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cyclohexane mixtures with facilitated transport membrane of poly(vinyl alcohol)–Co2+
Separation and Purification Technology 45 (2005) 103–108
Separation cyclohexene/cyclohexane mixtures with facilitated transport membrane of poly(vinyl alcohol)–Co2+ J.N. Shen a , L.G. Wu b,∗ , H.L. Chen a , C.J. Gao b a
Material Science and Chemical Engineering College of Zhejiang University, 310027 Hangzhou, PR China b Research and Development Center of Water Treatment Technology, SOA, 310012 Hangzhou, PR China Received 26 June 2004; received in revised form 3 January 2005; accepted 26 February 2005
Abstract Facilitated transport membranes based on the solid polymer electrolytes of poly(vinyl alcohol)–Co2+ (PVA–Co2+ ) blends were prepared. The membrane morphology was analyzed with X-ray scattering and DSC. The sorption behavior of cyclohexene, cyclohexane in the membranes were measured and the separation performance of the membrane were investigated for cyclohexene/cyclohexane mixtures. The result showed that Co2+ salts were highly soluble in PVA and disrupted the crystallinity of PVA. Compared with pure PVA, the cyclohexene saturated sorption capacity in the solid polymer electrolytes was improved a lot and reached 60 mg (cyclohexene)/g (dry membrane), while the cyclohexane saturated sorption capacity in the solid polymer electrolytes had little differences and was only 15 mg (cyclohexane)/g (dry membrane). Permeability flux of the membrane was 150 g/(m2 h) and a cyclohexene/cyclohexane separation factor was 60 when at 40 ◦ C and the optimal carrier content of the membrane ([Co2+ ]/[ OH]) was 0.75 (mol/mol) and cyclohexene mass fraction in the feed was 0.5. © 2005 Elsevier B.V. All rights reserved. Keywords: Facilitated transport; Membrane separation; Cyclohexene; Cyclohexane; Poly(vinyl alcohol)–Co2+ blend
1. Introduction Separation of organic–organic mixtures is one of the most important processes in chemical industry. Pervaporation membrane technology can be considered as an energyefficient alternative process to conventional technologies, such as distillation and extraction, especially the separation of azeotropic mixtures, isomeric components, close boiling point systems. For membrane processes must be costeffective, membranes should have high selectivity and high flux and also the membrane materials should be inexpensive. The membrane process using the concept of facilitated transport has been considered to be an energy-saving process [1]. Poly(vinyl alcohol) (PVA) membranes have been used for numerous separation process because of their low cost and mechanical stability. The poly(vinyl alcohol) cast with ∗
Corresponding author. Tel.: +86 571 88865762; fax: +86 571 88868427. E-mail address: wu [email protected] (L.G. Wu).
1383-5866/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2005.02.013
AgNO3 was reported by Ho and Dalrymple [2] for separation of 1-butene/n-butane. Bryant et al. [3] reported the facilitated transport membranes of poly(vinyl alcohol) containing silver ions as the carrier for separation of benzene/cyclohexane mixtures. Park et al. [4] investigated the chelate poly(vinyl alcohol)/poly(allyl amine) blend membrane for the separation of benzene/cyclohexane mixture. However, silver salts are sensitive to visible light and strongly oxidative, AgNO3 may reduce the Ag+ in the membrane to give particles of Ag. Co(NO3 )2 and Co(CH3 COO)2 is not sensitive to visible light, which may be more steady than the AgNO3 in the polymer and the Co2+ can also coordinate with olefin. Fig. 1 is a schematic diagram for describing the coordinating of cyclohexene in the feed and cobalt in the membrane. Metals are in the low oxidation state and often form a complex with acceptor ligands because of the opportunity they provide to lessen the electron density on the central metal atom. In this case, the interaction by a coordination can be occurred between the orbitals of the same symmetry and similar en-
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2.4. Swelling measurements The degree of swelling of membrane at a given time was defined by the following equation: Aτ =
Fig. 1. Schematic diagram of cobalt–cyclohexene coordination.
ergy. Therefore, the orbitals of cyclohexene can coordinate with 3d orbital of cobalt. In the present study, poly(vinyl alcohol) containing Co2+ facilitated transport for separation of cyclohexene from mixtures with cyclohexane was studied.
2. Experimental 2.1. Materials Poly(vinyl alcohol) (hydrolyzation degree 98–99%, average polymerization 1700), cobalt nitrate (Co(NO3 )2 ) and cobalt acetate (Co(CH3 COO)2 ) were of AR grade; cyclohexene and cyclohexane were of CP grade. CA ultra-filtration membranes were supported by the development center of water treatment technology of state ocean bureau China. 2.2. Membrane preparation Ten weight percent PVA was dissolved in boiling deionized water with stirring. Co(NO3 )2 and Co(CH3 COO)2 were added to PVA solution whiling stirring. The solution was cast on a CA ultrafiltration membrane to form composite membrane. Then, the composite membrane was dried at room temperature; the asymmetric membranes were treated in a dustless oven at 120 ◦ C for 2 h. The resulted membrane separation layer, which could be peeled off from the substrate, was measured to be ∼20 m.
mτ − m0 × 1000 (mg/g) mτ
where m0 and mτ are the weights of dry and solvent swollen membranes, respectively. The swelling experiments were carried out at constant temperature and the membranes were weighed at room temperature. The free liquid on the surface of the swollen membrane must be removed carefully by using filter paper before weighed.
2.5. Pervaporation measurements The pervaporation experiments were conducted with the apparatus is shown in Fig. 2. The membrane was placed in the stainless-steel permeation chamber, the membrane area in contact with the feed was 15.9 cm2 , the temperature was thermostatically controlled and the downstream pressure was 100 Pa. The permeation vapor was condensed by liquid nitrogen (N2 ), the composition of the permeation flux was analyzed by gas chromatograph. The permeation flux (J) and the separation factor (α) for all membranes were calculated according to the following equations: J=
g A × t
(2)
yi /yi xi /xj
(3)
αi/j =
where g is the permeation weight during the operation time, A the membrane area, and x and y are the weight fraction in the feed and permeate, respectively.
2.3. Membrane characterization X-ray scattering experiments were carried out for studying the solid polymer electrolytes morphology. The angle of diffraction (2θ) was varied from 0◦ to 65◦ . The effective distance between inter-segmental chains was determined by this technique. The spectrum was obtained by using a D/max-ra X-ray diffraction spectrometer. A Perkin-Elmer DSC-7 was used at a heating rate of 20 ◦ C/min under an N2 environment to measure the glass transition temperatures (Tg ) of the polymer electrolytes. Samples of PVA complexed with Co(CH3 COO)2 or Co(NO3 )2 were prepared with hydroxyl ( OH)-to-cobalt ratios of 20:1, 10:1, 6:1, 5:1, 4:1, 3:1 and 1:1.
(1)
Fig. 2. Schematic diagram of pervaporation experiment.
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3. Results and discussion 3.1. Membrane characterization X-ray scattering spectrum of different Co2+ concentration and various Co2+ salt kinds on the crystallinity are shown in Fig. 3a and b. The spectrum of pure PVA exhibits a strong crystalline peak centered at 2θ value round 20◦ . Weaker crystalline peaks are observed at 2θ value near 10◦ and 40◦ . The percent of crystallinity, calculated as the ratio of the area of the crystalline peaks to the total area under the crystalline peaks and the amorphous halo, is 52%. The crystallinity decreases strongly with increasing Co2+ concentration and is completely absent in the samples containing 50 wt% Co2+ or more. The inter-segmental d spacing in copolymer electrolytes was calculated using Braggs equation d = λ/2 sin θ with θ of the peak maximum. The d spacing of pure PVA film is 0.23 nm, which decreases gradually with the Co2+ salt concentration in both polymer electrolytes. The d spacing of the PVA–Co(NO3 )2 is smaller than that of the PVA–Co(CH3 COO)2 at a fixed amount of Co2+ salt in PVA. Another important fact is that NO3 − (62) group is heavier than CH3 COO− (59). Therefore, the main chain mobility will be reduced more in the PVA–Co(NO3 )2 system than in the PVA–Co(CH3 COO)2 . Fig. 4a shows DSC heating curves of PVA, PVA/Co(CH3 COO)2 and PVA/Co(NO3 )2 systems. As shown in Fig. 4a, the glass transition temperature (Tg ) of PVA increases with adding cobalt salt. Tg of the pure PVA is 68 ◦ C, the value of the glass transition temperature in the
Fig. 4. (a) DSC thermograms for PVA, PVA/Co(CH3 COO)2 and PVA/Co(NO3 )2 systems. (b) Variations of glass transition temperature as a function of salt concentration for PVA with Co(CH3 COO)2 and Co(NO3 ).
complexes of PVA with Co(CH3 COO)2 and Co(NO3 )2 for various mole ratios of [ OH]/[Co] 1:1 are, respectively, 80 and 90 ◦ C. The increase in Tg upon addition of metal salts is primarily attributable to both the transient cross-link of polymer segmental chains and the dangling of cobalt salts on the main chain. And the reduction in crystallinity may increase chain mobility, which may have less effect than the decrease in chain mobility due to the formation of Co2+ / OH linkages between neighboring chains and the dangling of cobalt salts on the main chain. So Tg increases a little and the crystallinity decreases upon addition of metal salts. The values of the glass transition temperature in the complexes of PVA with Co(CH3 COO)2 and Co(NO3 )2 for various mole ratios of [ OH]/[Co2+ ] are presented in Fig. 4b. In the presence of cobalt salts, the values of Tg in both systems increase gradually, indicating cobalt ions coordinating to the hydroxyl of PVA and restricting chain mobility of PVA. Tg reaches the maximum with an increasing amount of cobalt salts and then it keeps constant above a certain salt content, which is a common feature of polymer/metal salt complexes [5–8]. 3.2. Performance study of solid polymer electrolytes membranes
Fig. 3. X-ray diffraction spectrum of PVA membrane: containing (a) various cobalt concentration; (b) different Co2+ salt.
3.2.1. Degree of swelling The degree swelling (DS) of cyclohexene in pure PVA and PVA–Co2+ electrolytes were measured at a constant temper-
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linearly with τ 1/2 demonstrating the DS of cyclohexane in PVA and PVA–Co2+ electrolytes can be readily described by Henry mode. The DS of PVA/Co(CH3 COO)2 membrane is larger than PVA/Co(NO3 )2 . The DS of cyclohexene in the membrane of various Co2+ concentration is shown in Fig. 5c. The DS of cyclohexene in the PVA/Co(CH3 COO)2 membrane increases with increasing the Co2+ concentration. This is due to the fixed sites, which increases with increasing the Co2+ concentration.
Fig. 5. (a) The degree of swelling of cyclohexene in pure PVA and PVA–Co2+ electrolytes membrane. (b) The degree of swelling of cyclohexane in pure PVA and PVA–Co2+ electrolytes membrane. (c) The degree of swelling of cyclohexene in PVA/Co(CH3 COO)2 membrane of various Co2+ ions concentration.
ature, the results are presented in Fig. 5a. The cyclohexene solubility in pure PVA increases linearly with τ 1/2 , demonstrating the solubility of cyclohexene in PVA can be readily described by a simple Henry mode. The curves of the cyclohexene solubility in cobalt polymer electrolytes become concave to the ordinate with the τ 1/2 , indicating the presence of the second mode sorption. The second mode sorption is evidently due to the interaction between cobalt ions and cyclohexene molecules and it can also be described by a simple Langmuir model. And the DS of cyclohexene in PVA–Co2+ electrolytes membranes increases with increasing the Co2+ ions concentration. However, the mechanical power of the membrane decreases with further increasing the Co2+ ions concentration. The DS of cyclohexane in pure PVA and PVA–Co2+ electrolytes were shown in Fig. 5b. The DS of cyclohexane in pure PVA and PVA–Co2+ electrolytes increases
3.2.2. Pervaporation results The separation of cyclohexene/cyclohexane mixtures (1:1, v/v) was performed to evaluate the facilitated transport effect of cobalt ions in PVA. Fig. 6 shows the effect of molar ratio [Co2+ ]/[ OH] on permeation flux of cyclohexene and the selectivity for cyclohexene/cyclohexane. The permeation flux of cyclohexene increases with increasing molar ratio of [Co2+ ]/[ OH] up to 1:1. However, the selectivity of cyclohexene over cyclohexane increases with increasing molar ratio of [Co2+ ]/[ OH] up to 3:4 and decreases with further increasing the molar ratio of [Co2+ ]/[ OH] which may due to the salt solubility gets to saturated value at the molar ratio of [Co2+ ]/[ OH] up to 3:4. The membrane may form the defect with farther increasing the cobalt concentration. Such selectivity on the molar ratio of [Co2+ ]/[ OH] implies that the coordination environment of cobalt ions in PVA has a significant effect on the facilitated transport of olefins. Fig. 5 also shows the effect of anion size on cobalt ions electrolytes membrane for separation cyclohexene/cyclohexane mixtures. The size of CH3 COO− (0.213 nm) group is larger than NO3 − (0.189 nm). The larger size of anion has the tendency of cobalt salts to form complexes and the lattice energy of Co(CH3 COO)2 is smaller than Co(NO3 )2 , which indicates that interaction of cobalt ions with NO3 − is stronger than with CH3 COO− , consequently that the complexation of cobalt (Co2+ ) with cyclohexene molecules may be stronger in the Co(CH3 COO)2 membrane than that in the Co(NO3 )2 membrane, resulting in cyclohexene permeation flux and selectivity of PVA–Co(CH3 COO)2 electrolytes membrane is better than PVA–Co(NO3 )2 .
Fig. 6. Effect of carrier content on separation performance of membrane (testing temperature, 40 ◦ C; cyclohexane mass fraction, 50 wt%).
J.N. Shen et al. / Separation and Purification Technology 45 (2005) 103–108
Fig. 7. Effect of cyclohexene mass fraction in the feed on separation performance of membrane (testing temperature, 40 ◦ C; [Co2+ ]/[ OH], 0.75).
The effect of cyclohexene mass fraction in the feed on separation performance of membrane was performed. The results are shown in Fig. 7. The separation performance of the [Co2+ ]/[ OH] was 3:4 in PVA–Co(NO3 )2 and PVA–Co(CH3 COO)2 electrolytes membrane was investigated at constant temperature of 40 ◦ C. The permeation flux of the cyclohexene increases and the selectivity decreases with increasing cyclohexene fraction in the feed because more cyclohexene can coordinate with cobalt ions with increasing the cyclohexene mass fraction. The decline in the selectivity is explained by the plasticization effect of cyclohexene on the membrane. With increasing cyclohexene content, the membrane is swollen and the relaxed polymer chains arrows for enhanced permeability of cyclohexane. The effect of temperature on the separation characterization is shown in Fig. 8. The permeation flux of the cyclohexene increases and the selectivity decreases with increasing feed temperature. For non-facilitated transport membrane system, increasing temperature generally leads to an increase in the diffusivity and a decrease in the solubility and consequently the permeation flux may increase or decrease depending on the relative significance of the thermal effects of the two aspects. For facilitated transport membrane systems, it is anticipated that a temperature change will affect both kinetics and equilibria of the complexation reactions; it is difficult to formulate a simple explanation on the temperature depen-
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dence of permeability. Unfortunately, study on the temperature effects on the general performance of cobalt containing polymer electrolyte membranes for olefin separation are lacking the open literature, more studies are required to develop a better understanding of how the operating temperature influences the membrane performance. In this paper, as temperature increases, the cyclohexane permeation flux increases (the results is not shown in the manuscript) due to the increase in the diffusivity of cyclohexane, which is transported by the simple solution–diffusion mechanism. Although the cyclohexene flux also increased, the extent of the increase on permeation flux was lower than that of cyclohexane. This suggests that the coordination reaction between cobalt ions and cyclohexene may be exothermic, and the decrease in the equilibrium constant of the reaction with increasing temperature may compensate the increase in the diffusivity. So the flux of cyclohexene increases and α decreases with temperature.
4. Conclusions In the present study, PVA membranes containing cobalt salt were prepared for separation of cyclohexene from cyclohexene/cyclohexane mixtures. X-ray scattering experiments shows that the Co2+ salts can dissolve in the PVA membrane. And the optimal Co2+ concentration of ([Co2+ ]/[ OH]) is 0.75 (mol/mol). The complexation of Co2+ ion with cyclohexene molecules was stronger in the Co(CH3 COO)2 membrane than that in the Co(NO3 )2 membrane. As the Co2+ is coordination with cyclohexene in the feed rather than with cyclohexane, the degree of swelling of PVA membrane based on the polymer electrolytes in cyclohexene is greater than in cyclohexane. As the cobalt concentration of ([Co2+ ]/[ OH]) is 0.75 (mol/mol), the permeation flux of PVA membrane containing Co2+ increases 2.0–3.0 times and the separation factor increases 50 times compared with pure PVA membrane.
Acknowledgements The financial support of this project by the National Nature Science Fund of China (Grant No. 20276014) and the State Key Development Program for Basic Research of China (Grant No. 2003CB615700).
References
Fig. 8. Effect of feed temperature on separation performance of membrane ([Co2+ ]/[ OH], 0.75; cyclohexane mass fraction, 50 wt%).
[1] H. Matsuyama, K. Matsui, Effects of membrane thickness and membrane preparation condition on facilitated transport of CO2 through ionomer membrane, Sep. Purif. Technol. 17 (1999) 235. [2] W.S. Ho, D.C. Dalrymple, Facilitated transport of olefins in Ag+ containing polymer membranes, J. Membr. Sci. 91 (1994) 13. [3] D.L. Bryant, R.D. Noble, C.A. Koval, Facilitated transport separation of benzene and cyclohexane with poly(vinyl alcohol–AgNO3 ) membranes, J. Membr. Sci. 127 (1997) 161.
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[4] C.K. Park, B.K. Oh, M.J. Choi, Y.M. Lee, Separation of benzene/cyclohexane by pervaporation through chelate poly(vinyl alcohol)/poly(allyl amine) blend membrane, Polym. Bull. 33 (1994) 591. [5] J.Y. Kim, S.U. Hong, J.O. Won, Y.S. Kang, Molecular Model and analysis of glass transition temperatures for polymer–diluent–salt systems, Macromolecules 33 (2000) 3161. [6] S.U. Hong, C.K. Kim, Y.S. Kang, Measurement and analysis of propylene solubility in polymer electrolytes containing silver salts, Macromolecules 33 (2000) 7918.
[7] J.H. Kim, B.R. Min, C.K. Kim, Y.S. Kang, Role of transient crosslinks for transport properties in silver-polymer electrolytes, Macromolecules 34 (2001) 6052. [8] J.H. Kim, B.R. Min, J.O. Won, Y.S. Kang, Structure and coordination properties of facilitated olefin transport membrane consisting of cross-linked poly(vinyl alcohol) and silver hexafluoroantimorate, J. Polym. Sci. B: Polym. Phys. 42 (2004) 621.
Separation cyclohexene/cyclohexane mixtures with facilitated transport membrane of poly(vinyl alcohol)–Co2+ J.N. Shen a , L.G. Wu b,∗ , H.L. Chen a , C.J. Gao b a
Material Science and Chemical Engineering College of Zhejiang University, 310027 Hangzhou, PR China b Research and Development Center of Water Treatment Technology, SOA, 310012 Hangzhou, PR China Received 26 June 2004; received in revised form 3 January 2005; accepted 26 February 2005
Abstract Facilitated transport membranes based on the solid polymer electrolytes of poly(vinyl alcohol)–Co2+ (PVA–Co2+ ) blends were prepared. The membrane morphology was analyzed with X-ray scattering and DSC. The sorption behavior of cyclohexene, cyclohexane in the membranes were measured and the separation performance of the membrane were investigated for cyclohexene/cyclohexane mixtures. The result showed that Co2+ salts were highly soluble in PVA and disrupted the crystallinity of PVA. Compared with pure PVA, the cyclohexene saturated sorption capacity in the solid polymer electrolytes was improved a lot and reached 60 mg (cyclohexene)/g (dry membrane), while the cyclohexane saturated sorption capacity in the solid polymer electrolytes had little differences and was only 15 mg (cyclohexane)/g (dry membrane). Permeability flux of the membrane was 150 g/(m2 h) and a cyclohexene/cyclohexane separation factor was 60 when at 40 ◦ C and the optimal carrier content of the membrane ([Co2+ ]/[ OH]) was 0.75 (mol/mol) and cyclohexene mass fraction in the feed was 0.5. © 2005 Elsevier B.V. All rights reserved. Keywords: Facilitated transport; Membrane separation; Cyclohexene; Cyclohexane; Poly(vinyl alcohol)–Co2+ blend
1. Introduction Separation of organic–organic mixtures is one of the most important processes in chemical industry. Pervaporation membrane technology can be considered as an energyefficient alternative process to conventional technologies, such as distillation and extraction, especially the separation of azeotropic mixtures, isomeric components, close boiling point systems. For membrane processes must be costeffective, membranes should have high selectivity and high flux and also the membrane materials should be inexpensive. The membrane process using the concept of facilitated transport has been considered to be an energy-saving process [1]. Poly(vinyl alcohol) (PVA) membranes have been used for numerous separation process because of their low cost and mechanical stability. The poly(vinyl alcohol) cast with ∗
Corresponding author. Tel.: +86 571 88865762; fax: +86 571 88868427. E-mail address: wu [email protected] (L.G. Wu).
1383-5866/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2005.02.013
AgNO3 was reported by Ho and Dalrymple [2] for separation of 1-butene/n-butane. Bryant et al. [3] reported the facilitated transport membranes of poly(vinyl alcohol) containing silver ions as the carrier for separation of benzene/cyclohexane mixtures. Park et al. [4] investigated the chelate poly(vinyl alcohol)/poly(allyl amine) blend membrane for the separation of benzene/cyclohexane mixture. However, silver salts are sensitive to visible light and strongly oxidative, AgNO3 may reduce the Ag+ in the membrane to give particles of Ag. Co(NO3 )2 and Co(CH3 COO)2 is not sensitive to visible light, which may be more steady than the AgNO3 in the polymer and the Co2+ can also coordinate with olefin. Fig. 1 is a schematic diagram for describing the coordinating of cyclohexene in the feed and cobalt in the membrane. Metals are in the low oxidation state and often form a complex with acceptor ligands because of the opportunity they provide to lessen the electron density on the central metal atom. In this case, the interaction by a coordination can be occurred between the orbitals of the same symmetry and similar en-
J.N. Shen et al. / Separation and Purification Technology 45 (2005) 103–108
104
2.4. Swelling measurements The degree of swelling of membrane at a given time was defined by the following equation: Aτ =
Fig. 1. Schematic diagram of cobalt–cyclohexene coordination.
ergy. Therefore, the orbitals of cyclohexene can coordinate with 3d orbital of cobalt. In the present study, poly(vinyl alcohol) containing Co2+ facilitated transport for separation of cyclohexene from mixtures with cyclohexane was studied.
2. Experimental 2.1. Materials Poly(vinyl alcohol) (hydrolyzation degree 98–99%, average polymerization 1700), cobalt nitrate (Co(NO3 )2 ) and cobalt acetate (Co(CH3 COO)2 ) were of AR grade; cyclohexene and cyclohexane were of CP grade. CA ultra-filtration membranes were supported by the development center of water treatment technology of state ocean bureau China. 2.2. Membrane preparation Ten weight percent PVA was dissolved in boiling deionized water with stirring. Co(NO3 )2 and Co(CH3 COO)2 were added to PVA solution whiling stirring. The solution was cast on a CA ultrafiltration membrane to form composite membrane. Then, the composite membrane was dried at room temperature; the asymmetric membranes were treated in a dustless oven at 120 ◦ C for 2 h. The resulted membrane separation layer, which could be peeled off from the substrate, was measured to be ∼20 m.
mτ − m0 × 1000 (mg/g) mτ
where m0 and mτ are the weights of dry and solvent swollen membranes, respectively. The swelling experiments were carried out at constant temperature and the membranes were weighed at room temperature. The free liquid on the surface of the swollen membrane must be removed carefully by using filter paper before weighed.
2.5. Pervaporation measurements The pervaporation experiments were conducted with the apparatus is shown in Fig. 2. The membrane was placed in the stainless-steel permeation chamber, the membrane area in contact with the feed was 15.9 cm2 , the temperature was thermostatically controlled and the downstream pressure was 100 Pa. The permeation vapor was condensed by liquid nitrogen (N2 ), the composition of the permeation flux was analyzed by gas chromatograph. The permeation flux (J) and the separation factor (α) for all membranes were calculated according to the following equations: J=
g A × t
(2)
yi /yi xi /xj
(3)
αi/j =
where g is the permeation weight during the operation time, A the membrane area, and x and y are the weight fraction in the feed and permeate, respectively.
2.3. Membrane characterization X-ray scattering experiments were carried out for studying the solid polymer electrolytes morphology. The angle of diffraction (2θ) was varied from 0◦ to 65◦ . The effective distance between inter-segmental chains was determined by this technique. The spectrum was obtained by using a D/max-ra X-ray diffraction spectrometer. A Perkin-Elmer DSC-7 was used at a heating rate of 20 ◦ C/min under an N2 environment to measure the glass transition temperatures (Tg ) of the polymer electrolytes. Samples of PVA complexed with Co(CH3 COO)2 or Co(NO3 )2 were prepared with hydroxyl ( OH)-to-cobalt ratios of 20:1, 10:1, 6:1, 5:1, 4:1, 3:1 and 1:1.
(1)
Fig. 2. Schematic diagram of pervaporation experiment.
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3. Results and discussion 3.1. Membrane characterization X-ray scattering spectrum of different Co2+ concentration and various Co2+ salt kinds on the crystallinity are shown in Fig. 3a and b. The spectrum of pure PVA exhibits a strong crystalline peak centered at 2θ value round 20◦ . Weaker crystalline peaks are observed at 2θ value near 10◦ and 40◦ . The percent of crystallinity, calculated as the ratio of the area of the crystalline peaks to the total area under the crystalline peaks and the amorphous halo, is 52%. The crystallinity decreases strongly with increasing Co2+ concentration and is completely absent in the samples containing 50 wt% Co2+ or more. The inter-segmental d spacing in copolymer electrolytes was calculated using Braggs equation d = λ/2 sin θ with θ of the peak maximum. The d spacing of pure PVA film is 0.23 nm, which decreases gradually with the Co2+ salt concentration in both polymer electrolytes. The d spacing of the PVA–Co(NO3 )2 is smaller than that of the PVA–Co(CH3 COO)2 at a fixed amount of Co2+ salt in PVA. Another important fact is that NO3 − (62) group is heavier than CH3 COO− (59). Therefore, the main chain mobility will be reduced more in the PVA–Co(NO3 )2 system than in the PVA–Co(CH3 COO)2 . Fig. 4a shows DSC heating curves of PVA, PVA/Co(CH3 COO)2 and PVA/Co(NO3 )2 systems. As shown in Fig. 4a, the glass transition temperature (Tg ) of PVA increases with adding cobalt salt. Tg of the pure PVA is 68 ◦ C, the value of the glass transition temperature in the
Fig. 4. (a) DSC thermograms for PVA, PVA/Co(CH3 COO)2 and PVA/Co(NO3 )2 systems. (b) Variations of glass transition temperature as a function of salt concentration for PVA with Co(CH3 COO)2 and Co(NO3 ).
complexes of PVA with Co(CH3 COO)2 and Co(NO3 )2 for various mole ratios of [ OH]/[Co] 1:1 are, respectively, 80 and 90 ◦ C. The increase in Tg upon addition of metal salts is primarily attributable to both the transient cross-link of polymer segmental chains and the dangling of cobalt salts on the main chain. And the reduction in crystallinity may increase chain mobility, which may have less effect than the decrease in chain mobility due to the formation of Co2+ / OH linkages between neighboring chains and the dangling of cobalt salts on the main chain. So Tg increases a little and the crystallinity decreases upon addition of metal salts. The values of the glass transition temperature in the complexes of PVA with Co(CH3 COO)2 and Co(NO3 )2 for various mole ratios of [ OH]/[Co2+ ] are presented in Fig. 4b. In the presence of cobalt salts, the values of Tg in both systems increase gradually, indicating cobalt ions coordinating to the hydroxyl of PVA and restricting chain mobility of PVA. Tg reaches the maximum with an increasing amount of cobalt salts and then it keeps constant above a certain salt content, which is a common feature of polymer/metal salt complexes [5–8]. 3.2. Performance study of solid polymer electrolytes membranes
Fig. 3. X-ray diffraction spectrum of PVA membrane: containing (a) various cobalt concentration; (b) different Co2+ salt.
3.2.1. Degree of swelling The degree swelling (DS) of cyclohexene in pure PVA and PVA–Co2+ electrolytes were measured at a constant temper-
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linearly with τ 1/2 demonstrating the DS of cyclohexane in PVA and PVA–Co2+ electrolytes can be readily described by Henry mode. The DS of PVA/Co(CH3 COO)2 membrane is larger than PVA/Co(NO3 )2 . The DS of cyclohexene in the membrane of various Co2+ concentration is shown in Fig. 5c. The DS of cyclohexene in the PVA/Co(CH3 COO)2 membrane increases with increasing the Co2+ concentration. This is due to the fixed sites, which increases with increasing the Co2+ concentration.
Fig. 5. (a) The degree of swelling of cyclohexene in pure PVA and PVA–Co2+ electrolytes membrane. (b) The degree of swelling of cyclohexane in pure PVA and PVA–Co2+ electrolytes membrane. (c) The degree of swelling of cyclohexene in PVA/Co(CH3 COO)2 membrane of various Co2+ ions concentration.
ature, the results are presented in Fig. 5a. The cyclohexene solubility in pure PVA increases linearly with τ 1/2 , demonstrating the solubility of cyclohexene in PVA can be readily described by a simple Henry mode. The curves of the cyclohexene solubility in cobalt polymer electrolytes become concave to the ordinate with the τ 1/2 , indicating the presence of the second mode sorption. The second mode sorption is evidently due to the interaction between cobalt ions and cyclohexene molecules and it can also be described by a simple Langmuir model. And the DS of cyclohexene in PVA–Co2+ electrolytes membranes increases with increasing the Co2+ ions concentration. However, the mechanical power of the membrane decreases with further increasing the Co2+ ions concentration. The DS of cyclohexane in pure PVA and PVA–Co2+ electrolytes were shown in Fig. 5b. The DS of cyclohexane in pure PVA and PVA–Co2+ electrolytes increases
3.2.2. Pervaporation results The separation of cyclohexene/cyclohexane mixtures (1:1, v/v) was performed to evaluate the facilitated transport effect of cobalt ions in PVA. Fig. 6 shows the effect of molar ratio [Co2+ ]/[ OH] on permeation flux of cyclohexene and the selectivity for cyclohexene/cyclohexane. The permeation flux of cyclohexene increases with increasing molar ratio of [Co2+ ]/[ OH] up to 1:1. However, the selectivity of cyclohexene over cyclohexane increases with increasing molar ratio of [Co2+ ]/[ OH] up to 3:4 and decreases with further increasing the molar ratio of [Co2+ ]/[ OH] which may due to the salt solubility gets to saturated value at the molar ratio of [Co2+ ]/[ OH] up to 3:4. The membrane may form the defect with farther increasing the cobalt concentration. Such selectivity on the molar ratio of [Co2+ ]/[ OH] implies that the coordination environment of cobalt ions in PVA has a significant effect on the facilitated transport of olefins. Fig. 5 also shows the effect of anion size on cobalt ions electrolytes membrane for separation cyclohexene/cyclohexane mixtures. The size of CH3 COO− (0.213 nm) group is larger than NO3 − (0.189 nm). The larger size of anion has the tendency of cobalt salts to form complexes and the lattice energy of Co(CH3 COO)2 is smaller than Co(NO3 )2 , which indicates that interaction of cobalt ions with NO3 − is stronger than with CH3 COO− , consequently that the complexation of cobalt (Co2+ ) with cyclohexene molecules may be stronger in the Co(CH3 COO)2 membrane than that in the Co(NO3 )2 membrane, resulting in cyclohexene permeation flux and selectivity of PVA–Co(CH3 COO)2 electrolytes membrane is better than PVA–Co(NO3 )2 .
Fig. 6. Effect of carrier content on separation performance of membrane (testing temperature, 40 ◦ C; cyclohexane mass fraction, 50 wt%).
J.N. Shen et al. / Separation and Purification Technology 45 (2005) 103–108
Fig. 7. Effect of cyclohexene mass fraction in the feed on separation performance of membrane (testing temperature, 40 ◦ C; [Co2+ ]/[ OH], 0.75).
The effect of cyclohexene mass fraction in the feed on separation performance of membrane was performed. The results are shown in Fig. 7. The separation performance of the [Co2+ ]/[ OH] was 3:4 in PVA–Co(NO3 )2 and PVA–Co(CH3 COO)2 electrolytes membrane was investigated at constant temperature of 40 ◦ C. The permeation flux of the cyclohexene increases and the selectivity decreases with increasing cyclohexene fraction in the feed because more cyclohexene can coordinate with cobalt ions with increasing the cyclohexene mass fraction. The decline in the selectivity is explained by the plasticization effect of cyclohexene on the membrane. With increasing cyclohexene content, the membrane is swollen and the relaxed polymer chains arrows for enhanced permeability of cyclohexane. The effect of temperature on the separation characterization is shown in Fig. 8. The permeation flux of the cyclohexene increases and the selectivity decreases with increasing feed temperature. For non-facilitated transport membrane system, increasing temperature generally leads to an increase in the diffusivity and a decrease in the solubility and consequently the permeation flux may increase or decrease depending on the relative significance of the thermal effects of the two aspects. For facilitated transport membrane systems, it is anticipated that a temperature change will affect both kinetics and equilibria of the complexation reactions; it is difficult to formulate a simple explanation on the temperature depen-
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dence of permeability. Unfortunately, study on the temperature effects on the general performance of cobalt containing polymer electrolyte membranes for olefin separation are lacking the open literature, more studies are required to develop a better understanding of how the operating temperature influences the membrane performance. In this paper, as temperature increases, the cyclohexane permeation flux increases (the results is not shown in the manuscript) due to the increase in the diffusivity of cyclohexane, which is transported by the simple solution–diffusion mechanism. Although the cyclohexene flux also increased, the extent of the increase on permeation flux was lower than that of cyclohexane. This suggests that the coordination reaction between cobalt ions and cyclohexene may be exothermic, and the decrease in the equilibrium constant of the reaction with increasing temperature may compensate the increase in the diffusivity. So the flux of cyclohexene increases and α decreases with temperature.
4. Conclusions In the present study, PVA membranes containing cobalt salt were prepared for separation of cyclohexene from cyclohexene/cyclohexane mixtures. X-ray scattering experiments shows that the Co2+ salts can dissolve in the PVA membrane. And the optimal Co2+ concentration of ([Co2+ ]/[ OH]) is 0.75 (mol/mol). The complexation of Co2+ ion with cyclohexene molecules was stronger in the Co(CH3 COO)2 membrane than that in the Co(NO3 )2 membrane. As the Co2+ is coordination with cyclohexene in the feed rather than with cyclohexane, the degree of swelling of PVA membrane based on the polymer electrolytes in cyclohexene is greater than in cyclohexane. As the cobalt concentration of ([Co2+ ]/[ OH]) is 0.75 (mol/mol), the permeation flux of PVA membrane containing Co2+ increases 2.0–3.0 times and the separation factor increases 50 times compared with pure PVA membrane.
Acknowledgements The financial support of this project by the National Nature Science Fund of China (Grant No. 20276014) and the State Key Development Program for Basic Research of China (Grant No. 2003CB615700).
References
Fig. 8. Effect of feed temperature on separation performance of membrane ([Co2+ ]/[ OH], 0.75; cyclohexane mass fraction, 50 wt%).
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