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Correlation between anions of ionic liquids and reduction of silver ions in facilitated olefin transport membranes
Desalination 233 (2008) 327–332
Correlation between anions of ionic liquids and reduction of silver ions in facilitated olefin transport membranes Sang Wook Kanga, Jinkee Honga, Kookheon Chara, Jong Hak Kimb, Jungahn Kimc, Yong Soo Kangd* a
School of Chemical & Biological Engineering and NANO Systems Institute-National Core Research Center, Seoul National University, Seoul 151-744, South Korea b Department of Chemical Engineering, Yonsei University, Seoul 120-749, South Korea c Research Institute of Basic Sciences and Department of Chemistry, Kyung Hee University, Seoul 130-701, South Korea d Department of Chemical Engineering, Hanyang University, Seoul 133-791, South Korea Tel. þ82(2)22202336; Fax þ82(2)22962969; email: [email protected] Received 31 July 2007; accepted revised 28 September 2007
Abstract þ þ The correlation between the anions of the ionic liquids of BMIMþBF 4 , BMIM CF3SO3 and BMIM NO3
in poly(2-ethyl-2-oxazoline)/AgNO3 membranes and reduction of silver ions has been investigated. The anion type negligibly affected the initial separation performances, however it significantly affected the long-term operation stability. Additionally, UV and TEM confirmed the long-term operational stability was strongly associated with the reduction rate of the silver ions. The reduction rate of the silver ions in the polymer/silver salt/ionic liquid complex was observed in following order: BMIMþBF 4 þ þ > BMIMþCF3SO > BMIM NO , suggesting that among the ionic liquids investigated, BMIM NO 3 3 3 had the most improved separation performance. Keywords: Ionic liquids; Reduction; Anion; Silver ion; Facilitated olefin transport
*Corresponding author. Presented at the Fourth Conference of Aseanian Membrane Society (AMS 4), 16–18 August 2007, Taipei, Taiwan. 0011-9164/08/$– See front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2007.09.058
328
S.W. Kang et al. / Desalination 233 (2008) 327–332
1. Introduction Facilitated olefin transport membranes employing solid polymer electrolytes consisting of silver salt dissolved in a polar polymer matrix have shown high separation performance for olefin/paraffin mixtures by reversible complexation with olefin molecules due to the olefin carrier activity of silver ions [1–4]. Despite many advantages of the silver polymer electrolyte membranes, a problem for practical application is long-term stability of the separation performance as the silver ions can be reduced to form metallic silver, and consequently lose olefin carrier activity [5,6]. Among the silver salts capable of reversibly forming silver–olefin complexes, AgBF4 has commonly been used due to high carrier activity with respect to olefin molecules [7]. However, AgBF4 is easily converted to metallic silver, which diminishes the carrier activity. Alternatively, AgNO3, is not readily reduced to metallic silver, however it is rather inactive as an olefin carrier [7]. Therefore, it is desirable to significantly stabilize but inactivate AgNO3 for olefin complexation and carriers for facilitated transport membranes [5]. Recently, highly charged ionic liquids (ILs) were used to activate AgNO3 in silver polymer electrolyte membranes consisting of poly(2-ethyl2-oxazoline) (POZ) and AgNO3 [8]. The introduction of ionic liquids BMIMþNO 3 and into the POZ/AgNO complex BMIMþBF 3 4 membranes increased the Agþ activity in reversible complexation with olefins. The enhanced olefin complexation ability was presumably due to the interaction between BMIMþ and NO 3 of AgNO3, which weakened the interaction between Agþ and NO 3 and resulted in the formation of free silver ions. The effect of IL anions on the reduction behavior of silver cations and separation performance of olefin/paraffin mixtures through POZ/AgNO3/IL membranes was observed. The
ILs investigated were 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMþBF 4 ), 1-butyl3-methylimidazolium triflate (BMIMþCF3SO 3) and 1-butyl-3-methylimidazolium nitrate (BMIMþNO 3 ). 2. Materials and methods 2.1. Materials AgNO3 (99%) and POZ (Mw ¼ 5.0 105 g mol1) were purchased from Aldrich Chemiþ cal Co. BMIMþBF 4 , BMIM CF3SO3 and þ BMIM NO3 were purchased from C-TRI Co. All chemicals were used as received. 2.2. Separation performance The POZ/AgNO3/ionic liquid electrolytes were prepared by dissolving AgNO3 and each IL in an aqueous solution containing 20 wt.% POZ. The [C¼O]:[Ag] molar ratio was 1:1 and the amount of IL was 0.1 molar ratio. For fabrication of the separation membranes, the POZ/ AgNO3/ionic liquid electrolyte solutions were coated on polysulfone microporous support (Seahan Industries Inc., Seoul, Korea) using an RK Control Coater (Model 101, Control Coater RK Print-Coat instruments LTD, UK). After solvent evaporation in a convection oven at room temperature under nitrogen, the POZ/AgNO3/ ionic liquid complexes were vacuum dried for 2 days at room temperature. The thickness of the top polymer electrolyte layer was determined with SEM to be approx. 1 mm. Gas flow rate or gas permeance was measured with a mass flow meter. The unit of gas permeance is GPU, such that 1 GPU ¼ 1 106 cm3 (STP)/(cm2 s cm Hg). Mixed gas (50:50 vol% propylene/propane mixture) separation properties of the POZ/ AgNO3/ionic liquid complexes were evaluated using a gas chromatograph (Hewlett-Packard G1530A, MA) equipped with a TCD detector and unibead 2S 60/80 packed column.
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2.3. Characterization To observe silver nanoparticles produced from the reduction of silver, transmission electron micrographs (TEM) were obtained from a Philips CM30 microscope operating at 300 KV. TEM samples were prepared by casting a solution of 1/1/0.1 POZ (10 wt.%)/AgNO3/ionic liquid on a glass plate and drying for 2 h in a light-protected convection oven at room temperature under nitrogen. After drying, the samples were dissolved in a solvent and a drop of the resulting colloidal silver dispersion was placed on a standard copper grid. For UV–Vis spectroscopy, 1/1/0.1 POZ/ AgNO3/ionic liquid complex solutions with the same polymer concentration in 0.5 wt.% were dropped on each quartz window with a micropipette. The quartz windows were dried under nitrogen for 2 h at room temperature and further dried in a vacuum oven for 2 days. UV–Vis spectra between 190 and 900 nm was measured with a spectrophotometer (Hewlett-Packard).
329
Table 1 þ Effects of the ionic liquids (BMIMþNO 3 , BMIM BF4 þ and BMIM CF3SO3 ) on the mixed gas permeances and propylene/propane selectivities of 1/1 POZ/AgNO3 membranes Membranes
Total Selectivity permeance (GPU)
POZ/AgNO3 POZ/AgNO3/BMIMþNO 3 POZ/AgNO3/BMIMþBF 4 POZ/AgNO3/BMIMþCF3SO 3
0.1 5.6 5.4 5.1
0.95 32.0 31.8 33.2
olefin transport resulting from enhanced carrier activity of Agþ in the complexes with ILs [8]. Regardless of the type of IL used, POZ/AgNO3/ IL complex membranes had similar initial separation performances for the propylene/propane mixtures. 3.2. Long-term separation performance
3. Results and discussion 3.1. Mixed gas performance The effect of ILs on the separation performance was evaluated for propylene/propane mixtures with 1/1/0.1 POZ/AgNO3/BMIMþNO 3 , 1/1/0.1 and 1/1/0.1 POZ/ POZ/AgNO3/BMIMþBF 4 þ AgNO3/BMIM CF3SO3 complexes. As summarized in Table 1, the selectivity of propylene over propane, defined as the propylene concentration ratio of the permeate to feed, was approximately 0.95 for 1/1 POZ/AgNO3 without ILs and approximately 0.1 GPU permeance. However, the selectivities of the 1/1/0.1 POZ/AgNO3/ þ BMIMþNO 3 , 1/1/0.1 POZ/AgNO3/BMIM BF4 þ and 1/1/0.1 POZ/AgNO3/BMIM CF3SO 3 complex membranes were 32.0, 31.8 and 33.2, respectively, with mixed gas permeances of 5.6, 5.4 and 5.1 GPU, respectively. The improved separation performance was due to the facilitated
The stabilities of the long-term separation performances of the polymer/silver salt/ionic liquid complex membranes were investigated. Fig. 1 indicates that the 1/1/0.1 POZ/AgNO3/ membrane exhibited stable BMIMþNO 3 separation performance throughout continuous operation for 140 h, whereas the separation performance of the 1/1/0.1 POZ/AgNO3/ BMIMþCF3SO 3 membrane deteriorated. Furthermore, the separation performance of the 1/1/0.1 membrane rapidly POZ/AgNO3/BMIMþBF 4 decreased with time, suggesting there was a strong dependence on the stability of the olefin carrier activity of the silver ion on the nature of the counter anion. 3.3. Formation of silver nanoparticles The effect of the IL counteranions on the formation of silver nanoparticles in the POZ/
S.W. Kang et al. / Desalination 233 (2008) 327–332 40 6
35 30 25 20 15 POZ/AgNO3/BMIM+NO3–
10
+
–
POZ/AgNO3/BMIM Tf
POZ/AgNO3/BMIM+BF4–
5
Permeance (GPU)
Selectivity (Propylene /Propane)
330
5 4 3 POZ/AgNO3/BMIM+NO3– POZ/AgNO3/BMIM+Tf–
2
POZ/AgNO3/BMIM+BF4–
1
0 0
20
40
60
80
100 120 140 160
0
20
40
60
80
Time (h)
Time (h)
(a)
(b)
100 120 140 160
Fig. 1. Separation performance: (a) mixed-gas selectivity and (b) mixed gas permeance of 1/1/0.1 POZ/AgNO3/ þ þ BMIMþBF 4 , 1/1/0.1 POZ/AgNO3/BMIM CF3SO3 and 1/1/0.1 POZ/AgNO3/BMIM NO3 complex membranes with time. (40 psig and 20 C).
AgNO3/IL complex membrane was investigated using TEM. The TEM micrograph of the 1/1/0.1 POZ/AgNO3/BMIMþNO 3 membrane immediately after preparation indicated a few particles of approximately 5 nm (Fig. 2a). As shown in Fig. 2b, the size and number of the silver nanoparticles remained unchanged after 1 week, indicating the reduction reaction of silver ions to nanoparticles was mostly prohibited. Alternatively, the TEM micrographs of 1/1/0.1 POZ/AgNO3/BMIMþCF3SO 3 and 1/1/0.1 POZ/
complexes immediately AgNO3/BMIMþBF 4 after preparation indicated silver nanoparticles of approximately 10 nm in size (Figs. 3a and 4a), which was much larger than the 1/1/0.1 POZ/ AgNO3/BMIMþNO 3 complex. Furthermore, the size and number of silver particles in 1/1/0.1 POZ/AgNO3/BMIMþCF3SO 3 and 1/1/0.1 POZ/AgNO3/BMIMþBF com4 plexes increased significantly after 1 week (Figs. 3b and 4b). Specifically, compared to the other POZ/AgNO3/IL complexes, the particle
Fig. 2. Transmission electron micrographs of 1/1/0.1 POZ/AgNO3/BMIMþNO 3 membrane (a) right after preparation and (b) after storage time of 1 week in dark place.
Fig. 3. Transmission electron micrographs of 1/1/0.1 POZ/AgNO3/BMIMþCF3SO 3 membrane (a) right after preparation and (b) after storage time of 1 week in dark place.
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331
size of the 1/1/0.1 POZ/AgNO3/BMIMþBF 4 complexes increased rapidly, indicating that more silver ions were reduced to silver nanoparticles. The changes in particle size and number were consistent with the long-term separation tests indicating the poor stability of the POZ/ AgNO3/BMIMþBF 4 complex membrane.
3.4. UV–Visible spectroscopy
Fig. 4. Transmission electron micrographs of 1/1/0.1 POZ/AgNO3/BMIMþBF 4 membrane (a) right after preparation and (b) after storage time of 1 week in dark place.
UV–Visible absorption spectra are known to be quite sensitive to the formation of silver nanoparticles. Fig. 5 shows the absorption spectra
1.4
1.4
Absorbance
1.2 1.0
2h 1h 30 min 0 min
1.0
0.8 0.6 0.4
0.6 0.4 0.2
0.0
0.0 400
600
800
1000
2h 1h 30 min 0 min
0.8
0.2
200
POZ/AgNO3/BMIM+CF3SO3–
1.2
Absorbance
POZ/AgNO3/BMIM+NO3–
200
1200
400
Wavelength (nm)
600
800
1000
1200
Wavelength (nm)
(b)
(a) 1.4
Absorbance
2h 1h 30 min 0 min
POZ/AgNO3/BMIM+BF4–
1.2 1.0 0.8 0.6 0.4 0.2 0.0
200
400
600
800
1000
1200
Wavelength (nm)
(c)
Fig. 5. The UV–Visible spectra of (a) the 1/1/0.1 POZ/AgNO3/BMIMþNO 3 membrane, (b) the 1/1/0.1 POZ/AgNO3/ þ membrane and (c) 1/1/0.1 POZ/AgNO /BMIM BF with varying UV irradiation time. BMIMþCF3SO 3 3 4
332
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of 1/1/0.1 POZ/AgNO3/BMIMþNO 3 , 1/1/0.1 þ POZ/AgNO3/BMIM BF4 and 1/1/0.1 POZ/ AgNO3/BMIMþCF3SO 3 complexes with UV irradiation at 9970 mW cm2 and with varying time. A broad absorption maximum near 420 nm was attributed to the plasmon excitation of silver nanoparticles and it is generally accepted that the peak height correlates to the amount of silver nanoparticles [9]. UV irradiation experiments were performed to accelerate the reduction reaction to silver nanoparticles and thus investigate the formation kinetics of silver nanoparticles for polymer/silver salt/ionic liquid complexes. The intensity of the plasmon peak in the 1/1/0.1 POZ/AgNO3/BMIMþBF 4 complex was greater than 1/1/0.1 POZ/AgNO3/BMIMþNO 3 and 1/1/ þ 0.1 POZ/AgNO3/BMIM CF3SO3 , corroborating the higher concentration of silver nanoparticles observed with TEM.
4. Conclusion The correlation between IL anions and the reduction behavior of the silver ion in POZ/ AgNO3/IL membranes was strongly associated to the type of the IL anions used. In particular, the reduction rate of silver ions in a polymer/ silver salt/ionic liquid complex to metallic silver was observed by TEM and UV–Vis in the followþ ing order: BMIMþBF 4 > BMIM CF3SO3 > þ BMIM NO3 . Additionally, reduction to silver nanoparticles was observed for long-term operational stability. The reason why the BMIMþBF 4 among ionic liquid has the bad effect on long-term stability could be guessed by that since the stability among silver salts was as follows: AgNO3 > AgCF3SO3 > AgBF4 [8], it could be thought that the anion existed in polymer electrolyte affects on the reduction of silver ion. In the same way, POZ/AgNO3/BMIMþNO 3 membrane was more stable than POZ/AgNO3/BMIMþBF 4 membrane. However, the investigation on detail reasons is in progress and will be reported in near future.
Acknowledgments This work was supported by Energy Technology R&D program (2006-E-ID11-P-13) under the Korea Ministry of Commerce, Industry and Energy (MOCIE). KC acknowledges the financial support of NSI-NCRC and the Ministry of Education through the Brain Korea 21 Program at Seoul National University.
References [1] S.U. Hong, J. Won and Y.S. Kang, Polymer-salt complexes containing silver ions and their application to facilitated olefin transport membranes, Adv. Mater., 12 (2000) 968. [2] I. Pinnau, L.G. Toy and C. Casillas, Olefin separation membrane and process, US Patent, 5,670,051, 23 September 1997. [3] S. Sunderrajan, B.D. Freeman, C.K. Hall and I. Pinnau, Propane and propylene sorption in solid polymer electrolytes based on poly(ethylene oxide) and silver salts, J. Membr. Sci., 182 (2001) 1. [4] J.H. Kim, B.R. Min, C.K. Kim, J. Won and Y.S. Kang, Role of transient cross-links for transport properties in silver-polymer electrolytes, Macromolecules, 34 (2001) 6052. [5] B. Jose, J.H. Ryu, B.G. Lee, H. Lee, Y.S. Kang and H.S. Kim, Effect of phthalates on the stability and performance of AgBF4-PVP membranes for olefin/ paraffin separation, Chem. Commun., (2001) 2046. [6] J.H. Kim, B.R. Min, H.S. Kim, J. Won and Y.S. Kang, Facilitated transport of ethylene across polymer membranes containing silver salt: effect of HBF4 on the photoreduction of silver ions, J. Membr. Sci., 212 (2003) 283. [7] I. Pinnau and L.G. Toy, Solid polymer electrolyte composite membranes for olefin/paraffin separation, J. Membr. Sci., 184 (2001) 39. [8] S.W. Kang, K. Char, J.H. Kim, C.K. Kim and Y.S. Kang, Control of ionic interactions in silver polymer electrolytes using an ionic liquid and its implication in facilitated olefin transport, Chem. Mater., 18 (2006) 1789. [9] H.S. Kim, J.H. Ryu, B. Jose, B.G. Lee, B.S. Ahn and Y.S. Kang, Formation of silver nanoparticles induced by poly(2,6-dimethyl-1,4-phenylene oxide), Langmuir, 17 (2001) 5817.
Correlation between anions of ionic liquids and reduction of silver ions in facilitated olefin transport membranes Sang Wook Kanga, Jinkee Honga, Kookheon Chara, Jong Hak Kimb, Jungahn Kimc, Yong Soo Kangd* a
School of Chemical & Biological Engineering and NANO Systems Institute-National Core Research Center, Seoul National University, Seoul 151-744, South Korea b Department of Chemical Engineering, Yonsei University, Seoul 120-749, South Korea c Research Institute of Basic Sciences and Department of Chemistry, Kyung Hee University, Seoul 130-701, South Korea d Department of Chemical Engineering, Hanyang University, Seoul 133-791, South Korea Tel. þ82(2)22202336; Fax þ82(2)22962969; email: [email protected] Received 31 July 2007; accepted revised 28 September 2007
Abstract þ þ The correlation between the anions of the ionic liquids of BMIMþBF 4 , BMIM CF3SO3 and BMIM NO3
in poly(2-ethyl-2-oxazoline)/AgNO3 membranes and reduction of silver ions has been investigated. The anion type negligibly affected the initial separation performances, however it significantly affected the long-term operation stability. Additionally, UV and TEM confirmed the long-term operational stability was strongly associated with the reduction rate of the silver ions. The reduction rate of the silver ions in the polymer/silver salt/ionic liquid complex was observed in following order: BMIMþBF 4 þ þ > BMIMþCF3SO > BMIM NO , suggesting that among the ionic liquids investigated, BMIM NO 3 3 3 had the most improved separation performance. Keywords: Ionic liquids; Reduction; Anion; Silver ion; Facilitated olefin transport
*Corresponding author. Presented at the Fourth Conference of Aseanian Membrane Society (AMS 4), 16–18 August 2007, Taipei, Taiwan. 0011-9164/08/$– See front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2007.09.058
328
S.W. Kang et al. / Desalination 233 (2008) 327–332
1. Introduction Facilitated olefin transport membranes employing solid polymer electrolytes consisting of silver salt dissolved in a polar polymer matrix have shown high separation performance for olefin/paraffin mixtures by reversible complexation with olefin molecules due to the olefin carrier activity of silver ions [1–4]. Despite many advantages of the silver polymer electrolyte membranes, a problem for practical application is long-term stability of the separation performance as the silver ions can be reduced to form metallic silver, and consequently lose olefin carrier activity [5,6]. Among the silver salts capable of reversibly forming silver–olefin complexes, AgBF4 has commonly been used due to high carrier activity with respect to olefin molecules [7]. However, AgBF4 is easily converted to metallic silver, which diminishes the carrier activity. Alternatively, AgNO3, is not readily reduced to metallic silver, however it is rather inactive as an olefin carrier [7]. Therefore, it is desirable to significantly stabilize but inactivate AgNO3 for olefin complexation and carriers for facilitated transport membranes [5]. Recently, highly charged ionic liquids (ILs) were used to activate AgNO3 in silver polymer electrolyte membranes consisting of poly(2-ethyl2-oxazoline) (POZ) and AgNO3 [8]. The introduction of ionic liquids BMIMþNO 3 and into the POZ/AgNO complex BMIMþBF 3 4 membranes increased the Agþ activity in reversible complexation with olefins. The enhanced olefin complexation ability was presumably due to the interaction between BMIMþ and NO 3 of AgNO3, which weakened the interaction between Agþ and NO 3 and resulted in the formation of free silver ions. The effect of IL anions on the reduction behavior of silver cations and separation performance of olefin/paraffin mixtures through POZ/AgNO3/IL membranes was observed. The
ILs investigated were 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMþBF 4 ), 1-butyl3-methylimidazolium triflate (BMIMþCF3SO 3) and 1-butyl-3-methylimidazolium nitrate (BMIMþNO 3 ). 2. Materials and methods 2.1. Materials AgNO3 (99%) and POZ (Mw ¼ 5.0 105 g mol1) were purchased from Aldrich Chemiþ cal Co. BMIMþBF 4 , BMIM CF3SO3 and þ BMIM NO3 were purchased from C-TRI Co. All chemicals were used as received. 2.2. Separation performance The POZ/AgNO3/ionic liquid electrolytes were prepared by dissolving AgNO3 and each IL in an aqueous solution containing 20 wt.% POZ. The [C¼O]:[Ag] molar ratio was 1:1 and the amount of IL was 0.1 molar ratio. For fabrication of the separation membranes, the POZ/ AgNO3/ionic liquid electrolyte solutions were coated on polysulfone microporous support (Seahan Industries Inc., Seoul, Korea) using an RK Control Coater (Model 101, Control Coater RK Print-Coat instruments LTD, UK). After solvent evaporation in a convection oven at room temperature under nitrogen, the POZ/AgNO3/ ionic liquid complexes were vacuum dried for 2 days at room temperature. The thickness of the top polymer electrolyte layer was determined with SEM to be approx. 1 mm. Gas flow rate or gas permeance was measured with a mass flow meter. The unit of gas permeance is GPU, such that 1 GPU ¼ 1 106 cm3 (STP)/(cm2 s cm Hg). Mixed gas (50:50 vol% propylene/propane mixture) separation properties of the POZ/ AgNO3/ionic liquid complexes were evaluated using a gas chromatograph (Hewlett-Packard G1530A, MA) equipped with a TCD detector and unibead 2S 60/80 packed column.
S.W. Kang et al. / Desalination 233 (2008) 327–332
2.3. Characterization To observe silver nanoparticles produced from the reduction of silver, transmission electron micrographs (TEM) were obtained from a Philips CM30 microscope operating at 300 KV. TEM samples were prepared by casting a solution of 1/1/0.1 POZ (10 wt.%)/AgNO3/ionic liquid on a glass plate and drying for 2 h in a light-protected convection oven at room temperature under nitrogen. After drying, the samples were dissolved in a solvent and a drop of the resulting colloidal silver dispersion was placed on a standard copper grid. For UV–Vis spectroscopy, 1/1/0.1 POZ/ AgNO3/ionic liquid complex solutions with the same polymer concentration in 0.5 wt.% were dropped on each quartz window with a micropipette. The quartz windows were dried under nitrogen for 2 h at room temperature and further dried in a vacuum oven for 2 days. UV–Vis spectra between 190 and 900 nm was measured with a spectrophotometer (Hewlett-Packard).
329
Table 1 þ Effects of the ionic liquids (BMIMþNO 3 , BMIM BF4 þ and BMIM CF3SO3 ) on the mixed gas permeances and propylene/propane selectivities of 1/1 POZ/AgNO3 membranes Membranes
Total Selectivity permeance (GPU)
POZ/AgNO3 POZ/AgNO3/BMIMþNO 3 POZ/AgNO3/BMIMþBF 4 POZ/AgNO3/BMIMþCF3SO 3
0.1 5.6 5.4 5.1
0.95 32.0 31.8 33.2
olefin transport resulting from enhanced carrier activity of Agþ in the complexes with ILs [8]. Regardless of the type of IL used, POZ/AgNO3/ IL complex membranes had similar initial separation performances for the propylene/propane mixtures. 3.2. Long-term separation performance
3. Results and discussion 3.1. Mixed gas performance The effect of ILs on the separation performance was evaluated for propylene/propane mixtures with 1/1/0.1 POZ/AgNO3/BMIMþNO 3 , 1/1/0.1 and 1/1/0.1 POZ/ POZ/AgNO3/BMIMþBF 4 þ AgNO3/BMIM CF3SO3 complexes. As summarized in Table 1, the selectivity of propylene over propane, defined as the propylene concentration ratio of the permeate to feed, was approximately 0.95 for 1/1 POZ/AgNO3 without ILs and approximately 0.1 GPU permeance. However, the selectivities of the 1/1/0.1 POZ/AgNO3/ þ BMIMþNO 3 , 1/1/0.1 POZ/AgNO3/BMIM BF4 þ and 1/1/0.1 POZ/AgNO3/BMIM CF3SO 3 complex membranes were 32.0, 31.8 and 33.2, respectively, with mixed gas permeances of 5.6, 5.4 and 5.1 GPU, respectively. The improved separation performance was due to the facilitated
The stabilities of the long-term separation performances of the polymer/silver salt/ionic liquid complex membranes were investigated. Fig. 1 indicates that the 1/1/0.1 POZ/AgNO3/ membrane exhibited stable BMIMþNO 3 separation performance throughout continuous operation for 140 h, whereas the separation performance of the 1/1/0.1 POZ/AgNO3/ BMIMþCF3SO 3 membrane deteriorated. Furthermore, the separation performance of the 1/1/0.1 membrane rapidly POZ/AgNO3/BMIMþBF 4 decreased with time, suggesting there was a strong dependence on the stability of the olefin carrier activity of the silver ion on the nature of the counter anion. 3.3. Formation of silver nanoparticles The effect of the IL counteranions on the formation of silver nanoparticles in the POZ/
S.W. Kang et al. / Desalination 233 (2008) 327–332 40 6
35 30 25 20 15 POZ/AgNO3/BMIM+NO3–
10
+
–
POZ/AgNO3/BMIM Tf
POZ/AgNO3/BMIM+BF4–
5
Permeance (GPU)
Selectivity (Propylene /Propane)
330
5 4 3 POZ/AgNO3/BMIM+NO3– POZ/AgNO3/BMIM+Tf–
2
POZ/AgNO3/BMIM+BF4–
1
0 0
20
40
60
80
100 120 140 160
0
20
40
60
80
Time (h)
Time (h)
(a)
(b)
100 120 140 160
Fig. 1. Separation performance: (a) mixed-gas selectivity and (b) mixed gas permeance of 1/1/0.1 POZ/AgNO3/ þ þ BMIMþBF 4 , 1/1/0.1 POZ/AgNO3/BMIM CF3SO3 and 1/1/0.1 POZ/AgNO3/BMIM NO3 complex membranes with time. (40 psig and 20 C).
AgNO3/IL complex membrane was investigated using TEM. The TEM micrograph of the 1/1/0.1 POZ/AgNO3/BMIMþNO 3 membrane immediately after preparation indicated a few particles of approximately 5 nm (Fig. 2a). As shown in Fig. 2b, the size and number of the silver nanoparticles remained unchanged after 1 week, indicating the reduction reaction of silver ions to nanoparticles was mostly prohibited. Alternatively, the TEM micrographs of 1/1/0.1 POZ/AgNO3/BMIMþCF3SO 3 and 1/1/0.1 POZ/
complexes immediately AgNO3/BMIMþBF 4 after preparation indicated silver nanoparticles of approximately 10 nm in size (Figs. 3a and 4a), which was much larger than the 1/1/0.1 POZ/ AgNO3/BMIMþNO 3 complex. Furthermore, the size and number of silver particles in 1/1/0.1 POZ/AgNO3/BMIMþCF3SO 3 and 1/1/0.1 POZ/AgNO3/BMIMþBF com4 plexes increased significantly after 1 week (Figs. 3b and 4b). Specifically, compared to the other POZ/AgNO3/IL complexes, the particle
Fig. 2. Transmission electron micrographs of 1/1/0.1 POZ/AgNO3/BMIMþNO 3 membrane (a) right after preparation and (b) after storage time of 1 week in dark place.
Fig. 3. Transmission electron micrographs of 1/1/0.1 POZ/AgNO3/BMIMþCF3SO 3 membrane (a) right after preparation and (b) after storage time of 1 week in dark place.
S.W. Kang et al. / Desalination 233 (2008) 327–332
331
size of the 1/1/0.1 POZ/AgNO3/BMIMþBF 4 complexes increased rapidly, indicating that more silver ions were reduced to silver nanoparticles. The changes in particle size and number were consistent with the long-term separation tests indicating the poor stability of the POZ/ AgNO3/BMIMþBF 4 complex membrane.
3.4. UV–Visible spectroscopy
Fig. 4. Transmission electron micrographs of 1/1/0.1 POZ/AgNO3/BMIMþBF 4 membrane (a) right after preparation and (b) after storage time of 1 week in dark place.
UV–Visible absorption spectra are known to be quite sensitive to the formation of silver nanoparticles. Fig. 5 shows the absorption spectra
1.4
1.4
Absorbance
1.2 1.0
2h 1h 30 min 0 min
1.0
0.8 0.6 0.4
0.6 0.4 0.2
0.0
0.0 400
600
800
1000
2h 1h 30 min 0 min
0.8
0.2
200
POZ/AgNO3/BMIM+CF3SO3–
1.2
Absorbance
POZ/AgNO3/BMIM+NO3–
200
1200
400
Wavelength (nm)
600
800
1000
1200
Wavelength (nm)
(b)
(a) 1.4
Absorbance
2h 1h 30 min 0 min
POZ/AgNO3/BMIM+BF4–
1.2 1.0 0.8 0.6 0.4 0.2 0.0
200
400
600
800
1000
1200
Wavelength (nm)
(c)
Fig. 5. The UV–Visible spectra of (a) the 1/1/0.1 POZ/AgNO3/BMIMþNO 3 membrane, (b) the 1/1/0.1 POZ/AgNO3/ þ membrane and (c) 1/1/0.1 POZ/AgNO /BMIM BF with varying UV irradiation time. BMIMþCF3SO 3 3 4
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of 1/1/0.1 POZ/AgNO3/BMIMþNO 3 , 1/1/0.1 þ POZ/AgNO3/BMIM BF4 and 1/1/0.1 POZ/ AgNO3/BMIMþCF3SO 3 complexes with UV irradiation at 9970 mW cm2 and with varying time. A broad absorption maximum near 420 nm was attributed to the plasmon excitation of silver nanoparticles and it is generally accepted that the peak height correlates to the amount of silver nanoparticles [9]. UV irradiation experiments were performed to accelerate the reduction reaction to silver nanoparticles and thus investigate the formation kinetics of silver nanoparticles for polymer/silver salt/ionic liquid complexes. The intensity of the plasmon peak in the 1/1/0.1 POZ/AgNO3/BMIMþBF 4 complex was greater than 1/1/0.1 POZ/AgNO3/BMIMþNO 3 and 1/1/ þ 0.1 POZ/AgNO3/BMIM CF3SO3 , corroborating the higher concentration of silver nanoparticles observed with TEM.
4. Conclusion The correlation between IL anions and the reduction behavior of the silver ion in POZ/ AgNO3/IL membranes was strongly associated to the type of the IL anions used. In particular, the reduction rate of silver ions in a polymer/ silver salt/ionic liquid complex to metallic silver was observed by TEM and UV–Vis in the followþ ing order: BMIMþBF 4 > BMIM CF3SO3 > þ BMIM NO3 . Additionally, reduction to silver nanoparticles was observed for long-term operational stability. The reason why the BMIMþBF 4 among ionic liquid has the bad effect on long-term stability could be guessed by that since the stability among silver salts was as follows: AgNO3 > AgCF3SO3 > AgBF4 [8], it could be thought that the anion existed in polymer electrolyte affects on the reduction of silver ion. In the same way, POZ/AgNO3/BMIMþNO 3 membrane was more stable than POZ/AgNO3/BMIMþBF 4 membrane. However, the investigation on detail reasons is in progress and will be reported in near future.
Acknowledgments This work was supported by Energy Technology R&D program (2006-E-ID11-P-13) under the Korea Ministry of Commerce, Industry and Energy (MOCIE). KC acknowledges the financial support of NSI-NCRC and the Ministry of Education through the Brain Korea 21 Program at Seoul National University.
References [1] S.U. Hong, J. Won and Y.S. Kang, Polymer-salt complexes containing silver ions and their application to facilitated olefin transport membranes, Adv. Mater., 12 (2000) 968. [2] I. Pinnau, L.G. Toy and C. Casillas, Olefin separation membrane and process, US Patent, 5,670,051, 23 September 1997. [3] S. Sunderrajan, B.D. Freeman, C.K. Hall and I. Pinnau, Propane and propylene sorption in solid polymer electrolytes based on poly(ethylene oxide) and silver salts, J. Membr. Sci., 182 (2001) 1. [4] J.H. Kim, B.R. Min, C.K. Kim, J. Won and Y.S. Kang, Role of transient cross-links for transport properties in silver-polymer electrolytes, Macromolecules, 34 (2001) 6052. [5] B. Jose, J.H. Ryu, B.G. Lee, H. Lee, Y.S. Kang and H.S. Kim, Effect of phthalates on the stability and performance of AgBF4-PVP membranes for olefin/ paraffin separation, Chem. Commun., (2001) 2046. [6] J.H. Kim, B.R. Min, H.S. Kim, J. Won and Y.S. Kang, Facilitated transport of ethylene across polymer membranes containing silver salt: effect of HBF4 on the photoreduction of silver ions, J. Membr. Sci., 212 (2003) 283. [7] I. Pinnau and L.G. Toy, Solid polymer electrolyte composite membranes for olefin/paraffin separation, J. Membr. Sci., 184 (2001) 39. [8] S.W. Kang, K. Char, J.H. Kim, C.K. Kim and Y.S. Kang, Control of ionic interactions in silver polymer electrolytes using an ionic liquid and its implication in facilitated olefin transport, Chem. Mater., 18 (2006) 1789. [9] H.S. Kim, J.H. Ryu, B. Jose, B.G. Lee, B.S. Ahn and Y.S. Kang, Formation of silver nanoparticles induced by poly(2,6-dimethyl-1,4-phenylene oxide), Langmuir, 17 (2001) 5817.