Behavior of Inorganic Nanoparticles in Silver Polymer Electrolytes and

Aug 19, 2009 - Behavior of Inorganic Nanoparticles in Silver Polymer Electrolytes and Their Effects on Silver Ion Activity for Facilitated Olefin Tran...
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Ind. Eng. Chem. Res. 2009, 48, 8650–8654

Behavior of Inorganic Nanoparticles in Silver Polymer Electrolytes and Their Effects on Silver Ion Activity for Facilitated Olefin Transport Sang Wook Kang,† Wanki Bae,† Jong Hak Kim,‡ Jung Hyun Lee,§ and Yong Soo Kang*,§ School of Chemical & Biological Engineering, Seoul National UniVersity, Seoul 151-744, South Korea, Department of Chemical Engineering, Yonsei UniVersity, Seoul 120-749, South Korea, and Department of Chemical Engineering, Hanyang UniVersity, Seoul 133-791, South Korea

TiO2 and Al2O3 nanoparticles having different chemical properties were introduced into a poly(2-ethyl-2oxazoline) (POZ) membrane to make inactive AgNO3 chemically more active in olefin complexation, resulting in facilitated olefin transport. The interaction schemes were particularly emphasized to understand the activation of AgNO3 for facilitated olefin transport. The separation performances were similar in both cases, consistent with similar changes in the binding energy of silver ions and the relative concentrations of the added ionic species. However, the bond strength of CdO in POZ increased with TiO2 whereas it decreased with Al2O3, as confirmed by FT-IR spectroscopy. This difference was explained in terms of the difference in the zeta potential: TiO2 and Al2O3 displayed zeta potentials of -18 and +6.5 mV at pH 4, respectively. On the basis of these, the following interaction scheme was proposed: the TiO2 nanoparticles predominantly interact with silver cations, whereas the Al2O3 nanoparticles preferentially interact with NO3-. As a result, both nanoparticles help to loosen the bond strength between Ag+ and NO3-, resulting in the activation of the silver cations in olefin complexation; consequently, olefin transport is facilitated, which results in improved separation performance in the separation of propylene/propane mixtures. Introduction Addition of nanoparticles into a polymeric matrix is a good way to improve mass transport.1-11 For example, silica nanoparticles enable an increase in the penetrant permeability coefficients in glassy poly(4-methyl-2-pentyne) (PMP) primarily because of the generation of free volumes at the interface between the particle and polymeric matrix.1 MnO2, TiO2, and ZrO2 nanoparticles have also been used to improve the ionic conductivity in poly(ethylene oxide) (PEO) and polyaniline (PANI) nanocomposites.12-15 In addition, nanoparticles such as fumed silica nanoparticles were used to increase the chemical activity of silver ions for olefin complexation in poly(2-ethyl2-oxazoline) (POZ)/AgNO3 membranes, resulting in improved performance for separation of propylene/propane mixtures.16 Since the silver cation interacts or complexes with olefin reversibly, it can act as an olefin carrier for facilitated olefin transport and is known to be a powerful tool to increase both permeability and selectivity simultaneously. Silver polymer electrolyte membranes showed very good separation performance for olefin/paraffin mixtures because of the facilitated olefin transport brought about by the carrier action of silver cations dissolved in a polymeric matrix. In facilitated olefin transport membranes, the carrier action is critically important in determining separation performance as well as operational durability. Common silver salts used for silver polymer electrolyte membranes are AgBF4, AgCF3SO3, AgClO4, and AgNO3. AgBF4 is very active in olefin complexation and shows good separation performance, but readily loses its carrier activity via conversion into metallic silver, resulting in deterioration of the separation performance with time. On the other hand, AgNO3 is much less active chemically.17 Commonly, this * To whom correspondence should be addressed. Tel.: +82-2-22202336. Fax: +82-2-2298-4101. E-mail: [email protected]. † Seoul National University. ‡ Yonsei University. § Hanyang University.

means that the addition of AgNO3 provides no improvement in separation performance, but this salt is more attractive than AgBF4 in terms of its long-term stability. Therefore, it is desirable to increase the chemical activity of AgNO3 to provide both higher separation performance and long-term separation durability. POZ/AgNO3 membranes without nanoparticles such as silica displayed no separation of propylene/propane mixtures because of the inactivity of the silver cations with respect to olefin. Their inactivity in complexing with olefin was primarily due to the high lattice energy of the silver salt AgNO3, and the consequent difficulty in generating free silver cations, known to be the most active olefin carrier for facilitated transport. When fumed SiO2 particles were introduced into the POZ/AgNO3 membrane, the separation performance for propylene/propane mixtures was improved markedly. These enhanced silver ion activities were attributable to the Lewis acid-base interactions between counteranions of the silver ion and the SiO2 nanoparticles. In the present work, we incorporated acid-type inorganic nanoparticles such as TiO2 and Al2O3 having different chemical properties into POZ/AgNO3 complexes and investigated their effects on the separation performance of propylene/propane mixtures, particularly in terms of their charge nature. Especially, the primary focus is on the role of the zeta potentials of the nanoparticles in determining the chemical activity of the silver cations for facilitated olefin transport membranes. Experimental Section Materials. Silver nitrate (AgNO3, 99.9%), poly(2-ethyl-2oxazoline) (POZ, Mw ) 5.0 × 105 g/mol, and Al2O3 nanoparticles (20-nm primary particle size) were purchased from Aldrich Chemical Co. TiO2 nanoparticles (P 25, 21-nm primary particle size) were purchased from Degussa Co. All the chemicals were used as received. Membrane Preparation and Permeance Measurements. Polymer/silver salt complex solutions containing equimolar amounts of AgNO3 and polymer were prepared by dissolving

10.1021/ie9000103 CCC: $40.75  2009 American Chemical Society Published on Web 08/19/2009

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Figure 1. Separation performance of POZ/AgNO3 membranes with various molar ratios of TiO2 nanoparticles. (a) Mixed gas permeance. (b) Mixed gas selectivity.

0.34 g of AgNO3 in 0.2 g of POZ/0.8 g of water. Various mole ratios of TiO2 nanoparticles and Al2O3 nanoparticles with respect to the carbonyl oxygen of POZ were added to these solutions. The solutions were then coated onto polysulfone microporous membrane supports (Woongjin Chemical Co. and pore size ) 70 nm) 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 thickness of the top polymer membrane layer was ca. 1 µm, as determined by scanning electron microscopy (SEM). For sample preparation, after POZ/AgNO3/inorganic nanoparticles composite solutions coated onto polysulfone were dried, these membranes were frozen under liquid nitrogen. Then, the frozen sample was cut and the cut aspect was observed via SEM. Permeation tests were performed in a stainless steel separation module as described elsewhere. 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. The unit of gas permeance is GPU, where 1 GPU ) 1 × 10-6 cm3 (STP)/(cm2 s cmHg). Mixed gas (50:50 vol % of a propylene/propane mixture) separation properties of the membranes were evaluated by gas chromatography (Hewlett-Packard) equipped with a TCD and a unibead 2S 60/80 packed column. Characterization. IR measurements were performed on a Perkin-Elmer FT-IR spectrometer; 32 scans were signalaveraged at a resolution of 4 cm-1. Elemental analysis was used to detect atomic composition. The weight percent of carbon, hydrogen, and nitrogen in each sample was obtained from an elemental analyzer (Flash EA 1112, Thermo Electron Co., USA). Zeta potential was acquired using an OTSUKA ELS-8000. 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 for determining the binding energies of silver ions. Raman spectra were collected for POZ: silver salt films were analyzed at RT using a Perkin-Elmer System 2000 NIR FTRaman 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. Results and Discussion Separation Performance. The separation performance of propylene/propane mixtures using nanocomposite silver polymer electrolyte membranes, comprising AgNO3 dissolved in POZ containing dispersed nanoparticles, was evaluated for various concentrations of TiO2 and Al2O3 nanoparticles. The mole ratio of POZ monomers to silver ions was fixed to unity, i.e., [CdO]: [Ag] ) 1:1. POZ/AgNO3 membranes without inorganic nanoparticles exhibit low gas permeation and show no separation of propylene/propane mixtures; mixed gas permeance is ca. 0.1 GPU (1 GPU ) 1 × 10-6 cm3(STP) cm-2 s-1 cmHg-1) and the selectivity of propylene/propane is nearly unity. The introduction of TiO2 nanoparticles in POZ/AgNO3 membranes gives an abrupt increase in mixed gas permeance, e.g., up to 2 GPU from 0.1 by the addition of TiO2 in a 0.05 mole ratio (Figure 1). The mixed gas permeance decreased with increased molar ratios of added TiO2, with minimum permeance at 0.15. On the other hand, the selectivity of propylene/propane increased from nearly unity to 6.5 as the molar ratio of TiO2 was increased to 0.15 (Figure 1b). The increased permeance and selectivity may be explained by the facilitated olefin transport associated with the increased silver ion activity of AgNO3. However, the permeance increased and the selectivity decreased at mole ratios higher than 0.15, presumably due to TiO2 aggregation or defect formation by TiO2 nanoparticles. Consistent with these results, gas permeance in neat POZ membranes did not change very much with the mole ratio of TiO2 nanoparticles up to 0.15 mole ratio, above which it increased abruptly (Figure 2). The selectivity was nearly unchanged regardless of the mole ratio of TiO2. These results indicate that TiO2 may not play the role as olefin carrier, and the improved separation performance of POZ/AgNO3/TiO2 was attributable to the facilitated olefin transport by the carrier activation of AgNO3 by TiO2. The POZ/AgNO3 composite membranes also show a similar pattern in both the mixed gas permeance and selectivity of propylene/propane (Figure 3a,b). The POZ/AgNO3/Al2O3 composite membranes also show an abrupt increase in gas permeance at the Al2O3 molar ratio of 0.15, which is consistent with the composite membrane containing TiO2. Consequently, it could be concluded that both TiO2 and Al2O3 show similar behavior in activating AgNO3 for olefin complexation and facilitated olefin transport in terms of the mixed gas

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Figure 2. Separation performance of neat POZ membranes with various molar ratios of TiO2 nanoparticles. (a) Mixed gas permeance. (b) Mixed gas selectivity.

Figure 3. Separation performance of POZ/AgNO3 membranes with various molar ratios of Al2O3 nanoparticles. (a) Mixed gas permeance. (b) Mixed gas selectivity.

Figure 4. FT-IR spectroscopy of POZ/AgNO3 complexes with various molar ratios of nanoparticles. (a) TiO2 nanoparticles. (b) Al2O3 nanoparticles.

permeance and selectivity. In the following sections, detailed analysis will be provided to help explain the activation mechanisms for both cases. Interaction of Silver Cation with Carbonyl Oxygen. The coordinative interaction between Ag+ and the carbonyl oxygen of POZ was investigated using FT-IR spectroscopy (Figure 4). FT-IR spectra for the POZ/AgNO3/TiO2 system show that the CdO stretching band at 1641 cm-1 from neat POZ shifted to a band at 1605 cm-1 for POZ/AgNO3 without TiO2, presumably because of the loosened CdO double-bond strength by the coordinative interaction between the silver cation and carbonyl oxygen of POZ. The band at 1605 cm-1 shifts to 1621 cm-1 with the addition of TiO2. This new band can be explained by the fact that the interaction between the CdO of POZ and the silver ion becomes weaker and consequently the carbonyl bond becomes stronger.

When Al2O3 was added in POZ/AgNO3, the complexed CdO stretching band at 1605 cm-1 shifts to 1593 cm-1, in the opposite direction of the shift in the POZ/AgNO3/TiO2 system. The reversal shift of the carbonyl CdO is attributable to the fact that the interaction between the CdO in POZ and the silver ion becomes stronger. The IR spectroscopy suggests that the bond intensity of the carbonyl CdO in POZ/AgNO3 increased with addition of TiO2, whereas it decreased with Al2O3, although both TiO2 and Al2O3 showed similar results in separation performance by facilitated olefin transport. Binding Energy of Silver Ions by X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (XPS) spectra were obtained to investigate the binding energy of the silver ion in the POZ/AgNO3/inorganic nanoparticle complex. The binding energy of the d5/2 orbital in the Ag atom in the POZ/ AgNO3 system decreased gradually from 369.63 to 368.54 eV

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Figure 5. XPS results of binding energy for the silver ion in POZ/AgNO3 complexes with different mole ratios of (a) TiO2 nanoparticles and (b) Al2O3 nanoparticles to the silver ion.

Figure 6. FT-Raman spectra of POZ/AgNO3 complexes with different mole ratios of (a) TiO2 nanoparticles and (b) Al2O3 nanoparticles to the silver ion.

with increasing concentration of TiO2 nanoparticles, up to a 0.15 molar ratio (Figure 5a). This represents that the binding energy of the valence electrons in the silver atom is lessened because of the weakened interaction between the silver cation and its counteranion. However, at molar ratios of TiO2 higher than 0.15, the binding energy shifts back to the original value, representing aggregate formation and consequent loss of interactions between AgNO3 and TiO2 nanoparticles. In the POZ/AgNO3 system, the binding energy of the Ag atom d5/2 orbital decreased gradually from 369.63 to 368.76 eV with increasing concentration of Al2O3 nanoparticles (Figure 5b). The Al2O3 system also displayed shift intensity similar to that of the TiO2 system. Characterization of Ionic Species by FT-Raman Spectroscopy. FT-Raman spectroscopy was used to identify the NO3- ion in POZ/AgNO3 complexes with added TiO2 and Al2O3 (Figure 6). Note that the NO3- stretching bands at 1034, 1040, and 1045 cm-1 are assigned to free ions, ion pairs, and ion aggregates, respectively.18 Each peak was deconvoluted into free ions, ion pairs, and ion aggregates peaks to obtain quantitative information. For reference, note that the proportion of free ions, ion pairs, and ion aggregates of NO3- in the POZ/AgNO3 system was estimated to be 13%, 25%, and 62%, respectively.19 The POZ/ AgNO3/TiO2 system showed the fractions of free ions, ion pairs, and ion aggregates of NO3- are 13%, 33%, and 54%, respectively, upon addition of 0.15 molar ratio of TiO2 nanoparticles (Figure 7). The POZ/AgNO3/Al2O3 complex gave the fractions of free ions, ion pairs, and ion aggregates of NO3- as 13%, 29%, and 58%, respectively, upon addition of 0.15 molar ratio

Figure 7. Deconvoluted FT-Raman spectra for the 1:1:0.15 POZ/AgNO3/ TiO2 complex electrolytes. Open circles, squares, and triangles indicate ion aggregates, ion pairs, and free ions of AgNO3, respectively.

of Al2O3 nanoparticles. These results suggest that the interactions between Ag+ and NO3- are weakened by the addition of either TiO2 or Al2O3. Thus, we conclude that the concentration of ion pairs, known to be more active olefin carriers than ion aggregates, increases upon addition of TiO2 and Al2O3, to some extent, and presumably makes the silver ion more active in facilitated olefin transport. Surface Charge of the Nanoparticles and Proposed Interaction Scheme. The surface charges of both TiO2 and Al2O3 were measured by zeta potential at pH 4. Zeta potential showed that TiO2 had the zeta potential of -18 mV at pH 4, while Al2O3 had zeta potential of +6.5 mV at pH 4. This

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Scheme 1

suggests that TiO2 nanoparticles interact favorably with silver ions, whereas Al2O3 nanoparticles have a favorable interaction with NO3-.20 Scheme 1 proposes interaction schemes based on the experimental findings mentioned above. Both TiO2 and Al2O3 nanoparticles help to loosen the bond strength between Ag+ and NO3-, resulting in the activation of the silver cations in olefin complexation; consequently, olefin transport is facilitated and the separation performance of propylene/propane mixtures is increased. However, the TiO2 and Al2O3 nanoparticles act differently in bringing about these effects. The TiO2 nanoparticles predominantly interact with silver cations, whereas the Al2O3 nanoparticles preferentially interact with NO3- in decreasing the bond strength between Ag+ and NO3-. Conclusions We were able to activate the chemically inactive POZ/AgNO3 systems to create membranes for the separation of propylene/ propane mixtures, with simultaneously improved gas permeance and separation selectivity. Both TiO2 and Al2O3 nanoparticles help to decrease the bond strength between Ag+ and NO3- when these nanoparticles are added to silver polymer electrolyte membranes comprising AgNO3 dissolved in poly(2-ethyl-2-oxazoline). The addition of TiO2 and Al2O3, therefore, results in the activation of the silver cations in olefin complexation and consequently facilitates olefin transport for better separation performance of propylene/propane mixtures. However, the TiO2 nanoparticles predominantly interact with silver cations, whereas the Al2O3 nanoparticles preferentially interact with NO3- to loosen the bond strength between Ag+ and NO3-. Acknowledgment This work was supported by the Energy Technology R&D program (2006-E-ID11-P-13) under the Ministry of Knowledge Economics of Korea and also by the Korea Science and Engineering Foundation (KOSEF) grant funded from the Ministry of Education, Science and Technology (MEST) of Korea for the Center for Next Generation Dye-sensitized Solar Cells (No. 2009-0063369).

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ReceiVed for reView January 5, 2009 ReVised manuscript receiVed July 10, 2009 Accepted July 30, 2009 IE9000103