pentane separation using facilitated transport membranes

Journal of Membrane Science 233 (2004) 113–117

Isoprene/pentane separation using facilitated transport membranes Seoung Hyun Kim a , Dong Bum Kim a,b , Dae Ki Choi a , Hyunjoo Lee a , Hoon Sik Kim a,∗ , Jongok Won b a

Division of Environment and Process Technology, Korea Institute of Science and Technology, 39-1, Hawolgokdong, Seongbuk-gu, Seoul 136-650, South Korea b Department of Chemistry, Sejong University, Seoul, South Korea

Received 20 September 2003; received in revised form 23 September 2003; accepted 19 January 2004

Abstract The facilitated transport membranes, AgClO4 -CA (cellulose acetate) and AgBF4 -CA were prepared and tested for the separation of isoprene/pentane mixture. The maximum selectivities for isoprene over pentane were found to be 53 and 83 for AgClO4 -CA and AgBF4 -CA membranes, respectively. FT-IR and UV-Vis studies show that the silver ions in the membranes are coordinated by the carbonyl groups of CA. Reversible diene coordination to silver ions was observed by FT-IR and UV-Vis studies. Treatment of the AgBF4 -CA membrane placed in a gas cell with isoprene produces an isoprene-coordinated membrane in which the coordinated isoprene is easily replaced by other diene compounds such as trans-piperylene and 1,3-butadiene. © 2004 Elsevier B.V. All rights reserved. Keywords: Isoprene/pentane separation; Cellulose acetate; Facilitated transport membrane; Alkene/alkane; Silver salts

1. Introduction Alkene/alkane separation by facilitated transport membrane, using silver salts as carriers, has been considered as a promising alternative to the conventional energy intensive distillation process [1–10]. The basis for the separation is the ability of silver ions to react reversibly with alkenes forming silver–alkene complexes. There have been many reports on the facilitated transport of alkenes by using supported liquid membranes, ion exchange membranes, or dense polymer membranes containing copper ions or silver ions as carriers [11–17]. Accordingly, much information has been disclosed about phenomena occurring on the membrane such as reversible interactions of silver ions with alkenes, which is of pivotal importance in understanding the facilitated alkene transport across the membrane. Recently, it has been shown that Ag+ -exchanged perfluorosulfonated ionomer membranes are also effective for the separation of dienes from monoenes, but the spectroscopic investigation to characterize the interaction between diene

∗

Corresponding author. Tel.: +82-2-958-5855; fax: +82-2-958-5859. E-mail address: [email protected] (H.S. Kim).

0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.01.007

with silver ions in membranes has rarely been attempted [18,19]. In this paper, we report the performance of the cellulose acetate membranes containing silver salts for the separation of isoprene/pentane mixtures as well as the spectroscopic observations of the reversible diene coordination to silver ions confined to membranes. Isoprene/pentane mixture has been chosen as the feed mixture because isoprene is currently being obtained by energy-intensive extractive distillation from C5 fraction due to the formation of azeotrope with pentane [20].

2. Experimental method The cellulose acetate (CA) membranes containing silver salts, AgBF4 or AgClO4 were prepared as follows: a tetrahydrofuran (THF) solution of AgClO4 (Aldrich, 99.9%) or AgBF4 (Aldrich, >99.99%) was mixed with 10 wt.% cellulose acetate (Aldrich, acetyl content: 39.8 wt.%) in THF (Baker, >99%) solution. The resulting solution was then cast onto a polyester microporous membrane support (47 mm, 0.1 ␮m, Whatman Industries Inc.) using a doctor blade. The cast membrane was stored for 2 h in a light-protected convection oven at room temperature under a stream of nitrogen,

S.H. Kim et al. / Journal of Membrane Science 233 (2004) 113–117

3. Results and discussion Fig. 1 shows the IR spectra of AgBF4 -CA and AgClO4 -CA membranes at various conditions. The carbonyl peak of cellulose acetate at 1741 cm−1 in Fig. 1a, shifts to lower frequencies at 1713 and 1711 cm−1 by the incorporation of AgBF4 and AgClO4 , respectively. Fig. 1d–g demonstrates the rapid and reversible diene coordination to silver ions dissolved in the solid AgClO4 -CA and AgBF4 -CA membranes. When the AgBF4 -CA membrane (Ag+ /CA = 3) was exposed to isoprene for a couple of seconds and then purged with N2 , new IR absorption peaks appeared at 1560 cm−1 (Fig. 1d). The peak at 1560 cm−1 remains even after degassing at 133.32 × 10−5 Pa for 4 h at room temperature. However, exposure of the isoprene-coordinated AgBF4 -CA membrane to trans-piperylene and subsequent flushing with N2 gives three new peaks at 1555, 1606 and 1638 cm−1 with the concomitant disappearance of the peak at 1560 cm−1 (Fig. 1e and Eq. (1)), indicating that the coordination mode of trans-piperylene is different from that of isoprene. The single C=C stretching frequency for coordinated isoprene and 1,3-butadiene shows that isoprene and 1,3-butadiene coordinate to silver ion in an ␩4 -fashion. On the other hand, several bonding modes seem to exist for coordinated trans-piperylene including ␩2 -and ␩4 -fashions from the observed three C=C stretching frequencies (see Eqs. (1) and

(a)

Transmittance (arbitrary units)

and then in a vacuum oven for 2 h at room temperature to produce AgBF4 -CA or AgClO4 membranes. The thickness of the membrane and the top polymer electrolyte layer was ca. 30 and 1.0 ␮m, respectively, as determined by SEM [21]. Separation measurements were performed using the prepared membranes placed in a stainless steel separation module with isoprene/pentane (50/50) mixture. The sweep gas (helium) was controlled using a mass flow controllers. The total feed pressure of mixed gas was set at 106 cmHg by a back pressure regulator. The permeated gas was analyzed by using a gas chromatography (Younglin, Model 600D) equipped with FID, and unibead-1S capillary packed column. The samples for FT-IR measurements were prepared by coating 10 wt.% of THF solution containing CA and a silver salt (molar ratio of Ag+ /CA = 3) onto a 25 mm × 3 mm CaF2 window at room temperature for 6 h. The coated window was dried under vacuum and placed in a specially designed gas cell [22,23]. FT-IR spectra were recorded on a Mattson Infinity spectrophotometer equipped with an MCT detector. The samples for UV-Vis spectra were prepared by coating 1.0 wt.% AgClO4 -CA or AgBF4 -CA solution in THF onto a 25 mm × 3 mm quartz window. The coated quartz window was vacuum-dried for 2 h at room temperature and placed in the gas cell. UV-Vis spectra were taken on a Scinco UV S-2100 spectrophotometer.

1741

(b) 1713

(c) (d)

1711 1560

(e)

1606 1638 1555

(f) 1551

(g)

2000

1560

1850

1700

1550

1400

1250

1100

Wavernumbers (cm-1) Fig. 1. FT-IR spectra of AgClO4 -CA and AgBF4 -CA membranes as well as those of diene-coordinated membranes: (a) CA; (b) AgClO4 -CA; (c) AgBF4 -CA; (d) isoprene-coordinated AgBF4 -CA membrane; (e) trans-piperylene-coordinated AgBF4 -CA membrane; (f) 1,3-butadienecoordinated AgBF4 -CA membrane; (g) isoprene-coordinated AgClO4 -CA membrane.

(2)). Likewise, the three peaks disappeared and new peak at 1551 cm−1 corresponding to coordinated 1,3-butadiene appeared when 1,3-butadiene was introduced into the cell (Fig. 1f). Fig. 1g is the spectrum of isoprene-coordinated AgClO4 -CA membrane. The reversible diene coordination was also observed by the spectral changes in the UV-Vis absorption spectra (Fig. 2). Fig. 2e shows broad absorption band centered at 238 nm for isoprene-coordinated AgBF4 -CA membrane. 211

(a)

Absorbance (arbitrary units)

114

215

(b)

208

(c) 236 238

(e) 260 233

200

(d)

(f)

250

(g) 300

400 500 Wavelength (nm)

600

Fig. 2. UV-Vis spectra showing the effects of diene coordination on the CA-AgBF4 membrane: (a) isoprene; (b) 1,3-butadiene; (c) trans-piperylene; (d) AgBF4 -CA membrane; (e) isoprene-coordinated AgBF4 -CA membrane; (f) 1,3-butadiene-coordinated AgBF4 -CA membrane; (g) trans-piperylene-coordinated AgBF4 -CA membrane.

S.H. Kim et al. / Journal of Membrane Science 233 (2004) 113–117

of the molar ratio of Ag+ /CA on selectivity for isoprene over pentane. The selectivities for isoprene over pentane increase with increasing molar ratio of Ag+ /CA up to 3 and then rapidly decrease with further increase in the molar ratio. AgBF4 -CA membranes exhibit higher selectivity than AgClO4 -CA membranes at or below the molar ratio of Ag+ /CA = 3. This is possibly because the dissociation of anion is greater for AgBF4 , providing more room for isoprene to coordinate to silver ions. The maximum selectivity at the molar ratio of Ag+ /CA = 3 suggests that the coordination environment of silver ions in cellulose acetate membranes has a pronounced effect on the facilitated transport of isoprene. The monomer unit of cellulose acetate (acetyl content: 39.8 wt.%) has approximately 4.9 carbonyl groups and thus, in principle, each silver ion in the membrane with molar ratio of Ag+ /CA = 3 is coordinated by about 1.6 carbonyl groups [23]. Therefore, it is likely that the most favorable number of coordinated carbonyl groups to silver ions in Ag+ -CA membranes should be around 1.6 to be highly selective for separating isoprene from pentane mixture. The basis for the separation by facilitated transport membrane is the ability of silver ions

When the isoprene-coordinated AgBF4 -CA membrane (Ag+ /CA = 3) was exposed to 1,3-butadiene vapor for 2 min and followed by N2 purge for 10 min, the absorption band centered at 238 nm disappeared, and new band appeared at 260 nm for the coordinated 1,3-butadiene (Fig. 2f). In the same manner, the absorption band of the coordinated trans-piperylene centered at 233 and 250 nm appeared with the simultaneous disappearance of the absorption band at 260 nm upon exposure of the membrane to trans-piperylene and subsequent N2 purge (Fig. 2g). These FT-IR and UV-Vis spectral results strongly indicate that the coordinated diene is labile enough to be easily replaced by an incoming diene molecule, thereby diene can diffuse from the feed stream across the membrane, resulting in selective separation of diene from alkane mixtures (Eqs. (1) and (2))

Ag +O

P

+ O

+

O

P

CH3

O

CH3

Ag

Ag

Ag

+O

+O

P

(1)

Ag

O

P

CH3

O

115

+O

+

P

CH3

O

CH3

O

P

O

CH3

Cellulose Acetate

=

(2)

The separation of isoprene/pentane mixture was performed to evaluate the facilitated transport effect of silver ions in the cellulose acetate membrane. Fig. 3 shows the effect 100 AgBF4 -CA

Selectivity (isoprene/n-pentane)

90

AgClO4-CA

80 70 60 50 40 30 20 10 0 0

1

2

3

Molar ratio of

4

5

6

Ag+/CA

Fig. 3. Effect of the molar ratio of Ag+ /CA on the selectivity for isoprene over pentane.

to react reversibly with isoprene forming silver–isoprene ␲-complexes. Since the silver ions in CA membranes are already coordinated by certain number of carbonyl groups isoprene should compete with carbonyl groups for the coordination to silver ions. At lower molar ratio of Ag+ /CA, when silver ions are coordinated by larger number of carbonyl groups, the access of isoprene to silver ions becomes rather restricted. Furthermore, the electron density on silver ions will be increased due to the increased electron donation from carbonyl groups, which in turn reduces the ability of silver ions to interact with isoprene to form ␲-complexes. On the other hand, at higher molar ratio of Ag+ /CA, the silver ions become more electrophilic due to the decreased electron donation from carbonyl groups, forming stronger ␲-bonds with isoprene. As a result, the decomplexation of isoprene would be more difficult. Permeation properties of Ag+ -CA membranes at different molar ratios of Ag+ /CA were also investigated. As can be seen in Fig. 4, fluxes of pure isoprene increased with increasing molar ratio up to Ag+ /CA = 3 and thereafter gradually decreased, again emphasizing the importance of coordination environment of silver ions. Maximum fluxes

116

S.H. Kim et al. / Journal of Membrane Science 233 (2004) 113–117

4

10-5 cm3(STP)/cm2 s cmHg

[2]

3 [3]

2 [4]

1

[5]

AgBF4-CA AgClO4-CA

[6]

0 0

1

2

3

4

5

6

+

Molar ratio of Ag /CA

[7]

Fig. 4. Effect of the molar ratio of Ag+ /CA on the permeate flux for pure isoprene.

[8]

+

of pure isoprene at the molar ratio of Ag /CA = 3 and at room temperature were 1.8 × 10−5 for AgClO4 -CA and 3.9 × 10−5 cm3 cm−2 s−1 cm Hg−1 for AgBF4 -CA, respectively.

[9]

[10]

4. Conclusions The polymer electrolyte membranes, AgClO4 -CA and AgBF4 -CA have been prepared and applied to the facilitated transport membranes for the isoprene/pentane separation. FT-IR and UV-Vis spectra studies show that coordinated isoprene to silver ions in AgBF4 -CA membrane is labile enough to be easily replaced by other diene molecules. Therefore, isoprene can permeate the membrane, resulting in selective separation of diene from the isoprene/pentane mixture. The selectivity for isoprene over pentane increased with increasing molar ratio up to Ag+ /CA = 3, and rapidly decreased thereafter. The selectivity maximum is believed to originate from the difference in accessibility of isoprene to the binding sites of silver ions and ease of decomplexation.

[11]

[12]

[13]

[14] [15]

[16]

Acknowledgements This research was supported by a Grant (BC1-108) from Carbon Dioxide Reduction & Sequestration Research Center, one of the 21st Century Frontier Programs funded by the Ministry of Science and Technology of the Korean government. References [1] H.S. Kim, J.H. Ryu, H.G. Kim, B.S. Ahn, Y.S. Kang, Reversible olefin complexation by silver ions in dry poly(vinyl methyl ketone)

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