from the gas mixture before further utilization. Unfortunately, due to the similar molecular sizes and volatilities of ethylene and ethane, the traditional cryogenic distillation technology to separate ethylene from ethane has been criticized as the most energy and cost intensive process.3-4 Therefore, extensive efforts have been pursued to develop energy effective, sustainable and environment friendly alternatives for the ethylene/ethane separation.5-9 Very recently, membranes based on ionic liquids (ILs) or ionic liquid analogues have attracted particular interest for olefin/paraffin separations, which combine the advantages of membrane with the outstanding solvent properties of ILs, thus exhibiting great potential to afford much lower cost and energy consumption.4, 10-12 Aprotic ILs (AILs), as a subset of ILs, have been deeply explored as suitable separation media to carry out olefin/paraffin separations, which indicated that the gas selectivity was greatly affected by the anions of AILs and could be improved by the introduction of cyano groups.11-13 Despite their promising and green advantages, the selectivity of pure AIL based membranes is very low. Therefore, to enhance the olefin/paraffin selectivity, carrier-facilitated transport concept is introduced for designing novel AIL based membranes. Ortiz and co-workers reported AIL based membranes containing AgBF4 as the carrier for the transport of olefin, and then Matsuyama’s group investigated the potential of the AIL based gel membranes for the olefin/paraffin separation.10, 14 Unfortunately, the low solubilities of silver salts in AILs result in limited improvement in the olefin/paraffin selectivity.15 Moreover, the poor long-term stability of the carriers in AILs, the complicated preparation and high cost of AILs are also great obstacles for the practical applications of these membranes.10 On the other hand, creation of ordered and continuous nanochannels is perceived as a straightforward and effective strategy to achieve sufficient separation performance, which has been widely applied in designing advanced membranes.16 Currently, a common approach to construct nanochannels has been accomplished by the direct assembly of fictional or polymer-grafted 2D nanosheets, such as graphene oxide,16 MXene,17 molybdenum disulfide and so on.18 However, exercising precise control over the assembly process leading to nanochannels with long-range continuity is still a great challenge.19 Furthermore, the preparation of fictional or polymer-grafted 2D nanosheets may involve complex multistep synthesis to achieve the target architecture. Therefore, a simple and easy-operating approach to construct nanochannels is imperative. Based on above analysis, the development of IL composite membranes with high carrier concentration, long-term stability, and gas transport nanochannels may lead to a qualitative leap in olefin/paraffin separations compared to current research. PILs have aroused our great interest due to their distinct

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properties and unique nanostructures. Compared with the AILs widely applied in gas separation membranes, the PIL gas separation membrane is still in its infancy.20 PIL is typically synthesized by the stoichiometric neutralization reaction of a Brønsted acid and a Brønsted base, leading to the presence of proton-donor and proton-acceptor sites.21 The available proton-donor and proton-acceptor sites enable PILs to develop dense three dimensional hydrogen bond networks, which make PILs mirror a number of remarkable structural and solvent properties of water.22-23 The special hydrogen bond networks and water-like properties of PILs endow their high solvent polarity and good dissolving capacity for the silver salts, guaranteeing the high carrier concentration in the membranes. The useful Brønsted acidic properties of PILs are likely to stabilize the transport carriers, thus favoring for the long-term stability of these membranes.24-25 Moreover, PILs have the advantages of being easier to synthesize, cheaper and more biocompatible compared with AILs,26 which open access to practical applications of PIL composite membranes. Besides the enormous potential of PILs to overcome all the drawbacks of traditional AIL based membranes, the nanostructure of PILs offer a simple and effective approach to construct highly permeable and selective nanochannels in PIL composite membranes. PILs arrange into a sponge-like bicontinuous nanostructure, meaning that the bulk liquid is structurally inhomogeneous.27 The nanostructure in PILs results from the interactions between cations and anions, such as amphiphilicity, electrostatic and hydrogen bonding attractions. The electrostatic and hydrogen bonding interactions lead to the formation of polar domains. Cation alkyl groups are repelled from polar regions and forced to cluster together into apolar regions, which depend on ion amphiphilicity.28 Lengthening the cation alkyl chain leads to more segregated, better defined polar and apolar domains, and also increases the area of apolar domains.29 The nanostructure of PILs greatly influences the solvation of silver salts, which are forced to heterogeneously accommodate into the polar nano-domains of PILs,29 which constructs fast and selective ethylene transport nanochannels.30 Additionally, the structure of nanochannels can be fine-tuned simply by an appropriate choice of cation and anion of PIL or by adding specific functional groups.31-33 In this study, we report for the first time the fabrication of PIL composite membranes with fast and selective gas transport nanochannels through congregating the transport carriers (silver salts) within the polar nano-domains of PILs. The separation performances of composite membranes were investigated by the C2H4/C2H6 separation experiments systematically. Especially, the effect of PIL structure on the separation performance has been highlighted and the separation mechanism was probed by various characterizations, which offered insight for the smart design of new PILs based membranes for C2H4/C2H6 separation.

2. EXPERIMENTAL SECTION 2.1. Materials. Ethylene and ethane gases were purchased from Tianjin Tang Dynasty Gas Co., Ltd

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(China) with a minimum purity of 99.9 mol%. Propylamine (99 wt%), butylamine (99.5 wt%), tetrahydro pyrrole (99 wt%), 1-ethylimidazole (99.5 wt%), tributylamine (99.5 wt%) and nitric acid (65-68 wt%) were all supplied from Shanghai Aladdin Biochem Technology Co., Ltd. (China). Silver nitrate

(AgNO3,

≥99.0

wt%),

silver

tetrafluoroborate

trifluoromethanesulfonate (AgCF3SO3, ≥99.9 wt%)

(AgBF4,

≥99.5

wt%),

silver

were supplied by Beijing J & K Chemical

Technology Co., Ltd. (China). All chemicals were used as received unless otherwise stated. The microporous membrane support was hydrophilic polysulfone (PS) flat sheet membrane with porosity of 70%, the average thickness of 100 µm, a nominal pore size of 0.1 µm and effective area of 19.625 cm2, which was obtained from Haining Zhongli Filtering Equipment Corporation (China). 2.2. Synthesis of PILs. The PILs were synthesized following a similar procedure described elsewhere.34 In short, the nitric acid (0.2 mol) was slowly dropped into the methanol solution of 1-ethylimidazole (0.2 mol) at 0 oC or below with rapid stirring. The mixture was further reacted for 8 hours at room temperature and then the solvents (water and methanol) were removed by rotary evaporation to obtain transparent PIL. The water contents of the PILs were determined to be 0.3-0.8 wt% by Karl–Fischertitration method (DL37 KF Coulometer, Mettler Toledo). Figure 1 shows the chemical structures of PILs investigated in this study.

Figure 1. The chemical structures of the PILs investigated in this study. 2.3. Dissolution of Silver Salts. PILs were dried under vacuum at 65 °C for 4 hours prior to use. The desired amount of silver salt was added into PIL, and the mixture was agitated with a magnetic stirrer at 60 °C for several hours (0.5 ~ 3 hours). Then, homogeneous transparent reactive Ag/PIL liquids with molar fraction of silver salt between 0.5 and 4 mol/L were obtained directly. All the reactive Ag/PIL liquids remained as liquids after cooling down to room temperature and then stored in an argon-filled Mbraun Labmaster 130 glove box (H2O < 1 ppm). 2.4. Preparation of Composite Membranes. The composite membranes were prepared using spin coating method as described elsewhere.35 Firstly, the PS membranes were applied under vacuum ( PAN > BAN > EIMN > TBAN, which almost exhibited negative correlation with the sizes of cations. While the C2H6 permeability was listed as follows: TBAN > BAN > PAN = PyAN > EIMN. Interestingly, the BAN and TBAN based composite membranes obtained higher C2H6 permeabilities than that of PyAN and PAN based membranes although the viscosities of BAN and TBAN were much higher than that of PyAN and PAN, which suggested PIL viscosity did not provide a full description of gas permeability. This behavior has been reported by the Marrucho et al.39 using membranes based on IL mixtures for CO2 separation. As also seen from Figure 7, the C2H4/C2H6 selectivity significantly increased from 3.0 to 14.5 as the sizes of cations increased. Therefore, it can be concluded that the C2H4 permeability and C2H4/C2H6 selectively can be greatly tuned by the variation of cation sizes. The different sizes of cations meant different ionic amphiphilicity, electrostatic and hydrogen bond interactions in the PILs, thus resulting in different nanostructure and nanochannels of composite membranes.28, 30, 34 The different nanochannels determined the gas separation performances

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of the composite membranes.

Figure 7. The investigation of structure-performance relationships of composite membrane: (a) the gas permeability; (b) the C2H4/C2H6 selectivity (Conditions: 1.5 mol/L AgNO3 concentration, 0.1 bar transmembrane pressure, 298 K, 20 mL/min sweep gas and 30 mL/min: 30 mL/min feed stream). In order to obtain deep insight of the correlation between the nanochannels and gas separation performances, the structures of composite membranes have been investigated in great detail by FTIR and FT-Raman spectra. As shown in Figure 8a, the asymmetric and symmetric vibrations of N-H decreased with the sizes of cations decreasing, which indicated strength of hydrogen bonds gradually got stronger as the sequence of BAN < EIMN < PAN < PyAN. The stronger hydrogen bonds between cations of PILs and NO3- of AgNO3 weakened the interactions between Ag+ and NO3-, thus enhancing the carrier activity and improving the separation performances. Figure 8b-8f provides quantitative information on the dissociation behaviors of AgNO3. Note that the NO3- stretching bands at 1034, 1040, and 1045 cm-1 are assigned to free ions, ion pairs, and ion aggregates, respectively.40 The proportion of free ions gradually increased as the order of BAN < EIMN < PAN < PyAN, while the proportion of ion aggregates decreased. This behavior further confirmed that the polar domains increased with the sizes of cations decreasing, thus leading to the better disassociation and homogeneous distribution of carrier.

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The increased polar domains and the better disassociation of the carrier meant the more effective transport nanochannels for the C2H4 transport. Therefore, the composite membranes with smaller sizes of cations obtained good separation performances.

Figure 8. The investigation of separation mechanism: (a) FTIR spectra; (b) FT-Raman spectra, (c-f) the ionic constituents of PyAN, PAN, EIMN and BAN. In brief, with the sizes of cations increasing, the PILs possessed better defined polar and apolar domains. Meanwhile, the apolar domains increased and the polar domains decreased (Figure 9). The smaller polar domains led to more serious aggregation of carrier, and thus resulting in thus decreasing activity of the carrier and increasing invalid region for selective C2H4/C2H6 separation. Therefore, the PIL composite membranes with larger cations exhibited poorer C2H4/C2H6 separation performances.

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Figure 9. The separation mechanism of Ag/PIL composite membranes. 3.4. Role of the Carriers in Facilitated Transport. Facilitated C2H4 transport is strongly dependent upon the properties of the anionic constituents of the silver salts and the effect of the carriers on the gas separation performance were investigated (Figure 10). The C2H4 and C2H6 permeabilities increased following the order of AgNO3 < AgCF3SO3 < AgBF4. The C2H4/C2H6 selectivity also witnessed the same trend with the gas permeability and showed a negative correlation with the lattice energies of silver salts (data adopted from reference

41

). However, it was worthy of noting that when AgBF4 was

replaced by the AgNO3, the C2H4 permeability and C2H4/C2H6 selectivity of composite membrane decreased only by 28% (from 150 to 108 Barrers) and 12.9% (from 17 to 14.5), respectively. Therefore, compared with the silver salts containing large and weakly electronegative anions such as BF4- and CF3SO3-, AgNO3 exhibited comparable ability of facilitated olefin transport in spite of its high lattice energy, which also revealed that the PILs were good solvents for dissolution and activation of AgNO3 due to their hydrogen bond networks and water-like properties. Moreover, considering the low cost and facile availability of AgNO3, the AgNO3 is especially competitive in the industrial application. Therefore, AgNO3 was selected as the carrier in the following study.

Figure 10. The effect of carriers on the facilitated transport: C2H4 and C2H6 permeabilities and C2H4/C2H6 selectivity (Conditions: 1 mol/L silver salt concentration, 0.1 bar transmembrane pressure, 298 K, 20 mL/min sweep gas and 30 mL/min: 30 mL/min feed stream).

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3.5. Role of Silver Salt Concentration in Facilitated Transport. The effect of silver salt concentration on the gas separation performance was investigated (Figure 11). Interestingly, high solubility (up to 4 mol/L) could be achieved for AgNO3 in the PyAN, which indicated good compatibility between the carriers and PILs. As shown in Figure 11a, the C2H4 permeability was always much higher than that of C2H6 due to different transport mechanisms. The great improvement of C2H4 permeability suggested the carrier-facilitated transport mechanism took place in composite membranes, which has been further investigated by the 1H NMR spectra. As seen from Figure 11b, on addition of C2H4 into pure PIL, a new weak peak appeared at 5.626 ppm, which was contributed to the physical absorption of C2H4. After the addition of AgNO3, the peak shifted to up-field, which suggested the formation of chemical complex between Ag+ and C2H4. Based on the results of 1H NMR, we could detail the carrier facilitated transport mechanism: the carrier initially coordinated with C2H4 at the feed side, increasing the solubility and diffusivity of C2H4. Then, the resultant ethylene-carrier complex diffused through the membrane from the high-pressure feed side to the low-pressure permeate side, where the decomplexation occurred and released the C2H4. The regenerated carrier diffused back to the feed side and continually combined with C2H4. As also seen from Figure 11a, the C2H4 permeability increased significantly from 37 to 248 Barrers when the AgNO3 concentration increased from 0 to 4 mol/L, because more carriers were available for the transport of C2H4. It should be also noted that the C2H4 permeability almost exhibited a linear increase with the AgNO3 concentration increasing, which could be explained by the disassociation behavior of silver salt in the PIL. As shown in Figure 11c, in the pure PIL, the stretching vibration of NO3- occurred at 1041 cm-1. Interestingly, the stretching vibration of NO3- almost remained unchanged and slightly decreased with the AgNO3 concentration increasing, which meant the good disassociation of AgNO3 in the PIL.42 Compared with the C2H4 permeability, the C2H6 permeability exhibited a sharp decrease, changing from 15.5 to 4.3 Barrers in the same range of silver salt concentration, which was mainly due to that with the addition of AgNO3, the viscosity of PIL increased and the gas diffusion was suppressed (Figure 11a). Therefore, the C2H4/C2H6 selectivity obviously increased from 2.3 to 57.2, which was almost 30 times higher than that of PIL based membranes without the addition of AgNO3 (Figure 11d).

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Figure 11. The effect of silver concentration on the separation performance: (a) gas permeability; (b) C2H4/C2H6 selectivity (Conditions: 0.1 bar transmembrane pressure, 298 K, 20 mL/min sweep gas and 30 mL/min: 30 mL/min feed stream). The investigation of facilitated transport: (c) 1H NMR spectra; (d) FT-Raman spectra. 3.6. Optimization of the separation process. Figure 12 shows the effects of transmembrane pressure and operating temperature on the gas separation performances of composite membranes. The C2H6 permeability slightly decreased with the transmembrane pressure increasing, as was in accordance with our previous research.9 In contrast, the C2H4 permeability decreased significantly with the increase of transmembrane pressure, e.g. from 244 to 59 Barrers for PyAN based composite membranes and from 169 to 41 Barrers for EIMN based composite membranes. This is a typical feature of the carrier facilitated transport mechanism, namely, most of the carriers are saturated even at low pressure and fail to react with the excessive C2H4 at high pressure. Therefore, the C2H4 permeation is a diffusion-controlled process.3 As seen from Figure 12b, the C2H4/C2H6 selectivity decreased by 28% (from 57 to 41) and 26% (from 50 to 37) for PyAN and EIMN based composite membranes, respectively. Figure 12c provided further confirmation of positive correlation between gas permeability and the operating temperature. The gas permeability is dependent on the solubility along with the fast diffusion. As the temperature increased, the viscosities of PILs decreased and the gas diffusion was enhanced, thus resulting in the increase of gas permeability. It should be noted that the C2H4 permeability increased more slowly than that of C2H6. For instant, the C2H4 permeability for PyAN increased by 1.5 times (from 244 to 366 Barrers), while the C2H6 permeability increased by nearly 2 times (from 4.2 to 8.1 Barrers). As a result, the C2H4/C2H6 selectivity decreased from 57 to 45 for PyAN based composite

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membranes and from 50 to 38 for EIMN based composite membranes, respectively. This was supposed that the high temperature weakened the complexation between the carrier and C2H4 and greatly decreased the C2H4 solubility in the composite membranes, thus resulting in the slower increase of C2H4 permeability. Figure S4 presented a linear relationship between the logarithm of the permeability and the inverse of temperature, which was in agreement with our previous study.9 Therefore, the temperature influence on the gas permeability was well described in terms of an Arrhenius type relationship. As shown in Figure S4, the activation energies of permeations through PyAN based membrane were 15.97 kJ/mol and 25.27 kJ/mol for C2H4 and C2H6, respectively, which suggested the C2H6 permeability was more sensitive to temperature and quantitatively illustrated the effect of temperature on gas permeability.

Figure 12. The effect of transmembrane pressure on the gas permeability (a) and C2H4/C2H6 selectivity (b); the effect of operating temperature on gas permeability (c) and C2H4/C2H6 selectivity (d). (Conditions: 4 mol/L silver salt concentration, 20 mL/min sweep gas, 30 mL/min: 30 mL/min feed stream, 298 K or 0.1 bar transmembrane pressure). As shown in Figure 13a, the C2H4 permeabilities for PyAN and EIMN based composite membranes increased from 78 to 412 Barrers and from 54 to 286 Barrers, respectively, as the feed ratios of C2H4 to C2H6 changed from 10:50 to 50:10, whereas the C2H6 permeabilities exhibited the opposite trend. Therefore, the C2H4/C2H6 selectivities exhibited a sharp increase up to 215 and 192 at the feed ratio of 50:10 for PyAN and EIMN based composite membranes accordingly (Figure 13b). It should be noted that our membranes still possessed acceptable selectivity (8~10) at low C2H4 concentration, which suggested these membranes can be used for C2H4 recovery from the reactor purge gas of petrochemical processes or the synthesis of polyethylene. As shown in Figure 13c-13d, the C2H6 permeability was

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almost unaffected by the sweep gas. In contrast, the C2H4 permeability increased as the flow rate of sweep gas increased, which was because higher flow rates of sweep gas removed the C2H4 molecules more efficiently which arrived on the permeation side, allowing for operation at higher driving forces. The C2H4/C2H6 selectivity initially increased slightly from 4 to 12 mL/min and almost remained unchanged when the flow rate of sweep gas was higher than 12 mL/min.

Figure 13. The effect of feed ratio on the gas permeability (a) and C2H4/C2H6 selectivity (b); the effect of flow rate of sweep gas on the gas permeability (c) and C2H4/C2H6 selectivity (d). (Conditions: 4 mol/L silver salt concentration, 0.1 bar transmembrane pressure, 298 K, 20 mL/min sweep gas or 30 mL/min: 30 mL/min feed stream). 3.7. Stability of composite membranes. Figure 14a clearly showed the chemical stability of the composite membrane after storing for 30 days. As suggested by the FTIR spectra, no noticeable change of chemical structures of composite membranes could be detected after 30 days, which further proved the structural stability of these membranes. As presented in Figure14b, C2H4 and C2H6 permeabilities experienced a slight decrease and then remained almost unchanged during long-time run. The decrease in the startup was probably as a consequence of that the evaporation of trace water imperceptibly increased the viscosity of the Ag/PIL.36 The C2H4/C2H6 selectivity varied slightly in the initial stage of the experiment and then remained stable for test times ranging from 2 to 170 h. The results clearly showed that the prepared composite membranes have great potential for long-time operation.

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Figure 14. (a) The long-term chemical stability of composite membranes investigated by FTIR. (b) The long-term stability investigation of composite membranes (Conditions: 0.1 bar transmembrane pressure, 298 K, 4 mol/L silver salt concentration, 20 mL/min sweep gas and 30 mL/min: 30 mL/min feed stream). 3.8. Comparison with other studies. A summary of C2H4/C2H6 separation performances of IL or IL analogue based membranes was presented in Table 2 to make a comparison between this work and results of other groups. Some membranes for C3H6/C3H8 separation were also collected due to significant similarities between C2H4/C2H6 separation and C3H6/C3H8 separation. As seen in Table 2, the composite membranes prepared in this work exhibited high C2H4/C2H6 selectivities and excellent C2H4 permeabilities, which were superior to those results in literature. Furthermore, PILs and the carrier employed in this work were inexpensive and environmentally friendly, which demonstrated the practicability of composite membranes. Table 2. Comparison of olefin/paraffin separation performances of ILs or IL analogues based membranes. Gas Permeability ILs/DESs

Support

Olefin/paraffin Selectivity

Ref

Carrier C2H4 (Barrer)

C2H4/C2H6

C3H6/C3H8

PyAN

PS

AgNO3

243.9

57

this work

EIMN

PS

AgNO3

139.3

51

this work

[MOIM][NO3]

PS

2.3

[11]

[Bmim][BF4]

PS

2.8

[11]

[Bmim][BF4]

PS

Ag nanoparticles

17

[40]

[MOIM][BF4]

PS

Cu nanoparticles

2

[43]

[Bmim][BF4]

PVDF

AgBF4

19.5

[10]

[Bmim][Cl]

PVDF

CuCl

123.5

11.8

[4]

ChCl-G

Nylon

CuCl

13.2

19

[9]

Chcl-EG

Nylon

CuCl

121

13

[37]

4. CONCLUSIONS 17



Novel PIL based composite membranes were prepared using spin coating method. The synthesized PILs were characterized by 1H NMR, FTIR, TG and their physical properties were measured, which confirmed the existence of strong electrostatic and hydrogen bonding interactions. The SEM images confirmed the successful fabrication of composite membranes. The investigation of separation performances indicated that the increase of PIL polar domains with the sizes of cations decreasing led to larger naonochannels and better disassociation of the carrier, which significantly improved the separation performances of membranes. The effect of different carriers on facilitated transport was studied, which suggested AgNO3 exhibited comparable ability of facilitated olefin transport compared with the silver salts containing large and weakly electronegative anions such as BF4- and CF3SO3-. The C2H4 permeability and C2H4/C2H6 selectivity could be greatly boosted with the silver salt concentration increasing. The higher transmembrane pressure meant lower C2H4 permeability and C2H4/C2H6 selectivity. The increase of operating temperature increased the C2H4 permeability but decreased C2H4/C2H6 selectivity. In contrast, C2H4 permeability and C2H4/C2H6 selectivity increased together with the flow rate of sweep gas and the feed ratio of C2H4 to C2H6 increasing. In all, the long-term stability, high C2H4/C2H6 selectivity and excellent C2H4 permeability endowed our membranes with great potential in industrial application.

ASSOCIATED CONTENT Supporting Information FT-Raman Spectra of Pure PILs, SEM images of composite membranes, 1H NMR spectra of Ag/PILs, Arrhenius type relationship between operating temperature and gas permeability.

AUTHOR INFORMATION Corresponding Authors *E-mail address: [email protected] (L. Zhang) Tel./fax: +86 2227400199.

ORCID Luhong Zhang: 0000-0001-5190-2918 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is support from National Key R&D Program of China (No. 2016YFC0400406 and 2017B0602702).

REFERENCES (1) Li, P.; He, Y.; Arman, H. D.; Krishna, R.; Wang, H.; Weng, L.; Chen, B. A microporous six-fold interpenetrated hydrogen-bonded organic framework for highly selective separation of C2H4/C2H6. Chem. Commun. 2014, 50 (86), 13081-13084. (2) Tome, L. C.; Mecerreyes, D.; Freire, C. S. R.; Rebelo, L. P. N. Marrucho, I. M. Polymeric ionic liquid membranes containing IL-Ag+ for ethylene/ethane separation via olefin-facilitated transport. J. Mater. Chem. A.

2014, 2 (16), 5631-5639.

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GRAPHIC ABSTRACT

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Novel Protic Ionic Liquid Composite Membranes with Fast and

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