prospects of facilitated olefin transport membranes in

Systems Membranes-complex roadmaps towards functional devices and coupled processes SYSMEM

PROSPECTS OF FACILITATED OLEFIN TRANSPORT MEMBRANES IN THE SOLID STATE Sang Wook KANG2), Jung-Hyun LEE1) and Yong Soo KANG*1) 1) Department of Energy Engineering, Hanyang University, Seongdong-gu, Seoul 133-791, Korea

[email protected] 2) Department of Chemistry, Sangmyung University, Jongno-gu, Seoul 110-743, Korea

Abstract Facilitated transport membranes in the solid state have been of interest because they show extremely high separation performance for olefin/paraffin mixtures such as propylene/propane and ethylene/ethane mixtures.Therefore, the facilitated transport membrane process could be an energy-saving alternative separation technology to conventional energyintensive cryogenic distillation process. In the facilitated transport membranes,carrier activity play a major role in determining the separation performance, where olefin carriers are typically silver ions or silver nanoparticle in the solid state. Here it will be reviewed recent progress and critical issues in facilitated olefin transport membranes. Particular attention has been paid to the interaction of the carrier with olefin molecules, the transport mechanism, the separation performance of olefin/paraffin mixtures, and long-term membrane durability. These results are discussed along with recommendations for future research.

1. INTRODUCTION Facilitated transport membranes have been interesting mostly because they can provide both high permeability and high selectivity. Furthermore, the driving force for the facilitated transport is the product of the concentration gradient and the equilibrium reaction constant between the carrier and the specific solute. Therefore, facilitated transport membranes can be very effective even when the concentration gradient is low. Facilitated transport occurs when carrier-mediated transport occurs in addition to normal Fickian transport, where the carrier interacts with a specific solute reversibly. Thus the facilitated transport depends on the carrier activity significantly. The carrier activity mostly represents here the reversible interaction with the

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solute, and consequently plays a major role in determining the transport properties.

Fickian transport +

Carrier-mediated transport II

Facilitated transport

+

carrier Scheme 1. Facilitated transport in the solid state Initial researches on facilitated transport membranes have mostly focused on immobilized liquid membranes, solvent-swollen membranes, and ion-exchange membranes [1-5]. Since the facilitated oxygen transport phenomena were observed in the solid state by Professor Nishide and coworkers, facilitated transport membranes in the solid state become more interested because they do not have problems associated with utilizing liquid membranes: the loss of the liquid solvent and carrier, the limits of the operation temperature and the membrane thickness. Olefins such as ethylene and propylene are very important feed stocks in chemical industries. Currently, olefins are mostly produced by cryogenic distillation processes, which is one of the most energyintensive processes in petrochemical industries because of high capital investment and enormous operation cost. Therefore, the development of alternative energy-saving separation technology such as membranes or reactive absorption is urgently demanding [6-8]. Here, applications of facilitated transport membranes for separation of olefin/paraffin mixtures are reviewed. In particular, novel olefin carriers such as metallic nanoparticles are focused along with brief review for the ionic silver carrier of polymer–silver salt complex or silver polymer electrolyte membranes. 2. MATHEMATICAL MODEL

Mathematical models for the facilitated transport phenomena in the solid-state membrane such as dual sorption, effective diffusion coefficient, limited mobility of chained carriers and concentration

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fluctuation models will be described in brief and are compared each other. The first three models are called hopping model, where a solute is allowed to hop or jump from one carrier to another no matter how long the distance between two carriers. It is, therefore, understood that the hopping models are valid only when the solute concentration is high enough so that the diffusional jump distance is longer than the distance between two carriers. The concentration fluctuation model, however, is able to describe the facilitated transport phenomena exteremly accurately with only one adjustable parameter. The dual sorption model: The dual sorption model, originally developed to interpret the sorption behavior of gases or vapors in glassy polymers, has been commonly employed to explain facilitated transport properties because it is conceptually analogous to the mass transport in a facilitated transport membrane in solid state. The permeability, P, through facilitated transport membranes is given [6]: P = k D D D + D C C 'C

K 1 + Kp

(1)

where DD is Fickian diffusion coefficient and DC is effective diffusion coefficient between carriers. A fairly linear relationship between P and (1+Kp)-1 was obtained from the experimental oxygen permeabilities of PBMA/CoPIm membranes demonstrating the validity of the dual sorption model [9]. The simple dual sorption model was developed by assuming that only two independent diffusional pathways of the ordinary diffusion and the reversible complexation were present. However, it may not be valid in real system. Thus, this assumption was relaxed to adopt four modes of diffusional pathways: 1) diffusion between dissolved modes, 2) diffusion from the dissolved mode to the carrier mode, 3) diffusion from the carrier mode to the dissolved mode, and 4) diffusion for the hopping between the fixed carriers. The dual sorption model is simple and predicts the dependency of permeability on upstream pressure well and therefore has been frequently accepted to interpret facilitated transport behavior. However, it neglects the importance of the reversible complexation reaction kinetics between solute and carrier on facilitated transport, which is found to be very important in determining the facilitated transport. Effective Diffusion Model : A more rigorous analysis of the facilitated transport in solid state was presented by introducing a concept of “the effective diffusion coefficient” between fixed site

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carriers by Noble [10]. If the excess carrier is assumed, the concentration of the unreacted carrier is constant  D  K[B]0 1 +  AB  J  D A  1 + K[A ]1 = J0  D  K[B]0  tanh φ  1 +  AB     D A  1 + K[A ]1  φ 

(2)

where J0 and J are solute fluxes for Fickian transport and for facilitated transport with carriers, respectively; DAB is the effective diffusion coefficient, [A]1 is the solute concentration at the interface and

[{

φ = (1 / 2 ) k b L 2 / D AB

}{(1 + (D AB

]

/ D A )K [B ]0 + K [A ]1 ) / 1 + K [A ]1 }

1/ 2

Eq. (2) is identical to that of facilitated transport in liquid membrane developed by Smith and Quinn [11]. If reaction rate is assumed to be much faster than diffusion rate,

tanhφ ≈ 0 and then the model φ

reduced to the dual sorption transport model. D  K[B]0 J = 1 +  AB  J0  D A  1 + K[A ]1

(3)

The validity of the model was examined against the experimental data for PBMA/CoPIm, where E=J/J0-1 [7]. At low pressure, Eq. (3) fits them well, but starts to deviate gradually as the oxygen pressure increases. This is because the excess carrier assumption is violated at high oxygen pressures. Limited chain mobility model : Cussler et al. [12] analyzed fixed site facilitated transport with a concept of “limited mobility of chained carriers”. They assumed that no uncomplexed solute can exist in the membrane, and the reaction between a carrier and solute occurs only at the surface of the membrane, and is fast. The mobility of chained carrier, commonly pending on side chain, can allow the complex to encounter a second, uncomplexed carrier, resulting in a facilitated transport. However, diffusion is only allowed over a limited distance due to the limited mobility of the chained carriers, which, in turn, leads to a percolation threshold.

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J=

D[B]0  Kp  2λ / λ0     L  1 + Kp  3 − λ0 / λ 

(4)

where λ is the distance between carriers and λ0 is the distance for limited chain mobility. The existence of a percolation threshold owing to direct movement between two carriers is unlikely for many systems because facilitated transport has been experimentally observed with the low concentration of carriers, as low as 0.6 wt% [13]. Concentration fluctuation model: In facilitated transport membranes, reversible chemical reaction between carrier and solute continuously occurs. At a certain moment, the local solute concentration may be fluctuated slightly and instantaneously owing to the reversible reaction. However, the time-averaged concentration still maintains linear as shown in Figure 1. When a solute reacts with a carrier to make solute-carrier complexes, the local solute concentration at that specific site will be decreased instantaneously from its average value. In other hand, it will be increased when the complex releases the solute into the matrix. The concentration fluctuation induces the increase in the chemical potential of the solute according to the Cahn’s theory [14]. The increased chemical potential will result in a higher driving force for mass transfer and lead to the facilitated transport.

p = p0 pd

p=0 Fig.1. Concentration fluctuation profile across the membrane at the steady state [15-16].

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Based on above concepts and analogy between electron transfer in a series of parallel resistor-capacitor circuits and mass transport in a fixed site carrier membrane, Kang et. al [15,16] proposed a concentration fluctuation model to explain facilitated transport in a fixed site carrier membrane.  2πk L2 [B] ln(1 + Kp)  p Pf 2 = 1 + ( d ) n2 +   P0 p P0 p  

2

(5)

where P0 is permeability of the membrane matrix without carrier. pd is pressure fluctuation due to the reversible reaction, respectively. n = NA [B](πr 2L) , L is membrane thickness, r is the permeant radius and [B] the initial carrier concentration, k2 and K are the backward reaction rate constant and the equilibrium constant of the solute-carrier reaction, respectively. Equation (5) can be simplified when n is not large:

Pf ∝ k 2 [B] ln(1 + Kp)

(6)

The model predictions were compared with experimental data for facilitated oxygen transport membranes containing cobalt porphyrin compounds attached on poly(butyl methacrylate) (PBMA) chains [6]. Figure 2 demonstrates that the concentration fluctuation model predicts the experimental results remarkably well.

Permeability (barrer)

11 PBMA D u a l S o rp tio n M o d e l C u rre n t R C M o d e l Concentration Fluctuation Model

10

9

8 0

200 400 600 U p s tre a m P re s s u re (m m H g )

Fig.2. Comparison between theroretical predictions by the concentration fluctuation model and dual sorption model with experimental data [9, 15, 16].

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According to the concentration fluctuation model, Pf increases linearly with [B] demonstrating the importance of carrier concentration, and with decreasing p as observed experimentally. Above all, Pf increases with reverse reaction rate (k2), which have not been described in the other models. The concentration fluctuation model predicts that Pf increases much more effectively when K and k2 increases simultaneously. In most experimental data analysis, it was claimed that facilitated transport occurred primarily due to the increased solubility. However, the concentration fluctuation model suggests that the kinetic term k2 is much more effective in improving the facilitated transport, and the equilibrium solubility K plays a minor role. The concentration fluctuation model predicts the experimental results exceptionally well even at high pressure range where the “effective diffusion coefficient” model fails to predict. This suggests the validity of the concentration fluctuation model. 3. FACILITATED TRANSPORT MEMBRANES USING IONIC SILVERS

Facilitated transport is considered to be an effective approach to improve the permeability and selectivity simultaneously for olefin/paraffin mixtures as shown in Figure 3 [17-20]. Silver ions are known to interact with olefin reversibly and therefore they can act as an effective olefin carrier.

Fig.3. Separation performance of facilitated transport membrane containing silver ions and polymer membranes

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Recently, solid-state facilitated transport membranes consisting of polymer-silver salt complexes also exhibited high separation performance for propylene/propane mixtures. For example, the membranes containing AgBF4 dissolved in poly(2-ethyl-2-oxazoline) (POZ) or poly(N-vinyl pyrrolidone) (PVP) solvent gave the propylene permeance about 45 GPU (1 GPU = 1x10-6 cm3(STP) cm-2 sec-1 cmHg-1) at 140 kPa whereas the propylene permeance through membranes made of pure POZ and PVP was approximately 0.05 GPU [9]. These results clearly demonstrate the facilitated transport of propylene due to the presence of silver ion. This fact implies that polymer/silver salt complexes could be successfully used as a facilitated transport membrane material for separation of olefin/paraffin mixtures as shown in Figure 3. Interaction of silver cations with olefin and molecular mechanism for facilitated transport will be described biefly as follows. 3.1. Interaction of Carrier with Olefin Molecules

The solubility of olefin molecules such as propylene and ethylene in polymer-metal salt complexes is much greater than that of paraffin because of the π-complex formation between metal ions and the C=C bond of the olefins [21]. It was found that the solubilities of propylene in pristine POZ and PVP films were as low as 4-5 cm3 (STP)/cm3 at 370 kPa, and increased linearly with propylene pressure. This dissolution mode was well described by a simple Henry’s equation (C = kDP). In contrast, the solubility of propylene in polymer-silver salt complexes was much higher than that in the neat polymer film, which is probably due to the specific π-complex formation between silver ions and the C=C bond of the olefins. The curves for the propylene solubility in polymer-silver salt complexes become concave with increasing silver concentration, indicating that the Langmuir sorption mode was prominent. Thus, the solubility of propylene in polymersilver salt complexes can be analyzed using the dual sorption model (the sum of Henry’s mode and the Langmuir mode) as in Eq. (7). C = k Dp +

C 'c Kp 1 + Kp

(7)

where C is the concentration of the solute gas in the membrane, p is the applied gas pressure, kD is the solubility coefficient of the gas for Henry’s law, K is the gas binding equilibrium constant, and Cc’ is the saturated amount of gas reversibly bound to the carriers (silver ions).

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3.2. Facilitated transport mechanism

When carriers are dissolved in a polymeric matrix to form polymersalt complexes, carrier-mediated transport occurs in addition to Fickian transport, resulting in facilitated transport. The carrier must interact reversibly with a specific solute, such as the propylene in propylene/propane mixtures, therefore an interaction mechanism of silver ions with olefins is suggested. The most effective carrier among the three ionic constituents (free ions, contact ion pairs, and higher order ionic aggregates) is then identified. Finally, the coordination behavior of silver cations with both ligands and C=C bonds is analyzed.

K

M+

[intermediate] M+

+ CH2=CHR2

CH2=CHR2

M+

CH2=CHR1

M+ + CH2=CHR1

CH2=CHR1

Molecular mechanism of two-step interaction: The molecular mechanism for facilitated olefin transport through polymer-salt complex membranes involves a two-step reaction: the slow and irreversible process of forming olefin-silver ion complexes as an intermediate product, then the rapid and reversible olefin exchange reaction, which seems to be a key step in determining facilitated olefin transport. [22a]

+ CH2=CHR1

CH

2 =C

HR

2

M+ = CH 2

R1 CH

[transition state]

where R1 and R2 represent either hydrogen atom or alkyl group. Effective olefin carrier: Polymer-salt complexes may contain various ionic constituents including free ions, ion pairs, and higher order ionic

136


aggregates. A complexation mechanism of olefin molecules with carriers in polymer-salt complex membranes was proposed, which directly dictates the most effective olefin carrier. It was found that the permeance of the propylene increased almost linearly with total silver concentration above 0.25 of the silver mole fraction regardless of the ionic constituents, suggesting that either all three constituents showed the same carrier activity or only one constituent was effective. Because all three ionic constituents were converted into free ions upon exposure to propylene, the most effective olefin carrier was determined to be free ions. Therefore, a new complexation mechanism between propylene and silver salt in polymer-salt complexes has been proposed [22]. free ion :

C=O

Ag + + X-

ion pair :

C=O

Ag +

ion aggregate :

C=O + (Ag+ )m

X-

+ C3H6

C=O

Ag +

C3H6 + X-

( X-) n

Coordination behavior of silver ions: A threshold concentration is commonly observed in solid-state facilitated transport membranes, above which the propylene permeability increases with carrier concentration, but below which the permeability merely increases [20,22]. The selectivity of propylene/propane through polymer-salt complex membranes showed that the threshold concentration was observed at a silver mole fraction of 0.25 for the AgBF4 and AgCF3SO3 complexes [20]. The threshold concentration was found to be strongly associated with the coordination behavior of silver ions with both carbonyl oxygens and olefin molecules. The coordination number of silver ions (m) with carbonyl oxygens (C=O) can be calculated using the concentration ratio of the complexed carbonyl oxygen to the concentration of both the free silver ions and the ion pairs. The coordination number of silver ions (n) with olefin molecules can be obtained from the concentration ratio of propylene molecules coordinated with silver ions (Cc’) to the concentration of both the free silver ions and the ion pairs. It was found that m decreased exponentially with silver concentration, whereas n increased slightly. Interestingly, the total coordination number (m + n) was nearly invariant around a value of three. This result strongly indicates that the most favorable coordination number for the silver ion dissolved in a polymer matrix in a propylene environment is three [21].

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CH3 [ C=O ]m /// Ag+ /// [ ]n : m+n ≈ 3 3.3. Long-term stability of silver ion

Although polymer-silver salt complex membranes showed the high separation performance of olefin/paraffin mixtures in the solid state, a pending question for industrial applications is the stabilization of carriers and the long-term operational durability. The membranes frequently suffer from a lack of long-term stability mostly due to 1) the reduction of silver ions to silver nanoparticles and 2) the rapid agglomeration of nanoparticles during the separation process. The long-terma stability was improved markedly by retarding the reduction of silver cations into metallic silvers [27-31] In order to prevent the reduction of silver cations, a very stong interactive matrix such as poly(ethylene phthalate) with silver cations was employed to dissove AgNO3, which is not readily dissolved in common polymeric matrix and is not readily reduced to metallic silver either [27,28]. More interestingly, non-interactive polymers such as poly(dimethyl siloxane) (PDMS) and poly(ethylene-co-propylen) (EPR) with silver cations also showed good separation performance and also much enlarged long-term stability [29]. As you may expect, the silver salts were not dissolved in the non-interactive PDMS matrix but physically dispersed. When the PDMS membrane containing dispersed silver salts were exposed to propylene during separation experiments, the silver salts were dissolved to become free silver cations for olefin carriers due to the coordinative complexation of silver with propylene. Ionic liquids also resulted in the improved longterm stability [30]. It is also very important to note that feeds of propyelene/propane and ethylene/ethane mixtures may cotain even small amount of acetylene and/or sulfur compounds. It has been well known that silver cations react with acetylene in aqueous solution to produce silver acetylides, which is very sensitive and reactive [33]. Sulfur compounds also react with siver cations readily and lose their carrier activity with time. For practical applications of the polymer-silver salt complex membranes, therefore potential side reactions of ionic silvers with acetylene and sulfur compounds should be carefully treated.

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4. FACILITATED TRANSPORT MEMBRANES USING METALLIC SILVER NANOPARTICLES

It is anticipated that metallic silver may also interact with olefin reversibly. Since the interactions between the metallic silver and olefins may occur at the surface of the metallic silver, the surface area should be enlarged to improve the carrier activity of the metallic silver. Thus metallic silver nanoparticles are used as a novel olefin carrier. Metallic silver may be much less interactive with olefins than silver cations. In other words, the interactions between the silver nanoparticles and olefin may not be strong enough to show facilitated olefin transport. Therefore, the interactions were enhanced by inducing partial positive on the surface of the nanoparticles by electron acceptors or polarizers. It is expected that the reactivity of solid state metallic silver nanoparticles with acetylene could be negligible. Therefore, metallic silver nanoparticles may show much longer long-term stability than the silver cations. In these regards, metallic silver nanoparticles carriers could be much more appropriate than the facilitated transport membranes containing silver cations. Here, recent studies on metallic silver nanoparticles as novel olefin carrier will be described briefly. 4.1. Reversible Interactions The interaction possibility of metallic silver with olefin molecules was first investigated theoretically. The theoretical calculation showed the weak interactions between metallic silver with olefin molecules, which are not , however, strong enough to provide facilitated olefin transport. Therefore, the interactions should be strengthen for facilitated transport. In this regard, polarizers to interact with metallic silver nanoparticle surface and to induce partial positive charge on the surface were used. Experimentally, the metallic silver nanoparticles activated by polarizers were active in complexing with olefins and resulted in facilitated olefin transport. Theoretical calculation for interaction between nanoparticle and olefin: Specific interactions between the surface of silver nanoparticles and olefin molecules were elucidated using quantum mechanical ab-initio calculations [38]. Theoretical calculations were conducted with the 6-31G* basis set for ethylene molecules and the effective core potential (ECP) basis sets of type LANA12DZ for the transition metal element, Ag. The nature of all stationary species was verified by the vibrational frequencies. Figure 4(a) shows the 2dimentional structure of a silver nanoparticle consisting of 10 silver

139


atoms. It was found that ethylene molecules can interact with the upper edge, the side, or the palm side of the silver nanoparticle with corresponding complexation enthalpies (∆H) of -4.48, -3.65 and -1.42 kcal/mol, respectively, as shown in Figure 4(b-d). These calculations indicate the presence of the specific interactions between silver nanoparticle and olefin molecules, with the most probable interaction of ethylene with the upper edge of the silver nanoparticle. Thus it is expected that the interactions of the metallic nanoparticles with olefin molecules would be increased when positive charge is generated on the surface of the metallic silver nanoparticles, resulting in the enhanced carrier activity toward olefin molecules for facilitated olefin transport.

(a)

(b)

(c)

(d)

Fig. 4. Theoretical structure of a silver nanoparticle containing 10 atoms and the three different complexation structures and corresponding enthalpies (∆H) with ethylene. (a) Silver nanoparticle; complex structures with ethylene and the silver nanoparticle’s (b) upper edge, (c) side, and (d) palm side.

Chemical activation of surface of metallic nanoparticles toward olefin interactions: During the last few years, there have been significant interests in the synthesis and characterization of nanostructured materials including nanoparticles because of their intriguing optical, electrical, and mechanical properties [32,34,35]. It has been well recognized that the unique properties of nanomaterials stem from their smaller size and larger specific surface area. For example, a low temperature study of the interaction of elemental O2 with Ag nanoparticles of various sizes showed an increased capability of smaller nanoparticles to dissociate dioxygen to the atomic species, whereas the adsorbed oxygen species on bulk Ag

140


at 80 K became O2- [36]. Since it was known that aqueous silver nanoclusters have also been capable of transferring electrons to suitable acceptors [37], the surface of silver nanoparticles has a partial positive charge. Therefore it was expected that the chemically active metallic nanoparticles became more activated chemically toward olefin interactions when the positive charge density of the surface of metallic nanoparticles was enhanced by a proper electron acceptor. The chemically activated or positively charged surface of metallic nanoparticles may interact with olefin to some extent and consequently act as an olefin carrier. For instance, metallic silver and gold nanoparticles were chemically activated by a suitable electron acceptor such as p-benzoquinone (p-BQ), 4-dimethylaminopyridine (DMAP) or ionic liquids (ILs) to enhance specific interactions with olefin molecules [38-41]. Therefore, membranes containing chemically activated or polarized metal nanoparticles showed the facilitated olefin transport behavior and good separation performance in both selectivity and mixed gas permeance. The main objective of this book chapter is to provide the reader with a general overview of the present knowledge of facilitated transport membranes in the solid state for olefin/paraffin separation with reference to chemical activation of metallic nanoparticles, reversible interactions between polarized silver nanoparticle and olefin, and separation performance of olefin/paraffin mixtures. When metallic silver nanoparticles are chemically activated to have partial positive charge on their surface, the binding energy of the metallic silver atom will be increased. X-ray photoelectron spectroscopy (XPS) revealed that the introduction of suitable electron acceptors, i.e. p-BQ, ILs or DMAP caused the increase in the binding energy of the silver atoms, indicating the surface of the metal nanoparticles to become more positively polarized and the enhanced carrier activity of metal nanoparticles toward olefin comlexation [3841]. The change of the chemical environment around silver nanoparticles in poly(ethylene-co-propylene) (EPR)/Ago/p-BQ and ILs/Ago composite membranes was observed using XPS (Figure 5) [38]. The binding energy of the d5/2 orbital of the silver particle in the EPR/Ago/p-BQ system increased gradually from 368.26 to 368.89 eV with increasing p-BQ content. The binding energy of the 4f7/2 orbital of the gold nanoparticles (average 5.5 nm as shown in TEM) polarized by DMAP was 84.65 eV, while that for the neat gold nanoparticles without DMAP was

141


83.31 eV, as shown in Figure 6 [41]. Note that the binding energy was reported to be 83.5 eV (average size = 1.3 nm) [42].

368.73

Weight ratio of O EPR/Ag /p-BQ

374.67

368.89

1:1:1

374.85

1:1:0.85 368.68

374.75

1:1:0.5 368.35

374.33

1:1:0 368.26

374.26

Ag 364 366 368 370 372 374 376 378 380 382

Binding energy (eV)

Fig.5. XPS spectra for silver binding energies of the EPR/Ag/p-enzoquinone composites for varying weight ratios of p-benzoquinone (p-BQ) and + BMIM BF4 /Ag composite

84.65

88.32 (b)

83.31

87.01 (a)

78

81

84

87

90

93

96

Binding energy (eV) b)

a)

Fig.6. TEM image of Au nanoparticles stabilized by DMAP, and binding energy of (a) Au nanoparticles without DMAP and (b) Au nanoparticles stabilized by DMAP.

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In addition, the effect of the three different ionic liquids on the formation of polarized silver nanoparticle surfaces and their subsequent influence on facilitated olefin transport were also investigated [40]. Figure 7 showed that the binding energies of the d5/2 orbital of the silver particle in the 1/0.7 BMIM+BF4-/Ag, 1/0.7 1butyl-3-methylimidazolium triflate (BMIM+Tf-)/Ag metal and 1/0.7 1butyl-3-methylimidazolium nitrate (BMIM+NO3-)/Ag metal composites were measured to be 369.12, 368.63 and 368.26 eV, respectively, which were higher than 368.26 eV for the neat silver nanoparticles. These results indicated that the binding energy was increased upon addition of ionic liquids and the binding energy decreased in the order of BMIM+BF4-, BMIM+Tf-, and BMIM+NO3-. It was thus anticipated that the polarity of silver nanoparticles complexed with BMIM+BF4- was much higher than those with BMIM+Tf- and BMIM+NO3-, sexpecting that the carrier activity was also the same order with the binding energy.

+ BMIM BF4 /Ag

369.12

375.03

= 1/0.7

368.63

+ BMIM Tf /Ag = 1/0.7 + BMIM NO3 /Ag

374.61

368.47

374.45

= 1/0.7

368.26

374.26

Ag 362

364

366

368

370

372

374

376

378

380

Binding energy (eV) Fig.7. Binding energy of Ag in 1/0.7 BMIM+ BF4-/Ag, 1/0.7 BMIM+ Tf-/Ag, and 1/0.7 BMIM+ NO3-/Ag composites

These results suggested that the binding energies of silver and gold nanoparticles were increased with the polarizer such as p-BQ, DMAP and the ionic liquids and consequently the surface positive charge on the metallic nanoparticles was generated. The changes in the binding energies of silver or gold atoms with varying p-BQ, ILs or

143


DMAP may indicate partial electron transfer or shift from the metallic atoms to the electron acceptors, as suggested by Tripathi [37]. It is thus expected that the positively charged surface of the metallic nanoparticles may have interactions with olefin molecules. 4.2. Reversible interaction between nanoparticles and olefin molecules The reversible complexation of propylene with the surface of silver nanoparticles was investigated using FT-IR spectroscopy [37]. Figure 8(a) shows the FT-IR spectra of the EPR/Ago composite without p-BQ upon exposure to 40 psig of propylene for 10 min. The spectrum of pure propylene was included as a reference. Upon exposure to propylene, the EPR/Ago composite showed new shoulder peaks at 1664 and 1640 cm-1 representing the C=C stretching vibration of propylene (υ1 and υ2 of C=C in free propylene are 1664 and 1640 cm-1, respectively). The positions of the peaks remained unchanged as the exposure time was increased up to 30 minutes. The propylene peaks disappeared in the spectra following desorption of propylene for 5 minutes. These results suggest no interaction of propylene with the surface of the silver nanoparticles in the EPR/Ago composite without p-BQ and the propylene absorption occurs only in the EPR matrix. Therefore, the EPR/Ago composite membrane without p-BQ did not show facilitated olefin transport since the

1664

30minsorption

Absorbance

1640

5mindesorption

Absorbance

5mindesorption

1664 1640

5minsorption 0minsorption o EPR/Ag

30minsorption 5minsorption 0minsorption EPR/Ag/p-BQ Propylene

Propylene

a) 1800

1700

1600

1500

b) 1400

1300

1800

Wavenumber (cm-1)

1700

1600

1500

1400

1300

Wavenumber (cm-1) o

Fig.8. FT-IR spectra for (a) 1/1 EPR/Ag composite and (b) 1/1/0.85 o EPR/Ag /p-benzoquinone (p-BQ) composite as a function of propylene sorption or desorption time.

144


complexation of propylene on the surface of the silver nanoparticles was negligible. The EPR/Ago/ p-BQ composite membrane, on the other hand, also showed two new peaks for the C=C stretching vibration of propylene upon exposure to propylene (Figure 8(b)). Immediately following propylene exposure, i.e., zero exposure time, the two propylene peaks appeared at the same positions as those for the EPR/Ago composite without p-BQ. However, the peak shape of both 1664 and 1640 cm -1 was changed with increasing propylene exposure tim. In other words, the initial two propylene peaks at 1664 and 1640 cm-1 shifted to 1649 cm-1 presumably due to the partial electron transfer or shift from the C=C bond of propylene to the partially positively charged surface of the silver nanoparticles. These suggested the complexation of propylene on the surface of the metallic silver nanoparticles activated by p-BQ. Following propylene desorption for 5 minutes, the peak at 1649 cm-1 disappeared almost completely, indicating that the interactions between the partially polarized surface of the silver nanoparticles and propylene were very much reversible. Note that the propylene peaks in the EPR/Ago without p-BQ instantaneously disappeared following desorption as described previously. The FT-IR spectra suggest that propylene complexes with the surface of the silver nanoparticles activated by pBQ reversibly, expecting the olefin carrier activity for the facilitated propylene transport through the EPR/Ago/p-BQ composite membrane. 4.3. Separation performance Composite membranes were prepared by coating active selective layer onto an asymmetric polysulfone support. The separation performance of propylene/ propane mixtures through EPR/AgO/p-BQ membranes was evaluated at room temperature and 40 psig. EPR was used as a matrix polymer to prepare the composite membranes because EPR is amorphous, highly permeable to both propylene and propane gases. The EPR/AgO membrane without p-BQ exhibited low gas permeation and practically no separation of the propylene/propane mixtures. The mixed gas permeance was ca. 0.01 GPU and the ideal separation factor, defined as a pure gas permeability ratio, of propylene/propane was nearly unity. Figure 9(a) shows the ideal separation factor of propylene over propane through the EPR/AgO composite membranes containing pBQ. The weight ratio of EPR to the silver metal was fixed at unity, i.e. [EPR]/[AgO]=1/1, because the best separation performance was observed at this composition. As the weight ratio of the silver

145


nanoparticles increased, the separation performance also increased due to the increased number of olefin carrier. The ideal separation factor remained at nearly unity at low weight ratios of p-BQ, but increased sharply above the weight ratio of 0.5, and reached almost 170 at the weight ratio of p-BQ of 0.85. Composite membranes containing the weight ratios of p-BQ greater than 0.85 could not be prepared due to the formation of particle aggregation in the solution state. The low ideal separation factor at low weight ratios of p-BQ is presumably due to limited activation of the surface of the nanoparticles. It increases with the weight ratio of p-BQ, suggesting that facilitated propylene transport occurs because of the carrier action of the silver nanoparticles activated by p-BQ. From these data, it is suggested that p-BQ may cause the surfaces of silver nanoparticles to become olefin carriers for facilitated olefin transport, due to the partially positively charged surfaces of the silver nanoparticles, as confirmed previously.

8

Pure gas permeance (GPU)

Ideal separation factor

200

1/1 EPR/Ago

150

100

50

0

7 6 5 4 3

Propylene permeance Propane permeance

2 1 0

0.0

0.2

0.4

0.6

0.8

1.0

0.0

Weight ratioa) of p-benzoquinone

0.2

0.4

0.6

0.8

b) Weight ratio of Ag to BMIM+BF4-

1.0

Fig.9. Ideal separation factor and pure gas permeances through 1/1 (wt/wt) o + EPR/Ag /p-benzoquinone membranes and BMIM BF4 membrane with varying Ag content, respectively.

Figure 9(b) shows pure gas permeances of propylene and propane through the BMIM+BF4-/Ag nanocomposite membrane. The presence of the silver nanoparticles in BMIM+BF4- resulted in the increase in propylene permeance while the propane permeance remained nearly constant. In particular, the propylene permeance increased initially markedly with the increasing weight ratio of the silver nanoparticles up to a ratio of 0.7. The maximum propylene

146


Selectivity (Propylene/Propane)

permeance was obtained up to 7.8 GPU at a weight ratio of 0.7. The ideal separation factor, defined as the ratio of propylene flux to that of propane, had a maximum value of 780, also at a weight ratio of 0.7. At weight ratios higher than 0.7, the propylene permeance decreased with the increase in silver metal content, presumably due to aggregation of the silver nanoparticles which results in the loss of carrier activity and the blocking of the diffusion pathways. These increases indicate facilitated olefin transport due to the interactions of propylene with the partially positively charged surface of silver nanoparticles by suitable electron accepters such as p-BQ and ILs.

+

-

20

BMIM BF4 /Ag

18

BMIM TF /Ag + BMIM NO3 /Ag

+

16

-

14 12 10 8 6 4 2 0 0.0

0.2

0.4

0.6

0.8

1.0

Weight ratio of Ag to ionic liquid Fig.10. Mixed gas selectivity of ionic liquid/Ag metal composite membrane with various weights of Ag metal.

Figure 10 shows the mixed gas selectivity of propylene over propane through BMIM+BF4-/Ag, BMIM+Tf-/Ag and BMIM+NO3-/Ag composite membranes. Permeation test indicated that the separation performance for the mixed gas was in the following order: BMIM+BF4/Ag >> BMIM+Tf-/Ag > BMIM+NO3-/Ag. The selectivity pattern in three kinds of IL/Ag metal composite membranes was observed to be similar to the increased degree of binding energy, as confirmed by XPS [29]. These results indicated that as the particle surface was positively polarized, the facilitated olefin transport also increased. The mixed gas separation performance of poly(vinyl pyrrolidone) (PVP)/Au nanocomposite membranes have been evaluated. Neat PVP membrane showed the mixed gas permeance of 0.1 GPU and the selectivity of 1.2 for propylene/propane mixtures. On the other hand, when the gold nanoparticles were incorporated into PVP, both the mixed gas permeance and the selectivity increased to 1.2 GPU

147


and 22, respectively. This remarkable enhancement of the separation performance of the membranes is attributable to the olefin transport facilitated by the gold nanoparticles acting as effective carriers for olefins, which is possible due to the reversible interaction between the partially polarized gold nanoparticles and the olefin molecules. 4. 4. Long term stability The separation performance of the 50/50 (v/v) propylene/propane mixture was tested through the 1/1/0.85 EPR/AgO/p-BQ and BMIM+BF4-/Ag membrane with time for up to 100 hrs. Figure 11 shows the mixed gas selectivity as a function of the operational time, indicating that the selectivity was nearly invariant for the duration of the experiment up to 100 hrs. These results represent that the carrier activity by the polarized surface of silver nanoparticles is highly stable for a long time.

a)



20

12 10 8 6 4 2 0 0

20

40

60

80

100 120

15

10

b) 5

0

Time(hour)

0

20

40

60

80

100

Time(hour) o

Fig.11. Selectivity of propylene/propane mixtures of (a) EPR/Ag /pbenzoquinone and (b) BMIM+BF4-/Ago with time.

5. CONCLUSIONS

The recent researches on the development of solid-state facilitated transport membranes were described based on hybrid membranes containing metallic nanoparticles dispersed in polymeric matrix. The metallic nanoparticles should be properly polarized by electron acceptor, resulting in the activation of nanoparticles toward

148


olefin complexation and an olefin carrier for facilitated olefin transport. The activated surface of nanoparticles leads to the specific, reversible interactions or complexation with olefin molecules, such as propylene and ethylene, as supported by FT-IR spectroscopy and ab-initio calculations. The facilitated olefin transport membranes containing activated metal nanoparticles exhibited a long-term stability up to 100 hrs in separation performance of propylene/propane mixtures. For practical applications, however, potential side reactions of metallic silver with reactive compounds such as H2S and acetylene should be carefully treated. We hope that these prospects will be helpful in providing the insight of the facilitated transport phenomena in the solid state, the molecular structures of hybrid materials containing metallic nanoparticles and breaking the limit of the membrane technology for practical applications. Acknowledgements This work was supported by Energy · Resources Technology R&D program (2006EID11P101C-21-1-000) under the Ministry of Knowledge Economy, Republic of Korea REFERENCES [1]

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