Highly permeable poly(ethylene oxide) with silver nanoparticles for facilitated olefin transport

Abstract

We report a highly permeable poly(ethylene oxide) (PEO)/silver nanoparticles (AgNPs)/p-benzoquinone (p-BQ) composite for use in a facilitated olefin transport membrane. Surface-tuned AgNPs were generated using AgBF₄ as a precursor. The electronic structure of the AgNP surface was tuned by the electron acceptor p-BQ to induce positive charges. The interaction between olefins and the polarized surface of AgNPs in a permeable PEO matrix was expected to show excellent separation performance. Our results show that the PEO/AgNPs/p-BQ composite membranes possess a selectivity of 10 and a mixed-gas permeance of 15 GPU for durations longer than 240 h. The surface-activated AgNPs were characterized using UV-visible and X-ray photoelectron spectroscopies.

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1. Introduction

Nanomaterials such as nanoparticles, nanocomposites, and nanostructures are of great interest because of their size-related optical, electrical, mechanical, and transport properties.^1–6 These unique physicochemical properties and their large surface areas can be widely applied in several fields such as catalysis, drug delivery, sensors, and photonics.^1–3 In particular, nanoparticles have been successfully utilized to increase the mass flux through polymer matrices for gas separation.^4–9

A new strategy to design facilitated transport membranes has recently emerged, based on the discovery that a positive surface charge on metallic silver nanoparticles (AgNPs) enables complexation with olefins such as ethylene and propylene.^7–9 The so-called “facilitated transport” is defined as the transport phenomenon in a membrane containing carriers by means of coupled diffusion and reversible reactions.^10,11 The carriers form a reversible complex with a specific component, thereby increasing the transport rate of that component relative to the others. Therefore, the mass flux of the specific solute is accelerated by facilitated transport even in the solid state, resulting in a simultaneous increase in both the permeability and selectivity of the membrane, whereas these attributes are in conflict in common polymeric membranes. Consequently, the concept of facilitated transport is highly attractive for improving the separation performance of membranes.^7–9

Recently, we reported that positively polarized AgNPs were successful olefin carriers for facilitated transport. For example, poly(ethylene-co-propylene) (EPR)/AgNPs/p-benzoquinone (p-BQ) composite membranes demonstrated a propylene/propane selectivity of 11 and a total mixed-gas permeance of 0.5 GPU (1 GPU = 1 × 10⁻⁶ cm³ (STP)/(cm² s cm Hg)),⁸ whereas the selectivity with neat EPR membranes was nearly 1. Here, the positive surface charge of the AgNPs, induced by p-BQ as an electron acceptor, played a major role in the facilitated olefin transport. Furthermore, we also recently reported the remarkable separation performance of poly(vinylpyrrolidone) (PVP)/AgNPs nanocomposite membranes utilizing the interface of the dipole on the AgNP surfaces with a strong electron acceptor, 7,7,8,8-tetracyanoquinodimethane (TCNQ).⁹ This membrane showed unprecedented separation performance, with a selectivity of 50 and a permeance of 3.5 GPU. It was suggested that the positively charged AgNPs are effective and robust olefin carriers, thus explaining the separation performance of olefin/paraffin mixtures. It has also been proposed that for the membranes to be viable in industrial processes, a minimum propylene permeability of 1 Barrer (1 Barrer = 10⁻¹⁰ cm³ (STP) cm/(cm² s cm Hg)) and a propylene selectivity of 35 are required. More permeable polymer matrix membranes are needed for practical applications.

In this study, we report the enhanced permeance performance of a silver nanocomposite membrane by utilizing poly(ethylene oxide) (PEO) instead of PVP. p-BQ was used as a stabilizing agent and an electron acceptor. The formation of positive charge was confirmed by the positive shift in the binding energy of the silver atom using X-ray photoelectron spectroscopy (XPS). Furthermore, the size and dispersion of the AgNPs were characterized using UV-vis and TEM. These chemically activated AgNPs possessed olefin carrier properties and showed a significant improvement in the separation performance, including both the selectivity and permeance, for propylene/propane mixtures.

2. Results and discussion

The preparation of the nanocomposite membranes containing AgNPs activated by p-BQ is shown in Scheme 1. This scheme illustrates the interaction between the AgNPs and p-BQ, which generated an interfacial dipole on the metal surface. The AgNPs, stabilized and polarized by p-BQ, were well-dispersed into PEO, and then cast onto the microporous polysulfone membrane support.

Scheme 1 Preparation of PEO/AgNPs/p-BQ nanocomposite membrane.

In order to determine the effect of the electron acceptor on the separation of a propylene/propane mixture, AgNPs were generated utilizing AgBF₄ as a precursor in PEO without p-BQ. The separation performance for the PEO/AgNPs composite membrane without p-BQ showed a selectivity of 1.1 for a propylene/propane mixture and a mixed-gas permeance of 18 GPU (Table 1). The reason for the lack of selectivity is that positively polarized AgNPs were not generated because of the absence of electron acceptors such as p-BQ and TCNQ. The relatively high permeance, i.e., 18 GPU, was attributable to the permeable characteristics of PEO; in contrast, only 3.5 GPU permeance was observed with the PVP/AgNPs/TCNQ composite membrane. Alternatively, it can be proposed that the interfacial defects between the PEO and the AgNPs rapidly transported the gas molecules because of the absence of any stabilizing agent for the AgNPs.

Table 1 Membrane performance of PEO/AgNPs and PEO/AgNPs/p-BQ

	Permeance (GPU)	Selectivity (propylene/propane)
PEO/AgNPs	18	1.1
PEO/AgNPs/p-BQ	15	10

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On the other hand, when p-BQ was incorporated into the PEO/AgNPs composite membranes, the selectivity for the propylene/propane mixture increased to approximately 10, and the mixed-gas permeance was 15 GPU. The increased selectivity was attributable to the positive polarized surface of the AgNPs, induced by the electron-accepting p-BQ. Thus, the positively polarized AgNPs were able to interact reversibly with propylene molecules, resulting in the facilitated transport as shown in Scheme 2.

Scheme 2 Facilitated olefin transport membrane.

Interestingly, the separation performance of the membrane was observed to be better than that of the PVP/AgNPs/p-BQ membrane. Note that the PVP/AgNPs/p-BQ membrane had a mixed-gas selectivity of approximately 1.⁹ Furthermore, by dispersing the surface-tuned AgNPs into permeable PEO, a highly permeable membrane could be prepared, with 15 GPU. Because the membrane thickness was 20 μm, resulting in a permeability of 300 Barrer, the permeance can be greatly enhanced if a thin film is used. This high value was the best mixed-gas permeance value in a polymer nanocomposite membrane for the separation of propylene/propane; the earlier reported values are only 0.5 and 3.5 GPU (140 Barrer), respectively.^8,9

Thus, it can be proposed that the greatly enhanced permeance was caused by the uniform distribution of the AgNPs as an olefin carrier in the permeable PEO support.

The separation performance of a 50 : 50 (v/v) propylene/propane mixture was tested over time through the PEO/AgNPs/p-BQ membrane to confirm the long-term stability (Fig. 1). The selectivity of approximately 10 for propylene/propane and the gas permeance of 15 GPU remained constant for up to 240 h. This long-term stability indicated that the polarized AgNPs remained stable over time, even though the carriers could sequentially react with propylene molecules. The excellent separation performance and long-term stability of the PEO/AgNPs/p-BQ membrane increase the possibility of the membrane's practical applications.

Fig. 1 Separation performance for 50 : 50 (v/v) propylene/propane mixture using PEO/AgNPs/p-BQ composite membrane as a function of time.

TEM was used to investigate the effect of both PEO and p-BQ on the size of the AgNPs. In the TEM images, the stabilized AgNPs were well dispersed in the PEO support (Fig. 2a). The average diameter of the AgNPs was approximately 40 nm (Fig. 2b). The well-grown AgNPs in PEO matrix was attributable to the strong affinity between metal surface of Ag and p-BQ since the ether groups of PEO with high molecular weight couldn't stabilize metal NPs than amide groups of PVP.

Fig. 2 TEM images of AgNPs in PEO/AgNPs/p-BQ.

Furthermore, the uniformity of the stabilized AgNPs was confirmed using UV-vis spectroscopy. It is well known that a broad absorption maximum is observed at approximately 420 nm because of the plasmon excitation of AgNPs.¹² Fig. 3 shows that the peak maximum was observed at 437 nm in the UV-vis spectra, indicating the formation of AgNPs. Furthermore, the spectrum shows a symmetric peak, meaning monodisperse AgNPs were formed in the PEO/AgNPs/p-BQ composite.

Fig. 3 UV-visible absorption spectra of AgNPs in PEO/AgNPs/p-BQ composite.

XPS was used to observe the change of the chemical environment around the Ag atom in the PEO/AgNPs/p-BQ composite. As shown in Fig. 4, the binding energy of the d_5/2 and d_3/2 orbitals of the Ag atom in the PEO/AgNPs/p-BQ composite was observed at 368.78 and 374.78 eV, respectively, even though that of AgNPs appears at 368.26 eV for d_5/2 orbital.⁹ This result indicated that the binding energy of the Ag atoms increased because of the strong interactions between the Ag metal surface and p-BQ.⁸ Therefore, it could be concluded that the AgNP surface was partially positively polarized, and therefore, capable of interacting with propylene molecules.

Fig. 4 Binding energy of silver atoms in PEO/AgNPs/p-BQ composite.

3. Experimental

Materials

PEO was purchased from Aldrich Chemical Co. p-BQ was purchased from JUNSEI Chemical Co. Silver tetrafluoroborate (AgBF₄) was purchased from TCI Fine Chemicals. All the chemicals were used as received.

Characterization

The TEM images were obtained using a JEOL JEM-3000 operating at 300 kV. The UV-vis absorption spectrum was obtained using an Agilent Technologies Cary 5000 UV-Vis-NIR spectrophotometer for the PEO/AgNPs/p-BQ composite solution. The 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 operating at 300 W (15 kV, 20 mA). The carbon (C 1s) line at 285.0 eV was used as the reference for determining the binding energies of silver.

Preparation of membranes

Nanocomposite membranes containing AgNPs activated by p-BQ were prepared. The AgNPs were prepared at a fixed mole ratio of 1/0.4 PEO/AgBF₄. The solution was stirred for 30 min at 60 °C. Subsequently, “PEO-protected AgNPs” in 50% (v/v) ethanol in water, which were surface-activated by the electron acceptor p-BQ (mole ratio of 0.04), were cast with an RK Control Coater (Model 101, Control Coater RK Print-Coat instruments LTD, UK) on a commercial microporous polysulfone membrane support with an average surface pore size of 0.1 μm (Woongjin Chemical Co., Ltd, Republic of Korea) to fabricate the nanocomposite membranes.

Separation performance

The PEO/AgNPs/p-BQ composite membranes were oven-dried for one day at room temperature. Their selectivity toward propylene/propane (50 : 50 vol% propylene/propane mixture) was measured using gas chromatography (Young Lin 6500 GC system). The flow rates of the gas were controlled using mass flow controllers. Gas permeance values were measured with a bubble flow meter at upstream with various pressure (psig) and atmospheric downstream pressure. Gas permeance is expressed in units of GPU.

4. Conclusion

A highly permeable facilitated olefin transport membrane was synthesized by utilizing a PEO/AgNPs/p-BQ composite. The composite was characterized using TEM, UV-visible spectroscopy, and XPS. The selectivity and permeance performance of the PEO/AgNPs/p-BQ composite membrane were 10 and 15 GPU, respectively. The excellent separation performance was attributable to the polarized surface of the AgNPs, which were tuned by an electron acceptor, p-BQ, to induce high positive charges in the permeable PEO matrix, thereby resulting in the facilitated olefin transport.

Acknowledgements

This research was supported by a 2013 Research Grant from Sangmyung University. This work was also supported by the Energy Efficiency & Resources of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Ministry of Knowledge Economy (20122010100040), the Korea government.

References

1. H. J. Shin, R. Ryoo, Z. Liu and O. Terasaki, J. Am. Chem. Soc., 2001, 123, 1246.
2. M.-C. Daniel and D. Astruc, Chem. Rev., 2004, 104, 293.
3. Y. Sun and Y. Xia, Science, 2002, 298, 2176.
4. A. J. Haes and R. P. Van Duyne, J. Am. Chem. Soc., 2002, 124, 10596.
5. N. R. Jana, T. K. Sau and T. Pal, J. Phys. Chem. B, 1999, 103, 115.
6. A. Tao, F. Kim, C. Hess, J. Goldberger, R. He, Y. Sun, Y. Xia and P. Yang, Nano Lett., 2003, 3, 1229.
7. S. W. Kang, K. Char and Y. S. Kang, Chem. Mater., 2008, 20, 1308.
8. Y. S. Kang, S. W. Kang, H. Kim, J. H. Kim, J. Won, C. K. Kim and K. Char, Adv. Mater., 2007, 19, 475.
9. I. S. Chae, S. W. Kang, J. Y. Park, Y. G. Lee, J. H. Lee, J. Won and Y. S. Kang, Angew. Chem., 2011, 123, 3038.
10. Y. S. Kang, J. H. Kim, J. Won and H. S. Kim, in Material Science of Membranes for Gas and Vapor Separations, ed. Y. Yampolski, I. Pinnau and B. D. Freeman, Wiley, Chichester, 2006, ch. 16.
11. J. Won, Y. S. Kang and H. Nishide, in Metal Complexes and Metals in Macromolecules, ed. D. Wöhrle and A. D. Pomogailo, Wiley-VCH, Weinheim, 2003, ch. 9.
12. S. W. Kang, J. H. Kim, D. Ko, C. K. Kim, J. Won, K. Char and Y. S. Kang, J. Polym. Sci., Part B: Polym. Phys., 2004, 42, 3344.

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Footnote

† Equally contributed as the first author.

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