- Email: [email protected]
Nanocomposite silver polymer electrolytes as facilitated olefin transport membranes
Journal of Membrane Science 285 (2006) 102–107
Nanocomposite silver polymer electrolytes as facilitated olefin transport membranes Sang Wook Kang a , Jong Hak Kim b , Kookheon Char a , Jongok Won c , Yong Soo Kang d,∗ a
School of Chemical & Biological Engineering, Seoul National University, Seoul 151-744, South Korea b Department of Chemical Engineering, Yonsei University, Seoul 120-749, South Korea c Department of Applied Chemistry, Sejong University, Seoul 143-747, South Korea d Department of Chemical Engineering, Hanyang University, Seoul 133-791, South Korea Received 5 June 2006; received in revised form 31 July 2006; accepted 3 August 2006 Available online 14 August 2006
Abstract Solid-state silver polymer electrolyte membranes containing AgNO3 dissolved in poly(2-ethyl-2-oxazoline) (POZ) as an olefin carrier are not efficient for the separation of olefin/paraffin mixtures (selectivity ∼1). The addition of fumed silica nanoparticles into the POZ/AgNO3 complex membranes, however, resulted in a surprisingly significant enhancement of the permeance and selectivity (higher than 80) for propylene/propane mixtures. These enhancements are explained in terms of the silver ion activity in olefin coordination. FT-IR, X-ray photoelectron spectroscopy (XPS) and FT-Raman demonstrate that the silver ion activity continuously increases by Lewis acid–base interactions upon incorporation of the silica nanoparticles up to 0.1 mole ratio of silica, above which it abruptly decreases. Furthermore, the structural change of polymer electrolyte membranes took place by the presence of nanoparticles, supported by the Tg measurement. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanocomposite membrane; Silver; Silica nanoparticle; Olefin; Facilitated transport
1. Introduction Solid polymer electrolytes (SPEs), formed by dissolving salts in a polymer solvent, are popular materials for use in rechargeable lithium batteries [1]. In addition, silver polymer electrolytes comprising silver salts dissolved in a polar polymer matrix have attracted much attention for their application in solid state facilitated transport membranes. These silver SPEs have many advantages, including high separation performance, simple operation and low energy consumption [2,3]. For instance, solid polymer electrolyte membranes consisting of poly(2-ethyl-2-oxazoline) (POZ) and AgBF4 , or poly(vinyl pyrrolidone) (PVP) and AgCF3 SO3 , demonstrated a propylene/propane selectivity of 45 and a total mixed gas permeance of 12 GPU (1 GPU = 1 × 10−6 cm3 (STP)/(cm2 s cmHg)) [4,5].
∗
Corresponding author. Tel.: +82 2 2220 2336; fax: +82 2 2298 4101. E-mail address: [email protected] (Y.S. Kang).
0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.08.005
Recently, SPEs containing inorganic nanoparticles have demonstrated superior transport properties. For example, conductivity was significantly increased by adding nanoparticles 10% by weight to poly(ethylene oxide) (PEO)/LiClO4 electrolytes [6]. In addition, inorganic nanoparticles such as MnO2 , TiO2 , and ZrO2 have been used to increase the conductivity of polyaniline nanocomposites [7–9]. In a gas separation, the permeability coefficient of poly(4-methyl-2-pentyne) (PMP) increased substantially with increasing concentration of nonporous, nanoscale fumed silica particles [10,11]. In an effort to increase both permeability and selectivity, micron-sized porous zeolite particles have been added to organic polymers in the hope of combining the elasticity and processability of polymers with the size-selective characteristics of spatially well-defined zeolite pores [12]. Unfortunately, the silver polymer electrolyte membranes for olefin/paraffin separation have poor stability for long term operation, primarily due to the reduction of silver ions to metallic silver [13]. In particular, the olefin carriers AgBF4 and AgClO4 can be easily reduced to metallic silver and consequently lose
S.W. Kang et al. / Journal of Membrane Science 285 (2006) 102–107
their olefin carrier activity [14]. Therefore, it is desirable to employ a silver salt with a high lattice energy such as AgNO3 to prevent silver reduction, but AgNO3 has been found to be a rather inactive olefin carrier in solid polymer electrolytes [2]. Here we report nanocomposite silver polymer electrolyte membranes containing silica nanoparticles in POZ/AgNO3 complexes for the separation of olefin/paraffin mixtures. The silica nanoparticles were introduced to increase the activity of AgNO3 in olefin complexes, resulting in facilitated olefin transport and improved separation performance for olefin/paraffin mixtures. The potential interactions of AgNO3 with the nanoparticles and/or polymer matrix have been characterized by X-ray photoelectron spectroscopy (XPS), FT-Raman and FT-IR spectroscopy. 2. Experimental 2.1. Materials Silver nitrate (AgNO3 , 99.9%), silver tetrafluoroborate (AgBF4 , 99.9%), poly(2-ethyl-2-oxazoline) (POZ, Mw = 5.0 × 105 g/mol), and fumed silica nanoparticles (12-nm primary particles size) were purchased from Aldrich Chemical Co. All chemicals were used as received. 2.2. Membrane preparation and permeance measurements Polymer/silver salt complex solutions containing equimolar amounts of AgNO3 and a polymer were prepared by dissolving 0.34 g of AgNO3 in 0.2 g POZ/0.8 g water. Various mole ratios of fumed silica nanoparticles with respect to the carbonyl oxygen of POZ were added to these solutions. POZ/AgBF4 membranes were prepared by dissolving 0.39 g of AgBF4 in 0.2 g POZ/0.8 g water. The solutions were then coated onto a polysulfone microporous membrane support (SEAHAN Industries Inc., Seoul, Korea) using an RK Control Coater (Model 101, Control Coater RK Print-Coat instruments Ltd., UK). The solvent was evaporated in a light-protected convection oven at room temperature under a stream of nitrogen, and then the membranes were dried completely in a vacuum oven for 2 days at room temperature. The water remained nearly in the membranes, as confirmed by FT-IR. The thickness of the top polymer membrane layer was ca. 1 m, as determined by scanning electron microscopy (SEM). Permeation tests were performed in a stainless steel separation module as described elsewhere [4]. The flow rates of mixed gas and sweep gas (helium) were controlled using mass flow controllers. The gas flow rates represented by gas permeance were determined using a mass flow meter (MFM). The unit of gas permeance is GPU, where 1 GPU = 1 × 10−6 cm3 (STP)/(cm2 s cmHg). The membranes’ separation properties for mixed gas (50:50 vol.% of propylene/propane mixture) were evaluated by gas chromatography (Hewlett Packard) equipped with a TCD and a unibead 2S 60/80 packed column. Membranes were operated at the condition of 40 psig and 20 ◦ C.
103
2.3. Characterization The IR measurements were performed on a Perkin-Elmer FT-IR spectrometer; 64–200 scans were signal-averaged at a resolution of 4 cm−1 . XPS data were acquired using a Perkin-Elmer Physical Electronics PHI 5400 X-ray photoelectron spectrometer. This system was equipped with a Mg X-ray source operated at 300 W (15 kV, 20 Ma). The carbon (C 1s) line at 285.0 eV was used as a reference in determining the binding energies of silver ions. Raman spectra were collected for POZ: silver salt films at RT using a Perkin-Elmer System 2000 NIR FT-Raman, at a resolution of 1 cm−1 . This experimental apparatus included a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser operating at 1064 nm. Spectroscopic characterization was performed using a pressure cell equipped with CaF2 windows. A Perkin-Elmer DSC-7 was used to measure glass transition temperatures of POZ:Ag salt electrolytes containing fumed silica nanoparticles at a heating rate of 20 ◦ C/min under an N2 environment. 3. Results and discussion 3.1. Separation performance The separation of propylene/propane mixtures using nanocomposite silver polymer electrolyte membranes was evaluated with varying concentrations of the fumed silica nanoparticles. The POZ/AgNO3 membrane without silica nanoparticles exhibits low gas permeation and no separation of the propylene/propane mixtures; mixed gas permeance is ca. 0.1 GPU (1 GPU = 1 × 10−6 cm3 (STP)/cm2 s cmHg) and the selectivity of propylene/propane is nearly unity. Fig. 1 shows the total permeance and the selectivity of propylene over propane through POZ/AgNO3 membranes containing the fumed silica nanoparticles. The mole ratio of the monomeric unit of POZ to silver ion was fixed at unity, i.e. [C O]:[Ag] = 1:1. The mole ratio of 1/1/0.1 POZ/AgNO3 /SiO2 means 0.2 g/0.34 g/0.012 g by mass. The presence of the fumed silica nanoparticles in the POZ/AgNO3 membranes resulted in an increase in both the selectivity and the permeance. It was thought that the interactions between the silver ion and the counteranion would be reduced upon the addition of the fumed silica nanoparticles, allowing the silver ions to be more active for olefin coordination. The selectivity of propylene/propane and the mixed gas permeance increased to 88.0 and 1.02 GPU, respectively, for the POZ/AgNO3 /SiO2 composite membrane. Thus, the transport property of propylene will be significantly dependent on the activity of the silver ion as a propylene carrier. Another interesting feature is that the membrane performance shows a maximum at the mole ratio of fumed silica nanoparticles of 0.1. Above the 0.1 mole ratio, the separation performance in terms of both the selectivity and permeance were decreased. This is likely caused by the self-hydrogen bonding between OH groups in the silica surface. The separation performances of propylene/propane mixtures were measured with time up to 160 h. Fig. 2 shows the mixed gas permeance and the selectivity with respect to propylene/propane
104
S.W. Kang et al. / Journal of Membrane Science 285 (2006) 102–107
Fig. 1. Separation performance: (a) total permeance and (b) mixed gas selectivity of POZ/AgNO3 membranes with various mole ratios of fumed silica nanoparticles.
Fig. 2. Separation performance: (a) total permeance and (b) mixed-gas selectivity of 1:1:0.1 POZ/AgNO3 /fumed silica nanoparticles and 1:1 POZ/AgBF4 membranes as a function of time (40 psig and 20 ◦ C).
3.2. The interactions of the silver ion with carbonyl oxygens of the POZ/AgNO3 /fumed silica nanoparticles composite and POZ/AgBF4 membranes. The mole ratio of silver ions to the monomeric units of polymer was fixed at 1, and the mole ratio of fumed silica nanoparticles to silver ions was fixed at 0.1. The selectivity and the permeance of the POZ/AgNO3 membrane containing the fumed silica nanoparticles were nearly invariant for the duration of the experiment up to 160 h, which suggests that the action of the silver ions as olefin carriers is very stable in this system. On the other hand, both the selectivity of propylene over propane and the mixed gas permeance through the POZ/AgBF4 membrane decrease rapidly with time. This monotonic decrease in the separation performance of the POZ/AgBF4 membrane indicates the deactivation of the olefin carrier, i.e. the reduction of silver ion to silver metal nanoparticles [14]. These experimental results suggest that the fumed silica nanoparticles in POZ/AgNO3 complexes cause a favorable interaction between the silver ion and olefin, resulting in facilitated olefin transport and improved separation performance for 160 h. The increased carrier activity of the silver ions are characterized in the following sections in terms of the changes in the interactions of the silver ions with POZ and the fumed silica nanoparticles, as examined by spectroscopy.
The incorporation of the fumed silica nanoparticles in the silver polymer electrolyte membranes can lead to a change in the interactions of the silver ions with carbonyl oxygens. FTIR spectra for pure POZ, 1:0.1 POZ/SiO2 , 1:1 POZ/AgNO3 , 1:1:0.05 POZ/AgNO3 /SiO2 and 1:1:0.1 POZ/AgNO3 /SiO2 complexes are shown in Fig. 3. Upon incorporation of SiO2 into POZ, the C O stretching band of POZ at 1641 cm−1 hardly changes, indicating that interaction between POZ and SiO2 is negligible. When AgNO3 is complexed with POZ, the C O stretching band shifts from 1641 cm−1 to a lower wave number at 1605 cm−1 , presumably due to the weakened C O double bond strength by the coordinative interaction between the silver cation and carbonyl oxygen of POZ. When SiO2 was added in POZ/AgNO3 , the relative intensity of the complexed C O stretching band at 1605 cm−1 decreased and a new band at 1590 cm−1 became prominent. This peak shift from 1604 to 1590 cm−1 suggests a stronger interaction between the silver ion and the C O of POZ containing the SiO2 nanoparticles, as compared to that without SiO2 . These results imply that the interaction between NO3 − and Ag+ became weakened by a Lewis acid–base interaction between SiO2 and the NO3 − anion in
S.W. Kang et al. / Journal of Membrane Science 285 (2006) 102–107
105
ened interaction between the silver cation and its counter anion. However, above a 0.1 mole ratio of SiO2 , the binding energy increases. It is anticipated that at low silica concentrations, most of the Lewis acid silica nanoparticles interact with NO3 − , resulting in the decrease of interaction intensity between the silver ion and the NO3 − anion. However, at high silica concentrations, silica nanoparticles are not homogenously or uniformly dispersed due to the self-segregation of the nanoparticles resulting from the hydrogen bonding interactions between the OH groups in the silica surface [17,18]. Thus the silica nanoparticles barely affect the interaction intensity between the silver ion and NO3 − anion. Therefore, the silver ion activity only increases up to 0.1 mole ratio of silica nanoparticles, above which it decreases to some degree. Fig. 3. FT-IR spectroscopy of POZ/AgNO3 complexes with different mole ratios of fumed silica nanoparticles to the silver ion.
the POZ/AgNO3 /SiO2 nanocomposite electrolyte membranes. Since fumed silica nanoparticles (SiO2 ) are considered to be a weak Lewis acid [15,16], they are expected to interact with the NO3 − anion, resulting in the weakened interaction between the silver ion and the NO3 − anion. This reduced interaction causes the silver ions to complex with olefin, consequently leading to the facilitated olefin transport and the enhancement of the separation performance of propylene/propane mixtures. 3.3. Binding energy of silver atoms XPS was used to observe the change of the chemical environment around the silver ions in the silver polymer electrolyte membranes due to the incorporation of the silica nanoparticles. The binding energy of the d5/2 orbital of the silver atom in the POZ/AgNO3 system decreases gradually from 369.63 to 368.39 eV with increasing silica nanoparticles concentration, as shown in Fig. 4. This indicates that the binding energy of the valence electrons in the silver atom decreases due to the weak-
Fig. 4. XPS results for the binding energies of silver ion in POZ/AgNO3 complexes with different mole ratios of fumed silica nanoparticles to the silver ion.
3.4. Ionic species of AgNO3 The interactions between Ag+ and NO3 − in the silver polymer electrolytes upon addition of the fumed silica nanoparticles were investigated using FT-Raman spectroscopy. The Raman spectra of POZ/AgNO3 electrolytes with varying mole ratio of the fumed silica nanoparticles are shown in Fig. 5, in the regions of the NO3 − stretching bands. Note that the NO3 − stretching bands at 1034, 1040, and 1045 cm−1 are assigned to free ions, ion pairs, and ion aggregates, respectively [19]. To obtain quantitative information, each peak is deconvoluted into free ions, ion pairs and aggregates. As a result, the fraction of free NO3 − in the POZ/AgNO3 without fumed silica nanoparticles is estimated to be only 13%, but increases approximately two-fold up to 27% upon the addition of 0.1 molar ratio of the fumed silica nanoparticles. This result suggests that the interactions between Ag+ and NO3 − are weakened by the addition of the fumed silica nanoparticles. Therefore, we conclude that the concentration of free NO3 − increases markedly upon the addition of the fumed silica nanoparticles, presumably making the silver ion more active in facilitated olefin transport
Fig. 5. Raman spectra of POZ/AgNO3 complexes with different mole ratios of fumed silica nanoparticles to silver ion.
106
S.W. Kang et al. / Journal of Membrane Science 285 (2006) 102–107
leading to the facilitated olefin transport and enhancement of the selectivity of propylene/propane and propylene permeance. However, at high silica concentrations, silica nanoparticles are not homogenously and uniformly dispersed, presumably due to self-segregation of nanoparticles resulting from the hydrogen bonding interaction between OH groups in the silica surface. Thus, the silica nanoparticles may not effectively form the Lewis acid–base complexes, resulting in poor separation performance for olefin/paraffin mixtures. Acknowledgements
Fig. 6. Glass transition temperature (Tg ) of 1:1 POZ/AgNO3 complexes with different mole ratios of fumed silica nanoparticles.
membrane. However, above 0.1 mole ratio of SiO2 , the fraction of free NO3 − no longer increases significantly, indicating that the role of the silica nanoparticles becomes less prominent. This result is consistent with the XPS data. 3.5. Glass transition temperature (Tg ) The change in chain flexibility of the polymer electrolytes upon addition of the fumed silica nanoparticles was investigated by measuring the glass transition temperature (Tg ) with a Perkin-Elmer DSC-7 at a heating rate of 10◦ min−1 under an N2 environment. Fig. 6 shows that Tg is found to be 78 ◦ C for neat POZ/AgNO3 , but it decreases to 67 ◦ C with increasing silica concentration up to 0.1 mole ratio of silica. If bridges among the hydroxyl groups of the nanoparticles formed, Tg increased with the concentration of the nanoparticles, [20] leading to decreased chain mobility and increased Tg . However, the bridge formation could be restricted in the nanocomposite silver electrolytes at low nanoparticle concentrations due to the Lewis acid–base interaction between the silica particles and NO3 − . Therefore, the decrease in Tg at silica concentrations less than 0.1 may be due to the restricted bridge formation. At high silica nanoparticle concentrations, however, the bridges among the particles are likely to be formed and consequently the chain flexibility is decreased, resulting in a higher Tg . 4. Conclusion The POZ/AgNO3 membrane exhibits extremely poor separation performance for olefin/paraffin mixtures due to the low concentration of the free silver ion of AgNO3 . When fumed silica nanoparticles are introduced into the POZ/AgNO3 membrane, the separation performance for olefin/paraffin mixtures improved markedly. These results can be explained by Lewis acid–base interaction between the fumed silica nanoparticles and the NO3 − anion. This reduced interaction causes the silver ions to be more active for olefin coordination, consequently
We acknowledge the partial financial support from Hanyang University through an internal research fund. KC acknowledges the financial support of the National Research Laboratory Program (Grant M1-0104-00-0191) and the Ministry of Education through the Brain Korea 21 Program at Seoul National University. References [1] F. Croce, G.B. Appetecchi, L. Persi, B. Scrosati, Nanocomposite polymer electrolytes for lithium batteries, Nature 394 (1998) 456. [2] I. Pinnau, L.G. Toy, C. Casillas, Olefin separation membrane and process, U.S. Patent 5,670,051 (2005) (September 23, 1997). [3] S. Sunderrajan, B.D. Freeman, C.K. Hall, 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] S.U. Hong, J. Won, Y.S. Kang, Polymer-salt complexes containing silver ions and their application to facilitated olefin transport membranes, Adv. Mater. 12 (2000) 968. [5] J.H. Kim, B.R. Min, J. Won, Y.S. Kang, Complexation mechanism of olefin with silver ions dissolved in polymer matrix and its effect on facilitated olefin transport, Chem. Eur. J. 8 (2002) 650. [6] F. Croce, R. Curini, A. Martinelli, L. Persi, F. Ronci, B. Scrosati, R. Caminiti, Physical and chemical properties of nanocomposite polymer electrolytes, J. Phys. Chem. B 103 (1999) 10632. [7] M. Biswas, S.S. Ray, Y.P. Liu, Water dispersible conducting nanocomposites of poly(N-vinylcarbazole), polypyrrole and polyaniline with nanodimensional manganese (IV) oxide, Synth. Met. 105 (1999) 99. [8] S.J. Su, N. Kuramoto, Processable polyaniline-titanium dioxide nanocomposites: effect of titanium dioxide on the conductivity, Synth. Met. 114 (2000) 147. [9] S.S. Ray, M. Biswas, Water-dispersible conducting nanocomposites of polyaniline and poly(N-vinylcarbazole) with nanodimensional zirconium dioxide, Synth. Met. 108 (2000) 231. [10] T.C. Merkel, B.D. Freeman, R.J. Spontak, Z. He, I. Pinnau, P. Meakin, A.J. Hill, Sorption, transport, and structural evidence for enhanced free volume in poly(4-methyl-2-pentyne)/fumed silica nanocomposite membranes, Chem. Mater. 15 (2003) 109. [11] T.C. Merkel, B.D. Freeman, R.J. Spontak, Z. He, I. Pinnau, P. Meakin, A.J. Hill, Ultrapermeable, reverse-selective nanocomposite membranes, Science 296 (2002) 519. [12] S. Kulprathipanja, R.W. Neuzil, N. Li, U.S. Patent 4,740,219 (1988). [13] B. Jose, J.H. Ryu, B.G. Lee, H. Lee, Y.S. Kang, H.S. Kim, Effect of phthalates on the stability and performance of AgBF4-PVP membranes for olefin/paraffin separation, Chem. Commun. (2001) 2046. [14] S.W. Kang, J.H. Kim, K.S. Oh, J. Won, K. Char, H.S. Kim, Y.S. Kang, Highly stabilized silver polymer electrolytes and their application to facilitated olefin transport membranes, J. Membr. Sci. 236 (2004) 163. [15] W. Wieczorek, P. Lipka, G. Zukowska, H. Wycis’lik, Ionic interactions in polymeric electrolytes based on low molecular weight poly(ethylene glycol)s, J. Phys. Chem. B 102 (1998) 6968.
S.W. Kang et al. / Journal of Membrane Science 285 (2006) 102–107 [16] D. Swierczynski, A. Zalewska, W. Wieczorek, Composite polymeric electrolytes from the PEODME-LiClO4 -SiO2 system, Chem. Mater. 13 (2001) 1560. [17] J. Hou, G.L. Baker, Preparation and characterization of cross-linked composite polymer electrolytes, Chem. Mater. 10 (1998) 3311. [18] J.H. Kim, M.-S. Kang, Y.J. Kim, J. Won, N.-G. Park, Y.S. Kang, Dye-sensitized nanocrystalline solar cells based on composite polymer
107
electrolytes containing fumed silica nanoparticles, Chem. Commun. (2004) 1662. [19] J.H. Kim, B.R. Min, C.K. Kim, J. Won, Y.S. Kang, Spectroscopic interpretation of silver ion complexation with propylene in silver polymer electrolytes, J. Phys. Chem. B 106 (2002) 2786. [20] J.R. MacCallum, C.A. Vincent, Polymer Electrolyte Reviews, Elsevier, Amsterdam, 1987.
Nanocomposite silver polymer electrolytes as facilitated olefin transport membranes Sang Wook Kang a , Jong Hak Kim b , Kookheon Char a , Jongok Won c , Yong Soo Kang d,∗ a
School of Chemical & Biological Engineering, Seoul National University, Seoul 151-744, South Korea b Department of Chemical Engineering, Yonsei University, Seoul 120-749, South Korea c Department of Applied Chemistry, Sejong University, Seoul 143-747, South Korea d Department of Chemical Engineering, Hanyang University, Seoul 133-791, South Korea Received 5 June 2006; received in revised form 31 July 2006; accepted 3 August 2006 Available online 14 August 2006
Abstract Solid-state silver polymer electrolyte membranes containing AgNO3 dissolved in poly(2-ethyl-2-oxazoline) (POZ) as an olefin carrier are not efficient for the separation of olefin/paraffin mixtures (selectivity ∼1). The addition of fumed silica nanoparticles into the POZ/AgNO3 complex membranes, however, resulted in a surprisingly significant enhancement of the permeance and selectivity (higher than 80) for propylene/propane mixtures. These enhancements are explained in terms of the silver ion activity in olefin coordination. FT-IR, X-ray photoelectron spectroscopy (XPS) and FT-Raman demonstrate that the silver ion activity continuously increases by Lewis acid–base interactions upon incorporation of the silica nanoparticles up to 0.1 mole ratio of silica, above which it abruptly decreases. Furthermore, the structural change of polymer electrolyte membranes took place by the presence of nanoparticles, supported by the Tg measurement. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanocomposite membrane; Silver; Silica nanoparticle; Olefin; Facilitated transport
1. Introduction Solid polymer electrolytes (SPEs), formed by dissolving salts in a polymer solvent, are popular materials for use in rechargeable lithium batteries [1]. In addition, silver polymer electrolytes comprising silver salts dissolved in a polar polymer matrix have attracted much attention for their application in solid state facilitated transport membranes. These silver SPEs have many advantages, including high separation performance, simple operation and low energy consumption [2,3]. For instance, solid polymer electrolyte membranes consisting of poly(2-ethyl-2-oxazoline) (POZ) and AgBF4 , or poly(vinyl pyrrolidone) (PVP) and AgCF3 SO3 , demonstrated a propylene/propane selectivity of 45 and a total mixed gas permeance of 12 GPU (1 GPU = 1 × 10−6 cm3 (STP)/(cm2 s cmHg)) [4,5].
∗
Corresponding author. Tel.: +82 2 2220 2336; fax: +82 2 2298 4101. E-mail address: [email protected] (Y.S. Kang).
0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.08.005
Recently, SPEs containing inorganic nanoparticles have demonstrated superior transport properties. For example, conductivity was significantly increased by adding nanoparticles 10% by weight to poly(ethylene oxide) (PEO)/LiClO4 electrolytes [6]. In addition, inorganic nanoparticles such as MnO2 , TiO2 , and ZrO2 have been used to increase the conductivity of polyaniline nanocomposites [7–9]. In a gas separation, the permeability coefficient of poly(4-methyl-2-pentyne) (PMP) increased substantially with increasing concentration of nonporous, nanoscale fumed silica particles [10,11]. In an effort to increase both permeability and selectivity, micron-sized porous zeolite particles have been added to organic polymers in the hope of combining the elasticity and processability of polymers with the size-selective characteristics of spatially well-defined zeolite pores [12]. Unfortunately, the silver polymer electrolyte membranes for olefin/paraffin separation have poor stability for long term operation, primarily due to the reduction of silver ions to metallic silver [13]. In particular, the olefin carriers AgBF4 and AgClO4 can be easily reduced to metallic silver and consequently lose
S.W. Kang et al. / Journal of Membrane Science 285 (2006) 102–107
their olefin carrier activity [14]. Therefore, it is desirable to employ a silver salt with a high lattice energy such as AgNO3 to prevent silver reduction, but AgNO3 has been found to be a rather inactive olefin carrier in solid polymer electrolytes [2]. Here we report nanocomposite silver polymer electrolyte membranes containing silica nanoparticles in POZ/AgNO3 complexes for the separation of olefin/paraffin mixtures. The silica nanoparticles were introduced to increase the activity of AgNO3 in olefin complexes, resulting in facilitated olefin transport and improved separation performance for olefin/paraffin mixtures. The potential interactions of AgNO3 with the nanoparticles and/or polymer matrix have been characterized by X-ray photoelectron spectroscopy (XPS), FT-Raman and FT-IR spectroscopy. 2. Experimental 2.1. Materials Silver nitrate (AgNO3 , 99.9%), silver tetrafluoroborate (AgBF4 , 99.9%), poly(2-ethyl-2-oxazoline) (POZ, Mw = 5.0 × 105 g/mol), and fumed silica nanoparticles (12-nm primary particles size) were purchased from Aldrich Chemical Co. All chemicals were used as received. 2.2. Membrane preparation and permeance measurements Polymer/silver salt complex solutions containing equimolar amounts of AgNO3 and a polymer were prepared by dissolving 0.34 g of AgNO3 in 0.2 g POZ/0.8 g water. Various mole ratios of fumed silica nanoparticles with respect to the carbonyl oxygen of POZ were added to these solutions. POZ/AgBF4 membranes were prepared by dissolving 0.39 g of AgBF4 in 0.2 g POZ/0.8 g water. The solutions were then coated onto a polysulfone microporous membrane support (SEAHAN Industries Inc., Seoul, Korea) using an RK Control Coater (Model 101, Control Coater RK Print-Coat instruments Ltd., UK). The solvent was evaporated in a light-protected convection oven at room temperature under a stream of nitrogen, and then the membranes were dried completely in a vacuum oven for 2 days at room temperature. The water remained nearly in the membranes, as confirmed by FT-IR. The thickness of the top polymer membrane layer was ca. 1 m, as determined by scanning electron microscopy (SEM). Permeation tests were performed in a stainless steel separation module as described elsewhere [4]. The flow rates of mixed gas and sweep gas (helium) were controlled using mass flow controllers. The gas flow rates represented by gas permeance were determined using a mass flow meter (MFM). The unit of gas permeance is GPU, where 1 GPU = 1 × 10−6 cm3 (STP)/(cm2 s cmHg). The membranes’ separation properties for mixed gas (50:50 vol.% of propylene/propane mixture) were evaluated by gas chromatography (Hewlett Packard) equipped with a TCD and a unibead 2S 60/80 packed column. Membranes were operated at the condition of 40 psig and 20 ◦ C.
103
2.3. Characterization The IR measurements were performed on a Perkin-Elmer FT-IR spectrometer; 64–200 scans were signal-averaged at a resolution of 4 cm−1 . XPS data were acquired using a Perkin-Elmer Physical Electronics PHI 5400 X-ray photoelectron spectrometer. This system was equipped with a Mg X-ray source operated at 300 W (15 kV, 20 Ma). The carbon (C 1s) line at 285.0 eV was used as a reference in determining the binding energies of silver ions. Raman spectra were collected for POZ: silver salt films at RT using a Perkin-Elmer System 2000 NIR FT-Raman, at a resolution of 1 cm−1 . This experimental apparatus included a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser operating at 1064 nm. Spectroscopic characterization was performed using a pressure cell equipped with CaF2 windows. A Perkin-Elmer DSC-7 was used to measure glass transition temperatures of POZ:Ag salt electrolytes containing fumed silica nanoparticles at a heating rate of 20 ◦ C/min under an N2 environment. 3. Results and discussion 3.1. Separation performance The separation of propylene/propane mixtures using nanocomposite silver polymer electrolyte membranes was evaluated with varying concentrations of the fumed silica nanoparticles. The POZ/AgNO3 membrane without silica nanoparticles exhibits low gas permeation and no separation of the propylene/propane mixtures; mixed gas permeance is ca. 0.1 GPU (1 GPU = 1 × 10−6 cm3 (STP)/cm2 s cmHg) and the selectivity of propylene/propane is nearly unity. Fig. 1 shows the total permeance and the selectivity of propylene over propane through POZ/AgNO3 membranes containing the fumed silica nanoparticles. The mole ratio of the monomeric unit of POZ to silver ion was fixed at unity, i.e. [C O]:[Ag] = 1:1. The mole ratio of 1/1/0.1 POZ/AgNO3 /SiO2 means 0.2 g/0.34 g/0.012 g by mass. The presence of the fumed silica nanoparticles in the POZ/AgNO3 membranes resulted in an increase in both the selectivity and the permeance. It was thought that the interactions between the silver ion and the counteranion would be reduced upon the addition of the fumed silica nanoparticles, allowing the silver ions to be more active for olefin coordination. The selectivity of propylene/propane and the mixed gas permeance increased to 88.0 and 1.02 GPU, respectively, for the POZ/AgNO3 /SiO2 composite membrane. Thus, the transport property of propylene will be significantly dependent on the activity of the silver ion as a propylene carrier. Another interesting feature is that the membrane performance shows a maximum at the mole ratio of fumed silica nanoparticles of 0.1. Above the 0.1 mole ratio, the separation performance in terms of both the selectivity and permeance were decreased. This is likely caused by the self-hydrogen bonding between OH groups in the silica surface. The separation performances of propylene/propane mixtures were measured with time up to 160 h. Fig. 2 shows the mixed gas permeance and the selectivity with respect to propylene/propane
104
S.W. Kang et al. / Journal of Membrane Science 285 (2006) 102–107
Fig. 1. Separation performance: (a) total permeance and (b) mixed gas selectivity of POZ/AgNO3 membranes with various mole ratios of fumed silica nanoparticles.
Fig. 2. Separation performance: (a) total permeance and (b) mixed-gas selectivity of 1:1:0.1 POZ/AgNO3 /fumed silica nanoparticles and 1:1 POZ/AgBF4 membranes as a function of time (40 psig and 20 ◦ C).
3.2. The interactions of the silver ion with carbonyl oxygens of the POZ/AgNO3 /fumed silica nanoparticles composite and POZ/AgBF4 membranes. The mole ratio of silver ions to the monomeric units of polymer was fixed at 1, and the mole ratio of fumed silica nanoparticles to silver ions was fixed at 0.1. The selectivity and the permeance of the POZ/AgNO3 membrane containing the fumed silica nanoparticles were nearly invariant for the duration of the experiment up to 160 h, which suggests that the action of the silver ions as olefin carriers is very stable in this system. On the other hand, both the selectivity of propylene over propane and the mixed gas permeance through the POZ/AgBF4 membrane decrease rapidly with time. This monotonic decrease in the separation performance of the POZ/AgBF4 membrane indicates the deactivation of the olefin carrier, i.e. the reduction of silver ion to silver metal nanoparticles [14]. These experimental results suggest that the fumed silica nanoparticles in POZ/AgNO3 complexes cause a favorable interaction between the silver ion and olefin, resulting in facilitated olefin transport and improved separation performance for 160 h. The increased carrier activity of the silver ions are characterized in the following sections in terms of the changes in the interactions of the silver ions with POZ and the fumed silica nanoparticles, as examined by spectroscopy.
The incorporation of the fumed silica nanoparticles in the silver polymer electrolyte membranes can lead to a change in the interactions of the silver ions with carbonyl oxygens. FTIR spectra for pure POZ, 1:0.1 POZ/SiO2 , 1:1 POZ/AgNO3 , 1:1:0.05 POZ/AgNO3 /SiO2 and 1:1:0.1 POZ/AgNO3 /SiO2 complexes are shown in Fig. 3. Upon incorporation of SiO2 into POZ, the C O stretching band of POZ at 1641 cm−1 hardly changes, indicating that interaction between POZ and SiO2 is negligible. When AgNO3 is complexed with POZ, the C O stretching band shifts from 1641 cm−1 to a lower wave number at 1605 cm−1 , presumably due to the weakened C O double bond strength by the coordinative interaction between the silver cation and carbonyl oxygen of POZ. When SiO2 was added in POZ/AgNO3 , the relative intensity of the complexed C O stretching band at 1605 cm−1 decreased and a new band at 1590 cm−1 became prominent. This peak shift from 1604 to 1590 cm−1 suggests a stronger interaction between the silver ion and the C O of POZ containing the SiO2 nanoparticles, as compared to that without SiO2 . These results imply that the interaction between NO3 − and Ag+ became weakened by a Lewis acid–base interaction between SiO2 and the NO3 − anion in
S.W. Kang et al. / Journal of Membrane Science 285 (2006) 102–107
105
ened interaction between the silver cation and its counter anion. However, above a 0.1 mole ratio of SiO2 , the binding energy increases. It is anticipated that at low silica concentrations, most of the Lewis acid silica nanoparticles interact with NO3 − , resulting in the decrease of interaction intensity between the silver ion and the NO3 − anion. However, at high silica concentrations, silica nanoparticles are not homogenously or uniformly dispersed due to the self-segregation of the nanoparticles resulting from the hydrogen bonding interactions between the OH groups in the silica surface [17,18]. Thus the silica nanoparticles barely affect the interaction intensity between the silver ion and NO3 − anion. Therefore, the silver ion activity only increases up to 0.1 mole ratio of silica nanoparticles, above which it decreases to some degree. Fig. 3. FT-IR spectroscopy of POZ/AgNO3 complexes with different mole ratios of fumed silica nanoparticles to the silver ion.
the POZ/AgNO3 /SiO2 nanocomposite electrolyte membranes. Since fumed silica nanoparticles (SiO2 ) are considered to be a weak Lewis acid [15,16], they are expected to interact with the NO3 − anion, resulting in the weakened interaction between the silver ion and the NO3 − anion. This reduced interaction causes the silver ions to complex with olefin, consequently leading to the facilitated olefin transport and the enhancement of the separation performance of propylene/propane mixtures. 3.3. Binding energy of silver atoms XPS was used to observe the change of the chemical environment around the silver ions in the silver polymer electrolyte membranes due to the incorporation of the silica nanoparticles. The binding energy of the d5/2 orbital of the silver atom in the POZ/AgNO3 system decreases gradually from 369.63 to 368.39 eV with increasing silica nanoparticles concentration, as shown in Fig. 4. This indicates that the binding energy of the valence electrons in the silver atom decreases due to the weak-
Fig. 4. XPS results for the binding energies of silver ion in POZ/AgNO3 complexes with different mole ratios of fumed silica nanoparticles to the silver ion.
3.4. Ionic species of AgNO3 The interactions between Ag+ and NO3 − in the silver polymer electrolytes upon addition of the fumed silica nanoparticles were investigated using FT-Raman spectroscopy. The Raman spectra of POZ/AgNO3 electrolytes with varying mole ratio of the fumed silica nanoparticles are shown in Fig. 5, in the regions of the NO3 − stretching bands. Note that the NO3 − stretching bands at 1034, 1040, and 1045 cm−1 are assigned to free ions, ion pairs, and ion aggregates, respectively [19]. To obtain quantitative information, each peak is deconvoluted into free ions, ion pairs and aggregates. As a result, the fraction of free NO3 − in the POZ/AgNO3 without fumed silica nanoparticles is estimated to be only 13%, but increases approximately two-fold up to 27% upon the addition of 0.1 molar ratio of the fumed silica nanoparticles. This result suggests that the interactions between Ag+ and NO3 − are weakened by the addition of the fumed silica nanoparticles. Therefore, we conclude that the concentration of free NO3 − increases markedly upon the addition of the fumed silica nanoparticles, presumably making the silver ion more active in facilitated olefin transport
Fig. 5. Raman spectra of POZ/AgNO3 complexes with different mole ratios of fumed silica nanoparticles to silver ion.
106
S.W. Kang et al. / Journal of Membrane Science 285 (2006) 102–107
leading to the facilitated olefin transport and enhancement of the selectivity of propylene/propane and propylene permeance. However, at high silica concentrations, silica nanoparticles are not homogenously and uniformly dispersed, presumably due to self-segregation of nanoparticles resulting from the hydrogen bonding interaction between OH groups in the silica surface. Thus, the silica nanoparticles may not effectively form the Lewis acid–base complexes, resulting in poor separation performance for olefin/paraffin mixtures. Acknowledgements
Fig. 6. Glass transition temperature (Tg ) of 1:1 POZ/AgNO3 complexes with different mole ratios of fumed silica nanoparticles.
membrane. However, above 0.1 mole ratio of SiO2 , the fraction of free NO3 − no longer increases significantly, indicating that the role of the silica nanoparticles becomes less prominent. This result is consistent with the XPS data. 3.5. Glass transition temperature (Tg ) The change in chain flexibility of the polymer electrolytes upon addition of the fumed silica nanoparticles was investigated by measuring the glass transition temperature (Tg ) with a Perkin-Elmer DSC-7 at a heating rate of 10◦ min−1 under an N2 environment. Fig. 6 shows that Tg is found to be 78 ◦ C for neat POZ/AgNO3 , but it decreases to 67 ◦ C with increasing silica concentration up to 0.1 mole ratio of silica. If bridges among the hydroxyl groups of the nanoparticles formed, Tg increased with the concentration of the nanoparticles, [20] leading to decreased chain mobility and increased Tg . However, the bridge formation could be restricted in the nanocomposite silver electrolytes at low nanoparticle concentrations due to the Lewis acid–base interaction between the silica particles and NO3 − . Therefore, the decrease in Tg at silica concentrations less than 0.1 may be due to the restricted bridge formation. At high silica nanoparticle concentrations, however, the bridges among the particles are likely to be formed and consequently the chain flexibility is decreased, resulting in a higher Tg . 4. Conclusion The POZ/AgNO3 membrane exhibits extremely poor separation performance for olefin/paraffin mixtures due to the low concentration of the free silver ion of AgNO3 . When fumed silica nanoparticles are introduced into the POZ/AgNO3 membrane, the separation performance for olefin/paraffin mixtures improved markedly. These results can be explained by Lewis acid–base interaction between the fumed silica nanoparticles and the NO3 − anion. This reduced interaction causes the silver ions to be more active for olefin coordination, consequently
We acknowledge the partial financial support from Hanyang University through an internal research fund. KC acknowledges the financial support of the National Research Laboratory Program (Grant M1-0104-00-0191) and the Ministry of Education through the Brain Korea 21 Program at Seoul National University. References [1] F. Croce, G.B. Appetecchi, L. Persi, B. Scrosati, Nanocomposite polymer electrolytes for lithium batteries, Nature 394 (1998) 456. [2] I. Pinnau, L.G. Toy, C. Casillas, Olefin separation membrane and process, U.S. Patent 5,670,051 (2005) (September 23, 1997). [3] S. Sunderrajan, B.D. Freeman, C.K. Hall, 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] S.U. Hong, J. Won, Y.S. Kang, Polymer-salt complexes containing silver ions and their application to facilitated olefin transport membranes, Adv. Mater. 12 (2000) 968. [5] J.H. Kim, B.R. Min, J. Won, Y.S. Kang, Complexation mechanism of olefin with silver ions dissolved in polymer matrix and its effect on facilitated olefin transport, Chem. Eur. J. 8 (2002) 650. [6] F. Croce, R. Curini, A. Martinelli, L. Persi, F. Ronci, B. Scrosati, R. Caminiti, Physical and chemical properties of nanocomposite polymer electrolytes, J. Phys. Chem. B 103 (1999) 10632. [7] M. Biswas, S.S. Ray, Y.P. Liu, Water dispersible conducting nanocomposites of poly(N-vinylcarbazole), polypyrrole and polyaniline with nanodimensional manganese (IV) oxide, Synth. Met. 105 (1999) 99. [8] S.J. Su, N. Kuramoto, Processable polyaniline-titanium dioxide nanocomposites: effect of titanium dioxide on the conductivity, Synth. Met. 114 (2000) 147. [9] S.S. Ray, M. Biswas, Water-dispersible conducting nanocomposites of polyaniline and poly(N-vinylcarbazole) with nanodimensional zirconium dioxide, Synth. Met. 108 (2000) 231. [10] T.C. Merkel, B.D. Freeman, R.J. Spontak, Z. He, I. Pinnau, P. Meakin, A.J. Hill, Sorption, transport, and structural evidence for enhanced free volume in poly(4-methyl-2-pentyne)/fumed silica nanocomposite membranes, Chem. Mater. 15 (2003) 109. [11] T.C. Merkel, B.D. Freeman, R.J. Spontak, Z. He, I. Pinnau, P. Meakin, A.J. Hill, Ultrapermeable, reverse-selective nanocomposite membranes, Science 296 (2002) 519. [12] S. Kulprathipanja, R.W. Neuzil, N. Li, U.S. Patent 4,740,219 (1988). [13] B. Jose, J.H. Ryu, B.G. Lee, H. Lee, Y.S. Kang, H.S. Kim, Effect of phthalates on the stability and performance of AgBF4-PVP membranes for olefin/paraffin separation, Chem. Commun. (2001) 2046. [14] S.W. Kang, J.H. Kim, K.S. Oh, J. Won, K. Char, H.S. Kim, Y.S. Kang, Highly stabilized silver polymer electrolytes and their application to facilitated olefin transport membranes, J. Membr. Sci. 236 (2004) 163. [15] W. Wieczorek, P. Lipka, G. Zukowska, H. Wycis’lik, Ionic interactions in polymeric electrolytes based on low molecular weight poly(ethylene glycol)s, J. Phys. Chem. B 102 (1998) 6968.
S.W. Kang et al. / Journal of Membrane Science 285 (2006) 102–107 [16] D. Swierczynski, A. Zalewska, W. Wieczorek, Composite polymeric electrolytes from the PEODME-LiClO4 -SiO2 system, Chem. Mater. 13 (2001) 1560. [17] J. Hou, G.L. Baker, Preparation and characterization of cross-linked composite polymer electrolytes, Chem. Mater. 10 (1998) 3311. [18] J.H. Kim, M.-S. Kang, Y.J. Kim, J. Won, N.-G. Park, Y.S. Kang, Dye-sensitized nanocrystalline solar cells based on composite polymer
107
electrolytes containing fumed silica nanoparticles, Chem. Commun. (2004) 1662. [19] J.H. Kim, B.R. Min, C.K. Kim, J. Won, Y.S. Kang, Spectroscopic interpretation of silver ion complexation with propylene in silver polymer electrolytes, J. Phys. Chem. B 106 (2002) 2786. [20] J.R. MacCallum, C.A. Vincent, Polymer Electrolyte Reviews, Elsevier, Amsterdam, 1987.